干燥过程中木材内部含水率检测的X射线扫描方法

以红橡和黑胡桃木材为研究对象,采用X射线扫描方法测量常规热风干燥过程中木材厚度方向含水率分布和平均含水率的可行性。结果表明:采用X射线扫描无损检测方法可以实现干燥过程中同一木材试件含水率分布和平均含水率的动态检测;与称重法含水率测量值相比,在8%～68%的较宽含水率范围内,不论是在高含水率阶段还是在低含水率阶段,采用X射线扫描法测量木材含水率时,都具有较高的测量精度,测量值偏差最大不超过3%;在热风干燥过程中,所有试件含水率的称重法测定值与X射线扫描法测定值之间相关系数的平方都在0.97以上;X射线扫描方法具有扫描速度快、设备费用较低、测量精度高的特点,为木材含水率的动态检测提供了一种很好的技术手段。     

基于SVM的木材干燥过程含水率软测量研究

介绍木材干燥过程含水率主要测量方法,以及支持向量机回归原理。针对干燥过程木材含水率测量存在的问题,提出采用基于支持向量机的软测量方法测量干燥过程的木材含水率。通过对柞木试材干燥数据的仿真实验,结果表明支持向量机软测量模型具有良好的泛化性,能够准确预测干燥过程的木材含水率。     

基于粒子群优化支持向量机的时间序列木材含水率预测

木材干燥过程中对含水率的检测是一个复杂的非线性过程。为了提高含水率数据采集的精确性和可靠性，本文将粒子群优化算法（PSO）引入到含水率检测中，优化支持向量机（SVM）参数，对试验采集过程中失真数据时间点对应的实际值进行预测。通过粒子群优化支持向量机算法和拉格朗日插值法的仿真结果进行对比表明：PSO—SVM算法提高了误差精度，避免了拉格朗日插值法的龙格现象，得到了较好的预测结果。     

木材干燥过程含水率和温度变化的数学模型研究

木材干燥过程存在着热质互换和能量的转移,这为从热力学的角度研究木材干燥过程提供了前提条件,因此提出利用热力学原理分析木材干燥过程含水率和温度变化规律的数学模型.首先建立描述含水率和温度变化的高阶非线性偏微分方程组,然后采用有限元分析原理将方程组离散化,再经过一系列数学运算得到方程的收敛解,具体方法为牛顿迭代法,最后通过对木材含水率和温度变化情况仿真结果的分析,验证该方法的可行性及数学模型的准确性.     

电容式含水率测量仪在干燥室内测量准确性研究

室干过程中,测量木材的含水率是执行含水率基准和决定干燥终点的前提.木材易于产生弯曲,木材未干、板材含水率达不到标准若再次干燥,势必增加成本.监测干燥室内板材含水率有以下几种方法:用电阻和电容式水分仪测量;测量气流穿过材堆前后温度差,然后按经验公式推算含水率;用材堆的细     

利用SMS-2窑用木材含水率测湿仪计算多点平均值

木材干燥是木材深加工的第一步，而干燥时木材含水率的控制是一个关键问题。为确保在木材干燥过程中对木材含水率的准确测量，使其能够准确表达出材堆的实际情况，人们常用多点测量计算其平均值。这种方法可比较可靠地保证整个材堆的含水率情况，但在人工操作状况下测试比较麻烦。在计算机高度发达的今天，利用计算机对木材干燥过程的控制已经成为一项比较成熟的技术。可对现有的ＳＭＳ-２型窑用木材测湿仪稍加改造，使其能够对多个测点进行循环检测并计算其平均值。ＳＭＳ－２型窑用木材测湿仪是电阻式木材测湿仪，它具有检测迅速、精度高…     

木材干燥研究方向

1989年7月在美国西雅图召开的国际木材干燥学术讨论会上,代表们认为今后木材干燥的研究的重点应放在几个方面: 1.发展和完善木材干燥的测量技术如温度、压力传感器,以及能敏捷测量全范围含水率的传感器。 2.研究针叶材和阔叶材小径木的干燥。 3.探讨木材干燥降等的原因。 4.在木材干燥学科内,建立一系列标准,即在试验设计,试验过程,干燥降等和干燥质量和干燥质量的测量等方面。     

百度试验条件下木材含水率场的数学分析及其实用意义

为将控制工程理论中的系统辩识方法应用于木材干燥技术，本文建立起在进行百度试验时沿厚度方向木材含水率场随时间变化的数学模型并求出其数学解。这一数学解表述了在百度试验条件下木材初含水率、干燥时间、即时含水率和相应的扩散系数之间的动态函数关系。据此，可以通过试验的方法得到在此条件下不同树种木材水分在不同含水率条件下的扩散系数，从而为制定木材干燥基准给出必要的数据。本文试图为用控制工程理论研究木材干燥技术提供一条新的思路。     

樟子松木材高温干燥过程中水分的非稳态扩散和干燥能耗的研究

为更加高效广泛地利用樟子松材,对不同初含水率(IMC)的木材进行高温干燥试验。探究相同厚度下不同初含水率木材高温干燥能耗。并依据非稳态扩散理论,在一定的实验条件下探讨不同初含水率对水分径向扩散系数的影响。结果表明:初含水率越高,扩散系数越大,在纤维饱和点附近达到最大值。当含水率在纤维饱和点以下时,水分扩散系数随着初含水率的增加而增大;而当木材含水率在纤维饱和点以上时,其扩散系数基本保持恒定。樟子松材高温干燥过程中,干燥时间越长,能耗越大;不同初含水率试件干燥结束时的能耗不同,初含水率越高,高温干燥过程用时越长,能耗越大。木材干燥过程中依据含水率的差异进行分级干燥可以达到节能的目的。     

基于控制容积干缩模型的X射线法测算木材含水率分布

【目的】构建木材控制容积干缩模型，采用X射线法测量木材剖面密度，计算含水率分布，为科研和生产测定木材含水率及其分布提供技术支撑。【方法】将绝干前木材沿厚度方向划分控制容积，引入木材全干干缩率参数并依据木材干缩原理计算绝干后木材控制容积厚度，利用木材剖面密度确定控制容积密度，完整构建求解绝干前木材控制容积含水率计算模型。模型中绝干前木材控制容积含水率、绝干后木材控制容积厚度和密度等变量高度耦合，通过开发迭代算法并在Matlab软件中编制计算程序求解以上变量，该算法以绝干后木材实际厚度为约束条件不断修正木材全干干缩率，有效避免因全干干缩率引入值与真实值偏差带来的计算误差。同时，开展毛白杨木材对流干燥试验，利用本研究方法测算不同干燥阶段木材含水率分布，并与木材实际平均含水率（称重法）以及假设木材厚度方向均匀干缩法测算的含水率分布进行对比分析。【结果】本研究方法测算的木材平均含水率与称重法测量的木材平均含水率非常接近，二者相关系数的平方（R2）在0.999 2以上。本研究方法与均匀干缩法相比，在FSP（纤维饱和点）以上时2种方法测算的木材含水率分布一致；在FSP以下时本研究方法测算的木材含水率...     

小型除湿干燥机干燥乐器用材的实践

本研究使用小型除湿干燥机对提琴用材进行了干燥工艺生产性试验。结果表明，干燥过程中试材未产生内裂和表裂；干燥后板材的含水率在材堆内分布均匀，在板材厚度方向分布有一定差异，平衡后可以使用；干燥成本在100元/m^3左右；干燥后的木材质量符合乐器制作的要求。本试验确定的干燥工艺可适用楔形不规则乐器用材，还可进行不同规格乐器用材的混合干燥。     

Moisture content target optimization in lumber drying

Abstract

A Lumber Quality Model developed to predict lumber grade after drying was applied and validated in industry. The model was calibrated with a 198-piece lumber package that was dried at a local sawmill in British Columbia, Canada, and 30 sawmill grade reports were recorded from 2008 to 2010. The calibrated model was then used to predict the target moisture content that would result in the maximum lumber value. It was predicted that lumber value should increase by approximately 17 CAD (Canadian dollars) m^?3 if the target moisture content were reduced from the historic 17% to an optimum of approximate 13%. The sawmill implemented the recommendations and the predictions were validated with another seven industrial drying runs.     

Determination of diffusion coefficients for sub-alpine fir

Estimates of the drying rate for sub-alpine fir lumber and the effect of process variables on final moisture contents are valuable information for kiln operators. The diffusion approach offers one generalized way to estimate the drying time required and the final moisture content distribution. The diffusion coefficients of normal wood and wet wood of sub-alpine fir lumber were determined by two types of approaches in this study: the traditional method and the inverse moisture diffusion algorithm. The results from both approaches indicate that the diffusion coefficient of normal wood is greater than that of wet wood for sub-alpine fir.     

Cooling and steam conditioning after high-temperature drying of Pinus radiata board: experimental investigation and mathematical modelling

 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     

Simple analytical methods to predict one- and two-dimensional drying stresses and deformations in lumber

This study was conducted to develop analytical models for the prediction of drying stresses and defomations in lumber. Both one- and two-dimensional analytical models were developed with two assumptions: the visco-elastic creep could be neglected, and the diffusion coefficient is constant with moisture changes. The method developed in this study showed that the drying stress of lumber with symmetric moisture profile could be approximately predicted using one-dimensional (1D) and two-dimensional (2D) models. In the case of the 1D model, drying deformations could be determined rather easy and the drying stress can even be predicted by hand calculation. The results of this study might be used for lumber with an asymmetric moisture profile. In order to predict more accurately drying stress and deformation across overall moisture changes, however, this procedure should be incorporated with the other moisture transport models and might be extended to a 3D model.     

小径级柞木干燥工艺的试验研究

小径级柞木干燥试验结果表明:在干燥初期干球温度为35℃、干燥末期干球温度为70℃、干燥周期中进行两次中间处理和末期终了处理、干燥周期为15 d的工艺条件下,板材干燥前总平均含水率41.6%,干燥结束后总平均含水率11.7%,板材厚度上含水率偏差<3%;干燥结束后伸长应力和压缩应力甚微,板材表面的颜色轻微变暗,除髓心材和水纹材有少数裂纹和轻微变形外,其余木材并没有发生翘曲、变形、开裂等干燥缺陷,干燥质量满足了GB6491—86《国家锯材干燥质量标准》二级以上的指标要求。本次试验确定的干燥工艺满足了小径级柞木板材的干燥工艺要求。     

Analysis of moisture diffusivity of larch timber during convective drying condition by using Crank＇s method and Dincer＇s method

Two analytical procedures （Crank＇s method and Dincer＇s method） for porous solid materials were reevaluated and used to determine moisture diffusion coefficients and moisture transfer coefficients for larch lumber subjected to drying. A diffusion-like equation was used to describe drying process data. The lumber was idealized in the modeling as infinite plates. The moisture transport process inside the board was assumed to be one-dimensional. The macroscopic drying kinetics curves of larch timber at particular conditions were determined experimentally. Based on these data, calculation for both the moisture diffusion coefficients and moisture transfer coefficients by the Dincer＇s analytical procedure were made. The dynamic moisture diffusion coefficients by the traditional Crank＇s method were calculated. In general, diffusion coefficients calculated by the Dincer＇s method were all higher than those by Crank＇s method. These results could be due to the differences between two analytical methods and also different characteristics between solid moisture diffusion process and heat transfer process. Therefore the analysis and solution procedures of moisture diffusion differential equations need to be adapted in the future. With drying temperature＇s increasing moisture diffusion coefficient （D） and moisture transfer coefficient （k） increases accordingly. Also the relationships between diffusion coefficients and temperature as well as material moisture contents were analyzed by using Arrhenius equation and bound water transport theory.     

Studies on Measurement of Moisture Content by X-ray Scanning Method

In order to explore the feasibility of using X-ray to measure moisture content in the drying process and have a further study on the effect of precision of moisture content when scanning through different grain directions,X-ray scanning method and weighting method were used to measure average moisture content of Cunninghamia lanceolata during hot air drying.The results show that the moisture content values of two methods were very close to one another,the determination coefficients of both were highly correlated with a value over 0.99,and the absolute deviation was below 2%;when scanning along radial direction and longitudinal direction,the precisions of average moisture content were higher than that of the tangential direction,but the deviation can be negligible in the actual measurement;in the practical application,the effect of grain direction on precision of moisture content measurement could not be taken into account when using X-ray to measure the moisture distribution of wood.     

The temperature and moisture content in lumber during preheating and drying

Summary The time required to thoroughly preheat/presteam green 8/4 (51 mm) thick red pine and white birch lumber was determined. Thermocouples imbedded in the centre of samples were used to determine presteaming/preheating times for experimental material. These data were then compared to time determined by analytical methods (using formulae developed by Kollmann). No significant differences were found.During the subsequent drying period data were collected on interior wood temperature, mousture content and gradient. Results of the drying tests indicated: i) interior temperature increases in direct relation to elapsed time, ii) moisture content drops exponentially with elapsed time, and iii) an exponential relationship exists between average moisture content and the core temperature of the lumber.Results indicate that monitoring of the lumber drying process through measurements of interior wood temperature may be feasible.The first author was visiting Canada under the Chinese Scholar Program established by the Council of Ministers of Education, Canada. His research work was carried out at Forintek Canada Corp., Eastern Laboratory, OttawaThe authors would like to acknowledge the assistance and support of the following: Dr. M. Y. Cech, Manager, Lumber Manufacturing Technology, Forintek Canada Corp., Eastern Forest Products Laboratory-Senior Project Supervisor, and Mr. F. Pfaff, Research Scientist, Forintek Canada Corp     

Microwave and steam injection drying of CO2 laser incised Sugi lumber

To investigate the effect of CO₂ laser incising under five drying methods on drying characteristics of Sugi lumber, the squares (120 mm × 120 mm) of Sugi lumber with length of 650 mm were used. A half of samples were incised by CO₂ laser with incising density of 2,500 holes/m². Five types of drying methods were used: microwave drying, steam injection drying, and three combinations of microwave heating and steam injection drying. Steam injection drying was conducted by injecting superheated steam of 120 °C through a perforated plate heated to 140 °C of an injection press. Microwave was irradiated with the power of 3 kW at frequency of 2.45 GHz. The results indicated that incising helps heat through a specimen and thus the whole temperature raised rapidly, which was up to threefolds compared to that of no-incised one. Incised specimens dried by a combination of microwave heating for 1 h and steam injection showed the highest drying rate, which was up to 5.3 %/h. Incising and microwave heating contributed positively to dry lumber under more uniform distribution of moisture content and to reduce surface and internal checks. Incised specimen dried by microwave showed the most uniform distribution of moisture content without surface and internal checks.