Drying Kinetics of Pinus Rqdiata Boards at Elevated Temperatures

ABSTRACT

From the numerical results of a rigorous, mathematical model, the rate–o–drying curves for high–temperature seasoning of Pinus radiata board show that the drying process can be divided into three periods. Based on the mechanistic picture of moisture movement and differences in the movement in each period, this paper simplifies the model by using the concept of the haracteristic drying curve.

In heartwood drying, the first period is very short (5–15 minutes), hence it can be neglected, whereas for sapwood, the first period is substantial with the drying rate being taken as constant in the analysis. Since the physical characteristics of the second and third drying periods are different, separate characteristic drying curves have been developed for each period. It has been found that common normalised drying curves are able to describe the results generated from the earlier model for normal range of kiln conditions over the examined ranges in dry–bulb temperatures of 1 10–140°C, wet–bulb temperatures of 70–90°C and air velocity of 3–7 m s^-1. Therefore. the derived simplified model can directly be used to calculate kiln–wide variations in moisture content and drying rate.     

Drying at high temperature of sapwood and heartwood : theory, experiment and practical consequence on kiln control

This paper aims to relate the last results obtained in the field of drying theory with the industrial process. It deals particularly with the problem of the difference between sapwood and heartwood that always occurs in the whole stack of an industrial drier. The experimental procedure and the simulation code TRANSPORE are shortly presented. Experiments have been carried out on sapwood and heartwood (Spruce with superheated steam and pine with moist air). The code is used to compare sapwood and heartwood. In the simulations, only the initial moisture content and the permeabilities differentiate both parts of the tree. Once again, the code proves very efficient. For both experiment and theory, the main results are: i) longitudinal liquid migration occurs up to the end of the board only for sapwood ii) the first drying period disappears for heartwood iii) the drying kinetics cross each other at the end of drying These results are of great importance at the time of optimizing the process: among several possible control devices, the temperature loss through the stack seems to be a good choice combining relevant information and easy-to-use measurement.     

Drying at high temperature of sapwood and heartwood : theory,experiment and practical consequence on kiln control

ABSTRACT

This paper aims to relate the last results obtained in the field of drying theory with the industrial process. It deals particularly with the problem of the difference between sapwood and heartwood that always occurs in the whole stack of an industrial drier. The experimental procedure and the simulation code TRANSPORE are shortly presented. Experiments have been carried out on sapwood and heartwood (Spruce with superheated steam and pine with moist air). The code is used to compare sapwood and heartwood. In the simulations, only the initial moisture content and the permeabilities differentiate both parts of the tree. Once again, the code proves very efficient. For both experiment and theory, the main results are: i) longitudinal liquid migration occurs up to the end of the board only for sapwood ii) the first drying period disappears for heartwood iii) the drying kinetics cross each other at the end of drying These results are of great importance at the time of optimizing the process: among several possible control devices, the temperature loss through the stack seems to be a good choice combining relevant information and easy-to-use measurement.     

THE HIGH-TEMPERATURE SEASONING OF SOFTWOOD BOARDS

Pinus radiata, the commonest plantation-grown softwood in New Zealand, is increasingly dried in kilns under high-temperature conditions when the dry-bulb temperature exceeds100°C. The ease of drying depends upon the ways moisture can move through the boards, which differ in heartwood from those in sapwood. These considerations have led to a drying model which can describe the temperature and moisture-content variation of aboard as it dries. Local variations of the external mass-transfer coefficient (also incorporated in the model) have an influence which is greater as the air temperature is raised. Comparison with laboratory data on the drying of a single board give confidence in the applicability of the model. There are two principal stages in the drying process: an earlier one dominated by a receding front about the boiling point, and a later one in which cell-wall and vapour diffusion prevails. This suggests that two characteristic drying curves may correlate the behaviour of the drying board with a given thickness. This simplified model describes the results of the more detailed physical model adequately, and it can then be used as the basis for investigating kilnwide variations in moisture content and temperature and the impact of airflow reversals.     

THE HIGH-TEMPERATURE SEASONING OF SOFTWOOD BOARDS

ABSTRACT

Pinus radiata, the commonest plantation-grown softwood in New Zealand, is increasingly dried in kilns under high-temperature conditions when the dry-bulb temperature exceeds100°C. The ease of drying depends upon the ways moisture can move through the boards, which differ in heartwood from those in sapwood. These considerations have led to a drying model which can describe the temperature and moisture-content variation of aboard as it dries. Local variations of the external mass-transfer coefficient (also incorporated in the model) have an influence which is greater as the air temperature is raised. Comparison with laboratory data on the drying of a single board give confidence in the applicability of the model. There are two principal stages in the drying process: an earlier one dominated by a receding front about the boiling point, and a later one in which cell-wall and vapour diffusion prevails. This suggests that two characteristic drying curves may correlate the behaviour of the drying board with a given thickness. This simplified model describes the results of the more detailed physical model adequately, and it can then be used as the basis for investigating kilnwide variations in moisture content and temperature and the impact of airflow reversals.     

OPTIMIZATION OF DRYING SCHEDULES ADAPTED FOR A MIXTURE OF BOARDS WITH DISTRIBUTION OF SAPWOOD AND HEARTWOOD

A distributed optimization model for wood drying with several different boards simultaneously is presented. Optimization is performed with a gradient-based program. During optimization, convex subproblems are created and transformed to the dual problem and solved. Arbitrary outtakes and board dimensions are possible, as well as different material data and distribution of sapwood and heartwood. It is also possible to optimize drying schedules where drying of boards with variations in environmental conditions is simulated. A two-dimensional orthotropic drying model is used in the moisture transport and structural analysis, where the variation in radial and tangential directions are considered. The influence of temperature and moisture content on material data and mechanical properties is also taken into account. The drying schedules achieved are optimized to minimize drying time for a representative mixture of boards. A numerical example is presented where the drying schedule is optimized for two boards with different outtakes and distributions of sapwood and heartwood. Optimization is performed with two computers in a network. Drying starts from the fibre saturation point in these simulations.     

OPTIMIZATION OF DRYING SCHEDULES ADAPTED FOR A MIXTURE OF BOARDS WITH DISTRIBUTION OF SAPWOOD AND HEARTWOOD

ABSTRACT

A distributed optimization model for wood drying with several different boards simultaneously is presented. Optimization is performed with a gradient-based program. During optimization, convex subproblems are created and transformed to the dual problem and solved. Arbitrary outtakes and board dimensions are possible, as well as different material data and distribution of sapwood and heartwood. It is also possible to optimize drying schedules where drying of boards with variations in environmental conditions is simulated. A two-dimensional orthotropic drying model is used in the moisture transport and structural analysis, where the variation in radial and tangential directions are considered. The influence of temperature and moisture content on material data and mechanical properties is also taken into account. The drying schedules achieved are optimized to minimize drying time for a representative mixture of boards. A numerical example is presented where the drying schedule is optimized for two boards with different outtakes and distributions of sapwood and heartwood. Optimization is performed with two computers in a network. Drying starts from the fibre saturation point in these simulations.     

Effects of Wood Drying Schedules on Fluid Flow in Paulownia Wood

Effects of three drying schedules on fluid flow were studied in the sapwood and heartwood of Paulownia wood (Paulownia fortunei). Boards with a commercial thickness of 5 cm were randomly dried to a final moisture content of 8 ± 2% using a mild (T₆E₃), a moderate (T₆E₄), and a severe (T₇E₄) drying schedule. Permeability measurement was carried out when specimens reached the final moisture content. Results showed a significant difference in the specific gas permeability as well as liquid permeability of the boards dried under the three drying schedules. Furthermore, a significant difference was observed in the gas permeability of sapwood and heartwood, dried under the different schedules, but not much significant difference was seen in the liquid permeability between sapwood and heartwood. T₆E₃ had the highest liquid permeability; furthermore, it was reported to have resulted in the lowest warping and most homogeneous moisture profile. This mild schedule is therefore recommended for commercial drying of Paulownia wood when further preservation and impregnation processes are planned for the dried boards. In the meantime, it is concluded that the age and drying schedule have significant effects on the formation of tyloses, significantly affecting gas and liquid permeability in Paulownia wood.     

Intermittent and Continuous Drying of Red Beech Timber From the Green Condition

Intermittent and continuous schedules for drying New Zealand's indigenous red beech timber (Nothofagus fusca) from the green condition have been compared. Both schedules can be used to dry red beech with good timber quality. Continuous schedules are appropriate for use in conventional kilns, where better humidity control can be used to minimize timber degrade and equalize moisture contents. Intermittent schedules can be adapted for use in dehumidifiers, where the lower achievable humidities—often a result of poor insulation and air leakages—can be compensated for by the relaxation periods, when the fans and heaters are switched off intermittently to relieve drying-induced stresses. However, equalization cannot be easily applied in a dehumidifier. In this case, a presort of the green timber into two classes—a heartwood heavy class and a mixed heartwood light and sapwood class—would then prove beneficial to address the issue of significant timber property and green moisture content variations that occur in red beech timber.     

MOISTURE MOVEMENT ON DRYING SOFTWOOD BOARDS AND KILN DESIGN

ABSTRACT

This paper provides an overview of present understanding of how moisture can move through softwood boards, as a basis for determining kiln-seasoning strategies. Moisture in green wood is held essentially unbound, whereas below fibre saturation it is bound to a variable extent to the fibre walls. Sapwood, which is that part of the timber used for the transport of liquid nutrients, contains more moisture than physiologically inactive heartwood. Sawing the felled log creates a moisture-denuded layer at the damaged exposed surfaces. These features have a profound influence on the way that moisture can be removed on drying. Superimposed are differences arising from seasonal variations in the growth of wood between earlywood and latewood, which have different moisture permeabilities. When the width of the annual growth ring is relatively large compared with the board dimensions, moisture movement and the development of drying stresses depend markedly upon the sawing orientation relative to the grain direction. Quarter-sawn boards dry more uniformly (in the direction normal to the drying surfaces), but more slowly than flat-sawn boards. Most timber boards are stacked and then dried in box-shaped kilns. The uniformity of drying depends on the goodness of this stacking and on a uniform airflow being presented to the inlet face of the stack. Some non-uniformities can be mitigated by periodic reversals of the airflow direction through the stack and by overdrying the majority of boards to reduce wet spots, but there are limits, while overdrying reduces kiln capacity. Attention to aspects of the kiln geometry can reduce the fan-energy requirements and shorten the drying time, with a more uniform moisture content through out the kiln load.     

MOISTURE MOVEMENT ON DRYING SOFTWOOD BOARDS AND KILN DESIGN

This paper provides an overview of present understanding of how moisture can move through softwood boards, as a basis for determining kiln-seasoning strategies. Moisture in green wood is held essentially unbound, whereas below fibre saturation it is bound to a variable extent to the fibre walls. Sapwood, which is that part of the timber used for the transport of liquid nutrients, contains more moisture than physiologically inactive heartwood. Sawing the felled log creates a moisture-denuded layer at the damaged exposed surfaces. These features have a profound influence on the way that moisture can be removed on drying. Superimposed are differences arising from seasonal variations in the growth of wood between earlywood and latewood, which have different moisture permeabilities. When the width of the annual growth ring is relatively large compared with the board dimensions, moisture movement and the development of drying stresses depend markedly upon the sawing orientation relative to the grain direction. Quarter-sawn boards dry more uniformly (in the direction normal to the drying surfaces), but more slowly than flat-sawn boards. Most timber boards are stacked and then dried in box-shaped kilns. The uniformity of drying depends on the goodness of this stacking and on a uniform airflow being presented to the inlet face of the stack. Some non-uniformities can be mitigated by periodic reversals of the airflow direction through the stack and by overdrying the majority of boards to reduce wet spots, but there are limits, while overdrying reduces kiln capacity. Attention to aspects of the kiln geometry can reduce the fan-energy requirements and shorten the drying time, with a more uniform moisture content through out the kiln load.     

A Numerical Simulation for Solving the Dippusion Equations in the Drying of Hardware Timber

A numerical simulation is described for solving the thermal conduction and mass diffusion equations in boards of hardwood timber, and the Einite-volume method used here has been applied to the drying of Eucalypt timber, an Australian hardwood. The predictions of the variation in the average moisture content with time agree well with both ex~erimental data from the literature and analvtical solutions of the-diffusion equation. The nume ical simulation treats the boundary conditions more accurately than the analytical solutions when the moisture movement is two dimensional, as it is through the cross-section of a timber board. This feature makes the simulation useful when describing the drying process under intermittent drying conditions. These conditions are encountered in the drying of timber in solar kilns, and this simulation may be used to predict the distributions of temperature andmoisture content inboards of timber which are being dried intermittently inside conventional kilns. The numerical simulation for intermittent drying has shown, inthe example studied here, chat the same overall change in average moisture concentration can be achieved with 12-hour active drying and 12-hour relaxationperiodaas forcontinuousdryingbyincreasingthedry-bulb temperature by 10°C for this timber. In spite of the higher dry-bulb temperature used in the active drying period of intermittent drying. the moisture concentration profiles within the board are predicted to be more uniform than with continuous drying, because the internal moisture diffusion process continues during the relaxation period. These mare uniform moisture concentration profiles in intermittent drying are likely to result in lower stress levels within the timber than with continuous drying.     

A Study of Vacuum-Drying Characteristics of Sugi Boxed-Heart Timber

In this study, we evaluated the effects of drying under atmospheric and vacuum pressure on the drying time, checking, and color change of sugi boxed-heart timber dried at the same dry-bulb temperature and the same wet-bulb depression. The results obtained were as follows: Sugi boxed-heart timber specimens dried at any temperature under vacuum pressure had a shorter drying time than the specimens dried under atmospheric pressure. At moisture content above fiber saturation point and at the same dry-bulb temperature, the specimens dried under vacuum pressure had a drying rate that was almost twice as fast as that of the specimens dried under atmospheric pressure. Sugi boxed-heart timber specimens dried under both atmospheric and vacuum pressure at a higher drying temperature had a shorter drying time than the specimens dried at a lower drying temperature. Apart from the sugi boxed-heart timber specimen dried at a temperature of 100°C under atmospheric pressure, no surface checks were observed for the specimens dried under vacuum pressure or at the other temperatures dried under atmospheric pressure. Slight internal checks were observed in sugi boxed-heart timber specimens dried at a temperature of 100°C under both atmospheric and vacuum pressure. After planer shaving, there was no significant difference between kiln drying under atmospheric pressure and that done under vacuum pressure in terms of the color change (ΔE*) for both sapwood and heartwood of sugi boxed-heart timber specimens.     

INFRARED THERMOGRRPHY IN THE ANALYSIS OF MOISTURE FLUX FROM DRYING WOODEN SURFACES

Infrared thermography is used to visualize moisture evaporation from wooden surfaces. The initial moisture flux is shown to be dependent upon the growth ring structure as well as the heartwood and sapwood portions of a board.     

MODELLING OF STRESS DEVELOPMENT DURING DRYING AND RELIEF DURING STEAMING IN PINUS RADIATA LUMBER

A one-dimensional stress model was proposed for drying of radiata pine lumber, which has considered wood moisture shrinkage, instantaneous stress-strain relationships, mechano-sorptive creep, time-induced creep and temperature effects. In addition, wood hardening behaviour in the plastic region and differences between stress increase and decrease have been taken into account. The proposed Stress model can predict stress development and relief in a drying cycle once the required wood mechanical and Theological properties have been quantified.

Drying experiments were performed to dry Pinus radiata sap wood boards of 100×40×590 mm in a tunnel dryer. In the experiment, wood temperature, moisture content gradient and residual stress through board thickness were measured. The drying cycle included HT drying, cooling and final steam conditioning. The measured stress patterns were in agreement with the model predictions. However, more accurate calculations will be made once the detailed experimental data for radiata pine wood mechanical and rheological properties are available.     

INFRARED THERMOGRRPHY IN THE ANALYSIS OF MOISTURE FLUX FROM DRYING WOODEN SURFACES

ABSTRACT

Infrared thermography is used to visualize moisture evaporation from wooden surfaces. The initial moisture flux is shown to be dependent upon the growth ring structure as well as the heartwood and sapwood portions of a board.     

MODELLING OF STRESS DEVELOPMENT DURING DRYING AND RELIEF DURING STEAMING IN PINUS RADIATA LUMBER

ABSTRACT

A one-dimensional stress model was proposed for drying of radiata pine lumber, which has considered wood moisture shrinkage, instantaneous stress-strain relationships, mechano-sorptive creep, time-induced creep and temperature effects. In addition, wood hardening behaviour in the plastic region and differences between stress increase and decrease have been taken into account. The proposed Stress model can predict stress development and relief in a drying cycle once the required wood mechanical and Theological properties have been quantified.

Drying experiments were performed to dry Pinus radiata sap wood boards of 100×40×590 mm in a tunnel dryer. In the experiment, wood temperature, moisture content gradient and residual stress through board thickness were measured. The drying cycle included HT drying, cooling and final steam conditioning. The measured stress patterns were in agreement with the model predictions. However, more accurate calculations will be made once the detailed experimental data for radiata pine wood mechanical and rheological properties are available.     

Review of heat Conduction,Second Edition,S. Kakac and Y. Yener

In some species, such as subalpine fir (Abies lasiocarpa [Hook] Nutt), the water content of the confined zones in heartwood is as high as or greater than that of sapwood. Such wet zones of heartwood are referred to as “wetpocket” or “wetwood.” Wood products from subalpine fir forests are adversely affected by the wetwood-associated problems, particularly during the drying process. The objectives of the study were as follows: (1) to investigate feasibility of a high X-ray energy industrial computed tomography (ICT) scanner for imaging wetwood; and (2) to determine changes of the 2-D and 3-D moisture profiles (from core to shell) at different drying times.

Although medical CT scanning has been used for attaining signal intensity profiles of typical wood at different drying times, the technology has not, to date, been used for the study of wetwood phenomenon. This study presents, for the first time, results from the ICT imaging of the wetwood phenomenon. The results indicate that the ICT imaging system provides a powerful technique for imaging wetwood at different drying times. In addition, the results show that during the initial phase of drying, almost flat moisture profiles were observed in all wood types except for the wetwood, which showed a relatively higher moisture profile. A much slower (sluggish) drying development pattern at each increment from core to shell was found within the wetwood zone than normal wood regions along the width, thickness, and length of the board.     

PROBABILISTIC ANALYSIS AND DESIGN OF THE INDUSTRIAL TIMBER DRYING PROCESS

The distribution in the moisture content of dried planks is an important parameter for kiln operators. The evolution of variability in the moisture content of timber boards during a batch drying process is investigated. This random variability in moisture content arises from a distribution in plank initial moisture content and dispersion in plank drying rate. A simple deterministic model of timber drying is outlined. Theoretical probabilistic analysis is applied to this model to predict the mean and standard deviation in board moisture content as a function of time. The solution is assessed with representative industrial kiln drying data. The utility of the approach in suggesting strategies to promote uniformity in final moisture content is outlined using some design studies. These strategies include sorting of the timber by moisture content and adjusting the drying rate and equilibrium moisture content to reduce variability. It is also demonstrated that the probabilistic approach can yield a better estimation of kiln average moisture content.     

Moisture Distribution Changes and Wetwood Behavior in Subalpine Fir Wood during Drying Using High X-Ray Energy Industrial CT Scanner

In some species, such as subalpine fir (Abies lasiocarpa [Hook] Nutt), the water content of the confined zones in heartwood is as high as or greater than that of sapwood. Such wet zones of heartwood are referred to as “wetpocket” or “wetwood.” Wood products from subalpine fir forests are adversely affected by the wetwood-associated problems, particularly during the drying process. The objectives of the study were as follows: (1) to investigate feasibility of a high X-ray energy industrial computed tomography (ICT) scanner for imaging wetwood; and (2) to determine changes of the 2-D and 3-D moisture profiles (from core to shell) at different drying times.

Although medical CT scanning has been used for attaining signal intensity profiles of typical wood at different drying times, the technology has not, to date, been used for the study of wetwood phenomenon. This study presents, for the first time, results from the ICT imaging of the wetwood phenomenon. The results indicate that the ICT imaging system provides a powerful technique for imaging wetwood at different drying times. In addition, the results show that during the initial phase of drying, almost flat moisture profiles were observed in all wood types except for the wetwood, which showed a relatively higher moisture profile. A much slower (sluggish) drying development pattern at each increment from core to shell was found within the wetwood zone than normal wood regions along the width, thickness, and length of the board.