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554 IRVING HARTMANN (see Figure 2), and lower minimum explosive concentration. Furthermore, sin?' in a fine dust cloud there are more particles with a greater total surface area and the particles are spaced more closely than the particles in a coarse cloud of eqija weight concentration, the rate of flame propagation is greater and the heat losse are smaller; therefore in a fine dust cloud higher maximum pressures and high rates are developed in explosions. These effects are illustrated in several pap. by Hartmann and co-workers for coal8, for metal powders,5 for synthetic res! and molding compositions,8 and for cornstarch.7 An extreme example of the effe HAZARDS OF COMBUSTIBLE DUST 555 Jinple, that aluminum and magnesium powders produced by stamping, which Igelds flat, thin particles, have higher flammabilities than approximately spherical Jrticles of the same mesh size produced by atomization or by milling. S. Concentration of Dust Cloud In a dust cloud, just as in a gas-air mixture, ignition of one part of the cloud ill 'be propagated throughout the entire mixture and develop into an explosion ly when the concentration of the dust in the air is between certain lower and per limits. The minimum explosive concentration,.or lower limit of explosibility, that concentration at which there is just barely enough dust in the air to propa te flame throughout the cloud after ignition at a localized point. The upper Ifplosive limit is theoretically that dust concentration at which the heat genAted through complete combustion of a portion of the dust in the available , gen is insufficient to raise the entire dust cloud to the ignition temperature. 6,000 Figure 2. Effect of particle sice on the minimum energy required to ignite dust clou, cornstarch in air by electrical sparks. (Courtesy ^Bureau of Mines, tJ. S. Department o(; Interior.) ^ of fineness on flammability is manifested by the pyrophoric behavior of c.f magnesium, lead, iron, uranium, and other metal powders. When of verjjf pifticrOif^^fi^O"01!'r|iers_oxidize-so~rapidly--.uppn-exposui<e--in.-aiE7:tb^ ignite spontaneously. Dusts of a given material produced by different manufacturing prop often have different shapes and surface characteristics that affect their,;fl|| -hility. This mav^Tesult..from-differ,ences.in.sur.f.a.oe^ar.ea...deP8ityT-th;e^m^' adsorbed gas, or the formation of protective surface layers. It is knbwngfg "I. Hartmann, J. Nagy, and H. Brown, V. S. Bur. Mines Kept. Invest. No. 3722,;|9J I, Hartmann and J. Nagy, V. S. Bur. Mines Rept. Invest. No. 3751, 1944. , TI. Hartmann, A. R. Cooper, and M. Jacobson, U. S. Bur. Mines Rept. Invest. Nd.%" 1950. `His 3,000 Maximum pressure and rates of pressure rise developed during explosions of aluminum powder at various concentrations. etween these two limits there is an "optimum" concentration at which there Jp'"enoUgh''duit in the cloud for conipietd combustion in the available oxygen, -greeting dissociation, in a dust cloud of uniform concentration this optimum lire should correspond to the stoichiometric mixture; actually it is always some'' at higher. The pressure developed and the speed of the explosion at the i3mrnt:d.ust-concentration-tmder-^OBt'-Javorable-conditjonaLflrez^B*a.-f.t`r--t.han,, hwer or higher concentrations. The effect of concentration on the maximum insures developed and on the rates of pressure rise in explosions of dust clouds BShtamped aluminum powder can be seen in Figure 3. Similar data have been "ilstrated for other metal powders and metallic hydrides,8 for bituminous coal,8