Automatic weir(s) for controlling the bed height/ residence time in Fluid Bed Dryers: In an efficiently fluidized bed, the solid material like granules/powders or pellets, are fluidized by an upflow gas.
The wet solids are lifted from the bottom and causes the solids to behave as a fluid. The air velocity is adjusted to keep the moving layer of material fluidized.

At the end of the dryer, the same volume of solid runs over an overflow weir (this can also be a rotating weir) as wet solid material is added at the entry of the dryer.
The solid material flows in a countercurrent to the upward current of air.
The overflow weir is height adjustable and, by influencing of the height of the fluidized bed, enables adjustment of the solid-material residence time.