Naughton ("HVAC Systems… Part 1," 1990) discusses components for cleanroom cooling.
Chiller Polytropic Head:
When designing the chiller and chilled-water systems, understanding the factors that affect system operating costs will help in maximizing overall system efficiency. The total brake horsepower required by a centrifugal refrigeration machine is directly related to the polytropic head and refrigerant weight flow and inversely related to compressor efficiency. The polytropic head is determined by the suction and condensing temperatures where the difference between the suction pressure and the refrigerant condensing pressure roughly equals the polytropic head or lift of the compressor. Changes in polytropic head have the greatest effect on brake horsepower (Aloi 1976)Increasing cooler water temperature differentials or increasing the log mean temperature difference (LMTD) will result in higher suction temperature and a lower polytropic head. High system temperature differentials also benefit lower pumping costs.
The condenser water LMTD should be reduced to a minimum to reduce the condensing temperature and thus reduce the polytropic head. The refrigerant condensing temperature will decrease as the condenser water temperature is decreased; therefore, operating cooling towers and chillers at the lowest possible temperature will afford a more efficient chiller.
