How does it work?
Super heated steam drying kiln in The Netherlands During the warming-up phase all air initially present in the kiln will be replaced by steam. The relative humidity of the air is during the warming-up phase kept at a maximum level close to 100 %, air vents are closed, but venting will occur based on the overpressure in the kiln resulting from the increase of temperature and the supply of steam. If the temperature of the drying medium approaches 100 °C the climate has changed into saturated steam. If the saturated steam is heated by the heaters in the kiln, the steam will become super heated and the free water inside the boards will start to boil. The temperature of the boards will stay at the boiling point of water, 100 °C, as long as the MC is above fibre saturation point. This boiling effect will cause a slight overpressure in the wood, comparable with the overpressure in a pan with boiling water causing a rattling lid. This internal overpressure speeds up the moisture transport from the core to the surface and consequently increases the drying rate.

Is the drying rate under control? 
The drying rate is determined by the energy supply to the wood. The energy supply depends on the air velocity in the kiln as well as on the wet bulb depression. So for example, with a super heated steam temperature of 110 °C, the wet bulb depression will be 10 °C. If the control system allows accurate temperature control in the kiln, then the optimal drying rate can be chosen by adjusting the steam temperature.

What will be the reduction in drying time?
In general the drying time will be reduced to 50 % of the regular drying time, which is used by the sawmill to achieve the required drying quality.

What happens to the timber quality? 
Since the temperature of the boards is above or equal to 100 °C, the wood becomes a little bit plastic. This will help to reduce initial stresses present in the boards. In combination with a top load on the stack, the reduction of, for example twist, is substantial. At these temperatures there is less weight of top load required to realise a substantial reduction of the twist in comparison to conventional drying. This is investigated in detail in the EU-funded project titled STRAIGHT.

What happens to the strength?
Color difference between conventionally dried (left) and steam dried timber (right) As long as the temperatures used during the drying process are below 120 °C the strength of the wood will remain unchanged.

What happens to the color? 
The effect steam drying on the color depends of the type of wood species. The color of planed Spruce is a slightly more yellow, as shown in photo alongside.

Can we save energy?
In a steam drying kiln there is no external ventilation with fresh air necessary to maintain the required wet bulb depression. This results in a considerable reduction of the energy consumption. Furthermore the reduction of drying time will reduce the amount of electricity for the fans substantially.