DUST COXTROL IX GRAIN ELEVATORS 
45 
gathered up by the suction. Access to the interior of the trap is 
provided by a door, which can be opened only by the inspector. 
A trap designed and used successfully by the Underwriters Labora- 
tories (fig. 34) is usually installed in a horizontal run of the branch pipe 
to the hood. A baffle assists in making the separation. A trap of 
simple construction developed by the Bureau of Chemistry (fig. 35) 
can be installed instead of an elbow 
connecting horizontal and vertical 
runs of piping, or it may be placed 
in a vertical run of piping by the 
use of another elbow. 
AIR-VELOCITY CONTROL VALVES 
Valves are necessary for control- 
ling the air velocity in many of the 
branch pipes of a dust-collecting 
system, especially near the fan where 
the static pressure in the main pipe 
is very high. Valves make it pos- 
sible for a dust-collecting system to 
be so balanced that the velocity in 
all the branches will be uniform. 
They reduce the power consumption, 
because less air is handled by the 
system and a lower static pressure 
at the fan is required. Of equal 
importance is the fact that proper 
valves, placed in the branches, make 
it possible to control the air velocity 
in a branch pipe connected to the 
hood. Such valves should be so de- 
signed that they can be locked in 
position by the inspector who has supervision over the grain weights. 
A valve of the blast-gate type (fig. 36) has been used successfully. 
In one elevator the installation of these gates in the branches made 
Friction Cap 
with Hasp 
Fig. 35.— United States Department of Agri- 
culture inspection trap 
Lock 
Fig. 36.— Blast gate 
