A Simulation of Wet Pocket Lumber Drying

Abstract

In general, wood containing wet pockets is difficult to dry and to ensure uniformity of moisture content at the end of the drying process. Large variations of final moisture content and severe case hardening are common problems associated with the drying of wet wood. In order to devise optimal strategies for drying wood containing wet pockets, it is necessary to understand its complex moisture movement mechanisms and therefore predict drying times and final moisture content. Sub-alpine fir dimension lumber was used in this research because of its inherent issues related to wet pockets.

A two-dimensional mathematical drying model for wood containing wet pockets was developed. An effective diffusion coefficient (D _eff ) was utilized in the model and heat and mass transfer equations were solved using a control volume approach. The difficulties involved in the simulation of the drying process of wet pocket lumber are due to the differences in moisture content and physical properties between wet and normal wood. Thus, an adjustable D _eff based on the moisture content (for both below and above fiber saturation point) was used during the simulation.

Four drying runs involving green unsorted sub-alpine fir lumber were carried out in a 3-ft laboratory kiln and in an 8-ft pilot kiln. The results of the simulations were in agreement with the results obtained through the drying experiments.

Keywords:

• Drying model
• Effective diffusion coefficient
• Moisture movement
• Wet pockets

ACKNOWLEDGEMENTS

Forintek thanks its industry members, Natural Resources Canada (Canadian Forest Service), and the Provinces of British Columbia, Alberta, Saskatchewan, Manitoba, Quebec, Nova Scotia, New Brunswick, and Newfoundland and Labrador for their guidance and financial support for this research. The authors thank Mr. Changming Yu of Forintek for his comments and suggestions. Special thanks to Mr. Brad Wang of Forintek for preparing graphics for this article and Mr. Bill Mundie from West Fraser for providing the sub-alpine fir lumber.