Bulk material handling is more nuanced than people think. The flow behaviors of solids differ widely from those of liquids, so we cannot set up our processing equipment the same way and expect success. Dry bulk solids encompass a very diverse range of substances, but the one thing they have in common is that they're composed of individual particles.
The size, shape, and chemical composition of those constituent particles will all affect flowability and may have a role in creating potential problems. Let's talk about these flow issues, where they manifest, and how we can engineer bulk material handling solutions around them.
Common flow issues
Ultimately, the objective of any bulk material handling equipment system is mass flow — this means all of the material is in motion when it's supposed to be, and the first particles into the system are the first ones out. Most problems arise from a funnel flow condition, wherein only some of the material is in motion and the first particles into the system are the last ones out.
Bridging occurs when material starts to cohere and forms an arch over the hopper outlet, obstructing flow.
Ratholes are tunnels that form above discharge outlets, caused when the surrounding material has compacted into stabilized walls. Loose particles may trickle through the rathole, but most of it will stagnate.
Limited discharge rate
Limited discharge rate is a problem encountered with fine powders, attributable to counter-flowing air at the hopper outlet hanging up the discharge rate.
Flooding or flushing
Occurs with bulk materials with a tendency to aerate or fluidize, causing an overflow of systems not designed to handle such behaviors.
When particles of one size separate from particles of another, segregation is at play, and it's often bad news for product consistency. One of the most frequently-occurring segregation behaviors is sifting, wherein finer particles centralize under a fill point and coarser particles roll off to the fringes.
Particle degradation happens when stagnant particles start to lose their integrity, spoiling, oxidizing, or caking together to further compound flow issues (such as ratholing).
Problem areas (and engineering solutions)
Flow issues crop up most frequently in the various points of transition within a dry bulk handling system — namely between hoppers, feeders, and chutes.
Hoppers sit inside the bottom of a storage bin or silo, its walls converging toward a discharge outlet that feeds material to the equipment below. To prevent funnel flow, engineers must design hoppers with steeper, smoother walls (to encourage gravity and prevent friction) and a larger outlet.
Again, the material(s) to be handled will dictate the geometry of the hopper, as well as whether any flow aids (like vibration) need to be added to prevent bridges or ratholes.
Feeders help facilitate mass flow by extracting material from the full breadth of the hopper outlet. Either conveyor belts or screws can be used to help push that material out, usually paired with an outlet gate to either cut off or open flow from the above hopper.
Issues might arise if the outlet gate does not open or close fully, causing uneven discharge rates, or the feeder is designed incorrectly. For mass flow purposes, the feeder must be engineered for increasing capacity in the direction of flow. In the example of a screw feeder, that might mean tapering that screw from back (less capacity) to front (more capacity) under an elongated hopper outlet.
Chutes are sloping channels or slides conveying bulk materials from high to low and must be optimized against flow issues. The major components of the chute are:
- The chute hood: top portion where bulk materials begin their descent.
- The chute body: middle/connecting portion between the upstream and downstream conveyor
- The chute exit: Also known as the spoon.
Ideal bulk material handling chute design will prevent plugs/blockages (with adequate smooth/steep contact surfaces and sufficiently wide cross-sectional areas), minimize wear to chute surfaces and downstream belts, and keep materials from degrading, segregating, or aerating.
Achieve mass flow with ETS
Have you lost your flow? If your bulk materials are delayed in transit, Erie Technical Systems has your ticket out, with innovative hopper, chute, and feeder designs to get (and keep) things moving in the right direction. Smartly shake things up with our selection of vibratory conveyors, feeders, and spiral elevators, or schedule a consultation to discuss the best bulk transfer solution for your application.