The invention relates to a method and an apparatus for preventing formation of mist by means of an effect of negative pressure in the wire section of a paper machine which section comprises a carrying wire and a covering wire and a transfer suction roll inside the loop of the carrying wire over a sector of which the wires and the web travel together after which the covering wire is led to a turning roll while the web is retained on the carrying wire. In this context, the paper machine means a paper, board or tissue paper machine or similar.

When a twin-wire former is run at high speed (e.g. over 2,000 m/min), a considerable amount of mist is created on the turning roll of the covering wire following the twin-wire zone when a centrifugal force throws water out from the holes of the wire bending at a large angle around the turning roll. The draining of water is promoted by a positive pressure created in a closing nip between the wire and the turning roll which pressure is caused by air flows incoming to the nip along with the wire and the roll. Air is able to escape the gap exclusively through the wire which, after the twin-wire zone, is saturated with water and suspended solids originated from paper stock.

On the turning roll, a centrifugal force is applied to water travelling along with the wire which force promotes the draining of water from the wire. The magnitude of centrifugal force, F, is given by:

m -v

F = (D R

where m is the mass of water, v is the speed of the wire and R is the diameter of the turning roll.

Centrifugal force F can be converted into a pressure dependent on the layer thickness of water by utilising the following equations:

m = δ - h - A (2)

V — _A (3)

in which δ is the density of water, h is the thickness of water layer and A is the area in contact with the roll.

Substituting equations (2) and (3) into equation (1) gives us the following equation for pressure p:

S - h - A - v ² δ - h - v ²

P = (4).

A - R R

In order for water to be retained in the wire, the negative pressure provided by the centrifugal force cannot exceed the vapour pressure of water in the process temperature. The vapour pressure of clean water in 50°C is 50 kPa. The negative pressure forming on the surface of the water film increases when the thickness of the water film increases, run speed increases and the radius of the roll decreases.

The centrifugal force alone cannot drain water from the roll, if there is a hydraulic contact between the roll surface and the water film. The hydraulic contact can be retained when the forming of an air cushion between the roll surface and the wire is prevented.

On the turning roll, mist released from the wire makes the pick-up devices and trim squirts close-by very quickly dirty. The disadvantage is primarily cosmetic but formation of mist can also decrease the rannability and efficiency of the paper machine because of dirt dropping from the trim squirts to the web. With SSB wire types quickly becoming common, formation of mist is even stronger than with traditional double-layer wires. This is caused by the fact that the open volume of triple-layer SSB wires is much greater than that of traditional wires.

An object of the invention is to decrease formation of mist in the forming section of a paper machine and especially on the wire-turning roll following the twin- wire zone.

To achieve the afore-mentioned object and those which come out later, a method according to the invention is characterised in what is presented in the characterising part of claim 1. Correspondingly, an apparatus according to the invention is characterised in what is presented in the characterising part of claim 13.

According to the invention, an effect of negative pressure is applied to the covering wire at some point in an area which begins before separating the covering wire from the surface of the web and ends at the turning roll. The invention is based on the idea that formation of mist can be decreased by decreasing the water content of the wire before it comes onto the turning roll and by preventing the formation of positive pressure in the closing nip between the wire and the turning roll.

In a first embodiment of the invention, the effect of negative pressure is applied to the covering wire it being on the surface of the transfer suction roll. Advantageously, the effect of negative pressure is provided by blowing pressurised medium towards the wire which medium scrapes air and moisture from the boundary surface of the wire, and by leading the mist drained by the

pressurised medium from the wire to a collecting chamber which is connected to a source of negative pressure. The pressurised medium can be air or vapour. A jet of pressurised medium is directed against the wire so that is hits the wire at an angle of 45-90°. The collecting chamber can be connected to a blower with which is provided a relatively low negative pressure required which advantageously is 0.05-1.O kPa.

The air curtain blown towards the wire functions like a knife removing air, water and mist from the surface of the wire. Such a contact-free air knife is safe because it stays clean and incurs no damage to the wire. Blowing applied to the covering wire also promotes dewatering of the web when it presses the web against the negative-pressure zone of the transfer suction roll. A pressure pulse created by the jet of pressurised medium facilitates the separation of wire from the web.

Instead of the transfer suction roll, there can be a transfer suction shoe which has a curved surface and which comprises at least one negative-pressure zone which ensures that when the twin-wire zone ends the web is retained on the carrying wire.

In an other embodiment of the invention, formation of mist is decreased by applying the effect of negative pressure to the covering wire immediately before the closing nip between the wire and the turning roll. The effect of negative pressure is provided by preventing the access of air flows travelling along with the wire and the turning roll to the closing nip. The inlet side of the nip is closed with a housing arranged between the wire and the turning roll which housing comprises a first guide surface which extends close to the surface of the covering wire and prevents the access of air flow travelling along with it to the nip between the wire and the roll, and a second guide surface which extends close to the surface of the turning roll and prevents the access of air flow travelling along with it to the nip. Then, as the roll rotates, a low negative pressure is created on the inlet side of the nip. If required, the effect of negative pressure can be promoted by connecting the

internal space of the housing to the blower, whereby the magnitude of negative pressure is advantageously 0.05-2.0 kPa.

The effect of negative pressure on the inlet side of the nip can also be obtained by using a grooved roll or a suction roll as the turning roll. The suction roll is expensive because of which it has not been traditionally used as a wire-guiding roll. Now, a considerable improvement in the operational reliability of the wire section can be obtained by means of it, which compensates for the costs incurred by negative-pressure zones.

The invention can also be implemented by positioning a suction device inside the loop of the covering wire at some point between the transfer suction roll and the turning roll. E.g. a suction box provided with a ceramic slotted cover can be used as the suction device.

The effect of negative pressure can be the same over the whole width of the wire or, alternatively, a higher effect of negative pressure can be applied to the edge area of the wire than in the centre area of the wire. Formation of mist is usually a greater problem in the edge area of the fabric than at the centre of the fabric.

The effect of negative pressure can be further promoted by blowing air towards the wire from the opposite side of the wire.

The invention will now be described in detail with reference to the figures of the accompanying drawings, to the details of which the invention is, however, by no means intended to be narrowly limited.

Fig. 1 is a side view of a twin-wire former provided with an apparatus preventing formation of mist.

Fig. 2 is an enlarged side view of the apparatus preventing formation of mist of Fig. 1.

Fig. 3 shows an enlarged detail of the apparatus of Fig. 2.

Fig. 4 illustrates the formation of positive pressure in the closing nip preceding the turning roll.

Fig. 5 is a side view of an apparatus preventing formation of mist arranged into connection with the turning roll.

Fig. 1 shows a twin-wire former which comprises a first wire loop 10 which functions as a covering wire and a second wire loop 20 which functions as a carrying wire. The travel of the covering wire 10 is controlled by a forming roll 11, a turning roll 12 and guide rolls 13. The travel of the carrying wire 20 is controlled by a breast roll 21, a transfer suction roll 22 and guide rolls 25.

Pulp suspension is fed from a headbox 9 to a forming gap G which is defined by the covering wire 10 travelling over the forming roll 11 and the carrying wire 20 travelling from the breast roll 21 to the forming roll 11. In the twin- wire zone following the forming gap G, water is removed from pulp suspension through the wires 10 and 20 by means of dewatering elements of which only the forming roll 11 and the transfer suction roll 22 are shown in Fig. 1. At the end of the twin- wire zone, the wires 10, 20 and the web W formed between them travel together over a curved sector a of the transfer suction roll 22, after which the covering wire 10 is directed separate from the web W by means of the turning roll 12. The web W continues its travel carried by the carrying wire 20 to a pick-up point P in which it, by means of a pick-up roll 31, is transferred to a pick-up fabric 30 which takes the web W further to the press section of the paper machine.

The transfer suction roll 22 comprises a perforated shell inside of which there are two successive negative-pressure zones 23 and 24. To the negative-pressure zones 23 and 24, negative pressures P ₁ and p ₂ different from each other are arranged so that a higher negative pressure prevails in the latter negative-pressure zone 24 than in the former negative pressure zone 23, that is, p ₂ > p _1# The function of the latter negative-pressure zone 24 is especially to ensure that the web W is retained supported by the carrying wire 20 when the twin- wire zone ends.

During web formation, free spaces in the fabric structure of the covering wire 10 are filled with water and suspended solids originated from pulp suspension. When the wire 10 saturated with water after the twin-wire zone bends with a great speed and at a large angle around the turning roll 12, the consequence is the throwing of water out of the wire 10, which appears as mist 15 and splashes. If formation of mist on the turning roll 12 is not prevented, impurities included in mist 15 are splashed to pick-up devices and trim squirts causing them to become dirty.

Figs. 1-3 show a first embodiment of the invention in which above the transfer suction roll 22 is positioned an apparatus preventing formation of mist 40 which comprises a blow nozzle 41, a collecting chamber 42, which is connected via a draining channel 44 to a source of negative pressure, and a first and a second guide wall 43, 45. The blow nozzle 41 is arranged to blow pressurised air towards the surface of the covering wire 10 at a small angle α in relation to the travel direction of the wire 10. The air jet hits the wire advantageously at an angle of 45- 90°. The jet of pressurised air scrapes water and suspended solids from the surface of the wire 10 at the same time as a pressure impact created by it forces part of water and suspended solids to be transferred through the wire 10 to the web W below. The air jet also presses the web W more tightly against the surface of the transfer suction roll 22.

The blow nozzle 41 and the jet of pressurised air stop a boundary-layer flow 16 of air travelling along with the wire 10. The guide wall 45 directs the air flow 16 and

mist 17 drained from the wire 10 to the collecting chamber 42 which is connected via the draining channel 44 to a source of negative pressure. A centrifugal blower can function as the source of negative pressure by means of which a low negative pressure p ₃ is created to the collecting chamber 42. Negative pressure p ₃ prevailing in the collecting chamber 42 is considerably lower than negative pressures P ₁ and p ₂ used in two negative-pressure zones 23, 24 of the transfer suction roll 20. Typically negative pressure p ₃ is 0.05-1.0 kPa.

The blow nozzle 41 is advantageously arranged at the latter negative-pressure zone 24 of the transfer suction roll 22 so that the air jet hits the wire 10 just before the point in which the covering wire 10 separates from the web W. Thus, blowing provides a pressure pulse which facilitates the separation of the wire 10 from the web W at the same time as it promotes dewatering via the negative-pressure zone

24. However, the intent is not to blow the air totally through the layer structure formed by the covering wire 10, the web W and the carrying wire 20, but to have part of blown air to be retained inside the covering wire 10 from which it is directed to the collecting chamber 42. Because of this, the method is also suitable for manufacturing thick grades of paper and board and for high run speeds of the paper machine.

Instead of the transfer suction roll 22, there can be a transfer suction shoe provided with a curved surface. Instead of the blow nozzle 41, there can be a blow box which blows air with a low positive pressure towards the covering wire 10.

Fig. 4 illustrates factors affecting the creation of positive pressure p ₊ in the closing nip 14 between the covering wire 10 and the turning roll 12. The surfaces of the covering wire 10 and the turning roll 12 moving to the same direction convey border air flows along with them. These air flows are transported along with the surfaces to the nip 14 in which positive pressure p ₊ is created. From the nip 14, this additional air is able to exit mostly through the wire 10. The air flow penetrating the wire 10 pushes water and dirt particles in front of it, whereby mist

15 is created which spreads to a large area to the surroundings of the turning roll 12.

Fig. 5 shows a second apparatus 50 preventing formation of mist working in the principle of the invention. A housing 50 is arranged between the covering wire 10 and the turning roll 12 which housing prevents air from being transported along with the wire 10 and the roll 12 to the closing nip 14 and through that creates a low negative pressure p ₄ in the nip 14. A first guide wall 51 of the housing 50 stops air layers travelling with the wire 10, possibly drains water from the surface of the wire 10 and leads possible splashes to a trough 53. A second guide wall 52 of the housing 16 stops air layers travelling along with the roll 12, scrapes water from the surface of the roll 12 and leads splashes to the trough 53. By means of the guide walls 51, 52, a low negative pressure p ₄ is created to the closing nip 14 which negative pressure promotes the retention of water in the wire 10 when it travels around the turning roll 12. Negative pressure p ₄ is advantageously 0.05-2.0 kPa.

The guide walls 51 and 52 advantageously extend to a small distance from the surface of the moving wire 10 and, correspondingly, the roll 12. Alternatively, the guide walls 51 and 52 can be replaced with blow nozzles which stop air flows following moving surfaces.

Mist prevention can also be implemented with many other ways which are not shown in detail in the figures. The creation of positive pressure to the nip 14 between the wire 10 and the turning roll 12 can be prevented by using a grooved roll as the turning roll together with a suction box with which negative pressure is provided to the grooves. A suction roll, which is provided with at least one negative pressure zone, can be used as the turning roll. On a straight run of the wire 10 from the transfer suction roll 22 to the turning roll 12, a suction device which removes water from the wire 10 can be arranged. Dewatering can be further promoted by blowing air through the wire 10 at the point of the suction device.

The effect of negative pressure can be extended to the whole width of the wire or it can be focused only on the edge area of the covering wire 10.

The claims will now be presented and, within the inventive idea defined by the claims, many variations of the invention are possible.