Nonazeotropic mixtures as working fluids in large heat pumps - Heat transfer in a falling film evaporator and system simulations

Doktorsavhandling, 1986

The thesis is divided into two main parts, the first one dealing with heat transfer in the evaporator and the second one dealing with simulations of the whole heat pump cycle with nonazeotropic mixtures. One of the aims with the heat transfer studies was to investigate if the additional mass transfer resistance, introduced when mixtures are used, can be reduced in a thin falling film with high turbulence compared with corresponding one for a horizontal tube. The measurements were carried out in one smooth test tube with the mixtures on the inside. The investigated fluids are R22/R114, R12/R114 and R12/R11 and the corresponding pure fluids. For the mixtures the transition point between surface evaporation and nucleate boilng was depressed compared to pure fluids. This was found to be the main reason for the decrease of the heat transfer coefficients for mixtures. When all fluids were in the surface evaporating regime this decrease was small, as a maximum 21 %. The magnitude of the heat transfer coefficients varied between approximately 950 W/m2K and 4 000 W/m2K depending on Re number, heat flux and type of fluid. With aid of the computer program a parameter study on heat pump systems with nonazeotropic mixtures has been performed in the second part of this work. This study has been carried out for three case studies, two industrial ones and one space heating case. The results are shown as COP, heating capacity and condensing pressure. A special emphasis has been laid on the influence on these parameters of the heat exchangers size and a comparison between horizontal and falling film evaporators. The calculations have shown that, for given heat exchangers, there are in some cases possibilities for considerable increases of the COP. In favourable situations this improvement is in the order of 15-20 %. The heating capacity can be increased considerably in all cases where the "mean volatility" of the mixture is higher than that of the pure fluids, with which the comparison is made. Improvements of more than 100 % are possible to achieve. The comparison between falling film and horizontal evaporators has shown that the COP for a given area in some cases can be increased when the falling film type is used. In many cases the same COP can be achieved with a considerably smaller evaporator size, 50 % or less in the falling film case. This fact should make falling film evaporators economically interesting competitors to horizontal heat exchangers in the future.