Letters

(ProQuest Information and Learning: ... denotes formulae omitted.)

Calculating Pipeline Pressure Drop

To the Editor:

The article "Pipeline Pressure Drop: A New Design Correlation," written by Roger Gilmont in the June 2006 issue of CEP (pp. 34-41) and the response by Kevin Farrell

in the Oct. 2006 issue (p. 6) both tall short of the state-of-the-art in prediction of the pressure drop in turbulent flow through pipes. The article and letter are based primarily on the 1932 experimental data of Nikuradse, but those data have been superseded by improved measurements, most notably those of Zagarola (PhD thesis, Princeton Univ., 1996). The following expression of Churchill (Adv. Heat Trans., 34, pp. 255-357 (2001)) for the Fanning friction factor reproduces the new measurements almost exactly (within 0.5%) and incorporates the well-known correlating equation of Colebrook for the effect of commercial roughness:

...

The iterative solution of this expression for a specified value of Re converges rapidly, even with a handheld programmable calculator. The unfamiliar second and third terms on the right-hand side are a necessary consequence of the viscous boundary layer near the wall. They become significant in magnitude only near the lower limit of fully developed turbulent flow, namely Re = 4,000.

Stuart Churchill

Univ. of Pennsylvania

Clarification on Pharmaceutical Compound Solubilities

In the article "Predicting Solubilities of Pharmaceutical Compounds" (Aug. 2006, pp. 20-21), the fourth parameter in Eq. 3, d, was not defined. Due to page constraints, the equation that defined d was not included in article. It was, however, defined in the original paper:

...

where m is the number of components in the system. Note that A is also a function of the bulk properties:

...

where: d is a constant; d^sub solute^ and ... the solute and average dielectric constants. All of these parameters are at 25?C.

For more detailed information, please read the authors' original manuscript: Mirmehrabi, M., Rohani, S. and L. Perry, "Thermodynamic Modeling of Activity Coefficient and Prediction of Solubility: Part 1. Predictive Models," J. Pharm. Sci. 95 (4), pp. 790-797 (2006).

Readers who are interested in receiving the original manuscript may contact the authors and request copies. Mahmoud Mirmehrabi can be reached at (514) 748-4730 or mirmehma@wyeth.com. Please he sure to give your complete mailing address information (P.O. box numbers will not be accepted ). -Editors

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