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Understanding Vacuum Drying Technologies for Commercial Lumber individual system, there is a chance of over- or under-drying the kiln charge. Over-drying is especially problematic because its effects cannot be undone. This makes it difficult for the user of one system to be able to easily operate another without having in-depth knowledge of how it operates. Compared with conventional drying, there is a lack of available kiln schedules or instructions on how to dry different wood species in vacuum kilns. Most vacuum drying systems do not allow the operator to add moisture back into the wood at the end of the drying cycle. This process, called conditioning, is an essential part of traditional kiln drying in that it reduces drying stress in lumber. The inability to add a conditioning step in vacuum kilns can produce lumber with residual stress, which makes it difficult to maintain quality when machined into smaller components for use in furniture or flooring. However, a vacuum kiln with a radio-frequency system has been shown to produce stress-free red oak lumber by not allowing the internal wood temperature to go above 100 °F (37.7 °C) for the first 48 hours of the drying process (Harris and Taras 1984). In that system, the drying process took 88 hours to dry the samples from 67% to 7% moisture content. The wood core temperature reached 140 °F (60 °C) during the final stage. The permeability of wood species is an important consideration when drying in a vacuum kiln. Some species, such as white oak, are generally hard to dry, and it would not be economical to dry them in a vacuum system because of the limited improvement in drying time compared with conventional drying. Summary In summary, vacuum drying is gaining interest among wood product manufacturers with its fast drying rates and unique uses compared with conventional drying systems. However, final moisture content variation and residual stress have been significant issues of concern with vacuum drying. Therefore, proper training and applied research are still needed to develop standard practices to assist in reducing drying defects in vacuum drying of commercial lumber. Acknowledgments We are grateful to Bill Smith of State University of New York (SUNY), College of Environmental Science and Forestry, Syracuse, New York, USA, and Omar Espinoza of the University of Minnesota, St. Paul, Minnesota, USA, who gave valuable reviews that improved this report. JLB Design, LLC, Woodruff, Wisconsin, USA, prepared Figure 2, and Vacutherm, Inc., Barre, Vermont, USA, supplied Figure 7. Literature Cited Brenes-Angulo, O.; Bond, B.; Kline, E.; Quesada-Pineda, H. 2015. The impact of vacuum-drying on efficiency of hardwood products manufacturing. BioResources. 10(3): 4588-4598. Chen, Z. 1997. Primary driving force in wood vacuum drying. Blacksburg, VA: Virginia Polytechnic Institute and State University. 185 p. Ph.D. dissertation. Denig, J.; Wengert, E.M.; Simpson, W.T. 2000. Drying hardwood lumber. Gen. Tech. Rep. FPL-GTR-118. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 138 p. Espinoza, O.; Bond, B. 2016. Vacuum drying of wood— state of the art. Current Forestry Reports. 2: 223-235. Harris, R.A.; Taras, M.A. 1984. Comparison of moisture content distribution, stress distribution, and shrinkage of red oak lumber dried by a radio-frequency/vacuum drying process and a conventional kiln. Forest Products Journal. 34(1): 44-54. Lyon, S.; Bowe, S.; Wiemann, M. 2021. Comparing vacuum drying and conventional drying effects on the coloration of hard maple lumber. Res. Pap. FPL-RP-708. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 5 p. Ressel, J.S. 1994. State-of-the-art report on vacuum drying of timber. In: Haslett, A.N.; Laytner, F., eds. 4th IUFRO International Conference on Wood Drying. Rotorua, New Zealand: 255-262. 5PDF Image | Understanding Vacuum Drying Tech for Commercial Lumber
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