WO2002050478A1

WO2002050478A1 - Bi-thermal heat exchanger in particular for condensing boilers

Info

Publication number: WO2002050478A1
Application number: PCT/IB2001/002300
Authority: WO
Inventors: Ruggero Marchetti
Original assignee: Merloni Termosanitari S.P.A.
Priority date: 2000-12-20
Filing date: 2001-12-03
Publication date: 2002-06-27
Also published as: ITTO20001184A0; AU2002223971A1; ITTO20001184A1; IT1321119B1

Abstract

This document describes a bi-thermal heat exchanger, of the condensing type, comprising tubular components (1) formed by at least a first pipe (2), wherein a first fluid can flow, and by a second pipe (3), wherein a second fluid can flow, in which the two pipes (2, 3) are positioned one inside the other; the external surface of the first pipe (2) can be heated by means of the flues produced by a burner, so that the first pipe (2) yields heat to the first fluid, the first fluid yields heat to the second pipe (3) and the latter finally yields heat to the second fluid. According to the invention, - at least one pipe (2) is composed of two parts (2A, 2B), each made of different materials; -at least the surfaces of said pipes (2, 3) that are in contact with said first fluid are made of the same material; -at least the external surface of said first pipe (2) is made of a material having characteristics of high thermal conductivity and and high resistance to corrosion on the part of the acid condensates of said flues; -at least the internal surface of said second pipe (3) is made of a material compatible from an alimentary point of view with said second fluid.

Description

BT-THERMAL HEAT EXCHANGER IN PARTICULAR FOR CONDENSING BOILERS

DESCRIPTION

This invention relates to a heat exchanger particularly for condensing boilers and to a specific method for its construction.

Several types of boilers are known especially gas boilers for room heating only or for the combined heating of both the water of the heating system and of that for hygienic and alimentary purposes (sanitary water)

Then there are also known gas water heaters defined as instantaneous which work on the basis of the same principles as the aforementioned boilers but produce only hot sanitary water

In boilers for combined heating, the heating of the sanitary water can be carried out either indirectly in a dedicated heat exchanger and by way of the hot water of the heating circuit or directly inside the heat exchanger with the flues generated by a burner; this type of heat exchanger, which envisages the circulation and heating both of the heating circuit water and of the sanitary water inside the device is known as "bi-thermal".

As is known, in the bi-thermal exchanger the sanitary water circulates inside a first pipe which in turn is housed inside a second pipe through which the water of the heating circuit flows

Of course, boilers for combined heating equipped with a bi-thermal heat exchanger are less expensive than those for the indirect heating of the sanitary water circuit, as these require instead two separate heat exchangers. It is also known that the more efficient boilers are those of the "condensing" type, equipped with a heat exchanger employing the flues generated by the burner with a very large surface; particularly, the combustion flues have a temperature ranging between 800 and 1.000°C at the moment of their entry and exit at a temperature of approximately 30-50°C; in this way, inside the heat exchanger, the water vapor contained in the flues is condensed and hence both the sensible heat and the latent heat are recovered. The condensates thus obtained collect however many acid substances; therefore, heat exchangers need to be built with a material resistant to corrosion; for the above reason materials like normal steel or copper alloys are not suitable for the construction of condensing heat exchangers, while materials like stainless steel or aluminum-based alloys have proven to be more suitable. Moreover, the aluminum-based alloys have also proven to be more convenient and effective in terms of the heat exchange than steel.

In the light of as much heat exchangers suitable for contact with acid condensates have been proposed.

In accordance with a first type of solution, the heat exchanger wherein the fluid to be heated flows is composed of a series of finned tubular components made of aluminum alloys, wherein the external fins are obtained: either by the direct rolling of the aluminum alloy tubular component; or by way of separate fins made of aluminum alloy which are applied and braze welded on the tubular component, or by way of separate fins made of aluminum alloy which are applied by expansion on the tubular components.

A second type of heat exchangers suitable for contact with acid condensates, envisages instead the use of exchange surfaces shaped like tubular components made of stainless steel; these exchangers, however, are very expensive owing to the direct cost of the material and because the poor thermal conductivity of said material requires heat exchanger shapes that are more complicated to obtain and hence more costly.

It is also known that, with regard to an instantaneous condensing boiler, there is a third type of heat exchangers suitable for contact with flues and acid condensates composed of co- extruded tubular components, whose inner part (i.e. the part that comes into contact with the sanitary water) is made of a copper alloy admissible for alimentary use, while the external part (i.e. the part that comes into contact with the flues and condensates) is made of an aluminum alloy resistant to acid condensates and equipped with fins obtained by rolling the external part of the aluminum.

At the state of the art, save for the use of stainless steel only, it is impossible to build a condensing boiler equipped with a bi-thermal heat exchanger; in fact, the aluminum alloys suitable for contact with acid condensates are not suitable for contact with water for hygienic and alimentary use; moreover, the copper alloys that can be used for alimentary purposes are not resistant to acid condensates.

This invention aims at solving the drawbacks typical of the current state of the art. Generally speaking, this invention aims at the construction of a condensing bi-thermal heat exchanger that - is perfectly compatible with sanitary water from an alimentary point of view;

- is resistant to the corrosion of condensates;

- makes use of materials which are extremely suitable for heat exchange;

- is efficient and convenient to build; is not subject to galvanic type corrosions. These and other purposes will be illustrated more in detail below and are attained by this invention by way of a heat exchanger and of a relevant construction method with the characteristics laid down in the enclosed claims which are to be considered as an integral part of this description.

Other purposes, characteristics and advantages of this invention can be understood more easily thanks to the detailed description provided below and to the attached drawings provided by way of example and without posing any limitation whatsoever. The latter include:

- Fig. 1 which illustrates a schematic sectional view of a tubular component belonging to a heat exchanger built according to the principles of this invention; - Fig. 2 which illustrates a schematic axial view of a tubular component belonging to a possible embodiment of a heat exchanger as per this invention;

- Figures 3 and 4 which illustrate respectively the prospective and lateral view of a set of tubular components belonging to a heat exchanger built according to the principles of this invention. The heat exchanger as per this invention is of the condensing bi-thermal type.

It is composed of a bundle of tubular components, each of which is formed at least by a first pipe wherein a first fluid can transit and a second pipe wherein a second fluid can transit and which is located inside the first pipe. Each of the tubular components is connected to the others by way of suitable joints according to known techniques; these joints are not illustrated herein for the sake of simplicity. As is widely accepted by the state of the art, the external surface of the first pipe can be heated by means of the flues generated by a burner so that the first pipe yields heat to the first fluid, the first fluid yields heat to the second pipe and the latter finally yields heat to the second fluid. In Fig. 1, with 1 is indicated a tubular component belonging to the heat exchanger as per the invention taken as a whole.

Tubular component 1 is in fact formed by a first external tubular pipe indicated with 2 and by a second internal tubular pipe indicated with 3 which is inserted inside the first pipe. As it can easily be inferred, a first fluid can flow through external pipe 2. In the case of this invention said fluid is the water necessary to supply a domestic heating system. Instead, in the internal pipe 3 a second fluid can flow: in the case of this invention it is the sanitary water.

In compliance with a first important aspect of the invention in question, external tubular pipe 2 has a composite structure, since it is formed by at least two parts made of different materials and particularly:

- an external part 2A, directly exposed to the flues and made of a material having characteristics of extreme thermal conductibility and high resistance to corrosion;

- an internal part 2B, in direct contact with the first fluid, i.e. the water of the heating system, made of the same materials of the external surface of internal pipe 3 and having extreme thermal conductibility.

On the other hand, according to the invention: - at least the internal surface of internal tubular pipe 3 is made of a material compatible from an alimentary point of view with the sanitary water flowing through pipe 3, and

- at least the external surface of the internal tubular pipe 3 is made of the same material forming the internal part 2B of external tubular pipe 2.

In the recommended embodiment of the invention, the external part 2 A of pipe 2 is made of an aluminum alloy and particularly an aluminum-silicium alloy, while internal part 2B of pipe

2 is made instead of a copper alloy; internal tubular pipe 3 is made completely of the same material forming said internal part 2B of pipe 2, i.e. a copper-based alloy. This solution envisaged as per the invention is the result of the fact that the construction of a bi-thermal type heat exchanger whose tubular components are composed only of an external pipe made completely of aluminum alloy and of an internal pipe made on the inside of a copper alloy is not feasible owing to the risk of causing of corrosion of an electrolytic nature. From this point of view it is basically impossible to insert a copper alloy tubular component containing sanitary water inside and in contact with an aluminum alloy tubular component containing the water of the heating circuit, since corrosion owing to galvanic currents are liable to occur. This electrochemical type corrosion could be avoided in theory if the direct contact between the copper alloy and the aluminum alloy were avoided by means of a layer of a dielectric material on the internal surface of the aluminum alloy tubular component or on the external surface of the copper alloy tubular component; this solution, however, would be extremely unreliable, as it would be impossible to ensure the total absence of pores in the dielectric layer and its duration in the course of time; moreover, if the dielectric material were too thick, it would hinder the effective transmission of the heat.

The construction of the tubular components 1 as per the invention hence makes it possible to overcome the aforesaid problem. The shape of the section of internal pipe 3 can be basically that considered to be the most suitable to:

- increase the thermal exchange surface between the water of the heating system in external pipe 2 and the sanitary water in pipe 3;

- to receive at least a part of the heat even directly from external pipe 2 by conduction.

For instance, in the embodiment illustrated by way of example in the figures, the internal tubular pipe 3 has a cross or star-shaped section for the very same reasons mentioned above so that internal pipe 3 has a wider external surface exposed to the fluid in internal pipe 2 and internal pipe 3 can come into contact and, if joined in the four directrices of the external pipe

2, receive the heat by direct conduction.

The internal tubular pipe 3 can be made, of course, with any known technique. On the other hand, external tubular pipe 2 can be obtained in various ways.

A first option is that of obtaining pipe 2 through the co-extrusion of its two parts 2A and 2B, made of aluminum alloy and copper alloy respectively (please note that hereinafter the term "aluminum" is used to indicate for brevity's sake any alloy suitable for the purpose made of aluminum and likewise "copper" stands for any copper alloy suitable for the purpose). As an alternative, the external pipe 2 can be obtained by applying a thin layer of aluminum to be used to form part 2A on the external surface of a tubular component made of copper to be used to form part 2B, or by applying a thin layer of copper to be used to form part 2B on the internal surface of an aluminum tubular component to be used to form 2A. Another option for the making of the external tubular pipe 2 is that of plating copper tubular component with an aluminum tubular component. This process is already known and basically consists in the following: i) an aluminum tubular component for the making of part 2A is taken; its sectional dimensions, when cold, do not make it possible to insert in it a copper tubular component for the making of part 2B; ii) the external tubular element is heated to make it dilate enough to allow for the insertion of the internal tubular component; iii) the internal tubular component is inserted in the hot and dilated external tubular element; iv) the external tubular component, once it has cooled, locks onto the internal tubular component. Another option is that of expanding a copper tubular component inside an aluminum tubular component. This procedure, already known, is carried out basically as follows: i) an internal copper tubular component for the making of part 2B is taken; it is to be inserted in an aluminum tubular component to be used to make part 2A; ii) the copper tubular component is inserted in the aluminum tubular component; iii) a relevant plunger is pushed inside the copper tubular component to dilate it from the inside so that the copper tubular component is pressed against the internal surface of the aluminum tubular component to obtain the perfect coupling of the parts. In the recommended embodiment of the invention a series of fins are applied to the external surface of external tubular pipes 2 in order to improve the heat exchange with the flues produced by the burner of the heat exchanger.

For instance, these fins can be obtained by means of the aforementioned techniques, i.e. the rolling of the aluminum tubular component forming part 2A of pipe 2 which consists in the plastic deformation of this tubular component; another possibility is that of applying aluminum fins by way of braze welding or by way of expanding on part 2 A of pipe 2. In a particularly advantageous embodiment of the invention a series of appropriate aluminum components can be used for the making of said part 2A of the external pipes 2 and of said fins.

Fig. 2 shows a schematic sectional view of said embodiment of a pipe 2 of the heat exchanger as per the invention, wherein reference number 5 indicates some of said components. Each component 5 is made from a base 6 composed of a pre-perforated and pre-shaped aluminum plate so that a collar 7 provided with a hole protrudes from a surface; for the sake of the simplicity of the description, it is assumed that said collars 7 protrude from the upper surface of the respective plate 6.

The section and the dimensions of collars 7 and of the relevant holes are such that they can be fit on the copper tubular components 2B'for the making of the part 2B of external pipes 2 as per Fig 1 with a slight interference to ensure the proper contact between the parts (and in particular to ensure the proper contact between the internal surface of the collars 7 and the external surface of the copper tubular components 2B').

With regard to the making of pipe 2, according to the proposed embodiment, an adequate number of components 5 are fit in succession on a copper tubular component 2B' so that said tubular component 2B' is coated by the collars 7 overlapping the various components 5; of course, collars 7 of each component 5 are joined, for instance, by welding or another known technique to the lower surface of plate 6 belonging to the component 5 located on top of it in order to ensure that the external surface of the copper tubular component 2B' forming the internal part 2B of the pipes 2 are perfectly closed and sealed by the group of overlapping collars 7.

As it can be inferred, the solution illustrated in Fig. 2 in any case makes it possible to obtain a heat exchanger wherein a series of copper tubular components 2B' forming the internal part 2B of the external pipes 2 are completely protected against the aggressive condensates generated by the flues discharged by the burner associated to the heat exchanger by the collars 7 of the overlapping components 5, while the whole of said collars constitute in fact the external part 2 A of the pipes 2 of Fig. 1. Furthermore, by means of the proposed solution, the plates 6 of each component 5 form the fins of the tubular components 1 with the purpose of improving the heat exchange with the flues discharged by the heat exchanger's burner.

As regards the assembly of the internal tubular pipes 3 in the external tubular pipes 2 of Fig. 1, it can be carried out in various ways.

For instance, a first way envisages that the aluminum parts and the copper parts are made separately and then assembled together to obtain a composite external pipe 2. From this point of view, for instance, each copper tubular component to be used to make part 2B houses a star or cross-shaped copper tubular to be used to make pipe 3; the two said copper tubular components are hence joined together by way of braze welding.

On the other hand, a series fins made of aluminum and attached by way of braze welding are applied on a series of tubular components made of the same alloy and to be used to make part

2A.

This way all the copper parts can be braze-welded during a process separate from the braze welding of the aluminum parts; it must be borne in mind that the brazing of the copper parts in a brazing furnace for aluminum can lead to the contamination of the latter or vice versa.

Therefore, the copper tubular components comprising the parts 2B, equipped on the inside with the relevant pipe 3, are inserted in the aluminum tubular components forming the parts

2A already equipped with fins. Then follows the expanding of the internal parts of the copper tubular components composing the parts 2B on the internal surfaces of the aluminum tubular components composing the parts 2 A; this is obtained by means of expanding pins forced between the pipe 2 and the pipe

3, while a duly shaped male part inserted inside the pipe 3 avoids that the expanding pins crush the copper tubular component forming the internal pipe 3. Other assembly solutions envisage instead that the composite external pipe 2 be prepared before the final assembly.

From this point of view a possible way to obtain the tubular components 1 of Fig. 1 is the following.

The external pipes 2 with the parts 2A and 2B are already formed by means of one of the techniques illustrated above (plating, coating, expanding, etcetera). The fins are already mounted on these external pipes 2 by way of braze welding or expanding. The copper tubular components forming the internal pipes 3 are then inserted in pipes 2; then follows the expanding of the internal pipes 3 on the internal surfaces of pipes 2; this operation too can be carried out by means of an appropriate male piece which, inserted inside the tubular component forming the pipe 3, avoids that the expanding pins of the plunger crush the tubular component.

Another solution is that of obtaining by way of rolling the fins on the external pipe 2 previously formed with one of the techniques already illustrated above (plating, coating, expanding, etcetera).

In this case the fins of the tubular components 1 are not made of separate components inserted inside the composite pipe 2, but are obtained directly via the plastic deformation of its aluminum part 2A.

The form of the heat exchanger forming the subject matter of this invention can be either of the single tubular component set type or the separate tubular component bundle type.

In the former case, the various tubular components 1 are obtained with the procedures described above, but without any fins; the forming of the set of the heat exchanger's tubular components can hence be obtained by means of the relevant aluminum plates with holes for the various tubular components 1.

One of said embodiments is illustrated by way of example in figures 3 and 4.

In these figures 10 indicates the aluminum plates provided with eleven holes for the insertion of just as many tubular components 1; in the illustrated case said holes are arranged in three parallel rows and the holes of each row are staggered compared to those of the next row.

The use of these plates 10 for the construction of a tubular component set type heat exchanger is quite easy.

For instance, a first solution is that of fitting in succession a certain number of plates 10 duly spaced between each other on a series of eleven external pipes 2 without fins or on a series of eleven tubular components 1 completely formed and without fins.

In the first case, the plates 10 are attached to the pipes 2 by means of braze welding and then the internal pipes 3 are inserted in the respective external pipes 2 whereto they are fastened by means of expanding. In the second case, instead, the plates 10 are simply fastened into place on the tubular components 1, for instance, means of the expanding of the latter.

In the case in which the heat exchanger is of the type with bundles of separate tubular components, the fins can be made directly through the rolling of the aluminum parts 2A of the various tubular components or separately for each tubular component 1, for instance with the technique described above with regard to Fig. 2.

Of course, without prejudice to the principle inspiring the invention, those illustrated above are just some of the possible techniques for the construction of a heat exchanger according to the invention.

Once the set or bundle of the heat exchanger's tubular components is obtained, these are connected in a known way with one another either in series or in parallel for instance by means of appropriate joints or fittings; the boiler equipped with the heat exchanger as per the invention is connected so that the external pipes 2 receives water from a heating circuit and the sanitary water from the internal pipes 3.

As it can be inferred, the operating of the boiler incorporating the heat exchanger according to the invention is quite simple. The flues produced by the boiler's burner yield heat to the external pipes 2 and hence to the heating system's water; the water, in turn, yields heat to the sanitary water in the respective internal pipes 3.

The fact that the external part 2A of the tubular components 1 is made of aluminum avoids any risk of corrosion owing to the acid substances that may be present in the condensates produced inside the boiler; then, the fact that the internal surface of the pipe 3 is made of copper solves also any other problem regarding the compatibility of the heat exchanger with regard to the sanitary water for alimentary purposes; finally, the fact that no other metal alloys of a different nature are immersed in the water excludes corrosion of a galvanic nature. The use of materials having characteristics of great thermal conductibility ensures a highly effective operating of the heat exchanger despite the minor industrial costs for its construction.

The description above clearly illustrates the characteristics and advantages of the heat exchanger and of the relevant construction method forming the subject matter of the invention in question. These characteristics and advantages are also summarized in the enclosed claims which form an integral part of this description. It is clear that there are many possible variants that can be adopted by experts with regard to the condensing heat exchanger and to the relevant production method described as per the example without however transcending the novelty inherent to the idea inspiring the invention, as it is just as clear that in the practical implementation of the invention the various components described can have different shapes and materials and be replaced by technically equivalent components. For instance, possible variants can regard the way in which the internal pipe 3 is made.

For instance, reference was made above to the construction of an internal pipe 3 made on the inside of copper alloy, yet nothing hinders that the same component can be made with another material compatible with sanitary water for alimentary purposes, provided however that also part 2 A of the external pipe 2 is made of the same material. Another possibility is that envisaging an internal pipe 3 with a composite structure, i.e. formed by at least two parts made of different materials and similar to as much illustrated above with regard to the pipes 2; the internal of part of an internal pipe 3 of this kind can hence be made of a material compatible with sanitary water, while the external part can be made of the same material forming the material composing the internal surface of the external pipe 2.

Of course, in this case, the external pipe 2 can be made completely of a same material, i.e. without the need for a composite structure, having characteristics of high thermal conductivity and high resistance to corrosion; from this point of view, for instance, the internal pipe can comprise an internal part made of copper alloy and an external part made of aluminum alloy and the external pipe can be made completely of an aluminum alloy.

In compliance with yet another possible variant, a series of collar components having the same functions as those indicated with 6 in Fig. 2 can be applied directly on a series of plates of the type of those indicated with 10 in Fig. 3 and 4. In other words, near each of the holes through said plates a collar-like component can be envisaged; a number of these plates, obtained as described above, can be fit in succession on a series of internal parts 2A and the collar-like components of each plate can be fastened to a surface of a plate on top of it; in this way, said plates can form both the external part 2A of the pipe 2 of each tubular component 1 and the fins and a means binding a number of tubular components 1.

Claims

1. Bi-thermal type heat exchanger comprising one or more tubular components (1), wherein at least one of said tubular components (1) is formed by at least a first pipe (2) in which a first fluid can flow and by a second pipe (3) in which a second fluid can flow; wherein said second pipe (3) is located inside said first pipe (2); wherein the external surface of said first pipe (2) is heated by the flues produced by a burner so that the first pipe (2) yields heat to the first fluid, the first fluid yields heats to the second pipe (3) and the latter yields heat to the second fluid, and characterized by the fact that:

- at least one (2, 3) of said pipes is formed at least by two parts (2A, 2B) made of different materials and distinguished in an external part (2A) and an internal part (2B);

- at least the surfaces of said pipes (2, 3) that come into contact with said first fluid are made of the same material;

- at . least the external surface of said first pipe (2) is made of a material having characteristics of high thermal conductivity and high resistance to corrosion; - at least the internal surface of said second pipe (3) is made of a material compatible with the second fluid for alimentary purposes.

2. Heat exchanger according to claim 1, characterized by the fact that:

- said second pipe (3) is made on the inside of a material compatible with said second fluid for alimentary purposes and in particular of copper or an alloy comprising copper; - said first pipe (2) is formed by at least two parts (2 A, 2B) made of different materials, which are composed as follows:

- an external part (2A), directly exposed to said flues and made of a material having characteristics of high resistance to corrosion, particularly an alloy comprising aluminum;

- an internal part (2B) in direct contact with said first fluid and made of the same material of which said second pipe (3) is made.

3. Heat exchanger according to claim 1, characterized by the fact that: said first pipe (2) is made completely of a material having high resistance to corrosion, particularly an alloy comprising aluminum; said second pipe (3) is formed by at least two parts made of different materials, which are composed as follows: an internal part made of a material compatible from an alimentary point of view with said first fluid, particularly of copper or an alloy comprising copper; - an external part in direct contact with said first fluid and made of the same material of which said first pipe is made (2).

4. Heat exchanger according to at least one of the previous claims, characterized by the fact that one of said fluids, in particular the first, is the water needed to supply a heating system and that the other of said fluids, in particular, the second, is sanitary water.

5. Heat exchanger according to at least one of the previous claims, characterized by the fact that the overall section of said second pipe (3) is shaped to have one or more points of contact and/or connection with said first pipe (2) and that said section has a star or cross shape.

6. Heat exchanger, according to claim 1, characterized by the fact that the external surface of said tubular components (1) is provided with fins (6; 10).

7. Heat exchanger, according to the previous claim, characterized by the fact that said fins are an integral part of said first pipe (2), as it is obtained in particular by way of rolling.

8. Heat exchanger, according to claim 5, characterized by the fact that said fins are made of separate components compared to said tubular components (1) and attached to the external surface of the same in particular by way of braze welding or expanding.

9. Heat exchanger, according to claim 1 , characterized by the fact that said external part (2A) is formed by a number of components (5) provided with at least an opening which are staggered and overlapping with one another on said internal part (2B).

10. Heat exchanger according to claims 5 and 8, characterized by the fact that said components (5) define said fins.

11. Heat exchanger, according to claim 8 and/or 9, characterized by the fact that said components (5) are attached together in particular by way of braze welding.

12. Heat exchanger, according to at least one of the previous claims, characterized by the fact that it is of the tubular component set type.

13. Heat exchanger, according to at least one of the previous claims, characterized by the fact that it is of the tubular component bundle type.

14. Heat exchanger, according to claim 11, characterized by the fact that a series of perforated plates (10) forming said fins and joining together several of said tubular components (1) are envisaged.

14. Heat exchanger, according to claim 11, characterized by the fact that said perforated plates (10) form also the external part (2A) of said first pipe (2).

15. Method of construction of a heat exchanger of the type comprising one or more tubular components (1), with at least one of said tubular components (1) being formed by at least a first pipe (2), wherein a first fluid can flow and by a second pipe (3), wherein a second fluid can flow; with said second pipe (3) being located inside said first pipe (2), wherein the external surface of said first pipe (2) can be heated by means of the flues generated by a burner, so that the first pipe (2) yields heat to the first fluid, the first fluid yields heat to the second pipe (3), and the latter yields heat to the second fluid, characterized by the fact that: - at least one (2) of said pipes is formed by at least two parts (2 A, 2B) made of different materials, one of which is an external part (2 A) and the other an internal part (2B);

- at least the surfaces of said pipes (2, 3) which are in contact with said first fluid are made of the same material;

- at least the external surface of said first pipe (2) is made of a material having characteristics of high thermal conductivity and high resistance to corrosion;

- at least the internal surface of said second pipe (3) is made of a material compatible from an alimentary point of view with said second fluid.

16. Method, according to claim 15, characterized by the fact that at least one of said pipes (2,3), in particular said first pipe (2), is obtained by way of the co-extrusion of said two parts (2A, 2B), with said external part (2A) being made in particular of copper or an alloy containing copper and said internal part (2B) being made in particular of an alloy containing aluminum.

17. Method, according to claim 15, characterized by the fact that at least one of said pipes (2,3), in particular said first pipe (2), is obtained by applying on the external surface of a tubular component made of a first material, in particular copper or an alloy containing copper, used for the construction of said internal part (2B), a layer of a second material, in particular an alloy containing aluminum, used for the construction of said external part (2A).

18. Method, according to claim 16, characterized by the fact that at least one of said pipes (2,3), in particular said first pipe (2), is obtained by applying on the internal surface of a tubular component made of a first material, in particular an alloy comprising aluminum, to be used for the construction of said external part (2A), a layer of a second material, in particular copper or an alloy containing copper, to be used for the construction of said internal part (2B).

19. Method, according to claim 15, characterized by the fact that at least one of said pipes (2, 3), in particular said first pipe (2), is obtained by way of the plating of a tubular component made of a first material, in particular an alloy containing aluminum, on a tubular component made of a second material, in particular copper or an alloy containing copper.

20. Method, according to claim 15, characterized by the fact that at least one of said pipes (2,3), in particular said first pipe (2), is obtained by way of the expanding of a tubular component made of a first material, in particular copper or an alloy containing copper, inside a tubular component made of a second material, in particular an alloy containing aluminum.

21. Method, according to at least one of the previous claims, characterized by the fact that a series of fins are made directly on the external surface of said first pipe (2), in particular by way of rolling.

22. Method, according to at least one of the previous claims, characterized by the fact that:

- a second tubular component to be used for the making of said second pipe (3) is inserted in a first tubular component to be used for the making of said internal part (2B) and the two tubular components are then joined together;

- a series of fins is applied to a third tubular component to be used for the making of said external part (2A);

- the first tubular component, equipped on the inside of the second tubular component, is inserted in the third tubular component, already equipped with fins;

- the first tubular component is attached to the third tubular component, in particular by way of expanding, wherein the first and second tubular component are made of a first material and the third tubular component and the fins are made of a second material.

23. Method, according to at least one of the claims from 15 to 20, characterized by the fact that:

- a said first pipe (2) is provided with fins; - a tubular component to be used for the making of internal pipe (3) is inserted in said external pipe (2) and attached to it, in particular by way of expanding.

24. Method, according to claim 15, characterized by the fact that a set of said tubular components (1), formed by a number of perforated plates (10) inserted in succession on a series of said external pipes (2), is envisaged, that said plates (10) are then attached to said external pipes (2), in particular by way of braze welding and that the respective internal pipes (3) are then inserted in said external pipes (2) and attached therein, in particular by way of expanding.

25. Method, according to claim 15, characterized by the fact that a set of said tubular components (1), formed by a number of perforated plates (10), inserted in succession on a series of said tubular components (1) already formed is envisaged and that said plates (10) are attached to said tubular components (1), in particular by way of expanding.

26. Method, according to claim 15, characterized by the fact that said external part (2A) is formed by the overlapping of several components (5) provided of at least an opening, that said components (5) are fit in succession with one another on a tubular component composing internal part (2B), and that said components (5; 15) are then attached with one another.

27. Method, according to claim 15, characterized by the fact that a series of plates having a number of holes and provided with a collar-like component on each of said holes are envisaged, wherein several of said plates are fit in succession on a series of said internal parts (2B), that the collars of each plate is attached to a surface of a plate positioned on top of it, and that said plates form the external part (2A) of said first pipe (2), the fins, and a fastening means between several of said tubular components (1).

28. Heat exchanger and the relevant construction method, according to the principles of this description and of the attached drawings.