GREENHOUSE DRYING

GREENHOUSE DRYING

1. BIRSA AGRICULTURAL UNIVERSITY
Protected Cultivation and Secondary
Agriculture
LECTURE 11: GREENHOUSE DRYING
BY
DR. PRAMOD RAI
DEPARTMENT OF AGRICULTURAL ENGINEERING

2. Drying
Freshagriculturalproducehavethetendencyto
getspoiledaftertheharvest.
Dryingisoneoftheoldestandeasiestmethods
offoodpreservation.
Itistheprocessofremovingwaterormoisture
fromafoodproduct.
Theremovalofmoisturefromfoodsmakes
themsmaller,lighterandlessvoluminous.

3. Principle of Drying
Foodscanbespoiledbymicroorganismsor
throughenzymaticreactionswithinthefood.
Bacteria,yeast,andmoldsmusthavea
sufficientamountofmoisturearoundthemto
growandcausespoilage.
Reducingthemoisturecontentoffood
preventsthegrowthofthesespoilage-causing
microorganismsandslowsdownenzymatic
reactions.

4. The Process of Drying
Heatistransferredtothefood.
Wateristransferredfromthefoodintothe
dryingenvironment.
ItisprocessofHeatandMasstransfer.
Fig.1: Internal and external mass transfer in drying

5. PredryingTreatments
Selection ( Fresh ,
Undamaged)
Cleaning (Washing and Disinfection)
Preparation ( Peeling ,
Slicing)
Pretreatment (Sulfurization , Salting,
Blanching)
Drying
Packaging
Storage (Shelf life)
Fig.2: Flow chart of predryingtreatments

6. Factors affecting Drying Process
External parameters
Solar radiation
Temperature
RH
Air velocity
Internal parameters
Initial moisture content
Type of crops
Size and composition of food materials
Crop absorptivity
Exposed surface area

7. Slideshare.net
a b(critical moisture content)
Fig.3: Drying rate curve with time

8. Slideshare.net
Methods of Drying
Conventional drying
Tray drying
Vacuum tray drying
Freeze Drying
Spray drying
Micro wave /radio Frequency /infrared drying
Others
Solar Drying
Open sun drying.
Greenhouse Drying

9. Slideshare.net
Advantages and Disadvantages of Conventional Drying
Advantages
Continuousorbatchprocess,maximumproductcan
bedriedinlesstime.
Controloveroperatingconditionofdrying.
Canbeusedroundtheyear.
Disadvantages
High cost investment.
High energy consumption.
High maintenance cost is required etc.

10. Slideshare.net
SolarDrying
Adeviceusedforthepurposeofsolardrying
isknownassolardryer.
Solarenergy,renewablesourcecanplayan
effectiveroletomeetenergydemand.Among
all,solarenergyismostreliable,abundant,
inexhaustibleandenvironmentalfriendly.
Wecanuseforpumping,dryingofagricultural
produceetc.
.

11. Classification of Solar Drying Techniques
Fig.4: Classification of solar drying techniques

12. Itistheoldestandmostcommontraditional
methodtopreservefoodproductssuchasgrains,
fruits,vegetables,fishetc.
Foodproductsarespreadongroundanddirectly
exposedtosolarradiations.
Thesolarradiationsfallingonthefoodsurfaceis
partlyreflectedandpartlyabsorbed.The
absorbedradiationsandsurroundingairheatup
thefoodsurface.
Apartofthisheatisutilizedtoevaporatethe
moisturefromthefoodsurfacetothe
surroundingair.
Open sun drying (OSD)

13. Working ofOSD
Fig.5: Working of Open sun drying (OSD)

14. Advantages of OSD
No fuel or mechanical energy is required.
Operation is very simple and inexpensive.
Viability, germination, baking qualities are
fully maintained.
Disadvantages of OSD
Slow drying method, takes several days.
Dustcontamination,insectinfestation,
microbialcontaminationandspoilagedueto
rains.
Productdriedisunhygienicandsometimes
unfitforhumanconsumption.
Completely dependent on weather.

15. Greenhouse dryers
Allthesolardryeractsontheprincipleof
greenhouseeffect,soitisalsocalledgreenhouse
dryer.
Thegreenhouseeffectinsolardryerdueto(i)a
confinementeffect,resultingfromthedecrease
intheairexchangeswiththeoutside
environment;and(ii)aneffectcausedbythe
existenceofacovercharacterizedbyitslow
transparencytofarinfraredradiation(emittedby
thefoodmaterialsandtheinnergreenhouse
elements),butitshightransparencytosunlight.

16. Fig. 6: Working of GH Dryer

17. AdvantagesofGHdryer
ItisfasterthanOSD,freshproducecanbedriedinashorter
timeperiod.
Itismoreefficient,sincefoodstuffscanbedriedmore
quickly,lesswillbelostduetospoilage.
Itishygienic,sincefoodstuffsaredriedinacontrolled
environment.
Itishealthier,dryingfreshproduceatoptimumtemperatures
andinashorteramountoftimeenablesthemtoretainmore
oftheirnutritionalvaluesuchasvitaminC.
Itischeap,usingfreelyavailablesolarenergyinsteadof
conventionalfuelstodryproducts.
DisadvantagesofGHdryer
Smallcapacitylimitsittosmallscaleapplications.
Dependsuponopenfieldmicroclimatecondition.
Controloftemperature&RHisdifficult.

18. Passive (Natural convection) solar dryer
Theairmovementisoccursbynatural
circulation,andproductsgetheateddueto
directabsorptionofheatorduetohigh
temperatureintheenclosureandthenmoisture
removedfromthecropescapesoutofthe
chamberbynaturalcirculationofair.
Movementofairtakeplaceduetodensityor
thepressuredifference.

19. Fig.7: Working of Passive Solar Dryer

20. Fig.8: GH natural Solar Dryer

21. Active (Force Convection) Solar Dryer
Theairisforcedintooroutofthedrying
chamberusingablowerorfanwhichis
electricallyormechanicallyoperated.
Forcedconvectiondryersarebestsuitedfor
highmoisturecontentcropswhereasnatural
convectionisbestforlowmoisturecontent
crops.

22. Working of Active Solar Dryer
Fig.9: Working of Active Solar Dryer

23. Fig. 11: Active solar energy cabinet dryer
Fig.10: Forced-convection GH dryer

24. Indirectsolardryers,collectionofsolarenergy
anddryingofproducttakesplaceinanenclosed
insulatedstructure.
Themoistureistakenawaybytheairentering
intothedryerfrombelowandescapingthrough
atthetopexit
Thesedriershaveenclosuresandcoveredwith
glassortransparentLDPEetc.
Direct solar dryer (DSD)

25. Working ofDSD
Fig.12: Working of Direct Solar Dryer

26. AdvantagesofDSD
Simplerandcheapertoconstructthanthe
indirect-typeforthesamecapacity.
Offerprotectionfromrains,dews,debrisetc.
Productqualityobtainedisbetterthanopensun
drying.
DisadvantagesofDSD
Poorvaporremovalratesleadingtorelatively
slowoveralldryingrates.
Smallcapacitylimitsittosmallscale
applications.
Discolorationofcropduetodirectexposureto
solarradiation.

27. Thecropsaredriedintraysorshelvesinsidean
opaquedryingchamberandaseparateunit,solar
collectorisusedforheatingoftheenteringairintothe
cabinet.
Theheatedairisallowedtoflowthrough/overthewet
cropthatprovidestheheatformoistureevaporation
byconvectiveheattransferbetweenthehotairand
thewetcrop.
Dryingtakesplaceduetothedifferenceinmoisture
concentrationbetweenthedryingairandtheairinthe
vicinityofcropsurface.
Thebettercontrolledoftemperatureindryerresultsin
efficientoperation.
Indirect solar dryer (ISD)

28. Fig.13: Working of Indirect Solar Dryer

29. Fig.14: Indirect natural convection dryer

30. AdvantagesofISD
Canbeoperatedathighertemperature,
recommendedfordeepbeddrying.
Itismoreefficientthanthedirectsolardryer.
Maintainthequalityofproductbyavoiding
directexposureofsolarradiations.
DisadvantagesofISD
Morecomplexandexpensivethandirectsolar
dryer.
Incurlargermaintenancecoststhanthedirect
solardryer.

31. Mixed-mode solar dryer
Theproductsareexposeddirectlytosolar
radiationsandhotairfromsolarcollector.Itis
acombinationofdirectandindirectsolar
dryers.

32. Workingof mixed-modesolar dryer
Fig.15: Isometric illustration of the mixed-mode solar dryer

33. Advantagesofmixed-modesolardryer
Lessdamagefromtemperatureextreme.
Disadvantagesofmixed-modesolardryer
Morecomplexandexpensivethandirectsun
drying.
UVradiationcandamagefood.

34. Fig.16: Typical solar energy dryer designs

35. Low cost Multipurpose GH (MGH)
Duetohightemperatureandhighlightintensity
duringsummerseasoninopen,itisverydifficult
utilizeGHforcultivation/nurserygrowing.
TheMGHcanutilizehightemperatureforother
beneficialapplication.
WhentheGHisusedforcultivation/nursery
growinganddrying/soilsolarization,itiscalled
Multipurposegreenhouse(MGH).
Hereduringwinterandrainyseasonitisusefor
cultivation/nurserygrowingbutduringsummer
seasonitisusedfordrying/soilsolarization.

36. MidFeb-June:Utilizedfordrying&Soil
solarization.
Drying process: Passive (natural) & active (using
fan or chimney)
Fig.17: Low cost multipurpose
GH
Fig.18: Tomato cultivated under
low cost multipurpose GH

37. Fig.19a: Open field dried tomato (WB)
Fig.19b: Open field dried tomato (B)
Fig.20a: Low cost multipurpose GH
dried tomato (WB)
Fig.20b: Low cost multipurpose GH
dried tomato (B)

38. Advantages
Itcanbeefficientlyutilizedforcultivationof
vegetablesornurseryraisingduringJune-Mid.
February.
Itcanbeusedfordryingofcauliflower,
mushroom,mahuaetc.duringMid.Feb-Junedue
tohightemperature(40-55
0
C).
Itcanbealsobeutilizedforsoilsterilization
beforenextcrop.
Youtubevideo:Lowcostmultipurpose
greenhouse (MGH)
https://youtu.be/Xbyq8U9TUaE)

39. Multipurpose Greenhouse (MGH) vsConventional GH
Dryer
TheMGHcanbeusedfordryingaswellas
cultivationwhereasGHdryerusedforonlydrying
purpose.
TheconstructioncostoflowcostMGHisaround
Rs.300persquaremeterwhereasGHdryerit
variesbetweenRs.400to600persquaremeter.
ThecosteconomicsofGHdryerismajorhindrance
initsadaption.
TheMGHisusedfordrying&cultivationhas
bettereconomicsthenstandaloneGHdryerandit
canbeconstructedlocallyandusedbyfarmer
communitytoimprovedtherelivelihood.

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