Generalized equation of state for refrigerants with applications.

Abstract

Recent studies have indicated that chlorofluorocarbon compounds that are widely used as refrigerants in vapor-compression refrigeration cycles and as propellants are the major suspect to the depletion of the upper atmosphere ozone layer which protects the earth from the harmful radiation. Studies need to be made to replace these refrigerants with alternatives that are safe to our environment. In order to develop new refrigerants, we need to have a better generalized equation of state that can predict thermodynamic properties of the methane and ethane series refrigerants with a minimum number of characteristic parameters. In this work, a four-parameter generalized equation of state with three reference fluids is developed and compared with the experimental data. Significant improvement has been achieved over other well known generalized equations of state such as Lee-Kesler and Wu-Stiel. Some refrigerant mixtures have favorable thermodynamic properties and possess great potential as the working fluids in the vapor-compression refrigeration cycle. The generalized equation of state, combined with the mixing rules suggested in this study will be used in predicting thermodynamic properties of the mixture. The generalized equation of state requires only four characteristic parameters. For some refrigerants, however, even these parameters are not available. This problem arises if the refrigerant of interest has not yet been manufactured or the experimental data are unreliable or insufficient. To solve this problem, estimation of these parameters based on the group contribution method which only requires the information of the molecular structure is given. Coefficients from original work are recalculated to give better results for the refrigerants. Finally, the generalized equation of state is applied to an ideal vapor-compression refrigeration cycle. Small deviation from other existing equations of state which have been developed by fitting an extensive number of experimental data verifies the accuracy of this work. If the normal boiling temperature of a substance is known, other thermodynamic properties can be predicted with significant accuracy. Composition dependence of the coefficient of performance using binary non-azeotropic mixtures of R22 + R12 and R22 + R114 are calculated and the optimum composition is suggested. This work will provide a valuable tool for studying refrigeration cycles with different pure and mixed refrigerants. It can also be used to search for new alternative refrigerants that do not destruct the ozone layer.