Continuous Fast Drying of Boards*

Jarl-Gunnar SALIN
PhD - SP Swedish National Testing and Research Institute- Building Technology and Mechanics – Trätek
Address: P.O. Box 5609, SE-114 86 Stockholm, Sweden
E-mail: jarlgunnar.salin@sp.se

Abstract:
Sideboards represent a relatively low valued product at most Scandinavian softwood sawmills. A simple and efficient handling of these boards is thus desired. An improvement in that direction could perhaps be achieved if the drying process is integrated into a continuous production line. Preliminary considerations indicated that the target should be a drying time of two hours for Norway spruce sideboards with a thickness up to 22 mm. This requirement implies a high drying temperature and a forced contact drying air/wood. This brings the focus on veneer dryers of the roller type, where the boards are conveyed lengthwise between pairs of cylinders and subjected to hot air from high velocity jet blow boxes.

A series of drying tests have been performed in an almost full-scale veneer dryer adapted to this purpose. The main results from these tests can be summarized as follows:

• with a drying air temperature level of 135-140oC a drying time of two hours can be achieved for thicknesses up to 22 mm;
• the quality of the dried timber was better than expected, as far as color, knot reactions, resin flow and surface checking are concerned;
• at temperatures above 140oC a slight gray color starts to develop on the timber surface;
• the clearly biggest quality problem is a very pronounced case hardening (and moisture gradient) which has to be handled with some kind of conditioning before or in connection with the timber cooling during the final part of the drying process;
• another result that may constitute a problem is a high spread in the final moisture content.

A cost estimate for a full-scale dryer for a specific case has been obtained. Finally a feasibility assessment for a continuous production line is presented.

Key words: drying, sideboards, Norway spruce, continuous process.

*Note:
This paper was first presented within the 9th International IUFRO Wood Drying Conference – Nanjing (China), in August 2005 and selected to be published in PRO LIGNO journal.