Using the Concept of Thin Dry Layer to Explain the Evolution of Thickness, Temperature, and Moisture Content during Convective Drying of Norway Spruce Boards

Abstract:

This article presents an experimental device able to follow the wood temperature, moisture content, board thickness, and surface checking during convective drying. Tests have been carried out on Norway spruce (Picea abies) boards, with different sawing patterns and subjected to a more or less severe drying schedules. For some boards, we observed a first drying phase followed by a transient phase lasting several hours, during which the thickness reduces very slightly and the board temperatures depart from the wet bulb temperature. This period, which is not exactly a true constant drying rate period, can be analyzed using the concept of “thin dry shell.” In order to confirm our hypothesis, an analytical model was used and its results are compared with the experiment. This model takes into account external heat, mass transfer, and diffusion of heat and vapor between the exchange surface and the drying front. Very good agreement is obtained between calculated and experimental temperatures, which validates our equipment and brings physical explanations to different observed behavior; in particular, the existence of a transient first drying period with a thin dry layer is confirmed.

Keywords:

• Evaporation front
• Norway spruce
• Thickness change
• Thin dry shell
• Wood drying

ACKNOWLEDGEMENTS

The authors acknowledge the French Ministry for Agriculture and Fisheries for funding one part of this work in the framework of the research project no. 61.45.39/01 entitled “Development of drying schedules adapted to large thickness of spruce lumber.”