Compressed air system and method of operating same

A compressed air system comprising:

• an air distribution network;
• a plurality of fixed speed drive base compressors, each base compressor having an air outlet coupled to the air distribution network and operable to independently supply a flow of pressurized air to the air distribution network;
• at least one variable speed drive trim compressor having an air outlet coupled to the air distribution network and operable to independently supply a flow of pressurized air to the air distribution network;
• and
• a control system coupled to the air distribution network, the base compressors, and the at least one trim compressor, the control system monitoring the pressure of the air distribution network and controlling the operation of the base compressors and the at least one trim compressor to meet an air demand placed on the air distribution network;
• wherein when the control system determines that there is excess air production and seeks to stop operation of a base compressor, the control system initiates a pre-unloading unload test for an operating base compressor, the unload test including reducing the output pressure of the operating base compressor by a predetermined increment in a series of steps and, at each step, monitoring the pressure of the air distribution network and confirming the reductions by counting a pulse in the output pressure of the operating base compressor, the control system unloading the operating base compressor only when the reductions are confirmed and the pressure of the air distribution network remains above the a predetermined threshold, and maintaining operation of the operating base compressor when the reductions are not confirmed or the pressure of the air distribution network drops below the predetermined threshold.