Mixing equipment design: For a paddle agitator, what is the typical impeller-to-tank diameter ratio used as a design rule of thumb?

Introduction / Context:
Agitator geometry strongly influences flow patterns, power draw, and mixing time. A fundamental geometric parameter is the impeller diameter-to-tank diameter ratio (D/T). For paddle agitators (flat blades), standard proportions are used to ensure circulation and suspension without excessive power consumption.

Given Data / Assumptions:

• Cylindrical tank with standard baffles or unbaffled as commonly used with paddles.
• Ambient liquid mixing without gas dispersion considerations.

Concept / Approach:
Paddle agitators produce primarily radial flow. To be effective across the tank cross-section, a relatively large impeller diameter is used. Typical D/T values for paddles are around 0.4–0.5, whereas high-efficiency axial-flow impellers (e.g., pitched blade turbines, hydrofoils) often use smaller D/T ~0.3–0.35 with higher speed. Choosing D/T ≈ 0.5 for paddles provides robust circulation in viscous or moderate-viscosity services.

Step-by-Step Solution:

Verification / Alternative check:
Vendor literature and handbooks list D/T ≈ 0.4–0.5 for paddles depending on duty and viscosity.

Why Other Options Are Wrong:

• 0.1 and 0.25: impeller too small to sweep the cross-section effectively.
• 0.8: excessively large, causing high torque/power and mechanical issues.

Common Pitfalls:
Confusing paddle with axial-flow impellers that use smaller D/T but higher rotational speeds.

Final Answer:
0.5