Department of Building Services Degree in Building...

Degree in Building Services (DT026.4), Control Engineering (Section A)

Dr.JMcGrory & Dr.BCostello, DIT, cont_engineering_dt026_4_building_year_4_section_a_v2.0 of 124

Department of Building Services

Degree in Building Services DT026 Dublin Institute of Technology

Bolton Street Dublin 1

Control Engineering (Section A Only) Forth Year Course

Version 2.0

Lecturer: Dr. John McGrory School of Control Systems and Electrical Engineering, Dublin Institute of Technology, Room 10, Kevin Street, Dublin 8. Phone: +353-(0)1-402-2848 E-Mail: [email protected] Web Site: http://eleceng.dit.ie/jmcgrory/



Notes from the author. Each semester is 15 weeks in duration. This includes one week for revision and two weeks for exams. This equates to only 12 weeks teaching. Assuming this course is disseminated using four lectures per week, two for each section, this means that there are only twenty-four contact hours per section involved (not including laboratory time or your private study time for this subject). Therefore, onus is on you from the beginning to perform to the best of your ability. The term “luck” refers to that which happens beyond a person's control. Passing the exam and handing in satisfactory laboratory reports is not a matter of “luck” it is a matter of effort and work. Remember woulda coulda shoulda are the last words of a fool. As a caveat to students, the contents of these notes should not be considered the complete course. Items raised during the lectures are just as important and revenant and you should note them for yourselves. These notes are provided before the lecture takes place. This allows you read ahead and to make the best use of your contact time with the lecturer. Section B is contained in another document. Keep the notes separate, as both sections are to be attempted for the examination. In the diagram below you can see my office (Room KEG-010) location in Kevin Street. Beside my office in Room KEG-012 in Kevin Street is where the laboratory is located. So “yes” you have to get to Kevin Street for the laboratories once every three or four weeks. So let’s get through it and work hard,

John McGrory



Table of Contents

NOTES FROM THE AUTHOR. .................................................................................................................... 2

TABLE OF CONTENTS ................................................................................................................................. 3

OBJECTIVES OF THIS COURSE:............................................................................................................... 6

AIM: ................................................................................................................................................................ 6 OBJECTIVE: ..................................................................................................................................................... 6 COMPLETION TIME .......................................................................................................................................... 6 SYLLABUS: ...................................................................................................................................................... 6 METHOD OF INSTRUCTION: ............................................................................................................................. 6 ASSESSMENT PROCEDURES AND CRITERIA: .................................................................................................... 6

CHAPTER 1, ELEMENTARY CONTROL FUNCTIONS.......................................................................... 7

AIM OF CHAPTER ............................................................................................................................................ 7 OBJECTIVE OF CHAPTER.................................................................................................................................. 7 OUTSIDE AIR ................................................................................................................................................... 8 CONSTANT MINIMUM OUTSIDE AIR. ................................................................................................................. 8 OA ECONOMY CYCLE – TEMPERATURE CONTROL............................................................................................ 9 THE ECONOMY CYCLE SCHEDULE .................................................................................................................... 9 ENTHALPY CONTROL .................................................................................................................................... 14 ROOM PRESSURE CONTROL WITH DAMPER CONTROL..................................................................................... 19 STRATIFICATION ........................................................................................................................................... 21 QUESTIONS ON CHAPTER 1............................................................................................................................ 23

CHAPTER 2, HEATING AND COOLING COILS.................................................................................... 24

AIM OF CHAPTER .......................................................................................................................................... 24 OBJECTIVE OF CHAPTER................................................................................................................................ 24 CONTROL OF HEATING COILS........................................................................................................................ 25 REHEAT CONTROL ........................................................................................................................................ 25 JUST AS A REMINDER "ENERGY EFFICIENT" HUMIDIFICATION. ......................................................... 26 THE TWO SCHOOLS OF THOUGHT ON HUMIDIFICATION: ................................................................................. 26 SENSIBLE HEATING, AND COOLING ALONG COOLING, OR HEATING COIL........................................................ 29 FROST PROTECTION....................................................................................................................................... 30 AFTER HEAT COILS ........................................................................................................................................ 32 LIMITS OR OVERRIDE TO CONTROL THE SUPPLY AIR. ..................................................................................... 35 CONTROL OF COOLING COILS ........................................................................................................................ 35 CONTROL OF CHILLED WATER (CW) COILS ................................................................................................... 44 CONNECTING CHILLED WATER IN COILS IN PARALLEL OR COUNTER FLOW................................................... 44 THE AIR WASHER ......................................................................................................................................... 46 TWO STAGE EVAPORATIVE COOLING ............................................................................................................ 48 NON-ADIABATIC HUMIDIFICATION PROCESS.................................................................................................. 48 STEAM HUMIDIFIERS (ELECTRICAL) .............................................................................................................. 50 PRESSURE WATER HUMIDIFIERS (ELECTRICAL) ............................................................................................ 51 CHEMICAL DEHUMIDIFICATION..................................................................................................................... 51 CONTROL ...................................................................................................................................................... 53 CONTROL OF ELECTRIC HEATERS ................................................................................................................. 54 CONTROL OF REFRIGERATION SYSTEMS........................................................................................................ 56 CAPACITY OF SPEED CONTROL OF COMPRESSOR............................................................................................ 56 STAGED COMPRESSION .................................................................................................................................. 56 SCROLL COMPRESSOR. .................................................................................................................................. 56 CYLINDER UPLOADING. ................................................................................................................................. 57 INLET VANE CONTROL OPERATES AS PER FANS. ............................................................................................. 58 AIR COOLING CONDENSERS ........................................................................................................................... 60



WATER COOLED CONDENSERS....................................................................................................................... 63 CONTROL OF COOLING TOWERS..................................................................................................................... 63 QUESTIONS ON CHAPTER 2............................................................................................................................ 64

CHAPTER 3, COMPLETE CONTROL SYSTEM .................................................................................... 65

AIM OF CHAPTER .......................................................................................................................................... 65 OBJECTIVE OF CHAPTER................................................................................................................................ 65 THE SINGLE ZONE SYSTEM WITH VARIABLE OA QUANTITY FOR ECONOMY CYCLE. ....................................... 66 CONTROL OF RELATIVE HUMIDITY (RH) ...................................................................................................... 68 VARIABLE AIR VOLUME (VAV) CONTROL.................................................................................................... 70

Room control ........................................................................................................................................... 70 Discharge temperature control................................................................................................................ 70 Mixed Air Temperature control .............................................................................................................. 71 OA Variable Flow Rate (VFR) Control .................................................................................................. 71 Fan Control ............................................................................................................................................. 72

REHEAT SYSTEM ........................................................................................................................................... 75 Commercial application........................................................................................................................... 75 Industrial applications............................................................................................................................. 77

QUESTIONS ON CHAPTER 3............................................................................................................................ 77

CHAPTER 4, HEAT RECOVERY CONTROL SYSTEM ........................................................................ 78

AIM OF CHAPTER .......................................................................................................................................... 78 OBJECTIVE OF CHAPTER................................................................................................................................ 78 AIR-TO-AIR HEAT RECOVERY IN VENTILATION SYSTEMS ............................................................................ 79

CHAPTER 5, PACKAGED PRESSURISATION AND FILLING SYSTEM .......................................... 81

AIM OF CHAPTER .......................................................................................................................................... 81 OBJECTIVE OF CHAPTER................................................................................................................................ 81 PACKAGED PRESSURISATION AND FILLING UNIT ........................................................................................... 82

CHAPTER 6, BOILERS AND CHILLERS................................................................................................. 83

AIM OF CHAPTER .......................................................................................................................................... 83 OBJECTIVE OF CHAPTER................................................................................................................................ 83 PUMPS ........................................................................................................................................................... 84 CENTRAL BOILER AND CHILLERS CIRCUITS .................................................................................................. 85

Boiler control. .......................................................................................................................................... 85 Common Header single boiler................................................................................................................. 86 Frost protection........................................................................................................................................ 86 Multiple boilers in sequence (Parallel connection) ................................................................................ 87 Boilers connected in Series ..................................................................................................................... 88 Modular Boilers, Wall Hung type ........................................................................................................... 89 Based load ................................................................................................................................................ 90 Heating inertia......................................................................................................................................... 90

CHAPTER 7, SIZING VALVES FOR WATER SERVICE ...................................................................... 92

AIM OF CHAPTER .......................................................................................................................................... 92 OBJECTIVE OF CHAPTER................................................................................................................................ 92 SIZING VALVES FOR WATER SERVICE............................................................................................................. 93

Example 1 ................................................................................................................................................ 94 PUMPS ........................................................................................................................................................... 96 CIRCULATION SYSTEM CHARACTERISTICS..................................................................................................... 96

Example 2 ................................................................................................................................................ 96 Example 3 ................................................................................................................................................ 97

ACTUAL PERFORMANCE ................................................................................................................................ 98 THREE-PORT VALVE ...................................................................................................................................... 99 TWO-PORT VALVES ..................................................................................................................................... 102



VALVE AUTHORITY ..................................................................................................................................... 105 Example ................................................................................................................................................. 105 Example ................................................................................................................................................. 107

THREE-PORT CONTROL VALVES AND VALVE AUTHORITY ............................................................................ 110 CAVITATION AND FLASHING........................................................................................................................ 111

Cavitation in liquids............................................................................................................................... 111 Flashing in liquids................................................................................................................................. 113 Avoiding cavitation ................................................................................................................................ 113

CHAPTER 8, FLOW CHARACTERISTICS............................................................................................ 116

AIM OF CHAPTER ........................................................................................................................................ 116 OBJECTIVE OF CHAPTER.............................................................................................................................. 116 FAST OPENING CHARACTERISTIC ................................................................................................................. 119 LINEAR CHARACTERISTIC............................................................................................................................ 119

Example ................................................................................................................................................. 120

REFERENCE ............................................................................................................................................... 123

GLOSSARY OF TERMS ............................................................................................................................ 124



Objectives of this course:

Aim: The aim of the module is to achieve an appropriate knowledge and understanding of the principles of the subject matter and reach a level of appropriate academic competence in descriptive, analytical and computational elements and apply this knowledge in the optimisation of engineering solutions.

Objective: On completion of the course the student will be able to:

1. Appropriately comprehend the principles involved. 2. Apply the principles in a logical and appropriate manner. 3. Where appropriate, measure and evaluate empirically the major issues. 4. Analyse and compute some of commonly encountered models in a

manner, which leads towards a resolution of the situation presented. 5. Evaluate the often-competing potential solutions so as to formulate an

optimum and appropriate solution.

Completion Time Chapters 1 to 2 xx Hours Chapters 3 to 8 xx Hours Chapters 9 to 12 xx Hours Chapter 12 to 15 xx Hours

Syllabus: 1. Introduction of Control Theory 2. The principles of Operation of Electronic control equipment. 3. Direct Digital Control. 4. Building Management Systems 5. Communications Networks 6. Communications Systems

Method of Instruction: Instruction is by lecture sessions.

Assessment Procedures and Criteria: As per syllabus.



Chapter 1, Elementary Control Functions

Aim of Chapter

The aim of this chapter is to describe a variety of environment and structure characteristics of buildings and show how control must focus on measurable components, even though they are not always the exact aspects being controlled. By appropriately comprehending these characteristics improved solutions can be incorporated into designs from the beginning rather than retrofitting them later.

Objective of Chapter

On completion of this chapter the student will be able to: (a) Differentiate between constant minimum outside air and variable

outside air is discussed in terms of heating and the economy cooling cycle. This includes aspects of the controls used to provide these facilities.

(b) Distinguish between Temperature and Enthalpy from a sensing and control perspective in order to be able to provide an appropriate and cost effective control system.

(c) Apply the pressure control and stratification of air streams principles in a logical and appropriate manner.



Outside air Outside air (OA) is defined as “air that is brought into the ventilation system from outside the building and, therefore, has not been previously circulated through the system” [Cupton, 1989]. The use of OA within buildings varies from:

• Commercial buildings with 80% recirculated air (RA), and 20% OA,

• Laboratories with 100% OA. Some applications such as ventilation to toilet areas, special manufacturing processes may require 100% OA. Some areas such as electroplating shops and chemical laboratories may require to be kept at (-ve) pressure (partial vacuum) to prevent leaking of dangerous chemicals or substances in the air to other locations in the building.

Constant minimum outside air. An application of the constant minimum outside air is a simple on/off control interlocked with the supply fan as shown in Figure 1.1. The damper in the RA duct is a simple balancing damper. OA may also need a balancing damper.

Figure 1.1 Constant minimum outside air example. This example is a very crude way of ensuring the constant minimum OA, but should only be used where the OA requirement is fixed and the economy “free cooling cycle” is not required due to the size of the plant.

Balance

Manual

OA

Recirculation Manual

Panel

Filter

ON/OFF

Supply Fan Interlocked

to damper motor

MD

Motorised

Damper

T

Temperature

Balance

Manual

OA

Recirculation Manual

Panel

Filter

ON/OFF

Supply Fan Interlocked

to damper motor

MD

Motorised

Damper

T

Temperature



OA economy cycle – temperature control A typical ratio of OA to RA in a fixed or two positioned OA damper would be 20:80. A typical the supply air (SA) temperature would be 14oC, and RA temperature of 24oC. Hence the balance would be:

Sensible Heat (OA) + Sensible Heat (RA) = Sensible Heat (SA)

0.2 0.8

0.2 0.8 24 14

0.2 19.2 14

26

OA RA SA

OA

OA

OA

Ms CP T Ms CP T Ms CP T

T

T

T DegC

× × + × × = × ×

× + × =

× + =

= −

So, when the outside OA temperature is above -26°C the supply temperature of 14°C cannot be provided and the temperature of 14°C has to be provided by mechanical cooling of the air. This is very wasteful in energy. It is therefore clear that while the OA temperature is at or below 14°C the temperature of 14°C can be provided without mechanical cooling hence the so called economy cycle. The normal arrangement is for three dampers (i.e. the OA, the Exhaust Air (EA) and the RA. The OA and RA operate in unison with the recirculation air in the appropriate position.

The economy cycle schedule Based on the 20% OA and 80% RA with supply at 14°C and return at 24°C the following equation holds where X is the OA fraction, CP is the specific heat capacity, and Ms is the mass flow rate of the supply air.



( ) (1 )

( ) (1 )

(1 )

(1 )24 14

24 24 14

14 24 24

10 24

OA RA SA

OA RA SA

OA RA SA

OA

OA

OA

OA

OA

x Ms CP T x Ms CP T Ms CP T

x Ms CP T x Ms CP T Ms CP T

x T x T T

x T x DegC DegC

x T DegC x DegC DegC

x T DegC DegC x DegC

x T DegC x DegC

T

× × × + − × × × = × ×

× × × + − × × × = × ×

× + − =

× + − =

× + − × =

× = − + ×

× = + ×

10 24DegC x DegC

x

+ ×=

Using the above formula it is possible to calculate the OA temperature TOA for various mixing ratios of OA.

Table 1.1 Mixing ratio of OA

Consider for example 50% OA and 50%RA

0.5 0.5

0.5 0.5

0.5 0.5

(0.5 4 ) (0.5 ( 24 ) 14

OA RA SA

OA RA SA

OA RA SA



T T T

DegC DegC DegC

× × × + × × × = × ×

× × × + × × × = × ×

× + × =

× + + × + =

This is not a linear relationship but a curve relationship as shown graphically in Figure 1.2.

X 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

TOA -26.0 -9.3 -1.0 4.0 7.3 9.7 11.5 12.9 14.0



Figure 1.2 Mixing Ratio to temperature As it is possible to supply the building without mechanical cooling while the OAT remains below the design supply air temperature, the chillier can remain off. Therefore an “enabling stat” is fitted in the OA duct to enable the chillers to operate only when the OAT reaches the design supply air temperature typically 14°C in commercial buildings as shown in Figure 1.3. The SA temperature is sensed and transmits a signal to the controller. The controller is also supplied with the RA temperature (RAT) from the building and the OAT. While the OAT is below the set point of the supply air the OA damper modulates – opening incrementally to maintain the supply air temperature when OAT>RAT then return OA to the minimum position. Usually, if the OAT is < the design winter temperature of the OA approximately -2°C in Dublin it also modulates to the min OA position. During all these operations the EA damper modulates as the exact position of the OA and the RA damper modulates but in the opposite position.

Return Air

Temperature

24DegC

Supply Air

Temperature

14DegC

Design Air Temperature

OAT

Usual control modification at design OAT

MAX

MIN

20%OA

In this region the

OA makes some

contribution to mechanical cooling

100%OA

Return Air

Temperature

24DegC

Supply Air

Temperature

14DegC


OAT

Usual control modification at design OAT

MAX

MIN

20%OA

In this region the

OA makes some

contribution to mechanical cooling

100%OA



Figure 1.3 Enabling stat fitted in the OA duct

At any given time the controller is receiving in data on the:

• OAT

• RAT

• SAT If the OAT > RAT then Min OA Damper Min EA Damper Max RA Damper

If the OAT < Design Winter OAT then Min OA Damper Min EA Damper Max RA Damper

If the OAT < Set point SAT to building then Max OA Damper Max EA Damper Min RA Damper

RA

Exhaust

AirMotorised

Damper

MD

Recirculation Air Motorised

Damper

T MDMD

EA OA

OutsideAir

Motorised Damper

SA

T

T

Set Point =

SA 14DegC

OA Design Winter

Temperature

Controller

RA

Exhaust

AirMotorised

Damper

MD


Damper

TT MDMD

EA OA

OutsideAir

Motorised Damper

SA

TT

TT

Set Point =

SA 14DegC

OA Design Winter

Temperature

Controller



If the OAT > Design Winter < Set point SAT to building then Modulate OA Damper

Modulate EA Damper Modulate RA Damper

To give the set point of the SA temperature, generally all OA dampers are interlocked with the supply operation to close when the fan is off as shown in Figure 1.4. The operation of the minimum OA damper setting can be either built into the software – stops at a set angle, or use a separate minimum OA damper – on/off with the modulating damper going then to fully closed. In order to operate with software setting the correct angle of the blade must be determined during commissioning – by measurement. This might not always be that easy hence the alternative.

Figure 1.4 Interlocking damper to fan The control schedule described is to be used with a VAV system (VAV stands for Variable Air Volume or also known as VFR - Variable Flow Rate). VAV boxes provide constant or variable air depending on the temperature demands of the space. As the temperature rises the VAV damper opens to send a designed amount of airflow to the room. It supplies a constant (say 14°C) air temperature all year round.

RA

ON/OFF interlocked with

Supply FanMotorised Damper

ON/OFF

Supply Fan Interlocked to damper motor

MD

Mixing Air

Motorised Damper

MD

EA OA

Modulating with controller

Motorised Damper

SA

MD


Damper

RA

ON/OFF interlocked with

Supply FanMotorised Damper

ON/OFF

Supply Fan Interlocked to damper motor

MD

Mixing Air

Motorised Damper

MD

EA OA

Modulating with controller

Motorised Damper

SA

MD


Damper



Figure 1.5 VFR results overview However an economy cycle with a constant air volume (CAV) system can give problems with heating costs in winter. With a CAV system the supply temperature is varied at a constant VFR. With a CAV single zone system for example it may be required to supply air to the building above room temperature let alone above 14°C. In such cases the approach is to sense the room temperature (TRoom). If the TRoom < the room set-point, then heating is required, the dampers modulate to minimum. If the TRoom > the room set-point then cooling is required, the dampers modulate to the schedule previously described.

Enthalpy Control Heat sources internal to the building such as people, lights, computers, copy machines, motors, printers and other equipment causes the temperature inside a structure to continuously increase. Heat soaked up by the building structure may also continue to heat the building long after the OA temperature has dropped. There are times when the OA temperature is lower than the temperature inside. Whenever the cooling system is calling for cooling and the temperature outside is cool enough it is economical to shut off the compressor and bring in cool outside air to satisfy the cooling needs of the building. Consider when the OA temperature

Return Air

Temperature

24DegC

Supply Air

Temperature

14DegC


-2 DegC

OAT

MAX

MIN

20%OA

100%OA

MIN

VFRVFR

Return Air

Temperature

24DegC

Supply Air

Temperature

14DegC


-2 DegC

OAT

MAX

MIN

20%OA

100%OA

MIN

VFRVFR



is close to the desired SA temperature then the EA and OA dampers are fully opened, and the RA air damper is completely closed. Under these conditions, 100% OA enters the building and the chiller can be shut off, because there is no need for cooling the air. Such is the function of an air economiser system and night purging. Strictly speaking the economiser mode should be used whenever the enthalpy difference between the OA and the SA is less than the enthalpy difference between the RA and the SA. It is assumed for this module that you are familiar with psychometric chart as shown in Figure 1.6!!!! Seven different components shown on the one diagram is a lot of detail to take in.

Figure 1.6 Psychometric chart overview This is because the enthalpy is the energy and not just the temperature of the air as shown in Figure 1.7. The term “enthalpy” means, total heat. The enthalpy control measures both sensible and latent heat in the air and only allows outside air to be used for cooling if the air is both cool and dry enough to satisfy the space conditions. However, it is difficult to measure enthalpy so usually just the air temperatures are used if the humidity is not a large factor. The economiser mode is sometimes used at night to cool off a building mass in preparation for the next days cooling load. This is called “night purging” [Curtiss et al., 2002].



Figure 1.7 Temperature or Energy There is one drawback to this type of control system. Even though the thermostat acknowledges that the outside air temperature is low enough to cool the building, the OA may be too humid to provide adequate comfort for the building occupants. The occupants will feel cool but clammy. The solution is an economiser that adds a second control which works in harmony with the outdoor thermostat and measures the OA humidity. Such a control is called an “enthalpy” control. If the indoor thermostat calls for cooling and the outside air enthalpy (total heat) is low enough then the economiser brings in this cooler and less humid air and uses it for cooling instead of operating the compressor. Using the outside air for cooling is less expensive than operating the compressor to provide cooling. So an enthalpy control is a control which checks to see if both the temperature (sensible heat) and the humidity (latent heat) are low enough to be used for cooling. This combination provides for the greatest comfort at the least cost. Not all economisers use enthalpy controls. Some just check the outside air temperature and do not check the outside air humidity. Those controls do not provide the same levels of comfort as enthalpy controlled economisers.

Just refers to Temperature

This refers Energy

Just refers to Temperature

This refers Energy



Enthalpy or heat content is a description of thermodynamic potential of a system, which can be used to calculate the "useful" work. It is often contested that instead of sensing the TOA and the TRA the enthalpy of these air streams should be sensed instead.

Figure 1.8 Typical controls & sensors for economiser system In Figure 1.8 the typical controls and sensors used in an economiser system. This is able to provide the minimum OA during occupied periods when it is warm outside, to use outdoor air for cooling when appropriate by means of a temperature based economiser cycle and to operate fans and dampers under all conditions. The numbering system used in the figure indicates the sequence of events as the air-handling system begins operation.

1. The fan control system turns on when the fan is turned on. This may be by a clock signal or a low temperature space condition.

2. The space temperature signal determines if the space is above or below the set-point. If

above the economiser feature will be activated to control the outdoor and mixing dampers. If below, the OA damper is set to its minimum position.

3. The mixed air PI controller controls both sets of dampers (OA/RA and EA) to provide the

desired mixing air temperature.

4. The OA temperature rises above the cut-off point for economiser operation, the OA damper is returned to its minimum setting.



5. The supply fan is off the OA damper returns to its NC position and the RA damper returns to its NO position.

6. The supply fan is off the exhaust damper also returns to its NC position.

So in the case where the OA temperature is >RA temperature, the RA would be taken in preference to OA. However taking RA which has a higher enthalpy would put a higher load on the plant and we actually ought to take the OA instead. So it is suggested the sensors in these locations should be enthalpy rather than dry bulb temperature. The difficultly is that enthalpy sensors are more expensive and also may need more maintenance than the dry bulb temperature sensors. So are they really required? In Ireland probably not, the RA from the building is unlikely to exceed 24DecC and 60% RH which has an enthalpy of 53kj/kg. The summer time design external condition in Dublin is typically 24.5°C and 10g/kg moisture content. This has a specific enthalpy of 50kj/kg and a Wet Bulb (WB) temperature of 18°C. Hence while the OA temperature is < 24°C its enthalpy will be almost < the RA enthalpy and it is very seldom that the OA enthalpy would be above the RA enthalpy, while the OA temperature is > RA temperature. Hence in the Irish conditions temperature sensing alone is sufficient. Also cost effective. The problem generally only arises in very humid coastal regions. The most humid cities on earth are generally located closer to the equator, near coastal regions. Cities in South and Southeast Asia seem to be among the most humid such as Kolkata, India, Bangkok and Thailand.

Dublin Relative Humidity 2005Dublin Relative Humidity 2005



Room pressure control with damper control One effect of “free cooling cycle” is that pressure in the room may change. Room pressure can be controlled by cycling the dampers, but free cooling control is generally then not used. Usually with CAV systems room pressure is set up at commissioning in commercial applications. But in industrial applications where room pressure is critical then exclusive control of dampers by means of pressure sensors is generally adopted as shown in Figure 1.9.

Figure 1.9 Room pressure control with dampers Pressure in the space is generally controlled with reference to pressure in an adjacent space (+15Pa -15Pa) or with reference to outside pressure. If the room pressure is falling, the OA damper opens and the exhaust damper closes slightly, therefore more air is supplied than is removed from the space and the pressure rises in the room. Recirculation quantity will rise as the exhaust is restricted, which could reduce the effectiveness of the throttling of the exhaust.

RA

ExhaustAir

Motorised Damper

MD

Recirculation Air Motorised Damper

MDMD

EA OA

OutsideAir

Motorised Damper

SA

Controller

Set point of difference

pressure input

Pressure

Sensor

PressureSensor

Other room pressure or reference pressure

Room pressure sensed

Designated damper controllerStabiliser Delays Reaction

RA

ExhaustAir

Motorised Damper

MD


MDMD

EA OA

OutsideAir

Motorised Damper

SA

Controller

Set point of difference

pressure input

Pressure

Sensor

PressureSensor

Other room pressure or reference pressure

Room pressure sensed

Designated damper controllerStabiliser Delays Reaction



Throttling of the exhaust alone may not therefore reduce the extract from the room. Room pressure is corrected simply by increasing the supply fan. If the recirculation damper however is also throttled as the exhaust is throttled and the OA damper opened then the pressure is more effectively restored as shown in Figure 1.10 and as per air flow chart in Figure 1.11.

Figure 1.10 Room pressure control The capacity of the extract fan is more effectively reduced as both dampers are controlled.

Figure 1.11 Supply-Exhaust fan curves

RA

ExhaustAir

Motorised Damper

MD

Recirculation

Air Motorised Damper

MDMD

EA OA

Outside

AirMotorised

Damper

SA

Set point of difference pressure

input

Room

RA

ExhaustAir

Motorised Damper

MD

Recirculation


MDMD

EA OA

Outside

AirMotorised

Damper

SA

Set point of difference pressure

input

Room

Q

∆P 1

2Supply 1

2

Q

∆P

1

2Exhaust 1

2

Q

∆P 1

2Supply 1

2

Q

∆P

1

2Exhaust 1

2



Stratification In larger mixing boxes with colder OA intakes can be a problem. Air stratification is the tendency of two or more airstreams to remain separated. This commonly occurs in central air handling units in commercial and industrial building as return and outside air are introduced into the mixing box of the air handling unit. Whether a significant temperature difference exists between the two air streams or not, these two air streams tend to remain separated due to the inherent momentum/velocity of each stream. The presence of air stratification creates many challenges in proper design and operation of air handling units. Among the most notable problems are:

• Freeze-Stat Trips & Frozen Coils

• Poor Temperature Control Accuracy

• Insufficient Fresh Air Distribution

• Poor Economizer Operation

• Uneven Velocity Profile

The only way to positively address each of these issues related to air stratification is properly mixing the return and outside air streams.

Figure 1.12 Air stratification

RA at 24DegC

ExhaustAir

Motorised Damper

MD


TT MD MD

EA

OA at a low temperature

of 4DegC

OutsideAir

Motorised

Damper

SA

TT

TT

Supply Air in this branch is

not at the mixed temperature

Separation or Stratification of hot RA and cool OA. Incorrect Mixing



A solution to the stratification problem is to force the air streams to mix with baffling as in Figure1.13, or stirring fans in the chamber as in Figure 1.14 or rearranging the RA inlet to force the opposed air streams to mix as in Figure 1.15.

Figure 1.13 Baffling The concept of baffling is based on introducing a folded sheet metal plate structure that forces a turbulence and mixing of air streams.

Figure 1.14 Stirring fans Stirring fans are mechanical propellers that are placed into the mixing box of the AHU. As they rotate they mix the air flow streams. This is a similar concept to overhead cooling/mixing fans which are attached to light fittings. Figure 1.14 is a crude representation of the propeller. Industrial implementations are more efficient.

RA at

24DegC

ExhaustAir

Motorised

Damper

MD

Recirculation

Air Motorised

T MDMD

EA

OA at a low

temperature of 4DegC

Outside

AirMotorised

Damper

SA

T

T

Supply Air in

this branch is

now Mixed

Separation or Stratification of

hot RA and cool OA. Incorrect Mixing

Supply Air in

this branch is

now Mixed

RA at

24DegC

ExhaustAir

Motorised

Damper

MD

Recirculation

Air Motorised

TT MDMD

EA

OA at a low

temperature of 4DegC

Outside

AirMotorised

Damper

SA

TT

TT

Supply Air in

this branch is

now Mixed

Separation or Stratification of

hot RA and cool OA. Incorrect Mixing

Supply Air in

this branch is

now Mixed



Figure 1.15 Rearranging the RA inlet By rearranging the location of the RA inlet to the mixing box it is possible to force a mixing of the air streams. This does cause some inefficiency in the ductwork and fans but may not be excessive when compared to installing a mixing fan.

Questions on Chapter 1. Q1.1 Describe the term “free air cooling” and elaborate on the difficulty in

installing it. Q1.2 Explain why is enthalpy sensing and control is not normally implemented in

Ireland and describe conditions in which it should be used. Q1.3 One of the effects of “free cooling cycle” is room space pressures are

generally controlled with reference to adjacent spaces. Describe some of the consequences in not regulating this space pressure.

Q1.4 In air handling units with large mixing boxes and a very cold Outside Air

(OA) supply, stratification of the air can be a problem. Describe the stratification phenomenon and discuss three ways in which it can be reduced.

Q1.5 Describe using a process diagram (including sensors, actuators, controllers

and interconnections) and written description of the process a system with Supply Air (SA), Outside Air (OA), Exhaust Air (EA) and Return Air (RA) that can control and regulate the air pressure.

MD

MD

MD

MD

MD

MD

MD

Plan View of the system



Chapter 2, Heating and cooling coils

Aim of Chapter

This chapter focus on the heating and cooling of buildings, both for air quality improvement and for actual room temperature regulation. For air quality to be correct, the temperature and humidity of the environment of the air being supplied and the that of the air in the room needs to be carefully controlled. This chapter revises the heating and cooling aspects of building services with the addition of control systems.


On completion of this chapter the student will be able to: (a) Identify from a control perspective the key functions of equipment

used in HVAC systems and commercial refrigeration systems. (b) Distinguish between forms of humidification and discuss the energy and

cost issues relating to their selection. (c) Recognise and interpret the significance of frost to building equipment

and fabric and explain whys in which it can be reduced through careful design.

(d) Describe techniques of controlling the refrigeration cycle with a view to energy conservation verses controllability accuracy.

(e) Distinguish between electricity usage as a prime heating or complementary heating source.



Control of Heating Coils The normal medium for supplying hot water (HW) heating coils is Low Pressure HW (LPHW) typically 28-71°C. Although “Heat Recovery Water” at the about 45-40°C is increasingly being used. Direct fired gas may also be used occasionally e.g. swimming pools, but control and efficiency needs to be checked first. Generally steam coils should be avoided. If steam is the source of heat availability it is better to use this to generate LPHW at 82°C for use in the coils.

Figure 2.1 Heating Coils using steam

Reheat Control Occasionally the heating requirements are divided into PREHEAT and AFTERHEAT coils in the AHU. If adiabatic humidification is being provided then this separation is essential in countries where the OA temperature are not frequently < ODecC.

(a) Adiabatic humidification occurs when water vapour is added to the atmosphere.

(b) Specific humidity will increase, along the wet bulb temperature line. Reduction in dry bulb temperature will happen as, the

T

+ +

S.V.

S.T.

T

Set at 90DegC

HL

Steam Supply

Cold Air Warmed Air

Set at 82DegC

Control

S.V.

TT

+ +

S.V.

S.T.

TT

Set at 90DegC

HL

Steam Supply

Cold Air Warmed Air

Set at 82DegC

Control

S.V.



evaporated water will absorb heat. A humidifier will perform this function.

(c) Adiabatic dehumidification occurs similar to dehumidification, but on the opposite direction.

Figure 2.2 Adiabatic moisture addition or removal

Just as a reminder "ENERGY EFFICIENT" HUMIDIFICATION. There are often misconceptions that some humidification systems require significantly less energy to operate than others. While some energy savings between different types of humidifiers exist, large differences in overall facility energy consumption many times do not materialise. Often times, when two different humidification systems are compared, only the energy required to operate the humidifiers is considered. This is not the entire picture. Different humidification systems affect the entire HVAC system energy usage differently.

The two schools of thought on humidification: One type of humidification that most people are probably aware of is isothermal humidification. With isothermal humidification, steam is injected into an air-handling unit or directly into the space that is to be humidified as shown in Figure 2.3. Water can be boiled with electric resistance, natural gas, or some other heat source to make steam. The other method of humidification is called adiabatic humidification. This method of humidification atomizes water into very small droplets. This can be done by forcing pressurized water through very tiny nozzles (high pressure humidification) or by vibrating a pan of water at very high frequencies (ultrasonic humidification). The droplets are then injected into the air-handling unit or directly into the space that is to be humidified. The atomized water droplets are then evaporated into the air to increase the relative humidity.



Figure 2.3 Steam humidification Many companies are advertising that the adiabatic humidification technologies are extraordinary energy savers. The fact is, when strictly comparing only the humidifier, an isothermal (boiling) unit will always use more energy than an adiabatic (evaporating) unit of equal capacity. But the energy implications are significantly more complex than this. The small droplets of water introduced by an adiabatic humidification system are not evaporated into the air for free. The water droplets absorb heat from the air to evaporate. We have all experienced heat absorption from evaporation. When we sweat, the moisture on our skin evaporates absorbing heat from our body to keep us cool, just as the water droplets absorb heat from the air during adiabatic humidification. Kg for Kg, evaporating water droplets requires nearly as much energy input as boiling. Adiabatic humidification can offer an advantage if humidification is required at the same time that cooling is needed. Believe it or not, some type of cooling is needed most of the year for many air-handling systems in some particular countries. This type of humidification will provide a free cooling effect along with the humidifying. However, during the heating season adiabatic humidification can result in increased space-heating energy. More heat may be required to maintain the desired space temperature. Isothermal humidification, on the other hand, does not significantly affect the temperature of the surrounding air. There may be several reasons to choose one type of humidification over the other, including energy usage. Every system should be considered case-by-case to determine what is right for your application. Remember, when analysing the energy consumption of a humidification system, it is important to look at the whole picture. Focusing just on the humidifiers can lead to some misleading results. Generally preheat coils are used with all OA plants. Where plants use recirculation however with a “hold off stat” set at 18°C (senses the return air from the building)



preheat may not be required. The recirculation mixing chamber acts as a preheat coil. In this case only an after-heater is used. Where large quantities of OA are used, unless a preheat coil is used very cold air at <0°C is passing through the plant (and may damage it) until it reaches the heating coil. Freezing may occur in filters or in cooling coils themselves. It is common to place the last heating battery downstream of the cooling coil in order to allow some control of Relative Humidity (RH). The cooling battery can reduce the air temperature below the sensible cooling load, to remove sufficient moisture and then reheat to the ∆t between the supply and return air required for the sensible cooling load as shown in Figure 2.4. Figure 2.4 Cooling battery to reduce air temperature below sensible cooling

load

∆t Supply-Return

- +A B

C

A

B C

Dry Bulb

Wet B

ulbM

ois

ture

Con

ten

t

∆t Supply-Return

- +A B

C

A

B C

Dry Bulb

Wet B

ulbM

ois

ture

Con

ten

t



Sensible heating, and cooling along cooling, or heating coil

(a) Sensible cooling happens when the cooling or air, does not alter the specific humidity. However, the relative humidity will increase.

Sensible cooling can only happen under condition that, the cooling coil's temperature, is not lower than the dew point temperature of entering air

(b) Sensible heating is similar to sensible cooling (alteration in dry bulb temperature), but dry bulb temperature is increased in this case.

The condition that has to be met is, there should be no water within the heating system, to avoid increase in specific humidity

Figure 2.5 Steam humidification



Frost Protection Research has shown that water flowing in a coil at velocities > 0.7m/s will not freeze, once the air temperature is >-30°C. Hence the general principle of frost protection in heating systems is to use two sensors. An OA sensor which brings in the pumps at about +8°C if they are not already on and an immersion thermostat in the boiler return which brings on the boiler at about +4°C.

Figure 2.6 Control of preheat coil on all OA plant Normal control of preheating coils is to raise the temperature after the preheat coil to a minimum low temperature (the minimum design supply temperature for cooling, for example). For instance the preheat coil always heats the air to 12°C, all year round, while the plant is on. This temperature clearly needs to be below the min supply temperature for cooling, or equal to the low limit settings of the supply air, as shown in Figure 2.6. The frost protection of the coil is provided by a thermostat which is located downstream of the coil. This is set to +4°C and locks out the supply fan if the air temperature is < +4°C at this point. This means the heating system has failed for whatever reason. Another option is to use the temperature sensor to open the valve to provide circulation but this is not effective if the pump is off at the time or is faulty. It is also common in OA plants to interlock with the ON/OFF control of the intake damper. A wind velocity of 8m/s which is common on Ireland will produce a velocity Pressure of 38Pa. This is sufficient to move significant quantities of air through the plant and duct, and may cause the coils to freeze when the plant is off as shown in Figure 2.7. A solution other than just using isolation dampers is to use a “Penthouse” louver as shown in Figure 2.8.

Panel

Filter

MD

T

Temperature Set at 12DegC –

Or <= the design cooling supply air temperature to the

building

OA

+4DegC

SA+ +

Boiler

T

Panel

Filter

MD

T

Temperature Set at 12DegC –

Or <= the design cooling supply air temperature to the

building

OA

+4DegC

SA+ +

Boiler

T



Figure 2.7 Wind velocity

Figure 2.8 Penthouse louver

Q

∆P

VFR, possible if wind

only is driving force at

8m/s

38Pa

Q

∆P

VFR, possible if wind

only is driving force at

8m/s

38Pa



After heat coils All heating coils and chilled water (-) cooling coils may be controlled by:

(i) A diverting valve (common) Figure 2.9 (ii) A mixing valve (accurate) Figure 2.10

(iii) A two port valve Figure 2.11

Figure 2.9 Diverting valve arrangement

Figure 2.10 Mixing valve arrangement

Panel Filter

MD

T

OA

+4DegC

SA+

LPHW Supply

T

LPHW Return

Diverting Valve

Panel Filter

MD

T

OA

+4DegC

SA+

LPHW Supply

T

LPHW Return

Diverting Valve

Panel Filter

MD

T

OA

SA+

LPHW Supply

T

LPHW Return

Mixing Valve

Panel Filter

MD

T

OA

SA+

LPHW Supply

T

LPHW Return

Mixing Valve



Figure 2.11 Two-Port valve arrangement Each of these may have the usual possibility of:

(i) ON/OFF control (ii) Proportional control (iii) Proportional and integral control

(ii) and (iii) are referred to as modulating, and all of these methods of control have been discussed in section B of this course. Note: generally counter flow arrangements – water flowing back and air flowing forward. In a single zone system the after heat coil is controlled from a room temperature sensor as shown in Figure 2.12. Otherwise it is controlled from a plant temperature sensor as shown in Figure 2.13. In a reheat system the reheat coil is controlled from a room temperature sensor.

Figure 2.12 Two-Port valve arrangement

Panel Filter

MD

T

OA

SA+

LPHW Supply

T

LPHW Return

Two Port Valve

DPS

Panel Filter

MD

T

OA

SA+

LPHW Supply

T

LPHW Return

Two Port Valve

DPS

Room Sensor, Single Zone system

Panel Filter

MD

T

OA SA

+

Boiler

T

T

Room Sensor, Single Zone system

Panel Filter

MD

T

OA SA

+

Boiler

T

T



Figure 2.12 Single zone room sensor to valve directly arrangement

Figure 2.13 Multi zone room sensor to valve directly arrangement In a reheat system the reheat coil is controlled from a room temperature sensor as shown in Figure 2.14.

Figure 2.14 Preheat and reheat system control arrangement

Plant Sensor for Multi-zone system alternative OA. Sensor

on a schedule basesPanel

Filter

MD

T

OA SA

+

Boiler

T

Plant Sensor for Multi-zone system alternative OA. Sensor

on a schedule basesPanel

Filter

MD

T

OA SA

+

Boiler

T

Boiler

Panel Filter

MD

T

OA SA

+

T

T

+

T

+

T

+

Room Sensor Controls valve



Boiler

Panel Filter

MD

T

OA SA

+

T

T

+

T

+

T

+






Limits or override to control the supply air. It is sometimes necessary to put a limit on the minimum and maximum temperature which the plant delivers in response to controls located remotely from the plant, for example in a room or in a zone somewhere. Hence low limit (LL) and high limit (HL) may be set in the software to the temperature of the air delivered from the plant. A typical low level would be 12°C and a high level would be 50°C. If LPHW is at 82°C it would deliver air at up to 75°C. Which is far too high and would probably trip all the fire dampers? This could happen under fault conditions. A typical low level limit would be 12°C as the cold water flow temperature (CWFT) of 5°C could produce air at 7°C which would probably cause condensation in the room. Hence supply temperature must be sensed – even in plants in which coils are controlled from the room.

Control of cooling coils These are generally confined to the air handling unit. Very occasionally re-cooling coils are used, but these are certainly not at all as common as reheating coils. Occasionally when chemical dehumidifiers are use re-cooling may be used. Re-cooling deserves investigation in certain climates it may be a preferable approach to reheating. There is one argument for re-cooling – cooling is using mainly electrical energy – heating using fossil. Fossil is about 30% the cost of the electrical production, therefore it is better to use fossil directly. Therefore would it make more sense to heat everything up first and then trim with the cooling rather than the other way around!



Control of Direct Expansion (DX) type evaporator coils. In a similar fashion to heating coils evaporator coils could be controlled using a basic two position control or more sophisticated control. So let’s discuss the basic two position control aspect.

Figure 2.15 Direct Expansion (DX) Refrigerant flowing through the coil tubes is controlled by an expansion device, usually a Thermostatic Expansion Valve ("TEV" or "TXV"). The TXV is mounted at the coil just ahead of the refrigerant distributor, and automatically feeds just enough refrigerant into the coil to be completely converted (boiled) from liquid to gas as shown in Figure 2.15. The TXV is controlled by a temperature sensing bulb mounted on the coil outlet (suction) connection. Proper operation of the TXV depends on the bulb sensing the required amount of superheat in the refrigerant gas at the coil outlet (superheat = the number of degrees above the boiling point temperature of the refrigerant).



Figure 2.16 Cooling coil control TEV measures the refrigerant to the coil in order to maintain a specific temperature (generally 1°C). This ensures that only refrigerant in a gaseous state enters the compressor. The control of the most basic form is via an ON/OFF solenoid valve and the control arrangement is shown in Figure 2.16. In this basic system the LLTS is a low level temperature sensor is simply an on/off solenoid valve as shown in Figure 2.17. Otherwise the process can be accomplished with temperature sensors and a minimum limitation set at the supply air.

Figure 2.17 On/off solenoid valve

T

SA

Refrigerant Load

Room

T

TEV

TReturn Temperature

Refrigerant

Return

T

SA

Refrigerant Load

Room

T

TEV

TReturn Temperature

Refrigerant

Return

Spring

ElectricalCoil

LLTS

Room

temperature sensor

Spring

ElectricalCoil

LLTS

Room

temperature sensor



Controllability is limited and is best used at <85kw load (small load) and where load is stable (industrial load or internal load the building). Improved control is obtained, but at great expense, with multistage coils, initially two stages as shown in Figure 2.18.

Figure 2.18 Two-stage cooling coils Just to put an image to the equipment being used Figure 2.19 shows thermostats which have one, two, three and four stages. Alternatively a temperature sensor set at two different limits can be used in sequence.

Figure 2.18 Two-stage cooling coils

T

Refrigerant Load

TEV

T

Refrigerant Return

TEV

T

ON/OFF

ON/OFF

Two StageThermostat

T

Refrigerant Load

TEV

T

Refrigerant Return

TEV

T

ON/OFF

ON/OFF

Two StageThermostat



Room temperature sensed 22-24 = Valve No. 1 24-26 = Valve No. 2 In this case the lower limit can be override the output signal if the supply goes below say 12°C The alternative is to use a modulating Back Pressure Valve (BPV). The back pressure valve shown in Figure 2.19 is for fixed pressure settings but motorised versions of this valve are available which allow the modulating of the back pressure setting.

Figure 2.19 Back Pressure Regulator

BACK PRESSURE / RELIEF REGULATOR



When installed in the control system the valve works by raising the evaporator pressure and hence temperature, therefore reducing the heat transfer as shown in Figure 2.20 and Figure 2.21. As the room temperature falls, the BPV throttles thereby increasing the refrigerant temperature and reducing the heat output of the evaporator.

Figure 2.20 Back Pressure Regulator location

Figure 2.21 Back Pressure Regulator P-h diagram

Refrigerant Return

SA

Refrigerant Load

Room

T

TEV

TReturn Temperature

Modulating Back Pressure Valve

Refrigerant Return

SA

Refrigerant Load

Room

T

TEV

TReturn Temperature

Modulating Back Pressure Valve

P

h

∆P-Across BPV

P

h

∆P-Across BPV



Generally a small portion of the fluid entering the evaporator is in vapour form. By using Hot Gas Bypass Pressure valve (HGBP) this portion can be artificially increased as shown in Figure 2.22. Hot gas bypass used for capacity control is provided to maintain a constant evaporator pressure when the system load decreases. If the compressor does not have an unloading mechanism (reduced volume flow capability), the hot gas bypass allows the evaporator temperature to remain constant. This also provides a constant volume flow, at the appropriate evaporating temperature, to the compressor. When the system load decreases (less cooling or heating required) without a hot gas bypass valve or capacity control, the evaporating temperature will be reduced, since the compressor is still pumping the same volume (m^3/min). As cooling can only be provided by evaporating refrigerant, this reduces the coil capacity. Generally on/off solenoids are used on situations in which a series of parallel evaporators are in operation. The advantage is that the compressor can be kept running because of the constant volume flow.

Figure 2.22 Hot gas bypass valve

A

B

A

B



Keys to successful implementation for Figure 2.22 (A) only: (Note reference to OIL refers to refrigerant/oil).

(1) Position the HGBP valve above the discharge line, near the compressor. If the system includes pump-down, provide a means to shut off refrigerant flow.

(2) Pitch the line upstream of the HGBP valve to drain oil back into the discharge line.

(3) Pitch the line downstream of the HGBP valve toward the evaporator, away from the valve.

(4) If the HGBP line includes a riser, regardless of height, provide a drain leg of the same diameter as the riser. Add an oil-return line 25 mm from the bottom of the trap; use tubing that is 6 mm and at least 3000 mm long. Pre-charge the trap with oil.

(5) Divert hot gas to each active distributor at the expected operating points for hot gas bypass.

(6) If the HGBP line feeds multiple distributors, provide a check valve for each distributor.

(7) Insulate the entire length of the HGBP line. Keys to successful implementation for Figure 2.22 (B) only: (Note reference to OIL refers to refrigerant/oil).

(1) Position the HGBP valve above the discharge line, near the compressor. If the system includes pump-down, provide a means to shut off refrigerant flow.

(2) Pitch the line upstream of the HGBP valve to drain oil back into the discharge line.

(3) Pitch the line downstream of the HGBP valve toward the suction line, away from the valve.

(4) Assure that the evaporator and suction line freely drain to the suction-line HGBP connection.

(5) Site the suction-line HGBP connection upstream of the pilot-line tap for the HGBP valve and at least 5 ft (1.5 m) upstream from the compressor inlet. Angle the connection into the suction flow.

(6) Attach the remote bulb for the liquid-injection valve to the suction line, downstream of the HGBP connection.

(7) Provide a solenoid valve upstream of the liquid-injection valve. Synchronize the operation of the HGBP and liquid-line solenoid valves.

(8) Insulate the entire length of the HGBP line.



Control of chilled water (CW) coils Chilled water cooling coils are controlled as per LPHW heating coils by:

(1) Two port valves (2) Three-port diverting (3) Three-port mixing

The most common form of control is 3 port diverting although 2 port is becoming more common to save energy of the pump. 3 port mixing is rarely used on CW coils (more uncommon that with heating coils). As with heating coils it gives more accurate control of the air-off-temperature (i.e. the air coming off the cooling unit). However, it has the disadvantage of eliminating latent cooling at port load which may be a disadvantage in many situations where RH control is important.

Connecting chilled water in coils in Parallel or Counter Flow Does the direction of the flow into the coil matter? In Figure 2.32 the two types of flow are discussed.

Figure 2.23 Counter flow or Parallel flow

Temp

Distance

Counter Flow

ParallelFlow

6

1311

27Air

Water

Temp

Distance

11

13

6

27Air

Water

Temp

Distance

Counter Flow

Counter Flow

ParallelFlow

ParallelFlow

6

1311

27Air

Water

Temp

Distance

11

13

6

27Air

Water



Log mean temperature difference (LMTD)

For Counter Flow For Parallel Flow

(27 11) (13 6)

(27 11)ln

(13 6)

9

0.82

10.97

− − −=

−

−

=

=

(27 6) (13 11)

(27 6)ln

(13 11)

19

2.35

8.08

− − −=

−

−

=

=

The Heat Transfer in the parallel arrangement is 8.08

100 74%10.97

= × = of that in the in

the counter flow, therefore this is very significant. It has been shown that a change from parallel to counter flow can result in the CW temperature increasing by 3°C without any decrease in the heat transfer as shown in Figure 2.24.

Figure 2.24 Distance-Temperature Flow Hence more efficient coils can be produced and energy saved by increasing the coefficient of performance (COP) (i.e. temperature of the chilled water).

Where Q is the useful heat supplied by the condenser and W is the work consumed by the compressor. (Note: COP has no units, therefore in this equation, heat and work must be expressed in the same units). Note that counter flow has an impact with cooling coils as they are deep 6-14 rows, but not so much with heating coils which have 2-4 rows.

Counter

Flow

ParallelFlow

Temp

Distance

9

1314

27Air

Water

Temp

Distance

11

13

6

27Air

Water

Counter

Flow

ParallelFlow

Temp

Distance

9

1314

27Air

Water

Temp

Distance

11

13

6

27Air

Water



The Air Washer Most basic – the desert cooler as shown in Figure 2.25

Figure 2.25 Desert cooler Generally no thermostatic control other than fan and pump interlocked and manual on/off control of the circuit.

BA

Eliminator

Mw

100

100

Evapo

ration R

ate

BA

Eliminator

Mw

100

100

Evapo

ration R

ate

Mw

100

100

Evapo

ration R

ate

Figure 2.26 Desert cooler



Figure 2.27 Psychometric diagram for desert cooler

As the efficiency of the process is almost constant at all conditions the OFF condition depends on the ON condition as shown in Figure 2.27. Generally used for “relief-cooling”, what ever cooling can be obtained is taken. Generally used where cooling is required all year (for example North-Africa). In theory it might be thought that control could be exercised by putting a modulating control on the pump, but as the pump circulates many multiples of the evaporator, there would be little impact on the rescaling off condition. Speed control of the fan is an option but this is often ruled out based on cost. Figure 2.28 shows an operation diagram of the desert cooler.

Figure 2.28 Operational diagram for desert cooler

A

B

B’

A

B

B’



Two stage Evaporative Cooling With two stage operation you can switch from single to two stages and this gives some control and better efficiency flexibility. Another option is to add a heating coil to the process as shown in Figure 2.29, which initially seems incorrect. However as the degree of evaporation depends on the incoming condition wet bulb temperature the heating coil can control this and therefore control the degree of evaporation by controlling the coil condition

Figure 2.29 Pre-heating before desert cooler

Hence more or less cooling can be obtained. This is a more effective way then trying to control the circulation pump.

Non-adiabatic humidification process

BA

Eliminator

A B B’

+ C

Heating

Room

T

C

C’No Heating coil

BA

Eliminator

A B B’

+ C

Heating

Room

T

C

C’

BA

Eliminator

A B B’

+ C

Heating

Room

T

C

C’No Heating coil



In thermodynamics, an adiabatic process is a thermodynamic process in which no heat is transferred to or from the working fluid. The term "adiabatic" literally means impassable, corresponding to an absence of heat transfer. Non-adiabatic humidification is where heat does transfer to the passing medium as part of the humidification. The system shown in Figure 2.30 details the heating or cooling of the process air to obtain the desired supply air state.

Figure 2.30 Non-adiabatic humidification

E-summer

LE

Eliminator

+

Heating

Room

H

L

S

LPHW

CW

Controller

S

E-winter

Heat Added

Heat

Removed

Space Humidity Sensor

E-summer

LE

Eliminator

+

Heating

Room

H

L

S

LPHW

CW

Controller

S

E-winter

Heat Added

Heat

Removed


LE

Eliminator

+

Heating

Room

H

L

S

LPHW

CW

Controller

S

E-winter

Heat Added

Heat

Removed




Steam Humidifiers (electrical)

Figure 2.31 Steam humidification High level sensor as shown in Figure 2.31 is required in the duct as at high levels of humidity condensation can occur in the duct. Water treatment may be necessary with some systems to avoid the nozzles clogging as the nozzle aperture is quite fine as shown in Figure 2.32, for example areas where there is high calcium content.


RH

Heater

Room

RH

Relative Humidity SensorHigh Level

Room or more often the return air sensed

RH

Heater

Room

RH

Relative Humidity SensorHigh Level

Room or more often the return air sensed



Pressure Water Humidifiers (electrical)


An alternative to the steam humidifier is to use a combination of compressed air and water as shown in Figure 2.33. These combine in the nozzle, increases the liquids velocity and when passed through the fine aperture a very fine mist is produced. This system avoids the addition of heat to the air as is the case with the steam. But some people consider the lack of the steam heating to be a cost saving, but in reality the cost of running the compressor can be comparable and so no real saving will be achieved.

Chemical Dehumidification In some environments a combination of high temperature and high humidity defies a remedy by convectional air-conditional air-conditioning, which is biased towards temperature rather than humidity. Dehumidification is merely a by-product of bringing the temperature down below the due point of the air causing condensation. A desiccant is a hygroscopic material whether it is liquid or solid, which can extract moisture from humid air, gas, and liquids. Hygroscopy is the ability of a substance to attract water molecules from the surrounding environment through absorption. Liquid desiccants work by absorption where moisture is taken up by chemical action. Solid desiccants have a large internal area capable of absorbing significant quantities of water by capillary action. Examples of efficient desiccants are:

• Silica Gel

• Activated Alumina

• Lithium salts

• Triethylene glycol

Water

Room

RH

Relative Humidity SensorCompressed air

Water

Room

RH

Relative Humidity SensorCompressed air



This is generally used where low relative humidity areas are encountered, typically less than 40% are required. To achieve very low humidification using refrigerants requires very low evaporating conditions, hence the coefficient of performance (COP) falls dramatically and the capital cost is high. The chemical dehumidification process reduces the moisture content of the air by absorption, but increases the sensible temperature as shown in Figure 2.34. The vapour in the air is absorbed as water in the chemical. Hence it gives it latent heat and the dry bulb (DB) temperature rises as a result. This method of dehumidification requires a heating stage in the process. This is to dry or regenerate the desiccant material and requires a temperature range of 60-90°C. One option is to supply the heat by means of evacuated tube solar collectors, backed up by natural gas when insolation is inadequate (Insolation is a measure of solar energy received on a given surface area in a given time. It is commonly expressed in kilowatt-hours per square meter per day (kWh/m²/day)). Alternatively waste heat, for example from a combined heat and power (CHP) unit may be exploited. As an alternative to air-conditioning, desiccant dehumidification can be used in conjunction with evaporative cooling. After being dried by the revolving desiccant wheel the air passes through a heat exchange such as a thermal wheel for cooling as shown in Figure 2.35. If necessary, further cooling may be achieved by an evaporative cooler before the air is supplied to the building as discussed earlier.

Figure 2.34 Dehumidification using desiccant material

A

A

B

Dry Bulb

Wet B

ulb

Mo

istu

re C

on

ten

t

BA

A

B

Dry Bulb

Wet B

ulb

Mo

istu

re C

on

ten

t

B



Figure 2.35 Dehumidification using rotating desiccant material

The process needs to cool the air after drying, as the latent heat temperature rises. If the wheel is rotating the absorbed moisture in the wheel must be removed from the chemical so that the process can begin again a reprocessing cycle is used.

Control The room thermostat controls the cooling coil while the room RH stat controls the regenerated air heater, the approach being that drying the supply air falls off as the air is not regenerated. The desiccant wheel is usually interlocked with the fan such that it rotated only when the fan is on.

EA

Heat for regeneration

Return Air

Air Conditioning units

Desiccant Wheel

Interlocked with Fan

Air Inlet

Exhaust Air

Supply air to

the building

Room

T RH

EA

Heat for regeneration

Return Air

Air Conditioning units

Desiccant Wheel

Interlocked with Fan

Air Inlet

Exhaust Air

Supply air to

the building

Room

T RH

Sustainability at the Cutting Edge: Emerging Technologies for Low Energy by Peter F. Smith



Control of Electric Heaters These are often used for convenience in application which may have a short intermittent running time, for example ventilation of a meeting in use < 1000 hours/year, in which the capital cost of a LPHW coil and connections to boilers is not warranted. Special control is required, because if insufficient air passes the coil it could burn out the coil element. Hence

(i) Heater is interlocked with fan. (ii) Differential Pressure Switch (DPS) is used to confirm airflow is

adequate before the power is allowed to the heater element. The electrical diagram is shown in Figure 2.36.

Figure 2.36 Electric Heaters arrangement

+ Room

T

~

T

DPS

Contactor

High Limit

Interlock

+ Room

T

~

T

DPS

Contactor

High Limit

Interlock

+ Room

T

~

TT

DPS

Contactor

High Limit

Interlock



Figure 2.37 Electric heaters electrical arrangement The configuration of the heater elements can be either star or delta. The star needs an additional neutral to complete the circuit as shown. Various forms of control for the electrical elements are possible including:

(i) Staging the heater (possible across phases not recommended if large). (ii) Solid state controller, involving alteration of the voltage chopping the

cycle, frequency generated. (iii) Transformer type, reduces the voltage, hence current and wattage.

DeltaStar

Star or Delta configuration

Fan interlock

DeltaStar

Star or Delta configuration

Fan interlock



Control of Refrigeration systems. Capacity of speed control of compressor. This has now become very important with the development of compressor technology. Methods of controlling compressors are:

• Staging, where multiple compressors are used.

• Cylinder uploading

• Speed selection (scroll compressors)

• Inlet vanes (centrifugal)

Staged compression Since compression generates heat, the compressed gas is to be cooled between stages making the compression less adiabatic and more isothermal. The inter-stage coolers cause condensation meaning water separators with drain valves are present. The compressor flywheel may drive a cooling fan.

Scroll Compressor. The flow rate capacity for scroll compressors can be controlled by varying the speed at which the scroll rotated. Use the following hyperlink to see it in operation http://en.wikipedia.org/wiki/Scroll_compressor

Figure 2.38 Scroll Compressor



Cylinder uploading. Cylinder uploading is a method of holding the discharge valve open on one or two of the cylinders of a multi-cylinder compressor. The discharge stroke therefore does not build up the pressure on these cylinders and hence capacity is reduced.

Figure 2.39 Cylinder uploading Figure 2.39 illustrates the pistons of a car engine, but can also be used to demonstrate the operation of a cylinder compressor. The main difference is that no fuel is added so no need for a spark plug, and that the main crankshaft (C) is turned by an electric motor (not the combustion of the fuel). (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug which does not exist in the compressor approach, (V) Valves an inlet and outlet, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow because when a gas is compressed it heats up in accordance with the Combined Gas Law.

Whenever we are dealing with problems that have initial and final conditions. The link below will also show a moving image of the system.



http://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Cshaft.gif/300px-Cshaft.gif

Figure 2.40 multi-cylinder arrangement

Inlet vane control operates as per fans. Inlet guide vanes are more efficient than butterfly valves by pre-swirling the air to help minimize power consumption by allowing lower turndown. Figure 2.41 illustrates a standard inlet vane arrangement. The internals are sealed to prevent contamination from ambient conditions. The stainless steel vanes virtually eliminate corrosion.

Figure 2.41 Inlet vane arrangement



All refrigeration compressors operate between two pressure limits or settings or the high pressure and low pressure sensors. When considering control keep this in mind! For the condenser to maintain its pressure it must received gas to be condensed at the same rate as it has the ability to condense the gas.

Figure 2.42 Compressor HP and LP As the Variable Flow Rate (VFR) of the refrigerant falls the speed of the compressor is reduced to match the VFR of the refrigerant being generated. The capacity of the condenser must also be turned back to prevent the condenser pressure falling as volume of gas produced from the compressor falls. Likewise if the load on the evaporator falls then the volume of gas produced is reduced the compressor and condenser must now reduce in capacity to match the volume of gas being generated as illustrated in Figure 2.43. This is what the controls of the system are designed to do.

Figure 2.43 Compressor to cooling coil arrangement

Room

T

TEV

T

Return Temperature

T T T T

Evapo

rato

r

Condenser

CompressorSpeed Control

Room

T

TEV

TT

Return Temperature

TTT TTT TTT TTT

Evapo

rato

r

Condenser

CompressorSpeed Control

LPHP LPHP



Air cooling condensers Generally speaking the condenser fans start on/off with the compressor on/off. An illustration of the arrangement is shown in Figure 2.44 where the coils carry the refrigerant medium. The fins attached to the coils are used to maximise the contact of the air passing over the coils in order to cool the keep the refrigerant medium in the plates. The cool air entering the coil is heated up by the warm refrigerant medium and this warmer air leaves the system.

Figure 2.44 Air cooled condenser arrangement Heat is rejected to the ambient air the temperature of the ambient air varies from about -2°C to +25°C in Dublin. Hence a given condenser designed to reject its full load at 25°C (say refrigerant at 45°C) would have far greater capacity at say +5°C (∆t = 40k) than at ∆t = 20k design. So the capacity to reject heat of the condenser must be reduced to match the ambient conditions and load available. This is referred to Lead Pressure control.

Vapour from compressor

To evaporator:

� Condensed Vapour

� Liquid

Cool air

Warmed air after passing over plates

Conduction finsCoils


To evaporator:

� Condensed Vapour

� Liquid

Cool air





In its simplest form the fans are simply switched on in turn. In a more sophisticated form the speed of the fans is controlled using a variable speed drive as shown in Figure 2.45.

Figure 2.45 Forced air cooled condenser arrangement The basic from of control is to hold a particular set point pressure and reduce the air flow rate over the coil as the condenser pressure would start to fall. This form of control however does not give any energy benefits. This is possible as with falling ambient conditions it should be possible to maintain the design ∆t (20k) at lower condenser pressures. Hence if the ambient is 15 DecC then a condenser temperature of 35DecC should be possible to reject the same load. This would increase the coefficient of performance (COP) of the refrigerant process.


To evaporator:� Condensed Vapour

� Liquid

Cool air



Rotation, Increases air flow over the fins and coils

P



� Liquid

Cool air






� Liquid

Cool air





� Liquid

Cool air




P



Figure 2.46 Forced air cooled condenser arrangement with controller

The OA temperature sensor senses the OA condition and the controller “reads” a suitable set point for the pressure.

Figure 2.47 Change in pressure to OA temperature Hence the system can be controlled in this way.



� Liquid

Cool air




P

Controller

T

OA temperature



� Liquid

Cool air





� Liquid

Cool air




P

Controller

TT

OA temperature

7

8

9

10

11

12

5 10 15 20 25 30

Required pressure in Bar

OA Temperature

7

8

9

10

11

12

5 10 15 20 25 30

Required pressure in Bar

OA Temperature



Water cooled condensers An alternative to the air cooled condenser is the water cooled condenser. In this example the condenser is enclosed in a pocket of circulating water. Water enters the condenser coils at say 27°C and leaves the condenser at 33°C. The water at the higher temperature is delivered to the spray bars of the cooling tower and atomises the water. As the atomised water in the cooling tower falls the cool air comes in and cools down the droplets, thus cooling the water collecting at the bottom. The volume of air passing through the tower can be controlled by the fan speed. An alternative approach is to use a 3-port diverting valve arrangement which acts by maintaining the water temperature above the set point of 27DecC.

Figure 2.48 Water cooled condenser

Control of cooling towers. The degree of control necessary depends on the annual load pattern. Towers designed to operate only in the summer months may have sufficient control with fan speed alone. Summer towers are usually drained in the winter time. Where towers are designed to be operated in the winter then additional control and freeze protection may be required. As the minimum speed is limited by the necessity to reject heat from the motor the min speed is usually limited to 20%. Also when the fan is off some heat may be rejected. Hence the necessity to provide additional control such as the diverting circuit as this allows a full range of possible control.

T


To evaporator:� Condensed Vapour� Liquid

Cooled water circulates around the cooling fins

Fan to force air through tower

Cool OA

entering tower

Warm Air leaving

Warm water leaving the condenser

3-port valve

Spray Bar

Water droplets

Cool OA

entering tower

Cooled Water Say, 27DegC entering

Say, 33DegC leaving

Indirect cooling, where the cooling is

performed away from the condenser

T


To evaporator:� Condensed Vapour� Liquid

Cooled water circulates around the cooling fins

Fan to force air through tower

Cool OA

entering tower

Warm Air leaving

Warm water leaving the condenser

3-port valve

Spray Bar

Water droplets

Cool OA

entering tower

Cooled Water Say, 27DegC entering

Say, 33DegC leaving

Indirect cooling, where the cooling is

performed away from the condenser



Frost immersion heaters in additional to trace heating may be required. Glycol (Ethylene Glycol is an alcohol based chemical compound widely used as automotive antifreeze) is not used in towers as it is a recirculation system and is not an evaporating systems.

Questions on Chapter 2.

Q2.1 Air handling plant is susceptible to damage from frost. Describe at least two

ways using automated control how air handling plant can be protected against frost.

Q2.2 There are often misconceptions that some energy efficient humidification

systems require significantly less energy to operate than others. Take for example steam verses pumped atomised spray. While some energy savings between different types of humidifiers exist, the larger savings in the overall facility energy consumption never materialise. Discuss this argument.

Q2.3 Define what is meant by control valve authority and indicate how this

quantity is determined for a heating coil controlled by: � A mixing valve. � A diverting valve. � Two port valve.

Q2.4 Describe the operation and control of a direct expansion type evaporator

coil.



Chapter 3, Complete Control System

Aim of Chapter

Building on the fundamental aspects introduced in chapter 1 an 2, this chapter focus on system as a whole whilst tying into the natural environment data such as TRY data . It includes aspects that affect the systems design such as variable air volume and variable flow rate. Temperature control of the duct and/or rooms and issues relating to speed and accuracy of control are also developed.


On completion of this chapter the student will be able to: (a) Identify from a control perspective the key functions of equipment in

single zone HVAC systems. (b) Distinguish between variable flow rate and variable air volume systems. (c) Recognise and interpret the significance of constant speed verses

variable speed fan control. (d) Describe techniques incorporating Test Reference Year (TRY) data into

a systems design and distinguish between energy conservation verses controllability accuracy.



The single zone system with variable OA quantity for economy cycle. A variable OA system for a single zone is shown in Figure 3.1

Figure 3.1 Single zone system with variable OA

The normal arrangements for operating this type of system are: If the Room Temperature < Room Set point Temperature then,

(i) Stop cooling (ii) Decrease OA (iii) Increase Heating

If the Room Temperature > Room Set point Temperature then,

(iv) Stop heating (v) Increase OA (vi) Increase cooling

RA

ExhaustAir

Motorised Damper

MD


Damper

T MDMD

EA

OAOutside

AirMotorised Damper

SA

3 Output Sequence Controller

+

Heating

-

T

Cooling

Set point for Temperature

Delivery


Room

Room

TDuct sensor

Limiter

RA

ExhaustAir

Motorised Damper

MD


Damper

TT MDMD

EA

OAOutside

AirMotorised Damper

SA

3 Output Sequence Controller

++

Heating

-

TT

Cooling


Delivery


Room

Room

TDuct sensor

Limiter



Note the set point of the room temperature can be varied manually at the controller similar to that shown in Figure 3.2. This can also be done automatically in response to an OA temperature sensor. This may help to save energy.

Figure 3.2 Thermostat for room

Figure 3.3 Room Set-point to OA Temperature The duct temperature sensor as shown in Figure 3.1 is a limiter and typically it prevents the supply air from going below 12°C, even though the load on the room may be heavy. A point to note is the sequence of the response. If Room Temperature > Room Set point Temperature then the sequence is:

(i) Cooling coil valve opens (ii) Chilled water admitted (iii) Air is cooled (iv) Colder supply air will eventually reduce the Room Temperature and the

valve will close.

18

19

20

21

22

23

0 25 30

Set Point

of Room

OA Temperature

23

26

18

19

20

21

22

23

0 25 30

Set Point

of Room

OA Temperature

23

26



This process will take time. In the meantime the room air temperature will have risen still further. The time delay depends on

(i) The length of the ducting involved (ii) The location of the Temperature room sensor (iii) The thermal storage features of the room.

Remember the concept of PID in the lecture notes for section B. This means that the control could utilise the proportional, integral and derivative parts of the PID controller. In large spaces this delay can lead to a wide controlled band in the room. One way for shortening this band is to use the supply air temperature sensor as the controller (duct temperature as apposed to the room temperature). But to use the room temperature to reset the set point of duct temperature. Hence as follows:

Room set point temperature = 23°C Duct set point temperature = 14°C Room temperature = 25°C Duct temperature = 14°C

So with the duct set point temperature reset to 12°C, the cooling valve would open quickly to satisfy the duct temperature. The process is quicker because the control loop is very short and independent of the length of duct or absorption in the space. Therefore in effect what happens is that the room condition is not allowed to get as far out of control as it could.

Control of Relative Humidity (RH) This area is broadly divided into two applications.

(i) Commercial: minimum RH 30%, maximum 65% (ii) Industrial, narrow limits of say 40% to 50%.

In the case of the commercial application low and high limit humidity sensors are used usually in the return duct. The high limit humidity sensor is also an input to a sequence controller and overrides control of the cooler coil by the air temperature sensor using a selector relay. For most of the time there is no input signal form the RH sensor, therefore the selection relay simply puts the temperature sensor input as the output. When the RH rises above 65% the RH sensor now controls the cooling coil and dehumidifying the air. This removed moisture from the air but over cools the room. The reheat coil now activates to restore room temperature. So for this to occur the heating coil must be placed after the cooling coil.



With modern controllers this selector relay function is built into the sequence controller and does not appear as a separate “relay”. Methods of implementing this with the PLC or controller are also discussed in Section B under the parts relating to ladder logic and sequence diagrams. As the moisture for outside air in Dublin is 10g/kg and the moisture content for room air at 23°C is 11.5g/kg. The chances of having an RH above 65% is very rare unless the VFR is very low as discussed earlier. Addition of up to 1.5g/kg is possible. Also at a room temperature of 23DecC the minimum room moisture for 30% RH the moisture content is 5.2g/kg and at -1°C the moisture is 3.4g/kg, requiring an addition of 1.8g/kg is required. Now, if the ASHRAE standard of 25% RH is required then the minimum moisture in the room is 4.4g/kg and only an addition of 1.0g/kg is required to bring the system within limits. Except in high occupancy rooms humidity control in commercial buildings is not often provided. A scheduling of the room temperature from 20°C in winter to 24°C in summer is often a better approach. High limit humidity is provided by a steam humidifier generally as shown in Figure 3.4. Humidifier placed in the discharge duct at the fan exit.

Figure 3.4 Steam Humidity When sizing a humidifier do not size it to add enough steam for the design room condition, but just to add enough for the minimum condition at the design OA winter condition. Saving from moisture at design air condition of 3.4g/kg to about 5.2g/kg.

RH

Low Limit

in return air duct

Low Limit in return air controls the steam humidifier

Steam humidifier

using electrical element

Clouds of steam in air stream

RH

Low Limit

in return air duct

Low Limit in return air controls the steam humidifier

Steam humidifier

using electrical element

Clouds of steam in air stream



Variable Air Volume (VAV) control

Figure 3.5 Control of the VAV system Room control Temperature sensor T2 senses the room temperature and controller C2 controls the damper actuator. The set point of the room temperature and the high and low VFR is set at the controller C2. Discharge temperature control This would typically be controlled to 12°C set point. The chiller (-) and heater (+) are adjusted in sequence to do this. A common modification is to fit a discriminator relay. This receives an input from all the room temperature sensors and gives as an output the highest of the inputs. If no room is at the maximum setting say 25°C then the set point of T1 at C1 is adjusted upwards. This has the result:

(a) Saving energy (b) Avoiding low turndown, i.e. turnover quantities of air in the room. (c) Possible disadvantage of using more fan energy as the VFR is

greater than it would otherwise be.

RA

ExhaustAir

Motorised Damper

MD


MD MD

EA

OAOutside

AirMotorised Damper

SA+

Heating

+

T1

Cooling

RoomT2

MD

Controller 2

Controller 1Set point

T3

Controller 3

RA

ExhaustAir

Motorised Damper

MD


MD MD

EA

OAOutside

AirMotorised Damper

SA++

Heating

++

T1T1

Cooling

RoomT2

MD

Controller 2

Controller 1Set point

T3T3

Controller 3



Mixed Air Temperature control Temperature sensor T3 senses the condition of the mixed air prior to cooling or heating, and C3 controls the 3 dampers to give the required condition. The set point of C3 is equal to C1 as this is the minimum temperature required to be supplied, this gives free cooling up to typically 12°C OA temperature. The reset line from the discriminator relay can also be used here to raise the set point if possible. OA Variable Flow Rate (VFR) Control

Figure 3.6 Grid Sensor The grid sensor measures static and velocity pressure across the width of the duct as shown in Figure 3.6. From this by calibration the VFR can be inferred. The transducer converts this VFR to a 0-10Vdc signal. The controller has the set-point of the VFR set. This is the calculated required minimum VFR for effective ventilation.

Transducer

MDController

0-10Vdc signal

measures VFR

Set-point of VFR

Controls

Min Limit

Grid Sensor, also known as a Wilson Flow Grid

Transducer

MDController

0-10Vdc signal

measures VFR

Set-point of VFR

Controls

Min Limit

Grid Sensor, also known as a Wilson Flow Grid



The arrangement acts as a minimum limit on the OA damper. This will allow the VFR at the damper to go above this set point but not below. Hence the damper acts to mix the quantities to get the correct free cooling but only when it does not compromise the ventilation. Fan Control The speed of the return air fan is controlled to be a certain % of the supply fan. The measurement of air can be completed in many ways using a turbine (Figure 3.7), varying resistance (Figure 3.8) and pressure tube as shown in Figure 3.6 etc.

Figure 3.7 Turbine Sensor

Figure 3.8 Resistance Sensor

1/3

2/3

There are various measurements of air flow:

Turbine/vane

Pressure tubes,

etc.

Controller

1/3

2/3

There are various measurements of air flow:

Turbine/vane

Pressure tubes,

etc.

Controller



Control of the fan speed and air volume can be performed in a number of different ways (i.e. variable speed drives or adjustable vanes etc). In Figure 3.9 a comparison of the different types of ways which are used to vary the volume of air through a fan. The ideal relationship where Power proportional to Q is shown as a dashed line. (a) is where the fan speed is constant and the ductwork is changed in some way to achieve the change in volume. (b) is where the duct work is fixed and the speed of the motor is the only thing that changes.

Figure 3.9 Constant Speed Fan or Variable Speed Fan A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, micro-drives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives. Figure 3.10 shows an image of the VSD and also shows the AC incoming wave being sliced up into pulses. It is important to note that the motor acts on the principle of magnetism and it is not essential for the incoming wave to be a sine AC. It can be + and – pulses, with the amplitude and frequency of the wave being used to generate the equivalent magnetism affect in the motor.



Figure 3.10 Variable Speed Drive



Reheat system

Figure 3.11 Reheat System The reheat system is a lot simpler than the VAV control. There are two main options as shown in Figure 3.11.

(i) Systems use primarily for room sensible temperature control, usually in commercial applications.

(ii) System used primarily for room RH accurate control (industrial application)

Commercial application Sequence controller controls the supply temperature in the normal way. Typically supply temperature would be 14°C. The supply temperature can be reset upwards when no room is on a full load. The test reference year (TRY) was developed as a replacement for energy analyses. It produces a set of data which measures the temperature for every hour in a year and produces a cumulative % in relation to the temperature. The dry bulb temperature for Dublin is given in Figure 3.12 (a). The dry bulb temperature is shown under the Bin column ranging from -8 to 26Deg C, the number of hours is given under the Frequency column which sums up to 8760 Hours (24hr/day by 365 days of the year 24x365=8760Hrs). This shows for how many hours the temperature was reached. Similar TRY data is available for the Wet Bulb in a given year is also given in Figure 3.12(b).

RA

Exhaust

AirMotorised

Damper

MD

Recirculation


MD MD

EA

OAOutside

AirMotorised

Damper

SA+

Heating

+

Ts

Cooling

TR

Controller 1

T3

Controller 2

+

DiscriminatorController

TR1

TR2

TRx

Room

Room conditions Inputs

Reset line (highest of inputs chosen)

RA

Exhaust

AirMotorised

Damper

MD

Recirculation


MD MD

EA

OAOutside

AirMotorised

Damper

SA++

Heating

++

TsTs

Cooling

TR

Controller 1

T3T3

Controller 2

+

DiscriminatorController

TR1TR1

TR2TR2

TRxTRx

Room

Room conditions Inputs

Reset line (highest of inputs chosen)



Note from test reference year (TRY) data shown below that a supply temperature it design of 14DecC gives a cumulative % of 87.8%, while the 12°C gives 68.4% on a TRY bases. Hence if 14°C is chosen as a reheat supply temperature it is not necessary that energy inefficient. Also with a load analyser fitted the energy waste can be reduced still further. Hence the combination of the sequence controller and the discriminative/load analyser gives a more energy efficient system.

Figure 3.12 Dry Bulb and Web Bulb Test Reference Year

(a)

(b)



Industrial applications With this arrangement the sensor should be fitted downstream of the cooling coil and set to control the off moisture content at a constant level. This is done by setting the dry bulb sensor so called dew point sensor. This is based on the idea that the “off” coil condition is 95% saturation. Hence a dry bulb sensor can control the supply moisture content in this way. If the RH in the space is too high the duct sensor set point can be reset downwards. If RH in the space is too low then humidification is introduced.

Figure 3.12 Dew point sensor

Questions on Chapter 3.

Q3.1 Describe changes which can be incorporated into a single zone system to make better use of the economy cycle.

Q3.2 Explain why temperature control is used in AHUs in some countries, where

enthalpy controls is used in others to provide a higher efficiency system. Q3.3 Distinguish between a Constant Air Volume (CAV) and a variable Air

Volume (VAV) system. Elaborate on why one is more energy efficient that the other.

10 12 14

9.5

8.5

7.3

95%

10 12 14

9.5

8.5

7.3

95%



Chapter 4, Heat Recovery Control System

Aim of Chapter

In the majority of air conditioning systems low pressure hot water (LPHW) and refrigeration coolant are the primary ways in which heat is transferred to and from the air. This chapter focuses on ways in which heat can be recovered from normally exhausted warm air.


On completion of this chapter the student will be able to: (a) Identify key components and issues in air to air heat recovery systems

and complications associated with the dynamics from a control perspective.

(b) Recognise and interpret the significance of face and bypass for heat recovery control.



Air-to-Air Heat Recovery in Ventilation Systems A heat recovery unit, also known as a recuperator, transfers heat (some units also moisture) from the exhaust air stream over to the supply air stream, thus reducing the heat loss due to ventilation, and reducing the need to condition the cold supply air. Conversely, in hot and humid outdoor conditions, a heat recovery unit can keep heat (some units also moisture) outside, thus reducing air conditioning costs.

Figure 4.1Air-to-Air Heat Recovery

However the control of the air-to-air heat recovery unit as shown in Figure 4.1 is difficult because there is no way to limit the air flow and therefore the heat. An alternative arrangement is to add a damper as shown in Figure 4.2, where it acts as a bypass. It may not be accurate but is capable of allowing an element of control.

Figure 4.2 Air-to-Air Heat Recovery with bypass

This is only needed to divert the incoming stream, not the exhaust. A good linear type control depends in good damper authority sizing. The tendency with the system is that as the damper opens slightly most of the air tends to go through the damper.

MDMD



Figure 4.3 Damper position to flow rate

Figure 4.3 illustrates a scenario where 20% opening of the damper would result in an 80% flow through the damper. A better control of the incoming and exhaust air would be using a face and bypass arrangement, similar to that shown in Figure 4.4.

Figure 4.4 Face and Bypass arrangement

0% Open, Fully closed

100% Fully Open

100

Flow thro the damper

Damper position

20%

80%

0% Open, Fully closed

100% Fully Open

100

Flow thro the damper

Damper position

20%

80%



Chapter 5, Packaged Pressurisation and Filling System Aim of Chapter

From first year engineering and leaving cert physics the observable fact of water expanding when it gets warmer has been taught and demonstrated. Large commercial water distribution systems rely heavily on water as the primary conduit for heat distribution. However, with continuously adjusting, switching and modulating of valves on the system in order to keep control the amount of water and pressure careful control is needs to ensure this aspect of the system is kept in control. This chapter focuses on the pressurisation and filling of the water distribution systems.


On completion of this chapter the student will be able to: (a) Identify key components and issues pressurisation and filling system. (b) Recognise and interpret the significance of continuously varying

pressure systems.



Packaged Pressurisation and Filling unit Before describing further the common heating systems, it is important to discuss the controls used to ensure the water systems continue to have adequate water reserves. The system illustrated in Figure 5.1 is used only to fill and pressurise the water in the system. These systems are available off the shelf as a package plant. Some contain methods to dose the water with different types of chemicals and detergents. The mains water enters the storage tank through the ball valve. The pump pressurises the water but is controlled via the Low/High pressure sensors and the Operational Pressure sensor. There is a bypass loop also which is shown as an alternative to quickly fill the system is the pump is damaged or inoperable due to maintenance. In some cases there is a level sensor in the water storage tank to indicate if low or high water levels are present. The purpose of the expansion vessel is to maintain a constant pressure in the system.

Figure 5.1 Pressurisation system Typical settings would be:

PL set at 1 bar POP set at 2 bar PH set at 4 bar SV set at 5 bar

PL PH POP

Anti Gravity Loop

LSV

System

to be

filled

Low Pressure sensor

High Pressure sensorDiaphragm Type Expansion Vessel

Pump (perhaps multistage)

But not a circulating pump

Mains water supply

PL PH POP

Anti Gravity Loop

LSV

System

to be

filled

Low Pressure sensor




Mains water supply

PL PH POP

Anti Gravity Loop

LSV

System

to be

filled

Low Pressure sensor




Mains water supply



Chapter 6, Boilers and Chillers Aim of Chapter

All true air condition systems require conditioning of the air, either by cooling or heating the air. Not all of this conditioning is just to adjust temperature and some is to aid in removal or addition humidity to the system with more efficiency. This chapter focuses on the boiler and chillers pipe work arrangements and subsequent control aspects associated with altering the thermal/energy state of the air.


On completion of this chapter the student will be able to: (a) Identify key components and issues in boiler connection and control. (b) Recognise and interpret the significance of thermal inertia in selecting

and controlling boiler.



Pumps The flow of water around a heating system is maintained using pumps. The pumps can be arranged in series or in parallel. Both arrangements have different impacts on the flow and total head as shown in Figure 6.1.

Figure 6.1 Boiler and shunt pump

Pumps in Series Pumps in ParallelPumps in Series Pumps in Parallel



Central boiler and Chillers Circuits Boiler control. Some pointers to note:

(1) Distinguish between high heat inertia boilers and low heat inertia boilers. (2) Distinguish between the fuels used for firing. (3) Distinguish between single boilers and multiple boilers in a sequence. (4) Heat dissipation stats.

Cast iron sectional boilers have a very high heat capacity in metal and water content. Small natural gas fired wall hung boilers have a copper heat exchanger and have a very low heat capacity. Fire tube steel boilers have a less cast iron, but still have a massively high inertia. Shunt pumps are required if:

(1) Low return waste temp (2) Oil fuel with sulphur content

The shunt raises the return water temperature to prevent sulphuric acid from attacking the rear of the boiler. Shunt arrangement is sized to raise the return water temperature by diverting the water around the boiler as shown in Figure 6.2.

Figure 6.2 Boiler and shunt pump

TBoiler

Shunt

TBoiler

Shunt



Common Header single boiler

Figure 6.3 Single Boiler and multiple loads

Primary pump interlocked with the boiler. This primary pump is sized on the bases of the total simultaneous circuit demand +5%. This would typically result in

_/

(82 71) 4.2

Boiler kWkg s=

− ×

The specific heat capacity for water is 4.186 kj/kg ≈ 4.2 kj/kg and the 82 and 71 are the ring main temperatures in °C. More information is available on http://www.jbc-controls.co.uk/jbctrssn2.htm. Frost protection Immersion thermostat in the boiler return is usually set at 4°C and brings on the burner. Air thermostat outside set at 4-8°C at a set OA temperature to protect the building and system from frost damage when the boiler is off.

TBoiler

T

Package pressurisation

and filling unitSupply when needed

S.V.

LSV

TBoiler

T

Package pressurisation

and filling unitSupply when needed

S.V.

LSV



Multiple boilers in sequence (Parallel connection)

Figure 6.3 Boilers in Parallel The arrangement is shown in shown in Figure 6.3. Return water temperature is sensed. When the temperature of the return water falls below a set level the second boiler is brought on. Typically set at 71°C if the flow is set at 82°C. If the return temperature is 76°C then it is possible to meet demand with one boiler running. All boilers have their own operation and high level thermostat which maintains the outlet water temperature by controlling the burner. Operating thermostat is set typically at 82°C and the high level thermostat at about 90°C. In some cases boilers may have on/off two port valves to control the flow of water thro the idle boiler. Care is needed as this will change the pressures as the boiler is the main resistance in the primary circuit. (Remember it is the boiler in parallel not the pump)

TBoiler #2

T

S.V.

T

Boiler #1

T

Boiler Sequence Controller

TBoiler #2

T

S.V.

T

Boiler #1

T

Boiler Sequence Controller

∆P

Q

B1

B1+B2

∆P

Q

B1

B1+B2



Boilers connected in Series

Figure 6.4 Boilers in Series The return water thermostat is set at 71°C and the flow at 82°C. In this arrangement as shown in Figure 6.4 boilers may operate by a sequence controller in the normal way. Alternatively the boiler thermostat can be set differently with Boiler 2 being set at 82°C and Boiler 1 being set at 76°C. This latter method is not favoured as the boiler settings can be easily changed at the dial and the boiler cannot be used in isolation, except with the bypass as shown.

Boiler #2

T

S.V.

T

Boiler #1

T

Boiler Sequence

Controller

Manual Bypass valve

Normal direction of flow

Boiler #2

T

S.V.

T

Boiler #1

T

Boiler Sequence

Controller

Manual Bypass valve

Normal direction of flow



Modular Boilers, Wall Hung type

Figure 6.5 Modular Boilers Normally used with wall hung gas boilers. Modules fire in sequence in response to load demand. Low inertia system with good energy efficiency. This arrangement also has the advantage that further modules can be added if demand increases in the future. Be careful of the changes in system pressure as a result of the circuits being run in parallel. Advantages:

• Low inertia

• Flexibility

• Small space required Disadvantage Very little heat stored therefore not suitable for high based load applications say hospitals, it is more suited to an office application.

Boiler Sequence

Controller

T

T

Boiler Sequence

Controller

T

T



Based load

Figure 6.6 Base load

The base load refers to the on going minimum load experienced by the system. For example buildings such as hospitals and swimming pools have continuously long periods where the heat in the system needs to be maintained at a high level. The heating of pool water is ongoing 24hr/7 and the heating of the hospital space is 24hr/7. This causes the base load to be high. However office buildings are heated up by the sun during daylight and most of the workers would work 8-7 so the building heating load is not maintained throughout the day. Heating inertia Modern office buildings have insulation fitted to the inside of the building as shown in Figure 6.7 Case III. This means that the office space is the only heating load on the system not the heating of the building itself. However this means that the building will also lose it heat just as quick. This is termed heating Inertia. If the heating is placed on the outside of the building then the walls will have to be heated up before the space really heats up. This takes time and energy. The building will also take longer to cool as the heat has to return to the building from the walls.

Base Load

Time

Load

Base Load

Time

Load

High Base Load Low Base Load

Base Load

Time

Load

Base Load

Time

Load

Base Load

Time

Load

High Base Load Low Base Load



Figure 6.7 Heating Inertia



Chapter 7, Sizing valves for water service

Aim of Chapter

All true air condition systems require conditioning either by cooling or heating of the air. Not all of this conditioning is just to adjust temperature, some is to aid in removal or addition humidity to the system with more efficiency and other “conditioning” activities. This chapter focuses on the boiler and chillers pipe work arrangements and subsequent control aspects associated with altering the thermal/energy state of the air.


On completion of this chapter the student will be able to: (a) Select valves based on flow and pressure characteristics. (b) Identify the issues that Valve authority has to control and system

operation. (c) Recognise the difference between diverting and mixing valve

arrangement.



Sizing valves for water service In order to size a valve for a water application, the following must be known:

The volumetric flow rate through the valve.

The differential pressure across the valve.

The control valve can be sized to operate at a certain differential pressure by using a graph relating flow rate, pressure drop, and valve flow coefficients.

Alternatively, the flow coefficient may be calculated using a formula. Once determined, the flow coefficient is used to select the correct sized valve from the manufacturer's technical data.

For liquid flow generally, the formula for Kv is shown in Equation 7.1.

Equation 7.1 Where: Kv = Flow of liquid that will create a pressure drop of 1 bar (m3/ h bar)

= Flow rate (m3/h)

G = Relative density/specific gravity of the liquid (dimensionless). Note: Relative density is a ratio of the mass of a liquid to the mass of an equal volume of water at 4°C

∆P = Pressure drop across the valve (bar)

Sometimes, the volumetric flow rate needs to be determined, using the valve flow coefficient and differential pressure.

Rearranging Equation 7.1 gives:

For water, G = 1, consequently the equation for water may be simplified to that shown in Equation 7.2.

Equation 7.2



Example 1 10 m3/h of water is pumped around a circuit; determine the pressure drop across a valve with a Kv of 16 by using Equation 7.2:

Equation 7.2 Where: = 10 m3/h

Kv = 16

Alternatively, for this example the chart shown in Figure 7.1, may be used. (Note: a more comprehensive water Kv chart is shown in Figure 7.2):

1. Enter the chart on the left hand side at 10 m3/h.

2. Project a line horizontally to the right until it intersects the Kv = 16 (estimated).

3. Project a line vertically downwards and read the pressure drop from the 'X' axis (approximately 40 kPa or 0.4 bar).

Note: Before sizing valves for liquid systems, it is necessary to be aware of the characteristics of the system and its constituent apparatus such as pumps.

Figure 7.1 Extract from the water Kv chart



Figure 7.2 Water Kv chart



Pumps Unlike steam systems, liquid systems require a pump to circulate the liquid. Centrifugal pumps are often used, which have a characteristic curve similar to the one shown in Figure 7.3. Note that as the flow rate increases, the pump discharge pressure falls.

Figure 7.3 Typical pump performance curve

Circulation system characteristics It is important not only to consider the size of a water control valve, but also the system in which the water circulates; this can have a bearing on which type and size of valve is used, and where it should be positioned within the circuit. As water is circulated through a system, it will incur frictional losses. These frictional losses may be expressed as pressure loss, and will increase in proportion to the square of the velocity. The flow rate can be calculated through a pipe of constant bore at any other pressure loss by using Equation 7.3, where 1 and 2 must be in the same units, and P1 and P2 must be in the same units. 1, 2, P1 and P2 are defined below.

Equation 7.3 Where:

1 = Flow rate at pressure loss P1

2 = Flow rate at pressure loss P2

Example 2 It is observed that the flow rate ( 1) through a certain sized pipe is 2500 m3/h when the pressure loss (P1) is 4 bar. Determine the pressure loss (P2) if the flow rate ( 2) were 3500 m3/h, using Equation 7.3.



It can be seen that as more liquid is pumped through the same size pipe, the flow rate will increase. On this basis, a system characteristic curve, like the one shown in Figure 6.3.4, can be created using Equation 7.3, where the flowrate increases in accordance to the square law.

Figure 7.4 Typical system curve

Example 3 A manifold of PN6 flanged pipework was designed to distribute 3,000m3/h of water with a pressure loss of 5bar. However, during commissioning when the system was set up exactly as per the design the flow rate was measured as 4,900m3/h and the pipework flanges were leaking. Determine the pressure loss in the system and comment on why the flanges were leaking and make a brief comment on the design itself.



barP

barP

barP

barP

V

VPP

P

P

V

V

34.13

68.25

109

1024015

3000

49005

2

2

6

4

2

2

2

2

2

1

2

2

12

2

1

2

2

2

1

=

×=

×

××=

×=

×=

=

The flanges are rated to 6bar pressure. They should be able to hold for short durations 1.5 times the PN pressure of 6bar. That would be 9 bar, but only for short durations such as starting and stopping of a pump where some water hammer can occur. Specifying a the use of a PN6 when the design was near 5bar meant there was very little room for error, allowance for contamination, blockage, increased pipe resistance over time or future proof adjustment.

Actual performance It can be observed from the pump and system characteristics that as the flow rate and friction increase, the pump provide less pressure. A situation is eventually reached where the pump pressure equals the friction around the circuit, and the flow rate can increase no further.

If the pump curve and the system characteristic curve are plotted on the same chart - Figure 7.5, the point at which the pump curve and the system characteristic curve intersect will be the actual performance of the pump/circuit combination.



Figure 7.5 Typical system performance curve

Three-port valve A three-port valve can be considered as a constant flow rate valve, because, whether it is used to mix or divert, the total flow through the valve remains constant. In applications where such valves are employed, the water circuit will naturally split into two separate loops, constant flow rate and variable flow rate. Ensure to check CIBSE Guide H pg 3-13 section 3.3.4 for more information.

The simple system shown in Figure 7.6 depicts a mixing valve maintaining a constant flow rate of water through the 'load' circuit. In a heating system, the load circuit refers to the circuit containing the heat emitters, such as radiators in a building.



Figure 7.6 Mixing valve (constant flow rate, variable temperature)

The amount of heat emitted from the radiators depends on the temperature of the water flowing through the load circuit, which in turn, depends upon how much water flows into the mixing valve from the boiler, and how much is returned to the mixing valve via the balancing line.

It is necessary to fit a balance valve in the balance line. The balance valve is set to maintain the same resistance to flow in the variable flow rate part of the piping network, as illustrated in Figures 7.6 and 7.7. This helps to maintain smooth regulation by the valve as it changes position.

In practice, the mixing valve is sometimes designed not to shut port A completely; this ensures that a minimum flow rate will pass through the boiler at all times under the influence of the pump.

Alternatively, the boiler may employ a primary circuit, which is also pumped to allow a constant flow of water through the boiler, preventing the boiler from overheating. The simple system shown in Figure 7.7 shows a diverting valve maintaining a constant flow rate of water through the constant flow rate loop. In this system, the load circuit receives a varying flow rate of water depending on the valve position. The temperature of water in the load circuit will be constant, as it receives water from the boiler circuit whatever the valve position. The amount of heat available to



the radiators depends on the amount of water flowing through the load circuit, which in turn, depends on the degree of opening of the diverting valve.

Figure 7.7 Diverting valve (constant temperature in load circuit with variable

flow)

The effect of not fitting and setting a balance valve can be seen in Figure 7.8. This shows the pump curve and system curve changing with valve position. The two system curves illustrate the difference in pump pressure required between the load circuit P1 and the bypass circuit P2, as a result of the lower resistance offered by the balancing circuit, if no balance valve is fitted. If the circuit is not correctly balanced then short-circuiting and starvation of any other sub-circuits (not shown) can result, and the load circuit may be deprived of water.



Figure 7.8 Effect of not fitting a balance valve

Two-port Valves When a two-port valve is used on a water system, as the valve closes, flow will decrease and the pressure upstream of the valve will increase. Changes in pump head will occur as the control valve throttles towards a closed position. The effects are illustrated in Figure 7.9.

A fall in flow rate not only increases the pump pressure but may also increase the power consumed by the pump. The change in pump pressure may be used as a signal to operate two or more pumps of varying duties, or to provide a signal to variable speed pump drive(s). This enables pumping rates to be matched to demand, saving pumping power costs.

Two port control valves are used to control water flow to a process, for example, for steam boiler level control, or to maintain the water level in a feed tank. They may also be used on heat exchange processes, however, when the two-port valve is closed, the flow of water in the section of pipe preceding the control valve is stopped, creating a 'dead-leg'. The water in the dead-leg may lose temperature to the environment. When the control valve is opened again, the cooler water will enter the heat exchange coils, and disturb the process temperature. To avoid this situation, the control system may include an arrangement to maintain a minimum



flow via a small bore pipe and adjustable globe valve, which bypass the control valve and load circuit.

Two-port valves are used successfully on large heating circuits, where a multitude of valves are incorporated into the overall system. On large systems it is highly unlikely that all the two-port valves are closed at the same time, resulting in an inherent 'self-balancing' characteristic. These types of systems also tend to use +variable speed pumps that alter their flow characteristics relative to the system load requirements; this assists the self-balancing operation.

Figure 7.9 Effect of two-port valve on pump head and pressure



When selecting a two-port control valve for an application:

� If a hugely undersized two-port control valve were installed in a system, the pump would use a large amount of energy simply to pass sufficient water through the valve. Assuming sufficient water could be forced through the valve, control would be accurate because even small increments of valve movement would result in changes in flow rate. This means that the entire travel of the valve might be utilised to achieve control.

� If a hugely oversized two-port control valve were installed in the same system, the energy required from the pump would be reduced, with little pressure drop across the valve in the fully open position.

However, the initial valve travel from fully open towards the closed position would have little effect on the flow rate to the process. When the point was reached where control was achieved, the large valve orifice would mean that very small increments of valve travel would have a large effect on flow rate. This could result in erratic control with poor stability and accuracy.

A compromise is required, which balances the good control achieved with a small valve against the reduced energy loss from a large valve. The choice of valve will influence the size of pump, and the capital and running costs. It is good practice to consider these parameters, as they will have a bearing on the overall lifetime cost of the system.

These balances can be realised by calculating the 'valve authority' relative to the system in which it is installed.



Valve authority Valve authority may be determined using Equation 7.4.

Equation 7.4 Where:

N = Valve authority

∆P1 = Pressure drop across a fully open control valve

∆P2 = Pressure drop across the remainder of the circuit

∆P1 + ∆P2 = Pressure drop across the whole circuit

Valve authority expresses the ratio between pressure drop across the control valve compared to the total pressure drop across the whole circuit.

The value of N should be near to 0.5 (but not greater than), and certainly not lower than 0.2.

This will ensure that each increment of valve movement will have an effect on the flow rate without excessively increasing the cost of pumping power.

Example

A circuit has a total pressure drop (∆P1 + ∆P2) of 125 kPa, which includes the control valve.

a) If the control valve must have a valve authority (N) of 0.4, what pressure drop is used to size the valve?

b) If the circuit/system flow rate ( ) is 3.61 l/s, what is the required valve Kv?

DP1 DP2



Part a) Determine the ∆∆∆∆P

Equation 7.4

Consequently, a valve ∆P of 50 kPa is used to size the valve, leaving 75 kPa (125 kPa - 50 kPa) for the remainder of the circuit.

Part b) Determine the required KV

Equation 7.2 Where: = 3.61 l/s (13m3/h)

∆P = 50 kPa (0.5 bar)

Alternatively, the water KV chart (Figure 7.2) may be used.



Example A heating coil has a load of 100kW. The coil is supplied with hot water from a boiler at 82°C and which returns to the boiler at 71°C. The coil is controlled by a diverting valve and it has a water pressure drop of 30kPa at the design load. The control valve is required to have an authority of 0.5. Using the Landis and Staefa data sheet extract below select a suitable valve size for this application.

Modulating control valve PN16, M3P...FY c/w magnetic actuator for hot and chilled water with positioning control and position feedback



Q = m c ∆T

where Q is the heat energy put into or taken out of the substance, m is the mass of the substance, c is the specific heat capacity, and ∆T is the temperature differential.

So

Q = 100kW

m = ?

c = 4.194 at 75°C

∆T = 82-71 = 11°C

Rearranging this calculation so we can find m:

m = 2.168kg/s

Which is 0.00222m^3/s or 2.22 l/s

kPaPv

kPaPv

Pv

30

305.0

=∆∴

+∆

∆=

The flow rate is 0.0022m^3/s x3600(second to minutes to hours) = 7.992m^3/hr



7.99m^3/hr

M3P DN32

Kvs 12Or

M3P DN40

Kvs 20

30kpa

7.99m^3/hr

M3P DN32

Kvs 12Or

M3P DN40

Kvs 20

30kpa



Three-port control valves and valve authority

Three-port control valves are used in either mixing or diverting applications. When selecting a valve for a diverting application:

� A hugely undersized three-port control valve will incur high pumping costs, and small increments of movement will have an effect on the quantity of liquid directed through each of the discharge ports.

� A hugely oversized valve will reduce the pumping costs, but valve movement at the beginning, and end, of the valve travel will have minimal effect on the distribution of the liquid. This could result in inaccurate control with large sudden changes in load. An unnecessarily oversized valve will also be more expensive than one adequately sized.

The same logic can be applied to mixing applications.

Again, the valve authority will provide a compromise between these two extremes.

With three-port valves, valve authority is always calculated using P2 in relation to the circuit with the variable flow rate. Figure 7.10 shows this schematically.

Figure 7.10 Valve authority diagrams showing three-port valves



Note: Because mixing and diverting applications use three-port valves in a 'balanced' circuit, the pressure drop expected over a three-port valve is usually significantly less than with a two-port valve.

As a rough guide:

A three-port valve will be 'line sized' when based on water travelling at recommended velocities (Typically ranging from 1 m/s at DN25 to 2 m/s at DN150).

10 kPa may be regarded as typical pressure drop across a three-port control valve.

Aim for valve authority (N) to be between 0.2 and 0.5, the closer to 0.5 the better.

Cavitation and flashing Other symptoms sometimes associated with water flowing through two-port valves are due to 'cavitation' and 'flashing'.

Cavitation in liquids Cavitation can occur in valves controlling the flow of liquid if the pressure drop and hence the velocity of the flow is sufficient to cause the local pressure after the valve seat to drop below the vapour pressure of the liquid. This causes vapour bubbles to form. Pressure may then recover further downstream causing vapour bubbles to rapidly collapse. As the bubbles collapse very high local pressures are generated which, if adjacent to metal surfaces can cause damage to the valve trim, the valve body or downstream pipe work. This damage typically has a very rough, porous or sponge-like appearance which is easily recognised. Other effects which may be noticed include noise, vibration and accelerated corrosion due to the repeated removal of protective oxide layers.

Cavitation will tend to occur in control valves:

� On high pressure drop applications, due to the high velocity in the valve seat area causing a local reduction in pressure.

� Where the downstream pressure is not much higher than the vapour pressure of the liquid. This means that cavitation is more likely with hot liquids and/or low downstream pressure.

Cavitation damage is likely to be more severe with larger valves sizes due to the increased power in the flow. Figure 7.11 shows the damage to an impeller due to the



Figure 7.11 Damage to an impeller due solely to cavitation



Flashing in liquids Flashing is a similar symptom to cavitation, but occurs when the valve outlet pressure is lower than the vapour pressure condition. Under these conditions, the pressure does not recover in the valve body, and the vapour will continue to flow into the connecting pipe. The vapour pressure will eventually recover in the pipe and the collapsing vapour will cause noise similar to that experienced with cavitation. Flashing will reduce the capacity of the valve due to the throttling effect of the vapour having a larger volume than the water. Figure 7.12 illustrates typical pressure profiles through valves due to the phenomenon of cavitation and flashing.

Figure 7.12 Cavitation and flashing through a water control valve

Avoiding cavitation It is not always possible to ensure that the pressure drop across a valve and the temperature of the water is such that cavitation will not occur. Under these circumstances, one possible solution is to install a valve with a valve plug and seat especially designed to overcome the problem. Such a set of internals would be classified as an 'anti-cavitation' trim.

The anti-cavitation trim consists of the standard equal percentage valve plug operating inside a valve seat fitted with a perforated cage. Normal flow direction is used. The pressure drop is split between the characterised plug and the cage which limits the pressure drop in each stage and hence the lowest pressures occur. The multiple flow paths in the perforated cage also increase turbulence and reduce the pressure recovery in the valve. These effects both act to prevent cavitation occurring in case of minor cavitation, or to reduce the intensity of



cavitation in slightly more severe conditions. A typical characterised plug and cage are shown in Figure 7.13.

Figure 7.13 A typical two-port valve anti-cavitation trim

The pressure drop is split between the orifice pass area and the cage. In many applications the pressure does not drop below the vapour pressure of the liquid and cavitation is avoided. Figure 7.14 shows how the situation is improved.

Fig. 7.14 Cavitation is alleviated by anti-cavitation valve trim



Figure 7.15 A typical two-port valve with anti-cavitation trim



Chapter 8, Flow characteristics Aim of Chapter

The proportion of valve movement to the actual orifice opening is based on the profile of the plug and seat of the valve. There are a number of different options available and they part to play on the control characteristics of the system. This chapter focuses on the valve flow characteristics.


On completion of this chapter the student will be able to: (a) Identify the issues the shape of the plug and seat have on the

operational characteristics of the valve



All control valves have an inherent flow characteristic that defines the relationship between 'valve opening' and flow rate under constant pressure conditions. Please note that 'valve opening' in this context refers to the relative position of the valve plug to its closed position against the valve seat. It does not refer to the orifice pass area. The orifice pass area is sometimes called the 'valve throat' and is the narrowest point between the valve plug and seat through which the fluid passes at any time. For any valve, however it is characterised, the relationship between flow rate and orifice pass area is always directly proportional.

Valves of any size or inherent flow characteristic which are subjected to the same volumetric flow rate and differential pressure will have exactly the same orifice pass area. However, different valve characteristics will give different 'valve openings' for the same pass area. Comparing linear and equal percentage valves, a linear valve might have a 25% valve opening for a certain pressure drop and flow rate, whilst an equal percentage valve might have a 65% valve opening for exactly the same conditions. The orifice pass areas will be the same.

The physical shape of the plug and seat arrangement, sometimes referred to as the valve 'trim', causes the difference in valve opening between these valves. Typical trim shapes for spindle operated globe valves are compared in Figure 8.1.

Figure 8.1 The shape of the trim determines the valve characteristic



In this Tutorial, the term 'valve lift' is used to define valve opening, whether the valve is a globe valve (up and down movement of the plug relative to the seat) or a rotary valve (lateral movement of the plug relative to the seat).

Rotary valves (for example, ball and butterfly) each have a basic characteristic curve, but altering the details of the ball or butterfly plug may modify this. The inherent flow characteristics of typical globe valves and rotary valves are compared in Figure 8.2.

Globe valves may be fitted with plugs of differing shapes, each of which has its own inherent flow/opening characteristic. The three main types available are usually designated:

� Fast opening.

� Linear.

� Equal percentage.

Examples of these and their inherent characteristics are shown in Figures 8.2.

Figure 8.2 Inherent flow characteristics of typical globe valves and rotary valves



Fast opening characteristic

The fast opening characteristic valve plug will give a large change in flow rate for a small valve lift from the closed position. For example, a valve lift of 50% may result in an orifice pass area and flow rate up to 90% of its maximum potential.

A valve using this type of plug is sometimes referred to as having an 'on / off' characteristic.

Unlike linear and equal percentage characteristics, the exact shape of the fast opening curve is not defined in standards. Therefore, two valves, one giving a 80% flow for 50% lift, the other 90% flow for 60% lift, may both be regarded as having a fast opening characteristic.

Fast opening valves tend to be electrically or pneumatically actuated and used for 'on / off' control.

The self-acting type of control valve tends to have a plug shape similar to the fast opening plug in Figure 8.1. The plug position responds to changes in liquid or vapour pressure in the control system. The movement of this type of valve plug can be extremely small relative to small changes in the controlled condition, and consequently the valve has an inherently high rangeability. The valve plug is therefore able to reproduce small changes in flow rate, and should not be regarded as a fast opening control valve.

Linear characteristic

The linear characteristic valve plug is shaped so that the flow rate is directly proportional to the valve lift (H), at a constant differential pressure. A linear valve achieves this by having a linear relationship between the valve lift and the orifice pass area (see Figure 8.3).

Figure 8.3 Flow / lift curve for a linear valve



For example, at 40% valve lift, a 40% orifice size allows 40% of the full flow to pass.

Equal percentage characteristic (or logarithmic characteristic)

These valves have a valve plug shaped so that each increment in valve lift increases the flowrate by a certain percentage of the previous flow. The relationship between valve lift and orifice size (and therefore flow rate) is not linear but logarithmic, and is expressed mathematically in Equation 8.1:

Equation 8.1

Where:

= Volumetric flow through the valve at lift H.

x = (ln τ) H Note: 'In' is a mathematical function known as 'natural logarithm'.

τ = Valve rangeability (ratio of the maximum to minimum controllable flow rate, typically 50 for a globe type control valve)

H = Valve lift (0 = closed, 1 = fully open)

max = Maximum volumetric flow through the valve

Example The maximum flow rate through a control valve with an equal percentage characteristic is 10 m3/h. If the valve has a turndown of 50:1, and is subjected to a constant differential pressure, by using Equation 8.1 what quantity will pass through the valve with lifts of 40%, 50%, and 60% respectively? Where:

max = Maximum volumetric flow through the valve = 10 m3/h

H = Valve lift (0 closed to 1 fully open) = 0.4; 0.5; 0.6

τ = Valve rangeability = 50

Equation 8.1



The increase in volumetric flow rate through this type of control valve increases by an equal percentage per equal increment of valve movement:

When the valve is 50% open, it will pass 1.414 m3/h, an increase of 48% over the flow of 0.956 m3/h when the valve is 40% open.

When the valve is 60% open, it will pass 2.091 m3/h, an increase of 48% over the flow of 1.414 m3/h when the valve is 50% open.

It can be seen that (with a constant differential pressure) for any 10% increase in valve lift, there is a 48% increase in flow rate through the control valve. This will always be the case for an equal percentage valve with rangeability of 50. For interest, if a valve has a rangeability of 100, the incremental increase in flow rate for a 10% change in valve lift is 58%.

Table 8.1 shows how the change in flow rate alters across the range of valve lift for the equal percentage valve in the Example with a rangeability of 50 and with a constant differential pressure.



Table 8.1 Change in flow rate and valve lift for an equal percentage characteristic with constant differential pressure

Figure 8.4 Flow rate and valve lift for an equal percentage characteristic with constant differential pressure for the Example

A few other inherent valve characteristics are sometimes used, such as parabolic, modified linear or hyperbolic, but the most common types in manufacture are fast opening, linear, and equal percentage.



Reference

Cupton Guy W., (1989). “HVAC Controls, Operations & Maintenance”, published by Van Nostrand Reinhold, ISBN 0442237324. Curtiss Peter, S., Breth Newton, (2002). “HVAC Instant Answers”, published by McGraw-Hill, ISBN 0071387013. Ashrae Handbook, “Fundamentals”, published documentation of the American Society of heating, refrigeration and air conditioning engineers ASHRAE, ISBN 1-883413877



Glossary of terms

Outside air (OA) - air that is brought into the ventilation system from outside the building and, therefore, has not been previously circulated through the system. Recirculated air (RA) - air that is brought through the ventilation system from inside the building and, therefore, has been previously circulated through the system. VAV Variable Air Volume VFR Variable Flow Rate CAV Constant air volume SA supply air Enthalpy - total heat. The enthalpy control measures both sensible and latent heat in the air and only allows outside air to be used for cooling if the air is both cool and dry enough to satisfy the space conditions. Night purging –the economiser mode is sometimes used at night to cool off a building mass in preparation for the next days cooling load. Low limit (LL) High limit (HL) Thermostatic Expansion Valve ("TEV" or "TXV"). The LLTS is a low level temperature sensor.

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