Efficiency of pump

8/9/2019 Efficiency of pump

1/13

Pump ED 101

Centrifugal Pump Efficiency What, How, Why & When ?

Joe Evans, Ph.D http://www.pumped101.com

IntroductionInthistutorial,wewillinvestigateseveralaspectsofcentrifugalpumpefficiency. FirstI

willdefineefficiencyandgivesomeexamples. Nextwewillexaminesomeofthe

designcriteriathatultimatelydictatetheefficiencyexhibitedbyaparticularpump. We

willalsotrytomakethatsomewhatnebulasquantity,knownasspecificspeed,more

meaningful. Iwillalsoshowitseffectontheshapeofapumpsperformanceand

powercurves.

Finally,

we

will

discuss

the

importance

of

(or,

sometimes,

unimportance)

ofefficiencyasitrelatestoaparticularapplicationorprocess. Wewillalsoillustrate

therelationshipofefficiency,head,andflowastheyapplytobothsteepandflat

performancecurvesandtheirrolesinconstantandvariablespeedapplications. Wewill

endwithabrieflookatthecombinedefficiencyofapumpanditsdriver.

WhatisPumpEfficiency?Whenwespeakoftheefficiencyofaanymachinewearesimplyreferringtohowwell

it

can

convert

one

form

of

energy

into

another.

If

one

unit

of

energy

is

supplied

to

a

machineanditsoutput,inthesameunits,isonehalfunititsefficiencyis50%. As

simpleasthismayseem,itcanstillgetabitcomplexbecausetheunitsusedbyour

Englishsystemofmeasurementcanbequitedifferentforeachformofenergy.

Fortunately,theuseofconstantswillbringequivalencytothese,otherwise,diverse

quantities.

Acommonexampleofsuchamachineistheheatenginewhichusesenergyinthe

formofheattoproducemechanicalenergy. Thisfamilyincludesmanymembersbut,

theinternalcombustionengineisonewithwhichweareallfamiliar. Althoughthis

machineis

an

integral

part

of

our

every

day

lives,

its

effectiveness

in

converting

energy

isfarlessthanwemightexpect. Theefficiencyofthetypicalautomobileengineis

around20%. Toputitanotherway,80%oftheheatenergyinagallonofgasolinedoes

nousefulwork. Althoughgasmileagehasincreased,somewhat,overtheyearsthat

increasehasasmuchtodowithincreasedmechanicalefficiencyasincreasesinengine

efficiencyitself. Dieselenginesdobetterjob,butstillmaxoutaround40%. This

increaseisdue,primarily,toitshighercompressionratioandthefactthatthefuel,
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underhighpressure,isinjecteddirectlyintothecylinderatthetopofthecompression

stroke. Gasolineengines,ontheotherhand,arelimitedtolowercompressionratios

becausefuelentersthecylinderpriortothecompressionstroke.

Inthepumpindustry,muchourwork involvestwoextremelysimple,yetefficient

machines

the

centrifugal

pump

and

the

AC

induction

motor.

The

centrifugal

pump

convertsmechanicalenergyintohydraulic(flow,velocity,andpressure)energyandthe

ACmotorconvertselectricalenergyintomechanicalenergy. Manymediumandlarger

centrifugalsofferefficienciesof7590%andeventhesmalleronesusuallyfallintothe

5070%range. LargeACmotors,ontheotherhand,canapproachanefficiencyof97%

andanymotor,fivehpandabove,canbedesignedtobreakthe90%barrier.

Theoverallefficiencyofacentrifugalpumpissimplytheratioofthewater(output)

powertotheshaft(input)powerandisillustratedbytheequationbelow.

=PW/PS whereisefficiency,Pwisthewaterpower,andPsistheshaftpower.

IntheUS,PsisthepowerprovidedtothepumpshaftinbrakehorsepowerandPwis

Pw=(QxH)/3960 whereQisflowinGPMandHisheadinfeet.

Theconstant,3960,convertstheproductofflowandhead(footpoundsorthemore

politicallycorrecttermpoundfeet)intoBHP. Theseequationspredictthatapump,that

produces100GPMat30ofheadandispoweredbyamotorthatproduces1BHPwill

haveanoverallefficiencyis75.7%atthatpoint. Thesecondequationwillalsoallowus

tocomputetheBHPrequiredatanypointonapumpsperformancecurveifweknow

theefficiency. Wewillseesomeexamplesofthisinthelastsectionofthistutorial.

HowisPumpEfficiencyAttained ?Ifyouthinkaboutit,thecentrifugalpumphasalotincommontheinductionmotor

whenitcomestothedesignphase. Thatcommonalityisthatbothhaveonlytwomajor

componentsthatcanbemodifiedbythedesigner. Inthecaseofthemotoritistherotor

andthestatorandforthepumpitistheimpellerandthevolute(ordiffuser). Ofcourse

thefrictionproducedbybearingsandothermechanicalcomponents(seals,stuffingbox,

etc)also

affect

pump

efficiency,

but

the

impeller

and

volute

have

the

greatest

influence.

Letsstartourinvestigationofcentrifugalpumpefficiencywiththeimpeller.

Thelawsofaffinitytellusquiteabitabouttheinnerworkingsofanimpeller. We

knowthat,foranygivenimpeller,theheaditproducesvariesasthesquareofachange

inspeed. Doublethespeedandtheheadincreasesbyafactoroffour. Ifyoukeep

speedconstant,thesameruleholdstrueforachangeinitsdiameter. Theflowthrough

animpellerfollowsasimilarrulebut,inthiscase,itschangeisdirectlyproportionalto


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thespeedordiameterchange doublethespeedordiameterandflowisdoubled.

Actually,whenwetalkaboutachangeinrotationalspeedorimpellerdiameter,weare

reallyreferringtoitsperipheralspeedorthespeed,infeetpersecond,ofapointonits

outermostcircumference. Itisthisspeedthatdeterminestheabsolutemaximumhead

andflowattainablebyanyimpeller(seetheUPandDownPuzzlerforanexplanation

of

the

falling

body

equation

and

how

it

relates

to

centrifugal

pump

head).

Theheadproducedbyanimpellerisalmostentirelydependentuponitsperipheral

velocitybut,flowisinfluencedbyseveralotherfactors. Obviously,thewidthand

depth(crosssectionalarea)oftheflowpassages(vanes)andthediameterofthe

impellereyeareimportantconsiderationsastheydeterminetheeasewithwhichsome

volumeofwatercanpassthroughtheimpeller. Otherfactorssuchasvaneshapealso

influenceanimpellersperformance. But,ifyouwantedtodesignanimpellerfrom

scratchwheretheheckdoyoustart? Doyoujusttakeawildguessaboutdimensions

andshapes,makesomesamples,andthentestthem? Well,intheearlydaysthatis

exactlywhat

we

did.

Today,

however,

we

can

draw

on

years

of

experience

and,

at

least,

findasuitablestartingpointforourdesign. And,thatstartingpointissomethingcalled

SpecificSpeed.

SpecificSpeedisoftenconfusingtomanyofusbecausewhenweseethewordspeed,

weimmediatelythinkimpellerspeed. Actually,itisjustanumber(often

dimensionlessliketheReynoldsnumberwhichisusedtopredictturbulentflow)that

referstoaparticularimpellerdesignorgeometrywithoutrespecttoitssize(capacity).

Itusestheknowledgewehavegainedovertheyearstocategorizetheperformanceof

variousimpeller

designs

based

upon

our

application

requirements.

The

chart

below

showstherelationshipofthenumericalvalueofspecificspeedtoaparticularimpeller

design.

Thelowervalues(500to1000)ontheleftdescribethechanginggeometryoftheradial

vaneimpellerwhilethehighervalues(1000015000)ontherightequatetotrueaxial


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flowimpellers. Thoseinthemiddle(15007000)aretypicaloftheFrancisvaneand

mixedflow(whichshowbothradialandaxialcharacteristics)impellers. Thecross

sectionalpicturesonthechartshowthat,asspecificspeedincreases,theimpellerinlet

oreyediameterincreasesandeventuallyapproachesorequalsthatofthevaneoutlet.

Theflowpassagesalsoincreaseinsizeatacorrespondingrate.

Ithinkyouwillagreethatwhilethisisanicecomparison,whatuseisittothepump

designer? Well,therehappenstobeanequationthatrelatesspecificspeedandits

correspondinggeometrytothoserealapplicationvaluesofhead,flow,androtational

speed. Thatequationis

Ns=nxQ/H0.75

whereNsisthespecificspeed,nisthepumprotationalspeedinRPM,Qisflowin

GPM,andHisheadinfeet. Wecanusethisequationtodeterminewhichimpeller

designcanbestmatchtherequirementsofaparticularapplication.

Suppose,forexample,weneedanimpellerthatwillproduce1000GPMat200feetof

head. IfweenterthesevaluesinQandHandalsoenteramotorspeedof3600rpmwe

obtainaspecificspeedof2140. Theimpellerwouldhaveageometrysimilartothe

Francisvaneimpellerseenonthechartatthe2000point. An1800rpmmotorwould

lowerthespecificspeedto1070andwouldhaveageometrysimilartotheradialvane

impellershownbeneaththe1000point. At1200rpmspecificspeedis714andthe

impellerwouldlooklikeahybridofthetwoimpellersseentotheleftofthechart.

The

chart

below

shows

illustrates

how

specific

speed

can

provide

us

with

several

predictionsastotheperformanceofaparticularimpellerdesign. Asaruleofthumb,

impellerefficiencyreachesitsmaximumataspecificspeedbetween2000and3000

althoughfavorableefficiencycanoccuratalmostanyspeed. Alsotheareaaroundthe

BestEfficiencyPoint(BEP),ordesignpoint,tendstobeflatterandbroaderasspecific


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speeddecreases. (Impellerefficiencyalsoincreaseswithpumprotationalspeed,

especiallyhighspeeds,butthatincreaseisnotaspronouncedatspeedsof3600rpmand

below.) Specificspeedalsoeffectstheshapeoftheheadcapacitycurve. Lowspecific

speeds(5002000)producerelativelyflatcurveswhilehighspeeds(5000+)produce

extremelysteepcurves. Intermediatespeedsproducecurvesthatfallinbetweenthese

extremes.

These

results

are

due

to

the

vane

shape

(flat

versus

backwards

curved)

at

variousspecificspeeds. (Wewilldiscusscurveshapeinmoredetailinthenextsection.)

Finally,specificspeedprovidesuswithonemoreprediction thecharacteristicsofthe

powercurve. Atspecificspeedsbelow3500,powerdropsasflowisreducedandisat

itsminimumatshutoffhead. Thepowercurveremainsrelativelyflat,acrosstheentire

headcapacitycurve,between4000and4500andrisestowardsshutoffatspecific

speedsabove5000. Atspeedsabove9000thepowerandheadcapacitycurvesalmost

paralleloneanother. Stateddifferently,powerisgreatestatshutoffandisatits

minimumatfullflow.

Oncea

particular

impeller

geometry

is

chosen,

the

pump

designer

can

go

through

a

comprehensivemathematicalanalysisthatwillallowhimtoderivealloftheimpeller

dimensionsandanglesnecessarytomeetthedesignpoint. Tosaytheleast,thisisan

arduoustask. Ifyouwouldliketoreviewacomprehensiveexampleofhowthisis

done,seepages2.23 2.31ofthesecondeditionofPumpHandbook(McGrawHill).

Theshapeandspacingoftheimpellervanesobviouslyhavealargeeffectupon

efficiency. Althoughtheidealpumpwouldhaveaninfinitenumberofvanes,thereal

worldlimitsusto57fortypicalpumpsandevenfewerforpumpsthathandlelarger

solids.Also,

flow

would

be

exactly

parallel

to

the

vane

surfaces

but

that

doesnt

happeneither. Butoddlyenough,ifthedesignerfollowssomewelldocumentedrules,

impellervaneefficiencylossesremainrelativelyflat(about2.5%)acrossaspecificspeed

rangeof500to7000. Diskfriction,whichiscausedbycontactbetweenthepumpage

andtheimpellershroudsandhubsurfaces,canreduceimpellerefficiencyanother4to

15%atspecificspeedsbelow2000butdecreasesto2%orlessat3000andabove. So,

dependinguponitsdesign,theimpellercanreduceoverallpumpefficiencybyaslittle

as4.5%orasmuchas17.5%.

Tongue

Throat

Area

Thevolutealsoplaysaroleinpumpefficiency. Atspecific

speedsbelow

2000,

its

losses

range

from

1

to

2.5%

but

losses

can

approach10%atspeedsover5000. Typically,volutedesign

beginswiththethroat,asitscrosssectionalareawilldetermine

theflowvelocityoutofthevolute. Flowthroughthethroatand

otherportionsofthecasingfollowsthelawofconstantangular

momentumsothedesignerwilltrytoavoidabruptchangesits

nearlycirculargeometrywhilegraduallyincreasingitsvolume.


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Anothercriticalareaofthevoluteistheclearancebetweentheoutercircumferenceof

theimpellerandthatofthevolutetongueorcutwater. Asthisdistancebecomeslarger,

anincreasingvolumeofpumpageescapesentryintothevolutethroatandis

recirculatedintothevolutecase. Thesmallestdistancepossible,thatdoesnotgiverise

topressurepulsations,willproducethebestefficiency. Asaruleofthumb,5to10%of

the

impeller

radius

tends

to

be

a

safe

value.

In

the

next

section

we

will

discuss

this

in

moredetailwhenwecomparetheefficienciesthatresultfromtrimminganimpeller

versuschangingitsrotationalspeed.

Itisdebatableastowhetherthevolumetricefficiencyofacentrifugalpumpisa

functionofthevoluteortheimpeller(itisprobablyboth)butIwillincludeitseffect

here. Volumetricefficiencyrepresentsthepowerlostdueleakageflowthroughthe

wearrings,vanefrontclearances(semiopenimpeller),andbalancingholesintherear

shroudofanimpeller. Asaruleofthumb,leakageincreaseswithadecreaseinspecific

speed,flow,oracombinationofthetwo. Forexampleataspecificspeedof500anda

flowof

100

GPM,

leakage

can

account

for

as

much

as

7%

of

the

total

power

consumed.

At2000GPMitisreducedtoabout2%. Athigherspecificspeedsandflowsitcanbeas

lowas1%.

Thefinalpieceofthepumpefficiencypuzzleisthatofmechanicallosses,although

someoftheselossesarenotalwaysincludedinpublishedefficiencycurves. Inthecase

ofaframemountedpump,theselossesarecausedbytheshaftbearingsandthe

mechanicalsealorpacking. Forclosecoupledpumps,bearinglossesarefiguredinto

themotorefficiency. Againtheruleofthumbfollowsthatofvolumetricefficiency,and

lossesincrease

as

flow

and

/or

specific

speed

decrease.

If

we

use

the

same

values

of

specificspeedandflow,asinthevolumetricexampleabove,wecouldexpectlossesof

5%and1%foraframemountedpump. Athigherspecificspeedsandflows,

mechanicallossesdropwellbelow1%.

WhyandWhenisEfficiencyImportant?OK,nowweareallonthesamepageastothedefinitionofpumpefficiencyandwe

havesomeideaofthepumpdesignersabilitytocontrolefficiencyduringthedesign

phase. But,isitthemostimportantcomponentinpumpdesign? Shouldwealways

shootfor

the

best

possible

efficiency

when

we

design

a

pump?

Theimportanceofpumpefficiencyisentirelyrelatedtotheuseofenergy. Asthecost

ofelectricityandotherenergysourcescontinuetorise,itjustmakesgoodsensethatwe

useitasefficientlyaspossible. Wheneverpossible,weshouldselectthemostefficient

pumpavailableasitwillusuallyjustifyits,potentiallyhigher,firstcostduringitsuseful

life. NoticethatIdidsaywheneverpossible.


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Whenisitnotsoimportant?

Severalfactorscaninfluenceourdecisionabouttheimportanceofpumpefficiency.

Sometimesitispurelyeconomic themarketmaynotbewillingtopaythepricefor

higher

efficiency.

There

are

also

times

when

a

higher

efficiency

pump

may

not

perform

aswellasoneoflowerefficiency. And,thereareinstanceswherewejustcannotattain

areasonableefficiencybasedontheheadandflowrequired. Letstakealookatseveral

examples.

Agoodexampleoftheroleofeconomicsistheresidentialpump. Theefficiencyofmost

fractionalHPdomesticboosterandcirculationpumpsfallsintothe50%range(Their

motorsarenotmuchbetter,butwewilladdressthatalittlelater). Thesepumpscanbe

designedtooperateathigherefficienciesbut,thecostwouldscaremosthomeowners

away. And,iftheyareusedonlyoccasionally,theenergysavingsmaynotjustifythe

additionalcost.

Unfortunatelythis,lowfirstcost,mindsetoftenspillsoverintosectorsthatcouldeasily

justifybetterefficiency(certainsectorsoftheHVACmarketcometomind). For

example,atypical,lowcostpumpwillnotincorporatesuctionwearringsand,as

wearprogresses,moreandmoresuctionrecirculationoccursandefficiencydecreases.

Theonlywaytofixthisistoreplacetheimpellerandpossiblythevolute. Incorporation

ofsimple,flatwearringsintothevoluteandtheimpelleraddsonlyafewpercentage

pointstothepumpcostbutallowslowercostefficiencymaintenanceasitisneeded.

Small,twoportimpellersewagepumpsmustsacrificeacertainamountofefficiency

inordertopasssolidswithoutclogging. Athreeorfourvaneimpellerwouldprovide

farbetterefficiencybutthesizeofthesoliditpassedwouldbegreatlyreduced

comparedtoatwovaneimpeller. Inthiscase,efficiencybecomessecondarytothe

requirementsoftheapplication. Therecessedimpellersewagepumpalsoofferssome

realadvantagesincertaininstallations,butisregardedbymanyasapoorchoice

becauseofitsverylowefficiency(3550%). Itusesatwostepprocess(theimpeller

createsavortexandthevortexcreatesflow)thatwastesquiteabitofenergybut,itwill

passstringymaterialandlargerdiametersolidsthantwovanepumpsofequivalent

size.In

a

small,

commercial

or

municipal

application

(say

5

10

HP)

which

is

more

costly wasting15 20%moreenergyinthecaseofthevortexpumportheweekly

expenseofpullingastandardpumpforcleaning?

Finally,thereareapplicationdesignpointswherereasonableefficiencycannotbe

attainedbutapumpisstillrequired. Supposesomemilliondollarprocesslinecannot

useapositivedisplacementpumpbut,instead,requiresacentrifugalpumpthatcan


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deliver20GPMat3000ofhead. Wouldwereallycareifasinglestagepumphadtobe

drivenat23,000rpmandthatitsefficiencywaslessthan25%? Probablynot,andthere

arefarmoreofthesetypesofapplicationsthanyoumightsuspect.

Whenisitimportant?

Themajorityofmunicipalandcommercial,clearwater,applicationsarenotrestricted

bythelimitationsoutlinedaboveandthesepumpsshouldbeselectedwithefficiencyin

mind. Ifapumpisgoingtooperateataconstantflowandhead,itshouldbeselectedto

operateasclosetoitsBEPaspossible. Anexampleofthistypeofapplicationwouldbe

apumpthatfeedsamunicipalwatertank. Sinceelevationandpipelinefrictionremain

constant,thepumpcanbesizedtomeetitsrequirementatthebestpossibleefficiency.

But,notallpumpingapplicationshaveaconstantflowandtheshapeofthe

performancecurvecanoftenbeasimportantasBEPitself. Anexampleofthistypeof

applicationis

the

constant

speed

booster

pump.

In

these

applications,

a

pressure

reducingvalve(PRV)throttlesthepumpwhendemanddecreasesinordertomaintain

someconstantpressure. Pressureonthepumpsideofthevalvefollowsthe

performancecurvepressurewhileapreset,constantpressureismaintainedonthe

dischargesideofthevalve. Whatpumpdesignbestfitsthisapplication?

Thechartbelowshowstheperformancecurveforapumpthatcouldbeusedinthe

boosterapplicationdescribedabove. Theapplicationcallsforapumpthatcanprovide

aconstantpressureboostof130overaflowrangeof100to300GPM. Thenumber

aboveeach

of

the

flow

points

on

the

performance

curve

is

the

BHP

required

at

that

Constant Speed Pump 1

15.2

14.3

13.7

12.4

11.1

9.99.0

80

105

130

155

180

205

230

0 50 100 150 200 250 300 350 400

Gallons Per Minute

Head

in

feet

pointandtheredlineisthedesiredsystempressure. TheBHPrequiredtooperatethis


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10/13

Asyoureviewvariouspumpcurvesfortheirfitinaconstantspeedboosterapplication,

youwillnoticeanotherefficiencytrait. Theefficiencies

oneithersideofBEParemorestableforsomepumps

thanothers. Forexample,aparticularpumpwithaBEP

of77%at400GPMisabletomaintain70%efficiency

over

a

range

of

250

to

550

GPM.

Another

pump,

with

a

similarBEP,maydropbelow70%muchmorequickly.

ThechangeinefficiencyaroundBEPhasalot todowith

theimpellersvaneangleatitsentrance. AtBEP,flowis

nearlyparallelwiththevanebut,asflowincreasesor

decreasesitsentranceangleintothevanealsochanges.

Smallchangesintheentranceangledesigncanaffect

bothBEPandtheefficiencyvaluesoneitherside. The

figureontherightshowshowtheentryanglechangesundervariousflowconditions.

BEP HIGH

LOW

Now,suppose

that

these

same

two

pumps

were

candidates

for

installation

in

a

variable

speedboostersystem(SeeVariableFrequency101ifyouarenotfamiliarwithVFD

operation). Wouldourfinalselectionbedifferent? Thechartbelowshowstheresults

whenPump2isoperatedunderVariableFrequencycontrol. Theheadrisefromfull

flowtoshutoffisabout15%andwouldallowjusta4Hzreductioninspeedifweareto

maintainaconstantpressureof130. Unfortunately,thisreductionisnotnearlyenough

toachieveareasonablepowersavingsoverthatoftheconstantspeedbooster. At

VFD Control Pump 2

1714

1211976

1311

108765

108

76544

0

50

100

150

200

250

0 50 100 150 200 250 300 350 400

Gallons Per Minute

Head

in

feet

60hz

55hz

50hz

System

flowsof200and100GPMpowersavingswouldbejust2and1HPrespectively. Also,


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thislimitedfrequencyrangewouldnotallowprecisecontrolofthepumpoveritsrather

broadflowrangeandasignificantamountoffrequencyhuntingcouldoccur.

But,Pump1hasaheadrisetoshutoffthatismuchgreaterandwhenoperatedunder

VFDcontrol(shownbelow)itcanperformquitewell. From100GPMtofullflowitwill

operate

over

a

range

of

47

to

60

hz

and

the

power

savings

at

each

reduced

flow

point

is

significantlygreaterthanthatofPump2runningatconstantorevenvariablespeed.

Variablespeedoperationoffersanadditionalbenefit. Theunbalancedhydraulicforces

thatexistatlowerflowsintheconstantspeedboosteraregreatlyreduced. Sothesteep

curveloosestotheflatcurvewheninstalledinconstantspeedboostersbutwinsina

variablespeedapplication. Keepthisinmind whenevaluatingpumpsforbooster

applications.

VFD Control Pump 1

910

11

12

14

14

15

78

9

10

11

11

12

5 66

7

8

8

9

4 45

56

66

0

50

100

150

200

250

0 50 100 150 200 250 300 350 400

Gallons Per Minute

Head

in

feet

60hz55hz50hz45hzSystem

Thereis,however,avariablespeedapplicationwhereflatcurvesexcel. Closedloop

circulationisaverycommonapplicationintheHVACmarket. Intheseapplicationsthe

pump

sees

no

head

due

to

elevation

and

all

it

has

to

overcome

is

the

friction

in

the

loop.

Asflowisreduced,headduetofrictiontendstofallquicklyandpumpspeedcanbe

greatlyreduced.

Thechartonthefollowingpageshowstheenergysavingsthatcanbeattainedby

utilizingflatcurvesinclosedloopapplications. Thesystemcurve(inred)showsthat

thefrictioninthelooprangesfromabout55at1400GPMtoabout15at700GPM.

Thisequatestoaspeedrangeof60to31hertzandresultsinapowersavingsof


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Closed Loop VFD Controlled

2

5

11

19

29.7

41.7

55

70

0

10

20

30

40

50

60

70

80

0.0 200.0 400.0 600.0 800.0 1000.0 1200.0 1400.0 1600.0 1800.0

Gallons Per Minute

Head

in

Ft

60hz

55hz

50hz

45hz

40hz

35hz

30hz

System3.1 hp 86% 31 hz

22.4 hp 87% 60 hz

8.6 hp 86% 44 hz

approximately86%attheminimumflowof700GPM. Allpointsinbetweenshowa

similarsavings.

Anothercentrifugalpumpefficiencytraitisillustratedbythisclosedloopexample.

Notice

that

the

efficiency

at

700

GPM

is

only

1%

less

than

that

of

the

true

BEP

at

1400

GPM. Whenthespeedofacentrifugalpumpisreduceditsefficiency,atanycapacity

pointonthe60hertzperformancecurve,followsthatcapacityatthelowerspeed. In

otherwords,efficiencymovestotheleftwithcapacityasspeedisreduced. Thisalso

occursinconstantpressurevariableflowapplicationsbutitismoreapparentin

applicationswherebothheadandflowarevariable.

Toacertainextent,wewillseethesameresultwhenanimpelleristrimmed

efficiencywillmovetotheleftwithcapacity. But,atsomepoint,thedistancebetween

theimpellerperipheryandthecutwatercausesunacceptablerecirculationand

efficiencybegins

to

drop.

Although

small

impeller

trims

can

be

effective,

a

change

in

rotationalspeedisthemostefficientmeansofchangingapumpscapacityandhead.

And,itforthisreasonthatvariablespeedpumpingsystemswillcontinuetoevolve.

CombinedEfficiency

Finally,letstakealookatsomethingIcallcombinedefficiencyasitisprobablymore

importantthanpumpefficiencyalone. Idefinethisefficiencyasthecombinationofthe


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hydraulicefficiencyofthepumpandthemechanical,heat,orelectricalefficiencyofthe

devicethatisdrivingit. Inthecaseofanelectricmotordrivenpump,itiscalledthe

wiretowaterefficiencyandreferstohowwellthetwomachinesworktogetherto

producehydraulicenergyfromelectricalenergy. Thereasoncombinedefficiencyisso

importanthastodowiththemathematicalrelationshipbetweenthetwoindividual

efficiencies.

SupposewehaveapumpwithaBEPof80%thatisdrivenbyamotorwithan

efficiencyof90%. Ifyouweretoasktheaveragepersontocalculatethecombined

efficiencyofthetwomachinestheywouldtypicallyaddthetwoefficienciestogether,

dividebytwo,andgiveyouananswerof85%. Ifthisweretrue,combinedefficiency

wouldbeanonissuebutunfortunately, itisnottheaveragebuttheproductofthetwo

efficiencies. Individually,90%and80%lookprettydarngoodbutwhenyoumultiply

onebytheother,theresultingefficiencydropsto72%! Still,overthelifeofan

installation,arelativelysmallincreaseinthecombinedefficiencycanmakeabig

differencein

energy

costs.

TheComprehensiveEnergyPolicyAct(EPACT),thatbecamelawin1997,setsome

minimumefficienciesforgeneralpurposemotorsfrom1200HPandspeedsof1200

3600RPM. Higherhorsepower,lowerspeed,anddefinitepurposemotorswerenot

requiredtomeettheseminimumefficiencies. Interestinglyenough,oneofthese

definitepurposemotorsistheclosecoupledpump(CCP)motor. Thesemotorsare

someofthemostcommonpumpdriversinuseandwhytheywereexcluded,Idonot

know. Theirunregulated,efficienciesrangefrom80%at3HPtoabout89%at30HP.

Mostmanufacturers

do,

however,

offer

higher

efficiency

models

that

range

from

86

to

94%overthesameHPrangeandtheirusecanmakeabigdifferenceinpumpenergy

consumptionovertime.

Youcouldprobablymakeaprettygoodcasethatpumpefficiencyisnottooimportant

ifthepumpistobedrivenbyagasolineengine. Althoughan80%efficientpump

shouldsavequiteabitofenergyoveronethatis65%efficient,thegasengine

(approximately20%)bringstheirtotalsdownto16%and13%respectively. Itmaybe

hardtojustifyahigherinitialpumpcostforsuchasmallenergysavings,unlessthe

pumpisusedfrequentlyandforlongperiodsoftime.

JoeEvans November2005

Efficiency of pump

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