Comparative study of some equations of state for computing properties of refrigerant-12

Abstract

An extended form of the popular two-constant van der Waals and Redlich-Kwong equations of state is suggested. In this extended form, the second constant in each equation is replaced by a polynomial in powers of temperature. The accuracy of the resulting equations in describing the (p, v, T) behaviour of Refrigerant-12 (R-12) and in generating its enthalpy and entropy curves is investigated for various polynomial degrees. The Benedict-Webb-Rubin equation with a varying number of terms is also included in the investigation for comparison purposes. A general procedure has been developed to compute the polynomial coefficients from the p-v-T data at a limited number of 'base' points and to calculate the enthalpy and entropy using thermodynamic relationships. It has been found that the extended Redlich-Kwong equation, with four polynomial terms, produces results of comparable accuracy to the rather complicated Benedict-Webb-Rubin equation and covers the whole vapour region of R-12 up to the critical point.