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木材の減圧乾燥における乾燥機構を解析するためには,減圧および乾燥中における木材内部圧力の変動を明らかにする必要がある.本報では,これについての具体的資料を得る目的から,はじめに減圧雰囲気内に置かれた気乾材の繊維方向における気体の非定常流による内部圧力の減少経過について理論的検討と基礎実験を行ない,つぎに,供試材として気体透過性の著しく異なる3樹種生材の高周波加熱減圧乾燥を行なって,乾燥経過中の材内...各部位の圧力ならびに含水率の経時変化を測定した.その結果,内部圧力は材内水分の呈する蒸気圧の増加と外部からの減圧の差として表わされ,含水率の低下にともない,その材の気体透過性と材内の部位によって特徴ある3段階の減少経過を辿ることを明らかにした.
It is essential for investigating the mechanism of vacuum drying of lumber to obtain the quantitative information on the pressure behavior in the lumber during evacuating and drying. Hence the purpose of this work is to obtain these data. Green and air-dried square lumbers (10^2 x 100 cm^3) of three kinds of wood species having widely differing permeabilities were prepared for the tests. The evacuational property of air-dried lumbers in the longitudinal direction were tested in a vacuum chamber under reduced pressure of 40-400 mmHg. Drying tests of green lumbers were performed in a vacuum dryer under 40 mmHg, heating up to 40℃ at the center of the lumber with radio frequency. The pressure within lumber (p) was measured by use of the mercury manometers which were inserted into the lumber at the center and the other four positions. The results obtained were as follows. (1) Experimental curves of p versus evacuating time for air-dried lumber at the center and the other four positions show a certain degree of approximation with the theoretical curves calculated by substituting the best fitting coefficient value of air longitudinal permeability of the wood (k_g) into the unsteady-state diffusion differential equation. (2) The pressure at each position in a green lumber during vacuum drying decreases in a characteristic course with lower moisture content and the following three phases occur: The first phase; In the initial stage. the value of p decreases rapidly or gradually depending on the value of k_g and the distances from the measured positions to both the ends and the edges of the lumber. The second phase; The decreasing rate of pressure becomes to be equilibrated with the increasing rate of moisture vapor pressure generated in the lumber. and the value of /) maintains nearly a constant value. The third phase; After being dried under the fiber saturation point, the value of p begins to decrease again and approaches to the reduced pressure in the chamber.show more
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