Geometric design rule:\nFor a paddle agitator in a standard baffled tank, what is the typical design ratio of paddle diameter (D) to tank diameter (T) used as a starting point?

Introduction / Context:
Initial sizing of impellers relies on practical geometric ratios. For flat-blade paddles, the impeller diameter relative to tank diameter affects flow pattern, power draw, and shear. A common baseline helps designers estimate motor size and hydrodynamics before detailed optimization.

Given Data / Assumptions:

• Standard baffled, cylindrical tanks with flat-blade paddles.
• Moderate viscosity Newtonian fluids.
• Single impeller placed near the lower third of liquid height.

Concept / Approach:
Design handbooks typically recommend D/T around 0.2–0.33 for paddles, with 0.25 used frequently as a starting value. Larger D/T (e.g., 0.5) is typical for anchors/helical ribbons in viscous mixing; smaller D/T (0.1–0.2) is more common for propellers or multi-impeller shafts where multiple stages provide circulation.

Step-by-Step Solution:

Verification / Alternative check:
Vendor catalogs and mixing texts commonly present power/flow data normalized with D/T ≈ 0.25 for paddle examples, confirming the convention.

Why Other Options Are Wrong:

• 0.1: too small; characteristic of small propellers.
• 0.8: unrealistically large for paddles; near anchor scale.
• 0.5: large; more like anchors or scrapers.
• 0.02: non-physical for bulk mixing.

Common Pitfalls:
Assuming a single ratio fits all fluids; viscous or non-Newtonian systems may require different D/T and blade widths.

Final Answer:
0.25