Halon Fire Extinguishers

Halon fire extinguisher are a specialty type of fire extinguisher that contain a gas that interrupts the chemical reaction that takes place when fuels burn. This type of fire extinguisher is often used to protect valuable electrical equipment since they leave no residue.

Halons are fire extinguishing agents which are gaseous when discharged and are extremely popular in the aircraft industry, as well as in certain technology marketplaces because Halons are electrically non-conducting. Halons are in almost universal use in aircraft fire extinguishers.. They exist in two forms:

• Halon 1211 is used only in portable extinguishers and is a streaming agent.  A halon fire extinguisher has a limited range, usually 4 to 6 feet

• Halon 1301 is used only in fixed extinguisher installations and is a total flooding agent.This type of extinguisher is commonly found in computer server rooms and clean rooms.

According to SkyBrary, a wiki focused on aviation safety:

Both Halon variants work by a combination of chemical and physical effects. The chemical effects, which are dominant in their overall effect, are achieved by the atoms in the gas directly inhibiting combustion in two different ways:

• Bromine, Iodine and Chlorine atoms act catalytically so that each atom participates repeatedly in the scavenging of important free radicals from the combustion gases.
• Fluorine atoms react with free radicals and form strong chemical bonds which neutralise combustion but can only do so once and are then “consumed.” The physical effects are both temperature reduction and dilution.

Temperature reduction occurs, whenever a non-reactive gas is added to a flammable gas, because the heat liberated by the reaction of oxygen molecules with a fuel source must be distributed into the overall environment. The rate of the combustive chemical reaction decreases rapidly with reductions in temperature and, if the concentration of added inert gas is high enough, the flame chemistry fails altogether.

Halon gas mixtures are not only inert but of low temperature when released from their pressurised state. Dilution is a simple matter of reducing the collision frequency of the oxygen and fuel source so that there is a reduction in chemical reaction rates. The magnitude of this effect, however, is relatively small compared to chemical inhibition and thermal effects, the former of these being the predominant one.

The chemical consituents in Halon gases, and the products of the reactions they produce when used to fight fires, have been identified as causing damage to the Ozone layer.  Halon is still in use today, but is falling out of favor for many uses due to its environmental impact. Europe, and Australia have severely restricted its use, since the Montreal Protocol of 1987. It is however still in use in the United States, the Middle East, and Asia in limited ways.  Since the Montreal Protocol Halon is one of the only ozone depleting chemicals with concentrations still rising, due to the release of fire extinguishing equipment already deployed.

A few 'clean agent' alternatives have been appearing on the market as an alternative to Halon extinguishers, including:

• Halotron I extinguishers, like carbon dioxide units, are "clean agents" that leave no residue after discharge. Halotron I is less damaging to the Earth's ozone layer than Halon 1211 (which was banned by international agreements starting in 1994). This "clean agent" discharges as a liquid, has high visibility during dischage, does not cause thermal or static shock, leaves no residue and is non-conducting. These properties make it ideal for computer rooms, clean rooms, telecommunications equipment, and electronics. These superior properties of Halotron I come at a higher cost relative to carbon dioxide.
• FE-36^TM (Hydrofluorocarbon-236fa or HFC-236fa) is another "clean agent" replacement for Halon 1211. This DuPont-manufactured substance is available commercially in Cleanguard® extinguishers. The FE-36 agent is less toxic than both Halon 1211 and Halotron I. In addition, FE-36 has zero ozone-depleting potential; FE-36 is not scheduled for phase-out wheras Halotron I production is slated to cease in 2015. A 100% non-magnetic CleanGuard model is now available.