Air-cooled chillers have long been considered inefficient for comfort cooling, because they operate at a high condensing temperature in the vapour compression cycle. This paper discusses how the condensing temperature can drop to improve the efficiency of these chillers at part load. Based on the experimental results of an air-cooled reciprocating chiller, the schedule of staging condenser fans under head pressure control (HPC) is compared with that under the control of floating condensing temperature (FCT). The extent to which the condensing temperature has to rise above the outdoor temperature depends greatly on the condenser effectiveness. Using FCT means maximizing this condenser effectiveness to moderate the rise in the condensing temperature, and, in turn, to trade off a reduction in compressor power against an increase in condenser fan power. Compared to HPC, chiller efficiency in kW/ton can be enhanced by 8.6% to 40.2% under FCT. Furthermore, cooling capacity can enlarge by 2.3% to 14.3%, depending on weather-load conditions. The potential and difficulty of applying FCT to existing chillers are discussed. It remains to be seen how to couple the condensing temperature with weather-load conditions to precisely specify the efficiency of air-cooled chillers.
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