WO2020114820A1

WO2020114820A1 - Steam heated yankee drying cylinder for paper or tissue machines with condensate draining system

Info

Publication number: WO2020114820A1
Application number: PCT/EP2019/082419
Authority: WO
Inventors: Ola Jonsson
Original assignee: Valmet Aktiebolag
Priority date: 2018-12-05
Filing date: 2019-11-25
Publication date: 2020-06-11
Also published as: SE543082C2; SE1851513A1

Abstract

The invention relates to a steam condensate removal system in a steam-heated Yankee drying cylinder (3) for fiber webs. According to the invention is a continuous steam condensate catching groove (7) arranged in the inner surface with a length of the groove exceeding at least 5-10 turns over the inner surface. In one embodiment is a single continuous groove (7) running over the entire width of the steam-heated Yankee drying cylinder.

Description

STEAM HEATED YANKEE DRYING CYLINDER FOR PAPER OR TISSUE MA CHINES WITH CONDENSATE DRAINING SYSTEM

BACKGROUND OF THE INVENTION

The invention relates to steam heated Yankee drying cylinders for paper or tissue machines with condensate draining system. Yankee drying cylinders have been used frequently as the main drying step in both paper and tissue machines. On a Yankee drying cylinder the fiber web is subjected to drying where the dryness increases from about 40% dryness to about 90% dryness. These Yankee cylinders have traditionally been made of cast iron, and recently also welded steel Yankees have been made. The Yankee drying cylinders typically have a diameter up to 6 meters and a cylinder length of up to 8 meters, and with a gross weight of up to 200 ton.

Conventional Yankee drying cylinders have an outer jacket which on its inside has multiple circumferential grooves for collecting condensate that is formed as a result of steam condensing on the interior of the jacket and thus heating the jacket. A large steam chamber located radially inwardly of jacket is fed with hot steam at considerable pressure, typically 5-10 bar steam pressure. The Yankee drying cylinder rotates with peripheral speeds up to 2000 m/min, i.e. rather harsh conditions for a revolving pressure vessel. Some Yankee cylinders have even exploded due to deficiencies in

manufacturing or due to uneven heating of the Yankee.

In order to obtain a uniform temperature over the outer surface of the jacket, i.e. the surface where the paper or tissue web is running on for drying, the condensate must be evacuated from the multiple circumferential grooves. If a uniform temperature is not established, the paper is not dried uniformly, and stripes will occur in the dried web and cause uneven diameter in the final paper rolls produced. For that purpose, drain pipes are arranged with a condensate inlet in the bottom of each groove, and with a pipe outlet outside of the jacket, i.e. outside of the pressure vessel. The condensate may be sucked into the pipes and sometimes using steam ejectors around the pipe inlet that withdraw the accumulated condensate as droplets. But often is the suction effect obtained by establishment a pressure drop of about 0.3-0.7 bar, conventionally at 0.5 bar, from the high pressure inside of the Yankee cylinder and to the outer evacuation end of the drainage system located outside of the Yankee cylinder. I.e. in a Yankee pressurized with steam at 8,5 bars establishing about 8.0 bars in the evacuation end.

The heating effect from condensation could approach 1,8 MW and as much as 40 liter of condensate may be formed per minute, wherein this volume of condensate must be removed as condensate layers reduce heat transfer to the outer jacket. This is of outmost importance for equal heating over the tissue or paper web as condensate may be accumulated unevenly on the inner surface of the outer jacket.

In Yankee cylinders, the entire interior volume inside of the outer jacket is filled with hot pressurized steam with a temperature close to the saturation point. Hence, condensable steam is readily available for condensation on the inside of the outer jacket in equal amount over the entire length of the outer jacket, i.e. along the generatrix of the outer jacket.

Some heated drying cylinders used in early stages of the paper or tissue machine are heated with steam supplied into a narrow annular space between the outer jacket and an inner cylinder, wherein the steam is supplied into a gable end of the drying cylinder and residual steam is evacuated from the narrow annular gap from the other gable end. Most often, the heating effect is limited and heating of the outer surface is mainly caused by convection effect from the fast-flowing steam and not primarily by condensation.

Examples of such heated drying cylinders, or thermal rollers, may be seen in

US3425288, US4955268 or US2018202725. This kind of heating is not working in Yankee cylinders where final drying of the tissue or paper web occurs, and a heating effect of about 1,8 MW is needed. Heating by convection could only reach a fraction of the heating possible using condensation. Moreover, the heating roll will not be heated in a uniform manner over the lengthwise direction of the heating roll as the steam with highest heat value is fed in from one end and residual steam with lower heat value leaves the other end.

PRIOR ART US8.959.790 discloses a drain pipe positioning system for Yankee drying cylinders having a joint connection for the drain pipe enabling positioning of the drain pipe close to the bottom surface of the condensate groove. The draining system for condensate, comprises a plurality of elongated drain pipes 6 which commonly are called suction pipes and are arranged in a plurality of evenly distributed groups around the inside of the jacket 2, wherein at least one suction pipe is used for draining each individual groove 3 from condensate as efficiently as possible. A problem with this kind of draining system is that the drain pipe or pipes for an independent circumferential groove may become blocked and this may cause the condensate level in the groove to rise with uneven heating of the jacket as a result causing stripes in the dried web (the condensate accumulated in the groove decreases heating of the jacket in the area of the groove). This might lead to the above-mentioned explosion of the jacket due to uneven heating, but also to uneven heating of the paper or tissue web that results in streaks of differing thickness of the fiber web in the final take- up roll after the drying cylinder. A remedy to avoid this is to use 2-4 drain pipes for each individual groove, which may safeguard that at least one drain pipe functions properly. The number of draining pipes necessary is at least the same as the number of grooves in the jacket, which also increases the cost of the drying cylinder and adds complexity into the draining system. The number of draining pipes needed also increases operating costs as each draining pipe causes a pressure loss, if simple suction technique is used, and/or increased consumption of steam, if steam ejectors are used.

SUMMARY OF THE INVENTION

The invention relates to an improvement in steam condensate removal systems for steam heated Yankee drying cylinders, where the steam condensate removal capacity may be assured or even improved with a smaller number of suction pipes for the con densate.

In this context, a Yankee cylinder is the final drying cylinder in a tissue or paper machine where the fiber web running on the outer surface of the Yankee cylinder in creases in dryness from about 30-50%, typically 40% to about 80-95%, typically 90%, and where the outer surface is heated by steam condensing on the interior surface of the Yankee cylinder, and which Yankee cylinder has an extended steam chamber occupying more than 50% of the radius of the Yankee cylinder filled by pressurized hot steam preferably close to its condensation point. The invention further improves an even heat profile over the entire width of the Yankee drying cylinder, avoiding local heat sinks, caused by a single groove being flooded by condensate, that could cause heat stresses that could cause the Yankee drying cylinder to explode, or could cause uneven drying of the web.

The invention also enables less frequent stops for solving drain pipe blockage that re sults in flooding of single independent grooves and non-uniform heating of the Yankee drying cylinder. With a continuous groove the condensate may be prevented from local accumulation in a single groove and instead be smeared out over the bottom of the con tinuous groove.

In order to obtain these advantages, the steam condensate removal system is modified in a steam-heated Yankee drying cylinder for fiber webs in tissue or paper machines. The Yankee drying cylinder has a jacket which on the interior surface has circumferentially running grooves for collecting condensate that is formed on the interior surface as the steam condenses and transfers heat to the interior surface for further transfer of heat to the outside of the jacket where the fiber web is heated and residual water in the fiber web is evaporated from the fiber web increasing the dryness of the fiber web. The cir cumferentially running grooves are equipped with a draining system with multiple draining pipes having one suction end located in the bottom of a groove and an outlet end arranged outside of the Yankee drying cylinder for evacuation of condensate accu- mulating in said grooves during heating. The inventive modification lies in the fact that at least a part of the interior surface of the jacket has a continuous groove running over at least 5-10 turns over the interior circumference.

By this concept each condensate catching grove could be given an extended length but still with a number of draining pipes for this continuous groove, maintaining assured withdrawal of the condensate, while smearing out the accumulated condensate film over the entire length of the continuous groove.

According to the invention the continuous groove is of similar shape and size as inde pendent grooves of conventional design lying in a common vertical plane trough the Yankee drying cylinder. Hence, said continuous groove has a width in the range 9-18 mm and with intermediate notches between grooves with a width of said notches in the range 9-25 mm, and with a depth of said groove in the range 25-55 mm.

With these dimensions of the notch and grooves will the pitch angle, i.e. a = b, be less than 5 degrees and preferably lie in the range 0,1 to 2,5 degrees for Yankees with a cir- cumference from 3-6 meter. This is of outmost importance for the Yankee to withstand the applied steam pressure, as all notches functions like reinforcement ribs.

According to a preferred embodiment of the invention, the interior surface of the jacket may have one single continuous groove running over the entire width of the jacket. This embodiment may simplify machining processes of the groove, using multiple turning steel arranged one after the other and cutting off the material in the jacket until the con tinuous groove reaches the total depth required in the range 25-55 mm.

According to an alternative preferred embodiment of the invention the interior surface of the jacket may have at least two areas each with a single continuous groove running over the entire width of respective area of the jacket. In this embodiment the two areas may each have a different pitch angle of the groove. The pitch angles could be selected such in relation to the direction of rotation such that a thrust force on the condensate is generated by the friction between the condensate volume and the walls of the groove towards the outer ends of the Yankee drying cylinder, further improving evacuation of the condensate, and preventing any accumulation in the center of the Yankee drying cylinder.

According to a preferred embodiment utilizing the advantages with longer grooves the number of draining pipes may be less than the number of full turns of the continuous groove such that less than one suction end per full turn of the continuous groove is located adjacent to the bottom of the continuous groove such that the suction ends of neighboring suction ends are located at least 1.2 times turns apart in the continuous groove.

In order to improve evacuation of condensate from the continuous groove outwardly towards the ends of the Yankee drying cylinder the depth of said groove could also increase towards at least one end of the jacket such that a slanting bottom surface of said groove is formed. The effect from the centrifugal force on the condensate film will push the condensate over the slanting bottom surface and towards the ends of the Yankee drying cylinder.

The invention will be disclosed in more detail with reference to following figures. LIST OF FIGURES

In the following schematic drawings are details numbered alike in figures, and details identified and numbered in one figure may not be numbered in other figures in order to simplify figures.

FIG. 1; shows a schematic representation of a Yankee drying cylinder in

operation;

FIG. 2; shows a cross-sectional part of a cylindrical outer jacket of a conventional

Yankee drying cylinder;

FIG. 3; shows same view as in figure 2 and with suction pipes for withdrawal of condensate; FIG. 4a; shows a cross-sectional view of a Yankee drying cylinder with one kind of condensate removal system;

FIG. 4b; shows same view as in figure 4a but with an alternative condensate

removal system;

FIG. 5; shows same view as in figure 2 but with grooves according to the invention;

FIG. 6a shows a first example of the inside of the Yankee drying cylinder with grooves according to one embodiment of the invention;

FIG. 6b shows a second example of the inside of the Yankee drying cylinder with grooves according to another embodiment of the invention; FIG. 6c shows a third example of the inside of the Yankee drying cylinder with grooves according to another embodiment of the invention; FIG. 7 shows same view as in figure 4a and 4b, but utilizing only one suction pipe per each turn of the groove, as back up suction pipes are arranged in preceding and following turns of the continuous groove; and

FIG. 8 shows an alternative embodiment of the invention with an inner surface slanting towards the ends of the Yankee drying cylinder.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1 a typical paper or tissue machine set-up is shown where the in ventive Yankee drying cylinder could be implemented. In following parts, the machine is referred to as a tissue machine, but it should be clear that the inventive Yankee drying cylinder may be implemented in a paper machine as well. The tissue machine has a forming section 110 comprising an inlet box (head box) 120 arranged to feed a fiber suspension into a gap between two wire belts 130, 140. A fiber web W will be formed, and the fiber web supported by the wire belt 140 will be fed to the Yankee drying cylin- der 3 which is heated by steam. The fiber web will be dried when fed upon the outer surface of the Yankee drying cylinder as the jacket of the Yankee drying cylinder is heated to about 140°C by steam condensing on the interior surface of the jacket, result ing in that water content in the fiber web is evaporated. After drying of the fiber web W on the Yankee drying cylinder the fiber web will be removed by a doctor blade 150 and finally accumulated on a take up roll 160. The machine set up shown in figure 1 may be a soft tissue machine for production of sanitary paper or toilet paper but the inventive Yankee drying cylinder may also be used in conventional paper machines.

The Yankee drying cylinder 3 is heated by hot steam, pressurized to about 5-10 bar and fed to the interior of the Yankee drying cylinder 3. With reference to figure 2 the jacket 4 of the Yankee drying cylinder 3 is designed with conventional independent circumfer ential grooves 7, each groove lying in one vertical plane with grooves separated by notches 9. The reference number 8 in figure 2 is indicating the very bottom surface of each groove 7. These circumferential grooves are typically obtained from milling or turning operations on the inner surface, and in a typical Yankee drying cylinder the number of grooves may amount to about 100-200 grooves. The grooves are imple mented in order to increase heat transfer to the jacket as these grooves increase the total inner surface of the jacket on which the steam may condense. The intermediate notches 9 also provide for a structural strengthening of the Yankee cylinder, increasing the limit for the maximum pressurization of the Yankee cylinder interior. Some Yankee cylinders may have smooth inner surface but are used in application with lower steam pressures and lower heating capabilities.

Compared with a smooth inner surface of the jacket a Yankee drying cylinder with grooves could increase heating of the jacket to such an extent that the production could be increased almost in proportion to the increase of area of the inner surface, i.e. by in creasing the production by the increase of the circumferential speed of the Yankee dry ing cylinder and still obtain the same dryness of the fiber web.

When the Yankee drying cylinder 3 is used to dry a fiber web, hot steam is supplied into the inside of the Yankee drying cylinder 3. The steam condenses on the jacket 4 of the Yankee drying cylinder 3 and the outer surface 5 of the jacket will be heated. The con densate will accumulate as a thin film on the bottom surface 8 of the grooves as the re volving speed of the Yankee drying cylinder is more or less smearing out the conden- sate as a film on this bottom surface 8, under influence of the centrifugal forces. As this condensate film increases in height during heating the heat transfer will be reduced and hence should this condensate be removed in order to maintain a high heat transfer rate. The circumferentially running grooves 7 each has a width A in a direction orthogonal to the machine direction (i.e. the direction of the paper web through the machine). With reference to figure 2, the width A is the same as the distance between two neighboring notches 9. The width A is a standard width, made by same lathing steel or milling tool over the entire width of the drying roll. The notches 9 have a width B which conven tionally also is the same over the entire width of the drying roll.

For several types of Yankee drying cylinders 3 a standard width of the grooves 7 is about 12 mm, but may lie in the range 9 to 18 mm and the width of the notches 9 about 18 mm but may lie in the range 9 to 25 mm. The depth of the grooves 7 may vary but lies typically in the range 25-55 mm.

In figure 3, the principle of condensate removal is disclosed. In a gray section is dis- closed the entire interior volume that is pressurized with hot steam preferably close to the condensation point. This volume filled with hot steam has a radial length well ex ceeding 50% of the full radius of the Yankee drying cylinder, often in the range 60-80% of the full radius of the Yankee drying cylinder. Suction pipes 10a, 10b, 10c and lOd are arranged with a suction inlet 11 close to the bottom 8 of the groove 7. The suction pipes 10a- lOd are all arranged fixed with the jacket 4 of the drying roll and corotate with it.

As seen in figure 4a, showing a first embodiment of the condensate removal system, a number of suction pipes 10a could be arranged in each individual groove, here four pipes 10a for each individual groove 7, but any number of 1-5 suction pipes may be ar ranged for each individual groove. As seen in figure 3 from the left-hand side the next fourth groove may be evacuated by suction pipes 10b, the next eight groove be evacu ated by suction pipes 10c, the next twelfth groove be evacuated by suction pipes lOd and so on over the entire width of the jacket. All using suction pipes oriented as shown in figure 4a at clock positions 12:00, 03:00, 06:00 and 09:00. The intermediate grooves in figure 3 may be evacuated by suction pipes arranged slightly indexed from the clock positions shown in figure 4.

All suction pipes 10a- lOd are connected to a central evacuation channel (not shown per se) in the shaft 12 that leads the condensate to the outside EXT of the Yankee drying cylinder.

The suction inlet 11 may be of any kind of design and the design per se is not part of the inventive concept. In figure 4a, the suction inlet 11 is shown as a T-shaped member forming an inlet facing in the direction of rotation R of the Yankee drying cylinder. However, the suction inlet may alternatively be a pipe end with a radially directed inlet, facing the bottom of the groove, and/or with a coaxial steam ejector design that by the speed of steam may withdraw condensate as droplets in the steam flow through pipes 10a- lOd.

In figure 4b is an alternative embodiment of the condensate removal system, where in- stead 4 headers He are arranged at a distance from the interior surface of the jacket and with suction pipes la arranged at an angle towards the bottom 8 of the grooves. At the end of each header is a draining pipe connected from the header to an evacuation chan nel in the shaft (not shown per se). In figure 5, a first embodiment of the invention is shown in the same view as shown in figure 2. The difference here is that the grooves 7 are not strictly running in a circumfer- ential plane, but instead machined into the inside surface of the jacket 4 with a pitch an gle such that the grooves form one single continuous groove along the inside surface of the jacket, from one end of the jacket to the other end of the jacket. The pitch angle a is shown in figure 6a in a first embodiment where the groove forms one single continuous groove over the entire width of the jacket. In order to visualize the grooves 7 they are shown in slight grey tone, only the first 4 grooves 7 on the left side numbered, and the notches 9 are shown in white, only the first 4 notches 9 on the left-hand side numbered. In this embodiment the suction inlets 11, of which only the first 4 in the upper row R1 are numbered, may be arranged slightly different from conventional design with inde- pendent single grooves. Due to the fact that the groove is a continuous groove the num ber of suction pipes and associated suction inlets 11 may be reduced considerably, as complementary suction inlets will follow in the continuous groove and guarantee that condensate is withdrawn from the groove. Due to the centripetal acceleration the con densate layer on the bottom 8 of the groove 7 is smeared out and local accumulation of a condensate layers may be prevented. In this embodiment a subsequent suction inlet is arranged in a position at about 1.2 full turns of the jacket from a preceding suction inlet. This embodiment shows the principle in possible reduction of suction inlets, which when using a minimum of one suction inlet per independent groove according to prior art necessitates at least one suction inlet per groove and one single turn of the groove. But as indicated earlier, most often 2 or sometimes 3 suction inlets are used for one sin gle independent groove in order to maintain withdrawal capacity if one suction inlet be comes plugged. The back-up withdrawal capacity of withdrawing condensate when us ing a continuous groove may thus reduce the number of suction pipes and associated suction inlets by a factor of 2 or even 3, which dramatically reduces costs for the con- densate removal system. When using independent grooves according to prior art and with a backup withdrawal capacity with 2 suction inlets per groove as many as 400 suc tion pipes and associated suction inlets could be needed if the jacket has 200 grooves. With a continuous groove according to the embodiment of figure 6a, the num ber of suction pipes and associated suction inlets could be reduced to less than a 100, down to about 80 (if suction inlets are located 1,2 turns apart). If 200 independent grooves are machined into the interior surface of the jacket and 4 suction pipes are lo cated in each groove, the total number of grooves reaches a total number of 800 suction pipes which increases the cost for manufacturing and introduces a time-consuming ser vice job when plugs in blocked suction pipes needs to be removed, most often done by pressurizing each individual suction pipe inlet with pressurized air during shut down. In figure 6b, a second alternative embodiment of the invention is shown schematically. The difference here is that the grooves 7 are machined with two different pitch angles a and b in two areas A1 and A2 respectively, i.e. the left- and right-hand side. The pitch angle a may correspond to +30 mm per turn, and the pitch angle b may correspond to - 30 mm per turn, resulting in same pitch angle, i.e. a = b but opposed. The backup function from multiple suction inlets in each half will still be obtained, but in this em bodiment, a positive thrust force T from the rotation in the direction R may be subjected to the condensate layer in the grooves and forwarding condensate towards both ends of the Yankee drying cylinder. Yet an alternative design, shown in principle in figure 6c, may be a solution with multi ple areas like the left- and right-hand side parts of figure 6b, i.e. with for example 4 ar eas A1 to A4 with grooves arranged from left hand side to right hand side with a pitch a in a first area A1 followed by a pitch b in a second area A2, followed by a pitch a in a third area A3 and finally a pitch b in a fourth area A4. But the improvements in backup withdrawal capacity will be obtained also in such arrangements with multiple areas with differing pitch in these areas, as long as each area has a continuous groove running at least 5-10 turns over the inner surface of the jacket. In a Yankee drying cylinder with normally 200 independent grooves, a 4- area sectioning would include a single continu ous groove with about 50 turns in each area if similar notch and groove widths are im- plemented.

Figure 7 shows a cross section similar to that which is shown in figure 4b, but with the inventive continuous groove it may be fully sufficient to have only one suction pipe 10a per turn of the groove, as back-up withdrawal is obtained from suction pipes in follow- ing or preceding turns of the continuous groove. As indicated in Figure 6a, the headers He may be arranged in four rows Rl, R2, R3 and R4. The suction pipe shown in figure 7 may be the first suction pipe from the left-hand side in figure 61, connected to a header for row R4, and the following suction pipe may be arranged 1.2 turns after the first and connected to a header for row R3 and so on.

A further alternative design used with continuous grooves is shown in figure 8. Here the inner surface 9 of the jacket 4 has an inner diameter that increases towards both ends of the jacket, but at least towards one end of the jacket. This results in that a slanting inner surface 9 of said jacket 4 is formed that promotes transport of condensate towards ends of the jacket, or to the next outwardly located groove. The invention is related to the very principle with continuous grooves in condensate catching grooves in Yankee drying cylinders and may be combined with other principle draining pipes and evacuation systems than those schematically shown in the attached figures. This relates also to the form of the grooves that in attached figures are shown as grooves with parabolic shape. The number of grooves running over the interior surface may be more than one starting from the end of the jacket, and hence 2-4 grooves may be machined in parallel from the end of the jacket.

Claims

1. A steam condensate removal system in a steam-heated Yankee drying cylinder (3) for fiber webs in tissue or paper machines capable of increasing the dryness of the fiber web from about 30-50% to about 80-95%; using hot pressurized steam supplied to a steam chamber occupying more than 50% of the radius of the Yankee drying cylinder, said Yankee drying cylinder having a jacket (4) which on the interior surface (9) facing the steam chamber has circumferentially running grooves (7) for collecting condensate that is formed on the interior sur face (9) as the steam condenses and transfers heat to the interior surface for fur- ther transfer of heat to the outside (5) of the jacket where the fiber web (W) is heated and water in the fiber web is evaporated from the fiber web increasing the dryness of the fiber web; said circumferentially running grooves equipped with a draining system with multiple draining pipes (10a, 10b, 10c, lOd) having one suction end (11) located in the bottom of a groove (7) and an outlet end (EXT) arranged outside of the Yankee drying cylinder for evacuation of condensate ac cumulating in said grooves during heating; characterized in that at least a part of the interior surface of the jacket (4) has a continuous groove (7) running over at least 5-10 turns over the interior circumference and with a pitch angle (a or b) lying below 5 degrees.

2. A steam condensate removal system according to claim 1 wherein the pitch angle (a or b) lie in the range 0,1 to 2,5 degrees for Yankees with a

circumference from 3-6 meter.

3. A steam condensate removal system according to claim 1 wherein said groove

(7) has a width (A) in the range 9-18 mm and with intermediate notches between grooves with a width (B) of said notches in the range 9-25 mm, and with a depth (D) of said groove in the range 25-55 mm.

4. A steam condensate removal system according to claim 1 , wherein the interior surface of the jacket has one single continuous groove (7) running over the entire width of the jacket.

5. A steam condensate removal system according to claim 1, wherein the interior surface of the jacket has at least two areas (Al, A2) each with a single continuous groove running over the entire width of respective area (Al and A2 respectively) of the jacket.

6. A steam condensate removal system according to claim 5, wherein the two areas (Al, A2) each have a different pitch angle (a and b respectively) of the groove (7).

7. A steam condensate removal system according to claim 1, wherein the number of draining pipes (10a) are less than the number of full turns of the continuous groove (7) such that less than one suction end (11) per full turn of the continuous groove is located adjacent to the bottom (8) of the continuous groove (7), such that the suction ends of neighboring suction ends are located at least 1.2 times turns apart in the continuous groove.

8. A steam condensate removal system according to claim 3, wherein the depth (D) of said groove is increasing towards at least one end of the jacket such that a slanting bottom surface (8) of said groove (7) is formed.

9. A steam condensate removal system to claim 1 , wherein the inner surface (9) of the jacket (4) have an inner diameter that increases towards at least one ends of the jacket such that a slanting inner surface (9) of said jacket (4) is formed.