Thermal and hydraulic characteristics of brazed plate heat exchangers - Part II: Current research on evaporators at KTH

This paper summarizes recent research on plate heat exchangers used as evaporators in domestic heat pumps and refrigeration systems, carried out at the Royal Institute Of Technology, Sweden. The investigations have been focusing on issues relevant to the performance and to parameters influencing the performance for the application in mind. Thus, the adiabatic pressure drop in a plate heat exchanger has been investigated and a new correlation is suggested, based on the classical approach by Lockhart-Martinelli and the correlation by Chisholm. The boiling section heat transfer coefficient was experimentally determined using TLC measurements. The resulting heat transfer coefficient was then plotted against mass flux and heat flux in order to investigate the relative importance of the two parameters. As it was found that the heat flux correlated the data much better than the mass flux, the applicability of using the LMTD assuming a heat flux-governed flow boiling correlation was investigated next. In addition, since the heat transfer coefficient seemed dependent on heat flux (wall superheat) the impact of different brine mass flow rates and brine temperature profiles was investigated. Further the possible improvement of running the evaporator in co-current configuration was investigated. The influence of geometry (chevron angle) and of using different inlet refrigerant flow distributor devices on the performance of a plate heat exchanger was also investigated.