Recently interest has been shown in drying timber at very high temperatures (close to or above 100C, using superheated steam). These high temperature timber drying systems are therefore operating at temperatures much higher than used in conventional timber drying kilns This can be especially attractive when drying timber species which are very prone to distortion on drying, and consequently dried timber of high quality can be produced. High temperature timber drying kilns can dry suitable species up to twice as rapidly as can conventional “Heat and Vent” type kilns. It is however, 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 stock can occur when high temperature timber drying, although this may not be of serious concern for certain uses of the timber. (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: “Heat and Vent” type equipmentFactors Controlling the Drying of Wood).
The intention 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 equipmentdistorting 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 dried in these high temperature kilns are necessarily smaller than used in conventional drying kilns. In addition, accurate monitoring and control of the drying conditions within high temperature timber drying kilns is vital. The most popular high temperature timber drying kilns are designed to operate at less than 100C, since the construction of this type is cheaper than those designed to operate at greater than 100C which require expensive, pressurised containers.