Thermodynamic Performance Limit and Evaporator Design Considerations for Narm-Based Domestic Refrigerator-Freezer Systems

Non-azeotropic refrigerant mixtures (NARMs) are investigated for a twotemperature level heat exchange process found in a domestic refrigerator-freezer. Ideal (constant air temperature) heat exchange processes are assumed. The results allow the effects of intercooling between the evaporator refrigerant stream and the condenser outlet stream to be examined in a systematic manner. For the conditions studied, an idealized NARM system has a limiting coefficient of performance (COP) that is less than that of the best performing pure refrigerant component. However, for non-ideal heat exchange processes (gliding air temperature), the NARM-based system has a higher limiting COP than a system running on either pure NARM component. lntercooling significantly affects the COP of NARM·based systems: however, depending on the location of "pinch points' in the heat exchangers, only one intercooling heat exchanger may be needed to obtain a NARM's maximum refrigerator COP. Three pairs, R22/R142b, R22/R123 and R32/R142b, were studied, but only the results for R22/R123 will be presented because of its unique temperature glide curvature. Practical implementation of a Lorenz cycle constrains evaporator design. An evaporator module design is presented which meets the NARM system constraints. Preliminary NARM evaporator data are presented and compared to standard,: pure refrigerant evaporators. The air side efficiency criterion indicates the feasibility of the NARM design, and the direction of further research is suggested. INTRODUCTION The U.S. domestic refrigerator industry is currently interested in methods that improve refrigerator performance in order to meet recently announced u.s. government energy standards [1]. Simultaneously, the refrigerator industry must choose satisfactory replacements for R 12 (thermodynamic cycle fluid) and R 11 (blowing agent for urethane foam). One of the options proposed for improving thermodynamic cycle efficiency with non-ozone depleting compounds is the use of Non-Azeotropic Refrigerant Mixtures (NARMs). Figure 1 is a schematic of a refrigerator utilizing a NARM based on the L.orenz cycle [2]. The main features in the system are two evaporators and two refrigerant-torefrigerant heat exchangers (intercoolers). The heat exchange processes will be described in detail in a later section. An early study in this area by L.orenz and Meutzner [2] indicated significant performance gains in an experiment that utilized a NARM in a modified domestic refrigerator. Subsequent work by Stoecker and co-workers [3,4,5,6,7,8] examined NARMs through numerical simulation and experiment. While simulation results show improvements to cycle efficiency, the experiments were unable to quantitatively prove significant performance gains. Actual experiments are difficult because the compressor and other components have to be shifted to non-optimal conditions as mixture compositions are changed. The emphasis of this work is an idealized examination of NARMs in a refrigerator heat exchange configuration and develqpment of a preliminary design for a practical NARM evaporator. The heat transfer investigation is ideal in two ways. First, the air side of the evaporators is assumed to be at a constant temperature level. This is equivalent to assuming that the air flow rate through the evaporator is large (in terms of its energy capacitance) compared to the overall evaporator heat transfer coefficient. Second, it is assumed that all heat exchange processes continue to the limit allowed by the second law of thermodynamics (infinite heat exchange area). That is, heat exchange in a