Methods for predicting the condensation heat transfer of zeotropic mixtures of refrigerants

Zeotropic mixtures of refrigerants could be advantageously utilised in some industrial applications. In fact, as their isobaric phase-change processes develop under non-isothermal conditions, the use of high temperature glide zeotropic mixtures allows an optimization of the temperature profiles in the heat exchangers and thus higher efficiency in some industrial processes. A high temperature glide mixture can be obtained by blending together the fluids R-125 and R-236ea. This paper presents and critically reviews some methods to design a condenser working with the mixture R-125/236ea. When predicting the heat transfer coefficient during condensation of a zeotropic mixture, an additional mass transfer resistance has to be taken into account for, as compared the case of pure fluids. Three different procedures are applied and compared during an in-tube condensation process: the equilibrium method by Silver (1947), Bell and Ghaly (1973), the classical non-equilibrium method by Colburn and Drew (1937), and the recent non-equilibrium method by Koyama et al. (1998). They are applied to three mixtures R-125/236ea (30/70%, 50/50%, 70/30 % by mass), for the design of a tube-in-tube condenser for water heating application.