Patent ID: 7059837

Claim:
A screw compressor(s) comprising at least one stage of compression, each compressor stage comprising a pair of rotors, variable speed compressor drive means for driving at least one of said rotors to effect air compression, an oil reclaimer for extracting oil from the compressed air, cooling apparatus for cooling oil extracted from the compressed air, wherein said cooling apparatus comprises a heat exchange device and a fan driven by a motor which can be run at different speeds to provide cooling air to the heat exchange device, said motor being independent from said variable speed compressor drive means, the speed of the fan motor being controlled by a control unit, the control unit comprising processing means for processing signals generated by a plurality of devices monitoring operating parameters of the compressor, at least one of which monitoring devices monitors the speed of the variable speed compressor drive means, said processing means calculating the input power of the compressor using a combination of the speed measurement and torque of the variable speed compressor drive means, and adjusting the speed of the fan proportionally to the input power to balance the heat rejected to the cooling air with heat rejection to the oil.