SE520855C2 - Ways and devices for drying wood - Google Patents

Abstract

The disclosure relates to a method and apparatus for drying wood (3). The drying takes place in a closed drying chamber (1) with the aid of elements (2) which emit radiation energy. The radiation is of such wavelength that it is absorbed by the water molecules in the wood, while the remainder of the wood is substantially unaffected. In order to control the drying, a number of indicators (7, 10) are provided which sense temperature and moisture within at least one of the wood parts (3) as well as temperature and relative humidity in the drying chamber (1). The interior of the drying chamber (1) consists of a material displaying high reflectance. The drying chamber further has a circulation fan (4) and a ventilation damper (14).

Description

10 20 25 30 35 520 855 For wood, there is a general curve described in the literature, the equilibrium moisture ratio at different dry and wet temperatures / relative humidities. The equilibrium moisture ratio is the moisture ratio of materials that are in equilibrium with the environment. For wood, the equilibrium moisture ratio is thus affected by the relative humidity in the ambient air and its dry and wet temperature. The conditions are in principle such that if you lower the relative humidity of the air surrounding the wood, the equilibrium moisture ratio in it decreases. The lower the equilibrium moisture ratio, the "drier" the wood. In the timber dryers that exist today, the drying usually takes place by allowing heated air to circulate around the scattered timber. Then the temperature and humidity of the heated air change gradually so that the wood has the desired moisture ratio, while at the same time it is hoped that the wood will have as few cracks and deformations as possible. The air temperature is regulated with flue gas heating, steam or electric heating and the air circulation is regulated with a fan system. However, the temperatures used often lead to the portions closest to the surface of the wood forming a waterproof layer which prevents the moisture from the inner parts from reaching the surface of the wood.

Due to the relatively long drying time when using known technology, there is always a general desire to be able to shorten the drying time and still maintain the same quality or preferably improve the quality of the dried wood.

Another general wish is that the energy consumption during drying should be as low as possible.

The above advantages and desires are met by a method and an apparatus according to the independent claims. The dependent claims state advantageous embodiments of the invention. Additional advantages of the invention will become apparent from the detailed description below.

The present invention is based on using only radiant energy (heat radiation) to heat the wood and that the radiation used comprises a wavelength range within which water has its highest absorption coefficients and that the temperature of the air passing past the wood during the drying phase of the wood periodically has a temperature which is lower or equal to the temperature of the wood.

Heat radiation has the characteristic property that no medium is required to transfer energy between two bodies. This can be compared to the sun's energy being transported to the earth. In conventional wood drying, a heating coil is used to heat the air and fans to supply the air and thus the heat energy to the wood.

With radiant energy in a narrow wavelength band where the water has a high absorption capacity, the radiant energy is transferred directly to the water molecules in the wood. This leads to significantly shortened drying times, significantly less energy consumption and an improved quality of the dried wood.

Compared with previously used technology, the invention provides a technique which makes it possible to work with a much higher relative humidity in the air surrounding the wood during drying. By using this high relative humidity, the quality of the dried wood is improved.

The invention will now be described in more detail with the aid of a number of exemplary embodiments according to the accompanying figures. Fig. 1 shows an example of a drying device comprising a drying chamber according to the present invention, Fig. 2 a section along the line II-II in Fig. 1, Fig. 3 an example of a rack with radiation sources according to the invention, Fig. 4 a sectional line IV-IV in Fig. 3, Fig. 5 is a section through an embodiment of a radiation source, Fig. 6 is a perspective view of an alternative embodiment of a radiation source, Fig. 7 is a perspective view of a dryer stack and Fig. 8 is a block diagram showing an embodiment of the control function according to the invention.

Hereinafter, the term "element" 2 will be used to refer to a radiation source.

The element is designed as a means that emits radiation comprising a selected path length range. Figures 1-2 show an embodiment of a drying device comprising a drying chamber 1 in which the wood is dried. The walls of the chamber are usually clad on the inside with stainless steel, aluminum or similar high-reflection material for radiation within the selected wall length range specified above. In other words, the interior of the drying chamber is designed as a large reflector. The walls are usually heat insulated.

The drying chamber 1 is arranged to accommodate drying stacks 6 which consist of wood 3 (Fig. 7), hereinafter also referred to as "wood", stacked in the usual manner with intermediate bedding. Figures 3 and 4 show a schematic diagram of the construction of a rack 5 which includes a number of elements 2. In some applications the racks are fixed in the drying chamber (Fig. 2) while in other embodiments (Fig. 7) they are designed to be placed in the drying stacks Fig. 4 shows an embodiment of a rack where a reflector 20 is arranged behind each element 2.

To provide good reflection of the radiation, the reflectors are usually made of aluminum, stainless steel or other highly reflective material. The rack 5 is provided as required with handles, suspension devices, electrical outlets, recesses for forklifts, etc. Normally the elements 2 are arranged in any direction in relation to the longitudinal direction of the wood 3.

In the embodiment shown, the drying device is provided with a circulation fan 4 and a ventilation damper 14. The circulation fan 4 circulates the air in the drying chamber and thereby transports moisture which leaves the surface of the wood. The ventilation damper 14 is used to regulate the size of the flow of air that is to leave the drying chamber and thus the relative humidity in the drying chamber 1. The temperature in the drying chamber 1 is regulated by means of the elements 2. For accurate control of the humidity in the chamber 1, sensor 10 is used. the relative humidity. In the drying device, a sensor 9 is also arranged for measuring the temperature in the drying chamber and / or for the air which leaves and / or is supplied to the drying chamber. In addition, there are sensors 7 that measure the temperature of the wood 3. As a rule, the temperature inside the wood is measured and in some embodiments also sensors 7 which measure the temperature of the wood. In some embodiments, there are also sensors 8 that measure the moisture content of the wood. This is usually measured inside the wood.

The sensors are often placed in the center of the wood, but this is not necessary because the location of the sensors is taken into account when regulating the drying process. In order to measure the moisture in wood, in some embodiments a scale 27 is used, where the difference between the measured weight and the weight of an ideal, dry wood gives the current moisture ratio.

The task of the fan system is to circulate air around the wood 3 and thereby take moisture from the wood surface. In the present invention, a flow velocity of 1-5 m / s is normally used, when the flow velocity is measured, for example, at the end of one or more drying stacks 6 or between drying stacks 6.

By suitable placement of the fan system's inlets and outlets, the air flow can be passed through the dryer stacks. In addition, by regularly reversing the flow direction in the fan system at regular intervals, an even distribution of humidity and temperature in the chamber is obtained.

The drying of wood can in principle be divided into three phases, namely heating of the wood, drying of the wood and finishing of the wood. In reality, the first two phases overlap, because even before the wood reaches the temperature range within which the temperature is to be kept during drying, a transport of water vapor starts out of the wood despite the fact that during the heating stage the wood humidity is kept so high that the air is mainly saturated.

In general, when radiant energy is applied to the wood, the water molecules absorb the radiant energy in a surface layer of the wood. As a result, the water in the surface layer is heated.

Since the radiant energy energy that is not absorbed by a single molecule reaches other molecules after it has passed through the individual molecule or been reflected by it, the radiant energy is also absorbed by water molecules inside the surface layer.

As a result, water in wood also heats up inside the surface layer.

According to the present invention, periods during which the elements 2 are energized as a rule for alternating heat energy are alternated with periods during which the elements 2 are not energized. During the "rest periods" of the elements 2, a vapor pressure is equalized by evaporated water going towards the surface of the wood from the interior of the wood. This keeps the surface of the wood moist. As a rule, the supply of energy to the wood is controlled so that at least during a part of the drying phase the surface of the wood is covered by a thin layer of water. The drying process continues until the desired moisture content of the wood has been achieved.

When the water is heated, water evaporates and when the partial pressure (the pressure generated by the evaporated water) exceeds the vapor pressure of the surface of the surface layer, the moisture ratio decreases in the wood containing evaporated water, ie in the wood forming the surface layer. In the direction of the center of the wood, the surface layer (at least after some time) borders on wood with a higher moisture content. The water contained in the wood located inside the surface layer leads spring energy from the surface layer towards the center of the wood. This causes evaporation of water which is heated. When the partial pressure of the formed steam becomes higher than the partial pressure in the surface layer, a certain equalization of the steam pressures takes place inside (closer to the center of the wood) the surface layer and in the surface layer. In other words, steam is moved into the surface layer and out of it through the surface of the wood.

With continued regulated heating of water in the surface layer, energy is transported further and further into the wood, whereby the process just described is repeated in areas closer to the center of the wood. When the temperature of the wood is highest in its center, the partial pressure of the evaporated water is highest in the center of the wood (theoretically) and lowest closest to the surface of the wood. This results in a transport of evaporated water from the center of the wood. The transport of evaporated water to the surface of the wood is so efficient that the humidity in the drying chamber 1 is mainly maintained by water or evaporated water which has left the wood.

During the latter part of the heating phase and at least also during the initial part of the drying phase, in a preferred embodiment, the relative humidity of the ambient air is kept at a level which is close to 100% and is usually within the range of 99%. -100%. The care just given for relative humidity in the air is used, for example, when drying pine or spruce. For other types of wood, other levels are used, which are located in the range 85% to 100%.

After the process has been going on for some time, as already stated above, the temperature is lower on the surface of the wire than in its interior. When the wood has reached a predetermined control temperature, the elements 2 are normally switched off, after which the surface of the wood cools faster than its interior. Thereafter, the temperature of the wood is periodically maintained at a level which corresponds to or exceeds the temperature of the ambient air. This is done, for example, by controlled connection and disconnection of the elements 2. At the same time, the relative humidity of the ambient air is kept at a relatively high level. which corresponds to the relative humidity of the surrounding air. In this way a safe transport of V water molecules is achieved from the surface of the wood to the air surrounding the wood. Fig. 8 shows an example of a control device for a drying device according to the present invention. The signals from the various sensors 740,27 in the drying chamber 1 are received and processed in a recording and calculation means 12, hereinafter referred to as PLC system 12. Depending on the received signals, the type of wood and the dimensions of the wood which to be dried, the PLC system 12 then controls the elements 2, the ventilation damper 14 and the circulation fan 4 so that the drying process described in the previous paragraph takes place. Those skilled in the art will appreciate that in principle it is also possible to run the drying process manually by continuously monitoring the values of the sensors.

The safety limit position 26 in the control diagram is used to interrupt the power supply to the elements 2 if any of the doors 25 in the drying chamber 1 are opened.

As already stated, the chamber 1 and the rack 5 containing the elements 2 are usually provided with reflectors of, for example, aluminum or stainless steel. These materials have reflection coefficients in excess of 95% at the frequencies used. Radiation that hits the reflectors is directed by them back towards the wood. There is no requirement for reflectors to be used, but they help to reduce energy consumption.

According to the invention, drying of the wood 3 takes place by means of elements 2 each in an embodiment designed as an electric heating coil 22, a tube 17, plates 18 or as a similar member. These elements 2 emit radiation in a limited wavelength range adapted to the absorption of water. The radiation generated is absorbed directly or after reflection of water present in the surface layer of the wood. In this case, the water evaporates with high efficiency, while the wood is otherwise mainly not affected by the radiation.

In the exemplary embodiment according to Fig. 5, the element 2 consists of an electrical resistor 22 arranged centrally in the tube 17, which is heated when current from a voltage source 11 via lines (not shown) passes through the resistor. Fig. 6 shows a plate 18 which in principle has the same construction as the tube 17 according to Fig. 5.

In the plate 18 is embedded electrical resistor 22 which is connected via wires 19 to a voltage source 11. According to the prior art, there are several examples of how to obtain with a suitable choice of material and suitable current the operating temperature of the radiation source which results in the radiation being maximized within the wavelength range at which water best absorbs radiation.

The wavelength band has been chosen for the interval about 2-20 μm and usually about 5-20 μm, intervals that contain wavelengths at which the water's absorption of radiation is large. In this case, the fact that within these ranges water has peaks with an absorption coefficient higher than 1000 cm -1 is used.

In an embodiment shown in perspective in Fig. 7, a drying stack consists of a number of racks 5 interspersed with wood 3 to be dried. Each rack 5 includes one or more elements 2, which in the embodiment shown consist of the tubes 17. These tubes 17 are mounted shock-absorbing in the frame of the rack 5. The figure shows an embodiment where several pipes are placed in width. At least one of the racks 5 is provided with an electrical outlet 13 for energizing all elements 2 via internal connections in and between the racks 5. The electrical outlet 13 is connected to a voltage source 11 (Fig. 8). Furthermore, the rack 5 has recesses 15 for a forklift truck so that a drying stack can be easily driven in and out of the drying chamber 1.

Fig. 7 shows an example of how a dryer stack is built up. First, a rack 5 is placed in the bottom, after which a timber layer 16a is placed on top of the first rack 5. Depending on the thickness of the timber 3, a new rack 5 or another timber layer 16b is then placed on top of the previous timber layer. In the usual way, straw battens 21 are placed between the timber layers. The construction of the dryer stack then continues to the desired height with alternating application of rack 5 and timber layers.

Often only a single layer with planks, boards, etc. is placed between layers with elements 2. In other cases, such as with relatively thin details, it is possible with two or more wooden layers with a lath 21 in between. In other embodiments, the stated distances vary depending on the type of element 2, type of wood, dimensions, etc.

When a stacker is ready to be dried, it is driven into the drying chamber, for example by means of a forklift. The dryer stack usually ends with a rack 5 on top. Due to the design of the rack 5, it is possible to use the technology according to the present invention even with existing dryers without major alterations. If there is space, it is possible to place several dryer stacks next to each other. They can also be placed at an angle to each other.

In practical experiments, it has been shown that it is possible to obtain the desired drying of wood with good capacity and maintained good quality even in applications where element 2 is not included in the drying stacks but is only arranged on the walls and ceiling of the drying chamber.

When drying wood in a drying device according to Figs. 1-2 above, the process is regulated by means of the temperature in the wood 3. As a rule, the temperature is measured with an accuracy of 1 ° C, at the same time as the relative humidity of the air is regulated and the moisture content of the wood is measured. When the moisture content of the wood has dropped to fiber saturation, about 25-30%, the control temperature in the wood 3 is raised to a level which depends on the type of wood and quality. It is possible to maintain a high relative humidity. The moisture released by the wood is usually sufficient to keep the humidity at the desired level.

Using the temperature in wood 3 as the most important control parameter means that wood 3 is supplied with exactly the energy needed to achieve a certain drying temperature. Instead of measuring the temperature, you can in principle measure the pressure in the wood, since the 2nd derivative of the pressure is proportional to the temperature at constant volume. However, it is much easier to measure the temperature.

Depending on the type of wood and requirements for the quality of the dried wood, changes can be made with regard to, for example, heating times, drying temperature before fiber saturation, temperature increase before final temperature, final temperature or moisture ratios.

It is obvious to the person skilled in the art that the "constant" relative humidity in the drying chamber is adapted to the properties of the wood in question and as a rule within a relatively narrow range. For example, when drying oak, the humidity is kept at about 85% 14%. In the method according to the invention, it is important to pour this high humidity into the drying chamber 1 in order not to risk that the surface of the wood dries. During the heating phase and at least at the beginning of the drying phase, the relative humidity is higher, often it is 100% or close to 100%.

During drying, moisture from the surface of the wood 3 is taken up by the slowly circulating air in the drying chamber 1. When the wood moisture content has dropped to a level below about 10% or lower, in a short last part of the drying the humidity is raised for a short period, by that elements and fan are switched off. In this case, the humidity in the wood is raised slightly to the final desired moisture ratio of about 6-10%.

The above-mentioned example of temperatures and levels for moisture in wood and humidity is, as mentioned, primarily intended for oak. For other types of wood, other care applies, but the principle is largely the same. For conifers such as pine, the drying period is significantly shorter than for deciduous trees.

Those skilled in the art will appreciate that the above are merely examples of various regulatory alternatives and that there are many more possible alternatives and modifications within the scope of the appended claims. 520 855 By the method and apparatus of the present invention, the drying times have been shortened by 40-50% compared to those for conventional drying at the same temperature.

In addition, the wood shows less cracking and skew than with previously applied prior art. Because the invention makes it possible to dry the wood 3 at a lower temperature than according to the prior art, it is also largely avoided that the wood shows color changes and "kada pricking. Automate means that the drying process does not require the intervention of a machine operator, inspector or any similar person, but takes place from start to finish completely automatically.

Taken together, it can be stated that the invention also provides a technique which results in shorter drying time, lower energy consumption and reduced occurrence of crack formation and deformation of the wood during its drying.

The above detailed description has referred only to a limited number of embodiments of the invention, but it will be readily appreciated by those skilled in the art that the invention encompasses a large number of embodiments within the scope of the appended claims.

Claims (13)

W 20 25 35 520 855 1: @:. @ ¿F (a fi 2001- 12-04 \\ CURRENT \ DB \ PUBLIC \ DOC \ P \ S0 ll PATENTKRAV Method for drying wood (3) in a drying chamber (1) using air which passes the wood, where at least one element (2) arranged in the drying chamber emits heat radiation, characterized in that during the drying phase the air has periodically a temperature that is lower than or equal to the temperature of the wood and that the radiation includes the wavelengths at which the water has peaks for absorption of radiant energy with absorption coefficients that are greater than about 1,000 cm ”. Method according to claim 1, characterized in that the radiation has been selected to the interval cza 2-20 μm and as a rule cza 5-20 μm. Method according to Claim 1 or 2, characterized in that a path (27) and a calculation means (12) are used for determining the current moisture ratio of the wood. Method according to one of the preceding claims, characterized in that at least during the drying phase the humidity in the drying chamber (1) is mainly maintained by water or evaporated water leaving the wood (3). Method according to one of the preceding claims, characterized in that at least during the initial part of the drying phase, when drying spruce, pine or wood with corresponding properties, the relative humidity of the air in the drying chamber (1) is kept at a level which is close to 100% and is usually located in the range 99% -100%. Method according to one of the preceding claims, characterized in that the temperature and / or the moisture ratio in the wood (3) and / or the humidity in the drying chamber (1) are sensed to control the switching on or off of the elements (2). U 20 25 30 35 520 ass) g §jg ¿v 2001-12-04 \\ CURREN'I '\ DB \ PUBLIC \ DOC \ P \ 5 Il Method according to one of the preceding claims, characterized in that during parts of the drying process said element (2) is switched off and that the length of said element shutdowns and the time intervals between the shutdowns change as the wood moisture content changes. Device for drying wood (3) in a drying chamber (1) according to the method according to any one of the preceding claims, characterized in that the drying chamber comprises at least one element (2) arranged in the drying chamber for emitting heat radiation at wavelengths , in which the absorption of the water by a fan (4) lation of air in the drying chamber, radiation is large, is arranged for circulating - that sensors (7, 8) are arranged for sensing the temperature and / or the moisture ratio in (3) , by the temperature and / or relative humidity of the air wood that sensors (9, 10) are arranged for sensing in the drying chamber (1) and that a control system and the fan (4) are based on signals received from the sensors. (PLC system) is arranged to control the elements (2) base Device according to claim 8, characterized in that the elements (2) are mounted in racks (5) and that the racks (5) have surfaces with high reflectance. Device according to Claim 8 or 9, characterized in that the drying chamber (1) is built up of a chamber which is lined on the inside by plates of a high-reflectance material, stem (4) in that the drying chamber (1) is provided with a fan system. - (14), are arranged to sense temperature and humidity (27), to sense the weight of the wood and that the signals from all gi- (7-10, 27) and ventilation dampers to sensors (9, 10) in the drying chamber (1), that sensors are arranged to last (12). is transferred to a calculation and control body 10 520 855; §g¿§ {a§; = 2001- 12-04 \\ CU'RRENT \ DH \ PUBLIC \ DOC \ P \ 50400 (11 11. ll. Device according to one of Claims 8 to 10, characterized in that the rack (5) with elements (2) is arranged to be placed on the walls of the chamber, and / or between (6) with wood (3) stacks to be dried and / or or in such stacks that the rack (5) with the elements (2) and the wood (3) to be dried are arranged in alternating layers and that the elements (2) are tubular or platform-shaped. characterized in that (22) or the like. Device according to claim 11, (2), an electrical resistor (18) comprises each element surrounded by a tube (17), a plate characterized in that (22) is made of material having properties which give a radiation Device according to claim 12, the part surrounding the electrical resistance maximized within the wavelength range at which water best absorbs radiation.