To dry wood the surrounding air must be sufficiently dry so as to absorb its moisture. This can be accomplished either by ventilating or heating the kiln air. During the first stage of the drying cycle, air easily absorbs the moisture and relative humidity inside the chamber may keep close to 100%, with water often beading on the walls in a based-greenhouse structure. As the process evolves wood moisture expelling turns increasingly difficult, mainly due to the low diffusion speed of moisture in wood. At a final stage, when all free water has been lost, only cell bonded moisture is left to be extracted. This final stage is more time and energy consuming, since it requires additional energy supply to break the bonds. Quality regards are present in the intermediate and final drying stages. In this process temperature, relative humidity and wood moisture content are the most relevant quantities [2-3Conventional drying

The main purpose of lumber air drying is to evaporate as much water as possible before end use or transfer to a kiln drier. Air drying can usually proceed until wood moisture content attains 25% to 20%. Another drying methodology must follow if a lower target value is desired. Air drying saves energy costs and reduces required dry kiln capacity, but presents the usual limitations of an uncontrolled process: in winter months drying rates cold be very slow, particularly in raining periods. By other hand under summer hot dry winds wood quality may be degraded as a result of surface shrinking and end splitting, due to severe differential drying (surface vs. interior). Another drawback of this method is the space and long time storage costs of wood stacks, implying large immobilization periods [4]

In kiln drying processes, higher temperatures and faster air circulation are used to considerably increase drying rate. Specific drying schedules/profiles have been developed to control temperature and relative humidity in accordance with the moisture content and stress situation within the wood, in order to minimize shrinkage-caused defects and improving quality. Conventional drying is one of the most expensive processes in wood industry, due to the enormous thermal energy expenditure

[3].