Homer W. Carhart

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

Flame quenching by condensed or gaseous extinguishants and by external sources is exam ined. The quenching by extinguishants is due to heat-absorption sinks—dissociation, decompo sition, 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 sub stances at 298 K, but such limits and quenching quantities exhibited no systematic character. However, alternate limits and quenching quantities based on heats...

Flame extinguishment properties of dry chemicals: Extinction concentrations for small diffusion pan fires

An extensive experimental study has been made of the extinguishment of a smalln-heptane diffusion flame (14.7 cm diameter pan) by five common dry-chemical powders—potassium bicarbonate, sodium bicarbonate, potassium chloride, monoammonium phosphate, and Monnex. The fire extinguishing effectiveness (reciprocal of the observed minimum extinction concentration) of each dry-chemical substance, other than that due to chemical specificity, is fully explained on the basis of particle size and distribution. Literature correlations between effectiveness and particle surface area appear to be incidental and stem from the inherent relationship between particle size and surface area.An important finding of the study is a discontinuity in the extinguishing effectiveness of a powder as a function of particle size. This occurs for each substance at a unique particle diameter above which there is a dramatic five-to eight-fold decrease in effectiveness. For a given substance, all powders with particle sizes below the limiting value exhibit the same maximum effectiveness.

Effect of conductivity on charge generation in hydrocarbon fuels flowing through fiber glass filters

Abstract The quantity of electrostatic charge generated when hydrocarbon fuels are passed over a phenol-formaldehyde resin-coated fiber glass filter is dependent upon the conductivity of the liquid and the flow velocity through the filter. As the conductivity of the liquid is increased, the charging tendency, as measured by the filter current, increases, passes through a maximum, and then decreases. At higher flow velocities, the position of the maximum in the filter current vs. conductivity curve was found to shift to a higher conductivity range. In most cases, the sign of the charge on the fuel was found to be dependent upon the nature of the surface of the filter material. Thus, when phenol-formaldehyde resin-coated fiber glass, nylon, dacron, and Kel-F filters were used, the sign of the streaming current was usually positive. When ordinary glass wool or fiber glass filters, from which the resin coating had been removed by baking, were used, the current was negative. By dividing the flow of fuel from a storage tank into two streams each containing different filter materials, one causing the fuel to acquire a positive charge and the other a negative charge, and by carefully controlling the flow through each filter, it was possible to deliver fuel to the receiving tank with virtually no net charge on it. The results suggest a possible method of reducing the charge level on fuel in a practical system by employing two types of filter elements in a filter separator unit, each producing charges of the opposite sign in the fuel. Another possibility would be to fabricate the filter element using alternate layers of filter materials which produce charges of opposite sign in the fuel. Although the latter method appears to be less effective, it may offer some advantages from a manufacturing standpoint.

Flame Extinguishment Properties of Dry Chemicals: Extinction Weights for Small Diffusion Pan Fires and Additional Evidence for Flame Extinguishment by Thermal Mechanisms

An experimental study was made of the extinguishment of a small n-heptane diffusion flame (25.5 cm dia. pan) by a number of dry-chemical powders — KHCO3, NH 4H2PO4, Na2B4O7 . 10H2O, (NH4)2SO4, K2CO 3, K4Fe(CN)6 . 3H2O, Na2CO 3, K2SO4, NaCl, Na2C2O 4, K2C2O4 . H2O, and Monnex. The minimum weight of each required for flame extinction was determined as a function of particle size and sieve fraction. The sharp dis continuity previously observed1 is shown to be a common phenomenon. The experimental procedure was such that complete flame penetration was achieved for all particle sizes of each substance. Therefore, all results are latent, or maximum possible, effectiveness values. Flame extinguishment effectiveness is defined as the reciprocal of the extinction quantity.Substantial evidence, based on the new experimental data, is presented to support the propositions: (a) that particle sizes by sieve analysis below a limit size completely decom pose and/or vaporize in the flame; and (b) that the observed optimum...

Evidence for flame extinguishment by thermal mechanisms

This paper presents an analysis of experimental flame and detonation extinguishment data published by a number of authors, including those in a companion paper. The maximum effectiveness observed for each of five common dry chemicals at small particle diameters is shown to be related to heat extraction from the flame by active endothermic sinks—heat capacity, fusion, vaporization, and decomposition. Larger particles are more stable in the flame and the reduced level of effectiveness observed is due principally to the only active sink—heat capacity.Evidence is presented to support two propositions: first, that the strong chemical inhibiting effects exhibited by many substances in flame velocity studies are effectively confined to low-concentration regimes; and second, that regardless of chemical effects, diffusion flames of the type studied are largely extinguished by thermal or heat extraction mechanisms at extinguishant concentrations that are quantitatively predicted by a simple heat balance and a predictable limit temperature.

Pressurization with Nitrogen as an Extinguishant for Fires in Confined Spaces

Abstract Pressurization with nitrogen has been studied as a technique for suppressing fires in confined spaces. Liquid hydrocarbon-air fires of various sizes in a gastight experimental chamber were extinguished in approximately 30 seconds by increasing the total pressure of the enclosure from l atm. to 1.35 ± 0.03 atm. The post-fire atmospheres were found to contain sufficient oxygen for support of normal human activity, while the buildup of hazardous combustion products was well below established safety levels.

Impact of low O2 on fires

Abstract It is the thesis of this paper to show that through control of oxygen, fires can be markedly minimized or eliminated while still maintaining an atmosphere that will sustain human life. The driving force in this study is to decrease the fire hazard in manned capsules, such as in space, under the ocean, submarines, etc. which operate in an environment exceedingly hostile to life and from which escape is next to impossible. In most studies on fire, the earths atmosphere with its 21% ( v v ) oxygen is almost a given and is not usually varied. But it takes both a fuel and an oxidizer to have a fire, and though we study extensively the impact of structure, composition, concentration, flammability limits, energies of ignition, propagation rates, etc., for the fuel component of the combustion process there is a significant paucity of information on the impact of variations of the oxidizer. In this paper, the oxidizer of choice is oxygen because we also want to sustain life, and one of the beauties of a totally closed environment is that the oxygen can be varied at will, and we can take advantage of that fact to minimize or quench fires while not jeopardizing life.

Extinguishing Class A fires with multipurpose chemicals

We investigated the extinguishing properties of multipurpose dry chemicals using openregime cribs of two sizes. One measured 20.3 by 20.3 by 15.2 cm, and the other 50.8 by 50.8 by 38 cm. A crib fire, which has both flaming and smoldering contributions, is primarily extinguished when the burning surfaces are covered or smothered by a gas phase of the original multipurpose agent or a product of the agents decomposition. The overall effectiveness of each dry chemical depends on its particle size, the degree of its decomposition and/or vaporization, and the openness of the crib. All particles of an agent below a unique limiting size completely decompose and/or vaporize in the crib fire and have the same extinguishing effectiveness. Particles above the limit have no measurable effectiveness. Scaling is considered on the basis of the minimum extinction weight per unit of exposed wood surface.

Evaluation of Halon 1301 test gas simulants: Enclosure leakage

Full discharge tests used in verification and quality assurance of total flooding Halon 1301 fire protection systems represent a large portion of the Halon 1301 emissions annually. It is, therefore, desirable to use a suitable simulant in these tests in view of the stratospheric ozone problem and current and future regulation of Halon 1301.Sulfur hexafluoride, SF6, and chlorodifluoromethane, R-22, were identified as candidate simulants on the basis of their similarity in physical properties to Halon 1301. These two candidates were then evaluated on the basis of leakage from an enclosure.SF6 was determined to be an excellent simulant for Halon 1301 when considering leakage from an enclosure. Further testing of SF6 and R-22 is progressing on other important aspects of Halon 1301 systems; i.e., flow hydraulics and initial mixing.

Pressurization with Nitrogen as an Extinguishant for Fires in Confined Spaces II. Cellulosic and Fabric Fuels

Abstract Laboratory experiments on pressurization with nitrogen as a technique for suppressing fires in gastight spaces have been extended to solid fuel combustion. Flames from fabric and cellulosic fires in a 9.5 ft3 chamber were quenched by the addition of ≲ 0. 5 atm of nitrogen. Several samples of each fuel were allowed to burn to self-extinguishment in the closed chamber to ascertain the true effectiveness of the technique in halting the production of noxious gases. By comparison with these unattended fires, it was shown that the depletion of oxygen was terminated quickly and the gross buildup of noxious gases was significantly repressed by nitrogen pressurization, leaving a habitable postfire atmosphere.