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Title:
A METHOD AND AN APPARATUS FOR HEAT TREATMENT OF WOODEN OBJECTS
Document Type and Number:
WIPO Patent Application WO/2003/106123
Kind Code:
A2
Abstract:
A method for heat treatment of wooden objects, wherein the treatment of the wooden objects (1) takes place in an autoclave (2) so that the wooden objects (1) may be treated by pressure and/or vacuum, preferably to a pressure which is in the region of the pressure of the saturated steam, and that the amount of oxygen in the autoclave (2) may be reduced prior to heat treatment, following which a form of protective gas may be added for reducing carbonization of the wooden objects (1). The invention also comprises a system for the treatment of wooden objects (1) in a heated space, said space being formed by an autoclave (2), said autoclave (2) being connected with a pressure system (3) for adjusting the pressure in the system, said autoclave (2) being connected with a condensate container (4). The system comprises means for pressure equalization of the autoclave as well as means for clamping the wooden objects and/or clamping them together, so that these do not burst or twist during or after the treatment.

Inventors:
SKOVMAND ERIK (DK)
CHRISTENSEN PETER (DK)
Application Number:
PCT/DK2003/000368
Publication Date:
December 24, 2003
Filing Date:
June 06, 2003
Export Citation:
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Assignee:
IWOTECH AS (DK)
SKOVMAND ERIK (DK)
CHRISTENSEN PETER (DK)
International Classes:
F26B3/00; F26B7/00; F26B21/14; F26B25/00; (IPC1-7): B27K/
Domestic Patent References:
WO1990006840A11990-06-28
Foreign References:
DE3044221A11982-06-03
EP0956934A11999-11-17
DE4222178A11994-01-13
NL9201265A1994-02-01
Attorney, Agent or Firm:
LARSEN & BIRKEHOLME A/S (Banegådspladsen 1P.O. Box 362, Copenhagen V, DK)
Download PDF:
Claims:

PATENT CLAIMS 1. A method for heat treatment of wooden objects, wherein the wooden objects are arranged stacked and optionally with gaps between the individ- ual objects in a closed container, such as an autoclave, to which a heating medium, preferably saturated steam, is fed, which is discharged from the autoclave according to a predetermined heating period, c h a r a c t e r- i z e d by the following steps: - arranging the wooden objects in the autoclave, - providing a negative pressure in the autoclave, preferably of a size of about 100 mb, - feeding a protective gas, preferably CO2, to the autoclave, - feeding saturated steam to the autoclave at a temperature of about 160 - 230 degrees, preferably about 210 degrees, - discharging the saturated steam after a predetermined heating period from the autoclave with simultaneous pressure relief, - cooling the wooden objects, - removing the wooden objects from the autoclave.
2. A method for heat treatment of wooden objects according to claim 1, characterized by feeding C02 in such an amount that the pressure in the autoclave is about 1 bar.
3. A method for heat treatment of wooden objects according to claim 1 or 2, characterized in that the feeding of the saturated steam takes place such that the temperature increases according to a predetermined temperature/time curve, and that, following completed heat treatment, cooling also takes place according to a predetermined temperature/time curve.

4. A method according to claim 3, characterized in that cooling takes place by the supply of atmospheric air, which flows from below up through the gaps between the wooden objects.
5. A system for the treatment of wooden objects in the performance of the method according to claims 1-4, with an autoclave (2) which is adapted to receive the wooden objects (1), and to which a heating agent, such as satu- rated steam, may be supplied, and which is connected with a container (4) for the collection of condensate (5) which is generated during the heat treatment, characterized in that the autoclave (2) is connected with means for providing a negative pressure in the autoclave, such as a vac- uum pump (3), and with means for supplying COz to the interior of the auto- clave.
6. A system according to claim 5, c h a r a c t e r i z e d in that the auto- clave is connected with a control system with valves allowing the pressure in the autoclave to be equalized slowly after completed heat treatment.
7. A system according to claim 5 or 6, characterized in that the container (4) for the collection of condensate (5) is connected with a stor- age container (6) to which the condensate (5) may be fed for later use.
8. A system according to claims 5-7, characterized in that the system comprises means for clamping the wooden objects (1) and/or clamping them together.
Description:

A METHOD AND AN APPARATUS FOR HEAT TREATMENT OF WOODEN OBJECTS Prior art The invention relates to a method for heat treatment of wooden objects as stated in the introductory portion of claim 1.

The invention also relates to a system for heat treatment of wooden objects by means of this method, as stated in the introductory portion of claim 5.

In many connections, it is desirable to heat-treat wooden objects, such as boards or strips before they are used. For pressure-impregnated wood, this may take place by carrying out a so-called fixation of the agents of the pressure impregnation in the wood such that they do not penetrate out into the surroundings. Wooden objects may also be pressure-impregnated to give them better properties, to make them less sensitive to moisture, or to prolong their service life.

Heat treatment may take place by heating in open spaces, but small closed compartments, such as autoclaves, are frequently used, in which the ob- jects are arranged stacked and with gaps between the individual objects.

This reduces the consumption of heat, and the objects may be pressurized or be subjected to vacuum, which may be expedient in certain connections.

Usually, steam is used for the heat treatment of wood. The steam may be saturated and have a temperature of about 100 degrees.

A known system for heat treatment of wooden objects, in particular with a view to fixation after pressure impregnation, is described in PCT application no. WO 90/06840.

The system comprises an autoclave in which the wooden objects are ar- ranged stacked on carriages. The autoclave is in communication with a container for condensate which is generated during the heat treatment, and has means for the supply of hot, saturated steam.

According to this prior art, the autoclave is evacuated completely, or as much as possible, as the heat transfer from the steam may take place more rapidly when there is no air in the autoclave. Therefore, steam at a rela- tively low temperature may be used.

For some uses, however, it is desirable to heat-treat wooden objects at a high temperature. In case of treatment at temperatures above 160 degrees, expediently up to 210 degrees, it may be ensured that the wood gets some expedient properties. Thus, prolonged service life, better strength proper- ties and in particular better resistance to ingress of moisture may be achieved.

This effect is brought about by the fact that the glucose or sugar sub- stances in the wood are released and form a layer around the cells of the wood which so to speak hardens on the cell walls, such that the cells are less susceptible to ingress of moisture.

However, when so high temperatures are used, carbonization of the wood may occur, which causes discoloration. Therefore, measures to counteract this carbonization are required.

Obiect of the invention Accordingly, the object of the invention is to provide a method for heat treatment of wooden objects wherein it is possible to use high temperatures without risking that the wood carbonizes and is thereby discoloured.

This is achieved by the method which is defined in claim 1.

When the container or the autoclave is evacuated to a suitably low pres- sure, preferably around 100 mb, at the start of the process, a great part of the oxygen around the wood has disappeared. Then, a protective gas, preferably C02, is fed, and finally the saturated steam is fed, whose pres- sure and temperature are gradually raised to the mentioned high levels.

It has been found that the low amount of present oxygen together with the presence of COz eliminates the problem of carbonization of the wood, such that heat treatment may be performed with the desired high temperatures without problems.

When, as stated in claim 2, the vacuum is"filled"with C02 before the sup- ply of the steam, a particularly good effect is achieved.

Claim 3 defines an expedient way of performing the heat treatment at this high temperature. When the temperature is so high, neither the heating nor the subsequent cooling may take place too suddenly. Cooling expediently takes place in the manner stated in claim 4, viz. by blowing air into the stack of wooden objects from below.

The invention also relates to a system as stated in claim 5 for performing the method of claims 1-4. The low pressure of 100 mb is provided without problems with an ordinary vacuum pump, in contrast to what is the case if the vacuum is to be complete, as is the case in the above-mentioned prior art.

By equipping the system with an expedient device for equalizing the pres- sure slowly, as stated in claim 6, bursting of the wooden objects is avoided.

With the very hot saturated steam that is used, the pressure becomes high,

up to 29 bars. This pressure also prevails inside the structure of the wood, and a sudden relief might cause the wood to burst.

The condensate container itself, which is in communication with the auto- clave, may expediently be connected with a storage container to which the condensate may be supplied for later use, as stated in claim 7. The con- densate, which occurs at the high temperatures, may contain substances, such as turpentine, which it may be useful to collect.

Finally, the wooden objects in the stack may be clamped together and/or be clamped so that they do not warp during the heat treatment with the high temperatures, as stated in claim 8.

The drawing The invention will be explained more fully with reference to the drawing, which shows a schematic view of an embodiment of the system according to the invention.

Description of an embodiment In a preferred embodiment of the method according to the invention, wooden objects 1 are introduced into an autoclave 2. When the autoclave 2 is closed, it is evacuated to a suitable negative pressure or vacuum by means of a form of a vacuum pump 3. This evacuation generates a so- called pre-vacuum whereby some oxygen is removed from the autoclave 2, so that the risk of carbonization during treatment is reduced considerably.

In the event that the heat treatment proceeds at temperatures around 180 oc or above, COz in gas form may be supplied after the evacuation to counteract carbonization.

The actual heating takes place in that steam is supplied to the autoclave 2, during which supply the temperature is controlled automatically via a com- puter (not shown). The heating thus follows a predetermined curve with temperature increase and time. The heating may e. g. be set to between 160 °C and 230 °C, preferably 210 °C.

When the desired temperature has been reached, it is kept constant, and an air moisture essentially corresponding to the pressure of the saturated steam occurs.

The steam, which condenses on the wood 1, is fed to a container 4 for the collection of condensate 5. This container or tank 4, which, like the auto- clave 2, is also under pressure, may be emptied automatically by means of one or more float devices (not shown) as well as by means of the pressure existing in the container.

In addition to the condensate tank 4, the system may also comprise a fur- ther storage tank 6 for the storage of condensate 5 prior to further pumping.

The condensate 5 may subsequently be neutralized and/or pumped from an outlet 7 to an impregnation system (not shown) for mixing with impreg- nation liquid. Before the condensate 5 is conveyed from the condensate tank 4 and to the further storage tank 6, the condensate 5 may be passed through a heat exchanger 12, so that the condensate 5 is caused to release its residual heat for use in the heating of the medium for the heat treatment.

If it is advantageous in certain cases to heat the condensed liquid or the impregnation liquid before the feeding into the autoclave, this may likewise take place by flow through the heat exchanger 12.

The condensate 5 inter alia contains turpentine, which may be separated from the condensate, and the separated turpentine may optionally be used for other purposes. One of these purposes may e. g. be to use the turpen-

tine completely or partly as a fuel for the heating of the medium for the heat treatment.

The medium for heating the autoclave 2 is supplied to the system at 8, and the cooled medium is returned to the heating system (not shown) in the form of condensate at 9.

The time period which the wooden objects are to stay in the autoclave dur- ing heating and cooling is adjusted, optionally via automatics, on the basis of a predetermined course (program) in view of the dimensions of the ob- jects and the wood species or type concerned.

After the heat treatment has been completed, the pressure in the autoclave is neutralized slowly to prevent bursting in the wood, as the cells of the wooden objects will be burst by the outflowing residual moisture/air in case of too rapid equalization. The pressure from which equalization takes place may e. g. be of the order of 10-29 bars, which pressure is present in the autoclave. as well as in the cells of. the wood....

By keeping the wooden objects in a range around the pressure of saturated steam in the autoclave it is ensured that the wooden objects do not dry up, as the wood will keep a wood moisture of about 10% of the dry weight of the wood.

To prevent cracking from occurring in the wood, or twist of the wood from occurring during cooling, cooling takes place according to a predetermined temperature and time curve. Cooling takes place in that air, preferably at normal room temperature, is blown through the stack of wood from below and upwards by means of a cooling blower 10, whereby the heat and the steam are blown into the atmosphere via a steam chimney 11.

Residual heat, if any, from the autoclave may be discharged via the steam chimney 11 or be passed through a heat exchanger 12 to thereby recover part of the energy which was spent on heating the medium to steam.

It is not a necessity for the performance of the invention to use a heat ex- changer to recover part of the energy, but for economic reasons it may be very advantageous.