Capacity control system and method for multi-stage centrifugal compressor

1. 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.

2. The method of controlling the multiple stage compressor as recited in claim 1 , wherein adjusting the flow reduction device position for each stage of the compressor comprises controlling each flow reduction device by an identical signal.

3. The method of controlling the multiple stage compressor as recited in claim 1 , wherein calculating the saturated suction temperature and the saturated discharge temperature comprises extracting the saturated suction temperature and the saturated discharge temperature from a controller saturation table.

4. The method of controlling the multiple stage compressor as recited in claim 1 , wherein a speed increase value may be saved in a controller under a table of values for a discrete number of flow reduction device positions.

5. A system for controlling a multiple stage compressor, comprising:

a compressor having a first compressor stage and a second compressor stage, a condenser and an evaporator connected in a closed refrigerant loop, each compressor stage having a flow reduction device;

a control system configured to control a capacity of each stage of the compressor, the control system configured to:

calculate a minimum required motor drive frequency;

adjust a flow reduction device positon;

if the flow reduction device position reaches a fully open position and a leaving chilled water set point is not met, increase an actual motor drive frequency;

measure a suction pressure and a discharge pressure;

calculate a saturated suction temperature and a saturated discharge temperature;

calculate 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;

select a Mach ratio providing a margin free from surge conditions; and

controlling the compressor to operate free from a surge condition.