EP0760930B1 - Vacuum drier for sawn timber, and wood-drying method - Google Patents

Abstract

The invention concerns a vacuum drier (1) for sawn timber and a wood-drying method, the timber (3) being held in a cell (2), in the form of superposed layers of wood, with heating elements (8) between the layers and a compression device (7) being provided to exert pressure on the layers of wood. In order to improve a drier of this kind so that the pressure can be adjusted independently of the type of wood used, the invention proposes that the cell (2) can be sealed off from the surrounding atmosphere on all sides so that it is pressure-tight and the compression device (7) is located inside the cell.

Description

The invention relates to a vacuum dryer for sawn timber according to the preamble of claim 1, and a process for drying wood, especially under Use of the vacuum dryer for sawn timber after Preamble of claim 12.

Such a vacuum dryer is known from DE-A-35 36 595, with the wood to be dried one above the other in a cell is layered, being between the individual layers of wood Heating elements are arranged. The cell embraces two opposing walls, their one by the outside when evacuating the cell acting atmospheric pressure towards the other is so movable that the wood firmly against it fixed wall opposite the movable wall presses.

Such a vacuum dryer enables through the Evacuation of the cell both faster transportation of the water in the wood to be dried from the Wood (moisture transport), as well as a lowering of the boiling point of water, which has the consequence that the water at boils at a lower temperature than under atmospheric pressure. This also prevents that Wood discolored when drying.

The heating elements serve for optimal heat transfer on the dry goods. It has been shown that the Contact transfer is the best heat transfer. This heat transfer will be improved even more when on that Pressure is applied to dry goods. Another advantage " the pressure on the dry good is that the Wood does not warp when drying.

A disadvantage of the known vacuum dryer is, however, that the wood to be dried always has the same pressure, which is the difference between atmospheric pressure and the pressure in the cell is fixed, works, the pressure cannot be varied in other words. It is inevitable that it will dry the shrinkage of the wood that is too high the pressure exerted on the wood leads to cracking in the wood. This stems from the fact that the clamped under pressure Wood the friction between the wood and the Heating elements enlarged, and the shrinking of the wood like this is difficult. Such a cracking could occur with a lower pressure can be avoided. To reduce the pressure prevailing in the cell should However, the vacuum device inside the cell be reduced, which in turn has negative consequences for the Drying process of the wood.

From CH-A-665 271, US-A-4,343,095, US-A-4,233,752 and US-A-3,986,268 are also vacuum dryers known in which by means of a printing device pressure is exerted on the dry goods.

A disadvantage of these vacuum dryers, however, is that Pressure is not adjustable so that it on the Dry goods can be adjusted. Beyond that for example those from CH-A-665 271 and US-A-4,233,752 known vacuum dryers of the hydraulic cylinders on one wall of the vacuum dryer inside or arranged outside the cell. Such an arrangement of the Hydraulic cylinder is problematic because when exercising a relatively high pressure a high load the outer wall occurs on which the hydraulic cylinder is attached.

They are also connected to the hydraulic cylinder Pressure plates required to print as possible Apply evenly to the wood to be dried. This Pressure plates are one of the known vacuum dryers Solid metal plate that is rigidly attached to the lower end of the extendable piston of the hydraulic cylinder is attached. This results in particular then problems if the dry material is not a flat surface having. In this case the pressure plate is not plan on the dry goods and does not practice any even pressure on this. Rather, in Pad area exerted a very high pressure while where the pressure plate is not on the dry goods there is no pressure at all. this leads to unsatisfactory drying of the dry material.

The invention is therefore based on the object Vacuum dryer and a method for drying wood to improve in such a way that it is technically simple realizing an adjustable pressure on the Dry material can be exercised, whereby if possible low load on the entire vacuum dryer optimal drying result, especially when not layered in the vacuum dryer Dry material is achieved.

The task is in a generic vacuum dryer for sawn timber according to the invention by the features of Labeling part of claim 1 solved.

It is particularly advantageous that the printing device a arranged inside the cell, the dry material one side covering large, expandable Sack is the one with an adjustable pressure is acted upon. In this way, on the one hand, that the pressure on the dry goods on is easily adjustable, the other is at optimal for any layered dry material Drying achieved because the expandable sack Adapts unevenness of the dry goods well.

It is particularly advantageous that the pressure is continuous is adjustable.

A differential pressure medium is preferably used for its adjustment used, which is the setting of a Differential pressure with a value between that of atmospheric pressure and that of within the cell prevailing negative pressure varies. Furthermore, can also a pressure can be set that is greater than that Atmospheric pressure.

It is also advantageous if the pressure is pulsed is exercised. This puts a lot of pressure on the wood exercised, which enables high heat transfer. Then the pressure is removed or reduced, so the shrinking of the wood while avoiding cracks, which is reinforced by a pressure on the wood can be done. This process is then repeated.

The wall opposite the sack is preferably the Cell movable. In this way, the vacuum dryer easily to different thicknesses of the dry goods, i.e. of be adapted to dry wood.

The sack can hold the dry goods both on its top as well as covering it on the underside. Covers he removes the dry material from the underside, so it is advantageously a height compensation with different Heights of the dry material are not necessary because of the Sack the entire stack of stacked Wood pushes towards the opposite wall and so every height difference compensates. In this case moving the wall opposite the sack omitted.

These are preferably between the individual layers of wood arranged heating elements plate-shaped, from a Adjustable temperature heating medium (e.g. water, Oil, air, steam or the like) flowed through hollow aluminum bodies. In this way, a Heating temperature through temperature control of the heating medium be set on the one hand as large as possible Enables energy transfer to the wood, on the other hand but is so low that there are no cracks in the wood. In addition, heating elements can also be provided in the plates be.

So that the cell can be easily loaded with the dry goods can be sealed on one side of the cell Door provided through which the dry material in the Cell can be spent.

Claim 12 also conveys an inventive Process for drying wood, in particular using the vacuum dryer according to the Claim 1.

This method of drying wood enables one quick and effective drying of various Types of wood advantageously in that wood in one sealed against the environment The cell of the vacuum dryer is stacked, whereby heating elements between the individual layers of wood arranges, and that one by means of an expandable sack of the dry material on one side over a large area covers one on top of the stacked wood exerts adjustable pressure.

It is advantageous that this adjustable pressure can be varied and thus to the type of wood, Wood thickness and its specific dry data set can be. The pressure can take on a value that between that of the negative pressure prevailing in the cell and one that is greater than atmospheric pressure is varied.

Further advantageous refinements of the method are Subject matter of claims 19 to 22.

Fig. 1

schematisch eine Seitenansicht eines Vakuumtrockners für Schnitthölzer;

Fig. 2

eine Draufsicht eines Vakuumtrockners für Schnitthölzer; und

Fig. 3

eine Vorderansicht eines Vakuumtrockners für Schnitthölzer.

The following description of a preferred embodiment of the invention serves in conjunction with the accompanying drawings for further explanation. Show it:

Fig. 1

schematically shows a side view of a vacuum dryer for sawn timber;

Fig. 2

a plan view of a vacuum dryer for sawn timber; and

Fig. 3

a front view of a vacuum dryer for sawn timber.

As can be seen from the figures, includes a vacuum dryer 1 for sawn timber one on all sides compared to the environment cell 2 closed in a pressure-tight manner, which, for example is made of metal. A dry material is placed in this cell 2 3, for example wooden beams to be dried (see FIG. 1 and 3) introduced and stacked in layers, heating elements 8 between the layers of wood are arranged.

As is apparent from Fig. 2, the Dry goods 3 via one on the side of the vacuum dryer arranged door 4, which after the wood is introduced is pressure-tight sealable. Here, the entire stack of dry goods 3, for example on one mobile carriage 6 (see Figs. 1 and 3) inside the Cell 2 are inserted.

Instead of on the side, the door 4 can also be on the top of the Cell 2 may be arranged so that the dry material 3 directly can be inserted into cell 2 from above and into in this case the carriage 6 is unnecessary.

Inside the cell 2 is at the top of the dry material 3 an expandable bag 7 is arranged, which the Dry material 3 covers a large area at the top. The sack 7 advantageously extends over the total area of the top of the dry material 3.

To compensate for different heights of the dry material 3, can the wall of the cell 2 opposite the sack 7 in movable in a manner known per se, i.e. in the direction of the Sacks 7 be slidable (not shown). In this case, the dry material 3 is in for as long Moved towards the sack 7 until it is from this one optimal in terms of pressurization Distance.

But it is also possible and particularly advantageous that the bag 7 the dry material 3 on its underside over a large area covers and so when it expands to it opposite wall presses. This has compared to the in the embodiments shown in the figures have the advantage that different stack heights are not due to shifting balanced the wall in the manner described above balanced the wall in the manner described above Need to become. The sack 7 can for example already be provided on the carriage 6.

The cell 2 is via a vacuum pump, not shown evacuated so that an optimal negative pressure in the cell 2 prevails. This negative pressure has a value of approximately 100 - 200 mbar, so that an 80 - 90% vacuum is present.

The sack 7 is with a continuously adjustable Pressure can be applied, the pressure setting via a Differential pressure medium, not shown, occurs enables a differential pressure with a value that between that of atmospheric pressure and that of within pressure in the cell may vary.

To achieve a pressure higher than the maximum Differential pressure between the atmospheric pressure and the in the pressure prevailing in cell 2 is alternatively also possible in the sack 7 an overpressure, i.e., a pressure, which is greater than atmospheric pressure.

When the bag 7 is pressurized, it stretches is equally strong in all directions, being one pressure on the dry material directed along the arrows A. 3 exercises.

As already mentioned, it is particularly advantageous here that the bag 7 the dry material 3 over a large area covers, so that the pressure on the dry material 3 evenly over the entire area of one side of the dry material 3 is exercised. This creates a point pressure one place of the dry goods 3 avoided and a as As a result, the warping to be dried warps Wood almost impossible.

The pressure can also be applied in a pulsed manner by that repeats a high pressure that has good heat transfer allows an interval follows in which at all no or a significantly reduced pressure follows. In this pressure-free interval can be practically without the wood the risk of cracking disappears.

As already mentioned, within cell 2 are like 1 and 3, in particular in regular distance between the individual layers of the Dry goods 3 arranged the heating elements 8. This Heating elements 8 are plate-shaped aluminum hollow bodies, the one on two opposite each other Sides of the dry material 3 on (not shown) pipes are connected. Through these pipes as well as the cavity the heating elements 8 flows water with an adjustable Temperature. In this way, the heating elements 8 serve between the individual layers of wood 3 of warming each a single layer of wood 3. In addition to water can also oil, air or the like. can be used as heating medium. It it is also possible that 8 heating elements in the heating elements are arranged.

The vacuum dryer for sawn timber now becomes Drying wood used in the following ways:

First you layer the wood to be dried, the Dry goods 3, inside one opposite the environment Lockable cell 2 of the vacuum dryer 1 one on top of the other, one between the individual layers of wood Arranges heating elements 8. Then the Cell evacuated. Then the dry goods 3 by means of an expandable sack 7, which the dry goods 3 one side covers a large area, an adjustable pressure exercised while giving the wood thermal energy via the Feeds heating elements.

Here, the pressure, as described above, over a Differential pressure medium set. It can vary between the negative pressure prevailing in the cell and the Atmospheric pressure. It can also be larger than the atmospheric pressure, if that is for a special one Wood species is required.

The pressure is set in such a way that first a certain type of wood dries and with this Drying process experimentally determines how high the on The wood may be exerted pressure until it is in the wood Cracks result, which on the one hand by the with the Drying accompanying shrinkage of the wood, on the other hand but caused by the pressure exerted on the wood will. The pressure determined in this way can be used for any Wood type and wood thickness are determined, he represents a maximum pressure at which the wood in can be optimally dried without cracking and is always for future drying processes adjustable, for example by a computer control (not shown).

It can also be beneficial to avoid cracking be when the wood is only briefly under high pressure is applied. Through this pulsed pressure transfer is on the one hand due to the high on the wood pressure exerted a high heat transfer possible, on the other hand, the wood - after printing again has been taken away or reduced - wane without that there is pressure on him. This will increase the risk of Cracking in the wood further reduced.

The temperature of the Heating element set to a value on the one hand the greatest possible energy transfer to the wood enabled, but on the other hand is so low that no cracks in the wood.

In this way, you can use the most diverse types of wood by means of the vacuum dryer 1 and the Process for drying wood using this Vacuum dryer 1 optimal results when drying Wood can be achieved.

Claims (19)

1. A vacuum drier for sawn timber, comprising a cell (2) for holding material to be dried (3) in the form of superimposed layers of wood, comprising heating elements (8) arranged between the layers of wood, and comprising a pressing device for exerting pressure on the layers of wood, wherein the cell (2) is pressure-sealable on all sides from the environment and wherein the pressing device is arranged inside the cell (2), characterised in that the pressing device is an expandable bag (7) arranged inside the cell (2) and covering a large area of the material to be dried (3) on one side thereof, the bag (7) being subjectable to variable pressure.
2. A vacuum drier according to claim 1, characterised in that the bag (7) covers a large area of the upper surface of the material to be dried (3).
3. A vacuum drier according to claim 1, characterised in that the bag (7) covers a large area of the underside of the material to be dried (3).
4. A vacuum drier according to any one of claims 1 to 3, characterised in that the bag (7) is subjectoble to continuously variable pressure.
5. A vacuum drier according to claim 4, characterised in that the pressure is varied by means of a differential pressure means which permits a differential pressure to be set at a value which is variable between that of the atmospheric pressure and that of the negative pressure inside the cell (2).
6. A vacuum drier according to any one of claims 1 to 5, characterised in that the bag (7) is subjectable to a pressure having a value greater than that of the atmospheric pressure.
7. A vacuum drier according to any one of claims 1 to 6, characterised in that the bag (7) is subjectable to pressure in a pulsed manner.
8. A vacuum drier according to one or more of the preceding claims, characterised in that the heating elements (8) are flat, plate-shaped, hollow aluminium bodies which are arranged at regular intervals between layers of wood stacked one above the other and forming the material to be dried (3) and through which a medium of variable temperature flows.
9. A vacuum drier according to one or more of the preceding claims, characterised in that the wall of the cell (2) opposite the bag (7) is movable.
10. A vacuum drier according to one or more of the preceding claims, characterised in that the cell (2) has at least one sealable door (4) for filling the cell (2) with material to be dried (3).
11. A vacuum drier according to one or more of the preceding claims, characterised in that the material to be dried (3) is conveyable into the interior of the cell (2) by means of a carriage (6).
12. A method of drying wood, in particular using the vacuum drier according to the preceding claims, wherein wood as material to be dried (3) is first arranged in superimposed layers in a cell (2) of the vacuum drier (1), the cell (2) being pressure-sealable from the environment, wherein heating elements (8) are arranged at regular intervals between the individual loyers of wood and wherein the cell (2) is then evacuated, characterised in that variable pressure is exerted on these layers of wood by means of an expandable bag (7) covering a large area of the upper surface of the superimposed layers of wood, while thermal energy is supplied to the wood via the heating elements (8).
13. A method according to claim 12, characterised in that the variable pressure varies between the atmospheric pressure and the negative pressure in the cell.
14. A method according to claim 12 or 13, characterised in that a differential pressure means is used to vary the pressure.
15. A method according to claim 14, characterised in that the variable pressure is greater than the atmospheric pressure.
16. A method according to either one of claims 14 and 15, characterised in that the pressure is exerted in a pulsed manner.
17. A method according to any one of claims 12 to 16, characterised in that plate-shaped, hollow aluminium bodies, through which a medium of variable temperature flows, are used as heating elements (8).
18. A method according to claim 17, characterised in that the temperature of the medium is set at a value which allows as much energy as possible to be transmitted to the wood, but which is low enough to prevent cracks forming In the wood,
19. A method according to one or more of the preceding claims, characterised in that the material to be dried (3) is conveyed on a carriage (6) into the interior of the cell (2).

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