木材乾燥に対する意識の位相に関する研究(1) : 中九州の製材分野における乾燥木材の生産構造

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近年，住宅の供給構造や生活様式の変化，あるいは木材流通構造の変化にともなって乾燥木材に対する要求が強まりつつある．製材工場は木材乾燥への関心を高め，産地間競争に勝ち残るために地域的な課題として取り組みを強めている地域もある．しかし，木材の生産流通にかかわる担い手の木材乾燥に対する認識は，含水率概念の欠如や乾燥コスト概念の未成熟なこと等にみられるように，混乱しており，乾燥木材の需要と生産を拡大する上で大きな障害になっている．本研究は，このような木材の生産流通にかかわる担い手の木材乾燥に対する意識の位相を明らかにし，これによって乾燥木材の生産流通システムのあり方と価格形成の適正化の方向を明らかにすることを目的としており，本報告では，人工乾燥施設を持つ製材工場における乾燥木材の生産構造とその問題点を中九州地区の製材工場に対するアンケート調査によって明らかにした．その概要は次の通りである．1）1988年の林野庁の調査によると，全国の製材工場に設置されている人工乾燥施設数は1470室で，種類別では蒸気式59％，低温除湿式29％，その他12％の比率である．乾燥木材の生産量は945千m^3で，樹種別では国産材44％，外材56％であり，用途別では構造用材は29％でごく一部に過ぎない．九州では全国の20％が生産されており，国産材が61％を占めるが，構造材は12％に過ぎない．2）中九州（熊本，大分県）の乾燥施設設置製材工場は51工場で，年商額や従業員数は製材工場の中では上層に位置している．乾燥方式によって区分すると低温除湿式乾燥室を持つ低温除湿式類型が53％で最も多く，ついで蒸気式類型33％，その他類型24％である．樹種はヒノキやスギ等の国産針葉樹材が圧倒的に多く，外材を乾燥するところは少ない．3）乾燥木材の用途によって区分すると，一般建築内装材や役物柱角等の乾燥を行う建築用材型が69％で最も多く，ついでフローリング材や集成材等の集成材型33％，家具建具用材型31％，その他型28％の割合である．用途類型と乾燥類型には相関関係があり，低温除湿式では建築用材型が多く，蒸気式では集成材型が多い．また，前者は乾燥室が小さく，年間生産量も少ないが，後者は乾燥室生産量ともに規模が大きい．4）木材の人工乾燥の前処理としての天然乾燥を，蒸気式類型は70％が行っているが，低温除湿式類型は半分に達しない．また，人工乾燥の目標含水率は蒸気式では20％未満のところに集中しているが，低温除湿式では収斂点が認められない．低温除湿式類型には「含水率概念の欠如」というべき状況がみられる．5）1m^3当りの乾燥コストは，蒸気式類型では6～10千円を中心に4～10千円に集中しているが，低温除湿式類型では4～15千円まで分散し，収斂点がはっきりしない．標準的な場合であれ，最も効率的な条件を想定した場合であれ，エ場毎にばらばらで「コスト概念の成熟」とは程遠い状況にある．6）乾燥コストの軽減の方策として，製品の天然乾燥，ついで葉枯らしなどの原木乾燥を上げるものが多く，特に蒸気武類型では製品の天然乾燥を重視するものが多い．伐採から製材加エに至る木材の全生産過程を通して含水率管理を行う必要がある．7）乾燥木材の需要拡大のためには，乾燥施設の導入による供給数の増大と同時に，適正コストのコンセンサスの形成が必要であるが，「含水率概念の欠如」や「乾燥コストの未成熟」な状況では困難であり，知識，技術の普及によって木材乾燥に関する認識の成熟化が求められている．Recently, the demand for dried lumber has been increasing, as the structure of the housing construction and timber distribution have changed. Saw mill managers are taking more interest in wood drying and some timber districts have begun to dry wood as a regional measure to strengthen their competitive position in timber markets. But the increase in demand and supply of dried lumber is being hindered by confused approach on the part of lumber producers and timber distributors. This confuse can observed in the lack of attention to moisture content and immature approach to reasonable wood drying costs. In this paper, the author analyzes the structure of the production of dried lumber and the approaches to wood drying using a questionnaire given to saw mill managers who have wood drying kilns in central Kyushu. (1) According to research by the Forest Agency 1988, there were 1470 wood drying kilns attached to saw mills throughout Japan. Of these saw mills, steam type kilns were present in 59%, dehumidifier type kilns in 29%, and other types in 12%. Volume of dried wood production was 945 thousands m^3. Of this, domestic timber represented 44% and foreign timber 56%. Structural timber represented only 29%. Dried wood production in Kyushu was 20% of nationwide volume, domestic timber representing 60%, whereas structural timber only 12%. (2) In the central Kyushu (Kumamoto pref. and Oita pref.) there were 51 saw mills with drying kilns. There were in the upper range in terms of their annual sales and their number of employees. 50% of saw mills with drying kilns had a dehumidifier type, 33% had a steam type and 24% had other types. The greatest volume of dried lumber was hinoki(Chamaecyparis obtusa) and next was sugi(Cryptomeria japonica). Few saw mills were drying foreign timber. (3) Classifying saw mills acording to the utilization of dried lumber, there were four types. The 1st was construction lumber type, drying interior member, fancy colum and other construction timber. The 2nd was a laminated lumber type, drying board for flooring or glued -laminated lumber. The 3rd was a furniture lumber type, drying lumber for furniture, sliding doors, paper screens etc., And the 4th were ohter lumber types. The 1st was present in 69% of mills, the 2nd in 33%, the 3rd in 31%, and the 4th in 28% (this total is over 100% because some mills had two or three kiln types). There was a close correlation between utilization and drying method. Most dehumidifier types were of construction lumber type, and most steam kilns were of laminated type. Farmers used small drying types and small annual production volumes, the latter being comparatively larger. (4) 70% of the steam type used air seasoming as a pretreatment in artificial drying, whereas less than half of the dehumidifier type did. The final moisture content of the steam type aimed to concentrated under 20%, but dehumidifier types had no convergent level. Dehumidefier type had not considered final moisture content. (5) The lumber drying cost per m^3 of the steam type was between six and ten thousand-yen. But the drying cost of the dehumidifier type varied widely from four to fifteen thousand-yen, and convergent level of the cost was blurred. In the case of both the standard method of wood drying and most efficient method, drying costs were scatterd, because concept of a reasonable drying cost had 1101 developed. (6) As a means of reducing the cost of wood drying, most saw mills advocate air seasonig of sawn lumber and seasonig of logs in forests. Especially air seasoning of sawn lumber is considered to be important by steam type mills. It is therefore necessary that moisture content should be controlled throughout all processes in lumber production from felling to sawing. (7) In order to increase the demand for dried lumber, it is necessary not to increase the supply of dried by more investment in drying kilns but also to reach a consensus concerning reasonable drying costs between producers, dealers and end users. But this is impossible due to a lack of attention paid to final moisture content and an immature approach to reasonable drying costs. It is therefore necessary to make the wood drying industry more efficient through improvements in techniques and an illcrease in available knowledge.

著者

堺正紘
九州大学農学部

木材乾燥に対する意識の位相に関する研究(1) : 中九州の製材分野における乾燥木材の生産構造

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