Thermodynamic Properties of Difluoromethane on the Saturation Curve

Vapor pressures in the range of 256 to 331 K and saturated liquid and vapor densities from 315 to 331 K have been measured for difluoromethane (HFC-32) on the saturation line by means of variable volume and static experimental methods. Our results have been combined with literature values, and on the basis of all the measurements, accurate analytical correlations for saturated liquid and vapor densities and vapor pressures qave been established. The functional form of the correlations is accurate over a wide range of temperatures including those near the critical temperature. The uncertainty in these correlations are comparable to uncertainties in the experimental data on which the correlations are based. Comparisons of thermodynamic properties on the saturated line, calculated using these correlations, with generalized correlations for HFC-32 reported earlier are also discussed. INTRODUCTION Hydrofluorocarbon (HFC) compounds have received much attention as replacement working fluids for refrigeration processes due to their low ozone depletion and global warming potentials. HFC refrigerants are also attractive because of their higher energy efficiency, which reduces of their total global warming impact. However, no single HFC compound has been identified as a replacement for the widely used refrigerants, HCFC-22 and R-502 [1,2]. Alternatively, a number of binary and ternary mixtures containing HFC-32 as a constituent have been identified as potential replacements. Pure difluoromethane is not a feasible working fluid because of its high flammability and vapor pressure, but this HFC can form azeotropic and near-azeotropic mixtures with other compounds and has a higher efficiency than most other alternatives. Therefore, HFC-32 as an added constituent will improve the energy efficiency of refrigerant blends. In the present paper, we combine our measurements of vapor-liquid equilibrium (VLE) for HFC-difluoromethane with data available in the literature to provide thermodynamic properties for this HFC over a wide range of temperatures, including those in the vicinity of the critical point. EXPERIMENTAL RESULTS Two different experiments have been used to measure the thermodynamic properties of difluoromethane at saturation. A static method was applied to measure vapor pressures. In this method, a 75 cm stainless steel cell was immersed in a Neslab temperature bath which controls temperature to 0.01 K. The temperature of the bath was measured to the same accuracy using a resistance thermometer. The equilibrium cell was connected bY stainless steel tubing to one side of a Ruska differential pressure cell (Model 2413-705). The vapor pressure on this side of the differential cell was balanced by nitrogen pressure using a Ruska differential pressure null indicator (Model 2416-711 ). Nitrogen pressure was controlled and measured to an accuracy of 0.001 MPa with a Heise Precision pressure controller (Model PPC-159). The differential pressure cell, connecting lines, and valves were heated to at least 5 K higher than the bath temperature to ensure no condensation of the vapor phase. Further details of the apparatus and experimental technique are given elsewhere [3]. Vapor pressures have been measured at thirty-two temperatures between 255.95 and 291.21 K. Experimental uncertainties in the temperature are less than ±0.02 K, and the maximum uncertainty in pressure is less than 0.2% of the measured value at the lowest pressure. The purity of the sample was 99.95%. The measured vapor pressures are summarized in Table I.