Fire Extinguishing Method

The present invention relates to a method for extinguishing or "killing" fires of classes A, B, and C in enclosed spaces such as engine compartments and crew compartments of combat military vehicles. In the past, the most effective agents for fire killing have been Halons, and CFC and HFC type chemical gasses. Though effective against fires, these agents have adverse environmental effects and can be highly toxic. The present invention addresses the foregoing concerns by using water-based, relatively inert salts that do not have the toxicity or harmful environmental effects of Halons, CFC, or HFC gasses.

Presently, the most effective nontoxic fire extinguishing agents kill fires through a complex phenomenon that involves both cooling and chemically acting on reagents of the fire. The chemical aspect of the phenomenon is known as "free radical scavenging" or as a "free radical trap". Our scheme utilizes a halide salt or acetate salt dissolved in water that likewise both cools and chemically reacts to kill fires. Our extinguishant includes a nontoxic, nonflammable freezing point suppressant which makes the extinguishant useful in arctic climates. The extinguishant is also usable under extremely hot climatic conditions of deserts or tropical areas.

Our extinguishant has a surfactant to reduce the extinguishant's droplet size when the extinguishant is sprayed. In our overall fire extinguishing scheme, the extinguishant's chemical reactions occur only in the presence of heat from an incident fire, whereby these reactions do not unintentionally occur during accidental discharge of the extinguishant. Consequently, our fire extinguishing scheme is safer than many known fire fighting methods.

In some instances in our use of the extinguishant, it may flow in a line past a small phial or container whose mouth opens to the line. Across the mouth is a screen coated on one side with polytetrafluorethylene (TEFLON.RTM.), the screen retaining water in the phial unless extinguishant in the line flows past the screen. Water from the phial replaces water that may be vaporized downstream of the phial. The screen prevents deposits of the extinguishant from forming in the line and clogging the line. Articles such as boats and coffee cups have previously been made, at least for demonstration purposes, from the same material as the aforementioned screen, but we believe it is not known to use such a screen as we have.

FIG. 1 is an essentially schematic diagram of a fire extinguishing system used in conjunction with our fire killing scheme.

FIG. 2 is a cutaway view showing a typical placement of a fire extinguishing system in a crewman's compartment of a tank.

FIG. 3 is a sectioned view of a nozzle used in the system shown in FIG. 1.

FIG. 4 is a sectioned view of a cruse or phial opening into a feeder line of the FIG. 1 system.

The preferred composition of the fire extinguishant used in our system is a water based solution that can be described in a generic formula as follows:

In this formula, A.sup.- can be the acetate ion H.sub.2 CCOO.sup.-, but A.sup.- can also be chloride, bromide, or iodide. The term (M.sup.+ A.sup.-).sub.sat denotes that the amount of the chemical compound M.sup.+ A.sup.- used is saturated in water at a selected temperature T. The compound M.sup.+ A.sup.- is not necessarily saturated for all possible applications, but maximizing the amount of M.sup.+ A.sup.- dissolved in the water enhances the effectiveness of the extinguishant. We prefer a saturated level of M.sup.+ A.sup.- at T in our extinguishment scheme for killing fires in closed compartments of combat vehicles, since these vehicles typically carry live ammunition. The term fps denotes any suitable freezing point suppressant for water, and d is a surfactant for water, such as a soap or a detergent. Ordinary tap water can be used for the extinguishant, although purified water makes a slightly more efficacious extinguishant since purified water holds more M.sup.+ A.sup.- in solution.

It is preferred that the amount of M.sup.+ A.sup.- and fps not exceed their saturation levels in water at temperature T. Otherwise, the M.sup.+ A.sup.- or fps may possibly precipitate and may then obstruct lines or nozzles in the extinguishant delivery system shown in FIG. 1, assuming the lines or nozzles have relatively small inner diameters. Consequently, after the extinguishant is mixed, it is cooled to temperature T, or perhaps a few degrees lower, and then held at this temperature for approximately 15 minutes. The liquid extinguishant is then pipetted or siphoned off, whereby any excess salt or other precipitate is separated from the extinguishant.

Since personnel typically occupy the closed compartments of military vehicles about which we are concerned, it is desirable that the ingredients of the extinguishant be nontoxic to humans and have no damaging effects on human tissue. Sodium halide, sodium acetate, sodium iodide, lithium halide, lithium acetate, lithium iodide, potassium halide, potassium acetate, potassium iodide, ammonium halide, ammonium acetate, and ammonium iodide, in the concentrations described above, are nontoxic and nondamaging to humans; and these substances are also not unacceptably corrosive. There are likewise suitable freezing point suppressants which are nontoxic and not overly corrosive in the concentrations described above. Such suppressants include ferric halide, ferric chloride, ferric acetate, potassium carbonate, lithium chloride, lithium acetate, sodiumacetate, urea (NH.sub.2 CONH.sub.2) strontiumhalide, strontium chloride, strontium iodide, glycerol (CH.sub.2 OHCHOHCH.sub.2 OH), ethylene glycol, potassium acetate, potassium iodide, and potassium bromide; and these suppressants are nonflammable as well. Other freezing point supressants include sodium halide, lithium halide, potassium halide, ammonium acetate, amonium halide, ferric halide, strontium acetate, and strontium halide Suitable surfactants include water soluble oils such as cutting oils, detergents, stearic acids, aldic glycerides, and soaps such as commercially available dish washing soaps or hand soaps, preferably those that do not irritate human skin or eyes.

The most favored mixture effecting essentially instantaneous fire kills has 9.0 to 9.5 grams of potassium acetate for every 10 milliliters of water combined with 1 gram of soap. In this mixture, the potassium acetate will be saturated in water at a temperature T of -60 to -65 degrees Fahrenheit. T was chosen here to be -60 to -65 degrees Fahrenheit because -60 degrees Fahrenheit is the coldest temperature expected to occur in a military vehicle compartment, even in an arctic climate. T can be set at any temperature, but it normally is at or slightly below the lowest expected temperature of the places where our fire extinguishant is used. Potassium acetate not only acts as a fire killing agent by supplying an acetate radical to a combustion reaction but also suppresses the extinguishant's freezing point. In similar fashion, sodium acetate, lithium acetate, lithium iodide, sodium iodide, or potassium iodide could serve the dual purposes of fire killing agent and freezing point suppressant.

Regarding the above generic formula, the amount of detergent can be varied from 1 or more grams of detergent per 10 milliliters of water (a 10% solution) to as little as 1 gram of detergent per 12 liters of water. Adding more than 1 gram of soap or other detergent per 10 milliliters of water is possible, but has no appreciable fire killing benefit. The addition of at least 3% soap by weight approximately doubles the speed of the extinguishant's action. That is, a fire of a given size can be killed in about half the time if 3% soap is added. In the alternative, if at least 3% soap is added then about two-thirds as much extinguishant is needed to kill a given fire without slowing the extinguishant's action. In the most favored mixture described above, the combination of soap with potassium acetate approximately quadruples the speed of the extinguishant as compared to plain water. I.e., the most favored mixture kills fires in about one-fourth the time required by plain water.

FIG. 1 is a semischematic diagram of a fire fighting system 10 in which our extinguishant is used, where a conventional inverted extinguisher bottle 12 typically holds 1.5 to 8 liters of extinguishant at a pressure of 750 to 1000 psi. It is not necessary that the extinguishant be stored under pressure; rather, it is only necessary that pressure be supplied when bottle 12 is to be discharged. Suitable means of supplying pressure include pistons driving through cylinders, or breakable packets of propellant such as those found in air bags for passenger cars. Other kinds of extinguisher bottles besides bottle 12 may be used, but bottle 12 is preferred since it is the type already commonly installed on US Army combat vehicles. Bottle 12 is typically charged by first adding our extinguishant and then pressurizing the bottle with nitrogen gas. A gate device 14 controls the flow of extinguishant from bottle 12 to feeder line 16. Gate device 14 is shown as a valve in FIG. 1, but device 14 can be another known mechanism such as a membrane or a sheet of material together with componentry to puncture the membrane in response to an appropriate signal. Device 14 opens to release extinguishant from bottle 12 in response to a signal along line 18 from sensor 20. Sensor 20 can be a heat sensor, a photoelectric element, or optical sensor within military vehicle 8 in a compartment 22, the sensor designated by dashed lines in FIG. 1. Compartment 22 is also shown in the cutaway portion of tank in FIG. 2.

Opening into to line 16 is an inverted phial or cruse 24 at whose open end 26 (FIG. 4) is a fine mesh screen or barrier 28 having a polytetrafluorethylene coating on one side. Screen 28 intrudes slightly into line 16. Cruse 24 contains a small quantity of preferably pure water, the quantity typically being between half an ounce and sixteen ounces. The screen's mesh is sized so the screen's coating and the water's surface tension coact to keep the water from leaving cruse 24 through the screen if no extinguishant flows by the screen in line 16. When extinguishant does flow past screen 28, water from cruse 24 is drawn into the flowing extinguishant. Screen 28 normally has 200 to 300 meshes per square inch, and the polytetrafluorethylene coating is on the side of the screen facing toward cruse 24. Preferably, screen 28 has no more than 300 meshes per square inch. Otherwise the coating will keep water from going through screen 28 from its coated side even if the extingushant flows past screen 28.

The extinguishant flows from feeder line 16 to distribution lines 30 and 32, which lead back to compartment 22. At termini 34 and 36 of lines 30 and 32 are spray nozzles or nozzle-like ends such as that seen at 38. Termini 34 and 36 taper to a preferred diameter of 0.5 millimeters to 3 millimeter at orifice 40. In our tests, the droplet size of the sprayed extinguishant was 100 to 250 .mu.m and was effective against fires within compartment 22. Larger droplets would be needed for fires at ranges exceeding several feet, such ranges being greater than those encountered in compartment 22. Droplets bigger than 15 .mu.m are highly desired for human-occupied compartments since droplets 15 .mu.m or smaller can be absorbed by the human lung and salt abrasion of the lungs could occur.

Possibly, very hot fires can heat lines 30 and 32 and nozzles 38 enough to vaporize water from the extinguishant as the extinguishant first arrives in these lines or nozzles, whereby precipitate deposits there. Cruse 24 prevents the deposition of precipitate, which may obstruct subsequent flow of extinguishant in system 10. Water from cruse 24 is drawn into the extinguishant as the extinguishant begins to flow past cruse 24 through the lines. Water from cruse 24 offsets any vaporization of the extinguishant's water that occurs before the extinguishant exits the nozzles, whereby cruse 24 is a means to eliminate line or nozzle blockage due to precipitation. The vaporization of water in lines 30 and 32 does not present a problem at temperatures where the water in cruse 24 is frozen.

We do not desire to be limited to the exact details of construction or method shown herein since obvious modifications thereto will occur to those skilled in the relevant arts without departing from the spirit and scope of the following claims