Obtaining Uniform Final Moisture Content in the High Temperature Drying of Paper Birch Flitches

The high temperature (HT) drying of 7/4 paper birch flitches was evaluated in the context of saw-dry-rip (SDR) research. Our initial approach called for equalizing to commence, at a dry-bulb temperature (WB temp.) of 180?F, when the driest sample approached 7 to 8 percent moisture content (MC). Invariably this sample proved to be entirely of whitewood. When using 240?F DB temp. and 190? WB temp., the equalizing commenced after about 1 day. The switch from HT drying to equalizing dramatically reduced the drying rate of the darkwood. The independent determination of whitewood and darkwood MC in individual flitches showed that darkwood MC dropped only 15 percent during 6 days of equalizing, i.e., down from about 30 to 15 percent, while the MC of the whitewood remained essentially static at 8 percent. The dried flitches contained severe stresses and large MC differences between whitewood and darkwood. It was then discovered that drying at 240?F DB temp. with the kiln vents wide open for the first 36 hours approximately, i.e., no wet-bulb control, would reduce the darkwood and whitewood MCs, respectively, to about 10 and 4 percent. This, followed by 1 day of equalizing at 7 percent equilibrium MC, produced a 5 to 6 percent MC for both darkwood and whitewood. The flitches were also free of casehardening. In our HT drying of paper birch flitches, honeycomb and collapse has occurred only in the darkwood. However, the severity appears no worse than what we have observed in drying them by the recommended FPL schedule for 8/4 paper birch. A certain percentage of the darkwood has even demonstrated honeycomb in the air drying of flitches. Therefore, the occurrence of defect in the darkwood of paper birch is not considered to be the deciding factor in the choice of drying method.

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