Research On Preheating And Middle Treatment During Conventional Drying Of Oak Lumber With Thickness 35mm

The oak lumbers of 35mm in thickness were selected in this study, pretreated with different time, temperature, humidity and then dried with a same conventional drying schedule, when the Moisture content To the vicinity of the fiber saturation point in different paragraphs, middle treatment with different timing time, temperature, then continue drying with a same conventional drying schedule. The drying rate, distribution of moisture content, residual drying stress and visible defects were investigated. Based on the interaction among these parameters, the suitable preheating condition and its effect on middle treatment timing were determined. And the appropriate middle treatment timing, frequency and conditions were determined. The results provide a basis data for optimizing the preheating conditions and the timing and conditions for middle treatment in wood conventional drying. The main results are as follows:(1) In the first phase drying process(60%-40%), with the extension of preheating time, drying speed accelerated, the drying quality improved and middle treatment timing was delayed. After 8h, the influence on the drying effect above-mentioned is not obvious; with the preheating temperature increases, drying speed accelerated, drying quality improved, middle treatment timing was delayed. When the temperature exceed70℃, the color of wood surface became deeper, and quality declined. The results shows that for the oak lumbers of 35mm in thickness,70℃ temperature and 100% relative humidity with 8h was the more appropriately preheating conditions which can get a better drying effect.(2) If do only once middle treatment at the moisture content of 35% to 30% and 25% to 20%. Respectively, there were all drying defects before treatment, and the dry mass failed to meet the II standard after drying; Do once intermediate processing(record twice) at the moisture content 40% to 35% and 25% to 20%, there were no drying defects before treatment. After drying, there were 75% of dry mass index reach to I standard and 25% reach to Ⅱ standard. The results show that the boards were more appropriate for intermediate processing for 2 times, respectively in the timing of moisture content was 40% to 35% and 25% to 20%.(3) The impact of first middle treatment on drying effect from after-treatment to 20% moisture content:as time increased, the drying speed increased a little, the drying quality improved significantly, but the effect had became insignificant over 8h.Remperature of middle treatment increased, drying speed accelerated, moisture content deviation, residual stress index and visibility drying defects were increased. It is good to both aspects of the combined effect when 72 ℃.when the relative humidity increased, the visible defects decreased, but over 96%, quality improvement is not obvious and energy consumption increased and drying speed was slowed down. The results shows that for the oak lumbers of 35mm in thickness,72℃ temperature and 96% relative humidity with 8h was the more appropriately first intermediate processing conditions which can get a better drying effect.(4) The impact of second middle treatment on drying effect from after-treatment to 10% moisture content:as time increased, the drying speed reduced a little, but the effect had became insignificant over 8h.Temperature of middle treatment increased, drying speed accelerated, drying quality reduced, It is good to both aspects of the combined effect when 80 ℃; when the relative humidity increased, the visible defects decreased, but over 96%, quality improvement was not obvious and energy consumption increased and drying speed was slowed down. The results shows that for the oak lumbers of 35mm in thickness,80℃ temperature and 96% relative humidity with 8h was the more appropriately second intermediate processing conditions which can get a better drying effect.