共查询到20条相似文献，搜索用时 31 毫秒
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The first part of this contribution determined the demands on climate control in vacuum drying kilns that are necessary to achieve the final moisture content variation stipulated in the European Drying Group (EDG) proposal on wood drying quality standards. In this second part of the contribution, these demands are compared with measurements of actual climates and the resulting final moisture content in vacuum kilns during industrial production. The measurements show that none of the studied industrial vacuum kilns are capable of controlling climate with acceptable accuracy. The variations in drying climate lead to large variations in final moisture content and reduced production capacities. Drying quality and drying capacity would be greatly increased with improved kiln design and improved climate control systems. Critical factors in kiln design and climate control system design necessary to maintain a controlled drying climate are listed. 相似文献
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《Wood material science & engineering》2013,8(1-2):71-77
Abstract Discolouration during the drying of non-steamed beech is a major industrial concern. The generally preferred yellow-white colour can easily develop a reddish or dull grey appearance during drying. In this study, the influences of log storage time and kiln drying climate on the colour of non-steamed sawn beech have been investigated and quantified. Samples 27×87 mm in cross section were dried in laboratory kilns and the average colour, in CIELAB colour space was measured on dry planed surfaces using a photoelectric colorimeter. Log storage for 13 weeks under low-temperature conditions had no visible effect. The reddish discolouration is mainly temperature related while the greyish discolouration is mainly controlled by the equilibrium moisture content (EMC) during the initial drying. Within the investigated climate interval, the EMC was twice as important as temperature for the final colour. Regression models developed show that, as long as the EMC is kept below 15%, a temperature of up to 37°C can be allowed without any visually detectable discolouration. 相似文献
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Björn A. Källander 《Wood Science and Technology》2002,36(6):477-486
The European Drying Group (EDG) proposal on a wood drying quality standard defines demands on final moisture content variation of the dried wood. The final moisture content variation will depend on material parameters as well as the production process and the wood will always show a “natural” moisture content variation after drying. Thus the drying process has to be defined well enough to allow for the natural moisture content variation in order to fulfil the demands of the drying standards. As the average equilibrium moisture content of the wood in a vacuum drying kiln with pure steam atmosphere is determined by the pressure and the temperature, the demands on the climate control system to fulfil the demands of the drying standard can be calculated with regard to the natural moisture content variation of the wood. In the first part of this contribution the demands on climate control in vacuum dryers are calculated based on the EDG-standard and the natural moisture content variation. In the second part of the contribution the demands on climate control are compared with climate and moisture content measurements from industrial production in vacuum kilns. Critical factors in kiln design and climate control system design necessary to maintain a controlled drying climate are listed. 相似文献
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《Wood material science & engineering》2013,8(2):55-65
Abstract During drying, timber changes its shape, mainly as a result of specific properties such as shrinkage anisotropy, radial differences in longitudinal shrinkage and spiral grain. The distortion, causing severe downgrading, can be reduced by restraining the timber and by using special drying schemes. The research described here is related to a project on the improvement of shape stability of Norway spruce. In the present part, different larch species from three stands were dried at high temperatures (80, 120 and 170°C). The effects of restraint during presteaming, drying and steaming on short-term twist reduction were investigated. The permanency of drying distortions was investigated in subsequent moisture cycling. Results showed a clear dependency of twist on the distance to pith. Restrained specimens sawn close to pith experienced reduced twist. This twist reduction was permanent during subsequent moisture variations. Drying temperature did not significantly influence twist and twist amplitude in moisture cycling. 相似文献
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《Wood material science & engineering》2013,8(1):24-33
Abstract Siberian larch (Larix sibirica Ledeb.) timber is suitable for many uses in the mechanical wood industry. Drying it without any decrease in value, however, is difficult and slow. The purpose of this research was to compare the drying quality of Siberian larch timber dried with three different conventional schedules taking into account the density and annual growth of wood. Five drying tests were performed. The final moisture content (MC), MC gradient, cracks, deformations (bow, crook, twist and cup) and case hardening were measured from the dried timber. The basic density particularly affected the MC, as shown in the differences regarding dried wood, with the denser wood having higher final MC and MC gradient. It was also found that large annual growth increased some deformations. Most of the measured factors were best after drying at the highest temperatures used; however, a slightly different trend was observed for bow, twist and cup. MC factors and twisting were the most problematic properties in drying according to this study. Sorting Siberian larch timber, particularly according to density, would improve the MC properties of dried timber by ensuring sufficient drying time, as economically as possible, for each timber piece. 相似文献
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《Wood material science & engineering》2013,8(1):37-45
Abstract Plantation wood from tropical climate has been introduced rapidly in the national market; however, there is lack of knowledge about the process. The main objective of this study was to investigate the kiln drying behavior of 10 plantation-grown wood species from natural forest in Costa Rica using the recommended drying schedule. Initial moisture content (MCi), final moisture content (MCf), drying rate, drying time, and drying defects were evaluated. The drying schedule applied produced the following results: (1) high MCi (over 110%) in four species and lower values in two species. (2) The largest drying time was found in species with high MCi, and the shortest drying time in species with lower MCi. (3) Significant variations of target MCf were found in some species, despite equalization and conditioning. (4) Exponential relationship MC=a*??t*b was used to establish a moisture content (MC) decrease model, which is not a good indicator of drying time for fives species. (5) High incidence of drying defects was found in Alnus acuminata and Vochysia guatemalensis. According to the above results, it is necessary to test other drying schedule oriented to improve lumber quality and to decrease variability of MC in wood from plantation trees. 相似文献
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以菲律宾桃花芯木为研究对象,探索30 mm厚菲律宾桃花芯木锯材干燥工艺。通过百度试验法得知其初期开裂为3级、内裂为2~3级、截面变形为1级;通过密度测定实验得知其气干密度为0.562 g/cm^3、全干密度为0.517 g/cm^3和基本密度为0.465 g/cm^3。根据该木材密度和干燥特性制定3种30 mm厚菲律宾桃花芯木锯材干燥基准并分别进行常规干燥工艺试验。通过对3次工艺试验结果的综合分析表明：在3次工艺实验所用执行基准均能满足2级干燥指标。其中,第一次工艺实验所用执行干燥基准为本研究中最佳干燥基准。第一次工艺实验的初始温度为60℃,末期温度为80℃、初含水率为66.97%、终含水率为7.79%的锯材干燥周期为185 h。 相似文献
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马尾松木材在高温干燥中的水分扩散性 总被引：9，自引：0，他引：9
对马尾松木材在高温干燥过程中的水分非稳态扩散进行了研究 ,结果表明当含水率高于纤维饱和点时 ,水分扩散系数随含水率的降低而增加 ;当含水率低于纤维饱和点时 ,水分扩散系数随含水率的下降而减少。马尾松木材的径向扩散系数大于弦向扩散系数。随着温度的升高和相对湿度的降低 ,木材的横向水分扩散系数增大 相似文献
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《Wood material science & engineering》2013,8(3-4):162-172
Abstract A new model for predicting moisture content, distortion and shrinkage distribution after lumber drying has been designed, implemented and tested. The model was implemented using Monte Carlo simulation, and it involves three empirical equations that were developed on the basis of experimental data. The model is referred as the Lumber Quality Model, and it is designed to be calibrated by knowing the initial and final moisture content, distortion and shrinkage distribution for a reference drying run. After calibration, the model can be used to predict the same information for other hypothetical drying scenarios. The present study explains the theoretical aspects of the model and the methodology for implementation. The model was validated with experimental data measured in a laboratory kiln. A full-scale industrial validation will be reported in a future paper. 相似文献
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研究了竹材热压干燥过程中的水分迁移特性.结果表明:在整个干燥过程中,前期含水率降低较快,后期含水率降低较慢.竹材平均干燥速度与次表层竹材的干燥速度相近;在含水率较高的干燥初期,水分迁移的阻力在竹材表面,水分迁移主要靠毛细管张力作用;在含水率较低的干燥后期,水分迁移的阻力在竹材内部,水分迁移主要以扩散方式进行,干燥速度取决于木材内部水分移动的速度.竹材热压干燥过程中的水分移动,主要受温度梯度和含水率梯度的共同作用. 相似文献
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A numerical model that predicts the stochastic dispersion associated with industrial kiln drying of timber was adapted to conventional drying and evaluated with experimental data. The theoretical aspects of the model are briefly explained, a selection of the calibration parameters was carried out, and a new empirical dispersion factor is proposed to account for all unknown sources of random behavior. The model was calibrated with six experimental runs of western hemlock and amabilis fir (116mm2 timbers) to an average moisture content (target) of 14%–20%. It was found that with implementation of the dispersion factor, the number of required simulations is considerably reduced, the calibration results are consistent for all the experimental runs, and the target moisture content along with its standard deviation can be well reproduced using the all-run average parameters. 相似文献
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Ikuo Momohara Shuetsu Saito Wakako Ohmura Makoto Kiguchi 《Journal of Wood Science》2009,55(6):441-445
Green Japanese cedar logs 2 m in length and 18 cm in diameter were dried to a mean moisture content of less than 30% by either
air drying or kiln drying. Dried logs were impregnated with copper azole (CUAZ) solution according to Japanese Industrial
Standard (JIS) A9002. Preservative absorption was calculated from the log weight before and after preservative impregnation.
Impregnated logs were then dried in the air and cut at the center to determine preservative penetration. The penetration area
was determined after visualizing the preservative with chrome azurol S. The visualized area indicated that the preservative
solution penetrated into the sapwood portion of almost all the air-dried logs. However, the kiln-dried logs did not show full
penetration into the sapwood portion. The visualized area of some kiln-dried sapwood showed a penetration value of less than
80%, which is the minimum requirement set by the Japanese Agricultural Standard (JAS) for sawn timber. Statistical analysis
showed that penetration in the air-dried sapwood was significantly better than that in the kiln-dried sapwood. It was concluded
that air drying is more favorable than kiln drying as the predrying method for CUAZ impregnation. On the other hand, preservative
absorption was not affected by the drying method. 相似文献
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Steam conditioning of softwood boards after kiln drying is of critical importance for relief of residual drying stresses
and to improve distribution of final moisture content. The conditioning practice in New Zealand includes two steps: immediately
after high temperature (HT) drying the load is cooled until the core wood temperature is 75 to 90°C, and then the stack is
steam conditioned for a period of 1 to 4 hours depending on the lumber thickness and moisture content after drying. In this
work, experimental and theoretical studies were performed to better understand the conditioning process and to investigate
factors which influence its effectiveness. In the experiment, 50 mm thick Pinus radiata sapwood boards were first dried at 120/70°C for 11, 12, 13, 16 and 18 hours, respectively, to varying moisture contents,
and then cooled and steam conditioned for 1 hour. To assess the effectiveness of conditioning, moisture pick-up, moisture
gradient, and transverse residual drying stress (indicated by cup and strain) were measured. It was found that drying wood
to a low moisture content (below 6%) increased the conditioning effectiveness. A separate matched stack was conditioned for
4 hours after 13 hours drying which showed better results than 1 hour conditioning. A mathematical model for wood drying was
extended to include both the cooling and conditioning phases. The model was numerically solved to examine the wood temperature
and moisture content changes during the whole process of drying, cooling and final steam conditioning. Increase in wood temperature,
moisture pickup and moisture gradient during steam conditioning were predicted and validated by the experimental data. This
information is currently being used at the New Zealand Forest Research Institute in simulation of stress development and relief
for drying of Pinus radiata lumber.
Received 6 July 1998 相似文献
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《Wood material science & engineering》2013,8(2):45-51
Abstract A stochastic three-dimensional model for the behaviour of free water in wood, which was described in Part I, is used for the simulation and analysis of some further features of wood drying. The kiln brown stain mechanism is studied and the connection to the condition of the wood surface is pointed out. Several drying rate questions and related aspects are analysed and compared with experimental results. Finally the risk of surface checking in an early stage of sapwood drying is shown theoretically. 相似文献