Assessment of CO2/R600a blends in parallel compression refrigeration systems: A focus on fractionation

Medium to large refrigeration applications generally employ CO2 as a working fluid since it is environmentally friendly and ensures high security. The addition of a small proportion of another component to CO2, known as doping, has attracted interest from the scientific community because CO2-based mixtures introduce some operating benefits, such as pressure reductions and COP increments. This work studies the mixture CO2/R600a (10% mass fraction of additive) in one of the most-used architectures for warm climates, the parallel compression cycle. Fractionation takes place in this configuration, a phenomenon that causes the appearance of three different mixtures circulating along the cycle. This aspect is addressed in the study by determining circulating currents and mass flow rates of each cycle line. A comparison between experimental results and results obtained with REFPROP is conducted. Fractionation rules are validated for an open system working with CO2/R600a.