Comparing conventional and vacuum kiln-drying of hard maple lumber: board flatness and drying time
| Authors: | Alex Anderson, Michael C. Wiemann, Scott Lyon, Scott Bowe |
| Year: | 2025 |
| Type: | Research Paper |
| Station: | Forest Products Laboratory |
| DOI: | https://doi.org/10.2737/FPL-RP-723 |
| Source: | Research Paper FPL-RP-723. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory |
Abstract
Vacuum drying technology can produce kiln-dried lumber at rates faster than conventional steam kiln-drying, resulting in reduced operating costs. Because of this, wood products manufacturers are increasingly becoming interested in vacuum drying technology, but the effects of this technology on the flatness of the resulting lumber are not known. This study compared the flatness of hard maple lumber when dried in a vacuum kiln with that of the same type of lumber dried in a conventional steam kiln. The drying schedules were also compared to quantify the difference in amount of time needed to dry the samples to the same moisture content. Paired samples of 1-in.- (25.4-mm-) thick flat-sawn boards were compared in this study. Half the samples were dried in a conventional steam kiln, and half were dried in a vacuum kiln. After the drying stage was complete, flatness of the two groups was measured using a flatness gauge. Although the vacuum-dried boards had less warp than the steam-dried boards in the rough-sawn condition, there were no statistically significant differences after planing. When the planed boards were acclimated to 12% moisture content, the narrowest vacuum-dried boards showed less residual warp than did the steam-dried boards, but that was not the case for wider boards. Also, the vacuum drying was found to take only about one-fifth the amount of time of the conventional steam drying.