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CTC Machinery

Process Mechanism and Objectives

The equipment completes the leaf maceration process partly accomplished in the upstream Rotorvane equipment. Maceration is most critical in case of Black CTC tea, as the oxidation must encompass the complete leaf even in twisted condition right up to the core during the fermentation process to ensure the needed liquor characteristics to develop in the final product.

The word C.T.C. stands for the actions of ‘Crush, Tear and Curl’, since crushing or macerating is the main objective and tearing is the design-inherent action, which actually takes place in this equipment leading to the cutting of the leaves for sizing.

The severe maceration action also initiates the micronisation action, which is to be completed in a controlled manner by some mechanism to achieve the intended reduction in size(s), as best possible. This is also to be followed immediately with the curling action on each of the cut particle to attain the desired shapes. All these three actions are to be arranged to be conducted one after the other in the same equipment, before discharge.

The final levels of maceration, size reduction and shaping to be achieved is not generally attained in a single stage activity, but through several stages of similar actions with preferably gradual increase in the intensity of different stresses applied on the leaf, so that the prescribed limits of resultant strains are not exceeded during the operation of this process.

The Final Maceration process objectives in CTC can therefore be summarized as following:

  • To ensure, mainly, that the desired appearance, or in actual fact, the shapes and sizes, of the final product particles, “sans” the incidental processing generated wastes, are achieved at the end of the drying process.
  • The cells are to be ruptured to allow all the juice consisting of the solids in solution with water and the ppo enzymes are fully (or at worst mostly) released from the body of the whole shoot.
  • The leaf shoot is micronized into smaller sized particles, which are also rolled into the desired shapes.
  • The micronized particles are fully wrapped all round with the released juice and ppo enzymes without allowing any amount of these to run off as waste.
  • And, all these are to be done within the upper critical boundary limit of Temperature during this process, most preferably without any moisture loss, so as not to jeopardize the yet inherent quality values of tea wrt it’s liqor properties.

Design Objectives

From the study of the process mechanism, as mentioned in above, the following main design objectives had been arrived at, for the process equipment.

  • The leaves are to be subjected to some crushing action, as a direct or in-direct result of the applied force(s), to release the juice, water and the enzymes etc from the leaf cells to the desired extent
  • The juice and water mixture released due to the crushing action is to be thoroughly wrapped up and absorbed by all the leaf particles by the end of the process, i.e. before being discharged out of the concerned stage equipment.
  • Reduce the size of individual leaf to several smaller ones by inducing some sort of shearing/tearing or cutting action
  • To impart desired twisting or curling action on the torn/cut particles for each of these to acquire the intended shapes, to be acceptably attractive in the market after drying.
  • The main force applied on the Rotorvaned leaf in CTC is the hammering one. The severity of applied maceration impact, types of size reduction and twisting/curling effects to be achieved would vary with the quality of the plucked leaf and also for different varieties of tea manufacture.
  • The CTC machine in the CTC tea manufacturing process plays a very critical role for both the quantity and most of the physical quality aspects or the so called “appearance” of the final product. Hence the performance of CTC machine is critical in tea making of CTC variety of tea and “it’s use or abuse would naturally affect accordingly it’s sale realisation price”.

Salient Features / Technical Specifications

  • For any hammering action “there comes the necessity of a resisting body like an anvil on which the material to be hammered would need to be placed to get hit by a swinging hammer”. In other words, there have to be a relative difference between the surface speeds of the anvil and the hammer.
  • There are two rollers in a CTC, which are rotating at different speeds, which achieved by gear boxes with two output shafts with a speed difference ratio of 1:10. The slow speed roller serves the purpose of the anvil for the hammering actions relative to the high speed roller and also brings the leaf under the hammer for the crushing and tearing etc work to be undertaken by the hammers on the high speed roller.
  • The frequency of hammering action per unit time, which is dependent on the surface speed differential between the rotating anvil and the hammer, would naturally determine the productivity of the action as well.
  • The main working components or the heart of a CTC cutting unit are two Rollers. Each roller mainly consists of a cylindrical mandrel on the surface of which certain numbers of 2” thick AISI 316L grade circular stainless steel segments of two inches in width and of 13” outside diameter with it’s surface concentricity being precision machined to very close tolerance limits are securely assembled side by side, with proper ‘interference (hot) fit’ for it’s whole working length of 36”, with measures to prevent any axial displacement on the mandrel during the operation. Any relative rotational motion between the consecutive segments is also prevented with the use of concentric steel pins inserted in holes made on the two mating sides of the consecutive segments.
  • 8 TPI V shaped annular parallel grooves of ‘British Standard Whitworth’ (BSW) thread specifications would be cut throughout the entire width of the Roller. 80 numbers of helical grooves would also be milled on the surface of the segments cut with these annular grooves on each roller with 40” ‘lead’. These helical grooves cut across the annular grooves would create a number of hammer shaped tooth on the Rollers at nearly 1:1 ratio of length between the shoulder & slope of the tooth.
  • The production capacity is directly proportional and the quality aspects are inversely proportional to the Surface Speed difference between the pair of meshed rollers. The surface speed, in turn, is directly proportional to the product of the diameter and the rpm of the roller. A surface speed differential of 1442 ft/min between a pair of Rollers of any diameter is assumed to be the best compromised one between the quality & quantity of the final tea.
  • The three cutting units are connected with two nos of Intermediate Conveyors wit one Feed and one Discharge conveyors to complete the flow of leaf. The drives to the conveyors are taken from the slow Roller non-gearbox ends through sprocket & Roller chain.