【０００１】
【発明の属する技術分野】
本発明は、木材特に住宅構造用の柱や梁材等の心持ち材を割れや材色の変化を生じないよう乾燥させる乾燥方法に関する。
【０００２】
【従来の技術】
従来の木材乾燥方法は、被乾燥材を９５℃で８時間蒸煮した後、乾球温度１２０℃、湿球温度９０℃の条件下で７２時間乾燥させて、材面割れも少なく、短時間での乾燥が可能となるようにしている。（例えば、非特許文献１参照）
また、乾球温度を１１０℃以上の条件下で行なう第１乾燥工程の後、第１乾燥工程よりも低い乾球温度の条件下で第２乾燥工程を行なって、材面割れ及び材色の暗色化の抑制と、内部割れの抑制とを図るようにしているものもある。（例えば、特許文献１参照）
【０００３】
【非特許文献】
第４９回日本木材学会運営委員会編集「第４９回日本木材学会大会研究発表要旨集」日本木材学会発行、１９９９年３月１０日、Ｐ１３０
【特許文献】
特開２００１−２８７２０６号公報（第３〜第５頁）
【０００４】
【発明が解決しようとする課題】
しかしながら、前記木材乾燥方法のうち前者は、被乾燥材の材面割れは減少したものゝ、内部割れが多発して、材色が暗色化するという問題点を生じた。また、後者は、材面割れ及び材色の暗色化の抑制と、内部割れの顕著な抑制も可能とされているが、発明者の実験によると、第２乾燥工程を乾球温度１００℃以上で行なうと、被乾燥材の内部割れ及び材色の暗色化は依然として発生する。そこで、乾球温度を８５〜９５℃に下げて第２乾燥工程を行なうと、内部割れ及び材色の暗色化は殆ど認められなくなり、十分な抑制効果が確認された。しかし、８５℃で第２乾燥工程を行なうと、乾燥に８〜１２日という１００℃以上で第２乾燥工程を行なう場合の２倍近い期間を要して、生産性が悪いという問題点がある。
【０００５】
本発明は前記問題点を解消し、被乾燥材の材面割れ及び内部割れと、材色の暗色化とが十分に抑制された高品質の乾燥材を可及的短期間で生産し得る木材の高品質乾燥方法を提供することをその課題とする。
【０００６】
【課題を解決するための手段】
前記課題を解決するため、本発明に係る木材の高品質乾燥方法は以下の手段を採用することを特徴とする。
【０００７】
（１）常圧下で被乾燥材を８０〜１００℃で蒸煮した後に、乾球温度を１００〜１３０℃、湿球温度を乾球温度より２０〜５０℃低くした高温低湿な条件下で、被乾燥材の表層を急速に乾燥し、その後、乾球温度を１００℃以下に下げ、乾燥室内を５〜７０ｋＰａ程度の減圧状態として、被乾燥材の内層を速やかに乾操すること。
【０００８】
（２）被乾燥材を常圧下８０〜１００℃での蒸煮に引き続いて、乾燥室内を速かに５〜７０ｋＰａ程度の減圧状態にし、この状態をしばらく維持した後に、再び８０〜１００℃の蒸煮を行う操作を一度か、或いは複数回、繰り返し行ない被乾燥材の含有水分を減少させてから、請求項１記載の乾燥方法を実施すること。
【０００９】
【発明の実施の形態】
以下に本発明に係る木材の高品質乾燥方法の実施形態を説明する。
【００１０】
この乾燥方法は、被乾燥材を常圧下で蒸煮する工程と、蒸煮した被乾燥材を高温低湿な条件下で表層の乾燥を行なう工程と、表層乾燥の工程よりも温度を下げるとともに、減圧した条件下で被乾燥材の内部の乾燥を行なう工程とを順次に行なって、被乾燥材を高品質に可及的短期間で乾燥させるものである。
【００１１】
この乾燥方法を実施するには、図１に概要構成を示す乾燥装置Ａを用いる。この乾燥装置Ａは、被乾燥材１を収容してその蒸煮及び乾燥を行なう乾燥室２と、該被乾燥材１を桟積み（各被乾燥材を相互間に桟を挟んで隙間ができるように積む）にして乾燥室２へ入れ出しする台車３と、桟積みした被乾燥材１の隙間を通るように乾燥室２内に空気を循環流動させる送風機４と、送風機４で循環される空気を加熱するフィン付きヒーター５及び加熱、加湿する蒸気の噴射管６と、フィン付きヒーター５及び蒸気の噴射管６へ、制御弁７を備える配管８と、制御弁９を備える配管１０で蒸気を供給する蒸気発生装置１１と、乾燥室２へ制御弁１２を備える吸引管１３を接続して、乾燥室２内を減圧させる真空ポンプ１４と、乾燥室２の天部等に設けたダンパー１５を備える吸排気筒１６等で構成される。
【００１２】
前記乾燥装置Ａで実施する被乾燥材１を蒸煮する工程は、乾燥室２から台車３を引き出して、この台車３へ桟積みにした被乾燥材２を移載する。そして、台車３を乾燥室２に引き入れて蓋を閉じ、乾燥室２を密閉した後に、送風機４を運転して、乾燥室２内に空気を図１に矢印で示す通り循環流動させつゝ、蒸気の噴射管６へ蒸気発生装置１１より蒸気を供給噴射させる。こうすると、循環する空気中へ蒸気が混合して空気の加温と加湿とを行なうため、温度が８０〜１００℃で相対湿度が１００％の循環気流が被乾燥材１の隙間を貫流する。従って、所定の時間間隔で送風機４を反転させて循環気流の方向を変換しながら被乾燥材１の中心部が８０〜１００℃に達するまで蒸煮を行ない、中心部が蒸気温度に到達したとき蒸煮を終了するものであって、この蒸煮工程の所要時間は杉、桧材の場合、概ね４〜８時間である。
【００１３】
被乾燥材１の表層を乾燥する工程は、被乾燥材１の蒸煮工程の終了に伴い、蒸気の噴射管６よりの蒸気の噴射を止めて、フィン付きヒーター５に蒸気を流通させ、乾燥室２内を循環流動する空気を加熱する。そして、乾球温度が１００〜１３０℃程度、湿球温度が乾球温度より２０〜５０℃低い高温低湿（相対湿度３３〜３４％）の条件で、材間風速２．５〜３ｍ／秒で循環気流の方向を所定の時間間隔で変換しながら乾燥を行なう。こうすると、被乾燥材１は表層部を材面割れを生じないように急速に乾燥されて、乾燥の所要時間は被乾燥材の初期含水率により異なるが、概ね６〜３６時間であった。
【００１４】
被乾燥材の内層を乾燥する工程は、被乾燥材１の表層乾燥の工程終了に伴い、フィン付きヒーター５による空気加熱の温度を下げて、循環気流の乾球温度を５０〜９５℃に下げるとゝもに、真空ポンプ１４を運転して吸引管１３により乾燥室２より空気を吸出させ、乾燥室２内を５〜７０ｋＰａの減圧状態にして、循環気流の方向を所定の時間間隔で変換しながら乾燥を行なうか、または、所定時間を減圧状態を維持した後に常圧に戻し、この状態を所要時間維持してから減圧状態にする操作を繰り返して乾燥を行なう。こうすると、被乾燥材１の内層部の水分を速やかに表層部へ移動させることにより、内層部の乾燥を促進して内部割れや材色の暗色化を生じない乾燥が速やかに行われるもので、乾燥の所要期間は被乾燥材１の初期含水率により異なるが、概ね４〜８日であった。
【００１５】
また、被乾燥材の表層乾燥工程に先立ち、請求項２に示す通り、蒸煮減圧工程を追加実行することで、被乾燥材の初期含水率、特に重量材のそれを減少させる操作を行ってもよい。即ち、常圧下での８０〜１００℃の蒸煮に引き続き、速やかに乾燥装置内を５〜７０ｋＰａ程度の減圧状態にして、１〜３時間程度この状態を維持した後に、再び８０〜１００℃の蒸煮を行い材温を上昇させるか、あるいは、この操作を数回繰り返して行う。この減圧操作で、被乾燥材の内層部の飽和蒸気圧が乾燥装置内の圧力よりも相対的に高くなる。従って、被乾燥材の含有水分が押し出される現象が起って、乾燥の初期含水率を減少させることとなり、その後の乾燥工程で消費するエネルギーを削減させることができる。
【００１６】
実施例１
減圧状態に維持できる気密性の高い乾燥室に、横断面１３ｃｍ×１３ｃｍ、長さ３ｍ、含水率平均８２％の杉の心持ち材を搬入して、乾球温度９５℃、湿球温度９５℃で８時間蒸煮を行なった後に、乾球温度１２０℃、湿球温度９０℃の条件の表層乾燥工程に移り、１８時間にわたって表層を急速に乾燥させた後に、乾球温度８５℃（湿球温度は制御なし）、乾燥室内の圧力が３２ｋＰａ程度の条件下で内層乾操する工程を５日間維持した。その結果、材面割れも、内部割れも殆どなく、材色も杉特有の赤みが暗色化することなく残っていて、含水率も平均１８％に減少して適度の乾燥状態にあった。
【００１７】
実施例２
実施例１と同様の杉心持ち材の内層乾燥工程を、減圧せずに常圧下で乾球温度８５℃、湿球温度６５℃で８日間行なった。その際の仕上がり乾燥率は、平均で２４％であり、しかも、乾燥不十分の材が多数残っていた。以上の結果により、内部乾燥工程を減圧状態とすることで、乾燥時間を短縮する顕著な効果が得られることが確認された。
【００１８】
実施例３
実施例１と同じサイズの杉の心持ち材で、乾球温度９５℃、湿球温度９５℃での蒸煮を６時間だけ行なった場合と、乾球温度９５℃、湿球温度９５℃で蒸煮を５時間行なった後に、３５ｋＰａの減圧状態を１時間維持した場合とで、重量の変化を比較したところ、後者は、前者よりも２５％多い１．９６Ｋｇの脱水量が得られて、しかも、重量材ほど脱水量が多い傾向が見られたことから、蒸煮減圧による脱水効果の向上が確認された。
【００１９】
【発明の効果】
請求項１の効果 被乾燥材の材面割れ及び内部割れと材色の暗色化を殆ど生じない木材乾燥を従来の乾燥方法よりも短い期間で行って、乾燥材の品質並びに生産性の向上に顕著な効果を挙げ得る。
【００２０】
請求項２の効果 被乾燥材の内層の飽和蒸気圧が乾燥室の内圧力よりも相対的に高くなって、被乾燥材から含有水分を押し出す現象を起こすため、乾燥初期の含水率が減少することとなって、乾燥工程での所要エネルギーを大幅に低減させることが可能となる。
【図面の簡単な説明】
【図１】本発明に係る木材の高品質乾燥方法に使用する乾燥装置の構成図である。
【符号の説明】
Ａ 乾燥装置
１ 被乾燥材
２ 乾燥室[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a drying method for drying core materials such as timber, especially pillars and beams for a house structure, so as not to cause cracking or change in material color.
[0002]
[Prior art]
The conventional method of drying wood involves steaming the material to be dried at 95 ° C. for 8 hours, and then drying it at a dry bulb temperature of 120 ° C. and a wet bulb temperature of 90 ° C. for 72 hours. So that it can be dried. (For example, see Non-Patent Document 1)
After the first drying step performed at a dry-bulb temperature of 110 ° C. or higher, a second drying step is performed at a dry-bulb temperature lower than that of the first drying step, so that cracks in the surface of the material and color of the material can be reduced. In some cases, darkening and internal cracking are suppressed. (For example, see Patent Document 1)
[0003]
[Non-patent literature]
Edited by the 49th Steering Committee of the Japan Wood Research Society, “Abstracts of the 49th Annual Meeting of the Wood Science Society of Japan” Published by The Japan Wood Research Society, March 10, 1999, P130
[Patent Document]
JP 2001-287206 A (pages 3 to 5)
[0004]
[Problems to be solved by the invention]
However, in the former wood drying method, although cracks on the surface of the material to be dried are reduced, internal cracks occur frequently, and the wood color becomes dark. Further, the latter can suppress the cracking of the material surface and darkening of the material color, and can also remarkably suppress the internal cracking. However, according to the experiment of the inventor, the second drying step is performed at a dry bulb temperature of 100 ° C. or higher. In this case, internal cracking of the material to be dried and darkening of the material color still occur. Then, when the dry-bulb temperature was lowered to 85 to 95 ° C. and the second drying step was performed, internal cracks and darkening of the material color were hardly observed, and a sufficient suppressing effect was confirmed. However, when the second drying step is performed at 85 ° C., the drying takes 8 to 12 days, which is almost twice as long as when the second drying step is performed at 100 ° C. or higher, and there is a problem that productivity is poor. .
[0005]
The present invention solves the above-mentioned problems, and wood capable of producing a high-quality dried material in which the cracks in the surface and internal cracks of the material to be dried and the darkening of the material color are sufficiently suppressed in the shortest possible time. It is an object of the present invention to provide a high quality drying method.
[0006]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, a high-quality drying method for wood according to the present invention employs the following means.
[0007]
(1) After the material to be dried is steamed at 80 to 100 ° C under normal pressure, the material to be dried is heated and dried under a high temperature and low humidity condition in which the dry bulb temperature is 100 to 130 ° C and the wet bulb temperature is 20 to 50 ° C lower than the dry bulb temperature. The surface layer of the dried material is rapidly dried, and then the dry-bulb temperature is reduced to 100 ° C. or less, and the inside of the drying material is quickly dried by reducing the pressure in the drying chamber to about 5 to 70 kPa.
[0008]
(2) Subsequent to steaming the material to be dried under normal pressure at 80 to 100 ° C, the drying chamber is quickly brought into a reduced pressure state of about 5 to 70 kPa, and after maintaining this state for a while, steaming at 80 to 100 ° C again Is repeated once or more times to reduce the moisture content of the material to be dried, and then the drying method according to claim 1 is performed.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the high-quality wood drying method according to the present invention will be described.
[0010]
This drying method, the step of steaming the material to be dried under normal pressure, the step of drying the surface of the steamed material to be dried under high-temperature and low-humidity conditions, and lowering the temperature than the step of drying the surface, and reducing the pressure. And a step of drying the inside of the material to be dried under the conditions in order to dry the material to be dried with high quality in as short a period as possible.
[0011]
In order to carry out this drying method, a drying apparatus A whose schematic configuration is shown in FIG. 1 is used. The drying apparatus A includes a drying chamber 2 for storing and drying and steaming the dried material 1, and stacking the dried material 1 (a gap is formed between the dried materials by sandwiching the bars between each other). Trolley 3 for taking out into the drying chamber 2, a blower 4 for circulating and flowing air into the drying chamber 2 so as to pass through a gap between the materials 1 to be dried, and air circulated by the blower 4. Steam is supplied to a finned heater 5 and a steam injection pipe 6 for heating and humidifying the heater, a finned heater 5 and a steam injection pipe 6 through a pipe 8 having a control valve 7 and a pipe 10 having a control valve 9. A vacuum pump 14 for connecting the supply steam generator 11 to the drying chamber 2 with a suction pipe 13 having a control valve 12 to reduce the pressure inside the drying chamber 2 and a damper 15 provided at the top of the drying chamber 2 are provided. And an intake / exhaust cylinder 16 provided.
[0012]
In the step of steaming the material to be dried 1 performed by the drying device A, the truck 3 is pulled out from the drying chamber 2 and the material to be dried 2 stacked on the truck 3 is transferred to the truck 3. Then, the carriage 3 is pulled into the drying chamber 2 and the lid is closed, and after closing the drying chamber 2, the blower 4 is operated to circulate the air in the drying chamber 2 as shown by the arrow in FIG. The steam is supplied and injected from the steam generator 11 to the steam injection pipe 6. In this case, since the steam is mixed into the circulating air to heat and humidify the air, a circulating airflow having a temperature of 80 to 100 ° C. and a relative humidity of 100% flows through the gap of the material 1 to be dried. Accordingly, while the blower 4 is reversed at a predetermined time interval to change the direction of the circulating airflow, the steam is cooked until the center of the material to be dried 1 reaches 80 to 100 ° C. The time required for this steaming step is approximately 4 to 8 hours for cedar and cypress wood.
[0013]
In the step of drying the surface layer of the material 1 to be dried, the steaming from the steam injection pipe 6 is stopped and the steam is circulated through the finned heater 5 with the end of the steaming step of the material 1 to be dried. Heat the air circulating in 2. Then, under the condition of a dry-bulb temperature of about 100 to 130 ° C. and a wet-bulb temperature of 20 to 50 ° C. lower than the dry-bulb temperature at a high temperature and a low humidity (a relative humidity of 33 to 34%), the inter-material wind speed is 2.5 to 3 m / sec. Drying is performed while changing the direction of the circulating airflow at predetermined time intervals. In this way, the material to be dried 1 was rapidly dried so as not to cause cracks in the surface of the surface layer, and the required time for drying was generally 6 to 36 hours, depending on the initial moisture content of the material to be dried.
[0014]
In the step of drying the inner layer of the material to be dried, the temperature of the air heating by the finned heater 5 is reduced with the end of the step of drying the surface of the material 1 to be dried, and the dry bulb temperature of the circulating airflow is reduced to 50 to 95 ° C. At the same time, the vacuum pump 14 is operated to suck air from the drying chamber 2 through the suction pipe 13, and the inside of the drying chamber 2 is reduced in pressure to 5 to 70 kPa, and the direction of the circulating airflow is changed at predetermined time intervals. Drying is performed while maintaining the reduced pressure state for a predetermined time, and then returning to normal pressure, maintaining the state for a required time and then reducing the pressure to repeat the drying. In this case, the moisture in the inner layer portion of the material 1 to be dried is promptly moved to the surface layer portion, whereby the drying of the inner layer portion is promoted, and drying without causing internal cracks and darkening of the material color is performed promptly. The required period of drying varies depending on the initial moisture content of the material 1 to be dried, but is generally 4 to 8 days.
[0015]
Further, prior to the surface layer drying step of the material to be dried, as described in claim 2, by additionally performing the steaming pressure reducing step, the initial moisture content of the material to be dried, particularly the operation of reducing that of the heavy material, may be performed. Good. That is, following the steaming at 80 to 100 ° C. under the normal pressure, the inside of the drying device is promptly reduced in pressure to about 5 to 70 kPa, and after maintaining this state for about 1 to 3 hours, the steaming at 80 to 100 ° C. is performed again. To raise the material temperature, or this operation is repeated several times. By this decompression operation, the saturated vapor pressure of the inner layer portion of the material to be dried becomes relatively higher than the pressure in the drying device. Therefore, a phenomenon occurs in which the water content of the material to be dried is extruded, so that the initial moisture content of the drying is reduced, and the energy consumed in the subsequent drying step can be reduced.
[0016]
Example 1
A cedar core material having a cross section of 13 cm x 13 cm, a length of 3 m and an average moisture content of 82% is carried into a highly airtight drying chamber capable of maintaining a reduced pressure, and is dried at a dry bulb temperature of 95 ° C and a wet bulb temperature of 95 ° C. After steaming for 8 hours, the process proceeds to a surface layer drying step under the conditions of a dry bulb temperature of 120 ° C. and a wet bulb temperature of 90 ° C., and after the surface layer is rapidly dried for 18 hours, a dry bulb temperature of 85 ° C. The process of drying the inner layer under the condition that the pressure in the drying chamber was about 32 kPa was maintained for 5 days. As a result, there were almost no cracks on the surface and no cracks on the inside, the color of the wood remained reddish peculiar to cedar without darkening, the water content was reduced to an average of 18%, and the wood was in a moderately dry state.
[0017]
Example 2
The same inner layer drying process of the cedar core material as in Example 1 was performed for 8 days at a dry bulb temperature of 85 ° C. and a wet bulb temperature of 65 ° C. under normal pressure without reducing the pressure. At that time, the finished drying rate was 24% on average, and a large number of poorly dried materials remained. From the above results, it was confirmed that a remarkable effect of shortening the drying time was obtained by setting the internal drying step in a reduced pressure state.
[0018]
Example 3
The cedar heartwood of the same size as in Example 1 was steamed at a dry bulb temperature of 95 ° C and a wet bulb temperature of 95 ° C for only 6 hours, and steamed at a dry bulb temperature of 95 ° C and a wet bulb temperature of 95 ° C. When the change in weight was compared with the case where the reduced pressure state of 35 kPa was maintained for 1 hour after performing for 5 hours, the latter obtained a dehydration amount of 1.96 Kg which was 25% larger than that of the former, and the weight was also increased. Since the amount of dehydration tended to be larger for the material, it was confirmed that the dehydration effect was improved by reducing the steam pressure.
[0019]
【The invention's effect】
Effect of Claim 1 Wood drying that hardly causes surface cracks and internal cracks of the material to be dried and darkening of the material color is performed in a shorter time than the conventional drying method, and the quality and productivity of the dried material are improved. It can have significant effects.
[0020]
Effect of Claim 2 The saturated vapor pressure of the inner layer of the material to be dried becomes relatively higher than the internal pressure of the drying chamber, causing a phenomenon that water content is pushed out from the material to be dried, so that the water content in the initial stage of drying decreases. As a result, the required energy in the drying step can be significantly reduced.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a drying apparatus used for a high-quality drying method for wood according to the present invention.
[Explanation of symbols]
A drying device 1 material to be dried 2 drying room
