There are numerous applications that require the feeding of bulk solid materials. Virtually all manufacturing industries, from pharmaceuticals to mining, require some form of solid material handling. However, without proper planning, solid feeding can be the cause of equipment shutdowns, unscheduled maintenance and production delays.

The ultimate goal is to have a feeding system that operates on two principles: constant mass flow and first in, first out (FIFO). Good materials feeding will have a constant, predictable discharge rate. This will ensure that the same amount of product is being dispensed, improving quality control and batch to batch consistency. Properly designed feeding systems will have a good match between the material hopper and the feeding mechanism, and will discharge across the entire outlet. By doing so, there are fewer chances for materials to stagnate.

Materials considerations

When designing a solids handling system, engineers must examine the properties of the material carefully. The size, geometry and bonding properties of the material will help determine the best feeding mechanism, discharge rate, port size, hopper design and so on.

Particle size analysis will reveal the average particle size as well as the distribution of sizes throughout the material. In an ideal world, all feed particles would be identical in size, but this rarely happens. Understanding the distribution of particle size can help prevent settling, where the material segregates in the hopper by particle size, which can lead to mass flow rate fluctuations and selective discharging of one particle size over another.

Size, however, isn’t enough. The engineer should have a good understanding of feed geometry. Fibrous materials, or materials that are not isotropic in size can cause problems for some feeding devices, such as rotating valve feeders. They can lead to clogs in narrow or small-diameter ports. In general, if non-isotropic feed material is used, the ports should be larger.

Also, the engineer should have some understanding of the chemical nature of the material. In particular, they should pay close attention to whether the feedstock will agglomerate. If material can stick together, it can lead to bridging, rat-holing and other common feed problems. One indicator of the potential for agglomeration is whether the material is hydrophilic or not. Hydrophilic materials will absorb moisture from the air and will cause the material to clump. This can be particularly frustrating — the system may have good, consistent feed on a dry day, but then clump up on a humid one. Another indicator is whether the material is a good conductor of electricity. Insulating materials can build up a static charge, causing particles to stick together due to electrostatic forces. These materials will do the opposite of the hydrophilic materials, flowing poorly on dry days.

Feeding mechanisms

The three most common types of feed are screw feeders or augers, belt or apron feeders, and rotating valve feeders.

Screw feeders

Screw feeders have a hopper above a screw-shaped shaft that rotates, moving the material laterally. Typically, the hopper discharges through a rectangular outlet onto the screw. The outlet should be designed so that no material can be discharged outside of the screw channel.

This feeding mechanism can break up some agglomerated feed material. As the screw rotates, it can cause clumps to tumble and break apart, making it a good choice for hydrophilic or electrically insulative materials. For particularly difficult materials, there are dual screw feeders, where there are two separate screws that run next to each other. These can help break up agglomerations as well.

An Archimedes screw was one of the first screw feeders. It was used for moving water from lakes and rivers into ditches and troughs. Source: Kuroczynski/CC BY-SA 4.0 DEED

There are several variables for optimizing the screw feeder. First, the rotational speed can be varied. If the speed is too low or too high, the product will not be consistently discharged. Also, there are different screw geometries that can help improve feeding to maintain FIFO feed. Tapered shafts and variable pitch screws have an advantage over non-tapered shafts and fixed pitch screws.

Belt and apron feeders

Another common feeding mechanism is the belt and apron feeder. Material is dropped from the hopper onto a moving belt, typically through a rectangular port. An apron is used to guide and pile the material in a consistent fashion.

These systems are relatively simple to operate and maintain. There is a conveyor belt, a hopper and an apron, often with very few moving parts. However, these are subject to stagnation if improperly designed. Feed tends to occur more readily from the back of the outlet, meaning material at the front of the outlet can stagnate.

Incredibly long, outdoor belt feeder in Nigeria. Source: Fachab/CC BY-SA 4.0 DEED

To improve the design of belt and apron feeders, the outlet can be tapered. It is a little more complicated to manufacture, but the outlet is smaller at the back of the belt and gets larger toward the front of the belt. This improves FIFO flow. Furthermore, the distance between the belt and the apron can be adjusted. If the apron is scraping material from the top of the pile, it is too close to the belt. The belt should not be placed so far away as to generate unnecessary dust.

Rotating valve feeders

Another common solids feeding mechanism is the rotating valve. A rotating valve feeder has an internal wheel with multiple cavities that are filled with solids as the wheel turns. The cavities are filled either via gravity or by pneumatic pressure in the hopper. Often, the wheel fills at the 12 o’clock position and empties at the 6 o’clock position (for clockwise rotation). When properly configured, the volumetric or mass flow rates of material can be calculated from the rotational speed of the valve.

A well-designed rotating valve is fed by a variable frequency drive (VFD) so that the speed of rotation can be changed as process needs change. They will also have an isolation gate valve above the rotating valve so that maintenance can be performed on the rotating valve without emptying the hopper. Also, somewhere along the 6 o’clock to 12 o’clock position, the sections should be vented to improve the flow of material in and out of the valve.

The biggest variables for designing rotating valve feeders are the number of partitions (and sections), the rotational speed, and the volume of each section. If the rotational speed is too slow, the material can get caught between the wheel and the valve body, leading to premature wear and requiring more energy to turn the wheel. If the wheel is too fast, it will not fully fill the section.

Final thoughts

Feeding mechanisms are often overlooked in manufacturing processes. If the engineer has the opportunity to design the feed system from the beginning, some headaches can be avoided entirely. However, most of the time, the engineer must retrofit an existing system, or figure out ways to make an existing process more efficient. Feeding mechanism upgrades can alleviate inconsistencies and delays.

One of the most common methods to find feed problems is to look at the hoppers and feeders. Even if none of the common flow problems are observed, check the outside of the hoppers and feeders for small dents, scuffs and other marks. When there are feed problems, operators tend to perform “percussive maintenance,” striking the hopper to break up agglomerations, cave in rat holes and collapse bridges in material.