DUST CONTROL, IK GRAIN ELEVATORS 7 
condition about the boot therefore depends upon the removal of the 
surplus air in the boot, by means of an air suction connection from 
a fan system or a vent to the outside of the building. It is necessary 
only to exhaust the air from the boot as fast as it accumulates. No 
excessive velocities are required to draw out the dust. 
It has been found difficult to apply suction to elevator boots in 
such a manner as to remove all suspicion that the installation may 
be able to lift grain, chaff, and other light material, thus interfering 
with the grain weights. In practically every dust-collecting installa- 
tion at elevator boots, it is necessary to use a hood so designed that 
the air enters the hood, preferably in the form of a curtain, and is 
not of sufficient velocity to lift dust particles from the grain stream. 
The United States Department of Agriculture hood (p. 37) was 
designed to meet all the requirements for such an installation. 
Fig. 4. — Application of suction to elevator boot by horizontal hood 
Even a properly designed hood may prove unsatisfactory unless 
it is at a point where grain cannot be thrown into the hood opening 
and where there is no possibility that a " choke-up" may back up 
grain into the hood and give the suction an opportunity to draw out 
grain. When the suction connection can be made at some distance 
from the grain, it is sometimespossible, although not advisable, to 
use a direct pipe connection. When suction is applied to the boots, 
into which receiving belts discharge grain from cars, it is well to 
place a trap in the suction line to prove that the installation does not 
interfere in obtaining the correct grain weight. A velocity control 
valve (fig. 29), used in conjunction with the trap, makes it possible 
to regulate the air velocity in the branch line. 
Many methods of applying suction to elevator boots are in use. 
The suction connection may be made on the front leg, on the back 
