1. Field of the Invention
This invention relates to a method for extinguishing a fire and preventing re-ignition. More particular, a fire extinguishing agent is discharged at a first mass flow rate to extinguish the fire followed by discharge at a second mass flow rate that is effective to prevent re-ignition of the fire.
2. Description of Related Art
Fire involves a chemical reaction between oxygen and a fuel that is raised to its ignition temperature by heat. Fire suppression systems operate by any one or a combination of the following: (i) removing oxygen, (ii) reducing the system temperature, (iii) separating the fuel from oxygen, and (iv) interrupting the chemical reactions of combustion. Typical fire suppression agents include water, carbon dioxide, dry chemicals, perfluorocarbons (PFC's), hydrofluorocarbons (HFC's) and the group of halo-carbons collectively known as Halons.
The most efficient fire suppression agents are Halons. Halons are a class of brominated fluorocarbons and are derived from saturated hydrocarbons, such as methane or ethane, with their hydrogen atoms replaced with atoms of the halogen elements bromine, chlorine and/or fluorine. The most widely used Halon is Halon 1301, CF.sub.3 Br, trifluorobromomethane. Halon 1301 extinguishes a fire in concentrations far below the concentrations required for carbon dioxide or nitrogen gas. Typically, a Halon 1301 concentration above about 3.3% by volume will extinguish a fire.
Halon fire suppression occurs through a combination of effects, including decreasing the available oxygen, isolation of fuel from atmospheric oxygen, cooling and chemical interruption of the combustion reactions. The superior fire suppression efficiency of Halon 1301 is due to its ability to terminate the runaway reaction associated with combustion. The termination step is catalytic for Halon 1301 due to the stability of bromine radicals (Br.circle-solid.) formed when Halon 1301 is disposed on a combustion source.
When unreacted Halon 1301 migrates into the stratosphere, sunlight breaks down the Halon 1301 forming bromine radicals which react to consume ozone in an irreversible manner: EQU Br.circle-solid.+O.sub.3 .fwdarw.Br.circle-solid.+O.sub.2
In view of the current recognition that ozone depletion is a serious environmental problem, a move is on to: (i) identify fire suppression agents having a less severe environmental impact than Halon; and (ii) develop devices to deliver these more environmentally friendly agents.
Most agents identified as replacements for Halon 1301 are not as efficient extinguishants. Typically, these replacement agents require between two and three times the volume as compared to Halon 1301. The excess volume creates a retrofit problem when space is at a premium.
In addition to extinguishing the fire, it is necessary to suppress the fire as well. Suppression insures that the fire does not re-ignite and requires an inerting agent to remain in contact with the location of the extinguished fire for a time sufficient to either (1) reduce the system temperature below the temperature necessary to support combustion, (2) remove the fuel source, or (3) separate the fuel from the oxygen.
A fire suppression apparatus is frequently located in an aircraft engine nacelle, the aerodynamic structure surrounding the engine. An annular region between the engine and the nacelle presents a fire hazard. During flight, all the requirements of a fire--fuel, oxygen and heat--are present in the nacelle. Some aircraft engine components operate at elevated temperatures, in excess of 700.degree. F. (370.degree. C.), and are thus capable of igniting fuel. An airflow containing oxygen is routed through the annular region to cool the engine. Fuel and hydraulic fluids are supplied to the engine in lines that extend through the region and can leak. In combat, military aircraft can be exposed to unfriendly fire that can sever fuel or hydraulic lines as can other mechanical failures or damage.
Therefore, most commercial and military aircraft utilize an on-board engine nacelle fire detection and extinguishing/suppression system.
Conventionally, when a fire occurs in an engine nacelle, the pilot performs two tasks to save the aircraft: (1) fuel to the engine is shut off; and (2) an on-board fire extinguisher is activated discharging an agent into the nacelle. In some aircraft, the fuel is automatically shut off to the engine in question when the extinguisher is discharged. Generally, several seconds are required to de-pressurize or bleed the fuel lines, during which interval, they may continue to deliver fuel to the fire.
After the nacelle fire is extinguished, re-ignition must be prevented. Preventing an extinguished fire from re-igniting is called suppression. If the re-ignition source is a component operating at an elevated temperature, the suppression time is dependent on how long it takes to bleed the fuel out of the lines. If the re-ignition source is a surface heated by the fire, then the suppression time is dependent on the time it takes the air flow to cool the surface below the ignition temperature (if less than the time required to bleed the fuel line). In either instance, generally from about six to seven seconds are required to inert a fire, extinguish it, and suppress its re-ignition. Therefore, the inerting agent must be able to extinguish the fire and keep it out for a predetermined time, which is typically aircraft-specific.
When used as an inerting agent, Halon 1301 is discharged from a pressurized bottle. The bottle containing the Halon 1301 is supercharged with nitrogen to a predetermined pressure. When activated, the agent is discharged by a blowdown mode and routed to the nacelle via tubing. It is necessary to maintain a minimum concentration of 3.3%, by volume, of Halon 1301 over the entire time required to extinguish and suppress re-ignition of the fire. To compensate for the dissipation of inerting agent, in a conventional fire extinguisher the concentration of inerting agent is initially brought up to a level significantly higher than 3.3% to insure that an effective concentration will remain for suppression. The inventors have observed that this excess amount of inerting agent is not required to fight the fire and represents a significant penalty as to cost, weight and environmental impact.
There remains, therefore, a need for a system to economically inert a fire that does not suffer from the disadvantages of the prior art.