Plant and method for heat treatment of ligneous material

Abstract

 A plant and method for heat treatment of ligneous material in steam developing from evaporation of water therefrom are provided. Before loading into a heat chamber, the ligneous material (25) is arranged upon a carriage (24) so as to fill the work volume (2) of the heat chamber substantially mirror-symmetrically, close to the outline of the cylindrical inner face of the heat chamber, but without contact between the ligneous material (25) and pipes (6) or inner surface of the heat chamber and advantageously not higher than the uppermost pipe (6). Space between neighboring pieces of the ligneous material (25) being provided. Due to arrangement of pipes (6), conditions for natural convective circulation of the gaseous medium are provided within the heat chamber. A fluid heat-transfer medium flowing through the pipes (6) is used as a heating agent.

Description

[0001] The present invention relates to the field of heat treatment of ligneous material, particularly, to a plant and method for heat treatment of ligneous material. The invention may be used in wood-working industry for heat treatment of ligneous material such as wood of different species. The invention provides heat treatment of both greenwood and a pre-dried lumber with a thickness of 5-500 mm.

[0002] At present, different devices and processes for wood heat treatment are used in Europe, but all they are based on use of overheated steam, which is forced-circulated within a heat chamber, often at high speeds. E.g., the patent document Fl 117405 discloses a device and method for high-temperature drying or treatment of timber. One lot of timber to be heat-treated is divided into two groups spaced from each other and heat-treated in a heat chamber, in which hot air or steam is blown between the groups. This technology does not provide a uniform treatment of the timber, and fans or other equipment to blow hot air or steam is required in addition to heating the timber, which makes the device too complex. Furthermore, to prevent spontaneous inflammation of the wood, equipment for injecting water is used.

[0003] The patent document US 1,328,505 discloses a number of processes for heat treatment of wood, in the course of which the wood, in addition to heating, is impregnated with water and/or subjected to steam and high pressure and vacuum. This complex technology requires rather complex equipment.

[0004] Up to now, improvements in devices and processes for wood heat treatment have been coming along with their complication. But the more complex is a structure, the lower is its reliability and the worse is the reproducibility of results. Therefore, in the field of wood heat treatment there is a need for a simple and reliable plant for heat treatment of ligneous material, which would provide a high-quality treatment thereof by a simple method.

[0005] We reverted to well-known process and apparatus for treating wood or lumber disclosed in the US patent No. 217,022 issued on July 1, 1879, to Robbins, in respect of which we managed to make some significant improvements. These process and apparatus are taken as the closest prior art.

[0006] In the Robbins's apparatus, to heat wood or lumber in a heat chamber A, multiple curved heating tubes E permeable for heat-transfer medium are used within the heat chamber A, the heating tubes E being connected in parallel to one another with main tubes F intended for intake and offtake of the heat-transfer medium, the heating tubes E being curved according to the circumflexion of the cylindrical inner side face of the heat chamber A. The heating tubes E completely occupy the circumference of the inner side face of the heat chamber A except its bottom part, where there are longitudinally laid said main tubes F for intake and offtake of the heat-transfer medium, air-supply tubes M, and a rail-track to roll in and roll out a carriage loaded with wood or lumber. Steam or hot water can be used as the heat-transfer medium. In the apparatus there is provided drainage of water condensate from the heating tubes E (in case where steam is used as the heat-transfer medium) and process wastes runoff from the heat chamber A during the heat treatment. The apparatus includes a thermometer and a pressure gauge. In effort to make the heating uniform, forced circulation of the gaseous medium within the heat chamber is provided by injection of air through multiple holes distributed along the air-supply tubes M. To prevent spontaneous inflammation of the wood or lumber within the heat chamber A, injection of steam is provided.

[0007] According to the Robbins's patent teaching, the apparatus permits treating lumber at temperatures in the range of approximately 100 to 260°C (215 to 500°F).

[0008] However, the Robbins's apparatus is not free from essential shortcomings.

[0009] This technology does not provide such a quality of heat-treated material that would meet the up-to-date requirements.

[0010] A great number of parallel heating tubes E requires a great number of connections thereof to said main tubes F for intake and offtake of the heat-transfer medium. Being weak points, the connections need an additional attention. The connections being numerous, the MTBF (mean time between failures) deteriorates.

[0011] If water is used as the heat-transfer medium, air locks can form in the upper parts of the heating tubes E.

[0012] There is not provided a natural circulation of the gaseous medium within the heat chamber A, and injection of air is needed by additional equipment, which leads to additional costs.

[0013] To prevent spontaneous inflammation of dry-heated wood or lumber, steam is injected into the heat chamber A, but in case of failure to provide the steam, spontaneous inflammation of the wood becomes inevitable. In case water is used as the heat-transfer medium, additional equipment for steam generation is required.

[0014] Temperature control within the heat chamber A is not automatic, which requires a close attention and continuous presence of an operator during the treatment.

[0015] Cooling is natural in the Robbins's apparatus, and there is no possibility to accelerate it.

[0016] Process wastes are not collected and utilized, which makes the Robbins's technology environmentally detrimental.

[0017] The plant of invention provides essential improvement of the prior art.

[0018] The objects of the present invention are to increase the quality of heat treatment of ligneous material, to decrease power consumption, and to raise effectiveness of the heat chamber operation.

[0019] The objects are achieved by providing a plant for heat treatment of ligneous material in a medium of steam (in a "steam jacket") developing as a result of evaporation of water from the ligneous material, the plant comprising:

• a sealable heat chamber having a substantially cylindrical inner space forming its work volume, the central axis of the heat chamber being directed substantially horizontally, the heat chamber being adapted to operate using energy from an external heating source located outside the heat chamber;
• a pipeline system comprising pipes permeable for a fluid heat-transfer medium, the pipes being located within the heat chamber near its cylindrical inner face;
• means for measuring temperature of the gaseous medium within the heat chamber, and
• means for discharging excessive steam and process wastes comprising at least one process wastes collector communicating, through an intake pipe, with at least one intake opening located substantially in the lowest area of the bottom portion of the work volume of the heat chamber, the collector being located outside the heat chamber;

wherein,

• to provide a substantially symmetrical natural convective circulation of the gaseous medium within the heat chamber when the latter is loaded with ligneous material, said pipes are laid substantially mirror-symmetrically relative to the central longitudinal vertical plane in vicinity of the cylindrical inner face of the heat chamber, a clearance space between the pipes and the heat chamber cylindrical inner face being provided, the pipes occupying said cylindrical inner face so that its upper portion of about 20-30% of its total area is free from the pipes;
• the pipes are laid in meanders having straight portions extending in a nearly horizontal direction nearly in parallel with the generatrix of the cylindrical inner face of the heat chamber, said straight portions being connected in series by U-shaped rounded portions;
• to accelerate cooling the ligneous material upon completion of the heat treatment, the plant is adapted to be connected to a cooling system located outside the heat chamber;
• the pipes are alternatively switchable between said external heating source and said cooling system, for which purpose a switching means to switch the communication of the pipes between the external heating source and the cooling system is provided;
• to provide a natural runoff of the process wastes, the bottom portion of the work volume of the heat chamber is inclined relative to the horizontal plane towards said at least one intake opening communicating with the process wastes collector through the intake pipe;
• to control temperature of the fluid heat-transfer medium and the gaseous medium within the heat chamber, an automatic temperature control system is provided; and
• said means for discharging excessive steam and process wastes comprises a separate means for discharging excessive steam.

[0020] A service compartment comprising a control board can be arranged behind the heat chamber in the same housing. The control board is equipped with a display to monitor the temperature of the fluid heat-transfer medium and the temperature and humidity of the gaseous medium within the heat chamber, and a temperature controller relating to said automatic temperature control system. Any automatic temperature control system known in the art can be used.

[0021] A heat-stable oil can be used as the fluid heat-transfer medium. An electrically heated oil boiler can be used as the external heating source. The fluid heat-transfer medium can circulate through the pipeline system naturally, on account of ascent of masses of the fluid heat-transfer medium with a lower specific gravity from the external heating source and descent of masses of the fluid heat-transfer medium with a higher specific gravity upon transferring some of their heat energy to the gaseous medium within the heat chamber. Alternatively, the fluid heat-transfer medium can circulate through the pipeline system forcedly, e.g., by means of at least one circulation pump.

[0022] As mentioned above, the pipes, which during the heat treatment are used as heating pipes, are laid in meanders having straight portions, which extend nearly horizontally and nearly in parallel with the generatrix of the cylindrical inner face of the heat chamber, said straight portions being connected in series by U-shaped rounded portions. To prevent forming air locks in the pipelines, said straight portions are correspondingly inclined relative to the horizontal plane at about 1-1.5 cm per 1 m of pipe.

[0023] The transverse cross-section of the work volume of the heat chamber is substantially circular, which helps the gaseous medium within the heat chamber to circulate by a natural convection.

[0024] To provide a natural runoff of process wastes during the heat treatment, the bottom portion of the work volume of the heat chamber is inclined relative to the horizontal plane at an angle of about 1-5°, preferably about 3° towards said at least one intake opening communicating with the process waste collector.

[0025] To improve conditions for natural circulation of the gaseous medium within the heat chamber, the pipes are spaced from the cylindrical inner face of the heat chamber at a distance of about 2-5 cm.

[0026] A steam-discharge pipe may be used as the means for discharging excessive steam.

[0027] Besides that, the objects of the invention are achieved by providing a method for heat treatment of ligneous material in a medium of steam (in a "steam jacket") developing as a result of evaporation of water from the ligneous material, using the above-described plant, the method comprising the following operations:

• arranging the ligneous material on a transporting means so as to provide that the work volume of the heat chamber is filled substantially mirror-symmetrically relative to the central longitudinal vertical plane, substantially close to the outline of the cylindrical inner face of the heat chamber, but without contact between the ligneous material and heating pipes or inner surface of the heat chamber and advantageously not higher than the uppermost pipe, interspaces between neighboring indivisible pieces of the ligneous material being provided;
• loading the arranged ligneous material into the heat chamber and sealing the latter;
• heating the ligneous material so as to cause water to evaporate therefrom;
• by heating the ligneous material, which leads to evaporating water therefrom, forming a saturated or nearly saturated steam within the heat chamber at a preset temperature chosen dependent on the wood species, thickness of one indivisible piece of the ligneous material, and desired color of the ligneous material upon treatment in a range of about 140-200°C;
• heat treatment of the ligneous material at preset temperature conditions during a time period chosen dependent on the initial humidity of the ligneous material, wood species, thickness of one indivisible piece of the ligneous material, and desired color of the ligneous material upon treatment;
• during the heat treatment, maintaining the preset temperature conditions within the heat chamber;
• during the heat treatment, maintaining a saturated or nearly saturated state of the steam in the work volume of the heat chamber;
• during the heat treatment, discharging excessive steam and process wastes through means for discharging excessive steam and process wastes;
• upon completing the heat treatment, without unsealing the heat chamber, cooling the ligneous material using a cooling system until temperature within the heat chamber decreases to a preset value; and
• unsealing and opening the heat chamber and unloading the ligneous material therefrom.

[0028] Both greenwood and pre-dried lumber can be used as the ligneous material.

[0029] The initial humidity of the ligneous material before the heat treatment may be of any value. Preferably, it is within a range of about 6-10%. The higher the initial humidity, other things being equal, the longer the total time of heat treatment cycle, because a greater amount of excessive steam should be discharged.

[0030] The steam in the work volume of the heat chamber is maintained in saturated or nearly saturated state, in order to alleviate internal stresses in the ligneous material and prevent deforming thereof, as well as to prevent a spontaneous inflammation thereof, which is achieved by a proper choice of heat treatment parameters, such as temperature and time duration, dependent on the thickness of indivisible pieces of the ligneous material, wood species and desired color of the ligneous material upon treatment.

[0031] In order to provide natural convective circulation of the gaseous medium within the heat chamber, the ligneous material to be heat-treated is arranged on a transporting means, e.g., a carriage so as to provide that the work volume of the heat chamber is filled substantially mirror-symmetrically relative to the central longitudinal vertical plane substantially close to the outline of the cylindrical inner face of the heat chamber, but without contact between the ligneous material and the pipes or inner surface of the heat chamber and advantageously not higher than the uppermost pipe, interspaces between neighboring indivisible pieces of the ligneous material being provided. Said interspaces between horizontal layers of lumber are provided by spacer elements, dependent on the thickness of the lumber and its initial humidity is chosen in a range of about 1-5 cm. Because of the same considerations, interspaces between neighboring vertical piles of the lumber are chosen in a range of about 2.5-5 cm.

[0032] Upon treatment, the ligneous material can be cooled naturally. Preferably, it is cooled using a cooling system, which can directly contact the ambient air or a coolant medium and to which the pipes within the heat chamber are switched. The cooling system provides both uniformly cooling the ligneous material and saving time to cool same. Upon lowering the temperature of the gaseous medium within the heat chamber to about 30-40°, cooling may be stopped and the heat chamber may be unsealed and opened to unload the ligneous material therefrom.

[0033] For a better understanding of the invention, a non-limiting embodiment of the plant of invention will be further described in more details with references to the accompanying drawings.

Fig.1 schematically shows a longitudinal section of a plant according to the present invention, the arrows showing circulation of a fluid heat-transfer medium in the pipes.

Fig.2 schematically shows a cross section of the plant of Fig.1 loaded with ligneous material, the arrows showing streamlines of the gaseous medium within the heat chamber in course of the heat treatment.

[0034] The following reference signs are used in the accompanying drawings:

1 - housing;

2 - work volume of the heat chamber;

3 - circulation pump to pump the fluid heat-transfer medium;

4 - three-way cutoff valve to switch the pipes alternatively between the external heating source (electrically heated oil boiler) and the cooling system;

5 - electrically heated oil boiler used as the external heating source;

6 - pipes within the heat chamber;

7 - expansion tank;

8 - safety valve;

9 - inverted valve;

10-control board;

11 - cooling system to cool the heat chamber;

12,13 - rest elements for the housing;

14 - temperature transducers;

15 - humidity transducer;

16 - inner door of the heat chamber;

17 - outer door of the heat chamber;

18 - external heat-insulating layer of the heat chamber housing;

19 - intake pipe to collect excessive steam and process wastes;

20 - process waste collector;

21 - steam-discharge pipe;

22 - stop valve;

23 - drain valve;

24 - carriage used as the transporting means;

25 - arranged ligneous material (lumber).

[0035] The plant comprises a horizontal cylindrical housing 1 with a substantially circular traverse cross-section resting on the rest elements 12,13 and forming a work volume 2 of the heat chamber. In order to provide favorable conditions for natural convective circulation of the gaseous medium within the heat chamber, said work volume 2 has substantially circular transverse cross-section. Pipes 6 are laid within the work volume 2 near the cylindrical inner face of the heat chamber with a clearance space of about 3 cm between the pipes 6 and the heat chamber cylindrical inner face. The pipes 6 occupy about 80% of the total area of said cylindrical inner face, so that an upper portion (segment F in Fig.2) of the cylindrical inner face of about 20% of its total area is free from the pipes 6. The pipes 6 are connected to a single return pipe, which is conducted out of the heat chamber into the work compartment and connected to the circulation pump 3. Operating requirements being observed, no condensed steam precipitates on the inner face of the heat chamber, therefore, for cost-saving reasons, a time-proof housing 1 can be obtained even if made of a ferrous metal, e.g., an ordinary (not stainless) steel. The pipes 6 may be made of a ferrous metal, e.g., an ordinary steel as well. A heat-stable oil is used as the fluid heat-transfer medium circulating through the pipes 6. The fluid heat-transfer medium circulation system also includes an extension tank 7 and safety valve 8.

[0036] For the reasons of energy saving, the housing 1 of the heat chamber is furnished with an external heat-insulating layer 18.

[0037] The pipes 6, which are used as heating pipes during the heat treatment, are laid in evenly spaced meanders having straight portions extending in a nearly horizontal direction nearly in parallel with the generatrix of the cylinder inner face of the heat chamber, said straight portions being connected in series by U-shaped rounded portions (see Fig.1). To prevent forming air locks in the pipes 6, said straight portions are correspondingly inclined relative to the horizontal plane at about 1 cm per 1 m of pipe. To improve conditions for natural convective circulation of the gaseous medium within the heat chamber, the pipes are spaced from the cylindrical inner face of the heat chamber at a distance of about 3 cm.

[0038] In the bottom portion of the work volume 2 of the heat chamber, substantially at the lowest point thereof, there is located an opening of an intake pipe 19 (intake opening), which is connected, through a stop valve 22, to a process waste collector 20, which communicates with a steam-discharge pipe 21 and a drain tube furnished with a drain valve 23. To provide a natural runoff of process wastes during the heat treatment, the bottom portion of the work volume 2 of the heat chamber is inclined relative to the horizontal plane at an angle of about 3° towards said intake opening communicating with the process waste collector 20. The intake pipe 19 comes in contact with a corroding medium, therefore, it is made of a corrosion-resistant material, particularly, stainless steel.

[0039] Behind the heat chamber, a service compartment is arranged in the same housing 1. In the service compartment there are located: an electrically heated oil boiler 5 serving as the external heating source, which is furnished with asafety valve 8 and inverted valve 9, a circulation pump 3 to pump the fluid heat-transfer medium, a three-way cutoff valve 4 used as means for switching the pipes 6 alternatively between the external heating source (electrically heated oil boiler 5) and the cooling system 11, which can directly contact the ambient air or a coolant medium, and a control board 10 equipped with a display to monitor the temperature of the fluid heat-transfer medium and the temperature and humidity of the gaseous medium within the heat chamber by means of temperature transducers 14 relating to an automatic temperature control system and a humidity transducer 15.

[0040] One of the main distinctive features of the plant of invention is that the heat treatment is performed in a medium of a saturated or nearly saturated steam (in a "steam jacket") developing as a result of evaporation of water from the ligneous material. A saturated or nearly saturated steam being permanently present within the heat chamber, inner stresses of the ligneous material do not appear and it does not deform. Further, spontaneous inflammation of the ligneous material is prevented.

[0041] The plant operates as follows.

[0042] As soon as a carriage 24 used as the transporting means with ligneous material (lumber) 25 arranged thereupon so as to provide that the work volume 2 of the heat chamber is filled substantially mirror-symmetrically relative to the central longitudinal vertical plane substantially close to the outline of the cylindrical inner face of the heat chamber, but without contact between the ligneous material and heating pipes or inner surface of the heat chamber and advantageously not higher than the uppermost pipe, interspaces between neighboring indivisible pieces of the ligneous material being provided (interspaces between epy horizontal layers being provided by spacer elements not shown in the drawings), has been rolled into the work volume 2 of the heat chamber, the inner door 16 intended to increase the heat-insulation of the heat chamber and the external door 17 are closed, the latter being fixed by means of fastening screws to seal thereby the heat chamber. Then the circulation pump 3 and the electrically heated oil boiler 5 are successively switched on, which results in that the fluid heat-transfer medium starts to circulate through the pipes 6 (see Fig.1). An operator assigns a pre-determined treatment temperature on the control board 10, the temperature being selected within a range of 140-200°C dependent on the wood species, thickness of one indivisible piece of the ligneous material, and desired color of the ligneous material upon treatment. As the gaseous medium within the heat chamber is being heated, it rushes up to the upper portion of the work volume 2 of the heat chamber both along the cylindrical inner face of the heat chamber and through the spacedly arranged ligneous material 25. In the upper portion of the work volume 2 of the heat chamber, the gaseous medium becomes relatively cooled because of the absence of pipes 6 in this area, and therefore, it rushes down through the spacedly arranged ligneous material 25. In the bottom part of the work volume 2 of the heat chamber, it is heated again from the pipes 6 and rushes up again, as a result of which a natural convective circulation of the gaseous medium within the work volume 2 of the heat chamber is provided (see Fig.2).

[0043] Meanwhile, steam is evaporated from the ligneous material and gradually, the gaseous medium within the heat chamber becomes more and more saturated with the steam, partial pressure of the latter becoming higher and higher, and excessive steam leaving the work volume 2 of the heat chamber through the intake pipe 19, stop valve 22, process wastes collector 20, and steam discharge pipe 21 into the atmosphere.

[0044] Due to the incline of the bottom portion of the work volume 2 of the heat chamber relative to the horizontal plane towards the intake opening communicating with the process waste collector 20, the liquid process wastes, which run down to the bottom portion of the work volume 2 of the heat chamber during the heat treatment, run into said intake opening of the intake pipe 19 and through the intake pipe 19 and stop value 22 run into the process wastes collector 20, from which, through a drain tube furnished with a drain valve 23, the liquid process wastes are taken away to be utilized. This solution makes the plant of invention eco-friendly.

[0045] Due to a permanent presence of a saturated steam within the heat chamber, spontaneous inflammation of the ligneous material during the heat treatment is prevented.

[0046] As soon as the pre-determined temperature within the heat chamber is achieved, the ligneous material is heat-treated under this temperature during a period of time dependent on the initial humidity of the ligneous material, wood species, thickness of an indivisible piece of the ligneous material and desired color of the ligneous material (25) upon treatment. Upon completion of the heat treatment, the electrically heated oil boiler 5 is switched off, and the heat chamber is switched to a cooling mode by the three-way cutoff valve 4, which switches the pipes 6 off the external heating source (electrically heated oil boiler 5) to the cooling system 11 (see Fig.1), closing a fluid heat-transfer medium circulation circuit through the latter. The cooling system 11 is located outside the heat chamber and can directly contact the ambient air or a coolant medium, which provides rapidly but uniformly cooling the gaseous medium and ligneous material within the heat chamber.

[0047] As soon as the temperature within the heat chamber has decreased to about 30-40°C, the doors 16,17 are opened, unsealing the heat chamber, and the carriage 24 with the ligneous material 25 thereupon is rolled out.

[0048] Depending on the initial humidity of the ligneous material and its desired color upon treatment, the operating cycle takes about 40-70 hours in total.

[0049] The plant of invention has the following principal advantages:

• a natural convective circulation of the gaseous medium within the heat chamber, which enables to obviate the need for using additional equipment (e.g., fans);
• eco-friendliness, because there are no emissions of pollutants into the atmosphere;
• uniformly cooling the ligneous material upon treatment due to the cooling system used in the plant;
• a high quality of the ligneous material upon treatment due to the "steam jacket", which covers the ligneous material during all the period of the heat treatment, prevents it from crackling and deformation, as well as from spontaneous inflammation;
• an even distribution of humidity upon unloading the ligneous material from the heat chamber - unevenness of humidity within the lot of the heat-treated ligneous material does not exceed 1%.

EXAMPLES 1-11

[0050] Lumber of different wood species, different thicknesses, and different values of initial humidity was heat-treated in a plant of invention. The housing was made of a ferrous metal. The inner diameter of the heat chamber (diameter of its work volume) was 189 cm; the length of the work volume was 530 cm. The total length of pipes (made of a ferrous metal) was 106 m, with their external diameter 76 mm, and internal diameter 69 mm. The heat chamber was loaded with the lumber as described above.

[0051] To provide interspaces between horizontal layers of lumber, wooden spacer elements with cross-section dimensions 10 mm×10 mm were used. Circulation of the heat-transfer medium was provided by a circulation pump of 230 W; an electrically heated oil boiler of 18 kW was used as the external heat source.

[0052] The stage of heating the lumber to develop a saturated steam within the heat chamber took 20-30 hours; the stage of the heat treatment as such took 4-24 hours. The cooling system directly contacted ambient air; the stage of cooling the lumber took 12-17 hours at the ambient air temperature 20-25°C. The heat chamber was opened as soon as the temperature therein lowered to 40°C.

[0053] Other parameters and results are in the table below.

Example No.	Wood species	Thickness of lumber (mm)	Initial humidity of lumber	Heat treatment temperature (°C)	Duration of heat treatment stage (hours)	Color of heat treated lumber
1	Ash-tree	5	15%	140	4	Light brown
2	Aspen	50	10%	170	6	Dark brown
3	Birch	30	10%	160	4	Brown
4	Fir	200	10%	200	15	Brown
5	Locust	8	10%	150	6	Dark brown
6	Maple	100	8%	180	6	Dark brown
7	Oak	50	8%	180	6	Dark brown
8	Oak and Ash-tree together	50 (oak) 30 (ash)	8%	160	14	Brown
9	Pine-tree	50	7%	180	4	Light brown
10	Pine-tree	50	10%	180	4	Light brown
11	Pine-tree	500	10%	200	24	Light brown

Claims

1. A plant for heat treatment of ligneous material in a medium of steam developing as a result of evaporation of water therefrom, the plant comprising:

- a sealable heat chamber having a substantially cylindrical inner space forming its work volume (2), the central axis of the heat chamber being directed substantially horizontally, the heat chamber being adapted to operate using energy from an external heating source (5) located outside the heat chamber;

- a pipeline system comprising pipes (6) permeable for a fluid heat-transfer medium, the pipes (6) being located within the heat chamber near its cylindrical inner face;

- means for measuring temperature of the gaseous medium within the heat chamber, and

- means for discharging excessive steam and process wastes comprising at least one process wastes collector (20) communicated, through an intake pipe (19), with at least one intake opening located substantially in the lowest area of the bottom portion of the work volume (2) of the heat chamber, the collector (20) being located outside the heat chamber;

characterized in that

- to provide a substantially symmetrical natural convective circulation of the gaseous medium within the heat chamber when the latter is loaded with ligneous material (25), said pipes (6) are laid substantially mirror-symmetrically relative to the central longitudinal vertical plane in vicinity of the cylindrical inner face of the heat chamber, a clearance space between the pipes (6) and the heat chamber cylindrical inner face being provided, the pipes occupying said cylindrical inner face so that its upper portion of about 20-30% of its total area is free from the pipes (6);

- the pipes (6) are laid in meanders having straight portions extending in a nearly horizontal direction nearly in parallel with the generatrix of the cylindrical inner face of the heat chamber, said straight portions being connected in series by U-shaped rounded portions;

- to accelerate cooling the ligneous material (25) upon completion of the heat treatment, the plant is adapted to be connected to a cooling system (11) located outside the heat chamber;

- the pipes (6) are alternatively switchable between said external heating source (5) and said cooling system (11), for which purpose a switching means (4) to switch the communication of the pipes (6) between the external heating source (5) and the cooling system (11) is provided;

- to provide a natural runoff of the process wastes, the bottom portion of the work volume (2) of the heat chamber is inclined relative to the horizontal plane towards said at least one intake opening communicating with the process wastes collector (20) through the intake pipe (19);

- to control temperature of the fluid heat-transfer medium and the gaseous medium within the heat chamber, an automatic temperature control system is provided; and

- said means for discharging excessive steam and process wastes comprises a separate means for discharging excessive steam (21).

2. The plant according to claim 1, wherein the traverse cross-section of the work volume (2) of the heat chamber is substantially circular.

3. The plant according to claims 1 or 2, wherein the bottom portion of the work volume (2) of the heat chamber is inclined at an angle in a range of about 1-5°, preferably about 3° relative to the horizontal plane towards said at least one intake opening communicating with the process wastes collector (20).

4. The plant according to any of claims 1-3, wherein an electrically heated oil boiler is used as the external heating source (5).

5. The plant according to claim 4, wherein, to prevent forming air locks in the pipes (6), the straight portions thereof have corresponding inclines relative to the horizontal plane of about 1-1.5 cm per 1 m of pipe.

6. The plant according to any of claims 1-5, wherein the clearance space between the pipes (6) and the heat chamber cylindrical inner face is about 2-5 cm.

7. The plant according to any of claims 1-6, wherein, in order to force circulation of the fluid heat-transfer medium through the tubes (6) during the heat-treatment and cooling stages, at least one pump (3) is provided.

8. The plant according to any of claims 1-7, wherein a heat-stable oil is used as the fluid heat-transfer medium.

9. The plant according to any of claims 1-8, wherein a steam discharge pipe (21) is used as the separate means for discharging excessive steam.

10. A method for heat treatment of ligneous material in a medium of steam developing as a result of evaporation of water therefrom, using a plant for heat treatment of ligneous material according to any of claims 1-9, the method comprising the following operations:

- arranging the ligneous material to be heat-treated on a transporting means so as to provide that the work volume of the heat chamber is filled substantially mirror-symmetrically relative to the central longitudinal vertical plane substantially close to the outline of the cylindrical inner face of the heat chamber, but without contact between the ligneous material and heating pipes or inner surface of the heat chamber and advantageously not higher than the uppermost pipe, interspaces between neighboring indivisible pieces of the ligneous material being provided;

- loading the heat chamber with the arranged ligneous material into the heat chamber and sealing the latter;

- heating the ligneous material so as to cause water to evaporate therefrom;

- by heating the ligneous material, which leads to evaporating water therefrom, developing saturated or nearly saturated steam within the heat chamber at a preset temperature chosen dependent on the wood species, thickness of one indivisible piece of the ligneous material, and desired color of the ligneous material upon treatment, in a range of about 140-200°C;

- heat treatment of the ligneous material at preset temperature conditions during a time period chosen dependent on the initial humidity of the ligneous material, wood species, thickness of one indivisible piece of the ligneous material, and desired color of the ligneous material upon treatment;

- during the heat treatment, maintaining the preset temperature conditions within the heat chamber;

- during the heat treatment, maintaining a saturated or nearly saturated state of the steam in the work volume of the heat chamber;

- during the heat treatment, discharging excessive steam and process wastes through means for discharging excessive steam and process wastes;

- upon completing the heat treatment, without unsealing the heat chamber, cooling the ligneous material using a cooling system until temperature within the heat chamber lowers to a preset value; and

- unsealing and opening the heat chamber and unloading the ligneous material therefrom.

11. The method according to claim 10, wherein a ligneous material with initial humidity of about 6-10% is heat-treated.

12. The method according to claim 10 or 11, wherein a pre-dried lumber is used as the ligneous material.

13. The method according to claim 12, wherein interspaces between horizontal layers of lumber are provided by spacer elements, the thickness of which, dependent on the thickness of the lumber and its initial humidity, is chosen in a range of about 1-5 cm, and interspaces between neighboring vertical piles of the lumber are provided in a range of about 2.5-5 cm.

14. The method according to any of claims 10-13, wherein, upon completion of the heat-treatment stage, the ligneous material (25) is allowed to cool until the temperature of the gaseous medium within the heat chamber decreases to about 30-40°.

Drawing