US20170122680A1

US20170122680A1 - Tube bottom and heat exchanger

Info

Publication number: US20170122680A1
Application number: US15/407,519
Authority: US
Inventors: Harald Bronner; Volker Kurz; Stefan Weiss
Original assignee: Mahle International GmbH
Current assignee: Mahle International GmbH
Priority date: 2014-07-15
Filing date: 2017-01-17
Publication date: 2017-05-04
Also published as: EP3169965A1; JP2017520744A; CN106662413A; BR112017000736A2; DE102014213758A1; WO2016008855A1; EP3169965B1; CN106662413B; BR112017000736B1

Abstract

A tube bottom for a heat exchanger, with a bottom region having slot-type openings, wherein the slot-type openings are rimmed by eyelets and the tube bottom has an attached rim region. Formed between the eyelets and the attached rim region is a U-shaped trough as a receiving region for the foot of a box-like cover. The U-shaped trough has an inner first wall, the attached rim region and a second wall forming the trough bottom, wherein the inner first wall has at least one hump-like raised portion in the continuation of an eyelet on a side oriented toward the U-shaped trough, which bounds the breadth of the U-shaped trough between the attached rim region and the first wall and extends from the trough bottom to that end region of the eyelet oriented toward the U-shaped trough. A depression is formed in the hump-like raised portion.

Description

This nonprovisional application is a continuation of International Application No. PCT/EP2015/065989, which was filed on Jul. 13, 2015, and which claims priority to German Patent Application No. 10 2014 213 758.6, which was filed in Germany on Jul. 15, 2014, and which are both herein incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention
The invention relates to a tube bottom for a heat exchanger, having a bottom region which has slot-type openings, wherein the slot-type openings are rimmed by eyelets and the tube bottom has an attached rim region, a U-shaped trough being arranged between the eyelets and the attached rim region, the U-shaped trough being formed by an inner first wall, the attached rim region and a second wall forming the trough bottom. The invention also relates to a heat exchanger.
Description of the Background Art
Coolant coolers can have a plurality of tubes, which can be traversed by a medium and at the same time can be surrounded by a medium. In a known construction, the tubes are received at their ends in tube bottoms. The tube bottoms have a number of openings corresponding to the number of tubes, one tube each being inserted into an opening. The tube is closed by a box-like cover to form a collecting box. For this purpose, the box-like cover is inserted into the tube bottom and is connected to it.
A multi-chamber tube is known from DE 199 20 102 B4, which corresponds to U.S. Pat. No. 6,343,645, which is incorporated herein by reference. This multi-chamber tube can be inserted into correspondingly adapted tube bottoms so as to form a heat transfer block. Ribbed elements can be arranged between the multi-chamber tubes, which improve the heat transfer.
DE 10 2011 085 479 A1, which corresponds to US 2014/0262187, which is incorporated herein by reference, discloses a heat exchanger which has two tube bottoms, in each of which a plurality of tubes is received at their ends. Furthermore, a box-like cover is inserted into the tube bottoms, whereby a heat transfer block which can be flowed through is formed.
A heat exchanger is known from DE 103 43 239 A1, which corresponds to US 2007/0000657, which is herein incorporated by reference, and which has a heat transfer block having a plurality of tubes received at the ends in tube bottoms. A flow-through heat transfer block is formed by box-like covers, which are inserted into the tube bottoms and are connected to the tube bottoms.
A bottom plate of a collecting box for a heat exchanger is known from DE 10 2008 011 579 A1. The bottom plate forms the so-called tube bottom, which has a plurality of openings into which tubes can be inserted. A box-like cover can also be inserted into the tube bottom shown, whereby a flow-through heat transfer block can be produced.
A disadvantage of the devices in the art is, in particular, that the tube bottoms can be adapted only with difficulty to openings of different widths when the width of the tube bottom remains otherwise unchanged. Therefore, an increased adjustment effort is required in order to provide tube bottoms of the same width with openings of different widths. In particular, the receiving area of the tube bottoms, into which the box-like cover is inserted, must be specially adapted to achieve a sufficiently high tightness. For this reason, tube bottoms of different widths are used for openings of different widths, resulting in varying dimensions of the heat exchangers that are formed. This necessitates a higher assembly effort, which adversely affects the assembly time and costs.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a tube bottom which while maintaining the same width of the tube bottom can be adapted to openings of different widths in the tube bottom. Furthermore, the object of the present invention is to provide a heat exchanger with such a tube bottom.
An exemplary embodiment of the invention relates to a tube bottom for a heat exchanger, having a bottom region which has slot-type openings, wherein the slot-type openings are rimmed by eyelets and the tube bottom has an attached rim region, a U-shaped trough being arranged for the bottom of a box-like cover between the eyelets and the attached rim region, wherein the U-shaped trough is formed by an inner first wall, the attached rim region and a second wall forming the trough bottom, wherein the inner first wall has at least a hump-like portion in the form of an extension on the side oriented towards the U-shaped trough, which bounds the breadth of the U-shaped trough between the attached rim region and the first wall and which extends from the bottom of the trough to the end region of the eyelet facing the U-shaped trough, which serves to produce a lead-in chamfer for tubes.
In this case, it is advantageous if a material displacement towards the side of the first wall facing away from the U-shaped trough is formed through the molded-in depression,
By means of a hump-like portion from the inner wall, the breadth of the U-shaped trough can be particularly advantageously bounded. At the same time, the height of the U-shaped trough on the inside can be defined by the hump-like portion. The hump-like portion can in this case be adapted, in particular, as a function of the respective width of the eyelets. In this way, tube bottoms with an identical basic width can be produced which each have differently wide openings and eyelets. This is advantageous because the same production machines can be used for the different tube bottoms and only the respective thermoforming tools need to be adapted. In addition, a standardized width for the tube bottoms can thus be ensured even in tubes of different widths, whereby the space requirements of the heat exchangers produced can be standardized.
By means of a formed depression, a targeted material displacement can be achieved. A greater material thickness can be achieved, in particular, on the side of the inner wall oriented away from the U-shaped trough since, in addition, material is displaced there by the molding-in of the depression. This is particularly advantageous to be able to produce sufficiently large-dimensioned lead-in chamfers for mounting the tubes on the tube bottom.
The transition region between the U-shaped trough and the end region of the eyelet can be individually configured by adapting the hump-like portion, so that sufficient guidance and fixing is provided for the bottom region of a box-like cover, and at the same time, sufficient material thickness for the formation of lead-in chamfers to facilitate tube installation remains.
The deflection of the hump-like portion can increase from the inner wall outwards from the bottom of the trough towards the respective eyelet. This is advantageous since an oblique surface is produced which faces the U-shaped trough and can serve as a centering aid for the bottom region of a box-like cover.
The hump-like portion can extend into the bottom region and at least partly surrounds the eyelet. This is particularly advantageous to create higher stability and, at the same time, to achieve softer transitions between the eyelet and the hump-like portion or the inner wall.
It is also expedient if the first wall has a lead-in chamfer on its side facing away from the U-shaped trough, the lead-in chamfer being arranged on the hump-like portion, and the lead-in chamfer producing a tapering of the slot-type openings rimmed by the eyelets.
A lead-in chamfer is advantageous for facilitating the insertion of the tubes into the openings rimmed by the eyelets. In this case, it is particularly advantageous if the lead-in chamfers are formed in regions of the tube bottom, which have a sufficient material thickness to form a sufficiently long lead-in chamfer.
In addition, it is advantageous if the lead-in chamfer is formed on the molded-in depression. A lead-in chamfer on the molded-in depression is advantageous since a material accumulation is produced by the depression, which allows for a lead-in chamfer with sufficiently large dimensions.
An exemplary embodiment provides that the slot-type openings have two opposing narrow sides and two opposing wide sides, wherein a slot-type opening is tapered by lead-in chamfers on the narrow side and on the end regions of the wide sides facing the narrow side. It is particularly advantageous if lead-in chamfers are formed on both the narrow side and on the wide sides to enable a production that is as exact and simple as possible. In particular, the introduction of the tubes is facilitated in this way.
Furthermore, the hump-like portion and/or the depression is impressed in the hump-like portion into the tube bottom and/or can be formed by thermoforming. The impressing and/or thermoforming is advantageous to produce the hump-like portion and the depression with the same production methods as are used for the rest of the tube bottom. This simplifies production and thus reduces costs.
The first wall and the surface of the hump-like portion that are oriented towards the U-shaped trough can be oriented towards the trough bottom at an angle greater than 90 degrees. An angle greater than 90 degrees is advantageous to produce a slope which allows for an advantageous insertion of the bottom region of a box-like cover into the U-shaped trough.
The side of the hump-like portion facing the U-shaped trough can form a centering aid for the insertion of the box-like cover into the U-shaped trough.
An embodiment of the invention relates to a heat exchanger with two tube bottoms, wherein the heat exchanger has a plurality of tubes which are arranged parallel to each other and are accommodated at their end in each case in an opening of a tube bottom rimmed by an eyelet, wherein the tubes are inserted along the lead-in chamfers into the respective openings, wherein a box-like lid, which is connected to the respective tube bottom, is inserted into the U-shaped trough of each tube bottom.
Such a heat exchanger is particularly advantageous since it is particularly easy to install. The lead-in chamfers allow for easy positioning of the tubes in the openings. Furthermore, the centering aid on the hump-like portion allows for simple positioning of the box-like cover in the U-shaped trough.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes, combinations, and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

FIG. 1 is a sectional view of a tube bottom,

FIG. 2 is a sectional view of a tube bottom, wherein the transition between the eyelet and the U-shaped trough is formed by a hump-like portion,

FIG. 3 is a view of two contours each of a U-shaped trough,

FIG. 4 is a perspective view of a tube bottom, wherein the opening or the eyelet is formed very broad,

FIG. 5 is a perspective view of an alternatively designed tube bottom, the openings or the eyelets being designed less broadly than in the preceding FIG. 4, wherein the hump-like portion has a plateau,

FIG. 6 is a perspective view of an alternative tube bottom, wherein the plateau is designed flatter than the tube bottom shown in FIG. 5,

FIG. 7 is a sectional view through a tube bottom according to the invention, wherein a depression is formed in the hump-like portion from above, which depression forms a material accumulation on the downwardly directed side of the hump-like portion,

FIG. 8 is a perspective view of the tube bottom according to FIG. 7,

FIG. 9 is a view of the underside of the tube bottom according to FIGS. 7 and 8,

FIG. 10 is a sectional view through two tube bottoms, wherein one tube bottom with a less broad eyelet is shown, and one tube bottom with a broader eyelet, and

FIG. 11 is a perspective view of a tube bottom with two different embodiments of a lead-in chamfer.

DETAILED DESCRIPTION

FIG. 1 shows a cross-section through a conventional tube bottom 1. The tube bottom 1 has a plurality of openings 3 which are arranged on the flat bottom region 2. The openings 3 are each rimmed by eyelets 4. Right and left of the openings 3 are U-shaped troughs 5, which are essentially formed between an attached rim region 6 and an inner wall 7 extending parallel thereto.
The U-shaped troughs 5 are in each case terminated in the downward direction by the trough bottom 8. Tubes can be inserted into the openings 3 along the direction 9.
FIG. 2 shows a sectional view through an alternatively designed tube bottom 10, which in particular has a different geometry for the U-shaped trough 14. Specifically, the transition of the eyelets 13 toward the U-shaped trough 14 is designed differently.
The tube bottom 10 has a bottom region 11 in which a plurality of openings 12 are formed, which are bordered by collar-like eyelets 13. The end regions of the eyelets 13 located on the left and right are terminated by hump-like projections 22, which can be introduced into the inner wall 17 of the tube bottom 10, for example, by an impressing process. The eyelets 13 project into the U-shaped trough 14 through the hump-like portions 22. The U-shaped trough 14 is formed by the attached rim region 15, the inner wall 17 and the trough bottom 24. The upper region 16 of the attached rim region 15 is further angled as compared to the lower region 15.
In particular, the inner wall 17 is at an angle greater than 90° to the trough bottom 24. The hump-like portion 22, on its side facing outwards towards the attached rim region 15, forms a surface 21, which in particular can be used as a centering aid for inserting a box-like cover or the bottom region of the box-like cover. The surface 21 is inclined such that a box-like cover inserted from above is automatically deflected into the U-shaped trough 14 provided for this purpose.
The reference numbers 18 and 19 show different possible positions for a bottom region of a box-like cover. The limitation 20 shows the maximum possible inner position for a bottom region. A further displacement of the bottom region inwards would lead to faulty or incorrect mounting. Each arrangement of a bottom region, which is located to the left of the marking 20, with the aid of the centering aid 21 leads to an exact fit of the bottom region in the U-shaped trough 14. The tube bottom 10 of FIG. 2 is likewise already known from the prior art.
FIG. 3 shows two cross-sections of U-shaped troughs in each case of a tube bottom. A contour of a U-shaped trough with a hump-like shape is shown represented by the reference number 30, for example, in FIG. 2. A contour of a U-shaped trough is represented by reference number 31, as shown by way of example in FIG. 1. FIG. 3 illustrates the different construction, in particular the different width or the different height on the inner side of the different U-shaped troughs.
FIG. 4 shows a perspective view of a tube bottom 10 as has already been shown in FIG. 2. In FIG. 4 it can be seen in particular that the hump-like portion 22 has a flat surface 21, which is directed to the attached rim region 15, which also serves as a centering aid. The hump-like portion 22 is formed from the otherwise flat inner wall 17, for example, by thermoforming or impressing.
The hump-like portion 22 extends not only over the inner wall 17, but also into the bottom region 11, where in particular the partial region 23 of the hump-like portion 22 also rims the eyelet 13 along its wide sides.
FIG. 5 shows an alternative embodiment of a tube bottom 40. The tube bottom 40 has a bottom region 41, which has a plurality of openings 42, which are bordered by collar-like eyelets 43.
In the exemplary embodiment shown in FIG. 5, the eyelets 43 or the openings 42, respectively, have a shorter extent in the breadth of the tube bottom 40. The breadth of the tube bottom 40 is measured here as well as in the other exemplary embodiments in each case from an attached rim region 45 to the rim region situated opposite the latter.
To compensate for this shortening of the openings 42 or of the eyelets 43 in comparison with the embodiment of FIG. 4, the hump-like portion 48 has an extended region which is designed as a plateau 50. As in the preceding FIG. 4, a partial region 49 of the hump-like portion 48 also partially encloses the wide sides of the respective eyelet 43. The hump-like portion 48 also has a surface facing the attached rim region 45, which surface can be used as a centering aid.
The U-shaped trough 44 is formed by the attached rim region 45, the inner wall 47 and the trough bottom 51. The attached rim region 45 has slots which can serve for fastening the box-like cover in the tube bottom 40. The fastening of the box-like cover can be achieved, for example, by clamping, by providing corrugated slot crimping or with the aid of other known fixing elements.
The hump-like portion 48, in particular the plateau 50 and the partial region 49, are likewise formed by an impressing process or by a thermoforming process from the inner wall 47 or the bottom region 41.
The plateau 50 serves to bridge between the U-shaped trough 44 and the eyelet 43. It must preferably be sufficiently flat so that enough material remains on the underside of the tube bottom 40 to be able to form a sufficient lead-in chamfer and at the same time be high enough to provide sufficient height of the U-shaped trough 44 on the inside. The plateau 50 in the exemplary embodiment of FIG. 5 is designed high. This allows for sufficient height of the U-shaped trough 44 but is not optimal with respect to the lead-in chamfer on the underside. The following FIG. 6 shows a plateau 70 which is flatter and is thus more advantageous with respect to the formation of a lead-in chamfer, but at the same time requires lower height of the U-shaped trough, which is disadvantageous.
FIG. 6 shows a further alternative exemplary embodiment of a tube bottom 60, which has a bottom region 61 with an opening 62 arranged therein and a collar-like eyelet 63. The opening 62 or the eyelet 63 in FIG. 6 has a relatively short extent in the width of the tube bottom 60. For this reason, the hump-like portion 68 likewise has a plateau 70, wherein the plateau 70 is shorter and flatter than the plateau 50 of the preceding FIG. 5. For this purpose, the partial region 69, which comprises the wide sides of the eyelets 63, is formed even further from the plane of the bottom region 61 than the plateau 70. The partial region 69 forms a U-shaped bead which surrounds the eyelet 63 both on the wide sides and on the narrow side.
The tube bottom 60 also has a U-shaped trough 64. Like in the preceding figures, the U-shaped trough 64 is formed by an attached rim region 65, an inner wall 67 situated opposite the latter and a trough bottom 71. The attached rim region 65 likewise has an outwardly angled partial region 66 in the upper region. The hump-like portion 68 also has a surface directed toward the attached rim region 65, which surface can be used as a centering aid.
FIG. 7 shows an embodiment of a tube bottom 80 according to the invention. The tube bottom 80 has a bottom region 81. Openings 82, which are rimmed by collar-like eyelets 83, are formed in the bottom region 81. The openings 82 have two opposing narrow sides 98 and two opposing wide sides 97. Correspondingly, the eyelets 83, which surround the openings 82, each have two narrow sides 98 and two wide sides 97. The U-shaped trough 84 is formed by an attached rim region 85 and an inner wall 87 opposing the latter and the trough bottom 94.
A hump-like portion 88 is formed from the inner wall 87 and the bottom region 81, which, in particular with the surface 91 facing the attached rim region 85, forms a smaller angle with respect to the attached rim region 85 than the inner wall 87. In addition, the hump-like portion 88 has a depression 90 molded in from above, which is arranged immediately adjacent to the end region of the eyelet 83.
Through the depression 90, which can also be molded into the hump-like portion 88 by an impressing process or by thermoforming, a material displacement downwards takes place which allows for a formation of a lead-in chamfer in particular on the downwardly directed side 96 of the inner wall 87 or of the bottom region 81. Due to the material displacement, the lead-in chamfer can be made larger overall, wherein in particular, the extent of the lead-in chamfer along the insertion direction of the tube can be enlarged. This simplifies the insertion of the tubes and thus allows for a more stable installation process. The lead-in chamfer is not shown in FIG. 7.
In FIG. 7, a partial region 89 of the hump-like portion 88 also rims the wide sides 97 of the eyelet 83. Furthermore, the hump-like portion provides a sufficient height of the U-shaped trough 84, whereby a secure positioning of a box-like cover can be ensured.
FIG. 8 shows a further perspective view of the tube bottom 80, as has already been shown in FIG. 7. In particular, it can be seen in FIG. 8 that the depression 90 is formed by a rectilinear depression running parallel to the U-shaped trough 84. Furthermore, FIG. 8 shows that both the end region 99 of the wide sides 97 and the narrow side 98 of the eyelet 83 or the opening 82 are enclosed by the partial region 89 of the hump-like portion 88. Moreover, the hump-like portion 88 also forms a flat, inclined surface 91, which can be used as a centering aid for the bottom region of a box-like cover.
FIG. 9 shows a view of the tube bottom 80 of FIGS. 7 and 8, with a viewing direction from below onto the opening 82 or the eyelet 83. The side 96 of the inner wall 87 facing away from the U-shaped trough 84 can be seen. In addition, FIG. 9 particularly shows the lead-in chamfers 92 and 93, which run both on the narrow side 98 of the eyelet 83 as well as on the end regions 99 of the two wide sides 97. These lead-in chamfers 92 and 93 are formed, in particular, in the region of the material displacement produced by the depression 90. In particular, these serve for the simplified positioning and assembly of the tubes in the openings 82.
The embodiment according to the invention, as shown in FIGS. 7 to 9, can in particular also be combined with the preceding embodiments for less wide eyelets 43 and for wider eyelets 13 of FIGS. 4 to 6. In particular, the additional depression 90, which leads to a material displacement, is advantageous in this case to obtain the required material on the underside of the eyelets 83 to be able to form sufficiently dimensioned lead-in chamfers 92 and 93. This is due in particular to the fact that the tube bottoms, as shown in FIGS. 1 to 9, are preferably produced from a plate-like material by using several thermoforming processes. In each thermoforming process, material compression and material stretching take place, which ultimately lead to a thinning of the base material. By introducing additional depression, this partially disadvantageous thinning can be counteracted and, specifically, material can be displaced to defined regions.
FIG. 10 shows a comparative view of an embodiment with a tube bottom 80 with a less wide eyelet 83 and a wider eyelet 13 as already shown in FIG. 4. In particular, it can be seen in FIG. 10 that the U-shaped trough 14 or 84 is designed identical, irrespective of the width of the eyelet 13 or 83. In the case of the less-wide eyelet 83, the depression 90 is molded into the hump-like portion 88, as shown in FIGS. 7 to 9. In the case of the wider eyelet 13, this depression is not provided, as already shown in FIG. 4. In an alternative embodiment, however, the depression can also be combined with a wide eyelet 13.
FIG. 11 shows a comparative view of two lead-in chamfers 101 or 102, as can be formed as a function of the width of the eyelet, which is designated by the reference number 100. The lead-in chamfers 101 and 102 in particular form the front-side lead-in chamfers, which are formed on the respective narrow side of the eyelet 100. Preferably, the lead-in chamfer formed on the narrow side has an angle of approximately 40° with respect to a vertical, while the lead-in chamfers at the end regions of the wide sides preferably have an angle of approximately 30° to the vertical. These values are exemplary and do not particularly limit the design of the individual embodiments.
The embodiments of the preceding FIGS. 1 to 11 are exemplary and partially represent embodiments as are known from the prior art. In particular, a combination of the features of the embodiments known from the prior art, which are shown in FIGS. 1 to 6, and the features of the embodiments according to the invention, which are shown in FIGS. 7 to 9, is possible at any time. The individual embodiments of the figures have no limiting character, in particular with respect to the selection of the material, the arrangement of the individual elements and the dimensioning of the respective elements.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.

Claims

What is claimed is:

1. A tube bottom for a heat exchanger, the tube bottom comprising:

a bottom region with slot-type openings rimmed by eyelets, the tube bottom having an attached rim region; and

a U-shaped trough formed between the eyelets and the attached rim region as a receiving area for the bottom of a box-like cover, the U-shaped trough being formed by an inner first wall, the attached rim region and a second wall forming a trough bottom,

wherein the inner first wall has at least one hump-like portion in an extension of an eyelet on a side facing the U-shaped trough, which bounds a breadth of the U-shaped trough between the attached rim region and the first wall and extends from the trough bottom to the end region of the eyelet facing the U-shaped trough, and

wherein a depression is formed in the hump-like portion as a lead-in chamfer for tubes.

2. A tube bottom according to claim 1, wherein a displacement of the material towards the side of the first wall facing away from the U-shaped trough is formed by the molded-in depression.

3. A tube bottom according to claim 1, wherein a deflection of the hump-like portion from the inner wall increases from the trough bottom towards the respective eyelet.

4. A tube bottom according to claim 1, wherein the hump-like portion extends into the bottom region and at least partly surrounds the eyelet.

5. A tube bottom according to claim 1, wherein the first wall has a lead-in chamfer on its side facing away from the U-shaped trough, wherein the lead-in chamfer is arranged on the hump-like portion and the lead-in chamfer creates a tapering of the slot-type openings rimmed by the eyelets.

6. A tube bottom according to claim 5, wherein the lead-in chamfer is formed on the molded-in depression.

7. A tube bottom according to claim 5, wherein the slot-type openings have two opposing narrow sides and two opposing wide sides, wherein a slot-type opening is tapered by lead-in chamfers on the narrow side and on the end regions of the wide sides facing the narrow side.

8. A tube bottom according to claim 1, wherein the hump-like portion and/or the depression is impressed into the tube bottom in the hump-like portion or molded in by thermoforming.

9. A tube bottom according to claim 1, wherein the first wall and the surface of the hump-like portion oriented to the U-shaped trough are directed towards the trough bottom at an angle of more than 90 degrees.

10. A tube bottom according to claim 1, wherein the surface of the hump-like portion facing the U-shaped trough provides a centering aid for inserting the box-like cover into the U-shaped trough.

11. A heat exchanger with two tube bottoms according to claim 1, wherein the heat exchanger has a plurality of tubes, which are arranged parallel to each other and are accommodated at their ends in each case in an opening of a tube bottom rimmed by an eyelet, wherein the tubes are each inserted in respective openings along the lead-in chamfers, and wherein a box-like cover, which is connected to the respective tube bottom, is inserted into the U-shaped trough of each tube bottom.