Rotary screw compressors have been successfully applied in the industrial refrigeration industry since the late 1950s. However, one characteristic that has limited their application is that they are fixed-volume ratio compressors. A compressor is designed for one operating compression ratio; if applied at a different ratio, its efficiency will be reduced.

A practical method for varying the volume ratio of a screw compressor during operation has been developed and successfully applied. This advance in screw compressor design eliminates the losses associated with over- or undercompression and provides for reduced energy consumption, particularly on applications with variable operating pressure ratios or on applications requiring a compressor with a volume ratio that is not readily available.