Composition Shift of the Mixtures R-125/236fa (49.4/50.6) and R509A

A shift in the composition of a refrigerant mixture in a refrigerating circuit is a phenomenon related to the thermophysical properties of the mixture and lubricating oil, the refrigerant/lubricant mass ratio in the circuit, and the features of the plant concerned. This phenomenon may also be accentuated by any leakage or by unsuitable charging procedures. This paper gives a detailed description of a method for measuring the circulating composition of a mixture. The method calls for the use of a gas chromatograph cmmected directly to the circuit so as to enable in-line analyses. Measurements were taken using two mixtures with different behavior, i.e. a zeotropic mixture with a wide glide, R-125/236fa (49.4/50.6), and an azeotropic mixture, R-509A (R-22/218 (44/56]). The mixtures were charged in a circuit with a dry evaporator and no receivers. The compressor was lubricated with POE oil. The composition of the mixture in the liquid phase was measured at the condenser outlet and the composition in the vapor phase was measured at the compressor discharge. The concentration of the low boiling component shifted at most from 49.4% to 54.5% in the case of the zeotropic mixture and from 44.0% to 37.8% in the case of the azeotropic mixture.