WO2006077073A1

WO2006077073A1 - Method for drying wood

Info

Publication number: WO2006077073A1
Application number: PCT/EP2006/000364
Authority: WO
Inventors: Franciscus Antonius Van Der Heijden
Original assignee: Stichting Famecon
Priority date: 2005-01-21
Filing date: 2006-01-17
Publication date: 2006-07-27
Also published as: NL1028081C2

Abstract

Method for drying wood, wherein the wood (1) is enclosed in a flexible plastic pack (2) which is connected to a vacuum device (48) , the content of the plastic pack (2) is vacuum sealed, and wherein the wood (l) is heated by means of an energy source (5) where the heat is generated directly inside the pack (2) .

Description

METHOD FOR DRYING WOOD

The invention relates to a method for drying wood in which the wood is enclosed in a flexible plastic pack which is connected to a vacuum device and the content of the plastic pack is sucked in by a vacuum.

Such a method is disclosed in US - A -6.634.118.

In the method disclosed therein, packs are filled with wood and connected to a vacuum device immersed in a liquid bath, which liquid can be heated in order to heat the wood in the pack, thereby accelerating the evaporation process .

This method is not very efficient because on the one hand the liquid bath must be heated, and on the other the heat transfer between the liquid and the pack and the wood is very low . In particular, it takes a long time for the innermost parts of the wood load to be heated sufficiently for them to lose their moisture .

The obj ect of the invention is to provide a method of the type mentioned in which these problems are avoided .

This obj ect is achieved according to the invention in that the wood is heated by means of an energy source in which the heat is generated directly inside the pack .

Because of the use of such an energy source the heat is transferred directly to the wood load without intermediate heating of a liquid bath . Moreover, if heating is applied on the basis of microwaves , the heat is generated directly in the wood both on the outer wall and in the central parts of the load . On the other hand, if electronic resistance is applied, the stack of wood is kept under a certain pressure , thereby avoiding deformation . Planks or beams are sawn directly from a trunk to a desired wood dimension, with minimal wood loss . Stacks are preferably formed from one type of dimension, lathing being dispensed with or used only minimally. One stack of planks or beams can now be formed in a closed plastic bag and the bag then connected to a vacuum pipe . The evaporation heat can now be supplied in a manner comparable to a magnetron, except that the power and wavelength are adapted so that the entire stack of wood is heated inside the bag . The radiation unit may be mounted permanently or detachably . In the latter case the radiation is transmitted along a long row of stacks of wood so that there is periodic heating and cooling of the wood . The heat can be supplied to the wood much more directly and economically, in terms of energy, by installing an electrically heated metal plate in a stack of wooden planks or beams , preferably with the same distance between each or several layers of planks or beams , or by providing insulated resistance wire between two plastic films .

Because of the direct electric current fed through the metal plats , or because of the resistance wire , the stack of wood is heated uniformly as one whole and the wood will also dry very uniformly . There is therefore a much lower risk of contraction cracks whilst the metal plates counteract warping of forcing of the planks or beams .

In addition to the system mentioned, i . e . that of placing a stack of planks or beams in a plastic bag, it is possible to place a stack of wooden planks or beams of the same dimension on a plateau and covering this with a plastic film, whereupon the plastic is pressed against the plateau so that it seals it . The plateau is provided with openings onto an enclosed space below it , which can be connected by a condenser to a vacuum pump . In this system too, a radiation unit is able to move along several stacks of wood . The radiation unit preferably moves along both sides of the stacks of wood to achieve uniform through-heating . In this system of stacks of wood packed in plastic the wood may also be heated by direct electric current supply through metal plates or through insulated resistance wire .

Both in the bag system and the cover system the wood is now compressed by the vacuum within the stack and by the atmospheric pressure of the outside of the plastic cover, preventing the planks or beams from bending or cracking . Because the drying process takes place in a vacuum at a relatively low temperature , there is no discoloration of the wood . If the drying energy is supplied directly by electric current through metal plates , drying may take place in a fairly high vacuum, causing the wood to be pressurised between the plates , whilst the wood is pressed from the sides by fitting a plate with proj ections on the side , these proj ections pressing against the wood between all the metal plates . It is desirable to increase the vacuum periodically so that the friction between the metal plates and the wood is eliminated and the planks are able to settle evenly to prevent cracking in the wood. We may also increase the friction by applying a vibration from the side of the stack . If the drying energy is supplied directly by electric current through metal plates or through insulated resistance wire , drying may take place without a vacuum or with a very long vacuum, in which case it is desirable to pressurise the stack of wood to counteract cracking and warping of the wood and to ensure that the heat is transferred from the metal plate to the wood . At the end of the drying process the temperature of the wood may be increased to over 100⁰C for a relatively short time to render the wood stress-free . Here the vacuum is increased if necessary, and the pressure difference is applied to the wood in one or more directions with hydraulic cylinders , for example .

Other characteristics and advantages of the invention will be explained in the following description with reference to the attached drawings , in which :

Figure 1 : shows a diagrammatic representation of a longitudinal section through a stack of wood to be dried, packed in a plastic bag .

Figure 2 : shows a diagrammatic representation of a cross- section through the stack of wood in Figure 1.

Figure 3 : shows a diagrammatic representation of a plastic net on which thin laths are placed to promote the supply of steam.

Figure 4 : shows a diagrammatic representation of a stack of wood in a longitudinal section, this wood being heated electrically with electric current through metal plates applied after each layer of planks .

Figure 5 : shows a diagrammatic representation of a cross- section Through the stack of wood in Fig . 4 , the stack of wood being vacuum sealed with film and connected to a vacuum pump . Figure 6 : shows a diagrammatic representation of a longitudinal section through a stack of wood to be dried, which stack is covered so that it is vacuum-tight with a film of plastic and is sucked in at the bottom whilst heat is introduced from above .

Figure 7 : shows a diagrammatic representation of a cross- section through the stack of wood in Fig . 6 , with a connection to the vacuum pipe system.

Figure 8 : shows a diagrammatic representation of the vacuum pipe system.

Figure 9 : shows a diagrammatic representation of an elevation of a drying system in which the energy wave system moves alongside the stacks , suspended to a circulating rail system.

Figure 10 : shows a diagrammatic representation of a cross- section with a row of stacks of wood to be dried, which stacks are covered with a plastic film and connected to a condenser and vacuum pump .

Figure 11 : shows a diagrammatic representation of a stack of wood in cross-section, which stack is electrically heated with electric current through metal plates , and where plates with a proj ection are fitted to the sides , or where the wooden planks proj ect outside the metal , whereupon the vacuum stack is covered directly with a plastic film.

Fig . 1 shows diagrammaticalIy a representation of a longitudinal section through a stack of wood 1 to be dried, the dimensions of the planks or beams of which are equal , whilst the round sawn planks or beams are steel wet and are possibly of fresh wood, so that they are still free from contraction cracks and are not warped .

This stack of plans of beams 1 is placed in a closed plastic bag 2 and tightly bound to a cover plate 3 on which there is a connection 4 to a vacuum pump so that when the pump puts the stack in a vacuum, the plastic bag will lie tightly against the wood causing the atmospheric outside pressure on the bag to compress the stack of wood so that the planks or beams cannot warp during drying .

This closed bag containing a stack of planks or beams can be positioned in or near a type of magnetron whose power and wavelength are adapted and can be set , if necessary, so that the temperature in the wood is brought close to the boiling point of water .

It is appropriate to move energy source 5 backwards and forwards above one or more stacks to provide a favourable system of heating and cooling by water evaporation in the wood. No metal may be used within the working area of energy source 5. A condenser in which the water evaporated in the wood condenses is fitted between a stack of food and the vacuum pump, which only begins to operate if the vacuum is insufficient .

By placing a stack of wood in a plastic bag the expensive vacuum boiler is not required, whilst the power of the vacuum pump will be minimal due to the small space within a stack of wood in which vacuum is to be sucked in .

Fig . 2 shows diagrammatically a representation of a cross- section through the stack of wooden planks or beams 1 placed in a plastic bag 2 , which stack is brought to a desired temperature as shown in Fig . 1.

In order to discharge the water vapour more easily to the condenser, the stack may be divided into two or more blocks 6 and 7 , between which a discharge slot 8 is provided in which an element is fitted to absorb the lateral pressure in a vacuum.

Fig . 3 shows diagrammaticalIy a representation of an element for keeping two stacks of blocks 6 and 7 in the plastic bag 2 a short distance apart , e . g . 3 mm, in the form of a plastic net 9 on which are fitted thin laths 10.

If the stack of wood 1 in bag 2 is sufficiently dry, the stack of wood can e brought a temperature exceeding 100⁰C in a low vacuum, thus converting the wood to a more or less plastic state , enabling the wood to be delivered stress- free and packed directly in plastic bag 2 in which bag the stack of wood has been dried . If plastic bag 2 consists of shrink film, it will wrap itself up to 100% tightly around the wood on heating so that the wood can be stored or delivered to a user .

Fig . 4 shows diagrammatically a representation of a longitudinal section through a stack of wood 11 to be dried, which is or is not heated electrically in a vacuum with electric current through metal plates 12 , which can be fitted after each layer of planks or beams 3 , wherein the metal plates 12 may possibly be replaced by electric resistance wire which may be insulated between two plastic films . At the end of the stack of wooden planks 13 of equal length and thickness the metal plates are electrically connected to each other with spring elements 14 so that electric current is able to flow from connection 15 through plates 12 , which are connected in series to terminal connection 16 , although the plates or resistance wire may also be connected in parallel or series-parallel . A maj or advantage of this system of heating is the uniformity with which the entire stack 11 is heated, with or without a plastic film cover, so that if required, the stack can be connected to a condenser and vacuum pump in order to reduce the boiling point of water in wood 13 and to compress the stack of wood, although this can also be achieved by applying extra weight to the stack or by compressing with hydraulic cylinders or compressed air .

Fig . 5 shows diagrammatically a representation of a cross- section through stack of wood 1 in Fig . 4 , wherein stack of wood 11 is heated by an electric current which flows through metal plates 12 or insulated resistance wire, and wherein stack 11 is sealed vacuum tight with film 17 by wedge-shaped strips 18 in floor openings 20 lined with rubber-like material 19. In order to protect film 17 on the sides of stack 11 against breakage, wooden or plastic cover plates 21 and 22 are screened against metal plates 12 by the vacuum, whilst laths 23 are fitted on plate 22 to form steam discharge openings to floor space 24 and to discharge 25 leading to the condenser and vacuum pump if shutoff valve 26 is open.

Fig . 6 shows diagrammatically a representation of a longitudinal section through a stack of wood 27 to be dried, which is covered vacuum-tight with a plastic film 28 which secured around edge 29 with a wedge-shaped wooden or plastic strip, for example , which is pushed into a groove 30 , wherein groove 30 can be lined with a rubber-like corner piece 31. At the top an energy source 32 can bring the stack of wood to a desired temperature and can be moved backwards and forwards over rails 33 , the advantage of this being that many stacks of wood can be brought to temperature simultaneously, whilst the water vapour is discharged via the underlying vacuum space 34 through a vacuum pipe 35 to a condenser .

Fig. 7 shows diagrammatically a representation of a cross- section through stack of wood 27 in Fig . 6 , wherein the stack may possibly be divided into two stacks 36 and 37 , with a water vapour discharge slot 38 in between, these stacks being kept apart by the element shown in Fig . 3. Energy source 32 may periodically bring the water in the wood to boiling point , whereupon the water vapour is discharged via the underlying vacuum space 34 through vacuum pipe 35 that can be sealed with a valve 39 and through main discharge 40 to the condenser and vacuum pump . If stack of wood 27 is sufficiently dry, the wood can be heated to over 100⁰C in a low vacuum, whereby the wood, which is now more or less plastic , is rendered entirely stress-free so that after cooling only straight planks or beams are formed which, because of the low drying temperature , , will have not or hardly changed colour .

Fig . 8 shows diagrammatically a representation of a possible vacuum pipe system in which stacking points 41 - for large stacks 42 and small stacks of wood 43 , are sealably connected on main vacuum pipe 44 to valves 45 , as a result of which water vapour is fed through a condenser 46 which receives cooling water 47 , wherein vacuum pump 48 maintains the vacuum level if necessary .

Fig. 9 shows diagrammatically a representation of a possible elevation of a multiple drying system in which the vacuum packed stacks of wood 49 are heated on the side by an energy source 50 , whilst energy source 50 is moved on a rail system 51 along stacks of wood 49 , wherein it is possible to move in one direction over the rail so that all stacks of wood 49 are periodically heated or so that heat source 50 moves backwards and forwards along some of the stacks of wood, whilst the remainder are removed dry and are replaced by new stacks of wet wood .

The maj or economic advantage of this system is that the drying installation can be very large , but low cost , with the possibility of extending it so that the drying time of a stack of wood may possibly be increased at low extra cost but with improved drying quality .

Fig . 10 shows diagrammatically a representation of a possible cross-section I - I through a long row of stacks of wood to be dried, shown in Fig . 9 , which row is covered with a plastic film 52 and is connected to main vacuum pipe 55 by means of a pipe 54 that can be sealed with valve 53 , which pipe feeds water vapour to a condenser, the stacks being kept in a vacuum by means of a vacuum pump . Energy source 50 suspended on rail 51 heats stack of wood 49.

Fig . 11 shows diagrammatically a representation of a stack of wood 56 in cross-section, wherein planks 57 of equal thickness , preferably freshly sawn, are heated electrically by current to be fed through metal plates 58 , whilst the stack is covered vacuum-tight with a plastic film 59 , as shown in Fig . 5. Plates 60 protecting the film on the sides are provided with local proj ections 61 , which are preferably vertically displaceable in relation to plate 60 , and which in a vacuum do not press against metal plates 58 but onto plans 57 resting against each other, to prevent contraction cracking during drying . Instead of proj ections 61 plans 57 may proj ect outside metal plates 58 for the same purpose . It is desirable to increase the vacuum in the stack of wood periodically during the drying process so that the pressure on planks 57 is eliminated and planks 57 are able to settle in order to remove shrinkage stresses . The friction between metal plates 58 and planks 57 may also be increased by applying vibration to the stack from the outside, which also enables the wood to condense . Proj ections 61 may be laths with limited length so that water vapour can be discharged, according to arrow 62 , to the underlying space 63 and condenser connection box 64. Laths 65 , which are of course thinner than planks 57 , allow water vapour to be discharged at the ends of stack of wood 56.

Claims

1. A method for drying wood, wherein the wood is enclosed in a flexible plastic pack which is connected to a vacuum device, and wherein the content of the plastic pack is vacuum sealed, characterised in that the wood is heated by means of an energy source , wherein the heat is generated directly within the pack .

2. The method according to Claim 1 , characterised in that the energy source is a device for generating radiation, particularly microwaves .

3. Method according to Claim 1 , characterised in that the load of wood is applied between two conducting plates and en electric voltage is applied between these plates .

4. Method according to Claim 3 , characterised in that a number of electrically conducting plates are placed in the load and a voltage difference is applied between each pair of adj acent plates .

5. Method according to any one of the preceding claims , wherein the load consists of sawn wood, characterised in that openings are provided inside the stack of sawn wood for discharging the moisture .

6. Method according to any one of the preceding claims , characterised in that the temperature is first set to a low value in a first phase of the drying process , and a temperature of 100⁰C to 15O ⁰C is set in the final phase .

7. Method according to any one of Claims 1-4 , characterised in that the plastic pack is manufactured from shrink film.

8. Method according to any one of Claims 1-5 , characterised in that a number of packs are connected to a vacuum device , and in that a heating device can be moved along the packs .

9. Method according to Claim β , characterised in that the packs are placed on after the other in a system in which the heating device is able to move along a closed circuit along which the packs are arranged .