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LEV testing - design
The systems that pass their thorough examination and test are the ones designed to control exposure in the first place. Get the design right and testing confirms it; get it wrong and no amount of re-testing will save it. Here is what good design gets right.
The short answer
An LEV system passes testing when it is designed to control exposure: hoods positioned close to the source, capture velocity high enough to draw the contaminant in, ductwork sized to carry it away, a fan matched to the system, and commissioning to prove it works from day one. Access points and airflow indicators built in make it testable and maintainable. Design decides the result; testing only confirms it.
Principle one
The single most important design decision is getting the capture point close to where the contaminant is released. The effectiveness of a hood falls away rapidly with distance, so a hood placed near the source captures reliably while the same hood set back a little may barely reach the plume. Enclosing the source as far as the work allows - a booth, a partial enclosure, a close-fitting hood - is always better than trying to catch contaminant across open space.
This is where many systems fail before they are ever tested. A hood chosen for convenience rather than positioned for capture leaves the contaminant to escape into the operator's breathing zone, and no increase in fan power downstream fixes a hood that is simply in the wrong place. Capture at source is the foundation everything else rests on.
Principle two
A hood has to pull air fast enough at the source to overcome the way the contaminant is released and any air movement in the room. Different processes need different capture velocities - a fine, still-air dust is easier to catch than fume thrown off with energy or a source buffeted by draughts - and the design has to provide enough velocity for the specific job, at the actual distance the source sits from the hood.
Under-provide it and the system tests as under-capturing, however healthy the airflow looks elsewhere. Over-provide it wastefully and you pay to move more air than you need. Good design matches the capture velocity to the process and the geometry, which is exactly what a thorough examination later measures against.
Principle three
Beyond the hood, the ductwork has to carry the captured contaminant away without letting it settle. That means a transport velocity high enough to keep dust and mist moving rather than dropping out and blocking the run, ducting sized and routed to balance the branches, and a fan matched to the whole system's resistance. A system where one branch robs air from another, or where the fan is fighting an undersized duct, will under-perform somewhere no matter how good the hoods are.
Designing the hoods, ducts and fan as one balanced system - rather than bolting parts together - is what makes performance predictable and even across every capture point. It is also what stops a later addition, like an extra workstation, quietly starving the rest.
Principle four
A system designed to pass is a system designed to be tested and maintained. That means building in test and access points so airflow can be measured where it matters, fitting simple airflow indicators so a problem shows up between examinations, and commissioning the system when it is installed to establish a benchmark of how it performs when working correctly. Every future thorough examination and test is measured against that benchmark.
These are cheap to design in and expensive to retrofit. A system without accessible measuring points is hard to examine honestly; one with no commissioning baseline gives a first examiner nothing to compare against. Designing them in from the start is what makes the system verifiable for its whole life.
Getting it right
Put the principles together and the theme is consistent: an LEV system passes its test because it was designed to control exposure, not because it was tuned to satisfy a measurement. Capture at source, adequate velocity, a balanced and properly sized system, and access and commissioning built in - get those right and the thorough examination and test confirms what the design already delivers.
It also pays off over the system's life. A well-designed system is easier to maintain, cheaper to run, and far less likely to fail a future test, because it is doing the job properly rather than just scraping a pass. The best time to make sure an LEV system will pass is before it is installed - and involving competent advice at the design stage is what turns that intention into a system that genuinely protects people.
Questions
Design that controls exposure: hoods close to the source, adequate capture velocity, ductwork and fan sized as one balanced system, and commissioning plus built-in access so it is testable. Testing confirms good design; it cannot rescue bad design.
Because a hood's effectiveness falls off rapidly with distance. A hood close to the source captures reliably; the same hood set back may barely reach the plume, and no extra fan power downstream fixes a badly placed hood.
The speed at which a hood pulls air at the source - enough to overcome how the contaminant is released and any room air movement. It has to be matched to the specific process and the distance the source sits from the hood.
Because they work as a system. Ducts need enough transport velocity to keep contaminant moving, branches need balancing, and the fan must match the system's resistance - or one point under-performs however good the hoods are.
Test and access points so airflow can be measured, airflow indicators so problems show between examinations, and commissioning to establish a performance benchmark that future tests are compared against.
Far cheaper to design it right. Access and commissioning are cheap to build in and expensive to retrofit, and a well-designed system is easier to maintain, cheaper to run and less likely to fail a future test.
We advise at design and commissioning and test to COSHH and HSG258, so your LEV controls exposure and passes - by design.
