Comparative evaluation of the intermediate systems employed in two-stage refrigeration cycles driven by compound compressors

This work presents an experimental evaluation, from an energy analysis point of view, for two of the most used inter-stage systems in two-stage vapour compression cycles driven by compound compressors: the direct liquid injection and subcooler systems. The evaluation, considers the two-stage cycle with no intermediate systems as reference cycle and was performed on a plant, driven by a compound compressor using the refrigerant R404A for an evaporating temperature range of −36 to −20 °C and for a condensing temperature range of 30–47 °C. The analysis of the results shows that the inter-stage systems determine the performance of the energy parameters, which are discussed in the paper. The configuration yielding the best results was the two-stage compression cycle with subcooler, since the cooling capacity and COP values for this configuration were the highest. Conversely, the two-stage compression cycle working with direct liquid injection showed lower performance from an energy point of view, though this configuration allows a substantial reduction of the discharge temperature.