原木板材的窑式干燥

THE KILN SEASONING OF TIMBER BOARDS

本文综述了canterbury大学近年来关于新西兰木材窑式干燥的研究。有些硬木单纯窑式干燥是困难的,但利用太阳能窑式风干则有可能;轮回操作可以减小干燥应力从而减小产品性能的退化程度。种植生产为主的软木干燥较容易,从经济考虑应采用高温干燥,以减少干燥时间。高温操作时对流传递显得相当重要。实验表明,当空气流过板面时,不同点处的传质系数有显著差异。湿分在木板中移动的模型可假定为:随着干燥进行蒸发面向中心退缩。但心材和边材的行为有所不同:边材中的水份可在相邻脉管间迁移,但在心材中则不可能。正是这种差异导致干燥时间和干燥温度曲线不同。

The paper reviews recent works done at the university of Canterbury to under-stand the drying behaviour of certain New Zealand woods seasoned in a kiln. Some hard-woods are difficult to dry in this way,but the use of a solar kiln shows promise.The cyclic process conditions reduce drying strresses and thus degrade of the product. On the other hand,the principal plantation timber,a softwood,readily dries and increasingly high-temper-ature drying conditions are being adopted commercially ,with consequantial reduction in dry-ing time. Under these conditions ,the convective transfer within the stack of timber becomes important, and experiments have shown that there is a considerable variation in the local mass transfer coefficients as the air flows over the boards. The moisture movement through the boards can be modelled in terms of an evaporative front withdrawing into the wood as it dries,but the behaviour of heartwood and sapwood differs. Liquid transport between adjacent vessels is possible in sapwood ,but not in heartwood. This difference results in differenscs in drying times and temperature profiles during drying.