APPROACH FLOW SYSTEM

Abstract

The invention relates to an arrangement for feeding a fibrous suspension (1) to at least one headbox (8) of a paper or cardboard machine with at least one diluting device (5), in which the fibrous suspension (1) is produced by mixing a high-consistency suspension (6) with a diluting liquid, wherein the diluting liquid is formed at least in part by backwater (2) of the paper or cardboard machine, the backwater (2) is pre-degassed in at least one open channel (3) and subsequently is guided into a diluting device (5) via a degassing device (4). The expenditure is to be reduced thereby in that the potential energy of the fibrous suspension (1) in the channel (3) is not above the potential energy of the fibrous suspension (1) in the diluting device (5) and the pressure at the outflow of the degassing device (4) is between −0.3 and 1 bar or that the potential energy of the fibrous suspension (1) in the channel (3) is above the potential energy of the fibrous suspension (1) in the diluting device (5).

Description

The invention relates to an arrangement for feeding a fibrous suspension to at least one headbox of a paper or cardboard machine with at least one diluting device, in which the fibrous suspension is produced by mixing a high-consistency suspension with a diluting liquid, wherein the diluting liquid is formed at least in part by backwater of the paper or cardboard machine, the backwater is pre-degassed in at least one open channel and subsequently is guided into a diluting device via a degassing device.

Methods of this type are used to supply paper or cardboard machines with fibrous suspension. The stock feeding systems required thereto are known in principle. The fibrous suspension to be supplied receives the major part of the fibers from a high-consistency suspension provided in the stock preparation unit. The high-consistency suspension has, e.g., a typical consistency of between 2.5 and 5%. By the addition of a diluting liquid, in particular backwater of the paper machine, the consistency is lowered to a value that is favorable for operating the headbox of the paper machine. Although the backwater is optimally suited for this diluting task, there are, however, problems due to the high gas content, the predominant part being air.

The largest proportion of these gases escapes very quickly, however, the residual gases often have to be removed in a complex manner. Otherwise, the quality of the paper produced would be unduly lowered.

Known solutions are large degassing containers in which a permanent negative pressure is maintained by evacuation, which negative pressure corresponds to the steam pressure of the suspension to be degassed.

In DE 10 2004 051 327 an arrangement is therefore proposed that is similar to the preamble of claim 1.

The object of the invention is to reduce the expenditure in arrangements of this kind.

According to the invention, the object was attained in that the potential energy of the fibrous suspension in the channel is not above the potential energy of the fibrous suspension in the diluting device and the pressure at the outflow of the degassing device is between −0.3 and 1 bar or that the potential energy of the fibrous suspension in the channel is above the potential energy of the fibrous suspension in the diluting device.

If the potential energy of the fibrous suspension in the channel is higher than in the diluting device, a pump between the assemblies can be omitted, which considerably reduces the expenditure. The difference of the potential energy thereby essentially determines the throughput of fibrous suspension. If the difference of the potential energy is too great, an overpressure must be reduced via throttles or the degassing device, which is not advantageous in terms of energy.

The difference of the potential energy of the fibrous suspension in the channel and the diluting device should therefore correspond to no more than an equivalent of 8 m difference in height.

If the potential energy of the fibrous suspension in the channel is not above that in the diluting device, the degassing device must be arranged and/or designed such that at the outflow thereof a pressure between −0.3 and 1 bar is adjusted.

The height difference between the channel and the degassing device, but also the pressure build-up by the degassing device due to a pump action of the same have a decisive influence.

If a pressure of less than −0.3 bar is adjusted at the outflow of the degassing device, a small pump should be utilized between the degassing device and the diluting device.

Regardless of the embodiment, the pressure at the outflow of the degassing device should advantageously be higher than the pressure at the inflow of the diluting device, so that the pressure difference ensures the transport of the fibrous suspension between these elements.

To this end it is advantageous if the outflow of the degassing device is arranged above the inflow of the backwater into the diluting device. In this manner the flow of the backwater from the degassing device to the diluting device can take place entirely without conveyor elements.

In any case, the pressure at the outflow of the degassing device should be no more than 0.5 bar less than the pressure at the inflow of the diluting device. In this case, a small pump between the degassing device and the diluting device can take over the transport of the fibrous suspension.

For adjustment to the conditions of the installation, but also for optimization, the flow of the fibrous suspension between the channel and the degassing device and/or between the degassing device and the diluting device should preferably be restricted in a changeable manner.

Moreover, it is advantageous if the inflow of the degassing device is arranged above the channel.

In the interest of a degassing that is as efficient as possible, the degassing device should be connected to a vacuum source. Degassing devices with a rotating rotor are particularly suitable, in the interior of which the backwater is guided, wherein the gas collecting in the center due to centrifugal forces is suctioned by the vacuum source.

The diluting device can be embodied as a closed or as an open container.

After the diluting device, the fibrous suspension should be guided to the headbox via a stock pump if possible in a closed system.

In the event of the standstill of the degassing device, the liquid level in the degassing device falls to that of the diluting device. In order to generate the difference in level necessary for the transport of the backwater at the start of the degassing device, the stock pump should be started before start-up of the degassing device.

The method is to be used particularly favorably with paper machines, the screening rate of which is between 800 and 1600 m/min, since the quantity of the air contained in the backwater depends considerably on the screening rate.

The invention is explained in more detail below based on an exemplary embodiment. In the attached drawing, the figure shows a layout plan.

A fibrous suspension 1 is thereby generated in the usual manner by mixing a high-consistency suspension 6 with a diluting liquid and thereafter has essentially the desired consistency at this point for the operation of the headbox 8 of the paper or cardboard machine. As is known, headbox consistencies are in the range between 0.5 and 2%, generally around 1%.

The diluting liquid, which is fed in a diluting device 5, in the example shown here comes from the first backwater 2, that is, the water accumulated in the forming region 10 of the paper or cardboard machine. It is often referred to as backwater I, as opposed to the second backwater II accumulating on the paper machine later, which second backwater contains far fewer fines.

It is known that backwater 2 of this type is mixed among other things with a considerable proportion of air and possibly other gases. The backwater 2 is collected below the paper machine screen and drained off at the side. A large part of the air contained can already escape in the open channels 3 used for this. However, it is often expedient to eliminate a further gas proportion from the backwater 2 in a subsequent degassing device 4, as is described, for example, in DE 10 2004 051 327.

The degassing device 4 is constructed as a cylindrical or conical container and provided according to the invention with a rotor 13. The fibrous suspension 1 reaches the essentially cylindrical interior 14 of the rotor 13 and is put into rapid rotation. This principle corresponds to that of a solid bowl centrifuge. As a result of the centrifugal forces, the gases contained move inward and are suctioned off from the center by a vacuum source 7.

The centrifugal forces can be at least 5 times, preferably 10 times the acceleration of gravity. The vacuum source 7 requires no negative pressure that corresponds to or is similar to the steam pressure of the suspension. Values customary for the negative pressure here are approximately 0.8 to 0.9 bar.

The inlet 16 of the degassing device 4 can be arranged centrally or eccentrically. The liquid added through it is first greatly accelerated in the circumferential direction, for which, e.g., acceleration ribs 24 can be used. As a result of the centrifugal forces, the liquid bears against the interior wall of the rotor 13, wherein the gas contained therein, in particular the air, moves towards the center of the rotor 13. As a rule, a discontinuity surface forms between the liquid and the gas.

The gas can be suctioned off through a central degassing tube with the aid of the vacuum source 7.

In the area of the outflow 15, the wall of the rotor 13 is thereby embodied to be permeable to liquids, e.g., by placing openings in this area.

Advantageously, the outflow 15 is mounted tangentially such that the rotational flow of the degassed liquid causes a pressure build-up in the outflow 15. The openings in the wall of the rotor 13 are so big that they do not lead to a screening of the degassed liquid.

The degassing device 4 in addition can also be provided with an outer screen basket, via which the backwater 2 pressed radially outwards can be divided into accepts and rejects.

The degassed backwater 2 is guided via the outflow 15 of the degassing device 4 directly into the diluting device 5, which here is embodied as an open container. The degassing can be carried out more easily due to the far lower proportion of solids of the backwater 2 compared to the fibrous suspension 1. However, this requires at most a merely low gas content in the high-consistency suspension 6 added to the diluting device 5 in order to avoid losses in quality.

A mixing device described in DE 100 50 109 is particularly suitable as a diluting device 5. The important aspect here is that the intermixing of the flows takes place via an injection of the high-consistency suspension 6 into the backwater 2, wherein the inflow speed of the high-consistency suspension 6 is preferably 3 times to 15 times the flow speed of the backwater 2.

As is known, there are many paper or cardboard machines in which the headbox 8 is supplied not only with the fibrous suspension 1 already mentioned, but in addition with a diluting liquid which is added in a metered manner at various points seen over the width of the headbox 1. It is thus possible to influence, in particular optimize, e.g., the cross profile of the fibrous layer formed with the headbox 8.

The figure shows an option of this type, in which the backwater 2 is used not only as a diluting liquid for the high-consistency suspension 6. A part of the backwater 2 deaerated in the degassing device 4 is conveyed with the diluting water pump 20 as diluting liquid into a metering device 17 of the headbox 8, in which it is divided and the fibrous suspension 1 is added at different points.

In a paper mill there are often further points at which further diluting liquid deaerated by the method can be used.

A last screening stage 18 is often placed upstream of the headbox 8 when supplying a paper machine, in order to prevent contaminants from getting into the headbox which interfere with the operation of the headbox 8 or the paper machine.

This screening function can also be carried out in combination with the degassing function, as described above, in the same device.

After leaving the diluting device 5, the fibrous suspension 1 is guided via a stock pump 9 in a closed system (i.e., without any open containers or vats) via a cleaner unit 11, a further pump 12 and the screening step 18 to the headbox 8.

The potential energy of the fibrous suspension 1 in the channel 3 is here higher than the potential energy of the fibrous suspension 1 in the diluting device 5. As a result, pumps for conveying the backwater 2 can be omitted in this area, which has a considerable effect on costs. However, the energy difference should not be too great either, since the excess energy otherwise has to be reduced via adjustable throttle elements 21, 22 in the form of valves.

In order to be able to ensure a transport of the fibrous suspension 1 between the degassing device 4 and the diluting device 5 also without an additional pump, the pressure at the outflow 15 of the degassing device 4 is higher than the pressure at the inflow 19 of the diluting device 5. Moreover, the outflow 15 of the degassing device 4 is arranged above the inflow 19 of the backwater 2 in the diluting device 5.

At rest the liquid level in the degassing device 4 falls to the liquid level in the diluting device 5, which is why the level difference necessary for the transport of the backwater 2 must be reestablished before starting up the degassing device 4.

This can be achieved in that the liquid level in the diluting device 5 is reduced to the necessary amount with the stock pump 9 before starting up the degassing device 4.

Claims

1. An arrangement for feeding a fibrous suspension (1) to at least one headbox (8) of a paper or cardboard machine with at least one diluting device (5), in which the fibrous suspension (1) is produced by mixing a high-consistency suspension (6) with a diluting liquid, wherein the diluting liquid is formed at least in part by backwater (2) of the paper or cardboard machine, the backwater (2) is pre-degassed in at least one open channel (3) and subsequently is guided into a diluting device (5) via a degassing device (4), characterized in that the potential energy of the fibrous suspension (1) in the channel (3) is not above the potential energy of the fibrous suspension (1) in the diluting device (5) and the pressure at the outflow of the degassing device (4) is between −0.3 and 1 bar.

2. An arrangement for feeding a fibrous suspension (1) to at least one headbox (8) of a paper or cardboard machine with at least one diluting device (5), in which the fibrous suspension (1) is produced by mixing a high-consistency suspension (6) with a diluting liquid, wherein the diluting liquid is formed at least in part by backwater (2) of the paper or cardboard machine, the backwater (2) is pre-degassed in at least one open channel (3) and subsequently is guided into a diluting device (5) via a degassing device (4), characterized in that the potential energy of the fibrous suspension (1) in the channel (3) is above the potential energy of the fibrous suspension (1) in the diluting device (5).

3. The arrangement according to claim 2, characterized in that the difference of the potential energy of the fibrous suspension (1) in the channel (3) and the diluting device (5) corresponds to no more than an equivalent of 8 m difference in height.

4. The arrangement according to claim 1, characterized in that the pressure at the outflow of the degassing device (4) is higher than the pressure at the inflow (19) of the diluting device (5).

5. The arrangement according to claim 1, characterized in that the pressure at the outflow (15) of the degassing device (4) is no more than 0.5 bar less than the pressure at the inflow (19) of the diluting device (5) and a pump is arranged between the degassing device (4) and the diluting device (5).

6. The arrangement according to claim 1, characterized in that the fibrous suspension flow (1) between the channel (3) and the degassing device (4) is preferably restricted in a changeable manner.

7. The arrangement according to claim 1, characterized in that the fibrous suspension flow (1) between the degassing device (4) and the diluting device (5) is preferably restricted in a changeable manner.

8. The arrangement according to claim 1, characterized in that the inflow (16) of the degassing device (4) is arranged above the channel (3).

9. The arrangement according to claim 1, characterized in that the outflow (15) of the degassing device (4) is arranged above the inflow (19) of the backwater (2) into the diluting device (5).

10. The arrangement according to claim 1, characterized in that the degassing device (4) is connected to a vacuum source (7).

11. The arrangement according to claim 10, characterized in that the degassing device (4) has a rotating rotor (13), in the interior of which the backwater (2) is guided, wherein the gas collecting in the center due to centrifugal forces is suctioned by the vacuum source (7).

12. The arrangement according to claim 1, characterized in that the diluting device (5) is embodied as a closed container.

13. The arrangement according to claim 1, characterized in that the diluting device (5) is embodied as an open container.

14. The arrangement according to claim 1, characterized in that after the diluting device (5), the fibrous suspension (1) is guided via a stock pump (9).

15. The arrangement according to claim 14, characterized in that the stock pump (9) guides the fibrous suspension (1) to the headbox 8 in a closed system.

16. A method for starting up an arrangement according to claim 14, characterized in that the stock pump (9) is started before start-up of the degassing device (4).