Optimization of Internally Geared Screw Machine Geometry for Air Compression Application

Gerotor machines are commonly used as oil and fuel pumps, and as hydraulic pumps and motors. They also have the potential to be used as positive displacement compressors, which is the focus of current research. The mechanism consists of an inner and outer rotor which rotate in the same direction but are each centered on offset parallel axes. The rotor profiles are specified such that multiple continuous contact points occur between them forming several separate working chambers, whose volume varies from minimum to maximum and back to minimum during a single rotation of the outer rotor. For a gas or two-phase working fluid, varying the discharge port geometry allows internal compression to occur prior to discharge. Furthermore, adding helical twist to the rotors allows the forces and torques acting on the rotors to be modified in order to minimize contact forces and power transfer between the driven and idler rotors. Previous research has investigated the operation of these internally geared screw machines via characterization of key geometrical properties and simplified analysis of the compression process. To compare the performance of an internally geared screw machine with the performance of the conventional twin screw machine, it is important to provide optimum design for a specific application. The current paper is focused on providing initial multivariable geometry optimization of the internally geared screw machine for compression of air from 1 to 8 bar. Number of lobes, outer rotor diameter and wrap angle are optimized and the existing quasi one-dimensional chamber model within the in-house performance prediction software is used.