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.     

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

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.     

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.     

INFLUENCE OF DRYING/CONDITIONING SCHEDULES ON STRESS DEVELOPMENT, RELIEF AND PRONG TEST

The prong test often displays ambiguous responses as shown in recent research. The objective of this paper was to learn how different drying/conditioning schedules influence the stress development/relief of lumber and therefore the prong response. During each drying/conditioning schedule, residual stress analysis was used to obtain stress levels, moisture content gradients and prong responses. The analysis of the data revealed the following: stress levels are strongly influenced by different schedules; in contrast, prong responses are only slightly influenced by different schedules. Stress levels are more effectively relieved by using a multiple- step increase of the equilibrium moisture content than a single-step increase. It is concluded that the schedule governs the final stress level. Even though differences in stress level are measurable by the slice test, the prong test does not reliably display these differences.     

INFLUENCE OF DRYING/CONDITIONING SCHEDULES ON STRESS DEVELOPMENT,RELIEF AND PRONG TEST

ABSTRACT

The prong test often displays ambiguous responses as shown in recent research. The objective of this paper was to learn how different drying/conditioning schedules influence the stress development/relief of lumber and therefore the prong response. During each drying/conditioning schedule, residual stress analysis was used to obtain stress levels, moisture content gradients and prong responses. The analysis of the data revealed the following: stress levels are strongly influenced by different schedules; in contrast, prong responses are only slightly influenced by different schedules. Stress levels are more effectively relieved by using a multiple- step increase of the equilibrium moisture content than a single-step increase. It is concluded that the schedule governs the final stress level. Even though differences in stress level are measurable by the slice test, the prong test does not reliably display these differences.     

A THEORETICAL AND EXPERIMENTAL INVESTIGATION OF THE CONVECTIVE DRYING OF AUSTRALIAN PINUS RADIATA TIMBER

A series of experiments on the convective drying of Pinus radiata has been undertaken at the CSIRO Division of Forest Products in Australia. This paper uses the experimental results to compare predictions from both a comprehensive mathematical model, which includes wood temperature, moisture content and pressure distributions, and a simplified model (two versions) which assumes constant total pressure. From the- simulations, it is seen that both models adequately predict the overall kinetics of the wood drying process. For a complete understanding of the heat and mass transfer phenomena that occur in wood during drying the comprehensive model is necessary. However, it is computationally long and expensive, and as such, does not suit the practical drying needs of the timber industry in Australia. Consequently, the simplified model, which has acceptable computation time and sufficient accuracy for engineering purposes, enables die wood drying process to be optimised from both performance and economic perspectives.     

A THEORETICAL AND EXPERIMENTAL INVESTIGATION OF THE CONVECTIVE DRYING OF AUSTRALIAN PINUS RADIATA TIMBER

ABSTRACT

A series of experiments on the convective drying of Pinus radiata has been undertaken at the CSIRO Division of Forest Products in Australia. This paper uses the experimental results to compare predictions from both a comprehensive mathematical model, which includes wood temperature, moisture content and pressure distributions, and a simplified model (two versions) which assumes constant total pressure. From the- simulations, it is seen that both models adequately predict the overall kinetics of the wood drying process. For a complete understanding of the heat and mass transfer phenomena that occur in wood during drying the comprehensive model is necessary. However, it is computationally long and expensive, and as such, does not suit the practical drying needs of the timber industry in Australia. Consequently, the simplified model, which has acceptable computation time and sufficient accuracy for engineering purposes, enables die wood drying process to be optimised from both performance and economic perspectives.     

TEMPERATURE,DISTRIBUTIONS AND HEAT TRANSFER DURING THE DRYING OF LUMBER

ABSTRACT

This paper describes techniques that have been developed for accurately measuring the surface temperature of drying lumber using a radiation thermometer, and interior temperatures at various depths using 30 gauge thermocouples. Methods for calculating heat transfer coefficients during the drying of lumber are also described. Experimental results, showing surface and interior temperatures, and moisture content, as functions of drying time are presented.     

THE INFLUENCE OF COMPRESSION WOOD ON THE DRYING CURVES OF PINUS RADIATA DRIED IN DEHUMIDIFIER CONDITIONS

ABSTRACT

Measurements of drying rate of Pinus radiata at 55°C and 30% RH are presented. The data, which has been used to establish empirical models for P. radiata under dehumidifier drying conditions, was obtained in four drying runs in a drying tunnel, each yielding detailed drying curves for twelve sample sapwood boards of size 350 × 100 × 50 mm. Compression wood was found to have a significant effect on the drying rate curve, giving lower drying rates at 40–100% MC. This effect is interpreted through the use of a numerical multiple-mechanism two-zone model and quantified by using best-fit diffusion parameters from an isothermal diffusion model. A positive correlation was discovered between the moisture diffusion coefficient and initial moisture content, a strong indicator for the presence of compression wood. In the two-zone model, the compression wood effect was replicated by using a tenfold decrease in permeability to liquid flow. Attributes of compression wood that could cause reduced permeability include an increased proportion of latewood, narrower lumen, and a scarcity of bordered pits on the radial walls of longitudinal tracheids.     

THE INFLUENCE OF COMPRESSION WOOD ON THE DRYING CURVES OF PINUS RADIATA DRIED IN DEHUMIDIFIER CONDITIONS

Measurements of drying rate of Pinus radiata at 55°C and 30% RH are presented. The data, which has been used to establish empirical models for P. radiata under dehumidifier drying conditions, was obtained in four drying runs in a drying tunnel, each yielding detailed drying curves for twelve sample sapwood boards of size 350 × 100 × 50 mm. Compression wood was found to have a significant effect on the drying rate curve, giving lower drying rates at 40-100% MC. This effect is interpreted through the use of a numerical multiple-mechanism two-zone model and quantified by using best-fit diffusion parameters from an isothermal diffusion model. A positive correlation was discovered between the moisture diffusion coefficient and initial moisture content, a strong indicator for the presence of compression wood. In the two-zone model, the compression wood effect was replicated by using a tenfold decrease in permeability to liquid flow. Attributes of compression wood that could cause reduced permeability include an increased proportion of latewood, narrower lumen, and a scarcity of bordered pits on the radial walls of longitudinal tracheids.     

MODELLING OF PARTICULATE DRYING IN THEORY AND PRACTICE

The range of theoretical models used by SPS for the design and analysis of dryers, especially dispersion dryers, is reviewed. Different levels of complexity are appropriate at different stages of the design process. Models should use an appropriate level of rigour for the available data and the required purpose of the results. Scale-up from experimental design curves often yields better results in practice than a highly complex model requiring many parameters, and the type of drying kinetics measurements required also depends on the type of model to be used. Models at four different levels are presented in general terms and for a number of major types of dryers. Significant developments in the SPS methodology for fluidised bed, pneumatic conveying and cascading rotary dryers are noted. It is also important to consider the dryer in the context of the overall process flowsheet.     

MODELLING OF PARTICULATE DRYING IN THEORY AND PRACTICE

ABSTRACT

The range of theoretical models used by SPS for the design and analysis of dryers, especially dispersion dryers, is reviewed. Different levels of complexity are appropriate at different stages of the design process. Models should use an appropriate level of rigour for the available data and the required purpose of the results. Scale-up from experimental design curves often yields better results in practice than a highly complex model requiring many parameters, and the type of drying kinetics measurements required also depends on the type of model to be used. Models at four different levels are presented in general terms and for a number of major types of dryers. Significant developments in the SPS methodology for fluidised bed, pneumatic conveying and cascading rotary dryers are noted. It is also important to consider the dryer in the context of the overall process flowsheet.     

PRONG TEST AND STRESS DEVELOPMENT DURING CONDITIONING IN RED OAK

ABSTRACT

Lumber must be conditioned to relieve transverse drying stresses. The standard prong test has been used for many decades to determine the conditioning time needed for complete stress relief. However, no work has been directed at the proper procedures or interpretation of this test. The purpose of this study was to obtain initial information on the factors influencing the prong test and stress development. The major factors studied were prong thickness and surface moisture content Data on stress distribution, moisture gradient, and prong response were obtained periodically throughout conditioning of red oak. Slicing, a residual strain release method, was used to obtain the stress distributions. The results demonstrated that the prong response is a function of prong thickness and stress distribution. Additionally, an explanation of how the stresses are altered was proposed. Further studies concerning conditioning schedules need to be performed before standard test procedures can be suggested.     

MODELLING OF HEAT TRANSFER IN HOT PRESSING AND IMPULSE DRYING OF PAPER

A modelling and simulation study regarding the heat transfer in hot pressing as well as in impulse drying of paper has been performed. The study is focused on modelling the heat transfer from a hot surface to a moist paper sheet and the heat transfer inside the paper sheet. The model is based on solving the equation for unsteady heat transfer by conduction. The model was compared with experimental data from a heated laboratory platen press and data from the experimental paper machine EuroFEX at the Swedish Pulp and Paper Research Institute (STFI) in Stockholm. The results in this study showed that a rather simple heat transfer model could be used to simulate the heat transfer in cases in which the temperature was moderate and the applied pressure did not exceed 0.5 MPa. It was also concluded that a paper could be regarded as a semi-infinite body for basis weights above 110 g/m², considering the assumptions used in the model. It was also shown that a laboratory press could simulate heat transfer in hot pressing and impulse drying with a high degree of accuracy.     

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

Abstract

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.     

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.     

MODELLING OF HEAT TRANSFER IN HOT PRESSING AND IMPULSE DRYING OF PAPER

A modelling and simulation study regarding the heat transfer in hot pressing as well as in impulse drying of paper has been performed. The study is focused on modelling the heat transfer from a hot surface to a moist paper sheet and the heat transfer inside the paper sheet. The model is based on solving the equation for unsteady heat transfer by conduction. The model was compared with experimental data from a heated laboratory platen press and data from the experimental paper machine EuroFEX at the Swedish Pulp and Paper Research Institute (STFI) in Stockholm. The results in this study showed that a rather simple heat transfer model could be used to simulate the heat transfer in cases in which the temperature was moderate and the applied pressure did not exceed 0.5 MPa. It was also concluded that a paper could be regarded as a semi-infinite body for basis weights above 110 g/m², considering the assumptions used in the model. It was also shown that a laboratory press could simulate heat transfer in hot pressing and impulse drying with a high degree of accuracy.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D_e) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X_o). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.