CAF produces large volumes of CO2 that actually extinguishes the fire.
De-Gassing a CAFS Urban Myth - Throughout the history of the fire service an
informal passing of information has developed. This verbal passing of sometimes
critical data is so wide spread that many of us have jokingly coined the phrase
“Telegraph, telephone, tell a Fireman”! This has lead to the dilution of, or
many times, the exaggeration of facts that may be of crucial importance in decision
making. In the terminology of the layman this type of story telling is referred
to as an “Urban Myth”. One of the more recent myths that I have encountered
concerned a subject that I’m quite familiar with, Compressed Air Foam Systems
(CAFS). Like many of these myths they originate and then are perpetuated by
a small number of individuals and may thankfully be geographically isolated.
Sometimes they come about with hidden agendas such as sales, product misinformation
campaigns, or intentional misrepresentations. Granted, sometimes these myths
come from simple misunderstanding of the chemistry and physics involved with
state of the art firefighting technology.
The particular story that I’ve come across several times over the past couple of years concerns the actions on and byproducts of fires controlled with CAFS. The story as related to many firefighters, including myself, is that the application of CAF onto a working fire some how magically produces huge volumes of the inert gas Carbon Dioxide (CO2). Being somewhat familiar with chemistry as well as fire fighting I found this implausible, yet intriguing. Keep in mind that what is leaving the smooth bore nozzle of a CAFS is 99.7% water! The very idea of a small number of common elements coming together in the presence of heat and producing an inert atmosphere is very attractive and drove me to investigate more thoroughly.
These investigations involved reproducing the circumstances that would purportedly produce the CO2. The CO2 is said to not only extinguish the main body of fire being attacked, but would then migrate to other compartments within the structure, knocking down fire as it moved about. Replicating these circumstances not only involves setting up working fires in a structure, but also real time atmospheric monitoring of fire gasses produced during interior fire attacks. Personnel safety, NFPA recommendations, realistic temperatures and environments as well as the safe guarding of the involved equipment were all considerations.
All tests were conducted in a new regional Class “A” live fire training center to provide the safest possible yet realistic interior fire environment. Three burn rooms located on various levels in the building were all utilized. All recommended safety recommendations were adhered to including a safety officer monitoring temperature readouts and manning emergency ventilation actuators, R.I.C. teams with charged lines on the exterior and back-up crews with charged lines interior. This was set up as a fire attack and victim rescue scenario for local fire departments. Monitoring of the fire gasses was an added benefit to the local agencies for continuing education purposes. The fuels utilized were common wooden pallets and bails of dry straw to propagate the fires. No ignitable fluids were allowed or utilized on the training grounds.
The fire control equipment used consisted of a 500 GPM / 140 CFM Waterous CAFS utilizing 90 GPM at 110 PSI panel pressure with 0.3% of PhosCheck WD-881 Class “A” concentrate. The water to air ratio was at 2:1, producing “Wet” CAFS foam. Attack lines consisted of 150 feet of 1.75” double jacket structural hose with a 15/16” smooth bore nozzle. These are manufacturer recommendations for this type of fire attack. The atmospheric monitoring equipment consisted of a Drager pump with color metric tubes. These were operated per manufacturer recommendations and were temperature and humidity compensated during analysis of the readings. Fresh color metric tubes were used for each attack and gas level readings were ascertained immediately after each sampling. Multiple fire gasses were monitored but CO2 is the primary emphasis of this article.
Four live fire evolutions with multiple fire attacks were conducted and monitored. The conditions within the muti-story heavy gauge metal structure varied throughout just as all room and contents fires do. Temperatures measured at 1 foot from ceiling level ranged from 700 to 1150 degrees F. depending upon the particular burn room’s size and rating. This aspect was controlled by Safety, Vent and a crew of fire stokers. Fire gas sampling was conducted at a variety of locations and levels with the intent of capturing a sample of the cloud of CO2 gas that the myth is based upon. Samples were taken at the nozzle location and as near the seat of the fire as possible immediately after CAF application. Samples were also drawn at 5 foot and 1 foot from floor level. This low level was deemed important due to the fact that CO2 is heavier than air. The 5 foot location was used to account for any possible thermo dynamics occurring that could cause a lifting of the heavy gas. To account for possible room to room migration of the gas, as purported by the myth, additional samples were taken in adjoining compartments. This production and migration of CO2 is reported to be so predominating that large volumes are moved around creating a virtual “CAFS Push”. Monitoring in active burn rooms other than the one being attacked was done to account for the reported migration of the CO2 via convection and natural air flows as well as direct observation of fire behavior.
Evaluation of all samples showed that the maximum levels of CO2 detected never exceeded 15,000 parts per million (PPM) during any of the interior fire attacks. That is slightly less than one and half percent of the air by volume. This is no where near the volume needed to displace enough oxygen to extinguish a working fire. Even when combined with the other fire gasses generated by a room and contents type fire there is not enough volume of gasses outside their flammable ranges to extinguish a fire by themselves. Without a doubt CAFS has been utilized as a superior fire fighting agent many thousands of times around the globe, but this is attributed to its ability to alter the physical properties of the water into a product that reacts in a positive fashion with fire and Class “A” fuels. It’s the water doing the work, not a hypothetical production of an inert gas with mystical properties that defy physics.
After compiling the results of all the tests conducted along with comparisons to other data and observations during CAFS fire attacks in occupied dwellings, a series of conclusions have been reached. Foremost, that interior fire attack with CAFS is far superior to and safer than the same attacks with plain water due to the increased rate of heat absorption, fuels wetting and isolation, the ability to hold water on the target and the extended stream reach. Secondly, the urban myth of CO2 production to the point of being a major contributing factor in fire control is just that, a myth, now debunked through acquisition of valid data.
All aspects of fire fighting deserve review, especially a growing technology such as CAFS that that may be unfamiliar to a portion of the fire service community. I would encourage all to look at systems that make the job of protecting the internal customer (you) and the external customer (the public) safer. These technological investigations must be based upon sound scientific practices tempered with common sense. When hearing urban myths, such as this CO2 magic, that just sounds too good to be true or doesn’t stand up against science please take the time to de-gas the urban myth!
inFOAMation associates, LLC.
CAFS causes the temperature to rise within a structure as fire control
This myth is perpetuated by firefighters with little or incorrect
training. A brief application of dry foam into a super heated atmosphere will
cause extreme thermal disruption at ceiling level thus forcing high temperature
gases to floor level driving fire fighting forces out of the building and
worsening conditions for trapped occupants.
With proper application techniques and flow rates,
the temperatures in a burning structure will cool so rapidly that the atmosphere
transitions through the steam producing temperature range so quickly that very
little steam will be produced. Many cases of actual smoke inversion due to rapid
cooling have been seen throughout North America.
Using low GPM flows during fire attack will reduce over all water
Most firefighters are aware of “Critical Application Rates”. The use
of Class A foam, especially in the form of CAF does have a positive effect on
Critical Application Rates. Fire departments across North America report
dramatic water savings through the proper use of CAFS, but this savings is
achieved via quick fire control, reduced mop-up and shutting down of the nozzle
at the correct time, not low GPM flows. Low GPM flows will require attack teams
to remain in the fire area for a longer period of time to achieve control. Very
low flows are a safety concern and are not recommended.
The nozzle you select will make or break your CAFS fire attack.
TRUE ! When CAF bubbles reach the end of an appropriate length of hose line it’s construction is complete. The nozzle has a direct and dramatic impact on the foams physical form and thus it’s reaction to heat and Class A fuels. A basic rule of thumb to use is: Larger orifice = dryer foam and smaller orifice = wetter foam. This has to do with the expansion of or, compacting of the foam as it leaves the nozzle tip. The tactical needs drive the selection of tip size, which should be quickly interchangeable by the nozzle operator. The use of currently marketed fog nozzles with CAF will cause degradation of the bubble structure. The components of a fog nozzle that break water or foam solution into droplets (Fog) also breaks the CAF bubbles and releases the air forming the bubbles back to atmosphere. Thus, you are then discharging liquid foam solution with little to no bubble structure. All the time and funds expended on acquiring a CAFS capable engine are negated by selecting a nozzle that destroys the desired product. CAF is most effective and most versatile when applied via a smooth bore nozzle. A fog nozzle is an excellent and indispensable tool; it’s just not the tool for the task of discharging Compressed Air Foam.
#5 Flows can be diminished or completely stopped due to hose kinks.
If you have other concerns regarding Compressed Air Foam, please feel free to ask us!!!