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.     

MEASURED GAS VELOCITY AND MOISTURE CONTENT DISTRIBUTION IN A CONVECTIVE VACUUM KILN

Transferring the necessary heat of evaporation to the stack is the bottleneck in convective vacuum drying of wood. Higher gas velocities are applied to compensate for the lower gas density and to obtain similar heat and mass transfer characteristics as under normal pressure. Like in conventional kiln drying the region with the most unfavorable drying conditions determines drying time and product quality. To use the full potential of the meanwhile established superheated steam vacuum drying technology, it is therefore necessary to work on an improved uniformity of process conditions in the kiln.

To evaluate the fluid dynamics and its influence on the final moisture content, experimenls in a laboratory convective vacuum kiln were carried out. For different total pressures the profiles of dynamic pressure in the stack entry section were measured in a dry atmosphere. At normal pressure the profiles were determined between the board layers throughout the whole stack. For the same slack configuration vacuum drying tests were used to assess the impact of the velocity distribution in the slack on the final moisture content distribution-Regions of low gas velocities coincided well with regions of high final moisture content.     

MEASURED GAS VELOCITY AND MOISTURE CONTENT DISTRIBUTION IN A CONVECTIVE VACUUM KILN

ABSTRACT

Transferring the necessary heat of evaporation to the stack is the bottleneck in convective vacuum drying of wood. Higher gas velocities are applied to compensate for the lower gas density and to obtain similar heat and mass transfer characteristics as under normal pressure. Like in conventional kiln drying the region with the most unfavorable drying conditions determines drying time and product quality. To use the full potential of the meanwhile established superheated steam vacuum drying technology, it is therefore necessary to work on an improved uniformity of process conditions in the kiln.

To evaluate the fluid dynamics and its influence on the final moisture content, experimenls in a laboratory convective vacuum kiln were carried out. For different total pressures the profiles of dynamic pressure in the stack entry section were measured in a dry atmosphere. At normal pressure the profiles were determined between the board layers throughout the whole stack. For the same slack configuration vacuum drying tests were used to assess the impact of the velocity distribution in the slack on the final moisture content distribution-Regions of low gas velocities coincided well with regions of high 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.     

PARAMETERS INFLUENCING DRYING BEHAVIOR OF RUBBER WOOD (HEVEA BRAZILLIENSIS) AS DETERMINED FROM DESORPTION EXPERIMENT

In this study, the drying properties of rubber wood, which are the basic parameters for kiln scheduling design, were determined from desorption experiment. Equilibrium moisture content expression was developed. The diffusion coefficients at different drying environments were evaluated. It is more appropriate to determine the diffusion coefficients by the optimum scheme in comparison to other schemes; the logarithmic, square-root and half-fraction of evaporable moisture schemes. Finally, the diffusion coefficient of rubber wood was described by, instead of wood moisture content, the drying temperature and relative humidity, which are the parameters controlling the drying kiln operation.     

A COMMERCIALLY VIABLE SOLAR WOOD DRYING KILN SYSTEM

The purpose of the study was to create a totally passive solar wood drying kiln that would dry lumber to 9% moisture content in a reasonable amount of time. A series of modifications led to a kiln design that dried freshly-cut lumber to 8% in a 29-day period with no case hardening or cracking. Air speed, internal and external temperatures and relative humidity levels were measured at 5-minute intervals. The average temperature inside the kiln was 12% higher with relative humidity levels 19% lower than outside the kiln. It is hypothesized that the daily cycles of heating and cooling permitted the interior moisture of the wood to reach the surface through diffusion, thus lessening stress and speeding drying of the lumber.     

Preliminary tests for the application of an optical measurement system for the development of a kiln-drying schedule

The drying defects of refractory species are known to begin occurring in the early stages of kiln drying. Therefore, monitoring the strain distribution in wood at these early stages of drying is expected to be helpful for the reduction of possible defect development. In this study, the strain distribution of red oak specimens was progressively monitored on the tangential surface of the wood using an optical measurement system based on the digital image correlation (DIC) principle. The specimens were kiln-dried with three different initial conditions. The strain distribution maps obtained at various moisture contents clearly illustrated the progress of the strain development in the wood. In addition, strain velocities and strain ratios were calculated from the data extracted from the strain distribution maps. These parameters could be used for developing or modifying a kiln-drying schedule of a refractory species.     

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 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.     

PARAMETERS INFLUENCING DRYING BEHAVIOR OF RUBBER WOOD (HEVEA BRAZILLIENSIS) AS DETERMINED FROM DESORPTION EXPERIMENT

ABSTRACT

In this study, the drying properties of rubber wood, which are the basic parameters for kiln scheduling design, were determined from desorption experiment. Equilibrium moisture content expression was developed. The diffusion coefficients at different drying environments were evaluated. It is more appropriate to determine the diffusion coefficients by the optimum scheme in comparison to other schemes; the logarithmic, square-root and half-fraction of evaporable moisture schemes. Finally, the diffusion coefficient of rubber wood was described by, instead of wood moisture content, the drying temperature and relative humidity, which are the parameters controlling the drying kiln operation.     

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 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.     

The shrinkage properties of red pine wood assessed by image analysis and near-infrared spectroscopy

This work focused on the shrinkage and drying stress of red pine (Pinus densiflora) wood during kiln drying and investigated transverse shrinkage and moisture changes in thin specimens using digital image analysis and near-infrared spectroscopy, respectively. The effect of specimen thickness, which ranged from 1 to 10?mm on shrinkage, was analyzed under drying conditions with restrained stresses. The shrinkage due to moisture content was presented in the form of an exponential function and a linear function above and below the fiber saturation point, respectively. A shorter existence of the moisture gradient increased the transverse shrinkage. The tangential and radial shrinkage at the tissue level in 30-µm (longitudinal) thick slices was measured and compared with the shrinkage of above specimens with difference thicknesses. The transverse shrinkage of 1-mm-thick specimen is similar to the shrinkage measured using 30-µm-thick slice.     

Assessment of the Actual Performance of an Industrial Solar Kiln for Drying Timber

The aim of this study was to assess the actual performance of an instrumented industrial solar kiln for drying Australian hardwood timber (Eucalyptus pilularis) boards (270 × 43 mm). Ambient temperature and humidity, air temperature and humidity in the kiln, and wood moisture contents were recorded on site (Heron's Creek, NSW, Australia) using sensors and an electronic data acquisition and logging system. The average increases in air temperatures in the kiln compared with ambient conditions were 17.3°C (May-June), 13.8°C (July-August), 10°C (September-October), 8.2°C (November-March), and 7.5°C (March-May) for five runs monitored. Drying times were 2-4 months from initial moisture contents of 43 to 62% (dry-basis) to final moisture contents of 12 to 22%. Overall, the solar kiln has been shown to be an acceptable alternative to air-drying for pre-drying of Australian hardwood timber.     

COMPARISON OF EXPERIMENTAL RESULTS WITH ANALYTICAL SOLUTIONS AND TWO DIHENSIONAL MODEL OF OAK DRYING IN AN EVACUATED KILN

In continuation of a series of tests, the original results of oak drying in an evacuated kiln are presented here for different plate temperatures and for various pressures in the kiln. These results include more particularly the drying curves, the evolution of temperature, of moisture and of pressure in and on the wood. They evidence the pressure and the levels of temperature occurring in the wood during the drying period.

These results also allow the development of two types of drying models a simple monodimensional model of drying curves from the analytical solutions of the equations of water diffusion in the wood and, moreover, a model, in two dimensions, of temperature, moisture and pressure fields in the wood by applying the finite element method. The boundary conditions of the second model can be fixed with precision thanks to the results of the first model. In both cases, the proposed solutions are justified by experimental results.     

COMPARISON OF EXPERIMENTAL RESULTS WITH ANALYTICAL SOLUTIONS AND TWO DIHENSIONAL MODEL OF OAK DRYING IN AN EVACUATED KILN

ABSTRACT

In continuation of a series of tests, the original results of oak drying in an evacuated kiln are presented here for different plate temperatures and for various pressures in the kiln. These results include more particularly the drying curves, the evolution of temperature, of moisture and of pressure in and on the wood. They evidence the pressure and the levels of temperature occurring in the wood during the drying period.

These results also allow the development of two types of drying models a simple monodimensional model of drying curves from the analytical solutions of the equations of water diffusion in the wood and, moreover, a model, in two dimensions, of temperature, moisture and pressure fields in the wood by applying the finite element method. The boundary conditions of the second model can be fixed with precision thanks to the results of the first model. In both cases, the proposed solutions are justified by experimental results.     

Density and Moisture Profile Evolution during Timber Drying by CT Scanning Measurements

Timbers of 105 × 105 mm² in cross section and 1000 mm long were computed tomography (CT) scanned with a commercial unit in equal time steps during a drying cycle. The study aimed to convert the scanning numbers into wood density values, observe the changes that occur during the dehydration process, and, from these, differentiate moisture content as a function of time and space. The reference for calculating the moisture content of each image pixel was the oven-dry wood scan. Analysis of the results allowed visualizing the water distribution, the mechanism of water removal, as well as check formation. Although only a single drying run was analyzed and a limited number of timbers were scanned, the results revealed trends and thus show promise, so further research is urged because it may lead to a better understanding of water movement and check formation in wood during kiln drying.     

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.     

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.