Basic Documentation

Siemens Industry, Inc. Page 3 of 6
Document No. 149- 976
velocity rather than 100 fpm, an automatic face
velocity setba
ck provision will provide a near 40%
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reduction in fume hood ventilation energy for most of
the time.
However the actual energy savings are diminished
if:
1. Users close their sashes when they no
longer need immediate access to the fume
hood interior. VAV fume hood ventilation
energy savings will then attain the maximum
reduction of approximately 80% when the
sash is closed.
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2. Fume hoods are actively used for more than
the short usage times often claimed.
3. Facilities use a fume hood sash stop to
restrict how far sashes may be opened. If a
vertical sash can only be opened 18 inches
rather than 28 inches, the fume hood
ventilation energy is reduced by nearly 35%
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without the need for any face velocity
setback. In such situations, face velocity
setback does not add significant savings.
4. Room air currents in front of the fume hood
require that the automatic face velocity set
back needs to maintain a higher face
velocity than 60 fpm, thus reducing the
savings
9
.
Attaining Fume Hood Sash Closure
The argument for automatic face velocity reduction
is based upon the assumption that fume hood users
do not and will not close their sashes. However
implementing automatic face velocity reduction is
likely to only make this situation worse since it
6. 100 fpm – 60 fpm = 40% less.
7. The 80% air reduction figure is based upon the AIHA Z9.5
and NFPA 45 recommendation that VAV fume hoods
maintain a minimum exhaust airflow rate of 25 cfm per
square foot of internal work surface when the sash is fully
closed (Paragraph 3.3.1 in AIHA Z9.5 & Paragraph A.6.4.6
in NFPA 45). This enables reducing a VAV fume hood's air
consumption by approximately 80% of maximum when a
constant face velocity of 100 fpm is maintained.
8. For an 18-inch vertical sash opening rather than 28 inches,
the sash open area is reduced to about 65% (18 in. ÷ 28 in.)
and, thus, only requires 65% of the maximum ventilation
airflow. This results in the 35% reduction in air American
National Standard for Laboratory Ventilation ANSI/AIHA
Z9.5, 2002 5.2.2, consumption.
9. Since room air current velocities will not be known until after
the room is constructed and the ventilation system is put into
operation, the permissible face velocity setback value cannot
be determined beforehand.
communicates to the users that they are not
expected to close sashes. Compare this to a room
equipped with a lighting control arrangement that
shuts off the lights when no one is in the room.
Persons are less likely to make an effort to turn the
lights off when they know it will happen
automatically.
With fume hoods, the concern is primarily worker
safety and not just with energy savings. To obtain
the cooperation of fume hood users for keeping
sashes closed the following suggestions are offered:
 Avoid impersonal and authoritative approaches
since these are unlikely to generate enthusiastic
cooperation. An ineffective example would be a
sternly worded notice such as:
NOTICE TO ALL LABORATORY PERSONNEL:
Effective immediately, all fume hood sashes may be open
only when absolutely necessary. At all other times, sashes
shall remain closed!
 Conduct periodic safety training by a skilled
professional safety trainer or consultant,
preferably from outside of the facility. People are
more likely to heed an outside expert than
another facility person.
 Post instructional posters or signage to educate,
encourage and remind workers of safe work
practices. For VAV fume hoods also provide
information indicating the difference in energy
consumption between open and closed sashes.
Figure 2 provides examples of signs that help
reinfo
rce the importance of safe work practices
and energy consciousness.