SE540216C2

SE540216C2 - A yankee drying cylinder for drying a fibrous web and a method of making a yankee drying cylinder

Info

Publication number: SE540216C2
Application number: SE1650417A
Authority: SE
Inventors: Jonsson Ola
Original assignee: Valmet Oy
Priority date: 2016-03-31
Filing date: 2016-03-31
Publication date: 2018-05-02
Also published as: WO2017171602A1; SE1650417A1; PL128030U1; AT16483U1; BR212018070127Y1; CN209443296U; DE212017000093U1; BR212018070127U2

Abstract

The present invention relates to a Yankee drying cylinder (1) for drying a fibrous web (W) such as a tissue paper web. The Yankee drying cylinder (1) comprises a circular cylindrical shell (2) that has an axial extension and a first axial end (4) and a second axial end (5). The shell also has an internal surface (6) and an external surface (7). For each axial end (4, 5) of the circular cylindrical shell, there is an end wall (8, 9) joined to the circular cylindrical shell (2) such that the circular cylindrical shell (2) and the end walls (8, 9) define an enclosed space (13). A central a central tie member (14) extends between the end walls (8, 9) and is joined to the end walls (8, 9) by weld joints (19, 24, 25, 26). The invention also relates to a method of manufacturing the Yankee drying cylinder.

Description

A YANKEE DRYING CYLINDER FOR DRYING A FIBROUS WEB AND A METHOD OF MAKING A YANKEE DRYING CYLINDER FIELD OF THE INVENTION The present invention relates to a Yankee drying cylinder for drying a fibrous web such as a tissue paper web. The invention also relates to a method of making the inventive Yankee drying cylinder.

BACKGROUND OF THE INVENTION Yankee drying cylinders are used in the manufacturing of paper, in particular tissue paper. Such a cylinder is designed to be supplied with hot steam to the interior of the cylinder such that the Yankee drying cylinders is heated. When installed in a paper making machine such as a tissue machine, the Yankee drying cylinder is arranged to be rotated. In operation, a wet fibrous web coming from the forming section of the paper making machine is transferred to the outer surface of the Yankee drying cylinder and follows the surface of the Yankee drying cylinder as the cylinder rotates. When the fibrous web is in contact with the outer (external) surface of the Yankee drying cylinder, the fibrous web is dried since the external surface of the Yankee drying cylinder is hot due to the hot steam that is supplied to the interior of the cylinder. The dried fibrous web then leaves the external surface of the Yankee drying cylinder and is sent further to a following station, for example to a reel-up where the fibrous web is reeled into a paper roll. In some machines, other equipment may be located between the Yankee drying cylinder, for example measuring equipment for measuring various properties of the paper web, for example thickness of the paper web, dryness of the web or dryness variations of the web in the cross-machine direction.

For a long period, Yankee drying cylinders were commonly manufactured by casting and in which end walls are connected to a cast cylindrical shell by such elements as bolts. It has also been suggested that Yankee drying cylinders be manufactured by methods in which steel plates rather than cast iron is used. An example of such a manufacturing method is disclosed in, for example, US patent No. 4320582. It has also been suggested that Yankee drying cylinders of steel may be manufactured in a process in which the end walls of the Yankee drying cylinder are welded to the cylindrical shell and an example of such a method is disclosed in for example US patent No. 8438752.

A technical problem in connection with Yankee drying cylinders is that the heat energy that is supplied to the Yankee drying cylinder, and which normally comes in the shape of hot steam, must not be lost other than through heat transfer to the wet fibrous web. Another problem is that the Y ankee drying cylinder must have a design that contributes to a long life, i.e. that the Yankee drying cylinder can be in operation over many years. Furthermore, it is desirable that the Yankee drying cylinder can be manufactured in a way that is fast and time-efficient. These and other problems are solved by the present invention as will be explained in the following.

DISCLOSURE OF THE INVENTION The inventive Yankee drying cylinder is intended for the purpose of drying a fibrous web such as a tissue paper web. The Yankee drying cylinder according to the invention comprise a circular cylindrical shell with a first and a second axial end, an internal surface and an external surface. For each axial end of the circular cylindrical shell, an end wall is joined to the circular cylindrical shell such that the circular cylindrical shell and the end walls define an enclosed space. The inventive Yankee drying cylinder also comprises a central tie member that extends between the end walls and is joined to the end walls. According to the invention, the central tie member is joined to the end walls by weld joints. The tie member is joined to each end wall element not only by internal weld joints/weld seams but also by welding joints located outside the enclosed space.

Preferably, the end walls are also connected to the circular cylindrical shell by weld joints such that the entire structure comprising the circular cylindrical shell, the end walls and the central tie member is held together by weld joints.

In preferred embodiments of the invention, the internal surface of the circular cylindrical shell is provided with internal circumferential grooves. However, embodiments without such circumferential grooves are conceivable.

The central tie member may take many different forms but, in advantageous embodiments of the invention, it may be formed by or comprise a tubular structure that is connected to or connectable to connectable to a source of hot steam outside the Yankee drying cylinder. In embodiments including such a tubular structure, the tubular structure may optionally be provided with openings (through-holes in the wall of the tubular structure) such that hot steam can enter the Yankee drying cylinder through the tubular structure and reach the inner surface of the circular cylindrical shell through the openings in the tubular structure.

The invention also relates to an inventive method of making a Yankee drying cylinder. The method comprises the steps of providing a circular cylindrical shell with a first and a second axial end; providing a first and a second end wall element, each end wall element having a circular cylindrical outer perimeter and a central opening; providing a tie member extending between a first longitudinal end of the tie member and a second longitudinal end of the tie member and wherein the distance between the longitudinal ends of the tie member corresponds to the axial length of the circular cylindrical shell. The method further comprises the step of joining the circular cylindrical shell, the end wall elements and the tie member to each other such that the end wall elements close the axial ends of the circular cylindrical shell to define an enclosed space and the tie member extends between the end wall elements and connect them to each other.

According to the inventive method, the tie member is joined to the end wall elements in such a way that the longitudinal ends of the tie member are fitted into the central openings of the first and second end wall element respectively and welded to the end walls elements such that each end wall element is joined to the tie member by welds (weld joints/weld seams). The longitudinal ends of the tie member are joined to the end wall elements by welding joints located outside the enclosed space and by welding joints located inside the enclosed space.

In embodiments of the invention, the circular cylindrical shell, the end wall elements and the tie member are joined to each other in a sequence. The sequence comprises the steps of performing a machining operation on the first end of the tie member such that the external shape of the first end of the tie member can fit into the central opening of the first wall element and fitting the now machined first end of the tie member into the central opening of the first wall element. The sequence further comprises the steps of aligning the tie member with the first wall element and welding the first end of the tie member to the first wall element with a first weld between the tie member and the first wall element in the area where that first end of the tie member is inserted into the first wall element. These method steps are then followed by machining the second end of the tie member such that it can fit into the central opening of the second wall element and aligning the circular cylindrical shell with the first wall element. Thereafter, the second wall element is aligned with the circular cylindrical shell and with the tie member such that the now machined second end of the tie member fits into the central opening of the second wall member. The first wall element is then welded to the circular cylindrical shell with a second weld which is an internal weld. The second wall element is then welded to the circular cylindrical shell with a third weld which third weld is an internal weld such that the circular cylindrical shell, the end walls and the tie member will form a coherent structure.

In advantageous embodiments, the method further comprises the following steps: placing the coherent structure on turning rolls such that the axis of the circular cylindrical shell extends in a horizontal direction; rotating the coherent structure and welding the first wall element to the circular cylindrical shell with a fourth weld which is a circumferential external weld and welding the second wall element to the circular cylindrical shell with a fifth weld which is a circumferential external weld; welding the first wall element to the first end of the tie member with a sixth weld which is an external weld while the first weld is an internal weld; and welding the second end of the tie member to the second wall element by a seventh weld which is an internal weld and with an eighth weld which is an external weld.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view which gives a schematic representation of a Yankee drying cylinder on operation.

Figure 2 is a schematic representation of different components used in the manufacturing of the Yankee drying cylinder.

Figure 3is a schematic representation of one of the initial steps of the inventive method.

Figure 4 is a schematic representation of a part of the sequence following the method step shown in Figure 3.

Figure 5 shows a part of the sequence after the sequence step of Figure 4.

Figure 6 is a schematic representation a part of the manufacturing sequence immediately following the one of Figure 5.

Figure 7 is a view corresponding to Figure 6 and shows a subsequent step of the sequence.

Figure 8 is a view correspo9nding to Figure 7 and illustrating the next part of the manufacturing sequence.

Figure 9 is a schematic side view illustrating how the end walls are being provided with external welds.

Figure 10 is a view corresponding to Figure 8 and showing the Yankee drying cylinder after the external welding operation indicated in Figure 9.

Figure 11 is a view corresponding to Figure 10 and representing the following part of the manufacturing sequence.

Figure 12 is a view corresponding to Figure 11 and indicating one of the final steps in the manufacturing sequence.

DETAILED DESCRIPTION OF THE INVENTION With reference to Figure 1, a Yankee drying cylinder 1 is shown in operation. The Yankee drying cylinder 1 may in a Yankee drying cylinder according to the present invention and Figure 1 gives an example of how such a Yankee drying cylinder may be used in practice. As can be seen in Figure 1, a fibrous web W is carried by a fabric 50 to a nip between a roll 51 and the Yankee drying cylinder 1. The fabric 50 may be a felt which is permeable to air and water and capable of receiving water. The roll 51 may be, for example, a suction roll or a shoe press roll but it may also be some other kind of roll such as a deflection-compensated roll. In the nip between the roll 51 and the Yankee drying cylinder 1, the fibrous web W is transferred to the external surface 7 of the Yankee drying cylinder 1. The Yankee drying cylinder 1 has a smooth outer surface 7. In the nip between the roll 51 and the Yankee drying cylinder 1, the web W will be transferred from the fabric 50 the surface 7 of the Yankee drying cylinder 1. The fibrous web W has a strong tendency to follow a smooth surface rather than a rough surface and the smoothness of the outer surface 7 of the Yankee drying cylinder 1 results in a very strong tendency of the fibrous web W to follow the outer surface 7 of the Yankee drying cylinder. The outer surface 7 of the Yankee drying cylinder 1 is normally made smooth special surface treatment such as, for example, grinding while the fabric 50 normally has a surface that is rough, at least compared to that of the outer surface 7 of the Yankee drying cylinder 1. The fibrous web W follows the surface of the Yankee drying cylinder 1 as the Yankee drying cylinder rotates in the direction of arrow R. The Yankee drying cylinder 1 is heated and normally heated from within by a heating medium such as hot steam. A source of hot steam is symbolically indicated by the reference numeral 16. As the fibrous web W is in contact with the outer surface 7 of the Yankee drying cylinder 1, the hot surface 7 of the Yankee drying cylinder 1 will cause water in the fibrous web W to evaporate such that the fibrous web W is dried. The fibrous web W may then be removed from the surface 7 of the Yankee drying cylinder 1. For example, it may be creped away from the outer surface 7 by means of a doctor 53 as schematically indicated in Figure 1. It should be understood that the Yankee drying cylinder 1 is journalled for rotation and the reference numeral 52 indicates a journal for the Yankee drying cylinder 1. The Yankee drying cylinder of the present invention is intended for drying wet fibrous webs and may be used in the way explained with reference to Figure 1 and the Yankee drying cylinder of Figure 1 may be understood as a representation of the inventive Yankee drying cylinder. The Yankee drying cylinder may be followed by other equipment such as a reel-up.

The inventor of the present invention has found that, in known Yankee drying cylinders, there may be heat losses resulting from the leakage of steam from the Yankee drying cylinder. In a conventional Y ankee drying cylinder, a central tie member which is usually shaped as a tubular element is used to connect the end walls of the Yankee drying cylinder. The inventor has found that leakage of steam may occur in the area where the central tie member is joined to the end walls and that this is related to the way in which the central tie member is joined to the end walls. The conventional way of securing the central tie member to the end walls is by screws and/or bolts and this method of fastening has traditionally always been used but the inventor of the present invention has found that an alternative way of fastening the tie member to the end walls should be used.

The design of the inventive Yankee drying cylinder will now be explained with reference to Figure 12 and Figure 2. As can be seen in Figure 12 and Figure 2, the Yankee drying cylinder 1 according to the present invention comprises a circular cylindrical shell 2 that has an axial extension E (Figure 2), a first axial end 4 and a second axial end 5. The cylindrical shell 2 has an internal surface 6 and an external surface 7. For each axial end 4, 5 of the circular cylindrical shell, an end wall 8, 9 is joined to the circular cylindrical shell 2 such that the circular cylindrical shell 2 and the end walls 8, 9 define an enclosed space 13 (see Figure 12). A central tie member 14 extends between the end walls 8, 9 and is joined to the end walls 8, 9. According to the present invention, the central tie member 14 is joined to the end walls 8, 9 by weld joints 19, 24, 25, 26 (see Figure 12). The central tie member 14 is joined to the end walls 8, 9 by weld joints 19, 25 that are placed inside the enclosed space 13. The tie member 14 is additionally joined to each end wall element 8, 9 by welding joints 24, 26 located outside the enclosed space 13.

In advantageous embodiments, the end walls 8, 9 are connected also to the circular cylindrical shell 2 by weld joints 20, 21, 22, 23 such that the entire structure comprising the circular cylindrical shell 2, the end walls 8, 9 and the central tie member 14 is held together by weld joints 19, 20, 21, 22, 23, 24, 25, 26 (see Figure 12).

As can be seen in, for example, Figure 2 and Figure 12, the internal surface 6 of the circular cylindrical shell 2 may be provided with internal circumferential grooves 3. The circumferential grooves 3 serve to improve heat transfer to the outer surface 7 of the cylindrical shell 2 when the Y ankee drying cylinder 1 is being used. When a heating medium such as hot steam is sent into the enclosed space 13, the steam will condensate against the internal surface 6 and heat energy will be transferred to the cylindrical shell 2.

Preferably, the central tie member 14 is formed by or comprises a tubular structure that is connectable to a source 16 of hot steam outside the Yankee drying cylinder 1 (see Figure 1). As can be seen in for example Figure 12, the tubular structure is provided with openings 17 such that hot steam can enter the Yankee drying cylinder 1 via the tubular structure of the tie member 14, pass through the openings 17 in the tubular structure and reach the inner surface 6 of the circular cylindrical shell 2.

One possible sequence for making the inventive Yankee drying cylinder 1 will now be explained with reference to Figures 2 - 12. With reference to Figure 2, the initial steps of the method comprises: providing a circular cylindrical shell 2 with a first and a second axial end 4, 5; providing a first and a second end wall element 8, 9, each end wall element 8, 9 having a circular cylindrical outer perimeter 10 and a central opening 11, 12; providing a tie member 14 extending between a first longitudinal end 15a of the tie member and a second longitudinal end 15b of the tie member 14 and wherein the distance between the longitudinal ends 15a, 15b of the tie member 14 corresponds to the axial length (or extension E) of the circular cylindrical shell 2. With reference to Figure 3, the tie member 14 may be subjected to a machining operation at its first longitudinal end 15a. In this context, the term “machining” operation should be understood as an operation in which material is removed from the tie member 14. For example, it may consist of or comprise a turning operation. In Figure 3, the reference numeral 18 is used to indicate a machine tool such as a tool used for turning. The machining operation is carried out to such an extent that the external shape of the first end 15a of the tie member 14 is such that it fits into the central opening 11 of the first wall element 8. With reference to Figure 4 and Figure 5, the tie member 14 is aligned with the central opening 11 of the first wall element 8 and moved in the direction of arrow B such that the first end 15a of the tie member 14 is fitted into the central opening 11 of the first wall member 8. With reference to Figure 6, the tie member 14 is joined to the fist wall element by a first weld 19 which is preferably a circumferential weld joint/weld seam that goes all around the circumference of the tie member 14 such that the tie member 14 is joined to the first wall element 8 along its entire circumference. As can be seen in Figure 6, the first weld 19 is placed 8 in the area where the first end 15a of the tie member 14 is inserted into the first wall element 8. In yet another machining operation (e.g. by turning), the second end 15b of the tie member 14 is given such a shape that it can fit into the central opening 12 of the second wall element 9.

With reference to Figure 7 and Figure 8, the circular cylindrical shell 2 may be aligned with the first wall element 8 and the second wall element 9 may subsequently be aligned with the circular cylindrical shell 2 and with the tie member 14 such that the now machined second end 15b of the tie member 14 fits into the central opening 12 of the second wall member 9. With reference to Figure 8, the first wall element 8 is welded to the circular cylindrical shell 2 with a second weld 20 which is an internal weld. The second wall element 9 is welded to the circular cylindrical shell 2 with a third weld 21 which is an internal weld (i.e. a weld on the inside of that which is to become a Yankee drying cylinder 1) such that the circular cylindrical shell 2, the end walls 8, 9 and the tie member 14 now form a coherent structure 27.

It can now be seen that the circular cylindrical shell 2, the end wall elements 8, 9 and the tie member 14 have been joined to each other such that the end wall elements 8, 9 close the axial ends 4, 5 of the circular cylindrical shell 2 to define an enclosed space 13 and the tie member 14 extends between the end wall elements 8, 9. The tie member 14 will of course also be joined to the second wall element 9 as will be explained in the following such that each end wall element 8, 9 is joined to the tie member 14 by one or several welds.

In principle, the longitudinal ends 15a, 15b of the tie member could be joined to the end wall elements 8, 9 only by welding joints formed from one side. However, according to the invention, the tie member 14 is joined to the end walls (or end wall elements) 8, 9, by welding joints located both inside and outside the enclosed space 13.

In preferred versions of the inventive method, a continuation of the manufacturing sequence may be as follows. With reference to Figure 9 and to Figure 10, the coherent structure 27 may be placed on turning rolls 28 such that the axis of the circular cylindrical shell 2 extends in a horizontal direction. The coherent structure is then rotated and, while it is being rotated, the first wall element 8 is welded to the circular cylindrical shell 2 with a fourth weld 22 which is a circumferential external weld and the second wall element 9 is welded to the circular cylindrical shell 2 with a fifth weld 23 which is a circumferential external weld.

With reference to Figure 9 and Figure 10, the first end 15a of the tie member 14 may thus be welded externally to the first wall 8 with an external weld (weld seam/weld joint) 24 which is preferably a circumferential weld 24 that goes all around the circumference of the tie member 14 where it contacts the wall of the central opening 11. In Figure 9, the reference numeral 54 refers to a welding apparatus used for making the external welds 22, 23.

With further reference to Figure 11, a following step may include heat treatment of the coherent structure 27 which is now a Yankee drying cylinder in the making. In Figure 11, the arrows P symbolically represent heat energy transferred to the Yankee drying cylinder 1. The heat treatment may include heating the entire Yankee drying cylinder in the making to a temperature in the range of 580 °C - 680 °C. In embodiments of the invention, the heat treatment may include heating the Yankee drying cylinder 1 to a temperature in the range of 605 °C - 640 °C. For example, it can be heated to a temperature of 620 °C. The heating should preferably not be too quick and the inventor has found that heating should preferably take place at such a rate that the temperature of the Yankee drying cylinder is increased by 20 °C - 45 °C per hour (20 °C - 45 °C/hour). More preferred, the temperature is raised at a rate of 25° C - 35 °C/hour and a suitable rate of heating may be, for example, 30 °C/hour. The rate at which the temperature is increased is held low in order to avoid harmful tensions in the material, in particular in the weld seams. When the Yankee drying cylinder has reached its final temperature, it may be kept at that temperature for a period of time. As a rule of thumb, the Yankee drying cylinder should be kept at its final elevated temperature for about one hour per 25 mm thickness of the material. For example, if the material thickness is 75 mm, the Yankee drying cylinder should be held at the elevated temperature for about 3 hours. The Yankee drying cylinder is then allowed to cool. The Yankee drying cylinder should initially cool down at a slow rate, about 20 °C/h - 30 °C/h, until it has reached a temperature of about 200 °C after which it can be allowed to cool freely.

By raising the temperature to a level in the range of 580 °C - 680 °C, the advantage is attained that any tensions after the welding operations will be eliminated.

It should be noted that, in a preferred embodiment of the invention, the heat treatment is carried out while the second end 15b of the tie member 14 is still not welded to the second end wall 9. The reason is that if the second end 15b is welded to the second end wall 9 before heat treatment, shrinking in the weld seams might cause remaining mechanical tensions in the Yankee drying cylinder. If the second end 15b has not yet been welded to the second end wall 9 when the heat treatment is carried out, the second end 15b of the tie member is still free to move relative to the second end wall 9. It can then be welded to the second end wall 9 after heat treatment. In this way, the creation of mechanical tensions can be avoided.

With reference to Figure 12, following method steps include welding the second end 15b of the tie member 14 to the second wall element 9 by a seventh weld 25 which is an internal weld and an eighth weld 26 which is an external weld. In principle, the seventh weld 25 and the eighth weld 26 may be carried out before heat treatment but for reasons explained above, it is preferred that these welds are made after heat treatment of the coherent structure 27.

The welding of the second end 15b of the tie member 14 to the second end wall 9 may advantageously (but not necessarily) be followed by a local heat treatment of the area where the second end 15b is welded to the second end wall 9.

Thanks to the present invention, leakage of steam and thereby heat energy can be reduced. When the entire structure is welded, heat losses can be reduced even more.

The welding of the end walls to the circular cylindrical shell may optionally be carried out in such a way that the end result (with regard to the location of the welding joints connecting the end walls 8, 9 to the cylindrical shell) is as described in US patent No. 8438752 and as shown in, for example, Figure 3 of that patent.

To further reduce heat losses, the end walls may be provided with special insulation, for example as disclosed in US patent No. 8398822.

In use, the inventive Yankee drying cylinder may be used in combination with a hot air hood, for example such a hot air hood as disclosed in US patent No. 6079115 or as disclo sed in EP 2963176 A1.

If the inventive Yankee drying cylinder is provided with grooves 3, the Yankee drying cylinder may also have equipment for removing condensate water from the grooves. Examples of such equipment is disclosed in, for example, European patent No. 2614182.

While the invention has been described in terms of a Yankee drying cylinder and a method of making a Yankee drying cylinder, it should be understood that these categories only reflect different aspects of one and the same invention. It should thus be understood that the Yankee drying cylinder may comprise features that would be the inevitable result of manufacturing the Yankee drying cylinder by means of method steps described above, regardless of whether such features have been explicitly mentioned or not.

Claims

1. A Yankee drying cylinder (1) for drying a fibrous web (W) such as a tissue paper web, the Yankee drying cylinder (1) comprising: a circular cylindrical shell (2) having an axial extension and a first axial end (4) and a second axial end (5), an internal surface (6) and an external surface (7); for each axial end (4, 5) of the circular cylindrical shell, an end wall (8, 9) joined to the circular cylindrical shell (2) such that the circular cylindrical shell (2) and the end walls (8, 9) define an enclosed space (13), each end wall (8, 9) having a circular cylindrical outer perimeter (10); a central tie member (14) extending between the end walls (8, 9) and having longitudinal ends (15a, 15b) and being joined to the end walls (8, 9), and the central tie member (14) being joined to the end walls (8, 9) by weld joints (19, 24, 25, 26) characterised in that each end wall (8, 9) has a central opening (11, 12) into which the longitudinal ends (15a, 15b) of the tie member (14) are fitted and welded to the end walls (8, 9) by welding joints located inside the enclosed space (13) and by welding joints located outside the enclosed space (13).

2. A Yankee drying cylinder (1) according to claim 1, wherein the end walls (8, 9) are connected to the circular cylindrical shell (2) by weld joints (20, 21, 22, 23) such that the entire structure comprising the circular cylindrical shell (2), the end walls (8, 9) and the central tie member (14) is held together by weld joints (19, 20, 21, 22, 23, 24, 25, 26).

3. A Yankee drying cylinder (1) according to any of claims 1 or 2, wherein the internal surface (6) of the circular cylindrical shell (2) is provided with internal circumferential grooves (3).

4. A Yankee drying cylinder according to any of claims 1 - 3, wherein the central tie member (14) is formed by or comprises a tubular structure that is connectable to a source (16) of hot steam outside the Yankee drying cylinder (1) and wherein the tubular structure is provided with openings (17) such that hot steam can enter the Yankee drying cylinder (1) through the tubular structure and reach the inner surface (6) of the circular cylindrical shell (2).

5. A method of making a Yankee drying cylinder (1) which method comprises the steps of: providing a circular cylindrical (2) shell with a first and a second axial end (4, 5); providing a first and a second end wall (8, 9), each end wall (8, 9) having a circular cylindrical outer perimeter (10) and a central opening (11, 12); providing a tie member (14) extending between a first longitudinal end (15a) of the tie member and a second longitudinal end (15b) of the tie member (14) and wherein the distance between the longitudinal ends (15a, 15b) of the tie member (14) corresponds to the axial length of the circular cylindrical shell (2); and, joining the circular cylindrical shell (2), the end walls (8, 9) and the tie member (14) to each other such that the end walls (8, 9) close the axial ends (4, 5) of the circular cylindrical shell (2) to define an enclosed space (13) and the tie member (14) extends between the end walls (8, 9) and connect them to each other, characterized in that the tie member (14) is joined to the end walls (8, 9) in such a way that the longitudinal ends (15a, 15b) of the tie member (14) are fitted into the central openings (11, 12) of the first and second end wall (8, 9) respectively and welded to the end walls (8, 9) by welding joints located inside the enclosed space (13) and by welding joints located outside the enclosed space (13).

6. A method according to claim 5, wherein the circular cylindrical shell (2), the end walls (, 9) and the tie member (14) are joined to each other in a sequence that comprises the following steps: performing a machining operation on the first end (15a) of the tie member (14) such that the external shape of the first end (15a) of the tie member (14) can fit into the central opening (11) of the first end wall (8); fitting the now machined first end (15a) of the tie member (14) into the central opening (11) of the first end wall (8); aligning the tie member (14) with the first end wall (8); welding the first end (15a) of the tie member (14) to the first end wall (8) with a first weld (19) between the tie member (14) and the first end wall (8) in the area where the first end (15a) of the tie member (14) is inserted into the first end wall (8); machining the second end (15b) of the tie member (14) such that it can fit into the central opening (12) of the second end wall (9) and aligning the circular cylindrical shell (2) with the first end wall (8); aligning the second end wall (9) with the circular cylindrical shell (2) and with the tie member (14) such that the now machined second end (15b) of the tie member (14) fits into the central opening (12) of the second end wall (9); welding the first end wall (8) to the circular cylindrical shell (2) with a second weld (20) which is an internal weld; welding the second end wall (9) to the circular cylindrical shell (2) with a third weld (21) which third weld is an internal weld such that the circular cylindrical shell (2), the end walls (8, 9) and the tie member (14) now form a coherent structure (27).

7. A method according to claim 6, wherein the method further comprises the following steps: placing the coherent structure (27) on turning rolls (28) such that the axis of the circular cylindrical shell (2) extends in a horizontal direction; rotating the coherent structure (27) and welding the first end wall (8) to the circular cylindrical shell (2) with a fourth weld (22) which is a circumferential external weld and welding the second end wall (9) to the circular cylindrical shell (2) with a fifth weld (23) which is a circumferential external weld; welding the first end wall (8) to the first end (15a) of the tie member (14) with a sixth weld (24) which is an external weld while the first weld (19) is an internal weld; and welding the second end (15b) of the tie member (14) to the second end wall (9) by a seventh weld (25) which is an internal weld and an eighth weld (26) which is an external weld.