In an agitated tank with vertical baffles, what is the recommended minimum baffle height relative to tank size to control swirling and improve mixing?

Introduction / Context:Baffles suppress vortexing, increase turbulence, and enhance top-to-bottom mixing in stirred tanks. Their height is a key geometric parameter that determines liquid-surface stability and circulation patterns for a wide range of viscosities and impeller types.

Given Data / Assumptions:

• Cylindrical, baffled tank with a standard impeller (e.g., Rushton or pitched-blade turbine).
• Goal is to reduce swirling and improve mixing uniformity.
• Typical design practice for general chemical service.

Concept / Approach:Best practice extends baffles close to the liquid surface and down toward the bottom to intercept tangential flow. Heights significantly shorter than the liquid depth allow a strong surface vortex; excessively long baffles can be impractical or interfere with internal fittings. A commonly adopted guideline is a baffle height of roughly 70–80% of the liquid height (≈ 3/4 of tank height).

Step-by-Step Solution:Choose a baffle height that reaches most of the liquid column to damp swirl.Adopt 3/4 of tank height as a robust rule of thumb balancing performance and fabrication.Verify clearances near the bottom and freeboard near the top to avoid solids build-up or gas entrainment.

Verification / Alternative check:Textbook and vendor guidelines (with baffle width ≈ 1/12 to 1/10 of tank diameter) pair well with 3/4 height for general liquids, delivering predictable power draw and flow patterns.

Why Other Options Are Wrong:(a) and (b) tie height to impeller size rather than liquid depth; (c) is unrealistic; (e) is incorrect—most axial impellers still benefit from baffles.

Common Pitfalls:Stopping baffles far below the surface; ignoring freeboard and bottom clearances; assuming gas dispersion duties use the same geometry without checks.

Final Answer:About three-quarters (3/4) of the tank height