Basic Documentation

Ventilation Systems
Siemens Industry, Inc. 101
Topic Requirement(s) Commentary
Room Supply
Air
U.S. OSHA, 29 CFR, Part 1910, Occupational Safety and Health Standards,
Subpart Z, Title: Toxic and Hazardous Substances,1910.1450,C3,C4:
3. Usage - The work conducted and its scale must be appropriate to the physical
facilities available and, especially, to the quality of ventilation.
4. Ventilation - (a) General laboratory ventilation. This system should: Provide a
source of air for breathing and for input to local ventilation devices; it should not be
relied on for protection from toxic substances released into the laboratory; ensure that
laboratory air is continually replaced, preventing increase of air concentrations of
toxic substances during the working day.
National Fire Protection Association, Standard NFPA 45, 2011
8.3.5 The location of air supply diffusion devices shall be chosen so as to avoid air
currents that would adversely affect the performance of chemical fume hoods,
exhaust systems and fire detection or extinguishing systems.
A-8-3.5 Room air current velocities in the vicinity of fume hoods should be as low as
possible, ideally less than 30 percent of the face velocity of the fume hood. Air supply
diffusion devices should be as far away from fume hoods as possible and have low
exit velocities.
8.8.6 For auxiliary air fume hoods, auxiliary air shall be introduced exterior to the
hood face in such a manner that the airflow does not compromise the protection
provided by the hood and so that an imbalance of auxiliary air to exhaust air will not
pressurize the hood interior.
American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5-2003
5.2.2 Supply air distribution shall be designed to keep air jet velocities less than half,
preferably less than one-third of the capture velocity or the face velocity of the
laboratory chemical hoods at their face opening.
For most laboratory chemical hoods, this requirement will mean 50 fpm (0.25 m/s) or
less terminal throw velocity at 6 ft (1.8 m) above the floor.
For rooms with greater supply air requirements, either perforated ceilings or special
large capacity radial diffusers may be necessary.
Supply air diffusers where practical should be located as close to the personnel
corridor and entry door to the laboratory and as far from the major exhaust devices as
is practical.
The ideal arrangement is to group the hoods and exhaust devices as far as possible
from entry doors and exit corridors and locate supply air diffusers close to entry doors
and exit corridors.
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Research on fume hood containment has
indicated the importance of ensuring that room
air currents (supply air discharge and
infiltration air) near the fume hood face
opening are kept low, typically no more than
30%to 50% of the fume hood face velocity that
is necessary for containment.
Excessive cross drafts typically result from:
• Improper room supply air diffusers
• Too few supply air diffusers resulting in high
supply air velocity
• Improper diffuser location (such as directly
above fume hoods or on room side walls)
• Use of portable cooling fans and/or operable
windows in the open position.
• Rapid movement of persons working at or
passing by fume hoods.
See Fume Hoods: Room Air Cross Currents
for requirements.
Research has also indicated that better supply
air infusion will be attained in a laboratory
room by using multiple perforated ceiling air
diffusers located as far as practical from the
fume hoods or biological cabinets.
VAV ventilation systems that are based on
maintaining a constant fume hood face
velocity and a supply air volume based upon
actual room needs can enhance fume hood
containment since a reduction in supply air
reduces the terminal throw velocity.