Agitator size is a function of the temperature, density and viscosity of the liquid, the agitation power required, the shape of the agitator and the number of blades, and the shape and degree of baffling in the tank. Consult Perry, "The Chemical Engineer's Handbook", any edition, for correlations on vessel mixing, or consult a qualified Chemical Engineer for assistance on agitator sizing and selection - firms involved with the sale of agitators will also be able to assist, carrying out field trials if necessary to help select the right one if, indeed, the conditions of use have not been supplied already elsewhere.
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...oh, and also the size profile, the shape and density of the particulate matter that is needed to remain in suspension.
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