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Would you like to add information to this article? Interested in writing or submitting an article? Have a question about this article? The Importance of Air Velocity in Drying Reprinted with permission from Modern Woodworking. By Dr. Fred M. Lamb
It is one of those “fundamental principles” that to efficiently and effectively dry lumber, you must move air of a uniform and controlled temperature and humidity and that air must pass uniformly over the surface of the lumber. In this capacity, airflow performs two major functions. First, it transfers the heat energy to the wood in order to evaporate the water. Secondly, the air carries away the evaporated water vapor from the surface of the lumber. Air velocity and drying rate
Figure 1 shows the relationship between drying rate and air velocity for different levels of moisture content of the wood. As the graph shows, at 60 percent moisture content, the drying rate increases with increasing air velocity. At 20 percent moisture content, drying rate is generally unaffected by air velocity increases. The practical use of air velocity
As the air moves across a stack of lumber, its temperature decreases and its relative humidity increases. This temperature drop and relative humidity rise depends on the volume of airflow. As the velocity increases, there is an increase in volume of airflow through the sticker spaces. If there is more volume of air moving across the lumber surface, there will be a smaller temperature drop and a smaller relative humidity rise. With a lower volume of air, the reverse is true. The volume of air is related to both velocity and sticker thickness: the higher the air velocity, the more volume of air across the lumber, and the thicker the stickers, the more volume of air. However, it must be remembered, that for a constant volume of air supplied by the fans, an increase in sticker thickness will cause a reduction in the air velocity across the lumber. This will reduce the drying rate, but will make for a more uniform drying throughout the load. A lower relative humidity may be needed to offset the lower air velocity to maintain an appropriate drying rate. It is this relationship between relative humidity and air velocity that is extremely important and often overlooked. At a given air temperature, a low air velocity may require a lower relative humidity (larger wet bulb depression) to increase the drying rate to the required level. Conversely, a high air velocity may require a higher relative humidity (smaller wet bulb depression) to reduce the drying rate to the appropriate level. If the volume of air across the load is too low, the air quickly increases in relative humidity. This creates a drying gradient across the load. Long fan reversal times aggravate this problem and can result in an increase in checking. The solution is to use shorter fan reversal times: two-hour reversal is best, three-hour is satisfactory, and four-hour reversal is the maximum that should be used. Loading and baffling
What is the “best” air velocity?
Fred M. Lamb is Professor and Extension Specialist, Department of Wood Science and Forest Products, Brooks Forest Products Center, Virginia Tech, Blacksburg, Virginia. Dr. Lamb can be contacted at 540-231-7256; Fax: 540-231-8868 Reprinted with permission from Modern Woodworking. Have you reviewed the related Knowledge Base areas below? 
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