Extinguishment of Class B Flames by Thermal Mechanisms; Principles Underlying a Comprehensive Theory; Prediction of Flame Extinguishing Effectiveness

Abstract

Flame quenching by condensed or gaseous extinguishants and by external sources is examined. The quenching by extinguishants is due to heat-absorption sinks—dissociation, decomposition, vaporization, and heat capacity. External quenching by water-cooled metal surfaces or by radiation to surroundings is shown to have common properties with internal quenching by extinguishant particles or molecules.Flame-extinguishing mechanisms are effectively explained with thermal quenching concepts and a flame heat balance. New criteria for the extinguishment of Class B flames are postulated and, then, substantiated by a comprehensive analysis of extinguishment data for a large number of agents. Adiabatic limit temperatures were initially computed with the flame heat balance (Equation 1) using quenching quantities based on heats of formation of extinguishing substances at 298 K, but such limits and quenching quantities exhibited no systematic character. However, alternate limits and quenching quantities based on heats of formation of molecular parts (of extinguishing substances) exhibited exceptional ordering and internal consistency.This alternate analysis contributed to increased understanding of thermal mechanisms, concurrent exothermic reactions, and other processes involved in flame extinction. It is further demonstrated that the flame extinguishing effectiveness of dry chemicals and most gaseous halocarbons (agents containing C, H, F, CI, Br or I) can be reliably predicted from the additive properties of enthalpy, temperature, and quenching quantities. The flame-extinguishment model provides definitive criteria for selecting alternate agents with superior flame extinguishing properties.

Experimental Studies of Flame Extinction by Sodium Bicarbonate (NaHC03) Powder

“…The chemical and thermal aspects associated with the inhibition mechanism of various fire retardants are discussed by da Cruz et al (1988), Ewing et al (I989a), Birchall (1970), Ewing et al (1992), and Ewing et al (1994. Ewing et al (1989a) analyzed experimental data obtained by various investigators and concluded that the extinction of diffusion flames by most of the inhibiting agents including NaHC0 3 can be attributed to thermal heat absorption effects such as heat-capacity sinks and decomposition/vaporization sinks.…”

Water for Manual Fire Suppression

“…It can be used to hit the flames, the hot gases produced, the heated surfaces of the room, the fuel surface and the fuel not yet involved in the fire. Cooling of the fuel and stopping fuel gasification is the main mechanism when applying the water at the fuel surface [9,10], and cooling of the flames below the adiabatic flame temperature is the main mechanism when applying water droplets into the flames [11]. There are also other effects, e.g., the stirring of fire gases due to the water flow and to the generation of steam.…”

Flame Retardants, Phosphorus

“…Physical (endothermic) modes of action have been shown to be of dominant importance in the flame-retardant action of a wide range of non-phosphorus-containing volatile compounds (38,39).…”