The convex bladed mixed flow impeller and the marine propeller: a multipurpose agitator

Abstract

A new type of impeller for stirred tanks - the convex bladed mixed flow impeller (CBMF) is described. It is a compromise between the disc turbine (Rushton) impeller: the CBMF combines the swept-back blades with a propeller action. The swept-back feature gives lower energy dissipation in the vicinity of the impeller than the disc turbine; the propeller action gives a strong downward liquid motion through the impeller and hence good circulation in the stirred tank. Such a compromise is necessary for a multipurpose mixing vessel. The CBMF impeller was tested comprehensively in comparison with the disc turbine, the marine propeller and the pitched bladed turbine; the comparison includes published data. The tests included measurements of: (1) power consumption in water, glycerol and aqueous CMC solutions, (2) minimum power for gas dispersion, (3) 95% mixing time for a tracer in water, (4) gas hold up in water, (5) k_La for oxygen transfer from pure oxygen to water and CMC solutions and (6) minimum power and speed for solid suspension. The CBMF is not better than the above mentioned impellers in all tests but it does appear to have advantages in respect of: (1) lower power consumption, (2) shorter mixing time, (3) good mass transfer and (4) lower minimum power for solids suspension.