The present invention is directed to staging compressors in a multi-compressor refrigeration system, and more particularly, to a method of balancing run time between the multiplicity of compressors while minimizing the number of compressor starts.
In the past compressor staging has been based on continuous compressor run time. The compressor with the longest continuous on time is turned off whenever the refrigeration system calls for a decrease in capacity, and the compressor with the greatest amount of run time is turned on whenever the refrigeration system calls for an increase in capacity. U.S. Pat. Nos. 4,612,776; 4,679,404 and 4,580,947 are examples of this type of control system. This type of control system is intended to balance run time between operable compressors, thereby equalizing wear.
However, one of the largest causes of compressor failure is related to the stress undergone by the compressor during start up. It would be therefore advantageous to minimize the number of starts.
U.S. Pat. No. 4,535,602 alternately selects between equal capacity compressors to even the wear between them. This patent uses predetermined logic tables to determine when to "cut-in" or "cut-out" a compressor. Such an approach cannot compensate for periods of high capacity requirements where a single compressor is cycled on and off while the remaining compressors remain on. Nor can such a system compensate for compressor down time, where the available compressors must be turned on to maintain system refrigeration goals. Additionally, run-time is not equalized by such an approach.