Thermo- and fluid dynamics of lubricants with CO2 at transcritical conditions.

While fluorinated hydrocarbons such as HFCs in cooling circuits are in the subcritical state, carbon dioxide is both sub- and supercritical with respect to pressure and temperature. The viscosity decrease caused by the solubility of CO2 in lubricants under high pressure might cause wear in a compressor. For the proper design and operation of refrigeration circuits, knowledge of phase behaviour and viscosity behaviour of lubricants with CO2 is essential. Results of basic investigations on three different synthetic lubricants exhibiting different types of phase behaviour with CO2, are presented and discussed with respect to typical operating conditions.