Issue link: https://www.e-digitaleditions.com/i/1302781
34 / November 2020 powderbulk.com 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 compres- sion 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 drop- ping 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 tem- porarily 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 hop- pers don't have to be refilled very often. The good news is that controls have improved sig- nificantly 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 Fig- ure 4. This vibrating tray, known as a vibratory feeder, Continuous loss-in-weight feeders The most sophisticated and most expensive continu- ous 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 assem- bly — 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 con- trols 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 revo- lution of the feeder as described in Part I, Volumetric feeding (September issue). 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. FIGURE 3 Continuous loss-in-weight feeder with screw auger Courtesy of Coperion K-Tron. Hopper with material Load cell Screw auger rotates inside housing Material outlet