Lecture_1_Steam power plant _Ideal and Actual cycles.pdf

Steam Power Plant

1. Lecture 1
(MEEN-4164)
Introduction to Power Plants
Dr. Muhibullah
Khwaja Fareed University of Engineering and Information Technology

2. Textbook
•Thermodynamics An Engineering Approach
2

3. Grade Weightage
•Quiz 10%
•Project 10%
•Mid Term 30%
•Final Exam 50%
3

4. Electricity
Weareclearlydependentonelectricityformostofour
everydayactivities.Thisdependencealsodemonstrateswhy
electricityisregardedasoneofthemostsignificantsociological
inventionsofthe20thcentury.AlthoughtheUnited
Statesandherindustrializedneighborsenjoythefullbenefits
ofelectricity,theworld'sdevelopingnationsdonot.Supplying
electricitytotheseareasundervariouseconomic,environmental,
social,andpoliticalconstraintswillbeoneofthe
majorchallengesinthe21stcentury.
4

5. 5
Power Plant
•Powerplantisanindustrialfacilityusedtogenerateelectricpowerwith
thehelpofoneormoregeneratorswhichconvertsdifferentenergy
sourcesintoelectricpower.
•ConventionalandUnconventionalpowerplants
•Thepowerplantsproducinglargescalepowerandtheirtechnologiesare
wellestablishedareknownasconventionalpowerplantse.g.
•Thermalpowerplants
1.Steamturbinepowerplants,
2.gasturbinepowerplants
3.dieselpowerplants
4.petrolandgasengines
•Hydelpowerplants
(a)number1reactiontypehydraulicturbines(KaplanandFrancis)
(b)impulsetypehydraulicturbines(Peltonturbine)

6. 6
Power Plant
•Thepowerplantswhichproducepoweronsmallscaleandtheir
technologiesareprogressingarecallednonconventionalpower
plants,e.g.
1.Hydraulicpowerplantsatsmallscale(e.g.Kaplanturbinefor
maximum5Mhead)
2.Windmillsorwindturbine
3.SolarenergyusingPV
4.Oceanthermalenergy
5.Biomassfuelcells

7. 7
Steam Power Plant
•Asteampowerplantcontinuouslyconvertstheenergystoredin
fossilfuels(coal,oilornaturalgas)orfissilefuels(uraniumor
thorium)intoshaftworkandultimatelyintoelectricity
•Theworkingfluidiswaterwhichissometimesintheliquidphaseand
sometimesinthevaporphaseduringitscycleofoperation.
•Theenergyreleasedbytheburningoffuelistransferredtothewater
intheboilertogeneratesteamatahighpressureandtemperature
whichthenexpandsintheturbinetoalowpressuretoproduceshaft
workthesteamleavingtheturbineiscondensedintowaterinthe
condenserwherecoolingwaterfromariverorseacirculatescarrying
awaytheheatreleasedduringcondensationthewaterorcondensate
isthenfedbacktotheboilerbythepumpandcyclegoeson
repeatingitself.

8. Thermodynamics of Phase Change -
Reminder
8

9. 9

10. Carnot Cycle -Ideal Cycle
10

11. 11

12. 12

13. 13

14. 14

15. 15

16. 16

17. 17

18. 18

19. 19

20. 20

21. 21

22. 22

23. 23
Schematic Diagram of Steam Power PlantFurnace
T
CB
P
Generator Electricity
Electric
Motor
Cooling Water
From Ocean
Q2Q1
WT
WP
FuelAir
Flue Gases
Heat
Rejected
Heat
Supplied
TheworkingsubstancefollowsalongBoilertoTurbine,Turbineto
condenser,condensertoPump.

24. 24
Energy Balance net net
Cycle Cycle
QW= 12 TP
Q Q W W− = − 1 2 2
1 1 1 1
1
net TP
cycle
W W W Q Q Q
Q Q Q Q

−−
= = = = −

25. 25
Rankine Cycle : The Ideal Cycle for Vapor Power Cycles
(From Thermo-I Cengel (10-2))
•ManyoftheimpracticalitiesassociatedwiththeCarnotcyclecanbe
eliminatedbysuperheatingthesteamintheboilerandcondensingit
completelyinthecondenser.ThecyclethatresultsistheRankine
cycle,whichistheidealcycleforvaporpowerplants.Theideal
Rankinecycledoesnotinvolveanyinternalirreversibilityand
consistsofthefollowingfourprocesses:
•Isentropiccompression(pressurization)inapump
•Constantpressureheatadditioninaboiler
•Isentropicexpansioninaturbine
•Constantpressureheatrejectioninacondenser

26. 26
T-S Diagram
•1-2Isentropiccompression(pressurization)inapump
•2-3Constantpressureheatadditioninaboiler
•3-4Isentropicexpansioninaturbine
•4-1Constantpressureheatrejectioninacondenser

27. 27
•Waterentersthepumpatstate1assaturatedliquidandis
compressedisentropicallytotheoperatingpressureoftheboiler.
Theverticaldistancebetweenstates1and2ontheT-sdiagram
isgreatlyexaggeratedforclarity.
•Waterenterstheboilerasacompressedliquidatstate2andleaves
asasuperheatedvaporatstate3.Theboilerisbasicallyalarge
heatexchangerwheretheheatistransferredtothewateressentially
atconstantpressure.Theboiler,togetherwiththesectionwhere
thesteamissuperheated(thesuperheater),isoftencalledthe
steamgenerator.
•Thesuperheatedvaporatstate3enterstheturbine,whereit
expandsisentropicallyandproducesworkbyrotatingtheshaft
connectedtoanelectricgenerator.Thepressureandthe
temperatureofsteamdropduringthisprocesstothevaluesatstate
4,wheresteamentersthecondenser.Atthisstate,steamisusually
asaturatedliquid–vapormixturewithahighquality.Steamis
condensedatconstantpressureinthecondenser,whichisbasically
alargeheatexchanger,byrejectingheattoacoolingmediumsuch
asalake,ariver,ortheatmosphere.Steamleavesthecondenser
assaturatedliquidandentersthepump,completingthecycle.

28. 28
•Forthesakeofexplanation,the
heatadditioninRankineCycle
isfurthersubdividedas
•2-3:Reversibleheatingof
subcooledliquidatconstant
pressuretothesaturated
liquid(Preheating).
•3-4:Reversibleheataddition
intheboileratconstant
pressureandtemperature
(Phase Change Heat
Transfer)
•Heatingofdrysaturated
vaporsatconstantpressure
tosuperheatedstate
The Heat Addition in Ideal Rankine Cycle

29. 29
•2-3:Reversibleheatingof
subcooledliquidatconstant
pressuretothesaturated
liquid(Preheating).
•ThePreheatingiscarriedout
usingEconomizersorFeed
WaterHeaters.
•Economizersuseexhaust
gasesfromtheburningof
fueland,FeedWater
Heatersuseextractedsteam
fromtheturbine.
The Heat Addition in Ideal Rankine Cycle

30. 30
•AllfourcomponentsassociatedwiththeRankine
cycle(thepump,boiler,turbine,andcondenser)
aresteady-flowdevices,andthusallfour
processesthatmakeuptheRankinecyclecan
beanalyzedassteady-flowprocesses.
•Thekineticandpotentialenergychangesof
thesteamareusuallysmallrelativetothework
andheattransfertermsandarethereforeusually
neglected.
•Thenthesteady-flowenergyequationperunit
massofsteamreducesto
The Energy Analysis of Ideal Rankine Cycle

31. 31
•Theboilerandthecondenserdonotinvolveanywork,andthe
pumpandtheturbineareassumedtobeisentropic.Thenthe
conservationofenergyrelationforeachdevicecanbeexpressed
asfollows
The Energy Analysis of Ideal Rankine Cycle

32. 32
•TheThermalEfficiencyoftheRankinecycleisdeterminedas
The Energy Analysis of Ideal Rankine Cycle

33. 33
•TheactualvaporpowercyclediffersfromtheidealRankinecycle,asa
resultofirreversibilitiesinvariouscomponents.Fluidfrictionand
heatlosstothesurroundingsarethetwocommonsourcesof
irreversibilities.
Deviation of Actual Vapor Power Cycles From
Idealized Ones
Fluidfrictioncausespressure
dropsintheboiler,thecondenser,
andthepipingbetweenvarious
components.Asaresult,steam
leavestheboileratasomewhat
lowerpressure.Also,thepressure
attheturbineinletissomewhat
lowerthanthatattheboilerexitdue
tothepressuredropinthe
connectingpipes.Thepressuredrop
inthecondenserisusuallyvery
small.Tocompensateforthese
pressuredrops,thewatermustbe
pumpedtoasufficientlyhigher
pressurethantheidealcyclecalls
for.Thisrequiresalargerpumpand
largerworkinputtothepump.

34. 34
Deviation of Actual Vapor Power Cycles From
Idealized Ones

35. 35
•Theothermajorsourceofirreversibilityistheheatlossfromthe
steamtothesurroundingsasthesteamflowsthroughvarious
components.Tomaintainthesamelevelofnetworkoutput,moreheat
needstobetransferredtothesteamintheboilertocompensatefor
theseundesiredheatlosses.Asaresult,cycleefficiencydecreases.
•Ofparticularimportancearetheirreversibilitiesoccurringwithinthe
pumpandtheturbine.Apumprequiresagreaterworkinput,anda
turbineproducesasmallerworkoutputasaresultofirreversibilities.
Underidealconditions,theflowthroughthesedevicesisisentropic.The
deviationofactualpumpsandturbinesfromtheisentropiconescanbe
accountedforbyutilizingisentropicefficiencies,definedas
Deviation of Actual Vapor Power Cycles From
Idealized Ones

36. 36
•wherestates2aand4aaretheactualexitstatesofthepumpandthe
turbine,respectively,and2sand4sarethecorrespondingstatesforthe
isentropiccase.
•Otherfactorsalsoneedtobeconsideredintheanalysisofactualvapor
powercycles.Inactualcondensers,forexample,theliquidisusually
subcooledtopreventtheonsetofcavitation,therapidvaporizationand
condensationofthefluidatthelow-pressuresideofthepumpimpeller,
whichmaydamageit.Additionallossesoccuratthebearingsbetween
themovingpartsasaresultoffriction.Steamthatleaksoutduringthe
cycleandairthatleaksintothecondenserrepresenttwoothersources
ofloss.Finally,thepowerconsumedbytheauxiliaryequipmentsuchas
fansthatsupplyairtothefurnaceshouldalsobeconsideredin
evaluatingtheoverallperformanceofpowerplants.
Deviation of Actual Vapor Power Cycles From
Idealized Ones

37. 37
•Steampowerplantsareresponsiblefortheproductionofmost
electricpowerintheworld,andevensmallincreasesinthermal
efficiencycanmeanlargesavingsfromthefuelrequirements.
Therefore,everyeffortismadetoimprovetheefficiencyofthe
cycleonwhichsteampowerplantsoperate.
•Thebasicideabehindallthemodificationstoincreasethethermal
efficiencyofapowercycleisthesame:Increasetheaverage
temperatureatwhichheatistransferredtotheworkingfluidin
theboiler,ordecreasetheaveragetemperatureatwhichheat
isrejectedfromtheworkingfluidinthecondenser.Thatis,the
averagefluidtemperatureshouldbeashighaspossibleduring
heatadditionandaslowaspossibleduringheatrejection.
•Nextwediscussthreewaysofaccomplishingthisforthesimple
idealRankinecycle.
Increase in the Efficiency of the Rankine Cycle

38. 38
•Steamexistsasasaturatedmixtureinthe
condenseratthesaturationtemperature
correspondingtothepressureinsidethe
condenser.Therefore,loweringthe
operatingpressureofthecondenser
automaticallylowersthetemperatureof
thesteam,andthusthetemperatureat
whichheatisrejected.
•Theeffectofloweringthecondenser
pressureontheRankinecycleefficiency
isillustratedonaT-sdiagram.
Lowering the Condenser Pressure (Lowers T
low,avg)
•Forcomparisonpurposes,theturbineinletstateismaintainedthesame.
Thecoloredareaonthisdiagramrepresentstheincreaseinnetwork
outputasaresultofloweringthecondenserpressure.Theheatinput
requirementsalsoincrease(representedbytheareaundercurve2-2’),
butthisincreaseisverysmall.Thustheoveralleffectofloweringthe
condenserpressureisanincreaseinthethermalefficiencyofthecycle.

39. 39
•Totakeadvantageoftheincreasedefficienciesatlowpressures,the
condensersofsteampowerplantsusuallyoperatewellbelowthe
atmosphericpressure.Thisdoesnotpresentamajorproblem
sincethevaporpowercyclesoperateinaclosedloop.However,
thereisalowerlimitonthecondenserpressurethatcanbe
used.Itcannotbelowerthanthesaturationpressurecorresponding
tothetemperatureofthecoolingmedium.
Lowering the Condenser Pressure (Lowers T
low,avg)

40. 40
•Loweringthecondenserpressure
isnotwithoutanysideeffects,
however.Foronething,it
createsthepossibilityofair
leakageintothecondenser.
Moreimportantly,itincreasesthe
moisturecontentofthesteamat
thefinalstagesoftheturbine,as
canbeseenfromtheFig.The
presenceoflargequantitiesof
moistureishighlyundesirable
inturbinesbecauseitdecreases
theturbineefficiencyanderodes
theturbineblades.
•Fortunately,thisproblemcanbe
corrected,asdiscussednext.
Lowering the Condenser Pressure (Lowers T
low,avg)

41. 41
•Theaveragetemperatureatwhichheat
istransferredtosteamcanbeincreased
withoutincreasingtheboilerpressure
bysuperheatingthesteamtohigh
temperatures.Theeffectof
superheatingontheperformanceof
vaporpowercyclesisillustratedona
T-sdiagram.
•Thecoloredareaonthisdiagram
representstheincreaseinthenetwork.
Thetotalareaundertheprocesscurve
3-3’representstheincreaseintheheat
input.
Superheating the Steam to High Temperatures
(Increases T
high,avg)
•Thusboththenetworkandheatinputincreaseasaresultofsuperheatingthe
steamtoahighertemperature.Theoveralleffectisanincreaseinthermal
efficiency,however,sincetheaveragetemperatureatwhichheatisadded
increases.

42. 42
•Superheatingthesteamtohighertemperatureshasanothervery
desirableeffect:Itdecreasesthemoisturecontentofthesteamatthe
turbineexit,ascanbeseenfromtheT-sdiagram(thequalityatstate
4’ishigherthanthatatstate4).
•Thetemperaturetowhichsteamcanbesuperheatedislimited,
however,bymetallurgicalconsiderations.Presentlythehigheststeam
temperatureallowedattheturbineinletisabout620°C(1150°F).Any
increaseinthisvaluedependsonimprovingthepresentmaterialsor
findingnewonesthatcanwithstandhighertemperatures.Ceramics
areverypromisinginthisregard.
Superheating the Steam to High Temperatures
(Increases T
high,avg)

43. 43
•Anotherwayofincreasingtheaverage
temperatureduringtheheat-addition
processistoincreasetheoperating
pressureoftheboiler,which
automaticallyraisesthetemperatureat
whichboilingtakesplace.This,inturn,
raisestheaveragetemperatureatwhich
heatistransferredtothesteamandthus
raisesthethermalefficiencyofthecycle.
•Theeffectofincreasingtheboiler
pressureontheperformanceofvapor
powercyclesisillustratedonaT-s
diagram.
Increasing the Boiler Pressure (Increases T
high,avg)
•Noticethatforafixedturbineinlettemperature,thecycleshiftstotheleftand
themoisturecontentofsteamattheturbineexitincreases.Thisundesirable
sideeffectcanbecorrected,however,byreheatingthesteam,asdiscussedin
thenextsection.

44. 44
•Operatingpressuresofboilershavegraduallyincreasedovertheyearsfrom
about2.7MPa(400psia)in1922toover30MPa(4500psia)today,generating
enoughsteamtoproduceanetpoweroutputof1000MWormoreinalarge
powerplant.Todaymanymodernsteampowerplantsoperateatsupercritical
pressures(P22.06MPa)andhavethermalefficienciesofabout40percentfor
fossil-fuelplantsand34percentfornuclearplants.Thereareover150
supercritical-pressuresteampowerplantsinoperationintheUnitedStates.The
lowerefficienciesofnuclearpowerplantsareduetothelowermaximum
temperaturesusedinthoseplantsforsafetyreasons.TheT-sdiagramofa
supercriticalRankinecycleisshowninFig.10–9.
Increasing the Boiler Pressure (Increases T
high,avg)