Experimental study of plate heat exchangers in ammonia refrigeration unit

Ammonia is one of the alternative refrigerants to CFC502 used in commercial refrigeration system applications. For evaporators and condensers of such refrigerating systems, plate heat exchangers may be preferred, because of their high compacity and low hydraulic diameters which allow to reduce the quantity of ammonia in the refrigeration unit. The experimental work on plate heat exchangers, the results of which are presented in this report, has been performed on a two-stage compression system in which the evaporator can be used in two different ways: whether as a direct expansion evaporator or as a flooded evaporator with gravity circulation. Thus the inlet conditions such as the vapor quality, the saturation temperature, the mass flow rate... can vary. In this report, we present the experimental results obtained for both working conditions. Each of them has its specificity. The system working with a direct expansion evaporator shows clearly the importance of having an appropriate distribution device of the mass flow rate for the two phase flow at the inlet of the different channels because of its influence on the performance of the exchanger (moreover for fluids such as ammonia which have a very high latent heat of vaporisation and consequently a very low mass flow rate). The system working with a flooded evaporator fed by gravity circulation does not meet any problem of flow distribution between the different channels of the evaporator but shows several problems of flow instabilities.