3.4.3. Coating efficiency and uniformity
Coating efficiency tends to increase with size of equipment especially from 15 cm to 45 cm diameter units. This is important since often the actual coating weight or thickness is not measured directly due to the difficulty in making such measurements. Instead, the measured release rate (for coats that provide a modified release profile) is most important. Thus, the amount of coating is usually based on theoretical weight gain, which is simply the total mass of solids in the coating solution/suspension (solute but not solvent) sprayed into the bed. For the same theoretical coating, the larger equipment will tend to give higher actual coating weights because of increased efficiency. The coating efficiency tends to be higher in larger equipment because the solids spend more time in the draft tube, which is longer, and therefore the amount of coating solution lost in the exhaust air tends to be lower, as illustrated in Fig. 4. This increase in coating efficiency translates to shorter coating times than indicated in Table 1, for the larger equipment.