GAS-SOLIDS MIXING AND HEAT TRANSFER STUDIES IN INCIPIENTLY FLUIDIZED BEDS OF NONUNIFORM CROSS-SECTIONAL AREA SIR: I n Figure 10 on page 238 of the April 1969 issue of INDUSTRIAL A N D ENGINEERING CHEMISTRY PROCESS DESIGNAND DEVELOPMENT, Miller and Edwards show their experimental heat transfer coefficients compared with, among others, values predicted from the book by Zenz and Othmer, published in 1960. I don’t know what curve these authors used, but if they used the recommended Figure 13.14 on page 441 of my book, I fail to see how they could predict the low values shown in their Figure 10. For example, a t a modified R e of 1.0 the correlation in my book would give N u = 0.4 and based on a thermal conductivity of about 0.016 B.t.u./hr. x sq. ft. x (OF./ ft.) this would give for their smallest particle size an h equal to about 7 B.t.u./hr. x sq. ft. x OF. Similarly, their largest particle a t Re = 100 would give a calculated h of about 13. These values are different from what Miller and Edwards attribute to Zenz and Othmer. If they used the fixed-bed curves shown as Figure 13.9 on page 432 of my book, they would also have come up with coefficients considerably higher than they attribute to me. Unless I have missed something here, it might be worth considering a correction of their Figure 10, for this can divert readers to wrong conclusions. F . A . Zenz Frederick A . Zenz, Inc. Roslyn, L. I . , N . Y . 11576
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SIR: The information in our Figure 10 ascribed to Zenz and Othmer was obtained by the use of their Figure 13.9 on page 432, and involved some extrapolation of the solid line in that figure labeled “Correlation of Leva’s Over-all Coefficients.” I recognize that the particle diameters involved in Leva’s correlation are, even in the case of his smallest size, almost three times as large as our largest size. The use of the dashed line in Figure 13.9, labeled “Correlation of Wall Coefficients Data of Yagi and Wahoo, Plantz and Johnstone, Felix” would, indeed, give higher coefficients. If a line were plotted on our Figure 10 using this dashed line as a basis, I believe the line so plotted would be close, but slightly above, our line on Figure 10 labeled “Colburn (1931).” We used the correlations mentioned in order to demonstrate the range of values obtained by other investigators and I believe no adjustment of that line in our Figure 10 is called for. I concur fully with Zenz in the values he obtains using the dashed line in his Figure 13.9, which appears to be approximately the same as the solid line in his Figure 13.14.
R . M . Edwards University of Arizona Tuscon, A r k 85721
