WO2008099434A1

WO2008099434A1 - Tubeless heat exchanger and method for the manufacture thereof

Info

Publication number: WO2008099434A1
Application number: PCT/IT2007/000104
Authority: WO
Inventors: Raffaele Giannoni
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-02-15
Filing date: 2007-02-15
Publication date: 2008-08-21
Anticipated expiration: 2009-08-15

Abstract

A gas-liquid heat exchanger (1) comprises: a plurality of pack-tightened fins (2) each comprising at least one through hole (31) and at least one collar (32) coaxially . surrounding said through hole (31) and substantially extended perpendicular to a face of the fins (2) , and a plurality of first ducts (3) having an inner -wall and extended in a direction substantially perpendicular to the fins (2) , at least one of said first ducts (3) being defined by adjacent portions of the collars (32) of the fins (2) . Said at least one collar (32) comprises a first portion (32a) with larger inner cross-sectional area and a second portion (32b) with smaller inner cross-sectional area and having a respective inner wall, wherein said first portion (32a) defines a receiving seat for receiving the second portion (32b) of the collar (32) of an adjacent fin (2) such that the inner wall of said second portion (32b) extends longitudinally substantially flush with the inner wall of said at least one of said ducts (3) . A fin for the heat exchanger (1) is also described (2) along with a method for manufacturing the heat exchanger (1) -

Description

Tύbeless heat exchanger and method for the manufacture thereof

DESCRIPTION

Background of the invention

The present invention refers, in its main aspect, to a heat exchanger. In particular, the invention refers to a gas- liquid heat exchanger, which has a preferred although not exclusive use as gas-liquid heat exchanger in water-heating apparatuses intended both for domestic use and for utilization in residential areas.

In further aspects thereof, the invention also refers to a fin for the aforementioned heat exchanger and to a water- heating apparatus incorporating the same, as well as a to method of manufacturing such heat exchanger.

Prior Art ^'

In the field of gas-liquid heat exchangers in general, and in particular in the field of gas-liquid heat exchangers for water-heating apparatuses, there is a constant need of making thermally compact heat exchangers, i.e. characterised by a high ratio between the heat exchange surface and the total size of the heat exchanger. Such type of heat exchangers permits reducing the size, thermal power being equal, or increasing the thermal power, size being equal .

A known solution to meet this need, successfully adopted above all in the refrigeration apparatuses and automotive fields, is represented by the so-called ^λλtubeless" heat exchangers. In these heat exchangers, the ducts in which the heat carrier liquid flows are formed by the coupling in series of a plurality of conical or frustoconical collars extended from respective faces of a plurality of fins adapted to be pack-tightened together. The absence of tubes permits, thermal power being equal, making more compact heat exchangers and, at the same time, reducing the quantity of metal material needed, and hence the costs.

A heat exchanger of this type is for example disclosed in patent US 2,028,458, which describes a refrigeration apparatus comprising a condenser having a thermal exchange portion formed by a plurality of horizontal tubes and vertical fins. The tubes are formed by projecting tapered tubular portions, formed integrally with each fin, adapted to be inserted one into the other when the fins are pack- tightened.

Another heat exchanger of this type is disclosed m patent application FR 2 728 668, referred to a heat exchanger for use in particular as radiator in automobiles.

The attempt to use tubeless heat exchangers of known type in water-heating apparatuses for producing sanitary hot water and/or for room heating, rather desirable due to the abovementioned advantages, has nevertheless not been fruitful up to now. This because the water normally used as heat carrier liquid in these applications contains scale and suspended particles, in the following globally indicated with the term ^Λλimpurities", and often also suitable additives, which can give rise to deposits in the heat exchanger ducts, thus making the heat exchange worse. In practice, it has been found that deposit of impurities or additives in tubeless heat exchangers of known type when employed in this field was so high that it was necessary to clean or even substitute the heat exchanger after short operating periods.

Summary of the invention

The technical problem underlying the present invention is that of providing a gas-liquid heat exchanger, in particular a gas-liquid heat exchanger for a water-heating apparatus, of the tubeless type having both a high heat exchange efficiency and a sufficiently long operating life, even when the heat carrier liquid contains impurities and/or additives, as in the case of sanitary water or water for room heating.

The Applicant has perceived that the aforementioned problem can be solved by appropriately modifying the geometry . of the collars which form the ducts in the tubeless heat exchangers, so to ensure that the Inner walls of the ducts in which the heat carrier liquid flows are substantially free of unevenness or cavities which can promote precipitation and accumulation of impurities and/or additives present in the heat carrier liquid.

The Applicant deems that a possible explanation for the high accumulation of impurities and/or additives in the tubeless heat exchangers of known type used as gas-water heat exchangers in water-heating apparatuses is to be found in the irregularity of the inner walls of the ducts . in which the water flows. In fact, the overlapping of collars which results when they are associated to form the ducts causes the inner walls of the ducts to have projections. The niches identified between consecutive projections are trigger points for the precipitation and accumulation of impurities and/or additives present in the water. Once the precipitation phenomena is triggered, it goes on ever more rapidly, leading to a deposit on the inner walls of the ducts such to considerably deteriorate the heat exchange in a much shorter time than in the heat exchangers of traditional type.

This drawback, of course, is not encountered, or is encountered in a much smaller measure, when using pure fluids or, in any case, substantially without impurities and/or additives. This explains the possibility to use, without drawbacks, tubeless heat exchangers of known type for example in refrigeration apparatuses or as radiators for automobiles, as for example^' provided for by the abovementioned prior art documents.

According to a first aspect thereof, the invention therefore refers to a gas-liquid heat exchanger comprising: a plurality of pack-tightened fins, each comprising at least one through hole and at least one collar coaxially surrounding the through hole and extended substantially perpendicular to a face of said fins;

- a plurality of first ducts having an inner wall and extended in a direction substantially perpendicular to the fins, at least one of said first ducts being defined by adjacent portions of the collars of the pack-tightened fins, characterised in that said at least one collar comprises a first portion with larger inner cross-sectional area and a second portion with smaller inner cross-sectional area and having a respective inner wall, wherein said first portion defines a receiving seat for receiving the second portion of the collar of an adjacent fin, so that the inner wall of said second portion extends longitudinally substantially flush with the inner wall of said at least one of said first ducts.

Advantageously, the modified geometry of the collars permits having ducts whose inner walls are substantially smooth and free of unevenness. Since trigger points for the precipitation and accumulation of impurities in the ducts are substantially not present, the heat exchanger of the invention, still of tubeless type, can be employed without drawbacks even as gas-water heat exchanger in water-heating apparatuses wherein a not particularly "clean" heat carrier liquid, often rich with impurities and containing special additives, is used. The invention therefore permits obtaining the advantages of the tubeless . heat exchangers set forth above along with the performance and operating life of the tube heat exchangers commonly employed in water-heating apparatuses. A further advantageous consequence of having ducts with substantially smooth inner surfaces is that, unlike as in the prior art tubeless .heat exchangers, the fluid-dynamic conditions of the heat carrier liquid which flows in the ducts are substantially independent^' from the flow direction. In the heat exchanger of the invention, it is therefore possible to freely choose both a configuration with ^' flows in parallel and a configuration with flows in series, since even in the latter case heat exchange differences due to different fluid-dynamic conditions in ducts flown in opposite directions do not arise. Therefore, the heat exchanger of the invention offers great flexibility as far as its thermal fluid-dynamic design is concerned.

Further preferred features of the heat exchanger according to the invention are defined in the attached claims 2-12, whose content is herein fully incorporated by reference.

In a second aspect thereof, the invention refers to a heat exchange fin for a gas-liquid heat exchanger comprising a ■ plate-shaped body in which at least one through hole is formed, coaxially surrounded by a collar extended substantially perpendicular to a face of said plate-shaped body, characterised in that said at least one collar comprises a first portion with larger inner cross-sectional area and a second portion with smaller inner cross- sectional area.

Such features permit, once a plurality of fins is pack- tightened, forming in a tubeless heat exchanger ducts having inner ^' walls ' substantially smooth and without unevenness, thus obtaining the advantages set forth above with reference to the heat exchanger of the invention.

Further preferred features of the fin according to the invention are defined in the attached claims 14-20, whose content is herein fully incorporated by reference. In a third aspect thereof, the invention refers to a water- heating apparatus comprising a heat exchanger having all of the abovementioned features.

Of course, the advantages of the heat exchanger of the invention are transferred to the water-heating apparatus incorporating such heat exchanger. This water-heating apparatus, in addition to having a smaller size and cost, does not require specific or more frequent maintenance interventions than those required for water-heating apparatuses currently on the market.

In a fourth aspect thereof, the invention refers to a method of manufacturing a gas-liquid heat exchanger comprising the steps of: a) providing a plurality of substantially plate-shaped fins; b) forming at least one through hole in each of said fins; c) forming a collar coaxially to said at least one through hole, said collar being extended substantially perpendicular from said fin and comprising a first portion with larger inner cross-sectional area and a second portion with smaller inner cross-sectional area and having a respective inner wall, wherein said first portion defines a receiving seat for receiving the second portion of the collar of an adjacent fin; d) pack-tightening said fins such that the second portion of the collar of each fin is received in the receiving seat defined by the first portion of the collar of an adjacent fin and the inner wall of said second portion extends longitudinally substantially flush with the inner wall of said at least one duct.

Further preferred features of the method according to the invention are defined in claims 23-28_., whose content is herein fully incorporated by reference. Brief description of the figures

Further characteristics and advantages of the present invention will be clearer from the following description of some preferred embodiments thereof, made hereafter, for indicating and not limiting purposes, with reference to the attached drawings . In such drawings :

- Figure 1 is a schematic perspective view, partially exploded, of a heat exchanger according to a preferred embodiment of the invention; - Figure 2 is a schematic view in longitudinal section and in enlarged scale of a portion of a first duct of the heat exchanger of figure 1;

Figure 3 is a schematic perspective view of a fin of the heat exchanger of figure 1.

Detailed description of the preferred embodiments of the invention

In figure 1, with 1 is globally indicated a gas-liquid heat exchanger of the tubeless type according to the invention.

The heat exchanger 1 has a preferred but not exclusive use in a water-heating apparatus per se known, not illustrated here, for producing sanitary hot water and/or room heating.

The heat exchanger 1 comprises a plurality of fins 2 (only some of which are represented in fig. 1) , pack-tightened together, essentially consisting of suitably conformed plate-shaped bodies, and a plurality -of first ducts 3 extended perpendicularly to the fins 2, preferably substantially rectilinear and parallel to each other, for the passage of water in the heat exchanger 1, as will be' better described below.

In each fin 2, a plurality of through holes 31 is formed. Coaxially to each through hole 31, a respective collar 32 is formed having a longitudinal axis X-X and extending substantially perpendicular to a face of the fins 2 (figs. 1 and 3 ) .

The ducts 3 are formed by the mutual coupling of the collars 32 of adjacent fins 2, which is obtained, as in the tubeless heat .exchangers of known type, when the fins 2 are pack-tightened together (figs. 1 e 2).-

The ducts 3 have an inner wall 30 substantially smooth and without unevenness. According to the invention, this is achieved by the fact that the collars 32 comprise a first portion 32a with larger inner cross-sectional area and a second portion 32b with smaller cross-sectional area and with an inner wall 33b, and that the first portion 32a of each collar 32 defines a receiving seat 34 for receiving, inside the respective collar 32, the second' portion 32b of a collar 32 of an adjacent fin 2 such that the inner wall 33b ^' of this second portion 32b extends longitudinally substantially flush with the inner wall 30 of the ducts 3. In other words, the coupling between the collars 32 is made in a such a way that the inner walls 33b of the second portions 32b lie all substantially flush with each other, thus the inner walls 30 of the ducts 3 do not have projections or grooves (fig. 2) .

It should be noted that possible cavities 36, even of minimal size, which can arise at the inner wall 30 due to a not perfect contact in longitudinal direction between one collar 32 and the adjacent one upon pack-tightening the fins 2, are in any case filled with material (fig. 2) when the ducts 3 are subjected to braze-welding, as will be described below with reference to the method of the invention. The inner walls 30 of the ducts 3 are therefore substantially smooth in every respect.

In each collar 32, the first portion 32a has an inner wall 33a and the second portion 32b has an external wall 37b. Preferably, the first portion 32a (i.e. the receiving seat 34) of each collar 32 receives the second portion 32b of the collar 32 of an adjacent fin 2 so that the external wall 37b of said second portion 32b is in contact substantially along its entire length with the inner wall 33a of said first portion 32a (i.e. of said receiving seat 34) . Due to this feature, an engagement between the collars 32 substantially without radial clearances is ensured, and at the same time the structural strength of the ducts 3 formed by the assembly of the collars 32 is improved.

In the preferred embodiment illustrated here, the first portion 32a has an inner wall 33a which is slant with respect to the longitudinal axis X-X of the respective collar 32. This advantageously permits obtaining the aforementioned desired contact between inner wall 33a- of the first portion 32a and outer wall 37b of the second portion 32b even when the walls of the collars 32 are longitudinally tapered, for example due to a drawing manufacturing process, which naturally causes a stretching and thinning of the material.

The slant angle α of the inner wall 32a is determined based on the type and thickness of the material used, as well as on the length of the collar 32, and preferably is between about 0° and about 4°. The lower value of about 0° applies when it is possible to obtain collars 32 with wall thickness substantially constant longitudinally.

Preferably, the inner wall 33b of the second portion 32b is substantially parallel with respect to the longitudinal axis X-X of the respective collar 32. This permits advantageously obtaining ducts 3 with substantially constant inner section.

The first and second portion 32a, 32b of the collars 32 of each fin 2 are preferably joined together by means of a substantially^' step-like junction 35.

Preferably, the junction 35 defines an abutment stop for an adjacent fin 2. Due to this abutment stop, it is advantageously possible to control the pitch between the fins 2 in a simple, precise, as well as completely mechanical manner. The pitch is in fact automatically determined upon pack-tightening the fins 2, without the need to carry out additional controls for this purpose.

Preferably, the collars 32 are formed integrally with the fins 2, more preferably they are formed in the fins 2 by means of drawing. This permits making the fins 2 and collars 32 with the above described features in a relatively simple and fast way, easily automatable for mass production.

In the exemplar embodiment of the invention illustrated here, the- inner cross sections of the first and second portion 32a, 32b of the collars 32 are substantially elliptical. However, in order to satisfy specific technical needs, in alternative embodiments a man skilled in the art can choose cross sections of different shape, in particular ovoid or "drop-like", circular or rectangular cross sections .

Preferably, the heat exchanger 1 also comprises a plurality of second ducts 4 arranged inside the first ducts 3, so to advantageously obtain a heat exchanger structure of the bithermal type, i.e. suitable for simultaneously heating primary water or water for room heating, and sanitary water intended for domestic use. In particular, an hollow space 5 is defined between each duct 3 and the corresponding inner duct 4, inside which primary water is made to flow, while within the ducts 4 sanitary water is made to flow (fig. 1) .

The ducts 4 are preferably substantially coaxial with the ducts 3. In the embodiment illustrated here, the ducts 4 have a cross section with substantially lanceolate form, but alternative shapes can also be employed, in particular rhomboid or cruciform. Due to these features, the cross section of the hollow space 5 is longitudinally constant and has a form which permits optimising the heat exchange towards the sanitary water which flows in the inner ducts 4 .

It should be noted that the possibility to obtain a configuration of bithermal type which is efficient ^• both from the structural and heat exchange standpoint, even in the case of a tubeless heat exchanger, is linked to the possibility of having ducts 3 whose inner walls 30 are substantially smooth, contrary to what can be obtained in the known tubeless heat exchangers described above. Due to this feature, the contact zones between the outer walls of the ducts 4 and the inner walls 30 of the ducts 3 - zones which are defined by the shape of the respective cross sections - do not have unevenness in the longitudinal direction. This permits obtaining an optimal connection, for example by means of braze-welding, between two ducts and also to optimise the conductive heat exchange between these at the contact zones.

The heat exchanger 1 further comprises manifolds 6 and 7 of conventional type, which are associated to the ducts 3 at axially opposed sides and are in fluid communication with the hollow space 5 intended for the flow of primary water (fig. 1) .

A first inlet (or outlet) fitting 61 for the primary water and a second inlet (or outlet) fitting 62 for the sanitary water are associated with the manifold 6. Similarly, a first outlet (or inlet) fitting 71 for the primary water and a second outlet (or inlet) fitting 72 for the sanitary water are associated with the manifold 7.

With the manifolds 6 and 7 is further associated a plurality of U-shaped bends 8, adapted to connect the ducts 4 with each other. The ducts 4 thus^' form a single continuous duct, substantially coil-shaped and extended between the inlet (or outlet) fitting 62 and the outlet (or inlet) fitting 72 of the sanitary water.

Preferably, the fins 2 are provided with turbulence generating- elements, adapted to generate turbulence in the gas flowing among them, outside the ducts 3. In the preferred embodiment illustrated here (figs. 1 and 3), such turbulence generating elements comprise bridges 9 projecting from a face of the fins 2 and indentations 10 which, in the illustrated embodiment, are holed, so as to make possible gas flows also transverse to the fins 2. In alternative embodiments, the indentations 10 can also be blind.

Preferably, all the components of the heat exchanger 1 described above are made of a suitable metal material having good thermal conductivity, for example copper.

With reference in particular to figs. 1 and 2, a method for manufacturing the heat exchanger 1 will now be described.

In a first step of the method, a plurality of substantially plate-shaped fins 2 are provided.

In a subsequent step, through holes 31 are formed in each fin 2 in a number corresponding to the number of ducts 3 required in the heat exchanger 1.

In a subsequent step, coaxially to each through hole 31 and preferably on a same side of each fin 2, collars 32 are formed having the features set forth above while describing the heat exchanger 1.

Preferably, the collars 32 are formed integrally with the fins 2 and the aforementioned steps of forming the through holes 31 and of forming the collars 32 are carried out simultaneously by means of drawing of the plate-shaped bodies defining the fins 2. This permits conferring the desired shape to the fins 2 and the collars 32 in a relatively simple and economical manner. In particular, the working of the fins 2 by means of drawing can be easily automated for mass production..

In a subsequent step, the fins 2 thus shaped are pack- tightened, so that the second portion 32b of the collars 32 of each fin is received in the receiving seat 34 defined by the first portion 32a of the collars 32 of an adjacent fin 2 and that the inner wall 33b of each second portion 32b extends longitudinally substantially flush with the inner wall 30 of the respective duct 3.

Preferably, in this step each fin 2 comes into abutment with the substantially step-like junctions 35 of the collars 32 of the adjacent fin 2. In this manner, the pitch between the fins 2 is advantageously automatically determined in an completely mechanical manner at the time of the pack-tightening, without specific steps being required for determining the pitch by means of special mechanical or electronic devices, which would place a substantial limit on the production speed.

Preferably, the method of the invention further provides for a final step of braze-welding of the previously formed ducts 3 to ensure the stability of the connection between the collars 32 and the seal of the ducts 3.

Obviously, those skilled in the art may introduce variants and modifications in the heat exchanger and method described above, in order to satisfy specific and contingent requirements, which variants and modifications fall anyhow within the scope of protection as is defined by the appended claims.

Claims

1. Gas-liquid heat exchanger (1) comprising: a plurality of pack-tightened fins (2) each comprising at least one through hole (31) and at least one collar (32) coaxially surrounding said through hole (31) and extended substantially perpendicular to a face of said fins (2) ;

- a plurality of first ducts (3) having an inner wall

(30) and extended • in a direction substantially perpendicular to said fins (2), at least one of said first ducts (3) being defined by adjacent portions of the collars (32) of said pack-tightened fins (2), characterised in that said at least one collar (32) comprises a first portion (32a) with larger inner cross- sectional area and a second portion (32b) with smaller inner cross-sectional area and having a respective inner wall (33b), wherein said first portion (32a) defines a receiving . seat (34) for receiving the second portion (32b) of the collar (32) of an adjacent fin (2) such that the inner wall (33b) of said second portion (32b) extends longitudinally substantially flush with the inner wall (30) of said at least one of said first ducts (3) .

2. Heat exchanger (1) according to claim 1, wherein the first portion (32a) of said at least one collar (32) has an inner wall (33a) and the second portion (32b) of said at least one collar (32) has an outer wall (37b) , and wherein the receiving seat (34) of said at least one collar (32) receives the second portion (32b) of- the collar (32) of an adjacent fin (2) such that the outer wall (37b) of the second portion (32b) of said collar (32) of the adjacent fin (2)' is in contact substantially along its entire length with the inner wall (33a) of the first portion (32a) of said at least one collar (32) .

3. Heat exchanger (1) according _.to claim 2, wherein the inner wall (33a) of the first portion (32a) of said at least one collar (32) is slant with respect to a longitudinal axis (X-X) of the collar (32) .

4. Heat exchanger (1) according to claim 3, wherein the slant angle (α) of the inner wall (33a) of the first portion (32a) of said at least one collar (32) is in the range between about 0° and about 4°.

5. Heat exchanger (1) according to any one of the preceding claims, wherein the inner wall (33b) of the second portion (32b) of said at least one collar (32) is substantially parallel with respect to a longitudinal axis (X-X) of the collar (32) .

6. Heat exchanger (1) according to any one of the preceding claims, wherein the first and second portion

(32a, 32b) of said at least one collar (32) are joined together by means of a substantially step-like junction (35) .

7. Heat exchanger (1) according to claim 6, wherein said substantially step-like junction (35) defines an abutment stop for an adjacent fin (2) .

8. Heat exchanger (1) according to any one of the preceding claims, wherein said at least one collar (32) is formed integrally with the respective fin (2) .

9. Heat exchanger (1) according to any one of the preceding claims, wherein the shape of the inner cross sections of the first and second portion (32a, 32b) of said at least one collar (32) is selected from: circular, elliptical, ovoid, rectangular.

10. Heat exchanger (1) according to any one of the preceding claims, comprising a plurality of second ducts

(4) arranged inside said first ducts (3) .

11. Heat exchanger (1) according to claim 10, wherein the shape of the cross section of said second ducts (4) is selected from: lanceolate, cruciform, rhomboid.

12. Heat exchanger (1) according to any one of the preceding claims, wherein said fins (2) are provided with turbulence generating elements (9,10).

13. Heat exchange fin (2) for a gas-liquid heat exchanger (1) comprising a plate-shaped body in which at least one through hole (31) is formed, surrounded coaxially by a collar (32) extended substantially perpendicular to a face of said plate-shaped body, characterised in that said at least one collar (32) comprises a first portion (32a) with larger inner cross-sectional area and a second portion (32b) with smaller inner cross-sectional area.

14. Fin (2) according to claim 13, wherein the first portion (32a) of said at least one collar (32) has an inner wall (33a) slant with respect to a longitudinal axis (X-X) of the collar (32) .

15. Fin (2) according to claim 14, wherein the slant angle (α) of the first portion (32a) of said at least one collar (32) is in the range between about 0° and about 4' O

16. Fin (2) according to any one of the claims 13-15, wherein the second portion (32b) of said at least one collar (32) has an inner wall (33b) substantially parallel with respect to a longitudinal axis (X-X) of the collar (32) .

17. Fin (2) according to any one of the claims 13-16, wherein the first and second portion (32a, 32b) of said at least one collar (32) are joined together by means of a substantially step-like junction (35) .

18. Fin (2) according to any one of the claims 13-17, wherein said at least one collar (32) is formed integrally with the fin (2) .

19. Fin. (2) according to any one of the claims 13-18, wherein the shape of the inner cross sections of the first and second portion (32a, 32b) of said at least one collar (32) is selected from: circular, elliptical, ovoid, rectangular.

20. Fin (2) according to any one of the claims 13-19, wherein said fins (2) are provided with turbulence generating elements (9,10).

21. Water-heating apparatus comprising a heat exchanger (1) according to any one of the claims 1-12.

22. Method of manufacturing a gas-liquid heat exchanger (1) comprising the steps of: a) providing a plurality of substantially plate-shaped fins (2); b) forming at least one through hole (31) in each of said fins (2) ; c) forming a collar (32) coaxially to said at least one through hole (31) , said collar (32) being extended substantially perpendicular to said fin (2) and comprising a first portion (32a) with larger inner cross-sectional area and a second portion . (32b) with smaller inner cross- sectional area and having a respective inner wall (33b) , wherein said first portion (32a) defines a receiving seat (34) for receiving the second portion (32b) of the collar (32) of an adjacent fin (2); d) pack-tightening said fins (2) such that the second portion (32b) of the collar (32) of each fin (2) is received in the receiving seat (34) defined by the first portion (32a) of the collar (32) of an adjacent fin (2) and that the inner wall (33b) of said second portion (32b) extends longitudinally substantially flush with the inner wall (30) of said at least one duct (3) .

23. Method according to claim 22, wherein said first portion (32a) of the collar (32) has an inner wall which is slant with respect to a longitudinal axis (X-X) of the collar (32) .

24. Method according to claim 22 or 23, wherein said inner wall (33b) of the second portion (32b) of the collar (32) is substantially parallel to a longitudinal axis (X-X) of the collar (32) .

25. Method according to any one of the claims 22-24, wherein said steps^" of forming said at least one through hole (31) and said collar (32) are carried out simultaneously by means of drawing said fins (2) .

26. Method according to any one of the claims 22-25, wherein said step of forming said collar (32) comprises forming a substantially step-like junction (35) between said first and second portion (32a, 32b) of the collar (32).

27. Method according to claim 26, wherein, in said step of pack-tightening the fins (2) , each fin (2) comes into abutment with said substantially step-like junction (35) of the collar (32) of an adjacent fin (2) .

28. Method according to any one of the claims 22-27, further comprising the step of stably joining the collar (32) of each fin (2) with the collar of an adjacent fin (2) by means of braze-welding.