| 概要 |
近年,木材工業分野で注目されている高周波加熱減圧乾燥法について,二三の基礎実験を行い,つぎの結果を得た.(i)この乾燥法には,材内の水分移動を促進する二つの作用がある.すなわち,減圧下で生材を高周波加熱することにより,材内の蒸気圧を上昇させ繊維方向の水分移動を活発にする作用と,材の厚さ方向に含水率傾斜と蒸気圧傾斜を形成し,これによって水分の拡散移動を活発にする作用である.(2)前者の作用は,繊維方...向気体透過性のすぐれたアピトンの乾燥で顕著な効果を発揮し,乾燥時間は熱風乾燥に比し著しく短縮され,しかもその効果は板厚や材長の影響をあまり受けない.(3)後者の作用は,期待透過性の不良なべイスギの乾燥で認めることができるが,これによる乾燥時間の短縮は比較的少ない.
Recently, vacuum drying method combined high frequency heating has been applied to a certain extent in the field of wood industry. The purpose of this work is to obtain some fundamental information about the drying rate in this methord comparing with that in conventional kiln drying method. Green boards and dimention lumbers from the three kinds of wood species were used for the test material: Apitong (very high permeable wood), Matoa (less permeable wood) and Western redcedar (very low permeable wood). Some specimens were covered with thick epoxy-resin films on their cross sections or the other four faces to control water evaporation from the specimens. Vacuum drying was done in the vacuum drier under 30 or 40 mmHg and the test specimens were heated up to 40℃ with high frequency (6.7 MHz, 4000 V, 0.2 A). Kiln drying was performed in the oven drier basing upon the standard drying schedules used in Japan. The results obtained are summarized as follows. (1) The drying time for green board is reduced remarkably or slightly by application of high frequency heating-vacuum method comparing with drying time by kiln method. The reducing ratio is 1/13 for Apitong, 1/2.6 for Western redcedar and 1/1.4 for Matoa, and the efficiency of this method on drying time depends on wood species. (2) Based on the experimental results on the evaporating rates, it is considered that there are two principal effects on the moisture movement in high frequency heating-vacuum drying, that is, one is the promotional effect of the vapor pressure difference between inside and outside of specimen on the moisture movement in the vessels along fiber direction as observed in the case of Apitong, and the other is the accelerating effect of the moisture and pressure gradient on moisture diffuse from inside to outside of the specimen perpendicular to fiber direction as observed in the case of Western redcedar. (3) In air drying, the drying time does not depend on board length except under 0.5 m. But in high frequency hea ting-vacuum drying, the time varies slightly or greatly with board length depending on wood species. For instance, the drying time for a board of 1 m length increases by approximately 2 times that for 10 cm length in the case of Apitong, and about 10 times in the case of Western redcedar. (4) Although the drying time for boards and dimension lumbers in kiln drying increases exponentially with increasing their thickness, the time in high frequency heating-vacuum drying increases slightly with increasing thickness. For instance, the drying time for a dimention lumber of 10 cm thickness is about 2.5 times as long as that for a board of 1 cm thickness in the case of Apitong, and about 8 times in the case of Western redcedar.続きを見る
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