Theoretical equations are developed to relate the flow velocities in a tank with impeller power consumption, flow circulation efficiency index N-Q/P-0(1/3) and other parameters. A non-dimensional parameter eta = V/((P/(rho T-2))(1/3)) has been used to characterise the velocity energy efficiency. Laser Doppler velocimetry measurement studies were carried out on the flow field produced by a few commercial axial flow hydrofoil impellers and pitch bladed turbines operating in water and in a non-Newtonian shear-thinning fluid. The data were used to characterise the velocity energy efficiency. The results suggest that hydrofoil impellers are all equally efficient when operating in water. Varying impeller geometrical parameters such as number of blades, pitch angle had marginal effects on the flow energy efficiency. It is suggested that this principle be utilised to achieve some desirable plant improvement results. When operating in viscous non-Newtonian fluids, increased velocities at a given power input can be achieved via optimising impeller geometrical parameters. This effect is more pronounced at smaller Reynolds numbers. (c) 2006 Elsevier B.V. All rights reserved.