High-performance fume hoods take on different names throughout the industry, and therefore the meaning of the term “high-performance fume
hood” is often misunderstood. With all
the various definitions and industry lingo,
the terminology can be tricky. This article
will help set things straight.
Air’s relationship with fume hoods
Air is pulled through the fume hood via
a blower, creating a negatively pressurized
environment to protect the user from
harmful vapors. The air travels at a certain
speed, or velocity. The velocity of a fume
hood is measured in the plane of the sash,
and referred to as the face velocity, measured in feet per minute (fpm).
The face velocity is related to the
amount of air being pulled through the
fume hood. The amount of air is called the
volumetric flow rate, measured in cubic
feet per minute (CFM). The more air that
is pulled through the opening, the faster
the air will travel.
Using an example of a garden hose to
relate, you can place your thumb over the
opening of a hose to make the water spray
further because blocking that opening
speeds up the water’s velocity. The volume
of water being pumped through the hose
remains the same.
Since water and air are both fluids, the
same physics apply. The face velocity (fpm)
is dependent upon both the amount of air
going through the hood (CFM) and the
size of the opening the air passes through,
including the bypass area.
Face velocity set point
Different fume hood models will operate
at different published face velocities. Since
not all laboratories need the most advanced fume hoods, many manufacturers
offer a range from which to choose.
Specific face velocities should be chosen
based on several variables. The recommended operating face velocity is typically
dictated by the safety officer of the laboratory, not the fume hood manufacturer. A
safety officer may say it is acceptable to use
a fume hood at 60 fpm with an 18” sash
working height, or due to drafty lab conditions or hot plate use, the face velocity
requirement may rise.
Fume hoods cause an incredible strain
on energy in a laboratory. They take
expensive, tempered air and throw it out-
side – much like having a window open
year-round and forcefully blowing air out
of it. So if a lower face velocity can be
used, then it should. A lower face velocity
means lower volumetric flow, which
means energy conservation and savings to
the bottom line.
Velocity does not equal safety
Prescriptions range widely for required
face velocities, however, it has been proven
that faster is not necessarily safer. In fact,
many laboratory standards specifically
state that operating at high velocities
(above 150 fpm) can actually create a
safety risk due to turbulent air.
If a laboratory is balanced and follows
general guidelines for operating a constant
volume fume hood, a significant amount
of energy may be saved if the fume hood
operates lower than 100 fpm face velocity.
A lab can be starved of air, and if it is,
the fume hood will not exhaust properly.
Be sure there is enough supply air provided
to the lab before adding fume hoods or
choosing face velocity set points. That is
not to say all labs can operate their fume
hoods at as low as 60 fpm to relieve the
strain on their mechanical system. Fume
hood face velocity should be analyzed and
carefully specified because there are many
benefits to operating at a lower face velocity, and higher face velocities cause their
share of risks and unnecessary costs.
A real high-performance fume hood
High-performance fume hoods are
subjected to many marketing spins, and
therefore different names are thrown
around in an attempt to attract custom-
ers. If a fume hood is referred to as a low
velocity, high efficiency, energy efficient, or
low exhaust volume, it all references the
The True Definition of a
High-Performance Fume Hood
With so much industry jargon, true high-performance fume hoods can
be difficult to identify, but it’s important to conduct due diligence and
choose the hood that will get the job done safely and effectively.
by Beth Mankameyer, Sales Engineer, Labconco Corp., Kansas City, Miss.
The ASHRAE 110 test comprises a face velocity
profile, smoke visualization test, and a tracer gas
test using sulfur hexafluoride and a manikin.