Feeding Frenzy: Important Considerations for Selecting a Bulk Solid FeederFeeding Frenzy: Important Considerations for Selecting a Bulk Solid Feeder

There are many considerations when selecting the feeder including operation/process and material property specific.

McKinnon Ray

July 16, 2024

8 Min Read
Figure 1: An operator swinging a “flow influencer” and resulting “bin rash” Jenike & Johanson

Bulk solids handling is prevalent in almost every industry, including mining, agriculture, chemical, and pharmaceutical sectors. In fact, more than 70% of manufactured products involve bulk solids handling at some stage, whether as raw materials, intermediates, or final products. Unfortunately, bulk solids handling is rarely taught at the university level, leaving process engineers in the dark about the importance of achieving reliable solids flow.

When designing bulk solids handling storage vessels—such as bins, hoppers, silos, or stockpiles—there are two primary goals: storage capacity and reliable discharge. Achieving the first goal is relatively straightforward: With a known bulk density, you can calculate the necessary volume to meet storage requirements and design a geometry to accommodate it. It is also critical the storage vessel is designed so that the “live” storage capacity is maximized. For additional details on storage vessel design, I recommend checking out Eric Maynard’s article “10 Steps to an Effective Bin Design.”1

Getting the material out of the storage vessel is a different story. Ensuring effective discharge is much more challenging and can leave process engineers, operations, and maintenance teams pulling their hair out. Effective discharge from the bin is consistent, predictable, able to hit minimum and maximum flow rate requirements for the process, and activates the entire flow channel in the bin above. In many cases, operations teams are left with no other choice but to employ “flow influencers” to get material out of the storage vessels. These influencers are much more direct than the ones found on social media and can result in a nasty case of “bin rash.” Even with flow influencers, discharge is often not reliable nor consistent.

flow influencer

The good news is that reliable discharge without flow influencers is achievable if the bin and feeder are properly designed. Often, mass flow discharge is needed from the bin, which is influenced by both the design of the bin and the feeder. Mass flow discharge occurs when all material is in motion whenever any is withdrawn from the bin, illustrated in Figure 2. This contrasts with funnel flow discharge, in which an active flow channel forms above the hopper outlet, with material stagnating along the periphery.

Funnel flow

Image3-Jenike.jpg


Common Feeders

Conveyors and dischargers are often mistaken as feeders. Conveyors are used to transport materials from one point to another, often at higher speeds compared to feeders. Conveyors typically transfer at a constant speed, over longer distances and operate partially full. On the other hand, feeders operate at lower, variable speeds to modulate flow rate out of the bin and are choke-fed from the bin above. Dischargers are used to promote flow from the bin above but are not used to control flow like a feeder. A feeder has three primary objectives:

1. Provide reliable and uninterrupted flow of material from the bin above

2. Control discharge rate from a silo, achieving the required rate while preventing flooding (if the bulk solid is fine enough to behave like a liquid)

3. Remove material from the entire cross section of the hopper outlet, to promote a mass flow discharge of material

Three of the most common feeders include screw feeders, belt/apron feeders, and rotary valves. Each of these feeders must be designed to accomplish the three objectives mentioned above. Properly designed screw feeders are typically designed with tapered shaft, increasing pitch, or a combination of the two to provide reliable flow from the bin above. If a screw feeder was designed without these features (i.e., constant pitch and shaft), material will flow through one central channel, which will automatically create a funnel flow discharge pattern in the bin above. Likewise, belt and apron feeders should be designed with interfaces that are specifically designed to increase in capacity in the direction of flow. Rotary valves can act as both a feeder and a gas pressure lock in specific applications. A proper rotary valve feeder includes a proper interface that allows solids to be withdrawn uniformly over a hopper outlet’s entire cross section. Examples of properly designed feeders are illustrated in Figures 3 and 4.


screw feeder

interface


Operational Considerations

There are many operational considerations when selecting a feeder type. One important thing to consider is interfacing with equipment directly upstream and downstream of the bin. For example, a bin with a hopper that has an elongated outlet, a rotary valve is likely not a good candidate. Likewise, if a downstream process occurs at a different pressure relative to the bin, a belt feeder may not be the best feeder for that application. Additionally, downstream processes may involve vapor generation and/or operate at different temperatures, which could lead to buildup within the feeder. Additionally, the type of modulation - volumetric or gravimetric (weight) - must be selected. Volumetric modulation (feeding a specific volume per unit time) is typically acceptable in applications that do not require tight accuracy (+/-2%) nor handle materials with highly variable bulk densities. For applications that require tighter accuracy, gravimetric control (feed control based on a measured weight per unit time) is likely needed. Other important considerations for feeder selection include throughput required, dust enclosure, etc. Table 1 provides a general comparison of the three feeders described so far.

Table1-Jenike.jpg

In addition to operation considerations, flow and physical properties of the material must be measured. A material’s flow properties influence many things, including the flow pattern in the bin above, which materials should be used for construction, sizing of the hopper outlet and feeder, and more. Unfortunately, bulk solids flow property data cannot be found in textbooks like gases and liquids. Rather, testing is required to establish design criteria used for both bin and feeder design. Key flow and physical properties needed for feeder design include:

  • Particle size analysis: a bulk solids full particle size distribution must be measured to ensure feeder selected will operate as intended.

  • Cohesive strength: used to establish hopper outlet sizing to ensure reliable flow

  • Wall friction: used to evaluate solids flow along inside surfaces of hoppers, and feeders. Also used to evaluate ability of materials to build up/adhere to internal surfaces of the feeder. Wall friction also influences power required to operate the feeder.

  • Compressibility: measure of bulk density as a function of consolidation pressure. Compressibility data is critical for the evaluation of loads, operating speeds, and more.

  • Permeability: used to predict effects of two-phase flow effects on fine powders. Data is used to evaluate the size of the outlet required to achieve a specific flow rate.

  • Abrasive wear: used to estimate expected thickness loss in bins and feeders. Commonly this test is used to evaluate the effects of sliding wear, or abrasive wear due to a bulk solid sliding against a surface.  

Table 2 provides an additional comparison of the three feeders discussed so far, with considerations of flow and physical properties.

Table2-Jenike.jpg

It is important to note that Tables 1 and 2 give general guidelines when selecting between the three feeders, and there are exceptions that can be made. For example, with proper inventory control and sufficient residence time, belt feeders can be used with fine powders. Likewise, abrasion-resistant features (chromium overlays) can be added to allow for rotary valves to be used with abrasive materials. 

Material properties may indicate that none of the three feeders discussed so far is appropriate. In those cases, it may be necessary to consider forced extraction (extremely cohesive materials), vibration (fragile materials), fluidized discharge (potentially necessary to achieve high throughputs) and other feeding/discharging methods. In fact, there are many feeder types beyond the three traditional feeders discussed here, that cover a very wide range of materials and applications.

Conclusions

Feeders are a critical component to every storage vessel, whether material is stored in a bin, stockpile, dump hopper, or silo. Proper feeder design is accomplished when the feeder provides reliable, uninterrupted flow, controls the flow rate (either volumetrically or gravimetrically), and removes material across the entire hopper outlet cross section. Three of the most common feeders include screw feeders, belt/apron feeders, and rotary valves. There are many considerations when selecting the feeder including operation/process (throughput, upstream/downstream effects, etc.) and material property specific (particle size, cohesiveness, abrasiveness, etc.).

McKinnon Ray is senior project engineer, Jenike & Johanson (Tyngsboro, MA). For more information, email [email protected], call 978-649-3300, or visit jenike.com.

References

1 Maynard, E. (2013, November). Ten Steps to an Effective Bin Design. American Institute of Chemical Engineers; https://www.aiche.org/sites/default/files/cep/20131125_1.pdf

About the Author

McKinnon Ray

Jenike & Johanson

McKinnon Ray is senior project engineer, Jenike & Johanson

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