Basic Documentation

Exhaust Systems
Siemens Industry, Inc. 147
Topic Requirement(s) Commentary
Materials and
Fire
Protection
National Fire Protection Association, Standard NFPA 45, 2011
A.8.1 NFPA 90A, Standard for the installation of Air-Conditioning and Ventilating
Systems, and NFPA 91, Standard for Exhaust Systems for Air Conveying of Vapors,
Gases, Mists, and Noncombustible Particulate Solids, contain additional requirements
for general environmental ventilating systems.
8.2.1 Laboratory ventilation systems shall be designed to ensure that fire hazards and
risks are minimized.
A.8.2.1 For additional information on laboratory ventilation, see ANSI/AIHA Z9.5,
Laboratory Ventilation. For information on preventing the spread of smoke by means of
utilizing supply and exhaust systems to create airflows and pressure differences
between rooms or building areas, see NFPA92A, Standard for Smoke-Control Systems
Utilizing Barriers and Pressure Differences.
A.8.2.2 It is not the intent of this standard to require emergency or standby power for
laboratory ventilation systems
8.2.5 Exhaust and supply systems shall be designed to prevent a pressure differential
that would impede egress or ingress when either system fails or during a fire or
emergency scenario. This design includes reduced operational modes or shutdown of
either
the supply or the exhaust ventilation system.
8.2.6 The release of chemical vapors into the laboratory shall be controlled by
enclosure(s) or captured to prevent any flammable and/or combustible concentrations
of vapors from reaching any source of ignition.
8.5.1 Ducts from chemical fume hoods and from local exhaust systems shall be
constructed entirely of noncombustible materials except in the following cases:
(1) Flexible ducts of combustible construction shall be permitted to be used for special
local exhaust systems within a laboratory work area.
(2) Combustible ducts shall be permitted to be used if enclosed in a shaft of
noncombustible or limited-combustible construction where they pass through non-
laboratory areas or through laboratory units other than the one they serve. (See 8.5.2)
(3) Combustible ducts shall be permitted to be used if all areas through which they
pass are protected with an approved automatic fire extinguishing system. (See 8.5.2)
8.5.2 Combustible ducts or duct linings shall have a flame spread index of 25 or less
when tested in accordance with ASTM E 84, Standard Test Method for Surface
Burning Characteristics of Building Materials, or ANSI/UL 723, Standard for Test for
Surface Burning Characteristics of Building Materials. Test specimens shall be of the
minimum thickness used in the construction of the duct or duct lining.
8.5.3 Linings and coatings containing such fill as fiberglass, mineral wool, foam or
other similar material that could accumulate chemical deposits shall not be permitted
within laboratory exhaust systems.
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Laboratory exhaust systems are most
commonly comprised of welded stainless
steel; however this is not necessarily the
most appropriate material for all applications
(see the ASHRAE reference). When
selecting a material, consideration should
also be given to the usage factor. If highly
corrosive vapors will seldom be present and
the overall use of the hoods will not be
intensive, a lower cost and more easily
fabricated material may be a more cost
effective choice.
Metal ductwork has good fire rating
characteristics but in many instances has
inferior corrosion resistance to ductwork
comprised of non-metallic material such as
PVC. However non-metallic ductwork may
not comply with the local building code.
The design of the exhaust system and the
quality of the fabrication is often a more
important factor than the material itself. A
stainless steel exhaust system with welds of
marginal quality and comprised of poor
overall general workmanship may provide
less service life than a well constructed
exhaust system of galvanized steel. Duct
fittings and horizontal runs where
condensation can collect are most
susceptible to corrosion.