Basic study on the oil recovery in a hybrid heat pump using ammonia/water solution

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

In an ammonia/water hybrid heat pump system which is a vapor compression cycle with a solution circuit, lubricating oil is commonly used for the compressor. Since Poly-Alpha-Olefin (PAO) oil which is commonly used for ammonia is immiscible with the ammonia/water solution, a proper oil recovery method is required for a smooth operation of the compressor. Although the oil separator installed at the outlet of the compressor removes most of the oil from the refrigerant vapor, some oil droplets are carried over and accumulated in the solution reservoir. Unlike the pure ammonia vapor compression system, the density of PAO oil is smaller than that of ammonia/water mixture which has the ammonia concentration of 30-40%, and the oil tends to rise and gather near the liquid/vapor interface. In this study, a method for oil recovery from the solution reservoir is suggested. In the present method, the mixture of the oil and the solution is drained into an oil separator having a narrow cylinder at the top, if the oil in the reservoir is greater than a certain amount. The oil droplets in the solution rise by buoyancy and gather at this upper narrow cylinder. The gathered oil is extracted and returned to the compressor by an oil recovery pump. Since the solution has to be returned to the reservoir as soon as the separation process is finished, the process time for the separation should be as short as possible. To predict the time for the separation, experiments and simulations have been carried out. The model using the multiphase segregated flow (MSF) showed that a proper choice of droplet diameter is necessary to predict a correct separation time. Also, a simulation model which is able to consider the effect of surface tension and droplet merging is needed to be developed.