Emilio Navarro-Peris

Experimental study of a heat pump with high subcooling in the condenser for sanitary hot water production

The use of heat pumps in order to produce sanitary hot water have been demonstrated as a very efficient alternative to traditional boilers. Nevertheless, the high water temperature lift (usually from 10°C to 60°C) involved in this application has conditioned the type of used solutions. In order to overcome it, transcritical cycles have been considered as the most suitable solution. The current article analyzes a new heat pump prototype able to enhance the heat pump efficiency using a subcritical cycle. The proposed prototype is able to control the system subcooling and make it capable to work at different subcoolings in the condenser. That kind of mechanism has demonstrated its capability to increase the efficiency of the heat pump. The obtained results have shown that coefficient of performance depends strongly on subcooling. In nominal condition (inlet/outlet water temperature at evaporator is 20°C/15°C and the water inlet/outlet temperature in the heat sink is 10°C and 60°C), the optimal subcooling is 42 K with a heating coefficient of performance of 5.35, which is about 25% higher than the same cycle working without subcooling.

Analysis of the Condensate Carryover Phenomenon on Fin and Tube Evaporators

Possible carryover of the condensate from the surface of the evaporator has always been a problem that in practice has been solved by limiting the air velocity. However, the need for more compact solutions and especially for the reduction of the frontal area in many applications requires the increase of the air velocity and therefore new solutions to overcome this problem must be developed. In this contribution, the authors develop an analytical model to estimate the evolution of the condensing drops over the fin surface of a heat exchanger as a function of the fin surface properties and air velocity. This model allows the estimation of the drop size when it starts to move and its trajectory and evolution along the fin. The possibility of drops forming water bridges in between the fins is also analyzed with estimation of the minimum fin separation to avoid its formation depending on the air velocity and the wettability of the fin surface. Finally, the results of an experimental campaign performed with two fin and tube coils of exactly same dimensions and geometry but with different fin materials: one with the standard aluminum fin and the other one with a specially outer hydrophilic layer, are presented, showing that this kind of coating avoids the condensate carryover with no appreciable penalty on the heat transfer performance.