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Can the Mixed Refrigerant Heat Transfer Challenge Be Overcome?

DOI: 10.1615/ICHMT.1986.IntSympHMTinRefCryo.80
pages 105-116

Geoffrey G. Haselden
The University of Leeds, Leeds LS2 9JT, UK


In discussing mixed refrigerants in the context of either refrigeration or heat pumps I shall be concerned exclusively with mixtures which boil or condense over a temperature range. I suggest that it is better not to use the description 'non-azeotropic' mixtures because azeotropic mixtures are comparatively rare, and it is better to assume that mixtures behave normally unless otherwise stated.
Although the idea of using mixtures to achieve a close thermodynamic temperature match in continuous-flow cooling or heating duties has been known for a long timed,2), its only successful commercial application so far has been in the liquefaction of natural gas (3). Here the objective was reduction of capital cost due to fewer compressors and heat exchangers rather than a large power saving. Even so it was necessary to use multicomponent mixtures of very carefully controlled composition in cycles with several stages of expansion, and temperature pinches as close as 3°C at critical points in the exchangers. The resulting heat exchange units, each enclosing as many as 2000 miles of aluminium tubing, posed manufacturing problems of extreme severity. Perhaps most challenging was the management decision to risk building the first plant, because pilot scale demonstrations could not prove the designs.
In this paper I shall concentrate on applications of mixed refrigerants to refrigeration or heat pumping at power levels of up to a few hundred kilowatts and duties where the cooling or heating range spans between about 20-40°C. To exploit the advantages of mixtures without using multistage compression one wants a similar temperature span in both the condenser and the evaporator. Since one of these will normally involve heat exchange with the heat sink a 20° span is more likely than a 40° one.

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