Experimental Study and Modelling of a Two-Stage Compression R744 Refrigeration System with Vapour Injection and Inter-Cooling

Current trends in CO2 supermarket refrigeration have leaned towards systems including parallel compression and ejectors. However, the possibilities that 2-stage compression system architectures can offer remain out of the main spotlight, thus, system optimization and comparison with the other alternatives are less studied. In this work, a Condensing Unit for CO2 supermarket refrigeration is tested in a climatic chamber. The system includes a 2-stage rolling piston compressor with vapor injection and inter-cooling. The condensing unit is tested in several operating conditions to characterize the system COP, with a special focus on also assessing the compressor performance. On one side, a system-cycle model is applied and calibrated to theoretically optimize the system performance and to evaluate the impact of the intercooler. On the other side, the experimental results allow for calibrating a 2-stage rolling piston compressor semi-empirical model. This model is for the first time validated not only for main mass flow rate and power consumption prediction but also for discharge temperature and vapor injection mass flow rate calculation. Results show that the semi-empirical model can predict these four key compressor variables satisfactorily. The usefulness of the model is thus highlighted. On the other side, the cycle analysis shows that the intercooler has the double positive impact of enhancing compressor performance and increasing the injection mass flow by rising the density of the fluid before the 2nd stage of compression.