Class A Foam and CAFS Briefing: Structural Firefighting

Nov. 30, 2009
One of the most promising technological advances to occur within the fire service over the last 25 years was the technology associated with Class A foam and compressed air foam systems (CAFS). This technology, which primarily had its beginnings in wildland fire operations, represents a revolutionary breakthrough today for use in structural firefighting.

One of the most promising technological advances to occur within the fire service over the last 25 years was the technology associated with Class A foam and compressed air foam systems (CAFS). This technology, which primarily had its beginnings in wildland fire operations, represents a revolutionary breakthrough today for use in structural firefighting.

In the more than two decades that I have been involved in fire service training and education, I have seen a lot of innovations that held promise. Some worked and were adopted by the fire service; some worked and were not adopted; and others just didn't work. But few innovations have come along that represent such a significant step forward in our capability to control structure fires.

The intent of this three-part series is to share the basic concepts of Class A foams and CAFS and their benefits to the structural fire service, even though virtually all fire departments that must fight fires in other types of ordinary combustible fuel could reap the same benefits. For a much more comprehensive text about Class A foam and CAFS technology, obtain a copy of The Compressed Air Foam Systems Handbook at

The final product of the foam-generation process is finished foam, which is created when air has been added to foam solution. The Class B foams that we have historically used in the fire service are known as low-expansion foams.

Low-expansion foams generally expand at a ratio up to 20:1 when aspirated. This means that, at a 10:1 expansion ratio, 100 gallons of Class B foam solution would expand 10 times to 1,000 gallons of finished foam.

Finished foams are classified as:

  1. Low expansion
  2. Medium expansion
  3. High expansion

Low-expansion foams have expansion ratios of less than 20:1. Medium-expansion foams have ratios of 20:1 to 200:1. High-expansion foams, on the other hand, have expansion ratios above 200:1 and as high as 1,000:1.

The key to expanding foams is to produce the type of foam bubble that benefits the specific fire suppression application the most. Low-expansion foams have a greater amount of water contained in each bubble and thus tend to cool fuels more effectively than high-expansion foams. High-expansion foams have very little moisture in them and as a result resemble soap suds, which are very light. However, when we remember that the purpose of using high-expansion foams is to displace the air within a compartment with foam to effectively smother a fire, not much moisture is needed.

The Process

After the foam solution and plain water are properly proportioned to create foam solution, the foam solution is ready to be applied through a discharge device onto the burning fuel. The amount of aspiration and the type of discharge device are contingent on the fire problem at hand. This is where experience, training and available resources come into play.

The finished foam that provides the best knockdown and extinguishment characteristics for structure fire attack is a low-expansion, quick-draining foam comprised of small, uniform bubbles. A finished-foam with an expansion ratio of 7:1 of finished foam to foam solution produces a wet, frothing foam, which increases the water's surface area for efficient heat absorption, yet still holds a sufficient amount of water on the fuel to wet the surface and extinguish the fire.

Does the fire problem dictate a dense finished-foam? A quick-draining finished-foam? A very lightly aspirated foam? A deep-seated fire in a bale of paper, for example, may require using a foam solution in its raw form without aspiration. Protecting exposures will require an air-aspirating nozzle that forms a slower-draining finished-foam blanket that adheres to, insulates and wets the exposure for a considerable length of time.

Different low-expansion foam-generating devices, such as fog nozzles, air-aspirating nozzles or compressed air foam systems, produce different qualities of finished-foam bubble structures — bubble sizes, bubble durability and bubble drain time. These variables have a direct correlation to the efficiency and effectiveness of the foam for different tactical challenges.

Conventional Foam Generation

Foam solutions can be applied through conventional nozzles. Smooth-bore nozzles produce finished foam with little or no bubbles and are excellent for deep-seated fires in compacted fuels. Fixed gallonage, adjustable gallonage and automatic nozzles, depending on design and stream pattern, produce a finished foam with from little or no bubbles to a 2:1 to 4:1 low-expansion, quick-draining foam. These nozzles are excellent for producing foam for deep-seated fires in compacted fuels and for structure attack. The only aspiration with these conventional nozzles is that which occurs through natural air entrainment created by the fire stream and turbulence inside the nozzle itself. Applying foam solutions through conventional (smooth-bore or variable-pattern nozzles) is an ideal way to institute a Class A foam program, since the flow and firefighting techniques remain basically the same.

Nozzle-Aspirating Foam Systems

In a nozzle-aspirated foam-generating system, the fire apparatus pump provides the energy to move foam solution from the apparatus to an aspirating nozzle at the end of an attack hose or fixed or portable monitor. This energy is not only used to propel the fire stream, but also is used to draw atmospheric air into an aspirating foam nozzle or foam tube. As foam solution passes through the specially designed nozzle, the nozzle design draws atmospheric air into the nozzle and mixes it with the foam solution to create the finished foam.

Nozzle-aspirated foam generating methods produce low-expansion foams (20:1 or less) or medium-expansion foams (from 20:1 to 200:1). Low-expansion finished foams produced by air-aspirating nozzles, particularly on the lower end (below 10:1) of the low-expansion range, are excellent for direct structure fire attack. Choose a nozzle with a high gallons-per-minute (gpm) flow to maximize volume and reach. Medium-expansion foam generally is not a suitable choice for direct fire attack, since medium-expansion, air-aspirating nozzles use most of the fire stream energy to create the finished-foam and leave little energy to propel the fire stream.

Medium-expansion nozzles produce a drier finished-foam bubble that will fill most structural wall cavities, fill inaccessible/poorly ventilated fire compartments such as basements, and envelope burning tire or rubbish piles, cutting off oxygen while wetting and cooling fuels. Medium-expansion foams are also excellent for mop-up and overhaul.

Next: Compressed air foam systems

DOMINIC COLLETTI is the foam systems product manager for Hale Products and the author of the books The Compressed Air Foam Systems Handbook and Class A Foam — Best Practice for Structure Firefighters. Colletti is a former assistant fire chief and serves on the technical committee of the National Fire Protection Association (NFPA) 1500 Fire Department Occupation Safety and Health Program. He is a fire instructor with over 20 years of CAFS tactical firefighting experience. Colletti may be reached at [email protected].

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