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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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.
Photo Courtesy of East Northport Fire Department
East Northport, NY, firefighters use a high-expansion foam generator during a training session. When attack teams using manual hose streams have trouble reaching the seat of a basement fire, high-expansion foam generators are an attractive alternative. They generate high-volume Class A foam product that can effectively fill the fire compartment and arrive at the source to extinguish the fire.
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.