重齿铁线子木材高频真空干燥工艺研究
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摘要
高频真空干燥有效集成了高频快速加热与真空下水沸点降低的双重优势,能够使木材在低温下快速干燥,在干燥名贵和大断面木材上具有综合优势。探讨了高频真空干燥工业生产中干燥工艺参数的计算方法和选用原则,并对重齿铁线子(Manilkara bidentata)木材进行了生产试验验证。结果表明:单位体积输入功率(Pu)及脱水量的确定是干燥工艺实施及干燥过程控制的前提,Pu可以通过调整高频发振设备的电流(IP)、电压(EI)及发振率(R)来设置;干燥后期通过下调IP降低极板间的电压来避免干燥过程极板放电;干燥后木材含水率及干燥缺陷满足国标二级要求;干燥速度为1.2%/d,且随含水率的下降而减小。含水率在20.9%~10%之间,脱水效率与材温呈正相关。此外,脱水效率与含水率呈二次抛物线关系,在含水率为14.5%时取得峰值;材料升温能耗占全部能耗的11.2%,全部过程高频加热能耗占比为71.7%;升温阶段能量转换效率为44.5%,干燥阶段为27.7%,升温及干燥时间计算时应该考虑能量转换效率。干燥工艺参数计算方法、选用原则及试验结果可为高频干燥工业生产提供技术参考。
Radio-frequency/vacuum( RF/V) technology can dry wood with high drying rate and high quality because the radio-frequency can heat wood fast and the vacuum condition can lower water boiling point,accelerating water transfer rate. It has significant advantages for the drying of large dimension or high valued wood. Many studies of RF/V drying have been conducted to apply this technology to the drying industry. However,most of them conducted by using the lab-scale dryers,not the industrial scale. To solve the industrial production problems on drying technology and schedule development,this study investigated the drying related parameter calculation methods and selection rules. In addition,the hardwood lumber( Manilkara bidentata) RF/V drying tests were carried out according to the calculated parameters for comparison. The wood quality such as final moisture content( MC) distributions,cracks,power cost,energy conversion efficiency,drying rate and efficiency of dehydration after drying was explored. The results indicated that the unit input power( Pu) should be determined firstly according to the wood dry characteristics and size,then determined by other factors,such as,input current( IP),input voltage( EI) and oscillation efficiency( R). The quantity of dehydration by 1% MC was important for MC monitoring during the drying process. There were no cracks on ends,surface and inner of lumber,and the MC was even for all size of materials and the wood quality after R/FV drying can meet the grade 2 of the GB/T 6491-2012 standard. The power cost in the heating stage took 11.2% of the total energy consumption,and the energy cost of the RF heating counted 71.7% of the whole process. Since the energy conversion efficiency in heating stage and drying stage were 44.5% and 27.7%,respectively,the energy conversion efficiency should be considered in the parameter calculation. The total average drying rate was 1.2% each day,and decreased as the MC decreased. The efficiency of dehydration was affected by lumber temperature and the MC. When the MC was between 20.9% and 10. 0%,it was of positively related with the lumber temperature. The efficiency of
dehydration had quadratic relations with the MC and it reached the peak at an MC of about 14.5%.The parameter calculation method and selection rules as well as the results of this study can provide industrial RF/V drying productions with useful technical information.
Radio-frequency/vacuum( RF/V) technology can dry wood with high drying rate and high quality because the radio-frequency can heat wood fast and the vacuum condition can lower water boiling point,accelerating water transfer rate. It has significant advantages for the drying of large dimension or high valued wood. Many studies of RF/V drying have been conducted to apply this technology to the drying industry. However,most of them conducted by using the lab-scale dryers,not the industrial scale. To solve the industrial production problems on drying technology and schedule development,this study investigated the drying related parameter calculation methods and selection rules. In addition,the hardwood lumber( Manilkara bidentata) RF/V drying tests were carried out according to the calculated parameters for comparison. The wood quality such as final moisture content( MC) distributions,cracks,power cost,energy conversion efficiency,drying rate and efficiency of dehydration after drying was explored. The results indicated that the unit input power( Pu) should be determined firstly according to the wood dry characteristics and size,then determined by other factors,such as,input current( IP),input voltage( EI) and oscillation efficiency( R). The quantity of dehydration by 1% MC was important for MC monitoring during the drying process. There were no cracks on ends,surface and inner of lumber,and the MC was even for all size of materials and the wood quality after R/FV drying can meet the grade 2 of the GB/T 6491-2012 standard. The power cost in the heating stage took 11.2% of the total energy consumption,and the energy cost of the RF heating counted 71.7% of the whole process. Since the energy conversion efficiency in heating stage and drying stage were 44.5% and 27.7%,respectively,the energy conversion efficiency should be considered in the parameter calculation. The total average drying rate was 1.2% each day,and decreased as the MC decreased. The efficiency of dehydration was affected by lumber temperature and the MC. When the MC was between 20.9% and 10. 0%,it was of positively related with the lumber temperature. The efficiency of
dehydration had quadratic relations with the MC and it reached the peak at an MC of about 14.5%.The parameter calculation method and selection rules as well as the results of this study can provide industrial RF/V drying productions with useful technical information.
引文
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[8]刘洪海,杨琳,吴智慧,等.大断面欧洲赤松的真空变定—高频干燥[J].东北林业大学学报,2017,45(2):61-64.LIU H H,YANG L,WU Z H,et al.Effect of vacuum drying set and radio-frequency/vacuum drying on wood quality[J].Journal of Northeast Forestry University,2017,45(2):61-64.
[9]AVRAMIDIS S,ZWICK R L.Commercial-scale RF/V drying of softwood lumber.2.Drying characteristics and lumber quality[J].Forest Products Journal,1996,46(6):27-35.
[10]夏兴华,蔡英春.高频-对流木材干燥设备中高频发生器的选用[J].东北林业大学学报,2010,38(6):125-126.XIA X H,CAI YC.Selection of high-frequency generator for highfrequency convective drying of wood[J].Journal of Northeast Forestry University,2010,38(6):125-126.
[11]蔡英春.木材高频真空干燥机理[M].哈尔滨:东北林业大学出版社,2007.
[12]寺澤眞,金川靖,林和男,等.木材の高周波真空乾燥[M].大津:海青社,1998.
[13]YANG L,LIU H H,CAI Y C,et al.Real-time moisture content measurement of wood under radio-frequency/vacuum(rf/v)drying[J].Drying Technology,2014,32(14):1675-1682.
[14]李丽霞,张妍,张宏斌.日产高频真空干燥机的应用[J].林业机械与木工设备,1996(1):22-23.
[15]董丰,侍继梅,涂登云,等.水曲柳地板坯料气干与窑干联合干燥工艺的研究[J].林业科技,2011,36(5):43-45.DONG F,SHI J M,TU D Y,et al.Research on drying technology of Fraxinus mandshurica flooring blank[J].Foresty Science and technology,2011,36(5):43-45.
[16]高瑞清,李晓玲,黑田尚宏,等.人工林木材高频真空干燥与其他干燥方法的比较(英文)[J].北京林业大学学报,2005,27(S1):27-31.GAO R Q,LI X L,NAOHIRO K,et al.Comparison of RFV method with other drying methods for plantation timber[J].Journal of Beijing Forestry University,2005,27(S1):27-31.
[2]HUANG R F,WU Y M,LU J X,et al.Factors affecting the temperature increasing rate in wood during radio-frequency heating[J].Drying Technology,2013,31(3):246-252.
[3]AVRAMIDIS S,LIU F.Drying characteristics of thick lumber in a laboratory radio-frequeocy/vacuum dryer[J].Drying Technology,1994,12(8):1963-1981.
[4]张璧光.我国木材干燥技术现状与国内外发展趋势[J].北京林业大学学报,2002,24(5/6):262-266.ZHANG B G.Present status of wood drying technology in China and development tendency at home and abroad[J].Journal of Beijing Forestry University,2002,24(5/6):262-266.
[5]庄寿增.木材高频真空干燥及其应用前景[J].林业科技开发,1992,6(1):19-20.
[6]李晓玲,小林功,黑田尚宏,等.日本柳杉髓心方材高频真空干燥试验[J].北京林业大学学报,2005,27(S1):42-46.LI XL,KOBAYASHI I,KURODA N,et al.Radio frequency vacuum drying tests on Japanese sugi[J].Journal of Bei Ijing Forestry University,2005,27(S1):42-46.
[7]刘洪海,杨琳,吴智慧,等.大果紫檀等3种红木薄板的高频真空干燥[J].林业工程学报,2017,2(1):15-19.LIU H H,YANG L,WU Z H,et al.Study on radio-frequency vacuum drying of three rosewood thin boards[J].Journal of Forestry Engineering,2017,2(1):15-19.
[8]刘洪海,杨琳,吴智慧,等.大断面欧洲赤松的真空变定—高频干燥[J].东北林业大学学报,2017,45(2):61-64.LIU H H,YANG L,WU Z H,et al.Effect of vacuum drying set and radio-frequency/vacuum drying on wood quality[J].Journal of Northeast Forestry University,2017,45(2):61-64.
[9]AVRAMIDIS S,ZWICK R L.Commercial-scale RF/V drying of softwood lumber.2.Drying characteristics and lumber quality[J].Forest Products Journal,1996,46(6):27-35.
[10]夏兴华,蔡英春.高频-对流木材干燥设备中高频发生器的选用[J].东北林业大学学报,2010,38(6):125-126.XIA X H,CAI YC.Selection of high-frequency generator for highfrequency convective drying of wood[J].Journal of Northeast Forestry University,2010,38(6):125-126.
[11]蔡英春.木材高频真空干燥机理[M].哈尔滨:东北林业大学出版社,2007.
[12]寺澤眞,金川靖,林和男,等.木材の高周波真空乾燥[M].大津:海青社,1998.
[13]YANG L,LIU H H,CAI Y C,et al.Real-time moisture content measurement of wood under radio-frequency/vacuum(rf/v)drying[J].Drying Technology,2014,32(14):1675-1682.
[14]李丽霞,张妍,张宏斌.日产高频真空干燥机的应用[J].林业机械与木工设备,1996(1):22-23.
[15]董丰,侍继梅,涂登云,等.水曲柳地板坯料气干与窑干联合干燥工艺的研究[J].林业科技,2011,36(5):43-45.DONG F,SHI J M,TU D Y,et al.Research on drying technology of Fraxinus mandshurica flooring blank[J].Foresty Science and technology,2011,36(5):43-45.
[16]高瑞清,李晓玲,黑田尚宏,等.人工林木材高频真空干燥与其他干燥方法的比较(英文)[J].北京林业大学学报,2005,27(S1):27-31.GAO R Q,LI X L,NAOHIRO K,et al.Comparison of RFV method with other drying methods for plantation timber[J].Journal of Beijing Forestry University,2005,27(S1):27-31.