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Octobe r 2 001 ASHRA E Journa l 41

ASHRAE Journal Technology Award

S

About the Author

By Arthur G. Veneklase, P.E.Member ASHRAE

Arthur G. Veneklase, P.E., is princi-pal mechanical engineer, URS Corpo-

ration, Grand Rapids, Mich.

pect rum Heal th s Blodgett

Campus facility in Grand Rap-ids, Mich. has expanded to416,000 net ft2 (38 600 m2),which includes operating

rooms, an emergency department, labs,patient rooms and a medical office build-ing. The facilitys continued growth andits increase of internal equipment loadscreated a need for more cooling capac-ity. The facilitys cooling requirementson summer days with high wet-bulb tem-peratures exceeded the central chilledwater plants capacity.

The existing chiller plant consisted oftwo single-stage 1118-ton (3932 kW)steam absorption chillers, a 1,060-ton(3728 kW) two-stage absorption chiller,a 268-ton (943 kW) electric screw chiller,and a plate and frame heat exchanger forfree cooling. Four cooling tower cells,piped in parallel, provided common con-denser water for all chillers.

The primary chilled water pumps alsowere piped in parallel and provided flowto the parallel piped chillers. The cool-

ing towers were sized to handle only twoof the three running absorbers. The fourthcooling tower cell had been added to pro-vide cooler water to the condensers.While this helped, it was no longerenough to satisfy the facilitys demand.

The hospital decided to consider re-placing one of the existing 20-year-oldsingle-stage steam absorption chillers.

Replacement options were narrowed tothe following for analysis:

1. Continue to operate as is (baselinefor analysis).

2. New 1200-ton (4220 kW) electric

tric screw chiller was then brought on

followed by the new two-stage absorp-tion unit, and then the existing two-stageabsorption chiller.

Alt 4. The existing 268-ton (943 kW)screw chiller and new 1200-ton (4220kW) centrifugal chiller were combinedand modeled by a 1450-ton (5100 kW)centrifugal with an efficiency of 0.65kW/ton (a weighted average of the screwand centrifugal). The free cooler wasbrought on first when cold enough towerwater was available.

During off-peak periods, the centrifu-gal chiller was brought on f irst followedby the two-stage absorption unit. Duringon-peak periods, the two-stage absorp-tion unit was brought on followed by thecentrifugal chiller. This minimized themonthly on-peak demand charges whiletaking advantage of cheap off-peak elec-tricity. The on-peak electrical consump-tion rate with demand factored inaveraged $0.12/kWh, while the off-peakrate is only $0.0265/kWh.

Cost of OperationThe electric centrifugal chiller is theleast expensive machine to run off-peaksince it generates chilled water for$0.0159/ton-hour. This same chiller gen-erates chilled water on-peak for approxi-mately $0.072/ton-hour. Thesingle-stage absorption chiller generatesat a rate of $0.0765/ton-hour. The exist-

centrifugal chiller (0.61 kW/ton) oper-

ating as the primary chiller.3.New two-stage steam absorption

chiller.4. New 1200-ton (4220 kW) centrifu-

gal chiller (0.61 kW/ton) running as theprimary chiller during off-peak periodwith existing two-stage steam absorptionchiller running as primary chiller duringon-peak periods.

The computer analysis showed the fol-lowing annual energy cost savings:

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830,75$

The sequence of operation for the al-ternatives was as follows:

Alt 1. The free cooler was brought onfirst if cold enough tower water was avail-able. The existing 268-ton (943 kW)

screw chiller was then brought on, fol-lowed by the existing two-stage absorp-tion chiller and then a single-stageabsorption chiller.

Alt 2. The free cooler was brought onfirst if cold enough tower water was avail-able. The existing 268-ton (943 kW) elec-tric screw chiller was then brought onfollowed by the new 1200-ton (4220kW) centrifugal chiller , and then the ex-isting two-stage absorption chiller.

Alt 3. Again, the free cooler was usedfirst if cold enough tower water was avail-

able. The existing 268-ton (943 kW) elec-

Innovative ChillerLoading StrategyInnovative ChillerLoading Strategy

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ASHRAE Journal

ing two-stage absorption unit (at 12 lb/h [3.4 kW] of steam/ton) generates chilled water for approximately $0.0459/ton-hour. A new two-stage high-pressure machine (at 9.9 lb/h [2.8kW] of steam/ton) generates chilled water at about $0.0379/ton-hour. These rates are for full-load production.

The two alternatives with the lowest life-cycle costs wereAlternatives 2 and 4. These were the two centrifugal chilleroptions. The internal rate of return (IRR) on Alternative 4 is

24.4% while Alternative 2 showed an IRR of 11.2%. The dif-ference between Alternatives 2 and 4 showed the impact plantoperation has on costs.

Plant Operation

The basic premise for operation of the plant is to use thecentrifugal chiller to produce chilled water when no demandchanges are in effect, while minimizing running, or at least

loading of the centrifugal chiller, whendemand changes are in effect.

When the chilled water load during on-peak time s exce eded the two-stageabsorbers capacity, the hospital would haveto start the centrifugal chiller. Each chillerthen takes a share of the load proportionalto the percentage of the chilled water thatflows through it. Avoiding demand chargesrequires preferentially loading the ab-

sorber to minimize loading of the electricchiller.

Reducing the chilled water flow to thecentrifugal chillers evaporator would un-load the centrifugal chiller and have theadded benefit of reducing bypass flow, re-sulting in warmer water entering the evapo-rator. Traditional wisdom required theevaporator flow to be constant. Could flowthrough the evaporator be modulated andstill allow the chiller to operate reliablyand efficiently?

Computerized ratings were run for the1200-ton (4220 kW) chiller, from the origi-nal flow down to minimum allowable flow,assuming the entering water temperature(EWT) and leaving water temperature(LWT) remained as designed. The loadingof the chiller in this case dropped off dueto a drop in flow. In a constant flow setup,if the chiller unloads, there must be a cor-responding drop in temperature differencebetween entering and leaving chilled wa-ter temperature. If the discharge tempera-ture is controlled to a constant setpoint,

then that must mean the return water tem-perature is returning colder. This is whathappens in a constant flow evaporator sys-tem due to mixing of unused chilledwater, which bypasses the coils eitherthrough a three-way valve at the coil or abypass line. Table 2 shows the chillers ef-ficiency at part loading based on varyingthe temperature versus varying the waterflow.

As seen in Table 2, the part-load effi-ciency of the chiller is improved by vary-ing the flow to the evaporator. Based on

these findings, it was decided that vary-

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Octobe r 2 001 ASHRA E Journa l 43

Technology Award

ing flow through the evaporator ofthe centrifugal chiller would notonly preferentially load the absorber,but also improve the energy eff i-ciency of the centrifugal chiller. The

savings due to the preferential load-ing are beyond the operational sav-ings originally identified in the studythat assumed when both chillers ran,they shared the load equally and usedthe constant flow efficiency curve.

A modulating butterfly valve wasinstalled in the chilled water line atthe new centrifugal chiller to varyevaporator water flow. A speed adjust-ment built into the actuator was usedto ensure slow valve movement. Thisis necessary since a sudden reduction

in flow would create operating prob-lems internal to the chiller.

A flow sensor was mounted in thechilled water line to control thevalve. In addition, after testing andbalancing, a minimum valve positionwas set in software to avoid closingthe valve too far. A motorized valvealso was installed in the chilled wa-ter line serving the absorber. A differ-ential pressure sensor controls thisvalve across the absorption chillerevaporator barrel. If a higher thanmaximum pressure drop is sensed, thevalve will be trimmed to avoid ex-cessive flow through the evaporatordue to reduction of flow to the newcentrifugal chiller.

Flow is also measured in the twomain system loops. Temperatures aresensed in system supply and returnlines, chiller plant common lines, andat each chiller. The primary pumpsconvey chilled water to secondarypumps located in various areas of the

facility. Two-way valves control most coils. Variable-speeddrives had been installed on some of the secondary pumps andone primary pump (Figure 1).

Other ConsiderationsThree reduced bypass lines with control valves had been

installed years earlier across the primary chilled water supplyand return lines to bypass chiller flow not needed by the sec-ondary pumps. Determining how many valves to open duringoperation of the plant had proved difficult. If more than onevalve was open, there was a possibility of short-circuiting bothprimary and secondary flow. Under this project the controlvalves were removed and two check valves installed to allow

two bypasses to be active while ensuring that no short-circuit-

Figure 1: Chilled water flow schematic.

ing is taking place (Figure 1). The chiller plant pumps arecontrolled to minimize flow through the bypass while ensur-ing required minimum flow through the chillers, design chilledwater supply temperature to the system, and required systemflow.

Default or failure modes were determined upfront and pro-grammed to allow the system to continue to operate if a tem-perature sensor or flow meter should fail. These were written sothat electrical demand savings would not be compromised dueto a sensor failure, while ensuring delivery of chilled water tothe building.

Conclusions

Before this project was completed, the chiller plant could

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not produce 42F (5C) chilled water on hot, humidsummer days. As chilled water supply temperaturefloated up near 50F (10C), discharge air tempera-tures from air-handling units floated up. This resultedin many complaints regarding areas of the hospital

being warm and humid. Replacing the 1118-ton(3932 kW) steam absorption chiller with a 1200-ton(4220 kW) electric centrifugal chiller has increasedcooling capacity while the load on the cooling tow-ers has decreased. The chiller plant is able to main-tain 42F (5C) discharge chilled water under allambient conditions. This has resulted in the air-han-dling units being able to satisfy the spaces they serve.Complaints due to lack of chilled water capacityhave disappeared. The facility is now able to meetthe building cooling load on even the hottest days.

How a chiller plant is operated has a major effecton the cost of operation. The major aspects of operat-

ing the plant are:1. Preferential loading of two different types of

chillers to take advantage of the least costly energystream.

2. Modulating evaporator flow through the new electric cen-trifugal chiller to control its loading and maximize both chillerand system efficiency.

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not/Wk 07.0 26.0 85.0 65.0 45.0 55.0 75.0 95.0

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WkkaeP-ffO 428,2 999,2 315,2 680,3smrehTsaG 782,655 251,673 441,555 401,405

Table 1: Projected total facility energy consumption (annual).

Table 2: Effect of evaporator flow on efficiency. Ratings are for 85F

(29C) entering condenser temperature and 42F (5C) leaving evapo-rator temperature. Under the variable flow scenario, flow is at 50% of

design from 0% to 50% load, and matches the percent load above 50%.

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3. Changing chilled water bypass to self-regulating andeliminating the short circuit path to minimize unwanted mix-ing of supply and return water.