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英文原文EfficiencyAndOperatingCharacteristicsOfCentrifugalAndReciprocatingCompressorsByRainerKurz,BernhardWinkelmann,andSaeidiVIokhatabReciprocatingcompressorsandcentrifugalcompressorshavedifferentoperatingcharacteristicsandusedifferenteificiencydefinitions.Thisarticleprovidesguidelinesforanequitablecomparison,resultinginauniversalefficiencydefinitionforbothtypesofmachines.Thecomparisonisbasedontherequirementsinwhichauserisultimatelyinterested.Further,theimpactofactualpipelineoperatingconditionsandtheimpactonefficiencyatdifferentloadlevelsisevaluated.Atfirstglance,calculatingtheefficiencyforanytypeofcompressionseemstobestraightforward:comparingtheworkrequiredofanidealcompressionprocesswiththeworkrequiredofanactualcompressionprocess.Thedifficultyiscorrectlydefiningappropriatesystemboundariesthatincludelossesassociatedwiththecompressionprocess.Unlesstheseboundariesareappropriatelydefined,comparisonsbetweencentrifugalandreciprocatingcompressorsbecomeflawed.Wealsoneedtoacknowledgethattheefficiencydefinitions,evenwhenevaluatedequitably,stilldontcompletelyansweroneoftheoperatorsmainconcerns:Whatisthedriverpowerrequiredforthecompressionprocess?Toaccomplishthis,mechanicallossesinthecompressionsystemsneedtobediscussed.Trendsinefficiencyshouldalsobeconsideredovertime,suchasoff-designconditionsastheyareimposedbytypicalpipelineoperations,ortheimpactofoperatinghoursandassociateddegradationonthecompressors.Thecompressionequipmentusedforpipelinesinvolveseitherreciprocatingcompressorsorcentrifugalcompressors.Centrifugalcompressorsaredrivenbygasturbines,orbyelectricmotors.Thegasturbinesusedare,ingeneral,two-shaftenginesandtheelectricmotordrivesuseeithervariablespeedmotors,orvariablespeedgearboxes.Reciprocatingcompressorsareeitherlowspeedintegralunits,whichcombinethegasengineandthecompressorinonecrankcasing,orseparablehigh-speedunits.Thelatterunitsoperateinthe750-1,200rpmrange(1,800rpmforsmallerunits)andaregenerallydrivenbyelectricmotors,orfour-strokegasengines.EfficiencyTodeterminetheisentropicefficiencyofanycompressionprocessbasedontotalenthalpies(h),totalpressures(p),temperatures(T)andentropies(s)atsuctionanddischargeofthecompressoraremeasured,andtheisentropicefficiencyrthenbecomes:),(),(),(),(suctsuctdischdischsuctsuctsuctdischsTphTphTphsph(Eq.1)and,withmeasuringthesteadystatemassflowm,theabsorbedshaftpoweris:),(),(.suctsuctdischdischmTphTphmp(Eq.2)consideringthemechanicalefficiencyr.Thetheoretical(isentropic)powerconsumption(whichisthelowestpossiblepowerconsumptionforanadiabaticsystem)followsfrom:),(),(.suctsuctsuctdischtheorTphsphmP(Eq.3)Theflowintoandoutofacentrifugalcompressorcanbeconsideredassteadystate.Heatexchangewiththeenvironmentisusuallynegligible.Systemboundariesfortheefficiencycalculationsareusuallythesuctionanddischargenozzles.Itneedstobeassuredthatthesystemboundariesenvelopeallinternalleakagepaths,inparticularrecirculationpathsfrombalancepistonordivisionwallleakages.Themechanicalefficiencyr).,describingthefrictionlossesinbearingsandseals,aswellaswindagelosses,istypicallybetween98and99%.Forreciprocatingcompressors,theoreticalgashorsepowerisalsogivenbyEq.3,giventhesuctionanddischargepressureareupstreamofthesuctionpulsationdampenersanddownstreamofthedischargepulsationdampeners.Reciprocatingcompressors,bytheirverynature,requiremanifoldsystemstocontrolpulsationsandprovideisolationfromneighboringunits(bothreciprocatingandcentrifugal),aswellasfrompipelineflowmetersandyardpipingandcanbeextensiveinnature.Thedesignofmanifoldsystemsforeitherslowspeedorhighspeedunitsusesacombinationofvolumes,pipinglengthsandpressuredropelementstocreatepulsation(acoustic)filters.Thesemanifoldsystems(filters)causeapressuredrop,andthusmustbeconsideredinefficiencycalculations.Potentially,additionalpressuredeductionsfromthesuctionpressurewouldhavetomadetoincludetheeffectsofresidualpulsations.Likecentrifugalcompressors,heattransferisusuallyneglected.Forintegralmachines,mechanicalefficiencyisgenerallytakenas95%.Forseparablemachinesa97%mechanicalefficiencyisoftenused.Thesenumbersseemtobesomewhatoptimistic,giventhefactthatanumberofsourcesstatethatreciprocatingenginesincurbetween8-15%mechanicallossesandreciprocatingcompressorsbetween6-12%(Ref1:Kurz,R.,K.Bun,2007).OperatingConditionsForasituationwhereacompressoroperatesinasystemwithpipeofthelengthLuupstreamandapipeofthelengthLddownstream,andfurtherwherethepressureatthebeginningoftheupstreampipepuandtheendofthedownstreampipepeareknownandconstant,wehaveasimplemodelofacompressorstationoperatinginapipelinesystem(Figure1).Figure1:Conceptualmodelofapipelinesegment(Ref.2:Kurz,R.,M.Lubomirsky.2006).Foragiven,constantflowcapacityQstdthepipelinewillthenimposeapressurepsatthesuctionandpdatthedischargesideofthecompressor.Foragivenpipeline,thehead(Hs)-flow(Q)relationshipatthecompressorstationcanbeapproximatedby11112243skkdsppQCCTCH（Eq.4）whereC3andC4areconstants(foragivenpipelinegeometry)describingthepressureateitherendsofthepipeline,andthefrictionlosses,respectively(Ref2:Kurz,R.,M.Lubomirsky,2006).Amongotherissues,thismeansthatforacompressorstationwithinapipelinesystem,theheadforarequiredflowisprescribedbythepipelinesystem(Figure2).Inparticular,thischaracteristicrequiresthecapabilityforthecompressorstoallowareductioninheadwithreducedflow,andviceversa,inaprescribedfashion.Thepipelinewillthereforenotrequireachangeinflowatconstanthead(orpressureratio).Figure2:StafionHead-FlowrelationshipbasedonEq.4.Intransientsituations(forexampleduringlinepacking),theoperatingconditionsfollowinitiallyaconstantpowerdistribution,i.e.theheadflowrelationshipfollows:constHPssm（Eq.5）QconstHss1andwillasymptoticallyapproachthesteadystaterelationship(Ref3:Ohanian,S.,R.Kurz,2002).Basedontherequirementsabove,thecompressoroutputmustbecontrolledtomatchthesystemdemand.Thissystemdemandischaracterizedbyastrongrelationshipbetweensystemflowandsystemheadorpressureratio.Giventhelargevariationsinoperatingconditionsexperiencedbypipelinecompressors,animportantquestionishowtoadjustthecompressortothevaryingconditions,and,inparticular,howdoesthisinfluencetheefficiency.Centrinagalcompressorstendtohaveratherflatheadvs.flowcharacteristic.Thismeansthatchangesinpressureratiohaveasignificanteffectontheactualflowthroughthemachine(Ref4:Kurz,R.,2004).Foracentrifugalcompressoroperatingataconstantspeed,theheadorpressureratioisreducedwithincreasingflow.ControllingtheflowthroughthecompressorcanbeaccomplishedbyvaryingtheoperatingspeedofthecompressorThisisthepreferredmethodofcontrollingcentrifugalcompressors.Twoshaftgasturbinesandvariablespeedelectricmotorsallowforspeedvariationsoverawiderange(usuallyfrom40-50%to100%ofmaximumspeedormore).Itshouldbenoted,thatthecontrolledvalueisusuallynotspeed,butthespeedisindirectlytheresultofbalancingthepowergeneratedbythepowerturbine(whichiscontrolledbythefuelflowintothegasturbine)andtheabsorbedpowerofthecompressor.Virtuallyanycentrifugalcompressorinstalledinthepast15yearsinpipelineserviceisdrivenbyavariablespeeddriver,usuallyatwo-shaftgasturbine.Olderinstallationsandinstallationsinotherthanpipelineservicesometimesusesingle-shaftgasturbines(whichallowaspeedvariationfromabout90-100%speed)andconstantspeedelectricmotors.Intheseinstallations,suctionthrottlingorvariableinletguidevanesareusedtoDrovidemeansofcontrol.Figure3:Typicalpipelineoperatingpointsplottedintoatypicalcentrifugalcompressorperformancemap.Theoperatingenvelopeofacentrifugalcompressorislimitedbythemaximumallowablespeed,theminimumflow(surgeflow),andthemaximumflow(chokeorstonewall)(Figure3).Anotherlimitingfactormaybetheavailabledriverpower.Onlytheminimumflowrequiresspecialattention,becauseitisdefinedbyanaerodynamicstabilitylimitofthecompressorCrossingthislimittolowerflowswillcauseaflowreversalinthecompressor,whichcandamagethecompressor.Modemcontrolsystemspreventthissituationbyautomaticallyopeningarecyclevalve.Forthisreason,virtuallyallmoderncompressorinstallationsusearecyclelinewithcontrolvalvethatallowstheincreaseoftheflowthroughthecompressorifitcomesnearthestabilitylimit.Thecontrolsystemsconstantlymonitortheoperatingpointofthecompressorinrelationtoitssurgeline,andautomaticallyopenorclosetherecyclevalveifnecessary.Formostapplications,theoperatingmodewithanopen,orpartiallyopenrecyclevalveisonlyusedforstart-upandshutdown,orforbriefperiodsduringupsetoperatingconditions.AssumingthepipelinecharacteristicderivedinEq.4,thecompressorimpellerswillbeselectedtooperateatornearitsbestefficiencyfortheentirerangeofheadandflowconditionsimposedbythepipeline.Thisispossiblewithaspeed(N)controlledcompressor,becausethebestefficiencypointsofacompressorareconnectedbyarelationshipthatrequiresapproximately(f