An improved Wurster coater in which conditioned fluidizing gas, drawn through a coating chamber bowl from a plenum by a vacuum fan, is directed through a perforated gas distribution plate through one or more Wurster tubes. The perforated gas distribution plate defines three concentric gas flow zones: a central high-flow up-bed zone beneath the Wurster tube, a surrounding annular lower-flow down-bed zone, and an outer peripheral bypass zone defining a bypass cavity underlying the outer lower wall of the bowl. A controlled bypass flow of conditioned fluidizing gas is directed into the bypass zone, either around the edge of the distribution plate or through relatively larger holes in its plate's outer periphery, which causes that portion of the fluidizing gas to flow radially inward across the distribution plate, thereby sweeping particles away which would otherwise stagnate at the inner corners of the bowl due to low gas velocity in that region. The depth of the cavity is adjustable for varying the pressure drop, and thereby the volume and velocity, of the bypass gas stream.
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