Dryers.ppt

Type of grain dryers

1. Lecture 8
Graindrying–principles-advantages-types-
batchandcontinuous,mixingandnonmixing–
LSUdrier–constructionandoperation-heat
sources-performanceofdryer

2. Drying
Graindyingistheprocesstoreducethemoisture
fromgraintoasafelevelforstorageandhandling
followingharvest.
Itisacriticalstepformaintaininggrainqualityand
minimizestorageandprocessinglosses.
Inaddition,thelongerthedesiredgrainstorage
period,thelowertherequiredgrainmoisturecontent
mustbe.
Heatandmasstransferprocess

3. Grains are hygroscopic –gain or lose moisture as
per atmospheric conditions
Moisture migration into or from a product is
dependent on the difference in vapour pressure
between atmosphere and product.
Moisture remains inside grains in two forms –
bound and unbound

4. Methods of Drying
Threebasicmethodsofdryingareusedtoday
1)sundrying,atraditionalmethodinwhichmaterials
drynaturallyinthesun
2)hotairdryinginwhichmaterialsareexposedtoa
blastofhotairand
Thefundamentalnatureofalldryingprocessistheremoval
ofvolatilesubstances(mainlymoisture)frommixtureto
yieldasolidproduct.
Ingeneraldryingisaccomplishedbythermaltechniques
andthusinvolvestheapplicationofheat,mostcommonlyby
convectionfromcurrentofair.

5. Throughouttheconvectivedryingofsolid
materials,twoprocessesoccursimultaneously
namely,transferofenergyfromthelocal
environmentinthedryerandtransferofmoisture
fromwithinthesolid.
Thereforethisunitoperationmaybeconsidered
assimultaneousheatandmasstransfer
operation.
Thehotair/gassuppliestheenergyrequiredfor
dryingandalsocarriesawaythemoisture
releasedbythesolid.

6. Drying mechanism
Twofundamentalstepsoccurduringdrying
1.Watermigrationfromgraininteriortothesurface.
2.Waterevaporationfromthesurfacetothe
surroundingair.
Water
movement

7. Inpaddygrain,moistureispresentattwoplaces:atthe
surfaceofthegrain,surfacemoistureandinthekernel,
internalmoisture.
Surfacemoisturewillreadilyevaporatewhengrainis
exposedtohotair.
Internalmoistureevaporatesmuchslowerbecauseit
firsthastomovefromthekerneltotheoutsidesurface.
Asaresult,surfacemoistureandinternalmoisture
evaporateatadifferentrate.
Thisdifferenceresultsinadifferentdryingrate;therate
atwhichgrainmoisturecontentdeclinesduringthe
dryingprocess.

8. Drying rate periods
Divided into 3 periods
Constant rate period -fruits and vegetables
First Falling rate period -grains
Second falling rate period

10. Constant rate period
Somecropsathighmoisturecontent(>70%)
aredriedunderconstantrateperiodattheinitial
periodofdrying.Fallingrateperiodfollows
subsequently.
Thisperiodcontinuestilltherateofevaporation
ofwaterfromthesurfaceisequaltotherateof
moisturemigrationfrominteriorofproducttoits
surface.
Criticalmoisturecontentisreached

11. Falling rate period
Cerealgrainsareusuallydriedentirelyunderfalling-rate
period.
Falling-rateperiodentersaftertheconstantdryingrateperiod
Itcorrespondstothedryingcyclewhereallsurfaceisno
longerwettedandthewettedsurfacecontinuallydecreases,
untilattheendofthisperiodthesurfaceisdry.
Thecauseoffallingoffintherateofdryingisduetothe
inabilityofthemoisturetobeconveyedfromthecentreofthe
bodytothesurfaceataratecomparablewiththemoisture
evaporationfromitssurfacetothesurroundings.
Thefalling-rateperiodischaracterizedbyincreasing
temperaturesbothatthesurfaceandwithinthesolid.

12. Firstfallingrate
Becauseofdecreaseinwetsurfaceareathe
dryingratedecreases
Asdryingproceeds,thefractionofwetsurface
decreasestozero,wherefirstfallingrateperiod
ends

13. Second falling rate
Subsurfaceevaporationtakesplace&itcontinues
untiltheequilibriummoisturecontentisreached
Inthiscase,evaporationtakesplacefromwithin
thesolidandthevapourreachesthesurfaceby
moleculardiffusionthroughthematerial.
Theforcescontrollingthevapourdiffusion
determinethefinalrateofdrying,andtheseare
largelyindependentoftheconditionsoutsidethe
material.

14. Classification of dryers
Itcanbeclassifiedbasedonmodeofoperation
suchasbatchorcontinuous.
Incaseofbatchdryerthematerialisloadedinthe
dryingequipmentanddryingproceedsforagiven
periodoftime.
Whereas,incaseofcontinuousmodethematerialis
continuouslyaddedtothedryeranddriedmaterial
continuouslyremoved.

15. Based on mode of heat transfer
Heat always moves from a warmer place to a cooler
place.
Heat transfers in three ways:
Conduction
Convection
Radiation

18. Based on movement of material
inside the dryer
Mixing type –material flows in a different path
inside the drying chamber
Non –mixing type –material flows in a straight
path

19. Methods of Drying
Sun drying
Mechanical drying

20. Traditional methods of drying
Sun Drying
Sundryingisthetraditionalpracticefordryingwheregrain
isexposedtosunandwindandisstillpreferredinAsia
becauseofitslowcostcomparedtomechanicaldrying.
Sundryinghassomelimitations:
Itisnotpossibleduringrainandatnight.
Anydelayleadstoexcessrespirationandfungalgrowth
causinglosses
Itislaborintensiveandhaslimitedcapacity.
Temperaturecontrolisdifficult.Overheatingorre-wetting
ofgrainscanresultinlowmillingqualityasaresultof
cracksdevelopinginthekernels.

21. Methods of sun drying
Drying of standing crops
Drying of grains on stalk
Drying of threshed grains

22. Drying of standing crops or Field
drying
Grainsaredriedontheplanttill
propermoisturecontentisattained.
Then,thecropisharvestedand
threshed.
Butconsiderablequantityislostdue
toshattering.
Thisdryingprocessisslowand
takesabout2-3weeksafterthe
grainshaveattainedbiological
maturity.
Mayaccumulateheatandmoisture
whichcouldcausemoldgrowth
Rapidqualityreduction

23. Drying of grains on stalk or Panicle
drying
Cropisharvestedathigher
moisturecontentandisleftin
thefieldtillithasdriedtoproper
moisturecontent
Anothermethodistodrythe
harvestedcroponracks
Cropisbundledandhungona
ropeexposingtothesun
Lowcapacity
Labourintensive
Unevendrying:outsidedries
faster

24. Drying of threshed grains or Drying
on mat or canvas
Harvestedcropathighermoisturecontentisthreshed
andthegrainsarespreadonthefloorin1to3cm
thicklayer
Continuouslystirredmanuallytillithasattainedthe
desirablemoisturecontent
Graincanbeplacedonnets,matsorcanvas
Lesscontaminatedbystonesanddirt
Easymixingandcollection
Needsmallinvestmentonmatorcanvas

25. Crib drying
Cobmaizecanremainhighmoisturecontent(20%)with
naturalventilation.
Maizecribactsasbothadryerandastoragestructure
Dryingratesarecontrolledbyrelativehumidityand
movingspeedofthesurroundingair

26. Solar Drying
Solardryingusessunenergybutisperformedinasolar
dryerdesignedandbuiltforthispurpose.
Solardryersaredevicesthatusesolarenergytodry
substances,especiallyfood.
Theinsideofthedryerislinedupwithablackabsorbing
materialorpaintedblack.
Sunlightentersanenclosedchamber(suncollector)where
itstrikesadarksurfaceconvertingitsenergytoheat.
Coldairflowsinatthebottombutthesunrayswillheatup
thedryingchamberandtheairinit.Warmairrisesupand
leavesthechamber.

27. Airflowisaccomplishedbyanaturaldraft(warmairrises)
whichcanbecontrolledbyadjustingvents.
Thisnaturalairflowdependsonthedifferenceinheight
(pressure)betweenfreshairintakeandhotairexhaust.
Thebiggerthedifferencethefastertheairflow.
Thesuncollectorandthedryingchambercanbe
combinedinoneunitorseparated.Whenthecollectoris
freestanding,theheatitgeneratesisdirectedintothe
dryingchamber.
Solardehydratorsmayhavebackupelectricheatingto
provideheatwhenthesunisnotaround.

29. Tent dryer

32. Advantages of solar dryer
Thehighertemperature,movementoftheairandlowerhumidity
increasestherateofdrying.
Foodisenclosedinthedryerandthereforeprotectedfromdust,
insects,birdsandanimals.
Thehighertemperaturedetersinsectsandthefasterdryingrate
reducestheriskofspoilagebymicroorganisms.
Thehigherdryingratealsogivesahigherthroughputoffoodand
asmallerdryingarea(approximatelyonethird).
Thedryersarewaterproof,therefore,thefooddoesnotneedto
bemovedduringraining.
Dryercanbeconstructedfromlocallyavailablematerialsandare
relativelylowcost.

33. Limiting issues with solar dryer
Canbeonlyusedduringdaytimewhenadequate
amountofsolarenergyispresent.
Lackofskilledpersonnelforoperationand
maintenance.
Lessefficiencyascomparedwithmoderntypeof
dryers.
Abackupheatingsystemisnecessaryfor
productsrequirecontinuousdrying.

34. Mechanical drying depends on modes of heat
transfer
Convectivedryingmethod
Heatedgaseousmedium(usuallyair)isventilated
throughamassofwetmaterialsandcarrieswithitthe
water-vapourevaporatedfromthematerial
Tosaveenergy,sometimestheexitairfromdryerisre-
circulated.
Sometimes,thepartiallydriedmaterialisre-circulatedfor
furtherdrying
Insuchsituationthesystemisknownasre-circulatory
dryer.

35. Thin layer drying
Processinwhichallgrainsarefullyexposedtothedryingair
underconstantdryingconditionsi.e.atconstantairtemperature
&humidity.
Upto20cmthicknessofgrainbedistakenasthinlayer
Allcommercialdryersaredesignedbasedonthinlayerdrying
principles
RepresentedbyNewton’slawbyreplacingmoisturecontentin
placeoftemperature
M-M
e/M
o-M
e= e
-Kθ
M –Moisture content at any time θ, % db
M
e-EMC, %db
M
o–Initial moisturecontent, %db
K –drying constant
θ -time, hour

36. Deep bed drying
Indeepbeddryingallthegrainsinthedryerarenotfully
exposedtothesameconditionofdryingair.
Theconditionofdryingairatanypointinthegrainmass
changeswithtimeandatanytimesitalsochangeswiththe
depthofthegrainbed.
Overandabovetherateofairflowperunitmassofgrainissmall
comparedtothethinlayerdryingofgrain.
Allonfarmstaticbedbatchdryersaredesignedondeepbed
dryingprinciple.

38. 1Motor
2Panel Board
3Temperature Indicator
4Hot air outlet
5Tray
6Door
Tray or Cabinet
Dryer

39. Tunnel dryer
Theheateddryingairisintroducedatoneendofthetunneland
movesatanestablishedvelocitythroughtraysofproductsbeing
carriedontrucks.
Theproducttrucksaremovedthroughthetunnelatarate
requiredtomaintaintheresidencetimeneededfordehydration.
Theproductcanbemovedinthesamedirectionastheairflow
toprovideconcurrentdehydrationorthetunnelcanbeoperated
incountercurrentmanner.

41. Withconcurrentsystems,ahigh-moistureproductis
exposedtohightemperatureair,andevaporation
assistsinmaintaininglowerproducttemperature.
Atlocationsnearthetunnelexit,thelower-moisture
productisexposedtolower-temperatureair.
Incountercurrentsystems,alower-moistureproduct
isexposedtohigh-temperatureair,andasmaller
temperaturegradientexistsneartheproductentrance
tothetunnel.

42. Rotary dryer
Therotarydrierisbasicallyacylinder,inclinedslightlytothe
horizontal,whichmayberotated,ortheshellmaybestationary,
andanagitatorinsidemayrevolveslowly.
Ineithercase,thewetmaterialisfedinattheupperend,andthe
rotation,oragitation,advancesthematerialprogressivelytothe
lowerend,whereitisdischarged.
Indirect-heatrevolvingrotarydriers,hotairoramixtureofflue
gasesandairtravelsthroughthecylinder.
Thefeedrate,thespeedofrotationoragitation,thevolumeof
heatedairorgases,andtheirtemperaturearesoregulatedthat
thesolidisdriedjustbeforedischarge.

43. Astheshellrevolves,thesolidiscarriedupwardone-fourthofthe
circumference;itthenrollsbacktoalowerlevel,exposingfresh
surfacestotheactionoftheheatasitdoesso.
Theefficiencyisgreatlyimprovedbyplacinglongitudinalplates3or
4in.wideontheinsideofthecylinder.Thesearecalledlifting
flights.
Thesecarrypartofthesolidhalf-wayaroundthecircumferenceand
dropitthroughthewholeofadiameterinthecentralpartofthe
cylinderwheretheairishottestandleastladenwithmoisture.
Bybendingtheedgeofthelifterslightlyinward,someofthe
materialisdeliveredonlyinthethirdquarterofthecircle,producing
anearlyuniformfallofthematerialthroughoutthecrosssectionof
thecylinder.
Theheatedairstreamsthrougharainofparticles.Thisisthemost
commonformofrevolvingrotarycylinder.Ithashighcapacity,is
simpleinoperation,andiscontinuous.

44. Rotary dryer
Ideallysuited
for large
capacity
applications
anduneven
particlesize
distribution
Continuous
operationand
versatile
application
Lowoperating
&
maintenance

45. Desiccated air drying
Inthismethodofdrying,theambientairispassedthrougha
desiccatedmediumorsourcee.g.silicagel.
Thedesiccatorabsorbsmoisturefromtheair,asaresult,the
relativehumidityofairisreducedandatthesametimethereis
anincreaseinitstemperature.
Whensuchaircomesincontactwithwetgrains,transferof
moisturefromwetgrainstodryingairtakesplace.
Thedryingactionoccursduetoconvectiveheattransfer.

47. Fluidized bed drying
Fluidizedbeddryerconsistofasteelshellofcylindricalor
rectangularcrosssection.
Agridisprovidedinthecolumnoverwhichthewetmaterialis
rests.
Inthistypeofdryer,thedryinggasispassedthroughthebedof
solidsatavelocitysufficienttokeepthebedinafluidizedstate.
Mixingandheattransferareveryrapidinthistypeofdryers.
Sinceeverysurfaceofproductisincontactwithdryingair,
uniformdryingof'productstakesplace.

48. Thedryercanbeoperatedinbatchorcontinuousmode.
Fluidizedbeddryeraresuitableforgranularandcrystalline
materialsandnormallyusedforthematerialswhichhavehigh
initialmoisturecontentandarelighterandatthesametime
requirestobedriedquicklysuchasvegetableseeds.
Themainadvantageofthistypeofdryerare:rapidanduniform
heattransfer,shortdryingtime,goodcontrolofthedrying
conditions.

50. Incaseofrectangularfluid-beddryersseparatefluidized
compartmentsareprovidedthroughwhichthesolidsmovein
sequencefrominlettooutlet.
Theseareknownasplugflowdryers;residencetimeisalmost
thesameforallparticlesinthecompartments.
Butthedryingconditionscanbechangedfromonecompartment
toanother,andoftenthelastcompartmentisfluidizedwithcold
gastocoolthesolidbeforedischarge.

51. Bin dryer
On-farmgraindrying
Binisfilledwithgrainand
dryingairisforcedup
throughthegrainfroma
plenumchamberbeneathh
theperforatedfloorofthe
bin.
Thegrainonthebottomis
driedfirst.
Dryingfront-layerofdrying
grainseparatesthedried
grainfromtheundried
grain.
Thisdryingfrontprogresses
upwardthroughthebinof

52. Flat bed dryer
Similar to deep bed dryer except that the surface
area is more and depth is less
Capacity –1 to 2 tonnes
Designed for farm level operation
Grains are spread 0.6 to 1.2 m deep over the
perforated floor and dried.

54. Continuous flow dryers
Columnar type in which wet grains flow from the top
to bottom of the dryers
Types –mixing and non-mixing
If the grains flows in a straight path it is non –mixing
Grains are diverted in a zig zag manner it is mixing
type
Non –mixing : Flow rate –125 to 250 m
3
/ min-tonne
at 54
o
C
Mixing : Flow rate –50 to 95 m
3
/ min-tonne at 65
o
C

55. MIXING TYPE DRYER

56. NON -MIXING TYPE
DRYER

57. LSU dryer
Developed at Louisiana state university (LSU)
Continuous mixing type dryer
Developed specifically for rice to ensure gentle treatment,
good mixing & good air to grain contact
Consists of rectangular chamber, holding bin, blower with
duct, grain discharging mechanism and air heating system
Layers of inverted V shaped channels are installed in the
drying chamber; heated air is introduced through these
channels at many points
Alternate layers are air inlet & outlet channels; arranged
one below the other in an offset pattern

58. Contd…
Inlet port consists of few full size ports & two half size ports; all
ports are of same size arranged in equal spacing
Ribbed rollers are provided at the bottom of drying chamber for
the discharge of grain
Capacity varies from 2-12 tonnes
Recommended air flow rate is 60-70 m3/min/tonne
Air temp. are 60 &85°C for raw & parboiled paddy
Uniformly dried product can be obtained
Can be used for different types of grain
High capital investment

61. Recirculatory dryer
Multi pass continuous flow dryer
During each pass grains are exposed to a hot air
temperature for short time (15-30) min at a air
temp of 60-80
o
C
Between drying passes grain is stored in a
tempering bin for 4 to 24 hours

62. Recirculatory Batch dryer
Continuous flow non mixing type
Consists of 2 concentric circular cylinders, set 15-20 cm apart
Bucket elevator is used to feed & recirculate the grain
Centrifugal blower blows the hot air into the inner cylinder, acts as
a plenum
Grain is fed at the top of the inside cylinder; comes in contact with
a cross flow of hot air
The exhaust air comes out through perforations of the outer
cylinder
Grain is recirculated till it is dried to desired moisture content
Drying is not uniform as compared to mixing type

64. Recirculating Batch
Dryer Coupled with
Husk-fired Furnace
for Paddy Drying, IIT,
Kharagpur

65. Grain drying in bags
Used for drying small quantities
Requires large number of unskilled labourers
More space needed because bags of grains are put
on perforated racks
Hot air is forced through racks and bags
Flow rate of 4 709 m
3
/ min-bag at 45
o
C
Bags are inverted once during drying period to
accomplish drying on both sides

66. Method CropFlow DryingTechnology Characterization
Fielddrying Piles,racks Rapid quality reduction
SundryingBatch Drying pavements or matsCheap
Labor intensive
Typically poor milling quality
Heated air
drying
Batch Fixedbeddryer Inexpensive, small scale operation
possible
Local construction from various
materials
Operation with unskilled labor
Moisture gradient
Labor intensive
Re-circulating batch dryerMixing of grain
Large capacity range
Good quality
Skilled laborers required
Medium capital investment
After-sales service requirement
Wear of moving components
Continuous Continuousflowdryer Large capacity
Economics of scale
High capital investment
Not feasible for small batches of
different varieties
Complicated
In-Store
Drying
Batch Storage bin with aeration
components and pre-
heater for adverse weather
and nighttime
Excellent grain quality
Large capacity range
Pre-drying of high moisture grain
Risk of spoilage during power
failure

67. Performance of the dryer
Overall thermal efficiency = Amount of heat
utilised x 100
Amount of heat
supplied
Amount of heat utilised = (Amount of water
vapour removed, kg) x (Latent heat of
evaporation, kcal/kg)
Amount of heat supplied = (Amount of fuel, kg) x
(Net calorific value of fuel, kcal/kg)

68. Heat utilisation factor (HUF)
The utilisation factor of a drying system is the ratio
of drop in temperature of drying air by drying
process and increase in the temperature of
ambient air by heating
HUF = t
2–t
3
t
2–t
1
t1 = temperature of ambient air ,
o
C
t2 = temperature of hot air,
o
C
t3 = temperature of exit air,
o
C