Phoenix Journal · LEV Testing
Three velocities decide whether your extraction actually works - and they are not the same number. Here is what capture, face and transport velocity each mean, and how they show up on a thorough examination.
Reading the airflow · LEV fundamentals
Stand next to a running extraction hood with an anemometer in your hand and you quickly learn that airflow is not one number - it is at least three, and they behave very differently.
Picture a welding bay at the end of a shift. The fan is humming, the hood is drawing, and on paper the system passes. Yet a thin haze still drifts past the operator's face before it curls into the hood. A few metres away, inside the ductwork, a fine grey deposit is building along the bottom of a horizontal run where nobody can see it. Both problems are about velocity - the speed of the air doing the work - and both are invisible until someone measures the right thing in the right place.
This is where a lot of confusion starts. People talk about "airflow" as though a single reading tells the whole story, then wonder why a system with a healthy fan still fails to protect anyone. In truth, a competent examiner is chasing three separate speeds: how fast the air moves at the point where contaminant is released, how fast it moves across the opening of the hood, and how fast it moves along the duct. Capture velocity, face velocity and transport velocity. Get any one of them wrong and the whole chain of control breaks, whatever the headline flow rate says.
Most of the real control happens in the first few centimetres in front of a hood, and that is exactly where capture and face velocity live. They sound similar, they are often quoted in the same breath, and they are routinely muddled - so it is worth pinning down what each one actually describes before we follow the air into the ductwork.
Capture velocity is the air speed you need at the point where the contaminant is generated, far enough out in front of the hood to grab the cloud of dust, fume or vapour and drag it back in against whatever else is going on in the room. It is the figure that decides whether contaminant ever reaches the hood at all. The trouble is that suction falls away sharply with distance - move out from a plain opening by roughly one duct diameter and the velocity can drop to a fraction of what it was at the face. That is why a hood parked too far from the work fails even when the fan is more than powerful enough.
The right capture velocity depends on how energetically the contaminant is thrown off and how draughty the surroundings are. HSG258, the HSE guide to local exhaust ventilation, sets out indicative bands that most practitioners still work to:
Notice how much the target moves. A cross-draught from an open door or a passing forklift can wipe out a modest capture velocity in an instant, which is why good practice keeps the hood close and shields the working zone rather than simply reaching for a bigger fan.
Face velocity is the more literal measurement: the average air speed across the plane of the hood, booth or cabinet opening - the "face" itself. It is straightforward to check with a grid of readings across the aperture, which is why it appears on nearly every LEV report. For an enclosing hood such as a booth or a partial enclosure, a healthy, even face velocity is a decent proxy for the system pulling as intended, and for cabinets and cupboards it is often the primary control benchmark in its own right.
The key thing to hold onto is that face velocity and capture velocity are not interchangeable. Face velocity is measured at the opening; capture velocity is what survives out at the source where the work actually happens. A booth can show a textbook face velocity and still fail to capture, if the operator stands too far inside, if the piece being worked blocks the airflow, or if the contaminant is flung outward faster than the air can reclaim it. Reading them together - and reading them against how the job is really done, not how it looks on a quiet afternoon - is what separates a meaningful examination from a box-ticking one.
Once contaminant is inside the hood, a different problem takes over. Now the air has to keep whatever it has picked up in suspension all the way to the filter or discharge, and that is a job for transport velocity. It is the velocity most likely to be quietly failing inside a system that otherwise looks fine from the shop floor.
Transport velocity, sometimes called conveying or duct velocity, is the air speed along the inside of the ductwork. It has to stay high enough that particles do not lose momentum and settle out on horizontal runs and at the bottom of bends. When it drops - because a duct is oversized, a damper has drifted, or a system has been extended without recalculating - dust falls out and begins to build. That accumulation narrows the duct, chokes the flow further, and in the wrong materials becomes a genuine fire or explosion risk. Heavier and stickier contaminants need a faster duct; light gases and fumes need far less. Indicative HSG258 and ACGIH figures run along these lines:
This is the reading you cannot take by standing in front of a hood. It needs measurement inside the duct at the right points, which is one reason a proper examination involves test ports along the system and not just a wand waved at the opening.
The clean way to think about it is a relay. Capture velocity gets the contaminant into the hood. Face velocity confirms the opening is pulling evenly. Transport velocity carries the load safely to the filter. Drop the baton at any handover and the finish line means nothing - a perfect duct velocity is wasted if capture fails at the source, and flawless capture is undone if the material settles out two bends later. A single flow rate at the fan can hide all of this, because the same volume of air behaves completely differently through a wide slot, a narrow spigot and a long horizontal run.
That interdependence is exactly what a thorough examination and test under COSHH Regulation 9 is built to expose, and why it is a legal duty at least every fourteen months for most systems. It also explains why extraction alone is rarely the whole answer. Where a process genuinely cannot be brought within reach of an adequate capture velocity, the honest conclusion is often that the hood needs redesigning rather than the fan uprating - a theme we pick up in our guide on how to manage welding fume beyond just buying extraction. And where LEV is backed up by respiratory protection for residual exposure, that protection only counts if each mask actually seals to its wearer, which is the whole point of face-fit testing and why it is not optional. Read together, the three velocities stop being jargon and become what they really are: the difference between a system that looks busy and one that keeps people well.
Questions
Face velocity is the air speed measured right across the opening of a hood, booth or cabinet - the plane of the face. Capture velocity is the air speed you need further out, at the point where the dust or fume is actually released, to draw that contaminant back into the hood against room draughts. A hood can show a healthy face velocity yet still fail to capture at the source, which is why a competent examiner assesses both against how the job is really carried out.
Transport velocity is measured as part of the thorough examination and test required under COSHH Regulation 9, which for most local exhaust ventilation systems must be carried out at least once every fourteen months. Some higher-risk processes require more frequent testing. Because low transport velocity lets dust settle and build up inside ducting - a hidden efficiency loss and, with some materials, a fire or explosion risk - it should be measured at ports along the ductwork rather than assumed from the fan reading.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.