EP0076724B1 - Method of manufacturing a heat exchanger with annularly mounted water tubes - Google Patents

Description

The invention relates to a method for manufacturing a heating body of the sheet type of water tubes arranged around a cylindrical combustion chamber for domestic gas boilers, for example condensing.

In a known embodiment, the heating body consists of a sheet of longitudinal water tubes arranged all around a central burner whose combustion gases are emitted radially and pass through this sheet of tubes, the best conditions for exchange between water and gas is obviously sought by the shape and positioning of the tubes. These longitudinal water tubes of elongated section are generally brazed between two annular collectors to form a cylindrical combustion chamber around the burner. Such a boiler is known from FR-A-2 463 368.

The design of this kind of heating body requires a rather particular technology of materials, assembly and brazing of the tubes on the annular collectors and this because of the large number of tubes and their proximity necessary for a good coefficient of exchange .

The use of copper, a material having a good coefficient of conductivity and which is easily brazed, has drawbacks. In fact, the resistance to deformation, more particularly in the case of tubes with flattened faces, necessarily annealed during the brazing operation, is insufficient to withstand the water pressure normally used in a central heating boiler. But moreover and especially in the presence of the water of condensation in the fumes, the corrosion of the tubes is extremely fast, certain gas being able to even contain sulfur likely to produce sulfuric acid. Copper can obviously be protected by a coating based on lead, but this is difficult to apply because of the proximity of the tubes, lead running the risk of filling the intervals between these tubes. It is also known that this protection is limited in time.

To overcome these drawbacks, the use of thin stainless steel is known, which gives a good heat transfer coefficient and sufficient resistance to inflation under water pressure.

However, the use of such a steel for this kind of cylindrical layer heating body of water tubes leads to a particular assembly and brazing technique.

Indeed, we know the use of copper vase brazing, but as indicated above copper oxidizes quickly especially in condensates.

We also know the soft solder based on tin and lead, but these solder in addition to a limited mechanical resistance risk in case of accidental overheating of the heating body to bring about its destruction due to their low melting temperature.

Silver brazing is possibly possible but prohibitively expensive. The same is true of an electrical TIG or MIG welding under protective gas which requires torches to follow the entire contour of the assemblies to be carried out, which makes the operation very long and very expensive. As part of the manufacturing process indicated below, the present invention therefore relates to a method which avoids the aforementioned drawbacks by guaranteeing, after brazing, a rigorous sealing of the elements as well as an excellent mechanical resistance of them while reducing the cost of implementing the method.

The manufacturing process consists in putting your longitudinal tubes to the desired length, - deburring the ends of the tubes, - forming a boss on the external part and possibly internal at each end of the tubes to form supports for the annular collectors, -to fold the ends of the tubes inwards to give them a conical profile and thus facilitate their introduction into the collectors, -to arrange the tubes between the two annular collectors by introducing them into corresponding orifices, -to flare the protruding ends of the tubes to form a crimp on the annular collectors, -to simultaneously calibrate the interior of the tubes to obtain between the tubes and the collectors a minimum clearance less than 0.1 mm compatible with the solder, -to deposit in each annular collector tangentially to the ends tubes of circular brazing beads, and placing the compact assembly in a brazing oven.

According to the invention the stainless steel used for the tubes is a ferritic steel stabilized with niobium and the brazing is ensured in a vacuum furnace at a temperature less than or equal to 1000 ° C. for approximately 15 minutes using a brazing with a very high percentage of nickel and chromium plus fluxes compatible with the stainless steel used.

• - figure 1: une vue en perspective éclatée du corps de chauffe.
• - figures 2 à 5: des vues schématiques en coupe montrant les étapes du procédé d'assemblage des tubes sur les collecteurs annulaires.

Other particular characteristics and advantages of the invention will appear on reading the following description of an embodiment with reference to the appended drawings which represent:

• - Figure 1: an exploded perspective view of the heating body.
• - Figures 2 to 5: schematic sectional views showing the steps in the method of assembling the tubes on the annular manifolds.

There is shown in Figure 1 a heat exchanger body consisting of two annular manifolds 1 and 2 arranged coaxially and joined by longitudinal tubes 3 of stainless steel brazed on the manifolds and uniformly distributed between them according to the general trice a cylinder so as to form a cylindrical interior chamber 4 for the introduction of a tubular burner not shown.

These tubes 3 in which the fluid to be heated circulates have an elongated section so that the lateral faces of each tube are parallel to the lateral faces of the neighboring tubes in order to provide between them a gap 5 of small thickness intended for the passage of combustion products .

Referring to Figures 2 to 5, we will now describe the method of assembling such a heating body. After having put the tubes 3 to the desired length and deburred the ends, a boss 7 (FIG. 2) is produced by a big horn stamping process on the two external parts and at each end 6 of said tubes. These ends are folded inward to give them a conical profile (Figures 2 and 3) which facilitates their introduction into cer orifices 8 provided on each annular collector (1-2), these orifices 8 have a profile identical to that of tubes 3 so that the collectors 1 and 2 are supported on the bosses 7 of the tubes 3 (Figure 4). Then, the projecting ends 6 of the tubes 3 (figures: 4 and 5) are spread outwards to form a crimp on the annular collectors, the interior of the tubes being simultaneously calibrated in order to minimize the clearances.

This calibration must limit as much as possible the play between the tubes and the orifices cut out in the annular manifolds to allow correct brazing to be ensured. In addition, all of these operations contribute to giving the heating body sufficient rigidity for soldering, without additional assemblies which could require protection by product or special shapes to prevent the solder from leaking with the risk of this. involves brazing together the parts to be brazed and the mounting fixtures and would increase the cost of the brazing operation (greater mass to be heated-greater space requirement of the parts).

Once this assembly is achieved, is automatically deposited in each annular collector 1 and 2, around the ends 6 of the tubes 3, circular beads 9 (Figure 1) of solder generally in powder or paste. This solder must have a very high percentage of nickel and chromium with the addition of various fluxes. The actual brazing is carried out in an oven preferably under vacuum so as not to risk, in the presence of an atmosphere rich in hydrogen and nitrogen, of weakening the metal.

The exact choice of brazing and the corresponding temperature depends on the stainless steel used, a more economical ferritic steel and insensitive to stress corrosion will require a brazing temperature less than or equal to 1000 ° C for about 15 minutes to avoid any grain magnification. This choice is also a function of the specific qualities of this solder, its resilience and its flowability. Too little resilient brazing would be fragile and flowing brazing would require too little play and would risk going up between the tubes. In addition, if it flows too little, it risks giving leaky seals.

The ferritic steel of the tubes, which has the advantage of allowing the tubes to be welded at high speed by radio frequency, must, for corrosion reasons, be stabilized with niobium, the stabilization with titanium being incompatible with vacuum brazing.

Claims (1)

1. A method for the making of a heating unit for domestic gas boilers comprising a screen of longitudinal stainless steel tubes with an elongated cross section disposed along the generatrix of a cylinder between a lower annular header and an upper annular header to form an internal cylindrical combustion chamber, in accordance wherewith the tubes (3) are cut to the desired length, the ends (6) of tubes (3) are trimmed, a bulge (7) is formed on the external and possibly internal part at each end of the tubes, the ends (6) of tubes (3) are bent back towards the inside to give them a conical profile, tubes (3) are disposed between the two annular headers (1, 2) by inserting them in the corresponding openings (8), the projecting end (6) of tubes (3) are expanded towards the outside to form a crimped rim on the annular headers (1, 2), the bore of tubes (3) is simultaneously finished, circular brazing layers (9) are deposited in each annular header tangentially to the ends of the tubes and the consolidated unit is disposed without special tools inside a brazing furnace, characterised in that the stainless steel used for the tube is a niobium stabilised ferritic steel and in that the brazing is performed in a vacuum furnace at a temperature below or equal to 1000°C for approximately 15 minutes by means of a brazing metal with a very high percentage of nickel and chrome with flux additives compatible with the stainless steel used.

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