By Todd Messmer
Consideration of material properties and system design is key to choosing the right equipment
For those interested in automating a manual, dry bulk ingredient addition into their process, it can be difficult to know where to begin. Before starting to look at equipment, there are several key factors that need to be considered.
First it is important to determine why automation is desired in the first place. By doing so, an equipment supplier can calculate a return on investment on the equipment, which will ultimately be needed when filling out a Capital Appropriations Request to upper management.
Shown below are some initial items to consider when evaluating the need to automate ingredient additions into a process.
Ergonomics – One must decide if there any hazards that can be eliminated by manually adding materials.
Quality – The end product may have undesirable variations.
Raw Material Costs
Once a reason to automate is established, the next step is to understand the properties of the material(s) to feed. This starts the process of determining the ideal feeder for the specific application.
Free flowing: These materials are the easiest to feed and consist of items such as plastic pellets and certain powders or granules. They do not require any special provisions such as agitation or positive extraction in order to get them to feed. In addition, there is the added benefit of using gravity to assist in metering them.
Easy to feed plastic pellets
Abrasive: These materials have a tendency to wear out seals and material contact parts as energy is added to them to get them to feed. Materials such as sugar fall into this category, and special provisions need to be considered for abrasive materials.
Arching/ Bridging: These materials have a high angle of repose, which is the angle created between the flat surface that a pile of material sits on and the angle the pile of material makes. Materials with higher angles of repose tend to be more difficult to feed and can cause issues with arching or bridging inside of the feeder.
Floodable/ Aeratable: These materials can behave like a liquid when aerated. Special precautions need to be made to prevent these materials from flushing out of the feeder.
Cohesive: Materials with cohesive properties stick to themselves and can become packed in a “ball.”
Adhesive: These materials wear seals and material contact surfaces. While sugar is considered an abrasive material, depending on the particle size and type, it also can be an adhesive material.
Friable: These materials are very fragile and can break apart, causing them to lose some of their texture or aesthetic features. Various foods, such as rice crisps or fruits, will fall into this category.
Pressure sensitive: Materials such as certain waxes or beads can become solid if there is too much head load on them.
Hygroscopic: These materials wick up moisture and behave differently if special precautions are not taken to prevent this from happening.
Static generating – Materials with these properties create a static charge and will start clinging to both product- and non-product- contact parts. The need for special grounding features to help dissipate the static charge from the material may be required.
Once one understands how the material will behave, an environmental evaluation of the material-feeding equipment must be performed. Important considerations are how the material is delivered or packaged in its bulk form, how the material is transferred to or placed in the feeder, and any floor or height restrictions of the necessary equipment.
Volumetric and gravimetric feeding differences
Volumetric feeding is material dispensing at a given rate proportional to the RPM of the helix when utilizing a screw feeder (unit of volume per time). The feed rate must then be estimated based upon catch samples and manual calibration of the feeder speed. Volumetric feeding should be used:
A simple calculation for determining a volumetric feed rate is to divide the required feed rate by the bulk density of the material. This provides the volumetric flow rate required of the feeding device. A volumetric system is an “open-loop” system that does not give any feedback.
Gravimetric feeding is the controlling, batching-, or totalizing of material flow on the basis of weight versus volume. Gravimetric feeding should be used:
A gravimetric system is considered a “closed-loop” system that automatically adjusts for changes in material bulk density or flow rate.
There are two types of gravimetric systems – continuous and batching.
Continuous systems are often referred to as loss-in-weight (LIW) systems because they measure the amount of material weight exiting the feeder, which either sits on a scale or has built-in load cells. Continuous systems feed continually into a process and even continue feeding while the feeder is being refilled.
Batching systems can be either loss-of-weight (LOW) or gain-in-weight (GIW) systems.
LOW systems operate similarly to continuous LIW systems; however, they stop after a certain batch time. The material batch amount is measured as the feeder loses weight.
A GIW system consists of a volumetric feeder that feeds (batching) into a weighed vessel or device that sits on a scale or has built-in load cells. GIW systems measure the amount of material going into the vessel or device as it gains weight.
Which batching system should be chosen? The answer to this question depends on the individual process and budget.
Batching LOW systems offer quicker process times when dealing with multiple ingredients. Because all feeders are on scales (or have built in load cells) they can all batch simultaneously. The disadvantage of this system is the cost, which is high due to the need for load cells (or a scale) on each feeder.
For processes with multiple materials, it is important to note that, with GIW batching, it will be necessary to batch one material at a time and sequence to the next feeder (material) as each batch is completed, which creates a longer process time since simultaneous batching is not available. The advantage of this system, however, is reduced costs because it uses only one controller or scale.
A better understanding of how to measure feeder performance is necessary before turning to feeder design. As indicated earlier this is referred to as feeder accuracy. Volumetric feeders are typically accurate to approximately +/- 2 percent to 5 percent of the feed rate setpoint/batch, whereas gravimetric feeders are typically accurate to approximately +/- 0.25% or better.
Accuracy is determined by three factors:
We measure accuracy via standard deviation. Typically, 30 catch samples are taken from either a continuous or a batching feeder, and the standard deviation at 2 sigma (two standard deviations) is calculated from the average of those samples.
Feeders come in all different shapes and sizes. This section offers a brief overview of available feeding options.
Vibratory feeders feed via a continuous movement of particles along a vibrating tray or tube. The throughput of the vibratory feeder is calculated from the cross-sectional area of the feed tray/tube multiplied by the velocity of particle flow. The velocity is a function of amplitude and frequency of the vibration.
Advantages of vibratory feeders:
Disadvantages of vibratory feeders:
Screw feeders feed a continuous flow of material via a feed screw rotating inside a nozzle. The theoretical throughput is calculated by the water volume of one flight at 100 percent fill, multiplied by the rpm of the feed screw.
Advantages of screw feeders:
Disadvantages of screw feeders:
Screw feeder & agitation system
Some screw feeder manufacturers will offer a screw feeder with an agitation system that helps make difficult-to-feed materials to flow more easily from the feeders. There are basically two types of agitation systems: internal and external.
Internal agitation of a screw feeder
Internal agitation system advantages:
Internal agitation system disadvantages:
External agitation systems
Typically consist of flexible, plasticized PVC that is gently massaged by external paddles or rollers.
External agitation of a screw feeder
External agitation system advantages:
External agitation system disadvantages:
Most feeder manufacturers that offer external agitation systems also have a dual-drive option that includes a separate motor for the feed screw and agitation paddles, allowing for additional flexibility for multiple material types.
Twin-screw feeder & agitation system
Another screw feeder option is the twin-screw design. As the name implies, there are two screws housed in a common feed nozzle.
Twin Screw Feeder
Twin-screw feeder advantages:
Twin-screw feeder disadvantages:
Finally, there are weighbelt feeders that as the name implies, feed via a rotating belt that travels across a weigh deck or weigh bridge.
Weighbelt feeder advantages:
Weighbelt feeder disadvantages:
Remaining questions on choosing a feeder are best directed to an applications engineer from a feeder manufacturer. They can help determine the right equipment for specific applications and help facilitate material testing at their factory. Material testing will ultimately confirm the best approach for individual dry bulk solid feeding needs.
Author: Todd D. Messmer is the Applications Engineering Manager for Schenck Process located in Whitewater, Wisconsin. He has been with Schenck Process for 19 years. Messmer may be reached at firstname.lastname@example.org.