Reduc Cond Flow

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CTV-PRB015-ENMarch 2004

Engineering

Bulletin

Reduc ing C ondenser FlowRates for Systems withCenTraVac Chillers

or

3.0 GPMc /Ton is Too High


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2 Reducing Condenser Flow Rates for Systems with CenTraVac Chillers CTV-PRB015-EN

Run the numbersThe optimum condenser waterflow rate for minimum system

poweris less than 3 GPMC/ton.

The optimum flow rate willdepend on several variables,including the hydraulic distancefrom the chiller to the tower, thespecific chiller, the cooling tower,and other factors. The only wayto determine the optimum flowrate is to do an analysis, and onesuch way is presented here.

Minor chiller adjustmentsThe analysis which follows usesthe same size chiller/motor/heatexchangers, but the impellerdiameter may change. There isno impact on chiller cost.

Chiller replacementsFor a chiller replacement wherethe tower will also be replaced,there are some operating versusfirst cost choices to be made.When the existing tower will bereused, it is important to make

sure that the nozzles areappropriate for the flow rate.

Assumptions1. The size of the condenserw ater piping in t his analysiswill remain unchanged.However, the building owner canseparately examine reducing thecondenser water piping size,trading higher condenserpumping costs for lower installedcosts.

2. The tow er size and tower

performance will be heldconstant. A customer can use an existingtower for a larger tonnage chillerwhen using lower GPMC/ton (as

a tower is essentially a GPMC

heat exchanger). The buildingowner can separately examineusing a smaller tower to tradehigher system operating costsfor a lower tower first cost.

3. The benefit of reduced flowwill be taken as lower designentering condenser watertemperature (ECWT) to thechiller.Because we are asking itto create colder water, the coolingtower will provide morecondenser water temperaturerelief for the chiller whenoperating at part load.

To determine the balance enteringcondenser water temperature(ECWT) at design conditions, the

centrifugal chiller selectionprogram and the tower selectionprogram must be run to obtainthe same temperatures. Forexample, at 2 GPMC/ton, the

programs balanced at 83.3F.Figure 1shows the average ECWTas a function of condenser flowrate. The averages werecalculated by maintaining thesame cooling tower and runningthe tower hardest to obtain thelowest return condenser water

temperature. This temperaturewill vary with the specific coolingtower. Figure 1 shows the averageECWT from several cooling towerselections.

Figure 1: ECWT by Flow Rate

GPMC/ton Average Design ECWT

3 85

2.8 84.7

2.6 84.4

2.4 84.1

2.2 83.72.0 83.3

Related TopicsChiller-Tower optimization

The optimumhourly, not design,tower temperature varies withthe chiller, the ambientconditions, the load, and somesystem criteria. Generally, theoptimum temperature for bestchiller plant efficiency will varybetween 60 - 77F it is NOT at55F entering condenser watertemperature. This topic isaddressed separately in an

Engineers Newsletter:Tower Temperature...Control it How?,available from the web athttp://www.trane.com/commercial/library/vol241/v24a.asp and in:

CTV-SLB005-EN, Chilled WaterSystem Design and Operation,available on TraneNET.

EarthWise System DesignsNot covered in this analysis but ofsimilar value are lower orvariable evaporator flow (GPME)

for reduced evaporator pumpingcosts and lower leavingevaporator water temperature(perhaps 42 or 40 F or lower,depending on the specificinstallation) to reduce thedelivered air temperaturethereby reducing the first costand operating cost of the airhandlers. The building owner canseparately examine reducing theduct sizes, effectively tradinghigher air side operating costs forlower installed costs.

3.0 GPMC /Ton is Too High
http://-/?-http://www.trane.com/commercial/library/vol241/v24a.asphttp://www.trane.com/commercial/library/vol241/v24a.asphttp://www.trane.com/commercial/library/vol241/v24a.asphttp://www.trane.com/commercial/library/vol241/v24a.asphttp://tranenetlax1/LBU_Marketing/Home/Data/Literature/ctvslb005en.pdfhttp://tranenetlax1/LBU_Marketing/Home/Data/Literature/ctvslb005en.pdfhttp://-/?-http://www.trane.com/commercial/library/vol241/v24a.asphttp://www.trane.com/commercial/library/vol241/v24a.asphttp://tranenetlax1/LBU_Marketing/Home/Data/Literature/ctvslb005en.pdf


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CTV-PRB015-EN Reducing Condenser Flow Rates for Systems with CenTraVac Chillers 3

Calculations

Evaporator pump energy requirements1

Condenser pump energy requirements2

PD =pressure drop (ft) H =tower height (ft)

1.Pump efficiency used for all calculations is the assumed combinedefficiency of the pump and pump motor efficiencies: 71% pump effi-ciency and 94.5% pump motor efficiency (obtained from a water pumpmanufacturer). GPMEassumes a 12F water temperature change inthe evaporator.2.Condenser pump calculations assume 20 feet of water pressure dropbetween the chiller and tower. This pressure drop varies with thesquare of the flow rate (GPM2).

EvapPumpkW

0.000188 kW

G P M F t!-------------------------

" #$ % x G PME x PD chiller PD loop+& '& '

(pump (pumpmotor)-----------------------------------------------------------------------------------------------------------------------------------------------------------=

EvapPumpkW

0.000188 kW

G P M F t!-------------------------

" #$ % 1000GPME 24.65Ft 30Ft+& '))

0.67------------------------------------------------------------------------------------------------------------------------------------------------------------ 15.3kW==

CondPumpkW

0.000188 kW

G P M F t!-------------------------" #

$ % GP Mc PD chiller Htower PD chiller *tower+ +& '))

(pump (+ pumpmotor------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------=

CondPumpkW

0.000188 kW

G P M F t!-------------------------" #

$ % 1000GPMc 12.09Ft 12Ft 8.9Ft+ +& '))

0.67---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 9.3kW==

CondPumpkW

0.000188 kW

G P M F t!-------------------------" #

$ % 1100GPMc 14.38Ft 12Ft 10.8Ft+ +& '))

0.67-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 11.5kW==

CondPumpkW

0.000188 kW

G P M F t!-------------------------

" #$ % 1200GPMc 16.88Ft 12Ft 12.8Ft+ +& '))

0.67-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 14.0kW==

CondPumpkW

0.000188 kW

G P M F t!-------------------------

" #$ % 1300GPMc 19. 60F t 12F t 15F t+ +& '))

0.67-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 17.0kW==

CondPumpkW

0.000188 kW

G P M F t!-------------------------

" #$ % 1400GPMc 22.55Ft 12Ft 17.4Ft+ +& '))

0.67-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 20.4kW==

CondPumpkW

0.000188 kW

G P M F t!-------------------------

" #$ % 1500GPMc 25. 73F t 12F t 20F t+ +& '))

0.67-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 24.3kW==


2.2 GPMc/ton:

2.4 GPMc/ton:

2.6 GPMc/ton:

2.8 GPMc/ton:

3.0 GPMc/ton:

2.0 GPMc/ton:


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Trane has a policy of continuous product and product data improvement and reserves the right to

change design and specifications without notice.

Trane

A business of American Standard Companies

www.trane.com

For more information, contact your local Trane

office or e-mail us at [email protected]

Literature Order Number CTV-PRB015-EN

Date March 2004

Supersedes CTV-PRB015-EN 1/04

Stocking Location Electronic

Figure 2: Analysis for 500 ton chiller plant

Percent Load

Condenser GPMC/ton kW 100 75 50 25

2.0 chiller 302 195.8 124.5 79.8

evap. pump 15.3 15.3 15.3 15.3

cond. pump 9.3 9.3 9.3 9.3

tower fan 21 21 21 21

totals 347.6 241.4 170.1 125.4

2.2 chiller 297.4 192.8 122.1 78.5

evap. pump 15.3 15.3 15.3 15.3

cond. pump 11.5 11.5 11.5 11.5

tower fan 21 21 21 21

totals 345.2 240.6 169.9 126.3

2.4 chiller 293.6 189.8 120.3 77.7evap. pump 15.3 15.3 15.3 15.3

cond. pump 14.0 14.0 14.0 14.0

tower fan 21 21 21 21

totals 343.9 240.1 170.6 128

2.6 chiller 291 188.6 119.7 77.5

evap. pump 15.3 15.3 15.3 15.3

cond. pump 17.0 17.0 17.0 17.0

tower fan 21 21 21 21

totals 344.3 241.9 173.0 130.8

2.8 chiller 288.8 186.6 118.2 76.8

evap. pump 15.3 15.3 15.3 15.3

cond. pump 20.4 20.4 20.4 20.4

tower fan 21 21 21 21

totals 345.5 243.3 174.9 133.5

3.0 chiller 287.3 185.9 117.8 76.6

evap. pump 15.3 15.3 15.3 15.3

cond. pump 24.3 24.3 24.3 24.3

tower fan 21 21 21 21

totals 347.9 246.5 178.4 137.2

ConclusionsFigure 2shows the breakdown ofpower usage when varyingcondenser flow from 3.0 down to2.0 GPMC/ton. The optimum

condenser flow rate is in the 2.0to 2.5 GPMC/ton range to

minimize overall chiller plantpower. As the tower is locatedfurther from the chiller, thecondenser water pressure dropincreases and the optimumcondenser flow rate decreases.

In this particular analysis, systempower consumption is mini-mized between 2.2 and 2.4GPMC/ton.


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