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Wood Drying Procedures in a Kiln

Manufactured wood is used for a number of different products, from building construction plywood to a wood rocking chair. However, each wood piece contains a lot of natural moisture after being cut from a tree. That moisture must be removed before the wood is usable for consumers. Typically, wood is dried in a very large kiln with specific procedures for the best dry wood composition.

Drying Types
- Four kiln drying processes are available to wood manufacturers before distributing the wood product. Dehumidification kilns use hot air passing across the wood. The evaporating moisture from the wood exits the kiln as humid, hot air. The humid air passes across cold refrigeration coils. As the air passes, the humid air condenses, removing the majority of moisture. The treated, dry air enters the kiln again for more dehumidification. In contrast, a conventional kiln heats the wood by radiating heat from steam pipes within the kiln. The wood's moisture evaporates and exits through vents. Alternatively, solar drying uses the sun's rays for evaporation, whereas vacuum drying creates a high-speed drying process using concentrated air vacuums for removing moisture.
Moisture Removal Scheduling
- Regardless of the kiln drying process, moisture removal must be monitored throughout the entire procedure. Kiln temperatures must vary as the wood's moisture content decreases over time for preserving the wood's composition. A general drying schedule starts with green wood lumber within a relatively cool kiln, heated to about 110 degrees Fahrenheit. As the wood loses moisture, the temperature is slowly increased to approximately 160 degrees Fahrenheit for a 10-15 percent wood moisture level.
Wood Stress
- Wood stress occurs to both the internal and external fibers as the drying process removes moisture. Initially, the wood exhibits a stress-free composition since the moisture is evenly distributed within the lumber. However, the outside wood fibers lose moisture faster than the internal fibers. This dry exterior causes compression against the interior. As a result, the internal moisture is slowly drawn out, reducing the compression in the wood's center. The final dry wood has an even moisture level within all of its fibers.
Benefits
- Drying wood helps increase the lumber's strength, especially for construction building. Additionally, wood deterioration from insects and fungi is decreased with drying processes. The excess moisture adds weight to the wood. After the kiln drying process, the wood weighs less for easier transport.

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