IMPORTANT NOTE

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• Directly Heated Kilns - Direct heating can be used in either a compartment (see figure below) or progressive kiln. The hot gases of combustion from an oil, wood or gas burner are blown through a duct running along the length of the kiln roof, and is then distributed evenly into the drying chamber through a series of inlet ports in the ceiling. This type of kiln is the most common type of kiln in Australia, both for low temperature drying of eucalypts and high temperature drying of pine.

  

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

  It is unlikely anymore kilns will be built to use oil because of the present high cost of this fuel. However, some systems have been developed to use the combustion gas from sawmill residue burned in a furnace (Mills 1991). This has obvious economic advantages, but has to be weighed up against alternate uses of this sawmill residue, ie., chip for particleboard or MDF production (eg. CSR Tumut) and landscape material.

  

  A variation of the compartment kiln known as the "screen" kiln. In this design the air flow is horizontal and only one large fan is required. Screens are incorporated in the design to give uniform air flow along the length of the charge (Mills 1991).

• Indirectly Heated Kilns - In indirectly heated kilns some portion of the circulating air is withdrawn through ducts, heated and then returned to the kiln. This heating is usually done using an air to air heat-exchanger. Hot flue gases pass through one side of the heat-exchanger and cooler kiln air over the other. This type is going out of favour due to problems with equipment and air flows (Mills 1991)

• Dehumidifier Kiln - These kilns often use the heat pump concept in which latent heat of evaporation from the air outside of the kiln is extracted via a refrigeration (air conditioning) unit and is used to raise the kiln operating temperature.(Mills 1991)

  

  Dehumidifier kiln. (Mills 1991)

• Solar Kiln - These kilns are becoming more popular for low temperature drying of eucalypts because of the significant operating cost advantages.

• Compartment of Batch Kilns - These units are still the most common type of kiln Australia, especially for use in high temperature drying of pines. The concept of this type of kiln is a batch of timber is loaded into the drying chamber and left stationary for the length of the drying schedule. The temperature, humidity and air velocity are altered during to the schedule to achieve the required drying rate. Advantages of this type of kiln are they are more flexible when drying different sized material, you can achieve higher temperatures and in some cases, reconditioning can be done in the same chamber, without moving the timber.

  

  Windsor CT kilns at Mininco, Chile. (Windsor)

  

  Twin high temperature kiln and reconditioning chamber under construction at Morewell, Vic. (Windsor)

• Predriers - These units are commonly used to dry eucalypt timber from the green state to fibre saturation point (aproximately 30% MC). They are common in the southern states (TAS & VIC) as air drying isn't as effective due to low annual temperatures (see figure below)

  

  Sectional elevation of a six line predrier (Mills 1991).

• Progressive Kilns - A CSIRO designed kiln, the progressive kiln is commomly used to dry eucalypt timber from the green state to EMC (approxately 12% MC). Over 30 of these kilns have been built in Tasmania and Victoria. The stacks are placed in the kiln length ways (stickers aligned across the kiln) and with the use of baffles, the air is directed through the stack in a zig-zag direction (see figure below). The hotter, drier air enters from the end of the kiln with the drier timber and comes out cooler and wetter at the infeed end of the kiln. The stacks are periodically moved through the kiln as their moisture content comes down. Air velocity is usually in the range of 0.3 to 0.6 m/s.

  

  Progressive kiln. Baffles along the plenum space redirect air flow through the timber stacks (Mills 1991).
  For enlarged picture

• Tunnel Kilns - This type works on the same principles as a progressive kiln, except the stacks are placed across the length of the kiln (stickers aligned length ways) and the air passes straight down the length of the kiln.

  

  Tunnel Kiln (Mills 1991).
  For enlarged picture

• Microwave Kiln - These units are still in the research stage.

• Vacuum Radio Frequency Kiln - These units are still in the research stage.