FIELD OF THE INVENTION

The present invention relates to a Yankee drying cylinder for drying a wet fibrous web.

BACKGROUND OF THE INVENTION

Yankee drying cylinders are used to dry wet fibrous webs and they are used in particular for the manufacture of tissue paper grades, i.e. such paper grades that are used for such purposes as, for example, bathroom, kitchen towel or facial tissue. Such paper grades may have a basis weight in the range of, for example, 15 g/m ² - 40 g/m ² . When the tissue paper is manufactured, a wet fibrous web is dried on a Yankee drying cylinder which is filled with a hot medium, usually hot steam. Heat from the steam is transferred to the outer surface of the Yankee drying cylinder such that water in the wet fibrous web that contacts the outer surface of the Yankee drying cylinder is evaporated. For many years, such Yankee drying cylinders have been made of cast iron. However, it has also been suggested that Yankee drying cylinders can be made of steel and an example of such a Yankee drying cylinder is disclosed in, for example, US patent No. 8,438,752. Yankee drying cylinders are provided with end walls or end covers such that the inside of the Yankeed drying cylinder will be an enclosed volume that is capable of holding the hot steam. US patent No. 9,428,861 discloses a Yankee drying cylinder in which the end walls comprise an inner cover and an outer cover and a thermal-insulation layer is used which may take the form of a coating. The object of the present invention is to provide an improved Yankee drying cylinder of steel which has a high strength and effective heat insulation. These and other objects are achieved with the present invention as will be explained in the following.

DISCLOSURE OF THE INVENTION

The invention relates to a Yankee drying cylinder for drying a wet fibrous web. The inventive Yankee drying cylinder has a shell which shell is made of steel and has a circular cylindrical form with a central axis. The shell is symmetrical around its central axis, extends in an axial direction and has two axial ends and a middle section located between the axial ends. For each axial end of the shell, there is an end cover that is made of steel material and fixed to the shell such that the shell and the end covers form a hollow inside volume. Each end cover has an outer circumference fastened to the shell and an inner circumference fastened to an attachment ring. Each end cover comprises an inner wall and an outer wall and each of the outer and inner walls has an outer circumference adjacent the shell and a central opening defining an inner circumference adjacent the attachment ring. The central openings of the inner and outer walls are coaxial with each other and form together an opening in the end cover in which opening the attachment ring is fitted. The inner and outer wall are spaced apart from each other in the axial direction of the shell in at least one region between the outer circumference of the end cover and the inner circumference of the end cover. Each end cover is welded to the shell and to the attachment ring. The inner and outer walls are curved or slanting inwards in the axial direction of the shell such that, in a radial direction away from the outer circumference of the end cover and toward the central axis of the shell, the distance in the axial direction of the shell from both the outer and the inner walls to the middle section of the shell decreases. The distance decreases such that, at the inner circumference of the inner and outer walls, the distance in the axial direction from each of the inner and the outer walls to the middle section of the shell is smaller than at the outer circumference of the inner and outer walls.

In embodiments of the invention, the design of the end cover may be such that, for at least a part of the radial distance from the connection of the end cover to the shell toward the central axis of the shell, the inner wall is more slanting or curved inwards than the outer wall such that, in the radial direction toward the central axis of the shell, the distance between the inner wall and the outer wall increases (i.e. the distance in the axial direction of the shell i

In other embodiments, the Yankee drying cylinder may be designed such that, in the radial direction from the shell toward the central axis of the shell, the distance in the axial direction of the shell between the inner wall and the outer wall remains constant.

In advantageous embodiments of the invention, each of the outer wall and the inner wall are welded both to the shell and to the attachment ring and both the inner wall and the outer wall are curved or slanting inwards in the axial direction of the shell and the outer wall and the inner wall may be spaced apart from each other in such a way that, in a radial direction away from the outer circumference of the end cover and toward the central axis of the shell, the distance between the inner and the outer wall increases. In embodiments of the invention, at least one of the outer wall and the inner wall comprises at least two segments that have been welded together with welds extending in the radial direction from the outer circumference of the end cover to the inner circumference of the end cover.

In advantageous embodiments of the invention, both the inner and the outer walls comprise at least two segments that have been welded together.

In embodiments in which both the inner and outer walls comprise two or more segments that have been welded together, the segments of the outer wall are preferably (but not necessarily) rotated in relation to the segments of the inner wall such that the welds that connect the segments of the inner wall have angular positions other than the angular positions of the welds that connect the segments of the outer wall.

Preferably, both the outer wall and the inner wall comprise at least four segments that have been welded together. Embodiments of the invention are conceivable in which one or both of the outer wall and the inner wall comprise more than four segments that have been welded together. For example, the inner and/or the outer wall comprise(s) six segments that have been welded together or eight segments that have been welded together.

Preferably, at least one reinforcement element of steel is welded to both the outer wall and the inner wall and connects the outer wall with the inner wall.

According to one embodiment of the invention, the outer wall is welded to the shell and to the attachment ring, wherein the inner wall is divided into a radially outermost part adjacent the inner wall of the shell and a radially innermost part adjacent the attachment ring. In this embodiment, a reinforcement ring is placed between the inner wall and the outer wall and welded is to the outer wall, to the radially outermost part of the inner wall and to the radially innermost part of the inner wall and the inner wall is welded at its outer and inner circumference to the outer wall.

In embodiments of the invention, at least one of the end covers and preferably each end cover comprises a peripheral ring that extends around the outer circumference (outer periphery) of the at least one end wall, the peripheral ring having the same diameter as the shell. In such embodiments, at least one of the inner and outer walls is welded to the peripheral ring and the peripheral ring is welded to the shell. In some embodiments using a peripheral ring, the outer wall is directly welded to the peripheral ring and to the attachment ring, while the inner wall is divided into a radially outermost part adjacent the shell and a radially innermost part adjacent the attachment ring. The radially innermost part may then be welded to the attachment ring and the radially outermost part can be welded to either the peripheral ring or to the outer wall.

In all embodiments, at least one reinforcement element can be placed between the inner wall and the outer wall and welded to both the inner wall and the outer wall in such a way that at least one slot is formed in the outer wall and the outer wall is welded to the at least one reinforcement element through the at least one slot (34). The at least one reinforcement element that is welded to the outer all through at least one slot may be ring-shaped.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side view giving a schematic representation of the inventive Yankee drying cylinder in use in a paper making machine.

Figure 2 is a schematic cross-sectional view along the axis of a Yankee drying cylinder according to the present invention.

Figure 3 is a front view of a side wall without attachment ring.

Figure 4 is a front view similar to Fig. 3 but showing also the attachment ring.

Figure 5 is a front view of two elements that are parts of a side wall.

Figure 6 is a side view showing the same elements as in Fig. 5.

Figure 7 is a view similar to Fig. 4 but schematically indicating how the side wall may be formed by segments.

Figure 8 is a cross-sectional view along the axis of a Yankee drying cylinder according to a first embodiment of the present invention.

Figure 9 is a cross-sectional view corresponding to Fig. 8 but showing a part of the end cover 10 in greater detail.

Figure 10 is a cross-sectional view similar to Fig. 9 but showing a second embodiment of the invention. Figure 11 is a cross-sectional view along the axis of a Yankee drying cylinder according to a third embodiment of the present invention.

Figure 12 is a cross-sectional view similar to Fig. 11 but showing a fourth embodiment of the invention.

Figure 13 is a schematic front view of an end wall showing one form that the second embodiment of the invention may take.

Figure 14 is a front view of a segment forming a part of an inner or an outer wall.

Figure 15 is a side view of the segment shown in Fig. 14.

Figure 16 is a front view of two segments ready to be welded together to form an inner or an outer wall.

Figure 17 is a front view showing four segments ready to be welded together to form an inner or an outer wall.

Figure 18 is a cross-sectional side view similar to Fig. 11 but showing a fifth embodiment of the invention.

Figure 19 is a schematic representation of a front view of an end cover 10 for the embodiment shown in Fig. 18.

Figure 20 is a cross-sectional side view similar to Fig. 11 and Fig.18 but showing a sixth embodiment of the invention.

Figure 21 is a side view of an element in Figure 20.

DETAILED DESCRIPTION OF THE INVENTION

With reference to Fig. 1, a Yankee drying cylinder 1 is used in a paper making machine and the Yankee drying cylinder 1 may be a Yankee drying cylinder 1 according to the present invention and Fig. 1 may be understood as representing a possible configuration in which the inventive Yankee drying cylinder may be used. It should, of course, be understood that the inventive Yankee drying cylinder 1 may also be used in other configurations and Fig. 1 represents only an example of a possible arrangement. In Fig. 1, a wet fibrous web W is carried by a fabric 3 which may be, for example, a water absorbing felt. The web W is carried in the direction of arrow S by the fabric 3 to a nip formed between a press roll 4 and the Yankee drying cylinder 1. The press roll 4 is preferably be an extended nip roll/shoe roll, for example such a roll as disclosed in US patent No. 7527708, but other rolls may also be used, for example suction rolls or deflection compensated rolls. In the nip between the press roll 4 and the Yankee drying cylinder 1, the wet fibrous web W is transferred to the smooth outer surface of the Yankee drying cylinder which is rotating in the direction of arrow R about a central axis A of the Yankee drying cylinder 1. The Yankee drying cylinder 1 has a journal 29 which is placed in a bearing (not shown). The Yankee drying cylinder 1 is heated from the inside by a hot fluid which is normally hot steam under a certain overpressure. As the wet fibrous web travels on the surface of the Yankee drying cylinder, the web W is dewatered by evaporation as the heat in the steam is transferred through the shell of the Yankee drying cylinder to the wet fibrous web as is known in the art. The ready-dried fibrous web is then creped off the outer surface of the Yankee drying cylinder by a doctor 2. Thereafter, the web will travel to further processing such as reeling on a reel- up. The Yankee drying cylinder may optionally be provided with a Yankee hood 5, for example such as Yankee hood as disclosed in EP 2963176.

The design of the inventive Yankee drying cylinder will now be explained in greater detail.

With reference to Fig. 1 and to Fig. 2, the Yankee drying cylinder according to the present invention has a shell 6 which has a circular cylindrical form. The shell 6 is made of steel and it is symmetrical around its central axis A which is the axis about which the Yankee drying cylinder will rotate when it is in operation in a paper making machine. The shell extends in an axial direction (i.e. the direction of the central axis A) and it has two axial ends 7, 8 and a middle section 9 located between the axial ends 7, 8. It should be understood that the middle section 9 is not a separate element but an integral part of the shell 6. For the purposes of this patent application, the concept of a middle section is introduced merely for the purpose of defining a direction in relation to the axial ends 7,8 and it can be thought of as simply a part of the shell 6 that is equidistant to the axial ends, i.e. that part of the shell 6 that lies halfway between the axial ends 7,8 of the shell 6. The expression“middle section” can be understood referring to a part of the shell 6 that has an arbitrarily small width or being even just an imaginary line half-way between the axial ends 7, 8. For each axial end 7, 8 of the shell 6, the Yankee drying cylinder has an end cover 10, 11 that are made of steel material and fixed to the shell 6 such that the shell 6 and the end covers 10, 11 form a hollow inside volume V.

With reference to Fig. 3, each end cover 10, 11 has an outer circumference 12 which is of circular shape and matches the circular cylindrical shape of the shell 6. It should be understood that, while Fig. 3 only shows the end cover 10, that which is stated about the end cover 10 also applies to the end cover 11. Each end cover 10, 11 is fastened to the shell 6 and each end cover 10, 11 has a central annular opening 23 that defines an inner circumference 13 of the end cover 10, 11. With reference to Fig. 4, it can be seen that an attachment ring 14 is placed in the annular opening 23. The attachment ring serves for attachment of the journal 29 (see Fig. 1), i.e. for receiving the journal 29.

With further reference to Fig. 5 and Fig. 6, each end cover 10, 11 comprises an inner wall 15 and an outer wall 16. Each of the outer and inner walls 15, 16 has an outer circumference 17, 19 adjacent the shell 6 and a central opening 21, 22 defining for each of the outer wall 16 and the inner wall 15 an inner circumference 18, 20 adjacent the attachment ring 14 (the attachment ring 14 not shown in Fig. 5 and Fig. 6).

The central openings 21, 22 of the inner and outer walls 15, 16 are coaxial with each other and form together the central annular opening 23 in the end cover 10, 11 in which central annular opening 23 the attachment ring 14 is fitted.

An embodiment of the invention will now be explained in greater detail with reference to Fig. 8. It should be understood that what is in the following stated about the end cover 10 shown in Fig. 8 also applies to the end cover 11. In Fig. 8, it can be seen that the inner and outer walls 15, 16 of the end cover 10 are spaced apart from each other in the axial direction of the shell 6 in the region between the outer circumference 12 of the end cover 10 and the inner circumference 13 of the end cover 10. The end cover 10 is welded to the shell 6 and to the attachment ring 14. The inner and outer walls 15, 16 are curved or slanting/inclined inwards in the axial direction of the shell 6. Since the inner and outer walls are inclined inwards, the distance in the axial direction from both the outer and the inner walls 15, 16 to the middle section 9 of the shell 6 decreases in the radial direction from the outer circumference 12 of the end cover 10 toward the central axis A of the shell 6.

With reference to Fig. 2, it can be seen that, at the outer circumference of the end cover 10, there is a first distance dl from the end cover 10 to the middle section 9 and where the end cover 10 meets the attachment ring 14 (and is fastened to the attachment ring 14), there is a distance D2 in the axial direction from the end cover 10 to the middle section 9 of the shell 6. As can be seen in Figure 2, Dl is larger than D2 (Dl > D2).

With further reference to Fig. 8, it can be seen that, at the outer circumference of the end cover 10, there is a first distance dl in the axial direction between the inner wall 15 and the middle section 9 and, at the point where the inner wall 15 meets the attachment ring 14, there is a second distance in the axial direction from the inner wall 15 to the middle section 9 and the first distance dl is larger than the second distance d2 (dl > d2). It can thus be seen that, in the radial direction from the outer circumference of the inner wall

15 toward the central axis A, the distance in the axial direction from the inner wall 15 to the middle section 9 of the shell 6 decreases. The same is also true for the outer wall 16. It can thus be stated that, at the inner circumference 18, 20 of the inner and outer walls 15, 16, the distance in the axial direction from each of the inner and the outer walls 15,

16 to the middle section 9 of the shell 6 is smaller than at the outer circumference 17, 19 of the inner and outer walls 15, 16. In Fig. 8, a reinforcement element 26 is shown that is placed between the inner wall 15 and the outer wall 16. The reinforcement element 26 may take many different shapes. One or several such reinforcement elements 26 may be used. The reinforcement element(s) 25 is/are preferably made of steel and preferably welded to both the inner wall 15 and the outer wall 16. Conceivably, such reinforcement elements 26 could be made of another metallic material than steel. For example, cast iron or bronze could be considered. If the material of the reinforcement element(s) 26 is not suitable for welding, the reinforcement element(s) 26 can be fastened to the inner and outer walls 15, 16 by other methods such as screws or rivets. Non-metallic reinforcement elements 26 may also be considered, for example reinforcement elements made of a ceramic material. However, it is deemed preferable to use reinforcement elements 26 that are made of steel and that are welded to the inner and outer walls 15,

16.

As can be seen already in Fig. 8, the inner wall 15 is inclined with a larger angle than the outer wall 16 toward the middle section 9 of the shell 6. As a consequence, the distance between the inner wall 15 and the outer wall 16 increases in the radial direction from the outer circumference of the end cover 10 toward the central axis A and the attachment ring 14.

Reference will now be made to Fig. 9 which is an enlargement of some elements shown in Fig. 8. In the area where the inner and outer walls 15, 16 are connected to the attachment ring 14, there is a first distance Sl between the inner wall 15 and the outer wall 16. Further away from the central axis A in the radial direction R, there is a second distance S2 between the inner and outer walls 15, 16 and the second distance S2 is smaller than the first distance Sl. In the area where the inner and outer walls are connected to the shell 6, there is a third distance S3 between the inner and outer walls 15, 16 and the distance S3 is smaller than the distance S2 (Sl > S2 > S3). In the embodiment of Fig. 8 and Fig. 9, the distance S between the inner and outer walls 15,

16 decreases linearly in the radial direction R from the attachment ring 14 to the shell 6 but embodiments are conceivable in which the distance decreases in a non-linear way (and increases in the opposite direction). In Fig. 9, the distances Sl, S2 S3 may be selected such that Sl is in the range of 200 mm - 300 mm, S2 is in the range of 100 mm - 200 mm and S3 is in the range of 10 mm - 90 mm. In one embodiment contemplated by the inventor, the distance Sl close to the attachment ring 14 maybe about 250 mm, the distance S2 halfway from the attachment ring 14 may be about 150 mm and the distance S3 close to the shell 6 may be about 50 mm. The thickness of each of the inner and outer walls 15, 16 in Fig. 9 may be in the range of 20 mm - 45 mm. For example, the thickness of each of the inner and outer walls 15, 16 may be 25 mm or 40 mm.

The inner surface of the shell 6 is preferably provided with grooves 31 in which hot steam may condensate and transfer heat to the outer surface of the shell 6 to evaporate water from a wet fibrous web W.

In preferred embodiments, one or several reinforcement elements 26 may be welded to at least one of the inner wall 15 or the outer wall 16 or to both. Such reinforcement elements may increase the strength of the end cover such that the end cover better may resist the internal pressure in the Yankee drying cylinder when the Yankee drying cylinder is filled with steam that is pressurized. The at least one reinforcement element 26 may have an annular shape or it may be in the shape of pieces distributed around the circumference of the end cover 10.

It should be understood that embodiments are conceivable in which the distance S between the inner and outer walls 15, 16 does not change linearly in the radial direction R. It should also be understood that embodiments are conceivable in which - unlike the embodiment of Fig. 8 and Fig. 9 - the distance S (the distance in the axial direction of the shell) between the outer and inner walls 15, 16 increases for only a part of the way between the outer circumference/periphery of the end cover 10 and the attachment ring 14.

However, in preferred embodiments of the invention, the end cover 10 is designed such that, for at least a part of the radial distance from the connection of the end cover 10, 11 to the shell 6 toward the central axis A of the shell 6, the inner wall 15 is more slanting or curved inwards than the outer wall 16 such that, in the radial direction toward the central axis A of the shell 6, the distance S between the inner wall 15 and the outer wall 16 increases.

Preferably, each of the outer wall 16 and the inner wall 15 are welded both to the shell 6 and to the attachment ring 14 by welds 30. In all embodiments, both the inner and outer walls 15, 16 are curved or slanting inwards in the axial direction of the shell 6.

Preferably, the outer wall 16 and the inner wall 15 are spaced apart from each other in such a way that, in a radial direction away from the outer circumference of the end cover and toward the central axis of the shell, the distance S between the inner and the outer walls 15, 16 increases such that it is greater where the inner and outer walls 15, 16 are welded to the attachment ring 14 than it is where the inner and outer walls 15, 16 are welded to the shell 6.

A second embodiment of the invention will now be explained with reference to Fig. 10. The embodiment of Fig. 10 is substantially similar to that of Fig. 9. However, unlike the embodiment of Fig. 9, the distance Sl between the inner wall 15 and the outer wall 16 remains constant all the way from the shell 6 to the attachment ring 14. In Fig. 10, an angle ais indicated. This is the angle between the inner wall 15 and a plane which is perpendicular to the central axis A of the shell 6. In the embodiment of Fig. 10, the angle a is the same for both the inner wall 15 and the outer wall 16. In the embodiment of Fig. 9, the corresponding angle is different for the inner wall 15 and the outer wall 16 and, in the embodiment of Fig. 9, the angle a is larger for the inner wall 15 than for the outer wall 16. The angle a may vary depending on the requirements of the specific case but the angle amay be in the range of 5°- 40° or 10°- 30° in many realistic embodiments but other angles are also conceivable. For example, small angles of l°- 4°may be tested or, in some cases, even angles over 40°.

In many realistic embodiment, the distance Sl in Fig. 10 may be in the range of 50 mm - 300. In one embodiment contemplated by the inventor, the distance Sl in Fig. 10 may be 100 mm. However, other values for the distance Sl in Fig. 10 are also conceivable. The thickness of each of the inner and outer walls 15, 16 in Fig. 10 may be in the range of 20 mm - 45 mm. For example, the thickness of each of the inner and outer walls 15, 16 may be 25 mm or 40 mm.

A third embodiment of the invention will now be explained with reference to Fig. 11 and Fig. 13. In the embodiment of Fig. 11, the inner wall 15 is divided into a radially outermost part 27 adjacent the shell 6 and a radially innermost part 28 adjacent the attachment ring 14. The outer wall 16 is welded to the shell 6 and to the attachment ring 14, for example as indicated by the welds 30 in Fig. 11. A ring-shaped (annular) reinforcement element 26a is placed between the inner wall 15 and the outer wall 16 as can be seen in Fig. 11 and Fig. 13. The ring-shaped reinforcement element 26a is welded to the outer wall 16, for example by a weld 30 as indicated in Fig. 11. The ring- shaped reinforcement element 26a is also welded to the radially outermost part 27 of the inner wall 15 and to the radially innermost part 28 of the inner wall 15, for example as indicated by the weld 30 in Fig. 11. The inner wall 15 is welded at its outer and inner circumference 17, 18 to the outer wall 16. With reference to Fig. 7 and Fig. 13, at least one of the outer wall 16 and the inner wall 15 comprises at least two segments 24, 25 that have been welded together with welds extending in the radial direction from the outer circumference 12 of the end cover 10, 11 to the inner circumference 13 of the end cover 10, 11. In Fig. 13, the segments 25a, 25b, 25c, and 25d make up the outer wall 16.

A fourth embodiment will now be explained with reference to Fig. 12. The embodiment of Fig. 12 is largely similar to that of Fig. 11 but, unlike the embodiment of Fig. 11, the radially innermost part 28 of the inner wall 15 is welded to the attachment ring 14 and the radially outermost part 27 of the inner wall 15 is welded to the shell 6.

A further aspect of the invention will now be explained with reference to Fig. 7 and to Fig. 13. In preferred embodiments of the invention, both the inner and the outer walls 15, 16 can be made of segments that have been welded together. Each of the inner and outer walls 15, 16 preferably comprises at least two segments 24, 25 that have been welded together. Preferably, at least four segments are used for both the inner wall 15 and the outer wall 16. For larger Yankee drying cylinders, eight segments may be used for the inner wall 15 and eight segments for the outer wall 16. Preferably, the segments of each wall 15, 16 are of equal size and have the same shape (except for differences due to for example manholes).

With reference to Fig. 7, the outer wall 16 of the end cover 10 comprises four segments 25a, 25b, 25c and 25d that are welded together along weld lines indicated by X in Fig.

7. The inner wall 15 also comprises four segments 24a, 24b, 24c and 24d that are welded together along weld lines indicated by Y in Fig. 7. The segments 25 of the outer wall 16 are rotated in relation to the segments 24 of the inner wall 15 (i. e. rotated clockwise/anti-clockwise as seen in Fig. 7) such that the welds Y that connect the segments 24 of the inner wall 15 have angular positions (angular positions in the circumferential direction of the shell 6) other than the angular positions of the welds X that connect the segments 25 of the outer wall 16. The configuration with four segments in the outer wall 16 can also be seen in Fig. 13. With reference to Fig. 7, it should be understood that the segments 24a, 24b, 24c and 24d are not directly visible in Fig. 7 since they are hidden behind the segments 25a, 25b, 25c and 25d that form the outer wall 16. However, the reference numerals 24a - 24d and the weld lines Y indicate the position of the segments 24a - 24d.

In the embodiment of Fig. 11, more segments will normally be used for the inner wall 15 than for the outer wall 16 since the inner wall 15 is also divided into a radially outermost part and a radially innermost part. It should be understood that both in the embodiment of Fig. 8 and the embodiment of Fig. 11, both the inner wall 15 and the outer wall 16 can be made of a plurality of segments as indicated in Fig. 7 and Fig. 13.

Reference will now be made to Fig. 14, Fig. 15 and Fig. 16. In Fig. 14, a segment 24 b is shown in a front view and the segment 24b is intended to be a part of an inner wall 15. The segment 24b has been subjected to bending which is symbolically indicated by bending lines 32. Seen from the side, the segment 24b looks like a truncated cone as can be seen in Fig. 15. The segment 24b represents one half of a complete inner wall 15. With reference to Fig. 16, two such segments 24a and 24b can be combined with each other by welding to form a complete inner wall 15. Of course, the inner wall 15 may be formed by more than two segments 24a, 24b. As shown in Fig. 17, the inner wall 15 may comprise 4 segments 24a, 24b, 24c and 24d which segments can be joined by welding or conceivably by other methods. It should also be understood that the inner wall 15 may comprise six segments, eight segments, ten segments or twelve segments. The inner wall 15 may also comprise more than twelve segments. Embodiments are conceivable in which the inner wall 15 may comprise an uneven number of segments, for example three segments, five segments, seven segments or a number of segments larger than seven. However, the number of segments is preferably an even number. For smaller Yankee drying cylinders, a relatively small number of segments may be suitable while it may be suitable to use a larger number of segments for the inner wall 15 for a larger Yankee drying cylinder, i.e. a Yankee drying cylinder with a relatively large diameter. What has been stated above with reference to Figs. 14 - 17 about the inner wall is applicable also to the outer wall 16 which may in the same way comprise a number of segments that have been welded together. Preferably, the same number of segments are used for the outer wall 16 and the inner wall 15. For example, if the inner wall 15 has four segments, it is preferable that the outer wall 16 also has four segments. However, embodiments are conceivable in which the inner wall 15 and the outer wall 16 do not have the same number of segments. Embodiments are also conceivable in which both the outer wall 16 and the inner wall 15 have been shaped from one single piece, for example a piece of sheet metal. The material in the segments is preferably steel.

A fifth embodiment will now be explained with reference to Fig. 18. The embodiment of Fig, 18 is in many ways similar to the embodiment of Fig. 11. However, in the embodiment of Fig. 18, at least one of the end covers 10, 11 (preferably both end covers 10, 11) comprises a peripheral ring 33 that extends around the outer

circumference/periphery of the at least one end wall 10, 11. The peripheral ring 33 has the same diameter as the shell and at least one of the inner and outer walls 15, 16 is welded to the peripheral ring and the peripheral ring 33 is welded to the shell 6 by a weld 30. As illustrated in Fig. 18, the outer wall 16 is welded to the peripheral ring 33 while the inner wall 15 is welded to the outer wall 16 at its outer circumference instead of being welded directly to the peripheral ring 33. However, it should be understood that both the inner wall 15 and the outer wall 16 could be welded to the peripheral ring 33 at their radially outer circumference.

In the embodiment of Fig. 18, the outer wall 16 is directly welded to the peripheral ring 33 and to the attachment ring 14 while the inner wall 15 is divided into a radially outermost part 27 adjacent the shell 6 and a radially innermost part 28 adjacent the attachment ring 14 and the radially innermost part is welded to the attachment ring 14 while the radially outermost part is welded to the outer wall 16. However, the radially outermost part 27 could alternatively be welded directly to the peripheral ring 33.

With further reference to Fig. 18, at least one reinforcement element 26a is placed between the inner wall 15 and the outer wall 16 and welded to both the inner wall 15 and the outer wall 16. The at least one reinforcement element 26a is preferably annular, i.e. ring-shaped and symmetrical about the central axis A of the shell 6.

In the embodiment of Fig. 18, at least one slot 34 is formed in the outer wall 16 and the outer wall 16 is welded to the at least one reinforcement element 26a through the at least one slot 34. Preferably, more than one slot 34 is used, for example two slots 34, see also Fig. 19. It should be understood that welding the outer wall 16 to the reinforcement element 26, 26a may also be used in the embodiments of Fi. 8, Fig. 12 and Fig. 11. It is also possible to use one or several reinforcement elements 26, 26a (ring-shaped or not) in the embodiment of Fig. 10 and such reinforcement elements 26, 26 a may optionally be welded to the outer wall 16 through one or several slots 34. With further reference to Fig. 19, it can be seen how segments 25a, 25b, 25c and 25d that together form the outer wall 16 are provided with slots 34 (i.e. through-going openings in the segments) through which the segments 25a, 25b, 25c, 25d can be welded to a ring-shaped reinforcement element 26a that is placed between the outer wall 16 and the inner wall 15. In Fig. 19, the outer wall 16 is shown as having four segments 25a, 25b, 25c, 25d but it should be understood that more than four segments or less than four segments can be used. For example, the outer wall 16 may have four segments or eight segments. With reference to Fig. 20 and Fig. 21, a sixth embodiment of the invention will now be explained. The embodiment of Fig. 20 is substantially similar to the embodiment of Fig. 18 but the reinforcement ring 26a that is shown in Fig. 18 has been replaced by a number of reinforcement sheets 35 that extend in the radial direction of the Yankee drying cylinder 1 from the attachment ring 14 toward the shell 6. Such a reinforcement sheet 35 is shown separately in Fig. 21. The reinforcement sheet 35 is a reinforcement element that serves basically the same function s the is preferably made of steel and it has such a shape that it fits into the space between the inner wall 15 and the outer wall 16 while at the same time being in contact with both the inner wall 15 and the outer wall 16. More than one such reinforcement sheet 35 is used and preferably at least three such reinforcement sheets 35 are provided in the embodiment of Fig. 20. Even more preferred, at least four such reinforcement sheets 35 are used. Preferably, the reinforcement sheets 35 are distributed around the circumference of the end cover 10,

11 with the same angle between all reinforcement sheets. In one embodiment contemplated by the inventor, eighteen such reinforcement sheets 35 are used that are spaced from each other in the circumferential direction of the end cover 10, 11 by 20°. Each reinforcement sheet 35 may be welded to the inner wall 15 and the outer wall 16 through slots 34 in substantially the same way as explained with reference to Fig. 18 and Fig. 19. Obviously, the slots 34 in the embodiment of Fig. 19 may have a different shape than the slots 34 shown in Fig. 19. To allow reliable welding, the reinforcements sheets 35 should preferable have a thickness which is at least 20 mm and preferably at least 25 mm. For example, each reinforcement sheet 35 may have a thickness in the range of 25 mm - 40 mm. In one embodiment contemplated by the inventor, eighteen sheets are used that each has a thickness of 30 mm.

In all embodiments, the thickness of each of the inner and outer walls 15, 16 may be in the range of 20 mm - 45 mm but other values for the thickness may be contemplated.

For all embodiments of the invention, the following can be stated. Since the distance between the outer and inner walls 15, 16 is larger close to the central axis A than at is where the inner and outer walls are connected to the shell, the strength of the end cover 10, 11 will be optimized where it is most needed (i.e. close to the central axis A). At the same time, the inner wall 15 is connected to the shell 6 close to or at the axial end 7 of the shell 6 which means that hot steam inside the Yankee drying cylinder 1 can heat the shell 6 substantially all the way to the axial end 7 of the shell 6.

Another advantage of the present invention lies in the use of outer and inner walls 15, 16 which results in a sandwich construction of the end covers 10, 11. The sandwich construction improves the structural strength of the end walls. At the same time, a layer of air between the inner wall 15 and the outer wall 16 results in better heat insulation such that heat losses are reduced.

A further advantage of the present invention is that a welded Yankee drying cylinder can be more easily fitted into an existing machine without reducing the effective width of the Yankee drying cylinder since the inwardly bulging end covers 10, 11 can accommodate existing bearings and journals.

If a reinforcement element 26, 26a is welded to the outer wall 16 through one or several slots 34, the weld can be inspected from the outside which is preferable. In the embodiment of Fig. 18, it may be noted that all welds 30 can be inspected either from the outside or from the inside by a worker inside the Yankee drying cylinder 1.

It should be understood that, while the solution using a peripheral ring 33 is shown only in Fig. 18, the same solution (i.e. a peripheral ring 33 to which the end cover is fastened/welded may be used also in the embodiments of Fig. 8, Fig. 9, Fig. 10, Fig. 11 and Fig. 12. The use of a peripheral ring 33 entails the advantage that the end cover can be manufactured separately and then be welded as one single piece to the shell 6. The manufacturing process thereby becomes simpler. However, it should be understood that manufacturing methods in which the peripheral ring 33 is welded to the shell 6 before the other parts of the end cover are welded/connected to the peripheral ring 33.

In many realistic embodiments of the invention, the Yankee drying cylinder may have a diameter in the range of 3.5 m - 7 m. The opening 23 in the end cover may in many realistic embodiments have a diameter in the range of lm - 2.5 m. For example, the Yankee drying cylinder may have a diameter of 5.5 m (or about 5.5 m) while the opening 23 in the end covers may have a diameter of 2m (or about 2m).

While the different embodiments of Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 18 and Fig. 20 have been described above with reference to one axial end 7, 8 of the shell 6, it should be understood that what applies to one axial end 7, 8 of the shell 6 also applies to the opposite axial end of the shell 6 such that each axial end 7, 8 of the shell 6 has an end cover of the same design. In principle, embodiments are conceivable in which different designs of the end cover are combined with each other. For example, one axial end 7, 8 of the shell 6 could be provided with an end cover as shown in Fig. 8 while the end cover at the other axial end of the shell 6 could have an end cover as shown in Fig. 12. However, it is preferred that the end covers 10, 11 have the same design at both axial ends 7, 8 of the shell 6. Although not shown in the figures, a manhole for such purposes as inspection, service and repairs is preferably provided on at least one of the end covers 10, 11. This applies to all embodiments.

As can be seen in for example Fig. 10, the shell 6 can be designed to define a step that supports the outer wall 16 at the outer circumference (periphery) 19 of the outer wall 16 and a similar supporting step in the shell 6 may also be provided for the inner wall 15.

In the same way, the attachment ring 14 may have corresponding steps to support the outer wall 16 and the inner wall 15 (see Fig. 10).

In all embodiments of the invention, the Yankee drying cylinder 1 is preferably equipped with an arrangement for removing condensate water from the grooves 31, for example such an arrangement as shown in EP 2614182 Bl or in US patent No.

4476637.