Standard Geometric Ratio — In many STR designs, what is the typical ratio of impeller diameter to tank diameter (Da : Dt)?

Introduction:
Geometric similarity is commonly used when scaling stirred-tank reactors (STRs). One key dimensionless parameter is the ratio of impeller diameter to tank diameter (Da:Dt). Selecting an appropriate ratio ensures comparable flow patterns, power input per volume, and mixing performance across scales.

Given Data / Assumptions:

• Cylindrical, baffled tank with a standard radial or pitched-blade impeller.
• Impeller placed at conventional clearances from the bottom and liquid surface.
• Typical bioprocess fluids with moderate viscosities.

Concept / Approach:

Design handbooks and practice often adopt Da ≈ Dt/3 (that is, Da:Dt ≈ 1:3) as a baseline for many STR configurations. This ratio provides a balance between circulation capacity and power draw. Larger impellers (higher Da/Dt) can over-shear and over-power the system, while very small impellers (lower Da/Dt) may not provide adequate bulk flow without excessive speed.

Step-by-Step Solution:

Verification / Alternative check:

Vendor specifications and scale-up literature frequently use Da/Dt ≈ 0.33 as an initial design, later refined by CFD or pilot data.

Why Other Options Are Wrong:

B and C imply a larger impeller than the tank—impractical. D and E are smaller than common practice and may require high speeds to achieve circulation.

Common Pitfalls:

Applying a single ratio indiscriminately; non-Newtonian or gas-handling duties may warrant different Da/Dt.

Final Answer:

1 : 3