Large blade low speed agitators

Uhl and Voznick showed that the mixing effectiveness of a particular anchor agitator in a Newtonian liquid of dynamic viscosity 40 Pa s was the same as for a particular turbine agitator in a Newtonian liquid of dynamic viscosity 15 Pa s. 

Helical screws normally function by pumping liquid from the bottom of a tank to the liquid surface. The liquid then returns to the bottom of the tank to fill the void created when fresh liquid is pumped to the surface. A rotating helical screw positioned vertically in the centre of an unbaffied cylindrical tank produces a mild swirling motion in the liquid. Since the liquid velocity decreases towards the tank wall, the liquid at the wall of an 

Flow pattern in a baffled helical screw system 

Gray (1963) investigated the mixing times of helical ribbon agitators and found the following equation to hold  

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Agitators with a large blade area which rotate at low speeds. These include anchors, paddles and helical screws. 

Precoat slurries are prepared batchwise in a dedicated tank. This must have a working volume greater than that of the filter(s) to be treated along with the associated piping. An agitator is essential to keep the filter aid in suspension. Large-bladed turbines or propellers rotating at low speed are best. 

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