Basic Documentation

Chapter 6–Laboratory Containment Units - Ventilation
A laboratory ventilation system designer must know the size, type and the ventilation
characteristics of the fume hoods that will be in each individual laboratory room,
since they will have a major impact on the required functionality and capacity of the
laboratory ventilation system. Since fume hoods do not normally contain integral
exhaust fans
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, their ability to provide effective containment is mostly dependent upon
the proper functionality of the laboratory facility ventilation system. The ventilation
system designer must therefore know the operational characteristics of the fume
hoods in order to design a ventilation system that will provide the exhaust and
makeup air requirements necessary for proper fume hood containment.
Fume Hood Containment
Regardless of physical size and other amenities of a fume hood, it is of foremost
importance that the fume hood provide a high degree of fume containment and
therefore protection for the user. Considerable research has been done particularly
over the last decade in terms of determining ways to maximize fume hood
containment. Among professionals and organizations that have conducted research
associated with chemical hygiene and laboratory safety, it is commonly agreed that
maintaining an adequate fume hood exhaust airflow and proper face velocity are
highly important factors contributing to a fume hoods ability to provide good
containment. However, aside from an adequate fume hood exhaust airflow and face
velocity, other factors also have a very pronounced affect on an individual fume
hood’s ability to provide good containment. A very significant factor is the velocity of
room air currents that may be blowing across the front of the fume hood. In addition,
other factors such as the proper placement of chemicals within the fume hood interior
and the practice followed by the users all play an important role in fume hood
containment.
Fume Hood Face Velocity
Face velocity refers to the speed of the makeup air that enters the open area of the
fume hood and is normally expressed in terms of feet per minute (fpm).
Research indicates that the optimum face velocity of a fume hood is mostly
independent of the size of the sash opening and should not appreciably vary
regardless of the size of the sash opening. An average face velocity of approximately
100 feet per minute (0.5 m/s) is most commonly accepted as providing the best level
of containment. It must be understood that this is not an absolute minimum or
maximum value, but is the generally accepted value for most fume hood
applications
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. It is also very important that a fume hood’s face velocity be understood
in terms of it being an average value. The actual inward airflow characteristics of a
given fume hood typically result in a higher air velocity at the mid-point a of the sash
opening and the lowest velocity at corners of the opening.
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Ductless fume hoods usually contain an integral fan to draw air into the fume hood, force it through a filter and then
exhaust it back into the room. Ductless fume hoods have limited application in a laboratory environment since the
ability of a filter to remove all possible hazardous materials has not been substantiated. Such fume hoods are more
appropriately used for limited applications where the fumes released pose only a very slight hazard and the fume hood
usage is not intensive. Ductless hoods may be applied where the rate of fume release is very low and perhaps is more
of an annoyance (that is, unpleasant odor) rather than a potential toxic or flammable hazard.
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Note however that a specific fume hood may provide better containment at a slightly higher or lower average face
velocity value. Containment versus face velocity data is a part of the overall technical data that a fume hood
manufacturer should be able to provide as a result of standard containment testing (ASHRAE 110) performed on a
given model of fume hood as manufactured.
46 Siemens Building Technologies, Inc.