14 BULLETIN 681, U. S. DEPARTMENT OF AGRICULTURE. 
SERIES 4. 
Object. — 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 (PL III, 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 mill as rapidly as possible in order to get a 
large amount of dust into the 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 mill 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 mill. The 
flame was extinguished by merely turning off 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 mill 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. 
