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.

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

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.

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

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

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

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

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

Centrifugal Compressors- Working Afterreachingimpeller,fluidenteringmovesalong

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

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

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.

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

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

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

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

FLOW THROUGH CENTRIFUGAL & AXIAL FLOW COMPRESSORS

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 valveisfullyopenandallthepoweris