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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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 cafsinstitute.org.
In the past 25 years, Class A foam and CAFS have received a great deal of attention. Not since the start of the debate over which is the "best" nozzle — a smoothbore or a variable pattern — has there been such controversy over new technology in the fire service.
It would seem that today there are numerous opinions on the use of Class A foam and CAFS from many in the fire service. Since an opinion is nothing more than a statement unsubstantiated by fact (if an opinion were a "fact," it would no longer be an opinion), let's examine the facts about Class A foam and CAFS and see what they can do for your fire department involved in providing structural firefighting response and operations.
The concept of Class A foam and CAFS is simple: Add Class A foam concentrate to water, forming a foam solution that, when applied to the surface of a Class A fuel, will spread out, wetting the surface of the fuel and penetrating below the surface to absorb heat and cool the fuel faster than plain water. In a nutshell, adding Class A foam concentrate to water doubles the water's fire suppression effectiveness. Adding compressed air to Class A foam solution to make a foam bubble blanket makes the water up to five times more effective. How is this accomplished? Read on.
Surface tension and gravity impede firefighting water. When one gallon of water at 60 degrees Fahrenheit is heated to 212°F and vaporizes, it absorbs a maximum of 9,366 Btu of heat — or 100% efficiency. If each gallon of water applied to a structure fire achieved 100% efficiency, fire suppression would be much easier and cause far less water damage. However, such is not the case.
Walter M. Haessler's research in 1974 found that a solid stream of water achieved only 5% to 10% efficiency (The Extinguishment of Fire, National Fire Protection Association, 1974). This means that each gallon of water absorbed about 933 Btu, and that the majority of the water applied through a solid stream simply ended up on the floor and then ran out of the fire compartment. This was due to:
- The surface tension of water, and
- The force of gravity
Surface tension affects the ability of a liquid to spread across a given surface. This is why water tends to bead on horizontal surfaces and roll down vertical surfaces. Water's surface tension limits the surface area in contact with the fuel, as most of the water beads or runs off, limiting its ability to absorb heat under actual fireground conditions.
What Is Class A Foam?
While firefighting foams of various types have been around since the late 1800s, most of them have been designed for use on flammable liquids and other Class B fuels. The only foams that were intended for use on Class A fires were the high-expansion (200 to 1,000:1 expansion rates) types originally developed for fighting fires in coal mines. These foams are best used as smothering agents where they are forced into compartments to displace air. With such high expansion rates, they have little water within them to perform any significant cooling.
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.
Next: Finished foam, foam proportioning and foam generation
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 firstname.lastname@example.org.