Basic Documentation

Siemens Industry, Inc. Page 5 of 6
Document No. 149-989
Designer Responsibility
The ventilation system designer must ensure that
the individual fume hood exhaust provision, as well
as the supply make-up airflow for each laboratory
room, can provide somewhat greater airflow if a
fume hood’s face velocity must be increased beyond
what was originally anticipated. On average, a 10%
safety factor over the design conditions
13
should
usually suffice for this purpose.
VAV Advantage
VAV fume hoods will typically allow greater flexibility
in establishing the necessary average face velocity
for a particular fume hood due to the diversity factor
associated with VAV systems.
14
In other words if a
particular fume hood requires a higher than
anticipated average face velocity, it can usually be
attained without adding to the overall exhaust
system capacity. Also, the fume hood working height
for vertical sash fume hoods can usually be
restricted to 18 inches
15
by implementing a sash
stop. This reduces the sash opening to about 65% of
the normal full open sash height. As a result, the
average face velocity could then be increased by up
to 35% if necessary, based upon the exhaust airflow
rate for a fully open sash.
13. Fume hood design airflow should be based upon either the
owner’s stated face velocity requirements or the fume hood
manufacturer’s as manufactured (AM) test data.
14. Fume hoods that have their sashes closed enable the
exhaust air capacity (and the room makeup air) to be used
where more airflow is needed.
15. A typical maximum fume hood vertical sash opening is at
least 28 inches.
Conclusion
Most laboratory safety professionals hold the
position that even if a fume hood’s face velocity is
within safety standard recommendations, it will not
serve as a guarantee of safe fume containment.
Although one cannot argue against this position, it is
somewhat similar to saying that a good braking
system will not serve as a guarantee of vehicle
safety. As in many situations, ensuring total safety is
a more complex task and requires addressing
multiple issues.
If the necessary fume hood average face velocities
are determined by on-site fume hood containment
testing, then the face velocity should serve as a
benchmark for safe containment if other important
aspects - particularly the room ventilation attributes -
remain unchanged and safe work practices are
followed. Periodic re-testing of fume hood
containment on an annual basis is still advised.
Ventilation system designers should allow some
extra capacity in exhaust and supply makeup air
systems in case individual fume hood exhaust
airflows must be increased above a manufacturer’s
recommendation to attain the average face velocity
required for acceptable fume containment.