The tray dryer is the device used the drying of the wet products of the crude drugs, chemicals, powders or the granules etc.
The simplest form of the dryer in this category is a cabinet with a heater at the bottom that is laboratory oven.
These ovens are of very little value because there is no control of the heat transfer or humidity.
If a fan is fitted to the oven the forced hot air is circulated which helps in increasing the heat transfer and also in reducing he local vlour concentrations. Despite this there is no adequate control.
The best type of a tray dryer is that of the directed circulation form, in which the air is heated and is directed across the material in a controlled flow.
The material to be dried is spread on the tiers of the trays. The trays used have solid ,perforated or wire mesh bottoms.
The screen tras are lined with paper so that the air circulates across rather than through the drying materials.
The paper is used as a disposable tray liner to reduce cleaning time and also to prevent contamination of the product.
In a modern tray dryers a uniform temperature and air is maintained by the use of a well insulated cabinet with strategically placed fans and heating coils.
There is an alternate arrangement of the sheleves so that air can flow uniformly without any obstructions. Heater is fixed in such a way that the air is reheated before passing over each shelf.
When the air passes over each shelf a certain amount of heat is given up to provide latent heat of vaporisation. In such type of dryers there can be a good control of heat and humidity provided it is designed correctly.