Handbook of Pneumatic Conveying Engineering

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Handbook of Pneumatic Conveying Engineering

Transcript of Handbook of Pneumatic Conveying Engineering

Page 1: Handbook of Pneumatic Conveying Engineering
Page 2: Handbook of Pneumatic Conveying Engineering

Handbookof PneumaticConveyingEngineering

David MillsUniversity of NewcastleCallaghan, New South Wales, Australia

Mark G. JonesUniversity of NewcastleCallaghan, New South Wales, Australia

Vijay K. AgarwalIndian Institute of TechnologyHauz Khaas, New Delhi, India

M A R C E L

MARCEL DEKKER, INC. NEW YORK • BASEL

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Although great care has been taken to provide accurate and current information, neither theauthor(s) nor the publisher, nor anyone else associated with this publication, shall be liablefor any loss, damage, or liability directly or indirectly caused or alleged to be caused by thisbook. The material contained herein is not intended to provide specific advice or recom-mendations for any specific situation.

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MECHANICAL ENGINEERINGA Series of Textbooks and Reference Books

Founding Editor

L. L. Faulkner

Columbus Division, Battelle Memorial Instituteand Department of Mechanical Engineering

The Ohio State UniversityColumbus, Ohio

1. Spring Designer's Handbook, Harold Carlson2. Computer-Aided Graphics and Design, Daniel L. Ryan3. Lubrication Fundamentals, J. George Wills4. Solar Engineering for Domestic Buildings, William A. Himmelman5. Applied Engineering Mechanics: Statics and Dynamics, G. Boothroyd and

C. Poli6. Centrifugal Pump Clinic, Igor J. Karassik7. Computer-Aided Kinetics for Machine Design, Daniel L. Ryan8. Plastics Products Design Handbook, Part A: Materials and Components; Part

B: Processes and Design for Processes, edited by Edward Miller9. Turbomachinery: Basic Theory and Applications, Earl Logan, Jr.

10. Vibrations of Shells and Plates, Werner Soedel11. Flat and Corrugated Diaphragm Design Handbook, Mario Di Giovanni12. Practical Stress Analysis in Engineering Design, Alexander Blake13. An Introduction to the Design and Behavior of Bolted Joints, John H.

Bickford14. Optimal Engineering Design: Principles and Applications, James N. Siddall15. Spring Manufacturing Handbook, Harold Carlson16. Industrial Noise Control: Fundamentals and Applications, edited by Lewis H.

Bell17. Gears and Their Vibration: A Basic Approach to Understanding Gear Noise,

J. Derek Smith18. Chains for Power Transmission and Material Handling: Design and Appli-

cations Handbook, American Chain Association19. Corrosion and Corrosion Protection Handbook, edited by Philip A.

Schweitzer20. Gear Drive Systems: Design and Application, Peter Lynwander21. Controlling In-Plant Airborne Contaminants: Systems Design and Cal-

culations, John D. Constance22. CAD/CAM Systems Planning and Implementation, Charles S. Knox23. Probabilistic Engineering Design: Principles and Applications, James N.

Siddall24. Traction Drives: Selection and Application, Frederick W. Heilich III and

Eugene E. Shube25. Finite Element Methods: An Introduction, Ronald L. Huston and Chris E.

Passerello

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26. Mechanical Fastening of Plastics: An Engineering Handbook, Brayton Lincoln,Kenneth J. Gomes, and James F. Braden

27. Lubrication in Practice: Second Edition, edited by W. S. Robertson28. Principles of Automated Drafting, Daniel L. Ryan29. Practical Seal Design, edited by Leonard J. Martini30. Engineering Documentation for CAD/CAM Applications, Charles S. Knox31. Design Dimensioning with Computer Graphics Applications, Jerome C.

Lange32. Mechanism Analysis: Simplified Graphical and Analytical Techniques, Lyndon

O. Barton33. CAD/CAM Systems: Justification, Implementation, Productivity Measurement,

Edward J. Preston, George W. Crawford, and Mark E. Coticchia34. Sfeam Plant Calculations Manual, V. Ganapathy35. Design Assurance for Engineers and Managers, John A. Burgess36. Heat Transfer Fluids and Systems for Process and Energy Applications,

Jasbir Singh37. Potential Flows: Computer Graphic Solutions, Robert H. Kirchhoff38. Computer-Aided Graphics and Design: Second Edition, Daniel L. Ryan39. Electronically Controlled Proportional Valves: Selection and Application,

Michael J. Tonyan, edited by Tobi Goldoftas40. Pressure Gauge Handbook, AMETEK, U.S. Gauge Division, edited by Philip

W. Harland41. Fabric Filtration for Combustion Sources: Fundamentals and Basic Tech-

nology, R. P. Donovan42. Design of Mechanical Joints, Alexander Blake43. CAD/CAM Dictionary, Edward J. Preston, George W. Crawford, and Mark E.

Coticchia44. Machinery Adhesives for Locking, Retaining, and Sealing, Girard S. Haviland45. Couplings and Joints: Design, Selection, and Application, Jon R. Mancuso46. Shaft Alignment Handbook, John Piotrowski47. BASIC Programs for Steam Plant Engineers: Boilers, Combustion, Fluid

Flow, and Heat Transfer, V. Ganapathy48. Solving Mechanical Design Problems with Computer Graphics, Jerome C.

Lange49. Plastics Gearing: Selection and Application, Clifford E. Adams50. Clutches and Brakes: Design and Selection, William C. Orthwein51. Transducers in Mechanical and Electronic Design, Harry L. Trietley52. Metallurgical Applications of Shock-Wave and High-Strain-Rate Phenom-

ena, edited by Lawrence E. Murr, Karl P. Staudhammer, and Marc A.Meyers

53. Magnesium Products Design, Robert S. Busk54. How to Integrate CAD/CAM Systems: Management and Technology, William

D. Engelke55. Cam Design and Manufacture: Second Edition; with cam design software

for the IBM PC and compatibles, disk included, Preben W. Jensen56. Solid-State AC Motor Controls: Selection and Application, Sylvester Campbell57. Fundamentals of Robotics, David D. Ardayfio58. Belt Selection and Application for Engineers, edited by Wallace D. Erickson59. Developing Three-Dimensional CAD Software with the IBM PC, C. Stan Wei60. Organizing Data for CIM Applications, Charles S. Knox, with contributions

by Thomas C. Boos, Ross S. Culverhouse, and Paul F. Muchnicki

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61. Computer-Aided Simulation in Railway Dynamics, by Rao V. Dukkipati andJoseph R. Amyot

62. Fiber-Reinforced Composites: Materials, Manufacturing, and Design, P. K.Mallick

63. Photoelectric Sensors and Controls: Selection and Application, Scott M.Juds

64. Finite Element Analysis with Personal Computers, Edward R. Champion,Jr., and J. Michael Ensminger

65. Ultrasonics: Fundamentals, Technology, Applications: Second Edition,Revised and Expanded, Dale Ensminger

66. Applied Finite Element Modeling: Practical Problem Solving for Engineers,Jeffrey M. Steele

67. Measurement and Instrumentation in Engineering: Principles and BasicLaboratory Experiments, Francis S. Tse and Ivan E. Morse

68. Centrifugal Pump Clinic: Second Edition, Revised and Expanded, Igor J.Karassik

69. Practical Stress Analysis in Engineering Design: Second Edition, Revisedand Expanded, Alexander Blake

70. An Introduction to the Design and Behavior of Bolted Joints: SecondEdition, Revised and Expanded, John H. Bickford

71. High Vacuum Technology: A Practical Guide, Marsbed H. Hablanian72. Pressure Sensors: Selection and Application, Duane Tandeske73. Zinc Handbook: Properties, Processing, and Use in Design, Frank Porter74. Thermal Fatigue of Metals, Andrzej Weronski and Tadeusz Hejwowski75. Classical and Modern Mechanisms for Engineers and Inventors, Preben W.

Jensen76. Handbook of Electronic Package Design, edited by Michael Pecht77. Shock-Wave and High-Strain-Rate Phenomena in Materials, edited by Marc

A. Meyers, Lawrence E. Murr, and Karl P. Staudhammer78. Industrial Refrigeration: Principles, Design and Applications, P. C. Koelet79. Applied Combustion, Eugene L. Keating80. Engine Oils and Automotive Lubrication, edited by Wilfried J. Bartz81. Mechanism Analysis: Simplified and Graphical Techniques, Second Edition,

Revised and Expanded, Lyndon O. Barton82. Fundamental Fluid Mechanics for the Practicing Engineer, James W.

Murdock83. Fiber-Reinforced Composites: Materials, Manufacturing, and Design, Second

Edition, Revised and Expanded, P. K. Mallick84. Numerical Methods for Engineering Applications, Edward R. Champion, Jr.85. Turbomachinery: Basic Theory and Applications, Second Edition, Revised

and Expanded, Earl Logan, Jr.86. Vibrations of Shells and Plates: Second Edition, Revised and Expanded,

Werner Soedel87. Steam Plant Calculations Manual: Second Edition, Revised and Expanded,

V. Ganapathy88. Industrial Noise Control: Fundamentals and Applications, Second Edition,

Revised and Expanded, Lewis H. Bell and Douglas H. Bell89. Finite Elements: Their Design and Performance, Richard H. MacNeal90. Mechanical Properties of Polymers and Composites: Second Edition, Re-

vised and Expanded, Lawrence E. Nielsen and Robert F. Landel91. Mechanical Wear Prediction and Prevention, Raymond G. Bayer

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92. Mechanical Power Transmission Components, edited by David W. Southand Jon R. Mancuso

93. Handbook of Turbomachinery, edited by Earl Logan, Jr.94. Engineering Documentation Control Practices and Procedures, Ray E.

Monahan95. Refractory Linings Thermomechanical Design and Applications, Charles A.

Schacht96. Geometric Dimensioning and Tolerancing: Applications and Techniques for

Use in Design, Manufacturing, and Inspection, James D. Meadows97. An Introduction to the Design and Behavior of Bolted Joints: Third Edition,

Revised and Expanded, John H. Bickford98. Shaft Alignment Handbook: Second Edition, Revised and Expanded, John

Piotrowski99. Computer-Aided Design of Polymer-Matrix Composite Structures, edited by

Suong Van Hoa100. Friction Science and Technology, Peter J. Blau101. Introduction to Plastics and Composites: Mechanical Properties and Engi-

neering Applications, Edward Miller102. Practical Fracture Mechanics in Design, Alexander Blake103. Pump Characteristics and Applications, Michael W. Volk104. Optical Principles and Technology for Engineers, James E. Stewart105. Optimizing the Shape of Mechanical Elements and Structures, A. A. Seireg

and Jorge Rodriguez106. Kinematics and Dynamics of Machinery, Vladimir Stejskal and Michael

Valasek107. Shaft Seals for Dynamic Applications, Les Horve108. Reliability-Based Mechanical Design, edited by Thomas A. Cruse109. Mechanical Fastening, Joining, and Assembly, James A. Speck110. Turbomachinery Fluid Dynamics and Heat Transfer, edited by Chunill Hah111. High-Vacuum Technology: A Practical Guide, Second Edition, Revised and

Expanded, Marsbed H. Hablanian112. Geometric Dimensioning and Tolerancing: Workbook and Answerbook,

James D. Meadows113. Handbook of Materials Selection for Engineering Applications, edited by G.

T. Murray114. Handbook of Thermoplastic Piping System Design, Thomas Sixsmith and

Reinhard Hanselka115. Practical Guide to Finite Elements: A Solid Mechanics Approach, Steven M.

Lepi116. Applied Computational Fluid Dynamics, edited by Vijay K. Garg117. Fluid Sealing Technology, Heinz K. Muller and Bernard S. Nau118. Friction and Lubrication in Mechanical Design, A. A. Seireg119. Influence Functions and Matrices, Yuri A. Melnikov120. Mechanical Analysis of Electronic Packaging Systems, Stephen A.

McKeown121. Couplings and Joints: Design, Selection, and Application, Second Edition,

Revised and Expanded, Jon R. Mancuso122. Thermodynamics: Processes and Applications, Earl Logan, Jr.123. Gear Noise and Vibration, J. Derek Smith124. Practical Fluid Mechanics for Engineering Applications, John J. Bloomer125. Handbook of Hydraulic Fluid Technology, edited by George E. Totten126. Heat Exchanger Design Handbook, T. Kuppan

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127. Designing for Product Sound Quality, Richard H. Lyon128. Probability Applications in Mechanical Design, Franklin E. Fisher and Joy R.

Fisher129. Nickel Alloys, edited by Ulrich Heubner130. Rotating Machinery Vibration: Problem Analysis and Troubleshooting,

Maurice L. Adams, Jr.131. Formulas for Dynamic Analysis, Ronald L. Huston and C. Q. Liu132. Handbook of Machinery Dynamics, Lynn L. Faulkner and Earl Logan, Jr.133. Rapid Prototyping Technology. Selection and Application, Kenneth G.

Cooper134. Reciprocating Machinery Dynamics: Design and Analysis, Abdulla S.

Rangwala135. Maintenance Excellence: Optimizing Equipment Life-Cycle Decisions, edi-

ted by John D. Campbell and Andrew K. S. Jardine136. Practical Guide to Industrial Boiler Systems, Ralph L. Vandagriff137. Lubrication Fundamentals: Second Edition, Revised and Expanded, D. M.

Pirro and A. A. Wessol138. Mechanical Life Cycle Handbook: Good Environmental Design and Manu-

facturing, edited by Mahendra S. Hundal139. Micromachining of Engineering Materials, edited by Joseph McGeough140. Control Strategies for Dynamic Systems: Design and Implementation, John

H. Lumkes, Jr.141. Practical Guide to Pressure Vessel Manufacturing, Sunil Pullarcot142. Nondestructive Evaluation: Theory, Techniques, and Applications, edited by

Peter J.Shull143. D;'ese/ Engine Engineering: Thermodynamics, Dynamics, Design, and

Control, Andrei Makartchouk144. Handbook of Machine Tool Analysis, loan D. Marinescu, Constantin Ispas,

and Dan Boboc145. Implementing Concurrent Engineering in Small Companies, Susan Carlson

Skalak146. Practical Guide to the Packaging of Electronics: Thermal and Mechanical

Design and Analysis, All Jamnia147. Bearing Design in Machinery: Engineering Tribology and Lubrication,

Avraham Harnoy148. Mechanical Reliability Improvement: Probability and Statistics for Experi-

mental Testing, R. E. Little149. Industrial Boilers and Heat Recovery Steam Generators: Design, Ap-

plications, and Calculations, V. Ganapathy150. The CAD Guidebook: A Basic Manual for Understanding and Improving

Computer-Aided Design, Stephen J. Schoonmaker151. Industrial Noise Control and Acoustics, Randall F. Barren152. Mechanical Properties of Engineered Materials, Wole Soboyejo153. Reliability Verification, Testing, and Analysis in Engineering Design, Gary S.

Wasserman154. Fundamental Mechanics of Fluids: Third Edition, I. G. Currie155. Intermediate Heat Transfer, Kau-Fui Vincent Wong156. HVAC Water Chillers and Cooling Towers: Fundamentals, Application, and

Operation, Herbert W. Stanford III157. Gear Noise and Vibration: Second Edition, Revised and Expanded, J.

Derek Smith

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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158. Handbook of Turbomachinery: Second Edition, Revised and Expanded,edited by Earl Logan, Jr., and Ramendra Roy

159. Piping and Pipeline Engineering: Design, Construction, Maintenance, Integ-rity, and Repair, George A. Antaki

160. Turbomachinery: Design and Theory, Rama S. R. Gorla and Aijaz AhmedKhan

161. Target Costing: Market-Driven Product Design, M. Bradford Clifton, HenryM. B. Bird, Robert E. Albano, and Wesley P. Townsend

162. Fluidized Bed Combustion, Simeon N. Oka163. Theory of Dimensioning: An Introduction to Parameterizing Geometric

Models, Vijay Srinivasan164. Handbook of Mechanical Alloy Design, edited by George E. Totten, Lin Xie,

and Kiyoshi Funatani165. Structural Analysis of Polymeric Composite Materials, Mark E. Turtle166. Modeling and Simulation for Material Selection and Mechanical Design,

edited by George E. Totten, Lin Xie, and Kiyoshi Funatani167. Handbook of Pneumatic Conveying Engineering, David Mills, Mark G.

Jones, and Vijay K. Agarwal

Additional Volumes in Preparation

Mechanical Wear Fundamentals and Testing: Second Edition, Revised andExpanded, Raymond G. Bayer

Engineering Design for Wear: Second Edition, Revised and Expanded,Raymond G. Bayer

Clutches and Brakes: Design and Selection, Second Edition, William C.Orthwein

Progressing Cavity Pumps, Downhole Pumps, and Mudmotors, Lev Nelik

Mechanical Engineering Software

Spring Design with an IBM PC, Al Dietrich

Mechanical Design Failure Analysis: With Failure Analysis System Softwarefor the IBM PC, David G. Ullman

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 10: Handbook of Pneumatic Conveying Engineering

To the memory of our colleaguePredrag Marjanovic

who died suddenly in September 2001

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Preface

Pneumatic conveying is widely used for the transport of dry bulk particulate materials.However, there is considerable misunderstanding on exactly how materials are con-veyed and what velocity is required to prevent pipeline blockage. In this handbookdilute and dense phase conveying are discussed and a detailed review is given of vari-ous positive pressure and vacuum systems. Extensive consideration has also beengiven to the numerous means available for feeding materials into pipelines for high,low, and negative pressure systems, as well as to evaluation of major components suchas blowers, compressors, exhausters, filters, and the multitude of valves employed.

Specification of air requirements is critical to the successful operation of anysystem. Air, or any other gas used, is compressible and so great care must be takenin evaluating velocities. All the models required for system specification havebeen developed using U.S. engineering units. Chapter 9 is entirely devoted tostepped pipelines and includes a number of first approximation design methods.This material should be invaluable in feasibility studies where a quick check onpower requirements and operating costs may be required.

Many industries have processes that involve the transport of a wide varietyof materials conveyed in powdered and granular form. Bulk materials are con-veyed in the food, chemical, mining, agriculture, pharmaceutical, metals, paint andrubber industries, among others. A number of chapters are therefore devoted spe-cifically to different industries and typical conveying data for various materialsconveyed. Those materials include coal and fly ash, polyethylene and soda ash,flour and sugar, iron powder, cement, alumina, and drilling mud powders.

The issue of bends in pipelines has been addressed with extensive informa-tion on pressure and velocity profiles, equivalent length, location and geometry,and the influence of bends on material degradation and their vulnerability to ero-

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 12: Handbook of Pneumatic Conveying Engineering

vi Preface

sive wear. Similar consideration is given to flows in vertical pipelines, both up anddown, and the use of flexible hose. System capabilities in terms of maximum op-erating pressure, conveying distance, and material flow rates are also discussed.

Engineers who commission, operate, and maintain pneumatic conveying sys-tems will find this book to be a valuable resource. Pipeline blockages and systemsnot capable of achieving the desired material flow rate are common. Step-by-stepprocedures are presented to identify problems and which operating parameters needto be adjusted to optimize system performance. The conveying of friable and abra-sive materials is particularly problematic in pneumatic conveying systems, but thereare numerous ways these problems can be minimized.

Pneumatic conveying is a subject that tends to be neglected in educationalprovision. Engineers required to design, operate, and maintain these complex sys-tems may have received little more than an hour or two of lectures on the subjectduring a three- or four-year engineering degree course. Nevertheless, they are ex-pected to take on the responsibility for these systems when working in industry.There is clearly a need for a book on this subject and it is hoped that this text willhelp to fill the curriculum gap in this very important branch of engineering.

A vast amount of personal hands-on experience is required to address thissubject, and I have asked my former students, Mark Jones and Vijay Agarwal,who are actively involved in the field, to join me. The point of reference for us allwas Thames Polytechnic in London, where Stan Mason was head of the Depart-ment of Mechanical Engineering. He had the foresight to establish pneumatic con-veying as a major research area in the department.

The authors would have included Predrag Marjanovic, who also studiedpneumatic conveying with me at Thames Polytechnic and was later appointedProfessor at Glasgow Caledonian University. Sadly, Predrag died a few weeksbefore work was started on this book and it is dedicated to his memory.

A practical book, by necessity, has many diagrams and graphs, and we ac-knowledge Neeti Rajput for her excellent work in preparing the material. I wouldlike to thank my wife, Philippa, who contributed her IT expertise and enabled me toprocess this work. We also thank Vijay's wife, Sangeeta, Mark's wife, Jane, and ourfamilies for their forbearance.

David Mills

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 13: Handbook of Pneumatic Conveying Engineering

Contents

Preface

1. Types of Pneumatic Conveying Systems

2. Feeding Devices

3. System Components

4. Gas-Solid Flows

5. Air Requirements

6. Air Only Data

7. Conveyed Material Influences

8. Pipeline Material, Orientation, and Bends

9. Stepped Pipeline Systems

10. Pneumatic Conveying of Coal and Ash

11. Pneumatic Conveying of Food and Chemicals

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12. Pneumatic Conveying in the Aluminum Industry

13. Conveying of Cement and Drilling Mud Powders

14. Conveying of High Density and Other Materials

15. System Design Using Conveying Data

16. Quick Check Design Methods

17. Innovatory Conveying Systems

18. Fluidized Motion Conveying Systems

19. Commissioning and Throughput Problems

20. Erosive Wear Problems

21. Material Degradation Problems

22. Health and Safety Issues

23. Pneumatic Conveying Test Facilities

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 15: Handbook of Pneumatic Conveying Engineering

1Types of Pneumatic ConveyingSystems

1 INTRODUCTION

Pneumatic conveying systems are basically quite simple and are eminently suit-able for the transport of powdered and granular materials in factory, site and plantsituations. The system requirements are a source of compressed gas, usually air, afeed device, a conveying pipeline and a receiver to disengage the conveyed mate-rial and carrier gas. The system is totally enclosed, and if it is required, the systemcan operate entirely without moving parts coming into contact with the conveyedmaterial.

High, low or negative pressures can be used to convey materials. Forhygroscopic materials dry air can be used, and for potentially explosive materialsan inert gas such as nitrogen can be employed. A particular advantage is thatmaterials can be fed into reception vessels maintained at a high pressure if required.

1.1 System Flexibility

With a suitable choice and arrangement of equipment, materials can be conveyedfrom a hopper or silo in one location to another location some distance away. Con-siderable flexibility in both plant layout and operation are possible, such that multi-ple point feeding can be made into a common line, and a single line can be dis-charged into a number of receiving hoppers. With vacuum systems, materials canbe picked up from open storage or stockpiles, and they are ideal for clearing dustaccumulations and spillages. Pipelines can run horizontally, as well as vertically

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 16: Handbook of Pneumatic Conveying Engineering

Chapter 1

up and down, and with bends in the pipeline any combination of orientations canbe accommodated in a single pipeline run. Material flow rates can be controlledeasily and monitored to continuously check input and output, and most systemscan be arranged for completely automatic operation.

Pneumatic conveying systems are particularly versatile. A very wide range ofmaterials can be handled and they are totally enclosed by the system and pipeline.This means that potentially hazardous materials can be conveyed quite safely.There is minimal risk of dust generation and so these systems generally meet therequirements of any local Health and Safety legislation with little or no difficulty.

Pneumatic conveying plants take up little floor space and the pipeline can beeasily routed up walls, across roofs or even underground to avoid any existingequipment or structures. Pipe bends in the conveying line provide this flexibility,but they will add to the overall resistance of the pipeline. Bends can also add toproblems of particle degradation if the conveyed material is friable, and suffer fromerosive wear if the material is abrasive.

1.2 Industries and Materials

A wide variety of materials are handled in powdered and granular form, and alarge number of different industries have processes that involve their transfer andstorage. Some of the industries in which bulk materials are conveyed include agri-culture, mining, chemical, Pharmaceuticals, paint manufacture, and metal refiningand processing. In agriculture very large tonnages of harvested materials such asgrain and rice are handled, as well as processed materials such as animal feed pel-lets. Fertilizers represent a large allied industry with a wide variety of materials.

A vast range of food products from flour to sugar and tea to coffee areconveyed pneumatically in numerous manufacturing processes. Confectionery is anindustry in which many of these materials are handled. In the oil industry finepowders such as barite, cement and bentonite are used for dril l ing purposes. Inmining and quarrying, lump coal and crushed ores and minerals are conveyed.Pulverized coal and ash are both handled in very large quantities in thermal powerplants for the generation of electricity.

In the chemical industries materials include soda ash, polyethylene, PVC andpolypropylene in a wide variety of forms from fine powders to pellets. Sand is usedin foundries and glass manufacture, and cement and alumina are other materialsthat are conveyed pneumatically in large tonnages in a number of differentindustries.

1.3 Mode of Conveying

Much confusion exists over how materials are conveyed through a pipeline and tothe terminology given to the mode of flow. First it must be recognized that materi-als can either be conveyed in batches through a pipeline, or they can be conveyedon a continuous basis, 24 hours a day if necessary. In batch conveying the materialmay be conveyed as a single plug if the batch size is relatively small.

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Page 17: Handbook of Pneumatic Conveying Engineering

System Types 3

1.3.1 Dilute Phase

For continuous conveying, and batch conveying if the batch size is large, twomodes of conveying are recognized. If the material is conveyed in suspension inthe air through the pipeline it is referred to as dilute phase conveying. If the mate-rial is conveyed at low velocity in a non-suspension mode, through all or part ofthe pipeline, it is referred to as dense phase conveying. Almost any material can beconveyed in dilute phase, suspension flow through a pipeline, regardless of theparticle size, shape or density.

1.3.2 Dense Phase

In dense phase conveying two modes of flow are recognized. One is moving bedflow, in which the material is conveyed in dunes on the bottom of the pipeline, oras a pulsatile moving bed. The other mode is slug or plug type flow, in which thematerial is conveyed as full bore plugs separated by air gaps. Moving bed flow isonly possible in a conventional conveying system if the material to be conveyedhas good air retention characteristics. Plug type flow is only possible in a conven-tional conveying system if the material has good permeability.

1.3.3 Conveying Air Velocity

For dilute phase conveying a relatively high value of conveying air velocity mustbe maintained. This is typically in the region of 2400 ft/min for a very fine pow-der, to 3200 ft/min for a fine granular material, and beyond for larger particles andhigher density materials. For dense phase conveying, air velocities can be down to600 ft/min, and lower in certain circumstances. Because of the fine particle sizerequired to provide the necessary air retention, particle density does not have sucha significant effect on the minimum value of conveying air velocity in moving bedtype dense phase conveying.

1.3.4 Solids Loading Ratio

The solids loading ratio, or phase density, is a useful parameter in helping to visu-alize the flow. This is the ratio of the mass flow rate of the material conveyed di-vided by the mass flow rate of the air used to convey the material. It is expressedin a dimensionless form. For dilute phase, maximum values that can be achievedare typically of the order of 15, although this can be higher if the conveying dis-tance is short and the conveying line pressure drop is high.

For moving bed flows, solids loading ratios of well over 100 can be achievedif materials are conveyed with pressure gradients of the order of 10 lbf/in2 per 100foot of horizontal pipeline. For plug type flows the use of solids loading ratio is notso appropriate, for as the materials have to be very permeable, maximum values areonly of the order of about 30. Despite the low value of solids loading ratio,materials can be reliably conveyed at velocities of 600 ft/min and below in plugtype flow.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 18: Handbook of Pneumatic Conveying Engineering

Chapter 1

2 SYSTEM TYPES

A wide range of pneumatic conveying systems are available, and they are all gen-erally suitable for the conveying of dry bulk particulate materials. The majority ofsystems are conventional, continuously operating, open systems, in a fixed loca-tion. To suit the material being conveyed, or the process, however, innovatory,batch operating and closed systems are commonly used. Many of these systemscan be either positive or negative pressure in operation, or a combination of thetwo. In this review some of the more common systems are presented.

The problem of system selection is illustrated in Figure 1.1. This shows thecombinations that are possible for conventional pneumatic conveying systems witha single air source. Only system types are presented in detail, with positivepressure, vacuum, and combined positive and negative pressure systemsconsidered, in relation to both open and closed systems.

With such a wide range and choice of system types, a useful starting point isto consider the alternatives in pair groupings:

Open and closed systems Open systems are the norm for pneumaticconveying, particularly when conveying with air. Closed systems wouldonly be employed for very specific circumstances, such as with highlytoxic and potentially explosive materials.

Positive pressure and negative pressure systems Materials can besucked as well as blown and so either pressure or vacuum can be em-ployed for pneumatic conveying. This is often a matter of company or per-sonal preference.

l ~ i Fixed and mobile systems The majority of pneumatic conveying sys-tems are in fixed locations and so this is not identified as a particular case.A variety of mobile systems are available for specific duties.

SystemType

Mode ofOperation

OperatingPressure

Open Closed

Combined Negative Positive PositivePressure Pressure Pressure

Continuous

Low

Continuous BatchI

High Low

Figure 1.1 Diagram to illustrate the wide range of conveying systems available forconventional systems operating with a single air source.

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System Types 5

H High and low pressure systems In pneumatic conveying, high pres-sure typically means any pressure above about 15 lbf/in2 gauge. For sys-tems delivering materials to reception points at atmospheric pressure, 100lbf/in gauge is typically the upper limit, due to the problems of air expan-sion. Very much higher pressures (typically 300 to 400 lbf/in2) can be em-ployed if delivering materials to reception points maintained at pressure,such as chemical reactors and fluidized bed combustion systems.

Conventional and innovatory systems Conventional systems arethose in which the material is simply fed into a pipeline and either blownor sucked, and so this is not identified as a particular case since this is thenorm. Innovatory systems are those in which the material to be conveyedis conditioned in some way, either at the feed point or along the length ofthe pipeline, generally in order to convey the material at low velocity andhence in dense phase, if the material has no natural capability for low ve-locity conveying.

Batch and continuously operating systems Both of these types ofconveying are common in industry.

!"• Single and multiple systems The majority of conveying systems aresingle units. It is possible, however, to combine units for certain duties.

Dilute and dense phase systems Dilute and dense phase conveying donot relate to any particular type of system. Any bulk particulate materialcan be conveyed in dilute phase. It is primarily the properties of the mate-rial that determine whether the material can be conveyed in dense phase,particularly in conventional conveying systems.

,j Pipeline and channel flow systems In the vast majority of pneumaticconveying systems the material is conveyed through pipelines. Fluidizedmotion conveying systems generally employ channels having a porousbase, through which air is introduced, and they are very limited with regardto vertical conveying.

3 CLOSED SYSTEMS

For certain conveying duties it is necessary to convey the material in a controlledenvironment. If a dust cloud of the material is potentially explosive, nitrogen orsome other gas can be used to convey the material. In an open system suchenvironmental control can be very expensive, but in a closed system the gas can bere-circulated and so the operating costs, in terms of inert gas, are significantly re-duced.

If the material to be handled is toxic or radioactive, it may be possible to useair for conveying, but very strict control would have to be maintained. A closedsystem would be essential in this case. Continuous conveying systems are probablythe easiest to arrange in the form of a closed loop. A typical system is shown inFigure 1.2.

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Chapter 1

Heat Exchanger

SupplyHopper

Blower

Back-up Filter

Primary Filter

ReceptionHopper

Feeder

jf^Vx.'K'.v.'vjxtv.'V.'iH' •>.v.">vt*>>;».->-.«^*y

y*&$#\*»Wt'\V;Vjf;\v»*»*.>\"'.'ti

Figure 1.2 A closed loop pneumatic conveying system.

A null point needs to be established in the system where the pressure is ef-fectively atmospheric and provision for make up of conveying gas can be estab-lished there. If this is positioned after the blower the conveying system can operateentirely under vacuum. If the null point is located before the blower it will operateas a positive pressure system.

A back-up filter would always be recommended, because positive displace-ment blowers and compressors are very vulnerable to damage by dust. This issimply a precaution against an element in the filter unit failing. There will gener-ally be an increase in temperature across an air mover and so in a closed loop sys-tem it may be necessary to include a heat exchanger, otherwise there could be agradual build up in temperature. The heat exchanger can be placed either before orafter the air mover, depending upon the material being conveyed.

4 OPEN SYSTEMS

Where strict environmental control is not necessary an open system is generallypreferred, since the capital cost of the plant will be less, the operational complexitywill be reduced, and a much wider range of systems will be available. Most pneu-matic conveying systems can ensure totally enclosed material conveying, and sowith suitable gas-solid separation and venting, the vast majority of materials canbe handled quite safely in an open system. Many potentially combustible materialsare conveyed in open systems by incorporating necessary safety features.

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System Types

4.1 Positive Pressure Systems

Although positive pressure conveying systems discharging to a reception point atatmospheric pressure are probably the most common of all pneumatic conveyingsystems, the feeding of a material into a pipeline in which there is air at a positivepressure does present a number of problems. A wide range of material feedingdevices, however, are available that can be used with this type of system, fromverturis and rotary valves to screws and blow tanks, and these are considered indetail in Chapter 2. A typical low positive pressure pneumatic conveying system isshown in Figure 1.3.

With the use of diverter valves, multiple delivery to a number of receptionpoints can be arranged very easily with positive pressure systems. Although multi-ple point feeding into a common line can also be arranged, care must be taken,particularly in the case of rotary valve feeding of the pipeline, since air leakagethrough a number of such valves can be quite significant in relation to the total airrequirements for conveying.

4.2 Negative Pressure (Vacuum) Systems

Negative pressure systems are commonly used for drawing materials from multi-ple sources to a single point. There is little or no pressure difference across thefeeding device and so multiple point feeding into a common line presents fewproblems. As a result the rotary valve and screw can also be a much cheaper itemfor feeding a pipeline in a negative pressure system than in a positive pressuresystem. The filtration plant, however, has to be much larger as a higher volume ofair has to be filtered under vacuum conditions. Particular care, therefore, must betaken when specifying these particular components. A typical system is shown inFigure 1.4.

SupplyHopper

Diverter Valves

V

BlowerDischarge Hoppers

Figure 1.3 A typical positive pressure conveying system.

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Chapter 1

Storage Hoppers

Air

xp£

Filter

Air

t.'*.t s.»*'/<WAf

/Exhauster

DischargeHoppers

Figure 1.4 A typical negative pressure conveying system.

Negative pressure systems are also widely used for drawing materials fromopen storage, where the top surface of the material is accessible. This is achievedby means of suction nozzles. Vacuum systems, therefore, can be used most effec-tively for off-loading ships. They are also particularly useful for cleaning proc-esses, such as the removal of material spillages and dust accumulations. Anotherapplication is in venting dust extraction hoods.

If a very high vacuum is used for the conveying of a material, considerationshould be given to the stepping of the pipeline part way along its length. Air iscompressible and the rate of change of volume increases considerably with de-crease in pressure. If the pipeline is not stepped, extremely high values of convey-ing air velocity can occur towards the end of the pipeline. The situation is the samefor very high pressure positive pressure conveying systems. These issues are con-sidered in detail in Chapters 5 and 9.

Vacuum systems have the particular advantage that all gas leakage is inward,so that the injection of dust into the atmosphere is virtually eliminated. This isparticularly important for the handling of toxic and explosive materials, or anymaterial where environmental considerations have to be taken into account. It is notalways necessary to employ a closed system with these materials, therefore,provided that adequate safety measures are taken, particularly with regard toexhaust venting.

As a result of the conveying air being drawn through the air mover, it is es-sential that the exhauster should be protected from the possibility of the failure ofone or more of the filter elements in the gas-solids separation system. This can beachieved by incorporating a back-up filter, as shown in Figure 1.2 for the closedloop system. A negative pressure conveying system operating with a vacuum noz-zle is shown in Figure 1.5.

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System Types

Filter

Stockpile Exhauster

ReceivingHopper

Figure 1.5 Vacuum conveying from open storage.

4.3 Combined Negative and Positive Pressure Systems

Protection has to be provided for the exhauster/blower from the possible ingress ofmaterial, as with negative pressure and closed loop systems. It should be notedthat the available power for the system has to be shared between the two sections,and that the pipelines for the two parts of the system have to be carefully sized totake account of different operating pressures.

Some air movers, such as positive displacement blowers, operate on a givenpressure ratio, and this will mean that the machine will not be capable of operatingover the same pressure range with the combined duty as compared with their indi-vidual operation. This will mean that the system capability is limited in terms ofboth tonnage and distance. Although there is only one air mover, two filter unitswill be required. A typical system is shown in Figure 1.6.

Storage Hoppers Filter Diverter T- iuivericr TQ aiternatlve

discharges

Exhauster / Blower Pressure Line

Figure 1.6 Combined negative and positive pressure system.

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10 Chapter 1

4.4 Dual Vacuum and Positive Pressure Systems

If the conveying potential of a system requiring the vacuum pick-up of a materialneeds to be improved beyond that capable with a combined negative and positivepressure system, whether in terms of conveying distance or material conveyingrate, then a dual system should be considered. In this combination the two convey-ing elements are separated and two air movers are provided.

A typical system is shown in Figure 1.7. It should be noted that as there aretwo separate systems, two gas-solid separation devices also have to be provided.Filters and valves have been omitted from the sketch of the system for clarity.

As two air movers are provided this means that the most suitable exhaustercan be dedicated to the vacuum system and the most appropriate positive pressuresystem can be used for the onward transfer of material. If the vacuum off-loadingsection is only a short distance, it is possible that the material could be conveyed indense phase over the entire conveying distance. It is simply a matter of pressuregradient for materials that have good air retention properties, as considered inChapters 4 and 7.

The system shown in Figure 1.7 is typical of a ship off-loading system. Witha high vacuum exhauster a material such as cement could be off-loaded at a rate ofabout 900 to 1000 ton/h through a single pipeline. Twin vessels on the quaysidewould allow continuous conveying to shore based reception vessels, which couldbe some 2000 ft distant if a high pressure compressor was to be used. For the on-ward conveying two pipelines would probably need to be used to achieve the 1000ton/h.

Reception_Silo

Exhauster

Compressor

Figure 1.7 Typical dual vacuum and positive pressure system.

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System Types 11

5 BATCH CONVEYING

The systems considered so far have all been capable of continuous conveying. Inmany processes, however, it may be more convenient to convey one batch at atime. An additional classification of conveying systems, as indicated on Figure1.1, is based on mode of operation. Conveying can either be carried out on a con-tinuous basis or in a sequence of isolated batches.

Although a batch conveying system may be chosen for a specific processneed, the mode of conveying is, to a large extent dictated by the choice of pipelinefeeding device. The majority of batch conveying systems are based on blow tanks,and blow tanks are chosen either because of their high pressure conveying capabil-ity, or because of the abrasive nature of the material.

Two types of system are considered. In one, the batch size is relatively large,and the material is fed into the pipeline gradually, and so can be considered as asemi continuous system. In the other, the material is fed into the pipeline as a sin-gle plug. Particular features of blow tanks, as material feeding devices, are consid-ered in Chapter 2. In this chapter the emphasis is on types of conveying system.

5.1 Semi Continuous Systems

It should be noted that when batches of material are fed into the pipeline gradually,there is essentially no difference in the nature of the gas-solids flow in the pipelinewith respect to the mode of conveying through the pipeline. This is certainly thecase during the steady state portion or the conveying cycle, regardless of the valueof solids loading ratio.

The blow tanks used vary in size from a few cubic feet, to 1000 ft3 or more,generally depending upon the material flow rate required as well as a need tomaintain a reasonable frequency of blow tank cycling. The material can be con-veyed in dilute or dense phase, depending upon the capability of the material, thepressure available and the conveying distance, as with continuously operating sys-tems.

With a single blow tank it is not possible to utilize the pipeline while theblow tank is being filled with material or when the system is being pressurized.Since batch conveying is discontinuous, steady state values of material flow rate,achieved during conveying, have to be higher than those for continuously operat-ing systems in order to achieve the same time averaged mean value of materialflow rate. This means that air requirements and pipeline sizes have to be based onthe maximum, or steady state, conveying rate. The intermittent nature of the con-veying cycle is illustrated in Figure 1.8.

In comparison with a continuously operating system, therefore, the batchoperating system would appear to be at a disadvantage. Blow tank systems, how-ever, can operate at very much higher pressures to compensate, and they can beconfigured to operate continuously, as will be considered in the next chapter onpipeline feeding systems.

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12 Chapter 1

Maximum or steadystate conveying rate

mean or

continuous flow rate

Time

Figure 1.8 Sketch showing the transient nature of batch conveying.

It should be emphasized that blow tanks can be operated at low as well ashigh pressure, depending upon the system needs. If a material needs to be fed into achemical reactor or a boiler plant that is maintained at a pressure of 400 Ibf/in , forexample, the blow tank can be designed to operate at 410 Ibf/in2 for the duty. Whendelivering material to a reception point at atmospheric pressure, however, airsupply pressures greater than about 100 Ibf/in" are rarely used. This is mainlybecause of the problem of air expansion and the need for a stepped pipeline toprevent excessively high values of conveying air velocity.

A typical batch conveying system based on a single blow tank is illustratedin Figure 1.9.

StorageHopper Filter \ rn

DischargeHopper

Blow Tank

Figure 1.9 Typical single blow tank conveying system.

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System Types 13

5.2 Single Plug Systems

In the single plug conveying system the material is effectively extruded into thepipeline as a single plug, although the material is generally well aerated. It is typi-cally about 30 ft long. This plug is then blown through the pipeline as a coherentplug. A certain amount of material will tail off the end of the plug as it is con-veyed, but the front of the plug will sweep up material deposited in the pipeline bythe previous plug. Blow tanks are generally used as the feeding device and a typi-cal single plug conveying system is shown in Figure 1.10.

The air pressure has to overcome the frictional resistance of the plug of ma-terial in the pipeline. As a result blow tank sizes are rarely larger than 150 ft3,unless very large diameter pipelines are employed. In terms of system design, acycling frequency is selected to achieve the required material flow rate, whichdetermines the batch size. The pipe diameter is then selected such that the fric-tional resistance of the plug results in a reasonable air supply pressure to propel theplug at the given velocity.

The material will be conveyed at a low velocity, in what may be regardedas dense phase, but solids loading ratios have no significance here, and steady stateconveying, as depicted on Figure 1.8, does not apply either. Single plug systemsare capable of conveying a wide range of materials, and generally at much lowervelocities than can be achieved in continuously operating systems. Many coarse,granular materials are either friable or abrasive and can only be conveyed in dilutephase with conventional conveying systems, and so single plug systems can repre-sent a viable alternative, although it would always be recommended that tests becarried out to confirm this.

Filter

Vent

Conveying Line

/DischargeHopper

Blow Tank

Figure 1.10 Sketch of a typical single plug conveying system.

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14 Chapter 1

Material discharge often represents a problem with this type of system. Al-though the plugs of material are conveyed at a relatively low velocity, once theyare discharged from the pipeline the high pressure air released behind the plug cancause severe erosion of the pipeline on venting.

6 INNOVATORY SYSTEMS

Unless the material to be conveyed has natural bulk characteristics such as goodair retention or permeability, it is unlikely that it will be possible to convey thematerial at low velocity, and in dense phase, in a conventional continuous or semicontinuous conveying system such as those described above. Even if a high pres-sure system is employed it is unlikely that such a material will convey in densephase, unless the pipeline is relatively short. Dense phase conveying is not syn-onymous with high pressure, it is material property dependent.

For materials that are either abrasive or friable, alternatives to conventionalsystems may have to be considered, particularly if the materials are not capable ofbeing conveyed in a dense phase mode, and hence at low velocity. For friablematerials considerable particle degradation can occur in high velocity suspensionflow, and erosion of bends in the pipeline and other plant surfaces subject toparticle impact will occur if an abrasive material is conveyed in dilute phasesuspension flow.

For a material that is only slightly hygroscopic, successful conveying may beachieved if the material is conveyed in dense phase, without the need for air dryingequipment, since air quantities required for conveying can be significantly lowerthan those for dilute phase. For food products, that may be subject to a loss in flavorin contact with air, dense phase conveying might be recommended. If any suchmaterial is not capable of being conveyed in dense phase in conventional systems,however, alternative systems will also have to be considered.

With a need to convey many materials at low velocity, much developmentwork has been undertaken since the late 1960's to find means of conveyingmaterials with no natural dense phase conveying capability at low velocity. Theinnovatory systems produced as a result of these developments have centeredaround some form of conditioning of the conveyed material, either at the feed pointinto the pipeline or along the length of the pipeline. Since the modifications areessentially based on the pipeline, types of conveying system have not changedsignificantly.

6.1 Plug Forming Systems

The pulse phase system was developed in the late 1960's. A typical pulse phasesystem is shown in Figure 1.11. An air knife, positioned at the start of the pipeline,intermittently pulses air into the pipeline to divide the discharging material intodiscrete short plugs. Blow tanks are commonly used for the feeding of materials inthis type of system also.

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System Types 15

Figure 1.11 Pulse phase conveying system.

No further conditioning of the material occurs along the length of the pipe-line. The pulse phase system was initially proposed as a solution to the problem ofconveying cohesive bulk solids, but subsequent developments have shown that awider range of materials can be conveyed successfully.

6.2 By-Pass Systems

.The most common by-pass systems employ a small pipe running inside the con-veying line, having fixed ports, or flutes, at regular intervals along its length. Thisinner pipe is not supplied with an external source of air, but air within the convey-ing line can enter freely through the regular openings provided. In an alternativedesign the by-pass pipe runs externally to the pipeline and is interconnected atregular intervals. By this means pipeline bends can also be conveniently incorpo-rated.

If the material is impermeable the air will be forced to flow through the by-pass pipe if the pipeline blocks. Because the by-pass pipe has a much smallerdiameter than the pipeline, the air will be forced back into the pipeline through thenext and subsequent flutes because of the extremely high pressure gradient, andthis will effect a break up of the plug of material causing the blockage.

6.3 Air Injection Systems

A number of systems have been developed that inject air into the pipeline at regu-lar points along its length, as illustrated in Figure 1.12.

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16 Chapter 1

External bypass

Internal bypass

Porous tube

-» 8 Booster line

Figure 1.12 Various plug control systems.

While by-pass pipe systems artificially create permeability in the bulk mate-rial, air injection will help to maintain a degree of air retention within the material.Continuous injection of air into the pipeline, however, does mean that conveyingair velocities towards the end of the pipeline will be much higher as a result.

In some systems sensors are positioned between the parallel air line and theconveying pipeline and air is only injected when required. If a change in pressuredifference between the two lines is detected, which would indicate that a plug isforming in the conveying pipeline, air is injected close to that point in order tobreak up the plug and so facilitate its movement. Various plug control systems,including both by-pass pipe and air addition methods are shown in Figure 1.12.

Many of the innovatory systems are capable of being stopped and re-startedduring operation. With most conventional systems this is not possible, and wouldresult in considerable inconvenience in clearing pipelines if a blockage shouldoccur as a consequence. Since they are capable of conveying materials in densephase, operating costs for power are likely to be lower than those for a conventionaldilute phase system. Capital costs for the innovatory systems are likely to be higher,however, and so an economic assessment of the alternative systems would need tobe carried out.

7 FLUIDIZED MOTION CONVEYING SYSTEMS

The categorizing of fluidized motion conveying systems always represents a prob-lem. They are not generally recognized as pneumatic conveying systems because

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System Types 17

they only use very low positive pressure air, the material does not flow through apipeline, and they have been limited to flow down gradual inclines only. They are,however, clearly not in the mechanical conveying group of conveyors. Until recentyears their application was relatively limited because the main driving force wasgravity, and so they would only operate on a downward incline, although at a verylow angle.

The material is conveyed along a channel that has a continuous porous base.Air enters the material through the porous base and fluidizes the material. In thiscondition the material will behave like a liquid and flow down an inclined channel.The channel is generally closed to keep the system dust tight.

In early systems the channel ran with the material only partly filling thechannel. The fluidizing air escaped into the space above the flowing material andwas ducted to a filtration plant. In a recent development the channel operates fullof material and is capable of running horizontally. It is possible that the channelcould be made to operate at a higher pressure and so be able to convey material upan incline. As the channel can run full, negative pressure operation is another pos-sibility.

7.1 Air-Assisted Gravity Conveyors

In situations where the flow of a material can be downwards, the air-assisted grav-ity conveyor has a number of advantages over pneumatic conveying systems.Plant capital costs can be much lower, operating costs are significantly lower, anda wide range of materials can be conveyed at a very low velocity.

Air-assisted gravity conveyors can be regarded as an extreme form of densephase conveying. The conveyor consists essentially of a channel, divided longitu-dinally by means of a suitable porous membrane on which the material is con-veyed. Such a system is shown in Figure 1.13.

Supply Hopper

Air

Figure 1.13 Air-assisted gravity conveyor.

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18 Chapter 1

If a small quantity of low pressure air is fed through the membrane, the in-ter-particle and particle/wall contact forces will be reduced and the material willbehave like a liquid. If a slight slope is imparted to the conveyor, the material willflow. These conveyors are often referred to as 'air slides'. They have been in usefor over 100 years and are still widely used today for materials such as alumina,cement and fly ash.

Air-gravity conveyors, ranging in width from 4 in to 2 ft, can convey mate-rials over distances of up to 300 ft, and are suitable for material flow rates of up toabout 3000 ton/h. In general, most materials in the mean particle size and densityranges from 40 to 500 micron and 80 to 300 Ib/ft3 are the easiest to convey andwill flow very well down shallow slopes.

7.2 Full Channel Conveyors

Hanrot [1] describes a pressurized horizontal conveying system developed byAluminum Pechiney to convey alumina. The alumina was conveyed from a singlesupply point to more than one hundred outlets. Electrolysis pots on a modern alu-minum smelter were required to be filled and the distance from the silo to the fur-thest outlet was approximately 600 ft. Air at a pressure of about I'A lbf/in2 gaugeis required. A conveying channel is employed, as with the air-assisted gravity con-veyor, but the channel runs full of material. The system is illustrated in Figure 1.14and this shows the principle of operation.

Supply Hopper

De-dustingDuct

'i '-v'T^Tffii1^

Fan

Pot Hoppers

Figure 1.14 Principle of potential fluidization ducts.

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System Types 19

Balancing columns are positioned on the conveying duct and are used forde-dusting. This is not a continuously operating system in the application de-scribed. It is a batch type system and its object is to meet the demands of the in-termittent filling of the pot hoppers. The system, however, is clearly capable ofcontinuous operation and of significant further development.

8 MOBILE SYSTEMS

All of the systems described so far have been essentially fixed systems. The onlyreal flexibility in any of the systems has been the capability of moving vacuumnozzles in negative pressure systems. By the use of flexible hoses these can bemoved, and they find wide application in ship off-loading systems, and the clear-ing of material from stockpiles or spillages. Many road sweeping vehicles employvacuum conveying for their operation.

Many bulk particulate materials are transported from one location to anotherby road, rail and sea. Many materials, of course, are transported in a pre-packagedform, or in bulk containers, and can be transported by road, rail, sea or air, in asimilar manner to any other commodity. Many transport systems, however, arespecifically designed for bulk particulate materials and have a capability of selfloading, self off-loading, or both. These are generally mobile versions of the abovestatic conveying systems, depending upon the application and duty.

8.1 Road Vehicles

Road vehicles are widely used for the transport of a multitude of bulk particulatematerials, such as cement, flour, sugar and polyethylene. Road vehicles often havetheir own positive displacement blower mounted behind the cab and so can off-load their materials independently of delivery depot facilities. The material con-taining element on the truck can generally be tipped to facilitate discharge, whichcan be via a rotary valve, or the container might double as a blow tank which canbe pressurized.

8.2 Rail Vehicles

Rail cars or wagons generally rely on delivery depot facilities for off-loading. Be-cause of their length tilting is not an option and so multiple point off-loading isoften employed. They may be off-loaded by rotary valve, or the rail car may becapable of being pressurized so that it can be off-loaded as a blow tank.

Whereas road vehicles are typically designed to operate with air at 15 Ibf/in2

gauge for this purpose, rail vehicles are generally designed to 30 Ibf/in2 and canusually be off-loaded in about one hour. The base of the rail car is usually angledat about five degrees in herringbone fashion around each discharge point andfluidized to facilitate removal of as much material as possible.

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20 Chapter 1

8.3 Ships

Large bulk carriers usually rely on port facilities for off-loading and these are gen-erally similar to that depicted in Figure 1.7. Intermediate bulk carriers, however,often have on-board facilities for off-loading. Such vessels are often used for thetransfer of materials such as cement to storage depots at ports for local supply, orto off-shore oil rigs.

Materials are typically transferred from storage holds in the ship by acombination of air-assisted gravity conveyors and vacuum conveying systems, intotwin blow tanks. High pressure air is supplied by on-board diesel drivencompressors and materials are conveyed to dock-side storage facilities throughflexible rubber hose, which solves the problems of both location and tidalmovements.

9 MULTIPLE SYSTEMS

Two multiple systems have already been considered. These were the combinednegative and positive pressure system, illustrated in Figure 1.6, and the dual vac-uum and positive pressure system, illustrated in Figure 1.7. A third possibility isthe staging of pneumatic conveying systems, which would be required for verylong distance conveying. Materials are currently conveyed over distances of 5000ft in a single stage and flow rates of 40 ton/h over this distance are not unusual.

With much higher air supply pressures, conveying over longer distances ispossible, and with larger bore pipelines higher material flow rates can be achieved.For very much longer distance conveying, however, staging will have to be em-ployed. A problem with using very high pressure air is that of the expansion of theair and the need to step the pipeline to a larger bore part way along its length, butthis can be overcome to a large extent by discharging the material at the end ofeach stage at a pressure of about 45 lbf/in2 rather than to atmospheric pressure.

10 SYSTEM REQUIREMENTS

The uses, applications and requirements of pneumatic conveying systems aremany and varied. A number of system requirements were highlighted at variouspoints with regard to the systems. Some of the more common requirements ofsystems can be identified and are detailed here for easy access and reference, sincethese may feature prominently in the choice of a particular system.

10.1 Multiple Pick-Up

If multiple point feeding into a common line is required, a vacuum system wouldgenerally be recommended. Although positive pressure systems could be used, airleakage across feeding devices such as rotary valves represents a major problem.The air leakage from a number of feed points would also result in a significant

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System Types 21

energy loss. The air loss could be overcome by adding isolation valves to eachfeed point, but this would add to the cost and complexity of the system.

10.2 Multiple Delivery

Multiple delivery to a number of reception points can easily be arranged with posi-tive pressure systems. Diverter valves can be used most conveniently for this pur-pose. Such a system was illustrated in Figure 1.3. This shows a separate filter unitmounted on each reception hopper. If the situation allows, a common unit could beused, but care would have to be taken with the specification and layout.

The problem with vacuum systems performing this function is equivalent tothe problem of using a positive pressure system for the multiple pick-up ofmaterials, which is one of multiple point air leakage. In this case, however, it isingress of air into the system. Any air leaking into a conveying system pipelinealong its length will by-pass the material feed point and this could result in theconveying air velocity being too low to convey the material.

10.3 Multiple Pick-Up and Delivery

The suck-blow, or combined vacuum and positive pressure system, illustrated inFigure 1.6 is ideal for situations where both multiple pick-up and delivery is re-quired. The pressure drop available for conveying is rather limited with this typeof system and so if it is necessary to convey over a long distance, a dual systemwould be more appropriate. In this the vacuum and positive pressure conveyingfunctions are separated and a high pressure system can be used to achieve the dis-tant conveying requirement, as shown in Figure 1.7.

10.4 Multiple Material Handling

If it is required to handle two or more materials with the one system, referenceshould be made to the conveying characteristics for each material to be conveyed.It is quite likely that the air requirements for the materials will differ to a largeextent, and that different flow rates will be achieved for each material, for identicalconveying conditions.

In this case it will be necessary to base the air requirements, to be specifiedfor the air mover, on the material requiring the highest conveying line inlet airvelocity. Consideration will then have to be given to a means of controlling the airflow rate, to lower values, for the other materials, if this should be required. Alter-natively the same air flow rate can be used for each material but different pipelinebores will have to be used to achieve the conveying air velocity values required.

Since it is likely that the flow rate of each material will be different, thefeeding device, or devices, will have to meet the needs of every material, in termsof flow rate and control. The solutions will also differ between a system in whichthere is a change of material to be conveyed, and a system in which different mate-rials are to be fed from one or a number of different supply hoppers.

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22 Chapter 1

10.5 Multiple Distance Conveying

If it is required to convey a material over a range of distances, such as a roadtanker supplying a number of different installations, or a pipeline supplying anumber of widely spaced reception points, consideration will again have to begiven to differing material flow rates and air requirements. It is possible for bothof these to change if there is a change in conveying distance, for whatever reason.

For a given air supply, in terms of delivery pressure and volumetric flowrate, the material flow rate achieved will decrease with increase in conveying dis-tance. As a consequence the material feeding device will either need to be con-trolled to meet the variation in conveying capability, or the feed rate will have tobe set to the lowest value for the longest distance.

For materials capable of being conveyed in dense phase there is the addedproblem of the possibility that the air flow rate will also need to be increased ifthere is a significant change in conveying distance.

10.6 Conveying From Stockpiles

If the material is to be conveyed from a stockpile, then a vacuum system usingsuction nozzles will be ideal. The type of system required will depend upon theapplication and conveying distance. For a short distance a vacuum system willprobably meet the demand on its own.

Where access is available to a free surface, as in ship off-loading, vacuumnozzles can transfer material under vacuum to a surge hopper. If this is not the finaldestination for the material it could be the intermediate hopper in a combinedpositive and negative pressure conveying system, or the supply hopper for thesecond part of a dual system, from where the material could be blown onward.

For clearing dust accumulations and spillages, and surplus material depos-ited in stockpiles, mobile units are particularly useful. These are generally suck-blow systems with a vacuum nozzle. Although they can be small versions of acontinuously operating suck-blow system, they are more usually batch conveyingsystems with the transfer hopper acting also as a blow tank. Material is first drawninto the hopper/blow tank under vacuum, and when it is full it is pressurized andconveyed on to the reception point.

10.7 Start-Up With Full Pipeline

If there is likely to be a need to stop and start the conveying system while it isconveying material, a system capable of doing this will need to be selected. This israrely possible in conventional systems, and so consideration will have to be givento innovatory systems.

Many of these systems are capable of starting with a full pipeline, althoughtheir capabilities on vertical sections may need to be checked, particularly if thestoppage is for a long period. The possibility of power cuts, from whatever source,should also be taken into account here.

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System Types 23

11 MATERIAL PROPERTY INFLUENCES

The properties of the materials to be conveyed feature prominently in the decisionsto be made with regard to the selection of a pneumatic conveying system. As with'System Requirements', considered above, some of the more common materialproperties can be identified and are detailed here for easy access and reference.

11.1 Cohesive

Problems may be experienced with cohesive materials in hopper discharge, pipe-line feeding and conveying. If there is any difficulty in discharging a cohesivematerial from a rotary valve, a blow-through type should be used. If there is anydifficulty in conveying a cohesive material in a conventional system, then an in-novatory system should be considered. The pulse phase system, for example, wasdeveloped for the handling of such fine cohesive powders.

11.2 Combustible

There is a wide range of materials which, in a finely divided state, dispersed in air,will propagate a flame through the suspension if ignited. These materials includefoodstuffs such as sugar, flour and cocoa, synthetic materials such as plastics,chemical and pharmaceutical materials, metal powders, and fuels such as woodand coal. If a closed system is used the oxygen level of the conveying air can becontrolled to an acceptable level, or nitrogen can be used. If an open system is tobe used, then adequate safety devices must be put in place. One possibility is touse a suppressant system. Another is to employ pressure relief vents and othersafety features.

11.3 Damp or Wet

Materials containing a high level of moisture can generally be conveyed in con-ventional systems if they can be fed into the pipeline, and do not contain too manyfines. Most of the handling problems with wet materials occur in trying to dis-charge them from hoppers. Fine materials may not discharge satisfactorily from aconventional rotary valve and so a blow through type should be used.

Fine materials which are wet will tend to coat the pipeline and bends, andgradually block the line. Lump coal having a large proportion of fines is a particu-lar problem in this respect. Single plug blow tank systems and some of the innova-tory systems are capable of handling this type of material. If a conventional systemmust be used, the problem can be relieved by heating the conveying air, if the ma-terial is not too wet.

11.4 Electrostatic

If the build up of electrostatic charge is a problem when conveying a material, theair can be humidified. This process can be carried out on-line and does not usually

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24 Chapter 1

require a closed system. In dense phase the quantity of air which needs to be con-ditioned is much less than in dilute phase systems, and so for materials capable ofbeing conveyed in dense phase, the operating costs for air quality control will belower. The entire system and pipe-work network should be earthed.

11.5 Erosive

If the hardness of the particles to be conveyed is higher than that of the systemcomponents, such as feeders and pipeline bends, then erosive wear will occur at allsurfaces against which the particles impact. Velocity is one of the major parame-ters and so the problem will be significantly reduced in a low velocity system. If adilute phase system must be used, feeding devices with moving parts, such as ro-tary valves and screws, should be avoided, and all pipeline bends should be pro-tected.

11.6 Friable

If degradation of the conveyed material is to be avoided, a system in which thematerial can be conveyed at low velocity should be considered. The magnitude ofparticle impacts, particularly against bends in the pipeline, should be reduced asthis is one of the major causes of the problem. Damage caused by attrition as aconsequence of particle to particle and particle to wall surfaces must also be takeninto account. Pipeline feeding devices which can cause particle breakage, such asscrews, should also be avoided.

11.7 Granular

Granular materials can be conveyed with few problems in pneumatic conveyingsystems provided that they can be fed into the pipeline. Problems with feeding canoccur with top discharge blow tanks and conventional rotary valves. Air will oftenpermeate through granular materials in top discharge blow tanks and the materialswill not convey, particularly if the blow tank does not have a discharge valve.Granular materials containing a large percentage of fines, and which are not capa-ble of dense phase conveying, may block in a top discharge line. In rotary valves,shearing of granular materials should be avoided, and so a valve with an off-setinlet should be used.

11.8 Hygroscopic

If a material is hygroscopic the air used for conveying can be dried to reduce themoisture level to an acceptable level. This process can be carried out on-line anddoes not usually require a closed system. For a material which is only slightly hy-groscopic, successful conveying may be achieved if the material is conveyed indense phase, without the need for air drying equipment, since air quantities re-quired for conveying can be significantly lower than those for dilute phaseconveying.

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System Types 25

11.9 Low Melting Point

The energy from the impact of particles against bends and pipe walls at high ve-locity in dilute phase conveying can result in high particle temperatures being gen-erated. The effect is localized to the small area around the point of contact on theparticle surface, but can result in that part of the particle melting. The problem isaccentuated if the particles slide on the pipe wall and around pipeline bends. Plas-tic pellets such as nylon, polyethylene and polyesters are prone to melting whenconveyed in suspension flow.

Velocity is a major variable and so the problem will generally be signifi-cantly reduced for most materials in a low velocity, dense phase system. If suchmaterials have to be conveyed in dilute phase a roughened pipeline surface mayhelp to reduce the problem considerably as this will prevent the particles fromsliding.

11.10 Radioactive

Radioactive materials must be conveyed under conditions of absolute safety, andso it would be essential to employ a closed system so that strict control of the con-veying environment could be maintained. A vacuum system would also be neces-sary to ensure that no conveying air could escape from the system, or material inthe event of a bend eroding, for some of these materials do tend to be rather abra-sive.

11.11 Toxic

If toxic materials are to be handled, strict control of the working environment mustbe maintained. A vacuum system, therefore, would be essential to ensure that therecould be no possibility of material leakage. If the conveying air, after filtration,could be vented safely to the atmosphere, an open system would be satisfactory. Ifnot, a closed loop system would have to be used.

11.12 Very Fine

A problem of pipeline coating can occur with very fine powders in the low micronand sub-micron range, such as carbon black and titanium dioxide. These materialstend to adhere to the pipe wall when conveyed in conventional systems. The coat-ing gradually builds up and can cause a marked reduction in the pipe section area,and hence a reduction in conveying capacity. Many of the innovatory systems arecapable of handling this type of material successfully.

If a conventional system is to be used the material should be conveyedthrough a flexible pipeline or hose so that the material build-up can be shaken freeon a regular basis. It is quite likely that the natural pulsations that occur within thesystem would be sufficient to vibrate the material free to enable it to be re-entrained in the conveying line.

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26 Chapter 1

REFERENCES

1. J-P. Hanrot. Multi-point feeding of hoppers, mounted on aluminum smelter pots, bymeans of potential fluidization piping. Proc 115th An Mtg, The Metallurgical Soc ofAIME. pp 103-109. New Orleans. March 1986.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Feeding Devices

1 INTRODUCTION

All pneumatic conveying systems, whether they are of the positive or negativepressure type, conveying continuously or in a batch-wise mode, can be consideredto consist of the basic elements depicted in Figure 2.1.

Material In

Fee<

Clean

Air In

ier\ |

Air and Material

in Pipeline

Filter

/

Clean

Air Out

Material Out

Figure 2.1 Basic elements of a pneumatic conveying system.

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28 Chapter 2

Numerous devices have been developed to feed materials into pipelines. Invacuum systems the material feed is invariably at atmospheric pressure and so thepipeline can either be fed directly from a supply hopper or by means of suctionnozzles from a storage vessel or stockpile.

1.1 Air Leakage

In positive pressure systems, separation devices invariably operate at atmosphericpressure. Pipeline feeding in positive pressure systems represents a particularproblem, however, for if the material is contained in a storage hopper at atmos-pheric pressure, so that it is continuously available for filling, the material has tobe fed against a pressure gradient.

As a consequence of this there may be a loss of conveying air. In certaincases this air flow can interfere with the feeding process. Also, if the loss is sig-nificant, the air supply will have to be increased, for the correct air flow rate to thepipeline must be maintained for conveying the material.

1.2 Pressure Drop

Material flow rate through a pipeline is primarily dependent upon the pressuredrop available across the pipeline. A basic requirement of any feeding system,therefore, is that the pressure drop across the feeding device should be as low aspossible in low pressure systems, and as small a proportion of the total as possiblein high pressure systems.

If the feeder requires an unnecessarily high proportion of the total pressuredrop from the air source, less pressure will be available for conveying the materialthrough the pipeline, and so the material flow rate will have to be reduced to com-pensate. Alternatively, if a higher air supply pressure is employed to compensate,more energy will be required, and hence the operating cost will be greater.

1.3 Maintenance

Maintenance is another important factor. Very often air leakage has to be acceptedwith a particular feeding system, but it is essential that the rate of loss should notincrease unduly with time as a result of gradually increasing wear. If undue weardoes occur, insufficient air may ultimately be supplied to the pipeline and a block-age is likely to occur as a consequence. This usually happens just after the guaran-tee has expired!

1.4 Material Properties

Material properties are particularly important and have to be taken into account inthe selection of feeding devices. In feeding systems that have moving parts, carehas to be taken with both abrasive and friable materials. Material flow propertiesalso need to be taken into account with feeding devices, and particle size must be

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Feeding Devices 29

considered in all cases, particularly the two extremes of large particles and pellets,and very fine particles and powders.

1.5 Devices Available

Many diverse devices have been developed for feeding pipelines. Some are spe-cifically appropriate to a single type of system, such as suction nozzles for vacuumsystems. Others, such as rotary valves, screws and gate valves, can be used forvacuum and positive pressure systems. The approximate operating pressure rangesfor various pipeline feeding devices are given in Figure 2.2.

Developments have been carried out on most types of feeding device to in-crease the operating pressure range of the device and the range of materials capa-ble of being fed. Each type of feeding device, therefore, can generally be used witha number of different types of conveying system, and there are usually many alter-native arrangements of the feeding device itself.

7.5.7 Blow Tanks

For high pressure systems, and particularly where the material has to be fed into asystem that is maintained at a high pressure itself, blow tanks are commonly em-ployed. These are generally used for conveying batches, although they can quiteeasily be adapted for continuous conveying. This, of course, is the particular ad-vantage of all the other systems included in Figure 2.2.

The time averaged mean flow rate for a batch type system is somewhatlower than that of the equivalent continuous conveying system. There are, how-ever, numerous ways by which the operating performance of blow tanks can beimproved. Because they have no moving parts, blow tanks are often used in lowpressure applications. Since there are so many different types of blow tank ar-rangement, and because they are increasing in popularity, a large section of thischapter is devoted to this type of feeder.

Feeding Device

Blow TankVenturiScrewRotary ValveGate ValvesSuction NozzleTrickle Valve

-veSystem Pressure - lbf/in2 (gauge)

20 40 60 80 100

11

1 1 1 1 1 1 1 1 1 11n

1 I High pressure

t ! High pressure

1 t 1

Figure 2.2 Approximate operating pressure ranges for various feeding devices.

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30 Chapter 2

7.5.2 Vacuum Conveying

It will be noted that there is no scale on Figure 2.2 for feeding devices for negativepressure conveying systems. This is because in a vacuum conveying system mate-rial is normally fed into the pipeline at atmospheric pressure. Only if the inlet tothe feeding device was choked would there be a negative pressure. Choking isoften employed to help promote the flow of material into the pipeline but the orderof magnitude is generally very low.

1.6 Feeding Requirements

The air mover can be positioned at either end of the system shown in Figure 2.1. Ifthe air is blown into the pipeline, the air at the feed point will be at a pressure closeto that of the air supply. In this case the material has to be fed into the pipeline atpressure, and so consideration has to be given to the possibility of air leakageacross the material feeding device.

If the air mover is positioned downstream of the system, so that it acts as anexhauster to the separator/discharge hopper, the air at the material feed point willbe close to atmospheric pressure. In this case the effect of a pressure gradient onthe feeding device need not be taken into account. Consideration, however, willhave to be given to the possibility of air ingress into the system.

1.6.1 Material Surges

A further requirement of the feeding device is that it should feed the material intothe conveying line at as uniform a rate as possible. This is particularly so in thecase of dilute phase systems, for the material is conveyed in suspension and quitehigh values of minimum conveying air velocity have to be maintained. With amean conveying air velocity of 4000 ft/min, for example, it will only take aboutsix seconds for the air to pass through a 400 ft long pipeline.

If there are any surges in material feed, the pipeline could be blocked veryquickly. Alternatively, if the air mover has a pressure rating to make allowance forsuch surges, the output from the system could be increased if the flow rate, andhence the conveying line pressure drop, was kept constant at a higher value tomatch the rating more closely.

1.6.2 Flow Metering

Positive displacement feeding devices, such as screws and rotary valves, can servethe dual purpose of both metering the material into the pipeline, whilst effectingthe air-lock that is necessary for successful operation, in the case of positive pres-sure systems.

Some feeders act only as air locks and so require additional equipment tometer the material into the conveying line. Some feeders have no moving parts,and so particular attention is given to them, as their means of material flow controlmay not be obvious.

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Feeding Devices 31

2 ROTARY VALVES

The rotary valve is probably the most commonly used device in general industryfor feeding materials into pipelines. This type of feeder consists of a bladed rotorworking in a fixed housing. In many applications in which it is used its primaryfunction is as an air lock, and so it is often referred to as a rotary air lock. Thisbasic type of valve is generally suitable for free flowing materials. It is a positivedisplacement device and so material flow rate can readily be achieved by means ofvarying the speed of the rotor.

The traditional, or low pressure, rotary valve has an upper pressure limit ofabout 15 lbf/in2 gauge, which closely matches the delivery pressure capability ofthe positive displacement blower, and so the two are a common combination forpositive pressure pneumatic conveying systems. The upper limit on the pressurecapability of the rotary valve is dictated primarily by the problem of air leakageacross the valve.

Developments in the mid 1980's by a couple of companies, and since bynumerous others, have greatly extended the range of operating pressure for rotaryvalves, as indicated on Figure 2.2. The use of rotary valves is rather limited forabrasive materials, however, owing to problems of erosive wear. This is particu-larly the case for positive pressure conveying systems, since air leakage across thevalve aggravates the wear problem considerably.

2.1 Drop-Through Valve

The type of valve described above is usually referred to as a 'drop-through' feederand is depicted in Figure 2.3. This type of feeder is generally suitable for freeflowing materials. Material from the supply hopper continuously fills the rotorpockets at the inlet port which is situated above the rotor. It is then transferred bythe motor driven rotor to the outlet where it is discharged and entrained into theconveying line.

2.7.7 Valve Wear

By the nature of the feeding mechanism, rotary valves are more suited to relativelynon-abrasive materials. This is particularly the case where they are used to feedmaterials into positive pressure conveying systems. By virtue of the pressure dif-ference across the valve, and the need to maintain a rotor tip clearance, air willleak across the valve. Wear, therefore, will not only occur by conventional abra-sive mechanisms, but by erosive wear also.

Air leakage through the blade tip clearances can generate high velocityflows, which will entrain fine particles, and the resulting erosive wear can be farmore serious than the abrasive wear. Wear resistant materials can be used in theconstruction of rotary valves, and removable lining plates can be incorporated tohelp with maintenance, but wear can only be minimized, it cannot be eliminated ifan abrasive material is to be handled.

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32 Chapter 2

Material FeedMaterial Inlet Port

Housing

IDrive,Shaft

Rotor

z

7Material Outlet Port

Figure 2.3 Drop-through rotary valve.

2.2 Alternative Designs

As the rotary valve is probably the most common feeding device in use, it is notsurprising that much effort has gone into to developing it further. The improve-ment in materials and construction methods to make it more acceptable for han-dling abrasive materials is one such area.

The reduction in air leakage and the development of a rotary valve capableof operating at much higher pressures, and across much higher pressure differen-tials, has been another. Its capability for handling a wider range of materials wasan early development.

2.2.1 Off-Set Valve

Rotary valves that have an off-set inlet, and hence a corresponding off-set outletfor material feed, are often employed in applications where shearing of the mate-rial should be avoided. This is particularly a problem where the material has alarge proportion of large particles. A typical valve is given in Figure 2.4.

They employ a side inlet, generally with an adjustable flow control, so thatthe angle of flow of the material does not permit the rotor pocket to be completelyfilled. As the rotor rotates toward the housing, material flows into the trough of therotor and so prevents shearing. This type of valve is widely used for feeding pellet-ized materials.

2.2.2 Blow-Through Valve

Another variation of the standard type of feeder is the 'blow-through' valve, whichis also shown in Figure 2.4.

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Feeding Devices 33

Figure 2.4 Alternative rotary valve configurations, (a) Off-set and (b) blow-through.

With the blow-through valve the conveying air passes through and purgesthe discharging pockets such that the material entrainment into the conveyingpipeline actually takes place in the valve itself. These valves are primarily in-tended for use with the more cohesive types of material, since this type of materialmay not be discharged satisfactorily when presented to the outlet port of a 'drop-through' valve.

It should be borne in mind that for an eight bladed rotor, such as that shownin Figure 2.3, rotating at a typical speed of 20 revolutions per minute, a time spanof only 0-375 seconds is available for the material to be discharged from eachpocket. The importance of feeding material into a pipeline as smoothly as possiblewas mentioned above, and it was stated that in a dilute phase conveying systemthe air would traverse a 400 ft long pipeline in about six seconds. For the rotaryvalve being considered, about 13 pockets of material would be deposited into thepipeline in this period.

In fuel firing systems this is not likely to acceptable as it will result in sig-nificant pulsing of the flame in the furnace. If a rotary valve is to be used in thistype of application it would be recommended that a helical or twisted rotor be usedso that the material is deposited into the pipeline at a more uniform rate.

A blow-through valve would not be recommended for the feeding of abra-sive materials. This is a very turbulent region and the rotor blades, and rotor hous-ing near the point of entry to the pipeline, would be prone to very severe wear.End plates could not be employed on the rotor very conveniently with this ar-rangement and so the pressure capability would be limited.

2.3 Air Leakage

It is an unavoidable physical characteristic of the rotary valve that, in a positivepressure pneumatic conveying system, there will be a leakage of air across thevalve, via the returning empty pockets and the various rotor blade clearances.Typical air flows and leakage paths for a rotary valve system are shown in Figure2.5.

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34 Chapter 2

Housing

Blower

I

oo

Rotor

Supply Air - Vs

Hopper

Leakage Air - VL

Note : Vc = Vs- VL

Conveying Air - Vc

Figure 2.5 Air flows and leakage paths for rotary valve system.

For a 4 inch bore pipeline the air leakage, VL, could be as much as 15% ofthe air supplied. For a material such as plastic pellets it will be even higher, sincethe material itself offers little resistance to air flow, and in smaller diameter pipe-lines the percentage will be proportionally greater. For a valve operating across asmall pressure difference with a very fine material, however, air leakage will besignificantly reduced.

The magnitude of the loss will depend upon the pressure difference acrossthe valve, the valve size, the rotor tip clearance, the nature of the material beinghandled, and the resistance to air flow by the head of material over the valve. If airleakage across the valve is not taken into account, or if the anticipated leakage isincorrect for some reason, it can have a marked effect on the performance of theconveying line.

If insufficient air is available for conveying the material in the pipeline, as aresult of losses across a rotary valve, it is possible that the pipeline will block,'for aloss of 10 to 20% of the total air supply will significantly affect the velocity of theair in the conveying system. Also, if two or more rotary valves feed into a com-mon line, and there is no additional valve over each rotary valve to minimize airlosses from those not in use, the air, and hence energy loss, could be very consid-erable.

Rotor tip clearance is an important variable here. The gradual wear of avalve in use, such that the rotor clearances increase slightly over a period of time,will affect the balance of the air flows shown in Figure 2.5, and consequently af-

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Feeding Devices 35

feet the conveying line performance with respect to time. This is one of the rea-sons why rotary valves are not generally recommended for the handling of abra-sive materials. It is important, therefore, that rotary valves should be well main-tained, and if they are to be used for abrasive products they should incorporatewear resistant materials.

2.3.1 Air Venting

Unless the air leakage across the rotary valve is vented away, prior to the materialentering the valve, material flow into the valve may be severely restricted by theupward flow of air. This air flow may also result in a change in bulk density of thematerial. The magnitude of the problem depends very much upon the properties ofthe material being handled.

For plastic pellets and granular materials, venting may not be necessary, butfor fine cohesive materials and light fluffy materials the volumetric efficiency ofthe valve, in terms of pocket filling, may be very low. In this case material feed ata controlled rate might be difficult to achieve. A number of different ways of vent-ing rotary valves are presented in Figure 2.6.

Since the vented air will contain some fine material, this is normally di-rected back to the supply hopper, or to a separate filter unit. Because there will bea carry-over of material this filter must be a regularly cleaned unit, otherwise itwill rapidly block and cease to be effective. Indeed, the pipe connecting the vent tothe filter must be designed and sized as if it were a miniature pneumatic conveyingsystem, in order to prevent it from getting blocked.

Figure 2.6 Methods of venting rotary valves, (a) Internal vent, (b) external vent, and(c) pellet vent.

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36 Chapter 2

Air Air andMaterial

Air andMaterial

Figure 2.7 Entrainment sections for rotary valves, (a) Drop-out box and (b) venturi.

2.4 Entrainment Devices

Owing to the pulsating nature of the material flow at outlet from the valve, withthe individual pockets of material discharged from the rotor, the change in direc-tion of the material flow, and the flow of leakage air against the direction of mate-rial flow, the region beneath a rotary valve is particularly turbulent. In order toreduce the turbulence level, and hence energy loss, entrainment devices are oftenused under the rotary valve. A common device is a 'drop-out' box and this is illus-trated in Figure 2.la.

Another configuration is the venturi entrainment section, and this is shownin Figure 2.7b. Here the cross-sectional area of the air supply pipeline is reducedby means of a convergent section immediately prior to the rotary valve. As a resultthere is a corresponding increase in entrainment velocity and hence a decrease inpressure in the region beneath the valve. A consequence of this decrease in pres-sure is that there will be less air leakage through the valve to interfere with mate-rial feeding.

This should result in an improvement in performance when handling thefiner, free flowing types of material. A divergent section allows the kinetic energyof the high velocity air to be re-converted back to pressure. This type of devicewould not be recommended for abrasive or friable materials, however, because ofthe increase in air velocity and turbulence generated in the area.

2.5 Rotor Types

Rotors are either of the 'open-end' type or 'closed-end' type. With 'open-end'types the blades are welded directly to the driving shaft, whilst with the 'closed-end' type discs or shrouds are welded to the shaft and blade ends to form enclosedpockets. These two types of rotor are illustrated in Figure 2.8.

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Feeding Devices 37

(a) A A—/ (b)

Figure 2.8 Rotor types, (a) Open-end and (b) closed-end rotors.

Although open-end rotors are less expensive they have several disadvan-tages. With the more abrasive materials, wear of the rotor housing end plates ispossible since the material is in constant contact with them. Also, they are not asrigid as the closed-end type for they only have one edge secured to the drive shaft.They can not, however, be used in the blow-through type of feeder shown in Fig-ure 2.4b, as mentioned above.

2.5.1 High Pressure Designs

The closed-end type of rotor provides a very much more rigid construction, and itis with this type of rotor that developments to much higher pressure applicationshave been possible. With an end plate it is possible to provide a seal to signifi-cantly reduce the quantity of air that leaks across the valve by this route, and amore rigid construction allows rotor tip clearances to be reduced. The reduction inair flow, and more particularly material, past the rotor end plate also providesadded protection for the bearings.

Air leakage via the returning empty pockets remains a problem and leakagevia the blade tip clearances will still occur. By these various improvements, how-ever, the operating pressure differential has been improved to about 45 to 60lbf/in2, compared with about 15 lbf/in2 for the conventional rotary valve, as indi-cated earlier on Figure 2.3.

2.5.2 Pocket Types

There are two rotor pocket configurations in widespread use, and these are shownin Figure 2.9. The most common type has deep pockets and hence maximumvolumetric displacement. This is more suited to the handling of free flowing mate-rials.

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38 Chapter 2

Figure 2.9 Rotor pocket configurations, (a) Deep pocket rotor, (b) shallow pocket ro-tor, and (c) rotor with blade tips.

The second type has shallow, rounded pockets and so its volumetric capac-ity is reduced. This configuration is generally used with the more cohesive typesof material that tend to stick in deep pockets. All angles are eliminated by creatingthe more rounded profile. Blade tips are often employed, and a sketch of such arotor is given in Figure 2.9c. Many of these are adjustable to maintain operatingefficiency. They can be made of resilient, spark-proof, flexible or abrasion resis-tant materials.

2.5.3 Rotor Clearance

The rotor clearance can have a significant effect on valve performance, and in anattempt to minimize the effect of the leakage on the feed rate, manufacturers makethese clearances as small as possible. Clearances on new valves are typically of theorder of 0-003 to 0-006 inch. Clearances smaller than this would add considerablyto the cost of manufacture and may even lead to binding in the housing due todeflection of the rotor, or movement within the bearings, when subject to the ap-plied pressure gradient.

Particular care with respect to binding must be taken if the material to behandled is hot, because differential expansion of rotor and casing could cause thevalve to seize up. The fitting of flexible elastomer/polymer wipers to the rotorblades, such that they are in contact with the housing, is quite common. This ap-proach, however, is generally limited to low pressure applications, typically up toabout 4 lbf/in2 gauge, since the leakage at pressure gradients greater than this candeflect the wipers and so lose the advantage.

2.5.4 Blade Numbers

The number of blades on the rotor will determine the number of blade labyrinthseals that the air must pass before escaping from the system. From an air loss pointof view, therefore, a ten bladed rotor would be specified for applications withpressure differentials from 8 to 15 lbf/in2. Eight bladed rotors are commonly usedin applications with pressure differentials up to 8 lbf/in2, and six bladed rotorswhere the pressure differential is below 3 lbf/in2.

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There is obviously a practical limitation to the number of blades that can beused in a rotor when handling a given material. The number is largely dependentupon the material itself, since increasing the number of blades decreases the anglebetween them. A decrease in this angle is sufficient with some materials to preventif from being discharged when presented to the outlet port.

2.6 Feed Rate

The feed rate of a rotary valve is directly proportional to the displacement volumeof the rotor and its rotational speed. The displacement volume is simply the pocketsize or volume multiplied by the number of rotor pockets. If a mass flow rate ofmaterial is required this must then be multiplied by the bulk density of the mate-rial. The constant of proportionality here is the volumetric efficiency of the rotaryvalve.

2.6.1 Pocket Filling Efficiency

If air leakage impedes material flow, the pockets will not fill completely and so thevolumetric efficiency will be reduced. Air leakage may also have the effect ofreducing the bulk density of the material, for with some materials the fluidizedbulk density can be very much lower than the 'as poured' bulk density. It shouldbe noted that, because of air leakage, the volumetric efficiency of a rotary valvewhen feeding a negative pressure system will generally be much greater thanwhen feeding a positive pressure system.

2.6.2 Feed Rate Control

As the rotary valve is a positive displacement device, feed rate control can beachieved quite simply by varying the speed of the rotor. The pocket filling effi-ciency of a rotary valve, however, is also a function of rotor speed. Up to a speedof about 20 rev/min the filling efficiency is reasonably constant, but above thisspeed it starts to decrease at an increasing rate. Thus there is a limit on feed ratefor a given rotary valve. Rotary valves, however, do come in a very wide range ofsizes to meet almost any duty.

2.7 Feeding Negative Pressure Systems

With negative pressure conveying systems there is no adverse pressure gradientacross the material feeding device and so the leakage of air across the valve willnot be a problem, as it will not occur. The valve will not have to be designed towithstand a pressure difference, or be manufactured to provide fine blade tip clear-ances. As a consequence a rotary valve for a negative pressure conveying systemis likely to be very much cheaper that than for a positive pressure system.

It must be emphasized, however, that under no circumstances should a ro-tary valve designed for a negative pressure conveying system be used in a positivepressure conveying system. Since there is no air leakage across the valve, air vent-

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ing is not required, and erosive wear will not be a problem with the handling ofabrasive materials, although the abrasive wear element will remain.

2.7.1 Rotary A ir Locks

With negative pressure conveying systems the reception vessel will be maintainedunder vacuum. Rotary valves, or rather rotary air locks in this case, are often usedfor the discharge of materials from such reception hoppers. If discharge occurswhile conveying takes place, and the vessel is under vacuum, there will be a leak-age of air across the valve, in exactly the same way as air will leak across a rotaryvalve when feeding a positive pressure conveying system.

Although the leakage of the air across the rotary air lock into the receptionvessel will not influence the feeder in any way, the air that leaks across the rotaryair lock will result in the conveying line being starved of air. Exhausters are speci-fied for a given duty, and so if air leaks into the system such that it by-passes theconveying pipeline, the conveying line inlet air velocity will be reduced as a con-sequence. If the air ingress rate is too high, such that the conveying line inlet airvelocity falls below the minimum conveying air velocity for the material, the pipe-line is likely to block.

3 SCREW FEEDERS

Much of what has been said about rotary valves applies equally to screw feeders,with respect to both positive pressure and vacuum conveying systems. They arepositive displacement devices and so feed rate control can be achieved by varyingthe speed. They can be used for either positive pressure or vacuum pipeline feed-ing duties. Air leakage is a problem when feeding into positive pressure systems,and they are prone to wear by abrasive materials.

There are two basic types of screw feeder: the simple screw feeder and thevariable pitch device, and the two types have very different capabilities with re-gard to pipeline feeding.

3.1 The Simple Screw Feeder

A simple type of screw feeder is shown in Figure 2.10. Rotation of the screwmoves a continuous plug of material into the pipeline, where it is dispersed andentrained with the conveying air. A particular advantage of screw type feeders isthat there is an approximate linear relationship between screw speed and materialfeed rate, and so the discharge rate can be controlled to within fairly close limits.

The simple type of screw feeder, however, is rarely used for feeding positivepressure conveying systems. This is because there is little in their design to satisfythe basic requirement of feeding across an adverse pressure gradient. Air leakagerepresents a major problem with many materials, and so they are generally limitedto vacuum systems where operating pressure differentials are not a concern.

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MaterialFeed

Screw

CheckValve

Air and Material

Drive Shaft

Air

Figure 2.10 Simple screw feeder.

3.2 Variable Pitch Screw Feeder

The simple screw feeder has been developed by several companies into a devicethat can feed successfully into conveying lines at pressures of up to about 35lbf/in2 gauge. One such device, which was manufactured by the Fuller Companyof the USA, and known as a Fuller-Kinyon pump, or screw pump, is shown inFigure 2.11.

Material Feed

WeightedNon-return

Valve

Air and Material

Drive ShaftScrew t

Air Inlet

Figure 2.11 Commercial type of screw feeder.

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The main feature of these screw feeders is that the screw decreases in pitchalong its length. By this means the material to be conveyed is compressed to forma tight seal in the barrel. These feeders used to be widely used in the cement indus-try, and for fly ash conveying in power stations. The material is fed from the sup-ply hopper and is advanced through the barrel by the screw.

Since the screw pitch decreases towards the outlet, this compacts the mate-rial as it passes through the barrel. This is sufficient to propel the plug through thepivoted non-return valve at the end of the barrel and into a chamber into which airis continuously supplied through a series of nozzles. A pressure drop of about 7lbf/in2 must generally be allowed for the air across these nozzles, which adds sig-nificantly to the power requirement.

A screw having a decreasing pitch does, however, require a very high powerinput for the feeder. For a given feed rate a screw pump might require a 100 hpmotor for the screw drive, compared with about 5 hp for a rotary valve, and effec-tively zero power for a blow tank system.. For high pressure operation the deviceis only suitable for materials that can be compressed, which generally restrictstheir use to materials that have very good air retention properties, such as cementand fly ash.

Both fly ash and cement are abrasive materials and so maintenance of such afeeder is a problem in industries that demand long operating periods betweenplanned maintenance shut-downs. As a consequence of the high power demandand wear problems, the market for this type of feeder has reduced in recent years.It is often used, however, where high pressure, closed loop conveying of fine, po-tentially combustible materials, is required.

4 VENTURI FEEDERS

Since one of the basic problems with feeding positive pressure conveying systemsis that the air leakage arising from the adverse pressure gradient can interfere withthe flow of material into the pipeline, this situation can be improved by using ven-turi feeders. These basically consist of a reduction in pipeline cross-section prior tothe region where the material is fed from the supply hopper, followed by a diver-gent section, as shown in Figure 2.12.

A consequence of this reduction in flow area is an increase in the entrainingair velocity and a corresponding decrease in pressure in this region. With a cor-rectly designed venturi the pressure at the throat should be the same, or just a littlelower, than that in the supply hopper which, for the majority of applications, willbe atmospheric pressure. This then encourages the material to flow more readilyunder gravity into the pipeline, since under these conditions there is no leakage ofair in opposition to the material feed.

In order to keep the throat at atmospheric pressure, and also of a practicalsize that will allow the passage of material, and for it to be conveyed, a relativelylow limit has to be imposed on the air supply pressure.

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Material Feed

Air Air andMaterial

Inlet Throat

Figure 2.12 Basic type of venturi feeder.

These feeders, therefore, are usually incorporated into systems that are re-quired to convey free flowing materials at low flow rates over short distances.Since only low pressures can be used with the basic type of venturi feeder shownin Figure 2.12, a standard industrial type of fan is often all that is needed to pro-vide the air requirements.

4.1 Commercial Venturi Feeder

To fully understand the limitations of this type of feeder the thermodynamic rela-tionships need to be followed and these are presented in most standard textbookson the subject. In a 4 inch bore pipeline, with air supplied at 3 lbf/in2 gauge, forexample, the throat diameter would have to be about 1 !/2 inch. Although Venturiscapable of feeding materials into conveying systems with operating pressure dropsof 5 lbf/in2 are commercially available, the pressure drop across the venturi can beof the same order. Such a venturi is shown in Figure 2.13.

Material Feed

AirAir andMaterial

Nozzle Throat

Figure 2.13 Commercial type of venturi feeder.

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This means that the air supply will have to be at about 10 lbf/in2 gauge andconsequently the air will have to be supplied by a positive displacement blower.Since there are no moving parts these feeders are potentially suitable for abrasiveand friable materials. Care must be exercised in using Venturis to feed such mate-rials into the conveying line for the very high air velocity in the throat may lead toconsiderable erosion and particle degradation in this region. If abrasive materialshave to be conveyed, a verturi feeder fitted with a replaceable wear resistant linerwould be recommended. Experience has shown that these feeders are best suitedto the handling of free flowing materials. Care must be taken, however, to con-tinuously control the flow of materials, otherwise a blockage may occur.

4.2 Feed Control

It will be seen that there are no moving parts with this type of feeding device,which has certain advantages with regard to wear problems, but there is no inher-ent means of flow control either, and so this has to be provided additionally. Thismeans that the venturi could be fed from a belt, screw, rotary valve or vibratoryfeeder.

A screw or vibratory feeder could be very elementary devices since therewill be no problems of pressure differential and air leakage. Alternatively a supplyhopper could be used if fitted with a trickle valve (as illustrated in Figure 2.13), acalibrated orifice plate or a gate/slide valve, provided that this type of device pro-vides a suitable degree of control for the material to be fed.

5 GATE LOCK VALVES

These are probably the least used of all devices for feeding pneumatic conveyingsystem pipelines. They are variously known as double flap valves, double dumpvalves, and double door discharge gates. They basically consist of two doors orgates that alternately open and close to permit the passage of the material from thesupply hopper into the conveying line. The operating sequence is illustrated inFigure 2.14.

These gates may be motor driven, cam or air cylinder operated, or may workunder gravity. The air which passes the lower gate from the conveying pipeline isvented so that it does not interfere with the material about to flow through the up-per gate, in positive pressure systems. As with rotary valves, the blower or com-pressor should be sized to allow for this leakage, although this is not as effective inthis case, as there is an order of magnitude in difference in the operating fre-quency.

Like the venturi feeder, care must be taken to ensure that the material is me-tered into the gate lock since it will cease to function correctly under a head ofmaterial, as would be the case if it was situated directly beneath the outlet of thesupply hopper. A typical commercial type of gate valve feeder is shown in Figure2.15.

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Figure 2.14 Operating sequence of gate lock valves.

Although the device is capable of providing a controlled feed rate, it is verymuch an intermittent feeder. The gate lock feeder typically operates at between 10and 15 cycles per minute. In contrast, the rotary valve has approximately 150 to200 discharges per minute from its pocketed rotor.

Material

Flow ControlValve

Upper Hopper

Seat

Vented Air

Gate

AirAir andMaterial

Figure 2.15 Commercial type of gate valve feeder.

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Such a frequency is relatively slow and would result in a constantly fluctuat-ing conveying line inlet air pressure. In order to maximize material flow ratethrough a conveying pipeline the air supply pressure needs to remain reasonablyconstant. As a consequence the efficiency of conveying is reduced.

This reduction in the number of discharges also means that the air supply, interms of flow rate, and particularly pressure, must be correctly evaluated to pre-vent the possibility of line blockage. With few moving parts this type of feeder canbe used to feed friable materials, and with appropriate materials of construction itis also suited to the handling of abrasive materials.

They are often used to feed large granular materials, but they should be of arobust construction. Care must be exercised to prevent large particles from gettingtrapped in the gates and causing them to buckle or deflect. If the gates cease to seatcorrectly, a large proportion of the conveying air could be lost through the ventingsystem which could result in pipeline blockage.

The same situation will occur if large particles jam in the gate and preventits complete closure. Once again, this is only a serious problem with positive pres-sure conveying systems.

6 SUCTION NOZZLES

A specific application of vacuum conveying systems is the pneumatic conveyingof bulk particulate materials from open storage and stockpiles, where the top sur-face of the material is accessible. Vacuum systems can be used most effectivelyfor the off-loading of ships and for the transfer of materials from open piles tostorage hoppers.

They are particularly useful for cleaning processes such as the removal ofmaterial spillage and dust accumulations. In this role they are very similar to thedomestic vacuum cleaner. For industrial applications with powdered and granularmaterials, however, the suction nozzles are rather more complex.

It is essential with suction nozzles to avoid filling the inlet tube solidly withmaterial, and to maintain an adequate flow of air through the conveying line at alltimes. To avoid blocking the inlet pipe, sufficient air must be available at the ma-terial feed point, even if the suction nozzle is buried deep into the bulk solid mate-rial.

Indeed, the vacuum off-loading system must be able to operate continuouslywith the nozzle buried in the material at all times in order to maximize the materialflow rate. Sufficient air must also be available for conveying the material throughthe pipeline once it is drawn into the inlet pipe. In order to obtain maximum outputthrough a vacuum line it is necessary to maintain as uniform a feed to the line aspossible.

To satisfy these requirements two air inlets are required, one at the materialpick-up point and another at a point downstream. A typical suction nozzle for vac-uum pick-up systems is shown in Figure 2.16.

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Material Inlet

Figure 2.16 Suction nozzle for vacuum pick-up systems.

6.1 Feed Rate Control

The suction nozzle is provided with an outer sleeve at its end, and primary air formaterial feed is directed to the conveying line inlet in the annular space created.The length 'a' of this sleeve has to be long enough to ensure that it is not buried bythe movement of the material and so prevent the flow of primary air. This sleevemay be many feet long for a ship off-loading application. The position of the endof the sleeve relative to the end of the pipeline, 'b', is mostly material dependent,and could be positive or negative. This dictates the efficiency with which the ma-terial is drawn into the conveying line.

Secondary air for conveying the material is generally introduced via a seriesof holes in the pipeline. Some form of regulation of both the primary and secon-dary air is necessary, and the proportion of the total which is directed to the mate-rial inlet is particularly important. This is also material dependent, in a similar wayto the proportion of the total air supply which is used in a blow tank for control ofthe discharge rate into the pipeline. In a way, the vacuum nozzle is very similar toa blow tank. Neither of them have any moving parts, but by proportioning the airbetween primary and secondary supplies, total control can be achieved over mate-rial feed rate. This is discussed in more detail in a later section of this chapter onblow tanks.

6.2 Flow Aids

The end of the pipeline at the material inlet point is often fabricated into a rectan-gular shape for manual applications in order to facilitate more effective surfacecleaning. Many variations in shape and design are possible, including the use of

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multiple 'tails' to a common suction line. In the case of large scale vacuum sys-tems, such as ship off-loading, it is often necessary to attach mechanical dredgingand paddle devices to the end of the nozzle. This is particularly so if materials withpoor flow properties have to be unloaded, for it is essential to maintain a continu-ous supply of material to the nozzle to achieve the maximum potential of a vac-uum line.

6.3 Hopper Off-Loading

Although suction nozzles are generally associated with mobile systems such as forspillage clearance and ship off-loading applications, they can equally be used infixed systems for the emptying of hoppers and silos. In this application the nozzleis usually positioned in the bottom of the hopper and a typical arrangement is illus-trated in Figure 2.17

The vacuum nozzle is generally fitted into the hopper via a sleeve so that itcan be easily removed when required. The controls over the primary and secon-dary air are also arranged to be external to the hopper, and very often the locationof the inner tube with respect to the outer tube can also be adjusted external to thehopper. For these reasons a section of flexible hose is often incorporated into theconveying pipeline close to the hopper.

7 TRICKLE VALVES

These are only suitable for negative pressure conveying systems, since there is nopressure drop against which to feed. The greatest problem with this class of feederis that of flow rate control.

Supply Hopper

To Delivery

Flexible Hose

Figure 2.17 Application of vacuum nozzles to hopper off-loading.

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Control is generally achieved by calibration and adjustment on site, but thisis very material dependent. A slight change in particle size, particle shape or mois-ture content will affect the balance of the setting for the material and change theflow rate. With this type of feeder it is usual to have a short length of pipeline priorto the material feed point and to choke this at the inlet. By this means a slightnegative pressure will be generated beneath the hopper, which will mean that therewill be a slight pressure gradient in the direction of material feed, and this willencourage the flow of material into the pipeline.

8 BLOW TANKS

In recent years blow tanks have become more widely used, but there is a lot ofuncertainty as to how they operate, how they can be controlled and how theymight be specified for a given duty. There are also a large number of differenttypes and configurations available. They are rarely available as a standard piece ofequipment that can purchased from a catalog, like a rotary valve, and are generallysupplied as part of a package for a complete conveying system.

8.1 Introduction

Blow tanks are often employed in pneumatic conveying systems because of theircapability of using high pressure air. A high pressure air supply is necessary if it isrequired to convey over long distances in dilute phase, or to convey at high massflow rates over short distances through small bore pipelines. Blow tanks are nei-ther restricted to dense phase conveying nor to high pressure use.

Materials not capable of being conveyed in dense phase can be conveyedequally well in dilute phase suspension flow from a blow tank. Depending upontheir pressure rating, blow tanks generally have to be designed and manufacturedto an appropriate pressure vessel code, and are subject to insurance and inspection.They can, therefore, be more expensive than alternative feeding systems.

8.1.1 Low Pressure Systems

Low pressure blow tanks are often used as an alternative to screw feeders and ro-tary valves for feeding pipelines, particularly if abrasive materials have to be con-veyed. The blow tank has no moving parts and so both wear of the feeder and deg-radation of the material are significantly reduced. Low pressure blow tanks operat-ing with positive displacement blowers, for example, do not usually need to becoded vessels and so the cost of this type of blow tank can be much lower as aconsequence.

Another advantage of these systems is that the blow tank serves as both thehopper and feeder, and so the problems associated with feeding against an adversepressure gradient, such as air leakage, do not arise. There will, however, be a smallpressure drop across the blow tank in order to discharge material into the pipeline,

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and so this must be taken into account when evaluating air requirements. Therewill also be a need to vent the blow tank when it is filled with material.

5.7.2 Blow Tank Control

In most blow tank systems the air supply to the blow tank is split into two streams.One air stream pressurizes the blow tank and may also fluidize or aerate the mate-rial in the blow tank. This air stream serves to discharge the material from theblow tank. The other air stream is fed directly into the discharge line just down-stream of the blow tank. This is generally referred to as supplementary air and itprovides the necessary control over the material flow in the conveying line. Moredetailed information is provided later in this chapter.

8.2 Basic Blow Tank Types

There are numerous different types of blow tank, and for each type alternativeconfigurations are possible. The basic features of different blow tanks are essen-tially similar, but different arrangements can result in very different conveyingcapabilities and control characteristics.

8.2.1 Top and Bottom Discharge

The blow tank shown in Figure 2.18 is a top discharge type. Discharge is arrangedthrough an off-take pipe which is positioned above the fluidizing membrane. Withthis type of blow tank, however, it is not possible to completely discharge the con-tents, although with a conical membrane very little material will remain.

Conveying Line

Supplementary orConveying Air

Blow TankVent Line

Discharge Pipe

FluidizingMembrane

Fluidizing Air

Figure 2.18 Top discharge blow tank with fluidizing membrane.

Air Supply

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Vent

Balancing line

Air.

Hopper

Inlet Valve

Blow Tank

Discharge valve

Air and Material

Figure 2.19 Bottom discharge blow tank.

In a bottom discharge blow tank there is no membrane and material is grav-ity fed into the pipeline, and so the contents can be completely discharged. Such ablow tank is shown in Figure 2.19.

Top and bottom discharge generally refers only to the direction in which thecontents of the vessel are discharged. This simple classification, however, canbecome confused by the considerable number of different configurations that areused to admit air to the blow tank and to the conveying line.

A number of alternative top and bottom discharge blow tank types, with andwithout fluidizing membranes, are shown in Figure 2.20, and a number of alterna-tive bottom discharge arrangements are shown in Figure 2.21. This is by no meansa definitive group for there are a considerable number of other possibilities. Manycompanies like to adopt a particular configuration that is recognizable as a specificproduct of their company.

Figure 2.20 Alternative top discharge blow tank arrangements, (a) With fluidizingmembrane, (b) with conical off-take pipe and (c) with aerated base.

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Figure 2.21 Alternative bottom discharge blow tank arrangements, (a) With air inlet totop only, (b) with air to top and pipeline and (c) with two air supplies at base.

In Figures 2.20a and c, and 2.21c the material is aerated or fluidized, bymeans of a porous membrane in the first case, and via a narrow annular gap in theother two. In some cases an additional air supply is taken directly to the entranceof the off-take pipe to provide further fluidization in this region. This is sometimesnecessary for materials with very poor air retention, for they could block the dis-charge pipe if only a small percentage of the total air supply is directed to the blowtank for aerating the material and pressurizing the blow tank.

In some of the blow tanks illustrated only one air supply line is used. Theapplication of these types is strictly limited since little control can be exercisedover material flow rate, unless air is introduced into the pipeline downstream ofthe blow tank via trace lines or boosters. In general the top discharge type withfluidization of the material is most suitable for powdered materials, and bottomdischarge blow tanks are best suited to granular materials.

8.2.1.1 Fluidizing MembranesFluidizing membranes may consist of a porous plastic, a porous ceramic, or a filtercloth sandwiched between perforated metal plates. A perforated metal plate is re-quired beneath the membrane in order to support the mass of fluidized materialthat it has to carry. A perforated metal plate is needed above the membrane in or-der to provide support against the pressure of fluidizing air from beneath. If a po-rous membrane is used it is important that the fluidizing air is both clean and dry,for dust and moisture in the air will cause a gradual deterioration in performance.

In top discharge blow tanks it is not usually necessary for the discharge pipeto have a conical end, unless additional fluidization is required in this region. Atypical arrangement is shown in Figure 2.22. For powdered materials the off-takepipe needs to be spaced about two inches above the base or membrane. If it is fur-ther away the blow tank will simply discharge less material and hence reduce itseffective capacity. If it is too close it may adversely affect the discharge rate.

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Material-Air flow toConveying Line

Off-Take Pipe

- -

tAir Inlet from Plenum Chamber

Figure 2.22 Sketch of straight end discharge pipe.

5.2.2 Blow Tank Pressure Drop

The pressure drop across the blow tank represents a potential source of energy lossto the conveying system and so should be kept as low as possible. In the case oftop discharge blow tanks this is particularly important. The discharge pipe must bekept as short as possible because the pressure gradient in this line will be very highowing to the very high material concentration. Supplementary air should be intro-duced immediately above the blow tank, as shown in Figures 2. 1 8 and 2.20c.

With very large blow tanks the discharge pipe should be turned through 90°just above the membrane and be taken through the side of the vessel if necessary.Alternatively the supplementary air should be introduced within the blow tank,and be fed into the discharge pipe close to the membrane end in the style of a vac-uum nozzle. If the discharge pipe is kept to about six feet long the pressure dropacross the blow tank will be about 3 lbf/in2, which includes the membrane resis-tance. In the case of bottom discharge blow tanks, very short discharge lines canusually be arranged and so the pressure drop is generally no more than about 1 !/2lbf/in2.

8.2.3 Road and Rail Vehicles

Many road and rail vehicles used for the transport of bulk solids are essentiallyblow tanks. In the case of road tankers the vehicle usually has its own air supplyfor off-loading. These are generally rated at a pressure of about 1 5 lbf/in2 gaugeand positive displacement blowers are used for the purpose. Rail vehicles gener-ally rely on a site supply for off-loading, with a much higher pressure capability.

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8.3 Single Blow Tank Systems

A particular problem with single blow tank systems is that conveying is not con-tinuous, as it can be with rotary valve and screw feeding systems. In order toachieve an equivalent material mass flow rate, therefore, steady state values of theflow rate during conveying have to be somewhat higher. This point was illustratedearlier with Figure 1.8.

8.3.1 Blow Tanks Without a Discharge Valve

The simplest form of blow tank is one which has no discharge valve. Such an ar-rangement is illustrated in Figure 2.23. This is shown in a top discharge configura-tion with a fluidizing membrane, but could equally have been shown in any of thearrangements given in Figures 2.18 to 2.21. With abrasive materials the dischargevalve is particularly susceptible to wear and so the possibility of operating a blowtank without such a valve can be a considerable advantage.

Although there is no valve in the material discharge line, other valving isnecessary. A valve is required to isolate the blow tank from the material supplyhopper, so that the blow tank can be pressurized, and a vent line valve is neededfor venting the blow tank whilst filling from the hopper. These valves are eitherfully open or closed. Valves, or possibly flow restrictions or orifices, are requiredin the air supply lines in order to provide the necessary degree of control over thematerial discharge rate from the blow tank.

Conveying Line

Blow TankVent Line

Discharge Pipe

FluidizingMembrane

Supplementary orConveying Air

Air Supply

Fluidizing Air

2.23 Single blow tank without discharge valve.

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Operation without a discharge valve will present no operational problemsfor top discharge blow tanks but the possibility of such an arrangement with bot-tom discharge blow tanks depends upon the material being fed. Fine, free flowingmaterials are likely to flood feed, with the possibility of blocking the pipeline onstart up.

8.3.1.1 Conveying Cycle AnalysisWith the arrangement shown in Figure 2.23 the blow tank starts to pressurize assoon as the vent line valve is closed. Both the blow tank and conveying line haveto be pressurized before any material is delivered from the pipeline and this proc-ess can take a significant proportion of the total cycle time. Even when the mate-rial is first discharged from the conveying line, the pressure, and hence conveyingrate, have still to reach steady state values. The pressure builds up gradually asmore material is conveyed, but it is a relatively slow process.

Towards the end of the conveying cycle, when the blow tank has almostbeen discharged, the blow tank has to be de-pressurized and the entire conveyingline has to be cleared of material and vented. This process also takes a significantamount of time, particularly if the pipeline is long. The time required to fill theblow tank and set the valves has to be taken into account in addition. This type ofblow tank system, however, is very easy to operate and maintenance costs are verylow.

8.3.2 Blow Tanks With Discharge Valves

If there is a valve on the blow tank discharge line, and control valves on the sup-plementary and fluidizing air supply lines, the blow tank can be pressurized in ashorter space of time if all the air available is directed to the blow tank, and dis-charge is prevented until the steady state pressure is reached. This time can beshortened further if an additional air supply is available for the purpose, but thecost and complexity would be considerable, and the benefits obtained wouldprobably be marginal.

When the blow tank discharge valve is opened the control valves on thesupplementary and fluidizing air supply lines must be returned to their settings forconveying. This is essential, for the correct air flows must be maintained toachieve satisfactory blow tank discharge and material conveying at the desiredrate. In the blow tank without a discharge valve these settings are rarely changed,and this is why it takes so long to achieve steady state conveying, particularly ifthe material is conveyed in dilute phase.

8.3.2.1 Blow Tank VentingIf there is a vent line between the blow tank and the supply hopper it will also bepossible to reduce the time required for de-pressurizing the system. As soon as theblow tank is empty, the discharge valve should be shut and the vent line opened. Itwill also be necessary to shut the blow tank fluidizing air supply valve and fullyopen the supplementary air supply valve. By this means the blow tank can be iso-

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lated from both the air supply and the conveying line, and the processes associatedwith each can be carried out simultaneously.

By this means the blow tank can be de-pressurized very quickly in isolationfrom the conveying line. The total air supply will still be available to the pipelineso that this can be purged separately, and at the same time. This will also preventthe large volume of air in the blow tank from expanding rapidly through the con-veying line, thereby causing very high air velocities and possible severe pipelineerosion during the venting process if the conveyed material is abrasive.

Isolation of the blow tank will also reduce the loading on the filtration unitat this time in the conveying cycle. It is important that this surge of air at the endof the cycle is taken into account when sizing the filters for the plant, regardless ofthe mode of blow tank operation, but particularly if the blow tank is not vented inisolation. If the blow tank is vented to the supply hopper it is equally essential thatthe filter on the supply hopper is also correctly sized for the anticipated volumetricflow rate.

8.4 Twin Blow Tank Systems

If two blow tanks are used, rather than one, a significant improvement in perform-ance can be achieved. There are two basic configurations for twin blow tanks.One is to have the two in parallel and the other is to have them in series.

8.4.1 Twin Blow Tanks in Parallel

The ratio of the mean flow rate to the steady state material flow rate can bebrought close to unity if two blow tanks in parallel are used. While one is feedingmaterial into the conveying pipeline, the other can be de-pressurized, be filled withmaterial, and be pressurized, ready for discharging when the other blow tank isempty. By this means almost continuous conveying can be achieved through acommon pipeline.

This arrangement, however, requires a full set of discharge, vent and isolat-ing valves for each blow tank and an automatic control system to achieve the op-timum timing. In some plants three blow tanks are utilized. A typical twin blowtank arrangement is shown in Figure 2.24. The sequence of events would be asfollows:

Blow Tank A Blow Tank B

fill dischargepressurize

Change •> ———^__—_^_^^__Over vent

discharge fillpressurize

Change •> —————^—^———Over ventfill discharge

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Feeding Devices 57

Conveying Line

Air Supply

Figure 2.24 Typical twin blow tank arrangement.

From this it can be seen that the blow tank pressurizing process in one blowtank has to be carried out while the material is being discharged from the other.This would require additional air and, once again, it would probably not be eco-nomically viable for the marginal improvement obtained. To achieve a high mate-rial flow rate with a single blow tank, a fairly large blow tank would be needed,but with twin blow tanks the blow tank size can be smaller. The size can be basedon a reasonably short blow tank cycle, provided that the two sets of sequences canbe fitted into the time available.

8.4.2 Twin Blow Tanks in Series

If two pressure tanks are placed vertically in line beneath a hopper it is possible touse a high pressure air supply for the continuous conveying of a material. A typi-cal arrangement is shown in Figure 2.25. The vessel between the hopper and theblow tank transfers the material between these two, and is effectively a lock hop-per. The vent line is used to release the pressure in the transfer vessel, in additionto venting on filling.

The lock hopper, or transfer vessel, is filled from the hopper above. The lockhopper is then pressurized to the same pressure as the blow tank, either by meansof a pressure balance from the blow tank, which acts as a vent line for the blowtank while it is being filled, or by means of a direct line from the main air supply.With the transfer vessel at the same pressure as the blow tank, the blow tank canbe topped up to maintain a continuous flow of material.

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58 Chapter 2

SupplyHopper

Vent Line

Lock Hopper

PressureBalanceand Vent

ConveyingLine

Air Supply

2.25 Blow tank system capable of continuous operation.

The lock hopper, however, will have to be pressurized slowly in order toprevent a loss in performance of the system while it is conveying material. Theblow tank in Figure 2.25 is shown in a top discharge configuration, but without afluidizing membrane. The air enters a plenum chamber at the base, to pressurizethe blow tank and fluidize the material, and is discharged via an inverted cone intothe conveying line. Twin blow tanks, with one positioned above the other, do re-quire a lot of headroom, and so the blow tank arrangement shown in Figure 2.25 issometimes employed to minimize the head required.

8.4.2.1 Alternative Feeding ArrangementsIf a lock hopper arrangement is used, as shown in Figure 2.25, the pipeline feedingdevice need not be a blow tank at all, despite the use of high pressure air. With thetransfer pressure vessel separating the hopper and the pipeline feeding device, thefeeding device can equally be a rotary valve or a screw feeder, for there is littlepressure drop across the feeder. The pressure drop is, in fact, in the direction ofmaterial flow and so there are no problems of air leakage across the device, asthere are with conventional feeders of this type.

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Feeding Devices 59

A rotary valve or screw may be used in this situation to guarantee the feedof a steady flow of material into a pipeline. If a rotary valve or screw is to be em-ployed, designs to cater for high pressure differentials do not have to be used. Ero-sive wear problems associated abrasive materials are also significantly reducedwith this type of system. The most usual configuration is to mount the rotary valveor screw inside the blow tank. A sketch of a screw feeder based on this twin blowtank principle is given in Figure 2.26.

8.4.2.2 ApplicationsIn cases where there is a need for a high air supply pressure, either to convey amaterial in dense phase or over a long distance, and continuous operation is essen-tial, such a twin blow tank system is ideal. Although these systems do requiremore headroom than rotary valves, screw feeders and many single blow tank sys-tems, this need not be excessive. It clearly depends upon the material flow rate tobe achieved, but if a reasonable cycling frequency between the two pressure tanksis employed, the capacity of the vessels can be quite small and a compact systemcan be obtained.

A particular application of these systems is for the direct injection of pulver-ized coal (DIPC) into boilers and furnaces. In the case of furnaces the materialoften has to be delivered against a pressure. This, of course, presents no problemsince high air supply pressures can be utilized.

\ Supply >\Hopper / Vent Line

Lock Hopper

Air Supply

Material FeedVessel

Screw Feeder

Pipeline

Figure 2.26 Twin blow tank system with screw feeding.

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60 Chapter 2

A general requirement of DIPC systems is that the material should be con-veyed at a very uniform rate, and that it should also be capable of achieving a highturn-down ratio. An operating range of 10:1 on material flow rate is often re-quested in this respect. Blow tanks are capable of operating quite successfully overthis range and so they are ideally suited to this type of application.

8.4.2.3 Alternative Blow Tank ArrangementIf headroom is restricted, particularly in the case of an existing system, which mayrequire to be changed or up-rated, it is possible to design a series operating blowtank system such that only the lock hopper has to be located beneath the supplyhopper. A typical arrangement, with a screw feeder incorporated is shown in Fig-ure 2.27.

In this case the conveying blow tank is positioned alongside the lock hopperand the transfer has to be achieved by pneumatically conveying the material be-tween the two, instead of using gravity. The driving force for this particular devel-opment was the possibility of replacing screw pump feeding systems with suchblow tanks. The lock hopper fits into the existing space beneath the hopper, va-cated by the screw pump, and the blow tank is placed alongside. This requires thematerial in the lock hopper to be conveyed to the blow tank, but it does allow con-tinuous operation.

Vent Line

Vent Line \

Lock Hopper /Transfer Vessel

Blow Tank Feed Line

DischargeBlowTank

Screw Feeder

\A A A AVV \I

Pipeline

Figure 2.27 Sketch of side-by-side arrangement of twin blow tanks in series.

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Feeding Devices 61

8.5 Blow Tank Control

With rotary valves and screw feeders, material flow rate can be controlled, over alimited range, simply by varying the drive speed. Blow tanks, as it has alreadybeen mentioned, have no moving parts, and yet turn-down ratios of 10:1 can beachieved quite successfully.

8.5.1 A ir Proportioning

Control of a blow tank is achieved by proportioning the total air supply betweenthat which is directed to the blow tank and that which goes directly to the start ofthe conveying line. The total air supply is used to convey the material through thepipeline.

8.5.1.1 Blow Tank AirThe air directed to the blow tank is used to pressurize the blow tank. This air sup-ply may also aerate or fluidize the material, depending upon the bulk characteris-tics of the material. The blow tank air discharges the material from the blow tankinto the conveying line. The solids loading ratio of the material in the blow tankdischarge line can be very high, and hence there is a pressure drop associated withthis feeding. This is why supplementary air is necessary, unless the conveying lineis short and high pressure air is available.

8.5.1.2 Supplementary AirThe supplementary air passes directly to the start of the conveying line at the blowtank discharge point. The supplementary air effectively dilutes the flow of materialfor conveying through the pipeline. It is essential that the correct solids loadingratio is achieved at this point in order to match the capability of the air mover interms of pressure available.

If the solids loading ratio is too low, for example, the pressure drop over theconveying line will be low and the pipeline will be under-utilized. If, on the otherhand, the solids loading ratio is too high, the pressure drop required to convey thematerial through the pipeline may exceed the capability of the air mover, and thepipeline will probably block.

8.5.2 Discharge Rate Control

To show how the proportion of air that is used to fluidize the material in the blowtank can influence the discharge rate, a graph of material flow rate against total airmass flow rate has been drawn, and data in terms of the ratio of fluidizing air tototal air mass flow rate has been plotted. The resulting family of curves is shownin Figure 2.28.

This graph shows how the total air supply from the compressor should bedivided between the blow tank for fluidizing the material, and the supplementaryair line for conveying the material. Provision, therefore, must be made for thiscontrol facility on the plant, and this can be clearly identified as a point to observeduring the commissioning of a plant.

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62 Chapter 2

24

20

,-C!

S 16

12

Proportion of totalair flow directedto blow tank - %

Conveyinglimit formaterial

100

10

40 80 120

Air Flow Rate - ftVmin

160 200

Figure 2.28 Typical blow tank discharge characteristics.

Figure 2.28 was derived from the conveying of cement through a 330 ft longpipeline from a top discharge blow tank having a fluidizing membrane. Both thedischarge pipe and the pipeline were two inch nominal bore and contained seven-teen 90° bends. The 100% line represents the conveying limit for the blow tank,and would represent the only control available in a blow tank without supplemen-tary air.

8.5.2.1 Material InfluencesIt is well known that different materials can have totally different conveying

characteristics when conveyed through exactly the same pipeline. The same alsoapplies in terms of different materials with respect to their blow tank dischargecharacteristics. These characteristics will also differ with blow tank type, in par-ticular, top and bottom discharge configurations. If a higher discharge rate is re-quired for a blow tank, an improvement in the aeration of the material might help.Otherwise a larger discharge pipe will be needed. The discharge pipe does nothave to be the same diameter as the conveying pipeline.

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System Components

1 INTRODUCTION

In this chapter a brief review is given of system components and includes air mov-ers, filters, and pipeline. The correct choice of air mover is essential for the suc-cessful operation of any conveying system. Choices with regard to filtration plantare not so wide but correct specification is equally important. Pipeline also needscareful consideration for it is often with these that many plant operating problemsoccur.

2 AIR SUPPLY

The air mover is at the heart of the pneumatic conveying system, and the successof the entire system rests on correctly specifying the air mover, in terms of thevolumetric flow rate of free air required, and the pressure at which it must be de-livered. The choice of air mover to deliver the air is equally important, and there isa wide range of machines that are potentially capable of meeting the duty. Not allair movers are suited to pneumatic conveying, however, and so the operating char-acteristics must be understood and interpreted. Plant air may be available, but itmay not be economical to use. Some air movers have limitations, and some aremore suited as exhausters than compressors, and so the correct choice must bemade for vacuum and positive pressure duties [1].

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64 Chapter 3

There are also many peripheral issues associated with the supply of air forpneumatic conveying systems that are considered in addition to the basic hard-ware. Power requirements can be very high, and so a first order approximation ispresented to allow reliable estimates to be made early in the selection process.With most compressors the air is delivered at a high temperature, which may notbe desirable. When compressed air is cooled to ambient temperature it is oftensaturated with water, and this may not be suitable, and many compressors do notdeliver oil free air.

2.1 Types of Air Mover

Air movers available for pneumatic conveying applications range from fans andblowers producing high volumetric flow rates at relatively low pressures to posi-tive displacement compressors, usually reciprocating or rotary screw machines,capable of producing the higher pressures required for long distance or densephase conveying systems. The main features of some air movers typically em-ployed for both vacuum and positive pressure pneumatic conveying duties areoutlined here. The basic types of air mover that are available are categorized in achart of compressor types in Figure 3.1.

The approximate performance ranges for some of these machines are illus-trated in Figure 3.2. It should be emphasized that Figure 3.2 is intended only togive a guide to the range of operation of different types of machine. In most casesthere are substantial overlaps in their performance coverage. In particular, the re-ciprocating compressor is available in a very extensive range of sizes and types,and models could be found to satisfy almost any operating conditions, as shown inFigure 3.2. Many compressors are capable of being staged in order to deliver air atvery much higher pressures.

Compressors

Aerodynamic Positive Displacement

Radial Flow Axial Flow Rotary Reciprocating

Single Rotor Twin Rotor

I I I ISliding Liquid Blower ScrewVane Ring

Figure 3.1 Classification of air movers.

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System Components 65

200

100

50

20

10

5

2

1

-

J

«

i 1 1 1 1 i

RotaryScrew

LiquidRing — *-

PositiveDisplacem :

i , , 1 1 1 1

-f-

Reciproc

SidiVan

it

, , i 1 1 j

Siting >

'g

, i i i i i

10 100 1000 10,000

Volumetric Flow Rate - Free Air Delivered - ft3/min

Figure 3.2 Approximate ranges of operation of air movers.

2.1.1 Aerodynamic Compressors

For high pressure duties centrifugal compressors, and especially the multiple stageaxial flow types, are normally manufactured only in large sizes, handling veryhigh volumetric flow rates, and so they rarely find application to pneumatic con-veying installations. Axial flow compressors are widely used in aircraft engines,and centrifugal compressors are often used to provide the air for testing aircraftengines in wind tunnels. Fans, however, are often used for dilute phase systems asthese provide high volumetric flow rates at low pressures.

2.1.1.1 Constant Speed CharacteristicsThe main problem with fans is that they suffer from the disadvantage that the airflow rate is very dependent upon the conveying line pressure drop. The constantspeed operating characteristic tends to flatten out at high operating pressure. Thisis a fundamental operating characteristic for pneumatic conveying, and so thisclass of compressor cannot be used reliably for heavy duty conveying.

With a compressor having this type of operating characteristic, it means thatif the solids feed rate to the system should become excessive for any reason, caus-ing the pressure drop to increase significantly, the air flow rate may become solow that the material falls out of suspension, with the risk of blocking the pipeline.This is particularly a problem in dilute phase suspension flow systems where theconveying air velocity is relatively high. Because the residence time of the mate-rial in the pipeline is very short, a sudden surge in feed rate can quickly have asignificant effect on the pressure required.

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66 Chapter 3

00caU

I

120

100

80

60

40

Axial FlowCompressors

Radial FlowCompressors

Positive DisplacementCompressors —

40 60 80 100

Volumetric Flow Rate Change - %

120 140

Figure 3.3 Constant speed characteristics of aerodynamic and positive displacementcompressors.

Positive displacement machines, for which the volumetric flow rate islargely independent of the discharge pressure, are less likely to cause this type ofsystem failure. This point is illustrated in Figure 3.3, where the two classes ofcompressor are compared.

In order to convey materials reliably in pneumatic conveying systems aminimum value of conveying air velocity must be maintained. For dilute phaseconveying systems this is typically of the order of 3000 ft/min, and if it drops bymore than about 10 or 20%, the pipeline is likely to block. A small surge in thefeed rate into a pipeline of only 10% would cause a corresponding increase inpressure demand, and with either an axial or a radial flow machine, the reductionin the volumetric flow rate of the air would probably result in blockage of thepipeline.

2.1.1.2 FansIn pneumatic conveying applications, fans used are normally of the radial, flatbladed type. Fans are widely used on short distance dilute phase systems, wherethe chance of blocking the pipeline is small. Fans may be used on both positivepressure and negative pressure systems, and also on combined 'suck-blow' sys-tems, where, with light or fluffy, non abrasive materials, it is sometimes possibleto convey the material through the fan itself.

This is not a possibility with positive displacement machines, for with a'suck-blow' system an intermediate hopper must be provided, which will add verysignificantly to the cost, but the pressure capability is very much greater than thatof a fan system. On vacuum duties they are often used for cleaning operations.

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System Components 67

With waste and stringy materials, such as paper, and plastic and textile trim,the blades of the fan can be sharpened so that they cut or chop the material intosmaller pieces as it passes through the fan.

2.1.1.3 Regenerative BlowersThe performance curves for regenerative (side channel) blowers are generally bet-ter than those of the aerodynamic compressors shown in Figure 3.3, but are not asgood as those for positive displacement machines. There is a natural tendency tooperate a compressor at a pressure close to its maximum rating, but it is generallyin this area that the operating characteristics deteriorate. Particular care should betaken with regenerative blowers, therefore, and it is essential that the operatingcharacteristics of the machine are related to the extreme requirements of the sys-tem.

2.1.2 Positive Displacement Compressors

The constant speed operating characteristic for positive displacement machines,shown on Figure 3.3, provides a basis on which the design of heavy duty convey-ing systems can be reliably based. A pressure surge in the conveying system willresult in only a small decrease in the air flow rate delivered by the compressor, andthis can be incorporated into the safety margins for the system.

A pressure surge will cause a reduction in air velocity because of the com-pressibility effects, which must also be catered for in such safety margins. With apositive displacement compressor the percentage reduction in conveying air veloc-ity due to the constant speed characteristic will be no more than that caused by thecompressibility effect.

In the classification of compressors presented in Figure 3.1, five differenttypes of positive displacement compressor are included. The constant speed oper-ating characteristic of each of these is similar to that shown on Figure 3.3 for posi-tive displacement compressors.

A particular feature of most of these machines is that very fine operatingclearances are maintained between moving parts. As a result there is no possibilityof the conveyed material being conveyed through the compressor, as it can with afan. Indeed, if the material being conveyed is abrasive, even dust must be pre-vented from entering the machine or it will suffer severe damage.

2.1.2.1 BlowersIn 1854 Roots invented the original rotary positive displacement blower. They arenow widely used for pneumatic conveying applications where the operating pres-sure does not exceed about 15 lbf/in2 gauge. Blowers are probably the most com-monly used type of compressor for dilute phase conveying systems.

They provide an ideal match, in terms of pressure capability, with the con-ventional low pressure rotary valve, and is a typical working combination on manyplants. Positive displacement blowers are generally bi-rotational, so that they canbe used as vacuum pumps, or exhausters, as well as blowers.

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68 Chapter 3

Intake

Figure 3.4 Operating principle of the positive displacement blower.

The principle of the blower, which is available in sizes handling up to about20,000 ftVmin, is illustrated in Figure 3.4. Twin rotors are mounted on parallelshafts within a casing, and they rotate in opposite directions. As the rotors turn, airis drawn into the spaces between the rotors and the casing wall, and is transportedfrom the inlet to the outlet without compression.

As the outlet port is reached, compression takes place when the air in thedelivery pipe flows back and meets the trapped air. Due to this shock compressionthe thermodynamic efficiency of the machine is relatively low, and this is one ofthe reasons why these simple compressors are only used in low pressure applica-tions. In order to reduce the pulsation level, and the noise, three lobed rotors, aswell as twisted rotors, have been introduced.

The maximum value of compression ratio with these machines is generally2:1 when operating oil free. This means that for blowing, the maximum deliverypressure is about 15 lbf/in2 gauge, and for exhausting, the maximum vacuum isabout 7 or 8 lbf/in2. For combined vacuum and blowing duties these pressures willnaturally be much lower, and are typically between about 11 lbf/in2 absolute (3-7lbf/in2 vacuum) and 22 lbf/in2 absolute (7-3 lbf/in2 gauge), as a maximum. Evenwith a lubricated machine little improvement on this operating range can beachieved for suck-blow conveying systems.

2.1.2.].! CompressorsGears control the relative position of the two impellers to each other, and maintainvery small but definite clearances. This allows operation without lubrication beingrequired inside the air casing. The performance of the machine would be enhancedwith lubrication, but oil free air is a general requirement of these machines. Dou-ble shaft seals with ventilated air gaps are generally provided in order to ensurethat the compressed air is oil free.

Typical blower characteristics are shown in Figure 3.5 for a blower operat-ing as a compressor. Manufacturers of blowers rarely present operating character-istics in this form, but a plot of this type clearly illustrates the constant speed char-acteristics of the machine.

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System Components 69

12

a 1090 Power

absorbed80 - hp"]

70

£ 4

b£ 2 n13 Rotor speedQ - - rev, / min. . . . 1200

800

600 800 1000 1200 1400 1600

Volumetric Flow Rate - Free air conditions - ft3/ min

Figure 3.5 Typical operating characteristics for positive displacement blower config-ured as a compressor.

This particular plot is also useful in terms of making changes in performanceacross the range of rotor speeds and operating duties that the given model covers.Lines of constant power are particularly useful as these will indicate whether thechanges can be achieved with an existing drive. Performance data for compressorsis generally presented in tabular form, and so it would be recommended that a fewminutes be spent plotting the tabular data in the graphical form shown in Figure3.5 so that a clear picture of the performance of a machine is available.

A further development with this type of machine is to operate at very muchhigher speeds than those indicated on Figure 3.5. With an improvement in materi-als of manufacture, and greater accuracy of machining, high speed blowers havebeen developed, thereby producing a more compact machine. The thermodynamicefficiency is improved, and as a result the operating temperature is lowered, andthere is a reduction in power requirement.

2.1.2.1.2 ExhaustersPerformance characteristics for a similar positive displacement blower operatingas an exhauster are presented in Figure 3.6.

2.1.2.1.3 StagingAs with most aerodynamic and positive displacement machines, staging is alsopossible with positive displacement blowers, although it is probably less common,and is generally limited to a maximum of two machines in series. For blowing thecompression ratio is usually limited to about 1-7 for each machine, for oil freeoperation, and so a delivery pressure close to 28 lbf/in2 gauge can be achieved bythis means. With lubricated machines the compression ratio can be increased toabout 1-95, which means that a delivery pressure of 40 lbf/in2 gauge is a possibil-ity.

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70 Chapter 3

Volumetric Flow Rate - Free air conditions - ft3/min

600 800 1000 1200 1400 1600

-1

oo -2"fa

I -4

I -6

-7

Rotor speed- rev / min

Powerabsorbed....* hp

Figure 3.6 Typical operating characteristics for positive displacement blower config-ured as an exhauster.

If blowers are to be operated in series the air at outlet from the first stagemust be cooled before the second stage. Although heat exchangers are generallyused for this purpose, water sprays can also be used. Evaporation of water canhave a very significant cooling effect, because of the very high enthalpy of evapo-ration, which is typically over 1000 Btu/lb for water, and so the mass flow rate ofwater for this purpose would only need to be about 2% of the air flow rate. Con-sideration, of course, must be given to any subsequent problems with the con-veyed material and condensation.

2.1.3 Sliding Vane Rotary Compressors

For medium and high pressure systems the sliding vane type of rotary compressoris well suited. These generally produce a smoother flow of air at a higher pressurethan the blower, and a single stage machine is capable of delivering in excess of3000 ft3/min of free air at a maximum pressure of about 60 lbf/in2. Significantlyhigher operating pressures may be obtained from two stage machines. Oil injec-tion also permits higher working pressures (up to about 150 lbf/in2), but this typeof machine is generally not available in capacities greater than about 250 tf/min.

Figure 3.7 illustrates the operating principle of a single stage sliding vanecompressor. It is a single rotor device, with the rotor eccentric to the casing.Compression occurs within the machine, unlike the blower, and so the air is deliv-ered without such marked pulsations. The machine will operate equally well as anexhauster for vacuum conveying duties.

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System Components 71

F T T v / / / / / / / / / / / / / ,

Figure 3.7 Sketch of sliding vane rotary compressor.

It should be noted that some form of cooling is essential since quite hightemperatures can be reached as a result of the combined effect of the vanes rub-bing against the casing and the compression of the air. The cooling may be bywater circulated through an external jacket, or by the injection of oil directly intothe air stream just after the beginning of compression. As mentioned previously,the latter method does permit higher working pressures, but an efficient oil separa-tion system does add to the cost of the plant.

2.1.4 Liquid Ring Compressors

Most of the air movers described previously, or suitable variations of these, can beused on negative pressure conveying systems. However, the most commonly usedare probably positive displacement blowers, operating as exhausters, which arecapable typically of holding a continuous vacuum of about 15 in Hg. Higher vac-uums can be maintained by a blower fitted with water injection, but it would bemore usual to employ a liquid ring vacuum pump which can reach 23 in Hg gauge(7 in Hg absolute) in a single stage, and about IT/2 in Hg in two stages.

Liquid ring vacuum pumps having capabilities from about 80 ftVmin up to5000 ftVmin are available. The liquid ring compressor was developed around 1905from a self-priming rotary water pump, first built in 1817. As a compressor it isused for applications up to about 60 Ibf/in . This type of machine, however, isrelatively inefficient when operating as a compressor and so is more often used forlow pressure applications, generally as vacuum pumps.

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72 Chapter 3

LiquidRing

InletPort

Impeller

OutletPort

Figure 3.8 Sketch of liquid ring compressor/vacuum pump.

A particular advantage of the machine is that it produces oil free air and cantolerate a certain amount of dust. A typical form of liquid ring compressor is illus-trated in Figure 3.8.

As with the sliding vane rotary compressor, this is also a single rotor ma-chine in which the rotor is eccentric to the casing. As the impeller rotates, the ser-vice liquid (usually water) is thrown outwards to form a stable ring concentricwith the pump casing. As the impeller itself is eccentric to the casing, the spacesbetween the impeller blades and the liquid ring vary in size, so that air enteringthese spaces from the suction port is trapped and compressed before being dis-charged through the outlet port. The liquid ring also performs the useful functionsof cooling the compressed air and washing out small quantities of entrained dust.

2.7.5 Rotary Screw Compressors

A relatively recent innovation for medium to high pressure operation is the helicallobe rotary, or Lysholm, screw compressor. The rotary screw compressor waspatented in 1878, but in a form similar to the positive displacement blower, that is,without internal compression. The mathematical laws for obtaining compressionwere developed by the Swedish engineer, A Lysholm, in the 1930's.

In 1958 rotor profiles giving a high efficiency were developed but these re-quire oil injection into the compression chamber to reduce internal air leakage.The oil helps to cool the air during compression but, as with oil injected slidingvane machines, it is generally necessary to remove the oil from the compressedair. With large compressors the injection, separation and filtration equipment canrepresent a substantial proportion of the plant cost. In 1967 a much improved rotor

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System Components 73

profile was developed which allowed rolling motion between the rotor flanks withreduced air leakage, without the need for oil injection.

This machine consists essentially of male and female intermeshing rotorsmounted on parallel shafts. Inlet and outlet ports are at opposite ends of the com-pressor. Air entering one of the cavities in the female rotor becomes trapped by amale lobe, and as the rotors turn, this trapped air is compressed and moved to-wards the discharge end. Continuing rotation of the lobes causes the dischargeopening to be uncovered so that the trapped air, now at minimum volume, is re-leased into the discharge line.

Screw compressors are manufactured with capacities ranging from 120fVVmin to 25,000 ft3/min. With oil injection they can develop maximum pressuresof about 120 Ibf/in . Dry machines can reach 160 lbf/in2 with two stages, andabout 60 lbf/in with a single stage. As these machines are generally free from pres-sure pulsations it is not usually necessary to operate with an air receiver, and theydo not require special foundations for mounting, which is generally a requirementfor reciprocating compressors.

2.1.6 Reciprocating Compressors

The familiar reciprocating compressor, until recent years, was probably the mostwidely used machine for providing high pressure air for pneumatic conveyingsystems, but the screw compressor has been a serious competitor where large flowrates are required. Reciprocating compressors are available as single cylinder ma-chines, or with multiple cylinders arranged to give one or more stages of compres-sion. Reciprocating compressors probably have the best thermodynamic efficiencyof any air mover.

Where it is essential that there should be no material contamination with oil,reciprocating compressors can be provided with carbon filled ptfe (polytetra-fluoroethylene) rings, which eliminate the need for oil in cylinder lubrication, andhence additional separation equipment. A compressor of this type could thus befound to suit almost any pneumatic conveying application in the medium to highpressure range.

Even the disadvantage of a pulsating air flow, usually associated with recip-rocating machines, can be overcome by selecting one of the modern, small mobilecylinder compressors, such as that in which seven pairs of radially disposed op-posing pistons are made to reciprocate by the motion of a centrally placed wobbleplate.

2.1.7 Staging

The arranging of two or more compressors in series, in order to achieve higherdelivery pressures, is possible with most types of compressor, as mentioned earlierwith respect to the staging of positive displacement blowers. To improve the effi-ciency of compression it is usual to cool the air between stages. Isothermal com-pression of air is the most efficient method of compressing air thermodynamically

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74 Chapter 3

and so inter-cooling is an effective means of achieving this with adiabatic com-pressors.

Because of the high delivery temperature of air from compressors, whichwill be considered in detail later in this chapter, this cooling is generally essential.The lower volumetric flow rate of the air, as a result of the reduction in tempera-ture, will mean that the size of the next compression stage can be reduced, apartfrom improving conditions with regard to lubrication.

Inter-cooling by means of an air blast, or water based heat exchanger, arethe normal means of cooling the air between stages. With regard to the staging ofpositive displacement blowers, considered above, it was mentioned that watersprays could also be used.

Some compressors, however, are susceptible to damage by water drops andso it is generally recommended that the air between stages should not be cooled toa temperature below that of the prevailing dew point. The elimination of waterbetween stages will also minimize the problems caused by the possible rusting ofmaterials in this area.

2.2 Specification of Air Movers

The operating performance of compressors and exhausters, for a particular model,is generally in terms of the volumetric flow rate of the air and the delivery pres-sure, or vacuum, for a range of rotational speeds, similar to those presented in Fig-ures 3.5 and 3.6. Different models will cover a different range of duties, and thereis likely to be an overlap in volumetric flow rate capability between different mod-els.

Because air is compressible, with respect to both pressure and temperature,it is necessary to specify reference conditions for air movers, which are interna-tionally recognized. It is essential, therefore, that it should be realized that thevolumetric flow rate to be specified for the air mover will not be same as thevolumetric flow rate required to convey a material at the start of a pipeline. It willbe necessary to convert the volumetric flow rate required for the system to thevolumetric flow rate to be specified for the air mover.

The air mass flow rate will be exactly the same for the two, but this is nothow air movers are specified. It would provide a useful check, however, if the airmass flow rate was to be evaluated for the two cases. All the models required forthis type of analysis are presented in Chapter 5 on Air Requirements.

2.2.1 Blowers and Compressors

The specification of machines delivering air at positive pressures is in terms of thevolumetric flow rate of the air drawn into the machine and the delivery pressure atwhich the air is required. The pressure and temperature of the air, for which thevolumetric flow rate applies, is generally free air conditions. This is a pressure of14-7 lbf/in2 absolute and a temperature of 519 R (59°F). The situation is summa-rized in the following sketch:

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System Components 75

Intake tocompressor

LocationMaterial feed

si'Compressor Flow

Reference

Parameter:PressureTemperature

Flow rateAir velocity

p0 = 14-7 lbf//in2

T0 = 419 R

Va = specified

not appropriate

Pi - to be specifiedTI - will be given

V{ - to be calculatedC] - to be specified

2.2.1.1 PressureThe pressure to be specified for the compressor is/?/. This is the pressure of the airrequired at the material feed point into the pipeline. This will depend upon theflow rate of material to be conveyed, the conveying distance, pipeline routing andthe conveying characteristics of the material. An allowance will need to be madefor any losses in air supply lines, pressure drop across the feeder, possible surgesin feed rate, and a margin for contingencies and safety.

2.2.1.2 Volumetric Flow Rate

The volumetric flow rate to be specified is V0 . This is the volumetric flow rate of

free air that is drawn into the compressor. The critical design parameter for apneumatic conveying system is the conveying line air velocity, C/, at the material

feed point into the pipeline. This is the starting point in evaluating V0 and so a

value of Ci must be specified. This is a fundamental design parameter in pneu-matic conveying.

Equation 10 from Chapter 5 on Air Requirements can be used to determine

V0 , knowing Ch the pipeline bore, d, the air pressure, p,, and the air temperature,

T,. The constant, 0-1925, takes account of the reference, free air, values of pres-sure and temperature required at the compressor inlet:

V = 0-1925Pi d2 c\

T;ftYmin (1)

Note that an allowance must be made for any air leakage across the materialfeeding device, or any other loss of air from the system. This will have to be added

to the above V0 value, since this is only the quantity of air required for conveying

the material.

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76 Chapter 3

2.2.2 Exhausters and Vacuum Pumps

The specification of machines operating under vacuum conditions is also in termsof the volumetric flow rate of air at inlet to the machine, and the temperature hereis also 519 R. The vacuum capability of the machine is specified and this generallyrelates to the air being discharged at standard atmospheric pressure of 14-7 lbf/in2

absolute. The situation is summarized in the sketch below:

Pipeline Intake DischargeLocation Feed Discharge

|Exhauster -> Flow

Reference © ® ® ©

p4 = 14-7 lbf/in2Parameter:

PressureTemperature

Flow rate

Air velocity

atmosphericatmospheric

calculate

C, - specify

P3T3

^

- specify- 519 R

- specify

2.2.2.1 VacuumThe vacuum to be specified for the exhauster is p3. This will depend mainly uponthe pressure drop across the pipeline, p, - p2, necessary to convey the material atthe required flow rate over the given distance. An allowance will have to be madefor any other losses and margins that might need to be included, as for the com-pressors considered above.

2.2.2.2 Volumetric Flow Rate

The volumetric flow rate to be specified is V^. This is the actual volumetric flow

rate of air that will be drawn into the exhauster. The critical design parameter for apneumatic conveying system is the conveying line inlet air velocity, C/, at the ma-terial feed point into the pipeline.

This is the starting point in evaluating V^ and so a value for C/ must be

specified. Once again, this is the fundamental design parameter in pneumatic con-veying. Equation 10 from Chapter 5 on Air Requirements can be used once again

but it needs to be modified slightly to determine J'3, knowing C/, the pipeline

bore, d, the air pressures, pt and p3, and the air temperature, T,.

pt d2 C,F3 = 2-83 x —— fvVmin . . . . . (2)

A PT,

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System Components 77

The constant is now 2-83 because p, is included in the equation, as the pres-sure at the feed point may be a little below 14-7 lbf/in2 absolute. Note that an al-lowance must be made for any air leakage into the system that may occur acrossthe material discharge device or any other gain of air into the system. This will

have to be added to the above F^ value, since this is only the quantity of air re-

quired for conveying the material.

2.3 Air Compression Effects

When compressed air is delivered into a pipeline for use, the air will almost cer-tainly be very hot, and it may contain quantities of water and oil. Air delivery tem-perature and the problem of oil are considered in some detail at this stage, but thespecific subjects of moisture and condensation, and air drying, although intro-duced here, are considered generally and in more detail in Chapter 5. The powerrequired to provide the compressed air, and hence the operating cost, can be veryhigh, and this topic is also considered in some detail at this stage.

2. 3. 1 Delivery Temperature

Much of the work energy that goes into compressing air manifests itself in increas-ing the temperature of the air. For air compression to pressures greater than about30 lbf/in2 gauge air cooling is generally employed. The most efficient form ofcompression is to carry out the process isothermally, and so cylinders of recipro-cating machines are often water cooled, and if staging is employed for achievinghigh pressures, inter-cooling is generally incorporated as well. For most high pres-sure machines with some form of cooling, therefore, the influence of air tempera-ture can be neglected.

In the majority of dilute phase conveying systems, where a large volume ofair is required at a relatively low pressure, positive displacement blowers are gen-erally used. For this type of application they are not usually cooled and so the air,after compression, can be at a fairly high temperature. Thermodynamic equationsare available which will allow this temperature to be evaluated and these are pre-sented for reference. Compression can be based on an isentropic model for whichthe relationship between the absolute pressure, p, and the absolute temperature, T,is given by:

(3,

where y is the ratio of specific heats

= — = 1-4 for airCv

and subscripts 1 and 2 refer to inlet and outlet conditions

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78 Chapter 3

This is the ideal case. In practice the air will be delivered at a higher tem-perature than this due to thermodynamic irreversibilities. The compression processis adiabatic, partly because of the speed of the process, but it is far from being areversible process. As a result, the temperature of the air leaving a compressor canbe very high.

If, for example, air at a temperature of 60°F is compressed to 15 lbf/in2

gauge in a positive displacement blower, the minimum temperature after compres-sion, for a reversible process, would be about 176°F, and with an isentropic effi-ciency of 80% it would be 205°F. Irreversibility is taken into account by means ofan isentropic efficiency, which is defined as the ratio of the theoretical temperaturerise to the actual temperature rise, as follows:

T2 -(4)

where T7 = actual temperature of compressed air

A graph showing the influence of delivery pressure and isentropic efficiencyon delivery temperature is given in Figure 3.9. This covers the range of pressuresappropriate to positive displacement blowers.

If air at 60°F is compressed to 45 lbf/in2 gauge in a screw compressor it willbe delivered at a temperature of about 400°F, which is why for air compression topressures greater than about 30 lbf/in2 gauge, air cooling is generally employed.Whether or not the air can be used to convey a material without being cooled, willdepend to a large extent on the thermal properties of the material to be conveyed.

260n.o

g 220

I 180<U

I 140^>

Q 100

Inlet Conditions:1 emperature - t>0 tPressure- 14-71bf7in2abs

10 12 14 16

Delivery Pressure - lbf/in2 gauge

Figure 3.9 The influence of delivery pressure and isentropic efficiency of compressionon delivery temperature.

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System Components 79

If a conveyed material is not temperature sensitive, high temperature airmight be used for conveying, since the volumetric flow rate will be very muchhigher. This might allow a smaller compressor to be used for the duty, but caremust be taken if cold material is to be conveyed, for the chilling effect of the mate-rial might cause the air velocity to fall below the minimum conveying value.

2.5.2 Oil Free Air

Oil free air is generally recommended for most pneumatic conveying systems, andnot just those where the material must not be contaminated, such as food products,Pharmaceuticals and chemicals. Lubricating oil, if used in an air compressor, canbe carried over with the air and can be trapped at bends in the pipeline or obstruc-tions. Most lubricating oils eventually break down into more carbonaceous matterwhich is prone to spontaneous combustion and where frictional heating may begenerated by moving particulate matter.

Although conventional coalescing after-filters can be fitted, which arehighly efficient at removing aerosol oil drops, oil in the super-heated phase willpass straight through them. Super-heated oil vapor will turn back to liquid furtherdown the pipeline if the air cools. Ultimately precipitation may occur, followed byoil breakdown, and eventually a compressed air fire. The only safe solution, whereoil injected compressors are used, is to use chemical after-filters such as the car-bon absorber type which are capable of removing oil in both liquid droplet andsuper-heated phases. The solution, however, is very expensive and requires con-tinuous maintenance and replacement of carbon filter cells.

2.3.3 Water Removal

As the pressure of air is increased, its capability for holding moisture in suspen-sion decreases. As the temperature of air increases, however, it is able to absorbmore. If saturated air is compressed isothermally, therefore, the specific humiditywill automatically be reduced. If the air is not initially saturated, isothermal com-pression will reduce the specific humidity of the air and it may well reach the satu-ration point during the compression process, or in a following after-cooler.

Where air is compressed isothermally, therefore, quite large quantities ofwater vapor can be condensed, and in many cases the air leaving the compressorwill be saturated. In adiabatic compression the temperature of the air will rise, andbecause of the marked ability of warmer air to support moisture, it is unlikely thatany condensation will take place during the compression process.

2.3.3.1 Air Line FiltersAs compression occurs very rapidly, it is quite possible that droplets of water willbe carried through pipelines with the compressed air. Also, if additional cooling ofsaturated air occurs in the outlet line, further condensation will occur. The removalof droplets of water in suspension is a relatively simple process. Normal air linefilters work on a similar principle to a spin drier. Air flowing through the filter ismade to swirl by passing it through a series of louvers. This causes the water drop-

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80 Chapter 3

lets to be thrown outwards and drain to a bowl where it can be drained off. It isimportant, therefore, that such filters, and compressor and air receiver drains,should be carefully maintained, and be protected from frost.

2.3.4 Air Drying

If dry air is required for conveying a material, a reduction in specific humidity canbe obtained by cooling the air. When air is cooled its relative humidity will in-crease, and when it reaches 100% further cooling will cause condensation. Beyondthis point the specific humidity will decrease. If the condensate is drained awayand the air is then heated, its specific humidity will remain constant, but the rela-tive humidity will decrease. This process is adopted in most refrigerant types of airdryer. Alternatively a desiccant dehumidifier can be used for the purpose. If thematerial to be conveyed is hygroscopic, some form of air drying is usually incor-porated.

2.3.4.1 RefrigerantsRefrigeration drying is particularly effective when the air is warm and the humid-ity is high. Under these circumstances a cooling system can remove two to fourtimes as much energy (temperature and moisture) from an air stream as the ma-chine consumes in electrical power to accomplish this removal [2]. The air may bedried under atmospheric conditions, prior to being compressed or otherwise used,or it may be dried at pressure after it has been compressed. In the latter case refrig-eration units are used for the dual purpose of both drying and cooling the air.

These usually have two stages of heat exchange. In the first the warm air ispre-cooled by the cold, dry, outgoing air. It then passes to a refrigerant heat ex-changer where it is cooled to the required dew point. This is usually about 35°F.Drying down to this level of moisture avoids problems of ice formation and freez-ing. If any further drying is required, much lower temperatures would have to beachieved, and this would make a refrigerant unit very expensive.

Such units, however, are now available and these have the capability ofcooling the air down to -75°F. The process is generally staged, with three unitsarranged in series and parallel. The first is a conventional, continuously operatingunit, which reduces the temperature to 35°F, as above. In series with this are tworefrigeration units with the capability of cooling the air down to -75°F. Becauseice will form on these units they are arranged in parallel, with one operating, to drythe air, while the other is being de-frosted.

2.3.4.2 DesiccantsDesiccant dehumidifiers are particularly well suited to the removal of moisturefrom air at low temperature and low humidity. The driest possible air is obtainedfrom a desiccant dryer. Desiccant dryers are capable of reducing the moisture levelto an equivalent dew point temperature of -95°F if necessary. They should not,however, be used for drying warm, humid air unless absolutely necessary, for they

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System Components 81

are costly to operate. A refrigeration system will generally add 10% to the operat-ing costs, but this may be as high as 30% with a chemical type of dryer.

Typically 15% of the compressed air being dried is lost to the system as it isrequired for purging the saturated desiccant in regenerative types. An additionalproblem with this type of system is that dust can be carried over into the convey-ing line. Water droplets often result in the bursting of the desiccant granules and soit is necessary to provide a filter for these fragments.

There are two main types of desiccant dryer. In one, desiccant tablets arecharged, and when these decay they need to be topped up. In the other, a regenera-tive system is employed. For drying compressed air, two units operating in parallelare generally used. While the process air is passed through one unit for drying, thedesiccant in the other unit is being dried by heated reactivation air ready for re-use.For the drying of atmospheric air a slowly rotating (typically at about six revolu-tions per hour) device is generally used in which the process and re-circulation airstreams are kept separate by means of seals.

It should be noted that this is entirely a chemical process, and although ex-tremely low values of dew point can be achieved, there is no physical reduction intemperature of the air. The air temperature will, in fact, rise in proportion to theamount of water removed [2]. For positive pressure conveying systems these aregenerally used to dry the air after it has been compressed and cooled.

2.3.5 The Use of Plant A ir

If plant air is available it may be possible to use this rather than to purchase acompressor for the conveying system. If plant air is used it will certainly reducethe capital cost of the system, but careful consideration will have to be given to theoperating cost of this arrangement. If plant air is available at 100 or 150 lbf/in2,and the system only requires air at 15 or 30 lbf/in2, the cost of using plant air willbe significantly higher than that from an air mover dedicated to the conveyingsystem. In the long term it may well be more economical to provide the systemwith its own air mover.

2.3.6 Power Requirements

Delivery pressure and volumetric flow rate are the two main factors which influ-ence the power requirements of a compressor, blower or fan. For an accurate as-sessment of the power requirements, it will clearly be necessary to consult manu-facturer's literature. By this means different machines capable of meeting a givenduty can be compared. For a quick, approximate assessment, to allow a compari-son to be made of different operating variables, a simple model based on isother-mal compression can be used [3]:

P-)Power = 0-128 V0 In — hp - - - - - - - (5)

\P\J

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82 Chapter 3

or = 1-67 hp (6)

where V0 = air flow rate at free air conditions - ftVmin

ma = air mass flow rate - Ib/min

Pi = compressor inlet pressure - lbf/in2 absand p2 = compressor delivery pressure - lbf/in2 abs

Note:To convert hp (horsepower) to kW (kilowatts) multiply by 0-746

This will give an approximate value of the actual drive power required. Ifthis is multiplied by the unit cost of electricity it will give the cost of operating thesystem. Since power requirements for pneumatic conveying can be very high,particularly if it is required to convey a material at a high flow rate over a longdistance, this basic model will allow an estimation of the operating cost per ton ofmaterial conveyed to be made.

To give some idea of the power required for the compressor for a pneumaticconveying system, a graph is included in Figure 3.10 which shows how drivepower is influenced by delivery pressure and volumetric flow rate. Air pressuresof up to 100 lbf/in2 gauge are considered in Figure 3.10, and so will relate to highpressure systems, whether for dilute or dense phase conveying.

erf

IoOH

I

'xei

300

250

200

150

100

50

0

2000 1600 1200

0 20 40 60 80 100Air Delivery Pressure - Ibf7in2 gauge

Figure 3.10 The influence of delivery pressure and volumetric flow rate on compressorpower required.

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System Components 83

12

10

.S 8

ffl 6

Conveying line inlet air velocity = 3500 ft/min

40 80 120 160

Approximate Power Required - hp

200

Figure 3.11 Approximate power requirements for low pressure dilute phase conveying.

Figure 3.11 is drawn and included specifically for dilute phase conveyingsystems, with delivery pressures appropriate to positive displacement blowers, upto 15 lbf/in2 gauge. A conveying line inlet air velocity of 3500 ft/min has beenconsidered and so the vertical axis has been drawn in terms of pipeline bore.

It must be emphasized that the models presented in Equations 5 and 6 arestrictly only for first approximation purposes and are provided for guidance only.The power required will vary from one type of compressor to another, and it willvary across the range of operating characteristics for the machine, such as thoseshown in Figures 3.5 and 3.6. For an accurate value, therefore, manufacturer'sliterature must be consulted, as mentioned above, both for the type of compressorand the operating conditions.

In comparison with a reciprocating compressor, for example, a screw com-pressor would require approximately 10% more power to provide the same vol-ume at a given pressure. In the case of positive displacement blowers, the powerrequirements indicated on operating characteristics provided by manufacturers,such as those shown in Figures 3.5 and 3.6, do not always include transmissionlosses, etc. Values given are generally of absorbed power for the bare shaft only,and so filtration and transmission losses must be allowed for when selecting a mo-tor.

2.3.7 Idling Characteristics

All types of compressor are available in a wide range of models in order to coverthe range of volumetric flow rates indicated on Figure 3.2. The upper limit on flow

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84 Chapter 3

rate is clearly dictated by the size of the machine but the lower limit, for any givenmodel, is not so clearly defined.

For the blower shown in Figures 3.5 and 3.6 it will be seen that limits areprovided in terms of a range of rotor speeds and the turn down ratio, in terms ofvolumetric flow rate delivered for the particular model, is about 2:1 on volumetricflow rate.

If a compressor is operated at a value of volumetric flow rate below its rec-ommended lower limit, the efficiency of operation will fall. This will manifestitself by a marked change in the slope of the lines of constant power absorbed forthe machine, such as those shown on Figures 3.5 and 3.6, at air flow rates belowthe lower operating limit.

This is illustrated in Figure 3.12. These are operating curves for a screwcompressor, which have been extended beyond the normal operating range for themachine, right down to zero flow rate, and hence idling conditions.

Compressors are often left to idle, when not required to deliver air, so thatthey do not have to be re-started, and so are instantly available for use when re-quired. It will be seen from Figure 3.12, however, that there is a significant penaltyto pay in terms of power required for this operating stand-by duty.

Because of the change in slope of the lines of constant power absorbed, be-low the recommended range of operating, the power absorbed when idling, andhence delivering no air, is almost 70% of that required for full load operation.Thus when idling, at a given delivery pressure, there is a saving in power of onlysome 30%.

120

1100ot

r»:S 80VH.0

a 603t/D

£ 40

I 20Q

0Power Absorbed - hp

1 1 1

200 400 600 800

Volumetric Flow Rate - Free Air Conditions - fWmin

1000

Figure 3.12 Typical idling characteristics for a screw compressor.

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System Components 85

2.4 Pre-Cooiing Systems

In recent years, with increasing emphasis on power consumption, more considera-tion is being given to ways of reducing power. A European Union study hasshown that 15% of the world-wide energy consumption is used to produce com-pressed air.

A proposal, with regard to reducing the power requirement for compressedair, is that the air should be cooled to -60°C before being compressed. It was men-tioned earlier, in relation to refrigeration drying of air, that units were availablethat were capable of cooling air to a temperature of -75°F. Such units form thebasis of commercially available pre-cooling systems for compressors.

The idea is that all of the air to be compressed should be physically cooledto -75°F first. By this means the air will be extremely dry, so that there will be noneed for a further dryer on the pressure side, and there will be no possibility ofcondensation occurring anywhere in the subsequent system.

This will also eliminate the presence of water-oil emulsions that can occur inlubricated compressors. For air at standard atmospheric pressure the density is0-0765 lb/ft3 at a temperature of 59°F but at -75°F it is 0-1032 lb/ft3, which repre-sents a 35% increase.

In terms of the volumetric flow rate it means that this is reduced to 74% ofthe free air flow rate that would have to be compressed, and so a much smallercompressor can be used. Manufacturers of this type of system claim that up to a30% reduction in power consumption of compressors can be made by this means,and that plant maintenance is significantly reduced.

If atmospheric air at a temperature of 59°F is compressed to 30 Ibf/ingauge, the delivery temperature, assuming adiabatic compression and an isentropicefficiency of 70%, will be about 337°F. For air at -75°F, similarly compressed, thedeliver temperature will be about 131°F. In the first case the air would, for mostapplications, have to be cooled, and if it was not dried, condensation could welloccur. With the pre-cooling system the air would probably not need to be cooledafter being compressed, and being dry, there would be no possibility of condensa-tion occurring.

3 AIR FILTRATION

Filtration plant is an essential element of any pneumatic conveying system. Anyfaults in its operation can lead to shut down of the entire plant. It is not, however,the type of component that is generally provided with stand-by provision. It isimportant, therefore, that the filtration plant is correctly specified and adequatelymaintained. It is also important in terms of satisfactory operation because of healthand safety requirements that are becoming more and more stringent as a conse-quence of changing legislation.

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86 Chapter 3

3.1 Introduction

Gas-solid separation devices associated with pneumatic conveying systems havetwo functions. The first is to recover as much as possible of the conveyed material.The second is to minimize pollution of the working environment by the material.

3.1.1 Separation Requirements

The first of these functions is principally a mater of economics, in that the morevaluable the material, the more trouble should be taken to ensure total recovery.However, the avoidance of environmental pollution is potentially more important,particularly since the introduction of more stringent Health and Safety at Worklegislation [4J. Where the material is known to be potentially dangerous, extrememeasures must be taken to prevent its escape into the atmosphere from the convey-ing plant. This is particularly the case with toxic and explosive materials.

The choice of gas-solid disengaging system to be used on any given applica-tion will be influenced by a number of factors, notably the volumetric flow rate ofthe air, the amount of bulk particulate material involved, the particle size range ofthe material, the collecting efficiency required, and the capital and running costs.In general, the finer the particles that have to be collected, the higher will be thecost of a suitable separation system.

3.1.2 Separation Mechanisms

Where a bulk material consists of relatively large and heavy particles, with no finedust, it may be sufficient to collect the material in a simple bin, the solid materialfalling under gravity to the bottom of the bin, whilst the gas is taken off through asuitable vent. However, with a bulk solid of slightly smaller particle size it may beadvisable to enhance the gravitational effect, and the most common method ofachieving this is to impart velocity and spin to the gas-solid stream, so that thesolid particles are thrown outwards while the gas is drawn off from the center ofthe vortex created. This is basically the principle on which the cyclone separatoroperates.

Where fine particles are involved, especially if they are also of low density,separation in a cyclone may not be fully effective for the very fine fraction of thematerial, and in this case the gas-solid stream may be vented through a fabric fil-ter. Many different types of fabric filter are in use and selection depends mainlyupon the nature of the solid particles being collected and the proportion of solidsin the gas stream.

For materials containing extremely fine particles or dust, further refinementin the filtration technique may be necessary, using wet washers or scrubbers, orelectrostatic precipitators, for example. While this last group of gas-solid separa-tion devices are used in industry, they are generally used with a specific processplant, and are very rarely used in conjunction with a pneumatic conveying system,and so no further reference will be made to any of these devices. In power stations,for example, electrostatic precipitators are widely used.

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System Components 87

3.1.3 Pressure Drop Considerations

The separation device should not present a high pressure drop to the system ifmaximum material flow rate is to be achieved for a given overall pressure drop.This is particularly the case in low pressure fan systems, where the pressure dropacross the separation unit could be a significant percentage of the total pressuredrop available. Regular maintenance of separation equipment is important. Thepressure drop across fabric filters will increase rapidly if they are not cleaned regu-larly, or if the fabric is not replaced when cleaning is no longer effective. If cy-clones are use for separation, wear will reduce the separation efficiency.

3.2 Dust Control

In addition to the economic reasons for efficiently removing material from a con-veying gas stream, there are important considerations of product quality control,and health and safety. In this respect if is generally the very fine particles of dustthat pose the problems.

3.2.1 Particle Degradation

In some manufacturing processes, a bulk solid is actually required in the form ofultra fine particles. In many cases, however, the presence of dust in the product isundesirable for practical and commercial reasons. Much of the dust results fromparticle degradation and attrition in the conveying process and, for a given mate-rial, this is a function of the conveying conditions, in terms of material concentra-tion and conveying air velocity, and the pipeline geometry.

Plant operating difficulties can result if degradation causes a large percent-age of fines to be produced, particularly if the filtration equipment provided is notcapable of handling the fines satisfactorily. Filter cloths and screens will rapidlyblock if they have to cope with unexpectedly high flow rates of fine material. Thenet result is that there is usually an increase in pressure drop across the filter.

This means that the pressure drop available for conveying the material willbe reduced, which in turn means that the mass flow rate of the material will proba-bly have to drop to compensate. Alternatively, if the filtration plant is correctlyspecified, with particle degradation taken into account, it is likely to cost more as aresult. This, therefore, provides a direct financial incentive to ensure that particledegradation is minimized, even if it is not a problem with respect to the material.

3.2.2 Dust Emission

Excepting the potentially explosive and known toxic materials, the most undesir-able dusts are those that are so fine that they present a health hazard by remainingsuspended in the air for long periods of time. Airborne dusts which may be en-countered in industrial situations are generally less than about ten micron in size.Particles of this size can be taken into the body by ingestion, skin absorption orinhalation. The former is rarely a serious problem and, although diseases of the

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88 Chapter 3

skin are not an infrequent occurrence, it is inhalation that presents the greatesthazard for workers in a dusty environment.

3.3 Separation Devices

An assessment of the magnitude of a potential dust problem can be made by ex-amining the bulk material to be handled, paying special attention to the fines con-tent of the material. When making a decision about the type of gas-solid separationdevice to be used in a pneumatic conveying plant for a particular material, it isclearly important to know the particle size distribution of the bulk material, pref-erably after conveying, rather than at the feed point.

3.3.1 Gravity Settling Chambers

The simplest type of equipment for separating material from a gas stream is thegravity settling chamber in which the velocity of the gas-solid stream is reduced,and the residence time increased, so that the particles fall out of suspension underthe influence of gravity. Such a device is shown in Figure 3.13.

3.3.1.1 Collecting EfficiencyThe rate at which the solid particles settle, and therefore the efficiency of separa-tion, is very much dependent upon the mass of the particles, that is, upon their sizeand density. In general, settling chambers on their own would only be used fordisengaging bulk solids of relatively large size. Typically this would mean parti-cles greater than about 150 micron (100 mesh).

The limiting size obviously depends also upon the shape and density of theparticles, hence the value of tests and experience in this respect. For particles lar-ger than about 300 micron (50 mesh), a collecting efficiency in excess of 95%should be possible.

Gas out

Gas / Solids InScreen

Material Out(b) Material Out

Figure 3.13 Gravity settling chambers, (a) Basic system and (b) design incorporating a

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System Components 89

To improve the collecting efficiency of the basic gravity settling chamberwhen working with materials of low density, or of a fibrous nature, a mesh sepa-rating screen may be fitted at an angle across the gas flow, as shown in Figure3.13b. The screen should be provided with a rapping mechanism to shake col-lected particles free on a regular basis.

Although the gravity settling chamber is basically a very simple device, careshould be taken to ensure that its design allows, as far as possible, a uniform dis-tribution of the gas as it enters and leaves. Within the settling chamber the gasvelocity should generally be less than about 500 ft/min if excessive re-entrainmentof collected particles is to be avoided. Where a material consists essentially ofcoarse particles, but also has some dust content, it may be satisfactory to use asettling chamber with the gas vented through a suitable fabric filter. In this ar-rangement it is important that the filter is correctly sized to prevent over loading,and that an adequate cleaning routine is followed.

3.3.2 Cyclone Separators

In pneumatic conveying plants handling medium to fine particulate material, thegas-solid separator is often a cyclone-receiver. This may be combined with a fab-ric filter unit if the bulk material is dusty. The cyclone separator is also dependentupon the mass of the particles for its operation. The forces that disengage the solidparticles from the conveying gas, however, are developed by imparting a spinningmotion to the incoming stream so that the particles migrate outwards and down-wards under the influence of centrifugal and gravitational effects.

3.3.2.1 Reverse Flow TypeThe commonest form of cyclone is the so-called 'reverse flow' type, illustrated inFigure 3.14, in which the rotation of the gas is effected by introducing it tangen-tially to the cylindrical upper part of the device. The solid particles are then col-lected from the outlet at the base of the conical lower part whilst the cleaned gasflows in the opposite direction through the top outlet.

3.3.2.2 Collecting EfficiencyThe size of particles that can be separated in a cyclone, and the collecting effi-ciency, depend principally upon the difference in density of the solid particles andthe air, the solid concentration, and the dimensions (notably the diameter) of thecyclone. Increasing either the entry velocity or decreasing the cylinder diametershould normally result in an increase in the collecting efficiency of finer particles,but the practical lower limit on particle size if likely to be about ten micron.

It should be noted that decreasing the diameter will reduce the gas-solidsthroughput, and consequently more cyclones will be needed for a given applica-tion, and at greater cost. Also, operating at a higher gas inlet velocity may causeproblems if the particles are friable or abrasive. In contrast, operation at highersolids concentrations may be advantageous, as finer particles tend to be trappedand swept out by larger particles, resulting in an improved collecting efficiency.

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90 Chapter 3

Clean Gas Out

Gas/Solids In

Outer vortextakes particles

to wall

Figure 3.14 Principle of the cyclone separator.

Inner vortex takesgas out

Material Out

Performance data is normally presented in the form of a plot of collectingefficiency against particle size. Such a plot for two possible design extremes ispresented in Figure 3.15. One plot is for a high efficiency cyclone, and the other isfor a low efficiency cyclone having a high throughput capability. It is possible thattwo or more high efficiency cyclones would be needed to meet the flow rate capa-bility of the low efficiency cyclone.

100

80

.§ 60

eu

£ 40

Io

° 20

High Efficiency Cyclone

High Throughput Cyclone(Low Efficiency)

20 40 60Particle Size - ^m

80 100

Figure 3.15 Performance curves for typical cyclone separators.

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System Components 91

3.3.3 Filters

The fabric filter is now the 'industry standard' for gas solids separation duties inpneumatic conveying systems. This is particularly the case where there is an ele-ment of dust in the conveyed material. Considerable development has taken placeover recent years, with particular improvements in fabrics. In order to appreciatethe principles on which filter units are designed or selected it is helpful to under-stand the manner in which they operate.

3.3.3.1 Filtration MechanismsThere are two fundamental mechanisms by which particles can be removed from astream of gas passing through a porous fabric. The most obvious of these is a'sieving' mechanism in which particles too large to pass through the mesh of thefabric are caught and retained on the surface of the filter. The caught particlesgradually build up on the filter so that the labyrinthine nature of the gas flow pathcontinually increases whilst the effective mesh size decreases. The collecting effi-ciency of the filter will therefore tend to be improved by use, but the pressure dropacross the filter will increase, of course, and so regular cleaning is essential.

The less obvious, but for very fine particles, more important, mechanism offiltration is that in which the particles are caught by impingement on the fiberswithin the filter fabric. This is often referred to as 'depth filtration' to distinguish itfrom 'sieving'. It is for this reason that filters usually consist of a fibrous matrather than a single woven fabric screen. The actual flow paths followed by the gaspassing through a depth filter are thus extremely tortuous, and a particle unable tofollow these paths is given a trajectory which sooner or later brings it into contactwith a fiber where it adheres, largely as a result of Van der Waal's forces.

3.3.3.2 Collecting EfficiencyThe collecting efficiency of a fabric filter is mainly influenced by the gas velocitythrough the fabric, and the size of the particles to be collected. Where the particlesare relatively large, which means greater than about five microns, they are likely,because of their greater inertia, to come frequently into contact with the filter fi-bers. The tendency to 'bounce' off the fibers and escape from the filter, however,is also greater, especially where the gas velocity is high.

3.3.3.3 Filter MediaA wide range of materials is available for the manufacture of filter fabrics. Woolor cotton, the latter particularly having the advantage of low cost, may be used.For better resistance to abrasive wear or chemical attack, and a higher maximumoperating temperature, however, either glass fiber or one of a number of alterna-tive man-made fibers should be selected, such as Polyethylene, Polypropylene,Nylon (polyamide), Orion (acrylic), Dacron (polyester) and Teflon (PTFE).

Apart from the properties of the fibers themselves, specifications for filterfabrics should include the 'weight per unit area', which gives an indication of thethickness, and therefore the strength and durability of the fabric, and an indication

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92 Chapter 3

of its permeability. Filter surfaces may also be treated to increase their resistanceto combustion.

3.3.3.4 Selection CriteriaThe selection of a fabric filter for a given application should be made after consid-eration of a number of criteria. The first of these should be the mean particle size,and more particularly the particle size range, the nature of the solid material to becollected, and the temperature of the conveying gas, which wil l dictate the types offabric that would be acceptable. The size of unit required will depend principallyupon the maximum gas flow rate to be handled, and the maximum allowable pres-sure drop.

The size will also be influenced by the proportion of solid material carriedby the gas, and hence the solids loading ratio, the method of cleaning to be used,and the planned frequency of replacement of the filter fabric. Several of these cri-teria are clearly affected by cost factors, and so a careful balance must be struckbetween the capital cost of the equipment, normal running costs, and the cost ofroutine maintenance.

3.3.3.5 Bag FiltersIn pneumatic conveying systems handling fine or dusty material, the method offiltration that has become almost universally adopted is the bag type fabric filter.They may have application as bin vents in situations where all the solid material tobe collected is blown into a hopper, and the clean air is vented off at the topthrough the filter unit, whilst the collected material is discharged from the base ofthe hopper through a suitable air lock.

The actual configuration of the filter bags within the unit, and the method ofcleaning vary from one manufacturer to another. The bags are usually of uniformcross section along their length and the most common shapes are circular and rec-tangular. Rectangular bags probably provide a filter unit with the largest fabricsurface area to filter volume.

The cleaning process is of particular importance since it has a considerableinfluence on the size of filter required for a given application. Figure 3.16 illus-trates diagrammatically a typical form of bag filter unit. Although the filter bagsshown in Figure 3.16 are suitable for continuously operating systems, the methodof cleaning is only suitable for batch conveying operations. This is because filtersurfaces cannot be cleaned effectively by shaking, unless the flow of air across thefabric ceases.

The gas-solid stream should enter the device from beneath the fabric bags sothat larger particles are separated by gravity settling, provided that direct im-pingement of particles on the bags from the conveying line is prevented. Fine par-ticles are then caught on the outside of the fabric bags as the gas flows upwardsthrough the unit. These filters are available in a wide range of sizes, lengths,shapes and configurations. The shaking mechanism represented on Figure 3.16 isone of several methods of bag cleaning that may be employed.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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System Components 93

Shaking Mechanism

Clean GasOut

Gas/Solids In

*pitI

ii1*1

if i> ! ": ! :«*! i"

• • j*i : f; ! j

i*i :*

t

Filter Bag

Support Cage

ReceivingHopper

Figure 3.16 Sketch of typical shaken bag filter unit.

3.3.3.6 Filter SizeThe basic measure of filter size is the effective surface area of fabric throughwhich the gas has to pass. It is usual, in the case of pneumatic conveying systems,to specify the size of filter required on the basis of an assumed value of the socalled 'air to fabric ratio', which is defined as the ratio of the volumetric air flowrate divided by the effective area of the filter fabric. It should be noted that thisparameter is not, in fact, a ratio but has the dimensions of velocity. It is best re-garded as a superficial velocity of the air through the filter fabric.

The actual value of the air to fabric ratio to be used is difficult to assesstheoretically and so reliance must be placed on experience. The manufacturers offilter units should normally be able to advise on suitable air to fabric ratios for thebulk particulate material being handled. Typical values for felted fabrics would beabout 6 ft/min when handling fine particulate materials and up to 10 ft/min withcoarser or granular materials. For woven fabrics these figures should be halved,since the free area actually available for gas flow is much less.

3.3.3.7 Filter CleaningThe design of present day fabric filter units, with their multiple bags or envelopesand their complex cleaning mechanisms, has gradually evolved with increasingawareness of the need to conserve energy and to avoid atmospheric pollution. Theuse of multiple bags was simply a means of getting a larger area of fabric into asmall space, but a more important aspect of filter design concerned the method ofminimizing the proportion of fabric area out of action at any one time for cleaning.

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94 Chapter 3

This consideration led to the introduction of filter units having two or moreseparate compartments, each containing a number of bags. By this means onecompartment could be shut off for cleaning while the others remained in service,handling the full gas-solids flow. Modern filter units using pulsed air jets for fabriccleaning do not require the unit to be compartmentalized, but are still designed toensure that only a small number of the filter elements are out of service at thesame time.

Reverse air jet cleaning is now very much the industry standard. This isachieved by means of a system of high pressure jets, operating in sequence, whichinject air downwards through the bag walls in the reverse direction to the normalair flow. The pulsed reverse air jets last for only a very short period of time and socontinuous operation of the filter is possible, and maximum utilization of the fab-ric area can be achieved. Such a device is shown in Figure 3.17.

The air is generally pulsed through a venturi, positioned at the inlet to thebag, and the bags are usually supported by a wire cage. The high pressure airpulsed through the venturi creates a shock wave and it is this, in combination withthe reverse flow of air, that results in the cleaning of the filter bag. There is clearlya limit to the length of filter bag that can be effectively cleaned by this means, anda reduction in cleaning efficiency must be expected if very long bags are used.

The air needed for the high pressure jets is typically required at a pressure ofabout 80 psig. The volumetric flow rate, however, is quite low and so the powerrequired for these units is relatively low.

Solenoid Valve

Clean GasOut

Gas/Solids In

Compressed• Air Inlet

J

•;

v

;/;.

1VtrI

i

*ii

»

•1j.

r*

1=f

t

>ii

;

i *

1i-

1lr=j)!1 ' i

! 4^i "

• j* .1 iI PLi_

t

MV

-

1-

^^

_/ ~

I11111

Li

Air Nozzle

^^ ^ Venturi

Filter Bag

Support Cage

.

ReceivingHopper

Figure 3.17 Sketch of bag filter unit with high pressure pulsed air jets.

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System Components 95

3.3.3.8 System ConsiderationsBeing at the end of the conveying process, its importance is often overlooked, butincorrect design and specification can cause endless problems in the conveyingsystem. It is also important that the separation system is not considered in isola-tion. The influence that the system can have on the filter, and the influence that thefilter can have on the system need to be considered as well.

3.3.3.8.1 Blow Tank SystemsIf, at the end of a conveying cycle the pipeline and blow tank have to be ventedthrough the filter unit, the air flow rate will be considerably greater than the steadyair flow rating of the air mover. This is particularly the case if the blow tank oper-ates at a high pressure, for the transient nature of the air flow through the convey-ing cycle is significantly magnified at this time.

This will result in a considerable increase in the air velocity through the fil-ter, possibly resulting in blinded filters, giving higher filter resistance and subse-quent difficulty with cleaning. It is essential in these circumstances to reduce theair supply at the end of the conveying cycle in order to keep the total air flow rateto as low a value as possible. To cater for these surges by increasing the filter sizemay be a more expensive solution.

3.3.3.8.2 Vacuum Conveying SystemsIn vacuum conveying systems the clean air at outlet from the filter is drawnthrough an exhauster. Should a filter bag split, or otherwise fail, material will becarried over to the exhauster. Although a liquid ring vacuum pump can tolerate acertain amount of dusty air, provided that it is not abrasive, positive displacementblowers can not, and so some form of protection must be provided. A separate in-line filter is often used for this purpose, and although its efficiency with respect tofine particles is generally low, it will allow time for the system to be shut downbefore serious damage occurs.

In a negative pressure system the filter is under vacuum and this will have tobe taken into account. In comparison with a positive pressure system, employingthe same 'free air' flow rate, a vacuum system operating under 7 lbf/in2 of vacuumwill need to have a filter approximately twice the size of one required for a posi-tive pressure system.

4 PIPELINES

Decisions do have to be made with regard to the pipeline. Pipeline material, wallthickness, surface finish, steps, and bends to be used, all have to be given due con-sideration. One of the most critical parameters with regard to the successful opera-tion of a pneumatic conveying system is maintaining a minimum value of convey-ing air velocity for the material to be handled. For the dilute phase conveying ofgranulated sugar, for example, this is about 3200 ft/min. If the velocity drops to3000 ft/min the pipeline is likely to block.

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96 Chapter 3

4.1 Wall Thickness

The volumetric flow rate of the air required is obtained by multiplying the convey-ing air velocity by the cross sectional area of the pipeline, and making due note ofboth the pressure and temperature of the air. The diameter of a 4 inch nominalbore pipeline, however, is rarely 4 inches. If a conveying air velocity is based on adiameter of 4 inches, for example, and it is a schedule 10 pipeline, the actual borewill be 4-176 inch and not 4-000 inch. This difference will mean that the air veloc-ity will be about 9% lower. If 3200 ft/min is the velocity in a 4-000 inch bore pipe-line, it will only be 2935 ft/min in a 4-176 inch bore line and the pipeline is likelyto block.

If an abrasive material is to be conveyed, wear of the pipeline must be ex-pected. To give the pipeline a longer life, pipe having a greater wall thicknessshould be used. Schedule numbers are often used to specify wall thickness. If anabrasive material is to be conveyed schedule 80 pipeline would be recommendedas a minimum. Typical dimensions for 4 inch nominal bore pipeline are given inTable 3.1.

If the material to be conveyed is not abrasive at all, a thin walled schedule10 pipeline should be suitable. Pipeline weight in Ib/ft could be added to Table 3.1and this would show a marked difference. Lighter pipe sections will certainlymake construction of the pipeline easier, particularly if there are vertical sectionsto erect.

4.1.1 Pipeline Rotation

If a pipeline is to convey an abrasive material having a very large particle size, theparticles will tend to 'skip' along the pipeline and so wear a groove on the bottomof the pipeline. Erosive wear can be very severe with low angle particle impact(see Chapter 20). In this case a thick walled pipeline would be essential, but if thepipeline were to be rotated periodically, this would also extend the life of the pipe-line very considerably. For this purpose the pipeline must be located in a placewhere convenient access can be gained for the necessary changes to be made.

Table 3.1 Pipe Diameter and Wall Thickness for Four Inch Nominal Bore Pipe-line

Schedule Number

Dimensions

inches

Wall thickness

Pipe bore

Outside diameter

10

0-162

4-176

4-5

40

0-237

4-026

4-5

80

0-337

3-826

4-5

160

0-531

3-438

4-5

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System Components 97

4.2 Pipeline Material

Although steel is the most commonly used pipeline material, many other materialsare able to suit the conveyed material and the conveying duty. It was mentionedabove that thin walled pipe would be easier to handle and erect because it islighter. Aluminum pipe is often used for this purpose.

4.2.1 Hygiene

Because of problems of moisture and condensation in pipelines there is always thepossibility of steel rusting and contaminating the conveyed material, if air dryingis not employed. In cases where hygiene is important, such as with many food,chemicals and pharmaceutical products, the pipeline will need to be made fromstainless steel.

4.2.2 Hoses

Where flexibility is required in a pipeline, and this cannot be convenientlyachieved with a combination of straight pipe and bends, flexible hose can be used.Where a single line needs to feed into a number of alternative lines, and a flowdiverter is not wanted to be used, a section of flexible hose of the steel braidedtype can be used to provide the link.

Where road and rail vehicles and boats need to be off-loaded, flexible hoseis ideal. It is available in natural rubber and a variety of synthetic materials, andcomes in a wide range of sizes. The authors have experience of conveying variousdrilling mud powders through hoses at pressures of up to 80 lbf/in2 gauge to obtaindata for transferring these materials from boats to oil rig platforms, as well as test-ing flexible hose, rated at 250 atmospheres, for erosive wear resistance.

Flexibility is generally needed in ship off-loading applications with vacuumsystems, and hoses provide the necessary flexibility here. Care must be taken if thematerial is abrasive and has a large particle size, because the wear rate of rubberscan be excessive with such materials [5].

4.2.3 Erosive Wear

If a very abrasive material is to be conveyed in a pipeline, consideration must begiven to the use of schedule 80 pipeline or higher. An alternative to this, for veryabrasive materials is to use alloy cast iron or to line the pipeline with basalt. If abetter material is required, then alumina ceramics can be used, but this is likely tobe more expensive. A usual combination is to line the straight pipeline with basaltand use alumina for the bends.

4.3 Surface Finish

Most pipeline is supplied having a satisfactory surface finish with regard to fric-tional resistance to flow. For some materials, such as polyethylene, however, aparticular surface finish is required for the specific purpose of reducing the prob-

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98 Chapter 3

lem of 'angel hairs', or particle melting, with these materials. An artificiallyroughened surface is often specified for this class of material.

If abrasive materials are to be conveyed care must be exercised with thejoining of pipeline sections. Misalignment and poor welding can cause steps andridges in the flow and these can cause deflection of the gas-solid stream in thepipeline. A deflected flow of an abrasive material can cause rapid wear of thestraight pipeline down-stream.

4.4 Bends

Bends provide pneumatic conveying lines with their flexibility in routing, but theyare not all the standard items that one might expect from the handling of singlephase fluids. Some of the bends employed are shown in Figure 3.18.

Blind Tee Booth Bend

Vortice Ell Expanded Bend

Figure 3.18 Some special bends developed for pneumatic conveying systems.

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System Components 99

Although bends provide the necessary flexibility in routing, they are thecause of many problems, and hence the developments in this area. Each bend willadd to the overall resistance of the pipeline, and therefore to the conveying airpressure required. If the conveyed material is abrasive an ordinary steel bend couldfail within hours. Numerous different bends are available. Many of these are madeof, or lined with, basalt, cast iron, rubber, etc, and some have a constant bore and aconstant radius, as with conventional bends.

Another group of bends that have been developed have neither constant borenor constant radius. Some of these bends are shown in Figure 3.18. Care must betaken in selecting such bends, however, for account must be taken of their suitabil-ity for the material being conveyed and the pressure drop across the bend with thatmaterial.

4.4.1 Blind Tee

With an abrasive material, the simple blind tee bend shown in Figure 3.18 willprobably last fifty times longer than an equivalent radiused bend made of mild(low carbon) steel. It will ultimately fail around the inside corner due to turbu-lence. These bends can, however, be reinforced by increasing the wall thickness ofthe outlet pipe for a short distance and this will extend the life of the bend quiteconsiderably. For abrasive materials, therefore, it is extremely effective, and caneven be made out of scrap material.

The blind end of the bend traps the conveyed material and so the oncomingmaterial impacts against other material, instead of the bend, and thereby protectsit. The penalty is in the increased pressure drop that results. In a program of testswith a 165 ft long pipeline of two inch bore conveying fly ash, seven radiusedbends in the pipeline were changed with blind tee bends. With the radiused bendsand a 30 lbf/in2 pressure drop the fly ash was conveyed at 44,000 Ib/h. With theblind tee bends in place only 22,000 Ib/h could be achieved with a 30 lbf/in2 pres-sure drop [6].

4.5 Steps

If high pressure air, or a high vacuum, is used for conveying a material, it wouldgenerally be recommended that the pipeline should be stepped to a larger bore partway along its length. Figure 3.19 illustrates the case of a low velocity, densephase, conveying system.

This is to cater for the expansion of the air that occurs with decrease in pres-sure, and so prevents excessively high conveying air velocities towards the end ofthe pipeline. The minimum conveying air velocity that must be maintained for thematerial is about 1200 ft/min, and 350 ftVmin of free air is available to convey thematerial. The conveying line inlet air pressure is 45 lbf/in2 gauge. From Figure3.19 it will be seen that a 3 inch bore pipeline will be required for these conditions,and the resulting conveying line inlet air velocity will be 1755 ft/min. If a singlebore pipeline is used the conveying line exit air velocity will be 7125 ft/min.

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100 Chapter 3

6000 h

10 20 30

Air Pressure - Ibi7in2 gauge

40 50

Figure 3.19 Stepped pipeline velocity profile for high pressure dense phase conveyingsystem using 350 ftVmin of air at free air conditions.

Although this high velocity might be accepted in a dilute phase conveyingsystem, it is quite unnecessary in a dense phase system. Apart from reducing prob-lems of particle degradation and erosive wear, by reducing conveying air veloci-ties, a stepped pipeline is also likely to achieve an improved conveying perform-ance, compared with a single bore pipeline, for the same air flow conditions. Thevelocity profiles for a combination of 3, 4 and 5 inch bore pipes is shown super-imposed on Figure 3.19. This has resulted in the conveying air velocity being con-fined to a relatively narrow band, with the maximum value being limited to only2640 ft/min.

4.6 Rubber Hose

Rubber hose is widely used in conveying systems, for both pipeline and bends, asmentioned above. Its particular properties also make it ideal for use in systemswhere the material being conveyed may be friable, abrasive or ultra-fine and hencevery cohesive.

4.6.1 Erosive Wear and Particle Degradation

Rubber hose has the capability of withstanding erosive wear better than steel pipe-line in certain situations. Although the hardness of the surface material is generallymuch lower than that of alternative metal surfaces, and of the particles impactingagainst the surface, it derives its erosive wear resistance from the fact that it is ableto absorb much of the energy of impact by virtue of its resilience. By the same

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System Components 101

mechanism, the energy of impact of friable materials is also absorbed and so parti-cle degradation can also be reduced appreciably.

4.6.2 Pressure Drop

Problems of erosive wear and particle degradation are particularly severe in highvelocity dilute phase conveying. Unfortunately the pressure drop for gas-solidflows through rubber hose also increases with increase in velocity, and more sothan for steel pipeline.

4.6.3 Conveying Cohesive Materials

In steel pipelines, cohesive and sticky materials have a tendency to adhere to thepipeline wall and form a coating. This coating can gradually increase in thicknessuntil it builds up to such an extent that it results in the pipeline being blocked. Thisis particularly the case with ultra fine powders and materials that have a fat con-tent, or some other substance that makes the material sticky.

If such materials are conveyed through a thin walled rubber hose, the naturalmovement and flexing of the hose, resulting from the pulsations of the air underpressure and the material transfer, is generally sufficient to dislodge any materialthat has a tendency to adhere to the pipeline wall. The pipeline needs to be sup-ported so that it is free to move, but having sufficient support so that it is main-tained reasonably straight. With the requirement for a thin walled hose capable offlexing it is limited to low pressure dilute phase conveying, but it does provide asimple and effective means of conveying this type of material.

5 VALVES

A number of different valves may need to be used on pneumatic conveying plant,and a wide variety of different valves are available in the market place. Rotaryvalves have been considered at length, and are ideal for controlling the feed ofmaterial into or out of a system at a controlled rate. There is, however, a require-ment for many other types of valve, generally to be used for the purpose of isolat-ing the flow. Many of these have been included on sketches of conveying systemsearlier in these notes and include, discharge valves, vent line valves and divertervalves.

5.1 Discharge Valves

A valve in a conveying line that is required to stop and start the flow is an onerousduty. Although the valve is only used in either the open or closed position, and isnot used for flow control purposes, particulate material must be able to pass freelythrough when it is open. If the control surfaces of the valve remain in the flowpath, as they will with pinch valves and ball valves, they must provide a perfectlysmooth passage for the flow of material through the valve when open. Any small

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102 Chapters

protuberances or surface irregularities that could promote turbulence in the areawould result in a rapid deterioration in performance. This is particularly the casewhen the material to be conveyed is abrasive. This type of valve is also very vul-nerable during the opening and closing sequences, and so these operations shouldbe completed as quickly as possible.

5.7.7 Ball Valves

The authors have tested numerous ball valves in a 4 inch bore pipeline conveyingsilica sand in dilute phase at 30 lbf/in2. They did not perform very well in such aharsh environment. Because they have moving parts the very fine abrasive dust inthe conveyed material wrecked havoc. The valves soon lost their air-tightness, andthe torque required to operate the valves gradually increased and soon exceededthat available by the automatic control facilities provided with the valves.

5.7.2 Pinch Valves

Pinch valves are a much better proposition, as there is no relative movement be-tween surfaces in which fine abrasive dust can lodge. These can also be openedand closed rapidly. Rubbers and urethanes also have very reasonable erosive wearresistance, and so are well worth considering for this kind of duty. They will notlast for ever, and so periodic maintenance is essential, and will be required. Thesevalves must be located in an accessible position, and spares must be available.

5.7.5 Dome Valves

The dome valve is a more recent addition to the list of valves available, but it hasbeen specifically designed for this type of duty, and is now being widely used inindustry. The valve has moving parts, but these move completely out of the path ofthe conveyed material when the valve is open. On closing, the valve first cutsthrough the material and then becomes air-tight by means of an inflatable seal. Thevalve can be water cooled and so it is capable of handling hot materials.

5.2 Isolating Valves

There are many instances where material has to be transferred, usually under grav-ity, in batches. The valve is either open or closed and often has to provide an airtight seal. In the gate lock feeder, for example, a pair of valves are required to op-erate in sequence to feed small batches of material into a pipeline, often underpressure. Where batches of material have to be fed into blow tanks, the valve hasto be capable of withstanding the pressure subsequently applied to the blow tank.Another pressure situation is where isolating valves are used for flow diversion,considered below.

Of the valves considered above only the dome valve would be appropriatefor this type of duty. It finds wide use in this application, particularly with themore difficult granular and abrasive materials.

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5.2.7 Butterfly Valves

If the material to be handled is not abrasive, the butterfly valve is ideal. They arereasonably priced, require very little headroom, are not too heavy, and are rea-sonably air tight. They are widely used in the food and related industries, and ingate lock feeders. They are, however, much too vulnerable for use with abrasivematerials, since the valve remains in direct line in the flow when it is open.

5.2.2 Disc Valves

Disc valves, like butterfly valves, require very little headroom, but like domevalves, they swing completely out of the way of the flow of material. They cutthough the material on closing, but generally rely on the subsequent pressure in thevessel below to provide the necessary seal. Their suitability for use will dependvery much upon the material to be handled and the application.

5.2.5 Slide Valves

Slide valves are probably the oldest valves in the business, and although they havebeen improved over the years, the disc valve is a specific development from it.They take up little space and are relatively cheap. A particular application is interms of back-up. If any of the other more expensive and sophisticated valves fail,and need to be replaced, this can be a very difficult and time consuming task if thevalve is holding several hundred tons of material in a hopper, and this must bedrained out before the valve can be removed for repair or replacement.

Care must be taken if slide valves are used to isolate a line against pressure,particularly if the conveyed material is fine and abrasive. If the valve is not en-tirely air-tight, micron sized particles will flow with the leakage air. Because of thepressure difference the velocity of the leaking air will be high, even though theflow rate may be very small, and erosive wear will occur. Wear will then increaseexponentially once it has started and after a period of time can be severe. The au-thors have seen dramatic wear of such valves used for flow diverters in pipelinesconveying fly ash in power stations.

5.3 Vent Line Valves

This is a deceptively easy duty, but if it is on a high pressure blow tank handlingfly ash or cement, the valve will have to operate in a very harsh environment. Theair velocity will be very high, albeit for a very short period of time, but a lot ofabrasive dust is likely to be carried with the air. If the material is abrasive then thechoice is between a pinch valve and a dome valve. If the material is non abrasive,a diaphragm valve could be used.

5.4 Flow Diversion

Flow diverting is a very common requirement with pneumatic conveying systemsand can be achieved very easily. Many companies manufacture specific flow di-

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104 Chapter 3

verting valves for the purpose. Alternatively flow diversion can be achieved byusing a set of isolating valves. The most common requirement is to divert the flowto one of two alternative routes, typically where material needs to be dischargedinto a number of alternative hoppers or silos. In this case the main delivery linewould be provided with a diversion branch to each outlet in turn.

5.4.1 Diverter Valves

There are two main types of diverter valve. In one a hinged flap is located at thedischarge point of the two outlet pipes. This flap provides a seal against the inlet toeither pipe. The pipe walls in the area are lined with urethane, or similar material,to give an air tight seal, and this provides a very compact and light-weight unit.

One of the authors tested a Y-branched diverter valve of this design with sil-ica sand in dilute phase, but it was a disaster. After conveying only 12 ton of sandthe % inch thick bronze flap had a '/a inch diameter hole through it. The urethanelining, however, was in perfect condition. The problem was that the sand was al-ways impacting against the flap. A straight through design with a branch off wouldhave been better, but still not suitable for abrasive materials.

The other main design operates with a parallel tunnel section of pipe in aplug between the supply and the two outlet lines. This unit would not be recom-mended for abrasive materials either. This design, however, should provide a morepositive seal for the line not operating, which would probably make it a more suit-able valve for vacuum conveying duties. A typical parallel tunnel, or plug, typediverter valve is presented in Figure 3.20 to illustrate the method of operation.

Figure 3.20 Sketch of parallel tunnel type diverter valve.

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5.4.2 Isolating Valves

Flow diversion can equally be achieved by using a pair of isolating valves, withone placed in the branch, close to the supply pipe, and the other in the supply pipe,just downstream of the branch. This can be repeated at any number of points alongthe pipeline. The main disadvantage with this arrangement is that a plug of mate-rial will be trapped in the short section of pipeline not in use, which will have to beblown through when the flow direction changes.

If the conveyed material is abrasive, this method of flow diversion would berecommended. Either pinch valves or dome valves would need to be employed forthe purpose. With two separate valves, instead of one to operate, care would haveto be exercised with the sequencing when changing flow direction.

5.5 Flow Splitting

Multiple flow splitting is not a common requirement and so there are few devicesavailable. They are often required on boiler plant, where coal dust might need tobe sent to the four corners of a boiler, and on blast furnaces, where coal or lime-stone powder might need injecting at a dozen or more different points around itscircumference. The main requirement here is generally that all of the outletsshould be supplied with material, and at a uniform rate to each, despite the factthat the distance to each point will be different. The splitting is best achieved in thevertical plane, with the line sizes and geometries very carefully evaluated to pro-vide a uniform balance for each.

REFERENCES

1. D. Mills. Optimizing pneumatic conveying systems: air movers. Chem Eng. Vol 108,No 2, pp 83-88. February 2001.

2. K. Speltz. Dehumidification in manufacturing: methods and applications. Proc 23rd

Powder & Bulk Solids Conf. pp 83-93. Chicago. May 1998.3. D. Mills. Pneumatic Conveying Design Guide. Butterworth-Heinemann. 1990.4. D. Mills. Safety aspects of pneumatic conveying. Chem Eng. Vol 106, No 4, pp 84-91.

April 1999.5. D. Mills. Using rubber hose to enhance your pneumatic conveying process. Powder

and Bulk Eng. pp 79-87. March 2000.6. D. Mills and J.S. Mason. The influence of bend geometry on pressure drop in pneu-

matic conveying system pipelines. Proc 10th Powder & Bulk Solids Conf. pp 203-214.Chicago. May 1985.

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Gas-Solid Flows

1 INTRODUCTION

There is essentially no limit to the capability or a pneumatic conveying system forthe conveying of dry bulk particulate materials. Almost any material can be con-veyed and high material flow rates can be achieved over long distances. There are,however, practical limitations and these are mainly imposed by the fact that theconveying medium, being a gas, is compressible. The limiting parameters are thenmainly the economic ones of scale and power requirements.

Conveying capability depends mainly upon five parameters. These are pipebore, conveying distance, pressure available, conveying air velocity and materialproperties. The influence of many of these variables is reasonably predictable butthat of the conveyed material is not fully understood at present.

1.1 Pipeline Bore

The major influence on material flow rate is that of pipeline bore. If a greater ma-terial flow rate is required it can always be achieved by increasing the pipelinebore, generally regardless of the other parameters. In a larger bore pipeline a largercross sectional area is available and this usually equates to the capability of con-veying more material.

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108 Chapter 4

1.2 Conveying Distance

In common with the single phase flow of liquids and gases, conveying line pres-sure drop is approximately directly proportional to distance. Long distance con-veying, therefore, tends to equate to high pressure, particularly if a high materialflow rate is required. For the majority of conveying applications, however, it is notconvenient to use high pressures. As a consequence, long distance, with respect topneumatic conveying, means about one mile. This limitation, and means of ex-tending distance capability, are discussed at various points in this handbook. Inthis chapter the basic fundamentals are considered.

1.3 Pressure Available

Although air, and other gases, can be compressed to very high pressures, it is notgenerally convenient to use air at very high pressure. The reason for this is that airis compressible and so its volumetric flow rate constantly increases as the pressuredecreases. In hydraulic conveying, pressures in excess of 2000 lbf/in2 can be usedso that materials can be conveyed over distances of 70 miles and more with a sin-gle stage. With water being essentially incompressible, changes in the velocity ofthe water over this distance are not very significant.

In pneumatic conveying, air at pressures above about 15 lbf/in2 gauge isgenerally considered to be 'high pressure', as mentioned in Chapter 1. With air at15 lbf/in2 expanding to atmospheric pressure, for example, the conveying air ve-locity will double over the length of the pipeline. Although the air expansion canbe accommodated to a certain extent by stepping the pipeline to a larger bore partway along its length, this is a complex design procedure. As a consequence, airpressures above 100 lbf/in2 gauge are rarely used for pneumatic conveying sys-tems that deliver materials to reception points at atmospheric pressure.

Where pneumatic conveying systems are required to deliver materials intoreactors and vessels that are maintained at pressure, however, high air supply pres-sures can be used, and 300 lbf/in2 is not unusual. With a high back pressure theexpansion of the air is significantly limited and relatively few, if any, steps wouldbe required in the pipeline. It is on this basis that staged pneumatic conveying sys-tems would be designed for very long distance conveying.

1.4 Conveying Air Velocity

The parameter here is volumetric flow rate, for this has to be quoted, along withsupply pressure, when specifying a blower, compressor or exhauster for a pneu-matic conveying system. The critical design parameter with respect to pneumaticconveying, however, is conveying air velocity, and more particularly, conveyingline inlet air velocity or pick-up velocity. Since the air expands along the length ofthe pipeline it will always be a minimum at the material feed point at the start ofthe pipeline, in a single bore pipeline, regardless of whether it is a positive pres-sure or a vacuum conveying system.

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Gas-Solid Flows 109

In a single bore pipeline the velocity will be a maximum at the end of thepipeline. It is the value of the minimum velocity of the air that is critical to thesuccessful operation of a pneumatic conveying system. Volumetric flow rate, ofcourse, is given simply by multiplying conveying air velocity by pipe section area.In this process, however, the correct velocity has to be used and this is consideredin detail in the next chapter on 'Air Requirements'.

The minimum value of conveying air velocity depends to a large extent onthe properties of the bulk particulate material to be conveyed and the mode ofconveying. For dilute phase conveying this velocity is typically about 3000 ft/min,although this does depend upon particle size, shape and density, as will be dis-cussed.

For dense phase conveying the minimum velocity is about 600 ft/min. Forfine powders that are capable of being conveyed in dense phase the minimumvalue of conveying air velocity also depends upon the concentration of the mate-rial in the air, or the solids loading ratio, and this will be considered in detail in thischapter.

In dilute phase conveying the particles are conveyed in suspension in the airand this relatively high value of velocity is due, in part, to the large difference indensity between the particles and the air. In hydraulic conveying typical velocitiesfor suspension flow are only about 300 ft/min, but the difference in density be-tween water and particles is very little in comparison. The difference in densitybetween water and air is about 800:1. Since the difference in conveying mediumvelocity is only of the order of about 10:1 it will be seen that the pressure of theair, and hence its density, will not have a major effect on the value of minimumconveying air velocity for general pneumatic conveying.

1.5 Material Properties

The properties of the conveyed material have a major influence on the conveyingcapability of a pneumatic conveying system. It is the properties of the material thatdictate whether the material can be conveyed in dense phase in a conventionalconveying system, and the minimum value of conveying air velocity required. Forthis reason the conveying characteristics of many different materials are presentedand featured in order to illustrate the importance and significance of materialproperties.

Although it is the properties of the bulk material, such as particle size andsize distribution, particle shape and shape distribution, and particle density that areimportant in this respect, at this point in time it is the measurable properties ofmaterials in bulk that are more fully understood, These include air-material inter-actions, such as air retention and permeability, and are more convenient to use. Ingeneral, materials that have either good air retention or good permeability will becapable of being conveyed in dense phase and at low velocity in a conventionalconveying system. Materials that have neither good air retention nor good perme-ability will be limited to dilute phase suspension flow.

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110 Chapter 4

7.5.7 Dense Phase Conveying

There are two main mechanisms of low velocity, dense phase flow. For materialsthat have good air retention, the material tends to be conveyed as a fluidized mass.In a horizontal pipeline the vast majority of the material wi l l flow along the bot-tom of the pipeline, rather like water, with air above, but carrying very little mate-rial. At a solids loading ratio of about 150 the pipeline is approximately half full.For dense phase flows there is a distinct pulsing of the flow, with the materialflowing smoothly and then suddenly stopping for a second or two and then flow-ing smoothly again. In vertically upward flow, the flow of material also pulses,and for the second that the flow halts the material falls momentarily back down thevertical pipe.

For materials that have good permeability the material tends to be conveyedin plugs through the pipeline. The plugs fill the full bore of the pipeline and areseparated by short air gaps. As the conveying air velocity is reduced, the air gapbetween the plugs gradually fills with material along the bottom of the pipelineand the plug ultimately moves as a ripple along the top of an almost static bed ofmaterial. As the air flow rate reduces, to give very low conveying air velocities,the material flow rate also reduces.

Materials composed almost entirely of large mono-sized particles, such aspolyethylene and nylon pellets, peanuts, and certain grains and seeds, convey verywell in plug flow. In dilute phase conveying, nylons and polymers can suffer dam-age in the formation of angel hairs, and grains and seeds may not germinate as aconsequence of damage caused at the high velocities necessary for conveying.Because of the very high permeability necessary, air will readily permeate throughthe material while it is being conveyed and so maximum values of solids loadingratios will typically be about 30.

2 MATERIAL CONVEYING CHARACTERISTICS

If a pneumatic conveying system is to be designed to ensure satisfactory operation,and to achieve maximum efficiency, it is necessary to know the conveying charac-teristics of the material to be handled. The conveying characteristics will tell adesigner what the minimum conveying velocity is for the material, whether thereis an optimum velocity at which the material can be conveyed, and what pipelinediameter and air mover rating will be required for a given material flow rate andconveying distance.

Alternatively, for an existing pneumatic conveying plant, the appropriateconveying characteristics will tell a designer what flow rate to expect if it is neces-sary to convey a different material, and whether the air flow rate is satisfactory.Conveying characteristics can also be used to check and optimize an existing plantif it is not operating satisfactorily.

In order to be able to specify a pipe size and compressor rating for a re-quired duty it is necessary to have information on the conveying characteristics of

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Gas-Solid Flows 111

the material. If sufficient previous experience with a material is available, suchthat the conveying characteristics for the material are already established, it shouldbe possible to base a design on the known information.

If previous experience with a material is not available, or is not sufficient fora full investigation, it will be necessary to carry out pneumatic conveying trialswith the material. These should be planned such that they will provide data on therelationships between material flow rate, air flow rate and conveying line pressuredrop, over as wide a range of conveying conditions as can be achieved with thematerial.

The trials should also provide information on the minimum conveying airvelocity for the material and how this is influenced by conveying conditions. Thisis particularly important in the case of dense phase conveying, for the differencesin conveying characteristics between materials can be very much greater thanthose for dilute phase conveying.

If the investigation is to cover the entire range of conveying modes with thematerial, then the previous experience must be available over a similar range ofconveying conditions. Scale up in terms of air supply pressure, pipe bore, convey-ing distance and pipeline geometry from existing data is reasonably predictable,provided that the extrapolation is not extended too far. Scale up in terms of modeof conveying, into regions of much higher solids loading ratios and lower convey-ing air velocities, however, should not be attempted unless evidence of the poten-tial of the material for such conveying is available.

2.1 Conveying Mode

With high pressure air, conveying is possible in the dense phase mode, providedthat the material is capable of being conveyed in this mode. It is the influence ofmaterial properties on the possible mode of conveying, as well as differences inmaterial flow rates achieved for identical conveying conditions, that makes it es-sential for conveying trials to be carried out with an untried material before de-signing a pneumatic conveying system. In conveying tests with high pressure airthere is an additional need, therefore, to establish the limits of conveying and thismay be over a very wide range of conveying conditions.

In addition to material properties, conveying distance can have a significantinfluence on the solids loading ratio at which a material can be conveyed, andhence mode of conveying that is possible. The influencing factor here is simplypressure gradient, and this will limit conveying potential regardless of the capabili-ties of the material. This aspect of conveying pipeline performance is consideredin more detail in Chapter 8.

2.1,1 The A ir Only Datum

In order to illustrate how conveying characteristics can be used it is necessary toshow first how they are built up and to examine the influence of the main vari-ables.

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112 Chapter 4

30

40 80 120 160 200

Free Air Flow Rate - ftVmin

Figure 4.1 Air only pressure drop data for pipeline shown in figure 4.2.

The simplest starting point is to consider the air only flowing through thepipeline. If a graph is drawn of pressure drop against air flow rate for a conveyingline the result will be similar to that shown in Figure 4.1.

The data in Figure 4.1 relates to a 165 ft long pipeline of 2 inch nominalbore which includes nine ninety degree bends. Details of the pipeline are presentedin Figure 4.2. This pipeline was used for conveying many of the materials forwhich conveying characteristics are presented in the first part of this chapter, andseveral subsequent chapters. As a consequence, both the pipeline in Figure 4.2,and the air only pressure drop datum in Figure 4.1, will serve as a reference formuch of the data that follows.

The line representing the air only pressure drop on Figure 4.1 is effectivelythe lower limit for conveying and will appear on subsequent graphs with a zero toindicate that this is the datum for conveying and represents a material flow rate ofOlb/h.

It will be seen from Figure 4.1 that the air only pressure drop increasesmarkedly with increase in air flow rate. When material is added to the air in thepipeline, at any given value of air flow rate, there will be an increase in pressure.This is as a consequence of the drag force of the air on the particles to enable themto be conveyed through the pipeline.

The air, however, has to be at a velocity that is sufficiently high to conveythe material, otherwise the particles will not convey, and a build up of such mate-rial could cause blockage of the pipeline.

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Gas-Solid Flows 113

Pipeline:165 ft long2 inch nominal bore9 * 90° bends

D/d = 24

Figure 4.2 Details of pipeline used for conveying trials.

In some situations, when fine dust is fed into a pipeline, there will be a slightreduction in pressure drop, and this relates to modification of the boundary layer.The flow rates of material involved are very small and have no relevance to pneu-matic conveying. It will be seen from Figure 4.1 that if an air mover having a lowpressure capability is to be employed, the pressure drop available for conveyingmaterial will be very limited, particularly if a high air flow rate is required for di-lute phase conveying. Pipeline bore, of course, can be increased in order to com-pensate if the pressure available for conveying is limited.

2.1.1.1 Pressure Drop EvaluationFigure 4.1 relates to single phase flow and the analysis of such flows is well estab-lished and quite straightforward. The pressure drop, Ap, for a fluid of density p,flowing through a pipeline of a given diameter, d, and length, L, can be determinedfrom Darcy's Equation:

fLpC2

Ap a lbf/in2 - - (1)d

where / is the friction factor, which is a function of the Reynoldsnumber for the flow and the pipe wall roughness,

and C is the mean velocity of the flow - ft/min

It can be seen from this mathematical model, which is presented in more de-tail in Chapter 6 on 'The Air Only Datum', that pressure drop follows a square lawrelationship with respect to velocity. This means that if the velocity is doubled the

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114 Chapter 4

pressure drop will increase by a factor of approximately four. Velocity, therefore,is a very important parameter in this work and so in graphical representations ofexperimental results and data, velocity needs to be represented on one of the axes.

2.7.2 Conveying Air Velocity

A major problem with using velocity, however, is that it is not an independentvariable. Gases are compressible and their densities vary with both pressure andtemperature. Since density decreases with decrease in pressure, the velocity of theconveying gas will gradually increase along the length of a constant bore pipeline.In Figure 4.1 it will be noticed that free air flow rate has been used instead of ve-locity. Velocity, however, can be determined quite easily from the volumetric flowrate by use of the two following equations:

D V D V D Vr\ \ _ f T . 2 _ ^0 0 .-T*.

T T T•M *2 -'0

where p = absolute pressure of air - lbf/in2

V = volumetric flow rate of air - ftVmin

and T = absolute temperature of air - R(°F + 460)and the subscripts relate to:

1 = conveying line inlet2 = conveying line exit0 = free air conditions

and for a circular pipeline:

576 VC = — ft/min - - - - - . . . . - (3)

where C = conveying air velocity - ft/minand d = pipeline bore - inch

This shows quite clearly how velocity is influenced by both gas pressure andtemperature, for a given volumetric flow rate of free air, and that for any given setof conditions the gas velocity can be evaluated quite easily. These equations aredeveloped further in the next chapter.

In Figure 4.3 a graph is presented that will allow the conveying air velocityto be evaluated for any given free air flow rate and conveying air pressure for con-veying data relating to Figures 4.1 and 2. Conveying air velocity values up toabout 6000 ft/min have been considered as this is ideally the maximum value thatshould normally be employed in dilute phase conveying.

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Gas-Solid Flows 115

6000 L

c

IL 4000

_o>

•= 2000

coU

Conveying Air Pressure- Ibf/in2 gauge

Atmospheric Pressure= 14-7 Ibf7in2 absolute

Pipeline Bore = 2 in nominalAir Temperature = 60 F

40 80 120 160 200

Free Air Flow Rate - ft/min

Figure 4.3 The influence of air flow rate and pressure on conveying air velocity for testpipeline and data.

2.2 Pneumatic Conveying

If a small quantity of a granular or powdered material is fed into a gas stream at asteady rate there will be an increase in the conveying line pressure drop, above theair only value, if the gas flow rate remains constant. For a given material the mag-nitude of this increase depends upon the concentration of the material in the gas.As the material flow rate into the conveying line increases, therefore, the convey-ing line pressure drop will also increase.

In a two phase flow system consisting of a gas and solid particles conveyedin suspension, part of the pressure drop is due to the gas alone and part is due tothe conveying of the particles in the gas stream. In such a two phase flow the par-ticles are conveyed at a velocity below that of the conveying gas. There is, there-fore, a drag force exerted on the particles by the gas.

For dilute phase, suspension flow, this drag force is the main contributor tothe conveying line pressure drop, whether it is accelerating the particles from thefeed point or conveying them through straight pipeline or around bends, and so itis not surprising that different materials will behave very differently. These differ-ences will be highlighted in this chapter, and they will be a major theme throughthe handbook.

2.2.7 Slip Velocity

The difference in velocity between the conveying gas and the particles is called theslip velocity. The magnitude of the slip velocity will depend upon the size, shape

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116 Chapter 4

and density of the particles. For horizontal conveying, low density 20 micron sizedparticles are likely to be conveyed at about 90% of the velocity of the conveyinggas, and for high density 1000 micron sized particles the value will be about 50%.A typical representative value for the velocity of powdered materials is about 85%of the gas velocity for horizontal conveying and 75% of the gas velocity for con-veying vertically up.

2.2.2 Cases Considered

The influence of particle concentration on conveying line pressure drop over awide range of conveying air flow rates, and hence velocities, is illustrated withthree very different materials. These are ordinary portland cement, a sandy gradeof alumina and polyethylene pellets. They are representative of materials capableof the range of conveying modes discussed above and so are used to illustrate theconveying characteristics typical of these three groups of material.

Identical sets of axes have been used for presenting the conveying data foreach of the three materials so that direct visual comparisons can be made betweenthe conveying capabilities of the three materials. Each of the three materials con-sidered was conveyed through the pipeline shown in Figure 4.2. 200 ftVmin offree air was available at a pressure of 100 Ibf/in2 gauge, although the maximumvalue of pressure employed for conveying any of the materials was limited toabout 40 Ibf/in" gauge. A top discharge blow tank was used to feed each of thematerials into the pipeline.

It should be emphasized that the data presented here for the various materi-als relates only to the materials tested and to this particular pipeline. This aspect ofthe problem is considered in more detail in Chapters 7 and 8 where scaling pa-rameters are presented, which will allow the conveying data presented here to bescaled to any other pipeline required.

2.3 The Conveying of Cement

Pressure drop data for the cement is presented in Figure 4.4. This is a graph ofconveying line pressure drop plotted against free air flow rate, and lines of con-stant cement flow rate have been drawn as the family of curves. Within the limit ofthe 30 Ibf/in2 pressure drop the cement was conveyed at flow rates up to about35,000 Ib/h through this two inch nominal bore pipeline.

2.3.1 Conveying Limits

The zero line at the bottom of the graph is the curve representing the variation ofconveying line pressure drop with air flow rate for air only, which comes fromFigure 4.1 for the pipeline used. This, therefore, represents the lower limit withrespect to the material conveying capacity for the given system. Apart from thelower limit of zero for material conveying capacity, there are three other limita-tions on the plot in Figure 4.4.

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Gas-Solid Flows 117

30

Q 20

0.o

10

coU

0

Material Flow Rate- I b / h * 1000 30

0 40 160 20080 120

Free Air Flow Rate - ItVmin

Figure 4.4 Pressure drop data for cement.

The first is the limit on the right hand side of the graph, but this is set onlyby the volumetric capacity of the compressor or blower used. This was 200ftVmin, and by reference to Figure 4.3 it will be seen that conveying air velocitiesare up to about 8000 ft/min at the end of the pipeline. For the majority of pneu-matic conveying systems this is considered to be the upper limit.

This upper limit is partly influenced by problems of material degradationand bend erosion in the conveying line, but it is mainly due to the adverse effecton the conveying line pressure drop and hence material flow rate. This aspect ofthe problem is considered in more detail in the next section. In terms of the overallconveying characteristics, the shape of the curves is quite clearly establishedwithin this maximum limit.

The second limit is that at the top of the graph and this is set by the pressurerating of the compressor or blower used. Once again this is not a physical limit, forif air is available at a higher pressure, it can be used for conveying, but it wouldnormally be recommended that the pipeline be stepped to a larger bore in order tolimit the very high values of conveying air velocity. This aspect of system designis considered in Chapter 9.

The third is the limit on the left hand side of the graph and this representsthe approximate safe minimum conditions for successful conveying with the mate-rial. The lines actually terminate and conveying is not possible in the area to theleft at lower air flow rates. This limit is governed by a complex combination ofmaterial properties, material concentration and conveying distance, and is consid-ered in more detail later in this section.

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118 Chapter 4

Any attempt to convey with a lower air flow rate would result in blockage ofthe pipeline, in a conventional conveying system. This is because the air flow ratewould be below the minimum required to convey the material. The terminologyemployed for these situations is choking, when conveying vertically up, and salta-tion when conveying horizontally.

2.3.2 Conveying Air Velocity Effects

An alternative way of presenting the conveying data on Figure 4.4 is to plot thematerial flow rate against the air flow rate and to have a series of curves at a con-stant value of the conveying line pressure drop. Such a plot is presented in Figure4.5a. Although the air only datum is lost, this alternative plot shows the influenceof excessively high conveying air velocities very well.

The lines of constant pressure drop can be seen to slope quite steeply to theair flow rate axis, and hence to zero material flow rate at very high air flow rates,and hence velocities. This is because of the square law relationship of pressuredrop with respect to velocity, presented in Equation 1 for air only, but which ap-proximately applies to suspension flow for high velocity dilute phase conveying.

60

,50

40

_0

20

a

10

Conveying LinePressure Drop

- Ibt7in2

\35

30

(a)

0 50 100 150 200

Free Air Flow Rate - fVVmin

60

Solids LoadingRatio

Conveyingo 50oo

> 40

<uCS

30

_o

I 20O"o3

S10

0

-AREA

ConveyingPressure Dri

- Ibf/iiv

(b)

0 50 100 150 200

Free Air Flow Rate - ft3/min

Figure 4.5 Performance data for cement conveyed through the pipeline shown in Fig-ure 4.2. (a) Material flow rate data and (b) conveying characteristics.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Gas-Solid Flows 119

If the conveying system has a compressor or blower with a maximum ratingin terms of delivery pressure, a considerable amount of this available pressure willbe taken up by moving the air through the line if the air flow rate, and hence ve-locity, is too high.

Part of the pressure drop is due to the material being conveyed and thegreater the concentration of the material in the air, the greater the pressure drop. Ifthe conveying air velocity is too high, therefore, the concentration of the materialin the air will have to be reduced in order to match the available pressure drop, andso the resulting material flow rate will be much lower.

2.3.3 Solids Loading Ratio

Solids loading ratio is the term generally used by pneumatic conveying engineersto describe the conveyed gas-solids suspension flow. Solids loading ratio is theratio of the mass flow rate of the solids conveyed to the mass flow rate of the airused. The particular advantages over particle concentration are that it is a dimen-sionless quantity and its value does not vary with the conveying gas pressure. Withthe graph in Figure 4.5a being a plot of material flow rate against air flow rate,lines of constant solids loading ratio can be superimposed quite easily as they willbe straight lines through the origin. Such a plot is shown in Figure 4.5b.

The plot presented in Figure 4.5b is referred to as the conveying characteris-tics for the material and is, in effect, a performance map for the material in thegiven pipeline. A conveying limit for the material is also identified on this plot.From Figure 4.5b it will be seen that solids loading ratios up to about 140 havebeen achieved and this is quite clearly dense phase conveying. With a low airpressure and a high air flow rate, however, the cement is conveyed at solids load-ing ratios below ten and this is quite clearly dilute phase, suspension flow. It willbe seen that there is no transition between dilute and dense phase flow and so thedividing line between the two modes of flow is not clearly defined.

2.3.4 Minimum Conveying Air Velocity

The conveying limit represented on Figure 4.5b appears a little strange at firstsight. If reference is made to Figure 4.3, or if conveying air velocities are other-wise calculated, it will be seen that at the upper part of the conveying limit curvethe conveying air velocity is about 600 ft/min. This is where the solids loadingratio is about 140 and so a minimum conveying air velocity of 600 ft/min is con-sistent with that appropriate for dense phase conveying. At very low values ofconveying air pressure, and hence low values of solids loading ratio, the minimumconveying air velocity is about 2000 ft/min and this is consistent with that neces-sary for the dilute phase conveying of this type of material.

The slope of the conveying limit curve is positive in both of these extremeareas of dilute and dense phase conveying. This is due to the compressibility effectof the air. In these two regions the conveying air velocity is reasonably uniform,being about 2000 ft/min for the dilute phase conveying of the cement, and 600

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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120 Chapter 4

ft/min for the dense phase conveying. As the pressure of the conveying air in-creases, a greater volumetric flow rate of air is required to maintain the same valueof conveying air velocity, and hence the positive slope to the conveying limitcurve in these areas.

Between these two regions two opposing effects come into play. One is theproblem of compressibility, which means that a greater air flow rate is required asthe air supply pressure increases. The other relates to the considerable increase insolids loading ratio that is possible with an increase in conveying line pressuredrop. This means that the cement can be conveyed at a lower velocity, which inturn means that a lower air flow rate is required. The combination of these twoeffects dictates the shape of the transition between the dilute phase and the verydense phase portions of the conveying limit curve.

2.3.4.1 Solids Loading Ratio InfluenceThe relationship between the minimum conveying air velocity and the solids load-ing ratio at which a material is conveyed can be determined experimentally withthe material in a pipeline. This is typically derived during the conveying trials car-ried out with a material in order to determine the conveying characteristics for thematerial, since the determination of conveying limits is generally an integral partof the test work.

Pneumatic conveying trials with bulk particulate materials are considered inChapter 23. The approximate influence of solids loading ratio on the minimumconveying air velocity for the cement is presented in Figure 4.6.

3000

• 2000

enc

coU

1000

20 40 60 80 100

Solids Loading Ratio

Figure 4.6 Approximate influence of solids loading ratio on the minimum value ofconveying air velocity for the pneumatic conveying of ordinary portland cement.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Gas-Solid Flows 121

This curve is typical of the relationship between minimum conveying air ve-locity and solids loading ratio for air retentive materials that are capable of beingconveyed in the sliding bed mode of dense phase flow. This relationship has amajor influence on the operation and pneumatic conveying capability of this typeof material and will feature at many points throughout this Handbook. Possibly thegreatest effect is the change that occurs with increase in conveying distance, whichis considered in Chapter 7.

Since high solids loading ratios can only be achieved with a high value ofpressure gradient, an increase in conveying distance will mean that the value ofsolids loading ratio must be reduced if there is no increase in the air supply pres-sure. A reduction in solids loading ratio, as will be seen from Figure 4.6, will re-quire an increase in conveying air velocity and this will consequently require anincrease in air flow rate.

In the extreme the solids loading ratio will reduce to a value at which thematerial can only be conveyed in dilute phase. This relationship is introduced laterin this Chapter.

2.4 The Conveying of Alumina

The grade of alumina used and reported here is one that is generally referred to asbeing sandy or coarse. The alumina was conveyed through the pipeline shown inFigure 4.2 and the pressure drop data for the material is presented in Figure 4.7.This is a graph of conveying line pressure drop plotted against free air flow rate,and lines of constant alumina flow rate have been drawn as the family of curves.

D.eQu

IOJ

OH

DOC

CoU

30

20

10

Material Flow Rate- l b / h x 1000

40 80 120 160 200

Free Air Flow Rate - fr/min

Figure 4.7 Pressure drop data for sandy alumina.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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122 Chapter 4

Within the limit of the 30 Ibf7in2 pressure drop the alumina was conveyed atflow rates up to about 25,000 Ib/h through this two inch nominal bore pipeline. Ifthis is compared with the corresponding data for the cement in Figure 4.4 it will beseen that the maximum value of flow rate for the alumina is very much lower andthat the air flow rate required to achieve 25,000 Ib/h is significantly greater thanthat required to convey the cement at 35,000 Ib/h.

The same conveying limits, as discussed in relation to the conveying of ce-ment, apply to the alumina. It is the same pipeline and so the air only pressuredrop relationship is the same. It is the same air supply and so the air flow rate andpressure considered are also the same. It is the conveying limit for the material thatdiffers. Conveying capability and conveying limits, however, do differ widelyfrom one material to another, and this is why conveying data is so essential.

2.4.1 Conveying A ir Velocity Effects

An alternative presentation of the data, in terms of material flow rate plottedagainst air flow rate, with lines of constant conveying line pressure drop superim-posed, is presented in Figure 4.8a. Once again this graph is drawn with the sameaxes as that for the cement in Figure 4.5a so that a direct visual comparison of thetwo materials can be made.

60

50

40

g30

o

20od

10

(a)

Conveying LinePressure Drop

- lbf/in2

25

50 100 150 200

Free Air Flow Rate - ftVmin

60

50

o

* 40.o

I30•5

320

10

(b)

NO

GO

AREA

Solids LoadingRatio

ConveyingLimit

Conveying Line 20Pressure Drop

- lbf/in2

50 100 150 200

Free Air Flow Rate - ftVmin

Figure 4.8 Performance data for sandy alumina conveyed through the pipeline shownin figure 4.2. (a) Material flow rate data and (b) conveying characteristics.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Gas-Solid Flows 123

The comparison is striking in terms of the small area of the graph in whichthe data for the sandy alumina appears. It will be noted in Figure 4.8a that the linesof constant conveying line pressure drop terminate at progressively higher airmass flow rates as the material flow rate increases. This does not mean that theminimum conveying air velocity increases. This is entirely due to the influence ofair pressure and the compressibility of the air. By reference to Figure 4.3, it will beseen that the minimum conveying air velocity for this material is about 2600ft/min and that it changes little over this range of material concentration.

This slope of the minimum conveying limit on Figure 4.8b is a characteristicfeature of all materials conveyed in dilute phase and will be seen on the conveyingcharacteristics for most of the materials presented here. It applies equally to mate-rials capable of being conveyed in dense phase, if the pressure gradient is low, aswill be seen in the very low pressure area on Figure 4.5b for the cement.

With the cement it was possible to convey the material with higher air sup-ply pressures. From Figure 4.5a it will be seen that within the limit of 60,000 Ib/hof material, conveying line pressure drop values up to 40 Ibf/in2 were employed.From Figure 4.8a for the alumina it will be seen that 25 Ibf/in2 is close to the maxi-mum pressure that could be employed. Although the air pressure with the test fa-cility was available at 100 Ibf/in2 gauge, a pressure higher than 25 Ibf/in2 could notbe used because the volumetric flow rate of the air was limited to 200 ftVmin.

The locus of the conveying limit line is included on Figure 4.8b and it willbe seen that this passes through the 200 ftVmin air flow rate limit with an air sup-ply pressure of about 30 Ibf/in2. At these air supply pressures the minimum con-veying air velocity for the alumina is about 2600 ft/min compared with only 600ft/min for the cement, and so for a given air supply pressure the air flow rate ismore than four times greater.

2.4.2 Solids Loading Ratio

The conveying capability for the alumina is clearly illustrated with the conveyingcharacteristics presented in Figure 4.8b. The maximum value of solids loadingratio achieved is only just over 25 and this, together with the minimum conveyingair velocity of 2600 ft/min, equates to dilute phase, suspension flow for the mate-rial. Despite the fact that a high air supply pressure was available, the material isonly conveyed in dilute phase. It must be stressed, therefore, that high pressure isnot synonymous with dense phase conveying.

Solids loading ratios for dilute phase conveying are generally much lowerthan 25. The fact that a solids loading ratio as high as 25 was achieved in this caseis due to the fact that the pipeline was relatively short at 165 ft and the air supplypressure was relatively high at 25 Ibf/in2. To complete the picture for the alumina aplot of the minimum conveying air velocity versus solids loading ratio is presentedin Figure 4.9. This is simply a horizontal line at a value of 2600 ft/min over a lim-ited range of solids loading ratios. This is typical of materials that can not be con-veyed in dense phase, and there is generally little change in the value of the mini-mum conveying air velocity value over the range of solids loading ratios.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 137: Handbook of Pneumatic Conveying Engineering

124 Chapter 4

3000^

I

<5

2000

.S 1000

ooc

oO

20 40 60

Solids loading ratio

80 100

Figure 4.9 Approximate influence of solids loading ratio on the minimum value ofconveying air velocity for the pneumatic conveying of sandy alumina.

2.5 Comparison of Materials

To illustrate the influence of the material on conveying capability further, the con-veying characteristics for two more materials conveyed through the Figure 4.2pipeline are presented in Figures 4.10a and 4.1 Ob. The first of these is a fine gradeof pulverized fuel ash, obtained from the electrostatic precipitators of a boilerplant, and had a mean particle size of about 25 micron.

The second is a silica sand, obtained from a quarry, and air classified to givea mean particle size of about 70 micron. It will be seen that this pair of materialsare very similar to the cement and alumina in terms of overall characteristics, par-ticularly with regard to minimum conveying limits.

The conveying line pressure drop curves for the pulverized fuel ash, how-ever, are very much steeper than those of the cement and so very much highermaterial flow rates were achieved at low values of air flow rate. As a consequencevery much higher values of solids loading ratio were achieved.

Apart from differences in density, the shape of the particles are also verydifferent, with the cement coming from a grinding process and fly ash being de-rived from a combustion process. It is not surprising, therefore, that the conveyingcharacteristics are very different. Values of minimum conveying air velocity,however, are very similar.

The differences between the alumina and the sand are not so pronounced, al-though the sand has a slightly lower value of minimum conveying air velocity, buta much lower material flow rate was achieved for a given pressure drop.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 138: Handbook of Pneumatic Conveying Engineering

Gas-Solid Flows 125

60

50

40

ai

o20

S 10

(a)

300200 120 100 8060

50 100 150 200

Free Air Flow Rate - frYmin

40

.320

10

0

(b)

NO

GO

AREA

Solids LoadingRatio

ConveyingLimit

Conveying LinePressure Drop 25

\ 35 V.20

15

10

0 50 100 150 200

Free Air Flow Rate - ftVmin

Figure 4.10 Conveying characteristics for materials conveyed through the pipelineshown in figure 4.2. (a) A fine grade of pulverized fuel ash and (b) silica sand.

2.6 The Conveying of Polyethylene Pellets

Polyethylene pellets were also conveyed through the pipeline shown in Figure 4.2and the pressure drop data for the material is presented in Figure 4.11. This is agraph of conveying line pressure drop plotted against free air flow rate, and linesof constant material flow rate have been drawn as the family of curves. The axesused are the same as those employed for the cement in Figure 4.4 and the aluminain Figure 4.8 and so a direct visual comparison can be made.

Within the limit of the 30 lbf/in2 pressure drop the pellets were conveyed atflow rates up to about 30,000 Ib/h through this two inch nominal bore pipeline.This compares with 35,000 Ib/h for the cement and 25,000 Ib/h for the alumina butthe main point is that the overall conveying data is very different once again. Con-veying is possible at very low values of air flow rate, like the cement, but themaximum value of material flow rate was achieved at the highest value of air flowrate, like the alumina.

The same conveying limits, as discussed in relation to the conveying of boththe cement and alumina, apply to the polyethylene pellets. It is the same pipelineand so the air only pressure drop relationship is the same. It is the same air supplyand so the air flow rate and pressure considered are also the same. In this case it isthe conveying limit for the material that differs and the behavior of the material atlow values of conveying air velocity.

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126 Chapter 4

30MaterialFlow RateIb/h x 1000

80 120 160 200Free Air Flow Rate - ftVmin

Figure 4.11 Pressure drop data for polyethylene pellets.

2.6.1 Conveying Air Velocity Effects

An alternative presentation of the data, in terms of material flow rate plottedagainst air flow rate, with lines of constant conveying line pressure drop superim-posed, is presented in Figure 4.12a. Once again this graph is drawn with the sameaxes as those for the cement in Figure 4.5a and the alumina in Figure 4.8a so that adirect visual comparison of the three materials can be made. Once again the pat-tern of the data is totally different from that of the previous two materials.

With this material a distinct pressure minimum point is observed. The linesof constant conveying line pressure drop on Figure 4.12a change slope at the pointwhere the material flow rate is a maximum. The term 'pressure minimum' is actu-ally derived from Figure 4.11 where a minimum value of conveying line pressuredrop can be seen for each of the lines of constant material flow rate.

With the pressure drop lines changing slope below the pressure minimumpoint, material flow rates reduce considerably with further decrease in air flowrate. Although conveying is possible at lower air flow rates, unlike the alumina,material flow rates are significantly lower than those achieved with the cement. Itwill be seen that the pressure drop lines all merge below the pressure minimumpoints and this is why the curves on Figure 4.11 rise vertically below the pressureminimum point.

The material flow rate, however, was reasonably uniform over the entirerange of conveying conditions, although this is not always the case with this typeof material. With the pipeline being of relatively small bore it was not possible toseparate the lines of constant pressure in this area.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Gas-Solid Flows 127

60

50

ooo

40

« 30cd

as

FT 20

10

60

50

Conveying LinePressure Drop

- lbf/in2

NO

GO

AREA

\ Conveying LinePressure Drop

- lbf/in%

Solids LoadingRatio

(a)

0 50 100 150 200Free Air Flow Rate - ftVmin (b)

50 100 150 200Free Air Flow Rate - ftVmin

Figure 4.12 Performance data for polyethylene pellets conveyed through the pipelineshown in figure 4.2. (a) Material flow rate data and (b) conveying characteristics.

2.6.2 Solids Loading Ratio

The conveying characteristics for the polyethylene pellets are presented in Figure4.12b and from this it will be seen that the maximum value of solids loading ratioat which the material was conveyed is no different from that of the sandy alumina,at about 25. The polyethylene pellets, however, were successfully conveyed withconveying air velocities down to about 600 ft/min and so this is clearly densephase flow at low values of air flow rate.

The material, having a relatively large particle size, and being mono-sized,means that it is very permeable. As a consequence the air will pass through apacked bed of the material relatively easily. It is as a result of the material beingvery permeable that it will naturally convey in dense phase in a plug flow mode.With the material being so permeable it is possible that the material could be con-veyed with very much lower air velocities than 600 ft/min without blocking thepipeline. As the material flow rate decreases with decrease in air flow rate, belowthe optimum point, however, the benefits of ultra low velocity conveying need tobe carefully considered.

As with the cement, a natural transition from dilute phase conveying todense phase conveying occurs, but the value of the solids loading ratio provides noguidance in this case. It can, however, be determined by the value of the convey-ing line inlet air velocity and this is considered further in Chapter 7.

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128 Chapter 4

3 THE INFLUENCE OF MATERIALS

The conveying characteristics for different materials can vary significantly, asillustrated with the representative group of materials considered above. This isparticularly so for materials that are capable of being conveyed in dense phase. Atlow values of air flow rate the lines of constant conveying line pressure drop canhave a wide variety of slopes. There is also the added complexity of different ma-terials having different minimum conveying limits. Thus for a given air flow rateand conveying line pressure drop, material flow rates for different materials canvary considerably, and the air flow rate necessary to convey different materials canalso vary considerably.

Some of these differences were illustrated earlier with the materials used toshow how conveying characteristics are determined. These differences, however,are not just a feature of conveying with high pressure air but will be found in lowpressure systems also.

3.1 Low Pressure Conveying

If only low pressure air is available for conveying a material through a pipeline,such as that from a positive displacement blower or any vacuum conveying sys-tem, and with a pressure drop below about 15 lbf/in2, a material will only be con-veyed in dilute phase through a pipeline, unless the conveying distance is veryshort. Conveying at high values of solids loading ratio typically requires high val-ues of conveying line pressure drop.

The influence of solids loading ratio on pressure drop is illustrated in Figure4.13. This is an extension of the data presented in Figures 4.4 to 4.6 for the cementand is a plot of conveying line pressure drop against free air flow rate, with linesof constant solids loading ratio superimposed.

For the pipeline shown in Figure 4.2, for which the data relates, a conveyingline pressure drop in excess of 10 lbf/in" is required before the material can beconveyed in true dense phase and at low velocity. It will be seen that the volumet-ric flow rate of the air has a significant effect in this respect and helps to illustratewhy high values of solids loading ratio are not possible for materials that can notbe conveyed in dense phase.

If the pipeline is very much shorter, and with fewer bends, however, the aironly curve and the pressure drop axis on Figure 4.13 will be significantly reducedand low velocity dense phase conveying will be possible at much lower values ofpressure drop.

By positioning the reception vessel on the quayside close to bulk containerships, conveying distances can be kept very short and materials such as cementcan be off-loaded in dense phase, and at very high flow rates, with a vacuum con-veying system. Fly ash can similarly be transferred from electrostatic precipitatorson boiler plants to intermediate reception vessels in dense phase by vacuum con-veying systems.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 142: Handbook of Pneumatic Conveying Engineering

Gas-Solid Flows 129

30

o,eD

20

g ' l O

coU

Solids LoadingRatio

NO

GO

AREA

Conveyin:Limit

120 100 80 60 50 4030

20

40 80 120

Free Air Flow Rate - frVmin

160 200

Figure 4.13 Solids loading ratio data for cement.

Conveying data for four different materials is presented in Figure 4.14. Eachmaterial was conveyed in a positive pressure conveying system up to a limit of 8lbf/in2 in terms of conveying line pressure drop. All four materials were conveyedthrough the same pipeline, a sketch of which is given in Figure 4.15. Althougheach material could only be conveyed in dilute phase, because of the limit on pres-sure available, it will be seen that there are significant differences in their convey-ing capabilities.

The differences between materials are mainly in terms of the material flowrates achieved, varying from 8500 Ib/h for the pearlite to 3500 Ib/h for the ironpowder, for a pressure drop of 8 lbf/in2. Since all the materials were conveyed indilute phase, and they were all either powders or fine granular materials, suchmarked differences would not be expected in terms of minimum conveying airvelocities. With a 3 lbf/in2 pressure drop, these varied between 2400 ft/min for thepearlite and 3200 ft/min for the iron powder.

Although the iron powder achieved the lowest flow rate of the four materialspresented, it should be noted that the iron powder conveyed very well, regardlessof the fact that particle density was 355 lb/ft3 and the bulk density about 150 lb/ft3.Metal powders can be conveyed pneumatically; the main problem is that many ofthem are potentially explosive and so require to be conveyed with nitrogen.

Uranium with even higher density values is regularly conveyed in pneu-matic systems because of the safety aspects of the conveying system. At the otherextreme the pearlite had a bulk density of only 6 !b/ft' and a particle density of 50lb/ft'. With a higher pressure gradient available both the iron powder and pearlitehave dense phase conveying potential.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 143: Handbook of Pneumatic Conveying Engineering

130 Chapter 4

o 7

x 6X3

~T 5<L>"8

3ox,

2u

!2

i0

(a)

Conveying LinePressure Drop 8

- lbf/in2

Solids LoadingRatio

40 60 80 100 120Free Air Flow Rate - ft'/min

Solids LoadingConveying Line Ra(io

Pressure Drop- lbf/in2

(b)40 60 80 100 120

Free Air Flow Rate - frVmin

o2 6X

_c£ 5

8. 4

3

B 2

.Conveying Line- Pressure Drop

- lbf/in2

Solids LoadingRatio

oo

(c)

40 60 80 100 120

Free Air Flow Rate - ftVmin

1

0

(d)

Conveying LinePressure Drop

- lbf/in2

Solids LoadingRatio

40 60 80 100 120

Free Air Flow Rate - ftVmin

Figure 4.14 Conveying characteristics for low pressure conveying of materials, (a)Pearlite, (b) sodium chloride (salt), (c) iron powder, and (d) sodium carbonate (soda ash).

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 144: Handbook of Pneumatic Conveying Engineering

Gas-Solid Flows 131

Pipeline:length 115f tbore 2 inbends 8 x 90°D/d = 5

Figure 4.15 Details of pipeline used for low pressure conveying trials.

Many different materials have been tested in the pipeline presented in Figure4.15. To illustrate how the conveying characteristics of different materials canvary in such a low pressure system, the 8 lbf/in2 constant conveying line pressuredrop curves from a number of such materials are compared on Figure 4.16.

With additional materials it will be seen that the conveying performance, interms of material flow rate achieved, does not correlate with material density.Soda ash is little better than iron powder and pulverized fuel ash is better thanpearlite in terms of material flow rate achieved. Lump coal is better than finegranular salt, although a slightly higher value of conveying line inlet air velocity isrequired, and so performance does not correlate with particle size either.

(U^ .oi 4

• g 2

Pulverized Fuel Ash(fine grade)

Pearlite

Soda Ash

Silica Sand

Iron Powder

20 40 60 80 100 120 140

Free Air Flow Rate - fr/min

Figure 4.16 Comparison of performance of different materials conveyed through thepipeline shown in figure 4.15 with a conveying line pressure drop of 8 lbf/in2.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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132 Chapter 4

Different conveying capabilities and air requirements mean that particularcare must be taken if an existing system is to be used to convey another material,or if one system is required to convey a number of different materials. If the capa-bility of a system is dictated by the pressure rating of the air mover, then differentmaterial flow rates must be expected, and the feeding device must be capable ofmeeting the needs of any other material. A different air flow rate may also be re-quired, as shown by the different minimum values for conveying line inlet air ve-locity.

3.2 High Pressure Conveying

If high pressure air is available for conveying a material, and the pipeline is not toolong, then the material could be conveyed in dense phase if the material is capableof being conveyed in dense phase. Conveying data for a further four materials ispresented in Figure 4.17. All four materials were conveyed through the samepipeline once again, so that direct comparisons of performance can be made. Thepipeline is the same as that used for the earlier high pressure conveying trials, asketch of which was given in Figure 4.2. The compressor used for this high pres-sure work was capable of delivering 200 frVmin of free air at 100 lbf/in2 gauge.

The four materials presented include two food products and two metal prod-ucts, and from each group, one material could not be conveyed in dense phase andone could. The materials that could be conveyed in dense phase were conveyed atsolids loading ratios of well over 100 and were conveyed in the sliding bed modeof dense phase flow. These were wheat flour and iron powder and the conveyingcharacteristics for these materials are very similar in form to those for the cementin Figure 4.5b and the fly ash in Figure 4.10a presented earlier. With high pressureconveying air, and at high values of solids loading ratios, all four of these materi-als could be conveyed with conveying line inlet air velocities as low as 600 ft/min.

That high pressure is not synonymous with dense phase conveying is clearlyshown with the granulated sugar. A minimum conveying air velocity of 3200ft/min had to be maintained and, as a result, the maximum pressure that could beused was only 25 lbf/in2, because of the limit of 200 ftVmin on air flow rate avail-able. As a consequence the maximum solids loading ratio achieved was well be-low 20. Granulated sugar has both poor air retention properties and poor perme-ability.

Flour and sugar are often materials that are required to be conveyed with acommon system and often through the same pipeline. It will be seen that there aresignificant differences between the conveying capabilities of these two materials.The specification of air requirements represents a particular problem, apart fromchoice of feeder and controls.

Many different materials have been conveyed in the pipeline shown in Fig-ure 4.2. To show how the conveying characteristics of different materials can varyin such a high pressure system, the 20 lbf/in2 constant conveying line pressure dropcurves from a number of such materials are compared on Figure 4.18.

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Page 146: Handbook of Pneumatic Conveying Engineering

Gas-Solid Flows 133

50

I40

I30

° 20fcu

is'C0)

'S10

0

ConveyingLimit

Conveying Line

Solids LoadingRatio

50

>40

I730

Bi

.220~is<BJS10

(a)

0 40 80 120 160 200Free Air Flow Rate - ft3/min

NO

GO

AREA

Conveying LinePressure Drop -

- lbf/in2

Solids LoadingRatio

ConveyingLimit

(b)

40 80 120 160 200

Free Air Flow Rate - ftVmin

60

50ooo

^ 4 0£

£ 30

_oi 2°.5

'8S 10

ConveyingConveying Line Limit" Pressure Drop

Solids LoadingRatio

AREA

50 100 150 200

(C)Free Air Flow Rate - ft/min

60

050

X

=5 4Q

u

(S30s_oE|20

110

-

: NO• GO"

: AREA; c

• Conveying LinePressure Drop

; - lbf/in2 \

; 15

: 5.

SolidsRe

Convey!Limit

^^r=-j .

(d)

0 50 100 150 200Free Air Flow Rate - ftVmin

Figure 4.17 Conveying characteristics for high pressure conveying of materials, (a)Wheat flour, (b) granulated sugar, (c) iron powder, and (d) zircon sand.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 147: Handbook of Pneumatic Conveying Engineering

134 Chapter 4

50

ooo 40

30

FT 20

10

Pulverized Fuel Ash - fine

SugarGranulated

Barite

Iron Powder

Cement

Wheat Flour

PVC Powder

Magnesium/Sulfate

50 100 ^ 150Free Air Flow Rate - ft'/min

200

Figure 4.18 Comparison of performance of different materials conveyed through thepipeline shown in figure 4.2 with a conveying line pressure drop of 20 !bf/in2.

It will be noted that at the extreme right of Figure 4.18, at high air flowrates, all the materials are conveyed in dilute phase and the degree of scatter inmaterial flow rates is similar to that shown in Figure 4.16. All the pressure dropcurves have a negative slope in this area and each one will probably reach the airflow rate axis at a value of about 600 ft3/min.

As a result of the different slopes of the pressure drop curves, at low valuesof air flow rate, for the different materials, quite remarkable differences in materialflow rate can be obtained. This is for materials conveyed through exactly the samepipeline and under exactly the same conveying conditions. Differences in mini-mum conveying air velocities, for materials that will not convey in dense phase,significantly add to the problems of reliable system design, particularly for a newor unknown material.

3.2.7 Conveying L im its

Conveying limits in terms of minimum conveying air velocities and maxi-mum solids loading ratios vary widely for different materials. This point is clearlyillustrated in Figure 4.19 with the limits for three materials presented. Each mate-rial was conveyed through the 165 ft long pipeline shown in Figure 4.2. Althoughthe fine grade of fly ash could be conveyed at solids loading ratios in excess of200 and with minimum conveying air velocities close to 600 ft/min the copperconcentrate could not be conveyed above a solids loading ratio of about 55.

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Gas-Solid Flows 135

50

§40

o20

.3j§

10

Limit for Copper Concentrate

'100 /80

20

10

Solids Loading Ratio

50 100

Free Air Flow Rate - fr'/min

150 200

Figure 4.19 Comparison of material conveying limits for conveying under identicalconveying conditions.

The minimum conveying air velocity for the copper concentrate was about1600 ft/min. With the granulated sugar, however, conveying at a solids loadingratio of 20 could not be achieved and the minimum value of conveying air velocitywas about 3200 ft/min.

4 MATERIAL CHARACTERIZATION

Certain material characteristics can be used to predict the potential behavior of amaterial when pneumatically conveyed. These are mostly based on bulk propertiesof the material that relate to material-air interactions, such as fluidization, air re-tention and permeability.

The air retention capabilities of a bulk material are a good indicator ofwhether a material will convey in dense phase or not. Powdered materials such asfly ash, cement, and flour have very good air retention properties and are generallycapable of being conveyed at low velocities in a sliding bed mode of dense phaseflow. Large mono-sized particles having very good permeability, such as polyeth-ylene pellets are generally capable of being conveyed at low velocities in a plugmode of dense phase flow.

Coarse granular materials such as sand and alumina, that have very poor airretention and permeability are generally only capable of being conveyed in dilute

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136 Chapter 4

phase suspension flow in conventional pneumatic conveying systems, particularlyif they have a wide particle size distribution.

4.1 The Geldart Classification

The Geldart classification of materials is essentially in terms of two material prop-erties [1J. One is the difference in densities between the material particles and thefluidizing medium. For air this can simply be taken as the particle density, sincethe density of air is negligible in comparison.

The other property is the mean particle size of the material. This classifica-tion is shown here in Figure 4.20. It includes four broad areas that identify thebehavior of bulk materials when fluidized. It has often been considered that thisform of classification could be used to assess the suitability of materials for densephase conveying.

Group A materials retain air and the fluid bed collapses very slowly whenthe gas is turned off. These materials are generally capable of being conveyed indense phase. Group B materials do not retain air and the fluid bed collapses almostinstantaneously when the gas supply is turned off. These materials are not gener-ally capable of being conveyed in dense phase in a conventional conveying systemand so are restricted to dilute phase, suspension flow.

Group C materials are essentially cohesive and will behave in a similarmanner to Group A materials but are more difficult to handle. They will generallyconvey in dense phase but the main problem is often one of feeding them into thepipeline. Group D materials are likewise an extension of Group B in terms ofpneumatic conveying.

"o

I

500

100

50

10

10 50 100 500 1000

Mean Particle Size - micron

5000

Figure 4.20 Geldart's classification of fluidization behavior for fluidization with ambi-ent air.

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Gas-Solid Flows 137

This Group D classification, however, being in terms of mean particle size,is not capable of identifying materials that are capable of being conveyed in densephase in plug flow mode, for this is only appropriate to essentially mono-sizedparticles.

By the same reasoning the line separating Groups A and B is not particularlyreliable in identifying the division between dilute and dense phase conveying ca-pability.

4.2 Dixon's Slugging Diagram

Dixon [2], realized the importance of material type on the mode of conveying anddevised a classification known as the Slugging Diagram, which is shown in Figure4.21. The axes are the same as those for the Geldart classification and the samedivisions are identified. This classification, however, clearly identifies the capabil-ity of large mono-sized particles for conveying in the plug mode of dense phaseflow.

An understanding of the role of particle properties such as size, and size dis-tribution, shape or fractal properties and density will probably provide the ultimatesolution to the problem. It is, however, very difficult to quantify properties such asparticle shape and size distribution, and so measurable bulk properties relating togas-particle interactions offer the best short-term means of using property valuesto predict pneumatic conveying performance. Air retention, permeability and spe-cific surface are probably the best properties to consider for this purpose, althoughthe first two are probably the easiest to measure and determine.

500

100

t 50gQ

10

Group D

StrongAxisymmetric

Slugs

Group C

10 50 100 500 1000

Mean Particle Size - micron

5000

Figure 4.21 Dixon's slugging diagram.

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138 Chapter 4

4.3 Aeration Property Classification

Jones and Mills [3, 4] used a vibrated de-aeration constant and permeability factorto produce an empirical material classification for conventional pneumatic con-veying systems. The correlation that they produced is presented in Figure 4.22.For convenience the de-aeration rate was determined by vibrating the materialfrom the 'as poured' condition rather than measuring it from the fluidized state.

This clearly identifies the three main modes of conveying. Dense phase,moving bed flow, will naturally occur with materials that have very poor perme-ability and very low values of de-aeration. Dense phase, plug type flow, will natu-rally occur with materials that have very good permeability and a very rapid rateof de-aeration. The center grouping represents materials that are generally re-stricted to dilute phase flow in a conventional conveying system.

Materials that have very good air retention, and hence a low vibrated de-aeration rate value, such as cement, flour and fly ash, fall into the Group 1 cate-gory, and will convey very well in a conventional conveying system. A simple testto apply is to half fill a glass jar, preferably having a screw top lid, with a sampleof the material to be conveyed. Invert the jar a few times to aerate the material,place it on a surface, remove the lid, and drop a ball bearing or similar object intothe jar. If the ball bearing falls through the material and hits the bottom of the jar,the material is likely to have good air retention properties and be a potential candi-date for dense phase conveying.

With a material such as cement, the ball bearing will hit the bottom of thejar, even if it is dropped in the jar several minutes after the material has been aer-ated and left standing, as it has such good air retention properties.

T3& 0-5

S10

GROUP 2

DILUTE PHASE

GROUP 1 \ (SusPension Flow)Q

GROUP 3

PLUG TYPEFLOW

MOVING BEDTYPE FLOW

0-1 1 10 100Permeability Factor - ft3 in/lb x l(r6

Figure 4.22 Bulk material property classification for pneumatic conveying.

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Gas-Solid Flows 139

If the material is granular, the ball bearing is unlikely to penetrate the mate-rial and will simply come to rest on the top of the surface. In this case the materialis unlikely to have sufficient air retention to allow it to be conveyed in dense phasein a conventional conveying system.

If the material has good permeability, however, such that it falls into Group3, it is possible that the material will convey at low velocity in the plug type densephase mode of flow. Pelletized materials, such as polyethylene and nylon, areideal candidates and will convey very well in a conventional conveying system.Coarse granular materials having a wide particle size distribution, however, do notgenerally have sufficient permeability to be capable of dense phase conveying inthe plug phase mode.

5 CONVEYING SYSTEM CAPABILITY

For a given material a particular problem with pneumatic conveying systems is theevaluation of their conveying potential. The capability of a pneumatic conveyingsystem in terms of achieving a given material mass flow rate, depends essentiallyon the following three parameters:

D the diameter of the pipeline,D the distance to be conveyed, andD the conveying line pressure drop available.

Within normal limits, and for a given material, air flow rate is a secondaryfunction, being primarily dependent upon the pipeline bore and air pressure. It is,however, important with respect to achieving optimum conveying conditions in agiven pipeline. The properties of the material to be conveyed are also of para-mount importance. Their main influence, however, in terms of material mass flowrate, is in placing an upper limit on the solids loading ratio at which the materialcan be conveyed under particular conditions, as shown in Figure 4.19.

5.1 Solids Loading Ratio - </>

The solids loading ratio of a conveyed material is the dimensionless ratio of themass flow rate of the material being conveyed to the mass flow rate of the air usedfor conveying.

m<t> = — (4)

ma

where mp = mass flow rate of material - Ib/h

and ma = mass flow rate of air - Ib/h

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140 Chapter 4

Since air is a compressible fluid its density changes with pressure and so thevolumetric flow rate, and hence velocity, of the conveying air can increase quitesignificantly along the length of a pipeline. Solids loading ratio, therefore, is aparticularly useful parameter for describing the concentration of the material in theair in pneumatic conveying system pipelines, for it is a dimensionless quantity andits value remains essentially constant. This applies to stepped bore pipelines aswell as single bore lines.

5.2 The Influence of Pipe Bore

The mass flow rate of a material can be expressed in terms of the solids loadingratio at which the material is conveyed, by:

m = m Ib/h (5)

Note:To convert free air flow rate, in ftVmin, to a mass flow rate, in Ib/h,multiply by the density of the air, in lb/ff, and by 60 min/h:

x 0-0765 x 60 Ib/h (6)

Thus

Therefore

m,.

where F0 = volumetric flow rate of free air - ftVmin

ma cc

V =

and for a circular pipeline

n d2

576x C ft7min (7)

where d = pipeline bore - inand C = conveying air velocity - ft/min

oc Ccf (8)

As a first order approximation, for simplicity, conveying air velocity, C, canbe considered as being constant, so that:

m cf (9)

For a given system, therefore, throughput capability can be increased quiteconsiderably by increasing the pipe bore and so enable high material flow rates tobe achieved. The air requirements, of course, also have to be increased in the sameproportion in order to maintain an equivalent air velocity.

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Gas-Solid Flows 141

In terms of achieving a given material flow rate over a specified distance,pipeline bore is probably the main variable. Pressure drop is also important, but anincrease in air supply pressure is not always possible. Pipeline bore also has a sig-nificant effect on the air only pressure drop value, and this is particularly importantif a low pressure air supply is to be used. A significant proportion of the availablepressure could be used in getting the air through the pipeline. This aspect of sys-tem design is considered in Chapter 6.

5.3 The Influence of Pressure and Distance

The inter-relating effects of conveying line pressure drop and conveying distanceare illustrated for low pressure systems in Figure 4.23, and for high pressure sys-tems in Figure 4.24. The data is in terms of an approximate value of solids loadingratio that might be achieved for typical combinations of air supply pressure andconveying distance. It must be stressed that these figures are only approximationsfor the purpose of illustration and should not be used for design purposes. Pipebore, conveying air velocity and, more particularly, material type, all have an in-fluence on the overall relationship.

For very short distances it is quite possible to convey a material at high val-ues of solids loading ratio, even with the limited pressure drop available withnegative pressure systems, as will be seen in Figure 4.23, provided that the mate-rial is capable of being conveyed in this mode. Pressure gradient, therefore, is theparameter that will dictate the potential mode of conveying for a material that iscapable of being conveyed in dense phase.

150 100 80 60 40

o003

15

10

£ 5

.D.3

t/3

< -10

200 „ 300 400Conveying Distance"^

500

5

-10

100 80 60 40 30

Figure 4.23 Influence of air supply pressure and conveying distance on solids loadingratio for low pressure systems.

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142 Chapter 4

60

50

40

3 30cn

a0-

20

., ,0

30

500 1000 1500 2000

Conveying Distance - ft

2500

Figure 4.24 Influence of air supply pressure and conveying distance on solids loadingratio for high pressure systems.

Figure 4.24 shows that if very long conveying distances are required, thesolids loading ratio will be relatively low, even with a high pressure system. Witha low pressure system the maximum value of solids loading ratio that can beachieved will be very low, and then only with a large bore pipeline. It must bestressed once again that the high values of solids loading ratio are only applicableif the material being considered is capable of being conveyed in dense phase.

REFERENCES

1. D. Geldart. Types of gas fluidization. Powder Technology. Vol 7. pp 285-292. 1973.2. G. Dixon. The impact of powder properties on dense phase flow. Proc Int Conf on

Pneumatic Conveying. London. Jan. 1979.3. M.G. Jones and D. Mills. Product classification for pneumatic conveying. Powder

Handling and Processing. Vol 2. No 2. June 1990.4. D. Mills. Pneumatic Conveying Design Guide. Butterworth-Heinemann. 1990.

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Air Requirements

1 INTRODUCTION

The selection of a fan, blower or compressor is probably one of the most importantdecisions to be made in the design and specification of a pneumatic conveyingsystem. It is often the largest single item of capital expenditure and the potentialconveying capacity of the plant is dependent upon the correct choice being made.The output capability of the air mover is a major consideration in selection. Therating of the fan, blower or compressor is expressed in terms of the supply pres-sure required and the volumetric flow rate. Any error in this specification will re-sult in a system that is either over-rated or is not capable of achieving the desiredmaterial flow rate [1].

For an existing pneumatic conveying system it is often necessary to checkthe performance, particularly if operating problems are encountered, or changes inmaterial or conveying distance need to be considered. Here it is the conveying lineinlet air velocity that is important. Since the determination of conveying line inletair velocity and the specification of air requirements is so important for the suc-cessful operation of pneumatic conveying systems, all the appropriate models arederived and presented for reference purposes. In addition to the influence of pres-sure, temperature and pipeline bore, which are the primary variables, humidity isalso considered.

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144 Chapters

1.1 Supply Pressure

The delivery pressure, or vacuum, required depends essentially upon the workingpressure drop needed over the length of the conveying pipeline. The pressure dropacross the gas-solids separation device can usually be neglected, but if a blow tankis used for feeding the material into the pipeline then an allowance for the pressuredrop across the feeding device will have to be made. Consideration will also haveto be given to the pressure drop in any air supply and extraction lines, and to theneed for a margin on the value of conveying line pressure drop required to conveythe material through the pipeline at the specified rate.

The magnitude of the conveying line pressure drop, whether for a positive ora negative pressure system, depends to a large extent on the conveying distanceand on the solids loading ratio at which the material is to be conveyed. For shortdistance dilute phase conveying a fan or blower would be satisfactory, but fordense phase conveying or long distance dilute phase conveying, a reciprocating orscrew compressor would be required. The pressure drop is also dependent uponthe conveying gas velocity and a multitude of properties associated with the con-veyed material.

1.2 Volumetric Flow Rate

The volumetric flow rate required from the fan, blower or compressor dependsupon a combination of the velocity required to convey the material and the diame-ter of the pipeline to be used. Pipes and fittings are generally available in a rangeof standard sizes, but velocity is not so clearly defined.

For convenience the velocity at the end of the pipeline could be specified,for in the majority of cases compressors are rated in terms of 'free air delivered',and the pressure at the end of a pipeline, in positive pressure systems, in most ap-plications, will be sufficiently close to atmospheric for this purpose. It is, however,the velocity at the start of the line that needs to be ascertained for design purposes.The problem is that air, and any other gas that is used for the conveying of materi-als, is compressible and so its density, and hence volumetric flow rate, is influ-enced by both pressure and temperature.

In negative pressure systems the air at the start of the conveying line is ap-proximately at atmospheric pressure, and it decreases along the conveying line tothe exhauster. For this type of conveying system, therefore, the minimum velocitythat needs to be specified occurs at the free air conditions. Exhausters, however,are generally specified in terms of the volumetric flow rate of the air that is drawninto the air mover, and not free air conditions, and so it is essentially the sameproblem in evaluating air flow rates as with positive pressure conveying systems.

1.3 The Influence of Velocity

A conveying plant is usually designed to achieve a specified material flow rate.Material flow rate can be equated to the solids loading ratio and air mass flow rate.

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Air Requirements 145

Compressor Rating, | ,I I

Supply Pressure Volumetric Flow Rate

conveying LinePressure Drop

IMaterial Conveying

Properties Distance

IMaterial Concentrationor Solids Loading Ratio

I

IConveying

Gas VelocityI

PipBo

\Material Flow Rate

Figure 5.1 Parameters relating compressor rating with material flow rate.

The air mass flow rate is proportional to the volumetric air flow rate andthis, in turn, is proportional to the air velocity and pipeline bore. Since these threeparameters also have an influence on the compressor rating, it is extremely impor-tant that the correct air mover specification is made. The relationship between thevarious parameters that link the compressor rating and material flow rate is dem-onstrated with the path analysis shown in Figure 5.1.

Figure 5.1 also illustrates the importance of conveying air velocity in thisrelationship, as it influences both the supply pressure and the volumetric flow rateof the compressor. This helps to explain why conveying air is one of the mostimportant variables in pneumatic conveying, and why it need to be controlledfairly precisely.

If, in a dilute phase conveying system, the velocity is too low it is possiblethat the material being conveyed will drop out of suspension and block the pipe-line. If, on the other hand, the velocity is too high, bends in the pipeline will erodeand fail if the material is abrasive, and the material will degrade if the particles arefriable.

Velocity also has a major influence on the conveying line pressure drop, andhence on the mass flow rate of the material conveyed through a pipeline. Therange of velocity, therefore, is relatively narrow, particularly in dilute phase sys-tems, varying from a minimum of about 3000 ft/min to a maximum of around6000 ft/min. This includes the compressibility effect, for the 3000 ft/min relates tothe pipeline inlet and the 6000 ft/min relates to the pipeline outlet.

1.4 Air Movers

A wide range of air movers are available, but it is essential that the correct type ofmachine is chosen for the given duty. It is the characteristics of the air mover, interms of the variation of the air flow rate with change of delivery pressure, at agiven rotational speed, that are important for pneumatic conveying, as discussed inChapter 3 on System Components.

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146 Chapters

In the majority of pneumatic conveying systems the air mover is driven at aconstant speed, and design and operation is based on achieving a given conveyingline inlet air velocity. If the material feed rate into the pipeline was to increase by10%, there would be a similar increase in pressure demand. Even if the same airflow rate was delivered there would be a reduction in conveying line inlet air ve-locity because of the higher pressure. Motor sizes and air flow rate should bespecified to take this type of fluctuation into account.

Ideally an air mover is required that will deliver the same air flow rate at thehigher pressure. In practice a small reduction in air flow rate will result, and hencea further lowering in air velocity, and so this should be accommodated. With someair movers, a 10% increase in pressure demand will result in an even greater re-duction in air flow rate. Air movers with this type of operating characteristic areunlikely to be acceptable.

The vast majority of the power required by a pneumatic conveying system istaken by the air mover. If a conveying system requires a large bore pipeline and ahigh pressure air supply, the power required is likely to be very high. Power re-quirements, and hence the cost of operation for pneumatic conveying, does tend tobe much higher than for other conveying systems, particularly for materials con-veyed in dilute phase. It must also be recognized that as a result of the high speedof compression, the air will be delivered at an increased temperature and so a deci-sion will have to be taken on whether or not to cool the air.

1.5 Air Humidity and Moisture

Air is a mixture of gases. Oxygen and nitrogen are the main constituents, but it isalso capable of absorbing a certain amount of water vapor. There is, however, alimit to the amount of water that air can hold in gaseous form as vapor. Relativehumidity is a measure of the amount of moisture that air contains at a given pres-sure and temperature. It is expressed as a percentage. Relative humidity gives anindication of how dry the air is, and hence how much more vapor the air is capableof holding. 100% represents the limit for relative humidity and at this value the airis said to be saturated.

Specific humidity is a measure of how much moisture the air actually con-tains, and is usually expressed in terms of Ib of water per Ib of dry air. Relativehumidity cannot rise above 100% and so if changes occur such that saturationconditions are exceeded, condensation will occur. The amount of moisture that aircan support increases with increase in temperature and decreases with increase inpressure. Thus an increase in temperature will result in air becoming drier. A de-crease in temperature will result in the relative humidity increasing.

If saturation conditions are reached then condensation will occur with anysubsequent decrease in temperature. Compression of air is likely to result in con-densation if there is no change in temperature. Across a compressor there is usu-ally an increase in temperature, as well as pressure, and so at outlet the air is likelyto be dry, as temperature generally has an over-riding effect in this situation.

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Air Requirements 147

1.6 Compressibility Effects

The volumetric flow rate of air required to convey a material through a pipelinecan be evaluated from the cross sectional area of the pipeline and the air velocityrequired to convey the material. Consideration must be given, however, to the factthat air is compressible, and that it is compressible with respect to both pressureand temperature, and if the plant is not located at sea level, the influence of eleva-tion may also have to be taken into account. As a result of the compressibility withrespect to pressure, stepped bore pipelines are often employed and these are givendue consideration.

Although it is air that is generally referred to, materials can be conveyedwith any suitable gas. Constants are included in the equations that will correctlyaccount for the type of gas used when evaluating the volumetric flow rate re-quired. Air mass flow rate is also considered, as it is a useful working parameter,since its value remains constant in a pipeline, and is required for evaluating thesolids loading ratio.

1.6.1 Conveying Air Velocity

For the pneumatic conveying of bulk paniculate materials, one of the critical pa-rameters is the minimum conveying air velocity necessary to convey a material.For dilute phase conveying this is typically about 3000 ft/min, but it does dependvery much upon the size and size distribution, shape and density of the particles ofthe bulk material.

For dense phase conveying it can be as low as 600 ft/min, but this dependsupon the solids loading ratio at which the material is conveyed and the nature ofthe conveyed material. If the velocity drops below the minimum value the pipelineis likely to block. It is important, therefore, that the volumetric flow rate of air,specified for any conveying system, is sufficient to maintain the required mini-mum value of velocity throughout the conveying system.

1.6.2 Material Influences

It should be noted that in evaluating conveying air velocities and volumetric airflow rates in pneumatic conveying applications, the presence of the material isdisregarded in all cases, whether for dilute or dense phase conveying. The convey-ing air velocity is essentially the superficial value, derived simply by dividing thevolumetric flow rate by the pipe section area, without taking account of any parti-cles that may be conveyed.

In dilute phase conveying, and at low values of solids loading ratio, the in-fluence of the conveyed material will have negligible effect in this respect. At asolids loading ratio of 100, however, the material will occupy approximately 10%of the volume at atmospheric pressure and so the actual air velocity will be about10% higher. At increased air pressures and solids loading ratios the percentagedifference will be correspondingly higher.

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148 Chapters

It would be a very complex and time consuming process to evaluate actualair velocities and so for convenience the superficial air velocity is universally em-ployed. Critical values such as the minimum conveying air velocity and conveyingline inlet air velocity are mostly derived from experience and experimental work.In such cases it is the superficial air velocity that is used.

As with the flow of air only in a pipeline, or single phase flow, the flow of agas-solid mixture will also result if there is a pressure difference, provided that aminimum value of conveying air velocity is maintained. Material flow will be inthe direction of decreasing pressure, whether it is a positive pressure or a vacuumconveying system. Since air is compressible, the volumetric flow rate of the airwill gradually increase, from the material feed point at the start of the pipeline, tothe material discharge point at the end of the pipeline. In a single bore pipeline theconveying air velocity will also gradually increase over the length of the pipeline.

This means that it is the value of the conveying air velocity at the materialfeed point, or the start of the pipeline, that is critical, since the value of the convey-ing air velocity will be the lowest at this point, in a single bore pipeline. In deter-mining the necessary volumetric flow rate of air, therefore, it is the conditionsprevailing at the start of the pipeline, in terms of pressure and temperature, thatmust be taken into account.

2 VOLUMETRIC FLOW RATE

Volumetric flow rate in ftVmin has been chosen for use in all the mathematicalmodels developed and on all graphical plots presented in this Handbook. Althoughit is not the basic fps unit, ftVmin is more widely quoted in trade literature onblowers and compressors. It is also more compatible with the use of ft/min for airvelocity. Inches have been used for all pipeline bore references.

2.1 Presentation of Equations

The majority of the equations that follow are presented in terms of both volumetricflow rate and conveying air velocity. The reason for this is the need to providemodels that can be used for both the design of future systems and for the checkingof existing systems. In the design of a system a specific value of conveying airvelocity will generally be recommended, together with a pipe bore, and it is thevalue of volumetric flow rate that is required for specification of the blower orcompressor.

In order to check an existing system it is usually necessary to determine theconveying air velocity for the particular conditions. In addition to providing theappropriate models for the evaluation of air requirements and conveying air ve-locities, graphical representation of these models is also presented. With pro-grammable calculators and computers, models such as these can be handled quiteeasily and quickly.

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Air Requirements 149

Graphs, however, do have the advantage of showing visually the relative ef-fects of the various parameters, and in some cases can be used very effectively toillustrate particular processes, and have been adopted widely in this Handbook.The main equations that are developed are additionally presented in SI units, andreference to the equivalent SI units for all symbols and dimensions is given in theNomenclature at the end of this chapter.

2.2 The Influence of Pipe Bore

The diameter of a pipeline probably has the most significant effect of any singleparameter on volumetric flow rate. The volumetric flow rate through a pipelinedepends upon the mean velocity of flow at a given point in the pipeline and thepipe section area. The relationship is:

V =C x A

144frVmin (1)

where V = volumetric flow rate - ft /minC = conveying air velocity - ft/min

and A = pipe section area - in2

so that

for a circular pipe

where d = pipe bore - n

n d2CV = -^- ft/min (2)

or

C __ 576 Vft/min (3)

A graphical representation of the above models is presented in Figure 5.2.This is a plot of volumetric air flow rate against conveying air velocity, with aseries of lines representing the relationship for different sizes of pipe. Conveyingair velocities from about 500 ft/min to 10,000 ft/min have been considered in orderto cover the two extremes of minimum velocity in dense phase conveying andmaximum velocity in dilute phase conveying, although velocities as high as10,000 ft/min would not normally be recommended.

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150 Chapter 5

5000

4000

u 3000I

2 2000

1000

0

pfipetinerBore - in

.10

0 8000 10,0002000 4000 6000

Conveying Air Velocity fi/min

Figure 5.2 The influence of air velocity and pipeline bore on volumetric flow rate.

2.2.1 Reference Conditions

It should be noted that the volumetric flow rate on this graph is not related to anyreference condition. It is the actual flow rate at any given condition of air pressureand temperature. Equations 5.1 to 5.3 and Figure 5.2 can be used either to deter-mine the resulting velocity for a given flow rate in a given pipe size, or to deter-mine the required volumetric flow rate knowing the velocity and pipe bore.

Blowers and compressors are usually rated in terms of 'free air delivered'.This means that the volumetric flow rate is related to ambient conditions for refer-ence purposes - usually a pressure of 14-7 lbf/in2 absolute and a temperature of59°F (519 R). The influence of pressure and temperature on volumetric flow rate,and hence velocity, is discussed in the following sections.

2.2.2 Pipeline Influences

The air at the start of a conveying line will always be at a higher pressure than thatat the end of the line because of the pressure drop necessary for air and materialflow. Air density decreases with decrease in pressure and so, in a constant borepipeline, the air velocity will gradually increase from the start to the end of thepipeline. The air mass flow rate will remain constant at any section along a pipe-line, but as the rating of blowers and compressors is generally expressed in volu-metric flow rate terms, then knowledge of the air mass flow rate is of little value inthis situation.

2.3 The Ideal Gas Law

The relationship between mass and volumetric flow rate, pressure and temperaturefor a gas can be determined from the Ideal Gas Law:

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Air Requirements 151

144 p v = ma R T (4)

where p = absolute pressure of gas - Ib f / in

V = actual volumetric flow rateof the gas at the pressure, p - ftVmin

ma - mass flow rate of gas - Ib/min

R = characteristic gas constant - ft Ibf/lb Rand T = absolute temperature of gas - R

= ?°F + 460

Rearranging this gives:

P V

T 144

For a given gas and constant mass flow rate:

P V

T= constant

so that

Ti Ti

where subscripts , and 2 can relate to any twoanywhere along the conveying pipeline

or in terms of 'free air conditions'

P.(6)

where subscript 0 refers to reference conditionsusually pu = 14-7

T0 = 519Ibf/in2 absoluteR

and V0 = free air delivered in ft3/min

and subscript i refers to actual conditions, anywherealong the conveying pipeline

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152 Chapters

2.3.1 Working Relationships

Substituting reference values into Equation 6 and rearranging gives:

519 x p.V0 = — x F,

14-7 x T}

P^ V,= 35-3 x £1—L fWmin (7)

P^ V,= 2-843 x ^—1 mVs (7 si)

or alternatively

T, VF, = 0-0283 x ——- ft 3 /min - - - - - (8)

T v= 0-352 x -—- m3/s (8 si)

2.3.2 Gas Constants

The constant, R, in Equation 4 has a specific value for every gas and is obtainedfrom:

r>

R = — ftlbf/lbR - - - (9)M

where ,/?„ = universal gas constant - ft Ibf/lb-mol R= 1545 ft Ibf/lb-mol R= 8-3143 kJ/kg-molK SI

and M = molecular weight - mol

Values for air and some commonly employed gases are presented in Table 5.1:

Whichever gas is used, the appropriate value of R for that gas is simply sub-stituted into Equation 4 and the design process is exactly the same.

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Air Requirements 153

Table 5.1 Values of Characteristic Gas Constant

Gas

AirNitrogenOxygenCarbon dioxideSteamArgon

Equation

N2

02

C02

H20Ar

Molecular WeightM

28-9628-0132-0044-0118-0139-95

Gas ConstantR - ft Ibf/lb R

53-355-248-335-185-838-7

2.3.2.1 The Use of NitrogenIt will be noticed that there is little more than 3% difference between the values ofR for air and nitrogen. This is not surprising since about 78% of air, by volume, isnitrogen, and the two constituent gases have very similar molecular weights. As aconsequence little error would result if a system in which nitrogen gas was usedfor conveying a material, was to be inadvertently designed on the basis of air.

If carbon dioxide or superheated steam was to be used to convey the mate-rial, however, there would be a very significant error. Gases other than air andnitrogen are often used for specific pneumatic conveying duties.

3 THE INFLUENCE OF PRESSURE

The influence that air pressure has on volumetric flow rate is shown graphically inFigures 5.3 to 5.5 to highlight the influence of compressibility. These are plots ofvolumetric flow rate, at the reference atmospheric pressure of 14-7 lbf/in2 absolute,against actual volumetric flow rate.

To simplify the problem an isothermal situation has been assumed in orderto isolate the influence of pressure i.e. T, = Tu. Once again this is a linear relation-ship. A series of lines representing the relationship for different air pressures isgiven on each graph, and each one illustrates the relationship for a different type ofsystem.

3.1 System Influences

In Figure 5.3 the pressures considered range from 0 (atmospheric) to 12 lbf/in2

gauge and so is appropriate to low pressure, typically dilute phase, conveying sys-tems. If an air flow rate of 1500 ft3/min at free air conditions is considered it canbe seen from Figure 5.3 that the actual volumetric flow rate of the air at the mate-rial feed point, at the start of the conveying line, will be reduced to about 825fVVmin if the air pressure is 12 lbf/in2 gauge.

Pipeline bore is not included at this stage since it is simply the effect ofchanges in air pressure that are being illustrated.

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154 Chapter 5

2000

.3

1500

II 3000

&Hta|(2 500

o

Air Pressuregauge

O L400 800 1200 1600 2000

Volumetric Flow Rate - ftVmin at 14-7 Ibf7in2 abs

Figure 5.3 The influence of air pressure on volumetric flow rate for low pressure sys-tems.

Alternatively, the flow rate can be determined from Equation 5.8:

0-0283 x 519 x 1500

1 ( l 4 -7 + 12J

= 825 fVVmin

In Figure 5.4 the pressure ranges from 0 to 50 lbf/in2 gauge and so is rele-vant to high pressure conveying systems. If the air at the material feed point is at50 Ibf/in2 gauge, a free air flow rate of 1500 ft3/min will be reduced to about 340frVmin, as can be seen from Figure 5.4. In both of these cases the air will expandthrough the conveying line back, approximately, to the free air value of 1500ft /min, at the discharge hopper and filtration unit at the end of the pipeline.

In the case of a vacuum system, free air conditions prevail at the materialfeed point. The air then expands beyond this and so, if the exhaust is at -8 lbf/in2

gauge, 1500 ftVmin of free air will increase to about 3290 ftVmin, as can be seenfrom Figure 5.5. Alternatively, the air flow rate can be determined from Equation5.8 once again:

0-0283 x 519 x 1500

( l4 -7 - g)

= 3288 ftVmin

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Air Requirements 155

2000

.sJ 1500

3000

I 500

oE

0

Pipeline Exit-1500ft3/min

Flow

Air Pressure- lf>f/in2 .gjjuge j

Pipeline Inlet- 340 ftVrnin

400 800 1200 1600

Volumetric Flow Rate - ftVmin at 14-7 lbf/in2 abs

2000

Figure 5.4 The influence of air pressure on volumetric flow rate for high pressure sys-tems.

It can be seen from this range of values that it is extremely important to takethis compressibility effect into account in the sizing of pipelines, and particularlyso in the case of combined positive and negative pressure systems.

3500

3000

I 2500

u 20005t/i

(S 1500

| 1000

I 500

Air Pressure- lbf/in2 gauge

fipeiineiExit-]3290 ftj/min

400 800 1200 1600

Volumetric Flow Rate - ftVmin at 14-7 lbf/in2 abs

2000

Figure 5.5 The influence of air pressure on volumetric flow rate for negative pressuresystems.

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156 Chapters

An additional point to note is one of the many fundamental differences be-tween positive pressure and vacuum conveying systems. With positive pressureconveying systems the filtration plant can be sized on the basis of the free air flowrate value. For negative pressure conveying systems, however, this is not the case,as will be seen from Figure 5.5.

If the system exhausts at a vacuum of 8 Ibf/in2, for example, the flow rate tobe handled by the filter will be about 3290 ft '/mm which is more than double thefree air flow rate value, and the filter will have to be sized on 3290 and not 1500fVVmin.

3.2 Velocity Determination

If Figures 5.3 to 5.5 are used in conjunction with Figure 5.2, it will be possible todetermine the resulting conveying air velocities for given conditions. An alterna-tive to this procedure is to combine the models for actual volumetric flow rate andconveying air velocity.

3. 2. 7 Working Relationships

From Equation 2 the actual volumetric flow rate:

V] = ~576~

and from Equation 7 free air delivered:

v. - 35-3 * l\

and substituting Equation 2 into Equation 7 gives:

,2 f~<

V0 = 0-1925 x - ft3/min - - - - (10)J i

p. d2 C= 2-23 x -L- m3/s . . . . (10si)

•M

which is the form required for system design, and rearranging to the formrequired for checking existing systems gives:

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Air Requirements 157

C = 5-19d2 p}

ft/min (11)

= 0-448 d2 m/s (11 si)

3.2.2 Graphical Representation

It will be seen from these models that a total of five variables are involved and soit is not possible to represent them diagrammatically on a single graph. By neglect-ing the influence of temperature at this stage the models can be reduced to fourvariables, and so if particular values of volumetric flow rate are chosen, the influ-ence of the remaining three variables can be shown. This is presented for four val-ues of volumetric flow rate in Figures 5.6 to 5.9, the volumetric flow rates beingreferred to ambient conditions of temperature and pressure.

These are all graphs of conveying air velocity drawn against air pressure,with pipe bore plotted as the family of curves. The reason for this is that both con-veying air velocity and air pressure are infinitely variable in the system, but pipe-lines are only available in a number of standard sizes. They are drawn once againto illustrate the performance of different types of system. Figures 5.6 and 5.7 coverthe range of both positive and negative pressure systems and Figures 5.8 and 5.9are drawn for positive pressure systems only.

6000

J 5000<&£ 40001> 3000t-,

<g 2000'&g 1000ou

-10 -5 0 5 10 20

Air Pressure - lbf/in2 gauge

40

i 5.6 The influence of air pressure and pipeline bore on conveying air velocity fora free air flow rate of 1500 ft3/min.

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158 Chapter 5

Figure 5.6 clearly illustrates the influence of pressure on conveying air ve-locity in a single bore pipeline. The slope of the constant pipe bore curves increaseat an increasing rate with decrease in pressure. The reason for this can be seenfrom Equation 11. Conveying line inlet air pressure, ph is on the bottom of theequation, and so as its value gets lower, small changes in its value have a moresignificant effect. This is particularly so for negative pressure systems, and is quitedramatic at high vacuum, as shown on Figure 5.6.

3.2.3 Suck-Blow Systems

On Figure 5.7 the expansion lines for a typical combined positive and negativepressure system are superimposed. This illustrates the problems of both pipelinesizing, with this type of system, and the relative expansion effects at different airpressures.

With 1000 ft3/min of free air, a 6 in bore pipeline would be required for thevacuum line. This would give an air velocity of about 3560 ft/min at the materialfeed point and would expand to approximately 4900 ft/min if the exhaust was at -4Ibf7in2 gauge. If the pressure on the delivery side of the blower was 6 lbf/in2 gauge,a 5 in bore pipeline would be required. This would give pick-up and exit air ve-locities of about 3640 and 5130 ft/min respectively.

It will be noted that the pick-up and exit air velocities are very similar forthe two parts of the system, but different size pipelines are required.

7000

•| 600052i

•f 5000_o<ut 4000

3000

2000

-4 -2 0 10 12

Air Pressure - Iblfin gauge

Figure 5.7 Velocity profile for a typical combined positive and negative pressure(suck-blow) system with a free air flow rale of 1000 ftVmin.

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Air Requirements 159

The free air flow rate is clearly the same for the two parts of the system andso it will be seen that it is entirely due to the influence of the conveying line inletair pressure on the compressibility of the air.

In the above case it has been assumed that the material is conveyed in dilutephase suspension flow and that the minimum conveying air velocity for the mate-rial is about 3000 ft/min. If a 20% margin is allowed when specifying a conveyingline inlet air velocity, this would need to be about 3600 ft/min.

3.2.4 Low Pressure Systems

In Figure 5.8 a typical velocity profile for a low pressure dilute phase conveyingsystem is shown. In this case the minimum conveying air velocity for the materialis approximately 2700 ft/min and so with a 20% margin the conveying line inletair velocity needs to be about 3240 ft/min. With a free air flow rate of 900 ftVmin,and a conveying line inlet air pressure of 14 lbf/in2 gauge, a 5 in bore pipelinewould be required.

The resulting conveying line inlet air velocity is about 3380 ft/min, and itwill be seen that the air velocity gradually increases along the length of the pipe-line as the air pressure decreases. At the end of the pipeline, at atmospheric pres-sure, the conveying line exit air velocity will be about 6600 ft/min in this 5 in borepipeline.

The above is simply an example to illustrate the variation in conveying airvelocity from feed point to material discharge in a pipeline. For the design of aconveying system Equation 5.10 would be used to evaluate the free air require-ments.

.£7000

6000

I 5000<u

<j 40000£

^C

'& 3000

§u

2000

12 160 4 8

Air Pressure - Min2 gauge

Figure 5.8 Typical velocity profile for a low pressure dilute phase system for a free airflow rate of 900 ftVmin.

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160 Chapter 5

For a 5 in bore pipeline, and with a conveying line inlet air pressure of 14lbf/in2 gauge (28-7 lbf/in2 absolute) and a conveying line inlet air velocity of 3240ft/min, with air at 59°F, this would come to 862 ftVmin. If the influence of pres-sure was not taken into account, and the volumetric flow rate was evaluated on thebasis of an air velocity of 3240 ft/min, effectively at the end of the pipeline, theconveying line inlet air velocity that would result at a pressure of 14 lbf/in2 gaugewould be about 1660 ft/min, and the pipeline would almost certainly block.

The influence of air pressure on conveying air velocity is illustrated furtherwith Figure 5.9. This is a plot of conveying air velocity plotted against air pres-sure, and is drawn for a free air flow rate of 1000 ftVmin in a 6 in bore pipeline.During the operation of a pneumatic conveying system the conveying line inlet airpressure may vary slightly, particularly if there are variations in the feed rate of thematerial into the pipeline. If the feed rate increases for a short period by 10%, theconveying line inlet air pressure will also have to increase by about 10% in orderto meet the increase in demand.

If the minimum conveying air velocity for the material was 3000 ft/min, andit was being conveyed with a conveying line inlet air pressure of 8 lbf/in2 gauge,an increase in pressure to only 12 lbf/in gauge would probably be sufficient toresult in a pipeline blockage. At low values of air pressure, conveying air velocityis very sensitive to changes in pressure, and so due consideration must be given tothis when deciding upon a safety margin for conveying line inlet air velocity, andhence the volumetric flow rate of free air, to be specified for the system.

3600

B

1

3200

M 2800

Iu

2400

Pipeline Bore - 6 in|Air Temperature - 59° F

10 12 14

Air Pressure - lbf/in2 gauge

16

Figure 5.9 The influence of air pressure on conveying air velocity for a free airrate of 1000 ft3/min.

18

flow

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Air Requirements 161

3.2.5 Stepped Pipelines

In the low pressure case illustrated in Figure 5.8 the minimum conveying air ve-locity for the material was about 2700 ft/min and with a 20% margin this was3240 ft/min. With the blower available delivering 900 ft3/min of free air at 14lbf/in2 gauge, the resulting conveying line inlet air velocity in a 5 in bore pipelinecame to 3380 ft/min. As a pick-up velocity this is quite acceptable but the velocityat the end of the pipeline is quite unnecessarily high at 6600 ft/min.

The velocity profile for a 6 in bore pipeline is also included on Figure 5.8. Itcan be seen from this that if the pipeline was expanded from 5 to 6 inches at apoint in the flow where the pressure was about 5 lbf/in2 the maximum value ofconveying air velocity in the pipeline could be limited to about 5000 ft/min. TheFigure 5.8 pipeline is re-drawn in Figure 5.10 with such a step.

From Figure 5.10 it will be seen that the velocity profile has been main-tained between very much narrow limits as a result of the step to 6 inch bore. Thevelocity profile for an 8 inch bore pipeline has also been added and it will be seenthat expansion into such a bore would not have been possible. At the step into the6 inch bore line the velocity drops from 4920 to 3420 ft/min and this is quite ac-ceptable.

Problems arise when the step in bore is incorrectly positioned and the veloc-ity in the larger bore section of pipeline falls below the minimum value for thematerial. The fact that the velocity at exit from the pipeline is lower than that atentry to the step is of no consequence.

7000

.3

6000

8 5000I<! 4000

I'J?ct 3000

2000

Pipeline- in

4 8Air Pressure - Ibf7in2 gauge

12 16

Figure 5.10 The 5 inch bore pipeline velocity profile shown in Figure 5.8 modified bythe addition of a step to 6 inch bore.

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162 Chapters

Stepped pipelines are considered in more detail in Chapter 9. Equations forthe evaluation of pressure and velocity are developed and steps for both positivepressure and vacuum conveying systems are considered.

4 THE INFLUENCE OF TEMPERATURE

In the above figures the influence of temperature was not included, so that theinfluence of pressure alone could be illustrated, and so it was assumed that allflows and expansions were isothermal and at the standard reference temperature.In Equations 7 and 8 the influence of pressure and temperature on actual volumet-ric flow rate is presented. If the influence of pressure is neglected, in order to sepa-rate the effect of temperature, the equation reduces to:

V} = - - f t 3 / m i n - - - - - - - - - (12)

The influence that air temperature can have on volumetric flow rate isshown graphically in Figure 5.1 1. This is a plot of volumetric flow rate at the ref-erence temperature of 59°F, against actual volumetric flow rate at a given tem-perature. It should be noted that in Equation 12 and Figure 5.1 1 all pressures arestandard atmospheric so that the influence of temperature can be considered inisolation from that of pressure.

It can be seen from Figure 5.1 1 that changes in temperature do not have thesignificant effect on volumetric flow rate that changes in pressure can have. This isbecause the influence of temperature is in terms of the ratio of absolute tempera-tures and the 460 that has to be added to the Fahrenheit temperature has a consid-erable dampening effect. Figure 5.11 illustrates the influence of temperature overthe range of temperatures from -40°F to 200°F.

Air temperatures higher than 200°F can be experienced, however. Air at atemperature of 200°F will result from the compression of air in a positive dis-placement blower operating at about 14 lbf/in2 gauge, and from a screw compres-sor delivering air at 45 lbf/in2 gauge it could be more than 400°F. In some casesthe material to be conveyed may be at a high temperature and this could have amajor influence on the conveying air velocity.

It will be seen from Equation 1 1 that if the temperature is reduced, then thevelocity will fall. This is because the density of the air increases with decrease intemperature. The volumetric flow rate of air that is specified must be sufficient tomaintain the desired conveying line inlet air velocity at the lowest temperatureanticipated. Due account, therefore, must be taken of cold start-up and winter op-erating conditions, particularly with vacuum conveying systems which draw inatmospheric air. This point is illustrated quite forcefully in Figure 5.12.

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Air Requirements 163

1200

| 1000cI

I 800

I 600H•a

400

E 20°

0

.200

0 1000200 400 600 800

Volumetric Flow Rate at 59°F - ftVmin

Figure 5.11 The influence of air temperature on volumetric flow rate.

Figure 5.12 is drawn for a 6 in bore pipeline and an inlet air pressure of 15lbf/in2 gauge. It will be seen from this that conveying air velocity can be very sen-sitive to temperature. The average gradient on this plot is about 5 ft/min per °Ftemperature change, and so if the temperature of the conveying air was reduced forsome reason it could result in pipeline blockage in a system operating with a pick-up velocity close to the minimum conveying air velocity for the given material.

0 20 40 80 120Air Temperature - °F

160 200

Figure 5.12 The influence of air temperature on conveying air velocity for a free airflow rate of 1000 frVmin.

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164 Chapters

4.1 Conveyed Material Influences

The above analysis refers to the situation with regard to the air only. For the con-veying line, however, the material also has to be taken into account, and althoughthe air may be at 60°F, the material to be conveyed may be at 400°F or more. Inorder to determine the temperature of the conveyed suspension it is necessary tocarry out an energy balance. If a control surface is taken around the material feed-ing device and the immediate pipelines, an energy balance gives:

(m Cp t) + (m Cp t] = (m Cp t] ..... (13)V / p \ la \ 's

where m = mass flow rate - Ib/hCp = specific heat - Btu/Ib R

and t = temperature - °F

and the subscripts refer to:p = conveyed material or producta = air

and s = suspension

if heat exchanges with the surroundings, kinetic energies and other minor energyquantities are neglected.

It is the temperature of the suspension, ts, that is required and so a rear-rangement gives:

mt Cps > mx Cp, a

From continuity

ms = ma + mp Ib/h (14)

and by definition

mp = </> ma Ib/h . . . . . . . . (15)

where <j> is the solids loading ratio of the conveyed materialand

Cps = Btu/lbma + mp

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Air Requirements 165

Substituting these gives:

0 Cpp tp + Cpa ta

(16)

With so many variables it is difficult to illustrate the relationship graphi-cally. One case has been selected, however, for a conveyed material at a tempera-ture of 60°F, having a specific heat of 0-24 Btu/lb R and this is presented in Figure5.13. This illustrates the influence that conveying line inlet air temperature andsolids loading ratio can have on the resulting suspension temperature.

Figure 5.13 relates to the dilute phase conveying of a material with a posi-tive displacement blower, where the conveying line inlet air temperature might beup to about 220°F. This shows that the solids loading ratio has a dominating effecton the suspension temperature, even with dilute phase conveying. Unless the con-veyed material has a very low specific heat value, and is conveyed in very dilutephase, the temperature of the conveyed suspension will be close to that of the ma-terial to be conveyed. If cold air is used to convey a hot material, therefore, thecooling effect on the material of the cold air will be minimal. This is illustrated inmore detail in Figure 5.14 where material and air inlet temperatures of 1000°F and60°F respectively have been considered.

Solids Loading Ratio

120

I 100

90

80

70

60

Material: ! i :

Inlet Temperature - 60 °F \Specific heat - 0-24 Btu/lb R

60 100 140 180

Air Inlet Temperature - °F

Figure 5.13 The influence of air inlet temperature and solids loading ratio onthe equilibrium temperature of the suspension.

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166 Chapter 5

1000

' 800I

! 600I

I 400}

[ 200II!

0

Material:Inlet Temperature - iqOO°F"Specific Heat - b'24BtC7Ib R

12 16

Solids Loading Ratio

20

Figure 5.14 Influence of solids loading ratio on the equilibrium temperature of thesuspension.

Figure 5.14 is also drawn for a material having a specific heat value of 0-24Btu/lb R and shows the influence of solids loading ratio. It must be stressed thatthe suspension of material and air will only reach the equilibrium temperature atsome distance from the pipeline feeding point, for thermal transient effects have tobe taken into account.

The heat transfer process depends additionally upon the thermal conductiv-ity and shape and size of the particles. It is a time dependent process and with thehigh velocities required in dilute phase conveying, equilibrium will not be fullyestablished by the end of the pipeline with many materials. Since volumetric flowrate decreases with decrease in temperature, if there is any doubt with regard to thetemperature of the air at the start of a conveying line, the lowest likely valueshould be used for design purposes.

Particular care should be taken with vacuum conveying systems that are re-quired to convey hot materials. There are several points that need to be taken intoconsideration here. At the material feed point into the pipeline air at atmospherictemperature will generally apply. At any steps in the pipeline, however, the air willbe at a significantly higher temperature as a result of the heat transfer. Care mustalso be exercised with the specification of the exhauster, for this is generally basedon the volumetric flow rate of the air drawn into the exhauster.

4.1.1 Specific Heat

Specific heat is clearly an important property in this analysis and typical values aregiven in Table 5.2 and specific heat values for air and water are also added forreference purposes. That for air is a basic element in the model, of course.

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Air Requirements 167

Table 5.2 Typical Specific Heat Values

Material

Metals CopperNickelSteelAluminumMagnesium

Non-Metals Sand, dryFirebrickCoalCottonBakeliteCork

Note AirWater

Specific HeatBtu/lb R

0-090-110-110-210-24

0-190-230-310-310-38045

0-241-00

It will be noted that water has a very much higher specific heat value thanany of the other materials listed, and so if a material has a high moisture contentthis could have a considerable influence on the specific heat of the material.

5 THE INFLUENCE OF ALTITUDE

As elevation increases, pressure naturally decreases, and so the elevation of a plantabove sea level should always be noted for reference. With increase in elevationthere is a corresponding drop in the value of the local atmospheric pressure andthis will influence many of the velocities and volumetric flow rates in the calcula-tions. There is, of course, a direct influence on the performance of vacuum con-veying systems, since any reduction in atmospheric pressure automatically reducesthe available pressure difference. The variation of the local value of atmosphericpressure with the elevation of a plant above sea level is presented in Figure 5.15.

5.1 Atmospheric Pressure

Figure 5.15 shows that for a plant located 3000 ft above sea level there is a reduc-tion of more than 10% in atmospheric pressure, and equates to a reduction in pres-sure of about 1-6 lbf/in2 or 3-3 in Hg. It will be seen from this that the influence ofaltitude should be considered in detail for plants located above about 1000 ft, par-ticularly if a vacuum conveying system is to be considered. The normal atmos-pheric pressure at sea level can fluctuate quite naturally by ± 1 in Hg on a day today basis, which equates to a change in elevation of about 1000 ft.

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168 Chapter 5

0 2000 8000 10,0004000 6000

Plant Elevation - ft

Figure 5.15 The influence of plant elevation on the local value of atmospheric pressure.

6 MOISTURE AND CONDENSATION

Air naturally contains a certain amount of water vapor. The amount of water vaporthat air can contain depends upon both temperature and pressure. A decrease intemperature or an increase in pressure can result in condensation occurring whenthe air passes through the saturation point. The problem with condensation, how-ever, is that it can sometimes be very difficult to predict. The presence of moisturemay even be unknown if it cannot be seen, although its effects will certainly beevident.

The addition of water to a bulk solid can have a significant effect on itsflowability. Condensation usually occurs on the walls of containing vessels andsurfaces such as hoppers, silos and pipelines. Although the effect might be local-ized, the material/surface interface is critical to the smooth operation of most bulksolids handling plants. Some materials are hygroscopic and will naturally absorbmoisture from the air without condensation occurring. For these materials it isgenerally necessary to dry the air, that comes into contact with the material, to avalue of relative humidity below that at which the material is capable of absorbingatmospheric moisture.

Equations are derived and presented that will enable the amount of moistureassociated with air to be evaluated. Graphs and charts are also included, to illus-trate the influence of the main variables, and to give some idea of the order ofmagnitude of the potential problem. From the data presented it will be possible todetermine rates of condensation and evaporation in processes such as the compres-sion and expansion of air, as well as heating and cooling.

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Air Requirements 169

6.1 Humidity

The amount of water vapor that air can support is not constant but varies with bothtemperature and pressure. Once air is saturated, a change in either temperature orpressure can result in condensation occurring. The terms used here are relativehumidity and specific humidity, and the Ideal Gas Law, commonly used for air,provides the basis for modeling moist air.

Specific humidity is the ratio of the mass of water vapor to the mass of dryair in any given volume of the mixture. It is usually expressed in terms of Ib ofwater per Ib of dry air. Relative humidity is the ratio of the partial pressure of thevapor actually present to the partial pressure of the vapor when the air is saturatedat the same temperature. It is usually expressed as a percentage, with 100% repre-senting saturated air.

Thus, specific humidity is a measure of the moisture content of the air, andrelative humidity is a measure of the ease with which the atmosphere will take upmoisture. Relative humidity is usually obtained by means of wet- and dry-bulbthermometers, or some other form of hygrometer, and specific humidity can becalculated.

6.1.1 Specific Hum idity

Specific humidity, co, is the ratio of the mass of water vapor to the mass of dry airin any given volume of the mixture:

mvco = —- lbv/lba - (17)

ma

where mv = mass of vapor - Iband ma = mass of air - Ib

From the Ideal Gas Law:

144pa V = maRaT (18)

At low values of partial pressure, water vapor can also be treated as an IdealGas, and so:

144 pv V = mvRvT - - - (19)

where pa = partial pressure of air - lbf/in2

pv = partial pressure of water vapor - lb f / in 2

V = volume of mixture - f t 3

R = characteristic gas constant - ft Ibf/lb Rand T = absolute temperature of mixture - R

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170 Chapter 5

and note that V and T will be the same for both the air and the vapor, sincethe two constituents are intimately mixed.

The partial pressure of water vapor, pm varies with temperature. For refer-ence, values are given on Figure 5.16. The partial pressure of water vapor in-creases exponentially with increase in temperature and so the partial pressure axison Figure 5.16 is split in two. The axis on the right hand side, for high temperatureair, is magnified by a factor of ten, compared with that on the left hand side forlow temperature air. It will also be seen that at 32°F, the freezing point for water,that a significant quantity of vapor still exists in the air.

At temperatures below 32°F, therefore, water vapor will precipitate as iceonto cold surfaces, without passing through the liquid phase. By the same reason-ing, wet surfaces that are frozen can be dried, for the ice evaporates directly intovapor, without the surface becoming wet.

The characteristic gas constants for the two constituents can be obtainedfrom Equation 9 and values for various gases, including steam, are given in Table5.1. By substituting for R from Equation 9 into Equations 18 and 19 gives:

144 pa V Mamn = Ib (20)

and mv =144 pv V Mv

ITr lb (21)

I.0

0.1

o

EftJO

I-20 0 20 40 60 80 100 120 140

Saturation Temperature of Air - °F

Figure 5.16 The Variation of saturation vapor pressure with temperature.

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Air Requirements 171

Substituting Equations 20 and 21 into Equation 17 gives:

CO = " 1Mb. - - - - - - - - (22)Pa Ma

since V and T are common to both constituents.

From Dalton's Law of Partial Pressures:

P = Pa + Pv lbf/in2 - - - - - - - (23)

where p = total pressure, which for most applications= atmospheric pressure - lbf/in2 abs

Thus Specific Humidity, CO, is given by:

CO = -, r Ibv/lba (24)29 (p-pv)

Alternatively:

0'622 pvco = 1Mb. - - - - - - - (25)

P-Pv

6.1.1.1 The Influence of TemperatureA graphical representation of this equation is given in Figure 5.17. This is a graphof the moisture content of saturated air, in pounds of water per 1000 cubic feet ofair, plotted against air temperature. This graph is also plotted with a split moisturecontent axis in a similar manner to Figure 5.16.

The moisture content, in volumetric terms, is obtained simply by multiply-ing Equation 25 by the density of air. Figure 5.17 is derived for saturated air atatmospheric pressure, which means that this is the maximum value possible for agiven value of temperature. It is drawn with two sections, one covering cold airand the other warm air. It will be seen from these that the capability of air for ab-sorbing moisture increases very considerably with increase in temperature.

The moisture content of air can also be expressed in flow rate terms. This isdetermined simply by using the flow rate form of the Ideal Gas Law, as presentedin Equation 5.4, rather than the static form in Equations 18 and 19. Figure 5.18 issuch a plot and shows the magnitude of the potential moisture problem, of waterassociated with air, very well.

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172 Chapter 5

0.6

0.5

2 0.4X)

13 0.3

§u 0.2

0.1

0

Saturated Aid atAtmospheric Pressure

6

5

i3 ~

§

I

-40 -20 0 20 40 60 80 100 120 140

Air Temperature - °F

Figure 5.17 The influence of temperature on the moisture content of saturated air.

Figure 5.18 is drawn for saturated air at standard atmospheric pressure andshows how the quantity of water in the air is influenced by both the volumetricflow rate of the air and its temperature.

400 800 1200 1600 2000

Volumetric Flow Rate of Air - ftVmin at 14-7 lbf/in2 abs

Figure 5.18 The influence of temperature on the flow rate of moisture associated withsaturated air.

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Air Requirements 173

The influence of the volumetric air flow rate is linear, of course, but that oftemperature is not, as illustrated with Figure 5.17. For air at atmospheric pressurethis represents the worst case, in terms of the flow rate of water associated withair, since it is drawn for saturated air.

6.1.1.2 The Influence of PressureTwo further graphical representations of Equation 25 are given in Figures 5.19 and5.20. These are graphs of moisture content of air, in pounds of water per pound ofair, drawn to illustrate the influence of air pressure. Figure 5.19 is a graph of spe-cific humidity plotted against temperature, with lines of constant pressure drawn.The pressures cover a range from -10 to 50 lbf/in2 gauge and so are appropriate toboth positive and negative pressure conveying systems.

Figure 5.20 is a similar plot, but with the x-axis and the family of curves in-terchanged. Both plots are for saturated air. These show that pressure also has asignificant effect on the amount of water vapor that air can absorb, decreasing withincrease in pressure. Figure 5.20 shows the influence of pressure on the moisturecontent capability of air very well, particularly at low pressures and under vacuumconditions.

These plots can be used to determine whether condensation is likely to occurin processes such as the compression and cooling of air. For air that is not initiallysaturated, however, account has to be taken of the initial relative humidity of theair.

0-3

.o

.o0-2

33o

<a0-1

-10 -5 0 5 10 15

40 80 120 160Saturated Air Temperature - °F

200

Figure 5.19 The influence of temperature and pressure on the moisture content of satu-rated air.

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174 Chapter 5

-10 -5 10 20 30 40

Air Pressure Ibf/in2 gauge

Figure 5.20 The influence of pressure and temperature on the moisture content of satu-rated air.

6.1.2 Relative Humidity

Relative humidity, (p, is the ratio of the partial pressure of the vapor actually pre-

sent, to the partial pressure of the vapor when the air is saturated at the same tem-perature:

(26)

where pv = partial pressure of vaporand pg = partial pressure of vapor at saturation

This is usually expressed as a percentage.

This situation can be best represented with lines of constant pressure super-imposed on a temperature vs. entropy plot for H20. Such a plot is presented inFigure 5.21. This also shows the saturation lines for both liquid and vapor and howthese separate the various phases or regions. Air saturated with water vapor, andhaving a relative humidity of 100%, will lie on the saturated vapor line, g. Thevapor in air having a relative humidity less than 100% is effectively superheatedsteam and so the point will lie in the vapor region.

Point A represents the actual condition of the vapor in the air and it will beseen that it is in the superheated steam region. On the saturation line for the vapor,at the same temperature (point B), the pressure is/?y.

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Air Requirements 175

Io>P.

<L>

I

I

Liquid

Region

Constant Pressure Lines

P2

Liquid

plus

Vapor

SaturatedLiquid Line

Region

/ D r y BulbTemperature

Region

Bulb Temperature

Dew Point

SaturatedVapor Line

Entropy - s

Figure 5.21 Temperature vs. entropy plot for H2O.

If the air is cooled from point A it will follow the/?? curve to the saturationline at C, which is the dew point at this pressure. From Figure 5.21 the relative

humidity is given as:

<p =

The pressures/)/ and/>2 can be obtained from Figure 5.16, knowing the cor-responding saturation temperatures TK and Tc.

6.1.3 Psychrometric Chart

The above expression, in terms of pressures, and other equations that can be de-rived from the Ideal Gas Law, however, are of little practical use in the process ofdetermining relative humidity. For this we generally use wet- and dry-bulb ther-mometers or a hygrometer. The actual, or dry-bulb temperature, of the air is repre-sented by point B on Figure 5.21 and point D represents the approximate locationof the wet-bulb temperature for unsaturated air.

Since this method depends upon equilibrium between heat and mass transferrates, the equations are rather complicated, and so data is given in charts and ta-bles. The information is usually presented in a psychrometric chart. Such a chart,for air at atmospheric pressure, is shown in Figure 5.22. This is a graph of specifichumidity plotted against dry-bulb temperature.

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176 Chapter 5

0-025

« 0-020Xl

2i. 0-015

X 0-010

0-005

0

30 11040 50 60 70 80 90

Dry Bulb Temperature - °F

Figure 5.22 Psychrometric chart for air at atmospheric pressure.

The saturation line is presented on this chart and this represents a relativehumidity of 100%. This is the same line as that drawn on Figure 5.17. Dry air, orair with a relative humidity below 100%, is represented in the area to the right ofthe saturation line. Lines of both constant wet-bulb temperature and relative hu-midity are superimposed on the chart. Thus, if the wet- and dry-bulb temperaturesare known, for a given sample of air, both relative humidity and specific humiditycan be determined quite simply. On some psychrometric charts lines of constantspecific enthalpy and specific volume are also superimposed so that this data canalso be obtained quickly if required.

6.1.4 Universal Model

By combining Equations 5.25 and 5.26 an equation is obtained in which both rela-tive humidity and specific humidity appear. This is:

CO =0-622 <p Pg

P - <PPK

(27)

Thus, with relative humidity, (p, obtained from a hygrometer, the pressure,

p, obtained from a barometer or pressure gauge, and the saturation pressure, pK,obtained from Figure 5.16 or an appropriate set of tables, the specific humidity ofany sample of air can be readily evaluated.

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Air Requirements 177

NOMENCLATURE

ACCpdmm

MP

RRO

S

t

T

VV

Pipe section areaVelocitySpecific heatPipe boreMassMass flow rate

Molecular weightPressure

Characteristic gas constantUniversal gas constant

Specific entropyActual temperatureAbsolute temperature

VolumeVolumetric flow rate

. 7in"ft/minBtu/lb RinIbIb/h

-lbf/in2

Btu/lb RBtu/lb-mol R= l-986Btu/lb-molRBtu/lb Rop

R= t + 460ft3

ftVmin

SI

m2

m/skJ/kg Kmkgkg/s, tonne/h(1 tonne = 1000kg)-kN/m , bar(1 bar =100 kN/m2)kJ/kg KkJ/kg-mol K= 8-314kJ/kg-molKkJ/kg K°CK= t + 273m3

m3/s

Greekp Density Ib/ft3

</> Solids loading ratio= mplma

(p Relative Humidity %

co Specific Humidity lbv/lba

kg/m3

Subscriptsa AirffggPssat

Saturated liquidChange of phase (evaporation) (= g - f)Saturated vaporConveyed material or productSuspensionSaturation value or conditionsWater vaporReference conditions (free air)

= 14-7 lbf/in2 absolutePo

Tn = 519R

= 101-3kN/m2abs

= 288K

1,2 Actual conditions - usually inlet and outlet

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178 Chapters

REFERENCE

1. D. Mills. Optimizing pneumatic conveying. Chemical Engineering. Vol 107. No 13. pp74-80. Dec 2000.

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Page 192: Handbook of Pneumatic Conveying Engineering

Air Only Data

1 INTRODUCTION

Although few reliable or universal models currently exist for predicting the pres-sure drop for gas-solid flows in pipelines, models for the single phase flow of agas are well established. Once again, although discussion will generally be interms of air, the models presented will work equally well with the appropriatevalue of the specific gas constant for the particular gas being considered. Emptyconveying pipeline pressure drop values, for air only, will provide a useful datumfor both the potential capability of a system for conveying material and the condi-tion of the pipeline. Air only pressure drop values for the conveying pipeline alsoprovide a basis for some first approximation design methods for the conveying ofmaterials.

Air supply and venting pipelines can be of a considerable length with somesystems, whether for positive pressure or vacuum systems, particularly if the airmover or the filtration plant is remote from the conveying system. In these cases itis important that the air only pressure drop values in these pipeline sections areevaluated, rather than just being ignored, for they could represent a large propor-tion of the available pressure drop if they are not sized correctly. Air flow controlis also important, particularly if plant air is used for a conveying system, or if theair supply to a system needs to be proportioned between that delivered to a blowtank and that directed to the pipeline, for example.

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180 Chapters

2 PIPELINE PRESSURE DROP

The pressure drop in the empty pipeline is a major consideration in the design of apneumatic conveying system. If a positive displacement blower is used in combi-nation with a long distance, small bore pipeline, for the suspension flow of a mate-rial, for example, it is quite possible that the entire pressure drop would be utilizedin blowing the air through the pipeline and that no material would be conveyed.The pressure drop for air only in a pipeline is significantly influenced by the airvelocity that is required for the conveying of the material. Bends and other pipe-line features also need to be taken into account.

The value of the empty line pressure drop for any pipeline will provide auseful indicator of the condition of the pipeline. If a pressure gauge is situated inthe air supply or extraction line, between the air mover and the material conveyingpipeline, this will give an indication of the conveying line pressure drop. With anempty pipeline it will indicate the air only pressure drop. If this value is higherthan expected it may be due to the fact that the line has not been purged clear ofmaterial. It may also be due to material build-up on the pipe walls or a partialblockage somewhere in the pipeline.

2.1 Flow Parameters and Properties

In order to be able to evaluate the pressure drop for the air flow in the empty pipe-line, various properties of the air and of the pipeline need to be determined.Mathematical models and empirical relationships are now well established for thissingle phase flow situation, and so conveying line pressure drops can be evaluatedwith a reasonable degree of accuracy.

2.1.1 Conveying A ir Velocity

This is one of the most important parameters in pneumatic conveying, as discussedearlier, with the air velocity at the material feed point being particularly important.If the conveying air velocity is not specified, therefore, it will usually have to beevaluated from the volumetric flow rate, pipeline bore, and the conveying linepressure and temperature, as outlined in the previous chapter.

2.7.2 Air Density

The density, p, of air, or any other gas, is given simply by the mass of the gas di-vided by the volume it occupies:

m „p = — lb/ft3

V

where m = mass of gas - Iband V = volume occupied - ft3

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Air Only Data 181

The Ideal Gas Law, presented earlier in Equation 5.4, applies equally to aconstant mass of a gas, as to a constant mass flow rate of a gas, and so:

mP =

144 pib/fr

V RT

where R = characteristic gas constant - ft Ibf/lb R

P

(1)

RTkg/rri (1SI)

Gas constants for a number of gases were presented earlier in Table 5.1.

A particular reference value is that of the density of air at free air conditions:

For air R = 53-3 ft Ibf/lb R and so at free air conditions ofp,, = 14-7 lbf/in2

and T0= 519 Rits density p = 0-0765 lb/ft3

It will be seen from Equation 1 that air density is a function of both pressureand temperature, with density increasing with increase in pressure and decreasingwith increase in temperature. The influence of pressure and temperature on thedensity of air is given in Figure 6.1 by way of illustration.

0

-10 -5 0 5 10 20Air Pressure - Ibf7in2 gauge

40

Figure 6.1 The influence of pressure and temperature on air density.

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182 Chapter 6

>Ui

<

0-044 _ _

0-042

0-040 .-

475 500 525

Air Temperature - R

550

Figure 6.2 The influence of temperature on the viscosity of air.

2.7.3 Air Viscosity

The viscosity, ft, of gases can usually be obtained from standard thermodynamicand transport properties tables. In general the influence of pressure on viscositycan be neglected. The influence of temperature on the viscosity of air is given inFigure 6.2 [1].

2.1.4 Friction Factor

The friction factor,/ for a pipeline is a function of the Reynolds number, Re, forthe flow and the pipe wall roughness, e.

Note:Reynolds number

5 p CdRe =

where pCd

and fj

= density of air - lb/ft3

= velocity of air - ft/min= pipeline bore - in= viscosity of air - lb/ft h

Alternatively, by substituting p from Equation 1 and C from a combinationof Equation 5.4 into Equation 5.3 gives an alternative form:

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Air Only Data 183

2880 m..Re =

n d JLI

where rha = air mass flow rate - Ib/min

Note:The substitution of the volumetric flow rate, V , from Equation 5.4 into

Equation 5.3 gives:

4 ma R TC = — ft/min (2)

n d p

This provides a useful alternative expression for the evaluation of conveyingair velocity.

Typical values of wall roughness, 8, are given in Table 6.1 [2] and values offriction coefficient can be obtained from a Moody chart, a copy of which is givenin Figure 6.3. It should be noted that Figure 6.3 is a UK version of the Moody dia-gram and gives values of friction coefficient that are one quarter the value of thoseon an equivalent US version of the diagram. This difference is explained, andcompensated for, in the next equation to be presented.

It will be seen from Figure 6.3 that an accurate value of a surface roughnessis clearly not critical, for a 100% error in relative roughness will only result in a10% error in friction coefficient.

Table 6.1 Typical Values of Pipe Wall Roughness

Pipe Material Surface Roughness - S

(new) in

Glass 'smooth'

Drawn tubing 0-000,05

Commercial steel and wrought iron pipes 0-002

Asphalted cast iron 0-005

Galvanized iron 0-006

Cast iron 0-0!

Concrete 0-01 - 0-1

Riveted steel 0-05 - 0-5

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184 Chapter 6

sIco

0-02

0-01

0-006

0-003

0-002

0-001

Relative

Roughness

SmoothPipes

000,01

10J 10" 10"

Reynolds Number - Re

Figure 6.3 Friction coefficients for flow in circular pipes.

2.2 Pressure Drop Relationships

The pressure drop for straight pipeline, regardless of orientation, is derived interms of the pipeline friction coefficient. The pressure drop for bends and otherpipeline fittings and features is obtained in terms of a loss coefficient. For the totalpipeline system the two are added together.

2.2.7 Straight Pipeline

The pressure drop, Ap, for a fluid flowing in a straight pipeline can be determinedfrom Darcy's Equation:

4/1 pC2

Ap = xd

(3a)

This is the UK version of Darcy's Equation, which is in terms of d/4. This isderived in terms of a hydraulic mean diameter, to allow application to non circularpipes and open channels. Hydraulic diameter is the ratio of the flow section area tothe wetted perimeter, which for a circular pipeline running full is equal to d/4. Thisis the reason for the value of the pipeline friction coefficients on Figure 6.3 beingfour times lower than those on similar US charts.

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Air Only Data 185

If the pressure drop, Ap, is to have units of lbf/in2, and the other parametersin the equation are as follows:

/ = friction coefficient - -L = pipeline length - ftd = pipeline bore - inp = density - lb/ft3

C = velocity - ft/minand gc = gravitational constant - ft Ib/lbf s2

= 32-2 ftlb/lbfs2

then Darcy's Equation will appear as follows:

fL p C2

01 Ann A21,600 d gclbf/in ..... (3b)

For a compressible fluid such as air, the equation in this form is rather in-convenient, particularly if there is a large pressure drop, for average values of bothdensity and velocity need to be specified, as they are both very pressure depend-ent. Both density and velocity, however, can be expressed in terms of constantsand air pressure, which means that the expression can be easily integrated.

From Equation 1 :

144 pP ^ -

and from Equation 2:

4 ma R TC = - j - ft/min

n d p

Substituting these into Equation 3 and expressing in differential form gives:

frhl RTp dp = - r — - - dL - ..... (4)i * r\ T-J r 2 /5 ^ /

9-375 n d gc.

Integrating gives:

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186 Chapter 6

0-213 f Lm] R TPi - P2 (5)

since:

where subscripts , and 2 refer to pipeline inlet and exit conditions

This can be used to obtain the air only pressure drop for any straight pipeline

Ap = p} - p2

and noting that:

if T = p] - p\

0-5

then Apa = pl - | f - F

/ \ f l . sand Apa = ( p 2 + F

For a positive pressure system p2 will be specified (usually atmosphericpressure) and so a more useful form of Equation 5, which eliminates the unknownPi\s:

0-213 f Lm] R Tx O - 5

-p2 lbf/in2(6)

Similarly for a negative pressure system pt will be specified (usually atmos-pheric) and so an alternative form of Equation 5, which eliminates the unknown p2

is:

0-213 R TP^ lbf/in' (7)

2.2.1.1 The Influence of Air Flow RateThe velocity of the conveying air will be approximately proportional to the airflow rate, whether on a mass or volumetric flow rate basis. From Equation 3 it willbe seen that pressure drop is proportional to the square of the velocity, and so airflow rate will have a very significant effect on conveying line pressure drop. Theinfluence of velocity is considered in conjunction with pipeline length and borebelow.

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Air Only Data 187

6

5

| 4JD

§• 3

1 2COCO

£ 1

ol-

0Conveying Line Exit Air Velocity - ft/min

1000 2000 3000 4000 5000. ' i ' •• '. ' i •—t S—*-

6000

o 20 40 60Air Mass Flow Rate - Ib/min

,-1

80—r-100

Figure 6.4 The influence of pipeline length and air flow rate on the empty pipelinepressure drop.

2.2.1.2 The Influence of Pipeline LengthFrom Equation 3 it will be seen that pressure drop is directly proportional to pipe-line length. Typical values of pressure drop for a 6 in bore pipeline are given inFigure 6.4. This is a plot of conveying line pressure drop for the air against the airmass flow rate. Pipeline lengths of 500, 1000 and 1500 ft have been considered.

Conveying line exit air velocity values are also given on the air flow rateaxis of Figure 6.4. This clearly shows the adverse effect of velocity, and hence airflow rate, on pressure drop. It also shows that if a material has to be conveyed overa long distance, the proportion of the total system pressure drop due to the air onlyin the pipeline could be very significant.

2.2.1.3 The Influence of Pipeline BoreFrom Equation 3 it will also be seen that pressure drop is inversely proportional topipeline bore. Typical values of conveying line pressure drop for a 500 ft longpipeline are given in Figure 6.5. This is a similar plot to that of Figure 6.4. The airmass flow rate axis is proportional to pipe section area, hence the (d/3)2 term, andso conveying line exit air velocities are constant in each case.

It can be clearly seen from this plot that the air only pressure drop reduceswith increase in pipeline bore. If an air mover with a pressure limitation, such as apositive displacement blower, has to be used to convey a material over a long dis-tance, therefore, it should be possible to achieve reasonably high flow rates with alarge bore pipeline.

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188 Chapter 6

5 i_

I 3

2 2Q

Conveying Line Exit Air Velocity - ft/min

1000 2000 3000 4000 5000 6000 7000-4-

5 10 15

Air Mass Flow Rate - Ib/min x (d/3)2

20 25

Figure 6.5 The influence of pipeline bore on the empty pipeline pressure drop.

2.2.2 Bends

The pressure drop for bends in a pipeline can be expressed in terms of the 'veloc-ity head':

Ap = k xpC-.2

2 gc

(8a)

where k = the number of velocity heads lost for theparticular bend geometry and configuration

If the pressure drop, Ap, is to have units of lbf/in2, the other parameters willbe as follows:

k = constant - dimensionlessp = air density - lb/ft3

C = air velocity - f t /mingc = gravitational constant - ft Ib/lbf s2

and the bend loss equation wil l appear as follows:

4? = k1,036,800 gc

lbf/in" (8b)

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Air Only Data 189

20Ratio of D/d

Figure 6.6 Head loss for 90 degree radiused bends.

The pressure loss in such a bend will depend upon the ratio of the bend di-ameter, D, to the pipe bore, d, and the surface roughness. Typical values are givenin Figure 6.6 [3]. Data for radiused bends, showing the influence of bend angle, ispresented in Figure 6.7.

0-3

0-2 T-

0-1 -

00 30

Angle

Figure 6.7 Head loss for radiused bends.

90

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190 Chapter 6

1-2

1-0

0-8ou

• 0-6

0-4

0-2

030 60

Angle - 0 - degrees

Figure 6.8 Head loss for mitered or sharp angle bends.

From Figure 6.6 it can be seen that very short radius bends will add signifi-cantly to the empty line pressure drop. Minimum pressure drop occurs with bendshaving a D/d ratio of about 12. This is not a critical value, however, for a reasona-bly low value of head loss will be obtained with a D/d range from about 5 to 40.

A similar plot for sharp angled or mitered bends is given in Figure 6.8 [3].This shows that the mitered bend will result in the highest value of air only pres-sure drop for a ninety degree bend, particularly for smooth pipes. In terms of pres-sure drop, therefore, such bends should be avoided.

2.2.2.1 Equivalent LengthThe head loss for straight pipeline, as will be seen from Equation 3a,

is given by4 f L

d

The equivalent length of straight pipeline, Lc, of a bend, with a head loss ofk, will therefore be:

kd(9)

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Air Only Data 191

Taking a typical pipeline friction coefficient, / of 0-005, the equivalentlength of a 6 in bore 90° mitered bend of smooth pipe, for which k= 1-1, will beabout 27-5 ft. If there are a number of such bends in a short pipeline, the bendswill add significantly to the total air only pressure drop value.

2.2.3 Other Pipeline Features

Other pipeline features, such as branches and section changes, are treated in ex-actly the same way as any of the above pipeline bends, with the use of Equation8b. In Figures 6.9 to 6.11 similar head loss values are given for various pipelinefittings.

2.2.3.1 Expansion FittingsExpansion fittings are required in stepped pipelines, where the diameter of a line isincreased part way along its length in order to reduce the conveying air velocity.Figure 6.9 shows that the air only pressure drop will be a minimum if a taperedsection is used having an included angle of about six degrees.

Expansion and contraction sections often occur in association with pipelinefeeding systems such as rotary valves and screws. In venturi feeders the expansionsection is an integral part of the design. Where expanded bends are fitted into apipeline both expansion and contraction section are required. At the dischargefrom a pipeline into a reception vessel the expansion is effectively infinite. Figures6.9 and 6.10 show the importance of careful design in such devices.

30 60

Total Angle - 6 - degrees

Figure 6.9 Head loss for enlarging pipeline sections.

90 180

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192 Chapter 6

Abrupt Entrancek = 0-5

l/y i /l i 111 I 11 I I /

Abrupt exitk = 1-0

Gradual Entrancek = 0-05

II I I I 11II I 111 11

Gradual Exitk = 0-2K = u-z y

\ \ \ \ \ \ \ \ \ \ V\AA\ \J^

Figure 6.10 Entrance and exit head losses.

The head loss for various diverter sections, fabricated bends and 'dog-leg'sections, that are often used in air supply and exhaust pipelines, are given in Fig-ure 6.11. A comparison of the two 'dog-leg' sections shows just how importantcareful pipeline design and layout are in minimizing pressure drop.

k = 3-0

k = 1-0

= 0-16 smooth

= 0-30 rough

k = 040 smooth0-53 rough

30"

k = 0-40 smooth= 0-60 rough

Figure 6.11 Head loss for various pipe fittings.

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Air Only Data 193

2.2.4 Total Pipeline

The pressure drop for the total pipeline system is simply given by a summation ofall the component pressure drop values, so that:

^ ^21,600 d l,036,800j gc

where 2k = the sum of the head losses for all the bendsand fittings in the pipeline

Substituting p from Equation 1 and C from Equation 2 gives:

( f L 2 k\ ml R T

{9-375 d t juy n a' p gc

For convenience the head loss for the pipeline, bends and fittings can begrouped together using the term y/, such that:

fL S k(dimensionless) - - - (11)

9-375 d

Substituting and integrating, as with Equation 4, gives:

2i// m2 R T

' ' pl '-This can be used to obtain the air only pressure drop in any pipeline situa-

tion.

2.2.4.1 Positive Pressure SystemsFor a positive pressure system p2 will be specified, as mentioned earlier in connec-tion with Equation 6, and so a more useful form of Equation 12 is:

I• 2 D r\ °'5ma R T]

P2 + - r~T4 - - P2 lbf/i"2 - - (13)" Sc )

For air R = 53-3 ft Ibf/lb Rand if T = 519 R

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194 Chapter 6

and taking p2

substituting g,and for jf

14-7 Ibf/in" (atmospheric pressure)32-2 ft Ib/lbf s2

this gives:

216-1 +174^ ml

14-7 M/in2 - - (14)

1-0 +

x O - 5

x 105 -1-0 bar (14SI)

In many cases a value of the conveying line exit air velocity, C2, can be de-

termined, by using Equation 11, for example. A substitution of C? for ma can be

made from Equation 2:

m,.n d2 C2 p2

4 R TIb/min (15)

Substituting this into Equation 12 gives:

t~i~> ~>¥ Q p;Pi ~ Pi

from which:

Ap

8 gc

(16)

, 0-5

8 R T2 gc

Ibf/in2 (17)

Thus in a situation where the downstream pressure, p2, is known (commonlythis would be atmospheric pressure in a positive pressure system) and the convey-ing line exit air velocity can be determined, this expression allows the pressuredrop for the air alone to be estimated quite easily.

Alternatively, if the conveying line inlet air velocity, C/, is known, this can

be used instead. A substitution of C, for ma, from Equation 2, in Equation 12

gives:

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Air Only Data 195

P\ ~ P\

from which:

Apa =

¥

R Tt gc

(18)

8 R T, gc

.0-5

-1gc -

lbf/in2 - (19)

Note:The velocity, Ct, in Equations. 18 and 19, is not the conveying line inlet air

velocity that is specified for gas-solid flows in pneumatic conveying. It is the con-veying line inlet air velocity that will result when no material is conveyed. C2 inEquations 16 and 17, of course, is the same whether material is conveyed or not,since the pressure will always be the same at the end of the pipeline.

2.2.4.2 Negative Pressure SystemsFor a negative pressure system, p/, will be specified (usually atmospheric). A re-arrangement of Equation 18 gives:

0-5

1 - 1 - Ibf/in2 (20)

= Pi

,0-5

1 -R r, gc

N/m2 - (20si)

Note:In this case the conveying line inlet air velocity, C/, will be the same

whether the material is conveyed or not, since the pressure, pt, will be atmos-pheric in both cases. This is similar to Equations 16 and 17 for positive pressuresystems.

2.3 Air Only Pressure Drop Datum

The empty pipeline pressure drop relationships for a pipeline, such as those shownin Figures 6.4 and 6.5, provide a datum for material conveying characteristics andcapability. At a given value of air flow rate the pressure drop available must begreater than the air only pressure drop value, otherwise it will not be possible toconvey material.

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196 Chapter 6

At any value of conveying line pressure drop there will be a correspondingvalue of air flow rate at which the air only pressure drop will equal the conveyingline pressure drop. This value can be determined from Equation 13 by makingma the subject of the equation. Such a re-arrangement gives:

m.2 R T

Ib/min (21)

(PI

16 i/ RT

0-5

kg/s - - (21 si)

This is quite a useful relationship, for it allows an estimate to be made ofwhere the various lines of constant conveying line pressure drop on material con-veying characteristics will reach the horizontal axis.

For air R = 53-3 ft Ibf/lb Rand if T = 519 Rand with g, = 32-2 f t lb / lbfs 2

m,. = 0-0758P\ Pi

0-5

Ib/min (22)

2.4 Venturi Analysis

Particular advantages of using venturi feeders for positive pressure conveyinglines are that minimum headroom is required, there are no moving parts and, if thedevice is correctly designed, there need be no air leakage from the feeder, as thereis with nearly all other types of feeder. A venturi basically consists of a controlledreduction in pipeline cross-section in the region where the material is fed from thesupply hopper, as shown in Figure 6.12 and first presented in Figure 2.12.

A consequence of this reduction in flow area is an increase in the entrainingair velocity, and a corresponding decrease in pressure, in this region. With a cor-rectly designed venturi the pressure at the throat should be just a little lower, orabout the same, as that in the supply hopper which, for the majority of applica-tions, is atmospheric pressure. This then encourages the material to flow readilyunder gravity into the pipeline, and under these conditions there will be no leakageof air from the feeder in opposition to the material feed.

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Air Only Data 197

Air andMaterial

Inlet (i) Throat (t)

Figure 6.12 Basic type of venturi feeder.

In order to keep the throat at atmospheric pressure, and also of a practicalsize that will allow the passage of material to be conveyed, a relatively low limithas to be imposed on the air supply pressure. These feeders, therefore, are usuallyincorporated into systems that are required to convey free-flowing materials at lowflow rates over relatively short distances. Since only low pressures can be usedwith the basic type of venturi, a positive displacement blower or a standard indus-trial fan is all that is needed to provide the air.

To fully understand the limitations of this type of feeder, the thermodynamicrelationships are presented below. The two parameters of interest in venturi feed-ers are the velocity at the throat and the area, or diameter, of the throat. From thesteady flow energy equation, equating between the inlet (i) and the throat (t) gives:

Cp T, +C2

CpT,+2 gc

(23)

from which:

c, = 2 ge cp (T, - T,) + c?0-5

(24a)

If the velocities, C, and C,, are to have units of ft/min, and the other parame-ters in the equation are as follows:

Cp = specific heat - Btu/lb RT = absolute temperature - R

and gc = gravitational constant - ft Ib/lbf s2

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198 Chapter 6

= 32-2 ftlb/lbfs2

and noting that 1 Btu = 778 ft Ibf

the equation will appear as follows:

Ct = [l80 Cp(T,-T,)xl06+CfJ5 ft/mm - - (24b)

If an isentropic model of expansion is assumed for the venturi then:

Pt- = I —T, \ P,

Note that this appeared earlier in Equation 3.3

Substituting Equation 25 into Equation 24b gives:

0-5

(25)

Ct = \\WCpT, 6 2x!06+C ft/min - (26)

From the continuity equation:

ma = Pi AjCj = p,A,C, Ib/min

n d2

where A = section area = - in4

d = diameter - inand p = density of gas - lb/ft3

PRT

Substituting Equation 1 into Equation 27 gives:

(27)

(1)

x - x - x d, P,

in (28)

Substituting Equation 25 into Equation 28 gives:

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Air Only Data 199

dt = C' x 1X

c, 1

1

;

\

EL

P, }

7

-i 0-5

d, in (29)

If, for example,C, = 4000 ft/mindj = 4 inTi = 528 R = 68 °Fp, = 14-7 Ibf/in2absp, = 3 lbf/in2 gauge = 17-7 lbf/in2 abs

note that for air Cp = 0-24 Btu/lband y = 1-4

substituting into Equation 48 gives:

C, = j 180x0 -24x528

= 34,582 ft/min

1 4 - 7

1 7 - 7

0-286,0-5

x l O 6 +40002

and substituting into Equation 29 gives:

d, =4000

34,582

= 1-45 in

1 4 - 7

1 7 - 7

-0-714 0-5

x 4

Although Venturis capable of feeding materials into conveying pipelineswith operating pressure drops of 6 lbf/in2 are commercially available, the addi-tional pressure drop across the venturi can be of the same order. This means thatthe air supply pressure will have to be at about 12 lbf/in2 gauge and consequently,for this type of duty, it would be recommended that the air should be supplied by apositive displacement blower.

3 AIR FLOW RATE CONTROL

If the air to be used for conveying is taken from a plant air supply, or some centralsource, it will probably be necessary to put a flow restriction into the pipeline. Thiswill be needed in order to limit the quantity of air drawn to that of the volumetric

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200 Chapter 6

flow rate actually required. If this is not done an uncontrolled expansion will occurand very much more air than necessary will be used. It will only be limited by thevolumetric capability of the supply, or by the increased factional resistance of theflow in the pipeline. The increased air flow rate will almost certainly result in adecrease in the material flow rate through the pipeline. It will also add signifi-cantly to problems of erosive wear and particle degradation.

Flow restrictors may also be required in situations where the air supplyneeds to be divided, as in blow tank systems. For the control of many types ofblow tank it is necessary to proportion the air supply between the blow tank andthe conveying line. If the total air supply is set, a flow restrictor can be placed inone or both of the divided lines. This, however, can only be done if the blow tankis dedicated to a single material conveyed over a fixed distance. For systems han-dling more than one material, or conveying to a number of hoppers over varyingdistances, a variable flow control might be needed. In these cases special controlvalves would be required rather than fixed restrictors.

Nozzles and orifice plates are most commonly used for restricting the airflow in a pipeline. Under certain flow conditions they can also be used to meterand control the air flow.

3.1 Nozzles

For the single phase flow of fluids through nozzles the theory is well established,and for a gas such as air it is based on the use of many of the equations alreadypresented. Nozzles are either of the convergent-divergent type, as shown in Figure6.13a, or are convergent only, as shown in Figure 6.13b. Both types restrict theflow by means of a short throat section at a reduced diameter.

2 ) Direction

(b)

Figure 6.13 Nozzle types, (a) Convergent - divergent nozzle and (b) convergent nozzlein pipeline.

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Air Only Data 201

3.7.7 Flow Analysis

Assuming a steady one-dimensional flow, and equating the steady flow energyequation between inlet (1) and throat (t) gives:

CpT,C2

= Cp Tt +2 gc 2 g,

Neglecting the inlet velocity, Ci, and re-arranging gives:

(23)

c, = T,gcCPTA\ - -±

T,.

0-5

(30a)

For consistency in units, constants of 778 and 3600 have to be applied, aswith Equation 24, and this yields:

Ct = 13,430T.

CpTAl - ^

0-5

ft/min (30b)

Assuming isentropic flow, for which Equation 25 applies, the unknown tem-perature at the throat, T,, can be expressed in terms of the pressure at the throat, p,.Such a substitution gives:

C, = 13,430

Also for isentropic flow:

CpT, ft/min (31)

vt = v, x — fr/lb\Pt)

where v = specific volume - ft3/lb

Now, from the Ideal Gas Law (Equation 5.4):

144 Pl v, = R T,

and substituting this into Equation 32 gives:

- - (32)

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202 Chapter 6

R T}

144 Pl(33)

From the continuity equation (Equation 27):

A, C, A, C,lb/min

Substituting C, from Equation 31 and v( from Equation 33 into this gives:

m,.

4 x!3,430x- CpT,

y-\

-te)T_RT (pVr

144 p, p,

lb/min - (34)

Re-arranging this gives:

0-5

T;

where d, = nozzle throat diameter - in

lb/min - (35)

3.1.2 Critical Pressure

A peculiarity of the expansion of the flow of a fluid through a nozzle is that as thedownstream pressure, p2, reduces, for a given upstream pressure, ph the pressureat the throat,/),, will not reduce constantly with downstream pressure. The pressureat the throat will reduce to a fixed proportion of the inlet pressure, and any furtherreduction of the downstream pressure will not result in a lowering of the pressureat the throat.

Under these conditions the nozzle is said to be 'choked'. When critical flowconditions exist, the velocity at the throat will be equal to the local sonic velocity.The air mass flow rate through a nozzle is a maximum under choked flow condi-tions and no reduction of the downstream pressure, below the critical throat pres-

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Air Only Data 203

sure, will result in any change of the air mass flow rate. It can be shown [eg 4] thatthe ratio between the throat pressure and the supply or inlet pressure is given by:

(36)

For air y = 1-4R = 53-3 ftlbf/lbR

and Cp = 0-24 Btu/lb Rand so

— = 0-528P\

3.1.3 Nozzle Size and Capability

Substituting the above data for air into Equation 35 gives:

ma = 25-1 — j j- Ib/min (37)-'i

where p/ = inlet or supply pressure - Ibf/in2 abs

For the air flow rate in volumetric terms, Equation 5.4 gives:

maRTV = flrVmin

144 p

For the volumetric flow rate at free air conditions:

0V0 = 0-1743 x — — x - ftVmin ---- (38)

^1 Pa

and substituting for R and free air conditions gives:

P^ d?V0 = 328—^ ft-Vmin - ...... (39)

Alternatively, for a given air flow rate:

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204 Chapter 6

m,. -05 0-5

dt = 0-2 in (40)

The relationship between d,, pi and both ma and V0 , for air at a tempera-

ture, t,, of 68°F (T, = 528) is given in Figure 6.14.

3.1.4 Nozzle Types

The above analysis applies to either convergent-divergent or to convergent noz-zles. For convergent nozzles, however, the range of operation is limited to down-stream pressures that are less than 52-8% of the upstream pressure, that is, belowthe critical pressure ratio. With convergent-divergent nozzles this range can beextended significantly, and for a well made nozzle the downstream pressure can beas high as 90% of the upstream pressure, with little deviation from the predictedflow rate.

3.1.4.1 Orifice PlatesThese are frequently used for measuring the flow rate of gases through pipelinesbut can also be used to choke the flow and so apply a limit to the throughput. Theorifice is generally made from thin plate that is usually fitted into a flanged joint inthe pipeline. It has a sharp edged opening which is concentric with the pipe.

PlOO

I 90

80

70

60i fr/ntjn of Free Air

200 300 400

20 30Air Flow Rate - Ib/min

NozzleThrc(at Diameter

-inch

3/4

500 600

40 50

Figure 6.14 Influence of throat diameter and air supply pressure on choked air flowrate for nozzles.

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Air Only Data 205

The above analysis also applies to orifice plates. There is, however, a coeffi-cient of discharge associated with orifice plates and this has the effect of reducingthe flow rate to about 61 % of the theoretical value. This means that the constantsin Equations 34 to 39 would have to be multiplied by a factor of 0-61 and the con-

stant in Equation 40 would have to be divided by V O - 61 to take account of thiscoefficient of discharge. As with the convergent nozzle, the range of operation islimited to downstream pressures below the critical pressure ratio.

3.1.5 Flow Rale Control

It will be seen from Figure 6.14 that, for a given nozzle, the air flow rate can bevaried over a wide range simply by varying the air supply pressure. In a pipelinefrom a service supply, a diaphragm valve could be positioned upstream of the flowrestrictor, and this could be used to vary the inlet pressure and hence the air flowrate. Provided that critical flow conditions exist, only the inlet air pressure andtemperature, and the throat diameter, are needed to evaluate the air flow rate, aswill be seen from Equation 37.

It will be noticed that, apart from including a representative coefficient ofcontraction for orifices, no other coefficients have been included in the analysis toallow for friction and other irreversibilities in the flow. For most pneumatic con-veying applications it will not be necessary, as these losses are generally quitesmall. If these devices are to be used for flow measurement purposes, however,with a need for a high degree of accuracy, either the loss factors will have to betaken into account or the device will have to be calibrated.

NOMENCLATURE SI

A Pipe section area in2 m2

C Velocity ft/min m/sCp Specific heat at

constant pressure Btu/lb R kJ/kg KCv Specific heat at

constant volume Btu/lb R kJ/kg Kd Pipe bore in m/ Friction coefficientg Gravitational acceleration ft/s2 m/s2

= 32-2 ft/s2 = 9-81 m/s2

gc Gravitational constant ftlb/lbfs2 kgm/Ns 2

= 32-2 ftlb/lbfs2 = 1-0 kgm/Ns 2

k Bend loss coefficientL Pipeline length ft mm Mass Ib kgm Mass flow rate Ib/min kg/s

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206 Chapter 6

p Pressure lbf/in2

R Characteristic gas constant Btu/lb R/ Actual temperature °FT Absolute temperature R

= t + 460V Volume ft3

V Volumetric flow rate ftYminGreek

y Ratio of specific heats= Cp/Cv (adiabatic index)

8 Pipe wall roughness in// Viscosity Ib/ft hv Specific volume ftVlb

= \lpp Density Ib/ft3

if/ Total pipeline headloss coefficient

N/m2, kN/m2, bar(1 bar= 100 kN/m2)kJ/kg K°CK= t H

m3

m3/s

273

mkg/m snv/kg

kg/mj

Subscriptsa Airc Constante Equivalent value - usually lengthi Inlet conditionst Throat conditionso Reference conditions (free air)

= 14-7 lbf/in2 abs = 101-3 kN/m2 absPo

Tn = 519 R = 288 K

1,2 Actual conditions - usually inlet and outlet

SuperscriptsPer unit of time ie /min

Repeating value, eg 1/3 = 0-3

PrefixesA Difference in valueE Sum total

Non-Dimensional Groups

Re Reynolds Number5 p C d p C d

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Air Only Data 207

REFERENCES

1. Y.R. Mayhew and G.F.C. Rogers. Thermodynamic and Transport Properties of Fluids.Basil Blackwell. 1968.

2. J.M. Gasiorek and W.G. Carter. Mechanics of Fluids for Mechanical Engineers.Blackie and Son. 1967.

3. J.R.D. Francis. Fluid Mechanics for F^ngineering Students - 4th Ed. Edward Arnold.1975.

4. V.M. Faires. Applied Thermodynamics - 3rd Ed. MacMillan. 1957.

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Page 221: Handbook of Pneumatic Conveying Engineering

Conveyed Material Influences

1 INTRODUCTION

Although the performance of a pipeline with air only can be predicted reliably, theaddition of material to the flow of air changes the situation entirely. This was illus-trated in Chapter 5 where the conveying characteristics of a number of differentmaterials were presented. These were used to illustrate the differences in convey-ing capability between different materials, and the very wide differences that canexist between materials that can be conveyed in dense phase and those that cannot.

In this chapter these conveying characteristics are developed further to illus-trate the influence of conveying air velocity, and hence air flow rate, in more de-tail. Power requirements and specific energy are also considered, so that the influ-ence of velocity can be considered in more meaningful terms. This will also pro-vide a better basis for comparison between dilute and dense phase conveying ca-pability and provide a basis on which pneumatic conveying can be compared withalternative methods of conveying.

Pipeline bore and conveying distance are then considered. Pipeline bore isimportant because of the major influence that it has on the conveying capability ofa pipeline. Conveying distance is generally the most problematical of all the vari-ables. Conveying distance will nearly always be different from one situation to thenext, and hence the pressure gradient will also be different. It is essentially the

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210 Chapter 7

pressure gradient that will dictate the solids loading ratio at which a material canbe conveyed through a pipeline, as was illustrated in Figures 4.23 and 24. Then formaterials that have very good air retention properties, such as cement, the mini-mum conveying air velocity varies with solids loading ratio, as was illustrated inFigure 4.6.

2 MATERIAL COMPARISONS

Various materials were compared in Chapter 4 in terms of their conveying capabil-ity and the broad divisions that result between materials that can be conveyed indense phase and those that can not. In this section the differences are examined interms of conveying air velocities, power requirements and specific energy. Forcontinuity the three materials considered earlier are examined further. The materi-als were cement, sandy alumina and polyethylene pellets. Conveying characteris-tics for cement were presented in Figure 4.5b and are reproduced here in Figure7.1 for reference.

All three materials were conveyed through the Figure 4.2 pipeline whichwas 165 ft long of two inch nominal bore and included nine 90° bends. Similardata for the alumina and polyethylene pellets from Figures 4.8b and 12b are simi-larly reproduced in Figures 7.2 and 3. To allow visual comparisons to be made thesame axes have been used for all three materials and conveying line pressure dropvalues up to 25 lbf/in2 have been considered in each case.

Pressure Drop 160 120

Solids LoadingRatio

20

10

40 80 120

Free Air Flow Rate - itVmin

160

Figure 7.1 Conveying characteristics for cement.

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Material Conveying 211

30

20

t-3 10

Solids LoadingRatio

Conveying Line PressureDrop - lbf/in2

NO GO AREA

ConveyingLimit

40 80 120 160 200

Free Air Flow Rate - ft/min

Figure 7.2 Conveying characteristics for sandy alumina.

The data, therefore, relates to positive pressure conveying. A relatively highpressure has been used in order to accentuate the differences between the materialsconsidered. The same differences, however, will exist in negative pressure con-veying and so the analysis undertaken, and the results obtained, will differ littlebetween positive pressure and vacuum conveying.

ooo

cdOi

o

30

20

g 10'C

<Dta

Solids LoadingRatio

30

Conveying Line PressureDrop - lbf/in2

NO GO AREA

ConveyingLimit

40 80 120 160Free Air Flow Rate - ft'/min

200

Figure 7.3 Conveying characteristics for polyethylene pellets.

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212 Chapter 7

2.1 Conveying Air Velocity

Since conveying air velocity is such an important parameter this is consideredfirst. Conveying line inlet air velocity is one of the basic design parameters for apneumatic conveying system and so it is this value that is plotted. This is purely amathematical process.

The relevant model for plotting velocity on the conveying characteristicswas developed in Chapter 5 at Equation 10 and is re-presented here:

F = 0-1925pd2C

ftVmin (1)

where Vn = volumetric flow rate of free air - ftVmin

p = conveying air pressure - lbf/in2 absoluted = pipeline bore - inC = conveying air velocity - ft/min

and T = absolute temperature of air - R

Pipeline bore and air temperature will be known, and so for a given value ofconveying air velocity, the corresponding value of free air flow rate for given val-ues of conveying line inlet air pressure can be evaluated. By this means lines ofconstant value of conveying line inlet air velocity can be plotted. Such a plot forcement is presented in Figure 7.4.

160 120,100

Solids LoadingRatio

Conveying Line InletAir Velocity - ft/min

40 /

,3000

40 80 120 160

Free Air Flow Rate - ft3/min

200

Figure 7.4 Conveying air velocity data for cement.

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Material Conveying 213

At high values of solids loading ratio the minimum conveying air velocityfor the cement is about 600 ft/min. For dilute phase, suspension flow, the mini-mum velocity is about 2000 ft/min. Between these two extremes the conveyinglimit is dictated by the relationship between minimum conveying air velocity andsolids loading ratio presented in Figure 4.7. An extremely wide range of convey-ing conditions, therefore, are available for cement. To help in the decision makingprocess, power requirements and specific energy are developed in a similar man-ner below.

The lines of constant conveying air velocity help to illustrate the problemsof compressibility with air. As conveying air pressure increases, the value of thefree air flow rate must increase in order to maintain the same value of velocity. Inmany pneumatic conveying systems there is a limit on the volumetric flow rate ofair available and so great care must be taken if material feed rate into the pipelineis increased since this will require an increase in pressure for conveying.

Because exit from the pipeline in this case is always at atmospheric pres-sure, the conveying line exit air velocity only varies with air flow rate. Conveyingline exit air velocity can be determined simply by putting p = 14-7 lbf/in2 intoEquation 7.1 to determine this value. Similar data for the sandy alumina is pre-sented in Figure 7.5.

The range of conveying conditions for this material are very limited since itis only capable of being conveyed in dilute phase, suspension flow. The conveyinglimit, dictated by the combination of a fixed value of minimum conveying air ve-locity and the compressibility of the air, significantly reduces the operating enve-lope for this type of material.

ooo

30 Conveying Line Inlet AirVelocity - ft/min

Solids LoadingRatio

Conveying Line Pressuredrop - ibfin

c5B!_o

3 to.x>*^->Cl' ^—~'

..•'5000

..6000

0 40 80 120 160 200Free Air Flow Rate - fVVrnin

Figure 7.5 Conveying air velocity data for sandy alumina.

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214 Chapter 7

g30o

o

10

O03

Conveying Line Inlet AirVelocity - ft/min

Solids Loading/ Ratio

30Conveying Line Pressure

Drop - Ibf/in

40 80 120 160Free Air Flow Rate - ftVmin

200

Figure 7.6 Conveying air velocity data for polyethylene pellets.

As a consequence, changes in material feed rate, and hence pressure, have amuch greater effect in dilute phase conveying than they do in dense phase. Similardata for the polyethylene pellets is presented in Figure 7.6.

Although this material is capable of being conveyed at very low velocity,and hence in dense phase, the operating area available for dense phase conveyingis also very limited. The minimum conveying air velocity for this material for di-lute phase conveying will be about 3000 ft/min. Because of the positive slope tothe conveying limit curve only a narrow band, at low material flow rates, is avail-able for operation between these two limits. It is interesting that the 3000 ft/minvelocity curve approximately passes through the maximum value point on eachconstant pressure drop line.

2.2 Power Requirements

Pneumatic conveying has a certain reputation for high power requirements, cer-tainly with regard to dilute phase conveying, and so this is explored with regard tothe three materials being investigated. The relevant model for plotting power re-quirements on the conveying characteristics was developed in Chapter 3 at Equa-tion 6 and is re-presented here:

Power = 0-128 V hp (2)

where VQ = air flow rate at free air conditions - ftVmin

p2 = compressor delivery pressureand pi = compressor inlet pressure

- Ibf/in abs- Ibf/in2 abs

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Material Conveying 215

In order to plot lines of constant power, P, it is the volumetric flow rate of

free air, Va, that needs to be the subject of the equation and so a re-arrangement

gives:

• 7-81 P ,V0 = —, r ft3/min (3)

where P = power - hp

For a given value of power, P, the corresponding value of free air flow ratefor given values of conveying line inlet air pressure can be evaluated. By thismeans lines of constant value of power required can be plotted. Such a plot forcement is presented in Figure 7.7.

Power requirements for the cement on Figure 7.7 vary from a minimum ofabout 2 hp to a maximum of 25 hp. This shows the influence of air flow rate, andhence conveying air velocity very well. With a conveying line pressure drop of 25Ibf7in2, for example, 34,000 Ib/h of cement can be conveyed with 5 hp and 20,000Ib/h can be conveyed with the same 25 lbf/in2, but 25 hp. This represents a fivefold increase in power for a 40% reduction in cement flow rate. It is generally rec-ommended that a system be designed with a conveying line inlet air velocity about20% greater than the minimum conveying air velocity value.

Solids Loading Ratio

Power Required-hp

40 80 120 160 200

Free Air Flow Rate - ftVmin

Figure 7.7 Power requirements data for cement.

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216 Chapter?

This is usually a sufficient margin to allow for pulsations in material flowrate, compressor characteristics and compressibility effects. Although cement canbe conveyed at any point on the performance map it is clearly inefficient to do soat unnecessarily high air flow rates.

It is obviously necessary to know the value of the minimum conveying airvelocity and this is why conveying trials with a material are so important, particu-larly if previous experience with a material is not available. Similar data for thesandy alumina is presented in Figure 7.8.

Because of the very much higher minimum conveying air velocity with thismaterial only the bottom right hand corner exists, but it is essentially the samepattern of curves.

There is no longer any scope for the 5 hp curve to convey any substantialamount of material and capabilities are in a more ordered fashion. The slope of theconstant power curves is the same and so with 10 hp, for example, 10,000 Ib/h canbe conveyed with 140 ftVmin of free air and 2,500 Ib/h can be conveyed with 200ft3/min of free air. This represents a four-fold reduction in conveying capability fora 40% increase in air flow rate.

The 10 hp curve will ultimately reach the horizontal axis and convey noth-ing when the power is entirely taken up by transporting the air through the pipe-line. An explanation for this comes the pressure drop model for air only that wasfirst presented in a simplified form in Chapter 4 at Equation 1 and is re-presentedbelow for reference:

Solids Loading Ratio30

Power Required - hpooo

£ 20

sE.2 10

Conveying Line PressureDrop - lbf/in2

80 120 160 200

Free Air Flow Rate - If/min

Figure 7.8 Power requirements for sandy alumina.

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Material Conveying 217

Apf LpC2

dlbf/in2

(4)

It is the velocity term, C2, that dominates in this situation and is one of themain reasons why the constant power lines slope so steeply in this region. To con-vey more material the air flow rate needs to be reduced, but there is a conveyinglimit in the way to prevent this.

To convey more material the air pressure can be increased, provided that theair mover has the necessary capability and power, but if this is at the same air flowrate, the conveying limit is in the way once again. This is why a performance mapfor a material is so important, for it provides all the information necessary to makeall the decisions required for a successful system design. Similar data for the poly-ethylene pellets is presented in Figure 7.9.

There is little difference between the power requirements data for the poly-ethylene pellets and that for the sandy alumina. This is mainly because the operat-ing envelope for dense phase conveying with the polyethylene pellets is so small.Most of the performance data is in the dilute phase conveying region and this dif-fers little with regard to the properties of the material, regardless of whether thematerial can be conveyed in dense phase or not.

The main difference between the pellets and the alumina comes in systemoperation. If air flow rate is reduced, or pressure increased, with the pellets theconveying system will simply stall, and if the conveying conditions are changed itshould be possible to re-start with little problem.

30ooo

I 20

o

Power Required - hp

Solids Loading Ratio

\30

25

Conveying Line PressureDrop - Ibf/irv

80 120

Free Air Flow Rate - ftVmin

160 200

Figure 7.9 Power requirements data for polyethylene pellets.

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218 Chapter 7

The conveying limit for the alumina, and other similar materials that canonly be conveyed in dilute phase, suspension flow, is that the conveying limit gen-erally represent pipeline blockage, and once blocked it is often a time consumingprocess to clear the pipeline and re-start.

2.3 Specific Energy

In the above examples specific cases have been taken to illustrate particular points,such as the effect of air flow rate on performance. A problem with this is thatmany other parameters change and so global comparisons are difficult to make. Abasis on which direct comparison can be made is that of specific energy. This willprovide a reliable basis for comparing different materials, such as those being il-lustrated here, and with alternative mechanical conveying systems for the givenduty.

The units of specific energy are horsepower-hour per ton of material con-veyed or hp h/ton. Specific energy data superimposed on the conveying character-istics for the cement is presented in Figure 7.10.

Specific energy, E, is simply the ratio of power required, P, in hp, to material

flow rate, m „ , in ton/h:

8 = hp h/ton (5)

30

;, so

1b.

"3 101

25

10

0-5160 120

/, 100 80

Solids Loading/Ratio

40 80 120 160

Free Air Flow Rate - ftVmin

200

Figure 7.10 Specific energy data for cement.

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Material Conveying 219

Power requirement data was presented in Figures 7.7 to 7.9. To plot lines ofconstant specific energy simply divide power required by material flow rate andmark points on the graph that give rounded values of 0-5, 1-0, 1-5, etc. Thesepoints can then be joined to provide lines of constant specific energy. Such datafor the cement from Figure 7.7 is presented in Figure 7.10.

For the cement the specific energy data clearly identifies low velocity con-veying as being the most efficient. A wide range of specific energy values appearon Figure 7.10 but this is only because air flow rates up to 200 fWmin have beenincluded, to be consistent with the other materials being considered. For normalpurposes, and certainly for conveying, air flow rates above 80 ft3/min need not beconsidered for cement in the pipeline used. Similar data for the sandy alumina ispresented in Figure 7.11.

Specific energy data for alumina follows a similar pattern to that for the ce-ment. Values, however, are generally about five times higher and this typifies thedifference between dilute and dense phase conveying capability. The influence ofpressure is a little difficult to isolate.

Constant specific energy lines tend to run approximately parallel to the con-veying limit and so at first sight it would appear to have little effect. With highpressure air for conveying, however, stepping of the pipeline to a larger borewould be recommended.

All the data presented in this chapter is for the Figure 4.2 pipeline which issingle bore. At higher pressures, and with stepped bore pipelines, an improvementin performance would be expected.

30ooo

Solids Loading Ratio

Specific Energy

Conveying Line Pressure

80 120 160 200

Free Air Flow Rate - fVVmin

Figure 7.11 Specific energy data for sandy alumina.

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220 Chapter 7

30

20

_o 10

Solids Loading Ratio

Specific Energy - hp h/ton I

Conveying LinePressure Drop - Ibf7in

0 40 80 120 160 200

Free Air Flow Rate - ft7min

Figure 7.12 Specific energy data for polyethylene pellets.

With regard to pressure, comparisons across a given set of conveying char-acteristics are not likely to be made. They can certainly be used to investigate theimprovement in performance for a given system, but the alternative influence ofpipeline bore is more likely to be considered when designing a new system. Pipe-line bore will be considered as a separate issue later in this chapter.

Similar specific energy data for the polyethylene pellets is presented in Fig-ure 7.12. This follows a similar pattern to that of the alumina once again. At verylow values of air flow rate specific energy values are low, but material flow ratesare also low and so a much larger bore pipeline would probably be needed toachieve the desired material flow rate.

3 INFLUENCE OF PIPELINE BORE

Pipeline bore has a major influence on conveying capacity, as has been mentionedbefore. The influence of pipeline bore on conveying rate is reasonably predictableand so to illustrate the influence that pipeline bore can have two further materialshave been selected for this purpose. One is a very fine grade of dicalcium phos-phate, which is capable of being conveyed in dense phase. The other is a coarsegrade of magnesium sulfate which can only be conveyed in dilute phase in a con-ventional conveying system. Both materials were conveyed through a 310 ft longpipeline of three inch nominal bore having nine 90° bends. A sketch of the pipe-line is given in Figure 7.13 for reference.

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Material Conveying 221

Pipeline:LengthBoreBendsD/d =

310ft3 in nominal9x90°16

Figure 7.13 Sketch of three inch bore pipeline.

Conveying data for the dicalcium phosphate and the magnesium sulfate con-veyed through the above pipeline are presented in Figure 7.14.

Solids LoadingRatio

Conveying LinePressure Drop

- lbf/in2

NO GO AREA 25^* , ] 0

20-

(a)

0 100 200 300 400

Free Air Flow Rate - ftVmin(b)

0 100 200 300 400Free Air Flow Rate - ft3 / min

Figure 7.14 Conveying characteristics for materials conveyed through the pipelineshown in figure 7.13. (a) Dicalcium phosphate and (b) magnesium sulfate.

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222 Chapter 7

From Figure 7.14a it will be seen that the dicalcium phosphate could be con-veyed in dense phase and solids loading ratios up to about 120 were achieved. Themagnesium sulfate, however, had no dense conveying capability, the maximumvalue of solids loading ratio achieved was about 12, and the minimum value ofconveying air velocity was about 2500 ft/min.

It will also be noticed that conveying line inlet air pressures up to 30 lbf/in2

were used for both materials. The material flow rates achieved, however, werevery different and so a reduced scale has been used for the magnesium sulfate. 400fWmin of free air was available for conveying and it will be seen that within thislimit the maximum pressure that could be used for conveying was about 30 IbfVin2.Although the same horizontal axis has been used for both materials, it could wellhave been halved for the dicalcium phosphate.

3.1 Scaling Parameters

To illustrate the influence of pipeline bore on conveying capability, the conveyingdata presented in Figures 7.14a and b will be scaled to larger bore pipelines. Toisolate the influence of pipeline bore the length and geometry of the pipeline willremain the same in each case considered.

For the scale up of the conveying characteristics in respect of pipeline bore,the change in datum for the empty line will have to be taken into account. Thisprocess was considered earlier in Chapter 6 with Figure 6.5. For reference pur-poses a similar plot is presented in Figure 7.15, specifically for the Figure 7.13pipeline.

10

^ 8<4-

£

I

QJJ3c/s A

Cu

1 2l-

<

0

PipelineBore - i

0 100 200 300 400 500

Free Air Flow Rate - ftVmin x (d,/3)2

600

Figure 7.15 Influence of pipeline bore and air flow rate on empty pipeline pressuredrop for figure 7.13 pipeline.

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Material Conveying 223

The variation of pressure drop with air flow rate for the three inch bore pipe-line is included and so the change in datum can be obtained by taking the differ-ence between the three inch and the required bore of pipeline. It will be seen fromthis that the air only pressure drop element reduces significantly with increase inpipeline bore. For a given conveying line inlet air pressure this means that thepressure drop available for conveying material will increase slightly with increasein pipeline bore, and so it will be possible to convey more material as a conse-quence.

3.1.1 Scaling Model

Scale up of material flow rate, m „ , with respect to pipeline bore, d, can be carried

out with a reasonable degree of accuracy, if the extrapolation is not too great, onthe basis of pipe cross-sectional area, A:

mp x A oc d2 - - - - (6)

or alternatively:

m , m i

- - - c o n s t (7)

3.1.1.1 Working ModelThe working form of this scaling model is:

2x - Ib/h --- ..... (8)

where subscripts 1 and 2 relate to the appropriatepipe bores of the two pipelines

It is for this reason that the air flow rate axis on Figure 7.15 is in terms of airrequired for the three inch bore pipeline x (d2/3) . Conveying air velocities scaleup exactly and so a common axis can be used. For scaling up of the characteristicsin Figures 7.14a and b to larger bore pipelines the datum pressure drop should firstbe changed throughout by the appropriate values obtained from Figure 7.15. Mate-rial flow rates for a given air flow rate and pressure drop are then scaled in theratio of (d2/3)2.

The results of scaling the data in Figures 7.14a and b to larger bore pipelinesare presented in Figure 7.16 and 7.17. Scaling in each case has been carried out for4, 5, 6 and 8 inch bore pipelines.

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224 Chapter 7

.100

200 400 600

(a)Free Air Flow Rate - ft/min

200 80 6040

(b)

0 200 400 600 800 1000

Free Air Flow Rate - ft'/min

10030-^ -80 60

40

0 - , , , . ' , , , , , ,

20

0

(c)

0 400 800 1200 1600Free Air Flow Rate - ftVmin

(d)

0 1000 2000Free Air Flow Rate - ft3/min

Figure 7.16 Conveying characteristics for dicalcium phosphate in various bore pipe-lines relating to figure 7.13. (a) 4 inch bore, (b) 5 inch bore, (c) 6 inch bore, and (d) 8 inchbore pipeline.

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Material Conveying 225

100

,, § 8014 o

(a)

200 400 600Free Air Flow Rate - ftVmin

(b)

200 400 600 800 1000Free Air Flow Rate - ftVmin

800 1200 1600

Free Air Flow Rate - ft/min(C)

250

200

150

100

(d)

1000 2000

Free Air Flow Rate - ft/min

Figure 7.17 Conveying characteristics for magnesium sulfate in various bore pipelinesrelating to figure 7.13. (a) 4 inch bore, (b) 5 inch bore, (c) 6 inch bore, and (d) 8 inch borepipeline.

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226 Chapter 7

3.2 Scaling to Larger Bores

The scale up in terms of pipeline bore produces a set of curves that are basicallygeometrically similar for both materials, apart from the slight change due to theshift in datum for the empty line pressure drop relationship. There is, therefore,little difference in minimum conveying conditions for different pipeline bores,since similar solids loading ratios result. Air flow rates are totally different, ofcourse, as these have been scaled up in proportion to the pipeline cross-sectionalarea.

As pipeline bore increases there will be a need to increase the minimumvalue of conveying air velocity slightly because of the boundary layer effect. Asthe pipeline bore increases, the low velocity area in the boundary layer also in-creases and an increase in conveying air velocity is required to compensate and soprevent saltation.

The design department of a company installing a pneumatic conveying sys-tem are unlikely to go through this detailed process of scaling. They will knowwhat type of system they wish to supply and so will scale one or two data pointsonly. The detail is included here to illustrate the global changes, and to show howpipeline bore can influence the design and specification decisions.

If a range of pipeline bores is considered for a given material flow rate, theconveying line pressure drop required will decrease, and the air flow rate will in-crease, with increase in pipeline bore. This means that the pressure capability ofthe feeding device will reduce, but the size of the filtration plant will increase.

To illustrate the influence of pipeline bore on system design parameters, ma-terial flow rates of 80,000 Ib/h for the dicalcium phosphate and 25,000 Ib/h for themagnesium sulfate have been considered. Data has been taken from the varioussets of conveying characteristics presented and that for the dicalcium phosphate ispresented in Table 7.1.

Table 7.1 Conveying Parameters for 80,000 Ib/h of Dicalcium Phosphate

Pipeline

Bore

in

3

4

5

6

8

Air Inlet

Pressure

Psig

33

21

13

9

6

Free Air

Flow Rate

cfm

145

170

205

290

740

Solids

Loading

Ratio

-

110

102

85

60

24

Conveying Air Velocity

At Inlet

ft/min

800

800

800

900

1500

At Outlet

ft/min

2610

1950

1500

1480

2120

Power

Required

hp

22

19

17

18

32

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Material Conveying 227

Table 7.2 Conveying Parameters for 25,000 Ib/h of Magnesium Sulfate

Pipeline

Bore

in

3

4

5

6

8

Air Inlet

Pressure

psig

31

19

12

8

5

Free Air

Flow Rate

cfm

520

600

740

930

1400

Solids

Loading

Ratio

-

10-5

9-1

7-4

5-9

3-9

Conveying Air Velocity

At Inlet

ft/min

3000

3000

3000

3000

3000

At Outlet

ft/min

9370

6870

5420

4730

4010

Power

Required

hp

75

64

57

54

52

Similar data for the magnesium sulfate is presented in Table 7.2.

3.2.1 Influence on Pressure

These tables show that there is a wide range of air supply pressure and pipelinebore combinations that are capable of meeting any given duty for a material. Toillustrate the point with regard to the influence of pipeline bore on air supply pres-sure, the data from Tables 7.1 and 7.2 is presented graphically in Figure 7.18.

30

I01

* 20

3

•3 10

80,000 Ib/h ofDicalcium Phosphate

25,000 Ib/h ofMagnesium Sulfate

5 6Pipeline Bore - inch

Figure 7.18 Typical air pressure - pipeline bore relationships for conveying duties.

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228 Chapter 7

Figure 7.18 clearly shows that there is generally no one specific set of de-sign parameters for a pneumatic conveying system. With a wide range of pipelinebore and air supply pressure combinations being capable of achieving a given ma-terial flow rate, the obvious question is which pipeline bore or air supply pressureresults in the most economical design? Plant capital costs could vary considerably,for with different pipeline bore and air supply pressures there are correspondingdifferences in feeder types, filtration requirements and air mover types, apart fromwidely different pipeline costs, and so a major case study would need to be carriedout. Power requirements, and hence operating costs, however, are largely depend-ent upon the air mover specification and so these can be determined quite easily byusing Equation 3.6.

3.2.2 Power Requirements

The approximate power requirements for the cases considered are given in Tables7.1 and 7.2, and they are presented graphically in Figure 7.19. In most cases thepower required for the air mover represents the major part of the total systempower requirements, although for screw pumps a major allowance must be madefor the screw drive.

Figure 7.19 presents interesting trends for both materials considered. This isapart from the displacement of the curves, for very different conveying duties, butthis is primarily due to the fact that the magnesium sulfate could not be conveyedin dense phase.

80 I-

60

3cre* 40

o

20

25,000 Ib/h ofMagnesium Sulfate

80,000 Ib/h ofDicalcium Phosphate

5 6Pipeline Bore- inch

Figure 7.19 Influence of pipeline bore on power requirements for given conveyingduties.

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Material Conveying 229

For the dicalcium phosphate all of the smaller bore pipelines give reasona-bly low values of power requirement. This is because the material is conveyed indense phase in each case. It is only with the eight inch bore pipeline that there is amarked reduction in solids loading ratio and the power requirements start to risesteeply.

For the magnesium sulfate there is a gradual reduction in power require-ments with increase in pipeline bore. This is essentially due to the change in con-veying line exit air velocity. The minimum conveying air velocity for this materialis about 2500 ft/min and so a conveying line inlet air velocity of 3000 ft/min hasbeen taken in every case.

The minor influence of pipeline bore on minimum conveying air velocityhas not been taken into account in this case. Since all the pipelines considered aresingle bore, the conveying line exit air velocity is extremely high for the smallbore pipeline options, and this has a significant effect on pressure drop and henceconveying capability.

3.2.2.1 Stepped PipelinesFor the small bore pipeline/high pressure cases considered, stepped pipelineswould generally be recommended for both the dicalcium phosphate and the mag-nesium sulfate. This would have the effect of reducing the air supply pressureneeded, and hence the power required, for the smaller bore pipeline options.

In the case of the magnesium sulfate it would have the effect of making thepower requirement curve almost into a horizontal line at about 55 hp. For the di-calcium phosphate it would probably reduce the power requirements for all thesmall bore pipelines to about 15 hp. Chapter 9 of this Handbook is devoted en-tirely to stepped pipeline systems

4 INFLUENCE OF CONVEYING DISTANCE

Conveying distance also has a major influence on conveying capacity. If convey-ing distance is increased, the material flow rate will decrease, for the same convey-ing line inlet air pressure. If the air supply pressure is increased, and the air flowrate is also increased, to cater for the compressibility effect, it will be possible toachieve the same material flow rate. Increasing the air supply pressure, however, israrely an option.

The influence of conveying distance on conveying rate is reasonably pre-dictable and so to illustrate the influence that pipeline length can have, the datapresented in Figures 7.16d and 7.17d have been selected for this purpose. Theseare the conveying characteristics for the dicalcium phosphate and the magnesiumsulfate conveyed over 310 feet. This relates to the Figure 7.13 pipeline but havinga bore of eight inches. These two materials have been chosen once again becausethe influence of conveying distance is different between materials that are capableof being conveyed in dense phase and those that can only be conveyed in dilutephase.

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230 Chapter 7

4.1 Scaling Parameters

To illustrate the influence of conveying distance on conveying capability the setsof conveying data presented in Figures 7.17d and 7.18d are taken as the referencepoints and are scaled to longer length pipelines. For the scale up of the conveyingcharacteristics in respect of pipeline length, the change in datum for the empty linewill have to be taken into account.

This process was considered in Chapter 6 with Figure 6.4. For referencespurposes a similar plot is presented in Figure 7.20 for the conveying distances tobe considered.

The variation of pressure drop with air flow rate for the 310 ft long pipelineis included so that the change in datum can be obtained by taking the differencebetween the 310 ft and the required length of pipeline. It will be seen from this thatthe air only pressure drop element increases with increase in pipeline length. For agiven conveying line inlet air pressure this means that the pressure drop availablefor conveying material will reduce slightly with increase in pipeline length. Thismust be taken into account, as well as the influence of conveying distance on con-veying capability.

4.1.1 Scaling Model

Scale up of material flow rate, mp , with respect to conveying distance, L, can be

carried out with a reasonable degree of accuracy, if the extrapolation is not toogreat, on the basis of a reciprocal law model:

10

a.o

o 2

Pipeline Length -2500 1500

1000

310

1000 2000

Free Air Flow Rate - frVmin

3000 4000

Figure 7.20 Influence of pipeline length and air flow rate on empty pipeline pressuredrop for 8 inch bore pipeline.

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Page 243: Handbook of Pneumatic Conveying Engineering

Material Conveying 231

m oc — - - (9)P TA

or alternatively:

mplLc] = mp2Le2 = Const. . . . . . . . . (io)

For a constant air flow rate and pressure dropdue to the conveyed material.

where /»„ = mass flow rate of material

and Le = equivalent length of pipeline

Conveying distance, L, is expressed in terms of an equivalent length, Le, ofthe total pipeline. This comprises the three main elements of the pipeline routingand geometry. One is the length of the horizontal sections of pipeline, the secondis the length of vertically up or down sections of pipeline, and the third relates tothe bends in the pipeline. Horizontal pipeline is taken as the reference for equiva-lent length. The influence of distance, therefore, will ultimately depend upon therouting of the pipeline. For this exercise, to illustrate the typical influence of con-veying distance as a variable, the pipeline geometry in Figure 7.13 has been used.

4.1.1.1 Working ModelThe working form of this scaling model is:

m -, = mp]x-^ Ib/h - - - - - - - - ( i i )A2

where subscripts 1 and 2 relate to the appropriatelengths of the two pipelines

4.2 Scaling to Longer Distances

In this exercise the two materials are considered separately. With pipeline boreboth the air flow rate and material flow rate axes were scaled by the same parame-ter and so the results were approximately geometrically similar. For conveyingdistance only one of the axes has to be changed and this has a considerable distort-ing effect with regard to materials capable of dense phase conveying.

4.2.1 Magnesium Sulfate

The conveying characteristics for the sodium sulfate conveyed through the 310 ftlong pipeline of 8 in bore were presented in Figure 7.17d. Results of scaling tolonger length pipelines of 8 in bore are presented in Figure 7.21.

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232 Chapter 7

(a)

0 1000 2000

Free Air Flow Rate - ftVmin(b)

0 1000 2000

Free Air Flow Rate - ftYmin

1000 2000

(C) Free Air Flow Rate - ft/min

1000 2000

(a) Free Air Flow Rate - ft/min

Figure 7.21 Conveying characteristics for magnesium sulfate in pipelines of increasinglength, (a) 600 foot, (b) 1000 foot, (c) 1500 foot, and (d) 2500 foot pipeline.

Since there is no change in pipeline bore, and the same range of air supplypressures is considered, there is no change in the air flow rate axis for any of thefour conveying characteristics presented in Figure 7.21. The changes all relate tothe material flow rate axis, and hence also to the solids loading ratio values.

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Material Conveying 233

Over 310 ft, in Figure 7.17d, 180,000 Ib/h of material would be conveyedand the solids loading ratio would be about 15, with a conveying line pressuredrop of 30 lbf/in2. This is entirely dilute phase, suspension flow, as explained ear-lier, and the minimum conveying air velocity is about 2500 ft/min, almost regard-less of air supply pressure and conveying distance.

With the distance almost doubled to 600 ft in Figure 7.2la, and the scalingmodel being an inverse law relationship, it would be expected that the materialflow rate would drop to about half, for the same air supply pressure. It will be seenthat the material flow rate has, in fact, dropped to about 88,000 Ib/h. This slightreduction on half is mostly due to the increase in air only pressure drop, whichleaves less pressure available for the conveying of material.

If the conveying line pressure drop had been doubled to about 60 lbf/in2, inorder to compensate, and so maintain the same pressure gradient, a material flowrate close to 180,000 Ib/h would have been achieved. This is provided that the airflow rate was also increased in order to compensate for the compressibility effectof the air and thereby maintain 2500 ft/min as the minimum velocity.

It must be emphasized that if the conveying distance is doubled, the materialflow rate must be halved for the system to work within the capability of the sameair supply pressure, as illustrated with Equation 10. Double the distance for thesame material flow rate equates to double the energy required. This applies to bothdilute and dense phase conveying.

If a conveying system is extended to supply a storage silo that is furtheraway, a lower material flow rate must be expected. If a system has to supply anumber of silos at varying distances, by means of diverter valves, it is most impor-tant that this fact is taken into account. If there is no control over material feedrate, therefore, all silos will have to be fed at the lowest flow rate, corresponding tothe furthest silo, and so conveying to the nearest silo will be very inefficient.

With an extension in conveying distance to 1000 ft the maximum value ofmaterial flow rate drops further to about 52,000 Ib/h. If a much higher flow ratewere to be required over this distance there would be little option but to increasethe pipeline bore. Over a distance of 2500 ft the material flow rate drops to about18,000 Ib/h and it will be seen that the solids loading ratio is now only about l'/2which is very dilute phase. Conveying over this and very much longer distances,however, is possible and very much higher material flow rates can be achieved,but power requirements are relatively high.

4.2.2 Dicalcium Phosphate

Because of the changes that occur with materials capable of dense phase convey-ing, the reference conveying characteristics for the dicalcium phosphate conveyedover 310 ft from Figure 7.16d have been reproduced in landscape form in Figure7.22 in order to illustrate the nature of the changes more clearly for this type ofmaterial. Because the pipeline bore and air supply pressures remain the same inthis procedure there is essentially no change in air flow rate needed to maintain thesame conveying line inlet air velocity.

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234 Chapter 7

Conveying Line PressureDrop - lbf/in2

\

Solids LoadingRatio

500

400

~ 300

200

100

100

30.'.

20 1

1000 2000

Free Air Flow Rale - ftVmin

3000

Figure 7.22 Conveying characteristics for dicalcium phosphate conveyed over 310 ftin 8 in bore pipeline.

With a reduction in material flow rate, however, there will be a change insolids loading ratio, as was clearly illustrated in Figure 7.21 with the magnesiumsulfate. For powdered materials that can be conveyed in dense phase, however, theminimum value of conveying air velocity is influenced quite significantly by thevalue of the solids loading ratio. This concept was introduced in Chapter 4 withFigure 4.6.

The relationship between minimum conveying air velocity and solids load-ing ratio for dicalcium phosphate is presented on Figure 7.23. That for the magne-sium sulfate is also included on Figure 7.23 for reference and comparison. FromFigure 7.22 it will be seen that very high values of solids loading ratio wereachieved, and because the conveying distance was relatively short, solids loadingratios of almost 100 were achieved with a conveying line pressure drop of only 10lbf/in2. As conveying distance increases, however, and material flow rate de-creases according to an inverse law relationship, solids loading ratios reduce quitedramatically.

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Material Conveying 235

3000 h.Magnesium Sulfate

Dicalcium Phosphate

0 20 80 10040 60

Solids Loading Ratio

Figure 7.23 Minimum conveying air velocity relationships for materials used.

Conveying characteristics for the Dicalcium Phosphate conveyed over a dis-tance of 600 ft through the 8 inch bore pipeline are presented in Figure 7.24.

250

200

100

50

0

80 60 40so;

0 1000 2000 3000

Free Air Flow Rate - fiVmin

Figure 7.24 Conveying characteristics for dicalcium phosphate over 600 feet.

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236 Chapter 7

With an almost doubling in conveying distance to 600 ft there is a corre-sponding halving in material flow rate capability, and hence a similar reduction insolids loading ratio. The maximum value of solids loading ratio is now well below100 and only with a conveying line pressure drop of 20 lbf/in2 is the solids loadingratio above a value of about 70. For pressures below 20 lbf/in2 there is a dramaticchange in the conveying characteristics.

In this region the pressure gradient available is not high enough to supporthigh solids loading ratio conveying and much of the area around a pressure drop of10 lbf/in2 has changed entirely to dilute phase suspension flow. As a consequencethe air flow rate required to convey with a conveying line pressure drop of 10lbf/in2 changes from about 400 ftVmin over a distance of 310 ft to about 1200ftVmin over a distance of 600 ft.

It is the relationship between minimum conveying air velocity and solidsloading ratio in Figure 7.23 that dictates these changes. As the distance increases,the material flow rate decreases and hence the solids loading ratio also decreases.With a decrease in solids loading ratio below about 90 there will have to be anincrease in conveying air velocity. In order to increase velocity there must be anincrease in air flow rate. If there is an increase in air flow rate there will be a corre-sponding reduction in solids loading ratio.

This is a slowly converging cycle and explains why, for a pressure drop of10 lbf/in2, the air flow rate required can increase by a factor of three for a doublingin conveying distance. Extreme caution must be exercised in the design of densephase conveying systems in the region where conveying line pressure gradientsare in the region of 4 to 8 lbf/in2 per 100 ft of pipeline, particularly if operatingclose to the minimum value of conveying air velocity, for a reduction in materialflow rate could result in pipeline blockage. This aspect of system operation is con-sidered in more detail in Chapter 19.

This entire process is repeated, but at higher values of air supply pressure,with the extension of the pipeline to 1000 ft in Figure 7.25. From Figure 7.25 itwill be seen that there is no dense phase conveying capability over this distance atall. The minimum conveying air velocity is about 2100 ft/min for all pressuresconsidered. The transition is still there, but at higher pressures. At a pressure ofabout 45 lbf/in2 the material could be conveyed with a very low air flow rate andat low velocity. In this case the transition to dilute phase at 30 lbf/in2 would beeven more dramatic, but in terms of ratios of air flow rates it would be about threeto one again.

With further increase in conveying distance the changes are no differentfrom those for the magnesium sulfate in Figure 7.21. Conveying is only in dilutephase and so there are no further changes in air flow rate. Conveying characteris-tics for the dicalcium phosphate conveyed over 1500 ft are given in Figure 7.26.

The influence of conveying distance on material flow rate is illustrated inFigure 7.27. The maximum material flow rate achieved through an eight inch borepipeline with a conveying line pressure drop of 30 lbf/in2 has been taken as thebasis for both the dicalcium phosphate and the magnesium sulfate. The differencein conveying capability between the two materials is typical of the differences thatcan exist between different materials, as discussed in Chapter 4.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Material Conveying 237

160

o

§ 120

I 80

_gE

40C3

I I I 4 I I I I I I I I

0 1000 2000 3000Free Air Flow Rate - ftVmin

Figure 7.25 Conveying characteristics for dicalcium phosphate over 1000 feet.

100

80

60

40

20 •

0

30

I I I f I I ! I I I I

0 1000 2000 3000Free Air Flow Rate - ft3/min

Figure 7.26 Conveying characteristics for dicalcium phosphate over 1500 feet.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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238 Chapter 7

ooo

I

a!_ou,."§

500

400

300

200

100

DicalciumPhosphate

Air Supply Pressure = 301bf/in2|Pipeline Bore = 8 inch

500 1000 1500

Conveying Distance - feet

2000 2500

Figure 7.27 Influence of conveying distance on material flow rate for materials andconveying conditions considered.

This is approximately an inverse law relationship for both materials and so itwill be seen that changes are particularly pronounced over shorter conveying dis-tances. Conveying distance, therefore, is an important parameter to take into ac-count when designing a conveying system. It is even more important if materialsare required to be conveyed over a range of distances with a common conveyingsystem.

For the dilute phase conveying of materials little change in conveying air ve-locity is required with change in distance. For materials capable of being conveyedin dense phase, however, the specification of air flow rate is particularly important.Because low velocity dense phase conveying requires a relatively high pressuregradient, and because high pressure air is not convenient to use in systems thatexhaust to atmospheric pressure, the possibility of dense phase conveying rapidlyreduces with increase in distance.

This transition from dense phase to dilute phase conveying is illustrated forthe dicalcium phosphate in an eight inch bore pipeline in Figure 7.28. The verticalaxis is that of material flow rate, but scaled by the inverse law relationship withrespect to conveying distance. The horizontal axis is that of free air flow rate in aneight inch bore pipeline. Approximate lines of constant conveying line pressuredrop are also included. These are only approximate locations for reference sincetheir location will shift slightly with respect to conveying distance.

The sloping line at low air flow rate corresponds to a conveying line inlet airvelocity of about 600 ft/min and so represents the minimum conveying limit forthe dense phase conveying of the dicalcium phosphate.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Material Conveying 239

J 500or<->

^400'o

| 300

& 200

_oE 100cd

900 ft +

500 1000 1500

Free Air Flow Rate - fr/min

2000

Figure 7.28 Influence of conveying distance on air flow rate required for conveyingdicalcium phosphate.

Conveying down to this limit is possible with a high pressure gradient, typi-cally above about 10 lbf/in2 per 100 ft length of pipeline. This means that convey-ing in this region is possible with either a high conveying air pressure or with ashort conveying line.

The sloping line at high air flow rates corresponds to a conveying line inletair velocity of about 2100 ft3/min and so represents the minimum conveying limitfor the dilute phase conveying of the dicalcium phosphate. This is the minimumlimit for conveying if the conveying distance is long or the pressure available forconveying is low. These, of course, are relative terms, but Figure 7.28 illustratesthe situation with regard to dicalcium phosphate. Other materials, capable of beingconveyed in dense phase and at low velocity, will follow very similar patterns.

When conveying data for a material is extended down to the air only pres-sure drop datum, and hence zero material flow rate, as with the conveying charac-teristics presented here, most of the materials capable of being conveyed in densephase will include the transition from dense to dilute phase. That for the 310 ftlong pipeline starts the transition at a pressure of about 10 lbf/in and that for the600 ft long pipeline starts at about 20 lbf/in2. For pipelines above about 900 ft longthe transition occurs at a pressure above about 30 lbf/in . The transition generallyoccurs over a relatively narrow band of pressure drop values.

Conveying in the region between these two limits is perfectly safe, stableand viable. It is dense phase conveying. If changes in operating conditions with asystem, however, such as distance, pressure and material flow rate, result in theoperating point being close to the conveying limit that links the 600 ft/min and

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240 Chapter 7

2100 ft/min limit lines, the system could become unstable and likely to block thepipeline [1].

5 OTHER PIPELINE FEATURES

It was mentioned earlier, in relation to Equation 7.10, that the equivalent length ofpipeline comprised a number of elements and that horizontal conveying distancewas just one element. The other elements include vertical sections of pipeline andpipeline bends and these will be considered in the next chapter.

REFERENCE

1. D. Mills. An investigation of the unstable region for dense phase conveyingin sliding bed flow. Proc 4th Int Conf for Conveying and Handling of Particu-late Solids. Budapest. May, 2003.

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Page 253: Handbook of Pneumatic Conveying Engineering

8Pipeline Material, Orientation, andBends

1 INTRODUCTION

A major advantage that pneumatic conveying systems have over alternative me-chanical conveying systems is in flexibility in continuous pipeline routing. Pipe-lines can run horizontally, and with bends in the pipeline, flows can go verticallyup or vertically down, with little restriction on numbers of bends or distances.Pipelines inclined upwards are not generally recommended and so flow in inclinedpipelines is examined.

Up to now pressure gradient has been discussed in global terms of pressuredrop available and distance over which a material must be conveyed, with highpressure gradients being required for dense phase conveying. Data is included inthis chapter to show how pressure gradient varies with conveying parameters forhorizontal and vertical conveying in both dilute and dense phase flows.

Conveying parameters were introduced in the previous chapter for pipelinebore and conveying distance. In this chapter scaling parameters are presented forother pipeline features including vertical flow. The influence of conveying pa-rameters on pressure drop across bends is considered, for both dilute and densephase flow, and losses are presented in terms of both a pressure drop and anequivalent length. Pipeline material is also considered, with particular reference tothe use of flexible rubber hose.

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242 Chapter 8

2 PRESSURE GRADIENT DATA

All the conveying data presented so far has been for total pipeline systems. This isusually obtained from a test facility comprising a pipeline test loop that generallyincludes horizontal pipeline lengths, a number of bends and possibly an element ofvertical lift. The pressure drop data in the conveying characteristics presented hasbeen for the entire pipeline.

In order to isolate the effect of any individual element of pipeline, such as astraight section of horizontal or vertical pipeline, or a bend in the pipeline, pres-sure tappings must be fitted into the pipeline. Some of these issues are consideredin this chapter but are considered in more detail in Chapter 23.

2.1 Horizontal Conveying

Typical conveying data for flow in a horizontal section of pipeline is presented inFigure 8.1. The data is for barite, which is often used as a drilling mud powder.This material has a particle density of about 260 lb/ft3 but despite this it will beseen that the material could be conveyed at solids loading ratios in excess of 100and at low velocity. For drilling purposes it is used as a very fine powder and sohas very good air retention properties in this form. As a consequence of the airretention properties the material will convey in dense phase flow.

60 h

40

o

a

20

0

Solids Loading Ratio 120 JOO

40

20

'ressure GradientIb t7 in 2 per l00f t

0 50 100 150

Free Air Flow Rate - ftVmin

Figure 8.1 Pressure gradient in horizontal flow for barite in 2 inch bore line.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Material, Orientation, and Bends 243

The data in Figure 8.1 is presented in exactly the same form as the convey-ing characteristics, with material flow rate in Ib/h plotted against free air flow ratein fVVmin. The family of curves plotted is now of pressure gradient in lbf/in2 per100 ft length of pipeline rather than a pressure drop for the total pipeline. Lines ofconstant solids loading ratio are also included as these are simply straight linesthrough the origin as before. The juxtaposition of these two sets of curves on theone plot is particularly useful for illustrating once again the problem of maintain-ing flow in dense phase with increase in conveying distance.

2.1.1 Long Distance Conveying

As expected, it will be seen that as the solids loading ratio increases, the pressuregradient increases. At a solid loading ratio of about 100 the pressure gradient isapproximately 10 lbf/in2 per 100 ft length of pipeline. With a limit on air supplypressure because of air expansion problems, and the consequent need to step thepipeline, the scope for long distance dense phase flow is strictly limited. This doesnot take account of the additional pressure drop due to bends and sections of verti-cally upward pipeline that might need to be included either.

For longer distance conveying there must be a compromise and this is toconvey at a lower value of solids loading ratio where the pressure gradient islower. In Figure 7.2Id, magnesium sulfate conveyed over a distance of 2500 ft ispresented and the maximum value of solids loading ratio, with a conveying linepressure drop of 30 lbf/in2, is only about 1 '/2.

3 VERTICAL CONVEYING

Apart from the difficulty of finding a suitable wall or structure on which to mounta vertical pipeline for testing purposes, a test loop needs to be used, unless twoconveying systems are available, one conveying to the other. The former was usedfor the test work reported here [1]. An advantage of this method is that the pipelinemust go down as well as up and so data can be obtained for both sections of pipe-line in every test run.

A sketch of the test pipeline is given in Figure 8.2 together with dimensionaldetails. A high pressure top discharge blow tank was used to feed material into thepipeline. The layout of the test facility was such that the material was conveyedvertically down first and then vertically up. The fall and rise elements of the pipe-line were both 53 ft long. The total pipeline length was about 185 ft. Two pipe-lines were available; one of two inch and another of three inch nominal bore, bothfollowing an identical routing.

Typical conveying characteristics for the total pipeline system are presentedin Figure 8.3 [2]. These are for barite conveyed through the three inch bore pipe-line. Barite can be conveyed in dense phase, as was illustrated in Figure 8.1 and soconveying with air supply pressures up to 30 lbf/in2 was possible and solids load-ing ratios of well over 100 were achieved.

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244 Chapter 8

ooo

120

80

BendNumber

Figure 8.2 Details of test pipelineused for vertical conveying.

E 40

0

X3 0

'onveyingLine PressureDropIbf/in2

20

10

SolidsLoadingRatio

0 100 200 300 400

Free Air Flow Rate - ft3/min

Figure 8.3 Conveying characteristics forbarite in figure 8.2 pipeline of 3 inch bore.

Once again this illustrates the conveying potential of relatively small borepipelines in that material flow rates of over 100,000 Ib/h were achieved. For thetotal pipeline the form of the conveying characteristics is little different from thatfor other pipeline systems presented in earlier chapters.

In order to obtain pressure gradient data for the two test sections there were15 pressure tappings (seven along the down section and eight along the up sec-tion). The first and last tappings at each section were placed about five feet fromthe bends in order to ensure that any upstream or downstream effects would haveminimum influence on the pressure readings. At each location a ring of four tap-pings was used and all four were coupled to a common point.

Results from two tests carried out with a fine grade of pulverized fuel ash(fly ash) in the two inch bore pipeline are presented in Figure 8.4 [2, 3]. The hori-zontal axis represents the length of pipeline (see Figure 8.2) from the bend inwhich the flow is horizontal to vertically down (bend number 4), to the bend inwhich the flow is vertically up to horizontal (bend number 7).

The first section, therefore, represents the flow vertically down, along whichthere were seven pressure tapping locations, and the second section represents theflow vertically up, along which there were eight pressure tapping locations. Thevertical axis represents the pressure of the conveying air.

The solid lines drawn represent the linearized dependence, from the meas-ured values of pressure, while the dotted lines represent an approximate develop-ment of the pressure in the region where the pressure was not measured.

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Material, Orientation, and Bends 245

Ofl3taeo

20 \-

16

12

•OC

3 4

-a T3c cD U

CQ CQ 8 9 10 11 12 13 14

I

40 60 80 100 120Distance - feet

140 160

Figure 8.4 Typical pressure gradient results obtained with pulverized fuel ash.

It will be noticed that in one case the pressure gradient in the verticallydown section was negative, while in the other it was positive. In the vertically upflow the pressure gradient was negative in each case, but there was a significantdifference between the two tests. The influence that conveying conditions canhave on the values of pressure gradient are considered below.

3.1 Flow Vertically Up

Pressure gradient data obtained in this way for the vertically upward flow of baritein the three inch bore pipeline is presented in Figure 8.5. The barite was conveyedover a wide range of both air and material flow rates, and some forty to fifty indi-vidual tests had to be carried out in order to provide the necessary pressure gradi-ent data to obtain the plot or performance map shown in Figure 8.5. Once againsolids loading ratios well in excess of 100 were achieved with this material.

The data is plotted in terms of a pressure gradient in lbf/in2 per 100 feet ofvertically up pipeline. If the data is compared with that for the horizontal pipelinein Figure 8.1, which also relates to the conveying of barite, it will be seen that thepressure gradient values are significantly higher.

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246 Chapter 8

140 120 100

120

80

E 40

"c3

Solids Loading Ratio

PressureGradient - '"'

M/in 2 per lOOft

24

32

40

20

100 200

Free Air Flow Rate - ftVmin

300

Figure 8.5 Pressure gradient data for barite conveyed vertically up in 3 inch bore pipe-line.

Material flow rates are also very much higher but this is because the data isfor a larger bore pipeline. It is by comparing sets of data such as this that scalingparameters can be determined, but ideally they need to be of the same bore pipe-line, and so this is considered later in this chapter.

The data can also be compared with the conveying characteristics presentedin Figure 8.3. Figure 8.3 was generated from exactly the same test program as thatfor the data in Figure 8.5. One was plotted from the total pipeline pressure dropdata and the other from the pressure gradient data derived from the pressure tap-ping readings.

A comparison of the two will show that the slope of the pressure gradientlines on Figure 8.5 is very different from the slope of the lines of constant pressuredrop on Figure 8.3. Figure 8.5 is for the vertically upward section of pipeline inisolation, while Figure 8.3 is for the total pipeline system, including nine bends.The influence of bends is also considered later in this chapter.

Two additional sets of data are included to reinforce the nature of the curves.These are for cement and fly ash, both conveyed vertically up through a two inchbore pipeline. There are clearly differences between the two sets of data but fol-lowing the comparative data presented in Chapter 4 this will not come as a sur-prise.

The unknown factor is how the different elements of the pipeline contributeto the overall differences observed. Data for the cement is presented in Figure 8.6aand that for the fly ash in Figure 8.6b.

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Material, Orientation, and Bends 247

60

50

£ 400)a

oE

1 20o

10

Solids Loading Ratio

Pressure 140Gradient -

lbf/in2 per 100ft

Solids Loading Ratio

60

o:o

3*30_oE320

10

Pressure• Gradient -Ibf/m1 per 100ft

\

16

12

20

(a)

0 50 100 150

Free Air Flow Rate - fVVmin (b)

50 100 150

Free Air Flow Rate - ftVmin

Figure 8.6 Pressure gradient data for flow vertically up in a two inch nominal borepipeline, (a) Cement and (b) a fine grade of fly ash.

3.1.1 Scaling Parameter

In the majority of pneumatic conveying system pipelines the proportion of hori-zontal conveying is very much greater than that of vertical conveying. A scalingparameter, therefore, is required in terms of an equivalent length of straight hori-

zontal pipeline.In order to provide a comparison between the data for conveying vertically

up and conveying horizontally, and hence to obtain the necessary scaling parame-ter, a rectangular grid was placed on the various sets of pressure gradient data. Thegrid was set at corresponding values of air and material flow rates, and the ratio ofthe pressure gradient values obtained from the vertical and horizontal data were

determined.The results of this process, carried out for barite in two inch bore pipeline,

are presented in Figure 8.7a. They are presented on the same axes, together withthe solids loading ratio lines, so that any pattern in the values with respect to con-veying conditions could be determined. From this it will be seen that the ratio ofthe pressure gradient for vertically upward flow to that for horizontal flow variesfrom a minimum of about 1-9 to a maximum of about 2-4 and that the predominant

value is about two.

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248 Chapter 8

50

o8-

3(

I20ca'Blio

(a)

40 80 120 160Free Air Flow Rate - ftVmin

(b)

40 80 120 160

Free Air Flow Rate - ftVmin

Figure 8.7 Ratio of vertical to horizontal conveying line pressure drop data for flow intwo inch nominal bore pipeline, (a) Barite and (b) fly ash.

It can be seen that the relationship obtained covers a very wide range ofconveying conditions. A similar analysis, carried out with fly ash in a two inchbore pipeline is presented in Figure 8.7b. It will be noticed that there is very littlevariation in this ratio from minimum to maximum values of conveying air velocityand from minimum to maximum values of solids loading ratio. The only deviationfrom a mean value of about two would appear to be at the two extreme limits ofthe pressure gradient curves, where the data is least reliable. This, therefore, showsthat the pressure drop in conveying vertically up is approximately double that inhorizontal conveying, for given conveying conditions, over the entire range ofconveying conditions.

3.2 Flow Vertically DownIn the majority of pneumatic conveying systems, flow vertically down usuallyoccurs only when the pipeline is routed over some obstruction such as a road orrailway line. In these cases the influence of the vertically downward section isgenerally disregarded. It is essential, however, that the additional bends requiredare taken into account.

It is with mining that long vertical pipelines come into their own, both forconveying vertically down as well as vertically up. The removal of muck from theboring of vertical mine shafts is often undertaken pneumatically. Much of the coal

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Material, Orientation, and Bends 249

mined around the world is obtained from deep mines. With mechanization of coalface operations in the 1970's the mining capability exceeded the hoisting capabil-ity of winding gear and so additional means had to be found for extracting theadditional capacity. Pneumatic conveying was widely used for this purpose andthe conveying of coal 1650 feet vertically upwards at 110,000 Ib/h was quitecommon in the 1970's [4].

Back-filling of mined out areas is generally a requirement and cement andfly ash are widely used for this purpose. These materials, therefore, are often con-veyed vertically down mine shafts. Because of the vast quantities of fly ash beingproduced around the world from power generation with coal, and the environ-mental problems associated with the material, the disposal of fly ash in this way isbeing considered more widely [3]. The longest pipelines conveying material verti-cally down are probably in South Africa. Ice is used in many deep gold mines as aheat transfer medium for cooling ventilation air. Ice making plant is located at thesurface level and the ice produced is pneumatically conveyed over distances up tothree miles, with vertically down distances up to about 7900 feet [5].

Pressure gradient data for the pneumatic conveying of cement verticallydown in the Figure 8.2 pipeline of two inch nominal bore is presented in Figure8.8. Although the form of the data is similar to that for the other pressure gradientdata presented, it will be seen that signs have been added to the values.

60

40

O

20

Solids Loading Ratio

PressureGradient -

Ibf7in2perl00

\

20

50 100Free Air Flow Rate - ftVmin

150

Figure 8.8 Pressure gradient in vertically down flow for cement in two inch nominalbore pipeline.

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250 Chapter 8

At high values of solids loading ratio the pressure gradient is negative whichmeans that there is a rise in pressure along the length of the pipeline, rather than apressure drop. Where the pressure gradient is 16 lbf/in2 per 100 ft of pipeline, forexample, it means that at the bottom of the 53 ft vertical fall in the test facility thepressure will have risen by about 8'A lbf/in2. It will also be seen that some of thepressure gradients are positive which means that there is a pressure drop along thelength of the vertical fall.

The magnitude of the pressure gradient varies with solids loading ratio, andpressure rise for the flow vertically down increases with increase in solids loadingratio. At a solids loading ratio just below about forty the pressure gradient is zero,which means that the material is conveyed with no pressure drop whatsoever un-der these conditions. At lower values of solids loading ratio there is a pressuredrop and this covers the entire range of dilute phase conveying.

Two additional sets of data are included to reinforce the nature of the curves.These are for barite and fly ash, both conveyed vertically down through the sametwo inch bore pipeline. Data for the cement is presented in Figure 8.9a and that forthe fly ash in Figure 8.9b. It will be seen that all three materials follow a very simi-lar pattern, although material flow rates differ, as might be expected. The zeropressure gradient curve is also consistent in occurring at a solids loading ratio ofabout 35 in each case.

60

50

1 40

wa* 30oE

1 20

10

(a)

Solids Loading Ratio 60

Pressui- Gradientlbf/in2 per 100 ft

140 120,100 80oo .o •

Solids Loading Ratio

^ . 140Pressure

Gradient -Ibf/in2 per 100ft

120 100

20

0 50 100 150Free Air Flow Rate - ft3 / min

£40

_oE

120

10

(b)

80

\

-2+2

0 50 100 150Free Air Flow Rate - ft3 / min

Figure 8.9 Pressure gradient data for flow vertically down in a two inch nominal borepipe, (a) Barite and (b) fly ash.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Material, Orientation, and Bends 251

For conveying vertically down, therefore, materials capable of dense phaseconveying could be conveyed very long distances with a relatively low air supplypressure. A particular advantage is that in conveying materials such as cement andfly ash down a mine shaft, the pressure generated at the bottom could be highenough to automatically convey the material to underground mine workings an-other 5000 ft distant.

A problem with this, however, is in the sizing of the pipelines, for the veloc-ity of the material at the start of the horizontal run may be too low as a result of thehigh pressure generated. This point is considered further in the next chapter onStepped Pipeline Systems.

3.3 Minimum Conveying Air Velocity

Much has been said about minimum conveying air velocities and conveying lineinlet air velocities. The data presented so far has essentially been for total pipelinesystems comprising horizontal and vertical sections of pipeline, and bends. Mini-mum conveying air velocities in vertically upward flow are lower than those forhorizontal conveying but in a mix of orientations in the one pipeline it is usuallydifficult to take the benefit of this into account and so the worst case of velocityrequirements for horizontal conveying are usually specified.

In horizontal pipelines particles that drop out of suspension, or saltate, willcome to rest on the bottom of the pipeline. With an increase in the thickness of thesaltated layer the cross sectional area will reduce and there will be a correspondingincrease in conveying air velocity. Depending upon the nature of the material thismay result in a steady equilibrium situation. More often than not, however, thesaltated layer will be formed into dunes and these will be swept up and block thepipeline, often at a bend in the pipeline.

In vertically upward flow this process is referred to as choking. When parti-cles drop out of suspension, usually in the boundary layer at first, where the veloc-ity is lowest, they will enter into free fall. At velocities at which particles will set-tle on the bottom of the pipeline in horizontal flow, particles are likely to be re-entrained in flow vertically up because of impact with other particles moving upand the general turbulence. As a consequence minimum conveying air velocitiescan be lower for vertically upward flow. In mining situations, as discussed earlier,this can be used to advantage where there will be very long runs of vertical pipe-line. Where the majority of a pipeline runs horizontal it is more difficult to takeadvantage of this fact.

4 INCLINED PIPELINES

There is little published information on the advisability of using inclined pipelines.Much of it is anecdotal, but as it is generally experiential it would generally bewise to avoid pipelines that incline upwards. An inclined section of pipeline maywell reduce the overall length of a pipeline but their use is not recommended.

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252 Chapter 8

4.1 Upward Incline

The general consensus of opinion is that pipelines inclined upwards should beavoided and that for any vertical rise, a combination of horizontal and verticalsections only should be used. The problem relates essentially to low velocity con-veying and the influence that an inclined line might have on the minimum veloc-ity. There is, of course, the additional issue of pressure drop

When saltation occurs in a horizontal pipeline, particles will be deposited onthe bottom of the pipeline, as mentioned above. In a pipeline inclined upwards,however, particles dropping out of suspension will be more mobile and will tendto roll backwards. The saltated layer will readily form dunes and these will resultin pipeline blockage

Although the minimum conveying air velocity for vertically upward flow islower than that for horizontal flow, the minimum conveying air velocity for pipe-lines inclined upward is higher than that for horizontal pipeline. If it is known thatthe velocity in an inclined section of pipeline will be high there should be no riskof blockage.

It is also understood, however, that the pressure drop in a pipeline inclinedupwards is much greater and so on this basis it would be better to keep to horizon-tal and vertical sections for any vertical rise required. The scaling parameter is onefor horizontal flow and two for vertically upward flow. At an angle of inclinationof about 60° the scaling parameter is a maximum and is slightly greater than thatfor vertically up flow at 90° [6].

4.2 Downward Incline

The mechanism of flow in downward inclined pipeline is somewhat different andso there should be little difference in minimum conveying air velocity from that inhorizontal pipelines. Saltated particles will tend to roll in the direction of flow andbe re-entrained in the gas flow, rather than form dunes, at velocities just above theminimum conveying air velocity for horizontal flow.

5 PIPELINE BENDS

Although pipeline bends provide pneumatic conveying system pipelines with theirflexibility in routing, they do have an impact on the performance of a conveyingsystem. Determining the pressure drop due to bends in a pipeline, however, is nota simple matter.

Apart from the influence of the conveyed material, the location of the bendalong the length of the pipeline and the geometry of the bend are also likely tohave an influence on the pressure drop across the bend. Data on the influence ofbends may be required as an equivalent length rather than a pressure drop value.These issues are considered as well as the general influence of bends on conveyingperformance.

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Material, Orientation, and Bends 253

5.1 Classical Analysis

The difficulties of pressure measurement in pneumatic conveying system pipelinesare highlighted most effectively with the problem of measuring the pressure dropacross a bend in a pipeline. It is not just a matter of recording the pressure at inletto and outlet from the bend and subtracting the two readings. This will give a to-tally false recording, being significantly lower than the actual value. It is necessaryto record the pressure at regular intervals along the sections of pipeline both beforeand after the bend [3].

Part of the problem lies in the complexity of the flow in the region of abend. The conveyed particles approaching a bend, if fully accelerated, will have avelocity that is about 80% of that of the conveying air. This velocity, of course,depends upon the particle size, shape and density, and the pipeline orientation. Atoutlet from a bend the velocity of the particles will be reduced and so they willhave to be re-accelerated back to their terminal velocity in the straight length ofpipeline following the bend. The situation is depicted in Figure 8.10.

The pressure drop associated with this re-acceleration of the particles, there-fore, is not registered in the bend, but occurs in the pipeline following, and so itmust be taken into account as illustrated in Figure 8.10. The method by which thetotal pressure drop associated with a bend is determined is to instrument the pipe-line before and after the bend with pressure transducers. Typical data for a whitewheat flour is shown in Figure 8.11 [7].

IApproach | Bend

I

¥Ap in Bend *

Ap in Line f IFollowing Bend1 AP Total

^-^ \ 1

Following Straight

Distance

Figure 8.10 Pressure drop elements and evaluation for bends.

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254 Chapter 8

^c<£HX>

a,k*

<

17

16

15

14

13

Conveyed Material - Flour

Slope =2-7 lbf/in2

per 10ft

I Pressure DropDue to Bend = 2-0 lbf/in2

I = 75ft

Locationof Bend

>-20 -10 0 10 20 30

Distance From Bend - ft

40 50

Figure 8.1 1 Pressure profile in straight pipeline either side of a steel bend.

The bend was tested in a two inch nominal bore steel pipeline and had abend diameter, D, to pipe bore, d, ratio of 5:1. The bend was tested in the horizon-tal plane, with the flour conveyed at a solids loading ratio of about 32. The meanparticle size of the flour was 78 micron, and the particle and poured bulk densitieswere 87 and 30 Ib/ft3 respectively.

From Figure 8.1 1 it will be seen that the total pressure drop across the bendwas about 2-0 lbf/in2. Since the pressure gradient in the straight pipeline, both be-fore and after the bend was also available, the equivalent length of the bend couldbe determined. In the case presented this equivalent length was evaluated at about75 feet. Re-acceleration of the particles may require a significant distance down-stream of the bend, particularly if the particles have a large mass and density, andsomething of the order of a dozen pressure transducers would be required, asshown.

In practical terms this pressure drop is a little high. With eight such bends ina pipeline it would require the output of a positive displacement blower just tonegotiate the bends. The conveying air velocity at the bend was about 3500 ft/minand this is almost double that necessary to convey the flour at a solids loading ratioof 32.

The data was obtained from a test on an instrumented pipeline in a labora-tory facility and is used for illustration purposes. Systems, however, are frequentlyover designed, particularly if the designers are not certain of the minimum convey-ing air velocity value, and so there is often scope for improving the performance ofexisting conveying systems as a consequence.

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Material, Orientation, and Bends 255

5.2 Comparative Analysis

An alternative, and potentially quicker, means of determining the energy loss as-sociated with bends is to compare the conveying performance of two pipelines inwhich the same material has been conveyed. Ideally both pipelines should be ofthe same bore and preferably of a similar length and contain a different number ofbends of the same geometry. By comparing the performance data of materialsconveyed in the two pipelines it is possible to determine the influence of the addi-tional bends.

Conveying data obtained with barite conveyed through two such pipelines ispresented in Figures 8.12a and b. Both pipelines tested were two inch nominalbore and all the bends in the two pipelines had a bend diameter, D, to pipe bore, d,ratio of 24:1. One pipeline was 340 feet long and incorporated nine 90° bends andthe other was 330 feet long and incorporated seventeen 90° bends.

The bends were uniformly positioned along the length of the pipelines, andthere was sufficient length of straight pipeline before every bend to ensure that thematerial was fully accelerated to its terminal velocity [8].

50

40

30

Solids LoadingRatio

„ Conveyin- Line- PressureI Drop

- Ibf7in2

20

0

|'°

0

(a)

0 50 100 150

Free Air Flow Rate - ftVmin

Solids LoadingRatio

Conveying-Line

Pressure

0

(b)

.60

0 50 100 150

Free Air Flow Rate - ft3/min

Figure 8.12 Conveying data for barite in two inch bore pipelines of approximately thesame length. Pipeline with (a) 9 bends and (b) with 17 bends.

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256 Chapter 8

Conveying characteristics for the barite in the pipeline with nine bends arepresented in Figure 8.12a and for the pipeline with seventeen bends in Figure8.12b. Barite was chosen so that a very wide range of conveying conditions couldbe examined, from low velocity dense phase to high velocity dilute phase. Theconveying data has been presented on the same axes for both pipelines, with mate-rial flow rates up to 50,000 Ib/h considered for each, and it will be seen that for thepipeline with 9 bends, conveying line inlet air pressures up to 50 lbf/in2 were em-ployed, but this had to be increased to 60 lbf/in2 for the pipeline with 17 bends.The only essential difference between the two pipelines is eight bends and so thedifference between the two sets of data can reasonably be attributed to eight bends.

With complete sets of conveying characteristics obtained for the same mate-rial conveyed through two pipelines of approximately the same length, but withdifferent numbers of bends, it should be possible to compare the results and de-termine the influence that the bends have, since the influence of pipe bore andconveying distance have been isolated. The comparison is based on the mass flowrates of the barite achieved for given values of air flow rate and conveying linepressure drop.

A grid was drawn on each set of conveying characteristics and the ratio ofthe barite flow rates was determined for every grid point. In order to determinewhether there is any pattern in the value of this ratio, with respect to conveyingconditions, the values corresponding to the grid points have been plotted on theconveying characteristics for the pipeline with 17 bends. These are shown in Fig-ure 8.13.

0-85 0-80 0-7540

oo2 30

I 20?_o

fan

"310

a

Material - barite

mp in 330 ft x 17 bends

mp in 340 ft x 9 bends

0-70

0-65

0-69067

064

064

50 100 150Free Air Flow Rate - ftVmin

200

Figure 8.13 Ratio of material flow rates in pipeline with 17 bends, to pipeline withnine bends.

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Material, Orientation, and Bends 257

It will be seen that the material mass flow rates for the line with 17 bendsare lower in every case, varying from 88% to 64% of the value obtained with thepipeline with only nine bends. Figure 8.13 shows very clearly that bends haverelatively little influence when conveying at very high solids loading ratios withlow air flow rates, but have a very significant effect when conveying at low solidsloading ratios with high air flow rates. Solids loading ratios have not been shownon Figure 8.13 but this information is available from Figures 8.12a and b.

This also shows that no single value can be applied to allow for the influ-ence of bends in a pipeline. An allowance will quite clearly depend on the convey-ing conditions. Figure 8.13, however, shows that the influence of conveying con-ditions on the effect of the bends is very uniform and consistent, and so it shouldbe possible to determine a simple relationship between the allowance to be madeand some parameter that defines the conveying conditions.

5. 2. 1 Equivalent Length

The next stage in the analysis is to assign an order of magnitude, or value, to theallowance to be made for the bends. For this purpose an equivalent length isprobably the best way of allowing for the added resistance. An equivalent lengthof straight horizontal pipeline in feet is therefore required, so that this can beadded to the existing pipeline length to give the total equivalent length of the pipe-line [8].

As the equivalent length will vary with conveying conditions it is necessaryto superimpose regular grids on the two sets of conveying characteristics, as pre-sented earlier and to evaluate the value at every grid point established. The equiva-lent length of the bends can be determined with a model that relates material flowrate and equivalent conveying distance for a pneumatic conveying pipeline. Such amodel was presented in Chapter 7 with Equation 7.10:

...... - - 0)

where rhp = mass flow rate of material - Ib/h

Le = equivalent length of pipeline - ft

and subscripts 1 and 2 refer to differentpipelines of the same bore

The equivalent lengths of the two pipelines will be:

Le, = (340+ 9b) ftand Le2 = (330+176) ft .......... (2)

where b = equivalent length of straighthorizontal pipeline per bend - ft

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258 Chapter 8

Substituting Equation 8.2 into 8.1 and re-arranging gives:

mn, 330 + lib

m. 340 + 9 b(3)

It is this ratio that is plotted on Figure 8.13. The only unknown in this equa-tion, therefore, is b. The equivalent length, therefore, will increase in a patternsimilar to that shown for the ratios on Figure 8.13. An analysis of the data pro-duced the relationship shown in Figure 8.14. The entire program of test work andanalysis was repeated with cement, in place of the barite, and a very similar set ofresults was obtained.

The correlation is in terms of a single parameter, which is conveying lineinlet air velocity, which makes its application very convenient. This would indi-cate that the location of the bend along the length of the pipeline is only of secon-dary importance, despite the fact that the conveying air velocity will increasealong the length of the pipeline. It might, however, be that the difference in parti-cle velocities across the bends do not vary significantly with their position alongthe length of the pipeline.

From Figure 8.14 it will be seen that equivalent lengths of bends can be aslow as 5 ft per bend for low velocity dense phase conveying, with conveying lineinlet air velocities of 600 ft/min. For dilute phase conveying, however, with a con-veying line inlet air velocity of 4000 ft/min, for example, the equivalent length isabout 80 ft per bend. The curve continues to rise to higher values of equivalentlength with further increase in velocity.

1000 2000 3000

Conveying Line Inlet Air Velocity - ft/min

4000

Figure 8.14 Influence of conveying line inlet air velocity on equivalent length of longradius 90° steel bends for conveying barite.

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Material, Orientation, and Bends 259

5.2.1.1 Coefficient of RestitutionAlthough the data in Figure 8.14 relates to barite, the entire process was also car-ried out with cement, as mentioned above. There were obviously differences in theconveying characteristics between the barite and cement conveyed through thetwo pipelines but the analysis carried out produced an almost identical result interms of equivalent length [8]. It is suspected that many other materials will followthis same pattern in terms of equivalent length. However, it is believed that thevalue of the coefficient of restitution between the particles and the bend wall mightwell be an additional influencing parameter.

If materials having a high value of coefficient of restitution impact against abend the velocity of the particles on leaving the bend will not be as low as thosefor materials such as flour, barite and cement. As a consequence the energy lossacross the bend will not be as high, particularly for higher velocity flows. Thispoint is considered further, later in this chapter, when the analogous situation ofconveying materials through rubber hose is investigated.

5.2.2 Pressure Drop

An alternative presentation of the data in the form of conveying characteristics ispresented in Figure 8.15. The bend loss here is expressed in terms of lbf/in2 perbend. It will be seen that the most significant parameter is air flow rate, and henceconveying air velocity, with losses varying from about '/2 lbf/in2 per bend in lowvelocity dense phase flow to 2/4 lbf/in2 per bend in high velocity dilute phase flowover the range of air flow rates considered.

50

oo2 40

j5

i 30

I

S3a

10

Solids LoadinRatio x ' ^ , 1 0 0 ,80

60

PipelineBore

20

Bend Loss

= 2 inch -*•• ^ ""lbf/in /bend

0 40 80 120 160

Free Air Flow Rate - fWmin

Figure 8.15 Influence of conveying conditions on pressure drop for barite conveyedthrough long radius 90° steel bends.

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260 Chapter 8

From the pipeline and conveying parameters for the flour, presented in Fig-ure 8.11, the air flow rate was about 155 ftVmin and the solids loading ratio wasgiven as 32. If this data is plotted on Figure 8.15 for the barite it will be seen thatthe pressure drop would be about 2 lbf/in2 which is the same value as that reportedon Figure 8.11. This is despite the difference in bend geometry.

5.2.2.1 Comparative ValuesIt will be noted that in terms of equivalent lengths the spread of values over therange of conveying conditions considered is of the order of 20:1 from Figure 8.14,but in terms of pressure drop values from Figure 9.15 it is only about 5:1. Thevalues in terms of pressure drop are much closer because pressure gradient valuesin dense phase are very much higher than those for dilute phase. The use of scalingparameters for evaluating pneumatic conveying system performance and capabil-ity is very different from that of summing pressure drop values for individual ele-ments of the pipeline.

5.3 Bend Geometry

The majority of the work reported in this chapter has been undertaken with longradius bends having a D/d ratio of about 24:1. Tests with the wheat flour related toa bend with a D/d ratio of 5:1 but the data agreed quite closely with that for baritein the long radius bends. The influence of bend geometry on the air only pressuredrop for bends was considered in Chapter 6 with Figure 6.6 and this is reproducedhere in Figure 8.16 for reference. From this it will be seen that it is only with veryshort radius bends that pressure drops will be high for air only.

Rough Pipesf = 0-0075

Smooth Pipes/i= 0-0045

10 20

Ratio of D/d

Figure 8.16 Head loss for 90° radiused bends.

30 40

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Material, Orientation, and Bends 261

Bends having a wide range of geometries are employed in pneumatic con-veying system pipelines. Short radius bends and tight elbows are cheaper and eas-ier to install than long radius bends. Blind tees are often used in pipelines in whichabrasive materials are conveyed. In order to determine the influence of bend radiuson pressure drop and conveying performance a program of tests was carried outwith a range of bend geometries [9].

A pipeline was specially built with a double loop in the horizontal plane, inwhich the bends at the corners could be replaced. The pipeline included eleven 90°bends and seven of these could be conveniently changed. The pipeline was 165 ftlong and of two inch nominal bore. A fine grade of fly ash was used as the con-veyed material to ensure that tests could be carried out over as wide a range ofconveying conditions as possible. A sketch of the pipeline is given in Figure 8.17for reference.

The central group of seven bends, positioned in the corners of the doubleloop were arranged so that bends of different geometry could be conveniently in-corporated. The location of the bends is indicated on Figure 8.17 and were chosensince there was a reasonable length of straight pipeline before the bend to ensurethat the fly ash was accelerated to its terminal velocity before meeting the nextbend.

The group of seven bends, all having the same geometry, were all changedfor each test program. Tests were carried out with sets of long radius bends havinga bend diameter, D, to pipe bore, d, ratio of 24:1; with short radius bends (D/d =6); elbows (D/d = 2); and with blind tees. A proportioned sketch of the differentbends tested is given in Figure 8.22.

Return toHopper

SupplementaryAir t

Discharge fromBlow Tank

Figure 8.17 Pipeline used for bend geometry tests.

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262 Chapter 8

Elbow Shortradius

Longradius

Figure 8.18 Sketch of bends tested.

A complete set of conveying characteristics was obtained with the fly ashconveyed through the pipeline for each of the four different sets of bends. Theconveying characteristics obtained with the long radius bends and the blind tees

are presented in Figure 8.19.

Solids Loading. Ratio ^~-^

-Conveying Line• Pressure Drop'- lbf/in2" Conveying Li

_ Pressure Drop

10

\32

24

(a)

0 50 100 150Free Air Flow Rate - ftVmin (b)

0 50 100 150Free Air Flow Rate - ftVmin

Figure 8.19 Conveying characteristics for fly ash conveyed through the pipelineshown in figure 8.21 having, (a) Long radius bends and (b) blind tees.

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Material, Orientation, and Bends 263

If these two sets of conveying characteristics are compared it will be seenthat over a large area of conveying conditions an increase of about 50% in pres-sure drop is required in the pipeline with blind tees to achieve the same materialflow rate in the pipeline with long radius bends. If material flow rates are com-pared for a given value of conveying line pressure drop it will be seen that theflow rate achieved in the pipeline with blind tees is approximately half of thatachieved in the pipeline with long radius bends, particularly at high air flow rates.

It must be recalled that these two sets of conveying characteristics relate tothe same pipeline, for the 165 ft length of pipeline and four of the bends are ex-actly the same in each case. These differences, therefore, are due entirely to thechange in geometry of only seven of the bends in the pipeline.

To provide a full comparison of the different sets of bends the conveyingcharacteristics were compared over the entire range of conveying conditions. Thecomparison was based on the pressure drop required to achieve a specified mate-rial flow rate for a given air flow rate. To do this a grid was drawn on each set ofconveying characteristics at regular increments of both air and material flow rates,and the conveying line pressure drop at every grid point was noted [9].

The results of this analysis are presented in Figure 8.20 with both the blindtees and the short radius bends compared with the long radius bends. The longradius bends have been taken as the datum for reference.

10

(a)

Solids LoadingRatio

Solids Loading Ratiof APshoit rad

,00%p APlong radius | i QQ 60

0 50 100 150

Free Air Flow Rate - ftVmin (b)

0 50 100 150

Free Air Flow Rate - ftVmin

Figure 8.20 Comparison of performance of long radius bends, (a) With blind tees and(b) short radius bends.

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264 Chapter 8

The numbers on these plots are essentially the ratios of corresponding pres-sure drops, in terms of a percentage increase, or decrease where there is a negativesign. It will be seen that the pressure drop with the blind tees was about 40%greater than that for the line with the long radius bends, whether for the low veloc-ity dense phase or the high velocity dilute phase conveying of the material. Interms of energy considerations, therefore, blind tees could not be recommendedfor pneumatic conveying system pipelines with this type of material.

A comparison of the short radius with the long radius bends is given in Fig-ure 8.20b and from this it will be seen that there is little difference between thetwo, although at low values of both air and material flow rates the short radiusbends performed better. In terms of bend selection, therefore, long radius bendswould only be recommended if there were particular needs for such bends in termsof erosive wear resistance and the minimizing of material degradation.

A comparison of the elbows with the long radius bends showed an overallincrease in percentage ratios of about 15% over those shown on Figure 8.20b forthe short radius bends [9]. The data overall, therefore, shows a very close correla-tion with the data for air only in Figure 8.16, with respect to the influence of bendgeometry on pressure drop.

5.3.1 Pocketed Bends

Bradley [10] undertook a program of tests with a 90° pocketed bend and reportedthat the pressure drop was only marginally better than that for a blind tee. Thepocketed bend was of the vortice variety and was tested in a similar manner to thatdiscussed in relation to Figure 8.11.

5.4 Bend Location

The general recommendation is that a reasonable length of straight pipeline shouldproceed a bend in a pipeline, particularly the first bend in a pipeline followingmaterial feed into the pipeline.

This area in a pipeline is particularly critical because the conveying air ve-locity is at its lowest and the material is generally fed into the pipeline at zero ve-locity. Ideally the particles should be accelerated to their terminal velocity. Withlarge and high density particles this requires a relatively long distance. If this is notpossible, and particularly if the first bend is a blind tee or pocketed bend, it may benecessary to increase the conveying air velocity to compensate and this, of course,will increase the energy requirements for the system.

By similar reasoning no two bends in the pipeline should be spaced tooclosely together, particular in the low velocity area in a pipeline.

Long radius bends are also to be avoided if the first bend must feed verti-cally up. The problem here is analogous to inclined pipelines. A long radius bendin a horizontal to vertically up orientation will have a significant section of pipe-line on an incline, and a higher conveying air velocity may be required for materialto negotiate such a geometry.

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Material, Orientation, and Bends 265

6 PIPELINE MATERIAL

Although all the data so far has related to the pneumatic conveying of materialsthrough steel pipelines, not all pipelines are made of steel. Rubber hose is widelyused in pneumatic conveying systems, both for pipeline and bends, and in systemswhere a degree of natural flexibility is required, such as in vacuum off-loading andmobile systems.

By virtue of its natural resilience rubber hose can often be used to particulareffect in reducing erosive wear with abrasive materials and in minimizing degra-dation with friable materials. As a pipeline material it is particularly suited to theconveying of certain sticky and cohesive materials.

For the off-loading of ships, that have self-discharging facilities, high pres-sures are generally employed in order to keep the discharge time to a minimum.With materials such as cement, conveying air pressures up to 100 psig can be util-ized, and hose is available that will meet this requirement.

A particular application is the transfer of drilling mud powders, such as bar-ite, bentonite and oil well cement, from supply boats onto off-shore drilling plat-forms. As materials have to be off-loaded from boats in rough seas, a long lengthof hose is used to connect the discharge system on the boat with the fixed pipelineon the drilling rig.

Road trucks and rail tankers are most conveniently off-loaded throughlengths of flexible rubber hose, whether the vehicles are self off-loading or not. Inthese applications it would be impractical to use rigid metal pipelines because ofthe time required to achieve the necessary alignment. An unknown quantity, how-ever, is whether the pressure drop for rubber hose will be any different from thatof steel pipeline.

6.1 Pipeline Pressure Drop

In order to determine whether there is any difference in conveying performancebetween steel and rubber hose a program of tests was specifically undertaken. A140 ft long pipeline of two inch bore steel pipeline that incorporated five 90°bends was used.

Oil well cement was conveyed through this pipeline and its conveying char-acteristics were obtained. A 140 ft length of two inch bore rubber hose line wasthen strapped to the steel pipeline. By this means exactly the same routing andbend geometries were replicated. The oil well cement was then conveyed throughthis pipeline and its conveying characteristics were obtained.

The two sets of conveying characteristics are presented in Figure 8.21. Oilwell cement, like ordinary portland cement, is capable of being conveyed in densephase and at low velocity and so the two sets of data cover a very wide range ofconveying conditions. Tests were carried out with air supply pressures up to about28 lbf/in2 gauge and so, as the pipeline was relative short, solids loading ratios upto about 200 were achieved.

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266 Chapter 8

Solids LoadingRatio -*• 200 160 130

50

40

3 30oi

1320

S 10

0

100 8024

50

40

Conveying Line SoUds

Pressure Drop 18Q150]20 Loa(jjng

r .bt/in

- 16

(a)

0 50 100 150Free Air Flow Rate - ftVmin

S 10

0

(b)

\00 Ratio

20

40

0 50 100 150

Free Air Flow Rate - ftVrnin

Figure 8.21 Conveying characteristics for oil well cement conveyed through 140 ftlong pipeline of two inch bore of different materials, (a) Steel pipe and (b) rubber hose line.

From the two sets of conveying characteristics it will be seen that the natureof the curves is very different. With the steel pipeline there is a distinct pressureminimum point in the pressure drop curves. Conveying performance appears to besimilar at low values of air flow rate but are widely different at high values of airflow rate.

In order to compare the performance of the oil well cement in the two pipe-lines a grid was drawn on each of the sets of conveying characteristics, in muchthe same way as reported above for the program undertaken with bends of differ-ent geometry. The ratio of pressure drops for corresponding air and material flowrates were evaluated. The results of this exercise are presented in Figure 8.22.

From Figure 8.22 it will be seen that there is a gradual increase in pressuredrop for the rubber hose line, compared with that for the steel pipeline, with in-crease in air flow rate. The lines of constant percentage increase drawn on Figure8.22 slope in the same way as the lines of constant velocity on the conveying char-acteristics, as illustrated on Figure 7.4, and so it is clearly a conveying air velocityeffect. In dense phase flow at very low velocities there is little or no differencebetween the two pipeline materials, but with higher velocity dilute phase flow thepressure drop for flow through the rubber hose line is 50% greater than thatthrough the steel pipeline.

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Material, Orientation, and Bends 267

50

40

£ 30

.

20

10

+10 -1 x 100%

-10

Material - CementPipe Bore - 2 inchLength- 130 feet

1 I i 1 1 I L

0 40 80 120 160

Free Air Flow Rate - ft3/min

Figure 8.22 Comparison of pressure drop data for steel and rubber hose lines.

The program of tests was repeated with another drilling mud powder (barite)and a similar set of results was obtained [11].

6.2 Coefficient of Restitution

It is suspected that the coefficient of restitution between the particles and the pipe-line wall plays an important part. Rubber, being resilient, will have a lower coeffi-cient of restitution for impacting particles than steel. If the rubber absorbs more ofthe energy of impact of the particles than the steel, a greater pressure drop willresult with the rubber pipeline, due to having to re-accelerate the particles from alower velocity. This is why the pressure drop for flow through the rubber hose isgreater than that through the steel pipeline, and since pressure drop increases with(velocity)2, this is why it increases with increase in conveying air velocity [11].

7 EQUIVALENT LENGTH

Scaling, whether for system design or for undertaking a review of alternative con-veying systems for a given duty, is generally undertaken in two stages. The firststage is to scale to the length and routing required and the second is to scale withrespect to pipeline bore. Scaling with respect to length and pipeline routing is usu-ally in terms of an equivalent length of the pipeline. The equivalent length incor-porates vertical lift and bends, as well as horizontal pipeline, and is expressed in

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268 Chapter 8

terms of horizontal length. A factor of two is suggested for a scaling parameter forvertically upward sections of pipeline. Equivalent lengths for bends were pre-sented in Figure 8.14.

For non radiused bends and tight elbows an additional allowance will haveto be made. An additional allowance will also have to be made for rubber hose,but the data given here can be used in estimating appropriate values. Although theinformation presented relates to particular conveyed materials it must be appreci-ated that at this point in time there is no universal solution to the problem of de-signing pneumatic conveying systems and for determining the conveying capabil-ity of a pipeline. Different materials will behave differently, as was illustrated withtotal pipeline systems in Chapter 4.

REFERENCES

1. P. Marjanovic. An investigation of the behavior of gas-solid mixture flow propertiesfor vertical pneumatic conveying in pipelines. PhD Thesis. Thames Polytechnic (nowThe University of Greenwich) London. 1984.

2. D. Mills, J.S. Mason, and P. Marjanovic. The influence of product type on densephase pneumatic conveying in vertical pipelines. Proc Pneumatech 2, pp 193-210.Canterbury. Sept 1984.

3. D. Mills. Measuring pressure on pneumatic conveying systems. Chem Eng, Vol 108,No 10,pp 84-89. Sept 2001.

4. J. Firstbrook. Operation and development of the pneumatic coal transportation system.Proc Pneumotransport 5. BHR Group Conf. London. April 1980.

5. T.J. Sheer, R. Ramsden, and M. Butterworth. The design of pipeline systems for trans-porting ice into deep mines. Proc 3rd Israeli Conf for Conveying and Handling ofPaniculate Solids, pp 10.75-80. Dead Sea. May/June 2000.

6. D. Mills. A review of the research work of Professor Predrag Marjanovic. Proc 4th IntConf for Conveying and Handling of Paniculate Solids. Budapest. May 2003.

7. M.S.A. Bradley and D. Mills. Approaches to dealing with the problem of energylosses due to bends. Proc 13lh Powder and Bulk Solids Conf. pp 705-715. Chicago.May 1988.

8. P. Marjanovic, D. Mills, and J.S. Mason. The influence of bends on the performanceof a pneumatic conveying system. Proc 15th Powder and Bulk Solids Conf. pp 391-399. Chicago. June 1990.

9. D. Mills and J.S. Mason. The influence of bend geometry on pressure drop in pneu-matic conveying system pipelines. Proc 10th Powder and Bulk Solids Conf. pp 203-214. Chicago. May 1985.

10. M.S.A. Bradley. Pressure losses caused by bends in pneumatic conveying pipelines:effects of bend geometry and fittings. Proc 14th Powder and Bulk Solids Conf. pp 681-694. Chicago. May 1989.

11. P. Marjanovic, D. Mills, and J.S. Mason. The influence of pipeline material on theperformance of pneumatic conveying systems. Proc Pneumatech 4. pp 453-464. Glas-gow. June 1990.

12. D. Mills. Using rubber hose to enhance your pneumatic conveying process. Powderand Bulk Engineering, pp 79-87. March 2000.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 281: Handbook of Pneumatic Conveying Engineering

Stepped Pipeline Systems

1 INTRODUCTION

When either a high pressure or a high vacuum is used for pneumatic conveying, itis generally recommended that the pipeline should be stepped to a larger bore partway along the length of the line at least once. This is the case whether the materialis being conveyed in dilute or dense phase, and whether the pipeline is long orshort. Stepping of the pipeline is particularly recommended if the material beinghandled is either abrasive or friable. Problems of both erosive wear and particledegradation increase markedly with increase in velocity and so stepping the pipe-line can have a very significant effect on limiting conveying air velocity values,and hence in minimizing the magnitude of erosion and degradation.

For many materials it is possible that the lower velocity profile achieved in astepped pipeline will also bring benefits in terms of improved conveying perform-ance. A particular problem, however, is in the location of such steps, for if they areincorrectly located, pipeline blockage could result.

The capability of purging material from a stepped bore pipeline is anotherissue that might have to be taken into account. A situation in a continuous pipelinein which the pipeline may require to be reduced in diameter, rather than increasedwhich is generally the norm, is where the pipeline incorporates a long section ofvertically downward flow.

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270 Chapter 9

2 CONVEYING AIR VELOCITY

For the pneumatic conveying of bulk particulate materials, one of the critical pa-rameters is the minimum conveying air velocity necessary to convey a material.For dilute phase conveying this is typically about 3000 ft/mm, but it does dependvery much upon the size, shape and density of the particles of the bulk material.For dense phase conveying it can be as low as 600 ft/min, but this depends uponthe solids loading ratio at which the material is conveyed and the nature of theconveyed material. If the velocity drops below the minimum value the pipeline islikely to block. It is important, therefore, that the volumetric flow rate of air, speci-fied for any conveying system, is sufficient to maintain the required minimumvalue of velocity throughout the length of the conveying system.

2.1 Compressibility of Air

The following equations were presented in Chapter 5 and are presented below forfurther development. The first of these is from Equation 5.2 and relates volumetricflow rate with conveying air velocity:

TI d2 CV = ft3/min - - (1)

576

The second is the Ideal Gas Law from Equation 5.4:

144/7 V = ma R T - (2)

The third comes from Equations 5.5 and 5.6 and is the direct derivative fromthe Ideal Gas Law that equates any two points anywhere along the length of apipeline, and will also equate to free air conditions:

r, T2 TH

Using this group of equations the problem of compressibility with air in sin-gle bore pipelines was demonstrated with Figure 5.6 and this is presented here inFigure 9.1 for reference. A free air flow rate of 1500 ftVmin was selected and theinfluence of pipeline bore and pressure are clearly illustrated. The lines of constantpipeline bore represent the velocity profile through a pipeline in single bore pipe-lines. It will be seen that the slope of the lines of constant pipeline bore changeconstantly with pressure, and as the air pressure reduces the slope increases con-siderably. The problem of air expansion, therefore, is very marked in low pressuresystems and particularly so in negative pressure systems [1],

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Stepped Pipelines 271

6000

30 40

Air Pressure - Ibfin2 gauge

Figure 9.1 The influence of air pressure and pipeline bore on conveying air velocity fora free air flow rate of 1500 ft3/min.

3 STEPPED PIPELINE SYSTEMS

Figure 9.1 shows quite clearly the nature of the problem of single bore pipelineconveying, with respect to air expansion and hence conveying air velocities, par-ticularly where high pressures or vacuums are employed. For both long distance,and dense phase conveying, it is generally necessary to have a fairly high air pres-sure at the start of the conveying line. As the pressure of the conveying air de-creases along the length of the line, its density decreases, with a correspondingincrease in velocity, as illustrated above.

A simple means of limiting the very high velocities that can occur towardsthe end of a pipeline is to step the pipeline to a larger bore once or twice along itslength. By this means it will also be possible to keep the conveying air velocitywithin reasonable limits [2].

The ultimate solution, of course, is to use a tapered pipeline, for in this theconveying air velocity could remain constant along the entire length of the pipe-line. This, however, is neither practical nor possible, but it does provide the basisfor a model of what is required. A stepped pipeline, therefore, should be designedto achieve a velocity profile that is as close as practically possible to a constantvalue.

3.1 Step Location

The critical parameter in the design of any pipeline is the minimum value of con-veying air velocity required for the given material and conveying conditions.

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272 Chapter 9

Flow~* (f) di-2

Direction ^^

Figure 9.2 Stepped pipeline notation.

In the design of a stepped pipeline system it is essential to ensure that theconveying air velocity does not fall below the minimum value anywhere along thelength of the pipeline. In this respect it is the location of the steps to each largerbore section of the pipeline that are crucial. With the air expanding into a largerbore pipe the velocity will fall, approximately in proportion to the change in pipesection area, at the step. The location of the step, therefore, must be such that thepressure is low enough to ensure that the velocity in the larger bore section at thestep does not drop below the given minimum conveying air velocity.

A pipeline having two steps, and hence three sections of pipeline of differentbore, is shown diagrammatically in Figure 9.2. Reference numbers are assigned tothe start and end of each section, and provided that there is no leakage of air into or

out of the pipeline between the material feed point at © and the discharge point at

©, the air mass flow rate will remain constant and the continuity equation can beused to equate conditions at any point along the length of the stepped pipeline.

By combining Equations 1 and 2 and substituting V from Equation 3 gives:

576 Po V0 TC3 = - — f t / m i n - - - - - ( 4 )

n J _ /> T0

and substituting values for/?,, and T0 gives:

= 5-19 ~2 - — ft/mind3-4 Pi

This will give the conveying air velocity at the start of the second section ofthe stepped pipeline. By equating to the free air conditions in this way, the velocityat any section of the pipeline can be evaluated.

If it is the pressure at a step in the pipeline that is required Equation 4 can berearranged to give:

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Stepped Pipelines 273

P3

576 Po V0 T3

n 3—4 0 3(5)

= 5-19 Ibf/in absolute

It should be noted that since the end of one section of pipeline terminates atthe point where the next section of pipeline starts, the pressure difference betweenthese two points can be disregarded, and so in the above case: p2 = p} and/?./ = ps.It would generally be recommended that a tapered expansion section should beused to join any two sections of pipeline at a step. As a first approximation, theposition of the steps can be judged in terms of the ratio of the pressure drop valuesevaluated for the individual sections of pipeline, equating these in proportion tothe equivalent lengths of the pipeline, with due allowance for bends.

3.2 Dilute Phase Conveying

Figure 9.3 illustrates the case of a dilute phase conveying system. The minimumconveying air velocity that must be maintained for the material is about 3000ft/min, and 2000 ftVmin of free air is available to convey the material. The con-veying line inlet air pressure is 45 Ibf/in2 gauge.

12,000

10,000_g

^ 8000

.4

.3 6000u

I 4000

'&g 20003

o

14,660

10 20 30Air Pressure - Ibf/in2 gauge

40 50

Figure 9.3 Stepped pipeline velocity profile for high pressure dilute phase system using2000 ft3/min of air at free air conditions.

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274 Chapter 9

From Figure 9.3 it will be seen that a 5 in bore pipeline will be required forthese conditions, and the resulting conveying line inlet air velocity will be about3610 ft/min. If a single bore pipeline was to be used for the entire length of the linethe conveying line exit air velocity would be about 14,660 ft/min. The inlet airpressure is 45 Ibf/in gauge, which is approximately 60 lbf/in2 absolute, and so ifthe discharge is to atmospheric pressure, a near four fold increase in air velocitycan be expected.

If the material being conveyed is only slightly abrasive, severe wear will oc-cur at any bend towards the end of the pipeline, because of the excessive velocity,and significant degradation of the conveyed material will also occur, even if thematerial is not particularly friable.

If the velocity was allowed to rise to 7000 ft/min in this 5 in bore pipe achange to a 6 in bore pipe would only reduce the velocity to 5000 ft/min. The ve-locity in an 8 in bore pipe would be about 2800 ft/min, however, and this isunlikely to be acceptable. A 7 in bore pipe would probably be satisfactory, butcare must be taken that standard pipe sizes are selected. Even in a 7 in bore pipe-line the velocity at exit would be almost 7500 ft/min and so it is clear that twosteps and three different pipe sizes would be required.

The velocity profile for a possible combination of 5, 6 and 8 in bore pipes isshown superimposed on Figure 9.3, but even with this the exit velocity is about5725 ft/min, and the velocity at the end of the second pipe section reaches 6315ft/min. A plot similar to that shown in Figure 9.3, however, will give a clear indi-cation of what is possible. The velocities at the six reference points along the pipe-line are also presented on Figure 9.3 and these can be evaluated by using Equa-tions 4 and 5. It would always be recommended that a graph similar to that in-cluded in Figure 9.3 be drawn for any proposed stepped pipeline system.

3.3 Dense Phase Conveying

Figure 9.4 illustrates the case of a dense phase conveying system. The minimumconveying air velocity that must be maintained for the material is about 1200ft/min, and 350 ft /min of free air is available to convey the material. The convey-ing line inlet air pressure is 45 lbf/in gauge. From Figure 9.4 it will be seen that a3 in bore pipeline will be required for these conditions, and the resulting convey-ing line inlet air velocity will be about 1755 ft/min.

If a single bore pipeline is used the conveying line exit air velocity will beabout 7125 ft/min. Although this might be accepted in a dilute phase conveyingsystem it is quite unnecessary in a dense phase system. Apart from reducing prob-lems of erosive wear and particle degradation, by reducing conveying air veloci-ties, a stepped pipeline is also likely to achieve an improved conveying perform-ance, compared with a single bore pipeline, for the same air flow conditions. Thevelocity profile for a combination of 3, 4 and 5 in bore pipes is shown superim-posed on Figure 9.4. This has resulted in the conveying air velocity being confinedto a relatively narrow band, with the maximum value being limited to 2640 ft/min.

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Stepped Pipelines 275

6000

10 20 30

Air Pressure - Ibfin2 gauge

40 50

Figure 9.4 Stepped pipeline velocity profile for high pressure dense phase system us-ing 350 ftVmin of air at free air conditions.

3.4 Vacuum Conveying

Although negative pressure systems are naturally limited to a maximum convey-ing line pressure drop of less than 14-7 lbf/in2, stepping of the pipeline with vac-uum conveying systems is just as important as it is with high positive pressureconveying systems.

A typical vacuum conveying system is shown in Figure 9.5. It is drawn for adilute phase system, where a minimum conveying air velocity of 3000 ft/min mustbe maintained, using 500 ftVmin of free air at a temperature of 59°F and exhaust-ing to -9 lbf/in2 gauge (14-7 -9 = 5-7 lbf/in2 absolute). It must be rememberedthat absolute values of temperature and pressure must be used in all the equationsrelating to the evaluation of both velocity and pressure along the length of a pipe-line.

If the vacuum were a little higher than 9 lbf/in2, a step to a third section ofpipeline of 8 in bore would be required. Even with a conveying line exit air pres-sure of-7 lbf/in2 gauge, a step could be usefully incorporated in the case presentedin Figure 9.5.

Because the slope of the constant pipe bore curves increase at an increasingrate with decrease in pressure, steps are required more frequently at low air pres-sures. From Equation 9.4 it will be seen that pressure is on the bottom line and sowhen values get very low, as they will in high vacuum systems, a small change inpressure will result in a large change in conveying air velocity.

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276 Chapter 9

8000k

Air Pressure - Ibfin gauge

Figure 9.5 Stepped pipeline velocity profile for high vacuum system using 500 ft'/minof air at free air conditions.

3.4.1 Step Position

A practical problem that arises from this is the actual positioning of the varioussteps along the length of the pipeline. As a first approximation, in the absence ofany other information, pipeline lengths can be sized in proportion to the conveyingline pressure drop for each section, provided that a reasonably uniform value ofconveying air velocity is maintained along the length of the pipeline. It can be seenfrom Figures 9.3 to 9.5 that if there is a risk of the velocity being too low at thestart of the next section, and the pipeline blocking, then the transition to the largerpipe size should be moved a little further downstream, where the pressure will beslightly lower.

4 PIPELINE STAGING

With reference to Figure 9.1 and Equation 4 it will be seen that with increase inpressure the slope of the curves decrease. If a stepped pipeline system was to bedesigned on the basis of a doubling in conveying air velocity, for each section ofpipeline, the working pressure for each section of pipeline would increase signifi-cantly with increase in pressure, as shown in Table 9.1. If it were required to con-vey a material over a distance of the order of 100 miles, it would only be eco-nomical if an air supply pressure very much higher than 100 lbf/in2 was to be used.It would also be necessary to divide the system into stages, such that the materialwas discharged from one system, when the pressure had fallen to a given value,and be fed into the next system with high pressure air.

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Table 9.1 Typical Working Pressures Relating to a 2:1 Conveying Line AirVelocity Expansion Ratio

Air Inlet Pressure Air Outlet Pressure Pressure Difference

lbf/in2 absolute lbf/in2 gauge Ibf/in" gauge lbf/in2

14-7

294

58-8

117-6

235-2

470-4

0

14-7

44-1

102-9

220-5

455-7

-7-35

0

14-7

44-1

102-9

220-5

7-35

14-7

29-4

58-8

117-6

235-2

With a conveying line inlet air pressure of 455-7 lbf/in2 gauge, for example,the first step would not be necessary until the pressure had fallen to 220-5 lbf/in2

gauge, which gives a working pressure difference of 235-2 lbf/in2. If the systemdischarged to atmospheric pressure, the pressure at entry to the last section ofpipeline would be 14-7 lbf/in2 gauge and the working pressure difference wouldonly be 14-7 lbf/in2. This effect is shown in Figure 9.6, which illustrates the veloc-ity profile for the latter sections of a very high pressure stepped pipeline system inwhich the material is conveyed in dilute phase.

7000

6000

g 5000

I^ 4000M

3000

I2000

0 400100 200 300

Air Pressure - Ibfin2 gauge

Figure 9.6 Velocity profile for very high pressure stepped pipeline system.

500

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278 Chapter 9

It would be recommended, therefore, that for a very long distance conveyingsystem, at the end of each stage along the pipeline, and at the very end of the pipe-line, the material should be discharged at a pressure of at least 44 Ibfin2 gauge. Bydischarging at a high pressure, rather than atmospheric, the last two or three sec-tions of the largest bore pipeline can be dispensed with. The reduction in workingpressure drop would be very small in comparison and it would make for a verymuch simpler pipeline design and layout.

5 PIPELINE PURGING

In many applications it is necessary to purge the pipeline clear of material at theend of a conveying run, particularly with perishable commodities and time-limitedproducts. In single bore pipelines this is rarely a problem, even if the material isconveyed in dense phase, because the velocity at the end of the pipeline is usuallysufficiently high. There can, however, be problems with stepped pipelines. Acomparison of the velocity profiles for flow in single and stepped bore pipelines ispresented in Figure 9.7.

5.1 Dense Phase Conveying

Figure 9.7 is drawn for an air flow rate of 1000 ft3/min at free air conditions. Itrelates to the dense phase conveying of a material for which the minimum convey-ing air velocity is about 1000 ft/min. This is similar to the plot shown in Figure9.4, except that the flow of air is from left to right with the new figure.

PipelinbBore *• in

Air Flow5Q90f

2860

6000

5000

4000

3000

50 40 30 20Air Pressure - Ibf/in2 gauge

10

I

2000

Ia

Figure 9.7 Comparison of velocity profiles in single and stepped bore pipelines.

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Stepped Pipelines 279

Although this may be more conventional in terms of system sketching, itdoes mean that the air pressure axis is reversed, and is offered simply as an alter-native means of presentation.

Figure 9.7 is developed further in Figure 9.8 with empty line velocity pro-files added. This also provides a comparison between single bore and stepped borepipelines, with respect to purging, and clearly illustrates the problem towards theend of a stepped pipeline. At the end of a conveying run, with no material to con-vey, the pressure at the material feed point, at the start of the pipeline, will drop tothe air only pressure drop value.

For low velocity dense phase conveying the empty line pressure drop willonly be a fraction of the pressure drop required for conveying. Thus the velocity ofthe air through a single bore empty pipeline will be very high throughout itslength. At the end of the pipeline the air velocity will be exactly the same as in theconveying case, because the pressure here is always atmospheric. At the materialfeed point, however, the air velocity will only be slightly lower than that at the exitsince the air pressure at the feed point is so much lower when material is not beingconveyed.

With the stepped bore pipeline this same volumetric flow rate of air has toexpand into the larger bore section of pipeline, and so its velocity will reduce, asshown in Figure 9.9. At the end of the pipeline the situation is exactly the same asin the single bore pipeline case. The velocity for both conveying and purging willbe the same, because the pressure here is always atmospheric. Since the purgingvelocity will not be constant throughout the pipeline the potential for clearing ma-terial from the latter sections of stepped pipelines by purging, therefore, will beseverely limited.

Purging Mode

PipelineBore - in

40 30 20

Air Pressure - Ibfin2 gauge

10

Figure 9.8 Comparison of velocity profiles in single and stepped bore pipelines inconveying and purging modes.

both

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280 Chapter 9

5.2 Material Deposition

To illustrate the problem of material deposition in pipelines with low velocityconveying, data from a program of conveying trials carried out with a fine gradeof fly ash is presented in Figure 9.9 [3]. The fly ash was being conveyed through a425 ft long pipeline of 2'A inch nominal bore that incorporated nine 90° bends.

5.2.7 Fly Ash

In tests conducted with low air flow rates, and hence at low conveying air veloci-ties, it was observed that not all the batch of material in the blow tank was dis-charged into the receiving hopper. The material was, in fact, being deposited in thepipeline and remaining there at the end of the conveying run, when the conveyingair velocity used was too low to purge the pipeline clear.

The fly ash left in the pipeline did not represent a problem because it wasswept up with the next batch of fly ash conveyed. As a result the pipeline was onlypurged for a short time before starting the next test run. To give some indication ofthe potential problem of material deposition in a pipeline when conveying at lowvelocity, the data for every test carried out was analyzed to provide a figure for thepercentage of the batch conveyed that was discharged into the receiving hopper.

100% data points simply mean that the entire batch of 1000 Ib was con-veyed. For the very high velocity tests the data points have not been included. If80% of the batch was conveyed, then 20% of the batch remained in the pipeline atthe end of the test run, which amounted to 200 Ib of fly ash. The results and analy-sis are presented in Figure 9.9.

Percentage ofBatch Conveyed

'—! 1 1—i 1 1 1

40 80 120 160

Free Air Flow Rate - ftVmin

Figure 9.9 Analysis of pipeline purging data for fine fly ash.

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Stepped Pipelines 281

5.2.2 Cement

In an earlier program of work ordinary portland cement was similarly tested [4]. Itwas conveyed through a 310 ft long pipeline of 4 in nominal bore having nine 90°bends (Figure 7.13 pipeline). For this shorter pipeline of larger bore the batch sizeof the cement was 2750 Ib, since tests with material flow rates up to about 100,000Ib/h were undertaken. Testing was carried out with air supply pressures up to 40lbf/in2 gauge. An analysis of the test data obtained with the cement is presented inFigure 9.10.

The normal conveying characteristics for the material are presented in Fig-ure 9.10, together with conveying air velocity data. This is in terms of a full set ofcurves for the conveying line inlet air velocity and a parallel axis in terms of theconveying line exit air velocity.

From Figure 9.10 it will be seen that the cement could be conveyed withconveying line inlet air velocities down to about 500 ft/min and at solids loadingratios of over 100. Lines showing the percentage of the batch that was conveyedare also superimposed on Figure 9.10. In this case, when only 70% of the batchwas conveyed, 825 Ib of cement was left in the pipeline. As with the fly ash, thiscement was swept up by the next batch that was conveyed.

Conveying Line InletAir Velocity - ft/min

Solids Loading

14n i_ Ratio """-*. I^ Conveying m }QQ J

120 ~ x 10°

Conveying LinePressure Drop - lbf/in2

«c 100X>

i 80pi

I 60

I 40

20

0

80

NO GOAREA

Percentage ofBatch Conveyed

1600

2000

2400

100 200 300

Free Air Flow Rate - ftVmin400

1000 2000 3000 4000

Conveying Line Exit Air Velocity - ft/min

Figure 9.10 Conveying characteristics for cement in 4 inch bore pipeline.

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282 Chapter 9

6 DIVERSE MATERIAL CONVEYING

Not all pneumatic conveying systems are dedicated to the conveying of a singlematerial. There is often a need for a system to transport a number of different ma-terials. In many industries, such as food and glass, a wide variety of materials haveto be conveyed by a common system, since there is a requirement to deliver agiven 'menu' for a particular process [5]. In the case of packet soups, for example,it could involve more than twenty different materials. One of the authors cameacross a total of 78 different materials, ranging from iron powder to vermiculite, ina plant manufacturing welding rods.

Some of the materials to be transported may be capable of being conveyedin dense phase, and hence at low velocity, while others may have no natural densephase conveying capability and will have to be conveyed in dilute phase with ahigh conveying air velocity. The air requirements for the various materials, there-fore, could differ widely.

This is illustrated with the case of floury and sandy grades of alumina, con-veyed through the same pipeline, with conveying line inlet air pressures up to 45lbf/in2 gauge. The pipeline used was 155 ft long, of two inch nominal bore andincorporated six 90° bends. Conveying characteristics for these two materials arepresented in Figure 9.11.

_, . Conveying Line Pressure ^ ,. ,Conveying Drop. M/in2 SolidsLimit \ / Loading

Ratio

(a)

0

0 40 80 120 160 200Free Air Flow Rate - ftVmin

Conveying LinePressure Drop

- Ibffitf

ConveyingLimit

SolidsLoadingRatio \

\

(b)

0 40 80 120 160 200Free Air Flow Rate - ftVmin

Figure 9.11 Conveying characteristics for two grades of alumina conveyed through 155ft long pipeline of 2 in bore incorporating six 90° bends, (a) Floury and (b) sandy.

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Stepped Pipelines 283

6.1 Pipeline Selection

There is often a requirement for these two grades of alumina to be conveyedthrough a common pipeline. From Figures 9.1 la and b, however, it will be seenthat there are considerable differences in the conveying capabilities of these twomaterials. The floury alumina can be conveyed in dense phase and with conveyingair velocities down to about 600 ft/min, and with a conveying line pressure drop of40 lbf/in2 a material flow rate of 52,000 Ib/h can be achieved with a free air flowrate of approximately 55 ftVmin. The sandy alumina, however, can only be con-veyed in dilute phase and requires a minimum conveying air velocity of about2000 ft/min, and with the same pressure drop of 40 lbf/in2 a material flow rate ofonly 32,000 Ib/h can be achieved and this requires a free air flow rate of approxi-mately 170ft3/min.

If a 20% margin is allowed on minimum conveying air velocity, in order tospecify a conveying line inlet air velocity for design purposes, the value for thesandy alumina will be 2400 ft/min and for the floury alumina it will be 720 ft/min.To show how a common conveying system might be able to convey both materi-als, a graph is plotted of conveying air pressure and a series of curves for differentpipeline bore is superimposed in Figure 9.12. Onto this are drawn possible veloc-ity profiles for the two materials. Because of the extremely wide difference inconveying air velocities a single bore line is suggested for the floury alumina, andthree steps are required in the pipeline for the sandy alumina, but it will be seenthat the pipeline system meets the requirements of both materials.

PipelineBore - in

6000 h

e 5000

's^4000

1^ 3000

.5 2000

c

6 1000

20 30

Air Pressure - lbf/in2 gauge

Figure 9.12 Velocity profiles for sandy and floury alumina in a common positive pres-sure conveying system for a free air flow rate of 1000 ftVmin.

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284 Chapter 9

At entry to the reception hopper a common pipeline is possible in this case,as shown, but this is not necessarily a requirement. The use of two completelydifferent pipelines is not likely to be a problem. The pipeline used for the flouryalumina in Figure 9.12, therefore, could well be stepped part way along its lengthto 10 in bore, which could not possibly be used with the sandy alumina.

Consideration would have to be given in this case, however, to purging ofthe pipeline, since the maximum value of conveying air velocity in the pipelinewould be only 2000 ft/min. There are many alternative solutions to the problem ofconveying diverse materials, but the one illustrated is probably the simplest as itutilizes exactly the same air supply in terms of both pressure and volumetric flowrate. Material flow rates will clearly be different, but an extremely complex systemwould be needed to achieve this equality as will be seen from Figures 9.1 la and b.A sketch of a system relating to the data given in Figure 9.12 is presented in Fig-ure 9.13.

6.2 Low Pressure Systems

Although Figure 9.13 is drawn with a common pipeline feeding both materialsinto the reception silo, this is not a requirement, as mentioned above. Indeed, witha low pressure system this may not be a possibility. Two different pipelines, how-ever, could be utilized in exactly the same way. There would probably be no needto step any of the pipelines either.

8 in Bore

\

=0-1VCompressor

Common 8 in Bore

Hoppers forFloury Alumina

. vv/•

X

/

^ \ly"

Xif /C9 6 in Bore /| Hoppers for1 Sandy Alumina

RAA/ 1

r ReceptionSilo

\/vy

5 in Bore

^4 in Bore

Figure 9.13 Typical layout of a high positive pressure conveying system for conveyingdiverse materials.

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Stepped Pipelines 285

6.2.1 Vacuum Conveying Systems

Exactly the same principles apply to vacuum conveying systems. It may well bepossible to have a common pipeline delivering all materials into the reception siloand for materials with no dense phase conveying capability a stepped pipelinecould be utilized to provide the necessary pick-up velocity for the given air flowrate.

7 MATERIAL FLOW RATE

The influence that a stepped pipeline might have on material flow rate is not im-mediately obvious. For the flow of air only through a pipeline models are wellestablished. That for pressure drop takes the form:

LoC1

Apa °c — Ibf/in2 (6)d

where Apa = air only pressure drop - Ibfin2

L = pipeline length - ftp = density of air - lb/ft3

C = conveying air velocity - ft/minand d = pipeline bore - in

As pressure drop increases with increase in (velocity) , and decreases withincrease in pipeline bore, the pressure drop for a stepped pipeline will be signifi-cantly lower than that for a single bore pipeline of the same length, the same initialdiameter and for the same volumetric flow rate of air.

7.1 Fine Fly Ash

Comparative data for the performance of single bore and stepped pipelines israther limited but such work has been carried out with a fine grade of fly ash [6]. A380 ft long pipeline of 2 inch nominal bore and incorporating ten 90° bends wasbuilt for the purpose. A fine grade of fly ash was used, since it is capable of beingconveyed over a very wide range of flow conditions. A sketch of the pipeline ispresented in Figure 9.14 for reference. This also indicates where the steps in thepipeline were made to larger bore sections of pipe.

The conveying characteristics for the fly ash in the 380 ft length of singlebore pipeline are presented in Figure 9.15a. These are the reference set of convey-ing characteristics for the basis of comparison with the stepped pipelines exam-ined. From this it will be seen that the material could be conveyed at solids loadingratios up to almost 200, with conveying line pressure drop values up to 45 Ibf/in2,and over a very wide range of air flow rates.

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286 Chapter 9

Return to Hopper

First Step

Second Step

Figure 9.14 Pipeline used for stepped pipeline conveying tests.

In order to provide a comparison with the single bore pipeline, the secondhalf of the pipeline was changed from 2 inch to 21/2 inch bore pipe. At the transi-tion section the 2 in bore pipe was simply sleeved inside the 21/2 in bore pipe andwelded. The resulting conveying characteristics are presented in Figure 9.15b.

80

70

60

50

I 40

30

20

10

0

Conveying LinePressure Drop

- Ibf7in2

Solids Loading^—-"" Ratio

200 160 120

80

Conveying LinePressure Drop

lbf/in2

o 70

°60

200 160 120

SolidsLoading

Ratio

(a)

0 40 80 120 160 200

Free Air Flow Rate - ftVmin(b)

0 40 80 120 160 200

Free Air Flow Rate - ftVmin

Figure 9.15 Conveying characteristics for fine fly ash in 380 ft long pipeline of 2 inchinitial bore, (a) Single bore pipeline and (b) single step pipeline.

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Stepped Pipelines 287

By comparing Figures 9.15 a and b it will be seen that there is a very sig-nificant improvement in performance over the entire range of conveying condi-tions considered as a consequence of this single step. Much higher values of flyash flow rate were achieved, and with lower values of conveying line pressuredrop.

To illustrate the magnitude of the improvement a comparison of the singlestep and single bore pipelines is given in Figure 9.16a. For this purpose a grid wasdrawn on each set of conveying characteristics at regular increments of conveyingline pressure drop and air flow rate, and the value of the fly ash flow rate wasnoted at every grid point. The data points given on Figure 9.16a represent the ratioof the fly ash flow rates and this shows that the material flow rate achievedthrough the pipeline with the single step was about 1-9 times or 90% greater thanthat for the single bore pipeline for exactly the same inlet air conditions and hencepower required.

It is interesting to note that there is little change in the value of this ratioover the entire range of conveying conditions examined. The improvement appliesequally to low velocity dense phase conveying, and to high velocity dilute phaseconveying. Since there is no change in the air flow rate required to convey thematerial it is unlikely that there would be any need to change the filtration re-quirements for the conveying system either.

80

70

60

50

I 40

1 30

| 20<L>

1 10

0

(a)

Conveying LinePressure Drop- lbf/in2

-20

oaoi

_0

CL<

80

70

60

50

40

30

20

10

0

ConveyingLine PressureDrop

- lhf/in2

Solids Loading

200 160 120

0 40 80 120 160 200

Free Air Flow Rate - ftVmin(b)

0 40 80 120 160 200

Free Air Flow Rate - frVmin

Figure 9.16 Further conveying data and analysis for fly ash in 380 ft long pipeline of 2inch initial bore, (a) Single step comparison and (b) double step pipeline data.

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288 Chapter 9

For the second comparison the last quarter of the pipeline was changed from2'/2 inch to 3 inch bore. Thus the first 190 ft was of 2 inch, the next 95 ft was 2'/2

inch and the last 95 ft was of 3 inch nominal bore pipeline. It should be noted thatthese are by no means the ideal proportions. They were selected to illustrate thepotential improvement that might be achieved over a very wide range of convey-ing conditions. The optimum position of the pipeline steps will depend very muchupon the air supply pressure and pipeline bores available.

The resulting conveying characteristics for this pipeline with two steps arepresented in Figure 9.16b. It will be seen from this that a further improvementover the single step pipeline has been obtained. A similar analysis to that presentedin Figure 9.16a showed that the ratio of material flow rates between the doublestep and the single bore pipelines was about 2-2:1 [6].

7.2 Existing Systems

Since the diameter of the first section of the pipeline remains the same, the airflow rate also remains the same. This, therefore, has direct application to existingsystems, for if a single bore pipeline is used with a high pressure system, the onlychange may be in terms of stepping the pipeline. It is also unlikely that changesneed be made to either the compressor or to the filtration plant.

7.3 Other Materials

It is suspected that the very significant improvement in material flow ratesachieved with the fine fly ash are mainly due to the nature of the conveying char-acteristics for this type of material and are unlikely to be repeated to such amarked extent for materials having different conveying characteristics. A compari-son of constant pressures drop lines for a wide range of materials was presented inChapter 4 with Figure 4.18 and fly ash was clearly the steepest of the fifteen mate-rial included.

The lines of constant conveying line pressure drop on the conveying charac-teristics presented in Figure 9.15b, for example, have a steep negative slope overthe entire range of air flow rates and conveying capability. This means that as theair flow rate, and hence conveying air velocity, is reduced, there is always an in-crease in material flow rate, even at very low values of air flow rate.

This tends not to be the case for the low velocity conveying of polymericpowders and pelletized materials. These materials often exhibit a pressure mini-mum point in the conveying characteristics and at air flow rates below the pressureminimum point the lines of constant pressure drop have a marked positive slope.This was illustrated earlier with polyethylene pellets in Figures 4.12b and 7.3.

The conveying characteristics for terephthalic acid, which is a powder hav-ing a mean particle size of about 60 micron, and nylon pellets, which are monosized having a mean particle size of about 0-15 inch, are presented in Figures 9.17aand b to illustrate this point. The terephthalic acid was conveyed through a 165 ftlong pipeline of 2 inch nominal bore incorporating eight 90° bends.

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Stepped Pipelines 289

60

50

40

I30

o

10

0

c

(a)

Conveying LinePressure Drop- lbf/in2

Solids LoadingRatio

- 30

-25

50 100 150 200

Free Air Flow Rate - ft /min

Conveying LinePressure Drop Solids Loading

50

o 40oX

£30

I 20

I 10

- lbf/in2Ratio

15

(b)

0 100 200 300 400

Free Air Flow Rate - ft3/min

Figure 9.17 Conveying characteristics of materials exhibiting pressure minimumpoints, (a) Terephthalic acid and (b) nylon pellets.

The nylon pellets were conveyed through a 160 ft long pipeline of 3 inchnominal bore incorporating six 90° bends [7], It is suspected that the benefits ofstepping the pipeline will be very limited for the low velocity dense phase flow ofthese materials, but should be well worthwhile for dilute phase suspension flow.

8 EXPANDED BENDS

Since the magnitude of both erosive wear and particle degradation are influencedso markedly by conveying velocity, and that bends are the major cause or influ-ence, it has been suggested that the bends themselves could be stepped to a largerdiameter. The idea is to install bends into the pipeline that have a much larger borethan that of the pipeline [8].

The bends are fitted into the pipeline with tapered sections at inlet and out-let. There is no change in diameter of the pipeline either leading to the bend orfollowing the bend. A sketch of such a bend is given in Figure 9.18. From this itwill be seen that the step is only to and from the bend.

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290 Chapter 9

Solids Loading Ratio

PressureGradient' -Ibf/m2 14°-per 100ft

20

10

20

50 100 150

Free Air Flow Rate - ftVmin

Figure 9.18 Sketch of expanded bend Figure 9.19 Pressure gradient in verticallydown flow for fly ash.

In the expanding section prior to the bend the air velocity will fall and theparticles will be retarded. As a consequence the particles will impact against thebend wall at a lower velocity and so any erosive wear or particle degradation willbe reduced. Although the air velocity may fall below the minimum value for con-veying, the particles are decelerating and the turbulence in the region is so greatthat pipeline blockage does not appear to be a problem.

9 CONVEYING VERTICALLY DOWN

For the flow of bulk particulate materials through horizontal pipelines there is anassociated pressure drop. The situation is the same for flow vertically up, exceptthat the pressure gradient values are approximately double those for horizontalflow, as was discussed in the previous chapter. For flow vertically down, however,the situation is very different.

For materials that can be conveyed in dense phase there can be an increasein pressure as the material is conveyed vertically down the pipeline, if the materialis conveyed in dense phase. Data for cement, barite and a fine grade of pulverized

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Stepped Pipelines 291

fuel ash, conveyed vertically down through a two inch nominal bore pipeline waspresented in the previous chapter [9]. That for the fly ash is reproduced here inFigure 9.19 for reference.

Figure 9.19 is essentially the conveying characteristics for the material forflow vertically down, with the pressure drop being in terms of a pressure gradient,in lbf/in2 per 100 ft of pipeline, rather than the pressure drop for the entire pipelinesystem, as shown in Figures 9.16b to 17b, for example. From Figure 9.19 it will beseen that if the material is conveyed at a solids loading ratio of about 35 there willbe no pressure drop associated with the conveying.

At increasingly higher values of solids loading ratio there is a pressure re-covery, and hence the negative values on Figure 9.19, increasing with further in-crease in solids loading ratio, and hence material flow rate. At solids loading ratiosbelow about 35, and hence for the entire dilute phase region of conveying, therewill be a pressure drop associated with the flow of the material.

9.1 Underground Stowing

In situations where materials need to be conveyed long distances vertically down,very high pressures can be generated if the conveying conditions are carefullyselected. The transfer of fly ash and cement down mine shafts for undergroundstowing and roof support are particular examples.

In these cases it is possible for the materials to be conveyed over a distanceof several thousand feet horizontally from the bottom of the mine shaft by virtueof the pressure generated from the downward conveying of the materials. Providedthat the distance conveyed horizontally, prior to the vertical drop down the mineshaft, is kept relatively short, this could theoretically be achieved with a very lowair supply pressure.

A particular problem here, however, is that the pressure generated could beso high that the conveying air velocity in the following horizontal section of pipe-line could be too low to support conveying and the pipeline could block. In thiscase the pipeline would need to be reduced in diameter, rather than increased, inorder to increase the conveying air velocity.

The horizontal section of pipeline would need to be expanded to a larger di-ameter along its length in the usual way, as it would be discharging material atatmospheric pressure. A sketch of a pipeline for such an application is given inFigure 9.20. A sketch of a velocity profile, for a free air flow rate of 900 ft3/min,for the Figure 9.20 pipeline is presented in Figure 9.21.

A minimum conveying air velocity of about 800 ft/min for the verticallydown flow and approximately 1200 ft/min for the horizontal flow has been as-sumed. The dotted lines represent the flow vertically down and the dashed linesthe horizontal flow. It will be seen that the conveying line inlet air pressure isabout 10 lbf/in2 gauge and so a positive displacement blower is all that would berequired for the air supply, despite the fact that pressures of up to about 90 lbf/in2

are generated within the pipeline system.

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292 Chapter 9

MaterialFeed _Point

/-. -J 1 1

VerticallyDown

4 5 6

, 1 1 I

PipelineBore - in

8

I^ Material

Point

Figure 9.20 Proposal for a pipeline system for delivery of materials for undergroundstowing.

The arrows on the dotted and dashed velocity profiles indicate the actualflow direction through the various bore of pipeline utilized.

I 200°g'(3_o

>

oolOOO

I

me

• Vertically_ Down

Minimum Velocity

I I I 1 1 1 1 1 1 1 J l_J—L -I L

0 15 30 45 60

Air Pressure - lbf/in2 gauge

75 90

Figure 9.21 Velocity profile for conveying system delivering materials for under-ground stowing.

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Stepped Pipelines 293

10 AIR ONLY PRESSURE DROP

Stepped pipelines were discussed earlier to illustrate the problems of air expansionand velocity control along a pneumatic conveying system pipeline. The modelsnecessary to evaluate conveying air velocities and air only pressure drop were alsodeveloped earlier, particularly in Chapter 6, and so it is now possible to considerstepped pipelines further. A sketch of a two section stepped pipeline is given inFigure 9.22.

From Equation 6.12, for a single bore pipeline, the following expression wasdeveloped:

m R T(7)

_ r-<(8)

which gives:

4? = Pi ~P4 = dp 1-2+ AP3--I

Figure 9.22 Velocity and pressure profiles and notation for a stepped pipeline.

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294 Chapter 9

( Y'5either Apa = p - \ p2 - T lbf/in2 (9)

which is an expression in terms of the inlet pressure,/)/

or Apa = (pi + F) - p2 lbf/in2 (10)

which is an expression in terms of the outlet pressure, p2

For a stepped pipeline the total pressure drop will be equal to the sum of theindividual pressure drops for each section. For a two section pipeline the unknownpressure at the step can be eliminated by using both of the above expressions, andnoting that:

P2 = P3and

dpa = Pi ~ P4 = Ap,_2 + Ap3-4

For the first section:

\o - s

= P\ ~ \Pl ~ ri-2

and for the second section:

\o -52Jps-4 = (p, + r3_4\ i

adding these two expressions gives:

/ \0 -5 / \0 -5

PI - p4 = PI - P4 - \P\ - r ,_ 2 j + \p\ + r3_4

which reduces to:

P\-PI = r\.i + rw ( i i )This equation is of the same form as Equation 9.8 and so the solution can ei-

ther be in terms of the inlet pressure, plt as in Equation 9.9, or in terms of the exitpressure, p4, as in Equation 9.10. The choice will depend upon which value isknown, and whether the stepped pipeline is for a positive pressure or a vacuumsystem.

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Stepped Pipelines 295

It should be noted that if the pipeline comprises more than one step, addi-tional equations will be needed to solve the additional unknown pressures at thesteps.

10.1 Position of Steps

The position of the transition to a larger bore line must be such that the conveyingair velocity does not drop below that of the conveying line inlet air velocity em-ployed at the start of the pipeline. As the pressure drops along the length of thepipeline the velocity will increase, but a change in pipeline bore will significantlyalter the situation, as illustrated in Figure 9.22, and with the earlier examplesshown in Figures 9.3 to 9.8.

It was also mentioned earlier that as a first approximation, pipeline lengthscan be sized in proportion to the conveying line pressure drop for each section,provided that a reasonably uniform value of conveying air velocity is maintainedalong the length of the pipeline. With reference to Figure 9.22, the length of thefirst section of pipeline, Lt.2, would be:

P\ - PiLi.2 = x L ft (12)

P^ - PA

The process would be similar for other pipeline sections. The pressure at thesteps can be evaluated from Equation 5, developed earlier, and the velocity at theend of each section and along the length of the pipeline can be determined fromEquation 4.

10.2 Transition Sections

A tapered transition from one section to another would be recommended, in orderto recover as much of the energy as possible in the preceding high velocity flow.The included angle of the transition would need to be about 5 to 10 degrees, asshown in Figure 6.9.

REFERENCES

1. D. Mills. Optimizing pneumatic conveying. Chemical Engineering. Vol 107. No 13. pp74-80. Dec 2000.

2. D. Mills. Pneumatic Conveying Design Guide. Butterworth-Heinemann. 1990.3. D. Mills and V.K. Agarwal. Pneumatic conveying systems - design, selection, opera-

tion and troubleshooting with particular reference to pulverized fuel ash. 386 pp. TransTech Publications. 2001.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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296 Chapter 9

4. D. Mills, V.K. Agarwal, and M.D. Bharathi. The pneumatic conveying of fly ash andcement at low velocity. Proc 24'1' Powder and Bulk Solids Conf. pp 147-163. ChicagoMay 1999.

5. D. Mills. The use of stepped pipelines to enable different materials to be conveyedpneumatically by a common system. Proc 7lh Int Conf on Bulk Materials Storage,Handling and Transportation. The University of Newcastle, Australia. October 2001.

6. D. Mills and J.S. Mason. An analysis of stepped pipelines for pneumatic conveyingsystems. Proc 12th Powder and Bulk Solids Conf. pp 696-704. Chicago. May 1987.

7. M.G. Jones and D. Mills. Performance characteristics for the pneumatic conveying ofplastic pellets. Proc 21st Powder and Bulk Solids Conf. Chicago. May 1996.

8. V.K. Agarwal, N. Kulkarni, and D. Mills. The influence of expanded bends on wearand particle degradation in pneumatic conveying system pipelines. Proc IMechE Confon Powder and Bulk Solids Handling, pp 307-317. London June 2000.

9. D. Mills, J.S. Mason and P. Marjanovic. Pneumatic conveying - vertically down. Proc8th Powder and Bulk Solids Conf. pp 546-557. Atlanta. May 1983.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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10Pneumatic Conveying ofCoal and Ash

1 INTRODUCTION

Millions of tons of coal are burnt in thermal power plants around the world. Ther-mal power constitutes more than half of the world's electric power generation [I].The quality of the coal used varies widely from one country to another. It can varywith the location of the coal mine, and in some cases the quality of coal can varybetween the upper and lower seams in the same mine. This variation can be interms of both the calorific value of the coal and the quantity of un-burnt residueproduced when it is burnt in a boiler.

The quantity of ash generated, and its collection at various locations, is in-fluenced by the ash content of the raw coal, the boiler operating conditions, theexcess air used in the combustion process, and the soot blowing operations. Mil-lions of tons of ash are thereby produced and the ash can have a wide range ofproperties as a consequence, both in terms of chemical composition and particlesize.

It is important, therefore, that any system built to convey this ash should bereliably designed to take account of the properties of the conveyed material. Withfly ash having little or no commercial value, however, such conveying systems arenot always given the consideration that they require. A poorly designed conveyingsystem can result in repeated plant shut down, with a very significant loss in reve-

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298 Chapter 10

nue. With such a high production rate of ash it is essential that the material is re-liably and efficiently removed from the plant.

1.1 Ash Generation

The coal in the "As Received" condition is first pulverized in grinding mills toobtain Pulverized Fuel (pf) or Pulverized Coal. The resulting coal dust is blowninto the combustion chamber or furnace section of the boiler. In modern boilerplant the coal is required as a fine dust in order to achieve combustion rates similarto those of oil and gas.

During the burning of the coal, glassy droplets of ash are produced. Some ofthese particles impinge on the furnace wall, and at high temperatures the particlescan fuse together to form deposits of slag.

Build up of thick layers of ash on a furnace wall increases resistance to theheat transfer process, thus reducing the thermal efficiency of the boiler. In order tominimize the effects of the ash build up, these deposits are periodically removedby means of soot blowers. The dislodged lumps fall into the ash hoppers at thebottom and this is generally referred to as Furnace Bottom Ash (FBA) or simplyBottom Ash.

The bottom ash constitutes about 8 to 15% of the total ash and consists ofvery coarse particles and large lumps and agglomerates. These are generallycrushed to a smaller size before being mixed with water to be disposed of in slurryform.

1.1.1 Fly Ash

The remaining 85 to 92% (fly ash) is very much finer and the particles of ash arecarried away with the flue gases and get collected at various locations along theflue gas path. This ash is commonly referred to as Pulverized Fuel Ash (pfa) orsimply Fly Ash. The coarser fraction of this ash is collected in the economizer, airpre-heater and duct hoppers.

The finer fraction, and generally the largest percentage, is collected in theelectrostatic precipitator (ESP) hoppers. This ash typically has a mean particle sizevarying from about 150 micron in the economizer hoppers to about 30 micron inthe ESP hoppers. Figure 10.1 shows a typical layout of the ash collection points,and approximate percentages of ash collected at each location.

7.7.2 Ash Quality

The quantity of ash produced depends principally upon the quality of the coal usedand whether it has been cleaned in a coal washing plant after being mined to re-move shale, rock and debris. The quality is additionally influenced by the combus-tion process in the boiler, as well as the other operating variables mentionedabove. An inefficient combustion process, for example, may result in a high levelof un-burnt carbon in the ash produced.

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Coal and Ash 299

Chimney

fly ash

Figure 10.1 Ash accumulation points and typical ash distribution within a power plant.

Carbon in ash gives it a dark color, and as a result the ash becomes unsuit-able for certain applications. The ash content in superior grades of coal can be aslow as 6 to 8 %, but can be as high as 45% in poor grades.

India, for example, has abundant coal reserves, but the coal has such a highash content that to produce 55,000 MW of thermal energy, the quantity of coalburnt produces approximately 80 million ton of fly ash every year [3]. Proper utili-zation, or safe disposal, of such enormous amounts of fly ash is a challenge toengineers associated with power generation. It is, therefore, inevitable that powerplant requires an efficient and reliable ash handling system.

1.1.3 Ash Temperature

The temperature of the ash also decreases as it moves away from the furnace andthrough the gas passages [2], Due account of this must be taken in the design ofconveying equipment, not only in terms of materials of construction and for com-ponents, but in evaluating conveying air velocities and specifying air require-ments, for air is compressible with respect to temperature as well as pressure.

The influence of the temperature of the conveyed material on the volumetricflow rate, and hence velocity, of air was considered in detail in Chapter 5. Thistook account of the solids loading ratio of the conveyed material and the tempera-ture of the conveying air. The approximate variation of fly ash temperature withlocation within the boiler plant is given in Figure 10.2.

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300 Chapter 10

Bottom Ash

~ 700T

Ash

•700°F

Stack

•190°F

Figure 10.2 Typical ash temperatures at boiler plant hopper locations.

1.2 Properties of Fly Ash

It is important that the properties of any material that has to be conveyed should betaken into account, and that any variations in properties that are likely to occur,from any source, are also allowed for.

The chemical composition of coal, and hence of the resulting ash generated,will vary both globally and locally. This will also influence particle and bulk den-sity. The mean particle size will vary with respect to the location of the ash hopperon the boiler plant, as well as the air flow settings on the coal grinding mills. Parti-cle shape will be influenced to a certain extent by changes in the combustion proc-ess.

1.2.1 Typical Ash Composition

Silicon oxide (SiCy, or silica, and aluminum oxide (A1O2), or alumina, are the twomajor components in the chemical composition of fly ash. The percentage of silicacan be as high as 65%, and alumina can vary between about 15 and 30%. Bothalumina and silica are very hard materials, with silica having a hardness value ofabout 6 on the Mohs scale of hardness and that of alumina being close to 8. It isbecause of the high concentration of these constituents in fly ash that it is veryabrasive, and can cause damage to all surfaces against which it comes into contact,whether by abrasion or impact.

In some cases the ash may also contain trace elements, such as chromium,boron and arsenic. The ultimate safe disposal of such ashes may require additionalmeasures to be taken to prevent contamination of the soil, or the type of applica-tion to which the fly ash might be used.

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7.2.2 Size Distribution

As the flue gases pass through the boiler ducting, ash is collected at numerouslocations along its route from the boiler combustion area to the chimney. The par-ticle size of the fly ash decreases as the distance of the collection point from theboiler combustion zone increases.

The ash is first collected in the economizer hoppers, and then the air pre-heater hoppers, before it enters the series of electrostatic precipitator hoppers.About 85% of the total ash carried with the flue gas is collected in the ESP hop-pers. ESP's charge the dust particles and use electrostatic attraction to removeapproximately 99-5% of particles from the flue gas entering.

The average or mean particle size of the ash particles collected in theeconomizer and air pre-heater hoppers is about 125 microns. The size of the ashparticles collected in the ESP hoppers, however, is much finer. Within the variouszones of the electrostatic precipitator, ash collected in the initial row of hoppers, inthe direction of the gas flow, is of a higher average particle size as compared withthe ash collected in the last row of hoppers.

The design of any ash handling plant will also have to take these variationsin mean particle size into account. Although there is no change in the materialfrom one location to another, the variation in particle size distribution can have avery significant influence on the conveying capability of the material. Typicalvalues of the particle size of the ash collected in the various hoppers of a typical200 MW generating unit are given in Figure 10.3.

7.2.3 Shape of Fly Ash Particles

Since ash particles are produced as glassy droplets, as a result of combustion in theboiler, the majority of fly ash particles are spherical in shape.

Fly Ash

Electrostatic Precipitator I—

~7Flue Gas

Stack

~ lOum

•150um

Figure 10.3 Typical ash sizes at boiler plant hopper locations.

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302 Chapter 10

A considerable amount of fly ash that is collected in ESP hoppers is used inthe manufacture of cement. Cement, however, is produced by a grinding processand so the particles have an entirely different shape. Although the mean particlesize is very similar, the conveying characteristics of the two materials can be verydifferent as a consequence.

1.2.4 Particle and Bulk Density

In the case of materials that have to be handled in a large quantity, bulk densitycan be an important variable to consider. Since bulk density takes into considera-tion the particle density and voids in bulk storage, it is a useful parameter for thesizing of various system components.

Particle density will influence the slip velocity when the material is con-veyed pneumatically through pipelines in two-phase flow. It is important, there-fore, to have an idea of the typical range in which the particle density and bulkdensity of fly ash can vary. Most fly ashes have a bulk density of about 45 lb/ft3

and a particle density of around 110 lb/ft3. Bulk density will vary with locationwithin the boiler plant and the combustion process. Particle density will vary withcomposition.

1.3 Ash Collection Hoppers

Since close to 75% of the total ash produced in the combustion process is collectedin the ESP zone, it is necessary to consider the layout of these ash collection hop-pers. The electrostatic precipitators have several fields and each field has a numberof collection hoppers. A 200 MW generating unit will typically have six fields andfour hoppers in each field, thus making a total of 24 ash collection hoppers. Asketch showing the layout of a typical group of ESP hoppers, and the direction ofthe gas stream, is given in Figure 10.4.

ft*

Figure 10.4 Typical arrangement of electrostatic precipitator ash collection hoppers.

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Coal and Ash 303

The first field hoppers have the highest ash collection rate, which may varybetween 70 and 80%. The rate of ash collection in subsequent fields decreases insimilar proportions. As a result the ash collected in the hoppers of field 3 and on-wards is minimal. If, during a failure, however, field 1 is not operational, the field2 hoppers would have the same collection rate as the field 1 hoppers in normaloperating conditions.

The capacity of the ESP hoppers is generally selected so that they are capa-ble of storing as much ash as is generated in 24 hours of plant operation. The de-sign of the ash handling system has to consider the time cycle for the ash evacua-tion, keeping in view the differences in ash collection rate in the various hoppers.

1.3.1 Off-Loading Arrangements

The removal of ash from the ESP hoppers can either be in a direction parallel tothe gas flow, as shown in Figure 10.3, or across the direction of the gas flow. Inthe first case hoppers of various fields will be connected to each other so that theash collected in the receiving silo will have a mixture of 'coarse' and fine precipi-tator ash.

In the latter option, the hoppers of a particular field will be interconnectedthus making it possible to keep the 'coarse' ash of the initial two fields separatefrom that of the very fine ash of subsequent fields. Fly ash from the last few fieldsis generally preferred whenever it is required for use as a cement substitute in theconstruction industry.

In the case of the cross direction ash evacuation arrangement, however, theloading on the ash removal system would be non-uniform due to the large differ-ences in the ash collection rate in the hoppers of the various fields. This factormust be taken into consideration when designing the ash removal system for suchan arrangement. The choice of system depends largely upon the end utilization ofthe ash and the ESP plant layout.

1.4 Ash Transfer Systems

The selection of an ash removal system depends upon the nature of the ash, thequantity of ash to be handled, and if the ash has to be graded for the end utiliza-tion. Pneumatic conveying systems offer an ideal choice for the handling of fly ashin dry form. Both positive pressure and negative pressure conveying systems arewidely employed. Very often both are incorporated, and air slides are also used.

Because there can be between thirty and forty individual ash hoppers to beemptied on a 200 MW boiler unit, and obviously many more on larger units, theash handling is often split into two parts and intermediate silos are employed.Where the ash needs to be removed from the power station site, and long convey-ing distances are involved, this is almost essential. A sketch of a typical ash han-dling system for the removal of ash from the numerous ash hoppers on a boilerplant to intermediate storage is given in Figure 10.5.

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304 Chapter 10

Due

EconomizerHoppers _

DDD

t Hopper LJ

DP/P/

P*D0

XAir Pre-heater

Hoppers

/

O Intermediate1 O Storage Silos

v-/

D D a P Po p a P P

_P_

PElectrostatic Precipitator Hoppers

D P D P D D

P P P P P P

^Duct Hopper Stack Hopper

Figure 10.5 Sketch of pipeline layout for ash removal by vacuum conveying from a200 MW power plant.

The system shown in Figure 10.5 is a vacuum conveying system. Multiplelines would be used, with an exhauster dedicated to each, and stand-by machineswould also be available. The four pipelines shown would have cross-over connec-tions and valving so that virtually any hopper could be off-loaded through any lineso as to provide added security to guarantee that any hopper could be off-loaded.

Because different hoppers contain different quantities of ash, and everyhopper is at a different distance from the reception silo, the main design specifica-tion for this type of plant is often that the ash produced by the boiler in an eight-hour shift should be capable of being transferred to the silos in a four-hour period.This could amount to 700 ton of ash, and so although flow rates of 50 ton/h perline would be expected, it is also the sequencing of the off-loading of the hoppers

that is critical for the operation.With a common pressure drop being available to every hopper, the ash flow

rates will be very much higher for those hoppers that are close to the silo than forthose that are distant. In order to maximize performance, pipeline feeders need tobe able to meet the maximum potential for the location. Another problem is thatthe grade of ash varies from one hopper to another and this can have a markedinfluence on the conveying potential and capability of the system. This point isconsidered in more detail later in this chapter.

A sketch of a typical ash handling system for the onward conveying of theash from the intermediate silos to site disposal silos is given in Figure 10.6. Be-cause of the distances involved, which can be up to one mile and more, this type ofduty is mostly met by positive pressure conveying systems.

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Reception Silos

IntermediateStorage Silos

Figure 10.6 Sketch of a typical pipeline layout for the onward positive pressure con-veying of fly ash to reception silos

At some power stations the fine and coarse grades of fly ash are kept sepa-rate throughout the plant and one or two of the intermediate silos would be dedi-cated to the coarse ash. By this means separate systems can be installed to handlethe different grades for the onward transfer.

It would generally be recommended that the ESP ash be conveyed sepa-rately from the coarse ash. The difference in conveying capability between the fineand coarse grades of fly ash is such that serious consideration must be given to thissituation. If the grades of ash are mixed then the conveying conditions must becarefully selected. This issue will be considered in detail later.

If the conveying distance from the intermediate silo to the disposal silo ismore than about 3000 ft, high conveying air pressures will generally have to beused, or very large bore pipelines employed. Within the practical limits of thepressure drop, the material would be conveyed at a lower value of material to airratio.

In such a situation a higher conveying line inlet air velocity has to be used.The high pressure will result in a higher exit velocity. In such applications it isgenerally recommended that the pipeline should be stepped to a larger bore partway along its route. This helps to reduce the velocity and improve the performanceof the conveying system.

2 SYSTEM COMPONENTS

Because of the abrasive nature of the material, particular consideration has to begiven to the components of pneumatic conveying systems, and this includes the

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pipeline and the bends. This is certainly the case with ash, for as the abrasive ele-ments in coal do not take part in the combustion process, the ash has a signifi-cantly higher proportion of abrasive constituents. The ash can also be at a hightemperature, as was illustrated in Figure 10.2, and so this also has to be taken intoaccount.

2.1 Feeding Devices

Feeding devices that have no moving parts are the ideal choice, provided that theyare capable of providing the necessary control on feed rate. The choice is then forfeeders that have no pressure drop across the moving parts. Two mechanisms ofwear have to be considered. One is abrasive wear and the other is erosive wear. Ofthe two, erosive wear is the most serious and so consideration can be given tofeeders that have moving parts, provided that there is no pressure drop acrossthem.

2.1.1 Rotary Valves

Rotary valves are rarely used on boiler plant. They can not be recommended forpositive pressure conveying duties because of the pressure differential, despite thefact that they are available with wear resistant blades and liners. They can be usedfor feeding vacuum conveying systems, but are not popular for this duty, possiblybecause of the problems with hot ash. Differential expansion between movingparts and protection of bearings are particular problems.

2.1.2 Screw Feeders

The ordinary screw feeder is totally unsuitable for positive pressure conveying,because of the air leakage problem. Like the rotary valve they can be used forfeeding vacuum conveying systems but are rarely used for this purpose. The sim-ple screw feeder, however, has been developed by several companies into a devicethat can feed successfully into conveying lines at pressures of up to about 40lbf/in2 gauge.

One such device, that was manufactured by the Fuller Company in the USA,and known as a Fuller-Kinyon pump, was shown in Figure 2.11. This type offeeder is commonly used for the onward conveying of fly ash to reception silosFor high pressure operation, however, the device is only suitable for materials thatcan be compressed, which generally restricts their use to materials that have verygood air retention properties. Thus they are only suitable for materials such aspulverized coal and fine grades of fly ash.

This type of screw feeder would not be recommended for coarse grades offly ash, and certainly not for fluidized bed combustor ash or granular coal. As aresult of the high power requirements, and the fact that the screw is prone to wearand cause serious maintenance problems, this type of feeder is gradually beingreplaced by twin blow tank feeders operating in series and capable of continuousoperation.

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2.1.3 Venturi Feeders

Venturi feeders have limited pressure capability but are often used for the transferof fly ash from boiler plant hoppers to intermediate storage silos. They are some-times used also for the injection of pulverized coal into boilers, particularly onsmall-scale boiler plant where individual control over burners is required. Al-though they have no moving parts, wear in the throat area can be high because ofthe very high velocities and turbulence, and so wear resistant materials must beincorporated. Because they have no moving parts additional control must be pro-vided in order to adjust the material feed rate.

2.1.4 Trickle Valves

These are only suitable for negative pressure conveying systems, since there is nopressure drop against which to feed. They are widely used for the vacuum off-loading of ash from hoppers on boiler plant. With no moving parts or pressurerequirement they can be very cheap devices, and if thirty to forty are required on aboiler plant the overall saving can be very significant. The greatest problem withthis type of feeder is that of flow rate control. This is generally achieved by cali-bration and adjustment on site, but this is very material dependent. A slight changein particle size, particle shape or moisture content will affect the balance of thesetting for the material and change the flow rate.

2.7.5 Blow Tanks

Blow tanks are widely used in power plants for the pneumatic conveying of flyash. They can be used on individual ash hoppers for the transfer to intermediatestorage silos and for the onward transfer of ash to reception silos. Blow tanks canonly be used with positive pressure conveying systems, but they can be designedand built to almost any pressure capability. On boiler plants and in other situationswhere the material is delivered to a reception point at atmospheric pressure theyare generally limited to a maximum pressure of about 100 lbf/in" gauge, becauseof the air expansion problems.

For applications where material has to be fed into a reactor or vessel main-tained at pressure, there is essentially no limitation on operating pressure. Fluid-ized bed combustor boilers have been developed that can operate at a pressure of300 lbf/in2 for combined gas turbine and steam turbine generation cycles. Forthese to operate continuously, coal must be fed into the combustor while on load,and blow tanks are the only type of feeder capable of this type of duty. There arenumerous different types of blow tank, and for each type alternative configurationsare possible. They can be used individually or in pairs. Some of the types com-monly used on boiler plants are considered below.

2.1.5.1 Single Blow TanksSingle blow tanks can vary in size from a few cubic feet to 1500 ft3 and more. Onboiler plants small blow tanks are generally used to convey batches of material as

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a single plug. Large blow tanks may take 15 to 20 minutes to convey the batch andso the material is conveyed effectively on a continuous basis for the major part ofthe cycle.

2.1.5.1.1 Single PlugA sketch of a single plug conveying system was given in Figure 1.10. In this de-vice the material is effectively extruded into the pipeline as a single plug, typicallyabout 30 ft long. The discharged material is then blown through the pipeline as asingle plug. Coal and ash can be conveyed in this type of system, as well as millrejects.

The conveying mechanism is completely different from conventional diluteand dense phase conveying, and system performance is not so dependent upon thecharacteristics of the material. Wet coal, for example, can be conveyed, whichwould not be possible in any conventional conveying system. A sketch showingthis type of blow tank fitted beneath electrostatic precipitator hoppers is given inFigure 10.7. The blow tanks will generally feed into a common pipeline, as shown,and as illustrated earlier in Figure 10.5.

2.1.5.1.2 Single BatchBlow tanks are commonly used to convey fly ash from intermediate storage toreception silos. A 1350 ftj blow tank wi l l hold about 30 ton of fly ash and with a15 minute cycle a transfer rate of 120 ton/h can be achieved. A sketch of a typicalbottom discharge blow tank was given in Figure 2.19.

Figure 10.7 Sketch of blow tanks used for off-loading ash hoppers.

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The material in the blow tank needs to be fluidized, or aerated, close to thedischarge point, and material flow rate control is achieved by proportioning the airsupply between the blow tank and supplementary, or conveying air. Top dischargeblow tanks that have a fluidizing membrane across a flanged bottom section arealso widely used, but they are prone to maintenance problems as their performanceis susceptible to dust and moisture in the air.

2.1.5.2 Twin Blow TanksA particular problem with single blow tanks is that conveying is not continuous, asit can be with rotary valves and screw feeders. If two blow tanks are used, ratherthan one, a significant improvement in performance can be achieved when con-veying through a single pipeline. There are two basic configurations of twin blowtanks. One is to have the two in parallel and the other is to have them in series.These were considered in some detail in Chapter 2 with Figures 2.24 to 2.27.

Twin blow tanks arranged in series are now a common option for long dis-tance conveying. At many power stations, cement plants have been built alongsidethe power station in order to utilize the fly ash in the manufacture of cement. Thereception silos are generally located on the boundary of the two plants. This oftenrequires the pneumatic conveying of the fly ash over a distance of a mile or more.

2.7.6 Air Slides

Air slides are also used quite often for the off-loading of ash hoppers. Becausethese hoppers are generally at a high elevation, the headroom required to provide aslight slope is not generally a problem. A series of ash hoppers generally feed intothe one air slide, as illustrated in Figure 10.8. Several air slides can then feed intoa common air slide to transfer the ash to the intermediate storage silos.

Ash Hoppers

Air Supply

Figure 10.8 Application of an air slide to ash hopper off-loading.

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2.2 Air Movers

Because of the heavy duty requirements of coal and ash handling systems it isessential that positive displacement devices are used for all blowing and exhaust-ing duties. Because it is a potentially dusty environment, and the dust is extremelyabrasive, it is essential that air intake filters are fitted to all blowers and compres-sors. Exhausters need to be protected from the possibility of filter bag failure bythe provision of back-up filters.

The specification of exhausters is in terms of the volumetric flow rate of airdrawn into the machine. Some of the ash to be conveyed can be at a high tempera-ture, as illustrated on Figure 10.2, and so the temperature of the conveying air atentry to the exhauster must be taken into account in the specification to ensure thecorrect value of conveying line inlet air velocity is achieved at the material feedpoint.

2.3 Filters

Bag filters with reverse air jet cleaning are the industry standard for power plant.With a very high proportion of fines in fly ash, cyclone separators are not gener-ally a viable option. Care must be taken with vacuum conveying systems withrespect to their specification, for the volume of air to be handled is significantlyhigher than the free air value because of the reduced air pressure at which theyoperate.

Once again, with reference to Figure 10.2, it will be seen that the ash can beat a high temperature. Conveying air does not have a very significant effect interms of cooling the ash and so the air could be at a fairly high temperature at en-try to the filter. This will have to be taken into account in both the specification ofthe filter size, because of the reduction in air density, and the specification of thefilter material for the expected temperature.

At times when the coal mills need maintaining, and the velocity of the clas-sifying air is increased to compensate, the particle size of the coal will increase.The consequence of this is that combustion may not always be complete and it ispossible for glowing ash particles to be deposited in the economizer hopper. Whenthese are conveyed with air they continue to burn and these can cause seriousproblems with regard to filter fabrics if they come into contact.

2.4 Pipelines

Coal and ash are abrasive materials and so all pipelines need to be able to with-stand the wear. To minimize down time, plant is often required to operate for peri-ods of up to three years between planned maintenance periods. Fly ash conveyedthrough a normal mild steel pipeline would probably wear a hole through a 90°bend within one day of operation. Thick walled spun alloy cast iron is a normalspecification for pipeline. In extreme cases is may be necessary to line the pipelinewith basalt.

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2.4.1 Bends

Bends provide pneumatic conveying systems with considerable flexibility withregard to routing but they are very vulnerable to wear by impacting particles. Inextreme cases basalt and alumina ceramics may have to be used as lining materi-als. In recent years a number of alternative bend section profiles have been pat-ented and are widely available. These are generally very short radius and so havethe advantage of being relatively light and take up little space. They usually have abuilt-in pocket in which material is either trapped or 'circulates'. The pressuredrop associated with these bends, however, is generally very much higher thanradiused bends and so a penalty in energy may result if they are employed.

For normal operation either very thick wear back sections are cast into thematerial, or replaceable wear back sections are incorporated, and these are re-placed on a planned basis, typically every six months. A sketch of typical cast ironpipe bends and fittings is given in Figure 10.9.

2.4.2 Steps

If high pressure air, or a high vacuum, is used for conveying a material, it wouldgenerally be recommended that the pipeline should be stepped to a larger bore partway along its length. This is to cater for the expansion of the air that occurs withdecrease in pressure, and so prevents excessively high conveying air velocitiestowards the end of the pipeline. Stepped pipelines were considered in some detailin Chapter 9.

Figure 10.9 Typical cast iron pipe bends and fittings, (a) Integral and (b) replaceablewear-back fittings.

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2.5 Valves

A number of different valves are needed on pneumatic conveying plant and a widevariety of different valves are available in the market place. The requirement isgenerally for the purpose of isolating the flow. With abrasive materials, such ascoal and ash, valves should operate only in the fully open or fully closed position,and when open, the seating surfaces should be out of the flow path of the material,particularly in pipeline flow situations.

Valves should never be used in the partially open position to control theflow of material. This type of valve is also very vulnerable during the opening andclosing sequences, and so these operations should be completed as quickly as pos-sible.

Conventional butterfly valves, therefore, are not appropriate. Ball valves arenot generally recommended either, for as they have moving parts, fine abrasiveparticles can get between surfaces and they lose their air-tightness. The same situa-tion can apply with gate valves if they have to operate at pressure.

Pinch valves are a much better proposition, as there is no relative movementbetween surfaces in which fine abrasive dust can lodge. These can also be openedand closed rapidly. Rubbers and urethanes also have reasonable erosive wear resis-tance, and so are well worth considering for this kind of duty. They will wear,however, and so they must be located in accessible positions and spares must beavailable.

2.5.7 Dome Valves

The dome valve is a more recent addition to the list of valves available, but it hasbeen specifically designed for this type of duty, and is now widely used in the in-dustry. The valve has moving parts, but these move out of the path of the con-veyed material when the valve is open. On closing, the valve first cuts through thematerial and then becomes air-tight by means of an inflatable seal. The valve canbe water-cooled and so it is capable of handling hot materials.

2.5.2 Diverter Valves

There is often a need for a pipeline to deliver material to a number of differentreception points and this requirement can be conveniently met by means of di-verter valves. Conventional diverter valves, however, are not generally suitable forabrasive materials and so flow diversion is generally achieved by means of usingtwo flow isolating valves.

Dome valves are often used for this purpose, with one in each downstreampipeline. In the section of pipeline isolated, material will collect and so protect theclosed valve from particle impact. There will be considerable turbulence where thediversion involves a change of direction and so all pipe-work in the region shouldbe suitably protected from wear. The trapped material will be released when theflow direction changes and is unlikely to cause any subsequent flow problems inthe pipeline.

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3 CONVEYING CAPABILITY

It is well recognized that different materials can have very different conveyingcharacteristics, and that the conveying capability of different grades of the samematerial can differ widely. Coal and ash materials are no different in this respect.Fly ash, as was shown in Figure 10.3, can come in a very wide range of sizes, ef-fectively through a grading process.

Coal is a mined and quarried product and so will be found in a very widerange of sizes, depending upon the application. In boiler plant the raw coal willhave a mean particle size of about one inch. For combustion, however, a meanparticle size of about 50 micron is required and so it reduced in size in grindingmills.

In a large coal fired power station five million ton of coal per year might beused, but the raw coal is mostly conveyed by means of conveyor belt from thestockpiles to the boiler plant. The pulverized coal is conveyed pneumatically to theburners at the corners of the boiler, but the distance is generally very short sincethe grinding mills are generally located close to the boilers.

Because of combustion requirements the concentration of the coal in the airis very low and so high volume centrifugal fans are generally used for the purpose.The ash that is produced, however, is mostly cleared from the various boiler planthoppers by means of a variety of pneumatic conveying systems.

3.1 Pulverized Fuel Ash

A power station burning five million ton of coal in the USA is likely to produceabout 800,000 ton/yr of ash and, as will be seen from Figure 1, the vast majority ofthis will be fly ash. The same power station in India will produce about 2 millionton/yr because of the poor quality of the coal available. It is not surprising, there-fore, that much research on the subject has been undertaken in India [4].

Various grades of fly ash have been conveyed at the Indian Institute ofTechnology in New Delhi and data from these conveying trials is presented toillustrate the conveying capability of the material. A high pressure top dischargeblow tank conveying facility was used and details of the pipeline employed arepresented in Figure 10.10 for reference.

3.7.7 Fine Ash

Conveying characteristics for a fine grade of fly ash, obtained from the electro-static precipitator hoppers of a near-by power station serving the New Delhi areaare given in Figure 10.11. This fly ash had a mean particle size of about thirty mi-cron.

The conveying characteristics are presented in the usual form, of materialflow rate plotted against air flow rate, with conveying line pressure drop beingincluded as the family of curves.

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314 Chapter 10

Pipeline Details:Bore - 21/2 inLength - 435 ftBends - 10 x 90°

Figure 10.10 Sketch of pipeline used for conveying trials with fly ash.

o 50oo

,c 40£

I3 0

_o^ 20

O

I 10

0

Conveying Line Inletr Veloci-ft /min

Air Velocity \ 150 120 100

Solids Loading80 "* Ratio

60

ConveyingLimit

NO GOAREA

Conveying LinePressure Drop - lbf/in2

i i i i i i i f " i ••

0 50 100 150 200

Free Air Flow Rate - ft3/min

Figure 10.11 Conveying characteristics for a fine grade of fly ash.

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Coal and Ash 315

In addition to lines of constant solids loading ratio, lines of constant convey-ing line inlet air velocity are plotted, and the conveying limit for the material isalso identified.

It will be seen from Figure I O.I I that the fly ash could clearly be conveyedin dense phase. Solids loading ratios well in excess of 100 were achieved and con-veying was possible with conveying line inlet air velocities down to 600 ft/min.Fly ash flow rates of about 50,000 Ib/h were achieved with a pressure drop of 28lbf/in2 through the 435 ft long pipeline of 2'/2 inch bore.

3.1.2 Coarse Ash

Conveying characteristics for a coarse grade of fly ash are presented in Figure10.12. This fly ash had a mean particle size of about 110 micron and was obtainedfrom the air pre-heater hoppers of the same power station from which the fine flyash was obtained. It was conveyed through the same pipeline as the fine fly ash.From Figure 10.12 it will be seen that this material could only be conveyed in di-lute phase, suspension flow through the pipeline.

The maximum value of solids loading ratio that could be achieved was onlyjust 15 and the minimum value of conveying air velocity at which the materialcould be conveyed was about 2600 ft/min. With a conveying line pressure drop of28 lbf/in2 the maximum value of material flow rate achieved was only about18,000 Ib/h, compared with 50,000 Ib/h with the fine ash. To achieve 50,000 Ib/hwith the fine ash an air flow rate of about 55 ft /min was required, compared withabout 240 fVVmin for the coarse ash at 18,000 Ib/h.

o 24

x 20

I7161)

%£ 12_o

bu3 81>

Conveying Line InletAir Velocity - ft/min

Solids LoadingRatio

Conveying LinePressure Drop - lbf/in2

NO GOAREA

ConveyingLim

i oo ISO 200

Free Air Flow Rate - ft3/min

250

Figure 10.12 Conveying characteristics for a coarse grade of fly ash.

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316 Chapter 10

If these two sets of data are combined it will be seen that the specific energyrequired to convey the coarse ash is approximately twelve times greater than thatfor the fine fly ash under these conveying conditions.

3.1.3 Conveying Limits

The approximate influence of solid loading ratio on the minimum conveying airvelocity for the fine fly ash is presented in Figure 10.13. Data for the coarse gradeof fly ash is also given for comparison. For the coarse ash there is no significantchange in the minimum value of conveying air velocity over the range of solidloading ratios possible with the material.

To visually reinforce the differences in conveying capability between thesetwo grades of fly ash the conveying characteristics are presented side-by-side onsimilar axes in Figure 10.14. From Figure 10.5 it will be seen that there is often aneed to convey both grades of fly ash in the same conveying system. These twofly ashes from Figures 10.11 and 12, however, are essentially completely differentmaterials.

The minimum conveying air velocities differ widely and this means that theair flow rate requirements are very different. The material flow rates for differentconveying conditions also differ and so uniform flow rates can not be expected,and the pipeline feeding devices must be able to respond to these differences.

3000

^

I

•§2000

<c

1000SIH§

Coarse Grade

Fine Grade

20 40 60

Solids Loading Ratio

80 100

Figure 10.13 The influence of solids loading ratio on minimum conveying air velocityfor fly ash.

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Coal and Ash 317

'160 120' / / 100 80 50

oo2 40

I

i 30

03y!.\ 20

"ri

1 10

Solids LoadingRatio

Conveying LinePressure Drop - lbf/in2

0 100 200

Free Air Flow Rate - ftVmin

(a) (b)

100 200 300

Free Air Flow Rate - ftVmin

Figure 10.14 Comparison of conveying performance for (a) fine and (b) coarse grades

of fly ash.

From Figure 10.6 it will also be seen that a mix of the two grades often needto be conveyed through a common pipeline. It is unlikely that the two gradeswould be intimately mixed to give a material having a uniform size distribution.

3.1.4 Particle Size Influence

Since there is a large difference in the material flow rate, for a given conveyingline pressure drop, apart from the major influence on mode of conveying, a num-ber of fly ash samples having different mean particle sizes were conveyed in orderto investigate the influence of particle size on flow rate capability [5]. A singlereference point was taken for the subsequent comparison. Because the coarsegrades of fly ash could not be conveyed at low velocity, an air flow rate of 240ftVmin and a pressure drop of 23 lb/in2 were selected. The results from the rangeof fly ash grades tested are presented in Figure 10.15.

From Figure 10.15 it would appear that there could well be an optimumvalue of particle size at which the material flow rate is a maximum, and this couldwell occur in the range of mean particle size at which the transition from dilute todense phase conveying capability occurs. It is suspected that there will be littlefurther reduction in material flow rate with mean particle size above a mean parti-cle size of about 120 micron [4].

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318 Chapter 10

oo

10

o2 30

1

I 20*~ Conveying Conditions:

Air Flow Rate - 240ft3/minPressure Drop - 23 lbf/in2

20 40 60 80 100 120Mean Particle Size - micron

Figure 10.15 Influence of mean particle size of fly ash on material flow rate achievedfor given conditions.

The entire range of particles considered on Figure 10.15 is appropriate topower station fly ash. It is interesting to note that the conveying capability of thecoarse grades of fly ash change so markedly, and that even the different grades offine ash, from different fields of the electrostatic precipitator hoppers, also showdifferent conveying capabilities. Insufficient tests have been carried out with coalto say whether these trends are repeated, but it will be seen from the data presentedlater on coal that particle size does have a marked effect.

3.2 Material Characterization

Certain material characteristics can be used to predict the potential behavior of amaterial when pneumatically conveyed. These are mostly based on bulk propertiesof the material that relate to material-air interactions, such as fluidization, air re-tention and permeability [6]. This was considered in Chapter 4 with Figure 4.22.

4 CONVEYING DATA

Further conveying data for fly ash, and a number of other ash and coal products, ispresented below for comparison and reference. Data on both granular and pulver-ized coal is given, together with additional data on fine fly ash and fluidized bedcombustor ash. Fluidized bed boilers are becoming more popular because of theircapability of burning a wider range of fuels, with better control over emissions,

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Coal and Ash 319

and the very high pressure combustion capability. The ash produced, however, canhave a mean particle size of 0-1 in and above and has a very wide particle sizedistribution in addition.

4.1 Low Pressure Conveying

Details of a pipeline used to obtain low pressure conveying data are given in Fig-ure 10.16 for reference. The various materials tested were fed into the pipeline bymeans of a bottom discharge blow tank. A blow tank was used since it has nomoving parts, which is a definite asset when conveying such abrasive materials. Apositive displacement blower was used to provide the air supply. The blow tankwas a low pressure design, with a similar pressure rating to that of the blower, andso it did not need to be a coded vessel.

Data for a fine grade of fly ash, obtained from electrostatic precipitators, andconveyed through this pipeline is presented in Figure 10.17. This material is capa-ble of being conveyed in dense phase, but the conveying system only had a lowpressure capability. The conveying distance, however, was very short and so thepressure gradient available was sufficient to convey the fly ash at solids loadingratios of up to about sixty. As a consequence the conveying limit for the materialin this pipeline takes the form shown.

The locus of the conveying limit is dictated by the data for the fine fly ashpresented in Figure 10.13. If the data is checked it will be seen that the limit at apressure of 2 lbf/in2 is about 60 fiVYmin of air, because at a solids loading ratio ofseven the minimum velocity is about 2100 ft/min. At a pressure of 5 lbf/in2 thelimit is at about 35 fWmin, because at a solids loading ratio of 60 the minimumconveying air velocity is approximately 800 ft/min.

Pipeline Details:Length - 110f tBore - 2 inBends - 7 * 90°

D/d - 5

Figure 10.16 Details of pipeline used for low pressure conveying trials.

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320 Chapter 10

10

JO~ 6

ConveyingLimit

NO GOAREA

Conveying LinePressure Drop

- Ibf7in2

60 4032

50 100

Free Air Flow Rate - ftVmin

Solids LoadingRatio

150

Figure 10.17 Low pressure conveying characteristics for a fine grade of fly ash.

Data for fluidized bed combustor ash conveyed through the Figure 10.16

pipeline is presented in Figure 10.18.

10ooo

« 6cdoi

fc4

!3_§

NO GOAREA

Conveying LinePressure Drop

- lbf/in2

ConveyingLimit Solids Loading

Ratio

50 100

Free Air Flow Rate - fWmin

150

Figure 10.18 Low pressure conveying characteristics for fluidized bed combustor ash.

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Coal and Ash 321

With a high value of mean particle size and a very wide particle size distri-bution the bed ash will only convey in dilute phase, suspension flow. The maxi-mum value of solids loading ratio achieved was about 14 and the minimum valueof conveying air velocity was about 2600 ft/min, which dictates the conveyinglimit with a positive slope throughout, as is usual with materials that will onlyconvey in dilute phase.

If the conveying performance of the bed ash is compared with that of the flyash it will be seen that the material flow rate, for a given conveying line pressuredrop and air flow rate, is about half. This difference, of course, is consistent withthe data presented earlier on the high pressure conveying of fine and coarse gradesof fly ash. This does, therefore, reinforce the need for conveying trials to be car-ried out with a material, when designing a conveying plant, even for a dilute phaseconveying system.

Data for granular coal conveyed through the Figure 10.16 pipeline is pre-sented in Figure 10.19. The mean particle size of the coal was about Vi inch. Onceagain there was no possibility of conveying this material in dense phase, and cer-tainly not in a low pressure system. The minimum value of conveying air velocityfor this granular coal was about 2400 ft/min and it will be seen that higher materialflow rates were achieved with the coal than with the bed ash, despite the fact thatthe particle size was very much larger. At low values of solids loading ratioslightly higher values of conveying air velocity were required but this is probablybecause the top size of the material was about % inch. At higher material concen-trations this did not appear to be a problem.

1E•a 4

NO GOAREA

Conveying LinePressure Drop

- lbf/in2

ConveyingLimit Solids Loading

Ratio

J_50 100

Free Air Flow Rate - ftVmin

150

Figure 10.19 Low pressure conveying characteristics for granular coal.

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322 Chapter 10

4.2 High Pressure Conveying

In order to illustrate different points, and to provide data on different coal and ashmaterials, results from two separate sets of conveying trials are reported. In eachcase the materials were fed into the pipelines by means of top discharge blowtanks. Air was available at a pressure of about 100 lbf/in2 gauge but, because ofthe relatively short pipeline employed in each case, the conveying trials were lim-ited to a conveying line inlet air pressure of approximately 30 lbf/in2 gauge

4.2.1 Group One Trials

Details of the pipeline used to obtain the first set of high pressure conveying dataare given in Figure 10.20. Three different materials were tested and these were thesame materials that were tested in the low pressure system reported above andconveyed through the Figure 10.16 pipeline.

Conveying characteristics for the fine grade of fly ash are presented in Fig-ure 10.21. Because the pressure gradient was high in this test facility, solids load-ing ratios up to about 300 were achieved.

Conveying characteristics for the fluidized bed combustor ash are presentedin Figure 10.22. Despite the fact that a very high pressure gradient was availablefor conveying the material, there was no reduction in the value of the minimumconveying air velocity of 2600 ft/min that was reported above in Figure 10.18 inrelation to the conveying of this material in the low pressure test facility.

This reinforces the point that high pressure is not synonymous with densephase conveying. Although relatively high values of solids loading ratio wereachieved, the material was only conveyed in dilute phase suspension flow andwere simply a consequence of the very high pressure gradient.

Pipeline Details:Length - 140ftBore - 2 inBends - 6x90°

Figure 10.20 Details of pipeline used for high pressure conveying trials.

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Coal and Ash 323

Pressure Drop-lbf/in2 3.00. 200 160

50

- 40

• 30

I 20

E13

1 10

0

. NO

GO 15 (

Solids LoadingRatio

40

- AREA /

ConveyingLimit

0 150 20050 100

Free Air Flow Rate - ft3/min

Figure 10.21 High pressure conveying characteristics for fine grade of fly ash.

Conveying characteristics for the coal are presented in Figure 10.23. Onceagain this is only dilute phase conveying. With a slightly lower value of conveyingair velocity, and a much higher material conveying rate, compared with the bedash, for given conveying conditions, solids loading ratios are relatively high.

50ooo- 40

30

| 20E!3I 10

Solids LoadingRatio

Conveying LinePressure Drop - lbf/in

NO GOAREA

ConveyingLimit

50 100 150

Free Air Flow Rate - ft3/min200

Figure 10.22 High pressure conveying characteristics for fluidized bed combustor ash.

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324 Chapter 10

50

oo2 40

"730

20

Conveying LinePressure Drop - Ibf/in2

Solids LoadingRatio \

NO GOAREA

ConveyingLimit

50 100Free Air Flow Rate - frVmin

150 200

Figure 10.23 High pressure conveying characteristics for coal.

In a plant pipeline it would always be recommended that the pipeline shouldbe stepped to a larger bore part way along its length, if high pressure air is to beused to convey a material, particularly if the material is abrasive. All three of theabove materials were abrasive and the bed ash exceptionally so.

4.2.2 Group Two Trials

A sketch of the pipeline used for this set of conveying trials was given in Chapter4 at Figure 4.2. For reference, data on a fine grade of fly ash is included, as well asdata on pulverized coal. Granular coal was also conveyed through this pipeline.Coal, however, in addition to being very abrasive, is also very friable and addi-tional data on degradation and degraded coal is presented. Data for a fine grade offly ash conveyed through the Figure 4.2 pipeline is presented in Figure 10.24.Once again solids loading ratios up to 300 were achieved and the material wouldconvey reliably with conveying air velocities down to 600 ft/min [1].

Data for pulverized coal is presented in Figure 10.25. The mean particle sizeof this material was about 80 micron and so in terms of conveying capability itwas a borderline case for dense phase conveying. From Figure 10.25 it will beseen that the material could be conveyed at low values of air flow rate and thesecorresponded to a conveying line inlet air velocity of about 1400 ft/min, and sothis was clearly dense phase conveying.

The material, however, did not have the degree of air retention necessary toallow it to be conveyed over the range of conveying conditions achieved with thefly ash in Figure 10.24. This is often referred to as medium phase conveying, but itis clearly in the narrow transitional band, because of particle size, between diluteand full dense phase conveying capability.

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Coal and Ash 325

ooo

'§'ft

50

40

30

20

10

0

Pressure Drop-lbf/in2 300 200 160

: N120 100 80

Solids LoadingRatio /

"Conveyin;- Limit'' 5

0 50 100 150

Free Air Flow Rate - ft3/min

200

Figure 10.24 Conveying characteristics for a fine grade of fly ash.

Data for granular coal is presented in Figure 10.26. This coal had a meanparticle size of about 0-05 in (about 1 mm). As a consequence the material couldonly be conveyed in dilute phase, suspension flow.

24

I 12

Conveying LinePressure Drop

- lbf/in2 \12

Solids LoadingRatio

50 100

Free Air Flow Rate - ftVmin

150 200

Figure 10.25 Conveying characteristics for pulverized coal.

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326 Chapter 10

50 -Solids Loading

Conveying Line Ratio2 40

~ 3Q

fe 20

10

Pressure Drop- lhf/in2

NO GO AREA

ConveyingLimit

20

\10

i i i i i -r* i i i i i i i i i i i i i

0 50 100 150 200

Free Air Flow Rate - ftVmin

Figure 10.26 Conveying characteristics for granular coal.

The minimum conveying air velocity for the granular coal, however, wasabout 2200 ft/min, which is relatively low and so, with a very high pressure gradi-ent, solids loading ratios of up to about forty were achieved.

5 DEGRADATION OF COAL

Coal is a very friable material. Any handling operations with coal are likely toresult in degradation of the material. Pneumatic conveying, therefore, is likely tocause more damage to coal than any other bulk handling operation.

5.1 Free Fall Damage

To illustrate the potential damage that can result to coal as a consequence of han-dling, free fall tests were carried out with a sample of coal [8]. The coal was al-lowed to fall a distance of 20 ft onto a steel plate at an angle of 90°. The coal wasretained during its fall in a large diameter steel pipe. In other tests the pipe wasangled to the vertical so that the additional influence of pipeline surface effectscould be investigated.

Data for the coal is presented in Figure 10.27. The data is presented as acomparison of particle size distributions. The fresh 'as received' material had amean particle size of about 0-40 inch. After circulating the material three times themean particle size had fallen to about 0-34 inch.

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Coal and Ash 327

100

80

60

20

0-1 0-2 0-3

Particle Size - inch

04 0-5

Figure 10.27 Degradation data for coal in free fall test facility.

A particular problem is the generation of fines in this process. Apart fromthe health hazards associated with coal dust, there is always the potential of a dustexplosion with the -200 /urn (80 Mesh) fraction. In any safety survey on a plant,therefore, the potential changes that could result to a material, as a consequence ofhandling operations, must always be taken into account.

5.2 Pneumatic Conveying

The same coal, as reported above in the free fall tests, was pneumatically con-veyed in the low pressure conveying facility reported above. The pipeline wasshown in Figure 10.16. The coal was re-circulated a total of five times, underidentical conveying conditions, through the 110 ft long pipeline that incorporatedseven 90° bends. Material was collected for analysis at the end of each test run bymeans of a diverter valve in the pipeline just prior to the reception hopper [8].

A size analysis of the coal was undertaken on all five samples collected andthis data is presented in Figure 10.28, along with the particle size distribution forthe fresh 'as supplied' material. Despite the material being conveyed only oncebetween samples the lines for each sample on Figure 10.28 are widely spaced.

5.3 Conveying Characteristics

The influence of material grade on conveying performance was illustrated withrespect to fly ash in Figure 10.14 with the fine and coarse grades presented. Theconveying characteristics of coal are similarly influenced by grade.

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328 Chapter 10

100

•SD 60

20

Number of TimesCoal Circulated

0-1 0-2 0-3Particle Size - inch

0-4 0-5

Figure 10.28 Influence of pneumatic conveying on degradation of coal.

This will be seen by comparing the pulverized coal in Figure 10.25 with thatof the granular coal in Figure 10.26. With the mean particle size of coal changingso dramatically with re-circulation it is likely that the conveying characteristics ofthe coal could also change. Conveying characteristics for granular coal having amean particle size of 0'05 inch (1-2 mm) were presented in Figure 10.26. This coalwas conveyed through the Figure 4.2 pipeline. After the Figure 10.26 data wasobtained the coal was re-circulated many times until the mean particle size hadreduced to about 260 jum (60 Mesh). The conveying characteristics for this de-graded coal were then determined and they are presented in Figure 10.29.

If the data in Figures 10.26, for the 'as supplied' material, having a meanparticle size of about 0-05 in (1 -2 mm), is compared with the data in Figure 10.29,for the degraded material, having a mean particle size of about 260 /urn (60 Mesh),it will be seen that there has been a significant change in performance. With aconveying line pressure drop of 30 lbf/in2 the 'as supplied' coal could be conveyedat a maximum of about 26,000 Ib/h but this increased to almost 41,000 Ib/h withthe degraded coal. This influence of mean particle size on the conveying capabilityof the material is very similar to that reported for the fly ash in Figure 10.15.

The change in performance, however, is mainly with respect to the convey-ing capability of the material for given conveying conditions. There is little changein the value of minimum conveying air velocity required, but with the mean parti-cle size of the degraded coal being 260 ^m it is still very 'granular' and a signifi-cant change in this parameter would not be expected.

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Coal and Ash 329

Solids Loading50 \- Ratio

oS NO GO AREA"40

_o^20

10

0

Conveying LinePressure Drop,

>30

ConveyingLimit

0 50 100 150 200

Free Air Flow Rate - ftVmin

Figure 10.29 Conveying characteristics for degraded coal.

6 APPLICATIONS

Although power generation probably represents the greatest use of coal, and isresponsible for the greatest amount of ash generation, the coal has to be trans-ported to power stations and the ash has to be removed from site. Mining, there-fore, is a major industry worldwide that involves considerable conveying of coaland ash, and are quite likely to increase considerably during this century as oil andgas reserves diminish and alternative non fossil fuel power generation alternativesare slow to develop.

6.1 Mining

Much of the coal burnt in power stations has been obtained from deep mines. Withmechanization of coal face operations in the 1970's the mining capability oftenexceeded that of the hoisting capability of winding gear and so additional meanshad to be found of extracting the additional capacity. The alternatives consideredat that time were the sinking of additional shafts, hydraulic conveying and pneu-matic conveying. Pneumatic conveying was by far the cheapest option. Althoughthe operating cost was the highest, the capital cost of the equipment and its instal-lation was the lowest. This soon became widely adopted as a means of hoistingcoal from deep mines.

A particular problem with mining operations is that of subsidence of theground above. It is, of course, now a requirement that mined-out areas should beback-filled. An ideal material for this purpose is fly ash. Although it is a consider-

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330 Chapter 10

able added cost it is now being more widely considered as the best option on envi-ronmental grounds for the disposal of fly ash. The use of ash ponds for this pur-pose is gradually being restricted by governments on a world-wide basis.

6.1.1 Ash Disposal

Where power stations are located close to coal mines a logical solution to theproblem of disposal is to return the fly ash back underground for stowing. Thismay involve a vertical drop of 1000 ft or more down the mine shaft. If the fly ashis conveyed at a high solids loading ratio, the pressure generated at the bottom ofthe shaft can be high enough for the material to be automatically conveyed onwardto underground workings an equivalent distance horizontally [9].

Data on the conveying of fly ash vertically down was presented in Chapter8 with Figure 8.12. A particular problem here, however, is that the pressure gener-ated could be so high that the conveying air velocity in the following horizontalsection of pipeline could be too low to support conveying and the pipeline couldblock. In this case the pipeline would need to be reduced in diameter, rather thanincreased, in order to increase the conveying air velocity.

The horizontal section of pipeline would need to be expanded to a larger di-ameter along its length in the usual way, as it would be discharging material toatmospheric pressure [10]. Details of a possible conveying system were presentedin Chapter 9, with a sketch of a pipeline for such an application given in Figure9.20 and velocity and pressure profiles for the pipeline system given in Figure9.21.

6.1.2 Coal Hoisting

Onley and Firstbrook [11] reported on tests undertaken at Horden Colliery in theUK having an 8 in bore pipeline with a 1380 ft vertical lift. With minus one inchcoal, 90,000 Ib/h was achieved with a conveying line pressure drop of 25 lbf/in2,although with wet shale of the same size only 50,000 Ib/h could be achieved withthe same air supply pressure. 40,000 Ib/h of minus two inch dolomite was con-veyed with a conveying line pressure drop of 20 lbf/in2.

At Shirebrook colliery in the UK the pipeline bore was 12 in and the verticallift was 1070 ft [12]. In this case there were horizontal runs of 330 ft from the feedpoint and 175 ft to the reception point. 145,000 Ib/h of minus one inch coal wasconveyed with an air supply pressure of 11 lbf/in2. 7800 ftYmin of free air wasused and the motor power required to drive the blower was 700 hp.

Since the size of coal to be conveyed can vary from zero to four inch lumps,conveying is essentially in the dilute phase mode, although with the vertical dis-tances involved and air supply pressures employed, conveying could only be invery dilute phase because of the pressure gradient available. Systems operate at upto 20 lbf/in2 gauge, although 10 to 12 lbf/in2 gauge is more usual, with air pro-vided by positive displacement blowers. For air supply pressures of 20 lbf/in2

gauge, twin blowers in series are normally used.

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Coal and Ash 331

Rotary valves are generally used for pipeline feeding in this type of applica-tion but are built more substantially than rotating airlock feeders. The valves areusually powered by a direct drive hydraulic motor that can produce sufficienttorque to shear lumps of rock should they become jammed. Should, however, arock prove too strong to shear, or if tramp material should become trapped, therotor should instantly stop, with the hydraulic circuit by-passing to a tank.

6.2 High Pressure Coal Injection

The case of feeding coal into a high pressure fluidized bed combustor boiler wasmentioned earlier in relation to blow tanks operating in series and having essen-tially no limit on operating pressure. A much longer established application offeeding coal into a high pressure system has been that of feeding granular coal intoblast furnaces.

Coal is injected into blast furnaces in order to reduce the amount of coke re-quired to melt the iron ore. Coke is expensive to produce, but the quantity requiredcan be reduced significantly by injecting granular coal into the blast furnace in theregion of the tuyeres. This is the area where the hot air, typically at 1800 to2200°F, is blown in for combustion. The pressure in this region, however, is 20 to40 lbf/in2 gauge and so blow tanks are generally used for this purpose.

Apart from the temperature and pressure, a particular problem is that thecoal needs to be injected at multiple (typically 12 to 16) points around the perime-ter in this region. The hot combustion air is injected by means of nozzles from aring main. This is not appropriate for gas-solid flows and so a separate small di-ameter pipeline is used for each injection point. A common pipeline is generallyfed from the blow tank and then at a point conveniently close to the blow tank theflow is split into the 12 or 16 separate lines. It is necessary, therefore, to balancethe resistances in these lines such that a reasonably uniform flow of coal isachieved through each.

6.3 Long Distance Conveying

Both coal and ash are conveyed over long distances. With ash most long distanceduties are generally associated with power stations.

6.3.1 Fly Ash

The conveying of fly ash over distances of a mile or more has now become fairlycommon, particularly with the drive towards the utilization of the material and themove away from slurry conveying.

One of the early systems was at Ropar in India where a cement plant wasbuilt alongside the power station in order to utilize the fly ash in the manufactureof cement. The reception silos were located on the boundary of the two plants.This required the pneumatic conveying of the fly ash over a distance of 5100 ft.Four parallel pipelines were used, each with its own twin blow tank system, and

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332 Chapter 10

each one conveys about 85,000 Ib/h over this distance. Air at a pressure of about35 lbf/in2 gauge is used and the pipeline is stepped twice to a larger bore along itslength.

6.3.2 Coal

Mehring [13], reports on a system conveying pulverized coal over a distance of8330 ft. The coal is conveyed from a central grinding mill to a coal firing systemat a cement works and employs a parallel twin blow tank feeding system. The coalis conveyed in batches, with a 15 minute conveying cycle (ten minutes actual con-veying), at about 35,000 Ib/h. The conveying line pressure drop is 13 lbf/in2, theconveying line inlet air velocity 1500 ft/min and the pipeline bore is 10 in. Thesolids loading ratio was reported to be about five. The pipeline incorporated 14bends.

6.4 Multiple Grade Conveying

Not all pneumatic conveying systems are dedicated to the conveying of a singlematerial. There is often a need for a system to transport a number of different ma-terials. In power plant there is generally a requirement to convey different gradesof fly ash, as was illustrated in Figure 10.5. The conveying requirements of differ-ent grades of ash, however, can differ widely, as was clearly shown in Figure10.14.

There are many solutions to the problem but probably the simplest and mosteffective method is to use pipelines of different bore for the different materials.This technique was considered in general terms in Chapter 9 with Figures 9.12 and9.13.

By this means the same air mover and filtration plant can be used and eachmaterial can be conveyed with its own optimum conveying line inlet air velocity.It is possible that the two pipelines could be brought together for a common entryto the reception hopper if required.

The situation is illustrated with regard to different grades of fly ash with atypical plant layout sketch in Figure 10.30. A negative pressure conveying systemhas been chosen for the purpose to illustrate the fact that stepped pipelines are justas appropriate for high vacuum systems as they are for high positive pressure sys-tems.

The velocity profiles for the flow through the two pipelines, for a free airflow rate of 635 ftVmin, is presented in Figure 10.31. A vacuum of 11 lbf/in2 hasbeen taken and minimum conveying air velocity values of 1200 and 3200 ft/minhave been assumed for the fine and coarse grades of ash respectively [14].

Although this system is shown with a common pipeline entering the recep-tion silo, this is not a necessity. In some cases it might not be possible or appropri-ate for the two pipelines to join together, particularly if a step is required in thelow velocity pipeline. A very similar situation will exist for a positive pressureconveying system.

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Coal and Ash 333

Hoppers forFine Ash

Hoppers forCoarse Ash

Common 10 in Bore

10 in Bore

in Bore

- h i

ReceptionSilo

\Exhauster

8 in Bore

Figure 10.30 Sketch of negative pressure conveying system for conveying both coarseand fine grades of fly ash.

Pipeline Bore - in

6000 .

0

Coarse AshIvlinimum

Fine AshMinimum

-10 -8 -6 -4 -2

Conveying Air Pressure - lbf/in2 gauge

Figure 10.31 Velocity profiles for coarse and fine fly ash in common negative pres-sure conveying system.

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334 Chapter 10

REFERENCES

1. J. Harder. Dry ash handling systems for advanced coal-fired boilers. Bulk Solids Han-dling. Vol 17, No 1. Jan/Mar 1997.

2. A. Rengaswarmy, M. Kumar, and S. Chandra. National scenario on pfa production andutilization. Proc 2nd Int Conf on Fly Ash Disposal and Utilization. New Delhi. Feb2000.

3. Modern Power Station Practice. Vols B and E. Pergamon Press.4. D. Mills and V.J. Agarwal. Pneumatic Conveying Systems - Design, Selection, Opera-

tion and Troubleshooting with Particular Reference to Pulverized Fuel Ash. TransTech. 2001.

5. V.K. Agarwal, M.D. Bharathi, and D.Mills. The influence of material grade on pneu-matic conveying system performance. Powder Handling and Processing. Vol 12, No 3,pp 239-246. July/Sept 2000.

6. M.G. Jones and D. Mills. Product classification for pneumatic conveying. PowderHandling and Processing. Vol 2, No 2, pp 117-122. June 1990.

7. D. Mills. Pneumatic Conveying Design Guide. Bulterworth-Heinemann. 1990.8. D. Mills. The degradation of bulk solids by pneumatic conveying and its simulation by

small scale rigs. Pub by BMHB. 195 p. Feb 1989.9. D. Mills. Measuring pressure on pneumatic-conveying systems. Chem Eng, Vol 108,

No 10, pp 84-88. Sept 2001.10. D. Mills. Application of stepped pipelines in pneumatic conveying systems. Proc

Hydrotransport 15. BHR Group Conf. Banff, Canada. June 2002.11. J.K. Onley and J. Firstbrook. The practical application of pneumatic transport tech-

niques to the raising of mineral from deep shafts. Proc Pneumotransport 4. BHR GroupConf. USA. Jun. 1978.

12. J. Firstbrook. Operation and development of the pneumatic pipeline coal transportationsystem. Proc Pneumotransport 5. BHR Group Conf, London. April 1980.

13. B.F. Mehring. Recent Developments in long distance pneumatic conveying. Proc In-terbulk '89. NEC Birmingham. Trinity. Sept 1989.

14. D. Mills. The use of stepped pipelines to enable different materials to be conveyedpneumatically by a common system. Proc 7th Int Conf on Bulk Materials Storage,Handling and Transportation. University of Newcastle. Australia. Oct 2001.

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11Pneumatic Conveying of Food andChemicals

1 INTRODUCTION

A vast number of different materials are conveyed in both the food and chemicalsindustries. Probably as a consequence food and chemical products tend to have areputation for causing more problems in both the design and operation of pneu-matic conveying systems than any other group of materials. They can exhibit anextremely wide range of conveying capabilities; certainly wider than those of coaland ash considered in the previous chapter, and their conveying performance canalso vary during conveying. As with most materials, there is a dilute to densephase capability limitation, but with food and chemical products there is a morepronounced divide between moving bed and plug type flows, for those materialsthat are capable of being conveyed in dense phase.

These materials tend to come in a wide variety of forms, from fine powdersto granules and pellets, and the conveying performance of each can differ widely.The name of a material alone, in most cases, is not sufficient to define its convey-ing capability, for the same material can come in a number of different forms andgrades, and the performance of each can vary significantly. The main differencesare in the minimum conveying air velocity necessary for conveying, and in the airsupply pressure necessary to convey at a given rate. An adverse change in eitherone of these parameters is likely to result in pipeline blockage.

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336 Chapter 11

1.1 Systems and Components

In terms of the types of conveying systems employed for food and chemical prod-ucts the entire range of systems considered in chapter 1 are used. Probably themajority of these materials in finely divided form are potentially explosive andmany have very low values of minimum ignition level. As a consequence closedloop systems and the use of nitrogen for conveying is not uncommon.

The entire range of feeding devices considered in Chapter 2 are also em-ployed, although high pressure rotary valves are often preferred to blow tanks forhigh pressure conveying systems. Blow tanks are widely used for coal and ash,considered in the previous chapter, and there are no reasons why they could not bemore widely accepted in the food and chemicals industries. Other system compo-nents such as air movers, filters and valves are more or less common to all indus-tries.

1.2 Erosion and Degradation

Erosive wear tends not to be a problem of major concern, as it is with coal and ash,although with many harvested grains and seeds it does need to be given due con-sideration. Attrition and degradation of many materials, however, is often a majorconcern. As a consequence data is presented for a number of representative mate-rials, specifically to illustrate the effects that pneumatic conveying can have onthis group of materials. The problems of material degradation are considered inmore general terms in Chapter 21.

1.3 Conveying Data

To illustrate the nature of the problems of pneumatic conveying, and to show therange of conveying characteristics that can be obtained with different materials,performance data for a number of materials is presented. This conveying data willalso help to show that virtually any food or chemical product can be conveyed in apneumatic conveying system, although a large bore pipeline or a high air supplypressure may be required to achieve the desired flow rate with some materials.

2 LOW PRESSURE CONVEYING

Data is presented for a number of different materials conveyed through two differ-ent two inch nominal bore pipelines. Conveying characteristics for ammoniumchloride and PVC resin powder conveyed through the Figure 10.16 pipeline arepresented in Figures 11.1 and 11.2. In each case the materials were fed into thepipeline by means of a low pressure bottom discharge blow tank. A blow tank wasused because this one device is capable of feeding a very wide range of materialsover an extremely wide range of conveying conditions. A positive displacementblower was available, having a pressure capability of about 12 lbf/in2 and volu-metric flow rate of approximately 140 fWmin at free air conditions.

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Food and Chemicals 337

10

JO~ 6

1

I 2

0

Conveying LinePressure Drop - lbf/in2

Solids LoadingRatio

50 100

Free Air Flow Rate - ftVmin

150

Figure 11.1 Conveying characteristics for ammonium chloride conveyed through thepipeline shown in figure I O.I 6.

Sketches of the two pipelines were presented earlier in Figures 4.15 and10.16. These provide details of pipeline lengths and the number and geometry of

bends for reference.

10

ocSoi

|4

Conveying LinePressure Drop - lbf/in2

50 100

Free Air Flow Rate - ftVmin

Solids LoadingRatio

150

Figure 11.2 Conveying characteristics for PVC resin powder conveyed through thepipeline shown in figure 10.16.

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338 Chapter 11

Conveying LinePressure Drop

- lhf/in2

Solids LoadingRatio

10

I

i6

03oi

|4'—~3

00 50 100

Free Air Flow Rate - trYmin

Figure 11.3 Conveying characteristics for sodium chloride (salt) conveyed through thepipeline shown in figure 4.19.

Conveying characteristics for sodium chloride (salt), and a 'heavy' grade ofsoda ash (sodium carbonate), conveyed through the Figure 4.15 pipeline, are pre-sented in Figures 11.3 and 11.4. These two pipelines referenced here have exactlythe same pipe bore and are very similar in geometry. The Figure 4.15 pipeline isjust 5 feet longer and has one more 90° bend than the Figure 10.16 pipeline.

10

Conveying LinePressure Drop

- lhf/in2

Solids LoadingRatio

12

150Free Air Flow Rate - fr/min

Figure 11.4 Conveying characteristics for sodium carbonate (soda ash) conveyedthrough the pipeline shown in figure 4.19.

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Food and Chemicals 339

2.1 Conveying Capability

Because of the relatively high pressure gradient required to convey a material indense phase, as illustrated in Chapter 8, low pressure conveying is generally lim-ited to dilute phase conveying, unless the conveying distance is very short, as willbe seen from Figures 11.1 to 11.4. In dilute phase, however, almost any materialcan be pneumatically conveyed, regardless of the size, shape and density of theparticles. With low air pressures, positive displacement blowers and conventionallow pressure rotary valves can be used and simple systems can be built. As a resultdilute phase is probably the most common form of pneumatic conveying for thisgroup of materials.

A much higher conveying line inlet air velocity must be maintained for di-lute phase systems, even if the material is capable of being conveyed in densephase. Conveying line inlet air velocities are typically of the order of 2000 to 2400ft/min for fine powders, 3000 to 3400 ft/min for granular materials, and beyondfor larger particles and higher density materials, but provided that this minimumvelocity is maintained, most materials can be reliably conveyed. Differences inconveying capability, however, must be expected for different materials, evenwhen conveyed in dilute phase, suspension flow and this point is clearly illustratedwith Figures 11.1 to 11.4.

Although a diverse group of materials is included in Figures 11.1 to 11.4,there is not a lot of difference in their conveying capabilities with respect to airrequirements. Minimum values of conveying air velocity were about 2200 ft/minfor the ammonium chloride and 2300 ft/min for the PVC resin, salt and soda ash.Much greater differences in material flow rates were achieved, however, but this isto be expected following the comparative data plots presented in Figures 4.16 and4.18. Considering a conveying line pressure drop of 8 lbf/in2, for example, amaximum material flow rate of about 10,000 Ib/h could be achieved with the am-monium chloride in Figure 11.1. This reduces to 8,500 Ib/h for the PVC resin inFigure 11.2, to 6,500 Ib/h for the salt in Figure 11.3 and to only 5000 Ib/h for thesoda ash in Figure 11.4.

It will be noted that with the PVC resin there is a maximum value of mate-rial flow rate achieved for a given value of conveying line pressure drop. This doesoccur with certain materials and tends to be more marked in high pressure convey-ing, for materials that are capable of being conveyed in dense phase and hence atlow velocity, as will be illustrated later in this chapter. This is often referred to as apressure minimum point, for it also results in a minimum value of pressure dropfor a given material flow rate.

The conveying capability of some of these materials is considered furtherwhen data on the high pressure conveying capability of materials is presented laterin this chapter. For comparison, and reference purposes, a number of other materi-als conveyed through the Figure 10.16 pipeline are presented in Figures 10.17 to10.19. Other materials conveyed through the Figure 4.15 pipeline are presented inFigures 4.14 and 4.16.

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340 Chapter 11

2.2 Material Degradation

With the sodium chloride and soda ash, presented in Figures 11.3 and 11.4, pro-grams of conveying trials were undertaken to determine the level of degradationresulting from the pneumatic conveying of these materials [1]. Both materials wereconveyed through the Figure 4.15 pipeline for this purpose. Fresh material wasloaded into the test facility, it was circulated a total of five times and samples weretaken during each run.

Guaranteeing uniformity and accuracy in the sampling of bulk particulatematerials is always a problem and it is generally recommended that samplesshould be taken from a moving stream of the bulk material. In this case sampleswere taken by means of a diverter valve that was positioned near to the end of thepipeline.

For consistency an attempt was made to convey each material under similarconditions. It was not possible to employ identical conveying conditions for eachmaterial, of course, since the conveying characteristics differed, as will be seenfrom Figures 11.3 and 11.4. The approximate minimum and maximum values ofconveying air velocity were 3400 and 4400 ft/rnin and the solids loading ratio wasabout five. A size analysis of all the samples obtained from the fresh material, andeach of the five times the materials were re-circulated, was carried out and theresults are presented in Figures 11.5 and 11.6.

100 r

80Nt/3b-a5 60Cfl

a

" 40 r // ^ x\\x i-v; Clumber of times

Sjhaterial circulated

20

100 200 300 400 500 600

Particle Size - urn

Figure 11.5 Influence of conveying on the degradation of sodium chloride.

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Food and Chemicals 341

100 r

80

cD 60

a 40TO

20

0100 200 500 600300 400

Particle Size - u,m

Figure 11.6 Influence of conveying on the degradation of sodium carbonate.

In each case it will be seen that the material has degraded, and that a notice-able effect has been recorded every time each material was conveyed and re-circulated. In Figure 11.7 mean particle size data for the two materials is presentedso that a direct visual comparison can be made.

Sodium Chloride'(Salt)

HU8

u.0

3

0

Soda Ash

Fresh Material

200 350250 300

Mean Particle Size - um

Figure 11.7 Influence of material conveying on mean particle size.

400

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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342 Chapter 11

For the salt there was an overall reduction of about 78 jum from the freshmaterial, having a mean particle size of about 388 /jm. For the soda ash there wasan overall reduction of about 68 /um from the fresh material, having a mean parti-cle size of about 343 //m.

3 HIGH PRESSURE CONVEYING

All of the preceding data in this chapter has been for the low pressure (up to 8lbf/in2), and hence dilute phase, suspension flow of the materials considered,whether they had dense phase conveying capability or not. In this section, data ispresented for materials conveyed with air pressures of up to 30 lbf/in2 gauge.

With higher pressure air, for approximately the same length of pipeline,pressure gradients are now such that dense phase conveying is a possibility, butonly for materials that are naturally capable of being conveyed at low velocity,since this is a conventional type of conveying facility.

Although the data presented is derived from a high pressure conveying facil-ity, low pressure results are also included within the overall conveying characteris-tics, and so this area is equally appropriate for low pressure conveying systems.The data is simply compressed into a small area, rather than being magnified, aswith Figures 11.1 to 11.4.

The authors have conveyed a considerable number of different materialsthrough one particular pipeline and a sketch of this was presented earlier in Figure4.2. This is also a two inch nominal bore pipeline and materials were fed into thepipeline by means of a blow tank once again, for the same reasons as outlinedabove for the low pressure conveying data. In this case it was a high pressure, topdischarge, blow tank with a pressure rating of 100 lbf/in2 gauge. The air supplycame from a reciprocating compressor capable of delivering 200 ftVmin of free airat a pressure of 100 lbf/in2 gauge.

Conveying characteristics are presented for a copper-zinc catalyst, potas-sium chloride, magnesium sulfate and potassium sulfate in Figure 11.8. It will benoted that not one of these materials could be conveyed in dense phase and at lowvelocity, despite the availability of high pressure air. As with the group of materi-als considered above, that were conveyed in a low pressure conveying system,there was little difference in minimum conveying air velocities for these materialseither. Both the potassium chloride and magnesium sulfate required 2600 ft/min,the potassium sulfate 2800 ft/min and the catalyst 2900 ft/min.

For consistency, and ease of visual comparison, this set of conveying char-acteristics have been drawn to the same scale as those for other materials con-veyed through this same pipeline and presented earlier. From the group of materi-als presented in Figure 11.8 only the catalyst came close to being conveyed at20,000 Ib/h. It will be noted that the iron powder (Figure 4.17) was conveyed at40,000 Ib/h, and 55,000 Ib/h was achieved with both the cement (Figure 4.5b) andthe fly ash (Figure 4. lOa). For reference the other materials are alumna (4.8b), coal(10.25, 26 & 29), silica sand (4.1 Ob) and a group in Figure 4.17.

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Food and Chemicals 343

50

2 40X

J

1 30IBOH

1 20

."s'C

3 10

: 50

oo; Solids Loading ~ 40

Ratio jl

Conveying Line \ ^ 30Pressure Drop \ 3

I - Ibf/in2 \ ^ >X 25 ^^20 o 20

^"^ tu

: 2°^<W15 31S^^^^ o_^>^-«r-10 | 10

™^~~<^^ c ^• '-^^^^^r

"

; Solids LoadingRatio

\

Conveying Line \Pressure Drop \

". - Ibf/in2 ^\ ^2°

; V ._^-15

•jn " -J> '''*'

.^1^^^^^10 "V . * ^^^"^

: r ^^S"5

0 u0 50 100 150 200 0 50 100 150 200

50

0

2 40X

1"7 30u"3ai| 20

•1I 102

Q

• 50

oo2 40XJ3iB

i Solids Loading 7 39Ratio |

[ Conveying Line \ c£Pressure Drop \ | jn

". - Ibf/in2 i ^ E

: V 30 <^ 3 |~ *• i A ^ 1 0

20j,t f^... "S

"• 12_=--^==^IT~ 5

1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 n

:

-•; Solids LoadingI Conveying Line Ratio

Pressure Drop i• - Ibf/in2 1 1. j

' 2^_ ^- *£- ^J"L.' 4" , , , , , , r-rr ,*, , i , , , , 1-1 . ,

0 50 100 150 200 0 50 100 150 200

/_» Free Air Flow Rate - ft3/min / ji Free Air Flow Rate - ft3/min

Figure 11.8 Conveying characteristics for high pressure conveying of various materialsconveyed through the pipeline shown in figure 4.2. (a) A Cu-Zn catalyst, (b) potassiumchloride, (c) magnesium sulfate, and (d) potassium sulfate.

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344 Chapter 11

3.1 Conveying Capability

In order to complete the picture with regard to material conveying capability atthis point, two further materials conveyed through the Figure 4.2 pipeline are pre-sented. One is barite and the other is polyethylene pellets and this data is presentedin Figure 11.9.

These conveying characteristics are also drawn to the same scale as those forthe materials in Figure 11.8. Both of the additional materials could be conveyedwith air flow rates as low as 20 ftVmin and hence with conveying line inlet airvelocities of only 600 ft/min, which relates to dense phase flow. Although theconveying air velocity range is the same for both materials, values of solids load-ing ratios, and the slope of the constant pressure lines at low values of air flowrate, are completely different.

The barite had a mean particle size of about 15 micron and so had very goodair retention properties. As a consequence the material could be conveyed verywell in a sliding bed mode of flow, since the necessary pressure gradient wasavailable for conveying with the above test facility. These points are discussed insome detail in Chapter 4.

60

S10

Pressure Drop- lbf/in2

(a)

0 50 100 150 200

Free Air Flow Rate - ftVmin (b)

Solids LoadingRatio

Conveying LinePressure Drop

- lbf/in2

NO GO AREA

50 100 150 200

Free Air Flow Rate - frYmin

Figure 11.9 Conveying characteristics for materials capable of low velocity densephase flow conveyed through the pipeline shown in figure 4.2. (a) Barite and (b) polyethyl-ene pellets.

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Food and Chemicals 345

The polyethylene pellets had a mean particle size of about 0-15 in (4 mm)and a particle density of about 57 Ib/fT. The main feature, however, was that theparticles were all uniform and so there was virtually no separate particle size dis-tribution. As a consequence the material had very good permeability and so wouldconvey in dense phase, at low velocity, in a conventional pneumatic conveyingsystem, in plug type flow.

The very high permeability accounts for the relatively low maximum valueof solids loading ratio achieved. Powdered materials have almost no permeability,but have very good air retention properties, and so very high values of solids load-ing ratio can be achieved, particularly if high pressure gradients are available forconveying.

Although the material will convey with air velocities down to 600 ft/min,material flow rates are low at low air flow rates. There is a marked pressure mini-mum with this type of conveying characteristic, such that below the pressureminimum a decrease in air flow rate will result in a decrease in material flow ratefor a given air supply pressure.

If particle melting, and the formation of 'angel hairs' is a problem with thistype of material, however, low velocity conveying is an option for minimizing theproblem. The conveying characteristics of this type of material are consideredfurther in a later section in this chapter.

3.2 Dilute Phase Conveying

Two of the materials that performed very poorly in the Figure 4.2 pipeline weremagnesium sulfate and potassium sulfate. They were poor performers only interms of the material flow rates achieved, but presented no more difficulties inconveying than any other material. Further high pressure conveying data for thesetwo materials conveyed through the Figure 7.13 pipeline is presented in Figure11.10. This pipeline was 310 feet long and 3 inch nominal bore and 400 ft3/min offree air was available for conveying.

If the data for the magnesium sulfate from Figure 11.8 is compared with thatfrom Figure 11.10 it will be seen that the maximum material flow rate for a con-veying line pressure drop of 30 lbf/in2 has increased from about 13,000 Ib/h to25,000 Ib/h. The maximum material flow rate for the potassium sulfate, for a pres-sure drop of 25 lbf/in2 has increased from about 8,000 Ib/h to 15,000 Ib/h. Thisclearly shows the influence of pipeline bore in terms of increasing the flow rate ofa material in a pneumatic conveying system.

The increase in material flow rates in the above cases is approximately 90%.This is despite the fact that the three inch bore pipeline is about 90% longer thanthe two inch bore pipeline. The two pipelines, however, have exactly the samenumber of bends. Scaling parameters based on the use of conveying data are con-sidered in Chapter 15. These can take account of differences between the bore,length, pipeline orientation, and number and geometry of bends, such as that be-tween a test facility and a plant pipeline.

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346 Chapter 11

Conveying LinePressure Drop

- lbf/in2 v

NO GO AREA

Solids Loadingp RatioLine Ratio

0 100

(a)

1UU Z.UU JUU

Free Air Flow Rate

100

5 - ftVminFree

Figure 11.10 Conveying characteristics for materials conveyed through the pipelineshown in figure 7.13. (a) Magnesium sulfate and (b) potassium sulfate.

3.3 Material Grade

Great care must be exercised when a specific name is given to a material. Manymaterials are available in a variety of grades and it is quite possible for the convey-ing characteristics for different grades to be very different from one another. Aparticular case is that of pulverized fuel ash, and this was considered in some de-tail in Chapter 10. Although it is exactly the same material, the ash collected indifferent hoppers in a boiler plant will have very different particle size distribu-tions. That collected in the economizer hoppers, close to the combustion zone, willgenerally be very coarse and have no dense phase conveying capability at all. Thatcollected in the electrostatic precipitator hoppers, furthest from the combustionzone, will be a very fine powder and will convey in dense phase very well. Thedifferences in conveying capability were clearly illustrated in Figure 10.14.

Data for the conveying of two grades of dicalcium phosphate is included inFigure 11.11. These materials were also conveyed through the three inch nominalbore pipeline shown in Figure 7.13. One is referred to as 48% dicalcium phosphateand the other as 52% dicalcium phosphate. Although they have the same name theconveying performance of the two materials is widely different. The 48% materialwas a fine powder with very good air retention properties and the 52% was acoarse granulated material with neither good air retention nor permeability.

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Food and Chemicals 347

80

2 60

I 40

_o

20

0

30

120100 Solids Loading80 Ratio I

60

25

80

o0

2 60X

JD

iJ

_0

"s'£ 2003

n

; NO

" GOSolids Loading

• AREA Ratio

Conveying Line \Pressure Drop \

- lbf/in2 \

[ \ *^^1 -^

U-\^^'0

(a)

100 200 300 400Free Air Flow Rate - ftVmin (b)

100 200 300 400

Free Air Flow Rate - ft3/min

Figure 11.11 Conveying characteristics for (a) 48% grade and (b) 52 % grade of dical-cium phosphate conveyed through the pipeline shown in figure 7.13.

It will be seen that the axes for the material flow rate on the Figure 11.11axes had to be doubled from that employed on Figure 11.10 in order to accommo-date the data for the 48% dicalcium phosphate. The material flow rate achievedwith the 52% dicalcium phosphate was less than one-third of that for the 48%grade, and only marginally better than that for the potassium sulfate in Figure11.10. Maximum values of solids loading ratios achieved differ by a factor of

about ten to one.

3.4 Degraded Material

For materials that are very friable it is possible for them to degrade as a result ofbeing pneumatically conveyed. As a direct consequence of this it is possible forthe conveying performance of the material to change. This is essentially an exten-sion of the issue discussed above on the influence of material grade. Degradationof the material, as a result of conveying, can effectively cause a change in thegrade of the material.

This is particularly a problem with materials that border on the edge of thesliding bed mode of conveying capability. Such materials are typically fine granu-lar but do not have sufficient air retention to be capable of dense phase conveying.The fines generated while conveying the material, however, will cause an increase

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348 Chapter 11

in the air retention properties of the material and this, in turn, will result in a grad-ual lowering of the minimum velocity at which the material can be conveyed.

These issues are illustrated with Figure 11.12 which presents conveyingcharacteristics for sodium sulfate. Once again they relate to the 310 ft long pipe-line of three inch nominal bore shown in Figure 7.13. Figure 11.12a is for the ma-terial in the 'as supplied' condition and Figure 11.12b is for exactly the same ma-terial after it had been re-circulated a number of times. It will be seen that there aresignificant changes in both the minimum conveying air velocity at which the ma-terial could be conveyed and the material flow rate achieved for a given value ofconveying line pressure drop. As a result of these two changes there has also beena significant increase in the value of the solids loading ratio at which the materialcould be conveyed.

The conveying capability of the degraded material shown in Figure 11.12bis sometimes referred to as 'medium phase' conveying. This is not a commonsituation because of the specific material properties required. In true dense phaseconveying solids loading ratios of over 100 would be achieved with a conveyingline pressure drop of 30 lbf/in2 in a pipeline of this length, as demonstrated withthe dicalcium phosphate in Figure 11.1 la.

40

- 30X

|

u

(2 20

_0

u."sI 10

Conveying LinePressure Drop

- Ibf7in2

Solids LoadingRatio

Solids LoadingRatio /

Conveying Line /Pressure Drop \ /

-lbf/in2 30//14'12

25J""NO GO

40

2 30

ID

I 20

I 10Conveying

Limit

(a)

100 200 300 400

Free Air Flow Rate - ftVmin (b)

100 200 300 400

Free Air Flow Rate - ft3/min

Figure 11.12 Conveying characteristics for (a) fresh and (b) degraded sodium sulfateconveyed through the pipeline shown in figure 7.13.

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Food and Chemicals 349

If the material were to be re-circulated and degraded further it is likely thatconveying would be possible at progressively lower velocities and higher solidsloading ratios. For materials with no dense phase conveying capability it isunlikely that they could be conveyed at a solids loading ratio much higher than 25with a conveying line pressure drop of 30 lbf/in2 in a pipeline of this length.

If dramatic changes such as these occur to a material over relatively shortconveying distances, it is generally necessary to undertake conveying trials withfresh material every time it is conveyed. If this is not done serious errors can resultif conveying characteristics such as those presented here are to be produced. Theissue of material degradation as a result of re-circulating is considered in moredetail in relation to soda ash later in this chapter.

3.5 Plastic Materials

On Figure 11.2 the low pressure conveying characteristics for PVC resin powderwere shown to exhibit a pressure minimum point, with the material flow rate de-creasing with decrease in air flow rate beyond the optimum point. This type ofmaterial does appear to exhibit this particular characteristic, being a combinationof the conveying characteristics for conventional powders and those for plasticpellets. This particular material was also conveyed in a high pressure test facilityand a sketch giving appropriate details of this for reference is presented in Figure11.13 [2].

Data for two materials conveyed through this two inch nominal bore pipe-line is presented in Figure 11.14. The materials presented are the PVC resin pow-der and terephthalic acid (PTA). Both materials were conveyed with conveyingline pressure drop values of up to 30 lbf/in2 and both were clearly capable of beingconveyed in dense phase.

Pipeline:length = 165ftbore = 2 inbends = 8 * 90°

Figure 11.13 Details of pipeline used for the high pressure conveying of various plas-tic materials.

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350 Chapter 11

60

50

60

Conveying LinePressure Drop

Solids LoadingRatio

50ooo

1 Conveying LinePressure Drop

- Ibf/in2

Solids LoadingRatio

(a)Free Air Flow Rate - ft/min (b)

50 100 150 200

Free Air Flow Rate - ftVmin

Figure 11.14 Conveying characteristics for (a) PVC resin powder and (b) terephthalicacid conveyed through the pipeline shown in figure 11.13.

Both materials exhibited marked pressure minimum points, with that for theterephthalic acid being at a much lower air flow rate, and hence lower velocity,than the PVC resin. As a consequence much higher material flow rates and solidsloading ratios were obtained with the terephthalic acid than for the PVC.

The reason for the change in slope from negative to positive for the constantpressure lines at low air flow rates is not fully understood. The pressure dropcurves for the materials presented on Figure 11.14 may appear to be more logical,as it could be argued that the pressure drop curves might be expected to passthrough the origin, or close to it, as one would expect no material flow rate with noair flow. At very high air flow rates all of these curves have a negative slope andthese are consistent with a square law influence of velocity on pressure drop, andhence factional forces dominating in this region.

Although powdered materials of a non-plastic type are mostly depicted asmaintaining a negative slope throughout the range of conveying capability, as pre-sented in these notes, work with pulverized fuel ash has shown this same charac-teristic [3]. It generally occurs at extremely low values of conveying air velocityand so tends not to be included in conveying data, as it is generally too low forpractical application.

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Food and Chemicals 351

The pipeline shown in Figure 11.13 is very similar in terms of distance andgeometry to that of the Figure 4.2 pipeline and so the data for the materials pre-sented in Figure 11.12 can be compared reasonably well with that for the materialsshown in Figures 11.8 and 11.9.

It will be seen that the material flow rates obtained with both the PVC resinand the terephthalic acid were much higher than any of the materials presented inFigure 11.8. This is due, in part to the fact that the plastic materials could be con-veyed in dense phase, and hence at very much lower air flow rates. In comparisonwith the barite in Figure 11.9, however, material flow rates were significantlylower.

3.6 Pelletized Materials

Conveying characteristics for polyethylene pellets were presented earlier in Figure11.9. These were derived for flow through a two inch nominal bore pipeline. Inanalyzing the conveying data to produce the conveying characteristics it wasfound that the lines of constant conveying line pressure drop gradually mergedtogether as the air flow rate reduced and it was felt that this was a function of therelatively small bore pipeline employed.

In a similar program of conveying trials, carried out with nylon pellets in athree inch bore pipeline, it was possible to achieve an effective magnification ofthis area. A sketch of the pipeline used for the conveying trials with the nylon pel-lets is given in Figure 11.15 for reference. A high pressure, bottom discharge blowtank, was used for feeding the material into the pipeline. The nylon pellets had asimilar mean particle size to that of the polyethylene pellets, being about 0-15 in (4mm), and were essentially mono-sized once again.

Pipeline Details:length = 160ftbore = 3 inbends = 6 * 90°D/d = 5

Figure 11.15 Details of pipeline used for the high pressure conveying of nylon pellets.

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352 Chapter 11

Materials that will naturally convey in dense phase plug type flow require avery high value of permeability and so this mode of conveying is limited to mate-rials such as grains, seeds and pellets that have a very narrow particle size distribu-tion. The conveying characteristics for nylon pellets conveyed through the threeinch nominal bore pipeline shown in Figure 11.15 are presented in Figure 11.16a.This shows that the lines of constant conveying line pressure drop are very closetogether in the dense phase conveying region, at low air flow rates. Once again themaximum value of solids loading ratio is only about thirty.

In Figure 11.16b the conveying characteristics for the nylon pellets are pre-sented in terms of pressure drop plotted against air flow rate, with material flowrate as the family of curves. In many text books and articles on the subject ofpneumatic conveying this is commonly the form of presenting these relationships,either with logarithmic axes or no scales at all, and are generally for illustrationpurposes only. It is also more difficult to add lines of constant solids loading ratioto this plot. This form of presentation, however, clearly shows that a pressureminimum occurs with this material, such that there is a clearly defined value of airflow rate at which the conveying line pressure drop is a minimum for a given ma-terial flow rate.

Solids Loading

50 h Ratio \

40

30

20

10

,30

-Conveying LinePressure Drop

- Ibf7in2

(a)

0 100 200 300 400

Free Air Flow Rate - ft3/min

30

~ 25

20

15

CJc

co0

10

MaterialFlow RateIb/hxlOOO

40

(b)

0 100 200 300 400

Free Air Flow Rate - fr'/min

Figure 11.16 Nylon pellets conveyed through the pipeline shown in figure 11.15. (a)Conveying characteristics and (b) pressure drop data.

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Food and Chemicals 353

From Figure 11.16a it will be seen that the pressure minimum point occursat a gradually reducing value of air flow rate as the value of conveying line pres-sure drop decreases. This pressure minimum point does, in fact, occur at a convey-ing line inlet air velocity of approximately 2600 ft/min over the entire range ofpressures investigated. Above this velocity the conveying characteristics are verysimilar to those of any other material conveyed in dilute phase, suspension flow.2600 ft/min is approximately the minimum value of conveying air velocity for thedilute phase conveying of this material. This point is considered further later inthis chapter.

The material, however, is clearly capable of being conveyed at much lowervelocities than 2600 ft/min and it is also clear that at the pressure minimum pointthe mode of flow starts to change to one of plug flow, with reduction in air flowrate. Some materials have a smooth transition from dilute to dense phase flow withreduction in air flow rate, some show very erratic and unreliable behavior in thisregion, and others have a band of velocity values across which they cannot beconveyed, but when the velocity reduces to about 1000 or 1200 ft/min most mate-rials of this type will be capable of dense phase plug type flow in a conventionalconveying system.

3.7 Soda Ash

Light sodium carbonate (light soda ash) typically has a mean particle size of about115 micron and has something of a reputation for being a difficult material to con-vey. It is a friable material and slightly hygroscopic. In order to learn something ofits conveying capability a controlled program of conveying trials was undertaken[4]. A sketch of the pipeline used for this test work is given in Figure 11.17. Ahigh pressure, bottom discharge blow tank, was used for feeding the material intothe pipeline.

Pipeline Details:length =120 ft

Figure 11.17 Details of pipeline used for the high pressure convey ing of soda ash.

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354 Chapter 11

The normal procedure in obtaining a set of conveying characteristics for amaterial it to load a sample of the material into the supply hopper of the test facil-ity and to re-circulate the material. Each time the material is conveyed, the air flowrate, material flow rate and/or the air supply pressure are varied in order to achieveas wide a spread of test data as possible. It is normal practice to repeat tests on asequential basis in order to ensure that the recirculation of the material is not caus-ing any change in the conveying characteristics. This was observed with the sodaash, and it was so marked that it was decided that the conveying characteristicsshould be obtained on the basis of using fresh material for every test run.

The conveying characteristics determined for the soda ash on the basis of us-ing fresh material for every test are presented in Figure 11.18a. The conveyingcharacteristics obtained in the usual way, with material constantly re-circulated,are presented in Figure 11.18b for reference and comparison.

With many materials, when they become degraded, there is a tendency forthem to achieve a degree of air retention, and to have the capability of being con-veyed at a much lower velocity, as was the case with the sodium sulfate reportedearlier in Figure 11.12. With the soda ash the main influence was on the convey-ability of the material, in terms of the mass flow rate of the material achieved, for agiven value of conveying line pressure drop, and not to a lower velocity capability.

Solids LoadingRatio

40

Solids LoadingConveying Line Ratjo

Pressure Drop

30

(a)

0 50 100 150 200Free Air Flow Rate - ftVmin

(b)

0 50 100 150 200Free Air Flow Rate - ft3/min

Figure 11.18 Conveying characteristics for soda ash conveyed through the pipelineshown in figure 11.17. (a) Fresh material and (b) re-circulated material.

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Food and Chemicals 355

As will be seen from Figure 11.18a, the fresh material had a degree of densephase conveying capability since it could be conveyed at solids loading ratios ofup to about 60. Although higher values of solids loading ratio were achieved withthe degraded material this was due to the fact that higher material flow rates wereachieved. The fresh material showed a marked pressure minimum point in theconveying characteristics, whereas the degraded material showed no change inslope of the constant conveying line pressure drop curves with reduction in airflow rate.

3.7.1 Particle Size Changes

For reference purposes a batch of material was re-circulated and samples weretaken after every pass to show how the recirculation influenced the mean particlesize. For consistency the same air and material flow rates were maintained eachtime the material was re-circulated. A typical set of results is shown in Figure11.19. This is a plot of the mean particle size of the soda ash every time it wasconveyed.

The pipeline was only 120 ft long, with a total of five bends, but in the caseillustrated the material degraded from a mean particle size of about 117 micron to97 micron in the first pass. After ten passes the mean particle size had reduced toabout 73 micron. The maximum value of conveying air velocity was only 3500ft/min in this program of tests, in an attempt to minimize degradation. For the veryfirst pass the conveying line pressure drop was about 44 Ibf/in" and so the convey-ing line inlet air velocity in this case was about 900 ft/min.

120E

I'00

1 8°<X

I 6°

40

V0 = 80 frVmin

mp = 30,000 Ib/h

Cmax = 3,500 ft/min

4 6 8

Number of Passes through Pipeline

10

Figure 11.19 The influence of material recirculation on the mean particle size of thesoda ash.

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356 Chapter 11

3.7.2 Pressure Drop Changes

In the program of tests reported above, the material was conveyed with exactly thesame air flow rate, and the material was conveyed at a rate of 30,000 Ib/h eachtime. Every time the material was conveyed, therefore, the value of the conveyingline pressure drop was recorded.

The influence of material re-circulation on the conveying line pressure dropis shown in Figure 11.20. As mentioned above this was about 44 lbf/in2 for thefirst pass and it had reduced to about 27 lbf/in2 for the tenth and last pass. Fromthis it will be seen that there was a gradual, but very significant, reduction in pres-sure drop as the material was conveyed.

From the complete sets of conveying characteristics for the fresh and re-circulated materials the 15 lbf/in2 pressure drop lines have been compared on Fig-ure 11.21. This shows quite clearly the very significant differences that there arebetween the two samples of material. It is interesting that the difference in materialflow rates between the two increases with decreasing air flow rate, and hence withreducing conveying air velocity.

This, however, relates to the change in the nature of the conveying charac-teristics for the material, as discussed above. The fresh material clearly has a pres-sure minimum point, at a conveying line inlet air velocity of about 3000 ft/min,and the degraded material is typical of the majority of powdered materials in dis-playing no pressure minimum point. This is another aspect of pneumatic convey-ing that is not fully understood at the present time.

c 60„

m = 30,000 Ib/h

Qg 40

00.S 20

CoU

= 80 fr/min

P= 3,500 ft/min

0 2 4 6 8 1 0

Number of Passes through Pipeline

Figure 11.20 The influence of material recirculation on conveying line pressure dropfor the soda ash.

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Food and Chemicals 357

20ooo

110

_ou.

Re-circulatedMaterial

60 50

Solids LoadingRatio

15

/Pressure Drop

- lbf/in2

Fresh Material

I I I I I I I I

50 100 150

Free Air Flow Rate - ftVmin

200

Figure 11.21 Comparison of pressure characteristics for fresh and re-circulated sodaash.

4 MULTIPLE MATERIAL CONVEYING

Not all pneumatic conveying systems are dedicated to the conveying of a singlematerial. There is often a need for a system to transport a number of different ma-terials. In food related industries, in particular, a wide variety of materials have tobe conveyed by a common system, since there is a requirement to deliver a given'menu' for a particular process.

Some of the materials to be transported may be capable of being conveyedin dense phase, and hence at low velocity, while others may have no dense phaseconveying capability and will have to be conveyed in dilute phase with a highconveying air velocity. This is illustrated with the case of wheat flour and granu-lated sugar, conveyed through the same pipeline. Conveying characteristics forthese two materials, conveyed through the Figure 4.2 pipeline, are presented inFigure 11.22.

There is often a requirement for these two materials to be conveyed througha common pipeline. From the conveying characteristics presented in Figure 11.22,however, it will be seen that there are considerable differences in the conveyingcapabilities of these two materials. Wheat flour can be conveyed in dense phaseand with conveying air velocities down to about 600 ft/min, and with a conveyingline pressure drop of 25 lbf/in2 a material flow rate of about 24,000 Ib/h can beachieved with a free air flow rate of about 45 ftVmin.

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358 Chapter 11

50

40

- 30

I 20tu

10

ConveyingLimit , Solids Loading

i inn RatioConveying Line {//Pressure Drop VX/ ,80

- lbf/in2

50

ConveyingLimit

NO GO AREA

Solids LoadingRatio

Conveying LinePressure Drop '0

- lbf/in2 5

160 200

(a)Free Air Flow Rate - ft/min

(b)

0 40 80 120 160 200

Free Air Flow Rate - ftVmin

Figure 11.22 Conveying characteristics for (a) wheat flour and (b) granulated sugarconveyed through the pipeline shown in figure 4.2.

Granulated sugar, however, can only be conveyed in dilute phase and re-quires a minimum conveying air velocity of about 3200 ft/min, and with the samepressure drop of 25 lbf/in2 a material flow rate of only 15,000 Ib/h can be achievedand this requires a free air flow rate of about 185 ff/min.

There are a multitude of different possibilities for conveying both of thesematerials with a common system. Some of these are outlined below:

D One would be to control the volumetric flow rate of the air for the flourso that both materials are conveyed under the optimum conditions detailedabove. Changing air flow rates for each material is not always possible orconvenient, however, and if the surplus air had to be discharged to atmos-phere it would be a very significant waste of energy.

D If a larger bore pipeline could be used to convey the flour no changeneed be made to the common air supply. In the above case the diameter ofthe pipeline could be increased to 4 inches. This would reduce the convey-ing line inlet air velocity to 800 ft/min for the flour and increase the mate-rial flow rate to about 96,000 Ib/h.

0 If it was necessary to use the same pipeline bore and air flow rate forboth materials, the flow rate for the flour will reduce to about 13,000 Ib/h,

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Food and Chemicals 359

as will be seen from Figure 11.22a, which is less than that for the sugar,and is clearly a very inefficient option.

D If an air flow rate of 45 ftVmin was to be used for both materials 24,000Ib/h of flour would be conveyed, but as will be seen from Figure 11.22b,there would be no possibility of conveying any sugar. Only if the diameterof the pipeline for the sugar was reduced to one inch would it be possibleto convey the sugar with 45 ftVmin at 25 psig, but the material flow ratewould be reduced to about 3,000 Ib/h, which is unlikely to be acceptable.

D A further possibility is to use a smaller bore pipeline for the sugar and tostep the diameter to a larger bore along its length. By this means exactlythe same air supply could be used for both materials and a common pipe-line could be used to feed the materials into the reception hopper, if re-quired. A sketch of such a system is given in Figure 11.23. It is based onthe use of a 10 inch bore pipeline for the flour, with an air supply of 2250ftVmin of free air delivered at 30 lbf/in2 gauge.

For the given air supply specification of 2250 fVVmin of free air at 30 lbf/in2

gauge and the pipeline bores indicated on Figure 11.23, the velocity profiles forthe flow of the two materials through the two pipelines are presented in Figure11.24. By using a 6 inch bore pipeline for the sugar it will be seen that a pick-upvelocity of about 3770 ft/min could be achieved and by stepping up to 8 and then10 inch bore, as shown on Figure 11.24, the minimum conveying air velocitycould be kept at about this value throughout the pipeline. For the flour the pick-upvelocity would be about 1360 ft/min, expanding to about 4125 ft/min.

10 in Bore

Common 10 in Bore

Flour HoppersvvReception

Silo

8 in Bore

Compressor 6 in Bore

Figure 11.23 Sketch of a typical positive pressure conveying system for conveyingdiverse materials and utilizing a stepped pipeline.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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360 Chapter 11

_o"u

6000

4000

00c

2000coU

Pipeline Bore - in

Minimum conveying

air velocity for sugar

Possible velocityprofile for flour

line Inlet

0 10 20 30

Conveying Line Pressure - lbf/in2 gauge

Figure 11.24 Velocity profiles for conveying diverse materials in a common system.

If there was not a need for the two pipelines to be a common diameter tofeed into the reception hopper a separate 12 inch bore pipeline could be used, forwhich the pick-up and exit velocities would have been 940 and 2860 ft/min re-spectively.

Although 940 ft/min is satisfactory for the inlet velocity, the relatively lowexit velocity will mean that it will take a little longer to completely purge the pipe-line should this be a requirement for the material. This point was considered ear-lier with Figure 9.8 where purging of stepped pipelines was considered in somedetail. In the previous chapter a similar conveying duty was considered with re-spect to different grades of fly ash, but a vacuum conveying system was used inthat case.

5 CONVEYING AIR VELOCITIES

In order to illustrate further, and so reinforce the importance of conveying air ve-locity, the conveying characteristics for three representative materials presentedhere have been magnified and lines of constant conveying line inlet air velocityhave been superimposed. This is the minimum value of conveying air velocity inthe pipeline, which occurs at the material feed point into the conveying pipeline,whether the system is a positive pressure or vacuum conveying system, and iscommonly referred to as the 'pick-up' velocity.

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Food and Chemicals 361

30 Conveying Line Inlet AirVelocity - ft/min

Solids Loadingj. Ratio

7 20Conveying Line

Pressure Drop - lbf/in2

1 10<D

I

00 100 200 300 400

Free Air Flow Rate - ftVmin

Figure 11.25 Conveying characteristics for 52% dicalcium phosphate with pick-upvelocities superimposed.

Conveying characteristics for the 52% grade of dicalcium phosphate con-veyed through the Figure 7.13 pipeline, with pick-up velocities superimposed, arepresented in Figure 11.25. Lines of constant pick-up, or conveying line inlet airvelocity, can be superimposed on the conveying characteristics quite easily. This ispurely a mathematical process [5J. It is simply a function of the free air flow rate,the conveying line inlet air pressure and the pipeline bore, for a given air tempera-ture. The horizontal axis could also have been presented in terms of conveying lineexit air velocity, for as the conveying line exit air pressure in a positive pressureconveying system is atmospheric pressure, and hence constant, it is a direct con-version, unlike inlet air velocity.

These new conveying characteristics for the 52% dicalcium phosphate aretypical of all the materials presented earlier that are only capable of being con-veyed in dilute phase suspension flow. For this particular material the minimumvalue of conveying air velocity that could be used was about 2400 ft/min. For sys-tem design purposes a 20% margin would generally be suggested and so a convey-ing line inlet air velocity of about 2900 ft/min would be recommended.

Similar conveying characteristics for the 48% grade of dicalcium phosphatefrom Figure 11.11 a, also with pick-up velocities superimposed, are presented inFigure 11.26. These are also typical of all the materials presented earlier that arecapable of being conveyed at low velocity in dense phase in a sliding bed mode ofnon-suspension flow. The minimum conveying air velocity for this material wasabout 600 ft/min.

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362 Chapter 11

,„„„ ^ Conveying Line Inlet AirConveying Line 1000 Velocity - ft/min

gQ i Pressure Drop 'zu

" 2000 Solids LoadingRatio

~25////// X' ^* ^^w~x 60

I

0

a 20I

0

100 200 300 400Free Air Flow Rate - ft'/min

Figure 11.26 Conveying characteristics for 48% dicalcium phosphate with pick-upvelocities superimposed.

Once again a 20% margin on the minimum value would be suggested for thepick-up velocity for design purposes. The operating envelope for the conveying ofthis type of material at low velocity is very wide and so system control is not gen-erally a problem, provided that the minimum conveying air velocity is always areasonable margin above that dictated by the corresponding value of solids loadingratio. This was considered earlier at Figures 4.6 and 7.23.

Similar data for the nylon pellets presented earlier in Figure 11.16a areshown in Figure 1 1.27. Because of the positive slope to the lines of constant con-veying line pressure drop at low air flow rate, the operating envelope for the con-veying of this type of material at low velocity is very limited. As a consequenceconveying line pressure drop may be difficult to control. This particular aspect ofthe problem is shown more clearly with the pressure drop versus air flow rate plotin Figure 11.16b.

5.1 Minimum Conveying Air Velocities

The minimum values of conveying air velocity for some of the materials reportedhere are presented below in Table I l . l . Extreme caution must be taken in usingsuch data, for small changes in grade and characteristics of a given material canhave a significant influence on the value of minimum velocity. It would normallybe recommended that a conveying line inlet air velocity about 20% greater thanthe minimum conveying air velocity value reported should be used for the designof any conveying system, as considered above.

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Food and Chemicals 363

50

o2 40X

^ 30<u"cdOS5.2 20

"2

13S 10

Conveying Line Inlet AirVelocity - ft/min

Conveying Line ^Pressure Drop - lbf/in2

Solids LoadingRatio

1000

I I I

100 200 300Free Air Flow Rate - ftVmin

400

Figure 11.27 Conveying characteristics for nylon pellets with pick-up velocities super-imposed.

Table 11.1 Conveying Data

Material

Ammonium Chloride

Barite

Cu-Zn Catalyst

Dicalcium Phosphate:

48%

52%

Magnesium Sulfate

Location of Data

Figure Number

Dilute Phase Dense Phase

I I . I

11. 9a 11.9a

11. 8a

l l . l l a l l . l l a

11.2

11. l i b

11.25

11. 8c

l l . l O a

Minimum Conveying Air

Velocity -

Dilute Phase

2400

2400

2900

2100

2400

2600

ft/min

Dense Phase

X

600

X

600

x

x

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364 Chapter 11

Table 11.1 Conveying Data - Continued

Material

Location of Data

Figure Number

Minimum Conveying Air

Velocity - ft/min

Dilute Phase Dense Phase Dilute Phase Dense Phase

Nylon Pellets

Polyethylene Pellets

Potassium Chloride

Potassium Sulfate

PVC Resin

Sodium Carbonate:

Heavy Soda Ash

Light Soda Ash

Sodium Chloride

Sodium Sulfate

Sugar - Granulated

Terephthalic Acid

Wheat Flour

11.16a 11.16a

11.27 11.27

11. 9b 11. 9b

11. 8b

11. 8d

l l . l O b

11.2

11.14b ll.Hb

11.4

11.18a 11.18a

11.3

11.12a

11.22b

11.14a 11.14a

11.22a 11.22a

2600

2600

2600

2800

2300

2300

2100

2300

2300

3200

2300

1900

500

500X

X

600

X

/

X

x

X

600

600

REFERENCES

1. D. Mills. The degradation of bulk solids by pneumatic conveying and its simulation bysmall scale rigs. BMHB. 195 pp. Feb 1989.

2. J.S. Mason and D. Mills. The control of the flow of materials in a blow tank pneumaticconveying system. Proc 3rd Powder and Bulk Solids Conf. pp 392-400. Chicago. May1978.

3. D. Mills and V.K.. Agarwal. Pneumatic conveying systems - design, selection, opera-tion and troubleshooting with particular reference to pulverized fuel ash. 386 pp. TransTech Pub. 2001.

4. M.G. Jones, D. Mills, and N. Rolfe. The influence of product degradation on the pneu-matic conveying capability of light sodium carbonate. Proc Pneumatech 4. pp 427-446.Glasgow. June 1990.

5. D Mills. Optimizing pneumatic conveying. Chemical Engineering. Vol 107. No 13. pp74-80. December 2000.

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12Pneumatic Conveying in theAluminum Industry

1 INTRODUCTION

The aluminum industry employs pneumatic conveying widely for its materialshandling processes. As with many industries, there is economy in scale, and soindividual plants tend to be very large. It is still very much an expanding industry,and an industry that most large countries in the world like to have as part of theirindustrial infrastructure, particularly if cheap power is available from hydro-electric sources or from surplus gas reserves.

The economy of scale is such that alumina is one of the major bulk solidsthat is widely transported around the world by bulk carrier. At ports, ship off-loading systems based on pneumatic conveying of the material are commonlyemployed, such as that depicted in Figure 1.7. Over-land transport is generally byrail vehicles and these often have the capability of being pressurized to about 30lbf/in" gauge so that they can be off-loaded by positive pressure conveying sys-tems in a reasonably short period of time.

1.1 Systems and Components

The first point to note about alumina is that it is a very abrasive material. As aconsequence this must feature prominently in all decisions made with regard to theselection of systems and components.

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366 Chapter 12

This is one of the industries that tends to employ fluidized motion convey-ing systems. This type of conveying system was introduced in section 7 of Chapter1 and are considered in some detail in Chapter 18. Air-assisted gravity conveyorshave been quite widely used for fifty years or more, but the more recent innova-tion of full channel conveyors are gaining wider acceptance for alumina. A par-ticular advantage of this type of system is that the air requirements are very lowand the transport velocity is also very low, and so problems of wear associatedwith alumina are significantly reduced.

This is also an industry where innovatory pneumatic conveying systems areemployed. This type of system was introduced in section 6 of Chapter 1 and areconsidered in more detail in Chapter 17. Plug forming systems based on internalby-pass pipes are probably the most commonly used system. This, once again, isas a consequence of the abrasive nature of the materials conveyed.

This type of system, however, should only be used when actually required,and this is dictated by the grade of the material. If a material has good air retentionproperties it will convey quite naturally in dense phase and at low velocity in aconventional pneumatic conveying system, and an innovatory system would bequite unnecessary.

With regard to pipeline feeding devices, the ideal requirement is that thefeeder should have no moving parts, particularly if there is a pressure drop acrossthe feeder. Blow tanks, therefore, are widely used. If it is necessary to used a ro-tary valve then it will have to be made of appropriate wear resistant materials, forboth the rotor and casing, and an increase in air leakage with respect to time mustbe anticipated for the feeder.

2 MATERIAL GRADE

Alumina is another material that comes in a range of grades, and the grades aresuch that the material may be a powder having good air retention properties, inwhich case it may be capable of being conveyed in dense phase. Alternatively, if itcomes as a fine granular material with very poor air retention it will probably onlybe capable of being conveyed in dilute phase in a conventional pneumatic convey-ing system. It is a fine division between the two, as was considered in the previouschapter, with regard to the degradation of fine granular materials.

Alumina in fine powdered form is often referred to as floury alumina and isgenerally capable of being conveyed naturally in dense phase and hence at lowvelocity. Fine granular alumina is often referred to as sandy alumina and this isgenerally only capable of being conveyed in dilute phase suspension flow. Theconveying characteristics for two typical grades of alumina were presented in Fig-ure 9.11 in relation to the use of stepped pipelines for the conveying of diversematerials. In order to reinforce, at the outset, this important point of the influenceof material grade on pneumatic conveying performance, these conveying charac-teristics are reproduced here in Figure 12.1.

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Aluminum Industry Materials 367

Conveying LinePressure Drop - lbf/in2

ConveyingLimit

50

40

[200 120 100 80

30

20

1003

0

SolidsLoading

Ratio

ooo

50'

Solids LoadingRatio

Conveying LinePressure Drop

- lbf/in2 /NO

(a)

0 40 80 120 160 200

Free Air Flow Rate - ft3/min(b)

0 40 80 120 160 200

Free Air Flow Rate - ft3/min

Figure 12.1 Conveying characteristics for (a) floury and (b) sandy grades of alumina.

A sketch of the two inch nominal bore pipeline through which these twomaterials were conveyed is presented in Figure 12.2 for reference. A high pressurebottom discharge blow tank was used to feed the materials into the pipeline.

Figure 12.2 Details of pipeline used for the conveying of the two grades of aluminapresented in figure 12.1.

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368 Chapter 12

As a consequence of the relatively short length of the pipeline, and the highpressure air available for conveying, solids loading ratios of up to 200 wereachieved with the floury alumina. Values up to about 40 were achieved with thesandy alumina, but despite the high pressure air available, the material could notbe conveyed in dense phase and at low velocity.

2.1 Conveying Air Velocities

More detailed conveying data for this floury grade of alumina is presented in Fig-ure 12.3, with lines of constant conveying line inlet air velocity also superimposed.On this plot a second horizontal axis has been added. This is of conveying line exitair velocity. Since the conveyed material at the end of the pipeline is always atatmospheric pressure, conveying line exit air velocity is directly proportional tofree air flow rate and so both axes apply.

With both conveying line inlet and exit values of conveying air velocity rep-resented, the magnitude of the expansion of the air through the pipeline can beclearly seen. From points on the 45 lbf/in2 pressure drop curve, for example, it willbe seen that the conveying air velocity expands by a factor of approximately fourtimes between inlet and outlet. This is due to the fact that absolute values of pres-sure, and temperature, have to be used in all equations associated with the com-pressible flow of air.

200. 160 120 100Ratio

o 40^o50

40J 30

o

E20.2'C

I 10 Conveying LinePressure Drop

- lbf/in2

50 100 , 150 200Free Air Flow Rate - ftVmin

0 2000 4000 6000 8000

Conveying Line Exit Air Velocity - ftVmin

Figure 12.3 Conveying data for floury alumina conveyed through the pipeline shownin figure 12.2.

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Aluminum Industry Materials 369

50ooo'40

730

$20_o

I 10

Solids Loading RatioConveying

Limit

Conveying Line Inlet AirVelocity - ft/min

sure Drop - lbf/in2

0 50 100 , 150 200Free Air Flow Rate - fP/min

0 2000 4000 6000 8000Conveying Line E\\t Air Velocity - ftj/min

Figure 12.4 Conveying data for sandy alumina conveyed through the pipeline shownin figure 12.2.

Similar data for the sandy grade of alumina is presented in Figure 12.4. Withthis material the minimum conveying air velocity was always above 2000 ft/min,and although the minimum value of conveying air velocity reduced slightly withincrease in air supply pressure, it was only marginal.

3 LOW PRESSURE CONVEYING

As mentioned before, low pressure dilute phase conveying data is generally in-cluded in the conveying characteristics derived with high pressure conveying fa-cilities, and so this data is equally valid. Care must be exercised, however, in en-suring that the appropriate minimum conveying air velocity is used.

Both calcined alumina and hydrate of alumina have been conveyed throughthe Figure 10.16 pipeline of two inch nominal bore and 110 feet long. The lowpressure conveying characteristics for these two materials are presented in Figure12.5. In terms of conveying capability there is little difference between the twomaterials. The hydrate of alumina shows a tendency to a pressure minimum pointat low air flow rates and so the material flow rate in this region is slightly lowerthan that for the calcined alumina.

There is also a slight difference in minimum conveying air velocities be-tween the two materials. That for the calcined alumina is about 2300 ft/min andthat for the hydrate of alumina is about 2500 ft/min.

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370 Chapter 12

ooo7 6_c

'f -*uta 4oi

3 3fc"c3| 2

S 1

0

Conveying LinePressure Drop- Ihf/in2

SolidsLoading

Ratio

(a)

40 60 80 100 120

Free Air Flow Rate - ft3/min

0 7

1 6.Q7 5oC3 il

g

— 3

0

(b)

Solids/Loading

^ D<it;^Conveying Line 1Pressure Drop

- lbf/in2

40 60 80 100 120

Free Air Flow Rate - ftVmin

Figure 12.5 Conveying characteristics for (a) calcined and (b) hydrate of alumina con-veyed through the pipeline shown in figure 10.16.

Cumulative particle size distributions for the two materials are presented inFigure 12.6 for reference.

100

a soonJ-H

1>

I 60S

40

20

Hydrate ofAlumina

40 80

Particle Size - micron

120 160

Figure 12.6 Cumulative particle size distributions for alumina materials in figure 12.5.

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Aluminum Industry Materials 371

The mean particle size for the calcined alumina was about 66 /mi and thatfor the hydrate of alumina was about 60 jum. Granular materials, such as these,generally do not have sufficient air retention at this mean particle size for them tobe capable of being conveyed in dense phase in a conventional conveying system.It is unlikely, therefore, that either of these materials could be conveyed in densephase, even if a much higher air supply pressure were available. The particle den-sity for the calcined alumina was about 245 lb/ft j and that for the hydrate of alu-mina was about 150 lb/ftj.

4 HIGH PRESSURE CONVEYING

Both the calcined alumina and the hydrate of alumina have been conveyed in highpressure conveying facilities and it will be seen that they could not be conveyed indense phase and at low velocity, despite the high pressure. Data on a number ofother materials, such as aluminum fluoride, fluorspar and cryolite is also pre-sented.

4.1 Calcined Alumina

Conveying characteristics for calcined alumina conveyed through the Figure 7.13pipeline of three inch nominal bore are presented in Figure 12.7. The minimumvalue of conveying air velocity for the material was about 2300 ft/min.

25

§20

u

II10

OJ

I5

Solids Loading Ratio

Conveying Line PressureDrop - Ibf/in

0 100 200 300 400

Free Air Flow Rate - ft3 / min

Figure 12.7 Conveying characteristics for calcined alumina conveyed through thepipeline shown in figure 7.13.

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372 Chapter 12

It will be seen that the maximum value of solids loading ratio for the cal-cined alumina was just over 12 in Figure 12.7 and that a similar value was ob-tained for the material in the low pressure test facility in Figure 12.5a. This is dueto the fact that the material could only be conveyed in dilute phase suspensionflow in both cases, and that the pressure gradients for the two pipelines were verysimilar. The Figure 10.16 pipeline was 110 feet long and the maximum value ofpressure drop was 8 Ibf/in2, and the Figure 7.13 pipeline was 310 feet long with amaximum pressure drop of 25 Ibf/in2. The approximate factors of three in terms ofpipeline length and pressure drop cancel each other out.

For reference and comparison purposes, a number of other materials con-veyed through the Figure 7.13 pipeline were presented in Figures 11.10 to 11.12,including potassium sulfate and dicalcium phosphate. These show a very widerange of conveying capabilities.

4.2 Hydrate of Alumina

A sketch of the high pressure pipeline facility in which the hydrate of alumina wasconveyed is presented in Figure 12.8. It was a two inch nominal bore pipeline, 320

feet in length and incorporated thirteen 90° bends, having a bend diameter, D, topipe bore, d, ratio of about 24:1. Once again the material was fed into the pipelineby means of a high pressure, top discharge blow tank, having a fluidizing mem-brane.

The conveying characteristics for the hydrate of alumina conveyed throughthis pipeline are presented in Figure 12.9.

Pipeline :length = 320ftbore = 2 inbends = 13 x 90°

Figure 12.8 Sketch of pipeline used for the high pressure conveying of hydrate ofalumina.

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Aluminum Industry Materials 373

16

12

_ott,

ua 4S

Solids Loading Ratio

Conveying Line Pressuredrop - Ibf/in2

50 100 150

Free Air Flow Rate - ff / min

200

Figure 12.9 Conveying characteristics for hydrate of alumina conveyed through thepipeline shown in figure 12.8.

As with the calcined alumina, the hydrate of alumina could not be conveyedin dense phase either, despite the availability of high pressure air. The maximumvalue of solids loading ratio was about 16, which is similar to that achieved in thelow pressure conveying trials reported in Figure 12.5b, but this is due once againto the commonality of pressure gradients between the two sets of data. The mini-mum value of conveying air velocity was about 2500 ft/min once again.

4.3 Aluminum Fluoride

Similar data for aluminum fluoride conveyed through the Figure 12.8 pipeline ispresented in Figure 12.10. It will be seen that there is little difference between theconveying capability of the aluminum fluoride and the hydrate of alumina. Lowvelocity, dense phase conveying of this material was not a possibility in the con-veying system employed, as with the calcined alumina and hydrate of aluminareported above.

The minimum conveying air velocity for the aluminum fluoride was slightlyhigher at 2600 ft/min and material flow rates were slightly lower than those for thehydrate of alumina. The bulk density of the hydrate of alumina was about 75 lb/ft3

and that for the aluminum fluoride was about 90 lb/ft3. Such consistency in theconveying characteristics for a group of different materials is unusual.

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374 Chapter 12

ooo

16

12

_o

Solids Loading Ratio

Conveying Line Pressuredrop - lbf/in2

50 100

Free Air Flow Rate - ft3/min

150 200

Figure 12.10 Conveying characteristics for aluminum fluoride conveyed through thepipeline shown in figure 12.8.

4.4 Fluorspar

A sketch of the high pressure pipeline facility in which fluorspar was conveyed ispresented in Figure 12.11. It was a two inch nominal bore pipeline, 230 feet inlength and incorporated nine 90° bends, having a bend diameter, D, to pipe bore,d, ratio of about 24:1.

Pipeline :length = 230ftbore = 2 inbends = 9 x 90°

Figure 12.11 Details of pipeline used for high pressure conveying of fluorspar.

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Aluminum Industry Materials 375

The fluorspar was fed into the pipeline by means of a high pressure top dis-charge blow tank, having a fluidizing membrane. The fluorspar had a particle den-sity of about 230 Ib/ft3 and a bulk density of about 100 Ib/ft3, which is the highestamong the group of materials considered in this chapter. The mean particle size ofthe material was approximately 66 micron. At this particle size the alumina couldnot be conveyed in dense phase, but the fluorspar had a degree of air retention.The conveying characteristics for the fluorspar conveyed through the Figure 12.11pipeline are presented in Figure 12.12.

From Figure 12.12 it will be seen that the fluorspar was able to be conveyedat solids loading ratios of up to about 70. This is very much an intermediate valueof solids loading ratio and it is suggested that it is another case of 'medium phase'conveying. This was referred to in the previous chapter with regard to sodium sul-fate in Figure 11.12, and in Chapter 10 with regard to pulverized coal in Figure10.25.

The minimum value of conveying air velocity for the fluorspar was about1400 ft/min for conveying line inlet air pressures above about 20 psig. At lowpressure, and for the dilute phase suspension flow of the fluorspar, the minimumconveying air velocity was about 2500 ft/min.

30

£ 20

•g 10<5

.70 60

Solids LoadingRatio

Conveying LinePressure Drop

- lbf/in2

1 1 I

15

10

50 100

Free Air Flow Rate - ftVmin

150 200

Figure 12.12 Conveying characteristics for fluorspar conveyed through the pipelineshown in figure 12.11.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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376 Chapter 12

In the next chapter, conveying data for barite, bentonite and cement, eachconveyed through the Figure 12.11 pipeline, were all capable of being conveyed atsolids loading ratios of well over 100 and at conveying air velocities down to 600ft/min. Then in Chapter 14 data for silica sand having a mean particle size of ap-proximately 70 micron is presented and the maximum value of solids loading ratiois about 25, with a corresponding conveying line inlet air velocity of about 2300ft/min. These are typical of the operating limits for dense and dilute phase convey-ing for high pressure conveying in a pipeline of this length.

4.5 Cryolite

Cryolite is variously referred to as crushed bath and bath material. It has a meanparticle size typically about 0-1 inch but this depends upon the crushing process.The material generally has a very wide particle size distribution and often containsa high proportion of fines. The material reported here had a top size of/2 inch.

One of the pipelines through which this cryolite was conveyed is shown inFigure 12.13 for reference. It was two inch nominal bore, 165 feet in length andcontained eleven 90° bends, each having a D/d ratio of about 6:1. The materialwas fed into the pipeline by means of a high pressure blow tank. Since the materialhad such a large mean particle size and contained large lumps, in addition to beingvery abrasive, a blow tank was an ideal feeder for the cryolite.

Conveying characteristics for this cryolite conveyed through the Figure12.13 pipeline are presented in Figure 12.14. As expected, the material could onlybe conveyed in dilute phase suspension flow. Despite the large particles in thematerial, and a particle density of about 190 lb/ftj, the minimum conveying airvelocity was-about 2800 ft/min. This relatively low value of pick-up velocity, forsuch a material, is helped significantly by the fact that the material had a very wideparticle size distribution and a large proportion of fines.

Pipeline :length = 165ftbore = 2 inbends = 11 x 90'D/d = 6

Figure 12.13 Sketch of pipeline used for the high pressure conveying of cryolite.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Aluminum Industry Materials 377

Solids Loading Ratio

16

12

ooo

I

"eST, 4

Conveying Line PressureDrop - Ibfi'in

0 50 100 150 200

Free Air Flow Rate - itVmin

Figure 12.14 Conveying characteristics for cryolite conveyed through the pipelineshown in figure 12.13.

As a consequence of the high conveying air velocities required for convey-ing, and the abrasive nature of the material, it is generally recommended that wearresistant pipeline should be specified for conveying pipelines. Either alloy castiron or basalt lined pipe would be suggested. All bends in the pipeline would alsohave to be similarly reinforced, and possibly with alumina ceramic materials forthe greater wear resistance required.

Straight pipeline is not generally as vulnerable to erosive wear as the bendsin the pipeline, but when large particles have to be conveyed the problem is exac-erbated. In being conveyed through a horizontal pipeline the gravitational force onlarge particles is such that they tend to 'skip' along the pipeline and so low angleimpact of the particles against the pipeline occurs on a regular basis. This wearmechanism is considered in detail in Chapter 20.

This cryolite has also been conveyed through the Figure 7.13 pipeline,which is three inch nominal bore and 310 feet long. In order to convey the cryolitethrough this pipeline, with high pressure air, a free air flow rate of 600 ftVmin wasprovided, rather than the 400 frYmin that was used for other materials that havebeen conveyed through this pipeline and reported here. The conveying characteris-tics are presented in Figure 12.15. With a pick-up velocity of 2800 ft/min, convey-ing was possible with air supply pressures up to 35 psig with 600 ftVmin of freeair available.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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378 Chapter 12

40

ooo

30

(2 20

oE13| 10

CS

Solids Loading Ratio

Conveying Line PressureDrop - Ibt7in2

100 200 300 400

Free Air Flow Rate - ft3/min

500 600

Figure 12.15 Conveying characteristics for cryolite conveyed through the pipelineshown in figure 7.13.

Figure 12.15 clearly shows the influence of free air flow rate on the convey-ing capability of dilute phase conveying systems. With only 400 ftVmin themaximum value of air supply pressure that could be used would be about 20 psig,regardless of the fact that air was available at 100 psig. With only 200 ft3/min al-most nothing could be conveyed through the pipeline at all.

The conveying characteristics for calcined alumina conveyed through thissame pipeline were presented earlier in Figure 12.7. With only 400 ftVmin of freeair available for conveying, but with a lower pick-up velocity of 2300 ft/min, airsupply pressures up to about 25 psig could be employed. If the two sets of data arecompared it will be seen that a maximum of about 27,000 Ib/h of cryolite could beconveyed with a pressure drop of 25 lbf/in2, but only about 23,000 Ib/h of calcinedalumina could be conveyed with the same pressure drop.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 391: Handbook of Pneumatic Conveying Engineering

13Conveying of Cement and DrillingMud Powders

1 INTRODUCTION

Cement is another commodity that is manufactured on a large scale and, by thenature of the material, uses pneumatic conveying systems extensively for its con-veying. Because of its use in the construction industry it is distributed internation-ally, nationally and locally. Large bulk carriers are used for international transportand a wide range of ship loading and off-loading systems have been used and de-veloped over the years. Large and small scale storage depots are used for its distri-bution nationally, with inland locations generally supplied by rail wagons, andcoastal locations, often based at existing ports, supplied by self off-loading ships.Local distribution is normally by specialized road vehicles, generally capable ofbeing pressurized for self off-loading directly into storage silos.

Most large countries around the world have at least one cement manufactur-ing plant, and there are close to thirty countries with a manufacturing capability inexcess of 10,000,000 ton/year. Economy of scale is such that individual plants arerarely built to produce less than about one million ton per year.

1.1 Material Grade

Although the problem of material grade having an influence on the conveyingcapability of the material does exist with cement, it is not the major problem that it

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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380 Chapter 13

is with most other materials. Grade can vary with cement and it can have a secon-dary effect, but is rarely to a degree that the material cannot be conveyed in densephase. Cement is manufactured to standards and these are based on fineness. Aconvenient means of determining the degree of fineness is by measuring perme-ability, from which the specific surface of a material can be ascertained.

The most common device used is that devised by Blaine, and as a conse-quence the Blaine number is generally used as the international reference for thefineness of cement. The finished product is produced by grinding and so the finerthe cement the greater the cost. Cement is always manufactured to a fineness thatwill allow the material to be conveyed in dense phase and at low velocity in aconventional pneumatic conveying system.

The influence of Blaine number on the conveying capability of cement,however, is not known. In Chapter 10 it was shown that the conveying capabilityof fine fly ash could vary by a significant degree for just small changes in meanparticle size. It is suspected that cement will be similarly effected, but not neces-sarily in the same way, for the particle shape of cement is very different from thatof fly ash.

1.2 Materials Considered

Two types of cement are considered; ordinary Portland and oil well cement. Theoil and gas industry, in addition to using oil well cement also uses large quantitiesof barite and bentonite and so these materials are also included here. For drillingpurposes they are produced as fine powders and so these materials also have verygood air retention properties.

As a consequence these materials can also be conveyed in dense phase andat low velocity in conventional pneumatic conveying systems, provided that thepressure gradient available for conveying is sufficiently high to convey at thevalue of solids loading ratio required.

All four of these materials have been conveyed through the Figure 12.11pipeline and so their conveying characteristics are presented together for referenceand comparison in Figure 13.1. The pipeline was of two inch nominal bore, 230feet long and incorporated nine 90° bends.

The materials were fed into the pipeline by means of a high pressure topdischarge blow tank. 200 ftVmin of free air was available for conveying. The bar-ite had a mean particle size of about 12 micron, a bulk density of about 100 lb/ft3

and a particle density of approximately 265 lb/ft3. For the bentonite these figureswere 24 micron, 50 and 145 lb/ft' respectively.

It will be seen that there are only very slight differences between the ordi-nary Portland and the oil well cements, and the barite was also very similar in per-formance. Only with the bentonite is there any real difference in conveying per-formance. This material did not exhibit any pressure minimum effect, and so asthe material flow rate increased continuously with reduction in air flow rate, therewas a very significant difference in performance at low values of air flow rate.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Cement and Drilling Mud Powders 381

40

o230X.cX)

I20

_ofc.

$10a

Conveying Line.Pressure Drop

- lbf/in2 Solids LoadingRatio/

100 80 60 /40

Conveying Line_ Pressure Drop

- lbf/in2

10080

120160,

Solids LoadingRatio /

60

(a)Free Air Flow Rate - ft /min

40

oo230

20

o

io

Conveying LinePressure Drop

i - Ibf7in2

120

Solids LoadingRatio

30

- 20

(b)

40

oo230

I20

o

40 80 120 160 200Free Air Flow Rate - ftVmin

Conveying Line.Pressure Drop_ \ -!bf/in2120100

160^ / /80 60

15 10 a\

10

Solids Loading/ Ratio

50

(c)

0 40 80 120 160 200

Free Air Flow Rate - ft3 / min

0 40 80 120 160 200

(d)Free Air Flow Rate - ft / min

Figure 13.1 Conveying characteristics for (a) ordinary portland cement, (b) oil wellcement, (c) barite, and (d) bentonite conveyed through the pipeline shown in figure 12.11.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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382 Chapter 13

2 CEMENT

Conveying data for both ordinary portland and oil well cement, conveyed throughthe Figure 12.11 pipeline were presented above in Figure 13.1. Data on ordinaryPortland cement was presented earlier in Chapter 4 where the material was used toillustrate the conveying capability of materials capable of being convey in densephase in a sliding bed mode of flow. For this purpose data obtained with the mate-rial conveyed through the Figure 4.2 pipeline of two inch bore and 165 ft long wasused.

This same data was analyzed further in Chapter 7 to illustrate the influenceof conveying line inlet air velocity. Ordinary portland cement was also used inChapter 9 to illustrate the influence of conveying air velocities on pipeline purgingwith Figure 9.10. In this case the Figure 7.13 pipeline of four inch nominal boreand 310 ft long was used.

Data for oil well cement conveyed through the Figure 10.20 pipeline of twoinch bore and 140 ft long was presented earlier in Chapter 8. The oil well cementwas used to illustrate the influence of pipeline material on conveying performance.Identical pipelines of steel pipe and rubber hose were tested and the data was pre-sented in Figure 8.21 and 8.22. Barite was similarly conveyed through these pipe-lines and this data is presented in a section on rubber hose later in this chapter.

2.1 Ordinary Portland Cement

Conveying characteristics for ordinary portland cement conveyed through the rela-tively short Figure 10.20 pipeline of two inch nominal bore are presented in Figure13.2.

Since the pipeline was only 140 feet long and included six bends, conveyingat solids loading ratios in excess of 200 was possible with conveying air pressuresof 25 psig. Even with a conveying line pressure drop of 10 lbf/in2 the cementcould be conveyed at a solids loading ratio of 100 and with a conveying line inletair velocity down to about 600 ft/min.

With air supply pressures less than about 10 psig the pressure gradient wasnot sufficiently high to maintain such high solids loading ratios and so the mini-mum value of conveying line inlet air velocity had to increase as a consequence.Hence the increase in the volumetric flow rate of free air required at lower pres-sures, as shown on Figure 13.2.

The conveying limit for the material is dictated by the relationship betweenthe solids loading ratio and the minimum conveying air velocity. This was dis-cussed in Chapter 4, where the cement was used for illustrating purposes for mate-rials capable of dense phase conveying in a sliding bed mode of flow. The rela-tionship for ordinary portland cement is presented again in Figure 13.3 for refer-ence. From this it will be seen that if the cement is conveyed in dilute phase sus-pension flow a minimum conveying air velocity of about 2000 ft/min will have tobe maintained.

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Cement and Drilling Mud Powders 383

60

o50

J:40

SlO

Pressure Drop- Ibf/irv Solids Loading

Ratio

20

50 100Free Air Flow Rate - ftVmin

150 200

Figure 13.2 Conveying characteristics for ordinary portland cement conveyed throughthe pipeline shown in figure 10.20.

2400

20 40 60

Solids loading ratio

80 100

Figure 13.3 Influence of solids loading ratio on minimum conveying air velocity forordinary portland cement.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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384 Chapter 13

2. /. / Dilute to Dense Phase Transition

Situations in which it may only be possible to convey cement in dilute phase willoccur if the pressure available for conveying is very low or if the pipeline is verylong. The critical parameter is the pressure gradient available, and for cement thisneeds to be above about 2'/2 Ibf/in2 per 100 ft of horizontal pipeline. This is an'equivalent' length and so an allowance must be made for bends and any verticallift in the pipeline. To convey at a solids loading ratio of about 100 typically re-quires a pressure gradient of about 10 Ibf/in2 per 100 ft.

The full transition from dense phase conveying capability, with a conveyingline inlet air velocity of 600 ft/min, to dilute phase suspension flow, with a mini-mum inlet air velocity of 2000 ft/min for the cement, does not occur in Figure13.2. This is because the pipeline was too short. The effect, however, will be high-lighted with data for a longer pipeline, and this is presented below.

2.1.2 High Pressure Conveying

A sketch of a very much longer pipeline through which the ordinary portland ce-ment has been conveyed is shown in Figure 13.4. This pipeline was 535 ft longand incorporated seventeen 90° bends. The pipeline was two inch nominal boreand so the air only pressure drop was consequently very high. One would notnormally build a pipeline of such a geometry, being of a relatively small bore forthe length, and incorporating so many bends, but it is very useful for illustratingthe influence of the various parameters.

Pipeline:length = 535ftbore = 2 inbends = 17x90°D/d = 24

Figure 13.4 Details of pipeline used for the high pressure conveying of ordinary port-land cement.

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Cement and Drilling Mud Powders 385

Once again a high pressure top discharge blow tank was used to feed thematerial into the Figure 13.4 pipeline. 200 fVYmin of free air at a pressure of 100lbf/in2 was available for conveying. Since the pipeline was long and high pressureair was available the opportunity was taken to carry out test work on the cementwith conveying line inlet air pressures of up to 75 lbf/in2 gauge.

The maximum discharge capability of the blow tank used was about 55,000Ib/h, but as the pipeline was very long, and hence of high resistance, it would bepossible to carry out tests with very much higher air supply pressures. Air flowrate and control was made possible at these pressures by using convergent-divergent choked flow nozzles, as discussed in Chapter 6.

Conveying data for the cement in this pipeline is shown in Figure 13.5.Lines of constant conveying line inlet air velocity have been superimposed in ad-dition, in order to illustrate the nature of the dilute to dense phase conveying tran-sition. Conveying line exit air velocity has also been added as an additional axis.As this is a constant bore pipeline, the magnitude of the air expansion through thepipeline, with such high air supply pressures, can be clearly seen. With a convey-ing line inlet air pressure of 75 lbf/in~, for example, the expansion in conveying airvelocity is approximately 6:1.

For materials that can be conveyed at low velocity, high pressure air can beutilized quite conveniently, for the slope of the constant conveying line inlet airvelocity curves is very steep. From Figure 13.5 it will be seen that a very signifi-cant increase in material flow rate can be obtained through a pipeline by using ahigher air supply pressure, and the corresponding increase in air flow rate requiredis not proportionately large.

For dilute phase flow, however, where the conveying line inlet air velocitymay be 2000 or 3000 ft/min, the slope of the constant inlet velocity curves is notas steep and so considerably more air must be used if a higher air supply pressureis to be utilized. The conveying facility had 200 ftVmin of free air available and soif the conveying line inlet air velocity had to be 3000 ft/min the maximum pres-sure that could be utilized would only be about 25 psig. With an inlet air velocityof 600 ft/min only 90 ft3/min of air was required at 75 psig.

Since the pipeline was of small bore and relatively long it will be seen fromFigure 13.5 that a pressure drop of about 30 lbf/in2 was required before the cementcould be conveyed in dense phase. As a consequence a marked transition is shownbetween the minimum conveying limits for the dense and dilute phase areas on theconveying characteristics in this region. The transition from dilute to dense phaseconveying is very smooth. Indeed, with such high air supply pressures, for manyoperating points on Figure 13.5 the material is quite likely to be in dense phase atthe start of the pipeline and in dilute phase at the end of the pipeline.

An operating problem that does arise with this dilute to dense phase transi-tion relates to proximity to minimum conveying conditions. It is essential to avoidoperating a conveying system in dense phase in the region where the air supplypressure is marginal for dense phase conveying, as illustrated in Figure 13.5 [1].

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386 Chapter 13

40

30

utaoioE"2

20

10

Conveying Line Inlet AirVelocity - ft/min

Solids LoadingRatio

75080 / 70

1000

60 / 501250

40

J500

.30

Conveying LinePressure Drop

- Ibf7in2

3000

40 80 120 160 200

Free Air Flow Rate - ft /min

2500 5000Conveying Line Exit Air Velocity - ft/min

7500

Figure 13.5 Conveying data for ordinary portland cement conveyed through the pipe-line shown in figure 13.4.

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Cement and Drilling Mud Powders 387

2.1.3 Influence of Pipeline Bore

The data reported so far has been for two inch nominal bore pipelines. Despitethis, material flow rates have been typically up to about 40,000 Ib/h in the datapresented. Higher flow rates can be achieved by utilizing higher air supply pres-sures, as illustrated with Figure 13.5 but there is a limit to this. An increase inpipeline bore will allow a significant increase in material flow rate, being ap-proximately in proportion to the increase in pipe section area. Air flow rates alsohave to increase in proportion to the increase in cross sectional area, in order tomaintain the necessary conveying air velocities, and so this means that the convey-ing characteristics are almost geometrically similar.

To illustrate the influence of pipeline bore, three sets of conveying data arepresented for ordinary portland cement conveyed through the Figure 7.13 pipeline.This pipeline was 310 feet long and included nine 90° bends. Data for the cementconveyed through the Figure 7.13 pipeline of two inch nominal bore is presentedin Figure 13.6.

The maximum value of material flow rate here was just over 50,000 Ib/h be-cause the pressure gradient available (air supply pressure divided by equivalentlength of pipeline) was higher than that in the previous two inch bore pipelinesthat have been used to present data for the cement. Data for this same cement con-veyed through the same pipeline, but of three inch nominal bore, is presented inFigure 13.7.

60oo250.cjB740

:3o

CS

'£20

10

Solids LoadingRatio

" Conveying Linepressure Drop

- Ibt7in2

100 150

Free Air Flow Rate - frYmin

200

Figure 13.6 Conveying data for ordinary portland cement conveyed through the pipe-line shown in figure 7.13 of two inch nominal bore.

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388 Chapter 13

120

_g

I 40

Solids LoadingRatio

Conveying LinePressure Drop

- Ibt7in2

20

I I I I I L_

40 80 120 160

Free Air Flow Rate - ftj/min

200

Figure 13.7 Conveying data for ordinary portland cement conveyed through the pipe-line shown in figure 7.13 of three inch nominal bore.

It will be noted that the same range of air supply pressures was used in con-veying the cement through the three inch bore pipeline and so a direct comparisonwith the two inch bore pipeline data is possible. The material flow rate axis hasbeen doubled and it will be seen that the flow rate of cement has more than dou-bled, as would be expected, for the increase in cross sectional area from two tothree inch bore pipe is more than double.

It will be seen that the two sets of conveying characteristics are approxi-mately geometrically similar, as mentioned above. This is because the air flow rateaxis has been scaled in proportion to pipe section area and the resulting increase incement flow rate is approximately in proportion to this ratio also.

Solids loading ratios achieved are slightly higher with the three inch borepipeline and this is partly due to the fact that the air only pressure drop for thethree inch bore pipeline is much lower than that for the two inch bore pipeline.With a lower air only pressure drop, more of the pressure is available for convey-ing material.

Data for this same cement conveyed through the same Figure 7.13 pipelineof four inch nominal bore is presented in Figure 13.8. A further increase in airflow rate is required, as will be seen, but the same axis for material flow rate hasbeen maintained. As a consequence a lower maximum value of conveying linepressure drop has been employed. It will be seen, however, that for a given valueof conveying line pressure drop, the material flow rate achieved in the four inchbore pipeline is significantly greater than that achieved in the three inch bore line.

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Cement and Drilling Mud Powders 389

ooo

a

80

oE 40

U"S

Solids LoadingRatio

Conveying LinePressure Drop

20

0 100 200 300 400

Free Air Flow Rate - ftVmin

Figure 13.8 Conveying data for ordinary portland cement conveyed through the pipe-line shown in figure 7.13 of four inch nominal bore.

2.1.4 Industrial Installations

Because of the empirical nature of pneumatic conveying, little practical informa-tion on industrial plant finds its way into the literature. System design is generallybased on the scaling of data for the particular material to be conveyed. If previousexperience with a material is not available, the material will be conveyed througha test facility in order to generate the necessary data. The building, maintainingand operating of test facilities is an expensive support item for a company, and soany data that they obtain on materials has too much commercial value to publish.

Most reputable companies that manufacture pneumatic conveying systemshave such test facilities. Companies will request a representative sample of thematerial to be conveyed and undertake tests, even if they have previous experi-ence, simply because of the problems of different grades of the same material be-having very differently. Sometimes data does get published, particularly for adver-tising purposes, but very often it is incomplete so that it is not possible to extractuseful design information. Some interesting examples for cement are presentedbelow.

2.1.4.1 Positive Pressure ConveyingSchaberg and Mehring [2] reported on a pneumatic conveying facility for cementin England that was commissioned in 1986. The distance to the furthest mill wasabout 2800 feet and a material flow rate of 264,000 Ib/h was required. The con-veying route included twelve bends and incorporated five diverters. A single blow

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390 Chapter 13

tank with a 66,000 Ib batch capability was used. The total cycle time was fifteenminutes, giving the 264,000 Ib/h. The conveying time was twelve minutes, whichis equivalent to 330,000 Ib/h during the conveying phase of the cycle.

The pipeline bore was 10 inch (257 mm) and conveying line inlet and exitair velocities were quoted as being 732 ft/min (3-72 m/s) and 4468 ft/min (22-7m/s), with a conveying line pressure drop of 75 lbf/in2 (5-1 bar). The cement wasconveyed at a solids loading ratio of 32 and the compressor delivered 3425 ftVmin(97 nrVmin) of free air at a pressure of 100 lbf/in2 gauge (7 bar gauge), giving aspecific power consumption of 5-6 hp-h/ton (4-6 kWh/tonne).

From the above conveying air velocity data it wi l l be seen that a single borepipeline was used; for 732 x [(75 + !4-7)/14-7] = 4468 ft/min. Using the outletvelocity at atmospheric pressure, the free air flow rate used for conveying will beapproximately 4467 x n x 102/576 = 2436 frYmin which means that only about70% of the air available is used for conveying. Using this lower air flow rate andan air density of 0-0765 lb/ft3, a check on the solids loading ratio gives330,000/(2436 x 0-0765 x 60) = 30, which is close enough for a check. From Fig-ure 13.3 it will be seen that at a solids loading ratio of 32 the minimum conveyingair velocity suggested is approximately 850 ft/min and so for the cement in ques-tion this is obviously a conservative value.

2.1.4.2 Ship Off-loadingLigthart [3] reported in 1991 on a pneumatic conveying system for off-loadingcement from bulk carriers at 1,764,000 Ib/h (800 tonne/h), and its onward convey-ing to silos 1640 feet (500 m) distant through twin pipelines. The company had aneed to import up to one million tonne/yr of cement at a terminal 15 miles east ofLondon on the River Thames. Because the river is tidal (23 feet) it was necessaryto build a jetty in the river against which the ships could berth, and hence the longconveying distance.

A single vacuum nozzle was employed to off-load at 800 tonne/h, but it wasdecided to use two pipelines at 400 tonne/h each for the transfer to silos over 1640feet, as it was considered that a single bore pipeline would be more expensive tobuild. Four concrete silos of 10,000 tonne capacity were available for storage.

The single unloader is mounted on rails on the jetty to service the entireship. For onward conveying to the silos it is connected to the pipelines by flexiblehoses, through manifolds provided every 50 feet along the length of the ship dock-ing section. Air is blown into the pipelines at their start, at the end of the jetty, andthis dilutes the flow and transports the cement the 1640 feet to the silos. The un-loader has a single filter receiver vessel with four 20 tonne capacity blow tanksbeneath, arranged in two pairs.

It also has eight vacuum pumps, and two compressors to provide air to thestart of the two pipelines. All air movers are screw type with an 85% vacuum ca-pability for suction, and deliver oil free air, without cooling, at 44 lbf/in2 (3 bar)gauge for blowing. Although the total installed power is 4290 hp (3200 kW), only75% of this is required for conveying at 800 tonne/h over the 1640 feet.

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Cement and Drilling Mud Powders 391

A pneumatic system was chosen in preference to alternative mechanical sys-tems for the duty for cost, maintenance and environmental reasons. Although thecost of the actual pneumatic ship off-loading system was higher than the price fora mechanical un-loader, the cost for the overall system was lower for the pneu-matic system. This is because the pneumatic system required only two pipelines toconvey the cement the 1640 ft to the silos. A mechanical un-loader would haverequired long conveying belts, and vertical screw conveyors in addition, to bringthe cement to the top of the silos.

3 BARITE

As mentioned above, barite is a relatively dense material, having a bulk density ofabout 100 lb/ft3 and a particle density of about 265 lb/ft3. In drilling mud applica-tions, however, the material is typically ground down to a mean particle size ofabout 12 micron and at this value the material has very good air retention proper-ties.

As a consequence it is capable of being conveyed in dense phase at low ve-locity in a conventional conveying system, despite the high values of density. Thematerial will, of course, convey in dilute phase and a minimum conveying air ve-locity of about 2400 ft/min is required.

3.1 Low Pressure Conveying

Low pressure, dilute phase data for barite conveyed through the Figure 10.16 isshown in Figure 13.9. Although the conveying air pressure available was rela-tively low, the conveying distance was short, and so the start of the transition fromdilute to dense phase conveying is seen to occur with conveying line pressure dropvalues above about 6 lbf/in~.

This is the point, on Figure 13.9, where the solids loading ratio is already upto a value of 14 under minimum conveying conditions. Further increase in pres-sure results in an increase in pressure gradient to allow the material to be conveyedat a higher solids loading ratio. This, in turn means that the material can be con-veyed at a lower velocity, and hence with a slightly lower air flow rate.

It will be recalled that the influence of pressure gradient was illustrated inChapter 4 on Gas-Solid Flows with Figure 4.23 for low pressure conveying, andthis included both positive pressure and vacuum systems. It must be emphasizedthat the conveying distance on Figure 4.23 is an equivalent distance that takesaccount of vertical lift and the number of bends in the pipeline, as well as the hori-zontal conveying distance.

Although the pipeline for which the data relates was only 110 ft long, it didcontain seven 90° bends and it will be seen from Figure 8.16a that the equivalentlength of the bends, for which the conveying line inlet air velocity is 2400 ft/min,is about 40 ft each. Since the length of vertical lift in the pipeline was negligible,the equivalent length would be about 390 ft.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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392 Chapter 13

10ooo

aerf

Solids LoadingRatio

24

Conveying LinePressure Drop

- lbf/in2

ConveyinLimit

50Free Air Flow Rate - ftVmin

100 150

13.9 Conveying characteristics for barite conveyed through the pipeline shownin figure 10.16.

3.2 High Pressure Conveying

High pressure conveying data for barite conveyed through the 230 ft long, twoinch bore Figure 12.11 pipeline was presented earlier in Figure 13.Ic. A similartransition from dilute to dense phase conveying occurred, as shown in Figure 13.9above, but as conveying data was undertaken at pressures of up to 30 lbf/ingauge, the vast majority of the data presented related to dense phase conveying.Figure 13.9, in effect, provides a magnification of the very low pressure and mate-rial flow rate section of Figure 13.1c.

3.2.1 Influence of Pipeline Bore

As with the cement, reported above, barite has also been conveyed through two,three and four inch bore pipelines. In this case the pipeline was the 165 ft longFigure 4.2 pipeline. Data for the barite in the two inch nominal bore pipeline ispresented in Figure 13.10. With conveying line inlet air pressures of up to 30lbf/in2 gauge the vast majority of the data relates to dense phase conveying, withsolids loading ratios up to 200.

Similar data for the barite conveyed through the Figure 4.2 pipeline of threeinch nominal bore is presented in Figure 13.11. This shows an unusual anomaly.At low values of conveying line pressure drop, and low values of air flow rate, thematerial flow rate through the three inch bore pipeline is less than that through thetwo inch bore pipeline of identical geometry.

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Cement and Drilling Mud Powders 393

ooo

-O

I

60

50

40

30

20

10

0

Conveying LinePressure Drop- Ibf/in2

200 Solids LoadingRatio

20

1

'50 100Free Air Flow Rate - ft3/min

150

Figure 13.10 Conveying characteristics for barite conveyed through the pipelineshown in figure 4.2 of two inch nominal bore.

This is a rare occurrence, but is not unknown, and is yet another problemwith which to contend in pneumatic conveying.

120ooo

oi

o

80

Solids LoadingRatio

^ ,200150

Conveying LinePressure Drop

- lbf/in2

40 80 120 160

Free Air Flow Rate - ft3/min

200

Figure 13.11 Conveying characteristics for barite conveyed through the pipelineshown in figure 4.2 of three inch nominal bore.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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394 Chapter 13

The reason for this is not fully understood at the present time and it is doubt-ful whether any computer aided design program currently available would cor-rectly predict this reduction in performance with this particular material.

The fact that such data is recorded throws into doubt the use of small borepipelines to derive conveying data for system design purposes. In recent years,however, most companies that manufacture pneumatic conveying systems havebeen installing larger bore test facilities. A number of companies now have sixinch bore pipelines and four inch is typically the smallest bore of pipeline that iscurrently used.

Conveying data for the barite conveyed through the four inch bore pipelineis presented in Figure 13.12. Material flow rates here are significantly greater thanthose for the three inch bore pipeline, as would normally be expected.

3.2.2 Influence of Pipeline Material

Cement and drilling mud powders, as mentioned earlier, are regularly transportedby small ships to storage facilities at ports, particularly those ports that are used toservice off-shore drilling operations. Drilling mud powders are then loaded ontoservice boats to supply the off-shore drilling rigs. These vessels are generally selfoff-loading, usually by means of single or twin blow tanks, and flexible hoses arewidely used. At ports, flexibility is needed to overcome problems of tidal move-ment. For the loading of materials onto off-shore platforms flexibility is requiredto accommodate movement of the vessel on the open sea in bad weather, and thefact that the vessel must stand some distance off from the oil or gas rig.

200

x 160

1

k 120C3

ei

I 80

40

Solids Loadin;Ratio

Conveying LinePressure Drop

- Min2

200 300

Free Air Flow Rate - fWmin

400

Figure 13.12 Conveying characteristics for barite conveyed through the pipelineshown in figure 4.2 of four inch nominal bore.

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Cement and Drilling Mud Powders395

In these applications very long lengths of flexible hose, usually ot natural orsynthetic rubber, are used to connect the supply boat with the fixed pipeline on theplatform In these situations the hose naturally forms a catenary and so bends areof exceptionally long radius and the additional pressure drop is minimal. Pressuredrop for the hose, compared with that of a steel pipeline, however, must be taken

into consideration. . ,The influence of pipeline material on conveying performance was consid-

ered in detail in Chapter 8 with Figures 8.21 and 22. Oil well cement was the ma-terial considered and this was conveyed through identical pipelines of steel pipeand rubber hose (shown in Figure 10.20). Conveying characteristics for the ce-ment conveyed through the two pipelines were given and an analysis of the com-parative performance was presented. Barite has also been conveyed through thesesame 140 ft long Figure 10.20 pipelines and the two sets of data are presented in

xac the same axes have been used for the two sets of barite data in Fig-ure 13 13 as for the two sets of oil well cement data in Figure 8.21 so that directcomparisons can be made for both the different materials and the different pipe-

lines.

Conveying Line PressureDrop - Ibf/in2

, Solids Loading\ 200 Ratio*

50

8o

:40

_0

I 20

I10

50

Conveying Line PressureDrop - Ibf/in2

I Solids loading/ ratio

1 200 /160 /

130100

24

(a)

0 50 100 150

Free Air Flow Rate - ftVmin (b)

50 100 150

Free Air Flow Rate - frVmin

Figure 13.13 Conveying characteristics for barite conveyed through the pipelineshown in figure 10.20 pipeline made of (a) steel pipeline and (b) rubber hose.

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396 Chapter 13

The difference in conveying performance for the barite in Figures 13.13aand 13b follows a very similar pattern to that reported for the oil well cement andpresented in Figure 8.22. At very low values of conveying air velocity there islittle difference between the two sets of data and material flow rates for a givenconveying line pressure drop are very similar. As velocity increases there is a con-stant reduction in material flow rate for the barite conveyed through the rubberhose.

This is attributed to the fact that the coefficient of restitution for the materialimpacting against rubber pipe walls is much lower than that for material againststeel pipe walls. As a consequence the material, after impact with the rubber pipewall, will be at a lower velocity, compared with the steel pipe, and so additionalenergy will be lost in re-accelerating the material back to its terminal velocity.

4 BENTONITE

Both bentonite and barite have been conveyed through the 185 ft long Figure 8.2pipeline of two inch nominal bore. For comparison purposes the conveying char-acteristics for both materials are included in Figure 13.14.

60

50ooo

40

oE 20"s'Cu

I 10

(a)

SolidsLoading

Ratio ~

20

Conveying LinePressure Drop

iO 120,- Ibf7in2

0 50 100 150

Free Air Flow Rate - tf/min

60

50

Conveying LinePressure Drop

- Ibf'/in"

ooo

ooi

oE20.3

(b)

SolidsLoading

Ratio

50 100 150

Free Air Flow Rate - ft /min

Figure 13.14 Conveying characteristics for (a) bentonite and (b) barite conveyedthrough the pipeline shown in figure 8.2.

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Cement and Drilling Mud Powders 397

Figure 13.14 illustrates quite clearly the differences in conveying capabilitybetween the barite and bentonite, particularly for low velocity dense phase con-veying. While the conveying characteristics of barite are similar to those of ce-ment, the conveying characteristics of bentonite are similar to those of a fine gradeof fly ash. At low air flow rates, therefore, the differences in material flow ratesbetween these different powdered materials can be quite considerable.

Conveying data for bentonite was included in Figure 13.1 along with similardata for ordinary portland cement, barite and oil well cement, for comparison. Allfour materials were conveyed through the 230 ft long Figure 12.11 pipeline. Of thefour materials included, bentonite was the only one that showed a significant dif-ference in conveying capability in comparison with the other materials.

REFERENCES

1. D. Mills. An investigation of the unstable region for dense phase conveying in slidingbed flow. Proc 4th Int Conf for Conveying and Handling of Paniculate Solids. Budapest.May 2003.

2. F. Schaberg and B.F. Mehring. Dense phase conveying. Large outputs/long distances.Proc Pneumatech 4. pp 281-299. Jersey, UK. March 1987.

3. A. Ligthart. World's largest cement unloader. Bulk Solids Handling. Vol 11, No 3. pp671-676. August 1991.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 410: Handbook of Pneumatic Conveying Engineering

14Conveying of High Density andOther Materials

1 INTRODUCTION

In this chapter, data on materials that do not fall into the previous four categories ispresented. The main focus of the information provided here is on materials thathave a high density, since there is a lot of interest in this type of material, and thecapability of this class of materials for pneumatic conveying may not be fully ap-preciated. Several materials having fairly high densities have already been in-cluded in preceding chapters and so it may well be recognized that there is no dif-ficulty in conveying such materials.

In Chapter 12 on "Aluminum Industry Materials" fluorspar was includedand this has bulk and particle densities of about 100 and 230 lb/ft3. With a meanparticle size of about 66 micron the material did not have sufficient air retentioncapability to be conveyed in true dense phase flow. It did, however, achieve whatmight loosely be referred to as 'medium' phase conveying for at high values ofpressure gradient solids loading ratios of up to about 70 were achieved and thematerial could be conveyed with conveying line inlet air velocities down to ap-proximately 1400 ft/min.

Then in Chapter 13 on "Cement and Dri l l ing Mud Powders" barite was in-cluded and this has bulk and particle densities of about 100 and 265 lb/ft3. With amean particle size of about 12 micron, however, the material was capable of beingconveyed in dense phase, at very high values of solids loading ratio, and with con-

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400 Chapter 14

veying air velocities down to 600 ft/min very easily. In conveying barite verticallyup a distance of about 15 feet from a high pressure blow tank one of the authorsconveyed the material qui te steadily at a solids loading ratio of over 800. For thistype of material the value of solids loading ratio achieved is essentially only dic-tated by pressure gradient.

2 IRON POWDER

Although not the densest material to be considered here, it does represent a goodstarting point as the name itself generally conjures up high density with respect tobulk solids. The data presented here was obtained for iron powder having bulk andparticle densities of about 150 and 355 Ib/fr'. The mean particle size of the mate-rial was approximately 64 micron. The iron powder was conveyed in both lowpressure and a high pressure pneumatic conveying test facilities.

2.1 Low Pressure Conveying

Conveying characteristics for the iron powder conveyed in a low pressure system,and hence in dilute phase suspension flow, are presented in Figure 14.1. The mate-rial was fed into the pipel ine by means of a low pressure, bottom discharge blowtank. The Figure 4.15 pipel ine through which the material was conveyed was twoinch nominal bore, 1 1 5 feet long and included nine 90° bends. Air supply pres-sures up to 8 lbf/ in2 gauge were utilized and material flow rates up to about 3000Ib/h were obtained.

10

oo

_o

± 4

Solids LoadingRatio

Conveying LinePressure Drop

- lbf/ in2

0 50 100 150

Free Air Flow Rate - frVmin

Figure 14.1 Low pressure conveying characteristics for iron powder conveyed throughthe pipel ine shown in figure 4.15.

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High Density and Other Materials 401

Solids loading ratios up to about six were achieved and the minimum con-veying air velocity for the material was about 2800 ft /min. Provided that theminimum velocity was kept above this figure of 2800 ft/min, with due allowancefor the influence of air inlet pressure on the volumetric flow rate of free air re-quired, no operating diff icult ies were experienced with this material at all. Withthis particular material, however, there is no indication of whether there is anydense phase conveying potential at all.

2.2 High Pressure Conveying

This iron powder has also been conveyed in a high pressure conveying system andthe conveying characteristics obtained are presented in Figure 14.2. The materialwas fed into the pipeline by means of a high pressure, top discharge blow tank.The Figure 4.2 pipeline through which the material was conveyed was two inchnominal bore, 165 feet long and included nine 90° bends. Air supply pressures upto 30 lbf/in2 gauge were utilized and material flow rates up to about 40,000 Ib/hwere obtained. Solids loading ratios of over 140 were achieved and the minimumconveying air velocity for the material was about 750 ft/min.

Although the mean particle size of the material was about 64 micron, thematerial clearly had very good air retention properties, and better than those of thefluorspar mentioned above with a mean particle size of 66 micron, so that truedense phase conveying was achieved for the iron powder with the high pressuregradients available.

50

T30u"raerf

Conveying Line Solids LoadingPressure Drop 14° Ratio

- lbf/in2

5

0 50 100 150 200

Free Air Flow Rate - ft/min

Figure 14.2 Fligh pressure conveying characteristics for iron powder conveyed throughthe pipel ine shown in figure 4.2.

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402 Chapter 14

3 COPPER CONCENTRATE

The data presented here was obtained for copper concentrate having bulk and par-ticle densities of about 105 and 245 lb/ft'. The mean particle size of the materialwas approximately 55 micron. Conveying characteristics for the copper concen-trate, conveyed in a high pressure system are presented in Figure 14.3.

This is the same pipel ine as used for the high pressure conveying of the ironpowder, presented above, and the same high pressure blow tank was used to feedthe material into the pipeline.

A comparison of the conveying characteristics for the two materials willshow that the iron powder could be conveyed at a slightly higher flow rate than thecopper concentrate, for a given value of conveying line pressure drop. Air supplypressures up to 40 lbf/in2 had to be employed to convey the copper concentrate at40,000 Ib/h.

It should be noted that mean particle size is not a good indicator of the tran-sition from dilute to dense phase conveying capability for a material. Although itdoes tend to occur in the 50 to 100 micron size range, it is the air retention capabil-ity of a material that is a better indicator. Air retention is additionally influencedby particle size distribution and particle shape, and so this particular bulk propertydoes provide the best parameter for the purpose of assessing conveying capabilityat the present time.

50

oo240

JD

i 30<U

ttiK!

J20tu

Solids LoadingRatio

Conveying LinePressure Drop

-Ibf7in2 \ -^^^ 40

10

0 50 100 150 200

Free Air Flow Rate - IVYmin

Figure 14.3 High pressure conveying characteristics for copper concentrate conveyedthrough the pipeline shown in figure 4.2.

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High Density and Other Materials 403

4 ZIRCON SAND

The data presented here for this material was obtained for zircon sand having bulkand particle densities of about 160 and 285 Ib/ft ' . The mean particle size of thematerial was approximately 120 micron. Conveying characteristics for the zirconsand conveyed in a high pressure system are presented in Figure 14.4.

This is also the same pipeline and conveying system that was used for thehigh pressure conveying of the previous two materials and so direct comparisonsare possible. The first thing that wi l l be noticed, however, is that the material hasno natural air retention properties and so could not be conveyed in dense phaseand at low velocity. The m i n i m u m value of conveying air velocity was about 2600ft/min and the maximum value of solids loading ratio achieved was only about 16.Although a solids loading ratio of only 50 could be achieved with the copper con-centrate, the minimum value of conveying air velocity was about 1500 ft/min.

Material flow rates achieved are very much lower than those obtained withboth the iron powder and copper concentrate and as a consequence the materialflow rate axis was halved for these conveying characteristics in order to magnifythe data. In order to convey the material with high air supply pressures it was alsonecessary to use much higher air flow rates.

Despite these differences the material conveyed very well and high air sup-ply pressures can be employed if required. As with any material that can only beconveyed in dilute phase, however, power requirements will be very much higherbecause of these limitations.

24

o§20

i ^>; 12

_o± 8

Solids LoadingRatio

Conveying LinePressure Drop

- Ibf7 in 2

100 150 200

Free Air Flow Rale - ii'/min

Figure 14.4 High pressure conveying characteristics for zircon sand conveyed throughthe pipel ine shown in figure 4.2.

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404 Chapter 14

5 SILICA SAND

This material is widely conveyed in foundries and glass making. The most notablething about the material, however, is that it is extremely abrasive. Because it isrelatively cheap, and readily available in a wide range of particle sizes, it is idealfor research purposes. The authors have much experience of steel pipeline bendsfai l ing due to erosive wear while conveying this material. In dilute phase convey-ing, under quite normal conveying conditions of velocity and solids loading ratio,it is not unusual for a steel bend to wear through after just two hours of service.This subject is considered in some detail in Chapter 20.

5.1 Low Pressure Conveying

Silica sand typically has bulk and particle densities of approximately 90 and 160lb/ftj, and the data reported here is for sand having a mean particle size of about260 micron. Low pressure conveying data for the sand conveyed through the 115ft long Figure 4.15 pipeline of two inch nominal bore is presented in Figure 14.5.

The minimum conveying air velocity for the sand was about 2600 ft/min. Asa consequence of this, and the conveying line pressure drop being 8 lbf/in2 as amaximum, the maximum value of solids loading ratio achieved was only aboutten, despite this being a relatively short pipeline. Data on numerous other materialshas been presented for this pipeline, including a group of four in Figure 4.14 and acomparison of seven different materials in Figure 4.16 which showed the sand tobe one of the poorest performers.

10

oc

aOi

|4tu

Solids LoadingRatio

Conveying LinePressure Drop

- lbf/in2

0 50 100

Free Air Flow Rate - ftYmin

Figure 14.5 Low pressure conveying characteristics for silica sand conveyed throughthe pipel ine shown in figure 4.15.

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High Density and Other Materials 405

5.1.1 Material Degradation

This silica sand, conveyed through the Figure 4.15 pipeline, is another material forwhich data on degradation as a result of pneumatic conveying is available. Similardata on the degradation of sodium chloride and soda ash was presented in section2.2 of Chapter 11 , and for coal in section 5 of Chapter 10. As with these other ma-terials the approximate m i n i m u m and maximum values of conveying air velocitywere 3400 and 4400 ft/min, and the solids loading ratio was about five.

The sand was conveyed through the Figure 4.15 pipeline for the purpose ofdetermining the potential degradation of the material. Fresh material was loadedinto the test facility, it was circulated a total of five times and samples were takenduring each run. Samples were taken by means of a diverter valve positioned nearthe end of the pipel ine. A size analysis of all the samples, obtained from the freshmaterial and each of the five times the material was re-circulated, was carried outand the results are presented in Figures 14.6 and 14.7.

From Figure 14.6 it w i l l be seen that degradation of the si l ica sand is quitesignificant. A noticeable effect has been recorded every time the material wasconveyed and re-circulated. Despite the material being extremely hard and abra-sive it is clearly brittle and friable.

In Figure 14.7 the degradation is presented in terms of a change in meanparticle size. To provide a basis for comparison, data for degradation of both thesodium chloride and soda ash, from Figure 11.7, conveyed through the same pipe-line and under identical conveying conditions, has been added.

100

80

60

40

20

0

Number of timesmaterial circulated

100 200 300 400 500 600

Particle Size - //m

Figure 14.6 Influence of conveying on the degradation of silica sand.

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Page 417: Handbook of Pneumatic Conveying Engineering

406 Chapter 14

E£ 2o

1 1zs

'Z,

0

Sodium Chloride(Salt)

Soda Ash

Silica Sand

J L J L.

200 250 300

Mean Particle Size - //m

350 400

Figure 14.7 Influence of material conveying on mean particle size.

It is interesting to note that there appears to be little difference in the rate ofdegradation of any of these three materials. Material degradation is a subject thatis considered in some detail in Chapter 21.

5.1.2 Influence of Conveying Distance

Conveying characteristics for the sand conveyed through a longer pipeline are alsoavailable. A second loop was added to the Figure 4.15 pipeline to make the newpipeline 225 ft long, with the addition of four bends. A sketch of the pipeline isgiven in Figure 14.8.

Pipeline:LengthBoreBends

= 225ft= 2 in= 1 2 x 9 0 °= 5

Figure 14.8 Details of longer pipeline used for low pressure conveying trials.

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High Density and Other Materials 407

Conveying characteristics for the silica sand conveyed through the Figure14.8 pipeline are presented in Figure 14.9. The same conveying facility was used,as for the single pipeline loop data in Figure 14.5, and the conveying trials werecarried out over the same range of air supply pressures and air flow rates in orderto generate the data.

As expected, with an additional 110 feet of horizontal pipeline and four ad-ditional bends, the maximum material flow rates, for the same range of conveyingline pressure drop values, are significantly lower. It must be recognized that if apipeline is extended in this way then a reduction in conveying performance mustbe expected, if the same air supply pressure is to be ut i l ized.

By the same token, if material is fed into an extended pipeline at the samerate as in the original pipeline, then a very much higher conveying line pressuredrop wi l l result for the extended pipeline. If that higher pressure is not available,then it is likely that the pipeline will block as a consequence. This type of situationis considered in more detail in Chapter 19.

It wi l l be noted, by comparing Figures 14.5 and 14.9, that the material flowrates, for given values of conveying line pressure drop and air flow rate, have nothalved as a result of a doubling of the conveying distance. This is because scalingfrom one pipeline to another has to be in terms of equivalent lengths and so verti-cal l if t and pipel ine bends also have to be taken into account.

Solids LoadingRatio /

ooo

Conveying LinePressure Drop \ — / ^***^ 4

- Ibf/itr

0 50 100 150

Free Air Flow Rate - rr'/min

Figure 14.9 Low pressure conveying characteristics for silica sand conveyed throughthe pipel ine shown in figure 14.8.

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408 Chapter 14

The number of bends, for example, has increased from eight to twelve andso this has not doubled. The equivalent length of bends is relatively high, as wi l lbe recalled from Figure 8.14. Account also has to be taken of the difference in aironly pressure drop values between the two pipelines. This type of scaling is con-sidered in more detail in the next chapter.

5.2 High Pressure Conveying

Conveying characteristics for the silica sand conveyed through the 165 ft longFigure 4.2 pipeline of two inch nominal bore are presented in Figure 14.10. 200ftYmin of free air was available to this conveying facility and so it was possible touse conveying line inlet pressures up to about 35 psig.

With a mean particle size of 260 micron, however, this material was far toogranular to be capable of low velocity dense phase conveying in the conventionaltest facility used. The minimum value of conveying air velocity at which the sandcould be conveyed was about 2600 ft/min, and so this dictated the maximum con-veying l i m i t in terms of air supply pressure with the 200 cfm l imit on air flow rate,despite the fact that air at a pressure of 100 psig was available to the conveyingfacility.

With a conveying l ine pressure drop of 30 Ibf/irr the sand was conveyed atabout 17,000 Ib/h. The barite in Figure 13.10 was conveyed through this samepipeline and with the same pressure drop the material was conveyed at about50,000 Ib/h. The l imit with the barite was not pressure, but the fact that the dis-charge capability of the blow tank used had reached its l imi t at 50,000 Ib/h.

24

o§20

o

Solids LoadingRatio •

Conveying LinePressure Drop

12 h - Ib t / i n 2

0 50 100 150 200

Free Air Flow Rate - fr'/min

Figure 14.10 High pressure conveying characteristics for silica sand conveyed throughthe pipel ine shown in figure 4.2.

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High Density and Other Materials 409

6 COKE FINES

Petroleum coke is a material that is becoming widely available and is relativelycheap. As a consequence it is being used for firing furnaces and kilns in place ofnatural gas and oil, where energy costs are a high proportion of the cost of the finalproduct. It is available in a wide range of sizes from dust to coarse granular and so,like many similar materials such as fly ash and alumna, has an extremely widerange of conveying capabilities.

The coke fines, for which data is available here, are also those of petroleumcoke. The material was minus one mm in size (18 Mesh), with a very wide particlesize distribution and was very granular. As a consequence the material had verypoor air retention capability and a very low permeability. The material was con-veyed through the 165 ft long Figure 4.2 pipeline of two inch nominal bore and theconveying characteristics are presented in Figure 14.11.

The conveying characteristics are very similar to those for silica sand con-veyed through this pipeline and presented in Figure 14.10. The min imum convey-ing air velocity for the material was slightly higher at about 2900 ft/min and thishas meant that the maximum value of air supply pressure that could be util ized forconveying, wi th in the 200 cfm air flow rate l imi t , was 30 psig. For a given valueof conveying line pressure drop, maximum values of material flow rate were about25% greater for the coke fines than for the silica sand.

24

oo

* 12_ou.

Solids LoadingRatio

Conveying LinePressure Drop

- lhf / in 2

0 50 100 150 200

Free Air Flow Rate - itYmin

Figure 14.11 High pressure conveying characteristics for coke fines conveyed throughthe p ipe l ine shown in figure 4.2.

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410 Chapter 14

7 PEARLITE

Pearlite probably has the lowest density of any material reported in this Handbook.The data presented here was obtained for pearlite having bulk and particle densi-ties of about 6 and 50 lb/ft3. The mean particle size of the material was approxi-mately 158 micron. Pearlite is an exfoliated material and hence the large differ-ence between particle and bulk density values.

7.1 Low Pressure Conveying

Conveying characteristics for the pearlite conveyed in a low pressure system, andhence in dilute phase suspension flow, are presented in Figure 14.12. The datarelates to conveying through the 115 ft long Figure 4.15 pipeline of two inchnominal bore.

As a result of the low particle density and the shape of the particles, thepearlite conveyed very easily in dilute phase and the minimum value of conveyingair velocity was about 2100 ft/min. Because of this low value of velocity, solidsloading ratio values up to about 24 were achieved in this pipeline. This, however,is also partly due to the relatively high material flow rates achieved.

The iron powder in Figure 14.1 and the silica sand in Figure 14.5 were alsoconveyed through this Figure 4.15 pipeline. With a conveying l ine pressure dropof 8 Ibf/irr, 3500 Ib/h was achieved with the iron powder, 5000 Ib/h with the silicasand and, as wi l l be seen above in Figure 14.12, 8500 Ib/h was obtained with thepearlite.

Solids LoadingConveying Line s / 20 / RatioPressure Drop

- Ibl7in2

o 6

0 50 100

Free Air Flow Rate - ft'/min

Figure 14.12 Low pressure conveying characteristics for pearlite conveyed through thepipel ine shown in figure 4.15.

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High Density and Other Materials 411

This, once again, illustrates the wide difference in conveying capability thatcan be obtained in dilute phase conveying. In order to dispel the idea that this con-veying performance might simply be related to material density, the 8 lbf/in2 con-veying line pressure drop line for these three materials, together with a number ofother materials that have also been conveyed through this Figure 4.15 pipeline,were presented earlier for comparison in Figure 4.16. This is reproduced here inFigure 14.13 for reference.

With additional materials it wil l be seen that the conveying performance, interms of material flow rate achieved, does not correlate with material density.Soda ash, as mentioned before, has something of a reputation for being a difficultmaterial to convey, and this may be due, in part, to the fact that it is little betterthan iron powder. Soda ash has a bulk density of about 70 Ib/ft ' . Pulverized fuelash, with a bulk density of about 45 Ib/ft ' , had the best performance of the materi-als tested.

7.2 High Pressure Conveying

The pearlite has also been conveyed through the 165 ft long Figure 4.2 pipeline oftwo inch bore and the conveying characteristics are presented in Figure 14.14.

It wi l l be seen that with a much higher pressure gradient the material is nowcapable of being conveyed at very much higher solids loading ratios and lower airflow rates. Conveying with high pressure air, however, was not possible with thismaterial because the conveying l imit was about 25,000 Ib/h.

ooo

Pulverized Fuel Ash(tine grade)

Pearlite

20 40 60 80

Free Air Flow Rate - ftVtnin

100 120 140

Figure 14.13 Comparison of performance of different materials conveyed through thepipeline shown in figure 4.15 with a conveying l ine pressure drop of 8 lbf/in".

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412 Chapter 14

50

o§40

;f_g^20

10

Solids LoadingRatio

Conveying LinePressure Drop

- Ihf / in 2

50 100 150

Free Air Flow Rate - ftVmin

200

Figure 14.14 High pressure conveying characteristics for pearlite conveyed throughthe figure 4.2 pipeline.

It was reported above, in relation to the high pressure conveying of silicasand through this pipeline, that the conveying l imi t with barite was about 50,000Ib/h and that it was the discharge capability of the blow tank that was responsible.Because of the very much lower bulk density of the pearlite it is suspected that thel imi t is imposed by the blow tank once again.

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15System Design Using Conveying Data

1 INTRODUCTION

The design of pneumatic conveying systems is still very much based on the scal-ing of conveying data. Such data is generally obtained from a purpose built testfacility, similar to the data presented in the previous five chapters. It is not practi-cal, of course, that the plant pipeline should be replicated for test purposes. Forconvenience of testing different materials, the reception point at the end of thepipeline is generally located above the material feeding device to provide a con-venient loop, so that material can be re-circulated, where possible. As a conse-quence the test loop is l ikely to be mostly in the horizontal plane and may containa disproportionate number of bends.

1.1 The Use of Scaling Parameters

Scaling parameters, however, are available, as considered in Chapters 7 and 8, thatwi l l enable data obtained from one pipeline to be scaled to that for another pipe-line. In Chapter 7 scaling parameters were presented that wi l l allow for differencesin pipeline bore and conveying distance to be taken into account, between that ofthe test facility and that of the plant pipeline to be designed. In Chapter 8 similarconsideration was given to the influence of pipeline bends, both in terms of num-ber and geometry, and to pipeline orientation, including vertically up and verti-cally down routings. The influence of pipeline material was also considered. It is

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414 Chapter 15

for these reasons that fu l l details of all pipelines have been given, for which con-veying data has been presented in this Handbook.

1.2 Manufacturers'Approach

Most manufacturers of pneumatic conveying systems have bui l t such test facilitiesin order to obtain data for design purposes. They wi l l generally ask for a represen-tative sample of the material to be conveyed and demonstrate the conveying of thematerial to the client. This, of course, provides an element of protection for thevendor, should the client manufacture or source a slightly different grade of mate-rial when the system comes into use.

Manufacturers, however, are unl ike ly to derive entire performance maps fora material, such as those presented here. The cost of running and maintaining sucha test facility is very high, and most companies wi l l generally advertise the factthat they will undertake conveying tests at 'no charge'. Only a couple of tests arelikely to be undertaken, therefore, in order to establish the conveyability of thematerial. The vendor wi l l know what type of system it intends to offer a client andso only limited data wil l be required.

1.2.1 Previous Experience

Alternatively data obtained from previous experience of designing and bui lding asystem may be used, provided that the data relates to exactly the same material.Extreme caution must be exercised here, for different grades of exactly the samematerial can have very different conveying characteristics. It is also important tonote that data should never be scaled either to lower values of conveying air veloc-ity, or to higher values of solids loading ratios, than have previously been achievedwith the material.

1.3 Cases Considered

Since the geometry of the test facility, or a previous system installed, is unlikely tobe the same as that of the plant pipeline to be built, scaling parameters are used.Account may additionally have to be taken of changes in solids loading ratio, andhence conveying line inlet air velocity, particularly if the scaling is to a longerpipeline. A review of appropriate scaling parameters is presented, and these areillustrated with two case studies. One is for a material conveyed in dilute phase,suspension flow. Another is for a material capable of being conveyed in densephase, non suspension flow.

1.4 The Future

For single phase fluids, as is well known, the problem of analyzing the flow wasultimately solved empirically by the use of a friction coefficient in conjunctionwith Reynolds number and pipe wall roughness. The paral lel problem with two-phase, gas-solid flows wi l l no doubt be solved one day, and it is also likely to be

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Design Using Conveying Data 415

an empirical solution, but that day is not yet in sight. Meanwhile companies thatmanufacture pneumatic conveying systems are reluctant to publ ish any data thatthey have because it is of too high a commercial value, and there are very fewresearch groups around the world that are working in this area f I].

2 SCALING PARAMETERS

Scaling parameters for system design can be split into two main groups and thescaling process can be undertaken in two stages. The first stage relates to horizon-tal conveying distance. This is expressed in terms of an equivalent length and in-corporates pipeline bends, pipeline orientation and pipeline material. The secondstage relates to pipeline bore. In each case allowance must be made for the differ-ence in air only pressure drop for the given pipeline. This is because the scalingparameters relate only to the conveying of the material in the air.

Scaling relates to individual test points or data. If a complete set of convey-ing characteristics require to be scaled, a considerable number of data points willneed to be scaled and so it is a time consuming process. The best way of doing thisis to place a grid on the conveying characteristics and scale for every grid point.The most convenient grid for this purpose is probably one based on lines of con-stant conveying line inlet air velocity and lines of constant conveying line pressuredrop, at regular increments of each.

Both equivalent length and pipeline bore can have a very significant influ-ence on material flow rate through a pipeline. It must be recalled, however, thatany required material flow rate can generally be achieved, over any given convey-ing distance, with an appropriate combination of pipel ine bore and air supply pres-sure or vacuum. The l imitat ion, for any extreme value of either material flow rateor conveying distance, is generally power requirement.

2.1 Equivalent Length

The equivalent length of a pipeline, as mentioned above, incorporates straightpipeline sections and pipeline bends. The scaling parameter for equivalent lengthis an inverse law model. This was presented earlier in Chapter 7 and is re-produced here for reference.

1mp <x — lb/h - - - - - - - - - - ( i )

L'

or alternatively:

rhp}Lc] = mp2Lc2 = Const. - - - - - - - - ( 2 )

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416 Chapter 15

For a constant air flow rate and pressure dropdue to the conveyed material.

where m = mass flow rate of material

and Le = equivalent length of pipeline

The working form of this scaling model is:

m , = m , x —^- Ib/h (3)

where subscripts I and 2 relate to theappropriate lengths of the two pipelines

2.1.1 Conveying Capability

A graphical representation of this model is given in Figure 15.1 in order to illus-trate the very significant influence that equivalent length, and hence conveyingdistance and pipeline bends, can have on material flow rate. Figure 15.1 is drawnfor a coarse granular material conveyed through a 2 inch bore pipeline. It relates,therefore, to dilute phase suspension flow. A 2 inch bore pipeline over the longdistances considered, of course, is not realistic. It is simply extended that far toillustrate the nature of the model.

120

ooo

80

40

Conveying LinePressure Drop

- lbf / in 2

200 400 600 800

Equivalent Length of Pipel ine - feet

1000 1200

Figure 15.1 Influence of equivalent length and pressure drop on material flow ratethrough pipel ine.

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Design Using Conveying Data 417

2.1.1.1 Conveying OptionsWith this being an inverse law relationship it means that if the equivalent length isdoubled, the material flow rate wi l l be halved. The actual situation, however, isslightly worse than this, for as the conveying distance increases, the air only pres-sure drop also increases. As the air only pressure drop increases, less pressure isavailable to convey material. Thus if 60,000 Ib/h can be conveyed over an equiva-lent length of 200 feet, slightly less than 30,000 Ib/h wi l l be conveyed over a dis-tance of 400 feet.

This halving of the material flow rate for a doubling of equivalent lengthmust be appreciated in order to gain a f u l l appreciation of the capability of pneu-matic conveying systems. For the material represented in Figure 15.1 a conveyingline inlet air velocity of about 3000 ft/min would be required. With a conveyingline pressure drop of 40 lbf/in2, for example, equation 5.10 gives a free air flowrate of 243 ft'Ymin for these conditions. Then equation 3.6 gives a power require-ment of about 41 hp. This means that every point on the 40 lbf/in2 pressure dropline on Figure 15.1 wil l require 41 hp. Thus with 41 hp either 120,000 Ib/h can beconveyed through the pipeline with an equivalent length of 100 feet, or 11,000Ib/h with an equivalent length of 1200 feet.

If a pipeline is extended to a longer distance, and the same material flow rateis required, an increase in power will be required. If the conveying plant is alreadyoperating at maximum capability over the existing distance, it wi l l not be possibleto convey at the same rate over the longer distance unless major modifications tothe conveying system are made. If modifications are not made, then a lower mate-rial flow rate wi l l have to be accepted, and the feeder wil l have to be adjusted togive a lower flow rate for the longer distance.

2.1.1.2 Air PressureThe influence of conveying distance on the air only pressure drop was consideredearlier with Figure 7.20. For long distance conveying the use of a small bore pipe-line is not l ikely to be appropriate, unless that material flow rate required is verylow. In the case considered in Figure 15.1, the air only pressure drop for a 1500 ftlong pipeline wil l be of the order of 10 lbf/in". The influence of pipeline bore onthe air only pressure drop was considered in Chapter 7 with Figure 7.15.

The influence of conveying l ine pressure drop is addi t ional ly shown in Fig-ure 15.1. An increase in air supply pressure vvi l l always result in an increase inmaterial flow rate, provided that the required value of conveying air velocity ismaintained. Increasing the pressure to increase material flow rate, however, maynot be the best means of achieving an increased material flow rate. The alternativeis to increase the pipeline bore. It must always be remembered that a wide combi-nation of conveying line pressure drop values and pipeline bores are generallycapable of meeting any conveying duty. This point was considered earlier withFigures 7.18 and 19.

Although in the majority of cases scaling from existing data is generally tolonger pipelines, sometimes the scaling is to a shorter pipeline. In these cases the

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418 Chapter 15

air only pressure drop is l ikely to be lower and it wi l l be possible to achieve ahigher material flow rate for the same conveying conditions. Figure 15.1 can beused in either direction.

2.1.1.3 Dense Phase ConveyingFigure 15.1, as mentioned above, was specifically drawn for dilute phase convey-ing. Although a similar situation wi l l exist for dense phase conveying, particularattention has to be paid when the conveying distances are such that the transitionfrom dense phase to di lute phase conveying occurs. In many cases, when scalingto longer distances, there is little scope for increasing the air supply pressure. Thuswhen the distance increases, the pressure gradient decreases, and low velocitydense phase conveying requires a relatively high pressure gradient.

The transition from dilute to dense phase conveying was considered inChapter 13 at section 2 .1 .1 . It was suggested that to convey a material such as ce-ment at a solids loading ratio of about 100 typically requires a pressure gradient ofabout 10 lbf/in2 per 100 ft, and that the transition from dilute to dense phase startsto occur at a pressure gradient of about 2'/a lbf/in" per 100 ft. As the equivalentdistance increases for a given air supply pressure, therefore, the pressure gradientdecreases.

In dense phase the min imum conveying air velocity may be only 600 ft/min,but in dilute phase this can increase to 2000 ft/min. As a consequence the air flowrate may need to be increased. In d i lu te phase conveying the conveying air veloc-ity does not change, and so the air flow rate does not need to change either, unlessthe air supply pressure is increased. An increase in air flow rate wi l l mean that theair only pressure drop wi l l increase, and for many materials it will also mean areduction in material flow rate. The net effect of this is that the reduction in mate-rial flow with increase in equivalent length can be much greater than that predictedby the inverse law model given in Equation 3 and illustrated in Figure 15.1. Theseeffects were shown earlier in Figures 7.27 and 28.

2.7.2 Horizontal Pipeline

For the design of pneumatic conveying systems based on the use of conveyingdata and scaling parameters, horizontal pipeline is generally taken as the referencedimension for the process. The scaling parameter for horizontal pipeline, therefore,is unity.

Thus for horizontal pipeline the equivalent length, Lc, is simply given by:

Le = Lh f t . - . - . - - - . . . (4 )

where L/, = sum total of all horizontal pipeline sections

2.1.3 Vertical Pipelines

The pressure drop in vertical pipelines is very different from that in horizontallines and so they have to be treated separately. There are also significant differ-

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Design Using Conveying Data 419

ences in performance and pressure gradients between flow vertically up and flowvertically down, as illustrated earlier with Figure 8.4.

2.1.3.1 Flow Vertically UpMost pipelines wi l l contain an element of vertical l i f t , generally because of theneed to deliver material to the top of a silo or other elevated reception point. Re-sults from a comprehensive program of conveying trials with vertical and horizon-tal pipelines was presented in Chapter 8. Conveying characteristics with pressuredata in terms of pressure gradients were presented for a number of materials. Suchdata for barite conveyed through two inch nominal bore pipeline is presented inFigures 15.2.

The two sets of data for barite conveyed through two inch nominal borepipeline are presented together for direct comparison, with the data for horizontalflow in Figure 15.2a and the data for flow vertically up in Figure 15.2b. From thisit wi l l be seen that the pressure gradient for conveying a material vertically up isvery much greater than that for conveying the same material horizontally, andunder exactly the same conveying conditions. This is much as would be expected,since additional energy is required to overcome the gravitational force.

50

240

30

°20

10

Pressure Gradientlbf / in2 120 100 80

per 100ft

140

Solids Loading Ratio

(a)

0 50 100 150

Free Air Flow Rate - ftVinin

10

Pressure GradientIbf/ in2

per 100 ft120 100 80

Solids Loading Ratio

(b)

0 50 100 150

Free Air Flow Rate - ftVmin

Figure 15.2 (a) Horizontal and (b) vertically up pressure gradient dala for barite con-veyed through two inch nominal bore pipeline.

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420 Chapter 15

From this it wi l l be seen that a scaling parameter is required to take this intoaccount. In Chapter 8 a comparison of these two sets of data was presented. A gridwas superimposed on each set of data and the ratio of the pressure gradients wasevaluated for every grid point. The result was presented in Figure 8.7a. In Figure8.7b the result of a s imi lar analysis carried out with fly ash was presented. In eachcase the ratio was close to a value of two, and this was reasonably constant overthe entire range of conveying conditions examined. It is suggested, therefore, thatthe scaling parameter for vertically upward sections of pipel ine is 2-0.

Thus for vertical pipeline, in which the flow is vertically up, the equivalentlength, Le, of straight horizontal pipeline, is simply given by:

Le = 2 x Lv ft (5)

where Lv = sum total length of all vertically upward sec-tions in pipeline

2.1.3.2 Flow Vertically DownSimilar pressure gradient data to that given in Figures 15.2a and b, for barite con-veyed in two inch nominal bore pipe, is presented in Figure 15.3. This is for bariteconveyed vertically down.

60

ooo

_

T40

20

Pressure GradientIbf / in 2 per 100f t

-4

140 120 100

20

Solids Loading Ratio

50 100

Free Air Flow Rate - ft'Vmin

150

Figure 15.3 Pressure gradient data for vertically downward flow of barite in two inchnominal bore pipel ine.

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Design Using Conveying Data 421

With both pressure loss and pressure recovery occurring over the potentialrange of conveying conditions, this is clearly not going to produce a straightfor-ward scaling parameter. From a number of different materials tested, however,there was consistency in that the line of zero pressure gradient occurred at a solidsloading ratio of about 35 in each case [2. 3]. Thus for dilute phase conveying therewil l generally be a pressure drop in conveying vertically downwards and for densephase conveying there wil l generally be a pressure recovery.

As mentioned in Chapter 8, in the majority of pneumatic conveying sys-tems, flow vertically down usually occurs only when the pipeline is routed oversome obstruction such as a road or railway line. In these cases the influence of thevertically downward section is generally so small that it can be disregarded. Theadditional bends required, however, must be included in the total number of bendsin the pipeline.

If it is considered that the vertically downward section of pipeline should beincluded in the pipeline analysis it would be recommended that for dilute phaseflow the downward sections of pipeline should be treated as straight horizontalpipeline. For dense phase flow the length of pipeline involved could well be ne-glected.

For long distance transport vertically down it is clearly essential that datasuch as that presented in Figure 15.3 should be used. Dense phase conveying insuch a situation is an obvious advantage, for the pressure generated in the verti-cally downward section of pipeline could well be sufficient to convey the materialseveral thousand feet horizontally at the bottom. Care must be taken with convey-ing air velocity, and hence pipeline bore sizes, in this type of conveying situation,as considered in Chapter 9 with Figures 20 and 21.

2.1.4 Pipeline Bends

Pipeline bends are an essential element of any pipeline since they provide theflexibility in pipeline routing to allow any required geometry between the supplyand reception points. There is, however, a penalty to pay for this flexibility, foralthough they add essentially nothing to the actual conveying length, they can addsignificantly to the equivalent length of the pipeline. It is the equivalent length ofthe pipeline that dictates the conveying capability of a pipeline, as wi l l be seenfrom Equation 1 .

The nature of the problem, with respect to pipeline bends, was considered inChapter 8 with Figures 10 and 11. A detailed analysis was included of the influ-ence of bends and data in terms of an equivalent length for ninety degree bendswas presented in Figure 8.14. As this is a crucial element in the overall scalingprocess for pipelines this data is reproduced here as Figure 15.4 for reference pur-poses.

The scaling parameter is in terms of the equivalent length of straight hori-zontal pipeline. Conveniently the loss is in terms of a single parameter, which isthe conveying line inlet air velocity.

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422 Chapter 15

0 40001000 2000 3000

Conveying Line Inlet Air Velocity - ft/min

Figure 15.4 Equivalent horizontal length of bends for 90° radius bends.

This means that the total equivalent length for the bends in a pipeline is sim-ply given by the appropriate equivalent length from Figure 15.4 multiplied by thetotal number of bends in the pipeline.

Thus for bends in a pipeline the equivalent length, LL,, of straight horizontalpipeline, is simply given by:

Le = N ft (6)

where A' = sum total of all bends in pipelineand LL,f, = equivalent length for bends

From Figure 15.4 it w i l l be seen that bends in a pipeline can have a verysignificant influence, particularly for dilute phase conveying. With a conveyingline inlet air velocity of 3000 ft/min, for example, the equivalent length of just sixbends is approximately 300 feet. A definite recommendation, therefore, is that thenumber of bends in any pipeline should be kept to an absolute min imum.

The same recommendation also applies to dense phase conveying, for al-though the equivalent length for a bend may only be about five feet with a convey-ing line inlet air velocity of 600 ft/min, the corresponding pressure gradient in thepipeline is significantly higher. This is why the range of pressure drop values forbends, in terms of lbf/in2 presented in Figure 8.15, is over such a very much nar-rower range than that of equivalent length in Figure 15.4.

2.1.4.1 Position of Bends in PipelineIt will be noted that the equivalent length of all bends in a pipeline would appearto be the same. While this is certainly a convenient parameter for scaling, it might

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Design Using Conveying Data 423

be expected that the equivalent length would change with location, particularly asthe conveying air velocity increases along the length of a pipeline in a single boreline.

This, however, is the result of a comprehensive program of research workand analysis carried out [4J and although a change with location was expected itwas not observed. Since the energy loss is associated with the re-acceleration ofthe conveyed particles after the bend, it is possible that the reduction in velocity ofthe particles across a bend does not vary. This is yet another area where more re-search work needs to be undertaken.

2.1.4.2 Proximity of BendsIn undertaking research work on bend losses it is generally standard practice toensure that there is sufficient length of straight pipeline before the test bend toensure that the particles have fu l ly accelerated to their terminal velocity. In prac-tice, of course, some bends may have to be located relatively close to one another.A general recommendation is that bends should be spaced a sufficient distanceapart to allow particles to reach their terminal velocity.

This is particularly the case for the first bend in a pipeline, for this followsthe material feed into the pipeline where the material velocity is essentially zero. Ifthis bend is too close a higher conveying line inlet air velocity wi l l have to be em-ployed and this wi l l generally have the effect of reducing the conveying capacityof the pipeline.

Similarly, if bends are too closely spaced along the length of a pipeline itmay be necessary to increase the conveying air velocity to prevent pipeline block-age. The position in the pipel ine also needs to be taken into account here, for withbends located close to the end of the pipeline, where the conveying air velocity isvery much higher, it is not likely to be such a problem.

In a program of work carried out with the test facility associated with theFigure 4.15 pipeline two of the authors investigated this problem [5]. Silica sandwith a mean particle size of about 230 micron was used. Conveying characteristicsfor the sand conveyed through the Figure 4.15 pipeline are presented in Figure15.5 for reference. This is dilute phase suspension flow since the test facility waslimited to low pressure conveying, and with the material being granular it had littleair retention capability.

The Figure 4.15 pipeline was then modified by the addition of four extrabends. For convenience of support and examination these additional bends werelocated in the second half of the pipeline. A sketch of this part of the pipeline isgiven in Figure 15.6 for reference. This increased the number of the bends in thepipeline to twelve but the conveying distance remained at 115 ft. All twelve bendshad a bend diameter, D, to pipe bore, d, ratio of about 5 :1 . Thus two pipelineswere available with the number of bends being the only difference.

A further purpose of adding these bends to the pipeline, and locating themclose together, was to undertake an investigation into acceleration length for theconveyed material [6J.

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424 Chapter 15

a _oi 3

_o

I 'H

Solids LoadingRatio

Conveying LinePressure Drop

- Ibf/in2

4

0

Pipeline:Length = 115 ft

Bends 1 2 x 9 0 °D/d = 5

0 50 100

Free Air Flow Rate - ft'/min

Figure 15.5 Conveying characteristicsfor sand in figure 4.15 pipeline.

Figure 15.6 Modification to end offigure 4.15 pipeline.

Silica sand is an extremely erosive material and so an assessment of accel-eration length could be made by measuring the wear on the bends following eachstraight length of horizontal pipeline. This is why the pipeline incorporated a widerange of straight pipeline lengths before the various bends. Erosive wear increasesexponentially with increase in velocity and so provides a very simple yet sensitivemeans of assessing the velocity of the particles on impact with a bend. Fromanalysis of the various bends in the pipeline it was found that the accelerationlength for the 230 micron sand was about five feet.

Conveying characteristics for the same sand conveyed through the Figure15.6 pipeline are presented in Figure 15.7a. An additional loop was also added tothe Figure 4.15 pipeline and this is the Figure 14.8 pipeline. Thus the Figure 15.6and Figure 14.8 pipelines both have the same number of bends and it is the lengthof pipeline that differs. Conveying characteristics for this same silica sand con-veyed through the Figure 14.8 pipeline are presented in Figure 15.7b.

If the conveying characteristics for the sand in Figures 15.5 and 15.7a arecompared it w i l l be seen that there is a very significant reduction in performanceand this is due entirely to the fact that the sand represented in Figure 15.7a wasconveyed through four additional bends. An analysis of these two sets of data gavean equivalent length of the bends in the pipelines as about 32 feet. Although this isa little lower than might be expected, the acceleration length for many of the addi-tional bends was less than five feet.

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Design Using Conveying Data 425

o 5oo

* 3

_o

Solids LoadingRatio

Conveying LinePressure Drop 6

- Ibf/iir

Solids LoadingRatio

Conveying LinePressure Drop

- lbf/in2

(a)

50 100

Free Air Flow Rate - ft'/min(b)

0 50 100

Free Air Flow Rate - ftVmin

150

Figure 15.7 Conveying characteristics for 230 micron sand in (a) pipeline shown infigure 15.6 and (b) pipeline shown in figure 14.8, both pipelines having twelve 90° bends.

If the conveying characteristics for the sand in Figures 15.7a and 7b arecompared it wi l l be seen that there is a further reduction in performance and this isdue to the difference in length between the two pipelines of about 110 feet.

2.1.4.3 Bend GeometryIn all the data presented above for the sand conveyed in dilute phase, every bendin each of the pipelines had a D/d ratio of about 5:1. In the analysis undertaken toproduce the data presented in Figure 15.4, all the bends tested had a D/d ratio ofapproximately 24:1.

Bend geometry was considered in some detail in section 5.3 of Chapter 8.Tests were undertaken with a fine grade of fly ash and were carried out with bendsranging from long radius, through short radius to blind tees. It was found that bendradius had little influence on pipeline performance over a very wide range of val-ues. It was only with very short radius bends, and particularly blind tee bends thattheir effect on pressure loss was significant.

It is suggested that the equivalent lengths for bends presented in Figure 15.4will be appropriate for all radiused bends down to a D/d ratio of about 3:1. Blindtee bends would not be recommended for use unless specifically required for thehandl ing of abrasive materials. In this case it would be suggested that the equiva-lent lengths presented in Figure 15.4 be doubled.

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426 Chapter 15

2.1.4.4 Bend AngleNot all bends are 90° and a variety of other angles are used and are available asstandard pipeline fittings. There is a distinct lack of information on the perform-ance of such bends, possibly because their numbers are relatively small in com-parison. The energy loss associated with any bend is due to the retardation of theparticles as a result of impact and the consequent re-acceleration in the high veloc-ity conveying air flow. As a worst case it is suggested that all bends are simplytreated as standard 90° bends.

With short angle bends it is possible that the particles could pass through thebend with just a glancing blow, little more than a particle impacting against thestraight pipeline wall while being conveyed. In this case the influence of the coef-ficient of restitution of the particles wi l l play an important part, as discussed insection 5.2.1.1 of Chapter 8. Once again this is another topic in pneumatic convey-ing that st i l l requires a lot of research work to be undertaken.

2.1.5 Pipeline Wall Material

The scaling parameters and conveying data presented so far relate to steel pipeline.Pipeline material was considered in Chapter 8 at section 6 and specific data on theconveying performance of hose material was presented. With rubber hose it wasfound that there was a significant increase in pipeline pressure drop, particularlyfor high velocity dilute phase flow. Once again it is suggested that this relates tocoefficient of restitution between particles and surface.

It is suggested that if a conveying system does include a proportion of rub-ber hose line, the equivalent length of that section of pipeline could be increased.The data presented in Figure 8.22 could be used by way of guidance in this re-spect.

2. 1. 6 Material Feeding

Although material feeding wi l l be common to both pipel ines in the scaling proc-ess, an element for material feeding should be included as it w i l l influence theproportionality. A term is required here that wi l l account for the energy required toaccelerate the conveyed material from rest to its terminal velocity. This relatesessentially to the kinetic energy required to accelerate the material and so it can bea significant term for some systems.

From the steady flow energy equation the element for kinetic energy isgiven by:

Apacc = - lbf/in2 - - - - - ( 7 )2 gcx 3600x144

where p = density of conveyed material - lb/ft'C = velocity of conveyed material - tt/min

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Design Using Conveying Data 427

and gc = gravitational constant- 32-2 ftlb/lbfs2

The density term is that of the conveyed material and this is most conven-iently considered in terms of the solids loading ratio, 0, for the conveyed material.The energy term relates to the end of the pipeline and so in positive pressure con-veying systems, where the air density can be taken as the free air value of 0-0765lb/ft', the suspension density or the density of the conveyed material is approxi-mately:

p = 0-0765 <t> lb/ft3(8)

The velocity term is that at the end of the pipeline, but it is the velocity ofthe particles and not the velocity of the air. This is an important considerationsince it is the square of the velocity in this equation. Particles are typically con-veyed at about 80% of the conveying air velocity in horizontal flow and so it doeshave a significant effect on the energy term.

Particle velocity, Cp, is generally expressed in terms of a s l ip ratio, which isthe ratio of the velocity of the particle to the velocity of the conveying air, Ca. Thevalue of slip ratio wi l l depend upon the particle size, shape and density, as well aspipeline orientation. Typical data for spherical particles in horizontal flow is pre-sented in Figure 15.8 [7J.

1-0

0-8

• 0-7^oC3

oiJ5- 0-6

0-5

0-4 -

Particle Density - lb/lt3

20 40 60 100 200

Particle Diameter - micron

500 1000

Figure 15.8 Inf luence of particle size and density on slip ratio.

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428 Chapter 15

In terms of equivalent length the simplest way of taking particle accelerationpressure drop into account is to treat it in terms of two additional bends. In a blindtee the material has to be accelerated from near zero velocity, which is almost likefeeding the material into the pipel ine once again. The pressure drop across a bl indtee is about double that for a radiused bend and the recommendation above forbl ind tee bends was that the data in Figure 15.5 should be doubled.

2.1.7 Total Pipeline

The equivalent length of a pipeline, Le, is derived by simple addition of all theelements that need to be taken into account. Considering horizontal and verticallyup sections of pipeline, together with pipeline feeding and bends gives:

Le = Lh + 2LV + (N+2)Leb ft - - - - (9)

2.2 Pipeline Bore

The second element in the scaling process is to take account of differences in pipe-line bore. For a given pipeline layout this is the major means of increasing materialflow rate. The scaling parameter for pipeline bore relates to pipe section area. Thiswas presented earlier in Chapter 7 and is reproduced here for reference:

ril <x A oc d1 . . . . . . . . . (10)

where mi = mass flow rate of material - Ib/h

d = pipeline bore - in

or alternatively:

mn\ m,,2—'— = —'— = Const. - . . . . - - - - ( l l )

For the same conveying air velocity andpressure drop due to the conveyed material

The working form of this scaling model is:

/ \ 2

-M lb/h - - - - - - - (12)

Where subscripts 1 and 2 relate to theappropriate pipe bores of the two pipelines

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Design Using Conveying Data 429

2.3 Air Only Pressure Drop

In both scaling parameters (Equations 2 and 11), for conveying distance and pipe-line bore, reference was made to the fact that the scaling relates only to the pres-sure drop due to the conveyed material. This, of course, is generally the majorelement of the total system pressure drop. The pressure drop due to the air alone inthe pipeline has to be added separately to give the total pressure drop required forconveying.

Air only pressure drop values for pipelines were considered in Chapter 6.Typical data for pipeline length was presented earlier in Figures 6.4 and 7.20.With increase in length there is an increase in the air only pressure drop value andso this means that when scaling to longer pipelines less pressure will be availablefor conveying material.

Typical data for pipeline bore was presented in Figures 6.5 and 7.15. Withincrease in pipe bore there is a decrease in the air only pressure drop value and sothis means that when scaling to larger bore pipelines more pressure wil l be avail-able for conveying material.

3 CASE STUDIES

The influence of scaling with respect to conveying distance was considered indetail in section 4 of Chapter 7. Two different materials were considered, a mag-nesium sulfate that could only be conveyed in dilute phase, and a dicalcium phos-phate that could be conveyed in dense phase. The influence of increasing convey-ing distance was considered in each case, with the transition from dense phase todilute phase conveying being highlighted for the dicalcium phosphate.

The influence of scaling with respect to pipeline bore was considered in de-tail in section 3 of Chapter 7. The above two materials were considered onceagain, but since the scaling parameter for material flow rate is the same as that forair flow rate there is little change in solids loading ratio values, and so there is nochange in the transition from dense to dilute phase conveying. With the scalingcovering both pipeline bore and air supply pressure, an analysis of the relativeinfluences of these two parameters on system power requirements was also con-sidered.

Two further cases wil l be considered here to illustrate in more detail the useof the scaling parameters in pipeline design and system specification. The first isfor a low pressure dilute phase conveying system, for a material that can not beconveyed in dense phase, and the material considered is granular coal. The secondis for a high pressure dense phase conveying system and the material considered isordinary portland cement.

The conveying data to be used for these two materials was obtained fromconveying trials undertaken with the Figure 7.13 pipel ine. For convenience ofreference a sketch of this pipeline is given in Figure 15.9.

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430 Chapter 15

Pipeline:length = 3 l O f tbore = 3 in nominalbends = 9 x 90°D/d = 16

Figure 15.9 Sketch of figure 7.13 pipeline.

A sketch of the plant pipeline to which the data for coal is to be scaled ispresented in Figure 15.10.

DeliverySilo "

TransferHopper

40f t -

Figure 15.10 Details of plant pipeline for conveying coal.

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Design Using Conveying Data 431

For convenience the test pipeline is used in both cases. Since the conveyingconditions in the two cases wi l l be very different, equivalent lengths will also bevery different.

3.1 Low Pressure Conveying of Coal

Conveying characteristics for granular coal conveyed through the Figure 7.13pipeline are presented in Figure 15 .11 .

3.1.1 Air Supply Pressure

Although the conveying data in Figure 15.11 is available up to conveying line inletair pressures of 24 psig a low pressure system wil l be considered since the materialcan only be conveyed in di lu te phase suspension flow. A positive pressuredisplacement blower would be an ideal compressor for the duty but as thesetypically have a maximum operating pressure of about 15 psig, the pneumaticconveying system will be designed on a conveying line pressure drop of 12 lbf/in2 .

Thus the conveying l ine in le t air pressure, /?/, wi l l be 12 lbf/in" gauge or (forcalculation purposes) 26-7 lbf/in2 abs, assuming that the plant is located at or nearto sea level. A compressor with a 15 psig pressure capability would be specified inorder to provide to provide a safety margin on air supply pressure.

40

ooo

fL>

03

% 20_o

"rt

I 10

Solids LoadingConveying Line ^ | Ratio

Inlet Air Velocity- ft/min

Conveying LinePressure Drop

- Ibf/ i tr

Conveying

Limit

0 100 200 300 400 500 600

Free Air Flow Rate - fWmin

Figure 15.11 Conveying characteristics for granular coal conveyed through the pipe-l ine shown in figure 7.13.

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432 Chapter 15

3.1.2 Conveying Une Inlet Air Velocity

From Figure 1 5 . 1 1 it w i l l be seen that the min imum conveying air velocity for thematerial is about 2600 ft/min. A safety margin of about 20% on this for conveyingline inlet air velocity would be recommended. This gives 1-2 * 2600 = ^ 3 1 2 0ft/min. This wil l be the conveying line inlet air velocity, C/, for the plant pipeline.

The flow rate of the granular coal, achieved in the three inch bore test pipe-line, can be obtained from the vertical axis of Figure 15.11, and this is about

13,500 Ib/h. This is ril ^ which wi l l be required in the scaling process. The solids

loading ratio at which the coal is conveyed under these conditions is about 1 1 , andthe free air flow rate about 278 ft'Ymin.

3.1.3 Equivalent Lengths

In the scaling process equivalent lengths are required for both the test facility fromwhich the data is available, and the plant pipeline that requires to be designed.Now that the conveying line inlet air velocity has been established these can bedetermined. The relationship for the equivalent length of a pipeline was given withEquation 9:

Le = Lh + 2LV + (N+2}Leb ft - - - - (9)

Assuming that radiused bends wi l l be used throughout, the value of theequivalent lengths of all the bends can be obtained from Figure 15.4 since theconveying line inlet air velocity has been established as 3120 ft/min. The equiva-lent length is about 57 feet. This applies to both the pipeline from which the datawas obtained and to the plant pipel ine to which it wil l be applied.

Thus for the test pipeline:

Lgi = 310 + 0 + [(9 + 2) x 57]= 937 feet

And for the plant pipeline:

Le2 = 570 + (2 x 80) + [(8 + 2) x 57]= 1300 feet

3.1.4 Air Only Pressure Drop

Equations for evaluating the air only pressure drop were considered in Chapter 6.There are two possible approaches for this, depending upon the degree of accuracyrequired. For a quick approximation Equation 6.3b can be used for the pipelineand Equation 6.8b can be used for the bends and fittings.

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Design Using Conveying Data 433

Estimated mean values over the length of the pipeline for both air density, p,and air velocity, C, can be used. A representative value of 0-005 can be used forpipeline friction factor, /,' and a value of 0-2 for each bend in the bend loss coeffi-cient, k.

For a more accurate analysis Equation 6.14 is probably the most convenientof those presented. This combines pipeline and bend losses in the one equation.

This is in terms of the air mass flow rate, ma , and this can be evaluated by using

Equation 6.15 or alternatively, if the free air flow rate, VQ , is used:

tha = 0-0765 x Vn Ib/min - - - - - - - (13)= 0-0765 x 278= 21-3 Ib/min for the point being scaled on

the Figure 7. 1 3 pipeline

Pipeline friction and bend losses are included in Equation 6 . 1 1 :

f L Z kT = — : -- 1 -- (dimensionless)

9-375 d 450

Substituting appropriate values for the Figure 7.13 three inch bore test pipe-l ine gives:

0 - 0 0 5 x 3 1 0 9 x 0 - 2

9 - 3 7 5 x 3 450= 0-059

and similarly for the plant pipeline of three inch bore:

(/73P = 0 -119

Equation 6.14 for the air only pressure drop for a pipeline is as follows:

d4 Ibf/irr

and substituting values for the data point being scaled on the Figure 7.13 threeinch bore test pipeline gives:

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434 Chapter 15

,0-5

2 1 6 - 1 +1 7 4 x 0 - 0 5 9 x 2 1 - 3 "

- 1 4 - 7

V

= 1-84 lbf/in-

lbf/in2

This means that the pressure drop for conveying material for the data pointbeing considered is 12 - 1-84 = 10-16 Ibf/in2 .

Substituting values for the plant pipeline of three inch bore gives:

Apa3p = 3-52 Ibf/in2

This means that with an air supply pressure of 12 Ibf/ in2 for the plant pipe-line, the pressure drop for conveying material, for the data point being considered,will be 12-3-52 = 8-48 Ibf/in2 .

13,500 Ib/h was achieved in the test pipeline with an effective pressure dropof 10-16 Ibf/in2, but in the plant pipeline of the same bore only 8-48 Ibf/in2 is avail-

able and so fh ,( must be scaled in proportion to provide a modified value:

8 - 4 8

1 0 - 1 6= 11,270 Ib/h

3.1.5 Material Flow Rales

With all the necessary data evaluated for the test pipel ine, the first stage in thescaling process is to scale to the plant pipeline. The first stage is in terms ofequivalent length only and so there is no change in pipel ine bore. This scalingmodel was presented earlier at Equation 3:

Ib/h

and substituting values gives:

937= 11,270 x

1300= 8,120 Ib/h

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Design Using Conveying Data 435

The second stage is in terms of pipeline bore. This scaling model was pre-sented earlier at Equation 12. Re-arranging the equation to make the diameter ofthe plant pipeline the subject of the equation gives:

d? = d, (14)

In normal circumstances the material flow rate required for the plant pipe-line will be known. Let us suppose that this is 60,000 Ib/h. Substituting values inEquation 14 gives:

60.000x n

-15 in

3.1.6 Pipeline Bores

The above pipeline bore is not the final result. The calculation was necessary inorder to establish an approximate value, because an allowance has to be made forthe air only pressure drop. This is, in effect, an iterative process. The air only pres-sure drop for the plant pipeline having a bore of 8 in can be evaluated from Equa-tion 6.14 with the appropriate values:

The first of these values is that for the pipeline friction and bend losses forthe 8 in bore pipeline from Equation 6.11:

_ 0 - 0 0 5 x 6 5 0 8 x 0 - 2

P 9 - 3 7 5 x 8 450= 0-047

Substituting this, the new pipeline bore and a new air flow rate value, scaledon pipe section area, into Equation 6.14 gives:

2 1 6 - 1 +

•48 Ibf/in2

1 74 x 0 - 047 xf 8 f~

_2 i-3 iJ ,84

- 1 4 - 7 Ibf/ in2

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436 Chapter 15

This means that with an air supply pressure of 12 Ibf/irf for the plant pipe-line, the pressure drop for conveying material, for the data point being considered,wi l l be 12- 148 = 10-52 lbf/in2.

8,120 Ib/h was achieved in the plant pipeline, having a three inch bore, withan effective pressure drop of 8.48 lbf/in2 , but in the plant pipeline of eight inch

bore 10-52 lbf/in2 is now available and so m 3 must be scaled in proportion to

provide a modified value:

1 0 - 5 2

"'«"" ' 8J2° * -^= 10,070 Ib/h

Equation 15.12 can now be used to scale from the plant pipeline having athree inch bore to a plant pipeline having an eight inch bore:

9

mp&p = 10,070 x I -V J

= 71,600 Ib/h

The problem here is in selecting a standard pipeline bore. Six inch borewould be too small, seven inch is probably not available and so eight inch wouldbe required. Six inch bore would only be a possibility if a higher air supply pres-sure could be used and so this alternative could be explored if required. The capa-bility of an eight inch bore pipeline is almost 20% greater than required. It will , ofcourse, allow for future expansion and in the meantime the system could operatewith a lower air supply pressure.

3.1. 7 Summary

The scaling process may appear a little convoluted but it is an iterative process inpart. This is particularly the case when scaling a single data point. If the entireconveying characteristics in Figure 15.13 were to be scaled this would not be nec-essary, as an intermediate set of conveying characteristics for the three inch boreplant pipel ine would be constructed, but the scaling process would take very muchlonger.

The notation employed here has been as follows:Symbols

d Pipeline bore inL Pipeline length ftAp Pressure difference lbf/iny Pipeline loss coefficients -

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Design Using Conveying Data 437

Subscriptsa airm conveyed material

modifiedp conveyed product or material

plant pipelinet test pipeline3, 8 pipeline bore in inches

A summary of the results at the different stages is as follows:

Pipeline Test Plant

Bore in 3 3 »•

Length ft 310 > 650

ma Ib/min 21 -3 21-3 _+.

(// - 0-059 0 - 1 1 9

Apa Ibf / in 2 1-84 3-52

Apm Ibf/in 10-16 8-48

mp Ib/h 13,500

11,270 > 8,120

I10.070 >- 71,600

3.2 High Pressure Conveying of Cement

Conveying characteristics for cement conveyed through the Figure 7.13 pipelineare presented in Figure 15.12. This is the same pipeline that was used for thegranular coal presented in Figure 15.11 . A comparison of the two sets of data willshow the vast difference in conveying capability between the two materials. Withthe cement being capable of low velocity dense phase conveying, very much lowerair flow rates have been employed for conveying the material, and it has been pos-sible to use very much high conveying line inlet air pressures.

The conveying l imi t for the cement is about 600 ft/min for dense phase con-veying and 2000 ft/min for dilute phase conveying, compared with a single valueof 2600 ft/min for the coal as it had no dense phase conveying capability. With thecement, however, dense phase can only be maintained provided that the pressuregradient is high enough.

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438 Chapter 15

120

ooo

f_ou.

1 40

Conveying LiniPressure Drop

- Ibf/ in2

ConveyingLimit

Conveying LineInlet Air Velocity

- ft/min

Solids Loading"600 140 Ratio \

100

40 80 120 160Free Air Flow Rale - ft'/min

200

15.12 Conveying characteristics for ordinary portland cement in the pipelineshown in figure 7.13.

3.2.1 Air Supply Pressure

Conveying data in Figure 15.12 is available up to conveying line inlet air pressuresof 60 psig and a high pressure system would generally be recommended for thismaterial. For this case study, however, a conveying line inlet pressure of 20 psigwil l be considered since it wi l l incorporate the problems of the transition fromdense phase to dilute phase conveying of the material in the study. Thus the con-veying line inlet air pressure, pt, wil l be 20 Ibf/in" gauge or (for calculation pur-poses) 34-7 Ibf/ in2 abs, assuming that the plant is located at or near to seal level.

3.2.2 Conveying Line Inlet A Ir Velocity

From Figure 15.3 it wi l l be seen that the min imum conveying air velocity for thematerial is about 600 ft/min. A safety margin of about 20% on this for conveyingline inlet air velocity would be recommended. This gives 1-2 x 600 = 720 ft/min.Since the conveying line inlet air velocity, C/, depends upon the value of solidsloading ratio, as presented in Figure 13.3 for this material, this value may not beappropriate for the plant pipeline.

This part of the scaling process is exploratory in nature, and if it is foundthat a conveying line inlet air velocity of 720 ft/min is not possible for the plantpipeline, the process will have to start all over again. This is essentially a trial anderror solution and is an iterative process. From the result obtained a closer ap-proximation should be possible and convergence should occur quite quickly.

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Design Using Conveying Data 439

The flow rate of the ordinary Portland cement, achieved in the three inchbore test pipeline, can be obtained from the vertical axis of Figure 15.12, and this

is about 38,000 Ib/h. This is ril v which w i l l be required in the scaling process.

The solids loading ratio at which the cement is conveyed under these conditions isabout 87, and the free air flow rate about 95 ftYmin.

3.2.3 Equivalent Lengths

In the scaling process equivalent lengths are required for both the test facility fromwhich the data is available, and the plant pipeline that requires to be designed.Now that the conveying line inlet air velocity has been established these can bedetermined. The relationship for the equivalent length of a pipeline was given withEquation 9:

Le = Lh + 2LV + (N+T)Leb ft (9)

A sketch of the plant pipeline to be used for the conveying of ordinary Port-land cement is presented in Figure 15.13 for reference. This w i l l also need to beassessed in terms of an equivalent length for scaling purposes.

TransferHopper

ReceptionSilo

Figure 15.13 Details of plant pipel ine for conveying cement.

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440 Chapter 15

Assuming that radiused bends will be used throughout, the value of theequivalent lengths of all the bends can be obtained from Figure 15.4 since theconveying line inlet air velocity has been established as 720 ft/min. The equivalentlength is about 8 feet. This applies to both the pipeline from which the data wasobtained and to the plant pipeline to which it wi l l be applied.

Thus for the test pipeline:

Lei = 310 + 0 + [(9 + 2) x 8]= 398 feet

And for the plant pipeline:

Le-2 = 660 + (2 x 140) + [(8 + 2) x g]= 1020 feet

3.2.4 Air Only Pressure Drop

Equations for evaluating the air only pressure drop were considered in the previ-ous case study and Equation 6.14 wi l l be used here also.

The air mass flow rate, ma , is required and this can be evaluated by using

Equation 15.13:

ma = 0-0765 x VH Ib/min - - - - - - - (13)= 0-0765 x 95= 7-27 Ib/min for the point being scaled on

the Figure 7.13 pipel ine

Pipeline friction and bend losses for the test pipeline wi l l be exactly thesame as those evaluated above, and so:

!?3t = 0-059

and similarly for the plant pipeline of three inch bore:

,r/ 0 • 005 x 800 8 x 0 - 2

9 - 3 7 5 x 3 4 5 0= 0-146

Equation 6.14 for the air only pressure drop for a pipeline is as follows:

174 ;/ m2 V'52 1 6 - 1 + j^k _ 14.7 M/in2

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Design Using Conveying Data 441

and substituting values for the data point being scaled on the Figure 7.13 threeinch bore test pipeline gives:

,0-5

2 1 6 - 11 7 4 x 0 - 0 5 9 x 7 - 2 7 "

34

= 0-23 Ibf/iir

- 1 4 - 7 Ibf/in2

It wi l l be noted that this compares with a value of 1 -84 Ibf / in in the previouscase study for dilute phase conveying. In a high pressure, dense phase conveyingsystem, therefore, the allowance for and incorporation of air only pressure drop isnot so critical. In this case it means that the pressure drop for conveying materialfor the data point being considered is 20 - 0-23 = 19-77 Ibf/in2, which is only amarginal difference.

Substituting values for the plant pipeline of three inch bore gives:

Apa3p = 0-55 Ibf/in2

This means that with an air supply pressure of 20 Ibf/in2 for the plant pipe-line, the pressure drop for conveying material, for the data point being considered,wi l l be 20-0-55 = 19-45 Ibf/ in2 .

A modification of the material flow rate, due to this difference in pressuredrop values, as carried out above for the di lute phase conveying of the coal, re-duces the material flow rate of the cement from 38,000 Ib/h to about 37,380 Ib/h.For low velocity dense phase conveying, therefore, this element of the analysiscould well be omitted and particularly so with higher air supply pressures.

3.2.5 Material Flow Rates

With all the necessary data evaluated for the test pipeline, the first stage in thescaling process is to scale to the plant pipel ine. The first stage is in terms ofequivalent length only and so there is no change in pipel ine bore. This scalingmodel was presented earlier at Equation 15.3:

L.m. lb/h

and substituting values gives:

mp3p = 37,380 x398

1020= 14,585 lb/h

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442 Chapter 15

The second stage is in terms of pipeline bore. This scaling model was pre-sented earlier at Equation 15.14. Once again, in normal circumstances, the mate-rial flow rate required for the plant pipeline wil l be known. Let us suppose in thiscase that it is 140,000 Ib/h. Substituting values in Equation 15.14 gives:

d-, -142

, x O - 5[ 140 ,000 ]

"? -.' i Y)j x i inV 14,585 j

9-3 in

3.2.6 Pipeline Bores

From the above analysis it is clear that a ten inch bore pipeline would be required,and this might be capable of conveying 140,000 Ib/h of cement through the Figure15.13 pipeline with an air supply pressure of 20 psig. The diameter came to justover nine inches and the modification of the pressure drop was not taken into ac-count. However, a check needs to be made on whether this is a possibility beforeproceeding further.

3.2. 7 Solids Loading Ratio

The volumetric flow rate of free air required for this pipeline can be obtained fromEquation 5.10:

P\V,, = 0.1925 x— - ftYmin

and substituting values for the ten inch bore pipeline:

• 3 4 - 7 x l 0 2 x 7 2 0 ,K, = 0-1925 x - ft/min

520= 925 fVVmin

From ma = 0-0765 V{} Ib/min

= 0-0765 x 925= 70-8 Ib/min

Solids loading ratio

A r(p — (dimensionless)rh

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Page 454: Handbook of Pneumatic Conveying Engineering

Design Using Conveying Data 443

Substituting values gives:

140,000

7 0 - 8 x 60

= 33

The influence of solids loading ratio on the minimum conveying air velocitythat can be employed was first introduced in Chapter 4, and specific data for ce-ment was presented in Figure 13.3. This is reproduced here in Figure 15.14 forreference.

From Figure 15.14 it will be seen that a conveying line inlet air velocity of720 ft/min and a solids loading ratio of 33 is not a possible combination for con-veying. At a solids loading ratio of 33 the minimum conveying air velocity wil l beabout 860 ft/min, and then the 20% margin required makes the corresponding con-veying line inlet air velocity about 1030 ft/min.

1030 ft/min is a 43% increase on 720 ft/min and so a corresponding increasein air flow rate would be required. A 43% increase in air flow rate, however, wi l lmean a corresponding reduction in solids loading ratio to about 23. Because of theslope of the curve on Figure 15.14, the min imum conveying air velocity at a solidsloading ratio of 23 is 1360 ft/min, making the conveying line in le t air velocityabout 1630 ft/min. These points are illustrated on Figure 15.14 for reference.

2400 .

h

^'o

•§>

<toJJ

>~lo>c

Up

£

2

2000

1600

1200

snoouu

400

0

1630 < \\

1360* A

1030

860

720

; \

23

. ^ 720

\^_

8733 |

. 1 , 1 + ,

20 40 60 80 100

Solids Loading Ratio

Figure 15.14 Influence of solids loading ratio on m i n i m u m conveying air velocity forordinary portland cement.

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444 Chapter 15

1630 ft/min is an almost 60% increase on 1030 ft/min and so it will be seenthat with an air supply pressure of 20 psig convergence wil l only come when thecement is conveyed in di lute phase with a conveying l ine inlet air velocity of 2000ft /min and this wi l l be di lu te phase suspension flow. The slope of the curve onFigure 15.14, particularly between solids loading ratios of about 40 and 10, is suchthat conveying velocities must be at least 20% above the line otherwise the flowcould become unstable, particularly if the material flow rate should reduce forsome reason.

The data point of 20 psig and 2000 ft/min does not appear on the conveyingcharacteristics in Figure 15.12. It wi l l be seen, however, that the data point will beat a very much lower value of material flow rate and so a very much larger bore ofpipeline wi l l be required to convey 140,000 Ib/h. The increase in air flow rate w i l lbe even greater because this wi l l increase with both the increased conveying lineinlet air velocity and the increased pipeline bore.

3.2.8 Other Possibilities

The pipeline bore in the above case was evaluated to be about 9-3 inches. With astandard ten inch bore pipeline it would be possible to convey about 15% morematerial. The air flow rate was calculated for a 10 in pipe and so this means thatthe solids loading ratio would increase by the same percentage, from 33 to about38. This is now marginal but it could not be recommended.

Increasing the air supply pressure to 30 psig is another possibility. The oper-ating point on Figure 15.12 gives an increase in material flow rate to about 56,000Ib/h, and although the air flow rate increases, there is an increase in solids loadingratio. With the increase in pressure the 140,000 Ib/h can be conveyed through aneight inch bore pipeline. The solids loading ratio is now over forty and so the sys-tem should operate satisfactorily.

A further increase in air supply pressure to 40 psig results in further in-creases in conveying parameters. The pipeline bore required to convey the140,000 Ib/h is now down to seven inches. If seven inch pipeline was availablethis would be an ideal solution. If eight inch bore pipe had to be used, however,the material flow rate would have to be increased in order to maintain the neces-sary value of solids loading ratio. Selection of pipeline bore can be a major prob-lem and so it is always worth investigating the options in this way.

3.2.8.1 Stepped PipelinesWith most high pressure conveying systems the use of a stepped bore pipelinewould generally be recommended. This will usually result in an improvement inperformance. Although the case considered above, with an air supply pressure of20 psig, proved to be marginal in terms of operation, the use of a stepped pipelinecould well change the situation.

The procedure for designing or checking stepped pipeline systems is exactlythe same as that outlined above for single bore pipelines. Each section of pipeline,of a different bore, must be considered as a completely separate pipeline. A pipe-

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Page 456: Handbook of Pneumatic Conveying Engineering

Design Using Conveying Data 445

line having two steps, and hence three sections of pipeline of different bore, isshown diagrammatically below:

Flow

-^ ©Direction

Length

Pressure p\

Pressure drop

Air Velocity C/

iI

d,.2 ®® d3.4 ®© d5.6 di

i

Le/-2 Lej-j Les^P2-3 P4-5

Ap/_ ^ Ap3-4 dp 5.6

C2-3 C4.5

P6

When checking a system design, indiv idual p ipel ine lengths wi l l be speci-fied. When designing a system, however, these wil l have to be calculated. In eithercase it will be necessary to evaluate the conveying air velocities at the two steps,C2-3 and C/-J, to make sure that the velocity does not fall below the minimum con-veying air velocity value.

The air flow rate, V0 , and the conveying line inlet, p,, and outlet, p6, air

pressures will all be specified and so the air mass flow rate, ma , conveying line

pressure drop, Apj_6, and conveying l ine inlet, C/, and exit, C6, air velocities can allbe calculated directly, as illustrated above. These values will all be specified, evenif they are estimated values in the first place in order to start the calculation proce-dure.

The pressure at the pipeline steps, p2.3 and/^.j, the individual pipeline pres-

sure drops, dp 1.2, dp3_4 and dps.6, and the material flow rate, /»„ , wil l all be un-

known. Thus six equations will be needed to solve for the six unknown values.The scaling parameters applied to each pipeline section wi l l provide three of theequations, the continuity equation can be applied twice, and the fact that Ap,_6 =dpi_2 + dp 3.4 + dp5.6 provides the sixth equation. These can be solved simultane-ously in the usual way.

As with the single bore pipeline solution presented above, if a balance is notachieved between the initial estimate made for solids loading ratio, <p, and thevalue of conveying line inlet air velocity, C/, the procedure will need to be re-peated until convergence is achieved.

REFERENCES

1. D. Mills. Measuring pressure on pneumatic-conveying systems. Chemical Engineering.Vol 108. No 10. pp 84-89. September 2001.

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Page 457: Handbook of Pneumatic Conveying Engineering

446 Chapter 15

2. D. Mills. J.S. Mason, and P. Marjanovic. The influence of product type on dense phasepneumatic conveying in vertical pipelines. Proc Pneumalech 2. pp 193-210. Canter-bury. UK. Sept 1984.

3. P. Marjanovic. An investigation of the behavior of gas-solid mixture flow properties forvertical pneumatic conveying in pipelines. PhD Thesis. Thames Polytechnic (now TheUnivers i ty of Greenwich) London. 1984.

4. P. Marjanovic and D. Mil ls . The influence of bends on the performance of a pneumaticconveying system. Proc 15th Powder & Bulk Solids Conf. pp 391-399. Chicago. June1990.'

5. D. Mills, J.S. Mason, and V.K. Agarwal. An analysis of the dilute phase pneumaticconveying of sand. Proc Pneumatech 2. pp 258-278. Canterbury, UK. Sept 1984.

6. D. Mills, J.S. Mason, and V.K. Agarwal. An assessment of acceleration lengths througherosive wear measurements. Proc 10lh Powder & Bulk Solids Conf. pp 215-228. Chi-cago. May 1985.

7. M. Leva. Fluidizat ion. McGraw H i l l . 1959.

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16Quick Check Design Methods

1 INTRODUCTION

The design of pneumatic conveying systems is usually carried out on the basis ofscaling data obtained from the pneumatic conveying of the material to be trans-ported. If previous experience of conveying a given material is not available, datais generally derived for the purpose by conveying the material through a test facil-ity, as discussed in detail in the previous chapter. Most manufacturers of pneu-matic conveying systems have such test facilities for this purpose.

If it is required to make a quick check on the potential of an existing system,or to provide a check on design proposals, there is little information readily avail-able for the engineer to use. Pneumatic conveying does not lend itself to simplemathematical analysis, and it is l ikely that many engineers would not be able toundertake such a task easily, particularly if it were a low velocity dense phase sys-tem.

Since pneumatic conveying systems tend to have high power ratings, particu-larly for conveying in dilute phase suspension flow, it is useful to be able to obtaina rough estimate of air requirements at the feasibility stage of a project. Most of theoperating cost of a pneumatic conveying system is in the drive for the air mover. Ifan estimate can be made of the system air requirements, it is a simple matter toevaluate the operating cost in Cents per ton conveyed to see if it is at an acceptablelevel before proceeding further.

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448 Chapter 16

1.1 Methods Presented

A number of straightforward methods are presented that wi l l allow a check to bemade on the design of a pneumatic conveying system in a very short space of time,whether for a new or an existing system. Horizontal and vertical sections of pipe-l ine and bends can all be accommodated, as well as dilute and dense phase convey-ing in some cases. For high pressure systems the influence of stepped pipelines canalso be incorporated.

Three different methods are presented. One is based on the value of the aironly pressure drop for the pipeline, but this is strictly limited to dilute phase con-veying only. Another is based on the use of a universal set of conveying character-istics and the third uses the steady flow energy equation as a basis. Both of thesecan be used for di lute and dense phase conveying.

1.2 Potential Accuracy

It must be emphasized that all three of these methods are strictly quick checkmethods and will provide only a first approximation solution. Although some ofthe methods may appear to be mathematically correct, do not be lulled into a falsesense of security. There is no reference to conveyed material anywhere in any ofthe procedures. In this respect reference to Figures 4.16 and 4.18 will bring anyengineer back to reality with regard to the potential accuracy of these methods,whether for dilute or dense phase conveying.

For a given material to be conveyed it is possible that the accuracy of someof these quick check methods could be improved quite considerably. If conveyingdata is available for a particular material, fine tuning could be undertaken. Con-stants relating to individual pipel ine features such as bends, vertical lift and pipe-line bore could be changed or added. Once again it must be emphasized that theresulting models would only provide more reliable system design and checkinginformation for the material being considered, and only for that particular grade ofthe material.

2 AIR ONLY PRESSURE DROP METHOD

This method uses the value of the air only pressure drop for a pipeline as a basis forevaluating its conveying potential. This resistance due to the air is related to theadditional resistance resulting from the conveying of material. The pressure dropdue to the air can be readily measured, or simply calculated for any pipeline bymeans of the equations presented in Chapter 6.

2.1 Basic Equations

The Ideal Gas Law relates the volumetric flow rate of the air to the pressure andtemperature of the air, as considered in Chapter 5. The volumetric flow rate canalso be expressed in terms of the conveying line inlet air velocity and the pipeline

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Quick Check Methods 449

bore. In most conveying situations the volume occupied by the conveyed materialcan be neglected in comparison with that of the air.

These models, therefore, can be used quite reliably in gas-solid flow situa-tions. Material flow rate can be introduced in terms of the solids loading ratio ofthe conveyed material. The solids loading ratio is a parameter that is often knownapproximately, and in these cases quite simple equations can be derived equatingthe variables.

2. /. / Solids Loading Ratio

Solids loading ratio, $, is defined as the ratio of the mass flow rate of the materialconveyed, to the mass flow rate of the air used to convey the material and this wasfirst presented at Equation 4.5:

m(j) = (dimensionless) - - - - - - - (])

m

where «„ = mass flow rate of material - Ib/h

and ma = mass flow rate of air - Ib/h

It is a dimensionless ratio and is a particularly useful parameter since itsvalue remains constant along the length of a pipeline, regardless of the air pressureand conveying air velocity.

2.7.2 Ideal Gas Law

Air mass flow rate is not always a convenient parameter with which to work, andair flow rate is more usually expressed in volumetric terms. From the Ideal GasLaw, for a steady flow situation, however, one can readily be evaluated from theother, as first illustrated at Equation 5.4:

144 p V = ma R T - - - - - - - - - - - - (2)

where p = absolute pressure of gas - Ibf/in2

V = volumetric flow rate ofgas at pressure ;P - fVYmin

ma = mass flow rate of gas - Ib/min

R = characteristic gas constant - ft Ibf/lb Rand T = absolute temperature of gas - R

= / °F + 460

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450 Chapter 16

2.1.3 Volumetric Flow Rate

Volumetric flow rate is given by:

V = velocity * area

and for a circular pipe

n d2

576ft'/min - - - - - - (3)

where C = conveying air velocity - ft/minand d = pipe bore - in

This is the actual volumetric flow rate. Since air and other gases are com-pressible, volumetric flow rate will change with both pressure and temperature. Italso means that the conveying air velocity wi l l vary along the length of a pipeline.A full explanation and analysis of this was included in Chapter 5 on Air Require-ments.

2.2 Working Relationships

The three equations presented above are exact equations, and so any combinationof these equations will similarly produce precise relationships. Although theseequations include all the basic parameters in pneumatic conveying, they will notproduce design relationships. This is because they do not include the necessaryfundamental relationships between material flow rate, pressure drop and conveyingair velocity. Combinations of Equations 1 to 3, however, wi l l produce equationsthat can be usefully used to check system designs. They wil l also provide a goodbasis for the inclusion of design relationships.

2.2.1 Material Flow Rate

By substituting Equation 3 into 2 to eliminate V , making ma the subject of the

equation and substituting this into Equation 1 gives:

9n d p C

mn = <p x x ib/hp 4 RT

By putting R = 53-3 ft Ibf/lb R for air gives:

r /2p C dmn = 6 x ib/h - - - - - - - ( 4 )

P v 61.9T

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Quick Check Methods 451

2.2.2 Pipeline Bore

For a given material flow rate and conveying conditions, the diameter of the pipe-line is generally required. An alternative arrangement of the equations gives:

nO-5T

d = 8'24' P

1P

2.2.3 Conveying Line Pressure Drop

An alternative arrangement, in terms of the pressure required to convey the mate-rial gives:

mp Tp = 67-9 —-^— Ibf/in2 abs - - - - - (6 )

C dz 6

2.2.3.1 Reference ConditionsThe variables in these equations can be taken at any point along the pipeline. Inthe case of air pressure and velocity, however, these are generally only known,with any degree of accuracy, at the very start and end of a pipeline. Since the con-veying line inlet air velocity is probably the most critical parameter in system de-sign it is generally conditions at the material feed point, at the start of the pipeline,that are used for this purpose.

2.3 Empirical Relationships

It will be seen from Equations 4 to 6 that, for a given material and pipeline, thereare six variables relating the main conveying parameters. Of these, the conveyingair temperature wil l be known; solids loading ratio is a function of the conveyingair velocity, pipeline bore and material flow rate; and either the material flow raterequired, pipeline bore to be used, or conveying l ine pressure drop available willbe specified. This means that there are three basic variables in each of these equa-tions.

It wi l l be possible to provide solutions to Equations 4 to 6, therefore, if twofurther relationships can be provided. These wil l , by necessity, be empirical, andso the accuracy of any expressions developed wi l l ultimately depend upon theaccuracy of the empirical relationships used. The first of these is a relationship orsuggested value for conveying line inlet air velocity. The second is a relationshipfor solids loading ratio and this is in terms of conveying line pressure drop pa-rameters.

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452 Chapter 16

2.3.1 Conveying Line Inlet Air Velocity

The conveying line inlet air velocity to be employed depends upon the min imumconveying air velocity at which the material can be successfully conveyed. Thisdepends very much upon the material to be conveyed and the solids loading ratioat which it is to be conveyed.

A graphical representation of this relationship between minimum conveyingair velocity and solids loading ratio for typical powdered and granular materials ispresented in Figure 16.1. These relationships for different materials were first in-troduced in Chapter 4.

For a material that is capable of being conveyed in dense phase, such as amaterial having good air retention properties like barite, fine fly ash or cement, theconveying limits are defined approximately by:

Cmin = 2300 for ^ < 10

Cmin = (7330 $T°-3 - 1370) for 10 < </> < 80 ft/min - -

Cmin = 600 for <f> > 80

where Cmm = minimum conveying air velocity - ft/minand <b = solids loading ratio - -

(7)

3000

2000>

g 1000u

Material having dilute phaseconveying capability only

Material with dense phaseconveying capability

20 40 60

Solids Loading Ratio

80 100

Figure 16.1 Relationship between min imum conveying air velocity and solids loadingratio for different materials.

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Quick Check Methods 453

For a material that is not capable of being conveyed in dense phase, such asgranular materials having both poor air retention and poor permeability, the con-veying limits are defined approximately by:

Cmm = 2300 to 3200 ft/min (for all (8)

A graphical representation, for typical materials, of these relationships be-tween minimum conveying air velocity and solids loading ratio is also included onFigure 16.1. For most purposes, the use of this graph probably provides the quick-est means of determining the necessary value, but for anyone wanting to programthe analysis, Equations 7 and 8 are offered for this purpose.

Design would generally be based on a conveying line inlet air velocity, C/,20% greater than the min imum conveying air velocity, C,,,,,,:

C, = 1-2 ft/min (9)

2.3.2 Solids Loading Ratio

An approximate relationship between pressure drop and solids loading ratio, fordilute phase conveying, is presented in Figure 16.2. The relationship is based uponthe assumption that the curves on Figure 16.2 are equi-spaced with respect to con-veying line pressure drop. For many materials conveyed in dilute phase this is areasonable approximation.

Air Flow Rate - V0

Figure 16.2 Influence of solids loading ratio and air flow rate on conveying line pres-sure drop for d i lu te phase suspension How.

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454 Chapter 16

A mathematical expression for this is:

(10)

where Apc

and Ap,,

2.3.3 Material Flow Rale

Now from Equation I :

conveying line pressure drop - Ibf/irTair only pressure drop - Ibf/in2

mPm.

Directly equating these two expressions for solids loading ratio gives:

mp = Ib/h (11)

If air mass flow rate, ma , is not a convenient parameter, Equation 11 can be

expressed in an alternative form, in terms of air pressure, p, and velocity, C, bysubstituting a combination of Equations 3 and 5 to give:

mP

p n d C

4 R TIb/h

Another alternative is to substitute solids loading ratio, </>, from Equation 10into Equation 4, which gives:

mp =p C d2

67-9 TIb/h (12)

Pipeline inlet conditions are the most convenient to use here.

2.3.3.1 Negative Pressure SystemsFor vacuum systems the pressure, p, wil l be atmospheric.

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Quick Check Methods 455

2.3.3.2 Positive Pressure SystemsFor positive pressure systems the pressure, p, in Equation 12 w i l l be equal to theconveying line pressure drop, A/?c, plus atmospheric pressure, which is:

where A/? r= conveying line pressure drop - lbf/in2

and paln, = atmospheric pressure - lbf/in2

2. 3. 4 Pipeline Bore

By substituting the solids loading ratio, (/), from Equation 10 into Equation 5, theexpression can be in terms of the pipeline bore required:

d = 8-24A/?,,

Pin (13)

The situation for both positive and negative pressure systems is the same asabove.

2.3.5 A ir Supply Pressure

Alternatively, the expression can be in terms of the pressure required to convey the

material. Substituting the solids loading ratio, 0, from Equation 10 into Equation 6gives:

p = 67-9T

C d'lbf/in2 abs - (14)

Pipeline inlet conditions are again the most convenient to use.

2.3.5.1 Negative Pressure SystemsFor negative pressure systems the pressure, p, wi l l be atmospheric and hence Apc

can be determined, which is the value required in this case. Re-arranging Equation14 and expressing in terms of pipeline inlet conditions for this case gives:

67-9md~

.+ 1 | lbf/in2

(15)

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456 Chapter 16

2.3.5.2 Positive Pressure SystemsFor a positive pressure system:

A/7C = Pa,

Substituting this into Equation 14 and expressing in terms of pipeline in le tconditions gives:

PllPl = 67'9

mp

This is a quadratic equation, the solution to which is:

P Palm \Patm2 212mpTl&pa

(16)

Note:This wi l l give the correct root.

2.3.6 Air Only Pressure Drop

Since the air only pressure drop, Apa, features prominently in all of these models,a convenient expression for this pressure drop is required. An expression that willgive the air only pressure drop in terms of conveying line inlet, or exit, air velocityis needed. These models were derived in detail in Chapter 6 on The Air Only Da-tum.

2.3.6.1 Negative Pressure SystemsFor negative pressure systems the expression also needs to be in terms of the inletair pressure, p / , since this is generally known (usually atmospheric). Such an ex-pression was developed at Equation 6.20 and is:

,05

j _ lbf/in- (17)

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Quick Check Methods 457

2.3.6.2 Positive Pressure SystemsFor positive pressure systems the expression needs to be in terms of the exit pres-sure, p2, since this is generally known (usually atmospheric). Such an expressionwas developed at Equation 6.17 and is:

,0-5

1 + -1 Ibf/irT (18)

Pipeline Features

This quick check method does not take pipeline features such as vertical sectionsand bends into account very well. Bends are a particular problem because theequivalent length with material flow increases by an order of magnitude above theair only value. This is where the accuracy of the method can be improved, if con-veying data is available, and constants are applied to the component parts of thepipel ine for fine tuning.

2.3.7.1 Vertical ConveyingThe models presented so far relate essentially to horizontal pipelines. Most pneu-matic conveying systems, however, will include a vertical lift and so this needs tobe taken into account. The pressure drop in vertical conveying over a very widerange of solids loading ratio values, is approximately double that for horizontalconveying. Sections of vertical conveying in a pipeline, therefore, can most con-veniently be accounted for by working in terms of an equivalent length and allow-ing double for vertical lifts. This equivalent length needs to be incorporated in theactual pipeline length in the preceding equations.

2.3.7.2 Pipeline BendsA model to give the equivalent length of a bend in terms of straight pipeline waspresented in Chapter 6 at Equation 9:

k d

487(19)

For a radiused 90° bend, k is typically about 0-15 (see Figure 6.6) and an av-erage value of friction factor,/, is about 0-004 (see Figure 6.3). For a 6 inch borepipeline, therefore, the equivalent length of a bend is approximately 4-7 ft. Theperformance of bends within pneumatic conveying systems was considered inChapter 8 and equivalent lengths were presented in Figure 8.18. From this it isevident that a constant needs to be applied to the bend loss coefficient and a mult i-plying factor of three is suggested by way of compromise. With conveying datafor a particular material this is a particular area for fine tuning.

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458 Chapter 16

2.4 Procedure

To illustrate the method it is proposed to use the same example as employed in theprevious chapter to demonstrate the procedure with regard to scaling parametersfor dilute phase conveying. This was to investigate the conveying potential of thepipeline system illustrated in Figure 15.12 of 8 inch bore for the conveying ofgranular coal. The pipeline routing included a total of 570 feet of horizontal pipe-line, 80 feet of vertically up pipeline and eight 90° bends.

It was proposed that a conveying line inlet air pressure of 12 psig should beused, with atmospheric pressure at 14-7 psia. The minimum conveying air velocityfor the coal was given as 2600 ft/min and with a 20% margin the conveying lineinlet air velocity was taken as 3120 ft/min. It will assumed that the temperature ofthe air and coal are 520 R (60°F) throughout.

2.4.1 Air Only Pressure Drop

The starting point in the process is to evaluate the air only pressure drop, Apa, forthe pipeline and potential conveying parameters. Possibly the best equation for theair only pressure drop for the given situation is Equation 18 presented above, andthis is repeated below for reference:

P2

The terms in this equation are as follows:

Ibf/in" - - - (20)

. . . i P2 = 14-7 Ibf/in2 absolute. This is the conveying line exit air pressure,which is atmospheric pressure.

C? = 5660 ft/min. This is the conveying line exit air velocity. The con-veying line inlet air velocity is given above as 3120 ft/min and C2 can becalculated using Equations 5.1 and 5.5.

P R = 53'3 ft Ibf/lb Rand is the characteristic gas constant for air. See Ta-b l eS . l .

ij T2 = 520 R. This is the absolute value of conveying line exit air tempera-ture, given above.

gL. = 32'2 ft lb/lbfs~. This is the gravitational constant.\y = the constant relating to pipeline geometry:

The pipeline head loss coefficient, i//, was presented in Chapter 6 with Equa-tion 6.1 1 and is presented below for reference:

W = -- 1 -- (dimensionless) - - - - (21)9-375 d 450

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Quick Check Methods 459

The terms in this equation are as follows:

/ = 0-004. This is the pipeline friction coefficient. This is derived fromFigure 6.3, having evaluated the Reynolds number for the flow (see Chap-ter 6 section 2. 1 .4) in the usual way.

! L = 730 feet. This is the equivalent length of the straight sections ofpipeline, comprising 570 ft of horizontal pipeline and 80 ft of vertically uppipeline. [Lf = Lh + 2Lt]

d = 8 inch. This is the diameter of the pipeline.Zk = 3-6. This is the loss coefficient for all the bends in the pipeline. For

an individual bend the value of k for air is about 0-15 (see Figure 6.6).There are 8 bends in the pipeline and it was recommended above that thisloss coefficient should be multiplied by a factor of three.

Substituting the above set of values into Equation 21 gives \$i = 0-0469.

Substituting this value for if/ and the previous set of values into Equation 20gives:

= 14-7

= 1-65 lbf/in2

0-0469x56602

8 x 5 3 - 3 x 4 6 0 x 3 2 - 2

2. 4. 2 Material Flow Rate

Since the diameter of the pipeline is specified as 8 inch and the system is to oper-ate with an air supply pressure of 12 psig, it is the material flow rate that needs tobe evaluated. This is given by Equation 1 1, and requires a value for the air massflow rate in Ib/h. This can be determined by re-arranging Equation 5.4 and substi-

tuting V from Equation 5.2, since a value for volumetric flow rate has not yet beenevaluated:

m =p n d2Cx60

4RTIb/h (22)

Taking conveying line inlet air conditions and substituting the above valuesgives:

2 6 - 7 x ^ x 8 2 x 3 1 2 0 x 6 0-4x53-3x520

= 9065 Ib/h

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460 Chapter 16

Substituting these values into Equation 11 gives:

m - 9065 [ —-1" 11.65

= 56,900 Ib/h

In the case study presented in the previous chapter, using conveying data andscaling parameters, a material flow rate of about 71,600 Ib/h was evaluated for theconveying of granular coal. Since the method used here makes no reference to thetype of material being conveyed, a 20% error is to be expected.

3 UNIVERSAL CONVEYING CHARACTERISTICS METHOD

The pressure required to convey a material through a pipeline can be divided into anumber of component parts. The most important are the straight pipeline sectionsand the bends. For each of these elements there are a multitude of sub variablesthat can have an influence, but their incorporation necessarily adds to the compli-cation of the process. A compromise is clearly needed in order to provide a QuickCheck Method [1|.

3.1 Straight Pipeline

Figure 16.3 is a graph of material flow rate plotted against air flow rate, which isthe usual form for presenting conveying characteristics for materials. In this casethe family of curves that are drawn are lines of constant pressure gradient in Ibf/in2

per 100 ft of pipeline. The data was in i t i a l ly derived from conveying trials withbarite and cement, but has since been found to be reasonably close to that formany other materials. Data in this form has been presented for both horizontal andvertical pipeline runs in earlier sections, notably in Chapters 8 and 15.

3.1.1 Horizontal Pipeline

The data in Figure 16.3 represents the pressure gradient for conveying materialthrough straight horizontal pipeline of 2 inch bore. As will be seen, it covers bothdilute and dense phase conveying, with a smooth transition between the two. ThisQuick Check Method is based on the use of this data and so it w i l l be seen thatthere is no specific reference to material type.

It must be recognized, therefore, that this is also strictly a first approxima-tion method only, but it will provide an entirely different means of obtaining aquick check solution. To the pressure drop for conveying the material must beadded the pressure drop for the air, and this wi l l be considered later. The effect ofpipeline bore must also be considered, and this, of course, is also related to the airflow rate.

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Quick Check Methods 461

Solids LoadingRatio ---.

50

40

oi

o

30

20

10

0

240 200 160 120

\ll \i 15/14 13

100

PressureGradient- lbf/ in2

per 100 ft

20

10

0 40 160 20080 120

Free Air Flow Rate - ft'/min

Figure 16.3 Pressure gradient data for horizontal conveying in 2 ineh bore pipeline.

Straight vertical pipeline sections are another element that requires to betaken into account, but these can conveniently be incorporated with Figure 16.3, aswill be seen. Pipeline bends are a completely separate issue and wi l l be dealt withlater.

3. 1.2 Vertically Up Pipeline

For flow vertically up, it has been found that the pressure gradient is approxi-mately double that for horizontal conveying, as reported in Chapter 8, and that thisapplies over an extremely wide range of solids loading ratios. To take account ofvertically up sections of pipeline, therefore, the pressure gradient values on Figure16.3 simply need to be doubled for any operating point on the chart.

For flows in vertically down sections of pipeline the situation is very differ-ent, as discussed in Chapter 8. In dense phase flows there is a pressure recovery,such that the pressure gradient has a negative value. For dilute phase flows, how-ever, there is a pressure loss. The transition between the two occurs at a solidsloading ratio of about 35 and at this value materials can be conveyed verticallydown with no pressure drop at all. Figure 16.3, therefore, cannot be used in thiscase, as discussed in section 3.2 of Chapter 8.

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462 Chapter 16

3.2 Pipeline Bore

Material flow rate varies approximately in proportion to pipe section area, andhence in terms of (diameter)2. Air flow rate, to maintain the same velocity in apipeline of different bore, varies in exactly the same way. To determine the pres-sure gradient for flow in a pipeline having a bore different from that of the refer-ence data in Figure 16.3, both the material and air flow rates should be adjusted inproportion to (d2/2)2, where d2 is the diameter of the plant pipeline in inches. It willbe noted, therefore, that there will be no change in the value of the solids loadingratio.

It must be appreciated that along the length of a pipeline, as the pressuredrops and the conveying air velocity increases, the pressure gradient is likely toincrease. In Figure 16.3 a single value is given for the entire pipeline. This valuecan be taken to be an average for the pipeline, but it is another feature that rein-forces the point that this is only an approximate method.

3.2.1 Stepped Pipelines

When high pressure air is employed it is usual to increase the bore of the pipelineto a larger diameter along the length of the pipeline. This technique was consid-ered in some detail in Chapter 9. By this means the very high velocities that wi l lresult towards the end of a single bore pipeline, from the expansion of the air, canbe prevented.

By this means it is often possible to gain a significant increase in perform-ance of the pipeline. The pressure drop in a stepped pipeline can be evaluated inexactly the same way as outlined above. A critical point in stepped bore pipelinesis the location of the steps along the length of the pipeline. At each step in thepipeline the conveying air velocity must not be allowed to fall below a givenminimum value. The solution, therefore, will be an iterative one since the velocityof the air at the step depends upon the pressure at the step.

3.3 Pipeline Bends

Pressure drop data for bends in pipelines is presented in Figure 16.4. This is anidentical plot to that in Figure 16.3 and covers exactly the same range of convey-ing conditions. The pressure drop in this case is for an individual bend in the pipe-line and hence is in Ibf/in" per bend. The data given in Figure 16.4 relates to 90°radiused bends in a 2 inch bore pipeline. This is also data that was derived fromconveying trials with barite and cement that has since been found to be reasonably-close to that for other materials.

3.3.1 Bend Geometry

As mentioned earlier, it has been found that this pressure drop relationship varieslittle over a range of D/d (bend diameter to pipe bore) ratios from about 5 to 30.

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Quick Check Methods 463

Solids loading

60

50ooo

_0U,

13 20

a

10

ratio60

PressureGradient- tbf/in2

per 100ft

0 40 80 120 160 200

Free Air Flow Rate - ft ' /min

Figure 16.4 Pressure drop data for 90° radius bends in 2 inch bore pipeline.

It has been found that the pressure drop in very sharp or short radius bends,and particularly blind tee bends, however, is significantly higher and so an appro-priate allowance should be made if any such bend has to be used, or is found to befitted into an existing pipeline.

Little data exists for bends other than those having an angle of 90° and so itis suggested that the data in Figure 16.4 is used for all bends, since 90° bends arelikely to be in the majority in any pipeline. In the absence of any reliable data onthe influence of pipeline bore it is suggested that the data in Figure 16.4 is used forall bends, regardless of pipeline bore. For larger bore pipelines the material and airflow rates wil l have to be scaled in the same way as outlined for the straight pipe-line in Figure 16.3.

3.4 Air Only Pressure Drop

As mentioned earlier, the data in Figure 16.3 relates only to the conveying of thematerial through the pipeline, and so the pressure drop required for the air alonemust be added. In Figure 16.5 the influence of pipeline bore on this pressure dropfor 500 ft long pipelines is presented to illustrate the potential influence of thisvariable and is similar to that shown earlier in Figure 6.5.

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464 Chapter 16

PipelineBore - 1

Pipe!-500

Conveying Line Exit Air Velocity - ft/min

1000 2000 3000 4000 5000 6000 7000 8000 9000

40 80 120 160

Free Air Flow Rate - ftVmin * (d2/2)2

200

Figure 16.5 Influence of pipel ine bore and air flow rate on the empty pipeline pressuredrop.

Figure 16.5 shows the influence of air flow rate and pipeline bore on con-veying line pressure drop for a representative pipeline length of 500 ft. Since pipebore is on the bottom of Equation 6.3, pressure drop deceases with increase inpipeline bore. Figure 16.5 is presented by way of illustration. The air only pressuredrop for any pipeline can be evaluated as illustrated above in section 2.4.1 and themodels presented in Chapter 6.

It w i l l be seen that conveying line exit air velocity has been added to thehorizontal axis for reference. Conveying line inlet air velocity is the critical designparameter, but this cannot be added conveniently because it is also a function ofthe conveying line inlet air pressure. Because a range of pipeline bores are repre-sented on this plot, the air flow rate is in terms of that for the reference 2 inch borepipeline x (d2/2)2.

From Figure 16.5 it will be seen that the air only pressure drop can be quitesignificant, and particularly so for long, small bore, pipelines. As there are manyvariables in this pressure drop relationship it is probably best to evaluate the pres-sure drop mathematically on an individual basis, using the models presented inChapter 6, as mentioned above

Another graph, plotted for 4 inch bore pipelines, is presented in Figure 16.6to illustrate the influence of pipeline length, with the pressure drop relationshipbeing presented for 100 and 1000 ft long pipelines, as well as the 500 ft long pipe-line of 4 inch bore. This is similar to that shown above in Figure 16.5 and alsoincludes both air flow rate and conveying line exit air velocity on the horizontalaxis.

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10

,g

-O

T 6a

I§ 4

PipelineLength - ft

Pipeline Bfrre !- 4 inert

1000

-. 500

100

Conveying Line Exit Air Velocity - ft/min1000 2000 3000 4000 5000 6000 7000 8000 9000

40 80 120 160

Free Air Flow Rate - fVVrnin x (d2/2)2

200

Figure 16.6 Influence of pipeline length and air flow rate on empty pipeline pressuredrop.

3.5 Conveying Parameters

Many of the conveying parameters to be taken into account in the analysis aresimilar to those presented above for the previous method.

5.5.7 Pick-Up Velocity

System design decisions have always to be made with regard to a value of the con-veying line inlet air velocity to be employed. This is critical to the success of anypneumatic conveying system. The data presented in Figure 16.1 and Equations 7to 9 is equally valid here by way of guidance in determining a value for pick-upvelocity. Once again it must be emphasized that if the material is capable of beingconveyed at low velocity in dense phase, then the influence of solids loading ratiowill additionally have to be taken into account.

3.5.2 Influence of Distance and Pressure

The design method presented here is an iterative process, and particularly so fordense phase conveying where the conveying line inlet air velocity is a function ofthe solids loading ratio. Solids loading ratio is an important parameter in this proc-ess, and so the potential influence of conveying distance and air supply pressureon the solids loading ratio is presented in Figures 16.7 and 16.8. These graphswere presented earlier in Figures 4.27 and 4.28 to illustrate the potential capabilityof pneumatic conveying systems.

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466 Chapter 16

15

I °tx

-10

150 100 80 60 40

200 300 400Conveying Distance - ft

500

100 80 60 40 30

Figure 16.7 The influence of air supply pressure and conveying distance on solidsloading ratio for low pressure conveying systems.

60

<U

I50

r'c

§ 40

o

30

Q.

% 20C/3

10

ISO 100 80 60

500 1000 1500

Conveying Distance - ft

2000

30

2500

Figure 16.8 The influence of air supply pressure and conveying distance on solidsloading ratio for high pressure conveying systems.

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Figure 16.8 is drawn for high pressure, long distance conveying systems,with air supply pressures up to 60 lbf/in2 gauge and pipeline lengths of up to 2500ft. Figure 16.7 is drawn for shorter distance, low pressure systems, up to 15 lbf/in2

500ft.It should be noted that dense phase conveying is possible with low pressure

vacuum conveying systems, as will be seen from Figure 16.7. This is becausedense phase conveying is a function of pressure gradient and does not depend ondistance or pressure drop alone.

Pipeline bore, conveying air velocity, and material type, will all have an in-fluence on the overall relationship and so it must be stressed once again that thesefigures are only approximations for this purpose and on no account should they beused for design purposes alone, as mentioned earlier.

3.6 PROCEDURE

To illustrate the method it is proposed to use the same example as employed in theprevious chapter to demonstrate the procedure with regard to scaling parametersfor dense phase conveying. This was to investigate the conveying potential of thepipeline system illustrated in Figure 15.15 for the conveying of cement. The pipe-line routing included a total of 660 feet of horizontal pipeline, 140 feet of verti-cally up pipeline and eight 90° bends.

It was proposed that a conveying line inlet air pressure of about 30 psigshould be used, with atmospheric pressure at 14-7 psia. It was assumed that thetemperature of the air and cement were 520 R (60°F) throughout. A cement flowrate of 140,000 Ib/h was required and it was considered that an 8 inch bore pipe-line would be required.

The equivalent length of the Figure 15.15 pipeline can be obtained fromEquation 15.9:

Le = Lh + 2 Lv + (N + 2 Leb)

The terms in the equation are as follows:: Lh = total length of horizontal pipeline of 660 ft.i ! Lv= total length of vertically up pipeline of 140 ft._" N = total number of bends, which is eight.

Leh= equivalent length of bends, from Figure 8.18, assuming a convey-ing line inlet air velocity of about 900 ft/min, to be about 10 ft.

Substituting the above set of values into Equation 15.9 gives:

Le = 1040 ft

From Figure 16.8 a typical value of solids loading ratio would be about 40with an air supply pressure of 30 psig and an equivalent length of 1040 ft. FromFigure 13.3 the minimum conveying air velocity for cement conveyed at a solids

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468 Chapter 16

loading ratio of 40 is about 700 ft/min and so with a margin of 20% for conveyingline inlet air velocity, the value of 800 ft/min assumed above is satisfactory.

Since a pipeline bore and material flow rate are both specified, it is the ac-tual pressure drop that is required to be evaluated and this comes in three elements.

3.6.1 Straight Pipeline Pressure Drop

This can be obtained from Figure 16.3. This is drawn for a 2 inch bore pipelineand so the material flow rate needs to be scaled down by using Equation 15.12:

d,

= 140,000

= 8750 Ib/h

To use Figure 16.3 any two reference points are required and it will be seenthat for a material flow rate of 8750 Ib/h and a solids loading ratio of 40, the pres-sure gradient is approximately 4 Ibf/in2 per 100 ft of pipeline and the free air flowrate is about 55 ft'/min.

The pressure drop due to the material, therefore, is s imply given by the pres-sure gradient multiplied by the equivalent length:

x [660 + (2 x 140100

= 37-6 Ibf/in2

3.6.2 Pipeline Bends

From exactly the same location on Figure 16.4 the pressure drop due to the bendsis given as 0-6 Ibf/in2 per bend and so the total for the bends is:

Apb = 8 x 0 - 6= 4-8 Ibf/in2

3.6.3 Air Only

From Equation 6.10 the pressure drop due to the air is given by:

f L £ k p C2

21,600 d 1,03 6,800

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The terms in the equation are as follows:/ = 0-004. This is the pipeline friction coefficient. This is derived from

Figure 6.3, having evaluated the Reynolds number for the flow (see Chap-ter 6 section 2.1.4) in the usual way.

1 L = 800 ft. This is the actual length of the pipeline in this case.; ! d = 8 inch. This i s the diameter of the pipeline.. ZA= This is the loss coefficient for all the bends in the pipeline. For anindividual bend the value of k is about 0-15 (see Figure 6.6).

i p = 0-0765 Ib/fV. This is the approximate density of the air at the end ofthe pipeline.

i C = 3000 ft/min. This is the approximate velocity at the end of the pipe-line. This is only an estimate at this stage and may have to be re-considered if an iteration is required at the end of the calculation process.

gc = 32-2 ft Ib/lbf s2. This is the gravitational constant.

Substituting values gives:

0 - 0 0 4 x 8 0 0 8 x 0 - 1 5 ^ 1 0 • 0765 x 30002

121,600x8 1,036,800,1 3 2 - 2

= 0-42 lbf/in2

3.6.4 Total System

The total pressure drop for the pipeline, Apc, is the sum of the three elements:

Apc = App + Apt, + Apa

= 37-6 + 4-8 + 0-4= 42-8 lbf/in2

A check now needs to be made on the values obtained to ensure that they are

consistent. The free air flow rate, VQ , was 55 mVmin in the two inch bore pipe-

line. This equates to 880 ft /min in an eight inch bore pipeline. From Equation5.11:

C = 5-19 -^2- ft/min

The terms in this equation are as follows:

i . 71/ = conveying line inlet air temperature of 520 R.

T i V0 = volumetric flow rate of free air of 880 m3/min.

i d = pipeline bore of 8 in._! pi = conveying line inlet air pressure of 44-0 + 14-7 lbf/in2 abs

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470 Chapter 16

Substituting these values gives:

C, = 632 ft/mm

A check also need to be made on the value of solids loading ratio. For thiscalculation a value of air mass flow rate is required. This can be obtained quitesimply by multiplying the value of free air flow rate by the value of free air den-sity. Thus the solids loading ratio, $ is:

mp _ 140,000

p VH 0 • 0765 x 880 x 60

= 35

It will be seen from this check that both the solids loading ratio and the con-veying line inlet air velocity (and hence free air flow rate) are slightly below theinput values and so it would be recommended that the procedure be repeated onthe basis of the new data. Alternatively a higher air supply pressure could be usedor a larger bore of pipeline. However, since the first estimate is very close, it isalmost certain that with a stepped pipeline the conveying duty would be achievedsatisfactorily. There are many alternatives and possibilities.

Since a wide range of pipeline bore and air supply pressure combinationswill be capable of achieving the duty, it is always worthwhile investigating a num-ber of different options, as they are likely to lead to different system costs andoperating power requirements.

4 STEADY FLOW ENERGY EQUATION METHOD

The steady flow energy equation, in its fu l l form, includes a heat transfer term, aswell as a work transfer term and includes changes of both kinetic energy and po-tential energy, as well as changes in enthalpy. In this application heat transfer be-tween the system and the surroundings can be disregarded and so the only energyinput to the system that needs to be taken into account is that imparted by the airmover. The appropriate thermodynamic model is presented in detail below.

The energy input to the system from the air mover is transferred to both theconveyed material, and the conveying air, and so both must be included. It is con-sidered that changes in enthalpy can also be disregarded, but changes in both ki-netic and potential energy are included in the energy terms, and these are also pre-sented in detail below.

4.1 The Model

In this model the thermodynamic work imparted to the air by the compressor, orexhauster, Wa, is equated to the energies transferred to the conveyed material, Ep,

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and to the conveying air, Ea. Energies in this model are expressed in terms of a'head', h, or equivalent potential energy. Thus:

Wa = Ep + Ea hp - - - - - - - - - (23)

4.1.1 Applicability

The influence of conveying air velocity, pipeline bore, horizontal conveying, ver-tical lift, pipeline bends and plant elevation are all taken into account. The modelcan be applied equally to vacuum conveying and to positive pressure conveyingsystems. It will also apply to both dilute and dense phase conveying, provided thatthe conveying parameters are correctly specified. Although the model can theo-retically be applied universally it must be stressed once again that there is no refer-ence to material properties anywhere in the basic equations.

As with the previous methods considered, it is also possible to apply con-stants to the models presented, at appropriate points in the equations. These con-stants can be used to fine tune the model, for a given material, so that the modelcan be applied with a reasonable degree of reliability for the design of pipelinesystems for that specific material. Ideally, actual conveying data for the given ma-terial should be used for this purpose. By this means different sets of constants canbe determined to cater for a range of different materials, or for a number of differ-ent grades of a particular material.

4.2 COMPONENTS OF MODEL

The three basic terms of the model, presented in Equation 16.23 are detailed be-low. The energy input is just a single term as this relates to the work done by thecompressor or exhauster. The energy transfer to both the conveyed material andthe air are split into their component parts and it is with these than fine tuning canbe applied with the use of constants to improve the accuracy of the overall modelfor a given material.

4.2.1 Energy Input to Conveying Pipeline

The compression or expansion of the air, particularly within positive displacementmachines, is an adiabatic process and the useful work imparted to the air, Wa, isgiven by:

m \p v - p vw a v 4 4 3 3 • .Wa = -; r hp - - - - - (24)

229•2(n - l)

where ma = mass flow rate of conveying air - Ib/min

(see Equation 25)p = air pressure - lbf/in2 abs

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472 Chapter 16

v = specific volume of air - f t ' / lb(see Equation 27)

and n = adiabatic index for compression of air- 1-2

subscript 3 relates to the inlet conditions to the air moverand 4 relates to the exit conditions from the air mover

(see Figures 16.9 and 16.10at the end of this chapter)

4.2.1.1 Air Mass Flow Rate

The mass flow rate of the air, ma , used for conveying the particulate material can

be obtained from Equation 5.4, and is reproduced below once again:

\44pV = m R T - - - - - - - - - - ( 2 )

The volumetric flow rate of the air, V , can be obtained from:

n d2

V = C * fr'/mjn . . . . . . . (3)576

Substituting Equation 3 into Equation 2 and expressing in terms of convey-ing line inlet air conditions gives:

2P-, C, n d

mn = — Ib/min - - - - - - (25)a 4 R T,

where p/ = conveying line inlet air pressure - lbf/in2 absCt = conveying line inlet air velocity - ft/mind = pipe bore - mR = characteristic gas constant - ft lbf//lb R

and TI = conveying line inlet air temp - R

4.2.1.2 Specific Volume of AirThe specific volume of air, v, is given by:

V 1ft'/lb - - - - - (26)

ma Pawhere pa = density of air - lb/ft3

= 0-0765 Ib/ff for air at 'free air conditions'

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and from the Ideal Gas Law (Equation 2)

R Tftj/lb (27)

so that

144 p

R T^

144 p,etc

4.2.1.3 Adiabatic CompressionFor adiabatic processes we have the following basic relationship between/; and v.

P\v\ =

Note also that:

P\v\ _

'1Ti "i

(29)

andT1">

V2 (30)

4.2.2 Energy Transfer to Conveyed Material

As mentioned above, energies are expressed in terms of a head. For the conveyedmaterial, therefore, the vertical l i f t in the pipeline is taken as the reference, ratherthan the length of horizontal pipeline, as is generally common with most otherpneumatic conveying system design methods.

The energy transfer to the conveyed material, Ep, is given by:

m g hP P hp

where trip = flow rate of conveyed material - Ib/h

g = gravitational acceleration = 32-2 ft/s2

(31)

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474 Chapter 16

gt. = gravitational constant = 32-2 ft Ib/Ibf s2

and hp = total system head loss forconveyed material - ft

4.2.2.1 System Head LossThe total system head loss for the conveyed material, hp, is the sum of the compo-nent parts of the energy required to accelerate the particles to their terminal veloc-ity, hk, and to convey the material through the horizontal, h/,, and vertical, /!,,, sec-tions of the pipeline, and the bends, Nhb. Thus:

hp = hk + hh + hv + Nhb ft - - - - - (32)

where h/, = average head loss per bend due to particlesand N = total number of bends in pipeline

4.2.2.1.1 Acceleration Loss

0-8 C2]hk = J— f t - - - - - - - - (33)

7200 g

where C? = conveying line exit air velocity - ft/minand g = gravitational acceleration - ft/s2

Note:The constant (0-8) accounts for the fact that the particles will typically be

conveyed to a terminal velocity which is about 80% of that of the convey-ing line exit air velocity, C3 (see Figure 15.10).

4.2.2.1.2 Horizontal Line Loss

hh = lhLh ft - - (34)

where L/, = total length of horizontal pipeline - ftand /I/, = horizontal line constant - -

Note:The constant, 1;, accounts for the fact that the head loss is in terms of an

equivalent length of vertically upward pipeline.1 The value of this constant is clearly influenced by friction forces between

particles, and particles and pipe walls, and is likely to be affected quitesignificantly by material type. It is suggested that a value of 0-5 should beused in the first instance:

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4.2.2.1.3 Vertical Line Loss

hv = 1VLV ft (35)

where L,, = total length of vertically upsections of pipeline - ft

and A,, = vertical line constant - -

Note:The value of the constant A,, is also clearly influenced by friction forces

between particles, and particles and pipe walls, and is likely to be affectedquite significantly by material type. It is suggested that a value of 1 -0should be used in the first instance:

[J Vertically downward sections of pipeline can be disregarded, providedthat no individual section is longer than about 15 ft. For longer sections ofconveying vertically down refer to Chapter 8.

4.2.2.1.4 Bend LossesAverage bend loss:

hh = A,C'

7200 g(36)

where C = root mean air velocity

f ? -sO-5Cl+Cl

2

- ft/min

ft/min (37)

C/ = conveying line inlet air velocity - ft/minC2 = conveying line exit air velocity - ft/ming = gravitational acceleration - 32-2 ft/s2

and X\, = pipeline bend constant - -

Note:j The value of the constant ).h will be influenced by both the type of con-veyed material and the bend geometry. It is suggested that a value of 1 -5should be used in the first instance:

'..• The velocity of the air increases through the pipeline from C/ to C2 andso the head loss for every bend will be different. The root mean velocity istaken in order to provide an average value for the head loss. In general thebends are distributed uniformly along the length of the pipeline. Only ifthere is an unbalanced cluster of bends at the end of pipeline wil l this bendloss need to be reconsidered.

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476 Chapter 16

4.2.2.2 Total Energy TransferIf the individual elements of head loss are substituted back into Equation 16.32and this, in turn, is substituted into Equation 16.31, the result gives the total energytransfer to the conveyed material.

m 8

x l O 7200g

NC2

7200ghp (38)

4.2.3 Energy Transfer to Conveying A ir

The energy transfer to the conveying air, £„, needs to be considered in terms offriction losses. These are pipe wall friction and bend losses. This is given by:

m g ha a

33,000 ghp (39)

where ma = air mass flow rate - Ib/min

g = gravitational acceleration = 32-2 ft/s"hu = total system head loss for

conveying air - ftand gc = gravitational constant =32-2 ftlb/lbfs2

4.2.3.1 System Head LossThe total system head loss, /?„, is the sum of that due to the pipeline wall friction,h/, and pipeline bends, Nhha. Thus:

+ Nh.'ba ft

where hf = pipeline friction head loss - ftN = number of bends - -

and hf,a = head loss per bend due to air - ft

(40)

4.2.3.1.1 Pipeline Friction Loss

48 / L C2

xd

(41)

where / = pipeline friction coefficient

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L

dg

and C

4.2.3.1.2 Bend Loss

d khba =

where dk

f

= 0-0045 typically for p ipel ine~ total pipeline length= Lh + £,.= pipeline bore= gravitational acceleration

= root mean air velocity(see Equation 37)

- ft

- in- 32-2 ft/s2

- ft/min

(42)

- inpipeline borebend head loss coefficient0-15 typically for radiused bendspipeline friction coefficient0-0045 typically

Substituting values for k and/, the total head loss due to the bends wil l be:

Nh baN d

1-44(43)

4.2.3.2 Total Energy TransferIf the individual elements of head loss are substituted back into Equation 41 andthis, in turn, is substituted into Equation 39, the result gives the total energy trans-fer to the conveying air.

hp - - (44)ma g

c

/(V^v)c2

150 g d

N d

1-44

4.3 Procedure

The energy equation model presented in Equation 23 and the three components ofthe equation have been presented in detail above. It is possible to equate the workterm in Equation 24 to the energy terms for the conveyed material and the convey-ing air in Equations 38 and 44 in a single line and solve. This is not included hereas a further numbered equation since it is felt that a better understanding of theprocess will be gained by evaluating the constituent parts individually.

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478 Chapter 16

To illustrate the method it is proposed to use the same example as employedearlier in this chapter to illustrate the universal conveying characteristics methodwhich, in turn, was used to demonstrate the procedure with regard to scaling pa-rameters in the previous chapter. By this means the results of all three methods canbe compared, and in particular, these two quick check methods with each other,and with the more reliable method of using scaling parameters based on the use ofactual conveying data.

To recap, a sketch of the pipeline is given in Figure 15.15 and the routingincludes a total of 660 ft of horizontal pipeline, 140 ft of vertically up pipeline andeight 90° bends. It was proposed that a conveying line inlet air pressure of about30 psig (pi = 44-7 psia) should be used, with atmospheric pressure, p2, at 14-7 psia.A cement flow rate of 140,000 Ib/h was required and it was considered that an 8inch bore pipeline would be required.

For the convenience of calculation it was assumed that the temperature ofthe air and cement were 520 R (60°F) throughout, but for this particular method itmust be emphasized that the temperature of the air leaving the compressor must becalculated, since it will be at a very much higher temperature and the value of thespecific volume of the air at this point is a function of this temperature (see Equa-tion 27).

Once again it will have to be an iterative process, since the material is capa-ble of being conveyed in dense phase, and so a conveying line inlet air velocitywill have to be selected in order to allow the calculation to proceed. In terms of asolution it is proposed that the material flow rate achieved in an 8 inch bore pipe-line and with a conveying line inlet air pressure of 30 psig should be investigated.Any one of the three parameters can be chosen but this is probably the easiest interms of solving. Results can be obtained relatively quickly and so a wide range ofconveying parameters can be conveniently investigated.

4.3.1 Compressor Work

The expression for compressor work is given in Equation 24. This requires a valuefor air mass flow rate and so a conveying line inlet air velocity must be specified.This wi l l have to be an estimate to allow the calculation process to proceed, and soa value of 720 ft/min (20% margin on minimum value) is taken in anticipation ofthe solids loading ratio being greater than about 40 (see Figure 15.16). Air massflow rate is given by:

n d1 p Cm = Ib/min - - - - - - - - 6.15

4 R T

Substituting conveying line inlet air values presented above gives an airmass flow rate of 58-4 Ib/min. Specific volume values are also required in Equa-tion 24 and these can be evaluated using Equation 27. 7j, the temperature at inlet

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to the compressor can be taken as 520 R but the temperature at outlet, T4, wil l haveto be evaluated using the first part of Equation 30:

T4 = T; = 626 R

Substituting calculated values into Equation 24 gives:

5 8 - 4 [ ( 4 4 - 7 x 5 - 1 8 4 ) - ( l 4 - 7 x l 3 - 0 9 ) ]

2 2 9 - 2 x ( l . 2 - l )

= 50-07 hp

4.3.2 Energy Transfer to Conveyed Material

The expression for the energy transfer to the conveyed material, Ep, is given byEquation 38. This additionally requires a value for the conveying line exit air ve-locity which can most conveniently be evaluated by substituting Equation 9.1 intothe first part of Equation 9.3. Since it is a single bore pipeline and at constant tem-perature the expression reduces simply to pt x C, = p2

x C? and substituting val-ues gives C? = 2190 ft/min. With this value the root mean velocity can be deter-mined (Equation 37) as 1630 ft/min. Substituting these values, recommended val-ues of constants, and data required into Equation 38 gives:

mp x 3 2 - 2

l - 9 8 x 3 2 - 2 x ! 0 6

( 0 - 8 x 2 1 9 0 )

7 2 0 0 x 3 2 - 2,((, 5x66o )n-0 A v 1 Af)\ _i_

• 5 x 8 x l 6 3 0 2

7 2 0 0 x 3 2 - 2

m ,

1 - 9 8 x 1 0

= 3-13 x 10'4 m

-(13-2 + 330 + 140 + 137-5)

p

4. 3. 3 Energy Transfer to Conveying A ir

The expression for the energy transfer to the conveying air, Ea, is given by Equa-tion 44. All of the values required for this equation have been presented or evalu-ated and so substitution gives:

Ea =5 8 - 4 x 3 2 - 2

33 ,000x32-2

= 0-52 hp

1 5 0 x 3 2 - 2 x ! 1 - 4 4

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480 Chapter 16

4.3.4 Material Flow Rate

By substituting these component parts into the basic model, Equation 23, gives:

50-07 = 3-13 x 10"4 mp + 0-52

and mp = 158,300 Ib/h

Based on this value of material flow rate the solids loading ratio, for the airflow of 58-4 lb/min, comes to about 45. As a consequence the value of conveyingline inlet air velocity chosen is satisfactory and so the calculation is complete withno iteration required. The flow rate of cement obtained at 158,300 Ib/h is about13% greater than that derived by using the scaling parameters presented in theprevious chapter. If actual conveying data is available it would be recommendedthat the various constants incorporated into the equation should be 'fine tuned' inorder to increase the reliability of the method.

NOMENCLATURE

Symbol SI

A Pipe section area in~ m7

n d= for a circular pipe

4C Conveying air velocity ft/min m/s

C Root mean velocity ft/min m/s

2 2C, +C2

d Pipeline bore in mE Energy transfer hp kW/ Pipeline friction coefficientg Gravitational acceleration ft/s2 m/s2

32-2 9-81gL. Gravitational constant ft Ib/lbf s" kg m/kN s2

32-2 1-0h Head loss or gain ft mk Bend loss coefficientL Pipeline length ft m

ma Air mass flow rate lb/min kg/s

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m p Material flow rate

n Adiabatic indexA' Number of bendsp Air pressureP Power requiredR Characteristic gas constant

= for airl TemperatureT Absolute temperature

v Specific volume of air

V Volumetric flow rate of airW Work done

Greek

Air density= at free air conditions

Solids loading ratio

mp

Ib/h

lbf/in2 abshpft Ibf/lb R53-3op

Rt°F + 460ft3/lb

ftYminhp

Ib/ft'0-0765

m

X Conveying parameter constanti// Total pipeline head loss coefficient

Superscripts

n Adiabatic index

Subscripts

a Conveying airatm Atmospheric valueb Bendsc Material conveyinge Equivalent valuef Frictionh Horizontalk Acceleration or kinetic valuemin Minimum valuep Conveyed material or particlesv Vertically up

tonne/h

kN/nr abskWkJ/kg K0-287°CK.t°C + 273m3/kg

nrYskW

kg/m3

I-225

o Free air conditions

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482 Chapter 16

Pipeline inlet - material feed pointPipeline outlet - material discharge pointInlet to exhauster / compressorOutlet from exhauster / compressor

These numerical reference points are illustrated in relation to a vacuum con-veying system in Figure 16.9 and in relation to a positive pressure system in Fig-ure 16.10.

Material In

Feeder

Air In Material Out

Figure 16.9 Reference points in relation to a negative pressure or vacuum conveyingsystem.

Compressor

Air In

Material In

Air out

Filter,

Material Out

Figure 16.10 Reference points in relation to a positive pressure pneumatic conveyingsystem.

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Quick Check Methods 483

Note:;~ In a negative pressure system; pt will be slightly below atmospheric pres-

sure if an artificial resistance is added to the air pipeline inlet for the pur-pose of assisting the feed of material into the pipeline;/}? and Tj will gen-erally be equal to p3 and T3; but the mass flow rate of air at 3 might behigher than that at 2 if there is a leakage of air across the material outletvalve on the discharge hopper.

In a positive pressure system; p, wil l generally be equal \.op4 unless thereis a pressure drop across the feeding device; p2 and p3 will generally beequal to the local atmospheric pressure; and the mass flow rate of air at 1wil l be lower than that at 4 if there is a leakage of air across the feedingdevice.

Prefixes

A Difference in value1. Sum total

REFERENCE

1. D. Mills. A quick check method for the design of pneumatic conveying systems. Proc26th Powder & Bulk Solids Conf. pp 107-124. Chicago. May 2001.Subsequently published in Advances in Dry Processing. Cahners, pp 7-17. Nov 2001.

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17Innovatory Conveying Systems

1 INTRODUCTION

The motivation to use dense phase conveying technology arises generally from adesire to convey at low velocity in order to avoid a range of operational problems;in particular, the problems of erosive wear of pipelines and fittings or the attritionof the conveyed material. However, the ability or otherwise of a material to beconveyed in a dense phase flow regime depends on the particle and bulk propertiesof the material to be conveyed.

For materials that have natural dense phase performance in either of themajor modes of dense phase flow, no special equipment is required. For these ma-terials a standard pipeline and feeder may be used. In general, dense phase systemstend to use a blow tank to feed the conveying line since this device can operateover a very wide range of pressure conditions. For materials which do not exhibitnatural dense phase capability, there is often a need to use specialized techniquesand equipment to encourage the material to convey reliably in a dense phase modeof flow.

Three basic approaches are used in order to condition the material in theconveying system. The first method involves a form of plug creating device at thefeed point which aims to control the plug or slug formation in order to limit thesize of plug which is initially fed into the conveying line. The second approach isto use an air addition system, commonly known as boosters, to inject additional

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conveying air at various points along the pipeline in an attempt to ensure that thematerial in the pipeline is maintained in a fluidized condition. The third approachis to use an internal or external bypass l ine which aims to l imi t the maximum sizeof plug that wi l l form in the conveying line.

The exact design of these systems vary considerably, depending on the par-ticular manufacturer, but they have all been used with varying degrees of success.There is, however, a distinct lack of detailed technical literature underpinningthese systems, and hence the aim of this chapter is to approach these systems in ageneric manner and to explain their operation as far as is possible from a technicalpoint of view, rather than to review the systems of specific manufacturers.

2 PLUG CREATION SYSTEMS

In general, plug creation systems involve the use of a blow tank as a feeder inwhich the supplementary air supply is controlled in order to artificially createplugs of material of a given length. Typically, the supplementary air injectionpoint is located in the conveying pipeline just downstream of the blow tank dis-charge valve. The exact positioning is important since if the injection point is lo-cated too far downstream, the pressure drop across the extruded flow in the dis-charge pipe will lead to unacceptably high pressure drops across the dischargepipe.

This type of system was first developed following research undertaken bythe Warren Spring Laboratory in the UK [ I ] in the early 1970's. A sketch of atypical system, as originally developed, with a bottom discharge blow tank feedingdevice, is shown in Figure 17.1.

2.1 Principle of Operation

The pulse-phase system consists of a pressure vessel feeding a conventional pipe-line. The air supply to the blow tank is supplied both to the top of the vessel and toan aeration ring located around the conical section. The aeration ring providesfluidizing air which ensures that the material remains in a fluid-l ike state. Thisensures that, for powdered products, the material flows in a reliable manner intothe pipeline.

The aerated state of the material also ensures that the material can be moreeasily split-up into plugs. At the start of the conveying line, an 'air knife' device islocated. The air knife is essentially an annular device with a ring of small holesequally spaced around the pipeline. The air supply to the air knife is controlled tobe either on or off using a timer and a solenoid valve. When the air knife is operat-ing, a series of air cushions are created between the material plugs. The frequencyof the solenoid switching wi l l provide a degree of control over the plug length.Although this concept was originally created to handle fine powdered materials,the device has been used successfully for a wider range of materials includinggranular materials.

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Innovatory Systems 487

Air

110/240 v

Solenoid Valve Air Knife

Figure 17.1 Schematic of the 'Pulse-Phase' System.

This technology has been licensed to a number of vendor companies aroundthe world who have also developed and refined the basic concept. Many systemshave been operating around the world with solids loading ratios exceeding 300 invery short systems. Operating velocities have been reduced to values betweenabout 300 and 600 ft/mm.

2.2 Stress State in Slugs During Feeding

Despite the use of 'pulse-phase' type systems, there is much evidence to suggestthat the plug formation for many granular materials occurs quite naturally and thatfor coarse granular materials, with a high degree of permeability, no such condi-tioning is necessary. Hitt [2] found that for most free-flowing materials, no specialconditioning was required at all, and that material plugs formed spontaneously andsettled to a steady conveying condition during the steady state period of the blowtank cycle.

Considerable discussion has also taken place regarding the stress state exist-ing in the plug at the beginning of the pipeline and whether this is influenced bythe method of feeding and/or any conditioning of the material that takes place atthe feed point. Research undertaken by Li et al [3] suggests that when a ful l boreplug of material is formed at the feed point, the condition of the material duringthe plug formation does, in fact, influence the stress state in the slug and hence

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may influence the subsequent behavior of the material in the conveying line. Fur-ther work is required in this area to fully understand the operation of such systems.

3 AIR ADDITION SYSTEMS

Air addition along the length of the conveying pipeline is a method of condition-ing the gas-solid mixture during conveying. Two approaches are generally used.The first involves the continuous addition of small quantities of air at regular in-tervals along the length of the pipeline. The second approach aims to prevent apipeline blockage from occurring by injecting air into the pipeline at the pointwhere a blockage is about to occur.

The principle behind the first approach is to ensure that the material remainsin a fluidized condition and hence can be conveyed in the fluidized mode of densephase along the entire pipeline. In the second approach, air is only injected at thetime and position it is required in order to prevent or clear a blockage. Usually, thecontrol of this type of operation is based on a pressure signal.

3.1 Continuous Air Addition

The motivation to provide continuous air addition along the pipeline is generallyan attempt to keep the material in the pipeline in an aerated state. In practice, thisis very difficult to achieve and in general leads to velocities which are significantlyhigher than necessary.

The most critical velocity in pneumatic conveying is the pick-up velocity orthe velocity at the point where the material is fed into the pipeline. In most cases,and certainly for single bore pipelines, the velocity at the feed-point will be thelowest throughout the pipeline. Therefore, it is essential that the required mini-mum transport velocity is maintained at this point. In conventional systems, the airflow rate required for conveying is based on the m i n i m u m transport velocity forthe material concerned.

As the air expands along the pipeline, with the fal l in static pressure, a natu-ral consequence is for the air velocity to increase in proportion to the ratio of thestatic absolute pressures. Clearly the addition of air at various points along theconveying line will lead to a further increase in air velocity beyond that due to theexpansion of the air. Clearly, this is not desirable with the potential for increasederosion and/or attrition, in addition to the higher specific energy requirements forconveying.

Figure 17.2 shows the relationship between air velocity and pipeline length,both with and without air addition. The graph is based on a 4 inch single borepipeline and an air supply to the start of the conveying pipeline of 216 fV/min offree air (scfrn). In both cases the air velocity at the pick-up point is 1500 ft/minand the conveying line pressure drop is 10 Ibf/irr. At this low velocity, it is clearthat the mode of flow would be dense phase. In the case of no air addition, the airexpands to an air velocity of 2500 ft/min at the end of the pipeline.

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Innovatory Systems 489

Pressure Drop = 101bf/in2

10,000

8,000

I 6,000£I

'g 4,000

2,000

With Air Addition

No Air Addition

100 200 300

Length - feet

Figure 17.2 Air velocity versus pipeline length with and without air addition.

In the air addition case, boosters are located every 10 feet along the entirelength of the conveying pipeline and the air flow rate to each booster is 20 scfm. Inthis case, the air velocity reaches 9500 ft/min by the end of the pipeline. An impor-tant point to note is that in the case of a booster system of this type, the velocity atall points in the pipeline will be higher than in the case where the boosters are notused.

It is clear that the use of continuously operating boosters makes velocitycontrol in the pipeline very difficult. In the case illustrated above, it can be seenthat even if the system operates in dense phase initially, the velocity at the end ofthe pipeline indicates that the system wil l be operating in dilute phase by thatstage.

Even if the air flow rates to each booster are halved, the exit velocity wil lstill reach 6000 ft/min. A degree of control over the air velocity could be achievedby careful stepping of the pipeline at appropriate positions along the pipelinelength. This is always a useful technique for controlling the air velocity, asconsidered in Chapter 9.

A further consideration for continuous air injection systems is the level ofgas flow rate required at each injection point to ensure reliable conveying. Clearly,the quantity of air wi l l depend on the material being conveyed. Very littleinformation is available in the literature to provide guidance on this point withmost manufacturers of systems treating any information they have on this point ascommercially confidential.

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3.2 Non-continuous air addition systems

Many of the disadvantages of continuous air-addition systems can be overcome byusing non-continuous systems. In general, these systems are aimed at eitherpreventing a blockage from occurring, or breaking up a blockage that is in theprocess of forming. These systems generally consist of a main conveying pipelineand a parallel air only pipeline with a series of connections at some predeterminedspacing.

Associated with each of these 'boosters' is some form of pressuremeasurement. In general, the pressure measurement is aimed at determining whena significant pressure drop is detected between two boosters, which is assumed toindicate the location of a blockage, or a potential blockage. Once detected, theboosters inject air into the pipeline. The quantity of air injected, the injectionperiod and the overall control of the injection vary from manufacturer tomanufacturer and, once again, this information is generally considered to becommercially confidential.

The action of the air injection under these circumstances has manysimilarities to the action of the bypass system which is considered in the nextsection. However, a critical difference between the two generic systems is that, inthe case of the air addition systems, the air injected is additional to the air supplyprovided at the feed point to the system.

4 AIR BYPASS SYSTEMS

The air bypass system consists of two pipes; a main conveying pipeline and a sec-ond small bore pipeline which may be internal or external to the main pipeline.The small bore pipeline has openings into the main conveying pipeline at prede-termined intervals which allow the conveying air to move between the two pipe-lines. A schematic of an internal bypass arrangement is given in Figure 17.3. Fig-ure 17.4 shows a schematic of an external bypass arrangement.

Figure 17.3 Schematic of internal bypass arrangement.

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Innovatory Systems 491

Figure 17.4 Schematic of external bypass arrangement.

The work of Barton [4] is probably the most recent work dealing specifi-cally with bypass systems. The work was predominantly a global approach to theproblem which involved a direct comparison between two pipelines of the samegeometry; one containing an internal bypass pipeline and the other without. Arange of bypass lines were used, including copper pipelines with varying flute (orhole) spacing, as well as a totally porous pipeline constructed from a permeablepolymer. The conveyed material was alumina, and various grades were used in thetest program.

The test pipeline was 160 ft long with 6 x 90° bends each having a benddiameter to pipe bore ratio of 6:1. The pipeline was two inch nominal bore and themajority of the pipeline was in the horizontal plane, with only 12 ft being vertical.This pipeline could be operated as a conventional pipeline with no bypass line.Alternatively, an internal bypass pipeline could be inserted. Various designs ofbypass pipeline were tested during the course of this work.

To establish the effect of the bypass pipeline, Barton used a macro approachto pipeline testing, whereby the performance of various bypass systems werecompared directly with a conventional pipeline of the same geometry. The generaleffect of the internal bypass was two-fold; firstly, conveying at lower minimumconveying velocities was achieved when the bypass line was fitted; and secondly,the stability of the material flow was significantly enhanced.

However, as expected, the conveying rate for a given air flow rate and pres-sure drop was reduced, largely due to the reduction in flow channel cross-sectionalarea due to the presence of the internal bypass pipeline. In the case of the sandygrade of Alumina, the min imum superficial gas conveying velocity in the conven-tional pipeline was about 2000 ft/min. In the case where a bypass line was in-stalled, minimum conveying velocities were as low as 220 ft/min.

However, whereas maximum conveying rates of about 37,000 Ib/h wereachieved with a conveying line pressure drop of 45 lbf/in2 in the conventionalpipeline, maximum rates of about 28,000 Ib/h were achieved under similar con-veying conditions where the bypass line was installed. This is due to the reduction

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492 Chapter 17

in cross-sectional area of the main pipeline when the internal bypass line is in-stalled.

4.1 Analysis of the Operation of a Bypass Line

The primary function of the bypass pipeline is to prevent blockages of the mainconveying pipeline from occurring. The bypass line achieves this in two ways.Firstly, the bypass l ine provides an alternative route for the air to flow when ablockage occurs. Secondly, the provision of an alternative route for the air to flowprevents a build up of pressure behind the blockage, which reduces further com-paction.

The total air (or conveying gas) flow rate in the system is the sum of the gasflows in the main pipeline and in the bypass line. The ratio of these flow rates willdepend on the comparative resistance in each of the two flow channels. Understeady state conditions, the pressure profiles in each of the two flow channels willbe the same.

Clearly, if an increase in the flow resistance occurs in the main pipeline, agreater flow rate will occur in the bypass line in order to balance the pressures. Itis clear, therefore, that the design of such a system must be based largely on theresistance of the bypass pipeline.

The most likely reason for an increase in pressure in the main pipeline willbe due to the formation of a plug or slug of material. Hence, the pressure requiredto move a plug of material in the main pipeline is of critical importance to the de-sign of bypass systems. Barton [4] approached the problem by first attempting toestablish the relationship between the length of the material plug and the pressurerequired to move the slug. An example of the results reported in Barton's thesis isgiven in Figure 17.5 for alumina.

Knowing the relationship between slug length and the pressure required tomove the slug, Barton chose a critical slug length which corresponded to themaximum pressure available to move the slug. The bypass pipe diameter and theflute spacing was then selected to balance the resistance across a slug of criticallength. To establish the relationship between the slug length and the pressure re-quired to move the slug, Barton carried out experimental trials and compared theresults with the relationship developed by Konrad [5] given in equation I :

Ap

L D D ' " ° '"' D

- - - - - - ( 1 )

A comparison between the calculated values and the measured values ofpressure drop against plug length for sand are also given in Figure 17.5.

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Innovatory Systems 493

uu -

Ist;•g

Sp 40-CLoo5co" 20-QJ •*->-<

3LO[/]<L>L-Cu

0 -

/////

/

/0 10

/a\ Plug Length -

//

KonradModel

• Barton'sTest DataII

20 3(ft

Figure 17.5 Pressure drop versus plug

T 30-OBD

essu

re to

Mov

e I

to0

0

OH^/

/

/"

/

/

mKonradModel

" Barton'sTest Data

(b)

10 20

Plug Length - ft

30

Once the relationship between the plug length and the pressure required tomove the plug is determined, a decision can be made regarding the maximum de-sired plug length that wil l be allowed to occur in the pipeline. This decision wouldbe made based on the system pressure available with some margin of safety.

Barton's analysis focuses on determining the diameter of bypass pipelinerequired to ensure that a plug never exceeds the critical plug length. The analysisis based on a constant mass flow rate of gas supplied to the pipeline system suchthat:

or

m m<atal

(2)

(3)

Assuming a plug of critical length is created and the critical pressure is ap-plied across the plug, then the gas velocity through the plug can be determinedusing the Ergun Equation [6]:

TL 7^"T (4)

In quadratic form, this equation becomes:

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494 Chapter 17

• 7 5 - 150-A,

(5)

This quadratic can be solved for the mean gas velocity, Ugm. The mass flowrate of gas passing through the bed is obtained as follows:

(6)

where A is the cross-sectional area of the plug and the mean gas density is givenby:

'" (1\~RT - - - - - - - - - - - - - - ( I )

Knowing the mass flow rate of gas passing through the plug and the totalgas mass flow rate supplied to the system, equation 3 wil l give the mass flow rateof gas passing through the bypass line. Barton then determines the diameter ofbypass line required to provide the same pressure drop across the bypass line usinga differential form of the Darcy equation:

Pdp 4f 32fm2

KRT

dL D p 7T2 d5

Integrating between points 1 and 2 gives:

(8)

P\ ~ Pi64fm2

gRTL(9)

Re-arranging for bypass line diameter as the subject:

d =64/ m-RTL

(10)

The Darcy equation is for isothermal, incompressible flow, however, butprovided the Mach number does not exceed about 0-2, and mean density and ve-locity conditions are used, a reasonable approximation can be obtained.

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Innovatory Systems 495

Barton suggests that if equation 1 is evaluated using the maximum desiredplug length (which corresponds to the plug length used to calculate the mass flowrate of gas through the plug) this will yield the minimum bypass pipe diameterrequired to ensure that a plug or blockage wil l not exceed the critical plug length.That is, the resistance of the bypass pipeline balances the resistance through theplug of maximum desired length for the specified value of total gas mass flowrate. The effect of using a larger bypass pipe diameter would be to balance theresistances between the plug and the bypass line at a shorter plug length.

Figure 17.6 shows the air only pressure drop for different diameters of by-pass line compared to the pressure required to move a plug of material of variousplug lengths for a gas mass flow rate of 0-009 Ib/s. The diagram shows that, in thiscase, a bypass line diameter of 0-31 in (5/16" approx) or smaller is required inorder to ensure movement of the slugs of length up to about 30ft. To prevent slugsforming in excess of about 13 feet in length, a larger bypass line diameter of 0-35in (3/8" approx) would be required.

The work carried out by Barton provides a guide to the sizing of bypasspipelines. However, it does not quantify the effects of the flute (or hole) spacing,but Barton states that "The spacing between holes is important since spacing of theflutes 20 in apart gives better results than 40 in spacing." He goes on to say that "Itis unlikely, however, that this is the ideal distance between holes."

Although the plug lengths tested, and hence pressures generated, are greaterthan those encountered in commercial systems, the experimental work undertakenand modeling employed has helped to provide a better understanding of the flowmechanisms involved.

200 r

160

D.O

120

80

% 40cx

0-23 in

0-31 in

0-35 in0-39 in

10 15 20

Plug Length - ft

25 30 35

Figure 17.6 Air only pressure drop for different bypass line diameters against the pluglength for a gas mass flow rate of 0-009 Ib/s.

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496 Chapter 17

The work also shows that the system has the potential of being modeled sothat engineers should be able to evaluate pipeline sizes and air flow rates for agiven material in due course.

Work carried out by Jones and Soloman [7,8] argued that the bypass pipe-line has more of a transient effect whereby blockages are broken as they form. Themechanism behind this theory is that there will always be a residual pressure in thebypass l ine when a blockage is beginning to form. It is argued that this residualpressure wil l destroy blockages as they form.

The immediate effect will be to significantly reduce the air flow rate in themain conveying pipeline, as this will now be governed by the permeability of thematerial plug. This will lead to a very rapid change in the pressure profile in themain conveying line. However, the pressure profile in the bypass line will changemore gradually and will depend on how easily the gas can flow into the bypassline upstream of the blockage. This will be governed by the hole or flute diame-ters, their spacing or pitch and the diameter of the bypass pipeline. This situation isillustrated in Figures 17.7 and 8. Note that the exact shape of the pressure curve inthe main pipe is very dependent on the properties of the material being conveyed.

Bypass Pipe Pressure

— Main Pipe Pressure

Distance

Figure 17.7 Steady state flow conditions.

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Innovatory Systems 497

Distance

Figure 17.8 Instantaneous blockage conditions.

The dotted lines show the extremes of the possible pressure states in themain pipe. The upper limit of the pressure relationship is the situation where thepressure in the blockage decays linearly as indicated by the upper dotted line. Thiswould occur for materials that are highly permeable. The lower l imit of the pres-sure relationship would be where the pressure in the blockage drops off rapidly asindicated in the lower dotted line. This would occur in materials that have a lowpermeability. Most materials would display a pressure relationship in the blockagesomewhere between the upper and lower limits as indicated by the solid line.

It is argued that the pressure before the blockage in the main pipe will be-come constant as indicated by the flat line in Figure 17.8. This pressure will in-crease and rise to the limit of the air supply. The pressure after the blockage in themain pipe will also become constant and is indicated by a flat line. This pressurewil l decay to atmospheric pressure.

Clearly, air will flow from regions of higher pressure to regions of lowerpressure. In this case, air will flow from the main pipeline into the bypass pipelineupstream of the blockage and will flow from the bypass line into the main convey-ing pipeline at some point along the length of the blockage depending on the posi-

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tion of the cross-over of the pressure curves. The position of the pressure profilesand the magnitudes of the air flow rates into and out of the bypass line will dependon the diameter of the bypass line and the diameter and spacing of the flutes.

5 CONVEYING CHARACTERISTICS

It was mentioned earlier that when a sandy grade of alumina was tested in the 160ft long pipeline of two inch bore, a maximum flow rate of 37,000 Ib/h wasachieved and that 28,000 Ib/h was achieved when a fluted pipe was inserted in thesame pipeline. These two flow rates were obtained with a conveying line pressuredrop of 45 Ibf/in2 in each case and the difference was attributed to the reduction inpipeline material flow cross section area due to the presence of the internal pipe-line, since this should only transfer air.

It was mentioned above that in the conventional pipeline the minimum con-veying air velocity for the alumina was about 2000 ft /min and that with the flutedpipe it was as low as 220 ft/min. In terms of overall performance, however, con-veying characteristics are required. Those for the conventional pipeline were pre-sented in Chapter 12 on 'Aluminum Industry Materials' with Figure 12.Ib and aretypical of materials that can only be conveyed in dilute phase suspension flow.They are shown alongside those of a floury alumina in Figure 12.la to illustratethe differences between dilute phase and sliding bed dense phase modes of con-veying. Floury alumina naturally has very good air retention properties.

Conveying data for the alumina conveyed through the fluted pipeline waspublished in Reference 9 and this shows that the conveying characteristics are verysimilar to those for the polyethylene pellets presented in Figure 4.12b. This meansthat as the air flow rate is reduced, in the dense phase flow region, the materialflow rate reduces. Polyethylene pellets are naturally conveyed in plugs, and thepurpose of the fluted pipeline is to convey materials with no natural dense phasecapability in plugs, and so perhaps it is not to be unexpected that the conveyingcharacteristics should be very similar.

This does mean, of course, that at low velocity a very much reduced mate-rial flow rate wi l l be achieved. Thus a larger bore pipeline wi l l be required to con-vey a material at a given flow rate in the fluted pipel ine system at low velocity,than will be required to convey the material in dilute phase at high velocity in aconventional open pipeline. Many of the materials that are conveyed with flutedpipeline systems are highly abrasive and so the choice is possibly a decision inwhich the problems of pipeline wear and conveyed material contamination aretaken into account.

In the systems in which air is added to the material along the length of thepipeline, such as with 'boosters', it is often considered that this is an artificialmeans of giving a material a degree of air retention [10]. Air retentive materials,such as the floury alumina in Figure 12.la, suffer little or no reduction in convey-ing performance with reduction in air flow rate. It is not known, however, whether

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the conveying characteristics for air addition systems are similar to those for mate-rials that are capable of being conveyed in dense phase in the sliding bed mode offlow. The constant addition of air, as mentioned above, may have an over-ridingeffect. It is hoped that an answer to this question will be provided soon.

6 CONCLUDING REMARKS

There is little doubt that the innovatory systems discussed in this chapter have apart to play for those materials that do not have natural dense phase capability.However, it is essential to ensure that the material to be conveyed is suitable forthe class of system under consideration. At present this is still an empirical processand the only way to adequately determine the suitability of a material for a particu-lar class of system is by pilot scale testing.

It should be noted that the air only pressure drop curves for the various by-pass line diameters are theoretical and based on the Darcy equation. The curverepresenting the pressure required to move a plug of material of various lengths isbased on experimental data. Barton's experimental work involved plug lengths ofup to about 22 feet and pressures up to approximately 90 Ibf/in2.

It should also be noted that those materials that have natural dense phasecapability do not require the additional complexity and expense associated withthe innovatory systems described. However, commercial pressures in the marketplace can lead to these systems being offered for applications where they are reallynot required. It is clearly important for the user to ensure that, if these systems arebeing offered by vendors, they really are necessary for reliable operation.

NOMENCLATURE SI

A Plug section area in2 m2

c Inter-particle cohesion lb/in2 kg/ms2

d Bypass line diameter in mdp Particle diameter in mD Pipe bore in m/ Friction coefficientF Frontal Stress Ibf/in2 N/m2

g Gravitational acceleration ft/s2 m/s2

= 32-2 ft/s2 = 9-81 m/s2

KH. radial stress/axial stressL Pipeline length ft mm Mass flow rate Ib/min kg/s

p Pressure Ibf / in 2 N/m2, kN/m2, bar(1 bar= 100 kN/m2)

R Characteristic gas constant Btu/lb R kJ/kg KT Absolute temperature R K

= t + 460 = t + 273

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500 Chapter 17

U Velocity ft/min m/s

Greekt] Viscosity Ib/ft h kg/m ss Voidage(/> Angle of Internal Friction degrees degrees</>„. Angle of Wall Friction degrees degreesu Included angle on Mohr's Stress Diagramp Density Ib/ft3 kg/m3

Subscriptsb Bulk value

m Mean valuep Particlew Wall1 , 2 Reference points along pipeline

REFERENCES

1. J.H. Aspcy. The pneumatic pulse phase powder conveyor. Proc Joint Symposium onPneumatic Transport of Solids, SAIMechE, SA Institute of Materials Handling. 1975.

2. R.J. Hitt, A.R. Reed, and J.S. Mason. The effect of spontaneous plug formation indense phase pneumatic conveying. Proc 7th Powder & Bulk Solids Conf Chicago.1982.

3. J.Li and M.G. Jones. Towards the control of slug formation in low-velocity pneumaticconveying. Powder Handling & Processing. Vol 14. No 3. 2002.

4. S. Barton. The effect of pipel ine flow conditioning on dense phase pneumatic convey-ing performance. PhD Thesis. Glasgow Caledonian University. Scotland. 1997.

5. K. Konrad, D. Harrison, R.M. Nedderman, and J.F. Davidson. Prediction of the pres-sure drop for horizontal dense phase pneumatic conveying of particles. Proc 5th IntConf on the Pneumatic Transport of Solids in Pipes. BHRA Fluid Engineering Centre.Paper No El, pp 225-244. 1980.

6. S. Ergun. Fluid flow through packed columns. Chemical Engineering Progress. Vol48. No 2. pp 89-94. 1952.

7. M.G. Jones, X. Zhang, T. Krull, and R. Pan. Bypass systems in pneumatic conveying.Proc 15th Hydrotransport. BHR Group Conf. Banff Canada. June 2002.

8. M. Solomon. Bypass pneumatic conveying systems. Final project report. School ofEngineering, The University of Newcastle, Australia. 2002.

9. D. Mason and S. Barton. The use of air-bypass pipel ines to enable low velocity gas-solids flow in pneumatic conveying systems. Proc 8lh Int Freight Pipeline Soc Symp.pp 109-1 16. Pittsburgh. September 1995.

10. D. Mills. The artificial modification of material properties to achieve dense phasepneumatic conveying. Proc CJF-7. pp 249-254. 2000 China-Japan Symposium on Flu-idization. Xi 'an University, China. October 2000.

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18Fluidized Motion Conveying Systems

1 INTRODUCTION

Although these conveying systems have been in use for well over one hundredyears, they have been rather neglected. This is despite the fact that they are widelyused for materials such as cement, fly ash and alumina, and are very economical tooperate. The main problem is that they have, until recently, only been able to oper-ate on a downward incline and as a consequence have been referred to as air-assisted gravity conveyors or "air slides". They have now been developed to oper-ate horizontally and have considerable potential for further development.

Fluidized motion conveying can be regarded as an extreme form of densephase pneumatic conveying. It is essentially an extension of this method, with thebulk particulate material made to flow along a channel. In the air-assisted gravityconveyor the channel is inclined at a shallow downward angle and the predominantfactor causing flow is the gravitational force on the material. It this for this reasonthat air-assisted gravity conveying systems are potentially very economical to oper-ate.

1.1 Conveying Technique

The technique to achieve conveying is essentially to maintain an aerated state in thebulk particulate material, from the moment that it is fed into the upper end of thechannel, to the point at which it is discharged.

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Figure 18.1 The principle of air-assisted gravity conveying.

This is achieved by means of the continuous introduction of air, or other gas,at a relatively low flow rate and pressure, into a plenum chamber, in order tofluidize the material. The principle of operation is illustrated with the sketch inFigure 17.1.

The air passes through a false bottom, or membrane, made of a suitable po-rous material, which runs the entire length of the channel. With powdered materi-als, and those containing fine particles, the channel is generally enclosed, as shownin Figure 18.2 [1], By this means the entire conveying system can be totally en-closed. The fluidizing air, after passing through the bed of material sliding on themembrane, flows over the top of the bed of material and is vented through a suit-able filter unit.

Since the bulk solid material is kept live by means of the steady flow of air,the material flows freely down the slope, even when the angle of inclination isrelatively small. The quantity of air used is kept to the absolute minimum neces-sary in order to reduce both the inter-particulate forces, and the frictional forcesbetween the particles and the internal channel surfaces, sufficient to allow the ma-terial to flow [1]. The air requirements for fluidized motion conveying systems,therefore, are relatively low and so they do need to be maintained between rea-sonably close limits in order to optimize conveying conditions.

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PorousMembrane

SupplyHopper

ConveyingChannel

Section X-XPlenumChamber

Figure 18.2 Typical air-assisted gravity conveyor.

1.2 System Advantages

Fluidized motion conveying has all the advantages of pneumatic conveying, butwith few of the disadvantages. It provides a totally enclosed environment for thematerial, is very flexible in layout, and has no moving parts. With air-assistedgravity conveyors the only drawback is the fact that material can only be conveyedon a downward gradient, but as mentioned above, the system does have develop-ment potential that is making horizontal conveying, at least, a distinct possibility.

A particular advantage over pneumatic conveying is that the conveying ve-locity is very low. In dilute phase pneumatic conveying, solids loading ratios thatcan be achieved are very low and conveying velocities are consequently relativelyhigh. As a result, power requirements are much higher than almost any alternativemechanical conveying system. Operating problems associated with abrasive parti-cles, such as the erosive wear of system components, and degradation of friableparticles, can be so severe that pneumatic conveying, as a means of transport, isoften not considered for such materials.

If, in a pneumatic conveying system, the material can be conveyed in densephase, power requirements will be lower and operating problems will generally bereduced. In fluidized motion conveyors, however, solids loading ratios are evenhigher and conveying velocities are much lower than those in dense phaseconveying. As a result, power requirements are on a par with belt conveyors, and

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operating problems associated with abrasive and friable materials are almost non-existent.

1.3 Design Tolerance

The general principle of fluidized motion conveying is very simple and thismethod of conveying has a particular advantage of being essentially 'workable'.With pneumatic conveying systems it is critical that the conveying line inlet airvelocity is correctly specified. Because air is compressible, and very much higherair pressures are used in pneumatic conveying than in fluidized motion conveying,ensuring that the correct inlet velocity is achieved and maintained in a pneumaticconveying system is not a simple matter.

If this inlet air velocity is too high the material flow rate may be reduced, thepower requirements wi l l be excessive, and operating problems will be severe. Ifthis velocity is too low, the material may not convey at all, and the pipeline islikely to block. With fluidized motion conveyors a great deal of latitude is avail-able in the design of installations, and provided that a few basic requirements aremet, they will generally operate without trouble.

1.4 Historical

It is not known when aeration of a bulk solid material was first used as an aid toconveying, but one of the earliest relevant patents appears to have been that ofDodge, in Germany, in 1895 [2]. He proposed the use of air, entering an openchannel through slits in the base, to transport coarse grained materials, such asgrain. Real progress in the method of conveying was not made until some thirtyyears later when it was found that the gravity conveying of aerated powders wasideally suited to the conveying of cement.

The German company Polysius was a pioneer in the development of air-assisted gravity conveying. They were followed by the Huron Portland CementCompany of America. Huron's plant at Alpena, Michigan, was one of the first tomake extensive commercial use of this method of conveying. They employed"Airslides", as they came to be called, at various stages of the production process,from grinding mi l l discharge to finished cement. The third organization thatplayed a prominent part in developing and establishing air-gravity conveyors wasthe Fuller Company, which manufactured them under license from Huron [1 ].

Although the air-assisted gravity conveyor first came to prominence for thetransport of cement, and is still widely used for this material, many other types ofbulk paniculate material are now handled with relative ease. Pulverized fuel ash,from the power generation industry, and alumina, from the a luminum industry, arecommonly conveyed by this means, as well as diverse substances such as coal dust,sand, and numerous plastic and metal powders.

Typical of the large installations described in some detail in the publishedliterature are a 55,000 ton storage plant and an 88,000 ton ship loading plant, bothhandling alumina [3], and a Canadian aluminum smelter capable of handling

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175,000 ton of alumina per year [4J. Various sizes of conveying channel are usedin these installations, one of the largest being a 3 ft wide channel which couldtransport alumina from a surge hopper to a main silo at a rate of 1650 ton/h.

1.5 Conveying Principles

Considering the advantages that fluidized motion conveyors can offer over otherforms of bulk solids transport, particularly in terms of low power consumption, theuse of these conveyors is not as widespread as might be expected. To some extentthis may be the result of a lack of confidence on the part of the design engineer,since even air-gravity conveying remains something of an art.

In order to enable systems to be optimally designed, rather than over-designed, some understanding of the phenomena involved in air-float, or fluidizedmotion conveying, is necessary. Observation of a particulate bulk solid being con-veyed by this means along a duct will immediately suggest a similarity to a liquidflowing in an inclined channel. It will also be evident that the continuous supply ofair that is necessary to maintain the l iquid like state of the material has a close af-finity to the gas fluidization process.

The basic principles of static fluidization, therefore, are first extended todeal with the flow of fluidized bulk particulate materials. The design, constructionand operation of practical air-assisted gravity conveyors is then discussed at somelength. Consideration is finally given to a number of interesting variations on theconventional air-float conveyor in which the transported material flows along ahorizontal, or even an upward incl ined channel.

2 THE FLOW OF FLUIDIZED MATERIALS

When particulate materials become fluidized under the influence of a continuousupward flow of a gas, they tend to display many of the characteristics of liquids.One of these characteristics is the ability to maintain a horizontal free surface, andanother is the ability to flow from a higher to a lower level. This means that a flu-idized material wi l l flow from a hole in the side of a vessel. If a horizontal pipewas fitted to the hole the material would continue to flow, provided that the pipewas not so long that complete de-fluidization occurred along its length. If it werepossible to keep the material in its fluidized condition, as it passed along the pipe,the flow could be maintained almost indefinitely.

2.1 Pipeline Conveying

In pneumatic conveying, materials that have very good air retention properties cangenerally be conveyed in dense phase over a reasonable distance, quite naturally.A flow of high pressure air is all that is required to keep the material on the movethrough the pipeline. For materials with poor air retention properties, it is neces-sary to introduce air into the material by some means, continuously along the

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length of the pipeline, in order to achieve dense phase flow, as discussed in theprevious chapter.

2.1.1 The Gattys System

A patented method which can give a material artificial air retention properties isthe Gattys Trace Air ' system. In this system air at a relatively low pressure is sup-plied continuously to the material in the pipeline through an internal perforatedpipe which runs the whole length of the conveying line. The motive force comesfrom a pressure drop along the conveying line created by pumping air in at theupstream end, as in conventional pneumatic conveying.

An alternative, although unpractical, method is to have a continuous portionof the pipeline wall itself made of a porous material. From an external source ofair the material could be fluidized through the porous section of pipe, and the highpressure air within the pipeline would provide the motive force. This principleforms the basis of the more recent developments that allow the channel to runhorizontally. If the pipeline were to be inclined, gravity would provide the motiveforce and the air supply within the pipeline would not be needed.

2.2 Fluidized Flow

This combination of gravitational force with fluidization provides the basis of apotentially very economical method of transporting bulk solids. Figure 18.3 showsa different approach to the same concept of continuous fluidized flow. This illus-trates quite simply the fundamental principle on which the air-assisted gravityconveyor operates.

2.2.1 Angle of Repose

Most free flowing particulate materials display a natural angle of repose of around35 to 40 degrees, as illustrated in Figure 18.3a. In order to get such a material toflow continuously, under gravity alone, on an inclined surface, it would be neces-sary for the slope of the surface to be greater than this angle of repose, as shown inFigure 3b. Materials that exhibit some degree of cohesiveness have much largerangles of repose. Such materials will often not flow, even on steeply inclined sur-faces, without some form of assistance, such as vibration of the surface. The intro-duction of air to a bulk particulate material can also provide a means of promotingflow.

This can be achieved by supporting the material on a plate made of a suit-able porous substance and allowing air to flow upwards through the membraneinto the material. Air introduced into a material by this means can significantlyreduce the natural angle of repose. The material will then flow continuously fromthe plate when it is inclined at a very shallow angle. The angle of the plate needonly be greater than the fluidized angle of repose of the material, as shown in Fig-ure 18.3c. For most free flowing materials the fluidized angle of repose is betweenabout 2 and 6 degrees.

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Figure 18.3 Influence of aeration on angle of repose, (a) No aeration on horizontal pileof material, (b) no aeration on steep incline, and (c) with aeration on shallow incline.

2.3 Channel Flow

This phenomenon of fluidized flow can form the basis of a simple and reliablemethod of bulk solids transport. All that is required is a channel having a porousbase through which air can flow. The air must be available over the entire lengthof the channel and so a plenum chamber needs to be provided beneath, as shownin Figure 18.2.

2.3.1 Starling Flow

It is an essential requirement that sufficient air should pass into and through thematerial in the channel to cause it to flow. The porous base, therefore, must be ofhigh enough resistance to ensure that when part of it is covered by material, the airdoes not by-pass this section. Air flow will always have the tendency of taking thepath of least resistance. This is a particular problem on start up, as illustrated inFigure 18.4.

If the material on the channel becomes starved of air, it w i l l not flow any-further down the channel. On starting the flow, therefore, the air velocity into thestationary material must exceed the minimum value of fluidizing air velocity for

the material, Umf, even when a large part of the porous membrane is uncovered.

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Porous Membrane(Distributor)

Plenum Chamber

Figure 18.4 Starting flow of material on inclined channel.

2.12 Channel Slope

The other essential condition to be met is that the downward slope is sufficient topermit a steady continuous flow of the fluidized material. Provided that these con-ditions are satisfied, the air-assisted gravity conveyor will normally prove to be atrouble free and very economical method of transporting a wide range of pow-dered and granular materials.

The appearance of the flowing aerated powder in the channel can dependupon a number of properties that might together be termed the 'flowability' of thematerial. Thus a very free flowing dry material having a relatively low naturalangle of repose would be likely to fluidize very well. Such a material would havegood flowability and in this state would flow smoothly along a channel inclined atjust one or two degrees to the horizontal, as illustrated earlier in Figure 18.2.

Visual observation of the flowing material would show distinct l iquid likecharacteristics such as a smooth or slightly rippled surface. A partial obstruction tothe flow could set up a plume, and a more substantial obstruction could set up astanding wave. In contrast, a material that is cohesive can show a markedly differ-ent behavior in an air-assisted gravity conveyor [5, 6].

2.3.3 Cohesive Materials

Very cohesive materials are unsuitable for conveying in channels in this manner.Materials that are only slightly cohesive, however, can usually be conveyed pro-

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vided that the slope of the channel is greater than about 6 to 10 degrees. Observa-tion of these materials suggests that the particles are not fluidized, but move virtu-ally as a solid mass of material sliding along the channel, as illustrated in Figure18.5.

Irregular zigzag cracks in the flowing bed of material, and the craggy ap-pearance of its free surface, suggests similarities to the channeling and sluggingbehavior that can occur in stationary fluidized beds. These cohesive materialscould well be expected to exhibit such characteristics.

2.3.4 Flow Mechanisms

It is not clear as to what is the dominant factor that causes the improved flowabil-ity that results from the continuous aeration of materials. It could result from theair fil tering through the solid particles and reducing the contact forces betweenthem. Alternatively it could come from the formation of air layers between the bedof particles and the channel surfaces, with a consequent sharp reduction of theboundary shear stresses.

3 SYSTEM PARAMETER INFLUENCES

The main difficulty with the study of the flow of fluidized solids, and the reasonwhy it will be difficult to develop an exact mathematical analysis, is the largenumber of variables involved.

Cohesive Material

Layers

Air

Figure 18.5 Flow of cohesive material on inclined channel.

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With parameters such as particle shape, for example, there is a lack of a pre-cise and convenient definition that can be used for modeling. The shortage of ex-perimental data on the influence of these more complex parameters adds to thealready considerable problems.

In order to simplify the situation, it is as well in i t i a l ly to set aside all thevariables that contribute to what may be termed the 'nature' of the particulate bulksolid. Thus only a very general comparison will be made between the flow behav-ior of different materials, with no attempt to investigate, in depth, the effects ofmaterial characteristics such as particle density, particle size and shape, or theirdistributions.

3.1 Variables Considered

The study then becomes restricted to the flow of a given aerated material in aninclined channel. The main variables, therefore, are the mass flow rate of the mate-rial, the width and slope of the channel, the depth of the flowing bed of material,and the superficial velocity of the air. Note that the superficial air velocity is givenby the volumetric flow rate of the air, divided by the surface area of the porouschannel base. Most of the experimental work and theoretical analyses carried outby various researchers have concentrated on the relationships among these fiveparameters [7|.

3.1.1 Bed Depth Control

The majority of commercial installations involving air-assisted gravity conveyingrely on some form of flooded feed to the upper end of the conveying channel. Inthese the depth of the flowing bed is of little practical interest, provided that it doesnot increase to the extent that the channel becomes blocked. In experimental in-vestigations, however, it is likely that the bed depth would be the independentvariable, as the other parameters can usually be controlled without undue diffi-culty. A consideration of the influence of variables such as the channel slope, su-perficial air velocity, and solids mass flow rate, gives a useful insight into themechanism of fluidized flow in inclined channels.

3.2 Bed Depth and Channel Slope

In general, for a given solids mass flow rate and superficial air velocity, the depthof the flowing bed would tend to increase as the inclination of the conveyingchannel is reduced. At relatively steep slopes this effect is not very significant, butas the channel slope approaches the minimum at which flow can occur, the depthof bed increases rapidly. This effect is illustrated in Figure 18.6.

An increase in the solids mass flow rate would result in a shift of the curvesupwards. A similar result would be caused by a change in the superficial air veloc-ity. It is evident from the shape of the curves on Figure 18.6 that there is an ap-proximate m i n i m u m slope at which a fluidized material wi l l flow.

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CQ

For Constant SuperficialAir Velocity

Increasing MaterialMass Flow Rate

Channel Slope

Figure 18.6 Influence of channel slope and material flow rate on bed depth.

The actual value of this minimum channel slope depends mainly upon thenature of the material involved and, to a lesser extent, upon the solids mass flowrate and the superficial air velocity. Attempting to convey at a slope less than thisminimum value can result in a rapid thickening of the material bed to the point atwhich the channel becomes blocked. Conveying at slopes much greater than theminimum necessary does not yield a significant advantage, and does not make thebest use of available headroom.

3.2.1 Mathematical Analysis

It is not easy to express mathematically the relationship between the bed depth andthe channel slope, since so much depends upon the nature of the conveyed mate-rial. One form of expression that has been proposed is:

m = Chxb4'xSina

where m n = material flow rate

(1)

and

hbCX

a

= bed depth= channel width= a coefficient= an index= channel slope

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The index, x, has a value between 1 and 3, depending upon the aspect ratioof the flow. Aspect ratio is defined as the bed depth divided by channel width andis equal to h/b. Unfortunately the coefficient C is not constant, but depends uponthe nature of the conveyed material, and upon the flow conditions.

An alternative expression from Ref 7 is:

mp = K} phh2 (b g phSina~K2) ---- (2)

where PA = bulk density of materialg = gravitational acceleration

and KI and K2 are constants

KI and K2 are constants for a given bulk participate material. It has beenfound that Equation 2 can represent quite closely the form of the relationship be-tween bed depth and channel slope, as indicated on Figure 18.6. At the presenttime, however, insufficient information is available on the values of constants Kt

and K2. Whilst it seems probable that for different bulk solids, these constants willdepend primarily on particle size and density, much more experimental workneeds to be done to validate the suggestion.

3.3 Bed Depth and Superficial Air Velocity

A similar variation of bed depth occurs as a result of varying the superficial airvelocity, as shown in Figure 18.7. In this case the set of curves shown is plottedfor a constant solids mass flow rate, with each curve representing a different chan-nel slope.

Again there appears to be a tendency towards an optimum air flow which, aspreviously remarked, is related to the slope of the conveying channel. Reducingthe air flow rate to less than the optimum can cause the flowing material to be-come de-fluidized. This results in a sudden fall in the material flow velocity and aconsequent thickening of the bed, often to the point of total cessation of flow. Onthe other hand, an increase of the air flow rate much above the optimum produceslittle advantage, and is merely wasteful of energy.

3. 3. 1 Mathematical Analysis

No reliable mathematical model has yet been found that wi l l allow prediction ofthe variation of the depth of the flowing suspension with the superficial air veloc-ity. To make the problem even more difficult, there appears to be considerableinconsistency between different kinds of bulk solids with regard to the quantity ofair required in relation to minimum fluidizing velocity. This, perhaps, is not sur-prising since a very similar situation exists with regard to the pneumatic conveyingof bulk materials.

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For Constant Mass Flow Rate

Optimum Operating Band

ffl

IncreasingChannelSlope

0 Umf 2Umf 3Umf 4Umf

Superficial Air Velocity

Figure 18.7 Influence of superficial air velocity and channel slope on bed depth.

Thus, whilst most free flowing materials can be conveyed satisfactorily atsuperficial velocities of around 2 to 3 Umf, some bulk solids require air flows that

are much higher multiples of Umf. Again, further experimental work needs to beundertaken to investigate the possibility of predicting the influence of superficialair velocity on flow behavior from easily measured characteristics of a bulk solid.These may have to be in terms of air retention and permeability, rather than parti-cle size and density.

3.4 Bed Depth and Solids Mass Flow Rate

Observation of actual flows of fluidized materials in inclined channels suggest thatalthough the bed depth wil l increase if the solids mass flow rate is increased, asshown in Figure 18.6, the relationship is not one of direct proportionality. It has, infact, been found that the bed depth tends to vary as the square root of the solidsmass flow rate, as indicated by Equation 2.

This means that in most practical situations the relationship between the beddepth and the material mass flow rate is almost linear, but as the material flow rateis reduced towards zero, the bed depth begins to decrease sharply. This is illus-trated in Figure 18.8.

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uC2

IncreasingChannel Slope

For Constant SuperficialAir Velocity

Material Mass Flow rate

Figure 18.8 Influence of material flow rate and channel slope on bed depth.

3.4.1 Channel Clearing

It is a peculiarity of air-assisted gravity conveyors that when the solids feed is re-duced, the flow becomes unstable, and then stops, before the bed depth becomeszero. This means that the base of the channel cannot be completely cleared of theconveyed material simply by shutting off the solids feed.

3.5 Solids Flow Rate and Channel Width

There is no satisfactory experimental information in the literature on the relation-ships between channel width, bed depth and the mass flow rate of the conveyedmaterial. Some researchers have attempted to compare data collected from chan-nels of two and three different widths, but the range has been severely restrictedand the results, therefore, have been inconclusive.

Because of the relatively large quantities of material that can be transportedby the wider air-gravity conveyors, prohibitive costs have limited research rigs tochannel widths being a maximum of about six inches. Industrial installations arecommonly up to about two feet in width, and sometimes wider. Some caution,therefore, should be exercised when projecting data gathered on small experimen-tal rigs to these greater widths.

Naturally, if the channel slope, superficial air velocity and bed depth arekept constant, the material mass flow rate should be approximately proportional tothe channel width. It is more realistic, however, to recognize that conveying chan-

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nels are usually operated at a constant aspect ratio (bed depth/channel width) andtherefore the conveying capacity is normally taken to be proportional to the squareof the width. This is depicted graphically in Figure 18.9.

3.6 Other Influences

Although, for a given bulk particulate material, the main parameters influencing itsflow are those discussed above, there are several other system influences that cancause changes to occur during conveying. The most significant of these are mois-ture, electrostatic charging and particle segregation.

3.6.1 Moisture

It is well known that changes in the moisture content of powders can seriouslyaffect their handling characteristics, and this is especially true in the case of f luidi-zation and fluidized flow. Whilst a small increase in moisture may be beneficial inreducing the tendency of the material to hold an electrostatic charge, too muchmoisture can cause normally free-flowing materials to become so cohesive thatthey cannot be fluidized.

Although air quantities required for fluidizing are relatively small, it wouldbe recommended that the air for fluidizing should be dried, particularly if the con-veyed material is hygroscopic. Since the conveying system itself is almost totallyenclosed, materials are unlikely to absorb moisture from the atmosphere duringconveying.

a §OD u.c _'

Increasing Density

For Constant Aspect Ratio

Channel Width

Figure 18.9 Influence of channel width and density on conveying capacity.

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3.6.2 Electrostatic Charging

Electrostatic charging can have a similar effect on both flow and fluidizing asmoisture. Air drying, however, wi l l not help in this case, as mentioned above withrespect to moisture content. If the conveyed material is potentially explosive, elec-trostatic charging could present a considerable hazard. In this case the materialcould be fluidized with nitrogen.

3.6.3 Segregation

There is a natural tendency for segregation to occur in fluidized beds. This ten-dency for the coarser particles to drift down towards the porous membrane canalso occur in flowing fluidized materials.

Where the channel is short, and relatively steeply inclined, there would belittle opportunity for segregation to occur. In longer channels, however, the prob-lem may become significant. In extreme cases a deposit of coarse particles maycontinuously build up on the bottom of the channel until the material flow ceasesaltogether.

4 MATERIAL INFLUENCES

Almost any bulk particulate material having good fluidizing characteristics wil l ,when suitably aerated, flow easily down an inclined surface. Such materials can,therefore, be transported satisfactorily in an air-assisted gravity conveyor. Al-though it is often stated that being easily fluidizable is an essential requirement forconveying in this manner, many materials that are slightly cohesive can also beconveyed.

Very cohesive materials, however, are generally unsuitable for air-assistedgravity conveying. Materials that are cohesive by virtue of being damp or stickycome into this category, as do powders of extremely fine particle size that have atendency to smear over the channel surface, and hence bl ind the porous mem-brane.

4.1 Geldart's Classification

The work of Geldart in classifying bulk solids according to their fluidizationbehavior [8] provides a useful guide to the suitability of powders and granularmaterials for air-assisted gravity conveying. Geldart's classification is in terms ofthe mean particle size, and the difference in density between the particles and thefluidizing medium. Four different groups of material are identified.

The classification is presented in Figure 18.10. In this representation, parti-cle density rather than density difference is employed. This is because in fluidizingwith air, the density of the air can be disregarded as it is negligible compared withalmost any particle.

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500

200

'fe 100

|j 50<U

Q_CJ

I 20Q.

10

1111I\\

\\\\\\\\\\\\\\\

c N

\

A\\Nss

s.'

.

N

wV

\B

N\

\\\

D

\\

I20 50 100 500 1000

Mean Particle Si/.e - |am

5000

Figure 18.10 Geldart's classification of fluidization behavior for fluidi/ation withambient air.

4.1.1 Fine Granular Materials

In general, materials in Group B, which includes most fine granular materials, inthe mean particle size range from about 50 to 1000 micron, and density range of75 to 250 lb/ft3, are the easiest to convey. These will flow very well at very shal-low channel slopes. When the supply of fluidizing air is shut off, the bed collapsesrapidly and the flow stops. As a result there are unlikely to be any problems withair retention.

4.7.2 Large Granular Materials

Group D covers materials of larger particle size and high density granular materials.These materials can usually be conveyed in the same manner as the group Bmaterials. The quantity of fluidizing air required, however, tends to become ratherlarge. As a result other forms of transport, such as belt conveyors, might prove to bemore suitable.

4.1.3 Air Retentive Materials

Group A materials typically includes powders and very fine granular materials oflow particle density. These materials should convey well in an air-assisted gravity

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conveyor, but they may have a tendency to continue flowing for a time after thefluidizing air has been shut off. This property of air retentive materials must betaken into account when designing conveying systems, since the angle of reposefor a material, as illustrated in Figure 18.3a, cannot be relied upon to stop materialflow.

4.1.4 Cohesive Materials

Group C includes cohesive materials that are difficult to fluidize satisfactorily be-cause of high inter-particulate forces resulting from the very small particle size,electrostatic effects or high moisture content. The dividing line between groups Cand A is very indistinct and the only way of properly assessing the suitability ofdoubtful materials for air-assisted gravity conveying is by practical experiment ina small scale test rig.

As previously mentioned, it may be found that apparently unsuitable materi-als will , by a combination of flowing and sliding, move continuously along aninclined channel. The slope of the channel, however, will need to be greater thanthat for group A and B materials.

4.2 Material Suitability

At the present time it is still necessary to rely heavily on experience when makingan assessment of whether a given material is suitable for air-assisted gravity con-veying. Geldart's chart should help in this respect, however, and laboratory testsshould provide confirmation in case of doubt. Many different kinds of powdersand granular materials have, and are being, transported successfully by air-assistedgravity conveyor. Practical information obtained, unfortunately, tends to be jeal-ously guarded by the conveying equipment manufacturers. This does have theeffect of slowing system development and advancement of the technology.

5 PRACTICAL AIR-ASSISTED GRAVITY CONVEYING

As has been previously explained, conveying on a downward slope has the greatadvantage of gravity to assist the flow of the aerated bulk solid. This is thetraditional, low energy application of air-float conveying, and commercial units areavailable under a variety of trade names. Figure 18.11 represents a basic air-gravityconveyor in which the conveyed bulk solid flows continuously under gravity fromthe inlet to the discharge point.

In this form the device is also widely employed as discharge aids, or flowassistors, mounted on the floor inside silos, bunkers, railway wagons, trucks, etc.They enable containing vessels such as these to be made with a virtually flat base,and thus to have a substantially greater capacity [9, 10]. In these applications thechannel is generally very much shorter than when used for the transport of materi-als.

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Material Feed

InspectionCover

Vent toFilter Unit

PlenumChamber

Supply ofFiltered Air

Figure 18.11 Arrangement of a typical air-gravity conveyor.

5.1 Channel Construction

MaterialDischarge

The basic construction of a practical air-gravity conveyor is very simple, and thisis one of its main advantages over other methods of bulk solids transport. Fortransport applications the conveyor consists essentially of two U-section channels,with one inverted, and the porous membrane clamped between them. Figure 18.12shows a typical clamping arrangement for the duct sections and membrane.

ConveyingChannel

PorousMembrane

.-Menum Chamber

Inspection Cover(may be glazed)

Figure 18.12 Typical conveying duct section.

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The lower channel serves as a plenum chamber to which air is supplied atone or more points, depending upon the overall length of the conveying system.The presence of the covered top channel renders the conveyor virtually free fromproblems of dust leakage. It would, however, also operate quite satisfactorily as anopen channel, as it often must when operating as a discharge aid.

The conveyors can be manufactured in a range of standard bolt-togethercomponents, which include straight and curved sections of various widths. Arange of accessories are also available, such as flow diverters, inlet and dischargeports, inspection windows, gate valves, access ports and scrap traps.

5.2 Material Feeding

Where precise control of the material flow rate is not required, flooded feed fromthe supply hopper to the conveying duct should be satisfactory. The system is theneffectively self-regulating and, with free flowing powders, there should be littlerisk of the conveyor becoming choked, provided that the slope of the channel andthe flow rate of the fluidizing air are sufficient.

Some measure of solids flow control may be achieved with a gate or bafflein the conveying duct, positioned close to the inlet from the hopper. Placing a flowregulating gate near the outlet end of the conveyor is generally not advisable as thewhole channel could fill with material backing up from the gate. Problems wouldthen occur with venting of the fluidizing air and with erratic flushing of the mate-rial under the gate as it opens.

Solids flow control at the inlet end, although basically more reliable, doespresent a problem on long channels because of the considerable delay betweenmaking an adjustment to the control gate, and seeing the effect of this adjustmentat the lower end of the channel. In fact, where it is important to control the mate-rial flow rate within relatively close limits, it becomes almost essential to installsome form of buffer hopper close to the discharge point.

As an alternative to a regulating gate in the conveying duct, the materialflow control could take place at the hopper outlet. A conventional rotary vanefeeder or screw feeder would be ideal for the purpose. For a consistent free flow-ing material it is possible that a pinch valve or an iris valve would be suitable.

5.3 Porous Membrane

A variety of different materials may be employed as the porous membrane. Sometypical examples are woven cotton, polyester belting, sintered plastic, ceramictiles, and laminated stainless steel mesh.

5.4 Venting

When the conveyor is covered, it is necessary for the top channel to be adequatelyvented through suitable filters. With short conveyors it may be sufficient to rely onthe air escaping with the powder from the outlet end of the channel, and then

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through the vent system of the discharge hopper, if one is in use. If the conveyingsystem is long, or if there is a possibility of the channel outlet becoming chokedwith material, it is better to vent from two or more points between the inlet and theoutlet.

5.5 Access Ports

It is likely to prove useful to have inspection or access ports fitted at convenientpositions along the duct, especially in the region of the inlet and outlet, and in otherpositions where blockage may occur. An inspection point was shown earlier inFigure 18.12. These can be glazed if required.

5.6 Material Discharge

A wide choice of discharge arrangements is possible, ranging from a simple openend, to telescopic loading spouts. Some care should be taken with the venting ofair from the conveying duct to avoid excessive blowing through the dischargepoint, but otherwise there should be no problem with this part of the plant.

Controlling the location at which a material is discharged from an air-assisted gravity conveyor is likely to be more satisfactory than controlling the rateof discharge. Using appropriate bends, diverters and outlet ports it is possible toconstruct quite complex systems. Figure 18.13 illustrates an ingenious but simplesolution to the problem of automatically controlling the feed of material to astockpile.

Figure 18.13 Stockpile feeding through multiple discharge outlets.

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This shows an overhead air-gravity conveyor discharging the fluidized ma-terial down each of a succession of outlet spouts until the rising level of the stock-pile causes them to cease discharging. Material will automatically flow over thetop of any blocked discharge spouts, along the channel membrane, to flow throughthe next outlet available. Many functions, such as this, can be carried out auto-matically, and without the need of any valves or moving parts in the system, par-ticularly with Group B materials.

5.7 Components

One of the advantages of air-gravity conveying systems is their versatility, andmost manufacturers of these systems supply them as standard components whichbolt together to suit the user's particular requirement. In addition to the basicstraight channel units and intake and discharge sections, components normallyavailable include the following:

Q Bends: right hand and left hand.H Y-pieces to divide the flow from one channel into two or three, or to re-

combine into one.D Flow diverters, often used in conjunction with side discharge boxes, to

allow the operator to direct the flow as required.D Flow control gates or baffles: for either manual or automatic operation.0 Material traps for the collection and subsequent removal of heavy impu-

rities in the flow.

The construction of these components is basically quite straightforward. Asan example, a typical pattern of a flow diverter, or side discharge box, is illustratedin Figure 18.14.

6 DESIGN PARAMETERS

In terms of system design, the main parameter to consider, in order to achieve thedesired material flow rate, is the channel width. The correct specification of the airrequirements and channel slope, however, is essential in ensuring that the systemwill operate satisfactorily. These three parameters are considered in detail, alongwith the influence of conveying distance.

6.1 Channel Width

The main parameter governing the capacity of an air-gravity conveyor is the chan-nel width. In the literature published by manufacturers of these conveyors, and inother sources giving basic design data, quantities described as 'typical capacities'are given. Such capacities are generally given as a function only of the channelwidth, with little, if any, indication of how such data would be modified for differ-ent types of conveyed material, and for different slopes and air flow rates.

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Diverter Plate

Material DischargeFigure 18.14 Duct diverter facility.

This, however, is not as unreasonable as it first appears in view of the factthat, provided the slope and air flow rate exceed the required minimum or opti-mum values for the particular material being conveyed, they will have little influ-ence on the material flow rate. These points were illustrated earlier in Figures 18.6and 18.8.

6.1.1 Modeling

A useful preliminary estimate of the channel width required for a given applicationmay be made by regarding as constant the average velocity and the bulk density ofthe flowing suspension. The velocity and density are, in fact, both functions of thechannel slope and the fluidizing air velocity. By taking them as being constant, thewidth of a conveyor required to handle a given mass flow rate of a material isgiven approximately by:

b =f \^

r,, mn(3)

where b = channel width

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524 Chapter 18

= expansion ratio of bed

,„ = mass flow rate of material

ra = conveying aspect ratio

Pb = bulk density of materialand Up - velocity of the conveyed material

By taking suitable average values of the quantities Up re and ra and intro-ducing the particle density pp in place of the bulk density pt, a convenient 'rule-of-thumb' equation may be proposed as:

0-65f .

(4)

6.1.2 Capability Chart

If the mass flow rate of conveyed material, m „ , is given in ton/h and the particle

density, pp, in lb/ft3, the channel width, b, will be given in feet. This relationshiphas been used to plot the chart presented in Figure 18.15, which provides a quickreference for determining the approximate channel size for a given application.

It should be noted that normal industrial practice would be unlikely to per-mit the widest channels to operate with a conveying aspect ratio as high as 0-5, asused for the chart, and so caution should be exercised in this respect when usingthe above equations or chart.

6.2 Channel Slope

Equation 18.2 has been proposed to show the relationship between the channelslope and the other system parameters. However, it was pointed out that the use ofthis equation is restricted by a lack of information on the values of the constants K/and K2. At the present time, therefore, it is still necessary to resort to laboratorytests if an accurate indication of the optimum channel slope at which to convey agiven material is required.

In most industrial applications air-gravity conveyors are installed with aslope of 2 to 10 degrees. The lower level of inclination depends very much uponthe type of material being transported. The degree of initial aeration of the con-veyed material, and the nature of the porous membrane, may also influence theminimum slope that can be used.

In general about 1° is sufficient for very free flowing materials. Althoughsuch a low angle may suit the plant layout, it wi l l not necessarily be the optimumfor maximum flow, and a slope of around 3° may be more appropriate. More co-

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hesive materials may require a minimum slope of 7 to 10 degrees for satisfactorytransport, and continuous trouble free operation.

6000

4000

o 2000

'5 ioooI

500

100

Particle Density- lb/ft3

250

200

100

50

0 1 2

Channel Width - ft

Figure 18.1 Chart giving approximate conveying capabilities.

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526 Chapter 18

6.3 Conveying Distance

Provided that the continuous downward slope can be maintained, there is gener-ally no limit to the length of conveying channel that can be used. This is becausegravity provides the motive force and not pressure drop, as in pneumatic convey-ing, and horizontal fluidized motion conveyors. Air-assisted gravity conveyors of500 ft or more in length are not unknown.

It is necessary, of course, to arrange the air supply so that a uniform pressureexists beneath the distributor, or porous membrane, and in very long conveyors isusual to provide air inlets at several points along the length of the plenum cham-ber. It may also be advisable to vent the conveying channel at several points toprevent the bui ld-up of an excessive air velocity over the top of the material beingconveyed.

6.4 Air Requirements

In order to specify the air requirements of an air-assisted gravity conveyor it isnecessary to establish the volumetric flow rate of the air through the porous baseof the channel and the pressure within the plenum chamber.

6.4.1 A ir Supply Pressure

The pressure of the air in the plenum chamber is clearly a function of the resis-tance of the porous base of the channel, but it also depends upon the depth of theconveyed material in the channel. For the purpose of analysis it can be assumedthat the conveyed material is fully supported by the air. It is then possible to esti-mate the pressure on the upper surface of the porous membrane by simple fluidmechanics, for any required aspect ratio of the flowing bed.

Knowledge of the permeability of the porous base would then permit thepressure in the plenum chamber to be estimated. For the membrane, permeabilityis expressed as the volumetric air flow rate per unit area, per unit pressure dropacross it. If this information is not available for the membrane material, it can bemeasured quite easily in a permeameter. The flow resistances of the air from theair mover to the plenum chamber, and of the air through the venting system, mustthen be added to give the air supply pressure needed for the specification.

In practice, however, it is difficult to predict with any confidence an optimumvalue for this parameter because of the uncertainty over the actual pressure dropacross the flowing bed of material. As mentioned previously, it is essential that theporous membrane is of sufficiently high resistance to ensure a uniform distributionof air into the conveyed material. Typically the pressure of the air in the plenumchamber is found to be approximately 10 to 20 inch water gauge.

6.4.2 Volumetric Flow Rate

The flow rate of air that must be supplied to the air-gravity conveyor dependsprincipally upon the length and width of the channel and the nature of the bulk

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particulate material to be conveyed. The air flow rate can most conveniently beexpressed in terms of the volumetric flow rate per unit area of the porous channelbase. This, of course, is the superficial velocity of the air, from the plenum cham-ber into the conveyed material.

The value of the superficial velocity that is required, and must be main-tained, can be predicted approximately from a knowledge of the fluidization char-acteristics of the bulk particulate material. Both the channel slope and the materialflow rate, however, will also have an influence. The optimum value of superficialair velocity at which the conveyor can be operated economically, without unduerisk of stoppage of the material flow, is likely to be between two and three timesthe minimum velocity, Umf, at which the material could be fluidized, as illustratedearlier in Figure 18.7.

For very free flowing materials, on a relatively steep incline, an air velocityonly slightly in excess of the minimum fluidizing velocity may be sufficient. Forvery fine powders, however, air velocities up to ten times Umf may be needed. Inaddition to being wasteful of energy, operation at too high an air velocity cancause problems as a result of fine particles being entrained in the air stream leav-ing the top surface of the material being conveyed along the channel.

The designer, therefore, requires some knowledge, not only of the minimumfluidizing velocity of the material to be conveyed, but also of the air velocity atwhich entrainment can begin. Many methods of predicting Umf for bulk particu-late materials are to be found in the published literature. Figure 18.16 is a chartbased on one of these correlations for materials fluidized with air at a conditionclose to normal ambient [1], Also shown on this chart are approximate values ofUt, the terminal velocity of particles in free fall in still air. The air velocity atwhich particle entrainment can begin corresponds approximately to this velocity.

For a particulate material of known particle size and density, Figure 18.16allows a fairly reliable estimate to be made of the minimum fluidizing velocity.With a knowledge of the diameter of the smallest particles in the material, Figure18.16 also allows prediction of the air velocity at which these fine particles maybegin to be carried upwards from the surface of the bed.

Approximate ranges of the types of fluidization behavior, as given byGeldart's classification, are also shown on Figure 18.16. They are superimposedon the lines corresponding to the min imum fluidizing condition, thus helping toprovide a useful prediction of the likely behavior of a particulate material in an air-assisted gravity conveyor.

7 OPERATING PROBLEMS

It has been stated that air-assisted gravity conveyors are usually trouble free inoperation, and whilst this is true, there are one or two ways in which problemsmay arise.

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528 Chapter 18

1000

io_o4J

c.3

C/3

O- l wGroup C

I

20 50 100 200

Mean Particle Size - |am

500

Figure 18.16 Minimum fluidizing velocity and terminal velocity for a bed of particlesfluidized with air.

One potential source of trouble is the porous membrane that forms the baseof the conveying channel. There are many examples of installations in which thesame membrane has been in use continuously for a number of years. In othercases, however, replacement is necessary at quite frequent intervals. There isprobably little that can be done about blinding of the pores in the top surface of the

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membrane, but precautions can be taken against deterioration of the underside byensuring that the main air supply is adequately filtered.

A further precaution concerns the need for the porous membrane to with-stand a certain amount of ill use. It appears to be common practice for operativesto attempt to relieve suspected blockages with the aid of an iron bar, or similarimplement, wielded against the outside of the channel. If inspection ports are pro-vided these are often accessed with the not uncommon result that the porous dis-tributor is cracked, in the case of ceramic tiles, or punctured, in the case of wovenfabrics.

Blockage of the conveying channel is unlikely to occur unless the porousmembrane is damaged, or the nature of the conveyed material changes drastically,such as becoming wet. Both of these cases would tend to cause local, or completede-fluidization of the flowing material. Erratic flow in the conveying channel isunlikely to be caused by the air-gravity conveying system itself, unless the slope istoo shallow or the bed depth is too great. It is more probable that the feed to thechannel would be at fault, as a result of material arching in the hopper supplyingthe conveyor, for example.

8 HORIZONTAL AND UPWARD TRANSPORT

It has already been established, through the example of the air-gravity conveyor,that a fluidized material will flow along a channel, in the manner of a liquid, pro-vided that there is an input of energy to the material sufficient to maintain theflow. In view of the many positive features that air-gravity conveying has to offer,it is not surprising that there have been a number of attempts to devise modifica-tions to the basic system that would permit material to be transported horizontallyor on an upward slope.

One such example uses a series of stepped air-gravity channels, joined to-gether by vertical air lifts. Other methods, some of them exhibit ing considerableingenuity, rely on inclined air jets, or on the pressure gradient set up in a parti-tioned channel, to provide the forward motivation necessary for the material. Anumber of these systems are examined in detail below.

8.1 The Jet-Stream and Similar Conveyors

The simplest method of generating a flow of a fluidized material along a horizontalchannel is to introduce air through the base and/or sides of the channel in a series offorward facing jets. A number of interesting variations on this approach have beenproposed.

8.1.1 Distributor Types

The Jet-Stream conveyor, and similar types, have all the air entering the channelthrough louvers, or angled slits, in the base. Such a base is illustrated in Figure

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530 Chapter 18

18.17. Provided that a significant component of the velocity is in the horizontaldirection, the material to be conveyed will 'float' along on the resulting cushion ofair.

In the Jet-Stream itself [11, 12], air at very low pressure flows from a plenumchamber through a flat plate punched with a series of louvers, which forms thebase of the conveying channel. The louvers are laid out in rows, offset and stag-gered to ensure a uniform distribution of air jets, whilst maintaining adequate ri-gidity of the plate. The spacing of the perforations, their shape and the percentageof open area may be varied depending upon the material being conveyed. Withfine granular materials the depth of the louvers would be very small comparedwith the width, to minimize back flow of particles into the plenum chamber.

8.1.2 Operating Experience

The amount of published data available on the various methods of horizontal con-veying is somewhat limited. The first work on the multi-louvered base-plate ap-pears to have been that of Futer [12] who conveyed shelled corn, 17 Mesh (1100micron) sand and 35 Mesh (500 micron) aggregate, at rates of up to 165 ton/h, in achannel 12 in wide and 10 ft long. Kovacs and Varadi [13] using similar base-plates conveyed granulated sugar in a channel 6 in wide and 25 ft long.

From the published literature it appears that conveying channels fitted withlouvered or slotted base-plates have proved to be very successful for transportingcartons and packets, but rather less so for bulk particulate materials, especiallywhere these are of fine particle size. The main difficulty when conveying suchmaterials seems to be in maintaining a uniform flow of air, since the very lowresistance of the distributor plate means that the air flow is seriously affected by theamount of material in the channel.

Figure 18.17 Perforated plate distributor.

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The relatively high velocity of the air jets from the distributor, which can beup to 6000 ft/min [12J, may cause unacceptable degradation of friable materials. Ithas been shown [13] that backing the slotted plate with a porous fabric can givesome improvement in air distribution, but the reduction in air velocity from theslots largely destroyed the capability of the conveyor to operate on the horizontal.

8.1.3 Combination Conveyor

An interesting variant of the conveying channel with a slotted base is illustratedin Figure 18.18 [14]. This example again represents an attempt to overcome thedisadvantages of the simple slotted base by separating the fluidizing air from thepropulsion air, by the ingenious device of making angled slits in the porous base-plate. The resulting jets of air thus drive the fluidized material up the sloping sec-tions and along the channel.

8.2 The Pneumatic Escalator

Shinohara and Tanaka [15, 16] have described a device that has some affinity to theair jet conveyors and which they called a 'Pneumatic Escalator". In constructionthis device is similar to the conventional air-gravity conveyor, except that plasticplates are fitted across the conveying channel to form a series of inclined cells.The system is illustrated in Figure 18.19.

Air passing through the porous membrane lifts the particles up the inclinedplates from one cell to the next. In common with other forms of air jet conveyor,however, the quantity of air required tends to be large. Although the device ap-peared to work well on an upward slope of 3°, with conveying still possible at aslope of 26°, most of the channels tested were only 0-6 inches in width. It wouldnot appear to have been tried on a commercial scale.

Porous PlatesAngled with

Slits Between

PlenumAir Supply Chamber

Figure 18.18 A combination of porous distributor and directional air jets.

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532 Chapter 18

Air Supply

PorousMembrane

PlenumChamber

Figure 18.19 The pneumatic escalator.

8.3 The Isler Conveyor

A rather different approach to upward air-float conveying has been used by Isler[17]. He developed a system in which a pressure gradient was set up in the con-veyed material. This was achieved by dividing both the plenum chamber and thematerial flow channel into separate compartments, supplied with air at differentflow rates. This system is illustrated in Figure 18.20. The test channel was 20 ftlong and 10 in wide.

Air Inlet

Figure 18.20 The Isler conveyor.

Air ControlValves

MaterialDischarge

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Cement was conveyed at a rate of 20 ton/h, although this was claimed to bewell below the capacity of the channel. The maximum upward slope at which thechannel was operated was about 12°. Whilst this design of conveyor apparentlyrequires much less air than most of the others operating horizontally, and on up-ward inclines, this type of pressurized conveying channel is not so versatile, and itloses some of its simplicity in design. In order to operate at pressure, some form ofair lock type feeder is needed. Control valves are required on the air supply to theplenum chamber compartments, and these have to be carefully adjusted. They alsohave to be readjusted if the material flow rate needs to be changed. Because of thecompartmental nature of the device, the continual reduction of pressure and con-sequent expansion of the air means that the distance over which the conveyorcould practically operate is limited.

8.4 The Stepped Conveyor

Although the novel forms of air-float conveyor described above are interesting,and may be useful in certain specialized applications, they all fail to take ful l ad-vantage of the major characteristic of fluidized materials, which is their liquid-likebehavior in flowing from a higher to a lower level under gravity. It is this featurethat makes the air-gravity conveyor such an attractive proposition for the eco-nomic transport of bulk particulate materials at high rates and over long distances,provided that a continuous downward slope can be maintained.

It has been suggested [18] that where there is insufficient headroom for theinstallation of a single long conveyor of adequate slope, the distance requiredmight be achieved by using short lengths of inclined conveyor, joined by riserssupplied with air at a slightly higher pressure. Such an arrangement is illustrated inFigure 18.21.

PorousDistributors

A (IncreasedAir Inlets I Pressure) Plenum Chambers

Figure 18.21 Multi-section stepped conveyor.

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534 Chapter 18

8.5 Potential Fluidization

Hanrot [19] describes a pressurized horizontal conveying system developed byAluminum Pechiney to convey alumina. The alumina was to be conveyed from asingle supply point to more than one hundred outlets. Electrolysis pots on a mod-ern aluminum smelter were required to be filled and the distance from the silo tothe furthest pot hopper was about 600 ft. Air at a pressure of 1-5 psig is used. Asketch of the system is given in Figure 18.22 and this illustrates the principle ofoperation.

A conventional channel is employed, but the channel runs full of material.Balancing columns are positioned on the conveying duct and are used for de-dusting. This is not a continuously operating system in the application described. Itis a batch type system and its object is to meet the demands of the intermittentfilling of the pot hoppers. With several hundred such pot hoppers to fill, however,the system must be operating on a semi-continuous basis at least. Of all the systemspresented, this pressurized system probably has the greatest potential for futuredevelopment.

With the channel running full , flow up a slight incline is a logical extensionfor development, but this will start to require a significant increase in air pressure,and the channel will have to be designed to be more pressure-tight to meet thesedemands. Simple modeling, based on static fluid mechanics, can be applied here toillustrate the increase in pressure required.

Supply Hopper

De-dustingDuct I

1^•"<file«^el t'to^^8*!££5?{..v £&«£*>

' * I'jiiftTJ

Fan orBlower

Pot Hoppers

Figure 18.22 Principle of potential fluidization ducts.

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The hydrostatic relationship is:

A p = 2-^- - lbf/in2 - - - - - - - - - - - ( 5 )H4 gc

where Ap = pressure drop required - Ibf7in2

p = bulk density of fluidized material - Ib/ffg = gravitational acceleration - ft/s2

H = vertical lift - ft

and gc = gravitational constant - f t l b / l b f s "

Thus for a typical material having a bulk density of about 50 lb/ft3, thepressure drop required to lift the material a vertical distance of say 10 ft, would beabout 3 !/2 lbf/in2.

REFERENCES

1. C.R. Woodcock and J.S. Mason. Bulk Solids Handling - An Introduction to the Prac-tice and Technology. Blackie and Son Ltd. 1987.

2. J. Dodge. Procedure for transportation of materials in conveying channels using pres-surized air. DRP88402, 1895. German Patent.

3. R.E. Leitzel and W.M. Morrisey. Air-float conveyors. Bulk Materials Handling, Vol1, pp 307-325, Ed M.C. Hawk, Univ Pittsburgh, Sch Mech Eng. 1971.

4. E. Bushell and R.C. Maskell. Fluidized handling of alumina powder. Mech Handling,Vol 47, No 3, pp 126-131. March 1960.

5. C.R. Woodcock and J.S. Mason. Fluidized bed conveying - art or science? Proc Pncu-motransport 3, BHRA Conf paper E l . Bath, UK. April 1976.

6. C.R. Woodcock and J.S. Mason. The flow characteristics of a fluidized PVC powderin an inclined channel. Proc 2nd Powder and Bulk Solids Conf, pp 466-475. Chicago.May 1977.

7. C.R. Woodcock and J.S. Mason. The modeling of air-assisted bulk particulate solidsFlow in Inclined Channels. Proc Pneumotransport 4, Paper D2. BHRA Conf. Calif.June 1978

8. D. Geldart. Types of gas fluidization. Powder Technol, Vol 7, pp 285-292. 1973.9. R.E. Leitzel and W.M. Morrisey. Air-float conveyors. Bulk Materials Handling, Vol

1, pp 307-325. Ed M C Hawk, Univ Pittsburgh Sch Mech Eng. 1971.10. M.N. Kraus. Pneumatic Conveying of Bulk Materials. The Ronald Press Co. New

York. 1968.1 1 . D. Martin. No-transfer-point open conveyor. Process Engineering, p 39. July 1974.12. R.E. Filter. Conveying solids with co-operating series of air jets. ASME paper 68-

MH-31. 1968.13. L. Kovacs and S. Varadi. Conveying granulated sugar through aerated channels. Proc

2nd Conf on Pneumatic Conveying, paper BIO, pp 131-139. Pecs, Hungary. In Ger-man. March 1978.

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536 Chapter 18

14. W. Stegmaier. Horizontally conveying pneumatic chutes. Forden und Hebcn, Vol 26.No 6, pp 621-624. In German. 1976.

15. K. Shinohara and T. Tanaka. A new device for pneumatic transport of particles. JnlChem Kng of Japan, Vol 5, No 3, pp 279-285. 1972.

16. K. Shinohara, K. Hayashi, and T. Tanaka. Residence time distribution of particleswith pneumatic escalator. Jnl Chem Eng of Japan, Vol 6. No 5. pp 447-453. 1973.

17. W. Islcr. An air-slide type conveyor for horizontal and upward inclined transport.Zement-Kalk-Gips. Vol 10, pp 482-486. In German. 1960.

18. EEUA Handbook No 15. Pneumatic handling of powdered materials. Constable andCo, London. 1963.

19. J-P. Hanrot. Multipoint feeding of hoppers, mounted on aluminum smelter pots, bymeans of potential fluidization piping. Proc 115th An Mtg The Met Soc of AIME, pp103-109. New Orleans. March 1986.

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19Commissioning and ThroughputProblems

1 INTRODUCTION

The inability to convey a material, pipeline blockages, and systems not capable ofmeeting the required duty are, unfortunately, not uncommon problems to be ex-perienced with pneumatic conveying systems, and particularly so at the time ofcommissioning a system. The cause of a particular problem, however, is not al-ways obvious and it may require some 'detective' work to identify the cause of theproblem. It might relate to the type of system, the system components, or to thematerial being conveyed.

Problems often occur if a change is made to a system, the most common be-ing a change of conveyed material and distance transported. Pipeline blockages area major problem, and particularly so if they occur intermittently, or start to occurafter a period of reasonable operation. Explanations for these and many other oc-currences are given and solutions are proposed.

Although pneumatic conveying is essentially a very simple process, the fac-tors that influence system design are varied and complex. Most component speci-fications are based on data resulting from the design of the pipeline. Since the dataused in pipeline design is not always totally reliable, many systems incorporatemargins and factors to allow for uncertainties. This often leads to a mismatch be-tween components and over design in certain areas. Although over design willgenerally ensure that a system will work, it wi l l rarely work efficiently [ 1 1 .

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538 Chapter 19

If an unnecessarily high conveying air velocity is employed, for example,the system will almost certainly work, but it is quite likely that a higher materialflow rate could be achieved with a lower air flow rate, and hence a lower powerrequirement. In addition, the higher velocity could add to degradation, filtrationand erosive wear problems.

One of the major difficulties with pneumatic conveying systems is that it isnot always obvious what effect a change in operating conditions wil l have on sys-tem performance. A change of material or conveying distance, for example, mayrequire changes in both material feed rate and air flow rate. Problems relating tosystem throughput in terms of pipeline blockage are potentially the most seriousand so these are considered in some detail.

The inability to convey a material, frequent pipeline blockages, and systemsnot capable of meeting the required duty, are some of the problems that are alsoconsidered. Particular note of changes in performance that might occur with re-spect to time should be made, for these should not occur with a pneumatic convey-ing system, and could well lead to failure over a period of time.

2 PIPELINE BLOCKAGE

One of the most frustrating problems encountered in system operation is that of apipeline blockage. As there are so many different circumstances and possiblecauses, this section has been sub-divided for quick reference. The time that ablockage occurs and the nature of the blockage are useful indicators of the poten-tial cause and so it would be recommended that these details should be logged.

2.1 General

In any pipeline blockage situation the first thing to do is to check all the obvioussystem features:

Is the reception point clear?Are the diverter valves operating satisfactorily?Is the full conveying air supply available?

; Was the pipeline clear on start up?Has a pipeline bend failed?

L; Are feeder controls correctly set?

The problem may relate to system components, such as the feeding device,air mover or filter, and so reference should be made to the notes on the specificplant components. It may be a material related problem, such as particle size ormoisture, and so this section should also be checked. The appropriate section onTypes of System could also be consulted.

The time of the day and year that it occurs, together with the prevailingweather conditions, and the nature of the blockage, are useful indicators of the

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potential cause, and so the following notes wil l deal with these possibilities andcases.

2.2 On Commissioning

If the pipeline blocks during commissioning trials with the pneumatic conveyingsystem, it could indicate that there is either a serious design fault with the system,or some simple adjustment needs to be made to the plant.

2.2.1 Incorrect A ir Mover Specification

If it is the former, the most likely reason is that the air mover is incorrectly sizedfor the duty. If the volumetric flow rate of air available for conveying the materialin the pipeline is insufficient, it is unlikely that it will be possible to convey thematerial. A minimum value of conveying air velocity must be maintained at thematerial pick-up point at the start of the conveying line.

The value depends upon the material being conveyed and, for materials thatare capable of being conveyed in dense phase, in moving bed type flow, varieswith the solids loading ratio at which the material is conveyed. Since air is com-pressible it is extremely important that the pressure of the air at the material pick-up point, is taken into account in evaluating the free air requirements for the airmover specification.

2.2.1.1 Conveying Air VelocityBecause air is compressible, with respect to both temperature and pressure, thestarting point in the determination of any relationship for the determination ofconveying air velocity is the Ideal Gas Law. The appropriate model for checkingconveying air velocity is:

576 p() V, T,C, = f° ° ' ft/min (1)

n d p}T0

where C, = conveying line inlet air velocity -f t /minPi = conveying line inlet air pressure - Ibf/in2 absTI = conveying line inlet air temperature - Rd = pipeline bore - in

Vo = volumetric flow rate

at free air conditions - ftVminPo = free air pressure = 14-7 Ibf/in2

and TO = free air temperature = 519 R

The models from which this equation was derived were presented in Equa-tions 4 to 6 in Chapter 5. It should be noted that absolute values of both pressureand temperature must be used in this equation. The air flow rate is at free air con-

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540 Chapter 19

ditions, which are the reference conditions that are usually given as part of the airmover specification. Substituting values for free air conditions gives the modelderived at Equation 11 in Chapter 5:

T\ voC, = 5 - 1 9 1

2 f t / m i n - - - - - - - ( 2 )d p}

Equation 2 can be used to provide a check on the conveying line inlet air ve-locity, given the free air flow rate of the air mover and other parameters. If theconveying air velocity evaluated is too low for the given material and conveyingconditions then the pipeline is likely to block.

2.2.1.1.1 Pipeline Bore InfluencePipeline bores quoted are 'nominal ' sizes only since it is generally the outer di-ameter that is standardized because of the needs of flanging and threading. Thediameter of a 4 inch nominal bore pipeline, therefore, is rarely 4 inches. If a con-veying air velocity is based on a diameter of 4 inches, for example, and it is aschedule 10 pipeline, the actual bore will be 4-176 in (see Table 3.1) and not 4-000in. This difference will mean that the conveying air velocity will be about 9%lower. If 3000 ft/min is the velocity in a 4 inch bore pipeline, it wil l only be 2750ft/min in a 4-176 in bore line and the pipeline is likely to block if the minimumconveying air velocity for the material is 3000 ft/min.

2.2.1.1.2 Conveying Gas InfluencesAlthough air is used for the vast majority of pneumatic conveying systems, othergases such as carbon dioxide and superheated steam can be used for specific ap-plications. Nitrogen is often used if the material is potentially explosive. Theabove equation, in terms of velocity and volumetric flow rate will apply to anygas, but because the characteristic gas constant, R, for each is different, then thedensity of each gas wil l be different.

If densities or mass flow rates have to be used in any calculation, therefore,Equation 5.4 (The Ideal Gas Law) will have to be used and the appropriate valueof R will have to be applied (see Table 5.1 and Equation 5.9).

2.2.1.1.3 Solids Loading RatioIt is the velocity at the material feed point, at the start of the conveying line, that isimportant. If this velocity is too low the pipeline is likely to block. For materialsconveyed in dilute phase, or suspension flow, it is necessary to maintain a mini-mum velocity of about 2100 to 3200 ft/min, depending upon the size, shape anddensity of the particles, as mentioned in previous chapters. For materials capableof being conveyed in dense phase, however, the minimum velocity can be as lowas 600 ft/min.

For fine powders, such as cement, flour and fly ash, that are capable ofdense phase conveying in moving bed type flow, the value of minimum velocity isdependent upon the solids loading ratio at which the material is conveyed. Only at

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high values of solids loading ratio can the conveying air velocity be very low withfine powders.

If the material is conveyed in dilute phase, at a low value of solids loadingratio, velocities appropriate to the dilute phase conveying of a fine material mustbe used. There is, therefore, a gradual transition between dilute and dense phase,with respect to minimum conveying air velocity. These relationships, for typicalmaterials only, are illustrated graphically in Figure 19.1.

To ensure successful conveying, therefore, the conveying line inlet air ve-locity must be above these minimum values, whether the material is conveyed indilute or dense phase. Figure 19.1 shows quite clearly the transition from highvelocity suspension flow, at low values of solids loading ratio, and hence dilutephase flow, to low velocity dense phase flow at high values of solids loading ratio.

As a consequence of the much higher minimum values of conveying air ve-locity required for the dilute phase suspension flow of granular materials, and theadverse effect of velocity on pressure drop, it is not possible to achieve high valuesof solids loading ratio in di lute phase conveying, even if a high air supply pressureis available.

2.2.1.2 Air Mover ChangeIf pipeline blockages occur and it is found that the conveying line inlet air velocityis too low, then an air mover with a higher volumetric flow rate wil l have to beused.

3000 L.Suspension Flow-

Dense Phase Flow(moving bed type)

40 60

Solids Loading Ratio

80 100

Figure 19.1 Influence of solids loading ratio and material on minimum conveying airvelocity.

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542 Chapter 19

If it is replaced with one having a higher delivery pressure, as well as ahigher volumetric flow rate, Equation 2 must be checked again, because air supplypressure also has a significant influence on conveying line inlet air velocity.

It is equally important that any replacement is not over-rated. It is not gener-ally necessary for the conveying line inlet air velocity to be any higher than about20% greater than the minimum conveying air velocity value. If it is in excess ofthis it is likely to have an adverse effect on the material flow rate, as well as onpower requirements.

2.2.1.3 Conveying LimitationsAnother useful graph to illustrate the influence of minimum conveying conditionsis a plot of conveying line pressure drop drawn against air flow rate, with lines ofconveying line pressure drop superimposed. Such a graph for cement conveyedthrough the Figure 4.2 pipeline is presented in Figure 19.2.

The empty line, or zero material flow rate curve, provides a useful datum forthe relationship, for it shows just how much pressure is required to get the airthrough the given pipeline before any material is conveyed. This is a 'square law'relationship, and hence the gradual upward trend of pressure drop with increase inair flow rate, and hence velocity.

Material Flow Rate- l b / h x 1000

30

20

10

40 80 120 160

Free Air Flow Rate - ftYmin

200

Figure 19.2 The influence of material and air flow rates on conveying l ine pressuredrop for cement.

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The conveying limit identified represents the minimum conditions for suc-cessful conveying with the material. The lines of constant material flow rate actu-ally terminate, and conveying is not possible in the area to the left, at lower airflow rates. Any attempt to convey with a lower air flow rate would generally resultin blockage of the pipeline.

2.2.1.4 Influence of Material TypeThis limit to conveying is influenced very significantly by material type, as wasillustrated with Figure 19.1. The cement data in Figure 19.2 follows a similarminimum limit set by the lower curve in Figure 19.1. As the cement is capable ofbeing conveyed in dense phase, conveying with low values of air flow rate hasbeen possible with high values of conveying line pressure drop.

2.2.1.4.1 Dense Phase ConveyingAlthough Figure 19.1 shows that the minimum value of conveying air velocity canbe reduced, as the solids loading ratio increases, this reduction in velocity cannotbe directly related to air flow rate. An increase in solids loading ratio requires anincrease in air pressure and, as will be seen from Equation 2, an increase in pres-sure results in a decrease in velocity. The resulting conveying limit represented onFigure 19.2, therefore, is a complex shape resulting from an interaction of all ofthese variables.

2.2.1.4.2 Dilute Phase ConveyingIn Figure 19.3 similar data for a material not capable of being conveyed in densephase, and hence at low velocity, in a conventional pneumatic conveying system,is presented. The limit to conveying for this material is set by the upper curve inFigure 19.1. The material was a sandy grade of alumina and was also conveyedthrough the Figure 4.2 pipeline. A very significant difference in material flow ratecapability will be noticed.

The minimum material conveying l imit on Figure 19.3 is very regular. Thisis because it is defined only by a minimum conveying air velocity of about 2700ft/min. This means that the line drawn, representing the conveying limit , alsorepresents a line of constant conveying line inlet air velocity of 2700 ft/min. Thisillustrates the influence of air supply pressure on conveying very well, and showsthat great care must be exercised in operating such a conveying system, for it isvery easy to cross the conveying limit and block the pipeline.

For a given conveying system the air flow rate is generally fixed by thevolumetric capability of the air mover, and so with any change of delivery pres-sure the operating point for the system will be along a vertical line on either Figure19.2 or 19.3, at the appropriate air flow rate.

Another point to note with regard to Figure 19.3, and hence to dilute phaseconveying systems in general, is that the upper limit is not necessarily set by thepressure capability of the air mover. At a pressure of about 30 lbf/in2 gauge themaximum volumetric flow rate of air from the compressor is only just sufficient tomaintain the material in suspension flow. Because of the high velocity required,volumetric flow rate is the limiting factor in this case.

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544 Chapter 19

30

Q 20

coU

10

Material Flow Rate-Ib/h x 1000

40 80 120 160Free Air Flow Rate - fVVmin

200

Figure 19.3 The influence of material and air flow rates on conveying l ine pressuredrop for sandy alumina.

Air was available at a pressure of 100 psig on the test facility used, andcould have been used up to that l imit with the cement shown on Figure 19.2. Withsufficient air, conveying of the sandy alumina would be possible at much higherconveying line pressure drop values, but care must be exercised, for with singlebore pipelines very high conveying line exit air velocities can result.

2.2.1.5 Air Leakage Allowance

It is important that the VQ term in Equations 1 and 2 is the volumetric flow rate of

the air used to convey the material in the pipeline. If, in a positive pressure con-veying system, part of the air supply from the air mover is lost by leakage acrossthe material feeding device, this must be taken into account. A similar situationoccurs with negative pressure systems with ingress of air into the system, particu-larly through valving used to off-load material from the reception hopper. Thesepoints are discussed in more detail later.

2.2.2 Over Feeding of Pipeline

The pressure gradient in the conveying line is primarily dependent upon the con-centration of the material in the pipeline, or the solids loading ratio. If too muchmaterial is fed into the conveying line it is possible that the pipeline could becomeblocked. There are two possible reasons for this. One is related to compressor de-livery capability and the other is concerned with material conveying capability.

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System Capability Problems 545

These effects can be illustrated with reference to either Figure 19.2 or 19.3.The operating point on these curves will move vertically up as there is unlikely tobe a significant change in air flow rate. There will, in fact, be a slight decrease inair flow rate with increase in pressure as a consequence of the operating character-istics.

2.2.2.1 Compressor CapabilityIf a pipeline is over fed the pressure required may exceed that available from theblower or compressor and the line will block. It will, therefore, be necessary toreduce the material feed rate to match the capability of the air mover or its drivemotor. It could be either the compressor or the drive motor that imposes the limita-tion. The operating characteristics of the compressor and the specification of themotor could be checked to determine the exact cause of the problem.

This effect can be illustrated with reference to either Figure 19.2 or 19.3. Inthe case of cement in Figure 19.2 it will be seen that approximately 20,000 Ib/h ofcement can be conveyed with a conveying line pressure drop of about 19 lbf/in2 . Ifthe material flow rate was to increase by 25% to 25,000 Ib/h, a conveying linepressure drop of about 22 lbf/in2 would be required. If the compressor was onlycapable of delivering air at 20 psig, the extra 5000 Ib/h would probably result inpipeline blockage.

2.2.2.2 Material CapabilityWith a material such as the sandy alumina, shown in Figure 19.3, it will be seenthat an increase in pressure demand, due to over feeding of the pipeline, couldeasily result in pipeline blockage, particularly if the normal operating point for theconveying system was close to the conveying limit.

This effect can be illustrated on Figure 19.3. If a compressor delivers 140ft7min of free air and 10,000 Ib/h of material is being conveyed, the conveyingline pressure drop will be about 12 lbf/in2 and the system will operate satisfacto-rily. If the material flow rate increases to 15,000 Ib/h, there is unlikely to be anysignificant change in the air flow rate, and so the operating point will shift verti-cally up. This wi l l take the operating point across the material conveying limit,into the no go area, and the pipeline will probably block as a result. In such a casea compressor with a higher volumetric flow rate capability would be needed.

If, in the above case, the air was supplied by a positive displacement typeblower there might be some problem in determining the exact cause of pipelineblockage. Blowers typically have an upper l imit of about 15 psig as a deliverypressure and it is with a conveying line pressure drop close to about 15 psig thatthe system could be expected to fail, as this is where it would cross the materialconveying limit.

With any material conveyed in dilute phase, therefore, great care must beexercised when increasing the material feed rate, and hence air supply pressure,for if the air mover was under-rated in terms of either pressure or volumetric flowrate, the conveying limit could be crossed and the pipeline could block. For any

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546 Chapter 19

conveying system, and material to be conveyed, this l imit can be established quiteeasily by reference to Equation 19.2.

2.2.2.3 Feeder ControlEach type of pipeline feeding device has its own characteristic means of control-ling the material flow. With positive displacement feeders this is achieved directly:either by means of speed control, as in the case of rotary valves and screws; or byfrequency of operation, as in the case of double gate valves. In others, additionalflow control devices are required, as with venturi feeders. In the case of blowtanks and suction nozzles, control is achieved by means of air supply proportion-ing.

Feed control is very important at the time of commissioning a plant, particu-larly with conveying systems employing positive displacement feeders. Becauseof the expense, these feeders are not generally supplied with a variable speeddrive, unless there is a particular requirement during operation of the plant to beable to vary the feed rate. With rotary valves, for example, there is often a problemwith achieving fine control of feed rate, since a change of just one or two rev/mincan have a significant effect on material flow rate. On commissioning, therefore, itis essential that a means of obtaining a reasonable degree of speed control shouldbe provided, either side of the estimated value, so that fine control of the flow ratecan be achieved.

2.2.2.4 Performance MonitoringIt is often difficult to assess whether a pipeline blockage results from an incorrectair mover specification, or over feeding of the pipeline, as discussed above. For apositive pressure system this can be established quite easily if there is a pressuregauge in the air supply line just before the material feed point into the conveyingline [2], A typical arrangement is shown in Figure 19.4.

Compressoror Blower

Discharge""*" *• HopperAir Air-Solids Flow

Figure 19.4 Performance monitoring of a positive pressure conveying system.

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In a negative pressure, or vacuum conveying system, the pressure gaugewould have to be located in the discharge air pipeline between the filtration unitand the inlet to the exhauster, as shown in Figure 19.5. Such a pressure gauge, ineither the positive or negative pressure system, wil l give a reasonably close ap-proximation to the conveying line pressure drop, for the pressure drop in the shortsection of the air supply line will be small in comparison. The pressure gauge willalso work more reliably in the air line than it will in the material conveying line.

Note that many of the comments that follow refer only to positive pressureconveying systems, but they are generally equally applicable to negative pressuresystems. This is simply to avoid making the text unnecessarily long in referring totwo different cases at every juncture. The main difference between positive andnegative pressure conveying systems is in the specification of the volumetric flowrate of the air, since that for exhausters is generally in terms of exhauster inletconditions.

If, on starting a system, the pressure is seen to rise rapidly to the maximumcapability of the compressor, it will be clear that the pipeline is being over fed, andso the feed rate should be reduced. If the pressure is at the design value or below,and the pipeline blocks, it would indicate that the volumetric flow rate is insuffi-cient.

A pressure gauge in the air line is particularly useful for monitoring the per-formance of a system. If the pressure reading is below the design value, for exam-ple, it would indicate that the performance of the system has been under estimatedand that it would be possible to feed more material into the pipeline. Care must beexercised here, however, and the air velocities should be checked as mentionedabove, for an increase in air supply pressure will result in a lowering of the con-veying line inlet air velocity.

Filter

Storage Hoppers Air

AirDischarge

Hopper

Figure 19.5 Performance monitoring of a vacuum or negative pressure pneumaticconveying system.

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548 Chapter 19

The use of pressure gauges, such as those shown on Figures 19.4 and 19.5would also be invaluable in achieving the correct balance between material feedrate, and air supply pressure and flow rate, if a change in either conveying distanceor a change in material conveyed were to be made.

2.2.2.5 Influence of PressureThe influence of pressure, as it is such an important parameter, is illustrated fur-ther in Figure 19.6. This is a graph of conveying air velocity plotted against a nar-row band of air pressure. It is derived from Equation 2, for a fee air flow rate of1000 ftVmin at 59°F in a 6 in bore pipeline. It should also be noted that with mostpositive displacement air movers there is a slight reduction in volumetric flow ratewith increase in delivery pressure, and this will magnify the effect and make theslope of the curve slightly steeper.

Figure 19.6 is drawn on a magnified scale and shows quite clearly the verysignificant effect that changes in pressure can have on conveying air velocity.Some fine granular materials, such as sand, sugar and alumina, are very sensitiveto small changes in conveying air velocity. This can be the situation if a dedicatedair mover is used for a conveying system and is operated at constant speed.

Silica sand, for example, will convey very reliably with a conveying line in-let air velocity of 2700 ft/min, but if it drops to only 2600 ft/min the pipeline willblock within seconds. Granulated sugar, having a mean particle size of about 500micron (35 Mesh), is a similar material, that wi l l convey reliably with a conveyingline inlet air velocity of 3300 ft/min but will rapidly block the pipeline if the veloc-ity falls to 3200 ft/min. It only requires a small change in air supply pressure, for agiven air flow rate, to result in this change in conveying air velocity.

3600 '

3200

< 2800oc

2400

Pipeline bore - 6 inAir Temperature - 59°F

10 12 14Air Pressure - Ibf/irr gauge

16 18

Figure 19.6 Influence of pressure on conveying air velocity.

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2.2.3 Non Steady Feeding of Pipeline

If the pipeline blocks only occasionally, it is possible that this may be due tosurges in the material feed rate. For a system that is operating close to its pressurelimit, a momentary increase in feed rate could raise the material concentration to alevel that may be sufficient to block the line.

This can be seen by reference to Figures 19.2 and 19.3 once again. Any in-crease in material flow rate will require a corresponding increase in conveying linepressure drop. It is very approximately a linear relationship, and so a 10% increasein material flow rate will require a 10% increase in air supply pressure. If this pres-sure is not available, a momentary surge in feed rate could result in a blockedpipeline.

2.2.3.1 CommissioningIn addition to determining the mean flow rate on commissioning, the regularity ofthe flow rate over short periods of time should also be assessed. This is necessaryto ensure that these fluctuations will not overload the system. This is particularlythe case with dilute phase conveying systems and short pipelines. If the meanvalue of conveying air velocity in a pipeline system is 4000 ft/min, for example,and the pipeline is 300 ft long, it will only take 4'A seconds for the air to traversethe entire length of the pipeline. If the slip ratio of particles to air in the pipeline is0-8 the material will be conveyed through the pipeline in about six or seven sec-onds. It will be seen from this that only short momentary surges in feed rate couldoverload a pipeline.

It is essential, therefore, that both the compressor and the motor drive arespecified with adequate margins. The compressor should be capable of deliveringair at a pressure slightly higher than that required, and at a corresponding volumet-ric flow rate. Then the motor drive for the compressor should have sufficient sparepower capacity to meet the demand of any possible surges. This is where airmover operating characteristics presented in the form shown in Figures 3.5 and 3.6are particularly useful.

2.2.3.2 Control SystemsA useful aid is to fit differential pressure switches to all air movers and link theseto the material feeder so as to stop the feed in an over-pressure condition. Thisgives the system a chance to clear and it can be arranged to bring the feed back onagain automatically.

2.2.3.3 Feeding DevicesMaterial surges have to be considered in relation to the type of feeding deviceused. In this respect, positive displacement, volumetric devices need particularconsideration. A rotary valve, for example, with eight blades and rotating at 15rev/min will empty about two pockets of material every second. For most purposesthis frequency is sufficiently high, but with a short pipeline due care should betaken with such a feeder. Double flap valve type feeders, cycling at 10 to 20 times

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550 Chapter 19

a minute, clearly present a problem as this could be too coarse for many materialsand duties.

2.3 On Start Up

If a pipeline has a tendency to block when the system is started up after a shutdown period, some transient situation may be responsible. It is quite possible thatthe system will operate satisfactorily under normal load conditions [3].

2.3.1 Moisture in Line

If material is blown into a cold pipeline it is possible that the inside surface couldbe wet as a result of condensation. This is liable to occur in pipelines that are sub-ject to large temperature variations, particularly where there are pipe runs outsidebuildings. If air drying is not normally necessary, the problem can be overcomeeither by trace heating of exposed sections of the pipeline, or by blowing the con-veying air through the pipeline for a period to dry it out prior to introducing thematerial. Lagging may be sufficient in some cases.

This point is illustrated in Figure 19.7, which was presented earlier in Figure5.17. This is a graph that shows the variation, with temperature, of the mass ofwater that can be supported as vapor in saturated air. If the temperature rises, for agiven mass of water vapor, the humidity will decrease and air will become drier. Ifthe temperature falls, however, condensation will take place and the humidity willremain at 100%. For initially saturated air, therefore, Figure 19.7 can be used todetermine the mass of water vapor that will condense for a given change in tem-perature.

Moisture is often a problem in general high pressure plant air supplies. If aplant air used is used it would be wise to incorporate a moisture separating device.If the inside surface of a pipeline is wet, as a result of condensation, fine materialwill tend to stick to the wall surface. This is particularly a problem at bends priorto a vertical lift. Moisture condensing on the surface of the vertical pipeline willtend to drain down to the bend at the bottom and collect as a pool of water. It de-pends upon the nature of the material being conveyed, and its interaction with wa-ter, as to what wi l l happen when the material meets the water.

In many cases a hard scale wil l form, and this wi l l gradually accumulatewith successive cycles of condensation and conveying, to a point where the buildup adds significantly to the pipeline resistance. For a conveying system operatingclose to its pressure limit the added resistance could result in pipeline blockage.

2.3.1.1 Air Drying SystemsAir can be dried either by refrigerating or by chemical means. The decision de-pends upon the level of drying required. The quantity of water in air, as a functionof temperature, can be seen in Figure 19.7. The lower the air temperature (for re-frigeration), or the dew point (for chemical dryers), the less moisture there will bein the air. This particular topic was considered in detail in Chapter 3.

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0-6

r, 0-5<Koo2 0-425

i °-3oU Q-2

Saturated Air atAtmospheric Pressure

ooo

coU

o

-40 -20 0 20 40 60 80 100 120 140Air Temperature - °F

Figure 19.7 The influence of temperature on water vapor in air.

The capability of air for supporting moisture wi l l decrease with both a de-crease in temperature and an increase in pressure. If air is compressed isother-mally, or is compressed and allowed to cool before use, condensation will occur ifthe ambient air being compressed has a sufficiently high relative humidity. Provi-sion, therefore, must be made to drain this condensate.

The compression process, however, occurs very quickly and complete con-densation may not take place. Condensed water in the form of a fog or mist is of-ten conveyed with the air and can be transported through pipelines over long dis-tances. It is not always advisable, therefore, to rely on the compression process todry the air.

2.3.2 Cold Air

The density of air decreases with increase in temperature. In normal operation thedelivery temperature of the air from an air mover, such as a positive displacementtype blower, could be some 120 degrees F higher than the inlet temperature. Thismeans that the volumetric flow rate, and hence the conveying air velocity, couldbe 20% to 25% greater than the value at ambient temperature. On start up, how-ever, the air will initially be fairly cold for conveying the material, and so if theresulting conveying air velocity is below that necessary for the material, the pipe-line could block.

This point is illustrated in Figure 19.8. This is a graph of conveying air ve-locity plotted against a narrow band of air temperature. It is derived from Equation2 once again, for a free air flow rate of 1000 ft /min at a pressure of 15 psig in a 6

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552 Chapter 19

in bore pipeline. Figure 19.8 shows that conveying air velocity is quite sensitive totemperature, as well as pressure (Figure 19.6). It must never be forgotten that air iscompressible with respect to temperature as well as pressure.

Since air density increases with decrease in temperature, it is essential thatair requirements are based on the lowest temperature that is likely to be experi-enced. Thus a cold start up in winter with the lowest possible air and material tem-peratures must be catered for. Conveying line inlet air temperature alone shouldnot be used in evaluating conveying line inlet air velocity. Although the high tem-perature air delivered by a compressor might give a satisfactory value of velocity,the temperature of the material being fed into the pipeline must also be taken intoaccount The equilibrium temperature for conveying air and material at differenttemperatures was considered in Chapter 5.

2.3.2.1 Air Flow Rate ControlIf meeting the air flow requirements for the lowest temperature results in exces-sively high conveying air velocities during normal operation, then some means ofcontrolling the air flow rate to the conveying line must be incorporated. Variablespeed control of the air mover, choked flow nozzles in a by-pass air supply line,and the discharge of part of the air to atmosphere via a control valve, are some ofthe methods that could be considered for the control of the air flow rate to thepipeline for normal operation.

3200

I 3000

2800o

2600

2400

2200

2000

Pipeline Bore - 6 inAir Pressure - 15 psig

-20 0 20 40 80 120

Air Temperature - °F

Figure 19.8 The influence of temperature on conveying air velocity.

160 200

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System Capability Problems 553

2.3.3 Material in Pipeline

If, when the plant is shut down, the pipeline is not purged, a quantity of materialcould be left in the line. If the conveying line incorporates a long vertical lift sec-tion, sufficient material could accumulate in the bend at the bottom to prevent thesystem from being re-started. It is always a wise precaution on start up to blow airthrough the pipeline before material is introduced. If the pipeline was not purgedon shut down, there may be sufficient material left in the pipeline to cause block-age of the pipeline during start up. If the pipeline is already blocked it will consid-erably aggravate the situation if more material is blown into the pipeline.

2.3.3.1 Reference PressureIf there is a pressure gauge in the air supply or extraction lines, as shown in Figure19.9, the condition of the pipeline can be monitored. The reference value of pres-sure drop is that of the air only being blown through the pipeline. If this value isknown it can be compared with the air purge value. If the actual pressure drop issignificantly higher than the empty line value it would indicate that there is a largeamount of material still to be removed from the pipeline.

Regular monitoring of this air only pressure drop value will also help todetect whether there is any gradual increase in pipeline resistance. Some materialswill stick to bends when conveyed and form a hard scale which may be difficult toremove by purging. Any moisture in the air, or on the material, will tend to wet thesurface of the bends, as a result of the centrifugal force, and fines in the materialmay then adhere to the bend. Condensation occurring within the pipeline can havea similar effect, as mentioned earlier. These processes tend to be cumulative andwill ultimately result in pipeline blockage if the same material flow rate is main-tained, because of the increased pipeline resistance.

Filter

Compressoror Blower

Air OnlyValue

Storage Siloor Hopper

DischargeHopper

\Material Feed Device

X VerticalLift

Figure 19.9 Material purging from pipeline.

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2.3.3.2 Pipeline PurgingIt is also a wise precaution to purge the pipeline prior to shut down. The effective-ness of the purging can be determined by comparing the actual pressure drop withthe empty line value. If it is essential that the pipeline should be completelycleared of material, then the purging must continue until the empty line value isreached. If complete emptying is not required, then the purging process can beterminated when the pressure reaches a given set value.

2.3.4 After Unexpected Shut DownIf conveying stops unexpectedly due, for example, to a power supply failure, itmay not be possible to start the system again, particularly if the pipeline incorpo-rates a large vertical lift. If the bends at the bottom of any vertical sections aretaken out, to remove the material at these points, it may be possible to purge theline clear, if the pipeline is not too long. Should this be a common occurrence on aplant, an air receiver could be fitted between the air mover and the material feed-ing device. Such an arrangement is illustrated in Figure I9.10. If the material feedinto the pipeline is stopped at the instant the power supply fails, the air stored inthe receiver could be sufficient to purge the line clear of material.

It should be noted that in the various sketches used to illustrate the pointsbeing discussed, different types of system and material feeding arrangements areshown. This is simply to add variety to the notes and avoid repetition. In mostcases the modifications to the plant suggested can be applied to any type of pneu-matic conveying system and can be utilized with any type of feeder.

In Figure 19.10 a blow tank is shown to illustrate the point that considera-tion should also be given to the material feeder. With a rotary valve and screw, thematerial feed will automatically be stopped, but it may not be with a blow tank. Itis essential that the material feed should be stopped if the power to the air moverfails. In this event an outlet valve should be provided on the blow tank, with ar-rangements made for this to close in the event of a power failure.

MaterialFeed

System

Figure 19.10 Use of air receiver in air supply line for pipeline purging.

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2.4 After A Period of Time

If a system, that has operated satisfactorily for a long period of time, starts to givetrouble with blocked pipelines, wear of the feeding device could be the cause ofthe problem in the case of positive pressure systems. If the air leakage across thefeeding device increases, the air available for conveying the material will decrease.If the loss of air is too great, it is possible that the volumetric flow rate of air that isleft will be insufficient to convey the material and the pipeline will block.

Wear of screw flights, valve seatings of gate lock valves, and rotary valveblades and housings, will all result in a greater leakage of air across the respectivefeeding device. It is also possible that gradual deterioration in performance of theair mover will have a similar effect, both for positive and negative pressure con-veying systems. Positive displacement air movers operate with very fine clear-ances and generally cannot tolerate dust. If they are operated in a dusty environ-ment, and inlet filters are not maintained, wear will occur, particularly if the dust isabrasive. Exhausters on negative pressure conveying systems, and blowers usedwith closed loop conveying systems, are particularly vulnerable.

2.4.1 Component Wear

The situation with respect to a rotary valve feeding a positive pressure pneumaticconveying system pipeline is shown in Figure 19.11 by way of an example. Airwill leak across the rotary valve, via the empty pockets and the blade tip clear-ances, because of the pressure drop across the valve.

\ Material /

Housing

RotorFan orBlower

Supply Air - l's

Hopper

Leakage Air - VL

3 Note : Vc = Vs - Ks -> L* * * * * * *

Conveying Air - Vc

Figure 19.11 Air flow rate analysis For positive pressure conveying system having arotary valve feeder.

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In low pressure rotary valves, without end plate sealing, air will also leak be-tween the ends of the rotor blades, or the end plate, and the rotor housing. Thevolumetric flow rate of air delivered from the blower or compressor should bespecified to take this leakage into account, in order to ensure that there is sufficientair to convey the material through the pipeline.

Most manufacturers of rotary valve feeders provide data on air leakageacross their rotary valves so that this can be taken into account in the specificationof air requirements for air movers. The air flow rates to be taken into account areillustrated on Figure 19.11. The leakage rate depends primarily on the size of therotary valve, the blade tip clearance and the pressure drop across the valve. Rotorspeed and the nature of the material being handled can also have an influence. Theair leakage rate data, however, only relates to a new valve.

If there is wear, because of handling an abrasive material, blade tip clear-ances will increase, and there will be an increase in air leakage. If the air leakageincreases, less air will be available to convey the material. If the leakage is suchthat it results in the conveying air velocity falling below the minimum value, thepipeline will block.

These components should be checked for wear. The performance of the airmover should also be checked, as this might be responsible, as mentioned above.In the short term an increase in air loss across a feeding device can be compen-sated by increasing the air flow rate. In the long term, however, it is recommendedthat worn components should be replaced.

2.4.2 Pipeline Effects

The influence of a gradual increase in air leakage across a feeding device, or agradual reduction in performance of an air mover, is depicted on Figure 19.12.This is typical data for an abrasive granular material conveyed in dilute phase. Thesame situation will occur with a vacuum conveying system if the performance ofthe exhauster deteriorates or if there is an increase of air flow into the receptionhopper via the discharge valve.

From this it will be seen that the system would operate with a conveyingline pressure drop of about 14 Ibf/in2 , and 122 ftYmin of air was initially availablefor conveying the material. This provides the necessary margin of 20% on mini-mum air flow rate, and hence conveying line inlet air velocity, to ensure reliableconveying of the material. With gradual wear, of the feeder or air mover, however,the air flow rate available for conveying the material wi l l gradually reduce.

The material flow rate is likely to remain at a constant rate and as a conse-quence a slight reduction in conveying line pressure drop might be observed. Witha blow tank feeding the pipeline the pressure may remain constant and so an in-crease in material flow rate might be observed. It might be considered that thesystem is 'settling in', but this rarely happens with pneumatic conveying systems.Ultimately the air supply available will be insufficient to convey the material re-liably and the pipeline will block.

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ooo

_0

D-c

80

60

40

20

Conveying LinePressure Drop

- lhf/in2

NO GO AREA

ConveyingLimit

Solids LoadingRatio

50 100

Free Air Flow Rate - ft'/min

150

Figure 19.12 The influence of a gradual reduction in conveying air flow rate on sys-tem performance.

2.5 With New Material

It is quite possible that a system that operates satisfactorily with one material willbe completely unable to convey another material, or even a different grade of thesame material.

Minimum conveying air velocities differ markedly from one material toanother, as was illustrated with Figure 19.1. For given conveying conditions, of airflow rate and air supply pressure, different flow rates wil l be achieved with differ-ent materials. Great care must be exercised in designing any system in which morethan one material is to be conveyed.

2.5.1 Conveying Capability

This point is illustrated in Figure 19.13 which is an abbreviated version of Figure4.22. This is a plot of material flow rate drawn against air flow rate for a range ofdifferent materials. The curves represent the constant pressure drop line of 20lbf/in2 taken from the conveying characteristics for each material. They all relateto the material conveyed through the Figure 4.22 pipeline and so are all directlycomparable.

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558 Chapter 19

50

§ 40

30

•2 20

Pulverized Fuel Ash - Fine

Barite

Cement

Copper Concentrate,

PVC Powder

Polyethylene Pellets Magnesium SulfateI I I I 1 I t 1 L I 1 I I

50 100Free Air Flow Rate - ftj/min

150 200

Figure 19.13 Influence of material on conveying capability for identical pipel ine andconveying conditions.

The problem is illustrated very well with Figure 19.13. The materials testedcover a very wide range of material types, sizes and densities, and include repre-sentatives of each of the three main modes of conveying. Sandy alumina and mag-nesium sulfate are typical of materials that can only be conveyed in dilute phase,despite the fact that a high air supply pressure was used and the pipeline was rela-tively short.

Fine pf ash, barite and cement were all capable of being conveyed at verylow velocity in dense phase in a moving bed type flow. The polyethylene pelletswere also capable of being conveyed at very low velocity in dense phase, but inthis case it was plug type flow. The copper concentrate is typical of materials thathave limited dense phase conveying potential, and PVC powder is typical of manymaterials from the chemical industry.

2.5.2 Material Testing

It is also possible for different grades of the same material to give totally differentconveying line performances. A slight change in particle size distribution or parti-cle shape with some materials can result in a significant change in conveying ca-pability. As mentioned earlier, most manufacturers of pneumatic conveying sys-tems have test facilities so that they can convey a material for which a design is

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required, and so obtain the necessary data. Figure 19.13 will help to reinforce boththe need for such design measures, and the need for good troubleshooting proce-dure.

2.5.3 A ir Requirements

If a different material is to be conveyed its performance will depend very muchupon the air flow rate available, as will be seen from Figure 19.13. If there is in-sufficient air, it will not be possible to convey the material, unless the pressure isreduced, or more air is provided. In either case the material flow rate achieved islikely to be much lower and so consideration must be given to the capability of thematerial feeding device for the new duty. If the air flow rate is increased this mighthave an adverse effect on the performance of the filtration plant. As will be seen, achange of material can have an influence on many aspects of system design andoperation.

With conveying data on each material to be handled, however, it is a simplematter to design the system so that every material can be conveyed at the flow raterequired. If a diverse range of materials are involved, some compromises in per-formance may have to be made, but at least there will be an understanding of thecomplex interfacing problems involved.

2.6 With Change of Distance

If a system operates satisfactorily in conveying a material over a given distance itis quite possible that the pipeline wil l block if the pipeline is extended and it isrequired to convey the material over a longer distance.

2.6.1 Material Feed Rate

For a given value of conveying line pressure drop, the conveying capacity of apipeline wil l decrease with increase in distance. For a change in conveying dis-tance, therefore, there must be a corresponding change of material feed rate intothe pipeline. This is an inverse law relationship as considered with Equation 11from Chapter 7.

This point is illustrated in Figure 19.14 and comes from Chapter 7. This is aplot of material flow rate drawn against conveying distance. Two different materi-als are considered and it is drawn for an eight inch pipeline and an air supply pres-sure of 30 psig. One material is capable of dense phase conveying (dicalciumphosphate) and the other only has dilute phase conveying capability.

Figure 19.14 shows that for a given conveying line pressure drop the mate-rial flow rate is approximately inversely proportional to conveying distance. Thisis for illustrative purposes only, since it is the 'equivalent length' of a pipeline thatis the important parameter and this includes allowances for vertical l if t and num-ber and geometry of bends. A change in routing, therefore, involving the additionof bends, with no change in pipeline length, will have a similar effect.

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560 Chapter 19

500

o

2 400

200_oPL,

1 100

Air Supply Pressure = 30 Ibf/irrgPipeline Bore = 8 inch

500 1000 1500 2000 2500

Conveying Distance - feet

Figure 19.14 The influence of conveying distance on the conveying potential ofpneumatic conveying system pipelines.

From Figure 19.14 it will be seen that the lines slope steeply for shortconveying distances. For a given conveying line pressure drop, therefore, materialflow rate capability will change significantly for just small increases in conveyingdistance with short pipelines. To maintain the same material flow rate over alonger distance will require a significant increase in pressure drop. If a higherpressure is not available the pipeline will block if the material flow rate is not re-duced to compensate.

2.6.2 Air Flow Rate

If the conveying distance is increased, the material flow rate will have to decrease.This wil l result in the material being conveyed at a lower value of solids loadingratio. For a material capable of being conveyed in dense phase, in a conventionalsystem, such as the dicalcium phosphate, this wi l l mean that a slightly highervalue of conveying line inlet air velocity will have to be employed.

This, in turn, means that a higher flow rate of air will have to be used toconvey the material. The influence of solids loading ratio on conveying line inletair velocity for dense phase flow was illustrated earlier in Figure 19.1. This helpsto explain the differences in slope of the curves on Figure 19.14.

The specific effect on air flow rate is illustrated in Figure 19.15 and this isalso derived from Chapter 7. This is a plot of material flow rate against air flowrate for the same eight inch bore pipeline. The change in the limit to conveyingwith increase in conveying distance is due to the gradual change from dense todilute phase conveying which results from the gradual decrease in pressure gradi-ent available for material conveying.

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System Capability Problems 561

j1 500o

J 400oX

I 300

OJ

! 200

_o- 100cti

I o

Conveying Line

Pressure Drop - I b f 7 i r r ~ 10

0 500 1000 1500 2000Free Air Flow Rate - fWmin

Figure 19.15 The influence of conveying distance on conveying limits for materialscapable of dense phase conveying.

2.6.3 Conveying Potential

In terms of conveying potential it is conveying line pressure gradient and materialproperties that are the important parameters. To convey in dense phase requires ahigh pressure gradient, because of the high concentration of the material in the air.Because of the compressibility problems with air, and expansion effects in particu-lar, air supply pressures greater than about 80 psig are rarely employed, if thepipeline is to discharge to atmospheric pressure.

If it is required to convey over a long distance, therefore, the pressure gradi-ent must be reduced if it is not possible to utilize a higher air supply pressure tocompensate. If the pressure gradient has to be reduced it will not be possible toconvey in dense phase.

Thus even if a material is capable of being conveyed in dense phase and atlow velocity, the material will have to be conveyed in dilute phase, and at a muchhigher velocity, if it is required to convey the material over a long distance. If theproperties of the material are such that it can only be conveyed in dilute phase,suspension flow, the use of high pressure air for conveying will have no effect atall in changing this to dense phase, unless a totally different conveying system isemployed.

3 SYSTEM NOT CAPABLE OF DUTY

As with the problem of pipeline blockage, the inability of a system to achieve therated duty could result from an error in the system design. Alternatively, it is pos-sible that the problem could be rectified by some simple adjustment to the plant. It

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562 Chapter 19

is particularly important to determine whether the limitation on material flow rateis due to the material feeding device or to the pipeline and air supply.

3.1 Material Feeding

The first check to be made is on the conveying line pressure drop. If this pressureis below the capability of the air mover it is probable that insufficient material isbeing fed into the pipeline. This may be rectified by adjusting the controls on thefeeding device. If the maximum output of the feeder does not meet the conveyingcapability of the pipeline, however, it will probably be necessary to fit a largerfeeder.

In the case of feeders delivering material into positive pressure conveyingsystems, there will be a leakage of air across the device. It should be checked thatthe feeder is operating satisfactorily with the material before recommending a lar-ger size. In the case of rotary valves, for example, leakage air can restrict the flowof material into the valve.

The leakage air might also aerate the material to such an extent that there isa significant reduction in bulk density. The effectiveness of air vents and the clear-ances on all moving parts should also be checked. If the conveying line pressuredrop is at the design value, however, it would indicate that it is the pipeline or theair supply that is the main cause of the system not being able to achieve the re-quired material flow rate.

3.2 Air Filtration

Another check to be made should be on the filtration unit. If this is incorrectlysized for the duty it is possible that the pressure drop across the filter will be un-necessarily high, although this is only likely to be a problem in low pressure sys-tems. Under-sizing, however, will add to maintenance problems in all cases. Filtercloth surface areas are sized primarily on volumetric air flow rate. If it is incor-rectly sized an additional unit could be installed, if there is sufficient space. Ifthere is not sufficient room, then the filter unit will probably have to be replacedwith a larger unit. Before going to this length, however, a check should be madethat cleaning cycles are satisfactory, cleaning is effective, and that the filter clothsdo not need replacing.

3.3 Reduce Air Flow Rate

An improvement in system performance will often be obtained by reducing thequantity of air that is used for conveying the material, particularly if the system isover-rated in terms of the volumetric flow rate of air that is supplied. In the firstinstance this could be achieved by fitting a tee piece with a valve in the conveyingair supply pipe-work. In the case of a positive pressure system it would be posi-tioned between the air mover and material feed point into the pipeline as shown inFigure 19.16.

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System Capability Problems 563

Storage Siloor Hopper

Compressoror Blower

Air Air-Solids Flow

DischargeHopper

Figure 19.16 The use of an ofl'-take to monitor the performance of a positive pressurepneumatic conveying system.

In a negative pressure system the off-take would be positioned between thefiltration unit and the air mover as shown in Figure 19.17. Such an off-take or by-pass would enable the influence of a reduced air flow rate on system performanceto be accurately monitored, particularly if the quantity of air by-passing the con-veying system was measured. Ultimately it might be possible to change the speedof the air mover to achieve the desired air flow rate, and so benefit from a lowerpower requirement.

Air Intake

Filter

ControlValve

StorageHopper

Exhauster

DischargeHopper

Air

Figure 19.17 The use of a system by-pass to monitor the performance of a negativepressure pneumatic conveying system.

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3.3.1 Procedure on Plant

Although reducing the speed of the air mover will reduce the air flow rate, this isnot always convenient on a plant, and so the tee piece and valve is ideal. It wi l lalso allow changes to be made readily at any convenient time, and to be madegradually, which is important on a plant, particularly if continued and reliable op-eration is required while the system is being monitored.

If there is not a pressure gauge on the air supply or exhaust line, one shouldbe fitted, as indicated on Figures 19.16 and 19.17. This will be needed to recordthe air supply/exhaust pressure, or conveying line pressure drop. If a rotameter, orsimilar air flow rate measuring device, is also provided, it wil l be possible tomeasure the air flow rate by-passing the system in addition. As many valves havenon-linear characteristics, this would be particularly useful in ensuring that thedesired proportion of air was discharged or admitted.

3.3.1.1 Test DataIn most plants the supply or receiving hopper is mounted on load cells, or havesome other weighing mechanism, and so material flow rates can be determinedreasonably quickly and accurately. By gradually opening the off-take or by-passvalve a number of tests can be carried out, and if the air supply pressure and the airand material flow rates are recorded for each test, it will be possible to construct asmall part of the conveying characteristics. Depending upon the method of feedingthe material into the pipeline it may be necessary to make adjustments to the feedrate so that this can also be varied.

3.3.1.2 Sight GlassesIdeally some of the tests should be carried out with conveying air velocities asclose to the minimum as possible. For this purpose it would be a distinct advan-tage to have a short length of sight glass in the pipeline so that the material beingconveyed could be observed. With a sight glass in the line it would be possible todetect when conveying was being carried out close to the minimum conditions,and so tests could be carried out in this region with more confidence.

Such a sight glass should be positioned as close as possible to the point atwhich the material is fed into the pipeline, for as the air expands through the pipe-line the velocity will be a minimum at the material feed point. A sight glass in avertical line is probably better than one in a horizontal line, for minimum convey-ing conditions are easier to detect. In a vertical line the flow is across the full boreof the pipe, and at low velocities some of the material will be observed to drop outof suspension, fall down past the sight glass, and be re-entrained in the conveyingair. If the flow is so fast that it cannot be detected, this is confirmation that far toomuch air is being used.

3.3.1.3 Use of DataBy carrying out tests over as wide a range of conveying conditions as can beachieved, it should be possible to obtain a reasonable indication of the nature ofthe conveying characteristics in the region of interest. If an improvement in mate-

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rial flow rate was indicated with a reduced air flow rate, the off-take valve couldbe shut and the speed of the air mover reduced to provide the optimum value of airflow rate.

If the air supply to the conveying system was significantly over-rated, suchthat a considerable amount of air could be by-passed, it would be recommendedthat the speed of the blower should be reduced in stages. If the by-pass/off-takevalve was opened too far, a surge in material flow rate could cause prematureblockage of the pipeline. The surge would add to the resistance in the conveyingpipeline and alter the balance of resistances so that significantly more air by-passed the conveying system. The speed of the blower should be reduced as soonas about 10% of the air is by-passed in order to prevent such an occurrence.

If no improvement in material flow rate was achieved, the exercise would atleast confirm that the material was already being conveyed under optimum condi-tions and provide a check on the optimum value of air flow rate. This would alsoprovide useful information about conveying air velocities and pressure drops sothat the influence of alternative means of increasing the output could be assessed.

3.4 Change Components

If these modification are not sufficient to bring the system up to its rated output, itwill be necessary to either provide an air mover with a higher pressure rating, or toincrease the bore of the pipeline. In either case full consideration should be givento the influence that these changes can have of other system components.

An obvious help in any situation would be to reduce the conveying distance.A review of the pipeline routing would be well worthwhile, for a reduction in thenumber of bends would also help significantly. If there are any blind tees or sharpelbows in the pipeline it would be worthwhile changing these for short radiusbends.

If it is a high pressure or a high vacuum system, with a single bore pipeline,a stepping of the pipeline to a larger bore should also result in an increase inthroughput. Care must be taken with stepped pipelines, however, to ensure that theconveying air velocity does not fall below the minimum value at any step in thepipeline. This point was illustrated in some detail in Chapter 9.

4 SYSTEM OUTPUT GRADUALLY DECREASES

A gradual reduction in system output over a period of time is likely to be due ei-ther to plant wear or material build-up in the pipeline.

4.1 Plant Wear

If the material being handled is abrasive, wear of feeding devices such as screwsand rotary valves will occur. This will result in an increase in air leakage acrossthe feeder and hence a reduction in the air flow rate available to convey the mate-

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566 Chapter 19

rial. The effect of this on the material conveying rate depends very much on theproperties of the material being conveyed, but in some cases it will result in a de-crease in output. An increase in air leakage across the feeder is likely to reduce itsfeeding capability.

In vacuum, closed loop and combined systems, wear of the air mover mayoccur if the filtration system is not sufficiently efficient. This wi l l result in a grad-ual deterioration in performance of the air mover. A pressure gauge in the air linewill be useful in detecting any changes in pressure capability, but volumetric flowrate will also reduce and more specialist instrumentation will be needed to detectthis.

4.2 Pipeline Coating

Certain moist materials, pigments, hygroscopic materials, and food products witha high fat content, may have a tendency to coat the walls of a pipeline. If the coat-ing builds up it will gradually reduce the section area of the pipeline.

A decrease in pipeline bore will cause an increase in conveying air velocity.An increase in conveying air velocity and a decrease in pipeline bore will bothresult in an increase in the value of the empty pipeline pressure drop. If there is apressure gauge in the air supply line, in the case of positive pressure systems, andin the air extraction line, in the case of negative pressure systems, it should be pos-sible to recognize that this is occurring very easily. Sketches of both positive andnegative pressure systems with pressure gauges at these locations were shownearlier in Figures 19.16 and 19.17 with regard to the monitoring of system per-formance.

A comparison of the empty line pressure drop at the end of purging, for ex-ample, with that of a pre-recorded value when the pipeline was known to be clearof material, will give a good indication of the extent of the problem. At the end ofa conveying cycle this build up of material can generally be removed by vibratingthe pipeline during the air purge. The effectiveness can be judged by checking theempty pipeline pressure drop value. If it is not effective it may be necessary tochange the pipeline to one that can be more effectively cleared of material.

One possibility is to install a rubber hose capable of withstanding the con-veying air pressure. The natural flexing of the hose with the conveying of the ma-terial and pressurizing and de-pressurizing is often sufficient to dislodge any build-up of material.

5 SYSTEM OUTPUT GRADUALLY INCREASES

This is an item in a section on System Capability Problems, to which few Engi-neers are likely to refer. If an improvement in performance is obtained after a pe-riod of time, most Engineers will be more than happy with the situation, and at-tribute the improvement to 'settling in', as mentioned above. This rarely occurs

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with pneumatic conveying systems, however, and so the reasons for any change inperformance should be checked.

5.1 Plant Wear

This point was considered above in relation to system output gradually decreasing.If the system is over-designed it is quite likely that if the quantity of air used toconvey the material in the pipeline is reduced, there wi l l be an increase in materialflow rate, and a decrease in energy required will be achieved. As will be seen fromFigure 19.12, however, with further wear the pipeline could block and so conveynothing at all.

5.2 Pipeline Wear

In the case of nylons and polymers, special pipeline surfaces are often provided inorder to reduce the problem of angel hairs. The pressure drop through these pipe-lines is generally higher than that through a smooth bore pipeline. If the pipelinewears, therefore, there will either be a reduction in pressure drop for the samethroughput or an increase in material flow rate. In either case it is likely that theproblem of the angel hairs forming will return, and so it will indicate that the pipe-line will need to be re-treated.

REFERENCES

1. D. Mills. Identifying and solving material flow problems in pneumatic conveying sys-tems. Powder and Bulk Handling, pp 10-17. Vol 2. Oct-Dec. 1998.

2. D. Mills and J.S. Mason. The up-rating of pneumatic conveying systems. Jnl of Pipe-lines. Vol 2. pp 199-210. 1982.

3. D. Mills. Troubleshooting pneumatic conveying systems. Three part article in ChemicalEngineering: Part 1. Keeping pneumatic delivery up to speed, pp 93-105. June 1990.Part 2. Major systems and their key components, pp 101-107. July 1990. Part 3. Prob-lems from the outside, pp 157-163. Sept 1990.

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20Erosive Wear Problems

1 INTRODUCTION

Many bulk participate materials that have to be conveyed are very abrasive, suchas silica sand, alumina, cement and fly ash. As a consequence, the pipeline, bendsand various components that are exposed to impact by the gas-solid flows have tobe designed and specified such that the problem is minimized to an acceptablelevel. It is not uncommon for steel bends, installed in a pipeline conveying anabrasive material, to fail in a matter of hours. The problem relates to abrasive ma-terials and it is essentially the hardness of the particles that dictates the magnitudeof the potential problem.

It must be stressed that it is virtually impossible to eliminate erosive wear ifan abrasive material has to be conveyed. By the correct choice of materials of con-struction and design, and conveying conditions, however, the problems can gener-ally be reduced to an acceptable level, with the support of an appropriate mainte-nance program.

1.1 Erosive Wear

Abrasive wear is associated with sliding contact between surfaces. In bulk solidshandling plant abrasive wear is a major problem at hopper walls and in chutes,where materials slide over such surfaces. Erosive wear results from the impact of

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particles against surfaces. Typical erosive wear situations in bulk solids handlingplant are in the loading and off-loading of materials, and with free fall onto sur-faces. The blowing of materials into cyclones; their loading into hoppers and ontochutes; and off-loading from hoppers, conveyor belts and bucket elevators; arecommon examples. These are all cases where particles can impact against surfacesand cause erosive wear, rather than slide against a retaining surface and causeabrasive wear.

In pneumatic conveying systems, bulk particulate materials are physicallytransported by air. Bends in pipelines, therefore, are particularly vulnerable to ero-sive wear, as are diverter valves and any other surface against which particles arelikely to impact, including the pipeline itself to a limited extent. Where a pressuredifference might exist on a plant, in the presence of abrasive particles, erosivewear will also occur, if there is a flow of air. A particular example here is withrotary air locks and screws used to feed materials into positive pressure pipelines.Even isolating valves will wear if they are not completely air-tight.

1.2 Related Problem Areas

Erosion represents a major problem, not only in bulk solids handling plant, but inmany other areas. In thermal power plant pulverized fuel causes erosive wear ofsupply lines and nozzles, and the resulting fly ash is a problem with respect toboiler tubes. Both pneumatic and hydraulic conveying of particulate materials inpipelines can result in severe erosion problems, and aircraft, rockets and missilesare eroded by rain drops and ice particles. The area that has probably receivedmost attention, however, is aircraft engines, and in particular helicopters, for dustingestion can cause considerable damage, and has resulted in several catastrophicfailures in service.

7.2.7 Data Sources

Information on erosive wear comes from a very wide range of sources, therefore.Until recent years little was known of the fundamental mechanisms of the erosionprocess or of the variables that influence the problem. There are, in fact, so manyvariables that influence the problem that advances have only been made by thedevelopment and use of specially designed erosive wear testing rigs. In these awide range of powdered and granular materials have been impacted against a widerange of surface materials over carefully controlled conditions of velocity, particleconcentration, temperature, impact angle, etc.

Many studies have been of a general nature with a view to getting a betterunderstanding of the basic mechanisms of the process, and for this purpose nu-merous single particle impact investigations have been undertaken. Other studieshave been conducted for specific purposes, and so the range of variables investi-gated can be extremely wide. For particle impact velocity, for example, tests havebeen carried out at about 3 to 10 ft/s for hydraulic transport, from 3000 to 7000

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a/mm for pneumatic conveying, from 300 to 1500 mile/h for aircraft applications,and up to 18,000 mph for rockets.

2 INFLUENCE OF VARIABLES

There are many parameters associated with both the impacting particles and thesurface material that can have an effect on erosive wear. In some cases the vari-ables are inter-related and so need to be considered in groups in these situations. Areview of the most important variables is given to provide some guidance on theirinfluence on component specification and conveying conditions.

2.1 Impact Angle and Surface Material

A curve presented by Tilly [1] and shown in Figure 20.1 illustrates the variation oferosion with impact angle for two different surface materials and is typical of theearly work carried out to investigate the influence of these variables.

Both surface materials showed very significant differences in both erosivewear rate and the effect of impact angle. These materials do, in fact, exhibit char-acteristic types of behavior that are now well recognized. The aluminum alloy istypical of ductile materials: they suffer maximum erosion at an impact angle ofabout 20° and offer good erosion resistance to normal impact. The glass is typicalof brittle materials: they suffer severe erosion under normal impact but offer gooderosion resistance to low angle, glancing impact.

Al alloy

Glass

16

12

Particle

oPJ

Impactangle

Surface Material

30 60 90Impact Angle - <x - degrees

Figure 20.1 Variation of erosive wear with impact angle for various surface materials.

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The vertical axis on Figure 20.1 is in terms of the volume of surface materialeroded, in cubic inches, per ton of particles impacted against the surface, in orderto provide a more useful basis of comparison for the two materials. It will be notedthat the erosive wear rate for the glass is magnified by a factor of five. Impact an-gle is clearly defined in the particle/surface sketch alongside.

These particular tests were carried out with sand particles sieved to between60 and 125 jam and impacting at about 20,000 ft/min. The value of velocity is in-dicative that the tests were not carried out for pneumatic conveying, but aircraftapplications. That brittle and ductile materials respond to erosion in very differentways can be clearly seen from Figure 20.1, and it is obvious that different mecha-nisms of material removal must be involved.

The influence of impact angle and the different response of ductile and brit-tle materials to erosive wear is an aspect of the problem that will be considered atmany different points throughout this chapter. The relationships can be used toexplain a number of observed phenomena in erosive wear, and are particularlyuseful in predicting the possible behavior in new and untried situations.

2.1.1 Theories Proposed

From early thoughts on the matter it was suggested that for ductile materials (an-nealed low carbon steel, copper, aluminum, etc) material removal is predomi-nantly by plastic deformation. No cracks propagate ahead of the cutting particleand the volume removed is due entirely to the cutting action of the particle, ratherlike the cutting edge of a machine tool.

For brittle materials (glass, basalt, ceramics, cast iron, concrete, etc) it wasthought that material removal is in a large part due to the propagation of fracturesurfaces into the material.

These erosion processes, however, have subsequently proved to be not quiteas straightforward as this. Photographs taken of impact craters, produced as a re-sult of single particle impact studies, have shown clear evidence that melting hastaken place [2, 3]. The melting only occurs over a small part of the impact crater,but it must be considered as being contributory to the erosive wear process. This isconsidered further, in relation to heat treated surface materials, later in this chap-ter.

2.2 Velocity

Of all the variables that influence the problem of erosive wear, velocity is proba-bly the most important of all, not least because pneumatic conveying requires agiven minimum value in order to convey materials. It is generally recognized thaterosive wear is dependent upon a simple power of velocity, rather like the influ-ence of velocity on pressure drop, such as:

Erosion = constant x (velocity)"

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2.2.7 Surface Material

There has been much confusion as to the value of the exponent, and values of nranging from two to six have been reported. Tilly and Sage [4] tested a wide rangeof different materials and obtained very good agreement with respect to the expo-nent, n, in each case. Their results are reproduced in Figure 20.2. This is a log plotand the slope of all the lines was approximately 2-3. The velocities, of course, arewell above those generally encountered in pneumatic conveying systems, even atthe lower end of their range.

The velocity exponent is now generally considered to be approximately 2-5,and although erosive wear resistance varies widely for different surface materials,as shown in Figures 20.1 and 20.2, the value of the velocity exponent remainsreasonably constant at a value of about 2-5 for all surface materials, whether duc-tile or brittle.

2.2.2 Bend Wear

Few comprehensive erosion studies have been carried out exclusively in the veloc-ity range appropriate to pneumatic conveying. Results from one extensive researchprogram into the erosion of pipe bends in an actual pneumatic conveying system,at velocities appropriate to dilute phase suspension flow have been published [5].

Tests were carried out over a range of conveying air velocities from 3000 to7000 ft/min and at solids loading ratios from 0-5 to 8. Two inch nominal bore steelbends having a bend diameter, D, to pipe bore, d, ratio of about 5:1 were erodedby large batches of silica sand having mean particle sizes of 70 u.m (210 Mesh)and 230 u.m (65 mesh).

oDJ

1000

500

200

100

50

20

10

0

Glass

10 4015 20 30

Particle Velocity - ft/min x 1000

Figure 20.2 Variation of erosion with velocity for various surface materials.

50 60

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574 Chapter 20

Over the ranges of conveying air velocity tested the velocity exponent wasfound to be very consistent at a value of about 2-65. A graph showing the influ-ence of conveying air velocity on the specific erosion of the pipeline bends testedis given in Figure 20.3.

The erosion is in terms of the mass of metal eroded from a bend per ton ofsand conveyed through the bend. With a velocity exponent of 2-65 it means thatthe wear rate will increase by a factor of approximately six with a doubling of theair velocity. This explains why the curve rises so steeply on Figure 20.3.

If a positive pressure conveying system operates at a pressure of about 15psig, a doubling of the velocity wi l l be achieved in a single bore pipeline discharg-ing to atmospheric pressure. With a vacuum conveying system a doubling in ve-locity will be achieved with a system exhausting at about 7'/2 lbf/in2 absolute. Ineither type of system a bend at the end of the pipeline will wear six times as fast asa bend at the start of the pipeline.

If an abrasive material is to be conveyed, therefore, it would always be rec-ommended that the pipeline be stepped to a larger bore part way along its length inorder to l imit the maximum value of velocity that is achieved, in order to minimizethe erosive wear of bends towards the end of the pipeline. It is essential, of course,that the step to the larger bore pipeline is correctly positioned along the pipeline,for if the velocity falls below the minimum value of conveying air velocity at thestep to the larger bore pipeline, the pipeline is likely to block at this point.

I 0-8

oao

0-4a

c/o

Formild steel bends2 in bore6 in radius inhorizontal plane

Conveying70 urn sand at asolids loading ratio of 2

2000 4000

Conveying Air Velocity - ft/min

6000

Figure 20.3 The influence of velocity on the erosive wear of bends in a pneumaticconveying system pipeline.

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Figure 20.3 shows quite clearly that excessively high conveying air veloci-ties should be avoided. It also shows the benefits of conveying with low velocityair, and hence the potential of low velocity, dense phase flow in this respect. Withthe bends reported in Figure 20.3, tested at a solids loading ratio of two, bend fail-ure occurred when about 2 oz of metal was eroded from the bend. The bend wallthickness was about 0-15 inch.

In Figure 20.4 the conveying capacity of these bends, in terms of the mass ofsand that could be conveyed through the pipeline before bend failure occurred, ispresented. From this it will be seen that with a conveying air velocity of about6000 ft/min only 3 ton of sand could be conveyed before bend failure.

2.3 Particle Size

The general consensus of opinion with regard to particle size is that there is athreshold value of wear rate which, for velocities appropriate to pneumatic con-veying, occurs at a particle size of about 60 urn. Below this size wear rate reduces,but for particle sizes greater than 60 urn it remains constant. Results of work car-ried out by Tilly f6| are presented in Figure 20.5. This shows that the thresholdvalue increases with increase in velocity. The work was carried out for an investi-gation into the erosion of aircraft engines, which explains the high velocity range.A shot blast type of test rig, in which abrasive particles were impacted against flatplates, was used for the purpose.

eaT3

O(j

80

60

40

20

Formild steel bends2 in bore6 in radius inhorizontal plane

Conveying70 urn sand at asolids loading ratio of 2

2000 4000

Conveying Air Velocity - ft/min

6000

Figure 20.4 The influence of velocity on the conveying capacity of the bends shown infigure 20.3.

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60

o"5o

40

£ 20o

Particle Velocity- f t /min x 1000

600

500

260

50 100 150

Mean Particle Size - |im

200

Figure 20.5 The influence of particle size and velocity on erosion.

Wear rate here is expressed in specific terms, that is the mass of surface ma-terial eroded per unit mass of particles impacted. In a given mass of particles, thenumber of particles will reduce as the particle size increases, and so although thespecific erosion remains constant with increase in particle size, the erosive wearper particle will increase approximately with the cube of the particle size. Littlework has been undertaken with particles much larger than about 1000 micron (1mm or 18 Mesh) in size and so it is not known to what particle size the thresholdvalue remains constant.

2.3.1 Bend Wear

Work carried out on actual pipe bends in pneumatic conveying system pipelineswould tend to confirm this [7]. Batches of sand with mean particle sizes rangingfrom 70 to 280 u.m were used in a program of conveying trials. Six test bends inthe one pipeline were monitored for erosive wear, and the average mass erodedfrom each bend was found to be independent of particle size.

On an individual basis, however, the bends showed a very interesting trend.The degree of scatter in the results increased markedly with decrease in particlesize, as shown in Figure 20.6. With the larger particles the wear rates were re-markably consistent, but with the finer particles the spread of the results was verywide.

It is believed that the finer particles are influenced by the secondary flowsand turbulence that can be generated by the bends and that this causes acceleratedwear of some bends, although there is no obvious reason why some bends weremore vulnerable than others in the pipeline.

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0-6

> 0-4

<s>ewc 0-2'oOJ

Potential bandN

s width of resultss

X

MeanValue

XX

100 200

Mean Particle Size - u,m

300

Figure 20.6 The variation of individual specific erosion values with mean particle sizefor bends in a pneumatic conveying system pipeline.

This could well account for some of the premature failures that have beenreported in situations where very fine materials have been conveyed. It was alsofound that the depth of penetration of the particles into the bend walls was a factorof two greater for the 70 urn sand as compared with the 280 urn sand. Since failureoccurs when a given thickness of material is eroded, this parameter is potentiallyas important as specific erosion in pipe wear situations.

2.3.2 Fine Particle Wear

Some researchers have suggested that particles below about 5 urn will cause littleor no erosive wear. In pneumatic conveying this is probably a reasonable assump-tion, for particles below this size are likely to follow the air stream and not impactagainst a surface. The trend of the curves, representing the limits of the potentialspread of the results in Figure 20.6, with respect to particle size is not known. It issuspected that the upper limit may reach a maximum at about 50 urn and then rap-idly decrease.

2.4 Particle Hardness

The value of the particle hardness of the material being conveyed is the major in-dicator of the potential erosiveness of the material. Goodwin et al [8] investigatedthe influence of particle hardness on erosive wear with a rig in which abrasiveparticles were impacted against test plates. They found that erosion is related tohardness by the expression:

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Erosion = constant * H2-4P

where Hp = particle hardness

It is generally considered, however, that there is a threshold value of particlehardness beyond which erosion remains essentially constant. This occurs at a par-ticle hardness of about 800 kg/mm2, and so materials with hardness values muchgreater than this would not be substantially more erosive than sand particles.

2.4.1 Bend Wear

A sketch of the potential influence of particle hardness on the erosion of mild steelbends is given in Figure 20.7 [9]. It is derived for sharp angular particles, and theerosion is expressed in specific terms once again, that is oz/ton conveyed. Thehardness values of typical materials, both potential conveyed materials and bendsurface materials, have been superimposed for reference.

It will be noticed from this that coal is a very soft material and is unlikely tobe a problem with respect to erosion. In reality, of course, both pulverized andgranular coal are erosive materials. This, however, is due to the presence of non-combustible minerals, such as quartz and alumina in the coal, and not to the coalitself. With large tonnage flows, even small percentages of these highly abrasiveminerals will cause severe wear.

0-3

0-2co

0-1

o.00

90° mild steel bendsD/d = 6

Air Velocity = 5000 ft/min

en ogN O u

oO

3 c• - oc o^3 —

500 1000 1500 2000 2500

Particle Hardness - kg/mm"

Figure 20.7 The influence of particle hardness on the erosion of bends in a pneumaticconveying system pipeline.

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A similar situation applies to pulverized fuel ash, and other materials con-taining small percentages of similar contaminants, such as barite and wood chips.

2.4.2 Hardness Measurement

A knowledge of particle hardness is essential, therefore, particularly at the designstage of a plant, since it gives an indication of the need to take steps to avoid ex-cessive wear of key system components. Scratch hardness is the earliest knowntype of hardness test, and in its simplest form is the ability of one solid to scratch,or be scratched, by another.

The method was first proposed on a semi-quantitative basis in 1822 byMohs, who selected ten mineral standards, starting with the softest - talc (scratchhardness 1) and ending with the hardest - diamond (scratch hardness 10). Becauseof its simplicity it is still widely used today as a reference for potential erosivewear of plant by conveyed materials. This has become known as the Mohs hard-ness scale, but divisions along the scale are clearly not all of the same magnitude.

Since the Mohs scale proved too coarse for the measurement of the hardnessof general engineering metals, quantitative tests of the static indentation type weredevised, mostly based on the use of pyramids. Equipment is available for carryingout such tests with fine particulate materials, but because of its complexity, theMohs scale is still used today for many bulk solids handling applications. Metalhardness, of course, is usually referred to in terms of the value indicated by one ofthese indentation methods. Fortunately sufficient research has been undertaken torelate the hardness as measured by any of these methods to the Mohs scale num-ber. Such a relationship is shown in Figure 20.8.

10"

3U 75

1 50i$ 30(§10

750

500

250

100

Hardness Scales

103

2 3 4 5 6 7 8 9Mohs Number

10

Figure 20.8 Relationship between Mohs, Vickers, Brinell, and Rockwell hardnessscales.

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580 Chapter 20

2.5 Surface Material

A number of surface materials were included in Figures 20.1 and 20.2. In Figure20.2 it was shown that, for a given impact angle, the effect of velocity was similarfor each material. Figure 20.1, however, showed that impact angle could have avery different effect, with the ranking of different materials changing significantlywith impact angle. From these figures it is clear that surface hardness is not neces-sarily the main parameter to be considered in selecting materials for erosive wearresistance.

2.5. / Steels - Heat Treated

There is a wealth of information in the field of abrasive wear on the relationshipbetween surface material hardness and wear resistance for metals. One of the ear-liest of these [10] shows that the hardness value of annealed metals provides anapproximate estimate of their resistance to abrasive wear. Cold working fee metalsto higher hardness values has essentially no effect on abrasive wear resistance, andhardening and tempering carbon steels to achieve higher hardness levels does notresult in a corresponding increase in wear resistance.

Finnic [11] was the first to show that such a relationship might exist in thefield of erosive wear, but Finnie et al [12] were the first to produce a hardness towear resistance relationship similar to those presented for abrasive wear. Resultsof their work are presented in Figure 20.9.

<D 1 cu 15

oi

Tool Steel

200 400 600

Hardness - kg/mm2

800

Figure 20.9 Variation of erosive wear resistance with indentation hardness for varioussurface materials.

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The range of materials that they considered was rather limited but the shapeand trends of the curves were similar. Several researchers have commented on thepossibility of micro-melting occurring over a small part of the indented surface, asmentioned in relation to Theories Proposed in section 2.1.1 above. This could par-tially over-ride the effect of heat treatment and micro-structural changes.

2.5.2 Resilient Materials

Resilient materials, such as rubber and polyurethane, are commonly used in ero-sive wear situations. Although the hardness of the surface material is generally farlower than that of the particles impacting against the surface, they derive theirerosive wear resistance from the fact that they are able to absorb most of the en-ergy of impact by virtue of their resilience.

Mason et al [13] tested mild steel, nylon and Linatex (a proprietary materialcontaining 95% natural rubber) in a shot blast impact testing machine. Aluminaparticles were impacted at different angles over a range of velocities. They showedthat the nylon and rubber exhibited typically ductile behavior, with respect to im-pact angle, similar to mild steel. Their erosive wear results, with respect to air ve-locity, are shown in Figure 20.10.

These show that natural rubber is superior to mild steel at velocities belowabout 24,000 ft/min, but above this value the performance of the rubber rapidlydeteriorates. It is suspected that beyond a certain impact energy level the rubber isno longer able to absorb the energy. As a result the wear mechanism probablychanges to one of tearing and possibly burning because of the heat generation.

8000 16,000 24,000

Mean Air Velocity - ft/min

32,000

Figure 20.10 The influence of air velocity on wear rate for mild steel and rubber sur-face materials.

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This point is considered further in the section dealing with Industrial Solu-tions to the problem, where the use of rubber is considered as a bend wall material.

2.5.3 Hard Materials

Hard brittle materials are generally used in cases of severe erosive wear. Materialsused include Nihard, basalt and ceramics. Nihard is an abrasion resistant whitecast iron. It contains about 6% Ni, 8-5% Cr, 1-7% Si and 0-5% Mn and the struc-ture can be refined by chill casting. The material has a Brinell hardness of 550 to650, which is equivalent to a Vickers hardness of about 750 kg/mm2.

Basalt is a volcanic rock which can be cast into sections and used for l iningsurfaces, and although widely used for lining chutes and hoppers, it is often usedfor straight pipeline and bends. After casting, the material is heat treated to trans-form it from an amorphous into a crystalline structure. This is a naturally hardmaterial with a hardness, according to the Mohs scale of 7 to 8, which is equiva-lent to a Vickers hardness of about 720 kg/mm"". Basalt consists of approximately45% silica and 15% alumina, with the rest made up of oxides of iron, calcium,magnesium, potassium, sodium and titanium.

Of the materials used for providing erosion resistance, alumina based mate-rials are probably most common. A typical material consists of 50% aluminumoxide, 33% zirconium oxide and 16% silicon oxide. The general industry specifi-cation today is an alumina content of 85%, although higher alumina contents canbe supplied. It has a hardness of 9 on the Mohs scale, which is equivalent a Vick-ers hardness of about 2000 kg/mm2. Like basalt, these materials can also be castinto moulds of the required shape.

2.6 Particle Concentration

Particle concentration is a variable that has received little attention in basic re-search work on the subject, with the general opinion being that erosion decreasesonly very slightly for a large increase in concentration. Concentrations investi-gated, however, have generally been very much lower than those encountered inpneumatic conveying, even with dilute phase conveying. This is because mostresearch work on erosive wear has been with atmospheric dust loadings for aircraftapplications.

2.6.1 Bend Wear

The general explanation for the gradual reduction in erosive wear with increase insolids loading ratio is that as the particle concentration increases, fewer impactsoccur between the particles and the bend wall surface due to the interference of anincreasing number of other particles. From work on the erosive wear of pipebends, the following relationship for erosive wear has been derived [14]:

Mass Eroded = constant x (solids loading ratio)"

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It has also been found that the depth of penetration of particles into the bendwall surface varies with particle concentration, or solids loading ratio, as with par-ticle size reported earlier [14]. As the solids loading ratio increases, the particlesappear to focus on a smaller area of bend wall surface such that the rate of penetra-tion of the particles increases. In terms of the mass of metal that has to be erodedfrom a bend before failure occurs, the following relationship has been determined[14]:

Mass Eroded at Failure = constant x (solids loading ratio)-0-74

By combining the data on the mass eroded from the bends, with that on thedepth of penetration of the conveyed material into the bends, it is possible to de-termine a relationship for the mass of material that can be conveyed through abend before failure occurs [14]. Data for the bends investigated is presented inFigure 20.11.

This is similar to the data presented on the influence of velocity presented inFigure 20.3. It will be seen from Figure 20.11 that as the solids loading ratio in-creases the life expectancy of the bends reduces quite considerably. Although thespecific erosion rate decreases with increase in solids loading ratio, the influenceof the increase in penetration rate has an over-riding effect.

Formild steel bends2 in bore6 in radius inhorizontal plane

Conveying70 |am sand at5000 ft/min

0 1 2 3 4 5 6 7 8

Solids Loading Ratio

Figure 20.11 The influence of solids loading ratio on the conveying capacity of bendsin a pneumatic conveying system pipeline.

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It was reported earlier, with respect to particle size, and Figure 20.6, that thedegree of scatter of the results increased considerably with decrease in particlesize. A similar phenomenon was observed with regard to solids loading ratio, withthe degree of scatter increasing considerably with increase in solids loading ratio.In terms of component life, therefore, both particle penetration rate and possiblescatter in the results are potentially as important as mass eroded. Once again therewas no obvious explanation for the occurrence, but it is possible that eddies pro-duced and turbulence generated increase with increase in concentration.

2.7 Particle Shape

The influence of particle shape on mass eroded has been reported by many re-searchers. The result is much as one might expect, with smooth and rounded parti-cles causing much less erosion than sharp angular particles, under similar condi-tions of impact velocity, surface and particle hardness, etc. For test work on theerosive wear of pipe bends in pneumatic conveying system pipelines there is gen-erally a need to re-circulate the conveyed material.

As a result of re-circulating the material it degrades, and the sharp angularcorners and edges of the fresh material are gradually worn away, and so becomemore rounded and hence less erosive. Typical data on the erosive wear of pipelinebends is presented in Figure 20.12 [15].

0-4

g 0-3

•2 n-7vi U Z

Conveyed Material - SandMean Particle Size:O • Q - 230 |_im

x - 70 urn

Solids Loading Ratio

-i 1 1 1_

100 200 300

Number of Bends Conveyed Through

400 500

Figure 20.12 The effect of particle degradation and wear on the erosion of bends in apneumatic conveying system pipeline.

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The conveyed material was silica sand and the results from four differenttest programs are included. The average value of conveying air velocity in eachcase was about 5000 ft/min. Since it is likely that most of the wear and degrada-tion to the conveyed material in dilute phase conveying can be attributed to thebends in the pipeline, the horizontal axis is in terms of the accumulative number ofbends through which the sand was conveyed. The average spacing of the bends,for reference, was about 15 ft.

2.8 Surface Finish

It is generally thought that a highly polished surface will reduce the rate of erosivewear, but it must be emphasized that this is effectively just an incubation period. Itwill generally only have the effect of reducing the wear rate initially. Once thematerial surface starts to wear it will have little further influence on the steadystate erosion rate.

Brittle materials that have porous surfaces are particularly vulnerable to ero-sive wear. This can result from the casting process of these materials if gas bub-bles are allowed to form. If a gas bubble results in particles impacting at an angleclose to 90° extremely rapid wear will result in that area [16].

3 INDUSTRIAL SOLUTIONS AND PRACTICAL ISSUES

To a certain extent the problem of bend wear in pneumatic conveying system pipe-lines is a problem with which industry has learnt to live. There are a number ofways by which the severity of the problem can be reduced, but a number of factorsrelating to the material conveyed and the system itself have to be taken into ac-count. Expense is obviously a consideration, some methods may lead to a reduc-tion in the conveying capability of the plant, and if the material being conveyed isfriable then a solution that additionally minimizes the effect of degradation mustbe sought.

3.1 Pipeline Considerations

The volumetric flow rate of air specified and the pipeline bore chosen are of majorimportance, for the two have to be selected so that the resulting conveying air ve-locity is acceptable. The problem is that air is compressible and so its value is sig-nificantly affected by pressure. This represents a particular difficulty in high pres-sure systems, where the air pressure can drop from 20 to 40 psig at the start of thepipeline to atmospheric at the other end.

As the pressure of the conveying air decreases along the length of the pipe-line its density decreases, with a corresponding increase in volumetric flow rate,and hence velocity. In order to keep the velocity to within reasonable limits,stepped pipelines are often employed. A similar situation exists with negative

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pressure systems when operating under high vacuum. Stepped pipelines were con-sidered in some detail in Chapter 9.

If, for example, the air supply pressure in a positive pressure conveying sys-tem is 45 psig, and the conveying line inlet air velocity is 3000 ft/min, the convey-ing air velocity will approximately quadruple to about 12,000 ft/min at the end ofthe pipeline in a single bore line. A four fold increase in velocity will result in analmost forty fold increase in mass eroded from the bends.

This explains why bends near the end of a pipeline wil l generally fail in amuch shorter time than those near the start of the pipeline. If a dense phase systemwas specifically installed to overcome the problem of erosion, a stepped pipelinefor such a system would be almost essential if a high air supply pressure was used.

3.2 Bend Wear

By the very nature of the transport process, pipelines used for pneumatic convey-ing systems are prone to wear when abrasive materials have to be conveyed. Indilute phase, materials are conveyed in suspension in the air, and a relatively highconveying air velocity must be maintained in order to keep the material moving,and so avoid pipeline blockage.

The main problem relates to the wear of bends in the pipeline, and any othersurfaces where particles are likely to impact as a result of a change in flow direc-tion. Bends provide pneumatic conveying systems with their flexibility in routing,but if the material is abrasive and the velocity is high, rapid wear can occur.

3.2.1 Influence of Bend Geometry

Bends are available in a wide range of geometries, in terms of bend curvature,from long radius bends to tight elbows and mitered bends. Because bends are sovulnerable to wear there have been many developments and innovations for reduc-ing the problem.

The influence of bend geometry, for radiused bends, has been investigatedover a wide range of D/d values [17]. The results for 90° mild steel bends areshown in Figure 20.13. The bends were eroded by sand, conveyed at a solids load-ing ratio of two and with a conveying air velocity of 5000 ft/min.

The results can, to a certain extent, be predicted from the data presented inFigure 20.1 which shows the influence of impact angle on erosive wear for bothductile and brittle materials. It is possible to calculate the relationship between thebend geometry (D/d) and the impact angle and the results of such an analysis aregiven in Figure 20.14. This clearly shows the nature of the problem.

With sharp, or short radius bends, having a D/d ratio of 2:1, for example, itwill be seen from Figure 20.14 that the particles will impact against the bend wallat a fairly steep angle. At a high impact angle, as will be seen from Figure 20.1,erosive wear will not be too severe for a ductile material and so it can be expectedthat the bend will not wear too rapidly.

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0-3

I °'2t/1gwo

^ 0-1

0

For90° mild steel bendseroded by sand at asolids loading ratio of 2 withan air velocity of 5000 ft/min

0 204 8 12 16

Bend to Pipe Diameter Ratio - D/d

Figure 20.13 The influence of bend geometry on the erosive wear of pipeline bends.

Bends with a D/d ratio of 6:1 correspond closely to the worst case from thedata in Figure 20.13. The particles impact against the bend wall at an angle ofabout 30° and for a ductile material this wil l result in severe erosion. The particleimpact against the wall for the bends with a D/d ratio of 24:1 is at a much shal-lower angle. If the impact angle is relatively small erosion will not be too severe,and so it can be expected that the bend will not wear too rapidly.

60

bJ)oj-a

50

40

30

£ 20

10

0! 12 16 20

Bend to Pipe Diameter Ratio - D/d

24 28

Figure 20.14 Influence of bend geometry on particle impact angle.

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3.2.1.1 Long Radius BendsA very low impact angle is an essential pre-requisite for minimizing erosion, par-ticularly in the case of brittle surface materials. In the case of ductile materials,because of the remarkably steep increase in erosion with very small increase inimpact angle, as shown in Figure 20.1, exceptionally long radius bends would berequired. Bends with varying curvature have been proposed to overcome this par-ticular problem.

For ductile materials long radius bends are not likely to be a viable proposi-tion. For brittle materials, however, such as basalt and cast iron, they are essential,as mentioned above. A common method of providing a long radius bend is tomake the bend in segments. By this means the bend will be lighter and much eas-ier to fit into the pipeline.

Since the majority of the wear wil l be at the initial point of impact point,only one or two sections need be replaced should the bend fail. It is also possibleto reverse and inter-change segments and so extend the overall life of the bend.Segments can be made in 45°, 30° and 22'/2° sections. A four section long radiusbend is shown in Figure 20.15.

3.2.1.2 Short Radius BendsWith very short radius bends the angle at which the material impacts against thebend wall will be fairly high, as shown in Figure 20.14.

Figure 20.15 Long radius bend with replaceable and interchangeable 22'/20 segments.

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Although this will not be suitable for brittle surface materials, ductile mate-rials, because of their improved erosion resistance at high impact angle often givereasonable service in use, if the conveyed materials is not too abrasive.

Three major problems have to be taken into account, however, before usingvery short radius and similar bends. The more severe the impact of the materialagainst the bend wall the greater will be the problem of degradation if the materialis friable. The introduction of a very short radius bend will probably also increasethe conveying line pressure drop, which will mean that the material mass flow ratewill have to be reduced to compensate. A very short radius bend, and those thatare designed to trap the conveyed material, may require a slightly higher value ofconveying line inlet air velocity to ensure that the pipeline does not block.

A very cheap and often effective solution to the erosion problem is to use ablanked tee-piece or mitered bend (D/d = 0) made from regular pipe. Such a bendis shown in Figure 20.16. This gives a simple right-angled bend in the line, and soconsideration has to be given to the problems of purging the line clear, should thisbe necessary, added degradation (see next chapter) and pressure drop (see section5.3 in Chapter 8).

The material being conveyed fills the blanked section of the tee and part ofthe bend so that much of the material being conveyed impacts against itself andnot against the pipe wall. Should the line block at the bend, access can be gainedfrom the blanked section to facilitate clearing. Such bends generally fail at the startof the exit section of pipeline, due to the turbulence generated, and so it would berecommended that a thicker section of pipeline should be incorporated here.

Flow Direction

Figure 20.16 Sketch of tee piece bend.

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A more sophisticated version of this was developed in the early 1970's andis known as the Booth bend after its originator. This is a very short radius castbend which incorporates a shallow depression. This allows material to collect inthe bend and so subsequent material flowing through the pipeline will impactagainst itself. A sketch of the bend is given in Figure 20.17.

Another, more recent version, is a short radius bend with a large recessedchamber in the area of the primary wear point. It is claimed that this acts as a vor-tice and that material is constantly on the move in this pocket, thereby providing acushioning effect.

Consideration, however, has to be given to the location and orientation ofthe bend. Such a bend located close to the material feed point may require aslightly higher conveying line inlet air velocity. Limited published test work on thebend has shown the pressure drop across the bend to be quite high (see section5.3.1 of Chapter 8). A sketch ofthe bend is given in Figure 20.18.

5.2.2 Air Injection

A number of bend protection devices have been proposed that incorporate the in-jection of air into a bend. The object of these is to deflect the impacting particlesaway from the bend wall. The main problem with this type of device in a pneu-matic conveying line is that air injection has to be continuous at each bend.

Pipe Plug

Figure 20.17 Sketch of short radius bend with shallow depression.

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Flow Direction

Figure 20.18 Sketch of vortice pocket bend.

In a pipeline of constant bore this will result in a further increase in velocity,and since erosion is highly dependent upon velocity, this method does tend toaggravate the problem. The pipeline can be stepped to a larger diameter part wayalong its length, but this adds considerably to the cost of the plant and the com-plexity of its design, and so this method is rarely used.

3.2.3 The Use of Hard Materials

Hard, brittle materials are generally used in cases of severe wear. Materials usedinclude Ni-hard, basalt and ceramics, as discussed in the section on Hard Materialsabove. These materials can generally be cast or formed into sections, and in thecase of non-metals, are used for lining pipes and bends. Care must be taken withcast materials, however, as mentioned above in relation to Surface Finish, for if aporous surface if obtained, rapid erosion can result. Because of the impact angleeffect such bends do need to have a reasonably long radius.

3.2.4 The Use of Resilient Materials

Resilient materials such as rubber and polyurethane are widely used in erosivewear situations. Although the hardness of the surface is often far lower than that ofthe material being conveyed, and impacting against the surface, they derive theirerosion resistance from the fact that they are able to absorb much of the impactenergy, without being permanently damaged, by virtue of their resilience, as men-tioned earlier.

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Several programs of tests have been carried out to compare rubber and steelbends in pneumatic conveying pipelines [18]. In one such program a pipeline wasbuilt in which several rubber and steel bends were alternately positioned at thecorners of a test loop so that they could be tested at the same time for a directcomparison. Tests were carried out with lump coke and fine silica sand, each con-veyed at a solids loading ratio of 10 and with a conveying air velocity of about5000 ft/min. Figures 20.19 and 20.20 show the comparative wear effects of thecoke and sand on the rubber and steel bends very well.

These are pipe section profiles taken at the point around the bend where ei-ther the bend failed or where the penetrative wear was a maximum. Each bend wastwo inch nominal bore, with a pipe wall thickness of 0-16 inch in the case of thesteel bends and 0-40 inch in the case of the rubber bends. To illustrate the differenterosive wear profiles the pipe wall has been magnified by a factor of 1-5 in rela-tion to the pipe bore.

Figure 20.19 compares the pipe section profiles of the steel and rubberbends when eroded by coke. The rubber bends failed after about 50 ton of cokehad been conveyed through them, at which time about 2-0 oz had been erodedfrom the bends. Only 1 • 1 oz had been eroded from the steel bends, however, andthey would probably be capable of conveying another 50 ton before they wouldfail. In terms of potential service life, therefore, the steel bends could be expectedto last twice as long as the rubber bends for the conveying of the coke.

Figure 20.20 compares the pipe section profiles of the steel and rubberbends when eroded by the sand. In this case the steel bends failed after only 3-8ton of the sand had been conveyed through them, at which time about 1-9 oz hadbeen eroded from the bends.

Synthetic Rubber Steel Bend

Figure 20.19 Comparison of bend section wear profiles for bends eroded by coke.

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Synthetic Rubber Steel Bend

Figure 20.20 Comparison of pipe section wear profiles for bends eroded by sand.

Only 0-35 oz was eroded from the rubber bends at this stage, and they werequite clearly capable of handling considerably more sand before they would fail.In terms of potential service life, therefore, the rubber bends could be expected tolast about five times as long as the steel bends for the conveying of the sand.

Thus for bulk materials having a large particle size, such as lump coal, cokeand mined and quarried materials, rubber bends can not be recommended for ero-sive wear applications, and certainly not in high velocity, dilute phase conveyingapplications. It is believed that there is a threshold value of impact energy thatresilient materials such as rubber can withstand without suffering significant dam-age, as discussed in the section on Resilient Materials above. As either particlesize or impact velocity increase the impact energy of a particle will increase. Inrelation to velocity this effect was shown quite clearly in Figure 20.10.

3.2.5 Surface Coatings

A wide range of materials can be applied to existing surfaces, and in many casesthey are applied to erosion resistant surfaces, such as Ni-hard, to give added pro-tection. Polyurethane, which cures at ambient temperature, is often used. This canbe sprayed, or applied in putty form by trowel, which is particularly useful forrepairing eroded surfaces. Hard-facing metal alloys, tungsten carbide and a rangeof oxide ceramics can be applied to surfaces by means of flame spray coating.

Some of these materials have very high hardness values, and combined withthe fact that the surfaces can also be very smooth, they can provide good erosionresistance. The surface coatings, however, can generally be applied only in thinlayers and so once this is penetrated the bend will rapidly fail.

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3.2.6 Wear Back Methods

Wear back methods are potentially the cheapest and most effective means of sup-pressing bend erosion and are commonly used in industry. A channel welded tothe back of a bend and filled with concrete, as shown in Figure 20.21, is probablythe most common method adopted. When the outer surface of the original steelbend erodes, the concrete will generally extend the life of the bend for a reasona-bly long time. It is essential, however, that the wear back covers as much as possi-ble of the outer bend surface, for bends can be holed over a wide range of bothbend and pipe angles.

The only problem with this type of solution is that when a primary wearpoint is established in the concrete at the initial impact point, deflection of parti-cles can result, and these may cause erosion of the inside surface of the bend. Thebend may well fail through erosion of the inside surface long before the materialhas penetrated the concrete. Secondary and tertiary wear pockets in long radiusbends may also cause the material to be deflected against the wall of the followingstraight length of pipe and cause this to fail. These points are considered in moredetail below.

A similar method of prolonging bend life is to sleeve the main bend withanother pressure-tight bend, which is also shown in Figure 20.21. This providesprotection for both the inner and outer surfaces of the bend. When the inner bendfails the space will fill with the material being conveyed. Subsequent impact wil lmostly be conveyed material against conveyed material and so erosion of the in-side surface is not likely to cause failure of the bend.

This is not likely to work where the erosion at the secondary wear point is sogreat that very large areas of the bend are eroded away. In this case it might help ifthe annular gap was filled with a concrete, possibly made from the conveyed mate-rial itself.

Pressure-TightSleeve Over Bend

Concrete-FilledChannel Wear Back

Figure 20.21 Wear back methods of bend reinforcement.

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flL

Figure 20.22 Bend protection by use of sacrificial inserts in the preceding straightlength of pipeline.

3.2.7 The Use of Inserts

Considerable protection can be provided for a bend by positioning a sacrificialinsert in the pipeline just prior to the bend. An insert made of a flat strip andtwisted through 180°, for example, and shown in Figure 20.22, will ensure that thematerial impacts against the insert prior to impact against the bend. The velocity ofthe particles will be reduced after impact with the strip and the presence of thestrip will prevent the particles from focusing on a small area of the bend. Such astrip should offer little resistance to flow and should last for a reasonable period oftime, since the wear would be very evenly distributed over the entire surface of thestrip [19].

3.2.8 Ease of Maintenance

In terms of ease of maintenance, very short radius bends have the particular advan-tage of their much lighter weight. These can generally be removed and changed bytwo men without the use of special lifting equipment. Bends with the provision ofreplaceable wear backs are also very useful in this respect, such as that shown inFigure 20.23, as the bend itself does not have to be removed or replaced.

These are usually made of Ni-hard or similar material. The backs must bereplaced on a regular basis, however, and not when failure occurs. If they are leftuntil failure occurs, much of the body may have worn away and it may not be pos-sible to guarantee an air-tight seating. If the material being conveyed is potentiallyexplosive, the possibility of the bend wearing to a point where it will be incapableof withstanding the explosion pressure generated must also be considered.

With large bore pipelines square section bends are often fabricated, and insuch a manner that the outer wall can be removed, as illustrated in Figure 20.24.This allows for easy replacement. Alternatively the backing plate can be made of adifferent material, or be given a lining of a costlier material, to resist the erosivewear. Laminated backings of rubber and steel are often used.

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Figure 20.23 Bend with reinforced and replaceable back.

3.3 Wear Patterns and Deflecting Flows

Mason and Smith [20] carried out tests on one and two inch square section 90°bends with a flow of alumina particles from vertically up to horizontal. The bendswere made of Perspex and were constructed with substantial backing pieces.

Figure 20.24 Fabricated bend with square section and replaceable outer wall.

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Horizontal

Flow Direction

Figure 20.25 Wear and flow pattern for an eroded bend.

The substantial reinforcement was provided in order that the change in flowpattern and wear over a period of time could be visually observed. The resultsfrom one of their tests are given in Figure 20.25.

With a new bend the particles tend to travel straight on from the precedingstraight pipeline until they impact against the bend wall. After impact they tend tobe swept round the outside surface of the bend. They are then gradually entrainedin the air in the following straight length of pipe. In Figure 20.25 the flow patternis shown after substantial wear has occurred.

This shows quite clearly the gradual wearing process of a bend and the ef-fect of impact angle on the material in the process. Erosion first occurred at a bendangle of 21° which became the primary wear point. After a certain depth of wearpocket had been established, however, the particles were deflected sufficiently topromote wear on the inside surface of the bend, and then to promote a secondarywear point at a bend angle of 76°.

A small tertiary wear point was subsequently created at an angle of 87°. Ifsuch a well reinforced bend were to be used in preference to replacing worn bends,the deflection from the latter wear points would probably cause erosion of thestraight pipe section downstream of the bend. Because this pattern of particle de-flection in worn bends is now well recognized, some companies manufacture steelbends with thicker walls and a typical example is shown in Figure 20.26.

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Figure 20.26 Bend with reinforced walls.

This particular bend is also slightly thicker on the inside surface to allow forthe fact that particles can be deflected to the inside surface, as illustrated in Figure20.25, particularly in smaller diameter pipelines.

3.3.1 Influence of Impact Angle

The curve in Figure 20.1, of erosion against impact angle for the aluminum alloy,provides a means by which an interpretation of the type of wear shown in Figure20.25 can be obtained. The outer wall of the bend presents a surface at a low im-pact angle to the particles issuing from the preceding vertical straight pipe run, andas Perspex is a ductile material rapid erosion takes place. Gradually the impactangle at this primary wear point changes to almost 90°. From Figure 20.1 it will beseen that ductile materials suffer relatively little erosion under normal impact, andthis explains why little further erosion takes place at this point.

The conveyed material can be seen quite clearly to be deflected out of thisprimary wear pocket. Because of this abrupt change in direction, however, it is nolonger swept around the bend as before, but impacts on the inside surface of thebend. It is then deflected to the outer wall again, and because the low impact angleis maintained here, the erosion at this point is far greater than that at the primarywear point.

Mason and Smith [20] also mention that a conventional bend design, used toavoid plant shut down due to wear, is to reinforce the outside of the bend with amild steel channel backing filled with a suitable concrete, as illustrated in Figure20.21. They included a radiograph of such a four inch bore pipeline bend and thisshows a primary wear pocket developing in precisely the same manner as for the

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Perspex bend test. It is believed that the bend ultimately failed through erosion ofthe inner surface due to material deflection from the primary wear point.

3.4 Wear of Straight Pipeline

Straight pipeline is rarely a problem with regard to erosive wear, although thereare specific circumstances where it should be taken into account. Reference hasalready been made above to the deflection of particles issuing from a well rein-forced eroded bend. Similar deflections can be promoted from poorly aligned pipesections, and large abrasive particles present a particular problem.

3.4.1 Following Bends

It will be seen from Figure 20.25 that the straight section of pipeline, following awell reinforced bend, is liable to erosive wear. Although the angle of impact of theparticles is generally low, for a ductile material low angle impact is likely to resultin significant wear, because of the remarkably steep increase in erosion with verysmall increase in impact angle, as illustrated in Figure 20.1.

To extend the life of the pipeline following a bend, therefore, it is suggestedthat a short section of thick walled pipe should be placed between the bend and themain pipeline, as illustrated in Figure 20.27. The section of thick walled pipe fol-lowing a bend does not have to be very long for the deflecting flow is soon damp-ened out. A 6 ft section is generally long enough for small bore pipelines, andsomething of the order of twenty pipe diameters should be allowed for larger borepipelines.

Since the flow of deflected particles issuing from a bend will generally im-pinge constantly on the same area of the thick walled pipe it is also recommendedthat this short section of pipe should be connected by flanges to the bend and thefollowing section of regular pipeline so that it can be rotated on a regular basis.

Figure 20.27 Thick walled section of straight pipeline following reinforced bend.

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This will both help to extend the life of this section of pipe and prevent alarge wear pocket forming which could result in a further site for particle deflec-tion to occur.

Hot dust laden gases from boilers and reactors are often passed through heatexchangers for generating steam. The tubes through which the gases flow oftenwear, and are generally very expensive to repair. The wear is usually only at thestart of the tube. This is because the dusty gases on entry to the tube are in a veryturbulent state and numerous particle impacts occur.

After a short distance the flow is effectively straightened out and little fur-ther pipeline wear occurs. An effective solution to the problem is to provide a sac-rificial extension to the pipe array prior to the tube plate and the heat exchangesection for flow straightening purposes.

3.4.2 Pipe Section Joints

Misaligned flange joints, and welded joints with weld metal protruding inside thepipeline, as illustrated in Figure 20.28, can often lead to straight pipeline failure,particularly in small bore pipelines. It is a similar situation to the wear pocketsformed in bends, since the step produced can result in particle streaming. This isparticularly a problem if rubber hose is attached to steel pipe by means of pushingthe hose over the steel pipe or hose clamp fitting. A small step will be formed andthis can cause severe streaming of particles where the fitting presents a reductionin area to the oncoming flow.

3.4.3 Large Particles

Small particles will generally be conveyed through a pipeline with little contactwith the pipeline wall in dilute phase suspension flow, in the absence of flowstreaming and turbulence promoting sites. With large particles, however, gravita-tional force has a much greater effect. Large particles can be conveyed quite suc-cessfully, but in horizontal flow they will tend to skip along the pipeline.

(a) (b) IPFigure 20.28 (a) Welded and (b) flanged examples of erosion promoting sites atpoorly jointed pipeline sections.

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They will convey in suspension, but gravity wil l give them a low trajectoryin their flow, and hence they will impact fairly frequently with the pipeline wall.The impact angle will be very low but, as has been discussed before, wear of duc-tile pipeline materials can be significant as a result of glancing impact from abra-sive particles.

Erosive wear, as a result, will be concentrated along the bottom of the pipe-line. Since it is not generally very convenient to reinforce a pipeline along its en-tire length, in order to overcome this problem, it is recommended that the pipelineshould be rotated periodically. By this means the pipeline wil l last for a very muchlonger period of time. It is important to recognize this problem when the pipelinerouting is being planned, however, for the horizontal sections of pipeline need tobe located where convenient access can be gained to carry out the rotating process.

REFERENCES

1. G.P. Tilly. Erosion Caused by Impact of Solid Particles. Treatise on Materials Scienceand Technology, Vol 13, pp 287-319. Academic Press Inc. 1979.

2. B.J. Hockey and S.M. Wiederhorn. Erosion of ceramic materials: the role of plasticflow. Proc 5th ELSI Conf. Paper 26. Cambridge. Sept 1979.

3. E.E. Smeltzer. et al. Mechanics of metal removal by impacting dust particles. ASMEWinter An Mtg. Los Angeles. Paper 69-WA/Met-8. Nov 1969.

4. G.P. Tilly and W. Sage. The interaction of particle and material behavior in erosionprocesses. Wear, Vol 16, pp 447-465. 1970.

5. D. Mills and J.S. Mason. Conveying velocity effects in bend erosion. Jnl of Pipelines.Vol l .pp 69-81. 1981.

6. G.P. Tilly. Erosion caused by airborne particles. Wear, Vol 14, pp 64-69. 1969.7. D. Mills and J.S. Mason. The effect of particle size on erosion of pipe bends in pneu-

matic conveying systems. Proc Powtcch '79. NEC Birmingham. March 1979.8. J.E. Goodwin. W. Sage, and G.P. Tilly. Study of erosion by solid particles. Proc

IMechE, Vol 183, No 15, pp 279-292. 1969-70.9. K.N. Tong, D. Mills, and J.S. Mason. The influence of particle hardness on the ero-

sion of pipe bends in pneumatic conveying systems. Proc 6th Powder and Bulk SolidsConf. pp 281-293. Chicago. May 1981.

10. M.M. Khruschof. Resistance of metals to wear by abrasion, as related to hardness.Proc IMechE Conf on Lub and Wear, pp 655-659. 1959.

11. 1. Finnie. Erosion by solid particles in a fluid stream. Spec Tech Pub No 307 of AmSoc for Testing Materials, pp 70-82. June 1961.

12. I. Finnie et al. Erosion of metals by solid particles. ASTM Jnl of Mails, Vol 2, pp 682-700. Sept 1967.

13. J.S. Mason et al. The rapid erosion of various pipe wall materials by a stream of abra-sive alumina particles. Proc Pneumotransport 2. Paper E l . Guildford. 1973.

14. D. Mills and J.S. Mason. Evaluating the conveying capacity and service life of pipebends in pneumatic conveying systems. Jnl Powder and Bulk Solids Tech. Vol 3, No2, pp 13-20. 1979.

15. D. Mills and J.S. Mason. Particle size effects in bend erosion. Wear, Vol 44, pp 311-328. 1977.

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16. E. Raask. Impact erosion wear caused by pulverized coal and ash. Proc 5th ELS1 Conf.Paper 41. Cambridge. Sept 1979.

17. K.N. long. D. Mills, and J.S. Mason. The influence of bend radius on the erosion ofpipe bends in pneumatic conveying systems. Proc 5th Powder and Bulk Solids Conf.Chicago. May 1980.

18. V.K. Agarwal, D. Mills, and J.S. Mason. A comparison of the erosive wear of steeland rubber bends in pneumatic conveying system pipelines. Proc 6th ELSI Conf. Paper60. Cambridge. Sept 1983.

19. V.K. Agarwal and D. Mills. The use of inserts for reducing bend wear in pneumaticconveying system pipelines. Proc 14th Powder and Bulk Solids Conf. Chicago. May1989.

20. J.S. Mason and B.V. Smith. The erosion of bends by pneumatically conveyed suspen-sions of abrasive particles. Powder Technol. Vol 6. pp 323-335. 1973.

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21Material Degradation Problems

1 INTRODUCTION

Many materials that have to be conveyed are friable, and particles are liable to bebroken when they impact against retaining surfaces, such as bends in the pipeline.As a consequence there is often a reluctance to use pneumatic conveying systemsfor this category of materials, particularly if the material has to be conveyed indilute phase and hence at high velocity. There are, however, numerous means bywhich the problem can be reduced to an acceptable level.

1.1 Breakage Mechanisms

In some bulk solids handling processes intentional breakdown of the material isrequired, as in crushing, grinding and comminution. In many handling and storagesituations, however, unintentional breakage occurs. This is usually termed degra-dation or attrition, depending on the mechanism of particle breakage. Bulk materi-als, when pneumatically conveyed, will impact against bends in the pipeline, andthere may be a significant amount of particle to particle interaction in addition.There may also be frequent impacts against the pipeline walls, and in dense phaseflows particles wil l slide along the pipeline walls. These collisions and interactionswill produce forces on the particles that may lead to their breakage.

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If particle breakdown occurs readily the bulk solid is said to be friable. Ten-dency to particle breakdown covers three main situations. The first is a tendency toshatter or degrade when the bulk solid is subject to impaction or compressive load-ing. The second is the tendency for fines and small pieces to be worn away byattrition when bulk solids either rub against each other or against some surface,such as a pipeline wall or bend. The third is the tendency for materials such asnylons and polymers to form angel hairs when conveyed, as a result of micro-melting occurring due to the particles sliding against pipeline walls.

1.2 Magnitude of Problem

Of all conveying systems, dilute phase probably results in more material degrada-tion and attrition than any other. This is because particle velocity is a major vari-able in the problem and, in dilute phase conveying, high velocities have to bemaintained. The potential influence of a pneumatic conveying system on a mate-rial is demonstrated in Figures 21.1 and 21.2 [1], This is a consequence of convey-ing a friable material at an excessively high velocity in dilute phase suspensionflow in a conveying system with a large number of small radius bends.

Figure 21.1 shows the influence on the cumulative particle size distributionfor the material before and after conveying. The mean particle size, based on the50% value, has changed from about 177 to 152 urn. The really significant effect,however, is shown in the fractional size distribution plot in Figure 21.2. In thismagnified plot the effect of degradation on the material can be clearly seen. Aconsiderable number of fines are produced and even on a percentage mass basisthese cause a significant secondary peak in the particle size distribution.

100

80

60

3 40

20

Material beforeconveying

Material afterconveying

40 80 120 160

Particle Size - urn

240

Figure 21.1 Possible influence of pneumatic conveying on cumulative particle sizedistribution of a friable material.

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3 40oncC3

os§

c/5u 30

•S 20

a 10

Material beforeconveying

Particle size

Material afterconveying

40 80 120 160

Particle Size - u.m

200 240

Figure 21.2 Possible influence of pneumatic conveying on fractional size distributionof a friable material.

1.3 Operating Problems

Particle degradation can cause problems in a number of areas on account of thechanges in particle shape and particle size distribution that can result. It is a par-ticular problem with chemical materials that are coated, for it is the coating that isgenerally the friable element of the resulting material. Plant operating difficultiesare often experienced because of the fines produced, and problems in handlingoperations can also result after the material has been conveyed.

Apart from the obvious problems of quality control with friable materials,changes in particle shape can also lead to subsequent process difficulties with cer-tain materials. The appearance of the material may also change so that it is not soreadily sold. Changes in particle size distribution can affect flow characteristics,which in the extreme, can change a free-flowing material into one which will onlyhandle with great difficulty and, with materials for subsequent sale, this can lead tocustomer problems.

1.3.1 Filtration Problems

In pneumatic conveying systems plant, operating difficulties can result if degrada-tion causes a large percentage of fines to be produced, particularly if the filtrationequipment is not capable of handling the fines satisfactorily. Filter cloths andscreens wil l rapidly block if they have to cope with unexpectedly high flow ratesof fine powder. The net result is that there is usually an increase in pressure drop

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across the filter, and this could be a significant proportion of the total pressureavailable in a low pressure system.

This means that the pressure drop available for conveying the material willbe reduced, which in turn means that the mass flow rate of the material wi l l proba-bly have to be reduced in order to compensate. If this is not done there will be therisk of blocking the pipeline. Alternatively, if the filtration plant is correctly speci-fied, with material degradation taken into account, it is likely to cost very muchmore as a result. This, therefore, provides a direct financial incentive to ensure thatparticle degradation is minimized., even if it does not represent a problem withrespect to the material itself [2],

1.3.2 Flow Problems

In many systems there is a need to store the conveyed material in a hopper or silo.Flow functions can be determined for bulk particulate materials, from which hop-per wall angles and opening sizes can be evaluated, to ensure that the materialflows reliably at the rate required. A change in particle size distribution of a mate-rial, as a result of conveying operations, however, can result in a significantchange in flow properties. Thus a hopper designed for a material in the "as re-ceived" condition may be totally unsuitable for the material after it has been con-veyed. As a result it may be necessary to fit an expensive flow aid to the hopper torecover the situation.

1.3.3 Potential Explosion Problems

Many materials, in a dust cloud, can ignite and cause an explosion. Dust cloudsare clearly quite impossible to avoid somewhere in a pneumatic conveying system,and so this poses a problem with regard to the safe operation of such systems. Ofthose materials that are explosive, research has shown that it is only the fractionwith a particle size less than about 200 u.m that poses the problem. Degradationand attrition caused by pneumatic conveying, however, can result in the generationof a considerable number of fines, particularly if the material is friable. Even if thematerial did not represent a problem with respect to explosions in the "as re-ceived" condition, the situation could be very different after the material has beenconveyed.

1.4 Test Rigs and Data Sources

Little data is available on the degradation of materials in pneumatic conveyingsystems. This is partly due to the complexity of obtaining and analyzing the data,but mainly to the fact that so many variables are involved, together with the prob-lem of relating the data from one material and situation to another. A particularproblem with data obtained from a pneumatic conveying system pipeline is that itis very difficult to separate the individual contributions made by the bends and thestraight pipeline.

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A further problem is that in a pipeline there is a gradual expansion of theconveying air, which means that the particle velocity is constantly changing. Ve-locity is a major variable in particle degradation and so this makes attempts at de-vising experimental plans and analysis of results very difficult.

The major source of information is probably from the basic research that hasbeen undertaken with small bench scale test rigs in which particles have been im-pacted against test materials under controlled conditions. This work has often beencarried out to assist in an understanding of erosive wear problems. Although muchof this work cannot be related directly to pneumatic conveying situations, it canprovide valuable information of a comparative nature on a number of variables inthe process.

1.4.1 Acceleration Tube Device

Test facilities employed are very similar to those used in erosive wear research,such as whirling arm and acceleration tube devices. A device used by Salman et al[3] is shown in Figure 21.3 and consists of a linear air gun. One particle was testedat a time. Compressed air was used to accelerate the particles, and particle velocitycould be varied by adjusting the air pressure.

A cage was used to collect the particles and fragments after impact. The par-ticle impact velocity was determined by measuring the time required for a particleto travel from the end of the barrel to the target. A photodiode was located at theend of the barrel and a loudspeaker was mounted behind the target.

PressureRegulating

Valve

Cage

\

CompressedAir Supply

Particle Impact.Angle

Speaker

Computer High-SpeedElectronic Timer

Figure 21.3 Schematic arrangement of acceleration tube test apparatus and measuringsystem for particle impact studies.

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608 Chapter 21

In order to study the particle degradation process, brittle materials were usedto ensure that no plastic deformation should take place. Three types of particlewere used and tested. These were aluminum oxide, polystyrene and glass, and allthe particles were spherical. The majority of the work was carried out with 0-2 indiameter aluminum oxide particles, with particle velocities up to about 6000ft/min. For every test, 100 particles were impacted, and the number of unbrokenparticles was counted to provide an assessment of the degradation.

2 INFLUENCE OF VARIABLES

The variables in particle degradation are similar to those associated with erosivewear. Velocity, once again, is probably the most important, but particle size andconcentration also play a part. Particle impact angle is equally important, and has amajor influence with respect to the selection of pipeline bend geometry. The influ-ence of both particle materials and surface materials must also be given due con-sideration. As with erosive wear, much of the research work into the subject hasbeen carried out for various other purposes, and so the range of parameters inves-tigated is often beyond those associated with pneumatic conveying, but it doesprovide useful information on the general trends of the variables.

2.1 Velocity

The relative velocity between particles and surfaces has a major influence on thenature and extent of the degradation and is probably the most important variable inthe problem. In any collision the kinetic energy of the particles has to be absorbedand may provide sufficient energy for fracture. If the collision is elastic, with ahigh coefficient of restitution, much of the kinetic energy will reappear as particlevelocity. In plastic collisions much of the kinetic energy will be converted to heat.

Low velocity impacts tend to knock small chips from the edges of particles,whereas high velocity collisions are more likely to shatter particles. In general therate of damage has been found to be a power law function of velocity, in much thesame way as the erosive wear process. The range in value of the power coefficientis also large, and can vary between one and five, depending upon the conveyedmaterial and the system being considered. The possibility of there being a thresh-old value of velocity, below which no degradation occurs, is also a possibility.

2. /. / Peas

Agricultural products have been widely used in test work. Segler [4] investigatedthe effects of air velocity, moisture content, pipeline diameter and material con-centration on the damage of peas, as a result of pneumatic conveying. His test loopwas 240 ft long, 4'/2 in bore and contained 4 bends. The results of his tests on theeffect of aii- velocity are presented in Figure 21.4. These showed that the damageincreased approximately with the cube of air velocity.

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Particle Degradation 609

12

IX^Ho

S 4

2000 4000

Conveying Air Velocity - ft/min

6000

Figure 21.4 The influence of air velocity on the breakage of peas.

2.7.2 Quartz

Tilly et al [5] carried out impact studies with quartz particles against an alloy steeltarget in a rotating arm test rig. They found that the particles incurred a substantialdegree of fragmentation which was dependent upon the velocity of impact. Theirresults are presented in Figure 21.5.

I

Particle size - 125-150 urnTaraet Material - 11% Cr steel

100

80

60

40i-M

I 20g?o

00 20,000 40,000 60,000

Particle Velocity - ft/min

Figure 21.5 The influence of particle velocity on the degradation of quartz particles.

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610 Chapter 21

The velocity range comes as a result of their work being applied to aircraftengines. From this it would appear that for fragmentation to occur it is necessaryto exceed a threshold value of velocity. Below this velocity the particles may beconsidered to behave elastically. From Figure 21.5 this would appear to occur at avelocity of about 3000 ft/min for this material.

In work by Tilly and Sage [6] they impacted quartz particles in the sizerange of 100 to 225 urn at velocities of 12,000, 26,000 and 60,000 ft/min. Theirresults, in terms of particle size distribution, are presented in Figure 21.6. Al-though this data is for conveying velocities very much higher than those thatwould be encountered in a pneumatic conveying system, they relate to a singleimpact and so help to illustrate the nature of the problem, for many materials thatare conveyed are significantly more friable than quartz.

2.7.3 A lum inum Oxide

The results of a program of tests carried out with 0-2 inch aluminum oxide parti-cles impacted at 90° against a steel target are presented in Figure 21.7 [3"|. In thisplot the experimental data has been included to show how the relationship wasderived and to show the limits of scatter in the results. The relationship is typicalof the results obtained and so where families of curves are presented in subsequentfigures from this program of work, experimental data has been omitted for clarity.

It will be seen from Figure 21.7 that there is a very rapid transition in parti-cle velocity from zero breakage to total degradation. Below a particle velocity ofabout 1800 ft/min only elastic deformation occurs and there is no particle degrada-tion. Above a particle velocity of about 5000 ft/min, however, the stress inducedby the impact is always sufficient to damage every particle.

100

80

60

40

20

Particle Velocity- ft/min

BeforeImpact

Particles - QuartzTarget - 11% Cr Steel

50 100

Mean Particle size

150 200

Figure 21.6 Influence of particle velocity on size distribution generated with quartz.

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Particle Degradation 611

100

80

60

CD 40

X 20

Particle size = 0-2 inImpact angle = 90°Target material = Steel

2000 3000 4000

Particle Velocity - ft'min

5000 6000

Figure 21.7 The influence of particle velocity on the degradation of aluminum oxideparticles.

It is interesting to note that within the transition region the number of unbro-ken particles at any given velocity is very consistent and that a smooth transition isobtained from one extreme to the other over this range of velocity. This wasprobably one of the first research programs to focus on particle degradation in thevelocity range appropriate to pneumatic conveying.

2.2 Particle Size

Tilly et al [5] carried out impact studies with quartz particles against an alloy steeltarget in a rotating arm test rig. They found that the particles incurred a substantialdegree of fragmentation which was dependent upon the initial particle size. Theirresults are presented in Figure 21.8.

From this it would appear that for fragmentation to occur it is necessary forthe particles to exceed a threshold size of about 10 urn. Below this size the parti-cles probably behave elastically, for in their test rig the particles would have im-pacted the target since the tests were carried out in a vacuum.

The results of tests carried out with three different sizes of spherical alumi-num oxide particles are shown in Figure 21.9 [3], The data for the 0-2 in particles,which was the reference material in the work, was presented above in Figure 21.7.Results from similar tests with 0-12 and 0-28 in aluminum oxide particles, alsoimpacted at 90° against the same steel target are additionally presented in Figure21.9.

A very significant particle size effect is shown. As the particle size in-creases, the maximum velocity at which no degradation occurs decreases. Thetransition from no degradation to total degradation also changes, with the transi-tion occurring over a narrower velocity range with increase in particle size.

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612 Chapter 21

100

80

60S01)S

40

ob 20

Particle Velocity - 60,000 ft/minTarget Material - 11% Cr Steel

50 100 150

Initial Mean Particle size - u.m

200

Figure 21.8 The influence of initial particle size on the degradation of quartz particles.

2.2.7 Particle Velocity Influence

In more recent work on the influence of particle size, fertilizer particles, also hav-ing particle diameters of 0-12, 0-20 and 0-28 in, were pneumatically conveyed in atest facility to assess their degradation [7]. In this case the velocity used was that ofthe conveying air and not that of the particles. In terms of air velocity the 0-12 inparticles degraded the most and the 0-28 in particles the least.

100

0-

g 60^XJc

o!-~O

X)

40

20

Impact Angle-90°

Target Material- Steel

0-12

1000 2000 3000 4000

Particle Velocity - ft / min

5000

Figure 21.9 The influence of particle velocity and particle size on the degradation ofaluminum oxide particles.

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Particle Degradation 613

The reason for this is that when it is the air velocity that is held constant, thesmaller particles are accelerated to a higher velocity than the larger particles, asillustrated earlier with Figure 15.10. It is because particle velocity has a greaterinfluence on degradation than particle size that a reversal in the influence of parti-cle size has occurred.

2.3 Surface Material

With erosive wear of surface materials it has been found that the resilience of thesurface material can have a significant influence on erosive wear, and that rubbersand polymers can offer better wear resistance than metals having a very highhardness value in certain cases. Since the mechanisms of erosion and degradationhave many similarities, it is quite possible that resilient materials could offer verygood resistance to particle degradation.

2.3.1 Material Type

Further work by Tilly and Sage [6] showed that fragmentation is also dependentupon the type of target material. Figure 21.10 shows a comparison of their resultsfor quartz impacted against nylon and fiberglass, which with their earlier resultsfor alloy steel demonstrates the complex nature of the problem. Degradation interms of the influence of initial particle size is used for the comparison in this case.

The results of tests carried out on four different target materials are pre-sented in Figure 21.11 [3]. In each case the targets were 0-2 in thick and they wereimpacted by 0-2 in diameter aluminum oxide particles at an angle of 90°.

100xoo-

c 80

I 60

5hcd

'•o 40<L>

aQ 20

0

Target Material

Impact Angle - 90°

Particle,Velocity/

Steel - 60,000 ft/min

Fiberglass - 50,000 ft/min

50 100 150

Initial Mean Particle size -

200 250

Figure 21.10 The influence of initial particle size and target material on the degrada-tion of quartz particles.

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614 Chapter 21

100r •*—- •"•— —"•= — Plexiglass

80L ^ ^ . . .Aluminum

60

5 40<+-<o

I 20

J:

Particle Size - 0-2 inInroad Angle - 90°

1000 2000 3000 4000 5000

Particle Velocity - ft/min

Figure 21.11 The influence of particle velocity and target material on the degradationof aluminum oxide particles.

This also shows very clearly that target material can have a very marked ef-fect on degradation. Although there is little difference in the maximum value ofparticle velocity at which no degradation occurs, varying from 2000 ft/min forsteel to about 3000 ft/min for Plexiglas and aluminum, very significant differencesexist in the transition region between no degradation and total degradation. In thecase of the steel and glass targets the transition is very rapid. For the aluminumand Plexiglas, however, the transition is very slow, and so a high velocity impactagainst these materials would only result in limited damage occurring.

2.5.2 Surface Thickness

A similar program to that reported in relation to Figure 21.11 was carried out withsteel targets of varying thickness [7]. If the conveyed material is not abrasive, inaddition, a thin walled surface would also help reduce degradation, for the workshowed a significant reduction in degradation of the particles with an 0-04 in thicktarget as compared with an 0'08 in thick target.

The force acting on a particle is equal to its mass times the rate of decelera-tion. This force must be reduced in order to reduce the damage to particles on im-pact against a surface. This can be achieved to a certain extent by using either aresilient surface material or a surface material that wi l l flex on impact.

2.4 Particulate Material

In Figure 21.12 the data for the aluminum oxide of Salman et al [3J is presentedagain, together with the results from identical tests carried out with polystyreneand glass particles. It will be seen from this that polystyrene particles suffer a simi-lar transition from zero breakage to total degradation.

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Particle Degradation 615

100

Impact Angle - 90°Target Material - Steel5 60

o

1 20

40Aluminum11

Oxide

2000 3000 4000 5000 6000

Particle Velocity - ft/min

Figure 21.12 The influence of particle velocity on the degradation of various particu-late materials.

The only difference is that the transition occurs at a slightly higher velocityrange than the aluminum oxide. That different particulate materials can respond intotally different ways is clearly demonstrated by the glass particles. No damagewas observed to any of the particles tested up to the maximum particle velocityinvestigated of 6000 ft/min.

2.5 Particle Impact Angle

Particle impact angle, oc, was defined on the sketch of the acceleration tube testdevice shown earlier in Figure 21.3, and is the same as that used in erosive wearwork. Impact angle has been shown to be a major variable with regard to the ero-sive wear of surface materials, and hence is an important consideration in terms ofmaterial selection and the specification of components such as pipeline bends. Inrelation to particle degradation it is equally important, for as the impact angle re-duces, so the normal component of velocity decreases [8]. This will have a directbearing on the deceleration force on the particles, as discussed above in relation toSurface Thickness.

The results of a comprehensive program of tests carried out to investigatethe influence of particle impact angle are presented in Figure 21.13 [3]. 0-2 in alu-minum oxide particles were impacted against a steel target, which is the referencepoint in this particular program of work, and so the data for 90° impact is the sameas that presented earlier in Figures 21.7, 21.9, 2 1 . 1 1 and 21.12. It wi l l be seenfrom Figure 21.13 that there is little change in the response to degradation unti l theimpact angle is below about 50°. There is than a very marked difference in per-formance with only small incremental changes in impact angle.

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616 Chapter 21

100t/D

'4 80C3n_

jj 60o

| 40'o« 20£

Particle Size - 0-2 inTarget Material - Steel

2000 3000 4000 5000

Particle Velocity - ft/min

6000

Figure 21.13 The influence of particle velocity and impact angle on the degradation ofaluminum oxide particles

With a decrease in particle impact angle it would appear that there is littlechange in the particle velocity at which the onset of degradation occurs. The tran-sition from zero degradation to total degradation, however, becomes an increas-ingly more gradual process as the particle impact angle reduces. At impact anglesof 15° and 20° it would appear that this transitional process will be spread over avery wide range of velocity values. At an impact angle of 10°, however, there is asignificant change once again, in that no particle degradation was recorded at allup to 6000 ft/min. In Figure 21.14 an alternative plot of the data from this programof tests is presented.

Particle size - 0-2 inParticle velocity - 4600 ft/minTarget material - Steel

10 20 30 40 50 60Particle Impact Angle - a - degrees

80 90

Figure 21.14 The influence of particle impact angle on the degradation of aluminumoxide particles.

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Particle Degradation 617

This is effectively a slice taken from Figure 21.13 at a particle velocity of4600 ft/min. It will be seen from this that tests were carried out at regular incre-ments of impact angle of about 10° between 10° and 90°. This plot shows quiteclearly that at impact angles below about 12° no degradation occurs, and that atimpact angles above about 55° the degradation remains essentially constant at themaximum value for this particular impact velocity.

2.6 Other Variables

Segler [4] investigated the influence of moisture content on particle degradationand showed that degradation can increase dramatically with decrease in moisturecontent. The results of the following three tests with peas show the sensitivity tothis variable:

Moisture Content-% 17-1 16-1 15-4Broken Particles -% 0-1 1-1 1 1 - 1

Segler investigated the effect of particle concentration and found that thedamage decreased as the solids loading increased. The damage produced when thepeas were introduced individually was four times higher than in dense flow. Asimilar effect is found in erosive wear and can be attributed to the cushioning ef-fect of dense flows.

He also examined the damage to the peas in identical pipelines havingnominal bores of 2 and 10 inch. It was found that the damage in the 2 in bore pipe-line was two to three times greater than that in the 10 in bore pipeline. His expla-nation was that the frequency of pipe wall impacts, for such large particles, wouldbe more frequent for the small bore pipeline.

3 RECOMMENDATIONS AND PRACTICAL ISSUES

The results from the various programs of work reported here have produced somevery interesting relationships with respect to many of the variables investigated,and should provide useful guidance to the design engineer who has to ensure thatmaterial degradation is reduced to a minimum in pneumatic conveying systempipelines.

3.1 Particle Velocity

Particle velocity has been a major consideration in this work and it has beenshown quite clearly that there is a threshold value of particle velocity below whichno degradation occurs. The value of this particle velocity for the aluminum oxidewas about 2000 ft/min and was influenced only slightly by particle size, targetmaterial, and particle impact angle above about 15°.

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618 Chapter 21

3.1.1 Dense Phase Conveying

At velocities only slightly lower than this, however, the mode of conveyingchanges from dilute phase, suspension flow, to dense phase, non-suspension flow,for many of those materials capable of being conveyed in dense phase. In densephase conveying little impact occurs in horizontal pipelines and the mode of con-veying mostly involves sliding of the particles through the pipeline. With materialshaving good permeability, conveying is in plugs and slugs, and for materials hav-ing good air retention, it is as a moving bed along the bottom of the pipeline.

When particles slide through the pipeline the interaction results in attritionrather than degradation. In dilute phase there may be little particle to pipe wallinteraction, and it is suspected that most of the damage results from impact withpipeline bends. In dense phase, although the velocity is low, there is a significantamount of particle to pipe wall interaction and for certain materials this is likely tocause more damage to the particles than the bends.

As a consequence it is possible for some materials to suffer a greater amountof damage in low velocity dense phase flow than they would in higher velocitydilute phase flow. It is important, therefore, to examine the relative effects of deg-radation and attrition on the conveyed material before deciding upon the type ofpneumatic conveying system to be employed.

3.1.2 Dilute Phase Conveying

For many materials dense phase conveying is not an option, for the majority ofmaterials can not be conveyed at low velocity in a conventional conveying system.For these materials conveying has to be in suspension flow and so if the material isfriable, degradation must be limited. To this end the material should be conveyedat as low a velocity as possible, consistent with reliable conveying, and the pipe-line should be stepped to a larger bore part way along its length to reduce the highconveying air velocities that result at the end of the pipeline.

With a 15 lbf/in2 pressure drop in a positive pressure system, discharging toatmospheric pressure, the conveying air velocity wil l approximately double fromthe material feed point to discharge. For the situation presented in Figure 21.7 itwi l l be seen that at 2000 ft/min no damage occurs, but at 4000 ft/min 80% of theparticles are broken.

As the air expands through the pipeline, therefore, it is the bends at the endof the pipeline, in a single bore line, that are likely to cause the majority of thedamage. By stepping the pipeline the maximum velocity in the pipeline could pos-sibly be limited to 3000 or 3200 ft/min, at which the degradation would be limitedto only 30%.

3.2 Particle Impact Angle

For given conveying conditions, particle impact angle is probably the most impor-tant variable with respect to pneumatic conveying system pipelines. The angle ofimpact of particles against pipeline walls will generally be very low since particles

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Particle Degradation 619

will only suffer a glancing impact. From the data presented here it would appearthat little degradation will occur in straight pipeline, even for long pipelines andrepeated impacts.

It is clearly major changes in flow direction, and in particular bends, that arelikely to result in the majority of degradation occurring. In this respect, particleimpact angle can be related approximately to the radius of curvature of a bend. Ina short radius bend the particles will impact at a high value of angle, but in a longradius bend the impact angle will be much lower, as illustrated in the previouschapter with Figure 20.14. Since degradation reduces significantly with reductionin impact angle, the use of long radius bends would be recommended in any sys-tem where particle degradation needs to be minimized.

3.3 Bend Material

The choice of material for the pipeline, and in particular the bends, provides an-other means by which particle degradation can be minimized. Although there islittle change in the value of the lower threshold velocity, below which no degrada-tion occurs, with respect to target material, there is a very significant effect on theupper threshold value.

Thus, for a given particle impact velocity, very much less damage wil l resultto particles if they impact against a surface such as Plexiglas or aluminum, thanwill occur if they impact against steel or glass. If it is possible to use a more resil-ient material, such as rubber or polyurethane, an even more significant reductionin particle degradation may be achieved.

4 PNEUMATIC CONVEYING DATA

To provide some data on the potential order of magnitude of the problem of deg-radation, for materials conveyed in dilute phase suspension flow in a pneumaticconveying system, four different materials were pneumatically conveyed and theresulting degradation was monitored. A large scale pneumatic conveying facilitywas used and on-line samples were taken for analysis. Each material was re-circulated through the test loop a number of times so that the influence of convey-ing distance could also be investigated [9],

4.1 Material Degradation Data

The data has, in fact, been presented earlier, in the chapters dealing with convey-ing data on specific materials. Thus data on coal will be found in Chapter 10 atsection 5 and with Figure 10.28. Data on both sodium chloride (common salt) andsodium carbonate (soda ash) were included in Chapter 11 around Figures 11.5 to11.7. The fourth material was silica sand and this was included in Chapter 14 withFigures 14.6 and 14.7.

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620 Chapter 21

4.2 Conveying Characteristics

The influence that pneumatic conveying can have on the conveying characteristicsof a material has been considered for many different materials. As a result of re-circulating materials, in order to derive conveying characteristics, some of themore friable materials have degraded to such an extent that the conveying charac-teristics for the material have changed significantly. It must be recognized that thepneumatic conveying of a friable material can also change the conveying charac-teristics of a material.

5 PARTICLE MELTING

Particle melting is a form of material degradation that often occurs in pneumaticconveying plant handling plastic type materials, particularly in pelletized form. Ifconventional pipeline is used, materials such as polyethylene, nylon and polyesterscan form cobweb-like agglomerates. They are variously given names such as 'an-gel hairs', 'raffia', 'snake skins' and 'streamers'.

They frequently cause blockages at line diverters and filters which requireplant interruption to remove them. Equipment is generally installed at the termi-nating end of the system for this purpose. Such equipment is necessary becausethey also cause material rejection by customers, for the presence of these contami-nants in the product is undesirable.

5.1 Mechanics of the Process

The streamers are caused by the pellets impacting against the bends and pipewalls. A considerable amount of energy is converted into heat by the friction of thetwo surfaces when they touch. If the surface of the pipe is smooth, the pellet willslide. This contact, though momentary, decelerates the particle by friction which istransformed into heat. This is generally sufficient to raise the temperature at thesurface of the pellet to its melting point. To a certain extent this is analogous to thethermal model proposed for erosive wear.

5.2 Influence of Variables

The onset of the formation of these angel hairs or streamers is the result of a com-bination of conditions. Particle velocity is the most important, but it also dependsupon the temperature of both the pipe and the pellets, and the solids loading ratioof the conveyed material.

The influence of conveying line exit air velocity for low density polyethyl-ene is shown in Figure 21.15 and the influence of solids loading ratio for this samematerial is given in Figure 21.16 [10]. In each case the degradation of the materialis expressed in terms of the mass of streamers and fines produced, in ounces, perton of low density polyethylene conveyed.

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Particle Degradation 621

O

0

Material:Low Density Polyethylene

Pipeline:Bore - 4 inMaterial - AluminumSurface - Sandblasted

Conveying Conditions:Solids Loading Ratio = 8Material Temperature = 120°F

4000 12,0006000 8000 10,000

Conveying Line Exit Air Velocity - ft/min

Figure 21.15 Influence of velocity on the degradation of LDPE.

5.3 Pipeline Treatment

The formation of streamers and fines can be reduced quite considerably by suita-bly treating the pipe wall surface. A roughened surface is necessary in order toprevent the pellets from sliding. If the surface is too rough, however, small pieceswill be torn away from the pellets instead, and a large percentage of fines will re-sult. It will also have an adverse effect on the pressure drop, and hence on materialconveying capacity.

Conveying Conditions:Exit Air Velocity = 8000 fl/minMaterial Temperature = 120°Fo

o

O

Material:Low Density Polyethylene

Pipeline:Bore - 4 inMaterial - AluminumSurface - Sandblasted

4 6

Solids Loading Ratio

10

Figure 21.16 Influence of solids loading ratio on the degradation of LDPE.

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622 Chapter 21

Although the results presented in Figures 21.15 and 16 were obtained fromtests carried out with pipe surfaces roughened by sand blasting, this treatment isnot recommended as it will result in the generation of a large percentage of fines.Also, this roughness is relatively shallow in depth and an aluminum surface willwear so that the pipe must be retreated in six to twelve months [10].

REFERENCES

1. D. Mills and J.S. Mason. Problems of particle degradation in pneumatic conveyingsystems. Proc Pneumotransport 4. Paper F3. 10 pp. BHRA Conf. Carmel, California.June 1978.

2. D. Mills and J.S. Mason. The effect of pipe bends and conveying length upon particledegradation in pneumatic conveying systems. Proc 3rd Powder and Bulk Solids Conf.Chicago. May 1978.

3. A.D. Salman, M. Szabo, I. Angyal, A. Verba, and D. Mills. The design of pneumaticconveying systems to minimize product degradation. Proc 13th Powder and Bulk Sol-ids Conf. Chicago. May 1988.

4. G. Segler. Pneumatic grain conveying with special reference to agricultural applica-tions. Germany. 1951.

5. G.P. Tilly. Erosion caused by solid particles. Treatise on Materials Science and Tech-nology. Vol 13, pp 287-319. Academic Press Inc. 1979.

6. G.P. Tilly and W. Sage. The interaction of particle and material behavior in erosionprocesses. Wear, Vol 16, pp 447-465. 1970.

7. A.D. Salman, A. Verba, and D. Mills. Particle degradation in dilute phase pneumaticconveying systems. Proc 17th Powder and Bulk Solids Conf. Chicago. May 1992.

8. D. Mills. Particle degradation in pneumatic conveying, pp 31-48. Freight Pipelines. EdG.F. Round. Elsevier. 1993.

9. D. Mills. The degradation of bulk solids by pneumatic conveying and its simulationby small scale rigs. BMHB report. Feb 1988.

10. J. Paulson. Effective means for reducing formation of fines and streamers. Proc Confon Polyolefins. Soc of Plastic Engineers. Houston. 1978.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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22Health and Safety Issues

1 INTRODUCTION

Dust generated from most bulk solids poses a potential health problem, and manymaterials that have to be conveyed are potentially toxic. Pneumatic conveying isoften chosen for hazardous materials because the system provides a totally en-closed environment for their transport. It is also considered that the majority ofconveyed materials are potentially explosive, and this certainly applies to mostfood products, fuels, chemicals and metal powders. Pneumatic conveying systemsare basically quite simple and are eminently suitable for the safe transport of pow-dered and granular materials in factory, site and plant situations.

The system requirements are a source of compressed gas, usually air, a feeddevice, a conveying pipeline, and a receiver to disengage the conveyed materialand carrier gas. The system is totally enclosed, and if it is required, the system canoperate entirely without moving parts coming into contact with the conveyed ma-terial. High, low or negative pressure air can be used to convey materials. For po-tentially explosive materials, an inert gas such as nitrogen can be employed.

1.1 System Flexibility

Pneumatic conveying systems are particularly versatile. With a suitable choice andarrangement of equipment, materials can be conveyed from a hopper or silo in onelocation, to another location some distance away. Considerable flexibility in both

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plant layout and operation are possible, such that multiple point feeding can bemade into a common line, and a single line can be discharged into a number ofreceiving hoppers. With vacuum systems, materials can be picked up from openstorage or stockpiles, and they are ideal for clearing dust accumulations and spill-ages.

Pipelines can run horizontally, as well as vertically up and down, and withbends in the pipeline any combination of orientations can be accommodated in asingle pipeline run. A very wide range of materials can be handled, and they aretotally enclosed by the system and pipeline. This means that potentially hazardousmaterials can be conveyed quite safely, with the correct choice of system andcomponents. There is minimal risk of dust generation, and so these systems gener-ally meet the requirements of any local Health and Safety legislation with little orno difficulty.

1.2 System Integration

Dust, mess and spillage that are often found surrounding bulk solids handlingplant are not generally caused by pneumatic conveying systems. Feeders forpneumatic conveying systems, for example, usually fit under hoppers, and these inturn are fed from above by other systems, such as chain and flight (en-masse) con-veyors.

Dust and mess in the area often comes from poor integration of the me-chanical conveyor with the hopper, and not with the pneumatic conveyor. In termsof plant safety, therefore, due consideration must be given to the interfacing ofdifferent systems, particularly if they are operating in series [I].

Pneumatic conveying systems provide a totally enclosed environmentthroughout for the transport of materials, and along the conveying route there areno moving parts at all, unless diverter valves are employed for multiple point off-loading. Some feeding devices, such as blow tanks, Venturis and vacuum nozzleshave no moving parts, apart from valves opening and closing at the start and endof the process. Although pneumatic conveying systems are capable of releasingdust into the atmosphere, it generally occurs only as a result of a fault situation,and is not an endemic problem with the conveying system.

2 DUST RISKS

Many dusts represent a very significant health hazard. If these materials are to beconveyed it is essential that any dust associated with the material should remainwithin the conveying system throughout the entire transportation process. If anymaterial is deemed to be toxic to any degree there should be no possibility of anydust being released into the atmosphere. There is also a wide range of materials,which, in a finely divided state, dispersed in air, wi l l propagate a flame through thesuspension if ignited.

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These materials include foodstuffs such as sugar, flour and cocoa, syntheticmaterials such as plastics, chemical and pharmaceutical products, metal powders,and fuels such as coal and sawdust. If conveyed with air there is the possibility ofa dust explosion within the system. If the dust is released from the system there isthe possibility of a dust explosion external to the conveying system.

The potential magnitude of the problem can be illustrated by the fact thatduring the 17 years from 1962 to 1979 there were 474 recorded dust explosions inthe UK alone, resulting in 25 deaths [2]. This covers the whole area of bulk solidshandling, transport and storage and the number of explosions that could be attrib-uted directly to pneumatic conveying systems is not known.

In just two years (1976 and 1977), dust explosions in grain handling plant inthe United States claimed the lives of 87 workers and caused injuries to over 150more [3]. It is believed that most of these explosions were in bucket elevators andnot pneumatic conveying systems, but these statistics highlight the potential fordust explosions, regardless of the source.

2.1 Dust Emission

Excepting the potentially explosive materials, the most undesirable dusts are thosethat are so fine that they present a health hazard by remaining suspended in the airfor long periods of time. The terminal velocity of a 1 um particle of silica, for ex-ample, is about 1 mm in 30 seconds (0-006 ft/min), whereas that of a 100 urn par-ticle is about 100 mm/s (20 ft/min).

The terminal velocity of an object depends upon its density and size, and isapproximately proportional to the square of its size. Data on the terminal velocityof particles with respect to particle size and density was presented in Figure 18.16.Comparative size ranges of some familiar airborne particles are illustrated in Fig-ure 22.1 [1].

Particles falling in the size range of approximately 0-5 to 5 um, if inhaled,can reach the lower regions of the lungs where they may be retained. Prolongedexposure to such dusts can cause permanent damage to the lung tissues, symp-tomized by shortness of breath and increased susceptibility to respiratory infection.Legislation has been introduced in many countries, therefore, that specify maxi-mum exposure times to such dusts.

Prevention of the emission of these fine particles into the atmosphere is thusof paramount importance, regardless of source. Emissions of larger particles mayalso give rise to complaints, more often in a social context, created by the deposi-tion of the particles on neighboring properties or on vehicles belonging to a com-pany's own employees [4J.

2.1.1 Dust as a Health Hazard

When suspended in air the smallest particle visible to the naked eye is about 50 to100 um in diameter, but it is the particles of 0'2 to 5 um diameter that are mostdangerous for the lungs, as mentioned above.

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Paint Pigments Pollen

104

Mean Particle Size - um

Figure 22.1 Approximate size range of some familiar types of airborne particulatematerials.

Thus the existence of visible dust gives only indirect evidence of danger, asfiner invisible particles will almost certainly be present as well. The fact that nodust can be seen is no reliable indication that dangerous dust may not be present inthe air. The large visible particles in a dust cloud will quickly fall to the floor, butit will take many hours for the fine dangerous particles to reach the ground.

Airborne dusts that may be encountered in industrial situations are generallyless than about 10 urn in size and can be taken into the body by ingestion, skinabsorption or inhalation. The former is rarely a serious problem, but diseases ofthe skin are of not infrequent occurrence. Allergic reactions are known to becaused by powders containing, for example, metals such as chrome, nickel andcobalt. It is, however, inhalation that presents the greatest hazards for workers in adusty environment.

Relatively large particles of dust that have been inhaled and become depos-ited in the respiratory system will usually be carried back to the mouth by cilliaryaction and be subsequently swallowed or expectorated. At the other extreme, ultra-fine particles (less than about 0-2 urn) which become deposited are likely to passrelatively quickly, generally into solution in the extra-cellular fluids of the lungtissues. Much of this is excreted by the kidneys, either unchanged or after detoxi-cation by the liver. This is the fate of many systemic poisons, such as lead, whichgain entry to the body via the lungs [5].

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Inhaled particles with the approximate size range of 0-2 to 5 um can reachthe lower regions of the lungs where they will probably be retained. Prolongedexposure to such dusts can cause various diseases, most of them potentially seri-ous, and often resulting in permanent damage to the lung tissues. The best knownare probably the diseases collectively designated 'pneumoconiosis' and character-ized by chronic fibrosis of the lungs as a result of continuous inhalation of mineraldusts such as silica, asbestos and coal.

Generally the symptoms are chronic shortage of breath and increased sus-ceptibility to respiratory infection. Other dust-related diseases include pneumonitis(an acute inflammation of the lung tissue or bronchioles) and lung cancer. Therelative dangers of some common dusts are compared in Table 22.1 in which theminerals are conveniently classified in Groups I to IV [5].

2.1.2 Dust Concentration Limits

One of the criteria used in monitoring the compliance of companies with the 1974Health and Safety at Work Act (UK), and other relevant statutory provisions, isthe concentration of airborne dust. The measured concentration is compared withvariously defined 'threshold limit values' (TLV's) which are also functions of theduration of exposure of personnel to the dust. The most commonly used defini-tions of Threshold Limit Value are [6]:

TLV-TWA Time-weighted average concentration for a normal eight hourworking day or forty hour week, to which most workers canbe repeatedly exposed day after day, without adverse effect.

TVL-STEL Short term exposure limit. This is the maximum concentra-tion to which workers can be exposed for a period of up to15 minutes, provided that no more than four excursions tothis value occur each day.

TVL-C Threshold limit ceiling. This is the concentration that shouldnot be exceeded, even instantaneously.

For further information on actual threshold limit values Reference 7 shouldbe consulted. Different countries, of course, will have their own regulations andguidelines on these issues. This data is included to highlight the fact that such leg-islation is in place in most countries.

2.1.3 Dust Suppression

Where a test for dustiness, or previous experience with a material, indicates thatthe generation of dust is likely to present a problem, serious consideration shouldbe given to methods of reducing the material's dustability. It may be appropriate tore-examine the manufacturing process to see if the proportion of fines could belessened. Agglomeration of the particles, for example by pelletizing, should have asignificant effect

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Table 22.1 Relative Dangers of Some Common Dusts

GROUP 1

Very Dangerous

Expert advice should always be sought.

BerylliumParticularly as the oxide.

Silica (SiO2) which has been heated.In these circumstances silica undergoesmodification into biologically activeforms.

Calcined kieselguhr (diatomaceous earth)is also dangerous on this account.

Crocidolite (blue asbestos).Evidence associates this variety of as-bestos with the development of malig-nant tumors of pleura and peritoneum.

GROUP 2

Dangerous

A visible haze of any of these dusts isintolerable and no possible source ofsuch should be ignored whether ornot there is a visible cloud.

Asbestos, other than crocidoliteThe two important varieties in commerceare amosite (brown asbestos) and chry-sotile (white asbestos).

Silicaeg as quartz, ganister, gritstone, etc.

Mixed DustsContaining 20% or more of free silica,such as pottery dust, granite dust andsteel foundry dust.

Fireclay DustWith a total silicate (as silica) content inexcess of 60%.

GROUP 3

Moderate Risk

Emission of any of these dusts to form adense local cloud should cause concern.

Mixed DustsContaining some free silica but arbitrarilyless than 20%. In this group are includedthe dusts of Fe and non-ferrous foundries.

Coal Dust GraphiteSynthetic Silicas TalcKaolin (china clay, fullers earth)Non-crystalline silica

including unheated kieselguhrCarbides of some metalsCotton Dust

and other dusts of vegetable originAluminous Fireclay Mica

GROUP 4

Minimal Risk

Visible concentrations of these dusts,although inexcusable on general grounds,probably represent more danger to welfarethan to health.

Alumina ('aloxite', corundum)Glass (including glass fiber)Mineral Wool and Slag WoolPearlite and dusts from other basic rocks.Silicates, other than those already

mentioned.Tin Ore and OxidesZirconium Silicate and OxideMagnesium OxideCarborundumCementEmeryFerrosilicon

Zinc OxideBariteLimestoneIron Oxide

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If dust is generated during transport, it may be possible to change the trans-port routing or conveying parameters. Total enclosure of the processing and han-dling plant is probably the most desirable approach but, in addition to the highcost, there are obvious problems over accessibility.

A generally more satisfactory arrangement is to use some kind of partial en-closure or hood in conjunction with an exhaust system which wil l draw off thedusty air and so minimize the dispersion of solid particles into the atmosphere.Dusty air collected from a booth, hood or other type of partial or total enclosuremust then be filtered, or otherwise cleaned, before it can be released into the at-mosphere.

2.2 Explosion Risks

Apart from choking lungs, irritating eyes and blocking pores, some seeminglyinnocuous dusts can ignite to cause fires. Many materials, in a dust cloud, can ig-nite and cause an explosion which could be capable of demolishing a factory. Acorn starch powder explosion at General Foods, Banbury in the UK did just this in1981 and nine men were burned. The company pneumatically conveyed cornstarch, used in custard powder production, from a transfer hopper to feed bins, viaa diverter valve.

An accumulation of corn starch on the operating cylinder caused a malfunc-tion of this diverter valve. When one hopper was full, the flow should have beendiverted to the next hopper. An already full hopper, therefore, was over-filledcausing powder to be dispersed into the atmosphere. The actual explosion oc-curred outside the processing plant where the dust cloud was ignited by electricalarcing from nearby electrical switchgear, burning nine men and blowing outbrickwork and windows on all four walls [8].

When an explosive dust cloud is ignited in the open air there is a flash firebut little hazardous pressure develops. If the dust cloud is in a confined situation,however, such as a conveyor or storage vessel, then ignition of the cloud will leadto a build-up of pressure. The magnitude of this pressure depends upon the volumeof the suspension, the nature of the material, and the rate of relief to atmosphere.Research has shown that the particle size must be below about 200 urn for a haz-ard to exist.

At some point in a pneumatic conveying system, or time in the conveyingcycle, whether dilute or dense phase, positive or negative pressure, the materialwi l l be dispersed as a suspension. A typical point is at discharge into a receivingvessel and a common time is during a transient operation such as start-up or shut-down. Consideration, therefore, must be given to the possibility of an explosionand its effects on the plant, should a source of ignition be present.

Because of legal and Health and Safety Executive requirements it is advis-able for specialist advice to be sought on dust explosion risks. Authoritative litera-ture on the subject is widely available and there are many tests that can be carriedout to determine the seriousness of the problem. It is strongly recommended that a

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specialist in this field is consulted if there is any doubt about the potential explo-sion risk connected with the pneumatic conveying of any material.

2.2.7 Ignition Sources

For an explosion to occur two conditions must be satisfied. Firstly, a sufficientlyenergetic source of ignition must be provided and secondly the concentration ofthe material in the air must be favorable. Two sources of ignition frequently met inindustrial plant are a hot surface and a spark. Consequently, the minimum ignitiontemperature and the minimum ignition energy are the ignition characteristicscommonly measured in routine testing for explosibility.

Ignition temperature, however, is not constant for a given dust cloud, for itdepends upon the size and shape of the apparatus used to measure it. Minimumignition temperatures, therefore, are determined in a standardized form of appara-tus, which enables meaningful comparisons between materials to be made. Typicalvalues of minimum ignition temperature for sugar, coffee and cocoa are 660, 770and 790°F respectively [9].

The minimum energy relates to ignition by sparks, whether produced byelectricity, friction or hot cutting. A characteristic of any form of spark is that asmall particle or a small volume of gas at high temperature is produced for a shortperiod of time.

Since it is much easier for experimental purposes to measure the energy de-livered by an electric spark than by friction or thermal processes, the routine testfor determining this characteristic uses an electric spark ignition source. Typicalvalues of minimum ignition energy for titanium, polystyrene and coal are 10, 15and 60 mJ respectively [9],

2.2.2 Explosibility Limits

For a flame to propagate through a dust cloud the concentration of the material inair must fall within a range which is defined by the lower and upper explosibilitylimits. The lower explosibility limit, or minimum explosible concentration, may bedefined as the minimum concentration of material in a cloud or suspension neces-sary for sustained flame propagation. This is a fairly well defined quantity and canbe determined reliably in small scale tests. Values are usually expressed in termsof the mass of material per unit volume of air. Typical values for wood flour andgrain dust are 40 and 55 oz/103 ff respectively [9].

As the concentration of the material is increased above the lower explosibil-ity limit the vigor of the explosion increases. When the dust concentration is in-creased beyond the stoichiometric value, the dust has a quenching effect. Eventu-ally a concentration is reached at which flame propagation no longer occurs. Thisconcentration is the upper explosion limit. This limit is not as easy to determinebecause of the difficulty of achieving a uniform dispersion of the material.

From values that have been determined it would appear that for most com-mon materials the upper limit is probably in the range of 0-1 to 0-6 Ib/ff [4]. This

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is equivalent to solids loading ratios of about \Vi to 8, which covers a significantpart of the dilute phase conveying range. It is reasonable to conclude from this thatshould a favorable source of ignition be present in a pneumatic conveying system,then dilute phase systems are more of a problem than dense phase systems withrespect to explosions.

With truly dense phase systems, concentrations are well above the min imumexplosibility limit and so it is highly unlikely that an explosion wil l occur in thepipeline of such a system. Care should still be exercised with such installations,however, since it is possible for an explosive concentration to exist at entry to acyclone or receiver. Consideration should also be given to the start-up and shut-down transients associated with dense phase systems, for with certain modes ofoperation dilute phase situations may exist.

2.2.3 Pressure Generation

If a dust explosion occurs in industrial plant spectacular destruction can result if itis initially confined in a system that is ultimately too weak to stand the ful l force ofthe explosion. Two other characteristics of dust explosions, therefore, are alsoderived by means of tests.

One is the maximum explosion pressure generated, which would be requiredif it was desired to contain the explosion within the system. The other characteris-tic is the maximum rate of pressure rise, which would be relevant to the needs ofsuppressing an explosion within the system.

Typical explosion characteristics of some well known materials are pre-sented in Table 22.2 [9]. The data in the last two columns serves to illustrate themagnitude and rapidity of the sequence of events that follows such an explosion.Explosion pressures may be as high as 100 psig and the maximum rate of pressurerise may be in excess of 1000 atmospheres/second, or 15,000 Ibf/irf per second,which means that it could take less than 0-01 seconds to reach maximum pressure.

If ignition occurs within a pipeline, the pipeline may be capable of with-standing the full explosion pressure. If this is so, the resulting pressure wavewould pass along the pipeline and be relieved at the weakest point, which is usu-ally the collection hopper or cyclone. Because of their size these are generally onlycapable of withstanding pressures of 3 to 5 psig and, if exposed to higher internalpressures, may burst or disintegrate. Consequently, the collection unit is likely tobe the most vulnerable part of the system.

2.2.4 Expansion Effects

The combustion of a dust cloud will result in either a rapid build up of pressure orin an uncontrolled expansion. It is the expansion effect, or the pressure rise if theexpansion is restricted, that presents one of the main hazards in dust explosions.The expansion effects arise principally because of the heat developed in the com-bustion and, in some cases, to gases being evolved from the dust because of thehigh temperature to which it has been exposed.

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Table 22.2 Explosion Characteristics of Some Well Known Materials

Minimum Minimum Minimum Maximum MaximumMaterial ignition explosible ignition explosion rate of

temperature coneentration energy pressure pres. rise°F oz/103ft3 mJ lbf/in2 alm/s

Metal PowdersAluminumMagnesiumZinc

PlasticsNylonPolyethylenePolystyrene

AgriculturalCoffeeGrain dustSugarWheat Flour

MiscellaneousCoalWood flour

1180970

1110

930730910

770810660720

610810

4020

480

302015

85553550

5540

1540

650

201015

160303050

6020

909550

958090

50959095

85110

10001000

120

270510480

17190340250

150370

The pressure wave resulting from an uncontrolled dust explosion in a build-ing usually shakes down more dust that has settled over a period of time onto pipe-lines, roof beams and supports, ledges, etc. This makes an ideal condition for thesecondary explosion that almost always follows. It is this secondary explosion thatcan demolish a factory and kill the operatives. It is essential, therefore, than anexplosion occurring in a pneumatic conveying system is not allowed to be dis-charged into a building, and that good housekeeping procedures are adopted tominimize the build up of potentially explosive dusts on surfaces in such buildings.

2.2.5 Oxygen Concentration

Another characteristic of dust explosions, that can also be measured, is the per-centage of oxygen in the conveying gas at which an explosion will occur for agiven material. If the oxygen level in air is reduced, a point will be reached atwhich a flame cannot be supported. If a material is considered to be highly explo-sive it would generally be conveyed with an inert gas such as nitrogen, and not air.

For many materials, however, such an extreme measure is not necessary.The use of nitrogen will add significantly to the operating costs, particularly withan open system. If the oxygen content needs to be reduced by only a smallamount, a proportion of nitrogen can be added to the air to keep the oxygen levelbelow the required concentration.

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3 CONVEYING SYSTEMS

A wide range of pneumatic conveying systems are available to cater for an equallywide range of conveying applications. The majority of systems are generally con-ventional, continuously operating, open systems in a fixed location. To suit thematerial being conveyed or the process, however, innovatory, batch operating andclosed systems are commonly used, as well as mobile systems. To add to the com-plexity of selection, systems can be either positive or negative pressure in opera-tion, or a combination of the two [9].

3.1 Closed Systems

For the conveying of toxic or radioactive materials, where the air coming into con-tact with the material must not be released into the atmosphere, or must be veryclosely regulated, a closed system would be essential. A sketch of a typical systemis given in Figure 22.2. A closed system may also be chosen to convey a poten-tially explosive material, typically with an inert gas. In a closed system the gas canbe re-circulated and so the operating costs, in terms of inert gas, are significantlyreduced.

A null point needs to be established in the gas only part of the system, wherethe pressure is effectively atmospheric, and provision for make up or control of theconveying gas can be established here. If this null point is positioned after theblower the conveying system can operate entirely under vacuum. If the null pointis located before the blower it will operate as a positive pressure system.

Figure 22.2 Closed loop pneumatic conveying system.

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3.2 Open Systems

Where strict environmental control is not necessary an open system is generallypreferred, since the capital cost of the plant will be less, the operational complexitywil l be reduced, and a much wider range of systems will be available. Most pneu-matic conveying pipeline and channel systems can ensure totally enclosed materialconveying, and so with suitable gas-solid separation and venting, the vast majorityof materials can be handled quite safely in an open system. Many potentially com-bustible materials are conveyed in open systems by incorporating necessary safetyfeatures.

3.2.1 Positive Pressure Systems

Although positive pressure conveying systems discharging to a reception point atatmospheric pressure are probably the most common of all pneumatic conveyingsystems, the feeding of a material into a pipeline in which there is air at a highpressure does present a number of problems. A wide range of material feedingdevices are available that can be used with this type of system, from Venturis androtary valves to screws and blow tanks. With each type of feeder, however, thereis the potential of air leaking from the system, and carrying dust with it, as a resultof the adverse pressure gradient.

3.2.2 Negative Pressure (Vacuum) Systems

Negative pressure systems are commonly used for drawing materials from multi-ple sources to a single point. There is no adverse pressure difference across thefeeding device in a negative pressure system and so multiple point feeding into acommon line presents few problems. A particular advantage of negative pressuresystems, whether open or closed, in terms of potentially hazardous materials, isthat should a pipeline coupling be inadvertently left un-tightened, or a bend in thepipeline fail, air will be drawn into a system maintained under vacuum. With apositive pressure system a considerable amount of dust could be released into theatmosphere before the plant could be shut down safely.

3.3 System Components

The selection of components for a pneumatic conveying system is as important asthe selection of the type of conveying system for a given duty. Air movers, pipe-line feeding devices and gas-solid separation systems all have to be carefully con-sidered and there are multiple choices for each.

3.3.1 Blowers and Compressors

With air movers a positive displacement machine is generally required. If a bloweror compressor is incorrectly specified, in terms of either pressure or volumetricflow rate, the pipeline is likely to block, and with a toxic material this wi l l createits own hazards since the pipeline will have to be unblocked by some means. A

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minimum gas velocity must be maintained throughout the pipeline system to en-sure satisfactory conveying, and it must be remembered that all gases are com-pressible with respect to both pressure and temperature when it comes to evaluat-ing flow rates from specified velocities.

3.3.1.1 Oil Free AirOil free air is generally recommended for most pneumatic conveying systems, andnot just those where the material must not be contaminated, such as food products,Pharmaceuticals and chemicals. Lubricating oil, if used in an air compressor, canbe carried over with the air and can be trapped at bends in the pipeline or obstruc-tions.

Most lubricating oils eventually break down into more carbonaceous matterwhich is prone to spontaneous combustion, particularly in an oxygen rich envi-ronment, and where frictional heating may be generated by moving particulatematter. Although conventional coalescing after-filters can be fitted, that are highlyefficient at removing aerosol oil drops, oil in the super-heated phase will passstraight through them. Super-heated oil vapor will turn back to liquid further downthe pipeline if the air cools.

Ultimately precipitation may occur, followed by oil breakdown, and eventu-ally a compressed air fire. The only safe solution, where oil injected compressorsare used, is to use chemical after-filters such as the carbon absorber type which arecapable of removing oil in both liquid droplet and super-heated phases. The solu-tion, however, is very expensive and requires continuous maintenance, and re-placement of carbon filter cells.

3.3.2 Pipeline Feeding

There have been numerous developments in pipeline feeders to meet the demandsof different material characteristics, and ever increasing pressure capabilities forlong distance and dense phase conveying. Although the majority of systemsprobably operate with positive displacement blowers at a pressure below 15 psig,discharging to atmospheric pressure, there is an increasing demand for conveyingsystems to feed materials into chemical reactors and combustion systems that op-erate at a pressure of 300 psig or more.

With positive pressure systems the main problem is feeding the material intoa pipeline that contains air at pressure. Because of the adverse pressure it is almostimpossible to prevent air from leaking across the feeding device. This air will al-most certainly carry dust with it, and so if this air or dust must be controlled thensome means of containment must be incorporated into the conveying system.

3.3.2.1 Rotary ValvesThe rotary valve is probably the most commonly used device for feeding convey-ing pipelines. By virtue of the moving parts and a need to maintain clearancesbetween the rotor blades and the casing, air will leak across the feeder when thereis a pressure difference. Rotary valves are ideally suited to both positive pressureand vacuum conveying.

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The rotary valve is a positive displacement device and so feed rate can becontrolled fairly precisely by varying the speed of rotation. The situation with re-gard to screw feeders is very similar, as these are also positive displacement de-vices. Air leakage can be minimized by reducing blade tip clearances, increasingthe number of rotor blades, and providing seals on the rotor end plates, but it can-not be eliminated.

Air at pressure will always return with the empty pockets, apart from leak-ing past blade tip clearances. The air leaking across a rotary valve will often re-strict the flow of material into a rotary valve from the supply hopper above. Tominimize this influence it is usual to vent a rotary valve in some way. A commondevice is to provide a vent on the return side of the valve. Since the vented air willcontain some fine material, this is either directed back to the supply hopper, ductedto a separate filter unit, or re-introduced back into the conveying pipeline.

Because there will be a carry-over of material any filter used must be regu-larly cleaned, otherwise it will rapidly block and cease to be effective. If the air isvented into the supply hopper above, or to a separate filter, the pipe connecting thevent to the filter unit must be designed and sized as if it were a miniature pneu-matic conveying system, in order to prevent it from getting blocked. With lowpressure conveying systems a venturi can be used to feed the dusty gas from thevent directly back into the pipeline.

If the material to be conveyed is potentially explosive, the use of rotaryvalves will have to be questioned. With metal blades and a metal housing, ashower of sparks would result if the two were to meet, and a single spark wouldprovide an adequate source of ignition for many materials. With positive pressureconveying systems rotary valve blade tip clearances need to be very small and sodifferential expansion, resulting from the handling of hot material, or bearingwear, could cause the two to meet. Bearing failure on a rotary valve could wellresult in a surface at a sufficiently high temperature to provide a necessary ignitionsource, both within and external to the conveying system. In a fault situation dustcan leak from a pressurized conveying system and so bearings external to the sys-tem are vulnerable.

3.3.2.2 Blow TanksThe use of blow tanks has increased considerably in recent years and there havebeen many developments with regard to type and configuration. A particular ad-vantage with these systems is that the blow tank also serves as the feeding device,and so many of the problems associated with pressure differentials across thefeeder are largely eliminated.

Although continuous air leakage does not occur with blow tanks, as it doeswith rotary valves, consideration does have to be given to the venting of the blowtank at the end of the conveying cycle, as well as on filling. A similar situationexists with regard to gate valve feeders. Blow tanks generally form the basis ofmobile conveying systems, such as road and rail vehicles, and so special provisionmust be made for venting these during filling operations.

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Blow tanks can be de-pressurized through the conveying pipeline, but this isa slow process. If the blow tank is de-pressurized separately it would be recom-mended that it should be vented to the top of the supply hopper, provided that ithas a suitable filter. When the next batch of material is loaded into the blow tank,the air in the blow tank wil l need to be vented. If it is not vented the displaced airwill impede the flow of material into the blow tank as it will have to flow againstthe incoming material. The vented air is likely to carry dust with it and so thisshould be directed to the top of the supply hopper where it can be filtered. Withcontinuously operating blow tank systems the lock hopper has to be de-pressurizedeach cycle and this should be similarly vented.

3.4 Conveying Operations

Consideration must be given to some conveying operations and the conveying ofcertain materials with regard to safety provisions. Mention has already been madeof start-up and shut-down transients, for example. In most dense phase conveyingsituations, the concentration of the material will be well above the value at whichan explosion would be possible.

During transient operation, however, and plant shut-down in particular, theconcentration of the material in the air cannot be guaranteed to be above the re-quired value while the system is being purged. Regardless of the conveying sys-tem and the mode of conveying, however, the material will generally be dis-charged into a receiving vessel, where there is every possibility of the materialbeing dispersed in a low concentration cloud.

Pneumatic conveying is an extremely aggressive means of conveying mate-rials, and particularly so in dilute phase conveying where high gas velocities arerequired. As a result, abrasive particles can cause severe wear of the conveyingplant and friable particles can suffer considerable degradation. The consequencesof these influences must be given every consideration.

3.4.1 Tramp Materials

High conveying air velocities also mean that tramp materials can be conveyedthrough the pipeline with the material being conveyed. It is possible for nuts, boltsand washers to find their way into the conveyed material, somewhere in the sys-tem, and these will be conveyed quite successfully through the pipeline, with thepotential of generating showers of sparks, as they will inevitably make numerouscontact with the bends and pipeline walls in their passage through the pipeline.

3.4.2 Static Electricity

Whenever two dissimilar materials come into contact, a charge is transferred be-tween them. The amount of charge transfer depends upon the type of contactmade, as well as on the nature of the materials. Almost all bulk solids acquire anelectrostatic charge in conveying and handling operations. In a large number ofcases the amount of charge generated is too small to have any noticeable effect,

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but in many cases appreciable charge generation can occur, resulting in high elec-tric fields.

Very often these are just a nuisance, but occasionally they can attain hazard-ous levels. In all cases where dust clouds are present the build-up of an electro-static charge should be prevented. Pneumatic conveying systems are prolific gen-erators of static electricity. Frictional charging of the particles moving along thewalls of a pipeline can lead to a carry-over of net charge into the receiving hopper.

In the case of non-conducting materials a build-up of charge might occur inthe receiving vessel, because of the difficulties of leakage through an insulatingmedium. In the case of conducting solids, electrostatic problems can still arisewhen the particles are suspended in air. In such a case the air prevents the electriccharge on each individual particle from leaking away.

It is possible, therefore, for high electric fields to exist in receiving hoppers.In many cases the charge may reach the breakdown level for air and produce aspark. Such a spark may have sufficient energy to provide the necessary source ofignition for the dust cloud in the vessel, and hence cause an explosion. A 'rule ofthumb' value of 25 mJ is often taken, and materials with ignition energies lessthan this may be regarded as being particularly prone to ignition by static electric-ity. In these cases special precautions should be taken.

3.4.2.1 EarthingFrom an electrostatic point of view, pneumatic conveying lines should be con-structed of metal and be securely bonded to earth. All flanged joints in the pipe-work should maintain electrical continuity across them, to reduce the chance ofarc-over within the pipe.

Particular attention should be given to areas where rubber or plastic is in-serted for anti-vibration purposes, and where sight glasses are positioned in pipe-lines. Regular routine checks of the integrity of the earthing of all metal parts ofthe system should be carried out. The use of well grounded facilities can help toreduce these potential hazards.

Although certainly safer than systems that have plastic sections, wherecharge can build up, earthed metal systems will not ensure that the system is safe.Metal pipes provide a very effective source of charge for particles conveyedthrough them. The charge created on the pipe will flow instantly to earth, but thaton the particles may remain for long periods. The storage potential is particularlyimportant with regard to operations subsequent to conveying, for it is quite possi-ble for such a charge on a material to be transferred to operatives.

If this occurred in the presence of an appropriate concentration of the mate-rial, the spark could provide the necessary ignition energy to cause an explosion.In this case special precautions should be taken, including the use of anti-staticclothing and conducting footwear by all people in direct contact with a dust cloud.These, however, would be quite useless if they were to be used on a highly insu-lated floor, such as is often found in modern buildings. The operatives shouldstand on an earthed metal grid or plate at the point of operation.

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3.4.2.2 Humidity ControlStatic generation on a material increases as the relative humidity of the surround-ing air decreases, and since it is more difficult to generate and store charges undermore humid conditions, increasing the relative humidity of the conveying air to 60to 75% may also be used as a means of controlling the problem. The use of humid-ity for charge control is obviously not suitable for hygroscopic materials, and mustbe considered in relation to the possibility of condensation and freezing in anyapplication.

3.4.3 Particle A tlrition

Of those materials that are explosive, research has shown that it is only the fractionwith a particle size less than about 200 (im that poses the problem. If a size analy-sis of a material to be conveyed shows that there is no significant amount of mate-rial below this size, the possibility of an explosion occurring during its conveyingshould not be dismissed. Degradation caused by pneumatic conveying can resultin the generation of a considerable number of fines, particularly if the material isfriable. This point was illustrated in the previous chapter with Figure 21.2 thatshows the fractional size distribution of a material both before and after convey-ing. In terms of explosion risks the material after conveying could be a seriouscontender.

3.4.4 Erosive Wear

Many materials that require conveying are abrasive. These include some of thelarger bulk commodities such as cement, alumina, fly ash and silica sand. With aconveying air velocity of only 4000 ft/min silica sand is capable of wearing a holein a regular steel bend in a pipeline in less than two hours. Erosive wear can bereduced with wear resistant materials and special bends, but it cannot be elimi-nated. Even straight pipeline is prone to wear under some circumstances.

If an abrasive material has to be conveyed, therefore, consideration must begiven to the possibility of a bend or some other component in the system failing,with the consequent release of dust, particularly with a positive pressure convey-ing system. Bends are available that have detectors embedded into them so thatnotice can be given in advance of an impending failure.

3.4.5 Material Deposition

In long straight horizontal pipe runs, and large diameter pipelines, there is the pos-sibility of material coming out of suspension in dilute phase conveying and depos-iting on the bottom of the pipeline. Accumulations of material such as pulverizedcoal in a pipeline could result in a fire, through spontaneous combustion, and pos-sibly an explosion. An increase in conveying air velocity will generally help toreduce the problem but this is not an ideal solution. A disturbance to the flow witha turbulence generator usually cures the problem.

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Food products, of course, will deteriorate if left in pipelines, and contamina-tion of subsequent material could result. Since it is unlikely to be known whethersuch deposition occurs or not, it is necessary to physically clean all lines periodi-cally. For food and pharmaceutical products, pipelines and all valves and compo-nents that could possibly come into contact with the material being conveyed arelikely to be made of stainless steel. A particular problem with carbon steel is that itis liable to rust, as a result of condensation in the pipeline, and so contaminate thematerial.

3.4.5.1 Pipeline PurgingIf a pipeline is to be purged with the conveying air, in order to clear it of material,radiused bends should be used rather than blind tees. Blind tees are used in pipe-lines because they will trap the conveyed material and so provide protection to abend from abrasive particles, since the particles will impact against each otherrather than the bend wall. Material will require a much longer purging time to becompletely cleared from blind tees, however. If additional air is available for purg-ing, the process will be more effective. Air stored in a receiver will help here, par-ticularly if it is at pressure but care must be taken not to overload the filtrationplant during this operation.

In dense phase conveying, air velocities employed are very much lower thanthose required for dilute phase conveying. Pipeline purging can be a major prob-lem if additional air is not available. This point was considered in some detail inChapter 9 with data on fly ash with Figure 9.9 and cement in Figure 9.10.

If high pressure air is used for conveying a material it is common for thepipeline to be stepped to a larger bore along its length once or twice in order toallow the air to expand and so prevent excessive velocities from occurring towardsthe end of the pipeline. This does, however, create problems if such a pipelineneeds to be purged clear, for the purging velocity will decrease at each step to alarger bore and so considerably more air would be needed for the purpose. Thispoint was considered in Chapter 9 with Figure 9.8.

3.4.6 Power Failure

The consequences of a power failure on system operation need to be considered atthe design stage so that back-up systems and preventative measures can be incor-porated at the time of installation. With a pneumatic conveying system the plantwill generally shut itself down safely on loss of power, but whether it can bestarted up again will depend upon the type of conveying system, pipeline routing,mode of conveying and material properties.

In many cases the pipeline will block and the only method of restarting thesystem will be to physically remove the material from where the pipeline isblocked, usually at the bottom of a vertical lift. If this is not an option then a stand-by power system must be available to take over. Alternatively an air receiver canbe built into the air supply system, and this will provide air to purge the lines suf-

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ficiently clear of material so that the system can be restarted when power is rein-stated (see Figure 19.10).

If the possibility of the pipeline becoming blocked from any eventualitymust be avoided, consideration should be given to the use of an innovatory sys-tem. In 'conventional systems' the material is simply blown or sucked through thepipeline. In 'innovatory systems' the material is either conditioned as it is fed intothe pipeline, or along the length of the pipeline. There is no difference in any ofthe peripheral system components employed.

Air pulsing or trace air lines are generally employed. Parallel lines are usedeither to inject air into the pipeline, to give the material artificial air retention, or toallow the air within the pipeline to by-pass short sections of material, to give thematerial artificial permeability. Depending upon the properties of the material tobe conveyed, one or other of these innovatory systems will generally guaranteethat the pipeline can be restarted on full load.

4 EXPLOSION PROTECTION

Despite the fact that the potential for an explosion in a pneumatic conveying sys-tem is high, the demand for such systems remains high. This is partly due to thefact that the system totally encloses the material, such that dust generation externalto the system is virtually eliminated, and with a pipeline total flexibility in theconveying route is possible without material transfer or staging. There are also anumber of different means by which a pneumatic conveying system can quite eas-ily be protected.

Since the dispersion of powdered and granular materials in air is fundamen-tal to pneumatic conveying, it is evident that if a material is known or shown to beexplosive, then consideration should be given to the hazard that this presents at thedesign stage of a system, or when re-commissioning an existing system to conveya different material. Whilst it is equally obvious that the generation of sources ofignition should be minimized, unforeseen mechanical, electrical or human failuresmean that the complete elimination of ignition sources cannot be relied upon, par-ticularly where powered machinery is involved.

To avoid the potentially catastrophic effects of an explosion, therefore, reli-ance is normally placed on the adequate functioning of a means of protection forthe system. Such protection is normally based on one or more of the followingapproaches:

j Minimizing sources of ignition and prevention of ignition._: Allowing the explosion to take its full course but ensuring that it does so

safely by either containment or explosion relief venting.n Detection and Suppression.

The method of protection selected wil l depend upon a number of factors.These include the design of any associated plant or process, the running costs, the

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economics of alternative protection methods, the explosibility of the material, andthe extent to which an explosion and its consequences can be foreseen, togetherwith the requirements of any local regulatory authorities concerned.

4.1 Minimizing Sources and Prevention of Ignition

The first step in any explosion protection program is to minimize or eliminate, asfar as possible, all potential sources of ignition. The minimum ignition tempera-ture is relevant to ignition by hot surfaces. Rotary valve bearings have alreadybeen mentioned in this context, as an example, and welding operations on any partof the system should be prohibited while the system is operating. The possibilityof sparks must also be reviewed, with due consideration given to valve operations,friction with conveyed materials, and electrostatic generation.

4.1.1 Inert ing

Prevention of ignition can be guaranteed by using an inert gas such as nitrogen forconveying the material. Alternatively, nitrogen can be added to the air in order toreduce the percentage of oxygen present in the conveying air to a level at which aflame cannot be supported. The maximum oxygen concentration is one of themany standard tests that can be carried out with a material, as mentioned earlier.Since inert gases are rather expensive, these methods are generally used withclosed or semi-closed loop systems.

4.2 Containment

The combustion of a dust cloud will result in either a rapid build up of pressure orin an uncontrolled expansion. It is the expansion effect, or the pressure rise if theexpansion is restricted, that presents one of the main hazards in dust explosions.The expansion effects arise principally because of the heat generated in the com-bustion and, in some cases, to gases being evolved from the dust because of thehigh temperature to which it has been exposed.

If the presence of evolved gases is neglected, the situation can be modeledvery approximately with the thermodynamic relationship:

T-

(1)

If the explosion is confined, V '/ will equal V2 and so the resulting pressure,, will be given by:

T2Pi = Pi x - - - - - - - - - - - ( 2 )

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Flame temperatures are typically a couple of thousand degrees and so it canbe seen that explosion pressures can reach 100 psig quite easily. The critical in-formation from Table 22.2, however, is that this pressure can be reached in milli-seconds.

When a dust explosion occurs in industrial plant spectacular destruction canresult if it is initially confined in a system that is ultimately too weak to withstandthe full force of the explosion. The literature on this subject generally includesnumerous photographs of the resulting destruction to plant in order to reinforce thefact that explosions do occur and that they can be catastrophic. Blow tanks androtary valves, however, can be obtained that wil l withstand these pressures and ina positive pressure system the compressor or blower can be protected by means ofnon-return valves in the air supply line. Most pipelines are also capable of with-standing this order of pressure.

If this is so, the resulting pressure wave would pass along the pipeline andbe relieved at the weakest point, which is usually the reception vessel. Due to theirsize these are generally only capable of withstanding very low pressures, as men-tioned earlier, and so if exposed to higher internal pressure, would rupture or burst.Consequently the collection unit is likely to be the most vulnerable part of the sys-tem. It is unlikely to be an economic proposition to design the reception vessel towithstand the explosion pressure. There are, however, alternative means of pro-tecting the receiving vessel.

4.3 Explosion Relief Venting

The usual solution to the problem in situations where the risk of an explosion isonly very slight, is to allow an explosion to take its full course, whilst employingsuitable precautions to ensure that it does so in a safe manner. As an alternative tocontainment, the reception vessel can be fitted with appropriate relief venting. Thismay take the form of bursting panels, displacement panels or hinged doors thatoperate once a predetermined pressure has been reached.

In venting explosions to atmosphere strict attention must be paid to the safedissipation of the explosion products. It is a characteristic that the volume of flamedischarged from vents can be very large, and obviously must be directed to a safeplace away from operatives and neighboring plant. If this is necessary it is nor-mally achieved by attaching a length of ducting to the vent, or by installing deflec-tor plates. The duct attached to the vent should be short, free from bends (if at allpossible) and other restrictions to flow, and be kept clear of dust at all times.

The size of duct, in terms of flow cross-section area, for explosion venting isparticularly important. This is related to the maximum rate of pressure rise and themore vigorously explosive materials require larger areas of venting. The size ofvent is also dependent upon the volume of the receiving hopper or silo.

This can also be modeled very approximately from the above thermody-namic relationship. If the explosion is to be vented to prevent a pressure rise, p,will now equal p2, but V, will no longer equal V2. Thus:

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T,V2 = V, x — . . . . . . . . . . (3)•N

The volumetric flow rate of the gases now leaving the reception vessel willbe about seven times higher than normal, and this does not take account of gasesgenerated as a result of the explosion. There is no possibility of the existing filterplant being able to cope with this increased flow rate and so venting is essential.

To keep the pressure drop in the explosion relief ducting to as low a value aspossible, the duct will clearly have to be of a large section area. Since pressuredrop varies approximately with the square of velocity, the velocity of the gases inthe ducting will have to be very much lower than that of the incoming conveyingair. Combined with the seven fold increase in steady state flow rate, and the factthat this is a transient situation, duct sizing is a complex task and should only beassessed by an expert.

4.4 Detection and Suppression

If a system is inconveniently sited to allow for venting; a vent of the required sizecannot be fitted onto the existing hopper; or if the material is toxic, so that it can-not be freely discharged to atmosphere, the protection may be achieved by a detec-tion and suppression approach.

Although there may be only a few tens of milli-seconds between the ignitionof the material, to the build up of pressure to destructive proportions, this is suffi-cient for an automatic suppression system to operate effectively, as illustrated inFigure 22.3.

Maximum unsuppressedexplosion pressure

Suppressedexplosion / Unsuppressed

System / explosionactivationpressure

Maximum pressure that theplant can withstand

ssed explosion pressure

Time - ms

Figure 22.3 Comparison of pressure-time histories of unsuppressed and suppressedexplosions.

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Commercial equipment is available that is capable of both detecting the on-set of an explosion and of suppressing the explosion before it is able to develop.The sensing device, on detecting a rise in pressure, can send signals to switch offthe air supply and stop the feeding device in order to prevent the conveying of anyfurther material. A signal can also be sent to operate the automatic opening of aventing system. An automated opening has the advantage that vents are openedextremely rapidly and for very explosive materials this helps to reduce the maxi-mum explosion pressure. Alternatively a suppressant system can be triggered.Such equipment operates as illustrated in Figure 22.4.

Suppression involves the discharge of a suitable agent into the system withinwhich the explosion is developing. The composition of the agent depends upon thematerial being conveyed, and is typically a halogenated hydrocarbon, or an inertgas or powder. The suppressant is contained in a sealed receptacle attached to theplant and is rapidly discharged into the system by means of an electrically fireddetonator or a controlled explosive charge. Thus, as soon as the existence of anexplosion is detected, the control mechanism fires the suppressant into the plantand the flame is extinguished.

Alternatively the explosion can be automatically vented. When the explo-sion is detected a vent closure is ruptured automatically, thus providing a rapidopening of the vent. The vented explosion then proceeds as for cases in which thevents are opened by the pressure of the explosion.

The automatic method has the advantage that vents are opened extremelyrapidly, and for highly explosive materials this helps to reduce the maximum ex-plosion pressure. Since it is obvious that once an explosion has been initiated, nomore material should be fed into the system, plant shut-down can also be rapidlyachieved with the detector approach.

Action Signal

Blower/Compressor

Shutdown

Vent to Atmosphere

Detector

Feeder

ActionSignals

Suppressant

gmtionSource

Figure 22.4 Basic scheme for detection and suppression.

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4.5 Secondary Explosions

With positive pressure conveying systems there is always the possibility of a fail-ure or defect in the system resulting in the discharge of a dust cloud into the at-mosphere. Abrasive materials wearing holes in pipeline bends and neglecting totighten pipeline couplings have already been mentioned. Filters can also representa weak link. A pressure surge from a blow tank, or supplementary air from an airreceiver on purging a pipeline, may result in the release of dust, or even the failureof a filter element. A flammable dust cloud can be produced quite accidentally inmany different circumstances.

There must, therefore, be no possible sources of ignition external to the sys-tem. One of the major sources of ignition in this situation comes from electricalequipment. If the material being conveyed is potentially explosive, therefore, it isessential that all the lighting, switches and switchgear, contacts and fuses, andelectrical equipment in the vicinity, or within the same building, should be of astandard or class that would not be able to provide a source of ignition, whether aspark or hot surface. This is standard practice in chemical plant where fumes andvapors are likely to be present, but tends to be overlooked with respect to the pos-sible release of dusts.

The release of a dust explosion from a conveying system into a building, orthe explosion of a dust cloud released from a conveying system inside a building,are both clearly very serious situations. Little hazardous pressure is likely to de-velop from either of these sources of explosion within a large building, if short-lived.

The pressure wave generated, however, usually shakes down large quanti-ties of dust that has settled over a period of time onto pipe-work, roof beams andsupports, ledges, lighting, etc. This then makes an ideal condition for the secon-dary explosion that almost always follows. It is this secondary explosion that candemolish a factory and kill the operatives.

It is extremely important, therefore, that good housekeeping is maintained atall times within all areas within buildings, such that any dust release is not allowedto accumulate on any surfaces anywhere, and on lighting and electrical equipmentin particular.

4.6 Determination of Explosion Parameters

In most countries all tests concerned with assessing the explosibility or measure-ment of explosion characteristics of materials in suspension are methods agreed,typically with a National Factory Inspectorate, and are generally carried out in thesequence shown in Figure 22.5.

As a result of these established procedures, data regarding the explosioncharacteristics of many materials already exists. With a material that has not beenpreviously tested, the first step should be to determine whether it is potentiallyexplosive. The outcome of such a test will then indicate the necessity of incorpo-rating precautionary measures into the system design.

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Material Sample Explosion Characteristics Relevant I lazard orMethod of Protection

Classification Tests

Group A

Explosive —>

Group BNon-Explosive

Minimum Ignition Temperature

Maximum PermissibleOxygen Concentration

Material Concentration Limits

Minimum Ignition Energy

Maximum Pressure andRate of Pressure Rise

I lot Surfaces

Use of Inert Gas

Type of System

Static Electricity

Containment andExplosion ReliefVenting

Figure 22.5 Basic scheme of explosion tests.

4.6.1 Test Apparatus

Test apparatus used to measure explosion parameters is often classified as shownin Table 22.3.

Table 22.3 Classification of Test Apparatus

Apparatus Direction ofDispersionof Material

Ignition Source Application

Vertical Tube VerticallyUpwards

HorizontalTube

Horizontal

Inflammator VerticallyDownwards

Electric Spark orElectrically HeatedWire Coil

Electrically HeatedCoil at 2370°F

Electrically HeatedWire Coil orElectric Spark

All types of dust

Carbonaceous materials,especially of smallparticle size

Carbonaceous and metaldusts, especially largeor fluffy particles

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In the vertical tube apparatus the dust is placed in a cup and dispersed up-wards over the ignition source by a controlled air blast. Observation of the flamepropagation can then be made. Modification of the electrodes allow this device tobe used for the determination of minimum ignition energy. The Hartmann bomb isa strong version of this apparatus that can also be used for the measurement ofexplosion pressure and rate of pressure rise [11].

The horizontal tube apparatus also involves the dispersion by air of a dustsample over an ignition source. Since the residence time of a dust near the coil isshort, any material that is observed to propagate a flame must be regarded as pre-senting a serious explosion hazard. The inflammator is essentially a verticallymounted glass tube. A sample of dust, held in a horizontal tube, is blown by airand is directed downwards by a deflector plate.

Although convenient for the testing of explosion characteristics, the Hart-mann bomb has been criticized on the grounds that test results do not reliably scaleup to correspond to industrial plant. This has led to the development of the so-called 20-litre sphere apparatus. This consists of a spherical stainless steel vesselfitted with a water jacket. A dust cloud is formed in the vessel as the dust entersfrom a pressurized chamber through a perforated dispersion ring. 60 millisecondsafter the dust is released into the sphere the detonator is fired and the resultingpressure rise is monitored [ 1 1 ) .

4.6.2 Material Classification

Depending on the outcome of such tests the material is simply classified with re-spect to explosibility as follows:

Group A - Materials that ignited and propagated a flame in the apparatus.Group B - Materials that did not propagate a flame in the test apparatus.

Group A materials clearly represent a direct explosion risk and, as such, itwould be a wise precaution, or even a legal requirement, to incorporate protectionmeasures into the system. The range of materials which fall into this group iswide, as indicated earlier. Sand, alumina and certain paint pigments are examplesof Group B materials. Some Group B materials, although not explosible, may nev-ertheless present a fire risk.

If a material is shown to be of the Group A type, further information on theextent of the explosion hazard may be required when considering suitable precau-tions for its safe handling. The following parameters can be determined by use ofthe test methods described above:

_• Minimum ignition temperature.1 Maximum permissible oxygen concentration to prevent ignition.

".. Minimum explosible concentration.Minimum ignition energy.Maximum explosion pressure and rate of pressure rise.

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Since the explosion characteristics, in terms of these parameters, of manymaterials are well documented it is not appropriate to include this informationhere. In order to illustrate the magnitude of the quantities involved, however, de-tails regarding a few well known materials are given in Table 22.2. A summary ofthe applications of the results of these various tests to practical conditions is in-cluded in Figure 22.5.

REFERENCES

1. D. Mills. Safety aspects of pneumatic conveying. Chem Eng. Vol 106, No 4, pp 84-91. April 1999.

2. P. Field. Dust explosions. Handbook of Powder Technology. Vol 4. Hlsevier. 1982.3. J. Cross and D. Fairer. Dust Explosions. Plenum Press. 1982.4. HM (UK) Factory inspectorate technical data note 14. Health: dust in industry.

IIMSO 1970.5. C. Schofield. Dust: the problems and approaches to solutions. Proc Solidex '82 Conf.

Harrogate. March/April 1982.6. Health and Safety Executive. Guidance Note EH 15/80. Threshold l imi t values.

HMSO 1980.7. Corn starch dust explosion at General Foods Ltd, Banbury, 18 November 1981.

Health and Safety Executive Report. HMSO. London. 1983.8. HM (UK) Factory Inspectorate. Dust explosions in factories. Health and Safety at

Work Booklet No 22. HMSO. London. 1976.9. D. Mills. Pneumatic conveying: cost effective design. Chemical Engineering, pp 70-

82. February 1990.10. C.R. Woodcock and J.S. Mason. Bulk Solids Handling: an introduction to the practice

and technology. Chapman and Hall. 1987.

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23Pneumatic Conveying Test Facilities

1 INTRODUCTION

Since the use of test facilities for obtaining data for pneumatic conveying systemdesign is so important, consideration is given here to the requirements of such atest facility. A detailed specification for all the major components required isgiven, for a facility capable of testing materials over a wide range of conveyingconditions. The specification covers a range of pipeline bores from two to sixinches, and the use of such a test facility in determining test data is considered.Consideration is also given to service facilities and material characterizationequipment requirements.

The starting point here is to assume that no such facility exists and that thereis little experience in working with such a system. In the fist instance, therefore, asystem is required that will be capable of achieving as much as possible with asingle system. With experience derived from operating the system the facilitiescould be extended with additional and more specialist equipment.

1.1 Conveying Requirements

It is suggested that any system should be capable of both dilute and dense phaseconveying, at a reasonably high material flow rate, and over a reasonably longconveying distance. The conveying system should have a wide range of controls,

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and be suitable for test, development and research work, and conveying trialdemonstrations to potential clients.

1.2 Accommodation

Consideration must be given to the space required, and this includes areas for thetest rig and associated plant and equipment. A separate area for the compressorwould be recommended, because of the noise, and a separate room for any compu-tational and electronic equipment, because of the potential for dust generation withthis type of test facility.

1.3 Preliminary Decisions

A number of initial decisions need to be made which wil l affect the scope of thework that it will be possible to undertake, and the capital investment on the testfacility. In some cases a definite recommendation will be made, where it is feltthat the given parameter should be adopted for an initial facility. In other cases arange of values will be given so that the cost implications can be considered.

1.3.1 System Type

If a single pneumatic conveying system is to be installed initially, it would be rec-ommended that this should be a positive pressure conveying system. This isprobably the most useful and versatile of all conveying systems for test work. At alater date a vacuum conveying system could be considered.

1.3.2 Material Capability

If there is no previous experience of pneumatic conveying it would be suggestedthat testing should initially be limited to non explosive materials. At a later stage asuppressant system could be fitted, or provision could be made for explosion vent-ing or making the system into a closed loop and employing nitrogen. The convey-ing system should be capable of handling most powders and granular materials.

2 CONVEYING PARAMETERS

The conveying plant needs to be built to a scale that wi l l be seen by clients to besufficiently large to provide reliable scale up of test data, and to achieve the widestpossible range of conveying parameters.

2.1 Conveying Air Pressure

For dense phase conveying, and dilute phase conveying over a long distance, to acertain extent, air must be available at a high pressure. It is recommended that thecompressor for providing the air should be capable of about 100 psig. This willalso be useful for clearing pipeline blockages.

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2.2 Conveying Air Velocity

The test facility must be capable of conveying materials in both dilute and densephase. For dilute phase conveying a minimum conveying air velocity of at least3000 ft/min is often required. It is not envisaged, however, that air at a pressure of100 psig would be used for such materials. For dense phase conveying it would beexpected that tests would be conducted with conveying line inlet air velocitiesdown to about 600 ft/min. With a low velocity, and hence lower air flow rate re-quirement, it would be expected that tests could be carried out with high air supplypressures with materials capable of being conveyed in dense phase.

A compromise clearly needs to be made here. The appropriate model relat-ing these parameters was presented in Chapter 3 with Equation 1 and is:

,2 f~i

V0 = 0-1925 P] ' ftVmin (1)* 1

where V0 = volumetric flow rate of free air - ft7min

Pi = conveying line inlet air pressure - lbf/in2 absd = pipeline bore - in

C; = conveying line inlet air velocity - ft/minand Tt = conveying line inlet air temperature - R

An alternative form of this model, in terms of conveying air velocity, waspresented in Chapter 5 with Equation 11 and is:

T VC = 5-19 —5-2- f t / m i n - - - - - - - ( 2 )

d~ p

It is recommended, therefore, that the conveying system should be capableof achieving a maximum value of conveying line exit air velocity, C2, of about9000 ft/min.

This means that for dense phase conveying, with the maximum possible airsupply pressure of 100 psig, it will be possible to undertake tests with conveyingline inlet air velocities up to a maximum of about 1150 ft/min. For dilute phaseconveying, requiring a minimum conveying air velocity of 3000 ft/min, it will bepossible to undertake tests with conveying line inlet air pressures up to a maxi-mum value of about 30 psig. It is suggested that this is probably the best compro-mise in terms of selecting a single compressor for such a test facility.

The problems here can best be illustrated by reference to Figure 4.10. Con-veying characteristics for two different materials are presented, each conveyedthrough the 165 ft long Figure 4.2 pipeline of two inch nominal bore. They arereproduced here in Figure 23.1 for reference.

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654 Chapter 23

60

50ooo

40

B 30oi>oE 20

10

Solids Loading

/ Ratio

300200 120 100 80

(a)

50 100 150 200

Free Air Flow Rate - frVmin

60

_ 5 08o

10

0

(

(b)

NO

GO

AREASolids Loading

RatioConveying

Limit

Conveying LinePressure Drop

\-lbt7in2 15

50 100 150 200

Free Air Flow Rate - fVYmin

Figure 23.1 Conveying characteristics for (a) A fine grade of pulverized fuel ash and(b) a fine granular grade of silica sand conveyed through the pipeline shown in figure 4.2.

Figure 23.1 a is for a fine grade of pulverized fuel ash and Figure 23.1 b is fora fine granular grade of silica sand. These are typical of materials that might betested. For the cases illustrated it will be seen that within a pressure capability of100 psig, conveying is limited by a combination of the volumetric flow rate of airavailable and the conveying limit for the materials. The shape and slope of thecurves representing the conveying limits for the materials are both additionallydictated by the compressibility of the conveying air.

With the limit for the pulverized fuel ash being a conveying line inlet air ve-locity of about 600 ft/min, testing will be possible with air supply pressures up to100 psig if required. For the silica sand, however, with a minimum conveying airvelocity of about 2600 ft/min, conveying is limited to a maximum air supply pres-sure of about 35 psig within the limit of free air flow rate of 200 ft3/min.

2.3 Pipeline Bore

It would be recommended that the minimum diameter of conveying pipeline thatshould be considered should be 2 inch. Anything less than this would not be givencredibility by the industry. It is unlikely, however, that a pipeline bore greater than

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about 4 inch would be necessary for the vast majority of test work. In recent years,however, many companies have installed test facilities with 6 inch bore pipelinesand so the data presented here has been extended to that diameter of pipeline forreference.

Since pipeline bore has a very significant influence on the specification ofmany of the components that comprise the conveying system, a range of pipelinediameters are considered from 2 to 6 inch, so that cost implications can be takeninto account in the decision making process.

2.4 Free Air Flow Rate

In a positive pressure conveying system the velocity of the conveying air at theend of a pipeline, in which the material is discharged at atmospheric pressure, isapproximately at free air conditions. The recommended value of this velocity hasbeen set at about 9000 ft/min and so the values of free air flow rate for the range ofpipeline bores to be considered will be as follows:

Pipeline Bore - inch 2 2'/2 3 4 6

Free Air Flow Rate - ftVmin 200 300 450 800 1800

3 SYSTEM COMPONENTS

Some major pieces of equipment are required for a pneumatic conveying test facil-ity, and the size, and hence the cost of these items, is very dependent upon thepipeline bore selected.

3.1 Compressor Specification

Since the air supply pressure has been recommended, and the free air flow rateshave been evaluated, the compressor specifications for the range of pipelines boresconsidered are as follows:

Pipeline Bore

Air Supply Pressure

Free Air Flow Rate

Approximate Power

- inch

-psig

- cfrn

-hp

2

100

200

50

2'/2

100

300

80

3

100

450

120

4

100

800

210

6

100

1800

470Required

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It must be emphasized that these power requirements are approximate val-ues and are for guidance only. It would be recommended that the compressorshould supply oil free air. Since the conveyed material is to be re-circulated itwould also be recommended that the compressed air should be cooled for convey-ing purposes. If it is proposed that hygroscopic materials, such as alumina andsoda ash are to be conveyed, consideration will need to be given to the provisionof an air drier, although this could be a later addition.

3.1.1 Compressor Type

Positive displacement blowers are not worth considering here because of the pres-sure limitation on these machines. For any additional test facilities, however, sucha compressor would be ideal, particularly for dilute phase conveying with either alow pressure rotary valve or a low pressure blow tank. A screw or reciprocatingcompressor would be recommended for the duty.

3.1.2 Air Receiver

In the first instance an air receiver is not a necessity. With future development,however, it would be useful to have an air receiver located between the compres-sor and the conveying facility, particularly if further compressors and test facilitiesare added.

3.2 Pipeline Feeding Device

In order to utilize high pressure conveying air, and to test materials capable ofdense phase conveying, as well as dilute phase test work, a blow tank would berecommended for feeding materials into the pipeline. If it is envisaged that muchwork will be undertaken with abrasive materials, such as fly ash, cement and alu-mina, a blow tank would be ideal.

For test work a continuously operating pneumatic conveying system is notnecessary. Test work can conveniently be carried out on the basis of conveying abatch of material. A single blow tank fed from a hopper above, therefore, will beadequate and it will not be necessary to incorporate a lock hopper in the facility.Consideration, however, must be given to batch size and material flow rate to en-sure that a reasonable period of steady state conveying can be achieved during theconveying of the single batch.

The choice now is between top and bottom discharge types of blow tank.The best for dense phase conveying is top discharge, but can be unsuitable forgranular materials, for as they tend to be permeable it is often difficult to get themto discharge. Bottom discharge can be used to convey most materials. The idealsolution would be to have one of each. A typical solution to the problem is to havea common tank to which alternative bottom sections can be attached, one for topand another for bottom discharge. If only one blow tank is to be employed a bot-tom discharge blow tank would be recommended.

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3.2.1 Blow Tank Size

The batch size of material to be conveyed has to be large enough to ensure a rea-sonable period of steady state conveying during the conveying cycle. It would besuggested that the blow tank capacity be sized on the basis of a minimum of twominutes of conveying for the highest material flow rate to be expected. This willbe in the shortest pipeline to be tested. Blow tank sizes for the range of pipelinebores being considered here are approximately as follows:

Pipeline Bore - inch 2 2'/2 3 4 6

Maximum MaterialFlow Rate

Batch Size

Blow Tank Volume

- ton/h

- ton

- ft3

30

1

50

45

11/2

75

70

21/2

120

120

4

200

270

9

450

3.3 Supply/Reception Hopper

For test work it is necessary to re-circulate the conveyed material, and so it is mostconvenient to discharge the material from the end of the conveying pipeline backinto the supply hopper. Thus the supply hopper that feeds material into the blowtank doubles as the reception hopper.

Normally the entire batch of material in the supply hopper wil l be dis-charged into the blow tank to be returned to the supply/reception hopper. Since thematerial at the end of the pipeline will be in a highly aerated state, the size of thesupply/reception hopper typically needs to be about 20% greater than that of theblow tank, as follows:

Pipeline Bore - inch 2 2'/2 3 4 6

Supply/ReceptionHopper Volume - ft3 60 90 150 250 550

A conical or pyramid type section will be required on the bottom of the sup-ply hopper, depending on whether a square or circular design is adopted. In eithercase as steep a wall slope as possible would be recommended in order to minimizeflow problems in the filling process for the blow tank. If head room does not al-low, consideration must be given to the use of discharge aids, such as those basedon air, vibration, etc.

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658 Chapter 23

3.3.1 Support Structure

The supply hopper will need to be mounted above the blow tank and a supportstructure will be required for this purpose. It would be useful to incorporate a plat-form for access to the filtration plant in this structure, as well as the provision ofaccess from the ground.

3.4 Filtration Unit

The supply/reception hopper will need to be fitted with a filtration unit, probablymounted on top of the reception hopper. A standard reverse air jet type of bag fil-ter would be recommended for the duty. The size will be dictated by the free airflow rate to a large extent:

Pipeline Bore - inch 2 2!/2 3 4 6

Free Air Flow Rate - rf/min 200 300 450 800 1800

The filter should be sized on the basis of handling cement or very fine flyash, at these air flow rates.

3.5 Plant Layout

A typical layout of blow tank, supply/reception hopper and filtration unit is shownin Figure 23.2. With the filter mounted on top of the hopper, the conveyed mate-rial will remain within the conveying system. In this arrangement the filter unitdoes add to the overall height of the conveying plant. If this is too high, the aircould be ducted from the hopper to a filter unit positioned alongside, possibly onthe ground. This arrangement, however, will mean that much of the fine dust fromthe material will not be returned automatically to the bulk of the material. This,however, may be an advantage and so the alternatives must be considered.

3.5.1 Material Re-circulation

With a need to re-circulate the material, for the convenience of carrying out manytests, once the material is loaded into the conveying system, a decision will need tobe made on whether to keep the fine material within the system or to extract thismaterial. If the material being conveyed is friable, to the extent that a change inparticle size distribution might result in a gradual change in the conveying charac-teristics for the material, as illustrated with soda ash in Figures 11.18 and 19, itwould always be recommended that fresh material should be used for every test.

A vent line between the blow tank and the hopper should also be provided.This will need to be opened when loading the blow tank with material. It can alsobe used to de-pressurize the blow tank at any time, should this be necessary.

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Filter

Return toHopper

Figure 23.2 Sketch of typical conveying plant test facility.

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3.6 Conveying Pipeline

A convenient routing for the pipeline needs to be established, preferably with asingle loop, incorporating four bends, having a conveying distance of about 300 ft.The provision for additional loops also needs to be considered so that the convey-ing distance can be extended, possibly in units of 300 ft. The ratio of 4 bends in300 ft of pipeline should provide a typical pipeline balance. A larger bore linecould be added in future so that the performance of stepped pipelines can beinvestigated.

The pipeline should be reasonably accessible so that changes in bend typesand routing can be conveniently made. The possibility of having one or two sightglasses in the pipeline, for flow visualization purposes, should be considered. Thiswould be of particular value when demonstrating the operation of the test facilityto clients, and is always of value as a research facility since much can be learntfrom observation of the flow.

3.6.1 Orientation

For convenience it would be suggested that the pipeline loops be located entirelyin the horizontal plane. No attempt need be made at this early stage to incorporateany vertical lift into the pipeline, other than that necessary to accommodatechanges in elevation between the blow tank discharge and entry to the receptionhopper.

4 SERVICE FACILITIES

A number of service facilities will be required for the test facility, mainly centeredon the handling and storage of the materials to be conveyed. The size of some ofthese units will depend upon the batch size to be handled, and hence the pipelinebore selected.

4.1 Material Loading

A convenient means of loading a batch of material into the supply hopper wil l berequired. A small low pressure blow tank would be ideal for this purpose, whichneed not be large, since the load could be charged in small batches. Alternatively amechanical or aero-mechanical conveyor could be used for the purpose. The filtra-tion unit on the hopper should be sufficient for the purpose. If a dedicated line isemployed for material loading an isolating valve will be required.

4.2 Material Off-Loading

When test work with a particular batch of material is completed, the batch of ma-terial will need to be off-loaded from the test rig. The conveying system itself canconveniently be used for this purpose, possibly via a short section of the convey-

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ing pipeline, into an off-loading hopper. The off-loading hopper will need to be ofa similar size to that of the reception hopper and to be fitted with a filtration unit.A reduced quantity of conveying air could probably be used for this purpose sothat the filtration unit would not have to be as large as that mounted on the recep-tion hopper.

4.3 Storage Hoppers

If materials are to be stored for possible re-use a number of such storage hopperswill be required for the purpose. It would be an advantage to have these hopperselevated so that the contents could be discharged back into the supply hopper, bymeans of the loading facility, when required. Alternatively the material could beloaded back into sacks for subsequent disposal after use. A valve would also beneeded at the outlet for these purposes.

If provision needs to be made for the storage of a number of materials, thestorage hoppers could be inter-linked. By this means they could be loaded from acommon pipeline via diverter valves, and they could all be vented through a singlefilter unit.

5 INSTRUMENTATION

A number of measuring instruments will be required in order to take measure-ments of pressures, temperatures and flow rates.

5.1 Air Pressure

A minimum of two pressure measurements need to be recorded. These are of thepressure in the blow tank and of the air pressure at inlet to the conveying pipeline.These can be Bourdon type pressure gauges, with values recorded manually withrespect to time during each test. Alternatively pressure transducers can be em-ployed that give a digital display. If on-line computer analysis is to be employed,suitable pressure transducers should be used. The monitoring of pressure along thelength of the pipeline would not be recommended as an initial instrumentationrequirement, but it is suggested that it should be given high priority for future de-velopment, particularly if research work is to be undertaken.

In common with most plants that involve the flow of fluids, the measure-ment of pressure in pneumatic conveying systems is equally important to the effi-cient operation of any such plant. Gas-solids flows are not as amenable to mathe-matical analysis, as single-phase flows, and as a result the monitoring of pressureis a common requirement. Technical difficulties in measuring pressure in pneu-matic conveying system pipelines, however, tend to be much greater when com-pared with similar problems in single-phase flow [1].

Even the interpretation of the pressure readings obtained from gas-solid flowsystems requires specialized analytical techniques, as discussed in relation to the

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662 Chapter 23

measurement of pressure drop across bends with Figure 8.14. Since theoreticaldesign methods are severely limited by the complex behavior of gas-solid flows,the design of pneumatic conveying systems relies heavily on experimental meth-ods. This applies to both dilute and dense phase modes of conveying.

5.1.1 Pressure Tappings

The reliable measurement of pressure along pneumatic conveying system pipe-lines requires pressure tappings, and any connecting lines to a pressure measure-ment device, to remain unblocked by the conveyed material. Pressure tappingsinvariably block at the start of a conveying cycle, or as a result of pressure pulsa-tions that may occur during conveying. An increase in pressure will cause some ofthe fines in the material being conveyed to surge into the connecting lines, wherethe material may be deposited, and a gradual build-up is likely to result in a block-age.

The shortening of connecting lines will help to reduce the problem of mate-rial ingress. Another solution is to pass these lines vertically upward whereverpossible, so that particulate material will drain out of the lines, but this is not al-ways possible. In most cases filters are inserted near the tapping point. A typicalexample is illustrated in Figure 23.3 [1].

Filter pads will become covered and impregnated with conveyed material,and so it is usually necessary to provide a reverse flow of high pressure air in orderto purge all such pads clean periodically. It is also common practice to have morethan one pressure tapping at each location along a pipeline. Three typical ar-rangements are illustrated in Figure 23.4 [2J.

FilterPad

Figure 23.3 Typical pressure tapping point on a pneumatic conveying system pipeline.

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(a) (c)

Figure 23.4 Typical arrangements of static pressure tappings in pneumatic conveyingsystem pipelines, (a) All four inter-connected, (b) three inter-connected, and (e) separate.

The normal procedure is to link all three or four pressure tappings together,as shown in Figures 23.4a and b. The advantage of this arrangement is that if oneof the tappings becomes blocked, a valid pressure reading will still be obtained.Only for very specific research purposes would the individual tappings each beprovided with a dedicated pressure measuring device, as shown in Figure 23.4c.

5.7.2 Bend Pressure Drop Measurement

The difficulties of pressure measurement in pneumatic conveying system pipelinesare highlighted most effectively with the problem of measuring the pressure dropacross a bend in a pipeline, as illustrated with Figure 8 .11 . It is not just a matter ofrecording the pressure at inlet to and outlet from the bend and subtracting the tworeadings. It is necessary to record the pressure at regular intervals along the sec-tions of pipeline both before and after the bend.

Part of the problem lies in the complexity of the flow in the region of abend. The conveyed particles approaching a bend, if fully accelerated, wi l l have avelocity that is about 80% of that of the conveying air. This velocity, of course,depends upon the particle size, shape and density, and the pipeline orientation. Atoutlet from a bend the velocity of the particles will be reduced and so they willhave to be re-accelerated back to their terminal velocity in the straight length ofpipeline following the bend.

5.7.3 Particle Deflection Influences

Reliable pressure measurement at any given point requires the flow to be bothsteady and wholly axial. If this is not the case a dynamic element of pressure willexist, in addition to the static element, and inconsistent or false readings may re-sult. The dynamic element may add to or subtract from the static value depending

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664 Chapter 23

upon the geometry of the flow. This is a situation that can occur at outlet from abend in a pipeline. In a long radius bend centrifugal force will tend to take the par-ticles to the outer wall. In a short radius bend the particles may bounce through thebend. Following the bend the particles will gradually establish a steady flow re-gime some distance downstream.

In a horizontal pipeline, large or heavy particles will have a tendency to'skip' through the pipeline when conveyed in dilute phase. This is because thegravitational force on the particles is relatively high compared with the drag force.Poorly welded pipe joints and misaligned flanges can cause particles to stream anddeflect from the discontinuity in flow. This streaming of particles can be particu-larly pronounced in worn bends. Mason and Smith [3] carried out tests with a Per-spex bend in order that the change in flow pattern and wear over a period of timecould be visually observed. Alumina particles were conveyed and the flow wasfrom vertical to horizontal. The results of one of their tests was shown earlier inFigure 20.25.

Pronounced streaming of particles was observed from a number of wearsites that had formed, including the straight section of pipeline following the bend.Mason and Smith |2], monitoring pressures around 90° bends, and using the arrayof pressure tappings illustrated in Figure 23 Ac, recorded pressures at outlet from abend. Their work has shown that the upper tapping can record a pressure that isgreater than that at entry to the bend, from which it might be deduced that thepressure drop around the bend is negative.

The flowing suspension impacts on the wall surface at an angle of about 20°and the dynamic pressure contribution gives an apparent gain in 'static' pressure.A deflecting flow away from the surface can induce a suction effect, however,leading to an apparent excessive pressure loss. Such turbulence in pneumatic con-veying system pipelines is unavoidable, particularly after a change in direction, butits effects can be identified if pressure measurements are taken at regular intervalsalong a pipeline.

In straight pipeline without any fittings a reasonably regular pressure gradi-ent should exist and so if an isolated reading gives an inconsistent value it cangenerally be disregarded. It could also indicate that the pressure tappings at thispoint are blocked. Inconsistencies in pressure readings should not be dismissed,however, without examining all possible causes for, as mentioned earlier, gas-solid flows are very complex and measurement of pressure drop requires greatcare.

5.1.4 Straight Pipeline Pressure Gradient

Although the energy loss due to bends in a pneumatic conveying system pipelinecan be very significant, particularly if there are a large number of bends, the pres-sure drop in the straight pipeline generally dominates in most pipelines. Themethod of determining the pressure drop, or pressure gradient, in straight pipelineis much as shown in Figure 8.14.

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Because of all the problems enumerated above it is generally recommendedthat a long section of pipeline should be instrumented with at least six sets of pres-sure tappings. By this means reasonable data can be obtained even if one or two ofthe tapping points block. From Figure 8.14 it will be seen that the first of the seriesof pressure tappings should be located well down-stream of a bend in the pipeline,or any other fitting that is likely to cause an initial disturbance to the flow in thestraight section of pipeline.

With pressure tappings along a straight length of pipeline, data in the formof pressure gradients can be obtained, in isolation from the total pipeline. In Chap-ter 8 pressure gradient data was presented for a number of materials for flow inpipelines both vertically up and vertically down. The pipeline used to generate thisdata was shown in Figure 8.2 and the routing included long sections of verticalpipeline specifically for this purpose.

5.2 Conveying Air Temperature

The temperature of the air at inlet to the pipeline also needs to be recorded forreference. On a conveying plant the material could well be at a high temperatureand so the influence that this might have on the temperature of the suspension hasto be established. With a test facility it is unlikely that tests would be conductedwith material at an elevated temperature.

5.3 Air Flow Rate and Control

The air flow rate needs to be set and controlled at a reasonably precise given valuefor each test undertaken. The most convenient way of doing this with high pres-sure air is to use a set of convergent-divergent nozzles. Two sets wi l l be requiredfor a blow tank, one for the conveying air and another for the blow tank air supply.The modeling and use of such nozzles was considered in detail in Chapter 6 atsection 3.1.

A 2:1 progression in volumetric flow rate capacity is suggested for the noz-zles, starting at about 4 cfm, so as to give a very wide and uniform range of flowrates over which the air flow rate can be varied. It is suggested that for a 2 inchbore pipeline two sets of 6 nozzles would be required. Two sets of 7 nozzleswould be needed for the 21/2 and 3 inch bore lines and two sets of 8 nozzles for the4 inch bore pipeline.

5.4 Conveyed Material Flow Rate

The most convenient method of measuring the mass flow rate of the conveyedmaterial is to use load cells. These can be used on the blow tank to measure loss inweight, or on the receiving hopper to record gain in weight. The mounting of thereceiver on three load cells is generally the best configuration. The read out of thethree load cells is usually summed and then the values can either be displayed onan instrument for manual recording with respect to time; be recorded on a chart for

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666 Chapter 23

subsequent analysis; or be fed into a data logger or computer for possible on-lineanalysis, depending upon the level of sophistication required.

The rating of the load cells will depend upon the size of batch to be con-veyed and the weight of the reception hopper, both of which will depend upon thechoice of pipeline bore. For the 2 and 2'/2 inch bore pipelines it is suggested thatthree 1 ton load cells would be required, and for the 3 and 4 inch bore pipelinesthree 2 ton load cells would be needed.

5.4.1 Load Cells

The most commonly used device for the measurement of material flow rate is theload cell. A typical arrangement for a positive pressure pneumatic conveying sys-tem is illustrated in Figure 23.5 [4], The situation with regard to a vacuum convey-ing system would be essentially the same.

Whether the load cells are used in conjunction with the supply hopper or thereception hopper is mostly a matter of convenience. If the supply hopper is chosena loss in mass will be recorded and with the reception hopper there will be a gainin mass. In terms of steady state readings there should be little or no differencebetween the two.

Although Figure 23.5 is shown with both supply and reception hoppersmounted on load cells, only in special cases would it be necessary to mount bothhoppers on load cells. Such cases would include non-steady state conveying andthe need to monitor material deposition in the conveying pipeline.

FiltrationUnit

Figure 23.5 Sketch of typical positive pressure conveying system with hoppersmounted on load cells.

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5.4.1.1 FlexibilityFor load cells to provide repeatable and reliable recordings it is essential that thehopper should be allowed to 'float' as feely as possible on the load cells. No an-choring or restraining of the hopper should be employed that will apply any com-ponent of vertical force. Connecting pipelines often present a problem in this re-spect but this can be overcome quite reasonably by means of a flexible connectionin the pipeline, close to the hopper, with the pipe/hose connection furthest from thehopper being supported.

The pipeline feeding device and air supply/exhaust lines may also prove dif-ficult to accommodate, and for these reasons load cells are generally used on re-ception hoppers for positive pressure conveying systems and on supply hoppersfor vacuum conveying systems. Provided that they do not interfere with the verti-cal component of force, any filtration plant, feeders and offloading facilities asso-ciated with the hopper can be taken into account with the tare weight of the hopperitself. This weight, together with the maximum expected load of material in thehopper, will be used in determining the size of load cells to be employed for theduty.

5.4.2 Analysis of Data

The output from the load cells is either fed into a data logger or computer, or isrecorded on a chart. Either way the signal is integrated with respect to time to givethe material flow rate. A typical trace, with respect to time, for the conveying of a500 Ib batch of material is given in Figure 23.6.

25u3ctiM 20

15

10

<u"3 5

AirPressure

500

400

300

200

100

20 40 60

Time - seconds

80 100

o

Figure 23.6 Typical load cell and air pressure traces for conveying cycle with respectto time.

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668 Chapter 23

It is also useful to record the conveying line inlet air pressure in addition, forthis helps in identifying the nature of the steady state portion of the conveyingcycle, and the start-up and tail-out sections of the cycle [4],

A high pressure, top discharge, blow tank feeder was used to generate thedata in Figure 23.6. The material was cement and it was conveyed through theFigure 4.2 pipeline of 2 inch bore and 165 feet long. With a 500 Ib batch of ce-ment to convey, a reasonable period of steady state conveying was achieved forthe flow rate achieved. Pneumatic conveying is a relatively high speed process,even for dense phase conveying, and so the residence time of the material in thepipeline is generally short. For the cement the conveying line inlet air velocity wasapproximately 1100 ft/min, with a conveying line inlet air pressure of about 23psig, and so it would only take about five seconds for the air to traverse the lengthof the 165 ft long pipeline [5].

It should be noted that the blow tank did not have a discharge valve and soboth the start-up and tail-out transients are extended in time. Such a valve is proneto wear by abrasive materials and is not always necessary, but without the valvethe conveying efficiency is significantly reduced, as can be seen. The lack of sucha valve may also cause pipeline blockage in the case of low velocity dense phaseflow, as will be illustrated later. The material flow rate is determined from the loadcell trace, as illustrated on Figure 23.6.

6 TEST PROCEDURE

The design of pneumatic conveying systems is mostly based on empirical means.Existing data for a material will be scaled from the pipeline from which the con-veying data was derived, to the new pipeline that has to be designed. This processis carried out by using a series of scaling parameters, that will take account of thedifferences between the two pipelines in terms of pipeline bore, horizontal andvertical conveying distances, number and geometry of pipeline bends, and pipelinematerial. These procedures were considered in some detail in Chapter 15.

6.1 Performance Mapping

Ideally a performance map is required for the material in order to fully determineits conveying capability. This will identify whether it is possible to convey thematerial in dense phase, and at low velocity, or if the material will only convey indilute phase, and hence at high velocity, in a given conveying system. Tests needto be carried out over a wide range of air flow rates and conveying line pressuredrops, and for each combination the material mass flow rate needs to be recorded.Since the relationship between these parameters is unknown, it is necessary tocarry out a relatively large number of tests in order to identify both the minimumconveying limit and the relationship between the conveying parameters.

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Typical data obtained with granulated sugar conveyed through the 165 footlong Figure 4.2 pipeline of two inch bore is presented in Figure 23.7. This is agraph of material flow rate plotted against air flow rate, derived as illustrated inFigure 23.6, and the data superimposed is of conveying line inlet air pressure. Thepressure data has been rounded to }A Ibf/in2 and the decimal point represents thelocation of the data point on the graph [4].

There is always a certain amount of 'scatter' in the data obtained frompneumatic conveying systems, the degree of scatter depending upon the type ofmaterial being conveyed, but it is generally possible to identify the family ofcurves from the data plotted, as illustrated on Figure 23.7, if sufficient tests havebeen carried out.

6.1.1 Dilute Phase Conveying

A typical trace for the dilute phase conveying of granulated sugar is given in Fig-ure 23.8 [6J. It will be seen that the load cell trace is very smooth, showing that thematerial was conveyed very uniformly. The pressure trace, however, fluctuatesmarkedly, but within an acceptably narrow band in this case. For dilute phase con-veying the residence time of the material within the pipeline is very short, as dis-cussed above, and so it is particularly important that the material is fed into thepipeline at a reasonably uniform rate. A small pressure difference needs to be al-lowed between the pressure rating of the blower or compressor providing the air,and the maximum value of conveying line inlet air pressure expected. This is re-quired in order to prevent the possibility of pipeline blockage, due to a momentarysurge in feed rate, but the safety margin obviously does not want to be too great.

15oo

10

± 15

15

Conveying Line InletAir Pressure Data

- Ibf/in" gauge

10

20 140 160

Free Air Flow Rate - ft'/min

180 200

Figure 23.7 Conveying data generated for granulated sugar.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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670 Chapter 23

."3 on

CO ,

.£ u

300

200

coU

100

0 200100 150

Time - seconds

Figure 23.8 Typical trace for dilute phase conveying.

The slow start-up transient is due to the fact that a blow tank without a dis-charge valve was used for feeding the material into the pipeline, as discussed inrelation to Figure 23.6. The fact that the pressure trace does not return to zero atthe end of the conveying cycle is due to the fact that the pressure sensor is re-cording the air only pressure drop for the pipeline at this time. There is also asmall element of pressure drop due to the purging of residual material from theconveying line at the end of the conveying cycle.

6.1.2 Conveying Characteristics

In Figure 23.7 the experimental data for the conveying of the granulated sugar waspresented. This data can be improved in a number of ways, purely by mathemati-cal means. Lines of constant solids loading ratio, which give an indication of thesolids concentration in the air, can be added quite conveniently as these will bestraight lines through the origin. Solids loading ratio is the dimensionless ratio ofthe flow rate of the material transported to the flow rate of the air used, and theseboth relate to the axes used.

Knowing the air flow rate and the conveying line inlet air pressure it is astraightforward task to evaluate the conveying line inlet air velocity, as consideredin Chapter 5, and this, of course, is a major parameter in system design. The result-ing conveying characteristics for the granulated sugar in the given test pipeline arepresented in Figure 23.9 [4].

It will be noted that the lines of constant conveying line inlet air velocityhave a marked positive slope. This is due entirely to the compressibility of theconveying air.

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ooo

_o± 1 0

Solids LoadingRatio

NO GO

AREA

Conveying LineInlet Air Pressure

- lbf/in2 gauge

Conveying Line InletAir Velocity - ft/min

MinimumConveying

Limit

40 80 120 160

Free Air Flow Rate - ftVmin

200

Figure 23.9 Conveying characteristics for granulated sugar.

For a given free air flow rate supplied, the conveying line inlet air velocitywill gradually reduce as the material flow rate increases, since an increase in mate-rial flow rate will result in an increase in pressure required.

The minimum conveying conditions for the material, as identified in the testprogram, are also represented. It will be seen that the minimum conveying air ve-locity for the material was about 3200 ft/min and so the granulated sugar couldonly be conveyed in dilute phase in the conveying system. The maximum value ofsolids loading ratio achieved was only about 15, despite the fact that conveying airat 25 lbf/in" gauge was employed. This fact reinforces the point that high pressureconveying is not synonymous with dense phase conveying: it is the properties ofthe bulk material that dictate this capability.

6.2 Dense Phase Conveying

Materials that have either very good air retention properties, or very good perme-ability, are generally capable of being conveyed in dense phase, and hence at lowvelocity, in conventional pneumatic conveying systems [7]. To illustrate the simi-larities and differences between dilute and dense phase conveying two further ma-terials are included, and conveying characteristics and typical conveying cycletraces are included for each.

It should be noted that both of the additional materials were conveyed withthe same test facility and through the same pipeline as used for the granulatedsugar. In addition, the maximum values of both air flow rate and conveying lineinlet air pressure employed in the test work were the same for all three materials.

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672 Chapter 23

ConveyingLimit

Solids LoadingRatio

30ooo

20

•g 10

Inlet Air Velocity- ft/min

40 80 120

Free Air Flow Rate - frVmin

160 200

Figure 23.10 Conveying characteristics for ordinary portland cement.

6.2.7 Sliding Bed Flow

Conveying characteristics for cement, are presented in Figure 23.10. A compari-son of these two sets of data reveal striking differences between the capability ofthe two materials with respect to pneumatic conveying. Whereas the minimumconveying limit for the sugar occurred at a conveying line inlet air velocity ofabout 3200 ft/min, the cement could be conveyed at velocities down to approxi-mately 600 ft/min. The solids loading ratio at which the cement could be conveyedwas also significantly higher. This has meant that the 'no go area' for the convey-ing of the cement is significantly reduced in comparison with that for the granu-lated sugar [4].

For a given conveying line inlet air pressure material flow rates achievedwith the cement are significantly greater than those for the sugar. Not only is thematerial flow rate much higher but the air flow rate required for dense phase con-veying is significantly lower. The combined effect of these two factors is that for agiven material flow rate, specific energy requirements can differ by more than tento one between dense phase and dilute phase conveying (see Chapter 7).

It will be seen from Figure 23.10 that cement clearly has dense phase con-veying capability. This is due to the material's capability for air retention, and as aconsequence the cement is conveyed in a sliding bed mode of dense phase flow.

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When observed through a sight-glass in a horizontal section of pipeline it will beseen that the material flows along the bottom of the pipeline rather like a liquid.This mode of flow, however, requires a high pressure gradient and to convey at asolids loading ratio of about 100 typically requires a pressure gradient of about 10lbf/in2 per 100 feet of pipeline for horizontal conveying. For long distance trans-port or low pressure conveying, therefore, dilute phase conveying may be the onlyoption.

It will also be seen from Figure 23.10 that the minimum conveying limit forcement is very different from that of the granulated sugar. This is due to a com-bined effect of the minimum value of conveying air velocity decreasing with in-crease in solids loading ratio, the solids loading capability increasing with increasein pressure, and the increase in air density with increase in pressure. The potentialinfluence of solids loading ratio on the minimum value of conveying air velocityfor cement was presented earlier with Figure 4.7.

6.2.1.1 Conveying TraceA typical trace for dense phase sliding bed flow is given in Figure 23.11 [6]. Onceagain it will be seen that during the steady state period the load cell trace is verysmooth, showing that the material was conveyed very uniformly. The conveyingline inlet air velocity in this case was about 900 ft/min and the solids loading ratioabout 53. On the plot of minimum conveying air velocity in Figure 4.6 it will beseen that this particular data point is just above the operating curve for cement.The pulsations in the pressure trace were of a similar order as those for the dilutephase case above (see Figure 23.8).

48

< <ijOJ>

«-i pO 03 in

" M

£-Jon

16

.ZI300

200

100

50 100

Time - seconds

150

oCJ

Figure 23.11 Typical trace for dense phase sliding bed flow conveying.

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674 Chapter 23

It will be noticed, however, that the pipeline almost blocked after about 25seconds into the conveying run. No further material was delivered to the receivinghopper for about 30 seconds. During this time the air supply pressure graduallyincreased to about 40 Ibf/in gauge and conveying recommenced. At the point ofblockage the conveying line inlet air velocity was about 1800 ft/min and the solidsloading ratio about 10. On the plot of minimum conveying air velocity in Figure4.6 it will be seen that this point is below the operating curve for cement. Thiscorresponds to an air flow rate of about 55 ft3/min on Figure 23.10 and it will beseen that with a slow start up to the conveying cycle, without a discharge valve,the "no go area" had to be crossed.

In the test facility, air at a high pressure was available and so recovery of theconveying situation was possible. On a plant, air is generally available at a specificpressure and the operating point for a conveying system will be set at a value justbelow this. It is important, therefore, that this type of information is available atthe design stage of a plant.

6.2.2 Plug Flow

Conveying characteristics for polyethylene pellets, are presented in Figure 23.12.This performance map is very different from those of both the granulated sugarand the cement conveyed through the same pipeline. Conveying is possible withvery low air flow rates, and hence very low values of conveying air velocity, andso the material is capable of being conveyed in dense phase, despite the fact thatthe solids loading ratio values are not very high.

Solids LoadingRatio

30

ooo

!20

10

0

N0 G0 Conveying LineInlet Air Pressure

AREA - Ibf/in2 gauge

MinimumConveying Limit

Conveying LineInlet Air Velocity

- ft/min 10-

0 40 80 120 160

Free Air Flow Rate - ftVmin

Figure 23.12 Conveying characteristics for polyethylene pellets.

200

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As a consequence of the constant pressure lines having a positive slope inthe dense phase region the operating envelope here is very limited. Conveying alsotends to be unstable when operating in the area close to the minimum conveyinglimit.

The point at which the constant pressure lines change slope tends to definethe division between dense and dilute phase conveying on this performance map,and occurs at a conveying line inlet air velocity of about 3000 ft/mm. For thepolyethylene pellets tested there was no abrupt transition between dilute and densephase flow, as will be seen from the continuity in the data presented. With somematerials that have good permeability, however, there is often a band of conveyingline inlet air velocity values across which the flow tends to be unstable.

Conveying with inlet air velocities down to 600 ft/min and below is possi-ble, but may not be considered economical, due to the low material flow ratesachieved. If the material to be conveyed is very friable, however, this region ofconveying may well be a design option. Solids loading ratio values are low due thefact that the material is very permeable.

Good permeability is a specific requirement in order to achieve this mode offlow in a conventional conveying system. When observed through a sight-glass, ineither a horizontal or vertical section of pipeline, it will be seen that the materialflows through the pipeline in a series of plugs, each of which fills the pipeline, andare separated by short air gaps.

6.2.2.1 Dense Phase Conveying TraceA typical trace for dense phase plug flow conveying is given in Figure 23.13 [6].Both the load cell and pressure traces are very different from those for both thegranulated sugar and the cement.

300

200

oU

100

100 200 300

Time - seconds

400

Figure 23.13 Typical trace for dense phase plug flow conveying.

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676 Chapter 23

The load cell trace, however, with its characteristic 'staircase' profile istypical for this type of material. It will also be noted that the resolution obtainedfrom the load cell trace is such that the discharge of individual plugs of materialfrom the pipeline into the reception hopper can be clearly identified.

Because of the discharge of discrete plugs, the fluctuation in air pressure ismuch greater than that for either the sugar or cement, and will be particularlymarked in a short pipeline. In the case shown the conveying line inlet air velocitywas about 1000 ft/min and the solids loading ratio was 18, which is a combinationtypical of plug flow.

7 MATERIAL CHARACTERIZATION

Since the conveying characteristics of different materials, and even differentgrades of the same material, can vary very significantly, it is essential that smallrepresentative samples of all materials that are tested in the pneumatic conveyingtest facility are obtained and stored for reference. By recording property values ofmaterials tested it will be possible to gradually build up a data bank of materials,such that it will ultimately be possible to relate conveying performance to a set ofmaterial characteristics.

The retention of samples and the recording of property values is particularlyimportant for systems manufacturing companies. If at the time of commissioninga plant, it is not possible to achieve the stipulated material flow rate, a check on thematerial being conveyed might be appropriate. It is not unknown for a systemsuser company to change the source of material to be conveyed, particularly if thereare price implications, and the grade of the new material may be very differentfrom that for which the plant was designed.

7.1 Riffling Device

It is essential that a truly representative sample of the materials to be conveyed isobtained when recording material property values and so some form of rifflingdevice will be required. Several tons of material may be supplied for testing in thepneumatic conveying facility, but only a couple of ounces might be required forsieving to determine particle size distribution, and even less for laser diffractionparticle sizers. A riffling device will allow accurate sub-division of a sample thatmay weight several pounds when collected.

Samples will be required from the 'as supplied' material and possibly fromthe material during and after conveying, in order to check on material degradation.It would generally be recommended that sampling should be from a movingstream of the material, rather than a static pile. The sample, however, should betaken from the full cross section of the flow in order to avoid any segregationwithin the flow stream itself. It would also be suggested that the techniquesadopted should be employed for all samples taken.

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7.2 Particle Size

It is important that the mean particle size and particle size distribution are re-corded. A set of sieves, together with a mechanical shaker and an electronicweighing device is essential, but it must be recognized that sieving does have alimit with respect of fine particles, particularly with dry sieving, for which it isabout 45 micron (325 Mesh). Laser devices are ideal for fine particles, but arerather expensive, and so it would be suggested that samples be sent to a specialistlaboratory for analysis whenever required.

7.3 Particle Shape

This is not an essential property, but if a microscope is available it would be usefulto make a note of the particle shape for reference purposes. Differences in convey-ing capability between fly ash and cement having a similar particle size can berelated to differences in particle shape to a certain extent.

7.4 Particle Density

Particle density is clearly a useful reference parameter for any material. The aircomparison pycnometer is a convenient instrument for its measurement. For manymaterials, values will be found in appropriate reference books.

7.5 Permeability

A permeameter would be a useful facility. Not only can it be used to record mini-mum fluidizing velocity, fluidized bulk density, and permeability, it can also beused to observe fluidization behavior in general, and hence the potential value offluidization in terms of its employment as a flow discharge aid in hoppers, silosand chutes. A permeameter can also be used to determine the degree of air reten-tion for a material.

7.5.1 The Permeameter

This consists of a vessel of uniform section area, which is usually circular, having aporous membrane at the base. An air supply, which is capable of being varied overa very wide range of flow rates, is needed. A means of measuring the pressure dropacross the bulk particulate material is also required. A sketch of such a device isgiven in Figure 23.14.

7.5.2 Superficial Air Velocity

Although the volumetric flow rate of air is measured and controlled, it is thesuperficial air velocity that is the important parameter. This is the volumetric flowrate of the air divided by the cross sectional area of the fluidizing vessel whenempty. A program of tests with a material entails the determination of the variationof the pressure drop, across a bed of a given depth, with superficial air velocity.

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678 Chapter 23

Cross SectionalArea - A

L

PorouMembrane

PlenumChamber Air SuPP'y - ''

Figure 23.14 Sketch of atypical permeameter.

A typical relationship between pressure gradient and air velocity for flowthrough a bed of material is shown in Figure 23.15.

ID N

a. '

el**

Fixed Bed Fully Fluidized Bed

Slope =1/C

where C = Permeability Factor

Minimum FluidizingVelocity - Umf

Superficial Air Velocity - U

Figure 23.15 Typical relationship between pressure gradient and air velocity for flowthrough a bed of material.

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7.5.3 Permeability Factor

When air percolates through a bulk particulate material, a pressure drop results inthe direction of flow. The relationship between superficial air velocity, or air flowrate, and the pressure drop, for the fixed bed region, as shown in Figure 23.15, iscalled the permeability.

Referring to Figure 23.15:

u = Ap

L(3)

where U = superficial air velocity through bed

V

A

V = volumetric air flow rateA = cross sectional area of bedL = bed heightAp = pressure drop across bed

and C = permeability factor

The permeability factor, C, can be measured by means of a permeameter, asshown in Figure 23.14, which, in turn, enables the graph shown in Figure 23.15 tobe drawn, from which the permeability factor can be determined.

7.5.4 Pressure Drop

The pressure drop across the bed can be readily calculated. From fluid mechanicswe have the following hydrostatic relationship:

Ap = P § L

144 gc

where Ap

PgL

and gc

Ibf/in2 - - - - - - - - - ( 4 )

pressure drop across bed - Ibf/in2

bulk density of fluidized material - lb/ft'gravitational acceleration - ft/s2

bed height - ftgravitational constant - ft Ib/lbf s2

7.5.5 The Fluidization Process

The permeameter, if provided with a glass or Perspex container, can be used toillustrate the influence of superficial air velocity on fluidization behavior. At lowflow rates the air will merely filter through the interstitial voids without disturbingthe packing arrangement of the bed.

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680 Chapter 23

If the air flow rate, and hence the superficial air velocity, is graduallyincreased, the pressure drop across the bed will increase, as shown in Figure 23.15.For a given bed the pressure drop across it depends only upon the flow rate of theair, and in most cases the relationship is approximately proportional, as drawn onFigure 23.15, although a hysteresis effect is often observed if the plot is drawn withthe air flow rate both increased and decreased. This phase is termed a 'fixed' or'packed' bed.

7.5.5.1 Minimum Fluidizing VelocityIf the air flow rate is increased further, a stage is reached when the pressure dropapproaches the magnitude of the downward gravity force per unit of cross-sectionalarea of the bed of particles. If the bed is not restrained on its upper surface therewil l be a slight expansion of the bed, accompanied by a re-arrangement of theparticles as each one tends to 'float' separately in the upward flow of air.

This re-arrangement brings the particles towards a state corresponding to theloosest possible packing in the bed, which is now on the point of becoming

'fluidized'. The minimum fluidizing velocity, Umf, is defined as the point at whichthe bed of particles becomes fully supported from this loosest packing arrangement.The minimum fluidizing velocity, Umf, can be deduced from the plot shown inFigure 23.15.

7.5.5.2 The Porous MembraneIn all processes involving the fluidization of a bed of solid particles some form ofporous membrane or distribution device is needed to introduce the fluidizing air tothe bed. Although it would be possible to use a pipe grid at the base of the bed, forgas fluidization, the more widely used method is to construct the vessel with someform of gas plenum chamber at the bottom. This is separated from the maincontainer or column for the bed by a porous or perforated plate, as illustrated inFigure 23.14. The design of this distributor plate, particularly with regard to thematerial from which it is constructed, and the pressure drop across it, can vary overa wide range.

For example, some applications are suited to the use of metal platesperforated with a small number of relatively large holes, across which the pressuredrop would be very small. At the other end of the scale, however, will be foundporous distributors such as ceramics, sintered metals and plastics, and woven cottonand polyester materials, for which the pressure drop wil l be very much higher.

7.5.5.2.1 Minimum Membrane ResistanceConsiderable interest surrounds the influence of the pressure drop across the porousmembrane, or more specifically, the ratio of this pressure drop to that across thefluidized bed, on the quality of fluidization. The stability of the fluid bed system isan important criterion and it is generally expressed in terms of the ratio of thedistributor resistance to the bed resistance. Although there appears to be somedisagreement as to how the optimum pressure drop should be determined, the

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general consensus suggests that the pressure drop through the distributor should beat least 15% of that across the particle bed.

The stability of the fluidized bed may also be influenced by the size anddensity of the particles in it. Although very little experimental data is available,Figure 23.16 gives an indication of the way in which the minimum requireddistributor pressure drop would vary with these properties. Figure 23.16 is based onan equation by Siegel [8]. and is drawn for spherical particles fluidized with air at acondition close to normal ambient.

7.6 Air Retention

Some bulk solids, when fluidized or agitated in some way, have a tendency toretain air for a period, as mentioned in relation to Group A materials in Geldart'sclassification mentioned earlier (Figure 18.10). A measure of the air retentioncapability of a material can also be obtained by use of the permeameter.

7.6.1 De-aeration Constant

The air retention capability of a material is assessed in terms of the time it takes afluidized bed of material to return to a specified bulk density, or level in thepermeameter, after quickly shutting off the air supply. The starting, or reference,point for such a determination, is that the fluidizing should be at the point thatprovides a maximum volume increase of the material without severe bubbling atthe material surface.

2 CQ3 <n

3 <

24

20

e. %O u-i

I ° 12

350

Particle Density - Ib/ft3

100 200 300

Mean Particle Size - u.m

400 500

Figure 23.16 Minimum pressure drop required across distributor for a bed of sphericalparticles.

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682 Chapter 23

For convenience a scale should be provided on the permeameter. With somebulk solids the level of the material falls very rapidly, particularly in the earlystages, and so this is not a constant that can conveniently be recorded manually atthe time it is carried out.

7.6.1.1 AnalysisOne method of obtaining the necessary data is to use high speed photography.Another method is to use a video tape recording of the fall. Sutton and Richmond[9, 10] analyzed this transient fall by extending Pick's Law of Diffusion to thesituation. They obtained:

dp , ., AK>" — I" 1 t:\

dr L

where p = material bulk density - lb/ft3

T = time - mink" = de-aeration constant - ft/minA/? = pressure drop across bed - lbf/in2

and L = bed height - ft

Integration of this expression between suitable experimentally derived limitswill yield the de-aeration constant. High values of this constant indicate a highsettling rate and, therefore, poor air retention capability.

A further method of monitoring rapid transients is to use an electronicdifferential pressure transducer. If this is connected across the pressure tappings onthe column of material on the permeameter, it will provide a suitable trace of thepressure decay following the shut off of the air, for evaluation of the constant.

The value of the de-aeration constant obtained will give some indication ofthe capability of a material for dense phase pneumatic conveying, without the needfor air addition along the length of the pipeline. It will also give an indication of theeffect that aeration might have on the material, for aiding its discharge fromhoppers.

7. 6. 2 Vibrated De-aeration Constant

If the bed of material in the de-aerated condition is vibrated, the height wi l l fall in asimilar manner to that described above, in which the fluidized bed height falls whenthe air supply is cut off. A comparison of the two de-aeration plots of bed heightversus time is illustrated in Figure 23. 1 7 [7].

It is possible, therefore, that this vibration test could generally be of morevalue than the permeameter method. For materials that exhibit poor air retentioncharacteristics, and hence de -aerate rapidly, the rate of change can be slow enoughto observe visually. On the other hand, for some very air retentive powders, thesettling time can run into hours and even days, and vibration can speed up theprocess considerably.

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£P'SI

ffl

Fluidized

Compacted Condition

Settlement under theInfluence of Gravity

Condition

Settlement under theInfluence of Vibration

Time - T

Figure 23.17 Comparison of de-aeration curves.

It is also very much easier to apply to cohesive and other materials that aredifficult to aerate. Vibration is applied in the vertical plane, but only a narrow bandof frequencies have a settling effect on materials. If the frequency is too low it haslittle effect, and if it is too high dilation wil l occur instead of compaction. Also, thehigher the frequency, the lower the penetration of vibration.

7.6.2.1 AnalysisAn idealized graph showing the change in bed height with respect to time wasshown above in Figure 23.17. This compares settlement under the influence ofgravity and vibration. It can be seen that the relationship in each case is similar and,therefore, it is not unreasonable to apply the analysis proposed by Sutton andRichmond for the settlement of powders under the influence of gravity to thesettlement of powders under the influence of vibration. The application of theanalysis of Sutton and Richmond to this case yields:

JP - k"^~ ~ K vI

(6)

where k"v = vibrated de-aeration constant - ft/min

and Ap = py-, - p - lb/ft3

This expression can be put into a form where it can be integrated and thefollowing boundary conditions applied:

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684 Chapter 23

at r = 0, L = L,

T = oo, L = L,x

The result is:

In

L

= k" (7)

where £/ = initial bed height - ft

and LOO = final bed height - ft

This equation can be written in the form of a straight line graph, the slope ofwhich is the vibrated de-aeration constant.

Thus H = k" vr (8)

where

H = L In (9)

A detailed test procedure is given in Reference 7. These tests are relativelyeasy to undertake and take little time to carry out. A small sample of the material isall that is required and the equipment needed to carry out the tests manually isrelatively simple and inexpensive.

7.7 Permeameter Design

A permeameter, is an invaluable device both for determining the minimumfluidizing velocity of a bulk particulate material, and for observing the fluidizationbehavior of different materials. It can also be used to measure the permeability ofpowdered and granular materials, as well as the air retention characteristics of suchmaterials. It also provides an easy means of determining the resistance of porousmembrane materials.

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Air Mover

Membrane

Plenum Chamber

Air Flow Measurementand Control

Figure 23.18 Typical layout of permeameter and components.

The authors are not aware of any company that manufactures and marketspermeameters. Most companies and research organizations that find that they havea need for a permeameter, generally make their own. In order to provide somegeneral advice and guidance on the design and construction of a permeameter,notes are appended here.

For reference purposes a sketch of a permeameter is given in Figure 23.18.This shows the associated components in relation to the permeameter. The mainitems that are required are considered in the notes that follow. A range of sizes arealso considered.

7. 7 / Material Column

The heart of the device is the vertical column, or permeameter, in which the bulkparticulate material is fluidized. The behavior of the material in the permeameterrequires to be observed, and in particular the height of the free surface. For thisreason the column needs to be made of a clear material such as glass or Perspex.Perspex is the material most commonly used. The column is open to theatmosphere and so the pressure within the device is very low. A sketch is given inFigure 23.19.

7.7.1.1 DimensionsThe column can be square, circular, or of any other section, but it is usuallycircular and of constant diameter. The primary dimension of the device is theinside diameter, d, of the cylinder used to contain the particulate material.Diameters of 2, 4 and 6 in will be considered.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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686 Chapter 23

Cylinder

Material Bed

in

Pressure - 8 inTappings

Valve

2 f t

Membrane

Figure 23.19 Sketch of material column.

\Plenum

Chamber

Two inch is typically the smallest diameter used, and is probably the mostcommon, as only small quantities of material are required for testing purposes.With larger diameters, however, the wall effects are minimized and membraneinfluences on fluidization are easier to detect. Diameters not less than 4 in aregenerally recommended whenever possible. Larger diameters also help to increasethe accuracy of air flow measurement, and hence the determination of superficialgas velocity. This is particularly a problem with very fine powders since fluidizingvelocities can be very low.

Regardless of diameter, the column of material under test needs to be aboutone foot high. The height of the Perspex cylinder needs to be about double this attwo feet. The cylinder should be much higher than the bed of material in order toallow for expansion of the bed when fluidized, and possible violent agitation whenfluidized at high velocity. Also, some materials will rise en masse above themembrane when fluidized, and a reasonable column height will allow time toswitch off the air, or stir the material, before it discharges itself over the top.

7.7.1.2 Pressure TappingsIt is suggested that pressure tappings should be provided on the cylinder about 8 inapart, with the lower one about 2 in above the base. Depending upon the type ofpressure measuring device employed it may be necessary to add a gauze to filterdust from the device. It may also be necessary to cap the tappings, if the pressuremeasuring device is removed, such as when fluidizing in order to observe flowbehavior.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Conveying Test Facilities 687

PressureTappings

Clamps

-9t

AirSupply Dia

T

Material Column

Membrane

Hinges

ThGasket or

Seal

Gap

D

Depth

Figure 23.20 Sketch of plenum chamber.

7.7.2 Plenum Chamber

A plenum chamber is required for supporting the membrane and material column,and distributing the air to the bed of material. The plenum chamber can be squareor circular and needs to be vertical. A sketch of a typical plenum chamber is givenin Figure 23.20.

7.7.2.1 Hinged UnitThe plenum chamber can be hinged or entirely welded. In Figure 23.20 it is shownhinged. A hinged unit is very convenient in allowing the contents of the materialcolumn to be emptied simply by inverting the cylinder. A disadvantage is that thehinged top must be provided with an air-tight seal, since the air flow rate ismeasured upstream of the plenum chamber and needs to be determined accurately.Although an entirely welded unit eliminates air leakage, some means has to befound for removing the material from the permeameter. A vacuum cleaner istypically used for this purpose.

7.7.2.2 Pressure TappingA pressure tapping on the plenum chamber will allow the pressure drop acrossdifferent membrane types and materials to be measured directly. This, of course, iscarried out with no material in the permeameter. If the pressure measuring device isremoved, the pressure tapping must be capped to prevent air leakage.

7.7.2.3 DimensionsApproximate sizes for the plenum chamber, for the range of permeameter diametersconsidered, are as follows:

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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688 Chapter 23

PermeameterDiameter

din

Chamber Widthor Diameter

Din

Gap

in

Depth

in

Air SupplyPipe Diameter

in

246

81012

0-20-40-6

345

3/4

1%2

A short stub of pipe, of the same diameter as the permeameter, needs to befixed beneath the membrane, as shown in Figures 23.19 and 20. The spacingabove the base (gap) will help provide a uniform flow of air across the membranefor fluidizing the material in the permeameter. The other dimensions are also inproportion to the permeameter diameter, and hence air flow rate.

7.7.3 Membrane

A range of membrane materials and types may need to be tested and so ease ofchanging and testing needs to be incorporated into the design. A suggestion for afixing arrangement is given in Figure 23.21. Screwing to the top surface of theplenum chamber with studs is probably the most convenient, and wil l accom-modate a wide range of membrane thicknesses. Washers or gaskets wi l l have tobe provided on each side to prevent air leakage.

MaterialColumn

Top Surface ofPlenum Chamber

Y

SealingWashers / Gaskets

Membrane

Figure 23.21 Sketch of fixing arrangement for membrane.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Conveying Test Facilities 689

It is unlikely that any membrane material under test wil l need support in thepermeameter against either the air pressure beneath or the weight of material above.If it should be necessary, however, support can be provided on either side by meansof wire mesh.

7.7.4 A ir Supply

A small air mover is required to supply the air for fluidizing. Measurement andcontrol of the air flow are essential requirements.

7.7.4.1 RatingThe rating of the air mover is in terms of delivery pressure and volumetric flowrate.

7.7.4.1.1 PressureThe pressure required for the air mover is mainly that necessary to fluidize the bedof material. A small allowance will have to be made for the porous membrane andthe resistance of the air flow measuring device, together with all the connectingpipe-work.

The model for the fluidized bed was presented earlier in Equation 23.4:

Ap = £^±- lbf/in2

144 gc

taking p = 60 Ib/ft as a typical value (eg alumina)g = 32-2 ft/s2

L = } ftand gc = 32-2 ft Ib/lbf s2

givesAp = 0-4 lbf/in2

To enable the permeameter to be used with materials having a much higherbulk density, such as barite and metal powders, it would be advisable that the airmover have a pressure capability somewhat higher, at about 1-2 lbf/in (33 in wg).This will allow tests to be undertaken with materials having more than double thedensity of alumina, and also accommodate the pipe-work and flow measuringdevice losses.

Since the bed height remains constant, with increase in permeameterdiameter, the pressure required will also remain constant with the diameter of thepermeameter.

7.7.4.1.2 Flow Rate

The volumetric flow rate of air to be delivered, V , is given by air velocity timesflow area, which in this case is:

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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690 Chapter 23

,2

V = Umf x — f t 3 / m i n - - - - - - (10)576

where Umf = minimum fluidizing velocity - ft/min(see Figure 23.15)

and d = diameter of permeameter - in

From Figure 18.16 it will be seen that Umr can vary over an exceptionallywide range, depending mainly upon the mean particle size of the bulk solid. For 20micron sized particles having a particle density of 60 lb/ft3, for example, it is about0-04 ft/min, and for 500 micron sized particles having a particle density of 300lb/ft3 it is about 60 ft/min.

It is necessary for fluidization tests to be undertaken with air velocities muchhigher than the value of the minimum fluidization value and so it is recommendedthat a permeameter should be designed to provide a maximum fluidizing airvelocity of 100 ft/min. For the range of permeameter diameters being considered,the air flow rates required are as follows:

Permeameter Diameter - d Air Flow Rate

in ft3/min

2 24 86 19

7.7'.4.2 Air MoverFrom this pressure drop and flow rate rating it will be seen that a small fan orblower would be suitable. A power rating well below 1 hp would be required.

7.7.4.3 MeasurementAs discussed in relation to Figure 18.16, a very wide range of fluidizing velocitieshave to be catered for. With large particulate materials 100 ft/min will be requiredand the maximum air flow rate available will have to be used. With fine powders,however, the maximum fluidizing velocity required may be well below 1 ft/min. Inthis case less than 1% of the air flow rate will be required as a maximum, and it willbe necessary to accurately measure air flow rates to 1% accuracy below this value.It will be appreciated from this data, just why sealing of the membrane and plenumchamber are so important. A very small air leak can represent a very large error inthe value of the fluidizing velocity for a fine powdered material.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Conveying Test Facilities 691

For a permeameter to be capable of testing a wide range of powdered andgranular materials, therefore, the measurement of air flow rate is critical, and themeasuring device is a major feature of the permeameter. There is a need for the airflow rate to be reduced by a factor of at least 10,000:1. This is not likely to beachieved with a single flow meter, but will require staging and isolating valves.

A three stage device, based on rotameters, is shown in the sketch in Figure23.18. With a 10:1 turn down ratio on each rotameter, or flow measuring device,stage one could cater for fluidizing velocities from 0 to 1 ft/min, the second from 0to 10 ft/min and the third from 0 to 100 ft/min. By this means reasonable controland accuracy could be obtained in the testing of any material. Rotameters are idealfor this purpose as they provide a direct visual display, do not take up too muchspace, and can be easily plumbed into the system.

In terms of the three sizes of permeameter being considered, approximatevolumetric flow rates required, in ftVmin, for a three stage measuring device are asfollows:

PermeameterDiameter

d

in

246

Fluidizing Air Velocity Range

0 - 1

0-020-080-19

ft/min

0 - 10

0-20-91-9

0 - 100

2-08-4

19-0

The diameter of the air supply piping into the plenum chamber, for thedifferent permeameter diameters, was given earlier in the table in section 7.7.2.3.These same diameters can apply to the pipe-work throughout the entire air supplyand flow measuring system.

7.7.4.4 ControlIt is unlikely that either a fan or a blower would be capable of achieving such awide turn down ratio. To overcome this problem it is suggested that a tee piece withvalves should be fitted between the air mover and the flow measuring device, asshown on Figure 23.18, so that air not required can be discharged to atmosphere. Itis only loss of air downstream of the flow measuring device that must be prevented.

The valve on the air supply line, at entry to the plenum chamber, is not usedfor flow control. It is either fully open or fully closed. It does, however, need to becapable of rapid closure. This facility is required when the permeameter is used tomeasure the de-aeration constant for a bulk material, and in any emergencysituation, such as the bed of material rising en masse in the permeameter.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Page 700: Handbook of Pneumatic Conveying Engineering

692 Chapter 23

Air FlowMeasuring

Device

Discharge toAtmosphere

FluidizingColumn

Membrane

PlenumChamber

Figure 23.22 Possible working layout for permeameter.

7.7.5 Layout

It should be possible to mount the entire device on a small table. The air mover canbe placed on a shelf below, the flow measuring device can be mounted on a boardbehind, and the permeameter itself at the edge of the table, if it has a hinged top. Asketch of such a layout is given in Figure 23.22.

REFERENCES

4.

6.

D. Mills. Measuring pressure on pneumatic-conveying systems. Chemical Engineer-ing. Vol 108, No 10, pp 84-89. Sept 2001.J.S. Mason and B.V. Smith. Pressure drop and flow behavior for the pneumatic trans-port of fine particles around 90° bends. Proc Pneumotransport 2. BHRA Conf PaperA2, 16 pp. Guildford, England. Sept 1973.J.S. Mason and B.V. Smith. The erosion of bends by pneumatically conveyed suspen-sions of abrasive particles. Powder Technol, Vol 6, pp 323-335. 1972.D. Mills. Material flow rates in pneumatic conveying. Chemical Engineering. Vol109. No 4. pp 74-78. April 2002.D. Mills. Optimizing pneumatic conveying. Chemical Engineering. Vol 107. No 13.pp 74-80. December 2000.J.S. Mason and D. Mills. 20 years of pneumatic conveying with the Powder and BulkSolids Conference. Proc 20th Powder and Bulk Solids Conf. pp 3-40. Chicago. May1995.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Conveying Test Facilities 693

7. M.G. Jones and D. Mills. Product classification for pneumatic conveying. PowderHandling and Processing. Vol 2. No 2. pp 117-122. June 1990.

8. R. Siegel. Effect of distributor plate-to-bed resistance ratio on the onset of fluidizedbed channeling. AlChE Jnl. Vol 22. No 3. pp 590-592. 1976.

9. A.M. Sutton and R.A. Richmond. Improving the storage conditions of fine powdersby aeration. Trans Inst Chem Engrs. Vol 51. 1973.

10. A.M. Sulton and R.A. Richmond. How to improve powder storage and discharge inhoppers by aeration. Process Engng. Sept 1973.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.