Basic Documentation

Exhaust Systems
Siemens Industry, Inc. 141
Topic Requirement(s) Commentary
Stack Height
and
Discharge
Location
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National Fire Protection Association, Standard NFPA 45, 2011
8.4.12 Air exhausted from chemical fume hoods and special exhaust systems shall
be discharged above the roof at a location, height, and velocity sufficient to prevent
re-entry of chemicals and to prevent exposures to personnel.
A.8.4.12 Exhaust stacks should extend at least 3 m (10 ft) above the highest point on
the roof to protect personnel on the roof. Exhaust stacks might need to be much
higher to dissipate effluent effectively, and studies might be necessary to determine
adequate design. Related information on stack height can be found in Chapter 14,
Airflow Around Buildings, of the ASHRAE Handbook of Fundamentals.
National Sanitation Foundation, NSF 49 -2008, Biosafety Cabinetry: Design,
Construction, Performance and Field Certification
2.3 Roof Exhaust Systems
Roof exhaust systems serving biosafety cabinets should have a stack that extends
straight upward at least 10 ft (3 m) above the roof surface to avoid re-entrainment by
the building, and should be increased in elevation when necessary to avoid the
influence of surrounding structures. Raincaps or any other structure that deflects the
straight upward flow of the discharged air should be avoided. No precipitation can
enter the stack when air is being exhausted at normal stack velocities. To take care of
precipitation during periods when system is shut off, a 1 in. (2.5 cm) hole can be
drilled in the lowest point of the fan casing and the water allowed to drain onto the
roof
American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5-2003
5.3.4 The discharge of potentially contaminated air that contains a concentration
more than the allowable breathing air concentration shall be:
• Direct to the atmosphere unless the air is treated to the degree necessary for
recirculation (See Section 9.3);
• In compliance with federal, state, or local regulations with respect to air
emissions;
• Discharged in a manner and location to avoid reentry into the laboratory building
or adjacent buildings at concentrations above 20% of allowable concentrations
inside the laboratory for routine emissions or 100% of allowable concentrations for
emergency emissions under wind conditions up to 1%-wind speed for the site.
5.3.5 The exhaust stack discharge shall be in accordance with the ASHRAE 2003
HVAC Applications Handbook, Chapter 44. In any event the discharge shall be a
minimum of 10 feet (3 m) above adjacent roof lines and air intakes and in a vertical
up direction.
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