Trommel Screen Calculator

Critical speed equals square root of gravity over 4 pi squared times radius

Solution

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How It Works

A trommel screen is a rotating drum with holes that separates materials by size -- undersized pieces fall through the openings while oversized pieces tumble to the discharge end. The critical speed is the rotation rate at which centrifugal force pins material to the drum wall, stopping the tumbling action needed for effective screening.

Operators typically run trommels at 25–35% of critical speed to ensure good mixing and throughput. The critical speed depends only on drum radius and gravitational acceleration.

Example Problem

A trommel drum has a radius of 75 cm and operates under standard gravity (981 cm/s²). What is the critical speed?

  1. ηc = √(g / 4π²r) = √(981 / (39.48 × 75))
  2. ηc = √(0.331) ≈ 0.575 rev/s (34.5 RPM)

Operating at 30% critical speed gives about 10 RPM -- a common speed for municipal solid waste trommels.

Frequently Asked Questions

What is the critical speed of a trommel screen?

Critical speed is the rotational speed at which centrifugal force equals gravity at the drum surface. Above this speed, material sticks to the wall instead of tumbling. It equals √(g / 4π²r) and depends only on drum radius and gravity.

What percentage of critical speed should a trommel operate at?

Most trommels operate at 25–35% of critical speed. This provides enough tumbling action for material to contact the screen surface without excessive bounce or centrifuging. Too slow reduces throughput; too fast reduces separation quality.

What are trommel screens used for?

Trommel screens separate materials by size in solid waste recycling, compost production, mining, and aggregate processing. In waste management, they typically separate organics and fines from recyclable containers and debris.

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