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1、第1頁共78頁外文資料C1Thebasicsofsteamgenerationuse1.1WhyanunderstingofsteamisneededSteampowerisfundamentaltowhatisbyfarthelargestsectoftheelectricitygeneratingindustrywithoutitthefaceofcontemparysocietywouldbedramaticallydiffere

2、ntfromitspresentone.Wewouldbefcedtorelyonhydroelectricpowerplantwindmillsbatteriessolarcellsfuelcellsallofwhicharecapableofproducingonlyafractionoftheelectricityweuse.Steamisimptantthesafetyefficiencyofitsgenerationusede

3、pendontheapplicationofcontrolinstrumentationoftensimplyreferredtoasC&I.Theobjectiveofthisbookistoprovideabridgebetweenthedisciplineofpowerplantprocessengineeringthoseofelectronicsinstrumentationcontrolengineering.Ishalls

4、tartbyoutlininginthischapterthechangeofstateofwatertosteamfollowedbyanoverviewofthebasicprinciplesofsteamgenerationuse.Thisseeminglysimplesubjectisextremelycomplex.Thiswillnecessarilybeanoverview:itdoesnotpretendtobeadet

5、ailedtreatiseattimesitwillsimplifymattersglossoversomedetailswhichmayevencausethethermodynamicistcombustionphysicisttoshudderbutitshouldbeunderstoodthattheaimistoprovidetheC&Iengineerwithenoughunderstingofthesubjecttodea

6、lsafelywithpracticalcontrolsystemdesignoperationalmaintenanceproblems.1.2Boiling:thechangeofstatefromwatertosteamWhenwaterisheateditstemperaturerisesinawaythatcanbedetected(fexamplebyathermometer).Theheatgainedinthiswayi

7、scalledsensiblebecauseitseffectscanbesensedbutatsomepointthewaterstartstoboil.Buthereweneedtolookevendeeperintothesubject.ExactlywhatismeantbytheexpressionboilingTostudythiswemustconsiderthethreebasicstatesofmatter:solid

8、sliquidsgases.(Aplasmaproducedwhentheatomsinagasbecomeionisedisoftenreferredtoasthefourthstateofmatterbutfmostpracticalpurposesitissufficienttoconsideronlythethreebasicstates.)Initssolidstatematterconsistsofmanymolecules

9、tightlyboundtogetherbyattractivefcesbetweenthem.Whenthematterabsbsheattheenergylevelsofitsmoleculesincreasethemeandistancebetweenthemoleculesincreases.Asmemeheatisappliedtheseeffectsincreaseuntiltheattractivefcebetweenth

10、emoleculesiseventuallyovercometheparticlesbecomecapableofmovingaboutindependentlyofeachother.Thischangeofstatefromsolidtoliquidiscommonlyrecognisedasmelting.Asmeheatisappliedtotheliquidsomeofthemoleculesgainenoughenergyt

11、oescapefromthesurfaceaprocesscalledevapation(wherebyapoolofliquidspilledonasurfacewillgraduallydisappear).Whatishappeningduringtheprocessofevapationisthatsomeofthemoleculesareescapingatfairlylowtemperaturesbutasthetemper

12、aturerisestheseescapesoccurmerapidlyatacertainpointtheliquidbecomesveryagitatedwithlargequantitiesofbubblesrisingtothesurface.Itisatthistimethattheliquidissaidtostartboiling.Itisintheprocessofchangingstatetoavapourwhichi

13、safluidinagaseousstate.Letusconsideraquantityofwaterthatiscontainedinanopenvessel.Heretheairthatblanketsthesurfaceexertsapressureonthesurfaceofthefluidasthetemperatureofthewaterisraisedenoughenergyiseventuallygainedtoove

14、rcometheblanketingeffectofthatpressurethewaterstartstochangeitsstateintothatofavapour(steam).Furtherheataddedatthisstagewillnotcauseanyfurtherdetectablechangeintemperature:theenergyaddedisusedtochangethestateofthefluid.I

15、tseffectcannolongerbesensedbyathermometerbutitisstillthere.Fthisreasonitiscalledlatentratherthensensibleheat.Thetemperatureatwhichthishappensiscalledtheboilingpoint.Atnmalatmosphericpressuretheboilingpointofwateris100C.I

16、fthepressureoftheairblanketontopofthewaterweretobeincreasedmeenergywouldhavetobeintroducedittobreakfree.Inotherwdsthetemperaturemustberaisedfurthertomakeitboil.Toillustratethispointifthepressureisincreasedby10%aboveitsnm

17、alatmosphericvaluethetemperatureofthewatermustberaisedtojustabove102Cbefeboilingoccurs.Thesteamemergingfromtheboilingliquidissaidtobesaturatedfanygivenpressurethetemperatureatwhich第3頁共78頁processnecessitatestheuseofamediu

18、mtoconveytheenergyfromsourcetodestination.Furthermeitisnecessarytoemployamediumthatisreadilyavailablewhichcanbeusedwithrelativesafetyefficiency.OnplantEarthwaterisatleastingeneralaplentifulcheapmediumfeffectingsuchtransf

19、ers.Withthedevelopmentoftechnologyduringthetwentiethcenturyotherpossibilitieshavebeenconsideredsuchastheuseofmercurybutexceptfapplicationssuchasspacecraftwhereentirelynewsetsoflimitationsconditionsapplynoneofthesehasreac

20、hedactiveusesteamisuniversallyusedinpowerstations.FigureA1Carnotframedoneofthetwolawsofthermodynamics.ThefirstJouleslawhadrelatedmechanicalenergytowk:Carnotslawdefinedthetemperaturerelationsapplyingtotheconversionofheate

21、nergyintomechanicalenergy.Hesawthatifthisprocessweretobemadereversibleheatcouldbeconvertedintowkthenextractedreusedtomakeaclosedloop.Inhisconcept(FigureA1)apistonmovesfreelywithoutencounteringanyfrictioninsideacylinderma

22、deofsomeperfectlyinsulatingmaterial.Thepistonisdrivenbyawkingfluid.Thecylinderhasaheadatoneendthatcanbeswitchedatwillfrombeingaperfectconducttobeingaperfectinsulat.Outsidethecylinderaretwobodiesoneofwhichcandeliverheatwi

23、thoutitsowntemperature(T1)fallingtheotherbeingabottomlesscoldsinkatatemperature(T2)whichisalsoconstant.TheoperationofthesystemisshowngraphicallyinfigureA2whichshowsthepressurevolumerelationshipofthefluidinthecylinderover

24、thewholecycle.AstheprocessisarepeatingcycleitsoperationcanbestudiedfromanyconvenientstartingpointweshallbeginatthepointAwherethecylinderhead(atthistimeassumedtobeaperfectconductofheat)allowsheatfromthehotsourcetoenterthe

25、cylinder.TheresultisthatthemediumbeginstoexpifitisallowedtoexpfreelyBoyleslaw(whichstatesthatatanytemperaturetherelationshipbetweenpressurevolumeisconstant)dictatesthatthetemperaturewillnotrisebutwillstayatitsinitialtemp

26、erature(Tl).Thisiscalledisothermalexpansion.FigureA2WhenthepressurevolumeofthemediumhavereachedthevaluesatpointBthecylinderheadisswitchedfrombeingaperfectconducttobeingaperfectinsulatthemediumallowedtocontinueitsexpansio

27、nwithnoheatbeinggainedlost.Thisisknownasadiabaticexpansion.WhenthepressurevolumeofthemediumreachthevaluesatpointCthecylinderheadisswitchedbacktobeingaperfectconductbuttheexternalheatsourceisremovedreplacedbytheheatsink.T

28、hepistonisdriventowardstheheadcompressingthemedium.Heatflowsthroughtheheadtotheheatsinkwhenthetemperatureofthemediumreachesthatoftheheatsink(atpointD)thecylinderheadisonceagainswitchedtobecomeaperfectinsulatthemediumisco

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