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     

A global rheological model of wood cantilever as applied to wood drying

In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep. The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood, the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time. The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process.     

Experimental determination of the convective heat and mass transfer coefficients for wood drying

The knowledge of the convective heat and mass transfer coefficients is required for the characterization of the boundary conditions of the heat and mass transfer equations of a wood drying model based on water potential. A new experimental method for the determination of the convective mass transfer coefficient is presented. This method is based on the measurement of the moisture content, and indirectly the water potential, at the surface of a wood specimen at different drying times. Drying experiments were performed on red pine (Pinus resinosa Ait.) sapwood from nearly saturated to dry conditions at 56 °C, 52% relative humidity and air velocities of 1.0, 2.5 and 5.0 m s⁻¹. The results show that the convective mass transfer coefficient is constant until the wood surface moisture content reaches about 80% and then decreases more or less gradually as the moisture content decreases further. The convective mass transfer coefficient increases with air velocity. A regression analysis shows that there is no significant improvement in considering the water potential gradient near the wood surface when the difference in water potential between the surface and the surrounding air (ψ_s − ψ_∞) is used to determine the convective mass flux at the surface. Also, ψ_s − ψ_∞ is more appropriate than the water vapour pressure difference (pv_s − pv_∞) as the responsible driving force of the moisture flux leaving the wood surface. The convective heat transfer coefficient was determined during the same experiments. A plateau is observed at high values of moisture content corresponding to the constant drying rate period. Received 27 February 1998     

Determination of the moisture content in wood chips of Scots pine and Norway spruce using Mantex Desktop Scanner based on dual energy X-ray absorptiometry

There has been a longstanding interest in developing a fast and accurate method for measuring moisture content in wood. The gravimetric method that is commonly used today is a widely accepted industrial standard but it is time consuming. In this study, we tested and evaluated Mantex Desktop Scanner that is based on dual energy X-ray absorptiometry as a potential technique for fast and accurate determination of moisture content at different temperatures in wood chips of Pinus sylvestris, Picea abies and a variety of mixtures thereof. The results of the study give similar results as the gravimetric method when determining the moisture content in wood. They further show that there was no significant influence on the result if samples were measured while frozen or at room temperature.     

Experimental measurements of heat and mass transfer during convective drying of southern pine

Summary The transport of heat and moisture during the convective drying of southern pine was examined experimentally. Moisture distribution within the wood samples was measured using gamma attenuation. The accuracy of these measurements was found to be moisture content dependent with estimated uncertainties ranging from 29.5% at 10% moisture content to 6.8% at 120% moisture content. Local moisture content measurements reveal large inflections at high moisture contents. Peaks in moisture content are closely correlated with low fwood density and vice versa. Above the fiber saturation point the measured drying rate is not a well defined function of the moisture content. This is perhaps due to the biological variability of wood as well as grain orientation, although no definite conclusions could be drawn from the data with regard to the effect of grain orientation. The drying rate does not appear to be highly temperature dependent at high moisture contents. Below the fiber saturation point where diffusion is expected to be the dominant transport mechanism, the drying rate is less affected by biological variability and grain orientation, however, the effects of temperature are evident.The authors wish to acknowledge the support of this work by the National Science Foundation under Grant Number CME-7906367. We must also express our appreciation to staff members at Weyerhaeuser Technology Center who supplied us with wood samples. In addition to the authors, Mr. T. R. Brown, S. H. Moen, and D. Chow played a critical role in gathering much of the data     

A microwave applicator for on line wood drying: Temperature and moisture distribution in wood

Summary An especially designed open microwave applicator was analysed using wood as the material to be heated and dried. The idea was to develop an on line microwave construction consisting of several small open applicators, each fed by a small standard magnetron (for example 1.4 kW main power). The process was analysed by measuring the wood temperature during heating using an IR-camera and detecting the moisture distribution during drying by CT-scanning. Pine and birch wood samples were used in the experiments, mainly 40 mm in thickness. The experiments show that the power distribution differs between dry wood and moist wood. The analysis of the temperature fields captured by the IR-camera during the first minutes allows a rather accurate determination of the MW power. Consequently, the drying proceeds unevenly in the wood specimens, especially in the longitudinal direction. The dimensions of the applicator and its relation to the wood dimension are very important. However, the wood was not destroyed, the temperature and moisture gradients did not affect the wood in terms of checks or deformations. The drying rate in different positions of the specimen varied between 0.30 and 0.80 percentage moisture content/min. The uneven energy, meaning temperature and field distribution, is to be compensated in the future by a moving wood load and by alternating the position of each applicator in a larger scale microwave pilot plant. Received 25 February 1997     

New monitoring concept of moisture content distribution in wood during RF/vacuum drying

A new method for monitoring moisture content during radio-frequency (RF)/vacuum drying was developed by measurement of temperature and pressure in wood. Temperature and pressure inside the wood were measured simultaneously during RF/vacuum drying at the same point. The relative humidity (RH) and moisture content (MC) below the fiber saturation point (FSP) were calculated based on temperature and pressure, and the relationship between the temperature, RH, and equilibrium moisture content (EMC) at the measurement point. When the moisture content was below the FSP, the calculated MC was slightly greater than the value given by oven drying. The absolute error was within 0.8% near the open cross side, and was within 1.8% at another measurement point. Thus, we concluded that it was practicable to monitor the moisture content below the FSP according to the temperature and pressure inside the wood. Part of this study was presented at the 15th Annual Meeting of the Chugoku Shikoku Branch of the Japan Wood Research Society, Higashi-Hiroshima, Japan, September 2003     

Wood drying and Fick's second law

Summary The diffusion equation (sometimes referred to as Fick's second law) is derived in terms of water movement under the action of capillary forces. The mass diffusivity is thereby expressed in terms of the capillary diffusion coefficient. A numerical calculation is given for yellow poplar.Notations C diffusion coefficient for water in wood with capillary pressure as the driving force, kg/msPa - D diffusion coefficient for water in wood with moisture content as the driving force, kg/ms - F mass flux, kg/m²s - p_c capillary pressure, Pa - p_cf capillary pressure extrapolated linearly to fibre saturation, Pa - T absolute temperature, K - t time, s - x distance ordinale in the direction of flow, m - mass diffusivity, m²/s - density of liquid water, kg/m³ - _g basic density (dry mass/green volume), kg/m³ - _w density of wood substance, kg/m³ - moisture content of wood - _cls moisture content at continuous liquid saturation - _cs moisture content at complete saturation - _f moisture content at fibre saturation     

Kiln drying behavior of lumber from ten fast-growth plantation species in Costa Rica

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.     

非接触式无损检测木材干燥应力的方法

木材干燥应力是干燥过程中产生干燥缺陷的主要因素。文章根据数字散斑相关方法(DSCM)检测应力的原理设计出了一种非接触式无损检测木材干燥过程中应力的方法,用木材干燥表面测点位移的变化速率即应变速率来表示木材干燥过程中的应力状态,初步提出了建立木材干燥应力评价体系的设想。研究结果表明:木材表面测点位移变化量与时间存在着对数关系,与含水率变化量则呈线性关系。     

Reaction wood drying kinetics: tension wood in <Emphasis Type="Italic">Fagus sylvatica</Emphasis> and compression wood in <Emphasis Type="Italic">Picea abies</Emphasis>

The drying kinetics of reaction woods in Picea abies (compression wood) and Fagus sylvatica (tension wood) in comparison with their corresponding normal woods was investigated under constant convective drying conditions. Moisture profiles along the thickness of small flat-sawn boards taken from reaction and opposite wood zones were evaluated using a polychromatic X-ray system, a non-destructive method. The results revealed substantial differences in the drying behavior between the reaction and opposite woods. Both reaction woods represented slower drying rate than their matching normal woods mainly during the period of free water loss. However, the reaction and opposite woods reached the final moisture content (MC) of about 12% at the same time due to higher initial MC in the opposite woods. In the case of reaction wood, it took a longer time for the moisture profile to become approximately uniform. Overall, a more striking difference was observed in the drying behavior of compression and opposite wood in P. abies. Some important anatomical differences like the cell and pit dimensions and their proportion give some explanations for these drying behaviors.     

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.     

Modeling of cell wall drying stresses in wood

All applications of wood involve drying the material from the green state. The cell wall may be viewed as a laminate consisting of different layers. The layers have different orientations and therefore different moisture expansion characteristics. As a result, stresses will develop in the layers due to drying. Micromechanical models for fibre composite materials were used in combination with a laminate analogy in order to calculate these drying stresses in the cell wall layers S1, S2 and S3. Resulting stresses were very high. In reality viscoelastic effects will significantly reduce stresses at high moisture content. However, at lower moisture content irreversible cell wall damage is likely to form as a result of the stresses computed by the model. Received 20 October 1998     

基于时延神经网络的木材干燥温湿度建模研究

利用神经网络建立了木材干燥的温湿度模型,给出了其时延神经网络辨识结构。分别提出温、湿度控制模型(控制信号与温、湿度之间的关系模型)和木材干燥基准模型(温、湿度与木材含水率之间的关系模型),并利用实验干燥窑得到的实际数据进行了仿真研究。仿真结果表明,利用此方法建模是可行的,所建模型是有效的。图10参16。     

Stress-strain state of wood at kiln drying

Summary A model has been suggested which allows the calculation of stresses arising in kiln drying and humidification of wood, as well as the total residual strain, i.e. set strain _s, consisting of purely residual strain _r and the so-called frozen strain _f. Frozen strains arise under the operating influence of loading when the stiffness is increased because of a decrease in moisture content (or temperature) of the wood. The process of formation of set strains _s has been shown to depend on the history of loading, variations of the moisture content of the wood, as well as cooling of the section before the measurement of _s. The possibility of using set strain as a parameter of the state of stress of wood in kiln drying has been noted.Presented at the VIII. International Symposium on Fundamental Research of Wood. Warsaw, Poland, October 1990     

Modeling moisture absorption process of wood-based composites under over-saturated moisture conditions using two-part equations

The objective of this study was to investigate the moisture absorption process for wood-based composites subjected to over-saturated moisture conditions. Two stages are comprised in the moisture transfer process at the over-saturated moisture conditions, an initial stage which is the moisture transfer process mainly under fiber saturation point (FSP), and a second stage which is the moisture transfer process beyond the FSP. A model was developed based on two-part equations to describe the process, from which three coefficients (k ₁ , k ₂₁ , and k ₂₂) can be used to quantitatively describe the moisture transfer process under the conditions. Two different wood-based composites, wood fiberboard and wood fiber/polymer composites (polymer content: 30%), were used to test the model at four different ambient temperatures (30, 45, 62, and 80°C). It was shown that the two-part equation can accurately describe the moisture absorption process under over-saturated moisture conditions. The moisture absorption rate in the initial stage was about 30–60% greater than that in the second stage for most of the cases evaluated in this study. The higher the temperature, the greater moisture absorption parameters were obtained. At both moisture absorption stages (below FSP and above FSP), the calculated activation energy for the moisture absorption rate of wood fiberboard was very close to that of wood fiber/polymer composites.     

低循环风速下木材表层水分蒸发的试验研究

木材干燥过程中,介质循环速度是一个影响木材干燥的重要工艺参数.在木材各含水率阶段,通过试验分析研究不同介质循环速度对木材干燥速度的影响.结果表明,介质循环速度对干燥速度的影响显著,但其影响随木材含水率（MC）的降低而减弱.在低介质循环速度条件下,试件MC大于45％时,表现为木材干燥速度和木材含水率偏差（△MC）随循环风速的增加而增加,呈显著正相关关系;试件MC介于35％ ～ 45％之间时,正相关关系存在但不显著;试件MC小于35％时,干燥室内循环风速的大小不影响木材的干燥速度和木材含水率偏差（△MC）.对试件表层含水率分析,试件表层含水率大于25％时,试件表面循环风速对试件表层含水率的影响显著;试件表层含水率小于25％时,试件表面循环风速对试件表层含水率的影响很小,不同循环风速下试件表层含水率基本一样.     

Determination and use of moisture diffusion coefficient to characterize drying of northern red oak (Quercus rubra)

Summary Diffusion analysis can be used to estimate the time required to dry lumber. However, more accurate calculations require additional information on the relationship between the diffusion coefficient as a function of moisture content and on the effects of temperature, equilibrium moisture content, board thickness, and air velocity on drying time and moisture gradients. The primary objectives of this study were (1) to determine the diffusion coefficient of northern red oak (Quercus rubra) as a function of moisture content and (2) to compare experimentally determined sorption times and moisture content gradients with those calculated by the diffusion model. The diffusion coefficient was found to increase approximately exponentially with moisture content over a range of 6 to 30 percent at 43°C. This relationship was similar in both adsorption and desorption tests. Experimental adsorption and desorption times and desorption moisture content gradients were compared with finite difference solutions to the diffusion equation. Practical uses of solutions to the diffusion equation are illustrated for kiln drying wood that has first been predried to below the fiber saturation point. Drying time is also related to variable kiln conditions and board thickness.This research was partially funded under the U. S. Department of Agriculture Competitive Grant programThe Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time     

Effect of steam pretreatment on wood moisture content and characteristics of vacuum drying

The effects of high temperature steam pretreatment on the change in wood moisture content (MC) and characteristics of vacuum drying were investigated in this study. Poplar and manchurian walnut woods were pretreated with high temperature steam at 100°C and 140°C, prior to vacuum drying. A comparison of the characteristics of vacuum drying between steam pretreated wood and untreated wood was carried out. The results show that during steam pretreatment, the MC of wood decreased within a few hours. The reduction of MC varied with the temperature; the higher the temperature, the faster the MC dropped. During the vacuum drying stage, the rates of drying of pretreated samples were higher than those of untreated samples when MC was below the fiber saturation point. Furthermore, the total drying time of samples treated at a steam temperature of 140°C was lower than that of untreated samples. Therefore, a vacuum procedure after steam pretreatment can effectively shorten the drying time when drying wood.     

Determining moisture-dependent elastic characteristics of beech wood by means of ultrasonic waves

The present study investigates the influence of moisture content on the elastic characteristics of beech wood (Fagus sylvatica L.) by means of ultrasonic waves. A set of elastic engineering parameters (i.e. three Young’s moduli, three shear moduli and six Poisson’s ratios) is determined at four specific moisture contents. The results reveal the significant influence of the moisture content on the elastic behaviour of beech wood. With the exception of some Poisson’s ratios, the engineering parameters decrease with increasing moisture content, indicating a decline in stiffness at higher moisture contents. At the same time, wood anisotropy, displayed by the two-dimensional representation of the velocity surface, remains almost unchanged. The results prove that the ultrasonic technique is suitable for determining the elastic moduli. However, non-diagonal terms of the stiffness matrix must be considered when calculating the Young’s moduli. This is shown experimentally by comparing the ultrasonic Young’s moduli calculated without, and allowing for, the non-diagonal terms. While the ultrasonic technique is found to be reliable to measure the elastic moduli, based on the measured values, its eligibility to measure the Poisson’s ratios remains uncertain.