Modelled performance of non-chlorinated substitutes for CFC11 and CFC12 in centrifugal chillers

Abstract

A number of partially fluorinated alkanes and ethers were identified from earlier technical publications and from a joint Electric Power Research Institute/Environmental Protection Agency (EPRI/EPA) project as potential alternatives for chloroflurocarbon (CFC) and hydrogen-containing CFC refrigerants. These larger molecules, by virtue of their more elaborate structure, have larger vapour phase heat capacities ( C ps) and larger molecular weights than currently used refrigerants, which result in decreased volumetric capacities and greater flash gas losses in simple cycle applications. The compounds were fitted to the Lee-Kessler-Plöcker and Carnahan-Starling-DeSantis equations of state, and refrigerant property routines based on these equations were used to simulate their performance in a single-stage centrifugal chiller as pure refrigerants and nearly azeotropic refrigerant mixtures. Consideration was given to the effects of acoustic velocity in the refrigerant, rotational mach numbers, the application of superheat prior to the compressor inlet to avoid ‘wet isentropic compression’, and liquid subcooling before isenthalpic expansion. Results indicate that several chlorine-free compounds give modelled chiller performance comparable to R11 and R123 and better than R12 and R134a. Blends of these refrigerants may be required to mitigate the inflammability of alternatives that show the best performance. Modifications to the basic chiller cycle — such as liquid subcooling and suction gas superheat — may offer unique advantages for more complicated, larger refrigerant molecules. An algorithm based on molecular bond energies and vapour phase C ps was used to estimate the flammability of these alternatives and of blends made from them. Depending on desirable and permissible deviations from the currently used chiller operating conditions and on the acceptability of flammable refrigerant combinations, ideal coefficients of performance comparable to R11 and 5–10% better than R12 are indicated.