Operational ranges: industrial paddle agitators typically turn at what rotational speeds, and how does that relate to their mixing duty?

Introduction / Context:
Paddle agitators are classic low-to-moderate shear mixers used in many process industries for blending, heat transfer, and maintaining suspensions. Knowing typical speed ranges helps match impeller choice to process goals and motor sizing.

Given Data / Assumptions:

• Large tanks (hundreds to thousands of litres or more).
• Newtonian or mildly viscous liquids; limited gas dispersion duty.
• Standard two- or four-blade paddles with baffles in the vessel.

Concept / Approach:
Paddle impellers are designed to create bulk circulation at relatively low speeds, minimizing shear while still renewing heat-transfer surfaces. Typical industrial operation falls in the tens to low hundreds of rpm, commonly between about 20 and 150 rpm depending on diameter, viscosity, and duty. Much higher speeds are characteristic of small-diameter high-shear devices, not large paddles.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer curves and scaling rules (tip speed = π * D * N) show that at 20–150 rpm with large D, tip speeds and Re are appropriate for typical paddle duties.

Why Other Options Are Wrong:

• 150–300 rpm, 400–800 rpm, 1150 rpm: excessively high for large paddles; these imply high shear and power.
• 1–5 rpm: too low for effective circulation in most liquid tanks.

Common Pitfalls:
Confusing small-scale lab mixers (which may run at hundreds of rpm) with large industrial paddles where diameter is much larger.

Final Answer:
20 and 150 rpm.