14 BULLETIN 13^3, U. S. DEPARTMENT OF AGRICULTURE 
At first thought the quantity of air required by the discharging 
buckets may seem negligible, but an elevator handling 15,000 bushels 
of grain per hour requires 310 cubic feet per minute. Additional air 
is also carried down the back leg with the buckets. The only natural 
source of air supply outside the vent is up the front leg. As this 
leg is partially sealed by grain in the boot, the flow of air up the front 
leg is somewhat restricted and is insufficient to meet the air needs 
at the head. To make up this deficiency air enters through the vent. 
If no vent is provided, a slight negative pressure will exist in the 
elevator head. Therefore the quantity of air required to replace the 
grain in the buckets, added to the quantity which goes down the spout 
into the garners, plus that which goes down the back leg, must equal 
the quantity which is supplied by the front leg and through the vent 
to produce an equilibrium. 
Where the vent was placed over the discharge, to serve both the 
head and garner, a great deal of dust, chaff, etc., was discharged 
through the vent. Such an installation might create a decided short- 
age in weight because of the large quantity of chaff, dust, etc., 
exhausted. The reason that so much material is thrown out in this 
installation is that the air must flow from the garner to the vent 
through the discharge spout in a direction opposite to the flow of 
grain and natural flow of the air. Thus dust particles and chaff are 
picked up from the grain and discharged through the vent. 
Although when grain is being lifted the flow of air is inward 
through the elevator-head vent and outward through the garner vent, 
the use of an elevator-head vent in conjunction with a garner vent 
is advisable. A head vent, although not essential as a dust-control 
measure, serves as a ventilator, which will satisfy the air needs in 
the elevator head and promote a direct flow of apparently dust-free 
air with the grain from the elevator head to the garner. It also 
ventilates the basement when the elevator leg is not in operation. 
Readings have shown that more than 600 cubic feet of air per minute 
flows out through a 12-inch head vent over the front leg (Table 1). 
As a dust-explosion control measure a very large vent is even more 
valuable in relieving the pressure from an explosion in the leg. Ex- 
tending the elevator head directly through the roof and capping it 
with a large ventilator have been suggested as an explosion and dust- 
control measure. At least two elevators are now equipped in this 
manner. This method is especially recommended for new grain ele- 
vators. The diameter of a vent pipe installed on old equipment 
should be two-thirds the width of the leg and installed directly over 
the front leg (fig. 9). 
AT GARNERS AND SCALE HOPPERS 
The presence of dust on the scale floor is particularly objectionable, 
because of its effect on the scale mechanism. Grain entering the scale 
hopper displaces the air already there; and this air, carrying large 
quantities of dust, will escape into the building unless some provision 
is made to prevent it. The same conditions exist at the garners. 
Many attempts have been made to control the dust so that the 
weights would not be affected. Air ducts connecting the garners and 
scale hoppers have been developed to permit the passage of air from 
the scale hopper being filled into the garner being emptied and vice 
versa (fig. 11). A more general practice, however, is to inclose the 
