Conical screw compressor

In effect it can be thought of as a conical spiral extension of a gerotor, although the exact geometry is somewhat different due to the angular offset.

This theoretically allows much smaller rotors to have practical efficiency since at smaller sizes the leakage area does not become as large a portion of the pumping area as in straight screw compressors.

In conjunction with the decreasing diameter of the cone shaped rotor this also allows much higher compression ratios to be achieved in a single stage.

[1][2] A significant impediment to production is the machining of the outer rotor to the tolerances required.

Although the inner rotor can be manufactured with precise CNC machines using common methods, the outer rotor presents significant difficulties due to the restricted access to the interior for precision tooling, and assembling the outer rotor from easier to machine segments presents its own problems.