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Plantation

  • Vegetative
        Propagation
  • Planting
  • Pruning
  • Fertilizers
  • Weed Control
  • Pest Management


  • Plucking and
    Leaf Handling

    Withering

    Rolling

    Fermentation

    Drying

    Sorting


    Drying

    Definition

    Drying the process during which the enzyme activities are stopped and the moisture content is brought down to 2-3%.

    The load on dryers depends on wither.

    Kg of water to be evaporated in dryer/100 kg of fresh leaf (Containing different percentages of moisture.

                                                 100 Kg fresh leaf reduced to (Kg)

               Moisture content          i,e, Percent Wither  

    of fresh leaf (%) 90 80 75 70 60 50 45
    82 72 62  57 52 42 32 27
    78 68 58 53 48 38 28 23
    74 64 54 49  44 34 24 19  

    N.B. Respiration losses of solids and moisture remaining in dryer mouth tea more or less balance and have not been considered.

    With a soft wither on rains leaf, 3.77 times drying Capacity is required compared to a hard wither on dryish leaf (72 : 19) under average conditions.  A percentage wither of 85 instead of 75, means 17% more dryer hours.  A change from 75% to 65% wither will save one dryer in 5 or reduce drying hours by 1/5.

    Dryer adjustments  

    Variables Affected by
    Exhaust temperature Inlet temperature, spread, air-volume, tray speed
    Spread   Spreader, adjustment
    Time through dryer Tray speed adjustment,  
    Air volume Damper adjustment  
    Inlet temperature Heater adjustment
    % Moisture at dryer mouth Time through dryer, inlet temperature, air volume  

    Limiting factors for adjustments  

      Too high Too low
    Ehaust temperature Waste of heat  Low temperature stewing
    Spread Reduced air flow, incomplete drying Loss of air
    Time through Burniing dryer  Incomplete drying
    Air volume Leaf blow out Stewing
    Inlet temperature Burning/case-hardening   Exhaust too low
    Moisture %  atdischarge Incomplete drying Waste of heat,

    Dryer capacity

    a) It varies with :Dryer design and condition, 
                               Air volume
                               Available heat
                               Operational adjustments

    and Is measured m Kg of water evaporated/hr

    b) The weight of dryer mouth tea is a measure of capacity only if the moisture content of fermented leaf is stable and known.  

     

    Dryer mouth

    Water evaporated

    % Moisture in fermented leaf

    (Kg)

    (Kg)

    29 100 41 Darjeeling wither
    50 100 100 Darjeeling wither
    65 100 186 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 25-40 Kg made tea/hr. Fluid Bed Dryer (FBD) which is used for CTC manufacture only gives 280-400 Kg made tea/hr depending on the degree of wither and the design of the machine.

    c) Capacity Test:

    i)  Stabilize the exhaust temperature, inlet temperature and time through dryer.

    ii) Weigh all fermented leaf, before feeding it into dryer for 60 mms. starting and finishing without reserve leaf at spreader.

    iii) Weigh all dryer mouth tea for 60 mins.

    Dryer capacity = (ii) - (iii) Kg of water evaporated.  

    Dryer trays

    3 kinds of perforations:

    Bold 9-11  SWG
    2.8-3.2 mm
    Medium 13-14 SWG
    2.2-2.5 mm
    Fine 15-16 SWG
    1.6-1.8 mm

    Traditional designation, eg. 72/13, indicates 72 perforations/square inch, each of 13 SWG.

    Conventional Dryer :

      6 feet (180 cm) 4 feet (120 cm)
    Power for chamber drive HP 2 2  
                                           KW 1.5 1.5
    Fan                                  HP 12.5-15 10-12.5
                                           KW 9-11 7.5-9
    Floor space 4m x 13.5m 3m x 12.5m

    Fluid Bed Dryer

      3 feet (90 cm) 4 feet (120 cm)
    Power (Installed capacity)     
    Drying system      HP 23-28 33-56
                                KW 17-21 25-42
    Dust collection      HP 10 22.5
                                KW 7.5 16.8

    Moisture uptake

    a) At the dryer mouth  

    If not ventilated/cooled, the newly fired tea re-absorbs moisture, ‘sweats', 
    also called high temperature stewing.

    b) From ambient air during sorting etc:  

    In high temperature and high relative humidity (small wet bulb depression ) tea absorbs moisture rapidly to 8%, more slowly to 15%. Redrying, ‘final firing’ again reduce the moisture content to approx 3%.

    Fuel utilization

     a) Assuming leaf of average moisture content, 
         100 Kg = 78 Kg water, 22 Kg tea 
         Assuming 75% wither, the re water content is 
         53 Kg, or 2.40 Kg water for I Kg tea.

    Theoretically, it takes 600 kilo-calories to evaporate 1 kg water.

                        1440 kilo-calories x Kg tea made x 100 %
    %Efficiency= ----------------------------------------------------  
                         Kilo-cal/Kg fuel x Kg fuel used.

    b)Common fuel consumption figures per 1 Kg tea.

     i)   Conventional Dryer  

      Coal (Kg)
    Hand stoked
    Oil (1) Natural
    gas (m3)
      indirect Direct indirect Direct
    Drying only 1-1.10 0.3-0.4 0.5-0.6 -
    Including wither 1.15-1.25 0.4-0.5 0.6-0-7 0.50-0.85  

    ii)   Fluid Bed Dryer  

      Coal (Kg) Oil (1)  
    Drying only 0.39-0.70 0.17-0.20 0.17 Kg  

                                                    

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