The free surface model was introduced to describe the shell side fluid flow in a transverse flow hollow fiber membrane contactor, and a new method was developed to calculate the shell side hydraulic diameter, the effective average velocity, and the Reynolds number. An empirical shell side mass transfer correlation was presented for commercial Liqui-Cel (R) Extra-Flow contactors on the basis of the experimental data reported by Sengupta et al. The data were correlated very well with maximum discrepancies of +/- 10% between the predicted and observed results. (c) 2005 Elsevier B.V. All rights reserved.