EP0192207A2 - Veneer dryer - Google Patents

Abstract

Veneer dryer for sliced veneers with two conveyor belts lying one above the other (6.1, 6.2) which within a drying chamber jointly run round at least four deflection devices (5) mainly in loop form, and run between two deflection devices (5) in a straight line along a section the lenght of which is so dimensioned that a veneer passed along this section can shrink freely, at least 40 % of the conveying section in the part fitted with deflection devices running round deflections, characterized by the fact that at least two successive groups of, in each two deflection devices arranged one above the other (5), the conveyor belts (6) along the conveying section being alternately wrapped round the deflection devices (5) of one group.

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 between the pairs of rollers 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 easy. The disadvantage remains 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 in order to remove the waves that occurred during drying.

In the European patent application published under the number EP 0152576, a veneer dryer of the generic type is described with two superimposed conveyor belts, which essentially rotate around deflection devices, in particular rollers, drums or rollers. The conveyor belts run in a straight line between two deflection devices, the length of which is at least half the maximum veneer width.

While the veneer sheets alternately run between the two conveyor belts, they are smoothed by the pressure of the outer belt that occurs during the deflection. There is no pressure in the straight parts of the conveyor line, which means that the veneer sheets can shrink freely there. In addition, they are caused by the relative movement of the conveyor belts against each other mutual deflection is generated, permanently smoothed. With this veneer dryer it is possible to dry veneers with considerably improved quality. The veneers are very supple even when they are very dry - far below 12% residual moisture - which makes their processing much easier.

There, embodiments are described in which the conveyor belts alternately wrap around rollers, drums or rollers, which are arranged one behind the other in a horizontal plane. In some embodiments, a rectilinear conveying path is connected upstream of the first deflection device in the veneer dryer. The veneer dryers are built up in modules from successive fields, the rollers, drums or rollers being arranged in pairs in a field.

The present invention has for its object to make a generic veneer dryer space-saving and inexpensive.

This object is achieved with the characterizing features of patent claim 1.

It has been shown that in this way four and more deflection devices can be installed in a single drying field with a common heating and control system, without - with the appropriate design of this system - a reduction in the quality of the dried veneer sheets occurring.

Another advantage is that the arrangement of deflection devices one above the other makes it possible to increase the distance between two deflection devices reduce without the veneers starting to slide due to a steeper increase in the straight portions of the conveyor line. In addition, it has surprisingly been found that existing old systems with a straight conveyor line can be upgraded to higher performance without requiring additional space, while the veneer quality is significantly improved.

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 4, 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 differential speed according to claim 5 reduces the occurrence of cracks.

Veneers which are dried with the veneer dryers according to claims 6 or 7 are of particularly good quality, the performance being increased inexpensively in the embodiment according to claim 7.

The feature of claim 8 ensures free shrinking without the veneer slipping.

While the dryer performance is further increased by the feature of claim 9, claim 10 shows a simple construction for heating the surface of the rollers.

Two examples of preferred embodiments of a veneer dryer according to the invention are described below.

The drawings serve to further explain the invention.

Figure 1 and Figure 2 show simplified longitudinal sections through veneer dryer according to the invention.

A veneer dryer according to the invention is constructed in a modular manner from drying fields 1.1-1.2 or 1.3-1.8 arranged one behind the other, each dryer field 1.1-1.8 containing devices with which the required drying climate can be produced. These devices include circulating air fans 2, heating registers, regulating devices, etc. Following the respective last dryer field 1.2 or 1.8, a transfer field 3 leads to a cooling field 4. The last two drying fields 1.1, 1.2 and 1.7, 1.8 are each with deflection devices 5 in Form fitted with cylinders lying horizontally across the conveying direction; In the present embodiments, hollow rollers driven by chains are used, and the use of rollers or drums is also possible. The rolls 5 are arranged in successive groups of rolls 5 lying vertically one above the other, two groups in each case being installed in a dryer field 1.1, 1.2, 1.7, 1.8. The roller length corresponds to the maximum veneer length (approx. 5 m), its diameter is approx. 85 cm. The length of a dryer field equipped with rollers 5 is approximately 3 m.

In the embodiment according to FIG. 1, the dryer fields 1.1 and 1.2 each have two groups each of four rolls 5 lying vertically one above the other, that is to say a total of 16 rolls. In the embodiment according to FIG. 2, two groups of two rolls 5 lying vertically one above the other are arranged in fields 1.7 and 1.8, that is to say a total of 8 rolls.

A belt conveyor consisting of two endless conveyor belts 6.1, 6.2 runs through the dryer fields 1.1 - 1.2, 1.3 - 1.8. In the embodiment according to FIG. 2, the conveying strands of the belts 6.1, 6.2 run straight through the four fields 1.3-1.6 before they are deflected upwards by the first roller 5.1.

In the embodiments in which deflection devices are arranged along the entire conveying path through the dryer (FIG. 1), the proportion of the curved conveying path in the total conveying path is at least 40%, preferably between 50% and 70%.

In the example according to FIG. 2, the fields 1.3-1.6 are each approximately 2 m long. The proportion of the rectilinear part of the conveyor line in front of the first deflection roller is less than 50%, preferably between 20 and 35% of the total conveyor line; in the example according to FIG. 2 approx. 32%.

According to another advantageous embodiment of the invention, three fields are each equipped with four rollers. In front of these three fields, there are four fields with a straight line upstream of the conveyor line, which means that the straight line share in front of the first deflecting roller is almost 24% of the total conveyor line.

Arriving horizontally, the two belts 6.1, 6.2 are deflected upwards by an angle of approximately 150 ° by the first roller 5.1. They then alternately loop around the rollers of the first group one after the other from bottom to top until they are deflected horizontally by the top roller 5.2 of the first group to the top roller 5.3 of the second group. The conveyor belts 6.1, 6.2 are each deflected by approximately 120 ° from the middle rollers of a group (FIG. 1), while the bottom and top rollers 5.1 and 5.2 respectively deflect by approximately 150 °. The rollers of the second group are mutually wrapped around from top to bottom in accordance with those of the first group, the bottom roller 5.4 of this group then deflects the strips horizontally in the direction of the next field 1.2 or 1.8. The outlets 1.2 and 1.8 on the outlet side are passed through in an identical manner by the belts 6.1, 6.2 before they are separated in the transition zone 3 and returned to the dryer entrance.

The belts 6.1, 6.2 run straight along the conveyor path between two rollers 5 on a path which corresponds to at least half of the maximum veneer width; in the present example between two superimposed rollers 5 approx. 50 cm with a maximum veneer width of 1,000 mm. This is achieved in that two adjacent rollers 5 do not follow one another directly, but are arranged at a distance from one another. It has been shown to be particularly advantageous if the circumference and spacing of two rollers 5 lying one above the other are coordinated such that the rectilinear part is inclined at less than 45 °, preferably approximately 30 °, with respect to the horizontal. This prevents slippage even of the thinnest veneers.

The bands 6.1, 6.2 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.

Nozzle boxes, not shown in the drawings, are located in the dryer along the conveying path of the veneer, from which hot air is blown essentially vertically onto both veneer sides. In the fields 1.1, 1.2, 1.7 and 1.8, the nozzle boxes are arranged both to the right and to the left of the rollers 5 along an arc.

According to another embodiment of the invention - as already described in EP 0152576 - drums mounted externally are installed in the fields 1.1, 1.2 and 1.7, 1.8 instead of the rollers. Nozzle boxes are permanently installed inside the drums. The drums have no bottoms, and their walls are also perforated. The hot air can therefore be drawn through the drum walls and the internal conveyor belt with the inner nozzle boxes blow on the veneers. Other heating systems can also be installed inside the drums, for example oil or gas burners, with which the drums are heated directly.

In the embodiment according to FIG. 2, the conveying path of the veneers runs in a straight line through the first fields 1.3-1.6. 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 an inlet field. 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.

As soon as the first roller 5.1 has been 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 band (in the first roller 5.1 the lower belt 6.2) presses the veneer against the inner belt, which in turn rests on the roller 5.1 . The veneer is smoothed by the high pressure, free shrinking is not possible in this phase due to the high pressure. The tapes 6.1, 6.2 are designed so that no marks appear on the veneer. The contact pressure can be adjusted according to the respective requirements with tensioning devices via the tension of the belts 6.1, 6.2. A difference speed of 1% - 12%, preferably between 2% and 5%, is set between the bands 6.1, 6.2.

The belts 6.1, 6.2 in the transition piece between two rollers 5 each run straight for a distance. During the straight line guidance, no contact pressure occurs - with the exception of the pressure due to the component of the shroud weight perpendicular to the conveyor track; 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 30 degrees between two rolls of a group. 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 5 of a group are rotated alternately on the right or left 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 reciprocating movement of the belts 6.1, 6.2 against one another, since the respective outer belt remains as a function of the different rotation radii. Through this movement the Bands 6.1, 6.2, the veneers are smoothed. The strength of the relative movement relative to one another can be influenced, inter alia, by the thickness of the conveyor belts 6.1, 6.2, since their amount depends on the different radii of revolution and thus on the belt thickness. The smoothing is supported by the difference in speed between the two bands 6.1, 6.2.

To prevent a temperature difference between the outer side of a veneer sheet exposed to hot air and its inner side - it is covered by the roller wall - the rollers 5 are also blown with hot air from nozzle boxes on the side opposite the circulation and thus heated. They emit this heat through radiation to the inside of the veneer.

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

The veneers leave the dryer via the transfer field 3, from where they are conveyed into the cooling field 4 by the conveyor system. There they are cooled and then collected.

Claims (10)

1.Veneer dryer for sliced veneers with two superimposed conveyor belts (6.1, 6.2), alternately looping around the common deflection devices (5), in particular rollers, drums or rollers, the conveyor belts (6.1, 6.2) between two deflection devices (5) on one route run in a straight line, the length of which is at least half of the maximum veneer width, characterized by at least two successive groups, each consisting of at least two deflecting devices (5) arranged one above the other, the conveyor belts (6) alternately looping around the deflecting devices (5) of a group one after the other . 2. veneer dryer according to claim 1, characterized by at least four groups of two deflection devices (5) in the form of lying cylinders. 3. veneer dryer according to claim 1, characterized by at least two groups of four deflection devices (5) in the form of lying cylinders. 4. veneer dryer according to one of claims 1 to 3, characterized by heated deflection devices (5). 5. veneer dryer according to one of claims 1 to 4, characterized in that between the two conveyor belts (6.1, 6.2) there is a differential speed of 1% - 12%, preferably 2% - 5%. 6. veneer dryer according to one of claims 1 to 5, characterized by deflection devices (5) 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 1 to 5, characterized by a rectilinearly guided part of the conveying path in front of the first deflection device (5.1), the proportion of which in the total conveying path is less than 50%, preferably 20-35%. 8. veneer dryer according to one of claims 1 to 7, characterized in that the conveyor belts (6) between two deflection devices (5) of a group are less than 45 °, preferably about 30 ° inclined to the horizontal. 9. veneer dryer according to one of claims 1 to 8, characterized by nozzle boxes from which the continuous veneer is blown essentially vertically with a drying medium. 10. veneer dryer according to claim 9, with rollers (5) and / or rollers as deflection devices, characterized by nozzle boxes on the opposite side of the rotating conveyor belts (9) of the rollers (5) and / or rollers from which they are blown with a heating medium .

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