Performance Study of Cascade Refrigeration System using Natural Refrigerants (R290-R744)

In view of the concern over the use of synthetic refrigerants, CO2, a natural refrigerant with excellent thermo-physical properties, low toxicity, non-flammability and good availability, is a promising option. The objective of the present study is to investigate the potential of cascade systems using natural refrigerants. An experimental two stage Cascade system was developed, with Carbon-Dioxide as the refrigerant in Low Temperature Circuit (LTC) and Propane in the High Temperature Circuit (HTC). LTC uses a tube-fin evaporator, a hermetic reciprocating compressor and an Electronic Expansion Valve. The Electronic expansion valve maintained a constant superheat by controlling the flow in response to the pressure sensor at compressor inlet. The HTC has a thermostatic expansion valve, a reciprocating hermetic compressor and a fin-tube condenser. Experiments and Simulations were carried with two different cascade condensers, brazed Plate and Shell & tube type. The operation of the system was observed at different ambient temperatures and load conditions. Simulations were carried out using MATLAB as a mathematical modelling tool to predict system performance under varying conditions of load, ambient and cascade temperatures. The analysis showed that the cascade temperature has a dominating effect on the efficiency of the system. Simulation results showed that better performance can be obtained by using a plate heat exchanger than a shell and tube heat exchanger.