chiller, centrifugal compressor, variable speed
Capacity control of a chiller with multiple variable-speed centrifugal compressors running in parallel between a common evaporator and condenser under reduced load conditions can be achieved by speed variation and/or adjusting the number of compressors in operation. These different capacity control schemes result in modifications of chiller efficiency under part-load conditions. The decision in selecting the number of operational compressors matching the compressor rotational speed to reach a desired part-load capacity depends not only on the required partial load but also on the required pressure ratio at that part-load condition. To illustrate the control dilemma, the paper will start with an example that shows the need for a different control strategy for two different chiller pressure ratios at equal part-load capacity. This difference in partial load control can be planned by observing the topological variation of the efficiency islands on the variable-speed centrifugal compressor map. For a given refrigerant the efficiency of a compressor is a function of only two process variables: head (isentropic enthalpy rise) and volumetric flow rate on the compressor map. With that information a multiple compressor control algorithm has been developed that can estimate optimum chiller efficiency for any head/flow combination the chiller with multiple compressors might encounter. The improvement in chiller part-load efficiency using this algorithm can be substantial as will be illustrated in the paper for a two compressor chiller configuration.