CA1280634C - Method and device in the press section of a paper machine - Google Patents

Abstract

(57) Abstract of the Disclosure:

Method and device in the press section of a paper machine, in particular in a so-called closed press section provided with a smooth-faced press roll (10), for the control of the detaching of the paper web from the said press roll (10). In the method and by means of the device, the temperature of the face (10') of the said smooth-faced press roll (10) is adjusted. By means of this adjusting, the adhesion between the roll face (10') and the paper web (W) to be detached is affected, and thereby the detaching angle (0) and/or the detaching tension (T) of the paper web is set within an optimal range. The temperature profile of the smooth-faced press roll (10) in the axial direction of the roll (10) can also be adjusted in view of controlling the detaching process.

Description

0~34 METHOD AND D~VlCE IN TH~ PR~SS SECTION OF
A PAPER MAC~IN~
The present invention concerns a method in the press section of a paper machine, in particular in a so-called closed press section provided with a smooth-faced press roll, for the control of the detaching of the paper web from the said press roll.
The invention further concerns a device in the press section of a paper machine, which said press section includes a smooth-faced press roll, preferably a central roll, from whose smooth face the web is de-tached and passed preferably as an open draw to the drying section of the paper machine.
So-called closed press sections are commonly used in a paper machine wherein one press nip is formed or generally several press nips are formed in connection with the central roll. An example of such a prior-art press section is the press section marketed by the applicant under the trade mark "Sym-Press II", where a smooth-faced central roll having a larger diameter than the diameters of the other press rolls, is usually made of rock, as a rule of granite. Being an unhomogeneous natural material of low tensile strength, granite is quite problematic in machine construction. If it is desirable to heat a granite roll, the deformations thereof which are dependent on temperature, are non-linear and difficult to predict.
As a press roll material, granite has relatively good properties of adhesion, transfer and detaching of the web, which is at least one of the reasons for its popularity. However, the detaching properties could be better, in particular with unbleached paper qualities.
In the way known in prior art, the web is detached as an open unsupported draw from the face of the central roll in the press. This open draw is quite critical in view of the operation of the paper machine. In the open ~;
,,~

1 ~06~4 draw, a difference in speed is used which extends the web, which results in certain drawbacks. Moreover, the open draw forms a problematic point susceptible to breaks in a paper machine.
The prior-art technolo~y has not provided efficient means for controlling the open draw of a web taking place from a smooth-faced central rnll. The unfavourable properties of granite have, for their part, made the control of the open draw more difficult.
The open draw of the web has become an ever more important problematic point with continuously increa-sing running speeds of paper machines. Since different paper qualities are often manufactured by means of a single paper machine, with adhesion to the surface of a rock roll being different for different paper qualities, variations in the detaching tension required for the web result.
The p~esent invention is directed towards the pro-vision of a new and improved manner of detaching a web from the central roll in a press section and transfer-ring the web to the drying section, including a new and improved regulating system in which the detaching of the paper web from the smooth face of the central roll in a press section, can be controlled better than in prior art.
The present invention is particularly directed towards the provision of a sy~tem of regulating the detaching of the web from the smooth surface of a central roll, of the type noted above, in which the tension of detaching of the web can be set optimally irrespective of the dry solids content of the paper web, of surface energy of the substance, and of running speeds of the paper machine.
In accordance with one aspect of the present invention, there is provided a method for detaching a web from a roll, comprising the steps of adjusting J~

3~

temperature of a surface of the roll, whereby adhesion between the web and the roll surface is affected, and thereby setting at least one of a detaching angle of the web off the roll and detaching tension of the web within an optimal range. The roll is preferably a smooth-surfaced press roll, more preferably a central roll in a closed press section of a paper machine.
The present invention is also directed, in another aspect, to a device for detaching the web from a roll, which comprise~ means for adjusting temperature of a surface of the roll, and thereby controlling detachment of the web off the roll surface. The temperature adjusting means may comprise at least one heating device for applying heat to the roll surface. As noted a~ove, the roll may preferably be a smooth-surfaced press roll, more preferably a central roll in a press section of a paper machine~ The web is preferably passed from the central roll as an open draw to a drying section of the paper machine.
The method of the present invention is principally characterized by the temperature of the face or surface of a smooth surfaced press roll being adjusted, and adhesion between the roll face or surface and the paper web to be detached being influenced or affected by way of this adjusting. Thereby, the detaching angle and/or detaching tension of the paper web, are/is set within an optimal range.
Furthermore, a device in accordance with the pres-ent invention is principally characterized by heating devices being provided in connection with the smooth-surfaced press roll, by means of which temperature of the smooth face of the press roll, and thereby detaching of the web from the roll, are affected or influenced.
The present invention is based on the concept that the temperature at an interface between a paper web and a smooth roll face from which the web is to be detached 0~3~

affects the dry solids content of the web, the surface energies of the materials in contact with one another, and the viscosity of water. These parameters, in turn, affect or influence the adhesion between the paper web with the water contained therein and the smooth roll face. By establishing the interdependences of the said para-meters, by controlling the same, and by, on the basis of this information, adjusting the temperature of the roll face by means of a regulating system according to the present invention, it now is possible to set the detaching tension of the paper web at a suitable level, even within highly varying operating conditions. Thus, with the invention, it is possible to adjust the temperature of the smooth face of the roll to a certain set value, which provides an optimal detaching of the web and running quality when running different paper qualities and with different running speeds of the paper machine, with the web quality and machine speed that are used at each particular time.
In the present invention, the central roll of the press or any other corresponding smooth-faced roll from which the paper web is intended to be detached, may be a substantially metal-mantle roll coated with a metal, with a ceramic material, or with mixtures of these.
This roll may be a cast-iron roll or an uncoated metal roll. In palrticular, the roll is arranged to be heated by means of adjustable heating devices. Such heating may take place from inside and/or outside the roll at lea~t partly by means of techniques known in prior art.
The invention is by no means restricted for use for detaching a web from a central roll of a closed press section of a paper machine alone. Rather, the invention is well-suited and intended for controlling the detaching of a web from a smooth-faced roll in a press in general, i.e. also from a roll other than a central roll.

.

~0~34 ~ 5 The regulating system of the invention may be provided with a feedback, wherein the conduct of the web in the detaching draw is monitored either visually, by means of optical detectors, or by means of detectors that sense the location. In this feedback, it is pos-sible to use, as the measurement signal or adjustment signal, the difference in speed of the web between the drying group and the press, or a separate measurement roll by means of which the web ten~ion can be measured.
In a preferred embodiment of the invention, the temperature profile of the smooth face of the central roll or equivalent is provided to be adjustable in the axial direction of the roll. By means of this proce-dure, it i8 possible optimally to set the distribution of detaching tension in a transverse direction of the web and to prevent a curve formation in the detaching line in the lateral areas of the web, and thereby prevent breaks in the web, which usually start in these lateral areas of the web.
The advantages of the present invention are mani-fested with particular emphasis on thin paper qualities, with which it is possible to reduce the number of web breaks taking place in the said open draw to a substantial extent by way of the invention herein.
In some cases, owing to the invention, it is possible to shorten the open draw, or even to introduce a practically closed draw from the press section to the drying section.
The temperature of the roll face is preferably adjusted within the range of about 30D to 1507C, more preferably within the range of about 50 to 150 D C ~
In the following, the physical background of the invention and ~ome of its exemplifying embodiments will be described in detail with reference to the illustra-tions in the Figures in the attached drawings.

.d~ ' ~ , ~t~ 34 5a Figure 1 is a schematical view of a prior-art closed press section provided with devices making use of the method of the invention.
Figure 2 shows the rear end of the press section and the geometry of the open draw of the web as well as various parameters of same.
Figure 3 illustrates the dependence of the viscosity and surface tension of water on temperature.
Figure 4 i~ a graphic presentation of the te~t results with the invention, i.e. the detaching angle of the web from a smooth-faced roll as a function of the web temperature with newsprint.
Figure 5 is a schematical illustration of the principle of the induction heating apparatus suitable for an application of the present invention, seen in the machine direction.
Figure 6 shows an exemplifying embodiment of an induction heating apparatus in accordance with the invention as a block diagram.
Fig. 1 is a schematical side view of the appli-cant's "Sym-Press II" (TM) press section wherein a control system in accordance with the invention is applied. To begin with, as a background of the invention, the prior-art overall construction of the press section shown in Fig. 1 will be described. The paper web W i5 drained on the forming wire 50 of the paper machine, from which said wire the web W is detached on the downwardly inclined run of the wire 50 between the wire suction roll 51 and the wire drive roll 52 at the ~L~8~634 detaching point P and transferred within the suctio~
zone 53a of the pick-up roll 53 onto the pick~up felt 55, on whose lower face the web W is transferred into the first dewatering press nip N1.
The first nip N1 is formed between a press-suction roll 54 and a hollow-faced 57 lower press roll 56. Two felts run through the nip N1, viz. the lower felt 60 guided by guide rolls 58 and 59, and the pick-up felt 55, which acts as the upper felt in the first nip N1. After the first nip N1, the web W follows along with the upper roll 54 by the effect of the suction zone 54a of the press-suction roll 54, moving into the second dewatering press nip N2, which is formed between the said press-suction roll 54 and the smooth-faced 10' central roll 10. The diameter D1 of the central roll 10 is substantially larger than the diameters of the other press rolls 54,56,61. This is why there is space for various apparatuses to be fitted around the central roll 10, including the heating apparatuses 20,80,100 applied in the invention. Within the suction sector 54a of the suction roll 54 there is a steam box 81, which acts upon the outer face of the web W and raises the temper-ature of the web W and of the water contained therein, thereby lowexing the viscosity of the water.
Substantlally at the opposite side of the central roll 10, relative the second nip N2, there is a third dewatering press nip N3, through which the press felt 65 runs, guided by the guide rolls 63 and 64. The rolls of the nip M3 consist of the central roll 10 and the hollow-faced 62 press roll 61.
The adhesion properties of the smooth face 10' of the central roll 10 are such that, after the second nip N2, the web follows along with the face 10' of the central roll 10. On the lower free sector of the central roll 10, there is a doctor 69, which keeps the roll face 10' clean and detaches the paper web supposed to become broke from the roll face 10'. From the face 10' of the 63~L

central roll 10, the web is detached at the detaching point R as an open draw W0 and transferred onto the drying wire 70, whose loop has been brought to a distance as short as possible from the roll 10 face 10', being guided by the guide roll 66. After the guide roll 66, there are suction boxes 67 inside the loop of the drying wire 70, which said suction boxes ensure that the web W
adheres to the drying wire 70 and passes reliably to the drying section, whose first drying cylinder or a corres-ponding lead-in cylinder is denoted with the reference numeral 68.
In the following, with reference to Fig. 2, the detaching of the web W from the smooth face 10' of the central roll 10 and its transfer as an open draw W1 or W2 onto the drying wire 70 will be described. In Fig.

2, the detaching angle of the draw W1 is denoted with ~1 and the corresponding detaching point with R1. The detaching angle of the second draw W2 is denoted with ~2 and the detaching point with R2. The detaching orders of the open draws W1 and W2 are denoted with T1 and T2.
Generally speaking, it can be ascertained that the smaller the detaching angle ~, the higher is the detaching tension T that is required. On the other hand, the detaching tension T is determined by the difference between the speed v2 of the drying wire 70 and the speed v1 of the face ~0' of the central roll 10, i.e. by the so-called difference in draw Qv = v2 - v1 (v1 = web speed in the press section before detaching, v2 = web speed at the beginning of the drying section). As a rule, ~v/v1 is within the range of ~v/v~ ...3 %.
The web tension, i.e. the detaching tension T can be calculated as follows:
T = E S + ( 1 + ~ ) mv2 ( 1 ) 1 - cos~
WS = 8-42 [~vt(~v)]0 9s (2) ~063~

wherein T = web tension ~ = detaching angle WE = web elongation work WS = detaching work ~ = elongation m = mass v = speed ~ = viscosity ~ = surface energy From the above formulae (1) and (2), the followin~ circumstances essential in view of the inven-tion come out. When the temperature at the contact point between the web W and the roll face 10' rises, the de-taching tension T becomes lower, because the viscosity is reduced, and the surface energy ~ is also reduced, which, for its part, results therein that the detaching work Ws (formula (2)) is reduced and the dry solids con-tent is increased (owing to the last-mentioned fact, the term mv2 becomes lower).
Fig. 3 shows the dependence of the viscosity and surface tension of water on temperature. As is seen, the surface tension is lowered in a substantially linear way as the temperature is raised, whereas the viscosity is lowered within the temperature range of 0C to 80C
very steeply, and thereinafter substantially in the same proportion as the surface tension is lowered with a rising temperature.
From Fig. 3, and so also from the above equations (1) and (2), it can be concluded that with a rising temperature of the roll face 10' the web tension T required to detach the web W from the roll face 10' is lowered, i.e. the web W is detached from the roll face 10' more readily at higher temperatures.
As was stated above, a reduced web tension T results in an increased detaching angle 9.

1~30634 In the invention, the central roll 10 that is used is, as a rule, a roll with a metallic mantle, pre-ferably a roll of a ferromagnetic material, i.e. of a ro~
material that is preferable to rock material both con-structively and in view of the operation.
In the present invention, active use has been made of the, usually inverse, interdependence between the web tension T and the roll-face 10' temperature, which was described above. For this purpose, as is shown in Fig. 1, a steam box 80 is fitted in connection with the face 10' of the central roll 10 between the nips N2 and N3, the temperature of the web W and the temperature and viscosity of the water contained in the web as well as, indirectly, also the temperature of the face 10' of the roll 10 being affected by means of steam Sin passed into the said steam box 80.
As is shown in Fig. 1, before the detaching point R, inductive heating apparatuses 20 are placed substantially in the horizontal plane placed through the centre of rotation of the central roll 10, which said heating apparatuses act, free of contact, through an air gap V, substantially upon the temperature of the thin surface layer of the web face 10'.
As is shown in Fig. 1, a heating medium Fin is fed into the roll 10 through a plpe 91 and a connection 90, the $aid medium being taken out of the roll ~FoUt) through the same connection 90,or through another con-nection (not shown) placed in connection with the oppo-site roll shaft, and a pipe 92. The apparatuses for the circulation and heating of the heating medium are de-noted schematically with the block 100 in Fig. 1.
Even though, in Fig. 1, three different sets of equipment 20;80 and 100 are shown for the heating of the face 10' of the central roll 10 and of the web W in view of controlling the draw tension T,T1,T2 of the open draw WO,W1,W2, i.e. the steam box 80, the inductive heating apparatuses 20, and the devices 90,91,92 and 100 ~' 10 for the heating and circulation of the heating medium, in practice, as a rule, all of these means do not have to be used at the same time in one application.
As came out preliminarily above, when the method of the invention is being applied, a granite roll or any other rock roll is not used as the central roll in the press or as any other, corresponding smooth-faced roll, but a metal-mantle roll coated with a metal, a ceramic substance, or with a mixture of same, a cast-iron roll, or an uncoated metal roll, such a roll being construc-tively preferable to a rock roll of natural material.
The face 10' of a metal roll 10 or equivalent can be heated to a temperature optimal in view of the invention without uncontrolled phenomena of alteration.
In connection with the invention, as the central roll, it is favourably possible to use such a synthetic press roll as is described in the applicant's British Patent No. 2,180,624, according to which patent the basic level of the surface energy of the roll can be chosen appropriate from the point of view of the invention, in consideration of the adhesion between the web W and the roll face 10' and of the detaching process.
Instead of, or in addition to, the steam box 80 described above, it is possible to use a radiation heater, e.g. an infrared heater, whose construction is in itself known and an exemplifying embodiment of whose construction comes out, e.g., from the applicant's U.S.
Patent No. 4,854,052, true enough, applied in connection with an airborne web dryer.
In addition to the heating taking place by means of a heating medium circulating (Fin-FoUt) in the roll 10, shown in Fig. 1, it is possible to use electric heating apparatuses, such as resistance heating or inductive heating. An example of heating apparatuses fitted lOa inside the xoll and suitable for use in connection with the present invention is described in th~ applicant's ,, . ~

Finnish Patent No. 69,151, whereat the distribution of the temperature in the axial direction of the roll 10 can also be controlled by means of the apparatuses described in the said patent.
As condensed it can be ascertained that for the heating of the central roll before the last nip N3 it is possible to use either a steam box 80, infrared heating and/or inductive heating. For internal heating of the central roll 10, it is possible to use either a circulating medium, such as steam or water, and/or electric heating, such as inductive heating or resistive heating. Within the area of the detaching point R of the web W, as adjustable heating apparatuses of the roll 10, it is possible to use either infrared heating and/or an inductive heating apparatus 20.
Fig. 4 shows the effect of the steam box 80 on the detaching process. In the graphic presentation in Fig. 4, the vertical axis represents the detaching angle 0 of the open draw as degrees, and the horizontal axis represents the web temperature. The curve in Fig.
4 was obtained with the applicant's test paper machine and with newsprint when the web speed was 20 m/s. The measurement points in Fig. ~ were obtained with the web W speed of 20 m/s so that, e.g., the-difference in speed ~v was kept constant and only the web temperature was varied. From Fig. 1 it can be seen that the detaching angle 0 becomes larger when the web W temperature rises.
In the following, with reference to Figures 5 and 6, the inductive heating apparatuses 20 will be described, which are, according to the present estimate, the most advantageous embodiments of the invention both in view of efficiency and in view of a possibility to adjust the transverse profile oE the control and heating effect.
Th~ smooth-faced 10' press roll 10 shown in Fig. 5 is the roll from which the web W is detached. The roll 10 has a smooth and hard face 10', and it has a cylindrical mantle, which is made of a suitable ferro~
magnetic material, which has been chosen in considera-tion of the strength properties of the roll and of the inductive and electromagnetic heating in accordance with the invention. The roll 10 is mounted as revolving around its central axis K K by the intermediate of its ends 11 and axle journals 12. On the axle journals 12 there are bearings, which are fitted in bearing housings.
The bearing housings are attached to the supporting frame of the roll, which is placed on a base.
In the interior of the roll 10, it is possible to fit crown-variation or crown-adjustment devices in themselves known, for which there is plenty of room owing to the invention, because, in the interior of the roll 10, it is not necessary to use heating apparatuses operating with a liquid medium or other, corresponding heating apparatuses, which said heating apparatuses are, however, not excluded as apparatuses that may be used in connection with the present invention.
The roll 10 is arranged as inductively and electromagnetically heatable by means of eddy currents so that the temperature of the face 10' of the roll 10 is raised by means of this heating to a considerably high level, as a rule to about 70C to 100C. In view of accomplishing the inductive heating, at the proximity of the roll l0, in the same horizontal line with each other in the axial direction of the roll, component cores 201,202...20N of an iron core are arranged. These component cores 20n form a magnetic-shoe apparatus 20, which further includes an excitation winding 30, or a winding of its own for each component core (not shown).
The inductive heating is carried out free of contact so that a small air gap V remains between the iron core and the roll 10 face 10', the magnetic fluxes of the iron core being closed through the said air gap V via the roll 10 mantle, thereby causing a heating effect in it.

lZ~0634 According to Figures S and 6, all the component cores 201...20N (N = 16) have a common excitation winding 30, wherein there are two turns in Fig. 5, and only one turn in Fig. 6.
Each component core 20n is arranged separately displaceable in the radial plane of the roll 10 so as to adjust the magnitude of the active air gap V and, at the same time, the heating capacity. For this purpose, each component core is attached to the frame by means of an articulated joint. The displacing of the component cores 20n can be arranged by means of various mechanisms.
As a rule, the said air gaps may vary, e.g., within the range of about 1 to 100 mm. In respect of the mechanical devices for the adjustment of the air gaps, whose con-struction is not described in this connection, referencei8 made to the applicant's said Finnish Patent Appli-cation No. 833589.
In respect of the electrotechnical background of the in~ention, the following is ascertained. When a variable magnetic field is provided in a material that conducts electricity, as is well known, in the material eddy-current and hysteresis losses are produced and the material is heated. The power (P) of the eddy currents depends on the intensity (B) of the magnetic field and on the frequency (f) of variation of the magnetic field as follows:
P ~ B2 f2 (3) The variable magnetic field produced on the roll 30 is clo~ed ~etween the front face of the iron core and the air gaps V through the mantle of the roll 10. This magnetic field induces edddy currents in the surface layer of the roll mantle 10, which said eddy currents ge~erate heat owing to the high resistance in the roll mantle 10. The distribution of the eddy currents induced in the mantle 10 in the direction x of the radius of the roll follows the law Ix = IOe / (4) Ix is the current ~ensity at the depth x counted from the mantle face 10', Io is current density on the face 10' of the mantle 10, and ~ is depth of penetration. The depth of penetration has been defined as the depth at which the current density has been lowered to 1/e of the current density Io. For the depth of penetration, the following expression is obtained:
\

I f~ Qs (5) wherein p = specific resistance of the material, f = frequency of the magnetizing current, and ~ = relative permeability of the material.
The expression shows that with a higher frequency the depth of penetration is reduced. When steel is heated, both the electric conductivity and the permeability are reduced with a rising temperature.
In the invention, as a rule, heating capacities are used which are of the order of 1 to 30 kW/m. As is well known, the smaller the air gap V, the larger is the proportion of the electric power that is passed to the apparatus through the winding 30 which is transferred into the roll mantle 10 to be heated.
In accordance with Fig. ~, the electric power feeding the induction coil 30 is taken from a 50 Hz three-phase network (3 x 380 V). By means of a rectifier 33, the AC current is converted to DC current, which is, by means of an inverter in itself known, based on power electronics,converted to either constant-frequency or variable-frequency (f5~ AC current. The adjustment of the positions of the component cores 201...20N in the ~L~,8(~6~4 iron core 20 can be carried out, e.g., by means of the automatic closed regulating system shown in Fig. 6. The adjusting motors are stepping mctors 29, which receive their control signals S1 N from the regulating system 42.
The regulating system is controlled by a detector device 41, which is, e.g., an apparatus for the measurement of temperature, by means of which the factual values of the surface temperatures To1...Tok of the roll are measured at several different points in the axial direction K-K
of the roll 10. If the regulating system 42 includes a set-value unit, by its means it is possible to set the temperature profile in the axial direction K-K of the roll 10 so that an optimal detaching of the web W is obtained.
The output of the inverter 34 is fed through a matching transformer 35 into a LC resonance circuit.
In a way in itself known, the transformer 35 has a primary circuit 35a, an iron core 35b, and a secondary circuit 35c. The secondary circuit has n pcs. of taps 451~.~45n~ which can be connected via a change-over switch 36 to the resonance circuit 37, by means of which the power is fed into the induction coil 30. As is well known, the resonance frequency of a RLC circuit con-nected in series can be calculated from the formula:

f 1 (6) r 2~
In resonance, the current Ir = R~ wherein R is the resistance of the circuit 37.
The efficiency of the transfer of heating capacity is at the optimum when the operation takes place at the resonance frequency fr. It has, however, been noticed that, out of several reasons, it is not optimal to operate at the resonance frequency fr and/or simultaneously at both sides of it, but the frequency of operation is chosen within the areas fa1 to fy1 above the resonance frequency fr or correspondingly 1.~?.80634 within the area fa2 to fy2 below the resonance frequency fr Within the scope of the invention, the said frequency ranges are preferably chosen as follows:
fal'''fyi = (101...1.15) x fr or fa2...fy2 =
(0.85.. 0.99) x fr.
According to Fig. 6, in the RLC circuit a series capacitor Cs is used. The circuit 37 is tuned with basic tuning so that the transmission ratio of the transformer 35 is chosen by means of the switch 36 so that the resonance frequency fr calculated from the formula (6) becomes positioned correctly in accordance with the principles given above.
Fig. 6 shows a parallel capacitor Cr by means of broken lines, which said parallel capacitor can be used in stead of, or along with, a series capacitor Cs.
As is well known, the resonance frequency fr in a parallel resonance circuit, whose induction coil (L) has a resistance R, is calculated as follows:

f = / 1 - (7) 2~ L
The above equation (7) includes a factor dependent on the resistance R.
As a rule, however, a series resonance circuit is preferable, in particular in view of adjustment and control.
Within the scope of the invention, the reso-nace frequency is, as a rule, chosen within the range of f = 2...35 kHz.
r Depending on the dimensioning of the coil cores 20 and on the air gap V between the roll 10 and the cores 20n, the inductance of the resonance c1rcuit is, e.g. with a roll 10 of a length of 8 m, of an order of 10... 250 ~H. For example, if L = 60 ~H and fr =
20 kHz, the value of the capacitance of the capacitor becomes Cs = 1.06 ~F.

~8~i34 In order to keep the efficiency of the power supply high and to eliminate any phenomena of instability, i.e. "risk of runaway", the operating frequency fs is arranged automatically adjusted in accordance with the impedance of the resonance circuit 37 so that the operating frequency fs remains near the resonance fre-quency fr but, yet, at a safe distance from it, in view of the risk of runaway, i.e. within the areas fy1...f or fy2...fa2 shown in Fig- 6-The measurement of the impendance of the resonance circuit 37 may be based, e.g., on measurement of the current I passing in the circuit. This mode of measurement i5 illustrated in Fig. 6 by the block 46, from which the control signal b is controlled to the regulating unit 47, which alters the frequency fs ofthe frequency converter 34 on the basis of the control signal b. A further mode of measurement of the said impedance, which may be an alternative mode or which may be used in addition to the current measurement, is to pass a control signal c from the block 42, from which information can be obtained on the positions of the com-ponent cores 20n, i.e. on the air gaps V, which sub-stantially determine the said impedance by acting upon the inductance L. An alternative mode of adjustment is to pass feedback signals from the stepping motors 2 into the block 47 and further so as to act upon the output frequency fs of the frequency converter 34.
The mode of adjustment based on change in frequency, described above, can be used either alone in adjusting the temperature profile of the roll 10, or in addition to and together with air-gap adjustment to improve the accuracy and/or rapidity of adjustment.
In some cases, by using the above mode of adjustment based on change in frequency, it is possible to omit mechànical regulating devices acting upon the air gap V completely. In this way it is possible to increase the rapidity of the regulating system and, in some cases, to improve the accuracy of adjustment, even though, in such a case, it may be necessary to be content with a somewhat lower efficiency of the power supply.
In the following, the patent claims will be given, whereat the various details of the invention may show variation within the scope of the inventive idea defined in the said claims and differ from the details given above for the sake of example only.

Claims (23)

1. Method for detaching a moist paper web from a roll, comprising the steps of detaching the web from said roll, adjusting temperature of a surface of said roll, whereby adhesion between said web and said roll surface is affected, and setting at least one of a detaching angle of said web off said roll and detaching tension of said web as a result of said temperature adjustment, comprising the additional steps of pre-selecting at least one of said detaching tension and detaching angle, then determining at least one of surface tension and viscosity of water in the web, then selecting desired temperature of said roll surface based upon at least one of said determined viscosity and surface tension; and then carrying out said temperature adjustment step, whereby at least one of said detaching tension and angle are set.

2. The method of claim 1, wherein said roll is a smooth-surfaced press roll in a press section in a paper machine.

3. The method of claim 2, wherein said smooth-surfaced press roll is a central roll about which a plurality of press nips are formed in the press section of the paper machine which is closed by the web always being supported on at least one of a roll and supporting fabric through said press section.

4. The method of claim 1, comprising the additional step of adjusting temperature profile along the surface of said roll over an axial direction thereof.

5. The method of claim 1, wherein said temperature of said roll surface is adjusted by inductively heating said roll surface from outside thereof without contact.

6. The method of claim 1, wherein said roll surface temperature is adjusted within the range of about 30°C
to 150°C.

7. The method of claim 6, wherein said roll surface temperature is adjusted within the range of about 50°C
to 100°C.

8. The method of claim 1, wherein said temperature is adjusted by circulating a heating medium inside a mantle of said roll.

9. The method of claim 8, wherein said heating medium is water or steam.

10. The method of claim 1, wherein said temperature is adjusted by applying heat from outside said roll and from a steam box or an infrared radiation heater.

11. The method of claim 3, comprising the additional steps of passing said web through a first double-felt nip;
then passing said web through a second nip formed between said central roll and another press roll, prior to detaching said web from said central roll.

12. The method of claim 11, wherein said another press roll is a press-suction roll.

13. The method of claim 11, comprising the additional step of passing said web through a third press nip formed with said central roll, prior to detaching said web from said central roll.

14. The method of claim 1, wherein the web is detached as an open draw off said roll surface.

15. The method of claim 1, wherein the temperature of said roll surface is adjusted by at least one of the steps of (a) applying heat from outside said roll and from a steam box or an infrared radiation heater;
(b) circulating a heating medium inside a mantle of said roll; and (c) inductively heating said roll surface from outside thereof without contact.

16. The method of claim 15, wherein two of said three steps (a), (b) and (c) are applied.

17. The method of claim 16, wherein all three steps (a), (b) and (c) are applied.

18. The method of claim 14, wherein the web is unsupported by another fabric on a run of said roll between a final nip about said roll in a direction of web travel and a detaching point of the web off said roll surface.

19. The method of claim 18, wherein said run of said web about said roll surface between said final nip and said detaching point, is at least about 45°.

20. The method of claim 1, wherein at least one of said surface tension and viscosity is calculated according to the following two formulas:
DETACHING TENSION =

21. In a paper machine including a roll over a surface of which a moist paper web travels through at least one press nip formed in conjunction with said roll, a method for detaching the moist paper web from said roll comprising the steps of detaching the web from said roll at a detaching point after a final one of said at least one nip in a direction of web travel,

22 adjusting temperature of a surface of said roll, whereby adhesion between said web and said roll surface is affected, and setting at least one of a detaching angle of said web off said roll and detaching tension of said web as a result of said temperature adjustment, wherein said temperature of said roll surface is adjusted by inductively heating said roll surface from outside thereof without contact, and wherein said inductive heating is applied to said surface of said roll and web between said final press nip and said detaching point of the web off said roll surface.
22. In a paper machine including a roll over a surface of which a moist paper web travels through at least one press nip formed in conjunction with said roll, a method for detaching the moist paper web from said roll, comprising the steps of detaching the web from said roll at a detaching point after a final one of said at least one nip in a direction of web travel, adjusting temperature of a surface of said roll, whereby adhesion between said web and said roll surface is affected, and setting at least one of a detaching angle of said web off said roll and detaching tension of said web as a result of said temperature adjustment, wherein said roll is a smooth-surfaced press roll in a press section in the paper machine, wherein said smooth-surfaced press roll is a central roll about which a plurality of press nips are formed in the press section of the paper machine which is closed by the web always being supported on at least one of a roll and supporting fabric through said press section, and comprising the additional steps of

23 passing said web through a first double-felt nip, then passing said web through a second nip formed between said central roll and another press roll, and passing said web through a third press nip formed with said central roll and which is said final nip prior to said detaching point of said web from said central roll, and wherein said temperature of said roll surface is adjusted by inductively heating said roll surface from outside thereof without contact, after said third and final press nip and prior to said detaching point of the web from said central roll.
23. Method for detaching a moist paper web from a roll, comprising the steps of detaching the web from said roll, adjusting temperature of a surface of said roll, whereby adhesion between said web and said roll surface is affected; and setting at least one of a detaching angle of said web off said roll and detaching tension of said web as a result of said temperature adjustment, wherein at least one of said detaching angle and detaching tension is set according to the following two formulas:
DETACHING TENSION =
wherein at least one of said viscosity, surface tension, and web detaching work is dependent on the temperature of said roll surface.