Demonstration Lecture on Extinguishing Flame
I LIGHT a match, and blow it out— so. A fire has been extinguished. All of us have done that thousands of times, and yet I wonder how many have considered the phenomenon of extinguishment. To fully understand what happens when a match is blown out, it is first necessary to be familiar with combustion.
Combustion—What Is It?
Combustion is oxidation taking place at such a rapid rate as to produce luminous flame. Oxidation takes place all of the time in most materials. It is the combining of oxygen with the material in question. In the case of iron, the result of oxidation is rust or iron oxide. In an ordinary atmosphere containing about 21 per cent oxygen, rusting takes place slowly. In an atmosphere of pure oxygen, it is possible for oxidation to take place at such a rapid rate that the iron when ignited burns with incandescence, because oxidation in all things produces beat, and if it takes place rapidly enough, light is produced.
There are such curious conditions of flame that it requires some cleverness and nicety of discrimination to distinguish the kinds of combustion one from another. Today we will deal only with ordinary flames of common combustibles such as wood, textiles, etc., and of common flammable liquids, i.e., flames in the liberated flammable vapors.
Going back to the match which was extinguished, we note that the wood portion was ignited by the head, a heat generator. When the head has heated the wood sufficiently, a vapor is driven off, and flammable vapors are in a form which may oxidize rapidly. In the presence of flame, such as the burning head of the match, oxidation can be accelerated to the state of actual combustion. The flame from the burning vapors in turn generates more vapors from the wood, and flame continues until the wood is consumed or the match is extinguished.
There are several common ways of extinguishing fire. In practical importance their probable order is: cooling, smothering, separating the flame from the burning material.
In the case of the match, it was extinguished by blowing the flame away from the burning material so that it could not further heat the wood and form combustible vapors. If the match is blown out quickly, we simply separate the flame from the flammable vapors; if blown slowly, we push the flame aside to decrease the generation of flammable vapors.
Separation of Flame
Another experiment to show separation of flame can be conducted with this knife and this small can full of alcohol. I light the alcohol and very simply slice off the flame with this knife—thus.
A common method of separating flame from burning material was practiced by aviators in days past and perhaps is still employed when necessary. Fires frequently occurred at the carburetor while the plane was in the air. in which case the aviator would attempt to suck the flame into the engine by opening the throttle wide. If that failed the next step would be to dive and stunt in the hope of separating the flame from the carburetor. and many times that has been accomplished.
To demonstrate extinguishment by cooling, we will employ this small can of flammable liquid which has been burning so brightly at the end of the table. The can contains paraffin, heated until it will burn. If left undisturbed, it will continue to burn until consumed, because the heat of the flames serves to keep the paraffin at a temperature at which flammable vapors are released in sufficient quantity for combustion. By placing this can of burning paraffin in this pan of water, the fire can be extinguished by external cooling. The water serves to cool the paraffin in the can faster than the flames can heat it. and you will perceive that the flames are gradually receding, and, finally, are totally suppressed.
You can see fumes or vapor still coming from the hot paraffin; perhaps you wonder why they do not continue to burn. I will attempt with this match to relight them and you will note that time and again they flash, but will not hold their fire. The liquid is at a temperature which is called the flash point. By allowing it to cool just a little more you will see that I cannot ignite the paraffin with a match, although it is still in liquid form. To further demonstrate these various characteristics, I will again place the can above the Bunsen burner while we take up the next experiment.
The Kitchen Method
In this ten-quart pail we have one quart of alcohol. Alcohol is used in these larger tests since it burns without black soot and smoke. The alcohol is ignited readily by throwing a lighted match into the pail, for at room temperature it is well above its flash point. We would have to cool it below zero before it became difficult to ignite. Perhaps we might extinguish it by external cooling as we did in the case of paraffin, but to do that we would have to cool it far below zero since it is so very volatile at ordinary temperatures.
In this experiment, however, we choose to extinguish the flames by smothering, and will use what I will call the old fashioned kitchen method used by all busy cooks when the pork chops in the frying pan catch fire. We will use a cover. By placing the cover on the pail for an instant and then removing it, you see that the flames were out of sight, but persisted within the pail, for now, with the cover removed, the fire burns as bril liantly as ever. By holding the cover on the pail just a little longer we extinguish the fire. Removing the cover, you see that there are no flames. In the latter ease, we allowed sufficient time for the oxygen in the pail to be consumed, and without sufficient oxygen combustion cannot take place.
Ignition Without Flame or Spark
You have noted, that while conducting the last experiment, I have been repeatedly testing the little can of wax which is being heated by the burner. You saw that it was heated past the flash point and heating was carried on beyond the point at which burning would continue. We now have it at another critical temperature known as the apparent ignition point or autogenous ignition point. You notice that when I extinguish the fire by holding a cover over it, re-ignition takes place shortly after removing the cover and without adjacent flame of any kind. I do it again—-and again— and again. You ask: what trickery is this? How does fire occur without the addition of flame or spark for ignition? The answer is centered about that term autogenous ignition point. The paraffin has been heated to a point where ignition occurs automatically when the vapors come in contact with some equally hot surface such as the edge of the can. Oxidation takes place more rapidly at high temperatures than at low temperatures, generating heat in proportion to the rate of oxidation. When the autogenous ignition point is reached, flaming occurs automatically. Now again I place the can of burning paraffin in the pan of water for extinguishment while we go on with our further studies. You will notice that it will take considerably longer for extinguishment now that the paraffin has been heated more than it was in our initial trial.
Water Plentiful and Effective
Tt is one of those wonderful acts of providence that the most abundant liquid in this world should also be one of the very best fire extinguishing agents. To raise the temperature of a pound of water up to and past the steam point requires the addition of over 1,000 British thermal units. In other words, when one pound of water is changed to steam it takes over 1,000 Btu away from something else. When water is used to extinguish a deep-seated fire, the burning material is cooled to the extent of the heat taken by the water to form the steam. The heat required to raise the temperature of a pound of water one degree F is only one Btu, but the change from the liquid to the steam or vapor state takes 970 Btu, thus accounting for its great cooling effect. This is what makes water primarily suitable on deep-seated wood or textile fires which require cooling of large masses of glowing embers before extinction can finally be accomplished. Under certain conditions, the performance of water as an extinguishing agent can be improved somewhat by the addition of certain fireproofing chemicals such as salts of ammonium, potassium, or calcium, but that is a separate problem with which we will not deal now.
Effect of Water Spray
This experiment will demonstrate an additional method of employing water as an extinguishing agent. I have here an ordinary spray pump used in orchards and on shrubs for another purpose, but which also serves my purpose well in that it enables me to atomize the water so that I may apply it in finely divided form. I pour one quart of alcohol into the pail, ignite it and with very little water spray extinguish the fire. Transmission of heat from one thing to another depends in large part upon exposed area. Water in such finely divided form exposes a great total area to the flames into which it is directed, robs the flames of heat, cools the liquid surface, retards release of flammable vapors and combustion ceases. There have been recent developments of nozzles to make possible the spraying or atomizing of water. These are mainly in large sizes for fire department use. It is claimed that such nozzles, while not possessing the range of fire streams, often provide more efficient application with less water damage, and with this test considered, together with the one following, we will see such a difference.
Alcohol and water will mix in all proportions. I will relight the quart of alcohol in this pail and slowly pour in water. You notice that there seems to be no apparent immediate effect on the flames. As I continue, however, the flames diminish, and now that I have poured in a full gallon, the flames have finally been extinguished. The principle involved in this case is again very simple. It is a matter of diluting the mixture to a concentration at which alcohol vapors are no longer given off in sufficient quantity for combustion. This principle of dilution with water cannot be employed in the case of petroleum or other flammable liquids which float upon the water and are not soluble in it, and in general is not a practical method of extinguishing fire.
Types of Extinguishers Using Water
Hand extinguishers employing water as the main extinguishing agent, are made in a number of designs. The most common type is the soda-acid extinguisher, employing a solution of sodium bicarbonate in the main chamber which also holds a suspended acid bottle containing sulphuric acid. When the extinguisher is inverted, the acid and soda mix and react, forming carbon dioxide gas, which serves as the pressure means for discharging the solution through hose and nozzle in a continuous, easily directed stream. The main purpose of the chemicals involved is to provide pressure when wanted. They do not add a measurable fire extinguishing effect over that afforded by plain water.
Another type of extinguisher similar in appearance and in operation employs a carbon dioxide pressure cartridge which, when punctured, provides means for ejecting the water. An ordinary hand pump extinguisher is also available. Calcium chloride is employed in connection with water to form solutions for the latter type when freezing temperatures may be encountered. In such manner, the freezing point of the liquid can be depressed to as low a point as minus 50° F.
With the aid of this next pail containing alcohol, we will examine the extinguishing characteristics of a liquid employing carbon tetrachloride as a base. This liquid, known as vaporizing liquid, rapidly evaporates when played upon hot surfaces and forms a heavy, fire-smothering gas. Only about 100 Btu per pound are necessary to evaporate the liquid, while over 1,000 Btu are required for each pound of water. For this reason, in comparison with water, carbon tetrachloride has little cooling effect when applied to fires involving masses of glowing embers or coals. It is therefore generally recognized as not being suitable for such service.
Employing this one-quart pumptype extinguisher, I pump the vaporizing liquid into the alcohol, and you can see that there is no apparent extinguishing effect, although of course as in the case with water and alcohol, it would be possible to obtain extinguishment by dilution. Instead, however, I will direct the stream against the inside of the pail—-thus; and with a little squirt the fire is almost magically extinguished. The impinging stream splashes back over the flames in small droplets which are in a form which can be quickly evaporated. It is true that some cooling effect may here come into play, but by the difficulties I am encountering in re-igniting the alcohol you will observe that the heavy, fire-smothering gas is still hovering above the alcohol. As it again ignites, full flame is not re-established until the heavy vapors are dissipated. Vaporizing liquid type extinguishers are available in a number of designs and sizes, and are also especially suitable for use on fires involving electrical current as the liquid is an excellent dielectric or insulator.
Extinguishers have recently been developed which employ a dry compound. I have here a fifteen-pound size from which the compound is forcibly ejected in dust cloud form when a carbon dioxide pressure cart ridge within the extinguisher is punctured. The dust cloud of this special compound is effective in sweeping the flame from a burning surface, cooling and smothering the fire. As the extinguisher is too large for this little experiment. I will, in the case of this next pail of burning alcohol, apply the powder by hand. First, I will simply throw a large scoopful of dust into the pail. Notice that the fire was only slightly disturbed. However, in changing the method of attack by dispersing a much smaller quantity of dust (which I hold in my fingers) over the surface, the fire is suddenly quenched. To show you the difference between this compound and rock dust or cement, for example, which is equally ground, I will take a quantity of cement and with my very best efforts I can obtain no extinguishing effect.
Possible Catalytic Action
You may ask just what the difference is between these powders which makes one so effective and the other so ineffective. All of the answers are not known, and reference has been made to a catalytic action. The dry compound—the powder which was effective—is essentially sodium bicarbonate. Other things are added to keep it in powdery condition free from caking and packing. Sodium bicarbonate changes to sodium carbonate with the application of heat, and because of the heat requirements involved in such a change, a cooling action over large areas is available. During the change, carbon dioxide gas is liberated and this affords a certain smothering action. Thus cooling and smothering are used in combination, but it is difficult if not impossible to establish separately, the relative value of each.
Application of any extinguishing agent in a continuous, easily directed stream is generally the most efficient method of application. For this reason most hand extinguishers are designed to give a continuous stream by means of which any headway gained in extinguishing the fire can be followed up with greatest possible effectiveness. Many quenching agents are transient in their action, and if one fails to extinguish all of the fire, reflashes over the entire area often re-establishes the fire in its original intensity.
Foam is a quenching medium which provides a smothering blanket possessing a certain permanence. This foam extinguisher of 2 1/2-gallon size is too large for our demonstration purposes today. I will therefore discharge the device into this 25-gallon can. The extinguisher is very similar to the soda-acid device, a main difference being the addition of a foaming ingredient to the sodium bicarbonate solution. This makes a difference in performance much like the difference between drawing off beer from the spigot, and soda water from the siphon bottle. The beer has foam of a certain stability because of the malt and hops which act as a foaming ingredient, whereas the froth in soda water quickly dissipates because it consists only of water and carbon dioxide.
Now you see that I have practically filled the 25-gallon can with foam from this 2 1/2-gallon extinguisher. The foam is firm and stable and clings to my hand when I hold this handful upside down. After standing fifteen minutes it will shrink only approximately ten per cent. It is not rapidly broken down by flame as shown by this handful of foam which insulates my hand from the heat of the torch.
The extinguishing action of foam is principally that of smothering as it floats or rests upon flammable surfaces. There is however, a cooling action for in this twenty gallons of foam there are 2 1/2 gallons of water solution.
In demonstrating the smothering action of foam on this fire involving a pail holding one quart of alcohol, I am showing foam in an application in which it is not best suited. Foam readily dissolves in alcohol, acetones, etc., and is not as effective with these liquids as it is on petroleum products upon which it floats without reaction. However, by dumping this foam from a 1/2-gallon measure into the pail it forms a blanket and quenches the fire.
Foam streams should not be played into burning liquids for the foam then emerges covered with the blazing liquid. Foam streams should be directed against the inside of the container so that the foam drops gently on the surface of the liquid and forms a smothering blanket.
You have heard me mention carbon dioxide many times in this discussion, and perhaps you wonder just what effect carbon dioxide alone would have upon a fire. Carbon dioxide is a relatively inert gas which has many wonderful characteristics and uses. It is used in refrigeration, medicine, beverages, as an exterminator, and for blasting coal in mines. In its solid form it is the dry ice with which you are familiar. At normal temperature and pressure it exists in gaseous form and it is a part of the atmosphere, being liberated by combustion, fermentation and by exhalation of animals. It can be compressed and liquefied, and it is this latter characteristic which makes it adaptable to storage for fire extinguisher use.
I have here a two-pound size carbon dioxide extinguisher. The storage cylinders are made in many sizes. With sixty-eight per cent filling, the pressure in the cylinder is always 850 pounds at 70 F. The pressure varies greatly with change in temperature, and this characteristic is employed in a recent development of low pressure storage containers wherein the pressure is held at a constant low figure by refrigeration. Bulk storage in large quantity to the extent of tons of carbon dioxide is thus possible.
When I press the trigger of this hand extinguisher, CO2 in gaseous and snow form is discharged. Directing the discharge at my coat sleeve, a deposit of the snow is obtained. Regardless of the temperature of the atmosphere or of the extinguisher, the temperature of the snow is always exactly minus 110 F. When the snow is compressed it is dry ice.
Despite the low temperature of the discharge, a cooling effect in the order of only 100 Btu per pound is available, and accordingly the principal extinguishing action is smothering. I direct the discharge into the pail of burning alcohol, which is suddenly quenched in a most marvelous manner. The gas quenches in passing, and leaves without deposit. Because of the nature of the discharge, close approach to the burning area must be possible.
Carbon dioxide is also employed for total flooding of rooms to make an inert atmosphere in which combustion of ordinary materials cannot continue.
I would like to make just one more demonstration, not because of any practical significance in the present state of the art, but because of its oddity.
I have here an automobile horn, not of the air type but of the electrically vibrated diaphragm type. You see that when I sound it with a handkerchief hung over its bell the handkerchief does not blow away but simply vibrates, thereby showing rather conclusively that there is no appreciable draft or air current. I light this candle and place it near the horn. The horn, when sounded, sets up a terrific vibration in the flame and finally succeeds in extinguishing it. Why did the fire go out? I would like to leave you with something to think about. If you have followed me in what has been before discussed, I think you will come very close to having the right answers without becoming involved in complicated consideration of sound wave lengths, frequencies, and the like. The latter of course is another subject, theories on which are recorded in the literature.*
A. External Cooling
Paraffin (3/4 can full)
Lead weight in can (for weighting)
Bunsen burner, ring stand and gas supply
Water container (approximately same height as tin can)
Water in container (slightly higher than liquid in can after can is inserted)
B. Smotherinq (With Cover)
10 or 12-quart pail with cover to fit 1 quart alcohol
C. Smotherinq (Vaporizing Liquid)
10 or 12-quart pail
1 quart pump extinguisher (vaporizing liquid type)
1 quart alcohol
D. Cooling (Water Spray)
10 or 12-quart pail
2 1/2-gallon insect spray pump (with water)
1 quart alcohol
E. Extinguishing with Dry Powder
10 or 12-quart pail
1 quart alcohol
2 pounds sodium bicarbonate (sifted)
5 pounds Portland cement (sifted)
F. Smothering (With Carbon Dioxide)
10 or 12-quart pail
1 quart alcohol
2-pound carbon dioxide extinguisher
10 or 12-quart pail
1 quart alcohol
1 gallon water
H. Smothering (Foam)
10 or 12-quart pail
1 quart alcohol
2 1/2-gallon foam extinguisher (charged)
1/2-gallon wide-mouth container (see photograph)
Blow torch or Bunsen burner
I. Flame Separation (Difficult, requires practice and technique)
3-inch diameter heavy container with smooth and even rim
Alcohol to within 1/16 inch from top 2 1/2-inch wide spatula
About 1929 model auto horn (experiment before selecting)
Battery (wire connections) Candle
NOTE: Matches used for ignition. Small ball of asbestos fixed to end of wire makes useful torch after dipping in flammable liquid. Table should be sturdy and protected with an asbestos sheet. Use care in dispensing flammable liquids from safety can. Be careful, and see that there is no flammable material above demonstration fires. Provide ample fire protection equipment to take care of a major accident. Practice the entire series before making demonstrations. Several of the demonstrations may be eliminated for one one reason or another without detracting from the general interest and value of the whole demonstration. Dispose of used alcohol in drain, being careful to dilute it with at least four parts water.
Reprinted from “Bulletin of Education.” by permission of the Underwriters’ Laboratories.
*Those interested in theories on sound sensitive flames are referred to Lord Rayleigh’s “Theory of Sound.” Volume II (Macmillan & Company), pages 400-406, and the additional references therein noted.