Basic Documentation

Exhaust Systems
Siemens Industry, Inc. 139
Topic Requirement(s) Commentary
Air Velocity
National Fire Protection Association, Standard NFPA 45, 2011
8-6 Duct Velocities. Duct velocities of laboratory exhaust systems shall be high
enough to minimize the deposition of liquids or condensable solids in the exhaust
systems during normal operations in the chemical fume hood.
American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5-2003
5.3.5 A minimum discharge velocity of 3000 fpm (15.2 m/s) is required unless it can
be demonstrated that a specific design meets the dilution criteria necessary to reduce
the concentration of hazardous materials in the exhaust to safe levels (see Section
2.1) at all potential receptors.
Good design practice has generally
established a 1,000 fpm as an adequate
velocity within exhaust system ductwork. This
ensures that fine particulate is transported and
that excessive condensation will not occur. An
adequate stack discharge velocity should be a
minimum of 3,000 fpm.
Since chemical fume condensation droplets
will be carried upward at air velocities of 2,600
fpm, it is advisable to design and size exhaust
stacks so the internal upward velocity is no
more than 2,400 fpm in order to allow
condensate to trickle down the inside walls of
the stack to a roof drain. By also utilizing a
conical shaped top on the stack the discharge
velocity can be increased to at least 3,000 fpm
in compliance with the standards and good
design practice.