Hot-Gas-Bypass

providinginsightsfortoday’shvacsystemdesigner©2003AmericanStandardInc.AllrightsreservedVolume32,No.2■engineersnewsletterblessingorcurse?HotGasBypassIt’snotenoughtodeliveranHVACsystemthatmeetstheuniqueneedsofeachapplication.Theowneralsodeservesasystemthatisthemostreliableandtheleastcostlytooperate.Inspecificcases,addinghotgasbypasstotherefrigerationsystemmaybenecessaryinordertoachieveallthreegoals.Butmoreoftenthannot,theresponsibilitytodeliverareliable,cost-effectivesystemwillprecludeitsaddition.Hotgasbypasshasthepotentialtomodulatecapacityorimprovereliability,butthesebenefitswillnotberealizedwithoutcarefulevaluationofitsappropriatenessandpainstakingattentiontodesign,installation,control,andmaintenance.ThesuccessofanapplicationwithHGBPdependsonunfailingdiligenceateverystep.HGBPcanbeablessingwhenitiswarrantedandeffectivelyimplemented.Inallothercases,HGBPwillbeacurse.Tounderstandwhy,let’sbrieflyreviewwhathotgasbypassdoesandhowitisapplied.Theobjective.Whendiminishingloadsforcearefrigerationsystemtooperateatunstableconditions,compressorandevaporatorcapacitiesbalanceateverlowersuctionpressuresandtemperatures.Unchecked,theeventualresultiscoilfrostingandcompressorflooding.Hotgasbypasscanstabilizethesystembalancepointbydivertinghot,high-pressurerefrigerantvaporfromthedischargelinedirectlytothelow-pressuresideofthesystem.ThistacticreliableoperationbyintroducingproblemsstemmingfrominsufficientoilreturnandrefrigerantloggingintheHGBPline.Andinallcases,hotgasbypassinflatesthelife-cyclecostofthesystem:■HGBPincreasestheinitialcost.Itrequiresanadditionalrefrigerantline,whichalsoincreasesthelikelihoodofrefrigerantleaksandoil/refrigerantlogging.■HGBPgreatlyreducesoperatingefficiencybecausethebypassedvapordoesnousefulcooling.■HGBPcostsmoretooperate.Astheloadfluctuates,thecompressorconsumesmoreenergybecauseit’sforcedtooperateatacompressionstepthatislikelyonestagehigherthannecessary.Theblessing.Despitetheseshortcomings,hotgasbypasscanbeappropriateforcoolingapplicationsthatdemandtight,continuous,thermalcontrol—particularlyiflargeamountsofoutdoorair,widelyvaryingloads,orexcessivecompressoron/offdelaysareinvolved.Mostoftenthesecharacteristicsdescribeprocesscoolingratherthanstandardcomfortcooling.Ifappliedproperly,HGBPcan:■Preventexcessivecompressorcycling■Matchsystemcapacitytoload■AllowthesystemtooperateatsafebalancepointsduringunsafeloadsApplicationConsiderationsAlthoughitcanbeappliedinrefrigerant-to-watersystems,comfortcoolingapplicationsofhotgasbypassHGBPinchilledwaterapplicationsCertainprocesscoolingapplicationsmayrequirehotgasbypasstomatchchillercapacitytoaninstantaneousload.Insuchcases,thesystemisdesignedtooperatecontinuouslyandhotgasbypassneutralizestherefrigerationeffect.Forcomfortcooling,however,theadditionofhotgasbypassisseldomnecessary.That’sbecauseaproperlydesignedchilledwaterloopcanprovideaneffectivebufferbetweensystemloadandchillercapacity.Thekeyistoestablishalooptimethatequalsthegreateroftwovalues:eithertheminimum“compressoroff”timeforthelaststageofcooling,ortheminimumlooptimepermittedbythechillercontroller.Youcanaccomplishthisbychangingthedesignflowrateorbyaddingatank.Thesimplicityofthisapproachwillbenefittheowner,aswilltheoperating-costsavings.(Achilledwatersystemwithouthotgasbypassrequireslesspowerthanonethatoperatesthecompressorswithhotgasbypass.)■keepsthecompressormorefullyloadedwhiletheevaporatorsatisfiesthepart-loadcondition.Also,thedivertedvaporraisesthesuctiontemperature,whichpreventsfrostfromforming.FordecadesHGBPhasbeenappliedindirect-expansionrefrigerantsystemstocontrolcapacityatlowloads.Itwasprobablyconceivedtocorrectajob-specificproblembutwassubsequently,andindiscriminately,addedtoHVACsystemsasapreventivemeasure.Thecurse.True,HGBPhasprovidedfrostcontrolandsomesemblanceofcapacitycontrolinmanyapplications.ButnumerouscasesexistinwhichHGBPeitherfailstosafelystabilizethesystemor,worsestill,undermines■2TraneEngineersNewsletter—Vol.32,No.2mostofteninvolverefrigerant-to-airevaporators.Withcountlesscombinationsofequipmentandlayoutspossible,howcanyoubestandmostresponsiblyimplementhotgasbypass?ApplyHGBPasalastresort.Onlyusehotgasbypassifallotherdesignoptionsfailtomeetthedemandsoftheapplication.Tomakethisdetermination,itiscriticalto…Understandtheyear-roundloadsandrequirementsofthesystem.“Mostcomfortcoolingapplicationscanoperatebetweenstepsofloadingwithoutlossoftemperatureorhumiditycontrol.”Notonlydoesthisstatementholdtruebetweenthethirdandfourthstepsofcooling,whentheoutdoorlatentloadishigh,butitalsoholdstrueforthestepbetweenoffandfirst-stagecoolingwhentheoutdoorlatentloadiscomparativelylow.Choosetherightarrangement.Hotgasbypassisarrangedinoneoftwoways(seethecomparisoninTable1,p.4);bothrequirespecialcareduringdesignandinstallation.1HGBPtotheevaporatorinletdelivershotrefrigerantvaporbetweentheexpansionvalveandthedistributor(Figure1).DuringHGBPoperation,theexpansionvalvemetersenoughliquidrefrigeranttobothdesuperheatthebypassedvaporandsatisfytheevaporatorload.Theresultingrefrigerantflowrateissufficienttocarryoilthrough