Issue link: https://www.e-digitaleditions.com/i/1282913
September 2020 / 33 Editor's note: Part II, which will be published in November'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 Find more information on this topic in articles listed under "Weighing and batching" in Powder and Bulk Engi- neering's article index in the December 2019 issue or the article archive on PBE's website, www.powderbulk.com. Todd Smith (toddsmith@k-state.edu, 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 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 ingre- dient'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 pro- cess 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 ingre- dient 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 prac- tical 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 uncon- trolled 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. PBE