SE505655C2 - Process for drying wood - 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

505 655 10 15 20 30 35 wood is a drying damage that occurs early in the drying process. This is due to the fact that the mechanisms underlying the occurrence of a crack are strongly linked to the moisture and stress state of the wood surface. A crack occurs in principle when the tensile stresses across the direction exceed the breaking strength.

It is generally known that an interaction between strength and modulus of elasticity, which together give the property elongation at break, leads to the tendency to crack to decrease with increasing material temperature. In addition, wood will appear increasingly plastic at higher temperatures, ie wood has the ability to behave non-elastically. For example, wood can creep, especially under the influence of barrier and moisture variation. This creep, which is often referred to as mechanosportive creep, is in fact a prerequisite for being able to dry wood of a coarser dimension without crack formation.

In conjunction with the sawing of the timber before drying, timber surfaces are exposed, which will thereby emit water vapor to the surroundings, ie a surface drying is initiated. Demia drying is uncontrolled at this stage, ie external conditions in the form of "weather and wind" determine the drying intensity. It has been shown that wood that remains on a timber floor will have surface cracks after just a few days, probably earlier in dry and hot weather. These cracks then grow during the continued article drying due to the stress concentration that arises in the crack initials. In this case, there is thus no possibility of drying such wood without a considerable cracking.

However, it has been found that such surface cracks can also occur during the heating phase of the dryer. The purpose of this phase is to heat the wood batch to the starting temperature for the actual drying phase. If the heating is convective by circulating air through the wood, this air will be dehumidified fairly immediately by condensing on the cold wood surfaces. Then an uncontrolled drying is initiated by the increasingly warmer air absorbing moisture from the wood surfaces, which thus dries. The surfaces can already be so toned at this stage that tensile stresses arise across the direction, especially if the surface contains heartwood with a low water content, and a crack can occur. The wood also has a low temperature during the initial stage of heating, which is unfavorable based on the tendency to crack.

In summary, it can be stated that an uncontrolled pre-drying and heating of wood with dry air leads to initial cracking of the wood and that these cracks grow during the continued drying.

Furthermore, in connection with air circulation drying, a gradient in moisture ratio always arises from the surface. The surface will then be drier than the inner parts of a piece of wood, this difference (gradient) in moisture ratio corresponding to the driving force which leads to a moisture fl fate to the surface. The resulting voltage leads to a creep which results in a state of tension at the end of the drying. The surface will then settle in a state of accidental tensioning, while the inner parts of the timber will be in These stress differences lead to deformations, especially when the timber is split up.

In addition to the voltage difference which results from air circulation drying, as mentioned, a moisture ratio gradient will also be formed during drying. The highest moisture content will be found in the center of the wood while the surfaces are significantly drier. During cleavage, an undesired deformation also arises for this reason, as the wettest parts are exposed and dry further during shrinkage and thus deformation. In both the related cases, it can be stated that drying that ends without the moisture ratio gradient in the timber cross-section being evened out, leads to deformation problems during the further handling of the timber.

It can also be stated that an equalization of the moisture ratio in the wood individual at low temperature (room temperature) does not produce stress-free wood. This is because the necessary stress relaxation requires a high material temperature. Thus, a conditioning of the wood after the actual drying phase must be carried out at in principle as high a material temperature as technically possible, in order for the goal of "a stress-free and moisture-quota-balanced wood" to be realized.

According to the reasoning above, it is therefore important to heat the wood without initiating surface cracks and to finish the drying with an effective conditioning. Today, little attention is often paid to the programmatic control of the heating, which can mean that the heating is performed with too dry air. If, on the other hand, one wants to keep the humidity of the air high enough to avoid cracking, this results in extended heating times, which reduces the capacity of the drying plant.

As far as the conditioning phase is concerned, it is carried out today by raising the air humidity at the final stage of drying by supplying water from nozzles or directly as steam from special steam boilers. The most common problem in this respect is that water nozzles provide too low a moisture supply and that if steam is supplied directly, the steam boiler in known embodiments is greatly undersized. In both these cases, the steam supply aims to set an iron-weight climate in the drying atmosphere which results in a wetting of the wood surfaces corresponding to only a few percent above the intended final moisture ratio (average value). In practice, this conditioning procedure requires at least one day for plank dimensions. 10 20 30 35 505 655 A radically different technology is mainly applied in New Zealand, where after high-temperature drying of Pinus radiata (radiata pine), the wood is placed in special conditioning chambers, where steam is generated from open water tanks. In this case, a strong moistening of the wood surfaces is achieved, which evens out the moisture ratio and prevents internal cracking in connection with the cooling. To achieve this, however, the wood must be cooled to 70-80 ° C for some time in order for condensation to take place on the wood.

The object of the present invention is to provide such a method and such a device for drying wood in a chamber dryer that the heating time before drying is shortened while crack initiation during heating is prevented, and that the conditioning, which is done to equalize moisture ratio and stresses in the wood after drying phase, shortened and streamlined.

L ... This object is achieved according to the present invention with a method of the kind indicated in the introduction and characterized in that during the heating phase saturated water vapor at Athens sheep pressure is supplied to the hot air circulating around the wood in the drying chamber used for heating, which water vapor is supplied with a steam effect of 6 - 28 kW / m3, preferably with 10 - 15 kw / m3 sstsvslym sv the wood so that water vapor is condensed on the colder wood surfaces and thereby transfers condensation heat to the wood. According to a preferred embodiment of this process, the steam supply to the drying chamber takes place immediately from the start of the heating phase and is then preferably continued continuously without interruption and preferably until the end of the heating phase.

Optimal heating of the wood before drying aims to avoid cracking of the wood at the same time as the heating should take place in as short a time as possible. This is realized as mentioned according to the invention thus by saturating water vapor being supplied at atmospheric pressure to the drying chamber during the heating phase so that the water vapor is condensed on the colder wood surfaces. Thereby, these are protected from an uncontrolled drying and associated cracking. Furthermore, condensation heat is transferred when the water vapor condenses on the wood, which gives a significantly greater heat transfer than with convective heating only. In other words, this means a significantly faster heating of the wood before drying.

Thus, although the intention of the process according to the invention is to dry the wood, this drying process is surprisingly preceded by the wood being vigorously wetted with condensing steam during the heating phase. If the steam is supplied immediately from the start of the heating phase, a rapid heating and also comprehensive wetting of the working surfaces is obtained from the outset by transferring condensing heat to convective heating with the hot circulating air. This avoids crack initiation of the wood, which enables a drying process according to the invention for the production of largely crack-free wood.

As mentioned earlier, however, cracking can not be completely avoided if, after sawing carried out before the "arli fi cial" drying, the wood is stored in such a way in e.g. a timber plan that already where small surface cracks appear and then grow. With a properly managed storage of the wood after sawing, with the invention a ready-dried wood of very high quality can be produced with a time and thus cost-effective procedure. The heating rate in degrees per unit time is ultimately limited by the wood's thermal conductivity so that only so much heat can be applied to the surfaces that the wood is capable of leading on to the inner parts. In practice, this means that the heating speed becomes strongly dimension-dependent and ultimately depends on the surface volume ratio of the wood. However, this applies provided that a sufficiently large steam enthalpy is available- For a drying chamber with a batch volume of 150 m3, approx. 2 MW power Transferred to steam enthalpy is sufficient to give heating times of approx. 2 hours. This corresponds to a steam power of approximately 13 kW / m3 batch volume of the wood. According to the invention, the steam power should be kept in the range 6 - 28 kW / m3 batch volume, preferably between 10 - 15 kW / m3 batch volume. The duration of the heating phase is usually between 1.5 and 2.5 hours and at the end of the heating phase the wood has been brought to a temperature of between 50 ° C and 65 ° C. This heating time applies under normal conditions and it is realized that during the winter, when the wood can even be frozen, the heating time can be noticeably longer. i.

The batch volume can vary between different dryers, today's dryers often have a batch volume of 150 m3 while older dryers usually have a lower batch volume, eg 60 -70 m3.

After the heating phase, the wood is dried in one or more drying phases during which the hot drying air is circulated through the wood stacks. The drying phase can have a duration of between 2 and 14 days. During this time, dry air is supplied to the dryer and moist air is diverted. After the drying phase, when the wood has been brought to a final temperature of between 50 ° C and 90 ° C, the conditioning phase begins. An expedient embodiment of the process according to the invention is characterized in that during the conditioning phase water vapor is supplied at atmospheric pressure to the hot air circulating around the wood in the drying chamber used for the conditioning phase and in that this vapor supply begins immediately at the start of the main conditioning phase. immediately after the end of the drying phase (s) and that the water vapor supplied during the conditioning phase at atmospheric pressure is supplied with a steam power of 6 - zs kW / mß, preferably 10 - 15 kw / m3 eatevelym of the wood Föfeuadeevie, the same drying chamber should be used for the heating phase - drying phase phases) and the conditioning phase.

Effective conditioning of wood after drying should be carried out so that the wood temperature is as high as possible. In this way, the sperm relaxes quickly and moisture variations are evened out. With the supply of water vapor to the drying chamber immediately after the end of the drying, the steam will condense on the wood surfaces. As heat is released, the wood temperature rises, which is positive for the stress relaxation, at the same time as the dry wood surface is moistened and the moisture gradient is evened out.

Analogous to the condition in the heating phase, the steam power during conditioning is dimensioned on the basis of heating speed. It can be shown that the initial heating requires the greatest power / kg wood, so it can be stated that a steam power of approx. 2 MW / 150 m3 wood during the conditioning phase is sufficient to and tension-ironed timber in 1-2 hours. During the conditioning phase, the wood is brought to a final temperature of between 70 ° C and 98 ° C.

An embodiment of the method according to the invention is characterized in that the heating phase and / or the conditioning phase is controlled by the temperature smoke which during the conditioning phase can be measured with the ordinary temperature sensor of the drying chamber used and monitored to increase the average moisture content of the wood. It is the connection between temperature increase and condensate volume that makes this possible. Since the heat transfer is efficient, the temperature of the wood will thus be close to the temperature registered with the dryer's ordinary temperature sensor. By following this temperature development, it is possible to convert the temperature increase to an average moisture quotient of the wood, which enables an advantageous control of the conditioning process. The dryer thus does not need to be equipped with special sensor systems for this.

According to a preferred embodiment of the method according to the invention, it is characterized in that in a steam generator high-quality steam is generated and kept pressurized in an accumulator, and that the steam from the accumulator is distributed to connected drying chambers by operating valves. 10 20 25 30 35 505 655 According to another preferred embodiment of the process according to the invention, the primary energy for generating the used steam is taken from the sawmill's own solid fuel center. This means that the sawmill's waste products, mainly bark, can be utilized for the production of high-quality steam. The hot water is led from the solid fuel plant to a central steam generator. In the demia steam generator, the hot water is led through tubes in a cylindrical, partially water-filled container.

The heat is transferred to the water which is brought to a boil, whereby a volume of steam is generated in the upper part of the cylinder. The system is kept pressurized so that the steam can be discharged quickly into the stainless steel pipe network, which has the task of distributing the steam to the connected drying chambers. The distribution of the steam into each chamber is finally done by operating the valves. The advantage of this arrangement is that a central steam generator can serve several chambers sequentially for both the heating phase and the conditioning phase. Furthermore, the process means that the set-up times become short because the steam is supplied to the chamber as soon as the valve opens. The steam is introduced into the drying chamber in a saturated state or slightly overheated. In the event that the steam is supplied via a pipe system, it follows that a certain overheating (and thus overpressure) of the steam is necessary to provide the required driving pressure through the distribution system from the steam generator. The advantage can be between 0 - 1 atö. However, the steam must not be too overheated as too much dry heat will then be added. Crucial to the recovery is that the steam dew point both during the heating phase and the conditioning phase is above the dew point of the wood so that a condensation on the folding surfaces.

The method according to the invention will now be described in connection with the appended rimming clocks, where Fig. 1 schematically shows a vertical section of a chamber dryer seen from the side, and Fig. 2 schematically shows a horizontal section of the chamber dryer seen from above.

In Figure 1, 1 is a chamber dryer with a drying comb 2 intended for batch drying of wood, which settles in the drying chamber in the form of one or more piles of wood 3.

The timber stacks are arranged on a trolley 4 or the like, and means (not shown) are provided for moving one or more stacks 3 of the timber into and out of the drying chamber 2 through the openable port 5. In the drying chamber there is further means 6 (schematically shown) for 50 15 655 655 circulation of hot air in the drying chamber for carrying out all or at least one of the steps heating, drying and conditioning of the wood, and means (not shown) for supplying dry air and diverting moist air which is transported to and from the drying chamber by channels 7.8. The drying chamber further has means 9 for introducing and dispersing water vapor of atmospheric pressure in the drying chamber, and that a steam generator 10 is arranged to supply water vapor to these means through pipelines 11.

The means 9 can consist of adjustable valves with nozzles, and can be arranged at one or more places in the drying chamber for introducing the water vapor directly into the free air volume of the drying chamber. Alternatively, they may be arranged to introduce the water vapor into the hot circulating air in connection with the circulating means 6. They may also be arranged to supply water vapor both directly to the drying chamber and to the circulating means.

Fig. 2 shows an embodiment with fl your drying chambers 2 a-f, provided with means 9 a-f for supplying steam to the respective chamber. The steam generator 10 here serves all the drying chambers sequentially for both the heating phase and the conditioning phase. The steam generator should be equipped with an accumulator (not shown) for steam and designed to be kept pressurized in order to deliver steam to the respective drying chamber via the means 9 a-f.

The means 9 a-f suitably consist of nozzles with valves which can be remotely controlled.

Thanks to the steam accumulator, sufficient steam capacity can be available for each drying chamber during both the heating phase and the conditioning phase. The steam generator 10 can be arranged to supply energy via a hot water line 12 preferably from a solid fuel plant, schematically shown as 13, which is to be fired with the sawmill's waste products, mainly bark.

Claims (16)

10 25 30 35 505 655 9 Bar-arm A method of drying wood by circulating hot air around and through one or more stacks of wood used in batches in one or two closed drying chambers in a chamber dryer, wherein the circulating hot air during a heating phase is brought to heat the wood to the starting temperature for drying, thereafter a or fl your drying phases may bring about the actual drying and finally in a conditioning phase with a specially adapted humidity and air temperature than during the drying so that the procedure completely allows the moisture ratio and tension in the wood to be brought after drying, characterized in that during the heating steam the hot air circulating around the timber in the drying chamber used for the heating phase, which water vapor is supplied with an input of 6 - 28 kW / m3, preferably with 10 - 15 kW / m3 batch volume of the timber so that water vapor is condensed on the colder wood surfaces and thereby transmits condensation heat to vir ket. Method according to sea 1, characterized in that water vapor is supplied to the drying chamber substantially immediately from the start of the heating phase. Method according to sea 1, characterized in that water vapor is supplied to the drying chamber until essentially the end of the heating phase. Method according to Claim 1, characterized in that the supply of water vapor to the drying chamber during the heating phase takes place substantially continuously without interruption. Process according to seas 1 - 4, characterized in that the duration of the heating phase is between 1.5 and 2.5 hours. Process according to Claims 1 to 5, characterized in that the wood is subjected to a final temperature between 50 ° C - 65 ° C during the heating phase. Process according to seas 1 to 6, characterized in that during the conditioning phase saturated water vapor at atmospheric pressure is supplied to the hinged wood circulating hot air in the drying chamber used for the conditioning phase, and in that this steam supply begins immediately at the start of the conditioning phase and substantially immediately after the drying phase. phase) and that the water vapor supplied during the conditioning phase at atmospheric pressure is supplied with a steam power of 6 - 28 kW / m3, preferably with 10 - 15 kW / m3 batch volume of the wood Method according to sea 1 or 7, characterized in that the same drying chamber is used for the heating phase, the drying phase (s) and the conditioning phase. Method according to claims 7-8, characterized in that the duration of the conditioning phase is between 1-2 hours. Process according to seas 7 -9, characterized in that the wood is subjected to a final temperature between 70 ° C - 98 ° C during the conditioning phase. Method according to Claim 1 or 7, characterized in that the heating phase and / or the conditioning phase are controlled by following the temperature change which can be measured with the ordinary temperature sensor of the dryer used and converted to an increase in the average moisture ratio of the wood. Method according to sea 1 or 7, characterized in that in a steam generator high-quality steam is generated and kept pressurized in an accumulator, and that the steam from the accumulator is distributed to connected drying chambers by controlling valves. Method according to sea 12, characterized in that the primary energy for generating the steam used in the steam generator is taken from the sawmill's own solid fuel center from the sawmill's waste products, mainly bark. Method according to sea 13, characterized in that a central steam generator serves bet your drying chambers sequentially for both the heating phase and the conditioning phase. Process method according to claim 1, characterized in that during the drying phase (s), between the heating phase and the conditioning phase, the wood is maintained at a temperature between 50 ° C - 90 ° C and that the drying phase has a duration between 2 and 14 days. Process according to the preceding patent sea, characterized in that the steam is supplied with an excess of between 0 - 1.0 atm.