We have written several articles on proper feeder design. Over the years, we have stated ad nauseam that the feeder design is as important as the bin or silo design. They must work together to maintain reliable material flow.
An incorrectly selected or misused feeder that does not work in unison with your bin can cause a long list of flow problems. “A feeder is just a feeder, anything can be used!” While you probably know this is a dangerous statement, it’s easy to underestimate just how important feeder selection can be for reliable material flow. The selected feeder must not only control flow rate but work well with your bin or silo. Most powders require a mass flow pattern where all the material is in motion whenever any is withdrawn. If they flow in funnel flow, they will be susceptible to ratholing, flushing, segregation, etc.
Many of the feeders in use today are screws, belts, or rotary valves. Screw and belt feeders would typically be used to control discharge from slotted outlets (wedge-type hoppers). Whereas, a rotary valve feeder would typically be used for circular or square outlets.
Screws are a good choice when: an enclosed feeder is required; when space is restricted; when handling dusty or toxic materials; when attrition (particle breakage) is not a problem.
It is not unusual to use screws with constant pitches in an effort to feed material from a slotted opening. However, this approach typically causes a preferential flow channel to form at the back (over the first flight) of the screw. This type of flow would obviously destroy a mass flow pattern and exacerbate the problems created by funnel flow including structural issues.
A properly designed screw feeder allows material to be withdrawn over the entire cross-sectional area of the outlet, increasing in capacity in the discharge direction. This approach uses a combination of tapered-shaft diameter and increasing pitches to accomplish this. Half pitch screw flights are used over the tapered diameter which transitions to increasing pitches. This approach results in maintaining a mass flow pattern because the entire outlet cross-sectional area will remain live.
A belt feeder is also required to increase in capacity to ensure a fully live slotted outlet. A properly designed interface is required between the slot outlet and the belt will progressively discharge more product onto the belt along its length. This will make the full cross section of the hopper outlet live and therefore maintain mass flow.
A belt feeder interface increase is elevation from back to front of the belt. This allows more material to be deposited on the belt towards the discharge end of the bin. Additionally, the interface increase in width from back to front (in plain view) allows the material to stream onto the belt uniformly to maintain a fully live outlet.
A rotary valve feeder consists of a series of pockets attached to a rotating shaft. In many cases, rotary valves are used as airlocks to feed material from a bin into a pneumatic conveying system. This is to prevent countercurrent, high-pressure air from the conveying line from permeating up into the bin and interrupting material flow.
Rotary valves are notorious for feeding preferentially from one side of a bin or silo. The valves pockets (or vanes) fill with material from one side of the outlet such that it cannot receive material from the other side of the outlet. A common way to prevent this is by inserting a vertical section between the bin outlet and the rotary valve. The preferential flow channel will expand up through the vertical section, and material will be uniformly withdrawn from the bin outlet.
Additionally, rotary valves tend to drive excess air up through the outlet impeding flow. Most rotary valves include a vent line to direct the excess air to either the top of the bin or a dust collector.
The discussion of each of the above feeders emphasizes an important consideration when choosing or designing a feeder. The feeder must work in unison with the bin above to ensure uniform withdrawal of powder from the bin.
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