Thermodynamic calculations for CO2 refrigeration processes with single-stage or double-stage compression in air conditioning plants

摘要

For the design of an (automotive) air-conditioning system which employs R744 (carbon dioxide, CO2) as a refrigerant, thermodynamic calculations are performed for vapour compression refrigeration processes with single-stage or double-stage compression and throttle-valve expansion. The process parameters are varied systematically for an evaporation temperature of 0 A degrees C and a CO2-cooler exit-temperature (lowest CO2 temperature in heat transfer to the ambient) between 15 A degrees C and 55 A degrees C. Their influence upon the coefficient of performance, the specific and the volumetric refrigeration capacities is described. For the double-stage compression cycle, the ratio of the high-pressure and low-pressure mass flows is determined, too. The double-stage compression with interstage cooling and internal heat exchanger in the low- or high-pressure cycle shows the potential for a substantial increase in coefficient of performance relative to the single-stage compression cycle. Thermodynamically optimized intermediate pressure, high pressure and temperature change in the internal heat exchangers are presented for CO2-cooler exit-temperatures of 25 and 40 A degrees C. The increase in energy efficiency is paid for by additional invest and the more complex operation of the CO2 refrigeration unit.