Abstract: A control system is provided to determine when to use additional compressors in a multiple compressor refrigeration system during a pull down operation. The control system determines the rate of change of the difference between the leaving chilled water temperature and a setpoint temperature during the pull down operation of the refrigeration system. If the determined rate of change of the leaving chilled water temperature difference provided by the current configuration of the refrigeration system is not adequate, then an additional compressor of the refrigeration system can be started to obtain a better rate of change. The control system can repeat this evaluation of the determined rate of change of the leaving chilled water temperature difference until all compressors in the multiple compressor refrigeration system are used.