What's measured
The hood is where an LEV system succeeds or fails. Everything downstream - duct, filter, fan - only matters if the hood actually catches the contaminant at the point it is released. Get the hood type, size and position wrong and no amount of fan power will rescue it.
The short answer
The hood is where an LEV system succeeds or fails. Everything downstream - duct, filter, fan - only matters if the hood actually catches the contaminant at the point it is released. Get the hood type, size and position wrong and no amount of fan power will rescue it.
The detail
Hoods fall into three broad types, and they are not equal. An enclosing hood surrounds the source - a booth, a cabinet, a partial enclosure - and gives the best control because the contaminant is released inside the extracted space. A receiving hood sits in the path of a stream that the process throws or floats towards it, like a canopy over a rising heat plume. A capturing hood has to reach out into open air and pull the release back, and it is the least forgiving: for a plain opening the velocity collapses with distance, so it only works if it sits close to the source, and a flange or partial enclosure noticeably extends its reach.
Whichever type it is, the hood has to match the process and the tool. A hood that is incompatible with the tool it serves, or damaged, weakened or knocked out of position, loses control even if every other part of the system is sound. That is why a well-designed hood placed close to the source will out-perform a poorly placed one on a far bigger fan.
What it means for you
In a thorough examination the examiner checks the hood type and position against the process and the design, measures face velocity and judges capture at the source, and looks at how the hood is actually used - whether operators move it away or work outside its effective zone. A hood close to the source needs far less airflow to control than a distant one, so hood design is the single highest-leverage decision in the whole system.
If the hood is wrong, the test flags it, because everything else - the duct, the filter, the fan - is effort spent moving air that never catches the contaminant in the first place. Fixing the capture point is almost always cheaper and more effective than chasing airflow further down the system.
The service behind the guide
We check hood type, position and capture against the process the hood serves, then measure face velocity and capture to confirm it, with a clear report and remedial actions.
Questions
An enclosing hood, which surrounds the source so the contaminant is released inside the extracted space. Receiving hoods catch a stream thrown towards them, and capturing hoods reach out into open air and are the least forgiving.
For a capturing hood the air velocity falls off very quickly with distance, so it only works if it sits close to the source. A hood a short distance too far away can drop below the velocity needed to catch the release.
Yes. A flange or partial enclosure around a hood opening noticeably extends its effective reach for the same airflow, because it stops the hood drawing in useless air from behind it.
Yes. A hood that is incompatible with the tool or process it serves, or is damaged or mispositioned, loses control even when the duct, filter and fan are all sound.
The examiner checks hood type and position against the process and design, measures face velocity and capture at the source, and looks at whether operators use the hood correctly rather than working outside its zone.
Phoenix Duct Clean · by the numbers
We check hood type, position and capture against the process it serves, then measure to confirm it, with a clear report and remedial actions.