Drier Section

Abstract

Drier section for drying a material web, having two superimposed rows of cylinders; said material web travelling alternating from row to row, partially wrapping said cylinders; each two of said cylinders, following one another in the direction of travel of said material web being partially wrapped by different ones of contact webs; the first, in said direction of travel, of said contact webs and the second of said contact webs being so guided as to extend for a length on both the sides of said material web in the immediate vicinity to said material web.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a drier section for drying material which is supplied in web form, more especially a paper web, having at least two superimposed rows of cylinders, through which the material web to be dried travels from row to row with alternate wrapping of the cylinders, two cylinders which directly follow one another in the direction of travel of the material web being wrapped by contact webs which are different from one another.

When paper is produced by the conventional wet method, the water necessary for this purpose has to be removed again. This is effected mechanically first, in the wire section and in the press section. In the following drier section, those quantities of water which can no longer be removed mechanically are then extracted by drying by heat.

With the traditional machines, the guiding of the paper web in the drier section is effected substantially by the rotating cylinders, the paper web bearing on the cylinders. Where the paper web lies on the cylinder it is covered by a contact web, i.e. a so-called dry felt or dry screen. A so-called free passage of the distance of approximately 100 to 130cm lies between the successive cylinders, and in this free passage the paper web is without any guiding.

In this free passage, the paper web has a tendency to carry out oscillatory movements as the speed of transit increases, whereby in practice the said transit speed is limited to approximately 900 to 950 m/min.

Also in the paper industry, the general trend is to always higher outputs. These can be achieved by wider machines or by faster machines. As regards the width of the machines, however, limits are set by the size of the building or problems of constructive and technical nature. Therefore, existing installations can in practice only be brought to a higher output by increasing the speed of transit, whereby the drier section determines the transit speed.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a drier section in which substantially higher transit speeds than hitherto can be achieved without any fluttering of the material web to be dried.

For this purpose, the drier section according to the invention is characterised in that the contact web of the respective first cylinder and the contact web of the respective second cylinder, in the region in which the material to be dried travels from the respective first cylinder to the respective second cylinder, are guided at least in a part of the region so as to extend on both sides in the immediate vicinity of the material web.

With a drier section in which the material web is alternately guided over the cylinder surfaces of two rows of perforated cylinders, one row being arranged above the other, and the cylinders are exposed to a stream of heated air passing from outside inwardly through perforations in the cylinders, the contact web is perforated and bears immediately on the cylinder surface. The contact web of the respective first cylinder and the contact web of the respective second cylinder again are so guided that a guide gap for the material web to be dried, defined by two contact webs extending at least approximately parallel to one another, is formed between the succeeding perforated cylinders from the delivery point of one perforated cylinder to the receiving point of the following perforated cylinder, while the material web comes to bear freely on that part of the contact web which is bearing on the cylinder surface.

For this drier section with throughflown perforated cylinders, it is advantageous if the perforated cylinders are each enclosed by an air or gas supply hood on the side which is wrapped by the contact web. It is expedient for the air or gas supply hood to be provided with air or gas supply ducts directed radially towards the cylinder surface and with air or gas discharge ducts.

It has proved to be desirable for the spacing of the contact webs defining the guide gap to be at least twice as large as the thickness of the material web which is to be dried.

For regulating the speed of the cylinders independently of one another, it is advantageous if each of the cylinders is wrapped by a seperate contact web.

BRIEF DESCRIPTION OF THE DRAWING

The invention is hereinafter explained by way of example and by reference to the drawing, wherein:

FIG. 1 is a diagrammatic side elevation of a drier section according to the invention,

FIG. 2 is a diagrammatic side elevation of another drier section according to the invention, and

FIG. 3 is a diagrammatic side elevation of a further drier section.

The path of the material to be dried is indicated in the FIG. by means of arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drier section shown in FIG. 1 comprises two superimposed rows of perforated cylinders 1 and 2, a paper web 3 which is to be dried, travelling from row to row of the cylinders, alternately wrapping around them. Two cylinders 1 and 2 which directly follow one another in the direction of travel of the paper web are respectively wrapped by perforated belts 4 and 5 which are different from one another. As the paper web 3 to be dried is passing through the drier section, the web is exposed to a stream of heated air passing from outside inwardly through the cylinders 1 and 2.

Each of the perforated cylinders 1 and 2 is provided with a separate contact web, i.e. a perforated belt 4 and 5, respectively, which partially wraps around the cylinder and which extends on the cylinder from the running-on point 6 to the running-off point 7 of the perforated cylinder 1 or 2 and furthermore extends off the cylinder in an approximately tangential direction into the delivery region of the preceding perforated cylinder and into the receiving region of the following perforated cylinder, respectively. In this way, for the paper web 3 to be dried, a guide gap defined by two contact webs, i.e. the perforated belts 4 and 5 extending approximately parallel to one another is formed between the successive cylinders 1 and 2 from the delivery region of one perforated cylinder to the receiving region of the next following perforated cylinder, so that even with higher transit speeds of above 1,000m/min, there can be no fluttering of the paper web 3 to be dried as it passes from one cylinder to the next. Apart from this, the paper web 3 to be dried comes to bear freely on that part of the perforated belt 4 or 5 which is resting on the cylinder surface, so that the drying air flowing onto and through the paper web is impeded as little as possible.

The spacing of the contact webs 4 and 5 defining the guide gap is at least twice as large in the guide gap region as the thickness of the paper web 3 which is to be dried, so that the paper web is not squeezed between the two contact webs 4 and 5, which would produce an undesirably strong impression of the perforated belts 4 and 5 in the surface of the paper web 3 and abrasion because of different speeds of said belts. It has proved to be advantageous for the spacing of the contact webs 4 and 5 defining the guide gap to be larger on the entry side of the guide gap than on the exit side. Since each of the cylinders 2 and 1 is wrapped by a separate contact web 5 and 4, respectively, the separate cylinders can be driven at different speeds so as to allow for the shrinkage process occurring in the paper web being dried.

As can also be seen from FIG. 1, the perforated cylinders 1 and 2 are each enclosed by an air supply hood 8 on the side wrapped by the contact web 5 and 4, respectively, and the interiors of the perforated cylinders 1 and 2 are connected to an air-suction device for producing a vacuum in said cylinders. So as to avoid undesirable flow losses, the perforated cylinders 1 and 2 are each provided with a cover part 9 on that portion of the cylinder surface which is not covered by the contact web 5 or 4, respectively.

It is obvious that infra-red or microwave driers can additionally be provided, for example, inside the air supply hoods 8, so as to improve the drying efficiency. Since the paper web 3 to be dried is alternately flown through by the air in both directions, good drying effects are produced.

Each of the contact webs 4 and 5 is guided once out of the drying region, and when outside it is cleaned, guided and regulated for tension. The arrangement as described guarantees that the contact web, i.e. the perforated belt and its support construction is only contacted by air at low temperature, which obviously has a very desirable effect as regards the strength behaviour of the perforated belt.

In the embodiment shown in FIG. 2, a steam-heated solid wall drum 10 is arranged between the cylinder 1 or 2 and the transfer gap region which precedes or follows in the direction of travel of the paper web 3 to be dried, said solid cylinder being wrapped with contact by the paper web 3 to be dried. By means of this solid wall drum 10, it is possible to produce a very large wrapping angle around the perforated cylinders 1 and 2 and hence a long drying path per drier unit. These solid wall drums 10 serve as entry and exit guide rollers and as steamheated drying cylinders. By contact drying, these drums produce a surface-improving effect as compared with only throughflow drying. Since the paper web 3 to be dried alternately comes into contact with the solid wall drums 10 on both sides, the surface improvement occurs on both sides.

Also with this embodiment, the contact web of the respective first cylinder and the contact web of the respective second cylinder, in the region in which the paper web 3 to be dried runs from respectively the first to respectively the second cylinder, is guided in a part of that region so as to extend for a length on both sides in immediate vicinity to the paper web to be dried. Since in this way the paper web 3 to be dried always bears on at least one side on a contact web in the entire transfer region which is situated between the cylinders 1 and 2, substantially higher transit speeds of the paper web are possible than was hitherto the case, without there being any fluttering of the paper web in the transfer region.

So as to produce a combined impact flow/through-flow drying, the air supply hoods 11 associated with the perforated cylinders 1 and 2, are provided with air supply ducts 12 to be directed towards the cylinder surface, and also with air discharge ducts 13. For producing an impact flow drying, some of the supplied drying air after making impact with the surface of the paper web, is drawn off again through the ducts 13, while the remainder passes through the paper web 3 into the interior of the cylinder 1 or 2.

For the upper row and the lower row of internally heated solidwall cylinders 1 and 2, in the drier section which is shown in FIG. 3, a single contact web, i.e. a perforated belt 4 or 5 is used for each row. Such a constructional form is desirable in those cases where the shrinkage during the drying operation is relatively unimportant. Here also the contact web of the respective first cylinder and the contact web of the respective second cylinder, in the region in which the material to be dried travels from the said first to the said second cylinder, is guided in a part of this region on both sides in immediate vicinity to the paper web to be dried, so that a satisfactory transition of the paper web from one contact web to the other is obtained, and the material web to be dried consequently always bears at least on one side against a contact web.

Claims

We claim:

1. In a drier section of the type in which a traveling web of paper or like material to be dried partially wraps in sequence a plurality of cylinders arranged in two superposed rows and alternates between the rows as it transfers from each cylinder to the next in the direction of travel, and in which the cylinders are also partially wrapped by moving contact webs which are arranged so that a different contact web wraps adjacent cylinders in the direction of travel of said material web, the improvement which comprises guiding means for the contact webs which causes the contact webs associated with each pair of adjacent cylinders, in the direction of travel of said material web, to traverse paths immediately adjacent to and at opposite sides of the material web for at least a portion of the region in which the material web transfers cylinders, the contact webs overlapping in said portion of the transfer region so that they define a guide gap means through which the material web passes as it travels between cylinders and thereby serve to restrict fluttering movement of the material web in a direction normal to its path of travel.

2. A drier section as defined in claim 1 in which said guide gap means has a constant width.

3. A drier section as defined in claim 1 in which said guide gap means has a width which decreases in the direction of travel of the material web.

4. A drier section as defined in claim 1 in which said guide gap means has a width at least twice as great as the thickness of the material web.

5. A drier section as defined in claim 1 in which the cylinders are perforated wall cylinders; and the guiding means causes the contact webs to bear directly on the cylinders, whereby the material web rests on a contact web as it wraps each cylinder.

6. A drier section as defined in claim 5 including a solid wall drum which is associated with each of said pairs of adjacent cylinders and which is partially wrapped and in contact with the material web during the course of said transfer; and the guiding means causes one of the contact webs associated with each of said pair of cylinders to rest on the material web as the latter wraps the drum.

7. A drier section as defined in claim 6 in which there are two solid wall drums associated with each of said pairs of adjacent cylinders, the drums being located at opposite ends of said guide gap means and so arranged that they lead the material web off of and onto the two cylinders, respectively, of each of said cylinder pairs and contact opposite faces of the material web; and the guiding means causes the other contact web to rest on the material web as the latter wraps the second drum.