Quick answer: Dust ducts are sized to a minimum transport velocity, not a friction rate. Compute area from A = Q / V and diameter from D = √(4A/π), using about 12 m/s for fine dust, 18 m/s for wood dust and 23 m/s for heavy metal particles. Then — unlike HVAC — round the duct DOWN to the next standard size.
Dust ducts follow the opposite rule to HVAC
In HVAC you size for low velocity to cut noise and fan power, and rounding a duct up gives you margin. In dust collection, velocity is the design requirement: the air must move fast enough to keep particles suspended. Go too slow and material drops out of the airstream and settles along the duct.
Settled combustible dust inside ductwork is not just a blockage — it is the fuel for a duct fire or secondary explosion, which is why NFPA 652 and NFPA 654 treat accumulation as a hazard.
That is why rounding up is wrong here. A bigger duct means lower velocity, and lower velocity means the pipe silts up. Dust ducts round down, accepting more pressure drop as the price of a self-sweeping duct.
Minimum transport velocities (ACGIH)
| Material | Minimum velocity |
|---|---|
| Fine dust / fumes | ~12 m/s (2400 fpm) |
| Wood dust / sawdust | ~18 m/s (3500 fpm) |
| Grain / paper trim | ~20 m/s (4000 fpm) |
| Metal turnings / heavy | ~23 m/s (4500 fpm) |
Worked example
3,000 m³/h of wood dust at 18 m/s minimum: Q = 0.833 m³/s, A = 0.833 ÷ 18 = 0.0463 m², D = √(4 × 0.0463 ÷ π) ≈ 243 mm. Round down to a 200 mm standard duct → actual velocity ≈ 26.5 m/s, comfortably above the 18 m/s minimum. Rounding up to 250 mm would drop it to ~17 m/s — below transport velocity, and the duct would start collecting dust.
Design notes
- Use angled branch entries (30–45°), never square tees — square entries drop material.
- Check velocity in every branch, not just the main; lightly loaded branches settle first.
- Avoid horizontal dead legs and provide cleanouts at direction changes.
Duct diameter and transport velocity for wood, metal and fine dust. Free tool.
Open the Dust Collector Pipe Calculator →Frequently Asked Questions
What is transport velocity in dust collection?
Transport velocity is the minimum air speed that keeps particles suspended in the duct. Below it, dust falls out of the airstream and settles. Typical values are about 12 m/s for fine dust, 18 m/s for wood dust and 23 m/s for heavy metal particles.
Why do dust ducts round down instead of up?
A larger duct lowers velocity. In HVAC that is harmless, but in dust collection it drops air speed below transport velocity so material settles in the pipe, causing blockage and a combustible dust hazard. Rounding down keeps velocity high and the duct self-cleaning.
What velocity is needed for wood dust extraction?
About 18 m/s, roughly 3500 fpm, for typical sawdust and shavings. Heavier or damp material needs more; fine sanding dust can run slightly slower at around 12-15 m/s.
Is dust settled in ductwork actually dangerous?
Yes. Accumulated combustible dust inside ducts is a recognised fire and secondary-explosion hazard covered by NFPA 652 and NFPA 654. Holding transport velocity so ducts stay swept clean is a safety requirement.