Todd Smith | K-State Bulk Solids Innovation Center
Whether you’re manufacturing cement or seasonings, if you’re in the powder and bulk solids industry, you’re going to need to weigh and batch ingredients to come up with a consistent end product. This two-part article describes how to weigh and batch ingredients in an accurate and cost-effective manner. Part I explains the different ways that materials can be measured while Part II, which will appear in the November 2020 issue of PBE, explains how to incorporate the weighing and batching methods into your process.
Bulk solids are a critical ingredient in most of the products we all buy and use. In manufacturing processes, dry ingredients are combined according to a recipe in the correct proportions with the other ingredients. But how are bulk solids measured? And what are the best methods for weighing and batching? This article describes the various techniques for weighing and batching solid ingredients into your process. The methods are listed in general order from those that are the least expensive and have the least accuracy to those that are more expensive and have better accuracy.
The simplest method of dispensing and dosing ingredients involves a feeding device without weighing capabilities but with the ability to control the volume of dry solids being dispensed. This device is typically a volumetric feeder such as a screw conveyor or rotary valve, as shown in Figure 1 and Figure 2, respectively. The volumetric feeder uses mechanical means to create a repeating void space at the feeding device’s top. In other words, material ingredients fall into the empty void space at the feeder’s top. Then the volumetric feeder’s rotation carries material from the feeder’s top to the bottom discharge end. This process continues as long as the volumetric feeder is turning and there’s a supply of material falling into the device’s top.
The dispensing rate of a volumetric feeder is determined by the machine’s size and rotation speed. These mechanical volumetric methods are simple and low cost. They don’t require any controls other than a variable-frequency drive to adjust the rotation speed of the drive motor.
Volumetric feeders can be calibrated by catching a sample of material for a measured amount of time, then weighing the amount of dispensed material. These feeders can be reliable and consistent as long as the material properties, especially bulk density, are consistent. Production plants use volumetric feeders when the material is fairly consistent, a high level of accuracy isn’t required, and the material has a relatively low value so that an accurate accounting record isn’t required. An example of this might be feeding regrind or reprocessed material into an extruder when knowing the exact rate isn’t critical.
Be aware that material and environmental conditions can affect the accuracy and consistency of a volumetric feeder. Most importantly, if the material’s bulk density changes from what you typically process, then even if the volumetric displacement remains constant, the weighed amount will increase or decrease in direct proportion to the bulk density variations. Density change can occur without notice. For example, if the grinder becomes dull, this can change the particle shape and bulk density of the regrind material.
Other factors can also cause material changes that affect accuracy. For example, powders might flow consistently into the feeder on a dry day but less consistently when the weather is humid. In addition, aeration and fluidization can affect the bulk density of many powders — air is lighter, so mixing solids with air will lower their bulk density. Another factor is consolidation weight from the material above. Material in the top of the hopper pushes down, compressing the material at the hopper bottom, increasing its bulk density. Furthermore, this compression or consolidation factor will continue to increase the material’s bulk density over time. The material’s bulk density will also increase or decrease as the hopper’s material level goes up and down.
In summary, volumetric feeders are simple and low cost. But their accuracy may be inconsistent, and they don’t provide record keeping of the material usage. The next option in accuracy and price requires an actual weight measurement of the material being dispensed.
When a simple volumetric device isn’t adequate for the process, then load cells are usually added to the assembly to measure the material’s weight. A load cell is a type of transducer that converts force (weight pushing or pulling on it) into an electrical signal that can be measured or sent to a controller. As the force increases, the electrical signal changes proportionally.
In a simple, common configuration, several load cells are used to attach the hopper to a support frame, as shown in Figure 3. Proper selection of load cells is critical for a safe and accurate operation. Load cells are rated for the amount of weight they can handle. They have an electrical signal output that covers their entire range of weight. Therefore, you should choose the smallest one that covers your full range.
Because the load cells will support and weigh the entire assembly, not just the material contents, the load cell or cells need to be rated for the combined weight of the hopper as well as everything inside and everything firmly attached to the hopper. For example, if your total assembly weight will range from 0 to 1,200 pounds, then a good choice would be to support the assembly with three load cells rated at 500 pounds each. In this case, you will be using 1,200 of the rated 1,500 pounds. However, if you had chosen three 1,000-pound load cells for the same application, then your accuracy and scale resolution — the smallest increment in applied weight that the scale can detect or display — would be worse since you would only be using a small part of the range (1,200 pounds of the rated 3,000 pounds).
In addition, each of the load cells needs to support an equal portion of the weight. That’s why we used three load cells in the previous example because if you have three support points on which to balance an object, then each of them will always be supporting their portion of the load. In contrast, if you have four or more load cells and their alignment isn’t right, then one cell may carry more or less weight than the others.
Furthermore, the hopper’s support system should be designed so that the only force imparted on the load cells comes from the weight of the vessel and its contents. In other words, the assembly needs unrestrained movement up and down, so that nothing is supporting weight except the load cells. If another device, such as piping, is rigidly attached, then it’ll help support the hopper and take some of the load away from the load cells. That’s why a typical hopper assembly with load cells has inlets and outlets with flexible socks or boots. The flexibility of these parts keeps the material in the hopper but doesn’t impart extraneous forces such as side loads on the assembly. The same idea must be used with everything that’s connected to the hopper, including compressed air pipes, dust extraction ducting, and even electrical conduits.
Other factors, such as vibration and wind, can also cause problems with load cells. For best results, be sure to work with a load cell supplier that can provide advice and a good installation manual. Also, because the system includes a volumetric feeder and hopper with load cells added, those load cells and their more sophisticated controls mean this system will cost more than a feeder without load cells. However, gravimetric feeding is more accurate and preferred for processing potent and expensive ingredients.
Gain-in-weight batch hoppers
If you’ve determined gravimetric batch weighing is the way to go for your process, and you’ve designed and installed a weigh hopper into your system, you can proceed with weighing the ingredients. First, make sure the hopper is empty. When this happens, the load cells are reading the weight of the hopper and its assembly without material, which is called a tare reading. The tare reading “remembers” the weight of the empty assembly and sets the weight of the net contents at zero so that any additional weight measured by the load cells comes from the incoming material. The load cells will be measuring the critical value, which, in this case, is the gain-in-weight of the material being loaded into the hopper.
As shown in Figure 3, solid ingredients can be loaded into the top of the weigh hopper with any type of feeding device. These include a screw feeder, rotary valve, bucket elevator, or even a simple open-close valve. Normally, to achieve the best cycle time, the operator will load most of the material quickly at a high fill rate. Then, as the material approaches its desired weight, the controls will automatically switch from fast speed to dribble speed, which would be about 25 percent of the fast speed, to get better accuracy control during the last 5 to 10 percent of the weigh cycle.
Additionally, the controller mustn’t forget about material that’s falling but has not yet settled onto the scale. Once the feeder is stopped, some material will still be falling from the feeder to the hopper. Therefore, the feeder needs to stop at a pre-act point to allow for in-flight material. These factors can be learned during startup and tweaked in the control logic during the system’s startup commissioning.
The weighed material sits in the hopper until it’s called for by the downstream process. Emptying all of the material completely and getting back to the same tare weight with content equal to zero is important. However, this isn’t a simple matter with materials that are sticky or tend to bridge, like cocoa powder. Therefore, proper system design is essential. If it doesn’t empty each time consistently, then weights will vary from batch to batch, resulting in unwanted weighing errors.
Is it possible to weigh multiple materials in the same gain-in-weight hopper? The answer is yes and is shown in Figure 3. Doing so will reduce the complexity and the cost of the system because you wouldn’t need separate weighing systems for each ingredient. However, be aware that the hopper and load cell capacity must be sized for the total weighment and that the larger scale’s resolution will hurt each ingredient’s accuracy. A gain-in-weight batch hopper’s accuracy is fairly good when weighing one ingredient but decreases as ingredients are added, which is why you want to choose the smallest load cell for your weight range. Weighing multiple ingredients in the same gain-in-weight hopper is especially harmful if one of the ingredients is used in a much smaller amount than the others. The resulting accuracy may be good enough for the major ingredients but not the minor. In addition, only one ingredient can be weighed at a time, which will increase the cycle time. Finally, if several materials are added sequentially, they will be layered in the hopper on top of each other and will need to be mixed at some point in the process to make a homogeneous mixture.
Loss-in-weight batch hopper
Sometimes, you’ll want to use only a portion of the hopper’s contents without emptying it completely. In these cases, whether you’re working with material such as cement or flour, consider measuring the material as it discharges from the hopper rather than as it’s filling the hopper. First, fill the hopper with the solid ingredient and then stop the incoming material flow. Then, the material can be discharged from the hopper into a mixer (or other piece of equipment depending on the process) in a controlled manner. The hopper’s loss-in-weight is the critical figure being measured by the load cells. As the loss-in-weight approaches the desired amount, the discharge device can again switch from fast speed to dribble speed to achieve better control and accuracy.
After the batch of material has been dispensed, the hopper is refilled again. During refill, there’s no need to control the exact amount of material going into the hopper. You just need to make sure there’s more than enough to satisfy the next desired batch weight. The filled hopper then sits, waiting for the next desired discharge weighment.
Is it possible to weigh multiple materials in the same loss-in-weight hopper? No, it usually isn’t practical to empty the first material from the hopper in a loss-in-weight system. Since some of the first material remains in the hopper at the end of the weighment, it isn’t advisable to place another ingredient on top of it since the hopper would then contain an uncontrolled combination of both materials. However, when it comes to cost and accuracy, a loss-in-weight batch hopper is comparable in both categories to a gain-in-weight batch hopper.
Editor’s note: Part II, which will be published in November 2020’s issue, will conclude this article with methods for continuous gravimetric feeding, incorporating weighing into your conveying system, and weighing small quantities accurately.
For further reading
Todd Smith (316-350-5865) is the business and strategy manager for Kansas State University’s Bulk Solids Innovation Center. He’s spent more than 35 years in the bulk solids industry working in a variety of engineering and management positions. He has a mechanical engineering degree from KSU and an MBA from Kansas Wesleyan University.
Bulk Solids Innovation Center • Salina, KS
316-350-5865 • bulk-solids.k-state.edu
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