A loss-in-weight feeder accurately meters powders, granules, pellets, or solids-liquids mixtures into a downstream process at a desired feedrate. A LIW feeder can be designed for several industries including foods, chemicals, pharmaceuticals, and plastics.Components. The typical LIW feeder has an active or weighed, section that includes a material hopper (sometimes equipped with an agitator), discharge device (such as a screw feeder, belt feeder, or vibratory pan feeder), and motor and drive. The feeder also has an inactive, or nonweighed, section, including the frame, structural support, motor speed controller for the discharge device, and controller (typically remotely located and containing multiple displays for feedrate, motor speed, static weight, and other data). Together, a weighing mechanism (such as a load cell or digital weight-sensing device and its mounting hardware), wiring from the weighing mechanism to the controller, and wiring from the controller to the discharge device's motor speed controller form a bridge between the feeder's active and inactive sections.
How loss in weight is measured. The discharge device withdraws material from the hopper and meters it to a downstream process. The weighing mechanism typically weighs the material in the hopper, the hopper itself, and the discharge device, but doesn't weigh components in the inactive section. When the weighing mechanism's weight reading changes, the controller can direct the motor speed controller on the discharge device to deliver more or less material to the process. When the material level in the hopper drops, the controller signals an automatic refill device (or worker) to refill the hopper. The controller ensures that material continues to feed the process while the hopper is refilling. As the discharge device moves material from the hopper into the downstream process, the weight of material in the hopper decreases. The controller evaluates the amount of "loss in weight" sensed by the weighing mechanism over a given time interval to determine the feedrate; the controller adjusts the discharge device's speed via the motor speed controller to either increase or decrease the loss in weight and, hence, the feedrate.
