Comparison of Efficiency for Impellers Commonly Used in Anoxic Denitrification

The hydraulic efficiency of impellers operating in stirred vessels can be defined in the same way as a pump - the ratio of the flowing fluid's kinetic energy to the mechanical energy input by the mixing device.

In this presentation the hydraulic efficiencies of several impeller types commonly used in the process industries are compared to the efficiencies of impellers used for mixing in anoxic denitrification.  Values for the impellers' efficiencies are compared with values previously reported in the literature.

Results of a full-scale mixing trial in which the solids distribution was measured for three impellers will also be presented and results discussed in the light of the impellers' relative efficiencies.  The local solids concentration was measured at several locations within the vessel and the power input per volume determined when each local measurement was found to be within ± 5 % of the average.  Hydrofoil impellers were found to be 3 – 5 times more energy efficient than a Hyperboloid impeller in achieving this target.