Characterization of impeller types and evaluating agitator designs to suspend solids and pump liquid based on process and mechanical metrics in square vessels

Abstract

Suspension of solid particles in square cross section, low aspect ratio stirred vessels commonly found in the water and wastewater treatment (WWT) industry are studied for a range of impeller types. Four axial and four radial discharge impellers were investigated. The objective of the study was to characterize a wide range of impeller technologies and rank them based on energy and mechanical efficiency as these are primary evaluation principles in the WWT industry. To accomplish this goal each impeller was characterized to determine its design constants: power number, flow number, axial thrust coefficient, and radial thrust coefficient, using a variety of measurement techniques. Then, using a wastewater simulant particle, the just-suspended impeller speed was measured for each impeller type at 2 %v/v solids concentration. Finally, hypothetical designs were conceived for each impeller type operating at the just-suspended speed and were compared based on power usage, torque and bending loads, and shaft weight and length to determine which impeller(s) result in the most efficient agitator design. We find that generally, impeller design constants can be categorized and correlated based on primary pumping discharge classification and that axial discharge impellers are more energetically and mechanically efficient than radial impellers for suspending solids.