Capacity control system and method for multi-stage centrifugal compressor

A method of controlling a multiple stage compressor while maintaining compressor stability, comprising:

• providing a compressor having a first compressor stage and a second compressor stage disposed on opposite ends of a motor shaft, each of the first and second compressor stages having a respective flow reduction device for controlling flow of refrigerant through the respective compressor stage, and a control system;
• defining a Mach ratio and impeller outside diameter for each compressor stage;
• calculating, for each stage of the compressor, a minimum required motor drive frequency to operate free from surge conditions for a current head factor and flow reduction device position;
• adjusting, for each stage of the compressor, the flow reduction device position while maintaining an actual motor drive frequency at a lowest acceptable level to achieve a leaving chilled water temperature set point;
• if the flow reduction device position for each stage of the compressor reaches a fully open position and the leaving chilled water temperature set point is not met, and a maximum motor drive frequency has not been exceeded, increasing the actual motor drive frequency to increase motor speed;
• measuring a suction pressure and a discharge pressure for each stage of the compressor;
• calculating, for each stage of the compressor, a saturated suction temperature and a saturated discharge temperature;
• calculating an actual minimum motor drive frequency, wherein the actual minimum motor drive frequency is the greater of a first actual minimum motor drive frequency associated with the first compressor stage and a second actual minimum motor drive frequency associated with the second compressor stage;
• and
• selecting the Mach ratio which provides a desired margin free from surge conditions.