DEVELOPMENT OF A GENERALISED LOW-ORDER MODEL FOR TWIN-SCREW COMPRESSORS

Abstract

Twin-screw compressors (TSC) are commonly used in heat pump processes due to their robustness and flexibility. They exhibit two core properties, i.e. the swept volume and the built-in volume ratio (BVR), which heavily influence their capacity limits and off-design efficiency. Several models of vastly different computational costs have been proposed in literature to calculate the performance of TSCs. For applications that rely on large amounts of simulation runs, the computational cost of the compressor model becomes an essential factor. This work presents a new low-order model, which can accurately predict a TSC’s behaviour. First, a semi-empirical model from literature is slightly adapted and used to generate performance data for a large operational field. Then a polynomial model with a linearisation for high pressure ratios is fitted to this data. The model uses the external pressure ratio and volumetric compressor inlet flow to calculate isentropic efficiency and compressor speed. Both input parameters are normalised with a reference flowrate (calculated from the swept volume) and the BVR, respectively. This results in a generalised model of low numerical cost, which can be used for explorative studies independent of the specific machine size and BVR. A gain in computational speed by a factor of roughly 375 is achieved compared to a semi-empirical reference model. The model displays good predictive accuracy when used to predict the performance of machines with similar BVRs, but different sizes. When there is a difference in size and BVR, the prediction accuracy is still reasonable but significantly declines for small pressure ratios. Nevertheless, the proposed new approach extends the state-of-the-art by introducing a low-order model, which combines the advantages of low computational cost, good accuracy, physically correct predictions over a wide operational range and scalability to different machine capacities and BVRs.