Basic Documentation

Laboratory Ventilation Codes and Standards
Siemens Industry, Inc. 138
Topic Requirement(s)
Commentary
Manifolded
Systems
American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5-2003
5.3.2.2 Laboratory chemical hoods may be combined into a common manifold with the
following exceptions and limitations:
• Each control branch shall have a flow-regulating device to buffer the fluctuations in
pressure inherent in manifolds.
• Perchloric acid hoods shall not be manifolded with nonperchloric acid hoods unless
a scrubber is installed between the hood and the manifold.
• Where there is a potential contamination from hood operations as determined by the
Hazard Evaluation and Analysis of Section 2.4, radioisotope hoods shall not be
manifolded with nonradioisotope hoods unless in-line HEPA filtration and/or another
necessary air-cleaning system is provided between the hood and the manifold.
• Carbon bed filters shall be added for gasses.
5.3.2.3 Exhaust streams that contain concentrations of flammable or explosive vapors at
concentrations above the Lower Explosion Limit (LEL) as well as those that might form
explosive compounds (that is perchloric acid hood exhaust) shall not be connected to a
centralized exhaust system. Exhaust streams comprised of radioactive materials shall
be adequately filtered to ensure removal of radioactive material before being connected
to a centralized exhaust system. Biological exhaust hoods shall be adequately filtered to
remove all hazardous biological substances prior to connection to a centralized exhaust
system.
5.3.2.4 Unless all individual exhausts connected to the centralized exhaust system can
be completely stopped without having a hazardous situation, provision shall be made for
continuous maintenance of adequate negative static pressure (suction) in all parts of the
system.
5.3.2.12 Manifolds shall be maintained under negative pressure at all times and be
provided with at least two exhaust fans for redundant capacity. Emergency power shall
be connected to one or more of the exhaust fans where exhaust system function must
be maintained even under power outage situations.
ASHRAE, 2011 Handbook, HVAC Applications, Laboratories, Pg. 16.10 Types of
Exhaust Systems:
In addition, fume hoods or other devices in which extremely hazardous or radioactive
materials are used should receive special review to determine whether they should be
connected to a manifolded exhaust system.
VAV type fume hoods that have exhaust
control devices (that is, a damper or air valve)
will inherently regulate the airflow to respond
to static pressure fluctuations in the
manifolds.
Manifolded systems are also referred to as
centralized exhaust systems and combine all
laboratory room general exhausts and all
fume hood exhausts. They typically provide a
longer service life than an individual fume
hood exhausts because a centralized exhaust
system has higher overall dilution due to the
contribution of the room general exhaust to
the fume hood exhaust. In addition, the larger
and usually higher exhaust stack(s) of a
centralized exhaust system will provide higher
and better exhaust dispersion than a host of
smaller (individual fume hood) exhaust
stacks. Experience has demonstrated that
fewer larger exhaust fans provide more
reliability than a large number of small
(individual fume hood) exhaust fans
‘scattered’ over a large portion of the roof.
Many additional benefits are possible with a
manifolded exhaust system including:
• Lower ductwork cost.
• Fewer pieces of equipment to maintain.
• Fewer roof penetrations.
• Opportunity for energy recovery.
• Centralized locations for exhaust
discharge.
• Ability to take advantage of exhaust
system diversity.
• Ability to provide a redundant exhaust
system by adding a spare fan per
manifold.