Impeller placement in tall agitated tanks:\nWhen multiple impellers are used on one shaft in a tall tank, what should be the clearance between the tank bottom and the lowest impeller? (D = impeller diameter)

Introduction / Context:
In mixing design, axial spacing and clearances strongly affect circulation loops, dead zones, and power draw. For tall tanks requiring multiple impellers, proper bottom clearance ensures good sweeping of the floor region without excessive shear against the wall or risk of solids deposition (in slurries).

Given Data / Assumptions:

• Cylindrical tank with typical baffle configuration.
• Multiple impellers mounted vertically on one shaft.
• Impeller type is standard (e.g., 45° pitched blade or Rushton), not specialty close-clearance types.

Concept / Approach:
Rules of thumb: bottom impeller clearance ≈ 0.5 D provides adequate floor sweeping and promotes a lower circulation loop. Larger clearances risk stagnant bottom regions and solids settling; much smaller clearances increase hydraulic losses and mechanical risk. Vertical spacing between successive impellers is often 1.0–1.5 D depending on flow patterns sought (axial vs radial).

Step-by-Step Solution:

Verification / Alternative check:
Mixing handbooks specify 0.3–0.6 D as workable; 0.5 D is the most-quoted nominal clearance for general service.

Why Other Options Are Wrong:

• ≥ 1.0 D: too high; can leave a dead zone below the impeller.
• 0.25 D: overly close, risking mechanical issues and inefficient circulation.

Common Pitfalls:
Not adjusting clearance for bottom dish geometry; dished or conical bottoms may warrant modest adjustments.

Final Answer:
0.5 D