Description

Heat exchanger for domestic boilers, especially wall-mounted gas boilers

Technical Field

This invention relates to a primary heat exchanger for domestic boilers, especially open chamber and/or sealed chamber, wall-mounted gas boilers.

Background Art

At present, some types of heat exchangers for wall- mounted gas boilers are made of copper and consist of finned tube coils.

Some prior art coils consist of straight copper tubes, with circular or oval cross section, connected to each other by separate preformed curves .

The fins consist of stacked copper sheets each with a hole in it shaped to match the shape of the tubes in such a way that the tubes can be inserted into and attached to the stacked sheets.

The construction of the exchanger as a whole basically involves blanking the copper sheets and fitting them together in stacks that will subsequently form the fins.

The straight tube sections are then inserted into the holes in the stacked sheets and connected by the curves, placed on the outside of the stacked sheets in such a way as to form the tube coil . Water fittings are then applied and all the parts of the heat exchanger are joined together to form a single assembly and brazed in controlled atmosphere in a continuous furnace . The production process is completed by painting the heat exchanger with silicone resins and finally testing the end product.

The purpose of painting the heat exchanger is to coat it with a heat-resistant film designed to protect it against

acid corrosion caused by combustion fumes when the boiler is in operation.

Silicone coatings are a relatively recent development and replace the protective lead coatings, used in the past, in order to reduce harmful effects on the environment and on human health.

In terms of protective efficacy, however, silicone coatings have the disadvantage of not providing optimal protection. During the catalysing process, the thermosetting resin tends to form microcracks in the protective film through which the fumes can come into contact with the copper below.

This, added to the moist atmosphere, creates ideal conditions for particularly aggressive corrosion that will damage the heat exchanger in a relatively short space of time .

This is a very common problem in wall-mounted domestic boilers installed outdoors . Although this type of installation is very common, the problem does not concern all boilers installed since those installed indoors or in suitably protected places are much less affected by wet corrosion.

Further, ever-increasing limitations on energy consumption, posed also by stringent legal provisions, mean that wall-mounted gas boilers manufactured today are required to meet very high standards of energy efficiency involving a reduction in the temperature of exhaust fumes to values very close to 100 ⁰ C.

Under these circumstances, the vapours contained in the fumes are quite close to typical condensation temperatures.

For this reason, all wall-mounted boilers conforming with recent legislation on the subject of energy, irrespective of where they are installed, are open to attack by acid corrosion of their heat exchangers caused by combustion products. This corrosion causes rapid decay of the heat exchanger, especially at the water inlet end, where the temperature is lower.

Moreover, we should not forget that the cost of copper is climbing steadily as the demand for this metal increases more and more .

Disclosure of the Invention

This invention therefore has for an object to overcome these disadvantages by providing a heat exchanger for wall- mounted gas boilers whose heat exchanging efficiency is practically identical to that of copper heat exchangers but whose resistance to acid corrosion caused by combustion products is much higher than that of copper heat exchangers .

Another aim of this invention is to provide a heat exchanger that is much more inexpensive to manufacture than current heat exchangers . The technical characteristics of the invention according to the aforementioned aims may be easily inferred from the contents of the appended claims, especially claim 1, and any of the claims that depend, either directly or indirectly, on claim 1

Brief Description of the Drawings

The advantages of the invention will become more apparent from the detailed description which follows, with reference to the accompanying drawings which illustrate preferred embodiments of the invention provided merely by way of example without restricting the scope of the inventive concept, and in which:

Figure 1 is an assembly plan view of a heat exchanger according to the invention; Figure 2 is a side view of the heat exchanger of Figure 1;

Figure 3 is a cross section of the heat exchanger through the section line IH-III;

Figure 4 is a front view of the heat exchanger of Figure l;

Figure 5 is a cross section of the heat exchanger of Figure 2 through the section line V-V;

Figure 6 is a cross section of the heat exchanger through the section line VI-VT;

Figures 7 and 8 are, respectively, a plan view and a perspective view of an alternative embodiment of the heat exchanger;

Figures 9 and 10 are, respectively, a perspective view and a front view of a part of the heat exchanger;

Figure 11 is an enlarged detail from Figure 10.

Detailed Description of the Preferred Embodiments of the Invention

With reference to Figure 1, the numeral 1 denotes in its entirety a heat exchanger for domestic boilers, especially self-contained wall-mounted gas boilers. The heat exchanger 1 basically comprises a metal tube 2 -better illustrated in Figures 5 and 6 showing one particular embodiment- through which the water to be heated flows, and fins 4 associated with the outside of the tube 2 and designed, instead, to come into contact with the combustion products of the boiler.

The metal tube 2 and the fins 4 are made as separate parts, of aluminium alloy and are interconnected.

In a first embodiment that is not illustrated, the tube comprises a plurality of straight, separate tube sections placed side by side, operatively connected to each other in parallel and designed to have the water to be heated flowing through them. The fins 4 are associated with the outside of the straight tube sections . The fins 4 and the metal tube are made as separate parts, of aluminium alloy and, advantageously, the fins 4 and the tube are interconnected.

In an alternative embodiment, the metal tube 2 is essentially coil shaped. The coil extends in a plane 3. The fins 4 are designed to come into contact with the boiler combustion products in the form of gaseous flows 12 moving through the heat exchanger 1 in a direction transversal to the plane 3 in which the coil extends, as illustrated in Figures 4 and 6. The metal tube 2 and the fins 4 are

interconnected at least at the coil plane 3 in such a way as to form a single assembly, as clearly illustrated in Figure 5.

Advantageously, the coil consists of at least one continuous, single-part, suitably curved tube 2. The tube 2 consists of a single part for its entire length. Its structure is obtained by bending a single, initially straight tube 2, to obtain a sequence of straight sections 18 alternated with curved sections 21. In an alternative embodiment illustrated by way of example in Figures 7 and 8, the coil shaped tube 2 comprises straight sections 18 and one or more unions 20 fitted to the ends of the straight sections 18 in such a way as to put all the straight sections 18 in fluid communication with each other. Each union 20 forms a chamber 200 from which a first and a second straight section 18 of the tube 2 extend. Advantageously, the chamber 200 is delimited by the union 20 in combination with the wall 201 of the heat exchanger 1 to which the first and second straight sections 18 are directly attached. The fluid leaving the first straight section flows through the union 20 and into the second straight section. For constructional simplicity, the unions 20 are physically distinct from the straight sections 18 of the tube 2 and are joined to the straight sections 18. As illustrated in Figure 7, the fins 4 are interposed between the straight sections 18 of the tube 2.

The fins 4 are advantageously made from continuous metal strips 5 that are initially flat and straight and then corrugated into a sort of fretted shape. Each of the metal strips 5 extends in a direction 6. The direction 6 is preferably parallel with the straight sections 18 of the tube

2. If the tube 2 extends in coiled fashion, the direction 6 is within the plane 3 in which the coil extends . The straight sections 18 of the tube are connected to the fins 4 at the bends 9 of the corrugations in the strips 5. The connection can be accomplished according to any of several known methods falling outside the scope of this invention and therefore not described in this specification.

As shown in the accompanying drawings, the fins 4 are associated with the tube 2 only at two opposite sides 7 of it, said sides 7 being preferably parallel with each other.

The tube 2 has a flattened cross section, with opposite flat walls 8 abutting against the bends 9 of the strip 5.

As shown in Figures 1 and 6, the heat exchanger 1 also comprises a plate 10 that can be associated with a face 11 of the tube 2 located in distal position relative to the direction from which the fume flows 12 come. The fumes referred to in this specification are the combustion products of the boiler.

The plate 10 is oriented transversally to the fume flow 12 and is made in such a way as to control the dynamic behaviour of the flow in order to maximize heat exchange efficiency.

If the tube 2 is coil shaped, the plate 10 can be associated with a face 11 of the tube 2 located in distal position relative to the direction from which the fume flows 12 come. The plate 10 is oriented transversally to the fume flow 12 in such a way as to regulate the flow to maximize heat exchange efficiency.

More specifically, the plate 10 has apertures 13 and baffles 14 designed to redirect the fume flow 12 towards the walls 8 of the tube 2. The apertures 13 allowing the fumes to flow through the plate 10 are located at the straight sections 18 of the tube 2. The baffles 14, on the other hand, are located at the sides of the tubes 2 and are designed to slow the fume flow 12 and to redirect it towards the walls 8, constraining it to flow along the walls 8 and thus increasing heat exchange efficiency.

This effect is especially enhanced by the specific shape of the baffles 14 which have cusped surfaces 15 with vertices 16 pointed against the direction of the fume flow 12 and oblique walls 17 flared in the direction of the fume flow 12.

As illustrated by way of example in Figures 9 to 11, the fins 4 of the heat exchanger 1 constitute a plurality of

channels 40 for fume outflow. The heat exchanger 1 also comprises deflecting protuberances 41 extending from the fins 4. The protuberances 41 project into the channels 40 transversally to the fume flow in such a way as to control its dynamic behaviour, thus maximizing heat exchange efficiency. Advantageously, the deflecting protuberances 41 project into the channels 40 at right angles to the fume flow. The deflecting protuberances 41 impede the outflow of the fumes and redirect the flow towards the walls 8 of the tube 2. The corrugated strip 5 comprising the bends 9 comprises at least one first, one second and one third bend 91, 92, 93, the second and third bends 92, 93 being located immediately before and after the first bend 91; at least one deflecting protuberance 41 redirects the fume flow towards the first bend 91 and towards the gap 94 between the second and third bends 92 , 93. This enhances heat exchange between the fumes and the water flowing inside the tube 2. The fins 4 comprise a lateral face 96 that extends between two consecutive bends 9 in the strip 5; the at least one deflecting protuberance 41 extends from the lateral face 96 and is located at a predetermined non-zero distance from portions that are in contact with the tube 2 , said portions forming part of two consecutive bends 9 common to the lateral face 96. Advantageously, the at least one protuberance 41 extends along an imaginary line connecting two consecutive bends 9 of the lateral face 96. The predetermined non-zero distance makes it possible to form preferential lanes for the fumes redirected by the deflecting protuberances 41; these preferential lanes are located alongside the tube 2 and thus improves the heat exchange between the hot fumes and the water flowing inside the tube 2.

The heat exchanger also comprises a structural frame 19 surrounding the tube 2 and fin 4 assembly and providing stable support for fittings 20 at the ends of the tube 2 for connecting the tube 2 to the water supply, that is to say, for connecting the heat exchanger to the boiler water system.

The heat exchanger described fully achieves the above mentioned aims of having a high resistance to wet corrosion caused by combustion products, without requiring coats of protective substances, and of keeping the temperature of the combustion products well above condensation temperature.

Although the thermal conductivity of aluminium alloys is not as high as that of copper - used in prior art heat exchangers - the special shape of the parts of the heat exchanger 1 make heat exchange particularly effective, fully compensating for the lower thermal conductivity of aluminium alloys compared to copper.

Moreover, the geometrical shape of the heat exchanger 1 greatly simplifies its structure, providing a heat exchanger that is inexpensive to manufacture but capable of meeting market demand for energy efficiency and legislative requirements on the subject of energy saving.

Another advantage of the invention is the lighter weight of the boiler, a much appreciated feature in general and all the more so in a wall-mounted boiler, as in this specific case, designed to be hung on a wall.

The invention described has evident industrial applications and can be modified and adapted in several ways without thereby departing from the scope of the inventive concept. Moreover, all details of the invention may be substituted by technically equivalent elements.