DE69501588T2 - METHOD FOR WORKING WOOD AT RISED TEMPERATURES - Google Patents

Abstract

The invention relates to a method for processing wood at an elevated temperature. According to the method, the temperature of the wood is raised to a value at least exceeding 100 DEG C, whereby the temperature of the internal part of the wood and the temperature at the external surface of the wood, respectively, is measured during the treatment, and in raising the temperature, the difference between the internal and external temperatures of the wood is kept at least essentially constant at 10 to 30 DEG C. The treatment is preferably carried out in the presence of vapour. The invention can be used to effectively prevent the generation of internal cracks and the method can be applied for, e.g., rapid seasoning of wood as well as for modifying thermal processing of wood.

Description

The present invention relates to a method according to the preamble of claim 1 for treating wood at elevated temperature.

Wood is processed at elevated temperatures in conjunction with, for example, aging, thermal modification and generally various processing techniques. Wood expands when heated. However, the traditional view is that thermal expansion is a small-scale phenomenon and therefore it has not been considered in conjunction with thermal treatments. In fact, only the longitudinal expansion of wood occurs on a small scale; the expansion across the grain even exceeds that of aluminum. The significant thermal expansion in the cross-grain direction of wood easily causes internal cracks if it is not taken into account in the development of various types of processes.

The object of the present invention is to overcome the disadvantages hindering the prior art methods and to develop a concept for controlling the phenomena associated with the thermal expansion of wood, in particular to prevent the generation of internal cracks in wood when wood is heated to a temperature exceeding 90ºC, typically exceeding 100ºC.

The invention is based on the principle that, during the processes when wood is kept at an elevated temperature, the temperatures of the wood core and the outer surface of the wood, respectively, are continuously determined and the difference between the two temperatures is kept reasonably small, which also takes place on a continuous basis. These measures are carried out both during the heating step, where the temperature of the wood is increased for e.g. seasoning, and the cooling step, where the wood is cooled back to its base temperature. The temperature difference is preferably at most about 30ºC. Surprisingly, it has been found that this concept can even be used to completely prevent the generation of internal cracks in the wood material.

In particular, the method according to the invention is mainly characterized by the information in the characterizing part of claim 1.

In the context of the present invention, the term "internal temperature" of wood is used to refer to the temperature of the wood at a certain distance from the external surfaces of the wood. Depending on the shape of the piece of wood, the "internal temperature" is typically measured at a depth of at least about 2 to 5 cm.

The "external temperature" is the temperature measured at or slightly below the surface of the wood. In many cases, the "external temperature" essentially corresponds to the ambient temperature, for example the temperature in a drying oven.

The arrangement according to the invention can be carried out, for example, in a drying oven provided with an adjustable temperature and a device for steam emission.

When the prior art methods are used to dry, for example, room-dry wood at about 200ºC, it quickly reaches the same surface temperature, that is, about 200ºC. However, the heating of the internal parts of the wood stops at about 100ºC until the moisture content of the internal parts of the wood is reduced to a value below 5%. Only then does the temperature of the internal part of the wood begin to rise rapidly. As a result of the heating, the combined effect of the temperature differences and the degree of dryness of the wood causes internal cracks that are not visible on the outside but make the wood unusable.

According to the invention, the temperature is gradually increased so that the temperature difference between the surface and the inner part of the wood must not exceed 30ºC. The temperature difference is advantageously kept constant at a value between 10 and 30ºC. In practice, the temperature difference is kept constant within a limit of at least about 5ºC, depending on the thermostatic regulators used, the external temperature being varied together with the increasing internal temperature.

The concept according to the invention can be implemented by providing a test piece with at least two sensors, one sensor for measuring the internal temperature and the other sensor for the surface temperature of the wood. When storing larger quantities of timber, it is advisable to provide several test pieces with sensors. In industrial-scale arrangements, the measurement results are used as a basis for developing a suitable heating program for each type of wood, observing the effect of the initial moisture content on the process.

According to an alternative embodiment, the actual thermal treatment according to the invention is not started until the moisture content of the wood core has fallen to a value below 30%, advantageously below 10%, in some cases below the above-mentioned limit at 5%. Thus, during the first step of this alternative process, a piece of wood at room or factory temperature is placed in a drying oven at a temperature of about 110 to 140ºC to remove the majority of the water. Next, when the internal temperature of the wood has risen to at least about the said value, the ambient temperature, that is, for example, the temperature of the drying oven, is gradually increased while the difference between the internal and external temperatures of the wood is kept constant at a value below about 30ºC.

The inventive concept is particularly applicable in connection with hot drying and thermal processing of wood. During high-temperature drying, wood is dried until it has a moisture content of, for example, less than 15%, in some cases advantageously less than 10%.

When incorporating the process, it is advantageous to use steam throughout the treatment, keeping the relative humidity extremely high and the oxygen content in the ambient air low.

According to a particularly advantageous embodiment of the present invention, a solution is found for rapidly decaying wood.

Traditional solutions for seasoning wood were based on a "soft" deposition of wood to high quality joinery wood. Examples of these drying methods are air drying/oven drying, vacuum drying and condensation drying. In these drying methods, the aim was to control the drying process by monitoring, among other things, the difference between the wet and dry temperatures. For fear of internal cracks, it was not possible to increase temperatures to a large extent above 100ºC to speed up the seasoning process. In order to obtain faster drying methods, methods were developed in which the evaporation of water is intensified at lower temperatures or another type of method is used, e.g. the condensation drying method cited above, while no attention was paid to achieving a faster transfer of heat to the wood, nor was the wood surface kept wet for as long as possible.

The state of the art processes suffer from significant drawbacks, including the long drying times required (normal drying of spruce wood typically takes 5 to 7 days) and the high overall energy consumption.

An advantageous embodiment of the present invention can be combined with an elimination of the problems of the usual drying processes. The invention is based on the finding that, apart from joinery wood, color changes and resin exudates are acceptable for most practical wood applications, for example when wood is used as a building material. However, even in these applications it is necessary to prevent the generation of internal cracks in the wood.

Consequently, when applying the rapid drying process of the invention, which is used to dry wood effectively and rapidly, only the prevention of drying cracks is considered. It has been found that this objective can be achieved by using steam to protect the wood and to help warm it up during the seasoning, and by controlling the difference between the surface and internal temperatures of the wood in the manner described above.

The deposition process is based on the combination of the following three steps:

a) firstly, the temperature of the drying oven is increased to at least about 90ºC, advantageously to at least 100ºC, and then maintained at that value until the wood reaches at least approximately the same temperature,

b) then the temperature of the oven is increased progressively so that the difference between the internal temperature of the wood and the oven temperature remains constant and does not exceed 30ºC until the desired wood moisture content is reached, and

c) finally, the oven temperature is reduced so that the difference between the internal temperature of the wood and the oven temperature remains constant at 30ºC until the internal part of the wood reaches the desired temperature.

During the first step (step a) of the deposition process, the oven temperature is advantageously set at a temperature between 100 and 150ºC, preferably in the range of 100 to 130ºC. Heating is continued until the moisture content of the wood has been reduced to a value below at least 30%.

During the second step (step b), heating is stopped when the wood shows a sufficiently low moisture content with regard to the desired use. Generally, a moisture content of about 1 to 20% is desired, preferably below 15% and typically between about 10 and 15%. During step b, the temperatures inside the wood and on the outside surface of the wood can be determined accordingly continuously with e.g. sensors attached to the piece of wood, whereby the difference between the temperatures can be continuously kept moderately small on the basis of the measurement results.

During step b, the temperature can be increased to a value exceeding 130ºC, even 150ºC, whereby the time required to equalise the wood temperature can be reduced, since the drying of the inner part of the wood is faster at higher temperature is more effective, thereby reducing the differences in moisture content between the core and the surface.

The difference between the temperatures is monitored both during the heating step b, where the temperature of the wood is increased to dry the wood, and during the cooling step (step c), during which the wood is cooled back to its base temperature. The temperature difference is advantageously kept at a constant value of at most about 30ºC, preferably about 10 to 30ºC. Too small a difference in temperature prolongs the deposition process, while a large difference increases the risk of generating internal cracks. It has been found that the solution disclosed here (i.e. temperature difference < 30ºC) can be applied to even completely prevent the formation of cracks in the wood.

Step c involves reducing the oven temperature until the internal temperature of the wood has reached a value below 100ºC. As stated above, it is advantageous to keep the difference between the internal and external temperatures of the wood less than 30ºC even during the cooling step in order to prevent the generation of internal cracks.

During steps a, b and c, steam is introduced into the furnace and the so-called wet bulb temperature is maintained at about 80 to 120°C, preferably about 100°C, using this steam. The same introduction of steam is also applicable to other points of the process according to the invention.

The present invention offers considerable advantages. For example, the invention can be used to accelerate normal deposition, to control special deposition processes at temperatures exceeding 100ºC and to develop deposition applications under pressure. Importantly, the generation of internal fractures in the treated wood can be completely prevented in practice during deposition and heat treatment.

In the following, the invention is examined in detail with reference to the attached drawings.

Fig. 1 illustrates the temperatures measured at different points in the wood as a function of time during heating and during storage of fresh spruce wood at 220ºC for 5 hours according to the state of the art,

Fig. 2 illustrates the corresponding temperature measurements with non-seasoned spruce wood heated to the same temperature for 24 hours, whereby according to the invention the temperature difference between the inner and outer parts of the wood was kept at a value less than 30ºC,

Fig. 3 illustrates the measurement results corresponding to those of Fig. 1 (obtained using conventional methods) for the seasoning of aspen wood, and

Fig. 4 shows the temperature measurement results obtained for aspen wood using the method according to the invention.

example 1 Storing fresh spruce wood and treating it at elevated temperatures.

When wet spruce wood (50 x 100 x 1500 mm) with a moisture content of about 40% was heated for 5 hours according to Fig. 1 without restricting the temperature difference between the inner parts and outer surfaces, small internal cracks were observed when the test piece was split. As a result of the treatment a final moisture content of less than 5% after deposition was obtained.

When a test piece of wet spruce wood with the same dimensions was heated for 24 hours according to the invention, implementing the inventive concept of keeping the difference between the internal and external temperatures at 30ºC, no cracks were generated in the test piece (Fig. 2). The final moisture content of the aged test piece was less than about 5%.

Example 2 Storage of aspen wood and treatment of it at elevated temperatures.

When seasoned aspen wood (38 x 100 x 1500 mm) was heated for 7 hours, implementing the inventive concept, a rapid and controlled seasoning was obtained and no cracks were generated in the wood (Fig. 4). When the corresponding thermal treatment was carried out for 3 hours without in any way limiting the difference between the internal and external temperatures (Fig. 3), small internal cracks were found on the inner parts of the pieces, such cracks making the wood unsuitable for use in practice.

In both cases, the test pieces showed a final moisture content of less than 5%.

Claims (11)

1. Process for treating wood at elevated temperature, wherein the temperature of the wood is increased to at least over 100 ºC, characterized in that - during the treatment, the temperatures in the inner part of the wood or on its outer surface are measured, and - when the temperature increases, the difference between the internal and external temperatures of the wood is essentially kept at a constant value of 10 - 30 ºC. 2. Process according to claim 1, characterized in that the wood is treated in the presence of steam. 3. Method according to claim 2, characterized in that steam is used whose wet temperature is kept at about 80 - 120 ºC, preferably at about 100 ºC. 4. Method according to one of claims 1 - 3, characterized in that the temperatures of the inner part of the wood and the outer surface of the wood are continuously determined and when the temperature of the wood increases, the temperature of the outer surface of the wood is changed as a function of the inner temperature of the wood in such a way that the difference between the two temperatures is kept constant at 10 - 30 ºC with a margin of at most about 5 ºC. 5. Method according to one of claims 1 - 4, wherein the temperature of the wood is first increased and then reduced to a desired value, characterized in that the difference between the internal and external temperatures is kept constant at 10 - 30 ºC both during heating and cooling of the wood. 6. Process according to claim 1, characterized in that the wood is subjected to drying before the heat treatment in order to reduce the moisture content of the wood to below 30%. 7. Process according to claim 1, wherein the wood is dried in a drying oven in the presence of steam, characterized in that (a) the temperature of the drying oven is first raised to at least 100 ºC and then maintained at that value until the wood reaches at least approximately the same temperature, b) then the temperature of the oven is increased progressively so that the difference between the internal temperature of the wood and the oven temperature remains constant at 10 - 30 ºC until the desired wood moisture content is reached, and c) finally, the oven temperature is progressively reduced so that the difference between the internal temperature of the wood and the oven temperature remains at a constant value of 10 - 30 ºC until the internal part of the wood reaches the desired temperature. 8. Process according to claim 7, characterized in that during phase a) the temperature of the drying oven is set at 100 - 120 ºC and maintained at this value until the temperature of the wood reaches this temperature range and a moisture content of less than 30%. 9. Method according to claim 7 or 8, characterized in that during phase b) the heating is stopped when the wood has a moisture content of 1 - 20%. 10. Process according to one of claims 7 - 9, characterized in that during phase b) the wood is heated to a temperature of at least 130 ºC. 11. Method according to one of claims 7 - 10, characterized characterized in that during phase c) the temperature of the oven is reduced until the internal temperature of the wood reaches a temperature below 100 ºC.