Vacuum tray dryer is the most commonly used batch dryer. They are either box-shaped or cylindrical shaped loaded and unloaded via a door. Inside this contains several heating plates mounted one above the other on which the product is placed in trays. The Top of both heating plates (Shelves) and the bottom of heating shelves trays should be as smooth as possible to permit optimal heat transfer between plates and products. The medium flowing through the heating plates is water, steam, or thermal oil.
The distance between the heating plates is determined primarily by the surface loading and the foaming of the product. To avoid retrograde condensation, the cabinet walls are indirectly preheated by limpet coil provided on the outer side of the body (Optional). Only after all the product trays reach the same temperature, the cabinet is evacuated and drying can start. (The preheating phase is very important in order that the drying curve and the foaming of the product are identical throughout the cabinet). During the main drying phase, the vacuum is in the range of 40 to 80 mbar abs and in the final drying phase, the vacuum of only a few mbar abs is reached. The vacuum can be controlled through PLC in order to reduce the drying cycle. Nitrogen can also be purged in the VTD according to the product.