As a hygroscopic raw material, wood has the property of absorbing or releasing moisture depending on air humidity. Drying begins as soon as the trees have been felled. This drying process without the use of technically supplied energy is known as natural drying.
The thermal energy is provided by the outside temperature (sun). Ventilation is provided by wind. This requires the ambient relative humidity to be below 100%. The minimum wood moisture content that can be achieved by natural drying is approx. 13 - 15%.
As soon as technically supplied energy is used for wood drying, this is referred to as technical wood drying. This requires thermal energy for drying. In addition, it must be ensured that this thermal energy is supplied to the wood and that the moisture released in return is removed from the wood.
Depending on the temperature level during technical drying, a distinction is made between evaporation drying (below 100°C) or vaporisation drying (above 100°C).
In technical wood drying, thermal energy is supplied to the wood and the wood moisture is removed via a drying medium. The methods of wood drying differ in the way in which heat is supplied and moisture is removed. Heat is supplied either via the air or via a superheated steam.
The most important methods of drying wood are:
Fresh air/exhaust air drying uses air as the drying medium. The drying medium is then dehumidified by exchanging the warmer and more humid air from the drying kiln with colder, drier outside air. Variations in the management of fresh and exhaust air determine the efficiency of energy use.
Besides the thermally insulated kiln, the three main components in a fresh air/exhaust air drying kiln are:
In addition, the correct coordination of these assemblies in absolute terms and in relation to each other is essential for the overall performance of the system. The way in which the exhaust air is extracted and the fresh air is supplied determines the energy efficiency of the drying kiln.
Both thermal energy and electrical energy are used in a fresh and exhaust air kiln. The thermal energy is required on the one hand to heat the dry material and on the other hand to heat the fresh air drawn in. The majority of the electrical energy is required to operate the recirculated air (stack ventilation) and, in the case of active exhaust air, to operate the fresh and exhaust air fans.
While in wood processing companies the energy input is often related to the amount of dried wood (kWh / m³ wood), it would be more correct to relate the energy input to the amount of dried water. (kWh/kg water). Regardless of the amount of wood, energy is required for the actual amount of water to be dehumidified.
For example, drying dry main products requires less energy than drying much wetter side products.
Softwood typically dries more quickly than hardwood. The packages usually consist of edged (sharp-edged) boards and posts. In addition to the drying goal of "achieving a desired final moisture content" and "final moisture dispersion", the optimisation variants usually lie in the shortest possible drying time or the lowest possible energy consumption. Drying quality is not usually the highest priority. When drying softwood, the risk of drying errors is usually lower than when drying hardwood.
Typically, drying hardwood takes more time than drying softwood. The packages usually consist of unedged boards, posts or planks. In addition to the drying goal of "achieving a desired final moisture content" and "final moisture dispersion", the optimisation variants usually lie in achieving the highest possible drying quality. The drying time is usually not the highest priority. When drying hardwood, the risk of drying errors is usually higher than when drying softwood.