GEAIX-DUST EXPLOSIONS. 39 



DISCUSSION OF RESULTS AND CONCLUSIONS. 



SPARKS EB'SITTED BY FOREIGN SUBSTANCES. 



Many attempts were made, in both the prehminary and the regular 

 tests, to produce dust explosions in an attrition mill by introducing 

 with the feed various kinds of foreign substances, such as nails, small 

 pieces of stone and fhnt, and matches, but not once did an explosion 

 occur. Various kinds of grain products, with different degrees of 

 fineness, and both normal and minimum moisture contents, were fed 

 during these attempts. Evidently, however, the intensity and amount 

 of the heat generated by the sparks from these foreign substances were 

 not sufficient to ignite the dust, although an explosive mixture of air 

 and dust may have been present, as manifested by the regularity of 

 the explosions obtained by the electric arc under identical conditions. 

 In rare cases, and especially with very low humidity and moisture 

 contents, it might be possible for a nail or other hard substance to be 

 caught in such a way that a very long succession of sparks would be 

 emitted with sufficient intensity to cause ignition. The results of 

 these tests, however, point strongly to the following conclusions: 



1. Sparks emitted by foreign substances passing through an attri- 

 tion mill do not in general appear to be hot enough to ignite an 

 inflammable dust. 



2. Nevertheless, to take care of the exceptional case, every pre- 

 caution should be taken to keep all foreign substances from entering 

 the grinding machines. 



NAKED FLAME. 



Only two attempts were made to obtain explosions by means of a 

 naked or open flame, but both were entirely successful (p. 11). Safe 

 conclusions are as follows: 



1. Naked flames can readily ignite inflammable dust mixtures. 



2. A naked flame should never be allowed in the vicmity of dust. 



CARBON ARC IN ATTRITION MILL. 



The summary of all results obtained with the use of the electric 

 arc is shown in Table 16. It is to be noted that in practically two- 

 thirds of the attempts with materials which gave explosions at one 

 time or another, explosions were obtained by means of an electric 

 arc located near the grinding discs. In every one of the 201 explosions, 

 except Explosion 3 of Series 12, it was necessary to feed the material 

 in the amount of at least a handful to produce an explosion. Appar- 

 ently, a fairly dense cloud of dust was required to produce an ex- 

 plosion with even the most intense source of ignition available, the 

 electric arc. The following conclusions can be drawn : 



1. An attrition mill is capable of producing mixtures of dust and 

 air which can be easily ignited by a sufficiently intense source of heat. 



2. There is much less danger of an explosion, when the rate of feed- 

 ing is less than the amount which the mill can take care of. 



