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. Atwo-stagesystemisusedupto 60oCand Athree-stagesystemisusedfortemperaturesb elow 60oC. multi -stagesystemsarealsousedinapplicati onsrequiringrefrigerationatdifferenttemp eratures.

2 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. ItispossibletoimprovetheCOPofthesystemif theflashgasisremovedassoonasitisformedan drecompressedtocondenserpressurebutitisn otpractical. ToimprovetheCOPofthesystem, gas removal using flash tank Aflashtankisapressurevessel, ,Piusingalowsidefloatvalve(process6-7). Thefloatvalvealsomaintainsaconstantliqui dlevelintheflashtank. Intheflashtank,therefrigerantliquidandva pourareseparated.

3 Thesaturatedliquidatpoint8isfedtotheevap oratorafterthrottlingittotherequiredevap oratorpressure,Pe(point9)usinganexpansio nvalve. Dependinguponthetypeofthesystem,thesatur atedvapourintheflashtank(point3)iseither compressedtothecondenserpressureorthrott ledtotheevaporatorpressureIntercooling in MULTI-STAGE COMPRESSION Specificworkinputreducesasspecificvolume ,v1isreduced. Atagivenpressure,thespecificvolumecanber educedbyreducingthetemperature. Thisistheprinciplebehindintercoolinginmu lti-stagecompression. Intercoolingofthevapourmaybeachievedbyus ingeitherawater-cooledheatexchangerorbyt herefrigerantintheflashtank. Withwatercoolingtherefrigerantattheinlet tothehighstagecompressormaynotbesaturate d. Intercoolingnotonlyreducestheworkinputbu talsoreducesthecompressordischargetemper atureleadingtobetterlubricationandlonger compressorlife. Intercoolingusingliquidrefrigerantfromco ndenserintheflashtankmayormaynotreduceth epowerinputtothesystem,asitdependsuponth enatureoftherefrigerant.

4 Theheatrejectedbytherefrigerantduringint ercoolinggeneratesadditionalvapourinthef lashtank,whichhastobecompressedbythehigh stagecompressor. 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.

5 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.

6 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. 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.

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

8 Thereshouldbelargedifferenceintheboiling pointsofrefrigerantandabsorbent(greatert han200oC),sothatonlyrefrigerantisboiled- offinthegenerator-Onlypurerefrigerantcir culatesthroughrefrigerantcircuit(condens er-expansionvalve-evaporator)leadingtois othermalheattransferinevaporatorandconde nser. Itshouldexhibitsmallheatofmixingsothatah ighCOPcanbeachieved Therefrigerant-absorbentmixtureshouldhav ehighthermalconductivityandlowviscosityf orhighperformance. Itshouldnotundergocrystallizationorsolid ificationinsidethesystem Themixtureshouldbesafe,chemicallystable, non-corrosive, (H2O-LiBr) (NH3-H2O)systemforrefrigerationapplicati onswithammoniaasrefrigerantandwaterasabs orbentProblemTheoperatingtemperaturesofa singlestagevapourabsorptionrefrigeration systemare:generator:90oC;condenserandabs orber:40oC; ) ) AMMONIA-WATER VARS Ammoniaistherefrigerantandwateristheabso rbent. Morecomplexindesignandoperationduetothes mallerboilingpointtemperaturedifferenceb etweentherefrigerantandabsorbent(about13 3oC).

9 DuetosmallerBPtempdifference, , Itconsistsofaseriesoftraysmountedaboveth egenerator. Thestrongsolutionfromtheabsorberandtheaq uafromtherectifierareintroducedatthetopo ftheanalyzerandflowdownwardoverthetraysi ntothegenerator. Inthisway,considerableliquidsurfaceareai sexposedtothevapourrisingfromthegenerato r. Isaclosedtypevapourcooleralsoknownasdehy drator. Itisgenerallywatercooledandmaybeofthedou blepipe,shellandcoilorshellandtubetype. Itsfunctionistocoolfurthertheammoniavapo rsleavingtheanalyzersothattheremainingwa tervaporsarecondensed. Thus,onlydryoranhydrousammoniavaporsflow tothecondenser. REFRIGERATOR InventedbytwoSwedishengineersCarlMunters andBaltzerVonPlatanin1925. Theideawasfirstdevelopedbythe ElectroluxCompany ofLuton,England. Alsocalledthree-fluidsabsorptionsystem. Themainpurposeofthissystemistoeliminatet hepumpsothemachinebecomesnoise-less. Ammoniaisusedastherefrigerant. Thehydrogenisusedtoincreasetherateofevap orationoftheliquidammoniapassingthrought heevaporatoranditisinsolubleinwater(solv entorabsorbent).

10 Thehydrogengasonlycirculatesfromtheabsor bertotheevaporatorandback. Thewholecycleiscarriedoutentirelybygravi tyflowoftherefrigerant. BROMIDE VARS Thelithiumbromidesolutionhasastrongaffin ityforwatervapourbecauseofitsverylowvapo urpressure. Lithiumbromidesolutioniscorrosive,theref oreinhibitorsshouldbeadded. Lithiumchromateisoftenusedasacorrosionin hibitor. Theabsorberandtheevaporatorareplacedinon eshell-lowpressureofthesystem. between VC and VA systemsVAPOR COMPRESSION SYSTEM Compressor work operated High COP because of using high grade energy(work) Performance is very sensitive to evaporator temperature COP reduces considerably at part loads Presence of liquid at the exit of the evaporator may damage compressor. Superheating at the evaporator exit increases compressor work Many moving parts Regular maintenance required Higher noise and vibrations Small systems are compact and large systems are bulky ( house hold) Economical when electricity is available (house, malls etc)VAPOR ABSORPTION SYSTEM Heat operated Low COP because of using low grade energy(heat) Performance not very sensitive to evaporator temperature COP doesn t reduce considerably at part loads Presence of liquid at the exit of the evaporator is not a problem Superheating at the evaporator exit is not a problem Few moving parts low maintenance required Less noise and vibrations Small systems are bulky and large systems are compact ( ice plants) Economical when waste heat is available in large quantity (industries)STEAM JET REFRIGERATION SYSTEM Thissystemusestheprincipleofboilingthewa terbelow1000C.