CA2099379C - Method and device in the press section of a paper machine for detaching the web from the face of a press roll - Google Patents
Method and device in the press section of a paper machine for detaching the web from the face of a press roll Download PDFInfo
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- CA2099379C CA2099379C CA002099379A CA2099379A CA2099379C CA 2099379 C CA2099379 C CA 2099379C CA 002099379 A CA002099379 A CA 002099379A CA 2099379 A CA2099379 A CA 2099379A CA 2099379 C CA2099379 C CA 2099379C
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- transfer
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- suction
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- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
- D21F3/00—Press section of machines for making continuous webs of paper
- D21F3/02—Wet presses
- D21F3/0281—Wet presses in combination with a dryer roll
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
- D21F3/00—Press section of machines for making continuous webs of paper
- D21F3/02—Wet presses
- D21F3/04—Arrangements thereof
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- Replacement Of Web Rolls (AREA)
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 (N s) 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 (p s) 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 (p o) that has been set to prevail in said transfer zone (40a, 50a), said pressure (p o) being preferably lower than the atmospheric pressure.
Description
~1 rd Method and device in the press section of a paper machine for detaching the web from the face of a press roll The invention concerns a method in the press section of a paper machine for detaching the web from a smooth mantle face of a press roll, in which method a transfer zone is employed, over which a press fabric or a particular transfer fabric is passed so as to accomplish a closed draw of the web, and said transfer zone as well as said fabric are arranged to form a transfer nip or a transfer zone with said roll face.
Further, the invention concerns a press section of a paper machine, comprising 1 i an arrangement of press rolls, which form one or several press nips with one another, the web being detached from a smooth-faced roll in said press section by means of a roll or an equivalent transfer member and being transferred ' further to the drying section or into a separate nip in the press section, in which press section a revolving suction-transfer roll or a corresponding static suction-'~0 transfer box has been arranged to form a transfer nip or a transfer zone, which r accomplishes a closed draw of the web, in connection with the smooth roll face, in which nip or zone the web is subjected to a negative pressure which attempts to detach the web from the roll face.
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 men-tioned the applicant's "Sym-Press II" press section and the "Svm-Press 0"
press section (°"' = trade marks).
,() , .
In a "Svm-Press II" press section, the first twin-wire nip is formed between a hollow-faced lower press roll and an upper press-suction roll. Said suction rol ;~u~~~r~9 2 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.
J
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 problem-auc 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 's 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 contem-' plated. The open draw of the web from a smooth-faced roll in the press forms or v~ill form a bottleneck which for its part prevents increased runnin, speeds c~~
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 relied to the invention, reference is also made to U.S. Patents Nos. 4,919,759 and 4,489,596.
In U.S. Patent No. 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 U.S. Patent No. 4,889,598, a method and a device are described in S 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=pw2 arises, wherein F = tension [N/m]
p = grammage [kg/m2]
v = velocity [m/s]
Since the web has a certain finite breaking strength FM, the formula for the limit speed is FM
_ _____ P
Typically, the wet breaking strength of newsprint is FM ~ 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%.
~ p = 0.1 kg/m 200 m m 3 0 -» _ _ ~ _ 44.7 ___ limit -0.1 s 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:
m m $ ~ vlimit - __---_ ___ _ 23.7 _--0.1 s s The above velocity v~;m;~ 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 present invention is directed towards providing novel means in 1$ 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.
The present invention is specifically directed towards the provision of 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 present invention is directed towards the provision of 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 2$ section, as a rule onto its drying wire, or onto the press fabric of a separate nip.
It is an important goal of the invention to permit an increased running speed of the paper machine so that the degree of utilization of the machine can be kept high.
Accordingly, in one aspect of the present invention, there is provided a method for detaching a web from the smooth mantle face of a press roll in the press section of a paper machine, which comprises: employing a transfer zone over which a press fabric or a particular transfer fabric is passed, so as to accomplish a closed draw of the web; and arranging the transfer zone as well as the fabric to form a transfer nip with the roll face; wherein transfer-s suction zone is employed as the transfer zone in which an absolute pressure lower than the atmospheric pressure is set to prevail, and wherein at the transfer point the temperature level of at least one of the roll face and the web is set or regulated so high that the absolute pressure of saturated vapour of water present between the web and the roll face and partly in the structure of the web is equally as at least as high as the absolute pressure that has been set to prevail in the transfer-suction zone.
In accordance with a further aspect of the present invention, there is provided a press section of a paper machine, comprising an arrangement of press rolls, which form one or several press nips with one another, one of the rolls being a smooth-faced roll from which a web is detached by means of a transfer device which is a revolving suction-transfer roll or an equivalent static suction-transfer box, and transferred further to the drying section of the paper machine or into a separate press nip in the press section, wherein the transfer device is arranged to form a transfer nip, which accomplishes a closed draw of the web, in connection with the smooth roll face, the press section further comprising a heating device or devices by means of which the temperature of the web that enters the transfer point and/or the temperature of the smooth roll face is raised to such a high level that the water present between the web and the smooth roll face and partly in the structure of the web is vaporized, the revolving suction-transfer roll or equivalent suction-transfer box being provided with a suction zone at the transfer nip in which an absolute pressure lower than the atmospheric pressure is set to prevail and wherein the press section further comprises a heating device or device by means of which raise the temperature of the web that enters the transfer point and/or the temperature of the smooth roll face to such a high level that the absolute pressure of saturated vapour of 6a water vaporized from between the web and the smooth roll and partly from the structure of the web is as at least as high as the absolute pressure that is prevailing in the suction zone.
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 drawings, 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 10 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 Figure 1, an embodiment of the inven-., tion 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 a.re 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-la up point P by means of the press felt 12 on the suction zone lla 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,N2.1\3' through which the we iv WO 92/13133 PCT/FI92/00014 w ~f~~4J~~
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 mufti-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 trans-fer-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 inten sified in the nips M2 and N3. According to the invention, the temperature T o' 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 carned out successfully in the transfer nip Ns. 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. Instead of, or in addition to, the internal heating of the roll 18, the roll mangle 18a may be heated by means of the steam boxes 17 and 20, in particular by means of the steam 10 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 SO (Fig. 4). In Fig. 1, by means of the suction zones 40, 40b of the transfer-suction roll 40, 20 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 NS
25 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 30 around the transfer-suction roll 40, which fabric 44 is passed into the transfer nip Ns 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 5 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 10 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, instead, a steam pressure ps 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 po in the suction zone (ps > po). By means of the pressure difference ps - po, 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 Ns 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 l0a the first suction zone 40a are shown, which operate against the smooth inner face 41' of the roll mantle 41.
;~:~>.,' WO 92/13133 PCT/F192/00014 ~~~~~"~Lrs The paper web W enters into the transfer nip NS 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 NS, the temperature T of the face 18' and of the web W and of the water film that S 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 struc-ture of the web W is vaporized as its volume is increased in the transfer nip NS
when the web W is sucked towards the transfer fabric 44 by means of the negative pressure p~ 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 ps 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~ 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~
<
ps. The length of the transfer nip NS 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 NS 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 pS of saturated water vapour corresponding to the temperature T.
:, ~0 The temperature T of the face 18' of the centre roll 18 and of the web when they enter into the transfer nip NS is in the invention, as a rule, in the range of T ~ ~0...100~C, which corresponds to the range of pressure of saturated water .i vapour (absolute values) of pS ~ 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 ps ~ 0.25..Ø40 bar. Thus, for example, if the temperature T =
65°C
is employed, the corresponding vapour pressure pS ~ 0.25 bar, in which case, in a closed suction transfer, the absolute pressure p~ in the suction zone 40a must be p~ < 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 in stead of a revolving suction roll 40. Inside the suction box S0, two suction. chambers SOa and SOb 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 SOa, such a negative pressure pp prevails that in the transfer nip NS
the web W is separated from the roll face 18' as a closed draw. In the second suction zone SOb, there is a lower level of negative pressure (as a rule in a range of p~ ~ 0.1 bar), whereby it is ensured that the web W follows the transfer fabric 44. The suction chambers SOa and SOb communicate with the suction ducts SSa and SSb 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.
~S
Said temperature level T of the web W and the roll face 18' that enter into the y transfer nip NS 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.
?. to employ a heater placed before the transfer nip MS, preferably an induction WO 92/13133 Pcr/F192/ooota 13 1~~~~'l~
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 NS 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 inven-,..
tion 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 ?5 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 l6 forms two nips N.~ 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 lb, there is a separat.
W0 92/13133 PCTlF192100014 t""' ~~~~v~~~~~ 14 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,S0a of the suction roll 40 or of the suction box SO 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 satu-rated vapour of 0.30 bar, in which case the negative pressure that is required is preferably 1-0.30 bar = 0.7 bar. According to what was stated above, in particu-lar 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~ employed in the suction zone 40a;50a might be, at the minimum, of an order of 0.800.7 bar =
0.56 bar.
The scope of the invention also includes such embodiments differing from the above in which the temperature level T of the face 18' of the centre roll 18 is set . at the level of T >_ 100°C, in which case, in the transfer of the web W, a zone with negative pressure is not necessarily needed inside the transfer fabric 23;44.
According to the present-day estimation, however, the method, and the device of the invention can be accomplished optimally so that expressly a suction zone 2~ 40a,50a is employed' in which a pressure p~ lower than the atmospheric pressure has been set to prevail.
..
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.
w. WO 92/13133 laCT/F192/00014 15 ~~i~~~~~~
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.
Further, the invention concerns a press section of a paper machine, comprising 1 i an arrangement of press rolls, which form one or several press nips with one another, the web being detached from a smooth-faced roll in said press section by means of a roll or an equivalent transfer member and being transferred ' further to the drying section or into a separate nip in the press section, in which press section a revolving suction-transfer roll or a corresponding static suction-'~0 transfer box has been arranged to form a transfer nip or a transfer zone, which r accomplishes a closed draw of the web, in connection with the smooth roll face, in which nip or zone the web is subjected to a negative pressure which attempts to detach the web from the roll face.
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 men-tioned the applicant's "Sym-Press II" press section and the "Svm-Press 0"
press section (°"' = trade marks).
,() , .
In a "Svm-Press II" press section, the first twin-wire nip is formed between a hollow-faced lower press roll and an upper press-suction roll. Said suction rol ;~u~~~r~9 2 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.
J
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 problem-auc 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 's 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 contem-' plated. The open draw of the web from a smooth-faced roll in the press forms or v~ill form a bottleneck which for its part prevents increased runnin, speeds c~~
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 relied to the invention, reference is also made to U.S. Patents Nos. 4,919,759 and 4,489,596.
In U.S. Patent No. 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 U.S. Patent No. 4,889,598, a method and a device are described in S 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=pw2 arises, wherein F = tension [N/m]
p = grammage [kg/m2]
v = velocity [m/s]
Since the web has a certain finite breaking strength FM, the formula for the limit speed is FM
_ _____ P
Typically, the wet breaking strength of newsprint is FM ~ 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%.
~ p = 0.1 kg/m 200 m m 3 0 -» _ _ ~ _ 44.7 ___ limit -0.1 s 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:
m m $ ~ vlimit - __---_ ___ _ 23.7 _--0.1 s s The above velocity v~;m;~ 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 present invention is directed towards providing novel means in 1$ 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.
The present invention is specifically directed towards the provision of 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 present invention is directed towards the provision of 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 2$ section, as a rule onto its drying wire, or onto the press fabric of a separate nip.
It is an important goal of the invention to permit an increased running speed of the paper machine so that the degree of utilization of the machine can be kept high.
Accordingly, in one aspect of the present invention, there is provided a method for detaching a web from the smooth mantle face of a press roll in the press section of a paper machine, which comprises: employing a transfer zone over which a press fabric or a particular transfer fabric is passed, so as to accomplish a closed draw of the web; and arranging the transfer zone as well as the fabric to form a transfer nip with the roll face; wherein transfer-s suction zone is employed as the transfer zone in which an absolute pressure lower than the atmospheric pressure is set to prevail, and wherein at the transfer point the temperature level of at least one of the roll face and the web is set or regulated so high that the absolute pressure of saturated vapour of water present between the web and the roll face and partly in the structure of the web is equally as at least as high as the absolute pressure that has been set to prevail in the transfer-suction zone.
In accordance with a further aspect of the present invention, there is provided a press section of a paper machine, comprising an arrangement of press rolls, which form one or several press nips with one another, one of the rolls being a smooth-faced roll from which a web is detached by means of a transfer device which is a revolving suction-transfer roll or an equivalent static suction-transfer box, and transferred further to the drying section of the paper machine or into a separate press nip in the press section, wherein the transfer device is arranged to form a transfer nip, which accomplishes a closed draw of the web, in connection with the smooth roll face, the press section further comprising a heating device or devices by means of which the temperature of the web that enters the transfer point and/or the temperature of the smooth roll face is raised to such a high level that the water present between the web and the smooth roll face and partly in the structure of the web is vaporized, the revolving suction-transfer roll or equivalent suction-transfer box being provided with a suction zone at the transfer nip in which an absolute pressure lower than the atmospheric pressure is set to prevail and wherein the press section further comprises a heating device or device by means of which raise the temperature of the web that enters the transfer point and/or the temperature of the smooth roll face to such a high level that the absolute pressure of saturated vapour of 6a water vaporized from between the web and the smooth roll and partly from the structure of the web is as at least as high as the absolute pressure that is prevailing in the suction zone.
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 drawings, 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 10 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 Figure 1, an embodiment of the inven-., tion 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 a.re 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-la up point P by means of the press felt 12 on the suction zone lla 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,N2.1\3' through which the we iv WO 92/13133 PCT/FI92/00014 w ~f~~4J~~
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 mufti-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 trans-fer-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 inten sified in the nips M2 and N3. According to the invention, the temperature T o' 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 carned out successfully in the transfer nip Ns. 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. Instead of, or in addition to, the internal heating of the roll 18, the roll mangle 18a may be heated by means of the steam boxes 17 and 20, in particular by means of the steam 10 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 SO (Fig. 4). In Fig. 1, by means of the suction zones 40, 40b of the transfer-suction roll 40, 20 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 NS
25 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 30 around the transfer-suction roll 40, which fabric 44 is passed into the transfer nip Ns 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 5 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 10 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, instead, a steam pressure ps 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 po in the suction zone (ps > po). By means of the pressure difference ps - po, 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 Ns 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 l0a the first suction zone 40a are shown, which operate against the smooth inner face 41' of the roll mantle 41.
;~:~>.,' WO 92/13133 PCT/F192/00014 ~~~~~"~Lrs The paper web W enters into the transfer nip NS 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 NS, the temperature T of the face 18' and of the web W and of the water film that S 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 struc-ture of the web W is vaporized as its volume is increased in the transfer nip NS
when the web W is sucked towards the transfer fabric 44 by means of the negative pressure p~ 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 ps 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~ 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~
<
ps. The length of the transfer nip NS 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 NS 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 pS of saturated water vapour corresponding to the temperature T.
:, ~0 The temperature T of the face 18' of the centre roll 18 and of the web when they enter into the transfer nip NS is in the invention, as a rule, in the range of T ~ ~0...100~C, which corresponds to the range of pressure of saturated water .i vapour (absolute values) of pS ~ 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 ps ~ 0.25..Ø40 bar. Thus, for example, if the temperature T =
65°C
is employed, the corresponding vapour pressure pS ~ 0.25 bar, in which case, in a closed suction transfer, the absolute pressure p~ in the suction zone 40a must be p~ < 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 in stead of a revolving suction roll 40. Inside the suction box S0, two suction. chambers SOa and SOb 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 SOa, such a negative pressure pp prevails that in the transfer nip NS
the web W is separated from the roll face 18' as a closed draw. In the second suction zone SOb, there is a lower level of negative pressure (as a rule in a range of p~ ~ 0.1 bar), whereby it is ensured that the web W follows the transfer fabric 44. The suction chambers SOa and SOb communicate with the suction ducts SSa and SSb 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.
~S
Said temperature level T of the web W and the roll face 18' that enter into the y transfer nip NS 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.
?. to employ a heater placed before the transfer nip MS, preferably an induction WO 92/13133 Pcr/F192/ooota 13 1~~~~'l~
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 NS 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 inven-,..
tion 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 ?5 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 l6 forms two nips N.~ 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 lb, there is a separat.
W0 92/13133 PCTlF192100014 t""' ~~~~v~~~~~ 14 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,S0a of the suction roll 40 or of the suction box SO 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 satu-rated vapour of 0.30 bar, in which case the negative pressure that is required is preferably 1-0.30 bar = 0.7 bar. According to what was stated above, in particu-lar 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~ employed in the suction zone 40a;50a might be, at the minimum, of an order of 0.800.7 bar =
0.56 bar.
The scope of the invention also includes such embodiments differing from the above in which the temperature level T of the face 18' of the centre roll 18 is set . at the level of T >_ 100°C, in which case, in the transfer of the web W, a zone with negative pressure is not necessarily needed inside the transfer fabric 23;44.
According to the present-day estimation, however, the method, and the device of the invention can be accomplished optimally so that expressly a suction zone 2~ 40a,50a is employed' in which a pressure p~ lower than the atmospheric pressure has been set to prevail.
..
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.
w. WO 92/13133 laCT/F192/00014 15 ~~i~~~~~~
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 (10)
1. A method for detaching a web from the smooth mantle face of a press roll in the press section of a paper machine, which comprises:
employing a transfer zone over which a press fabric or a particular transfer fabric is passed, so as to accomplish a closed draw of the web;
and arranging said transfer zone as well as said fabric to form a transfer nip with said roll face, wherein transfer-suction zone is employed as the transfer zone in which an absolute pressure lower than the atmospheric pressure is set to prevail, and wherein at the transfer point the temperature level of at least one of said roll face and the web is greater than about 50°C and less than about 100°C so that the absolute pressure of saturated vapour of water present between the web and the roll face and partly in the structure of the web is equally as at least as high as the absolute pressure that has been set to prevail in said transfer-suction zone.
employing a transfer zone over which a press fabric or a particular transfer fabric is passed, so as to accomplish a closed draw of the web;
and arranging said transfer zone as well as said fabric to form a transfer nip with said roll face, wherein transfer-suction zone is employed as the transfer zone in which an absolute pressure lower than the atmospheric pressure is set to prevail, and wherein at the transfer point the temperature level of at least one of said roll face and the web is greater than about 50°C and less than about 100°C so that the absolute pressure of saturated vapour of water present between the web and the roll face and partly in the structure of the web is equally as at least as high as the absolute pressure that has been set to prevail in said transfer-suction zone.
2. The method of claim 1, wherein the level of subatmospheric pressure in said transfer-suction zone is chosen such that it is at least about 80% of the pressure of saturated water vapour corresponding to the temperature of said roll face or of the web to be transferred.
3. The method of claim 1 or 2, wherein in the method a transfer-suction roll is employed, over which the press fabric of the preceding nip in the press section or a transfer fabric of its own is passed and said transfer-suction roll is arranged to form a transfer nip with the smooth roll face from which the web is transferred as a closed draw without a substantial pulling or speed difference.
4. The method of claim 1 or 2, wherein in the method a static transfer-suction box is employed, whose suction zone or zones is/are arranged to form a closed web transfer area in connection with the smooth roll face.
5. The method of any one of claims 1 to 4, wherein the temperature level of said roll face, from which the web is detached as a closed draw without substantial pulling, is set or regulated within the range of approximately 65 to 75°C, and the absolute pressure in said transfer-suction zone is regulated or set within the range of approximately 0.25 to 0.4 bar.
6. The method of any one of claims 1 to 5, wherein in the method the web, which has been transferred from the smooth roll face (18') on the press fabric or on the particular transfer fabric, is carried as a closed draw into a subsequent transfer nip or equivalent, from which the web is transferred onto a press fabric of a separate press nip in the press section or onto a drying wire of the drying section.
7. A press section of a paper machine, comprising an arrangement of press rolls, which form one or several press nips with one another, one of the rolls being a smooth-faced roll from which a web is detached by means of a transfer device which is a revolving suction-transfer roll or an equivalent static suction-transfer box, and transferred further to the drying section of the paper machine or into a separate press nip in the press section, wherein said transfer device is arranged to form a transfer nip, which accomplishes a closed draw of the web, in connection with the smooth roll face, said press section further comprising a heating device or devices by means of which the temperature of the web that enters the transfer point and/or the temperature of said smooth roll face is raised to such a high level that the water present between the web and the smooth roll face and partly in the structure of the web is vaporized, said revolving suction-transfer roll or equivalent suction-transfer box being provided with a suction zone at the transfer nip in which an absolute pressure lower than the atmospheric pressure is set to prevail and wherein said press section further comprises a heating device or device by means of which raise the temperature of the web that enters the transfer point and/or the temperature of said smooth roll face is greater than about 50°C and less than about 100°C so that the absolute pressure of saturated vapour of water vaporized from between the web and the smooth roll and partly from the structure of the web is as at least as high as the absolute pressure that is prevailing in said suction zone.
8. The press section of claim 7, wherein the transfer-suction device is a transfer-suction roll, in which, at the transfer nip, there is a suction zone followed by a second suction zone, and that a press fabric of the preceding press nip or a particular transfer fabric is passed through said transfer nip.
9. The press section of claim 7 or 8, wherein in said smooth-faced roll, which is preferably the centre roll of a compact and closed press section, there is an internal heating arrangement, and/or a steam box and/or an induction heater or equivalent is arranged in connection with said roll, by whose means the temperature level of the roll face can be raised to a level sufficiently high in view of the closed suction transfer of the web.
10. The press section of any one of claims 7 to 9, wherein, after said transfer-suction roll or static transfer-suction box, the web is passed on a fabric into a further transfer nip or equivalent, by whose means the web is transferred onto the lower fabric of a separate press nip in the press section or onto a drying wire of the drying section of the paper machine or onto some other, equivalent fabric.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI910288A FI92941C (en) | 1991-01-18 | 1991-01-18 | Method and apparatus in the press portion of a paper machine for detaching the web from the surface of a press roll |
| FI910288 | 1991-01-18 | ||
| PCT/FI1992/000014 WO1992013133A1 (en) | 1991-01-18 | 1992-01-20 | Method and device in the press section of a paper machine for detaching the web from the face of a press roll |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2099379A1 CA2099379A1 (en) | 1992-07-19 |
| CA2099379C true CA2099379C (en) | 2000-04-25 |
Family
ID=8531762
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002099379A Expired - Fee Related CA2099379C (en) | 1991-01-18 | 1992-01-20 | Method and device in the press section of a paper machine for detaching the web from the face of a press roll |
Country Status (6)
| Country | Link |
|---|---|
| EP (1) | EP0567484B1 (en) |
| AT (1) | ATE155837T1 (en) |
| CA (1) | CA2099379C (en) |
| DE (1) | DE69221123T2 (en) |
| FI (1) | FI92941C (en) |
| WO (1) | WO1992013133A1 (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FI88812C (en) * | 1992-03-05 | 1993-07-12 | Valmet Paper Machinery Inc | Device for controlling the web's tip drawing tape in a paper machine |
| DE4322644C1 (en) * | 1993-07-07 | 1994-09-29 | Billhofer Maschf Gmbh | Device for applying dispersions containing wax |
| DE4427281A1 (en) * | 1994-08-02 | 1996-02-08 | Voith Gmbh J M | Threading up fibrous material lines esp. paper making machines |
| DE19741517A1 (en) * | 1997-09-20 | 1999-03-25 | Voith Sulzer Papiermasch Gmbh | Process for reducing the adhesion of a moist fibrous web to a rotating roller |
| FI117981B (en) * | 2002-12-20 | 2007-05-15 | Upm Kymmene Corp | Pinching arrangement for a paper machine or the like |
| FI120239B (en) | 2008-06-19 | 2009-08-14 | Metso Paper Inc | A web forming machine and a method in a web forming machine |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FI89085C (en) * | 1987-01-23 | 1993-08-10 | Valmet Paperikoneet Oy | Regulating procedure for a paper machine press section |
| FI75890C (en) * | 1987-01-23 | 1988-08-08 | Valmet Paperikoneet Oy | FOERFARANDE OCH ANORDNING I PRESSPARTIET I EN PAPPERSMASKIN FOER LOESGOERING AV BANAN FRAON YTAN AV EN PRESSVALS. |
| FI83895C (en) * | 1988-04-13 | 1991-09-10 | Valmet Paper Machinery Inc | Method and apparatus for press processing a paper web |
-
1991
- 1991-01-18 FI FI910288A patent/FI92941C/en not_active IP Right Cessation
-
1992
- 1992-01-20 EP EP92902194A patent/EP0567484B1/en not_active Expired - Lifetime
- 1992-01-20 DE DE69221123T patent/DE69221123T2/en not_active Expired - Fee Related
- 1992-01-20 CA CA002099379A patent/CA2099379C/en not_active Expired - Fee Related
- 1992-01-20 AT AT92902194T patent/ATE155837T1/en not_active IP Right Cessation
- 1992-01-20 WO PCT/FI1992/000014 patent/WO1992013133A1/en active IP Right Grant
Also Published As
| Publication number | Publication date |
|---|---|
| EP0567484B1 (en) | 1997-07-23 |
| EP0567484A1 (en) | 1993-11-03 |
| CA2099379A1 (en) | 1992-07-19 |
| FI910288A0 (en) | 1991-01-18 |
| FI92941C (en) | 1995-01-25 |
| DE69221123T2 (en) | 1997-11-20 |
| FI92941B (en) | 1994-10-14 |
| ATE155837T1 (en) | 1997-08-15 |
| WO1992013133A1 (en) | 1992-08-06 |
| DE69221123D1 (en) | 1997-08-28 |
| FI910288A (en) | 1992-07-19 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| MKLA | Lapsed |