Impact of the filling level on the global heat transfer coefficient of a plate cross section for sorption heat pumps

Abstract

Compact evaporator like plate heat exchangers can play a significant role in reducing the investment cost of low cooling power sorption systems. However, when water is used as refrigerant, the working pressure is very low and vaporization phenomena are really different than vaporization phenomena occurring at higher pressures. Few studies focus on this subject and there is a lack of knowledge about vaporization (boiling or evaporation) phenomena occurring in compact evaporators at low pressure. The design of such evaporators remain manly empirical. There is thus a need of better characterization of the influence of the driving parameters in order to optimize the evaporator design. The objective of this article is thus to go further in the understanding of phenomena occurring in compact plate-type evaporators. In that goal, an experimental campaign was conducted to study continuously the performance of a smooth plate type evaporator as a function of the filling levels. The influence of the saturation pressure and the secondary fluid temperature on an overall heat transfer coefficient is studied. It is show that there is a dependence of the maximal overall heat transfer coefficient to these parameters. It is also shown that there seems to be a strong dependence between phenomena observed and phenomena that happens before. Thus, dynamic and inertia effects must be taken into account and model developed in absorption configuration cannot be applied for this study.