GB2063948A - Paper-making machines - Google Patents

Abstract

The forming section of a paper- making machine includes first and second looped forming wires 10, 11 trained to provide a forming throat 14 followed by a forming and dewatering zone with the inner wire 10 trained over a roll 12 and the outer wire 11 trained over the inner wire. A saveall 27 is located opposite the forming and dewatering zone of the outer wire and an air jet 22 is positioned immediately after the forming and dewatering zone and is directed against the oncoming outer wire 11 to doctor water off the wire and towards the saveall 27. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the forming section of a paper making machine The present invention relates to improvements in paper making machines, and more particularly to an improved twin wire former and an improved method and apparatus for removing water expressed from a web being formed between travelling twin wires.

Developments in paper making machines have progressed to where the travelling fourdrinier wire has a successor in a structure which employs twin forming wires. In this type of structure, two looped wires are wrapped over a curved forming surface and a jet of stock is delivered into the throat between the two wires. As they travel together over the forming surface, water is expressed through the outer wire due to the pressure between the wires normal to the web of paper being formed therebetween.

Such twin wire machines are capable of being run to make a wide range of grades of paper and at speeds which are unattainable by a simple fourdrinier machine. Also, they require less space and use less energy and make a better sheet for many grades. Twin wire machines are being developed to make tissue, or to make multi-ply board, and to make other grades, and the present invention is particularly applicable to twin wire tissue machines, although it may also be employed in twin wire machines for making other grades.

In twin wire machines the wires generally travel over an imperforate curved surface such as a travelling roll, although in certain circumstances, the supporting roll may be provided with suction means. Twin wire formers that do not use vacuum for the drainage appear to consume less power than those that do. A problem encountered in the twin wire machines where the wires are trained over a solid surface is getting rid of the water which passes through the outer wire as the web is squeezed between the wires. Savealls or means for catching the water outside of the outer wire have been provided, but a certain amount of water is carried along with the outer wire to interfere with the travel of the wire through additional portions of the machine in that the water tends to depart from the wire in uncontrolled locations.

Once the outer wire is separated from the web, it should travel in a manner so that water which might remain thereon does not disturb the newly formed web. Further, removal of the water from the outer wire has a cleaning function and fibres otherwise tend to stick to the outer wire as the water drains from the web through the openings in the outer wire.

It is accordingly an object of the present invention to provide an arrangement wherein a twin wire machine is provided with improved means for removing water from the outer wire.

A further object of the invention is to provide an improved twin wire machine which is capable of making improved paper web and wherein improved means are provided for cleaning and removing the water from the wire.

According to the invention there is provided a forming section for a paper making machine comprising: first and second looped forming wires trained to converge and form a stock receiving throat and then to travel in parallel relationship to provide a forming and dewatering zone; means within the first wire guiding the wires through a forming and dewatering path with said second wire wrapped over the first for the extent of said path; means positioned to direct a flow of stock into the throat between said wires; and an air jet for directing a jet of air towards the oncoming outer surface of the second wire for doctoring water off the surface.

The invention also provides a method of handling water expressed from a web being formed in a paper making machine having first and second looped forming wires trained over an arcuate surface to provide a stock receiving throat followed by a forming and dewatering zone, the method comprising: directing an air jet towards the oncoming exposed surface of the outer second wire doctoring the water off the surface.

The following is a detailed description of an embodiment of the invention, reference being made to the accompanying drawing which is a front elevation, with parts shown in schematic form, of a twin wire forming section employing the features of the present invention.

As shown in the drawing, an outer wire 11 passes upwardly along with an inner wire 10. The two wires wrap the arcuate forming surface of a rotating roll 12. The wires taper together to define a forming throat 14 with the wire 11 being guided towards the throat over a roll 1 3. As the throat closes, the wires come together onto the supporting roll 12 to form a forming and dewatering run throughout which the fibres are first formed into a web and then continue to be dewatered until the wires are separated from each other, with the outer wire 11 being guided over a roll 16 followed by a roll 1 7.

In operation of the machine, a jet of stock 26 is, in known manner, delivered into the forming throat 14 through the slice opening 25 of a head box 23, 24.

Each of the wires 10 and 11 is in the form of a continuous loop, and within the loop of the inner wire 10 is a saveall 15.

Within the loop of the outer wire 11 are a group of deflectors or savealls 1 8, 19, 20 and 21. These deflectors are positioned with their leading edges in close running relationship to the outer wire 11 and are disposed to pick water which is thrown off the wire 11.

At the finished end of the forming and travelling run is positioned an air nozzle or air knife 22. The air nozzle 22 is positioned so that it will direct its flow of air towards the oncoming web. The air knife thus doctors off the layer of water which tends to remain on the surface of the forming wire as it is leaving the forming and dewatering zone.

The air knife 22 extends continuously across the length of the roll, and the air jet is directed relatively uniformly towards the oncoming wire so that the excess water can be doctored from the surface of the outer wire 11. As the air knife doctors the layer of water, the water tends to fly upwardly and outwardly to be deflected by the deflector 21 or to be thrown upwardly and to be caught by the collector housing 27. Suitable varying arrangements can be added for deflecting the water which is doctored off by the nozzle 22, and the nozzle is principally effective in removing the layer of water on the wire without danger of contact with the wire and without any substantial resistance to wire travel so as to increase power requirement.Further, the doctor is adjustable by the provision of a nozzle width adjustment 22a which adjusts the size of the nozzle opening for optimum effectiveness with respect to water carried on the surface of the wire 11. Also, the position of the nozzle is adjustable by a suitable mechanism, shown schematically by the arrowed line 22b, to set the angle of the nozzle for maximum effectiveness in doctoring off the water.

A supply valve 28 is provided which is capable of controlling the flow of air into the nozzle and thus the quantity and velocity of air being emitted from the nozzle opening is controllable. Air is supplied to the valve from a suitable air pressure source which is normally available in any paper mill installation.

While the twin wire former with the air doctor arrangement may be employed in formers for all grades, an example of operation with a tissue former is as follows. In the formation of tissue, the arrangement encourages gentle drainage and superior profile and formation. There is an easy control of tensile ratio at any speed which attains a better than 2:1 ratio. The basis weight of the tissue is in the range of 12 to 56 g/m2, but operations were run up to 78 g/m2. The operating speeds conducted were in the range of 1280 m/min on 12 git2, 760 m/min. The surface output per metre width was 75 m/tons/day/metre on heavier grades.

The aforementioned collector housing 27 is provided within the wire loop 11 for receiving and draining off water which is removed from the deflectorsavealls 18, 19,20and21 and for receiving water which is doctored off by the air knife 22. The housing 27 will be provided with a suitable drain so that the water will be continually removed and returned as white water, and the white water from the saveall 1 5 and the saveall housing 27 will usually be recirculated back to be mixed with stock.

Claims (14)

1. A forming section for a paper making machine comprising: first and second looped forming wires trained to converge and form a stock receiving throat and then to travel in parallel relationship to provide a forming and dewatering zone; means within the first wire guiding the wires through a forming and dewatering path with said second wire wrapped over the first for the extent of said path; means positioned to direct a flow of stock into the throat between said wires; and an air jet for directing a jet of air towards the oncoming outer surface of the second wire for doctoring water off the surface.

2. A forming section according to claim 1, further comprising saveall means positioned within the second wire loop opposite the forming and dewatering zone for receiving water expressed from a web being formed between the wires, said air jet also being positioned within the second wire loop for doctoring off water from the wire towards said saveall means.

3. A forming section according to claim 2, wherein said saveall means is in the form of a plurality of concave water receiving savealls with the concave portions facing away from said air jet.

4. A forming section according to any of claims 1 to 3, wherein said air jet is positioned substantially at the end of the forming and dewatering path.

5. A forming section according to any of claims 1 to 4, wherein the means for guiding the wires through the forming and dewatering path comprise an arcuate member within the first wire.

6. A forming section according to claim 5, wherein said arcuate member is in the form of a roll with the wires travelling with the surface of the rotating roll.

7. A forming section according to claim 6, wherein the throat faces substantially downwardly and the forming dewatering zone is on the uprunning side of the roll with the air jet positioned at the end of the forming dewatering run at the uppermost portion of the roll.

8. A forming section according to any of claims 1 to 7, including means for regulating the relative angle of the air jet projected onto the outer surface of the second wire.

9. A forming section according to any of claims 1 to 8, including means for regulating the width of the jet of air being directed at the oncoming wire surface.

10. A forming section according to any of claims 1 to 9, including means for controlling the velocity of said air jet.

11. A forming section for a paper making machine, substantially as hereinbefore described with reference to the accompanying drawing.

12. A paper making machine, including a forming section according to any of the preceding claims.

13. A method of handling water expressed from a web being formed in a paper making machine having first and second looped forming wires trained over an arcuate surface to provide a stock receiving throat followed by a forming and dewatering zone, the method comprising: directing an air jet towards the oncoming exposed surface of the outer second wire doctoring the water off the surface.

14. A method of removing water from a forming section according to claim 13, including providing savealls for receiving the water doctored off the surface of the second wire by the air jet.