Issue link: https://www.e-digitaleditions.com/i/1302781
November 2020 / 33 feeding while others are still conveying. Overall, this method has several advantages. It's simple and incor- porates 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 disadvan- tage 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 cin- namon 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 sta- bilizer, 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 ingre- dient 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 con- tinuous 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 con- tinuous process and can be used to weigh even small amounts accurately. some carryover material will swirl back to the hop- per'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 hoppers (September issue). 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 simul- taneously, this method has the added advantage of mixing the materials as they are being conveyed. Mix- ing 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. Other- wise, some material weighments will finish and stop FIGURE 2 Scale valve conveying system Material silo Scale valve (on each hopper) Pneumatic conveying pipeline Load cell Weigh hopper To mixer 1 To mixer 2 To mixer 3 Clean air from positive-displacement blower