Air heat exchanger of improved type, method and apparatus for the manufacture thereof

Abstract

The invention relates to an air heat exchanger (1) comprising a pack of juxtaposed metal fins (2) and supported mechanically by metal covering plates (3), and a circuit of metal heat exchange tubes (4) that perpendicularly cross said fins (2) and the aforesaid covering plates (3) and that are placed in hermetic fluid communication with each other and with an inlet distributor (5) and an outlet manifold (6) for the passage of a work fluid. The fins (2) are provided with connection holes (18) for connection with the heat exchange tubes (4) and the connection between said heat exchange tubes (4) and the aforesaid fins (2) is obtained by means of pneumatic expansion of the heat exchange tubes (4) in the connection holes (18) of the fins (2). The invention also concerns a method and an apparatus for obtaining pneumatic mandrel expansion of the heat exchange tubes (4) of an air heat exchanger (1) completely pre-assembled in its parts.

Description

[0001] AIR HEAT EXCHANGER OF IMPROVED TYPE, METHOD AND APPARATUS FOR THE MANUFACTURE THEREOF

[0002] DESCRIPTION

[0003] BACKGROUND OF THE INVENTION

[0004] The present invention relates to an air heat exchanger of improved type, and also to a method and an apparatus for the manufacture thereof.

[0005] A known air heat exchanger of generic type comprises a pack of juxtaposed metal fins mechanically supported by metal covering plates, and a circuit of metal heat exchange tubes that perpendicularly cross the fins and the covering plates and are placed in hermetic fluid communication with each other and with an inlet distributor and an outlet manifold for the passage of a work fluid. In the areas of the fins in which the heat exchange tubes are inserted, tubular collars are formed, integral with the fins, that serve as interconnection elements between the fins and the heat exchange tubes.

[0006] Since transmission of heat between the heat exchange tubes and the collars of the fins occurs through conduction, it is desirable for there to be a good contact between the inner surface of the collars of the fins and the outer surface of the portions of heat exchange tube inserted into the collars of the fins, in order to guarantee an efficient heat exchange.

[0007] For this reason, in the traditional technique the heat exchange tubes are subjected to a mechanical operation that allows the inner diameter thereof to be increased, so as to make them adhere to the collars of the fins. This mechanical operation is performed by means of the so-called “mandrel expansion” technique, which consists of expanding the heat exchange tubes by means of a specific mandrel, which is introduced into and made to slide in each heat exchange tube, in order to enlarge said tube so that it is in close contact, through interference, with the inner surface of the collars of the fins. Although this is an established and widely used technique, the traditional mechanical mandrel expansion described above is not free of drawbacks that could undermine the efficacy of the heat exchanger. It is, in fact, known that, depending on the operating characteristics of the heat exchanger, it can use heat exchange tubes having a smooth or corrugated inner surface. A corrugated surface allows the heat exchange coefficient in evaporation and in condensation to be increased. During the mechanical mandrel expansion operation, the corrugations on the inner surface of the heat exchange tubes can undergo considerable deformations that cause a deterioration of the heat exchange coefficient.

[0008] Furthermore, with the traditional mechanical mandrel expansion technique, it is difficult to guarantee a close contact between the outer surface of the heat exchange tubes and the inner surface of the collars of the fins. In practice, in the interconnection area between the heat exchange tubes and the collars of the fins, a relatively large annular hollow space can remain in which air is present that reduces the heat exchange between the heat exchange tubes and the collars of the fins of the heat exchanger.

[0009] Furthermore, mechanical mandrel expansion is an operation that requires a great deal of time, since the heat exchange tubes must be expanded one at a time before being able to assemble the pack of finned tubes with the other components of the heat exchanger.

[0010] Publication US 2022 / 0032358 A1 describes methods for performing the expansion of a tube, which use the pulses of a fluid under pressure charged inside the tube. Publications US 5 765 284 A, US 2012 / 036718 A1 , US 2005 / 061494 A1 , EP 0 237 761 B1 and WO 2019 / 058514 A1 disclose methods for the manufacture of a heat exchanger with a single charge and discharge cycle of a fluid under pressure inside the tubes of the exchanger.

[0011] SUMMARY OF THE INVENTION

[0012] The main aim of the present invention is to provide an improved air heat exchanger that has a greater heat exchange capacity and that can be manufactured more easily and in a shorter space of time compared to the traditional air heat exchangers with mechanically mandrel expanded heat exchange tubes.

[0013] Within this aim, an object of the invention is to provide an air heat exchanger on which the heat exchange tubes are made to adhere to the inner surface of the collars of the fins, without causing damage to the corrugations on the inner surface of the heat exchange tubes.

[0014] Another object of the invention is to provide an air heat exchanger that has a larger area of contact between the inner surface of the collars of the fins and the outer surface of the portions of heat exchange tube housed in the collars.

[0015] An object of the invention is also to provide a method and an apparatus for the manufacture of an air heat exchanger having an improved structure and, in particular, a greater heat exchange capacity.

[0016] In particular, an object of the invention is to provide a process and an apparatus that allow the mandrel expansion of the heat exchange tubes to be performed on the heat exchanger completely pre-assembled in its parts.

[0017] A further object of the invention is to provide a process and an apparatus that also allow the hermetic seal of the exchanger to be checked before and after mandrel expansion of the heat exchange tubes.

[0018] This aim, as well as these other objects which will appear more clearly below, are achieved with the air heat exchanger and the method of claims 1 and 3, respectively. Preferred embodiments of the invention are described in the remaining claims.

[0019] Compared to air heat exchangers having heat exchange tubes expanded using the traditional mechanical mandrel expansion technique, the air heat exchanger according to the invention has the advantage that the shape of the corrugations obtained inside the heat exchange tubes to improve the efficiency of heat exchange is not substantially altered as a result of expansion of said tubes inside the connection holes of the fins. In practice, the Applicant has found that deformation of the inner corrugations of the heat exchange tubes is limited and the height of the crests of the corrugations undergoes a crushing of less than 5%.

[0020] Another advantage of the air heat exchanger according to the invention is given by the fact that the expanded heat exchange tubes have a larger area of contact with the collars of the fins. Through the experiments performed, the Applicant was able to confirm that the area of contact between the inner surface of the collars of the fins and the outer surface of the portions of heat exchange tube housed in the collars is not lower than 60% of the available one.

[0021] Compared to the traditional systems of mechanical mandrel expansion of heat exchanger tubes, the method of the invention offers the advantage of allowing mandrel expansion of the heat exchange tubes to be performed on the heat exchanger completely pre-assembled in its parts.

[0022] In particular, according to the invention, mandrel expansion can be performed simultaneously for all the heat exchange tubes, with a considerable time saving compared to traditional mechanical mandrel expansion.

[0023] Yet another advantage of the pneumatic mandrel expansion method consists of the fact that the same mandrel expansion apparatus can also be used to perform testing of the seal of the circuit of heat exchange tubes, before and after mandrel expansion.

[0024] Lastly, a further advantage of the pneumatic mandrel expansion method according to the invention is given by the fact that it allows the thickness of the hollow space between the non-expanded heat exchange tubes and the collars of the fins to be increased, thus facilitating the insertion of the heat exchange tubes into the collars of the fins.

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Further objects, advantages and features of the invention will become more apparent from the following description of a preferred, but not exclusive, embodiment thereof, illustrated by way of a non-limiting example with the support of some examples of applications and the appended drawings, in which:

[0027] - Fig. 1 is a perspective view of an air heat exchanger; - Figures 2 and 2A are views of the detail of a first embodiment of a heat exchange tube of the heat exchanger of Fig. 1 ;

[0028] - Figures 2B and 2C are views of the detail of a second embodiment of a heat exchange tube of the heat exchanger of Fig. 1 ;

[0029] - Fig. 3 shows a stress and strain diagram of the tensile strength test of a typical material used to manufacture the heat exchange tubes of an air heat exchanger;

[0030] - Fig. 4 schematically shows the trend, as a function of time, of the pressure of the gaseous fluid introduced into the circuit of heat exchange tubes during application of the method of the invention;

[0031] - Fig. 5 shows the general diagram of an apparatus for performing the method of the invention;

[0032] - Fig. 6 schematically shows the trend, as a function of time, of the pressure of the gaseous fluid introduced into the circuit of heat exchange tubes in a first example of application of the method of the invention;

[0033] - Fig. 7 schematically shows the trend, as a function of time, of the pressure of the gaseous fluid introduced into the circuit of heat exchange tubes in a second example of application of the method of the invention;

[0034] - Fig. 8 is a cross-sectional view of the interconnection area between a heat exchange tube and a fin of an air heat exchanger with heat exchange tubes expanded by means of the traditional mandrel expansion technique,

[0035] - Fig. 9 is a cross-sectional view of the interconnection area between a heat exchange tube and a fin of an air heat exchanger with heat exchange tubes expanded by means of the mandrel expansion technique according to the present invention; and

[0036] - Fig. 10 is a cross-sectional view of a heat exchange tube provided with internal corrugations, shown before and after the traditional mechanical mandrel expansion operation. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] With reference to Fig. 1 of the drawings, a known air heat exchanger of the generic type is shown, indicated as a whole with the number 1 . The heat exchanger comprises a pack of juxtaposed metal fins 2 mechanically supported by metal covering plates 3, and a circuit of metal heat exchange tubes 4 that perpendicularly cross the fins 2 and the plates 3. The tubes 4 are placed in hermetic fluid communication with each other at respective curved couplings 41 and with an inlet distributor 5 and an outlet manifold 6 for the passage of a work fluid, provided with respective valves.

[0038] As can be observed in more detail in Figures 2 and 2A, the fins 2 are interconnected with heat exchange tubes 4, arranged inside respective holes 18 of said fins 2. Preferably, the fins 2 are arranged folded outside the tubes 4 so as to form, on their outer surface, respective reinforcing collars 7 on the section of engagement of the tubes 4 with the fins 2 (Figures 2B and 2C). The arrows F indicate the direction of the airflow that crosses the pack of fins 2 of the heat exchanger 1 .

[0039] It is important to note that, with the traditional mandrel expansion technique, it is difficult to guarantee that the outer surface of the heat exchange tubes 4 matches precisely the shape of the collar 7 of the fins 2. In practice, as shown in Fig. 8 of the drawings, in the interconnection area between the outer surface of the heat exchange tubes 4 and the inner surface of the collars 7 of the fins 2, a relatively large annular hollow space 19 can remain, in which a specific volume of air is trapped that reduces the heat exchange between the heat exchange tubes 4 and the collar 7 of the fins 2 of the heat exchanger.

[0040] In order to obtain a good contact between the heat exchange tubes 4 and the tubular collars 7 of the fins 2 in the interconnection area, the invention proposes subjecting the heat exchange tubes 4 of the heat exchanger 1 in the pre-assembled condition of Fig. 1 to a pneumatic mandrel expansion procedure, which produces a widening of the heat exchange tubes 4 in order to make them adhere as much as possible to the inner surface of the tubular collars 7, solving the problem of the formation of hollow spaces on the coupling surfaces between these parts, as shown in Fig. 9 of the drawings. In particular, the method of the invention provides for introducing a gaseous fluid into the circuit of heat exchange tubes 4 through the inlet distributor 5 and the outlet manifold 6 of the heat exchanger 1 and cyclically increasing and decreasing the pressure of the gaseous fluid within a range of yield values of the metal material of the heat exchange tubes 4, for example copper, steel, aluminium and similar, in order to obtain a residual expansion deformation of the heat exchange tubes 4 that produces a contact through interference between them and the inner surface of the tubular collars 7 of the fins 2 of the heat exchanger 1 . Lastly, the gaseous fluid is evacuated from the circuit of heat exchange tubes 4 of the heat exchanger 1. The gaseous fluid is preferably a gas, for example air, helium, nitrogen, argon, or other gases.

[0041] In particular, the method according to the invention provides for preassembly of the air heat exchanger 1 , in which the pack of metal fins 2 is mechanically supported by metal covering plates 3 and the metal heat exchange tubes 4 perpendicularly cross the fins 2 and the covering plates 3. In turn, the tubes 4 are placed in hermetic fluid communication with each other at respective curved couplings 41 and with an inlet distributor 5 and with an outlet manifold 6 for the passage of a work fluid.

[0042] After attachment through brazing of the tubes 4 onto the manifolds 5,6 and onto the curved couplings 41 , the method of the invention provides for simultaneous mandrel expansion of the tubes of the heat exchanger, in order to obtain their attachment inside the holes 18 of the fins 2.

[0043] In particular, according to the invention, mandrel expansion is performed through expansion of the heat exchange tubes 4, through compression of a fluid charged inside them. The method is performed by applying a succession of at least two charge and discharge cycles of the fluid present inside the tubes 4, in which the aforesaid charge and discharge cycles are performed by gradually varying the pressure of the fluid.

[0044] The charge cycles are performed by gradually increasing the pressure of the gas in a time interval from 10 to 500 seconds, from ambient pressure up to the expansion pressure that is a function of the exchanger in question; in example 1 , said pressure is 150 bar. Similarly, the discharge cycles are performed by gradually decreasing the pressure of the gas in a time interval from 10 to 500 seconds, from a maximum pressure equal to the expansion pressure that is a function of the exchanger in question (in example 1 , said pressure is 150 bar) down to a minimum pressure that is a function of the exchanger in question (in example 1 , said pressure is 90 bar).

[0045] By means of the method described above, it was surprisingly found that the annular hollow space 19 between the inner surface of the collars 7 of the fins 2 and the outer surface of the portions of heat exchange tube 4 housed in the collars 7 is significantly reduced and the area of contact between these surfaces is not less than 60% of the available one.

[0046] In addition to guaranteeing a better contact between the heat exchange tubes 4 and the tubular collars 7 of the fins, pneumatic expansion of the heat exchange tubes 4 also has the advantage of not substantially altering the shape of any corrugations 20 that are obtained on the inner surface of the heat exchange tubes in order to increase the efficiency of thermal exchange. In fact, while the traditional mandrel expansion technique causes a considerable deformation of the corrugations on the inner surface of the heat exchange tubes, the height of which passes from an initial value hi (Fig. 10(A)) to a final value h2 considerably lower than hi (Fig. 10(B)), the pneumatic mandrel expansion method according to the invention leaves the shape of the corrugations substantially unaltered, limiting the reduction in height of the corrugations to a value lower than 5% of the initial one.

[0047] Insofar as concerns the mechanism of pneumatic expansion of the heat exchange tubes 4, it must be observed that the residual expansion deformation of the aforesaid tubes is fundamentally the result of the fact that they are made of a material that plastically deforms in the yield zone. This can be understood by referring to Fig. 3 of the drawings, which shows the stress and strain diagram of the copper, used to manufacture the heat exchange tubes of an air heat exchanger. The method of the invention essentially provides for cyclically alternating steps of charge (increase in the pressure in the circuit of heat exchange tubes 4) in the material yield zone and steps of discharge (decrease in the pressure in the circuit of heat exchange tubes 4) until reaching a residual deformation of the heat exchange tubes 4 necessary to have a complete contact with the inner surface of the tubular collars 7 of the fins 2. In particular, Fig. 3 shows the case of repetition of three charge and discharge cycles. In the first cycle, the material is charged beyond the elastic limit up to a first yield point osihigher than the conventional yield point oso of the material (section ODi of the diagram) and then discharged (section D1L1 of the diagram), obtaining a residual deformation EI of the material. In the second cycle, the material is charged up to a second yield point oS2 higher than the first yield point osiof the material (section L1D2 of the diagram) and then discharged (section D2L2 of the diagram), obtaining a residual deformation £2 of the material. In the third and last cycle, the material is charged up to a third yield point oS3 higher than the second yield point Os2 of the material (section L2D3 of the diagram) and then discharged (section D3L3 of the diagram), obtaining a residual deformation S3 of the material. In this manner, the total plastic deformation of the material is s = si + 82 + £3.

[0048] The heat exchange tubes 4 of the heat exchanger 1 can thus be expanded gradually by means of application in succession of charge and discharge cycles of the gas under pressure that substantially reproduce the plastic deformation mechanisms described above. Fig. 4 of the drawings schematically illustrates the trend as a function of time of the pressure of the gas in the circuit of heat exchange tubes 4 during application in succession of three charge and discharge cycles. The pressure values Pmax and Pmin, the duration t, as well as the number of cycles, can vary based on the characteristics of the heat exchange tubes and the other components of the circuit of the heat exchanger 1. It is important to note that the application of a single charge and discharge cycle is not sufficient to obtain the desired plastic deformation of the heat exchange tubes 4 that ensures a close contact between them and the inner surface of the tubular collars 7 of the fins 2. In order to obtain the desired effect, there must be at least two, preferably three, charge and discharge cycles. Furthermore, due to the initial deformation acquired during the first charge and discharge cycle, the material of the heat exchange tubes 4 acquires the capacity to withstand higher charges during subsequent cycles without risking breakages. The method of the invention can optionally also provide, before and after expansion of the heat exchange tubes 4, for a step of initial and final testing of the circuit of heat exchange tubes 4. In this step, as shown in Fig. 4, the pressure at the inlet of the circuit of heat exchange tubes 4 is taken to a predetermined test value Pc and it is checked that the pressure at the outlet of the circuit of heat exchange tubes 4 has the same value.

[0049] Fig. 5 of the drawings schematically illustrates an apparatus for implementing the method of the invention, denoted as a whole with the number 8. The apparatus comprises a control unit 9, a compression unit 10, a storage tank 11 for the gas under pressure and couplings 12, 13 for connection to the inlet and the outlet, respectively, of the circuit of heat exchange tubes 4 of the heat exchanger 1 that has to be expanded and tested. The apparatus 8 further includes valves 14 and 15 for regulating the pressure of the gas in the storage tank 11 and further valves 16 and 17 for regulating the pressure of the gas upstream and downstream, respectively, of the circuit of heat exchange tubes 4 of the heat exchanger 1 . As can be noted, since the apparatus 8 is connected to the inlet and the outlet of the circuit of heat exchange tubes by means of the manifolds 5 and 6, it is possible to perform pneumatic mandrel expansion simultaneously on all the heat exchange tubes 4, with a considerable saving of time with respect to traditional mechanical mandrel expansion.

[0050] In particular, the control unit 9 is capable of charging the tank 11 with compressed fluid by operating the compressors 10 and suitably modulating the valves 14 and 15. It is necessary to have a high consistency of pressure inside the tank 11 . Furthermore, the control unit 9 must deliver the fluid under pressure to the heat exchanger 1 in order to obtain expansion of the tubes 4, following with the utmost care the pressures cycle indicated in Figure 4, suitably acting on the valves 16 and 17. Lastly, the control unit 9 measures the pressure drop between point 13 and point 12, in order to check the integrity of the exchanger 1 .

[0051] Two examples of application of the pneumatic mandrel expansion method according to the invention are described below.

[0052] Example 1

[0053] In this example, an air heat exchanger with heat exchange tubes having a diameter of 9.5 mm and fins with tubular collars having a diameter of 10 mm was connected to an apparatus according to the invention and air at an initial test pressure Pc of 30 bar was introduced into the circuit of heat exchange tubes, in order to check the seal of the circuit. Subsequently, two charge and discharge cycles were performed (Fig. 6), gradually increasing the pressure of the gas for 120 seconds up to a value Pmax of 150 bar and then gradually decreasing it for 60 seconds down to a value Pmin of 90 bar, with a duration of each cycle equal to 100 s. The widening of the heat exchange tubes was measured after each cycle. The diameter of the heat exchange tubes had increased by 10.1 mm after the first cycle and by 10.2 mm after the second cycle. Subsequently, the pressure of the gas in the circuit of heat exchange tubes was decreased down to a test value Pc of 30 bar, in order to perform final testing of the seal of the circuit of heat exchange tubes.

[0054] Example 2

[0055] In this example, an air heat exchanger with heat exchange tubes having a diameter of 12 mm and fins with tubular collars having a diameter of 13 mm was connected to an apparatus according to the invention and air at an initial test pressure Pc of 30 bar was introduced into the circuit of heat exchange tubes, in order to check the seal of the circuit. Subsequently, three charge and discharge cycles were performed (Fig. 7), gradually increasing the pressure of the gas for 100 seconds up to a value Pmax of 120 bar and then gradually decreasing it for 50 seconds down to a value Pmin of 70 bar, with a duration of each cycle equal to 100 s. The widening of the heat exchange tubes was measured after each cycle. The diameter of the heat exchange tubes had increased by 13.2 mm after the first cycle, by 13.4 mm after the second cycle and by 13.5 mm after the third cycle. Subsequently, the pressure of the gas in the circuit of heat exchange tubes was decreased down to a test value Pc of 30 bar, in order to perform final testing of the seal of the circuit.

[0056] It was confirmed in practice that the invention fully achieves the intended aim and objects. In particular, the invention provides an air heat exchanger having an improved structure and, in particular, a greater heat exchange capacity. Furthermore, the pneumatic mandrel expansion method allows the limits of traditional mechanical mandrel expansion to be overcome, as it allows better results in expansion of the heat exchange tubes of an air heat exchanger to be obtained compared to those obtained with the mechanical mandrel expansion technique, and in a shorter time. Although the invention has been described with reference to a preferred embodiment thereof and to some examples of application, it is clear that a person skilled in the art can make numerous modifications and variants to it, all falling within the scope of the inventive concept.

[0057] In practice, the number of charge and discharge cycles and the maximum and minimum pressure values of the gas used for pneumatic expansion of the heat exchange tubes could vary as a function of the characteristics of the heat exchange tubes and of other components of the heat exchanger that are placed under pressure during implementation of the method of the invention.

[0058] Furthermore, the presence of the collars 7 on the heat exchange tubes 4 could be omitted.

Claims

CLAIMS1. Air heat exchanger (1 ) comprising a pack of juxtaposed metal fins (2) and mechanically supported by metal covering plates (3), said exchanger further comprising a circuit of metal heat exchange tubes (4) that perpendicularly cross said fins (2) and the aforesaid covering plates (3) and that are placed in hermetic fluid communication with each other and with an inlet distributor (5) and an outlet manifold (6) for the passage of a work fluid, said fins (2) being provided with connection holes (18) for connection with said heat exchange tubes (4), characterised in that the connection between said heat exchange tubes (4) and said fins (2) is obtained by means of pneumatic expansion of the heat exchange tubes (4) in said connection holes (18) of the fins (2), wherein the aforesaid fins (2) are arranged folded outside the heat exchange tubes (4) so as to form, on their outer surface, respective collars (7) on the section of engagement of the tubes (4) on the fins (2), and the contact area between the inner surface of said collars (7) of the fins and the outer surface of the portions of heat exchange tube (4) housed in the collars (7) is at least 60% of the available one.

2. The heat exchanger according to claim 1 , characterised in that said heat exchange tubes (4) have corrugations (20) on their inner surface and the reduction in height of the crests of said corrugations following pneumatic expansion of the heat exchange tubes (4) is lower than 5%.

3. Method for the manufacture of an air heat exchanger (1) pre-assembled with a pack of juxtaposed metal fins (2) and mechanically supported by metal covering plates (3), said exchanger further comprising a circuit of metal heat exchange tubes (4) that perpendicularly cross said fins (2) and the aforesaid covering plates (3) and that are placed in hermetic fluid communication with each other and with an inlet distributor (5) and an outlet manifold (6) for the passage of a work fluid, said fins (2) being provided with connection holes (18) for connection with said heat exchange tubes (4), characterised in that it further provides for- brazing of said tubes (4) on the aforesaid manifolds (5,6) and on the connecting curves (41 ) between said tubes (4) of the heat exchanger,- and subsequently the simultaneous mandrel expansion of the tubes (4) to obtain their attachment inside the aforesaid holes (18) of the fins (2), said mandrel expansion being obtained through expansion of said heat exchange tubes (4), determined by placing under compression a fluid charged into them, with application of a succession of at least two cycles of charge and discharge of said fluid present inside said tubes (4), wherein said charge and discharge cycles are performed by gradually varying the pressure produced inside said tubes (4).

4. The method according to claim 3, characterised in that it comprises the steps of:- providing for brazing of said tubes (4) on the aforesaid manifolds (5,6) and on the connecting curves (41 ) between said tubes (4) of the heat exchanger,- subsequently performing the simultaneous mandrel expansion of the tubes (4) to obtain their attachment inside the aforesaid holes (18) of the fins (2), by introducing a gaseous fluid into said circuit of heat exchange tubes (4) through said inlet distributor (5) and said outlet manifold (6) of the heat exchanger (1 ),- cyclically increasing and decreasing the pressure of said gaseous fluid within a range of yield values of the metal material of said heat exchange tubes (4), in order to obtain a residual expansion deformation that produces a contact through interference between said heat exchange tubes (4) and said fins (2) of the heat exchanger (1 ), wherein the aforesaid charge and discharge cycles are performed by gradually varying the pressure produced inside said tubes (4), and- evacuating said gaseous fluid from said circuit of heat exchange tubes (4).

5. The method according to claim 4, characterised in that said gaseous fluid is a gas such as air, helium, nitrogen, argon, or another gas with similar characteristics.

6. The method according to claim 4, characterised in that, before and after the application in sequence of the cycles of increasing and decreasing the pressure of the gaseous fluid introduced into the heat exchange tubes (4), a stepof testing the circuit of heat exchange tubes (4) is optionally provided, wherein the pressure at the inlet of the circuit of heat exchange tubes (4) is taken to a predetermined test value and it is checked that the pressure at the outlet of the circuit of heat exchange tubes (4) has the same value.

7. The method according to one or more of the preceding claims, characterised in that the aforesaid fins (2) are arranged folded outside the tubes (4) so as to form, on the outer surface thereof, respective collars (7) on the section of engagement of the tubes (4) on the fins (2), the aforesaid residual expansion deformation producing a contact through interference between said heat exchange tubes (4) and said collars (7) of the aforesaid fins (2) of the heat exchanger (1 ), wherein the contact area between the inner surface of said collars (7) of the fins and the outer surface of the portions of heat exchange tube (4) housed in the collars (7) is at least 60% of the available one.

8. Apparatus (8) for the manufacture of an air heat exchanger (1 ) preassembled with a pack of juxtaposed metal fins (2) and mechanically supported by metal covering plates (3), said exchanger further comprising a circuit of metal heat exchange tubes (4) that perpendicularly cross said fins (2) and the aforesaid covering plates (3) and that are placed in hermetic fluid communication with each other and with an inlet distributor (5) and an outlet manifold (6) for the passage of a work fluid, said fins (2) being provided with connection holes (18) for connection with said heat exchange tubes (4), said apparatus being characterised in that it comprises a control unit (9), a compression unit (10), a storage tank (11 ) for storing gaseous fluid, couplings (12, 13) adapted for connection to said inlet distributor (5) and, respectively, to said outlet manifold (6) of the circuit of heat exchange tubes (4) of the heat exchanger (1 ), first and second valves (14,15; 16,17) for regulating the pressure of said gaseous fluid in the storage tank and, respectively, upstream and downstream of the circuit of heat exchange tubes (4) of the heat exchanger (1 ).

Images