For solids mixing, the baffles should end (lower end) half the baffle width off the bottom for flat bottom tanks and at the lower tangent line for dished bottom vessels.  Some clearance below the end of the baffles does eliminate a point of potential stagnation.  The main reason for baffles starting below the liquid surface is to increase surface motion if the solids are added on the surface.  Shortened baffles also eliminates a dry surface to which solids can adhere, although the tank wall has the same problem all of the way around the tank.  The upper end of the baffles should extend above the top impeller.

Best advice is start below the liquid surface and end above the bottom of the tank.

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