Method and apparatus for controlling a system of compressors to achieve load sharing

A compressed air system comprising:

• a plurality of centrifugal compressors, each one having an inlet valve and a bypass valve;
• means for storing compressed air at a predetermined system pressure, the compressed air storage means being disposed in fluid receiving relation with the plurality of centrifugal compressors;
• at least one control means for controlling a set point pressure for each centrifugal compressor;
• means for sensing the system pressure communicating with the at least one control means;
• means for identifying a predetermined centrifugal compressor to be unloaded from the compressed air system, the identifying means having a logic routine which ranks each centrifugal compressor with respect to the total time of compressor operation, and wherein the logic routine determines the lowest ranked centrifugal compressor having a bypass valve which has a position exceeding the bypass valve maximum position set point, and identifies said centrifugal compressor as the centrifugal compressor to be unloaded;
• and
• computer means for testing the identified centrifugal compressor to determine whether said centrifugal compressor should be unloaded from the compressed air system, the computer means communicating with the at least one control means and including a timing means which is activated upon identifying said predetermined centrifugal compressor, and whereupon deactivation, the computer means causes the at least one control means to decrease the set point pressure of said compressor until a predetermined test significant event occurs.