closes with the committee's findings regarding (1) the status of activities directed toward finding alternatives for halon as a fire suppressant and (2) the potential for discovering an environmentally acceptable equivalent.

Chemical and Physical Considerations in Evaluating Alternative Fire Suppression Agents

Several large chemical manufacturing concerns have invested heavily in the search for economically viable halon replacements, although the compounds investigated tend to be related to existing commercial products or their precursors. The National Institute of Standards and Technology (NIST) survey of candidate agents encompasses a somewhat broader range of compounds,10 including compounds whose study will help reveal the chemistry and physics of extinguishment.

Fire Suppression Models—Chemical and Physical Mechanisms

There are a number of ways in which a fire can be extinguished. The simplest introductory concept for understanding the behavior of a fire is the fire triangle, which has three sides: fuel, oxygen, and heat. If any leg of the triangle can be removed, the fire can be extinguished. Fires are categorized as being either flaming combustion or smoldering. The former is predominantly a gas-phase phenomenon and is characterized by the emission of visible and infrared (heat) radiation. The latter type of fire occurs when solids such as plastics burn, and heterogeneous reactions at or near the surface are important. Flaming combustion, the primary target of halon 1301, generates more heat and consequently is more dangerous in the short term. Smoldering often generates more toxic gas emissions and can be more difficult to extinguish.

A fire can be extinguished by either physical or chemical mechanisms. Physical suppression mechanisms involve removing at least one leg of the fire triangle. (1) By smothering or blanketing the fire, the fuel and air are separated. An example of such a method is the use of foam extinguishers. (2) If the heat source is removed, the fire can also be suppressed. Thus, methods that cool the flame are important extinguishing techniques. For example, an agent with a high heat capacity can cool the flame by absorbing heat or can undergo a phase change that also requires heat. The most important parameters are the heat capacity and/or the latent heat of vaporization; experiments have shown that the thermal conductivity (how fast heat is transferred) is of lower importance. (3) Mechanical means such as forcing a gas over the flame at high velocity can extinguish a fire by separating the fuel and the air or the fuel and the heat. (4) For liquid or solid fuels, it is possible to place an agent that absorbs thermal radiation between the surface of the fuel and the flame. This prevents the generation of gaseous fuel and is known as flame radiation blockage.

It is also possible to have chemical suppression of a flame. This occurs when an agent or its degradation products interfere with the chain reaction that is critical to sustaining combustion. When chain carriers in or near the reaction zone are removed, the chains are disrupted and the fire cannot sustain itself.

It is possible to combine both physical and chemical effects in an agent, and in fact many of the best suppressants operate by both mechanisms. It is difficult to separate out the physical and chemical aspects of flame suppression. For example, an agent can remove heat by undergoing an endothermic decomposition process. If the decomposition products are inert, this is considered to be a physical process. However, if the products are reactive, then such products can contribute to a chemical suppression mechanism. The most likely radical chain species to be removed by a reagent or its degradation products are atomic hydrogen and oxygen and the OH radical.

There are three types of fuels for fires: gas, liquid, and solids. Gaseous fuels are the easiest to understand as they can readily participate in chain reactions. For liquids, the fuel, in general, needs to be vaporized from the liquid surface, and so the amount of fuel is determined by the rate of vaporization and the surface area of the available liquid. However, because the flame generates heat that can affect the

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