The occurrence of pinch point and its effects on the performance of high temperature heat pump

Abstract

Zeotropic mixtures are popular alternatives to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) in the high temperature heat pump system. Zeotropic mixtures exhibit two major characteristics during phase change: temperature gliding and a nonlinear relationship between temperature and enthalpy. The theory proposed by Venkatarathnam that the nonlinear relationship between temperature and enthalpy in the two phase region for zeotropic mixtures can cause pinch points were verified experimentally. Results also show that the variations of the maximum temperature difference and the minimum temperature difference change the mean temperature difference in the same way, and further change the exergy loss in the same way. Therefore, when selecting zeotropic mixtures as working fluids in the high temperature heat pump, it is of great importance to check the pinch points occurring in condenser and evaporator. The zeotropic mixture that has a smaller maximum temperature difference in condenser and a smaller minimum temperature difference in evaporator can give a higher COP.