By Mark J. Cotter
In this second installment, I will go back to the basics, looking at two myths related to the products of combustion, specifically heat and smoke. I address a false assessment of fire dynamics that fire service authors and speakers cite repeatedly that is based largely on an accurate, but limited analysis of the combustion process. A half-myth is one that is true, but at the same time virtually useless, adding little to our knowledge and more importantly, changing nothing in our practices. First, though, an update on the myth example used in the introductory column, water curtain nozzles.
It seems that even these devices, long ago discarded by most in the fire service as a method to prevent the transfer of radiant heat from a fire to an exposure, still have their proponents. I heard of a chief who had just purchased such an appliance despite advice to the contrary from his junior officers. But to his credit, after reading my column (in which I described how useless these devices were), he shared this information with others. Also, as testament to this myth’s persistence, I recently received a new fire equipment catalog that advertised just such a device, but now called it a “water wall.”
One reader actually listed several valid uses for such appliances: using chocks to tilt the appliance and direct the spray to strike, and thereby directly cool, the exposure, essentially creating an unmanned protective monitor. You could also position the appliance to block the movement of toxic fumes, such as an acid cloud from a leaky vessel, although he noted the new problem this raised–controlling runoff. A firefighter in Australia reported that he and his fellow “firies” use water curtains to wash the salt water from their personal vehicles after a trip to the beach.
In an appropriate turnabout, this same firefighter from Down Under requested that I cite the research that showed that water is not effective in blocking radiant heat. He asked for this partly because of his department’s routine use of fog nozzles to provide protection in fighting flammable liquid fires (see photo above). My subsequent attempts to accumulate evidence about what I believed to be a “common sense” fire science “fact” was enlightening in its own right.
Water sprays do provide some protection from radiant heat; most absorption occurs when the water is in the form of tiny droplets. This is why water sprays are still used to protect oil-drilling platforms as they burn off natural gas. In fact, “Water Curtain” is now a registered trademark of Burner Fire Control (http://www.americanfireequipment.net/ServiceAreas/ServiceAreas.asp), describing their high-volume, fine-mist devices (this explains why that new catalog I mentioned used the term “water wall”). This also partly accounts for the benefit seen from the “branch” (nozzle) used by our brothers in Australia, although the effects of the entrained air cooling the nozzle crew and blocking convection is difficult to quantify without experimentation (anyone looking for a research project?).
Reportedly, it was Underwriters Laboratories that showed that cooling improved when water was applied directly to a surface, though I have as yet been unsuccessful in getting my hands on the specific experiment that was performed, relying instead on a reference in the Fire Protection Handbook, 20th Edition, pages 17-35. The research reported that water sprayed between a heat source and a metal surface did little to block radiant heat transfer, but the same flow directed onto the surface cooled it sufficiently to prevent damage. Another paper, “Thermal Shielding by Water Curtains,” by Jean-Marie Buchlin, Professor, von Karman Institute, Applied and Environmental Fluid Dynamics Department, Belgium (http://www.iitk.ac.in/che/jpg/papersb/full%20papers/B-%2071.pdf), demonstrated that a vertical array of misting nozzles could block up to 50 to 75 percent of radiation emitted from a source, whereas the same flow applied to the exposure blocked 90 percent of the heat, thanks to the added effects of direct cooling by evaporation. This improved protection provided by water sprays applied to an exposure, rather than merely nearby, explains why National Fire Protection Association 13, Standard for the Installation of Sprinkler Systems, requires that sprinklers installed on the exterior of buildings be positioned so that the spray strikes, and thereby wets, the exposure, protecting the building from radiant heat from potential nearby fires.
Keep in mind that the water curtain nozzles that I referred to in the last column, once common in the fire service (at least in North America), produced a coarse, fan-like stream, rather than the high-velocity, small-droplet mist used in the research referenced above. That difference in the quality of the spray explains the relative inefficiency of these ancient devices in blocking heat. On the other hand, although the dense clouds of fine water droplets described in the experiments cited provide real protection from radiant heat, they require multiple nozzles with overlapping patterns, several deep, to protect a large surface. Their usefulness is also hampered by their limited reach and inability to withstand the effects of wind. These qualities confine their outdoor use to fixed systems on high-hazard occupancies (e.g., flammable liquid storage tank farms), or a relatively small area as illustrated in the Australian gas fire attack photo above. Anything even a few feet to either side of the fog-mist nozzle would not be protected from radiant heat. (Indoor use of “mist” nozzles is another story altogether, with a lot of proponents, supporting research, and success, and something that I hope to explore in future columns.)
The point of the water curtain example was to show how a device that looked good, visually and theoretically, and was widely adopted by the fire service, was later shown experimentally to be less effective than just using the devices (nozzles) we already had on hand to spray water on exposures for their protection. I did not intend to argue that water does not absorb radiant heat. It certainly does, just not as efficiently as when applied directly to an exposure.
On to this column’s subject: How many times have you read or listened to this statement from a seasoned fire service “expert”: “The fires we are fighting these days are hotter than those of our predecessors?” The logic is that hydrocarbon-based materials (plastic, polyesters, etc.), which give off heat much more quickly when burned, have replaced cellulose-based materials (wood, cotton, etc) as the primary component of modern buildings and their contents. For example, an old-fashioned cotton mattress has a peak heat release rate (HRR) of up to 970 kilowatts (kW)–one kW equals the production of 1,000 joules (or 0.949 Btu) per second. A modern polyurethane mattress, even at a third of the weight of its cellulose-based ancestor, can generate up to 2,640 kW (For reference, an HRR of about 2,000 kW (or two megawatts (MW))–is commonly considered necessary to heat a “typical” residential room to flashover.)
The mistake comes when the fact that these “modern” materials burn more quickly leads to the assumption that the fires we now face are therefore hotter, which, surprising as it may seem, is false. Structure fires, at least indoors, burn no hotter today than they probably did 200 years ago. The reason for this is relatively simple, was first described more than 90 years ago, and has been repeatedly proven by fire service researchers. This myth has been used to justify using larger diameter handlines and flows on interior fires, as well as to attempt to debunk tested and validated fire flow formulas. Another attraction to this falsehood, at least for some, is that it makes us feel like we are battling fires that are stronger than those faced by the legendary firefighters of yore.
Let’s go back to basics–the fire triangle–for an explanation of how something made of essentially of solidified petroleum burns no hotter than a hunk of wood, at least in the settings in which we will most likely be trying to effect its extinguishment. As we know, combustion requires heat, fuel, and oxygen (and, for you newfangled fire tetrahedron proponents, a series of interrelated chemical reactions). Any change in the quantity of any of these components will affect the intensity of the fire. Well, it seems that oxygen is a significant limiting factor in compartment fires, and that describes most structure fires that we have any chance of controlling with an offensive attack.
W.M. Thornton reported in 1917 that the amount of heat generated by burning most organic compounds (which are those that include carbon in their molecular structure, including cellulose and hydrocarbon materials) is constant per unit of oxygen consumed, later found to be 13.1 mega joules (MJ) per kilogram (kg) of oxygen, with about a five-percent margin of error. Termed Thornton’s Rule, the validity of this phenomenon has formed the basis for determining the heat output of the combustion of various materials using an oxygen depletion (or consumption) calorimeter. These devices do not measure the heat given off from burning a test sample–quite difficult to do, given the need to account for heat added for ignition; lost through convection, radiation, and conduction; and remaining in the unburned material. Instead, they measure the amount of oxygen consumed in the combustion process of a particular sample, which is then used to calculate the amount of heat produced.
Although burning a pound of gasoline and a pound of lumber outdoors, or anywhere there is no limit to the inflow of fresh air (containing oxygen), will generate remarkably different amounts of heat, specifically because of the greater amount of oxygen the gasoline fire will consume, the same does not hold true for different fuels indoors. Such fires are inevitably regulated by the amount of oxygen present to support combustion. In other words, since you can only fit so much oxygen (as a component of air) into a space at once, you can only get so much heat out at time.
True, a fire in a room filled with material with a high HRR will burn faster, consuming the available oxygen more quickly, than a fire involving low HRR materials, indicating that the high HRR fuel is certainly more dangerous to occupants. Despite this head start, however, the heat outputs will eventually equilibrate based on the amount of air in the room, at least until enough time passes that the fire burns its way out of the compartment and overcomes that limitation. Since offensive interior fire attacks are, almost by definition, undertaken only on compartmented fires, this cap on heat production is valid during such activities. If more air is introduced, as a result of the windows failing or firefighters venting the structure, the increase in heat generated will be proportional to the amount of air flow, not the type of fuel.
This concept was recognized by Keith Royer and William Nelson of the University of Iowa when they performed their research in the 1950s that calculated and proved the amount of water required for fire knockdown. Regardless of the fire load present, they determined that the volume of the room (i.e., how much oxygen it could contain) was the determining factor. Essentially, they demonstrated that if water was flowed in sufficient quantity to overcome the amount of heat that the amount of oxygen in the compartment could produce, the fire would stop burning (more on this, too, in a future column).
The point is not that we should return to the practice of stretching booster hoses on structure fires, or to minimize the amount of heat generated at a fire. More water is always a good thing (though always having more than we need is better than always flowing all that we have–water damage is a preventable error). Also, fires involving even a roomful of antique furniture can get hot enough to reach flashover temperature (about 600°C), which is far beyond the limits of our personal protective equipment (modern turnouts will begin charring at about 300° C, and the polycarbonate that is used on SCBA facepieces will begin to soften at 140° C). My intent is to show that we need to look at all of the facts before we proclaim a need to change our tactics (e.g., use bigger hoselines) or generally declare that the sky is falling.
The half-myth I want to touch on is actually just annoying, at least to me, and is an example of information that is, as far as firefighters are concerned, useless: “Smoke is more toxic these days.” I’m reminded of the movie, “No Country for Old Men,” when Woody Harrelson’s character, about to embark on the search for a psychopathic killer on the loose in Texas, is asked, “How dangerous is he?” His answer “Compared to what? The bubonic plague? He’s dangerous.” Smoke is dangerous too, and always has been.
Consider the known components of the smoke from the combustion of good, old-fashioned, organically-grown wood:
- Carbon monoxide: a toxin that needs no introduction;
- Nitrogen dioxide: capable of causing respiratory distress;
- Polynuclear aromatic hydrocarbons: carcinogenic (cancer-causing) agents;
- Formaldehyde: keeps a dead body from decaying, but damages the lungs of the living;
- Acid gases (hydrochloric, sulfuric, nitric): not good to even touch, much less inhale;
- Phosgene: once used as a chemical weapon, and still effective; and
- Benzene: carcinogen and asphyxiant (smothering agent);
From “The Breath from Hell,” by Steve Bernocco, Mike Gagliano, Phil Jose, and Casey Phillips, Fire Engineering, March 2006, (http://emberly.fireengineering.com/articles/article_display.html?id=251465
True, the universal and extensive presence of synthetic building products and contents has introduced a wide variety of newly identified, deadly components in smoke, most notoriously, hydrogen cyanide. Still, a major producer of that particular poisonous gas is the combustion of wool–hardly a space-age material. My point is that smoke has always been toxic, and we firefighters have always needed to wear our SCBA anytime we were in a situation that might lead to inhaling products of combustion, whether the combustion had ceased or not. The fact that it is “more deadly” changes nothing in our practice (at least as far as prevention is concerned–there is recent progress in treating the results of any smoke inhalation that addresses the presence of hydrogen cyanide in its victims).
Maybe some firefighters resistant or unused to training, supervision, and discipline will be persuaded by this “new” hazard and increase their use of SCBA; that would be a good thing. Still, if they weren’t afraid of the deadly stuff in “old-fashioned” smoke, it’s hard to imagine that longer lists of longer chemical names will get their attention.
Now, all fire is hot, and all smoke is deadly, so we have no reason to let our guard down. We just need to keep in mind that the foe we face is, in many ways, the same as that of our predecessors. We need to learn from and improve on their methods, not dismiss them for lack of relevance to today’s fire environment. The danger of this myth, like others, is when we base decisions on it.
Mark J. Cotter has more than 30 years experience in emergency services and is currently a volunteer lieutenant with the Salisbury (MD) Fire Department. He will be presenting “Emergency Service Myths” at FDIC 2009 in Indianapolis, Indiana, and can be reached at markjcotter@comcast.net.
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Subjects: Firefighting myths, water curtains, heat release rate, compartment fires, modern building components and materials, smoke toxicity, firefighter tactics
