The document discusses tray dryers, which are commonly used for drying solid materials by removing moisture through thermal processes. These dryers operate by circulating heated air, which aids in moisture evaporation from various types of materials, and are suitable for small production rates despite their labor requirements. A typical tray dryer consists of a metal chamber with racks holding shallow trays where the drying process occurs through forced convection heating and continuous air exchange.

1. Tray dryer
Muhammad Zunair
Chemical Engineer
Comsats University
Date: 29-sept-18

2. Definition:
 Drying refers to the final removal of water, or any other solute from the solid
material to reduce the content of residual liquid to an acceptably low value.
 Drying is usually the final step in the series of operations, and the product from
the dryer is often ready for final packaging.

3. Process:
 Water or other liquids may be removed from solids mechanically by presses or
centrifuges or thermally by vaporization of the liquid or water content.
 A source of heat and an agent to remove the vapor produced by the process are often
involved in thermal drying.
 The solids to be dried may be in many different forms: flakes, granules, crystals,
powders, slabs or continuous sheets and may have widely differing properties.
 The liquid to be vaporized may be on the surface of the solid, it may be entirely inside
the solid or it may be partly outside or partly inside of the solid.
 There is a wide range of dryers available in the market for drying different products.

4. Mostly used in Lab scale:
 Tray Dryer is one of the most commonly used dryers.
 It is widely used and still being used where the moisture content is high
in the product and where the product has to be dried at a low
temperature for a long duration of time.
Tray dryers are useful when the production rate is small. Tray dryer can
dry almost everything but due to the requirement of labor for loading
and unloading, tray dryer is bit expensive.

5. Dimension of tray dryer:
 A typical tray dryer consists of a rectangular chamber of sheet metal containing two trucks that
support racks.
 Each rack carries a number of shallow trays around 750mm square and 50 to 150mm deep that
are loaded with material to be dried.
 Heated air is circulated at 2 to 5 m/s between the trays by fan and motor and passes over the
heater.
 Baffles distribute the air uniformly over the stack of trays. Some moist air is continuously vented
through the exhaust duct, makeup fresh air enters through the inlet. The racks are mounted on
truck wheels so that at the end of drying cycle the truck can be pulled out of the chamber and
taken to a tray dumping station.

6. Working:
 In tray dryer, hot air is continuously circulated.
 Forced convection heating takes place to remove moister from the solids placed in trays.
 Simultaneously the moist air is removed partially. Wet solid is loaded into the trays. Trays are
placed in the chamber.
 Fresh air is introduced through the inlet, which passes through the heaters and gets heated
up. 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.

7. Diagram:

8. Process and Instrumentation
diagram: