Method and apparatus for removing water from a fibrous web in a papermachine

Abstract

 The present invention relates to a method and apparatus for removing water from a fibrous web. The invention is intended especially for applications to be used in a twin wire former of a paper or a board machine.
A problem earlier experienced in foudrinier machines has been fiber flocking in the web surface, which results from a Iong and tranquil period of flowing, during which period fibers have enough time to stick to one another and form flocks that deteriorate the paper quality.
This problem has been solved by arranging rows of dewatering pressure elements (24, 32, 33) as part of the covers (25, 34, 33) of both upper and lower dewateri::g units (20, 30) so as to match or be aligned at least partly, whereby they generate an intensive turbulence field into the web surface to break the flocks that have already been formed.

Description

[0001] The present invention relates to a method and apparatus applicable to paper machines for removing water from a fibrous web. The method employs a so-called twin wire former (two endless, opposed mesh belts) where drainage is effected both upwards and downwards. The method involves production of flows in the fibrous web, said flows being longitudinal with respect to the web and bringing the fibers into a turbulent state in the surface of the web. Said flows are caused by arranging the foils of the dewatering units to be adjustable in the direc­tion of the web and by disposing them at least partly opposite to one another on different sides of the web.

[0002] In a known twin wire apparatus such as that disclosed in US patent 3994774, water removal from the web is effected by dewatering elements positioned alternately with respect to one another. In realization of one embodiment disclosed in said patent publication, conventional foils are used below the web with said foils being spaced relatively far apart from one another and characteristically removing water downwardly. Above the web and inside the other wire loop, there is another dewatering unit, the single dewatering elements of which are disposed between the foils of the lower dewatering unit. The upper dewatering unit is so disposed that, during operation, its single elements press the web together with the foils in slightly wavy manner. After the foils, water is separated from the web through the top wire by, pressure and centrifugal force and this water wi11 be removed from the top wire by means of a leading surface of said upper dewatering unit and led to a discharge pipe.

[0003] Another prior art patent publication is DE patent 3138133 which discloses different alternatives of arranging drainage at a twin wire former. The only features common to the vari­ ations are that, in the dewatering zone, the web is wavy and the dewatering elements disposed both above and below the web are positioned alternately. Furthermore, it has to be noted that in said device disclosed in the patent, pressure on the web is effected by tension of wires.

[0004] In spite of the fact that the draining capacity of such de­watering zone is relatively good, it also has some disadvan­tages. At each stage of drainage, water has free access straight upwards or downwards through the wire. Consequently, the fibers of the web tend to turn in the direction of the flow, which results in deterioration of the paper quality. On the other hand, filler loss is also bigger due to said fillers being discharged straight through the wire.

[0005] To eliminate the disadvantages described above, a new type of former has been developed wherein the dewatering elements on different sides of the web are adjustable with respect to each other in the direction of movement of the web and rows of pressure elements of the opposite dewatering units at least partly match. The latter characteristic ensures that the direction of water flow in the web surface is not solely perpendicular to the level of the web but is also in con­formity with the direction of movement of the web or opposite to it.

[0006] This arrangement has the advantage of effecting pulses and heavy turbulence in the surface layer of the web, which prevents flocculation on the surface of the web, thus im­proving the web quality. The dewatering units being adjustable with respect to each other in the direction of movement of the web ensure that in each case of web formation, it is possible to optimize the intensity of turbulence in the web surface, thus obtaining the best possible surface quality.

[0007] According to our invention, the web is primarily pressed by adjusting the height of the lower dewatering unit, whereby the stresses on wires diminish when compared with for example the above-mentioned DE patent publication.

[0008] The method according to the present invention is characterized in that the position of the cover of one dewatering unit is adjusted relative to the the cover of the other dewatering unit, thereby affecting both the intensity of the turbulence generated in the web surface and the distribution of the components of said turbulence, said components moving in different directions.

[0009] The apparatus according to the present invention is char­acterized in that web forming is effected in a twin wire former, said former having two dewatering units, the covers of which units or a side portion thereof comprise rows of press­ure elements, which elements at least partly match or are at least partially aligned on opposite sides of the web.

[0010] The invention will be described further, by way of example, with reference to the accompanying drawings, in which

Fig. 1 is an overall schematic elevation of the wet end of a paper machine to which the present invention is applied;

Fig. 2 is a fragmentary, detailed schematic elevational view of one embodiment of the apparatus according to the present invention;

Figs. 3 and 4 are schematic illustrations on enlarged scale of the disposition of the dewatering elements with respect to each other in different embodiments;

Fig. 5 is a schematic illustration of the operating principle of the method according to the present invention; and

Fig. 6 is a fragmentary enlarged end elevation of the wire support means.

[0011] The method and apparatus of the present invention are pri­marily intended to be applied to a fourdrinier machine 1 as shown in Fig. 1. The wet end of the fourdrinier machine 1 comprises in its simplest form a headbox 2 and a fourdrinier wire section 3, said fourdrine:r wire section comprising wire 4, breast roll 5, other rolls 6 and dewatering elements 7. The rear or downstream end of the fourdrinier wire section 3, i.e. a so-called web forming section 8, comprises an on-top unit 9 above the web and a dewatering unit 30 below the fourdrinier or the bottom or lower wire 4.

[0012] In accordance with Fig. 2, the on-top unit 9 includes, for example, a dewatering unit 30 and rolls 11, 12, 13 and 14, all of which, except for roll 13, are adjustable either vertically or, by means of a turning arm, in a swinging manner. The upper dewatering unit 20 mainly comprises three chambers 21, 22 and 23, which have a side part or cover 25 composed of rows of constricting or pressure elements 24, said cover being common to all of said chambers. The height of the slice opening of the first chamber 21 is adjustable. Water drained straight through top run of wire 10 without the help of dewatering elements above the wire is led to said chamber 21. All chambers of the dewatering unit 20 function as suction boxes the suction capacity of which is adjustable. Furthermore, the dewatering unit 20 itself is adjustable both vertically and in the longitudinal angle with respect to the machine.

[0013] In the embodiment shown in Fig. 2, the lower dewatering unit 30 comprises two covers 34 and 35 formed by pressure elements 32 and 33, said elements being united by a joint 31. The covers 34 and 35 are supported by beams 36 which are trans­verse in view of the machine direction through pressure-medium filled hose-like elements 37 so as to enable the longitudinal adjustment of the angular position of the covers 34 and 35 with respect to the machine. In other words, the pressure caused by the covers 34 and 35 on the bottom wire 4 can be changed. The adjustability is ensured by the fact that the pressure adjustment of each hose-like pressure element 37 is independent of the other elements 37. Furthermore, to improve the adjustability of the dewatering unit 30, the covers 34 and 35 are supported by an element 38 adjustable longitudinally relative to the machine, said support element being attached to the machine frame at one end and to the cover 34 at its other end. Any other elements relating to the longitudinal adjustment are unnecessary because hose-like pressure elements 37 provide a sufficient adjusting allowance. If required, dis­tances between various tables can be adjusted.

[0014] A structure characteristic of a preferred embodiment of the invention is shown in Fig. 3, which illustrates a twin wire former where the shape and opposed location of consecutively arranged constricting or pressure dewatering elements 24 and 33 of the upper and lower dewatering units 20 and 30 match. taking into account the longitudina1 adjustability relating to the cover 34, we come to a possible arrangement as shown in Fig. 4 in which the pressure elements 24 and 33 only partly match i.e. they are partly out of vertical alignment. A cor­responding result is also achieved if the distance between the covers 34 and 35 does not correspond to that between the dewatering elements 24 of the dewatering unit 20 above the web or to a multiple of said distance, in which case at least one of the covers 34 or 35 is disposed in accordance with Fig. 4.

[0015] Removal of water from a fibrous web in accordance with the invention is effected as shown in Fig. 5. The pressure of the lower dewatering unit on the web through pressure elements 33 makes the wires 4 and 10 swell a little outwards from the web in between the pressure elements 33 and 24. At the same time, the pressure compresses the web between the opposite elements 24 and 33 so as to make water flow in the machine direction so that first, when entering the range of the rows, it flows opposite to the moving direction of the web whereas on the other edge of the rows it flows at an accelerated speed in the moving direction of the web. Exactly on the level of the wire edge and in an area slightly outwards of that, water is pressed out of the web in a direction nearly perpendicular to the wire surface. All of these flows in different directions that quickly change result in an intensive vortex field which breaks fiber flocks, thus improving the surface quality of paper. By adjusting the oppositeness of the pressure elements 24 and 33 i.e. by partly overlapping said elements in one direction or the other, it is possible to affect the intensity of turbulence as desired. Turbulence can be affected also by changing the profile of the pressure elements 24 and 33. Together said ways of affecting turbulence give nearly unlimited possibilities of adjusting the operation of the web forming section and the surface of the web, especially when taking into consideration the other possibilities mentioned above of adjusting the upper and lower dewatering units.

[0016] The preferred embodiment as described above is not intended to limit the invention of what is presented in the accompanying claims. It is therefore possible, whilst the adjustment in the direction of the web is effected by means of a pressure-medium filled cylinder 38 in Fig. 2, that the adjustment is per­formed, for example, by an adjusting screw or equivalent mechanical device. Likewise, although Fig. 2 illustrates a two-part cover of the dewatering unit below the web, it is possible that there are several covers. The most substantial feature in the covers is that the pressure elements are secured to the covers and that the covers are substantially plane-like.

Claims

1. A method for removing water from a web wherefrom water has already been removed on a horizontal dewatering part, in which method the web is led in between two wires for compression and water is removed from the web through both wires, char­acterized in that an intensive turbulent flow field is generated in the web by means of rows of pressure elements of dewatering units. said units being disposed on opposite sides of the web, said flow field being composed of flow pulses to and fro that are substantially in the direction of the web level and of flows that are substantially perpendicu­lar to the web level, said flow field breaking fiber flocks in the surface of the web.

2. A method in accordance with claim 1, character­ized in that the intensity of the flow field and the distribution of its components are controlled by displacing one dewatering unit with respect to the other to a different alignment.

3. In an apparatus of the so-called twin wire former kind for removing water from a web, there are provided dewatering units (20, 30) both above and below the web, character­ized in that substantially planar side parts or covers (25, 34, 35) of the dewatering units (20, 30) mainly comprise rows of spaced apart pressure elements (24, 32, 33) on the opposite sides of the pair of wires (4, 10) entraining the web, with said pressure elements being at least partially aligned with pressure e1ements in the opposite row.

4. An apparatus according to claim 3, character­ized in that the cover (34, 35) of at least one dewatering unit (30) comprises at least two parts.

5. An apparatus according to claims 3 and 4, charac­terized in that the covers (25, 34, 35) of the de­watering units (20, 30) are themselves longitudinally ad­justable with respect to the web.

6. An apparatus according to claims 3 and 4, characte­rized in that the distance between the last pressure element of the cover (34) of one dewatering unit (30) and the first pressure element of the next cover (35) of the same dewatering unit is unequal to the distance, or a multiple t|:ereof, between the pressure elements (24) of the cover (25) of the opposite dewatering unit.

7. An apparatus according to claims 3 and 4, charac­terized in that the distance between the last pressure element of the cover (34) of one dewatering unit (30) and the first pressure element of the next cover (35) of the same dewtering unit is equal to the distance, or a multiple there­of, between the pressure elements (24) of the cover (25) of the opposite dewatering unit.

8. An apparatus according to claims 3 and 4, charac­terized in that the distance between the covers (34, 35) of at least one dewatering unit (30) is adjustable.

Drawing