Tray dryer - Pharmaceutical Engineering B. Pharm Third Semester PDF Notes

Tray dryer

Principle

In the tray dryer, hot air is continuously circulated. Forced convection takes place to remove moisture from the solids placed in trays. Simultaneously, the moist air is removed partially. (Shelf, cabinet or compartment dryer)

Construction

• It consist of rectangular chamber whose walls are insulated. Trays are placed inside the heating chamber. Laboratory dryers contains 3 trays minimum & in industry 20 trays

• Each tray is rectangular or square & about 1.2 to 2.4 meters square in area. Trays are usually loaded from10 to 100 mm deep

• Distance between bottom of upper tray & surface of the substance loaded in the subsequent tray must be 40mm.

Dryer is fitted with a fan for circulating air over the trays in the corner of chamber, direction vanes are placed to direct air in the expected path.

Working

• Wet solids is loaded into trays. Fresh air is introduced through inlet, which passes through the heaters & gets heated up.

• Hot air is circulated by means of fans at 2 to 5 meter per second. Turbulent flow lowers the partial vapour pressure in the atmosphere & also reduces the thickness of air boundary layer. As water evaporates from the surface, the water diffuses from the interior of solid by capillary action. These occur in single pass of air.

• The time of contact is short & amount of water is picked up in a single pass is small. Therefore discharged air to the tune of 80 – 90 % is circulated back through fans only 10-20% of fresh air is introduced.

Advantages

1. Handling of materials can be done without losses

2. It can be operated batch wise, for following reasons:

a) Each batch can be handled as separate entity

b) Equipment is readily adjusted for use in drying of variety of materials

c) Valuable products can be handled efficiently

Disadvantage

1.    It requires more labour to load & unload

2.    Time consuming

Uses

Following substances can be dried;

1. Sticky materials

2. Plastic substances

3. Precipitates

4. Chemicals

5. Tablet granules & powders