Vapor compression refrigeration and heat pump systems

which use the natural refrigerant R718 as refrigerant

usually work at sub-atmospheric pressure. In order to study the water boiling heat transfer characteristics at sub-atmospheric pressure

one test system was built

and the heat transfer performance was studied under the pressures of 1.8~3.3 kPa. The results show that the coefficient of heat transfer increases with the increase of pressure and heat flux. Experimental data were compared with the calculated results by cooper correlation at the same conditions. Regression equations of the coefficient of heat transfer

which apply to the conditions of this experiment

were fitted using the least square method for the sake of practical application. This experiment can offer a useful reference for the study of evaporator heat transfer and heat transfer enhancement in water vapor compression refrigeration and heat pump systems.