On the Analysis of End Cracking During Drying of Pine Wood
Abstract Generated abstract
This paper examines the formation and development of end checks during accelerated drying of pine lumber, with attention to their impact on drying quality and technological rejects. The authors describe experiments on pine specimens dried in a cabinet at about 100 degrees Celsius, using an automatic checking signal based on the total thickness of developed end cracks reaching the side surfaces, and relate the observed cracking behavior to calculated changes in the temperature field. The results indicate that small end cracks occur during the initial heating stage, while more developed end checks appear at the transition to the constant drying rate period, when capillary moisture removal becomes intensive. The study identifies this transition as the most critical stage for monitoring and possible control of drying schedules.
Full Text
ON THE ANALYSIS OF END CHECKING DURING THE DRYING OF PINE WOOD
G. I. Bandaevsky, A. A. Gurchenok
(Presented by the scientific seminar of the Department of Automation of Thermal-Power Processes of Industrial Enterprises)
Drying wood as a valuable raw material is an important national-economic task, since the industrial use of wood always requires a reduction in its moisture content. Accelerated kiln drying of lumber is finding wide application; in this process the question of the influence of wood drying on its quality arises especially sharply.
At all-Union conferences on drying [1, 2], the low quality of atmospheric and kiln drying of lumber was noted, as was the need to improve drying techniques in the direction of increasing the quality of the products made from it. The quality of wood drying is characterized by the presence of visible and hidden defects. Visible defects include: checking, warping, and collapse, of which the principal one, especially for structural materials, is checking. Nonuniform moisture content across the cross section of the lumber and internal stresses, which are invisible defects, are determined by laboratory tests. The most widespread drying defect, significantly affecting the percentage of technological rejects in the production process and reducing the mechanical strength and quality indicators of the manufactured articles, is considered to be checking. According to GOST 2140-61 [3], drying checks are divided into end, face, and edge checks, and it is indicated that face and edge checks may extend to the ends.
The main cause of the formation of drying checks lies in the very mechanism of moisture removal by evaporation from the surface of the lumber. The moisture gradient thereby arising across the cross section causes the occurrence of a volumetric stress state in the lumber, with the development of which checks are formed. To study the physico-mechanical nature of checks and the dynamics of their development during the drying process, it is necessary to study not only their direct relation to critical internal stresses, but also their relation to the thermodynamic parameters of the lumber.
The few studies carried out on the checking of lumber being dried, based on the method developed by B. N. Ugolev [4], were directed toward studying quantitative estimates of dangerous stresses as a function of the moisture gradients of the material.
S. I. Akishenkov [5] conducted a review of works on the question of end ...
checking of wood during drying and practical measures for its reduction have been outlined. To reduce checking in the lumber being dried, it is necessary to monitor the appearance of cracks so that, if necessary, the drying schedule can be changed in good time. For this purpose, modern drying chambers are equipped, for visual observation of the lumber being dried, with special sight windows developed by LTA-Giprodrev and installed in the end wall or in the door of the drying chamber.
To study the physico-mechanical characteristics of checking, it is necessary not only to establish the time at which checking begins, but also to give a quantitative assessment of its dimensions and development dynamics.
This article presents some results of work carried out at the Department of Automation of Thermal-Power Processes of the Tomsk—
Fig. 1. Curves of change in the relative checking parameter of dried specimens as a function of time
Polytechnic Institute on automatic control of checking in lumber being dried. With automatic control of lumber checking, the kiln operator is freed from the need to carry out visual monitoring of the material being dried itself, and, in addition, a signal proportional to the magnitude of checking can be fed into the automatic control system for the drying schedule.
Experimental drying in a drying cabinet at \(t = 100 \pm 5^\circ\mathrm{C}\) was performed on specimens of pine lumber measuring \(200 \times 50 \times 120\) mm. The initial temperature of the specimen was \(t = 20^\circ\mathrm{C}\), and the initial moisture content \(W = 58\%\).
On the chart of a recording instrument, curves were obtained for the change in the relative checking parameter of the dried specimens as a function of time (Fig. 1). The checking signal is formed according to the total thickness of those end cracks that emerge on the side surfaces of the specimen, i.e., the most developed ones. The time of crack formation is recorded by the change in the steepness of the curves. The sec—
the slope of the steep part of the graph characterizes the intensive appearance of cracks.
Observations showed that the quality of drying of sawn timber depends to a considerable extent on the sizes and types of cracks. Rejected cracked end sections, going to waste or to be burned, may be from 2–3 cm to 1 m long. It is natural to suppose that poorly developed hairline end cracks that do not reach the lateral surfaces give an insignificant percentage of rejects compared with wide end cracks that extend to the lateral surface. It is considered that end and external surface cracks are formed in the initial stage of the drying process, and that their primary cause is the moisture gradient through the volume of the material. The first stage of the drying process consists of a heating period, characterized by an insignificant change in the moisture content of the material, and a period of constant drying rate, when the temperature of the material is constant and equal to the wet-bulb temperature, while moisture evaporates intensively from the material.
To determine the nature and regularities of the development of small, purely end cracks that extend to the lateral surfaces, we calculated the rate of change of the temperature field of a sample of sawn timber.
We have
\[ K = f\left(F_0;\ Bi;\ \frac{x}{R}\right) \tag{6} \]
where \(K\) is the temperature criterion.
The calculation of the temperature field is carried out with corrections for the influence of the width, length, and moisture content of the sample being dried, for the middle \(\left(\frac{x}{R}=0\right)\) and the surface \(\left(\frac{x}{R}=1\right)\). It is seen in the figure that cracking begins 45 minutes after the start of drying. For this time, the table gives the results of calculating the temperature and the rate of its change at the center of the sample.
Table
| \(t_{\mathrm{ц}}\) | \(\dfrac{dK}{dF_{0x}}\) | \(\dfrac{dK}{dF_{0y}}\) | \(\dfrac{dt_x}{d\tau}\) | \(\dfrac{dt_y}{d\tau}\) |
|---|---|---|---|---|
| 40 | 0.3 | 1.7 | 2.3 | 2.8 |
From the table presented and the nomogram of the relative rate of change of the temperature field [6], it is evident that heating of the material through the thickness is already proceeding slowly. The slowed heating process of the material corresponds to the transition region from heating of the material to the period of constant drying rate.
The calculation of the temperature field and the rate of its change, carried out to determine the regularities of cracking, showed that in the first drying period, characterized by an intensive change in the temperature field, only small end cracks appear in pine wood, not extending to the surface of the sawn timber. In the second drying period—the period of constant rate, characterized by a weak change in the temperature field and intensive evaporation of moisture—developed end cracks appear, extending to the lateral surfaces and producing a large percentage of rejects.
Conclusions
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Specially conducted experiments made it possible to obtain a picture of the formation of end checks extending onto the side surfaces (Fig. 1), and to determine, by calculation, at what stage of the drying process they appear.
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The investigations showed that the most critical and dangerous moment for checking during the drying of pine is the transition from the period of heating the material to the period of constant drying rate, when intensive removal of capillary moisture begins. At this moment it is especially necessary to monitor the drying process.
References
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All-Union Scientific and Technical Conference. New equipment and progressive technology in the drying process. Moscow, 1969.
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Jubilee All-Union Conference on Wood Drying. Arkhangelsk, 1961.
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B. N. Ugolev. Internal stresses in wood during its drying. Moscow, Goslesbumizdat, 1959.
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GOST 2140-61. Wood. Defects. Moscow, Standards Publishing House, 1964.
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Improvement of kiln drying of wood. LTA. Scientific Works, No. 138, Leningrad, 1969.
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B. S. Chudinov. Theory of thermal treatment of wood. Moscow, “Nauka,” 1968.