9 PRINCIP The timber is dried through water vaporisation induced by boiling in an environment with temperatures of 100°C (212°F) and above. The timber is loaded onto trolleys that are then either pushed or pulled on rails into the kiln. Via fans, air is blown through the timber in the direction that is opposite to that of transport. Spraying with saturated steam ensures rapid heating. The moisture gradient (dry surfaces and a relatively moist centre) is equalised by conditioning. Photo: Martinsons, Jonas Westling Building system and fans. To withstand the high temperatures, steam pressure and evaporation rate, the drying facility is built in well-insulated steamtight stainless steel. All sensitive equipment is located in the building’s ventilated “cold attic”. The sturdily housed fan motors are air-cooled. Furthermore, the kiln has a loading/ unloading solution for streamlined pulling – timber is loaded onto trolleys that are fed in via a rail-based system. Control of drying. Drying is primarily controlled by regulating the heat input and selecting a drying temperature. This means that evaporation can be controlled to achieve the desired final results. In the final phase of drying, the process can also be controlled via hygrometric differences. Moisture equalisation through conditioning. After drying in a high-temperature kiln, timber has a relatively large moisture gradient. The surfaces are dry and the centre relatively moist. There is compression stress at the surface and tensile stress in the centre. When drying has finished, these differences are equalised through conditioning. Because of the short drying time, the air-blow depth in a high-temperature kiln should not be more than 4 metres. This allows two normal-size timber packages to be dried side by side. Volume and target moisture content. Valutec’s high-temperature kilns are particularly adapted for planks and poles, but can also be used for boards. When requirements in respect of moisture content variation and the final quality of the timber permit, an alternative drying method that offers extremely high productivity can also be chosen. Drying in a high-temperature kiln is considerably faster than in other timber kilns. To give just one example, the drying time for 50 mm timber is around 24 hours. With drying temperatures up to 140°C (285°F), water is vaporised through boiling and thus dissipates more quickly than in ordinary drying.
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