During the pre-heating phase in the steam drying kiln all air initially present within the kiln will be replaced by steam. The relative humidity of the air during this phase is kept at a maximum level close to 100% saturation. An overpressure valve prevents overpressure in the kiln, which occurs due to the increase of temperature and the supply of steam.
If the temperature of the drying medium approaches 100°C, the climate will have changed into saturated steam. If the saturated steam is heated within the kiln, the steam will become super heated and the free water inside the timber will start to boil. The temperature of the timber will stay at the boiling point of water (100°C), as long as the moisture content is above the timbers fibre saturation point. This boiling effect will cause a slight overpressure within the wood. This internal overpressure speeds up the moisture transport from the core to the surface of the timber and consequently increases the drying rate.
In general the drying time will be reduced with approx. 50% in comparison to the regular drying process. The drying quality will be maintained at the same level.