The Influence of Rotor Geometry on Power Transfer Between Rotors in Gerotor-Type Screw Compressors

The configuration of a twin-screw positive displacement machine is proposed, consisting of an internally geared outer rotor meshing with an externally geared inner rotor. Helical rotors with constant profile and pitch are used with parallel rotor axes and stationary end plates incorporating inlet and discharge ports to achieve internal compression or expansion. The focus of this paper is to understand the effect of rotor geometry on two key performance indicators; the swept volume of the machine and the proportion of input power transferred between the inner and outer rotors. This requires a detailed analysis of the limitations on rotor profile generation, the formation of working chambers, and the forces exerted on the rotors. The choice of the rotor for power transfer to or from the machine is shown to be an important consideration, and helical rotors are found to enable lower power transfer between rotors during operation when compared with straight-cut rotors, but with reduced swept volume for the same machine size. For particular applications, this compromise is characterized through multi-objective optimization of the rotor profile and wrap angle in order to identify appropriate configurations for the proposed machine.