Rotary screw compressors are positive displacement compressors. The principle of compression in oil-less rotary screw compressors is similar to that of oil-injected models, but without oil being introduced into the compression chamber. Two distinct types are available - the dry type and the water-injected type.
In the dry type oil-free rotary screw compressors, the inter-meshing rotors do not touch and their relative positions are maintained through lubricated timing gears outside the compression chamber. In the water injected type, similar timing gear construction is used, but water is injected into the compression chamber to act as a seal in internal clearances, and to remove the heat of compression.