Thermodynamic analysis of cascade refrigeration systems working with synthetic and natural refrigerants.

The use of natural fluids as refrigerants have attracted renewed interest during the last decade but they must observe precise conditions. They must be no less energy efficient than the fluids that they replace. They must be proven to be safe, both for the immediate neighbourhood and for the global environment. They must be simple and cost-effective to use, they must be readily available and ideally they must not require any significantly new or unfamiliar technology. This paper presents a thermodynamic analysis of different twostage cascade refrigeration systems using as refrigerant carbon dioxide (R744) in low-temperature circuit, and respectively ammonia (R717), propane (R290), butane (R600), R404A, R410A and R134a in hightemperature circuit. Two-stage cascade refrigeration systems are suitable for applications where the evaporating temperature ranges from -30 °C to -50 °C. The operating parameters considered in this study include condensing and evaporating temperatures in high-temperature circuit, temperature difference in the cascade heat exchanger, and evaporating and condensing temperatures in the low-temperature circuit. The results obtained show that a cascade refrigeration system using natural refrigerants is an interesting alternative to systems using synthetic refrigerants for energetic, security and environmental reasons.