DUST CONTROL IN GRAIN ELEVATORS 41 
The velocity at which oats are not lifted, 684 feet per minute, is 
o( the greatest value in proportioning a dust-collecting hood. Oats 
is the lightest grain handled in an elevator, and the maximum velocity 
at which it is not lifted is the determining value in proportioning a 
hood which will not lift grain. 
Experiments had shown that the air velocity at the opening of a 
satisfactory hood should never exceed 684 feet per minute. Obser- 
vation had shown that the most satisfactory air velocity for the 
main pipe lines and branches is about 4,000 feet per minute. The 
velocity in some of the branches near the fan, however, is often much 
higher because of the high static pressure in the main pipe. For 
that reason it was considered that a hood of standard dimensions 
for general use in grain elevators should be of such proportions that, 
with an air velocity of approximately 6,000 feet per minute in the 
pipe line, an air velocity of more than 684 feet per minute can not 
exist at the hood intake. A hood with an intake area nine times 
the area of the pipe connection was considered adequate for this 
purpose; but tests made with such a hood showed that, because of 
small eddies at the opening, light grains were picked up from a screen 
inserted in the mouth of the hood when a velocity of only 4,000 feet 
per minute was maintained at the pipe connection. Further experi- 
ments showed that a hood having an intake area equal to 12 times 
the area of the pipe connection would not pick up the lightest grains 
placed on a screen and inserted directly in the mouth of the hood at 
velocities up to 7,275 feet per minute. The intake of the United 
States Department of Agriculture hood, therefore, has an area 
equal to 12 times the area of the pipe connection. 
The dimensions of the United States Department of Agriculture 
hood (fig. 32) were calculated in terms of the pipe diameter and some 
other required dimension of the hood. In establishing dimensions for 
the length and width of the intake for piping of various diameters, so that 
the area of the intake is always 12 times the area of the pipe connec- 
tion, algebraic expressions were used. With D the diameter of the pipe 
connection, L the length of the rectangular hood opening, and W 
the width of the rectangular hood opening, then, >. 
WL= 12 — r-> = 9.425 D 2 , or approximately L = — '-^ — » and W=^-^-y — . 
The height of the hood was determined in terms of the length of 
the rectangular opening. Consequently the flare of the hood is a 
definite angle for openings of all sizes and shapes. The most sat- 
isfactory height is one and one-fourth times the length of the hood 
opening, H=±iyiL, measured from the apex of the extended cone to 
the mouth of the hood. The angle made by the sides of the hood 
with the horizontal is an angle whose tangent is 2.5, or an angle of 
68° 12'. Experiments have shown that within reasonable limits a 
uniform flow of air enters the mouth of a hood having these pro- 
portions. 
Dimensions for pipe connections of different diameters and for 
hood openings of various sizes are given in Table 6. When one or 
two dimensions of the hood are known, the others can easily be cal- 
culated. For instance, let it be assumed that a hood with a mouth 
30 inches long and a pipe connection as small as is satisfactory is 
desired for use over a conveyor belt. The length, 30 inches, is given 
