34 BULLETIN 1373, U. S. DEPARTMENT OF AGRICULTURE 
INTERSTICE BIN FANS 
Where only one interstice bin in the center of the storage section 
is being used as an air shaft, mechanical ventilating equipment has 
been used to increase the natural ventilation. The air is drawn from 
the basement and forced outside through this large air shaft by means 
of a fan large enough to effect at least one air charge per hour. The 
air forced out of the basement through the air shaft is replaced by 
fresh air coming in through the windows and doors at the side of the 
storage section. 
DUST-CONTROL EQUIPMENT 
FANS 
The steel-blade exhauster fan, of the most rigid construction, is 
generally used. The blades should be so well fixed that they can 
never work loose to produce a spark. This is best done by fastening 
the sides of the blades to heavy steel-plate flanges. The blast-wheel 
should be overhung, so that the material passing through the fan 
does not come in contact with the ball or roller bearings. 
The diameter of the fan inlet should be at least as large as that of 
the main suction pipe, which should have an area equal to the com- 
bined area of the largest number of branches it serves at one time. 
A fan and a main duct from 10 to 20 per cent oversize will provide 
for additional branch lines which may be attached to the system. 
The speed of the fan depends upon the static or suction pressure 
required at the fan inlet to overcome the friction losses resulting from 
the air traveling through the longest pipe line at a velocity of at least 
4,000 feet per minute. The speed of the fan and the power require- 
ments must be accurately determined in selecting a motor. The 
pipe friction losses plus the losses in the collector and grain catcher 
must be computed to determine the working pressure and speed of 
the fan. Information concerning the method of calculating the static 
or suction pressure required at the fan, together with data on the 
performance of fans, can be obtained from fan catalogues. Fan manu- 
facturers will give advice as to the proper size and type of fan and 
motor. 
PIPING SYSTEM 
Short, direct pipe lines, with as few elbows as possible, are most 
efficient. 
The area of the duct at the fan inlet should be equal to the combined 
areas of the branches served; and, immediately beyond each .point 
where a branch joins it, the area of the duct should be decreased by 
an area equal to that of the branch line. A main duct with an area 
from 10 to 20 per cent greater than the sum of branch areas, some- 
times recommended, reduces the friction losses in the pipe lines; 
but it also reduces the velocity in the main duct, so that the dust is 
not carried through the main duct when only a few of the branches 
are being used. The dust which settles on the bottom of the main 
duct is an explosion hazard. 
