DUST CONTROL IX GRAIX ELEVATORS 35 
The radius of each elbow in both branch and main pipes should be from 
one and one-half to two times the diameter of the pipes. In practice 
the friction of such an elbow is considered equivalent to 10 diameters 
in length of straight pipe. The friction in an elbow with a short 
radius may cause a loss as great as that in 60 to 70 diameters of straight 
pipes. The elbows should be of metal several gauges heavier than 
that used in the straight pipes. All joints should be soldered to 
prevent leakage and loss of power. 
The piping should be smooth and free from dents. The lap at 
the joints should be so formed that the flow of air is not obstructed. 
The piping should be at least 6 inches from the wall, so that the 
outside of the pipe can easily be kept clean. Handholes should be 
provided at frequent intervals in the duct, and traps should be placed 
wherever there is a tendency for material to settle. In vertical lines 
a trap or boxlike chamber should be placed at the base of the line to 
catch heavy particles, which can not be lifted by the air. All trunking 
should be grounded at 50-foot intervals for the removal of static 
electricity. 
The branches should enter the main duct at as small an angle as 
possible, never more than 45°. The branches should join at the side 
or the top of the main, never at the bottom. They should never 
enter the main opposite each other. The branch connection to the 
main should be as smooth as possible. 
DUST COLLECTORS 
The metal cyclone dust collector is the most satisfactory type of 
collector for either a dust-collecting or low-pressure dust-removal 
system. These collectors separate the dust from the air by means of 
centrifugal force and air expansion. Baffle plates are usually placed 
within the collector to control the conditions within the collector 
necessary to effect a more perfect separation. The most suitable 
collector makes the best separation with the least back pressure od 
the fan. The larger the collector, the better will be the separation 
and the less the back pressure on the fan and the power consumed. 
Under no circumstances should a collector be installed inside an 
elevator. It should be mounted on the roof of the dust house or at 
some other outside point. Each collector should have its individual 
exhaust pipe; a common exhaust line permits flame communication 
from one system to another and so constitutes an explosion hazard. 
The exhaust of the collector should be covered with a hood designed 
to prevent the entrance of sparks. The dust-discharge opening 
should be large, so that the collector will not choke. A manhole 
should lead to the interior of a large collector. 
GRAIN TRAPS 
A grain trap (figs. 27 and 28) is simply an air-expansion chamber 
in the main duct, so designed that the air velocity through it is too 
low to carry grain. The grain, stopped by baffles within the trap, 
falls into the hoppered bottom of the trap. Tandem flap valves 
covering the bottom permit the continuous discharge of the material 
without affecting the pressure within the system. 
