2-PHASE HEAT-EXCHANGE FOR NEW REFRIGERANTS AND THEIR MIXTURES

Most of the studies on boiling and condensation heat transfer of new refrigerants have been done with pure substances. They show that existing correlations, for instance those presented in the VDI-Warmeatlas, adequately describe the nucleate boiling and condensation heat transfer of pure refrigerants. Boiling and condensation heat transfer coefficients of mixtures are often considerably lower than those of the pure components. The reduction depends mainly on the difference between dew- and bubble-point temperature for a given concentration. It compensates often to a great extent the advantage of a gliding temperature difference. In forced convection boiling these effects are well described by a model recently presented by Steiner and Taborek, tested with over 13000 data points. As shown in this paper it applies also for refrigerant mixtures. When vapours are condensed the usual assumption of film condensation often does not hold for mixtures, particularly at condensation rates that are not too large. Measured heat transfer coefficients are then larger by up to a factor of 4 than those of film condensation. In the case of larger condensation rates, subcooling of the vapour phase with subsequent fog formation can occur.