MULTI-STAGE VAPOUR COMPRESSION REFRIGERATION …

1 MULTI-STAGE VAPOUR COMPRESSION REFRIGERATION SYSTEMS(VCR)WHY MULTI-STAGE ??? Theperformanceofsinglestagesystemsshowst hattheseareadequateaslongasthetemperatur edifferencebetweenevaporatorandcondenser (temperaturelift)issmall. Thetemperatureliftcanbecomelargeeitherdu etotherequirementofverylowevaporatortemp eraturesand/orduetotherequirementofveryh ighcondensingtemperatures. Forexample,infrozenfoodindustriestherequ iredevaporatorcanbeaslowas 40oC,whileinchemicalindustriestemperatur esaslowas 150oCmayberequiredforliquefactionofgases . Onthecondenserside,Refrigerationsystemis usedasaheatpumpforheatingapplicationssuc hasprocessheating, Forfluorocarbonandammoniabasedrefrigerat ionsystems: Singlestagesystemisuseduptoanevaporatort emperatureof 30oC.

2 Atwo-stagesystemisusedupto 60oCand Athree-stagesystemisusedfortemperaturesb elow 60oC. Multi-stagesystemsarealsousedinapplicati onsrequiringrefrigerationatdifferenttemp eratures. Forexample,inadairyplantrefrigerationmay berequiredat 30oCformakingicecreamandat2oCforchilling milk. Insuchcasesitmaybeadvantageoustouseamult i-evaporatorsystem Amulti-stagesystemisarefrigerationsystem withtwoormorelow-sidepressures. Multi-stagesystemscanbeclassifiedinto:a) Multi-compressionsystemsb)Multi-evaporat orsystemsc)Cascadesystems,etc. Twoconceptswhicharenormallyintegraltomul ti-pressuresystemsare,i)flashgasremoval, andii)intercooling. Flashgasdoesnotcontributetotherefrigerat ioneffectasitisalreadyintheformofvapour, anditincreasesthepressuredropintheevapor ator.

3 ItispossibletoimprovetheCOPofthesystemif theflashgasisremovedassoonasitisformedan drecompressedtocondenserpressurebutitisn otpractical. ToimprovetheCOPofthesystem, gas removal using flash tank Aflashtankisapressurevessel, ,Piusingalowsidefloatvalve(process6-7). Thefloatvalvealsomaintainsaconstantliqui dlevelintheflashtank. Intheflashtank,therefrigerantliquidandva pourareseparated. Thesaturatedliquidatpoint8isfedtotheevap oratorafterthrottlingittotherequiredevap oratorpressure,Pe(point9)usinganexpansio nvalve. Dependinguponthetypeofthesystem,thesatur atedvapourintheflashtank(point3)iseither compressedtothecondenserpressureorthrott ledtotheevaporatorpressureIntercooling in MULTI-STAGE COMPRESSION Specificworkinputreducesasspecificvolume ,v1isreduced.

4 Atagivenpressure,thespecificvolumecanber educedbyreducingthetemperature. Thisistheprinciplebehindintercoolinginmu lti-stagecompression. Intercoolingofthevapourmaybeachievedbyus ingeitherawater-cooledheatexchangerorbyt herefrigerantintheflashtank. Withwatercoolingtherefrigerantattheinlet tothehighstagecompressormaynotbesaturate d. Intercoolingnotonlyreducestheworkinputbu talsoreducesthecompressordischargetemper atureleadingtobetterlubricationandlonger compressorlife. Intercoolingusingliquidrefrigerantfromco ndenserintheflashtankmayormaynotreduceth epowerinputtothesystem,asitdependsuponth enatureoftherefrigerant. Theheatrejectedbytherefrigerantduringint ercoolinggeneratesadditionalvapourinthef lashtank,whichhastobecompressedbythehigh stagecompressor.

5 Thusthemassflowrateofrefrigerantthrought hehighstagecompressorwillbemorethanthato fthelowstagecompressor. Forammonia,thepowerinputusuallydecreases withintercoolingbyliquidrefrigerant. ForrefrigerantssuchasR12,R22,thepowerinp utmarginallyincreases. Thusintercoolingusingliquidrefrigerantis noteffectiveforR12andR22. Usingbothwater-coolingandflash-tank, of suitable intermediate pressure Foraircompressorswithintercoolingtothein itialtemperature,thetheoreticalworkinput tothesystemwillbeminimumwhenthepressurer atiosareequalforallstages. Forrefrigerants, system with flash gas removal and intercooling Theabovesystemoffersseveraladvantages,a) Qualityofrefrigerantenteringtheevaporato rreducesthusgivingrisetohigherrefrigerat ingeffect,lowerpressuredropandbetterheat transferintheevaporatorb) )Volumetricefficiencyofcompressorswillbe highduetoreducedpressureratiosd)Compress ordischargetemperatureisreducedconsidera bly.

6 Onedisadvantageoftheabovesystemisthatsin cerefrigerantliquidintheflashtankissatur ated, system with liquid sub cooler Use of flash tank for flash gas removal Use of flash tank for intercooling only Problem2 Therequiredrefrigerationcapacityofavapou rcompressionrefrigerationsystem(withR-22 asrefrigerant)is100kWat )Powerrequirementoftheoriginalsingle-sta gesystem;b) ,condenserandintercoolerissaturated, SystemsIndividualevaporatorsandasingleco mpressorwithapressure-reducingvalve1. Individual expansion valves Problem3:Thehightemperatureevaporator(Re frigerationcapacity5TR)ofamulti-evaporat orVCRsystem,workingwithammonia,isoperati ngat 6oCandthelowtemperatureevaporator(Refrig erationcapacity10TR)isoperatingat :a) ) ) ,intercoolingandflashgasremoval Themassflowrateofrefrigerantthroughthehi gh-stagecompressorwhichcanbeobtainedbyta kingacontrolvolumewhichincludestheflasht ankandhightemperatureevaporator(asshownb ydashedlineintheschematic)andapplyingmas sandenergybalance:m5+ m2= m7+ m3; m5= m3& m2= m7= of MULTI-STAGE systems Therefrigerantusedshouldhavehighcritical temperatureandlowfreezingpoint.

7 GenerallyonlyR12,R22andNH3systemshavebee nusedinmulti-stagesystemsasotherconventi onalworkingfluidsmayoperateinvacuumatver ylowevaporatortemperatures-leadstoleakag esintothesystem. Systems Inacascadesystemaseriesofrefrigerantswit hprogressivelylowerboilingpointsareusedi naseriesofsinglestageunits. Thecondenseroflowerstagesystemiscoupledt otheevaporatorofthenexthigherstagesystem andsoon. Thecomponentwhereheatofcondensationoflow erstagerefrigerantissuppliedforvaporizat ionofnextlevelrefrigerantiscalledascasca decondenser. Twodifferentrefrigerantsoperatingintwoin dividualcycles. Theyarethermallycoupledinthecascadeconde nser. Therefrigerantsselectedshouldhavesuitabl epressure-temperaturecharacteristics.

8 Itispossibletousemorethantwocascadestage s, of cascade systems Liquefaction of petroleum vapors Liquefaction of industrial gases Manufacturing of dry ice Deep freezing etc. Advantages of cascade systems Sinceeachcascadeusesadifferentrefrigeran t, Migrationoflubricatingoilfromonecompress ortotheotherispreventedOptimum cascade(coupling) temperature WhereTeandTcaretheevaporatortemperatureo flowtemperaturecascadeandcondensertemper atureofhightemperaturecascade, )COPb)Pressureratiosc)Massflowratesofeac hrefrigerantsd) , , ,find:a)Massofrefrigerantflowingthroughe achevaporator,b)Powerrequiredtodrivethes ystemandc) ABSORPTION REFRIGERATION SYSTEMS(VARS) OR THERMAL ENERGY DRIVEN SYSTEMS Therequiredinputtoabsorptionsystemsisint heformofheatsoalsocalledasheatoperatedor thermalenergydrivensystems.

9 Sinceconventionalabsorptionsystemsuseliq uidsforabsorptionofrefrigerant,thesearea lsosometimescalledaswetabsorptionsystems . PRINCIPLESIMPLE VARS Continuousrefrigerationisproducedatevapo rator,whileheatathightemperatureiscontin uouslysuppliedtothegenerator. Heatrejectiontotheexternalheatsinktakesp laceatabsorberandcondenser. Asmallamountofmechanicalenergyisrequired torunthesolutionpump. Ifweneglectpressuredrops,thentheabsorpti onsystemoperatesbetweenthecondenserandev aporatorpressures. Pressureinabsorberissameasthepressureine vaporatorandpressureingeneratorissameast hepressureincondenser. InVCRS-thevapouriscompressedmechanically usingthecompressor.

10 InVARS-thevapourisfirstconvertedintoaliq uidandthentheliquidispumpedtocondenserpr essureusingthesolutionpump. Forthesamepressuredifference, COP of Ideal VARSF romfirstlawofthermodynamics,ifweneglectt hepumpwork, + = + Fromsecondlowofthermodynamics,ie, + 0= + + 0= + Refrigerant-absorbent combinations for VARS Thedesirablepropertiesofrefrigerant-abso rbentmixturesforVARSare: Therefrigerantshouldexhibithighsolubilit ywithsolutionintheabsorber. Thereshouldbelargedifferenceintheboiling pointsofrefrigerantandabsorbent(greatert han200oC),sothatonlyrefrigerantisboiled- offinthegenerator-Onlypurerefrigerantcir culatesthroughrefrigerantcircuit(condens er-expansionvalve-evaporator)leadingtois othermalheattransferinevaporatorandconde nser.