WO2024167450A1

WO2024167450A1 - Screw conveyor for transport of lignocellulosic material

Info

Publication number: WO2024167450A1
Application number: PCT/SE2024/050039
Authority: WO
Inventors: Benny Tenggren; Johan Carlsson
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2023-02-10
Filing date: 2024-01-18
Publication date: 2024-08-15
Anticipated expiration: 2025-08-10
Also published as: SE2330075A1

Abstract

The present invention relates to a screw conveyor comprising a housing (20) with an inlet opening (13) and an outlet opening (14), a conveyor screw (30) having a central shaft (31) with a helical blade (32) on a first axial portion (A) and a plurality of paddles 33 on a second axial portion B, the paddles being between the helical blade and the outlet opening (14), and wherein the helical blade (32) forms a first helix (H1) on the central shaft and the plurality of paddles are arranged such that a middle point of each of the paddles form a second helix (H2), wherein one of the first helix and the second helix forms a right-handed screw and the other forms a left-handed screw on the central shaft.

Description

SCREW CONVEYOR FOR TRANSPORT OF LIGNOCELLULOSIC MATERIAL

TECHNICAL FIELD

The present invention relates to a screw conveyor for transporting lignocellulosic material, suitably during production of ethanol, pulp or similar.

BACKGROUND

During treatment of lignocellulosic material, screw conveyors are typically used to transport lignocellulosic material between treatment steps and comprise a conveyor screw with a helical blade that transports the material from an inlet towards an outlet. The treatment of lignocellulosic material may be aimed at production of ethanol, pulp, or similar.

For many treatment steps, it is desirable that the lignocellulosic material is delivered continuously at a steady rate to ensure that the material is thoroughly processed. When using screw conveyors, however, the material discharged through the outlet may be unevenly distributed and arrive in large quantities at each revolution of the helical blade. This is a problem in particular when handling lignocellulosic material having a higher dry solids content or containing larger objects such as straw, chips or knots.

One solution to this problem is to end the helical blade before the outlet to avoid the blade delivering the lignocellulosic material in discreet amounts. Although this does spread out the output of material to some extent, it also leads to buildup of lignocellulosic material on internal walls of the screw conveyor. To avoid buildup, EP 1084808 provides a scraping out zone where the lignocellulosic material is mixed by scrapers that also serve to scrape the walls.

Another solution to achieve an even flow is to divide the helical blade into blade segments in a zone near the outlet so that the flow of material is evened out. This is disclosed by US4206841. Also, KR101188740 discloses shear bars or shear panels provided between revolutions of the helical blade to break up the flow of material. Furthermore, CN 114313836 discloses a set of paddles provided between the helical blade and the outlet.

The solutions mentioned above all attempt to solve the problems, but they are not able to provide a uniform flow while at the same time avoiding buildup of lignocellulosic material on the wall of the screw conveyor. Using them therefore risks leading to clogging of material that is then delivered in clumps to the outlet or that provide the material at intervals rather than uniformly.

There is therefore a need for further improvements within this area.

SUMMARY

The object of the present invention is to eliminate or at least to minimize the problems discussed above. This is achieved by a screw conveyor according to the appended independent claim.

The screw conveyor of the present invention comprises an elongated housing, a conveyor screw rotatably arranged in the housing and extending from a first end of the housing to a second end of the housing, and an inlet opening and an outlet opening in the housing, wherein the inlet opening is arranged closer to the first end than the outlet opening is. Also, the conveyor screw comprises a central shaft having a first axial portion on which a helical blade is arranged for transporting a material in an axial direction from the inlet opening towards the outlet opening, and the central shaft also has a second axial portion on which a plurality of protruding paddles is arranged, said protruding paddles being separated from each other in the axial direction, and said first axial portion being closer to the first end than the second axial portion. The helical blade further forms a first helix on the central shaft and the plurality of paddles are arranged such that a middle point of each of the paddles form a second helix, wherein one of the first helix and the second helix forms a right-handed screw and the other forms a left-handed screw on the central shaft. By providing the helical blade and the plurality of paddles as screws of which one is right-handed and the other is left-handed, the lignocellulosic material in the screw conveyor is broken up into an even flow and can be delivered through the outlet in a truly uniform manner regardless of the dry solids content of the lignocellulosic material. Thus, the screw conveyor is suitably both for slurries having a low dry solids content and for lignocellulosic material that is dry or that comprises larger pieces such as straw, chips, knots or twigs that are not yet disintegrated into smaller pieces or particles. By ensuring the uniform and controlled delivery of the lignocellulosic material through the outlet, the screw conveyor is also suitable for use to deliver material to any treatment step in the production of ethanol, pulp, or similar. This is highly advantageous in ensuring a desired performance of all components for treatment or further transportation of the lignocellulosic material to which the screw conveyor may be connected.

Suitably, each paddle comprises a contact surface and the contact surface is arranged at a contact surface angle to the transport direction. Thereby, the paddles are more efficient in transporting the lignocellulosic material towards the outlet. The contact surface angle may be in the range 100° - 140°, preferably in the range 105° - 130°. Thereby, transport of the lignocellulosic material is rendered even more efficient to ensure that the material is both uniformly distributed and efficiently transported towards the outlet.

In some embodiments, the contact surface angle is within 20°, preferably within 10°, of a blade angle of the helical blade to the axial direction. Thereby, transport of the lignocellulosic material is rendered more efficient since the paddles contact the material at a similar angle to the helical blade.

Also, the contact surface of each paddle suitably has an axial length that covers an axial portion of the central shaft, and the plurality of paddles is arranged such that a combined axial length of the paddles covers the second axial portion of the central shaft. Thereby, the entire inner wall of the housing is covered by the paddles to ensure that build-up of lignocellulosic material on the inner wall is prevented. Suitably, the helical blade has an end point at an end closest to the second axial portion and the paddle closest to the first axial portion is arranged with its middle point offset in a circumferential direction from the end point of the helical blade, said offset being in the range 90° - 270°, preferably 150° - 210°, more preferably 180°. Thereby, a suitable interval is achieved between the contact of the helical blade and the contact of the first of the paddles, and this is advantageous in distributing the lignocellulosic material in the screw conveyor.

In some embodiments, the plurality of paddles comprises at least three paddles. Thereby, transporting and distribution of the lignocellulosic material towards the outlet is improved.

Also, the second axial portion may extend from the outlet opening a first distance towards the first axial portion, said first distance being at least 10 % of a diameter of the conveyor screw, preferably at least 20 % and more preferably at least 25 %. Thereby, transporting and distribution of the lignocellulosic material towards the outlet is improved.

Suitably, the outlet opening is in a bottom of the housing and the second axial portion extends over the outlet opening at least 25 % but not more than 50 % of a width of the outlet opening. Thereby, the paddles extend over the outlet opening and continue to transport and distribute the lignocellulosic material as the material falls into the outlet opening.

The plurality of paddles may further comprise a first set of paddles and a second set of paddles, wherein each set of paddles is distributed evenly on a circumference of the central shaft such that a set angle from one paddle to an adjacent paddle of the same set is substantially equal, and wherein further the second set of paddles is offset in a circumferential direction from the first set of paddles. Thereby, the paddles within a set contact the lignocellulosic material at regular intervals and a longer or shorter interval is provided between the sets. This ensures an improved distribution of the lignocellulosic material as it is transported towards the outlet opening. In some embodiments, the paddles are distributed on the central shaft without overlap in the circumferential direction. Thereby, the distribution of lignocellulosic material is rendered smooth and even.

Suitably, the central shaft comprises a third axial portion arranged between the first axial portion and the second axial portion, said third axial portion lacking both helical blade and paddles. Thereby, the lignocellulosic material transported by the helical blade is distributed evenly by the paddles and a risk of co-rotation of the lignocellulosic material or of plugs forming in the lignocellulosic material is minimized. This is particularly suitable where the lignocellulosic material is less likely to adhere to walls of the housing.

Many additional benefits and advantages of the present invention will be readily understood by the skilled person in view of the detailed description below.

DRAWINGS

The invention will now be described in more detail with reference to the appended drawings, wherein

Fig. 1 discloses a planar view from the side of a screw conveyor according to a first embodiment of the invention;

Fig. 2 discloses a planar view from above of the screw conveyor of Fig. 1 ;

Fig. 3 discloses a perspective view of the conveyor screw of the first embodiment;

Fig. 4 discloses an enlarged planar view of paddles of the conveyor screw of the first embodiment;

Fig. 5 discloses an enlarged planar view of a second end of the housing showing the helical blade, paddles and outlet opening in more detail; and

Fig. 6 discloses a planar view from the second end showing the central shaft and paddles in more detail. All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the respective embodiments, whereas other parts may be omitted or merely suggested. Any reference number appearing in multiple drawings refers to the same object or feature throughout the drawings, unless otherwise indicated.

DETAILED DESCRIPTION

The present invention will now be described with reference to the Figures in which a first embodiment is disclosed. Further embodiments are described in the text below, and it is in particular to be noted that features of such further embodiments may freely be combined with each other or with the first embodiment, as long as such combinations are not explicitly stated as undesirable or unsuitable.

The term “lignocellulosic material” as used herein is to be understood as materials containing lignin, cellulose and hemicellulose. One example of such materials is wood, others include other agricultural or forestry wastes. Lignocellulosic material thus refers to any such material, both treated and untreated.

The term “substantially equal” as used herein is to be understood as equal within manufacturing tolerances, or at least not differing more than 10 %. Thus, when one length or distance is stated as “substantially equal” to another, they are to be understood as not differing in length or distance more than 10 % of a total length of one of them.

The term “screw conveyor” is used herein to denote an assembly for transport or treatment of a lignocellulosic material in which a conveyor screw is used to transport material from an inlet to an outlet. The screw conveyor of the invention may be used only for transporting lignocellulosic material but may alternatively be a bioreactor or form part of a bioreactor; or be a cooking tube or digester, or form part of a cooking tube or digester.

Fig. 1 discloses a screw conveyor 10 according to a first embodiment of the invention, with an elongated housing 20 that extends from a first end 11 to a second end 12. The housing 20 has an inlet opening 13 and an outlet opening 14 for receiving and discharging lignocellulosic material, respectively, and the inlet opening 13 is closer to the first end 11 than to the second end 12 so that lignocellulosic material introduced into the housing 20 is transported in a transport direction D from the inlet opening towards the second end 12 and the outlet opening 14. In the first embodiment, the outlet opening 14 is arranged in a bottom b of the housing 20, but in other embodiments the outlet opening 14 could instead be at the second end 12 so that lignocellulosic material is discharged in the transport direction D. The inlet opening 13 is suitably in an upper end of the housing 20 but could in some embodiments instead be at the first end 11 so that introduction of lignocellulosic material into the housing 20 takes place in the transport direction D. The transport direction D is a direction along a rotational axis of the central shaft 31.

Inside the housing 20, a conveyor screw 30 is rotatably arranged. The conveyor screw 30 extends from the first end 11 to the second end 12, and may in some embodiments extend from a first end wall 11A to a second end wall 12 A, but in other embodiments simply extend from a vicinity of the first end wall 1 1A to a vicinity of the second end wall 12A as long at the conveyor screw 30 extends from the inlet opening 13 to the outlet opening 14. The conveyor screw 30 may be journalled in the housing 20 or may be held in another way such that it is rotatable.

The conveyor screw 30 comprises a central shaft 31 with a helical blade 32 arranged on a first axial portion A and a plurality of protruding paddles 33 arranged on a second axial portion B. The first axial portion A is closer to the inlet opening 13 than to the outlet opening 14, whereas the second axial portion B is closer to the outlet opening 14 than to the inlet opening 13. In this way, lignocellulosic material introduced through the inlet opening 13 will be contacted by the helical blade 32 before being contacted by the paddles 33 on its way towards the outlet opening 14. The helical blade 32 is arranged for transporting the lignocellulosic material in the transport direction D that is an axial direction along the conveyor screw 30 from the first end 11 to the second end 12, i.e. from the inlet opening 13 towards the outlet opening 14.

In the first embodiment, the second axial portion B extends along the central shaft 31 such that the paddles 33 are able to contact the lignocellulosic material both before it reaches the outlet opening 14 and as it falls into the outlet opening 14 from the housing 20. Thus, the second axial portion both extends from an edge 14A of the outlet opening 14 towards the first end 11 of the housing 20 and from the edge 14A of the outlet opening 14 towards the second end 12 of the housing 20. Suitably, the second axial portion B extends a first distance D I from the outlet opening 14 towards the first end 11, i.e. towards the first axial portion A. The first distance D I is at least 10 % of a diameter d of the conveyor screw 30, preferably at least 20 % and more preferably at least 25 %. It is advantageous for the paddles 33 to start at the first distance D I of at least 10 % of the diameter d since this enables them to act on the lignocellulosic material well before the lignocellulosic material reaches the outlet opening 14. Similarly, it is advantageous that the second axial portion B with the paddles 33 extends over the outlet opening 14, in particular when the outlet opening 14 is provided in the bottom of the housing 20 as in the first embodiment. This ensures that the lignocellulosic material is spread out evenly when it is discharged. Good results are achieved when the second axial portion B extends over the outlet opening 14 at least 25 % but not more than 50 % of a width w of the outlet opening 14. The first distance D I is thus a distance from the edge 14A of the outlet opening 14, said edge being the edge that is closest to the inlet opening 13. In the first embodiment, the outlet 14 is circular but in other embodiments other shapes may alternatively be used.

The paddles 33 are separated from each other in the transport direction D, and suitably at least three paddles 33 are provided to ensure that the distribution and transport of the lignocellulosic material is uniform. However, in some embodiments only two paddles 33 may be provided, and in the first embodiment eight paddles are provided as will be explained further below with particular reference to Fig. 6. The helical blade 32 forms a first helix H l on the central shaft 31, and the first helix H l suitably has a fixed pitch but in some embodiments the pitch may differ along the helical blade 32 so that the pitch increases or decreases along the helical blade 32 towards the second end 12 of the housing 20.

The paddles 33 are arranged on the central shaft 31 so that they form a second helix H2. The second helix H2 may be seen as a curve that winds around the central shaft 31 and that joins middle points P of each of the paddles 33 so that a helix shape is formed. This is indicated by a dashed curve in Fig. 1. The middle point P of a paddle 33 is defined as a point at a center of the paddle 33 in the circumferential direction.

One of the first helix H 1 and the second helix H2 forms a right-handed screw on the central shaft 31 and the other of the first helix H l and the second helix H2 forms a left-handed screw on the central shaft 31. Thus, the helices form threads that are wound in opposite directions as they travel along the central shaft 31 from the first end 11 of the housing 20 towards the second end 12 of the housing 20. When rotating the conveyor screw 30, a rotational direction that enables the helical blade 32 to transport the lignocellulosic material towards the outlet opening 14 is denoted as a forward direction, whereas a rotational direction that results in transport of the lignocellulosic material towards the inlet opening 13 is denoted as a backwards direction.

One particular advantage of providing the second helix H2 in the opposite direction to the first helix H l is that the interval between paddles 33 acting on the lignocellulosic material is increased. Thus, with an offset between the paddles of 90° as in the first embodiment (see below with reference to Figs. 5-6), the central shaft 31 rotates 270° between one paddle 33 contacting the lignocellulosic material and the next paddle 33 doing the same. This is particularly beneficial since it improves the distribution and transport of the lignocellulosic material in the conveyor screw 30.

Fig. 2 shows the screw conveyor 10 of Fig. 1 from above, to illustrate how the inlet opening 13 is arranged in a top t of the housing 20 to enable lignocellulosic material to fall into the screw conveyor 10 and be transported by the helical blade 32 towards the outlet opening 14 (see Fig. 1).

The conveyor screw 30 is operated by any suitable drive unit, suitably comprising an electrical motor.

In some embodiments, a third axial portion C may be provided between the first axial portion A and the second axial portion B. The third axial portion C is a portion of the central shaft 31 that lacks both the helical blade 32 and the paddles 33 and allows the lignocellulosic material to gather after passing the helical blade 32 and before being transported and distributed by the paddles 33. Including the third axial portion C is advantageous in distributing the lignocellulosic material evenly and in avoiding or at least minimizing co-rotation of the lignocellulosic material or formation of plugs inside the screw conveyor.

In other embodiments, the first axial portion A and the second axial portion B are arranged immediately adjacent to each other to allow the lignocellulosic material to be distributed and transported by the first paddle 33 immediately after leaving the helical blade 32. This is particularly advantageous for lignocellulosic material comprising softwood that may cling to an internal wall 20A of the housing 20.

Fig. 3 shows the conveyor screw 30 in a perspective view to disclose more clearly the arrangement and orientation of the paddles 33 as compared with the helical blade 32.

Fig. 4 discloses the paddles 33 in more detail, showing the paddles 33 each comprising a contact surface S that is arranged on the central shaft 31 at a contact surface angle a to the transport direction D. The contact surface angle a is larger than 90° to ensure that the paddles 33 transport the lignocellulosic material in the transport direction D. It is advantageous to arrange the paddles 33 with the contact surface angle a in the range 100° - 140°, preferably in the range 105° - 130°, since this ensures a smooth transportation of the lignocellulosic material towards the outlet opening 14. The helical blade 32 is arranged at a blade angle p to the transport direction D (see Fig. 5), and in the first embodiment the contact surface angle a is within 20°, preferably within 10° of the blade angle 3 of the helical blade 32 to the transport direction D. By arranging the paddles 33 with a contact surface angle a that is close to the blade angle 3, each of the paddles 33 will transport the lignocellulosic material at substantially the same rate as the helical blade 32 and this ensures a smooth transport in the screw conveyor 10. The flow of material in the second axial portion may be at another speed than the flow of material in the first axial portion, but it is advantageous that each of the paddles 33 transports the material at substantially the same rate as the helical blade 32 since this ensures smooth handling of the material.

The contact surface S of the paddles 33 is thus angled in relation to the transport direction D, and the contact surface S has an axial length L in the transport direction D. This axial length L corresponds to a portion of the internal wall 20A (see Fig. 2) of the housing 20 that the paddle 33 is able to scrape during rotation of the conveyor screw 30 so that deposits or build-up of lignocellulosic material on the interior wall 20A is prevented. In the first embodiment, the paddles 33 are arranged in such a way that they together are able to scrape the internal wall 20A along the entire length of the second axial portion B, and this is particularly beneficial in preventing deposits of material on the internal wall 20A. This is achieved by the axial length L of each paddle 33 forming a combined axial length in the second axial portion B that is equal to or substantially equal to the length of the second axial portion B and thereby covers the second axial portion B. In some embodiments, the axial length L of one paddle 33 may overlap the axial length L of another paddle 33 so that a portion of the internal wall 20A is scraped by two paddles 33, but in other embodiments the paddles 33 may instead be arranged so that the axial lengths L of the paddles 33 are adjacent in the axial direction but do not overlap. That the axial length L of a paddle 33 corresponds to a portion of the internal wall 20A is the same as the axial length L of the paddle 33 covering an axial portion of the central shaft 31. In some embodiments, the contact surface angle a of all paddles 33 is substantially equal, but in other embodiments the contact surface angle a may instead vary from one paddle 33 to another.

From an end point 32A of the helical blade 32 (see Fig. 1), the first of the paddles 33 is suitably offset in a circumferential direction to allow the lignocellulosic material to leave the helical blade 32 before being contacted by the paddle 33. Suitably, the offset of the middle point P of the paddle 33 closest to the helical blade 32 is in the range 90°- 270°, preferably 150° - 210°, more preferably 180°. This ensures that a suitable time passes before the paddle contacts the lignocellulosic material that has just left the helical blade 32.

In the first embodiment, the paddles 33 are provided in a first set 331 of paddles and a second set 332 of paddles and this is shown in particular in Figs. 5-6. The first set 331 is provided closer to the first end 11 of the housing 20 whereas the second set 332 is provided closer to the second end 12. Within each set, the paddles 331, 332 are distributed evenly on the circumference of the central shaft 31. This means that an angular distance from one paddle 33 in the first set 331 to an adjacent paddle 33 in the first set 331, i.e. a set angle y between the paddles 33 seen from the first end 11 as in Fig. 6, is substantially equal to an angular distance between any other adjacent paddles in the first set 331. Similarly, the angular distance or set angle y between two adjacent paddles 33 in the second set 332 is substantially equal to the angular distance between any other pair of adjacent paddles 33 in the second set 332. For the first embodiment with four paddles in each set 331, 332, that set angle y is 90°, but in other embodiments other set angles are of course also possible. The second set 332 is offset with an offset angle 6 from the first set 331 in the circumferential direction such that no paddle 33 of the second set 332 overlaps a paddle 33 of the first set 331. In the first embodiment, the offset angle 6 is 45 ° but other offsets are of course also possible.

In some embodiments, more than two sets 331, 332 of paddles 33 can be used to further improve the spread of the lignocellulosic material. It is in particular to be noted that the second helix H2 formed by the plurality of paddles 33 may have variable pitch so that the pitch in one set can differ from the pitch in another set or the pitch between sets where the offset is provided. Thus, the second helix H2 as disclosed herein is to be understood as a helix formed by any number of sets of paddles 33 with any offset between them.

It is to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.

Claims

1. Screw conveyor (10) for transport of lignocellulosic material, comprising:

- an elongated housing (20),

- a conveyor screw (30) rotatably arranged in the housing and extending from a first end (11) of the housing to a second end (12) of the housing,

- an inlet opening (13) and an outlet opening (14) in the housing, wherein the inlet opening (13) is arranged closer to the first end (1 1) than the outlet opening (14) is, wherein the conveyor screw (30) comprises a central shaft (31) having a first axial portion (A) on which a helical blade (32) is arranged for transporting a material in a transport direction (D) that is an axial direction from the inlet opening (13) towards the outlet opening (14), and the central shaft (31) also having a second axial portion (B) on which a plurality of protruding paddles (33) is arranged, said paddles (33) being separated from each other in the transport direction (D), and said first axial portion (A) being closer to the first end (11) than the second axial portion (B), and wherein further the helical blade (32) forms a first helix (H l) on the central shaft (31) and the plurality of paddles (33) are arranged such that a middle point (P) of each of the paddles form a second helix (H2), wherein one of the first helix (H l) and the second helix (H2) forms a right- handed screw and the other forms a left-handed screw on the central shaft (31).

2. Screw conveyor according to claim 1, wherein each paddle (33) comprises a contact surface (S) and wherein the contact surface (S) is arranged at a contact surface angle (a) to the transport direction (D) .

3. Screw conveyor according to claim 2, wherein the contact surface angle (a) is in the range 100° - 140°, preferably in the range 105° - 130°.

4. Screw conveyor according to claim 2 or 3, wherein the contact surface angle (a) is within 20°, preferably within 10°, of a blade angle (P) of the helical blade to the transport direction.

5. Screw conveyor according to any of claims 2-4, wherein the contact surface (S) of each paddle (33) has an axial length (L) that covers an axial portion of the central shaft (31), and wherein the plurality of paddles (22) is arranged such that a combined axial length of the paddles covers the second axial portion (B) of the central shaft (31).

6. Screw conveyor according to any previous claim, wherein the helical blade (32) has an end point (32A) at an end closest to the second axial portion (B) and wherein the paddle (33) closest to the first axial portion (A) is arranged with its middle point (P) offset in a circumferential direction from the end point (32A) of the helical blade (32), said offset being in the range 90° - 270°, preferably 150° - 210°, more preferably 180°.

7. Screw conveyor according to any previous claim, wherein the plurality of paddles (33) comprises at least three paddles (33).

8. Screw conveyor according to any previous claim, wherein the second axial portion (B) extends from the outlet opening (14) a first distance (D I) towards the first axial portion (A), said first distance (D I) being at least 10 % of a diameter (d) of the conveyor screw (32), preferably at least 20 % and more preferably at least 25 %.

9. Screw conveyor according to any previous claim, wherein the outlet opening (14) is in a bottom (b) of the housing (20A), and wherein the second axial portion (B) extends over the outlet opening (14) at least 25 % but not more than 50 % of a width (w) of the outlet opening (14).

10. Screw conveyor according to any previous claim, wherein the plurality of paddles (33) comprises a first set of paddles (331) and a second set of paddles (332), wherein each set of paddles (331, 332) is distributed evenly on a circumference of the central shaft (31) such that a set angle y from one paddle (33) to an adjacent paddle (33) of the same set (331, 332) is substantially equal, and wherein further the second set of paddles (332) is offset in a circumferential direction from the first set of paddles (332).

11. Screw conveyor according to any previous claim, wherein the paddles (33) are distributed on the central shaft (31) without overlap in the circumferential direction.

12. Screw conveyor according to any previous claim, wherein the central shaft (31) comprises a third axial portion (C) arranged between the first axial portion (A) and the second axial portion (B), said third axial portion (C) lacking both helical blade (32) and paddles (33).