DUST CONTROL IX GRAIN ELEVATORS 
25 
shut off by a swinging valve in the main duct leading from the fan 
and another trunk line caring for the branches in another section of 
the plant thrown in, so that the suction of the fan may be concen- 
trated at the point desired. 
In some installations a dust-collecting system is incorporated in 
the floor-sweep system (fig. 20). This is not adapted for applying 
suction to boots, elevator heads, garners, and scale hoppers; it can 
be used in caring for the tails of conveyor belts and, in some cases, at 
belt loaders. 
As much grain is handled by a floor-sweep system, grain traps are 
placed in the main duct ahead of the fan to recover the grain from 
the dust. In some installations the material from these traps is 
spouted or elevated to a screening bin, which feeds into a special 
Fig. 22. — Improved floor sweep 
grain cleaner, to separate the various grains of value. In most 
installations, however, the grain falls from the trap upon the floor, 
where it is bagged at frequent intervals. 
Floor-sweep design. — A poorly designed sweep can greatly reduce 
the efficiency of tne floor-sweep system because it can not rapidly 
gather up the material swept into the hood. In sweeps of the type 
usually installed (fig. 21) a hinged cover is placed over the end to 
shut off the suction not in use and a blast gate in the branch line is 
sometimes used instead of the cover. A sweep designed in the 
Bureau of Chemistry (fig. 22) to remove material very rapidly with- 
out choking operates on the principle of having the air come in 
more intimate contact with the material than it does in the usual 
installation. 
