JPH03181777A - Drying method for wood - Google Patents

Abstract

PURPOSE:To reduce a difference of moisture rate between a central part and a surface layer and shorten a drying time by a method wherein the central part is set to a high temperature state at a pre-drying through a high frequency heating operation and inner moisture is pushed to the surface layer, thereafter the moisture collected at the surface layer is evaporated out of the layer. CONSTITUTION:An inner part is heated by a high frequency wave to increase a vapor pressure and inner moisture is moved from a central part to a surface layer, resulting in that a moisture content rate is low at the central part and high at the surface layer under a pre-drying operation. Then, the moisture content is dispersed from the surface layer and a post-drying is carried out until the moisture rate at the surface layer becomes lower than that of the central part.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for drying wood in which wood is dried in two steps: pre-drying and post-drying.

[Conventional technology]

Conventionally, as a method for drying wood, an external heating method has been adopted in which hot air is applied from the outside into the shell to evaporate moisture from the surface layer of the wood.

In addition, high-frequency heating methods, which heat wood from the inside by applying high-frequency waves, are also used in some cases.

[Problem to be solved by the invention]

However, according to the above external heating method, the temperature distribution of the wood is high at the surface layer and low at the center, making it difficult for moisture to move from the center to the surface layer. A situation was occurring in which the water remained undried. For this reason, there has been a problem in that, after being put on the market as a dried product, it tends to shrink or become distorted when used in buildings or the like.

Furthermore, since the difference in moisture content between the surface layer and the center tends to be large, there is also a drawback that cracks tend to occur in the surface layer.

On the other hand, in the high-frequency heating method, contrary to the above-mentioned external heating method, the wood is heated from the center, so moisture moves actively from the center to the surface layer. According to this method, the center tends to become over-dry, which tends to cause internal cracks and charring. Therefore, in order to prevent the difference in moisture content between the center and the surface layer from becoming extremely large, high frequency waves are applied for a long time at low energy levels, which results in a longer drying time and poor energy efficiency. There was a problem of high cost.

SUMMARY OF THE INVENTION Therefore, the present invention provides a method for drying wood that can obtain a good drying condition with a small difference in moisture content between the central part and the surface layer, shorten drying time, and reduce drying costs. It is.

[Means to solve the problem]

In the present invention, the wood is internally heated by high frequency to increase the steam pressure, and the internal moisture is moved from the center to the surface layer. After pre-drying the wood until the moisture content is low in the center and high in the surface layer, By an external heating method, moisture is evaporated from the surface layer, and post-drying is performed until the moisture content of the surface layer is lower than that of the center.

(Function) In this way, in the pre-drying process using high-frequency heating, the central part is kept in a high temperature state and the internal moisture is pushed out from the central part to the surface layer to make the surface layer a high moisture content state, and then in the post-drying process by external heating. Since the moisture collected in the surface layer is evaporated to the outside, both the high-frequency heating time and the external heating time are shortened, improving drying efficiency and achieving a uniform drying state with a small difference in moisture content between the center and surface layers. Obtainable. Therefore, shrinkage and distortion are less likely to occur during use in buildings, etc.

Furthermore, compared to the conventional method of drying using only high-frequency heating, the drying time is shorter, less energy is required, and the drying cost is lower.

[Example] The present invention will be explained in more detail based on the following example.

Example 1 90aaX 90iigzX 4000 as material to be dried
The preservative injection I
Four types of samples with different (initial water content)
2. Dry several pieces at a time, and check the drying state of each (
moisture content distribution) was measured.

The results are shown in Table 1.

(Other drying conditions) ■ High-frequency heating - Five types of square lumber immediately after injecting preservatives were brought into a drying room equipped with a high-frequency shield, and 10
Rise to 0℃ (m degrees inside the square material) for 2 hours, 100℃
High frequency heating was performed while holding at 2 hours.

(2) External heating - Following high-frequency heating, the sample was transferred to another drying chamber and heated with hot air at a room temperature of about 50° C. for three consecutive days.

■Measurement method After finishing the external heating process, take a piece of each sample and heat it for 90 minutes.
In the cross section of jw +
The moisture content of the 90-am-thick back surface layer portion was measured by a total dry method. The initial average moisture content of each sample after injection is 113% for sample A, 110% for sample opening, and 129% for sample C.
Met.

As shown in the table above, despite the short time of 4 hours for high-frequency heating and 3 days for external heating, the maximum moisture content was 25% in the front surface layer (23% on average) and 26% in the back surface layer (
24% on average) and 33% in the center (32% on average), indicating that each sample had a highly dry state suitable as a dried product.

Further, the difference in moisture content between the surface layer and the center was within a small range of 8 to 9% on average, and a uniform dry state was obtained as a whole sample. This means that in conventional external heating methods, 2 to 4
This means that the drying time, which used to take weeks, has been shortened to 4 to 5 days.

Example 2 Same as Example 1 Hemlock 90MX 90MX 4000
Regarding the square timber of M, the same drying was carried out as in Example 1 with slightly different conditions of high frequency heating and preservative injection amount, and in addition, after high frequency heating (pre-drying) and subsequent external heating (post-drying). The drying state of each was investigated.The results are shown in Table 2.

Drying conditions (1) High frequency heating High frequency heating was performed by rising to -100°C for 3 hours and holding for 2 hours.

■ External heating - Same as Example 1.

(2) Measuring method - A piece of each sample was taken after the high-frequency heating and after the external heating, and the water content was measured in the same manner as in Example 1.

The initial moisture content after the preservative injection was as follows.

Sample 2...Front side average is 120%, back side average is 127%
, the median average is 100%.

Sample E...Front side average is 172%, back side average is 138%
, the median average is 160%.

Surface treatment: After high-frequency heating ■: After external heating As is clear from the table above, at the end of high-frequency heating, the internal moisture content is low in the center and high in the surface layer, and this relationship is reversed after the end of external heating. However, as in Example 1, it was relatively low at the surface layer and high at the center.

In order to understand this drying situation more clearly, the present inventor conducted more detailed measurements of sample 2 immediately after preservative injection, after high frequency heating, 1 day after external heating, 2 days after that, and 3 days after. The results (moisture content transition) are shown in the figure.

As shown in the figure, immediately after the preservative is injected, the moisture content is 100 to 120 in both the central and surface areas (only the surface side is shown).
% and has a high water content.

Next, due to high frequency heating, the moisture content decreases in both the center and surface layers, but internal heating removes moisture from the center to Table II.
By being extruded toward the center, the degree of decrease in moisture content was greater in the center, and water tended to collect in the surface layer.

Thereafter, with the start of external heating, the moisture accumulated in the surface layer evaporates, and the moisture content of the surface layer decreases in a sharp curve. On the other hand, the moisture content in the center decreases more slowly.

Then, after two days of external heating, the relationship of moisture content is reversed, being higher in the center and lower in the surface layer.

Furthermore, on the third day of external heating, the degree of dryness slowed down in both the surface layer and the center, and the final moisture content was 28% in the surface layer and 41% in the center. Further, according to this method, the drying stress generated in the center portion and the surface layer portion is small, and damage such as cracking and distortion can be suppressed to a small level.

For comparison, Table 3 shows the drying conditions obtained by the conventional method using only external heating using hot air.

In addition, all samples a, b, and c are Japanese hemlock 90 m x 90
Using ai+X40001w+ square lumber, each was dried on a drying schedule of 3 days of curing, 20 days of summer sun drying, and 3 days of hot air drying after injecting a preservative.

Table ■ = After 3 curing: After 2 drying in the sun, the unit of preservative injection amount is Ky/d) As a result, after complete drying, the average moisture content of each part is 23%, and 49% in the central region.

As is clear from this, according to the conventional method, it takes an extremely long time of 26 days for the surface layer to be sufficiently dried, and even after complete drying, the center still has a high moisture content and there is no difference between the surface layer and the surface layer. becomes larger.

Incidentally, in each of the above embodiments, wood that has been preservative treated before drying is taken as an example of the most common case, but the present invention is applicable to the case where the wood is dried from raw material without this preservative treatment. Of course, it is also applicable to

〔Effect of the invention〕

As described above, according to the present invention, the process is divided into two steps: a pre-drying step using high-frequency heating and a post-drying step using external heating using hot air. After that, the internal moisture that was difficult to remove is pushed out from the center to the surface layer, making the surface layer a high moisture content state. In the post-drying process, the moisture collected in the surface layer is evaporated, resulting in high drying efficiency and drying throughout the wood. as a highly dry condition, and
A uniform dry state with a small difference in moisture content between the center and surface areas can be obtained. Moreover, compared to the conventional method of drying using only high-frequency heating, this method can significantly shorten the drying time, reduce drying costs, and increase yields because there is less damage to the wood. .

[Brief explanation of drawings]

The figure is a graph for explaining the drying status of wood samples in experimental examples using the method of the present invention.

Claims (1)

[Claims] 1. By internally heating the wood using high frequency waves, the steam pressure is increased and internal moisture is moved from the center to the surface layer.
After pre-drying until the moisture content is low in the center and high in the surface layer, external heating is used to evaporate moisture from the surface layer until the moisture content in the surface layer is lower than that in the center. A method for drying wood characterized by post-drying.