Low molecular weight esters as hybrid fluids for R744 sublimation cooling circuits.

Up to now, for the temperature range between -50 °C and -90 °C mainly refrigerants like trifluoro methane (R23) or hydrocarbons (e.g. ethane or ethylene) are used. The phase-out of the fluorinated refrigerants with high GWP and the flammability of the hydrocarbons as refrigerants requires new technical solutions. In the presentation, considerations are given regarding the application of the carbon dioxide phase change from solid to gaseous state for low temperature cooling. Unfortunately, the triple point of CO2 does not allow a phase change between liquid and gas below -56 ° C. Thus, lower temperatures are only possible by means of sublimation. However, this requires new concepts for their technical implementation. CO2 gives us an alternative as a non-flammable, environmentally friendly, low temperature refrigerant for a temperature range down to -80 °C, if we succeed in using the sublimation of the CO2 for cooling applications. This requires the use of new compressor lubricants, which also serve as a heat transfer fluid during the sublimation of the CO2 in a refrigeration circuit. For this reason, we looked for suitable substances that could be used. We examined some suitable compounds with regard to their thermodynamic and tribological properties as well as material compatibility.