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 on Lyon's Publishing's Web page at www.classafoam.com or by telephone at 610-792-3115.
Two types of foam bubble generating systems produce Class A foam. These are Nozzle Aspirated Foam Systems (NAFS) and Compressed Air Foam Systems (CAFS). Let's overview NAFS and CAFS including how they mechanically operate, the foam quality types that they produce and their firefighting utilization.
NAFS consist of a water source, a fire pump, a foam proportioning system, fire attack hose and an air-aspirating nozzle. In a typical structural pumper equipped with NAFS, the fire pump drafts water from a municipal, static, or booster tank supply and discharges it into piping connected to a foam proportioning system. The foam-proportioning system injects or educts foam concentrate into the discharge forming foam solution. The Class A foam solution is pushed out the discharge through a hoseline and to an air-aspirating nozzle. Along with directing the stream, the air-aspirating nozzle provides turbulence or agitation to aspirate the foam solution to create finished foam.
NAFS are often called "low-energy" generation systems because they rely on the energy created by a fire pump to not only propel the foam stream from the nozzle, but also to aspirate the foam solution. Therefore, as nozzle expansion ratios increase, less energy becomes available to propel the fire stream (such as when using medium expansion air-aspirating nozzles) and vice versa. This is due to the fixed hydraulic energy available from the fire pump at a given foam-solution delivery rate. Since energy is expended when air is sucked into the nozzle, the higher the nozzle's expansion ratio, the shorter the distance the foam can be propelled at a given delivery rate.
Expansion ratio is the term used to measure the final volume of finished foam produced, as compared to the original volume of foam solution. For example, if an air-aspirating nozzle draws in nine units of air for each unit of foam solution discharged, this equates to a 10:1 expansion ratio of finished foam to foam solution.
EXPANSION RATIO DEFINITIONS
Low expansion - Up to 20:1
Medium expansion - 21:1 to 200:1
High expansion - Above 200:1
When the term NAFS is used, it designates the following discharge devices that use atmospheric air pressure for foam bubble generation:
- Air-aspirating nozzles (low and medium expansion).
- Variable-gallon/constant-flow fog nozzles and automatic fog nozzles.
- High-expansion foam generators.
There are many different nozzle types available that provide low and medium-expansion foam. These can be quickly changed on the end of the hose to create a wide range of foam qualities and expansion ratios for various tactical uses.
Air-aspirating nozzles. Air-aspirating nozzles use the energy of the fire stream passing through the nozzle to draw air into a foam tube at the base of the nozzle. The air then aspirates the foam solution to create finished foam as it leaves the nozzle. Available in low- and medium-expansion ratios, air-aspirating nozzles are available as complete stand-alone units or as tube-type attachments that clip onto variable-gallon/constant-flow and automatic fog nozzles.
Low-expansion finished foams produced by an air-aspirating nozzle, particularly on the lower end of the low-expansion range, are excellent for exterior structure fire attack. Each air-aspirating nozzle has its own gpm flow rate. For structure firefighting, be sure to choose the nozzle with the appropriate flow rate (such as 125 gpm). Many air-aspirating nozzles are made for low-flow wildland firefighting usage (such as 20 gpm).
Air-aspirating nozzles can be awkward for use in interior fire attack. Depending upon their length, they can be difficult to maneuver into position while advancing down a tight hallway or when making a sharp turn through a narrow doorway.
Medium-expansion air-aspirating nozzles are not suitable for structure fire attack because of their short discharge distances. Since these nozzles use most of the fire stream energy to create finished foam, little energy is left to propel the stream. However, in fire situations such as overhaul, where discharge distance is not an issue, medium-expansion foam nozzles produce a drier finished foam that works well to fill wall cavities and other inaccessible or poorly ventilated fire compartments such as basements, attics, and pipe-chases. The foam produced by these nozzles also works well to blanket burning rubbish and tire piles to cut off oxygen while wetting and cooling the fuels. Medium-expansion foam is excellent for the overhaul and mop-up of structure fires.
Caution! A misapplication of Class A foam that has occurred often is one in which firefighters use air-aspirating nozzles to apply foam to burning fuels such as baled hay. While the fuel is blanketed with foam, there is little penetration. As a result, the fire continues to burn under the foam blanket. We then hear "the foam did not work," when, in fact, the application was incorrect. In deep seated fires the use of a smooth-bore nozzle provides the fastest penetration and extinguishment.
Variable-gallon/constant-flow and automatic fog nozzles. One option used by many departments just beginning the implementation of Class A foam, is foam application through their existing fog nozzle. This is ideal in that it means little change in firefighting technique; essentially the same delivery rates and application methods used for water application are used for foam applications.
Fog nozzles aspirate foam solution because their streams draw in air after leaving the nozzle. Depending upon nozzle design and stream pattern, these nozzles produce low-expansion, sloppy foams with expansion ratios of about 2:1 to 4:1. These nozzles work well for a structure fire attack as the foam they produce clings to and then quickly drains out to penetrate gypsum wallboard and other interior surfaces.
Most firefighters are familiar with the idea of a fog stream directed through a window to ventilate an area during overhaul. Inside the building, a narrow fog stream sprayed through an exterior window moves air to remove smoke and heat from the interior. When we use a fog nozzle to deliver Class A foam, the same spray pattern acts to draw air into the foam-solution discharge creating a low-expansion, sloppy foam.
High-expansion foam generators. The ever-increasing number of fires occurring in warehouses, storage buildings, basements and tunnels has firefighters coming across fire situations where the use of traditional tactics - the advancement of hoselines to deliver water or foam streams - are not effective because hose teams have trouble reaching the seat of the fire. Often, in these fires, the combustibles are Class A and B types, with hazardous materials sprinkled into the mix. In these situations, high-expansion foam generators offer an alternative fire extinguishing procedure that, when properly deployed in many instances, will automatically seek out and extinguish the fire at the source. The high expansion foam blanket acts to transport the water to the fire, suffocates and cools it, while suppressing escaping vapors and encapsulating particulate.
High-expansion foam generators deliver expansion ratios between 200:1 and 1,000:1. The foam is capable of totally flooding large rooms and enclosures allowing it to reach the source and effectively extinguish horizontal and vertical (three dimensional) fires. After the foam dissipates, there is minimal water damage to the fire compartment and contents because of the foam's low water content.
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.
Using no nozzle on an attack hose is tough for structure firefighters to accept. This is understandably so, because in our previous discussion of NAFS, the type and design of each nozzle are very important factors in the quality of the finished foam produced. With CAFS, the hoseline nozzle does not create the finished foam. The CAFS apparatus creates the foam inside its piping and/or the attack hoseline - not at the nozzle! All that is required is a control valve so the hose team can shut off foam flow.
If desired, in place of using only a 1/4-turn ball control valve on a CAFS attack hose, you can attach an appropriate size smooth-bore nozzle to the valve (for example, for a 1 3/4-inch attack hose, use a 1 3/8-inch smoothbore nozzle attached to a 1/4-turn ball valve with the same inside diameter). Adding a smoothbore nozzle to the control valve will not improve stream quality or reach. It will, however, make firefighters "feel better" because there is some sort of nozzle in their hands.
In using CAFS for interior structure firefighting, some departments take issue with using a smoothbore nozzle for fire attack. This is because most have previously trained with and now use fog nozzles on a narrow fog pattern for interior attack with water. In these instances, a less desirable alternative is to install a variable-gallon/constant-flow or fog nozzle adjusted to the "flush" position on the hose in place of the smooth bore nozzle.
Most 1 1/2-inch variable-gallon fog nozzles have manual flow adjustments at 60-, 95-, 125-gpm, and so on. If some manufacturer's nozzles are adjusted further, into the "flush" position, the internal waterway opens up to over 300-gpm. This "flush" opening is large enough to allow a marginal compressed air foam stream through, with breakup of a portion of the finished foam bubble structure.
Even though some variable-gallon nozzles can be operated in the "flush" position as an alternative to a smoothbore nozzle to apply compressed air foam, nevertheless some compressed air foam bubbles are destroyed as they pass through the nozzle reverting to foam solution, reducing foam effectiveness. The only advantage of using the variable-gallon nozzle on "flush" is that the nozzle operator has the option to deliver either a CAFS straight stream or fog pattern for interior attack.
Each department using CAFS must evaluate their nozzle options and identify which works for them. High-quality training is the key to success when implementing the use of CAFS.
CAFS foam consistency is flexible for different tactical firefighting uses. Adjusting the volumes of foam solution and compressed air entering discharge lines alters finished foam product consistency. For example, a low-expansion ratio of air to water of 7:1 forms a quick-draining, runny Class A foam blanket used for quick knockdowns on direct structure attack. A medium expansion ratio of 30:1 produces a dry, shaving cream-like discharge used for exposure protection applications.
Dominic J. Colletti is the national OEM accounts manager at Hale Products Inc. in Conshohocken, PA. Colletti is a volunteer firefighter with the Royersford, PA, Fire Department and has served with Engine Company 3 of the Coram, NY, Fire Department. He has over a decade of experience in the research and development of Class A foam and CAFS application for structure firefighting. His e-mail address is: [email protected]
