Generally, feeders are required to have feed rate control. This is often achieved through exhaust port sizing and/or variable speed motors. The feeder and bin are closely related and have to work in unison to achieve the feeding objectives. A good understanding of bin configuration and material flow through the bin are important first steps.
Funnel Flow
In a funnel flow hopper, some material moves, while the rest of material remains static. This could occur because the walls in the conical part of the hopper are not steep enough and/or smooth enough for the given material (see figures below) to flow:
In the drawing on the left above, arching at the hopper outlet causes the flow of material to be erratic or stop. In the drawing on the right, the formation of a stable rathole causes an undesirable phenomenon known as "first in – last out". Material that entered the hopper first, is often the last material to leave it.
Mass Flow
In mass flow hoppers like those in the illustrations below, the walls in the conical section are steep and smooth enough to prevent material from stopping. When material is discharged from the hopper, all the material in the hopper moves at the same velocity and in the order that it is layered. This ensures "first in- first out" flow.
Types of Feeders
There are numerous types of feeders on the market today. Some feeders are industry specific; however, the majority of them fall into one of the following general categories:
- Screw Feeders
- Vibratory Feeders
- Rotary Valves
- Belt Feeders
Many articles have been written on the common feeders listed above. In addition, their dominant use in the industry proves their versatility for most materials. However, what do you do if your material does not fall into this category? In the past, operators and production personnel had to live with the problem and deal with it in the "classical" fashion: beat the hopper with a mallet and hope for the best!
A more "scientific" approach may be possible: Table Feeders
Table feeders can often be the solution for moving those "difficult to flow" materials.
