26 R&DMagazine December 2013 www.rdmag.com
Tips for Choosing In-Cabinet
losses and threaten life and property, which is
why in-cabinet fire protection has become a
popular option. The ability to quickly detect
and suppress a fire inside a fume hood, right
at its source, can reduce a major incident into
a minor event. However, the speed and direction of the airflow inside a fume cabinet makes
detection by traditional methods difficult.
Single-point fire detectors, such as glass
bulbs and fusible links, are typically mounted
in-cabinet above the workspace. These types of
detectors may or may not sense the route the
heat is following. Optical-type detectors that
“see” the energy given off by a fire can be effective in fume hoods, but are extremely expensive.
Various other types of detectors are available,
but very few are suitable for in-cabinet applications.
One practical and cost-effective type of
in-cabinet fire detection is linear heat detection.
Linear heat detectors are available in both electric and pneumatic designs and provide multi-point detection at any point along their length.
Linear heat detectors can be installed behind
the fume hood baffles and across the exhaust
duct opening, which provides fast and reliable
detection of heat and flames.
Once a fire has been detected it should be
suppressed using a fire-extinguishing agent
appropriate for the types of chemicals used in
the fume hood. Unfortunately, no one extinguishing agent is suitable for all applications.
In fact, applying the wrong agent can actually
cause a fire to worsen. Several commercially
available fire-suppressing agents are available to
protect fume hoods, with each having advantages and disadvantages. These extinguishing
agents include dry chemical powders, carbon
dioxide, foam and clean fire-suppressing agents
For most fume hood applications,
multi-purpose (ABC) dry chemical agent is
the fire suppressant of choice. Dry chemical
is widely accepted as the most effective agent
available for extinguishing flammable liquid
fires. It is recommended for fume hood applications because it won’t react negatively with
An alternative to dry chemical powder is
carbon dioxide, which is electrically non-con-ductive and effective on Class A, B and C fires.
Carbon dioxide is desirable for fume hood
applications because it won’t harm sensitive
equipment and will suppress a fire without
leaving behind any residue. Note, however, that
carbon dioxide may interact negatively with
some laboratory chemicals.
If laboratories have concerns about which
fire-extinguishing agent is best, they can refer to
the Material Safety Data Sheet (MSDS) for the
material(s) being used in the fume hood. This
will likely dictate, or at least help narrow, the
choice for best extinguishing agent.
The extinguishing agent must be discharged
long enough to offset the amount of the air/
agent mixture flowing through the fume hood
and out the exhaust. Therefore, the discharge of
the agent must be forceful and targeted to cover
the entire volume of the fume hood including
In addition to quick and reliable detection
and suppression of a fire, a good in-cabinet fire
detection and suppression system should also be
capable of shutting off the gas and electric supplies, sounding an alarm and closing the fume
hood sash (where applicable). As per NFPA 45,
the system shouldn’t shut off the extraction.
— David Wilhite
Marketing Development Manager
Virtually every laboratory has areas with elevated fire risks, with fume hoods being a primary concern. The presence of igni- tion sources, such as hot plates and Bun-
sen burners, the use of pyrophoric materials
and the inherent volatility of the various chem-
icals and compounds that are commonly found
in fume hoods all add up to a serious fire risk.
Traditionally, fire protection for laboratory
fume hoods has been limited to a code-man-dated alarm system, which does nothing to
suppress a fire, and an overhead water sprinkler
system. Overhead sprinklers can do a good job
of containing a fire, but they can also damage
property and equipment as much as—or more
than—the fire itself.
If laboratories rely on sprinklers for fire
protection, they must be aware that when a
fire occurs in a fume hood, the heat and flames
follow the airflow through the cabinet and out
the exhaust, which greatly reduces the ability of
an overhead sprinkler system to react. By the
time a fire grows large enough to overcome the
airflow and activate the sprinklers, the amount
of damage could be staggering.
Even a short delay in the detection and suppression of a fume hood fire can lead to serious
Interior of fume hood after the discharge of ABC
dry chemical powder.
Fire suppression system using pneumatic linear
heat detection tubing. Images: Firetrace