WO2011088888A2

WO2011088888A2 - Low energy head box

Info

Publication number: WO2011088888A2
Application number: PCT/EP2010/050572
Authority: WO
Inventors: Janne Lappi; Sami Anttilainen
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2010-01-19
Filing date: 2010-01-19
Publication date: 2011-07-28
Anticipated expiration: 2012-07-19
Also published as: WO2011088888A3

Abstract

There is described a head box for a paper machine or the like, wherein a nozzle (1) is adapted to apply a web-forming, fiber- containing pulp to a wire arrangement. The head box comprises a header (3) to which said pulp is supplied, a tube bank (4) including a plurality of tubes arranged side by side in the cross machine direction and connected to the header (3). Flow control elements (5) are arranged in the tube bank (4) for controlling flow passing through the individual tubes of the tube bank (4). Downstream of the tube bank (4) is a turbulence generator (2) which connects the tube bank (4) to the nozzle (1). The turbulence generator (2) has pulp channels (231-235) having a cross-sectional shape which changes gradually between inlet and outlet of said turbulence generator (2). Preferably, the total internal wall surface as defined by the channel axial length and internal circumference length is set to generate a turbulent boundary layer of the pulp flow causing breaking of fiber flocks in the pulp so that e.g. the passage of each channel may be of stepless and aerodynamic shape.

Description

LOW ENERGY HEAD BOX DESCRIPTION

The invention relates to a head box for a paper machine or the like, and in particular to a low energy head box having low energy consumption. A head box is the element in a fibrous web machine, like a paper machine, with which the pulp is introduced into the web forming part, i.e. the former of a fibrous web machine. Various types of formers are known; the basic principle is application of a fiber containing pulp with high water content on a wire mesh material through which the water flows out while the fibers are held on the wire mesh material to form the initial fibrous web. A head box usually comprises a nozzle with a slit shaped opening extending over the machine width in machine cross

direction, which nozzle is adapted to apply the web-forming and fiber-containing pulp evenly in machine cross direction (CD-direction) to the wire arrangement on which the initial web is formed. In order to obtain a uniform fibrous web, it is important to obtain an even or uniform distribution of the fibers over the wire arrangement. Thus, the head box evenly distributes the pulp in machine cross direction over the wire arrangement by suitable adjustment of the flow from the slit shaped nozzle. In order to obtain homogenous paper in machine cross direction, it is required to provide for a uniform fiber distribution in the pulp itself, which pulp is then evenly distributed on the wire. For improvement of material (fiber) distribution in the pulp, turbulence is generated in the pulp before it flows out from the nozzle, so as to break up fiber flocks which may occur in the pulp and to generate mixing flows in the pulp. Besides, the pressure loss of the turbulence generator is used to level out local pressure differences in the pulp in order to improve the uniformity of the pulp pressure along the nozzle in CD-direction.

In a known head box as is described in WO 01/96658 Al, the turbulence generator has tubes or channels which have a step in their inner diameter forming a sudden and sharp widening of diameter in the channels through which the pulp flows from an inlet manifold of the head box to the nozzle. In EP 1 033 436 Al a channel design is shown in which design the diameter of the channel increases in several steps .

Both designs of the channels form a sudden expansion which generates considerable turbulence, the vortices of which break up flocks of fibers in the pulp. This breaking up of flocks contributes to avoiding streaks in the paper web. On the other hand, the generated pressure loss homogenizes the pressure distribution in the pulp at the nozzle.

The inventors have studied the present design of known head boxes as described above. It was found that the turbulence vortices contribute to a reliable breaking up of fiber flocks. However, it was also found that the generation of vortices in the sudden expansion causes a pressure loss, which larger than required for improving the pressure distribution in the pulp. The pressure loss has a

considerable impact on the energy balance of the paper machine . Starting from the above findings, a head box with an acceptable pulp distribution performance and an improved energy balance shall be suggested. This result is achieved with a head box having the features of claim 1.

According to the invention, a head box is provided with a turbulence generator, the pulp channels of which have a cross-sectional shape which changes gradually. With the gradual change of the channels' internal shape, the

pressure loss at the sudden expansion is avoided. On the other hand, it was found that at a sufficient pulp

velocity, which is given in actual paper machines, and under the condition that the pulp is at rest at the channel walls, the flow of the pulp generates considerable shearing forces in the pulp. It was found that these shearing forces provide sufficient breaking up of fiber flocks. The invention brings a considerable reduction of the pressure loss at the head box as compared to a typical known machine, while maintaining the quality of the

manufactured fibrous web. Bearing in mind that such paper or board machines are designed to run almost permanently throughout the year, the invention allows considerable savings in energy consumption, which not only reduce the manufacturing cost of the paper or board, but also

contributes to protecting the environment. The invention thus suggests a head box for a paper machine or the like, which head box comprises a nozzle which is adapted to apply a web-forming, fiber-containing pulp to a wire arrangement, a header to which said pulp is supplied, a tube bank including a plurality of tubes arranged side by side in the cross machine direction and connected to said header and flow control elements arranged in the tube bank for controlling flow passing through the individual tubes of the tube bank. The turbulence generator connects the tube bank to the nozzle, wherein the turbulence generator has pulp channels with the above described internal shape, i.e. having a cross-sectional shape which changes gradually between inlet and outlet of said turbulence generator.

Preferably, the total internal wall surface as defined by the channel axial length and internal circumference length is set to generate a turbulent boundary layer of the pulp flow causing breaking of fiber flocks in the pulp.

Also preferably, the cross-sectional shape of the channels of the turbulence generator changes from a circular shape at the tube bank side end to a rectangular shape at the nozzle side end.

Further, the passage of each channel may be of stepless or smooth and aerodynamic shape. Preferably, the nozzle is a slice lip nozzle. Also preferably, at least one turbulence vane is arranged at the nozzle end side of each channel, said turbulence vane dividing the pulp flow in the nozzle in a direction crossing the flow direction of the pulp to the wire

arrangement .

Preferably, the slice lip may be locally adjustable towards and away from the nozzle opening for adjusting the base weight profile of the pulp flow in machine cross direction. Preferably, each flow control element may be adapted to have a maximum control range of 4- 4 % of the main flow.

Preferably, the flow control element has an aerodynamic design with regard to the flow which passes through the control element.

Also preferably, the pressure loss of the pulp in the head box is 1 to 25 kPa from the header to the nozzle.

The invention will now be explained in more detail by way of an embodiment using the drawings for illustration purpose. In the drawings:

Fig. 1 shows sectional view as seen in CD-direction of an embodiment of a head box according to the invention in which a turbulence generator is installed with a slice nozzle; and

Fig. 2 shows a turbulence generator similar to that of Fig. 1 having several pulp channels in detail.

Fig. 1 shows an embodiment of a head box for a paper machine to which the invention is applied. In Fig. 1 the head box is shown diagrammatically for the sake of clarity. The head box of Fig. 1 is a head box type useable in paper machines which produce paper from pulp which contains large part of recycled paper.

The nozzle 1 of the head box has a slit-shaped opening 11 which is defined by a lower lip 14 and an adjustable upper lip or slice lip 12. The position of the slice lip 12 is locally adjustable to adjust the CD-profile of the pulp flow from the opening 11 of the nozzle 1.

The nozzle 1 is a multilayer nozzle which has a turbulence vane 13 inside the nozzle downstream of a turbulence generator 2 which will described in detail with reference being made to Fig. 2. It is noted that the turbulence generator shown in Fig. 1 has two pipes for the pulp flow stacked in vertical direction in Fig. 1 while the

turbulence generator 2 of Fig. 2 has five pipes stacked in that vertical direction in that drawing figure. It is noted that there are plural of such stacks of pipes arranged next to each other along the width of the nozzle in machine cross direction (CD-direction) and form an array of pipes. In fact a turbulence generator may have several hundred pipes arranged in the array extending over the width of the nozzle in CD-direction.

The turbulence vane 13 divides the inside volume or nozzle chamber 15 of the nozzle 1, while the upstream edge of the turbulence vane 13 is connected to or at least arranged at the outlet 22 of the turbulence generator. The pulp flow exiting the turbulence generator 2 is divided (or more precisely: maintained divided) into separate layers of pulp flow. Here, in Fig. 1, only a single turbulence vane 13 is shown, but multiple layer nozzles with two or more, even up to seven, vanes are known. Separation into layers may not only be applied if a multi-layered product (web) having layers of different materials shall be made, uniformity of the pulp and the pulp flow may be improved by using the separation technology even if the pulp is the same for all layers. The turbulence vane 13 forms a wall along which the pulp flow has to pass. With the condition that the pulp must be at rest at the wall, a turbulent boundary layer is formed which generates turbulence in the pulp flow

contributing to a mixing and flock-breaking effect inside the nozzle.

The turbulence generator 2 is with its upstream end or inlet 21 connected to a tube bank 4. The tube bank is an row of tubes 41 which are arranged to connect the inlet 21 of the turbulence generator 2 to a header 3 which supplies the pulp to the nozzle 1. As is shown in Fig. 1, the header 3 comprises a tube of large diameter from which the tubes 41 of the tube bank 4 branch off. In the course of each tube 41 of the tube bank 4, there is provided a flow adjuster 5 with which the pulp flow through the individual tube can be adjusted. It is noted that the flow adjuster 5 is not a valve in the classic sense (providing an on/off- function) but it was found sufficient if the flow adjuster 5 is capable to change the flow within a range of 4- 4%. In some solutions it may even be sufficient if the adjustment range allows to change the flow in a range of +_ 2,5%. With such adjustment range requirements, it is possible to select design which penetrate into or disrupt the pulp flow only to a minimum extend (only small constrictions of the flow path are required) , so that an overall design is possible which generates only a low pressure loss at the tube bank. Also, designs can be implemented which avoid agglomeration or clogging of fibers due to shapes which do not cause severe deviations of the pulp flow in the tube bank .

Fig. 2 shows a turbulence generator 2 which is similarly structured as that of Fig. 1 in more detail. In Fig. 2, one stack of pipes forming the pulp channels as defined in the claims is shown, which stack is only a single layer or row of vertically stacked pipes. It is noted that a plurality of such stacks is arranged side by side in a direction perpendicular to drawing plane of Fig. 2 i.e. in the CD- direction (cross machine direction) of the paper machine to form an array of several tenths (or hundereds) of pipes X five pipes. The individual pipes 231 to 235 are stacked one on top of the other and they are held in position by means of a holder 61, 62 having suitable holes for receiving the pipes. Each pipe 231 to 235 has an inlet 211 to 215 which is of substantially circular cross-sectional shape. On the outlet end of the pipes 221 to 225 the circular shape of the cross-section of the pipes changes to a rectangular or square shape, so that the individual pipe outlets 221 to 225 each have a rectangular or square cross-section. It is noted that there are no sharp edges or bends involved in this change of shape.

Similar to the structure shown in Fig. 1, a slice or nozzle chamber may be is arranged downstream the turbulence generator of Fig. 2 as well (not shown) . According to the description of Fig. 1 turbulence vanes (here four vanes, each of them being arranged between two outlet openings) may be provided (not shown) . From Fig. 2 it becomes clear that each individual pipe 231 to 235 has no step or steps when changing its cross- sectional shape over its length. There is a certain bend of the pipes for properly directing the individual flows of pulp into the slice or nozzle chamber (not shown) . In particular, the central pipe 233 is straight, while the adjacent pipes 232 and 234 are slightly bent towards the outlet 223 of the central pipe 233. similarly to outer pipes 231 and 235 are bent slightly more towards the central pipe 233 to bring the outlets 221 and 225 closer to the outlet 223 of the central pipe 233. It is noted that these bends are not sharp, so as to not generate edges inside the pipes.

The diameters of the pipes are set such that there is a trade-off between the pressure loss in the so formed pulp channels on the one hand, and, on the other hand,

sufficient fluid friction is generated at the channel walls to have sufficient shearing force in the pulp for reliably breaking-up of fiber flocks.

In a calculated model of a head box, the pressure loss of the pulp in the head box is 1 to 25 kPa from the header to the nozzle. In a comparison between the head box of the invention and a typical head box which is nowadays commonly used in a typical paper machine it was found that there is significant energy saving potential of about 25% which may make up an annual energy saving of about 1000000 kWh.

Claims

1. A head box for a paper machine or the like, said head box comprising a nozzle which is adapted to apply a web- forming, fiber-containing pulp to a wire arrangement, the head box comprising

a header to which said pulp is supplied;

a tube bank including a plurality of tubes arranged side by side in the cross machine direction and connected to said header;

flow control elements arranged in the tube bank for controlling flow passing through the individual tubes of the tube bank, and

a turbulence generator connecting the tube bank to the nozzle, wherein the turbulence generator has pulp channels having a cross-sectional shape which changes gradually between inlet and outlet of said turbulence generator.

2. A head box according to claim 1, wherein the total internal wall surface as defined by the channel axial length and internal circumference length is set to generate a turbulent boundary layer of the pulp flow causing

breaking of fiber flocks in the pulp.

3. A head box according to claim 1 or 2, wherein the cross- sectional shape of the channels of the turbulence generator changes from a circular shape at the tube bank side end to a rectangular shape at the nozzle side end.

4. A head box according to claims 1, 2 or 3, wherein the passage of each channel is of stepless and aerodynamic shape .

5. A head box according to claims 1, 2, 3 or 4, wherein the nozzle is a slice lip nozzle.

6. A head box according to claims 1, 2 or 3, wherein at least one turbulence vane is arranged at the nozzle end side of each channel, said turbulence vane dividing the pulp flow in the nozzle in a direction crossing the flow direction of the pulp to the wire arrangement.

7. A head box according to claim 5, wherein the slice lip is locally adjustable towards and away from the nozzle opening for adjusting the base weight profile of the pulp jet in machine cross direction.

8. A head box according to any of claims 1, 2, 3 or 4, wherein each flow control element is adapted to have a maximum control range of 4- 4 % of the main flow.

9. A head box according to claim 7, wherein the flow control element has an aerodynamic design with regard to the flow which passes through the control element.

10. A head box according to any one of the preceding claims 1 to 8, wherein the pressure loss of the pulp in the head box is 1 to 25 kPa from the header to the nozzle.