Transport Phenomena

Properties, Handling and Mixing of Particulate SolidsBySidra Jabeen

Department of Chemical Engineering,University of Engineering & Technology Lahore 2SCREENING

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Screen / SieveScreen is an open container usually cylindrical with uniformly spaced openings at the base. It is normally made of wire cloth, the wire diameter and interspacing between wires is carefully controlled.

4Screening is a method of separating particles on the basis of their size.

Aperture size of screenMesh numberScreen intervalMaterial flow through screen

Screening and its Terminology

5Aperture Size of screen

The size of a square opening (length of clear space between individual wires) is called the aperture size of screen.

Screening Terminology

6Mesh number of screen

Screens are designated by their mesh number. Mesh no. indicated the number of apertures/openings per linear inch of the screen. E.g. A screen having 10 square openings per inch is said to have mesh no. 10Higher the mesh no., smaller the aperture size of screen.

Screening Terminology

7Screen Interval

Screen interval is a factor by which aperture size of a screen is to be divided to get the aperture size of next successive screen. The ratio of actual aperture size of any screen to that of the next smaller screen is 2 = 1.41.The area of openings in any one screen in the series is exactly twice that of the openings in the next smaller screen.

Screening Terminology

8FeedUnderflowOverflowMaterial flow through Screen

Overflow: The material that is rejected by the screen.Underflow: The material that is passed through the screen. Screening Terminology

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Material flow through Screen

Overflow (oversize particles) Underflow (undersize particles) Oversize particles -> Plus material Undersize particles -> Minus Material Two numbers are needed to tell the size range of the increment; one for the screen through which the material passes and the other on which it is retained. E.g. 14 / 20 or -14 + 20 The average size of the particles in the increment will be the arithmetic average of the aperture sizes of two screens. Screening Terminology

10Tayler Standard Screen Series

Square shape opening The area of the openings in any one screen in the series is exactly twice that of the openings in the next smaller screen. The ratio of the actual mesh dimension of any screen to that of the next smaller screen is = 2 = 1.41. Based on 200 mesh No. screen Usually 5-6 screens are arranged in a stack for closer sizing intermediate screens are available (with mesh dimension 1.189) Shaken mechanically for a definite time

11Tayler Standard Screen Series

Size of the upper screen must be larger by a factor 2 and that of lower screen must be smaller by a factor 2

i 1 - - - - - - - - - - - - - Di 1 i - - - - - - - - - - - - - Di i + 1 - - - - - - - - - - - - - Di + 1

Minimum particle diameter = Dpi (min) = Di Maximum particle diameter = Dpi (max) = Di-1 = 2 Di Particle Size Range = 2 Di Di = 0.4142 DiMean particle size = Dpi, mean = (Di + Di-1) / 2 = (2+1)Di / 2

12Screen analysis is carried out using number of screens so that aperture size reduces for lower sieve

Screens are arranged serially in a stack The smallest mesh at the bottom and the largest at the top Materials are loaded at top and then shacked for a period of timeMaterials are collected from every screen and weighed. How Screen / Sieve Analysis is done?

13Particle Sizing for top most screen

1. If the particles are large enough with appreciable concentration (mass fraction) so that their average size could easily be measured with the help of thread and meter rod. Then few representative particles are chosen 5 to 6 prominent dimensions of each particle is measured so that its average size is known. 14Particle Sizing for top most screen

If concentration (mass fraction) is appreciable and particles are small, then imaginary sieve immediately above the screen under consideration in T.S.S.S is used and the arithmetic mean of clear opening of two screens is used as representative size of material present over top most screen.

If concentration is negligible , the top most screen may be neglecting for sizing

15Particle Sizing for top most screen

4. If concentration is large enough with relatively wide variation in sizes of particles then 2 to 3 imaginary screens are assumed and then material is distributed over those screen(equal weight distribution, experience based distribution, graphical approach, computer simulation) 16Particle Sizing for bottom most screen (Pan)

1. If concentration is negligible , the bottom most screen may be neglecting for sizing

2. If concentration is small, however particles are nearly of same size then arithmetic mean of clear opening of the screen above pan and imaginary screen below it is taken.

17Particle Sizing for bottom most screen (Pan)

3. If concentration is large enough with relatively wide variation in sizes of particles then 2 to 3 imaginary screens are assumed and then material is distributed over those screen(equal weight distribution, experience based distribution, graphical approach, computer simulation) 18 Differential Analysis

Cumulative AnalysisRepresentation of results of Screen Analysis

19Differential and Cumulative Analysis

Mesh no. of ScreenAperture size of screenDpi (mm) Average Dia. of the particle Dpi (Avg.) (mm)

Mass Fraction x iCumulative Mass Fraction smaller than Dpi44.50-0163.193.840.020.9882.262.720.050.93101.61.930.10.83141.131.360.180.65200.80.960.250.4280.570.680.250.15Pan--0.15020

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23Factors affecting Screening

Orientation of particlePresence of near mesh particles Shape of particles Size of ParticlesMotionNature of particlesHumidity or moisture

241. Orientation of Particle

Overall probability of passage of one particleNo. of times particle strikeProbability of passage during single strike Angle of approachPerpendicular - larger the chance

Endwise min. contact area high probability of screeningSidewise max. contact area low probability of screening

252. Presence of near mesh particles

Particles having size very close to the aperture size of the screen.They may pass through the screen in any particular configuration.They may cause clogging or blinding of the screen.263. Shape of particles

Regular shape (spherical) screening is easy and efficientIrregular shape screening is difficult (they may pass through the screen in one particular direction or may retain on the screen in any other direction)

274. Size of particles

Coarse particles screening is difficult

Fine particles screening is easy

Ultrafine particles loss as dust

285. MotionThe purpose of induced motion is to enhance the probability of particles to strike on screen surface

Jolting actionSifting action

Too high reduces efficiency as particle bounce backIt reduces the screen blindnessHigh vibration at high feed rate is used

296. Nature of feed particlesSoft / porous particles cohesive in nature size enlargementHard / rigid better screening impact may cause screen failure

307. Humidity / moistureGreater the moisture cohesiveness size enlargement reduction in available screen surface

31 Screen BlindnessBridgingCohesiveness Size enlargement reduction in available screen surfaceClogging Irregular shape reduction in flow area of screen

32Comparison of IDEAL & ACTUAL screen

An ideal screen would sharply separate the feed mix in such a way that the smallest particle in overflow would be just larger than the largest particle in underflow

Ideal separation defines a cut diameter Dpc , the point of separation between oversize and undersize fractions and is equal to aperture size of the screen.

Actual screens dont perform a perfect separation about the cut diameter