Initial Parametric Results Using Cyclez- An LMTD-Specified, Lorenz-Meutzner Cycle Refrigerator-Freezer Model

A computer model representing a two-evaporator, two-intercooler refrigerator-freezer opera ting at steady-state with nonazeotropic refrigerant mixtures (CYCLEZ) has been developed at Oak Ridge National Laboratory (ORNL). This model is being used to assess the effects of system desig n and operating parameters on the cycle performance of a refrigerator-freezer designed around the L orenzMeutzner (L-M) circuil Separate evaporators for the freezer and fresh-food compartments are mo deled, as well as two intercoolers that subcool liquid refrigerant from the condenser by heat transfer wi th lowpressure refrigerant. The CYCLEZ refrigerator/freezer model is derived from the CYCLE7 heat -pump model developed originally by the National Institute of Standards and Technology (NIST). C YCLEZ currently uses the Camahan-Starling-DeSantis (CSD) equation-of-state to compute refr igerant thermodynamic properties, so that new refrigerants can easily be added. Condenser and evaporator heat-exchanger performance are defined by user-specified overall LMTDs which allow equivalent heat-exchanger sizing per unit refrigeration load to be maintained for different refrigerant mixtures. A more consistent formulation of overall heat-exchanger LMTD is applied across the condenser superheated and two-phase regions as well as over the two evaporators. Sour ce and sink conditions are specified in terms of inlet and outlet temperatures of the external fluid st reams. Intercooler high-side (subcooling) t:. Ts and relative fresh-food-to-freezer load ratio are also userspecified. These features make this model well suited for evaluating the optimal thermodynami c cycle requirements of the five heat exchangers used in the L·M refrigerator/freezer circuit. Parametric investigations involved nineteen ozone-safe refrigerant combinations which are under consideration for this appli~tion. Effects of the distribution of heat-exchanger area. exte nt and distribution of intercooler subcooling/superheat, refrigerant mixture composition, and r elative refrigerator/freezer loading were investigated. The modal indicates that improvements of 1 0 t o 20% in COP (coefficient of performance) are possible using mixed refrigerants in this cycle configu ration compared with the standard refrigerator/freezer circuit using R12. Most of this improvement Is due to closer matching between the air and refrigerant temperature profiles across the evaporators. Ho wever. intercooler subcoollng also results in a decreased pressure ratio across the compressor. INTRODUCTION Global environmental concerns have served to place restrictions on the production and sale of ch lorinecontaining, tully halogenated compounds (CFCs) [1 ,2]. Commercial production of refrigerant 12 (R12), which is extensively used in the refrigerating circuit of household refrigerator-freezers (RFs ), and refrigerant 11 (R11), used as a blowing agent tor the insulating foam, is likely to be phased out by the year 2000. Replacements for the refrigerants and blowing agents used in this application must be foun d, and energy efficiency has to be a primary consideration. Using nonazeotropic refrigerant mixtures (N ARMs) as refrigeration fluids can improve the efficiency of vapor-compression refrigeration equipment at the expense of circuit and hardware redesign [3]. The application of NARMs in domestic refrigerator-freezers has been suggested and experim entally investigated with varied results [4,5]. An RF circuit described and tested initially by A. Lorenz and K. Meutzner [6] in 1975 and by H. Kruse [7] in 1981 has verified the promise shown by NAAM refrigerants. The CYCLEZ AF program was developed at ORNL for modeling the Lorenz-Me utzner circuit with newer, environmentally acceptable NARM refrigerants. Background . . . . A fundamental paper on methods for the analytical companson of pure and m1xed refngerants 1n vaporcompression cycles was published in 1987 by Mc~inden and Radermacher [8]. They concluded that meaningful comparisons between pure and m1xed refr1gerants should Include the appli catiOn