Issue link: https://www.e-digitaleditions.com/i/507774
Tablets & Capsules May 2015 17 In operation, the feeding device discharges powder from the hopper at a speed determined by the controller. The weight-sensing device continuously reports the pow- der weight in the hopper to the controller. In a continu- ous feeding application, the controller determines the actual rate of weight loss and compares it to the feed rate set-point; in a batching application, it determines the measured (or absolute) weight loss and compares it to the batch weight set-point. The controller then increases or de creases the feeding device's speed to increase or decrease the powder weight change in the hopper, matching the feed rate or batch weight to the set-point. The controller regulates the feeding device's speed to compensate for non-uniform material flow characteristics and bulk density variations, providing high feeding or batching accuracy. The LIW feeder is most accurate when it uses high-resolution, high-speed load cells that are immune to vibration and temperature fluctuations. [Editor's note: For information on how a LIW feeder can be better suited to a batching application than a gain-in- weight (GIW) feeding system, see the sidebar "Com - paring LIW batching with GIW batching."] Selecting a LIW feeder to handle a toxic or reactive powder Let's consider two examples showing how LIW feeders can be selected for dispensing a toxic or highly reactive powder. Single LIW feeder dispensing powder from vacuum conveying system. This batching application requires one LIW feeder, as shown in Figure 2a. A large quantity of a highly reactive powder in a supply vessel on one floor is transferred by a vacuum conveying system to the LIW feeder on the floor above. The powder is com- pletely contained as it discharges from the conveying sys- tem's vacuum receiver into the feeder's hopper. Figure 2 LIW feeders in two batching applications Frame Load cells Vacuum receiver LIW feeder Blender, granulator, reactor, or other process vessel a. Single LIW feeder dispensing powder from a vacuum conveying system Intermediate bulk container discharger for major ingredient or active Blender, granulator, reactor, or other process vessel b. Two LIW feeders dispensing two powders into one vessel Large LIW feeder suspended from frame with three load cells Small LIW feeder on platform scale for minor ingredient Figure 1 LIW feeder Feeding device Hopper Weight-sensing device