Backdrafts— What they are and what to do about them
BACKDRAFTS are a constant threat to firemen and every fireman should know something of their nature, causes, forewarnings, and the measures that may be taken to prevent them and protect personnel against them. They are not uncommon at fires, and in many instances it is only a matter of good fortune that more firemen are not injured by their occurrence. A baekdraft is a rapid, almost instantaneous, combustion of flammable gases, carbon particles and tar balloons emitted by materials which are burning under conditions of insufficient oxygen.
Except under carefully controlled conditions, no combustion is complete; that is, all the fuel elements of the burning materials are not consumed but pass off into the atmosphere. It is not necessary that a material be aflame to emit these substances. Many combustibles begin to smoke before they actually burst into flame. In such cases there is either not sufficient oxygen to support fire, or the ignition temperature of the material has not been reached. Nevertheless, the gases and carbon being emitted are flammable.
We have all seen, after fires, blistered paint and blackened or partially charred wood, paper and cloth. We are aware that these were never actually afire because we saw no fire at their location, nor was it necessary to apply water to them. Yet it is obvious that they had undergone a change. This was a chemical change wherein the heat from the burning materials distilled gases and carbon.
When we consider this condition along with the fire itself, there is little wonder that flammable gases and carbon are free to some extent at all fires. The degree to which they exist depends upon how much oxygen was present to mix with them (physically) and to combine with them (chemically) by the time the fire department arrived at the scene.
Carbon and tar balloons
In thinking of backdrafts, only the flammable gases are usually considered, and the flammability of the carbon particles and tar balloons is more or less disregarded. We know that any flammable substance when finely divided and dispersed in the atmosphere in sufficient quantity will, if provided with some source of ignition, burn with a rapidity that is almost instantaneous. We know this as a dust explosion.
Certainly carbon particles emitted from a fire are flammable and they are finely divided. In many instances they are present in tremendous quantity. Some of the force of a baekdraft may be attributed to the almost instantaneous combustion of these highly flammable specks in the smoke, and it is possible that some backdrafts are a combination of a gas explosion and a dust explosion. It is altogether possible and probable that a good percentage of backdrafts are caused by the ignition of the carbon particles which, in turn, ignite the gases, since the ignition temperature of such gases is usually higher than that of the carbon particles (Ignition temperature of carbon monoxide is 1204°F; ignition temperature of carbon particles is 412°F.).
Most fires to which the department responds are in buildings. In cool and inclement weather, and even in some instances in warm fair weather, mans’ of these buildings are closed up. A fire burning in a closed building uses up the oxygen content of the atmosphere in the building fairly rapidly, depending upon the amount and kind of material afire and the heat produced. When the oxygen content is reduced to about 16 per cent, flame (in ordinary materials) cannot exist and the fire is reduced to a smouldering state. However, even in this state, it continues to produce flammable gases and carbon particles.
If buildings are completely airtight, smouldering will become progressively less as the oxygen is used up, until eventually, the fire goes out. Unfortunately, no building is airtight and, although the oxygen is consumed by the fire, enough oxygen-bearing air leaks into the building to nourish a smouldering fire, and the production of flammable gases and carbon particles goes on.
If the fire is hot enough, some of the fuel will be vaporized. When such vapor leases the immediate vicinity of the fire and cools off even slightly, it condenses back into solid fuel in the same manner in which water, vaporized into steam, condenses back into droplets of water. When fuel vapor condenses, it too takes the shape of droplets, but they are hollow rather than solid and are therefore referred to as tar balloons. These being actual fuel they add to the backdraft just as carbon particles do.
Eventually such a fire will provide itself with an opening by burning through the roof or wall, or its heat will break a window. If this does not happen before the arrival of the department, firemen will provide an opening in attempting to ventilate or in entering the building. Once such opening exists, a new supply of oxygen enters, mixes with the flammable atmosphere in the building and creates the possibility of a backdraft.
The backdraft may be of an incipient nature or of a severe nature, depending on such factors as: The proximity of the opening to the fire; the size of the opening, and the resultant rapidity with which the fresh air is entering the building; the temperature of the gases in the building, and the temperature of the air which is entering; the amount of turbulence in the building caused by wind, hose streams directed into the building, or firemen entering.
Importance of temperature
The temperature of the entering air and the temperature of the gases in the building are important factors in the degree of a backdraft. If the incoming air is not cold enough to cool the gases below the ignition temperature of the mixture, then all that is necessary for an explosion to occur is that a quantity of gases and air reach the upper explosive limit of the gases concerned. When this happens, the mixture will ignite spontaneously.
It is most likely that under these conditions a backdraft will not be too severe because it will take a comparatively short time for some of the gases and air to reach the explosive limit and the mixture will be comparatively small in volume. Only part of the room or building will contain an explosive mixture since there is not enough time for most (or all) of the gases to mix with air. The explosion will blow the unmixed gases out of the building either through windows which may be broken, or out of the veiy opening which admitted the air.
If the incoming air is cold enough to cool the gases below the ignition temperature of the mixture, then any resultant explosive mixture will not ignite spontaneously but will need a source of ignition. In this case, more and more gases will mix with the incoming air until a great quantity of explosive mixture exists. If the smouldering fire is on the extreme opposite end of the building from where the air is entering, then it is likely that the entire building or room will be filled with an explosive mixture before any of it reaches the fire and is ignited. Because such a great quantity of explosive mixture exists in the building, the backdraft will probably be one of extreme force.
Fortunately, backdrafts do not occur without forewarning. Their very nature creates certain conditions which are discernible to firemen. One indication is unusually thick, black smoke at an ordinary building fire where common materials such as wood, paper, cloth and other cellular materials are burning. The smoke is thick and black because of an extreme lack of oxygen, which causes carbon particles to find their way into the atmosphere in great quantity. This also indicates that generous amounts of flammable gases are being emitted.
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Caution is required if upon arrival at a fire it is found that the weather boards or other outside wall materials are hot, or even fairly warm, and little or no fire is in evidence. This indicates that a smouldering fire has been burning in the building for a considerable length of time and has driven off large quantities of gases which are still within. It is assumed that the building is still closed up, or that the surface heat is not due to fire in the wall where the heat is detected.
If upon arrival, the windows of the building can be heard rattling, very much like in a heavy wind storm, it is usually an indication that backdraft conditions exist and that an explosion can be expected momentarily. This sound can be heard at only a short distance (5 to 6 feet). The actual cause is not definitely known but experience indicates that the condition is a definite warning sign.
If upon attempting to look into a building through window glass, it is found that visibility inside is practically nil, and yet no fire is in evidence (or perhaps a faint orange glow can be seen through the smoke), it is reasonable to assume that a fire has been smouldering for some time in order to produce sufficient smoke to result in such poor visibility, and that dangers exist.
It is possible that backdraft conditions may not exist upon arrival but develop when firemen are in a building, or that the above warning signs are overlooked in the excitement and hurry to get into the building and extinguish the fire. At any rate, firemen may be in a building when backdraft conditions begin to develop.
Fortunately, there are also warnings which are discernible on the inside. Every fireman knows the approximate appearance of fire. If asked, he may describe the color of the flames as orange yellow or reddish orange and will say that the flames appear lively. If inside the building the flames assume a pale or sickly yellow hue, and seem to lose their liveliness, appearing somewhat like a slow motion picture, it is an indication that the fire is becoming oxygen-starved and that flammable gases are being emitted.
It may also be noticed by firemen that suddenly one becomes aware that things do not sound normal. Voices lose their resonance and words seem to be clipped off short. Bells of manning apparatus do not seem to have a clear tone and sirens have a strangely muffled sound. This condition is probably due to the combustible gas content of the building, and to the fact that these gases do not transmit sound as readily as the normal atmosphere.
A further development of this condition may become apparent in that part of the fire on which no water has yet been directed diminishes in intensity or the flames subside completely. This indicates that oxygen in the atmosphere is extremely low and backdraft conditions will develop quite rapidly.
Assuming the conditions are recognized upon arrival, every effort will have to be exerted to prevent the backdraft from occurring. Prompt and proper ventilation is in order, but even this cannot be relied upon to absolutely prevent the backdraft. In fact, there is no sure, guaranteed method of prevention. We can only ventilate in accordance with prescribed methods, and perhaps attempt to cool down the atmosphere and drive out flammable gases by use of fog streams directed into the building from the outside; then hope for the best.
So long as we have no positive method of knowing whether or not such efforts will be successful, it is foolhardy to enter a building until one of three things happens: (1) Ventilation working in accordance with theory, whereby the heat, gases and carbon particles are escaping through top openings and being replaced gradually by fresh air entering the other openings. In this case it will be apparent that smoke is clearing out, visibility is becoming good, and the fire is burning normally; (2) gases are reduced to the point where they do not explode but fresh air is entering in sufficient quantity and with sufficient rapidity to cause the fire to suddenly burn with great intensity and vigor; (3) a backdraft occurs. An explosive gas and air mixture takes place in spite of preventive measures.
Once one of these things has happened, it is safe to enter that portion of the building. It is possible that other portions are still ripe for a blow. Each enclosed section of the building must be treated as a separate, potential backdraft area.
In the interest of safety, while waiting for one of the above to occur, personnel should not stand in, or directly in front of door, window or other openings. It is safest if they do not stand within the V-shaped force pattern which will emanate from such openings. The gaseous products of the backdraft will expand as they come through the opening due to the lesser pressure of the atmosphere outside the building.
If hose streams must be played into such openings, men on the nozzles should position themselves alongside the opening and close to the wall of the building, or direct the hose streams from a distance of at least 25 feet, at which distance the force of the backdrafts will have diminished considerably. Window glass, window frames and other debris are usually not thrown more than this distance from the building.
If firemen are inside a building when backdraft conditions develop, they should leave as quickly as possible after they have recognized one of the forewarnings. If it is too late to leave because the explosion has begun, drop to the floor in a prone position. Men who have had the experience of being caught in a backdraft testify they could see it coming. They invariably describe the occurrence as begining with what appears like a reddish, rolling mass of fire which rapidly increases in size and spreads throughout the entire enclosure, overtaking them in its course of travel. While the force is evident from floor to ceiling, the fire itself usually comes only to within a few feet of the floor. Men who drop prone are usually not burned nor are they thrown about by the force, whereas those who remain standing are not only burned on exposed skin surfaces, but thrown considerable distances, sometimes through doorways, down shafts or stairways.
Backdrafts should be ever in the “minds of firemen at fires. All warning signs should be heeded promptly and all possible precautions taken to protect personnel, for even with the utmost care an occasional accident will occur. When it happens, it will at least be gratifying to know that everything possible was done to prevent it and to protect ourselves.