In a living tree, vast amounts of water are transported from the roots to the leaves or needles by a complex system of fibers. Fibers make up more than 90 percent of the volume of the tree and when they are fully developed they die. This means that almost all the cells in the tree are dead. Water and water vapor then move into the fiber cavities. A live tree can actually hold more water than wood.
If you cut down the tree, water disappears through various natural processes. However, there is always a certain amount of water in the timber, which affects the wood’s raw material density and thereby properties such as density, strength, hardness, shrinkage and swelling. Wood used in furniture, joinery, and construction products is normally hygroscopic, meaning that it can absorb and release water.
When manufacturing wood products, it’s important that the material’s moisture content is consistent with that of its future environment. If the moisture is lower or higher than the surrounding environment, the wood can deform with time.
Let’s return to the timber, which has now been cut. It contains a lot of water, and if drying isn’t properly controlled it will be damaged. If the wood is too damp, microorganisms may attack. If it’s too dry, it will shrink and crack, and its mechanical properties may be affected.
In the past, wood was dried outdoors or in airy halls. The planks were layered over some form of bedding to permit good air circulation. Air-drying is of course dependent on weather and season. If it’s rainy and cold, drying takes place too slowly, and if it is too hot it can result in cracks and other quality problems. Air-drying can take months or years.
These days, it’s common for wood to be dried under carefully controlled conditions in special kilns at high temperatures. The goal is to lower the moisture content of the wood without it cracking, and to achieve optimal moisture content so that it can be stored or transported without damage. The key issue with drying is to ensure that evaporation from the surface does not occur faster than the water seeps out from the center. To dry wood in this fashion achieves better quality and avoids the unnecessary transportation of water to the end user.
A kiln chamber is a common way of drying wood. In Sweden there are approximately 1,500 such facilities. Normally, wood with a moisture quotient of 60 to 120 percent is dried to a level of 8-18 percent. And 120 cubic meters of timber, with a moisture quotient of 75 percent, needs to evaporate 32 cubic meters of water to reach a moisture quotient of 12 percent. This is equivalent to 80 bathtubs full of water that must evaporate during a five-day period. The amount of energy consumed is equivalent to the annual consumption of two single-family houses.
Major electricity consumers
A drying chamber has large fans that blow heated air through the timber load. These fans use a lot of electricity, often half as much as an average a sawmill. Alent Drying has highlighted energy issues around the drying process, and the company has developed a patented technology to dry wood.
“Several hundred drying operations have shown that electricity consumption in the kilns can be reduced on the order of 100,000 kWh per dry year,” said Eric Björkman, CEO of the company. “In Sweden, there is a potential to save 0.1 TWh of electricity per year.”
“The method is based on exposing the wood to hard drying for a limited amount of time, and then to stop the circulation fans in the dryer chamber to allow the timber to recover. Moisture migration from the timber’s interior to the surface continues as the fans are standing still, while the air layer closest to the surface is saturated with moisture. This keeps the wood surface fresh and waterways open. Drying is both gentler and quicker. This is an oscillating drying method. We call it interval drying, or the Alent Pump. We can reduce electricity consumption by half and at the same time increase production while maintaining quality wood,” says Björkman.
“In our system, a number of dryers are synchronized smoothly, which limits electricity usage and provides cheaper electricity contracts.”
Alent Drying has patented condensation panels that use the incoming fresh air as a coolant for the precipitation of moisture from the chamber in the dryer circulating air. The need for ventilation decreases while the incoming air is heated.
“We are now working to strengthen the Alent Pump with condensation panels and advanced control technology, which will save more energy and lift drying technology one step further,” concludes Björkman.
Article published in May 2011