Berllcksichtigung veranderlicher Warmetibergangskoeffi-zienten und Warmekapazitaten by der Bemessung von Warmeaustauschern

Colburn model. If the nonlinearities are pronounced, the three point method of Roetzel as presented in this paper is not always a better choice compared to the Colburn method. The surface area predicted by the Roetzel method can be considerably higher or lower compared to the exact numerical results. In most shell-and-tube heat exchangers, the shell-side flow undergoes changes from crossflow to parallel-flow between and across baffle plates. Thus, the shell-side coefficient undergoes cyclic and not necessarily monotonic changes through the exchanger. In this regard, data set #5 was selected to demonstrate this influence on the surface area requirement. From the review of the results for a single data set #5 in Based on the findings of this work, it appears that none of the approximate methods considered here will accurately predict the exchanger surface area requirement when the variation of the heat transfer coefficient on one of the two-fluid sides is highly nonlinear and the corresponding thermal resistance is controlling. If the nature of variation in h is unknown, as in practical applications, the reliability of the approximate methods • is even more questionable. The best approach is to conduct the numerical integration to take into consideration the actual variation of the heat transfer coefficient. In many applications, the heat transfer coefficient on one fluid side may not be controlling (i.e., the heat transfer coefficients on both sides may be of the same order of magnitude). In this case, the conclusions derived from the example of this paper are not necessarily applicable. Hence, the best solution is to conduct exact numerical integration to take into account the variations in the heat transfer coefficient on one or both fluid sides. It should be emphasized that the most published correlations are not accurate to better than 10-15 percent, and also the variation in fluid properties and fouling factors may introduce additional uncertainties in h or U. Hence, even the "exact" numerical method may not have real validity. The approximate methods of overall heat transfer coefficient averaging may or may not have poor accuracy. In light of many commercial computer programs available for the design and analysis of heat exchangers that use some method of averaging U values, the results presented in this paper provide some guidelines on the averaging methods, particularly which ones should be avoided. However, unfortunately, none of the approximate methods came out superior to others in terms of accuracy and reliability. Conclusions The methods for incorporating the influence of nonuniformity in the overall heat transfer coefficient in conventional design procedures have been presented. Various definitions of mean overall heat transfer coefficients have been introduced (see Table 1) depending on the pertinent nonuniformity effect. The influences of the length effect (developing thermal boundary layer influence), the temperature effect (changes in thermophysical properties due to fluid temperature variations), and the combined effect have been included in the analysis by defining an area average mean overall heat transfer coefficient, a temperature average mean overall heat transfer coefficient and their combination. A step-by-step procedure is presented for evaluation of the mean overall heat transfer coefficient. Five different methods of averaging U were compared to determine how they rank in accuracy in predicting exchanger surface area requirement for an example where the heat transfer coefficient on one fluid side was controlling; hence, the variations in h corresponded to the variations in U. For a significant nonlinear variation in h, none of the five methods yielded accurate results. The only plausible method in such a case is the numerical approach. The overall heat transfer coefficients using various reference temperatures in most cases underestimated the heat transfer surface and should be avoided. Copyright © 1998 by ASM