For a propeller agitator installed in a mixing tank, the impeller diameter is typically what fraction (percent) of the tank diameter, and the normal peripheral speed is about 300–500 m/min?

Introduction / Context:
Propeller agitators are high-speed axial-flow impellers used widely for blending, solid suspension, and gas dispersion in low-to-moderate viscosity fluids. Correct impeller sizing as a fraction of tank diameter is a standard design starting point that influences flow pattern and power number.

Given Data / Assumptions:

• Propeller-style impeller (marine or hydrofoil variants).
• Peripheral speed guideline: 300–500 m/min.
• Baffled cylindrical tank, typical process fluids.

Concept / Approach:
Axial-flow impellers rely on tip speed and diameter to generate circulation. Industry practice sets the impeller diameter D_i at a moderate fraction of the tank diameter D_t to balance pumping capacity against shaft stresses and mechanical constraints. The common range for D_i/D_t is 0.15–0.30 (15–30%).

Step-by-Step Solution:

Verification / Alternative check:
Vendor selection charts and empirical design texts confirm D_i/D_t ≈ 0.2–0.3 for propellers; high-speed service uses smaller diameters, while higher pumping may push to the upper end of the range.

Why Other Options Are Wrong:

• 5–10%: Too small to deliver adequate axial pumping in many tanks.
• 40–50% or greater: More characteristic of low-speed turbines and paddles, not high-speed propellers.
• 55–75% or 80–90%: Uncommon and mechanically impractical for propellers.

Common Pitfalls:
Omitting baffles (leading to swirl and poor mixing) or choosing excessive rpm causing vibration and cavitation at the blade tips.

Final Answer:
15 to 30 percent of tank diameter