Agitator design (for economy and performance) is a multi-faceted matter.
The size and proportions of the vessel are critical, as are the properties of the fluid. The best impeller choices may be axial flow, radial flow, high shear … it all depends on the vessel geometry, required shear at the impeller, and circulation strategy chosen by the mixer designer.
Provide the material of construction you require. For tanks greater than 3 diameters high, seriously consider if an in-tank bearing is permitted. In-tank bearings greatly reduce agitator size and loads applied to the mixer mounting nozzle. Batch vs. continuous. Inlets and outlets. For processes where an in-tank bearing aren't practical (abrasive solids in the process fluids for example), keep the vessel height in check.
Make sure your tank supplier receives all structural loads and rigidity requirements from the mixer vendor. Mixers apply heavy bending loads to the mounting nozzle, and your tank needs to be suitably designed for it. Also make sure the anti-swirl baffle geometry is transferred to the tank design.
There is an agitator specification published by PIP. Feel free to read it for reference, however in my experience, one must be extremely selective using that content or else costs and negotiations will skyrocket. Very few agitator designs will benefit from applying all of the PIP standard, and most agitators require very little of it.
