Reduced Condenser Water Flow Rate Energy Saving Miracle or Mirage The conventional design of air conditioning plants using large tonnage chillers calls for a condenser water flow rate of 3 gallons perminute per ton of refrigeration GPM TR For years this standard has been thought to offer a good balance between first cost and energy cost Recently this conventional design has been challenged by proponents of a 2 GPM TR design These proponents suggest that reducing condenser water flow rate to 2 GPM TR will save both first cost and energy cost They posit that the cooling tower condenser water pump and piping can all be downsized thus reducing first cost Furthermore although the lower flow rate will penalize energy performance of the chiller the proponents suggest that the energy savings on the tower and pump will be greater than the energy penalty on the chiller and that these savings are available in most applications The legitimacy of these claims was recently scrutinized by Wayne Kirsner P E an independent consulting engineer based in Atlanta Ga He conducted an impartial analysis of the impact of 3 versus 2 GPM TR condenserwater flow rate which was published in the February 1996 issue of the ASHRAE Journal His analysis forms the basis upon which this discussion is founded Fallacies of First Cost Savings The 2 GPM TR proponents imply that a lower flow rate simultaneously offers first cost and energy cost savings We ll examine those claims separately starting with first cost The first cost claim is this lowering the condenser water flow rate will allow cost effective reductions in the condenser water piping condenser water pump and cooling tower But is this assumption generally correct Relying on Kirsner s analysis the following examination shows the answer is no That s because either smaller equipment can t be specified or where it can the first cost savings are too small to offset greater energy usage Piping First Cost Savings In many cases there can be no size reduction in condenser water piping because piping is only available in specific sizes Often the same diameter pipe used for a 3 GPM TR system must be used for 2 GPM TR For example we can refer to the 500 TR chiller used in the example offered by some 2 GPM TR proponents At 3 GPM TR the water flow is 1500 GPM With pipe sized according to the ASHRAE guideline of 1 to 4 feet of pressure drop per 100 feet of pipe this would require 8 inch pipe At 2 GPM ton water flow is 1000 GPM Can 6 pipe be used No because friction loss would exceed the ASHRAE recommended upper limit of 4 feet as shown in Table 1 So 8 inch pipe must still be used Thus in this and many other instances there can be no pipe size reduction and no first cost savings Table 1 Pipe Sizing GPM 1500 1000 6 13 57 06 19 8 3 37 1 56 10 1 07 0 50 Even if the pipe size could be reduced imagine that pipe components came in an infinite range of sizes it is important to realize that the increase in frictional pressure drop caused by smaller piping will negate 30 to 60 of the potential pump energy savings contribution depending upon the percentage that the piping system friction contributes to the total pump head the greater the magni tude of the piping WPD the greater the potential energy savings contribution lost by downsizing the pipe Even if upsizing is limited to only the motor and starter it could offset 30 of the first cost savings available from downsizing the piping tower and pump Tower First Cost Savings In most cases maximum downsizing of the cooling tower eliminates all of the tower energy savings from a 2 GPM TR design It is true that reducing condenser water flow rate will improve a tower s thermal efficiency because the entering water will be warmer and will increase its thermal driving potential This will allow the airflow on the original tower to be reduced 12 to 15 to do the same load and approach and result in 30 to 40 less fan energy for the same duty However if the tower is downsized to save first cost the increased thermal driving potential will be offset by the reduction in heat transfer surface and the airflow must be increased through a smaller face area The increased air pressure drop will eliminate the energy savings gained by a 2 GPM TR design Pump First Cost Savings Theoretically a condenser water pump handling 33 less flow could be reduced in size to provide first cost savings without running into an energy cost penalty But the theory usually doesn t hold up in practice For example a pump handling a condenserwater loop with 1500 GPM 3 GPM TR for a 500 TR chiller at 45 of pressure drop would be selected at 1750 RPM and 81 pump efficiency All selections are based on Aurora pumps At 1000 GPM 2 GPM TR with the same size pipe and condenser pass arrangement the head falls to 26 5 and the least expensive selection would be a smaller 1750 RPM pump resulting in a first cost savings of about 400 However the pump efficiency would fall to 70 resulting in a net energy cost increase of 600 year assuming 08 kWh and 5000 hours annual operation The best pump to handle 2 GPM TR would be the same pump used to handle 3 GPM TR but selected at 1150 RPM That s because it would perform at 84 efficiency paying back the added first cost of 400 compared to the smaller less efficient pump in less than a year This performance phenomenon is not isolated to this case Pump efficiency is a function of the specific speed of centrifugal impellers and lower heads usually result in lower efficiencies Thus there is no first cost benefit to switching to a smaller pump In fact in this example pump cost would rise by 100 because of the more expensive 1150 RPM motor Chiller First Cost Savings The first cost of the chiller does not decrease with a 2 GPM TR design In fact it generally increases A larger motor starter and in some cases a larger compressor are required Higher amp draw of a larger motor may also increase electrical installation costs by requiring larger wiring breakers and other components 2 First Cost Conclusions You cannot simultaneously reduce first costs and energy costs with a 2 GPM TR design If the goal is to maximize first cost savings then 70 to 83 of the energy cost savings from the tower and pump will be lost And without the energy cost savings to offset the chiller penalty 2 GPM TR does not normally make good economic sense Therefore if the goal is to maximize energy savings the piping pump and cooling tower cannot be downsized and first cost savings will be lost
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