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.
In tray dryer hot air is contiinuously circulated. Forced convection heating takes place to remove moister from the solids placed in trays.
Simuilaneously the moist air is removed partially.
- The construction of the tray dryer consists of a rectangular chamber whose walls are insulated.
- Tray are palced inside the heating chamber.
- The number of trays may vary with the size of the dryer.
- Dryers of the laboratory size may contains a minimum of the three trays, where as dryers of industry size may contain more than 20 trays.
- Each trays is rectangular or square and about 1.2 to 2.4 meters square in area.
- Trays are usually loaded from 10 to 100 mm deep.
- The distance between the bottom of upper tray and surface of the substance loaded in the subsequent tray must be 40 mm.
- Alternately the trays can be placed in trucks on wheels, which can be rolled in to and out of the chamber.
- Two such trucks can be arranged in side the dryers. Dryers I fitted with a fan for circulating air oven the trays.
- Electrically heated elements are provided inside rather than outside to heat the air steam also can be used as it is cheaper.
- In the corner of the chamber direction vanes are placed to direct air in the expected path.
Wet solid is loaded in to the trays. Trays are placed in the chamber.
Fresh air is introduced through in let, which passes through the heaters and gets heated up.
The hot air is circulated by means of fans at 2 to 5 metre per second.
Turbulent flow lowers the partial vapour pressure in the atmosphere and also reduces the thickness of the air boundary layer.
The water is picked up by the air. As the water evaporates from the surface, the water diffuses from the interior of the solids by the capillary action.
These events occur in a single pass of air. The time of contact is short and amount of water picked up in a single pass is small.
Therfore the discharged air to the tune of 80 to 90 % is circulated back through the fans. Only 10 to 20% of fresh air is introduced.
Moist air is discharged through outlet. Thus constant temperature and uniform air flow over the materials can be maintained for achieving uniform drying.
In case of the wet granules as in tablets and capsules drying is containued until the desired moister content is obtained.
At the end of the drying trays or trucks are pulled out of the chamber and taken to a tray dumping station.
- Tray dryer is used in the drying of the sticky materials.
- Tray dryers are used in the drying of the granular mass or crystalline materials.
- Plastic substances can be dryed by the tray dryers.
- Wet mass preparations, precipitates and pastes can be dried in a tray dryer.
- In the tray dryers the crude drugs, chemicals, powders and tablet granules are also dryed and shows free flowing of the materials by picking up the water.
- Some types of equipments can also be dryed in the tray dryers.
- In the tray dryers the handling of the materials, loading and unloading of the materials can be done without losses.
- Tray dryer is operated batch wise. Batch drying isused extensively in the manufacture of the pharmaceuticals for the following reasons
- Each batch of the materials can be handled as as separate entity.
- The batch sizes in the pharmacetical inustry are relatively small 250 kg or less per batch compared with the chemical industry 1000 kg or more per hour.
- The same equipment is readily adjusted for the use in drying a wide variety of the materials.
- Valuable products can be handled efficiently.
- Tray dryers requires more labour to load and unload.
- Hence increases in the cost.
- The process is time consuming.
Tray dryers can be operated under the vaccum often with in direct heating, this is done in special vaccum tray dryers for the drying vitamins and the other heat sensitive products.
- Mehta RM, Pharmaceutics-I: introduction to the drying process (1996), Page no: 187.
- C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices-filtration, 2001, page no. 390 - 392.