CASCADE REFRIGERATION MACHINES WITH R744 AS THE WORKING FLUID FOR THE HIGH-TEMPERATURE CASCADE

The cascade refrigeration machines are used for producing the cold at temperature level ?110…-50℃. The thermodynamic cycle of the cascade refrigeration machine consists of one-stage refrigeration cycles in series. In earlier time, R12 or R22 were used as the working fluid for the high-temperature cascade as well as R13 or R14 as the working fluid for the low-temperature cascade. In modern refrigeration engineering these working fluids are forbidden. This fact requires to select another working fluids taking into account energy saving and ecological issues. R744, R717 and R290 (different combinations between high-temperature and low-temperature cascades) are the working fluids that can be used for generating the cold at temperature level -30…-60℃. They have the following advantages: the ODP values are equal to zero as well as GWP values tend to zero. In the paper, the thermodynamic analysis of the cascade refrigeration machines with R744/R744, R744/R717, R744/R290 has been conducted with R744 transcritical cycle for the high-temperature cascade. The thermodynamic analysis is based on the method of the comparison of the thermodynamic cycles proposed by R. Plank. For each cascade machine the operation parameters have been optimized to solve the “energetic problem”. For the R744/R744 cascade refrigeration machine the “transport” and the “unification” problems are also solved. The parametric study of effect of the temperature within the condenser-evaporator on effectiveness of the thermodynamic cycles has been reported. The influence of the thermodynamic properties of the working fluids to the energetic and volumetric characteristics of the cascade refrigeration machines has also been evaluated.