This article is the third of four excerpts from a new book, Class A Foam - Best Practice for Structure Firefighters , by Dominic J. Colletti. Larry Davis is the technical editor of this 240-page educational textbook (© 1998 by Lyon's Publishing, Royersford, PA). Ordering information is available...
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Although high expansion foam generators have various designs, most use a fixed spray nozzle in a large enclosure that has a screen or net on the end. The spray nozzle draws air into the enclosure and mixes it with foam solution. Both are forced through the screen or net to create high expansion foam. A number of high-expansion foam generators use a water motor (water turbine) turning fan blades to induct more air through the netting. High-expansion foam generators that use gasoline engines to power the fan are also available.
Non-Aspirating Smoothbore Nozzles
While smooth-bore nozzles produce little bubbling action, they are still used to apply Class A foam solution and are especially well-suited when the fire calls for immediate foam solution penetration - in other words in those instances where ultra-deep-seated fire is the problem and where a foam bubble blanket is more a hindrance than an asset.
Smooth-bore nozzles provide little to no aspiration; the only agitation to produce bubbles comes about when the solid fire stream hits objects and breaks apart. This provides a minimal amount of turbulence and very little foaming action. Where Class A foam application through smooth-bore nozzles comes in handy is at barn, dump, trash and other fire scenarios where deep-seated fires can occur. The advantage of smooth-bore streams is favorable in these situations because the Class A foam solution immediately penetrates deep inches or even feet into the fuel. In situations like these, where deep and rapid penetration of the foam solution is critical to extinguishment, we want to minimize foam bubble production.
Conversely, smooth-bore nozzles are poor for structure attack. Without aspiration, much of the foam solution fire stream winds up being wasted as it rolls off interior surfaces, and runs out the front door or into the basement.
Compressed Air Foam Systems
A CAFS consists of a water source, a fire pump, a foam proportioning system, an air compressor, and ancillary controls that tie all the components together for effective pump operation.
How do CAFS work? In CAFS, an air compressor injects air into foam solution within the fire pump discharge piping. The air and foam solution mix as they move through a mixing chamber and into an attack hoseline or pre-piped monitor. Unlike low or medium-expansion air-aspirating nozzles that mix air with foam solution in the foam tube, CAFS use the scrubbing action of the turbulence within the mixing chamber and the attack hoseline to create the finished foam.
Foam bubbles produced by CAFS are high quality - very small, consistent in size, dense and tightly packed. Therefore, they interact with fire differently than foam produced through NAFS and have much longer 25% drain times. For a wide range of fire requirements, CAFS can provide foam consistencies ranging from wet, runny solutions, to thick, dry foam, similar to shaving cream. CAFS benefits include reduction in the weight of the hoseline (the hose is filled with approximately 52% compressed air) and increased foam stream discharge distance. Using CAFS, hose handling is easier, stream reach is excellent, and flame knockdowns are quick.
CAFS stream discharge distances are enhanced because of the additional energy added by way of compressed air. This means increased penetration from the exterior of a fully involved dwelling, giving the foam a better chance to reach the seat of the fire, where it's required. There is a noticeable difference between CAFS and NAFS upon direct attack. CAFS dramatically reduces knockdown time, generates little smoke and steam, and minimizes water damage.
CAFS Are Different From NAFS
Here are a few high points about CAFS that help explain just "how different" they are as compared to NAFS.
Foam bubble production occurs inside CAFS apparatus piping. Therefore, no nozzle is required on the end of a CAFS attack hose to create the finished foam bubbles. The only requirement for a discharge device is a ball valve alone, and this is placed there only to be able to shut down the hose stream.
A common CAFS question is: Even though bubbles are made in CAFS apparatus, isn't a nozzle still required to enhance a CAFS fire stream? A nozzle is not needed because the finished foam is ready for application as it leaves the last section of hoseline. Compressed-air foam leaves the hose as a high-quality product and does not need improvement by being sprayed into a pattern, expanded with more air or shaped by any type of nozzle. I will say it again, it is done, finished - nothing further is needed! The optimum way to discharge compressed air foam is as a solid stream through an appropriately sized turn ball valve.