Patent Publication Number: US-6669820-B2

Title: Twin-wire former

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims priority on Finnish Patent Application No. 20010129, Filed Jan. 22, 2001, the disclosure of which is incorporated by reference herein. 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a twin-wire former in a paper machine, which former includes two forming wire loops defining between themselves a twin-wire zone, and at least one dewatering box located inside one of the wire loops to remove water through said wire from the web being formed, and at least one loading blade located opposite to the dewatering box inside the other wire loop and in contact with said wire, said dewatering box including at least three successive dewatering zones. 
     In a gap former of a paper machine, a pulp suspension is fed into a forming gap between two forming wires, which wires are guided to curve over a forming roll and/or a forming shoe, which is equipped with a curved blade deck. In the narrowing gap, water is removed from the pulp suspension, among other things, by the action of a pressure caused by the tension of the wires, on the one hand, through the outer wire boosted by the centrifugal force and, on the other hand, through the inner wire boosted by the suction acting in the forming roll or shoe. 
     In the twin-wire zone after the forming gap, various kinds of web forming and dewatering elements are used, the purpose of which is to bring about pressure pulsation in the fiber layer being formed in order to promote dewatering of the web being formed and to improve its formation at the same time. U.S. Pat. No. 5,798,024 describes a state-of-the-art gap former in which a forming shoe and a loading blade unit function as dewatering and web forming elements after a forming roll. The blade deck of the forming shoe may be straight or curved. The loading blade unit includes a dewatering box provided with dewatering blades and a set of blade elements the loading of which can be controlled and which are placed inside opposite wire loops and in alternating positions with respect to one another so that the pressure pulses applied to the web by the dewatering blades and the loading blades alternate in the running direction of the web. The dewatering achieved by the loading blade unit is boosted by arranging a vacuum at at least one blade set and, preferably, at both blade sets. 
     The formation of the paper produced is improved by using a loading blade unit but, at the same time, it increases the porosity of paper, sometimes even to a harmful degree. Especially when making fine paper, high porosity may be a property which is attempted to be avoided. For this reason, loading blade units are not generally used when making paper grades containing highly beaten pulp and an abundance of filler. It has been found that in such cases it is preferable to use, for example, a forming shoe as the dewatering element, because it brings about lower pressure pulses in the fiber layer and affects the porosity of paper considerably less than a loading blade unit. 
     An aim of the invention is to reduce the above-noted problems associated with the prior art. A particular aim is to provide a novel twin-wire former by means of which efficient dewatering is achieved even at high running speeds and which makes it possible to produce paper having uniform formation and suitable porosity. 
     SUMMARY OF THE INVENTION 
     The twin-wire former according to the invention includes one or more dewatering boxes by means of which vacuum zones and vacuum-free zones are arranged in the web forming and dewatering zone, which zones alternate in the running direction of the web. One or more loading blades are arranged inside the wire loop situated opposite to the dewatering box, the loading blades being disposed such that opposite to each loading blade on the opposite side of the web there is a vacuum-free zone, which is preceded and followed by a vacuum zone. Since the vacuum zones and the loading blades are arranged to alternate in the running direction of the web in the proposed manner, the vacuum boosting dewatering and the loading blade producing a pressure pulse will never simultaneously affect the web that is being formed. 
     Alternation of the vacuum zones and vacuum-free zones is provided, for example, by dividing one continuous dewatering box with partitions into compartments which form several successive dewatering zones. Of these zones, every second zone is connected to a vacuum source in order to make dewatering more effective, and from every second zone water is removed mechanically without any significant vacuum. 
     Different vacuums can be advantageously arranged in the different vacuum zones so that the vacuum may be increased in the running direction of the web as the solids content of the web increases and dewatering becomes more difficult. 
     The number of loading blades and vacuum zones may vary in different applications. What is essential is that opposite to the loading blade there is always a vacuum-free zone, which is preceded and followed by a vacuum zone. In addition to this, the twin-wire former may include a pre-loading blade which precedes the dewatering box proper and which is also most advantageously placed at a location where it is opposed by a zone working without vacuum. 
     The invention combines characteristic features of known dewatering elements in a totally new way, so that by means of the new twin-wire former it is possible to simultaneously achieve good formation produced by the loading blades and moderate paper porosity produced by the suction forming shoe. 
     In the following, the invention will be described in greater detail with reference to the figures shown in the appended drawings, but the invention is not meant to be exclusively limited to the details of the figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a roll gap former in which vacuum zones and vacuum-free zones alternate, and at the latter zones the web is loaded with the aid of loading blades. 
     FIG. 2 shows an arrangement similar to that of FIG. 1, in which the surface of the dewatering box in contact with the wire is curved. 
     FIG. 3 shows a blade gap former. 
     FIG. 4 shows a modification of the former shown in FIG.  1 . 
     FIG. 5 shows another modification of the former shown in FIG.  1 . 
     FIG. 6 shows an arrangement with a minimum number of loading blades and vacuum zones. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The twin-wire former shown in FIG. 1 includes a first forming wire loop  10 , the running of which is guided by a first forming roll  11  and by guide rolls  12 ,  12   b , and a second forming wire loop  20 , the running of which is guided by guide rolls  21 ,  21   a  and by a second forming roll  22 . A pulp suspension is fed from a headbox  9  into a forming gap, which is defined by the first forming roll  11  on the side of the first wire  10  and by a breast roll  21   a  on the side of the second wire  20 . The joint run of the wires  10  and  20  extends substantially in the vertical direction from a suction sector  11   a  of the first forming roll  11  all the way to a suction sector  22   a  of the second forming roll  22 , after which the first wire  10  is guided by a guide roll  12   b  to separate from a web W, which will run with the second wire  20 . 
     In the twin-wire zone, that is, in the joint run of the wires  10 ,  20 , there is, in addition to the above-mentioned forming rolls  11 ,  22 , also a set of dewatering elements, of which a dewatering box  30  is located inside the first wire loop  10  immediately after the first forming roll  11 , and a suction box  42  is located inside the second wire loop  20  immediately before the second forming roll  22 . In addition, in the area of the dewatering zone there are a pre-loading blade  41 , which is located inside the second wire loop  20  opposite to the area remaining between the forming roll  11  and the dewatering box  30 , and loadable blade elements  40 , which are located inside the second wire loop  20  opposite to the dewatering box  30 . 
     On the surface of the dewatering box  30  in contact with the wire  10  there are dewatering blades  34  extending in the cross direction across the web and controlling the running of the wire, and between which dewatering blades water is removed from the web W in the direction of the dewatering box  30 . The internal space of the dewatering box  30  is divided by partitions  33  into five successive zones  30   a ,  30   b , of which three are connected to a vacuum source while a substantially normal pressure prevails in two. The vacuum zones  30   a  and the vacuum-free zones  30   b  alternate in the running direction of the web in such a way that each vacuum-free zone  30   b  is preceded and followed by a vacuum zone  30   a . When required, different vacuums p 1 , p 2 , p 3  can be arranged in the different vacuum zones  30   a . The vacuums are preferably arranged to increase in the running direction of the web (p 1 &lt;p 2 &lt;p 3 ). Water drained through the wire  10  is also guided away through the vacuum-free zones  30   b , which dewatering is boosted by the pressure pulses applied to the web by the dewatering blades  34  and the loading blades  40  alternately. 
     At the dewatering box  30  inside the second wire loop  20  there are two loadable blade elements  40  which extend in the cross direction across the web supporting and loading the wire  20  and doctoring water from its surface. The loading blades  40  also produce pressure pulses in the web W being formed, in consequence of which dewatering becomes more effective and the formation of the web is improved. The loading blades  40  are placed, in a manner known in itself, in alternating positions with the dewatering blades  34  of the deck of the dewatering box  30 . In the arrangement according to the invention, the loading blades  40  are placed specifically in those spaces between the dewatering blades  34  in which there is a vacuum-free zone  30   b  on the side of the dewatering box  30 . In that connection, the suction effect applied by the vacuum zones  30   a  on the web and the pressure pulse applied by the loading blades  40  to the web alternate in the running direction of the web and they never occur at the same time. 
     FIG. 2 shows a twin-wire former similar to the one shown in FIG. 1, in which the dewatering blades  34  are arranged to curve the wires  10 ,  20  while these travel over the dewatering box  30 . This increases the pressure applied by the wires  10 ,  20  to the web and thus improves the efficiency of dewatering. The vacuum zones  30   a  and the vacuum-free zones  30   b  are arranged to alternate in the running direction of the web, and the loading blades  40  are located opposite to the vacuum-free zones  30   b.    
     FIG. 3 shows a blade gap former in which in the forming gap area the running of the wires  10 ,  20  is guided by two breast rolls  12   a  and  21   a  and by a suction forming shoe  35  having a curved surface. After the forming shoe  35 , a dewatering box  30  is located inside the first wire loop  10 , opposite to a set of loading blades  40  located inside the second wire loop  20 . The dewatering box  30  is divided by partitions  33  into sections in such a way that a total of four vacuum zones  30   a  and three vacuum-free zones  30   b  is formed, which zones alternate in the running direction of the web W. Three loading blades  40  are located inside the second wire loop  20 , each one opposite to a vacuum-free zone  30   b.    
     FIG. 4 shows a roll gap former whose dewatering box  30  includes five vacuum zones  30   a  and four vacuum-free zones  30   b , four loading blades  40  being placed opposite to the vacuum-free zones inside the second wire loop  20 . 
     In the roll gap former shown in FIG. 5, the first zone  31  of a dewatering box  30  after a forming roll  11  is vacuum-free, and opposite to it there is a pre-loading blade  41  inside the second wire loop  20 . After the first zone  31  there are three further vacuum zones  30   a  and two vacuum-free zones  30   b , and opposite to the vacuum-free zones there are two loading blades  40  on the side of the second wire loop  20 . 
     In the example shown in FIG. 6 there is only one loading blade  40  and it is located opposite to a vacuum-free zone  30   b  remaining between two vacuum zones  30   a . The dewatering box  30  is followed further by a suction box  42  placed before the second forming roll  22 . The loading blade  40  is located in the same wire loop  10  as the first forming roll  11 , and the vacuum box  30  is located in the opposite wire loop  20  before the second forming roll  22 . 
     Above, the invention has been described with reference to a few examples only. However, the invention is not intended to be limited to these examples only, but many other modifications of the invention are also possible with the scope defined by the appended claims.