14 BULLETIN 681^ U. S. DEPARTMENT OF AGRICULTURE. 



SERIES 4. 



Ohject. — ^The object of this series of tests was to attempt to ob- 

 tain an explosion in the dust room (located outside the building, 

 at the end of the screw conveyor) and to determine if such explosion 

 would propagate the flame back to the mill. 



Conditions. — This dust room (5 by 10 feet by 7 feet high) was 

 made as tight as possible, and the door was replaced by a heavy 

 muslin curtain to retain the dust, at the same time permitting free 

 expansion in case of an explosion. The electric arc was located at 

 first in a small box built above the end of the conveyor, and later 

 just outside this box. In both cases the arc was regulated by means 

 of a long wooden pole extending to a safe point outside the building. 

 The duct leading from E to the vent at F was removed (PI. Ill, fig. 1). 

 The hopper was placed under the grinding disks, and all holes, in- 

 cluding B, were closed, in order to retain the dust as far as possible. 

 Both disks of the attrition mill were run at their normal speed of 

 2,200 revolutions per minute. The arc was turned on^ and various 

 kinds of grain products, which had given explosions in previous 

 tests, were run through the miU as rapidly as possible in order to get a 

 large amount of dust into tlie dust room. After various grains had 

 been fed for 15 minutes, a blast of burning dust was observed issuing 

 from the outlet in the box above the conveyor. It was evident 

 that the rapidly revolving attrition miE produced enough draft to 

 force its dust out at the conveyor end without the use of a special 

 fan. In fact, the force of the blast was so great that it did not ap- 

 pear possible for the flame to get back to the attrition miU. The 

 flame was extinguished by merely turning ofif the arc, and then more 

 elevator dust was rapidly fed into the mill, the arc having been 

 turned on again. Within 2 minutes, another heavy blast of flame 

 was observed at the end of the conveyor. In this instance the flame 

 could not be extinguished by turning off the arc, because the wooden 

 box on the end of the conveyor had caught fire. The mill was 

 stopped immediately, and the burning box saved by means of a fire 

 extinguisher. After the arc had been placed just outside this box 

 the miU was run again. Elevator dust and ground oat hulls were 

 fed for 15 minutes, during which time the room became very dense 

 with dust. Although the arc had been burning during this time, no 

 explosion occurred, but, as before, due to the intense heat radiating 

 from the air, the box over the end of the conveyor was again on fire. 

 The mill was stopped, and the fire easily extinguished with a pail of 

 water. 



Results. — No explosions were obtained in the dust room by means 

 of an electric arc in this series of tests. 



