Isentropic Mixtures and Their Application in Heat Pumps in Cold Climate Region

Organic fluids could be classified into wet, dry and isentropic types according to the slope of the saturated vapor envelope on a temperature-entropy diagram. Due to the limited availability of pure isentropic refrigerants, attempts are made to make isentropic mixtures by mixing wet and dry refrigerants with an appropriate composition in the present work. A simple principle for selecting potential component fluids for making isentropic mixtures is proposed. Based on the specified heat pump application with heat source inlet and outlet temperature being -8 oC and -13 oC respectively, and heat sink inlet and outlet temperature being 45 oC and 55 oC, respectively, 30 combinations of component refrigerants exhibiting characteristics of an isentropic fluid are obtained. The method to reduce the effect of pressure on superheat parameter of isentropic mixtures by adding a third component and adjusting the circulating mass fractions is discussed. Finally, the influence of superheat parameter on temperature glide matching in condenser as well as cycle performance is discussed for three different types of mixtures.