A rigorous argument based on the characteristic of a monotonously increasing function is presented to establish the well-known fact of higher LMTD value of counter flow heat exchangers as compared to parallel flow ones.

Incropera. F. P., and Dewit, P. D., 1994, Introduction to Heat Transfer, 3rd ed., Wiley, New York.
Folland, G. B., 1999, Real Analysis: Modern Techniques and Their Applications (Pure and Applied Mathematics), second edition, Wiley Intersciences, New York.
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