WO1998004777A1

WO1998004777A1 - Method for drying a paper to be surface-treated, in particular fine paper, in an after-dryer in a paper machine, and an after-dryer in a paper machine for carrying out the method

Info

Publication number: WO1998004777A1
Application number: PCT/FI1997/000459
Authority: WO
Inventors: Antti Kuhasalo; Pasi Ahonen
Original assignee: Valmet Corporation
Priority date: 1996-07-31
Filing date: 1997-07-29
Publication date: 1998-02-05
Also published as: DE19781902T1; FI963024L; FI963024A0; FI101487B; AU3544697A; FI101487B1

Abstract

The invention concerns a method for drying a paper to be surface-treated, in particular fine paper, in an after-dryer in a paper machine. In the method the paper web (W) is first dried in the forward dryer section of the paper machine by means of a number of successive groups (R1 ...RN) with single-wire draw that are open downwards on support of a drying wire (115), after which the paper web (W) is finished in a finishing section (D2), in which finishing section (D2) the web is surface-sized or coated. After the surface-sizing or coating, the paper web (W) is dried by means of an upwards open inverted group (Rk) with single-wire draw, in which connection the tendency of curling formed in the paper web (W) in the forward dryer section (D1) can be substantially eliminated and/or compensated for. Further, the invention also concerns a method in which, in the after-dryer, into connection with the dryer groups (R21, R22), hot moist air is fed in order to restrain the evaporation, or hot dry air is fed in order to promote the evaporation from the side of the web that is desirable in view of the control of curling, the air being fed into the desired locations, in which connection the tendency of curling formed in the paper web (W) in the forward dryer section (D1) can be substantially eliminated and/or compensated for. Further, the invention concerns a finishing section in a paper machine for drying a paper to be surface-treated, in particular fine paper, for carrying out the method.

Description

Method for drying a paper to be surface-treated, in particular fine paper, in an after-dryer in a paper machine, and an after-dryer in a paper machine for carrying out the method

The invention concerns a method for drying a paper to be surface-treated, in particular fine paper, in an after-dryer in a paper machine, in which method the paper web is first dried in the forward dryer section of the paper machine by means of a number of successive groups with single-wire draw that are open downwards on support of a drying wire, after which the paper web is finished in a finishing section, in which finishing section the web is surface-sized or coated.

Further, the invention concerns an after-dryer that makes use of the method of the invention for drying a paper to be surface-treated, in particular fine paper, in which paper machine, before the after-dryer, there is a forward dryer section, in which there are a number of successive groups with single-wire draw that are open downwards, and after which forward dryer section a finishing section is placed, in which there are devices for surface-sizing or coating of the paper web.

As is known from the prior art, in multi-cylinder dryers of paper machines, twin- wire draw and/or single-wire draw is/are employed. In twin- wire draw the groups of drying cylinders comprise two wires, which press the web one from above and the other one from below against heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular so when the web is still relatively moist and, therefore, of low strength. This is why, in recent years, ever increasing use has been made of said single-wire draw, in which each group of drying cylinders includes just one drying wire, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, whereas on the reversing cylinders or rolls between the drying cylinders the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop, and the reversing cylinders or rolls inside said loop.

In the what is called normal groups with single-wire draw, known from the prior art, the heated drying cylinders are placed in the upper row and the reversing cylinders are placed in the lower row, which rows are, as a rule, horizontal and parallel to one another. In the following, when the terms "normal (dryer) group" and "inverted (dryer) group" are used, what is meant is expressly groups with single-wire draw in multi-cylinder dryers, of the sort mentioned above.

When paper is dried by means of normal groups with single-wire draw from the side of its bottom face and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, seen from above.

As is known from the prior art, the tendency of curling of paper is already affected in connection with the web formation, in particular at the sheet formation stage by means of selection of the difference in speed between the slice jet and the wire, and by means of other running parameters. As is known from the prior art, for example, in the case of copying paper, by means of unequalsidedness of drying in the after- dryer a suitable initial curl form is regulated for the sheet in order that the curling of the paper after one-sided or double-sided copying could be optimized. In the case of copying paper, the reactivity of curling, i.e. the extent to which curling occurs per unit of change in moisture content, is affected to a greater extent by means of a multi-layer structure of the paper, which is produced in connection with the web formation in the wet end.

The most recent technology related to the present invention in high-speed paper machines, in particular in fine-paper machines, has been based on dryer sections in which there is single-wire draw over the major part of the length of the machine, and, in view of controlling the tendency of curling of paper, in practice, an inverted group has also always been used in order that the drying could be made sufficiently symmetric in the z-direction.

From the prior art, solutions are known for an after-dryer for paper to be coated, in particular for fine paper or equivalent, in which dryer there is first an upper cylinder and a lower cylinder and after this one group that employs normal single-wire draw, and after that a necessary number of dryer groups that make use of twin- wire draw. In these applications, it is a problem that, in view of the tendency of curling of paper, the ratio of the upper and lower cylinders is wrong if the curling is supposed to be regulated efficiently. It is an object of the present invention to provide a solution in which the curling of paper is controlled so that the after-dryer does, however, not become substantially longer.

In prior-art after-dryers, in particular for fine paper, in which the drying has been regulated so that the emphasis is on the lower cylinders, problems are often also encountered in relation to the moisture of air. The hood of the after-dryer and in particular the pocket spaces of the draw are often excessively dry for the control of the curling. The problems described above cannot be controlled by means of an increased moisture level in the hood alone, but the moisture levels in the upper and lower pocket spaces in the draw should also be separately adjustable.

Groups of the sort mentioned above for finishing of paper to be coated, in particular of fine paper, have been described, among other things, in the applicant's FI Patent Application No. 950434, filed February 1, 1995. Thus, the object of the present invention is further development of the solutions suggested in said application so that the tendency of curling of paper can be controlled more efficiently in the after-dryer.

It is a further object of the present invention to provide such a solution for an after- dryer as is suitable for use in particular in such dryer sections in which it has not been possible or desirable to control the curl of the paper web in the forward dryer section.

Further, it is an object of the present invention to provide an arrangement in which, in the after-dryer of the paper machine, the ratio of moisture between the upper and lower pockets can be regulated so that the moisture levels in the pocket spaces can be used for controlling the curling of the paper web.

In view of achieving the objectives stated above and those that will come out later, the method in accordance with the present invention is mainly characterized in what is stated in the characterizing part of claim 1 , 2 or 3.

On the other hand, the after-dryer of a paper machine that carries out the method in accordance with the present invention is mainly characterized in what is stated in the characterizing part of claim 10, 11 or 12.

According to the present invention, the after-dryer is started with a dryer group that applies an inverted single-wire draw, in which case, in view of the tendency of curling of paper, the desired ratio of upper cylinders to lower cylinders is obtained. After the coating device, size press or equivalent, the drying is started by means of an inverted group with single-wire draw, and said group is followed by either normal or inverted groups with single-wire draw or by dryer groups based on twin- wire draw. The paper web is brought from the coating device or equivalent onto an upper or lower cylinder, and after this there follows the inverted dryer group. Before the inverted group, there can be a spreader roll or spreader rolls, a reversing dryer, an airborne, infra or combination dryer. The most important advantage of this arrangement in accordance with the present invention is formation of a ratio of upper cylinders to lower cylinders that is correct in view of the tendency of curling of paper, without additional length of the dryer section. Owing to this, no additional arrangements are needed to supply additional heat to provide unequalsidedness. In accordance with the present invention, in the after-dryer of the paper machine, moist air is introduced in appropriate locations inside the hood, in which case the moisture levels in the different pocket spaces formed by the cylinders, rolls and wires in the dryer groups can be controlled. In the supply of moisture, it is possible to use air supply devices, ventilators, blow pipes, etc. fitted in the after-dryer. Through these devices, hot moist air is supplied into the desired upper/lower pockets, in which connection the control of curling takes place so that evaporation taking place from the wrong side of the paper is restrained by means of the supply of hot moist air. The necessary moist air is obtained, for example, from the exhaust air from the hood or from suction rolls. According to preferred additional embodiments of the invention, in the devices that supply air to the after-dryer, appropriately selected moisture levels of air are employed in order to achieve the desired distribution of moisture. The air moisture levels at all of the different moisture supply points can be regulated separately, and so also the exhaust quantities of all the air exhausts can be regulated separately if necessary. Also, the supply of hot dry air onto the side of the web from which evaporation is to be promoted is included in the scope of the present invention.

In the following, me invention will be described in more detail with reference to the figures in the accompanying drawing, the invention being, however, not supposed to be strictly confined to the details of said illustrations.

Figure 1 is a schematic illustration of an embodiment of a solution of the dry end of a paper machine, showing the forward dryer section and the following finishing section.

Figure 2 is a schematic illustration of an after-dryer of the present invention.

Figures 3A...3C are illustrations in part of different variations of the after-dryer as shown in Fig. 2, which variations are included in the scope of the invention. Figures 4A...4B are further illustrations of an embodiment of an after-dryer in accordance with the present invention.

Figures 5A...5B are schematic illustrations of some test results related to curling of the paper web in the after-dryer.

Figure 6 is a schematic illustration of a further embodiment of the invention.

As is shown in Fig. 1, the paper web is passed into the forward dryer section Dl from the press section onto the drying wire of the first group R_j with single-wire draw, to which wire the web is made to adhere by the effect of the vacuum in the suction boxes. In the forward dryer section Dl there are normal groups R_{j N}, as a rule N = 4...11, preferably N = 6...9, and typically N = 9. All the single-wire groups R, ...R_N are so-called normal groups, in which the, for example, steam- heated smooth-faced drying cylinders 110 are placed in the upper horizontal row and the reversing suction cylinders 111 are in the lower horizontal row. The web has closed draws over the gaps between the dryer groups.

Each normal group RJ ...R_N has a drying wire 115 of its own, which is guided by guide rolls 118. The drying wires 115 press the web W to be dried on the drying cylinders 110 against the smooth heated cylinder faces, and on the reversing cylinders 111 the web W remains on the outer face of the wire 115 at the side of the outside curve. On the reversing cylinders 111 the web W is kept reliably on support of the wire 115 against the effect of centrifugal forces by the effect of the vacuum present on the grooved faces of the reversing cylinders 111 or on the perforated mantle of an equivalent suction roll, by means of which effect cross-direction shrinkage of the web W is also prevented. As the reversing suction cylinders 111, particularly favourably the suction cylinders are used which are marketed by the applicant with the trade mark "VAC-ROLL"™ and which have no inside suction box, and with respect to the details of the constructions of said cylinders reference is made to the applicant's FI Patent No. 83,680 (equivalent to US Patents Nos.

5, 022, 163 and 5, 172, 491). In the forward dryer D 1 , the support contact between the web and the drying wire 115 is also kept adequate on the straight runs between the drying cylinders 110 and the reversing cylinders 111 by, at least on the runs from the drying cylinders 110 to the reversing cylinders 111, employing blow suction boxes, by whose means formation of pressures induced by the wire 115 in the closing wedge-shaped nip spaces between the wire 115 and the cylinder 111 mantles is prevented. Blow suction boxes are understood as blow boxes whose blowing of air produces a vacuum, and said boxes do not communicate with sources of vacuum.

In the forward dryer Dl, in the groups RJ ...R_N with single-wire draw, blow boxes are also used in the gaps between the reversing cylinders 111 , by means of which boxes said intermediate spaces are air-conditioned and evaporation from the web is promoted. The faces of the drying cylinders 110 can be kept clean by means of doctors. In the forward dryer section Dl, broke removal by means of gravity can be applied, for the groups R_j...R_N with single-wire draw are open downwards so that, in the event of a web break, the removal of paper broke can be carried out below the dryer groups Rι ...R_N that are open downwards, substantially by the effect of gravity, onto the broke conveyor placed underneath. Fig. 1 shows the conveyor belt 119 of the broke conveyor and its drive rolls 119a, 119b. On the belt 119 of the broke conveyor the paper broke is carried into the pulper 119c placed at one end of the broke conveyor. At the rear end of the forward dryer section Dl, there is a finishing unit D2, which includes, among other things, a surface-treatment or surface-coating device, an after-dryer, a calender, and a machine reel-up, for example a Pope type reel-up. The machine reel that is being produced by means of the reel-up 150 is denoted with the reference MR_Q, and one complete machine reel is denoted with the reference MR.

After the forward dryer section Dl, the paper web W, which has been dried to a dry solids content of k₂ = 96...99 %, is passed over paper guide rolls 125 and over a measurement beam 126, which measures the property profiles of the paper and is placed between said paper guide rolls, into the coating device, which is, for example, a coating device marketed by the applicant with the name Sym-Sizer™. The coating device includes two opposite coating rolls 11 and 12, in connection with each of which there are size feed devices so that the paper web is coated from both sides in the coating nip between the rolls 11 and 12. After this the web W is passed into the after-dryer.

As is shown in Fig. 2, after the coating rolls 11,12 the web W is passed over the guide roll 13 onto the first drying cylinder 14 in the after-dryer, on which cylinder the top side of the web W is placed against the cylinder face of the drying cylinder 14, and after this there follows a second drying cylinder 15, on which the bottom face of the web is placed against the cylinder face of the drying cylinder 15. After this the web is passed into the inverted dryer group R_k with single-wire draw, in which the drying cylinders 20 are placed in the lower row and the reversing rolls or cylinders, preferably rolls 25 that are marketed by the applicant with the trade mark VAC-ROLL and that have no inside suction box, in the upper row. The wire 24 that supports the paper web W to be dried enters from below the group R_k, guided by the guide rolls 21,22, onto the drying cylinder 20, and after this the wire runs meandering from the drying cylinders 20 onto the reversing roll 25 in the upper row so that on the drying cylinder 20 the web W is placed between the heated cylinder face and the wire 24. After the last reversing roll 25 in the group R_k the web W is passed on the wire 24 of the inverted group onto the drying cylinder 31 in the lower row in the following dryer group R₂2, with twin-wire draw, in which group the web W runs from the drying cylinder 31 in the lower row onto the drying cylinder 32 in the upper row so that between the rows of cylinders the web has a free draw W, and both the drying cylinders 32 in the upper row and the drying cylinders 31 in the lower row have drying wires 36,35, respectively, of their own as well as wire guide rolls 34,33, respectively, of their own.

In the exemplifying embodiment shown in Figs. 3A...3C, after the coating rolls 11 , 12 there are two guide rolls 13, by whose means the web W is passed onto the drying cylinder 15 placed in the upper row, after which the web is passed onto the first drying cylinder 20 in the inverted group R_k, which cylinder 20 is, thus, in the lower row in the group. In this respect, the inverted group is similar to the draw illustrated in Fig. 2, but after the inverted group R_k the web W is passed from the last suction roll 25 in the inverted group R_k onto a suction cylinder 37 fitted in connection with the upper wire in the group R₂₂ with twin-wire draw, which suction cylinder 37 is placed substantially at the same level as the lower cylinders 31 in the group R₂₂ with twin-wire draw. In the embodiment shown in Fig. 3C, the web is passed from the last drying cylinder 20 in the inverted group R_k directly onto the first cylinder 32 in the upper row in the group with twin-wire draw.

In the embodiment shown in Fig. 4A, after the coating rolls 11, 12 the web is passed over the guide roll 13 in a way similar to the exemplifying embodiment shown in Fig. 2, but in this embodiment the after-dryer is composed of one inverted dryer group R_k only. After the dryer group R_k, in the upper row, a drying cylinder 41 is placed, in connection with which cylinder a calender nip has been formed by means of the roll 44. Below the cylinder 41 there can be a doctor 42.

In the embodiment shown in Fig. 4B, in the portion after the inverted group in the after-dryer, in connection with the drying cylinder 41 a holding wire 47 is fitted, which has guide rolls 48 of its own.

The schematic test results illustrated in Figs. 5A...5B are related to a test in which the effects of modes of running of the forward dryer and after dryer on the tendencies of curling of paper were examined. The machine that was used in the test comprised a former, a press, a forward dryer section, in which there was, in the beginning, one single-wire group followed by three twin-wire groups, a coating device, and an after-dryer, which consisted of two twin- wire groups. The paper grade was copying paper 76 grams per sq.m. The three points of comparison in the test were:

R23 normal running mode of the machine: all cylinders in the forward dryer are open; in the after-dryer slightly more heat is supplied to the top face of the paper than to the bottom face,

R20 the supply of steam to all lower cylinders in the forward dryer in the machine is closed; the after-drying was normal, i.e. as in R23, R21 the supply of steam to all lower cylinders in the forward dryer in the machine was closed; the supply of heat in the after-dryer had been changed so that the emphasis was significantly on the bottom face of the paper.

During the test points R₂₃, R₂₀ and R₂₁, no other changes affecting the curl of the paper were made except said regulations in the dryer sections.

The results of one method of curl measurement are illustrated in Figs. 5 A and 5B. In the method, from a sample taken in the cross direction of the web, a number (in this case 16) of small pieces of paper sample are cut off, and the curling of said pieces is examined and measured under conditions constructed for the purpose. Based on the results, among other things, graphs that illustrate the cross-directional curl profile, as shown in Figs. 5 A and 5B, can be drawn. When the profiles of the points R20 and R23 are compared with one another, it cannot be said that there is a difference between them. Between these points, the difference in the running mode is present in the forward dryer section: in R20 the steam supply into the lower cylinders is closed, in R23 open. The difference in the running mode between R20 and R21 is present both in the forward drying and in the after-drying. In the former test, it was, however, noticed that the difference in the running mode in the forward dryer section is insignificant from the point of view of curling, so that the profiles in Fig. 5B indicate the considerable effect of the after-dryer on the curling.

In the after-dryer shown in Fig. 6, whose frame constructions are denoted with the reference numeral 100, there is first one dryer group R₂ι with normal single-wire draw, which group is followed by a group R₂₂ with twin-wire draw. The group R₂₁ with single-wire draw comprises heated drying cylinders 230 in the upper row and reversing rolls 231 in the lower row. The wire of the group with single-wire draw is denoted with the reference numeral 235. The wire guide rolls 238 guide the run of the wire 235. The group R₂₂ with twin-wire draw comprises two horizontal rows of steam-heated drying cylinders 230A and 230B, and the web has free draws W₀ on the runs between said rows. The group R₂₂ includes an upper wire 235A, which runs as guided by the guide rolls 238 and by the guide rolls 239 placed in the gaps between the upper cylinders 230A. Similarly, the group R₂₂ includes a lower wire 235B, which runs as guided by the guide rolls 238 and by the guide rolls 239 placed in the gaps between the lower cylinders 230B.

As is shown in Fig. 6, in the vicinity of the wire guide rolls 239, at the inlet side of the web W and the drying wire 235A and 235B, air blow boxes 237 are employed. Out of the blow boxes 237 fitted in the gaps between the drying cylinders 230A, 230B, air jets of appropriate directions and blow velocities are applied into connection with the runs of the drying wire 235A,235B and with the free sectors of the wire guide rolls 239, placed in the vicinity of said blow boxes, by means of which air jets the support contact between the drying wires 235A,235B and the web W is promoted, the formation of detrimental differences in pressure is prevented, and fluttering of the web W on the free draws W₀ is prevented. Said blowings can also be applied through the drying wires 235A,235B, by means of which ventilation of the pocket spaces P formed in the gaps between the drying cylinders 230A,230B can be promoted.

On the runs from the drying cylinders 230 onto the reversing cylinders 231 , there are blow-suction boxes 217, by whose means formation of pressures induced by the wire 235 is also prevented in the closing wedge-shaped nip spaces between the wire 235 and the cylinder 231 mantles. The blow-suction boxes 217 are understood as blow boxes whose air blowing produces a vacuum, and said boxes 217 do not communicate with sources of vacuum. With respect to the details of the constructions of these blow-suction boxes 217, which are marketed by the applicant with the trade mark "UNO RUN BLOW BOX"™, reference is made to the applicant's FI Patents Nos. 59,637, 65,460, and 80,491 (equivalent to US Pats. Nos. 4,441,263, 4,516,330, and 4,905,380. After the introduction of the "UNO RUN BLOW BOX"™ in the market, the applicant's competitors have also suggested some blow-box solutions, with respect to which reference is made to the US Patents 4,502,231 (J.M. Voith GmbH) and 4,661, 198 (Beloit Corp.), whose applications in positions of blow boxes are also included in the overall concept of the present invention. In the gaps between the reversing cylinders 231 there are blow boxes 216, by whose means said intermediate spaces are air-conditioned and evaporation from the web W is promoted.

In connection with each of the dryer groups R_2j ,R₂2 ^{m tne} after-dryer, additional supply of moisture has been arranged for the purpose of controlling the curling. Moist, hot air is blown into connection with the dryer groups, in particular into the pocket spaces P. The values of moisture and temperature to be chosen depend to a very great extent on the conditions; for example, the moisture limits can be 80...400 grams of H₂O per kilogram of dry air, and the temperature 60...95 °C. The moisture can be introduced in connection with the dryer group by means of the above blow boxes 237, blow-suction boxes 217, blow boxes 216 and/or by means of separate air supply devices 222, and in connection with each group it is also possible to arrange a separate air removing device (devices) 221. If necessary, the moisture content and the temperature of each air supply device 237,217,216,222 can be regulated separately, and so also the evacuation efficiency of the moisture removing points 221 can be regulated separately. In Fig. 6, the arrangement of control of the moisture state of the dryer groups in this after-dryer is illustrated quite schematically, but it can be accomplished by means of principles and devices in themselves known to a person skilled in the art.

Above, the invention has been described with reference to some exemplifying embodiments of same only, the invention being, however, by no means supposed to be strictly confined to the details of said embodiments.

Many modifications and variations are possible within the scope of the inventive idea defined in the following patent claims.

Claims

1. A method for drying a paper to be surface-treated, in particular fine paper, in an after-dryer in a paper machine, in which method the paper web (W) is first dried in the forward dryer section of the paper machine by means of a number of successive groups (R^ . -R_j^) with single- wire draw that are open downwards on support of a drying wire (115), after which the paper web (W) is finished in a finishing section (D2), in which finishing section (D2) the web is surface-sized or coated, characterized in that after the surface-sizing or coating, the paper web (W) is dried by means of an upwards open inverted group (R_k) with single- wire draw, in which connection the tendency of curling formed in the paper web (W) in the forward dryer section (Dl) can be substantially eliminated and/or compensated for.

2. A method for drying a paper to be surface-treated, in particular fine paper, in which method the paper web (W) is first dried in the forward dryer section of the paper machine by means of a number of successive groups (R_j ...R_N) with single- wire draw that are open downwards on support of a drying wire (115), after which the paper web (W) is finished in a finishing section (D2), in which finishing section (D2) the web is surface-sized or coated, characterized in that, in the method, in the after-dryer, into connection with the dryer groups (R₂₁,R₂₂), hot moist air is fed in order to restrain the evaporation, or hot dry air is fed to promote the evaporation from the side of the web desirable in view of the control of curling, to the desired locations, in which connection the tendency of curling that was formed in the paper web (W) in the forward dryer section (Dl) can be substantially eliminated and/or compensated for.

3. A method for drying a paper to be surface-treated, in particular fine paper, in which method the paper web (W) is first dried in the forward dryer section of the paper machine by means of a number of successive groups (R_j...R_N) with single- wire draw that are open downwards on support of a drying wire (115), after which the paper web (W) is finished in a finishing section (D2), in which finishing section (D2) the web is surface-sized or coated, characterized in that after the surface- sizing or coating, the paper web (W) is dried by means of an upwards open inverted group (R ) with single-wire draw, and that in the method, in the after-dryer, into connection with the dryer groups (R_k»R₂ι ,R₂₂)_> hot moist or dry air is fed to the desired locations, in which connection the tendency of curling formed in the paper web (W) in the forward dryer section (Dl) can be substantially eliminated and/or compensated for.

4. A method as claimed in any of the claims 1 to 3, characterized in that, in the finishing section (D2), the paper web (W) is passed through a double-sided coating device (11 , 12), in which both sides of the web are coated and moistened under control, asymmetrically if necessary, after which the web is passed through the after- dryer, in which the web is first passed through a dryer group with inverted single- wire draw, and in which after-dryer the web is dried preferably from the sides of both of its faces, after which the web is passed through a machine calender (40) or directly into the machine reel-up (50).

5. A method as claimed in any of the claims 1 to 4, characterized in that from e coating device (11,12) the paper web (W) is passed onto an upper or lower drying cylinder (15; 14), after which the web is passed into a dryer group (R_k) with inverted single- ire draw.

6. A method as claimed in any of the claims 2 to 5, characterized in that, in the method, hot moist air is fed by means of blow boxes (237,216) and/or blow-suction boxes (217) in the dryer groups (R_k,R₂₁,R₂₂).

7. A method as claimed in any of the claims 2 to 6, characterized in that, in the dryer groups (R_k, _{21 *}^₂₂) ^{m me} after-dryer, hot moist air is fed by means of separate air supply devices (222) into connection with the dryer group, and that air is removed by means of separate moisture removing devices (221).

8. A method as claimed in claim 7, characterized in that the supply of moisture is regulated in order to achieve the desired moisture content.

9. A method as claimed in any of the claims 2 to 8, characterized in that, in the method, moist air is fed into the pocket spaces (P) between the drying wires, cylinders and rolls.

10. An after-dryer in a paper machine for drying a paper to be surface-treated, in particular fine paper, for carrying out the method as claimed in any of the preceding claims, in which paper machine, before the after-dryer, there is a forward dryer section, in which there are a number of successive groups (R....R_N) with single-wire draw that are open downwards, and after which forward dryer section a finishing section (D2) is placed, in which there are devices for surface-sizing or coating of the paper web (W), characterized in that, after the surface-sizing or coating devices, an after-dryer is placed, and that the after-dryer first includes an upwards open inverted group (R_k) with single-wire draw, in which the tendency of curling that was formed in the paper web (W) in the forward dryer section (Dl) can be substantially elimin- ated and/or compensated for.

11. An after-dryer in a paper machine for drying a paper to be surface-treated, in particular fine paper, for carrying out the method as claimed in any of the preceding claims, in which paper machine, before me after-dryer, there is a forward dryer section, in which there are a number of successive groups (R ■ ...R_N) with single-wire draw that are open downwards, and after which forward dryer section a finishing section (D2) is placed, in which there are devices for surface-sizing or coating of the paper web (W), characterized in that, in the after-dryer, in connection with the dryer groups (R₂₁,R₂₂), air supply devices have been fitted so as to feed hot moist or dry air to the desired locations, in which connection the tendency of curling that was formed in the paper web (W) in the forward dryer section (Dl) can be substantially eliminated and/or compensated for.

12. An after-dryer in a paper machine for drying a paper to be surface-treated, in particular fine paper, for carrying out the method as claimed in any of the preceding claims, in which paper machine, before the after-dryer, there is a forward dryer section, in which there are a number of successive groups (R_j...R_N) with single-wire draw that are open downwards, and after which forward dryer section a finishing section (D2) is placed, in which there are devices for surface-sizing or coating of the paper web (W), characterized in that, after the surface-sizing or coating devices, an after-dryer is placed, that the after-dryer first includes an upwards open inverted group (R_k) with single-wire draw, and that in the after-dryer, in connection with the dryer groups (R_k,R₂₁,R₂₂), air supply devices have been fitted so as to feed hot moist or dry air to the desired locations, in which connection the tendency of curling that was formed in the paper web (W) in the forward dryer section (Dl) can be substantially eliminated and/or compensated for.

13. An after-dryer as claimed in any of the claims 11 to 12, characterized in that the air supply devices for the feed of hot moist air into connection with the dryer groups (R_k,R_2],R₂₂) are blow boxes (237,216) and/or blow-suction boxes (217).

14. An after-dryer as claimed in any of the claims 11 to 13, characterized in that the air supply devices for the feed of hot moist air comprise separate air supply devices (222) and separate air removing devices (221).

15. An after-dryer as claimed in claim 14, characterized in that the air supply devices include an arrangement for regulation of the desired moisture content.