Drying

The main objectives of drying are:

  • To arrest enzymic reaction as well as oxidation,
  • To remove moisture from the leaf particles and to produce a stable product with good keeping quality.

Technology of tea drying

On an average 100 kg of fresh leaf produces 22.5 kg of dried tea containing residual 3% moisture. The difference of 77.5 kg between the figures represents the moisture evaporated during the process. Of the 77.5 kg, about 20-25 kg are evaporated during withering and around 20-50 kg are evaporated during drying. A leaf particle has to undergo a moisture change from around 70% to 3% during drying.

ECP dryerDuring the early stage of drying the solid is so wet that a continuous layer of moisture exists over the entire surface. The temperature of the solid particles will be near the wet bulb temperature of the drying air. Removal ofthis layer of film of moisture is easier and such evaporation rate is independent of the moisture content of the particle and the moisture will be evaporated at the constant rate. This stage of drying is known as the ‘constant rate drying’. With the freely available water from the surface of the solid particle evaporated the drying no more takes place at the constant rate. Under this condition diffusion process controls the drying rates.The moisture trapped inside the drying particle travels to the surface before being evaporated. With reduction of moisture the drying now takes place at a falling rate. The quantity of moisture removed during the “Falling rate period” is small but the time taken is quite long. Hence the falling rate period has a very important effect on the time of drying and is dependent on size of the particle, thickness of spread and external variables like volumetric flow, temperature and humidity of air.

A tea dryer consists of the dryer, the air heater, heat exchanger and a fan. The fan is used to introduce the clean hot air at the bottom of the dryer. In the conventional dryer, the fermented leaf falls over a series of moving perforated trays where the hot incoming air-dries the leaf and the moisture is reduced from around 70% to 3%.

Factors that influence drying

The following factors influence drying:

  • Temperature of inlet and exhaust air
  • Volume of air
  • Quantity of leaf fed (i.e. thickness of spread)
  • Period of drying (through put time)

Temperature of inlet air

Drying of tea involves both physical and chemical aspects. Temperature at which tea is dried, therefore, has to be selected judiciously. Too high temperature at the initial stage may cause case hardening and blistering or scorching. Even if these effects are taken care of a faster rate of evaporation may impart the teas an undesirable harshness. On the other hand, too low a drying temperature slows down the rate of drying and high temperature oxidation is allowed to proceed for a longer period resulting in a ‘dull’ and ‘soft’ product. This affect is known as stewing. For conventional drier, an inlet temperature between 82-99°C (180-210 °F) has been found to be satisfactory. However, for Darjeeling manufacture where the wither is also high, a high inlet temperature of around 115°C (240 °F) is used to bring out the desired flavour. Exhaust temperature is equally important, for, it indicates how much heat has been extracted from the incoming hot air. For conventional drier an exhaust temperature of 49-54°C (120-130°F) is advocated as at this range the oxidation of the leaf is brought nearly to a stop.

Volume of air

Volume of air for drying depends on two factors – moisture to be removed and temperature selected. If the volume of air is below the normal requirement, the temperature will have to be increased to produce the same amount of heat. Increasing air volume can increase the capacity of a dryer to a certain extent. On the other hand lower fan speed, small exhaust duct, partly closed fan valve (damper) result in lower volume of airflow. There is a limitation in increasing the airflow as excessive airflow may result in blowing away of the small/dust leaf particles.

Thickness of spread

In a conventional drier in particular, the spread of the leaf should be to a reasonable depth to prevent the air from escaping freely through it. Overloading the dryer requires higher drying temperature and longer period of drying. In general, finer material should be spread thinner. Big bulk requires slightly longer period of drying and should be spread thicker. The thickness of spread of leaf will also depend on the degree of wither as well as inlet temperature index. A drier should never be operated with fixed spreader position.

Period of drying

Time required for drying tea varies with degree of wither, temperature, thickness of spread and volume of air. It is apparent that the two main objectives of drying viz., final moisture content (3%) and arrest of ‘oxidation’ can be achieved even in shorter residence time by increasing the drying rate. It does not necessarily mean that evaporation rate is also high in this case. A fast drying rate carries the risk of case-hardening or scorching. On the other hand longer period of drying which, may be due to higher thickness of spread, produces uneven teas, as the contact between the leaf particles and hot air supply is not proper.

Major components of a tea dryer

Three essential components of a dryer are:

  • A furnace in which fuel whether coal, oil,gas or wood is burnt
  • A heat exchanger where the heat of the furnace gases is transferred to clean air
  • The drying chamber in which the clean hot air is passed through the leaf and evaporates the moisture therein. Furnace and heat exchanger form one unit known as the stove or air heater.

Air heater

In tea both indirect and direct air heaters are used. The heater is an independent or self contained type of air heater. The type is known as self contained as both the aspects of generation and transmission of heat is incorporated in the body of the heater.

Direct fired heater

In direct fired heaters the products of combustion, mixed with atmospheric air, are directly drawn into the dryer chamber. Complete combustion must be ensured so that the heated air does not have any ill effect on the made tea.

Oil at requisite pressure and temperature is atomised and burnt in the combustion chamber. An induced draught is created by the centrifugal fan of the drier, the inlet of which is connected to the end of the mixing chamber. Atmospheric air on entering the mixing chamber is mixed with the products of combustion from the combustion chamber. This mixed air is subsequently forced into the drier chamber.

The essential parts of a DF heater are combustion chamber, a self proportioning oil burner, oil circuit with preheater and oil circuit with a blower (working at 22-28 inch wg).

The following factors are important:

  • Proper atomisation of the fuel oil. Oil should be free from deposits or any non-combustible particles as much as possible.
  • Thorough mixing of the atomised oil with proper amount of air for combustion
  • Keeping the furnace temperature high to sustain combustion
  • The flame action is not localised to avoid over heating of the furnace
  • Purging of all inflammable and explosive gases before ignition.

Dryer

Two types of dryers are in use today. Conventional pressure chamber type and the Fluid Bed Type (FBD).

Conventional dryer

Conventional dryerThis type consists of a rectangular chamber with conveyors carrying the leaf through it, while hot air is being blown through it at a pressure. The conveyors are arranged in the form of three tray circuits run usually at a speed ratio of 10:14:18.The dryer chamber is open at the top and closed on all sides except for the hot air inlet duct. The chamber has chain guides and sprockets fitted to the side walls. Air baffles are fitted at the ends to prevent air leakage. A scrapper circuit is fitted at the bottom to scrape the floor continuously to prevent accumulation. A spreader is fitted above the top tray to spread out the material evenly.

The trays are 4 ¼ inch wide of which 2 7/8 inch is perforated and are made of mild steel sheets with aluminium or stainless steel coating to assure absence of lead or zinc flakes which might come loose to become mixed with leaf particles. The trays, stiffened properly, are fixed to the driving chains at two ends by pins. The tray sides called lips are so arranged that they overlap the next tray’s lip and prevent air leakage. The trays must have maximum perforated area per square inch. Since both surfaces are used for leaf carrying, the both sides of the trays must have very good finish. For proper spreading of leaf, the stiffeners must be of small diameter or of flat construction so that a minimum spread can be set for teas such as in CTC manufacture. There may be an extra plain tray circuit at the top to act as a conveyor to feed the dryer.

Size

The size is expressed in terms of the length of the tray. Normal sizes are 4 feet and 6 feet.

Capacity

Both quality and the ECP driers have moisture evaporation capacity of 400-450 kg/hr. Therefore, under North East Indian conditions a six feet drier will give about 180 kg of CTC tea per hour.

Working

Fermented leaf fed to the top are carried forward and at the end of he run, the trays tilt one by one and discharge the leaf to the lower run. The process being repeated till the leaf in discharged by the bottom run onto a valve (discharge valve), which delivers the leaf outside the chamber. While the trays are carrying the leaf, hot air fed at the bottom pass through the trays and dries the leaf.

Air Requirement in Dryer

On an average a six feet drier has the capacity to remove 400-450 kg of water/hour. However, all the dryers do not have similar volumetric capacity. Volumes of some of the standard driers are given below:

Size

Type of Dryer

LxHxW

Volume

4/

Quality

12/ x 6/ x 4/

288 cft

6/

ECP

12/ x 6/ x 6/

432 cft

6/

Quality

16/ x 6/ x 6.25/

600 cft

6/

Quality

18/ x 4/ x 6.25/

450 cft

For drying, it is required that the volumetric content of the drying chamber is emptied every 2 seconds i.e., 30 changes per minute. A 6/ quality dryer will thus require to remove 600 x 30 = 18,000 cft of air per minute. This air is moisture laden and, therefore, moisture must be removed from the drying room, otherwise ‘back pressure’ effect will take place. Air comes out of the dryer chamber because of pressure differential between the inside of the drier and the room. If the room pressure increases, less air will come out of the dryer. Another aspect is of important consideration. The dryer is a pressure chamber. When the air from the drier is released to the atmosphere the pressure drops and air expands. With the drop in pressure, velocity also drops. This means the openings of the dryer room through which the exhaust air must escape must be larger than the top cross section of the drier. It is, therefore, essential to have 1.5 to 3 times the total dryer top area is exhaust outlets.

Back pressure within the drier is caused by blanketing and incorrect tray perforation. Blanketing in the top circuit is commonly encountered with soft withered leaf. The soft withered leaf particles get stuck to the trays and blocks of the airflow. This blocking of airflow may take place up to the third row of the tray circuit. The fermented leaf thus may not come in contact with the hot air up to about 10 minutes after being put into the drier. This results in underfired teas with poor liquor characteristics. The problem of blanketing is further aggravated by inadequate air flow. Fitting of scrappers at the start of the second row helps in obviating the problem of blanketing.

Few Important Practical Points in conventional Drying

  • Rate of loss of moisture

    An average loss of more than 4% moisture per minute leads to bitterness and harshness in made tea. Moisture loss at 2.8-3.6% per minute has been found to produce teas with good quality.

  • High temperature stewing

    Slow drying and blanketing results in stewing. High temperature stewing is a phenomenon encountered in faulty double fired tea, which may have to be carried out to negate the effect of blanketing. Quite often teas from first fire are kept in heaps before being taken for second firing. Teas from such heaps particularly at the bottom sweats. Such teas, if delayed by 7-10 minutes before being put for second fire, will result in a ‘stewed’ tea with soft and flat liquor. Therefore, teas with incomplete drying after first fire must be sent for second fire with least possible delay. In case there is delay, such teas should be spread thinly so that the teas can continue to dry at its own heat.

  • Air leakage

    Air being a fluid follows the path of least resistance. When the dryer is full, the layers of leaf provide resistance to air flow. If there is any leakage in the dryer, the hot air will escape and drying will affected. On the other hand, if there is over blowing of air or there are some empty trays in the circuit, valuable heat will escape. This will also send a wrong signal as far as the inlet as well as exhaust temperature are concerned. The discharge valve is the worst offender of air leakage.

Other important points are:

  • Fan speed should be correct
  • The fall through should be frequently collected
  • The automatic spreader should be at a uniform height and it should be adjusted from time to time
  • The time required by the leaf to pass through the dryer should be checked against each step pulleys
  • The thermometer should be periodically checked
  • The exhaust temperature measured on the top tray must be the temperature of true sample of the air that has actually been passing through the drying leaf and not the temperature of the leaking air
  • The dryer trays should be cleaned before day’s work is started

After overhauling, the air baffles should be correctly repositioned.

Fluidised Bed Dryer (FBD)

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 is the onset of fluidisation. Further increase in velocity causes the individual particles to separate from one another and float. The particles under this condition are said be under fluidisation. A fluid bed drier consists of a grid through which hot air is forced out. Fermented leaf with not too high moisture content is suspended by the air streams resulting in fluidisation. The leaf particles dry simultaneously. The air also acts as a carrier of the particles through the drier, making the bed of leaves move forward until the dried leaf is discharged at the opposite end. To facilitate fluidisation of wet fermented leaf particles, the grid is vibrated or the material is raked or an extra circuit is provided as in case of the hybrid dryer.

The fluid bed dryer 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 the grid plate of the drying chamber. The top of the drying chamber is closed and two sets of centrifugal exhaust fans provided with cyclones – one for refiring and the other for dust extraction.

Below the drying chamber where in the air pressure is uniform, the direction of the hot air entering into the grid plate is controlled by the flow control dampers. In each zone, the required volume and pressure of air is maintained by independent air valves.

In the first zone of the dryer, the very high moisture content of the fermented leaf is reduced rapidly. Hence maximum volume of air is introduced at this stage. With the loss of moisture the density of the particles is reduced. These low density particles tend to move away from the feed end and are replaced by fresh fermented leaf particles having high moisture content and higher density. The movement of the particles within the drying chamber is thus governed by the principle of displacement. When the material is fully dried, it is expelled into a cooling chamber where in ambient air is introduced.

In the “Vibro Fluid Bed Dryer” (VFBD) mechanical forces are added to the pneumatic forces. These help in :

  • Fluidisation accomplished with lower pressure and lower gas velocities. Quantity of air for fluidisation is reduced.
  • Drier can handle non fluidisable solid fractions
  • Start up of the drier is easier due to reduction in channelisation at the beginning of fluidisation.

The drying chamber of the VFBD is divided into three zones. The first two zones have one dust extractor each. The dust extractor collects the mixture of tea dust and sir; the entrained tea dust is separated in the attached cyclone and is added into the next zone for redrying. A cyclone blower is mounted on the third zone to provide adequate pressure for suction of the exhaust air in this zone and to meet the pressure drop in the cyclone separator. The tea dust and air are admitted into a cyclone to separate the tea dust. A cold air blower is provided to supply ambient air to a special mixing chamber for two temperature drying. Two temperature drying ensures uniform drying and obviates the need of a cooling section.

Hybrid dryer

This type of drier combines useful features of both the conventional as well as fluidised bed dryers. The dryer consists of three stages arranged one above the other. The first stage comprises of a 6 feet tray circuit where fermented leaf is quickly predried. At the second stage the tea is made to bubble over 4 feet grid trays that carry the tea to the third and the final stage – the fluidised zone. The dried tea is swept onwards and discharged – the fluff moving up is trapped by the first circuit on the top. The system does not require a separate fluff collection system. The temperature used for drying is around 110 – 115°C – much lower than that of the FBDs, while fuel consumption and capacity remain the same as those for FBD, the power requirement is much lower at 17 KW (23 HP).

Dryer capacity

Dryer capacity varies with design and condition of dryer, air volume, available heat and operational adjustment. It is measured in kg of water evaporated/hour.

% Moisture in fermented leaf

Dryer mouth (kg)

Water evaporated (kg)

Remarks

30

100

43

Darjeeling wither

50

100

100

Darjeeling wither

65

100

188

Plains Orthodox

70

100

223

CTC optimum

75

100

288

Soft wither

80

100

385

Wet leaf

Depending on wither 30 cm of dryer width can give from 25
to 40 kg of made tea/hour.

Common fuel consumption per kg of tea

1. Conventional drier

Coal (kg)

Hand stoked

Oil (l)

Natural gas

(m2)

Direct

Indirect

Drying only

1-1.10

0.3-0.4

0.5-0.6

0.60

Including wither

1.15-1.25

0.4-0.45

0.6-0.65

0.69

2. Fluid Bed Driers

Coal (kg) Oil (l)
Drying only 0.39-0.7 0.17-0.20

Chemistry of drying

In the final stage of black tea processing following chemical changes take place:

  • By the deactivation of enzyme like Polyphenol Oxidase (PPO), Peroxidase (PO), etc. almost all the biochemical reactions are brought to an end. However, as the temperature rises to the level for destroying enzymes, the particular enzymes get activated and faster reaction takes place in the initial stage of drying. If the temperature is raised slowly, TR is likely to be formed.
  • Chlorophyll is degraded to pheophytin and pheophorbide at elevated temperature during drying contributing towards the blackness of made tea.
  • At elevated temperature, by binding with proteins, Polyphenols make complex chemicals, which brings down the level of astringency.
  • Interaction between carbohydrates with amino acids at elevated temperature leads to the formation of flavour components.