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Kiln Schedules
For a given temperature and relative humidity, the final EMC of wood will be achieved slowly, but by periodically changing (increasing the temperature and decreasing the relative humidity) the final EMC of wood can be achieved much more rapidly. Kiln drying of timber therefore involves a group, usually between 5 and 8, of temperature and humidity conditions designed to maximise the rate of drying without causing drying degrade. A group of such temperature and humidity conditions is termed a kiln schedule. One definition of a kiln schedule is as follows;

" A drying schedule is a set of directions for the operation of a kiln during the drying period and is usually presented in the form of tables showing the temperatures and humidities to be used at various stages of the process. These schedules vary with species, size and grade of timber being dried; they are also influenced by local production and selling practices, by the degree of care in kiln operation and in kiln characteristics".(Typical Kiln Schedule)

The decision to move from one set of temperature and humidity conditions to another is usually based on experience or on measurements of the moisture content of samples placed within the kiln.

Kiln schedules for hardwoods are governed by the need to minimise drying degrade. As a result of this the pattern of drying of hardwoods is as follows (see figure below);

The drying pattern of a green hardwood

The drying pattern of a green hardwood. (Pratt 1974)
For enlarged picture

  • A period of degrade control when the wet bulb temperature is high. These conditions are maintained until the wood has lost about one third of its moisture.
  • A period of accelerated drying. This period begins when all of the free water has evaporated (FSP) and is signified by the raising of the dry bulb temperature, (dry bulb temperature raised as high as possible, wet bulb temperature low).
  • An equalising period to secure uniformity of moisture distribution. During this period the wetter material is allowed to go on drying, whereas the dry material stops drying, (dry bulb temperature decreased, wet bulb temperature raised).
  • A conditioning period to obtain stress free wood (re wetting period to avoid or relieve case hardening). A typical stress relief treatment will be carried out at a relative humidity of 90% with the temperature increased 10-15 degrees centigrade above the final schedule.

Certain hardwoods notably the eucalypts are prone to collapse. Collapse leads to abnormal shrinkage and a form of surface degrade called washboarding. The degrade resulting from collapse can be recovered or partially recovered if the wood is reconditioned after drying. Reconditioning which was first used commercially in Australia consists of heating the timber to a temperature of 100 degrees centigrade at a relative humidity of 100%. The treatment if applied for 4-8 hours, depending on species, thickness and degree of collapse, will usually cause most of the shrunken cells to resume their normal shape.

Permeable softwoods such as radiata pine can be dried much more rapidly without the need for a preliminary period of degrade control


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