Highlights
This type of system is usually the preferred control method if:
- Air contaminants pose serious health risk
- Large amounts of dusts or fumes are generated
- Increased heating costs from ventilation in cold weather are a concern
- Emission sources are few in number
- Emission sources are near the workers’ breathing zones
In a general way, a local exhaust system operates similar to a household vacuum cleaner with the hose as close as possible to the place where dirt would be created.
Mechanism
Many industrial processes release airborne contaminants into the workplace. The inadequate control of these can allow them to enter the breathing zone of workers’ resulting in inhalation exposure. One method of minimizing exposure is to apply extraction at the source of the contaminant generation, thereby removing the hazard before it enters the workplace air. This technique is usually referred to as local exhaust ventilation (LEV).
Local exhaust ventilation involves the capture of air contaminants at a source. A local exhaust ventilation system consists of a hood or enclosure to capture a contaminant, an air pollution control device to clean the air, and an air mover to provide air flow through the system. These systems range from small portable units, such as a high-efficiency particulate Air filter (HEPA)-vacuum system to extensive, permanent installations typical of large industrial facilities.
LEV systems comprise of the following main elements
- Hood – this is the point where the contaminant laden air enters the LEV system. The hood design varies considerably from one system to another.
- Ducting – the ducting transports the contaminant laden air from the hood to the air cleaner, fan and finally the discharge point.
- Air filters – this filters or cleans the air.
- Blower – this is usually a fan and moves the air through the system from hood to discharge point.
- Discharge – exhaust air should be discharged to a safe place. The most common method is vertical discharge to the outside of the building.