FI103834B - Method for drying wood - Google Patents

Description

Method for drying wood 103834

The present invention relates to a method 5 for drying wood according to the preamble of claim 1.

According to such a method, the wood is placed in a drying oven where it is dried at elevated temperature and in the presence of steam.

10 Traditional wood drying solutions have been based on the "gentle" drying of wood into demanding carpentry products. Said drying methods are represented by e.g. export drying, vacuum drying, condensation drying. Wood drying techniques have sought to control the wood drying process, e.g. by monitoring the difference between dry and wet temperatures. In order to speed up the drying, it has not been possible to raise the temperatures much above 100 to 15 ° C for fear of internal cracks.

To provide faster drying techniques, methods have been developed which promote evaporation of water at lower temperatures or utilize other techniques, such as the condensation drying mentioned above. Instead, the faster heat transfer: ': 20 to the wood and keeping the wood surface moist for as long as possible is not:: paid attention.

. · I · el »· 1: There are significant drawbacks to known solutions. • · long drying times (typically 5-7 days for normal spruce drying) and a high total energy requirement.

The object of the present invention is to eliminate the drawbacks of the prior art and to provide a completely new solution for the rapid drying of wood.

The invention is based on the idea that, with the exception of carpentry products, most: · '·: in practical wood applications, e.g., when using wood as a building material, '' · 'dried wood may be allowed to discolour and resin. However, even in these applications':: internal cracks in the wood must be avoided. Accordingly, it is an object of the invention to provide a 103834 2 rapid drying method for efficient and rapid drying of the wood, the method being limited to preventing drying cracks.

It has been surprisingly found in the invention that the desired object is achieved by using steam to protect wood and promote its warming in drying, and by controlling the difference between wood surface and indoor temperature.

The invention is therefore based on a combination of the following three steps: a) first the temperature of the drying furnace is raised to at least about 90 ° C, preferably at least 100 ° C, and maintained at this value until the wood temperature reaches at least approximately the same temperature; gradually increasing so that the temperature difference between the internal temperature of the wood and the furnace temperature does not exceed 30 ° C until the desired humidity of the wood is reached; and c) finally reducing the temperature of the furnace gradually so that the temperature of the wood temperature.

More specifically, the solution according to the invention is essentially characterized by the character '-...' as set forth in the characterizing part of claim 1.

!!! ! 20 '·' · '·' In the first step (a-step) of the process of the invention, the temperature of the drying oven 4 4 ··· t / · is preferably set at about 100-150 ° C, preferably at 100-120 ° C.

• 44 «• 4 1 * ···· * · 4 4 * · 4

In the second step (step b), heating is stopped when the moisture content of the wood is low enough for the desired use. Generally a humidity of about 1-20%, preferably less than about 15% humidity, typically about 10-15% humidity, is sought. In step B, the temperatures of the inner wood and the outer surface of the wood can be determined continuously by means of sensors, e.g. , the difference in bumps can be kept consistently low based on the measurement results.

In practice, the temperature control in the drying furnace can be implemented by means of a heating program determined on the basis of preliminary sensor measurements.

ί • 4 4 4 3 103834 The temperature difference is monitored both in the heating step b, where the wood temperature is raised to dry the wood, and in the cooling step (c), where the wood is cooled back to the base temperature. Preferably, said temperature difference is maintained at a maximum of about 30 ° C, most preferably about 10-30 ° C. Too little temperature difference prolongs the dry-drying process, while a large difference increases the risk of internal cracks. It has surprisingly been found that the proposed solution (i.e., a difference <30 ° C) can even completely prevent the formation of cracks in the wood.

The "indoor temperature" of a wood refers to the temperature of the wood some distance 10 from the outside of the wood. Depending on the shape of the wood body, the "inside temperature" is typically measured in the center of the body or at a depth of at least about 2 to 5 cm. "Outdoor temperature", in turn, refers to the temperature on or slightly below the wood surface (typically not more than 1 to 2 cm). Thus, in many cases, the "outdoor temperature" substantially corresponds to the ambient temperature.

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In step c, the furnace temperature is lowered until the internal wood temperature has dropped below 100 ° C. As noted above, the difference between the internal and external temperatures of the wood should be maintained at 30 ° C even during the cooling phase in order to avoid the formation of internal cracks.

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20 • I I «: · During the steps a, b and c, the steam is supplied to the furnace. wet bulb temperature

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'; 1; is maintained at about 80-120 ° C, preferably about 100 ° C. Preferably saturated: ·. '' 1 'water vapor is used.

The invention provides significant advantages, the most important of which is cost savings in the form of rapid drying. In addition, adjustments are achieved by making dry wood material that meets the requirements of end use and is not over-quality.

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In the following, the invention will be further examined with reference to the accompanying drawings and

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30 with a few application examples.

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Figure 1 shows the temperatures measured at different points of the wood as a function of time drying 4,103,834 fresh six hours according to the prior art, Figure 2 shows the corresponding temperature measurements of fresh spruce dried according to the invention, Figure 3 shows the measurement result Figure 5 shows the temperature measurement results for the wound obtained by the method according to the invention.

Example 1 10 Drying of fresh spruce

When wet spruce (50 x 100 x 1500 mm), initial humidity of about 40%, was heated for 5 hours as shown in Figure 1 without any limitation of the temperature difference between the inner and outer surfaces, small internal cracks were observed in the specimen splitting.

After drying, the humidity of the specimen was less than 5%.

When a similar sized specimen from wet to dry was heated for 24 hours in accordance with the invention, with the aid of a control system so that the temperature difference between the inside and the surface was 10-20 degrees, there were no cracks in the specimen; %.

Example 2 *: 1 '·: Drying of the Wound (15% Humidity) 25 When Dry Ash (38 x 100 x 1500 mm) was dried for 4 hours using a drying solution according to the invention, resulting in a fast and controlled drying, with no cracks in the lumber (Fig. 4). When similar drying was carried out without paying attention to the difference between the internal and external temperatures of the timber (Figure 3),

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';;; small cracks inside.

.30 · '·: In both cases the final moisture content of the specimens was less than 5%.

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Claims (5)

5, 103834 A method for drying wood comprising placing the wood in a drying oven at which it is dried at an elevated temperature, wherein the temperature of the drying oven is first raised to at least 100 ° C and maintained at this temperature until the wood temperature reaches at least approximately the same temperature, when the temperature of the wood has reached at least approximately the temperature of the drying furnace, the furnace temperature is gradually increased so that the difference between the wood core temperature and the furnace temperature does not exceed 30 ° C until the desired wood humidity is reached; the difference exceeds 30 ° C until the internal temperature of the wood reaches the desired temperature, characterized in that the wood is dried in the presence of steam by supplying steam to maintain the wet temperature at about 80-120 ° C. 15 Method according to Claim 1, characterized in that the furnace is fed with saturated water vapor, which maintains the wet temperature at about 100 ° C. Method according to Claim 1 or 2, characterized in that the heating of '· ·' 20 is stopped when the moisture content of the wood is from 1 to 20%. «· · '; 4. Method according to one of Claims 1 to 3, characterized in that the furnace temperature is reduced until the internal temperature of the wood has dropped below 100 ° C. «* ·« ·. · · • · · · V * 1 · f I * «t • ·· · · A method according to any one of the preceding claims, characterized in that said temperature difference is maintained at 10-30 ° C. • · · · · · · · · · · · t <<II · I I «« ·; V. i 6 103834