An empirical relation has been developed which correlates and predicts the 6re-suppression effectiveness of a wide variety of gaseous, liquid and solid agents. The flame-extinguisbment model is based on the premise that extinction is dominated by heat-absorption processes and that a flame is extinguished when suBcient heat has been removed by the extinguishant to reduce the temperature to a limit value. This limit is the minimum temperature at which the effective rate of the combustion reactions is s d a e n t to maintain flamepropagation, and it depends in a predictable way on the properties of the suppressant and flame system. The heat-balance equation describing tbis is derived in two stages. In the first, a preliminary equation is obtained by considering only those substances which are thermally stable and act only as heat-capacity sinks. In the second, the equation is generalized by consideration of all endothermic reaction sinks, e.g. vaporization, dissociation and decomposition. The general equation correlates most of the extinction data found in the literature. The results suggest that for most substances the extinguishing capacity is related to heat-extraction and that many of the effects previously attributed to chemical mechanisms may be thermodynamic in nature rather than kinetic.
In a continuing study of flame extinguishment, ~, 2, 3, 4 we report on scaling studies for dry chemicals on larger heptane diffusion flames (0.29 m 2 and 2.32 m 2 pans). We demonstrate again that small particle sizes extinguish most effectively. Extinguishment is related to heat absorption by decomposing or vaporizing particles. We show that the limiting particle size for each dry chemical--that is, the maximum size which completely decomposes or vaporizes in the flame--is independent of flame size for the systems studied. We broaden and apply the concept of latent or maximum effectiveness 2,3 to pan fires of all sizes. Finally, we describe and characterize an aerodynamic effect in the transport of powders, where large particles with their higher momenturn entrain and drag smaller, more effective particles into the flame.We also show that extinction curves, involving the ratio of real-to-latent extinction weight and the proportion of small to large particles, have predictable shapes and predictable quantitative levels for most dry chemicals. We have developed the real-to-latent concept along with scaling equations for agent mixtures and for a wide spectrum of agents and particle sizes.
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