High temperature predrying of wood

Reduction in energy consumption is a prime consideration of the forest products industry. Since drying consumes up to 70 percent of the total energy in manufacturing most wood products, savings in energy during drying would have a significant impact on the total energy required to produce wood products. Lumber is usually dried in large kilns at temperatures below 180°F. Energy is provided indirectly by steam or directly by fuel fired in the kiln. Humidity is controlled in these kilns by venting the humid air directly to the atmosphere. This vented air contains considerable energy, but because of its low temperature only limited attempts have been made to recover energy from it. At the Forestry Sciences Laboratory in Carbondale, Illinois, we have been investigating new methods for drying lumber. One such method, jet drying, uses impinging jets of air at high temperatures to rapidly remove moisture from wood (6). This paper will discuss a drying method whereby green lumber is jet-dried at a high temperature for a short period before it is kiln-dried to a final desired moisture content. We will show how heat from the vent air of the jet predryer can be recovered in the kiln to save energy in the drying process. Predrying of wood (the removal of moisture from green lumber prior to kiln drying) has generally been done at temperatures below 100°F. The simplest type of predrying is air drying, which is slow and unpredictable and necessitates keeping large lumber inventories. Several companies have installed predryers, which are basically large sheds with a circulation fan and sufficient heat capacity to keep the temperature from falling below 70 to 80°F in the shed (1,4). In some cases these sheds are heated by exhaust from conventional kilns. The lumber of certain species such as soft maple, yellowpoplar, and southern pine is capable of being jet-dried at temperatures above 250°F for a short time without significant degrade to the wood (6). A typical drying curve for 1.75-inchthick southern pine dried at 300°F (Figure 1) shows that almost half of the moisture in the wood is removed in only 1 hour. The rapid rate of moisture removal from the wood enables the wet-bulb temperature of the air in the jet dryer to be maintained at a high level (above 180°F). Thus, vent air from the jet dryer with temperature above 250°F and a high wet-bulb temperature is a good source from which to recover energy.