Fume Hood Operating Performance Factors
The location of the fume hood affects its efficiency and should be located in an area of minimal traffic. First, turbulence can be created just by someone walking past the face opening, causing contaminants to be drawn outside the hood. Next, if the air diffuser is located directly above the fume hood, air turbulence may be created causing contaminants to escape into the room. Finally, air flow into the room has an effect on the fume hood. All corridor entry doors should be kept closed to maintain the negative pressure of the lab. This ensures that any contaminants will be exhausted through the fume hood and not escape into the corridor.
Face velocity* is a measurement of the average velocity at which air is drawn through the face to the hood exhaust. The acceptable range of the average face velocity is generally 60-100 feet per minute (fpm). The ideal average face velocity is 80 fpm for most operations in newer hoods. Older design hoods are set at 100 fpm. Studies have demonstrated that flows greater than 125 fpm cause the creation of turbulence that causes contaminants to flow out of the hood and into the user's breathing zone.
Air Flow Indicators
Air flow indicators are small pieces of tinsel that are taped to the bottom corner of the sash. Inward movement of the tinsel indicates air is being drawn into the hood. Air flow indicators do not determine face velocity but only indicate that air is being exhausted through the fume hood.
Hood Monitor Alarm: is a more sophisticated indicator and control device that monitors the air flow exhausted by the hood. If the face velocity falls below an acceptable work range, the hood sensors will trigger an alarm to notify personnel. Hoods usually go into alarm mode either because the sash has been raised above the normal working height of 18" and/or no longer exhausting a sufficient amount of air.
*Face Velocity: air velocity at the plane of and perpendicular to the opening of an exhaust hood. (Brookhaven National Laboratory)