Experimental Comparison of the Performance of Refrigerants R134a and R32/R134a in Dry Expansion and Liquid Recirculation Refrigerating Systems

The ability of zeotropic mixtures with a temperature glide to operate in liquid recirculation systems is investigated in this paper, and the preliminary results of an experimental comparison between the performance of R134a and of zeotropic mixture R32/Rl34a, (25/75 by mass) are presented. The perfonnance of Rl34a operating in dry expansion mode are also presented as baseline data. INTRODUCTION Conventional refrigerating systems operate in the so called "dry expansion" configuration: the same refrigerant flow rate is encountered throughout the system and a superheated vapor exits the evaporator, under the control of the expansion valve. In liquid recirculation systems a liquid/vapor separator supplies saturated liquid to the evaporator and saturated vapor to the compressor; the low pressure side of the system and the high pressure side work with different refrigerant flow rates. The evaporator is overfed with respect to the compressor and the condenser: as a consequence the refrigerant flowing through the evaporator does not evaporate completely and, at the outlet of that heat exchanger, the vapor quality is always less than one. The ratio between the refrigerant flow rate in the low pressure side (evaporator) and the flow rate in the high pressure side (compressor and condenser) is called "recirculation ratio". When operated with pure refrigerants, liquid recirculation systems take advantage of a better utilization of the evaporator surfaces, so that the heat exchange coefficients are higher. Furthermore, the compressor is fed with saturated vapor, allowing for lower discharge temperatures, whilst the presence of the liquid/vapor separator prevents the compressor from receiving liquid in the vapor [1]. If the same systems operate with a zeotropic mixture as working fluid, the vapour and the liquid inside the liquid/vapor separator will have different compositions. T-herefore the mixture flowing in the high pressure side and the mixture flowing in the low pressure side have different compositions. In addition, the liquid at the inlet of the evaporator is richer of the less volatile components of the mixture, while the vapor at the inlet of the compressor is enriched with the more volatile components of the mixture. This change in composition has been claimed responsible for the reduction in any of the advantages of using a refrigerant with a temperature glide during the phase changes [2], [3], [4). Also there is an implication in the design of such systems in that there are unknown circulating compositions and therefore the properties of any such fluids are unknown. The purpose of this research is to study and compare the behavior of two different refrigerants, one characterized by a temperature glide during the phase changes and the other showing constant temperature (at constant pressure), when they operate in dry expansion and liquid recirculation systems. A customized test facility has been built to evaluate the relative performances of R134a and of the mixture R32/R134a (in the composition 25/75 by mass). The comparison is based on the. evaluation of four parameters: the compressor discharge temperature, the pressure ratio across the compressor, the coefficient of performance and the refrigerating capacity. Baseline testing results with Rl34a are presented in this paper, but only preliminary results with R32/134a mixture are reported. Experimental work is still in progress and involves gaschromatographic analyses of mixture composition within the liquid/vapor separator.