Some departments view compressed-air foam systems as extensions of standard apparatus water pump systems. They assume that all the performance ratings and conditions that apply to water pump systems also apply to CAFS. That is not true.
Here is a summary of some of the factors used to determine CAFS performance.
Flow and pressure ratings. NFPA 1901 rates pumps used on structural apparatus in terms of water flow rates at pressures of 150, 200 and 250 psi. A minimum rating of 250 gpm at 150 psi is required. NFPA 1906 rates pumps used on wildland apparatus in terms of water flow rates at pressures ranging from 100 psi to 400 psi. A minimum rating of 10 gpm at 100 psi is required.
In contrast, both NFPA standards rate CAFS in terms of simultaneous flowrates of water and air at pressures of 125 psi. The old ratings at 100 psi are no longer used. There are no requirements for minimum flow rates or for flowrates at higher pressures — one rating point is all that is required. The water and air must be discharged simultaneously at 125 psi in a ratio of at least 2 gpm of water for every 1 scfm (standard cubic feet per minute) of air. Some manufacturers rate the water and air separately without stating the simultaneous rating, but that is not correct. The system must be able to discharge both water and air at the required pressure and ratio at the same time — if you don’t see the word “simultaneous” in the rating, beware and ask.
Note that the water pump must still meet the requirements for either structural apparatus or wildland apparatus when discharging water without the compressed air flowing. This may be a problem for some slip-on CAFS units with separate gasoline or diesel engines. If you need a minimum water flow rate and pressure, especially if you want to use the unit to gain points on your ISO rating, you need to consider the water-only ratings as well as the CAFS rating.
Operation of other equipment. NFPA specifies that water pump systems must be able to produce the rated flow rates and pressures while the vehicle electrical system is delivering the total continuous electrical load and any engine-driven line-voltage generator is delivering 50% of its rated capacity. That’s pretty specific. In contrast, compressed-air foam systems must “be capable of supplying power for operating the CAFS at its rated capacity while simultaneously providing power to all other power-dependent systems installed on the apparatus.” That’s not quire as specific because it doesn’t state the load levels of the other equipment. The general requirement is still the same — the manufacturer cannot shut off other equipment to have enough horsepower to meet the CAFS performance requirements. This factor primarily applies to CAFS operated by vehicle engines through power take-offs.
Water-to-air ratios. The NFPA testing and rating requirements specify a minimum 2-to-1 ratio of water to air for CAFS — at least 2 gpm water for every 1 scfm air. The old ratio of 1-to-1 produced a less effective foam solution for most fires and is no longer required. Departments that like to use a wetter foam solution should select a system with a higher range of water-to-air ratios.
Nozzle selection. Nozzle selection for use with compressed-air foam is left to each department’s preference. There is no requirement or standard of performance. Those departments that use CAFS a lot often specify large-diameter, smooth-bore nozzles for maximum reach with pistol-grip handles for maximum control. Remote-control bumper monitors with smooth-bore tips provide an excellent way to hit fires fast without having the crew pull a handline.
Overall firefighting effectiveness. When CAFS was first introduced, there were claims that it was 10 times more effective in fighting fires than plain water. In general, most people now agree that ordinary nozzle-aspirated Class A foam can be roughly two times more effective than water, and compressed-air foam can be four times more effective. Effectiveness is measured in many ways, but the overall consensus is that compressed-air foam will knockdown fires in less time with less total water, but at the same application flow rate as water. Compressed-air foam will also lower interior temperatures faster, help prevent rekindles, and reduce water damage. Handlines filled with compressed-air foam are lighter and therefore easier to maneuver, but have greater nozzle reaction forces and are harder to control. Finally, compressed-air foam can be pumped to higher elevations with the same pump pressure, which is helpful in fighting high-rise fires and wildland fires in steep terrain.
If your department is thinking about adding CAFS to an apparatus, you need to understand the ratings and operating conditions to get the best performing system for your needs. Start by selecting a system that meets your requirements for water flowrates and pressures, then make sure it also meets the special requirements for CAFS. Once you have the system in service, train with it frequently to make sure firefighters know how to use it effectively and safely.
