Refilling continuous loss-in-weight feeders
Loss-in-weight feeders have evolved from the crude devices of the late 1940s to very sophisticates machines capable of deeding at continuous accuracies down to 0.2 percent of sample size at a 2 sigma confidence level [ depending on material]. Due to the nature of their operation, loss-in-weight feeders require periodic refill from an external device such as a screw conveyor, bin activator, pneumatic conveyor, slide gate valve, or rotary valve. Capacity requirements of the refill device and desirable refill frequencies are often overlooked by process engineers, yet they can have a major impact on the overall process accuracy. This article addresses these refill requirements and serves as a guide for specifying loss-in-weight feeders.
A loss-in-weight feeder consists of three components: a scale with a dedicated controller to weigh material, a feed mechanism [screw, vibrating tray, etc.] with a variable- speed drive to introduce material into the process; and an integral weigh hopper for material storage.