Equipment for processing bulk material is designed to save manufacturers both time and money. Machinery in any manufacturing environment is meant to optimize production, including equipment used to handle bulk material. Flow rates are a significant part of this process, as they indicate the speed at which products can be processed and shipped to customers, who are often also manufacturers themselves. To understand how to optimize bulk material flow rates, however, it’s important to understand their makeup and how best to move different types of raw products.

Flow problems are some of the most commonly faced by processing plants when handling bulk material. As bulk materials vary considerably – coming in powder, granular or lumpy form – how they flow through the system during processing depends on their characteristics. Equipment that controls bulk material flow rates must thus be used to encourage their movement through the processing system. In other words, it’s not just the equipment that affects how bulk material flows but also the processes and manner in which these are handled.

Improving Bulk Material Flow

Several properties affect flow rates during the processing of any bulk material. Flow behavior for a material determines how quickly the product moves through the processing system. It affects the amount of wear on machinery, hazards they present to workers, and whether it sticks to surfaces. Engineers who design equipment and processing systems understand these material characteristics directly affecting bulk material flow.

Ways in which bulk material flow rates are affected include: 

  • Adhesion: This occurs when forces cause material to adhere to equipment during processing that can then form what’s known as a rathole, which commonly results from bin outlets that aren’t steep enough to ensure efficient bulk material flow.
  • Angle of repose: Bulk material flowability must consider the physics behind the angle of repose, which relates to the highest angle at which a substance is a stable relative to the horizontal plane on which it is piled.
  • Bulk density: As density increases in a bulk material, the flow becomes smoother as gravity applies greater force to denser particles.
  • Cohesion: When particles are highly cohesive, they tend to bond and stick to each other, which affects bulk material flow.
  • Hydrophilicity: The more moisture within a material, the stickier it becomes, affecting the bulk material flow.
  • Particle size: The larger the particles, the less likely they will be to stick together, which is why flow rates for bulk material in granular form tend to be higher than for powdered material.
  • Shape of particles: Particle shape affects bulk material properties; flowrates tend to quicken the closer the particles to a spherical shape.
  • Temperature: Bulk material flowability can change due to heat or cold, both of which can affect a product’s cohesiveness in different ways; this depends on the type of material and application for which it’s used, though typically increased temperatures amplify cohesive strength.
  • Moisture: In silos and certain other locations that store bulk material, flowability can be affected by a damp or wet storage environment, which is made even worse if the material is hydrophilic.

By understanding both the properties and methods for processing a bulk material, flowability problems can be better addressed. When the product isn’t being transported steadily, it often means downtime for a processing facility while maintenance staff works to identify what’s obstructing the flow. For equipment operators, resolving difficulties with flowability habitually means manually hammering or shaking the storage container to break loose, jammed, or caked product, shortening the equipment lifecycle, and leading to additional maintenance.

Besides added maintenance, bulk material stuck inside a silo or other storage container can reduce storage capacity. In the chemical or food processing sector, leftover material may contaminate the next batch, leading to lower product quality. Even more serious are bulk materials that produce combustible dust during processing, which can explode if agitated. This includes materials like coal, coffee, flour, grains, powdered metals, sawdust, or sugar, which can cause fires or dust explosions if not handled correctly.

Enhancing Bulk Material Flow 

To prevent problems with bulk material flowability, here are a few tips.

Particle Size Distribution & Geometry

The size distribution and shape of particles within a bulk material directly affect how well it’s transported through a processing system. Industrial designers understand this, so equipment designs promote consistency in bulk material flow. This is why shear protectors are used with coarse materials like pellets or flakes. It’s also why larger inlets are used for processing equipment for lighter materials, while machinery used to process heavier material features sturdier shafts.  

Bulk Density & Throughput

How bulk material flows depends on a substance's density, which in turn affects throughput. Bulk densities determine throughput rates through material handling systems, with materials of lower densities tending to have lower throughputs. Materials under about 15 to 18 pounds per ft³ (240 to 288 kg per m³) don’t flow well but rather stick to equipment as their particles clump together. Flow characteristics should be tested for handlers of lighter bulk materials to see how best to improve flowability.

For lower-density substances, measures to increase bulk material flow rates include: 

  • Controlling temperatures to prevent clumping, contracting, or expanding to keep bulk material flowing steadily through the system.  
  • Mitigating air entrapment through pneumatic or other systems for conveying and drying material.
  • Monitoring how bulk material flows through a system with monitoring devices like flow meters and sensors to prevent bridging and clogging.
  • Pre-treating damp materials with hot or compressed air or other drying techniques.
  • Utilizing specialized feeders and ancillary equipment in a well-designed system to ensure greater throughput.

While plenty of mechanical solutions are available to enhance flowability, manufacturers may also be able to alter bulk density by adding amendments to improve bulk material flows. Density also affects how bulk materials are stored, as lighter materials will require large containers to store the same weight.

Facility Layout

How well a bulk material flows also depends on the design of the processing facility. For example, if a facility stores raw products at a considerable distance from the processing machinery, it will increase processing times. Planning a facility so that materials can be efficiently transported from one stage of production to another will make a big difference in how productive the facility will be, increasing profitability.

Material Characteristics 

The composition, moisture content, weight, or other aspects of the raw material prior to processing affect flowability. For example, a lighter-weight substance like wood dust will usually flow more uniformly through a bigger rotary airlock valve, preventing material buildup at the inlet flange. However, it’s important to ensure proper venting and keep air from escaping from a pressurized conveying line so that the pressurized air from the outlet doesn’t block material going through the inlet.

Hopper Angle & Friction

There isn’t a single “perfect” angle at which the hopper should be placed to optimize bulk material flow. The best hopper angle depends on the type of material and the particle size distribution, so it often requires experimentation to determine the best angle to avoid friction from slowing the flow of bulk material. Flowability is also affected by the friction between the product and the walls of the hopper, with steeper and smoother hopper walls reducing friction to make bulk material flow more efficiently.

Minimum Outlet Dimension

Depending on the bulk material, there’s a minimum dimension for hopper outlets and other outlets within a system. These are required to prevent stagnation around the outlets, though this depends on the material’s cohesive strength and consolidation under normal conditions.

Valve Rotation Speed

Rotary airlock valves are integral in the processing of bulk materials. Flow rates depend on the maximum speed at which valves rotate, with greater throughput possible for larger rotary valves. For abrasive bulk material, flows going through bigger valves allow for a decrease in speed without affecting throughput. This helps reduce rotary valve wear, extending its lifecycle. 

To size rotary airlocks for an application, manufacturers should know where the airlock is located in relation to the material, whether it needs to act as a metering device, if it feeds a pressurized conveying line, and how the bulk material flows.

Assessing Processes

When there’s a problem with bulk material flow, the design of the handling system may require revisiting. A step-by-step assessment of system design will help determine what additional components may increase bulk material flow. This is especially true if the system was designed to process only a set amount of product and the current need is now greater. 

To ascertain what can be done to optimize a material handling system, it’s important to evaluate equipment performance and other production data. Often, companies that sell material handling equipment will be a good resource for helping optimize production, with some even running their test labs for evaluating the best configurations for an application and material.

Material Batching

Material batching systems are relatively unsophisticated, though they affect bulk material flowrates. The two types of systems used for material batching are known as gain-in-weight and loss-in-weight. The former involves calculating weight gain from material as it goes into batch containers, while the latter measures weight loss in the feeder once materials are dispensed. Gain-in-weight systems move batches of material in more modest quantities and at a less hurried rate, ensuring accurate weighing while also allowing for thorough sanitation. 

Conversely, loss-in-weight batching systems can move large quantities quickly, though they’re neither as accurate nor allow cleaning time; however, gain-in-weight batching systems that use pneumatic conveying methods for loading batches can do so with precision.

Prater Equipment for Improving Bulk Material Flow

Along with grinding, metering, milling, and sifting equipment, Prater Industries also has considerable expertise in integrating our equipment into systems to optimize bulk material flow. Our in-house process controls team has the capabilities to completely build and manage a project to ensure work on your processing system is done correctly, punctually, and within the stated budget. Prater’s project management services are designed to meet our customers’ needs, from Gantt charting to complete system planning. For more tips on increasing bulk material flow rates in your processing system, contact the material handling experts at Prater today.