8.0 Chemical Fume Hoods

8.1       Built-in Fire Extinguisher

If a fire starts in a hood, pull the sash down (if it can be done safely), and pull the fire alarm. If the fire is small and you have been trained to use a fire extinguisher, do so. If the fire is large or there are flammable solvents in the hood, do not attempt to fight the fire. Evacuate the building and let the built-in extinguisher do its job.

8.2       Face Velocity Meters

8.3       Alarm and Purge

The face velocity meter is also equipped with a purge feature that can be used to increase the air velocity to the maximum level in case of a volatile chemical spill in the hood or in the lab.

If a spill occurs in the hood, close the sash and press the red purge button. If a spill occurs in the lab, open the sash all the way and press the red purge button before evacuating the lab. In either case, notify EH&S of the spill.

8.4       Adjustable Baffles

Under most conditions, contaminates generated within a chemical fume hood mix quickly within the hood and the mixture will have a density nearly the same as air. In this case, the baffle position shown in Figure III.1 (A) gives the best performance as it provides good flow in both the lower and upper parts of the hood work area.

Figure III.1: Chemical Fume Hood Baffles

                                                A                                             B                                                             C

When a hot plate or other hot process is used in the hood, the heat will cause contaminates to rise quickly. In this case, the baffle position shown in Figure III.1 (B) allows for more air to be exhausted from the upper part of the hood and gives the best performance.

If large volumes of dense vapors (e.g., chlorinated solvents) are being generated within the hood, these vapors will tend to sink. In this case, thebaffle position shown in Figure III.1 (C)gives the best performance as it increases the volume of air exhausted from the lower area of the hood.

8.5       Maintaining the Air Barrier

When you stand in front of a chemical fume hood, the air passing around your body forms a zone of low air pressure directly in front of you which extends into the hood about four inches (see Figure III.2 below). Since contaminates may enter this turbulent area from inside the hood and expose you outside the hood, keep hazardous materials at least six inches inside the hood, behind this protective air barrier.

Figure III.2: Chemical Fume Hood Protective Air Barrier

The farther behind the protective air barrier you place the source of contaminants, the greater protection the hood provides you. Therefore, you should place contaminates as far back in the hood as you can without blocking the bottom baffles or creating an ergonomic hazard when reaching.

8.6       Large Equipment in the Fume Hood

8.7       No Storage in the Fume Hood

8.8       Closed Sashes Save Money

The chemical fume hoods in the Center’s buildings are part of a variable air volume system. This system increases the volume of exhausted air as the sash opening increases. By closing the sash when the hood is not in use, you are helping the Center save money on utilities.