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Compressed-air foam (CAF) is generated by injecting air under pressure into a foam solution stream.

The process of moving the solution and air mixture through the hose or piping, if done correctly, forms a foam.

The energy for the CAF comes from the combined momentum of the foam solution and air.

One significant advantage of such systems is the increased momentum of the foam, enabling it to penetrate flames and reach the fuel surface.

Another advantage of CAF is that it possesses greater stability with respect to drainage than air-aspirated foams, since it is characterized by a narrow distribution of bubble sizes.

Early attempts to adapt CAF to fixed installations failed, owing to two fundamental technical difficulties: first, traditional sprinkler-type nozzles cannot distribute compressed-air foam without collapsing it, and second, the foam itself degenerates in fixed piping.

NRC-IRC overcame these difficulties and developed a means of producing CAF using Class A and Class B foam concentrates in a fixed-pipe system, using a new and innovative foam distribution nozzle.

Foam break-up, which prevented the development of this technology in the past, was avoided by careful engineering design of the nozzle and the piping system.

NRC-IRC conducted full-scale fire tests to evaluate the performance of a prototype CAF system.

The tests demonstrated the superior performance of the system in extinguishing both liquid fuel and wood crib fires with a small amount of water.

Also, CAF requires a smaller amount of foam concentrate to provide effective suppression, compared to systems based on air-aspirated nozzles.

In the NRC-IRC tests, less than one half of the normally recommended (for air-aspirated systems) Class A and Class B foam solutions were used without compromising the extinguishment efficiency of compressed-air foams [Kim, A.K. and Dlugogorski, B.Z. (1997), “Multipurpose Overhead Compressed Air Foam System and its Fire Suppression Performance,” Journal of Fire Protection Engineering, Vol. 8, No. 3, p. 133.].

NRC-IRC has developed a prototype CAF system for practical application in providing fire protection to very large structures, such as aircraft hangars and power transformers.

Full-scale experiments carried out with the prototype proved the superior performance of CAF in extinguishing simulated fuel spill and transformer fires.

Currently pre-engineered CAF systems are being developed for a variety of commercial applications.

A CAF system works best in extinguishing liquid fuel fires. It can suppress fires in an open area and does not require an enclosure.

It also requires far less water than conventional water-based systems.

However, a CAF system will have difficulty extinguishing 3-D fires, such as a spray fire.

Source : National Research Council of Canada

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