This article highlights some critical issues regarding the flow of solid materials as well as an overview of some material flow systems.
Material Flow Issues
Compared to fluid flow, solids flow requires additional considerations that complicate the equipment design process. In general, two major factors must be considered when designing material flow systems: cohesive strength and consolidation stress.
The cohesive strength is the bonding between particles, while the consolidation stress is the stress applied on solid particles that affect the volume those particles occupy. In combination, the cohesive strength of a material increases as the consolidation stress increases (as there are more particles in contact for consolidated materials).
Competing Flow Patterns
In solids handling systems, there are two major types of flow: mass flow and funnel flow. Mass flow occurs when the entire bed of material travels from inlet to outlet, while funnel flow occurs when stagnant regions exist along the flow path; funnel flow can lead to erratic flow behaviors (for example, when a stagnant region is dislodged, it can lead to a higher-than-normal input to downstream equipment) and material caking and degradation.
To ensure a consistent solids flow that follows a first-in, first-out flow sequence, the equipment must be designed with sufficiently vertically sloping walls and adequately low friction.
Inefficient Equipment Design
As mentioned above, inefficient equipment design can lead to undesirable material flow patterns that can result in material degradation and process inefficiency. However, many manufacturers are only vaguely familiar with solids handling concepts, and because solid materials are more complex than fluids regarding material flow, the design of solids handling equipment is complex.
Solids Handling Design Considerations
There are three major design aspects to consider when designing solids handling equipment: Hopper angle, minimum outlet dimension, and discharge rate.
The Hopper angle is the wall angle that can be used to adequately overcome the friction between the material and the wall; in general, the Hopper angle must be determined via experimentation and is dependent on the material and the particle size distribution.
Minimum Outlet Dimension
The minimum outlet dimension must be determined to prevent stagnant regions from forming at the outlet; minimum outlet dimension is thus dependent on the cohesive strength of the material and the consolidation of the material due to the normal force.
Discharge rate, which is determined according to process need, is dependent on outlet dimensions, velocity, and the bulk density of the material; thus, to achieve the desired flow rate, the velocity at the outlet and the bulk density of the material must be determined, and the outlet velocity must be set accordingly.
Material Flow Systems at General Kinematics
At General Kinematics, we offer a number of solutions for solids handling, such as our HULA-HOPPER® Bin Activator and our UN-COALER® Activator/Feeder, both of which incorporate vibration to ensure materials continue flowing smoothly. Additionally, GK engineers can work with you to design custom equipment to ensure process optimization.
HULA-HOPPER® Bin Activator
The HULA-HOPPER® Bin Activator is a single low-headroom vibratory unit with a simple design that eliminates the problems associated with more complex machinery. It allows for variable flow rate control, positive flow shut-off, and material activation. Moreover, the HULA-HOPPER® Bin Activator’s low-horsepower motor has low power requirements, thus minimizing operating costs.
The UN-COALER® Activator/Feeder, which combines the benefits of enclosed vibratory feeding with those of bin activation, is a solution for the feeding and metering of a wide variety of bulk materials, such as potash, pellets, sand, and limestone. The UN-COALER® Activator/Feeder can be used to provide consistent, and well-distributed solids flow to silos, barns, open-air storage, and material crushers.