SE541307C2 - Pressure relief drum washer and a method for manufacturing the drum washer - Google Patents

Abstract

A drum washer (10) for washing pulp comprises a cylindrical drum (2), a stationary casing (11), a pressurizing arrangement (30) and a motor arrangement. The cylindrical drum comprises a penetration layer (7) at an outer surface allowing liquids to penetrate. The cylindrical drum has a plurality of compartments (4) for pulp arranged at a periphery. The cylindrical drum further comprises a drainage system (25) inside, in radial direction, of the penetration layer. The drainage system comprises intermediate channels (22), main channels (24) and drainage openings (26). The intermediate channels are situated directly inside the penetration layer and the outlet from the drainage system is connected to the main channels. The main channels together have a cross-section area that is at least two times larger than a sum of a respective cross-section area in axial direction of the intermediate channels. The motor arrangement is configured to rotate the cylindrical drum.

Description

PRESSURE RELIEF DRUM WASHER AND A METHOD FOR MANUFACTURING THE DRUM WASHER TECHNICAL FIELD The present invention relates in general to cellulose pulp processing, and in particular to methods and devices for washing and dewatering of cellulose pulp.

BACKGROUND Pulp is produced by processing of different kinds of fibers. During this processing of fibers into pulp, the process typically comprises different steps where liquors are added to or removed from the pulp. For instance, the digestion liquor or different types of bleaching liquors have to be removed from the pulp in different stages of the process. In connection with such dewatering or de-liquoring, the pulp is typically also washed by adding and subsequent removing of a washing liquor. There are a number of different types of washing equipment operating according to different principles.

One well-known type of washing arrangement is the drum washer, where the pulp is dewatered on a rotary filter drum. The dewatering takes place after the addition of washing liquid, which operates by displacing the liquor remaining on the pulp web after preceding process stages, for example a digestion stage or bleaching stage. An under-pressure within the drum and/or an overpressure outside the drum forms a pressure difference over the pulp positioned on the rotary drum. This pressure difference causes the displaced liquid to pass through a penetration layer, typically a perforated metal sheet, located at a periphery of the rotary drum. An increased pressure difference typically leads to a faster displacement of the filtrate.

In an often used design of a pressurized displacement washer, the drum is provided with compartments, extending in the axial direction of the drum and intended to be filled with pulp. The compartments are defined by walls in the form of bars arranged axially along the entire drum shaft, as well as a bottom that consists of the penetration layer. The subdivision in compartments of the drum ensures that the pulp cake does not break up and move, but instead maintains the form produced upon application of the pulp. A drainage system is typically provided. The penetration layer, e.g. the perforated metal surface, on which the pulp deposits, is typically located at a distance from the main surface of the drum, so that filtrate channels or conduits are formed in a space between the actual drum and the perforated metal surface. In such a design, the filtrate channels or conduits form the drainage system.

During operation, pulp to be washed is entered into a forming zone through a pulp inlet. The pulp is allowed to fill the compartments when the compartments are rotated through the forming zone. The pressure difference over the perforated metal sheet extracts filtrate liquid from the pulp, thereby forming a pulp web in the respective compartments before the compartments pass into a next zone.

If the displacement of the filtrate through the pulp is fast enough, a multistage drum washer can be designed. In a multi-stage drum washer, a plurality of different washing stages can be carried out, with separate addition and subsequent removal of washing liquid to the different stages. The filtrates from the respective zones are separated by seals in a peripheral end valve arranged at one or both of the end walls of the drum. When the drum rotates, the compartments successively pass the different zones. Typically, the filtrate removed from a later stage is used as washing liquid in a previous stage, i.e. in a counter-flow manner, in order to maximize the use of the liquids.

In a discharge stage, carried out in a discharge zone of the drum, the washed and dried pulp is released from the penetration layer in the compartments. This is typically performed by removing the pressure difference over the penetration layer, thereby removing the force pressing the pulp towards the penetration layer. The release is typically also assisted by gravity, by positioning the discharge zone, or at least a main part of the discharge zone, below a level of the axis of the drum. In particular designs, the application of an overpressure inside the filtrate channel in the discharge zone may further improve the release function.

The operation of the drum washer is in general very efficient. However, minor problems still exist. In order to increase the capacity of the drum washer, the length of the drum may be increased. However, since the outlet from the drainage system typically is provided at one or both the axial ends, long drums may introduce pressure differences along the axis within each compartment. Such pressure differences may result in that the washing may exhibit variations, i.e. even within the pulp of one and the same compartment. Different fractions of the pulp web are thereby washed differently, and the overall washing efficiency becomes lower than expected.

Another minor problem with drum washers occurs in the discharge zone. When the pressure difference over the penetration layer is removed, or even reversed, liquid still being present in the drainage system may re-penetrate the penetration layer and re- wet the pulp in the compartments. This is also enhanced in designs when the discharge zone is positioned below a level of the axis of the drum, since the liquid is influenced by the gravity in the direction towards the pulp.

In the US patent 3,306,460, a wire wound filter with underlying peripherally spaced cover elements is disclosed. Drainage channels are drained through axially extended slots.

The US patent 1,892,306 illustrates a rotary drum filter for a continuous sheet of material where drainage channels are drained through closely provided drainage cuts in a corrugated sheet.

In the published US patent application US 2008/0087392 A1, a vacuum washer drum is disclosed. One inner part of the filtrate passing through a mesh screen is drained through a set of inner filtrate channels, while an outer part is drained through a set of outer filtrate channels. The efficient length of the filtrate channels are thus reduced to half.

SUMMARY A general object of the technology presented here is to provide a drum washer with improved filtrate displacement through the pulp and with reduced tendency for re-wetting.

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

In general words, in a first aspect, a drum washer for washing pulp comprises a cylindrical drum, a stationary casing, a pressurizing arrangement and a motor arrangement. The cylindrical drum is rotatably disposed around an axis. The stationary casing encloses the cylindrical drum. The cylindrical drum comprises a penetration layer at an outer surface. The penetration layer allows liquids to penetrate. The cylindrical drum has a plurality of compartments for pulp arranged at a periphery. The compartments are defined by axial compartment walls and the penetration layer. The cylindrical drum further comprises a drainage system inside, in radial direction, of the penetration layer. The drainage system is arranged for receiving liquids penetrating the penetration layer. The pressurizing arrangement is arranged for applying a pressure difference between a volume outside and in contact with at least a part of the plurality of compartments and at least a part of an outlet from the drainage system. The drainage system comprises intermediate channels, main channels and drainage openings between the intermediate channels and the main channels. The intermediate channels are situated directly inside the penetration layer and the outlet from the drainage system is connected to the main channels. The intermediate channels are axially disposed channels. The main channels together have a cross-section area in a flowing direction of the liquids that is at least two times larger, preferably four times larger, than a sum of a respective cross-section area in axial direction of the intermediate channels. The motor arrangement is configured to rotate the cylindrical drum.

In a second aspect, a method for manufacturing a drum washer for washing pulp comprises providing of a rotatable cylindrical drum, rotatably disposed around an axis. The cylindrical drum comprises a penetration layer at an outer surface. The penetration layer allows liquids to penetrate. The cylindrical drum further comprises a drainage system inside, in radial direction, of the penetration layer and the drainage system is arranged for receiving the liquids penetrating the penetration layer. Intermediate channels, main channels and drainage openings between the intermediate channels and the main channels are created in the drainage system, wherein the intermediate channels are situated directly inside the penetration layer and where the outlet from the drainage system is connected to the main channels. The intermediate channels are axially disposed channels. The main channels together have a cross-section area in a flowing direction of the liquids that is at least two times larger, preferably four times larger, than a sum of a respective cross-section area in axial direction of the intermediate channels.

One advantage with the proposed technology is that an improved filter drainage is achieved and re-wetting is reduced. 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 is a schematic perspective view of an embodiment of a cylindrical drum; FIG. 2 is an axial cross-section through an embodiment of a drum washer; FIGS. 3A-B are diagrams illustrating pressure situations along paths in a drum washer with a single channel as drainage system; FIG. 3C is a diagram illustrating the pressure difference over a pulp volume; FIG. 4 illustrates a part cross-section view of a compartment and underlying drainage system of an embodiment of a cylindrical drum; FIG. 5 illustrates a schematic cross-sectional view of one compartment of an embodiment of a cylindrical drum; FIGS. 6A-B are diagrams illustrating pressure situations along paths in an embodiment of a drum washer with a divided drainage system; FIG. 6C is a diagram illustrating the pressure difference over a pulp volume in an embodiment of a drum washer with a divided drainage system; FIG. 7 is a schematic illustration of a re-wetting situation in a compartment; FIG. 8 illustrates schematically an embodiment of a supporting delimitation arrangement to be fitted into an embodiment of a drainage system; and FIG. 9 illustrates a flow diagram of steps of an embodiment of a method for manufacturing a drum washer for washing pulp.

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 the basic design and operation of a drum washer.

Fig. 1 is a schematic perspective view of an embodiment of a cylindrical drum 2 that is one main part of a pressurized displacement washing arrangement, a so-called drum washer. The cylindrical drum 2 is by a shaft 6 rotatably disposed around an axis 3. A (not shown) motor arrangement is configured to rotate the cylindrical drum 2 at an operation rotational speed. The cylindrical drum 2 is provided with a plurality of compartments 4 arranged at a periphery 9 of the cylindrical drum 2. The compartments 4 are also commonly called pulp compartments or cells. The compartments 4 are intended to be filled by paper pulp to be washed. Each compartment 4 in the present embodiment is at least partially defined by a bottom 5 made of a penetration layer 7, e.g. a perforated surface, preferably made by metal, placed at a distance from the main peripheral surface of the cylindrical drum 2. A space is thereby defined between the peripheral surface of the cylindrical drum 2 and the penetration layer 7, which is used for extraction of filtrate liquid. This drainage system is described in more detail further below. The compartments 4 are further defined by two compartment walls 8 arranged axially with reference to the shaft 6 of the cylindrical drum 2. The compartment walls 8 of the cylindrical drum 2 of the present embodiment are evenly distributed along the circumference of the cylindrical drum 2. The cylindrical drum 2 is in general rotatably supported by a stationary support (not shown in the figure) in the washing arrangement.

In Fig. 2, an axial cross-section through an embodiment of a washing arrangement in the form of a drum washer 10 with a compartmented cylindrical drum 2 is shown. The cylindrical drum 2 is rotatably arranged with respect to the shaft 6 around the axis 3 within a stationary casing 11. The stationary casing 1 1 thus encloses the cylindrical drum 2 and defines a space 13 between the casing and the cylindrical drum 2. The drum washer 10 comprises a plurality of seals 12 placed between the cylindrical drum 2 and the surrounding stationary casing 11 in a longitudinal axial direction. These seals 12 seal particularly between the stationary casing 11 and the compartment wall 8 and serve as separating members between different zones of the drum washer 10 in the space between the casing and the cylindrical drum. One function of the seals 12 is to ensure that washing liquid intended for a specific washing stage is not moved to a subsequent washing stage, in particular since there may be a difference in pressure between different washing stages. More specifically, the seals 12 define at least a forming zone F for forming of the pulp in the compartments 4 of the cylindrical drum 2.

The illustrated embodiment of a drum washer 10 is provided with four longitudinal seals 12 that consequently divide the space 13 of a substantially annular shape into four zones. Besides the forming zone F, a first and second washing zone T1, T2 for washing the formed pulp, and a discharge zone U for discharge of the washed pulp are provided.

A drum washer 10 of the above described design operates with a continuously rotating cylindrical drum 2. This implies that the compartments 4 successively pass the different zones F, T1, T2, U. More particularly, each compartment 4 pass the forming zone F from an entrance part 16 to an exit part 17 of the forming zone upon rotation of the cylindrical drum 2. Pulp for washing in a porous state is fed into the forming zone F by at least one pulp inlet 19. Typically, the entire forming zone is filled with pulp, whereby the pulp is entered into the compartments 4 of the cylindrical drum 2. The pulp 18 is in the present embodiment placed as, in the axial direction of the cylindrical drum 2, long and narrow rectangles against the perforated surface, i.e. the penetration layer 7, in this embodiment a metal sheet that constitutes the compartment bottom 5. The division of the periphery of the cylindrical drum 2 into compartments ensures that the shape of the pulp cake or pulp web is maintained during rotation of the cylindrical drum 2.

Washing liquid can be supplied through inlets 15 to the annular space 13 and filtrate liquid is squeezed out of the pulp by a pressure difference applied over the penetration layer 7, i.e. between the space 13 and the drainage system inside the penetration layer 7. The filtrate liquid thereupon passes through the penetration layer 7 and leaves a more densely packed pulp. This occurs in the present embodiment with an overpressure in the space 13 in order to achieve an improved dewatering of the pulp. Consequently, the drum washer 10 comprises a pressurizing means 30 for applying a pressure difference over the perforated surface, i.e. the penetration layer 7. The pressurizing means 30 comprises in the present embodiment means 41 for providing an overpressure of the pulp entering the forming zone F and means 51 for providing an underpressure of the filtrate liquid having passed the penetration layer 7. In a most simple design, the overpressure is simply provided as the overpressure caused by the pump feeding the pulp into the forming zone F or the washing liquids into the washing zones T1 and T2. The means 51 for providing an underpressure is in this embodiment fluidly connected to the volume between the penetration layer 7 and the main peripheral surface of the cylindrical drum 2, i.e. inside the penetration layer 7, as indicated by the dashed line 52. In other words, the underpressure is applied to an outlet from the drainage system. The pressurizing means 30 thereby provides a pressure difference over the penetration layer 7.

In alternative embodiments, either the means 41 for providing an overpressure or the means 51 for providing an underpressure may be omitted. The pressure difference will in such case be a difference between the over/under-pressure and atmospheric pressure.

When the washing liquids are provided into the space 13 between the penetration layer and the stationary casing, the pressure difference will cause liquid to be drawn through the penetration layer into the space below, i.e. into the drainage system. The available pressure difference will be partitioned in a pressure drop within a path in the space 13 between the penetration layer and the stationary casing, a pressure drop through the pulp volume and penetration layer and a pressure drop within the drainage system. The first component is typically small and is neglected in the coming discussion. The drainage system will during operation contain liquid that continuously is drawn out from the drum washer, and this stream of liquid causes a pressure distribution within the drainage system. Parts of the drainage system that are positioned far away from the drainage system outlet, where the low pressure is applied, will experience a considerably higher pressure than parts of the drainage system that are positioned closer to the drainage system outlet. The remaining pressure difference is the pressure difference that cause the liquid to pass the pulp volume and the penetration layer. This means that there will be a lower available pressure difference at positions far away from the drainage system outlet compared to positions near the drainage system outlet. This in turn leads to that a larger part of the liquid is caused to pass the pulp volume close to the drainage system outlet, leading to an uneven washing. In order to achieve a more even washing efficiency, it would be required to provide for more evenly applied pressure differences over the pulp volume within each compartment, which in turn calls for a more evenly distributed pressure within the drainage system. In other words, the pressure drop within the drainage system should be minimized or at least the pressure differences between different parts of the drainage system that are in contact with the penetration layer should be minimized.

Fig. 3A is a diagram illustrating a pressure situation along a path in a drum washer with a single channel as drainage system. The total available pressure difference is divided between a pressure difference ?Ppulpover the pulp volume and a pressure difference ?Pdrainwithin the drainage system. The pressure difference ?Ppulpis thus the pressure difference giving rise to the washing flow through the pulp. Fig. 3A illustrates the situation for a flow path passing the pulp volume at an axial position close to the outlet from the drainage system. The main pressure drop occurs here within the pulp and the flow there through is therefore relatively high.

Fig. 3B is a diagram illustrating a similar pressure situation for a flow path passing the pulp volume at an axial position far away from the outlet from the drainage system. The pressure drop within the drainage system ?Pdrainis consequently larger than in the previous case. The pressure drop ?Ppulpover the pulp volume is therefore considerably reduced, and so is the flow rate through the pulp at this position.

Fig. 3C is a diagram illustrating the pressure difference ?Ppulpover the pulp volume, i.e. the pressure drop available for pushing the flow of washing liquid through the pulp, as a function of the position relative to the outlet from the drainage system. At the left side of the diagram, close to the outlet of the drainage system, the available pressure difference is relatively high, whereas the available pressure difference in the right part of the diagram, i.e. closer to the inner end of the drainage system, is considerably lower. This large difference in pressure difference ?Ppulpbetween different axial positions along the pulp compartments gives rise to an uneven flow. Most of the washing liquid will select a path with a high pressure difference ?Ppulpover the pulp volume, which leads to that pulp closer to the outlet from the drainage system is washed more than pulp situated further away.

At the same time, the conditions in the discharge zone have to be considered carefully. In one embodiment, the stationary casing defines a forming zone, configured for allowing received pulp suspension to settle in the compartments. Furthermore, the stationary casing defines a discharge zone, in which it is intended that dried pulp is released from the compartments. A main part of the discharge zone is provided below a level of the axis of the cylindrical drum.

In the discharge zone, the applied pressure difference is removed, or it may even be reversed. The liquid that still is present in the drainage system will by the reversed pressure difference, if any, or simply by gravity tend to flow back through the penetration layer and re-wet the pulp. This is an unwanted drawback of an efficient discharge of the pulp from the compartments. Arrangements for mitigating re- wetting is therefore requested. This is often in contradiction with possible solutions of the pressure difference problem discussed previously.

The presently presented technology gives some solutions to this complex problem situation.

The basic features of a suitable embodiment of a drum washer are similar as described above. In one embodiment, a drum washer for washing pulp comprises a cylindrical drum, rotatably disposed around an axis and a stationary casing enclosing the cylindrical drum. The cylindrical drum comprises a penetration layer at an outer surface. The penetration layer allowing liquids to penetrate. The cylindrical drum has a plurality of compartments for pulp arranged at a periphery. The compartments are defined by axial compartment walls and the penetration layer.

Fig. 4 illustrates a part cross-section view of a compartment 4 and underlying drainage system 25 of an embodiment of a cylindrical drum 2 seen mainly along the shaft of the cylindrical drum 2. The annular space 13 is defined by the compartment walls 8, the surrounding stationary casing 1 1 and the penetration layer 7. An inlet 15 for washing liquid or pulp suspension (c.f. 19, Fig. 2) may be provided. The washing liquid is typically distributed substantially evenly over the annular space 13 without causing any substantial pressure differences.

The cylindrical drum 2 further comprises a drainage system 25. The drainage system 25 is provided inside of the penetration layer 7 and is arranged for receiving liquids penetrating the penetration layer 7. The term “inside” refers here to the radial direction.

A pressurizing arrangement 30 is arranged for applying a pressure difference over the penetration layer 7, and more particular between a volume outside and in contact with at least a part of the plurality of compartments and at least a part of an outlet 23 from the drainage system 25. The volume outside and in contact with at least a part of the plurality of compartments is e.g. the annular space 13. The actual application of the pressure difference can be performed in any way known, as such, in prior art. The details of such application of a pressure difference are not of crucial importance for the technical effect of the present technology and is therefore not further discussed.

By increasing the space that is available for liquid having passed the penetration layer, the pressure drop in the drainage system can be reduced. This means that a larger part of the total pressure drop instead will take place over the pulp being present in the compartments and being effective in drawing the liquid through the pulp. The variation of the pressure drop over the pulp in different parts of the compartment will thereby also be reduced. However, having a large volume in direct contact with the penetration layer will also open up for large volumes of liquid that potentially could re-wet the pulp upon discharge, as the channel in direct contact with the penetration layer 7 will be emptied of essentially all liquid remaining in such a channel.

Therefore, the drainage system 25 comprises intermediate channels 22, main channels 24 and drainage openings 26 between the intermediate channels 22 and the main channels 22. The intermediate channels 22 are situated directly inside the penetration layer 7. The intermediate channels 22 are axially disposed channels. The outlet 23 from the drainage system 25 is connected to the main channels 24. This means that the pressure difference applied by the pressurizing arrangement for all possible paths is preferably applied partly over the main channels 24. In other words, in one embodiment, there is no direct path between the intermediate channels 22 and the outlet 23, which means that any drained liquid has to pass the main channels 24. The main channels 24 together having a cross-section area A in a flowing direction F of the liquids. This cross-section areas A is at least two times larger than a sum of a respective cross-section area a in axial direction of the intermediate channels 22. In a preferred embodiment the cross-section area A is at least four times larger than the cross-section area a.

Fig. 5 illustrates a schematic cross-sectional view of one compartment 4. A pulp volume 18 is provided on top of a penetration layer 7. An intermediate channel 22 is formed between the penetration layer 7 and delimitation sheets 28. Drainage openings 26 are formed in the delimitation sheets 28 allowing liquid to pass from the intermediate channel 22 into the main channel 24 and further towards the outlet 23. A number of flow paths 70A-G are indicated, exemplifying the paths that the liquid can select through the drum washer.

The area difference between the main channel 24 and the intermediate channel 22 results in a favourable pressure distribution. The main part of the pressure difference being present within the drainage system 23 will be present in the intermediate channels 22. Due to the larger cross-sectional area in the main channels 24, the pressure drop in the main channel 24 will be small compared to the pressure drop within the intermediate channels 22. By furthermore having drainage openings 26 connecting the intermediate channels 22 and the main channel 24 relatively close, also the pressure drop within the intermediate channels 22 can be kept at a relatively low level. The distance that the liquids will travel within the intermediate channels 22 becomes short. The entire design thus results in a generally low pressure drop within the entire drainage system 25, which means that the main part of the available pressure difference applied by the pressurizing arrangement 30 will occur over the pulp volume provided in the compartments 4 and thus contribute to the washing liquid flow through the pulp. The differences caused by the differing pressure drops within the drainage system 25 will therefore be reduced. The washing efficiency will therefore be more even across the entire compartment 4.

Fig. 6A is a diagram illustrating a pressure situation along a path 70B in a drum washer according to the embodiment of Fig. 5. The total available pressure difference is divided between a pressure difference ?Ppulpover the pulp volume, a pressure difference ?Pintwithin the intermediate channel and a pressure difference ?Pmainwithin the main channel. Fig. 6A illustrates the situation for a flow path 70B passing the pulp volume at an axial position close to one of the drainage openings of the drainage system and close to the outlet of the drainage system. The main pressure drop occurs here within the pulp.

Fig. 6B is a diagram illustrating a similar pressure situation for a flow path 70F passing the pulp volume at an axial position almost in the middle between two drainage openings of the drainage system and far away from the outlet from the drainage system. The pressure drop ?Ppulpover the pulp volume is here reduced compared to Fig. 6A, but with a considerably smaller amount compared with the situation of Fig. 3B. The pressure difference ?Pintwithin the intermediate channel and the pressure difference ?Pmainwithin the main channel are still relatively small.

Fig. 6C is a diagram illustrating the pressure difference ?Ppulpover the pulp volume as a function of the position relative to the outlet from the drainage system. This difference in pressure difference ??pulpare here relatively small. The washing liquid will thus be distributed relatively evenly over all paths through the pulp volume, which leads to that pulp becomes relatively evenly washed. Furthermore, since the dominating part of the total available pressure drop occurs over the pulp, the flow of washing liquid between the pulp becomes fast.

From Figs. 6A-C it is also easily understood that the fluctuations in the pressure difference over the pulp volume becomes smaller if the total pressure drop within the drainage system is kept small. The largest pressure drop contribution within the drainage system comes typically from the intermediate channels and for a point half way between two consecutive said drainage openings. By having the drainage openings closer, this maximum contribution can be lowered. However, as will be discussed further below, the drainage openings cannot be provided too close either.

In one embodiment, the distance L (Figs. 4 and 5) between the drainage openings is selected to cause a pressure difference between a point in the intermediate channels half way between two consecutive said drainage openings and a point in one of the two consecutive drainage openings that is less than 25% of a pressure drop over an intended pulp volume, when the pressure difference is applied. Preferably, this pressure drop is less than 15%.

The intended pulp volume is supposed to fill the compartments and is thereby defined by the geometrical size of the compartments. In other words, an intended pulp volume has a thickness equal to a height of the axial compartment walls.

Furthermore, in order not to cause any unnecessary pressure drop within the drainage system, in one embodiment, the drainage openings 26 (Figs. 4 and 5) should be wide enough to manage the flows in the intermediate channels 22. In such an embodiment, the width w, in the axial direction, of the drainage openings 26 should be at least two times larger than a height h, in the radial direction, of the intermediate channels 22.

The intermediate channels 22 and the main channel 24 are separated by the provision of delimitation sheets 28. This means that the volumes that are in direct contact with the penetration layer 7, i.e. the volumes that directly contributes to re-wetting, is kept small. Since it is mainly the liquid that is present in the absolute vicinity of the penetration layer 7 that is the main source for re-wetting liquid, the amount of liquid being present in the intermediate channels 22 is kept small. This reduces the amount of re-wetting. Liquids from the main channel 24 may also participate in the re-wetting, but since such liquid has to re-enter the intermediate channels 22 through the drainage openings 26, this amount is significantly reduced.

Fig. 7 is a schematic illustration of a re-wetting situation in a compartment. The compartment is for illustration purposes turned up-side down compared with e.g. Fig. 5, since this will be closer to an expected actual situation. The pulp volume 18 is placed immediate below the penetration layer 7 and liquid being present within the intermediate channel 22 may easily re-penetrate the penetration layer 7 and re- wet the pulp volume 18. Also some liquid being present in the main channel 24 may re-wet the pulp, in particular liquids that are present close to drainage openings 26. Liquid that is present further away have to flow a certain distance within the main channel 24 before it can reenter a drainage opening 26. One possible way to mitigate re-wetting would therefore be to increase the distance between drainage openings 26.

In order to provide a substantial reduction in re-flow, a distance in axial direction between two consecutive drainage openings 26 should therefore not be too short. In one embodiment, the axial direction between two consecutive drainage openings 26 is at least 0.2 m. In a preferred embodiment, the axial direction between two consecutive drainage openings 26 is at least 0.4 m.

Another means for reducing re-wetting is to provide flow restrictions 27 within the main channel, in particular in axial direction. Since the flow in the main channel during the main operation should be as easy as possible, the effect of such flow restrictions should be minimized during such circumstances. Therefore, in one embodiment, the extension d of a flow restriction 27 in the radial direction should not exceed 50% of the height H, in the radial direction, of the main channel.

Since the position of the compartment during inlet and washing phases typically is mainly sidewards or upwards, in relation to the cylindrical drum axis. Axial-flow restriction arrangements in an outer, in radially direction, wall will influence the flow mainly when the compartment faces downwards, i.e. during the discharge of the pulp. Therefore, in one embodiment, the main channels have axial-flow restriction arrangements in an outer, in radially direction, wall. Examples of such flow restrictions 27 are illustrated in Figs. 4, 5 and 7 in the shape of tangentially extended lips at the inner side of the delimitation sheets 28. In other words, the lips protrude towards the center of the cylindrical drum 2.

In one embodiment, the extension d of a flow restriction 27 in the radial direction is preferably at least 10% of the height H, in the radial direction, of the main channel.

The possibilities for re-wetting are not only dependent on compartment geometry in absolute numbers, but is also dependent on parameters such a rotational speed and width of the discharge zone in relation to the radius of the cylindrical drum. A wide discharge zone will provide a long time for the liquid to re-wet the pulp. A fast rotational speed will instead lower the available re- wetting time.

Fig. 8 illustrates schematically an embodiment of a supporting delimitation arrangement 80 to be fitted into a drainage system. The supporting delimitation arrangement 80 comprises a number of plates 82 provided side by side in the same plane with a small separating distance. The plates 82 form the delimitation sheets 28. The edges of the plates 82 in the intended axial direction are bent to give rise to the flow restrictions 27. In the plates 82, there is an array of openings 88 or slits.

The intended radial position of the delimitation sheets 28 is achieved by use of a set of distance sheets 85. In the present embodiment, the distance sheets have tabs formed in the upper part, which tabs fit though the slits of the delimitation sheets 28. When the tabs are put into the slits, the tabs will protrude a certain distance above the delimitation sheets 28, forming outer distance elements 86. The height of the outer distance elements 86 thereby define the distance between the delimitation sheets 28 and the intended penetration layer. The remaining part of the distance sheets 85 remains below the delimitation sheets 28, or radially inside with reference to the intended final use. This part of the distance sheets 85 constitutes inner distance elements 84. The height of the inner distance elements 84 thereby define the distance between the delimitation sheets 28 and the intended surface of the cylindrical drum.

The distance sheets 85 are also preferably provided with attachment structures 87 that are formed in such a way that they lock the axial ends of the delimitation sheets 28 in well-defined positions relative each other. In this way, the size of the drainage openings 26 can be defined.

The manufacturing of the delimitation sheets 28 and the distance sheets 85 are easily based on simple metal sheet processing. The intended shapes are cut out according to any suitable prior art metal cutting processes. The mechanical assembly is also simple, by just inserting the tabs of the distance sheets 85 into the openings 88 and fitting the attachment structures 87 around the edges of the delimitation sheets 28. If further fixing is necessary, the tabs forming the outer distance elements 86 can be slightly bended or deformed in any other way, in order to make a de-assembly more difficult. The assembling can thereby be made avoiding use of any welding, soldering or other attachment techniques that may introduce sources for fatigue problems. The entire supporting delimitation arrangement 80 can then be put directly onto the surface of the cylindrical drum and the penetration layer can be provided directly on top of the supporting delimitation arrangement 80.

In one embodiment, the intermediate channels and the main channels are defined in a drainage volume by a supporting delimitation arrangement 80 dividing the drainage volume into the intermediate channel 22 and the main channel 24. The supporting delimitation arrangement 80 has a number of delimitation sheets 28 separating the intermediate channel 22 and the main channel 24, and inner 84 and outer 86 distance elements. The inner distance elements 84 being supported on an inner, in radially direction, surface of the drainage volume. The outer distance elements 86 being supported on an outer, in radially direction, surface of said drainage volume. The delimitation sheet 82 is supported on the inner and outer distance elements.

In a further embodiment, the inner 84 and outer 86 distance elements are provided by common distance sheets 85. Protruding parts of the distance sheet 85, forming the inner 84 or outer 86 distance elements, are provided through openings 88 in the delimitation sheets 28.

In yet a further embodiment, the distance sheets 85 have attachment structures 87 conformal to ends of the delimitation sheets 82, enabling holding of the delimitation sheets 82 separated to form the drainage openings 26 there between.

Fig. 9 illustrates a flow diagram of steps of an embodiment of a method for manufacturing a drum washer for washing pulp. In step S 10, a rotatable cylindrical drum is provided. The rotatable cylindrical drum is rotatably disposed around an axis. The cylindrical drum comprises a penetration layer at an outer surface. The penetration layer allows liquids to penetrate. The cylindrical drum further comprises a drainage system inside, in radial direction, of the penetration layer and the drainage system is arranged for receiving the liquids penetrating the penetration layer. In step S 12, intermediate channels, main channels and drainage openings between the intermediate channels and the main channels are created in the drainage system. The intermediate channels are situated directly inside the penetration layer and the outlet from the drainage system is connected to the main channels. The intermediate channels are axially disposed channels. The main channels together have a cross-section area in a flowing direction of the liquids that is at least two times larger, preferably four times larger, than a sum of a respective cross-section area in axial direction of the intermediate channels.

In a further embodiment, the creation of the intermediate channels, the main channels and the drainage openings comprises, in step S 14, insertion of a supporting delimitation arrangement into the drainage volume. This insertion divides the drainage volume into the intermediate channel and the main channel. The supporting delimitation arrangement has a number of delimitation sheets separating the intermediate channel and the main channel, and inner and outer distance elements. The inner distance elements are placed to be supported on an inner, in radially direction, surface of the drainage volume. The outer distance elements are placed to be supported on an outer, in radially direction, surface of the drainage volume. The delimitation sheets are placed to be supported on the inner and outer distance elements.

In a further embodiment, the creation of intermediate channels, main channels and drainage openings further comprises, in step S16, forming the inner and outer distance elements as protruding parts of common distance sheets. In step S18, the protruding parts of the distance sheet are provided through openings in the delimitation sheets. In step S20, the inner and outer distance elements are locked relative the delimitation sheets by deforming the protruding parts after the provision of the protruding parts through the openings.

In yet a further embodiment, the creation of intermediate channels, main channels and drainage openings further comprises step S22, in which attachment structures of the distance sheets are formed to be conformal to ends of the delimitation sheets. This enables the distance sheets to hold the delimitation sheets separated to form the drainage openings there between.

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.

Claims (15)

1. A drum washer (10) for washing pulp, comprising: - a cylindrical drum (2), rotatably disposed around an axis (3); - a stationary casing (11) enclosing said cylindrical drum (2); said cylindrical drum (2) comprising a penetration layer (7) at an outer surface, said penetration layer (7) allowing liquids to penetrate; said cylindrical drum (2) having a plurality of compartments (4) for pulp arranged at a periphery (9), said compartments (4) being defined by axial compartment walls (8) and said penetration layer (7); said cylindrical drum (2) further comprising a drainage system (25) inside, in radial direction, of said penetration layer (7) and arranged for receiving liquids penetrating said penetration layer (7); - a pressurizing arrangement (30) arranged for applying a pressure difference between a volume outside and in contact with at least a part of said plurality of compartments and at least a part of an outlet (23) from said drainage system (25); wherein said stationary casing (11) defining a forming zone (F), configured for allowing received pulp suspension to settle in said compartments (4); and wherein said stationary casing (11) defining a discharge zone (U), in which it is intended that dried pulp is released from said compartments (4), a main part of said discharge zone (U) being provided below a level of said axis (3); and - a motor arrangement configured to rotate said cylindrical drum (2), characterized in that said drainage system (25) comprising intermediate channels (22), main channels (24) and drainage openings (26) between said intermediate channels (22) and said main channels (24), wherein said intermediate channels (22) are situated directly inside, in said radial direction, of said penetration layer (7) and where said outlet (23) from said drainage system (25) is connected to said main channels (24); said intermediate channels (22) being axially disposed channels; said main channels (24) together having a cross-section area in a flowing direction (F) of said liquids that is at least two times larger, preferably four times larger, than a sum of a respective cross-section area in axial direction of said intermediate channels (22).

2. The drum washer according to claim 1, characterized in that the pressure difference applied by the pressurizing arrangement (30) for all possible paths is applied partly over the main channels (24).

3. The drum washer according to claim 1 or 2, characterized in that a distance in axial direction between two consecutive said drainage openings (26) is at least 0.2 m, preferably at least 0.4 m.

4. The drum washer according to any of the claims 1 to 3, characterized in that a width (w), in an axial direction, of said drainage openings (26) is at least two times larger than a height (h), in said radial direction, of said intermediate channels (22).

5. The drum washer according to any of the claims 1 to 4, characterized in that said main channels (24) have axial-flow restriction arrangements (27) in an outer, in radially direction, wall.

6. The drum washer according to claim 5, characterized in that an extension (d) of said axial-flow restriction arrangements (27) in a radial direction is at least 10% of a height (H), in said radial direction, of said main channel (24).

7. The drum washer according to claim 5 or 6, characterized in that an extension (d) of said axial-flow restriction arrangements (27) in a radial direction is at most 50% of a height (H), in said radial direction, of said main channel (24).

8. The drum washer according to any of the claims 1 to 7, characterized in that said distance between said drainage openings (26) is selected to cause a pressure difference between a point in said intermediate channels (22) half way between two consecutive said drainage openings (26) and a point in one of said two consecutive drainage openings (26) that is less than 25%, and preferably less than 15%, of a pressure drop over a pulp volume with a thickness equal to a height of said axial compartment walls (8), when said pressure difference is applied.

9. The drum washer according to any of the claims 1 to 8, characterized in that said intermediate channels (22) and said main channels (24) are defined in a drainage volume by a supporting delimitation arrangement (80) dividing said drainage volume into said intermediate channel (22) and said main channel (24); said supporting delimitation arrangement (80) having a number of delimitation sheets (28) separating said intermediate channel (22) and said main channel (24), and inner and outer distance elements (84, 86); said inner distance elements (84) being supported on an inner, in radially direction, surface of said drainage volume; said outer distance elements (86) being supported on an outer, in radially direction, surface of said drainage volume; and said delimitation sheet (28) being supported on said inner and outer distance elements (84, 86).

10. The drum washer according to claim 9, characterized in that said inner and outer distance elements are provided by common distance sheets, wherein protruding parts of said distance sheet, forming said inner or outer distance elements, are provided through openings in said delimitation sheets.

11. The drum washer according to claim 10, characterized in that said distance sheets having attachment structures conformal to ends of said delimitation sheets, enabling holding of said delimitation sheets separated to form said drainage openings there between.

12. A method for manufacturing a drum washer for washing pulp, comprising the step of: - providing (S10) a rotatable cylindrical drum (2), rotatably disposed around an axis (3); said cylindrical drum (2) comprising a penetration layer (7) at an outer surface, said penetration layer (7) allowing liquids to penetrate; said cylindrical drum (2) further comprising a drainage system (25) inside, in radial direction, of said penetration layer (7) and arranged for receiving said liquids penetrating said penetration layer (7); said cylindrical drum (2) being enclosed by a stationary casing; said stationary casing (11) defining a forming zone (F), configured for allowing received pulp suspension to settle in said compartments (4); said stationary casing (11) defining a discharge zone (U), in which it is intended that dried pulp is released from said compartments (4), a main part of said discharge zone (U) being provided below a level of said axis (3), characterized by the further step of: - creating (SI 2), in said drainage system (25), intermediate channels (22), main channels (24) and drainage openings (26) between said intermediate channels (22) and said main channels (24), wherein said intermediate channels (22) are situated directly inside, in said radial direction, of said penetration layer (7) and where said outlet (23) from said drainage system (25) is connected to said main channels (24); said intermediate channels (22) being axially disposed channels; said main channels (24) together having a cross-section area in a flowing direction of said liquids that is at least two times larger, preferably four times larger, than a sum of a respective cross-section area in axial direction of said intermediate channels (22).

13. The method for manufacturing according to claim 12, characterized in that said step of creating (SI 2) intermediate channels, main channels and drainage openings comprises insertion (S14) of a supporting delimitation arrangement (80) into said drainage volume, dividing said drainage volume into said intermediate channel (22) and said main channel (24); said supporting delimitation arrangement (80) having a number of delimitation sheets (28) separating said intermediate channel (22) and said main channel (24), and inner and outer distance elements (84, 86); whereby said inner distance elements (84) are placed to be supported on an inner, in radially direction, surface of said drainage volume; whereby said outer distance elements (86) are placed to be supported on an outer, in radially direction, surface of said drainage volume; and whereby said delimitation sheets (28) are placed to be supported on said inner and outer distance elements (84, 86).

14. The method for manufacturing according to claim 13, characterized in that said step of creating (S12) intermediate channels (22), main channels (24) and drainage openings (26) further comprises: forming (S16) said inner and outer distance elements (84, 86) as protruding parts of common distance sheets; providing (S18) said protruding parts of said distance sheet through openings in said delimitation sheets; and locking (S20) said inner and outer distance elements (84, 86) relative said delimitation sheets (28) by deforming said protruding parts after said step of providing (S18) said protruding parts through said openings.

15. The method for manufacturing according to claim 14, characterized in that said step of creating (S12) intermediate channels (22), main channels (24) and drainage openings (26) further comprises: forming (S22) attachment structures of said distance sheets to be conformal to ends of said delimitation sheets (28), enabling holding of said delimitation sheets (28) separated to form said drainage openings (26) there between.