The performances of a refrigeration unit relying on compressors working in parallel have been investigated considering the influence of the compressor volumetric efficiency and isentropic efficiency on the compression ratio. Moreover, the following influential factors have been taken into account: evaporation temperature, condensation temperature and compressor suction-exhaust pressure ratio for different opening conditions of the compressor. The following quantities have been selected as the unit performance measurement indicators: refrigeration capacity, energy efficiency ratio (COP), compressor power consumption, and refrigerant flow rate. The experimental results indicate that the system refrigeration capacity and COP decrease with a decrease in evaporation temperature, increase of condensation temperature, and increase in pressure ratio. The refrigerant flow rate increases with the increase in evaporation temperature, decrease in condensing temperature and increase in pressure ratio. The compressor power consumption increases with the increase in condensing temperature and increase in pressure ratio, but is not significantly affected by the evaporation temperature.

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The thermal transmission coefficient for a micro-ribbed tube has been determined using theoretical relationships and the outcomes of such calculations have been compared with experiments conducted using a R1234yf refrigerant undergoing condensation. In particular four theoretical single-phase flow and three multi-phase flow models have been used in this regard. The experimental results show that: the Oliver et al. criterion equation overestimates the experimental results as its accuracy is significantly affected by the specific conditions realized inside micro-fin tubes; the Miyara et al. criterion equation prediction error is less than 15%; the Cavallini et al. approach gives the highest prediction accuracy; the Goto et al. model overestimates the test data. Such results are critically discussed and some indications for the improvement of such models are provided.

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