New phase separation phenomena in refrigeration plants working with CO2-based mixtures. Experimental approach

Use of CO2-based mixtures results in COP improvements for simple cycle layouts, however, mixtures working in the parallel compression cycle where fractionation process takes place have not been deeply explored yet. This work provides an experimental analysis of phase separator inner behavior using pure CO2 and three CO2-based mixtures with R600a and R290 as additives. With pure CO2 we identify and define a process of internal evaporation in the phase separator, which occurs with both pure CO2 and mixtures, leading to subcooling in the liquid phase and an increased vapor mass flow rate. Circulating compositions for the mixtures were calculated using a model which does not rely on REFPROP v.10.0 fractionation rules and significant deviations compared to it have been observed. These deviations are caused by three simultaneous phenomena in the phase separator: fractionation, internal evaporation, and partial density-layering. The latter two phenomena effects are contrary to that of fractionation, causing significant changes in the circulating compositions. With a self-developed model, internal evaporation mass flow rate, composition and capacity are then calculated. All results show consistency across all phase separator conditions and complete the explanation for the deviations between measured and REFPROP-predicted compositions. Consequently, the real behavior of the phase separator is described as well as its implications.