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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
It is effective because:
Good design, correct installation and commissioning are essential, or the system may not work.
When choosing a supplier:
After the LEV is installed:
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.
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.
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:
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.