Limitations in Feeder SelectionLimitations in Feeder Selection
A feeder must handle the bulk solid it is presented with, and not all bulk solids are the same.
August 4, 2023
Scott Miller, consultant, Solids Handling Technologies Inc.
The feeder stands as a final sentry for material exiting a bin. Bin performance hinges on this device. For the feeder to succeed it must work in unison with the bin, requiring proper design of both. Feeders come in a wide range of sizes, types, and styles. As well, the feeder can serve a range of functions beyond metering material at a certain rate.
With so much dependence on this device, the selection can be difficult. In many cases there may be multiple options for a feeder that will work. As priorities change, we may arrive at a different feeder. However, the possible type of feeder is not limitless. A feeder must handle the bulk solid it is presented with, and not all bulk solids are the same.
The feeder must impose some sort of order on the material. This often requires an ability to contain the material and some type of movement: the feeder must control the material. To achieve this the device relies on something such as the weight of the material, mechanical forces, vibration, some type of air injection, or some combination of the above. Depending on the material, these factors may not all be workable.
Take for example a very low-density powder discharging from a bin into a drop-through rotary valve. It is not uncommon with a material like this for this feeder style to see little to no product discharge. The drop through rotary valves require the material to act under the force of its own weight to fall into the pockets of the rotary valve and then on the discharge side drop out again, acting under the force of gravity. This may be impractical for a very light material, particularly at higher speeds of rotation where the rotor serves as something of a fan. The problem arises from the very low bulk density of the material, as well as the impermeable nature of the powder.
Take for another example a bulk solid with much larger particles and greater bulk density that attempts to employ a fluidized bin bottom for feeding. The approach of aeration, where low quantities of air is admitted through a permeable surface may work well for the previous powder but would be ineffective here. These heavier particles are highly permeable and allow for the air to pass through the bed of product without any discharge occurring. Here the problem is a function of the voids between the particles, the size of the particles, and density of the particles.
Finally, take for example a highly sticky product that clings to surfaces and is discharged by a vibratory pan feeder. Vibratory pan feeders will vibrate to a distinct stroke and frequency to achieve the movement of product along the trough. For a highly adhesive product, buildup may occur on the surface making rate control difficult with, at times, only a thin top layer in motion (if at all). The principle of the feeding device relies on a series of small “throws,” where product it “tossed” forward again and again to the point of discharge. When a product sticks to the trough this becomes a difficult task at best. Here the problem is a function of the high adhesion between the bulk solid and the trough surface.
None of the above feeder types is bad, rather in each instance above there is a mismatch between the material characteristics and the feeding device. The method the feeder seeks to employ in each case is ineffective due to the nature of the material. It should be noted that there are unique innovations for many feeding devices. These features in some cases allow for the use of a feeder that might otherwise not work. Steering away from a particular feeder or deviating from the standard design requires knowledge; both how the feeder interacts with the material and how a specific material at hand will respond.
Scott Miller has been providing consulting services for Solids Handling Technologies Inc. since 2016. For more information, visit www.solidshandlingtech.com.
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