A symmetry property which relates the thermal behavior of a heat exchange process to that of the reverse process, in which the directions of flow of both fluids are reversed, is demonstrated for the ideal case of no change of phase, no heat losses, and temperature-independent heat transfer coefficients and heat capacities. The symmetry is formulated in terms of a relation between the matrices that describe the direct and reverse process, which can be used to obtain one of them if the other is known. The formalism used does not require the inlet and outlet streams to be completely mixed, but when this is the case, both matrices coincide, and therefore, the effectiveness is the same for both processes. The consequences of the symmetry are ilustrated by the discussion of a three-row, two-pass air cooler, and the TEMA G split flow shell-and-tube heat exchanger.

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