Flour milling: flow problems affect bin design

by Emily Wilson
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The field of bulk solids handling was developed mainly due to the work of Andrew W. Jenike, who pioneered the theory of bulk solids flow. Dr. Jenike in the 1950s developed a scientific approach to the storage and flow of bulk solids that is still relevant today. The Jenike approach to shear testing is the ASTM standard in the U.S. and Europe for determining bulk solids cohesive and wall friction properties.

Knowledge of how materials flow directly affects the design of bulk flour bins and silos. An improperly designed bin or hopper can cause flow problems, resulting in work stoppages, equipment failure, down time and poor quality product.

This article will discuss the different types of flow problems and what happens when they occur, ways to resolve flow problems and, finally, how flow affects silo design.

 

FLOW PROBLEMS. There are several types of flow problems. No flow usually occurs when opening a gate or starting a feeder. Two problems can occur immediately. An arch (also called a bridge or dome) may form over the outlet that supports the entire contents of the silo above (see figure 1a). Extreme methods, such as sledgehammers, vibrators and air blasters, are commonly used to re-start the flow.

The second no-flow condition occurs when a stable rathole (also called a pipe or core) forms (see figure 1b). Some flour discharges but because of the material's cohesive strength, the flow channel empties out, resulting in a stable rathole. When this happens, the flow stops, and extreme measures are usually needed to reinitiate it.

Erratic flow consists of a rathole and an arch occurring in the same silo. What usually happens is that flow is initiated and a stable rathole develops. When the rathole is collapsed, by use of some flow aid device, the collapsing material arches as it impacts the outlet. Erratic flow can affect solids discharge rates, bulk densities and the structural integrity of the bin.

Flooding can occur if a stable rathole forms and additional flour is added or falls into the channel from above. As the flour falls in the open channel, it becomes entrained in the air in the channel and fluidized (aerated). The feeder designed to discharge de-aerated flour cannot meter a fluid and the material floods uncontrolled.

Limited discharge rates can occur typically because of counter-current airflow. A flow rate limitation is a function of the flour's ability to aerate or de-aerate.

Rate limitations typically develop because of a vacuum that is created as the flour flows in a hopper. Air or gas from the outlet flows counter to the material being discharged, causing a flow rate limitation. The usual approach to solve this type of flow problem is to increase the speed of the feeder. However, there is a limit to how fast flour will flow through certain openings.

Segregation occurs when a product composed of different particle sizes, such as grain with fines or dust, separates. The major cause is sifting, where fine particles sift between coarse particles. As an example, upon forming a pile of material with differing particle sizes, the fine particles typically would concentrate under the fill point while the coarse particles would roll or slide to the outside.

So, what happens when flow problems occur?

Limited live storage occurs when a bin is designed to store a certain amount of flour. If a rathole develops, only a small portion of the entire contents is actually live. The rest of the material is left stagnated in the bin. This can result in even further problems.

Spoilage or caking can occur as a result of stagnated flour that sits in the bin for days, weeks, months, and even years. Product that does not move in the bin remains stagnant, allowing bacteria to grow. Product that gains cohesive strength after storage can cake or agglomerate, creating an undesirable product.

Shaking, or vibration, occurs as ratholes collapse in the bin. Imagine that flour has ratholed in a bin and the rathole collapses either on its own or due to some external force. The volume of flour that impacts within the bin can cause significant vibrations. Eventually, this may affect the structural integrity of the bin.

Structural failure due to vibration is just one area of concern. A preferential flow channel can expose the silo to asymmetrical loads that can easily be great enough to cause dents in the silo sidewalls or even collapse the vessel.

Excessive power required to operate the feeding device that controls discharge can require high torque and large motor sizes to achieve the required discharge rates. Consider that if a product such as flour flows in a channel directly over the outlet, the feeder below is supporting the entire column of product.

 

AVOIDING FLOW PROBLEMS. Typical flow patterns include funnel flow and mass flow. Funnel flow occurs when some of the material in a silo moves while the rest remains stationary. The walls of the hopper are not sufficiently steep or smooth enough to force the material to flow along them.

The friction that develops between the silo walls and material inhibits sliding, which results in the formation of a narrow flow channel. The first material that enters the silo is usually the last material out. If the material has sufficient cohesive strength, it may bridge over the outlet. Or if the narrow flow channel empties out, a stable rathole will form.

The major benefits of a funnel flow silo are reduced headroom requirements and lower fabrication costs. That usually means that shallow cones (60 degrees or less), pyramidal hoppers and flat-bottomed silos are used.

However, funnel flow bins are suitable for coarse, free-flowing materials that do not degrade, such as plastic pellets. Fine powders such as flour do not flow well in a funnel flow silo.

Mass flow occurs when all the material in a silo is in motion whenever any is withdrawn. The material slides along the hopper walls because the walls are steep and smooth enough to overcome the friction that develops between the wall surface and bulk solid.

The hopper outlet must be large enough to prevent arching. Mass flow will not prevent arching, but stable ratholes cannot form in a mass flow silo. As a result, mass flow silos are suitable for cohesive solids, fine powders such as flour, materials that degrade or spoil, or solids that segregate.

The flow sequence is first-in, first-out, which allows mass flow silos to store solids that degrade with time. Powders cannot flood as long as the material's residence time is sufficient for de-aeration.

Particle segregation is minimized as the fines and coarse particles are reunited at the outlet. Typically, steep cones and wedge-shaped hoppers are used to ensure mass flow.

Flow property tests can be used to identify a flow problem in an existing bin or silo. These tests measure a material's cohesive strength, wall friction properties, compressibility and permeability.

 

AFFECT ON SILO DESIGN. Measurement of a material's cohesive strength yields a "flow function," which is a representation of the material's pressure/strength relationship. The wall friction values and compressibility are used along with the flow function to calculate outlet sizes required to prevent arches and ratholes in silos.

Outlet size determination yields the minimum opening size required to prevent a cohesive arch from forming. It can be calculated from the flow function and its interaction with a flow factor.

Flow factor values are used in outlet size analysis. The data obtained from the intersection of the flow function with a flow factor allows the user to calculate a minimum outlet dimension for both circular and slotted openings.

Hopper angles for mass flow for a particular bulk solid and wall surface can be calculated from the results of a wall friction test. This test generates a wall friction angle, the tangent of which is the coefficient of sliding friction that can vary with the pressure acting on the wall surface.

Hopper angles required for mass flow are designed for conical or wedge-shaped hoppers. Oftentimes, 70 degrees is considered the magic angle for mass flow. This is certainly not the case, as the hopper angle for mass flow is dependent on both the smoothness and steepness of the hopper wall and the properties of the bulk solid.

Structural considerations include knowledge of the material's flow properties, flow patterns, silo loads and dynamic effects. Silo failures can range from catastrophic structural collapse to denting of a steel shell.

If a bulk solid other than the one for which the silo was designed is deposited in the silo, the flow patterns and loads may be completely different. Side discharge outlets put in a center discharge silo can impose asymmetric loading on the silo, which can cause failure.

Another common problem is the development of mass flow in silos designed structurally for funnel flow. Mass flow loads are greater than funnel flow loads, and the structural integrity of the silo will be in jeopardy.

It is easy to underestimate just how important feeder selection can be for reliable material flow. A feeder must be selected that not only controls flow but also works with the silo.

You can go to great lengths to have your material's flow properties evaluated and develop the proper design. This can be expensive when using an expensive liner or steep hopper, and as such you can destroy this effort simply by using an improperly designed feeder.

The silo and feeder design go hand-in-hand, and for the feeder to work in unison with the silo it must:

•Suit the material's flow properties.

•Work with the silo outlet's shape to withdraw material uniformly across the outlet's entire cross-sectional area.

•Minimize the loads the material applies to the feeder.

•Accurately control the discharge rate.

It is imperative that the material's flow properties be identified and the silo and feeder designed to reliably handle the product. Experience is important when recognizing existing flow problems and predicting potential problems so that practical, cost-effective solutions can be developed.

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