It’s important to design feeders and hoppers so that bulk material flows properly during the processing of bulk solids. Flowrates in bulk material handling systems determine processing speed, which in turn establishes the amount of end product generated. For this reason, it’s imperative that bulk material handling companies utilize devices that are designed optimize flowrates of raw product during processing, especially when feeding raw product from storage.
Understanding & Managing Bulk Material Flowrates
Bulk material flowrates are central to all bulk material handling. When processing bulk solids, flowrates differ for various reasons. This includes whether raw material is in lumpy, powdery or granulated form, as these characteristics affect bulk material flow. Equipment design has an outsized role on how bulk material flows through processing systems.
Funnel Flows
The funnel flow effect occurs when hoppers are either not sufficiently steep or the sides aren’t smooth enough for gravity to pull material down the container’s walls, which also occurs when the hopper’s outlet doesn’t function properly. This issue with flowrates causes a channel to form above the discharging outlet, with the material nearer the hopper walls remaining largely immobile.
When bulk material flows less freely, this channel can expand to the extent that its diameter or diagonal is approximately as large as the dimensions of the hopper’s effective outlet. With circular outlets, it’s the diameter, while for rectangular or square outlets this distance is measured diagonally. The flow channel stays nearly vertical higher up the bin where it forms a pipe, with a diameter or diagonal that’s less than the minimum size of the channel or outlet through which material flows. This prevents the formation of a stagnant area within the hopper known as a rathole.
With bulk material flowing more freely this channel expands, with an angle dependent on the internal friction from the material’s effective angle. This flow channel is generally circular, but with a larger diameter than the outlet’s diameter or diagonal. As bulk material flows from a silo or stockpile, a flow channel forms directly above the outlet before crumbling slowly away as material slides into it.
This slow crumbling halts once sufficient cohesion causes the channel to form a stable rathole in the bulk material. Handling product at this point may require external poking or vibration to break up the material around the rathole. Smoother and steeper hopper walls increase the likelihood that it will empty completely. Generally, silos and stockpiles are only suitable for processing bulk solids that are coarse, free-flowing or only marginally cohesive.
Mass flow
When processing bulk solids, mass flow contrastingly occurs when the sides of a hopper are sufficiently smooth and steep to allow bulk material to slide downwards effortlessly. This means that during bulk material handling, product keeps moving throughout processing. Bulk solids don’t form ratholes in mass flow bins as product keeps moving, while the hopper’s geometry won’t allow shallow valleys to form. These bins are used for cohesive and easily degradable materials as well as for powders and applications requiring minimal separation.
Expanded flow
A mix between funnel and mass flow, expanded flow occurs when a funnel flow silo’s lower part or a stockpile functions in mass flow. When processing bulk solids in a mass flow hopper, the flow channel should expand so that it’s equal or greater to the outlet’s diameter, which reduces ratholing. Mass flow hoppers are sometimes placed close enough together to initiate a combined flow channel that helps prevent ratholing. Silos and stockpiles that operate via expanded flow are used primarily for modifying existing funnel flow silos to deal with issues like arching, flushing or ratholing, along with storing quantities of nondegradable materials.
Feeder and Gate Design
The feeder and gate design in bulk material handling systems directly affects bulk material flow into bins, hoppers, silos and stockpiles. For outlets in bins and hoppers, interfaces between the feeder and container must be fully effective to ensure sufficient bulk material flow through the outlet. There are various types of feeding devices, with rotary valve feeders one of the more commonly used in bulk material handling systems. Other types include apron, belt, screw and vibratory feeders.
For more information on bulk material flow and the processing of bulk solids, contact the bulk material handling experts at Prater Industries.