The effect of impeller-to-tank diameter ratio (D/T) on the draw down of solids was investigated using a mixed flow impeller (pitched blade turbine) and a narrow blade hydrofoil (LE-20) of D = T/2 and T/3. Operational conditions (impeller speed and power consumption) at which solids do not remain at the liquid surface for more than 2-4 s were determined for a given solid type (polyethylene particles) at a given concentration (X = 1%). Under selected conditions, liquid velocity values were obtained from LDA measurements and CFD simulations to better interpret the findings.Both upward and downward pumping modes were studied with the impeller mounted over a range of submergences, and different mechanisms of draw down were noted. For LDA measurements and CFD simulations, only the upward pumping PBT was considered. These were carried out under conditions where draw down did not occur through air ingesting vortices.When solids were drawn down without air being entrained from the surface, a smaller diameter impeller (D = T/3) was more energy efficient, although it required higher speeds to achieve draw down. Results from LDA measurements and CFD simulations showed that the discharge flow from a larger impeller has a stronger radial component of flow, and as a result the liquid is directed towards the vessel walls rather than the surface which supported this finding. The only exception to this was when solids were drawn down along with air through vortices. In this case, the power demand could be lower with a larger diameter impeller. (C) 2003 Elsevier Science Ltd. All rights reserved.