Variable Heat-Transfer Rate Correction in Multipass Exchangers, Shell-Side Film Controlling

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Abstract
The change in temperature of the fluid streams within a heat exchanger, through the consequent alteration of their physical properties, may cause the over-all heat-transfer coefficient to vary between rather wide limits; this represents the chief source of error in mean temperature difference formulas, most of which are derived on the assumption of constant over-all coefficient. Colburn (3) based a simple method of correcting for this effect in counter- and parallel-flow exchangers on the assumption of a linear variation of rate with fluid temperature. On this same basis, equations are developed in the present paper for multipass exchangers in which the shell-fluid film controls. It is shown that the results are practically independent of the number of tube passes, and the numerical data given are therefore those for the limiting case of crossflow with both fluids mixed, since this proves to be more amenable to direct mathematical treatment. These data are presented graphically and in tabular form as correction factors by which the arithmetic-average over-all heat-transfer coefficient and logarithmic mean temperature difference may be multiplied to obtain the true mean heat-transfer coefficient - temperature difference product; an example of their use is given.

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