Analysis of pinch point in liquid-vapour heat exchanger of R134a-DMAC vapour absorption refrigeration system

R134a (1,1,1,2 tetrafluro ethane)-DMAC (N,N Dimethyl Acetamide) vapour absorption refrigeration system can be used for sub-zero temperature applications and in industries where ammonia is forbidden. But it needs rectification of vapour from generator and draining of residual R134a-DMAC liquid from evaporator. As such, owing to the comparatively low ratio of latent heat of vapourisation to vapour specific heat of R134a, liquid vapour heat exchanger (LVHX) is required and the residual liquid further enhances its prominence in sub-cooling the incoming condensate to improve COP. In this paper LVHX is analyzed in detail by varying operating parameters like rectifier efficiency and evaporation and generator temperatures. Heat capacity rate of the cold stream (vapour and residual liquid) changes continuously due to the progressive phase change of the residual liquid. Depending on the rectifier efficiency, the maximum temperature difference shifts from one end of LVHX to the other, while at certain efficiencies it occurs within the heat exchanger indicating that normal design procedure would lead to its design oversize. The importance of LVHX increases with a decrease in both rectifier efficiency and evaporator temperature.