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.
Finished foam is a foam solution that has been "aspirated" to make the solution bubble. Children blowing bubbles dip a ring in a form of foam solution and then aspirate the bubble when they blow on the film of solution suspended on the plastic ring. In making firefighting foam, aspiration is achieved by mechanically agitating the foam solution to add air, which creates a bubbly mass of finished foam. It is the bubbles in the aspirated or finished foam that let the agent cling to the vertical surfaces of fuels and hold the water-based liquid in place until it has absorbed enough heat to evaporate.
You can perform a simple demonstration of the difference between foam solution and finished foam. Take a soda bottle and fill it about halfway with water, then add a couple of teaspoons of liquid detergent to the water. The water and detergent are essentially a foam solution. Cover the top of the bottle and shake it vigorously. You will see that the solution is bubbly — the solution has expanded into finished foam. If you really want to see a high-quality foam bubble structure, pour the solution into a blender. Since the blender creates much more agitation than shaking the soda bottle, the bubble mass will be much denser.
Finished-foam bubbles also provide a temporary "vapor seal" for Class A fuels that assists in fire extinguishment by affecting both the fuel and oxygen components of the fire tetrahedron. Foam bubbles create dead air spaces that "insulate" the fuel from heat and flames, thereby slowing heat transfer to the insulated fuel and flame spread.
The most important fact to remember about Class A foam is that regardless of the type of foam-generating system used, it is the water within the finished foam that extinguishes the fire. All the foam concentrate does is make the water work better. Under ideal conditions, 100% of the finished foam will cool the fuel or penetrate the fuel to which it is applied with no runoff. However, achieving 100% efficiency on the fireground is unlikely. This is because the efficiency of a foam application is affected by variables such as foam production methods, application methods, application rates, as well as the fire situation itself.
While a number of foam proportioning devices are available, we will look at three basic methods. These are tank or batch mixing; eduction; and direct injection.
• Tank or batch mixing — One of the easiest ways to mix a Class A foam solution is to pour foam concentrate directly into the water tank of a pumper. This is known as "tank" or "batch" mixing. Many departments that want to experiment with Class A foam use this method, since all that is needed is a container of foam concentrate. Mixing 2½ gallons of foam concentrate with 497½ gallons of water in a 500-gallon booster tank produces 500 gallons of foam solution with a 0.5% concentration.