(601e) Suspending Solids Using An Equal Area Draft Tube

Obtaining a nearly uniform suspension of solids in tall tanks is difficult. Typically more than one axial-flow impeller is required. The impeller spacing will depend on the type of impeller, and more impellers may be required for less axially efficient pumping impellers. Using a series of axial flow impellers ensures that there is good top to bottom flow and that short-circuiting is eliminated. However, in such a system a series of n identical impellers do not pump n times the pumping rate of a single impeller. This fact reduces the efficiency of uniformly distributing solids in taller, standard baffled tanks.

Using an equal area draft tube for uniformly distributing solids is a more efficient technique. A comparison of 4 different impellers, both in an equal area draft tube and in a standard baffled tank, will be made in a tank with a 1.9:1 Z/T (liquid level/tank diameter). The impellers will be a P-4 (standard pitched blade impeller), a Chemineer HE-3 impeller, a Chemineer SC-3 impeller and a Chemineer Maxflo W impeller. These impellers will be examined for two different settling rate solids of about 15 feet/min and 7 feet/min. Efficiency comparisons based on torque and horsepower will be made.