What are compressed air foam systems (CAFS)?
CAFS are systems that mix foam solution with compressed air to generate a homogenized mixture of foam bubbles that are dense and tightly packed. The basic components of a CAFS are a fire pump, a rotary air compressor and a foam proportioning system.

CAFS Structure Attack Experience: Manchester, NH
The Manchester, NH, Fire Department is a good example of a fire organization protecting an urban environment that has adopted CAFS specifically for structure fire control. Manchester received its first CAFS-equipped pumper in June 1995 and took delivery o f two more in April 1996. Each is equipped with a midship pump and a compressed air foam system.

Manchester Fire protects a New England mill city of 32 square miles with a population of 100,000 fire protection customers. The department has nine engines, six ladders and one heavy rescue. The annual department budget is approximately $12 million. Curre nt staffing consists of three-person engine companies and two-person ladder companies, with four firefighters assigned to the heavy rescue. In a report to the National Fire Academy, Captain Gary Simpson, Manchester's CAFS Training Program Leader, discusses a fire response that occurred in December 1996:

"...The last fire I would like to comment on is a fire we responded to in late December 1996. The initial call was for a bed fire. Our initial response turned out to be three engines, two trucks, a rescue and a district chief. Two of the initial engines t o respond were CAFS pumpers.

"The first-arriving pumper, Engine 7, found a three-story wood-frame apartment building at the alarm location. The dwelling had a flat roof with overall dimensions 60 feet wide by 50 deep. Heavy fire was showing from the backyard; all three floors with re ar overhang porches were fully involved.

"Two other apartment buildings in the complex, exposures adjacent to the main fire building, had fire showing with extension on the second and third floors. As if that weren't enough, in the backyard there were also two fully involved motor vehicle fires. Continuing, yet another exposure in the backyard was found steaming from the heat of the main fire building.

"Upon arrival, the lieutenant of Engine 7 ordered an LDH (large-diameter hose) supply line dropped from a nearby hydrant (left uncharged) and positioned his truck and engine companies in a parking lot at the rear of the fire building. Initially one 13Ú4-i nch and one 21Ú2-inch CAFS lines were pulled for fire attack. Using both these CAFS handlines from the exterior of the building, Engine and Truck 7's crew was able to extinguish the two car fires and knock down the exterior autoexposure fires adjacent to the main fire building. Foam application from the backyard also significantly darkened down the main body of interior fire within the original fire building's first and second floors.

"The Engine 3 crew then pulled another 13Ú4-inch CAFS line and continued to advance it through the front door of the main fire building. They advanced the hose to the third floor and quickly knocked down the body of fire there.

"This was a fire that easily should have had a sustained fire flow in excess of 2,000 gpm with the potential of keeping fire crews on scene all night. It potentially could have done serious damage to adjacent buildings. It was brought under control using only the tank water carried on two engines through two 13Ú4- and one 21Ú2-inch CAFS hoselines."

CAFS Features And Benefits For Structure Fire Control
What do departments find when they use compressed air foam for structure fire control in place of water? They find:

1. Flaming and non-flaming combustion is extinguished in less time and with less water. Available water supply and personnel resources are used much more effectively.
2. Compressed air foam clings well to threatened exposures. Less water is wasted during exposure protection applications.
3. Compressed air added to a foam solution stream provides extra energy for stream propulsion. Compressed air foam streams are projected further than water streams.
4. Finished foam produced by CAFS is dense; it clings better to burning or ready-to-burn materials and has a longer drain time. This allows superior fuel moisture penetration and cooling.
5. Hoselines are filled with a partial volume of compressed air (about 52%), therefore they are much lighter and easier to carry and advance.
6. CAFS produce foam types ranging from "wet" to "dry." By providing these options, the ideal foam type can be applied to each fire to maximize the effectiveness of water and personnel for each tactical fire control purpose.
7. With CAFS, foam bubble generation is very efficient, therefore less foam concentrate is required to generate foam streams as compared to using nozzle aspirated foam systems (NAFS).

These seven items collectively contribute to a significant gain over using water or Class A foam application by NAFS to control structure fire. This results in:

• Increased operational efficiency of available fire control resources such as water supply, personnel and apparatus.
  
• Increased firefighter safety.
  
• Reduced property damage.
  

CAFS And The Structure Fire Attack
Let's discuss previous experience and some of the issues involved in applying CAFS for the structure fire attack. (Note: All discussion regarding fire attack from this point forward refers to using a one-quarter-turn ball valve with, or without, a smooth- bore nozzle attached as the foam application device.)

Applying compressed air foam at working structure fires extinguishes a greater volume of fire as compared to water or Class A foam generated by NAFS. Using the specific compressed air foam consistency for the specific fire control purpose means the absolu te most fire-stopping power is garnered from the available water supply. While this benefits every type and size of fire department, it is most beneficial to rural and suburban departments with water availability restrictions and which must use tanker shu ttles. Also, the amount of overhaul labor and time required to mop up deep-seated combustion in building contents is reduced.

CAFS hoselines – normally 50% lighter in weight than those delivering water – can be advanced throughout structures with less effort, reducing firefighter stress. Engine company members adjust quickly to pulling the lightweight CAFS hose through hallways and over stairway banisters to upper floors. Once accustomed to handling lightweight CAFS hoselines, it's not easy for firefighters to go back to hauling water-filled lines.

With the additional energy of compressed air propelling hose streams, they penetrate farther into a building's interior. This comes in handy when combating a fully involved structure – since fire attack has to begin from the exterior anyway, CAFS str eams clearly reach and extinguish more of the core fire inside. Crews entering to finish up find less fire remaining, which translates to a less hazardous working environment.

Since exterior CAFS applications do such a good job at flame knockdown, some fire instructors say that they eliminate the need for an interior attack. This is untrue.

Compressed air foam hose streams do not change the reasons for a coordinated aggressive interior attack. Single-room interior fires must still be located, confined and extinguished in the same manner as that used when applying water. And just as with wate r hoselines, attack teams should position CAFS streams to push heat and toxic gasses away from victims, firefighters and non-fire involved portions of the building. What are conditions like during an aggressive CAFS interior fire attack? The most notable, besides quick flame knockdown, is the virtual absence of steam generation associated with conventional fire streams.

Therefore, provided adequate ventilation, interior visibility is enhanced when teams move through fire areas, after the fire is darkened.

High quality compressed air foam clings to both horizontal and vertical surfaces – walls, ceilings, doors, drapes, couches, tables and so on. It is believed that more of the foam clings to and penetrates surfaces than evaporates into the room's atmos phere. The benefit to firefighters is that, with no excessive steam production, the foam application does not disturb the thermal balance of gases inside burning rooms. Undisturbed layering of combustion gases means more effective natural horizontal and v ertical ventilation and better visibility. With the steam generation associated with a plain water attack reduced, interior crews are not driven to the floor and the "cooked lobster" feeling of superheated steam penetrating turnout gear is lessened.

As an added bonus, the overhaul process of extinguishing deep-seated fire begins upon initial compressed air foam application because foam hangs on to room materials. As such, there is less labor involved later on, during the mop-up phase.

Adjusting The Interior Fire Attack For CAFS
Departments that plan to use CAFS for structure fire attack must realize that adjustments to the nozzle handling tactics used for conventional water application are required.

In the past, using water, we have learned that very high water flow rates, applied for a short duration, effectively stop interior fires. When using these high water delivery rates, we have also learned to immediately shut the nozzle control valve when fl ame darkening occurs. This prevents excess water damage.

While we also use the same principle of "high flow-short duration" application with CAFS, nozzles must not be shut as soon as flames darken. If a CAFS hose stream is immediately shut after an interior fire is darkened, the fire will most likely rekindle a nd/or the atmosphere will remain untenable for firefighters because of high heat.

Why does this happen? Because compressed air foam is a very efficient flame-extinguishing agent. It is believed that it absorbs enough heat from solid fuels to reduce vapor distillation to the point where flaming combustion ends in a very short time. Howe ver, even though the flames are quickly darkened, there remains considerable heat radiating from a room's interior finish and contents.

To account for this CAFS subtlety, a tactic that works well is to continue applying foam to interior surfaces – even though visible flames are gone – for the same amount of time it initially took for blackout to occur. For example, if it takes 1 0 seconds for blackout to take place while combating a well-involved room and contents fire, continue applying foam, uninterrupted, for an additional 10 seconds to the room's interior surfaces. This coats fuels with a layer of foam and provides enough add itional agent to absorb residual heat from the compartment.

The trick is not to "over apply" foam and defeat the water-stretching benefit it provides. How will you know if you apply foam for too long a time? One way to tell is when foam begins to collect and flow over the floor.

Exposure Protection
Compressed air foam is not a "silver bullet" for exposure protection – some firefighters expect that "one coat does it all." While CAFS foam is not a long-term heavy-duty barrier, it still works well but may require replenishment after evaporating of f the exposure surfaces.

Applying a dry shaving-cream-type foam onto vinyl- or aluminum-sided dwellings provides good radiant heat insulation. Depending upon the proximity of the exposure to the flames, the foam blanket lasts for a time before evaporating and then needs replenish ment. The more intense and closer the flame source to the exposure, the quicker the foam evaporates. The farther away the flame source, the longer the foam lasts.

Steady streams of water are not required. And when the foam evaporates off the exposure, high foam visibility upon reapplication insures no more than needed is applied. As such, total water requirements are trimmed for exposure protection applications.

When applying foam onto a home with unsealed wood, a tactic that works well is to apply a wet Class A foam followed up by a dry foam covering. The dry foam acts to "cap" the wet and gives it time to penetrate the unsealed wood. The dry foam, with its long er drain time, is a cap to prevent the wet foam from evaporating.

This keeps the ambient air from drying out the wet foam and the fuel surface. Whenever the wet foam or fuel surface is exposed to air, moisture evaporates off the fuel. Fuel moisture always tries to reach equilibrium with the air's relative humidity. Usin g a dry foam cap prevents fuel moisture from evaporating and keeps the fuel wet for a longer time, delaying ignition.