Basic Documentation

Chapter 6–Laboratory Containment Units - Ventilation
Note that the primary purpose of this fume hood control arrangement is to
maintain an average face velocity in accordance with current safety
standards and regulatory requirements.
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Precise average face velocity
control is attained by the control arrangement of Figure 9, since it
mathematically determines the required total exhaust airflow rate necessary
to yield the desired average face velocity for any size of fume hood face
opening.
Aside from maintaining the required average face velocity, a second function
of a VAV fume hood controller is to always maintain a minimum fume hood
exhaust rate when the sash is totally closed. Since a fume hood always has
a certain amount of fixed open area, an adequate airflow should always be
maintained through these openings to ensure proper containment. In
addition, corrosive fumes must not be allowed to build up and remain within
the fume hood since this would be detrimental to the fume hood interior itself
as well as any experimental apparatus or instrumentation within the fume
hood. Lastly, the evaporation of volatile organic chemicals and the presence
of flammables without adequate dilution can result in a flammable or
explosive atmosphere within the fume hood. Thus, good laboratory
ventilation system design in accord with current fire protection standards,
requires maintaining a minimum fume hood exhaust rate of at least 20% of
the maximum fume hood exhaust rate.
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The ventilation system designer must coordinate the VAV fume hood control
system with control of the overall room ventilation system. An increase or
decrease in fume hood exhaust to maintain the required average face
velocity necessitates a corresponding change in laboratory room supply air
to maintain the correct amount of makeup air for the fume hood and air
balance within the room. Details on these factors are provided in the Room
ventilation, makeup air and pressurization control systems section.
• Two position, constant volume bypass fume hoods–This type of fume
hood really consists of a control arrangement that is applied to a regular
constant volume bypass type fume hood to save energy during unoccupied
periods if the sash is closed. A two-position constant volume bypass fume
hoods provide a substantial savings in energy in comparison to the regular
constant volume bypass fume hood
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. However, it should be noted that
during the regular occupancy period of a facility, a two position constant
volume fume hood still operates exactly as a regular constant volume bypass
fume hood. Since both types of fume hoods (one- and two-position constant
volume) do not have any provision to control face velocity or reduce the fume
hood exhaust airflow with respect to the sash opening, no reduction in fume
hood exhaust airflow is attained during occupied periods. The reduction in
fume hood exhaust airflow only is in effect during a facility’s unoccupied
period provided that the fume hood sash is fully closed.
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The American Industrial Hygiene Association’s (AIHA) American National Standard for Laboratory Ventilation
AIHA/ANSI Z9.5 states “Each hood shall maintain and average face velocity of 80 to 120 fpm with no face velocity
measurement more than or minus 20% of the average.”
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The National Fire Protection Association’s Standard NFPA 45 “Fire Protection for Laboratories Using Chemicals”
requires maintaining a minimum fume hood exhaust of 50 cfm per lineal foot of interior width. For a 100 fpm face
velocity, this equates to approximately 20% of the maximum fume hood exhaust airflow rate.
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Since the unoccupied period of a facility may be 50% to 70% of the total hours in a year, a significant energy savings
can be achieved by reducing the fume hood exhaust airflow down to only about 20% of what the exhaust airflow rate is
during the occupied period.
52 Siemens Building Technologies, Inc.