Performance Evaluation of R134a-DMAC VARS with Rectifier and Liquid-Vapour Heat Exchanger

R134a (1, 1, 1, 2 tetrafluro ethane) – DMAC (N, N Dimethylacetamide) vapour absorption refrigeration system (VARS), can be used for sub zero temperature applications and where ammonia cannot be used. This system needs rectification of vapour from generator and also draining of residual R134a-DMAC liquid from evaporator. Further, a temperature glide exists in the evaporator due to the presence of DMAC and the system can be operated at different low side pressures for the given evaporator temperature. This paper discusses the impact of low side pressure and the influence of rectifier and liquid – vapour heat exchanger (LVHX) on the overall performance of R134a-DMAC VARS. Without rectifier and LVHX, system COP decreases from 0.19 to 0.14 if evaporator pressure is reduced from 2 to 1.25 bar at cooling, sink and source temperatures of -10, 30 and140°C respectively, while it decreases from 0.328 to 0.181 with the rectifier and LVHX, each of efficiency 0.9. The use of LVHX enhances the COP by regaining the cooling available from both R134a in the residual liquid and vapour from the evaporator. The pinch point shifts from one end to the other of LVHX. At cooling, sink and source temperatures of -10, 25 and 140°C respectively, the system COP increases from 0.24 to 0.29 by using rectifier and LVHX of efficiency 0.9. The corresponding increase is from 0.132 to 0.24 at sink temperature of 35°C. Solution heat exchanger (SHX) is found to be a vital component of the system due to its large heat duty. The importance of rectifier increases with increase in efficiency of SHX.