Load Sharing Strategies in Multiple Compressor Refrigeration Systems

Abstract

Many refrigeration systems have multiple compressors that operate in parallel to meet the aggregate cooling load requirements of the system.Often, individual compressors are equipped with a means of modulating their capacity to match the prevailing cooling demand.Since the efficiency of a compressor tends to decrease as it is unloaded, compressor part-load characteristics influence the efficiency of the entire system.For this reason, it is desirable to identify operating strategies that properly account for compressor unloading characteristics and maximize the efficiency of the entire system.In this paper, we show that when two identical screw compressors are operating in parallel, there exists an optimum point at which it is best to switch from each compressor equally sharing the load to one compressor operating at full load and the other unloaded to match the remaining system cooling demand When two compressors of different sizes are operating, an optimal control compressor operating map can be developed which maximizes the efficiency of the entire system over a complete range of cooling loads.These optimum operating maps are shown to depend on the characteristics of the individual compressor's unloading performance and the relative sizes of compressors.An optimum control strategy for systems having multiple compressors can be implemented using the concept of cross-over points introduced in this paper.