EP0152576A2 - Method and apparatus for drying blade veneers - Google Patents

Abstract

In the case of a veneer dryer for sliced veneers with two superimposed conveyor belts (9.1, 9.2) that circulate deflection devices, in particular rollers (10), drums (11) or rollers together essentially in a loop, the conveyor belts (9.1, 9.2) run between two deflection devices on one route (b) straightforward. The length of the straight line (b) is at least half of the maximum veneer width.

Description

The invention relates to a veneer dryer for sliced veneers according to the preamble of patent claim 1.

When drying veneers, it is known to convey the veneers between two belts lying one above the other through a drying chamber. Depending on the design of the tapes, e.g. as a wire mesh tape, the weight of the top tape (cover tape) can be used to smooth the veneers. In many cases, however, this smoothing effect is not sufficient to prevent the appearance of waves during drying.

In roller dryers, a higher pressing pressure - and thus a better smoothing effect - can be achieved by passing the veneer between rollers arranged one above the other, which also promote it. However, roller dryers have disadvantages that have a particularly serious effect on thin, sensitive and high-quality sliced veneers: the veneer requires a certain degree of rigidity so that it can be conveyed from pair of rollers to pair of rollers. This is not the case with thin and / or too wet veneers. Furthermore, the veneers are between see the roller pairs firmly clamped. Since the veneers shrink up to 12% during the drying process, this leads to tensions, which in particular destroy thin veneers.

With belt dryers, the veneer can shrink between the belts, thus avoiding cracks. At the same time, guiding thin, wet veneers between tapes is problem-free. The disadvantage of this is that the weight of the cover tape is not sufficient to smooth the veneer satisfactorily. Thin sliced veneers are therefore often dried in belt dryers and then additionally smoothed in a smoothing press to remove the waves that occurred during drying.

From DE-AS l'266 233 a veneer dryer of the generic type is known, in which the veneers are guided in a loop between belts around rollers which are offset in zigzags. The goods are stored between two endless, smooth-rolled wire mesh belts, which are put under considerable tension by an articulated arrangement of the rollers, which are mounted in zigzags. This tension exerts a smoothing pressure on the veneer lying between the tapes.

With this veneer dryer, the shrinkage of the veneer during drying is not taken into account: the smoothing pressure is permanently applied to the veneer. It is constantly pinched between the belts, making it impossible to shrink without tears. This may be the reason why such a veneer dryer has not yet appeared on the market.

The invention has for its object to provide a veneer dryer for sliced veneers, which makes it possible to dry sliced veneers with significantly improved quality.

This object is achieved by the characterizing features of patent claim 1. These features ensure that each veneer sheet between the deflection devices is freed from the pressing pressure and thus released. This allows the veneer sheets to shrink freely between the ribbons and prevents cracks from occurring.

The subclaims contain preferred embodiments of the invention. Their features, individually and in combination, improve the veneer quality, operational safety and costs and / or the construction of the veneer dryer.

Thus, by the feature of claim 2, the side of the veneer sheet lying against the deflection devices is also dried during circulation. This prevents thermal tension and the curling of the veneer sheet (curling effect).

The feature of claim 4 ensures free shrinkage without the veneer slipping. The differential speed according to claim 5 reduces the occurrence of cracks.

Veneers that have been dried with the veneer dryers according to claims 3, 6, 7 and 8 have a particularly good quality, with the Embodiments according to claims 7 and 8 cost-effectively increased performance. The claims 9, 18 and 19 contain structurally particularly advantageous embodiments.

While the dryer performance is further increased by the feature of claim 13, claim 14 shows a simple construction for heating the surface of the rollers. The embodiment according to claim 15 is particularly advantageous since not only the bottoms of the drum are saved by the external storage, but also the fixed heating system inside the drum is much easier to install and to supply. The feature of claim 16 additionally enables the drum surface facing side of the veneer sheet to be directly loaded with drying medium during the circulation.

Furthermore, it is very advantageous that a periodic displacement relative to one another is achieved by the mutual deflection of the conveyor belts. This movement of the conveyor belts against each other means that the veneers can be smoothed out every time.

It has been shown that the veneers dried with a veneer dryer according to the invention are very smooth even when they are very dry - to far below 12% residual moisture - which considerably simplifies their further processing.

• Figur 1 zeigt als Längsschnitt einen Ausschnitt aus einem erfindungsgemäßen Furniertrockner.
• Figur 2 zeigt einen Ausschnitt aus einem anderen erfindungsgemäßen Furniertrockner im Längsschnitt.
• Figur 3 stellt den Längsschnitt durch einen kompletten Furniertrockner dar.

The drawings serve to explain the invention on the basis of simplified exemplary embodiments.

• Figure 1 shows a longitudinal section of a section of a veneer dryer according to the invention.
• Figure 2 shows a section of another veneer dryer according to the invention in longitudinal section.
• Figure 3 shows the longitudinal section through a complete veneer dryer.

The veneer dryer according to the invention consists of a sealed drying chamber 1, which is constructed in a modular manner from several fields 1.1 - 1.9. The number of fields varies depending on the operational use. At the dryer entrance there is an inlet field 2, and a transfer field 3 connects to the last dryer field 1.9. The transfer field 3 is connected to a cooling field 4 via a conveyor system 5, followed by a collecting device, not shown.

Each dryer field 1.1 - 1.9 has a circulating air fan 6 and a heat exchanger, not shown, with controllable heating medium supply; Heating medium is e.g. Hot water, steam etc. There is also an exhaust air chimney 7 on the drying chamber 1, which is connected to the interior of the chamber via a controllable shut-off valve 8. With these devices, the required dry climate can be established in each field 1.1 - 1.9.

The conveyor system for the veneers through the drying chamber 1 is a belt conveyor which consists of two endless belts 9.1, 9.2. The bottom one Run of the upper band 9.1 (= shroud) and the upper run of the lower band 9.2 (= support band) lying directly one above the other through the first five fields 1.1 - 1.5 in a straight line, then through the fields 1.6 - 1.9 in a loop around rollers 10.1 - 10.8 (Figure 1 , Figure 3) or drums 11.1 - 11.8 (Figure 2) out. Coming from the field 1.5 in a horizontal direction, lay down on the roller 10.1 at the bottom and wrap it around an arc of around 60 °. This is followed by a straight, obliquely upward free path, the length of which is denoted by b and, in a practical exemplary embodiment, is approximately 50 cm. This distance is tangent to the roller 10.1 and to the next roller 10.2. On the upper part of the roller 10.2, the two belts 9.1 and 9.2 run through an arc of approximately 120 °, which corresponds to a distance of around 90 cm for the roller diameter selected. Behind the roller 10.2 is again a straight line of length b, which is directed obliquely downwards and is tangential to both the roller 10.2 and the adjacent roller 10.3. The roller 10.3 is also wrapped in an arc of approximately 120 ° at the bottom. Accordingly, in the further course, the rollers 10.4 to 10.8 are wrapped alternately at the top and bottom, the carrier tape 9.2 and the cover tape 9.1 alternately resting on the surface of the respective roller. The belts run obliquely downwards from the last roller 10.8 to the deflection rollers 13.1 and 13.2. In the area of fields 1.6 to 1.9, the conveying path is zigzag, for example like a rounded sawtooth curve or similar to a sine curve. The length of the entire conveyor line in the dryer is slightly more than 20 m in the exemplary embodiments shown. The proportion of the straight conveyor section in front of the first roller 10.1 is approximately 10 m, ie almost 50% of the total distance. In the area of the zigzag path in the fields 1.6 to 1.9, the curved conveying sections (without the straight-line sections in between) add up to just under 7 m of track length, i.e. around 30% of the total conveying section and around 60% of the conveying section in this area.

Embodiments are also possible in which deflection devices are arranged along the entire conveying path. Then the share of the diverted conveyor section increases to over 40%. A proportion of 50-70% has proven to be favorable in these forms.

In the transfer field 3, where there is also a common drive 12 for the conveyor system, the belts 9.1, 9.2 are deflected away from one another by deflection rollers 13.1, 13.2 and, supported on rollers 14.1, 14.2, are returned to the dryer entrance. There, together with further deflection rollers 15.1, 15.2, they are part of the inlet field 2, in which each belt 9.1, 9.2 also passes through a tensioning device 16.1, 16.2, with which the tensile stress acting on it can be adjusted. The tensioning device 16 consists of a deflection roller, the position of which can be changed hydraulically.

The straps 9.1, 9.2 are made of non-stretchable material. It is entirely possible that they have a certain elasticity if it is ensured that they are able to produce the desired pressing pressure during the deflection by means of a sufficiently high tensile stress. They have openings for the drying medium - hot air in the present examples - they are as small as possible so that the largest possible contact surface on the veneer is available. Furthermore, the tapes are designed so that they can slide back and forth on the veneer without creating markings. Flat spiral wire mesh tapes and spiral tapes made from round wire have proven themselves in practice.

The belts 9.1, 9.2 are driven by the rollers 10 or drums 11, which are connected to the drive 12. In operation, a differential speed of 2% is set between the belts 9.1, 9.2. The rollers 10 or drums 11 are all of the same size, their number varies depending on the operational use; in the present exemplary embodiments, there are eight each. The arrangement takes place one after the other, the axes of rotation lying in a horizontal plane. Two rollers 10 or drums 11 are located in one field. To save space, it is also possible to arrange the rollers 10 or drums 11 at different heights; e.g. so that the axes of rotation are partly in a second, higher horizontal plane.

The exemplary embodiments according to FIGS. 1 and 2 have different drive systems: pins are fastened to the end faces of the rollers 10 and are rotatably mounted in a stand. Every second roller 10.2, 10.4, 10.6, 10.8 - they are each rotated on the upper side by the belts 9.1, 9.2 - is driven by a common chain 17 engaging the pin (FIG. 3). In the embodiment according to FIG. 2, the drums 11 rest on partially driven rollers 18, the drums 11.1, 11.3 etc., which are covered by the belts 9.1, 9.2 on their underside, being supported on their upper side by additional rollers 19. The rollers 18, 19 roll on races, not shown, which are attached to the drum ends. The non-driven rollers 10.1, 10.3 etc. or drums 11.1, 11.3 etc. can be provided with braking devices with which both the belt tension and the differential speed between the belts 9.1, 9.2 can be regulated by braking. In the example according to FIG. 3, two rollers 10.3, 10.5 have such braking devices in the form of disc brakes 20.1, 20.2.

It is essential to the invention that the belts 9.1, 9.2 run in a straight line for a piece b during their mutual mutual rotation around the rollers 10 or drums 11 on the transition path from one roller 10 or drum 11 to the next. This distance b must be at least half of the maximum veneer width, in the present example it is 500 mm with a 1,000 mm maximum veneer width. This is achieved in that two successive rollers 10 or drums 11 in the transport direction do not directly adjoin one another, but are arranged at a certain distance from one another.

In the drying chamber 1 there are nozzle boxes 21 along the conveying path of the veneer, from which hot air is blown essentially vertically onto both sides of the veneer. In the exemplary embodiment according to FIG. 1, the nozzle boxes 21 are arranged in the fields 1.6-1.9 both above and below the rollers 10 along an arc of a circle. It is different in the exemplary embodiment according to FIG. 2: there, the nozzle boxes 21 are fixedly attached inside the drums 11. The drums 11 have no bottoms, and their walls are also provided with holes. Therefore, the hot air can be blown with the inner nozzle boxes 21 through the drum walls and the inner conveyor belt on the veneers. Other heating systems can also be installed inside the drums 11, e.g. oil or gas burners with which the drums are heated directly.

In both embodiments, the conveying path of the veneers runs straight through the first fields 1.1-1.5. In these fields, the nozzle boxes are arranged next to each other on both sides of the conveyor line to achieve a high drying performance. This configuration of the dryer enables its overall performance to be increased cost-effectively without the veneer quality being significantly impaired.

The sliced veneers are introduced individually into the veneer dryer via the inlet field 2. As they pass, they lie between the carrier and cover tape, the weight of the cover tape ensuring a certain smoothing effect. The weight of the cover tape and the difference in speed between the carrier and cover tape are dimensioned so that the veneers are not prevented from shrinking during drying are. When passing through the fields 1.1-1.5, the veneers are subjected to hot air from the nozzle boxes 21 and thus pre-dried, preferably to the point of fiber saturation.

From field 1.6 onwards, that is to say as soon as the first roller 10.1 or drum 11.1 is reached, the veneers are guided on a loop-shaped path through the remaining dryer. The forced guidance along an arcuate path creates a radial pressing pressure that presses vertically on the veneer surface: the outer belt (in the first roller 10.1 or first drum 11.1, the lower belt 9.2) presses the veneer against the inner belt, which in turn lies on the roller 10.1 or drum 11.1. The veneer is smoothed by the high pressure, free shrinking is not possible in this phase due to the high pressure. The tapes 9.1, 9.2 are designed so that there are no marks on the veneer. The contact pressure can be adjusted according to the respective requirements with the tensioning devices 16.1, 16.2 via the tension of the belts 9.1, 9.2. The differential speed between the belts 9.1, 9.2 and the distribution of the tension in the dryer can be adjusted with the help of the braking devices 20.1, 20.2.

The belts 9.1, 9.2 in the transition piece between two rollers 10 or drums 11 each run straight for a distance b. There is no contact pressure during the straight line guidance - with the exception of the pressure caused by the component of the shroud weight vertical to the conveyor - on; the veneer can shrink freely. The component of the weight perpendicular to the conveyor track can be varied via the angle of inclination of the conveyor track to the horizontal; in the present example it is approximately 60 degrees. It has been shown that because of the tendency of the veneer sheets to contract, this angle cannot be increased arbitrarily. The minimum distance for the straight course ensures that each fiber of a veneer sheet can shrink freely at least for a moment, thus preventing cracks between adjacent fibers.

The rollers 10 or drums 11 are rotated alternately at the top and bottom with an alternating sense of rotation. This guide exposes both sides of a veneer sheet to the same treatment. In addition, this type of rotation produces a periodic back and forth movement of the belts 9.1, 9.2 against one another, since the respective outer belt remains as a function of the different rotation radii. This movement of the belts 9.1, 9.2 smoothes the veneers. The strength of the relative movement against each other can be influence by the thickness of the conveyor belts 9.1, 9.2, since their amount depends on the different orbital radii, and thus on the belt thickness. The smoothing is supported by the difference in speed between the two belts 9.1, 9.2, which can be influenced by the braking devices 20.1, 20.2.

In the exemplary embodiment according to FIGS. 1 and 3, in order to prevent a temperature difference between the outer side which is exposed to hot air, a Veneer sheet and its inner side - it is covered by the roller wall - the rollers 10 on the opposite side of the circulation also blown with hot air from nozzle boxes 21 and heated. They emit this heat through radiation to the inside of the veneer.

In the embodiment with drums 11, the inside of the veneer is blown directly through the perforated drum wall. With the nozzle boxes 21 mounted in the drums 11, precise metering and guidance of the hot air flow is possible.

The veneers leave the drying chamber 1 via the transfer field 2, from where they are conveyed into the cooling field 6 by the conveyor system 5. There they are cooled and then collected.

Claims (19)

1. veneer dryer for sliced veneers, with two superimposed conveyor belts that run around the conveyor section deflection devices, in particular rollers, drums or rollers together in a substantially loop-like manner, characterized in that the conveyor belts (9.1, 9.2) run in a straight line between two deflection devices on a route, the Length is at least half of the maximum veneer width. 2. veneer dryer according to claim 1, characterized by heated deflection devices. 3. veneer dryer according to claim 1 or 2, characterized in that the proportion of the deflected conveyor section of the total conveyor section is at least 20%. 4. veneer dryer according to one of claims 1 to 3, characterized in that the conveyor belts (9.1, 9.2) between two deflection devices between 30 degrees and 75 degrees, preferably about 60 degrees, inclined to the horizontal. 5. veneer dryer according to one of claims 1 to 4, characterized in that between the two conveyor belts (9.1, 9.2) there is a differential speed of 1% - 12%, preferably 2% - 5%. 6. veneer dryer according to one of claims 3 to 5, characterized by deflection devices along the entire conveying path, the proportion of the deflected conveying path being at least 40%, preferably 50% -70%. 7. veneer dryer according to one of claims 3 to 5, characterized by a straight-line part of the conveyor line in front of the first deflection device, whose share in the total conveyor line is at least 20%, preferably 35% - 55%. 8. veneer dryer according to claim 7, characterized in that the proportion of the deflected conveyor section of the total conveyor section is between 25% and 50%. 9. veneer dryer according to one of claims 1 to 8, characterized by at least four, preferably six to twelve deflection devices in the form of rollers (10), drums (11) or rollers along the conveyor line. 10. veneer dryer according to one of claims 1 to 9, characterized by endlessly rotating conveyor belts (9.1, 9.2) and a device (16.1, 16.2) for changing the tensile stress acting on them. 11. veneer dryer according to one of claims 1 to 10, with rollers, drums or rollers, characterized by a braking device (20) for at least one roller (10), drum (11) or roller. 12. veneer dryer according to claim 11, characterized by at least two braking devices (20.1, 20.2), each acting on rollers (10), drums (11) or rollers rotating in the same direction. 13. veneer dryer according to one of claims 1 to 12, characterized by nozzle boxes (21) from which the continuous veneer is blown essentially vertically with a drying medium. 14. veneer dryer according to claim 13, with rollers (10) and / or rollers as deflection devices, characterized by nozzle boxes (21) on the opposite conveyor belts (9) opposite side of the rollers (10) and / or rollers, from which these with a Blown heating medium. 15. veneer dryer according to one of claims 1 to 12, characterized in that at least one of the deflection devices is an outside on rollers (18, 19) mounted drum (11), in the interior of which a heating system is fixed. 16. veneer dryer according to claim 15, characterized in that the drum (11) is provided with holes. 17. veneer dryer according to claim 16, characterized in the inside of the drum (11) fixed nozzle boxes (21), from which the veneers are blown through the holes with drying medium. 18. veneer dryer according to one of claims 9 to 18, characterized in that the axes of rotation of the rollers (10), drums (11) or rollers lie in a horizontal plane. 19. veneer dryer according to one of claims 9 to 18, characterized in that it is built up in modules from successively arranged fields (1.1 - 1.9), which are at least partially equipped with two rollers (10), drums (11) or rollers.

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