Have you been wondering what is meant by the phrase, "Compressed Air Foam System?" Does it generate some sort of magical fire extinguishing foam? Is there truth to the claims that it is 10 times more effective that plain water? I will try to answer these and other questions in order to provide an understanding of compressed air foam mechanics and to separate some truth from fallacy about performance.

1. WHAT IS A COMPRESSED AIR FOAM SYSTEM? * In the simplest terms it is a standard water pumping system that has an entry point where compressed air can be added to a foam solution to generate a foam. The added energy from the compressed air turbo-charges the fire stream. The agent (foam) being expelled are extremely small bubbles. These bubbles give the fire stream a very large surface-to-mass-ratio thus absorbing heat very effectively.

A brief history lesson is necessary to crack the mystique the compressed air foam system, or CAFS, has acquired. CAFS is old technology. The Royal Engineering Handbook (Provisional) on the Use of Foam Fire-fighting Equipment of 1941 describes in detail a compressed air foam system used to combat fires on floating bridges (covering, coating, insulating, and suppressing flammable liquid vapor).

The U.S. Navy explored the concept in 1947. By using an air compressor that produced the same pressure (at static flow) as that of the water pump, the Navy found that two agents, foam solution and air, would readily merge at the mix point. An infinitely variable foam generating system was created that could make a small-bubbled foam with a full range of consistencies, from shaving cream to melted ice cream. The consistency was easily changed by adjusting the air to solution ratio.

Then and now it is critical that pressures at static be equal or slightly heavy on the solution side. If there is a difference in static pressure when the discharge valves are shut off the agent with the higher pressure will dominate. If the air is at a higher pressure then it will over-ride the water. The greater the difference in pressure the greater the volume of air stored for release through the discharge valve when it is opened. Just what a firefighter does not want is to discharge a large volume of air while fighting a compartment fire! The concept was dropped as too complex in the forties. Today, equal static pressure combinations should be a basic function of any CAFS unit.

During the 1960's the car wash industry adopted an idea for maximizing the effectiveness of detergent laden water for cleaning autos and trucks. You've seen them: "The Jiffy Do-it-Yourself Carwash." You put several quarters in the slot, select "Wash, Rinse, Wax, or Foam Brush." Yes, this is a compressed air foam system running at low pressure through a small diameter line where the foam oozes through the brush fibers. Or the truck washers with the handheld straight tipped wand which projects a stream of foam 25 feet using 4 gallons per minute (gpm) of solution and 4 cubic feet per minute (cfm) of air.

The Texas Forest Service re-introduced the concept of compressed air foam to the fire service in 1972 as the "Texas Snow Job," the "Water Expansion System" (WES) (figure 3), and the "Water Expansion Pumping System" (WEPS) (figure 4). The concept was promoted as a water saving or water expansion device. You could convert 250 gallons of foam solution into 2500 gallons of a water-based product for extinguishing fire. These systems were simple and inexpensive with water flows of 20 to 30 gpm.

In the mid-1980's the Bureau of Land Management and the US Forest Service increased the pumping scale by adding a reciprocating air compressor to a standard heavy fire engine, with a centrifugal water pump. This enabled the engine to maintain its water pumping capabilities and the air compressor to augment the fire stream reach. A larger variety of applications became available to meet objectives of any given incident.

Current technology advances reflect a competitive industry that believes compressed air foam systems are part of future standards. Engineering is replacing marketing. CAFS is no longer being validated with equipment designed for other uses. Components such as rotary screw air compressors are being coupled to all types of centrifugal water pumps, automatic foam proportioning systems and motionless mixers, are matched to the task. Let it be wildland, structural, vehicle, or flammable liquid fires.

2. IS CAFS MORE EFFECTIVE THAN OTHER FOAM GENERATING SYSTEMS? * Yes. With CAFS you can control and vary the volume of foam solution and compressed air at will. This flexibility permits structuring foams of varying densities and drain rates for various incidents (hazardous materials, structure, wildland, etc.). The air compressor also provides energy which, gallon for gallon, propels compressed air foam farther than aspirated or standard water nozzles. A well designed CAFS operates easily¼.engage the water pump, air compressor, turn on foam proportioner, open fully the water and air valves to charge attack line and discharge the product.

3. WHY IS THERE SUCH AN INTEREST IN CAFS? * The interest is high for several reasons. First, the system's versatility and success as a fire suppression tool are dramatic. In an age of shrinking budgets and increasing fire hazards, efficiency is in demand. Second, the system is not well understood. The fire equipment industry is promoting the concept to cash in on a naive market. CAFS is a technology learned at the nozzle, not at the video tape player. An investment in CAFS education will prevent a bad investment in equipment.

4. DOES CAFS INCREASE THE VOLUME OF WATER IN MY TANK? * NO! The foaming agent does make the water more effective and the structure of the foam generated appears to be superior for some applications, but compressed air foam does not miraculously create more water. If you have a 1000 litre water supply and a CAFS producing a 10 to 1 expansion ratio, you will generate 10000 litres of foam. The volume of water is still only 1000 litres.

5. WHAT ARE THE ADVANTAGES OF A CAFS? * The system produces an infinitely variable array of foam types. This is accomplished by changing the volume of air to water. Instant flexibility enables you to tailor the foam to suit the mission such as a dry foam (shaving cream) for vapor suppression or a wet, foam (froth) for optimum heat absorption and extinguishment or anything in between.

* The energy provided by the air compressor gives compressed air foam systems greater discharge distance per gallon of water flowed than other foam generating devices.

* Hose lines will float.

* The air compressor can be used separately to run any type of pneumatic tool. The air can be delivered many thousands of feet through any type of fire hose at pressures less than 150 psi.

* The system can not water-hammer. CAFS foam is compressible. Energy can be stored in stand pipes and hoses, to be used like a fire extinguisher when needed.

* CAFS foam can be pumped twice as high as plain water at the same pressure.

* The compressed air foam system pumps foam through the hose to the nozzle. Foam-filled hose is significantly lighter than hose filled with water. For example, an inch and a half cotton jacket-rubber lined hose filled with 10 to 1 expansion foam weighs less than half the weight of a one inch hose filled with water.

6. WHAT ARE THE DISADVANTAGES OF A CAFS? * The large amount of energy stored in the hose by the air compressor is "hidden" by the light weight of the hose. The initial discharge of compressed air foam can be difficult to control if the nozzleman is unprepared. This is caused by operating pressures being to high. Pressures in excess of 100 psi are needed only for extended hose lays or to impress you at a vendor demonstration.

* CAFS is more complex than pumping water alone. Obtaining the appropriate discharge of three inputs (water, air, concentrate) rather than one (for water) or two (for aspirated foam) requires education and training. Maintaining a system with more components requires more expertise and time.