Lecture 1 - Cleaning and types of screens.ppt

cleaning and types of screen used in grain processing

1. LECTURE -1
Cleaning and grading, aspiration, scalping, size
separation –screens –screen openings –aperture
and their selection

2. Winnowing
Winnowing is an operation done by blowing a current
of air through grain in order to remove the chaff.

3. Classification
Wind
Manual
Power operated

4. Wind winnowing
Anagriculturalmethoddevelopedfortheseparationof
grainsfromchaff.
Itisalsousedtoremoveweevilsorotherpestsfrom
storedgrain.
Winnowinginvolvesthrowingofthreshedmaterialsinto
theairstream,sothatthewindblowsawaythelighter
chaff,whiletheheaviergrainsfallbackdownfor
recovery.
Techniquesincludedusingawinnowingfan(ashaped
basketshakentoraisethechaff)orusingatool(a
winnowingforkorshovel)onapileofharvestedgrain.

5. Manual winnowing
In the manual winnowing, the throwing of threshed
materials will be done by manually and the natural air
stream is used to blow away the light denser
materials.
Some times winnowing fans are used to force the
air, but the throwing of materials will be done
manually.

6. Power operated winnowers
Inpoweroperatedwinnowers,thethreshedmaterials
willbefedintotothewinnower.
Theairstreamwillbeproducedthroughthe
winnowingfanorblowerswillbeused.
Thishelpsinthesegregationofgrainsfromthelighter
materialssuchaschaff,straw,hullsetc.
Inthissystem,theairvelocitycanbeadjusted
accordingtotherequirementofremovaloflighter
materialsthroughtheadjustmentofspeedofthe
winnowingfansorblowers.

7. Operations carried out in
winnowing
Cleaning
Grading
Sorting / Separation

8. Cleaning and grading
Important post harvest operations
Toremoveforeignandundesirablematerial
Toseparategrainsintovariousfractions
Commercialvalueofagricultureproductsdependson
gradefactors
These grade factors further depend upon
Physicalcharacteristics-Size,shape,moisture
content,coloretc.
Chemicalcharacteristics-Odour,freefattyacid
Biologicalcharacteristics–Germination,Insect
damage

9. Cleaning, Grading, Sorting &
Scalping
Cleaningmeansremovalofforeignandundesirable
mattersfromthedesiredgrains
Gradingreferstotheclassificationofcleanedproducts
intovariousqualityfractionstoenhancecommercial
value
Sortingreferstotheseparationofcleanedproductinto
variousqualityfractionsthatmaybedefinedonthebasis
ofcolour,size,shape,density,textureetc.,
Scalpingreferstotheremovaloffewlargeimpuritiesin
aninitialprocess

10. Physical characteristics of grains for
cleaning and grading
Physicalpropertiesofseeds-Size,Shape,specific
gravity,surfaceroughness,colour
Designingofairscreencleaner-shapeandsizeof
graindeterminethescreenopenings,angleof
inclinationandvibrationamplitudeandfrequencyof
screens
Sizeofscreeningsurface-determinedbydensityof
grains
Frontalareasandrelateddiameters-determinationof
terminalvelocity

11. Screening
Itisamethodofseparatinggrain/seedintotwoor
morefractionsaccordingtosizealone
Mostwidelyuseddeviceisscreen
Schematic Diagram of a screen

12. Screen motions
Purposes
Tospreadthematerialoverthesurfaceofscreen.
Tocausethefineparticlestosettleatthesub-
surface.
Todischargetheoversizeparticles.

13. Screen fractions
Materialretainedoverascreeniscalledoversize
material
Materialpassesthroughthescreeniscalled
undersizematerial
Materialpassesthroughaparticularscreenand
retainedoverasubsequentscreeniscalled
intermediatematerial

14. Mesh
Thisisthenumberofaperturesperlinearinch,i.e.number
ofaperturesin25.4mmalongthewire.Ifyoucountthe
numberofopeningsfromonewirealongtheinch
perpendiculartothewire,thenumberyougetisthemesh
ofthatscreen.Screenanalysisdataisgivenineithermesh
oraperturesizes.

15. Aperture
Itistheminimumclearspacebetweentheedgesof
theopeninginthescreeningsurfaceandisgenerally
specifiedininchesormillimeters.
Theopenareaofsquaremeshwireclothmaybe
determinedbythefollowingequation

16. Screen openings
Constructed using
Perforated sheet metal
Woven wire mesh
Perforated sheet
metal
Wire mesh

17. A. Round opening
Openings are measured by the diameter of the opening.
Eg. 1/18 screen has round perforation of 1/18 inch in diameter or 2
mm.
B. Oblong opening
Openings are designated by two dimensions.
i.eby width and length. eg. 1.8 (width) x 20 (length) mm
C. Triangular opening
Openings are measured by the length of each side of the triangle
in mm. Eg. 9 mm.
Perforated sheet metal

18. Oblong Round Triangular

19. Wire mesh
Opening in the wire mesh may be
Square mesh
Rectangular mesh
Square mesh
Openings are measured by the number of openings per
inch in each direction.
Rectangular mesh
Openingsaremeasuredbythenumberofopeningsper
inchineachdirection.Therectanglesareparalleltothe
directionofgrainflow.Eg.3x6mesh

20. Screen motions cause the grains
to
Travel downward to the screen
Stir thoroughly during the passage
Spread the material over the surface of the
screen
Settle the fine particles at the sub-surface
Discharge the oversize particles

21. Physical characteristics used for
separation
Whenaseedlotenterstheprocessingplantforcleaning,
contaminantsareremovedbytheuseofspecial
equipmentthattakesadvantageofdifferencesin
physicalcharacteristicsofvariouscomponentsinthe
mixture.
Thechiefcharacteristicsusedinmakingseparationsare
size,shape,density,surfacetexture,terminalvelocity,
electricalconductivity,color,andresilience.
Manytypesofseed-cleaningmachinesareavailablethat
exploittheabovephysicalpropertiesofseed,either
singlyorinsomecombination.

22. Physical characteristics used for
separation
Sizegrader:airscreencleaner
Length:discorindentedcylinderseparator
Weight:specificgravityseparator
Shape:spiralordraperseparatorforroundandflat
seeds
Surfacetexture:roughfromsmoothsurfaceseed–
doddermill
Colour:electroniccolourseparator
Electricalconductivity:electronicseparator
Affinitytoliquid:magneticseparator

23. Particle motions
Reciprocatory
Horizontal reciprocating motion
Inclined reciprocating motion

24. Vertical and Horizontal reciprocating motion

25. Combined Horizontal and reciprocating motion
Horizontal rotary motion

26. VIBRATORY OR GYRATORY?
Refers to movement of the screener (not the
screen)
Vibratory = shaking (vibrating)
Gyratory = movement in a specific pattern
(gyration in a circular motion)

27. Types of screens
Grizzly
Revolving
screen/Cylindersorter
Shakingscreen
Rotaryscreen
Vibratoryscreen
Horizontalscreen

28. Air screen cleaner
Performstheseparationbasedon
sizealone
Screensusedincombinationwith
airblastperformssatisfactory
cleaningandseparationoperations
formostofthegranularmaterials
Usesthreecleaningsystems;
blowingoraspiration,scalping
screenandgradinglowerscreen
Classifiedintotwotypesbasedon
themovementofthescreening
surface
oVibratoryscreen

29. Working Mechanism of Air-Screen Cleaner
Scalped off material (Trash)
Under size material
Lighter impurity
Cleaned Material

30. AIR SCREEN CLEANER

31. Vibratory screen
Vibratoryscreensareagitatedbyaneccentricunit,materials
areseparatedbyvibration,materialsarealsoagitatedand
separatedduringtheirtransitoverthescreen.
Eccentricityisusuallyoftwotypes,(1)ashafttowhichoff
centreweightsareattached,and(2)Ashaftthatitselfis
eccentricoroffcentred.
Inthelattercasetheeccentricityisbalancedbyaflywheelfor
providinguniformvibration.
Mostvibratingscreensareinclineddownwardfromthefeed
end.
Vibrationisprovidedtothescreenassemblyonlyandthebody
andothersurroundingstructuresareisolatedfromvibration.

32. Thevibrationofscreenisresponsibleforperformanceofthe
followingfunctions.
ithelpsinprovidingpassagetoparticlesthroughthe
openingsofscreen
itrestrictscloggingofthescreenbyparticlesthatbecome
trappedintheopening
becauseofvibrationtheparticlesarestratifiedoverthe
screensurfaceandeachparticlehasachancetomeetthe
screenopeningand
acontinuousflowofparticlesalongthescreenispossible.

34. Rotary screen
Rotaryandgyratoryscreensareeithercircularor
rectangulardecked.
Theirmotionisalmostcircularandaffectssiftingaction.
Thesearecapableofaccurateandcompleteseparation
ofveryfinesizes.
Itscapacityislimited.
Rotaryscreensareclassifiedintotwocategories.
a.Gyratoryscreenb.Circularscreen
a.Gyratoryscreens
Generallysingledeckedscreen.
Ithashorizontalplanemotionwhichiscircularatfeedend
andreciprocatingatthedischargeend.
b.Circularscreens:Itsmotioninhorizontalplaneis
circularovertheentiresurface

35. Double drum rotary screen cleaner

36. Horizontal screen
Thepatternofvibrationforthehorizontalscreen
deckisbackandforthwhiletheinclinedvibrating
screeniscircular.

38. Gyratory screen

39. Shaking screen
Shakerisarectangularsurfaceoverwhichmaterialmoves
downonaninclinedplane.Motionofthescreenisbackand
forthinastraightline.
Althoughinsomecasesvibrationisalsogiventothe
screen.
Unlikethevibratingscreen,theshakerdoesnottumbleor
turnmaterialenrouteexceptthatsomeshakingscreens
haveastep-offbetweensurfaceshavingdifferentsize
openings,sothattheremaybetwoorthreetumblesover
thefulllengthofscreen.
Theshakeriswidelyusedascombinedscreenand
conveyorformanytypesofbulkmaterial.

40. Grizzly

41. Revolving screen/cylinder
sorter
Trommelorrevolvingscreenisacylinderthatrotatesabout
itslongitudinalaxis.
Thewallofthecylinderismadeofperforatedsteelplateor
sometimetheclothwireonaframe,throughwhichthe
materialfallsasthescreenrotates.
Theaxisofcylinderisinclinedalongwiththefeedendtothe
dischargeend.
Sizingisachievedbyhavingsmallestopeningscreenatthe
feedendwithprogressivelylargeropeningscreenstowards
thedischargeend.

42. Thespeedofrotationofthetrommelbekeptwithinthe
limitatwhichthematerialiscarriedfrombottomtoa
distanceequaltotheradiusofcylinderbeforeitstarts
tumbling.
Effectivescreeningarea(notthetotalsurfaceofcylinder)
iscalculatedbymultiplyingthelengthofcylinderby1/3of
thediameter.

43. Ideal and actual screens
Ascreencanbetermedasidealscreenwhichseparates
thefeedmixtureinsuchawaythatthelargestparticleof
underflowisjustsmallerthanscreenopening,whilethe
smallestparticleofoverflowisjustlargerthanthescreen
opening.
Butinpracticenoscreengivesperfectseparationas
statedabove,andiscalledactualscreen.
Theunderflowmaycontainmaterialcoarserthanscreen
size,whereastheoverflowmaycontainsmallerparticles
thanscreensize.

44. Factors influencing the design of air
screen cleaner
Material factors
Compaction or bed density
• Free flow of material is greatly affected by compaction
Particle size and shape
• Deciding factors towards the identification of proper sieve
configuration
Distribution of various fraction sizes
• Straw, chaff, node etc in the mixture greatly influence the
separation
Stickiness and abrasiveness
• Material abrasiveness affects the selection of screen

45. Machine factors
Feeding mechanism
Feeding should be done properly in order to obtain maximum capacity
and efficiency
Size of screen surface
Width of screen affects capacity whereas its length affects efficiency
Length of screen should be between 2-3 times its width
Grain bed depth
All particles should be in contact with the screen
Thickness of bed should not exceed 4 times the size of screen aperture
Thickness of bed should not exceed 2.5 -3 times the size of screen
aperture
Shape of the opening
Depends upon the shape of the grain to be handled
For round shaped grain, top screen of round holes and bottom screen of
slotted holes are used
For elongated grains, slotted top screen and slotted bottom screen are
used
Percentage open area
Higher percentage open area increase both capacity and efficiency

46. Vibration amplitude and frequency
Objective is to properly stratify the feed over the screen
Fast shake speeds are very effective in cleaning chaffy materials
When the speed is too fast, round seeds tend to bounce over the
screen and are not sifted properly
Number of screens
Most efficient screening results when a series of single deck
screens are used
Rate of forward travel
Indicates the travel of material in m/min
Useful in determining the capacity
Air velocity
For removing the lighter particles based on difference in density of
the material
8m
3
/min per ton of paddy upto8 t/h & 5 m
3
/min for capacity more
than 10 t/h are recommended