How to size a fume extractor for a laser

Most undersized extraction systems were not bought carelessly. They were bought on a single number — airflow — without anyone checking the other three that determine whether that airflow arrives where the cutting happens.

Step 1: Establish required airflow

On a zoned downdraft table, airflow is not calculated across the whole bed. Only the zone under the cutting head is open, so the requirement is set by the area of a single zone and the face velocity you need through it, plus an allowance for leakage through the closed zones.

The variables that push the requirement up:

• Larger bed and larger zones — more open area to draw through.
• Higher laser power — faster cutting and more material processed per hour means more fume per hour.
• Thicker material — more kerf volume vaporised.
• Galvanised, coated or painted stock — substantially heavier fume generation.
• Poor bed sealing — leakage through zones that should be shut.
• A tube attachment — a separate capture problem with its own requirement, not a rounding error on the plate figure.

Step 2: Build a static pressure budget

This is the step that gets skipped, and it is the reason installations disappoint. The fan has to overcome every resistance in the system simultaneously:

• Losses through the table zone and its dampers
• Friction along every metre of straight duct
• Losses at every bend, transition and take-off — a tight radius bend can cost as much as several metres of straight duct
• Any flexible duct in the run, which is far more resistive than smooth rigid duct
• Resistance across the filters — and critically, across loaded filters, not clean ones

That last point matters more than any other. If you size the fan on clean-filter resistance, the system performs on day one and then degrades below requirement for the rest of the filter’s life. Size on the loaded-filter condition, and the system works all the way to changeout.

Step 3: Get duct velocity right

Duct velocity is a Goldilocks problem, and both failure modes are expensive.

Too slow and particulate drops out of suspension and settles inside the duct. It accumulates, progressively restricts the duct, and in some materials becomes a genuine fire load. Too fast and friction losses rise sharply — roughly with the square of velocity — consuming static pressure you needed elsewhere, and increasing noise and abrasive wear at bends.

For fine metal fume and dust in industrial ventilation practice, transport velocities in the general range of 18–23 m/s in branch ducts are commonly used, with the higher end favoured where heavier or coarser particulate is present. Main ducts and plenums may run lower. This is standard industrial ventilation territory — if you are designing a new duct run from scratch, it is worth having someone size it properly rather than matching whatever diameter the flange happens to be.

Step 4: Check filter area and air-to-cloth ratio

Airflow divided by total filter area gives the air-to-cloth ratio — effectively the velocity of air through the filter media. It determines how hard the fine particulate is driven into the media, and therefore whether pulse cleaning can get it back out.

Run the ratio too high and the dust cake is forced into the depth of the media rather than sitting on the surface. Pulse cleaning then cannot recover it, differential pressure climbs and stays there, and the cartridge is finished long before it should be. For fine metal fume, a more generous filter area is one of the most reliable predictors of long cartridge life.

Step 5: Sanity-check the rest

• Electrical supply — motor kW, three-phase availability, starter and overload rating, and cable size for the run. Fixed electrical work must be done by a licensed electrical worker.
• Compressed air — the pulse system needs the specified pressure and enough volume, delivered clean and dry. Add the collector’s demand to your existing shop load before assuming the compressor copes.
• Floor space, access and floor loading — a mid-size collector can weigh close to a tonne. Plan the route in, and the clearance to change cartridges and empty the bin.
• Noise — where the unit sits relative to workstations, and whether an attenuator is warranted.
• Discharge — inside or outside, and whether make-up air is needed if outside.
• Material hazard — if you cut aluminium or titanium, combustible dust needs proper assessment for your installation. Do not size it off a website.

The mistakes that cost the most

1. Sizing on clean-filter performance. The system then underperforms for most of its operating life.
2. Ignoring static pressure. Airflow ratings assume nothing much is connected to the inlet.
3. Not resizing after a machine upgrade. The extractor that suited a 3 kW machine will not suit the 12 kW machine that replaced it on the same footprint.
4. Adding a tube attachment without reassessment. Different capture problem, different requirement.
5. Comparing units on airflow alone. Ask for filter area and static pressure too, or you are comparing one number out of four.
6. Forgetting the material mix. A shop that has drifted from mild steel to mostly galvanised has quietly changed its extraction duty.
7. Treating compressed air as free. Inadequate or contaminated air destroys cartridges regardless of how well the collector is sized.

Already have a unit and want to know if it stacks up?

You do not need to guess. Measured airflow at the machine, differential pressure across the filters, and motor current draw against nameplate will tell you in one visit whether the system is performing to specification or quietly under-delivering. That is part of a standard service →

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Keep reading

• Plasma cutting fume extraction
• Laser fume extraction, explained properly
• Filter life, differential pressure and when to change
• Extraction notes by machine brand

General guidance based on field experience with industrial extraction equipment. It is not a substitute for a proper engineering assessment of your specific installation, the machine manufacturer’s stated requirements, or advice from a suitably qualified professional.