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LEV testing - hood design
Fans and ductwork get the attention, but the hood - where the contaminant is meant to be captured - is where LEV most often succeeds or fails. Its type and its position decide whether the airflow the system produces actually reaches the contaminant, and because capture velocity falls away so steeply with distance, the hood is the single biggest variable in whether the whole system works.
The short answer
The hood is the single biggest variable in LEV performance because it determines whether the airflow the system produces actually reaches the contaminant. Capture velocity falls off very steeply with distance from a hood - roughly with the square of the distance - so a hood placed too far from the source captures almost nothing, however powerful the fan. The type of hood matters just as much: an enclosing hood that surrounds the source is far more effective than a capturing hood that must reach out to draw the contaminant in. Get the hood type and position right and the system works; get them wrong and no amount of fan power rescues it.
Where LEV really succeeds or fails
When people think about extraction, they tend to think about the fan - the power that moves the air - and the ductwork that carries it. But the part that actually decides whether an LEV system controls exposure is the hood: the inlet at the process where the contaminant is supposed to be captured. Everything else exists to serve the hood; if the hood does not capture the contaminant, the power of the fan and the quality of the ducting are beside the point. The hood is where the system meets the hazard, and where it succeeds or fails.
This is why hood design is the single biggest variable in LEV performance. A well-designed hood in the right place captures the contaminant efficiently, and the rest of the system carries it away. A poorly designed or badly positioned hood fails to capture it in the first place, and nothing downstream can recover what was never drawn in. So while a weak fan or a leaky duct can certainly undermine a system, the most common and most decisive way LEV fails is at the hood - which makes getting the hood right the most important design decision of all.
Why distance is so decisive
The reason the hood's position matters so much is the way capture velocity behaves with distance. Capture velocity is the air speed the hood generates at the point where the contaminant is released - the speed that has to be enough to draw the contaminant in against the surrounding air. The crucial fact is that this velocity falls off very steeply as you move away from the hood: roughly with the square of the distance. Double the distance from the hood and the capture velocity drops to something like a quarter. The pull weakens dramatically with even a small increase in distance.
This is why a hood placed too far from the source captures almost nothing, no matter how powerful the fan behind it. The fan can move a great deal of air, but if the hood is too far from where the contaminant is released, the velocity at that point has fallen away to almost nothing, and the contaminant simply is not drawn in - it escapes into the room instead. Conversely, bringing the hood close to the source keeps the capture velocity high where it matters. So the single most important thing about a hood's position is how close it is to the point of release - because distance, through that steep falloff, is what makes or breaks the capture.
Why the type matters as much
Hood type matters as much as position, and it is really about how much the hood relies on reaching out for the contaminant. The most effective type is an enclosing hood - one that surrounds or encloses the source, so the contaminant is generated inside the extraction rather than outside it. An enclosure barely has to reach at all; the contaminant is released where the air is already being drawn away, so capture is highly effective and much less sensitive to draughts and distance. Wherever a process can be enclosed, this is the strongest approach.
Where full enclosure is not possible, a receiving hood positioned to catch a contaminant that is already moving towards it - like a canopy above a process that sends fume upwards - can work well, because it uses the contaminant's own motion. Least effective is the capturing hood that must reach out into open air to draw a contaminant in from a distance, fighting the steep velocity falloff and any cross-draughts. It can work, but only if kept close and shielded. So the hierarchy is clear: enclose if you can, receive if the contaminant moves predictably, and capture from open air only as a last resort - because the more the hood must reach, the more easily it fails.
What this means for testing
Because the hood is where LEV succeeds or fails, it is where a thorough examination and test looks hardest. The test measures the capture velocity at the point the contaminant is actually released - not just the airflow the fan produces, but whether that airflow translates into enough velocity at the source to draw the contaminant in. This is the measurement that matters, because it tests the thing the hood exists to do. A system can have a healthy fan and still fail here if the hood is the wrong type or too far from the source.
It also means hood problems are a common finding. A hood that has been moved, or a process that has crept away from its hood over time, or a hood that was never close enough to begin with, shows up in the test as capture velocity below what the contaminant needs - even when the rest of the system is sound. The fix is often at the hood: bring it closer, enclose the source better, reposition it to catch the contaminant's movement. So the test does not just confirm the fan and ducts are working; it confirms the hood - the decisive part - is actually capturing the contaminant, which is the whole point of the system.
The takeaway
The hood is the single biggest variable in whether LEV works, because it determines whether the airflow the system produces ever reaches the contaminant. Capture velocity falls off very steeply with distance - roughly with the square of it - so a hood too far from the source captures almost nothing however powerful the fan, and hood type matters just as much: an enclosing hood that surrounds the source beats a capturing hood that must reach out into open air.
So the hierarchy for hood design is clear - enclose the source if you can, use a receiving hood if the contaminant moves predictably towards it, and reach out from open air only as a last resort, kept close. And because the hood is decisive, it is where a thorough examination looks hardest, measuring the capture velocity at the point of release to confirm the airflow is actually translating into capture. Get the hood type and position right and the system works; get them wrong and no fan can rescue it. The hood is where LEV is won or lost.
Questions
Because it determines whether the airflow the system produces actually reaches the contaminant. Everything else serves the hood - if the hood does not capture the contaminant, the fan power and ductwork are beside the point. The hood is where the system meets the hazard and succeeds or fails.
Because capture velocity falls off very steeply with distance - roughly with the square of it. Double the distance from a hood and the capture velocity drops to about a quarter. So a hood too far from the source captures almost nothing, however powerful the fan, because the velocity there has faded away.
An enclosing hood that surrounds the source, so the contaminant is generated inside the extraction rather than outside it. It barely has to reach, so capture is highly effective and much less sensitive to distance and draughts. Wherever a process can be enclosed, this is the strongest approach.
A receiving hood is positioned to catch a contaminant already moving towards it - like a canopy above a process that sends fume upwards - using the contaminant's own motion. A capturing hood must reach out into open air to draw a contaminant in, fighting the steep velocity falloff and cross-draughts. The receiving type is more reliable.
It measures the capture velocity at the point the contaminant is actually released - whether the airflow translates into enough velocity at the source to draw the contaminant in. This is the decisive measurement, because a system can have a healthy fan and still fail if the hood is the wrong type or too far away.
Hood problems are a common finding - a hood moved, a process that has crept away from it, or one never close enough. The fix is usually at the hood: bring it closer to the source, enclose the source better, or reposition it to catch the contaminant's movement, restoring the capture velocity.
Our thorough examination measures the capture velocity at the point your contaminant is released - testing the hood, the decisive part, to confirm the airflow is actually translating into capture where it matters.
