Occurrence of pinch points in condensers and evaporators for zeotropic refrigerant mixtures

Abstract

Many zeotropic refrigerant mixtures are being proposed as alternatives to some CFCs and HCFCs. One of the advantages of zeotropes is the possibility of reduction in entropy generation by matching the temperature glides of refrigerant and heat-transfer fluid in both condenser and evaporator. However, perfect glide matching may not be possible with some zeotropes. Also, pinch points may occur with some zeotropes. In this paper the conditions that cause pinch points to occur are identified. It is shown that there exists certain limits for the temperature glide of the heat-transfer fluid in condensers and evaporators to avoid pinch points. The limits are unique for each mixture and are independent of the heat-transfer fluid. These limits can be used to evaluate quickly the suitability of a zeotropic refrigerant mixture for glide matching. These methods are illustrated with examples.