Tea World

Lesson 23

Firing or Drying

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Drying is the final stage of manufacturing of tea. During drying the moisture is removed from the fermented leaf particles in a suitable chamber by vaporization of water in a stream of hot air as the carrier fluid. Drying is a simultaneous heat and mass transfer· process - gain in temperature or heat, loss in moisture or mass. Air in warmed up condition and in adequate quantity is the most convenient medium for heat and mass transfer during tea drying. The objective of drying of tea is two-fold: One, to arrest the fermentation and to fix the desirable properties. Second to remove the moisture from the leaf particles and to obtain a finished product that is stable which can be handled and transported.

Technology of Tea Drying

When a particle is surrounded by moisture molecules in its surface, the removal of the moisture is relatively easy and such evaporation rate is independent of the properties of the particles. This rate of evaporation is governed by the mass flow rate and drying potential of air only. In the drying process, this stage is known as ' Constant Rate.

Period of Drying   

With the removal of freely available water from the surface of the solid particle, a stage is reached where some portions of the particles surface is devoid of any moisture molecules and the balance would still have some. The rate of drying of such particles will continuously decrease and drying will become increasingly more difficult. Once the surface drying is complete, diffusion processes control the drying rates. Those moisture molecules entrapped inside the particles have to come out to the surface either in liquid or in vapour form before leaving the tea granule. As the moisture level of the particles decreases, the concentration gradient decreases too reducing the rate of drying. This stage of drying is known as the ‘Falling Rate Period' of drying. The critical moisture content at which the rate of drying slows down depends on the size and shape of the solid particles and their texture. Early types of dryers were simple batch types, in which the leaf was spread on a perforated mesh or tray and heated air blown through until the tea was dried. A semi-continuous system employing a series of trays mounted horizontally on a vertical stack was constructed. Sequential mechanical tripping was employed to drop the contents of each tray at selected intervals onto the tray immediately below. Hot air is fed from below and the dried tea eventually emerging at the lowest point. These dryers known as Venetian dryers are not in use now-a-days.

The modem machine consists of two or three endless chains where perforated trays are mounted. In the present design, the drier is situated on the pressure side of the fan, but in the earlier versions the fans sucked air through the drier. Fermented leaf is dropped on the top tray of the drier by a conveyer. The leaf particles Tea Dryer falling on the perforated tray is taken through the drier by the moving trap. At the end of each tray level, the leaf is dropped to the tray immediately below. Hot air is sent from the bottom of the drier and is made to flow up through the perforations. The arrangement ensures progressively higher temperature during the course of drying and the air is made to come in contact with the increasingly moist leaf particles. A tea drier consists of the dryer unit, the air heater and the fan. The hot air is provided by a furnace to which are connected the heat exchangers. An induced drought is maintained by the fan. The fermented tea particles, when ready for drying, are regularly fed into a hopper with automatic spreader. Thickness of spread, speed of trays and the volume of air flow through the trays are regulated as desired. As is clear from the design at each stage of drying, the leaf is subjected to a different temperature. The exhaust temperature should be such that the fermentation process is brought to a stop immediately after the leaf has entered to the top tray of the dryer. However, in actual practice the fermentation, which is enzymic in nature, continues for some time in the dryer.

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