Effects of pressure and temperature in the chamber during vacuum drying on the relative humidity and evaporation of wood surface were investigated by using the vacuum chamber.The setting temperature during vacuum drying included dry-bulb temperature td, the wet-bulb temperature tw and the temperature difference between the air in the vacuum chamber and the cooling water in the condenser.Results indicated that relative humidity during vacuum drying was affected by the dry-bulb temperature td, the wet-bulb temperature tw and the temperature difference between the air in the vacuum chamber and the water in the condenser.Relative humidity of wood decreased with the increase in temperature at the given temperature of the water in the condenser.The relative humidity was affected slightly by pressure in the vacuum chamber pA, and it decreased from 70% to 65% with pA increased from 50 kPa to 101 kPa.Moreover, there was nearly no evaporation under the vacuum without external heating.
XIAO Hui, CAI Ying-chun Key Laboratory of Bio-based Material Science and Technology, Northeast Forestry University, Harbin 150040, P.R China
为了研究木材常规干燥过程黏弹性应变的发展模式与相对数量级,并为准确区分黏弹性应变与机械吸附应变提供理论依据,该文在实验室条件下对50 mm厚兴安落叶松板材进行常规干燥,使用切片法测定沿厚度方向的横纹弦向干缩应变、弹性应变、黏弹性应变的一维分布情况与变化趋势。基于高聚物与复合材料黏弹性理论,重点定性分析了木材干燥过程中干燥介质温度、蠕变恢复时间、干燥阶段等因子对木材厚度方向不同位置黏弹性应变特性的影响。结果表明:在环境平衡含水率保持在特定水平下,干燥介质温度水平与测定的拉伸弹性应变极值间具有一定正相关性;木材干燥过程中黏弹性应变的发展模式与弹性应变类似,经充分恢复后黏弹性应变的数值略大于弹性应变;木材表层、芯层在干燥过程中具有不同的黏弹性演化模式,干燥温度对木材表芯层黏弹性应变转换有一定影响,这种作用主要是由于不同温度条件下木材含水率梯度所导致的;根据试验测定的木材表层、芯层在3、62、4 h 3个应力释放时间内所达到的黏弹性应变极值,分别给出了干燥过程各阶段落叶松板材表层与芯层黏弹性应变恢复时间的推荐值。
