Variation in Drying Behavior and Final Moisture Content of Wood during Conventional Low Temperature Drying and Vacuum Drying of Betula pendula Timber

Abstract

Conventional and vacuum drying experiments were conducted on Betula pendula timber, which was sawn from trees felled during three different seasons. The influence of the wood procurement season on drying behavior differed, on the one hand, between the drying phases above and below 30% moisture content in the conventional drying, and, on the other hand, between the conventional and vacuum drying methods. During the first steps of the conventional drying process, relative humidity in the kiln, as well as drying time and drying rate, varied according to the felling season. Variations in environmental conditions outside the kiln and the seasonal variation in the physical properties of the wood were presumed to be the reasons for differences in drying behavior. The difference in moisture content gradient, i.e., the difference in final moisture content between the inner wood and the surface layer of boards, was greater in conventionally dried timber than in vacuum-dried timber. In conventionally dried timber there was a clear seasonal variation in the gradient of final moisture content, which was greatest for winter-felled wood. The premature drying of the surface layer during the first steps of the conventional drying process of winter-felled wood was the reason for the higher gradient of moisture content. Storage of wood as logs decreased the standard deviation of the final moisture content.

Keywords:

• Betula pendula
• Drying rate
• Drying time
• Wood drying
• Wood properties

ACKNOWLEDGMENTS

This study was funded by the Academy of Finland through The Finnish Forest Cluster Research Programme (Project 43098), which is gratefully acknowledged. The author also thanks Professor Erkki Verkasalo from the Finnish Forest Research Institute for reading and commenting on the manuscript.

Notes

T: dry bulb temperature (°C); D _f : drying force (D _f  = M/M _e when M < 30% and D _f  = 30/M _e when M ≥ 30%); p: air pressure (mbar); —: not included in the schedule.

^a Pressure leakages in vacuum chamber during drying below 30% M.

^b Data not available.

^a Pressure leakages in vacuum chamber during drying below 30% M.

^b Data not available for vacuum drying.

Significance levels: **p < 0.01

*p < 0.05.