JPH0343396B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The subject matter of the invention is to eliminate positive and negative pressures detrimental to the supporting contact between these webs and the fabric in the pits or nips defined by the fabrics/webs in the press or drying section of a paper machine. It is a method intended to be used in the above departments to prevent the formation of

A further subject of the invention is an apparatus for applying the method of the invention, comprising a blow box associated with the nip so as to extend over substantially the entire width of the web. The device is connected to a means for generating a jet of gas and is further provided with one or several nozzle slots.

It is well known that a thin layer of air follows all planes of movement. Although no slippage occurs between the air and the moving surface, the air particles that come into contact with the moving surface travel at the same speed as that surface.

The pit defined by the fabric (felt or wire screen) and the cylinder surface (cylinder or roll) is called the entrance nip when the fabric enters this pit;
Also, when Fabrik leaves this pit, it is called an exit nip.

If all confining surfaces of the nip are air impermeable, air flow will occur between each confining layer at both the inlet nip and the outlet nip, with each direction of flow being opposite for each layer. becomes. There is therefore a positive pressure across the fabric at the inlet nip because the flow is dammed in the confining layer, and a negative pressure across the fabric at the outlet nip due to the suction effect of the flow in the confining layer. There is a reason.

When air-permeable fabrics, i.e. wire screens, are used in a manner known in the art, the flow in the confining layer allows the wire to
The pressure difference created across the screen creates a flow of air through the wire screen.

Some prior art pocket ventilation systems in paper machines are based on the pumping effect of open drying screens.

As is well known, the first and second drying groups of a paper machine are usually provided with so-called single fabric guide means, which means that in the upper cylinder the paper web is positioned intermediate the fabric and the cylinder. It is also often done in the lower cylinder as it is on the fabric.

Single fabric guidance systems are associated with several problems resulting from pressure differences on different sides of the fabric caused by flow in the confined layer. Air flows through the wire screen from the high pressure side to the low pressure side, thereby tending to disrupt the supportive contact between the fabric and the paper web. The location in question is the exit nip and the entrance nip. At both of these points, the pressure differential across the wire screen tends to pull the paper web away from the wire screen. After this separation effect occurs, the paper web begins to behave erratically due to the flow generated within the pocket. The spaced apart webs are often stretched in the lower cylinder by the effect of centrifugal force. The result is severe wrinkling at the entrance nip and, in the worst case, web breakage.

Various seals have been developed to overcome the problems described above, the purpose of which is to prevent flow in the boundary layer following surface movement from entering the nip. For example, a single seal structure for this purpose is suggested in DE-A-2712184.

Finnish Patent Application No. 771056 suggests a method for passing the web as a sealed guide in the drying section of a paper machine, in which the initial stage of the drying section, in particular the first group of drying cylinders, , a drying screen or fabric is used in which the cylinders in one row are outside the loops of the fabric, and the cylinders in the other row are inside, so that the web is not exposed to the drying screen or fabric provided with the fabric. From the first cylinder group to the last cylinder group, the cylinders are always supported by the same fabric, making turns from one cylinder row to another. In the above-mentioned application, in at least some cylinders in those parts of the cylinder where the web is outside the fabric, a pressure difference is applied to the web via the hollow surface of these cylinders, so that the pressure on the outside of the web is is higher than the pressure inside the cavity of this hollow surface,
This has been considered novel in that it is intended, inter alia, to prevent the web from separating from the fabric and to ensure consistent operation of the paper machine.

Finnish Patent No. 54954 suggests a method for reliably transporting the web from a press station to a drying station; a separate guiding drying cylinder which does not belong to the To remain in contact, either an upper drying belt of width approximately equal to the width of the web or a lower drying belt running over the web and supporting the web is placed over the upper cylinder. said upper drying belt is adapted to press said web against a lower drying belt and said upper drying belt is brought into contact with a guiding drying cylinder by a guide roll; Its main novelty lies in that the position is adjustable, at least when the paper machine is started and the web is transferred from the press station to the drying station.

Furthermore, regarding the current state of the technology related to this invention,
Reference is made to the applicant's Finnish patent application no. Some are placed outside the loops of the drying fabric; others are placed outside the loop of the drying fabric;
Others are placed on the inside so that the web is adapted to pass from one cylinder or roll to another, supported by said fabric over the entire distance, and is connected to a drying fabric. It has been suggested that, in associated operation, the device also includes one or more suction boxes operatively fitted to the drying fabric. In the above application, the one or more suction boxes are fitted so as to extend from one cylinder or roll to another over substantially the entire length of the integral run of the web and drying fabric. and the suction of this suction box is configured to extend at least on the inlet side of the drying fabric and the web into the wedge-shaped space between the drying fabric and the drying cylinder or roll sheath. It is believed to be new.

A limitation of prior art mechanical seals is that they cannot be brought sufficiently close to a moving web or fabric because of the risk of damage to the web and wear of the fabric. For example, it is not possible to seal the exit nip by mechanical means since the moving paper web requires a distance of at least 10 mm from the seal. At the inlet nip, it is theoretically possible to place the seal close to the fabric, but this would move the problem point forward of the seal, where the flow in the confining layer would be affected. A positive pressure caused by the weir pressure is now created.

The efficiency of a mechanical seal will further decrease over time if the seal is placed so close to a moving surface that wear occurs on the seal.

It is an object of this invention to provide a method and apparatus that prevents the formation of harmful positive or negative pressures in the associated nips. A further object is to provide an apparatus that is not sensitive to abrasion and does not cause damage to the paper web or fabric during movement.

In order to achieve these objects and the following objects, the method of the present invention provides for a blowing means placed in relation to said nip to provide a blower in the direction of travel of a web/fabric running in close proximity to said blowing means. limiting the pressure level in said nip so as to prevent or reduce said pressure differential by injecting air in the opposite direction to the direction of travel of the cylinder face and in some cases also to the direction of travel of the cylinder face; thing,
and that air is blown out from said pressurized nip by said air jet or from a space located facing said nip under negative pressure on the opposite side of the run of the web and fabric. It is a characteristic.

On the other hand, the device of the present invention has a curved nozzle face associated with the nozzle slot, the angle of curvature of which is about 30 to 70 degrees, and the air ejected from the nozzle slot by the nozzle face is Its main characteristics are that the direction of the jet is rotated by the so-called Coanda effect, and that there is a substantially planar surface downstream of the curved nozzle surface which acts as the support surface.

In the following, the invention will be explained in detail with reference to some exemplary embodiments of the invention shown in the drawings, but the invention is not limited to the details of these embodiments.

As shown in FIG. 1, the fabric/web combination W/F comes to the cylinder and thereby the cylinder surface and said combination W/F have between them the flows T+ and K+ in the boundary layer.
This effect limits the narrowing of the pit. The positive pressure thus created in the pit N+ becomes discharged as a flow _F1 .

As shown in FIG. 2, the fabric/web combination W/F exiting the cylinder creates a boundary layer flow K-, which together with the boundary layer flow T- of cylinder 1 creates a negative pressure in the nip N-. generate,
This negative pressure becomes charged as flow F ₂ .

In the situation shown in Figures 1 and 2,
Fabric F is air impermeable. On the other hand, Figures 3 and 4 illustrate a situation where an air permeable fabric or wire screen is used. However, in that case, the mechanism of formation of nip N+ with positive pressure is similar to that of FIG.
The positive pressure at + has a tendency to be discharged as flow F ₃ through the fabric/web combination W/F. In a corresponding manner, as shown in FIG. 4, a flow passing through the fabric and web W/F is generated in the nip N ₁ with negative pressure, and these flows are indicated by the arrow F ₄ in FIG. Illustrated in

Figure 5 shows one of the multi-cylinder drying plants.
A section is shown in which the web W is supported by the fabric F and the upper cylinder 1
and the lower cylinder 2. In that case, a pit or nip N- ₁ , with a negative and positive pressure associated with the contact between the free surface of the cylinders 1, 2 and the web/fabric combination W/F;
N+ ₂ , N- ₃ , N+ ₄ , N- ₅ and N+ ₆ are generated alternately. As confirmed above, the most problematic points are nips N- ₁ , N+ ₂ and nip N
− ₅ and N+ ₆ , where the pressure difference across the fabric F
In the worst case, this leads to web breakage as identified above.

Figures 6, 7 and 8 suggest several possible solutions to the various problems discussed above.

As shown in FIG. 6, the multi-cylinder drying plant consists of a row of upper cylinders 13 and a row of lower cylinders 14. The upper cylinder 13 and the lower cylinder 14 are provided to interlock with each other. The wire screen 15 passes from the upper cylinder 13 to the next lower cylinder 14 and vice versa, making turns. On the upper cylinder the web W lies between the wire screen 15 and the cylinder 13, but on the lower cylinder 14 it lies on the wire screen 15.

The free surfaces of the drying cylinders 13 and 14, the wire screen 15 and the web W define a pit or nip N, within which a negative or positive pressure (N+ or N-) is created depending on the direction of movement. be done. In order to eliminate harmful positive or negative pressure, fume boxes 16, 17, 18 and 19 according to the invention are provided in association with the nip, by means of which the wire screen 15 (fabric) is removed. An air jet is blown in some manner to eliminate the deleterious pressure differential that tends to separate the web W.

One side of the blow box 17, which is provided with one nozzle slot 20, is sealed against the face of the cylinder 14 by a mechanical seal spatula 22 in the manner shown in FIG.

The air jet discharged from the nozzle slot 20 facing the web W or wire screen 15 is rotated by the Coanda effect such that the direction of the jet S ₁ is opposite to the direction of movement of the wire screen 15 or web W. However, this is an essential feature of the method and apparatus of the present invention.

The air jet _S2 directed away from the wire screen 15 exits from the nozzle slot 20a of the fume box 18, which is provided with two nozzle slots and is placed in the upper cylinder 13. It is turned so that it becomes parallel to the auxiliary plate 23 placed next to the auxiliary plate 23. The purpose of this auxiliary plate 23 is to increase the jetting effect of jets _S1 and _S2 .

The pressure level in the nip N+; N- is varied by the ejection effect of the air jet _S1 discharged from the nozzle slot 20; 20a, 20b. The desired pressure level within the nip is obtained by adjusting the amount of air within the fume box. As an example, experimental measurement results are shown. Nip was originally 2~
It had a positive pressure of 5Pa. By using the fumarole boxes 16, 17, and 18 according to this invention, a negative pressure of up to 700 Pa can be obtained in the nip. Fumarole box 16;1
7; By changing the amount of air in 18,
Nip N+; N- can have any pressure between the pressure levels mentioned above.

As confirmed above, the fumarole box 16,1
7, 18, and 19 are nip N where the above pressure difference tends to separate the web W from the fabric 15.
+: Installed in relation to N-. Fumarole boxes 17, 18 installed in connection with the nip N+ with positive pressure discharge their air from the nip with positive pressure.
S ₁ and S ₂ blow out air, and according to the embodiment described above, the fume boxes 16 and 19 are connected to the nip N- on the opposite side of the web W and the wire screen 15 from the nip N-. The associated air jet S ₁ generates a vacuum effect in a self-evident manner, by which the negative pressure existing at this point on the opposite side of the wire and web to the nip N- is compensated and no longer harmful. Such a large pressure difference cannot be formed. Associated with the fumarole boxes 16 and 19 is an auxiliary plate 23 which is connected to the wire screen 15 on one side.
The space of the box is defined by the common running of the web W and the web W. Since the air jets S ₁ and S ₂ eject air from their spaces, the pressure developed in them is approximately equal to the negative pressure existing on the opposite side of the web and wire screen in the nip N-. . Furthermore, the effect of the flow S ₁ is that the nozzle slot 20;
A negative pressure is generated on the foil surface after a, which causes the web W and the wire screen to assume a certain equilibrium position.

7 and 8 are cross-sectional views of two exemplary embodiments of a fumarole box applicable to the present invention. The illustrated fume boxes 117 and 118 extend the entire width of the web, and air is introduced into the boxes either from both ends of the boxes or only from one end.

As shown in FIG. 7, fumarole box 118 includes two nozzle slots 120a and 120b.
The fumarole box 118 is symmetrical with respect to the center plane in the vertical direction. Nozzle slot 120 as described above
a and 120b are defined between the auxiliary plate 123 and the curved walls 101a and 101b of the fume box. Nozzle slot 120a, 120b
After the opening, the walls 101a and 101b are curved, for example, in the shape of an arc R at an angle α=approximately 30 to 70°, and after that, that is, after point K, the walls 101a and 101b are flat. 102a;
102b, which are connected to each other by walls 103. Due to the Coanda effect, the air jets ejected from the nozzle slots 120a, 120b bend the curved wall 101 up to an angle α=approximately 70°.
a; Follows 101b.

As shown in FIG. 8, the fume box 117 includes one nozzle slot 120, which
04 and the curved wall 101. After the opening of the nozzle slot 120 there is a curved wall 101, the radius of curvature of which is R and the angle α = approximately 30-70°, after which a plane wall 102 starts from the point K. There is.
Further, the fumarole box 117 has flat walls 105 and 106
Contains.

The method of the invention does not involve the problem of wire screen or web friction and therefore does not cause wear, due to the fact that the nozzle used in the invention is a so-called foil nozzle; The fact that the foil nozzle is characterized in that the surface which is placed against the support surface (in the present invention a wire screen or web) occupies a so-called equilibrium position with respect to the support surface due to the flow of air to be ejected. This is due to There is always a gap of a few millimeters between the bearing surface of the nozzle and the wire screen or web. The nozzle face is so shaped that the jet is rotated at least approximately 30° from its initial orientation, so that the portion of the nozzle carrying face that rests against the wire screen or web has no sharp edges. It has no edges and is curved.

In the claims appended hereto, various details of the invention may be subject to various changes within the scope of the inventive concept as defined by these claims.

[Brief explanation of drawings]

FIG. 1 shows the formation of positive pressure within the pit defined by the cylinder surface and the web/fabric combination. FIG. 2 shows, in a manner corresponding to FIG. 1, the formation of negative pressure in the associated pit. FIG. 3 shows the flow of air through the wire screen created by positive pressure in the case where the web/fabric combination is air permeable. FIG. 4 shows the flow of air through the wire screen in a manner corresponding to FIG. 3 in the case where the web/fabric combination is air permeable. FIG. 5 shows a portion of a multi-cylinder drying plant provided with a single fabric guiding means to indicate the location of the nip with positive and negative pressures. FIG. 6 is a schematic illustration of a section of a multi-cylinder drying plant provided with a single fabric guide means for which the method of the present invention is applied to adjust the nip pressure level; Indicates what is applied. FIG. 7 is an enlarged view of a fumarole box provided with two nozzle slots. FIG. 8 is an enlarged view of a fumarole box provided with one nozzle slot. Explanation of symbols, 1, 2, 13, 14...Cylinder; 15...Fabric; 16, 17, 18, 1
9... Fumarole box; 20, 20a, 20b, 120,
120a, 120b...nozzle slot; 23
...Auxiliary wall, 101, 101a, 101b...
Wall; 117, 118... Fumarole box; W... Web;
F ₁ , F ₂ , F ₃ ...flow.

Claims (1)

[Claims] 1. In the press or drying section of a paper machine, for supporting contact between the web W and the fabric 15 at pits or nips N+, N- defined by the fabric/web and the cylinder surface. A method for preventing the formation of harmful positive and negative pressures, comprising blowing means 16, 17, 18, 19 provided in connection with the nips N+, N-.
to blow out air in a direction opposite to the running direction of the web/fabric W, 15 passing near the blowing means and in a direction that may also be opposite to the running direction of the cylinder surface. The pressure levels of the nips N+ and N- are affected by the above-mentioned nip N+ and N- in such a way that the difference between the two pressures is eliminated or reduced, and the air is discharged from the nip N+ by the above-mentioned jet of air, or A method characterized in that air is discharged from a space facing the nip N-, sandwiching the running path of the web and fabric W, 15. 2. The blowing means 16, 17, 18, 19 act on the side surface of the fabric 15 that supports the web W in conjunction with the nip that creates a pressure difference that tends to separate the web W from the fabric 15. The method according to claim 1, characterized in that the method is arranged to do so. 3. If the method is applied to a nip N+ with positive pressure, a blowing means 18 having nozzle slots on both sides is used, and the blowing air from the nozzle slot 20a on one of the two sides is It is oriented parallel to the fabric 15 with respect to its direction of movement, and the jet S ₃ from the other nozzle slot 20b is parallel to the plane of the cylinder 14 that defines the nip N+, and 3. Method according to claim 1, characterized in that it is oriented in a direction opposite to the direction of movement. 4. If the method is applied to a nip N+ with positive pressure, it is noted that the wedge-shaped space of this nip is limited by a sealing spatula 22 directed towards the face of the adjacent cylinder 14. A method according to claim 1 or 2, characterized in that: 5. If the method is applied to a nip N- with negative pressure, the space defined by the blowing means 16, 19 as well as the particular auxiliary wall 23 is in its interior under negative pressure by the blowing means. said web W on which pressure is generated and which limits said nip N-;
and the nip N- of the fabric 15.
3. A method according to claim 1, characterized in that the method is arranged on the side opposite to the . 6 In the blowing means 16, the auxiliary wall 2
6. A method according to claim 5, characterized in that an air jet _S1 is used in such a way that air is discharged from the space limited by 3 on one side only. 7 In the blowing means 19, air jets S ₁ and S ₂ are used on both sides, and one air jet
S ₁ is oriented from one side of the blowing means in relation to the fabric 15 and opposite to the direction of movement of this fabric, and the other air jet S ₂ is oriented from the other side of the blowing means, 6. A method as claimed in claim 5, characterized in that this air jet is also directed in such a way that air is discharged from the space defined by the auxiliary wall. 8. So-called foil nozzles are used in the blowing means 16, 17, 18, 19,
These foil nozzles are characterized by an adjacent wire screen 15 or a supporting surface mounted in connection with the running of the web and by an air stream ejected in conjunction with said supporting surface. Claims 1 to 7 are characterized in that a pressure effect is achieved which fixes the running web or wire in a certain equilibrium position relative to the support surface. Any method described. 9 In the press or drying section of a paper machine, the formation of positive and negative pressures which are detrimental to the supporting contact between the web and the fabric in the pits or nips defined by the fabric/web and the cylinder surface shall be avoided. In order to prevent this, one or several nozzle slots 120 are arranged in relation to said nips N+, N- extending over substantially the entire width of the web and connected to the means for generating a jet of air. Fumarole box 1 having 120a and 120b
17,118, wherein the nozzle
Associated with the slots 120; 120a, 120b are curved walls 101; The jet stream of air ejected from the nozzle slot is swirled by the so-called Coanda effect, and the curved wall 101;
After a, 101b is a substantially flat wall 102; 10
2a, 102b, the flat wall surface of which is adapted to act as a support surface. 10 The fumarole box 118 is substantially symmetrical about its center plane, and it includes two nozzle slots 120a, 120b ejecting air in opposite directions, which slots 10. Device according to claim 9, characterized in that it is defined by a particular, preferably curved auxiliary plate on the side facing the walls 101a, 101b.