Silos are large structures usually used to store bulk materials. The word "bulk" is used to define materials that are composed of a large number of small particles, such as sand, grains, pellets, and nuts. The stored particles must not be affected by external weather phenomena or other contaminants such as bacteria or dust; with longer storage.
Without the use of additional equipment such as drainage pumps and flow aids, silos are ideal for storing non-aromatic solids such as grain. Such devices can increase the cost of silo construction and maintenance. In addition, flow aids must be used with extreme care as they can put additional stress on the structure, potentially leading to failure.
Silos are primarily characterized by the fact that they are usually filled from above and discharged from below, with bulk materials ejected by gravity. When dispensing bulk materials, silo materials and hopper angles and openings play the most crucial role.
Image from Labster's silo design simulation.
Read on for some thoughts on why this can be a challenging topic for instructors and college students alike, 5 hints to change that, and ideas on why a virtual lab should make matters easier.
Why can silos design be difficult for students to understand?
There are three reasons why designing silos can be difficult for even the most diligent students.
1. Limitations of a practical session
Silos are complex structures with high construction and maintenance costs. Limited knowledge of avoiding silo problems such as bridging, funneling, potholes, roll-ups, segregation, stagnant zones, and product decay can discourage students from engaging in live sessions.
2. This is content heavy
The hopper is the bottom of the silo, which is responsible for the discharge of bulk material. The angle of the wall and the funnel's opening determines its geometry. Regardless of which hopper geometry best suits the user's requirements, all designs must provide for the mass flow of bulk solids.
For practical silo models, the equations or diagrams by Jenike (1964) are usually used to determine the openings and angles of the hopper walls. Jenike's calculations apply to both conical funnels and wedge-shaped funnels. For the basic shape of the funnel, a need to determine the funnel opening and the maximum angle of inclination of the walls to achieve mass flow.
Although there are many types of hoppers, most silos are equipped with a conical bottom hopper as shown in the first image:
Figure 2: Hopper type.
3. It's complicated
When removing bulk material from silos, two different flow regimes can be observed:
With mass flow, the entire contents of the silo move during unloading. Mass flow is only possible if the hopper wall is steep enough and the bulk material uses the entire outlet.
With funnel flow, the funnel wall is not steep enough. Bulk material more or less moves in the zone above the outlet, the remaining particles rest in the "dead" or "stagnant" zone.
The funnel angle can be determined using the following diagram:
Figure: Recommended wall angle to ensure mass flow in the conical bottom hopper.
5 ways to make silo design a more accessible topic
With those points in mind, here are five things you can incorporate into your silo design lessons to make them more engaging, accessible, and fun for you and your students.
1. Discuss concept challenges
Bridging occurs when a stable arc forms above the outlet and the flow of bulk material is interrupted. To avoid bridging, we need to calculate the critical diameter of the funnel opening. The opening is a function of bulk density, silo shape, wall friction, internal friction, and product cohesion. Funnels should be designed with openings larger than this critical diameter to prevent bridging.
2. Use grafts to emphasize key points
Particle fluidity: Flow is more often associated with liquids than solids. Since bulk solids are large collections of small particles, the term can also be used when the particles are in motion. Bulk solids should be considered Hookean solids and not Newtonian liquids. In contrast to liquids, bulk solids at rest can transfer shear stresses. Newtonian fluids have a linear relationship between stress and strain rate: the ratio of stress to strain rate indicates the fluid's viscosity.
A Hookean solid has a linear relationship between stress and strain: The ratio of stress to strain is the modulus of the solid. Many materials have Newtonian fluid properties and Hooke bodies.
3. Emphasize practical aspects
Soil mechanical test is carried out to determine soil properties such as shear stress, dilatancy, and creep. A direct shear test is carried out to determine the shear strength parameters for certain soil types. A normal load is applied to the soil sample and the soil is shifted across a predetermined horizontal plane between the two sections of the shear. Shear stress, shear displacement, and normal displacement are recorded during shearing. The test ends when the shear stress remains stable or begins to decrease. This is an indication that the soil cannot support the additional load because the sample has most likely been crushed.
The test shall be repeated on two or more identical specimens under different normal loads. From these results, we can determine the parameters of the shear strength. The shear strength is expressed as
S= c+ σ tanφ
Where c = effective cohesion, σ= effective stress, φ= effective angle of shearing resistance.
Soil grading is a classification of soil particle size distribution. A gradation test (also called a sieve test) is performed on an aggregate sample in a laboratory. A typical sieve analysis involves the interleaved columns of a wire mesh screen (sieve).
4. Seeing is believing
Pneumatic conveyors are a simple method for conveying powdery and granular materials at high, low, or negative pressures. The system requires a pressurized gas source, usually air, a feeder, a transport line, and a receiver to separate the transported material from the carrier gas.
Pneumatic conveyor systems are often used in agriculture, mining, chemical industries, pharmaceuticals, paint production, finishing, and metal processing. In various manufacturing processes, various kinds of food, drugs, and even explosives are transported pneumatically. These products are dry because long-distance transportation can cause changes in the saturation state of the wet material
5. Encourage the use of virtual lab simulations
A unique way to teach silo design is through a virtual laboratory simulation. At Labster, we’re dedicated to delivering fully interactive advanced laboratory simulations that utilize gamification elements like storytelling and scoring systems inside an immersive and engaging 3D universe.
Check out the Labster silo design simulation that allows students to learn about silo design through active, inquiry-based learning. In the simulation, students will learn how to design huge constructions called silos and perform soil mechanics tests to obtain the properties of powdered material.