A Dual-Scale Computational Model of Kiln Wood Drying Including Single Board and Stack Level Simulation

This article proposes a dual-scale computational model of wood drying in a batch lumber kiln. The stack may consist of a large number of boards (typically 100) arranged in layers. Each single board of the stack is simulated using one module of TransPore, a comprehensive computational model for heat and mass transfer in porous media. Timber variability is taken into account by a Monte-Carlo method. Such a dual-scale model allows the drying simulation of a 100-board stack to be completed in less than 30 s on a PC with a 2.8-GHz Xeon processor. Sample simulations are presented to depict the great potential and prospects of this new tool.     

Optimization of Solar Kiln for Drying Wood

Theoretical investigation of the physical process of solar drying of timber based on conventional heat and mass transfer equations is presented. The governing equations and boundary conditions of the mass diffusion through the wood timbers are derived; also the governing equations of the components of the solar kiln are presented. The finite difference technique is used to solve the set of these equations by means of a simulation program that is based on object-oriented approach. The simulation program is used to investigate the effect of several design parameters on the drying rate and duration of the wood timbers in order to accomplish the drying process with minimal drying defects. These parameters include the ventilation conditions that control the drying schedule inside the solar kiln, wood volume as a ratio to the solar kiln absorber area, wood timber thickness, season of drying, the drying air velocity, and the stresses that formed on the timber boards due to drying with these several parameters, leading to derive the limit of damage for a selected local wood type. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries.     

Optimization of Solar Kiln for Drying Wood

Abstract

Theoretical investigation of the physical process of solar drying of timber based on conventional heat and mass transfer equations is presented. The governing equations and boundary conditions of the mass diffusion through the wood timbers are derived; also the governing equations of the components of the solar kiln are presented. The finite difference technique is used to solve the set of these equations by means of a simulation program that is based on object-oriented approach. The simulation program is used to investigate the effect of several design parameters on the drying rate and duration of the wood timbers in order to accomplish the drying process with minimal drying defects. These parameters include the ventilation conditions that control the drying schedule inside the solar kiln, wood volume as a ratio to the solar kiln absorber area, wood timber thickness, season of drying, the drying air velocity, and the stresses that formed on the timber boards due to drying with these several parameters, leading to derive the limit of damage for a selected local wood type. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries.     

Energy Saving in Industrial Wood Drying Addressed by a Multiscale Computational Model: Board,Stack, and Kiln

This article proposes a multiscale computational model able to calculate energy consumption in a batch lumber kiln. A dual-scale computational model of wood drying deals with the boards/stack interaction and serves as a basis for the present work. A new module was added here that calculates heat losses through kiln walls (convection, condensation) and the energy used by each kiln component (fans, heating elements, humidifier, vacuum pump, etc.). The corresponding mathematical formulation is presented and then theoretical results are compared to those collected in an industrial vacuum kiln. As application example, the effect of air reversal, air velocity, and kiln insulation are exhibited, which depicts the great potential and prospects of this new tool for energy savings in relation to the product quality.     

Energy Saving in Industrial Wood Drying Addressed by a Multiscale Computational Model: Board, Stack, and Kiln

This article proposes a multiscale computational model able to calculate energy consumption in a batch lumber kiln. A dual-scale computational model of wood drying deals with the boards/stack interaction and serves as a basis for the present work. A new module was added here that calculates heat losses through kiln walls (convection, condensation) and the energy used by each kiln component (fans, heating elements, humidifier, vacuum pump, etc.). The corresponding mathematical formulation is presented and then theoretical results are compared to those collected in an industrial vacuum kiln. As application example, the effect of air reversal, air velocity, and kiln insulation are exhibited, which depicts the great potential and prospects of this new tool for energy savings in relation to the product quality.     

Optimization of Kiln Drying for Softwood through Simulation of Wood Stack Drying, Energy Use, and Wood Color Change

An integrated modeling system was developed to simulate the drying processing, energy use, and wood color change in kiln drying of softwood timber. The model has been applied for a temperature range from 50 to 70°C and an airspeed from 3 to 9 m/s. The model is based on theoretical analysis and contains components such as kiln configuration and practical operations. From the model simulation, optimized drying schedules for minimizing color change and energy use are recommended with dry bulb temperature of 60 to 70°C and wet bulb depression of 15 to 20°C.     

Optimization of Kiln Drying for Softwood through Simulation of Wood Stack Drying,Energy Use,and Wood Color Change

An integrated modeling system was developed to simulate the drying processing, energy use, and wood color change in kiln drying of softwood timber. The model has been applied for a temperature range from 50 to 70°C and an airspeed from 3 to 9 m/s. The model is based on theoretical analysis and contains components such as kiln configuration and practical operations. From the model simulation, optimized drying schedules for minimizing color change and energy use are recommended with dry bulb temperature of 60 to 70°C and wet bulb depression of 15 to 20°C.     

Experimental Evaluation of Solar Kiln for Drying Wood

Semi-greenhouse solar dryer was designed and constructed in the Solar Energy Department, National Research Center, Cairo, Egypt. It consists of steel frame structure and all the sides were insulated and covered by galvanized iron sheet. The roof faces south with a tilt angle of 30° and covered by glass sheets. Corrugated iron sheets, painted black on the top, were fixed beneath the glass roof as an absorber. The humidity inside the solar kiln is controlled by inlet vent at south wall and two outlet vents at north wall. Data Capture System was designed to ease the measurements and to capture the measured data automatically and continuously for 24 h a day. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries. The experimental results with this type of wood show considerable agreements with the simulated ones. A comparison was made between experimental results of solar wood drying and traditional air-drying. The results show that timber boards were dried inside solar kiln to moisture content of 12% within 17 days, where air dying moisture content was limited to 20% for the same period.     

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.     

Performance Comparison of Two Solar Kiln Designs for Wood Drying Using a Numerical Simulation

The predicted performances of the two different timber-drying solar kiln designs (Oxford and Boral kilns) have been compared by using the climatic and geographical conditions of Brisbane (27.46°S) in Australia. The dimensions of the original Boral kiln have been scaled down in order to compare the kilns’ performances on the basis of same load capacity. The comparison was made in terms of the key performance indicators, including drying rates, drying qualities (i.e., the level of strains developed within the timber boards during drying), energy gains, and major elements of the energy losses (i.e., the convection and radiation losses) while drying timber from an average initial moisture content of 53% (dry basis) to a final moisture content of 15% (dry basis) in both kilns. The relative performances for the two designs were discussed in more detail by using the typical climatic conditions (spring season 2013), when the days and nights are of approximately equal length. Finally, an analysis of the seasonal effects on the kiln performances was also carried out in this article. More effective orientation of the solar absorbers with respect to sun, together with faster heat transfer rates between the circulating air and the timber stack for the Oxford design, were likely to give consistently better predicted performance for the Oxford design than for the Boral kiln throughout the year.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

Numerical Simulation of a Solar Kiln Design for Drying Timber with Different Geographical and Climatic Conditions in Australia

A numerical simulation for the modeling of solar kilns for hardwood timber drying with different boundary conditions has been developed in this work. This simulation has been used to predict the key behavior of the wood and the kiln itself under different geographical and weather conditions in Australia. This work, therefore, enables a site to be suggested that is likely to give the best kiln performance both in terms of the productivity and the quality of the end-use product. As test cases, realistic climatic conditions for three representative geographical locations in Australia, including Sydney (latitude 33.86°S), Melbourne (37.81°S), and Brisbane (27.46°S), have been considered in this simulation. Out of the three sites considered in this work, the fastest drying rates (and highest diffusion coefficients) were predicted in Brisbane. The highest level of instantaneous strain in the timber for this location was also predicted to be 0.0075 m/m, which is much less than the failure value (0.02 m/m). The moisture content distribution profile within the boards toward the end of the drying period was found to be almost uniform, which is likely to cause low degrade in the dried timber.     

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.     

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.     

Probabilistic Analysis of Timber Drying Schedules

Timber drying schedules are primarily chosen to achieve a certain final moisture content accompanied by a minimum amount of board degrade. However the schedule adopted also influences the dispersion in moisture content between boards at the end of drying. A simple double set point schedule consisting of two distinct and sequential equilibrium moisture contents is selected to illustrate the concept. Theoretical expressions that predict mean and standard deviation in board moisture content vs. time are developed. The predictions of these equations are compared with the output of a Monte Carlo model of timber drying and with experimental measurements from a laboratory kiln. The advantages of a double set point over a single set point schedule are explained and an optimum double set point schedule is determined. The behavior of a commercial drying schedule is analyzed and its performance with respect to the optimum schedule is quantified. Finally some inherent characteristics of the variability in timber drying systems are outlined.     

Intelligent Wood Drying Control: Problems and Decisions

Common drying schedules are intended for different species with average properties and frequently they are too conservative. The algorithm allowing the drying optimization of a board batch in real-time scale using computer simulation is described in this article. The method of calculation of simulation model factors using interrelation of parameters of wood properties is described. Allowable reduction of residual wood strength was used as a criterion to find temperature levels at optimization of drying of a board batch. The proposed algorithm for the computation of the optimal drying parameters was tested experimentally in an industrial kiln. Results of tests have confirmed the reliability of the algorithm and the opportunity of its use for the creation of intelligent control systems (ICS).     

Probabilistic Analysis of Timber Drying Schedules

Abstract

Timber drying schedules are primarily chosen to achieve a certain final moisture content accompanied by a minimum amount of board degrade. However the schedule adopted also influences the dispersion in moisture content between boards at the end of drying. A simple double set point schedule consisting of two distinct and sequential equilibrium moisture contents is selected to illustrate the concept. Theoretical expressions that predict mean and standard deviation in board moisture content vs. time are developed. The predictions of these equations are compared with the output of a Monte Carlo model of timber drying and with experimental measurements from a laboratory kiln. The advantages of a double set point over a single set point schedule are explained and an optimum double set point schedule is determined. The behavior of a commercial drying schedule is analyzed and its performance with respect to the optimum schedule is quantified. Finally some inherent characteristics of the variability in timber drying systems are outlined.     

Microwave Drying of Wood Strands

Abstract

Characteristics of microwave drying of wood strands with different initial moisture contents and geometries were investigated using a commercial small microwave oven under different power inputs. Temperature and moisture changes along with the drying efficiency were examined at different drying scenarios. Extractives were analyzed using gas chromatography/mass spectrometry (GC/MS). The results showed that the microwave drying process consisted of three distinct periods (warm-up period, evaporation period, and heating-up period) during which the temperature, moisture change, and drying efficiency could vary. Most of the extractives were remnant during microwave drying. It was observed that with proper selections of power input, weight of drying material, and drying time, microwave drying could increase the drying rate, save up to 50% of energy consumption, and decrease volatile organic compound (VOC) emissions when compared with the conventional drying method.