METHOD AND APPARATUS FOR DRYING A FIBER WEB The method relates to a method of drying a fiber web by an appa- ratus comprising two endless bands that are impermeable to air, first turning rolls around which the first band is arranged to turn, and second turning rolls around which the second band is arranged to turn; the first band and the sec- ond band being arranged to run part of the way in parallel such that they de- fine between them a drying zone, the first band being heated and the second band being cooled, and the fiber web and at least two felts or wires, of which at least one is a coarse wire and at least one is a fine wire being arranged to run between the bands in such a manner that the fiber web is in contact with the first, heated band, the coarse wire is in contact with the second, cooled band, and the fine wire, correspondingly, is between the fiber web and the coarse wire.

The invention further relates to an apparatus for drying a fiber web, the apparatus comprising two endless bands that are impermeable to air, first turning rolls around which the first band is arranged to turn, and second turn- ing rolls around which the second band is arranged to turn; the first band and the second band being arranged to run part of the way in parallel such that they define between them a drying zone, the first band being heated and the second band being cooled, and the fiber web and at least two felts or wires, of which at least one is a coarse wire and at least one is a fine wire being ar- ranged to run between the bands in such a manner that the fiber web is in contact with the first, heated band, the coarse wire is in contact with the sec- ond, cooled band, and the fine wire, correspondingly, is between the fiber web and the coarse wire.

Many patent publications, such as WO 96/11300 ja US 4 461 095, teach the drying of a fiber web between two parallel metal bands moving in the same direction such that the fiber web is in contact the heated metal band and there is a wire between the fiber web and the second, cooled metal band, whereby steam separated from the fiber web by heating condenses to the wire by the effect of the cold metal band. The basic idea is that two endless metal bands are arranged to turn around turning rolls and that against the inner sur- face of the loops formed by the bands are provided pressure chambers con- taining hot steam and water, respectively, such that the pressure produced presses the hot and cold bands, respectively, against the fiber web and the

wire running between them. Together with seals, the bands located between the pressure chambers define one side of the pressure chambers, whereby the steam and the water can directly affect the bands. The operation of the apparatus is fully known per se and has been disclosed, for example, in the above patent publications, which are incorporated herein by reference.

The basic idea of the operation of the drying apparatus is that be- cause of the heat of the heated band, the water in the web is vaporized and transferred through the wire or wires towards the cooled band. The steam pro- duced on the surface of the cooled band condenses into water and is removed with the cooled band and the wire against it. US 5 594 997 teaches the re- moval of water from the wire against a cooled band by means of blow and/or suction boxes. In the drying apparatuses described above, the fiber web is guided to and from the apparatus by means of a fine wire. The fine wire is placed between the coarse wire located against the cooled band and the fiber web, and the fine web further aims to prevent the wiremark on the surface of the web caused by the coarse wire. The small apertures of the wire against the fiber web are susceptible to being clogged, which causes the wire to ad- here to the surface of the fiber web. This, again, renders the surface of the web uneven. In addition, clogged apertures prevent the water from passing from the web to the coarse wire, resulting in a lowered drying efficiency of the web.

An object of the present invention is to provide a method and an apparatus which can avoid the above disadvantages.

The method of the invention is characterized by cleaning the fine wire.

Further, the apparatus of the invention is characterized by compris- ing a cleaning device for cleaning the fine wire.

The essential idea of the invention is that a fine wire is arranged between the wire against the cooled band and the fiber web, and the fine wire is cleaned. In a preferred embodiment, the cleaning is performed by spraying water onto the wire by a water nozzle. In another preferred embodiment, after the water nozzles, warm or hot air is blown through the wire from the opposite side to the water nozzles. In a third preferred embodiment, after the water nozzle, warm or hot air is blown through the wire also from the same side of the water nozzle.

The advantage of the invention is that the small apertures of the

wire can be kept unclogged and the wire does not adhere to the surface of the wire. Further, as the apertures are unclogged, water from the web passes easily through the wire; in all, the surface of the web can be made even. The water nozzles allow the cleaning to be performed extremely efficiently; the air blowing allows harmful moisture to be discharged from the wire. The moisture would also show in the moisture profile of the web and in the characteristics of finished paper and board. By blowing air also from the opposite side of the wire to the water nozzles allows moisture from the wire to be removed ex- tremely efficiently.

The invention will be described in closer detail with reference to the attached figure, which is a schematic, sectional side view of a drying appara- tus of a fiber web in accordance with the invention, taken in the travel direction of the web.

The figure is a schematic sectional side view of an apparatus of the invention in the travel direction of the web. The apparatus comprises a drying device comprising a first band 1, i. e. an upper band, and a second band 2, i. e. a lower band, that are endless, impermeable to air, have good thermal con- ductivity, and are preferably made of metal. A fine wire or felt 3, a coarse wire 4 and a fiber web 5 pass between those surfaces of the bands that face each other. The fiber web 5 moves in the direction indicated by arrow A. The fiber web is a paper web or a board web, for instance. The first band 1 is arranged to turn around first turning rolls 6a and 6b located at the ends of the drying apparatus. Correspondingly, the second band 2 is arranged to turn around second turning rolls 7a, 7b located below the first turning rolls 6a and 6b like- wise at the ends of the drying apparatus. Wires 3 and 4 are supported and guided by means of guide rolls 8. Since the pressure prevailing in the drying zone in the area between the bands 1 and 2 is usually different from the pres- sure prevailing outside or on the sides of the bands 1 and 2, seals are ar- ranged on both sides of the apparatus between or at the edges of the bands 1 and 2, said seals preventing liquid or vapour from escaping from the space between the bands 1 and 2 through the sides, or vice versa. To effect the va- pour heating required by the drying process, the drying apparatus comprises a pressure chamber 9, which is located above the first band 1. The first band 1 is sealed with seals 9a in respect of the body 9b of the pressure chamber 9 such that the steam in the pressure chamber 9 is maintained at a suitable pressure. Below the second band 2 there is a water chamber 10, which con-

tains a cooling medium, such as water, that cools the second band 2. At the edges of the water chamber 10 there are seals 10a, with which the second band 2 is sealed in respect of the body 10b of the water chamber 10.

The operation of the drying apparatus is based on heating the first band 1, which comes into contact with the web 5, with hot steam contained in the pressure chamber 9, whereby the water in the web 5 is vaporized and transferred through the wires 3 and 4 toward the second band 2 by the effect of the temperature of the first band 1. The second band 2, in turn, is continu- ously cooled with the water located below it, whereby the steam produced on the surface thereof condenses into water and is removed with the band 2 and the wire 4.

The fine wire 3 against the fiber web 5 is cleaned by a cleaning de- vice 11. The cleaning device 11 comprises water nozzles 12 for spraying water from the side of the surface against the fiber web 5 of the fine wire 3 onto the fine wire 3 and through it. After the water nozzles 12 in the travel direction of the fine wire 3, there is a first air nozzle 13 arranged on the opposite side of the fine wire 3 in the cleaning device 11. Warm or hot air is blown through the fine wire 3 at a high rate by means of the first air nozzle 13. Further, in the travel direction of the fine wire 3 after the first air nozzle 13, a second air noz- zle 14 is arranged on the same side of the fine wire 3 with the water nozzle 12.

By means of the second air nozzle, warm or hot water is also blown through the fine wire 3 at a high rate. The first air nozzle 13 and the second air nozzle 14 can also be placed in an opposite order in the travel direction of the fine wire 3, but most preferably the order is as shown in the figure, whereby the surface of the fine wire 3 against the fiber web 5 more readily remains suffi- ciently dry.

A plurality of water nozzles 12 and air nozzles 13 and 14 can be provided in the machine and/or cross direction of the fine wire 3. The water nozzles 12 and the air nozzles 13 and 14 can be arranged to traverse, or move back and forth, in the cross direction of the fine wire 3. The nozzles are then, however, arranged with respect to each other such that they process the same point of the fine wire 3. For the sake of clarity, traversing means 15 moving the nozzles back and forth in the cross direction of the fine wire 3 is shown as greatly simplifie in the attached figure; for the same reason, the feeding arrangements for feeding air and water to the nozzles are not shown here. The air nozzles 13 and 14 can, however, be so wide that they blow air

through the fine wire 3 along the entire width of the wire 3. The fine wire 3 moves also during the cleaning in the direction indicated by arrow B, in other words the fine wire 3 is being cleaned while it is moving.

The air nozzles 13 and 14 are most preferably arranged at an angle of approximately 45° with respect to the weft threads and warp threads of the fine wire 3. The air jets can thus be made to flow more efficiently through the apertures between the weft threads and warp threads in the wire.

The figure and the accompanying description are only intended to illustrate the idea of the invention. The details of the invention may vary within the scope of the claims. It is thus not essential what pressure medium is used in the pressure chamber 9 and the water chamber 10. The pressure medium in the pressure chamber 9 can thus be, for instance, steam, air, hot fuel com- bustion products or water. The cooling medium in the water chamber 10 can be, for instance, air as well as water.

In addition to the heating by the pressure chamber 9, the first band 1 can also be heated at other places in manners known per se. Further, the first band 1 can also be heated entirely outside the pressure chamber 9, or the fiber web 5 can be dried even without the pressure chamber 9.

Furthermore, the second band 2 can be also cooled outside the water chamber 10 or it can also be cooled even without the water chamber 10 in a manner fully known per se.