Kilns and Kiln Firing
Technical Data Part II

Industrial Firing Processes

This section comprises a very brief overview of industrial ceramic firing methods. The information mostly relates to larger plants firing bricks and tiles. The most up to date information is available from your local industrial kiln manufacturer or technical ceramic sites on the web, listed on the Industrial Ceramics page.

If you are require faster turnaround times and high throughputs, the best approach is to fire with forced air burners. This system utilizes sealed, usually high velocity, burners fed with clean gas and pressurized air from an air blower. There is a design in a well known ceramic paperback that uses a single forced air burner on a small batch type studio kiln but typically, for capital cost recovery, the kiln would need to be a minimum of 3-5 m3 or was fired every second day to consider this application.

• Forced air burners
  produce:
  • Fast firings
  • Accurate control of the air and gas ratio through the firing. (This is due to non reliance on atmospheric primary air for initial combustion and secondary air through flue draw to complete it.)
  • High efficiencies through preheating combustion air, reuse of flue gasses for dryers and rapid firings
  • High velocity convection forcing the water from the product and evenly heating all sections of a load
    
    
  Forced air burners, due to the capability of forcing relatively high volumes of combustion products into the kiln space in a short time, must have rapid shut down safety systems. It is this requirement that can increase the cost together with fully automatic start, programmable temperature controllers and accurate burner control valves that are usually specified for maximum benefit.
  
  
• Die Composition
  As expected ceramic compositions and development of ceramics is advancing apace. Covering of wear prone parts such as auger and mixer blades and press dies with ceramic materials is showing promise. Fully ceramic parts or more widespread use of PSZ should be commercially viable by now. Compositions of zircon and alumina silicates show promise in certain areas.
  
  Dies are either the usual high chrome steel or newer ceramic coated for longer life. Steel dies would be able to be machined 5 or 6 times prior to replacement, the ceramic dies usually have several pieces and can last much longer.
  
  
• Dryers
  Thorough drying of the product is crucial for fast firing. Most industrial processes use waste heat from the kiln however a heat boost is required in the drying section. It would follow that the efficiency of the kiln process might be lower than is possible if a boost is not required as there would be excessive heat loss from the kiln section. If excessive water vapor is formed in the drying, increase the heat to overcome or increase the recirculating air volume.
  The usual drying arrangement uses air fans and gas ducted burners to move the heated air (typically 110° C) through the product. Jet drying is favored by some and this consists of higher pressures (typically 8 meter/sec.) forcing the heated air through a slotted side wall in the dryer. In some the slotted wall moves back and forth to distribute the air even more thoroughly. An ideal drying system would have the air delivery arranged to suit the product i.e. horizontal slots for thinner tiles.
  
  

• Fast Firing
  Most modern high throughput tile or brick kilns are tunnel kilns. These have been around for a while now and replaced the earlier "Hoffman" kilns (continuous kilns relying on continuous draw of combustion products through the load) as these did not give the efficiencies or evenness of temperature required for modern processes.
  
  The German Keller company uses tunnel kilns with multiple high velocity burners, special "cassettes" and cars designed to hold tiles and other products to allow efficient heat transfer and convection. Keller provides a total approach with savings on staff, work area taken into the total savings figure. Typical firings with multilayer stacks (typically 12 tiles high and wide) are approx. 8 hours versus the usual 20 or more hours in previous tunnel kiln plants although these vary from product to product.
  
  The Italian Mori company pioneered the "roller kiln" using ceramic rollers to propel the product through the kiln in a single layer at fast speeds. This is a common concept in the steel industry but has developed in the heavy clay area as cheaper and stronger ceramics have become available. Typically 2-3 hour firings for tiles would be expected with 30 minute firings for domestic ware. The key is in the kiln design, the number of burners and placement for extremely even temperatures (the product must be heated at the same rate) and, most importantly, the composition of the body. This is formulated to withstand the stress and the rapid cool to hot to cool cycle. Mori would supply two kilns if more product was required but the newer Mori concept has several layers of shelves moving on rollers located on the kiln side in between three burner layers. To support the weight across the 1.5 or 2 meter width the tile cassette is made from exotic alloys. This idea may not be commercially viable yet due to the high alloy cost.
  
  Both applications utilize many high velocity burners located above and below the stack for minimum temperature differential. Typically a kiln of this design would have 2-4° C difference in any part of the firing area at 1160° C. These tunnel kilns also have quite low stacks typically 600-750 mm high to achieve rapid transfer to the load. Low thermal mass insulation or interlocking refractory tiles are used and the impression of the kiln is quite small due to the smaller casing size and the reduced length of the tunnel. There is rapid forced cooling at the tunnel end with an air curtain to contain the heat loss. Flues are typically under the cars towards the drying end with facility for recirculating.
  
  

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