4. Rotating drum (perforated pan) coaters
In general, rotating drums or perforated pans are currently used primarily to coat tablets. Fig. 6 illustrates the basic elements of a rotating drum coating device. As the drum is rotated, solids cascade down the moving top surface of the bed. A dual-fluid nozzle distributes the coating material (solute and solvent) onto the moving particles. Simultaneously, hot air is drawn through the bed of solids to facilitate evaporation of the solvent. The repeated coating and drying cycle causes a coherent film of coating to be built up on the surface of the solids. The tumbling action of the solids is less abrasive than the circulating movement in a fluid bed coater, especially for large tablets that would require high air velocities that lead to high erosion and breakage in a fluid bed coater. However, heat and mass transfer are poorer in a rotating drum and the coating process tends to take significantly longer. In addition, the variability (tablet-to-tablet variation) tends to be higher for rotating drums compared to fluid beds. Thus, if active ingredients are contained in the coating material then it is more difficult to obtain the desired uniformity in a rotating drum coater than a fluid bed unit.