Unlike the volumetric screw feeder as detailed in Part 1 of this series, a continuous loss-in-weight (LIW) feeder is a gravimetric feeder that directly measures the material’s weight to achieve and maintain a predetermined feed rate that’s measured in units of weight per time. The LIW feeder consists of a hopper, refill device, weight-sensing device (typically either a digital or analog scale or load cells), material-discharge device (typically a volumetric screw feeder powered by a variable-speed motor), and controller. Before operation, an operator programs the controller to discharge material at a predetermined feed rate (or setpoint) measured in units of weight per time (such as lb/hr).
The bulk material (or liquid) is discharged from a hopper with a constant weight per unit time by weighing the hopper and regulating the speed of the feeding device depending on the rate of weight loss. The weighing control system compensates for non-uniform material-flow characteristics and variations in bulk density, thus providing a high degree of feeding accuracy. When the hopper reaches a predetermined minimum weight level, the LIW control is briefly interrupted and the hopper is refilled. With Coperion K-Tron feeders, during the refill period, the controller regulates the speed of the feeding device based upon the historic weight and speed information that was accumulated during the previous weight-loss cycle. This prevents overfeeding of material during the refill cycle due to changes in headload of material and filling of material into the screws. This is also critical for maintaining feed-rate performance within specification on a second-to-second basis. The LIW feeding principle is most accurate when using a high resolution, fast-responding, and vibration- and temperature-immune weighing system.