ReducedCondenser-WaterFlow Rate: Energy-SavingMiracle or Mirage?The conventional design of air-conditioningplants using large-tonnage chillers calls for acondenser-water flow rate of 3 gallons-per-minute-per-ton-of-refrigeration (GPM/TR). For years, this standard has been thought tooffer a good balance between first cost andenergy cost.Recently, this conventional design has beenchallenged by proponents of a 2 GPM/TRdesign. These proponents suggest that reduc-ing condenser-water flow rate to 2 GPM/TRwill save both first cost and energy cost. Theyposit that the cooling tower, condenser-waterpump and piping can all be downsized, thusreducing first cost. Furthermore, although thelower flow rate will penalize energy perfor-mance of the chiller, the proponents suggestthat the energy savings on the tower andpump will be greater than the energy penaltyon the chiller — and that these savings areavailable in most applications.The legitimacy of these claims was recentlyscrutinized by Wayne Kirsner, P.E., an inde-pendent consulting engineer based in Atlanta,Ga. He conducted an impartial analysis of theimpact of 3 versus 2 GPM/TR condenser-water flow rate, which was published in theFebruary 1996 issue of the ASHRAE Journal.His analysis forms the basis upon which thisdiscussion is founded.Fallacies of First-Cost SavingsThe 2 GPM/TR proponents imply that alower flow rate simultaneously offers first-costand energy-cost savings. We’ll examine thoseclaims separately, starting with first cost.The first-cost claim is this: lowering the con-denser-water flow rate will allow cost-effectivereductions in the condenser-water piping, con-denser-water pump, and cooling tower. But isthis assumption generally correct? Relying onKirsner’s analysis, the following examinationshows the answer is “no.” That’s becauseeither smaller equipment can’t be specified, orwhere it can, the first-cost savings are toosmall to offset greater energy usage.Piping First-Cost Savings?In many cases, there can be no size reduc-tion in condenser-water piping, because pipingis only available in specific sizes. Often thesame diameter pipe used for a 3 GPM/TR system must be used for 2 GPM/TR.For example, we can refer to the 500-TRchiller used in the example offered by some 2 GPM/TR proponents. At 3 GPM/TR, thewater flow is 1500 GPM. With pipe sizedaccording to the ASHRAE guideline of 1 to 4feet-of-pressure-drop-per-100-feet-of-pipe, thiswould require 8-inch pipe. At 2 GPM/ton, waterflow is 1000 GPM. Can 6” pipe be used? No,because friction loss would exceed theASHRAE-recommended upper limit of 4 feet,as shown in Table 1. So 8-inch pipe must stillbe used. Thus, in this and many otherinstances, there can be no pipe-size reductionand no first-cost savings.Even if the pipe size could be reduced(imagine that pipe components came in an infinite range of sizes), it is important to realizethat the increase in frictional pressure dropcaused by smaller piping will negate 30 to60% of the potential pump energy-savingscontribution, depending upon the percentagethat the piping-system friction contributes tothe total pump head — the greater the magni-GPM150010006" 8" 10"13.57'06.19'3.37'1.56'1.07'0.50'Table 1: Pipe Sizing2tude of the piping WPD, the greater the poten-tial energy-savings contribution lost by down-sizing the pipe. Tower First-Cost Savings?In most cases, maximum downsizing of thecooling tower eliminates all of the tower ener-gy savings from a 2 GPM/TR design. It is truethat reducing condenser-water flow rate willimprove a tower’s thermal efficiency, becausethe entering water will be warmer and willincrease its thermal “driving potential.” This willallow the airflow on the original tower to bereduced 12 to 15% to do the same load andapproach, and result in 30 to 40% less fanenergy for the same duty. However, if thetower is downsized to save first cost, theincreased thermal “driving potential” will be off-set by the reduction in heat-transfer surface,and the airflow must be increased through asmaller face area. The increased air-pressuredrop will eliminate the energy savings gainedby a 2 GPM/TR design.Pump First-Cost Savings?Theoretically, a condenser-water pump handling 33% less flow could be reduced insize to provide first-cost savings without running into an energy-cost penalty. But thetheory usually doesn’t hold up in practice.For example, a pump handling a condenser-water loop with 1500 GPM (3 GPM/TR for a500-TR chiller) at 45’ of pressure drop would beselected at 1750 RPM and 81% pump efficiency.(All selections are based on Aurora pumps). At 1000 GPM (2 GPM/TR) with the samesize pipe and condenser-pass arrangement,the head falls to 26.5’, and the least expensiveselection would be a smaller 1750 RPM pump,resulting in a first-cost savings of about $400.However, the pump efficiency would fall to70%, resulting in a net energy-cost increase of$600/year (assuming $.08/kWh and 5000hours annual operation).The best pump to handle 2 GPM/TR wouldbe the same pump used to handle 3 GPM/TR,but selected at 1150 RPM. That’s because itwould perform at 84% efficiency, paying backthe added first-cost of $400 (compared to thesmaller, less-efficient pump) in less than a year.This performance phenomenon is not isolat-ed to this case. Pump efficiency is a function ofthe specific speed of centrifugal impellers, andlower heads usually result in lower efficiencies.Thus, there is no first-cost benefit to switchingto a smaller pump. (In fact, in this example,pump cost would rise by $100 because of themore expensive 1150 RPM motor.)Chiller First-Cost Savings?The first-cost of the chiller does not decreasewith a 2 GPM/TR design. In fact, it generallyincreases. A larger motor, starter, and, in somecases, a larger compressor are required. Higheramp draw of a larger motor may also increaseelectrical installation costs by requiring largerwiring, breakers, and other components.Even if upsizing is limited to only the motorand starter, it could offset 30% of the first-costsavings available from downsizing the piping,tower and pump.First-Cost Conclusions You cannot simultaneously reduce firstcosts and energy costs with a 2 GPM/TRdesign. If the goal is to maximize first-cost savings, then 70 to 83% of the energy-costsavings from the tower and pump will be lost.And without the energy-cost savings to offsetthe chiller penalty, 2 GPM/TR does not normal-ly make good economic sense. Therefore, ifthe goal is to maximize energy savings,
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