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Case study · Joinery manufacturing · Wood dust
A factory making fire doors ran three dust extractors serving a shop full of woodworking machines. On paper the extraction looked ample. Under real conditions - with several machines running at once - one extractor could not keep the dust moving, and settled dust in a duct is both a blockage and a fire risk. Here is what the examination uncovered.
The site
The factory manufactures fire doors - high volumes of timber and engineered board worked through saws, planers, sanders and routers. Three dust extractors serve the shop between them, each feeding a network of ducting and blast gates that connect to the machines. When the shop is in full flow, several machines run at once, all drawing on the same extraction.
That shared arrangement is normal and efficient, but it has a catch that a single-machine system does not: the air is finite and gets shared out. An extractor that pulls strongly when one blast gate is open has to spread the same airflow across every gate that opens after it. The question for the examination was not "does it capture" in the abstract, but "does it still capture, at every machine, when the shop is actually working".
Fire doors mean a lot of engineered board as well as timber, and that matters: the dust is fine, it is produced in volume, and - like all wood dust - it is combustible. So the examination had a second dimension beyond health: the condition of the ductwork as a fire and explosion consideration.
The hazard and the law
The first duty is the familiar one. Wood dust is hazardous to health - hardwood dust can cause cancer of the nose, and both hardwood and softwood dust cause occupational asthma, with workplace exposure limits of 3 mg/m³ and 5 mg/m³ respectively. The engineered board common in fire doors adds fine dust and its own binder chemistry. Where extraction controls that dust it is LEV under COSHH, due examination and test at least every 14 months.
The second duty rides in the same ducts. Fine wood dust is combustible, and a dust cloud at the right concentration can explode if ignited - which brings the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) alongside COSHH. The link between the two is transport velocity: the air in the ductwork has to move fast enough - for wood dust, in the region of 20 m/s - to keep dust airborne all the way to the collector. Let the velocity drop and dust settles inside the duct, where it both blocks the run and becomes a fuel load. A degraded extraction system is therefore a health failure and a fire-safety failure at once.
What we tested
The key to examining a shared system honestly is to test it as it is actually used. We measured capture at the machines and the branch velocities, but crucially we did so under realistic simultaneous use - with the combination of blast gates open that the shop runs in practice - so we could see whether the extractors held their velocity when the air was shared out, or only when a single branch was open.
We measured the transport velocity in the duct runs against the benchmark for wood dust, inspected the fans and filters for capacity and loading, and - because of the DSEAR dimension - looked inside the ductwork for settled dust and reviewed the explosion-protection and housekeeping around the system. A single-machine test would have missed the point entirely; the whole issue here only appears when the shop is busy.
What we found
| Check | Finding | Result |
|---|---|---|
| Extractor 1 (fewer machines) | Capture and branch velocities within benchmark | Pass |
| Extractor 2 - under full load | Branch velocity low at 2 machines when several gates open | Fail |
| Duct transport velocity | Below benchmark in one run - dust settling inside the duct | Fail |
| Filters / fan capacity | Filters loaded; fan serving more open branches than it can keep clear | Remedial |
With one machine running, everything looked fine. Under real load - several blast gates open together - the picture changed: one extractor could not maintain branch velocity at two of its machines, and the transport velocity in a duct run fell below benchmark, so dust was settling inside the ductwork rather than being carried to the collector. Loaded filters were part of the capacity problem. That settled dust is the finding that matters most: it is a blockage forming, and in a combustible-dust system it is a fuel load accumulating where it should never be. A mixed result on health control, and a clear DSEAR flag.
The remedial actions
The outcome
With the blast gates managed, the fan capacity addressed, the settled dust cleared and the filters serviced, the extraction was re-tested under realistic load and held its branch and transport velocities. The factory came away understanding its system not as three extractors on a drawing but as a shared resource that has to be matched to how many machines run at once - and with the duct cleared of a growing fire risk it had not known was there.
Shared extraction is only as good as the number of machines drawing on it at any moment, and the failure is invisible from the shop floor - capture seems fine, while deep in a duct the velocity has dropped and dust is piling up. For wood dust that pile is two problems at once: a blockage and, under DSEAR, a combustible fuel load. The defences are to test under real load rather than a single-machine best case, to keep transport velocity up by managing the gates and fan capacity, and to keep the ducts clear. It is health and fire safety in the same maintenance task.
Sources and further reading
The duties, exposure limits and control standards described here come from the Health and Safety Executive. For your own process, always work from the current HSE guidance:
Questions
Because a shared system's performance depends on how many machines draw on it at once. One extractor serving several machines may give good capture when one branch is open, but as more blast gates open the air is shared out and the velocity at each drops. The only honest test is under realistic simultaneous use - testing one machine in isolation can pass a system that fails the moment the shop gets busy.
Transport velocity is the air speed inside the ductwork that keeps the dust airborne and moving to the collector. For wood dust it typically needs to be high - around 20 m/s for shavings and coarse material, and not much lower for fine dust. If the velocity drops below that, dust falls out of the airstream and settles inside the duct, which blocks the run over time and - because wood dust is combustible - builds a fuel load inside the system.
Fine wood dust is combustible, and a cloud of it in the right concentration can explode if ignited. Dust that settles inside ductwork because the transport velocity is too low is both a blockage and a reservoir of that fuel. This brings the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) into play alongside COSHH - the system has to control the health risk and manage the explosion risk, and a degraded, dust-laden duct works against both.
Capture that is fine with one machine running but falls away as several blast gates open together; branch velocities below benchmark at some machines; and - the tell-tale - settled dust accumulating inside a duct run where the velocity has dropped. The fan has simply been asked to serve more open branches than it has the capacity to keep clear at once.
Usually a combination: manage the blast gates so only the branches in use are open (interlocks help), review whether the fan has the capacity for realistic simultaneous use, clear the settled dust from the ductwork, service loaded filters, and check the explosion-protection measures. The aim is to keep the transport velocity up across the branches that are actually open at any time.
At least every 14 months under COSHH, and the examination should reflect real operating conditions, not a best-case single-machine run. Because wood dust also carries a DSEAR dimension, keeping the ductwork clear and the system in good order is part of fire and explosion safety as well as health - two reasons to stay on top of it.
Tell us your capture points and the process they control, and we will quote the thorough examination and test to COSHH and HSG258.
