A look at redry

Lumber that is not adequately dry after leaving the kiln has typically been treated as a nuisance in most kiln drying operations. Since few mills are adequately equipped to remove wet lumber and rehandle it, its nuisance status is understood. As a result of this situation, it is common practice to dry the lumber low enough so that little or no redry is generated. Let us look at the consequences of these drying procedures. While the redry problem is eliminated, we are simultaneously affecting the quality of the lumber. Lumber degrades in relationship to its loss of moisture so that as the moisture content is reduced, the amount of degrade increases. Extensive studies over the past several years within the Weyerhaeuser Company have shown that the rate of degrade for softwood dimension lumber is in the order of magnitude of 1 to 3 dollars per thousand board feet for every 1% of moisture lost in our normal drying range. That means that if a thousand board feet of dimension lumber were dried from 20 to 10% M.G. , the value could be reduced 10 to 30 dollars. These numbers are based on lumber prices almost a year old. In today's higher priced market, the losses could be even greater. Minimizing total costs including the indirect costs of degrade. should be of concern to the kiln operator. With this magnitude of loss in value, are we perhaps fooling ourselves in drying an entire charge of lumber so that few or no pieces exceed the 19% moisture content limit? Eventually the lumber must be dried below 19% M. C. in order to meet the grading rule specifications. If lumber all started out at a uniform moisture content and dried at a uniform rate, the maximum return could be achieved by drying it to just 19% M. C. Unfortunately, lumber neither starts at uniform conditions nor dries uniformly. Consequently, we are always faced with a range of moisture contents in our final product. A typical moisture content distribution after drying is shown in Figure 1. Given enough time, a very much tighter distribution could be achieved by allowing the lumber to equalize using a long equalizing kiln schedule. For dimension lumber, such an approach is impractical. The moisture content distribution is usually similar from run to run for a particular kiln and a set of conditions which includes the drying schedule, the species, the type of lumber, and the lumber stacking procedures. A significant change in any of these conditions can produce a change in the shape of the moisture distribution. Since it is the upper part of the distribution above 19% M. C. that represents the redry material, it is important that we see how this changes as the lumber dries. By sampling moisture content from a good many units dried to various final average moisture contents, a curve of the type shown in Figure 2 can be generated. This particular curve represents data on Douglas-fir dimension of various widths dried at one