As a consultant to the bulk solids handling industry for so many years, I have provided solutions to many solids handling problems. Typically, the problems range from arching or bridging, ratholing, flooding, or erratic flow. Much of the time these problems are caused by improper bin or silo design. Typically the first approach is to add a flow aid device to encourage flow from the troublesome bin. This may be a problem for at least two reasons:

1. A flow aid device may in fact cause more problems
2. Consider that perhaps it is not the bin design at all, but the feeder configuration

Whenever I provide solutions to solids handling problems, the process usually begins with knowledge of the bulk solid's flow properties. This requires flowability tests that describe just how the material is flowing in the existing equipment. Once I know how the material flows, I can provide either a new design or retrofit of the bin, but also the design of the feeder. When I lecture on solids handling I repeat this statement many times to ensure that it is recognized as an important part of the design process: The feeder design is just as important as the bin design!

Feeders are divided into two categories: volumetric and gravimetric.

Volumetric Feeders
A volumetric feeder discharges a volume of material as a function of time, such as cu ft/hr. Volumetric feeders include screws, belts, rotary valves, etc.

Screw feeders will:
* provide discharge from a slotted outlet
* work well when an enclosed device (dust, toxicity, etc.) is required
* cause a preferential flow channel to form at the back (over the first flight) of the screw if simply comprised of constant pitch flights
* withdraw over the entire cross-sectional area of the outlet by using a combination of tapered shaft diameter and increasing pitch flights to increase the screws capacity in the discharge direction   

Belt feeders will:
* provide discharge from slotted outlets
* handle friable, coarse, fibrous, elastic, sticky, or very cohesive solids
* experience flushing if the capacity of the material fed onto the belt does not increase in the discharge direction
* work properly if an interface between the bin and belt is used to provide the increase in capacity

Rotary valve feeders will:
* discharge materials to a pneumatic conveying system by providing a seal against an adverse pressure gradient
* typically develop a preferential flow channel if mounted directly to the bin outlet
* work well if a vertical section is installed to alleviate the formation of this preferential flow channel
* create counter current air flow issues
* work well if a vent line is used to send the counter current flowing air back to the top of the bin or to a dust collector

Gravimetric Feeders

A gravimetric feeder relies on weighing the material to achieve a required discharge rate or batch. Feed accuracy of 1/4% is sometimes obtainable with a properly designed gravimetric feed system.

There are basically two ways to feed gravimetrically: continuous and batch. A continuous gravimetric system controls the weight/unit time such as lb/hr. A batch system controls simply the weight of material, such as 50 lb of material to a mixer. Examples of gravimetric feeders are: weight belts, loss-in-weight systems, and gain-in-weight systems

Weigh belt feeders will:
* typically be used in continuous feeding applications
* typically weigh the belt, drive, and material on the belt
* use a prefeeder so that the weigh belt basically acts as a conveyor
* require constant calibration
* requires flexible connections to isolate the feeders from upstream

Loss-in-weight feeders will:
* typically use load cells that are attached to the bin or hopper to weigh the bin, feeder, and their contents
* measure the loss in weight of material discharged from the system
* typically be used both in continuous and batching systems
* typically require a fast refill time to maintain accuracy as it cannot weigh while they are being filled, so it switches to volumetric feeding while the filling process continues and back to gravimetric after filling

Gain-in-weight feeders will:
* typically be used only for batching applications
* use receiving container resting on a scale or on load cells
* will be limited due to headroom requirements, the fact that only one material can fill the GIW hopper at a time and the GIW hopper must empty completely with little or no buildup

If you would like more detailed information on feeders, please email  [email protected] to receive a pdf of his feeders paper.

Joseph Marinelli is a consulting engineer and president of Solids Handling Technologies. He has been providing testing and consulting services since 1972. As a former consultant with Jenike & Johanson Inc., he has years of experience testing powders and designing bins and feeders for reliable flow. He lectures frequently on the topic of powder handling, and has published several papers, including an article in a chemical encyclopedia and two in a food powder book. For more information, contact Joe at 803-802-5527 or [email protected].

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