High temperature timber drying is a kiln drying method that uses temperatures close to or above 100 °C, usually with superheated steam, to dry timber much more rapidly than conventional kiln drying.
When used correctly it can significantly reduce drying times and minimise distortion in suitable timber species, although it requires careful control and is only suitable for certain types of wood.
High temperature drying is particularly attractive for timber species that are prone to distortion during conventional drying because the wood can be dried more rapidly while still producing high-quality timber.
High temperature timber drying kilns can dry suitable species up to twice as rapidly as can conventional “Heat and Vent” type kilns.
The idea is to use the high temperatures to rapidly remove the water from the wood and also, importantly, plasticise the material – so that rather than “Heat and Vent” type equipment distorting in response to the drying stresses generated, the material “flows” in the manner of a heated plastic. The high operating temperatures and high humidities used in these high temperature timber drying kilns must be rapidly produced to reduce the chances of the timber being dried rapidly before plasticisation temperatures are attained. For similar reasons good air circulation must be ensured and the sizes of timber stacks are necessarily smaller than those used in conventional kilns.
Accurate monitoring and control of the drying conditions within high temperature timber drying kilns is vital.
It is vital that very high humidities are retained throughout the high temperature stage of the drying process otherwise the timber will be seriously damaged - water sprays may be used in addition to steam injection to produce the required relative humidities at the high temperatures used. In addition, only very permeable (predominantly softwood) species can be successfully dried by this method (See The Structures of Softwoods and Hardwoods and their effect on Wood Drying). Some discolouration of the timber can occur during high temperature drying, although this may not be of serious concern for certain uses of the timber. The sizes of timber stacks dried in these high temperature kilns are necessarily smaller than used in conventional drying kilns.
The most popular high temperature timber drying kilns are designed to operate below 100 °C, since the construction of this type is cheaper than those designed to operate at greater than 100 °C which require expensive, pressurised containers.
For an introductory understanding of the factors determining the removal of water from wood, and how these factors can be controlled to successfully dry timber without reducing the quality of the timber see: Factors Controlling the Drying of Wood).
If you'd like to explore other drying methods and their applications, these guides are a good next step.
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