Final Moisture Content Distribution Research Articles

Kiln-drying effectiveness as influenced by moisture content and density variation within a Scots pine timber batch

ABSTRACT Sorting of sawn timber batches is crucial for efficient kiln-drying. This study aimed to verify the hypothesis of different drying attributes of 25 mm thick sapwood and mixed Scots pine boards sawn from a group of butt, second, and top logs and assess the effects of kiln-drying. Three options dry-bulb temperature were taken into account. The drying results were evaluated based on the final moisture content distribution, changes in the cross-sectional dimensions of the sawn timber, drying time, and unit heat consumption. Correlations between sawn timber features, drying temperature, and drying results were analyzed using non-parametric statistical tests. The study proved the dependence of the initial moisture content and wood density variation of Scots pine sawn timber on the log type and the position on the round timber cross-section. The variation of drying attributes of the sapwood and mixed 25 mm Scots boards is insignificant in the final moisture content distribution and cross-sectional dimensions. However, green sorting may have only a beneficial effect on drying time and heat consumption.

Impact of Wood Variability on the Drying Rate at Different Moisture Content Levels

The impacts on drying behavior of basic density, initial moisture content, percentage and position of heartwood, ring count, rings from the pith, growth ring angle, and earlywood and latewood coverage on board faces were investigated for western hemlock (Tsuga heterophylla) lumber. Mathematical relations for the effects of wood properties and board geometric features related to position within its parental log were developed to predict the drying rate at four moisture content levels for drying temperatures of 80°C and 115°C. These relations were used for simulating wood drying as well as for predicting drying times of individual boards, which were used as a criterion for sorting green boards prior to entering their properties in the drying simulation. Drying rate was affected by basic density, initial moisture content, percent heartwood, and growth ring angle at 80°C and only initial moisture content at 115°C. Presorting into two groups based on these wood properties reduced the standard deviation of the final moisture content distribution by 40 percent.

Determination of diffusion coefficients for sub-alpine fir

Estimates of the drying rate for sub-alpine fir lumber and the effect of process variables on final moisture contents are valuable information for kiln operators. The diffusion approach offers one generalized way to estimate the drying time required and the final moisture content distribution. The diffusion coefficients of normal wood and wet wood of sub-alpine fir lumber were determined by two types of approaches in this study: the traditional method and the inverse moisture diffusion algorithm. The results from both approaches indicate that the diffusion coefficient of normal wood is greater than that of wet wood for sub-alpine fir.

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.

A comparison between cold water and steam for SPF lumber conditioning

Conditioning is an effective method to release drying stresses and improve final moisture content (MC) distribution for spruce-pine-subalpine fir (SPF) lumber during drying. There are currently two methods used in the industry to add moisture to lumber during conditioning, namely low-pressure steam spray and cold water spray. Sixteen kiln drying runs were carried out in a laboratory kiln. Two types of conditioning treatment were investigated: cold water spray and steam spray. Temperature profiles, final MC distribution and drying stresses obtained for lumber conditioned by the two methods were evaluated and then compared. Results indicated that there were no significant differences on drying temperature profiles and lumber quality between the two humidification systems. Therefore, both humidification systems can be used for controlling humidity during kiln drying and for conditioning treatments.

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air. The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.

Monte Carlo modelling of a vegetable tray dryer

The objective in food dehydration is to dry the product to a specified uniform moisture content. In practice, a distribution in final moisture content is unavoidable, arising from the intrinsic variability of food properties and the probabilistic nature of the food drying process. For commercial dryers there are significant costs associated with this deviation in final moisture. To investigate the variability in the dehydration of vegetables, a model of a tray dryer was constructed, based on the empirical Page equation. It was solved using the Monte Carlo technique by describing the initial moisture content and the drying rate with probability distribution functions. A series of experiments were conducted to provide constants and statistical data for the model and to validate it. The utility of this approach is illustrated by applying the probabilistic drying model to quantifying and optimising the final moisture content distribution.

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

ABSTRACT 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

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

ABSTRACT 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.

Management of Ambient Air Drying Systems

ABSTRACT MANAGEMENT strategies for continuous and humidistat controlled operation are evaluated for single-fill procedures using simulation. Results also are presented for delay-fill procedures and bins equipped with stirring devices. Performance measures include grain deterioration (dry-matter-decomposition), final moisture content distribution, fan operation time and energy required.