36 Drying of Wood

FIGURE 36.36 Final section curvature of oak samples dried on their upper face with conditions n 2 (T d ¼ 808C, T w ¼608C) and n3 (T d ¼ 808C, T w ¼ 768C), respectively.(Schedule B) is recommended for hardwoods. ScheduleB proposes lower temperature levels and higherrelative humidity values. In this drying schedule,EMC decreases significantly only at the end of theprocess, when the board is supposed to be dry with alow moisture content gradient. As the first consequenceof these drying conditions, one can notice amuch longer drying time for Schedule B (130 hagainst 50 h). The first drying period lasts also for alonger time for the second procedure (30 h instead of10 h). It may be noted that the first drying–periodduration represents about the same percentage of thetotal drying time for both schedules. This remark stillstands for the stress level. One can easily consider arelative time (current time over total drying time) atwhich all curves have the same shape and the samestress magnitude.In the first example, the advantages to be gainedfrom using a high relative humidity level (low moisturecontent gradient, high hygroactivation of theviscoelastic behavior) hardly offset the negative effectof the low temperature levels (slow moisturemigration and low thermoactivation of the viscoelasticbehavior). A careful analysis of these approachesis very promising. New rules can bederived to improve existing drying schedules or todevise innovative drying procedures.Schedule AAverageMoistureContent (%)Dry-BulbTemperature(8C)Wet-BulbTemperature(8C)RelativeHumidity(%)Green 71 66 8050 76.5 68.5 7030 82 70.5 6020 88 67.5 40Schedule BAverageMoistureContent (%)Dry-BulbTemperature(8C)Wet-BulbTemperature(8C)RelativeHumidity(%)Green 40.5 38 8560 40.5 37 8040 43.5 39 7535 43.5 38 7030 46 39.5 6525 51.5 43 6020 60 47.5 5015 65.5 49 40ß 2006 by Taylor & Francis Group, LLC.