6 BULLETIN 681^ U. S. DEPAfeTMENT OE AGRICULTURE. , 



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might be expected that the velocity of the flame through a cloud of very fine dust ■ 



would more nearly approach the velocity attained in gas explosions. jVs a matter of i 



fact, this is true. Taffanel has brought this out in tests made at the French Experi- i 



ment Station. Results obtained by the Bureau of Mines lead to the same conclusion. ' 



The rate at which inflammation travels through a gas mixture is dependent upon i 



at least two factors, the inflammability of the gas and the percentage of the gases in ■ 



the mixture. The rate at which inflammation travels through a dust cloud seems to i 



be dependent upon two similar factors, namely, the inflammal^ility of the dust and \ 



the amount of dust in suspension. A third important factor is the fineness of the dust. ■ 



It will be seen, therefore, that gas and dust explosions are similar in many ways, ' 



and that a gas explosion is only a limited case of a dust explosion. j 



A thorough knowledge of the nature of dust explosions and of the accompanying j 



phenomena is necessary in order to devise means for the prevention of such explosions | 



and for the stoppage of explosions if by mischance they should start. It is, therefore, ' 



necessary to know how dust ignites, the various means by which it may be ignited | 



or inflamed, and the chemical processes that take place. Another important factor : 



to know is the ease with which the dust ignites and propagates a flame. This property ■ 



we may call the "inflammability" of the dust. The term " explosil)ility " has also j 



been applied in this connection.. ! 



It is important to note that investigations of dust explosions i 



indicate that, in general, two reports are heard, the first being sharp \ 



and quick, and the second of a loud, rumbling nature and accom- •; 



panied by more flame. It is thought that the first report is due to - 



the ignition of a small quantity of fine dust in suspension, and that j 



the resulting concussion is sufficient to disturb the dust that has | 



settled at places farther away, thus forming an additional explosive \ 



mixture. The flame from the first explosion furnishes, the source of | 



ignition for this newly formed dust mixture, so that the big explosion \ 



with its loud roar is propagated throughout the entire dust zone. j 



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APPARATUS USED IN THE INVESTIGATION. 



For the purpose of conducting the experiments on grain-dust i 



explosions a small, light frame building (15 by 15 by 24 feet), covered i 



with galvanized iron, was erected by the Department of Mechanical ") 



Engineering, Pennsylvania State College (PL I). A small dust room I 



was afterwards built at the side of the main structure at the dis- • 



charge end of the screw conveyor. The grinding floor is elevated ■ 



about 6 feet above the basement, which is level with the ground. ] 



A 16-inch attrition mill (PI. II) was installed on a platform 14 



inches high, with a removable hopper underneath. Two elevators, ' 



one dump bin, a stock hopper over the mill, and a small bin for -. 



receiving ground materials were also provided. A screw conveyor ; 



10 feet long was placed 2 feet below the floor to receive the material J 



from the mill, and to carry it to the chop elevator or out into the \ 



dust room. The attrition miU, screw conveyor, and elevator are ; 



driven by a 15-horsepower direct-current motor, located in the i 



basement. ' j 



The elevator legs and feed bin can be seen on the left of figure 1, 



Plate II. Immediatelv over the mill hoDoer are the spouts extend- ; 



