Kilns and Kiln Firing
Technical Data Part II

• Total Plant Concepts
  Shorter firing cycles, better heat distribution, better insulation, better car and cassette design, better loading and unloading facility design etc.. all contribute to savings at the expense of higher initial capital cost. Throughputs of 20,000 to 45,000 pieces or more per day justify intense research on these concepts. Annual rates of 200,000 tons are noted.

  Of course it is critical for economy of scale that these plants are run frequently without stoppages. The burners are typically turned down not off although rapid heat ups are possible from cold in the furnace zone but the extra heat shock and heat loss in the cars etc.. can be costly if the plant is stopped. Shifts of 7 day, 20 hours per day are common. Cars are typically good RI bricks with fiber covering wherever possible. In some countries, bricks would need to be redesigned due to the lack of surface area available to exhaust the water and the carbon. Maximum carbon content would be around 1.5 to 3% with free silica less than 25%. Handling of the product deserves attention to minimize breakage's and excessive handling. The loading and unloading from the extruded to the drying car is done by either suction pods or silicon type "fingers".

  There is a "buffer" section between the manufacture, dryer and kiln and this provides a safety net to ensure the process is not stopped or all the parts are sufficiently dried. The buffer can be extra heating or a mechanism to off load a faulty cart. To ensure the products are not out of line, infrared detectors are used to line up the cars and reject loads before entering the dryer.

  The modern plant and both roller hearth and tunnel kilns are very expensive and also call for a continuous expenditure program on the maintenance of the apparatus. High tabor costs justify this investment in high wage countries. Minimum throughputs are required and many smaller plants are shutting, merging or committing to this upgrade as their competitors achieve greater economies of scale and higher efficiencies.
  
  

• Remote Area Plants
  A plant in a remote area needs to have the following desirable features:

  • Minimal capital cost
  • Low maintenance equipment
  • Low thermal mass
  • Low transport costs
  • Easy serviceability
  • High efficiency
  • Greater flexibility than larger plants firing a variety of products, with shorter working cycles
    
    
  As tabor costs will be expected to be lower, it is possible to achieve similar economies with modern plants by utilizing"older" technologies. Intermittent kiln designs such as the single or dual truck or portable cover would fulfill the above requirements. The burners would still need to be sealed higher velocity types to achieve consistent product and accurate temperature control and combustion product monitoring would be desirable. Recirculation of the exhaust heat to recuperators, regenerators and/or for drying must be installed. Gas firing should be considered as oils or other products would increase maintenance, produce inefficient firing and be more difficult to control. A typical batch type firing using high velocity burners could be to 950° C in 2-3 hours, to 1150° C in 3 hours, hold for 1 hour then down to 650° C. The efficiency of the plant is paramount and if a second car can be loaded and brought back to temperature as quickly as possible there will be less heat loss. Flue exits at 120° C with kiln exit at 40° C. Loading and unloading facilities and conveyor systems are critical for more consistent product and rapid kiln firing turnarounds. Rapid die changes for various product type should be considered.

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