EP1163481A2

EP1163481A2 - Method and apparatus for drying and/or thermally treating wood

Info

Publication number: EP1163481A2
Application number: EP00906391A
Authority: EP
Inventors: Arvo Ilmavirta
Original assignee: Lahden Ammattikorkeakoulu
Current assignee: Lahden Ammattikorkeakoulu
Priority date: 1999-02-22
Filing date: 2000-02-21
Publication date: 2001-12-19
Also published as: FI991443A; AU2807700A; FI110891B; WO2000050207A2; WO2000050207A3; FI991443A0

Abstract

The invention relates to a method and an apparatus for drying and/or thermally treating wood, wherein a batch of wood (2) to be processed is placed in a drying chamber (1) that is divided off from its surroundings, and energy is supplied to the chamber to dry the wood. In order to provide rapid drying, a negative pressure is generated in the drying chamber (1), and the energy used in the drying is supplied to the drying chamber (1) in the form of high frequency energy. When the moisture content has almost dropped to the final moisture level, the negative pressure is eliminated from the drying chamber (1), whereupon the final drying and/or thermal treatment are carried out by means of heat.

Description

METHOD AND APPARATUS FOR DRYING AND/OR THERMALLY TREATING

WOOD

The invention relates to a method of drying and/or thermally treating wood, wherein a batch of wood to be processed is placed in a drying chamber that is divided off from its surroundings, and energy is supplied to the chamber in order to dry the wood. The invention also relates to an apparatus for drying and/or thermally treating wood.

The drying of wood into a suitable production and manufacturing moisture content, and thermal treatment of the wood are central in the wood industry and usually constitute a very long process. The drying of timber can often take several weeks, wherefore large amounts of timber are preferably treated at once. Processing such large amounts of timber in one go requires great timber yards, which in turn call for a great deal of capital. Furthermore, it is not always possible to utilize all the advantageous manners of acquiring timber, such as buying batches of wood that would be otherwise suitable but that are small in quantity. Another drawback of the prior art is that not all the material that is dried will be actually utilized. When large amounts of wood are dried, also the volume of such material that will not be utilized in any way is substantial in the drying process. An object of the invention is to provide a method and an apparatus which eliminate the prior art drawbacks. This is achieved with the present invention. The method according to the invention is characterized by generating a negative pressure in the drying chamber, and supplying the energy used in the drying to the drying chamber in the form of high frequency energy by means of sheet-like or shaped electrodes, and after the moisture content has almost dropped to the final moisture level, eliminating the negative pressure from the drying chamber, whereupon the final drying and/or thermal treatment are carried out by means of heat or warm air. The apparatus according to the invention is in turn characterized in that the apparatus comprises first means for generating a negative pressure in the drying chamber, and second means for eliminating the negative pressure from the drying chamber, and sheet-like or shaped electrodes arranged on opposite sides of the batch of wood to be processed in order to supply high frequency energy to the wood. A drawback of the present vacuum drying technique is that at low pressures the energy required for evaporation cannot be transferred rapidly to the batch of material to be dried, especially to the inner parts of the wood This problem does not exist in the high frequency technique

A primary advantage of the invention is that the drying process is substantially faster than in the prior art methods This means in practice that in the present invention the time of drying is counted in hours, whereas the prior art methods require days and even weeks for the drying The rapid drying process enables fast circulation of the raw material which requires only a little capital Also, due to this rapid circulation it is possible to acquire timber that is exactly suitable for the purpose of a customer The fast drying of timber in the form of blanks that are ready for production also reduces costs, since only such wood that can be utilized is dried Drying of timber in the form of blanks requires that the drying is complete which has proved easy to achieve in the drying method according to the invention It is also advantageous that due to the invention, blanks for different products can be manufactured before the drying Another advantage of the invention is that the volume of the drying chamber is substantially smaller than in the conventional methods, which also reduces costs compared to the prior art

The invention will be described below in greater detail by means of the accompanying drawing, in which Figure 1 is a general side view of an apparatus according to the invention,

Figure 2 is a general view of an advantageous electrode arrangement in the apparatus according to the invention

Figure 3 shows a preferred embodiment of a side of an electrode in the apparatus according to the invention, and

Figure 4 shows a preferred embodiment of a corner of the electrode in the apparatus according to the invention

Figure 1 shows generally the apparatus according to the invention In the example of Figure 1 , a drying chamber which is divided off from its surroundings is denoted by reference numeral 1 and the batch of wood to be processed is correspondingly denoted by numeral 2 The batch of wood 2 to be processed can consist for example of boards, or blanks or some other prefabricated pieces formed before the drying In such a case all tne elements provide sufficient finishing allowance for manufacturing components of a final product The blanks or the prefabricated pieces are preferably stacked in the drying chamber such that either all the blanks or pieces in the same batch are alike, or that one batch comprises a sufficient number of all the blanks or prefabricated pieces required to manufacture ten products, for example. The batch of wood 2 to be processed can be inserted into the drying chamber via a suitable openable filling hole, which can be formed for example by making one end of the chamber openable. The filling hole is not shown in Figure 1.

The aforementioned blanks or prefabricated pieces can be sawn into shape. They are for example curved blanks for back legs of a chair, or other corresponding shaped pieces. The blanks or components are made from either unseasoned wood, wood that has been predried below the grain saturation point, or dry wood. As described above, the batch of wood to be processed can also consist of boards, for example. The batch 2 to be processed is piled inside the drying chamber 1 so that there are either no gaps or only small gaps between the pieces compared to a conventional load to be dried. According to the basic idea of the invention, a negative pressure is generated in the drying chamber 1 , and the energy required for the drying is supplied to the chamber in the form of high frequency energy. When the moisture content is near the final moisture level, the negative pressure is eliminated from the drying chamber 1 , whereafter the final drying and/or thermal treatment are carried out by means of heat. After the negative pressure has been eliminated, the pressure in the drying chamber 1 can also be increased to an excess pressure in order to reduce the evaporation of water and/or for the entire duration of the thermal treatment.

A negative pressure is generated in the drying chamber 1 by means of a vacuum pump 3. Reference numeral 4 denotes a control valve, which is controlled automatically, for example. Reference numeral 5 in turn denotes a cooler. High frequency energy is generated by means of sheet-like or suitably shaped electrodes 6, 7 and a high frequency generator 8. The electrodes 6, 7 are arranged on opposite sides of the wood to be treated. The negative pressure is eliminated from the drying chamber 1 by means of a loaded back pressure valve 9. This valve can be connected to a control valve 10, which can be controlled similarly as valve 4. In order to produce a higher excess pressure, the apparatus can be provided with an excess pressure pump 11 , which is also connected in the figure to a control valve 12 controlled similarly as valves 4 and 9. The apparatus of Figure 1 basically operates in the following manner. A batch of wood to be treated is inserted into the drying chamber 1 , which is thereafter closed and provided with a negative pressure by means of the vacuum pump 3. The batch of wood is dried by means of high frequency energy generated by the high frequency generator 8 and supplied to the drying chamber by the electrodes 6, 7. Since a negative pressure prevails in the drying chamber 1 , the water that is removed from the batch of wood boils at a temperature that is below the normal boiling temperature of water. The energy required for the evaporation of water is supplied with the electrodes 6, 7 to the parts of the wood that contain water, which is thereafter vaporized and removed from the wood in the form of steam. The moisture that is removed from the wood is conducted out of the drying chamber in a suitable manner, for example after condensation in the form of water via a discharge pipe or the like placed at the lower part of the drying chamber. When the final moisture content has almost been reached in the drying process, the vacuum pump 3 is first stopped and the cooler 5 is turned off. After the pressure has risen in the drying chamber, valve 4 is closed and valve 10 is opened, whereafter heat drying beings. By adjusting the load on the back pressure valve it is also possible to utilize a pressure that is higher than the atmospheric pressure in the drying chamber 1 , thus slowing down the evaporation of water. If a very high excess pressure is required, an excess pressure pump is used. In such a case the excess pressure must naturally be taken into account when designing the structure of the apparatus. The heat drying stage can also constitute only the stabilization phase of the process. The apparatus can also be used for combined thermal treatment and drying of timber. Since the apparatus is made to withstand an excess pressure as described above, the apparatus can be used after the drying for the thermal treatment of the timber, which can be carried out either at a normal pressure or under an excess pressure. The final moisture content can be stabilized in different manners by means of a suitable drying pattern. For example the following drying patterns are possible. The drying is first carried out by means of a high frequency field at a negative pressure of 0.01 to 0.3 bar. When the moisture content is almost at the target value, the pressure in the drying chamber is increased to 0.6 to 1.0 bar, and the process is continued. At the end the pressure can also be increased in one or more steps to 1.0 bar. Higher pressures and the resulting higher temperatures improve the uniformity of the high frequency field and slow down the drying at the final stage, thus stabilizing the final moisture content. The pressure in the drying chamber 1 can also be raised to an excess pressure, which expedites the stabilization during the final drying. The embodiment shown in Figure 1 utilizes electrodes 6, 7, the size of which corresponds to the dimensions of the batch of wood to be processed. However, electrodes that are of the same size as the batch of wood do not always provide a uniform high frequency field. Therefore in some situations it is advantageous in the invention to use electrodes which are larger in every direction than the batch of wood 2 to be processed. Such an arrangement is shown generally in Figure 2. The electrodes 6, 7 can be for example 50 to 200 mm greater in each direction than the batch of wood to be processed. The electrodes can also be smaller.

The electrodes 6, 7 can be shaped suitably to provide an advantageous final result. The corners and/or sides of the electrodes can be bent outwards, i.e. away from the batch of wood 2 to be treated, as shown in Figure 3. The corners of the electrodes can also be rounded, as shown in Figure 4. Naturally, the aforementioned details can be combined in one and the same construction, if required. The invention is particularly suitable for drying round timber and it provides a perfect drying result. Round timber elements, such as blocks, logs and other round blanks are stacked between the electrodes 6, 7 perpendicularly to or in parallel with the electrodes.

Due to the rapidity of the drying it has been difficult to control and especially terminate the drying at the correct degree of dryness, since the moisture content of the wood cannot be measured in any way in a high frequency field. The method according to the invention can be controlled as follows. The loss factor of the high frequency field for timber and thus the power consumption depend on the type of wood, the moisture content of the wood, the grain direction, and the amount of the wood. When a drying house is started up, the values of anode and gate currents of the high frequency generator with certain amounts and moisture levels of timber are measured at a particular negative pressure and temperature for each grain direction and with all the necessary electrode alternatives. When the amount of the timber and the grain direction are known in the drying process, and the anode and gate currents at a certain negative pressure and temperature are detected, the moisture content is also known. The apparatus thereafter automatically calculates the amount of water to be removed and the end time of the drying on the basis of the aforementioned data, and it also controls the drying so that no excess current or electric breakdown occurs. It is also possible to use in the invention changeable electrodes that are changed according to the dimensions of the batch of wood to be processed. The electrodes can also be changed during the drying process for example by stopping the drying for a while either manually or automatically without interrupting the actual drying process in the wood. The interruption of the drying process during the changing of the electrodes can be prevented for example by altering the pressure in the drying chamber 1. The final stabilization of the moisture remaining in the material to be dried can also be carried out in the drying chamber, which is provided with optimum conditions for the stabilization. Another possible place for final stabilization of the moisture content is a separate final stabilizing chamber, to which the batch of wood 2 is transferred at some point during the process.

The embodiments described above are not intended to restrict the invention in any way, but the invention can be modified freely within the scope of the claims. For example, the apparatus according to the invention or the details thereof do not have to be exactly as shown in the figures, but other arrangements are also possible.

Claims

1. A method of drying and/or thermally treating wood, wherein a batch of wood (2) to be processed is placed in a drying chamber (1) that is divided off from its surroundings, and energy is supplied to the chamber in order to dry the wood, characterized by generating a negative pressure in the drying chamber (1), and supplying the energy used in the drying to the drying chamber (1) in the form of high frequency energy by means of sheet-like or shaped electrodes (6,7), and after the moisture content has almost dropped to the final moisture level, eliminating the negative pressure from the drying chamber (1), whereupon the final drying and/or thermal treatment are carried out by means of heat or warm air.

2. A method according to claim 1 , characterized by increasing the pressure in the drying chamber (1) to an excess pressure after the negative pressure has been eliminated, in order to decrease the evaporation of water and/or for the duration of the thermal treatment.

3. A method according to claim 1 or 2, characterized by eliminating the negative pressure from the drying chamber (1) step by step according to a drying pattern.

4. A method according to claim 3, characterized by performing the drying at first at a pressure of 0.01 to 0.3 bar, and after the moisture content has almost reached the final moisture level, increasing the pressure to 0.6 to 1.0 bar.

5. A method according to claim 4, characterized by increasing the pressure to 1.0 bar in a single step.

6. A method according to claim 4, characterized by increasing the pressure to 1.0 bar in several steps.

7. A method according to any one of claims 1 to 6, where the batch of wood to be processed consists of round timber, characterized by piling the round timber elements between the electrodes (6, 7) either perpendicularly thereto or in parallel therewith.

8. A method according to claim 1 or 7, characterized by at first measuring the values of anode and gate currents of a high frequency generator (8) connected to the electrodes (6, 7) with certain amounts of wood and at certain moisture levels at a given negative pressure and temperature for each grain direction and with the required electrode alternatives, and determining the moisture content on the basis of said data, calculating the amount of water to be removed and the end time of the drying, and controlling the drying on the basis of said data in order to prevent the occurrence of excess current and electric breakdown.

9. An apparatus for drying and/or thermally treating wood, wherein a batch of wood (2) to be processed is arranged to be placed in a drying chamber (1) that is divided off from its surroundings, and energy is arranged to be supplied to the chamber in order to dry the wood, characterized in that the apparatus comprises first means (3, 4) for generating a negative pressure in the drying chamber (1), and second means (9, 10) for eliminating the negative pressure from the drying chamber (1), and sheet-like or shaped electrodes (6, 7) arranged on opposite sides of the batch of wood (2) to be processed in order to supply high frequency energy to the wood.

10. An apparatus according to claim 9. characterized in that the electrodes (6, 7) are larger in every direction than the batch of wood (2) to be processed.

11. An apparatus according to claim 9, characterized in that the electrodes (6, 7) are smaller in every direction than the batch of wood (2) to be processed.

12. An apparatus according to any one of claims 9 to 11, characterized in that the corners and/or sides of the electrodes (6, 7) are bent away from the batch of wood (2) to be processed.

13. An apparatus according to any one of claims 9 to 12, characterized in that the corners of the electrodes (6, 7) are rounded.

14. An apparatus according to any one of claims 9 to 13, characterized in that the electrodes (6, 7) are elements arranged to be changed according to the dimensions of the batch of wood (2) to be processed.

15. An apparatus according to claim 14, characterized in that the electrodes (6, 7) are arranged to be changed during the process.