An elemental NTU-ε model for vapour-compression liquid chillers

This paper presents a steady-state model for predicting the performance of vapour-compression liquid chillers over a wide range of operating conditions. The model overcomes the idealisations of previous models with regard to modelling the heat exchangers. In particular, it employs an elemental NTU-ε methodology to model both the shell-and-tube condenser and evaporator. The approach allows the change in heat transfer coefficients throughout the heat exchangers to be accounted for, thereby improving both physical realism and the accuracy of the simulation model. The model requires only those inputs that are readily available to the user (e.g. condenser inlet water temperature and evaporator water outlet temperature). The outputs of the model include system performance variables such as the compressor electrical work input and the coefficient of performance (COP) as well as states of the refrigerant throughout the refrigeration cycle. The methodology employed within the model also allows the performance of chillers using refrigerant mixtures to be modelled. The model is validated with data from one single screw chiller and one twin-screw chiller where the agreement is found to be within ±10%.