DUST CONTROL IN GRAIN ELEVATORS 
29 
baffles or a cone-shaped collector, which separates the heavier 
material by centrifugal action. Screens may be substituted for 
baffles in the expansion chamber, or a combination centrifugal 
separator with a cloth filter may be used. Data to indicate which 
type is preferable are not available, The separator in one system 
consists of a metal tank divided into compartments, with a hand- 
operated valve, which diverts the stream of dust-laden air to one 
comj artment as the other is being emptied. The filter consists of 
a tank, with a number of filtering screens which separate the dust 
from the air. An automatic device shakes the dust from these 
screens to the cone-shaped bottom of the tank, where it is discharged 
through a rotary air lock. This lock, which is simply a paddle wheel 
Fig. 23.— Vacuum-cleaning nozzle 
revolving in a tight-fitting cylinder, permits the continuous discharge 
of the dust while a partial vacuum is maintained in the filter. 
Dependability is the first consideration in selecting the vacuum- 
producing equipment to supply the required suction. In a plant 
operating 24 hours a day most of the equipment is operated inter- 
mittently, but the cleaning system must operate continuously. 
Even on Sundays and holidays and during shutdown periods, at- 
tention is given to cleaning. The equipment should operate con- 
tinuously over long periods with the minimum of attention. Multi- 
stage vacuum producers of the centrifugal fan type and positive 
pressure blowers have been used satisfactorily. When the producers 
are electrically driven, the motors should operate continuously with- 
