Fume hoods protect lab workers from hazardous chemicals by preventing toxic fumes from spreading. They are not interchangeable with biosafety cabinets. The velocity of air passing through a hood depends on the toxicity of the materials used inside. Standard, bypass, VAV, and auxiliary hoods are available.
Fume hoods, often called fume hoods, are used to protect laboratory workers from hazardous chemicals. The flow of air through a hood prevents toxic chemical fumes from wafting into the room. It is important to note that fume hoods may look similar to another laboratory safety device, the biosafety cabinet, but the two are not interchangeable: fume hoods should only be used for chemicals and biosafety hoods only for materials. biohazardous.
The typical extractor hood is a completely enclosed cabinet with a ventilation system. A door in the front of the cabinet, usually made of glass, allows users to access the inside of the hood. The doors can move horizontally or vertically, or in some cases in both directions. Some substances, such as radioactive materials and perchloric acid, require specialized fume cupboards and should only be used in such fume cupboards. Any chemicals and equipment inside a hood should be placed at least 6 inches (about 15.25cm) back from the opening for user protection.
A continuous stream of air flows through a fume hood, capturing hazardous fumes and venting them outside the building. Once the hazardous fumes are outside, their concentration in the outside air is so small that they pose no risk. The effectiveness of a hood depends on the speed of the air passing through it. The speed of the air at the front of the hood, the point at which the door is lifted to allow the user access to the materials inside, is regularly tested to ensure adequate performance.
The air velocity needed for a fume hood depends on the toxicity of the materials used inside. Common and less hazardous laboratory chemicals, such as ethanol, require only a Class C cabinet with a face velocity of 75 to 95 feet per minute (fpm) or 23 to 29 meters per minute (mpm). More hazardous materials, such as most acids, require a Class B enclosure with a face velocity of 95 to 110 fpm (29 to 33.5 mpm). Extremely toxic chemicals, including carcinogens, should be operated in a Class A cabinet with an average face velocity of 115 to 125 fpm (35 to 28 mpm). No point on the face of a Class A enclosure may have a speed lower than 100 fpm (30.5 mpm).
Standard fume hoods discharge a constant volume of air, which causes the face velocity to increase as the hood is lowered. A bypass hood uses constant air volume (CAV) technology, which always removes the same volume of air, even when the sash is closed, but doesn’t cause an increase in face velocity like a standard hood does. Variable Volume Air (VAV) and Auxiliary Hoods are designed to decrease energy consumption while maintaining the protective value of fume cupboards. VAV hoods do this by reducing the exhaust volume when the sash is down, while auxiliary air hoods draw in outside air as part of the exhaust.
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