Brazable Component And Heat Exchanger Comprising Same

Abstract

The brazable component for the circulation of a fluid in a heat exchanger, in particular for a motor vehicle, according to the invention, comprises an aluminum alloy core 4. It further comprises a pure aluminum protective layer 8 placed against the core 4.

Description

The present invention relates to the field of heat exchangers for motor vehicles which are intended to exchange heat between two fluids, and more particularly, but not exclusively, to aluminum alloy heat exchangers used to cool the exhaust gases of the heat engines of these vehicles.

Circuits for recirculating exhaust gas of the heat engine of a motor vehicle (called EGR for “Exhaust Gas Recirculation”) are known. The exhaust gases are recirculated therein to the intake of the engine where they are completely burned.

The recirculation circuit extends between the exhaust circuit of the heat engine and the intake circuit and generally includes a heat exchanger for cooling the exhaust gases since they are at an extremely high maximum temperature (400° C.-900° C.). It also includes an uncooled by-pass channel, a by-pass valve for selectively directing the gas stream into the heat exchanger or into the by-pass conduit and a control valve for controlling the throughput of the gas stream in the recirculation circuit.

These exhaust gases are particularly acidic and condense into acidic liquid in particular when the engine of the vehicle is cold. Since this acidic liquid particularly contains sulfates, nitrates, organic acids, chlorides and/or hydrocarbons, these cause a large amount of damage to the heat exchangers through corrosion. It is therefore common to note condensates of a pH less than 2 with chloride values of 50 ppm. On some vehicles, even pH 0.8 and chlorides at 200 ppm are noted.

Furthermore, the assembly of the various components of these exchangers is by means of brazing such that these elements are previously covered with a layer of solder. During brazing, diffusion of the silicon of the layer of solder towards the core of the elements is observed. This modification of the composition of the core makes the elements all the more vulnerable to corrosion.

There is hence a need for exchanger components that are corrosion-resistant.

Document WO 2006034876 is known, which proposes forming a hydrophobic layer on the surface of the exchange circuits such as to remove as quickly as possible the condensation of acid water without it having the time to attack the exchanger but this solution is not satisfactory as it does not completely prevent corrosion and significantly reduces the heat exchange capacities of the exchanger.

The aim of the invention is to propose components for such exchangers, which can be resistant to corrosion, for example, due to condensation of theses gases and which are particularly simple and economic to manufacture.

To this end, the invention proposes a brazable component for the circulation of a fluid in a heat exchanger, in particular for a motor vehicle, which comprises an aluminum alloy core, characterized in that it further comprises a pure aluminum protective layer placed against the core.

Therefore, after brazing, the pure aluminum layer is in contact with the corrosive gases and not the core of the component. The aluminum layer has a better resistance to corrosion than the core of the component and the presence thereof prevents the diffusion of the silicon of the solder towards the core. The component is resistant to corrosion whether due to the circulation of the exhaust gases, of a liquid such as glycol water or of another fluid.

According to embodiment features that are particularly simple and convenient, both at manufacture and use:

• • the protective layer comprises an aluminum weight percentage which is greater than or equal to 99.5%;
  • the core has a first surface on which the protective layer extends evenly;
  • the core has a second surface, opposite the first surface, and the component comprises a second pure aluminum protective layer which is placed against the second surface;
  • the two protective layers have the same composition;
  • the product of the mechanical strength after brazing of the component by the component thickness is greater than or equal to 46800 MPa.μm;
  • the component has a thickness of between 200 μm and 530 μm;
  • the thickness of the protective layer is between 25 μm and 80 μm;
  • the protective layer is placed between the core and a layer of solder for the assembly by brazing of the component to another component, the ratio between the thickness of the layer of solder and the thickness of the protective layer being between 0.3 and 1;
  • the thickness of the layer of solder is between 4% and 10% of the total thickness;
  • this component is a tube or a plate.

The invention also relates to an exchanger, in particular for motor vehicles, for the exchange of heat between a first fluid circulating in a first circuit and a second fluid circulating in a second circuit that is separate from the first circuit, characterized in that it further comprises a plurality of components as previously disclosed, the surface of the component comprising the protective layer belonging to the first circuit.

According to embodiment features that are particularly simple and convenient, both at manufacture and use:

• • the first fluid comprises recirculated exhaust gases;
  • after brazing, the product of the mechanical strength of the component by the thickness of the core of the component is greater than or equal to 46800 MPa.μm;
  • the components are stacked plates or tubes placed in parallel.

The features and advantages of the invention will emerge from the following description, given by way of preferred example, but which is in no way limiting, with reference to the appended drawings, wherein:

FIGS. 1 a and 1b are a pair of cross-sectional partial views of a tube according to the invention with a layer of pure aluminum on the outer surface only, before and after brazing;

FIGS. 2 a and 2b are a pair similar to that of FIG. 1 wherein a layer of pure aluminum and a layer of solder are placed on the inner surface and on the outer surface of the tube;

FIG. 3 is a perspective exploded view of an exchanger according to the invention;

FIG. 4 is a view similar to that of FIG. 3, the exchanger being assembled;

FIG. 5 is a perspective view of a plate before brazing according to the invention with a layer of aluminum and a layer of solder placed on the inner surface and on the outer surface of the plate;

FIG. 6 is a view similar to that of FIG. 5, after brazing;

FIG. 7 is a perspective view of a plate according to the invention with a layer of pure aluminum on one surface only of the plate and a layer of solder on each side, before brazing.

A component according to the invention is described with reference to FIGS. 1 and 2. In this case, it is a flat tube 1 intended to be part of a heat exchange bundle 2 as shown in FIG. 3. The tube 1 comprises a wall wherein the section transverse to the longitudinal axis thereof is oblong with two flat portions alternating with two rounded portions, for example semi-circle shaped. The rounded portions have not been shown in the figures. Two parallel segments 3 of the flat walls are seen in the figures.

The thickness of the wall of the tube 1, i.e. the thickness of the segments 3 is between 200 μm and 530 μm.

The wall of the tube 1 comprises a core 4 which is made of 3000 series aluminum alloy.

The inside 5 of the tube 1 is, in FIGS. 1 and 2, the space extending between the two segments 3. Each segment 3 has an inside face 6 which faces the inside 5 of the tube 1 and an outside face 7 opposite the inside face 6.

On the side of the outside face 7, the core 4 comprises a first surface against which a protective layer 8 of pure aluminum extends. Pure aluminum is taken to mean an aluminum alloy having an extremely high content of aluminum, a content which is greater than a given value of 99%. In this case, this is a 1000 series aluminum alloy. The layer 8 has a thickness of 40 μm and in this case an aluminum percentage of 99.5%.

Prior to brazing the tube 1, the protective layer 8 extends between the core 4 and a layer of solder 10. The thickness of the layer 10 is 30 μm. The solder is used for brazing the tube 1 to another component of the bundle 2, for example dividers placed between the tubes 1 or the collector plates of the bundle 2. As brazing is well known and is not the subject matter of the invention, no detailed description will be given in this case for brazing or for the composition of the solder, given that the solder has a melting temperature less that that of the components of the exchanger in order to allow the various components to be fixed to one another.

On the inside face 6 side, the core 4 comprises, prior to brazing, against the surface thereof, another layer 10 of solder. This layer 10 is used for brazing the tube 1 with disturbing elements (not shown) placed inside the tube 1.

Generally, the thickness of the component, i.e. the thickness of the wall of the tube 1, the thickness of the layer 8 and the thickness of the layer of solder 10 are chosen in the ranges and with the relationships as follows:

• • the ratio between the thickness of the layer of solder 10 and the thickness of the layer of aluminum 8 is between 0.3 and 1 for preventing the solder from migrating towards the core 4;
  • the thickness of the layer of solder 10 is between 4% and 10% of the thickness of the wall of the tube 1;
  • the thickness of the protective layer 8 is between 25 μm and 80 μm;
  • the thickness of the wall of the tube 1 is between 200 μm and 530 μm.

Following brazing of the exchanger 11 (FIGS. 3 and 4), the metal of the layer of solder 10 has migrated towards the areas to be brazed such that outside of these areas, the protective layer 8 is exposed.

The layer 8 forms a coating intended to be in contact with the fluid and to act like a sacrificial protective layer. Therefore, even if this layer 8 starts to be corroded, the core 4 of the wall of the tube 1 is not attacked.

On the tube 1 in FIGS. 1a and 1b, there is therefore a protective layer 8 on the outside of the tube 1 so as to protect the tube 1 against corrosion on the outside which could occur under the effect of a fluid passing by.

On the tube 1 in FIGS. 2a and 2b, the second surface of the core on the side of the inside face 6 of the tube 1 is coated with a second protective layer 8. This second layer 8 extends between the core 4 and another layer of solder 10. Brazing takes place on the outside of the tube and on the inside of the tube, for example with disturbing elements between the tubes and inside the tubes. Thanks to the presence of the protective layers 8 on the outside and on the inside of the tube 1, the latter is protected against corrosion which can occur under the effect of fluid passing between the tubes and in the tubes.

Following brazing, the mechanical strength of the wall of the tube 1 multiplied by the thickness of the core 4 is greater than 46800 MPa.μm.

An example for use of the tubes of FIGS. 2a and 2b is illustrated in FIGS. 3 and 4. The illustrated exchanger 11 comprises two rows of flat tubes 1. The bundle is placed in a housing 12.

The exchanger 11 includes a circuit for the circulation of the EGR gases with exchange of heat with a coolant.

The housing 12, which has a substantially rectangular section and is closed at one of the ends thereof, houses on the inside thereof the bundle 2 of tubes 1 which, in the present case, is U-shaped, i.e. the inlet 13 and outlet 14 of the tubes 1 are placed on the same side at the open end 15 of the housing 12.

The end of the bundle of tubes 1 is fixed to a support plate 16. The support plate 16 has a plurality of openings 17 for the installation of the respective tubes 1.

The housing 12 has an inlet channel 18 and an outlet channel (not shown) for the EGR coolant, in this case the glycol water of the engine cooling circuit.

The EGR gases enter the exchanger 11 through the inlet 13, circulate in the tubes 1 where they are cooled, then go back out through the outlet 15. The glycol water enters through the inlet channel 18, bathes the bundle 2 and exits through the outlet channel. An exchange takes place between the EGR gases and the glycol water so as to allow cooling of the EGR gases.

The acidity of the EGR gases can be around pH 2 and can, in certain circumstances, reach a pH of 0.8 and contain up to 200 ppm of chlorides. With possible condensation in the course of the circulation in the exchanger, the inside of the tubes is subjected to corrosive conditions against which they are protected through the presence of the protective layers 8.

The glycol water also acts corrosively and the outside of the tubes is protected against this thanks to the protective layers 8 located on the outside.

According to an alternative embodiment corresponding to FIGS. 1a and 1b, only the fluid circulating outside the tubes acts corrosively such that only the outside of the tubes is protected by a protective layer 8.

According to another alternative, protective layers are provided only on the inside of the tubes with layers of solder only on the outside of the tubes. Generally, a protective layer is provided on at least one of the inside or outside surfaces of the tubes.

According to another alternative, no solder is provided inside the tube.

As regards the manufacture of the tubes 1, firstly the core 4 is obtained in the form of a 3000 series aluminum alloy strip. Then, the 1000 series aluminum layer 8 is evenly sprayed on the outside (FIG. 1a) or on the outside and inside (FIG. 1b). Finally, a layer of solder is deposited on the outside and on the outside, for example, by steeping the tube 1 in a bath of solder. The solder can also be injected to form the layer inside the tube 1. The tube 1 is formed by closing the two edges of the flat core onto themselves. The tubes 1 are made watertight during brazing of the exchanger 11.

According to an alternative embodiment, the core 4 of the tube 1 is produced by extrusion. Then, the 1000 series aluminum layer 8 is evenly sprayed onto the outside of the tube 1 or onto the outside and inside. Finally, a layer of solder is deposited on the outside and inside of the tube 1.

In both cases, disturbing elements or turbulators can be placed inside the tubes 1, which disturb the circulation of the fluid in the tubes in order to improve the transfer of heat. Another embodiment will be described now with reference to FIGS. 5-8. For similar elements, the same numbering, increased by 100, will be used. The component according to the invention is, in this case, a plate 101 which comprises a core 104, the two faces of which are coated with a protective layer 108 (FIGS. 5 and 6). According to an alternative, only one surface of the plate 101 is coated with a protective layer (FIG. 7).

As with the previous embodiment, a layer of solder 110 is placed on the protective layer(s) 108.

Following brazing (FIG. 6), the protective layers 108 are exposed.

The exchanger 111 is a plate exchanger. The plates 101 are stacked, one above the other, such as to create two fluid circuits, for example an EGR gas circuit and a coolant circuit circulating glycol water of the cooling circuit of the engine. In this case, these are the plates 101 of FIGS. 5 and 6 which comprise protective layers on each side.

The stack of the plates 101 allows a circuit to be created for the EGR gases and a circuit to be created for the glycol water. As in the previously described embodiment, there are high-risk conditions in terms of corrosion such that the protective layers 108 play a role in the protection against corrosion.

According to an alternative embodiment (FIG. 7), the plates 108 only comprise one protective plate 108 on one side. In this case, the plates 108 are positioned such that the protective layers 108 of two successive plates face one another. This creates therefore a fluid circuit wherein the faces have protective layers alternating with a circuit wherein the faces do not have any specific protection. This layout is suitable for an exchange of heat between a corrosive fluid and a fluid which is only slightly corrosive or not corrosive at all.

As regards the manufacture of the plates 101, firstly the core 104 is obtained in the form of a 3000 series aluminum alloy strip. Then the 1000 series aluminum layer 108 is sprayed. Finally, a layer of solder 110 is deposited. Optionally, relief is created on the plate 101 in particular to create circulation channels (not shown) and bosses 120 or dimples in order to increase the exchange surface.

By way of example, a component with a thickness of 480 μm comprises a core of aluminum alloy 3916 having a thickness of 360 μm, a protective layer of aluminum alloy 1050 having a thickness of 48 μm and two layers of solder 4343 (one per face) having a thickness of 36 μm (i.e. by percentage, 75% for the core, 10% for the protective layer and 7.5% for each layer of solder). The mechanical strength of the component is 130 MPa.

Another example is a component with a thickness of 400 μm which comprises a core of aluminum alloy having a thickness of 300 μm, a protective layer of aluminum alloy 1050 having a thickness of 40 μm and two layers of solder 4343 (one per face) having a thickness of 30 μm (i.e. by percentage, 75% for the core, 10% for the protective layer and 7.5% for each layer of solder). The mechanical strength of the component is 156 MPa.

A third example is a component with a thickness of 400 μm which comprises a core of aluminum alloy having a thickness of 260 μm, two protective layers of aluminum alloy 1050 having a thickness of 40 μm (one per face) and two layers of solder 4343 (one per face) having a thickness of 30 μm (i.e. by percentage, 65% for the core, 10% for each protective layer and 7.5% for each layer of solder). The mechanical strength of the component is 180 MPa.

For each example, the product of the mechanical strength by the thickness of the core after brazing is 46800 MPa.μm.

As an alternative to the described embodiments, the component, tube or plate, can be obtained in the form of a rolled multi-layer sandwich.

Alternative embodiments cover the various combinations that can be obtained by providing a protective layer on one face only, a protective layer on each face, a layer of solder on one face only, and a layer of solder on each face.

The optional presence of a layer of solder is particularly linked to the need arising from the optional presence of dividers between the tubes or of turbulators in the tubes.

According to another embodiment, the claimed component (tube or plate) comprises the protective layer(s) while it is the dividers or disturbing elements placed between the tubes or the plates (or turbulators placed inside the tubes) which have the layers of solder.

According to another embodiment, the component according to the invention is a collector plate comprising an aluminum alloy core assembled by brazing with tubes for example. This collector plate comprises a pure aluminum protective layer placed on the surface of the core on the side of the tubes.

According to an alternative, the component is the support plate 16 of the exchanger 11. The tubes 1 and the plate 16 then comprise protective layers.

According to another embodiment, the heat exchanger according to the invention comprises a first circuit wherein there circulates a first fluid to be cooled, for example glycol water, by means of a second fluid fluid, for example air. The tubes or the plates of the exchanger comprise protective layers only on the surfaces which are part of the first circuit.

The present invention is not limited to the described and illustrated embodiment but covers any alternative embodiment.

Claims

1. A brazable component for the circulation of a fluid in a heat exchanger for a motor vehicle, the brazable component comprising an aluminum alloy core, wherein the brazable component further comprises a pure aluminum protective layer placed against the core.

2. The component as claimed in claim 1, wherein the protective layer comprises an aluminum weight percentage which is greater than or equal to 99.5%.

3. The component as claimed in claim 1, wherein the core has a first surface on which the protective layer extends evenly.

4. The component as claimed in claim 1, wherein the core has a second surface, opposite the first surface, and wherein the component comprises a second pure aluminum protective layer which is placed against the second surface.

5. The component as claimed in claim 1, wherein the two protective layers have the same composition.

6. The component as claimed in claim 1, wherein the product of the mechanical strength after brazing of the component by the component thickness is greater than or equal to 46800 MPa.μm.

7. The component as claimed in claim 1, wherein the component has a thickness of between 200 μm and 530 μm.

8. The component as claimed in claim 1, wherein the thickness of the protective layer is between 25 μm and 80 μm.

9. The component as claimed in claim 1, wherein the protective layer is placed between the core and a layer of solder for the assembly by brazing of the component to another component, the ratio between the thickness of the layer of solder and the thickness of the protective layer being between 0.3 and 1.

10. The component as claimed in claim 9, wherein the thickness of the layer of solder is between 4% and 10% of the total thickness.

11. The component as claimed in claim 1, wherein the component is a tube.

12. The component as claimed in claim 1, wherein the component is a plate.

13. A heat exchanger for motor vehicles, for the exchange of heat between a first fluid circulating in a first circuit and a second fluid circulating in a second circuit that is separate from the first circuit, wherein the heat exchanger comprises a plurality of components as claimed in claim 1, the surface of at least one of the plurality of components comprising the protective layer belonging to the first circuit.

14. The exchanger as claimed in claim 13, wherein the first fluid comprises recirculated exhaust gases.

15. The exchanger as claimed in claim 13, wherein, after brazing, the product of the mechanical strength of the component by the thickness of the core of the component is greater than or equal to 46800 MPa.μm.

16. The exchanger as claimed in claim 13, wherein the components are stacked plates.

17. The exchanger as claimed in claim 13, wherein the components are tubes placed in parallel.

18. The exchanger as claimed in claim 17, wherein the tubes are separated by dividers.

19. The exchanger as claimed in claim 17, wherein disturbing elements are placed inside the tubes.