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• Kiln shells Most older kilns are of brick or concrete construction, but prefabricated kilns constructed of insulated aluminium panels are being increasingly used (see figure below). Aluminium built kilns offer the advantage that being light in weight they do not require heavy foundations.

  

  Prefabricated, aluminium high temperature kiln for AFI, NSW. (Windsor)

• Kiln Heating There are two ways of supplying the heat required for drying; indirect and direct. Indirect heating involves passing steam, hot water or oil through heating pipes, heat exchangers or radiators within the kiln. In NSW the majority of plants use indirect steam heating. Another form of indirect heating which is seldom used commercially due to its cost is electrical heating. With direct heating the air for the kiln is ducted through a controlled gas or oil flame in a burner. The air is therefore heated directly and will contain the products of combustion. Direct heating is however claimed to be more energy efficient than indirect heating where a proportion of the heat is lost at source. In NSW 12.5 % of plants use direct heating.

  

  Direct gas fired kiln at Tasman Lumber Co., NZ. (Windsor)

• Kiln Humidification The humidity in a kiln is raised by injecting steam through perforated pipes or by using atomised water sprays. Roof ventilators open when the humidity of the kiln is too high, ie., when the wet bulb temperature is exceeded, vents are opened to let some of the damp hot air escape and humidity is therefore lowered with the intake of cold air. Some kilns use heat pumps or dehumidifiers rather than venting to control kiln humidity

  

  High Temperature kiln chamber with reconditioning capability for Carter Holt Harvey, NZ. (Windsor)

• Air Circulation All modern compartment kilns contain an internal fan and nearly all of these fans have a reversible pitch allowing frequent reversal of air flow. Kilns are often classified as follows on the basis of the fans used to circulate air in the kiln;
  
  
  • Overhead Fans
  • Side Fans