FI84658B - FOERFARANDE OCH ANLAEGGNING FOER TORKNING AV ETT SKIVFORMIGT MATERIAL, TILL EXEMPEL ENKELT FANER. - Google Patents

Description

84658

The present invention relates to a method for drying a planar material, for example plywood veneer, according to the preamble of claim 1.

The invention also relates to an apparatus for carrying out the method.

In previous solutions, especially for drying plywood, roller dryers or mesh dryers have been used, in which the veneer has been dried with the help of hot water vapor. The moisture of the dried veneers has been measured and the wettest veneers have been separated and re-dried.

A disadvantage of the prior art is that the drying result obtained with a roller or net dryer alone is uneven, which increases costs and slows down the production of the final product. Current dryers cannot be adjusted to account for veneer-specific moisture differences. As a result, the veneers with the driest initial moisture become over-dried, i.e. they have to stay in the dryer for too long. Due to over-drying, the quality of the veneers deteriorates so that they become brittle, which easily leads to breakage in the processing steps after drying. Although some veneers are too dry, some may remain too moist. In factory runs, over-wet veneers should be about 15% dry. The necessary re-drying slows down the process.

In addition, U.S. Pat. No. 4,565,917 and FI Patent Publication No. 80099 describe apparatus using successive infrared dryers for drying planar material. However, the radiation wavelengths of successive dryers have been chosen unfavorably in terms of maximum drying efficiencies.

It is an object of the present invention to obviate the disadvantages of the technique described above and to provide a completely new type of method and apparatus for drying a planar material, for example plywood.

The invention is based on the use of two consecutive types of infrared emitters on the drying line, the wavelength of the maximum radiation emitted by the first emitter type being at least approximately equal to the absorption maximum of the water (and possibly the material to be dried) and the maximum emitter intensity in the short-wave range (about 1 pm) and the radiant energy applied by both types of radiators to the material to be dried is approximately equal.

More specifically, the method according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The apparatus according to the invention, in turn, is characterized by what is set forth in the characterizing part of claim 3.

The invention provides considerable advantages.

With the solution according to the invention, the total energy consumption of the drying has in some cases been able to be calculated. Fluctuations in the moisture content of the product will be reduced and the drying time will be shortened, which will increase the capacity of the dryer.

The invention will now be examined in more detail by means of an application example according to the accompanying drawings.

Figure 1 shows the definitions of the wavelengths used in the invention.

Figure 2 shows graphically the absorption curves of wood and water as a function of wavelength.

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Figure 3 shows a partially schematic side view of a drying apparatus according to the invention.

Figure 4 shows a cross-sectional top view of a radiating element according to the invention.

The measurement results are based on small-scale experiments investigating the effect of medium-wave and short-wave infrared radiation on plywood veneer in preheating and post-drying situations. In practical experiments, both the optimal drying event and the equipment structure were sought.

IR (Infrared) radiation is divided into short, medium and long wave radiation according to Figure 1. Each substance has its own characteristic absorption spectrum. It can be seen from Figure 2 that with water and wood the absorption maxima are in the same mid-wave range at about 3 μm. Based on this, both wood warming and water evaporation could be expected to be most effective in this wavelength range. However, since in practice the radiation encountered by the body can be completely or partially reflected, absorbed or displaced, the most effective drying effect is not achieved by the radiation adjusted to the absorption maximum alone, but by the combination according to the invention.

The IR device solution is based on the successive use of medium and short wave radiation. At a suitable distance, first the medium-wave and then the short-wave radiation are applied to the veneer so that the energy applied to the veneer by both wavelengths is approximately equal. In addition, a back reflector that improves IR efficiency is placed in the conveyor used in connection with the IR device solution.

According to Figure 3, the IR device according to the invention can be combined with the conventional drying of plywood veneers either before or after the roller dryer 6. Simultaneous use of a pre- and post-dryer is also possible. According to a certain moisture distribution, too wet veneers are passed through the IR device 4 84658 to either preheating or post-drying. The wet veneer sheets are brought to the moisture measuring equipment 8 by a conveyor belt 1, and the sheets are directed to a suitable drying line 15 to 18 by means of a control unit 8. The wettest veneers are pre-dried at a temperature of approx. 180 ° C at four different levels. After steaming, the veneers are cooled. After the roller dryer 6, the moisture of the veneers is re-measured by a measuring device 7, the measurement data is fed to a control device 9 which controls the separating device so that the directed to further processing line 19.

In the radiant heater according to Figure 4, the conveyor 22 with its veneer to be dried passes under the heater from left to right. The speed of the conveyor 22 is typically 70 to 80 m / min. The size of the veneers to be dried is 1.5 * 1300 * 1410 mm. The heater is divided into two zones of approximately equal area, namely the medium wave zone A and the short wave zone B. The total length of the zones A and B in the direction of travel of the conveyor 22 is 1500 mm and the total width of the heater is about 1550 mm. The midwave zone A consists of groups of three double tubes 21 installed at intervals of about 55 mm. In Medium Wave Zone A, the total number of twin tubes is 12. In Shortwave Zone B, there are six twin tubes and they are installed at approximately 65 mm intervals. Since all infrared heating sites must have sufficient air circulation to remove vapors and enhance drying, the heater according to the figure is equipped with two fans (not shown). The fans are located on the top surface of the heater approximately in the middle of each zone. The flow rate used was 50 l / s per pu control. With such a flow, the temperature of the blow loop was about 30-35 ° C. In addition, to increase radiation efficiency, the heater is provided with an aluminum, not shown, backplate designed to reflect radiation passed through the conveyor 22 back to the veneers. The conveyor 22 moves between the radiator and the backplate.

In the practical tests performed with the device according to Fig. 3, infrared heaters were used as preheaters in connection with the roller dryer 6. The material to be dried was birch veneer. Spruce veneer has also been tried, in which case the power requirement has proved to be lower. The moisture content of most of the material was 80%. About 15 to 18% of the material had a moisture content of more than 90%. In the initial infrared drying, either medium-wave infrared alone, short-wave infrared alone, or combinations were used, the first of which uses medium-wave infrared first and then immediately short-wave infrared, and the second the same combination in reverse order. The power used and the exposure time were kept constant in each case.

The best combination of medium-wave and short-wave lamps proved to be the best combination in the experiments, in which the material to be dried first encounters medium-wave radiation with a wavelength corresponding to its maximum intensity approximately equal to the joint absorption maximum of veneer and water. The shortwave radiation then performs the final drying. The powers of both drying units were approximately the same, averaging 41.5 kW per square meter of veneer, and the radiation exposure time in both areas was 5 s. However, due to practical arrangements, 46% of the square power consists of average power and the rest of shortwave power. The distance of the infrared emitters from the veneer was 140 mm. The radiators used were clear quartz glass gold-plated double tubes manufactured by Hera-eus. The average wavelength of the double-wave tubes used is 1300 mm, the cross-sectional area 33 * 15 mm2, the power 3250 W and the wavelength range 1.5 ... 6.0 μm, with a maximum intensity wavelength of about 3 μm. 3 * 11 mm2, power 7000 w 6 84658 and wavelength range 0.5 ... 2.5 μm with a maximum intensity wavelength of approx. 1.2 μm The mentioned wavelength values are achieved with maximum minimum powers. The maximum exposure time used was 6 s / zone.

After the initial drying, the moisture content of the veneer batch decreased by 5.0 ... 9.7 percentage points. On average, the humidity percentage decreased by 7 units. Combined drying achieved a 10% increase in capacity compared to conventional drying. The shelf life was reduced by 10%. Electricity accounted for about 14% of energy consumption. Total energy consumption decreased by about 10%.

The second experiment tested a combination of a roller dryer and infrared post-drying. When using infrared drying for post-drying, all veneers with a moisture content of more than 5-6% should be directed to infrared drying.

The IR post-drying experimented with the same wavelength combinations and arrived at the same combination as the pre-drying. The average power was 43.3 kW / m2, of which 48% was average total power. The maximum exposure time used was 6 s / zone.

In the post-drying, the veneers with an initial moisture content of either about 10% or about 15% were dried. In infrared drying, the humidity decreased by an average of 6.0 and 7.9 percentage points, respectively. The tested power / speed combination is enough to dry over-wet veneers in the actual factory run, so the capacity increase is 15%. Electricity accounts for about 13% of total energy consumption. Energy consumption decreased by about 30%.

Compared to the medium-wave-short wave combination, evaporation was less efficient with the other combinations. In addition, with the short-medium wave combination using the same run values, veneer co-compression was observed.

The present invention is also applicable to a network dryer.

Claims (5)

7 84658 Patented applications: A method for drying a sheet-like material, for example wood veneer, in which - the main drying is carried out by applying thermal energy to the material to be dried, and - the moisture of the material to be dried is measured before and / or after the main drying, characterized by - applying a first infrared radiation to the wettest material. the wavelength is approximately equal to the maximum water absorption, - immediately after the first irradiation the material to be dried is exposed to a second infrared radiation with a maximum wavelength less than the maximum wavelength of the first radiation, and - the energies of the first and second radiation to the drying material are matched at least approximately. A method according to claim 1, characterized in that the wavelength of the maximum intensity of the first infrared radiation is adjusted to about 3 μm and the wavelength of the maximum intensity of the second infrared radiation is adjusted to about 1 μm. 3 · Apparatus for drying sheet-like material, such as wood veneer, comprising - a roller (6) or a net dryer in which the material to be dried can be dried by steam, 8 84658 - a conveyor (1) in which the material to be dried can be fed to a roller dryer (6), (2, 7) with which the moisture content of the material to be dried can be measured, and - control and separation units (4, 8) with which the material found to be the wettest can be distinguished from other material, known from infrared radiators (5, 11, 12) a first infrared radiation having a maximum intensity wavelength approximately equal to the water absorption maximum and then a second infrared radiation having a maximum intensity wavelength less than the maximum intensity wavelength such that the energies of the first and second radiation applied to the material to be dried are at least approximately equal. Apparatus according to Claim 3, characterized in that reflectors are arranged on one side of the conveyor (1) relative to the infrared radiators (5, 11, 12) in order to improve the radiation efficiency. Apparatus according to claim 3 or 4, characterized in that the infrared emitter is formed by a center wave zone (A) comprising a plurality of clear quartz glass center wave emitters arranged substantially perpendicular to the direction of travel of the conveyor (1) and adjacent to the center wave zone (A). a matched short-wave zone (B) comprising a plurality of bright quartz internal short-wave radiators (23) arranged substantially perpendicular to the direction of travel of the conveyor (1). 9 84658