WO2011039410A1

WO2011039410A1 - Method for transferring a fibrous web from forming section to press section and a fibrous-web machine

Info

Publication number: WO2011039410A1
Application number: PCT/FI2010/050664
Authority: WO
Inventors: Jyrki Savela
Original assignee: Metso Paper, Inc.
Priority date: 2009-09-30
Filing date: 2010-08-25
Publication date: 2011-04-07
Also published as: FI121818B; DE112010003870T5; FI20095995A; FI20095995A0

Abstract

On a fibrous-web machine there is a metal belt (10) arranged between a forming section (8) and a press section (9) supported by which the fibrous web is transferred from the forming section (8) to the press section (9). The transfer of the fibrous web comprises steps in which the fibrous web lying on a forming wire (11) is made to adhere to the surface of the metal belt (10), the fibrous web is separated from the forming wire (11) and passed supported by the metal belt (10) into contact with a press felt (12) and, finally, the fibrous web is separated from the metal belt (10) and made to follow the press felt (12). The fibrous web is made to adhere to the metal belt (10) in a pre-press nip (No) which has been provided by guiding the metal belt (10) to rotate over a wire guide roll (14) such that the forming wire (11) and the web lying on it are sandwiched between the metal belt (10) and the wire guide roll (14).

Description

Method for transferring a fibrous web from forming section to press section and a fibrous-web machine

The invention relates to a method for transferring a fibrous web from the forming section to the press section of a fibrous-web machine.

The invention also relates to a fibrous-web machine which comprises a forming section and a press section. In this context, fibrous-web machine refers to a paper, board, tissue or pulp-drying machine.

Conventionally, the fibrous web is transferred from the forming section to the press section at the so-called pick-up point by means of a transfer suction roll or a transfer suction box. The web lying on the forming wire is subjected to an underpressure through a press felt which makes the web separate from the forming wire and follow the press felt. The suction roll is an expensive component and, due to its poor efficiency, its use also becomes expensive. The suction boxes again wear the fabrics and increase the driving output of the wires and felts. Increasing the run speed from the existing is not very possible with the current technique.

Arrangements are also known of prior art in which a transfer belt not receiving water is utilised for the transfer of the web. For example, specification FI 98843 C describes a closed draw of the web from the forming section to the press section by means of a transfer belt. The web travelling on the forming wire is made to adhere on the outer surface of the transfer belt substantially not receiving water in a transfer and pre-press zone. After the pre-press zone, the web is substantially immediately separated from the wire and passed supported by the transfer belt onto the next press felt of the press section. The transfer belt is usually of synthetic material and it can be reinforced by metal, composite or fabric. It is smooth of its surface and substantially inelastic. In known arrangements, the web is made to adhere on the surface of the transfer belt in a pre-press zone formed by two rolls through which the web passes sandwiched between the wire permeable to water and the transfer belt impermeable to water. Such a pre-press nip wears the wire and shortens its lifetime. It can also easily cause marking in the web. Improving dewatering efficiency by increasing nip pressure decreases the bulk and flexural rigidity of the fibrous web. This can be avoided by using an extended nip, which is still an expensive arrangement.

Of prior art is known the use of a metal belt in the calendering of the web. For example, from specification EP 1470290 Bl is known a paper-making method in which the web is passed through a metal-belt calender comprising a metal belt arranged to travel around at least one guide element and at least one counter element located outside the belt for forming a contact surface with the belt such that the belt and the counter element form between them a calendering zone for passing the web being calendered through it. The length of the calendering zone is determined by the layout/positioning of the belt guide elements and/or the structure of the counter elements. Such a metal-belt calender can operate by pressure solely created by the tension of the belt and it can employ press elements pressing the belt in addition to the pressure created by the tension of the belt. By means of compression caused solely by the tension of the belt, it is possible to provide nip pressure of about 0.01-5 MPa. The object of the invention is to provide a reliable closed draw of a fibrous web from the forming section to the press section in a cost-effective way.

The method according to the invention is characterised by what is presented in the characterising part of claim 1. Equivalently, the fibrous-web machine according to the invention is characterised by what is presented in the characterising part of claim 6.

In the method according to the invention, the fibrous web is transferred from the forming section to the press section by means of a smooth metal belt impermeable to water. At the end of the forming section, the fibrous web lying on the forming wire is made to adhere to the surface of the metal belt in a transfer nip which is formed by a wire guide roll and the metal belt rotating around it. Nip pressure and nip length can be adjusted by changing the tension and overlap angle of the metal belt on the wire guide roll.

By means of the metal belt, it is possible to provide an extended nip by positioning the guide rolls of the metal belt such that the magnitude of the tension and the overlap angle of the metal belt on the wire guide roll can be achieved as desired. Advantageously, the metal belt is arranged above the web on the wire guide roll, whereby water is removed from the web downwards from the effect of gravity and compression applied by the metal belt on the web. The wire guide roll can be e.g. a grooved roll, a perforated roll without underpressure or a suction roll. The grooves, notches or holes in the wire guide roll receive water exiting the web in the pre-press nip.

The metal belt is substantially impermeable to water and air and smooth of its surface. Advantageously, a steel belt is used which can then be covered with a suitable coating which supplies the web with optimal adhesion. The coating is chosen such that the web can first be made to adhere durably on the metal belt and then to separate from it after the transfer to the press section. To intensify dewatering, the metal belt can be preheated e.g. by steam.

At the beginning of the press section, the fibrous web is separated from the metal belt e.g. by means of a transfer suction box which is located within a loop formed by the press felt at the point of a turning roll controlling the travel of the metal belt.

As a component, the price of a transfer suction box is a tenth of the price of an equivalent transfer suction roll. The vacuum efficiency of the suction box is considerably better than the efficiency of the suction roll, because underpressure is not applied on the web via the suction holes of the rotating roll, which would drop the efficiency to less than half, but through a static suction slot. For transferring the web, considerably lower underpressures than previously are sufficient, whereby great energy savings are possible. The transfer can be low- friction or frictionless depending on the magnitude of underpressure and flow (distance) and the number and location of the slots.

By means of the steel belt, the surface properties and dry content of paper can be improved to a higher level than currently. Dewatering is provided with more symmetry. Underpressure can be maximally utilised in dewatering without wearing the wires extensively.

Due to the invention, an effective concept is also provided in the narrow machine category. The width of the wire can vary in the range of 3.5-1 1.5 m. The arrangements are as independent of width as possible. By employing the metal belt, it is possible to provide closed draws, whereby the speed potential is sufficient for the speeds of over 2,000 m/min. The arrangement according to the invention provides the dry content of the web so high (even over 20%) that it is advantageously possible to use a one-nip press section as the press section.

The concept has a good energy balance. The structure is modular and designed in advance. The usability is developed and the maintenance requirement decreased. Fewer breaks than previously are expected. No couch pit is required. The web is stretched for the first time after the first drying group, whereby the web is durable enough to endure the measure without breaks. In the other transfer points, low draws only exist through fabric tensions because the overlap angles are large, whereby they enable run without breaks and steadying the web before the transfer point. Advantageously, web transfers utilise transfer suction boxes which have a cover adjustable during run with an elastic property.

Next, the invention will be described with reference to examples shown in the figures of the accompanying drawings to which the invention is, however, by no means intended to be narrowly defined.

Fig. 1 schematically shows the transfer of a web from the forming section to the press section by means of a metal belt. Fig. 2 shows an alternative arrangement for transferring the web.

Fig. 1 shows a part of a forming section 8 and a press section 9 of a paper machine between which is arranged a metal belt 10 via which a web is passed in a closed draw from a forming wire 1 1 to a press felt 12. On the press section 9, the web travels sandwiched between two press felts 12, 17 through a press nip Ni . In this embodiment, the press section 9 is a one-nip press section. After the press section 9, the web is passed in a closed draw supported by a drying wire 25 to a first drying cylinder 22 of the dryer section. The metal belt 10 forms an endless loop the travel of which is controlled by three guide rolls 13a, 13b and 13c. Into connection with the first guide roll 13a is arranged a hydraulic stretcher controller with which the tension of the belt 10 can be adjusted. The second guide roll 13b is located above the horizontal run of the forming wire 1 1 such that the metal belt 10 travelling from the second guide roll 13b to the third guide roll 13c rotates at a short distance around a wire guide roll 14. The third guide roll 13c operates as a turning roll which turns the travel direction of the metal belt 10 from one directed obliquely downwards to one directed obliquely upwards. It is possible to use in connection with the metal belt loop 10 a heating device 24 to heat the metal belt. The forming section 8 comprises the forming wire 1 1 supported by which the web comes to the wire guide roll 14. The metal belt 10 is arranged to travel over the wire guide roll 14 such that between the metal belt 10 and the wire guide roll 14 is formed a pre-press and transfer zone i.e. a pre-press nip No in which the metal belt 10 presses the web against the wire guide roll 14. The pre-press nip No removes water from the web in the direction of the wire guide roll 14 downwards and simultaneously the compression pressure makes the web adhere to the surface of the metal belt 10.

After the wire guide roll 14, the web separates from the forming wire 1 1 and follows the metal belt 10 adhered to its surface directed downwards. If required, the transfer of the web can be ensured by blowing which is applied between the forming wire 1 1 and the web after the pre-press nip No. The web follows the metal belt 10 until the turning roll 13c. The first press felt 12 of the press section 9 touches the turning roll 13c such that the web comes into contact with the press felt 12 in connection with the turning roll 13c. At the point of the turning roll 13c, there is within the first press felt loop 12 a transfer suction box 15 by means of which the web is transferred from the support of the metal belt 10 to the support of the press felt 12. After the turning roll 13c, the web follows the press felt 12 into the single press nip Ni of the one- nip press section 9.

The first press felt 12 of the press section 9 forms a loop the travel of which is controlled by guide rolls 23a, 23b and a first press roll 18. The second press felt 17 forms a second loop the travel of which is controlled by guide rolls 21 and a second press roll 19. The web is passed sandwiched between the press felts 12 and 17 through the press nip Nj . Advantageously, one of the press rolls, in this case the upper press roll 19, is a shoe roll, whereby the rolls 18 and 19 form an extended nip. After the press nip Ni, the web is separated from the second press felt 17 at the point of the second guide roll 21 by means of a transfer suction box 20 arranged within the press felt loop 12. The web follows the first press felt 12 onto the guide roll 23b after which the web is passed in a closed draw supported by the drying wire 25 to the first drying cylinder 22 of the dryer section.

The transfer suction boxes 15, 20 are substantially similar of their structure. Both suction boxes 15, 20 comprise one or more suction slots, advantageously two suction slots, via which underpressure is applied on the web travelling over the turning roll 13c or the felt guide roll 21 through the press felt 12. The overlap of the felt 12 at the transfer point is arranged in the direction of the turning roll 13c or the felt guide roll 21 such that the felt 12 does not rub the transfer suction box 15, 20 without underpressure. The web is separated from the metal belt 10 and transferred onto the first press felt 12 by means of a suction slot which starts at the separation point of the felt 12 and the metal belt 10. Equivalently, the web is separated from the second press felt 17 and transferred back onto the first press felt 12 by means of a suction slot which starts at the separation point of the felts 12 and 17.

It is possible to provide the pre-press nip N₀ forming between the metal belt 10 and the wire guide roll 14 solely with the tension of the metal belt 10. The nip N₀ is longer than the nip provided by means of two rolls and lower nip pressure can be applied in it, whereby it also wears the wire 1 1 less than a roll nip. In the nip No, water is removed from the wet web in the direction of the wire 1 1 downwards and simultaneously the web is made to adhere to the surface of the metal belt 10.

The metal belt 10 is advantageously a steel belt which is covered with suitable material affecting the adhesion of the web. Advantageously, the metal belt 10 is heated by the heating device 24, which improves the adhesion of the web, intensifies dewatering and pre-heats the web for the press section. The metal belt 10 can be replaced as an endless loop. Replacing the metal belt by welding is also possible but it is slower and considerably more demanding. The replacement occurs best by a run-in device and an external 'form' which is handled by an overhead crane. The steel belt is liftable to the floor with its frame as one package by the overhead crane, whereby the replacement of the belt occurs on the floor.

The wire guide roll 14 can be e.g. a grooved roll or a perforated roll without underpressure. In the most effective and expensive versions, the wire guide roll 14 is a suction roll.

An oblique drawing device and the cutting of edge trimmings 16 are located in Fig. 1 between the forming section 8 and the press section 9 at a point in which the fibrous web travels supported by the metal belt 10 adhered to its surface directed downwards. The droppings of trim squirts do not then cause web breaks which is a risk of the current concept. Advantageously, there is no couch pit between the forming section 8 and the press section 9.

Fig. 2 shows an alternative arrangement in which the same or equivalent parts are designated with the same reference numbers as in Fig. 1. Also in this case, the web is transferred at the end of the forming section 8 from the forming wire 11 to the metal belt 10 in the pre-press nip N₀ which is provided by pressing the metal belt 10 against the wire guide roll 14. Then, the web rotates along with the metal belt 10 around the turning roll 13c. The web is not transferred from the metal belt 10 onto the press felt 12 until on the run of the metal belt 10 directed upwards following the turning roll 13c. The transfer of the web is ensured by the transfer suction roll 23a located within the first press felt loop 12. The press section 9 is provided with one nip as in Fig. 1. After the press section 9, the web is transferred in a closed draw onto the fabric 25 of the dryer section. Many different variations of the invention are possible within the scope defined by the enclosed claims.

Claims

1. A method for transferring a fibrous web from a forming section (8) to a press section (9) of a fibrous-web machine, characterised by transferring the fibrous web from the forming section (8) to the press section (9) supported by a smooth metal belt (10) impermeable to water and making the fibrous web adhere to the metal belt (10) in a pre-press nip (No) which is provided by guiding the metal belt (10) to rotate around a wire guide roll (14) such that a forming wire (1 1) and the web on it are sandwiched between the metal belt (10) and the wire guide roll (14).

2. A method according to claim 1, characterised by comprising steps of

- making the fibrous web on the forming wire (1 1) adhere to the surface of the metal belt (10),

separating the fibrous web from the forming wire (11) and passing it supported by the metal belt (10) into contact with a press felt (12) and separating the fibrous web from the metal belt (10) and making it follow the press felt (12).

3. A method according to any one of preceding claims, characterised by transferring the web from the metal belt (10) onto the press felt (12) by means of a transfer suction box (15) which is arranged within the press felt loop (12) at the point of a turning roll (13c) that guides the travel of the metal belt (10).

4. A method according to any one of preceding claims, characterised by transferring the web from the metal belt (10) onto the press felt (12) by means of a transfer suction roll (23a) which is arranged within the press felt loop (12) at a point where the metal belt (10) has passed the turning roll (13c).

5. A fibrous-web machine which comprises a forming section (8) and a press section (9), characterised in that between the forming section (8) and the press section (9) is arranged a smooth metal belt (10) impermeable to water and that the web is arranged to be transferred supported by the metal belt (10) from the forming section (8) to the press section (9) and that means for making the web adhere to the metal belt (10) comprise a pre-press nip (No) which is provided by guiding the metal belt (10) to rotate around a wire guide roll (14) such that a forming wire (1 1) and the web lying on it are sandwiched between the metal belt (10) and the wire guide roll (14).

6. A fibrous-web machine according to claim 5, characterised in that into connection with the forming section (8) is arranged the pre-press nip (No) for making the web adhere on the metal belt ( 10) and that into connection with the press section (9) are arranged means (15, 23a) for separating the web from the metal belt (10).

7. A fibrous-web machine according to claim 6, characterised in that the means for separating the web from the metal belt (10) comprise a transfer suction box

(15) which is arranged within a press felt loop (12) at the point of a turning roll (13c) guiding the travel of the metal belt (10).

8. A fibrous-web machine according to claim 6, characterised in that the means for separating the web from the metal belt (10) comprise a transfer suction roll

(23a) which is arranged within the press felt loop (12) on the run of the metal belt (10) directed upwards following the turning roll (13c).