Theoretical and experimental investigations on the changing regularity of the extreme point of the temperature difference between zeotropic mixtures and heat transfer fluid

The application of zeotropic mixtures may lead to the occurrence of the EPTD (extreme point of temperature difference), i.e. the Pinch Point, between the working fluid and the HTF (heat transfer fluid) in heat exchangers. The positions of the EPTD are related to the system operating conditions. In this paper, the influence of the system parameters on the temperature profile of the fluid and the location of the EPTD in countercurrent heat exchangers are investigated theoretically. When the type of the mixture is determined, the position of the EPTD is mainly affected by the flow ratio of zeotropic mixture to HTF. Afterwards, experimental studies on the MTD (maximum temperature difference) are carried out in a counter-flow tube-in-tube evaporator using R290/R600 (mass fraction 0.85/0.15) as the working fluid to verify the proposed theory. The selected mixture 85% R290/15% R600 has a temperature glide of 7.63 K at normal pressure. The experimental results show that the temperature distribution, the position and value of the MTD and the length of the two-phase region in the evaporator are influenced by the HTF inlet temperature and the flow rate ratio of zeotropic mixture to HTF.