Maintaining Lumber Quality in Press Drying by Manipulating Sawing Patterns

Lumber is traditionally dried in kilns by processes that often take several weeks to complete. Considerable research has been conducted on more rapid processes. Press drying can dry common 25-mm-thick lumber in 1 to 2 hours instead of several weeks. However, little success has been achieved with this technique because intolerable drying defects usually accompany such rapid drying. This paper reports on a press-drying technique that offers considerable promise in avoiding honeycomb that usually develops. In this technique the sawing pattern of boards from logs is changed from the usual flatsawn pattern to quartersawn. The honeycomb that usually develops occurs at interfaces of ray tissue and the surrounding tissue and is caused by internal tensile forces that develop during drying. With flatsawn boards the wide face, which makes contact with the platens during drying, is parallel with the critical internal tensile forces. By using a quartersawn pattern, the direction of the critical internal tensile forces is effectively rotated by 90 degrees. The critical internal tension is then perpendicular to the wide face of the board.The hypothesis of the study was that the compressive pressure of the press therefore opposes the internal stresses and prevents honeycomb. In this study honeycomb in press-dried quartersawn red oak was significantly less than in press-dried flatsawn red oak. Small compressive forces of the platens were less effective than large forces in reducing honeycomb in quartersawn oak, thus verifying the hypothesis

Relationship Between Speed of Sound and Moisture Content of Red Oak and Hard Maple During Drying

The transit time for sound was measured both perpendicular and parallel to the grain of red oak and hard maple during drying under constant temperature (27°C (80°F)) and varying relative humidity conditions. Transit time perpendicular to the grain increased slightly with decreasing moisture content initially during drying but then decreased with decreasing moisture content. Transit time parallel to the grain remained nearly constant with moisture content initially during drying but then decreased. Transit time became sensitive to moisture content change at average moisture contents well above 30%. The relationship between relative transit time and moisture content showed three linear regions that could be characterized by a three-component regression equation. These results suggest that speed of sound measurements have good potential for control of hardwood kiln schedules in which changes in kiln conditions are made at moisture contents above 30%

Importance of Relative Humidity and Temperature Control in Conditioning Wood Products

Equilibrium moisture content of solid wood and reconstituted wood is analyzed to determine the level of relative humidity and temperature control necessary to maintain EMC within desired limits during conditioning. The necessary degree of control is found to be highly dependent on relative humidity and temperature

Two-Dimensional Heat Flow Analysis Applied to Heat Sterilization of Ponderosa Pine and Douglas-Fir Square Timbers

Equations for a two-dimensional finite difference heat flow analysis were developed and applied to ponderosa pine and Douglas-fir square timbers to calculate the time required to heat the center of the squares to target temperature. The squares were solid piled, which made their surfaces inaccessible to the heating air, and thus surface temperatures failed to attain the temperature of the heating air. The surface temperatures were monitored during heating and related to time by an empirical equation. When this equation was used as the boundary condition in the finite difference solution, calculated time estimates required to heat the center to target temperature agreed favorably with experimentally observed heating times

Using Acoustic Analysis To Presort Warp-Prone Ponderosa Pine 2 By 4s Before Kiln-Drying

This study evaluated the potential of acoustic analysis as presorting criteria to identify warp-prone boards before kiln-drying. Dimension lumber, 38 by 89 mm (nominal 2 by 4 in.) and 2.44 m (8 ft) long, sawn from open-grown small-diameter ponderosa pine trees, was acoustically tested lengthwise at green condition. Three acoustic properties (acoustic speed, rate of wave attenuation, and acoustic modulus of elasticity (MOE)) were then determined through waveform analysis. Boards were then kiln-dried, and warp was measured immediately after drying and after equilibrating to about 13% equilibrium moisture content. Crook and bow measured after drying decreased as acoustic speed and acoustic MOE of green boards increased and rate of wave attenuation of green boards decreased. Twist was found to have no relationship with any acoustic properties of green 2 by 4s. The results also show a statistically significant correlation between acoustic properties of green 2 by 4s and the grade loss caused by exceeding warp limits. As the number of Structural Light Framing grade losses increased, the acoustic speed and acoustic MOE decreased significantly, whereas rate of wave attenuation increased significantly. However, no relationship was found between green board density and warp and grades lost

Perpendicular-To-Grain Rheological Behavior of Loblolly Pine in Press Drying

To predict the thickness loss of loblolly pine lumber during press drying, a model was developed to describe the perpendicular-to-grain rheological behavior as a function of pressure, temperature, and drying time. The strain-time curve was divided into four parts—initial elastic deformation, viscoelastic deformation, final elastic springback, and time-dependent springback—according to the characteristic responses of these behaviors to pressure, temperature, and drying time. The model was fitted to experimental data by nonlinear regression. Good agreement was obtained between the predicted and experimental thickness loss during press drying

Evaluating Humidity At Dry Bulb Temperatures Above The Normal Boiling Point of Water A Research Note

Several methods were compared for evaluating relative humidities at temperatures between 210 and 300 F from psychrometric relations. An explicit method of calculating relative humidity, assuming that the wet bulb temperature is equivalent to the adiabatic saturation temperature, is shown to predict relative humidities with a maximum deviation of only 1% from those values predicted by more theoretically rigorous equations

Effect of Thickness Variation on Warp in High-Temperature Drying Plantation-Grown Loblolly Pine 2 by 4'S

Currently, an increasing proportion of southern pine dimension lumber comes from plantations; therefore, an increase in grade, value, and volume loss from warp is expected. One factor that has not been fully explored is the effect of lumber thickness variation on warp. The primary objective of this study was to increase quantitative understanding of the effects of thickness variation on crook, bow, and twist during high-temperature kiln-drying of plantation-grown loblolly pine to determine the importance of its control on the development of warp. Plantation-grown, 2 by 4 (nominal 50- by 100-mm) loblolly pine were kiln-dried at high temperature after surfacing them in such a way as to produce certain patterns of thickness variation. One group was not surfaced, i.e., left as mill run. All boards in a second group were surfaced to the same thickness. In a third group, the boards were divided into thirds, and each third surfaced to a different thickness. In this group, boards of the same thickness were stacked in vertical alignment to exaggerate the effect of the thickness variation. The fourth group differed from the third group in that the three thicknesses were randomly placed in the package. The extreme thickness variations did aggravate warp, especially twist. As a result of better sticker contact, thick boards warped less than did thin boards. However, even with perfect sticker contact, a substantial amount of warp developed, indicating that control of thickness variation can reduce but will not eliminate warp. Correlation of warp with board characteristics suggests that boards containing pith warp more than ones without pith, and boards sawn from near the center of the tree warp more than boards farther from the center of the tree

Equilibrium Moisture Content of Wood At High Temperatures

In answer to increased interest in using high-temperature (above 212 F) drying of wood, a method is described to extrapolate low-temperature equilibrium moisture content (EMC) data to high temperatures. The results are compared with data in the literature, and EMC data from 212 F to 300 F are presented in a form useful for kiln control as well as for other uses