EP0567484B1

EP0567484B1 - Method and device in the press section of a paper machine for detaching the web from the face of a press roll

Info

Publication number: EP0567484B1
Application number: EP92902194A
Authority: EP
Inventors: Juhani Niskanen
Original assignee: Valmet Oy
Current assignee: Valmet Technologies Oy
Priority date: 1991-01-18
Filing date: 1992-01-20
Publication date: 1997-07-23
Anticipated expiration: 2012-01-20
Also published as: FI92941C; CA2099379C; DE69221123D1; EP0567484A1; ATE155837T1; CA2099379A1; FI910288A0; DE69221123T2; FI910288A; WO1992013133A1; FI92941B

Abstract

Method and device in the press section of a paper machine for detaching the web (W) from a smooth mantle face (18') of a press roll (18). In the method and in the device, a transfer zone (40a; 50a) is employed, over which a press fabric (23) or a particular transfer fabric (44) is passed so as to accomplish a closed draw of the web (W). The transfer zone (40a, 50a) as well as said fabric (23; 44) form a transfer nip (NS) or a transfer zone with the roll face (18'). At the transfer point, the temperature level (T) of the roll face (18') and/or of the web (W) is set or regulated so high that the pressure (pS) of saturated vapour of the water present in connection with the web (W) and with the roll face (18') which corresponds to said temperature level (T) is substantially equally high as, or just little higher than, the pressure (pO) that has been set to prevail in said transfer zone (40a, 50a), said pressure (pO) being preferably lower than the atmospheric pressure.

Description

The invention concerns a method for detaching a web from the smooth mantle face of a press roll in the press section of a paper machine in the press section of a paper machine according to the preamble of claim 1.
Further, the invention concerns a press section of a paper machine according to the preamble of claim 7.
In prior art, various compact and closed press sections are known, in which there is a combination of press rolls, whose press rolls form, as a rule, at least three press nips with one another. Of these prior-art press sections should be mentioned the applicant's "Sym-Press II" press section and the "Sym-Press 0" press section ("" = trade marks).
In a "Sym-Press II" press section, the first twin-wire nip is formed between a holler-faced lower press roll and an upper press-suction roll. Said suction roll forms the second nip with the smooth-faced centre roll of the press, and the third press nip is formed in connection with said centre roll. Further, this press section may include a separate nip, as a rule a fourth nip in the press, after which the web is transferred to the drying section.
In a "Sym-Press 0" press section, the first twin-felt horizontal nip is formed between a lower hollow-faced roll and an upper press-suction roll, after which the web follows the upper fabric as a vertical run into the second nip, which is formed between a hollow-faced roll separate from the rolls in the first nip and the smooth-faced centre roll. In connection with said centre roll, a third press nip, which is provided with a felt, is formed by means of a hollow-faced press roll.
In the way known in prior art, the web is detached from the smooth face of the centre roll in the press by means of a pulling force parallel to the running direction of the web as a free unsupported draw. This free draw is quite critical in view of the operation of a paper machine. In said free draw, to produce the detaching force, a speed difference is employed which stretches the web, which results in certain drawbacks. Moreover, said unsupported draw forms a problematic point susceptible of breaks in the paper machine.
The prior art has not offered efficient means for controlling the detaching of the web from the smooth-faced centre roll and the subsequent free draw. The unfavourable properties of granite, which has been used commonly as the material of the centre roll, have, for their part, also restricted the control of the detaching of the web and of the free draw.
With ever increasing running speeds of paper machines, the free draw of the web has become an ever more important problem point. In the near future, paper machine running speeds of an order of as high as about 2000 m/min are contemplated. The open draw of the web from a smooth-faced roll in the press forms or will form a bottleneck which for its part prevents increased running speeds of paper machines. It is a further problem that, in a paper machine, different paper qualities are often produced, whose adhesion to the smooth face of the centre roll is different, which results in variations in the necessary detaching tension of the web.
In a "Sym-Press II" press section, in the second and third press nip, the surface properties of the centre roll must be such that the moist web adheres to the roll face as well as possible. On the other hand, the web should be detached from the roll face readily for the transfer to the drying section. The meeting of these paradoxical requirements has not been successful in every respect by the prior-art means.
In prior art, some transfer-suction rolls have been suggested which have been meant to form a transfer nip against the smooth-faced centre roll in the press section. It is a feature common of these prior-art transfer-suction rolls that they do not operate in the intended way in practice. This is due to the fact that, by means of the prior-art transfer-suction rolls, it has not been possible to separate the paper web from the centre roll by suction, because, at the transfer point, the web adheres to the smooth face of the centre roll tightly and, as a rule, even by the intermediate of a water film. Thus, the negative pressure applied to the web in the suction zone of the transfer- suction roll produces a corresponding negative pressure between the web and the smooth face of the centre roll, which has the consequence that the web is not detached but follows the smooth face of the centre roll. Owing to the above, one has been content with considering it almost as a law of nature that the web cannot be detached from a smooth roll face by suction by means of negative pressure.
In respect of the prior art related to the invention, reference is also made to US-A- 4 919 759 and US-A-4 889 598.
In US-A-4 919 759, a method and a device are described for detaching of the web by means of an unsupported draw from a smooth-faced press roll, in which method the temperature of said press roll is regulated and, by means of this regulation, the adhesion between the roll face and the paper web to be detached is affected, and thereby the detaching angle and/or the detaching tension of the paper web is set within an optimal range.
In US-A-4 889 598, a method and a device are described in which the web is detached from the smooth roll face as an unsupported draw or as a closed draw and, in the area of the detaching point, a momentary and local induction heating effect is applied to the web from outside the smooth-faced press roll, by means of which heating effect the water present between the web and the roll face is heated, preferably vaporized, locally within the area of the detaching point to detach the web from the roll face.
In the following, typical tensions that occur in free draw of the web will be examined in the light of an example.
In free draw of any web-like material, the tension ${F = ρ·v}^{2}$
arises, wherein

F = tension [N/m]
ρ = grammage [kg/m²]
v = velocity [m/s]

Since the web has a certain finite breaking strength F_M, the formula for the limit speed is $v = \sqrt{\frac{F_{M}}{ρ}}$
Typically, the wet breaking strength of newsprint is F_M ≈ 200 N/m, and the corresponding grammage is 42 g/m², and the moisture content in the first free gap in the nip is ≈ 42 %. $→ ρ = 0.1 kg/m$
${→ v}_{limit} = \sqrt{\frac{200}{0.1}} \frac{m}{s} = 44.7 \frac{m}{s}$
On the other hand, the typical strength values are average values, but the web breaks at the weakest point. If there is an occasional, local strength of 50 N/m in the web, the following velocity is obtained: ${→ v}_{limit} = \sqrt{\frac{56}{0.1}} \frac{m}{s} = 23.7 \frac{m}{s}$
The above velocity v_limit is at present a typical running speed employed in paper machines, for which reasons the web breaks now and then at said open draw.
If it is desirable to increase the speed of a paper machine, the web must be made generally stronger or more homogeneous (no weak points), or the free gaps must be eliminated. The present invention endeavours to solve said problems by means of elimination of the free gaps.
The general object of the present invention is to provide novel means in the detaching of the web from the centre roll or equivalent in the press section and in the transfer of the web into a separate nip or to the drying section.
It is an object of the present invention to provide a method and a device for detaching the web from a smooth-faced press roll wherein the web need not be stretched, i.e. it is unnecessary to employ so-called draw and speed difference. Thus, the object of the invention is to provide a press section in which, when necessary, it is possible to use a fully closed draw when the web is transferred from the centre roll or from a corresponding other roll in the press to the drying section, as a rule onto its drying wire, or onto the press fabric of a separate nip.
It is an important goal of said objectives to permit an increased running speed of the paper machine so that the degree of utilization of the machine can be kept high.
In view of achieving the objectives stated above and those that will come out later, the method of the invention is characterized by the features of the characterizing clause of claim 1.
On the other hand, the press section of the invention is characterized by the features of the characterizing clause of claim 7.
In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being in no way strictly confined to the details of said embodiments.
Figure 1 is a schematic side view of the environment of application of the invention and of a first embodiment of the invention, in which the press fabric of the third nip is used as the transfer fabric in the transfer from the centre roll.
Figure 2 shows, in a way corresponding to Fig. 1, an embodiment of the invention in which a separate transfer fabric is used that runs around the transfer-suction roll.
Figure 3 is a vertical sectional view in the machine direction of a transfer nip in accordance with the invention as accomplished by means of a transfer-suction roll.
Figure 4 shows an exemplifying embodiment of the invention in which the suction transfer takes place by means of a static transfer-suction box that comprises two parts.
Figures 1 and 2 are schematic illustrations of a press section of a paper machine, in which the paper web W, which comes from the forming wire 10 of the former, is dewatered by pressing. The web W is transferred from the wire 10 at the pick-up point P by means of the press felt 12 on the suction zone 11a of the pick-up roll 11. On the pick-up felt 12, the web W is transferred into the first dewatering nip N₁, which is formed between the press-suction roll 16 and the hollow-faced 14' lower press roll 14. The first nip N₁ is a two-felt nip, and it includes a lower felt 13 guided by the guide rolls 15.
In Figs. 1 and 2, owing to the suction zone 16a of the suction roll 16, after the nip N₁ the web W follows the felt 12 into the second nip N₂, which is formed between said suction roll 16 and the smooth-faced 18' centre roll 18 in the press. Between the nips N₁ and N₂ there is a steam box 17, which heats the web W directly. The third nip N₃ in the press is formed between said centre roll 18 and the hollow-faced 21' press roll 21. The press felt 23, which is guided by the guide rolls 22, runs through the third nip N₃. On the lower sector of the centre roll 18, there is a doctor 19, which keeps the smooth face 18' of the centre roll 18 clean and detaches the web passing to broke from the face 18'.
In Figs. 1 and 2 the press rolls 14,16,18, and 21 form a compact combination of rolls in which there are three subsequent nips N₁,N₂,N₃, through which the web W runs as a closed draw, being constantly supported by a felt or roll face. A fourth, separate nip N₄ is formed between a smooth-faced 30', large-diameter press roll 30 and a lower hollow-faced 31' press roll 31. After the fourth nip N₄, the web W follows the smooth face 30', from which it is detached on the guide roll 39, being transferred onto the drying wire 36 of the drying section, which is guided by the guide roll 35. The web W is made to adhere to the drying wire 36 by means of a suction box 37, being passed on the wire 36 into a multi-cylinder dryer (not shown). On the upper sector of the upper press roll 30 of the separate nip N₄, there are jet means 29 followed by a doctor 28, which keeps the roll face 30' clean.
According to Fig. 1, after the fourth nip N₄, the press fabric 26 is separated from the web W by means of the guide roll 38, being passed back into connection with the web W into the transfer nip 30/39 over the transfer-suction roll 39. On the first suction zone 39a of the transfer-suction roll 39, the web W is detached from the roll face 30', and the web W is transferred over the roll 39 on the press fabric 26, being secured by the negative pressure in the suction zone 39b. The web W is detached from the fabric 26 by means of the suction box 37 and is transferred onto the drying wire 36. Differing from the above, in Fig. 2, the press fabric 26, being guided by the guide roll 32b, is separated from the web W, which is transferred on the roll face 30' onto the suction zone 39a of the transfer-suction roll 30. Over the transfer-suction roll 39, a particular transfer fabric 46 is passed, on which the web W is transferred onto the drying wire 36, being secured by the second suction zone 39b of the roll 39.
On the suction sector 16a between the first and the second nip, there is a first steam supply box 17. In a corresponding way, between the second and third nip N₂ and N₃, there is a second steam supply box 20 operating against the web W. By means of the hot steam fed out of the boxes 17 and 20, it is also possible to affect the elastic properties of the fibres in the web and the viscosity and the surface tension of the water present in the web so that the dewatering is intensified in the nips N₂ and N₃. According to the invention, the temperature T of the web W and of the face 18' of the centre roll 18 is raised to such a level that the detaching of the web W and its transfer from the roll face 18' can be carried out successfully in the transfer nip N_S. The raising of the temperature level T of the web W takes place by means of internal heating of the centre roll 18, an example of which is given in Fig. 4 by the axial bores 18b made into the roll mantle 18a, into which bores a heating medium, such as hot water or steam, is passed. In stead of, or in addition to, the internal heating of the roll 18, the roll mantle 18a may be heated by means of the steam boxes 17 and 20, in particular by means of the steam box 20, and also by means of various electric heaters, of which an induction heater 60 is shown as an example in Fig. 2.
The press construction described above is substantially known in prior art, and it is described above just as a background and as an environment of application for the present invention.
After the third nip N₃, the web W follows the smooth face 18' of the centre roll 18, from which it is detached as a fully closed draw in accordance with the invention by means of a transfer-suction roll 40 provided with a drive or by means of a transfer-suction box 50 (Fig. 4). In Fig. 1, by means of the suction zones 40a,40b of the transfer-suction roll 40, the web W is transferred on the suction zone 25a of the second lower transfer-suction roll 25 onto the press felt 26, which carries the web W on its upper face into the fourth, separate nip N₄ in the press section.
According to Fig. 1, the press felt 23 of the third nip N₃ is passed after the nip N₃ onto the guide roll 22a and from it into the transfer nip N_S so that said press felt 23 operates as a transfer fabric, which runs around the transfer-suction roll 40 and on which the web W is transferred on the suction zone 25a of the second transfer-suction roll 25 onto the lower fabric 26 of the fourth press nip N₄.
According to Fig. 2, a transfer fabric 44 of its own is arranged to run around the transfer-suction roll 40. which fabric 44 is passed into the transfer nip N_S and separated from the transfer-suction roll 40 so that the web W follows the lower fabric 26 of the fourth press nip N₄. Said transfer is secured, e.g., by means of choice of the surface properties of the fabrics 26 and 44 and possibly by means of the suction box 33. In connection with the loop of the transfer fabric 41, there are guide rolls and conditioning devices known per se, not shown in Fig. 2.
As is shown in Figs. 1 and 2, the transfer of the web W from the smooth face of the centre roll 18 takes place as a fully closed draw on the transfer-suction roll 40 aided by the negative pressure present in its interior and effective on the suction zone 40a. The suction zone 40a is followed by the second suction zone 40b of the transfer-suction roll 40, on which the level of negative pressure is lower than that on the zone 40a, ensuring that the web W remains on support of the transfer fabric 23;44. It is an important feature in the embodiment of the closed draw of the invention that, before the transfer point, the smooth face 18' of the centre roll 18 and/or the web W has been heated to such a temperature level T that on the suction zone 40a of the suction roll 40, when the web W is being sucked off the face 18' of the centre roll 18, no vacuum is formed in the area between the face 18' and the web W but, in stead, a steam pressure p_S corresponding to the temperature T is formed, which steam pressure is, in the present invention, arranged so that its absolute value is higher than the absolute pressure p_O in the suction zone (p_S > p_O)_. By means of the pressure difference p_S - p_O, a force field is produced which presses the paper web W from the roll face 18' onto the transfer fabric 23;44.
In the following, mainly with reference to Fig. 3, the physical background of the method in accordance with the invention will be described in more detail.
Fig. 3 shows the environment of the transfer nip N_S between the centre roll 18 and the transfer-suction roll 40. The transfer-suction roll 40 includes a perforated 42 roll mantle 41, in whose interior there are suction boxes. In Fig. 3, in respect of the suction boxes, just the sealing ribs 43 of the first suction zone 40a are shown, which operate against the smooth inner face 41' of the roll mantle 41.
The paper web W enters into the transfer nip N_S on the smooth face 18' of the centre roll 18. As a rule, the paper web W adheres tightly to the face 18' by the intermediate of a non-unified water film. Before the transfer nip N_S, the temperature T of the face 18' and of the web W and of the water film that makes the web adhere to the face 18' has been raised to such a level that the water present between the roll face 18' and the web W and partly in the structure of the web W is vaporized as its volume is increased in the transfer nip N_S when the web W is sucked towards the transfer fabric 44 by means of the negative pressure p_O in the suction zone 40a of the roll 40. When there is water at the temperature T between the web W and the face 18', the maximum negative pressure that can arise here is the pressure p_S of saturated vapour prevailing at said temperature T.
Air starts flowing into the bottom of the wedge space A opening between the web W and the roll face 18' after the nip N_S as soon as the gap formed between the roll face 18' and the outer face of the web W is wide enough in comparison with the velocity of the faces. As the resistance to flow of the paper web W is high enough, a negative pressure can be sucked between the paper web W and the transfer fabric 44, the absolute value of said negative pressure being p_O < p_S. The length of the transfer nip N_S has been dimensioned such that there is time enough for generation of full negative pressure between the paper web W and the transfer fabric 44. For this purpose, the outer face of the mantle 41 of the transfer-suction roll 40 may be provided with a soft coating, by whose means the length of the transfer nip N_S can be extended to make it large enough. In Fig. 3, the reference S denotes the point or line at which the paper web W is separated from the face 18', because in the gap A that is opened, no higher negative pressure can arise than the pressure p_S of saturated water vapour corresponding to the temperature T.
The temperature T of the face 18' of the centre roll 18 and of the web when they enter into the transfer nip N_S is in the invention, as a rule, in the range of T ≈ 50...100°C, which corresponds to the range of pressure of saturated water vapour (absolute values) of p_S ≈ 0.13...1.0 bar. A temperature range that is particularly favourable and practical in view of the objectives of the invention is the range T ≈ 65...75°C, which corresponds to the range of pressure of water vapour of p_S ≈ 0.25...0.40 bar. Thus, for example, if the temperature T = 65°C is employed, the corresponding vapour pressure p_S ≈ 0.25 bar, in which case, in a closed suction transfer, the absolute pressure p_O in the suction zone 40a must be p_O < 0.25 bar, i.e., in the suction zone 40a, there must be at least a negative pressure of (1.00 - 0.25) bar = 0.75 bar.
Fig. 4 shows an embodiment of the invention in which a static suction box 50 is employed instead of a revolving suction roll 40. Inside the suction box 50, two suction chambers 50a and 50b are fitted, whose open ends are defined by horizontal sealing ribs 53, over which the transfer fabric 44 is passed. In the first suction zone 50a, such a negative pressure p_O prevails that in the transfer nip N_S the web W is separated from the roll face 18' as a closed draw. In the second suction zone 50b, there is a lower level of negative pressure (as a rule in a range of p_I ≈ 0.1 bar), whereby it is ensured that the web W follows the transfer fabric 44. The suction chambers 50a and 50b communicate with the suction ducts 55a and 55b of the suction box 50 by the intermediate of the pipes 54a and 54b. After the suction box 50, the web W is transferred on the transfer fabric 55 into a transfer nip formed by the rolls 32 and 45, after which the web W is separated from the transfer fabric 44 and is transferred onto the support of the press felt 26, to which it is made to adhere by means of the negative pressure in the suction box 33.
Said temperature level T of the web W and the roll face 18' that enter into the transfer nip N_S is regulated to a level suitable in view of the invention by means of internal heating systems 18b in the roll 18 or by means of steam boxes 17 and/or 20. The internal heating systems in the roll 18 may also include heating media passed into the interior of the roll mantle 18a, such as steam or hot water. In addition to the devices mentioned above, it is also possible, as is shown in Fig. 2, to employ a heater placed before the transfer nip N_S, preferably an induction heater 60, by whose means the temperature T of the roll face 18' is raised to a level suitable in view of the suction transfer and the closed draw of the web W in accordance with the invention. The induction heater 60 is connected with a unit 61 for the supply of electric power. If necessary, the induction heater 60 and the other heaters may also be arranged so that, by its means, the transverse temperature profile of the roll face 18' and the web W before the transfer nip N_S and in the nip area is controlled so that said profile is optimal in view of the transfer of the web W, as a rule as uniform as possible, e.g., in view of the different properties of adhesion to the roll face 18' possessed by different pulp qualities.
It is a requirement for a successful closed transfer in accordance with the invention that the negative pressure present in the suction zone of the transfer-suction roll 40 is not effective for an excessively long time, in which case the vapour arising at the transfer point between the web W and the roll face 18' would have time to cool the space between the web W and the roll face 18' by the effect of the vaporization heat.
Even though, above, the invention has been described with reference to an environment of application in which a separate nip N₄ is employed, the invention can also be applied to a press section in which there is no separate nip N₄, but the web W is transferred as a closed draw from the fabric that runs over the transfer-suction roll 40 into the drying section, e.g. onto its drying wire, so that the press fabric 26 shown in Figs. 1 and 2 corresponds to the drying wire in the first wire group, on which fabric the web W is passed as a single-wire draw through the first drying group.
Moreover, Figs. 1 and 2 show such a "Sym-Press-II" press section in which the suction roll 16 forms two nips N₁ and N₂. The invention can also be applied to other press geometries, such as the applicant's "Sym-Press-O" press section, in which, after the press-suction roll 16, the web W has a vertical draw supported by the fabric 12 and, in stead of the press-suction roll 16, there is a separate press roll in connection with the centre roll 18.
It is not always necessary that the negative pressure that prevails in the suction zone 40a,50a of the suction roll 40 or of the suction box 50 is exactly equal to the vapour pressure corresponding to the temperature T of the roll face 18 or of the web W, but in some special situations a lower level of negative pressure may also be adequate, which level is, however, chosen such that it substantially promotes the detaching of the web W without a detrimentally high pulling tension applied to the web. This is the case, e.g., with thicker paper qualities and/or with stronger pulps. For example, if the temperature of the web W to be detached and of the roll face is T ≈ 70°C, it corresponds to a pressure of saturated vapour of 030 bar, in which case the negative pressure that is required is preferably 1-030 bar = 0.7 bar. According to what was stated above, in particular cases, the invention can operate at a "level of negative pressure" estimated at about 80 %, i.e., in the above example, the negative pressure p_O employed in the suction zone 40a;30a might be, at the minimun, of an order of 0.8>0.7 bar = 0.56 bar.
Even though, above and in the exemplifying embodiments illustrated in the figures, the invention has been described with reference to so-called compact and closed press sections, the invention can also be applied to presses in which separate press nips are employed, between which the web W has. e.g., a draw supported by a transfer fabric.
In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above for the sake of example only.

Claims

Method for detaching a web (W) from the smooth mantle face (18') of a press roll (18) in the press section of a paper machine, in which method a transfer zone (40a, 50a) is employed, over which a press fabric (23) or a particular transfer fabric (44) is passed, so as to accomplish a closed draw of the web (W), said transfer zone (40a, 50a) as well as said fabric (23, 44) being arranged to form a transfer nip (N_S) with said roll face (18'), and in which method at the transfer zone the temperature level (T) of said roll face (18') and/or of the web (W) is set or regulated so high that the water present between the web (W) and the roll face (18') and partly in the structure of the web (W) is vaporized ,
characterized in that as transfer zone a transfer-suction zone (40a, 50a) is employed in which an absolute pressure (p_O) lower than the atmospheric pressure is set to prevail, and that at the transfer point the temperature level (T) of said roll face (18') and/or of the web (W) is set or regulated so high that the absolute pressure (p_S) of saturated vapour of the water which corresponds to said temperature level (T) is equally as high as or higher than the absolute pressure (p_O) that has been set to prevail in said transfer-suction zone (40a, 50a).
Method as claimed in claim 1, characterized in that the level of subatmospheric pressure (p_O) in said transfer-suction zone (40a;50a) is chosen such that it is at least about 80 % of the pressure (p_S) of saturated water vapour corresponding to the temperature (T) of said roll face (18') or of the web (W) to be transferred.
Method as claimed in any of the claims 1 or 2, characterized in that in the method a transfer-suction roll (40) is employed, over which the press fabric (23) of the preceding nip (N₃) in the press section or a transfer fabric (44) of its own is passed and that said transfer-suction roll (40) is arranged to form a transfer nip (N_S) with the smooth roll face (18') from which the web (W) is transferred as a closed draw without a substantial pulling or speed difference.
Method as claimed in any of the claims 1 or 2, characterized in that in the method a static transfer-suction box (50) is employed, whose suction zone (50a) or zones (50a,50b) is/are arranged to form a closed web (W) transfer area in connection with the smooth roll face (18').
Method as claimed in any of the claims 1 to 4, characterized in that the temperature level T of said roll face (18'), from which the web (W) is detached as a closed draw without substantial pulling, is set or regulated within the range of T ≈ 65...75°C, and that the absolute pressure p_O in said transfer-suction zone is regulated or set within the range of p_O ≈ 0.25...0.4 bar.
Method as claimed in any of the claims 1 to 5, characterized in that in the method the web (W), which has been transferred from the smooth roll face (18') on the press fabric (23) or on the particular transfer fabric (44), is carried as a closed draw into a subsequent transfer nip or equivalent, from which the web (W) is transferred onto a press fabric (26) of a separate press nip (N₄) in the press section or onto a drying wire (36) of the drying section.
Press section of a paper machine, comprising an arrangement of press rolls (14, 16, 18, 21), which form one or several press nips (N₁, N₂, N₃) with one another, one of these rolls being a smooth-faced roll (18) from which a web (W) is detached by means of a transfer device (40, 50) and transferred further to the drying section of the paper machine or into a separate press nip (N₄) in the press section, wherein said transfer device (40, 50) is arranged to form a transfer nip (N_S), which accomplishes a closed draw of the web (W), in connection with the smooth roll face (18'), said press section further comprising a heating device or devices (20, 60) by means of which the temperature (T) of the web that enters the transfer point and/or the temperature of said smooth roll face (18') is raised to such a high level that the water present between the web (W) and the smooth roll face (18') and partly in the structure of the web (W) is vaporized, characterized in that the transfer device is a revolving suction-transfer roll (40) or an equivalent static suction-transfer box (50), said revolving suction-transfer roll (40) or equivalent suction-transfer box (50) being provided with a suction zone (40a, 50a) at the transfer nip (N_S) in which an absolute pressure (p_O) lower than the atmospheric pressure is set to prevail and that the heating device or devices (20, 60) raise the temperature (T) of the web (W) that enters the transfer point and/or the temperature of said smooth roll face (18') to such a high level that the absolute pressure (p_S) of saturated vapour of the water which corresponds to said temperature level is equally as high as or higher than the absolute pressure (p_O) that is prevailing in said suction zone (40a, 50a).
Press section of a paper machine as claimed in claim 7, characterized in that the transfer-suction device is a transfer-suction roll (40), in which, at the transfer nip (N_S), there is a suction zone (40a) followed by a second suction zone (40b), preferably with a lower subatmospheric pressure, and that a press fabric (23) of the preceding press nip (N₃) or a particular transfer fabric (44) is passed through said transfer nip (N_S).
Press section of a paper machine as claimed in claim 7 or 8, characterized in that in said smoothfaced roll (18), which is preferably the centre roll (18) of a compact and closed press section, there is an internal heating arrangement (18b), and/or a steam box (20) and/or an induction heater (60) or equivalent is arranged in connection with said roll (18), by whose means the temperature level (T) of the roll face (18') can be raised to a level sufficiently high in view of the closed suction transfer of the web (W).
Press section of a paper machine as claimed in any of the claims 7 to 9, characterized in that, after said transfer-suction roll (40) or static transfer-suction box (50), the web (W) is passed on a fabric (23;44) into a further transfer nip or equivalent, by whose means the web (W) is transferred onto the lower fabric (26) of a separate press nip (N₄) in the press section or onto a drying wire (36) of the drying section of the paper machine or onto some other, equivalent fabric.