Critical Speed Explained: How to Optimize Rotary Drum Scrubber Performance

When assessing or designing a rotary drum scrubber, engineers often model its rotational speed as a function of critical speed. Expressing the operating speed as a percentage of critical speed offers a dimensionless number that allows direct comparison between scrubbers of any size. This normalised value describes the attritioning behaviour of a scrubber.

What is Critical Speed?

In any rotating mill or scrubber, critical speed is the theoretical speed at which the centrifugal force acting on particles at the drum shell equals the force of gravity. At this speed, particles are pinned against the wall of the drum and cease to fall back into the load. As a scrubber approaches this speed, relative movement between particles slows down. Grinding, scrubbing, and cascading actions diminish, and the unit stops working correctly

Mathematically, critical speed is defined by balancing the gravitational force against centrifugal force at the drum’s inner surface:

Operating at a Percentage of Critical Speed

In practice, scrubbers and mills are operated at a fraction of their calculated critical speed, typically between 50% and 75% depending on the duty:

• Below 50% – The cascading action is weak, and material may simply slide with insufficient lifting and attrition.

• 50–70% – The sweet spot for most scrubbers: material is lifted and dropped onto itself, creating good particle–particle interaction without excessive centrifuging.

• Above 75% – Centrifuging begins to dominate. The charge tends to stick to the shell, reducing the washing and disintegration effect.

You also do not want material to be lifted and then dumped past the floor material and straight onto the scrubber shell. Reaching a sweet spot wear material is lifted and falls back onto itself is where your rotary speed should be.

For scrubbers specifically, the goal is agitation and washing, not grinding. Therefore, they are usually operated in the low to mid 60% range of critical speed. This ensures strong cascading and adequate retention time without generating excess power draw or wear.

⚠️ Important Note: Critical speed must always be calculated using the internal diameter of the shell — not the external diameter. Using the external dimension will give a falsely high value, leading to an incorrect % critical speed calculation. For operators double-checking their own scrubber performance, it’s essential to take measurements inside the shell lining or from design drawings to ensure accuracy.

This equation highlights that larger-diameter scrubbers have lower critical speeds, while smaller drums can be operated at higher rpm.

Why It Matters

• Scrubbing Efficiency: Correct % critical speed ensures particles lift and fall, breaking down soft clays and liberating fines.

• Energy Utilisation: Operating too close to critical speed wastes energy as particles “plaster” the shell instead of tumbling.

• Liner & Shell Life: Higher % critical speed increases impact intensity, accelerating liner wear.

  
  

Conclusion

Critical speed is a deceptively simple concept with significant implications for rotary drum scrubber design and operation. By calculating the drum’s theoretical critical speed and selecting an appropriate operating fraction—usually 60–70%—plants achieve the best balance of throughput, disintegration, and equipment life.