UDC 674.047.3:53.083.8

PNEUMATIC METHOD FOR MONITORING CRACKING OF LUMBER IN THE INITIAL STAGE OF DRYING

G. I. Bandaevskii, A. A. Gurchenok

(Presented by the scientific seminar of the Department of Automation of Thermal-Power Processes of Industrial Enterprises)

Wood, when used industrially, requires a reduction of its moisture content to 6–20%, depending on its intended use. At present, of all the proposed and tested methods of drying wood, the kiln method of drying boards has found application in industrial practice. Now, throughout the country, lumber-drying kilns provide drying for no more than \(1/3\) of all sawn lumber [1], and there is a decline in wood quality during the drying process due to drying defects under intensive evaporation of moisture. To ensure the fastest possible and highest-quality drying of wood, TsNIIMOD has developed and, recently, improved standard schedules for kiln drying of lumber, which are calculated for a certain average quality of the lumber being dried. In practice, however, even with strict observance of the standard schedule, it often proves nonoptimal, and part of the material is rejected. Therefore, for each batch of lumber, in order to reduce rejects, it is necessary to adjust the standard schedule according to the condition of the lumber being dried.

The task of adjusting the standard schedule according to the condition of the lumber being dried consists in selecting and substantiating the type of corrective signal, taking into account its weight, in determining the quality of drying, and in developing a method and functional apparatus for implementing it. We note that the functional device carrying out the process of measurement or monitoring must operate automatically, since the ultimate goal of these efforts is the creation of an automatic control system for wood drying that takes the condition of the material into account.

Let us name several known solutions.

V. V. Sakharov and A. G. Koblikova [1] proposed using, as the corrective signal, the value of the current moisture content of the lumber averaged over the stack, with various implementations of the functional device. The results of tests of functional devices for measuring shrinkage, as a quantity functionally related to moisture content, are set forth in [3].

L. O. Leparskii [4] recommends, as a corrective signal in controlling the drying process, measuring the shrinkage of lumber over the entire width or over part of the width of a board.

GDR patent No. 81380 (cl. F 26 b) is known, in which a signal corresponding to the moisture content in the board being dried is supplied to the system for regulating the parameters of the drying agent.

In the work of B. N. Ugolev [5], for industrial application it is recommended to use a sensor of differential shrinkage through the thickness of the board in order to monitor the moment at which maximum stresses are reached in the process of drying lumber.

All the types of signals listed above—the average moisture content of the lumber, the moisture-content gradient through the thickness of the lumber, its shrinkage, and the magnitude of internal stresses—characterize invisible defects of lumber drying and are considered from the standpoint of the danger of visible defects arising during lumber drying, the principal one of which is cracking. The investigations carried out have established limiting values of the rate of change of the parameters at which cracking of the lumber occurs.

The general theoretical basis of the investigations is the analysis of the development of the stressed state of lumber during drying, in particular the dependence of the cracking criterion and tensile forces on the mean value and gradient of the moisture content of the lumber being dried [6].

\[ K=\frac{\overline{U}-U}{U_0}, \tag{1} \]

where

\(K\) — cracking criterion;
\(\overline{U}\) — average moisture content;
\(U\) — local moisture content;
\(U_0\) — initial moisture content.

\[ P_n=\frac{\beta_l \cdot E \cdot R}{3(1-\mu)\cdot(1+\beta_l\cdot U_0)}(\Delta U)_n, \tag{2} \]

where

\(P_n\) — tensile force;
\(\beta_l\) — coefficient of linear shrinkage;
\(E\) — modulus of linear deformation;
\(R\) — size of the plate;
\(\mu\) — Poisson’s ratio;
\(U_0\) — initial moisture content.

In these works, the limiting values of the parameters (or the rates of their change) are presented as the causes of lumber cracking, and limiting them should mean the complete elimination of lumber cracking during the drying process.

In the works of P. V. Sokolov and S. I. Akishenkov [1], [7], direct visual monitoring is carried out of the magnitude of end cracking in the lumber being dried, which the authors regard as one of the methods for reducing end cracking in dried lumber. The functional devices proposed by them (an illuminated viewing window and an industrial television installation) do not make it possible to automate the process of monitoring and measuring and to obtain a reliable correcting signal determined by the number and dimensions of cracks and by their dynamics.

Taking into account that one of the main parameters characterizing the quality of lumber dried in a chamber is its crack…

cracking, and at present there are no monitoring methods acceptable for production. We set ourselves the task of developing a direct method for automatic monitoring of the current amount of cracking, with a sufficiently simple implementation of the functional device.

In the drying process, small end cracks appear first, since the intensity of heat and moisture exchange on the end surfaces is greatest in the first period of drying. Then, under a severe regime, the developed end cracks extend to the edges and faces, which determines the amount of reject material when the dried boards are trimmed, in the form of cracked end offcuts.

As a result of experimental-design and research work carried out in the laboratory of the Department of Automation of Thermal-Power Processes of Industrial Enterprises of the Tomsk Polytechnic Institute, an installation was created that makes it possible to automatically monitor the dimensions of end, edge, and face cracks by measuring the pressure drop of air passing through the cavities of the cracks. The functional diagram of the installation is shown in Fig. 1.

Fig. 1

A box-type sensor (2) is attached (driven in or pressed) to the end surface of the board (1) located in the chamber; in this sensor, by means of an ejector (3) and a regulating valve (4), a constant vacuum \(\Delta p = 80\) mm water column is created. The pneumatic signal is converted into an electrical signal in an induction-type pressure sensor DT-250 (5). A secondary self-recording instrument (6) of the DSR1-07 type carries out measurement and recording of the measured quantity.

The sensitivity of the instrument is such that, when end, edge, and face cracks form, the pressure drop in the system decreases, and the pressure sensor (5) sends an electrical signal to the measuring instrument (6). The regulating valve (4) is installed for adjusting the “zero of the instrument,” which is performed after the box-type sensor (2) has been fastened to the end of the lumber. The possibility of creating a monitoring installation based on a change in the air pressure drop in the system at a constant initial pressure is due to the fact that the permeability of cracked wood is many times greater than the permeability of sound, uncracked wood. For our installation, with a large length of the lumber being dried (4–6 m), it is necessary to use in the calculation the permeability across the fibers, which is 15–20 times less than the permeability along the fibers [8].

During drying, a change in the moisture content and temperature of the lumber entails a small change in permeability in magnitude.

and the pressure drop in the system, which, from the standpoint of the measurement process, gives the systematic component of the instrument error.

Air leakage at the point of contact between the box-type sensor and the end surface depends mainly on the geometric dimensions of the sensor and the clamping pressure (or the depth of indentation into the end of the lumber). With a certain skill in fastening, this air leakage becomes sufficiently small and commensurate with the leakage caused by permeability through the lumber. In the installation, both of these leakages produce a zero error and determine the degree of opening of the regulating valve (4), Fig. 1.

In our installation the pointer of the measuring instrument deviates from the zero position depending on the magnitude of the measured pressure as a function of the air flow rate through the cavity of a crack of definite geometric dimensions. In the general case, the problem of monitoring and regulating the lumber being dried in the kiln can also be satisfactorily solved when it is not the magnitude of the crack itself that is monitored, but rather some function rigidly related to it (air pressure). With a certain skill, the kiln operator will be able to monitor cracking of the lumber sufficiently effectively and, on this basis, correct the drying schedule.

However, in order to obtain the most accurate and objective estimate of the degree of cracking of the lumber being dried, it is desirable to calibrate the scale of the instrument in units that determine the geometric characteristics of cracks. Such characteristics may be the length of cracks, depth, width, or certain generalized quantities, such as crack opening, effective flow area, hydraulic radius, and crack volume. In calibrating the instrument, allowance was made for the fact that generalized characteristics more fully determine the cracking of lumber and constitute certain generalized analogues for different geometric forms, whereas the individual characteristics of cracks have a large variance within a single species of lumber and differ substantially for different species.

Taking the above considerations into account, in calibrating the instrument a throttling spool valve with triangular windows was used, with each transition section of the valve being assigned an effective flow area of a crack equivalent in its hydraulic resistance.

In selecting the initial pressure and the structural elements of the hydraulic network, we sought to obtain a laminar flow regime, so that the linear law of dependence of the flow-resistance force on its velocity would be observed.

For the section of air outflow from the slot to the ejector, the following calculation formula (1) was obtained:

\[ \Delta p = 0.141 \nu_4 \nu_p + 0.54 \xi_4 \frac{\nu_4^2 \nu_p^2}{m^2} + 0.54 \xi_p \frac{\nu_4^2 \nu_p^2}{m_p^2}. \tag{1} \]

Here: \(\Delta p\) is the total pressure drop (mm water column);

\[ \nu_4 = \frac{G_{4i}}{G_{\max}}, \]

\(G_{4i}\) is the current value of the flow rate through the regulating valve (4), Fig. 1 (kg/sec);

\(G_{\max}\) is the maximum flow-rate value with the valve fully open. \(G_{\max}=1.41\cdot10^{-4}\ \mathrm{kg/sec}\);

\[ \nu_p=\frac{G_{pi}}{G'_{\max}}, \]

\(G_{pi}\) is the current value of the flow rate through the spool valve \((\mathrm{kg/sec})\);

\(G'_{\max}\) is the maximum value of the flow rate with the valve fully open \((\mathrm{kg/sec})\), which depends on the degree of opening of the regulating valve (4), Fig. 1.

\[ m_4=\frac{F_{4i}}{F_{\max}}, \]

where

\(F_{4i}\) is the current area of the flow cross section of the regulating valve, \(\mathrm{m}^2\);

\(F_{\max}\) is the maximum area of the flow cross section of the regulating valve, \(F_{\max}=2.8\cdot10^{-5}\ \mathrm{m}^2\);

\[ m_p=\frac{F_{pi}}{F_{\max}}, \]

where

\(F_{pi},\ F_{\max}\) are the current and maximum areas of the flow cross sections for the spool valve, \(F_{\max}=2.8\cdot10^{-5}\ \mathrm{m}^2\).

\(\zeta_4\) and \(\zeta_p\) are the coefficients of hydraulic resistance of the regulating valve and the spool valve.

The first term in formula (1) determines the pressure drop in the pulse connecting tubes.

When calculations were made by formula (1), the degree of opening of the regulating valve (4) was specified and the pressure drops were determined over the entire operating range of the spool valve. The relationship between the pressure drop and the area of the flow cross section of the spool valve, constructed from formula (1) at \(\nu_4=1,\ m_4=1\), is presented in Fig. 2.

Fig. 2

From the curve in Fig. 2, the values of the instrument “setting” are determined as the permissible effective flow cross section of the crack. To pass from the effective flow cross section of cracks to their unit geometric characteristics, in addition to determining the static characteristics of cracks, it is also necessary to estimate the wedge shape and surface roughness for various wood species, as factors that may significantly affect the hydraulic resistance to air motion. These questions require special studies.

Using the installation we developed for automatic monitoring of end cracks by the air-pressure drop under production conditions, data were obtained on the dynamics of the development of edge and face end cracks in lumber being dried and on the influence of drying-agent parameters on these cracks.

Conclusions

The described installation for automatic monitoring of end cracking made it possible to automate experiments on the study of cracking in lumber during drying and to conduct them under production conditions. Further development of the installation is being carried out in the direction of improving its structural elements as applied to production conditions.

References

1. Improvement of kiln drying of wood. Scientific Papers, No. 138, L., LTA, 1969.

2. V. V. Sakharov. Telemetry of the current moisture content of wood during its drying. Dissertation. Moscow, MLTA, 1970.

3. E. S. Bogdanov, A. I. Isakov. Analysis of systems for monitoring the moisture content of lumber during drying by stack shrinkage. Derevoobrabatyvayushchaya promyshlennost’, 1972, No. 3.

4. L. O. Leparsky. Study of shrinkage and stresses in wood under conditions of high-temperature drying in the manufacture of building components. Abstract of Candidate’s dissertation. Moscow, MISI named after V. V. Kuibyshev, 1962.

5. B. N. Ugolev. Deformability of wood and stresses during drying. Moscow, Lesnaya promyshlennost’, 1971.

6. A. V. Lykov. Theory of drying. Moscow, Energiya, 1968.

7. S. I. Akishenkov. Study of end cracking of lumber and methods for eliminating it during kiln drying. Dissertation. L., LTA, 1971.

8. P. S. Sergovsky. Hydrothermal treatment and preservation of wood. Moscow, Lesnaya promyshlennost’, 1968.