Top-mounted and side-entry mixers have entirely different mixing characteristics and the choice is often more related to the tank than the mixer.  Side-entry mixers are usually chosen because the top or shell of the tank will not support the weight and bending loads of a top-mounted mixer.  The top-mounted mixer not only requires that the tank or other structure support the mixer, but when center mounted, baffles on the wall of the tank are need to prevent swirling of low viscosity liquids.
 
With all of the additional tank requirements for top-mounted mixers, the advantages of top-mounted mixers are more intense and rapid blending, more capabilities for solids suspension, and generally more uniform mixing.  The net result is that a top-mounted mixer is likely to be more expensive than a side-entry mixer, but will usually give better mixing.  You must pick the mixer that is necessary for the application.
 
Both top-mounted and side-entry mixers can be effective when applied in appropriate situations.  Each has advantages and disadvantages.

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