Local exhaust ventilation
Local exhaust ventilation (LEV) captures air contaminants at their source.
LEV is necessary in circumstances where you are not able to eliminate the hazard, substitute a less hazardous substance, or isolate the process from people, and
- the contaminant is harmful, toxic or corrosive (such as lead fumes, acid mist, solvent vapour);
- contaminant levels are high;
- contaminants must be removed before release into the air; or
- the process gives off heat.
It is effective because:
- it minimises employee exposure to contaminants;
- the volume of exhaust air is much less than for general ventilation;
- the contaminant can be collected for disposal or recovery;
- equipment in the workplace is protected from heat and corrosive substances; and
- employees may not have to wear respiratory protection.
Supply, installation and commissioning
Good design, correct installation and commissioning are essential, or the system may not work.
When choosing a supplier:
- Ask potential suppliers about their qualifications and experience in supplying, installing and commissioning LEV and whether they can supply references.
- Ask potential suppliers how they will demonstrate the system works as designed and whether they can provide your staff with information and training on how to use and check the system.
- Provide the potential suppliers with details of the work and the contaminant, and photographs or drawings of the work area.
- Ask potential suppliers to visit the work area.
- Provide information about any special requirements, eg. flammable gases or combustible dusts.
- Provide information about the level of control you require.
- Get three or more quotes (consider upfront and ongoing costs).
After the LEV is installed:
- The supplier should commission the LEV to check it works as designed, and provide you with information (commissioning report) to demonstrate this.
- Ensure you have enough information to use and maintain the system correctly (user manual).
- Create a log book to schedule and record inspection and maintenance.
- Train the relevant workers on how to correctly use, inspect and maintain the LEV.
Inspection and maintenance
Inspection, maintenance and cleaning are vital to the efficiency of local exhaust ventilation systems. Easy access for cleaning and removing captured contaminants and test points to carry out checks are both important considerations when installing a system.
Checks and maintenance should include moving parts (eg. fan bearings), non-moving parts (eg. ducts, hoods) that may wear, and parts that require routine replacement or cleaning (eg. filters). An anemometer is useful to check that the face velocity (airflow at the front of the hood) meets the design requirement, or smoke tubes can be used to see if the system is working.
Major parts of LEV systems
Hoods come in a variety of designs. The one you choose should enclose or be located as close as possible to the contaminant source and be matched to the particular process. The hood should be designed so it captures contaminants as they are given off, and does not draw the air past the worker’s breathing zone. A hood does not have to be above the contaminant source; it may also be behind the contaminant source, below the contaminant source (eg. downdraft table) or attached to a tool.
Ducts carry the contaminants from the capture point to an outlet point. Air velocity in the ducting must be high enough to prevent contaminants settling in the system, but not so high that it causes vibration and noise problems. To ensure the correct balance is obtained, consult an occupational hygienist or ventilation engineer.
The fan pulls air into the system. Centrifugal fans are generally best for high pressures, and axial fans are best for low pressure/high volume applications. The fan must have enough power to capture the contaminant. Ask for data on noise levels before choosing a fan.
Filters or air cleaners
Air cleaning equipment captures contaminants in the extracted air and lets clean air continue through. The air cleaning equipment you install will depend on the type and volume of contaminants.
The discharge stack releases exhaust gas into the air. It must be high enough to avoid gas re-entering the workplace and make sure contaminant levels on the ground are within clean air standards. Stacks should be at least two metres above the highest roof or adjacent building and away from air inlets. Weather caps aren't a good idea as they can hinder vertical discharge; however an external 'sleeve' around the stack can be used to prevent water coming in. Contact the Department of Environmental Regulation to find out if there are environmental regulations for contaminant discharge for your industry.
Regulation 5.20 of the Occupational Safety and Health Regulations 1996 requires that, as far as practicable, employers must reduce the risks from hazardous substances. The regulation also requires that control measures such as ventilation be used in preference to personal protective equipment where practicable.
The Occupational Safety and Health Act 1984 requires employers to provide employees with a safe and healthy work environment, and employees to take reasonable care with their own safety and health and the safety and health of others at work.
An employer shall:
- ensure employees are not exposed to workplace safety and health hazards;
- provide employees with adequate information, instruction and training;
- consult with safety and health representatives and other employees about workplace safety and health; and
- where it is impracticable to prevent exposure to hazards, provide employees with adequate personal protective clothing and equipment.
- take reasonable care of their own health and the health of others; and
- co-operate with employers in carrying out safety and health requirements.
Health and Safety Executive (2011) - Clearing the air.
ACGIH (2013). Industrial Ventilation: A manual of recommended practice for design, 28th edition. Available for purchase from www.acgih.org.
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