Theoretical analysis of compressing CO2 in scroll compressors

A preliminary theoretical analysis on the feasibility of compressing the refrigerant carbon dioxide (R-744) in a scroll compressor has been carried out. The attention has been on an open scroll compressor type applied in an automotive A/C system. An operating condition for the R-744 compressor was first established based on an R-134a baseline operating condition. A specific R-134a scroll compressor (80cm(3)) applied in medium-sized car was used as a baseline compressor. The design target for the CO2 scroll profiles was at least equal or less bearing load, weight and physical overall dimensions at a comparable temperature operating condition. This was also fulfilled. Parameters of main interest were gas forces, rotating masses, geometrical parameters and gas leakage. Gas forces acting on the scroll profiles, on the thrust- and the tap bearing are larger in the R-744 compressor but should not cause any problem if self-adjusting back-pressure mechanism is applied. The self-adjusting back-pressure keeps the thrust bearing load on an acceptable level. Tip- and flank gas leakage was calculated (without influence of lubricant) for both compressors. The gas leakage is larger for the R-744 for equal clearance gaps. Equal isentropic- and volumetric efficiency could be achieved if the clearance gap i.e the flank- and the tip was reduced by 56 and 67% respectively compared to the gap in the R-134a compressor. Small clearance gap might be achieved by using axial- and radial compliance. Axial compliance together with a self-adjusting back-pressure might keep the tip leakage at acceptable level. The flank leakage might be reduced with radial compliance if the net radial force (also called sealing force) is larger for the R-744 compressor.