Paper machine having a long transport wire for making tissue paper

Abstract

A paper machine for making tissue paper includes a combination of the following: a headbox with outlet slit; a forming section having a curved initial part and a forming wire, onto which pulp is supplied from the headbox; a return roll for the forming wire, with dewatering action due to centrifugal force; a principal transport wire carrying the web from said return roll through the drying section of the unit and substantially up to a winding apparatus; a transfer roll and a second support and transport wire for particularly specified action in connection with introduction of the web into the drying section; a perfusion-impingement drier combination with a cylinder having a permeable mantle and the web being conducted over this cylinder with the principal transport wire interposed; and a pick-off roll for detaching the web from the cylinder of the drier and conducting it to the winding apparatus.

Description

The present invention concerns a paper making machine intended for the manufacturing of tissue paper.

According to prior art, tissue paper is commonly manufactured by forming a web on a comparatively short wire section resembling a normal plane wire and wherein a headbox supplies the pulp slurry onto a breast roll, which is frequently open or provided with internal vacuum. Thereupon the web passes on support of the wire past conventional dewatering means, such as table rolls, deflectors, foils, suction boxes and a suction roll, each of which removes water from the pulp web, and which means have been omitted in some instances in the newest designs. At the ultimate end of the wire section the partly dried web is transferred to a so-called pick-up felt, on the support of which it goes to continued drying in the press and drying sections.

The plane wire mentioned above has been omitted in some of the designs of prior art in order to simplify the machine and to reduce the space requirements. The web-forming is then accomplished on a suction breast roll completely covered with wire, from which the web is directly transferred to the pick-up felt.

The press section is composed either of conventional presses with two press rolls and a press felt and/or one of the press rolls used is a large diameter, so-called Yankee cylinder constructed for evaporation drying.

In tissue paper machines of prior art, the drying section consists of the Yankee cylinder either alone or in combination with a subsequent, so-called after-drier. In both cases the drying of the web is by effect of evaporation when the water-containing web is brought into contact with the surfaces of the Yankee cylinder and after-drier, if any, which are heated by internal pressurized steam. From the smooth surface of the Yankee cylinder, the web adherent to it is detached at the end of the drying zone by means of a doctor, which causes in the web, owing to crowding, a creping effect. The creping increases the dilatability and softness of the paper, both desired properties in tissue paper. On being creped in this manner the paper goes to the after-drier and possibly to a marking process, prior to winding on a roll. The after-drier consists of a number of normal, steam-heated drying cylinders of a paper machine. Especially in the case of the thinnest tissue paper brands, the after-drying may be omitted.

The aim of the present invention is to create a total concept of a tissue paper machine where the use of the expensive, so-called Yankee cylinder can be omitted and which has the fundamental principal of achieving the softness and elasticity required of a tissue paper, by maintaining the web in its original soft condition after the web has been formed, and by avoiding powerful pressing of the web between the rolls of the press section and between the Yankee cylinder and the pressure rolls urged against this cylinder.

The above-mentioned objects, and other objects of the invention, are attained by means of a combination characterized in that this combination comprises:

a. a headbox with outlet slit,

b. a forming section, having a curved initial part, constituted either by a suction breast roll or by a forming shoe, and furthermore a forming wire, onto which the web is supplied from the outlet slit of the headbox,

c. a return roll, over which the forming wire passes, and which return roll produces a centrifugal force dewatering the web,

d. a principal transport wire, which passes over its rolls in a manner interspacing at the return roll the web between the forming wire and the principal transport wire, and in transport by which same principal transport wire the web is carried through the drying section substantially up to the winding apparatus,

e. a transfer device and a second supporting and transport wire passing through same, and which have been so disposed that in connection with the transfer roll the web runs between the principal transport wire and the second supporting and transport wire,

f. a perfusion-impingement drier combination, onto the cylinder of which, having a permeable mantle, the web is brought by the aid of the transfer roll so that the principal transport wire comes to lie between the web and the mantle of the drier cylinder, and

g. a detaching means, by the aid of which the web is detached from the cylinder of the drier and conducted to the winding apparatus.

In the invention the following advantages are relaized: the wire section can be made short, which is due to the efficient initial dewatering achievable by means of the curved forming zone and of the curved upper lip confining the forming zone and on the other hand by means of the efficient dewatering action of the return roll producing a centrifugal force.

Subsequent to the web forming process the drying of the web has to take place with very great caution, and for this purpose the centrifugal force and so-called perfusion and impingement drying are efficiently employed in the invention.

It is also essential in the invention that the web is transferred from the forming section to the transport wire with great efficiency by the aid of centrifugal force and differential pressure. In several designs in present use the web is transferred from the forming wire to the press felt by the aid of a special suction roll or suction box, but this is in frequent instances a factor limiting the speed.

Furthermore, in the design according to the invention the creping of the web is completely avoided, because the web is not compacted by pressing at earlier stages of the process. In the creping process the fibre structure of the web is crowded by means of a particular creping doctor. As a result, part of the fibres are broken and become detached from the paper web, causing a powerful formation of dust. The use of pulp components with short fibres, in particular, is difficult in the creping method on account of heavy dust formation. In the design according to the invention, these drawbacks have been eliminated and therefore it offers possibilities for manufacturing various kinds of tissue paper from pulp types which are less expensive and have a higher yield (groundwood, semichemical pulps, etc.).

Depending on the output of the machine, it is naturally also possible to use two or several perfusion drying cylinders.

The ultimate base weight of the web is already determined in the wire section, because no creping, which affects the base weight, is required at later stages.

The transfer of the web from the wire section up to the vicinity of the winding apparatus is as uninterrupted as possible on the support of the principal transport wire.

The principal transport wire produces in the water-containing web a marking consistent with its texture, without any presses, whereby the softness of the web is preserved because the web is not pressed with great force between rolls at any stage.

The lack of presses is made up for, with regard to dewatering, by the perfusion-impingement drying combination, which at the same time replaces the expensive Yankee cylinder in evaporation drying with a less expensive and substantially more efficient arrangement, which is highly appropriate for tissue paper, considering the high porosity of this type of paper.

The replacement of creping by marking produced by the principal transport wire implies certain advantages in economy of the manufacturing process and it reduces the generation of dust from the completed product, which is the result when a creping doctor and Yankee cylinder are used; at the same time the soft structure of the web can be preserved up to the end.

The invention is described in detail with reference to the schematic figures of the attached drawing.

FIG. 1 presents a tissue paper machine according to the invention, in elevational view.

FIG. 1a shows a detail of FIG. 1, at the slit aperture of the headbox.

FIG. 2 presents an alternative design for the webforming section, where a particular forming shoe has been used.

FIG. 3 shows an alternative course of the transport and support wire.

Referring now to FIG. 1, the headbox supplies the pulp slurry onto the permeable cylinder 3 covered by the forming wire 2 (depicted with interrupted lines), within which cylinder there are vacuum chambers 4 for boosting the dewatering of the web 5 (depicted with dot-and-dash lines). The vacuum chambers 4 may be two or several; in FIG. 1a two chambers 4a and 4b have been shown. A design example of the cylinder 3 is seen in FIG. 1a, according to which the cylinder has a foraminous mantle 47 and upon this a honeycomb or ribbed structure 48, by the effect of which the water removed from the web is uniformly distributed. From the cylinder 3 the web passes into the wedge-shaped throat 10 constituted by the forming wire 2 and the principal transport wire 6 (depicted with a solid line). Interposed between these two wires 2 and 6, and supported by them, the web 5 arrives at the return roll 7, on the curved surface of which the web 5 is subjected to the effect of a powerful centrifugal force, and this in its turn efficiently dewaters the web 5 through the wire 6. If required, the roll 7 is provided with a permeable mantle and it may in its interior have an overpressure chamber 8 for boosting the dewatering in the contact zone of the roll and wire. By means of the return roll 7, the direction of travel of the web 5 is changed most appropriately by 120.degree. to 180.degree., while the web is interspaced between the wires 2 and 6. The forming section is furthermore provided with save-alls 9 and 11, in which the water flung off by the rolls 3 and 7 is collected. The save-all 11 is sealed so that, if required, a vacuum may be produced within it for boosted dewatering action, by connecting it with a suitable vacuum source. For the guidance and tensioning of the forming wire 2, a guiding and tension roll 12 has been provided on its return run. At the point where the wires 2 and 6 separate, a blow pipe 13 has been placed, which makes sure that the web follows along with the principal transport wire 6.

The principal transport wire 6 carries the web 5 formed in the forming section over the planar suction boxes 14, which continue the dewatering process, into the wedgeshaped throat 44 constituted by the second transport and support wire 15 and the wire 6, which throat terminates at the press defined by the rolls 16 and 17 and the vacuum chamber 18. The purpose of this press is to ensure an appropriately efficient contact of the second transport and support wire 15 with the upper surface of the web 5. According to FIG. 1, the second and support wire 15 runs in support of its particular roll 43 outside the drier space 23a. From the press, the web 5 continues between the wires 6 and 15 and supported by them, to the transfer roll 19, the purpose of which is to transfer the web 5 together with the wires 6 and 15 into the drying section, which operates according to the perfusion-impingement principle. The transfer roll 19 has a permeable mantle and an interior overpressure chamber 20, which partly assists the drying of the web 5 and, particularly, forces the web 5 to follow along with the wire 6 after the separation of wires 6 and 15.

The drying section operating according to the principle of a perfusion-impingement drier combination consists of a cylinder 21 with permeable mantle and an enclosed hood 22 surrounding this cylinder. Into the space 23a and 23b between the hood 22 and cylinder 21, pressurized air that has been heated in a radiator 45 is conducted through pipes 41a and 41 b by effect of a blower 40. Within the cylinder 21, again, a vacuum is created by connecting it to a suitable vacuum source 42. The wire 6 and upon this wire the web 5, both of which are permeable to air, pass over the surface of the cylinder 21. This provides the opportunity for drying of the web with good efficiency by two modes of action, one of which is the evaporation of water from the web 5 owing to impingement of the hot air in the space 23, while the second effect is the passage of hot air, caused by the differential pressure prevailing between both sides of the web 5, through the web from the spaces 23a and 23 b into the cylinder 21 and the heat transfer by convection into the web 5 taking place in this connection, with attendant evaporation of water. These events in combination imply a very rapid drying of the web. The wire 6 and web 5 circle around the cylinder 21 most appropriately through an arc of 310.degree. to 340.degree. and their detachment from the cylinder 21 is accomplished by the pick-off roll 24, which together with the transfer roll 19 and the sealing plate 25 forms an enclosed space 26 with a central partition 49 and within which space a vacuum consistent with that within the cylinder 21 prevails. Consequently, the transfer roll 19 and pick-off roll 24 define with the cylinder 21 of the drier such tangential points which are positioned with reference to each other in a central angle extending from the center of the cylinder 21, the size of which is most appropriately 20.degree. to 50.degree. .

The pick-off roll 24 has been provided with a permeable mantle, and with an internal overpressure chamber 27 opening into the space 26. There is, furthermore, another overpressure chamber 28 within the roll 24, beginning immediately at the point of contact of the sealing plate 25 and the roll 24 and serving the purpose of ensuring that the web 5 goes along with the wire 6 to the pick-off roll 29, where the web 5 is detached from the wire 6. The pick-off roll 29 has a permeable mantle and internal overpressure chambers 30 and 31, whereby the chamber 30 serves to detach the web 5 from the wire 6 and the chamber 31 serves to clean the wire 6. Subsequent to its detachment, the web 5 travels in a free run past the guiding roll 32 to the winding cylinder 33 and to the roll 34 of completed product. The wire 6 runs over returning rolls 35 back to the forming section. Of the returning rolls 35, the roll indicated by 35a has been provided with wire tensioning means and the roll 35b has been provided with wire guiding means. It is further possible to provide wire cleaning means 46 on the return run of the wire, as required.

FIG. 2 shows an alternative design of the webforming section. The rotating suction breast roll of FIG. 1 has now been replaced by a stationary forming shoe 36 with curved surface, and which has been divided with a partition into at least two parts 36a and 36b, which constitute two or several suction zones. The curved upper surface of the forming shoe 36 is composed of separately placed foils 37 transversal to the direction of travel of the wire. Said foils are preferably made of a hard and abrasion-resistant material, e.g. a ceramic material. The water emerging from the web can pass through the interstices of the foils 37 into the save-all of the forming shoe 36, whence it is returned to the process. In all its other most essential parts the forming section of FIG. 2 is equivalent to that shown in FIG. 1. The design of FIG. 2 represents a more simple construction, but it involves higher abrasion of the forming wire 2, because the wire now passes over abrading surfaces, which do not occur in the forming section of FIG. 1.

The FIGS. 1, 1a and 2 present, as regards the headbox, in particular the common feature that the upper, curved confining surface of the pulp delivery slit, or the so-called upper lip 38, extends substantially farther in the direction of flow than the lower confining surface, or the so-called lower lip 39. The purpose of this arrangement is to create a situation wherein the pressure and kinetic energy of the pulp suspension in the headbox is utilized towards the object of dewatering. The web formation takes place in a tapering segregation space having the curved upper lip 38 as one of its confining surfaces. In the event that the running conditions of the machine are changed, the segregation conditions may be correspondingly altered by adjusting the position of the upper lip 38.

In FIG. 3 a favourable alternative example of the course for the second support and transport wire 15 is seen. The wire 15 passes, as in FIG. 1, over a transfer roll 19 provided with overpressure chamber 20 and likewise over the roll 16 of a press. In this embodiment the course of the second wire 15 has been arranged with the aid of rolls 43a and 43b to be such that the wire 15 passes through one of the two pressurized spaces, 23a, of the perfusion drier. Hereby the web 5 is interposed between the principal transport wire 6 and the second support and transport wire 15 during the initial stage of the drying on the cylinder 21; this ensures the proper conducting of the web 5 on the drier and promotes the formation of a softening marking of the upper side of the web 5. The respective central angle of the cylinder 21 has been indicated with .alpha., and within the sector defined by the angle .alpha. the web 5 runs between two wires, 6 and 15.

Claims

I claim:

1. Paper machine for manufacturing tissue paper from a web, comprising, in combination:

a. a headbox with an outlet slit for fluid stock from which a web is formed,

b. a forming section including a curved initial part and a forming wire receiving the web from said outlet slit,

c. a return roll producing a centrifugal force dewatering the web, said forming wire passing over said return roll,

d. a principal transport wire carried by a plurality of rolls and passing over said return roll, which deflects the web 120.degree. to 180.degree. while the web is interposed between said forming wire and said principal transport wire,

e. a transfer roll and a second transport and support wire passing over said transfer roll, the web being interposed between said principal transport wire and said second transport and support wire,

f. a perfusion-impingement drier assembly having a cylinder with a permeable mantle, said transfer roll conducting the web onto said cylinder, said principal transport wire being interposed between the web and said mantle, and

g. a pick-off roll and a reeling apparatus, said pick-off roll detaching the web from said cylinder and transmitting it to said reeling apparatus, said principal transport wire traveling back to said return roll.

2. Paper machine according to claim 1, characterized in that before the tranfer roll in connection with the principal transport wire and the second support and transport wire there has been arranged a press composed of rolls and a vacuum chamber, which press ensures that the upper surface of the web is pressed against the second support and transport wire.

3. Paper machine according to claim 1, characterized in that the transfer roll and pick-off roll define points of tangential contact with the cylinder of the drier assembly, and that these points of tangential contact form with the center of a cylinder a central angle which is within the range of 20.degree.-50.degree..

4. Paper machine according to claim 1, wherein said transfer roll and said pick-off roll have overpressure chambers, characterized in that the cylinder of the drier, the transfer roll, the pick-off roll and a sealing plate constitute an enclosed space carrying a vacuum and into which drying air is drawn through the web from the overpressure chamber of the transfer roll and from the overpressure chamber of the pick-off roll.

5. Paper machine according to claim 4, characterized in that the pick-off roll contains a second overpressure chamber, by effect of which the web is detached from the pick-off roll and conducted, supported by the principal transport wire to a second pick-off roll having two overpressure chambers, wherein the first serves the purpose of detaching the web from the principal transport wire, and the second has the purpose of cleaning the principal transport wire, and that it is conducted back into connection with the return roll, returned by the second pick-off roll.

6. Paper machine according to claim 1, characterized in that the web travels in the drier assembly, after the transfer roll and upon the mantle of the cylinder of the drier, between the principal transport wire and the second support and transport wire during the initial stage of drying on said cylinder.

7. Paper machine according to claim 1, characterized in that the outlet slit of the headbox is confined by a curved upper lip which extends to lie over the curved initial part of the forming section, producing an efficient initial dewatering action.