Effects caused by the internal heat exchanger at the low temperature cycle in a cascade refrigeration plant

Abstract

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 %.