工程热力学(英文版)第10单元课件

1Chap9SummaryBasicconsiderations:actualcycle,idealcycle,carnotcycle,P-V,T-SReciprocatingenginesAssumptionsofgaspowercyclesTDCairstandardassumptions(空气标准假设):1)air=idealgas,Cv=const;2)internalreversibleprocess;3)combustionheatadditionprocess;4)exhaustheatrejectionprocessSIengines-OttocycleFourstrokes：compressionstroke,expansionorpowerstroke,exhauststroke,intakestroke.

(压缩冲程、做功（燃烧、膨胀）冲程、排气冲程和吸气冲程)CIengine-DieselCycleTheidealDieselcycle:1-2isentropiccompression(等熵压缩);2-3constantPheataddition(定压吸热);3-4isentropicexpansion(等熵膨胀);4-1constantVheatrejection(定容放热)Reciprocatingenginesareclassifiedas

spark-ignition(SI)engines(点燃式内燃机)compression-ignition(CI)engines(压燃式内燃机)BDCStrokeBoreIntakevalveExhausevalveMEPTheidealOttocycle:

1-2isentropiccompression(等熵压缩);2-3constantVheataddition(定容吸热);3-4isentropicexpansion(等熵膨胀);4-1constantVheatrejection(定容放热)1Chap9SummaryBasicconsiderat2Chap10SummaryCarnotcycleisnotasuitablemodelforvaporpowercycle,RankineCycleWaystoIncreaseη

ofRankineCycleRankincycles(朗肯循环)Rankinecycle:1-2Isentropiccompression(pump);2-3ConstPheataddition(boiler);3-4Isentropicexpansion(turbine);4-1ConstPheatrejection(condenser)Reheatrankinecycle（再热循环）Reheatcycle:Expandthesteamintheturbineintwostagesandreheatitinbetween.regenerativerankinecycle(回热循环)Regenerativecycle:extractingsteamfromtheturbinetoheatthefeedwaterbeforeboilerBasicidea:increaseThigh,avgforQin,anddecreaseTlow,avgforQout1,Loweringthecondenserpressure(lowerTlow,avg)2,SuperheatingthesteamtohigherT(increaseThigh,avg)3,Increasetheboilerpressure(increaseThigh,avg)Cogeneration(热电合供循环)Cogenerationistheproductionofmorethanoneusefulformofenergy(suchasprocessheatandfromthesameenergysource2Chap10SummaryCarnotcycleis3Chapter10VaporPowerCycles(蒸汽动力循环)3Chapter10VaporPowerCycles4Steamisthemostcommonworkingfluidusedinvaporpowercyclesbecauseofitsmanydesirablecharacteristics:lowcost,availability,andhighhfg.Thischapterismostlydevotedtothediscussionofsteampowerplants.Steampowerplantsarecommonlyreferredtoascoalplants,nuclearplants,ornaturalgasplants,dependingonthetypeoffuelusedtosupplyheattothesteam.Thesteamgoesthroughthesamebasiccycleinallofthem.4Steamisthemostcommonwork510-1TheCarnotVaporCycleCarnotcycleisthemostefficientcycleoperatingbetweentwospecifiedtemperaturelimits;Itwouldbeperfectifwemakeitastheidealcycleforvaporpowerplants.However,itisnotasuitablecycleforpowercycles.Considerasteady-flowcarnotcyclewithinthesaturationdomeofapuresubstance.510-1TheCarnotVaporCycleCaDomeDuomoDomeDuomo71-2:heatedinaboiler.Isothermalexpansion(等温膨胀)

2-3:expandedinaturbine.Adiabaticexpansion(定熵膨胀)；3-4:condensedinacondenser.Isothermalcompression(等温压缩).4-1:compressedinacompressor.Adiabaticcompression(绝热压缩).Severalimpracticalitiesinthiscycle(a)For1-2and3-4,isothermalprocessareeasytoachievebykeepthemaconstantP.But,thisisonlyapplicablefortwophasesystems,andMAXTHis373.95℃forwater.2-3isentropicisclosetoawell-designedturbine.Butqualityofsteamisdecreasingduring2-3.Lowquality(highmoistureisbadforturbineblades,becauseoferosionproblem)4-1involvescompressionofliquid-vaportoasaturatedliquid.Itisnotpracticaltodesignacompressorthathandlestwophases.71-2:heatedinaboiler.Isot8Foracarnotcyclelike(b),problemsare:4-1Isentropiccompressiontoextremelyhighpressures1-2isothermalheattransferatvariablepressures.So,weconcludethat:theCarnotcyclecannotbeapproximatedinactualdevicesandisnotarealisticmodelforvaporpowercycles.8Foracarnotcyclelike(b),p910-2Rankinecycle:theidealcycleforvaporpowercyclesRankinecycle（朗肯循环）

istheidealcycleforvaporpowercycles.Itdoesnotinvolveanyinternalirreversibilitiesandconsistsoffourprocesses.1-2Isentropiccompressioninapump2-3Constantpressureheatadditioninaboiler3-4Isentropicexpansioninaturbine4-1Constantpressureheatrejectioninacondenser910-2Rankinecycle:theideal101-2Isentropiccompressioninapump(泵，等熵压缩)2-3ConstantPheatadditioninaboiler(锅炉，定压加热)3-4Isentropicexpansioninaturbine(汽轮机，等熵膨胀)4-1ConstantPheatrejectioninacondenser(冷凝器，定压放热)饱和水过冷水过热蒸汽湿饱和蒸汽101-2Isentropiccompressioni11EnergyAnalysisoftheIdealRankineCycleAllfourcomponentsassociatedwiththeRankinecycle(thepump,boiler,turbine,andcondenser)aresteady-flowdevicesRankinecyclecanbeanalyzedassteady-flowprocesses.Thekineticandpotentialenergychangesofthesteamareusuallysmallandcanbeneglected.So,steady-flowenergyequationperunitmassofthesteamis11EnergyAnalysisoftheIdeal121-2:Pump(q=0)2-3:Boiler(w=0)3-4:Turbine(q=0)4-1:condenser(w=0)ThermalefficiencyEnergyAnalysisoftheIdealRankineCycle121-2:Pump(q=0)EnergyAnalysi13State1:P1,satliquidh1,v1State2:P2,s2=s1

h2State3:P3,T3h3,s3State4:P4,satMix,s4=s3h4Known:P1=P4=75kPa;P2=P3=3MPa,T3=350℃=0.2613State1:P1,satliquid1410-4Howcanweincreasetheefficiencyoftherankinecycle?Thebasicideaofallthemodificationstoincreasethethermalefficiencyofapowercycleisthesame:Increasetheaveragetemperature

atwhichheatistransferredtotheworkingfluidintheboiler(heataddition),ordecreasetheaveragetemperature

atwhichheatisrejectedfromtheworkingfluidinthecondenser(heatrejection).1410-4Howcanweincreasethe1510-4Howcanweincreasetheefficiencyoftherankinecycle?1.Loweringthecondenserpressure(lowerTlow,avg)Loweringthecondenserpressurelowersthetemperatureofsteam,andthusthetemperatureatwhichheatisrejected.1510-4Howcanweincreasethe1610-4Howcanweincreasetheefficiencyoftherankinecycle?1.Loweringthecondenserpressure(lowerTlow,avg)Effects:

(redforpositive,bluefornegative)Increasethenetworkoutput.Area1-4-4’-1’-2’-2-1.Increaseheatinputalso.Areaunder2’-2.Thermalefficiencyofcycleisincreased.Increasethemoisturecontentofsteaminturbine,whichwoulderodestheblades.x4’<x4(canbecorrected)ThereisalowerlimitforthisP:thePsatcorrespondingtoTofthecoolingmedium.P≥Psat@(Tsat=Triver+△T).E.g:Triver=15℃,△T=10℃,P≥

Psat@(Tsat=25℃)=3.2KPaAirleakageintothecondenser.1610-4Howcanweincreasethe1710-4Howcanweincreasetheefficiencyoftherankinecycle?2.SuperheatingthesteamtohigherT(increaseThigh,avg)SuperheatingthesteamtohighTincreasetheTofsteam,andthusthetemperatureatwhichheatisabsorbed.1710-4Howcanweincreasethe1810-4Howcanweincreasetheefficiencyoftherankinecycle?2.SuperheatingthesteamtohigherT(increaseThigh,avg)Effects:

(redforpositive,bluefornegative)Increasethenetworkoutput.Area3-3’-4-4’-3.Increaseheatinputalso.Areaunder3-3’Thermalefficiencyofcycleisincreasedasoveralleffectdecreasethemoisturecontentofsteaminturbine.x4’>x4ThereisalimitforthisT,becauseofmetallurgicalproblem.Current,thehighestTsteamatturbineinletis620℃.ToimproveorreplacethematerialsisonewaytoincreasethisTsteam.Likeusingceramics.1810-4Howcanweincreasethe1910-4Howcanweincreasetheefficiencyoftherankinecycle?3.Increasetheboilerpressure(increaseThigh,avg)Increasingtheoperatingpressureoftheboilerisanotherwayofincreasingtheaveragetemperatureduringtheheatadditionprocess.1910-4Howcanweincreasethe2010-4Howcanweincreasetheefficiencyoftherankinecycle?3.Increasetheboilerpressure(increaseThigh,avg)Effects:

(redforpositive,bluefornegative)Increaseanddecreaseofnetworkoutput.Area2-2’-3’-2andarea3-4-4’-3Increaseanddecreaseheatinput.ThermalefficiencyisincreasedasThigh,avgincreased.increasemoistureofsteaminturbine,butcanbecorrectedbyreheating.Manysteampowerplantsoperateatsupercriticalpressure(P>22.06MPa)andhavethermalefficiencisof40%.2010-4Howcanweincreasethe21212210-5theidealreheatrankinecycle(再热循环)Then,aquestionis:Howcanwetakeadvantageoftheincreasedefficienciesathigherboilerpressureswithoutfacingtheproblemofexcessivemoistureatthefinalstagesoftheturbine?“3.Increasetheboilerpressure(increaseThigh,avg)”increasesmoistureofsteaminturbineaswellasthermalefficiency2210-5theidealreheatrankin2310-5theidealreheatrankinecycle(再热循环)Twopossibilitiescometomind:1.Superheatthesteamtoveryhightemperaturesbeforeitenterstheturbine.Butthisisnotaviablesolution,sinceitrequiresraisingthesteamtemperaturetometallurgicallyunsafelevels.2.Expandthesteamintheturbineintwostages,andreheatitinbetween.Inotherwords,modifythesimpleidealRankinecyclewithareheatprocess.itisapracticalsolutiontothemoistureprobleminturbines,andiscommonlyusedinmodernsteampowerplants2310-5theidealreheatrankin2410-5theidealreheatrankinecycle(再热循环)Intheidealreheatrankinecycle,theexpansionprocesstakesplaceintwostagesFirststage(highpressureturbine),steamisexpandedisentropicallytoanintermediatepressureandsentbacktotheboilertobereheatedatconstantpressure,untilreachtheinletToffirstturbinestage.Secondstage(lowpressureturbine)steamisexpandedisentropicallytothecondenserpressure.2410-5theidealreheatrankin2510-5theidealreheatrankinecycle(再热循环)Effects:theincorporationofsinglereheatinapowerplantimprovesthecycleefficiencyby4-5%.(Thigh,avgisincreased)Decreasethemoistureofthesteaminturbine.Morethan2reheatisnotpracticalbecause:Efficiencyimprovementgainedby2ndreheatisonlyhalfof1stone;butcausePturbinedecreaseandsuperheatedexhaustthenTlow,avgincrease.AddedcostandcomplexityDoublereheatisonlyusedonsupercriticalpressurepowerplants.IfmaterialscouldwithstandsufficientlyhighT,reheatcycleisnotnecessary.

2510-5theidealreheatrankin2610-6theidealregenerativerankinecycle(回热循环)InT-sdiagramoftheRankinecycle,heattransferduringprocess2-2’isfoundatarelativelylowT,solowercycleefficiency;raiseToftheliquidleavingthepumpbeforetheboilerisonesolution;Apracticalprocess(regeneration,回热)isaccomplishedbyextractingsteamfromtheturbineatvariouspointsandheatthefeedwaterbeforetheboiler.Thedevicewherethefeedwaterisheatedbyregenerationiscalledaregenerator(回热器),orfeedwaterheater（给水加热器）.2610-6theidealregenerative2710-6theidealregenerativerankinecycle(回热循环)Effectsofregeneration:Improvecycleefficiency.Provideaconvenientmeansofdeaerating(除气)thefeedwaterandpreventcorrosionintheboiler.Controlthelargevolumeflowrateofthesteamatthefinalstagesoftheturbine(duetolargespecificvolumesatlowpressures)Regenerationhasbeenusedinallmodernsteampowerplantssince1920s.thefeedwatercouldbeheatedbymixingwithsteam(openfeedwaterheaters)orwithoutmixing(closedfeedwaterheaters)2710-6theidealregenerative2810-6theidealregenerativerankinecycle(回热循环)Anopen(ordirect-contact)feedwaterheaterisbasicallyamixingchamber,wherethesteamextractedfromtheturbinemixeswiththefeedwaterexitingthepump.2810-6theidealregenerative2910-6theidealregenerativerankinecycle(回热循环)Intheclosedfeedwaterheater,heatistransferredfromtheextractedsteamtothefeedwaterwithoutanymixingtakingplace.Thetwostreamscanbeatdifferentpressures,sincetheydonotmix.2910-6theidealregenerative3010-6theidealregenerativerankincycle(回热循环)ComparisonOpenfeedwaterheaters

aresimpleandinexpensivehavegoodheattransfercharacteristics.Theybringthefeedwatertothesaturationstate.Foreachheater,however,apumpisrequiredtohandlethefeedwater.

Theclosedfeedwaterheaters

morecomplexbecauseoftheinternaltubingnetwork,andmoreexpensive.Heattransferislesseffectivesincethetwostreamsarenotallowedtobeindirectcontact.However,closedfeedwaterheatersdonotrequireaseparatepumpforeachheatersincetheextractedsteamandthefeedwatercanbeatdifferentP3010-6theidealregenerative10-6theidealregenerativerankinecycle(回热循环)Moststeampowerplantsuseacombinationofopenandclosedfeedwaterheaters.3110-6theidealregenerativera3210-8Cogeneration(热电合共循环)Ingeneral,

cogeneration

istheproductionofmorethanoneusefulformofenergy(suchasprocessheatandelectricpower)fromthesameenergysource.Anidealcogenerationplant:120KWenergyisinputtotheboiler,20KWpowerisproducedinturbine,100KWheatissuppliedasprocessheat.Utilizationfactor(热量利用系数):3210-8Cogeneration(热电合共循环)In3310-8Cogeneration(热电合共循环)Anidealcogenerationplantisnotpracticalsincethepowerandprocess-heatisnotadjustable.Andamorepracticalcogenerationplantisshowninfig10-22.Cogenerationplants(热电厂)haveprovedtobeeconomicallyveryattractive.Moreandmorehavebeeninstalledandarebeinginstalled.Especiallyindistrictheating(区域集中供热).3310-8Cogeneration(热电合共循环)An10-310-710-9自学10-335Chap10SummaryCarnotcycleisnotasuitablemodelforvaporpowercycle,RankineCycleWaystoIncreaseη

ofRankineCycleRankinecycles(朗肯循环)Rankinecycle:1-2Isentropiccompression(pump);2-3ConstPheataddition(boiler);3-4Isentropicexpansion(turbine);4-1ConstPheatrejection(condenser)Reheatrankinecycle（再热循环）Reheatcycle:Expandthesteamintheturbineintwostagesandreheatitinbetween.regenerativerankinecycle(回热循环)Regenerativecycle:extractingsteamfromtheturbinetoheatthefeedwaterbeforeboilerBasicidea:increaseThigh,avgforQin,anddecreaseTlow,avgforQout1,Loweringthecondenserpressure(lowerTlow,avg)2,SuperheatingthesteamtohigherT(increaseThigh,avg)3,Increasetheboilerpressure(increaseThigh,avg)Cogeneration(热电合供循环)Cogenerationistheproductionofmorethanoneusefulformofenergy(suchasprocessheatandfromthesameenergysource35Chap10SummaryCarnotcyclei3610-1C10-2C10-7C10-8C10-9C10-10C10-12C

10-29C10-30C10-39C10-40C3610-1C37Chap9SummaryBasicconsiderations:actualcycle,idealcycle,carnotcycle,P-V,T-SReciprocatingenginesAssumptionsofgaspowercyclesTDCairstandardassumptions(空气标准假设):1)air=idealgas,Cv=const;2)internalreversibleprocess;3)combustionheatadditionprocess;4)exhaustheatrejectionprocessSIengines-OttocycleFourstrokes：compressionstroke,expansionorpowerstroke,exhauststroke,intakestroke.

(压缩冲程、做功（燃烧、膨胀）冲程、排气冲程和吸气冲程)CIengine-DieselCycleTheidealDieselcycle:1-2isentropiccompression(等熵压缩);2-3constantPheataddition(定压吸热);3-4isentropicexpansion(等熵膨胀);4-1constantVheatrejection(定容放热)Reciprocatingenginesareclassifiedas

spark-ignition(SI)engines(点燃式内燃机)compression-ignition(CI)engines(压燃式内燃机)BDCStrokeBoreIntakevalveExhausevalveMEPTheidealOttocycle:

1-2isentropiccompression(等熵压缩);2-3constantVheataddition(定容吸热);3-4isentropicexpansion(等熵膨胀);4-1constantVheatrejection(定容放热)1Chap9SummaryBasicconsiderat38Chap10SummaryCarnotcycleisnotasuitablemodelforvaporpowercycle,RankineCycleWaystoIncreaseη

ofRankineCycleRankincycles(朗肯循环)Rankinecycle:1-2Isentropiccompression(pump);2-3ConstPheataddition(boiler);3-4Isentropicexpansion(turbine);4-1ConstPheatrejection(condenser)Reheatrankinecycle（再热循环）Reheatcycle:Expandthesteamintheturbineintwostagesandreheatitinbetween.regenerativerankinecycle(回热循环)Regenerativecycle:extractingsteamfromtheturbinetoheatthefeedwaterbeforeboilerBasicidea:increaseThigh,avgforQin,anddecreaseTlow,avgforQout1,Loweringthecondenserpressure(lowerTlow,avg)2,SuperheatingthesteamtohigherT(increaseThigh,avg)3,Increasetheboilerpressure(increaseThigh,avg)Cogeneration(热电合供循环)Cogenerationistheproductionofmorethanoneusefulformofenergy(suchasprocessheatandfromthesameenergysource2Chap10SummaryCarnotcycleis39Chapter10VaporPowerCycles(蒸汽动力循环)3Chapter10VaporPowerCycles40Steamisthemostcommonworkingfluidusedinvaporpowercyclesbecauseofitsmanydesirablecharacteristics:lowcost,availability,andhighhfg.Thischapterismostlydevotedtothediscussionofsteampowerplants.Steampowerplantsarecommonlyreferredtoascoalplants,nuclearplants,ornaturalgasplants,dependingonthetypeoffuelusedtosupplyheattothesteam.Thesteamgoesthroughthesamebasiccycleinallofthem.4Steamisthemostcommonwork4110-1TheCarnotVaporCycleCarnotcycleisthemostefficientcycleoperatingbetweentwospecifiedtemperaturelimits;Itwouldbeperfectifwemakeitastheidealcycleforvaporpowerplants.However,itisnotasuitablecycleforpowercycles.Considerasteady-flowcarnotcyclewithinthesaturationdomeofapuresubstance.510-1TheCarnotVaporCycleCaDomeDuomoDomeDuomo431-2:heatedinaboiler.Isothermalexpansion(等温膨胀)

2-3:expandedinaturbine.Adiabaticexpansion(定熵膨胀)；3-4:condensedinacondenser.Isothermalcompression(等温压缩).4-1:compressedinacompressor.Adiabaticcompression(绝热压缩).Severalimpracticalitiesinthiscycle(a)For1-2and3-4,isothermalprocessareeasytoachievebykeepthemaconstantP.But,thisisonlyapplicablefortwophasesystems,andMAXTHis373.95℃forwater.2-3isentropicisclosetoawell-designedturbine.Butqualityofsteamisdecreasingduring2-3.Lowquality(highmoistureisbadforturbineblades,becauseoferosionproblem)4-1involvescompressionofliquid-vaportoasaturatedliquid.Itisnotpracticaltodesignacompressorthathandlestwophases.71-2:heatedinaboiler.Isot44Foracarnotcyclelike(b),problemsare:4-1Isentropiccompressiontoextremelyhighpressures1-2isothermalheattransferatvariablepressures.So,weconcludethat:theCarnotcyclecannotbeapproximatedinactualdevicesandisnotarealisticmodelforvaporpowercycles.8Foracarnotcyclelike(b),p4510-2Rankinecycle:theidealcycleforvaporpowercyclesRankinecycle（朗肯循环）

istheidealcycleforvaporpowercycles.Itdoesnotinvolveanyinternalirreversibilitiesandconsistsoffourprocesses.1-2Isentropiccompressioninapump2-3Constantpressureheatadditioninaboiler3-4Isentropicexpansioninaturbine4-1Constantpressureheatrejectioninacondenser910-2Rankinecycle:theideal461-2Isentropiccompressioninapump(泵，等熵压缩)2-3ConstantPheatadditioninaboiler(锅炉，定压加热)3-4Isentropicexpansioninaturbine(汽轮机，等熵膨胀)4-1ConstantPheatrejectioninacondenser(冷凝器，定压放热)饱和水过冷水过热蒸汽湿饱和蒸汽101-2Isentropiccompressioni47EnergyAnalysisoftheIdealRankineCycleAllfourcomponentsassociatedwiththeRankinecycle(thepump,boiler,turbine,andcondenser)aresteady-flowdevicesRankinecyclecanbeanalyzedassteady-flowprocesses.Thekineticandpotentialenergychangesofthesteamareusuallysmallandcanbeneglected.So,steady-flowenergyequationperunitmassofthesteamis11EnergyAnalysisoftheIdeal481-2:Pump(q=0)2-3:Boiler(w=0)3-4:Turbine(q=0)4-1:condenser(w=0)ThermalefficiencyEnergyAnalysisoftheIdealRankineCycle121-2:Pump(q=0)EnergyAnalysi49State1:P1,satliquidh1,v1State2:P2,s2=s1

h2State3:P3,T3h3,s3State4:P4,satMix,s4=s3h4Known:P1=P4=75kPa;P2=P3=3MPa,T3=350℃=0.2613State1:P1,satliquid5010-4Howcanweincreasetheefficiencyoftherankinecycle?Thebasicideaofallthemodificationstoincreasethethermalefficiencyofapowercycleisthesame:Increasetheaveragetemperature

atwhichheatistransferredtotheworkingfluidintheboiler(heataddition),ordecreasetheaveragetemperature

atwhichheatisrejectedfromtheworkingfluidinthecondenser(heatrejection).1410-4Howcanweincreasethe5110-4Howcanweincreasetheefficiencyoftherankinecycle?1.Loweringthecondenserpressure(lowerTlow,avg)Loweringthecondenserpressurelowersthetemperatureofsteam,andthusthetemperatureatwhichheatisrejected.1510-4Howcanweincreasethe5210-4Howcanweincreasetheefficiencyoftherankinecycle?1.Loweringthecondenserpressure(lowerTlow,avg)Effects:

(redforpositive,bluefornegative)Increasethenetworkoutput.Area1-4-4’-1’-2’-2-1.Increaseheatinputalso.Areaunder2’-2.Thermalefficiencyofcycleisincreased.Increasethemoisturecontentofsteaminturbine,whichwoulderodestheblades.x4’<x4(canbecorrected)ThereisalowerlimitforthisP:thePsatcorrespondingtoTofthecoolingmedium.P≥Psat@(Tsat=Triver+△T).E.g:Triver=15℃,△T=10℃,P≥

Psat@(Tsat=25℃)=3.2KPaAirleakageintothecondenser.1610-4Howcanweincreasethe5310-4Howcanweincreasetheefficiencyoftherankinecycle?2.SuperheatingthesteamtohigherT(increaseThigh,avg)SuperheatingthesteamtohighTincreasetheTofsteam,andthusthetemperatureatwhichheatisabsorbed.1710-4Howcanweincreasethe5410-4Howcanweincreasetheefficiencyoftherankinecycle?2.SuperheatingthesteamtohigherT(increaseThigh,avg)Effects:

(redforpositive,bluefornegative)Increasethenetworkoutput.Area3-3’-4-4’-3.Increaseheatinputalso.Areaunder3-3’Thermalefficiencyofcycleisincreasedasoveralleffectdecreasethemoisturecontentofsteaminturbine.x4’>x4ThereisalimitforthisT,becauseofmetallurgicalproblem.Current,thehighestTsteamatturbineinletis620℃.ToimproveorreplacethematerialsisonewaytoincreasethisTsteam.Likeusingceramics.1810-4Howcanweincreasethe5510-4Howcanweincreasetheefficiencyoftherankinecycle?3.Increasetheboilerpressure(increaseThigh,avg)Increasingtheoperatingpressureoftheboilerisanotherwayofincreasingtheaveragetemperatureduringtheheatadditionprocess.1910-4Howcanweincreasethe5610-4Howcanweincreasetheefficiencyoftherankinecycle?3.Increasetheboilerpressure(increaseThigh,avg)Effects:

(redforpositive,bluefornegative)Increaseanddecreaseofnetworkoutput.Area2-2’-3’-2andarea3-4-4’-3Increaseanddecreaseheatinput.ThermalefficiencyisincreasedasThigh,avgincreased.increasemoistureofsteaminturbine,butcanbecorrectedbyreheating.Manysteampowerplantsoperateatsupercriticalpressure(P>22.06MPa)andhavethermalefficiencisof40%.2010-4Howcanweincreasethe57215810-5theidealreheatrankinecycle(再热循环)Then,aquestionis:Howcanwetakeadvantageoftheincreasedefficienciesathigherboilerpressureswithoutfacingtheproblemofexcessivemoistureatthefinalstagesoftheturbine?“3.Increasetheboilerpressure(increaseThigh,avg)”increasesmoistureofsteaminturbineaswellasthermalefficiency2210-5theidealreheatrankin5910-5theidealreheatrankinecycle(再热循环)Twopossibilitiescometomind:1.Superheatthesteamtoveryhightemperaturesbeforeitenterstheturbine.Butthisisnotaviablesolution,sinceitrequiresraisingthesteamtemperaturetometallurgicallyunsafelevels.2.Expandthesteamintheturbineintwostages,andreheatitinbetween.Inotherwords,modifythesimpleidealRankinecyclewithareheatprocess.itisapracticalsolutiontothemoistureprobleminturbines,andiscommonlyusedinmodernsteampowerplants2310-5theidealreheatrankin6010-5theidealreheatrankinecycle(再热循环)Intheidealreheatrankinecycle,theexpansionprocesstakesplaceintwostagesFirststage(highpressureturbine),steamisexpandedisentropicallytoanintermediatepressureandsentbacktotheboilertobereheatedatconstantpressure,untilreachtheinletToffirstturbinestage.Secondstage(lowpressureturbine)steamisexpandedisentropicallytothecondenserpressure.2410-5theidealreheatrankin6110-5theidealreheatrankinecycle(再热循环)Effects:theincorporationofsinglereheatinapowerplantimprovesthecycleefficiencyby4-5%.(Thigh,avgisincreased)Decreasethemoistureofthesteaminturbine.Morethan2reheatisnotpracticalbecause:Efficiencyimprovementgainedby2ndreheatisonlyhalfof1stone;butcausePturbinedecreaseandsuperheatedexhaustthenTlow,avgincrease.AddedcostandcomplexityDoublereheatisonlyusedonsupercriticalpressurepowerplants.IfmaterialscouldwithstandsufficientlyhighT,reheatcycleisnotnecessary.

2510-5theidealreheatrankin6210-6theidealregenerativerankinecycle(回热循环)InT-sdiagramoftheRankinecycle,heattransferduringprocess2-2’isfoundatarelativelylowT,solowercycleefficiency;raiseToftheliquidleavingthepumpbeforetheboilerisonesolution;Apracticalprocess(regeneration,回热)isaccomplishedbyextractingsteamfromtheturbineatvariouspointsandheatthefeedwaterbeforetheboiler.Thedevicewherethefeedwaterisheatedbyregenerationiscalledaregenerator(回热器),orfeedwaterheater（给水加热器）.2610-6theidealregenerative6310-6theidealregenerativerankinecycle(回热循环)Effectsofregeneration:Improvecycleefficiency.Provideaconvenientmeansofdeaerating(除气)thefeedwaterandpreventc