FI117353B - Method and apparatus for drying wood and wood products - Google Patents

Description

117353

METHOD AND APPARATUS FOR DRYING WOOD AND WOOD PRODUCTS

The present invention relates to a method for drying wood or wood products, wherein the wood or wood products are transferred to a pressure-tight closure drying chamber, wherein the wood or wood products in the drying chamber are heated with hot compressed air at a pressure above normal pressure. the pressure difference moves the water in the wood to the outside of the wood, where the water evaporates or boils and the water-vapor and water formed are removed from the drying chamber. The invention also relates to apparatus for drying wood or wood products, comprising a drying chamber having a pressure-tight closure lid to which the wood or wood products to be dried can be moved during drying, and a vacuum pumping apparatus connected to the drying chamber to In addition, the apparatus includes a compressed air blowing and heating device coupled to the drying chamber for generating and blowing hot compressed air into the drying chamber so that the drying chamber pressure can be raised above normal pressure during the heating step, and a pressure accumulator for storing warm compressed air

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•; The wood dries when the partial pressure of the water vapor of the water bound by the tree is greater than the partial pressure of the water vapor of the surrounding air, i.e. the tree discharges the water as water vapor to the air. Conventional wood drying methods and equipment are based on the technique * * 25 where wood is heated with humid air, steam or some other method, usually at a temperature of about 60-90 ° C in a drying chamber. This temperature is maintained until the desired moisture content of the wood is achieved. The higher the temperature used, the higher the pressure of the water vapor: within the wood, the faster and more powerful the flow of water from the interior of the wood to the wood surface. In addition, all known methods require strong air blasting past the exterior surfaces of the wood material to be dried, since the water exiting the wood has to be evaporated from the wood surface to the surrounding air and completely removed from the drying chamber.

♦ · »· · ψ ·: ···. The presently known methods of rapid drying of wood are generally based on the promotion of the evaporation of water in the wood by high temperatures (above 100 ° C) or by lowering the boiling point of water by reducing the pressure in the drying space. At the same time, the moist air or steam in the drying chamber is circulated past the surfaces of the wood material to be dried, in order to evaporate the water passing to the surface of the wood material into the air flowing past it. Usually, the air moistened with moisture exiting the wood is dried with the aid of a condenser or similar air imaging device. For example, U.S. Patent No. 1,763,070 (Forrest Streator Shinn) discloses a method for rapid drying of wood, wherein wood in a pressurized drying chamber is first heated by hot steam supplied to the drying chamber at a pressure above normal atmospheric pressure. The steam supply to the drying chamber is then stopped and the pressure in the drying chamber is further increased by pumping compressed air into the drying chamber with a compressed air compressor. Overpressure creates a higher than normal air and vapor pressure inside the wood. After these steps, the compressed air and steam in the drying chamber are removed and vacuum is applied to the drying chamber. This creates a pressure difference between the inside of the heated wood and the outside, which transfers moisture from the inside of the wood to the surface of the wood. The warm water rising to the surface of the wood boils and evaporates to the air in the drying chamber, because under reduced pressure the boiling point of water is lower than the boiling point of the water rising to the surface of the wood. The moist air formed is discharged from the drying chamber to a condenser between the vacuum pump and the drying chamber. By this method, wood is obtained much faster than conventional air-drying of dry-soil.

Current instant drying methods work well as such, but still require high energy, high heat and high air or steam recycling. In addition, hot air and hot steam are released to the outer sides of the apparatus, e.g., in a pressure reduction step such as that described in U.S. Patent No. 1,763,070. - After falling below 30% below 30%, the wood begins to re-wet during the heating phase due to the absorption of the moist steam used in the heating.

. Thus, such rapid drying methods significantly slow down the drying of wood when the moisture content of the wood has dropped below 30%.

»·« · ·· *:! ·. It is an object of the present invention to provide a method for drying wood or wood products which eliminates the above drawbacks associated with existing methods for drying wood or wood products * 1. In particular, it is an object of the present invention to provide a method for drying wood or wood products, also faster at temperatures below 100 ° C and more energy efficient, and for drying wood at a much faster rate than conventional air drying. - 3 117353 even when the wood has less than 30% moisture. It is a further object of the invention to provide an apparatus according to the method of the invention.

The object of the invention is achieved by a method characterized by what is set forth in the characterizing part of independent claim 1 and by an apparatus 5 which is characterized by what is stated in the characterizing part of independent claim 6.

In the process of heating the wood or wood products in the heating step, the compressed air is blown into the drying chamber at a temperature of about 60-160 ° C so that the drying chamber pressure rises above the normal pressure and the subsequent evaporation step The advantage of such a process is that when hot compressed air is used to heat wood or wood products, the heating step avoids the re-wetting of the wood or wood products to be dried at a moisture content of less than 30% due to the use of hot steam. In addition, since the compressed air used in the heating step is recovered during the evaporation step in the pressure accumulator included in the apparatus, the residual heat after the heating step is mainly recovered so that it can be reused during the next heating step (e.g., the next drying cycle). This saves the drying energy used in drying by 20% compared to previously known methods: ** [: because of the hot steam used for heating and the heat associated therewith: it is let out of the plant so that it can no longer be used in the following * * *.:. In the heating step.

···· mmm * · * The apparatus according to the invention comprises a compressed air device connected to a drying chamber • 9: 25 blowing and compressed air heater for forming and blowing compressed air at a temperature of about 60-160 ° C so that the drying chamber pressure can be increased above during the heating phase. In addition, the equipment includes a pressure accumulator for storing warm compressed air: * "<sex during the evaporation phase. This type of compressed air blasting apparatus is provided. \ 30 warm air and 9 pressures of excess air are introduced into the drying wood during the heating phase. · * Above normal pressure and heats wood from inside.

Thanks to the pressure accumulator, a negative pressure is obtained inside the wood during the evaporation phase following the heating step, so that the mm · 35 warm compressed air in the drying chamber of the previous step is recovered. If necessary, the pressure accumulator and the pressure ducts conducted therein can be insulated with a suitable thermal insulator to further reduce heat evaporation.

In the following, the invention will be explained in more detail with reference to the accompanying drawing, which shows an apparatus for drying wood or wood products according to the invention.

The drying apparatus for wood or wood products according to the method of the invention shown in Fig. 1 includes a drying chamber 1 with sealing lids 2, a heating and humidifying system 17 in the drying chamber, a vacuum pump 3 connected to a drying a condenser 7 10 and a pressure accumulator 6 connected to the outlet side of the vacuum pump 3 by means of a compressed air duct 9.

The drying chamber 1 is, in a manner known per se, a pressure vessel made of, for example, a metal plate or the like, capable of withstanding a temperature of at least 160 ° C, 95% (0.05 bar) vacuum and 16 bar overpressure. Inside the drying chamber there is a necessary substrate or, for example, a suitable grid-like shelf, in which the timber or wood products to be dried can be placed so that the necessary air gaps are left between the individual pieces to allow moist air to escape. In addition, the dry: chamber 20 has a heating and humidification system manufactured in a manner known per se. * ·. The heating and humidification system includes heating radiators, as well as a water mist or steam supply system. The drying / · * 'front of the chamber provides a system for heating and humidifying the timber and timber, if necessary. - and at the rear there are openable closures of the drying chamber lids 2, which include the necessary sealing and locking means for sealing the lids in a pressure-tight manner so that the pressure and temperature conditions required for the drying process can be formed inside the drying chamber.

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The vacuum pump 3 is a vacuum pump in itself supported by a suction duct 8 connected via a condenser 7 to a drying chamber 1. The power of the vacuum pump is minimized to provide the above mentioned pressure of 0.05 bar. The condenser 7 is a surface condenser known per se.

: whose condensation surfaces are cooled by heat sinks from the surfaces of the coolant pipes ♦ · / tolu, where cold water is circulated. The condenser surfaces of the condenser are shaped to allow water accumulating therein to drain into a collecting vessel 3 5 which is discharged through the drain channel 15 into the drain.

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The compressed air blowing device 4 comprises a compressed air compressor known per se, which is connected to the pressure accumulator 6 by the compressed air duct 5 and thence to the drying chamber 1 by the compressed air duct 10. The blowing of hot compressed air into the drying chamber can be distributed. Further, the compressed air blowing device 4 comprises a compressed air valve 13 connected to the compressed air duct 10, by means of which the pressure supply from the pressure accumulator 6 and the compressor to the drying chamber can be controlled as desired. The pressure accumulator 6 is a heat-insulated pressure vessel separate from the dry chamber and substantially resistant to a pressure higher than that of the drying chamber, in which the pressure nylon supplied to the drying chamber can be stored and heated. For heating the compressed air, the accumulator is fitted with a suitable heating element, known per se, using electric or hot water.

With the apparatus shown in Fig. 1 according to the method according to the invention, for example, when drying a 19x100 mm pine board, the boards to be dried are moved to the drying chamber 1 of the apparatus and the lid 2 of the drying chamber is closed. In the first drying step (heating step), excess pressure is applied to the drying chamber by introducing hot compressed air from the pressure accumulator 6 and / or by blowing with a compressed air blower 4. In this case, the compressed air is heated so that the temperature of the compressed air at the end of step I is about 110 ° C and the pressure at the drying chamber is 20 bar at a maximum of about 4 bar. The duration of the heating phase is'. 90 minutes during the first drying cycle. In a second step (evaporation-4 4 4: · '), a vacuum is created in the drying chamber by pumping the hot compressed air in the drying chamber with a vacuum pump 3 back to the pressure accumulator 6. Thus, most of the heat trapped in the hot compressed air. At the beginning of the shaking: a *: 25 drying step, the pressure in the drying chamber is rapidly lowered so that it is mainly about 0.5 bar during this step. However, the temperature in the drying chamber is still kept as high as possible in order to • boil and evaporate the water rising to the surface of the boards to be dried. During the evaporation:, \ dehumidifying step, the humid air formed by the water rising from the boards to the boards * ··· [30 is continuously pumped out of the drying chamber by a vacuum pump 3. This causes the humid • 4 * "air to enter condenser 7. !: to the condensation surfaces and drains into the collection tray This prevents moisture from entering the vacuum pump and pressure accumulator The heating and evaporation phase: drying cycles are repeated in this case successively, ··· * 35 so that the desired moisture content of the wood is reached. for boards, the holding time of the overpressure in the heating phase is gradually reduced in subsequent dry cycles to about 20 minutes in the last drying cycles.

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Vacuum hold time is constant. The moisture content of the wood is measured at intervals between drying cycles as needed to see when the target dryness of the boards has been reached.

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Of course, the method and apparatus according to the invention are not limited to the exemplary embodiment described above, but the method and the apparatus according to the invention can be implemented in many respects differently from it. During the heating step of the process, the temperature of the compressed air blown into the drying chamber, the overpressure of the drying chamber and the holding time of the overpressure may vary. The temperature of the compressed air and the amount of pressure in the drying chamber as well as the holding time of the overpressure depend, among other things, on: the type of wood and material thickness of the dried wood or wood products. The temperature of the compressed air is usually between 60 and 160 ° C and the pressure in the drying chamber is between 1.1 and 16 bar. The purpose of the evaporation step is to provide the drying chamber with as little pressure as possible to achieve the greatest possible pressure difference between the wood surface and the interior. This also results in the most potent boiling possible, allowing the moisture transferred to the wood surface to evaporate as quickly as possible. However, the strong evaporation of moisture tends to increase the amount of gas molecules in the drying chamber, so the amount of vacuum that is achieved will depend to some extent on the amount of wood in the drying chamber, moisture and moisture transfer ability (wood species) and vacuum power. Normally, the pressure is reduced during the evaporation step to between 0.05 and 0.9 bar. Of course, the number of drying cycles required can also be varied.

* * * * · * Their amount depends on the moisture content of the wood pieces to be dried, the thickness of the material and the type of wood. The above drying parameters may also be modified for other reasons. For some wood species, it may be preferable to use a more complex and slower treatment than another wood species.

* · »# · ·) !! / The apparatus according to the method is described above only with respect to the essential equipment parts required by the method. For use in industrial production. . in the application, the hardware may include e.g. necessary automatic measuring, control * * · (: diverting and regulating equipment, whereby the temperature and air pressure of the drying chamber 30 can be adjusted according to a pre-planned drying program so that the wood or wood products are dried to the desired degree of dehumidity. In addition to these devices, the equipment may also include, for example, a blower unit located in the drying chamber to enhance the circulation of air within the drying chamber ** '', which increases the transfer of 35 compressed air heat to the wood to be dried during the heating step. and, on the other hand, increases the evaporation of the water on the surface of the wood during the evaporation step. Further heating of the compressed air may be effected instead of / in addition to the heating resistor in the pressure accumulator shown in the exemplary embodiment an electric heater with an electric heater or hot water heat exchanger.

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Claims (9)

A method for drying wood or wood products, wherein the wood or wood products are transferred to a pressure-tight closure drying chamber (1), and wherein the wood or wood products in the drying chamber (1) are heated at a pressure above normal pressure and lowered to normal atmospheric pressure. wherein the pressure difference between the inner parts of the wood and the outer surface transfers water in the wood to the outer surface of the wood, where the water evaporates or boils and the water vapor and water formed are removed from the drying chamber, characterized in that hot air is blown into the drying chamber (1) 60-160 ° C so that the pressure in the drying chamber rises above the normal pressure, and in the evaporation step following the heating step, a pressure lower than normal is applied to the drying chamber (1) by pumping the pressure air in the drying chamber 15 a to the accumulator (6). Process according to Claim 1, characterized in that the pressure of the drying chamber (1) is raised to about 1.1 to 16 bar in the heating step. Method according to claim 1 or 2, characterized in that after the evaporation step, the pressure in the drying chamber (1) is lowered to a pressure of about 0.05 to 0.99 bar in the evaporation step. • «« • »I t» »• Method according to claim 1 or 2, characterized in that the air in the drying chamber (1) is dehumidified before pumping the compressed air: i * i into the pressure accumulator (6). • »• ♦ ·· * Method according to one of Claims 1 to 3, characterized in that the compressed air in the pressure accumulator (6) is heated by a heating device (16) in the accumulator and that, when the pressure in the drying chamber (1) is increased, the heated air * * '* * the compressed air is discharged from the pressure accumulator (6) to the drying chamber (1). • »• ♦ t • i · M * *« · 6. Equipment for drying wood or wood products, comprising: a drying chamber (1) with a pressure-tight closure to be dried. ··· '35 wood or wood products are movable during drying, and a vacuum pumping apparatus connected to the drying chamber (1); 3) for forming a vacuum in the drying chamber, characterized in that the apparatus comprises a compressed air blowing and heating device (4, 6, 16, 17) connected to the drying chamber (1) 9 117353 for generating and blowing compressed air at a temperature of about 60-160 ° C. that the drying chamber pressure can be raised above normal pressure during the heating step, and that the apparatus includes a pressure accumulator (6) for storing a warm pressure nozzle during the evaporation step. Apparatus according to claim 6, characterized in that the pressure accumulator (6) is a heat insulated pressure vessel. Apparatus according to claim 7, characterized in that the compressed air heating apparatus (16, 17) is a heating resistor mounted on the drying apparatus for heating the compressed air. 8 117353 Apparatus according to claim 7 or 8, characterized in that the apparatus comprises an air drying device (7) for removing moisture from the compressed air pumped into the pressure accumulator (6). n «• * 9 9 9 • 9 •» «·» · · I ·· * • ·· i • 99 9 9 • # 999 • 9 • 99 ··· 999 9 999 9 9 9 9 9 * 9 9 9 9 * 9 9 9 9 • ·> 9 999 9 9 9 9 9 * 9 9 9 9 9 9 9 9 9 9 999 9 • 99 9 9 9 9 999 9 9 9 9 * 9 9 99 9 9 999 9 9 9 9 ··· 117353 10