FI75925C - Roll dryer for drying veneer sheets - Google Patents

Description

75925

Tumble dryer for drying plywood veneer Valstork för torkning av fanerark

The present invention relates to a so-called roller dryer for drying plywood veneers.

This type of dryer is known per se. It consists of an elongate, substantially closed cabinet with devices for conducting and removing drying air or gas, and devices for passing the sheets of veneer to be dried through the cabinet from one end to the other in contact with the drying, heated air. The devices for guiding the veneer sheets in the roller dryer consist specifically of several successive pairs of rollers, through the nip between which the sheets are guided from one pair of rollers to another. The pairs of rollers are located relative to each other so that the nips between them are located in the same, substantially horizontal plane in which plane the sheet to be dried will pass.

The diameter of the rolls of each pair of rollers is usually about 100 mm and the pairs of rollers have a pitch of about 300 mm in the longitudinal direction of the dryer. Such drying tracks are usually 3 or 4 superimposed in the dryer.

Such a roller dryer mainly dries cut, substantially intact and full-size veneer sheets. The device can also be used for drying joints with varying widths (so-called jonts). The sheets are fed into the dryer in the autumn direction.

The roller dryer is suitable for several veneer thicknesses, because in it the veneer runs well under forced control during its drying and transport. However, in known roller dryers, a disadvantage has been found in the tendency of the veneer to cup in a direction perpendicular to the fiber direction, i.e. the veneer will be quite wavy in the fiber cross direction when dried.

A substantial improvement in this known problem of a roller dryer is provided by the arrangement according to the invention, characterized in that the pairs of rollers of each drying track are arranged vertically relative to each other in such a way that every other pair of rollers is slightly lower or higher. somewhat undulating path in its course.

Blower drying devices in which the web-like product to be dried is guided in the direction of travel via stepped roller nips are known e.g. British Patent No. 1,206,201 and Finnish Patent Application No. 802580. Of these, the former publication describes a drying apparatus for drying various film webs, in which the stepping of roller nips is intended to ensure that the rotational force of the drying rollers is transferred to the film material without the film material.

The apparatus described in Finnish patent application no. 802580 relates to a veneer web drying apparatus in which the veneer is guided transversely to the fiber direction. The purpose of the arrangements made in the dryer is to prevent fiber-direction shrinkage cracks in the veneer during or after drying. As one such measure, it has been proposed to guide the veneer between the flexible guide members in a tightly controlled manner along a sharply meandering path as closely as possible in accordance with the jacket surfaces of the drying cylinder set. In order to prevent the drying shrinkage cracks, the running speed of the web is slowed towards the end of the drying, whereby the swelling effect thus obtained has been shown to compensate for the cracking tendency caused by the shrinkage. The effect can be enhanced by "softening" the veneer, i.e. by making fiber-oriented incisions in the surface of the veneer. The effects proposed in the publication have not been shown to have any effect on the bubbling of the veneer in a direction transverse to the fiber direction.

In the apparatus according to the invention, the veneer in each drying path has to run alternately upwards or downwards in its fibrous direction, i.e. along a wavy path resembling a blue curve, in which the curvature of the veneer has been found to be very effective in the transverse direction of the veneer. As a side result, this bending operation also has the effect of promoting the drying of the veneer, since the veneer then presses more tightly against the rollers of each pair of rollers. At the same time, it is conceivable that the wood pores on the ul-arch side of the veneer open more strongly than if the veneer ran without bending along its straight path.

The height shift between successive pairs of rollers does not have to be very large to provide sufficient veneer deflection, and is, for example, about 100 mm in diameter for pairs of ice rolls spaced about 300 mm apart, only about 20 mm. Although this distance can vary within rather wide limits to achieve the result of the invention, oversized displacements do not provide a corresponding additional benefit.

The invention will be explained with reference to the accompanying drawing, in which 4 75925

Figure 1 shows a schematic implementation of a roll arrangement of one part of a drying track of a conventional roll dryer, and

Figure 2 shows a corresponding situation in which the pairs of rollers are arranged according to the invention

The figures show a veneer sheet 1 guided through a plurality of nips of a pair of rollers formed by rollers 2, 2 '. As it passes between the pairs of rollers, the veneer 1 is subjected to a flow of drying, hot air or gas, which is led to the veneer by blowing nozzles, not shown, in each pair of rollers. The rollers 2, 2 'are also hot due to said hot air blowing, advancing the drying of the veneer.

In the situation according to Figure 1, corresponding to the known equipment, the pairs of rollers 2,2 'are placed in substantially the same plane, whereby the veneer 1 has a substantially horizontal straight trajectory. In this situation, however, the veneer tends to squeak strongly in its transverse direction.

To prevent this, in the embodiment of the invention according to Figure 2, the pairs of drying rollers are arranged in vertically displaced positions so that every other pair of rollers will be higher or lower than the pair of rollers between them. In this case, the veneer sheet 1 advances along a gentle sinusoidal path, bending in its path alternately upwards, respectively, downwards, which bending movement substantially reduces the possibilities of bending the veneer sheet in its transverse direction. At the same time, the sheet has to be pressed harder against the hot rollers, which has a beneficial effect on the drying of the veneer. flexural

Claims (2)

5,75925 also opens the outer arc-side pores, resulting in a freer escape of water vapor from the veneer. The reciprocating bending also in a way causes a pumping movement in the veneer, which may have its own effect on the observed enhanced drying effect. In addition to the alternative shown, in which the axes of each pair of rollers are in the same vertical plane, the pairs of rollers can also be mounted so that the planes containing the axes of each roll pair deviate by a certain parallel angle from the vertical plane. in the opposite direction to the vertical. A similar installation is also possible in which the planes containing the axes of both rolls of all roll pairs deviate in parallel and by the same angle from the vertical plane. Such an inclined installation of the axes of the rolls of the pairs of rollers makes it possible to influence the natural guidance of the veneer in its undulating trajectory. When using this inclined mounting method, the vertical arrangement of the pairs of rollers can also be adapted to provide a natural trajectory with respect to the inclination, for example for veneers of different thicknesses. 1. A plywood veneer roll dryer in which successive pairs of rolls form at least one substantially horizontal