are used for precise control of volume feed rate from larger openings. Rotary feeders are also known as rotary vane feeders or star feeders. The feed rate can be controlled by adjusting the speed of the rotary valve (star valve). The rotary valve can be designed to provide a positive seal between a gas atmosphere in the process and air in the hopper, and the feeder and hopper can be purged with nitrogen if the designer needs to prevent entry of air into the process. Rotary vane feeders are particularly useful when feeding into contained systems such as pneumatic conveyors or fluidized beds. Rotary vane feeders can be prone to jamming when the tolerances between the vanes and casing are small.