Effective wall shear rates were investigated experimentally in an external loop circulating bubble column made from transparent acrylic resin. The riser (D-R) and downcomer (D-D) diameters were 0.19 m and 0.14 m, respectively. The column working volume (V-R) was 170 dm(3), with a scale-up factor (A(D)/A(R)) of 0.54, and a dispersion height (H-D) of 2.25m. Polymer solutions of xanthan gum and carboxymethyl cellulose were used to simulate non-Newtonian behavior of biological systems. Effective wall shear rates for the non-Newtonian solutions were found by analogy with Newtonian glycerol solutions, employing downcomer liquid velocity as the measurable and comparable parameter. The experimental shear rate results were found to fit between those of the literature data. A new single correlation taking into account all the relevant data in the literature and the results of this work is proposed. The new correlation is an improvement over the other correlations because it includes two important design constants: scale-up factor and dispersion height. (c) 2005 Society of Chemical Industry.