RELATIONSHIP OF PROCESS PARAMETERS AND STACK WIDTH IN TIMBER DRYING

Drying schedules applied in kiln drying, especially for hardwood species should be supplemented with values of air velocity. When dryers have no air velocity control their drying schedule can be suitably corrected. The correction should take into account factors related to dried material, i.e. wood moisture content, timber thickness, dried timber volume and primary width of a stack. Drying efficiency may be the criterion of modifications. The applied procedure of drying efficiency calculations lets to determine quantitative relationships between process and material factors.     

RELATIONSHIP OF PROCESS PARAMETERS AND STACK WIDTH IN TIMBER DRYING

ABSTRACT

Drying schedules applied in kiln drying, especially for hardwood species should be supplemented with values of air velocity. When dryers have no air velocity control their drying schedule can be suitably corrected. The correction should take into account factors related to dried material, i.e. wood moisture content, timber thickness, dried timber volume and primary width of a stack. Drying efficiency may be the criterion of modifications. The applied procedure of drying efficiency calculations lets to determine quantitative relationships between process and material factors.     

STACK-WIDE EFFECTS IN THE MODELING OF SOLAR KILNS FOR DRYING TIMBER

This study examines the stack-wide effects due to the humidification and cooling of air as it passes through a 6 m wide stack of Australian ironbark timber for conditions that are representative of those for solar drying (dry and wet-bulb temperatures of 60 and 50°C, respectively). A solar kiln model for a greenhouse-type design has been modified to account for the drying of timber boards and the possibility of stack-wide effects, in terms of moisture-content differences in the streamwise direction of air flow through the stack. The maximum difference between the moisture contents of the leading and trailing boards is predicted to be 0.011 kg kg^-1 for these conditions, compared with timber moisture contents of 0.15-0.35 kg kg^-1. Hence, the stack-wide effect is insignificant for these conditions in this greenhouse kiln design and may be ignored, reducing the simulation time by over 50%. In addition, 14 elements within a finite-difference model for the drying of the timber boards (25 mm thick) gives predictions of the drying time that are acceptably accurate, while minimizing the computational time.     

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.     

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

Abstract

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.     

OPTIMISATION OF HARDWOOD DRYING SCHEDULES ALLOWING FOR BIOLOGICAL VARIABILITY

The amount of biological variability in timber creates considerable problems in producing timber of adequate and reproducible quality with a predictable amount of variability in final moisture contents. The development of optimised drying schedules for addressing these problems is therefore desirable. Previous methods (largely of a stochastic type) are reviewed in this work and their limitations assessed. The physical parameters which have the greatest impact on the stress levels (and hence quality) of the timber have been assessed using a diffusion model for the drying of Australian hardwood timber. This deterministic model is then used, together with statistical methods for quantifying the confidence regions for the variability in the physical parameters with the greatest impact, in a systematic technique to develop a new optimised schedule for grey ironbark timber (Eucalyptus paniculaia). This new schedule is then compared with a previous optimised schedule, which did not take this variability into account. The productivity (amount of good quality timber divided by drying time) appears to be maximised when the schedule is such that 90% of the timber produced is good quality.     

OPTIMISATION OF HARDWOOD DRYING SCHEDULES ALLOWING FOR BIOLOGICAL VARIABILITY

ABSTRACT

The amount of biological variability in timber creates considerable problems in producing timber of adequate and reproducible quality with a predictable amount of variability in final moisture contents. The development of optimised drying schedules for addressing these problems is therefore desirable. Previous methods (largely of a stochastic type) are reviewed in this work and their limitations assessed. The physical parameters which have the greatest impact on the stress levels (and hence quality) of the timber have been assessed using a diffusion model for the drying of Australian hardwood timber. This deterministic model is then used, together with statistical methods for quantifying the confidence regions for the variability in the physical parameters with the greatest impact, in a systematic technique to develop a new optimised schedule for grey ironbark timber (Eucalyptus paniculaia). This new schedule is then compared with a previous optimised schedule, which did not take this variability into account. The productivity (amount of good quality timber divided by drying time) appears to be maximised when the schedule is such that 90% of the timber produced is good quality.     

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.     

Model Predicted Effect of Process Variables on Kiln Drying of Pinus radiata Boards

The effects of kiln-drying process variables on drying time and final moisture content (MC) variability were assessed using a mathematical drying model (a kiln-wide model) developed earlier. Drying time was predicted to decrease by using higher air velocities and temperatures and, to a less extent, by increasing fan reversal frequencies. The drying time extended as board thickness or stack width increased. Increase in air velocity from 5 to 8 m/s tended to minimize the final MC variability. At least three reversals in the early stages of drying were required to reduce final moisture contents variation. The final MC variability increased with increasing stack width and with increasing temperature.     

Model Predicted Effect of Process Variables on Kiln Drying of Pinus radiata Boards

The effects of kiln-drying process variables on drying time and final moisture content (MC) variability were assessed using a mathematical drying model (a kiln-wide model) developed earlier. Drying time was predicted to decrease by using higher air velocities and temperatures and, to a less extent, by increasing fan reversal frequencies. The drying time extended as board thickness or stack width increased. Increase in air velocity from 5 to 8 m/s tended to minimize the final MC variability. At least three reversals in the early stages of drying were required to reduce final moisture contents variation. The final MC variability increased with increasing stack width and with increasing temperature.     

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.     

Real-Time Moisture Content Measurement of Wood Under Radio-Frequency/Vacuum (RF/V) Drying

Hinoki timber was dried under radio-frequency at 6.7 kPa using two drying schedules, schedule A and schedule B. Moisture content (MC) was measured at 58 points in various locations of the timber using a new in-process monitoring concept. This concept uses the relationship between temperature, pressure, and equilibrium moisture content (EMC). Factors affecting the accuracy of MC measurement were also investigated in this study. The results showed that small wood pieces reached equilibrium at constant conditions within 1.5 h of the fiber saturation point (FSP) and that using the mean value of temperature and pressure within 30 min during radio-frequency/vacuum (RF/V) drying for MC measurement was an efficient method. The accuracy of moisture content measurement was the same for both drying schedules A and B. It can be concluded that air in wood was removed completely with drying schedule B and that below the FSP, pressure in the wood was maintained only by water vapor pressure during drying. It was possible to obtain accurate MC measurement. Above or near the FSP, MC cannot be measured using this method, whereas below the FSP, whatever the MC is, it can be measured practically anywhere in the timber.     

Comparing Continuous and Cyclic Drying Schedules for Processing Hardwood Timber: The Importance of Mechanosorptive Strain

This work compares a conventional continuous drying schedule with a solar cyclic drying schedule for the seasoning of an Australian hardwood timber, Eucalyptus grandis, focusing on the simulated stresses and strains developed during drying as a measure of timber quality. The cyclic drying schedule has been found to give lower instantaneous strains, due to the effect of mechanosorptive strains in relieving stresses both in the initial stages of drying and over the entire drying period. The gentler initial drying conditions during cyclic drying are also beneficial compared to the harsher and unmodulated nature of conventional drying schedules. Without the modulation of the external drying conditions in intermittent or cyclic drying, the mechanosorptive strains are unable to relax or mitigate the stresses that are caused naturally by timber drying. There is some support for these conclusions by comparison with industrial experience and previous laboratory practice for intermittent and cyclic drying.     

THE SIGNIFICANCE OF THE GAPS BETWEEN BOARDS IN DETERMINING THE MOISTURE CONTENT PROFILES IN THE DRYING OF HARDWOOD TIMBER

A numerical simulation (CFX 4.1) of the airflow patterns around timber boards has been used to assess the significance of gaps between boards in terms of the mass-transfer coefficients across both side and top faces of 50 mm square pieces of hardwood timber. These gaps, which are the distances between board edges in the streamwise direction, are inevitable consequences of both imperfect sawing and shrinkage, and are typically of the order of 1-20 mm. However, for the laminar flow conditions which are typical of the air velocities used in hardwood drying (0.5 m ^-1), the simulations suggest that the air in the gaps quickly becomes almost saturated, even for 20 mm wide gaps, since there is no net air flow through the gaps. This situation means that the effective mass-transfer coefficients from the narrow faces of the boards are likely to be less than 1% of those from the broad faces (which are exposed to the airflow). This in turn suggests that the moisture-content profiles in stacked timber will be considerably less two-dimensional than those for the drying of single boards during laboratory testing, unless precautions are taken to simulate typical kiln stacking arrangements.     

SIMULATION OF THE EFFECT OF AIR VELOCITY IN THE DRYING OF HARDWOOD TIMBER

The effects of different air velocities on the drying of Australian hardwood timber have been investigated using a drying model based on Fickian diffusion. The air velocities studied were 0.05, 0.5, and 2 m.s^-1, corresponding to typical velocities covering the range used from pre-drying to normal kiln conditions. Decreasing tbe air velocity from 2 m.s^-1 to 0.05 m.s^-1 reduces the maximum strain experienced with an optimised drying schedule by 34%, although if the lower velocity is used throughout the drying period, the drying time is predicted to be 40% longer. Explorations with a program to optimise drying schedules suggest that there may Dot be any significant advantage in moving from a low air velocity of 0.05 m.s^-1 to a higher one (2 m.s^-1) in terms of reducing drying time, for the same maximum strain during drying, compared with using a constant air velocity of 0.5 m.s^-1.     

THE SIGNIFICANCE OF THE GAPS BETWEEN BOARDS IN DETERMINING THE MOISTURE CONTENT PROFILES IN THE DRYING OF HARDWOOD TIMBER

ABSTRACT

A numerical simulation (CFX 4.1) of the airflow patterns around timber boards has been used to assess the significance of gaps between boards in terms of the mass-transfer coefficients across both side and top faces of 50 mm square pieces of hardwood timber. These gaps, which are the distances between board edges in the streamwise direction, are inevitable consequences of both imperfect sawing and shrinkage, and are typically of the order of 1–20 mm. However, for the laminar flow conditions which are typical of the air velocities used in hardwood drying (0.5 m ^?1), the simulations suggest that the air in the gaps quickly becomes almost saturated, even for 20 mm wide gaps, since there is no net air flow through the gaps. This situation means that the effective mass-transfer coefficients from the narrow faces of the boards are likely to be less than 1% of those from the broad faces (which are exposed to the airflow). This in turn suggests that the moisture-content profiles in stacked timber will be considerably less two-dimensional than those for the drying of single boards during laboratory testing, unless precautions are taken to simulate typical kiln stacking arrangements.     

Mathematical Modelling of Solar Kilns for Drying Timber: Simulation and Experimental Validation

Abstract

A complete solar kiln model (including the drying of hardwood timber) has been developed with particular reference to the seasoning of blackbutt (Eucalyptus pilularis) timber. The predicted internal air temperatures, relative humidities, and timber moisture contents have been compared with experimental data. The maximum difference between the actual and predicted moisture contents was 0.05 kg kg^?1. The agreement between the predicted and measured temperatures of the internal air is reasonable, and both the predictions and measurements have a similar cyclical pattern. The generally good agreement between the model prediction of the final moisture content and its measurement may be due to the careful measurement of the boundary conditions such as the solar energy input. The main uncertainties were identified as the heat exchanger output, the measurement of the initial moisture content, and the estimation of sky temperature. The significant uncertainty (18%) in the estimation of the initial moisture content is a key reason for the mismatch between the model predictions and the measurements.     

An Improved Drying Achedole for Australian Ironbark Timber: Optimisation and Experimental Validation

A continuous schedule for the drying of Australian ironbark timber has been optimised using non-linear model-predictive control techniques. Initially, an experimental study was carried out using a conventional schedule to dry nine 600 mm long × 250 mm wide × 25 mm thick boards in order to obtain information on the drying behaviour and the extent of timber cracking. A diffusion model accurately fitted the average moisture contents observed when using this conventional drying schedule. The fitted coefficients in the diffusion model were used to optimise the drying schedule with the aim being to keep the strain below 0.02 mm/mm and the surface moisture above 7%. The resulting optimised schedule set gentler conditions at the start of drying and more aggressive ones towards the end than the conventional schedule. The new schedule was tested in an experiment using the same number of boards from the same tree, and was found to reduce the number of small and medium-sized cracks to less than 25% of the number observed from the conventional schedule with a significant improvement in the value of the timber produced. The fitted diffusion model predicted the drying behaviour observed during the optimised schedule adequately. This optimisation technique has considerable scope for improving drying schedules and, thus, productivity rates, for other timber species. It should be noted that a significant requirement for the use of     

Experimental and Simulated Performances of a Batch-Type Longan Dryer with Air Flow Reversal Using Biomass Burner as a Heat Source

This article presents experimental performance of a batch-type longan dryer using a biomass burner with air flow reversal and also presents modeling of the longan dryer for drying of whole longan. The dryer essentially consists of a biomass burner and a drying bin with an arrangement for periodic air flow reversal. Three drying runs with loading capacity of 2,000, 1,500, and 1,000 kg of whole longan were carried out. There was no significant difference in temperatures in different positions (except inlet and outlet) inside the dryer (p < 0.05) or moisture content inside the dryer (p < 0.05). Whole longan was dried from an initial moisture content of 74% (wb) to a final moisture content of 14% (wb). The drying time of whole longan in the longan dryer was 60, 54, and 48 h for 2,000, 1,500, and 1,000 kg loading, respectively. The quality of dried product was also good in comparison to the high-quality product in markets.

To simulate the performance of the longan dryer for drying of whole longan, a set of partial differential equations was developed and the equations were solved using the finite difference technique. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5 (Compaq Computer Corp., TX). The simulated moisture contents agreed well with the experimental data. This model can be used to provide the design data and it is also essential for optimal dryer design.