A Next Generation Refrigerant Properties Database

A completely revised version of the REFPROP refrigerant properties database is described. This program is based on the most accurate pure fluid and mixture models currently available. It implements three models for the thermodynamic properties of pure fluids: the modified Benedict-Webb-Rubin (MBWR) and Helmholtz equations of state, and an extended corresponding states (ECS) model. Mixture calculations employ a new model which applies mixing rules to the pure-fluid Helmholtz energies. Viscosity and thermal conductivity are modeled with either fluid-specific correlations or a new variation on the ECS method. These models are implemented in a suite of subroutines written in standard FORTRAN. A separate graphical user interface provides a convenient means of accessing the models. It will generate tables and/or plots for any user-specified mixture. Numerous options to customize the output are available as well as copy and paste capabilities to and from other applications. INTRODUCTION Knowledge of the thermophysical properties is essential for the evaluation of alternative refrigerants and the design of equipment using them. The REFPROP computer database from the National Institute of Standards and Technology (NIST) (Huber er al. 1995) has been one of the more widely used tools designed to provide these data. In the initial versions of REFPROP (Gallagher et al. 1993), the intent was to provide data on a wide variety of fluids to allow screening studies of possible replacements for the CFC or HCFC refrigerants. For many of these fluids, only sparse data were available, and, consequently. the database relied primarily on a simple model with few adjustable parameters-the Camahan-Starling-DeSantis (CSD) equation of state. As the alternative refrigerants move from the laboratory to use in commercial equipment, highly accurate properties are required for a more limited set of fluids. We describe a completely revised REFPROP program (designated as Version 6) based on the most accurate pure fluid and mixture models currently available. THERMODYNAMIC MODELS REFPROP 6 calculates the thermodynamic properties using comprehensive equations of state. This approach ensures thermodynamic consistency and allows calculations at all conditions. Other approaches, such as the combination of a vaporphase model with vapor pressure and liquid density equations may not be applicable in the compressed liquid and supercritica! regions and do not always give reliable results for derived properties such as heat capacity and speed of sound. Pure Fluid Models Three models are used for the thermodynamic properties of pure components, depending on the availability of data. The first is the modified Benedict-Webb-Rubin (MBWR) equation of state. This model was first proposed by Jacobsen and Stewart (1973) and has been applied to a wide variety of fluids, including hydrocarbons, cryogenic fluids, and refrigerants. It is capable of accurately representing the properties of a fluid over wide ranges of temperature, pressure, and density. The MBWR equation is the basis for the current international standard for the properties of R123 (Younglove and McLinden, 1994). The MBWR equation expresses pressure as an explicit function of temperature and molar density and is of the form, 9 15 P= LYnPn +exp[(pfpcrirt].Lanp2n-!7 , n=! n=!O (1) where the ai are simple functions of temperature resulting in a total of 32 adjustable parameters. For a complete description of the energy quantities (enthalpy, entropy, etc.), the MBWR equation is combined with an expression for the molar heat capacity of the ideal-gas state, that is, vapor in the limit of zero pressure. A form combining polynomial and theoretical terms is used: All of the thermodynamic properties can be computed from Eqs. (1-3), as detailed by Younglove and McLinden (1994). !Contribution of the National Institute of Standards and Technology, not subject to copyright in the United States. 2Pennanent address: Solar Energy Laboratory, University of Wisconsin, Madison, Wisconsin 53706 USA