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EnergySavings: VariableSpeedDrivesinChilledWaterSystems

ThefollowingarticlewascontributedbyCarrierCorporation. ArticleshighlightingnovelHVAC

technologiesshouldbesubmittedtoChapterTechnologyTransferCommitteeChairMark

Maguire(c021bog4@ashrae.net)forconsiderationinfuturenewsletters.

VariablefrequencydriveusehasrapidlygrowninthepastdecadeinHVACsystemsas

advancementsinVFDtechnologyandreductionsinfirstcosthaveincreasedreturnon

investment. VFDshaveincreasedinperformanceandreliability,leadingtoVFDsbeing

standardinVAVandinsecondaryandprimarypumpingapplications.

WhydoVFDssaveenergy?

Theenergysavingsoncentrifugaldevices(fans,pumps,compressors)coupledwithavariable

speeddrivearedictatedbytheidealfanlaws(orpumpaffinitylaws).

Powerisproportionaltospeedcubed; thisiswhyVFDssaveenergyandhavebecome

prevalentincentrifugaldevices. Ascanbeseenfromthegraphsabove,loweringthespeed

reducesthepowerrequirementgreatly. Therelationshipbetweenliftandspeedisalsovery

importantinapplyingVFDstocentrifugalchillers. Thecentrifugalcompressorliftisnota

mathematicalfunctionofspeedasitisforductandpipesystems; thisrelationshipisexamined

furtherlater.

HowdoVFDswork?

VFDsconvertACpowertoDC,thenconvertbacktoACatadesiredfrequency. Incoming3

phaseACpowerpassesthrougharectifier,separatingthealternatingcurrent. Capacitors

converttherectifiedcurrenttoDC. AninvertermadeupofIGBTsswitchtocreatethedesired

electricalcurrenttomatchthedesiredmotorspeed.

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VariableSpeedPumpBasics

VFDsareappliedtopumpstoreduceenergycostsandincreasecontrol. Designingtheoptimal

systemforenergyefficiencyandreliabilityrequiresafundamentalunderstandingofthePump

AffinityLaws. Speedcontrolallowsthesystemtosupplytherequiredflowatthelowest

possiblesystem

pressure

and

power,

generating

operational

costs

savings.

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VariableCondenserWaterFlow

Iftheflowrateisvariedthroughachillercondenser,itiscriticaltorespecttheonboardchiller

safetiessuchashighcondenserpressure. Variablecondenserflowisnotthenorm,especially

forcentrifugalchillersthatdependonheadpressurereductionsatpartload. Incentrifugal

chillers,reducedcondenserwaterflowsmayelevateheadpressures,resultinginalossof

efficiencyandpossiblechillerinstability. Examinetheeffectontheentiresystem(chillers,

pumps,coolingtowers)whenconsideringvariablecondenserwaterflow.

VariableSpeed

Drives

in

Chiller

Applications

Acentrifugalchillerisaturbomachinethatfollowsthesamelawsofoperationaspumpsand

fans. Similarly,centrifugalchillerscanunloadwitheitherinletguidevanecontrolorby

reducingcompressorspeedwithvfds.

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Acentrifugalcompressormustmoveenoughcompressedrefrigerantflowtoproducetonsof

capacity,and

produce

enough

lift

to

overcome

system

head

pressure

(difference

between

condensingandsuctiontemperature). Thekeydifferencesbetweenpumpsandchillers:

Pumps: thesystemheadisafunctionofflow

Chillers: thesystemheadisafunctionofcondenserwatertemperatureandnotdirectlyrelated

toflow.

Whencanacentrifugalcompressorreducespeed?

Whenlessflowisrequired: vfdscanslowdowntomovelessmassflow

orrefrigerantwhenlessthandesigncapacityisrequired.

Whenlessliftisrequired: forafixedductorpipingsystem,theliftor

systempressureisafunctionoftheflow. However,forchillers,wheneverthecondensing

temperatureislessthandesign(e.g.,85Ftowerwater),thevfdcanreducedspeed,saving

energy. Centrifugalchillerswithvfdssaveenergyevenatfulltonnagewhenentering

condenserwatertemperatureisnotatdesign(e.g.,85F). Reducedheadpressureallowsa

vfd/compressortomovemorecapacityatlowerspeedswhileconsuminglessenergy.

How

much

energy

does

a

vfd

save

on

water

cooled

chillers?

IsfullloadkW/tonagoodindicatorofkWhconsumption? Nochillerplantsrarelyoperateat

100%capacityanddesigncondenserwatertemperatures. Philadelphiaweatherdatashows

thatthemajorityofchillerrunhoursoccurataverage6575Fenteringcondenserwater

temperatures. Variablespeedoperationisidealbecauseofthelargeamountofoperating

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hoursthatoccuratlessthan85Fenteringcondenserwatertemperature. Optimizethechiller

formaximumefficiencyatthemaximumnumberofrunhoursbyusingavfd.