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Trainer’s Corner: Compressed-air foam here to stay

Over the past 10 years a vast amount of research has been focused on the ability of Compressed Air Foam Systems (CAFS). Certainly, there is little argument that a tiny amount of foam additive entrained into a water/air mix outperforms plain water.

December 6, 2007
By Ed Brouwer

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edbrouwerOver
the past 10 years a vast amount of research has been focused on the
ability of Compressed Air Foam Systems (CAFS). Certainly, there is
little argument that a tiny amount of foam additive entrained into a
water/air mix outperforms plain water.

The addition of a fire
fighting foam concentrate reduces the surface tension of water,
allowing the formation of small uniform bubbles. CAF systems produce a
much more stable foam than an eductor system.

An article on the
front page of the Osoyoos Times, in British Columbia, on July 18,
quoted Fire Chief Joe Simoes saying the Anarchist Mountain Fire
Department's new fire truck is equipped with a CAF system that
multiplies the 1,000 gallons of water 100 times, equalling 100,000
gallons of water. That's a bit of an exaggeration, but his excitement
is warranted.

Compressed-air fire fighting foams expand water
to between five and 15 times its original volume (i.e, one gallon of
water turns into five to 15 gallons of foam). A common mix ratio for
compressed-air foam is 0.2 per cent concentrate by volume (compared to
0.5 per cent for eductor generated foam). Assuming an average expansion
of 10 times, it follows that the foam consists of 0.02 per cent
concentrate, 9.98 per cent water and 90 per cent air.

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Through
evaporation of the water in compressed-air foam, the fire is cooled. As
the foam collapses, water is released and increases in temperature by
absorbing the heat and eventually turning into steam. The water is
released from foam, either through ruptures in the bubbles caused by
the heat from the fire expanding the entrapped air, or through the
effects of gravity distorting the bubble walls. As this is a gradual
process, the foam acts as a water reservoir, releasing the water at a
rate that allows the fuel to absorb it, rather than running off and
forming useless puddles.

An added benefit of foam concentrates
is that they contain surfactants, or wetting agents, that reduce the
surface tension of the water and allow for deeper penetration into
hard, woody fuels, deep forest litter, etc.

This "wet water"
effect allows the firefighter to get the optimum use out of the
available water. The deep penetration of the "wet water" into the fuels
inhibits rekindling for prolonged periods of time.

It is proven
that CAFS attacks all three sides of the fire triangle simultaneously:
the foam blankets the fuel, thereby reducing the fuel's capacity to
seek out a source of oxygen; the CAFS solution adheres to ceilings and
walls, more readily aiding in rapid reduction in heat; and the opaque
surface of the foam, as it adheres to walls and ceilings, shields the
fuel source from radiant energy.

In the 1930s, the British navy
experimented with agents that were made into foam by means of
compressed air and the U.S. navy was using compressed air foam systems
in the 1940s for flammable liquid fires.

In the mid 1970s, the
Texas Forest Service developed a water-expansion system known as the
Texas Snow Job. This pioneering Class-A CAFS used a pine-soap
derivative – which was readily available as waste from local paper
manufacturing industries – as a foaming agent, mixed as eight to nine
parts agent to 91 to 92 parts water, flowing up to 30 gpm. The duration
was limited by the use of compressed-air cylinders rather than
compressors. CAFS received national attention in 1988 during the
Yellowstone Park wildfires when the four-storey Old Faithful Lodge was
successfully protected by blanketing it with compressed-air foam.

In
the spring of 1994, a compressed-air foam demonstration vehicle
manufactured by W.S. Darley & Co. of Melrose Park, Ill., was driven
from coast to coast in Canada and the U.S., with the purpose of
spreading the word about CAFS to the Canadian and U.S. fire services.

Until
the CAF system development became available, fixed-pipe foam
fire-suppression systems used aspirating nozzles, blowers and
sprinklers. Each had its advantages and disadvantages.

In remote
areas or areas with substandard water supplies, CAF systems provide a
proven means to suppress flammable liquids fires. In these situations,
fire suppression systems would seldom be installed due to the
significant cost or local conditions and, hence, the hazard would not
be protected. CAF systems provide a means to reduce the hazard. As a
result of the significantly reduced water and foam use, CAF systems can
be installed in situations where environmental damage from fire
suppressants and the fire itself must be minimized.

CAFS is able
to deliver a range of useful foam consistencies, labelled from Type 1
(very dry) to Type 5 (wet), which are controlled by the air-to-solution
ratio, and, to a lesser extent, by the concentrate-to-water percentage.
Type 1 and 2 foams have long drain times (i.e., the bubbles do not
burst and give up their water quickly) and long duration. Wet foams,
Type 4 and 5, drain more quickly in the presence of heat.

Scientific
and engineering studies have led to significant advances in the
understanding of the dynamics and fire suppression mechanisms of CAF.

These
studies have also resulted in improved technology to generate foam and
increase its ability to flow through pipe and be dispersed for
successful fire suppression. CAF itself has been shown to perform
better than air-aspirated foam and unexpanded foam-water solution. CAF
systems have been demonstrated to successfully extinguish challenging
fires with less water and less foam than current fire-suppression
systems using foam and water.

The Snuffer, now manufactured by
Tuff Built Products Inc. of Dufresne, Man., boasts, "CAFS are up to 30
times more effective than water by itself. Our 42 cfm Snuffer system
can deliver 375 gpm of foam and has knocked down a fully involved fire
in less than 30 seconds, and oil-pan fires in less than 15 seconds."

Until
now, attempts to adapt the compressed-air foam approach to fixed
installations have failed because of two fundamental technical
problems: first, traditional sprinkler-type nozzles cannot distribute
compressed-air foam without collapsing it; and, second, the foam itself
degenerates in fixed piping.

Current systems are also unable
to provide foams with high-injection velocity, which is an especially
important requirement in the case of high-ceiling storage warehouses
and hangars, where the injected foam needs to travel relatively long
distances and penetrate fire plumes before reaching the seat of the
fire.

The National Research Council of Canada has overcome
these difficulties with an improved technical understanding of foam
behaviour, and by engineering sophisticated air-injection hardware with
a special nozzle. The result is a compressed-air foam system whose
performance has been proven in full-scale tests. Compressed-air foam
technology is a new development with good potential for fire
suppression because of its good suppression capability, low water
requirement and easy cleanup afterwards.

It seems that CAFS is
going to play a major role in the future of fire suppression given that
it's been widely referred to as the fire suppression equipment of the
future.

Until next time stay safe and remember to train like their lives depend on it because they do.

Sources:

Technical
Report 98: Compressed Air Foam Systems in Limited Staffing Conditions,
by the USA Fire Administration, the National Research Council Of
Canada, the National Interagency Fire Center, the National Defence
Canada, IFSTA, NFPA 11, and Fire Technologies of Ottawa.


Ed Brouwer is the Fire Chief/Training Officer for Canwest Fire and a
member of the Osoyoos (B.C.) Fire Dept. The 18-year veteran fire
fighter is also a Fire Warden with Ministry of Forests, a First
Responder III instructor/evaluator, Local Assistant to the Fire
Commissioner and a fire service motivational speaker and chaplain.
E-mail ed@thefire.ca .


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