A feeder is a device that maintains control of material discharge from a silo. This is not the only service that a feeder provides. More importantly this is not the paramount job of a feeder. The main role of a feeder is to maintain reliable material flow in your bin or silo. Or stated conversely, a feeder must not negatively affect the flow of a bulk solid from the silo. This applies to both mass flow and funnel flow silos.
A feeder may be providing all the following functions:
* Maintaining reliable material flow in the silo
* Metering the discharge of the bulk solid
* Serving as an isolation device at the silo outlet
* Conveying material to a desired destination
These are the more common tasks that a feeder is capable of, but depending on the material, application, and feeding device, may not be able achievable with a single piece of equipment. Two common temptations when selecting a feeder are to treat feeders and conveyors as interchangeable, and to assign too many jobs to a single feeder.
Feeders and conveyors exist as dual counterparts to one another. For example, screw feeders, vibratory pan feeders, and belt feeders each have conveying counterparts. A conveyor transports material from one point to another and operates at lower capacity over longer distances. A conveyor is capable of moving material at higher velocities and can elevate material. A conveyor also has lower pressures acting upon it, typically with lower power requirements. When these distinctions between conveyor and feeder blur, several problems can occur.
First, material flow in the bin may negatively be impacted. This frequently occurs when a conveyor is interfaced with a bin outlet. A properly designed feeder will ensure that the bin outlet is fully live. To maintain mass flow the feeder must be specifically designed to discharge over the entire outlet cross-section. In funnel flow, the feeder must discharge over an area sufficiently large enough to create a flow channel minimizing the formation of stable ratholes.
A second problem that may occur is inefficient conveying and excess power requirements. If the feeder meters flow and then conveys material some distance upwards, it will likely perform both tasks poorly. For example, using a long-inclined screw to feed a fine powder from a bin would not work well. The powder would be prone to sliding backwards resulting in inefficient conveying and would also be prone to packing in the screw, resulting in high torque and power requirements. In applications like this it is best to have a feeder to meter the powder to a conveyor (operating at low capacity) to elevate and convey material. Typically, a feeder should only be designed to convey materials short distances horizontally.
A third danger is excess feeder wear if it is operated at very high speeds. A belt conveyor can run at 400 ft/min, but if a belt feeder was operated at this velocity, high wear would result. Wear in material handling is a function of both pressure and velocity for a given bulk solid. Feeders are exposed to the pressures at the bin outlet and operate with more material. The higher material loading causes more pressure between the bulk solid and the contact surfaces. Material on a conveyor is exposed to less head pressure, due to lower capacity loading and can operate at higher speeds without the same magnitude of wear on a feeder.
The distinction between feeders and conveyors and the functions which they serve are not always clear. At times, the boundaries between the two blur and intersect. When designing a feeder, the list of possible jobs it can perform is not limitless. There may be times when a separate piece of equipment (such as a conveyor) is also needed to accomplish additional duties. A feeder may be used to meter discharge, serve as a means of isolation from upstream equipment, and convey material from one point to another. But the primary role of any feeder is to work in unison with the bin to maintain reliable flow.
Scott Miller is a consultant with Solids Handling Technologies Inc. For more information, visit solidshandlingtech.com.