Face velocity is the pull of air that moves fumes from the fume hood through the ductwork. One important consideration during the facility planning process is how to maintain a consistent, safe face velocity. Recommended face velocity ranges from 60 to 100 feet per minute (fpm), depending upon the contaminants being exhausted. Anything below the recommended face velocity is considered dangerous to the worker.
Factors Affecting Face Velocity
There are several factors that affect face velocity, including the type of hood installed and the room’s heating, ventilation, and air-conditioning systems. These areas should be carefully considered by engineers and architects during the fume hood laboratory design phase.
Sash position will also affect face velocity of a fume hood. Face velocity will decrease as the sash is closed. While a partially closed sash may save you money in energy costs, safety can be compromised if face velocity is not adequately measured. As a rule of thumb, be sure that the measured face velocity of the fume hood when the sash is partially closed is similar to what the velocity should be if the sash were fully open.
Room Air Patterns and Face Velocity
Room air patterns are one of the biggest factors affecting face velocity. As a matter of fact, room air patterns account for up to 50 percent of inadequate face velocity and unsafe hood performance.
Designers and architects should plan fume hood placement with this in mind. Because the opening of doors can suck large amounts of air out of the laboratory, fume hoods should be placed well away from entrances and exits. In addition, people walking can create a significant push and pull of air. Be sure that fume hoods are placed away from traffic patterns. The worker should be generating the only movement in front of the fume hood.
Face Velocity: Bigger Isn’t Necessarily Better
When working with particularly dangerous contaminants, follow the most recent safety regulations instead of increasing fpm without basis. Face velocities larger than 100 fpm are often not cost-effective, and can even reduce safety. Your fume hood design specialist can help you determine the correct face velocity for your laboratory, and can help you achieve your goal.