Heat exchanger - Week 7

POWEREQUIPMENTINSTRUCTOR:ROBERTA.MCLAUGHLINZAILITHEOZHAOFUNDAMENTALSOFHEATEXCHANGERS06:111LEARNINGOBJECTIVESUnderstandingofvarioustypesofheatexchangersusedinpowerproduction.Identifyoperatingprinciples,features,applicationsandlimitationsofvarioustypesofheatexchangers.DiscusstheSecondLawofThermodynamicsandhowitappliestoheatexchangers.Identifythethree(3)modesofheattransferandthe“gremlins”thatpreventtheprocess.IdentifythetworulespertainingtoefficientmaincondenseroperationandexamineandidentifythemainheatexchangersusedintheRankineCycle.Determinemethodofcalculatingheattransferintubetypeexchangers.06:112FIRSTLAWOFTHERMODYNAMICSInternalenergyfromonebodywillequalthatenergytakenbyasecondbody.Anythermodynamicsysteminanequilibriumstatepossessesastatevariablecalledtheinternalenergy(E)Betweenanytwoequilibriumstates,thechangeininternalenergyisequaltothedifferenceoftheheattransferintothesystemandworkdonebythesystem.06:1132NDLAWOFTHERMODYNAMICSHeatflowsfromhightemperatureregions(calledthesource)tolowtemperatureregions(calledthesink).Thereexistsausefulthermodynamicvariablecalledentropy(S).Anaturalprocessthatstartsinoneequilibriumstateandendsinanotherwillgointhedirectionthatcausestheentropyofthesystemplustheenvironmenttoincreaseforanirreversibleprocessandtoremainconstantforareversibleprocess.06:114TRANSFERSINTHREEWAYSConductionDirectcontactheatexchangeConvectionHeatexchangewithinafluidTwotypesFreeconvectionwheredensitydifferenceiscreatedwithinthefluid,causingmovementwithinthefluid.Heatedwaterorairmoveswithinthefluidduetothedensitydifference.Forcedconvectionwhereapressuredifferencecreatedbypumporfancausesthefluidmotion.RadiationHeattransferbywaves.06:115HEATTRANSFERTHROUGHPIPE06:116HEATEXCHANGERSAmechanicaldevicethatallowsthetransferofheatfromonesubstancetoanother.HeattransferistheflowofthermalenergyformonelocationtoanotherTsignifiesthetemperaturedifferenceofthecoolingfluidsinaheatexchangerTofthecoolingfluidisthedifferenceintemperatureenteringandleavingaheatexchangerTofthecooledfluidisthetemperaturedifferencebetweentheenteringandtheexitingheatexchanger.Whenaphasechangetakesplace,theremaynotbeaTacrosstheheatexchanger–latentheattransferThatmeansthattemperatureisnotandindicatorofheattransfer,butisonlyanindicatorofpotentialforheattransfer.06:117HEATEXCHANGERSThefluidflowthroughaheatexchangerincludesCounterflowThetwofluidsflowinoppositedirections06:118HEATEXCHANGERSParallelflowRarelyusedbecauseofthelengthoftimeitwouldtaketocreatethenecessaryheatexchange–makingthetransfersurfaceimpossiblebig06:119HEATEXCHANGERSCrossflowPrimarilyusedwherethemainpurposeistoremovelatentheat–likeinamaincondenser.TheTemp.variabletendencycurvessameasparallel06:1110HEATEXCHANGERSHeatexchangersfallintooneofthreecategoriesShellandtubealsoknownassurfacetypeheatexchangers.06:1111HEATEXCHANGERSPlatetype06:1112HEATEXCHANGERSPlatetypeSmall&highheatloadExpandable–retractableEasytoclean&maintaininspectionSmallpassagecaneasyclogEasytodamagewhileassemblingGenerallylimited290PISGandlower300°Fandless06:1113HEATEXCHANGERSDirectcontactheatersWhenthetwomediumsareindirectcontact06:1114WATERTOSTEAMTheheat-o-meterwouldbecalibratedinBtuOneBritishthermalunitistheheatrequiredtoraiseonepoundofwateronedegreeFahrenheitAsthewatertemperaturereaches212℉,theheat-o-meterreads180Btu.06:1115WATERTOSTEAMWATERINTOSTEAM06:1116SATURATEDPRESSUREMoleculesattheliquidsurfaceareinequilibriumbetweentheliquidandvapourphases;thevapourattheliquidsurfaceissaidtobesaturated.Atanygivenliquidsurfacetemperature,thevapourexertsapressureknownasthesaturationpressure.Moremoleculesescapeasthetemperatureincreasesandthisincreasesthevapourpressure06:1117RANKINECYCLERankinecycleisaheatenginewithvaporpowercycle.Thecommonworkingfluidiswater.Thecycleconsistsoffourprocesses:1-2:Isentropicexpansion(Steamturbine)2-3:Isobaricheatrejection(Condenser)3-4:Isentropiccompression(Pump)4-1:Isobaricheatsupply(Boiler)06:1118BOILERStartattheboiler(theheatsource)SaturatedsteamisproducedintheboilerthensuperheatedTheturbineusesthermalenergythenexhaustsorrejectssomethermalenergytothecondenser.Thatisenergytheturbinecannotuse.06:1119CONDENSERThecondenserisashellandtubeheatexchanger(alsoknownasasurface)thatuseswaterasacoolingmedium.SaltwaterforshipsFreshwaterforshoreplantsThecoolingmediumflowsthroughthetubesandtheexhauststeamsurroundsthetubes06:1120CONDENSERThecondensertakesawaythelatentheatofthesteamcausingittocollapsetowaterorcondensate.ThecondensatetemperatureshouldbeatornearthesaturationtemperatureIfitisbelowthesaturationtemperature,thenthecondensatehasbeensubcooledoritisdepressed.06:1121CONDENSER–RULENO.1TemperatureRiseisthedifferencebetweenthecirculatingwatertemperatureinlettothecondenserandtheoverboarddischargetemperature.Onasinglepass,marinetypecondenser,thetemperatureriseshouldbeabout10ºF.Thereisadirectrelationshipbetweentemperatureriseandcondensatedepression,andasanoperatingengineerthisisoneofthefundamentalrelationshipsthatyouhavecontroloverandsho