In small, dryers the material is placed on trays, which slide into the drying cabinet, while in large installations the interior may be designed for the wheeling in of trolleys containing the trays.
The simplest form of heating places the sources of heat (e. g. a steam coil or electrical coil) at floor level and relies on natural convection. Fans provide a forced circulation of air across the trays. Air flows in axle direction over each shelf in turn.
The wet material spread directly on the shallow trays resting on the shelves. Electrical elements (heater) or steam heated pipes are positioned so that air is periodically reheated after it has cooled by passage over the wet material on one shelf before it passes over the material on the next shelf. Tray dryer is a type of Fixed Bed Dryers i.e., the bed of the material to be dried is stationary.
The vacuum oven consists of a jacketed vessel sufficiently stout in construction to withstand vacuum within the oven and steam pressure in the jacket. In addition, the supports for the shelves form part of the jacket, giving a larger area of conduction heat transfer.
The oven can be closed by the door that can be locked tightly to give an airtight seal. The oven is connected through condenser and receiver to a vacuum pump, although if the liquid to be removed is water and the pump is of the ejector-type that can handle water vapor, the pump can be connected directly to the oven.