R.B. Keey

Kiln-Drying of Lumber

The Structure of Wood Wood-Water Relationship Evaporation and Humidification Wood Drying Kinetics Moisture Diffusion Multiple-Mechanism Models Lumber Quality Stress and Strain Behaviour Airflow and Convection Kiln Operation Pre-treatments of Green Lumber Less-Common Drying Methods Subject Index.

MOISTURE MOVEMENT IN SOFTWOOD TIMBER AT ELEVATED TEMPERATURES

ABSTRACT The anatomy of a common plantation-grown softwood in New Zealand, Pinna radiata, is reviewed and the differences between the drying behaviour of softwoods and hardwoods are discussed. These differences have significant effects on the paths for moisture movement, and this review of the timber physiology has been used as the basis for a model of the moisture movement at elevated temperatures which distinguishes between the drying behaviour of sapwood and heartwood. It is postulated for the drying of heartwood that liquid flow does not occur within a board due to the aspirated state of the pits, so an evaporative plane at the boiling point of water sweeps through the timber until the centre of the board is reached. In the case of the much wetter sapwood where the pits are not initially aspirated, we assume that liquid flow initially maintains an evaporative plane just below the surface until the moisture is no longer funicular. Thereafter, drying proceeds in a similar way to heartwood. Independent e...

A high-temperature drying model for softwood timber

The high-temperature drying of a softwood board with relatively high permeability was modelled on a one-dimensional basis. The moisture-transport mechanism between fibre saturation and irreducible saturation was elucidated, and moisture flow within the thin layer of damaged cells at the sawn surfaces of the timber board was examined. The predicted temperature and average moisture-content profiles with respect to time were in good agreement with some experimental data from the literature. Comparison between the experimental and predicted within-board moisture-content profiles illustrated the importance of accounting for the orientation of the growth rings with respect to the drying surfaces and the effect of density variations on liquid permeability at high core moisture contents.

IMPROVING THE QUALITY OF TIMBER FROM RED BEECH (N. FOSCA) BY INTERMITTENT DRYING

ABSTRACT The effect of cycling the external conditions on the timber quality and drying time during seasoning has been studied for red beech (Nothofagus fusca), a difficult-to-dry New Zealand hardwood, inalaboratorybatchdryer. Intermittent drying at anair temperature of 45°C and a wet-bulb depression of 10°C takes longer to yield a given moisture content than continuous drying under the same conditions, but produces timber without honeycomb or severe case-hardening as observed in continuous drying. The experiments also indicate that the timber can tolerate a higher air temperature (55°C) in intermittent drying at the same wet-bulb depression with less case-hardening and without showing honeycomb compared with continuous drying at 45°C, having a similar total drying time. Intermittent drying at a slightly higher dry-bulb temperature (60°C) and a much higher wet-bulb depression (18°C) gives a similar degree of case-hardening to that found in samples dried continuously at a dry-bulb temperature of 45°C.

Modelling the temperature profiles within boards during the high-temperature drying of Pinus radiata timber: the influence of airflow reversals

Profiles of the surface and centre temperatures in the high-temperature drying of Pinus radiata have been predicted as a function of time for heartwood and sapwood boards, using a receding evaporative plane model, which incorporates equations for local heat and mass balances, internal and external heat transfer, and internal moisture fluxes due to bound-water diffusion and flow of water vapour and liquid water. The model predictions have been compared with independent measurements of the temperatures at the surface and centre of sapwood, heartwood and mixed-wood boards. The simulated and measured temperature profiles are in good agreement except that the outer temperatures are predicted to rise more swiftly when the evaporative plane starts to recede in the wood, while the inner temperatures are estimated to rise more sluggishly once the first period is over. However, the maximum discrepancies in these three cases are less than 6°C, and may be due to difficulties in getting accurate measurements of local temperatures when these are changing. This model is used to predict local temperature and moisture-content profiles in high-temperature kiln drying of this timber for conditions with airflow reversals. Variations have been reported in the external mass-transfer coefficients in the streamwise direction which lead to differential drying of the boards. The differences in the extent of drying across each board can be reduced by reversing the airflow periodically. Airflow reversals every 4 h are adequate to give a high degree of uniformity in the final temperature and moisture-content profiles for heartwood after 24 h of drying, and may be sufficient for sapwood if a small degree of variation is acceptable. Flow reversals every 8 h yield essentially the same result.

Surface colour change in wood during drying above and below fibre saturation point

A technique, useful for studying the formation of kiln brown stain in wood drying, has been developed to measure the surface colour change in a single board wood sample during drying. The wood sample is planed carefully in the green state to remove any surface wood that was damaged during cutting. The intact tracheids at the surface cause the evaporative front to remain at the surface during drying and therefore colour formation also occurs right at the surface. In this way, the colour can be measured using a spectrophotometer at various stages during drying without having to slice the sample. Experiments were carried out to measure the change in colour of wood from green to EMC corresponding to the drying schedule used. At the end of each schedule the boards were held at the EMC to determine how the colour changed below fibre saturation point. The results show that the colour of the wood continues to change below the fibre saturation point and the nature of the colour change indicates an increase in the complexity of the coloured compounds present. Further experiments were done to measure the rate of colour development at different temperatures using the technique developed. The results have shown a correlation between temperature and colour development over the range 50oC to 70oC with the rate increasing significantly above 60oC.

Model fitting for visco-elastic creep of Pinus radiata during kiln drying

Abstract This work examines the applicability of mathematical models for correlating the visco-elastic strains during kiln drying of radiata pine (Pinus radiata D. Don) sapwood at various temperatures and moisture contents. The eventual aim is to use a mathematical model incorporating these strains to optimise the drying schedules and minimise the degradation. Data sets from previous experiments (Keep 1998) obtained at temperatures from 20 to 140 °C for sapwood at 5, 15 and 20% moisture contents (dry basis) were analysed. The data were fitted for various theoretical models, namely the Maxwell, Kelvin and Burgers models, and the Bailey-Norton equation. The parameter values and standard errors for the above models over the range of experimental data have been determined. The results indicate that the Maxwell model did not fit the experimental data well, having only one parameter. In most cases, the Bailey-Norton equation was inadequate, as it is a power-law model with a predicted continuous increase in creep with time and does not predict a plateau in the creep strain, as has been observed for most of Keeps (1998) data. The Kelvin model gave a better fit than the Bailey-Norton equation for most of the data sets. From visual inspection of the plots for the experimental data and the model predictions with time, it was found that both the Kelvin and Burgers models fitted the data satisfactorily. However, the three-parameter Burgers model was not a significant improvement over the two-parameter Kelvin model at the 0.01 level of significance, as shown by an analysis of variance.

MEASUREMENT OF COLOUR DEVELOPMENT IN PINUS RADIATA SAPWOOD BOARDS DURING DRYING AT VARIOUS SCHEDULES

Colour changes, such as kiln brown stain, that develop in Pinus radiata boards during kiln drying can reduce the value of the final wood products and result in significant losses due to downgrade or waste of dried wood by removing the darkened surfaces. This study measured how colour developed in Pinus radiata sapwood boards under different drying schedules. The boards used in these experiments were 40x100x800mm, cut from the same log and were endand edge-matched. Boards were dried at eight different schedules using temperatures from 50°C to 120°C and relative humidities from 14% to 67%. Separate boards were dried for 5 equal intervals through each schedule and colour profiles measured through the boards using a surface reflectance spectrophotometer. Lightness, L*, on a greyscale was used as an indication of colour change (darkening). The results show that there is generally a greater decrease in lightness with higher temperature schedules and also with slower, higher relative humidity, schedules. This suggests that both temperature and drying time are significant factors in the formation of colour during drying. The most significant changes in colour occurred near the board surfaces, indicating kiln brown stain.

Time-dependent flow in arrays of timber boards: Flow visualization, mass-transfer measurements and numerical simulation

Abstract A numerical simulation of the airflow patterns around an array of boards in a timber kiln has been modified to include the prediction of time-dependent flows, and the predictions of the simulation have been compared with airflow patterns recorded in a previous flow visualization study and experimental measurements of the mass-transfer coefficients along the boards. The numerical simulation solves the two-dimensional Navier—Stokes equations on a finite-difference grid. The simulation predicts that the enhancement of the asymptotic values of the mass-transfer coefficients above those predicted by conventional flat-plate correlations is due to the effects of the gaps between the boards in generating additional turbulence throughout the array of boards. The enhancements at the leading edges of the boards are also predicted by the simulation to be a consequence of the time-dependent nature of the flow. Periodic oscillations in the flow patterns around the gaps with a period of 1–7 s were observed in the flow visualization study, in agreement with the period of 2 s predicted by this simulation. The variation in the experimentally observed time period may be due to the sensitivity of the vortex-shedding phenomenon to small differences in alignment within the geometry between experiments. The enhancement in the mass-transfer coefficients at the leading edge of the first board is not predicted well, because the treatment of the flow near the wall requires improvement in the simulation.

THE INFLUENCE OF KILN GEOMETRY ON FLOW MALDISTRIBUTION ACROSS TIMBER STACKS IN KILNS

ABSTRACT A one-dimensional mathematical model is evaluated for flow through a timber-drying kiln. This model is used to develop recommendations for the design of the plenum chambers on either side of the timber stack. The variation of the vertical air velocity was explored for a sticker spacing of 20 mm and a board thickness of 50 mm. When the width of the plenum chambers is at least equal to the sum of the thickness of the stickers, flow maldistribution is substantially reduced in single-track kilns. The width of the plenum chambers of double-track kilns may be reduced to three-quarters of the sum of the thickness of the stickers, because the increase in timber-stack resistance to airflow mitigates the effect of pressure variations down the length of the plenum chambers.