The invention relates to an apparatus for dry¬ ing a fibre web, the drying apparatus comprising a first and a second endless band that are impermeable to air and conduct heat well, 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 and the second band being arranged to run part of the way parallel with each other so that they form a drying zone between them, the first band being heated and the second band being cooled, and a fibre web and at least one fabric that forms an endless loop being conducted between the bands so that the fibre web is in contact with the heated first band and the fabric is be¬ tween the fibre web and the cooled second band, as well as guide rolls for guiding endless abrics.

Finnish Patent 59439 discloses a method of dry¬ ing a fibre web. In this method, the fibre web is con- ducted between two endless metal bands, both being ar¬ ranged to run around turning rolls at the opposite ends of the apparatus. On the sides facing each other, the bands run parallel with each other and both the fibre web and one fabric or more fabrics, such as a felt or a wire, at the same speed between the bands in such a way that the fibre web is in contact with one band and the felts or wires are between the fibre web and the second band. To dry the fibre web, the band bearing against it is heated, due to which the water in the fibre web evaporates and moves towards the felts or wires. Simultaneously, the band facing the felts or wires is cooled, due to which the water steam cools to a condensate onto the band and thus allows a steam flow from the fibre web towards the cooled band. The conden- sate formed on the cooled band is removed after the

drying zone of the apparatus in such a way that there is substantially no water on the surface of the lower band at the inlet end of the apparatus .

During the drying procedure, the amount of con- densate accumulated on the cooled band increases from the inlet end of the apparatus towards the outlet end of the apparatus. This influences naturally to some ex¬ tent the capability of the apparatus to move steam from the fibre web towards the cooled band. Especially when thick webs containing plenty of water are dried, the drying apparatus can become rather long, which increases the costs of the apparatus as far as machine and build¬ ing investments are concerned.

The object of the present invention is to pro- vide a drying apparatus that is capable of drying a fibre web more efficiently, to make it possible to use shorter drying apparatuses and to achieve a better dry¬ ing result compared with previous apparatuses . The dry¬ ing apparatus according to the invention is character- ized in that the second band is arranged to run parallel with the first band only part of the length between the turning rolls of the first band, that the drying appara¬ tus comprises a third endless band that is impermeable to air, conducts heat well and is arranged to turn around third turning rolls and to run after the second band part of the way parallel with the first band to form together with it a drying zone, and that it com¬ prises a first fabric, which is conducted with the fibre web through the entire drying apparatus . The essential idea of the invention is that two or more cooled bands are used for drying a fibre web, the bands being positioned one after another in the lon¬ gitudinal direction of the drying apparatus and each band being freed separately from the condensate accu u- lated on the surface of the band. In this way, it is

possible to decrease the effect of the condensate weak¬ ening the drying so that the drying of the web will take place efficiently.

The invention will be described in further de- tail in the attached drawings, where

Figure 1 shows a schematic side view of a dry¬ ing apparatus according to the invention,

Figure 2 shows schematically a detail of a point where cooled bands are changed in the apparatus according to Figure 1,

Figure 3 shows a schematic side view of another drying apparatus according to the invention and

Figure 4 shows a schematic general view of a third embodiment of the drying apparatus according to the invention.

Figure 1 shows a drying apparatus according to the invention, the apparatus comprising endless drying bands that conduct heat well and are impermeable to air, i.e. a first band 1 as well as a second band 2a and a third band 2b; the first band 1, the second and the third band 2a and 2b being located substantially upon each other so that the second band 2a and the third band 2b are mutually one after another. The bands are ar¬ ranged in such a way that the first and the second band as well as the first and the third band, respectively, run part of the way parallel with each other so that they form two by two a drying zone between themselves . A fibre web 3 located against the first band runs be¬ tween the surfaces of the bands 1 and, on the other hand, the bands 2a and 2b, facing each other, and a first fabric 4, such as a felt or a wire, bearing against the fibre web runs between the fibre web 3 and the second bands 2a and 2b. There are additional fab¬ rics, i.e. preferably a coarser second and third fabric 5a and 5b between the fabric 4 and the second and the

third band 2a and 2b. The first band 1 is arranged to turn around first turning rolls 6a and 6b and the second band 2a is arranged to turn around second turning rolls 7a and 7b, respectively, and the third band 2b around third turning rolls 8a and 8b. The coarse fabrics 5a and 5b are guided by means of guide rolls 9a and 9b and the fabric 4 by means of guide rolls 10, respectively.

To dry the fibre web, heating chambers Ila and lib are arranged against the first band, into which chambers is fed hot pressurized steam heating the first band 1. On the opposite sides of the heating chambers Ila and lib, inside the second and the third band, there are cooling chambers 12a and 12b, into which is fed cooling medium, such as water or the like, so that the second and the third band 2a and 2b remain cool . Then the heat of the first band makes the water contained in the fibre web 3 evaporate and move through the fabric 4 to the coarse fabrics 5a and 5b and condensate on the surface of the second and the third band 2a and 2b, re- spectively. To remove water from the fabrics 5a and 5b, suction boxes 13a and 13b are positioned against them, the boxes sucking air through the fabrics and thus the water out of them. Further, the condensed water is re¬ moved from the surface of the second and the third band by means of doctor blades 14a and 14b, for instance. Subsequently, the bands and the fabrics return, in order to turn to run parallel with the first band again. The heating chambers Ila and lib and the cooling chambers 12a and 12b, respectively, are sealed with seals against the surface of the corresponding band in a manner known per se, in order to prevent the pressurized medium therein, such as steam or water, from flowing outside the chamber. A sealing like this is generally known per se and not essential for the invention, due to which it is not described here any further.

Because a pressure differing from the surround¬ ing pressure is generally produced between the bands 1 and the bands 2a and 2b when the fibre web becomes hot and the steam is cooling, there are seals between the bands on the sides thereof, which seals prevent the air from flowing between the bands or the steam from flowing out of there. Such a sealing between the bands is gener¬ ally known per se and not essential for the invention, due to which it is not described here any further. More- over, the inlet end of the apparatus comprises in a man¬ ner known per se a steam exhaust chamber 15 exhausting the air contained in the fibre web 3 and the fabrics 4 and 5a by feeding steam through them, whereby air mole¬ cules are replaced by steam molecules. Further, there is a steam chamber 16 in the space between the second and the third band, which chamber has a pressure some¬ what higher than the indoor air pressure so that no air can flow into this space. In addition, when the third fabric 5b comes into the steam chamber 16, steam flows through the fabric into an air exhaust space 17 between the fabric and the third band, due to which not even the third band brings air into the drying zone. Hot steam can also be fed into the steam chamber 16 separately to keep the pressure therein as desired and to intensify the air exhaust.

The fabric 4 runs through the space between the bands 2a and 2b and supports the fibre web 3 during this time. Because the fabric 4 and the fibre web 3 are not supported from below at this point, no pressurized heat- ing chamber is used at the corresponding point above either, but the heating chambers Ila and lib are posi¬ tioned in such a way that the cooling chambers 12a and 12b between the second and the third band 2a and 2b are located essentially opposite to the heating chambers Ila and lib.

Figure 2 shows schematically a detail of a dry¬ ing apparatus of the invention at a point between the second and the third band more accurately. Reference numerals used are in accordance with Figure 1. As appears from Figure 2, the second band 2a and the second fabric 5a part from each other after the turning rolls 7b, a doctor blade 14a scratching from the surface of the band 2a the condensate accumulated there¬ on. Correspondingly, there is a suction box 13a on one side of the second fabric 5a, which box sucks water therefrom, and on the other side, there can additionally or instead be a blow box 18 blowing air through the fab¬ ric to remove water from it . Between the fabrics 5a and 5b, there is a steam chamber 16, into which can be fed steam in such a way that the pressure therein remains higher than the pressure of the surrounding room. Steam flows from this chamber through the third fabric 5b into an air exhaust space 17, from where air is sucked all the time so that the fabric 5b moves around the roll 8a substantially air-free.

Figure 3 shows a schematic side view of another drying apparatus according to the invention. In this embodiment, not two separate additional fabrics run around the second band 2a and the third band 2b, but one additional fabric 5, which forms a closed loop around the two bands 2a and 2b. To remove water from the addi¬ tional fabric, the fabric is conducted to run around a separate guide roll 9 between the bands 2a and 2b and to return from between the bands 2a and 2b to run be- tween the band 2b and the first fabric 4. Here, a suc¬ tion box 13a is arranged against the additional fabric 5, by which box water is sucked from the fabric. If desired, a blow box can be used in the manner shown in Figure 2, either together with the suction box 13a or without it. Likewise, to remove the water from the addi-

tional fabric 5, another suction box and air blow or steam blow box can be used before the additional fabric returns into contact with the third band 2b. Further, when the additional fabric 5 parts from the fibre web after the third band, water is removed from it in the manner shown in Figure 1, for instance, so that this fabric may contain as little water as possible when it comes to the inlet end of the drying apparatus. Besides one common additional fabric, additional fabrics can, of course, also be used as a fabric forming a closed loop around the second and the third band 2a, 2b or around either one only, as shown in Figure 2. Figure 3 shows also a support member 19 bearing against the first fabric in the steam chamber 16, which support member can be for instance a foil box or a shoe, a suction box or the like, supporting the first fabric 4 and through that the fibre web 3.

Figure 4 shows a schematic general side view of a third embodiment of the drying apparatus according to the invention. For the sake of clarity, everything that has been explained earlier in connection with the other figures has been excluded from this figure, if not associated with this embodiment. In this embodiment, the second band 2a and the third band 2b are arranged to form an angle with each other, with the turning rolls 7a, 7b and 8a, 8b mounted in such a way that the midmost turning rolls 7b and 8a are lower than the outermost turning rolls 7a and 8b in the height direction of the apparatus. Then the bands 1 and 2a as well as 1 and 2b, respectively, run a distance parallel with each other, but the bands 2a and 2b form an angle with each other. Between the rolls 7b and 8a, the drying apparatus com¬ prises a support member 20 bearing against the surface of the band 1, which support member can be, as shown in the figure, a gently curved shoe, roll or the like,

around which the band 1 runs and against which the band 1, the fibre web 3 and the first fabric 4 are bearing. In this embodiment, the fibre web 3 can be supported by adjusting the tightness of the first fabric 4 suitably. The invention has been described in the above specification and in the drawings only by way of example and it is by no means restricted to that . Various known seal and air exhaust solutions are available for the im¬ plementation of the invention without being of essential significance for the very invention. Likewise, water re¬ moval from the fabrics, i.e. felts or wires, can take place in various ways. Also more than two wires or felts on each other can be used and the number and structure of the wires or felts running around the second band and the third band may be different. Likewise, heating the first band and cooling the second and the third band may be performed differently in manners known per se . If de¬ sired, even three or more cooling bands can be used instead of two cooling bands according to which kind of fibre web is dried and in what way it shall be dried.