Impact of High-Temperature Schedules on Drying of Spruce and Pine

Abstract

The impact of high-temperature drying (HTD) for spruce and pine lumber of 5 × 10 × 244 cm was investigated in this study. Four drying schedules with dry-bulb temperatures from 60 to 110°C were evaluated including Control 1 (conservative), Control 2 (accelerated), HTD1 (104°C) and HTD2 (110°C). In each of HTD1 and HTD2 schedules, two runs were performed, one humidified with steam and the other applied without steam. Drying rates, internal stresses (prong test), warp (bow, crook, and twist), modulus of elasticity (MOE), and modulus of rupture (MOR) were assessed for the dried lumber.

Six drying runs were carried out in a laboratory kiln and the results of this research indicated: (1) The drying rates in HTD1 and HTD2 increased by 2.2 to 3.5 times in comparison to the conservative schedule. (2) Both HTD1 and HTD2 schedules with steam resulted in greater internal stresses in the dried lumber compared to Control 1, but these stresses were smaller than those in Control 2. The standard deviations of final moisture content from HTD1 and HTD2 with steam were higher than those of Control 1 but similar to the values obtained for Control 2. (3) Warp was, in general, reduced by using the HTD schedules except for HTD2 without steam. Less crook was observed for HTD1 and HTD2. (4) Reductions in MOE and MOR were found for both HTD1 and HTD2 schedules but were not statistically significant.

Keywords:

• Drying rates
• High-temperature drying
• Internal stresses
• Warp

ACKNOWLEDGEMENTS

Forintek thanks its industry members, Natural Resources Canada (Canadian Forest Service), and the Provinces of British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, Nova Scotia, New Brunswick, Newfoundland, and Labrador for their guidance and financial support for this research.

Notes

^a Lumber value (U.S. dollar/Mfbm) determined using Table 2.

^b Devalued = Values reduced from the values of green lumber listed in column 2.

^a Change = Reduction comparing with the values from conservative schedule.

^b SD = Standard deviation.