COOLING AND HEATING OF GREENHOUSE�

COOLING AND HEATING OF GREENHOUSE�

1. BIRSA AGRICULTURAL UNIVERSITY
Protected Cultivation and Secondary
Agriculture
LECTURE 8: COOLING AND HEATING OF GREENHOUSE
BY
DR. PRAMOD RAI
DEPARTMENT OF AGRICULTURAL ENGINEERING

2. Why Greenhouse cooling is needed
Solarradiationisthe“heatinput”fortheearth.
Upto85%ofthisradiationmayenterthegreenhouse(mostoftheIRheat
becomestrappedinsideandgreatlyincreasesthegreenhousetemperature).
Aneffectcausedbytheexistenceofacovercharacterizedbyitslow
transparencytofarinfraredradiation(emittedbythecrop,thesoilandthe
innergreenhouseelements),butitshightransparencytosunlight.
Aconfinementeffect,resultingfromthedecreaseintheairexchangeswith
theoutsideenvironment
Mechanismisneededtoremovethistrappedheat.
Fig. 1: Working of GH

3. GH cooling
GH Cooling Systems
1. Ventilation 2. Evaporative Cooling 3. Heat Prevention 4. Composite System
1.1. Natural
Ventilation
1.2. Forced
Ventilation
2.1. Fan-Pad System
2.2. Fog/Mist System
2.3. Root
Evaporative Cooling
3.1. Shading
3.2. Radiation Filters
3.2.1. NIR-
Reflecting Film
Covers
3.2.2. Fluid
RoofCovers
4.1. Earth-to-Air Heat
Exchanger System
(EAHES)
4.2. Aquifer Coupled
Cavity Flow Heat
Exchanger System
(ACCFHES)

4. 1. Ventilation
HightemperatureinsideGHrequireconstantheat
removalfromtheGH.
Thismaybeaccomplishedbyreplacingtheexistingair
intheGHwithcoolerairfromoutsidethestructure.
Theupwardandoutwardmovementofwarmairpulls
incoolairfromsideorendvents.Thissystemismost
effectiveinthewinter,springandfall.
Itislimitedinitseffectivenessforsummercooling
sincetheincomingsolarloadandtheoutsideair
temperaturemaybetoohighforthecapabilitiesofthis
systemduringsummermonths.

5. 1.1 Natural Ventilation
Iftheoutsidetemperatureislowenough,andifthe
temperatureinsidetheGHisnottoohigh,warmairis
exhaustedpassivelythroughthegreenhousevents
(naturalventilation).
Theeffectivenessofthissystemdependsonthe
temperaturedifferencebetweeninsideandoutsidethe
GH(bouncyeffect)andonthewindspeedoutsidethe
structure(windeffect).
TheexternalcoolairenterstheGHthroughthelower
sideopeningswhilethehotinternalairexitsthroughthe
roofopeningsduetodensitydifference,loweringof
temperatureintheGH.

6. Fig. 2: Different types of ventilation opening

7. 1.2 Forced ventilation
Atlowwindspeed,exhaustfansareneededtoinduceaircirculation
throughthevents(forcedventilation).
Systemslikeexhaustfan,blower,etc.cansupplyhighairexchange
rateswheneverneeded.
Thesearesimpleandrobustsystemsandsignificantlyincreasetheair
transferratefromtheGHandallowmaintaininginsidetemperatureto
alevelslightlyhigherthantheoutsidetemperaturebyincreasingthe
numberofairchanges.
Fig. 3: Fans for GH forced ventilation

8. 2. Evaporative cooling (EC)
Itisbasedontheconversionofsensibleheatintolatentheatof
evaporatedwater.
Aswaterevaporatesittakesawayheatfromtheairthusreducingits
temperature.
Duringtheprocess,thetotalheat(enthalpy)oftheairremainsthe
same.
Fig. 4: PsychrometricChart

9. 2.1 Fan-pad system
Thissystemconsistsofafanononesidewallandpadon
theothersidewalloftheGH.
Theprincipleofevaporativecoolingisappliedby
runningawaterstreamoverthepadandconsequent
withdrawalofairthroughitbyfansontheoppositeside.
Theairbecomescooleranditshumidityisalsoraised.
Moreeffectivewhenoutsideairhumidityislow.
Fig. 5: Pad (left) and fan (right) greenhouse cooling system

10. Evaporative cooling
GenerallythesystemperformanceofEC
processcanbebasedonthesaturationefficiency
Where
T
dbin= The dry bulb T at inlet,
0
C
T
dbout= The dry bulb T at outlet,
0
C
T
wbin= The wet bulb T at inlet,
0
C
Leaving air condition
T
dbout= T
dbin–η
sat(T
dbin–T
wbin)

11. Air flow rates
ThemassflowrateofairthroughtheECisa
functionoftheairvelocityandiscalculatedon
thebasisoffrontalareaofthecooler.Thedensity
andvelocityofairattheentry.Themassflow
rateofair,m
a
m
a= ρAV
a
Where,
ρ = density of air at the entry of the cooler, kg/m
3
A = frontal area of the cooler’s opening, m
2
V
a= air velocity at the entry of the cooler, m/s

12. Cooling capacity
Thecoolingcapacity
Q
c = m
ac
p(T
dbin–T
dbout)
Where,
c
pis the specific heat capacity of air
Water consumption
Thewaterconsumptionisessentialbecauseitindicates
theamountofwaterneededtooperatethesystem
Q
Co=m
a(H
2­ H
1)
Where,
H: Humidity ratio

13. 2.2 Fog/misting system
Itisbasedonsprayingwaterassmalldroplets(dropletdiameterof2–
60micrometers)withhighpressurenozzles.
Coolingisachievedbyevaporationofdroplets.Freefallvelocityof
thesedropletsisslowandtheairstreamsinsidetheGHeasilycarry
thedrops.
Thiscanresultinhighefficiencyofwaterevaporationcombinedwith
keepingthefoliagedry.
ProvidesmoreuniformspatialairtemperatureandRHthanfan-pad
system.Lessexpensivetoinstallandoperate.
Fig. 6: Fog system used for greenhouse cooling

14. 2.3 Roof evaporative cooling
Itissprinklingofwaterontoasurfaceoftheroof
soastoformathinlayer,whichresultsinan
increaseofthefreewatersurfaceareaand
consequentlyincreasestheevaporationrate.
Thiscausesthewatertemperaturetofalltothe
wetbulbtemperatureofthecloselysurrounded
air.
Fig. 7: Roof sprinkling of water

15. 3. Heat prevention
Thespectraldistributionofglobalsolarradiationflux,thatisincident
onortransmittedintoaGHcanbedividedintoultravioletradiation
(UV:200–400nm;about5%ofglobalsolarradiation),visiblelight
orphotosyntheticallyactiveradiation(PAR:400–700nm;about
45%),andnearinfraredradiation(NIR:700–2500nm;about50%).
TheNIRislessabsorbedbytheplantsbutitisabsorbedmainlyby
theGHfloorsoil,installations,andconstructionelementsofthe
greenhouse.ThenitisreleasedagaintotheGHairasconvertedheat,
thatincreasestheGHairtemperature.
Accordingly,NIRisthemainsourceofheatloadthatshouldbe
removedtopreventtheoverheatingprobleminsummerinmany
sunnyareasworldwide.
Throughheatpreventionmethods,theradiativeheatloadcanbe
eliminatedorreducedbeforeenteringtheGHbyeitherabsorbing
and/orreflectingaportionoftheincidentradiationontheGHcover.
Thisisaccomplishedbyusingcommercialshadingdevices(curtains,
clothes,orplasticnets)orbyusingaradiationfilteringroof(blocking
theNIRviareflectionorabsorptionandtransmittingthePAR).

16. 3.1 Shading
ShadingtheroofofaGHisusuallyperformedbyvarious
conventionalmethodssuchaswhiteningtheroof,externalshade
cloths,deployingplasticnetsofvariouscolors,andmovable
refractivescreensorcurtains.
Whitening(whiteshadingpaint)canbeachievedbysprayingthe
exteriorcoversurfacewithanaqueoussolutionofhydratedCalcium
oxide[Ca(OH)
2].WhiteningtheGHroofisinexpensive,haspositive
effectsonbothmicroclimateandcropbehavior,andcanbe
consideredanefficientmeansforalleviatingthelargeheatload
duringsummer.
However,itreducedtheaverageGHtransmittancetosolarradiation
from0.62to0.31.Thewhiteningiswashedawayifrainsfalloverthe
GHanditsshadingdensitycannotbechangedonceapplied.
Theexternalshadeclothisusuallyappliedbydeployingwetordry
shadeclothsontheoutersurfaceoftheGHroof.
Anexternalorinternalshadecanalsobeobtainedbyusingmovable
plasticnets,curtains,orrefractivescreensappliedaboveorbelowthe
roofoftheGH.

17. Allshadingmethodsregulatetheamountofsolarenergy
enteringtheGHandreducetheheatingloadinsummer.
Disadvantageofshadingsystemthatusecurtainorscreen
belowtheroofoftheGHisthatwhenfullydeployed,itwill
decreasetheeffectivenessofthenaturalroofventilationand
negativelyaffecttheGHmicroclimate.
Moreover,presenceofshadingmaterialsdeployedintheGH
absorbsaportionofsolarradiation,reemitsitagainintheGH,
andreflectsbackaportionalsoinsidetheGH.Therefore,the
effectofinternalshadingonreducingtheGHairtemperature
isexpectedtobesmall.
Alltheaforementionedshadingmethodssignificantlyreduce
solarradiationacrossthewholesolarspectrumincludingthe
PAR(400–700nm)whichisessentialforplantgrowth.
Therefore,recentstudieshavefocusedondevelopingmore
selectivecoversthatcantransmitPARandblockNIR.

18. Control of light intensity in GH using shading
Shadingshouldbedoneverycarefully,especiallypermanent
shading.
Itreducesboththetemperature&thelightintensity.
Thereforeitisimportanttoconsiderbothfactorswhen
decidingwhichshadingtechniquetobeutilized.
Applicationofexternalshadingcompounds:Itwilldiffuse
lightrays&reflectheat,comeeitherinwhiteorgreenand
canbethinnedusingpaintsolvent,difficulttoapply&to
removetheuniformcoating,semipermanentandduring
cloudy&darkdaysofsummer;theplantsintheGHmay
receiveinsufficientlight.
Installingashadingscreenornetover/undertheGH
frame:reducelightlevelistoblockoutlightwithsome
shadingscreensmadeofcloth,polypropylene,polyesteror
aluminum-coatedpolyester,insidescreenservetwopurposes;
reducelightinthesummerandactasathermalblanketinthe
winter.

19. Fig. 8: Application of external shading compounds
Fig. 9: Shading screen or net over/under the GH

20. 3.2 Radiation filters
ForGHinhotandsunnyregions,scientistsand
companieshaveworkedformanyyearstodevelopGH
coveringsystemsthatisabletoreducetheheatloadas
wellastheairtemperatureintheGH.
Twosystemshavebeenintroducedforfilteringoutthe
incidentsolarradiationattheGHcover:
NIR-reflectingfilmcovers
Fluidroofcovers

21. Infrared (IR) Additive
Minimizetemperaturefluctuation:
Duringtheday,slightlydecreasestemperatureinsideGHby
blockingnearinfraredradiation(NIR:700-3000nm).Itisthepartof
thesolarspectralthatishardlyusedbytheplantsforphotosynthesis;
itismostlysubstitutedintoheat(sensibleandlatent)intheGH.This
canbeanadvantageinacountrywithacolderclimateanda
disadvantageinaGHlocatedinwarmcountry.
Duringthenight,increasestemperatureinsideGH,bycreatinga
barriertofarinfraredradiation(FIR:3000-100000nm)reflectedby
thesoil.Itisnotcausedbydirectsunradiation,butitisheatradiation
transmittedbyeachheatbodyintheGH.Thisradiationisvery
importantinGH;sinceitcausesapartofthegreenhouseeffect.
Fig. 10: Working of IR additive in GH film

22. 3.2.1 NIR-reflecting film covers
TheNIR(700-3000nm)canberejectedbyapplying
absorption,reflection,orinterferencepigmentstothe
polymerduringmanufacturingthecoveringmaterials.
Fig. 11: Working of NIR-reflecting film cover

23. Fig. 12: Low cost natural ventilated GH with NIR-reflecting film cover

24. 3.2.2 Fluid roof covers
Oneofthefirstattemptstoeliminatethemaximum
temperatureofairinaGHwascarriedoutbyflowinga
waterfilmof0.5mmthicknessontheroof,andadropof
4-5
0
Cintheinsideairtemperaturecouldbeachieved.
Awaterfilmupto10mmthicknessdidnotreducethe
PARtransmissionsignificantly;however,itblocks(via
absorptionandreflection)onlyabout5%oftheNIR.
Aconcentrationof1.5%∼2%CuSO4-watersolutionas
LRF(liquidradiationfilter)flowingthroughahollow-
channeled,rigidplastic(polycarbonateoracrylic)roofof
semiclosedGHhasbeenexamined.
Basedonthesestudies,thefluid-roofcoverscanremove,
viaabsorption,morethan50%ofsolarenergyincident
ontheGHcover;thustheradiationheatloadintheGH
canbereduced.

25. Fig. 13: Working of fluid roof cover

26. 4. Composite system
Anumberofstudieshavebeenreportedinliterature
whereadvancedcompositesystemareintegratedwitha
GHforcoolingandheatingofGH:
Earth-to-airheatexchangers(EAHES)
Aquifer-coupledcavityflowheatexchangersystems
(ACCFHES)

27. 4.1Earth-to-airheatexchangesystem(EAHES)
Usingtheearthmass,theearth-to-airheatexchanger
systemiswellestablishedincooling&heating.
Thegroundpotentialoftheearthcanalsobeusedfor
coolingtheGHinsummerconditionsduetoits
constantyearroundtemperature(26–28
0
C).
Thehotairiscirculatedthroughtheburiedpipe(2–4m
depth)fordissipationofheattotheundergroundsoil.
Thoughearth-to-airheatexchangersystemcanlower
theinsideairtemperaturetoaremarkableextent,but
themajordisadvantageofusingEAHESistheinitial
costinvolvementandlesslongevityofthemetallic
pipesduetocorrosion.

28. Undergroundheatexchanger
Alsocalled:
Earth-AirHeatExchangers
Air-to-soilHeatExchangers
EarthCanalsetc.
Fig. 14: Earth-to-air heat exchange system

29. Earthactsasourceorsink
HighthermalInertiaofsoilresultsin
airtemperaturefluctuationsbeing
dampeneddeeperintheground
UtilizesSolarEnergyaccumulatedin
thesoil
Cooling/Heatingtakesplaceduetoa
temperaturedifferencebetweenthe
soilandtheair
Earth-to-airheatexchangesystem(EAHES):Principle
Fig. 15: Variation of temperature with soil depth

30. EAT can be used in either:
Closed loop system
Open loop system
Open Loop system:
Outdoor air is drawn into tubes and delivered to
AHUs or directly to the inside of the building
Provides ventilation while hopefully cooling or
heating the building interior
Improves IAQ
Closed Loop system:
Interior air circulates through EATs
Increases efficiency
Reduces problem with humidity condensing
inside tubes.
Tube Arrangement
Fig. 16: Tube arrangement in Earth-to-air heat exchange system

31. Calculating benefits from EAT is difficult due to:
Soil Temperatures
Conductivity
Performance of EAT can be calculated as:
where;
T
o= Inlet Air Temperature
T
o(L) = Outlet Air Temperature
T
s= Undisturbed ground temperature
EAHES Efficiency

32. COP based on:
Amount of heating or cooling done by EAT (Heat
Flux)
Amount of power required to move the air through
the EAT
Q = Heat flux
W = Power
COP decreases as system is operated
COP can be integrated into system control strategies
When COP down to a certain point, EAT should be shut
down and conventional system should take over
Co-efficient of performance (COP)

33. 4.2 Aquifer coupled cavity flow heat exchanger system
(ACCFHES)
ItisusedforcoolingandheatingofGH.
Thesystemusedeepundergroundaquiferwaterfrom
irrigationtubewellatthegroundsurfaceatalmost
constanttemperatureofaround24
0
C(yearround).
TheintegrationofACCFHESwithGHhelpsin
maintainingtheinsideGHtemperature6-7
0
Cbelow
thatofambient.

34. Conclusion
ThenaturalventilatedGHisnormallyusedonly8to9months
duringtheyearduetohighinsidetemperatureofGHduring
summerseason.
Thecriticalfactorsaffectingtheperformanceofnaturalventilated
GHarerateofairexchangethroughnaturalconvectionandwhich
dependsontotalareaofvents,windspeedandtemperature
differencebetweeninsideandoutair.
Theselectionandoperationofthesystemisbasedonvarious
parameterssuchastypeofclimate,croptobegrown,cost,
maintenance,easeofoperation,reliability,lifeofthesystem,
dependencyonelectricity,etc.
NormallyforcoolingtheGHthecombinationofnatural
ventilation,insideshadenetmaterials/thermalscreenand
fogging/mistingisused.
SothemostsuitabletechnologyforGHcoolingisthatwhich
meetsmostofthedesiredconditionsofthefarmertogrow
offseasoncropsinordertofetchmaximumreturns.

35. GH Heating
GH Heating Systems
1. Passive 2. Active
1.1. Water Storage
1.2. Latent Heat Storage
Material
1.3. Rock Bed Storage
1.4. North Wall Storage
2.1.Heating 2.2. Radiant Heat
System
2.3. Composite System
2.1.1. Local
2.1.2. Central
2.3.1. Earth-to-Air Heat
Exchanger System (EAHES)
2.3.2. Aquifer Coupled Cavity
Flow Heat Exchanger System
(ACCFHES)

36. Need for GH Heating
Temperatureisoneofthemostimportantfactors
intheproductionofhorticulturalcrops.
Solarenergyonsunnydaysisoftenenoughto
keepaGHwarm,evenincoldweather.
Duringthenighttime,airtemperatureinsideGH
decreases.
TheheatisalwayslostfromtheGHwhenthe
surroundingsarerelativelycooler.
TherequirementsforheatingGHdependonthe
rateatwhichtheheatislosttotheoutside
environment.

37. Totimetheproductionforaspecificmarketandtohavesome
controlovercropquality&yield,growersneedtoheattheirGH
wheneveritstemperaturedropsbelowtherecommended
temperaturefortheirspecificcrop.
Exceptofraisingtheairtemperaturestothedesiredlevel,heating
isalsoappliedincaseswherethereisaneedtoreduceair
humidity(e.g.toreducetheprobabilityofcondensationonplant
organsandthusreducedevelopmentoffungaldiseases).
IncoldcountriesGHareheatedduringmostoftheyear,whilein
mildclimatestheheatingperiodisshorterandheatingisusually
appliedduringthewinter.
Heatingcostsarenotonlydirectlyconnectedtoprofitability,but
inthelongtermtheymaydeterminethesurvivaloftheGH
industry.
Inadditiontothecostsofhighenergyconsumption,heatingis
associatedwithenvironmentalproblemsthroughtheemissionof
noxiousgases.

38. Mechanism of Heat Loss
Mostheatislostbycoveringmaterialbyconduction.
Differentmaterials,suchasaluminumbars,glass,polyethylene,
andcementpartitionwalls,varyinconductionaccordingtothe
rateatwhicheachconductsheatfromthewarminteriortothe
colderexterior.
Spacesbetweenpanesofglassandventilatorsanddoorspermit
thepassageofwarmairoutwardandcoldairinward.
About10%oftotalheatlossfromastructurallytightglassGH
occursthroughinfiltrationloss.
AthirdmodeofheatlossfromaGHisthatofradiation.
Fig. 17: Heat loss from GH

39. Control of Heat Loss
Variousmethodsareadoptedtoreducetheheatlosses,
viz.,usingdoublelayerpolyethylene,thermopane
glasses.
Thereareonlylimitedwaysofinsulatingthecovering
materialwithoutblockingthelighttransmission.
Adeadairspacebetweentwocoveringsappearstobe
thebestsystem.Asavingof40%oftheheat
requirementcanbeachievedwhenasecondcovering
inapplied.
ForexampleGHcoveredwithonelayerof
polyethyleneloses,6.8Wofheatthrougheachsquare
meterofcoveringeveryhourwhentheoutside
temperatureis1
o
Clowerthantheinside.

40. 1.1 Water Storage
TheheatstoragesystemcanbeplacedinsidetheGH,inplastic
bagsfilledwithwater.Watercontainersusedassolarcollectorand
heatstorage.
Thesystemabsorbandtraptheincidentsolarradiationduringthe
day.Duringthenight,thestoredheatisreturnedtotheinteriorby
naturalconvectionorradiation.
Fig. 18: Passive solar GH with water storage in (a) plastic bags
and (b) water containers

41. 1.2 Latent heat storage material
Latentheatmaterialsareanalternativeheatstoragemediumandlike
CaCl
2
.
6H
2O(withameltingtemperatureof29.7
o
Candalatentheatof170
kJ/kg)havebeensuccessfullyusedinmanyGH.
Heatisabsorbedbythelatentheatstoragematerialandstoredforlater
use.Thematerialchangesphaseduringthisprocess.Atnight,coldair
frominsidetheGHiscirculatedthroughthestorageandisheatedbefore
returningintotheGH.Thelatentheatmaterialthenreturnstoitsinitial
solidphase.Thisprocessmayresultinahumidityincreaseduringthe
nightperiods.
Fig. 19: GH with rock bed storage

42. 1.3 Rock Bed Storage
Apopularandeconomicalheatstoragematerialisarockbed,
whichconsistsof20-100mmdiametergravel.Thestorageareais
placeundertheGHatadepthvaryingbetween40-50cm.
Duringtheday,excessheatistransferredfrominsidetheGHtothe
undergroundstore.AventilatorcanbeusedtotransportGHair
(usingafanatarateof5m
3
/minm
2
)totheheatstoragearea.
Atnight,theprocessisreversed.Thecoolairismovedthroughthe
store,whereheatistransferredfromthegraveltothecolderairand
thenreturntotheGH.
Fig. 20: Passive solar GH with latent heat storage material

43. 1.4 North wall storage
Toreduceconstructioncostresultingfromtheimplementationof
theprevioussystems,itispreferabletouseinsulatedsidesfor
reducingheatlossesandanorthstoragewall.
Thiswallisexternallyinsulatedandinternallypaintedblackwhich
operatesasaheatstorage.
Asimple,northsidestoragewallisofsmallcostandapplicableto
commercialapplicationwheretheheatingneedsarenotveryhigh.
Fig. 21: Passive solar GH with north wall storage

44. Heating needs
TherearevariouswaystocalculateGHheatingneeds(Hg)(W).
Thesimplestis
H
g= UA (T
i-T
o) (1)
Where
U = heat loss coefficient (W m
-2
K
-1
)
A = exposed greenhouse surface area (m
2
)
T
i= inside air temperature (K)
T
o= outside air temperature (K)
NotethattheestimationofGHheatingneedsusingEquation1did
nottakeintoaccountheatlossduetoleakage.Howeveritisa
simpleformulawhichcanbeusedinordertoestimateheating
needsaccordingtotheGHcoveringareaandthedesired
temperaturedifferencebetweeninsideandoutsideair.

45. Types of Heating system
2.1.1Localheatingsystems:Itareusuallyplaced
atoneendoftheGH.Theycanbeunitheaters,
convectionheatersorradiantheaters.Localheating
systemsaremoresuitableforsmallerGH.
2.1.2Centralheatingsystems:Itconsistofa
boilerinacentrallocation.Boilersheatwithsteam
orhotwaterandcanburnavarietyoffuels.Itis
usedmostcommonlyinlargercommercialGHdue
tocost.

46. Function of GH heating
TherearefourfunctionsthatmustoccurtoheataGH:
Conversionoffueltoheatenergy:Theconversionoffuel
toheatenergyistypicallyaccomplishedthrough
combustionwithaburnerinstalledinaboilerorheater
combustionchamber.
Distributionoftheheatenergy:Theheatenergyisthen
distributedthroughtheGHthroughpipes,ducts,tubes,
orair.
Transferoftheheatenergy:Oncetheenergyis
distributed,itmustthenbetransferredtotheplantsand
soilbyconvection,conduction,orradiation.
Conversionoftheheatenergyintouseableheatbythe
plant:Finally,oncetransferredtotheplantsandsoil,
theymustinturnabsorbitsenergyandconvertitto
usableheat.

47. Unit heaters
Theunitheaterisafanequippeddevicewithameanstoheattheair.
Themostcommonandleastexpensiveistheunitheatersystem.
Warmairisblownfromunitheaterswithself-containedfireboxes.
HeatersarelocatedthroughouttheGH,eachheatingafloorareaof
180–500m
2
.
Unitheatersareavailableinoilfired,electric,hotwaterorsteam,and
gasfired.Themostpopularbeingthegasfiredunit.Unitheatersare
typicallysuspendedfromtheGHframing.Floormountedunitsare
alsoavailable.
Therearetwomaintypesofunitheatersthatareusedforspace
heatinginGH:ventedandunvented.
Thetraditionalvented,gasfiredunitheatertransfersheatfromthe
combustiongasestotheairthroughaheatexchangerandexhausts
thecombustiongasesoutsidetheGHthroughafluepipe.
Anunventedunitheaterburnsthegasandexhaustsallcombustion
gasesdirectlyintotheGH,sovirtuallyalltheheatfromthefuelis
usedtoheattheair.

48. Reason for using Unit heaters
Theyprovidetheaircirculationneededandcanbe
usedinconjunctionwithventilationsystems.
Theycanprovideuniformbenchtoptemperatures
andunderthebenchtemperature.
Theyarecomparativelytheleastexpensiveand
quickresponsetotemperaturechanges.
Theyareeasytoinstallandofferinexpensive
expansionforadditions.

49. Fig. 22: Unit heaters in GH

50. Central heating
Steamorhotwaterisproduced,plusaradiatingmechanisminthe
GHtodissipatetheheat.
Unlikeunitheatersystems,aportionoftheheatisdeliveredtothe
rootandcrownzoneofthecrop,resultinginimprovedgrowthand
toahigherlevelofdiseasecontrol.
Placementofheatingpipesisveryimportantasitisdirectlyrelated
toheatloss;forexample,theplacementofpipesinthewallsresulted
inhighlossesthroughthesides.
Boiler components
Firebox:wherefuelisburned.
Flue:providesaway forsmoke,fromthe
firebox,toventtotheoutsideair.
Heatexchanger:networkoftubeseitherfilledwithorsurroundedby
water.

51. Fig. 23: Arrangement of Heating Pipe Coils

52. 2.2 Radiant heat systems
Theseheatersemitinfraredradiation,whichtravelsina
straightpathatthespeedoflight.Theairthroughwhichthe
radiationtravelsisnotheated.
Afterobjectssuchasplants,walksandbencheshavebeen
heated,theywillwarmtheairsurroundingthem.Air
temperaturesininfraredradiantheatedGHcanbe3-6°C
coolerthaninconventionallyheatedGHwithequivalent
plantgrowth.
Growerreportsonfuelsavingssuggesta30–50percentfuel
reductionwiththeuseoflowenergyinfrared-radiantheaters,
ascomparedwiththeunitheatersystem.
Fig. 24: Radiant heater in GH

53. Comparison to GH heating system
Type of system Advantages Possible disadvantages
Steam Cantransferheatthroughoutlarge
ranges(GHphysicalplants)
withoutcooling
Requiressmallerpipingthanhot
watersystemsreducinginstallation
cost
Makessteamavailableforheating
soilinGH
ControlofGHtemperaturesnotas
subtleaswithhotwaterandGH
maytemporarilyoverheated
Costofmaintenancegreaterwith
steamduetodamagingeffectthat
highpressureonpipes
Hot waterPermitsmoreaccurateand
responsivethermostaticcontrolof
GHtemperatures
Distributesheatmoreevenlywith
fewerhotspotstoinjureplants
Lesspotentialfordangerto
workersifahotwaterlineruptures
thanifapressurizedsteamline
breaks
Largerpipingrequired,increasing
costofinstallation
Steamstillrequiredforsoil
pasteurization
Unit heaterAdaptabletosmallGHareas
Excellentback-upsystemsinthe
eventofboilerbreakdownor
powerfailureoflargesystem
Fuelcost(gasoroil)maybegreater
thancostofexpandingexitingsteam
orhotwatersystem
Heatdistributionunevenbutcanbe
improvedbyattachingplastic
sleevesthatextendlengthofhouse.
Benchleveltemperaturesmaystill
becoolerthannearerroof.

54. Conclusion
Themaximumtemperatureduringsummermonthis
majorchallengeinIndiaforroundtheyearutilizationof
GHnotminimumtemperature.
Theminimumtemperatureisissuebutitisconfinedto
fewareasofcountry.
Theselectionofheatingmethodsdependsuponmany
factors,someofthemaresizeofGH,airandorsoil
heating,efficiencyofheatingsystemselected,
applicationdurationduringyear,passiveoractivesystem
etc.
Thealternativetechnologiesavailableforcultivation
duringlowtemperatureseason.
ThemostsuitabletechnologyforGHheatingwhich
allowfarmertogrowoffseasoncropsinordertofetch
maximumreturns.

55. Ifyouhaveanyquestion/suggestion
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