18 BULLETIN 379, U. S. DEPARTMENT OF AGEICULTUEE. 



agreed, however, that the dust must be fine and dry and in a state 

 of suspension in the atmosphere, so that upon being brought in con- 

 tact with sufficient heat or flame an ignition is caused. It generally 

 is conceded that there must be a proper proportion in diffusion in 

 order for the explosive mixture of dust and air to ignite with suffi- 

 cient force to propagate an explosion. 



The conditions under which these thresher fires and explosions 

 occur appear somewhat similar to those with other cereal dusts. 

 During the threshing process the smut, whi<;h is a form of very 

 fine, dry dust, is thrown into suspension in the air and forms a 

 dangerously explosive mixture, which readily would produce an ex- 

 plosion or fire if ignited. The mixture of smut dust and air may 

 have limits of explosibility, and it is quite possible at times to have 

 toO' much dust present, and at other times not sufficient, for an igni- 

 tion. For this reason explosions may occur at a given time and under 

 certain conditions and not occur at other times or under different 

 conditions. 



AMOUNT OF DUST REQUIRED FOR EXPLOSION. 



Explosions have been produced at the Pittsburgh Testing Station 

 of the United States Bureau of Mines when there was only 0.032. 

 ounce of coal dust suspended in each cubic foot of air, or 1 pound 

 in 600 cubic feet. In order to produce complete combustion it takes 

 all of the oxygen in 1 cubic foot of air to burn completely 0.12 ounce 

 of the coal dust used.^ Preliminary experiments have shown that 

 smut dust is highly inflammable, and also that many of the cereal 

 dusts have relatively a lower ignition temperature and produce 

 higher pressures than coal dusts. It might be concluded that the 

 explosive limits of cereal dusts would be lower than those of the 

 coal dusts. 



In some of the thresher explosions at least two distinct reports 

 were heard; the first being sharp and quick and the second resem- 

 bling a loud roar and lasting longer than the first and accompanied 

 by more flame. An explosive mixture consisting of sufficient quan- 

 tity of smut dust in suspension, ignited by sufficient source, would no 

 doubt cause the sharp report usually heard first. This original igni- 

 tion, possibly only an inflammation, might produce sufficient concus- 

 sion to shake the dust that had settled on various parts of the sepa- 

 rator into suspension in the air, thus forming an additional explosive 

 mixture. The heat or flame from the first small puff or inflammation 

 would cause an ignition of this newly formed mixture,, and the 

 explosion would propagate through the entire dust zone. This may 

 serve to explain the loud rumbling sound sometimes heard, accom- 

 panied by a large body of flame causing expensive damage. 



^ U. S. Dept. Interior, Bureau of Mines. Miners' Circular No. 3, Coal-Dust Explosions, 

 (1911), p. 7. Geo. S. Rice. 



