The science for sizing submersible mixers is about the same as for side-entering mixers.  In either case, tank geometry and mixer type both have some effect on the mixer size, but not specific for a typical geometry such as this one.  Most design procedures reduce to a simple estimate for minimum power input per unit volume.  For a typical aqueous system the minimum power input is 0.0014 kW / m3 (0.0014 kilowatts/cubic meter).  The 12 m x 12.5 m x 4 m tank contains 600 m3, suggesting a minimum mixer power of 0.84 kW.  A 1.5 kW mixer should be adequate to create motion through the tank.

The mixer should be mounted and angled such that flow is across the bottom of the tank to keep settling solids suspended, or at least not permanently on the bottom.  The motion along the bottom should carry up the opposite side of the tank.  Since the discharge direction of most submersible mixers can be adjusted, periodic changes in direction should help keep the entire bottom cleared, with the possible exception of the corners.

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