EP2061929B1

EP2061929B1 - Wet forming paper machine with a system to reduce turbulence in the water-slurry circuits

Info

Publication number: EP2061929B1
Application number: EP07827654A
Authority: EP
Inventors: Piero Celli; Marco Celli
Original assignee: A Celli Paper SpA
Current assignee: A Celli Paper SpA
Priority date: 2006-09-06
Filing date: 2007-08-31
Publication date: 2010-03-03
Anticipated expiration: 2027-08-31
Also published as: CN101517161B; CN101517161A; WO2008029427A1; ATE459749T1; EP2061929A1; DE602007005147D1; ITFI20060216A1

Abstract

The machine comprises: at least a first continuous flexible member (3), and a second continuous flexible member (1) defining a forming area for forming a web of cellulose fibers; a headbox (17), to feed a slurry of water and cellulose fibers to said forming area; a collection vessel (22) for the water eliminated from the web forming area; an arrangement to slow said water eliminated from the web forming area. A curved guide surface (19) is also provided, disposed at the side of the web forming area, on the side from which the water is eliminated. The surface extends gradually away from the forming area and is oriented downward, so that the water forms a flow adhering to the guide surface. Moreover, a rotating damping member (23) is provided, arranged between said guide surface and said collection vessel, below the guide surface, to receive the water coming from said guide surface and discharge it into said collection vessel at a substantially reduced speed.

Description

Technical Field

The present invention relates to improvements to continuous wet forming paper machines and more in particular, but not exclusively, to highspeed continuous machines to produce tissue paper.
Specifically, the invention relates to improvements to systems for reducing turbulence in the water-slurry circuit of these machines.

State of the Art

The most recent machines to produce paper in continuous sheet form have reached extremely high production rates, with speeds exceeding 1000 m/min now being common and, for some types of paper, speeds can even exceed the limit of 2000 m/min.
These speeds are reached in particular in the production of tissue paper, i.e. thin paper mainly for domestic use, as well as for other uses, such as to replace textile cloths for polishing and cleaning.
One of the problems that has arisen in recent systems for forming the sheet of paper is the generation of a voluminous flow of water at high speed, and consequently with considerable turbulence, in the approach system.
This fast and violent circulation in the approach system causes air to be incorporated in the water, which is extremely detrimental to the performance of the cycle as it causes the formation of foam, which must consequently be removed. Moreover, the air bubbles incorporated in the water are detrimental to good formation of the sheet of paper to be produced.
Another phenomenon caused by the violent circulation of water is the production of mist and small water droplets, which remain suspended in the air around the sheet forming area and which must be removed using suitably shaped collection hoods, connected through pipes to powerful air suction fans and subsequent separators of the suspended water.
The flow of water and fibers is delivered from the headbox at the same or almost the same speed as the feed speed of the filter fabric (v = up to 40 m/sec).
In forming machines for thin papers <50 g/m², the so-called "crescent formers", the jet of fluid is directed at the point in which a felt of textile nature that will receive the wet sheet formed comes into contact with the filter fabric on which the required sheet will be formed. This contact takes place on the generatrix of a large diameter roller called "forming cylinder", which rotates at the peripheral speed at which the sheet is produced.
It is evident that the water pressed between the felt and the fabric is delivered from the latter at the same speed as the feed speed of the fabric on the forming cylinder and, by means of pressing force and above all centrifugal force, is ejected against a curved wall suitably shaped to allow the water to be recovered and conveyed to the collection vessel below, from where it is sent to the liquid return channel to reach the silo that acts as reservoir and once again feeds the pump that supplies the circuit.
Management of the water recirculation phase has become of fundamental importance in order to reach high production speeds. This leads to the need to make use of means to reduce the speed at which the water falls into the collection vessel and is conveyed therefrom towards the discharge channel, so as to reduce the negative phenomena of entrainment and mixing of air in the circuit to a minimum, preventing the formation of foam that floats on top of the water and the formation of mists that disperse into the air in the entire sheet forming area.
US-A-4.028.174 describes a system for reducing turbulence in the water delivered from a paper forming area in continuous machines. In this system deflectors are positioned to collect the water ejected from the forming cylinder and feed it through holes produced in the deflectors. The water that passes through the holes in the deflectors then falls into the collection vessel below, from where it is recovered and recycled. Other systems to reduce the speed of the water eliminated from the paper web forming area are described in US patents nos. 4,267,017 ; 4,501,040 ; 4,790,909 ; 4,714,522 .

Object and summary of the invention

An object of the present invention is to provide a machine and a method that allow a more effective reduction of turbulence in the water-slurry circuits in continuous paper forming machines, in particular for forming tissue paper.
It is also an object of the invention to provide a machine and a method that allow at least partial reduction of the problems deriving from the formation of mists and/or incorporation of air in the water discharged from the paper web forming area.
The continuous wet forming paper machine according to the invention comprises the features defined in claim 1. Additional features of the machine are defined in the dependent claims.
US-A-6,398,913 and US-A-2001/0018958 describe continuous machines for wet forming a paper web, wherein a cross-flow type Banki turbine is arranged in the web forming area, the sole purpose of which is to recover energy from the flow of water eliminated from the web forming area. There is no mention of a particular configuration to reduce the problem of turbulence of the flow of water and the consequent drawbacks described above.
Vice versa, according to the present invention a rotating member is provided to slow the flow of water, receiving the flow itself in a blading and discharging it at a speed practically equal to zero, so as to greatly reduce the energy in the flow and consequently the turbulence thereof, preventing or in any case reducing the formation of mists and also reducing the turbulence and therefore the quantity of air incorporated in the water.
Further advantageous characteristics and embodiments of the machine and of the method according to the invention are indicated in the appended claims and will be described below with reference to non-limiting embodiments of the invention.

Brief description of the drawings

The invention will be better understood by following the description and the accompanying drawing, which shows non-limiting practical embodiments of the invention. More in particular, in the drawing:

figure 1 shows a side view of the web forming area in a continuous wet forming paper machine of the crescent former type incorporating the present invention in a first embodiment;
figure 2 shows an analogous view to the one in figure 1 in a second embodiment; and
figure 3 shows a schematic cross section of the rotating damping member with relative blading.

Detailed description of embodiments of the invention

Figure 1 shows a schematic side view of the approach system of a continuous paper machine, in particular for tissue paper, in a crescent former configuration. The machine comprises a first continuous flexible member 1, preferably constituted by a felt that is fed around a closed path, only partly represented here. Cooperating with the felt 1 is a second continuous flexible member 3, preferably constituted by a fabric, which follows a closed path defined by guide rollers 5, 7, 9, 11 as well as by a roller 15 constituting the forming cylinder. The felt 1 is also fed around the forming cylinder 15.
In the nip between the rollers 11 and 15 in the web forming area, a headbox 17 feeds, for the entire width of the machine, a slurry of water and cellulose fibers between the felt 1 and the fabric 3. This slurry is pressed between the members 1, 3, and most of the water, present in percentages exceeding 99% in weight, is eliminated by centrifugal force from the forming area between the felt 1 and the fabric 3 in the arc of contact α with the forming cylinder 15.
A curved guide surface 19 is arranged in front of the forming cylinder 15 for guiding the water eliminated from the fabric 3 through centrifugal force, as indicated schematically by the arrows A. The quantity of water eliminated through the fabric 3 is substantial, considering that the production speed of these machines is in the order of 1000-2000 m/min and that the dry percentage in the slurry fed from the headbox 17 is often below 1%, for example in the order of 0.2-0.4% in weight.
Below the guide surface 19 a collection vessel 21 for the discharged water is located, extending for the entire width of the machine, i.e. for the entire axial extension of the cylinder 15 and for the entire width of the felt 1 and of the fabric 3. This vessel is connected to a side discharge channel 22 from which the water is recovered to be conveyed to a circuit that once again feeds the headbox 17.
The guide surface 19 has a scored configuration, i.e. geometrically constituted as locus of straight lines parallel to one another and parallel to the axis A-A of the forming cylinder 15. The curvature of the guide surface 19 is such that it extends gradually away from the cylindrical surface of the cylinder 15 (and consequently from the paper web forming area defined between the felt 1 and the fabric 3 in the contact area with the forming cylinder 15) extending downward.
The surface 19 extends downward and terminates inferiorly with an edge 19A above a blading of a rotating damping member 23. The rotating damping member 23 rotates in the direction of the arrow f23 due to the kinetic energy of the water discharged from the surface 19 into the blading of the rotating damping member itself. The arrangement is such that the flow of water generated along the surface 19 (in substantially laminar flow conditions) is tangent to the rotating damping member 23 and does not cross it, as is instead the case in machines specifically provided with turbines for the sole purpose of recovering the kinetic energy of the water. The water discharged from the surface 19 into the blading 23P of the rotating damping member 23 is discharged substantially at zero speed at the lower part of the rotating damping member towards the collection vessel 21. The rotation axis 25 of the rotating damping member 23 is supported by the structure 27, inside which the collection vessel 21 for collecting the water is located.
The guide surface 19 is carried by a pair of side panels 31 between which, as well as the guide surface 19, a wall 33 also extends, interposed between the cylinder 15 and the rotating damping member 23. Therefore, between the wall 33 and the guide surface 19 a duct 35 is defined, inside which the blading 23P of the rotating damping member 23 projects. The water that flows in a substantially laminar fashion along a guide surface 19 is made to fall onto the portion of blading 23P projecting inside the duct 35.
The arrangement described above allows the flow of water discharged, i.e. eliminated through the fabric 3, to be "calmed", reducing turbulence and transforming it into a substantially laminar flow along the guide surface 19, to then slow it substantially in the rotating damping member 23 before discharging it into the collection vessel 21. This prevents, or in any case drastically reduces, the formation of mists and also substantially reduces the quantity of air incorporated due to the turbulence in the water collected in the tank 21. This substantially facilitates the recirculation of the water and reduces the need for suction of the mist that forms around the water discharge area, which conventionally must be filtered in specific purifiers to separate the water containing fiber residues from the air in which it is suspended.
The assembly formed by the walls or side panels 31, by the guide surface 19, and, if necessary, by the wall 33, is adjustable with an oscillating or sliding movement with respect to the position of the forming cylinder 15. This adjustability allows optimization of the position of the guide surface 19 with respect to the cylinder 15 and with respect to the rotating damping member 23, which is supported on the axis 25 fixed with respect to the structure 27. The assembly 31, 19, 33 can also be moved away from the cylinder 15 by a degree sufficient to facilitate operations for replacing the fabric 3 or the felt 1 when this becomes necessary.
To allow adjustment and movement of the side panels 31 with the surface 19 away from/towards the wall 33 with respect to the forming cylinder 15, specific adjustment and movement means are provided, described below. In figure 1, the side panels 31, the surface 19 and the wall 33 are pivoted around pivot pins 90 and can be made to oscillate by means of a jack 92 around this pivot pin to be moved away from the forming cylinder 15 and allow insertion or removal of the fabric and/or of the felt. The position of the pivot pins 90 is adjustable, for example, by mounting said pivot pins on supports (not shown) provided with slots for screw fastening to the fixed load bearing structure 27. By loosening the screws it is possible to adjust the position of the pivot pins 90 and consequently of the edge 19A of the surface 19 with respect to the blading of the rotating damping member 23.
In figure 2 the assembly formed of the side panels 31, the surface 19 and the wall 33 is movable with respect to the forming cylinder 15 by means of jacks 100 that allow it to be moved towards and away from said cylinder. The operating position is defined by adjustable stops 102, the function of which is again to adjust the position of the surface 19 and of the discharge edge 19A thereof with respect to the blading of the rotating damping member 23.
Figure 3 shows in greater detail a cross section of the profile of the rotating damping member 23 with the blading 23P thereof. In section the blading has a bucket configuration similar to the profile of the blading of a Pelton wheel, which allows the flow of water to be slowed efficiently, almost to zero speed.
In substance, the rotating damping member takes the configuration of a roller constituted by a cylindrical envelope, the periphery of which is provided with channels whose shape and dimension are suitable to receive the flow of water coming from the paper web forming area and divert the orientation thereof, so as to take the speed of the jet of water to almost a zero value, this jet being made to fall from the rotating damping member into the collection vessel or channel at the same speed as the fall speed caused by the difference in level between the rotating damping member and the collection channel itself.
The rotating damping member 23 can be used solely as a means to "calm" the flow of water and reduce the turbulence thereof. In this case, a braking member will advantageously be arranged on the axis of the rotating damping member 23 to absorb the energy transferred from the flow of water to the rotating damping member 23. The braking member can be of the same type as those commonly used in paper mills, for example to slow the reels on paper unwinders in winding machines. Alternatively, this energy can be used, for example, by mounting a specific electrical generator on the axis 25 of the rotating damping member 23, or in any other suitable manner.
Figure 2 shows a modified embodiment of the machine according to the invention. The same reference numerals indicate the same or corresponding parts to those already described with reference to figure 1. These parts will not be described again with reference to figure 2.
The main difference between the configuration in figure 2 and the configuration in figure 1 consists in the direction of rotation of the rotating damping member 23. In the configuration in figure 1 the rotating damping member 23 rotates (arrow f23) in the opposite direction with respect to the direction of rotation (arrow f15) of the forming cylinder 15. Vice versa, in the embodiment in figure 2, the arrangement of the rotating damping member 23 is such that the direction of rotation f23 thereof is the same as the direction of rotation f15 of the forming cylinder 15. For this purpose, the rotating damping member 23 is positioned with the rotation axis 25 thereof between the ideal extension of the guide surface 19 and the forming cylinder 15, while in figure 1 the rotation axis 25 of the rotating damping member 23 is positioned, with respect to the ideal extension of the guide surface 19, on the opposite side to the forming cylinder 15.
In the configuration in figure 2 it is possible to arrange a direct mechanical transmission, for example represented by a belt 41, between a pulley 43 mounted on the axis 25 of the rotating damping member 23 and a pulley mounted on the shaft A-A of the forming cylinder 15. A motor, which is responsible for movement of the felt 1 and of the fabric 3, acts on the axis of the forming cylinder 15. The feed speed of the flexible members 1, 3, and therefore also the paper forming speed, are controlled by means of said motor. The mechanical connection represented by the belt 41 between the forming cylinder 15 and the rotating damping member 23 allows the angular speed of the rotating damping, member 23 to be controlled by the same drive motor as the forming cylinder 15, thereby allowing optimization of the slowing function of the flow of water entrusted thereto.
In the configuration in figure 2, the rotating damping member 23 is substantially housed completely in the channel 35 defined between the surface 19 and the ideal downward extension thereof and the wall 33. Above the rotating damping member 23 a deflection surface 36 is also provided, to prevent water, directed to the rotating damping member 23, from accidentally falling in areas in which it could obstruct the rotation of the rotating damping member.
It is understood that the drawing only shows an example provided by way of practical arrangement of the invention, which can vary in forms and arrangement without however departing from the scope of the appended claims. Any reference numerals in the appended claims are provided to facilitate reading of the claims with reference to the description and to the drawing, and do not limit the scope of protection represented by the claims.

Claims

A continuous wet forming paper machine, comprising: at least a first continuous flexible member (1) and a second continuous flexible member (3), defining a forming area for forming a web of cellulose fibers and guided around a forming cylinder (15); a headbox (17), to feed a slurry of water and cellulose fibers to said forming area; a collection vessel (21) to collect the water eliminated from the web forming area; an arrangement to slow said water eliminated from the web forming area, said arrangement including a curved guide surface (19) arranged laterally to the web forming area and to the forming cylinder, on the side from which said water is eliminated, said water forming a flow adhering to said guide surface, and a rotating damping member (23) arranged between said guide surface and said collection vessel, said rotating damping member being arranged below the guide surface, to receive a flow of water generated along said guide surface and discharge it into said collection vessel a reduced speed;
characterized in that:
- said surface extends in a downward direction gradually away from the cylindrical surface of the forming cylinder and from the forming area defined between said first and second continuous flexible members in the contact area with the forming cylinder; and

- said surface and said damping member are designed and arranged so that said flow of water directed from said surface onto said damping member flows substantially tangentially around said damping member to be discharged into said collection vessel.
Machine as claimed in claim 1, characterized in that said guide surface extends around said forming cylinder and gradually away from the rotation axis of the forming cylinder, and that a protecting wall (23) is arranged between said rotating damping member and said forming cylinder, a descending duct (35) being defined by said protecting wall and said guide surface, at least part of said damping member projecting inside said duct.
Machine as claimed in claim 1 or 2, characterized in that said rotating damping member is arranged with its rotation axis substantially parallel to the rotation axis of the forming cylinder and there under.
Machine as claimed in claim 1, 2 or 3, characterized in that said rotating damping member has an approximately cylindrical extension and a blading (23P) to receive the flow of water.
Machine as claimed in one or more of the preceding claims, characterized in that the position of said guide surface with respect to said forming area is adjustable.
Machine as claimed in claim 5, characterized in that said guide surface is supported by a moving structure (92;100) to move it away from the forming area and adjust its position with respect to said forming area.
Machine as claimed in one or more of the preceding claims, characterized in that said rotating tamping member is mechanically connected to a guide roller of at least one of said continuous flexible members.
Machine as claimed in claim 7, characterized in that said rotating damping member is mechanically connected to said forming cylinder.
Machine as claimed in claim 7 or 8, characterized in that said rotating damping member rotates in the same direction as said guide roller and is connected thereto by means of a direct transmission, preferably by means of a flexible element (41).
Machine as claimed in one or more of the preceding claims, characterized in that said continuous flexible members comprise a felt and a fabric.