EP1373816A1

EP1373816A1 - Method for drying timber

Info

Publication number: EP1373816A1
Application number: EP02708399A
Authority: EP
Inventors: Paavo Kontinen
Original assignee: Tekmawood Oy
Current assignee: JAERME WOOD Oy
Priority date: 2001-03-26
Filing date: 2002-03-26
Publication date: 2004-01-02
Also published as: FI20010611A0; CA2441553A1; WO2002081992A1

Abstract

A method for drying solid-wood timber (3), wherein the timber is compressed between pressing elements (10, 12) during the drying to reduce cracking, in which method the timber is compressed by an amount at least equal to the drying shrinkage of the timber, and in which method the timber consists of pieces of timber placed between the pressing elements and dried at the same time. By means of the pressing elements, the timber (3) is only compressed. The pieces of timber (3) are arranged between the pressing elements in one or more tiers in the pressing direction so that the pieces of timber in the same tiers are pressed against each other during the drying process.

Description

METHOD FOR DRYING TIMBER

The present invention relates to a method for drying solid-wood timber as defined in the preamble of claim 1 , wherein the timber is compressed during the drying process.

The drying of solid-wood timber, such as planks, is carried out by heating the timber. Typically, the drying is implemented by first heating the timber at a lower temperature, e.g. 60°C, whereupon the temperature is raised gradually to a drying temperature, which may typically be e.g. 70°C.

In the drying of timber, a big problem is the cracking of the timber. In prior-art methods, a "cautious" drying scheme is used in order to avoid cracking. In connection with the drying of timber, 'drying scheme' refers to the values to which the humidity of the drying air (the difference between dry-bulb and wet-bulb temperatures) is adjusted. A feature characteristic of the prior-art cautious drying scheme is that, at the beginning of the drying process, the relative humidity of the air is high and it is reduced with the progress of the drying, the drying efficiency being thus increased. During the drying process, the humidity of the drying air is monitored by measuring the so-called "dry-bulb temperature" and "wet-bulb temperature". For example, for pine or fir timber having a thickness of 50-mm, the difference between these temperatures is at present typically 4 - 5°C at the beginning of the drying process and about 15°C at the end of it.

As it is necessary in drying processes generally used at present to employ a relatively "cautious" drying scheme to avoid producing cracks in the timber, the drying periods are long. For example, if pine planks having a cross-section of 50 mm x 100 mm are dried to a 13-% final moisture level, then the drying period is typically 7 - 8 days.

When the drying temperature is raised to a level exceeding 100°C, the process is called hot drying, and clearly shorter drying periods are possible, but in this case the problem with current methods is relatively heavy cracking of the timber, and especially internal cracking. The risk of cracking is the higher the thicker is the timber being dried. One solution for reducing cracking in connection with the drying of timber is to compress the timber while it is being dried. As the timber is under compression during the drying process, it cannot undergo any warping or bending. Furthermore, it retains its rectangular cross-sectional form, and e.g. a plank cannot be- come convex, which means that less raw material is wasted at planing time. Compression drying also allows a sawing procedure in which the core of the log remains in the middle of a plank. In this case, the core of the log and the juvenile wood near it, which is the weakest part of the wood material in respect of its properties, will remain completely hidden inside the plank and will not cause any harm in later use.

Specification WO 94/26485 discloses a method for form-drying of timber, especially planks, in a hot press comprising an upper press plate, upper supporters supporting the upper press plate, a lower press plate and hydraulic cylinders for moving it. The surfaces of the press plates are heatable. Both press surfaces are covered with screens permeable to steam. The planks to be pressed are placed between the screens, and the pressing stroke is limited by stoppers placed at the ends of the press plates. In the method, moist timber is pressed into a desired shape while the timber is heated at the same time. The method consists of two stages. During the first stage, the moist timber is pressed quickly into the desired shape under a high pressure against the gas-permeable surfaces until the final dimensions are reached, and after this pressing stage the timber is allowed to expand towards its original size. After this, the pressing is continued using a low pressure, which produces a change at least as large as the shrinkage resulting from the drying of the timber.

A drawback with the solution according to the specification in question is especially the fact that the heat is conducted into the timber via the press plates, the compression and heating of the timber being thus both performed by means of the press plates, i.e. as it were from the same source, which limits the usable heat sources to those in which the heat can be conducted via the press plates. It is not possible to place several pieces of timber side by side or one over the other in the pressing direction between the press plates. In addition, the method is relatively slow, and when it is used, cracking still occurs in the timber, espe- cially in the drying of thicker timber, so the method is not applicable for the drying of e.g. building logs. The object of the invention is to achieve a method for the drying of solid-wood timber that can be used to dry sawn timber as well as other timber quickly and so that the cracking occurring in the timber can be minimized. The method of the invention is based on the idea that the timber is compressed at least as much as the wood would shrink even otherwise when drying, and that the compression and drying of the timber are performed by means of separate devices. The drying heat is supplied from a different source and not via the press plates as in specification WO 94/26485, so the drying of the timber can be performed using any drying method, to which only devices for producing the necessary compression are added. For drying, several pieces of timber are placed one over the other in the pressing direction by so that they are pressed against each other.

The details of the features characteristic of the method of the invention are pre- sented in the claims below.

This allows a decisive expansion of the possibilities of application of the method. Different embodiments of the invention can be utilized in various processes, allowing economical solutions to be achieved when large amounts of timber are to be dried.

The drying method disclosed in the present invention provides good possibilities of achieving more efficient drying of timber. The essential point is that the invention makes it possible to prevent cracking, both surface and internal crack- ing, allowing the drying to be effected using a drying scheme as efficient as possible, thereby substantially reducing the drying period required. For example, when the drying temperature is 70°C, then the wet-bulb temperature is 35°C or less, and when the drying temperature is 150°C, then the wet-bulb temperature is 90°C or less. The drying can also be performed using high tem- peratures, which further reduces the drying periods. In respect of control of the drying, the method disclosed also simplifies the drying process as it allows continuous use of a simple drying scheme as efficient as possible. Using the method of the invention, effective drying of even large pieces of timber, such as balks, is possible.

In the following, the invention will be described in detail by the aid of examples with reference to the attached drawings, wherein Fig. 1 presents a chamber drier designed for the drying of solid-wood timber, in which a method according to the invention for the drying of building logs is use,

Fig. 2 presents an apparatus used for testing the method of the invention and illustrates the implementation of drying tests using planks placed edge to edge,

Fig. 3 presents an apparatus used for testing the method of the invention and illustrates the implementation of drying tests using planks placed with their flat sides against each other,

Fig. 4 presents an embodiment of the method of the invention wherein several logs placed one over the other can be dried simultaneously, and

Fig. 5 presents an embodiment of the method of the invention wherein several logs placed side by side can be dried simultaneously.

The drier presented in Fig. 1 comprises a chamber 2 placed in a drying plant building 1 , which chamber is used for the drying of timber, such as balks 3 (not shown in scale), planks or boards. At least one end of the chamber is provided with a door 4. The chamber is provided with radiators 5 together with the associated piping and control valves for heating the air, and parallel fans 6 controlled e.g. by a frequency converter and serving to create an air circulation of a desired type in the chamber. In addition, the chamber 2 may be provided with a moistening apparatus 7 and exhaust and fresh air ducts 8.

According to the invention, timber is dried in a drying plant by heating the timber and simultaneously compressing it by means of a pressing apparatus 9 (not in scale). The compression pressure of the pressing apparatus is adjusted by means of a control system (not shown) to a level such that it produces a contraction of a magnitude, optimally 4 - 6 %, that is equal to the shrinkage resulting from drying of the timber to prevent cracking, cupping, bending and warping of the timber.

The pressing apparatus comprises a press plate 10, hydraulic cylinders 11 and their control system for moving the plate, a fixed press plate 12 and a supporting structure 13. The timber 3 to be pressed is placed between the press plates 10 and 12 in one or more tiers and pressed by means of the hydraulic cylinders

11 from the direction of the arrow while being simultaneously heated. If there are several tiers, laths 18 are placed between tiers. The pressing can be performed in one stage, i.e. the timber is compressed continuously until the final dimension is reached, without letting it resile towards its original dimension before that. The compression can be started with a lower pressure, and the pressure is then increased during the drying process.

Fig. 2 and 3 present a laboratory-size apparatus, which has been used to carry out drying tests. The apparatus comprises a framework 21 , a hydraulic press 22 producing the required pressure, and timber 23 to be dried in the press, the pieces of timber being packed with their edges 24 (Fig. 2) or their flat sides 25 (Fig. 3) placed against each other. The drying is implemented by pushing the entire framework into a drier, applying a pressure and starting the drying proc- ess. In the following, two practical examples will be given to describe drying tests.

Example 1. The sawn timber to be dried consisted of pine planks measuring 50 mm x 100 mm. The planks were placed in the drier in the manner shown in Fig. 2, with their edges 24 against each other. The drying temperature was 70 °C. The drying process was started by a heating stage, during which the temperature was increased to 70°C. The duration of the heating stage was 2 h, and during this stage steam was supplied into the drier, so that no drying occurred at this stage. After the heating stage, a drying scheme as efficient as possible was used. The control values were such as to keep the temperature at 70°C, and the exhaust and fresh air slides were fully open. Upon the lapse of 2 h after the start of the drying, the "wet bulb temperature" was about 45°C, so the difference between the dry-bulb and wet-bulb temperatures was 25°C, and after a nearly full day of drying the wet-bulb temperature was 35°C, so the difference between the dry-bulb and wet-bulb temperatures was 35°C, and it remained so until the end of the drying process. The total drying time was 76 h. The compression pressure used was 3,3 bar. The average final moisture level was 13.5%. The dimensions of the planks before and after the drying are shown in Table 1.

Table t

No surface or internal cracks had appeared in the planks.

When a conventional drying method is used, drying a plank dimensioned 50 mm x 100 mm at 70°C to a 13% final moisture level takes about 179 h. Thus, by the drying method of the invention, the drying time can be reduced to less than half the time needed in conventional drying.

The thickness and width of the planks have changed so that the thickness has been slightly reduced and the width has been reduced by 5%. As compared with conventional drying, the total shrinkage is of the same order, but shrinkage has only occurred in one direction (i.e. in the direction in which the plank was compressed), which has to be taken into account when determining the sawing dimension of the plank.

Example 2. Pine planks dimensioned 50 mm x 100 mm were dried using the compression method of the invention. The planks were placed in the drier in the manner shown in Fig. 2, with their edges against each other. The drying temperature was 150°C, and the temperature was increased right from the start as fast as possible. A drying scheme as efficient as possible was used. At the final stage of the drying process, the wet-bulb temperature was about 90°C. The drying time was 20 h. The pressure used was 2.0 bar.

The actual drying was followed by a cooling stage, during which the pressure was still maintained until the internal temperature of the planks fell below 100°C. This procedure prevents the occurrence of a steam explosion at the time of de- pressurization. The average final moisture level of the planks was 4.7%. No surface or internal cracks appeared in the planks. Example 3. Pine planks dimensioned 50 mm x 100 mm were dried using the compression method of the invention. The planks were placed in the drier in the manner shown in Fig. 3, with their flat sides against each other. The drying temperature was 150°C, and the temperature was increased right from the start as fast as possible. A drying scheme as efficient as possible was used. At the final stage of the drying process, the wet-bulb temperature was about 90°C. The drying time was 20 h. The pressure used was 2.0 bar.

The actual drying was followed by a cooling stage, during which the pressure was still maintained until the internal temperature of the planks fell below 100°C. The average final moisture level of the planks was 15.2%. The dimensions of the planks before and after the drying process are shown in Table 2.

Table 2.

When the planks are placed in the drier with their flat sides against each other, the compression takes place in the thicknesswise direction and the planks must have a corresponding oversize in their thickness dimension.

On the surface of the flat sides of the planks, small cracks of a length of about 10 cm had appeared at the ends. There were no cracks in the core parts of the flat sides nor in the edges. The planks had no internal cracks.

Example 4. The sawn timber to be dried consisted of pine planks measuring 50 mm x 100 mm. The planks were placed in the drier in the manner shown in Fig. 2, with their edges against each other. The drying temperature was 70 °C. The drying process was started by a heating stage, during which the temperature was increased to 70°C. The duration of the heating stage was 2 h, and during this stage the drier was supplied with steam, so that no drying occurred at this stage. After the heating stage, a drying scheme as efficient as possible was used. The drying time was 91 h. The compression pressure was initially 6 bar, and it was increased gradually at intervals of one full day, and at the end of the drying process the pressure was 18 bar.

The average final moisture level of the planks was 11.9%. The planks had been compressed in their widthwise direction by 14.3%. Their thickness had been increased by 2.4%. The Brinell hardness of the flat side surfaces of the planks had been increased by 18%.

When the timber to be dried is compressed with a high pressure, typically in the range of 6 - 20 bar, the resulting shrinkage is greater than the shrinkage occurring due to drying of the wood. In this way, the hardness of the timber can be increased. The increase of hardness is the greater the greater is the compression shrinkage.

The drying of timber can be carried out either by placing the pieces of timber with their edge surfaces against each other, in which case the drying takes place via the flat sides, or by placing them with their flat sides against each other, in which case the drying takes place via the edge surfaces. This makes it possible to have the timber dried via desired surfaces, which in some cases has an advantageous effect on the properties of the timber.

Fig. 4 illustrates a case where several balks 3 are placed one over the other in the direction of compression with their edges 14, 15 against each other. The surfaces to be placed together can be machined so that steam can escape from between them, thus accelerating the drying process (not shown). Screens 16 may be inserted between the surfaces to be placed together, which also accelerates the drying process, as shown in Fig. 5, which illustrates the drying of several balks placed side by side with their lateral surfaces 17 against each other or against the pressing surfaces.

The practical examples described above relate to normal drying and hot drying, but that does not constitute any limitation of the application of the invention. The method of the invention, whereby timber is compressed during drying by an amount at least equal to the shrinkage, is applicable for use regardless of the drying method itself. The drying method may be vacuum drying, high-frequency drying, micro-wave drying, steam drying or any other method or their combination. The embodiments may vary in the scope of the claims presented below. The invention can also be used to dry e.g. a balk or a similar blank before its being planed to form a building log or sawn to produce sawn timber.

To describe an embodiment in which the timber to be dried consists of balks, which are sawn after the drying to produce boards and planks, the following practical example is presented.

Example 5. Pine balks measuring 105 mm x 182 mm were dried with their flat sides placed against each other. The drying temperature was 70°C and an effi- cient drying scheme was used. The drying period was 14 days, at the end of which the balks had an average moisture level of 14.9%. After being dried, a balk can be cut e.g. by sawing two boards measuring 28 mm x 100 mm from either side of it, the portion left between them forming a plank of 63 mm x 100 mm.

Claims

1. Method for drying solid-wood timber (3, 23), wherein the timber is compressed between pressing elements (10,12,22) during the drying to reduce cracking, in which method the timber is compressed by an amount at least equal to the drying shrinkage of the timber, and in which method the timber consists of pieces of timber placed between the pressing elements and dried at the same time, characterized in that by means of the pressing elements, the timber (3, 23) is only compressed, and that the pieces of timber (3, 23) are arranged between the pressing elements in one or more tiers in the pressing direction so that the pieces of timber in the same tier are pressed against each other during the drying process.

2. Method as defined in claim 1 , characterized in that the drying is performed in a drying space (2) provided with a pressing apparatus (10-14) for compressing the timber during the drying process.

3. Method as defined in claim 1 , characterized in that the timber is compressed with an increasing compression force during the drying process.

4. Method as defined in claim 1 , characterized in that the compression is per- formed in a single stage so that the timber is compressed continuously until the final dimension is reached, without interruptions allowing the timber to resile.

5. Method as defined in claim 1 for the drying of especially sawn timber, characterized in that the timbers are placed with their edge surfaces (24) against each other, so that the drying takes place via the flat sides.

6. Method as defined in claim 1 for the drying of especially sawn timber, characterized in that the timbers are placed with their flat sides (25) against each other, so that the drying takes place via the edge surfaces.

7. Method as defined in claim 1 , characterized in that the surfaces to be placed together have been machined to accelerate the drying process so that steam can effectively escape from between the surfaces.

8. Method as defined in claim 1 for the drying of timber, characterized in that screens (16) are fitted between the surfaces placed together to accelerate the drying process.

9. Method as defined in claim 1 , characterized in that the timber consists of a plank, beam or balk, which is machined further after the drying to produce e.g. sawn timber and/or a building log.

10. Method as defined in claim 1 , in which method the timber is dried by adjusting the difference between its dry-bulb and wet-bulb temperatures, charac- terized in that the difference between the dry-bulb and wet-bulb temperatures is so adjusted that it exceeds 15°C at least at the final stage of the drying process.