DUST CONTROL IX GRAIN ELEVATORS 27 
The air pressure and velocity needed are important questions. A 
few tests have shown that ordinarily a vacuum-cleaning system 
should be operated at a negative pressure sufficient to produce a 
static pressure or head of at least 2 inches of mercury at the inlet or 
tap farthest from the vacuum-producing equipment, when this tap 
and a number of other taps corresponding to the number of hose 
lines to be operated at one time by the system are open. With a 
static head of about 2 inches of mercury, the air entering a 2-inch 
tap opening had a velocity head of 13.5 inches of water, or a velocity 
of 14,700 feet per minute. Owing to friction, this air velocity is 
greatly reduced after a hose is attached to the tap. For example, 
assuming that one velocity head is lost in 50 pipe diameters of hose 
length, six velocity heads are lost in a 2-inch diameter smooth-interior 
hose 50 feet long. No tests were made to determine the velocity of 
air entering the hose; but, with a static pressure of 2 inches of mer- 
cury at an open tap, a well-designed cleaning nozzle connected to a 
2-inch hose 50 feet long, operated satisfactorily. If a 2-inch static 
head is maintained at the farthest tap, a higher velocity will advan- 
tageously exist at the taps nearer the vacuum-producing equipment. 
At least 325 cubic feet of air per minute is required for each 2-inch 
hose connection used at one time. Based on a velocity of 15,000 feet 
per minute at the tap openings, this value may appear excessive, but 
an even higher value may be advisable to compensate for the wearing 
of the equipment and to permit the cleaning of the piping by the 
opening of clean-out valves in the main pipe lines. 
The working pressure at the vacuum-producing equipment must 
be the pipe friction losses from the most distant taps to the producer 
plus the loss in the collector plus the static pressure at the tap. The 
friction of the ah, even in straight pipe, is a source of loss, Since 
the friction loss varies as the square of the air velocity and the 
roughness of the pipe and as the length divided by the diameter of 
pipe, care should be taken to make the main pipe lines of sufficient 
diameter to give a velocity which does not result in an excessive loss 
and to have the small-diameter branch lines as short as possible. 
The main lines should be so designed that the velocity of the air 
will not drop below 5,000 feet per minute under any possible operating 
condition. To maintain this minimum velocity and prevent ex- 
cessively high velocities, it may be necessary to operate the system 
with one or more taps open or a like number of nozzles always in 
use. Bends and turns in the pipe line will produce a drop in pressure, 
and the friction loss caused by them is usually based on the equivalent 
loss in length of straight pipe. The loss due to a bend with a radius 
equal to the diameter is equivalent to the loss in about 15 diameters 
of straight pipe. All fittings should be of the long-radius type, and 
45-degree laterals or 90-degree long turn "y" branches should be 
used wherever possible. It is best to have all fittings of the cast- 
iron drainage type, with recessed threads providing for the butting 
of the ends of the pipe against the shoulders of the fittings, thus 
making a smooth interior with no pockets or obstructions. 
Only piping and fittings with smooth interior surfaces should be 
selected. In making a joint, the pipe should be cut square and care- 
fully reamed, so that no obstruction is offered to the flow of air. 
