FI112696B - Method and apparatus for drying wood products - Google Patents

Abstract

Method and arrangement for drying of wood by circulating hot air around and through one or more piles (3) of the wood, arranged in batches in one or more closed drying chambers (2 a-f) in a chamber drier (1), the circulating hot air being first caused, in a heating phase, to heat the wood to the starting temperature for drying, subsequently in one or more drying phases accomplishing the actual drying, and finally, in a conditioning phase having a specially adapted air moisture content and air temperature other than during the drying, water vapour being supplied at atmospheric pressure by means (9 a-f), during the heating phase and the conditioning phase, to the hot air circulating around the wood in the drying chamber (1). The water vapour, which is generated in a vapour generator (10) and stored in an accumulator connected thereto, is supplied at a vapour generating capacity of 2-28 kW/m<3>, preferably 10-15 kW/m<3>, batch volume of the wood. The vapour generator (10) can be arranged to be supplied, preferably via a hot water conduit (12), with energy from a solid fuel central boiler plant (13) which is adapted to work by burning sawmill refuse, mainly bark.

Description

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Method and apparatus for drying timber

FIELD OF THE INVENTION This invention relates to a method for drying wood by circulating hot air around and through one or more wood piles disposed in one or more closed drying chambers of the chamber dryer, wherein the circulating air first causes the wood to warm during drying. 10 during subsequent drying of one or more drying steps, and finally, in a conditioning step having a specially adapted air humidity and air temperature different from the drying step, the post-drying moisture ratio and stress equalization of the wood as perfectly as allows. The present invention also relates to apparatus for carrying out the process of the present invention. This includes a chamber dryer having at least one drying chamber for batch drying wood stock, an apparatus for moving one or more wooden stacks in and out of the drying chamber, an apparatus for circulating hot air in the drying chamber and an apparatus for stabilizing hot circulating air, and for transporting in and out of the drying chamber.

Background of the Invention

Artificial chamber drying of timber nowadays utilizes hot air as a medium for conducting heat and humidity. This means that heat is transferred to the wood surfaces through the heating step as well as during the drying step. Experiments have also been conducted with alternative drying methods, for example, by supplying the heat energy required for heating and evaporation as microwave energy. Today, however, the air circulation method is. ; 35 drying process prevailing in the sawmill industry, among other things, because bark and sawdust can be used as inexpensive fuel in the sawmill's own heating plant.

As more and more timber is dried to low humidity, which is adapted to the requirements of the carpentry industry 5, it is also necessary to complete the drying with a so-called conditioning step. This drying finishing step aims to smooth the moisture differences between the "wood individuals" as well as within them, and to eliminate the stresses in the wood that develop during the drying step. If this conditioning step is omitted or performed incorrectly, the wood will deform during processing and splitting.

Product development to date to improve chamber drying has mainly focused on developing programmed control of the climate during drying and conditioning with certain basic properties such as maximum heat effect, batch volume, wood dimensions, etc. On the other hand, much less effort has been invested in 20 in terms of loss and time use / drying capacity to optimize this method. In this context, it can be demonstrated that the heating step and the conditioning step have been particularly neglected.

, Technical Problem 25 When drying timber, the wood may: · cause a certain quality impairment associated with the drying process. For example, crack formation is a type of drying damage that typically occurs in very thick timber. The formation of cracks in the wood is a drying damage that occurs early in the drying process. This is a consequence of the fact that the mechanisms that are present in the cracks are; the cause of the wood is strongly related to the moisture content of the wood surface; stress conditions. The crack appears in principle s *

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3 when the tensile stress transverse to the direction of the fibers exceeds the tensile strength.

It is generally known that the interaction between strength and modulus of elasticity, which together produce the fracture-5 core property, leads to a tendency to crack, which decreases as the temperature of the material increases. In addition, wood behaves more and more plastically at higher temperatures, i.e. wood tends to behave inflexible. For example, wood can crepe, especially under stress and humidity fluctuations. This type of creping, commonly referred to as mechano-sorption creping, is in fact a drying space for very thick wood with no crack formation.

When sawing wood prior to drying, the wood surfaces are bare, whereby they radiate water vapor into the environment, i.e. drying of the surface begins. At this stage, this drying is uncontrolled, i.e. external conditions in the form of "wind and weather" determine the drying efficiency. It has been found that wood that is left in the wood store will appear 20 cracks in just a few days, possibly earlier in hot and dry weather. These cracks then. grow during the next artificial drying of the initial,,; the stress concentration caused by inherent cracks. SIO. In this case, it is thus not possible to dry such timber without the presence of significant cracks.

However, it has been found that such surface cracks may also occur in the dryer during the heating step. The purpose of this step is to heat the batch of wood to the • initial temperature of the actual drying stage. If the heating is carried out by conduction recirculating air through the trees; around, this air dries almost immediately by condensation on cold wood surfaces. Thereafter, uncontrolled drying begins as the warmer air absorbs moisture from the wood surfaces, which thus dries. At this stage, the surfaces of 1 iL U> Ο 4 are already so dry that tensile stress is transverse to the direction of the fibers, especially if the surface is low water content heartwood, and a crack may form. Wood also has a low temperature during the early stages of heating, which is disadvantageous in terms of the tendency to form cracks.

In summary, uncontrolled preheating and heating of timber causes elemental cracking of the wood and the growth of these cracks as drying continues.

In air circulation drying there is always a moisture gradient on the surface. The surface is thus drier than the interior of the wood block and this difference in moisture ratio (gradient) corresponds to the driving force that causes the moisture to flow towards the 15 surfaces. The resulting tension causes creping, which leads to a tension state at the end of drying. Thereby, the surface is in a compression stress state, while the interior of the wood is in a tensile stress state. These stress differences lead to deformation, especially when splitting wood.

20 In addition to stress differences, which are a result of air circulation drying, moisture also develops during drying. ratio gradient as mentioned above. The highest moisture ratio is in the center of the wood, while the surfaces are significantly * drier. Unwanted deformation when splitting wood ... 25 is also caused by this because the humid parts are exposed and dry further during shrinkage and hence deformation. In both of the cases presented, it can be stated that drying, which is performed without playing the moisture gradient in the cross-section of the wood, leads to deformation problems in the further treatment of the wood.

It can also be noted that leveling the moisture ratio inside the wood individual at low temperatures (room temperature) does not produce stress-free wood. This ai- •; * 35 comes from the fact that the necessary voltage c 1 c,:. 6.6 rustling requires a high temperature of the material. Thus, the standardization of the timber after the actual drying must in principle be carried out at as high a temperature as is possible for this material in the production technology 5, thus enabling the achievement of the goal of "stress-free timber with balanced moisture".

Objective of the Invention

As discussed above, it is important to heat the timber without causing surface cracks and to complete the drying-10 event with effective conditioning. Nowadays, very little attention is paid to the programmed control of heating, which may mean that the heating is performed with air that is too dry. On the other hand, if it is desired to keep the air humidity at a high enough level to avoid the formation of cracks, this results in extended heating times, which reduces the capacity of the drying plant.

Nowadays, the conditioning step is performed by raising the final stage of drying without moisture by feeding water through the nozzles or directly as steam from special evaporation boilers. In this case, the most common problem is that the water nozzles supply too small amounts of water. tees, and if steam is fed directly, they exist. : evaporation boilers are very low power. In these two; * 25 cases, the steam supply is aimed at setting a balanced climate in the drying environment, which results in the wood surfaces being wetted only a few percent above the desired final moisture ratio (average). In practice, this standardization event requires at least 24 hours: 30 for the board.

:! Above all, a completely different method is used. , New Zealand, where, after drying Pinus radiata (radiata pine) .. at high temperature, the wood is fed into special conditioning chambers where 35 vapors are generated from open water tanks. Significant wetting of the wood surfaces is achieved, which smooths the moisture ratio and prevents the formation of internal cracks during cooling. However, in order to achieve this, the wood must be cooled for some time to cause condensation at a temperature of 70-80 ° C.

It is an object of the present invention to provide a method and apparatus for drying wood in a chamber dryer that decreases the heating time prior to drying while preventing the formation of cracks during the heating, and that stabilizing the wood moisture and tension after drying steps more efficient.

Problem Solving According to the present invention, this object is achieved by a method of the type described and characterized in that during the heating step, normal pressure water vapor is introduced into the hot air circulating in the drying chamber used for heating. A preferred embodiment of the present invention. accordingly, the supply of steam to the drying chamber occurs immediately from the beginning of the heating step and is continued; preferably continuously continuously and preferably ... until the end of the heating step.

'; Optimal heating of wood prior to drying aims to avoid the formation of wood cracks at the same time when heating should be applied for as short a time as possible. According to the present invention, this is achieved, as mentioned above, by supplying a normal pressure; of these, water vapor into the drying chamber during the heating step.

. The water vapor then condenses on the colder wood surfaces. As a result, they are protected against uncontrolled drying and, as a result, are protected from the formation of cracks. In addition, 35 the heat of condensation is transferred as the water vapor condenses on the wood. f.

y 1 ', which gives a significantly higher heat transfer compared to heating by conduction alone. In other words, this means substantially faster heating of the timber before drying.

Even though the object of the process of the present invention is to dry the wood, this drying process is unexpectedly started by watering the wood vigorously by compressing the steam during the heating step. If the steam is supplied immediately from the beginning of the heating step, rapid heating is achieved by the circulation of hot air and, in addition to the heating caused by condensation heat transferred from the very beginning, complete wetting of the wood surfaces. As a result, the formation of cracks in the wood is prevented, which allows the drying method of the present invention to produce wood which is as crude as possible.

However, as mentioned above, it is not possible to completely avoid the formation of cracks if, after sawing prior to "artificial" drying, the timber is stored, for example, in a wood store such that small surface cracks are formed at this stage, which then grow. If the timber ..,; Properly stored after sawing, pre-dried very high quality lumber can be produced according to the present invention by a process that is time-saving and thus cost-effective.

The heating rate in degrees per unit time is largely limited by the thermal conductivity of the wood so that only such amount of heat can be applied to the surfaces as the wood 30 can conduct to its interior. In practice, this means that the heating rate is strongly dependent on the outside dimensions and determines the surface to volume ratio of the wood. However, it is a condition that sufficient vaporization enthalpy is available.

For a drying chamber having a volume of 150 m3, the power of about 35 to 2 MW, converted to a vaporization enthalpy, is sufficient to provide a heating time of about 2 to 6, 6: 6. This corresponds to a steam generation capacity of approximately 13 kW / m3 per batch volume of timber. The vapor generation capacity according to the present invention should be in the range of 6-28 kW / m3 per batch volume of timber, preferably 10-15 kW / m3 per batch volume of timber. The duration of the heating step is generally between 1.5 and 2.5 hours, and at the end of the heating step the temperature of the wood is between 50 and 65 ° C. This heating time is valid under normal conditions, and it is understandable that in winter, when the wooden house reserve may even be frozen, the heating time is considerably longer.

The batch volume may vary between different dryers, the current one having a batch volume of 150 m3 in many cases, while older dryers generally have a smaller batch volume, for example 60-70 m3.

After the heating step, the drying of the timber is carried out in one or more drying steps, during which the hot drying air is allowed to circulate through the timber stacks. The drying process can take 2-14 days. During this period, hot air is supplied to the drying chamber and damp air is removed. After the drying step, when the final temperature of the wood is between 50 and 90 ° C, a stabilization step is initiated. A suitable embodiment of the method of the present invention, i>; the shape is such that, during the conditioning step, water vapor of normal pressure is fed around the timber at a constant rate; the hot air circulating in the drying chamber used for the drying step, and that this steam supply is started V * immediately at the beginning of the conditioning step and substantially immediately after the completion of the drying step (s). Preferably, the same drying chamber should be used for heating; the drying step (s) and the conditioning step. a.

Effective conditioning of the wood after drying should be: · carried out at a temperature as high as 35 ° C. This causes the tensions to release quickly and the fluctuations in humidity to equalize. By supplying water vapor to the drying chamber immediately after completion of drying, the steam condenses on the wooden surfaces. As heat is thereby released, the temperature of the wood rises, which is favorable for the release of the tensions 5, while the dry wood surface becomes wet and the moisture gradient is leveled.

Analogous to the state of the heating phase, the steam generation capacity during the conditioning phase is determined by the heating rate. It can be shown that the initial heating weighs 10 kilos per kilo of wood with the highest capacity, so that a steam generation capacity of about 2 MW / 150 m3 of wood during the conditioning phase is sufficient to equalize the moisture ratio and stresses in the wood within 1-2 hours. The water vapor supplied during this step should be fed at a capacity of 6 to 28 kW / m3, preferably 10 to 15 kW / m3 per batch volume of timber. During the conditioning phase, the wood is allowed to reach a final temperature of between 70 and 98 ° C.

An embodiment of the method of the present invention is that the heating step and / or the conditioning step 20 is controlled by a temperature rise that can be measured during the conditioning step with the usual V of the drying chamber used; with a temperature sensor that is monitored and converted to an average increase in the humidity ratio of the wood. The connection between the rise in heat and the amount of condensate makes. . 25 make this possible. Because heat transfer is efficient,; the temperature of the wood is thus close to the temperature recorded by the conventional dryer temperature sensor. By following this '' temperature evolution, it is possible to convert the temperature rise to the average humidity increase of the wood, which allows f V 30 to effectively control the conditioning event. Thus : ; the dryer does not need to be equipped with special sensor systems for this purpose.

A preferred embodiment of the present invention is! such that the steam generator develops high quality steam and maintains it under pressure in the accumulator; and 10

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that steam from said accumulator is fed to the combined drying chambers by operation of the valves.

According to another preferred embodiment of the method of the present invention, the main energy for generating the steam 5 is obtained from the sawmill's own solid fuel heating plant. This means that sawdust, mainly bark, can be used to produce high quality steam.

The hot water is led from the solid fuel boiler plant to the main steam generator. In this steam generator, the hot water is led through the tubes to a cylindrical tank partially filled with water. The heat is transferred to the water, which begins to boil, producing steam at the top of the cylinder. The system is kept pressurized so that steam can be rapidly removed through a stainless steel main line which operates by supplying steam to the combined drying chambers. The supply of steam to each chamber is finally accomplished by operation of the valves.

The advantage of such an apparatus is that the main steam generator can serve several chambers intermittently during both the heating phase and the conditioning phase. In addition = this method means that the changeover times are short because steam is introduced into the chamber as soon as the valve opens. · 25 inches.

Steam is supplied to the drying chamber in a saturated thi; in slow motion or slightly overheated. If the steam is supplied through a '' 'pipeline, a certain superheat of the steam (and thus excess pressure) is necessary to provide the necessary pressure from the steam generator throughout the supply system.

The overpressure may be between 0 and 101 kPa (0-1 atm) of valve pressure. However, the steam is not allowed to overheat,. because too much dry heat is being supplied. Crucial to this invention is that the dew point of the vapor, both during the heating phase and during the conditioning phase, is above the 22 1 ic 1: 1 dew point of the wood, thereby ensuring sealing to the wood surfaces. In the case of steam being generated directly in the drying chamber in an open evaporation system, the steam is fed in a saturated state.

An embodiment of the present invention will be described below with reference to the accompanying drawings, in which:

Figure 1 is a schematic vertical sectional side view of the chamber dryer; and

Fig. 2 is a schematic horizontal sectional view of the chamber dryer 10, viewed from above.

Figure 1 illustrates a chamber dryer 1 having a drying chamber 2 for drying wood in batches, wherein the wood is in the drying chamber in the form of one or more stacks 3. The wood stacks are located in a cart (4) or the like and apparatus (not shown) for moving one or more wood stacks 3 inside and out of the drying chamber 2 through a door 5 is opened. The drying chamber 5 further includes an apparatus 6 (schematically shown) for circulating hot air in the drying chamber for performing all, or at least one of the steps of heating, drying, and conditioning the timber, and an apparatus (not shown) for supplying hot air; is moved into and out of the drying chamber through channels 25 7, 8. The apparatus of the present invention is such that the drying chamber further comprises apparatus 9 for supplying and distributing normal pressure water vapor to the drying chamber, and that the steam generator is arranged to supply water vapor to these apparatus through ducts 11.

The apparatus 9 may consist of adjustable valves with nozzles and may be located in one or more locations within the drying chamber for direct injection of water vapor into the free air space of the drying chamber. Alternatively, they can be arranged to supply water vapor 12 1 i i "0 ^ v .." to hot recirculated air in connection with the recirculation apparatus 6. They can also be placed to supply water vapor both directly to the drying chamber and to the recycling apparatus.

Figure 2 illustrates a structure having a plurality of drying 5 chambers 2 a-f provided with means 9 a-f for supplying steam to a corresponding chamber. The steam generator 10 serves all the drying chambers intermittently, both during the heating step and during the conditioning step. The steam generator must be equipped with a steam accumulator (not shown) and designed to be kept pressurized to supply steam through the equipment 9a-f to the corresponding drying chamber. The apparatuses 9 a-f suitably consist of nozzle valves which are remote-controlled. The steam generator ensures that sufficient steam 15 is available in each drying chamber during both the heating step and the conditioning step.

The steam generator 10 can be provided, preferably by means of a hot water line 12, with energy from a solid fuel boiler plant 20, schematically represented by 13, which operates to burn sawdust, mainly bark.

Claims (23)

A method of drying wood by circulating hot air around and by one or more stacks of the wood, in batches, arranged in one or more closed drying chambers in a chamber dryer, wherein the circulating hot air is first brought into a heating phase. hot timber to the starting temperature for drying, then in one or more drying phases can effect the drying itself and finally in a conditioning phase with a different specially adapted air moisture content and air temperature than during the drying, so as to allow the process to equalize the moisture after drying, during the heating phase, saturated water vapor at atmospheric pressure, the circulating hot air is supplied to the wood in the drying chamber used for the heating phase, and the water vapor is supplied with a steam power of 6-28 kW / m3, preferably 10 - 15 kW / m3 of batch volume. , so that the water vapor is condensed on the cooler wood surfaces and The condensation heat is then transferred to the wood. Method according to claim 1, characterized in that water vapor is supplied to the drying chamber substantially; ·: immediately from the start of the heating phase. Method according to claim 1, characterized in. . By adding water vapor to the drying chamber in the housing; , presumably the end of the heating phase. Method according to claim 1, characterized in that the supply of water vapor in the drying chamber during the heating phase is carried out substantially continuously without being kept. The method according to claims 1-4, characterized in that the duration of the heating phase is between 1.5 and 2.5 hours. 6. A process according to claims 1 to 5, characterized in that the wood during the heating phase produces an final temperature between 50 - 65 ° C. 18 1, ... υ 7. A process according to claims 1-6, characterized in that during the conditioning phase water vapor at atmospheric pressure is supplied to the drying chamber circulating hot air in the drying chamber used for the conditioning phase, and that this meadow supply is started immediately at the start of the conditioning phase. and substantially immediately after the end of the drying phase (s), and the water vapor supplied during the conditioning phase at atmospheric pressure is supplied with a meadow power of 6 - 28 10 kW / m3, preferably 10 - 15 kW / m3 batch volume of the wood. Method according to claim 1 or 7, characterized in that the same drying chamber is used for the heating phase, the drying phase (s) and the conditioning phase. 15 9. A method according to claims 7 - 8, characterized in that the duration of the conditioning phase is between 1 and 2 hours. Method according to claims 7 to 9, characterized in that during the conditioning phase the wood is brought to a final temperature of between 70 and 98 ° C. 11. A method according to claim 1 or 7, characterized in that the heating phase and / or the conditioning phase is controlled by the temperature increase which can be measured with the regular drying chamber temperature sensor used, followed and converted to the average moisture ratio increase in: *: the wood. . Method according to claim 1 or 7, characterized in that in a meadow generator high-quality meadow is generated and poured under pressure into an accumulator, and that the meadow "... from the accumulator is distributed to the connected drying chambers by control of valves. A method according to claim 12, characterized by -; characterized in that the primary energy for generating the used meadow in the meadow generator is taken from the sawmill's own solid fuel plant from the sawmill's waste products, mainly bark. 19 1 · ί.: Υ> 1 · 14. A method according to claim 13, characterized in that a central steam generator serves several drying chambers sequentially for both the heating phase and the conditioning phase. 5 15. A method according to claim 1, characterized in that the wood during the drying phase (s), between the heating phase and the conditioning phase, is imparted a temperature between 50 and 90 ° C and that the drying phase has a duration between 2 and 14 days. 10 Process according to the preceding claim, characterized in that the steam is supplied with an overpressure of between 0 and 101 kPa (0-1 atm). Device for carrying out the method according to claim 1, comprising a chamber dryer (1) having at least one drying chamber (2) and intended for batch drying of wood, means for moving one or more stacks (3) of the wood into in and out of the drying chamber (2), means (6) for circulating hot air in the drying chamber for carrying out all or at least one of the steps of heating, drying and conditioning the wood, and means for spreading the warm circulating air and transport -To the tiling and from the drying chamber, characterized in that the drying chamber (2) additionally has means (9) for introducing and spreading saturated water vapor of atmospheric pressure: 25 in the drying chamber (2), and that a steam generator ( 10) is arranged to deliver water vapor to these agents (9). . Device according to claim 17, characterized in that the means (9) for insertion and spreading of; The water vapor is arranged at one or more places in the drying chamber (2) for introducing the water vapor directly into the air volume of the drying chamber. v Device according to claim 17, characterized in that the means (9) for introducing and spreading water vapor are arranged to introduce the water vapor into the connection to the means (6) for circulation of the hot air. 20 1ΐ-ϋ ^ · Device according to claim 17, characterized in that the steam generator (10) is provided with an accumulator for meadow and designed to be poured under pressure and to deliver meadow to the air via ducts (11) and means (9 af) respectively. drying chamber (2 of). Apparatus according to claim 17, characterized in that the steam generator (10) is arranged to be supplied preferably by means of a hot water pipe (12) from a solid fuel center (13) designed to be fired with the waste products of the sawmill 10, primarily bark. Apparatus according to claim 21, characterized in that the steam generator (10) is designed to operate multiple drying chambers (2a-f) sequentially for both the heating phase and the conditioning phase. Device according to claims 17 - 22, characterized in that the means (9 a-f) are made of valve means provided with nozzles.