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Wood Drying Methods If the major objective of drying lumber is to make money, then the lumber must be dried with minimal development of degrade. Degrade includes splits, checks, warp, discoloration, and any other change in wood that lowers the wood's usefulness and therefore its value. Often degrade losses exceed all other operating costs in drying! For example, it costs approximately $3 per day per MBF to operate a dry kiln with oak lumber - saving one day in drying time will save $3 per MBF in operating costs. But if degrade increases $3 per MBF due to faster drying (and a $3 increase is certainly a small amount of degrade), there is no savings in faster drying. There are basically five conventional methods of drying in use today - air drying, forced-air air-drying, low temperature drying (including both warehouse predrying and dehumidification drying), conventional kiln drying, and high temperature drying. These methods each have their own characteristics, which in turn affect performance and costs for difference species and thicknesses. These characteristics are briefly summarized below for all but high temperature drying; high temperature does not have any practicality for hardwoods today. In addition to these five methods, vacuum drying also has some industrial practicality, especially for drying squares and for smaller production requirements. It is important to appreciate the characteristics of a drying system, as many times drying quality and costs are limited by the system and not by the controls or operator's procedures. With the proper equipment and tools and knowing how to use them, drying costs can be minimized and drying degrade will be under 2%. The following discussion presents a summary of the various drying methods. These data were obtained from the author's experience and from several publications, including Drying Oak Lumber by Gene Wengert from the University of Wisconsin, Drying Eastern Hardwood Lumber, USDA Agricultural Handbook No. 520, and Opportunities for Dehumidification Drying from the Virginia Lumber Manufacturers Association. Air Drying Air drying is the exposure of lumber to outside environmental conditions, with no artificial heating, control of velocity, or control of humidity. The lumber may be protected from the rain and sunlight with a small roof on each pile of lumber or by sheds. Sheds greatly reduce degrade. The details of air drying are covered very well in USDA Handbook No. 402, Air Drying of Lumber written by Ray Rietz. High degrade risks make this method very expensive for degrade prone species and thicknesses. However, sheds can reduce the degrade substantially and make it possible to economically dry species such as oak. In fact, shed air-drying is often the best (most economical) method of drying thick oak. 8PDF Image | Principles and Practices of Drying Lumber
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