Why Isn't the Extraction Arm 'Pulling'? Capture Velocity, Flange Geometry, and the Physics Behind It

Stand in front of a fan and you can feel the discharged air metres away; it travels as an organised jet and loses energy slowly. Suction works the opposite way: a hood opening doesn't pull air from one direction, it draws from the entire spherical volume surrounding it at once. That's why capture velocity falls off far more sharply with distance than blown air does.

The Dalla Valle Equation

For a plain, unflanged round or rectangular hood, the classic Dalla Valle equation gives an approximate centreline capture velocity:

v_x = Q / (10·x² + A)

where Q is flow rate (m³/s), x is distance from the source to the hood (m), and A is the hood's face area (m²). This equation applies where the hood's aspect ratio is 0.2 or greater and is a well-established approximation in industrial ventilation references (ACGIH; CDC/NIOSH handbooks). Its practical implication: at a distance equal to the hood diameter (x = D), centreline velocity drops to roughly a tenth of the face velocity.

What a Flange Adds — and How Wide It Needs to Be

A flange around the hood opening blocks air from being drawn in uselessly from behind it. For a flanged round or rectangular opening, the equation becomes:

v_x = Q / (0.75 · (10·x² + A))

The 0.75 factor is the standard figure used in industrial ventilation references for the difference between flanged and unflanged openings: to reach the same capture velocity at the same distance, a flanged hood needs roughly 25% less flow than an unflanged one.

For the flange to deliver that benefit, it needs to be wide enough. References such as the U.S. Navy's industrial hygiene field manual recommend a flange width approximately equal to the square root of the hood's face area — for a round hood, that works out to roughly the hood's own diameter (commonly cited in the region of 75–90% of D). A narrower rim than that gives a partial benefit but doesn't reliably deliver the full 0.75 factor used in the equation.

What Capture Velocity Should Actually Be

ACGIH's reference figures for welding fume aren't a table broken down by process (TIG vs MIG vs FCAW); they vary with fume toxicity: roughly 0.5 m/s (100 fpm) for low-toxicity fume, and roughly 0.6–0.75 m/s (120–150 fpm) for moderately toxic fume. Some references cite a broader range up to about 0.87 m/s (170 fpm). Operations that release particulate with much more energy — heavy grinding, for instance — sit in a separate, higher band of ACGIH's general contaminant-release table, but that's a classification by release energy, not a fixed per-welding-process table.

Ambient Air Movement

Field results rarely match the calculation exactly, because a capture cone is fragile. An open door, a nearby make-up air fan, or a cross-draft from a neighbouring extraction point can disrupt a low-velocity capture cone. Ambient air movement should be accounted for when choosing a capture velocity target.

Getting It Right on the Floor

Three rules matter in practice: keep the hood as close to the source as the process allows (without flange assistance, aim to stay within one hood diameter); remember that datasheet flow figures are almost always measured at the hood face, not the working distance; and verify with an actual anemometer reading at the real working position during commissioning.

Conclusion

An extraction arm that "won't pull" is usually a matter of physics, not a fault. The fix is rarely a bigger motor — it's bringing the hood closer to the source, fitting a properly sized flange, and accounting for ambient air movement; none of that can be confirmed from a flow-rate figure on a label, only from an actual velocity reading at the working distance.

Frequently Asked Questions

How close should the hood be to the source? — Without a flange, aim to keep the distance within one hood diameter. How wide does the flange need to be? — Roughly equal to the hood's diameter; a narrow rim doesn't deliver the full benefit.

References

●        ACGIH — Industrial Ventilation: A Manual of Recommended Practice for Design (capture velocity tables; welding fume range of 100–170 fpm / 0.5–0.87 m/s).

●        CDC/NIOSH local exhaust ventilation references (Dalla Valle equation and the 0.75 flanged-hood factor).

●        U.S. Navy Industrial Hygiene Field Operations Manual — flange width as the square root of hood area.

●        ISO 21904-1:2020 — scope and general principles.