A new functional form and new fitting techniques for equations of state with application to pentafluoroethane (HFC-125)

A widely used form of an equation of state explicit in Helmholtz energy has been modified with new terms to eliminate certain undesirable characteristics in the two phase region. Modern multiparameter equations of state exhibit behavior in the two phase that is inconsistent with the physical behavior of fluids. The new functional form overcomes this dilemma and results in equations of state for pure fluids that are more fundamentally consistent. With the addition of certain nonlinear fitting constraints, the new equation now achieves proper phase stability, i.e., only one solution exists for phase equilibrium at a given state. New fitting techniques have been implemented to ensure proper extrapolation of the equation at low temperatures, in the vapor phase at low densities, and at very high temperatures and pressures. A formulation is presented for the thermodynamic properties of refrigerant 125 (pentafluoroethane, CHF2-CF3) using the new terms and fitting techniques. The equation of state is valid for temperatures from the triple point temperature (172.52 K) to 500 K and for pressures up to 60 MPa. The formulation can be used for the calculation of all thermodynamic properties, including density, heat capacity, speed of sound, energy, and saturation properties. Comparisons to available experimental data are given that establish the accuracy of calculated properties. The estimated uncertainties of properties calculated using the new equation are 0.1% in density, 0.5% in heat capacities, 0.05% in the vapor phase speed of sound at pressures less than 1 MPa, 0.5% in the speed of sound elsewhere, and 0.1% in vapor pressure. Deviations in the critical region are higher for all properties except vapor pressure. (C) 2005 by the U.S. Secretary of Commerce on behalf of the United States. All rights reserved.