Laminar Mixing in Mechanically Stirred Vessels

Power for laminar mixing can be derived based on Stokes drag and written as 

This correlation does not include the effects of blade number and blade width as expected from Stokes drag. Using the definition of Reynolds number in eq. (6-4) and turbulent power number Np in eq. (6-5), this power expression can be rearranged to 

The power draw can be very high in laminar mixing compared to turbulent mixing. In addition, these mixers are operated at low speeds and the torque on the shaft can be extremely high. The mixer drives designed for high torque require high investment costs. Since most of the power consumed by the mixer is dissipated into heat, removal of heat may be required to avoid possible adverse effects on the process and product quality. 

Pumping numbers for the helical ribbon and screw impellers are available in the literature. They range from 0.04 to 0.5 and are highly dependent on the geometry. The anchor impeller only pumps along its radial arms, and pumping numbers are not readily available for this impeller. 

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