EP1919650A2

EP1919650A2 - A device for the construction of heat exchanger elements provided with protection materials

Info

Publication number: EP1919650A2
Application number: EP06780270A
Authority: EP
Inventors: Eliseo Visca; Claudio Testani; Aldo Pizzuto; Stefano Libera; Andrea Mancini
Original assignee: Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA; Centro Sviluppo Materiali SpA
Current assignee: Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA; Centro Sviluppo Materiali SpA
Priority date: 2005-08-05
Filing date: 2006-08-01
Publication date: 2008-05-14
Also published as: WO2007017798A3; WO2007017798A2; ITRM20050105U1

Abstract

A device (1) for obtaining a junction between at least a first (2) and a second (3) element, which introduces a greater flexibility and cost-efficiency into this operation, comprises: a first main container (4), characterised in that it is open at least at one of its portions, able to be heated to high temperature, which in cooperation with closing means (5) defines an area for the junction of said first and second element to be joined (2, 3); one and/or a plurality of means (6, 7) for the containment of said first and second element to be joined (2, 3) and a spacer (8) apt to enclose said first element (2), which in conjunction with said closing means (5) and a plug (9) define a second main container (10), internal to said first main container (4) and structurally independent therefrom, provided with at least one access (11) for the introduction of a fluid under pressure such as to induce, on the surface opposite to the coupling surface of said first element (2), such a pressure as to press said first (2) and second element (3) against each other at a contact area.

Description

A DEVICE FOR THE CONSTRUCTION OF HEAT EXCHANGER ELEMENTS PROVIDED WITH PROTECTION MATERIALS

DESCRIPTION

The present invention relates to a device for the construction of heat exchanger elements provided with protection materials. In particular, it relates to a device for obtaining a junction between at least two elements to be joined; ceramic

(ceramic compounds)/metal (metal alloy) junctions and/or metal (metal alloy)/metal

(metal alloy) junctions.

In different fields of the art, it is necessary to effect junctions between materials exhibiting different chemical-physical properties which are able to withstand high thermal stresses. In said junctions, the heat flows whereto the materials in contact are subjected, both while achieving the coupling and in the actual operation of the component, induce high mechanical stresses on said materials, above all because the different thermal expansion coefficient causes different deformations in them, temperatures being equal.

The aforesaid problems are particularly felt in the sector of fusion nuclear reactors, and specifically in the construction of the divertors of so-called "Tokamak" reactors.

As it is well known to those skilled in the art, divertors are conduction heat exchangers apt to transfer heat from the plasma to a coolant which flows in a metallic tube internal to the divertor itself. The divertor of the Tokamak reactor therefore is the component under the greatest thermal stresses, with very high heat flows and specifically in the order of 20 MW/m². Therefore, the aforesaid metallic tube must be protected against direct contact with plasma by means of an outer coating made of refractory ceramic material such as graphite fibre composites (CFC). In particular, divertors are generally divided in two areas: in a first area, where a higher heat flow is expected, the metallic tube is protected by the Carbon Fibre Composite (CFC), whilst in a second area, in which a lower heat flow is expected, the tube is protected by blocks or single-piece blocks of tungsten (W). As mentioned above, in particular in the case of ceramic material/metal junctions, because of the different coefficient of thermal expansion, the heat flows induce considerable mechanical stresses in the materials of the junction. For example, if the tube is made of copper or copper alloy, it has a coefficient of thermal expansion (TEC) of about 2OxIO^"6 at 500 ⁰C, whereas the TEC of the refractory ceramic coating is practically nil.

The mechanical stresses induced by the different expansion of the materials, also during the construction of the junction, can lead to the formation of flaws, cracks or other mechanical damages that compromise the operation of the component. Said problems are further aggravated by the fact that the junction systems used for the applications described above generally provide for the injection of a pressurised fluid, such as to increase the pressure exerted by the metallic surface on the corresponding coupling surface of the ceramic (or metallic) material, so they are intimately in contact with each other.

The obtainment of junctions between metallic components (metal/metal) also has several problems, in particular when the junction has to meet highly strict qualitative requirements. Other sample cases of particularly burdensome welds are represented by tungsten-copper connections in thermo-technical applications at high temperatures, as stated previously, where, in addition to the poor mutual wettability of the metals, there is also the need to obtain a junction that does not constituted a thermal barrier.

Another example is constituted by connections between metals, such as aluminium-aluminium, aluminium-iron, aluminium-steel, which easily develop surface oxides and can make the weld less effective than required. In particular, aluminium-aluminium finned tubes are required in numerous thermo-technical applications to exploit the high thermal conductivity of aluminium, but they have the limitation of the heat barrier between fin and tube. A junction system that has been used for the applications described above is the so-called Hot Isostatic Pressing (HIP) Diffusion Bonding. According to this system, a tubular component is fitted with fins keyed on its outer surface according to the following operating procedures: enclosing the bodies to be welded in a metallic container that keeps them in position and that has to transmit external pressure; placing the metallic container in an autoclave at high pressures and temperature (pressure in the order of 1000 bar and temperatures ranging between 500⁰C and 2000⁰C); separating the piece obtained from the metallic container, which is often deformed by isostatic pressure.

Within the general HIP technique, examples of systems whereby this technique can be executed are described in US 6,077,476 (Quinlan et al.) and US 6,331,271 Bl (Bergman).

This procedure has numerous drawbacks. Among them, it is necessary to mention the difficulty in finding suitable autoclaves, in particular to obtain welded pieces with complex shapes or large sizes; the same drawback emerges with regard to the metallic container.

Moreover, the work processes for separating piece and container may cause the piece to be damaged and during the execution of the weld it is not possible to monitor the process parameters (pressure and temperature, local deformation), because of the barrier constituted by the autoclave. Moreover, said procedure is generally indicated for obtaining metal/metal junctions, but it has several drawbacks when it is necessary to obtain ceramic/metal junctions where the protection material has low mechanical strength. Lastly, the junction systems employed for this type of applications have additional and greater drawbacks when it comes to effecting mixed junctions of composite materials with part of the protection material being ceramic and part metallic, with additional difficulties when the junctions are for the construction of composite materials and with internal metallic element, having curvilinear longitudinal geometry (e.g.: a tube).

The technical problem constituting the basis for the present invention is to provide a device for diffusion bonding that enables to overcome the drawbacks mentioned with reference to the prior art.

This problem is solved by a device according to claim 1. Preferred characteristics of the present invention are present in the dependent claims thereof.

The present invention provides some considerable advantages. The main advantage of the device according to the present invention is that it enables to obtain a reusable device to achieve a junction between two or more elements to be joined, even with different shapes and dimensions, which can be heated with any means and in which it is possible to exercise a controlled pressure.

An additional advantage of the device is that it enables to construct components that entail the obtainment of junctions between material with low mechanical strength and metal, such as ceramic/metal junctions. Other advantages, characteristics and manners of using the present invention shall become readily apparent from the following detailed description of a preferred embodiment thereof, provided purely by of non limiting example with reference to the accompanying drawings, in which:

^■ Figure 1 shows a schematic view, in longitudinal section, of the device according to the present invention;

^■ Figure IA shows an enlarged detail of the device of Figure 1;

^■ Figure 2 shows a partially sectioned perspective view of a device according to the present invention;

^■ Figure 3 shows an exploded perspective view of the containment means with related elements to be joined of the device according to a preferred embodiment;

^■ Figures 3A and 3B respectively show the assembled containment means of Figure 3 and a variant of said assembled containment means relating to elements to be joined having a different geometry, according to the present invention;

^■ Figure 4 schematically shows a step of the method for using the device in Figure 1 ;

■ Figure 5 shows the device of Figure 1 in a step of said method. The embodiment example of the device described below shall be illustrated within the context of a preferred application thereof, i.e. the construction of components for a divertor of a Tokamak reactor. With reference initially to Figures 1 and 2, a device for obtaining a junction between at least two elements to be joined according to the present invention is globally designated by the number 1.

As mentioned in the introduction, a divertor requires the obtainment of a junction between an inner metallic tube 2 (generally made of copper) and an outer coating 3 comprising blocks or tiles made of ceramic or ceramic composite material

31 (generally, carbon fibre CFC) and block or tiles made of metallic material or metallic alloy 32 generally, tungsten alloy).

In the present embodiment example, the device 1 comprises a first main container 4, preferably made of stainless steel, with substantially tubular shape and characterised by being open at both its ends.

Each end has a flange 41 whereon are removably fastened closing means 5 which, together with the main container 4, define an area destined to the junction of the two elements to be joined; first element 2 and second element 3.

As shown in Figures 2 and 5, the main container 4 has, in central position, an opening 42 whereto it is possible to fasten a conduit through which the washing fluid can be sent, e.g. an inert gas such as argon, or to extract gases formed by the inner atmosphere to achieve a high degree of vacuum.

With reference to Figures 1 and 2, internally to the first main container 4 and structurally independent therefrom, is present a second main container 10. Said container 10 is defined by a plurality of means for the containment 6 and 7 of the two elements to be joined 2 and 3, by a spacer 8, apt to enclose the first element 2, and by the closing means 5 in collaboration with a plug 9.

According to a preferred embodiment, the closing means 5 comprise a counter-flange 51 Conflat^® apt to be removably fastened to the flange 41 and a closing flange 52 interposed between the spacer 8 and the inner surface of the counter-flange 51, as shown in Figure 1.

The closing flange 52 has an opening 53 with such inner diameter that it can be fitted on the tube 2, without an appreciable inter-space, and such as to be able to receive the plug 9 when fastened to the tube 2.

The flange 41 and the counter-flange 51 are vacuum and/or ultra- vacuum closing means and they comprise teeth, whose function is to block a first gasket 43, preferably made of copper, when flange and counter-flange are fastened by means of appropriate bolts 54.

The closing flange 52 and the counter-flange 51 are also vacuum and/or ultra- vacuum closing means and they comprise a second gasket 43', preferably made of copper, interposed between the two when fastened by means of additional bolts 55, as shown in Figure 1.

Lastly, the plug 9 is also fastened through adequate bolts 91 to the closing flange 52 and secured to the tube 2 by means of mechanical deformation with no need for bonding.

The plug 9 is provided with an access 11 for the introduction of a fluid under pressure, which is positioned in such a way as to induce, on a surface opposite to the coupling surface of the first element 2, such a pressure as to press the two elements to be joined (2 and 3) against each other at the contact area. In particular, as shown in

Figures 4 and 5, the access 11 is suitable to be connected to a high pressure circuit, whilst the opposite access 11^' can be used to connect an instrumentation for measuring the pressure inside the tube 2.

As stated previously, the second main container 10 comprises at its ends, in contact with the closing flange 52, a spacer 8 made of inert material, obviously not bonded on the first element 2, which in the present embodiment comprises multiple tubular elements with such an inner diameter that they can be fitted on the tube 2. According to the invention, the spacer 8 supports at least one thermal shield 12 which extends from the spacer in order to thermally shield a portion of the first main container positioned in proximity to the closing means 5.

At the centre, the second main container 10 comprises, flanking the spacer 8, the means for the containment, 6 and 7, of the two elements to be joined, 2 and 3, as shown in Figures 1 and 2.

The containment means, in a preferred embodiment, comprise first means for containment 6 apt to contain the coating of ceramic material 31 and the metallic tube

2 to effect the ceramic material/metal junction, and second means for containment 7 apt to contain the coating of metallic material 32 and the metallic tube 2 to effect the metallic material/metal j unction.

With reference in particular to Figures 3, 3A and 3B, the first containment means 6 comprise an outer armour 61, with preferably cylindrical shape, made of material with low heat expansion coefficient, e.g. a molybdenum alloy, and a plurality of elements 62 with greater coefficient of thermal expansion than the material composing the outer armour 61. The elements 62 can be inserted inside the armour 61 in order to define an inner space 63, suitable shaped to contain the elements 2 and 31 to be joined. Said elements 62 have in proximity to their contact surface and matching the edges of the parallelepiped constituted by the ceramic material 31, grooves 64. As shown in Figures 3 A and 3B, the elements 62 are in contact with the ceramic material 31 in such a way as to exercise, during the process for effecting the junction between the tube 2 and the material 3, an opposite pressure to the one exercised by the pressurised fluid inserted in the second main container 10. The operation of the first containment means 6, described above, shall be readily apparent from the description of the method for their employment.

The second containment means 7 comprises an outer armour, preferably made of steel, with inner space formed on the basis of the measurements and geometry of the coating metallic material 32 and closed by means of appropriate bolts 71, Figure 2.

The operation of the apparatus described above shall become readily apparent from the description of the method of its use.

The method is executed thanks to a furnace F that has a heated compartment 13, which is advantageously shaped to measure on the first main container, containing the area of the junction of the first and second element to be joined.

To make the method possible, the device 1, as described above, and the elements to be joined, i.e. tube 2 and tiles 3, are assembled.

In the case of ceramic/metal joints, the tiles are pre-brazed and subjected to an appropriate preparation process.

In this assembly, the elements to be joined are initially arranged in the first containment means 6, as shown for example in Figure 3.

After inserting said plurality of elements 62, in which are housed the tiles 31 and the tube 2, in the outer armour 61, the first containment means 6 are aligned by means of a dowel pin 65 passing through a hole 66 drilled in one of the elements 62, Figures 1 and 3B. Said dowel pin 65 is then removably fastened to the second containment means 7 once the latter has been assembled with the respective metallic tiles 32 and the tube 2, as shown in Figure 1.

The exact centring of the tiles relative to the tube 2, internally to the first main container 4, is obtained by means of said spacer 8.

Subsequently, a surface of the elements to be joined is isolated, whereon it is possible to exert such a pressure as to press said elements against each other at their contact area; in the present embodiment example, the inner surface of the tube 2 at the tiles 3.

Said isolation is obtained by said closing means 5 and the plug 9. In particular, the connection of the counter-flange 51 with the flange 41 and the closing flange 52 through the respective bolts, as well as the fastening of the plug 9, assure said isolation. At this point, it is possible to form a vacuum within the first main container 4. It is a so-called dynamic vacuum, obtained through continuous pumping through the opening 42 by means of an appropriate pumping system.

Through the opening 42 it can also be positioned an appropriate thermocouple or other equivalent measuring instrument to monitor the temperature of the contact area.

The degree of vacuum will preferably be between 10^"3 and 10^" Pa. The interior of the tube 2 is subjected to gaseous washing with argon before the complete closure of the device, with the connection to the plug 9 of a circuit under pressure 14 and of a pressure gauge 15 (Figure 5).

Subsequently, the device is placed with the first main container 4 in said heated compartment 13. The activation of the furnace F allows a first slow heating up to a temperature of 350°C-400°C. Once equilibrium is reached, the heating continues to a predetermined temperatures, which enables to favour the atomic inter-diffusion between the materials to be joined.

Said inter-diffusion temperature depends on the materials involved and it may generally vary between 500°C and 2000⁰C. The initial melting temperature of the first main container 4, made of stainless steel in the present example, is an upper limit to the maximum operating temperature, because the heating of the container is effected from the exterior and hence the armour 4 is at a higher temperature than the one measurable at said contact area.

The further to promote the obtainment of the junction, the elements to be joined are appropriately pressed against each other increasing the pressure inside the second main container 10 of the device, i.e. within the copper tube 2. For this purpose, a fluid under pressure, preferably an inert gas, is injected into the second container, until reaching a predetermined pressure apt to achieve an adequate contact between the surfaces to be joined. Preferably, said pressure shall be between 50 and 100 MPa.

Said pressure and temperature status must be controlled through appropriate temperature and pressure up and down ramps. In this way, the elements 62 of the first containment means 6 can compensate, through their heat expansion coefficient, and hence the related volume increase as temperature rises, the pressure exerted on the inner contact surface of the ceramic tiles 31 by the metallic tube 2, as a result of the pressurised fluid injected into it. The purpose of this system is to avoid that in a material with low mechanical strength like the ceramic one, the inner pressure exerted by a system like the one described above may cause it to be damaged. Said risk of damaging the ceramic tiles is present both in the step of increasing temperature and pressure and in the step in which temperature and pressure are returned to ambient values. Therefore, in both cases, the pressure and temperature up and down ramps must be controlled. Naturally, the duration of the entire process will be a function of pressure and temperature and of their up and down ramps. From the above description, it is readily apparent that the heating can be obtained in very simple fashion, e.g. by an non-pressurised resistance furnace without controlled atmosphere, hence at highly reduced costs.

Moreover, it is possible to heat only that part of case that contains the elements to be joined, not necessarily the entire case. This eliminates plastic deformations of the case and of the elements to be joined.

Control of the vacuum inside the first container is continuous, since the vacuum is dynamic, and the same holds true for the pressure inside the second container.

This method is shown schematically in Figure 4. At the end of the bonding method, the pressure state is appropriately decreased according to the temperature drop ramp and of the coefficient of thermal expansion of the elements 62.

At this point, it is particularly simple to separate first and second main container using the related bolts. Only the part of tube 2 housed in the closing flange 52 is eliminated together with the flange itself.

From the above description, it is clear that the device can be varied in dimensions and shape to be adapted to the shapes and dimensions of more complex pieces to be joined, even with hollow bodies of any shape which require a complete or partial external covering, made of a different material. It is not necessary to destroy the device to free the bonded pieces. Therefore, the device can be reused.

With regard to materials, for the containers of the device, dimensionally stable materials with high heat resistance are suitable: AISI 316L, Nickel or Tungsten based alloys or otherwise appropriate ceramic materials. On the other hand, for the construction of the plurality of elements 62, materials with rather high coefficient of thermal expansion are preferred, such as: Stainless Steel and Inconel®.

It will be appreciated, therefore, that the first containment means 6 can comprise a plurality of elements 62 of different material in order to compensate, through the specific heat expansion coefficient, any pressure imbalances, present during the execution of the junction, due to the anisotropy present in the materials or ceramic compounds.

Thanks to the ease of access to the pieces to be joined, it is possible to keep under control during the process, in addition to the pressures and to the temperature of the furnace, also the temperature at the contact area.

Moreover, it is possible to join even more than two pieces seamlessly. To the above described device for obtaining a junction between two elements to be joined and related method for its employment, a person skilled in the art, to meet further and contingent requirements, may make numerous additional modifications and variants, without thereby departing from the scope of protection of the present invention, as defined by the appended claims.

Claims

1. A device (1) for obtaining a junction between at least a first (2) and a second (3) element to be joined, comprising: a first main container (4), characterised in that it is open at least at one of its portions, able to be heated to high temperature, which in co-operation with closing means (5) defines an area for the junction of said first and second element to be joined (2, 3); one and/or a plurality of means (6, 7) for the containment of said first and second element to be joined (2, 3) and a spacer (8) apt to enclose said first element (2), which in conjunction with said closing means (5) and a plug

(9) define a second main container (10), internal to said first main container

(4) and structurally independent therefrom, provided with at least one access

(11) for the introduction of a fluid under pressure such as to induce, on the surface opposite to the coupling surface of said first element (2), such a pressure as to press said first (2) and second element (3) against each other at a contact area.

2. The device (1) as claimed in claim 1, wherein said first main container (4) has an inner space formed on the basis of the measurements and/or of the longitudinal curvature of said second main container (10).

3. The device (1) as claimed in either one of the claims 1 and 2, wherein said first main container (4) has substantially tubular shape and characterised in that it is open at one of its ends whereon said closing means (5) are removably fastened.

4. The device (1) as claimed in either one of the claims 1 and 2, wherein said first main container (4) has substantially tubular shape and characterised in that it is open at both of its ends whereon said closing means (5) are removably fastened.

5. The device (1) as claimed in any of the previous claims, wherein said first main container (4) has at least one opening (42) to extract the internal atmosphere to form a high degree of vacuum.

6. The device (1) as claimed in the previous claim, wherein through said opening (42) is positioned an appropriate thermocouple or other equivalent measuring instrument to monitor the temperature of the contact area.

7. The device (1) as claimed in any of the previous claims, wherein said main container (4) has at each end, characterised in that it is open, a flange (41) whereon are removably fastened said closing means (5).

8. The device (1) as claimed in any of the previous claims, wherein said closing means (5) comprise a counter-flange (51) Conflat^® apt to be removably fastened to said flange (41) and a closing flange (52) interposed between said spacer (8) and the inner surface of said counter-flange (51).

9. The device (1) as claimed in the previous claim, wherein said closing flange (52) is characterised in that it has an opening (53) of such diameter as to be able to receive said element (2) and said plug (9) when fastened to each other.

10. The device (1) as claimed in any of the claims 7, 8 and 9, wherein said flange (41) and counter-flange (51) are for vacuum and/or ultra- vacuum and characterised in that it comprises a first gasket (43), preferably made of copper, positioned between the teeth thereof, able to lock said gasket (43).

11. The device (1) as claimed in either of the claims 8 and 9, wherein said closing flange (52) is a vacuum and/or ultra- vacuum flange and characterised in that it comprises a second gasket (43% preferably made of copper, positioned between said counter-flange (51) and closing flange (52).

12. The device (1) as claimed in any of the claims 1, 2, 3 and 4, comprising a first access (11) connectable to a high pressure circuit (14) and a second access (H¹) usable to connect thereto an instrumentation for measuring interior pressure (15).

13. The device (1) as claimed in any of the previous claims, wherein said first element (2) is a metallic element.

14. The device (1) as claimed in the previous claim, wherein said metallic element is made of a copper alloy.

15. The device (1) as claimed in any of the previous claims, wherein said first element (2) has a coupling surface with substantially cylindrical geometry.

16. The device (1) as claimed in the previous claim, wherein said first element is a tube.

17. The device (1) as claimed in any of the previous claims, wherein said first element (2) has rectilinear and/or curved longitudinal extension.

18. The device (1) as claimed in any of the previous claims, wherein said second element (3) is a plurality of second elements characterised in that they are made of ceramic material and/or ceramic compound (31) and/or of metallic material and/or of metallic alloy (32).

19. The device (1) as claimed in claim 18, wherein said ceramic material and/or ceramic compound (31) is characterised in that it was pre-brazed at the coupling surface.

20. The device (1) as claimed in any of the previous claims, wherein said means (6, 7) for the containment of said first and second element to be joined (2, 3) comprise: first containment means (6) apt to contain said ceramic and/or ceramic composite material (31) and said first element (2) to be joined; and second containment means (7) apt to contain said metallic material and/or metallic alloy (32) and said first element (2) to be joined.

21. The device (1) as claimed in the previous claims, wherein said first containment means (6) are characterised in that each of them comprises an outer armour (61) made of material with low coefficient of thermal expansion and a plurality of elements (62) with greater coefficient of thermal expansion than the one associated with the material of the outer armour (61) which are inserted inside said armour (61) in order to define an inner space (63) suitable shaped to contain said elements to be joined.

22. The device (1) as claimed in the previous claim, wherein said outer armour (61) has substantially cylindrical shape.

23. The device (1) as claimed in claim 22, wherein said plurality of elements (62) are inserted inside said armour (61) in such a way as to obtain a series of grooves (64) in proximity to their contact surface.

24. The device (1) as claimed in the previous claim, wherein said plurality of elements (62) are in contact with said ceramic material and/or ceramic compound

(31) in order to exert, during the process for obtaining the junction between said first element (2) and said second element (3), a pressure opposite to the pressure exerted by said fluid under pressure.

25. The device (1) as claimed in the previous claim, wherein said plurality of elements (62) have different coefficient of thermal expansion in order to compensate for the anisotropy of said ceramic and/or composite ceramic material

(31) with which they are in contact.

26. The device (1) as claimed in claim 20, wherein said second containment means (7) is characterised in that it comprises an outer armour, preferably made of steel; said armour has an inner space that is appropriately shaped to contain said elements to be joined.

27. The device (1) as claimed in any of the previous claims, wherein said spacer (8) comprises one or more tubular elements whose inner diameter is such that they can fitted on the respective first element (2) to be joined.

28. The device (1) as claimed in claims 1 or 27, wherein said spacer (8) supports at least one heat shield (15) that extends from said spacer to thermally shield a portion of the first main container (4) positioned in proximity to said closing means (5).