By Bruce I. Nelson, P.E., President, Colmac Coil Manufacturing, Inc.

OptimizingHotGasDefrostIntroductionSeveralmethodsarecommonlyusedtoremoveaccumulatedfrostfromaircoolingevaporatorswhichoperatebelowfreezing.Theyinclude;water,electric,andhotgasdefrost.Ifdesignedandoperatedproperly,hotgasdefrostofferstherefrigerationsystemoperatoramethodwhichis:

Effective Automatic Reliable,and Safe

Whyisusinghotgasaneffectivemethodofdefrostingevaporators?

1. Theevaporatorbecomesacondenser.Duringthehotgasdefrostprocess,highpressurehotgasfromthedischargesideofthecompressorisintroducedintotheevaporatorinacontrolledfashionwhereitcondensesbacktoitsliquidstate.

2. Thelatentheatoftherefrigerantisused.Theprocessofcondensingreleasesalargeamountofenergy,equaltothemassflowrateofthehotgasenteringtheevaporatortimesthelatentheatofvaporizationoftherefrigerant.Theheatreleasedduringcondensingiscalledlatentheatsincethereisnochangeintemperatureduringthecondensingprocess(thetermlatentmeanshidden).Ifthecondensingpressureisheldconstant,thecondensingprocesswilltakeplaceataconstanttemperature.Theamountofheatreleasedduringthecondensingprocessismuchgreaterthantheamountofheatreleasedwhensuperheatedgasiscooledwithoutcondensing(calledsensiblecooling).

3. Thecondensedliquidisrecycledandsentdirectlybacktootherevaporators.ThecondensedliquidfromthedefrostingevaporatorisexpandedintothewetsuctionlineandreturnedtotheLowPressureReceiver(LPR)orIntermediatePressureReceiver(MPR)whereitisrecycledandpumpeddirectlybackouttoevaporators.

4. Hotgasdefrostactslikeaheatpumptomoveheat.Aheatpumpmovesheatuphillbygatheringenergyatalowtemperaturelevelintheevaporator,compressingtheevaporatedrefrigeranttoahigherpressure,thenreleasingtheenergyatahighertemperaturelevelduringthecondensingprocess.Thisprocessis7to8timesmoreenergyefficientthanburningfossilfuelorelectricitydirectlytoproducethesameheatingeffect.Inthesamewaytheheatusedforhotgasdefrostinghasactuallybeengatheredfromtherefrigeratedspacebytheoperatingevaporators,thenmovedtothedefrostingevaporatorsbythecompressionprocessatarefrigerantpressureandtemperaturehighenoughtomeltthefrost.Hotgasdefrostingisveryenergyefficient!

Technical Bulletin

DefiningDefrostEfficiencyAgenerallyaccepteddefinitionofdefrostefficiencyisshownbelow:Theabsoluteminimumamountofheatrequiredforanidealdefrost(100%efficient)wouldequaljustenoughtowarmandmeltthefrostitself.Anyadditionalheatappliedtotheevaporatorreducesthedefrostefficiencytolessthan100%.Unfortunately,heatmustbeappliedatthestartofthedefrostcycletoheatthemetaloftheevaporatorfromtheevaporatingtemperatureupto32F(0C).Thisheatmustthenagainberemovedattheendofdefrostwhentherefrigerationsystemisrestarted.Heatmustalsobeaddedtothedrainpantokeepthemeltedfrostliquidlongenoughtoescapetherefrigeratedspacethroughthedrain.Thisheating(andcooling)ofthecoilanddrainpanmetalisunavoidableandresultsinareductionintheidealmaximumdefrostefficiency.Defrostefficiencyisalsoreducedwhensomeofthedefrostheatislosttotheroomasheatedair(convection)andradiation.Finally,hotgasbypassingthedefrostregulatorattheendofthedefrostcyclerepresentsanotherlossbyimposingafalseloadonthecompressor,andfurtherreducesdefrostefficiency.Improvingdefrostefficiencybyreducingtheselasttwotypesofdefrostheatlossesisthesubjectofthefollowingdiscussion.Howefficientisatypicalfreezerdefrost?Cole(1989)observedthatmostfreezerevaporatorsoperatewithdefrostefficiencyofonly15%to20%.Ofthetotaldefrostenergyinputhedeterminedthat:

15to20%wasutilizedtomeltthefrost, 60%waslosttotheroomviaconvectionandradiation, 20%wasrequiredtoheatandcoolthemetalintheevaporator,and about5%waslostduetohotgasbypassingthedefrostregulatorattheendofdefrost.

Colefurthersuggestedthatthemaximumtheoreticaldefrostefficiencywasprobablyintherangeof60%to70%.Defrostefficiencywillbereducedasenergylosttotheroomduringdefrostincreases.Theamountofheatlosttotheroomisdirectlyaffectedbyroomtemperature(acolderroomwillhavelargerconvectivelosses),thedurationofthedefrost(alongerdefrostwillresultinmoreconvectiveheatloss),andthetemperatureofthehotgas(highertemperaturehotgaswillresultinmoreconvectivelosses).

Thefrequencyofdefrostsandamountofaccumulatedfrostwillalsoaffectdefrostefficiency,thatis,moreaccumulatedfrostwilldirectlyincreasedefrostefficiencybytheequationshownabove.Aheattransfermodelwaswrittenforatypicalindustrialevaporatortoexaminehowdefrostefficiencyisaffectedby:

Roomtemperature, Hotgastemperature, Durationofdefrost, Frostthickness,and Materialsofconstruction

RoomTemperatureAsroomtemperatureisreduced,thedefrostheatlosttotheroomduetoconvectiveheatingofairunavoidablybecomesgreater.Thismeansthatdefrostefficiencyinafreezerroomwillalwaysbelessthandefrostefficiencyinamediumtemperatureroom.Figure1belowillustratesthegreaterconvectiveheatlossinthefreezer(63%)comparedtothemediumtemperatureroom(46%),andtheresultinglowerdefrostefficiencyinthefreezer(17%)versusthemediumtemproom(32%).NotethatthedefrostefficiencyisequaltotheMeltFrostpercentagesshowninthecharts.Thishighlightstherelativelylargeamountofheatthatislosttotheroomduringdefrostduetoconvectiveairheatingregardlessoftheroomtemperature.Reducingthisconvectiveheatlossbychangingthedesignoftheevaporatorcabinetthereforerepresentsanopportunitytosignificantlyimprovedefrostefficiencyandwillbediscussedlater.

FIGURE1

HotGasTemperatureSincetheamountofconvectiveheatlosstotheroomisdirectlyaffectedbythetemperaturedifferencebetweenhotgastemperatureandroomtemperature,anyincreaseinhotgastemperatureabovetheabsoluteminimumrequiredtomeltthefrostresultsinaproportionalincreaseintheconvectiveloss.Itisgenerallyacceptedthatthepracticalminimumhotgastemperatureforeffectivedefrostingisaround50F(adefrostregulatorsettingof75psig).Figure2illustrateshowanincreaseinthehotgastemperatureresultsinanincreaseinconvectiveheatlossandreductionindefrostefficiency.Itistheauthorsobservationthatinmanyfacilities,thehotgastemperatureisraisedabovetheminimumrequired50Finanattempttoclearthecoiloficeduetosomedesignrelatedissue(s)suchasicebuildupindrainpans,orimproperdefrostpiping.

FIGURE2Inaconventionalhotgascontrolvalvearrangement,hotgaspressure(andthereforedefrosttemperature)isdeterminedbythedefrostregulatorsetting.Itisimportanttorecognizethatsomeminimumpressuredifferencebetweenhotgassupplypressureandthedefrostregulatorsettingmustbemaintainedinordertoprovideenoughpushtokeepclearingthecondensedrefrigerantoutofthe

coil.Apressuredifferentialof15to20psigshouldbesufficienttokeepthecoilclearofcondensedrefrigerant.Ifthispressuredifferencebecomestoosmall(eitherhotgassupplypressurefallstooloworthedefrostregulatorsettingistoohigh)thencondensedliquidrefrigerantcanaccumulateinthecoiltubesandbecomesubcooled,typicallyinthebottomrows.Oncetherefrigerantliquidbecomessubcooleditlosesitsabilitytomeltthefrostandicewillaccumulate.Also,coilmanufacturersmustproperlydesignevaporatorstocontinuouslydrainandclearcondensedrefrigerantfromthe:

Hotgaspanloop, Coilcircuits,and Liquidheaderandconnection

Designingtheliquidheadertoeffectivelytrapcondensedrefrigerantandformaliquidsealbelowthelowesttubeinthecoilisparticularlyimportanttoavoidtheproblemofaccumulatingsubcooledliquidinthebottomcoiltubesmentionedabove.DurationofDefrostCole(1989)confirmedbyhisownmeasurements,andbytheobservationofothers,thattheminimumtimerequiredtomeltthefrostonevaporatortubesandfinsisonlybetween8and10minutes.However,itistheobservationoftheauthorthatmostevaporatorsinindustrialrefrigerationfacilitieshavehotgasdefrostdurationsettingsinexcessof30minutes,thatis,theperiodoftimethehotgassolenoidisopen.Figure3showsthesignificantreductioninconvectiveheatloss,andtheincreaseindefrostefficiency,resultingfromshorteningthedurationofdefrostfrom30minutesto10minutes.

FIGURE3

Defrostdurationoflongerthantheminimum10minutesshouldnotbeneeded,however,itisquitecommontoseedefrostdurationsof30minutesorlonger.Thisbeingaresultofdeficienciesineitherthedesignoftheevaporator(iceindrainpansorimproperlytrappedcoiloutletconnection),orinthedefrostpipingand/orcontrols.FrostThicknessThedefinitionofdefrostefficiencyimpliesthatincreasingtheamountoffrostmeltedduringdefrostwilldirectlyincreasetheefficiency.Reducingthenumberofdefrostsperdaywillincreasefrostthicknessandincreaseefficiencyofdefrosting.Figure4showstheeffectofincreasingfrostthicknessfrom1mmto2mm,andconfirmsasignificantincreaseinefficiency.

FIGURE4Reducingthenumberofdefrostsperdaymayormaynotbepossiblewithexistinginstallations,dependingontheevaporatordesign.Inorderforevaporatorstocarrymorefrostonfinsurfacesbetweendefrosts,twodesigncharacteristicsareneeded:

1. Widefinspacing.Afinspacingof3fpi(8.5mm/fin)willallowmoreaccumulatedfrostbetweendefrostscomparedto4fpi(6.4mm/fin)withlessrestrictionofairflowandlessreductioninevaporatorperformance.

2. Alargeratioofsecondary(fin)toprimary(tube)surface.Evaporatorshavingveryclosetubespacingwillhavereducedtotalsurfaceareaforagivencoolingdutyandreducedfrostcarryingcapability.Evaporatorshavingtubesspacedfartherapartwillhavegreatertotalsurfaceareaandgreaterfrostcarryingcapability.Forexample,anevaporatorwith50mmtubespacingand3fpiwillallowlongerruntimebetweendefroststhananevaporatorwith38mmtubespacingand4fpi.Moretotalsurfaceareaforagivencoolingdutyallowsfewerdefrostsperday.

MaterialsofConstructionNelson(2003)showedthatmoreenergyisrequiredtoheatandcoolthemetalinagalvanizedsteelevaporatorcomparedtoanaluminumtube/aluminumfinevaporatorduringadefrostcycle.Thisisdueprimarilytothegreatermassofmetalinthegalvanizedsteelconstruction.Figure5showsthereductionindefrostefficiencyforagalvanizedevaporator(Stl/Zn)comparedtoanallaluminum(Al/Al)one.

FIGURE5

Summary:DefrostEfficiencyFromtheprecedingdiscussionwecansummarize:

1. Colderroomtemperatureswillunavoidablyhavelowerdefrostefficiencies.2. Defrostefficiencyimprovesas:

a) Hotgastempislowered,and/orb) Defrostdurationisshortened,and/orc) Timebetweendefrosts(frostthickness)isincreased.

3. Convectiveheatlossisasignificantpenaltyinallcases. 4. Reducingdurationandincreasingfrostthicknessimproveddefrostefficiencyfrom17%to44%in

thefreezer.5. Allaluminum(Al/Al)constructionimproveddefrostefficiencyfrom29%to34%inthefreezer

comparedtogalvanizedsteel(Stl/Zn).Notethatallaluminumconstructionwillalsodefrostfasterthangalvanizedsteelduetothemuchhigherthermalconductivityofaluminum.

ReducingConvectiveHeatLossAsshownabove,reducingconvectiveheatlossesduringdefrostrepresentsasignificantopportunitytoimprovetheenergyefficiencyofhotgasdefrosts.Figure6belowfromCole(1989)showsfieldmeasuredairmovementpatternsandvelocitiestakenduringdefrost.

FIGURE6CONVECTIVEAIRMOVEMENTDURINGDEFROST

Theuseofreturnairhoods,andfandischargesocksisarecentdevelopmentnowavailableasanoptionfromseveralevaporatormanufacturers.Returnairhoodsincombinationwithfandischargesocks

Takenfrom:Cole,R.A.1989.RefrigerationLoadsinaFreezerDuetoHotGasDefrostandTheirAssociatedCosts.ASHRAETransactions,V.95,Pt.2.

effectivelyeliminateconvectiveairmovementandheatlossduringdefrost.Figure7showstypicalreturnairhoodsandfandischargesocksinstalledonanevaporator.

FIGURE7EVAPORATORWITHRETURNAIRHOODANDDISCHARGESOCKINSTALLED

Returnairhoodssuchasthoseshownareveryeffective.However,ifcareisnottakento(a)insulatethehood,and(b)activelyheattheinsidesurfacesofthehoodduringdefrost,thenhoarfrostandicecanbuildupontheinsidesurfacesofthehoodandeitherblockairfloworfalltothefloorbelow.Also,fandischargesocksmayrequireperiodicremovalforcleaninganddeicing.OptimizingHotGasDefrost:ConclusionsFromtheabovediscussion,itcanbeseenthathotgasdefrostingofevaporatorscanbemadesignificantlymoreefficientbydoingthefollowing:

1. Minimizeconvectiveheatloss. Uselowestpracticaldefrostregulatorsetting.75to90psig(50to60F)shouldbe

adequate.Note:Ifhigherpressuresareneeded,lookforproblemselsewhere. Capturedefrostheat(i.e.installReturnAirHoods).

2. Shortendefrostduration. UsetopfeedorDX(directexpansion)evaporatorfeedtoreducetimerequiredforpump

out. Openthehotgassolenoidonlylongenoughtoclearcoil(810minutes). Installaseparatehotgassolenoidanddefrostregulatorforpreandpostheatingofthe

panloop.Alternately,installelectricresistancedrainpanheating.3. Reducethenumberofdefrostsperday.

Reducethenumberofdefrostsperdaytomatchthefrostload. Chooseevaporatorswithwidefinspacing(3fpiinsteadof4fpi)andlargesecondary

(fin)surfaceareatomaximizefrostcarryingcapacity.

CalculatingtheCostofDefrostIthasbeenshownthatdefrostefficiencycanbesignificantlyimprovedbyreducingtheamountofenergylosttotheroombyconvectionduringdefrost.Thenextlogicalquestionbecomes:HowmuchmoneycanIreallysavebyoptimizinghotgasdefrost?Toanswerthisquestion,thedefrostmodeldescribedintheprecedingsectionswasusedtocalculatethecostsavingsresultingfrom:

1. Reducingdefrostdurationfrom30minutesto10minutes,and2. Increasingfrostthicknessfrom1mmto2mm(reducingthenumberofdefrostsperdaybyhalf).

Thecalculationsassume:

Evaporatorcapacity:100TR Compressorruntime:16h/day CostofElectricity:$0.10/kWh

Table1showscalculatedcostsavingsforfourdifferentroomtemperatures.Asroomtemperatureislowered,lessmoistureisheldintheair(thehumidityratioisreduced).i.e.Coldairisdrierthanwarmair.Hence,atfreezertemperaturesthelatentcomponentofthetotalrefrigeratingloadislower(lessfrostaccumulatesforeachtonofrefrigeration)comparedtohigherroomtemperatures.ThisisreflectedintheSensibleHeatRatio(SHR)showninthetable.Tomakethecostsavingscalculation,systemCoefficientofPerformance(COP)alsoneedstobeassumed.TheCOPvaluesinthetableassumesinglestagecompressionforthe0C(+32F)and18C(0F)roomtemperatures,andtwostagecompressionforthe23C(10F)and34C(30F)roomtemperatures.

TABLE1CALCULATEDCOSTSAVINGS($/y/100TR)FOROPTIMIZEDVSCONVENTIONALDEFROST

SmartHotGasDefrostPipingConventionalammoniaevaporatorsaretypicallyarrangedforbottomfeedwiththehotgaspanlooppipedinserieswiththecoil.Asmentionedabove,asmarthotgaspipingarrangementresultinginhigherdefrostefficiencyandreducedoperatingcostsutilizestopfeed(orDX)withthehotgaspanlooppipedseparatelyfromthecoil.ThisSmartHotGasDefrostpipingarrangementisshowninFigures8and9.Withconventionalbottomfeedandhotgasdefrostpipingarrangement,duringdefrost,hotgasisfirstsentthroughthedrainpanloopandtheninseriesthroughthecoilblock.Thiscommonlyusedarrangementiseffectiveandsimple,however,itrequiresthatthehotgassolenoidremainsopentokeepthedrainpanheatedlongenoughforallwatertocompletelydrainandexitthroughthedrainpiping.Convectiveheatlosstotheroomcontinuesafterthecoilisclearoffrostwhilethepanisdraining.Amoreefficient(andcosteffective)arrangementistocontrolhotgastothecoilblockandtothedrainpanloopseparatelythroughtwoseparatelytimedhotgassolenoidvalves.Thisseparatecontrolofpanheatingcanalsobeaccomplishedbyelectricallyheatingthedrainpan.Thisarrangementshortenstheamountoftimehotgasisflowingthroughthecoilblock,minimizingtheconvectiveheatlossandmaximizingdefrostefficiency.

0 (+32) -18 (0) -23 (-10) -34 (-30)SHR 0.66 0.89 0.93 0.97System COP: 3.2 2.5 2.2 2Frost Removed, kg/day: 2,778 899 572 245Frost Removed, kg/y: 1,014,096 328,090 208,784 89,479I. Baseline (30 min, 1 mm)Defrost Efficiency, % 32% 18% 17% 14%Defrost Convective Losses, %: 46% 61% 63% 65%Defrost Convective Losses, kWh/y: 1,012,438 753,334 545,922 283,071Baseline Cost of Defrost (Convective), $/y: $31,639 $30,133 $24,815 $14,154II. Optimized (10 min, 2 mm)Defrost Efficiency, % 61% 46% 43% 40%Defrost Convective Losses, %: 15% 26% 27% 30%Defrost Convective Losses, kWh/y: 168,740 125,556 90,987 47,178Optimized Cost of Defrost (Convective), $/y: $5,273 $5,022 $4,136 $2,359

Savings Optimized vs Baseline, $/y: $26,366 $25,111 $20,679 $11,795

Room Temp, C (F)

ItisinterestingtonotethatthecontrolvalvesfortheSmartHotGasDefrostpipingarrangementshowninFigures8and9arelessexpensivethanaconventionalbottomfeedhotgasdefrostpipingarrangementwithdefrostregulator.

FIGURE8SMARTHOTGASDEFROSTPIPINGDIAGRAM(ELECTRICALLYHEATEDPAN)

HGS

HPL

LPS

DRAIN

PAN HEATER

EEV LLS

HGS

PSV

HEVSSV

LEGEND:- HAND EXP. VALVE- SUCT. STOP VALVE- PILOT SOLENOID VALVE- HOT GAS SOLENOID VALVE- ELECTRONIC EXP. VALVE- LIQ. LINE SOLENOID VALVE- LOW PRESS. SUCTION- HOT GAS SUPPLY- HIGH PRESS. LIQUIDHPL

HGSLPSLLSEEVHGSPSVSSVHEV

AIR

(or HEV)

FIGURE9SMARTHOTGASDEFROSTPIPINGDIAGRAM(WITHPANLOOP)

TypicalSmartHotGasDefrostSequenceofOperation

1. Defrostisinitiated.Defrostinitiationcanbetimedorondemandbyanairpressuredifferentialsensor(indicatesfrostedcondition)

2. LiquidLineSolenoid(LLS)closes3. Timedpumpoutfor510minutes4. Fan(s)stop5. Panheaters(orpanloopsolenoid)energizedfortimedpanpreheat(23minutes)6. Coilhotgassolenoidandpilotsolenoid(closessuctionstopvalve)open7. Timeddefrost(810minutes)8. Coilhotgassolenoidcloses9. Aftercoilpressureisequalizedtosuctionpressure(35minutes),SuctionStopValveopens

HGS

HPL

LPS

DRAIN

EEV LLS

HGS

PSV

HEVSSV

LEGEND:- HAND EXP. VALVE- SUCT. STOP VALVE- PILOT SOLENOID VALVE- HOT GAS SOLENOID VALVE- ELECTRONIC EXP. VALVE- LIQ. LINE SOLENOID VALVE- LOW PRESS. SUCTION- HOT GAS SUPPLY- HIGH PRESS. LIQUIDHPL

HGSLPSLLSEEVHGSPSVSSVHEV

AIR

(or HEV)

HGS

HEV

10. OpenLLS11. Panheater(orpanloopsolenoid)deenergized12. After5minutecooldowndelayfansrestart

DesignforReliabilityReliableoperationofthehotgasdefrostsystemdependsonanadequatesupplyofhotgasthroughoutthedefrostcycle.Rememberto:

1. CorrectlysizeandinsulatehotgaslinesaccordingtoIIARguidelines(IIAR2004).2. Makesure2coilsarerunningforeverycoilthatisdefrosting.Thisisbecausetheevaporatorhas

approximatelytwicethecondensingcapacityasevaporatingcapacityduringdefrost.Reliablehotgasdefrostoperationalsodependsoncorrectselectionandsizingofcontrolvalves.Controlvalvemanufacturersliteratureandguidelinesshouldbeconsulted.DesignforSafetySafetymustalwaysbeaprimaryconsiderationwhendesigningandoperatinganammoniahotgasdefrostsystem.Remember,asaminimum,todothefollowing:

1. UsegoodpipingpracticepertheIIARPipingHandbook(2004).2. Keephotgaslinesclearofliquidbypitchingdowntowardliquiddrainers.3. UseatwopositionSuctionStopValvetoallowthecoiltoallowthecoiltograduallycomeback

downtosuctionpressureattheendofdefrost.4. DevelopandmaintainacompletePSMRMP(ProcessSafetyManagementRiskManagement

Program)foryourammoniarefrigerationsystem.5. Developandmaintainacultureofsafetytrainingandpreparednessthroughoutalllevelsofyour

organization.ReferencesCole,R.A.1989.RefrigerationLoadsinaFreezerDuetoHotGasDefrostandTheirAssociatedCosts.ASHRAETransactions,V.95,Pt.2.IIAR.2004.AmmoniaRefrigerationPipingHandbook.InternationalInstituteofAmmoniaRefrigeration.ColmacCoilManufacturing,Inc.2003.BulletinENG00014424:UnitCoolers,Installation,Operation,andMaintenance.ColmacCoilManufacturing,Inc.Colville,WA.Nelson,B.I.2003.MadeforAmmonia.ProcessCooling&Equipment.July/August2003.

Formoreinformation,pleasecontactColmacCoilManufacturing,Inc.Phone:800.845.6778or509.684.2595

P.O.Box571,Colville,WA.991140571;www.colmaccoil.comCopyright2011ColmacCoilManufacturing,Inc.