The scientific foundation for fighting fires; the author proposes a solid scientific foundation...

DAVID P FORNELL IN HIS BOOK. Fire Stream Management Handbook, (Fire Engineering, 1991), proclaims: 'Most firefighters have blindly accepted as scientific truth, fire service lore, which has been handed down as sound tactics through generations of officers, instructors, and firefighters, even

Thus the scientific foundation for fighting fires is either not known, or is grossly misunderstood.

Most of the information in this paper was obtained from the NFPA Handbooks, 17th and 18th Editions, and the Society of Fire Protection Engineering Handbook, 2nd Edition. It expands upon the Chapter on 'Fire Behavior' in the 'Essentials of Fire Fighting', 4th Edition, and is done so by adding new information, reorganising all this, and in some cases correcting misinformation.

The result is a solid scientific foundation for firefighting that leads to safe and sane strategy and tactics. Firefighting is, or should be, simple and easy enough to perform. Certainly the principles are easy enough. It is only in extraordinary circumstances that difficulties arise that complicate the fighting of fires.

Conserving Energy

The most fundamental scientific principle, that is the foundation for everything else, is the Law of Conservation of Matter-Energy, in simple terms, this law states that matter, like energy, cannot be created or destroyed. If you will think for a moment, you will realise that many times statements are made that do not appear to conform to this law.

After a fire, for example, frequently it may be said that a house was destroyed. You have to be careful about the exact meaning of this statement. While it is true that the house no longer exists, it is also true that all of the materials that made up the house have not been destroyed. The solid materials that made up the house have either been transformed into gases or ashes, or have been converted to energy. Of course, this fact is little comfort to the homeowner. From the owner's point-of-view, everything is gone.

Likewise you may hear the statement: "We've got to conserve energy". In fact it is impossible not to conserve energy. We can not do otherwise. Energy is always conserved. The problem is that when energy is used, it is transformed in such a way that it no longer is available to be used. The correct name for this problem is enthalpy, and unfortunately enthalpy always increases.

Chief Lloyd Layman in Attacking and Extinguishing Interior Fires (NFPA, 1955) has stated the following principle: 'The control and extinguishment of interior fires is based upon the principle of removing the excessive heat from the involved building'.

Heat, of course, is the outstanding feature of fires, and the principal problem in fighting fires. Heat is energy, and as such cannot be destroyed. So just how is it possible to 'remove' heat from a building that is on fire?

In the 19th century scientists thought that heat was a substance, a colorless, mass-less, odorless, substance that flowed from one object to another. In other words, it was a substance that flowed from a body with a higher temperature to a body with a lower temperature. This substance was called "caloric".

Scientists now know better. Heat is not a substance. It is a process, a form of energy. There are two types of chemical reactions that involve this process. In one type, heat is absorbed by the chemical reaction. In the second type, heat is released by the reaction. The first type is called 'endothermic'. The second type is called 'exothermic'. 'Endo' means 'in'. 'Exo' means 'out'. 'Thermic' means 'heat'.

The combustion process is an exothermic chemical reaction. The fires that we fight are a rapid combustion process that releases heat and light. An accurate definition is: Fire is a hydrocarbon air diffusion flame process.

'Hydrocarbon' identifies the fuels as substances containing hydrogen and carbon. 'Air' indicates the oxygen source--the 21 per cent of air that is oxygen. 'Diffusion flame' indicates the type of combustion--the flames where oxygen and fuel gases unite to release heat and light.

We know that heat (energy) cannot be destroyed. It is always preserved, or conserved. So how do we get rid of the "excessive heat" that Chief Layman referred to? Obviously we do this by applying water to the fire. So how does water get rid of this excessive heat? It is safe to say, I believe, that many firefighters do not understand exactly how this is done.

Heat Reduction

One critical fact that forms a large part of the foundation for fighting fires is what is commonly called the 'Latent heat of vaporisation of water'. This is not a good name because of the phrase 'Latent heat'. This phrase indicates that heat is a substance of which there are two types: (1) sensible heat that we can feel, and (2) latent heat that we cannot feel. Heat is not a substance, so there cannot be two kinds of something that does not exist. The correct scientific name is 'enthalpy'. Thus the correct name for this scientific fact is: 'The enthalpy of vaporisation of water'.

What we are talking about, no matter what name is used, is a simple fact. When liquid water undergoes a physical change at 100[degrees]C from liquid water to a gas (steam), this process absorbs 2,198 J/g of water. This amount of heat must be retained by the steam to remain a gas.

Now we come to the critical fact about the vaporisation of water. This process, this change of state, does not increase the temperature of steam above 100[degrees]C. I do not believe that many firefighters realise the extreme importance of this fact. In other words, if the right amount of water is applied to a fire, then this water will absorb all the excessive heat being produced by the fire. Further the temperature of the steam remains at 100[degrees]C, it is not superheated.

Where has all this heat gone? Certainly it has not been destroyed, in reality it has not gone anywhere. It is still there but transformed into the energy needed to vaporise the water.

The beauty of this whole process is that since 100[degrees]C is well below fire temperatures and also well below ignition temperatures of hydrocarbon fuels, the temperature in the fire area will rapidly drop to around 149[degrees]C, thereby cooling the fuels below their ignition temperatures. This brings the fire under control. No other substance besides water has the power to do this so quickly and so efficiently.

Better yet, there is a second scientific fact that plays a critical role in controlling fires. In the process of vaporisation of water, one cubic meter of liquid water is expanded to 1,700 cubic meters of steam. This blast of steam is created very rapidly. In a confined space, all the contaminated atmosphere created by the fire will be blasted out of the structure. This deprives the fire, momentarily, of the oxygen needed for combustion. So the fire is smothered as well as cooled.

Thus a small amount of water, at most a few litres, is all that is needed to control or extinguish a room size structure fire. Using equipment that we have today, a single attack line flowing less than 378 liters per minute for a few seconds provides all the power needed for fire control for 75 per cent of all structure fires fought in the USA. The Enthalpy of Vaporization of Water provides a truly powerful weapon for fire combat. It is really simple, and easy enough, to match the endothermic power of water with the exothermic power of a fire, at least most of the time.

There is a second aspect of firefighting that has become a part of fire service lore. The idea is that the foundation of firefighting has changed in recent years. Specifically, the nature of fires has changed because of presence of new types of fuels. What we are talking about is the creation and widespread use of plastics in the last half of the 20th century. The lore is that these new fuels have changed the nature of fires for a simple reason: the heat of combustion of most plastics is, on average, about twice that of ordinary wood based fuels. Thus fires have become hotter today, hence require greater water flows and even new strategy and tactics in fighting

False Assumption

This idea is based upon one simple assumption. The rate of heat release in a given fire depends upon, or correlates closely with, the heat of combustion (heat content) of the burning fuels. However, this assumption is false. All of the authors of articles in the SFPE Handbook agree that the rate-of-heat release does not depend in any way upon the heat content of the fuels involved. Instead the rate-of-heat release depends directly upon the fuel surface area available to the fire. Further, there is no relation between the fuel surface area and the heat content of a given fuel.

This assumption is also false for a second reason. It ignores the fire triangle, or the fire tetrahedron. Besides fuel, oxygen is essential for combustion to occur. If the amount of oxygen available to a fire is limited, then the heat content of the fuels is not the limiting factor. It is a scientific fact that during most fire development, the rate-of-heat release is ventilation controlled, or oxygen limited. John A Campbell writing in the NFPA Handbook, 17th Edition, states that: 'Considerable ventilation is required for a fully developed fire to burn at a fuel surface controlled rate ... Many, if not most building fires will be ventilation controlled at least during the period of time in which containment is a consideration ... The maximum intensity of post-flashover room fires occurs when the ventilation is just sufficient to permit fuel surface-controlled combustion'.

Thus a structure fire is ventilation controlled on up to the time of maximum, or peak, intensity. Dr Campbell cites as an example, a 6 x 6 meter room with a 2.4 meter ceiling height with an exposed combustive surface of 244 square meters of ordinary combustibles. For such a room, over one-fourth of the wall area would have to be open to shift to fuel surface area controlled combustion

Another scientific fact is that ventilation controlled fires involve incomplete combustion. That is there is not enough oxygen to combust all the fuel vapors being formed. There is a very simple proof of this fact that you as firefighters have observed countless times. Incomplete combustion produces smoke, usually a great deal of smoke. Besides the soot (particles of carbon) contained in smoke, there are fuel gases leaving the fire area without being burned.

Thornton's Rule

There is another truly amazing scientific fact about oxygen limited fires. In Appendix A of the 17th Edition of the NFPA Handbook, Dr Vyetnis Braubaskas makes the following observation: 'Recently, however, increasing engineering use is made of the observation that the heat of combustion per kg of oxygen consumed is nearly constant for most organic fuels. It can be shown that ?llc/ro = 13.1 Mj/kg for O2 is near constant'.

Dr Frederick Clark in the same Handbook explains: 'Examination of the heat of combustion tables in Appendix A will show that while the heat of combustion is quite different for different organic materials, the heat produced per equivalent of oxygen consumed is the same within about 10 per cent. This fact, sometimes called Thornton's Rule, allows one to use oxygen consumption as a reasonable measure of the heat produced by a burning organic material'.

Thus Dr Clark is saying that the heat of combustion of plastics is irrelevant as far as determining the rate-of-heat release for a structure fire. Such a fire is limited by the amount of oxygen available and the rate-of-heat release is constant no matter what type of organic fuel is burning.

I will illustrate Thornton's Rule by using cellulose, the common element of all wood products, and ethylene, a plastic.

Cellulose: C6H1005

Heat of Combustion: 16.12 Mj/kg

Ratio O2 mass/fuel mass: 1.184

16.12/1.184 = 13.6 Mj/kg for O2

Ethytene: C2H4

Heat of Combustion: 47.17 Mj/kg Ratio of O2 mass/fuel mass: 3.422 47.17/3.422 = 13.78 Mj/kg for O2

Note that 13.6 is very close to 13.78, and that both are very close to the average of 13.1. Also please note that ethylene requires considerably more oxygen for complete combustion compared to cellulose. In fact, on the average, five grams of air is required to burn one gram of fuel

Thornton's Rule is the key scientific fact that is being used in fire engineering research today. Besides that, Thornton's Rule provides a solid scientific foundation for fighting fires. Not only is the rate-of-heat release controlled by the amount of oxygen available, but also this is a near constant for each unit of oxygen consumed. Thus there has been no radical change in the nature of fires that would call for any change in strategy or tactics to cope with such changes. Thornton's Rule means that all fires are more alike than they are different. With respect to their rate-of-heat release, it will always be a constant 13.1 Mj/kg of molecular oxygen consumed. This is the single most remarkable scientific fact about fires, and firefighting.

The Window Factor

Are there any other changes that could possibly affect the nature of fires, or firefighting? Certainly the structures that are being built today are quite different from structures built 100 years ago. A big change has been the use of insulation. There are, of course, numerous other changes, such as heating, lighting, and so on.

However, there is one building feature that plays a crucial role in firefighting. This is the presence, or absence, of windows. It is still universally true that almost every room in a house has at least one window. That has not changed. What is different is that instead of single pane plate glass windows, we now have double pane insulated windows. How does this change affect the nature of fires?

First, it is not commonly known that plate glass windows (single pane) break from thermal stress early in the development of a structure fire. This begins at a temperature range of from 260[degrees]C to 315[degrees]C. This temperature range is well below the temperatures needed to produce flashover (593[degrees]C). This is indeed fortunate. If windows did not break until much higher temperatures, then the possibility would exist for a backdraft explosion in every structure fire. Instead, a hot smoldering fire with backdraft potential is a rare occurrence and is limited to those structures that do not have plate glass windows in every room.

Now, what effect do double pane insulated glass windows have on fire behavior? Two instructors at the Rockland County Fire Training Center (NY, USA) conducted a side by side testing of a single pane window and a double pane window. Their test results were published in Firehouse magazine. Much to their surprise, the double pane window broke earlier than the single pane window. In fact the entire double pane window fell out because of the melting of the vinyl frame. The authors did conduct a thorough search for any data or testing done on the behavior of widows in a fire but they were unable to find any.

However, there is some information published in the 17th Edition of the NFPA Handbook in an article by Robert W Fitzgerald, entitled Structural Integrity During Fires. The following statements are made about window glazing (glass): 'It quickly cracks because of the temperature difference between the surfaces. Double glazing does not provide much improvement ... No glazing should be relied upon to remain intact in a fire'.

In fact the Rockland testing detected no improvement whatsoever. So here again there is no change that would have any impact upon fire behavior in a structure fire.

There is one glazing that does remain intact in a fire longer than ordinary plate glass. This is wired glass, a glass sheet containing a net of steel that helps distribute heat which lowers thermal stress. So wired glass remains intact until about 799[degrees]C when it begins to weaken. It will drop out at about 871[degrees]C. If you happen to notice wired glass in a building, watch out! A backdraft is possible.

To summarise, perhaps you were not thoroughly familiar with the Law of Conservation of Matter-Energy, the Enthalpy of Vaporization of Water, Thornton's Rule, or the fire behavior of plate glass windows before reading this article. If not, I hope that you have learned something by becoming familiar with these scientific facts. Take them, and add them to your already substantial knowledge about firefighting (from Essentials of Fire Fighting) and you will have a solid foundation for safe and sane strategy and tactics for fighting fires.