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