Hydrocarbons as Substitutes for Halogenated Refrigerants in Refrigerating Systems

The paper deals with the use of R-290/600a(50/50) and R-600a, that are now considered as alternative to R-134a to substitute R-12 in domestic refrigeration. A theoretical analysis of the mixture performance shows that both hydrocarbon refrigerants may be used. Tests of certain hermetic compressors confrrm that the mixture can be used both for old and new equipment, while the isobutane must be used for new equipment only . INTRODUCTION The need to substitute traditional CFCs with environmentally safe refrigerants is opening the refrigeration market to hydrocarbons, though strong and justified doubts still remain about the risks connected with their flammability. Hydrocarbons and their mixtures may be considered a good natural alternative to R-12 in all technical applications involving hermetic circuits with a low refrigerant charge. Their use as a substitute for R-12 is thus particularly promising in small domestic and commercial refrigerators, where it has been demonstrated that replacing R-12 with hydrocarbons raises no particular problems of material compatibility and solubility with mineral oils. At present, the opportunities for using hydrocarbons mainly concern the R-290/600a(50/50) mixture and R-600a. In both cases, certain changes have to be made to the refrigerating systems designed for use with R-12: the mixture calls for a proper design of the heat exchangers, whereas the isobutane, other conditions being equal, requires the use of compressors with a larger displacement. There are currently some doubts among those proposing the use of hydrocarbons in domestic refrigeration as to the choice between the R-290/600a(50/50) mixture and R-600a. This paper attempts to throw light on these doubts by testing the performance of small-displacement hermetic compressors operating with the two alternative refrigerants. Testing was also done using R-134a which still remains a feasible solution for the replacement of CFCs in domestic refrigeration. This study does not deal with the problem of flammability, which would have to be based on a statistical evaluation of the risks involved which goes beyond the scope of our present investigations. THEORETICAL PERFORMANCE OF THE R-290/600a MIXTURE To theoretically compare the performance of a refrigerating zeotropic mixture and pure refrigerant calls for the definition of objective criteria that are still not well-established. Though the problem has been amply discussed, there is no universally accepted solution. A simplified method has therefore been adopted, which consists in comparing ideal cycles with pure or mixed refrigerants operating between the same evaporation and condensation temperatures. As far as zeotropic mixtures are concerned, the condensation temperature is assumed to be the arithmetic mean of boiling and dew point temperatures in the condensation process (t4-t5 in Fig. 1), while the evaporation temperature is assumed to be the arithmetic mean of the dew point temperature and evaporator inlet temperature at the evaporating pressure (t 1 -~ in Fig. 1). 231 The performance of the R-290/600a mixture, as a function of the R-290 mass fraction, has been calculated Ill wit h reference to ideal cycles with a condensation temperature of sscc and two evaporating temperatures: -30°C and -23.3°C. w a: :J ~ a: w a. ::ii w 13