Fluid Bed Coater - High Velocity Gas Entrained Wurster Column Design, Enhanced Coating Uniformity

In Fluid Bed Coater the particles are entrained in a high velocity gas, they pass through the Wurster column where coating is applied via a two-fluid atomizer,they then dry in the expansion chamber and fall back down to the bed to repeat the process.
Of particular importance for the equipment geometry is the wurster gap a small passage at the base of the unit between the inner and outer (annular) columns. During operation, the particles in the bed are fluidized and coated primarily in the inner column, where the fluidization velocity is produced,and after reaching the upper region of the device,these particles stream down the walls of the outer section prior to being entrained through the gap and re-entering the primary fluidization region.
The results indicated that there is a steady solids flux through the larger gap,but in the case of the smaller gap,the flow of solids through the gap is chocked periodically and the bed does not maintain a steady fountain. This creates a non uniform solids flux in the spray region,which could lead to problems with coating uniformity.