Centrifugal and Axial Flow Compressors!

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NITISH BHUSHAN IIT DELHI Tutor – Prof S. Sarkar Flow through Centrifugal & Axial Flow Compressors

Transcript of Centrifugal and Axial Flow Compressors!

Flow through Centrifugal & Axial Flow CompressorsNITISHBHUSHAN IITDELHI Tutor ProfS.Sarkar

Outline of the Presentation Compressors WhatisaCompressor? WhyincreaseaFluidsPressure? Classification

CentrifugalCompressors Configuration Working BladeTypes

AxialFlowCompressors Configuration BasicOperation

Efficiency CentrifugalCompressorPerformanceCurveFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Outline of the PresentationCentrifugalCompressors Analysis AxialFlowCompressors Analysis SizingParameters CentrifugalCompressors AxialFlowCompressors

LossesinCompressors Centrifugalcompressorsvs.AxialFlowCompressors AdvantagesofAxialFlowCompressors AdvantagesofCentrifugalCompressors

Summary References AcknowledgementsFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Compressors - What is a compressor?

Machinetoraisepressureofafluid Usesseveralenergytransformations1. 2. 3.

Inputenergyconvertedtorotatingmechanicalenergy Rotatingimpellerincreasesfluidskineticenergy(velocity) Decreaseinkineticenergyduetoflowareaexpansion& increaseinpressureenergy

Energyinputs:electricity,highpressuresteam,fuel

oil,compressedair,etc.

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Compressors Why increase a Fluids Pressure?1. 2.

StaticElevation

Ex:fromonefloorofabuildingtoahigherfloor Fluidmovingthroughpipingsystemexperiencesfrictional losses Pressureincreasedtoovercometheselosses Pressureincreasedforprocessreasons.Ex:tomovefluid intopressurizedvessel VelocityleavingcompressorhigherthanenteringvelocityFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Friction

3.

Pressure

4.

Velocity

Compressors - ClassificationA. PrincipleofEnergyAddition 1. Kinetic

Energycontinuouslyaddedtoincreasevelocity Pressureincreasedwithreductioninvelocity Mostimportantpart:CENTRIFUGALCOMPRESSORS Energyadditionisperiodic(notcontinuous) Directapplicationofforcetofluid CausesanincreaseinpressuretorequiredvalueFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

2.

PositiveDisplacement

Compressors -ClassificationB. HowEnergyAdditionisAccomplished

Secondlevelofclassification Kinetic:Centrifugalpumps,regenerativeturbines& specialcompressors PD:Reciprocating&RotarycompressorsThirdlevelofclassification Centrifugal:Supportofimpeller,rotororientation,pump bearingsystem,no.ofstages PositiveDisplacement:manytypesofrotary&reciprocating pumps,eachwithauniquegeometryFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

C. GeometryUsed

Centrifugal Compressor

Acentrifugalcompressorisaradialflowrotodynamicfluid machinethatusesmostlyairastheworkingfluidandutilizes themechanicalenergyimpartedtothemachinefromoutside toincreasethetotalinternalenergyofthefluidmainlyinthe formofincreasedstaticpressurehead.Itisbestsuitedto smallunitsofcomparativelylowpressureratiowhereoverall diameterisnotarestrictingcriterion.

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Centrifugal Compressors ConfigurationA.Impeller:impartshighvelocitytothefluid Theimpellerinletiscalledtheinduceroreye Theimpellerhassealsrelativetoabackplate Theimpelleroutletiscalledtheexducer Theimpellervanesatexducermayberadialor backswept Freevortexflowuntilleadingedgeofdiffuservanes Flowhashighdegreeofswirl(50o) axialstraightener vanesFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressors - ConfigurationSchematic views of a centrifugal compressor

Rotating Impeller

Single Sided Impeller

Double Sided Impeller

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Centrifugal Pumps - ConfigurationB.CasingI.

II.

VoluteCasing:Singlecutwaterwhereflowis separated.Flowmovingaroundvolutecasing producesnetradialforcethatmustbecarriedby shaft&radialbearingsystem.Doubleortwin compressorvolutesproducenearradialsymmetry& balancethehydraulicradialloadsonpumpshaft DiffuserCasing:Morecomplexcasingarrangement consistingofmultipleflowpaths.Liquidentersthe nearestflowchannelinthecasing.Multiple cutwatersarethere,evenlyspacedaroundthe impeller.FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

AdvantageofDiffusercasing:Resultsinnearbalancingof

radialforces,thuseliminatingtheneedforheavyduty radialbearingsystem.Thus,theradialbearingloadis MINIMIZED DisadvantageofDiffusercasing:Diffusercasinghas generallymorecomplexpartsthanvolutecasing.Thus, dependingonthesizeofthecompressor,economics oftendonotjustifyuseofdiffusercasingFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressors - Working Impeller rotatingwithshaft&casing thatencloses

impeller Fluidforcedintoinletbyupstreampressure Fluidmovestodischargesideasimpellerrotates Thiscreatesavoidorreducedpressureatimpeller inlet Pressureatcompressorcasinginletforces additionalfluidintoimpellertofillthevoid

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Centrifugal Compressors- Working Afterreachingimpeller,fluidenteringmovesalong

impellervanes,increasinginvelocityasitprogresses FluidatimpelleroutlettipisatMax.Velocity Fluidenterscasingwhereexpansionofcross sectionalareaoccurs Diffusionprocessoccurs fluidsvelocitydecreases Pressureoffluidincreases(Bernoullisequation)

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VELOCITY&PRESSURELEVELSOFFLUIDINA CENTRIFUGALPUMP

PRESSURE

PRESSURE OUTLETTIPOF IMPELLERVANE INLETTIPOF IMPELLERVANE VELOCITY

VELOCITY

SUCTION

FLOWPATH

DISCHARGE

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Centrifugal Compressors - Blade Types

Therearethreeimpellervanetypesdefinedaccordingtothe exitbladeangles(DischargeVaneAngles) Impellerswithexitbladeangleequalto90degreesareradial vanes Impellerswithexitbladeanglelessthan90degreesare backwardcurvedor backwardswept Vaneswithexitbladeanglegreaterthan90degreesare knownasforwardsweptvanes Theforwardcurvedbladehasthehighesttheoreticalhead. Radialvanesrepresentacompromisebetweenmaxpressure ratio,maxefficiency&sizeFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressors - Blade TypesFORWARDCURVED

HEAD

RADIAL

BACKWARDCURVED

FLOWRATE

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Axial Flow Compressors

Axialcompressors arerotating,aerofoilbasedcompressorsin whichtheworkingfluidprincipallyflowsparalleltotheaxisof rotation.Thisisincontrastwithcentrifugal,axicentrifugaland mixedflowcompressorswheretheairmayenteraxiallybut willhaveasignificantradialcomponentonexit.

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Axial Flow Compressors Axialflowcompressoriscapableofhigherpressure

ratio onasingleshaft. Theenergytransferinasinglestageisverylimited (stagepressureratioofabout1.2) Buteaseofcombiningaxialflowstagesleadsto pressureratiosofupto6/1orevenhigher Thusaxialflowcompressorisconsideredasconsisting ofmanystages Singlestageisconsideredasafan Formostaircraft&industrialgasturbine,axialflow compressorisusedinpreferencetoradialflowtypeFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Axial Flow Compressors- Configuration Onestagecomprisesarowofrotor bladesfollowed

byrowofstator vanes Ano.ofsuchstageswithrotorsonacommonshaft formthecompressor OftenarowofOutletGuideVanes(OGVs)are requireddownstreamtocarrystructuralload VariableInletGuideVanesmaybeemployed Thesearearowofstatorvaneswhoseanglemaybe changedtoimproveoffdesignoperation

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Axial Flow Compressors Basic Operation

Workingfluidinitiallyacceleratedbyrotorblades,then deceleratedinstatorbladeswherekineticenergy transferredinrotorisconvertedtostaticpressure Manystagesnecessaryforrequiredoverallpressureratio Flowalwayssubjecttoadversepressuregradient Processconsistsofseriesofdiffusionsinbothrotor& statorbladepassages Carefuldesignofcompressorbladingnecessarytoprevent wastefullossesandminimizestalling Flowreversalsmayoccuratmassflowconditionsdifferent frombladedesignconditionsFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Flow through stages in Axial Flow Compressor

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Efficiency Isentropicefficiencyisratioofidealspecificwork

input,ortotalTrise,forgivenpressureratiotoactual Definitionofisentropicisadiabatic+reversible TotalTrise&powerinputtosustaingivenPratiois proportionaltoinlettotaltemperature Polytropicefficiency isisentropicefficiencyofan infinitesimallysmallcompressionstep,suchthatits magnitudeisconstantthroughout Isentropicefficiencyfallsaspressureratioisincreased forsamepolytropicefficiencyFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

EfficiencyIsentropic efficiency c c = (T03s T01)/(T03 T01)

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Centrifugal Compressor Performance Curve

Letsanalysewhatwilloccurwhenavalveplacedinthe deliverylineofacompressorrunningatconstantspeedis slowlyopened ThevariationinpressureratioisshownaboveFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Performance Curve PointA occurswhenthevalveisshut&massflowis

zero.Itcorrespondstocentrifugalpressurehead producedbyactionofimpellerontheairtrapped betweenthevanes. AtpointB,efficiencyandpressureratioapproach maximumvalue.Furtherincreaseinmassflowwill resultinfallofpressureratio. Formassflowsgreatlyinexcessofdesignmassflow, airangleswillbewidelydifferentfromvaneangles leadingtobreakawayofair&fallinefficiency. Thepressureratiodropstounityat'C',whenthe valveisfullyopenandallthepowerisabsorbedin overcominginternalfrictionalresistancesFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Performance Curve - Surging Theoperatingpoint'A'couldbeobtainedbutapartofthecurve

between'A'and'B'couldnotbeobtainedduetoSurging. Surgingisassociatedwithsuddendropindeliverypressure& withviolentaerodynamicpulsationwhichistransmitted throughoutthemachine ForanyoperatingpointD onthepartofcharacteristicscurve havingapositiveslope,adecreaseinmassflowwillbe accompaniedbyafallindeliverypressure. Ifthepressureoftheairdownstreamofthecompressordoes notfallquicklyenough,theairwilltendtoreverseitsdirection andwillflowbackinthedirectionoftheresultingpressure gradient. Whenthisoccurs,thepressureratiodropsrapidlycausinga furtherdropinmassflowuntilthepoint'A'isreached,where themassflowiszero.FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Performance Curve - Surging Surgingstartstooccurinthediffuserpassageswhere

flowisretardedbyfrictionalforcesnearthevanes Tendencytosurgeincreaseswithnumberofdiffuser vanes Severaldiffuserchannelstoeveryimpellerchannel tendencyforairtoflowuponechannel&downanother (conditionsconducivetosurging) Onlyinonepairofchannelsthedeliverypressurewillfall &increaselikelihoodofsurging Thusnumberofdiffuservanesislessthanno.ofimpeller vanes SurgingisthennotlikelytooccurFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Performance Curve- Rotating Stall

Itisanotherimportantcauseofinstability&poor performancewhichcanexistinthenominallystable operatingrange. A,B&Carethreeconsecutiveflowchannels Whenthereisnonuniformityinfloworgeometryof channelsbetweenvanesorblades,breakdowncanoccur inonechannel(saychannelB) AirdeflectsinsuchawaythatCreceivesfluidatreduced incidence&Aatincreasedincidence ChannelAstallswhichreducesincidenceinBenabling flowinthatchanneltorecover Rotatingstallmayleadtoaerodynamicallyinduced vibrationsleadingtofatiguefailuresinotherpartsFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Performance Curve

Thereisanadditionallimitationtotheoperatingrange, between'B'and'C'.Asthemassflowincreasesandthe pressuredecreases,thedensityisreducedandtheradial componentofvelocitymustincrease. Atconstantrotationalspeedthismeansanincreasein resultantvelocityandhenceanangleofincidenceatthe diffuservaneleadingedge. Atsomepointsay'E',thepositionisreachedwhereno furtherincreaseinmassflowcanbeobtainednomatter howwideopenthecontrolvalveis CHOKING Thispointrepresentsthemaximumdeliveryobtainableat theparticularrotationalspeedforwhichthecurveis drawn.FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Analysis

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Analysis

Noworkisassumedtobedoneinthediffuser Energyabsorbedisdeterminedbyinletandoutlet conditionsattheimpeller Airenterstheimpellerinaxialdirection,soinitialangular momentumiszero. Vaneshaveacurvedaxialportionforsmoothentryofair. Nomenclature:

anglemadebytheleadingedgeofthevanewiththe tangentialdirection. Vr1 relativevelocityofairattheinlet V2 absolutevelocityofairattheimpellertip Vw2 tangential/whirlcomponentofV2 U ImpellerspeedatthetipFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Analysis

UnderidealconditionswhirlcomponentofV2 isequalto theimpellertipspeedU Duetoinertia,airtrappedbetweentheimpellervanes doesntmoveroundwiththeimpeller. Thisresultsinahigherstaticpressureattheleadingface thanthetrailingface. SlipFactor, takesintoaccountthiseffect; =Vw2/U Awidelyusedexpressionfor suggestedbyStanitzwhich isthemostsuitabletoradialvanedimpellers =1 (0.63/n) wherenisnumberofvanesFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Analysis

Theoreticalworkdone:U2 Duetofrictionbetweencasingandaircarriedroundby thevanesandotherlosseslikewindage,actualwork inputisgreaterthantheoretical Powerinputfactor takesthisintoaccount actualworkdone=U2 Typicalvaluesfor liebetween1.035 1.04 StagnationTemperaturerepresentsthetotalenergyheld bythefluid. Noenergyisaddedinthediffuser,so,stagnation temperatureriseacrosstheimpelleristhatequaltothe wholecompressor.FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressor Analysis Adiabaticworkdoneisgivenby

w=Cp(T02 T01) Ifstagnationtemperatureattheoutletofthediffuser isT03 thenT03 =T02 p03/p01 =[1+c(T03T01)/T01]/1

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Axial Compressor Analysis Velocity Triangles

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Axial Compressor Analysis AirapproachesrotorwithvelocityV1 atangle1

inaxialdirection CombiningV1 vectoriallywithbladespeedU givesvelocityrelativetobladeVr1 atangle1 FluidleavesrotorwithrelativevelocityVr2 at angle2 Airleavingrotoratangle2 thenpassestostator whereitisdiffusedtovelocityV3 atanangle3 Typicaldesignissuchthat V3 =V1 &3=1FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Axial Compressor Analysis AssumingVf=Vf1=Vf2 thefollowingequationsemerge

U/Vf =tan1 +tan1 U/Vf =tan2 +tan2 Powerinputisgivenby W=mcp(T02 T01) &W=mU(Vw2 Vw1) Theexpressioncanbeputintermsofvelocity&air anglestogive W=mUVf(tan2 tan1) orW=mUVf(tan1 tan2)FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Axial Compressor Analysis Thisinputenergywillbeabsorbedusefullyinraising

pressureofair&wastefullyinovercomingvarious frictionallosses Regardlessoflosses,thewholeofinputwillrevealas riseinstagnationtemp.ofair

T0S =T03T01 =(UVf/cp)(tan2 tan1) Pressureratioisthengivenby p03/p01 =[1+s(T03T01)/T01]/1

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Sizing Parameters - Centrifugal1. MeanInletMachnumber ThisistheMachnumberatcompressorface Whileitisdesirabletohavehighinletmachno.to minimizefrontalarea,thisleadstohighrelative velocitiesatfirststagebladetip,&inefficiency. Valuesbetween0.4 0.6arecommon 2. TiprelativeMachnumber Conservative&ambitiousdesignlevelsare0.9&1.3 Foracentrifugalrearstageofanaxicentrifugal compressorevenlowervaluesmightbeinevitableFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Sizing Parameters - Centrifugal3. RotationalSpeed

Mustbesettomaximizeefficiencybyoptimisingspecific speed,keepingotherparameterswithintargetlevels Highestpressureratiofromasinglestageis9:1,andfrom twostages15:1 Owingtoductingdifficulties,unusualtousemorethan twocentrifugalstagesinseries Formaxefficiency,backsweepangleof40o ispractical Howeverthisresultsinincreaseddiameterforgivenmass flow&pressureratioFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

4. Pressureratio&no.ofstages

5. Backsweep

Sizing Parameters - Centrifugal6.

RimSpeedExducerrimspeedshouldnotexceedaround500m/s for aluminium&625m/sfortitanium

7.

ExducerHeightInitiallysettoachievetargetrelativevelocityratiofrom inducertiptoexitof0.50.6 Thisshouldbeideallyoptimizedbyrigtesting

8.

ExitMachnumber&SwirlAngleWherebend&axialstraightenersareemployed,exitmach no.&swirlanglemustbelessthan0.2&10o Ifbend&axialstraightenersarenotemployed,swirlangle willbeoforderof50oFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Sizing Parameters - Axial1.

MeaninletMachnumberCommonvaluesliebetween0.4 &0.6 Highestlevelforaeroenginesinsupersonicapplications

2.

TipRelativeMachnumberThehighesttiprelativemachno.willoccuronthefirststage Inletabsolutegasvelocitywillusuallybeaxial&maybe consideredconstantacrosstheannulus Conservative&ambitiousdesignlevelsare0.9 &1.3 Thelatterrequireshighdiffusionrelativetobladetoachieve subsonicconditions,whichincreasespressurelosses

3.

StageLoadingMeasureofhowmuchworkisdemandedofthecompressoror stage Itistheenthalpyincreaseperunitmassflowofair,dividedby bladespeedsquared(Dimensionless)FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Sizing Parameters - Axial Efficiencyimprovesasloadingisreduced,butmorestagesare

requiredforgivenpressureratio Loadingalongthepitchlineshouldbe0.25 to0.54.

Pressureratio&numberofstages Achievablepressureratioforgivenno.ofstagesgoverned

mostimportantlybygoodefficiency Highertheoverallpressureratioinagivenno.ofstages,& henceloading,lowertheefficiency5.

HadeAngle Angleoftheinnerorouterannuluslinetotheaxial Ahadeangleofupto10o ,butpreferablylessthan50 maybe

usedforouterannulus Innerannuluslineangleshouldbekepttolessthan10oFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Sizing Parameters - Axial6.

AxialVelocity&AxialVelocityRatioAxialvelocityratioistheaxialvelocitydividedbyblade speedonthepitchline Axialcomponentofvelocityisnormallykeptconstant throughoutthecompressor Axialvelocityratioisnormallybtw0.5 &0.75

7.

AspectRatioDefinedasheightdividedbyvaneorbladechord Typicaldesignlevelsare1.5 3.5

8.

ExitMachnumber&SwirlAngleMustbeminimizedtopreventdownstreampressureloss Machno.shouldntbehigherthan0.35 (ideally0.25) Exitswirlangleshouldbelessthan10O (ideally0)FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Losses in Compressors FrictionalLosses Majorportionofthelossesisduetofluidfrictioninstationaryand

rotatingbladepassages Flowinimpelleranddiffuserisdeceleratinginnature Frictionallossesareduetobothskinfrictionandboundarylayer separation Dependonthefrictionfactor,lengthoftheflowpassageand squareofthefluidvelocity IncidenceLosses

Duringtheoffdesignconditions,thedirectionofrelativevelocityof

fluidatinletdoesnotmatchwiththeinletbladeangle Hence,fluidcannotenterthebladepassagesmoothlybygliding alongthebladesurface Thelossinenergythattakesplacebecauseofthisisknownas incidenceloss

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Losses in Compressors Thisissometimesreferredtoasshocklosses. However,thewordshockinthiscontextshouldnotbe

confusedwiththeaerodynamicsenseofshock

Clearanceandleakagelosses Certainminimumclearancesarenecessarybetweenthe

impellershaftandthecasingandbetweentheoutlet peripheryoftheimpellereyeandthecasing Theleakageofgasthroughtheshaftclearanceisminimized byemployingglands. Theclearancelossesdependupontheimpellerdiameterand thestaticpressureattheimpellertip. Alargerdiameterofimpellerisnecessaryforahigher peripheralspeedanditisverydifficultinthesituationto providesealingbetweenthecasingandtheimpellereyetip.FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Losses in Compressors Dependence of various losses with mass flow

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Losses in Compressors

Theleakagelossescompriseasmallfractionofthetotal loss. Theincidencelossesattaintheminimumvalueatthe designedmassflowrate. Theshocklossesarezeroatthedesignedflowrate. However,Incidencelossescomprisebothshocklossesand impellerentrylossduetoachangeinthedirectionoffluid flowfromaxialtoradialdirectioninthevanelessspace beforeenteringtheimpellerblades. Theimpellerentrylossisverysmallcomparedtoother losses. Thisiswhytheincidencelossesshowanonzerominimum valueatthedesignedflowrate.FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal vs. Axial Flow Compressors Advantages of Axial Frontalareaislowerforgivenmassflow&pressure

ratio.Forex.atpressureratioof5:1,axialcompressor diameterwouldbehalfofcentrifugal Weightislessbecauseoflowerenginediameter Formassflowrates>5kg/s,axialflowcompressor hasgreaterisentropicefficiency Magnitudeofaboveadvantageincreaseswithmass flowrate Owingtomanufacturingdifficultiesthereispractical upperlimitofaround0.8mondiameterofcentrifugal impeller,hencemassflow&pressureratiocapabilityFLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal vs. Axial Flow Compressors Advantages of Centrifugal Over9:1pressureratioachievableinasinglestage.Foraxialflow

compressorthismaytakebetweensixtotwelvestages Centrifugalcompressorsaresignificantlylowerinunitcostfor same massflowrate&pressureratio Atmassflowrates