Development of novel wet sublimation cascade refrigeration system with binary mixtures of R744/R32 and R744/R290

Abstract

The use of carbon dioxide (R744) in refrigeration systems is usually limited by its triple point temperature of −56.5 °C; an objective of this study was to extend the range of this natural refrigerant to include low-temperature applications. A novel wet sublimation cascade refrigeration system was developed using low-GWP mixtures of R744 with hydrocarbons (HCs) and hydrofluorocarbons (HFCs) as low-temperature refrigerants. With propane (HC-290) and difluoromethane (HFC-32) serving as solvents for solid carbon dioxide, temperatures as low as −72 °C were obtained in a closed cycle system with mixtures containing 67% CO2 by mass. Visualisation experiments were performed to illustrate the crystallisation of a supersaturated carbon dioxide solution. The excess solute was visible as clouding or crystals, depending on the saturation level of the solution. High heat transfer rates, up to 3465 W m–2K−1, were obtained in the sublimator/evaporator section, resulting in low wall superheat values, indicating potential industrial applicability of the carbon dioxide-based slurry-like fluid. The pressure–mole fraction and temperature–mole fraction diagrams were constructed to study the system parameters working with binary mixtures. Moreover, vapour pressures of the binary mixtures of R744/R290 and R744/R32 at the temperature of –72.5 °C were experimentally obtained.