Geometry and performance modelling of internally geared screw machines with comparison to conventional twin screw compressor

Internally geared screw machines (IGSMs) are a class of positive displacement compressors introduced in recent years. Their design involves two intermeshing rotors, denoted as the main (inner) and gate (outer), which rotate in the same direction on parallel but offset axes. Unlike conventional twin screw machines, which implement two externally geared rotors positioned side by side, IGSMs feature internal gearing, with one rotor enclosed within the other. While conventional twin screw compressors are widely used and extensively studied, IGSMs remain relatively novel. Previous research has primarily focused on their geometric feasibility and potential advantages using simplified performance models. This paper contributes by presenting a detailed geometrical model of an internally geared screw machine, enabling the calculation of geometry relevant parameters and integration with a well-established one-dimensional chamber model for performance prediction. The chamber model is compared to Computational Fluid Dynamics (CFD) for a single IGSM design operating under both oil-free and oil-injected conditions. Furthermore, the IGSM design is compared with a conventional twin screw configuration to provide a preliminary assessment of relative performance for a small-scale air compression application. The results are analysed, and directions for future work are outlined to support continued development and validation of this novel machine concept.