Diffusivity and Surface Emissivity in Wood Drying

Edge and end-coated sweetgum (Liquidambar styraciflua L.) and redwood (Sequoia sempervirens (D. Don.) Endl.) 6 X 10 cm samples, of thicknesses ranging from 0.5 to 3.0 cm in either the longitudinal or tangential directions, were dried, from either the water-soaked condition or from slightly below fiber saturation, to equilibrium in circulated air (300 ft/min) at 100 F and 75% relative humidity.A constant-rate drying period was observed during the early stages of drying for the initially water-soaked samples but not for those initially below fiber saturation. The apparent diffusion coefficient D', calculated for the samples initially below fiber saturation on the assumption that surface resistance to drying was negligible, that is D' ≈ 0.2 a2/t0.5 (where a is the half-thickness and t0.5 is the half-drying time), increased with increasing wood thickness. The true diffusion coefficient D and the surface emission coefficient S (which is inversely proportional to the surface resistance to drying) were calculated from the linear relationships observed between t0.5/a2 (or 0.2/D') and 1/a, or between t0.5/a (or 0.2a/D') and a, using Newman's solution to the diffusion equation.The diffusion coefficient D was higher for longitudinal than for tangential drying for both woods, and higher for sapwood than for heartwood of redwood. The surface emission coefficient S for redwood was found to be 60% greater than for sweetgum, presumably because redwood is less hygroscopic and also less dense than sweetgum

Effect of Specimen Length on Longitudinal Gas Permeability in Hardwoods

A study was carried out to determine whether the longitudinal gas permeability of hardwoods is affected by specimen length. The results indicate that in most woods tested, the permeability remains constant as length is reduced, except for lengths below 0.75 inch. Thereafter, with decreasing specimen length, permeability increases drastically. The large differences in permeability of samples of different length are attributed to random blockages in the capillary structure of the wood

Effect of Extractives on Moisture Sorption and Shrinkage in Tropical Woods

Samples of sixteen tropical wood species from Indonesia were selected to undergo desorption and adsorption in the unextracted and extracted form. The anisotropic shrinkage values of these samples were also determined. At high humidities, the extracted woods exhibit higher equilibrium moisture contents than the unextracted woods. However, the isotherms of extracted and unextracted woods coincide at relative humidities below 70% for both desorption and adsorption. This phenomenon indicates that the hygroscopicity of wood is affected at high humidities through the extractives bulking the amorphous region in the cell wall. The shrinkage intersection point ranges from 18.0 to 34.1%, with an average of 24.8%, which is below the predicted 30% for temperate-zone woods. The linear relationship between volumetric shrinkage and specific gravity was significant, but the correlation (r2 = 0.40) is quite low. The removal of extractives with hot water and organic solvents caused excessive shrinkage. All the woods tested showed partial collapse, indicating the plasticization of the extractives at high temperature and high moisture content in the cell wall structure

Effect of Steaming and Hot-Water Soaking on Extractive Distribution and Moisture Diffusivity in Southern Pine During Drying

Samples of southern pine sapwood and heartwood were treated five different ways: steaming in the saturated condition for 1 h and 5 h, respectively, steaming at a moisture content near the fiber saturation point (FSP) for 1 h, hot-water soaking for 10 h, and steaming near 95% relative humidity at an equilibrium moisture content (EMC) slightly below the FSP for 1 h at 100°C. The samples were dried from near saturation condition to an EMC slightly below the FSP, and then to final 12% EMC. The results indicate that the amount of extractives removed tended to be evenly distributed along the flow direction before drying and after drying to near FSP, which suggests that extractives move with water in wood in response to moisture gradient during drying. Hot-water soaking and prolonged steaming increased the moisture diffusivities above and below the FSP. The variation in diffusion coefficient was partially due to changes in the extractive distribution profile

Evaluation of Diffusion Coefficient and Surface Emission Coefficient by an Optimization Technique

The rate at which wood dries can be expressed by two factors:(1) the diffusion coefficient, as an indication of internal resistance to moisture removal, and (2) the surface emission coefficient, as an indication of external resistance to moisture removal. An optimization technique was applied to calculate these two coefficients. Unlike existing methods where these coefficients are evaluated at only one point, usually at one-half of the evaporable water remaining in wood, this optimization technique used the entire drying data. The technique searches for the optimum pair of coefficients based on the least squares principle. The performance of this method was demonstrated by reanalyzing published data using a digital computer. The results indicate that a more accurate prediction of the drying process can be achieved by the optimization method than by existing methods. The dependence of the diffusion coefficient on moisture content and sample thickness is discussed

Effect of Cultural Treatment and Wood-type On Some Physical Properties of Longleaf and Slash Pine Wood

Wood was studied from relatively mature longleaf and slash pine trees that had been growing under experimental treatments including cultivation, two levels of thinning, and four levels of fertilization in various combinations. From discs taken at five heights and separated into three wood-types, green moisture content, radial shrinkage, tangential shrinkage, volumetric shrinkage, latewood percentage, number of rings per inch, and specific gravity were determined. In longleaf pine, there were indications of treatment effect on latewood percentage and the number of rings per inch, with the more intensive treatments generally leading to fewer rings and higher amounts of latewood. In slash pine, a similar trend was found with these two variables. The species exhibited an almost identical trend in the interrelationships among properties. Green moisture content was negatively correlated with specific gravity and moisture content of the innerwood and middlewood increased with height. Shrinkage was found to be negatively correlated with height and positively correlated with specific gravity, except in the innerwood. Latewood percentage accounted for much of the variation in specific gravity in all wood-types. In the outerwood only, there was a positive but weak correlation of number of rings with specific gravity

Optimum Average Diffusion Coefficient: An Objective Index in Description of Wood Drying Data

In the existing schemes for estimating average diffusion coefficients, the equations are approximate because of the use of only the first term in an infinite series and the subjective nature of the methods. The method described here takes into account all data points, and provides a systematic and objective way of analyzing wood drying data. Using the formula of the theoretical Fourier series solution, a series of theoretical Ē values, representing the fractional amounts of water in wood during drying, were coupled with experimental data, and the sum of squares minimized. The method sets up upper and lower expected bounds for diffusion coefficients, and then locates the optimum average diffusion coefficient by using a FORTRAN program based on the golden section search principle. Using data from a previous drying study on six hardwoods, it was found that the theoretical curves in the longitudinal direction fitted the data points satisfactorily. This suggests that diffusion coefficients in the longitudinal direction are virtually constant. This method, however, depends upon the assumption that the value of Ē at the surface drops immediately to zero as drying starts

Variation in Shrinkage Properties of Second-Growth Baldcypress and Tupelo-Gum

Trees of baldcypress and tupelo-gum aged from 55 to 75 years, growing in a naturally regenerated stand in the Atchafalaya Basin in south Louisiana, were felled to provide samples for determination of shrinkage, specific gravity, and extractive content. Samples were taken from six heights and from three radial positions (=wood-types) along three radii. Differences among trees and among plots (which had different basal areas) were found for all measured variables. There were also variations in unextracted specific gravity with height in the tree and wood-type in baldcypress, and, except for specific gravity, with wood-type in tupelo-gum. After extraction, shrinkage effects of height and wood-type were reduced or disappeared. Specific gravity was generally lowest in the lower portion of the stem and in the outerwood. Correlation of shrinkage with specific gravity was highest in the outerwood, and extractive content was correlated with shrinkage in tupelo-gum but not in baldcypress