Fume hoods are designed to remove vapors from work performed in the hood thus preventing fumes from entering the laboratory. The type of hood chosen depends upon the work to be performed inside the hood. A standard fume hood consists of an enclosure, a blower, baffles, and a sash. An air face velocity must be maintained whether the sash is all the way up or most of the way down. In the basic hood all the air entering the hood comes from under the sash, flows through baffles in the back and at the top of the hood and leaves the building through the ductwork via the blower. The size of the blower will depend not only upon the size of the hood but the length of the ductwork and the path, eg., how many turns the duct-work takes before it exhausts outside. A measure of the duct static pressure will be useful in choosing the proper size blower motor. ANSI/AIHA Z9.5 American National Standard for Laboratory Ventilation guidelines recommends a face velocity of 80 to 120 feet per minute for fume hoods. ASHRAE* Standard 110-95 is a test which should be performed at the time of installation. This test determines whether the hood is functioning properly and whether it will protect the workers in the lab.
The type of blower also depends upon the chemicals used in the hood. Flammable chemicals require a fiberglass blower with an explosion proof motor. Perchloric acid hoods require a blower which can handle very corrosive atmospheres. These blowers have a PVC housing with PVDF impellers.
Consideration for hood placement will include the availability of sufficient room air to maintain the face velocity required. The national Fire prevention Association (NFPA) Code 45 calls for replacing the room air with slightly less air than required by the hood. The resulting negative pressure in the room with the hood will pull air from surrounding corridors and prevent fumes from entering the building.
Hood suppliers such as Kewaunee and Labconco have guides for choosing the correct fume hoods and blowers for your application.
*American Society of Heating, Refrigerating and Air-Conditioning Engineers