A screw feeder is a screw that is mounted directly under the opening of a hopper. The product in the silo therefore rests on a part of the screw, and the screw flights are filled completely. This is a clear difference with a transport screw or screw conveyor, where the product is fed with a capacity that is smaller than the capacity of the screw.
A screw feeder is used a lot for the feeding of powders from a (wedge shaped) hopper. The reasons for this are:
* an even withdrawal of the powder is well possible, resulting in mass flow.
* the screw is closed, so there is no risk of pollution of the powder or surroundings.
* proper feeding is possible, in a broad range of capacity.
* the life of the screw; a wear resistant type is possible.
* the screw can be made well resistant against for example high temperature,
aggressive environment, etc.
In a screw transport depends on the friction between product and 1) screw blade, 2) trough or tube and 3) product itself (in the part beneath the silo). The capacity depends on the blade angle, defined by the pitch and the screw diameter. Furthermore the core diameter is of importance because this determines the content of a pitch.
In the capacity profile above of a screw feeder there are 3 characteristic points:
1) the minimum pitch. Below this pitch no transport occurs.
2) the pitch where optimum transport occurs. Below this pitch not all product is moving yet.
3) the maximum pitch. The pitch is so large now, that a further increase will affect the capacity in a negative way.
The position of these points depend on core diameter, screw diameter and product characteristics.
1) Minimum pitch
When the pitch is smaller than the minimum pitch, the product will be jammed between the blades, and it rotates with the screw. Here the core diameter plays an important role. If this diameter is bigger, the surface of the blade is smaller, resulting in a smaller minimum pitch.
Also the wall friction and the internal friction are of importance. In table 1 values of the minimum pitch (see lower boundary) are calculated. The friction between product and trough and product and screw blade is supposed to be equal. In the area between the lower boundary (1) and upper boundary (2) of the minimum pitch the effectiveness of transport is expressed in factor Z (see figure 3).
2) Optimum pitch
Above this pitch the functioning of the screw is best. In the table above this pitch is called the upper boundary of the minimum pitch. Just as the lower boundary the optimum pitch depends on the screw dimensions and the product characteristics. For a pitch above the optimum pitch the capacity increases almost linearly with the pitch. This is the area that is used for the design. When the pitch gets bigger, the capacity increase is less. At a certain point the capacity increase is so small that further increase of the pitch is not favourable.
3) Maximum pitch
Above this pitch the capacity decreases. The screw blade slips beneath the product. In a screw a pitch greater than the maximum pitch may not be present, because this leads to jamming of the screw. The power needed shoots up and if the drive is not damaged, severe damage of the screw will result.
See Wiki item Design of a screw feeder for the design procedure.