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.     

A Simulation Tool for the Optimization of Lumber Drying Schedules

Abstract

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.     

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.     

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.     

MATHEMATICAL MODELLING OF MDF FIBRE DRYING: DRYING OPTIMISATION

ABSTRACT

In the production of MDF, wet resinated fibre must be dried to its target moisture content, normally 9 to 11%, before compaction into a board by hot pressing. Fibre drying can be interpreted as an incorporated process involving gas-solid two phase-flow, inter-component transfer, and heat and mass transfer within the fibre. Based on these mechanisms, a mathematical model has been developed to simulate the MDF fibre drying process. From the model, fibre moisture content, air temperature and air humidity along the dryer length can be predicted and factors affecting the drying rate examined. The model can be employed to optimise drying conditions and to evaluate improvements in dryer design. A case study of drying improvement in reduction of dryer emissions and heat consumption is given to demonstrate the potential application of the developed dryer model.     

Using the Concept of Thin Dry Layer to Explain the Evolution of Thickness,Temperature, and Moisture Content during Convective Drying of Norway Spruce Boards

Abstract:

This article presents an experimental device able to follow the wood temperature, moisture content, board thickness, and surface checking during convective drying. Tests have been carried out on Norway spruce (Picea abies) boards, with different sawing patterns and subjected to a more or less severe drying schedules. For some boards, we observed a first drying phase followed by a transient phase lasting several hours, during which the thickness reduces very slightly and the board temperatures depart from the wet bulb temperature. This period, which is not exactly a true constant drying rate period, can be analyzed using the concept of “thin dry shell.” In order to confirm our hypothesis, an analytical model was used and its results are compared with the experiment. This model takes into account external heat, mass transfer, and diffusion of heat and vapor between the exchange surface and the drying front. Very good agreement is obtained between calculated and experimental temperatures, which validates our equipment and brings physical explanations to different observed behavior; in particular, the existence of a transient first drying period with a thin dry layer is confirmed.     

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.     

RELATIVE IMPORTANCE OF VAPOUR DIFFUSION AND CONVECTTVE FLOW IN MODELLING OF SOFTWOOD DRYING

ABSTRACT

The influence of vapour diffusion on the drying rate of a softwood board has been examined for drying temperatures varying from 60°C to 140°C. It is found that for very low temperature drying a model which considers both vapour convective and diffusion in wood predicts dry-rate curves matching the experimental data closely. For high temperature drying, both of the above drying model and a drying model which considers only vapour convective flow give predictions in agreement with the observed data. This illustrates that the diffusion of vapour and air is important in low temperature drying panicularly during the late stages of drying. However, for high temperature drying, the convective flow of moisture vapour is dominant and the diffusion component is negligible. The observation provides evidence for simplifying a drying model for high temperature drying without reducing its credibility in predicting drying rate curves.     

EFFECT OF CREEP AND MECHANO-SORPTIVE EFFECT ON STRESS DEVELOPMENT DURING DRYING

ABSTRACT

Constitutive equations to quantify wood deformation under combined mechanical loading and moisture content change (1] were coupled with the moisture distribution developed during drying to predict stress and strain in 50 by 190-mm Douglas-fir heartwood lumber.

Two combinations of temperature and relative humidity were used to dry the wood. The overall board shrinkage and the immediately released and set strains were measured as a function of time. Those strains were compared with analytic results, which showed good agreement.

The roles that four strain components played in the development of stress-both at board surface and center were compared for different drying conditions. The significance of creep and mechano-sorptive strain in relieving the stress was demonstrated by varying the model parameters.     

a rview of: UNIFIED ANALYSIS AND SOLUTIONS OF HEAT AND MASS DIFFUSION

ABSTRACT

The objective of this study is the formulation of a finite element model that could be used to analyze the stress crack formation in a viscoelastic sphere resulting from temperature and moisture gradients during the drying process. Numerical solutions to the simultaneous moisture and heat diffusion equations describing moisture removal and heat intake process for the sphere are obtained. The distribution and gradients of temperature and moisture developed inside the sphere during drying are established. The calculated temperature and moisture gradients are used in a finite element analysis of the thermo-hydro viscoelastic boundary value problem to simulate the stresses in the body.

The model is used to solve a sample problem of drying a soybean kernel. The simulated drying curve for the soybean model is obtained and compared favorably with the experimental results reported in the literature. Tangential stress, as a criteria for failure, is shown to change from compressive to tensile stress as it approaches the surface. It reaches its peak value at the surface in one hour and then decays slowly. The effect of different drying conditions is studied and the results are discussed.     

Experimental determination and mathematical modeling of the drying kinetics of a single droplet of colloidal silica

ABSTRACT

Colloidal silica has been used frequently as a model material of drying in the past two decades. Several models of single droplet drying have been validated against the sole experimental evidence by Ne?i? and Vodnik (Kinetics of droplet evaporation. Chemical Engineering Science 1991, 46(2), 527–537), in which relatively scattered experimental data on drying of single droplet of colloidal silica were provided. Due to the importance of this sort of data, the drying of single droplet of colloidal silica was determined more accurately under more extensive conditions in this work. The effect of air temperature on the drying of single droplet of colloidal silica was probed as well as the evolution of particle morphology. The droplet of colloidal silica was found to shrink irregularly during drying due to uneven exposure of droplet surface to air stream. The moisture within the droplet appears to transfer freely to the surface, keeping the surface highly moist. For a large part of drying process, drying of single droplet of colloidal silica is similar to the evaporation of water droplet, which can be predicted well using a simple mathematical model.     

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.     

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.     

Drying of Transformer Board

ABSTRACT

The internal insulation in shell type power transforms is usually ensured by the stacking of plates of board impregnated with oil. A board is in equilibrium with the atmosphere at a moisture content varying from seven to eight percent by weight, but, when the insulating function is required, the water content must be lower than 0.5 %. The aim of the present work is the understanding and the modelling of transformer boards drying.

The first part of this work presents experimental studies showing that the drying of transform board follows two regimes :

- a fast regime in the fim period

- a slow regime, up to 99 percent of dryness.

The effect of heating and the influence of the board thickness on the drying rate also studied. In the second part of this paper, we presnt a model capable to represent the observed kinetics of transformer board drying. This model is based on the diffusion of water vapour in the gaseous phases combined with the transfer of water vapor from the fibres to the -us phase. The due of the average global transfer coefficient can be deduced from the slope of the curve 1 f(drying time), where x     

OPTIMIZED WOOD DRYING

ABSTRACT

Nonlinear programming methods are used to create effective drying schedules for boards. A two-dimensional, orthotropic drying model is connected to an optimization routine that solves convex sub-problems. The iterative optimization program calculates the drying schedule, i.e. the variation of temperature and humidity with time, that gives the shortest drying time for a board with prescribed upper and lower level of the moisture content. To demonstrate the capability of the technique, numerical results are presented.     

Evaluating a Solar Dryer for In-Shell Drying of Split Pistachio Nuts

Abstract

A thin-layer forced air solar dryer was designed to study the feasibility of drying pistachio nuts. The dryer was tested during the 2001 and 2002 drying seasons. The maximum temperature in the solar collector reached 56°C, which was 20°C above the ambient temperature. The required drying time was 36 h. During the first day of drying (0800 to 1700 h) the moisture content dropped to about 21% (wb). The final moisture content of the dried nuts was 6% wb, which was 1% below the recommended storage moisture. The drying constant of the pistachio nuts during solar drying was determined using two mathematical models, a one-term series solution of Fick's diffusion equation and an exponential decaying model. There was no significant difference between the two models (α = 0.05). In general, the quality of solar dried nuts was better than the conventional heated air due to slower drying rates.     

THIN-LAYER DRYING CHARACTERISTICS OF ROUGH RICE AT LOW AND HIGH TEMPERATURES

Abstract

Thin-layer drying characteristics of rough rice were determined at temperature ranging from 11.8 to 51 °C and for relative humidities ranging from 37.1% to 91.3%, with initial moisture contents in the range of 24.7 to 41.6% dry basis. An oven, a self contain air conditioning unit, recently developed in Japan, was used for this experiments. With this apparatus, very smooth drying curves were obtained. The data of sample weight, and dry and wet bulb temperatures of the drying air were recorded continuously throughout the drying period for each test. The drying process was terminated when the moisture content change in 24 h was less than 0.2 % d.b. (weight change was less than 0.05 g). The final points were recorded as the dynamic equilibrium moisture contents.

The drying data were than fitted to the Page model. The model gave a very good fit for the moisture content with an average standard error of 0.294 % d.b? Both the drying parameters, K and N, are function of drying air temperature and relative humidity. The effect of variable initial moisture content was also described effectively by the empirical Page model. The drying time employed had a large effect on the K and N values. The results presented here, over typically 5 day drying, will be useful in the long term moisture transfer process occurring during ventilated storage.     

THE EFFECT OF HETEROGENEITY ON WOOD DRYING,PART I: MODEL DEVELOPMENT AND PREDICTIONS

ABSTRACT

A two dimensional model which can predict the effects of the anisotropy and heterogeneity on the transport phenomena which occur in wood during drying is developed. It is shown that the appropriate driving potential for moisture transport is the ratio of the moisture content to the driving potential. In its one dimensional form, die model results compare favorably with experimental measurements for drying in the radial direction. In its two dimensional form the model is used to predict drying in a direction midway between the radial and the tangential. In this case free water moves in a diagonal direction because the low density earlywood dries faster than the latewood during the early stages of drying. The result is significant gradients in moisture content, not only in the drying direction, but also in the direction parallel to the drying surface.     

PERFORMANCE ANALYSIS OF A DOUBLE-DECK CONVEYOR DRYER - A COMPUTATIONAL APPROACH

ABSTRACT

This paper illustrates the use of numerical simulation models for evaluating the performance of a moving bed dryer. A finite-volume method is employed in developing a steady state, two-dimensional numerical model for a double-deck conveyor dryer. Using this numerical model, variations in the product moisture content and temperature along the length and across the height of the product beds are predicted. Similarly, the resulting variations in the temperature and relative humidity of the drying air are predicted in the entire two-dimensional domain of a dryer. Effect of air-to-product mass flow ratio and product residence lime on the average moisture content of the outgoing product are also evaluated for three different drying air temperatures.     

Vacuum Drying of Oak Wood

ABSTRACT

Vacuum drying, j,e drying under absolute gas pressure of about 10? Pa. is an efficient means of reducing the process period and of producing good quality wood. We will examine here continuous vacuum drying where the plank surfaces are kept at a constant temperature, which remains above the boiling point, and moisture flowing to the surface is extracted from the kiln.

We have carried out an experimental study of oak drying under such conditions. The drying rate and moisture content profile of the sample (40 mm thick) are recorded during the whole drying period.

A model of continuous drying is established from general conservation equations with the main approximation that the air is rapidly extracted. The two constitutive equations of the model which describe temperature and water content fields are of a diffusive type and coupled through coefficients. The adequate boundary equation is not a convective one, but expresses a hygroscopic equilibrium between the vapour in the chamber and the wood surface. The mass diffusive coefficient can be adjusted to the drying rates through capillary pressure and bound water diffusion functions. The wood heterogeneity (seasonal growth) is the main factor of discrepancy in these functions. The simulated drying rates correspond with the experimental ones.