Reprinted with permission from Modern Woodworking.
By Dr. Fred M. Lamb
The control of hardwood lumber drying in a dry kiln or similar drying chamber is generally accomplished by the use of drying schedules. Dry bulb and wet bulb temperatures (or some equivalent system depending on the equipment) are manipulated based on the moisture content of the controlling samples. Many operations use the traditional drying schedules as is, that is, without any adjustments or modifications. Other operations modify the traditional schedules somewhat to fit their operational or drying requirements.
However, although the second approach (modifying the schedules) is preferred, both are still conservative approaches to lumber drying. In fact, I believe that many operations dry too conservatively. This occurs because the following requirements are lacking:
• A clear vision by the company of what it expects of its drying operation.
• “Specific” guidance to the drying operation by the company regarding the required levels of drying quality and expected drying times.
• A clear definition of drying quality and of an effective, consistent quality control program to insure that the required quality is being met.
• Proper auxiliary equipment (ovens, balances, meters, etc.) required to operate an optimum drying operation.
• Proper and well maintained drying equipment (dry kilns, predryers, air-dry yards, sheds, etc).
• Proper drying records and record keeping procedures.
The traditional drying schedules are only guidelines. They are meant to provide the operator with “a place to start” or a procedure that “will generally work.” The real task for a good dry kiln operator is to take the traditional schedules and modify them to meet the needs and quality requirements of his or her operation. One way to effectively “adjust” the schedule or more correctly “optimize” the drying operation is to use drying rates to monitor and control the drying.
The concept of drying rates is based on the premise that for every species and thickness there is a safe rate at which the moisture can be removed. In other words, a rate at which the lumber can be dried with little or no significant degrade or damage. This rate is usually expressed in terms of percent moisture content loss per day. Exceeding the maximum safe daily rate increases the risk of additional drying defects.
Regarding drying rates and defects, generally everyone understands that “drying too fast” (that is, exceeding the safe drying rate) causes an increase in the risk of more drying defects especially splits, cracks, and checks. However, “drying too slow” (drying at a rate substantially below the safe rate) also causes a risk of drying defects. In this case, the risk is for an increase in warp, stain and uneven drying.
Thus, drying rates offer a gauge by which an operator can measure the performance of the drying operation. Table 1 shows some maximum daily safe drying rates for 4/4 lumber of several species. Drying rates also provide a method of estimating drying times. For example, if an operation is drying 4/4 upland red oak at a rate of 3 percent moisture loss per day and it is drying a load from 30 percent MC down to 7 percent, it would take almost eight days plus equalizing and conditioning to dry the load. Conversely, if it takes 23 days to dry that load (this is, 1 percent moisture loss per day), the operation may wish to re-evaluate its drying practices. It may be drying too slow and too conservatively. But… if the load contains lowland red oak, that may indeed be an appropriate drying rate.
This leads to the question of how to use drying rates. In general, drying at a rate greater than the maximum daily safe rate increases the risk of “drying too fast” and the associated drying defects. Conversely, drying too far below the maximum daily safe rate increases the risk of “drying too slow” and its associated drying defects. Many operations adjust their drying schedules to maintain an appropriate drying rate that is somewhere between these two extremes, depending on their drying philosophy.
4/4 Maximum Drying Rate
% Moisture Loss per Day
|Red Oak Upland||3.5|
|Red Oak Lowland||1.0 to 3.0|
|White Oak Upland||2.5|
|Table 1. Some daily maximum safe drying rates|
The quality of the lumber and the condition of the kilns also are important factors in determining the appropriate rate to use. Poor quality material (that is, pre-existing drying defects or damage) and poor drying equipment are not conducive to maintaining an aggressive drying rate. Furthermore, the appropriate drying rate may vary by load, depending on the quality and uniformity of the lumber. An operator may use a slightly lower drying rate on a load of lower quality, more variable, and more “unknown” lumber than he or she would on a load of higher quality, highly uniform material.
Fred M. Lamb is a professor and extension specialist at the 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.