柞木单板高频真空干燥工艺
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摘要
以5 mm柞木单板为研究对象,对其实施不同干燥工艺(高频发振与停歇时间,木材控制温度(Tc)、环境压力(Pa))的高频真空干燥,测算其温度分布、干燥速度、干燥周期、终含水率及其标准偏差、脱水比、开裂和翘曲度等参数。通过对这些参数的对比分析,确定了其较适宜的高频真空干燥工艺。结果表明:干燥过程中单板材堆的温度分布变化,长度方向近端部略高,其它部位相近;宽度方向,呈现内部高侧边低的分布趋势;高度方向,呈现中心层高、近接地极板层低的分布趋势。底层与接地极板间设置已干单板后虽能减小温度梯度,但不能使其消除。单板干燥速度取决于温度(T)、Pa、水分渗透性、扩散系数、迁移距离(L),Pa降低,T升高,水分迁移驱动力(ΔP/L)增大,干燥速度加快;T升高还能使渗透性和扩散系数增大,因而对干燥速度影响显著,但单板易开裂;单板终含水率分布均匀性主要由材堆中温度分布均匀性、干燥工艺条件等决定。在材堆与电极板间设置隔热材料,能使材堆高度方向温度分布均匀性提高,含水率差异减小。5 mm柞木单板较适宜的高频真空干燥工艺确定为,高频发振7 min/停歇1 min、Tc为54.5℃、Pa为6.5 k Pa、ΔP为8.5 k Pa。
With the 5-mm oak veneers dried with different technique(high frequency vibration on and stop time,wood controlled temperature(Tc),environmental pressure(Pa) of radio-frequency vacuum drying),we measured the temperature distribution,drying speed,drying cycle,final moisture content and its standard deviation,dehydration ratio,crack,and warp.Through comparative analysis of these parameters,we found the suitable radio-frequency vacuum drying technique. In terms of the variation of temperature distribution during drying process,there was slightly higher near end and the other parts were similar along length direction. With respect to the length width direction,the interior of the veneer pile was high and the side edge was low. The height direction showed the distribution trend of high level of center layer and low level close to ground electrode plate. The temperature gradient can be reduced by placing the dried oak veneer between the bottom of the pile and the ground electrode plate,but it can't be eliminated. Veneer drying speed depended on the temperature(T),Pa,moisture permeability,diffusion coefficient,and the moisture migration distance L,the reduced Paand the upward T led to increased moisture migration driver ΔP/L,which speeded up the drying process. The upward T could also increase the permeability and diffusion coefficient,so it had a significant influence on the drying speed,but it was easy for veneers to crack. The veneer final moisture distribution uniformity was mainly determined by the uniformity of temperature distribution in the veneer pile and the drying technique conditions. The heat insulation material between the veneer pile and the electrode plate can be used to increase the temperature distribution uniformity along the height direction and reduce the difference of moisture content. The suitable radio-frequency vacuum drying technique was determined as the high frequency on 7 min/off 1 min,Tcof 54.5 ℃,Paof 6.5 k Pa,and ΔP of 8.5 k Pa.
With the 5-mm oak veneers dried with different technique(high frequency vibration on and stop time,wood controlled temperature(Tc),environmental pressure(Pa) of radio-frequency vacuum drying),we measured the temperature distribution,drying speed,drying cycle,final moisture content and its standard deviation,dehydration ratio,crack,and warp.Through comparative analysis of these parameters,we found the suitable radio-frequency vacuum drying technique. In terms of the variation of temperature distribution during drying process,there was slightly higher near end and the other parts were similar along length direction. With respect to the length width direction,the interior of the veneer pile was high and the side edge was low. The height direction showed the distribution trend of high level of center layer and low level close to ground electrode plate. The temperature gradient can be reduced by placing the dried oak veneer between the bottom of the pile and the ground electrode plate,but it can't be eliminated. Veneer drying speed depended on the temperature(T),Pa,moisture permeability,diffusion coefficient,and the moisture migration distance L,the reduced Paand the upward T led to increased moisture migration driver ΔP/L,which speeded up the drying process. The upward T could also increase the permeability and diffusion coefficient,so it had a significant influence on the drying speed,but it was easy for veneers to crack. The veneer final moisture distribution uniformity was mainly determined by the uniformity of temperature distribution in the veneer pile and the drying technique conditions. The heat insulation material between the veneer pile and the electrode plate can be used to increase the temperature distribution uniformity along the height direction and reduce the difference of moisture content. The suitable radio-frequency vacuum drying technique was determined as the high frequency on 7 min/off 1 min,Tcof 54.5 ℃,Paof 6.5 k Pa,and ΔP of 8.5 k Pa.
引文
[1]程瑞香.三层结构实木复合地板表板发展趋向[J].中国人造板,2007(12):5-7.
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[6]LOPATIN V V,GORESHNEV M A,SEKISOV F G.Moisture transport in birch lumber at low radio-frequency and contact vacuum drying[J].European Journal of Wood&Wood Products,2014,72(6):779-784.
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[8]FANG F,RUDDICK J N R,AVRAMIDIS S.Applacation of radiofrequency heating to utility poles.Part 1.Radio-frequency/vacuum drying of roundwood[J].Forest Products Journal,2001,51(7):56-60.
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[10]蔡英春.高频真空干燥机理[M].哈尔滨:东北林业大学出版社,2007:47-49.
[11]赵小矛,张帝树.桦木、柞木小径材刨切薄板热压干燥的研究[J].木材工业,1998,12(2):12-15.
[12]邵小桃,成超,张静文.平板电容器边缘电场特性探究[J].电气电子教学学报,2011,33(4):41-43.
[13]李贤军,张璧光,李文军,等.微波真空干燥过程中木材内部的温度分布[J].北京林业大学学报,2006,28(6):128-132.
[14]SAHIN H S,AY N.Dielectric properties of hardwood species at microwave frequencies[J].Journal of Wood Science,2004,50:375-380.
[15]LIU H H,YANG L,CAI Y C,et al.Distribution and variation of pressure and temperature in wood cross section during radiofrequency vacuum(RF/V)drying[J].Bioresources,2014,9(2):3064-3076.
[16]刘冬,彭晓峰,王补宣.微尺度沸腾的团聚过程分析与压力扰动模型[J].航空动力学报,2000,15(1):85-88.
[2]吴智慧.复合地板的生产工艺技术[J].林产工业,1997,24(2):41-43.
[3]吕蕾,时兰翠,黄海兵,等.柞木干燥预热时间的影响因素探讨[J].木材工业,2013,27(3):51-53.
[4]江京辉.过热蒸汽处理柞木性质变化规律及机理研究[D].北京:中国林业科学研究院,2013.
[5]郭明辉,赵西平,闫丽,等.干燥室内锯材摆放位置对干燥缺陷的影响[J].木材工业,2004,18(5):27-31.
[6]LOPATIN V V,GORESHNEV M A,SEKISOV F G.Moisture transport in birch lumber at low radio-frequency and contact vacuum drying[J].European Journal of Wood&Wood Products,2014,72(6):779-784.
[7]JUNG S K,EOM C D,SO B J.Comparison of vacuum drying characteristics of radiata pine timber using different heating method[J].Drying Technology,2004,22(5):1005-1022.
[8]FANG F,RUDDICK J N R,AVRAMIDIS S.Applacation of radiofrequency heating to utility poles.Part 1.Radio-frequency/vacuum drying of roundwood[J].Forest Products Journal,2001,51(7):56-60.
[9]HUANG R F,WU Y M,ZHAO Y K,et al.Factors affecting the temperature increasing rate in wood during radio-frequency heating[J].Drying Technology,2013,31(2):246-252.
[10]蔡英春.高频真空干燥机理[M].哈尔滨:东北林业大学出版社,2007:47-49.
[11]赵小矛,张帝树.桦木、柞木小径材刨切薄板热压干燥的研究[J].木材工业,1998,12(2):12-15.
[12]邵小桃,成超,张静文.平板电容器边缘电场特性探究[J].电气电子教学学报,2011,33(4):41-43.
[13]李贤军,张璧光,李文军,等.微波真空干燥过程中木材内部的温度分布[J].北京林业大学学报,2006,28(6):128-132.
[14]SAHIN H S,AY N.Dielectric properties of hardwood species at microwave frequencies[J].Journal of Wood Science,2004,50:375-380.
[15]LIU H H,YANG L,CAI Y C,et al.Distribution and variation of pressure and temperature in wood cross section during radiofrequency vacuum(RF/V)drying[J].Bioresources,2014,9(2):3064-3076.
[16]刘冬,彭晓峰,王补宣.微尺度沸腾的团聚过程分析与压力扰动模型[J].航空动力学报,2000,15(1):85-88.