More specifically, the present invention relates to a heat exchanger for a gas boiler for producing hot water.

BACKGROUND ART A gas boiler for producing hot water normally comprises a gas burner, and at least one heat exchanger through which combustion fumes and water flow. Some types of gas boilers, known as condensation boilers, condense the steam in the combustion fumes and transfer the latent heat in the fumes to the water. Condensation boilers are further divided into a first type, equipped with a first exchanger close to the burner, and a second exchanger for simply condensing the fumes; and a second type, equipped with only one heat exchanger which provides solely for thermal exchange along a first portion, and for both thermal exchange and fume condensation along a second portion. Condensation or dual-function exchangers of the

above type normally comprise a casing extending along a first axis and through which combustion fumes flow; and a tube along which water flows, and which extends along a second axis and coils about the first axis to form a succession of turns. The combustion fumes flow over and between the turns to transfer heat to the water flowing along the tube. In some exchangers, the spiral tube has fins extending perpendicularly to the tube axis. This technical solution provides for a high degree of thermal exchange, but the fins are expensive to produce. In other exchangers, the spiral tube has a depressed flow section, which is a cheaper technical solution, though much less effective in terms of heat exchange than the finned-tube solution.

DISCLOSURE OF INVENTION It is an object of the present invention to provide a heat exchanger for a gas boiler for producing hot water, which is extremely effective in terms of heat exchange, while at the same time being cheap to produce.

According to the present invention, there is provided a heat exchanger as claimed in Claim 1.

The present invention also relates to a method of producing a heat exchanger.

According to the present invention, there is provided a method of producing a heat exchanger, as claimed in Claim 16.

The present invention also relates to a gas boiler.

According to the present invention, there is

provided a gas boiler as claimed in Claim 20.

BRIEF DESCRIPTION OF THE DRAWINGS A number of non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a schematic front view, with parts in section and parts removed for clarity, of a gas boiler equipped with a heat exchanger in accordance with the present invention; Figure 2 shows a larger-scale section of a detail of the Figure l heat exchanger ; Figure 3 shows a view in perspective of a tube used to produce the Figure 1 exchanger; Figure 4 shows the Figure 3 tube partly coiled ; Figure 5 shows a larger-scale view in perspective of a component part of the Figure 1 exchanger; Figure 6 shows a view in perspective, with parts removed for clarity, of the heat exchanger being assembled; Figure 7 shows a lateral view of the Figure 6 heat exchanger; Figure 8 shows a section of a variation of the Figure 3 tube.

BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in Figure 1 indicates as a whole a gas boiler. Boiler 1 is a wall-mounted condensation boiler, i. e. in which the vapour in the combustion fumes is condensed, and comprises an outer structure 2 in which

are housed a burner 3; a heat exchanger 4; a gas supply conduit 5 ; a pipe 6 for supplying an air-gas mixture to burner 3; a combustion gas exhaust pipe 7; a fan 8 connected to supply pipe 6, and which performs the dual function of supplying the air-gas mixture to burner 3, and expelling the combustion fumes; and a water circuit 9. Structure 2 comprises a rear wall 10 fixed to a supporting wall (not shown) and supporting the component parts of boiler 1; a top wall 11; two lateral walls 12; and a front wall not shown in the accompanying drawings.

Burner 3 is connected to pipe 6, is cylindrical in shape, and comprises a lateral wall with holes (not shown) for emitting the air-gas mixture and feeding the flame.

Burner 3 is housed inside exchanger 4 which, in fact, also acts as a combustion chamber. Heat exchanger 4 is substantially cylindrical in shape, extends along a substantially horizontal axis Al parallel to rear wall 10, and comprises a casing 13, through which the combustion products flow; a finned tube 14, along which water flows; and a disk 15 for directing the burnt gases along a given path inside exchanger 4. Casing 13 comprises a cylindrical lateral wall 16 about axis Al ; an annular wall 17 connected to lateral wall 16, to supply pipe 6, and to burner 3; and an annular wall 18 connected to lateral wall 16 and to exhaust pipe 7. Burner 3 extends, coaxially with exchanger 4, inside of exchanger 4 for a given length. Tube 14 coils about axis Al to form a succession of adjacent turns 19, each located close to

lateral wall 16, and has two opposite ends with known fittings (not shown) for connecting tube 14 to water circuit 9 outside exchanger 4. Disk 15 has a thin lateral edge 20 engaging turns 19. That is, disk 15 is screwed to turns 19 into the desired position along axis Al and in a position substantially perpendicular to axis Al.

Exchanger 4 comprises three comblike spacers 21 for keeping turns 19 a given distance apart and a given distance from lateral wall 16. As shown more clearly in Figure 5, each spacer 21 comprises a straight portion 22 parallel to axis Al, and from which project teeth 23, each of which is interposed between two adjacent turns 19. Tube 14, disk 15, and spacers 21 define, inside casing 13, a region B1 housing burner 3; a region B2 communicating directly with exhaust pipe 7; and three regions B3, each extending between two spacers 21, turns 19, and lateral wall 16. Combustion of the air-gas mixture takes place in region Bl ; and the resulting fumes, being prevented by disk 15 from flowing directly to region B2, flow between turns 19, in a direction D1 substantially perpendicular to axis Al, to regions B3, along which they flow in a direction D2 substantially parallel to axis Al. On reaching regions B3, the fumes flow between turns 19 in direction D1 to region B2 and then along exhaust pipe 7.

Tube 14 is preferably made of aluminium or aluminium-based alloy. With reference to Figure 3, finned tube 14 is extruded, extends along an axis A2, and

comprises an oval-section wall 24; two fins 25 on one side of tube 14; two fins 26 on the opposite side to fins 25; a fin 27 between fins 25; and a fin 28 between fins 26. The cross section of tube 14 has a major axis X and a minor axis Y. Fins 25,26, 27,28 are all parallel to axis A2 of tube 14 and to major axis X, and are therefore parallel to one another. Fins 27 and 28 are coplanar with each other, and substantially lie in the same plane as axis A2 of tube 14 and major axis X. Fins 25 and 26 are arranged so that each fin 25 is coplanar with an opposite fin 26, and wall 24 of tube 14 forms a slight convexity between the coplanar fins 25 and 26. The maximum extension of fins 25 and 26, in a direction parallel to major axis X, is roughly a quarter of the length of major axis X.

With reference to Figure 4, once extruded with fins 25, 26, 27 and 28, tube 14 is coiled about axis Al, so that axis A2 of tube 14 also assumes a spiral shape. This operation actually comprises calendering tube 14, with the minor axis Y of the section of tube 14 maintained substantially parallel to axis Al. The relatively small size of fins 25,26, 27,28 does not hinder the calendering operation, and does not call for notching fins 25,26, 28,28. The three spacers 21 are then fitted between fins 26 of adjacent turns 19, and arranged 120 degrees apart, so as to form, with the coiled tube 14, an assembly which is inserted inside cylindrical wall 16 of casing 13 (Figures 6 and 7). Annular walls 17 and 18 are

then fitted to the opposite ends of cylindrical wall 16.

Tube 14 is coiled with a constant pitch and radius, so that fins 25 and 26 of each turn 19 face and are parallel to fins 25 and 26 of the adjacent turns 19, as shown in Figure 2. A gap is thus formed between each two adjacent turns 19, is of constant width at fins 25 and 26, and narrows at the convexities of walls 24. The fumes flow from region Bl to regions B3 in direction Dl towards wall 16, then flow in direction D2 between turns 19 and wall 16, flow between turns 19 in direction D1 from regions B3 to region B2, and are finally expelled by exhaust pipe 7. The successive gaps therefore define compulsory fume paths, and are so shaped as to produce a venturi effect which rapidly accelerates the fumes. As they flow from region B1 to regions B3, the accelerated fumes collide with the fumes flowing in direction D2 and with wall 16, thus increasing turbulence. And, as they flow from regions B3 to region B2, the accelerated fumes collide with the relatively slow-flowing fumes in region B2 to again increase turbulence. Turbulent motion of the fumes is generally desirable by improving heat exchange.

Fins 25 and 26 therefore not only increase the exchange surface of tube 14, but also accelerate and so produce turbulent motion of the fumes. In this connection, it should be pointed out that fins 27 and 28 provide mainly for enhancing heat exchange, and play no part in accelerating the fumes.

With reference to the Figure 8 variation, when tube

14 is relatively small, one fin 25 and one fin 26, on opposite sides of tube 14, are sufficient to produce the venturi effect. As tube 14 gets larger, at least two fins 25 and two fins 26 are required to avoid too marked a convexity on wall 24 between one fin 25 and a coplanar fin 26, which would prevent the gap from producing a venturi effect.

According to tests conducted by the Applicant, the gap provides for optimum heat exchange when given dimensional ratios of the gap are conformed with. With reference to Figure 2, Z1 indicates the distance between two facing fins 25 or 26 of two adjacent turns 19, and Z2 the minimum distance between the facing walls 24 of the same two adjacent turns 19. Tests conducted by the Applicant show the best Z2/Z1 ratio, in terms of thermal efficiency of the exchanger, to be 1/3, whereas Z2/Z1 ratios of 0.2 to 0.4 are considered acceptable.

Exchanger 4 as described above may also be used in condensation boilers comprising a main exchanger, and in which exchanger 4 provides solely for condensing the fumes, as opposed to acting as a combustion chamber as in the example described.

Exchanger 4 as described above has numerous advantages, by combining straightforward construction- as a result of the fins being formed directly by the tube extrusion process, as opposed to being added on after extrusion-with a high degree of thermal efficiency-by virtue of the fins increasing the exchange surface, and

the dynamic effect on the fumes of the fins in combination with the tube.

Moreover, exchanger 4 is relatively easy to produce, and can be produced in different lengths to meet different power requirements. For which purpose, lengths along axis Al can be produced which are multiples of a base length, and spacers 21 of a length equal to the base length can be produced to standardize spacer manufacture.