Blends of carbon dioxide and HFCs as working fluids for the low-temperature circuit in cascade refrigerating systems

This paper describes an analysis on the performances of a cascade refrigeration cycle operated with blends of carbon dioxide (CO₂, or R744) and hydrofluorocarbons (HFC) as the low-temperature working fluid. The aim of this work was to study the possibility of using carbon dioxide mixtures in those applications where temperatures below CO₂ triple point (216.58 K) are needed. The analysis was carried out by developing a software based on the Carnahan–Starling–De Santis (CSD) equation of state (EoS) using binary interaction parameters derived from our experimental data. The properties of the investigated blends (R744/R125, R744/R41, R744/R32, R744/R23) were used to simulate the behavior of a cascade cycle using ammonia (R717) as the high-temperature-circuit working fluid and operating at evaporating temperatures down to −70 °C. The use of a suction–liquid heat exchanger on the low-temperature side of the circuit was also investigated. Results show that the R744 blends are an attractive option for the low-temperature circuit of cascade systems operating at temperatures approaching 200 K.