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...
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While some work was done in attempting to use high-expansion Class A foam for manual application in structural firefighting, the concept was limited to fires in basements and other compartments with limited openings. Like any other foam, high-expansion foam is another specialized weapon in our firefighting arsenal. It is, however, effective in fixed-building fire protection systems for some special hazards.
The Class A foam technology of today is totally different from that of the past and has come about because of work in the area of wildland fire control. The most important Class A Foam concepts are:
- How Class A foams enhance water's capability to control structural fires
- How the technology, including CAFS, used to produce and deliver these Class A foams work
- How the new technology can enhance structural firefighting operations
Class A foam is nothing more than a formulation of chemicals mixed with water and air to form an expanded extinguishing agent enhancing the ability of plain water to suppress burning Class A fuels. Unlike Class B foams, which are intended to alter water's properties so it can float on a flammable liquid and thus extinguish the fire primarily by smothering, Class A foam alters water's properties to let it spread over and penetrate Class A fuels.
Anyone who has fought an interior structural fire knows that water tends to run off fuel surfaces. This runoff is essentially wasted water, and occurs even when fog streams are used to maximize steam conversion. For many years, wetting-agent additives have been used to create "wet" water. Wet water is simply a solution of water and a wetting agent that has a reduced surface tension to let the solution spread and penetrate much the same as the detergent added to water in the laundering process lets water penetrate fabrics to remove dirt.
Class A foams not only provide water with the properties of wetting agents, but provide it with the ability to form bubbles that tend to cling to Class A fuels. This clinging ability holds the water in the form of bubbles on the fuel surface. Rather than running off, the water applied in the form of foam remains on the fuel surface and continues to absorb heat until it is all gone. Thus, the water is held in position so that more of it is effectively used for cooling the fuel.
Class A foams were originally developed for use in wildland fires and had their origins in the "Texas Snow Job," first used in 1977 by wildland firefighters in Texas. The Texas Snow Job made use of pine soap as the foaming agent. Since that time, work has been done to develop the optimum Class A foam, with the first being developed in the early 1980s.
While these agents were originally intended to make the water available to wildland firefighters more effective and to essentially stretch it, they demonstrated great value in the exterior protection of structures in wildland fire situations. This was vividly demonstrated in the fires in Yellowstone National Park in 1988.
Class A Foam Concentrate and Water
Class A foam concentrate is a synthetic detergent hydrocarbon surfactant that, when mixed with water, reduces the water's surface tension. A solution of 99.7% water and 0.3% Class A foam concentrate will reduce the surface tension of the solution by about two-thirds. Unlike plain water, a drop of this foam solution will flow across a horizontal Class A fuel surface and penetrate it. Because of this, more of the solution's surface area contacts the fuel, thus increasing the altered water's rate of heat absorption.
In addition, Class A foams have an affinity for carbons and form a cooling foam blanket that is "oil-loving." The foam solution has a physical attraction to the charred carbon layer on most burning Class A fuels, which lets water first cling to and cool the surface, and then drain out of the bubble blanket, to spread, wet and penetrate the fuel.
A mixture of water and foam concentrate is called a foam solution. Class A foam solutions generally consist of from 0.1% to 1.0% foam concentrate. They have excellent spreading and penetrating properties because of their low surface tension. However, they do not reduce runoff due to gravity. Thus, while foam solution as an extinguishing agent is suited to some tactical applications such as deep-seated wildland fuels or garbage dump fires, foam solution, itself, is not in the optimum form for structure fire attack situations because it does not cling to vertical surfaces.