ASHRAE Journal - June 2014 - 50

Technical FeaTure

was under continuous tension, and multiple detectors
could be connected in series. Significantly, protection for
appliances was added at this time.
Another improvement was the development of sealable
metal cabinets to hold the dry chemical cylinder, gas cylinder, and pressure-regulated release assembly. The cabinet would be unsealed and accessed only during periodic
maintenance by an authorized technician. Cabinets were
mounted on walls, the ends of exhaust hoods, or inside
special appliance cabinets, which also provided locations
for manual pull stations. At some point, electric contacts
were added to release systems to facilitate code requirements for operating fuel shutoffs and controlling makeup
air and exhaust fans during fires.

Fryer Reignition Issue
In the late 1970s, one rapidly growing quick service
restaurant chain noted that occasional fires in its deep
fat fryers were often extinguished initially but soon reignited. With no further suppression available after the
gas cylinder and dry chemical cylinder had emptied, the
re-ignited cooking oil fires were difficult to extinguish.
Often, fryers burned vigorously until extinguished by
firefighters, causing major damage to structures and
contents, as well as business interruption.
Fires need fuel, heat, oxygen, and a continuous chemical reaction. Extinguishing a fire depends on removing
one or more of these elements. With dry chemical systems, oxygen was removed initially to extinguish fires,
but heat was not removed sufficiently to limit reignition
from continued oil vapor generation. Boiling and turbulence of the burning cooking oil often caused disruption
of the dry chemical "barrier" on the oil. Also, heat from
the cooking appliances would continue to support the
fire if the suppression system's automatic fuel shutoffs
did not operate properly or were not part of the system.
Dry chemical systems were also inconvenient for the
cleanup problem they presented, particularly from accidental activations.

Early Wet Chemical Systems
Two companies released the first wet chemical systems
for commercial kitchens in the mid 1960s. This development was spurred in part by the U.S. Navy's concern
with fires in shipboard galleys. CO2 or dry chemical
systems were problematic, not only for reignition and
cleanup issues, but also for the risk of contaminating
50

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J u n e 2014

Fire Detection Devices
Fusible Links
A fusible link assembly is installed within each
exhaust duct opening and above each protected cooking
appliance per NFPA 17A. Fusing (melting) during a fire
allows the links to separate, typically relaxing a small
diameter, spring-tensioned cable to activate the release
assembly.
Linear Fusible Links
Multiple fusible links are cabled in series at fixed
intervals over the entire length of the hazard zone.
There also must be a fusible link in each duct opening.
Fusing of any one device activates the system.
Pneumatic Tubing
Pressurized pneumatic tubing is placed along the full
length of the hazard zone. Heat from a fire causes the
tubing to deform or melt, reducing the pressure sensed
by a pressure switch that activates the system.
Electronic Fire Detectors
Listed electronic temperature sensors are installed
within each exhaust duct opening, and optionally, other
locations. A variety of temperature settings is available,
and some sensor products also activate fire suppression
based on the rate of temperature rise.

shipboard environments, particularly in submarines.
A sprinkler company pioneered one of the early wet
chemical systems, which featured a misting nozzle. This
was also the first system to include multiple wet chemical cylinders.

Expansion of Wet Chemical Systems
Wet chemical systems gained widespread acceptance
in the early 1980s when one large manufacturer's system was adopted for new restaurant construction by
the growing quick service chain mentioned previously.
Wet chemical systems are mechanically similar to dry
chemical systems, except for differences in piping type,
size, and nozzles. Additional tanks can be added for
larger hazard coverage, and optional accessories such as
alarms and warning lights can be added.
The wet chemical agents are proprietary mixtures of
potassium carbonate, potassium acetate, potassium
citrate, or a combination, in water with other additives
to form an alkaline fire suppressant, which enhances
saponification. This is the reaction between a strong

ASHRAE Journal - June 2014

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