JP5779747B2 - How to dry wood - Google Patents

Description

  The present invention relates to a method for drying wood, and more particularly to a drying method capable of preventing cracking of wood by appropriately setting the heat treatment conditions at the initial stage of drying in high-temperature drying of a support material.

  Wood contains a lot of moisture in the state of “trees” growing from the ground. In particular, cedar contains 0.5 to 2 times or more of the weight of the whole wood (total dry weight) after removing all moisture. It is called “moisture content” using the percentage of the weight of moisture contained in the total dry weight, but in the case of cedar, the moisture content is 50 to 200% or more.

Trees come to be called timbers by felling, but timbers are not replenished with moisture from the ground, so if left unattended, moisture will naturally drain and dry. And it shrinks by drying.
The degree of contraction of wood differs between the direction along the annual ring (tangential direction) and the direction along the radius (radial direction), and the tangential direction is about twice the radial direction.

Wood with a marrow in the cross section (the center of the annual ring ) is called “heart-holding material”. When this heart-supporting material is dried, cracking occurs due to the difference in shrinkage between the tangential direction and the radial direction. If broken, structural materials such as pillars, beams and girders will be reduced in value in the market and will be bought cheaply. Therefore, it has been the most important proposition for a lumberer to dry the heartwood without breaking.

  Until now, many methods have been adopted to avoid this cracking. However, in recent years, high temperature drying is performed by performing heat treatment (high temperature and low humidity treatment) at a dry bulb temperature of 120 degrees Celsius and a wet bulb temperature of 90 degrees Celsius in the initial stage of drying. The law has become widespread. This high-temperature and low-humidity treatment in the initial stage of drying is very effective in suppressing surface cracking. However, if the treatment time is too long, there is a drawback in that cracks do not appear on the surface but cracks appear inside.

Therefore, for example, research results have already been issued that the processing time should be about 12 to 18 hours in the case of cedar wood, and lumberers throughout the country are drying based on this. However, there are many varieties of cedar, and even in the same varieties, there are large variations in materials such as density and moisture content in each production area. Therefore, grasping the appropriate heat treatment time suitable for each region's cedar at each lumber mill is an important issue that leads to quality improvement and cost reduction.
However, to understand the appropriate processing time in the field, trial and error are currently required, and in some cases there is a risk of losing a considerable amount of products. As a result, examination of optimal processing conditions remains insufficient. In the current situation, the above processing time must be adopted.

Therefore, there is a need for a simple method of monitoring for any factors during the drying process and determining when the risk of surface cracking is low without destroying the wood. The possibility of surface cracking of wood during drying using high-temperature and low-humidity treatment of wood increases from the moment when the moisture content on the surface falls below the fiber saturation point (water content at which shrinkage begins, approximately 30%). Beginning and then the risk is gradually reduced when a drying set is formed on the surface. When the internal contraction starts, the tensile stress on the surface starts to weaken, and the possibility of surface cracking at this point is considered to be very small. Therefore, if it is possible to grasp the time when the internal contraction starts roughly even if it can be grasped in a non-destructive manner, it will be a point for solving the problem, but until now there has been no technology to realize this.

Techniques related to the present invention are disclosed in the following documents.
Japanese Patent Publication No.8-10116 JP-A-8-5237 Japanese Patent Laid-Open No. 11-237179 JP 2009-000879 A JP 2009-241265 A JP 2007-144866 A JP 2001-287206 A JP 2004-190957 A Japanese Patent No. 2681808

In the drying method using high-temperature and low-humidity treatment of wood, especially cedar and other dry wood, it is possible to set an appropriate heat treatment time according to the material of each wood, and to suppress the surface cracks and internal cracks appropriately. Provide a drying method.

The present invention provides a method for drying a heart-supporting material,
B: Process of steaming heart-holding material b: Process of high-temperature and low-humidity treatment of wood that has undergone the steaming process,
C: Next, a main drying process is performed in which the heart-holding material that has been subjected to the high-temperature and low-humidity treatment is dried by lowering the temperature until the moisture content reaches a predetermined moisture content. Measure and compare the shrinkage rates at the center and end of the surface on either side of the heart support material, and when the shrinkage rate at the center of the surface of the heart support material exceeds the shrinkage rate at the end The configuration is intended to solve the above-described conventional problems.

In the drying method of the heart support material according to paragraph 0011, a strain gauge is installed at each of the center portion and the end portion of the surface of the heart support material, and the dimensional change in the direction orthogonal to the fibers is monitored. It may be configured to determine the time point when the shrinkage rate at the center portion of the surface of the support material exceeds the shrinkage rate at the end portion by measuring and comparing the respective shrinkage rates.

In the method for drying a heart support material according to paragraph 0011, the strain gauge may have a configuration function of measuring a distance between two points by inserting a needle between any two points on the surface of the heart support material.

In the drying method of the heart support material according to paragraph 0012, the steaming process of the heart support material is continued at 90 ° C. for 12 to 20 hours, and then the process of high temperature and low humidity treatment is performed at a contraction rate at the center of the surface of the support material. In some cases, the main drying step may be continued under the conditions of a dry bulb temperature of 90 ° C. and a relative humidity of 25% until the moisture content of the heart support material reaches a target.

In the present invention, the end time of heat treatment (high-temperature and low-humidity treatment) for suppressing surface cracking in high-temperature drying, which has been performed so far by relying on the experience and intuition of wood drying equipment operator engineers in sawmills, is constant. This makes it possible to clearly discriminate on the basis of the above, and to determine an appropriate treatment time according to the production area and variety individually, and as a result, dry wood without cracks can be obtained efficiently.

In practice so far, standard drying schedules based on known high temperature drying methods have been applied to locally produced cedar wood. That is, for example, heat treatment for suppressing surface cracking of wood, particularly cedar, is required at a dry bulb temperature of 120 ° C. and a wet bulb temperature of 90 ° C. for about 12 to 18 hours.
However, there are many varieties of cedar, and even if the varieties are the same, the density and other materials vary greatly depending on the growth environment. Despite the need to seek time, this could not be set clearly. For this reason, there is a possibility that problems such as inability to dry well and cracking may occur, in which case the wood for one pot may be damaged. The current situation is that lumber mills operate with great risk.

For this reason, there are a limited number of companies that can find appropriate conditions through trial and error in the field, and often fail to stop drying or use extremely safe conditions. . If drying is stopped, undried materials will be shipped, and problems due to being undried will occur during or after the construction of houses, spreading distrust to wooden houses in the market. Also, if high temperature and low humidity treatment is performed for an extremely long time considering safe processing conditions, it may cause darkening and internal cracking, and the desiccant will be bought and damage to the lumberers will increase.

In the invention of the present application made to eliminate such inconvenience, the above-mentioned constitutional action makes it possible to clearly set the processing time suitable for the wood material to be used, and therefore it is firmly dried, less discolored, and In addition, a desiccant having no cracks on its surface and inside, that is, a desiccant that does not cause a complaint in the market, can be produced at a minimum necessary processing time, that is, at an appropriate cost.

By monitoring data related to the surface strain of the cedar heart holding material, the high temperature and low humidity treatment time required at the beginning of drying is determined, and a pair of strain gauges are used for data monitoring. The surface strain is a dimensional change in a direction perpendicular to the fiber on the surface of the support material.
In addition, the said supporting material is a square material used as a pillar or a beam material among the timber used as a structural material for houses, and has a marrow near the center of the cross section.

  The strain gauge needs to be able to measure the distance between any two points on the wood surface by inserting a needle into two points on the wood surface and measuring the distance between them. And since it installs and uses in a dryer during drying of a timber, it must be able to endure high temperature (maximum 130 degreeC).

In the high-temperature and low-humidity treatment process following the initial cooking, the heart-supporting material is first dried from the corners.
When processing at a high temperature, such as a dry bulb temperature of 120 ° C and a wet bulb temperature of 90 ° C, with a low humidity, a drying set is formed on the surface of the wood. Formed in advance.

In the vicinity of the corner, the drying proceeds while being pulled by the central portion of the material surface that has not yet sufficiently dried, so a difference occurs from the original shrinkage of the wood, and a so-called drying set is formed. However, since it is a corner, it is relatively free to contract because there is a cut (annular ring cut), and a very large set is not formed.

On the other hand, the central part of the wood surface shrinks in the early stages of drying after being delayed around the corner (edge of the material), and at the same time, the drying progresses while being pulled by the corners at both ends where the drying has already progressed. A set is formed and drying proceeds without much shrinkage. Thereafter, when the drying reaches the inside of the wood, the central portion of the material surface is pulled by the shrinkage of the inner layer, and the shrinkage due to the shrinkage of the inner layer at the corner is small due to the breakage. As the internal drying progresses, the strain (shrinkage) between the end of the material and the center of the material surface will eventually be reversed.

When this reversal point is reached, it can be determined that the inside has surely started shrinking and the risk of surface cracking has become extremely low. Accreditation at this time is performed by observing data by attaching one of the pair of strain gauges to the center of the material surface and the other to the end.

  The span of the strain gauge at the end of the material is best to reflect the effect of the corners, and it can be said that it is best to determine it according to the position of the break, but if it is a column material with a width of 120 mm, it is generally A width of about 20-30 mm is desirable. However, determination is possible even at 50 mm.

Further, the measurement span at the central portion does not need to be equal to the end of the material, and may be as long as it can be measured inside the cut position. Depending on the material of the individual, such as the annual ring width, about 30-60 mm is appropriate for a width of 120 mm. In this way, in the high-temperature drying process (high-temperature and low-humidity process), it becomes possible to determine when the drying set is formed on the surface of the material, the internal drying starts, and the risk of cracking on the surface is reduced. The low-humidity-treated wood can be transferred to a step of drying until reaching a predetermined moisture content without any timing.

  Next, an embodiment of the wood drying method according to the present invention will be described. The material to be dried in this example is a cedar regular material, and as shown in FIG. 1, a known dryer is used, and all the drying steps are (1) an initial steaming step, and (2) a high temperature and low humidity treatment. And (3) It is until it reaches a predetermined moisture content through this drying process. (1) The initial steaming step and (2) the high temperature and low humidity treatment are heat treatments for forming a drying set on the wood surface layer and suppressing cracks on the surface. In addition, (3) A drying set is not formed in this drying process.

(1) Initial steaming process Steaming at 90 degrees Celsius for 12-18 hours to soften lignin and the like. Steaming is performed by filling the dryer with high-temperature steam. At this time, the air in the dryer is exhausted by steam filling.

(2) High temperature and low humidity treatment Heat treatment is performed at a dry bulb temperature of 120 ° C. and a relative humidity of 35% for 18 to 24 hours. This forms a drying set on the wood surface and prevents surface cracking. The time when the drying set is formed is also the starting point of the internal drying of the wood, and this time is determined by monitoring a strain gauge attached to the surface of the wood as described later. That is, by monitoring, the time when the shrinkage rate at the center of the wood surface exceeds the shrinkage rate at the edge of the wood is determined as the time when the drying set is formed or the starting point of internal drying of the wood. It is stopped and the wood is transferred to the next drying step.

(3) Main drying process Drying is performed at a dry bulb temperature of 90 ° C. and a relative humidity of 25% until the drying is completed. That is, until the moisture content of wood reaches the target, for example, when used for structural materials (such as structural members of houses, pillars, etc.), the moisture content is 15%.

  In this example, as shown in FIG. 1, the wood was treated in the initial steaming process for 18 hours, and then transferred to the next high-temperature and low-humidity treatment, where the treatment was performed for about 20 hours. As will be described later, this 20 hours means a time point at which the high-temperature and low-humidity treatment should be finished for the dried cedar material. In other words, at the time point of 20 hours, the shrinkage rate at the center of the surface of the cedar material to be dried exceeds the shrinkage rate at the end of the wood, and a drying set is formed on the surface. It has begun.

  In this embodiment, a pair of strain gauges are attached to the center and end of the wood surface. And the time which should complete | finish the high temperature / humidity process described in the said paragraph 0036 is determined with the data which a strain meter shows. FIG. 2 is a graph showing the state of strain (shrinkage) at the center and end of the cedar surface by a strain gauge.

  As shown in FIG. 2, when steaming is performed for 18 hours and a transition is made to a high-temperature and low-humidity treatment (dry bulb temperature of 120 degrees Celsius, wet bulb temperature of around 90 degrees Celsius), when about 20 hours have passed after the transition, It turns out that the strain amount at the center exceeds the strain amount at the end. That is, in FIG. 2, the graph line 1 representing the relationship between the strain amount at the end of the material surface and the time and the graph line 2 representing the relationship between the strain amount at the center of the material surface and the time are points surrounded by ○ 3 in the figure. After this intersection 3, the amount of strain (shrinkage) at the center exceeds the amount of strain (shrinkage) at the end of the material surface, and a drying set was formed on the wood surface. Becomes clear. In the figure, reference numeral 4 is a graph line showing the difference in strain between the central portion and the end portion. Then, the time point at which the graph lines 1 and 2 intersect with each other is determined as the time point at which the high-temperature and low-humidity treatment should be finished for the wood to be dried and the continuation of the drying set formation on the surface should be stopped. Next, the wood enters the main drying step at a reduced temperature, and the operation is continued until a predetermined moisture content is reached while preventing internal cracking.

  Next, the contents of paragraph 0037 will be described in detail with reference to FIGS. FIG. 3 to FIG. 5 are schematic diagrams showing drying and shrinkage progress schematic diagrams of a material end and a central portion of a surface portion of a material to be dried (shown by a cross section of the end) and a graph of a shrinkage rate corresponding thereto.

  FIG. 3A shows the initial state of the corner 51 in the high temperature and low humidity treatment process of the material 5 to be dried, and FIG. 3B shows the shrinkage rate of the corner (material end) 51 and the surface center 52. It is a schematic diagram which shows the relationship of elapsed time. In the initial stage of the process, the moisture content starts to decrease at the corners as shown in the figure, and a drying set is formed while the corners shrink to some extent. At this time, as shown in FIG. The shrinkage rate of is higher than the shrinkage rate at the center of the surface.

  FIG. 4 (a) shows the situation when a certain amount of time has elapsed from the initial stage of the process (as shown in FIG. 2, 10 hours). In the figure, 6 is a strain gauge installed at the center of the material surface, and 7 is the material. It is a strain gauge attached to the surface edge. FIG. 4B is a graph showing the relationship between the shrinkage rate of the corner (material edge) 51 and the surface center 52 and the elapsed time. At this time, the corner 51 is a drying set and does not shrink so much, but the shrinkage proceeds along the annual ring at the center of the material surface, and the shrinkage rate of the material surface center 52 is as shown in FIG. It increases rapidly and the degree of increase exceeds the corner (material end) 51, but the shrinkage rate itself is still large at the corner (material end) 51.

  FIG. 5 (a) shows the situation at the end of the process, and FIG. 5 (b) is a graph showing the relationship between the shrinkage rate of the corner (material end) 51 and the surface center 52 and the elapsed time. In this situation, the shrinkage of the interior 53 of the material has started, while the shrinkage of the material surface has almost finished. However, when the cells in the material interior 53 start to contract, the cells in the material surface central portion 52 that do not need to contract anymore are also contracted because they are connected to the internal cells along the annual rings. As a result, the contraction rate of the material surface center portion 52 is added to the contraction rate of the material surface center portion 52, and the contraction rate of the material surface center portion 52 at a certain point in time as shown in FIG. The shrinkage rate exceeds 51.

As described above, in the present invention, in the high-temperature and low-humidity process, a drying set is formed on the surface of the material to be dried to prevent the surface cracking of the material. I will add to this significance. When moisture comes out of the wood (drys), the wood shrinks, more precisely, shrinkage begins when the moisture content is about 30% and continues until it reaches 0%.

  When the wood is freely shrunk during drying, the relationship between the moisture content and the shrinkage is always constant, and this is the original shrinkage. However, if the wood is dried while being pulled, the shrinkage is smaller than the original shrinkage. If the wood is dried while being compressed, the shrinkage is larger than the original shrinkage. In this way, when the original shrinkage rate is not shown, it is said that a drying set was formed there, and the difference between the original shrinkage rate and the shrinkage rate when dried while pulling or compressing is the set amount It is called.

  On the other hand, when wood is dried, unless the wood is sliced very thinly, the moisture content on the surface and inside is usually different, and the surface is dried first. Then the surface will try to shrink, but the interior that is not yet dry, i.e. not yet shrinked, will act to prevent the surface from shrinking, so the surface is relatively sideways (along the annual rings) Dry while being pulled. This force is strong and at normal temperatures (below 80 ° C.) this causes cracking. However, if drying is carried out at a sufficiently high temperature (high-temperature and low-humidity treatment), the wood becomes soft, the force to shrink becomes weak, and the moisture content decreases without much shrinking. That is, a drying set is formed here, and it does not shrink even when the moisture content falls, and the inside is soft and the force to shrink is weak, and cracking is prevented.

  As described above, in the present invention, when the drying set is sufficiently formed on the surface of the material to be dried and the surface crack can be suppressed, the high temperature and low humidity treatment process is performed based on the time when the internal shrinkage starts. The detection of this “time point” is specifically regarded as a time point when the strain (shrinkage) of the center portion and the end portion of the material is reversed, and is determined as the time when the high temperature and low humidity treatment should be ended.

  In this example, the set amount of the surface of the material to be dried is sufficiently formed in 20 hours from the start of the high temperature and low humidity treatment as shown in FIG. 6, and the surface release strain is 10 hours as shown in FIG. Overall, the point at which the strain (shrinkage) at the center of the material surface and the edge of the material surface reverses is determined as the end point of the high-temperature and low-humidity treatment, taking into account that the risk of surface cracking is low. Is done.

  FIG. 6 is a graph showing the progress of the set amount of the surface of the material to be dried, and FIG. 7 is a graph showing the progress of the release strain on the surface of the material to be dried, both of which are experimental data (high temperature and low humidity treatment process). ). As shown in FIG. 6, the number of drying sets on the surface starts to increase after a certain period of time. The fact that the drying set starts to increase indicates that the surface has started to compress due to the internal contraction. In FIG. 7, the release strain is the stress in the material remaining at that time, and FIG. 7 shows the release strain on the material surface. If the release strain is positive, the compression force remains, indicating that the risk of surface cracking is extremely small.

  In FIG. 6, the change to the set increase is about 20 hours from the start of the high-temperature and low-humidity treatment process, and almost coincides with the reversal of the strain at the material end and the center of the material to be dried. Further, as shown in FIG. 7, the release strain turns to plus in 10 to 15 hours from the start of the high temperature and low humidity treatment process. As a result, the high temperature and low humidity treatment time of 10 to 15 hours is considered to be sufficient. However, considering the progress of the drying set and the safety factor, about 20 hours, that is, determination based on strain was appropriate. Is recognized.

It is a graph which shows the relationship between time passage, temperature, and humidity in all the steps of the drying method according to the embodiment of the present invention. In the drying process of FIG. 1 , it is a graph which shows the condition of the distortion | strain (shrinkage rate) of the edge part and center part of a wood surface. (A) is sectional drawing which shows the shrinkage | contraction state of a material surface center part from a corner | angular part (material end) in drying of a timber, (b) is a graph which shows the shrinkage | contraction rate of both. (A) is sectional drawing which shows the shrinkage | contraction state of a material surface center part from a corner | angular part (material end) in the drying of the timber after predetermined time progress from the time shown in FIG. 3, (b) shows the shrinkage ratio of both. It is a graph. (A) is a cross-sectional view showing the contraction state of the center of the material surface from the corner (material end) in the drying of the wood after a predetermined time has elapsed from the time shown in FIG. It is a graph which shows the contraction rate of both. In the process shown in FIG. 1, it is a graph which shows progress of the set amount of a wood surface layer. In the process shown in FIG. 1, it is a graph which shows progress of the release strain in the timber surface.

51. . . . . . . . . . . . . Corner (material end)
52. . . . . . . . . . . . . Central surface portion 53. . . . . . . . . . . . . Inside the material

Claims (4)

A method for drying heart-holding materials,
B: Process of steaming heart-holding material b: Process of high-temperature and low-humidity treatment of wood that has undergone the steaming process,
C: Next, a main drying process is performed in which the heart-holding material that has been subjected to the high-temperature and low-humidity treatment is dried by lowering the temperature until the moisture content reaches a predetermined moisture content. Measure and compare the shrinkage rate at the center and end of the surface on either side of the heart support material, and when the shrinkage rate at the center of the surface of the heart support material exceeds the shrinkage rate at the end A method for drying a heart-supporting material, characterized by 2. The method for drying a heart support material according to claim 1, wherein strain gauges are respectively installed at the center portion and the end portion of the surface of the support material, and the dimensional change in the direction orthogonal to the fibers is monitored to thereby monitor the center portion and the end portion of the heart support material surface. A method for drying a heart-supporting material, characterized in that the respective shrinkage rates are calculated and compared to determine when the contraction rate at the center of the surface of the support material exceeds the contraction rate at the end. 3. The method for drying a heart-supporting material according to claim 2, wherein the strain gauge has a configuration function of measuring the distance between two points by puncturing a needle between any two points on the surface of the heart-supporting material. How to dry the heart. 4. The method for drying a heart support material according to claim 3, wherein the steaming process of the heart support material is continued at 90 [deg.] C. for 12 to 20 hours, and then the high temperature and low humidity treatment step is performed at a contraction rate at the center of the surface of the support material. After continuing to a time exceeding the rate, the main drying step is continued under the conditions of a dry bulb temperature of 90 ° C. and a relative humidity of 25% until the moisture content of the heart support material reaches the target. How to dry the heart.

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