United States Department of Agriculture Method to Estimate Forest Service Forest Dry-Kiln Schedules Products Laboratory Research and Species Groupings Paper FPL–RP–548 Tropical and Temperate Hardwoods William T. Simpson Abstract Contents Dry-kiln schedules have been developed for many wood Page species. However, one problem is that many, especially tropical species, have no recommended schedule. Another Introduction................................................................1 problem in drying tropical species is the lack of a way to Estimation of Kiln Schedules.........................................1 group them when it is impractical to fill a kiln with a single Background .............................................................1 species. This report investigates the possibility of estimating kiln schedules and grouping species for drying using basic Related Research...................................................1 specific gravity as the primary variable for prediction and grouping. In this study, kiln schedules were estimated by Current Kiln Schedules ..........................................1 establishing least squares relationships between schedule Method of Schedule Estimation...................................2 parameters and basic specific gravity. These relationships were then applied to estimate schedules for 3,237 species Estimation of Initial Conditions ..............................2 from Africa, Asia and Oceana, and Latin America. Nine drying groups were established, based on intervals of specific Estimation of Schedule Step Changes.......................4 gravity where drying times were similar, with the appropriate Calculation of Kiln Schedules .................................5 schedule applied to all members of the group. Estimation of Tropical and Temperate Schedules ............5 Keywords: tropical species, kiln schedule, kiln drying, dry kiln Species Grouping Method.............................................6 Background .............................................................6 Estimation of Drying Time........................................7 Kiln Schedules for Groups .........................................7 Conclusions .............................................................10 Literature Cited.........................................................12 Appendix A—Specific Gravity Conversion....................14 Appendix B—Method to Estimate Drying Time.............14 Appendix C—Species Data for Kiln Drying...................14 May 1996 Simpson, William T. 1996. Method to estimate dry-kiln schedules and species groupings: Tropical and temperate hardwoods. Res. Pap. FPL–RP–548. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 57 p. 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USDA is an equal employment opportunity employer. Method to Estimate Dry-Kiln Schedules and Species Groupings Tropical and Temperate Hardwoods William T. Simpson, Research Forest Products Technologist Forest Products Laboratory, Madison, Wisconsin Introduction Related Research Hisada and Sato (1976), Hisada and others (1986), and The large number of tree species in the world, especially in Durand (1985) conducted analyses to relate known kiln the tropics, presents two problems in drying lumber. Some schedules of Southeast Asian and African species to physical species have been used for timber products for decades, and properties. They developed multiple linear regressions be- dry-kiln schedules for these species have been recommended tween several kiln schedule parameters as the dependent based on experience and research . However, recommended variables and physical properties as the independent vari- kiln schedules are not available for many less-utilized spe- ables. Specific gravity was found to have the largest influence cies, especially tropical species. on schedule parameters. In general, they found good agree- ment between the recommended and estimated schedules, but A second problem stems from the sheer number of tropical noted that the estimated schedule for some species deviated species and their heterogeneous occurrence in the forest. As a considerably from the recommended schedule. Jankowski consequence, it is not always practical to fill a dry kiln with (1992) developed a rapid and simple test to indicate the a single species. The problem is the lack of a method to probable performance of wood species in kiln drying by group species, based on similar drying characteristics, so that comparing the test performance of six known species in species can be mixed and dried together in the same dry kiln. relation to their recommended schedule. Results showed that the selected schedules were adequate for the species tested. The purpose of this study was to develop and evaluate a method to solve these two problems. The first part of the Current Kiln Schedules report develops a method to estimate a kiln schedule for a species when a recommended schedule is not available; the For many years, kiln schedules for temperate and tropical second part of the report builds on the first part to develop a hardwoods have been developed by numerous people in species grouping method. research institutions and industry throughout the world. Many schedules are summarized in Hildebrand (1970), Pratt and Turner (1986), Boone and others (1988), and USDA Estimation of Kiln Schedules (1991). These schedules are recommended as conservative Background starting points—a safe reference to be adjusted upward in severity with experience. A typical kiln schedule consists of The strategy in selecting a kiln schedule is finding the right several steps. An initial dry-bulb temperature and initial wet- compromise between the desire to dry as fast as possible, bulb depression begin the drying at some green moisture which calls for severe drying conditions in the form of high content. These conditions are held until a predetermined dry-bulb temperatures and large wet-bulb depressions (low moisture content level is reached; then, the first change relative humidity), and the desire to minimize drying defects, (increase) is made in the wet-bulb depression. At subsequent such as surface checks and honeycomb, by using lower tem- moisture content levels during drying, additional increases in peratures and smaller wet-bulb depressions. Several wood the wet-bulb depression are made. During the latter stages of properties are associated with the allowable severity of a kiln the kiln schedule, increases are made in both the dry-bulb schedule, properties such as specific gravity, shrinkage, and temperature and the wet-bulb depression. A typical kiln mechanical properties perpendicular to the grain. Specific schedule for meranti (Shorea spp.), consisting of eight steps, gravity is the only property that is widely reported in the is given in Table 1 (Boone and others 1988). literature, so our analysis is confined to specific gravity. Table 1—Typical kiln schedule for hardwood lumber, in appropriate even though it is not optimized. For these this case meranti (Shorea spp.) (Boone and others 1988) reasons and the general lack of knowledge about criteria that Temperature Temperature led to schedule recommendation, it does not seem justifiable (°C) (°F) to apply statistical analyses that lead to probability-based Moisture Relative inferences on the credibility of schedule estimates. The pro- content Dry- Wet- EMCa humidity Dry- Wet- cedure we use is simple least squares curve fitting. Also, Step (%) bulb bulb (%) (%) bulb bulb keep in mind that conservative schedules are used as the base for establishing the relationship to specific gravity. The 1 >50 49 45 14.4 80 120 113 intent of these schedules is to serve only as a starting point 2 50 to 40 49 44 12.1 72 120 110 to be adjusted upward in severity as experience is gained 3 40 to 35 49 41 9.6 60 120 105 with the species in question. Even though all we can hope 4 35 to 30 49 35 6.5 40 120 95 for in schedule estimation are approximate guidelines, we feel 5 30 to 25 55 32 4.0 22 130 90 that this is a substantial improvement over the current situa- 6 25 to 20 60 32 2.9 15 140 90 tion of having no starting point in schedule selection. 7 20 to 15 66 38 3.2 18 150 100 8 15 to final 82 55 3.5 26 180 130 Method of Schedule Estimation aEquilibrium moisture content. Schedule estimation is based on relationships established between basic specific gravity (green volume and ovendry Our knowledge of the process that led to recommended kiln weight) and known recommended kiln schedules for schedules for different species is incomplete. For hardwoods 268 species or species groups