Local Exhaust Ventilation Design

LEV is an extract ventilation system that takes dusts, mists, gases, vapour or fumes out of the air so that they can’t be breathed in. Properly designed LEV will:

• collect the air that contains the contaminants;
• make sure they are contained and taken away from people;
• clean the air (if necessary) and get rid of the contaminants safely.

A typical LEV system will have:

• Hood(s) – to collect airborne contaminants at, or near, where they are created (the source).
• Ducts – to carry the airborne contaminants away from the process.
• Air cleaner – to filter and clean the extracted air.
• Fan – which must be the right size and type to deliver sufficient ‘suck’ to the hood.
• Discharge – for the safe release of cleaned, extracted air into the atmosphere.

If your work produces dust (eg flour dust in bakeries), mist (eg paint mist from spraying), fume (eg from welding), gas (eg carbon monoxide from furnaces) or vapour (eg solvents from painting), there may be a risk to the health of your employees.

The law says you must control the risks from hazardous substances (the Control of Substances Hazardous to Health Regulations (COSHH)). Installing LEV may help you to do this.

When developing exposure control measures, ‘process’ means the way airborne contaminants are generated, for example, in woodworking the processes would be cutting, shaping and sanding. The source is where the contaminant is generated by a process. Understanding the process means understanding the creation of ‘sources’. This can suggest ways to modify the process to reduce the number or size of sources, and contaminant clouds. The effective application of LEV requires a good understanding of the process and the sources.

It is crucial that the LEV system designer understands how processes generate sources and how contaminant clouds flow away from source.

General ventilation involves replacing contaminated workplace air with cleaned or fresh air. Dilution or mixing ventilation (clean air dilutes contaminated workplace air by mixing with it) and displacement ventilation (clean air pushes contaminated air away with minimal mixing) are two forms of general ventilation.

LEV involves capturing the contaminant at source before it can mix with the workplace air.

LEV might not be the right control solution when:

• there are a large number of widely-spaced sources;
• the source is large and LEV is impossible to apply over the entire source;
• the source position is not fixed;
• the source emits relatively small amounts of contaminant;
• the contaminant is offensive but not harmful.

(Contains public sector information published by the Health and Safety Executive and licensed under the Open Government Licence).