In a silo, the solids are withdrawn through chutes at the bottom. Previous dis­cussions examined solids flow through the silo. Now, we will look at the flow out of the silo through the chute, which connects the silo to the feeder. A proper chute design ensures uninterrupted flow from storage to feeder. Improper

design results in nonuniform flow. Figure 8.9 illustrates the problem, showing partial solids flow with a uniform-area chute and full solids flow with a properly designed chute. As the solids accumulate on the belt, their uniform flow through the hopper prevents them from accumulating at the chute’s downstream section. The chute’s expanded and lifted opening helps the solids spread well, allowing uniform withdrawal. For this reason, the modified design of Figure 8.9(b) shows the skirt on the chute to be lifted and expanded (in plan view) to facilitate uniform solid discharge from the hopper. These angles (slope and discharge) should be in the range of 3 to 5 degrees.

Figure 8.10 is another illustration of this phenomenon, this time with a rotary feeder. Here the design on the left (Figure 8.10a) is without the short vertical section like that on the right (Figure 8.10b). Solids are compressed in the direction of rotation and pushed up through the hopper. The design on the right uses a short vertical chute that limits this backflow only to the chute height, giving a relatively steady flow.

The two key requirements for chute design are: (1) the entire cross-section of the outlet must be active, permitting the flow of solids; and (2) the maximum discharge rate of the chute must be higher than the maximum handling rate of the feeder to which it is connected.

A restricted outlet, caused by a partially open slide gate, results in funnel flow with a small active flow channel regardless of hopper design. A rectangular outlet ensures that feeder capacity increases in the direction of the flow. With a belt feeder, the increase in capacity is achieved by a tapered interface. The

capacity increase along the feeder length is achieved by the increase in height and width of the interface above the belt.

Poor feeder design is a common cause of flow problems, as it prevents smooth withdrawal of solids. If the discharge rate of the chute is lower than the maximum designed feeding rate of the feeder, the feeder can be starved of solids and its flow control will be affected.