MEASUREMENTS OF DELAYED WARP IN EASTERN SPRUCE STUDS

Nearly eighty percent of the lumber manufactured in the United States is softwood dimension lumber used for the manufacture of housing. The principal quality related problem with softwood dimension lumber is that it is subject to warp related degradation. Much of the degradation occurs during the initial drying process, however, residual stresses and moisture content changes that occur after kiln drying can either reduce or increase the amount of warp. The magnitude of post kilning warp change, here termed delayed warp, has not been well quantified. Measurements of twist, bow, crook and moisture content were taken from eastern spruce (Picea spp) studs immediately after drying and planing. The wood was allowed to sit without restraint for a period of two weeks after which the measurements were repeated. Of the 1224 studs measured, over 76 percent experienced changes in twist, over 84 percent experienced changes in bow and over 73 percent experienced changes in crook. Increases in warp generally exceeded decreases, although a surprising percentage of the studs showed decreases in warp. Attention was focused on finding a correlation between moisture content and the changes in warp, but the correlation coefficients were low for both the increases and the decreases. The most likely cause of the warp was the high percentage of juvenile wood present in the raw material. The most likely causes for the changes in warp during the two-week period were stress relaxation aggravated by a change in the moisture gradient.     

Impact of High-Temperature Schedules on Drying of Spruce and Pine

The impact of high-temperature drying (HTD) for spruce and pine lumber of 5 × 10 × 244 cm was investigated in this study. Four drying schedules with dry-bulb temperatures from 60 to 110°C were evaluated including Control 1 (conservative), Control 2 (accelerated), HTD1 (104°C) and HTD2 (110°C). In each of HTD1 and HTD2 schedules, two runs were performed, one humidified with steam and the other applied without steam. Drying rates, internal stresses (prong test), warp (bow, crook, and twist), modulus of elasticity (MOE), and modulus of rupture (MOR) were assessed for the dried lumber.

Six drying runs were carried out in a laboratory kiln and the results of this research indicated: (1) The drying rates in HTD1 and HTD2 increased by 2.2 to 3.5 times in comparison to the conservative schedule. (2) Both HTD1 and HTD2 schedules with steam resulted in greater internal stresses in the dried lumber compared to Control 1, but these stresses were smaller than those in Control 2. The standard deviations of final moisture content from HTD1 and HTD2 with steam were higher than those of Control 1 but similar to the values obtained for Control 2. (3) Warp was, in general, reduced by using the HTD schedules except for HTD2 without steam. Less crook was observed for HTD1 and HTD2. (4) Reductions in MOE and MOR were found for both HTD1 and HTD2 schedules but were not statistically significant.     

Establishing Kiln Drying Schedule for Beech (Fagus orientalis) at 5-cm Thickness from the Neka Region

To establish a kiln drying schedule for beech (Fagus orientalis) lumber, 5-cm-thick boards were kiln dried down to a final moisture content of 8%. Three replications were made utilizing three kiln schedules of T5-C3, T5-C4, and T6-C4. With due attention to the effect of thickness on wood drying intensity, the t-test showed no significant difference between the thicknesses of the three drying schedules at a significance level of 99%. Therefore, the results of this study can be applied for 5-cm-thick boards.

The primary dry bulb temperature in each of the three schedules was adjusted to 41°C and the final dry bulb temperatures were adjusted to 71, 71, and 82°C, respectively. The schedule offering the shortest drying time for the desired quality was chosen. Specific gravity and dry specific gravity were measured as 0.52 and 0.61, respectively. Longitudinal, radial, tangential, and volumetric shrinkage were 0.46, 5.8, 10.2, 16.48%, respectively. The extent of defects including crook, bow, twist, and three longest surface checks of the lumber was determined for each drying schedule. Quality control graphs were used to analyze the lumber defects in order to determine the best drying schedule.

Analysis of the results indicates that with either of three kiln schedules the extent of defects before and after drying was not statistically different. However, the distribution of defects in the third schedule (T6-C4) was more uniform with respect to the average line compared to other two schedules. At the end of this schedule, a 17-h equalization and 24-h conditioning treatment is recommended.     

Some Considerations In Simulation Of Superheated Steam Drying Of Softwood Lumber

ABSTRACT

A mathematical model for high-temperature drying of softwood lumber with moist air has been modified and extended to simulate wood drying with superheated steam. In the simulation, differences between the two types of drying are considered, these include: external heat and mass transfer processes and calculation of equilibrium moisture content. The external mass transfer coefficient in the perheated steam drying was found to be much higher than that in the moist air drying, however, the heat ransfer coefficients for these two cases were of the same order. The predicted drying curves and wood temperatures from the superheated steam drying model were compared with experimental data and there was close agreement. Further studies will apply the model to development of commercial drying schedules for wood drying with superheated steam.     

A Simulation Tool for the Optimization of Lumber Drying Schedules

Abstract

A two-dimensional wood drying model based on the water potential concept is used to simulate the convection batch drying of lumber at conventional temperature. The model computes the average drying curve, the internal temperature and moisture content profiles, and the maximum effective moisture content gradient through board thickness. Various scenarios of conventional kiln-drying schedules are tested and their effects on drying time, maximum effective moisture content gradient, final moisture content distribution within and between boards, and energy consumption are analyzed. Simulations are performed for two softwood species, black spruce (Picea mariana (Mill.) B.S.P.) and balsam fir (Abies balsamea (L.) Mill.). The simulation results indicate that the predictive model can be a very useful tool to optimize kiln schedules in terms of drying time, energy consumption, and wood quality. Such a model could be readily combined with intelligent adaptive kiln controllers for on-line optimization of the drying schedules.     

A Simulation Tool for the Optimization of Lumber Drying Schedules

A two-dimensional wood drying model based on the water potential concept is used to simulate the convection batch drying of lumber at conventional temperature. The model computes the average drying curve, the internal temperature and moisture content profiles, and the maximum effective moisture content gradient through board thickness. Various scenarios of conventional kiln-drying schedules are tested and their effects on drying time, maximum effective moisture content gradient, final moisture content distribution within and between boards, and energy consumption are analyzed. Simulations are performed for two softwood species, black spruce (Picea mariana (Mill.) B.S.P.) and balsam fir (Abies balsamea (L.) Mill.). The simulation results indicate that the predictive model can be a very useful tool to optimize kiln schedules in terms of drying time, energy consumption, and wood quality. Such a model could be readily combined with intelligent adaptive kiln controllers for on-line optimization of the drying schedules.     

Canadian Developments in Kiln Drying

The objective of this paper is to summarize recent trends related to industrial kiln drying with especial emphasis on the drying of softwood dimension lumber in Canada. The paper explores future trends by analyzing the current market condition and wood products requirements. The paper also summarizes current research developments in the area of kiln lumber drying that was published by Canadian universities and research institutes. This includes basic research, such as wood physics and computer simulation, and applied research, such as non-conventional drying strategies and new technologies and sensors. A last note is also included to mention a kiln supervisor tool that was developed in Canada to analyze information related to kiln performance indicators. The kiln supervisor provides important trends for mills aiming to optimize their drying processes and realize gains without substantial investments in capital and other resources.     

太阳能-除湿-常规分段组合木材干燥工艺优化

为降低木材干燥能耗,通过对杨木进行太阳能-除湿-常规分段组合干燥,即含水率40%以上采用太阳能进行预干,25%—40%采用除湿干燥,25%以下采用常规干燥,并与常规干燥进行干燥能耗和木材质量的对比。结果表明,分段组合干燥比常规干燥节能19%左右,而且干燥质量和常规干燥基本一致。分段组合干燥充分利用了三者各自的优势,从而达到节能的目的。     

Modeling Shrinkage Response to Tensile Stresses in Wood Drying III. Stress—Tensile Set Correlation in Short Pieces of Lumber

This article reports on the correlation between tensile stresses, temperature, and target moisture content in short pieces of lumber based on relationships developed on experiments made on small wood strips subjected to different types of restraints. Linear motion position sensors were placed around a tested specimen using a frame connected to a support body. A wireless moisture and temperature monitoring system was used to record wood moisture and temperature changes. All measurements were performed perpendicular to fiber grain while drying at 40, 60, and 80°C in an environment set for a target moisture content of 5%; the experiments were stopped when the average moisture content of the lumber was around 10%. High stress values were obtained for specimens dried at 60°C and a low relative humidity, whereas a high reduction in stress level could be obtained for 80°C. The findings are intended to be used in further studies of the shrinkage process as an indicator of the tensile stresses generated in the early stages of wood drying.     

VACUUM-PRESS DRYING OF THICK SOFTWOOD LUMBERS

ABSTRACT

In vacuum-press drying of softwood species, wood lemperaiure exhibited a low temperature gradient, and plateau temperature of core lasied during all stages of drying. The drying curves were close to “linear”. The drying rates of the short lumbers were higher for red pine and western hemlock, lower for white pine and similar for larch lumber compared to the long ones. Transverse and longitudinal moisture gradients were small for western hemlock and red pine, and great for larch and white pine lumber. Energy consumption curves were split into three sections: increasing moderately, fairly constant and increasing rapidly. Casehardening stress of dried lumber occurred very slightly. Dried lumbers exhibited strong tendency for fine end checking, slight surface checking and no internal checking. Shrinkage appeared to be low.     

Review of Wood Drying Research in Brazil: 1984-2004

In Brazil, research on wood drying has been more focused on applied aspects than on fundamentals ones, and results have been published almost exclusively in Brazilian journals. The study of lumber deformation under aggressive drying conditions resulted in methods to group species and to define kiln schedules. Relationship between moisture content and electrical resistivity was used to improve quality control of dried lumber as well automatic control of the kiln drying process. Conventional kiln drying is the most common method for industrial drying, but seasoning and solar drying were also studied. The biggest research effort was directed to improve the drying of eucalypt lumber.     

A Simulation of Wet Pocket Lumber Drying

In general, wood containing wet pockets is difficult to dry and to ensure uniformity of moisture content at the end of the drying process. Large variations of final moisture content and severe case hardening are common problems associated with the drying of wet wood. In order to devise optimal strategies for drying wood containing wet pockets, it is necessary to understand its complex moisture movement mechanisms and therefore predict drying times and final moisture content. Sub-alpine fir dimension lumber was used in this research because of its inherent issues related to wet pockets.

A two-dimensional mathematical drying model for wood containing wet pockets was developed. An effective diffusion coefficient (D _eff ) was utilized in the model and heat and mass transfer equations were solved using a control volume approach. The difficulties involved in the simulation of the drying process of wet pocket lumber are due to the differences in moisture content and physical properties between wet and normal wood. Thus, an adjustable D _eff based on the moisture content (for both below and above fiber saturation point) was used during the simulation.

Four drying runs involving green unsorted sub-alpine fir lumber were carried out in a 3-ft laboratory kiln and in an 8-ft pilot kiln. The results of the simulations were in agreement with the results obtained through the drying experiments.     

A Simulation of Wet Pocket Lumber Drying

In general, wood containing wet pockets is difficult to dry and to ensure uniformity of moisture content at the end of the drying process. Large variations of final moisture content and severe case hardening are common problems associated with the drying of wet wood. In order to devise optimal strategies for drying wood containing wet pockets, it is necessary to understand its complex moisture movement mechanisms and therefore predict drying times and final moisture content. Sub-alpine fir dimension lumber was used in this research because of its inherent issues related to wet pockets.

A two-dimensional mathematical drying model for wood containing wet pockets was developed. An effective diffusion coefficient (D_eff) was utilized in the model and heat and mass transfer equations were solved using a control volume approach. The difficulties involved in the simulation of the drying process of wet pocket lumber are due to the differences in moisture content and physical properties between wet and normal wood. Thus, an adjustable D_eff based on the moisture content (for both below and above fiber saturation point) was used during the simulation.

Four drying runs involving green unsorted sub-alpine fir lumber were carried out in a 3-ft laboratory kiln and in an 8-ft pilot kiln. The results of the simulations were in agreement with the results obtained through the drying experiments.     

Simulation of Drying Using a Kiln Model

A mathematical model was developed for simulating a convective batch lumber drying process. The model incorporates mass and heat transfer relationships within the lumber stack, as well as thermodynamic properties of the wood and drying air. It takes into account the change of air properties along the stack and its effect on the mass and heat transfer parameters. The model relies on a drying rate function that is an empirical correlation based on single-board tests. A drying rate function for western hemlock (Tsuga heterophylla) lumber was developed. The drying rate function was obtained based on experiment results from 500 small boards dried over a range of conditions used in commercial practice. The model was first validated against data available in the literature and then against large batches of hemlock dried in a laboratory kiln. In both cases, the model output was in good agreement with the average moisture content, the drying rates, and the temperatures measured in the larger batches.     

PREDICTING THE CROOK STABILITY OF LUMBER WITHIN THE HYGROSCOPIC RANGE

ABSTRACT

The crook instability of lumber at moisture contents below the fiber saturation point was studied using both experiments and mathematical modeling. The changes in overall length and in crook that were observed with environmental humidity cycling at ambient temperature suggest that crook instability at such conditions is determined largely by elastic phenomena. Consequently, a simplified mathematical model of crook instability based on only elastie paramelers was created and verified using property data and crook measurements from selected boards. Results indicate that the elastic model accurately describes crook instability in the hygroscopic moisture content range near ambient temperatures. Experimental observations also show that board-to-board differences in crook instability can be quite complex. A better fundamental understanding of wood shrinkage behavior would help to explain these differences.     

PREDICTING THE CROOK STABILITY OF LUMBER WITHIN THE HYGROSCOPIC RANGE

The crook instability of lumber at moisture contents below the fiber saturation point was studied using both experiments and mathematical modeling. The changes in overall length and in crook that were observed with environmental humidity cycling at ambient temperature suggest that crook instability at such conditions is determined largely by elastic phenomena. Consequently, a simplified mathematical model of crook instability based on only elastie paramelers was created and verified using property data and crook measurements from selected boards. Results indicate that the elastic model accurately describes crook instability in the hygroscopic moisture content range near ambient temperatures. Experimental observations also show that board-to-board differences in crook instability can be quite complex. A better fundamental understanding of wood shrinkage behavior would help to explain these differences.     

High-Frequency Energy–Assisted Vacuum Drying of Fresh Eucalyptus globulus

Worldwide, eucalyptus tree plantations have been established in appropriate climates because of fast growth and wood qualities suitable mainly for pulp. A potential exists of converting eucalyptus trees into lumber that may be of higher value than pulp. Conventional drying of lumber of Eucalyptus globulus is often difficult because of the occurrence of drying stresses, leading to collapse and checking. The special method of vacuum drying while heating the wood with high-frequency energy (75–77 mbar, 46–51°C) was used to obtain short drying times (5–13 days from green state to 10% final moisture content) and low crack amount.     

Optical Measurement of Deformations in Drying Lumber

None of the destructive or nondestructive testing techniques for monitoring wood behavior during drying allow continuous quantitative measurements of the deformation and strain distribution in a section undergoing nonuniform moisture content changes in a kiln drying regime. In this study, a noncontact optical measurement technique based on digital image correlation (DIC) principles was used to visualize and measure the progressive deformation of wood subjected to simulated kiln drying conditions. A test apparatus was designed to simulate kiln drying conditions in such a way that a section of the specimen was accessible for optical measurements, and a hardwood and a softwood were dried at several temperatures. Full-field maps of principal strains were developed to illustrate the progress of the deformation gradients. The principal observations are that (1) the deformation of wood starts in the beginning of drying when the average moisture contents are well above the fiber saturation point, (2) the difference between the average deformations in the tangential and radial directions is much larger than expected, and (3) the samples swell a little in the radial direction early in drying.     

High-Frequency Energy-Assisted Vacuum Drying of Fresh Eucalyptus globulus

Worldwide, eucalyptus tree plantations have been established in appropriate climates because of fast growth and wood qualities suitable mainly for pulp. A potential exists of converting eucalyptus trees into lumber that may be of higher value than pulp. Conventional drying of lumber of Eucalyptus globulus is often difficult because of the occurrence of drying stresses, leading to collapse and checking. The special method of vacuum drying while heating the wood with high-frequency energy (75-77 mbar, 46-51°C) was used to obtain short drying times (5-13 days from green state to 10% final moisture content) and low crack amount.     

Drying kinetics of poplar lumber during periodic hot-press drying

The drying kinetics of poplar lumber was experimentally investigated as a function of drying temperature (115, 135, 160, 185 and 205°C) during a periodic hot-press-drying process. Poplar lumber was dried under contact (compression ratio of 10%) and high-press states (compression ratio of 44%). Compared with the contact-state, the high-press-state showed higher drying rate and higher efficiency of removing free water than bound water in wood. Eight mathematical models from the literature were established to analyze the drying behavior. The Weibull model, with an average determination coefficient R² of 0.9958, fitted well for all applied drying conditions. The scale parameter decreased with increasing drying temperature and was lower for high-press-state drying compared with that for contact-state drying. Moisture diffusivity and activation energy were calculated according to the Weibull model. Diffusivity increased with increasing drying temperature, with the average value of 1.734?×?10^?6 and 3.313?×?10^?6?m²/s and activation energy of 34.79 and 32.85?kJ/mol for contact-state drying and high-press-state drying, respectively. Hot-press drying created an M-shaped curve of density distribution, with high density at the two surface regions gradually decreasing toward the core region. The contact state-dried wood showed increased density near the wood surface. Both average density and peak density improved in the case of high-press-state-dried wood. Furthermore, the hydrophilic index of wood for high-press-state drying was lower than that of the contact-state drying, and the opposite was true regarding crystallinity index. The hygroscopicity of high-press-dried poplar decreased with lower equilibrium moisture content and higher moisture excluding efficiency, compared with contact-state-dried poplar. The rapid, high-quality drying of poplar lumber through periodic hot-press was more potentially achieved by the high-press-state compared with contact-state drying.