Sizing multipass heat exchangers with non-uniform heat transfer coefficients



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In the thermal analysis of heat exchangers, a uniform overall heat transfer coefficient acting over the entire heat transfer surface is usually assumed. However, if even one of the working fluids demonstrates substantial changes in thermophysical properties (such as viscosity) with variation in fluid temperatures, the character of the flow may vary substantially as the fluid proceeds through the heat exchanger. The change in fluid properties along the heat exchanger handling petroleum fractions and other viscous fluids results in a variation in the character of heat transfer and in the overall coefficient U. Single pass heat exchangers have been successfully analyzed in the past by allowing variations in the overall coefficient with different approaches taken by various authors. Nelson tried to develop formulas for 1-2 heat exchangers where U depends linearly on the temperature difference between hot and cold fluid. Kahl analyzed 1-2 heat exchangers more effectively by assuming U varied with cold fluid temperature and developed graphs similar to those of Ten Broeck. But so far, no effort has been made to develop a technique which includes variable U in 2-4 exchangers. The LMTD process (also called the F[subscript T] process for multipass exchangers) based on the assumption of constant U is still used for sizing such exchangers. In this thesis an analysis is made and sets of graphs are presented which include the effects of a non-uniform overall heat transfer coefficient which can be used to size 2-4 exchangers. The results are used to determine the accuracy of a modified F[subscript T] process which takes the variation in U into account. In addition, the error associated with the classical F[subscript T] process (constant U) is also predicted. Kahl's investigation for 1-2 exchangers is verified and extended by the construction of correction factor graphs (also called F[subscript T] graphs) for 1-2 exchangers with variable U.