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,
mailto:[email protected]://www.pumped101.com/http://www.pumped101.com/mailto:[email protected]8/9/2019 Efficiency of pump
2/13
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
8/9/2019 Efficiency of pump
3/13
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
8/9/2019 Efficiency of pump
4/13
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
8/9/2019 Efficiency of pump
5/13
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.
8/9/2019 Efficiency of pump
6/13
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.
8/9/2019 Efficiency of pump
7/13
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
8/9/2019 Efficiency of pump
8/13
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
8/9/2019 Efficiency of pump
9/13
8/9/2019 Efficiency of pump
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,
8/9/2019 Efficiency of pump
11/13
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
8/9/2019 Efficiency of pump
12/13
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
8/9/2019 Efficiency of pump
13/13
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