Inlet into the twin-wire zone of a hybrid former for a paper machine

Abstract: An inlet into the twin-wire zone in a hybrid former for a paper machine. The former comprises a lower-wire loop (10), in which there is a single-wire initial portion (10a) of the forming zone, in which, inside the lower-wire loop (10), there are draining elements (13) and, after that, inside the upper-wire and lower-wire loops (10,20), wire-guide, forming and draining elements. The former includes an upper-wire unit, in which there is an upper wire (20), which is guided by the rolls (21,21a,30) and which is guided by the breast roll (30) onto the pulp layer (W₀) that has been formed on the single-wire initial portion (10a) of the lower wire (10). In the former, in the twin-wire zone, there is a dewatering and forming unit (35;40) or units. At the inlet into the twin-wire zone, there is a breast roll (30) provided with an open face (31), which roll is fitted in such a position that the area (a) of the breast roll (30) that reaches contact with the pulp web (W₀) is pressed slightly into the upper face of the pulp web (W₀) without curving the lower wire (10) to a substantial extent.

Description: The invention concerns a hybrid former for a paper machine, which former comprises the features of the pre-characterizing portion of claim 1. A hybrid former of this kind is known from document EP-A2-0372815. A gap former is mostly better than a hybrid former both in view of the quality of the paper and in view of the runnability, but for many a paper mill a gap former, which also requires modernization of the existing headbox, is an excessively costly solution. In a number of cases, a more advantageous solution would be an upper-wire unit placed close to the headbox, by means of which solution at least a part of the favourable properties of a gap former are obtained. In modernizations of paper machines, this would permit the use of the existing fourdrinier headbox. In web formers of paper machines, a number of different forming members are used. The primary function of these members is to produce compression pressure and pressure pulsation in the fibre layer that is being formed, by means of which pressure and pulsation the draining of water is promoted out of the web that is being formed, while the formation of the web is improved. Said forming members include various forming shoes, which are usually provided with a curved ribbed deck, over which the forming wires placed one above the other and the web placed between them are curved. In the area of these forming shoes, water is drained through the wire placed at the side of the outside curve by the effect of its tensioning pressure, and this draining is aided further by a field of centrifugal force. Draining of water also takes place through the wire placed at the side of the inside curve, which draining is, as a rule, intensified by means of a vacuum present in the chamber of the forming shoe. The ribbed deck of the forming shoe produces pressure pulsation, which both promotes the dewatering and improves the formation of the web. The hybrid former according to document EP-A2-03 72 815 comprises an initial single-wire portion of the forming zone and, downstream thereof, a twin-wire zone defined between the upper wire and the lower wire. The breast roll of the upper-wire unit of the known hybrid former has a closed surface and guides the upper wire such that the upper wire and the lower wire together form a wedge-like gap at the inlet of the twin-wire zone. The pulp web supported on the lower wire enters this gap such that the upper wire contacts the pulp web downstream of the closed breast roll. A substantial portion of draining takes place in the initial single-wire portion of the forming zone through the lower surface of the pulp web before the web runs through the dewatering and forming unit at the twin-wire zone. The inlet of the twin-wire forming zone has proved a critical point. It has been noticed that the initial part of the twin-wire zone has a substantial effect, e.g., on the retention and on the porosity of the paper. Problems are produced in particular because, when the upper wire enters into contact with the top face of the pulp web that is being formed, the fibre structure "freezes", in which case any unevenness present in the upper wire or in the top face of the pulp layer is seen as flaws in the finished paper. Said unevenness is more likely to occur when the upper wire is brought into contact with the top face of the pulp layer in a curved area while the upper wire runs unsupported at said location. Document US-A-2 881 670 discloses a hybrid former which comprises an open-faced breast roll at the beginning of a twin-wire zone. The breast roll covered by an upper wire is arranged such that the lowermost mantle portion of the breast roll is depressed slightly into the upper surface of the pulp web without curving a lower wire. The breast roll cooperates with a doctor to define an inclined portion of the upper wire. The space above the inclined portion is under a reduced pressure. The pulp web is forcibly dewatered by pressure and suction within a very short length of the forming wires. In other words, in the hybrid former according to document US-A-2 881 670, the breast roll is an element of a pressure mechanism which generates a high dewatering pressure and forms the pulp web within a very short run of the forming wires, namely within a wedge-shaped zone which extends from the breast roll to the doctor. Downstream of the wedge-shaped forming zone, the web is already formed so that it can be pressed by press roll couples. The forced dewatering and forming within the short wedge-shaped forming zone results in a poor quality of the paper, and particularly in a poor retention of fines and fillers, in a high porosity and in flaws in the finished paper. It is an object of the present invention to improve the hybrid former comprising the features of the pre-characterizing clause of claim 1 such that an increased amount of water can be removed upwards so that a more symmetric web is provided without the quality of the paper being impaired. This object is achieved by the hybrid former according to claim 1. The curved forming shoe of the hybrid former according to the invention effects increased dewatering through the upper surface of the pulp web and thereby improves the symmetry of the formed web. The open-faced breast roll is pressed slightly into the top face of the pulp layer. Thereby, dewatering is produced through the upper wire, and as a result of this, a thin fibre layer is couched on the upper wire. The fibre layer that has been couched in the area of the open-faced breast roll guarantees good retention on the following forming shoe. Also, the open-faced breast roll produces a shear force of the desired magnitude in the pulp layer, which force, when it disintegrates any flocks that have been formed in the pulp layer, improves the base formation of the web. Owing to the little coverage of the open-faced breast roll fitted in accordance with the invention, a paper can be produced in which the ratio of the tensile strengths in the machine direction and in the cross direction is lower than in the prior art and typically in the range of 1.5...2, which is particularly advantageous, for example, in the case of fine papers. The inlet into the twin-wire zone of the hybrid former in accordance with the invention can be made stable, and in its area no detrimental sharply pulsating dewatering pressure is applied to the pulp web. With a hybrid former in accordance with the invention, it is also possible to run paper grades thicker than in the prior art, typically of a grammage higher than 170 g/m 2 , whose production has not been possible earlier with corresponding hybrid formers while, at the same time, retaining a good base of the paper. The open-faced breast roll is typically placed in a position in which it presses a depression of about 1...5 mm into the upper face of the pulp web. The breast roll that is used is quite an open breast roll, so that the proportion of open face is about 50...80 % of the active area of the roll mantle of the breast roll. The open-faced breast roll is fitted preferably in a position in which the consistency of the fibre layer is of an order of k 0 ≈ 0.7...1.7 %. In the following, the invention will be described in detail with reference to an exemplifying embodiment of the invention illustrated in the figures in the accompanying drawing. Figure 1 is a schematic side view of an embodiment of the invention. Figure 2 is an enlarged vertical sectional view in the machine direction of an inlet into the twin-wire zone in accordance with the invention. Fig. 1 illustrates a hybrid former, which is also suitable for modernizations of existing fourdrinier wire parts. In such a case, an existing fourdrinier wire part has been modernized by to it adding a new upper-wire unit 50 constructed on support of its frame part. The former shown in Fig. 1 may, of course, also be a new construction. The lower wire 10 is guided by rolls 11, 11a,11b,11c. Through the slice 12 of a headbox, a pulp suspension jet J is fed to the area of a breast roll 11a to the inlet end of a single-wire initial portion 10a. In this initial portion 10a, there are draining elements in themselves known, such as foils and suction foils. A twin-wire zone, which is defined between the lower wire 10 and an upper wire 20, starts in the area of a breast roll 30 of the upper wire 20, said breast roll having an open face 31. After the open breast roll 30, inside the lower-wire loop, there is a curved forming shoe 13a with a ribbed deck, which shoe may be provided with one or several suction chambers. After said ribbed shoe, inside the upper-wire loop, there are drain chambers 22a,22b and 22c of a dewatering and forming unit 340, in which chambers there are drain ducts 24. The unit 40 comprises an upper set of stationary ribs 25 a disposed within the upper-wire loop. Through the drain ducts 24, the water that is drained through the upper-wire loop is passed in the direction of the arrows F into the drain ducts 23, which are connected to suction legs (not shown). The drain chambers 22a,22b and 22c communicate with vacuum sources (not shown). Below the initial part of the upper set of ribs 25a, inside the lower-wire loop, there is a loading unit 14. In the loading unit 14, placed facing the gaps between the ribs 25a and before said ribs, there are loading ribs 15, which are pressed by means of the pressures of a medium passed into hoses 16 against the stationary ribs 25a so as to apply a dewatering pressure and shear forces to the pulp web W. As is shown in Fig. 1, the opposite sets of ribs 15,25a are followed by a stationary set of support ribs 25b, which is placed below the third drain chamber 22c without opposite loading ribs. The last set of ribs 25b guides the twin-wire zone upwards with a curve radius R 1 ≈ 3...8 m. The surrounding of the sets of loading ribs 15 can also be connected with sources of vacuum. In the final portion of the twin-wire zone, inside the lower-wire loop there are suction boxes 17a and 17b, the upper wire 20 being separated from the paper web W at the location of the latter one of said suction boxes while guided by the guide roll 21a. The web W is separated from the lower wire at the pick-up point P between the rolls 11b and 11c, and, being aided by the suction zone of a pick-up roll (not shown), is transferred onto a pick-up fabric (not shown), which carries the web W to a press section (not shown). Of the draining taking place in the twin-wire zone, for example, about 80 % takes place through the upper wire 20 into the drain chambers 22a,22b,22c while intensified by a vacuum or vacuums. In the following, mainly with reference to Fig. 2, an exemplifying embodiment of the construction of the inlet into a twin-wire zone and the operation of said construction will be described. In the single-wire initial portion 10a of the lower wire 10, by the inlet, the layer W h of the pulp web W that is placed against the lower wire 10 has been couched to a certain extent, but in the top face and layer W y of the pulp web W 0 there is still stock approximately of the headbox consistency in the area of the breast roll 30, over which breast roll the upper wire 20 is guided onto the pulp web W 0 . The height position of the breast roll 30 is set and/or adjusted, in the direction of the arrow A, precisely in such a position that its lowest mantle portion is depressed into the pulp web W 0 slightly while, nevertheless, not curving the lower wire 10 substantially. In Fig. 2, the "depression" Δh of the pulp web W 0 is typically Δh ≈ 1...5 mm. In this way, the breast roll 30 couches a fibre layer against the upper wire, as a result of which an improved retention is obtained on the subsequent ribs 15,25a,25b. As the breast roll 30, a breast roll is used that has a very open mantle 31, in which the proportion of the open face out of the entire active mantle face is typically 50...80 %. Favourable is, for example, an open-faced breast roll 30 that is covered with a wire sock. As the breast roll 30, it is also possible to use a suction roll provided with a mantle with through perforations, whose suction zone is at least partly placed against the web. The diameter of the breast roll 30 is typically D ≈ 500...1400 mm, depending on the width of the machine. At the inlet point defined by the breast roll 30, the average fibre consistency k 0 of the pulp web W is preferably in the order of k 0 = 0.7...1.7 %, thus, the breast roll 30 can be placed favourably in an area of a fibre consistency lower than the inlet consistencies in other formers without a risk of breaking or crushing of the structure of the pulp layer in the web W 0 . Owing to the fitting of the breast roll 30 in the described manner, no excessively sudden dewatering pulse is produced in the area of the breast roll 30, so that the described hybrid former can also be used for thicker grades than in the prior art (grammage > 170 g/m 2 ), in which the thickness of the pulp web at the inlet is typically h ≈ 8...25 mm.

Claims: A hybrid former for a paper machine, said hybrid former comprising an upper-wire unit (50) and a lower wire (10) defining an initial single-wire portion (10a) of the forming zone, wherein said upper-wire unit (50) comprises an upper wire (20), guide rolls (21, 21a) and a breast roll (30) for guiding the upper wire (20), wherein said upper wire (20), together with said lower wire (10), defines a twin-wire zone of the forming zone between the wires, wherein said breast roll (30) guides said upper wire (20) onto the pulp web (W o ) that has been formed on the initial single-wire portion (10a), wherein draining elements are arranged inside the loop of the lower wire (10) at the initial single-wire portion (10a), and wherein a dewatering and forming unit (40) is arranged inside the loops of the lower wire (10) and of the upper wire (20) at the twin-wire zone, characterized in that a forming shoe (13a) is arranged at the twin-wire zone downstream of its inlet, said forming shoe (13a) being provided with a curved ribbed deck and curving the twin-wire zone downwardly, in that said breast roll (30) covered by said upper wire (20) has an open face, and in that said breast roll (30) is fitted in such a position that the lowermost mantle portion of the breast roll (30) is depressed slightly into the upper surface of the pulp web (W o ) without curving the lower wire (10) to a substantial extent. A hybrid former as claimed in claim 1, characterized in that the depth of said depression by said breast roll (30) into the upper face of the pulp web (W 0 ) is of an order of Δh ≈ 1...5 mm. A hybrid former as claimed in claim 1 or 2, characterized in that said breast roll (30) comprises a roll mantle, in which the proportion of the open face is about 50...80 % of the active area of the roll mantle. A hybrid former as claimed in any of the claims 1 to 3, characterized in that said breast roll (30) is a suction roll whose roll mantle is provided with through perforations. A hybrid former as claimed in any of the claims 1 to 4, characterized in that said breast roll (30) is placed in a position in which the fibre consistency of the pulp web (W 0 ) is of an order of k 0 ≈ 0.7...1.7 %. A hybrid former as claimed in any of the claims 1 to 5, characterized in that said forming shoe (13a) is followed by opposite sets of ribs (15,25a). A hybrid former as claimed in any of the claims 1 to 5, characterized in that said forming shoe (13a) curves the twin-wire zone downwards with a curve radius of R 0 ≈ 3...10 m. A hybrid former as claimed in any of the claims 1 to 7, characterized in that said dewatering and forming unit (40) comprises inside the loop of the upper wire (20) one or several drain boxes (22a,22b,22c), below which there is a preferably stationary set or sets of support ribs (25a,25b), against which, inside the loop of the lower wire (10) there is a loading unit (14), in which there is a set of ribs (15) that can be loaded with a medium or with an equivalent force. A hybrid former as claimed in claim 8, characterized in that, downstream of said dewatering and forming unit (40) and inside the loop of the lower wire (10), there is one or several suction flatboxes (17a, 17b), in or after whose area the upper wire (20) is separated from the paper web (W), which is passed on support of the lower wire (10) to a pick-up point (P). A hybrid former claimed in any of the claims 1 to 9, characterized in that, depending on the wire width, the diameter D of said breast roll (30) has been chosen in the range of D = 500...1200 mm.

Inventors: SAVIA RISTO, SAVIA, RISTO
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1995
Main classification: D21F9
IPC codes: D21F1, D21F9