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.

Experimental results will be given that show the capability for the use in CO₂-sublimation cooling systems.