Basic Documentation

compromising effective fume hood containment.
Figure 1 illustrates this phenomenon.
Figure 1. Room Air Currents Can Draw Fumes
Out from Fume Hood Interior when Sash Open.
ANSI/AIHA Z9.5 states that room air cross
currents should be less than one-half and
preferably less than one-third of the fume hood
face velocity.
3
NFPA 45 also states that room air
currents should ideally be less than 30% of the
fume hood face velocity.
4
Some laboratory
rooms have many fume hoods and, therefore,
require a correspondingly large supply makeup
airflow. This can result in unavoidable room
cross currents that are substantially higher than
recommended by the aforementioned standards.
 Fume Hood Usage–Manufacturers normally
test their fume hoods without chemicals or
laboratory equipment inside the hood (except for
the tracer gas ejector). In actual use, a fume
hood is likely to contain all sorts of apparatus
and equipment, including: support racks,
beakers, hoses, heaters, chemical containers,
analyzers, etc. Some items may be quite large
Page 2 of 6 Siemens Industry, Inc.
Document No. 149-989
3. American National Standard for Laboratory Ventilation
ANSI/AIHA Z9.5, 5.2.2: Supply air distribution shall be
designed to keep air jet velocities less than half, and
preferably less than one-third of the capture velocity or the
face velocity of the laboratory chemical hoods at their face
opening.
4. National Fire Protection Association, Standard NFPA 45: A-
6-3.5: Room air current velocities in the vicinity of fume
hoods should be as low as possible, ideally less than 30% of
the face velocity of the fume hood.
and positioned in a way that adversely affects
the internal airflow pattern that is necessary for
optimum fume containment. Experiments or
chemical processes may give off substantially
greater amounts of fumes than the quantity of
tracer gas used in the ASHRAE 110 test.
5
Also,
the actual chemical fumes generated may be of
much different buoyancy than sulfur hexafluoride
tracer gas and, therefore, behave differently.
Heaters and electrical equipment within a fume
hood’s interior generate convection air currents.
All of these factors–either individually or
combined–usually have an adverse effect on
fume containment especially in comparison to a
nearly empty fume hood
6
tested under optimum
room conditions.
Face Velocity–Cause and Effect
Let’s consider how fume hood face velocity relates
to fume hood containment. It should be noted that
what primarily keeps fumes within the interior of a
fume hood is the pressure difference that exists
between the fume hood interior and exterior (the
laboratory room). The laws of physics do not allow
fumes or air to flow from an area of lower static
pressure to an area at a higher static pressure. Thus
by applying a constant exhaust to the fume hood, an
area of lower pressure is created within the fume
hood interior and this establishes the basis for fume
containment.
As a result of the pressure difference between the
room and the fume hood interior, room air flows into
the fume hood. The greater the pressure difference
the greater the inward airflow face velocity will be.
Since fume hood face velocity is more easily
detected and measured than the small pressure
difference
7
between the room and fume hood
interior, face velocity has become, by default, a
means to quantify the existence of a pressure
difference. It is also important to note that using face
velocity as a means to quantify the hood pressure
differential requires that the face velocity
measurement be the average face velocity.
A fume hood’s face velocity, like that of any air
current, will vary throughout its cross section with the
5. The ASHRAE 110 tracer gas test requires a release rate of
4.0 liters per minute of sulfur hexafluoride.
6. The ASHRAE 110 test tracer gas ejector occupies far less
space than the equipment and apparatus typically found in
actual laboratory fume hoods.
7. A face velocity of 100 fpm results from a pressure difference
of just 0.000623 in. WC.