Analysis of Propane + CO2 mixture as a working fluid in a vapor compression refrigeration system.

This paper explores feasibility of C3H8+CO2 mixtures for refrigeration applications. CO2 in the mixture suppresses flammability, while the presence of propane helps in reducing the critical pressure of CO2 to facilitate operation in subcritical regime. Performance of C3H8+CO2 mixture is analyzed for various molar compositions of C3H8 permissible within the flammability suppression envelope to arrive at an optimum molar fraction of propane in the mixture. Analysis is performed using a MATLAB based program coupled with REFPROP database for obtaining the thermodynamic mixture properties at various state points. Simulations are performed for various system configurations viable for cooling applications with evaporating temperatures ranging between -7°C to 12°C. The analysis shows that 15% CO2 in the mixture yields the performance equivalent to pure CO2 with a 70% reduction in optimum discharge pressure. For the operating range considered, the current mixture composition offers the flexibility to operate entirely in the subcritical regime.