A thermodynamic efficiency based on the second law of thermodynamics is defined for heat exchange devices. The efficiency can be simply written in terms of the mean absolute temperatures of the two fluids exchanging heat, and the appropriate environment temperature. It is also shown that for a given ratio of hot to cold inlet temperatures, the efficiency and effectiveness for particular heat exchange configurations are related. This efficiency is compared to second-law efficiencies proposed by other authors, and is shown to be superior in its ability to predict the effect of heat exchanger parameter changes upon the efficiency of energy use. The concept is applied to typical heat exchange cases to demonstrate its usefulness and sensitivity.

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