For many years, Canadian lumber producers have known about and dealt with problematic, hard-to-dry “wet pockets” in subalpine fir (predominantly in western Canada) and balsam fir (predominantly in eastern Canada). Both species contain “pockets” with high moisture content. In recent years, the proportion of subalpine fir in the SPF (spruce, pine, and fir) mix has risen significantly in British Columbia as a substitute for pine damaged by the mountain pine beetle. The result is significantly longer drying times and a much larger variation of moisture content in the final product. This leads to a higher percentage of wet lumber coming out of the kiln, and this wet lumber is usually downgraded to economy. Conversely, over-drying the lumber also increases the risk of developing drying defects such as warp which affects grade recovery and reduces planer productivity. The increased cost to the industry can be significant. Depending on the annual volume of subalpine fir and balsam fir being produced at a typical mill, it is estimated that the financial impact of wet pockets ranges between $600,000 and $1,300,000.
To tackle this problem, FPInnovations initiated a project to study and define the drying characteristics of wood affected by wet pockets, and devise appropriate wood sorting and drying management strategies to alleviate the problem and reduce the impact on quality of the final product. Drying rates of wood with and without wet pockets were determined under various drying schedules which led to a better understanding of how normal wood and wood with wet pockets dry. Preliminary results of this investigation indicate the potential of using green density relationships for sorting lumber with and without wet pockets, and drying them in separate batches using appropriate drying schedules.
Understanding the drying mechanisms better leads to the next phase of the project, which will look into options for wet pocket identification and sorting, including measuring technology to quantify and locate wet pockets. After sorting out wood with wet pockets, drying strategies such as pre-drying and/or a combination of drying technologies could be devised to employ flexible drying schedules to increase productivity and quality of the final product.
For more information, contact Luiz Oliveira, Research Leader of FPInnovations’ Drying and Energy group.
This article was originally published in the 2016-2017 IMPACT magazine.