CONVENTIONAL DRYING The principle involved in the conventional driers is that fermented leaf is subjected to a blast of hot air in such a manner that the hottest air first comes in contact with the tea having the least moisture content. In these driers, the fermented leaf falls on a series of moving perforated trays on which it is passed and repassed through a moving stream of hot air.

The perforated trays are mounted on an endless chain and arranged in a tier of six or eight units which alternate in the direction of motion. The Design is such that at each stage of the drying operation, the leaf is subjected to a different temperature. As the leaf passes from tray to tray, it progressively comes into contact with higher temperatures. When the air takes up moisture, the dry bulb temperature falls.

A final moisture content of between 2.5 and 3.0 per cent should be the aim. If the tea is dried below 1.0 per cent, it loses some quality. Tea dried to 3.5 per cent moisture content and above does not keep well.

The optimal inlet temperature for CTC processed leaf is 100 ± 5°C. The exhaust temperature should be maintained at 54.4 ± 2.7°C (130±5°F). If the exhaust temperature is less than 49°C (120°F), the post fermentation process will continue for a considerable time and will soften the liquor. This condition is referred to as "stewing". If the exhaust temperature is greater than 57.2°C (135°F) the rate of moisture removal is too rapid and results in case hardened tea in which the particles are hard on the outside but incompletely dried within; such teas yield harsh liquors and do not keep well. So it is of paramount importance to ensure that temperatures are kept under control to the extent possible.

FLUIDIZED BED DRYING The tea industry presently enjoys a variety of fluidized bed drying equipments like vibrobed, five zones and three zones cross flow fluid bed driers. All of them strive to get increased fuel economy without affecting quality.

When a fluid flows upwards through a bed of granular particles, the pressure drop is initially proportional to the rate of flow: At a certain increased air velocity, the frictional drag on the particles becomes equivalent to the apparent weight and the bed begins to expand. This stage is known as the onset of fluidization or incipient fluidization. Further increase in velocity causes the individual particles to separate from one another and float. Under these conditions the system is said to be fluidized. In fact the relative movements of the individual particles in the air stream acquire many properties of liquid and have analogous flow characteristics. Hence the term 'fluidized bed'.

One of the virtues of fluidized systems is that they have high rates of heat and mass transfer while maintaining uniform temperature characteristics on the bed. Consequently conditions such as case-hardening are seldom encountered with fluidized systems. Good thermal contact between the tea particles and the drying medium results in improved fuel efficiency. Particle to particle attraction in a fluidized drier is minimized because each particle is surrounded by its own fluid cushion. In practice, too, this expectation is realized by the production of blacker teas with better appearance and bloom.

The fluid bed drier essentially consists of a drying chamber, plenum chamber, dust collectors and flow control dampers. The drying chamber normally consists of three drying zones and one cooling zone. Fermented leaf is loaded on a grid plate of the drying chamber. The top of the drying chamber is totally closed and two sets of centrifugal exhaust fans are provided with cyclones; one for refiring and the other for dust extraction. Beneath the drying chamber is a plenum chamber where the air pressure is equalized. The direction of the hot air entering into a grid plate is controlled by the flow control dampers which can be operated independently. The flow control dampers have dual purposes - during the operation their direction determines the residence time of tea particles in the drier and at the end of manufacture, they serve to evacuate the drier completely. In each zone, the required volume and pressure of air is maintained by independent air valves. In some commercial driers, a blow-hole suppressor is provided in the drying chamber to facilitate easy cleaning of the grid plate.

When the fermented leaf enters the drying chamber, it has very high moisture content which is rapidly reduced in the first zone. At this point, maximum volume of air is introduced since rapid evaporation is required. As the moisture loss takes place, density of the material is reduced. This material tends to move away from the feed end as it is being displaced by fresh materials which contain more moisture and hence have high density. The movement of the tea particles within the drying chamber is governed by the principle of displacement. When the material is fully dried, it is expelled into a cooling chamber wherein ambient air is introduced by a forced draft fan.

The desirable inlet temperature ranges from 140° to 150°C. Firing at this temperature resulted in improved leaf appearance and better bloom. The exhaust temperature has to be maintained at 71.1°C(160°F) to 76.7°C(170°F) in the third section. In some driers, exhaust temperature is measured at the centre of the drying zone along the length, and kept at 57.2 ±2.8°C(135± 5°F).