TECHNICAL FIELD

The present invention relates in general to washing and filtering equipment and in particular to arrangements in connection with pulp filters.

BACKGROUND

In a pulp production, fibers are typically treated in different steps with different liquors. Within the process, different used liquors or unwanted products have to be removed from the pulp at different stages. It is thus common to use different kinds of washing equipment, typically in connection with different kinds of filters.

One well-known type of pulp filter is a drum vacuum filter. Such a filter comprises a rotatable drum mounted in a vat provided with pulp suspension. The outer surface of the drum is perforated and allows penetration of liquids. A“vacuum”, or at least an underpressure, is applied on the inside of the outer surface of the drum, causing a suction of liquid through the outer surface. This causes pulp to settle onto the outer surface. When the drum is rotated, the pulp cake will follow the drum up from the vat and the water content in the pulp cake is reduced by further suction of liquids through the outer surface. Different washing steps can also be provide here.

When the pulp cake is as dry as requested, it is removed from the drum. In order to facilitate this removal, the vacuum is no longer applied during that stage. This is arranged for by a cylindrical valve. In other words, in a vacuum filter with a cylindrical valve, there are in general two zones. One zone where the“vacuum” is applied over the pulp web and drain the pulp web and one zone without vacuum for enabling the release of the pulp web. The zones are defined by the cylindrical valve. A problem with this setup is that the cylindrical valve is designed so that the transition from non-vacuum to“full vacuum” occurs fast, which leads to that fibers may densely fill up the closest layer on the drum surface. This is particularly common for short fiber pulp. The pulp web then becomes dense, which deteriorate the dewatering and the release of the pulp web, which in turn deteriorate the washing.

SUMMARY

A general object is to improve the density distribution in the pulp web in a pulp-filter arrangement.

The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims.

In general words, in a first aspect, a pulp-filter arrangement comprises a rotatable drum enclosed by a non-rotating casing and a non-rotating valve arrangement. The rotatable drum has a cylindrical water-permeable pulp supporting shell, a plurality of drain pipes and a rotating hollow shaft. The plurality of drain pipes connects the rotating hollow shaft with a collecting channel. The collecting channel is situated radially directly inside the cylindrical water-permeable pulp-supporting shell, thereby enabling a drain of liquid, passing radially inwards through the cylindrical water-permeable pulp-supporting shell, through the rotating hollow shaft. The non-rotating valve arrangement is disposed within the rotating hollow shaft. The non rotating valve arrangement is arranged for sealing off the hollow part of the rotating hollow shaft from a radially inner outlet of the plurality of drain pipes when respective radially inner outlet during rotation of the rotatable drum in a rotation direction is situated within a sealed zone. The non-rotating valve arrangement is further arranged to, in a transition zone immediately following the sealed zone in the rotation direction, open a throttled passage between the rotating hollow shaft and the radially inner outlet of the plurality of drain pipes. The throttled passage is narrower than a fully open passage between said rotating hollow shaft ( 18) and said radially inner outlet (24).

One advantage with the slower application of vacuum provided in the proposed technology is that the dewatering and the release is improved. Other advantages will be appreciated when reading the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 schematically illustrates an embodiment of a vacuum filter in an elevational view;

FIG. 2 schematically illustrate a cross-sectional view of an embodiment of a vacuum filter;

FIG. 3 schematically illustrate an embodiment of a non-rotating valve arrangement in a vacuum filter; and

FIG. 4 schematically illustrates a top view of an embodiment of a valve plate of a non-rotating valve arrangement in a vacuum filter;

FIG. 5 schematically illustrates a top view of another embodiment of a valve plate of a non-rotating valve arrangement in a vacuum filter;

FIG. 6 schematically illustrates a top view of yet another embodiment of a valve plate of a non-rotating valve arrangement in a vacuum filter;

FIG. 7 schematically illustrates a top view of yet another embodiment of a valve plate of a non-rotating valve arrangement in a vacuum filter; and

FIG. 8 schematically illustrates a top view of yet another embodiment of a valve plate of a non-rotating valve arrangement in a vacuum filter.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements. For a better understanding of the proposed technology, it may be useful to begin with a brief overview of a vacuum filter in general.

Figure 1 shows an embodiment of a vacuum filter in an elevational view. Some parts are broken away to allow illustration of the inner parts of the pulp-filter arrangement 1. The pulp-filter arrangement 1 comprises a rotatable drum 10 enclosed by a non-rotating casing 12. The rotatable drum 10 has a pulp supporting shell 14, a plurality of drain pipes 16 and a rotating hollow shaft 18. The pulp-supporting shell 14 is cylindrical and water-permeable. In this particular embodiment, the pulp-supporting shell 14 comprises a mantle decking 13 and a filter wire 15.

The drain pipes 18 connect the rotating hollow shaft 18 with a collecting channel 22. The collecting channels 22 are situated radially directly inside the pulp-supporting shell 14. The collecting channels 22 thereby enable a drain of liquid, passing radially inwards through the pulp-supporting shell 14. The liquid is drained through the rotating hollow shaft 18.

An underpressure-providing arrangement 38, constituted by a drop leg 36, is arranged to provide an underpressure within a hollow part 37 of the rotating hollow shaft 18. The drop leg 36 is thus connected to the rotating hollow shaft 18. Alternatively, the underpressure-providing arrangement 38 may comprise any type of pump.

As will be discussed further below, pulp suspension is provided in a vat 32, surrounding the lower part of the rotatable drum 10. The underpressure of the hollow part 37 of the rotating hollow shaft 18 is provided to the collecting channel 22 via the drain pipes 16, resulting in that liquids from the pulp suspension is drawn through the pulp-supporting shell 14. This causes pulp to settle against the pulp-supporting shell 14, starting to build up a pulp web 20. When a portion of the pulp-supporting shell 14, upon rotation of the rotatable drum 10 in a rotation direction R, leaves the vat 32, the pulp supporting shell 14 is covered by a pulp web 20.

In the upper part of the pulp-filter arrangement 1 , different washing steps can be implemented. In order to simplify the understanding of the basic process, such arrangements are omitted in Figure 1.

When the pulp web 20 is drained from unnecessary liquids, the pulp web 20 is removed from the pulp-supporting shell 14 by a take-off device 28 comprising e.g. a doctor table 26. The removed and possibly cut pulp web is transported out from the pulp-filter arrangement 1 by a screw conveyor 34. The removal of the pulp web is facilitated by providing a vacuum free zone or a sealed zone at the sectors where the removal is performed.

To this end, the pulp-filter arrangement 1 further comprises a non-rotating valve arrangement 30 disposed within the rotating hollow shaft 18. The non rotating valve arrangement 30 comprises a valve shaft 31 parallel to the rotating hollow shaft 18. The non-rotating valve arrangement 30 is arranged for sealing off the hollow part 37 of the rotating hollow shaft 18 from a radially inner outlet 24 of the drain pipes 16. This sealing off is provided when respective radially inner outlet 24, during rotation of the rotatable drum 10 in the rotation direction R, is situated within a sealed zone. The sealed zone will be discussed more in detailed below.

The non-rotating valve arrangement 30 comprises a sealing plate 40 positioned against an inner surface 17 of the rotating hollow shaft 18 at the radially inner outlets 24 of the drain pipes 16 in the sealed zone. The sealing plate 40 has a shape of a part of a cylinder with an outer radius equal to an inner radius of the rotating hollow shaft 18. In this embodiment, the valve shaft 31 supports the sealing plate 40.

By configuring the non-rotating valve arrangement 30 in such a way that the vacuum is not fully applied at once when the drain pipes 16 leaves the sealed zone, the fibers from the pulp suspension will be placed on the pulp supporting shell 14 of the rotating drum 10 in a less dense manner. When the “full vacuum” is applied, a pressure drop over the existing pulp web 20 will counteract the formation of a dense pulp web 20 in the continued process. The careful opening in the non-rotating valve arrangement 30 can be realized e.g. by slits, holes or oblique cutting, as will be further discussed below.

In order to provide the careful opening the non-rotating valve arrangement 30 is further arranged to open a throttled passage between the rotating hollow shaft 18 and the radially inner outlet 24 of said drain pipes 16. The throttled passage is narrower than a fully open passage between the rotating hollow shaft 18 and the radially inner outlet 24. This careful opening takes place in a transition zone immediately following the sealed zone in the rotation direction R.

Preferably, the non-rotating valve arrangement 30 is arranged to gradually open the throttled passage when the rotatable drum 10 rotates in the rotation direction R.

In this embodiment, the non-rotating valve arrangement 30 comprises a valve plate 42 positioned against the inner surface of the rotating hollow shaft 18 at the radially inner outlet 24 of the drain pipes 16 in the transition zone. The valve plate 42 has a shape of a part of a cylinder with an outer radius equal to an inner radius of the rotating hollow shaft 18. The valve plate 42 presents at least one opening with an opening area that is smaller than a fully open passage.

Preferably, the least one opening 60 has an opening area that averaged over a width of the radially inner outlet 24 of the drain pipes 16 gradually increases in the rotation direction R. In this embodiment, the valve shaft 31 also supports the valve plate 42. Figure 2 illustrates an embodiment of a vacuum filter in a cross-sectional view. A pulp source 35 provides pulp suspension 33 into the vat 32. Upon rotation of the rotatable drum 10 in the rotation direction R, pulp will settle on the pulp-supporting shell, forming the pulp web 20. This action is supported by the vacuum provided by the rotating hollow shaft 18 and the drain pipes 16. The vacuum is applied in an open zone 52. When the pulp web leaves the pulp suspension 33 in the vat, the pulp web 20 does not grow in thickness any more, but instead the vacuum can be utilized to remove parts of the liquids of the pulp. Furthermore, different washing steps (not illustrated) can be applied within the open zone 52. These washing steps are preferably performed according to well-known prior art and are not influencing the main topics of the presently presented ideas. Therefore, such additional operations and arrangements are not further discussed in order to simplify the description.

When a portion of the pulp web 20 is ready to be removed, a corresponding part of the rotatable drum 10 enters into the sealed zone 50. In this sealed zone 50, the non-rotating valve arrangement 30 is arranged for sealing off the hollow part 37 of the rotating hollow shaft 18 from the radially inner outlet of the drain pipes 16. This sealing off is provided when respective radially inner outlet, during rotation of the rotatable drum 10 in the rotation direction R, is situated within the sealed zone 50. In this embodiment, the sealing off is performed by the sealing plate 40, sealing against the inner surface 17 of the rotating hollow shaft 18. The take-off device 28 removes the pulp web 20 and the bare pulp-supporting shell 14 can once again be brought down in the pulp suspension 33 of the vat 32.

When the vacuum once more is applied to the inside of the pulp supporting shell 14, this is performed gradually within the transition zone 54, in order to avoid a too dense pulp web closest to the pulp-supporting shell 14. In the transition zone 54, a passage is gradually opened between the rotating hollow shaft 18 and the radially inner outlet of the drain pipes 16 when the rotatable drum 10 rotates in the rotation direction R. The transition zone 54 is situated immediately after the sealed zone 50 in the rotation direction R. The valve plate 42 is in this embodiment positioned against the inner surface 17 of the rotating hollow shaft 18.

A close-up view of an embodiment of the non-rotating valve arrangement 30 and the parts cooperating therewith is illustrated in Figure 3. A valve support

41 mechanically connects the valve shaft 31 of the non-rotating valve arrangement 30 with the sealing plate 40 and the valve plate 42. In this embodiment, the valve plate 42 presents an opening 60, which gradually widens in the rotational direction R. In this particular embodiment, the valve plate 42 presents a V-shaped opening 61.

Also, in the embodiment of Figure 3, the sealing plate 40 and the valve plate

42 are built-up in a two-layer configuration, having a radially outer layer of a sealing material 46 provided in a close vicinity of the inner surface 17 of the rotating hollow shaft 18, however, typically with a very small slit therebetween. This radially outer layer of a sealing material 46 is supported by a radially inner rigid layer 44. This configuration can alternatively also be provided just to one of the sealing plate 40 and the valve plate 42.

It is preferably if the transition zone 54 covers at least two, more preferably at least four, and most preferably at least six of the radially inner outlets of the plurality of drain pipes 16 at a time. In this particular embodiment, the transition zone 54 covers six of the radially inner outlets of the plurality of drain pipes 16 at a time.

The valve plate 42 can be configured in many different ways. Common for the preferred embodiments is that an average total area, e.g. averaged over the width of a drain pipe, of the holes through the valve plate 42 gradually increases in the rotational direction. This gradual increase may be continuous or discrete.

In Fig. 4, a top view of an embodiment of a valve plate 42 of a non-rotating valve arrangement 30 in a vacuum filter is schematically illustrated. This design is essentially the same as shown in Figure 2, where the opening 60 is a V-shaped opening 61.

In Fig. 5, a top view of another embodiment of a valve plate 42 of a non-rotating valve arrangement 30 in a vacuum filter is schematically illustrated. This embodiment presents a series of holes 62 with increasing area. In other words, valve plate 42 presents holes 62, wherein an average total area of the holes 62 increases in the rotation direction.

In Fig. 6, a top view of yet another embodiment of a valve plate 42 of a non rotating valve arrangement 30 in a vacuum filter is schematically illustrated. This embodiment presents a number of slits 63 with different lengths along the rotation direction R. In other words, the valve plate 42 presents slits 63, an average total area of the slits 63 increases in the rotation direction R.

In Fig. 7, a top view of yet another embodiment of a valve plate 42 of a non rotating valve arrangement 30 in a vacuum filter is schematically illustrated. This embodiment also presents a number of holes 64, but with the same area. Instead, the number of holes increases in the direction along the rotation direction R, giving rise to a gradually increasing average total area.

In Fig. 8, a top view of yet another embodiment of a valve plate 42 of a non rotating valve arrangement 30 in a vacuum filter is schematically illustrated. This embodiment presents oblique cuts 65. In other words, the valve plate 42 presents oblique cuts 65. An average total area of the valve plate 42 that is removed by the oblique cuts 65 increases in the rotation direction.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.