Basic Documentation

highest velocity occurring near the center. The
lowest air velocity will normally occur at the
periphery (outer boundary) of an air current.
8
Obtaining the average fume hood face velocity
requires that a measurement traverse be made in
the plane of the sash opening. An alternate
procedure consists of mathematically calculating
average face velocity based upon a measurement of
the total fume hood exhaust rate and the total fume
hood open area.
9
Challenges to Containment
As stated, the laws of physics do not allow fumes to
flow from an area of lower static pressure to an area
of higher static pressure. Why then can fumes flow
out from a fume hood interior that is (presumably) at
a lower static pressure as evidenced by a proper
face velocity? Note that room air currents (like any
airflow) have a lower static pressure than room air
that is essentially at rest. And, as a room air current
passes an open sash, the pressure difference
between the fume hood interior and the static
pressure at the periphery of the air current can be
considerably less than the pressure difference
between the fume hood interior and room air that is
essentially at rest.
Figure 2 illustrates the relationship between the
static p
ressure (lower value) and the total pressure
(higher value) of an air current. The higher value of
total pressure is always in the direction of travel
while the lower static pressure is always
perpendicular to the direction of travel. As the
velocity of an air current increases even more, its
total pressure also increases while its static pressure
becomes even lower. Thus, as higher velocity air
currents pass crosswise to an open fume hood sash,
the reduced static pressure difference between the
fume hood interior and the periphery of the air
current creates a tendency for fumes to flow out from
the hood interior. This is especially true if air currents
within the fume hood travel toward the sash
Siemens Industry, Inc. Page 3 of 6
Document No. 149-989
8. A traverse consists of taking a series of individual air velocity
measurement at defined locations throughout a cross-
section of the full airflow area. The larger the airflow area the
more individual measurements are required.
9. The total fume hood open area consists of the sash opening
plus all additional openings, including the airfoil slot and the
bypass opening.
opening,
10
since they then have a higher total
pressure in the direction of travel.
To overcome the effect of room air currents, the
static pressure difference between the fume hood
interior and the room must usually be increased by
increasing the fume hood exhaust airflow. Increasing
the exhaust airflow will be manifested by a higher
inward airflow and thus a higher average face
velocity. In other words, the face velocity must often
be increased to overcome the effects of higher
velocity room air currents.
AIR CURRENT
TOTAL
PRESSURE
(HIGHER)
STATIC
PRESSURE
(LOWER)
T
S
Figure 2.
Static Pressure is Lower for Air in Motion.
10. Air currents or turbulence within a fume hood interior are
caused by multiple factors including the geometry of the
fume hood interior, local convection, fume generation, and
even excessive face velocity.