Impeller Characteristics Pumping and Power

Power numbers, pumping numbers, shear levels, and flow patterns characterize the various impellers described above. All the power applied to the mixing system produces circulating capacity, Q, and velocity head, H, given by 

Q represents internal circulation and H provides the shear in mixing. In a sense, the velocity head, H, provides the kinetic energy that generates shear through the jet or pulsating motion of the fluid. Both expressions have not included the effects of the number of blades and blade width. Head results in shear and is dissipated by turbulence. Equation (6-1) can be rewritten as 

It is important to note that some impellers and mixer configurations do not pump weU. For example, the retreat curve blade impeller in an unbaffled vessel creates solid body rotation and poor pumping. Pumping with close-clearance impellers such as anchors and helical ribbon can be very high or, sometimes, very poor, depending on conditions and the materials being pumped. Turbulent impellers in laminar applications only pump locally. Often, the rest of the tank goes unmixed. 

2 Power and Power Number. The power consumed by a mixer can be obtained by multiplying pumping, Q, and head, H, and is given by 

Using another viewpoint, power, generated by an individual section of an impeller, is equal to the drag, F, multiplied by the impeller velocity, V, for that section or 

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