Effect of scale on the draw down of floating solids

In this study, the draw down of floating solids from the liquid surface has been investigated using vessels of 0.61 and 2.67 m diameter. The importance of impeller type (mixed flow pitched blade turbine (PBT) and the narrow blade hydrofoil LE-20), pumping mode and position and the effect of varying liquid height have also been studied. Impeller speed and power consumption at which no solids remain at the surface for more than 2–4 s are determined by visual observations.Results from different scales are discussed in relation to the way in which solids are drawn down from the liquid surface. It has been shown that over a wide range of conditions the power required for drawing down solids can be reduced by operating in the upward rather than downward pumping mode and using an axial flow narrow blade hydrofoil rather than a mixed flow pitched blade turbine. Different scale up criteria, power per unit volume, tip speed and Froude number, are discussed for these systems. For scale up, specific power input is shown to be the most appropriate criterion for upward pumping impellers.