OF A HEAT EXCHANGER

Scope of the invention

The present invention concerns a heat exchanger and a procedure for the creation of a heat exchanger. The invention has been developed with particular regard to a heat exchanger for use in a gas boiler, of the type used for example for domestic heating and/or the production of domestic hot water. The heat exchanger is of the serpentine type, the latter term signifying that one or ^" more pipes of relatively small cross- section are wound in a cylindrical spiral, with coils of relatively much larger diameter with respect to the dimensions of the cross-section of the pipe. The heat exchanger is particularly suitable for use in a gas boiler of the condensing type.

Technological background

A heat exchanger comprises one or more pipes within which a fluid, for example water, circulates. In the case of a gas boiler, the pipes of the exchanger allow the water to be heated using the gases produced by combustion in the burner. During the operation of the boiler, the pipes are exposed to heating by a second fluid, for example air and combustion gas. The heat is transferred from one fluid to the other through the pipes.

Serpentine heat exchangers are known in which a pipe is wound in a cylindrical spiral. Such an exchanger is known, for example, from the document EP 0678186, which has a pipe with ^' a flattened oval transverse cross-section. To create such an exchanger, a pipe of circular cross-section is first wound in a spiral and then placed in a hydraulic press in order to create the flattened oval cross-section. During the phase of creation of the oval cross-section, projections are created on the outer wall of the pipe, which hold the coils at a predefined distance. These projections make it possible to improve the thermal exchange with a fluid circulating externally to the serpentine. In order to maintain the compact form of the exchanger, tack welds are created in the outermost area of the cylindrical serpentine.

Serpentine heat exchangers are also known in which a second pipe is inserted inside the pipe wound in a spiral. These exchangers are used, for example, in condensing boilers and allow the production of domestic hot water, which circulates normally in the inner pipe, and of hot water for the heating system, which circulates normally in the outer pipe. The document EP 2199703 shows this type of exchanger, in which the outer pipe of the exchanger has a cross-section defined by the combination of two adjacent circumferences arranged at a predetermined distance and connected to each other.

These known exchangers have certain disadvantages.

The exchangers, consisting of two pipes one inside the other, are created from two rectilinear pipes that are bent together to create the cylindrical serpentine form. When performing this bending, despite constant controls, it is difficult to ensure the centring of the inner pipe with respect to the outer pipe. This is because the inner pipe tends either to deform during the bending, causing obstruction of the passage of the fluid from one part of the outer pipe to the next, or to shift out of the centred position with respect to the outer pipe. This results in a sub-optimal thermal exchange.

More generally, the production of the known exchangers consisting of ^' two pipes one inside the other is rather difficult and does not guarantee optimal results in terms of thermal exchange. As already mentioned, the main problem is that of the correct and uniform positioning of the inner pipe inside the outer pipe along the whole of its spiral extension. The cross-section of each of the two pipes and their reciprocal positioning are aspects that have a particular influence on the performance and lifespan of the heat exchanger. In this regard, it should be noted that the production of water for heating, which runs inside the outer pipe, and the production of domestic water, which runs inside the innermost pipe, are independent from each other because they take place according to the user's demands. This means that one of the two pipes may contain water that remains still because it is not demanded by the user, while water runs inside the other pipe that is heated by the burner of the gas boiler. The risk is that the still water may reach boiling point, and this not only causes noise but also produces steam and raises the temperature of the walls of the pipe. Moreover, the still water in one of the two pipes produces a thermal flywheel effect, slowing the raising of the temperature of the water demanded by the user that runs inside the other pipe.

It must be ensured that the inner pipe is centred inside the outer pipe in both the radial and the axial direction of the serpentine. In the known art, the centring of the two pipes is not ensured with certainty, and disadvantageous contact may occur between the metallic walls of the two pipes, with the consequent formation of a thermal bridge that adversely affects the efficiency of the exchanger and may lead to localised overheating, with possible damage to the pipes.

In the creation of the outer pipe of oval cross-section, the problem of making the coils of the serpentine closely packed also arises. In the known art, this problem is solved by creating tack welds on the coils, but this introduces the problem of an alteration of the thermal exchange of the heat exchanger .

Summary of the invention

The aim of the present invention is to create a serpentine heat exchanger that provides an optimal thermal exchange. In particular, the aim of the invention is to reduce the risks of overheating of fluids inside a serpentine pipe. A further aim of the present invention is to simplify the creation of the heat exchanger, and to provide a heat exchanger that is reliable, long-lasting and economical to produce and maintain.

These aims are achieved by means of a heat exchanger and a for its creation as defined in the claims that to a first aspect, the heat exchanger comprises a first pipe, wound in an essentially cylindrical spiral, within which a first fluid may circulate, and a second pipe, arranged inside the first pipe and also wound in an essentially cylindrical spiral, within which a second fluid may circulate. The first pipe has a transverse cross-section that is essentially oval, preferably flattened, while the second pipe has a transverse cross-section that is essentially unflattened oval in shape. This combination of pipe cross-sections has been found to be particularly advantageous for obtaining an optimal thermal exchange, together with good fluid circulation. The pipe surface with an oval cross-section produces a better thermal exchange than that obtained with a circular cross-section, and also provides less resistance to the passage of the fluid around the pipe. Moreover, in the event of accidental contact between the inner and outer pipes, the area of contact is substantially reduced in comparison with pipes that both have flattened surfaces facing each other.

Preferably, the essentially oval cross-section is obtained from a rectilinear cylindrical pipe before the bending phase. This makes it easier to accurately control the geometry and tolerances of the spiral.

According to an advantageous implementation, the end portions of the second pipe are brought into a rectilinear configuration after completion of the insertion of the second pipe inside the first pipe. Preferably, the end portions of the first pipe are cut after the insertion phase and a terminal is applied to the ends of the first pipe and/or the second pipe, for example by braze welding. This solution makes it possible to simplify the procedure of creating the complete serpentine consisting of two pipes.

Preferably, the second, inner pipe is wound in an essentially cylindrical spiral whose median diameter is less than the median diameter of the essentially cylindrical spiral into which the first, outer pipe is wound. Thus, the second pipe is shifted inwards with respect to the centre line of the cross-section of the first pipe. This position makes it possible to achieve a good result in terms of speed of heating of the fluid inside the second pipe, since the mass of fluid in the first pipe that divides the second pipe from the hottest area of the heat source is reduced. Moreover, the greater surface area of passage of the fluid in the first pipe into the coolest area of the heat source makes it possible- to extract the heat from the second pipe more quickly in the event that the fluid inside it is still, thus avoiding the risks . of overheating and boiling.

Advantageously, the first pipe is bent in such a way as to create a first essentially cylindrical spiral; the second pipe, which has a transverse cross-section with a surface area smaller than the cross-section of the said first pipe, is also bent in such a way as to create a second essentially cylindrical spiral; ,the second pipe is then inserted inside the first pipe by performing a rotation of the said first spiral in one direction and a rotation of the said second spiral in the opposite direction.

Advantageously, the essentially cylindrical spiral of the first pipe has a median diameter greater than that of the essentially cylindrical spiral of the second pipe. In this way, the second pipe is inserted inside the first pipe in a non-centred position, but instead close to the inner wall of the first pipe.

Advantageously, the heat exchanger comprises means for distancing the coils of the first pipe from each other. According to advantageous forms of implementation, these means may be comb-shaped. The means for distancing the coils from each other make it possible to maintain constant the cross-sections of passage of the fluid contained inside the serpentine.

Advantageously, the procedure for the creation of the heat exchanger comprises the phases of bending a first pipe in such a way as to create a first essentially cylindrical spiral with closely packed coils and separating the coils from each other by distancing means, preferably comb-shaped. This makes it unnecessary to provide tack welds, which alter the thermal exchange. . ^' Advantageously, the second pipe is created with coils having the nominal pitch of the serpentine of the finished exchanger, while the first pipe is created with closely packed coils. After inserting the second pipe inside the first pipe, the coils of the first pipe are separated from each other by distancing means, preferably comb-shaped, in order to obtain an outer serpentine with a nominal coil pitch. The" inner serpentine elastically recovers the nominal pitch between the coils, and is thus axially centred with respect to the serpentine of the first pipe that contains it.

These solutions therefore make it possible to obtain optimal centring of the inner pipe with respect to the outer pipe, maintaining constant the cross-sections of passage of the fluid between one pipe and the other and therefore optimising the thermal exchange.

Brief description of the drawings

Further characteristics and advantages of the present invention will become clear from the detailed description that follows of some examples of the invention, given purely by way of non-limitative example, with reference to the annexed drawings in which:

Figure 1 shows a perspective view of a form of implementation of a heat exchanger according to the present invention;

Figure 2 shows an exploded perspective view of the exchanger of Figure 1 ;

Figure 3 shows a front view of the exchanger of Figure

1;

Figure 4 shows a cross-section along the line ^' IV-IV of Figure 3;

Figure 5 shows a detail of the cross-section of the pipes or the exchanger of Figure 4;

Figure 6 shows a cross-section of the pipes of the exchanger according to a second form of implementation;

Figure 7 shows a view from above of the exchanger created with the pipes of Figure 6;

Figure 8 shows a partially cross-section view of the inner pipe of the exchanger according to a variant;

Figure 9 shows a view from above of another form of implementation of the heat exchanger;

Figure 10 shows a front view along the line of the arrow X of Figure 9;

Figure 11 shows a cross-section along the line XI-XI of Figure 10; and

Figure 12 shows a magnified cross-section of the pipes of the exchanger of Figures 9 to 11.

Detailed description

Figure 1 illustrates a heat exchanger 1. The heat exchanger 1 comprises a first pipe 2 wound in such a way as to create a first cylindrical spiral 3 with an axis XX'. The first pipe 2 has a transverse cross-section that is essentially oval in shape, preferably flattened (Figure 5) . Advantageously, the major axis of the second transverse cross-section of the first pipe 2 is essentially perpendicular to the axis XX' of the cylindrical spiral 3. At the ends 4, 5 of the first pipe 2 wound in a spiral are connected, preferably by induction braze welding, a first and a second terminal 9, 10. As illustrated in' Figure 2, each terminal 9, 10 has a first portion 9A, 10A with the same cross-section as the first pipe 2 connected to a second portion 9B, 10B with a circular cross-section .

As illustrated in Figures 4 and 5, a second pipe 22 is inserted inside the first pipe 2. According to a first form of implementation, the second pipe 22 has an unflattened essentially oval cross-section and has a surface area smaller than that of the first pipe 2. The second pipe 22 is wound in such a way as to create a second cylindrical spiral 33. The second pipe 22 is essentially centred inside the first pipe 2, in such a way as not to impede the circulation of the fluid inside the first pipe 2. In this variant, the second pipe 22 is centred inside the first pipe 2 both horizontally and vertically, i.e. with respect to both axes of the essentially elliptical cross-section of the first pipe 2.

As illustrated in Figure 2, the end portions 24, 25 of the second pipe 22 each have a rectilinear portion 241, 251 to which a respective terminal 29, 30 is attached.

Advantageously, the coils 21 of the first pipe 2 are kept separate at a predefined distance by preferably comb-shaped distancing means 11. The comb 11 has an elongated plate 12 with an extension essentially equal to the height of the cylindrical spiral 3. More specifically, the elongated plate 12 may have an extension equal to the height of the cylindrical spiral 3 minus the height of the two end coils. From the plate 12 extend, perpendicularly to the said plate 12, teeth 13 arranged at staggered heights, in such a way as to be inserted between one spiral 21 and the other and hold them in position. Preferably, the combs 11 are angularly distanced in a regular fashion. Preferably, the combs 11 are eight in number. This ensures a constant cross-section of passage of the fluid situated outside the serpentine.

Figures 6 and 7 illustrate a variant of the heat exchanger described above. In particular, according to a second preferred form of implementation, the exchanger consists of a first outer pipe 20 and a second inner pipe 220, both wound xn sucn a way as to create an essentially cylindrical spiral. The first outer pipe 20 has a transverse cross-section that is essentially oval, preferably flattened. The wall 6 of the first pipe 20 has indentations 7 that make it possible to limit the movement of the inner pipe 220 during the bending phase and therefore to provide better control of the thermal exchange. According to this form of implementation, the second inner pipe 220 has an essentially circular cross- section. The inner pipe 220 may, however, be created with an essentially elliptical cross-section, with or without flattening. In particular, the inner pipe 220 may be created with the unflattened elliptical cross-section illustrated with reference to the second inner pipe 22 in Figure 5. As can be seen in Figure 6, the second inner pipe 220 is arranged in such a way that the centre of its cross-section does not correspond with the centre of the cross-section of the first outer pipe 22, but is shifted towards the inside of the first spiral formed by the first outer pipe. This characteristic .makes it possible to bring the second inner pipe 220 close to the hottest area of the heat source and also improve the thermal exchange between the inner pipe 220 and the outer pipe 22, in such a way as to avoid overheating and boiling of any still fluid in one of the two pipes during the heating of the fluid in the other pipe.

Figure 8 illustrates a variant according to which the second inner pipe 221 has a corrugated outer surface 26. Advantageously, on the outer surface 26 of the second pipe 221 is created a helical incision 28. with a total length L.l, a pitch P- and a depth Dpt predefined according to the dimensions of the second pipe 221. Preferably, the pipe 22 has a smooth portion of length L3 at each end 241, 251. Preferably, the second pipe 221 has an essentially circular transverse cross-section. The second pipe 221 ^' may, however, toe created with an elliptical transverse cross-section, with or without flattening, as described and illustrated previously with reference to the variant of Figure 5. The first outer pipe, not illustrated, may have a circular or oval cross-section, preferably flattened.

The corrugated surface makes it possible to limit the tendency of the pipe to deform during bending and therefore to impede the passage of the fluid.

Figures 9 to 12 illustrate another variant of the heat exchanger. As can be clearly seen in Figures 11 and 12, a second pipe 22 is inserted inside the first pipe 2. The second pipe 22 has an essentially oval cross-section that is not flattened and has a surface area smaller, than that of the first pipe 2. The second pipe 22 is wound in such a way as to create a second cylindrical spiral 33'. The second pipe 22 is essentially centred vertically inside the first pipe 2, i.e. along the axis XX' . The second pipe 22, however, is not centred radially inside the first pipe 2, but is shifted towards the inside of the cylindrical spiral 3. We define as the median diameter of the cylindrical spiral 3 the diameter of the geometrical cylinder that passes through the centre of the cross-sections of the first pipe 2. Similarly, we define as the median diameter of the cylindrical spiral 33' the diameter of the geometrical cylinder that passes through the centre of the cross-sections of the second pipe 22. In the example of Figures 11 and 12, the median diameter D of the first cylindrical spiral 3 is greater than the median diameter D' of the second cylindrical spiral 33'. Preferably, the second pipe 22 is positioned entirely in the innermost half-cross-section of the first pipe 2. The second pipe 22 is configured and arranged inside the first outer pipe 2 in such a way that it does not cross the centre line H-H. According to the present invention, a procedure for creating the cylindrical spiral 3 provides for a phase of bending of a rectilinear pipe in such a way as to create the cylindrical spiral 3 with closely packed coils 21 and then to maintain the said coils 21 at a certain distance from each other by holding them in position by means of combs 11.

In order to create the described exchangers, a first creation procedure according to the present invention provides for a first pipe 2 to be bent in such a way as to create an essentially cylindrical first spiral 3 and for a second pipe 22 having a transverse cross-section with a smaller surface area than the cross-section of the first pipe 2 to be bent in such a way as to create an essentially cylindrical second spiral 33, 33'. The second pipe 22 is then inserted inside the first pipe 2 by performing a rotation of the first spiral 3 in one direction and a rotation of the second spiral 33, 33' in the opposite direction.

In order to obtain the correct spacing of the cylindrical spiral 33, 33' inside the cylindrical spiral 3 along the axis XX', it is possible to create the cylindrical spiral 33, 33' with the coils correctly distanced at the nominal design pitch, i.e. the resulting inter-coil pitch in the finished configuration of the heat exchanger. Since the cylindrical spiral 33, 33' has a certain elastic flexibility along the axis XX', it is possible to insert the second pipe 22 inside the first pipe 2 by performing, as mentioned above, a rotation of the first spiral 3 and the second spiral 33, 33' in opposite directions. During this operation, the coils of the second pipe 22 are compressed to assume the pitch of the coils 21 of the first pipe 2, which, are created in ^' closely packed form. When the second pipe 22 is fully inserted into the first pipe 2, the coils of the latter- are. distanced by means or the combs 11 to obtain the nominal design pitch of the cylindrical spiral 3. The inner cylindrical spiral 33, 33' therefore recovers its original form of the nominal design pitch. By performing a centring between the terminals 9, 10 of the first pipe 2 and the corresponding terminals 29, 30 of the second pipe 22, it is thus possible to centre the whole of the second pipe 22 along the axis XX' inside the first pipe 2.

Advantageously, the first pipe 2 and the second pipe 22 have an essentially oval cross-section. The essentially oval cross-section of the first pipe 2 is preferably flattened, while the cross-section of the second pipe 22 is not. The cross-sections of the first pipe 2 and the second pipe 22 are preferably formed by respective rectilinear cylindrical pipes, with different diameters from each other, which are made oval and then bent into a spiral.

Since the serpentine must have two essentially rectilinear end portions, according to an advantageous implementation of the procedure the end portions of the first outer pipe 2 are cut after the bending is completed. A portion 241, 251 of the end portions 24, 25 of the second outer pipe 22 is brought into a rectilinear configuration. To the ends 4, 5 of the first outer pipe 2 are then connected a first and a second terminal 9, 10, preferably by means of induction braze welding .

In order to create the exchanger illustrated in Figures 5 and 6, an alternative procedure according to the present invention provides for the insertion inside a first pipe 20 of a second pipe 220 having a cross-section with a surface area smaller than the cross-section of the first pipe 20. Indentations 7 are created on the wall 6 of the ^■ first pipe 20. The first pipe 2d is bent in such a way as to create an essentially cylindrical first spiral 30 consisting of the first pipe 2 and a second essentially cylindrical spiral consisting of the second pipe 220.

Advantageously, the pipes have initially an essentially circular transverse cross-section. In this case, the outer pipe 20 is made oval by means of a press according to the known procedure once the bending has been performed to create the cylindrical serpentine 30.

The indentations 7 may be created on the wall 6 of the first pipe 20 either before the insertion of the second pipe 220 or after the insertion of the second pipe 220, before the bending or during the bending. These indentations 7 make itpossible to keep the inner pipe 220 essentially centred with respect to the outer pipe 20 during the bending phase.

An alternative procedure for the creation of a heat exchanger according to the present invention provides for the insertion inside a first pipe 2 of a second pipe 221 having a corrugated outer surface 26 and subsequently bending the first pipe 2 in such a way as to create a first essentially cylindrical spiral 3 consisting of the first pipe 2 and a second essentially cylindrical spiral consisting of the second pipe 221. The first pipe 2 may have an essentially circular or oval cross-section.

According to this procedure, the outer pipe 2 may also have an essentially circular initial portion and be made oval before the insertion of the second pipe 221 or after bending by means of a press according to the known procedure.

The procedure according to the invention therefore allows the creation of a serpentine exchanger that provides optimal thermal exchange. The position of the inner pipe is centred with respect to the outer pipe along the axis XX' of the serpentine. The radial or diametral position of the inner pipe with respect to the axis X'X may be either centred or non-centred with respect to the cross-section of the outer pipe. In the latter case, the position of the inner pipe is shifted closer towards the axis XX' with respect to the centre line of the cross-section of the outer pipe, for a more efficient thermal exchange. In addition, the closely packed form of the serpentine subsequently separated by the combs makes it possible to maintain a constant distance between the coils, thus improving the thermal exchange.