We present the experimental evaluation of a R134a/CO2 cascade refrigeration plant designed for low evaporation temperature in commercial refrigeration applications. The test bench incorporates two singlestage vapour compression cycles driven by semi hermetic compressors coupled thermally through two brazed plate cascade heat exchangers working in parallel and controlled by electronic expansion valves.The experimental evaluation (45 steady-states) covers evaporating temperatures from -40 to -30ºC and condensing from 30 to 50ºC. In each steady-state, we conducted a sweep of the condensing temperature of the low temperature cycle with speed variation of the high temperature compressor. Here, the energy performance of the plant is analysed, focusing on the compressors' performance, temperature difference in the cascade heat exchanger, cooling capacity, COP and compressors discharge temperatures. KEYWORDS

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We present the experimental evaluation of an internal heat exchanger or suction-line/liquid-line heat exchanger in a CO2 subcritical refrigeration plant with gas-cooler. The plant, driven by a 1.5kW CO2 semi hermetic compressor, uses brazed plate heat exchangers as condenser, evaporator and internal heat exchanger, an air finned tube gas-cooler and an electronic expansion valves. The evaluation (77 steadystates) covers evaporating temperatures from -40 to -25 ºC and condensing temperatures from -15 to 0 ºC, always at the nominal speed of the compressor. Here, the effect of the internal heat exchanger on the main energy parameters is analysed, i.e., cooling capacity, COP and heat rejection at gas-cooler and condenser. Also, the effect of the internal heat exchanger in a cascade cycle is analysed theoretically. It has been concluded that the internal heat exchanger does not improve the performance of the subcritical cycle, but it could improve the energy performance if it is used inside a cascade refrigeration system. KEYWORDS

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This work analyses and quantifies the effects caused by the use of an internal heat exchanger (IHX) at the CO2 subcritical cycle in an HFC134a/CO2 cascade refrigeration plant that incorporates a gas-cooler at the exit of the low temperature compressor. Previous theoretical and experimental studies showed that the IHX reduces the refrigeration capacity and COP of the subcritical cycle, however, it has been seen that it also lowers the heat to be rejected at the condenser. This reduction, when the cycle is a part of a cascade system, allows reducing the heat load of the high temperature cycle, modifying the working conditions of the cascade plant. The modifications result in an increment of the overall coefficient of performance of the cascade system. The analysis here presented is based on the evaluation of an experimental HFC134a/CO2 refrigeration plant, which has been analysed with and without internal heat exchanger in an evaporating temperature range from -40 to -30 ºC and in a condensing one from 30 to 50 ºC. The plant incorporates a gas-cooler at the exit of the CO2 compressor. The experimental results confirm that the IHX slightly reduces the cooling capacity but it can increment the overall COP up to 3.7 %.

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