A local exhaust ventilation system usually consists of a hood to capture contaminants, a system of ducts, and an exhaust fan to transport impurities outside. Air cleaners can also remove harmful types of dust or particulates from the air. If done appropriately, the cleaned air can often be recirculated, saving money on heating and cooling.
• Sample I – Local Exhaust Ventilation • Sample II – Local Exhaust Ventilation
Hood types for local exhaust ventilation systems vary in design, depending on specific use. Canopy hoods are used over electric kilns where heated contaminants often rise. Slot exhaust hoods are used for cleaning etching plates where contaminants can be captured at the workbench. Enclosed hoods are often used for acids that are dangerous to nearby users and materials. Spray booths are used for spraying paint and glazes, as these contaminants can easily float in the air to distant sites. Movable exhaust hoods are used for welding at multiple sites. Sawdust-collecting hoods are designed for woodshops. Many art practices can use either a slot exhaust hood or an enclosed hood to capture hazardous materials and eliminate them from the air supply.
Basic rules for operating local exhaust systems:
• Provide enough clean air to replace exhausted air. • Enclose the process as much as possible to ensure effective capture of contaminants. • Place the hood as close to the toxin-producing activity as possible. • Have as few bends in the ductwork system as possible. Make sure the bends are gradual, not sharp. • Locate fans outside the room so that all ducts are under negative pressure (like a vacuum), which draws air and contaminants into the exhaust system, and removes them to the outside. • Do not recirculate any of the exhausted air, unless you are using a dust collection system designed for recirculation. • Make sure exhausted air cannot reenter the room or the intake vents for other air systems. • Schedule regular maintenance. • When exhausting solvents or other flammable materials, use spark-proof (aluminum) fan blades and place fan motors outside the airflow stream so that sparks from the motor do not ignite the flammables.
Checking a Local Exhaust System
A local exhaust system, such as a spray booth or slot hood, should capture the contaminants before they get into the air that you breathe. If you can smell gases or vapors, or see contaminant dusts or mists floating in the air or settling on surfaces, the hood is not working properly. You can also use commercial-grade smoke tubes to generate a haze that will help you see the direction of airflow in a room. A less expensive alternative is a child's soap bubble kit. A properly working system will draw the bubbles (or smoke) steadily into the hood. If bubbles pass your face while you are in a working position, the toxic contaminants are being pulled into your breathing zone, where you can inhale them. If tests indicate that the local exhaust ventilation system is not working correctly, then look for these common problems:
• Is there adequate makeup air? One of the most common problems with local exhaust systems is insufficient fresh air from outside ("makeup air") to replace the air being exhausted. Insufficient makeup air weakens the performance of the exhaust system. • Is the makeup air source positioned properly? Placing the makeup air source too close to the local exhaust hood can create turbulence and/or blow contaminants out of the hood and by your face. Use soap bubbles or smoke to detect turbulence. • Are there cross-currents? Local exhaust hoods are often very sensitive to cross-currents caused by heavy traffic around the hood, nearby air conditioners, doorways that are opened or closed, etc. Check the movement of the bubbles or smoke to ensure they flow into the hood in common work conditions. • Is the activity adequately enclosed? A hood generally performs more effectively when it more completely encloses a work process. • Is the exhaust air being recirculated? Recirculation of exhaust air means that hazardous contaminants are being dispersed back into the space. Again, soap bubbles can be used to follow the path of exhausted air away from the fresh air intakes.
Following these simple tests and guidelines can help you ensure that your ventilation system is working properly. Check the Resource section of this chapter for recommended books and organizations that can also assist in addressing your ventilation concerns. In addition, for a full evaluation of your ventilation system, it is best to consult with a ventilation engineer.