Carbon dioxide as a possible low temperature refrigerant.

In this paper considerations will be given regarding the use of the carbon dioxide phase change from solid to gaseous state for low temperature cooling. Up to now, for the temperature range between -50 °C and -90 °C mainly refrigerants containing trifluoromethane (R-23) or hydrocarbons (e.g. propane or ethane) are used. So far, the triple point of carbon dioxide is limiting the application of the evaporation process used in conventional compression refrigeration. Below -56.6 °C carbon dioxide is only present either as a solid or as a gas without the liquid state. The most significant disadvantage of fluorinated low temperature refrigerants is their very high global warming potential. On the other hand, hydrocarbons - having very low global warming potentials - are flammable and thus have limited applications. A method will be described for the use of carbon dioxide in the form of dry ice for low temperature cooling as a climate-friendly alternative to HFC and flammable refrigerants. A TEWI analysis will show the energy advantage of the use of sublimation cooling in comparison to a cascade cooling system with the refrigerants R-404A and R-23.
Experimental results will be presented that demonstrate a solution of a sublimator with a solvent as heat exchanger fluid. This approach results in an improved cooling efficiency due to the higher heat transfer rate of sublimated dry ice in a liquid. Non-flammable per-fluorinated ketones and hydro-fluorinated ethers with low global warming potentials are shown to be suitable fluid classes for this task. Experimental results obtained with two different fluids will be given.