JPH0226531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for mixing at least two partial streams with different state quantities.

When mixing multiple partial streams with different state quantities,
If the flowing material is a liquid, mechanically driven agitation devices are used or, especially in gaseous media, a deflection of the flow or flow sections is used to transfer the partial flows into one another by reversing the flow direction and thereby A fixed guide surface is used which provides sufficient mixing.

The known methods and devices not only have the disadvantage of high constructional outlays, but also because of the high degree of blocking and the diversion of the partial flows, large pressure losses occur, which are dependent on the respective device. The associated high energy consumption during operation results in economic disadvantages that are even greater than the high construction costs.

The object of the invention is to make it possible to mix at least two partial streams with different state quantities in a short flow section with low losses and effectively, without this requiring large constructional outlays or large energy consumption. The object of the invention is to provide a method and a device of the kind mentioned at the outset, which do not.

In order to solve this problem, according to the method of the invention, at least one of the at least two sub-streams having different state quantities is arranged in the flow cross section of at least one of the sub-streams in a downstream direction at right angles to the flow direction. A vortex pulse is generated that spreads out to form a discontinuous vortex system, and causes a vortex system component perpendicular to the main flow direction of the one partial flow to enter the flow cross section of the other partial flow. By vortex pulse is meant here a vortex that is generated in a pulsed manner by the built-in element and automatically expands. According to another feature of the invention, the vortex pulses can be generated by at least one curved surface and/or by at least one edge of the surface or object. Another preferred configuration of the present invention is
This is caused by the two sharp edges of the delta-shaped built-in element extending at an acute angle to each other.

With the method according to the invention, an almost complete mixing of the partial streams takes place within short flow paths and with considerably lower pressure losses. In the known method, a mixing distance whose length is approximately four times the diameter of the main passage is required in order to mix sufficiently uniformly the partial streams with different state quantities;
In the process according to the invention, after approximately half the mixing distance, the pressure loss is already 10 to 10% lower than that of the known process.
Remarkably good mixing is achieved with a pressure drop of around 20%. These significant advantages are obtained in the following manner. In other words, in the method according to the invention, no driven mixing devices are used, nor are built-in pieces with large guide surfaces and a high degree of blocking, which forcefully divert portions of the flow, and only a large degree of blocking is used. A small fixed built-in element generates vortex pulses which, by means of downstream expansion perpendicular to the flow direction, produce a low-loss mixing of the partial flows. This is because the components of the vortex system extending at right angles to the main flow direction enter the flow cross section of the other substreams and thus cause intensive mixing. The method according to the invention can be used for both liquids and gases and is particularly suitable for solving different mixing tasks when mixing flue gases, waste gases and steam.

According to the device for carrying out the method according to the invention, the built-in element for generating the discontinuous vortex system has at least one separation edge with a component oriented at right angles to the main flow direction of one of the partial flows. Thereby, the built-in element generates a vortex pulse that spreads downstream at right angles to the flow direction and becomes a discontinuous vortex system, and the vortex-shaped components that enter the flow cross section of other partial flows are is guaranteed to mix intensely and with little loss.

The device according to the invention not only performs intensive mixing over short distances and with low pressure losses, but also requires extremely low construction costs, which makes it possible to integrate the device according to the invention even later into existing channels. is open. Since the built-in element according to the invention is not used for deflecting or diverting the flow section, but for generating pulses, the built-in element has a very low blocking degree and is resistant to dirt.

If the device has a continuous main channel which is at the same time an additional channel for the partial flow and at least one additional channel connected to the side of the main channel, according to the invention at least one built-in element is provided in the flow cross-section of at least one partial flow in the additional channel and/or the main channel. An integrated element can be provided in the main channel in the region of the merging opening of the additional channel, and this integrated element penetrates into the flow cross-section of all partial streams. In particular in the case of additional channels connected laterally to the main channel, at least one built-in element can alternatively be provided in the flow cross section of each partial flow. Furthermore, at least one built-in element can be provided in the main channel after the confluence of the additional channel, which mixes the partial flows by means of a vortex system expanding downstream.

If the device has a plurality of additional channels parallel to each other and opening into a common main channel, according to the invention one built-in element in each case is provided for controlling the boundary flow between the flow cross sections of adjacent partial flows. provided on the surface. In this case, the additional channels can open into the main channel next to each other or extend concentrically to each other, in the latter case a plurality of built-in elements being uniformly distributed in an annularly closed boundary flow surface. It is provided. In order to reduce the mixing distance, to increase the mixing intensity and to prevent undesired swirls, according to another feature of the invention, in the main channel downstream of the built-in element provided at the boundary flow surface, To,
Further integrated elements with opposite directions of incidence can be provided.

In particular in installations with a plurality of additional channels opening into a common main channel parallel to each other, as in the case of a flue gas supply line leading to a chimney, according to the invention, the confluence of each additional channel is provided. The range is provided with built-in elements. By forming the vortex system according to the invention, these built-in elements ensure a reliable and intensive mixing of flue gas streams with different state quantities, especially sulfur content, even when the flow ratio between the substreams changes. .

If several additional channels are connected to the sides of the narrowing main channel, it is proposed according to the invention to provide one built-in element in each of the boundary flow surfaces of the sub-flows, in order to control the direction of the resulting vortex. Can be adapted to the nozzle action in narrow areas.

In a device with a plurality of additional channels opening laterally to the main channel, a further proposal of the invention provides that the tube sections of the additional channels can project into the main channel and that the tube sections of the additional channels can project into the main channel. The bends can each be configured to generate one discrete vortex system. In this case, the built-in element is formed by a tube section of the additional channel, so that instead of generating the vortex pulses by a surface or an edge of the object, as in the present invention, a curved part is used in this case for generating the vortex pulses. The length of the tube piece projecting into the main channel is preferably between 10% and 25% of the diameter of the main channel, so that the partial flow leaving the tube section acts on the part of the medium flowing through the main channel. to form a vortex. The tube piece can have a circular or square cross section (triangular, quadrangular or more polygonal) and the curved surface of the tube piece can be provided with sharp-edged strips to increase the pulse for vortex formation. You can prepare.

If vortex pulses are to be generated by the method according to the invention by at least one edge of the surface, the built-in element has a component extending in the main flow direction and a component extending at right angles to the main flow direction. According to the invention, it is proposed to design it as a surface with an edge extending as follows. Preferably, the built-in element has a plane of symmetry extending preferably in the direction of the main stream and has symmetrically extending edges. According to the present invention, the built-in element can be circular, oval,
It can be constructed in the basic shape of an oval, a parabola, or a rhombus. A particularly good effect occurs if, according to another characteristic of the invention, the built-in element is configured in the form of a delta, with the tip facing away from the main flow direction. In order to increase the stability of the built-in element, the built-in element can have a V-shaped cross-section and/or be provided with bent edges.

Embodiments of the present invention will be described below with reference to the drawings.

The first embodiment shown in FIGS. 1 and 2 shows a main channel 1 of square cross section, to which an additional channel 2 is connected at a connection angle β. A partial flow Q ₁ enters the main channel 1 and is supplemented with a partial flow Q ₂ via an additional channel 2 .

A delta-shaped built-in element 3 is provided in the area of the confluence of the additional channel 2, which integrates the partial flow.
It is inclined at an incident angle α with respect to the flow direction of Q ₁ , and its tip faces opposite to the flow direction. This built-in element 3 can be fixed, for example screwed, to the wall of the channel 1 or 2 by means of a holding element, for example a rod, which is not shown. This tip of the built-in element 3 extends into the main channel 1 at a distance b to the continuous wall of the main channel 1 and by a recess depth h relative to the lower edge of the merging additional channel 2 in front of the merging point.
Immersed inside.

Two leading edges extending symmetrically with respect to the main flow direction and having a component extending in the main flow direction and a component extending at right angles to the main flow direction are used as flow separation edges forming a flow separation line. , generates a vortex pulse which spreads downstream of the main passage 1 at right angles to the flow direction into a vortex system, as schematically shown in FIG. The components of this vortex system enter the flow cross section of the partial stream Q ₂ at right angles to the main flow direction of the partial stream Q ₁ , so that the partial streams Q ₁ and Q ₂ are intensively mixed after gathering. In this case, the delta-shaped built-in element 3 does not carry out any significant turning or deflection of the partial flow Q ₁ , but instead carries out the mixing by means of the aforementioned vortex system, which, as in the case without the built-in element 3 , carries out the mixing of both partial flows Q _{1 .} and Q ₂ from flowing alongside each other in the upper part of the main passage 1. In order to also generate vortex pulses in the partial flow Q ₂ and to prevent the residual flow of the partial flow Q ₂ from flowing upwards along the walls of the main channel 1 without mixing, the upper edge of the built-in element 3 is is relatively close to the confluence of the additional passage 2. The diagram is
This shows that such residual flow is prevented by the vortex system expanding downstream.

In the second embodiment according to FIG. 3, the lateral additional channel 2 is connected to the main channel 1 at a connecting angle β. However, in this case two delta-shaped integrated elements 3 are present in the main channel 1 and these integrated elements create a vortex in the main channel 1, so that the partial flow Q ₂ added from the additional channel 2 is , is intensively mixed with the partial flow Q ₁ added to the lower part of the main channel 1.
In this case, the lower of the two integrated elements 3 essentially generates vortex pulses in the partial flow Q ₁ ,
In contrast, the upper built-in element 3 essentially generates an expanding vortex system in the partial flow Q ₂ .

In the third embodiment according to FIG. 4, the additional channel 2 is also connected to the main channel 1 at an acute connection angle β. A vortex system is generated in the partial flow Q ₁ by the built-in element 3 . In contrast to the embodiment according to FIG. 3, the vortex system in the partial flow Q ₂ is generated by a built-in element 3 which partly projects into the additional channel 2 and is tilted oppositely to the upper built-in element according to FIG. Ru. In this embodiment, an additional third built-in element 3 can be provided behind the confluence of the additional channel 2 in the main channel 1, as indicated by dashed lines in FIG. This additional built-in element 3 reduces the mixing distance due to additional vortex formation.

In the embodiment according to FIG. 5, two additional channels 2
a, 2b form connection angles β ₁ and β ₂ , respectively, and are shifted by a distance a in the longitudinal direction of the main passage 1,
It is connected to a continuous main passage 1. A delta-shaped built-in element 3 is provided in the confluence region of each additional passage 2a, 2b, and this built-in element
As shown in the figure, a vortex system is generated which intensively mixes the partial streams Q _2a and Q _2b into the partial stream Q ₁ . These built-in elements 3 partially protrude into the respective additional channels 2a, 2b, so that the mixing distance is shortened.

According to a fifth embodiment according to FIG.
When three additional passages 2a, 2b, 2c are connected spatially in common, a plurality of delta-shaped built-in elements 3 that generate a vortex system extending over the entire cross section of the main passage 1 are installed in the confluence area. It is sufficient to place it at the back. In the embodiment according to FIG. 6, the partial flows Q _2a , Q _2b and Q _2c are mixed in the main channel 1, in which case the additional channels 2a, 2b, 2c are connected to the main channel 1 with different connection angles and They have different distribution cross sections.

In another embodiment according to FIG. 7, the main channel 1 has two additional channels 2 which meet symmetrically and at an acute angle.
a and 2b, through which the partial flows Q _2a and Q _2b respectively flow into the main channel 1. In this embodiment, two delta-shaped built-in elements 3 are provided in the confluence region of the additional channels 2a and 2b, these built-in elements forming an intersecting vortex system slightly behind the confluence region and causing a partial flow. Mix Q _2a and Q _2b intensively.

The embodiment according to FIG. 8 shows two additional channels 2a and 2b of mutually oppositely directed partial flows Q _2a and Q _2b , which flow into the main channel 1 which branches off at right angles. do. A delta-shaped built-in element 3 is provided for mixing the partial streams Q _2a and Q _2b , the edges of which generate a vortex system which mixes the partial streams Q _2a and Q _2b with low losses. Can be mixed.

In the embodiment according to FIGS. 9 and 10, the main channel 1 has three additional channels 2a, 2b with a rectangular cross section.
and 2c, these additional channels open into the main channel 1 parallel to each other and side by side, but converge in front from different directions. In this embodiment, a plurality of delta-shaped built-in elements 3 are connected to adjacent substreams Q _2a and Q _2b or Q _2b
and Q _2c at the boundary flow surface. These built-in elements are lower built-in elements 3 in the vertical cross-sectional view.
, which are four central built-in elements 3 in plan view. In order to facilitate the action of these built-in elements 3 and shorten the mixing distance, in the embodiment according to FIGS. 9 and 10, in addition to the built-in element 3 provided at the boundary flow surface, another built-in element 3a is mainly used. Located in the lower part of the channel 1, these built-in elements are inclined in the flow direction opposite to the lower built-in element 3 and cause the partial flows Q _2a and Q _2c to mix more quickly.

Instead of the additional channels according to FIGS. 9 and 10 opening into the main channel 1 parallel to each other and alongside each other, the embodiment according to FIGS. 11 and 12 provides additional channels 2 into the main channel 1. It is also possible to add the substream Q ₁ to the substream Q 2 by providing the substream Q 1 concentrically with respect to the substream Q ₂ . In this case as well, the delta-shaped built-in elements 3 are arranged in a uniformly distributed manner on the annularly closed boundary flow surface, as shown in the plan view of FIG. The formation of the vortex system is shown in FIG. 11, whereas the rotationally symmetrical arrangement of the built-in elements 3 is best seen in FIG.

The embodiment according to FIGS. 13 and 14 shows a main channel 1 with a circular cross section, which is, for example, the lower part of a chimney, into which from below a circular flow cross section of the same size as one another is introduced. Three additional passages 2a, 2b, and 2c are opened in parallel to each other. The flow cross section of the main passage 1 is the additional passage 2a, 2
It is larger than the sum of the flow cross sections b and 2c. In order to intensively mix the partial flows Q _2a , Q _2b and Q _2c in the main channel 1, a delta-shaped built-in element 3 is provided in the area of the confluence of each additional channel 2a, 2b, 2c.
The orientation of these integrated elements 3 at the junction of the additional channels 2a, 2b, 2c is determined depending on the respective flow rates of the partial streams Q _2a , Q _2b and Q _2c .

In FIG. 18c, the model measured smoke density according to the embodiment of FIGS. 13 and 14 is shown diagrammatically. 18a and 18b show various characteristic quantities for a model experiment in which a substream Q _2a with smoke is observed, whereas substreams Q _2b and Q _2c without smoke are added. aisle 2b
and 2c. With the measuring element, the smoke concentration is measured along the diagonal line s--s of FIG. 18a and at a height H corresponding to 2.5 times the diameter of the main channel 1.

The almost horizontally extending curve of the diagram in FIG. 18c shows that, even though the smoke is introduced eccentrically into the main channel 1 through the additional channel 2a, the built-in element 3 causes the partial flows Q _2a , Q _2b and Q _2c are completely mixed; in this case, the relatively strongly appearing peaks and troughs of the almost horizontally extending curve are locally vortexed by the delta-shaped built-in element 3 in the measurement plane. indicates that it is formed.
In the diagram of FIG. 18c, the S-shaped and diagonally extending curve shows the situation without integrated element 3. In this case, it can be seen that there is a smoke concentration of almost 100% in the left part of the main passage 1 and at a distance of a height H corresponding to 2.5 times the diameter of the main passage 1.
The action of the built-in element 3 which mixes the partial streams Q _2a , Q _2b and Q _2c intensively and with low losses is therefore
This can be clearly seen from the diagram in Figure 18c.

In the embodiment according to FIG. 15, the main channel 1 narrows downstream behind the connection of several additional channels 2 into an outflow channel 1a, each of these additional channels 2 being significantly smaller than the main channel 1. It has a cross section. In this case, one built-in element 3 is provided in the area of each boundary flow surface indicated by the dashed line, so that a vortex system is created which is oriented in the direction of the narrowed outflow channel 1a.

In previous embodiments, the built-in element is configured as a delta-shaped surface with sharp edges;
In the embodiment according to the figures and FIG. 17, a curved surface is used to generate a vortex pulse, which spreads downstream at right angles to the flow direction into a discontinuous vortex system. The figure shows a main channel 1 with a circular cross-section, which has a total of six additional channels 2.
are connected radially and at right angles. Each tube piece 4 of these additional channels 2 projects into the main channel 1 . The curved surfaces of these tube sections 4 serve to generate a respective discontinuous vortex system, which spreads downstream and converts the partial flow Q ₂ introduced through the additional channel 2 into a partial flow Q ₁ . Mix. The tube piece 4 is circular, as shown in FIGS. 25 to 27.
It can have a rectangular or triangular cross section.
The tubular piece 4, which is circular or oval in cross-section, is advantageously provided with two preferably sharp-edged strips 4a on its outer surface, as shown in FIG. Make the pulse larger to help and generate the vortex system.

Finally, FIGS. 19 to 22 show four different embodiments of flat built-in elements 3. FIG. It can be seen from these figures that instead of the delta-shaped configuration described so far, it is also possible to configure the built-in element 3 in a circular, oval, parabolic or rhombic configuration. According to FIG. 23, the built-in element 3 can be constructed with a V-shaped cross section in order to increase its stability. Furthermore, the built-in element 3 can be provided with a folded edge 3b, as shown in FIG.
These folded edges increase stability on the one hand and generate sufficiently strong vortex pulses on the other hand.

[Brief explanation of drawings]

1 is a longitudinal sectional view of a first embodiment with a continuous main channel and an additional channel connected at an acute angle; FIG. 2 is a plan view of the device according to FIG. 1; 3 is a longitudinal sectional view of a second embodiment with a continuous main passage and an additional passage connected at an acute angle on the side, and FIG. FIG. 5 is a longitudinal sectional view of a third embodiment with additional passages connected at an acute angle on one side, FIG. FIG. 6 is a longitudinal section through a main passage in which three additional passages are connected at different angles; FIG. 7 is a symmetrical FIG. 8 is a longitudinal sectional view of a main passage leading to two additional passages which are arranged at an acute angle and converge at an acute angle. FIG. 9 is a longitudinal sectional view of the embodiment, and FIG. 10 is a longitudinal sectional view of a main passage consisting of three rectangular additional passages having a constant flow cross section and opening at different angles. 9 is a plan view of the main channel, FIG. 11 is a longitudinal section through the main channel which is constituted by two additional channels of constant cross-section and running concentrically with respect to each other, FIG. 12 is a plan view of the main channel, FIG. A plan view of the main channel according to FIG. 11, and FIG. 13 shows a longitudinal section of the lower part of the main channel with a circular cross section but opening into three additional channels with a small cross-sectional area. 14 is a plan view of the main channel according to FIG. 13; FIG. 15 is a longitudinal sectional view of the main channel with a narrowing cross section and several laterally connected additional channels; FIG. Figure 16 is a longitudinal sectional view of a main passageway with a circular cross section in which a tube piece projects into the main passageway and six additional passageways with arbitrary cross sections are connected; A plan view of the passage, FIG. 18a is a plan view of the device as a target for model measurement of smoke concentration, FIG. 18b is a longitudinal sectional view of the device as a target for model measurement of smoke concentration, and FIG. 19. Diagram of model measurements of smoke concentration measured at height h on the diagonal line s-s of the model shown in plan view in FIG. 18a and in longitudinal section in FIG. 18b. 22 to 22 are plan views of four basic shapes of built-in elements, FIGS. 23 and 24 are cross-sectional views of the built-in elements, and FIGS. 25 to 28 are cross-sectional views of four tubular built-in elements. It is a diagram. 1... Main passage, 2... Additional passage, 3... Built-in element.

Claims (1)

[Claims] 1. At least two partial flows Q ₁ with different state quantities
At least one built-in element 3 or 3a creates a vortex in the flow cross-section of at least one of the partial flows Q ₁ or _{Q 2 of} at least two with different state quantities, characterized in that a pulse is generated to cause a vortex component perpendicular to the main flow direction of the one partial flow to enter the flow cross section of the other partial flow Q ₂ or Q ₁ ; How to mix partial streams. 2. Method according to claim 1, characterized in that the vortex pulses are generated by at least one curved surface of the built-in element. 3. Claim 1, characterized in that the vortex pulse is generated by at least one edge of the built-in element 3, 3a formed as a surface or a solid.
The method described in section. 4. The method according to one of claims 1 and 3, characterized in that the vortex pulses are generated by two flow separation edges of the delta integrated elements 3, 3a extending at an acute angle to each other. . 5 At least two partial flows Q ₁ with different state quantities
At least one built-in element 3 or 3a creates a vortex in the flow cross-section of at least one of the partial flows Q ₁ or _{Q 2 of} Built-in elements 3, 3a that generate a discontinuous vortex system in the device that generates a pulse and causes a vortex component perpendicular to the main flow direction of the one partial flow to enter the other partial flow Q ₁ or Q ₁ is characterized in that it has at least one separation edge with a component oriented at right angles to the main flow direction of the one of the partial streams Q ₁ or Q ₂ , mixing at least two partial streams with different state quantities; device to do. 6. The continuous main channel is an additional channel for one partial flow, at least one additional channel for the other partial flow is connected to the side of the main channel, and at least one built-in element 3, 3a is an additional channel for the other partial flow. 2, 2a,
6. The device according to claim 5, characterized in that it is provided in the flow cross-section of at least one partial flow Q ₁ , Q ₂ in 2b, 2c or in the main channel 1. 7 Built-in element 3 connects main passage 1 to additional passage 2
and all partial flows Q ₁ , Q ₂
Device according to one of claims 5 and 6 (FIGS. 1 and 2), characterized in that it penetrates the flow cross-section of the flow section. 8. Device according to claim 5, characterized in that at least one built-in element 3 is provided in the flow cross-section of each partial flow Q ₂
Figures 4 and 7). 9. Claims 5 and 6, characterized in that each additional channel 2a, 2b connected laterally to the main channel 1 is provided with an integrated element 3 in each flow cross section. The apparatus described in one of these (Fig. 5). 10 Additional passages 2a, 2b, 2 in main passage 1
The device according to claim 5 (FIGS. 6 and 8), characterized in that at least one built-in element 3 is provided behind the junction of c.
figure). 11 A plurality of additional channels open parallel to one another into a common main channel, each with at least one built-in element 3 at the boundary flow surface between the flow cross-sections of adjacent partial flows Q _2a , Q _2b , Q _2c . Device according to claim 5 (FIGS. 9 and 10), characterized in that it is provided. 12. Device according to claim 11 (FIGS. 9 and 10), characterized in that the additional channels 2a, 2b, 2c open into the main channel 1 side by side. 13. Claim 11, characterized in that the additional channels 2 extend concentrically with respect to each other and a plurality of built-in elements 3 are provided uniformly distributed on the annularly closed boundary flow surface. (FIG. 11 and FIG. 12). 14. In the main channel 1, downstream of the built-in element 3 provided at the boundary flow surface, a further built-in element 3a with an opposite direction of incidence is provided. Apparatus according to one of clauses 13 (FIGS. 9 and 10). 15 A plurality of additional passages, in particular flue gas supply lines leading to the chimney, open parallel to each other into a common main passage, and an integrated element 3 is provided in the area of the confluence of each additional passage 2a, 2b, 2c. The device according to claim 5, characterized in that:
(Fig. and Fig. 14). 16 A plurality of additional channels open to the sides of the main channel in front of the narrowing, each with one built-in element 3
The device according to claim 5 (FIG. 15), characterized in that the is provided at the boundary flow surface of the partial flow _Q2 . 17 A plurality of additional passages open to the sides of the main passage,
Claim 1, characterized in that the tube segments 4 of the additional channel 2 protrude into the main channel, the curved surfaces of the tube segments 4 being configured in each case to generate one discontinuous vortex system. The device according to item 5 (No. 16)
(Fig. and Fig. 17). 18 Claim 17, characterized in that the tube piece 4 has a circular or square cross section.
The device according to one of paragraphs and paragraphs 18 (second
Figure 8). 19 Claim 17, characterized in that the tube piece 4 is provided with a sharp-edged strip 4a on its curved surface.
The device according to one of paragraphs and paragraphs 18 (second
Figure 8). 20 Patent, characterized in that the built-in elements 3, 3a are configured as surfaces with edges extending in such a way that they have a component extending in the main flow direction and a component extending at right angles to the main flow direction Device according to one of claims 5 to 16. 21. Claim 2, characterized in that the built-in elements 3, 3a have a plane of symmetry extending in the mainstream direction and have symmetrically extending edges.
The device according to item 0. 22. Claim 20, characterized in that the built-in elements 3, 3a have a basic shape of circle, oval, oval, parabolic or rhombus.
The device according to one of paragraphs and paragraphs 21 (first
Figures 9 to 22). 23. According to one of the claims 20 and 21, characterized in that the built-in elements 3, 3a are configured in the form of a delta with the tips pointing opposite to the main flow direction. The device described. 24. According to one of the claims 20 to 23, characterized in that the built-in elements 3, 3a are V-shaped in cross section and/or are provided with bent edges 3b. apparatus (Figs. 23 and 24).