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Lecture3.

December172007

KamaruddinAbdullahLaboratoryofSolarConversionTechnology

FacultyofEngineering

DarmaPersadaUniversity

[email protected]

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Outlines(Lecture3) Introduction

Basicdryingtheory

Thermophysicalpropertiesasbasisfordryingsystemdesign

Solardrying

Typesofdevelopedsolardryers TestPerformances

Conclusions

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Introduction Highairtemperatureandhumidityulturaland inthe

tropicsmakemanyagriculturalandmarineproductsaresusceptibletorapiddecompositionandtherefore,arenotsuitableforhumanconsumption.

Duepoorpostharvesthandlingaround1220%ofharvestarereportedlosseveryyear

Dryingcanextendshelflifeofproductsbyreducingitsmoisturetoalevelsaveforlongstorage

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Dryingtheory Thetermdryingrefersgenerallytothemoisture

removalfromasubstance.Forexampleawetsolidsuchaswood,grainssuchascoffee,cocoa,roughriceetc. maybedriedbyevaporationofthemoistureeitherintogasstreamorwithoutthebenefitofthegastocarryawayvapor,butmechanicalremovalofsuchmoisturebyexpressionorcentrifugingisnotordinarilyconsidereddrying.

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Terminology(Treybal,1968)Moisturecontent,wetbasis,X, Massofmoisture

withinasubstancedevidedbymassofthatsubstance,expressedin%w.b.

Moisturecontent,drybasis,M, istheratiobetweenthemassofwaterwiththemassofdrymatterinsolid

and

is

expressed

in

percent.(%

d.b.) Equilibriummoisture,Me,expressedcommonlyin

termsof%drybasisisthemoisturecontentofasubstancewhenatequilibriumwithagiventemperatureandRHsurroundingasubstance.

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Terminology Boundmoisture.Isthemoisturecontainedbyasubstancewhich

exertsanequilibriumvaporpressurelessthanthatofpureliquidatthesametemperature. Boundmoistureisthetypeofmoistureheldbychemicalsolutionandincapillarywithinthesolid.

Unboundmoisture.Isthemoisturecontainedbyasubstancewhichexertsanequilibriumvaporpressureequaltothatofpureliquidatthesametemperature. Suchconditioncanbeintheformfreewateronthesurfaceofsubstance.

Freemoisture,istheportionofmoisturenotbeingheldbychemicalreactionwithinthesubstance.Onlyfreemoisturecanbeevaporated, andthefreemoisturecontentofasoliddependsuponthevaporconcentration inthegas.

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ThePsychrometric chart Psychrometric chartdescribes theallimportant

propertiesofdryandmoistairusedduringdrying.Usingthepsychrometric chartonecanstudythedryingprocessinasimpleway.

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12

3

AbsoluteHumidityKg/kgdryair

Drybulbtemperature

Wetbulbtemperature

EnthalpykJ/kg

R

H

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Dryingrate(%/h)

Moisturecontent(%wb)

Constantrateperiod

Fallingrateperiod

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Moisturecontent(%db)

Dryingrate(%/min.)

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S

Basic equationMMe/MoMe=Aexp( kt)..(10a)M(A,Me,K)=A(MoMe)exp( kt)+Me.(10b)

LinearizationusingTaylorexpansion

Mi(A,k,Me)=Mi(A,k,Me)+dMi/dA (Ai)+dMi/dk (Ki)+dMi/dMe (Mei)..(10c)i=1,2.3..nwheredM/dA=(MoMe)exp(kt)dM/dk= Ak(MoMe)exp(kt)dM/dMe=Aexp(kt)+1

Least Square Method

The 1st iteration Let A=Ai, Me=Mei, K=Ki

Substituting into eqs. (10a, 10c) will give the respective values of Ai,Ki,andMei.Me,new=Mei

Mei,A new=AiAiKnew=KiKi

Ai,Ki,andMei

0.004?

No Print Final value of A,K, and Me

End

Determination of drying parameters(Nishiyama, 1973)

yes

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Dryingparameters(cont.) EquilibriumM.C.

Thedryingconstant

&


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Commodity Specific heat,Cp (kJ/kg)

Latent heat of evaporation,Hfg (kJ/kg)

Equilibrium m.c.Me (%,db)

Drying constant, k(1/min.)

1. Coffee berriesDyah W. (1997) Jusuf

(1990)

Cp=0.02125 M + 1.8175For 0.58%

Me =

3.7045+0.11716 t+0.007679 t2

K= exp( 15.432-5976.4t )

2. Cocoa berriesNelwan, 1998.

Hfg/Hfgw=(1+ 0.7297 exp(-0.1361 t Me))at t =55 C and 7%


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Solardryingsimulation

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Energy balance in heat collector to calculate the value of plate temperature Tp,

mp Cpp dTp/dt = Ap(600 sin( t/8)+300) - 1.5 h Ap (Tp - Tr) (1)

Drying chamber temperature, Tr

mr Cpa dTr/dt =mu Cp (Ta-Tr)+1.5 h Ap (Tp -Tr)+hw Aw(Tw-Tr)

-U Ad (Tr-Ta)-Wd (dM/dt) Hfg+hf Af (Tf-Tr) .. (2)

Temperature rice of the floor , Tf, which is painted black

mf Cpf dTf/dt= Ap(600 sin( t/8)+300)-hf Af (Tf-Tr) -k Af (Tf-Tso)/xf (3)

Temperature of hot water tank

mw Cpw dTw/dt= [d(mb)/dt] CV-Uw At(Tw-ta)-hw Aw (Tw-Tr) (4).

The ambient temperature change, Ta

Ta = 4 sin ( t/8) + 28 (5)

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Daytime drying: W=300 kg; mu=0.05

kg/s

0

20

40

60

80

100

0 1

1.5 2

2.5 3

3.5 4

4.5 5

5.5

Time (h)

Tp(C)

Tr(C)

Ta(C)I(t)/10

Series1

GHESolardryingsystem

Integratedthefunctionofsolarheatcollectoranddryingchamberintoonecompartmenttoreduceconstructioncost

Canbedesignwithdifferentconfigurationsaccordingthegeometryandproductholder:

Stationary:

Housetypewith:flatbed,cabinet,drums,trolleys,etc.

Venturytypeortruncatedpyramidsorcones

Recirculationtype:bunker,inclineddryingchambercollector

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Recirculationdryer Dryerwithinclinedcollectordrying

chamber

Airin $

$$

Grain

in

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Workingprinciple:inclined

collectordryingchamber Usespneumatic conveyortorecirculatethegrains

Dryingprocessoccurswhenthegrainfallsintothecollectordryingchambersectionofthedryer

Severaldryingcyclesareneededtoaccomplishdryingprocess

Biomassstovecanbeoperatedforday andnightdrying

Goodforsmalltomediumcapacitydryingofgrains(roughrice,corn,soybeans,etc.)

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Integratedcollectordryingchamber

TheEnergybalance

mg Cpg dTg/dz=hrW(TrTg) m HfgdM/dz IC2...(1)

dTg/dz=z1(TrTg)+z2dM/dzz3I........................................(2)

WaCpadTr/dz= hcW(TcTr)+(Ua+Ub)W(TrTa)+

hrW(TrTg)+mHfgdM/dz......(3)

dTr/dz=z4(TcTr)+z5(TrTa)+z6(TrTg)+z7dM/dz)............(4)

Massbalance

mM|zmM|z+z= (maCpT|z maCpT|z+z)/Hfg or

Limz>0

dmM/dz=madTr/dz/Hfg ....................................................(5)

Mdm/dz= madTr/dz/Hfg mdM/dz............................(6)

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z

z

z+z

Ua

Ub

I(t)mg

ma


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Dryingofcorninrecirculationdryerinclined

dryingchambercollectorsystem

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Solarrecirculationdryer

GHEtypewithbunkerfortemporaryinstoredryer

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Airin

Workingprinciple: Bunkertype

Usespneumatic conveyortorecirculatethegrainsandvortex

Dryingprocessoccurswhenthegrainfallsintoacyllinricaltypeheatexchangerlocatedatthecenterofthedryingchamber

Severaldryingcyclesareneededtoaccomplishdryingprocess

Biomassstovecanbeoperatedduringthenightofbadweather

Goodformediumtolargecapacitydryingofgrains(corn,roughrice,soybeans,etc.)

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Experiments: Temporaryinstoredryer

Capacity :

266.4

kg/h Dimension : 2,28m,height 3,09m Conveyor :CentrifugalBlower

Model :CZR 200 Voltage : 220Volt, 50Hz Volume :450m3/h Size : 60mm Pressure :1200Pa Power Type :ElectricMotor,0.25kW Model :YY632 2 RPM :2840rpm Voltage :220Volt,50Hz,1,9A,

Auxiliaryheater Type :Biomassstove Fuel :Charcoal,sawdust

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Sample:RoughriceIR64 AverageMC : 23,6 %wb

Degreeofcleanliness : 97,2 %

Dimension(LxWxth) : 9,87 1,94 2,37mm

Bulkdensity : 463,92 gr/cm3

Intactgrains : 97,8%

Persentageofcrackedgrains : 2,2%

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TestresultsconductedbyMinistryofAgriculture:

Bunkertypesolardryer(Prototype)

1. Intial mass of grain

2. Drying time (hr)

3. Mass of grain after 10 hr drying

4. Drying rate

5. Average temperature at top section

6. Average temperature at the middle section

7. Temperature of polycarbonate wall

8. Increase in cracked grains

9. Homogenity in m.c.

10. Fuel use

~ charcoal (18 kg)

~ Solar irradiation (1252,67 Wh)

: 155 kg

: 10

: 14 0 kg

: 0,74 %/hr

: 40,34 0C

: 38,12 0C

: 40,65 0C

: 1,4 %

: 0.05 % 0.51 %

: 4371 0,19 kJ/hr

: 99%

:1%

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Experiment2:inclinedcollectordrying

chamber(Prototype) OverallDimension: : L:2630mm,W:1530mm,H:2520mm

Holdingcapasity :93,31kg

Recirculationcapacity :42,12kg/h

Pneumaticconveyor :CentrifugalBlower,0,25kW,22V,ACTypeYY6322,RPM:2840

Coveyorpipe:

Diameter :420mm

Length :1080mm

Dryingchamber:

Dimension :2270 1080mm

Policarbonatethickness:1,2mm

Blackenedmetalsheetthickness :0,5mm

Inclinationfromh or izo nta l :200

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Testresultsinclinedcollectordryingchamber

1. Initial mass

2. Effective drying time

3. Final mass after 7 hrs drying

4. Drying rate

5. Temperature in heating chamber

6. Temperature of drying air

7. Temperature of absorber

8. Increase of cracked grains

9. Homogenity in m.c.

10. Energy consumption

~ Charcoal (12 kg)

~ Solar radiation (161,26 Wh)

: 24 kg

: 7 hr

: 12 kg

: 1,03 %/hr

: 67,41 0C

: 47,11 0C

: 51,56 0C

: 2,4 %

: 0.04% 0.41 %

: 34690,63 kJ/hr

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Conclusions

1) SeveraldesignconfigurationsofGHEsolardryerhavebeendevelopedinIndonesiaandmanyhavebeendistributedthroughoutthecountry

2) ThedryersmaybeusedasthemaincomponentofaSPU

3) LaboratoryandfieldtestresultshaveshownthatthedevelopedGHEsolardryerscanbeusedtodryfoodcrops(roughrice,corn,soybeans),estatecrops(coffeeandcocoabeans, cloves, pepper,etc.,marineproducts(avarietyoffish,seaweeds,fishcrackers)

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