Local exhaust ventilation

The hood

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 (e.g. downdraft table) or attached to a tool. 

The ducts

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

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. 

Discharge stack

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.  

Good design, correct installation and commissioning are essential, or the system may not work. When choosing a supplier:

• ask about their qualifications and experience in supplying, installing and commissioning LEV and whether they can supply references
• ask 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 details of the work and the contaminant, and photographs or drawings of the work area
• ask to visit their work area
• provide information about any special requirements, e.g. 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, 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 (e.g. fan bearings), non-moving parts (e.g. ducts, hoods) that may wear, and parts that require routine replacement or cleaning (e.g. 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.