CHAPTER - 2 Steam Power Plant Cycles

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Power Plant Engineering Steam Power Plant Cycle

11 Created by: Assistant Professor Dr. Mishaal AbdulAmeer AbdulKareem ... [email protected]

Chapter(2)SteamPowerPlantCyclesSimplevaporcycle: Steam power plant is an example of a systemoperatinginacyclewhichincludesaseriesofseparatesteadyflowprocessesateachunitofthispowerplantasshowninfigure(2.1).

1. Boiler(B): Where the water is converted to steam at constant

pressurebytheheatenergyreceivedfromthefuel.

2. Turbine(T): Inwhichthesteamexpandstoalowerpressure.Causingworkenergytobeavailable.

3. Condenser(C): Inwhichheatenergyflowsfromthelowpressure

steam into the condenser coolingwater, resulting the steam tobecondensed.

4. Pump(P): Itreturnsthewaterintotheboiler.



CarnotCycleforSteam:

This cycle is composed of two isothermal and two isentropicprocesses,asshowninfigure(2.2).Therefore,theapplicationofthiscycletovaporconsistsofthefollowingprocesses:12:isentropicexpansioninturbine23:isothermalheatrejection,(T1=T2),(Partialcondensation),qC=Tmins34:isentropiccompressionofinitiallywetsteamincompressor41:isothermalheatadditioninboiler,(T4=T1,P4=P1),qB=Tmaxs

1

DisadvantagesofCarnotCycleapplication:AlthoughtheCarnotcycleisthemostefficientcycle.Therearereasonswhyitisnotusedinpractice,suchas:

1 Ithasalowworkratio2 Itisdifficulttostopcondensation(atstate3)3 It is difficult to startCompression of a verywet vapor (at state 3)

efficiently.The liquid tendstoseparateout from thevaporand thecompressorwilltransfertwophasemixture.

4 Thevolumeof fluid (atstate3) isveryhigh.Therefore,avery largecompressorisrequired.

Therefore, the Carnot cycle is modified to overcome the abovedifficulties.ThemodifiedcycleisknownasRankinecycle.



RankineCycle



Itisanidealcycleforasteampowerplantcycles,asshowninfigure(2.3).Foranygivenpressure, the steamapproaching the turbinemaybesuperheated(state1),drysaturated(state1),orwet(state1),butthefluidapproaching the pump in each case is saturated liquid (state 3). Steamexpandreversiblyandadiabaticallyintheturbinefrom(state1)to(state2),or(1to2),or(1to2).Thesteamleavingtheturbinecondensestowaterinthecondenseratconstantpressurefrom(state2,or2,or2)to(state3),the water is then pumped to the boiler to produce steam at constantpressurefrom(state4)to(state1,or1,or1).ThesteamflowaroundthecyclemaybeanalyzedusingtheSteadyFlowEnergyEquationandneglectingthechangesinK.EandP.Eterms.Therefore,eachprocesscanbeconsideredasfollows:

1 SteamGenerator(BoilerB): FromtheS.F.E.E.: 0 (Noshaftwork)

(kj)(kj/kg)

2 Turbine(T): FromtheS.F.E.E.:

BSteam

Water

1

4

12 12

0

12 12

0



0 (Insulation)

(kj)(kj/kg)

3 Condenser(C): FromtheS.F.E.E.:

0 (Noshaftwork)

(kj)(kj/kg)

4 FeedPump(P): FromtheS.F.E.E.: 0 (Insulation)

(kj)

(kj/kg)

Butsince =>

12 12

0

Steam

12 12

0



(Volume)(kj)(kj/kg)Thequantityof the feedpumpwork (WP) is small if compared to theturbine work (WT), it is usually neglected especially when the boilerpressureislow.

CycleNetwork(Wnet):

(kj)

(kj/kg) 0 Andiffeedpumpworkisneglected,then:

(kj/kg) 0

WorkRatio(WR):

ItistheratiooftheNetWorkoutputtotheGrossWorkoutputofthesystem.

if 0 1



SteamRate(SpecificSteamConsumptionS.S.C):It is thesteam flow in (kg/hr) required toproduce (1kW)ofpoweroutput.Therefore,theunitsof(S.S.C)is .

HeatRate(H.R):Itistherateofheatinputtotheplantrequiredtoproduce(1kW)ofpoweroutput.Therefore,theunitsof(H.R)is .

EfficienciesinSteamPowerPlant:

1 Cycleefficiencyorthermalefficiency2 Mechanicalefficiency3 Isentropicefficiency

. .

.

. 3600

. . 3600

=Steammassflowrate,(kg/hr). =TurbineBrakePower,(kW)=actualNetWorkobtainedattheshaftofturbine,(kj/kg)

. . .

. 3600 S. S. C

=Boilerheatadded,(kj/kg)



1. Cycleefficiencyorthermalefficiency:

0 0 2. Mechanicalefficiency:Since 3. Isentropicefficiency:

Example (1):ASteampowerplantoperatesbetweenaboilerpressureof(3bar)andacondenserpressureof(0.025bar).Calculatethecycleefficiency,theworkratio,andthespecificsteamconsumption:

a ForaCarnotcycleusingwetsteam.b ForaRankinecyclewithdrysaturatedsteamatentrytotheturbine.c For a Rankine cycle of (b), when the expansion process has an

isentropicefficiencyof(80%).

. 3600

3600 .



Solution:

a Carnotcycle:Fromstemtable:@3bar=>[email protected]=>tsat.=21.1oCTmax=133.5+273.15=406.65KTmin=21.1+273.15=294.25K

406.65 294.25

406.65 0.2764 27.64%

@3 2164/

0.2764 2164 598.13/ @3 2725/

From(hs)chart=> 2050/

2725 2050 675/

598.13675 0.886

. . 3600

3600598.13

6.0188/.



b Rankinecycle: @3 2725/From(hs)chart=> 2050 @0.025 88/ @0.025 0.001 0.001 3 0.025 10

10 0.2975/ 2725 2050 675/

675 0.29752725 88 0.2975

0.256 25.6%

675 0.2975675 0.99956

. . 3600

3600675 0.2975

5.335/.



c Rankinecycle(Actualcycle):

0.8

540/

540 0.29752725 88 0.2975

0.205 20.5%

540 0.2975540 0.99945. . 3600

3600540 0.2975 6.67/. MeanTemperatureofHeatAddition:IntheRankinecycle,heatisaddedreversiblyataconstantpressure, if is themean temperatureofheataddition,asshowninfigure2.4,sothattheareaunder4and1isequaltotheareaunder5and6,thenheataddedis;

Andheatrejectedis;

2



1 1 1

Where; =thetemperatureofheatrejection.Foragiven ,theloweristhe ,i.e.loweristhecondenserpressure,thehigherwillbetheefficiencyoftheRankinecycle.Butthelowestpracticabletemperature limit of heat rejection is the temperature of thesurroundings .Thesaturationpressurecorrespondingtothistemperatureistheminimumpressuretowhichsteamcanbeexpandedintheturbine.Thisbeingfixedbytheambientconditions, OnlyThehigherthemeantemperatureofheataddition,thehigherwillbethecycleefficiency.Effect of Superheat: The effect of increasing the initial temperature atconstantpressureoncycleefficiencyisshowninfigure2.5.Whentheinitialstatechangesfrom1to1, between4and1ishigherthan between4and1.Soanincreaseinthesuperheatatconstantpressureincreasesthemean temperature of heat addition and hence, the cycle efficiency.Moreover,with increase in superheat, theexpansion lineof steam in theturbineshiftstotheright,asaresultofwhichthequalityofsteamatturbineexhaustincreasesandperformanceoftheturbineimproves.



Effectofinletpressure: Theoperating temperatureof thematerialsusedforthemanufactureofsuperheaters,valves,pipelines,turbinebladesattheinletstages,andsoon,islimitedduetometallurgicalconsideration.Whenthemaximum temperature is fixed by this limit, as the operating steampressureatwhichheat isadded in theboiler increases from ,asshowninfigure2.6,themeantemperatureofheatadditionincreasessincebetween7and5ishigherthan between4and1.Moreover,withincreaseinturbineinletpressure,theexpansionlineofsteamintheturbineshifts to the left and themoisture content of steam at turbine exhaustincreases .Ifthemoisturecontentofsteam inthe laterstagesoftheturbine ishigh,thewaterparticlesthatiscarriedalongwiththevaporcomingoutofthenozzleswithhighvelocitystrikethebladesanderodethereedges,asaresult,theperformanceoftheturbineandthelifeofitsbladeswilldecrease.Themaximummoisturecontentattheturbineexhaustisnotallowedtoexceed12%.Reheat:Inthereheatcycle,asshowninfigure(2.7),thevaporatstate(1)isexpandedpartiallyinahighpressure(H.P)turbine,thenitisreturnedbackto the steam generator to be reheated at constant pressure (ideally) totemperatureatstate(3).Thereheatedsteamexpandsinalowpressure(L.P)turbine to the condenser pressure. As can be seen, reheat allows heatadditiontwice:(from6to1)and(from2to3).Itresultinincreasingtheaveragetemperatureatwhichheatisadded,whichresultinimprovingthecycleefficiencyandadriersteamatturbineexhaust(state4instead4).



Fig. 2.7 Reheat Cycle

Theanalysisofreheatcycleis:

But

0



Example (2):Anidealreheatsteampowerplantoperatesbetweenaboileroutletat(150bar,500oC),reheatoutletat(20bar,500oC)andacondenserpressure(0.1bar).Calculatethefollowing:

a Steamqualityatturbineexhaust.b Cycleefficiency.c Steamrate.d Heatrate

Solution:

From (h-s) chart: @ 150, 500 3309/ @ 20, 500 3116/ @ 20 2799/@ 0.1 0.9, 2344.8/From steam table:@ 0.1 192/

0.1 10/ 0.1 10 150 0.1 10 15/



3309 192 13 3116 2799 3421/ 3309 2799 3116 2344.8 1281.2/ 1281.2 15 1266.2/

1266.23421 0.37 37%

. . 3600

36001266.2 2.84/.

. 3600 . . 2.84 3421 9715.64/. Regeneration: The mean temperature of heat addition can also beincreasedbyreducingtheamountofheatadded inthe lowtemperaturesregion(intheliquidphase)intheeconomizersectionofthesteamgenerator.Thisprocesscanbeaccomplishedifthefeedwatercouldbeenteredtothesteamgeneratoratsaturatedliquid(state5)ratherthan(state4),asshownin(figure2.5).Thisispossiblebytheprocessofregenerationinwhichenergyisexchangedinternallybetweentheexpandingfluidintheturbineandthecompressedfluidbeforeheataddition.TheStirlingCycle:A wellknown gas cycle that uses regeneration is theStirlingcycle.Thiscycle iscomposedof two reversible isothermsand tworeversible isochoric (constantvolume)processes,asshown in figure (2.8).Thiscycleconsistsofthefollowingprocesses:12:isochoricexpansion23:isothermalheatrejection34:isochoriccompression41:isothermalheataddition



Theareasunder12and34denotingheatlostbytheexpandedfluidandgainedbythecompressedfluidareequal.Therefore,alltheheatisaddedreversiblyat andalltheheatisrejectedreversiblyat .Therefore,theidealStirlingcyclehavethesameefficiencyastheCarnotcycle.Theidealregenerativecycle:In the ideal regenerative cycle for steam, as shown in figure 2.9, thecondensateafter leavingthepumpcirculatesaroundtheturbinecasingsothat heat is transferred from the vapor expanding in the turbine to thecondensate circulating around it. The process 12 represents reversibleexpansion of steam in the turbinewith reversible heat rejection to thesurroundingliquidheatedreversiblyintheprocess(4s5).Since and , also area (5ba4s5) is equal andidenticaltoarea(1dc21).Therefore,alltheheataddedfromtheexternalsource isatconstanttemperature ,andalltheheatrejected isatconstanttemperature ,bothbeingreversible,then;

=> => 1 1



Therefore, the efficiency of the ideal regenerative cycle and that of theStirlingcycleareequaltothatoftheCarnotcycle.Thiscycleisnotpracticalbecause;

1 Reversibleheattransfercannotbeachieved.2 Heatexchangerintheturbinecasingisnotpractical.3 Themoisturecontentofthesteamintheturbineishigh.

Opentyperegenerativefeedwaterheaters(Directcontact):Inthiscycle,asshowninfigure(2.10),aportionofsteamisextractedataselectedpointsalongtheturbinetoheatthefeedwaterinaseriesoffeedwaterheaters.Theextractionsteam ismixeddirectlywith the incomingsubcooled feedwatertoproducesaturatedwaterattheextractionsteampressure.Thisriseintemperatureofthefeedwaterenteringtheboilercausesan increaseinthermalefficiencyofthecycle.

Fig.2.10Regenerativecyclewithtwodirectcontactfeedwaterheaters

Amassbalancefor(1kg/s)atturbineinletgives:

Massflowbetween1and2=1Massflowbetween2and9= Massflowbetween2and3=1 Massflowbetween3and7= Massflowbetween4and7=1 Massflowbetween7and9=1 Massflowbetween9and1=1Where , aresmallfractionsof1



TheenergybalanceforFeedWaterheater(1)(FWH1): 1 1 TheenergybalanceforFeedWaterheater(2)(FWH2): 1 1 1 1 1 1 1 1 1 1 NumberofPumps=Numberofopentypefeedwaterheaters+1

Example (3):AnidealRegenerativesteampowerplantthatusesingleDirectcontacttypefeedwaterheater,steamenterstheturbineat(30bar,400oC)and the exhaust pressure is (0.1 bar). The feedwater heater is a directcontacttypewhichoperatesat(4bar),calculatethefollowing:

a Steamqualityatturbineexhaust.b Cycleefficiency.c Steamrate.d Heatrate. Neglectpumpwork



Solution:

From (h-s) chart: @ 30, 400 3231/@ 4 2753/@ 0.1 0.836, 2191.7/From steam table: 0@ 0.1 192/ @ 4 605/TheenergybalanceforFeedWaterheater: 1 1 2753 605 1 605 192 0.1612/boilersteam 3231 605 2626/ 1 3231 2753 1 0.16122753 2191.7 948.818/

948.8182626 0.3613 36.13%

. . 3600 3600

948.818 3.8/. . 3600 . . 3.8 2626 9978.8/.



Closed type regenerative feed water heaters with drain cascadedbackward: Inthistype,asshowninfigure(2.11),thecondensateisfedbacktothenextlowerpressure feedwaterheater.Thecondensateof the lowestpressurefeedwaterheaterisfedbacktothemaincondenser.

Fig.2.11Regenerativecyclewithtwoclosedtypefeedwaterheaterswith

draincascadedbackward

Amassbalancefor(1kg/s)atturbineinletgives:Massflowbetween1and2=1Massflowbetween2and3=1 Massflowbetween3and10=1 Massflowbetween10and1=1Massflowbetween2and12= Massflowbetween3and12= Massflowbetween12and10= Where , aresmallfractionsof1Theenergybalancefor(H.P)FeedWaterheater(FWH1):



Theenergybalancefor(L.P)FeedWaterheater(FWH2): Sinceprocesses(910)&(1112)arethrottlingprocesses.Therefore; , .Also , 1 1 56 5 1 Closedtyperegenerativefeedwaterheaterswithdrainpumpedforward: Inthistype,asshowninfigure(2.12),thedrain,insteadofbeingcascadedbackward,ispumpedforwardintothemainfeedwaterline.

Fig.2.12Regenerativecyclewithtwoclosedtypefeedwaterheaterswith

drainpumpedforward



Amassbalancefor(1kg/s)atturbineinletgives:Massflowbetween1and2=1Massflowbetween2and12= Massflowbetween2and3=1 Massflowbetween3and14= Massflowbetween3and7=1 Massflowbetween13and14= Massflowbetween8and9=1 Massflowbetween11and12= Massflowbetween10and1=1Where , aresmallfractionsof1Theenergybalancefor(H.P)FeedWaterheater(FWH1): 1 Theenergybalancefor(L.P)FeedWaterheater(FWH2): 1 Assume => , 11 13 56 5 1 1 1 1 1 1 1 56 5 13 11 and 1 and



Example (4):Anidealregenerativesteamcycleusingtwoclosedfeedwaterheaterswithdrainscascadedbackward,thesteamissuppliedtotheturbineat (40bar,500Co),and isexhaustedto thecondenserat (0.035bar).Theintermediate bleed pressures are obtained such that the saturationtemperature intervalsareapproximatelyequal,givingpressureof (10and1.5bar).Calculatethefollowing:(neglectpumpwork)

a Theamountofsteambledateachstage.b Theworkoutputoftheplantin(kj/kg)ofboilersteam.c Thethermalefficiency.

Solution:

From (h-s) chart: @ 40, 500 3445/@ 0.035 0.824, 2120.912/@ 1.5 0.977, 2641.8/@ 10 3032.9/From steam table: @ 0.035 112/@ 1.5 467/ @ 10 781/Sinceprocesses(910)&(1112)arethrottlingprocesses.Therefore; , .Also , 0 =>



Theenergybalancefor(H.P)FeedWaterheater(FWH1): 3032.9 781 781 467 781 4673032.9 781 0.14/boilersteamTheenergybalancefor(L.P)FeedWaterheater(FWH2): 2641.8 467 0.14781 467 467 112

467 112 0.14781 4672641.8 467

0.143/boilersteam 3445 781 2664/ 1 1 3445 3032.9 1 0.143032.9 2641.81 0.14 0.1432641.8 2120.912 412.1 336.346 373.4767 1121.923/

1121.9232664 0.421 42.1%



Cogeneration: Industries such as paper mills, textile mills, chemicalfactories, sugar factories, ricemillsand soonuse saturated steamat thedesiredtemperaturetoutilizethe latentheatreleasedforheating,Dryingetc.Inaddition,afactoryalsoneedspowertodriveitsmachines.Therearetwotypesofcogeneration:

1 BackpressureTurbine: TheexhauststeamfromtheturbineisusedforheatingintheprocessheaterthatisreplacingthecondenserintheRankinecycle,asshowninfigure(2.14).Suchaturbineiscalledabackpressureturbine.Aplantproducingbothelectricalpowerandprocessheat simultaneously is called a Cogeneration Plant. This plant iscalledaByproductpowercycleonlyifitisusedtoproduceprocesssteamasamajorproduct,andproduceelectricalpowerasaminorproduct.

Fig.2.14Cogenerationplantwithabackpressureturbine

and

, Or ,



TheCogenerationplantefficiencyisgivenas;

Forseparategenerationofelectricityandsteam,Theheataddedperunittotalenergyoutputis

1

WhereE=Electricalfractionoftotalenergyoutput.

Electricplantefficiency

Processheatefficiency

Therefore,theCombinedefficiencyforseparategenerationisgivenas;

1

1

Cogenerationplantisusefulonlyif .

2 PassOutTurbine:PassOutturbineisusedwhenthepoweravailable

fromthebackpressureturbineislessthanthatrequiredinafactory.Steam is extracted from the turbine at an intermediate stage forheatingprocessatthedesiredpressureandtemperature,asshowninfigure(2.15).

and



Example (5):Atextilefactoryrequires(10ton/hr)ofdrysaturatedsteamforprocessheatingat(3bar)and(1MW)ofpower,forwhichabackpressureturbineof(70%)isentropicefficiencyistobeused.Findthesteamconditionrequiredatinletoftheturbine.Solution:Fromsteamtable:@ 3 2725/ 561/ 2164/ 6.993/. 1.672/. 5.321/.

1 10 100003600 2725 1000 3085/

0.7

3085 27253085 2570.714/

2570.714 561 2164 0.9287 1.672 0.9287 5.321 6.6136/. From(hs)diagram:Since 3085/ 6.6136/. 37.3 345

Createdby:AssistantProfessorDr.MishaalAbdulAmeerAbdulKareem [email protected]

PowerPlantEngineering TUTORIALSHEET2 SteamPowerPlantCycles

40

1 Drawasimplesketchthenshowitsprocessesonthe(Ts)and(hs)diagramsforeachidealcyclelistedinthetablebelow.Cycle Type SteamCondition ClosedFWH Direct

ContactFWHTurbineinlet Reheat Outlet Condenser DrainCascaded

BackwardDrain

PumpedForward

A Rankine 4bar,Dry 0.035bar B Rankine 60bar,

410Co 0.07bar

C Reheat 100bar,410Co

60bar,410Co 0.07bar

D Regenerative 60bar,410Co

0.07 bar 20 bar10 bar

E Regenerative 60bar,410Co

0.07bar 20 bar 10bar

F Regenerative 60bar,410Co

0.07bar 20bar 10bar

G Regenerative 60bar,410Co

0.07bar 20 bar 10bar

H Regenerative 60bar,410Co

0.07bar 20bar10bar2bar

I Regenerative 60bar,410Co

0.07bar 20bar 2bar10bar

J Regenerative 60bar,410Co

0.07bar 20bar 2bar10bar

K Regenerative 60bar,410Co

0.07bar 20bar 10bar 2bar

L ReheatRegenerative

100bar,410Co

25bar,410Co 0.07bar 12 bar2bar0.5 bar

M ReheatRegenerative

100bar,410Co

25bar,410Co 0.07bar 12 bar 0.5 bar2 bar

N ReheatRegenerative

100bar,410Co

25bar,410Co 0.07bar 12bar 0.5 bar2bar

O ReheatRegenerative

100bar,410Co

25bar,410Co 0.07bar 12bar 2bar 0.5bar



41

2 Drawallthedetailsofthe(Ts)diagramoftheidealsteampowerplantlayoutshowninthefigurebelow.

3 Forthefollowingsteamcyclesthatarelistedinthetablebelow,find(a)in(kj/kg)(b)in(kj/kg)(c)in(kj/kg)(d)(e)S.S.Cin(kg/kW.hr)and(f)moisturefractionattheendoftheturbineprocess.Showtheresultsintabularformwithyourcomments.Cycle Type BoilerOutlet Condenser

PressureA IdealRankine,NeglectWP 10bar,Saturated 1barB Assume 75% 10bar,Saturated 1barC IdealRankine 10bar,Saturated 0.1barD IdealRankine 10bar,300Co 0.1barE IdealRankine 160bar,600Co 0.1barF Reheatto600CoatmaximumI.Ptolimitendmoistureto15% 160bar,600Co 0.1barG Reheatto600CoatmaximumI.Ptolimitendmoistureto15%

butwith 75%160bar,600Co 0.1bar

H Singleopenfeedwaterheaterat110Co 10bar,Saturated 0.1barI Twoopenfeedwaterheatersat90Co and135Co 10bar,Saturated 0.1barJ Twoclosedfeedwaterheaterswithdraincascadedbackwardat

90Coand135Co10bar,Saturated 0.1bar



42

4 Inanidealsteamcycle,steamat(20bar,360Co)isexpandedinaturbineto(0.08bar).It thenentersacondenserwhere it iscondensed tosaturated liquidwater.Then thepump feedsback thewater into theboiler.Findperkgof steam (a) (b) (c)if 80%,findthepercentagereductioninand.Ans.(a)969.61(kj/kg)(b)32.5%(c)20.1%(d)20.1%

5 Neglectthefeedpumpterm,comparetheRankinecycleefficiencyofahighpressuresteamplantoperatingat(80bar)withthatofalowpressureplantat(40bar).Inbothcases,themaximumtemperatureis(400Co)andthecondenserpressureis(0.07bar).Explainyourresultbriefly.Alsofindtheratiooftheheatrejectedfromthecondensingplants.Assumethattheturbinesaretoproducethesamepower.Ans.0.391,0.357,0.83(H.P/L.P)

6 An ideal reheat cycleworksbetweenpressuresof (120and0.035bar).The steam issuperheatedto(570Co)andafterexpansiontothedrysaturatedstate isreheatedto(500Co).Calculatethethermalefficiencyofthecycletakingaccountofthefeedpumpwork.Ans.0.453

7 Asteamturbineissuppliedwithsteamat(40bar,450Co)andexhaustedat(0.035bar).Neglectingthefeedpumpterm,findtheidealRankinecycleefficiencyoftheplant,andcompareitwiththeidealefficiencywhichwouldbeobtainedifregenerativefeedwaterheatingwereemployed.Assumetheuseofopenheatersand(a)Onebleedingpoint(3bar),(b)Threebleedingpointsat(15,3,and0.5bar).Ans.0.391,(a)0.173kg,0.415,(b)0.114,0.083,0.08kg,0.429

8 Saturatedsteamat(30bar)entersahighpressureturbineandexpandsisentropicallytoapressureatwhich itsdryness fraction is (0.841%).At thispressure, themoisture isextractedandreturnedtotheboilerviafeedpump.Theremainder,assumedtobedrysteam, is expanded isentropically to (0.04 bar) in a low pressure turbine, and thecondensate is returned to the boiler via a second feed pump. Calculate the cycleefficiencywhentheisentropicefficiencyofeachturbineandfeedpumpis(0.8).Whatisthenthedrynessfractionattheexitofeachturbine?Ans.0.357,0.291,0.88,and0.871.



43

9 Asteamturbinegetsitssupplyofsteamat(70bar),(450Co).Afterexpandingto(25bar)inhighpressurestages, it isreheated to (420Co)at theconstantpressure.Next, it isexpanded in intermediatepressurestages to theextractionpoint foradirectcontactfeedwaterheatertoanappropriateminimumpressuresuchthatpartofthesteambledatthispressureheatsthefeedwatertoatemperatureof(180Co).Theremainingsteamexpands fromthispressuretoacondenserpressureof (0.07bar) inthe lowpressurestage.The isentropicefficiencyofthehighpressurestage is(78.5%),whilethatoftheintermediateandlowpressurestagesis(83%)each.Drawasimplesketchofthecycleandshowitsprocessesonthe(Ts)and(hs)diagrams,thendeterminethefollowing.(a)Theminimumpressureatwhichbleedingisnecessary.(b)Thequantityofthesteambledperkgofflowattheturbineinlet.(c)Thethermalefficiencyofthecycle.(Neglectpumpswork).Ans.(a)10bar,(b)0.206kg/kgsteamflowattheturbineinlet,(c)35.92%.

10 Inan ideal reheatcycle,steam initiallyat (150bar,550Co)expands ina turbine toacondenser pressure of (0.1 bar), and the moisture in the condenser inlet is (5%).Determine(a)Thereheatpressure,(b)Thecycleefficiency,and(c)Thesteamrate.Ans.(a)13.5bar,(b)43.65%,(c)205kg/kW.hr.

11 Thenetpoweroutputofanidealreheatregenerativecycleis(80MW).SteamenterstheHPturbineat(80bar)and(500Co),andexpandstillitbecomessaturatedvapor.Someofthesteamthengoestoanopenfeedwaterheaterandthebalanceisreheatedto(400Co),afterwhich itexpands in the LP turbine to (0.07bar).Determine (a)The reheatpressure,(b)ThesteammassflowratetotheHPturbine,and(c)Thethermalefficiency.Ans.(a)6.5bar,(b)58.4kg/s,and(c)43.7%.

12 Thenetpoweroutputofanidealreheatregenerativecycleis(100MW).SteamenterstheHPturbineat(90bar)and(550Co).Afterexpansionto(7bar),someofthesteamgoestoanopenfeedwaterheaterandthebalanceisreheatedto(400Co),afterwhichitexpandsintheLPturbineto(0.07bar).Determine(a)ThesteammassflowratetotheHPturbinein(ton/hr),(b)Thetotalpumpwork,and(c)Thethermalefficiency.Ans.(a)256.186(ton/hr),(b)9.767(kj/kg),and(c)44.36%.

CHAPTER-2 Steam Power Plant CyclesTutorial Sheet-2 Steam Power Plant Cycles

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CHAPTER - 2 Steam Power Plant Cycles