What's measured
Ductwork is the least glamorous part of an LEV system and the most common cause of failure found at test. A leak, a crushed section or a blockage quietly robs the hood of the airflow it needs - and it does it out of sight, behind the machine.
The short answer
Ductwork is the least glamorous part of an LEV system and the most common cause of failure found at test. A leak, a crushed section or a blockage quietly robs the hood of the airflow it needs, and it does it out of sight, behind the machine where nobody looks.
The detail
The key number for a duct is its transport velocity - the minimum air speed inside the duct needed to keep particulate suspended and moving rather than settling out. For most industrial dusts that sits between roughly 15 and 25 metres per second, with the exact figure depending on particle size and density: light welding fume needs less, heavy or coarse dust needs more. Drop below it and the dust starts to fall out of the air stream and collect along the bottom of the duct.
That is where a leak does its worst damage. A gap upstream lets the fan draw easy air through the hole instead of through the hood, so capture at the hood falls. At the same time the velocity in the duct drops, dust begins to settle, the settled dust narrows the bore, the narrower bore drops the flow again - a self-feeding spiral that ends in a blocked duct and a hood pulling almost nothing. Flexible hose is the usual weak point, crushing where it bends or splitting at the cuffs, and a poorly made joint leaks air inward the whole time the fan runs.
What it means for you
Degraded ductwork is the single most common cause of LEV underperformance found during a thorough examination, which is why the examiner reads velocity and static pressure along its length to find where air or speed is being lost, checks joints, seams and flexible hose for leaks and splits, and looks for the tell-tale settled dust that signals low velocity. Short runs, smooth bores, gradual bends instead of sharp elbows and properly sealed joints are what keep transport velocity up.
The lesson is simple: a hood can be perfectly designed and perfectly placed and still fail its test because the duct behind it leaks. The ductwork is not passive plumbing - it is an active part of the control, and it deserves the same scrutiny as the hood and the fan. In practice the examiner will often traverse the duct at several points, because a reading that looks healthy near the fan can hide a branch upstream that has all but stopped pulling.
The service behind the guide
We trace velocity and static pressure along your ductwork to find leaks, crushed sections, blockages and settled dust, with a clear report and remedial actions.
Questions
It is the minimum air speed inside the duct needed to keep particulate suspended and moving rather than settling out. For most industrial dusts it is roughly 15 to 25 metres per second, depending on particle size and density.
A leak lets the fan draw easy air through the gap instead of through the hood, so capture at the hood falls. It also lowers the velocity in the duct, which can let dust settle and gradually block it.
When velocity drops below the transport velocity, dust settles along the duct. Settled dust narrows the bore, the narrower bore drops the flow further, and the spiral can end in a fully blocked duct.
Very. Degraded ductwork is the single most common cause of LEV underperformance found during a thorough examination and test.
The examiner reads velocity and static pressure along the duct to find where flow is lost, checks joints, seams and flexible hose for leaks and splits, and looks for settled dust that signals low velocity.
Phoenix Duct Clean · by the numbers
We trace velocity and pressure along your ductwork to find leaks, blockages and settled dust, with a clear report and remedial actions.