You install a fume extractor arm above the welding station. The welder still complains about smoke. The arm is on, the fan is running, but the plume drifts across the shop. You moved the arm closer. It didn’t help. The problem isn’t the arm position; it’s the airflow.
Capture velocity, not volume
Fume extraction is rated by CFM or m3/h. That number is the total airflow. What matters is the capture velocity at the plume: how fast the air moves at the point where the weld is happening. An arm that moves 500 m3/h but sits 500 mm from the arc has low capture velocity. A smaller arm that sits 150 mm away captures more smoke.
The rule of thumb is 0.5 to 1 m/s capture velocity at the plume for manual welding. That means the hood needs to be within 150 to 200 mm of the weld. Any farther, and cross-drafts from doors, fans, and HVAC blow the plume away before the hood gets it.
The cross-draft problem
A fume arm fights every air movement in the shop. A door opening, a forklift passing, or an overhead fan creates a draft that pushes the plume sideways. The arm can’t overcome a 0.5 m/s cross-draft. This is why a system that works on a quiet Saturday fails on a Monday when the dock doors open.
If the station is near a door or an aisle, add a curtain or a partial enclosure around the weld zone. You don’t need a fully enclosed booth; a side curtain on the draft side cuts the cross-flow enough for the arm to work.
Source capture vs ambient
Source capture (the arm right at the weld) is the right approach for manual welding. It pulls the fume at the source, before it spreads. Ambient systems (ceiling ducts, whole-vehicle booths) clean the whole shop air, but they don’t protect the welder’s breathing zone.
A welder leans over the arc. Their face is 300 mm from the plume. An overhead duct that pulls from 2 m above the floor doesn’t help. The fume rises past the welder’s face before it reaches the duct. Source capture is the only thing that protects the operator.
Filter maintenance
A fume extractor with a clogged filter pulls less air. The pressure drop climbs, the fan moves less CFM, and the capture velocity drops. Operators don’t notice until the smoke gets worse, because the degradation is gradual over weeks.
Monitor the differential pressure across the filter. When it exceeds the manufacturer’s limit, replace or clean the element. A dirty filter isn’t just weak; it can release accumulated fume back into the air when the fan cycles. For welding on stainless or galvanized steel, use a HEPA-grade filter; the standard cartridge doesn’t catch the fine metal oxide particles.
Flexible arm reach
A 2 m arm reaches 2 m. But a flexible arm droops when extended, and the hood position shifts as the welder moves. If the welder has to reach around the arm, the arm isn’t working. Mount the arm on a counterbalanced balancer or a wall-mounted jib so it stays where you put it without needing a lock.
Position the hood 150 mm above and 100 mm behind the weld direction. The plume rises and drifts back; the hood catches it there. Don’t point the hood directly at the arc; it disrupts the shielding gas and creates a bad weld.
Bottom line
Fume extraction works when the hood is close to the plume, the cross-drafts are blocked, and the filter is clean. Arm position matters more than CFM. A 300 m3/h arm at 150 mm beats a 1000 m3/h system at 500 mm every time.
