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.     

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.     

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.     

高温除湿与常规蒸汽联合干燥的试验与分析

讨论了高温除湿与常规蒸汽联合干燥过程中干燥室内空气温度、相对湿度及木材含水率的变化趋势，分析了干燥过程中除湿量及能耗的变化规律。实验结果显示，高温除湿与常规蒸汽联合干燥节能效果显著，干燥质量好。高温除湿机可以直接放在木材干燥室内部工作，无需对干燥室窑体进行改装。     

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.     

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.     

THE EFFECT OF DRYING TEMPERATURE ON THE MECHANO-SORPTIVE BEHAVIOR OF RED OAK LUMBER

ABSTRACT

Matched units of 3.2 cm thick red oak lumber were dried simultaneously in a steam heated and dehumidification kiln. The two pairs of runs are designated Set I and Set II. The objective was to compare the mechano-sorptive behavior and board shrinkages while using the recommended U.S. Forest Products Laboratory schedule with the steam kiln and comparatively low temperature drying in the dehumidification kiln.

Drying rates in Set I and Set II were comparable for the two kilns up to approximately 450 hours, which illustrated the dependence of the drying rate on the relative humidity of the kiln air rather than its temperature. Subsequently, the stepwise increases in dry bulb temperature for the steam kiln were accompanied by accelerated drying.

Less compression set developed in the interior mechano-sorptive slices for the dehumidification kiln runs. On an average, the maximum compression set for the core slices from the dehumidification kiln was about S0% of that for core slices from the steam heated kiln. Simultaneously the surface slices from the dehumidification kiln developed more tension set than those from the steam heated kiln. Board width shrinkage at the end of drying, at the same average moisture content, was greatest for the steam kiln. The greater shrinkage is attributed to greater compression set due to the higher drying temperatures. These results support McMillen)s explanation for the effect of drying temperatures upon sets and the shrinkage of red oak lumber.     

THE EFFECT OF DRYING TEMPERATURE ON THE MECHANO-SORPTIVE BEHAVIOR OF RED OAK LUMBER

Matched units of 3.2 cm thick red oak lumber were dried simultaneously in a steam heated and dehumidification kiln. The two pairs of runs are designated Set I and Set II. The objective was to compare the mechano-sorptive behavior and board shrinkages while using the recommended U.S. Forest Products Laboratory schedule with the steam kiln and comparatively low temperature drying in the dehumidification kiln.

Drying rates in Set I and Set II were comparable for the two kilns up to approximately 450 hours, which illustrated the dependence of the drying rate on the relative humidity of the kiln air rather than its temperature. Subsequently, the stepwise increases in dry bulb temperature for the steam kiln were accompanied by accelerated drying.

Less compression set developed in the interior mechano-sorptive slices for the dehumidification kiln runs. On an average, the maximum compression set for the core slices from the dehumidification kiln was about S0% of that for core slices from the steam heated kiln. Simultaneously the surface slices from the dehumidification kiln developed more tension set than those from the steam heated kiln. Board width shrinkage at the end of drying, at the same average moisture content, was greatest for the steam kiln. The greater shrinkage is attributed to greater compression set due to the higher drying temperatures. These results support McMillen)s explanation for the effect of drying temperatures upon sets and the shrinkage of red oak lumber.     

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.     

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.     

Modeling and Microscopic Analysis of Vaporization During High-Temperature Drying of Sub-Alpine Fir

Mills experience reduced kiln productivity in drying sub-alpine fir lumber. High-temperature drying is an effective approach to accelerate the process. Thermal modification improves the dimensional stability and decay resistance. During these processes, two distinct stages, wood heating and water vaporization, were encountered. This research was aimed at developing a method to evaluate the vaporization process using an approximate analytic solution. The heating process was predicted by combining the analytic solution with numeric simulation. Using a scanning electron microscope, the impact of vaporization on wood micro-structure was examined. The vaporization process was able to open the aspirated pits and break the membranes. Laboratory drying experiments confirmed that the program could obtain satisfactory estimations of heating rates and times. The results indicated that the vaporization time is a function of the moisture content and size of the lumber, the temperature and air velocity in the kiln.     

THE INFLUENCE OF STORAGE/TRANSPORTATION CONDITIONS ON DRYING STRESSES AND THE PRONG TEST

ABSTRACT

From the time lumber is removed from the kiln to the time the remanufacturer uses it, it can be exposed to many different humidities With exposure to severe humidities, cupping in the final product can occur. This paper considers if humidity conditions during storage and transportation can significantly influence drying stresses and prong lest response. To determine the influence of exposure to different humidities, dried lumber was slowly equilibrated to 7,5% moisture content. It was than cut into blocks. The blocks were exposed to various humidity scenarios lo simulate actual storage and transportation conditions. Stress levels, moisture content and prong response were recorded. The analysis showed that stress level and prong response were strongly influenced by exposure to different humidities. The analysis also indicated that returning lumber to 7.5% equilibrium moisture content did not return the stress level or prong response to the original values. These combined results confirm that the prong test cannot be used as an indicator of how well the lumber has been dried or conditioned after it has been stored and transported. This makes seller/buyer disagreements difficult to resolve.     

THE INFLUENCE OF STORAGE/TRANSPORTATION CONDITIONS ON DRYING STRESSES AND THE PRONG TEST

From the time lumber is removed from the kiln to the time the remanufacturer uses it, it can be exposed to many different humidities With exposure to severe humidities, cupping in the final product can occur. This paper considers if humidity conditions during storage and transportation can significantly influence drying stresses and prong lest response. To determine the influence of exposure to different humidities, dried lumber was slowly equilibrated to 7,5% moisture content. It was than cut into blocks. The blocks were exposed to various humidity scenarios lo simulate actual storage and transportation conditions. Stress levels, moisture content and prong response were recorded. The analysis showed that stress level and prong response were strongly influenced by exposure to different humidities. The analysis also indicated that returning lumber to 7.5% equilibrium moisture content did not return the stress level or prong response to the original values. These combined results confirm that the prong test cannot be used as an indicator of how well the lumber has been dried or conditioned after it has been stored and transported. This makes seller/buyer disagreements difficult to resolve.     

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.     

Visual Method to Assess Lumber Sorting Before Drying

This article explores the possibility of using a simplified but intuitive method to quickly assess the potential benefits of sorting lumber before industrial kiln drying. The method consists of using scatter plots to visualize the probability of obtaining a certain drying result, such as final moisture content, as a function of a property of the green lumber that can be measured in practice. The method was first validated with four drying runs of 116 mm × 52 mm hemlock lumber: one run contained unsorted lumber and the others contained the same type of lumber but sorted into low, medium, and high groups depending on the electrical capacitance of the green wood. After validation, the scatter plots were used to assess the benefits of two typical industrial sorting strategies, namely, sorting by electric capacitance and sorting by weight. It was found that both methods have the potential to increase lumber production and reduce over dried lumber in approximately the same magnitude. For a typical industrial schedule, sorting into three groups reduced the drying time by approximately 10% and over dried lumber to practically zero.     

SIMULATION OF DRYING IN A BATCH LUMBER KILN FROM SINGLE-BOARD TESTS

A deterministic model was developed to perform a board-by-board simulation of a forced convective batch lumber kiln. Individual board properties may be input and dryer operating parameters varied. The drying rates are empirical correlations based on single-board laboratory tests. The model incorporates the thermodynamic properties of the wood and gas, as well as mass and energy balances within the lumber stack. It also accounts for differences in heat and mass transfer resulting from position and changing gas properties throughout the dryer. The rate of drying predicted by the model and the final moisture content distribution were verified by weighing boards in a batch kiln before, during, and after drying. The application of the model is illustrated by simulating four common scenarios.     

Drying Lumber In Kilns: A Methodology for Stochastic Analysis Using Markov Modelling

The value of dried lumber to the kiln operator is to a large extent dependent on the final moisture content distribution of the batch. This variation in moisture content arises from the intrinsic heterogeneity of wood and the stochastic nature of the timber drying process. Theoretical analyses of wood drying have hitherto relied upon deterministic models and have limited practical applicability. A methodology is proposed herein whereby the inherent probabilistic dynamics of batch drying can be modelled efliciently using Markov chains. The discretization of the variables of moisture and time that is necessary for this approach is discussed. By simulating the drying process with the model, the progressive transformation of the batch moisture distribution throughout the drying cycle can be studied. Output from the model is compared with experimental drying results to test its validity. There are     

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.