The purpose of a chemical fume hood is to contain and exhaust chemical and/or radioactive material so the user and other lab occupants are not exposed. Chemical fume hoods are certified annually according to the SEFA-1 Standard. Copies of certification reports are available in EH&S.
8.1 Built-in Fire Extinguisher
If a fire starts in a hood, pull the sash down (if it can be done safely), andpull 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.
Chemical fume hoods located in the Hutchinson Center buildings are equipped with digital face velocity meters. The meter should read 100 ± 10 linear feet per minute of airflow into the hood. As the sash is moved up or down on these hoods, the airflow increases or decreases to maintain a constant face velocity.
The face velocity meter is equipped with an alarm. If the alarm sounds, use the silence button to turn it off and call Facilities Engineering to report an airflow problem. Be sure that chemical containers or waste containers left in the hood are not causing the airflow problem. These do not belong in the hood unless being actively used.
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.
Chemical fume hoods in Center lab buildings are equipped with baffles that users may adjust easily.
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 (seeFigure 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
Large containers or equipment, such as shaker tables or water baths, will interfere with airflow inside the chemical fume hood by causing reverse flows and dead spots. These dead spots may allow contaminants to escape from the hood. Placing large equipment on legs or blocks will help reduce a reverse airflow by allowing air to circulate beneath the equipment.
The chemical fume hood should not be used for storage of chemicals, waste, equipment, or other materials. Stored material in the hood interferes with airflow and may contribute to fueling a fire or block air flow. Only the containers, materials, and equipment you are actually using should be in the chemical fume hood.
When working at the chemical fume hood, open the sash only as far as necessary to access the work area. The lowered sash helps contain contaminates in the hood and the smaller hood opening makes the hood less susceptible to room drafts and other external air disturbances.
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.