Underwriters’ Laboratories Conduct Water Fog Tests
In the recent years there has been a considerable increase in the use of finely divided water, discharged from spray nozzles or automatic sprinklers, for extinguishing certain types of fires. Since water in the form of small drops appears to be very effective on many fires, this development has become of increasing importance to both the fire services and fire insurance interests.
Recently, Underwriters’ Laboratories, Inc., in a report prepared for the National Board of Fire Underwriters, describes the fundamental factors involved in fire extinguishment by means of water sprays. According to the study, tests show that fire extinguishment is due primarily to smothering effects of water vapor produced by evaporation of the water drops. Cooling effects of the water supply are also important.
The laboratory-scale tests were run to determine the droplet size distribution in experimental sprays and then to determine the extinguishing effects of water sprays of known droplet size.
According to the National Board, many successful applications of water spray in putting out fires had been reported previously, but data on the fundamental processes involved in extinguishment and the factors which influence them were somewhat meager. In order that this method of attacking fires may be employed most advantageously and its limitations understood, a knowledge of the way in which water droplets act to put out various fires is essential.
It is generally recognized that fire is a chemical reaction proceeding with the evolution of heat and light. The most common unwanted fires involve reactions between a substance and the oxygen of the surrounding air. Two essential requirements for the initiation and maintenance of ordinary combustion are a fuel and an adequate supply of oxygen. The combustion processes will cease and the fire be extinguished if either reactant is no longer present in sufficient proportions.
The extinguishment of fire by removing the fuel is not always practical, although in some circumstances, such as closing a valve on a burning gas line, it may be the simplest method.
Curtailment or depletion of the oxygen supply is the basis of many fire fighting techniques, commonly referred to as smothering or blanketing a fire. In order to put out a fire, elimination of all oxygen is generally not necessary; flaming combustion of most common materials can no longer continue after oxygen is reduced to 12 to 15 per cent by volume.
Fires can be extinguished by diluting the oxygen content of the surrounding air with an inert gas or vapor. Carbon dioxide and nitrogen are commonly used for this purpose as are the various vaporizing extinguishing agents.
Similarly, it is well known that water vapor or steam may be used under appropriate conditions to dilute the oxygen in the atmosphere so as to suppress fire. In extinguishing fires by smothering, sprays of finely divided water as a source of water vapor have certain advantages.
In addition to fuel and oxygen, heat is necessary to start and maintain a fire. A combustible substance must be heated to its ignition temperature if combustion reactions are to proceed. Once started, the reaction is usually maintained by the heat liberated by combustion. If the seat of a fire can be cooled to temperatures below the ignition temperature, the fire will be extinguished. Sprays of water droplets apparently have certain advantages as a cooling medium for fire extinguishment.
Nineteen different spray nozzles or combinations of nozzles were tested on a variety of fires during the investigation. Fuels used included wood, gasoline, kerosene, and ethyl alcohol.
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According to the NBFU report, the extinguishing action of sprays of finely divided water applied to commonly encountered fires appears to be due predominantly to dilution of the air supply in the zone of burning with vapor resulting from evaporation of water droplets in the heated area surrounding the fire. The cooling effects of the water may also be important factors in extinguishment in many cases. In order to obtain extinguishment, the water droplets comprising the spray must be relatively small, and the amount of water applied must be adequate in relation to the specific fire.
The marked increase in water surface per gallon where water is dispersed as droplets in a spray is an advantage since the rate of evaporation of the water and the cooling effects are directly proportional to the surface area, other factors being constant.
There is a lower limit to the size of the droplets which are of practical use in fighting fire, however, since the droplets must be large enough to travel from the source of the spray to the heated area surrounding the fire, and to arrive there with sufficient energy to overcome the currents of hot and turbulent gases moving upwards and away from the fire.
Droplets too large to evaporate completely on passing through the zone of combustion and its immediate surroundings may contribute to extinguishment in some eases by cooling the unburned fuel, Even though no extinguishment occurs, the application of water spray may control the rate of burning to some extent.
In tests where experimental water sprays having known droplet size distribution were applied to fires, extinguishment occurred if the droplet size was within certain limits and the total water reaching the test fire exceeded a minimum volume in relation to the drop size. The results of these tests suggest that the optimum average volume diameter of the water droplets for extinguishment is of the order of 300 microns (0.012 inch).
The tests also showed that kerosene and ethyl alcohol fires appear to be more readily extinguished by water spray than are gasoline fires.
In tests with water sprays applied to gasoline fires, it appeared that extinguishment was due primarily to dilution of the oxygen supply with water vapor, The absorption of heat during evaporation of the droplets may also contribute to extinguishment, particularly when it occurs in or near the zone of burning.
Extinguishment by cooling the fuel with water spray is not possible in the case of gasoline fires since the fire point and flash point of gasoline are well below the freezing point of water.
Copies of this report, “The Mechanism of Extinguishment of Fire by Finely Divided Water,” may be obtained from the Research Division of the National Board of Fire Underwriters, 85 John Street, New York 38, N. Y.