Thermodynamic Calculations for Mixtures of Environmentally Safe Refrigerants Using the Lee-Kesler-Plocker Equations of State

The imminent phaseout of R-11, R-12, and R·llS because of their high ozone depletion potentials has prompted many independent investigations of alternative or subst:i t:uee refrigeranes, ei eher as pure compounds or as mixtures of t:wo or more environmentally acceptable fluids. There are several motivations for investigating mixtures. There are very few pure compounds which can be used for refrigerants that are non-eoxic:, non-flammable, and also have very low ozone depleeing pot:eneials (ODP) and greenhouse warming potentials (GWP). Mixeures composed of three or more component:s offer a diseinct: advaneage in ehat: careful selection of the components could permit a flammable refrigerant to be used in a blend that remains non-flammable under all conceivable conditions. Also, component: concentrations in ternary or higher order mixtures can be adjusted to linearize ehe temperature-enthalpy profile of a mixture making it more suitable for applications where secondary fluids are heaeed or cooled through large temperacure glides (i.e., refrigeration and air conditioning). · The computer model LKPSIM was constructed from subroutines written by Ulf PlOcker and incorporated into a Lorenz cycle refrigerator/freezer simulation model for binary refrigerant mixtures developed at the University of Hannover. Modifications were made to the Hannover routines eo compute ehermodynamic properties for mixtures of three or more components, the lise of available refrigerants was expanded from 11 co 17 possible choices, and a new correlation was developed for compucing che inceract:ion coefficients for mixtures of halocarbon refrigerants. Tables of refrigerant molecular weights, critical temperatures and pressures, normal boiling points, acem:ric factors, and coefficiem:s for fits co the ideal gas heae capacity for each refrigerant are buile into a menu-driven data input: routine for ease of use. Results of calculaeions of saturated and superheated vapor propereies from the LKPSIM program were compared with experimental data and with computations using the Garnahan-Starling-DeSantis (GSD) equation of state (EOS) for pure refrigeranes and for binary and ternary mixtures of refrigerants. There is excellent agreement between t:he calculations and the published data for pure refrigerancs that are well characcerized (e.g., R-12, R-22) with greater discrepancies between LKPSIM and data from other sources for newer compounds (e.g., R-143a, R-124). Calculacions were also performed eo validate ehe program for mixtures. LKPSIM calculations agreed more closely wieh observed saturaeion properties for mixtures of R-143a and R-124 over a range of operacing conditions and mixture compositions t:han did those of the CSD EOS. It: also gave very close agreement wich data from a manufacturer of a ternary blend of refrigerants. · The CYCU:-7 program from the National Inst:it:ute of Standards and Technology (NIST) was adapced to use the Ll<P subroutines with mixtures of ehree or more components (McLinden). CYCLE· 7 is a thermodynamic model for a Rankine cycle with couneerflow heat: exchangers using specified pressure drops and log mean at's for ehe condenser and evaporaeor. The modified program (CYCLE-10) was tested against: known sets of daca for pure refrigerants and blends of two and three components and will be used in screening eest:s of blends of fluids with low OOPs and GWPs for applications in heat: pumps and refrigerating equipment.