(211c) Solids Suspension by Hydrofoil Impellers Operating Close to the Vessel Bottom at Low Liquid Levels

Typically top entry agitators with turbine type high efficiency axial flow impellers are used to suspend and re-suspend slurry or blend fluids to continuously feed a process. The turbine impeller used by most agitator suppliers is an axial flow device pumping down to sweep the bottom of the tank and push solids up the side to maintain the suspension and blend well for a continuous process. While the axial flow impeller is currently the most efficient device to accomplish the task, the impeller requires a minimum distance off the tank bottom in order to perform, plus a minimum amount of fluid coverage above the impeller to establish impeller pumping. The resultant minimum fluid height in the tank can be as much as 30 percent of the tank diameter, potentially rendering 30% of the tank volume unusable, in a batch process, because the slurry suspension or blending cannot be maintained for pumping. To compound the issue, if the level gets too close or at the impeller, excessive hydraulic load will develop leading to mechanical failures. This presentation will present the problem and solution with calculations and video taped laboratory demonstrations.