In most process applications, the flow rate of solids leaving a bin or hopper is governed by a device that ensures a constant flow rate on a volume flow or mass flow basis, and that permits the flow to be turned off when necessary. The design of the feeder is an integral part of the design of the bin and hopper. For mass flow to occur in the bin, the feeder shape and opening must allow material to discharge uniformly across the open area at the base of the hopper. The feeder must also allow flow of solids to be interrupted and resumed, and must prevent the stationary solids from wicking moisture from downstream fluid operations.