Cascade refrigeration, R744, R134a, R404a
This study evaluates the performance of a cascade system in subcritical operation using the pair R744 / R134, as an option to conventional systems in supermarkets, which usually uses R404A, or R22. The experimental apparatus consists of a variable speed reciprocating compressor for R744 and an electronic expansion valve that promotes direct evaporation of the CO2 inside a cold room (2.3m x 2.6m x 2.5m) to maintain the internal air temperature stable. The high-temperature cycle consists of a reciprocating compressor for R134a, a thermostatic expansion valve, and an air-cooled condenser. A plate heat exchanger, which is at the same time, the condenser for the R744 and evaporator to R134a completes the setup. Two parameters were manipulated: The superheating degree of the R744, 5-20 K, and the R744 compressor operation frequency, 40-65 Hz. In order to contribute to the improvement of the cooling processes, mainly about the sustainability and energy efficiency, a drop-in has been made at the high-temperature cycle, whose R134a load has been replaced by R404A. The alternative cooling system (R744 / R404A) was subjected to two stages of tests which allowed the energy comparison between the two refrigerant sets. Through the obtained results, it was estimated maximum COP equivalent to 1.36 and a minimum value of 1.06 for the R744 / R134a pair, demonstrating the applicability of this cascade system of variable thermal load conditions, the values â€‹â€‹of capacity cooling, settled between 4.09 and 5.13 kW. The minimum value of the air temperature within the cold room was - 28 Â°C and -5 Â°C the maximum. Finally, it was found that the results obtained for R744 / R404A pair attended the air temperature condition inside the cold room with similar COP values. However, R744 / R404A pair operated at lower refrigeration capacities, these settled between 2.16 and 3.00 kW. This reduction is due to the large difference between the volumetric cooling capacity values of these HFCs in question, resulting in adaptation problems of the R404A to the compressor and the expansion mechanism.