Performance modelling of air-cooled twin-circuit screw chiller

A large variety of chiller models are available in the public domain but none can model chillers that comprise multiple and separate refrigerant circuits, despite that chillers of this type are already widely used for their good part-load performance. Presented in the paper is a mathematical model for an air-cooled twin-circuit chiller, with two screw compressors per circuit. The chiller model comprises a series of linked mathematical modules, each made up of a set of thermodynamic and empirical equations for modelling the major chiller components. The coefficients in the component models were evaluated using rated operating conditions obtained from the manufacturer and measured performance data of an existing chiller. The chiller model had been applied to simulate the performance of another set of chiller of the same make and model. Comparison of the predicted and measured performance of the chiller showed that the model could yield accurate energy use predictions over a wide range of operating conditions. The model could also provide good predictions of the variation in chiller performance due to staged operation of the separate refrigerant circuits in the chiller and of compressors in each circuit, which matched with observations made with measured chiller operation data.