Processes involving the movement of bulk solid materials require careful consideration of the feeder equipment design, including how the feeders work with various bins and hoppers
The importance of feeder design in maintaining reliable material flow in solids handling processes can easily be underappreciated and underestimated, but ignoring the feeder will likely result in costly and disastrous outcomes. Feeders are designed to control solids flow, but they also need to work well with the bin, hopper or silo system associated with the process. Engineers can expend great effort and resources to evaluate a material’s flow properties, design the best bin for those properties and spend signficant funds on liners or steep hoppers, only to have all the money and effort wasted by an improperly designed feeder.
The bin and feeder must work together to ensure uninterrupted flow from the bin. To foster a deeper understanding of the importance of feeder design, this article discusses material flow basics, flow patterns, the key role of bin design, the types of feeders commonly used and devices available in the marketplace. In addition, it includes a list of feeder-related “dos and don’ts” and finally a flow chart used to determine the correct feeder for an application (see flowchart, below).
Several flow problems can occur in bins and silos, including the following:
- No flow due to arching and ratholing
- Erratic flow
- Flooding or flushing
- Flowrate limitations
These problems are discussed further below.
Arching and ratholing. In some situations, flow is initiated from the bin, and it either does not flow at all or perhaps flows for a short time and then abruptly stops flowing. When this occurs, the material is in a no-flow condition due to the formation of an arch or a rathole (Figure 1). If an arch forms, consider that this arch is strong enough to support the entire contents of the bin or silo and stop the flow. An arch is sometimes referred to as a bridge or dome. When material arches, sometimes the only way to address it is to employ a drastic means of flow promotion, such as vibrators, fluidization and unfortunately, sledgehammers.
Typically, a rathole develops when flow is initiated and the material flows for a short time, then stops. Consider that friction develops between the material and the hopper wall surface such that, if the hopper wall is too rough or too shallow (or both), the material will not slide. When this occurs, a flow channel develops. Usually this channel forms straight up into the material, but it can also travel off to one side. If the solid material has cohesive strength, the flow channel will empty out and form a stable “pipe” that is referred to as a rathole. Like arching, collapsing a rathole requires extreme measures. The effects of these measures on the structure of the bin or silo should be considered. Collapsing ratholes can release tremendous amounts of material, which can cause silos to fail.
Erratic flow. The problem of erratic flow is a combination of arching and ratholing. If the material in a bin ratholes, and a flow-aid approach is used to collapse the rathole, a bridge may still form. Consider that when a rathole collapses, the material impacts the outlet with tremendous pressure. In some cases, the force of the collapsing material is so great that a stable arch forms due to the impact pressure. This type of flow problem can adversely affect flowrate, cause unwanted variations in bulk density and can potentially affect the structural integrity of the bin.
Flooding or flushing. Fine powders are easily fluidized. When a rathole forms and material is knocked off the top of the flow channel, this material will be highly fluidized and uncontrollable. Also, if the level detector indicates a low level (due to ratholing), it will call for more material to fill the bin. As this material enters the rathole, it will become highly fluidized. When one of these two conditions occurs, the feeder at the outlet of the bin, which is designed to handle a solid, will be overcome with fluidized product and will likely flush uncontrolled out of the feeder.
Limited discharge rate. Fine powders are less permeable than coarser materials. This creates a problem with air passing through the particles easily. For example, let’s say a discharge rate of 10 ton/h is required and when the feeder is turned on, the rate is actually 2 ton/h. Normally, discharge rates can be raised by increasing the feeder speed. However, when this is done, the discharge rate increases to 3 ton/h no matter how fast the feeder speed is increased. What has happened? When the bin is filled, the air within the voids of the solid material is squeezed out through the bin filter at the top. As flow is initiated, more air is squeezed out of the material voids. Also, when material enters the hopper, it dilates or expands, creating a vacuum. Nature tries to satisfy this condition by bringing air in from the outlet below. This counter-current airflow slows the material down and limits the flowrate.
Segregation. Separation of particles, or segregation, occurs when a product composed of different particle sizes or densities (for example, grain with fines or dust) separates. The major cause is sifting, where fine particles of a solid sift between coarse particles. As an example, upon forming a pile of material with differing particle sizes, typically, the fine particles would concentrate under the fill point, while the coarse particles would role or slide to the outside (Figure 2). There are several other mechanisms of segregation that can be troublesome if uniform density or mixed material is required for a process.
There are two major types of flow patterns for solids in a bin or silo: funnel flow and mass flow.
Funnel flow. A material flows in funnel flow mode when only some of the material is flowing, while the rest remains stagnant along the walls of the vessel. Suppose that the hopper walls slope at 45 deg or even 60 deg. In most cases, these hopper walls at these angles are not steep enough to ensure solids flow along them. The hindrance of solids flow occurs because of the friction that develops between the bin wall and the material. When the walls are too shallow or too rough to overcome friction, many of the flow issues described previously can result (Figure 3). The flow sequence for funnel flow can be characterized as “first-in-last-out,” where the first material in the bin is typically the last to discharge. If the material is cohesive, it may bridge or rathole.
Funnel flow bins are suitable for coarse, free-flowing materials that do not degrade. Additionally, they can also be used with solids that are not susceptible to segregation problems. The bin’s live or usable capacity is reduced with funnel flow, and ratholes also can potentially cause structural failure.
Material such as plastic pellets or coarse, dry sand would typically be suited to funnel-flow applications. The major benefit of funnel flow is that the headroom requirements for the bin are reduced, as are fabrication costs. However, solids flow problems usually far outweigh the benefits, as most fine powders do not flow well in funnel flow conditions.
Mass flow. A material flows in mass flow mode when all the material in the bin moves when any of it is withdrawn. The solid is capable of flowing along the walls because they are steep and smooth enough to overcome the friction that develops between the solid and the wall surface (Figure 4). A rathole cannot form, simply by definition — all the material is in motion when discharging. Note that arching is not eliminated by mass flow. If a material has sufficient cohesive strength, it may arch over the outlet.
Mass flow bins are suitable for cohesive solids, fine powders, materials that degrade or spoil, and solids that segregate. Particles that have segregated by sifting (side to side segregation) flowing in a mass flow bin will be remixed as they discharge through the outlet. Fine powders that tend to flood are allowed to de-aerate in the bin and flow in a controlled manner.
Bins designed for mass flow develop a “first-in-first-out” flow sequence and the entire contents of the bin are fully live.
It is critical that the feeder pull material from the entire cross-sectional area of the bin outlet. If this does not occur, a funnel-flow pattern will develop in a bin that was designed and modified for mass flow.
Hopper shape: wedge versus conical
Conical shaped hoppers are commonly used to store and handle bulk solids. Conical hoppers required to ensure mass flow usually have steep hopper slopes and smooth surfaces. On the other hand, wedge hoppers require less steep hopper walls to ensure mass flow. Examples of wedge hoppers are transition and chisel hoppers. Each of these incorporates a slotted or elongated opening (Figure 5). Typically, a wedge-type hopper can be about 11 deg less steep than a conical hopper and still promote flow along the walls (that is mass flow).
Some advantages of wedge hoppers over conical hoppers are the following:
- Wedge hoppers do not have to be as tall as conical hoppers to ensure mass flow
- Wedge-shaped hoppers require a smaller slot width than the diameters of conical hoppers to prevent arching. Consider also, that this allows the use of a smaller feeder
- Wedge-shaped hoppers also allow material to flow at higher discharge rates because of the increased cross-sectional area created by the slotted
- opening on this type of hopper
Disadvantages of wedge-shaped hoppers that must be considered include the following:
- Wedge-shaped hoppers may cost more to fabricate than one with a conical geometry
- The feeder used to discharge material must be capable of discharging material over the entire cross-sectional area of the slotted outlet. This requires a specially designed screw or belt for use with the hopper
- This feeder may be more expensive than one used for a circular opening
- This feeder will discharge offset at its end. Centerline discharging from the bin may better be served by a conical configuration
Volumetric feeder types
A volumetric feeder allows material to be fed on volume-per-time basis. Typically, volumetric feeders will provide accuracies within 2 to 5%, which may be more than adequate. Volumetric feeders work well when a material has a fairly consistent density, meaning the weight per volume remains constant. In controlling the feedrate, the volumetric feeder assumes the density remains the same and relies on feeder speed to control the rate. In mass flow situations, material will be discharged with a fairly uniform density due to the consistent pressure history the material will have experienced.
With most feeders, gravity flow from a bin discharges to the feeder. Even the best feeder will not work if the material arches above the bin outlet. And even the best feeder will not overcome a poor bin design. Common volumetric feeder types include screw feeders, belt feeders, rotary valves, pan feeders, louvered feeders, circle feeders, and Laidig-type unloaders.
Screw feeders. A screw feeder uses a rotating auger contained in a trough. The auger is a shaft with flights wrapped around it. As the screw rotates, material shears or slips on itself, and on the surface of the flights, advancing forward. A screw pitch is made up of one 360-deg wrap of the flight. The distance from the start to end of the pitch will determine the volume of material that can be transported for every revolution of the screw. The discharge rate of a screw feeder can then be changed by adjusting how fast the screw rotates, which is typically measured in revolutions per minute (rpm).
A screw feeder is well suited to a wide range of both fine and coarse materials. Because screw feeders are enclosed, they are well suited to handle dusty or hazardous materials. In addition, screw feeders are not ideal for friable materials, where particle attrition is undesirable. However, screw feeders require very little headroom, which is helpful when there are process-height restrictions.
A screw feeder is typically used with wedge-type hoppers with slotted outlets. To maintain mass flow, a screw feeder must have an increasing capacity in the discharge direction. This can be accomplished by varying the flight pitches and altering the screw shaft by use of a conical shape (Figure 6). The goal is to ensure a uniform withdrawal of material from the bin, achieving a fully active outlet. The screw shown on the right side of Figure 6 however, is a constant-pitch screw, which develops a preferential flow channel over the back flight. The material over the front of the screw is stagnated.
A mass-flow screw feeder has a conveying section, which means that its discharge placement is offset from the centerline of the bin. Additionally, exceptionally long conveying sections may result in unacceptable shaft deflections and will increase the power requirements to turn the screw.
Belt feeders. A belt feeder consists of an endless moving belt, idlers that support and guide the belt, one or more pulleys to control the belt tension, a drive to power the belt, and the supporting structure. The top of the belt will carry material from the bin outlet to an offset discharge point. The material volume conveyed by the belt is often set by an adjustable gate below the bin outlet. This gate will shear the material to a constant depth and width. The speed of the belt is used to adjust the material feedrate.
Like the screw feeder, a belt feeder is often required for bins with slotted outlets. To maintain a mass-flow pattern, a belt feeder must increase in capacity in the discharge direction. This can be accomplished through a properly designed interface between the bin outlet and the belt. This interface uses gradually rising interface from back to front. This allows more material to be deposited on the belt toward the discharge end of the bin. Additionally, the interface increases in width from back to front, allowing the material to stream onto the belt uniformly and maintaining a fully live bin outlet (Figure 7).
The belt feeder can handle a wide range of materials, including friable materials, very cohesive materials and sticky or fibrous materials. A belt feeder is not well suited for fine powders or other materials that are prone to flooding or those that are extremely dusty. Some materials may require a belt scraper or some other cleaning device to deal with material buildup on the belt. Belt feeders also demand regular maintenance and cleaning.
Rotary valve feeders. A rotary valve feeder (also known as a rotary airlock) consists of a series of pockets attached to a rotating shaft. The feeder has a driven rotor with vanes attached to it that form the pockets, and is enclosed in a cylindrical fixed housing. As the rotor spins, material from the bin fills the pocket by gravity. Once the pocket rotates 180 deg to the bottom of the valve, gravity causes material to drop out of the valve. Typically, rotary valves are used as airlocks to feed material from a bin into a pneumatic conveying system. They are capable of providing an air seal due to the tight clearance between the vanes and the housing. This seal prevents countercurrent, high-pressure air from permeating up into the bin and interrupting material flow.
As the pockets begin filling with material, air displaced from formerly empty pockets may cause some erratic material flow from the bin. Typically, this is observed as a reduction in material flow or as material flooding. To alleviate this problem, newer rotary valves include a vent line designed to send the displaced air to either the top of the bin or to a dust collector. A vent line may need to be added if the rotary valve does not have one. Rotary valves typically feed preferentially from one side of a bin, as the pocket fills with material and cannot take any material from the other side of the outlet. To restore mass flow, the bin outlet must be made fully live, where the material is uniformly withdrawn from the bin outlet. This is typically accomplished by adding a vertical section between the bin outlet and the rotary valve. The preferential flow channel will expand upward through the vertical section, and uniform material flow will be established (Figure 8).
A rotary valve will be sized based on the outlet and required feedrate. Feedrate can be adjusted by changing the pocket size and the rotary valve rotations per minute. Rotary valves are suited for bins with square or circular outlets. They are typically not applicable for very cohesive materials, friable materials, fibrous materials, or materials with large chunks. Any material that can jam or stick inside of the pockets may prove problematic. Another concern is very abrasive materials that can wear away at the clearance between the vanes and housing. Wear will diminish the air seal and change the accuracy of the feeder over time, because material could leak and bypass the rotating pockets.
Pan feeders. Pan feeders (also called vibratory pan feeders), use a pan with a vibratory drive attached to vibrate and feed the material. As the pan feeder vibrates, material is thrown very slightly up and forward. Material feeds from the outlet into the tray and can be conveyed a short distance to some discharge point (Figure 9a).
Vibratory feeders can handle a range of materials and achieve a wide range of flowrates. Increasing or decreasing the vibration is typically used to control the feedrate. Very fine materials may not be suited to vibratory feeders since they are prone to flooding. Dusty materials can be accommodated by an enclosure over the pan. Actual flowrates for a given material will be influenced by particle size, cohesive strength, wall friction and a variety of other factors. Depending on the material, additional testing may be required to ensure proper feeding and control.
Louvered feeders. A louvered feeder attaches directly to an outlet and uses a series of internal louvers (or slats) that gently vibrate to discharge material. The louvers will be spaced and angled such that when not vibrating, material will find its angle of repose and not discharge. A vibratory drive attached to the outer housing of the enclosed louvers provides discharge. Beneath the louvers is a chute that directs the material stream to the desired outlet.
The housing of the feeder is supported by cables attached to the bin. The feeder has a flexible skirt to seal and isolate the bin from vibrations (Figure 9b).
Circle feeders. A circle feeder is a unique table-style feeder that uses a rotating vane to push material to an outer perimeter and one or more discharge points. It consists of a slow rotating vane, an enclosed housing, discharge ports, and a driving mechanism (Figure 9c). The feeder mounts directly on the outlet and uses a number of flat blades spaced equidistantly, making up the vane, to work material to an outer-perimeter chamber and then to outlet ports. These feeders have excellent precision and work well with a large variety of materials.
Cone-bottom or auger-type unloaders. A cone-bottom or auger-type unloader consists of an internal auger that sweeps around the cone bottom of a bin. The auger undercuts material in the bin, feeding it to the center outlet. The auger drive is located outside of the hopper, keeping it out of the material stream and accessible. These types of feeders are ideal for feeding difficult-flowing or very cohesive materials that might otherwise bridge over the outlet (Figure 9d).
Feeder Dos and Don’ts
The following represent practices to employ, and others to avoid, when designing and choosing feeders and related equipment.
Do measure flow properties. Material flow properties are required to determine your material’s cohesive properties, wall friction properties and compressibility. These properties provide necessary information for proper design of your feeder.
Don’t choose the wrong feeder. You would not want to use a belt feeder when handling floodable materials. Likewise, you would not use a screw feeder to handle friable products.
Do design for mass flow. The bin and feeder must work in unison. If the bin is not designed to ensure mass flow, the feeder will be dealing with funnel flow and ratholing, erratic flow, flooding and segregation issues. Likewise, if the feeder is designed improperly, flow from the bin will be affected negatively.
Don’t assume that one feeder is suited to all solids. You may need to discharge at high flowrates. This would likely require a slotted opening with either a belt or screw. If you need to discharge to a pneumatic conveying system, a rotary airlock will be required to seal against a positive pressure gradient. You would not want to use a belt or screw to do this.
Do consider material consistency. If you are handling a low density product or require low discharge rates, you may want to consider a pan feeder to provide uniform discharge. If your material is dusty or toxic, a properly designed screw feeder will be useful.
Don’t mistake a conveyor for a feeder. Often, conveyors are assumed to be feeders. A feeder is designed to operate nearly 100% full; whereas, a conveyor would typically operate at less than, say 40% capacity to provide conveyance.
Do account for loads on a feeder. Feeder loads can be tricky. For instance, consider a bin with a properly designed mass-flow screw feeder that has initially been filled with your material. Be aware that initial fill loads require increased torque to turn the screw. This initial fill pressure can require 2.5 times the running torque to start the screw.
Don’t use constant-pitch flights. Constant-pitch screws are notorious for developing preferential flow channels. The back flight (away from the discharge end) fills with material and the succeeding flights being constant pitch, do not have the capacity to take any more material and material does not flow over the entire bin outlet, only the back.
Do ensure increasing-capacity geometries. A screw or belt’s success is dependent on increasing the capacity of the equipment in the discharge direction. With a screw, this is done by varying the flight pitches, and on a belt, by using an interface design that provides the increase in capacity required for uniform flow.
Don’t sacrifice quality for low cost. We all have to work within budget constraints. But purchasing an incorrectly designed feeder because it was less expensive can lead to severe handling problems down the road.
Gravimetric feeder types
A gravimetric feeder allows for material to be fed on a weight-per-time basis (rather than volume-per-time). Gravimetric feeders achieve a greater degree of accuracy than volumetric feeders. If a system or process requires accuracy closer than±5%, a gravimetric feeder will be able to achieve 0.25 to 0.5% accuracy. Unlike volumetric feeders, gravimetric feeders will allow for density variations within a material. Common gravimetric feeders are weighbelt feeders, loss-in-weight (LIW) feeders, and gain-in-weight (GIW) feeders.
Weighbelt feeder. A weighbelt feeder is much like the volumetric belt feeder. The weighbelt feeder incorporates some type of weighing platform. Typically this will be one or more load cells with one of the belt’s support rollers mounted to it. The weighbelt may be directly under a bin or be fed by a prefeeder. The feeder uses both the belt speed and the continuous weight measurement to determine the amount of material being delivered on a weight basis.
Weighbelts work well for both continuous or batch processes. Weighbelts are sensitive to material buildup, which will result in a loss in accuracy and may require maintenance to clean, adjust and recalibrate the feeder. Weighbelts can handle a similar range of materials as that of belt feeders. Also, they are typically not suited to materials that are sensitive to flooding.
Loss-in-weight system. An LIW system controls the feedrate based on the weight of material taken away from a hopper. LIW systems comprise three components: a refill device, a LIW hopper and a feeder. The refill device, which could be a hopper with a slide gate or a volumetric feeder, will quickly fill the LIW hopper. Once the hopper is filled, weighing begins. As discharge begins, the load cells measure the loss in weight of the product being discharged to the process.
A LIW system accommodates both continuous and batch processes. For a continuous process, when the hopper is refilling, the feeder will switch to volumetric mode. Once the hopper has been filled again, the feeder reverts to gravimetric mode. Another option is to use dual LIW hoppers (no freeze). While one hopper is discharging, the other is filling, so no volumetric mode is necessary.
Gain-in-weight system. A GIW system is strictly a batch system. It weighs the total material filling a container. Material is fed into a container on a scale or load cells, until the desired amount is reached. Once the amount is reached, filling will stop and the GIW system will be emptied to a process. Most materials will work with a GIW system; however, it is vital that the GIW container be completely emptied to maintain accuracy.
Edited by Scott Jenkins
Joseph Marinelli is president of Solids Handling Technologies, Inc. (1631 Caille Ct., Fort Mill, SC 29708; Phone: (803) 802-5527; Email: firstname.lastname@example.org; Website: www.solidshandlingtech.com). Marinelli is a bulk materials handling expert who has taught hundreds of highly acclaimed engineering seminars. Since 1972, he has been active in testing bulk solids and consulting on materials handling systems design. Marinellii has previously worked with Jenike & Johanson, Inc., world-renowned experts on solids handling. Marinelli received his B.S.M.E. degree from Northeastern University in Boston, Mass. Marinelli has also worked for manufacturers of solids handling equipment, such as feeders and silos. This background provides a unique blend of consulting and manufacturing experience to solve solids flow problems. He lectures frequently, teaching courses on solids flow principles and flow property testing, and has authored several papers and an encyclopedia section on the subject. Since 1997, he has been involved with very popular seminars at the University of Wisconsin in the areas of bin and feeder design and solids flow property testing. He is also a columnist (“Powder Perspectives”) for the website, www.powderbulksolids.com
Scott Miller is senior consultant for Solids Handling Technologies, Inc.(same address as above; Phone: (803) 517-0058; Fax: (803) 802-0193; Email: email@example.com). Miller works closely with clients to provide the expertise required to evaluate and solve their solids flow problems. He analyzes flow properties test data produced by his employer’s testing laboratory, and writes flow reports describing the parameters necessary to resolve flow problems. Miller is also actively involved in providing practical, conceptual design recommendations to clients. Miller received his B.S.M.E. degree, with a minor in mathematics, from Geneva College in Beaver Falls, Pa. His background as a plant engineer at a power-generation facility in Pennsylvania provides valuable experience handling anthracitic waste, planning outages and providing solutions to typical coal-handling problems.