Degrade due to drying was studied at seven industrial drying operations. Three high-temperature, three conventional and one dehumidification kiln were selected to evaluate differences in degrade by type of drying system. Lumber from various regions of the kiln was graded before and after drying. Seasons for change in grade from green to dry were recorded.
At 17 to 19 percent moisture content (MC), there is very little difference in degrade between the three drying systems studied. As moisture content dropped, degrade losses increased. The rate of increase was similar for the dehumidification and conventional dried material; however, the high-temperature dried material showed drastic increases in degrade at lower MC’s. For example, high-temperature dried material at 12% MC showed 2.5 times as much degrade as at 19%.
Average degrade for the three high-temperature kilns studied was $16.51/Mbf versus an average of $9.80/Mbf for the conventional systems. Most of the difference is caused by the higher degrade levels in the over-dried material.
Degrade losses in the uppermost ten rows of the kiln load were almost twice as high as in lower regions. The information provided could be used to evaluate the economics of placing weight on the kiln load to reduce warpage.
In each drying system, crook was the most common cause for down-grading, while twist was the second most prevalent cause. Poor piling and handling practices were identified as factors that contributed to the amount of warpage.
Further work is warranted on dehumidification drying. At the time of this study, there were no other mills with this equipment drying a S-P-F species mix suitable for comparison with this work.