Centrifugal and 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
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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