EP0448991B1 - Heat exchanger - Google Patents

Abstract

The invention relates to a heat exchanger 1 consisting of at least one heat exchanger module which is characterised by improved efficiency and a high permissible pressure difference between the two flowing media. Improvement in efficiency is achieved by means of profile elements 2 which are welted in a zig-zag fashion and have side walls 18 and intermediate elements 3 having cambers 4, 8 in each case. Furthermore, the heat exchanger 1 is characterised in that the profile elements 2 and intermediate elements 3 are welded to one another at their respective lines of contact 16, so that a high pressure difference is permitted. <IMAGE>

Description

The invention relates to a heat exchanger for exchanging heat between two separate media, consisting of at least one heat exchanger module with connecting and connecting elements, with layered profile elements in sandwich construction and intermediate layer elements for separating the different flow directions, the side walls of the profile elements and the intermediate layer elements each one Form flow chamber and common lines of contact and the edge areas are sealed by end strips and the modules by end plates.

Heat exchangers are required, for example, to exchange heat with the same or different media in order to carry out heat recovery or heat transfer. From today's environmental perspective, this process is becoming increasingly important and is required in a variety of ways in industry. It may also be necessary that the greatest safety precautions can be taken with aggressive or reacting media.

From CH-PS 193 732 a device for the heat exchange of flowing media is known, which has a crossed sandwich structure of flat and corrugated leaves. The interleaf sheets can be reinforced by embossing and are held together with the corrugated sheets by external contact angles.

From DE-PS 12 59 362 a heat exchanger is known, which is constructed from flexible plastic films as storage chambers. These are closed at the edge and a heat-exchanging medium flows through the corresponding connection elements. In the inflated state, these plastic films form a flat, hollow-plate-like element, which is arranged in a flow around one another by a further medium.

DE-A 30 03 573 shows a heat exchanger element which consists of metallic profile foils or plastic foils which are alternately layered on smooth and corrugated foils. A disadvantage of this type of heat exchanger is the low efficiency, because adequate mixing of the flowing media within a flow chamber is not ensured and therefore no great heat transfer takes place. Furthermore, no measures were taken to meet high tightness or high safety requirements.

A disadvantage of these two types of heat exchangers is the low permissible pressure difference between the heat-emitting and heat-absorbing medium. Furthermore, the efficiency of the heat exchanger is very limited and in need of improvement. Due to the simply designed partition walls, use with high security requirements such as excluded with toxic, aggressive or expensive media.

The invention is therefore based on the object of providing a heat exchanger which has a high degree of efficiency and a low weight and can be used for high pressures and, where appropriate, meets high safety requirements, and also specifies a method for its production.

According to the invention it is provided that the thin-walled profile elements are folded in a zigzag shape and the side walls of adjacent flow chambers of the profile elements and / or the thin-walled intermediate layer elements have curvatures to at least one adjacent flow chamber (s) and that the profile and intermediate layer elements are connected to one another at their respective contact lines.

Due to the special shape of the zigzag-shaped profile elements, an extremely stable structure is achieved, the walls of the profile elements being reinforced by curvatures and having great rigidity in order to absorb large pressure and buckling forces. The intermediate layer elements each separate the different media that flow in the crossed profile elements and are also reinforced by bulges. These measures allow the heat exchanger to be made from extremely thin-walled sheets. Furthermore, the heat and transfer areas are increased by the curvatures and the resulting turbulence in the profile chambers results in an increase in the Reynold number and thus an increase in the k value, so that the heat exchanger has a large ratio between exchange surface and space requirement and Efficiency of the heat exchanger is significantly improved.

Because the contacting surfaces and lines of the profile and intermediate layer elements are connected to one another, it is also possible to allow a high pressure load by the media, which are separated from the intermediate layer elements. In addition, the necessary security requirements for high-risk media are met in that the intermediate layer elements are made of two layers.

The curvatures of the profile elements and intermediate layer elements can be arranged transversely or in the flow direction, the curvatures of the intermediate layer elements being arranged on one side or alternating with the profile elements. The curvatures of the profile elements can be designed in a wave shape overall. In special cases, the formation of parallel beads in the profile elements can be provided. The curvatures of the intermediate layer elements can be chosen to be oblong, spherical or pyramid-shaped. The profile elements are layered crosswise rotated by 90 degrees, so that good heat transfer is achieved as a cross-flow heat exchanger.

In a special embodiment of the invention it is provided that the flow chambers of the profile elements one Flow plane are angled in a U-shape, the central region and the angled end regions each being stacked parallel to one another with the other flow chambers. The course of the flow direction can be chosen in the same direction or in opposite directions.

This configuration and arrangement of the profile elements creates a countercurrent heat exchanger in the second case, which has a further improved efficiency.

In a further embodiment of the invention it is provided that the intermediate layer elements are made of two layers and the arches are arranged one inside the other.

The safety requirements are met by the double-layer design of the intermediate layer elements. Wherever toxic, aggressive or very expensive products require increased safety technology for heat transfer, a complex construction of this type is justified. The basic module with increased security technology is constructed with the same construction elements as the normal module, whereby only two intermediate layer elements are used and the edges of these elements are tightly connected to one another. This creates an intermediate space that catches the medium in the event of any leaks in one or the other flow level and can generate a corresponding warning message, for example, via a leak detector. For this special purpose it is provided according to a further subclaim that the spaces formed by the double-layer intermediate layer elements are connected to one another via side boundary profiles which have at least one connecting channel.

Through the connecting channel, all or part of the gaps are connected to each other, i. H. it is possible to monitor the gaps with one or more common leak detection devices. Arises on an intermediate layer element e.g. a leak, the medium can get from the profile chambers into the space and from there via the channel or channels to the leak detector. If necessary, the connection to a collecting container can be made on the channels of the side boundary profile via a connected pipeline, this container being equipped with a leak indicator. In order to accelerate the escape of the medium into the leak detection container in the event of damage, the container and the channel system can be placed under vacuum. After reaching the final pressure, the vacuum connection is welded, so that the container and the channels thus remain under vacuum until the intermediate layer elements are damaged. The heat transfer is maintained here by the connection of the intermediate layer elements.

The heat exchanger remains fully functional even if an intermediate layer element is damaged, and the damage is indicated in good time by the warning via the leak indicator.

Depending on the intended use, built-in vortex generators are provided to increase the swirling in the profile chambers. The crossed profile elements can have different heights so that the heat exchanger modules can be used not only as a heat exchanger but also as a heat store. Tolerance compensation of the profile elements or the different heights can be achieved, for example, by triangular strips that can be pushed together and connected. When used as The profile planes of a flow direction are designed to be correspondingly high, so that a larger amount of heat-absorbing materials can be embedded. A liquid or gaseous medium can flow through the profile level with a low height for heating or cooling, which, for example, emits its heat to the storage medium when it is heated and is heated again after the storage medium has cooled. Paraffin or Glauber's salt, for example, can be used as storage media.

To avoid build-up and deposits on viscous, sticky or cracking products, the surfaces that are touched by these media can be electropolished.

A heat exchanger is constructed using modules that can be connected to each other via pipe segments. The pipe segments are used on the one hand as connecting elements between the profile planes and have disks in the pipe segments which serve to direct the flow and, on the other hand, the pipe segments stiffen or reinforce the entire structure of the heat exchanger. In particular, further attached pipe segment quarters and ribs are provided, which increase the stiffening of the housing. Furthermore, the pipe segments are provided with flattened connecting pipes and connecting elements so that a flow circuit of the media can be implemented.

Metallic materials are preferably used to manufacture the heat exchangers with profile elements, intermediate layer elements, end plates and triangular strips.

A method for producing a heat exchanger according to one or more of the features already mentioned before that the profile, intermediate layer elements, end plates and end strips, as well as contact lines and / or surfaces of the sandwich structure are welded to one another, in particular by means of hidden seams, completely, point-wise or at intervals.

The square all-round welding of the individual profile and intermediate layer elements with those of the elements above and below creates stable configurations that are suitable for transmitting high pressures with very thin walls.

In the construction with double-layered intermediate layer elements, it is provided that the intermediate layer elements around the bulges are also welded to one another at points. It is also provided that the triangular end strips are welded.

According to a further feature of the method, it is provided that the welding process is carried out by means of an electron beam, laser and / or plasma welding process with a low energy transfer in order to weld the structure in a stress-free, smooth and distortion-free manner.

To avoid intergranular corrosion in the event of a chemical attack in the weld heat affected zone, welding is carried out under vacuum or inert gas blanketing.

In order to achieve chemical resistance of the weld seams in stainless steels, all seams are pickled and passivated after the welding process. The end strips of the modules and the pipe segments of the heat exchanger are welded tight, so that no flowing Medium can leak. For the same reason, the double-walled intermediate layer elements are welded tight.

The design of the heat exchanger according to the invention is illustrated by means of several figures.

Fig. 1

einen Wärmetauscher in teilweise geschnittener perspektivischer Darstellung ohne Anschlußrohre und Seitenbleche,

Fig. 2

einen Wärmespeicher in teilweise geschnittener perspektivischer Darstellung ohne Anschlußrohre und Seitenbleche,

Fig. 3

eine Explosionszeichnung eines Wärmetauschers in perspektivischer Darstellung mit U-förmig abgewinkelten Profilelementen,

Fig. 4

eine Explosionszeichnung des Wärmetauschers nach Fig. 1,

Fig. 5

eine Explosionszeichnung des Wärmetauschers nach Fig. 1 mit doppellagig ausgeführten Zwischenlagenelementen,

Fig. 6

einen Ausschnitt des Wärmeübertragers mit Profil- und Zwischenlagenelementen,

Fig. 7

einen Ausschnitt gemäß Fig. 5 mit zusätzlichen Wirbulatoren,

Fig. 8

ein Profilelement in einer perspektivischen Draufsicht mit unterschiedlichen Wölbungen,

Fig. 9

ein Zwischenlagenelement in einer perspektivischen Draufsicht mit unterschiedlichen Wölbungen,

Fig. 10

eine verschweißte Profilebene mit Zwischenlagenelementen und Abschlußleisten,

Fig. 11

eine Anordnung von Wärmeübertragermodulen mit Rohrsegmenten,

Fig. 12

eine geschnittene Seitenansicht zweier Wärmeübertragermodule mit angeschweißten Rohrsegmenten und Strömungsscheiben,

Fig. 13

eine perspektivische Seitenansicht eines Wärmeübertragers mit zusätzlichen Gehäuseversteifungen,

Fig. 14

einen vergrößerten Ausschnitt der Rohrsegmente mit Gehäuseversteifungen,

Fig. 15

eine perspektivische Seitenansicht mit verrippten Rohrsegmenten,

Fig. 16

einen Wärmeübertrager mit einem oder mehreren Modulen und unterschiedlichen Anschlußrohrsegmenten.

It shows

Fig. 1

a heat exchanger in a partially sectioned perspective view without connecting pipes and side plates,

Fig. 2

a heat store in a partially sectioned perspective view without connecting pipes and side plates,

Fig. 3

an exploded view of a heat exchanger in perspective with U-shaped angled profile elements,

Fig. 4

2 shows an exploded view of the heat exchanger according to FIG. 1,

Fig. 5

1 with double-layered intermediate layer elements,

Fig. 6

a section of the heat exchanger with profile and intermediate layer elements,

Fig. 7

5 with additional turbulators,

Fig. 8

a profile element in a perspective top view with different curvatures,

Fig. 9

an intermediate layer element in a perspective top view with different curvatures,

Fig. 10

a welded profile level with intermediate layer elements and end strips,

Fig. 11

an arrangement of heat exchanger modules with pipe segments,

Fig. 12

a sectional side view of two heat exchanger modules with welded pipe segments and flow disks,

Fig. 13

a perspective side view of a heat exchanger with additional housing stiffeners,

Fig. 14

an enlarged section of the tube segments with housing stiffeners,

Fig. 15

a perspective side view with ribbed pipe segments,

Fig. 16

a heat exchanger with one or more modules and different connecting pipe segments.

1 and 2 each show a module of a heat exchanger 1 in a partially sectioned and perspective representation, which is constructed from cross-layered profile elements 2 with intermediate layer elements 3 arranged in between in a sandwich construction. The profile elements 2 consist of a zigzag-folded sheet, which is provided on the side walls 18 with curvatures 4. In this embodiment variant, the curvatures 4 are arranged transversely to the flow direction A, B and consist of corrugated impressions or beads. On the end faces 5, 6, openings of the zigzag-shaped flow chambers 7 of the profile elements 2, each offset by 90 °, can be seen.

In Fig. 1, the height of the profile elements 2 for use as a heat exchanger between two different or the same media in both flow directions A, B is the same and in Fig. 2, the height of the profile elements 2 for use as a heat store between two different media formed differently in both flow directions A, B. The intermediate layer elements 3 are flush with the profile elements 2 on the end faces 5, 6 and separate the two flow directions A, B from each other. For better heat transfer and swirling of the media, the intermediate layer elements 3 also have bulges 8, which consist of elongated, spherical or pyramid-shaped impressions. The lines of contact 16 of the profile 2 and intermediate layer elements 3 are welded together for reasons of stability.

The lateral closure of the profile elements 2 parallel to the flow direction A, B is carried out by end strips 9, which have a square cross section and are tightly welded to one another and to the intermediate layer elements 3 in the corner points 10. The upper and lower profile level is sealed by a tightly welded end plate 11, so that a basic module is formed which has flow directions A, B offset by 90 °, so that the media through the flow chambers 7 in the direction A, A 'or B, B 'Can flow. The existing curvatures 4, 8 ensure a swirling of the media and thus an improved efficiency of the heat exchanger. In FIG. 2, rectangular end strips 9 are used to complete the higher profile levels.

FIG. 3 shows a heat exchanger 1 in an exploded view, which has an elongated extension compared to FIGS. 1 and 2 and is equipped with profile elements 2 which are angled in a U-shape. The middle region of the profile elements 2 and the angled end regions 12 are layered in such a way that the flow chambers 7 of each plane run parallel, the flow chambers 7 of the individual flow planes being flowed through in opposite directions, so that the principle of a countercurrent heat exchanger is achieved. The two openings of the angled end regions 12 of the flow directions A, A 'and B, B' each point to a longitudinal side of the heat exchanger 1 and allow the two media to flow in and out.

FIGS. 4 and 5 each show an exploded view of the heat exchanger 1, from which the structure can be seen particularly clearly. An intermediate layer element 3 and a profile element 2 with lateral end strips 9 are alternately arranged on the square end plate 11, both elements 2, 3 being provided with curvatures 4, 8 in the form of beads or elongated or spherical impressions. The intermediate layer element 3 in this case has spherical impressions in the center of the surface and elongated impressions on the two edges towards the end strip 9. The end strips 9 consist of two triangular strips 20 which are mutually displaceable and are used for tolerance and height compensation of the profile elements 2. All components of the heat exchanger 1 are welded, the contact lines 16 and surfaces 19 located at the edge in particular being welded tightly.

In FIG. 5, the intermediate layer elements 3 are double-layered and layered with the curvatures 8 so that a high safety standard is achieved.

By the curvature 8 intermediate layer elements 3 spaces 15 are formed, which are connected by holes 14 in the corners 13 of the end strips 9 and the end plate 11, so that the spaces 15 are connected to a collecting container, not shown, for the media and / or a leak detector can.

In FIGS. 6 and 7, three intermediate layer elements 3 and two profile elements 2 are shown in an enlarged and partially openwork representation. The spherical impressions in the intermediate layer elements 3 and the wave-shaped impressions or the beads in the walls 18 of the profile elements 2 can be seen very well. The lines of contact 16 of the individual elements 2, 3 are welded at points or fully.

In FIG. 7, additional wave-shaped turbulators 17 are welded into the flow chambers 7, which lead to a further swirling of the media and thus to a further improvement in the efficiency. The turbulators 17 are spot welded to the folded profile element 2 and protrude into the flow chamber 7, the turbulators 17 also being corrugated.

FIG. 8 shows a single profile element 2 which is folded in a zigzag shape and has elongated beads on the walls 18.

FIG. 9 shows an intermediate layer element 3 which has alternative embodiments of the curvatures 8 in the form of elongated, spherical and pyramid-shaped impressions. These impressions can be aligned alternately to the two adjacent profile elements 2.

In Figure 10, three end strips 9 are shown with two intermediate layer elements 3 and a profile element 2, which are welded at the dashed lines 16 and surfaces 19, the middle end strip 9 consists of two triangular strips 20, which are also welded together.

FIG. 11 shows three modules of the heat exchanger 1 which are arranged next to one another and are connected to a tube segment 21 which is semicircular with the flat open side 22 against the modules and is welded to them. The pipe segment 21 is closed with an end piece 23.

FIG. 12 shows three modules of the heat exchanger 1 in a sectional side view with two tube segments 21 which rest with their flat open sides 22 on the end faces 5, 6 of the heat exchanger 1. The pipe segments 21 serve as inlet and outlet pipes 28 and have at their ends connection flanges 24 which serve to connect the inlet and outlet lines. In the pipe segments 21, disks 25 are arranged, which are used to direct the flow of the medium.

FIG. 13 shows a compact and complete heat exchanger 1, each with two inlet and outlet pipe segments 21, each pipe segment 21 being connected flat to the module and each having a connecting flange 24. The flow directions are characterized by A, A 'and B, B'. The pipe segments 21 are welded together by further pipe segment quarters 26 and reinforce and stiffen the heat exchanger 1.

FIG. 14 shows the attachment of the attached tube segment quarters 26 to the tube segments 21 again in a sectional drawing.

FIG. 15 shows two modules with a tube segment 21 and a connecting flange 24, with additional reinforcement of the construction being brought about by ribs 27.

FIGS. 16 a - d each show a heat exchanger 1 which is made up of one or more modules with connecting elements in the form of tube segments 21 and connecting flanges 24. While FIG. 16a is based on one module, in FIG. 16b there are three modules arranged side by side and in FIG. 16c six modules, three of them next to each other and two above each other. FIG. 16d shows twelve modules, each of which is arranged three in a row in a package of four and is equipped with double connecting segments 21 and connecting flanges 24.

Reference list

1

Heat exchanger / module

2nd

Profile element

3rd

Liner element

4th

Curvature of the profile elements

5, 6

End faces

7

Flow chamber

8th

Curvature of the liner elements

9

End strips / side boundary profile

10th

Corner point

11

End plate

12th

End area

13

Corner point

14

Bore / connecting channel

15

Space

16

Contact line

17th

Wirbulator

18th

Side wall of the profile element

19th

Touch area

20th

Triangular molding

21

Pipe segment

22

page

23

Tail

24th

Connecting flange

25th

disc

26

Pipe segment quarter

27

rib

28

Inlet and outlet pipe

Claims (31)

1. A heat transfer unit (1) for exchanging heat between two media guided separately from one another, consisting of at least one heat transfer module with connecting and attaching elements, having layered profiled elements (2) arranged sandwich-like and intermediate layer elements (3) for separating the different directions of flow (A, B), the side walls (18) of the profiled elements (2) and the intermediate layer elements (3) each forming a flow chamber (7) and joint contact lines (16), and the edge regions being connected to one another by end strips (9) and the individual modules being connected to one another by end plates (11),
   characterised in
   that the profiled elements (2) are folded in a zig-zag-like way and, towards the flow chambers (7), on the side walls (18) are provided with curved portions (4); that the intermediate layer elements (3) comprise curved portions (8) towards one or two adjoining profiled elements (2); and that the profiled (2) and intermediate layer elements (3) are connected to one another at their respective contact lines (16).
2. A heat transfer unit according to claim 1,
   characterised in
   that the curved portions (4, 8) are arranged so as to extend transversely to, or in, the direction of flow.
3. A heat transfer unit according to any one or several of claims 1 to 2,
   characterised in
   that the curved portions (8) of the intermediate layer elements (3) are arranged so as to extend towards the profiled elements (2) in alternating directions.
4. A heat transfer unit according to any one or several of claims 1 and 3,
   characterised in
   that the curved portions (4) of the profiled elements (2), in their entirety, are corrugated or consist of stamped beads extending parallel relative to one another.
5. A heat transfer unit according to any one or several of claims 1 to 4,
   characterised in
   that the curved portions (8) of the intermediate layer elements (3) consist of stamped regions being oblong, shaped like a spherical segment or pyramid.
6. A heat transfer unit according to any one or several of claims 1 to 5,
   characterised in
   that the profiled elements (2) are each turned by 90° and layered cross-wise.
7. A heat transfer unit according to any one or several of claims 1 to 5,
   characterised in
   that the flow chambers (7) of the profiled elements (2) of one plane of flow are angled in a U-shaped way and that the central part and the angled end parts (12) each extend parallel to the other flow chambers (7) and are layered so as to be positioned one above the other.
8. A heat transfer unit according to any one or several of claims 1 to 7,
   characterised in
   that the intermediate layer elements (3) are double-layered and that the curved portions are arranged so as to be positioned one inside the other.
9. A heat transfer unit according to any one or several of claims 1 to 8,
   characterised in
   that the intermediate layer elements (3) form spaces (15) which are connected to one another by means of side delimiting profiles (9) with at least one connecting channel (14).
10. A heat transfer unit according to any one or several of claims 1 to 9,
    characterised in
    that the spaces (15) are connected to a collecting container and a leakage indicator.
11. A heat transfer unit according to any one or several of claims 1 to 10,
    characterised in
    that additional whirling units (17) are incorporated into the profiled elements (2).
12. A heat transfer unit according to any one or several of claims 1 to 11,
    characterised in
    that the heights of the profiled elements (2) differ from one another.
13. A heat transfer unit according to any one or several of claims 1 to 12,
    characterised in
    that, for the purpose of adapting to the height of the profiled elements (2), the end strips (9) consist of triangular strips (20) which are movable relative to one another and which are connected to one another.
14. A heat transfer unit according to any one or several of claims 1 to 13,
    characterised in
    that the surfaces of the profiled elements (2) and intermediate layer elements (3) are electro-polished.
15. A heat transfer unit according to any one or several of claims 1 to 14,
    characterised in
    that the connecting elements consist of tubular segments (21) which connect the flow chambers (7) and modules (1) to one another.
16. A heat transfer unit according to any one or several of claims 1 to 15,
    characterised in
    that the connecting elements consist of supply and discharge pipes (28).
17. A heat transfer unit according to any one or several of claims 1 to 16,
    characterised in
    that, for the purpose of reinforcing the modules (1), there are provided further tubular segment quarters (26) arranged in the longitudinal direction of the tubular segments (21).
18. A heat transfer unit according to any one or several of claims 15 to 17,
    characterised in
    that the tubular segments (21) contains plates (25) for directing the flow of the medium.
19. A heat transfer unit according to any one or several of claims 15 to 18,
    characterised in
    that the tubular segments (21) are extended by flattened supply and discharge pipes (28) and comprise connecting flanges (24).
20. A heat transfer unit according to any one or several of claims 16 to 19,
    characterised in
    that the flattened supply and discharge pipes (28) and end plates (11) of the modules (1) are reinforced by ribs (27).
21. A heat transfer unit according to any one or several of claims 1 to 20,
    characterised in
    that all profiled elements (2), intermediate layer elements (3), connecting plates (11) and triangular strips (20) are produced of thin-walled metallic materials.
22. A heat transfer unit according to any one or several of claims 1 to 21,
    characterised in
    that the heat transfer modules (1) may be used as heat exchangers.
23. A heat transfer unit according to any one or several of claims 1 to 21,
    characterised in
    that the heat transfer modules (1) may be used as heat stores and that the flow chambers (7) of one direction are filled with a stored medium.
24. A heat transfer unit according to claim 22,
    characterised in
    that the stored medium may be paraffin or Glauber salt.
25. A method of producing a heat transfer unit according to any one or several of claims 1 to 24,
    characterised in
    that the profiled elements (2), intermediate layer elements (3), end plates (11) and end strips (9) as well as the contact lines (16) and/or sandwich-like contact faces (19) are welded to one another, especially welded continuously, spot-welded or welded with spaces between welds.
26. A method of producing a heat transfer unit according to any one or several of claims 1 to 25,
    characterised in
    that around the curved portions (8), the intermediate layer elements (3) are spot-welded.
27. A method of producing a heat transfer unit according to any one or several of claims 1 to 26,
    characterised in
    that the end strips (20) are welded to one another.
28. A method of producing a heat transfer unit according to any one or several of claims 1 to 27,
    characterised in
    that the welding process is an electron beam welding, a laser welding and/or a plasma welding process with a low energy transfer.
29. A method of producing a heat transfer unit according to any one or several of claims 1 to 28,
    characterised in
    that the welding process is carried out under vacuum or in an inert gas atmosphere.
30. A method of producing a heat transfer unit according to any one or several of claims 25 to 29,
    characterised in
    that, after welding, the welds are pickled and passivated.
31. A method of producing a heat transfer unit according to any one of claims 2 to 30.
    characterised in
    that the end strips (9) of the modules (1) and the tubular segments (21) as well as the intermediate layer elements (3) are tightly welded all round.

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