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 explained the different ways of how materials can be measured, while Part II explains how to incorporate the weighing and batching methods into your process.
Weighing and conveying
Frequently, you can build gain-in-weight or loss-in-weight methods into equipment that’ll be needed for processing your bulk solids. This reduces capital costs and the amount of equipment required for your ingredient handling system.
System 1. A typical vacuum conveying system consists of one or more ingredient sources with piping to a filter-receiver, as shown in Figure 1. From pet food and plastics to chemical and pharmaceutical ingredients, the filter-receiver collects the material, and its self-cleaning filter media separates the dry material from the conveying air.
If the receiver is mounted on load cells, then it can act as a gain-in-weight hopper and all of the previous considerations for a gain-in-weight hopper would still apply [Editor’s Note: See “Solids Weighing and Batching Overview — Part I”]. In this case, the feeder at the ingredient source is the control device, and better accuracy will again be achieved if the feeder has a fast speed and a dribble speed. In addition, the conveying system’s source feeder will require a quicker pre-act point for shutting off the feeder since there will be more material in flight between the source and the receiver. But fortunately, the in-flight quantity — the amount of material that’s been discharged from the feeder but has not yet settled onto the scale — should be repeatable from batch to batch with most materials. And the in-flight time is negligible since a typical dilute-phase system operates at a conveying speed of several thousand feet per minute.
Is it possible to weigh multiple materials in the same filter-receiver? Yes, and a filter-receiver on load cells operates similar to how a gain-in-weight hopper does. This means that each ingredient must be conveyed and controlled one at a time.
Sometimes, the gain-in-weight filter-receiver is used as the primary weighing device, as previously described. However, in other cases, the primary weighing of each ingredient is done earlier in the process, such as at a bag-dump station. When this is the case, the filter-receiver’s load cells are used as a “check-weigh” or verification point to make sure the material weight or amount is correct before dispensing it into the process. No matter where weighing takes place in the system, adding a gain-in-weight method to your process’s material handling equipment can be done at a modest cost, resulting in a fairly inexpensive system.
System 2. Another conveying and batch weighing system also uses a receiver on load cells as a gain-in-weight hopper but eliminates the hopper’s filter section through the use of a special scale valve, as shown in Figure 2. The scale valve has two positions. When in the “fill” position, the valve directs material and air into the weigh hopper below. Most of the material is retained in the hopper, but the conveying air and some carryover material will swirl back to the hopper’s top and exit through the outlet port of the scale valve. When the weight reaches the desired setpoint, then the valve switches from the “fill” position to the “through” position in which all of the material and air bypass that particular weigh hopper and go on to the next one. In this manner, each hopper gets filled to its desired weight one at a time. All of the conveying air and some of the material get carried away, usually back to the material silo via a return loop, as shown in Figure 2. The advantages of this system are that it’s relatively inexpensive and requires less head-height in the process area. The filter section has been removed, thus making each receiver much shorter. However, this method’s accuracy is typically not as good as other methods’ accuracy, so this method is usually only used for major ingredients, like flour and sugar, for example, and not ones that require high accuracy.
System 3. In addition, one final system of weighing and conveying deserves mention. In this system, load cells support each material source, and loss-in-weight scaling is used to control each one, as described in Part I, Loss-in-weight batch hopper. For example, if the material sources of Figure 1 were placed on load cells, then each material could be weighed individually. In fact, they could all be weighed and then conveyed simultaneously, which dramatically reduces the cycle time. Furthermore, since all ingredients are in the conveying line simultaneously, this method has the added advantage of mixing the materials as they are being conveyed. Mixing will be most effective if you size the source feeders so that each source is adding material to the conveying line for approximately the same time period. Otherwise, some material weighments will finish and stop feeding while others are still conveying. Overall, this method has several advantages. It’s simple and incorporates load cells into equipment that’s needed in the process anyway. This method has a fast cycle time and mixes the materials in the process. Accuracy will be good for large weighments of at least a few hundred pounds, but accuracy will decline for smaller amounts and probably be unacceptable for minor ingredients, depending on the equipment’s size. Another disadvantage is that this system is more expensive than other weighing and conveying methods since it uses several sets of load cells.
Small-quantity weight accuracy
The methods previously described are appropriate for batch processes in which each ingredient is weighed and then dispensed into the process according to a recipe. For example, a Kansas school cafeteria’s cinnamon roll mix has 100 pounds of flour, 10 pounds of brown sugar, 1 pound of shortening, 2.5 pounds of dry milk powder, and 1.25 pounds of cinnamon — yum! A much less tasty batch of PVC might include 200 pounds of resin, 30 pounds of mineral filler, 12 pounds of stabilizer, and 2 pounds of stearic acid. Each ingredient is weighed and then dispensed into a process machine such as a mixer.
As shown in these examples, most recipes call for a large variance in the ingredient weights. While the number of ingredients will vary, most recipes have several small minor ingredients and one or two major ingredients. Many of the scaling systems previously described provide acceptable accuracy for large amounts, but what about smaller amounts? How do you measure small minor ingredients, especially when precise control is more important for minor potent ingredients that are quite expensive? Using more ingredient than is needed would waste money but providing less than required would harm the final product quality since the minor ingredients, such as colors or flavorings, are often potent.
When the previous weighbatching methods aren’t adequate for weighing minor ingredients or when a product is manufactured in a continuous process such as in an extruder, then most bulk solids processors will use a continuous loss-in-weight feeder. In continuous loss-in-weight feeding, the feeder accurately weighs the discharge rate as material is continuously dispensed. The following continuous loss-in-weight feeders can accurately dispense materials into a continuous process and can be used to weigh even small amounts accurately.
Continuous loss-in-weight feeders
The most sophisticated and most expensive continuous loss-in-weight feeders use a feeding device such as a screw auger attached to the bottom of a hopper of material, as shown in Figure 3. The entire assembly — the combined weight of the hopper as well as everything inside and everything firmly attached to the hopper — is supported on one or more load cells, and the assembly and its contents are weighed. A sophisticated scaling and process controller is required to measure the material weight as it’s dispensed over time; the controller adjusts the feeder’s speed faster or slower to control the discharge rate. The weighing controls are sensitive and precise enough to weigh even small amounts accurately.
The concept is straight forward, but the controls are quite sophisticated. This is important because the controller must overcome several issues. A common example is that as a material’s bulk density changes, it directly affects the amount being dispensed per revolution of the feeder as described in Part I, Volumetric feeding. Accordingly, when the bulk density increases, more weight gets discharged per revolution of the feeder, so the feeder’s speed must be slowed. The opposite is true if the bulk density decreases, and more revolutions per minute will be needed to dispense more volume of material to achieve the same weight.
Material pushing down on itself will increase its bulk density as material at the bottom is being pushed down upon and compressed by the material above it. Consolidation force presses material together at the bottom of the hopper, which increases the material’s bulk density. Furthermore, when material height in the hopper changes, it’ll change the amount of compression weight, and thus affect the bulk density. When the hopper is full, the downward consolidation force is much greater than when the hopper’s nearly empty.
Yet another phenomenon can alter a material’s bulk density each time the hopper refills. When powder material falls into a nearly empty hopper, it can pick up entrained air and become fluidized. The resulting mixture of powder and air has a much lower bulk density than nonaerated material, so the screw auger has to turn faster to provide more material to achieve the same weight.
Another issue the feeder controller has to account for is when the hopper needs to be refilled, material dropping into the hopper causes the scale reading to jump around. Moreover, material weight gets added at the top while simultaneously being dispensed from the bottom, and the controller cannot resolve between the two. Therefore, the controls traditionally ignore the weight signal during refill, and the screw auger operates temporarily in volumetric mode rather than gravimetric. However, it’s undesirable to run in volumetric mode because this method is less accurate. Because of the negative side effects that come with refilling the hopper, many suppliers require large hoppers so that the hoppers don’t have to be refilled very often.
The good news is that controls have improved significantly in the past few years. For example, instead of just running the hopper at average speed in volumetric mode during a refill, the control program “learns” how rotation speed changes during each refill-and-empty cycle. The controls then use that information to automatically adjust the speed to accommodate the expected material bulk density changes as material height changes during the refill cycle. Furthermore, the controls are increasingly able to distinguish between and compensate for regular but undesired inputs such as vibration and impact of falling material.
Sophisticated controls and drives also allow for a large turndown ratio. Turndown refers to the range of flowrates a feeder can accurately dispense material. Enhanced controls and drives can operate at a very fast speed, a very slow speed, or anywhere in between without overheating or stalling the system.
While screw augers are the most common feeding devices used in continuous loss-in-weight feeders, they don’t work well with some materials, such as flakey ingredients. In lieu of a screw auger, some feeders use other devices such as a vibrating tray, as shown in Figure 4. This vibrating tray, known as a vibratory feeder, weighs and conveys material within a process. The vibratory feeder works similarly to a screw auger in that the tray assembly is on a load cell and has a weigh controller. The material is then weighed as the vibratory tray conveys it to the next point in the process.
Yet another type of continuous loss-in-weight feeder uses a conveyor belt to dispense the material. In a continuous weighbelt feeder, as shown in Figure 5, one section of the belt is supported by load cells. If the weight of the material on the belt and the belt speed are both known, then the two figures are multiplied to derive the gravimetric feedrate. The controller compares this rate to the desired rate and adjusts the belt speed faster or slower to achieve the desired throughput. The continuous weighbelt feeder typically requires less head-height than a loss-in-weight screw auger or vibratory feeder with a hopper and can be less expensive than other options when high material throughput is required.
While continuous loss-in-weight feeders are the most expensive option for weighing and batching equipment, the price difference between screw augers, vibratory feeders, and weighbelt feeders isn’t huge.
The right weighing method
This article has described many weighing methods. How do you know which one to use? In general, you will want to use the least expensive method that meets your accuracy and process needs. In many cases, the best answer might mean you need a variety of methods within your plant to handle the various materials and parts of your process.
For example, many recipes include a couple of large volume major ingredients with several smaller minor ingredients. The major ingredients are often relatively inexpensive commodity materials, so less accurate weighing methods such as a gain-in-weight hopper will be fine for batch operations. On the other hand, minor ingredients, such as vitamins, might be very expensive or the exact ingredient quantity might be critical and greatly affect the end-product result, such as the amount of foaming agent added to rubber. In these cases, an accurate continuous loss-in-weight feeder is the best answer.
Remember that whenever accuracy and weight recording aren’t required in your process, you can use a volumetric feeder to measure your materials. However, when a high degree of accuracy is called for, you’ll need to use a gravimetric feeder such as a continuous loss-in-weight feeder.
The good news is that many weighing and batching options are available for your bulk solids processing. You can choose the one that best suits your needs and you can use any combination of methods to achieve the desired result at the lowest cost.
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.
K-State Bulk Solids Innovation Center • Salina, KS
785-404-4918 • https://bulk-solids.k-state.edu
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