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.     

Opportunities to Reduce Energy Consumption in Softwood Lumber Drying

While a few years ago the cost and availability of energy was not an important issue for the Canadian lumber industry, this situation has recently changed. Today, companies that have operated trouble-free for many years using the same technologies and practices may now have to adapt to the rising cost of fossil fuels and concerns about greenhouse gas emissions. This article addresses this problem for the particular case of the softwood lumber industry. The study describes the most important sources of energy losses in conventional drying and proposes a number of strategies and technologies to reduce kiln energy demand.     

Variation in Drying Behavior and Final Moisture Content of Wood during Conventional Low Temperature Drying and Vacuum Drying of Betula pendula Timber

Conventional and vacuum drying experiments were conducted on Betula pendula timber, which was sawn from trees felled during three different seasons. The influence of the wood procurement season on drying behavior differed, on the one hand, between the drying phases above and below 30% moisture content in the conventional drying, and, on the other hand, between the conventional and vacuum drying methods. During the first steps of the conventional drying process, relative humidity in the kiln, as well as drying time and drying rate, varied according to the felling season. Variations in environmental conditions outside the kiln and the seasonal variation in the physical properties of the wood were presumed to be the reasons for differences in drying behavior. The difference in moisture content gradient, i.e., the difference in final moisture content between the inner wood and the surface layer of boards, was greater in conventionally dried timber than in vacuum-dried timber. In conventionally dried timber there was a clear seasonal variation in the gradient of final moisture content, which was greatest for winter-felled wood. The premature drying of the surface layer during the first steps of the conventional drying process of winter-felled wood was the reason for the higher gradient of moisture content. Storage of wood as logs decreased the standard deviation of the final moisture content.     

Variation in Drying Behavior and Final Moisture Content of Wood during Conventional Low Temperature Drying and Vacuum Drying of Betula pendula Timber

Conventional and vacuum drying experiments were conducted on Betula pendula timber, which was sawn from trees felled during three different seasons. The influence of the wood procurement season on drying behavior differed, on the one hand, between the drying phases above and below 30% moisture content in the conventional drying, and, on the other hand, between the conventional and vacuum drying methods. During the first steps of the conventional drying process, relative humidity in the kiln, as well as drying time and drying rate, varied according to the felling season. Variations in environmental conditions outside the kiln and the seasonal variation in the physical properties of the wood were presumed to be the reasons for differences in drying behavior. The difference in moisture content gradient, i.e., the difference in final moisture content between the inner wood and the surface layer of boards, was greater in conventionally dried timber than in vacuum-dried timber. In conventionally dried timber there was a clear seasonal variation in the gradient of final moisture content, which was greatest for winter-felled wood. The premature drying of the surface layer during the first steps of the conventional drying process of winter-felled wood was the reason for the higher gradient of moisture content. Storage of wood as logs decreased the standard deviation of the final moisture content.     

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.     

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.     

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.     

ZKY-01型箱式木材真空干燥机及干燥工艺的试验研究

介绍了ZKY-01型箱式木材真空干燥机的结构，采用浮压手段对榆木和酸枝木进行真空干燥试验。试验结果表明，ZKY-01型箱式木材真空干燥机的干燥效果与常规干燥相比，干燥速度大大提高，其干燥时间仅为常规干燥时间的15％左右；干燥质量好，没有发生端裂、表裂与变形等干燥缺陷；被干锯材的应力试片其齿条基本通直，应力指标为0．42％-0．76％。箱式木材真空干燥机具有一定的发展潜力。     

Green Kerfing Treatment for Improving Softwood Lumber Drying: A Review

Kerfing is a lumber-drying improvement technique that consists of cuts along both sides of boards, transversal to the longitudinal axis. The rationale behind this procedure is that, by cutting the fibers, the moisture loss through the end-grain is increased. The aim of this paper is to evaluate the effects of kerfing on drying time, warp occurrence, and bending properties, based on different experiments conducted during the last seven years. In most cases, kerfing reduced the drying time by half of that of the control samples. Results were inconclusive regarding reduction of warp. Regarding the bending strength of treated pieces, kerfing caused a minimum loss of moment of inertia in the piece, resulting in a slightly lower bending strength and higher stiffness.     

Analysis of the Vacuum Drying Rate for Red Oak in a Hot Water Vacuum Drying System

Red oak boards of 76.2 cm (long) × 7.62 cm (wide) × 2.54 cm (thick) were dried from green moisture content (MC) to 7% MC in the hot water vacuum-drying system. These boards were dried at the pressure of 12 mm Hg and the temperatures ranging from 30 to 50°C within 25 to 70 h. Drying rates were measured and drying curves were calculated. The results showed that the drying rate was higher at higher temperatures. The vacuum drying was faster when wood MC was above 30% than when it was less than 30%. The individual samples did not dry at the same drying rates even at the same drying conditions because of anatomical variations between boards.     

Analysis of the Vacuum Drying Rate for Red Oak in a Hot Water Vacuum Drying System

Red oak boards of 76.2 cm (long) × 7.62 cm (wide) × 2.54 cm (thick) were dried from green moisture content (MC) to 7% MC in the hot water vacuum-drying system. These boards were dried at the pressure of 12 mm Hg and the temperatures ranging from 30 to 50°C within 25 to 70 h. Drying rates were measured and drying curves were calculated. The results showed that the drying rate was higher at higher temperatures. The vacuum drying was faster when wood MC was above 30% than when it was less than 30%. The individual samples did not dry at the same drying rates even at the same drying conditions because of anatomical variations between boards.     

Experimental Analysis of Low-Temperature Bed Drying of Wooden Biomass Particles

This report investigates the low-temperature bed drying of biomass particles for use in a gasification process. Experimental drying equipment capable of drying up to 0.25 m³ of biomass batchwise was constructed. The experimental result indicates that the drying equipment and applied method are appropriate to evaluate different drying parameters and their influence on the drying course. Drying parameters such as air temperature, air velocity, bed height, and type of wood are studied regarding the drying rate, final moisture content in the material, and bed pressure drop. The constant-rate and falling-rate drying period are studied to obtain an energy efficient drying process. Measurements show that the drying zone (where the actual drying occurs) progresses irregularly through the bed.     

Experimental Analysis of Low-Temperature Bed Drying of Wooden Biomass Particles

This report investigates the low-temperature bed drying of biomass particles for use in a gasification process. Experimental drying equipment capable of drying up to 0.25 m³ of biomass batchwise was constructed. The experimental result indicates that the drying equipment and applied method are appropriate to evaluate different drying parameters and their influence on the drying course. Drying parameters such as air temperature, air velocity, bed height, and type of wood are studied regarding the drying rate, final moisture content in the material, and bed pressure drop. The constant-rate and falling-rate drying period are studied to obtain an energy efficient drying process. Measurements show that the drying zone (where the actual drying occurs) progresses irregularly through the bed.     

Microwave-Vacuum Drying of Wood: Model Formulation and Verification

Based on the mechanism of moisture and heat transfer in wood during microwave-vacuum drying (MVD), a one-dimensional mathematical model to describe the process of wood MVD was established and verified by experiments in this research. The results showed that the process of MVD of wood experienced three distinct periods: (1) accelerating rate with rapid warming-up drying period, (2) a constant temperature and constant rate drying period, and (3) a heating-up with falling rate drying period. Compared with conventional hot air drying, the total drying process is almost governed by a constant rate period in vacuum-microwave drying of wood. The predicted temperature and moisture content in wood match well with the experimental data, the square of the relevant coefficient of the values of simulation and test is above 0.9, and the simulation precision of the change rule of the moisture is higher than that of the temperature.     

Comparative Analysis of Three Methods for Stochastic Lumber Drying Simulation

Abstract:

This article describes a novel stochastic model designed to simulate systems that cannot be analyzed as a unit, but as a collection of a large number of similar components. In order to state advantages and disadvantages, the proposed method is compared with two other published models. The first is a symbolic mathematical relationship designed to predict average moisture content and standard deviation after conventional drying of lumber. Since this model is exact, it was used as reference to evaluate the accuracy of the other approximate numerical methods. The second model is entirely random, and it emulates a real system behavior in which the parameters and conditions randomly change from one component to the other. The proposed method is based on numerical integration of the parameter's frequency distribution curves, which always produce the same and most probable result for the same parameters and conditions. The three methods were applied for simulation of conventional lumber drying, and the results were compared both qualitatively and numerically.     

Application of Wood Drying Simulation Models in Commercial Lumber Manufacturing

Drying models are created both to develop a better understanding of the governing heat and mass transfer phenomena and to assist drying practitioners in achieving commercial goals. This article is a review of the application of wood drying and kiln simulation models to commercial lumber drying challenges. Examples from the literature are briefly reviewed and four selected applications in commercial lumber manufacturing are described. They include development of equalization schedules, assessment of green lumber sorting, development of alternative drying schedules, and assessment of fan reversal frequency and timing.     

Drying Related Strain Development in Restrained Wood

The objective of this study was to investigate elastic, viscoelastic, mechanosorptive, and plastic strains developed by the drying process in perpendicular-to-grain restrained western hemlock specimens. Drying tests were performed on small, clear wood specimens sawn in such a manner that either the tangential or the radial direction was parallel to their length, where stress driven deformation rates were determined using resistive transducers. Different mechanical restraint schemes were designed by using weights for partly and a load cell for totally restrained specimens. The difference from previous research done in this domain was the restraining technique, which allowed stress to be induced by the drying process, thus, wood was forced to lift the weight as it reached the fiber saturation point. Drying rates were determined gravimetrically on free specimens having similar wood structure. Experiments with free and restrained specimens were carried out simultaneously while drying at 40, 60, and 80°C, down to 17, 12, and 6% target moisture contents. Overall, the magnitude of restrained shrinkage was successfully quantified and separated in free, combined viscoelastic and mechanosorptive and plastic strains. In all these preliminary experiments the dimensional change started at high overall moisture contents, temperature and environmental conditions being key factors. The results can be used to assess the stress indirectly by studying the restrained strain together with moisture content value.     

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.