The chemical and physical properties of some solid materials render them prone to restricted flow and flow stoppages when discharging from bins and hoppers. This one-page reference outlines common solids-flow problems, as well as strategies and devices that can aid discharge from bins and hoppers.
Common solids flow problems
When solids flow is hindered, one or more of a set of common solids-flow problems is typically observed.
FIGURE 1. Cohesive bridging (a) and ratholing (b) are two common no-flow conditions that can occur when discharging solids with certain properties
Cohesive bridging. Cohesive bridging is a “no-flow” condition where material forms a stable arch over bin outlets, due to the material adhering to itself (Figure 1a; [1]).
Ratholing. This is a “no-flow” condition where a stable, open channel forms above the outlet and stagnant material outside of the channel does not empty (Figure 1b).
Mechanical interlocking. This is a “no-flow” condition where material forms a stable arch over bin outlets, due to the ability for coarse, anisotropic materials to interlock and form strong physical bridges.
Limited discharge rate. This is a “not-enough-flow” condition when interactions between fine material and air restrict the solids discharge rate.
Caking. Caking is a condition where stagnant material agglomerates into lumps, potentially disrupting flow.
Buildup. This is a condition where material is allowed to build up within bins and chutes (Figure 2; [1]).
Flooding. This condition involves interactions between fine material and air that can cause the solid material to behave like a liquid, resulting in the solid material overwhelming downstream equipment.
FIGURE 2. Fine material can be a problem if it builds up on the inside of bins and chutes
Active flow aids
Mechanical devices for ensuring solids flow include the following:
Vibrating bin discharger. This type of device is hung from a storage bin by rubber bushed links. It incorporates a rubber skirt to prevent leakage and to isolate the bin from the vibrations. Vibration is transmitted through an outer shell and into an internal dome or cone-shaped baffle by a motor with eccentric weights. Vibrating dischargers can accommodate hopper openings from about 3 to 15 ft and are intended to keep material completely live over a hopper outlet’s entire cross-sectional area [2].
Agitation. Devices that agitate a solid product are typically composed of multiple segmented helical sections that slowly rotate within the body of the discharger. This produces a downward flow into a discharge auger that controls the rate of withdrawal. For agitator-type flow aids, the size of the unit is usually based on the arching dimensions of the material [2]. These flow aids can have difficulty with flaky or cohesive materials.
Vibrator. Vibrators can be mounted on the side of a bin or chute to initiate flow. They can be air- or electrically operated and are available in many shapes and sizes. Types of vibrators include rotary, piston, turbine, linear, electromagnetic, eccentric and others. Some types are designed to provide high-frequency, low-amplitude vibration, while others generate lower-frequency, high-amplitude vibrations. Vibrators may cause packing of the material inside the bin if the material is pressure-sensitive, or if the device is operated with the gate closed.
Cone unloader. Cone unloaders have an internal cone that is raised into the product at the bottom of a hopper or bin and vibrated to initiate flow. It can be used as a gate as well as a discharging device.
Aeration. Air pads work by discharging air along the bin and hopper walls. They provide localized fluidization to aid flow, and require several pads to be effective. Users must be careful that pads do not obstruct flow.
Fluidizers. Fluidizers work by undercutting solids with localized fluidization. Air blasters inject high-pressure air or nitrogen into a bin or silo.
Non-mechanical flow aids
Several types of non-mechanical aids can also improve flow.
Flow-aid chemicals. Flow-aid chemicals, such as fumed silica, can improve flowability, reduce caking and improve storage stability.
Cone-in-cone. This involves a conical hopper mounted within a larger conical hopper. The design is intended to minimize hopper height and promote mass flow. The inner cone, which is open at the top and bottom forces the material to flow along the walls of the shallow outer cone.
Letdown chute. Letdown chutes minimize attrition for fragile solids. The material is deposited in the top of the spiral chute, and is lowered to the bottom of the bin, where it gently spills out of the openings provided.
Editor’s note: The text and images in this column are adapted from the following articles: 1. Ray, M. and Maynard, E., Impact of Particle Size Control on Bulk Solids Flow Behavior, Chem. Eng., August 2023, pp. 30–34; and 2. Marinelli, J., Overcoming Solids Caking with Flow Aids, Chem. Eng., April 2014, pp. 38–41.