Recycled Gypsum Front-Loaded Metering Bin
This case study is an example of a retrofit of a front-loaded metering bin handling recycled gypsum – a very sticky, cohesive material. The retrofit was completed in 1995.
The Existing System Prior to the Retrofit
Prior to the retrofit, the front-loaded bin was discharged via three screw augers. The augers suffered from chronic plugging and the gypsum plant was keen to find a solution. The challenge with this application is that not only did the plant need to find a feeder that could reliably handle the recycled gypsum but that could also deliver a very consistent discharge.
The plant receives recycled gypsum for free but is limited in the amount of recycled gypsum that it can add to its wall-board because of the recycled gypsum’s paper content. Hence, to maximize the recycled gypsum content without exceeding the prescribed limit, the front-loaded bin needs to deliver a very accurate and even discharge. What makes the application further challenging is that the recycled gypsum is stored outside in the rain, making it particularly sticky.
Why the Existing System Suffered From Chronic Plugging
The augers struggled handling the recycled gypsum for two reasons:
- The conveying action of the screws, which are shearing material from the storage bin, was compacting the recycled gypsum against the front of the bin wall. As the gypsum is compacted, the material gains significant shear strength. Eventually, the compacted gypsum would have sufficient shear strength to bridge over the Feeder.
- The screws have a propensity to withdraw material from the rear of the bin. This selective withdrawal of material is inducing a funnel flow, or first-in, last-out discharge pattern in the bin. In principle, there is nothing wrong with funnel flow as long as the effective opening of the bin exceeds the bulk solid’s piping dimension (or distance over which the bulk solid can form a stable rat-hole). Unfortunately, the piping dimension for recycled gypsum is quite large – in fact, it is larger than the front-loaded bin. To handle recycled gypsum reliably without hang-ups it must be discharged in mass flow, or a first-in, first-out discharge pattern. To do so, the Feeder must withdraw material evenly from the full width and length of the bin discharge outlet.
The solution to fixing this problem storage and feed system was quite simple: Kamengo raised the storage bin and slipped in a Kamengo Feeder between the bin outlet and screw feeders.
First, Kamengo was able to re-use the existing front-loaded bin because the geometry of the bin, including width and length of the discharge opening, and angle of the bin sloping walls were sufficient to deliver a mass flow or first-in, first-out discharge pattern. Good bin geometry is essential to reliable discharge. The measure of a good bin is that, if the feeder were removed, it should completely self-empty with only the aid of gravity.
By inserting the Kamengo Feeder between the screw feeders and the front-loaded bin, the screw augers were no longer acting as feeders but instead only operated as conveyors. This is because the head of material in the front-loaded storage bin was placed on the Kamengo Feeder, which in-turn delivered a metered discharge onto the augers such that they were never more than one-third full. With no head of material on the screw augers, they are plenty reliable acting as conveyors.
Adding a Kamengo Feeder was necessary to achieve a mass flow discharge in the front-loaded bin. By definition, to achieve mass flow, where the stored material comes down as a single body, the feeder must withdraw material evenly from its entire opening. If the Feeder withdraws material selectively from the bin discharge outlet, sections of material in the bin will be stagnant and funnel flow will ensue.
To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please download our white paper entitled: The Design of Reliable Storage Bins and Feeders for the Gypsum Industry.