What kind of feeder?
A slot shaped opening helps to create mass flow. This will pose additional demands on the feeder.
A feeder must provide an equal discharge over the area of the opening. If this is not the case then the feeder will cause funnel flow, even if the silo has build for mass flow.
This is illustrated in the following situation from practice:
A silo for wood dust and gypsum powder regularly gave flow problems. This resulted in stagnation of the flow, especially after a weekend. Under the silo a screw feeder with a varying pitch was mounted.
After checking of the silo geometry, it appeared that here mass flow would occur.
Check of the screw made it clear that it had not been designed properly. The occurring downward velocity of the product and the capacity of the screw were as shown in the figure opposite:
From this it can be seen that the first pitch of the screw takes up almost 50% of the total capacity. In other words: only 10% of the screw length is responsible for 50% of the capacity.
This causes a relatively very high product velocity above the first 10% of the screw. In this situation the vertical velocity above the other 90% of the screw was so low that flow above this part did not occur at all.
Based on the internal and wall friction of the products a screw has been designed, so that product will be discharged over the length of the screw. With this new screw flow problems no longer occurs.
Directives for feeder design
There is no overall theory for the design of feeders. There are however a few points of attention, that must be taken into account when designing feeders.
The feeder must:
- be able to yield all demanded capacities;
- be adequate for all products handled;
- supply a flow as constant as possible, at every capacity;
- be well controllable over the whole range;
- discharge product evenly over the whole area of the opening;
- when using a screw feeder, the pitch must tuned to the product characteristics, so equal drawdown is ensured.
When vibratory feeders are used in combination with other feeders or transport devices, the vibratory unit must determine the capacity. For example in the combination of vibrating bottom and transport screw, the latter must have a significantly greater capacity, to prevent the bottom from blocking.