EP1561075A1 - Condensation heat exchanger with plastic casing - Google Patents

Abstract

The invention concerns a heat exchanger comprising at least one helically bound bundle (2) of tubes, whereof the thermally conductive wall has a flattened and oval cross-section, whereof the main axis is perpendicular to that (X-X') of the helix, while the width of the interstice separating two adjacent turns is small and constant, said bundle being fixedly mounted inside a casing (1) fitted with a pipe for evacuating gases generated by a burner (6), means being provided for circulating cold water inside said bundle (2), said exchanger being further designed so that the hot gases pass radially through the bundle via the insterstices of the turns. Said exchanger is characterized in that the casing (1) is made of heat-resistant plastic material and means (5; 3-30) are provided for ensuring mechanical containment of the beam in the axial direction, and for damping the thrust loads resulting from the fluid internal pressure, preventing their being transmitted to the casing (1). The inventive condensation heat exchanger is particularly designed for domestic use and is inexpensive to produce.

Description

 CONDENSING HEAT EXCHANGER

PLASTIC

The present invention relates to a condensing heat exchanger, associated - directly or indirectly - with a burner, in particular gas or oil.

This exchanger is intended in particular to equip a gas boiler for domestic applications, in order to supply a central heating circuit and / or to supply water for sanitary use.

The heat exchanger which is the subject of the invention, more specifically, is of the type comprising an envelope which delimits an enclosure inside which is housed at least one bundle of tube (s) of flattened section, of the genre described in document EP-B-0 678 186, which can be referred to if necessary.

In document EP-B-0 678 186 is described a heat exchanger element which consists of a tube of thermally good conductive material, in which a heat transfer fluid, for example water to be heated, is intended to circulate. This tube is helically wound and has a flattened and oval cross section whose major axis is substantially perpendicular to the axis of the helix, and each turn of the tube has flat faces which are spaced from the faces of the adjacent turn of a gap of constant width, this width being substantially smaller than the thickness of said cross section, the spacing between two neighboring turns being further calibrated by means of spacers, which are formed by bosses formed in the wall of the tube .

This document also describes heat exchangers comprising several elements as described above, which are arranged in different ways in the various embodiments described. An exchanger element thus designed is capable of ensuring a very efficient heat exchange between, on the one hand, very hot gases, which can be generated directly by a burner mounted in the enclosure, or come from an external source, which lick the tubular element, and, on the other hand, the fluid to be heated, such as water, which circulates inside of it. Indeed, during its passage through the gap between the turns, in an approximately radial direction, the flow of hot gases comes into contact with a relatively large surface of the wall of the exchanger element. The object of the present invention is more particularly to propose a condensing heat exchanger, of the general type described above, the heat exchange elements of which are bundles of flat tubes such as those known from EP-B- 0 678 186 mentioned above. The envelope making up the known condensing apparatuses of the kind exposed above, like the tube (s), is made of metal, generally of stainless steel.

The use of metal, and in particular stainless steel, is indeed suitable for resisting both mechanically the stresses due to the expansions occurring within the tube (s) winding and chemically to the corrosion emanating from the fumes ( burnt gases) and condensate.

In this regard, it should be noted, by way of indication, that the pressure of the fluid to be heated, and in particular of the water, inside the tube (or tubes) in use can be relatively high, of from 2.5 to 3.5 bars, i.e. 2.5.10 ⁵ to 3.5.10 ⁵ Pa.

For security reasons, the ribbon bundle is advantageously designed to be able to withstand a pressure of 4.5. 10 ⁵ Pa.

The side walls, initially flat, of the tubes tend to bulge, the amplitude of the deformation being an increasing function of the value of the internal pressure.

This deformation propagates axially, from a wall to the adjacent wall, via the bosses forming spacers which separate them.

As an indication, if we consider a winding of four juxtaposed tubes with a wall thickness of 0.6 mm, whose axial dimension is initially 128 mm, this dimension, due to the deformation of the tubes, will be brought to a value of the order of 129.2 mm for a pressure of 2 bars and of the order of 129.8 mm for a pressure of 3 bars.

The total elongation is proportional to the number of windings mounted end to end which constitute the bundle of the exchanger. Of course, by increasing the wall thickness of the tubes, the amplitude of the deformation can be reduced. Unfortunately, oversizing the thickness significantly increases the weight of the device. It also poses problems in manufacturing the tubular elements, by hydroforming, a process which requires extremely high working pressures. To oppose the elongation and resist axial thrusts resulting from the internal pressure of the fluid circulating in the bundle, the solution hitherto used is to adopt a metal envelope (serving to support the two ends of the beam), the thickness and mechanical strength of which are chosen so that they prevent the axial expansion of said beam under the effect of the internal pressure, or at least restrict it to an acceptable amplitude, compatible with the limit of elastic deformation of the envelope.

This type of exchanger is satisfactory from a technical point of view, especially in terms of performance.

However, it is relatively heavy, which can pose difficulties for the operator during its transport and its handling during its installation, and its cost price is relatively high, because it is necessary to resort ( in order to withstand the mechanical stresses and chemical attack from smoke and condensate) to an envelope made of high quality metallic material, such as stainless steel.

The basic objective of the present invention is to significantly reduce both the weight and the cost price of the device, by proposing to provide it with an envelope which, although made of substantially less noble and less material expensive, in this case the plastic, does not pose a problem of resistance of a chemical or mechanical order, in consideration of the problem of axial expansion recalled above. Another objective of the present invention is, in a variant, to ensure that the plastic envelope is insulated as best as possible from the heat generated by the burnt gases passing through the turns of the winding and, correspondingly, to substantially lower the level of temperatures to which the envelope is exposed, this by implementing simple, light and inexpensive means, in this case a ferrule playing the role of thermal shield.

The condensing heat exchanger which is the subject of the invention is intended to be associated with a gas or oil burner.

It comprises at least one bundle of tubes, which consists of a tube, or a group of tubes arranged end to end, forming a helical winding, in which the wall of the tube (s) is made of a thermally good material. conductor and has a flattened and oval cross section, the major axis of which is perpendicular, or approximately perpendicular, to that of the helix, while the width of the gap separating two adjacent turns is constant and considerably less than the thickness of said cross section, this bundle being fixedly mounted inside a gas-impermeable envelope, means being provided for circulating a fluid to be heated, in particular cold water, inside the constituent tube (s) of said bundle, this envelope having a flue gas discharge sleeve, this exchanger being so arranged that the hot gases generated by the burner pass radially, or approximately radially, said beam passing through the interstices separating its turns. According to the invention:

on the one hand, said envelope is made of heat-resistant plastic and:

- on the other hand, the exchanger includes means for mechanically restraining said bundle in its axial direction, capable of absorbing the thrust forces resulting from the internal pressure of the fluid which circulates therein and which tends to deform the walls thereof, avoiding that these efforts are not transmitted to the envelope.

The two roles hitherto assigned to the envelope are thus dissociated, namely serving as an enclosure for the circulation and evacuation of hot gases, as well as for the collection and evacuation of condensates, and, on the other hand , ensure the mechanical strength of the tube bundle.

Furthermore, according to a certain number of advantageous, but not limiting, characteristics of the invention:

- The exchanger includes a temperature probe carried by said casing, capable of controlling the shutdown of the burner when the temperature prevailing inside the casing, in the vicinity of this probe, exceeds a predetermined threshold;

- Said restraining means comprise a set of tie rods which extend outside the bundle, parallel to the axis of the propeller, and the ends of which are secured to support elements applying against the two faces beam opposites;

- The support element located at one end of the set of tie rods is a thin plate, for example disc-shaped, which is perforated in the central part, therefore annular in shape;

- Said plate acts as a facade, which partially seals an open face of the envelope, and is fixed to the latter at its periphery, for example by crimping;

- The end portions of the tie rods pass through said facade so as to project slightly outwards and these end portions are threaded so that they allow a removable mounting of a door against the facade by means of nuts;

- Said door is integral with the burner; - said tie rods are four in number, substantially arranged in a square, and the support elements located on the side opposite to said facade consist of a pair of arched or bent flanges, shaped to closely match the outline of the beam in s 'applying against two diametrically opposite zones thereof, each flange being fixed to a pair of neighboring tie rods;

- The plastic material constituting the envelope is a composite material based on resin loaded with fibers or glass flakes;

- Said resin is a compound of polyphenilene oxide, polystyrene and polypropylene; the exchanger comprises two bundles of coaxial tubes, located end to end, and connected to each other, one of which acts as a primary exchanger and the other as a secondary exchanger, a deflector member being interposed between these two bundles, and thus arranged, that the hot gases generated by the burner first pass through the primary exchanger, crossing the interstices separating its turns from the inside to the outside, then the secondary exchanger, passing through the interstices separating its turns from the outside to the inside;

- The deflector is integral with said bundles of tubes;

the burner being mounted inside the bundle which acts as a primary exchanger, said deflector has a discoid shape and is integral with the end of the burner, this deflector being provided at its periphery with a thermally insulating seal which s 'applies against the inside of the beam;

- Said envelope consists of two molded half shells joined and secured to each other, for example by welding;

the exchanger comprises a ferrule disposed outside the bundle of tube (s) and inside said plastic casing, this ferrule providing a heat shield function capable of isolating the latter from the heat emitted by burnt gases;

- This ferrule is made of a thin stainless steel sheet; - The ferrule is pressed against the internal surface of the plastic envelope, but is kept at a certain distance from the latter, for example by means of a series of bosses stamped in the wall of the ferrule;

- The ferrule is made up of two complementary curved parts joined against one another so as to form an annular envelope which fits against the internal surface of said plastic envelope; - the facing edges of said curved parts have a row of approximately semi-circular, or semi-ovalized notches, capable of enclosing the rectilinear end portions of the tube, or of the constituent tubes) of the winding, when these parts curved are joined against each other. Other characteristics and advantages of the invention will emerge from the description and the appended drawings which represent, by way of non-limiting examples, possible embodiments. In these drawings:

- Figure 1 is a schematic front view of a first embodiment of the invention, cut by the vertical plane referenced I-I in Figure 2;

- Figure 2 is a schematic left view of the apparatus of Figure 1;

- Figures 3 and 4 are views similar to Figures 1 and 2 respectively, showing the bundle of tubes and its restraining means only;

- Figure 5 is a view similar to Figure 1, showing a second possible embodiment of the exchanger whose axial size is smaller;

- Figure 6 is a side view of the exchanger of Figure 5, illustrating the beam containment mode which is implemented there;

- Figure 7 shows these compression means seen from the front, and schematically;

- Figure 8 is a detail view showing a possible variant of the temperature detector capable of being implemented, replacing that illustrated in Figure 5;

- Figure 9 illustrates the operation of the apparatus of Figure 5;

- Figures 10, 11 and 12 are views similar, respectively, to those of Figures 1, 2 and 3, showing a third embodiment of an exchanger according to the invention, devoid of burner; - Figures 13 and 14 are schematic views, respectively from the front and side of an exchanger according to the invention, cut by a vertical plane passing through the axis of the winding, this exchanger being similar to the embodiment of Figure 5, but having a ferrule providing a heat shield function; - Figures 15 and 16 show, still schematically, the two elements (not yet bent) in the form of bands, constituting the ferrule.

The exchanger shown in FIGS. 1 and 2 comprises a shell, or envelope, 1 which delimits an enclosure inside which is fixedly fixed a tubular bundle 2, which consists of a helical winding, of axis XX 'of a group of tubes arranged end to end and connected in series.

They are tubes of flattened cross section, the long sides of which are perpendicular to the axis XX '. Bosses 200 provided on the large faces of the tubes act as spacers, making it possible to delimit between each turn an interstice of calibrated value, substantially constant.

This winding is intended to be traversed internally by the fluid to be heated, which is for example water. In the illustrated embodiment, three contiguous helical tubular elements are provided, connected in series, in which the fluid to be heated circulates from left to right.

Collectors 15, 16, which are fixed to the casing 1 allow the device to be connected, in a conventional manner, to a pipe for supplying the cold fluid, which must be heated, and for discharging the hot fluid.

These collectors also transfer the circulating fluid from a tubular element to the neighboring winding.

Each tubular element has straight end portions, that is to say with a rectilinear axis, and of progressively variable cross section, the through end portion of which is circular.

In the example illustrated in Figure 2, the two end portions are arranged in parallel and located on the same side of the winding.

It can be noted that a similar arrangement is also provided for the third embodiment illustrated in FIGS. 10 and 11. On the contrary, for the second embodiment of the invention illustrated in FIGS. 5 and 6, the two portions d end of a tubular winding extend in the same plane, their mouths being directed opposite one another, according to an arrangement conforming to that illustrated in FIG. 24 of the European patent 0 678 186 already cited . The inlet and outlet mouths 20, 21 of the tubular elements are suitably crimped and sealed in ad hoc openings provided in the envelope 1, as can be seen in FIG. 2; the collectors 15, 16 are fixed at this level.

According to an essential characteristic of the invention, the envelope 1 is made of plastic. It is for example obtained by rotational molding or injection molding.

The envelope is made of two half shells which are heat sealed to each other after the tube bundle has been installed inside one of them. The envelope 1 is open on one of its sides, in this case on the side located on the left, if we consider FIG. 1.

During use of the device, part of the water vapor contained in the burnt gases condenses on contact with the walls of the tubes.

The reference 10 designates the bottom wall of the enclosure; in known manner, this bottom is sloping, which allows the evacuation of the condensates towards an outlet orifice 13.

The rear wall of the envelope bears the reference 11; this has a recess 110 which, as will be seen below, forms a channel allowing the passage of the burnt gases and the fumes and channeling them towards an evacuation sleeve 12.

Of course, the orifice 13 is connected to a condensate discharge pipe, while the cuff 12 is connected to a smoke discharge pipe, for example a chimney pipe. These conduits are not shown in the figures. The open side of the envelope is closed by a facade element 3.

The latter is fixed over its entire periphery by a flange 30 which is crimped hermetically to gases on a peripheral bead 14 bordering the inlet of the envelope. A seal, for example made of silicone (not shown) can advantageously be provided at this level. The front plate 3, which is for example made of stainless steel, is normally closed by a removable door 4.

In the embodiment shown, the door 4 is in two parts; it is composed of an external plate 40, made of metal or of heat-resistant plastic, and an internal plate 41 made of insulating material, for example based on ceramic. These two plates are traversed in the central part by an opening which is crossed by a burner 6, for example a gas burner, which is secured to the door 4 by means not shown.

Appropriate means connected to the burner 6 make it possible to supply the device with a mixture of gas fuel and air, such as propane + air.

These means may consist in particular of a fan fixed on the door, capable of blowing the gas mixture into the burner, or of a flexible duct connected to the door.

The burner 6 is a cylindrical tube with a closed end, the wall of which is pierced with a multitude of small holes which allow the passage of the combustible mixture, radially towards the outside of the tube.

The outer surface of this wall constitutes the combustion surface. An ignition system of known type, not shown, comprising for example an electrode generating a spark, is obviously associated with the burner.

The latter is located coaxially in the middle of the winding 2, but it does not extend over its entire length.

Indeed, the tube bundle 2 is subdivided into two parts, one 2a located to the left of a deflector 7, and the other 2b located to the right of it. The deflector 7 is a disc made of thermally insulating material, for example based on ceramic; it is carried by a thin plate-shaped frame 70, of stainless steel, the peripheral edge of which is inserted between two adjacent turns of the bundle.

We are dealing here with a double exchanger, as shown in Figure 8 of the aforementioned European patent, which provides excellent performance.

Part 2b of the beam preheats the fluid, which flows from right to left if we consider Figure 1. Part 2a performs the actual heating.

According to an essential characteristic of the invention, the turns of the tube bundle 2 are firmly held applied against one another by means of a mechanical restraint system.

It is, in this case, a set of four tie rods 5, constituted by cylindrical rods of stainless steel, and which are associated with support elements for each of the two opposite ends of the bundle. As seen in Figure 2, the tie rods 5 are arranged at the four vertices of a fictitious isosceles trapezoid. On one side (on the right of Figures 1 and 3), their end 51 is fixed - for example by welding - to a discoid annular plate 30, made of stainless steel, in the center of which is formed an opening 300.

On the opposite side, which corresponds to the left of Figures 1 and 3, the tie rods 5 are fixed to the front 3 which has been mentioned above.

On this side, the end portions of the tie rods 5 are threaded; they pass through suitable orifices provided at the periphery of the front plate 3.

Nuts 500 screwed onto these threaded portions 50 ensure the tensioning of the tie rods, so as to apply with force (from right to left) the plate 30 against the last turn of the bundle 2 and, correspondingly, (in the opposite direction) the facade 3 against the first turn of this beam.

The bundle 2 is thus axially compressed with force between the support elements 3 and 30.

Note that the end portions 50 are relatively long; they project beyond the nuts 500 over a considerable length, as can be seen in FIG. 3.

In fact, the portions 50 also have the function of ensuring the centering and fixing of the door 4 against the facade 3.

To this end, the plate 40 constituting the door, the diameter of which is greater than that of the insulating part 41, is traversed by four holes allowing the engagement of the portions 50.

The fixing is ensured by nuts 400, which are advantageously self-locking nuts, to reduce the risk of untimely loosening, in particular under the effect of vibrations. An annular lip seal 42 housed in an appropriate groove made in the plate 40 allows the latter to be applied in a smoke-tight manner against the external face of the facade 3.

As can be seen in FIG. 2, the tie rods 5 are arranged outside of the bundle 2. On observation of FIG. 3, it is clear that the assembly constituted by the facade 3, the tie rods 5 and the elements end support 3, 30, constitutes an autonomous assembly.

The expansions which tend to occur under the effect of the internal pressure prevailing in the tube of the winding 2 are thwarted by the tie rods and the support elements which fully absorb the forces of the axial thrust. There is no repercussion of this push against the wall of the envelope containing this assembly.

The tube bundle can be held in place inside the envelope simply as a result of the interlocking connection of the end portions of the tubes 20, 21 in the housings provided in the envelope to receive them.

It will also be noted that a deflecting partition 8 is provided above the rear zone of the winding 2, which partially covers the rear annular plate 30, up to its central opening 300. This partition advantageously contributes to the good maintenance of the beam inside the envelope.

It is fixed to the internal wall of the envelope and extends obliquely under the cuff 12. It preferably has an arcuate shape, of contour in an arc of a circle, surrounding the upper zone of the bundle. The hot gases generated by the burner 6 first pass through the first part 2a of the bundle 2 (located on the left of the deflector 7), passing between the interstices of the tubes radially, from the inside to the outside.

Thanks to the presence of the partition 8, they cannot escape immediately through the cuff 12. They must pass through the rear part 2b of the exchanger (located on the right of the deflector plate 7), this time from the outside. inward, preheating the water flowing in the tube bundle.

Finally, the cooled gases escape via the rear channel delimited by the wall 110, to join the evacuation sleeve 12. The plastic material constituting the envelope is chosen to continuously resist temperatures of the order of 150 ° to 160 ° C.

It is advantageously a composite material based on resin loaded with fibers or glass flakes.

As a particularly suitable type of resin, mention may be made of a polyphenilene oxide, polystyrene and polypropylene compound, such a material being suitable for resisting chemical attack by hot smoke and condensates.

The wall of the envelope 1 can be relatively thin, for example with a thickness of between 2 and 4 mm, because it is not exposed to significant mechanical stresses. For maintenance purposes, it is easy to access the interior of the front part of the exchanger, which is the only one actually exposed to fouling due to smoke; it suffices to unscrew the nuts 400 and axially remove the assembly constituted by the door 4 and the burner 6 which is integral therewith. After cleaning, the installation of this set is just as easy.

These disassembly and reassembly operations have no effect on the compression operated by the tie rods 5, which remain active despite the temporary removal of the door. In an alternative embodiment of this device, it would be possible to fix the discoid deflector 7 at the end of the burner 6.

In this case, the door 4, the burner 6 and the deflector 7 would form a dismountable unit in block, which would allow access for cleaning to the entire interior space of the winding, including in the portion rear which provides preheating.

Of course, in this hypothesis, it would be necessary to provide all around the deflector disc 7 an annular seal, highly heat resistant, coming to rest against the internal surface of the bundle to avoid the direct passage of gases at this level, towards part 2b. In the second embodiment of the invention which is illustrated in FIGS. 5 to 1, there is a configuration similar to that which has just been described, the apparatus however being turned 180 degrees (front panel located on the right of Figure 5).

Elements identical or similar to those of the first embodiment have been assigned the same reference numbers, and no further explanation will be given as to their nature and their function.

It will be noted that this exchanger has a greater axial compactness than that of the first embodiment.

As already said, the rectilinear end portions of the tubes extend tangentially to the winding, their axes being contained in the same longitudinal plane, arranged laterally (see FIG. 6).

Furthermore, on the side opposite to the facade 3, the tie rods 5 are fixed not to an annular plate 30, but a pair of bent flat rods 30a, 30b, the central zones of which bear against an angular sector, of relatively large area. limited, of the corresponding end turn. As seen in Figure 6, the tie rods are this time arranged in a square, and the bent rods 30a, 30b connect these sides two by two, closely matching two diametrically opposite areas of the winding.

Note (see Figure 5) that the partition 8 has a recess 80 located above the tubular winding, near the tubes located at the outlet of the part 2a constituting the main exchanger.

In this recess is mounted a temperature probe 9.

It is a thermal circuit breaker, which is mounted in a sealed manner with respect to the casing. To this end, the probe 9 is advantageously held in place by means of a circlip in a stainless steel bowl fitted into the recess 80, which is open at the bottom, an appropriate seal ensuring the seal between the bowl and the wall. of the recess 80.

This probe is connected to the burner control, and is adapted to cause the burner to stop when the detected temperature exceeds a predetermined threshold, for example 160 ° C.

Abnormal overheating can occur accidentally, for example in the absence of water in the tubes or in the event of poor circulation of water in the tubes, for example due to blockage of one of them.

In the absence of any security, there would be the risk of a very significant rise in the temperature of the fumes leaving the tubes placed around the burner, and which come into contact with the interior of the plastic casing. In fact, the fumes would no longer sufficiently transmit their heat to the tubes.

There could then be a problem with the mechanical strength of the plastic and serious deterioration, or even an inflammation of the envelope.

In the variant illustrated in FIG. 8, the probe, referenced 9 ', includes a fuse element 92', sensitive to heat.

The electric supply circuit of the boiler is connected to two terminals 90 'and 91' which are connected via this thermo-fuse 92 '. In the event of an abnormal rise in temperature, for example beyond 160 ° C, the melting of this element 92 'breaks the electrical circuit between the two lugs 91', 90 ', causing the burner control to stop .

FIG. 9 illustrates the circulation of the hot gases generated by the burner 6, which is supplied with fuel mixture G + A. After lighting the burner, it generates hot gases, for example at a temperature of 1000 ° C, which propagates radially outwards as symbolized by the arrows F_.

These hot gases pass through the interstices of the first part of the exchanger 2a, radially, from the inside to the outside (arrows F ₂ ).

During this passage, a large part of the heat of the hot gases is transmitted via the wall of the tubes to the water which circulates there, so that the temperature of the hot gases at the outlet of the beam part 2b is -to indicative - of the order of 110 to 140 ° C. It will be noted that the presence of the deflector 6 prevents the axial escape of the burning gases Fi.

The partially cooled gases then pass through the second part 2b of the exchanger, this time from the outside to the inside, as symbolized by the arrows F ₃ . An additional part of the heat is thus transmitted to the water circulating in the tubes. The temperature of the gases escaping from the device (arrows F ₄ and F ₅ ), for information is of the order of 65 to 70 ° C.

As for water, it is generally warmed from room temperature to a temperature of the order of 80 ° C. Of course, the flow of water takes place against the flow of smoke, preheating taking place in the area 2b of the exchanger and the actual heating in zone 2a.

In the embodiment which is represented in FIGS. 10 to 12, the exchanger does not have a burner. The envelope includes a sleeve E for admission of hot gases, which come from an external source.

This cuff opens out inside the tube winding 2.

This is a provision similar to that which is the subject of Figure 19 of the aforementioned European patent. The same reference numbers were used to designate elements identical to those of the first embodiment, possibly assigned the "prime" index when the elements are similar but not identical.

This is a simple exchanger (without preheating).

The hot gases which penetrate into the interior enclosure of the envelope, via the sleeve E, escape radially from the interior towards the outside of the tubular bundle 2, heating the fluid which circulates therein; the cooled gases escape through the sleeve 12.

The tubular elements constituting the winding can be arranged in parallel, the inlet and outlet manifolds 15 'and 16' respectively ensuring their collection and their distribution either at the inlet or at the outlet of the tubes.

The envelope is made of plastic.

The means of mechanical containment of the bundle are similar to those of the first embodiment.

They include a set of four tie rods which are fixed at their ends, for example by welding, to plates 30, 3 '.

The plate 30 located on the side of the intake cuff E is a disc whose center has an opening 300 coming in correspondence with the gas inlet passage delimited by the cuff E.

The bottom plate 3 'is a non-perforated disc. It closes the rear part of the winding, forcing all of the hot gases to exit through the interstices of the turns.

To prevent the bottom wall of the envelope located opposite the plate 3 ', which is exposed to hot gases, a clearance j is provided between these two elements. Of course, this device can also be equipped with a temperature probe adapted to stop the admission of hot gases when the probe detects a predetermined excessive temperature.

To return to the first two embodiments, it should be noted that the burner used is not necessarily cylindrical in shape; it could have a flat or hemispherical shape while remaining integral with the door.

The weight gain obtained by the use of a plastic envelope is of the order of 20% compared to a similar device, having the same performance, but whose envelope is metallic. The variant exchanger illustrated in Figures 13 and 14 is similar in structure to that already described with reference to Figures 5 to 7 and this is why this structure will not be described again here.

However, as will be explained, it has a ferrule which acts as a heat shield. Indeed, the annular wall portion of the envelope 1 which surrounds the winding 2 is internally lined with a ferrule 100. This is made in thin sheet of stainless steel, the thickness of which is for example of the order of about 0.3 to 0.4 mm.

This ferrule bears against the internal face of the envelope, with a certain spacing j (see FIG. 13), of the order of 2 mm for example. This spacing is ensured by a plurality of support pads 101 constituted by small-sized cups stamped in the sheet so as to form bosses projecting towards the outside of the shell. As shown in Figures 15 and 16 which show a developed sheet in two constituent parts of the shell, these bosses 101 have a regular geometric distribution in the surface of the sheet, in this case in an arrangement according to equal equilateral triangles .

The spacing j and the presence of the bosses 101, the support of which against the casing 1 is done by zones of very small surface - almost pointwise - makes it possible to considerably reduce the transmission of the heat absorbed by the ferrule 100 to the wall which surrounds it.

At its ends, this ferrule bears, on the front side against the front 3, and on the other side against the partitions 8-8 '.

Its axial length, which corresponds substantially to that of the winding 2, is referenced K in FIG. 13. In the illustrated embodiment, the ferrule 100 consists of two separate parts, initially planar, shown in FIGS. 15 and 16 , and referenced 100a, respectively 100b.

These are strips of stainless steel sheet of width K and length L_, respectively L ₂ . On its longitudinal edges, each of the strips 100a, 100b, has a series of four notches 102, of substantially semi-circular or semi-oval shape, complementary to the shape of the section of the end portions of the tubes at the level of the wall 1 they are crossing.

The length ι of the strip 100a is notably greater than that L ₂ of the strip 100b.

The sum Lt + L ₂ corresponds approximately (taking into account the spacing j) to the circumference of the internal wall of the casing 1 against which the strips 100a and 100b come to be applied after being bent to accommodate the curvature of the wall of the envelope 1. As can be seen in FIG. 14, this has a cross section whose contour is intermediate between a circle and a square with rounded angles. The short element 100b is placed on the side where the mouths 20 ', 21' of the tubes are located, outside of the latter (on the left of FIG. 14), while the long element 100a is placed from the 'other side.

They are joined by their longitudinal edges (parallel to X-X ') and enclose with little play by their notches 102 - suitably shaped and positioned for this purpose - the end portions, or mouths, of the tubes constituting the winding 2 .

Because of their elasticity, the two sheet metal strips are applied intimately, by means of their bosses 101 against the internal face of the envelope, without the need to resort to specific fixing means. They thus form a ferrule which insulates in a relatively leaktight manner said internal face of the envelope from the hot gases circulating in the exchanger, playing the role of a thermal or isofhermic screen.

In the case where, as in the embodiment illustrated in FIG. 13, the wall of the envelope 1 has a recess 80 directed inwards, which receives a temperature probe 9, it goes without saying that the ferrule is crossing in this zone of a suitable opening into which the recessed wall portion is inserted. In this zone, the wall of the envelope, not thermally protected, is therefore exposed to a temperature higher than that of the rest of the wall, which is protected by the shell.

In practice this does not pose any difficulty since this zone has a very limited surface, and the excess heat which appears there is evacuated by thermal transfer towards the region of the neighboring wall, which is less hot.

The presence of the ferrule has the effect of lowering the temperature to which the wall of the envelope is exposed by a value of the order of 15 to 20 ° C., which makes it possible to use less plastic noble and therefore less costly than that which can be used with the embodiments previously described (devoid of ferrule), and / or to improve its resistance over time as well as its longevity.

Claims

1. Condensing heat exchanger, associated with a gas or fuel burner (6), which comprises at least one bundle (2) of tubes, which consists of a tube, or a group of tubes arranged end to end, forming a helical winding, in which the wall of the tube (s) is made of a thermally good conductive material and has a flattened and oval cross section, the major axis of which is perpendicular, or approximately perpendicular, to that (X-X ') of the propeller, while the width of the gap separating two adjacent turns is constant and appreciably smaller than the thickness of said straight section, this bundle being fixedly mounted inside an envelope (1) impermeable to gases, means being provided for circulating a fluid to be heated, in particular cold water, inside the constituent tube (s) of said bundle (2), this envelope (1) having a flue gas evacuation sleeve (12), t exchanger being so arranged that the hot gases generated by the burner (6) pass radially, or approximately radially, said bundle passing through the interstices separating its turns, characterized in that, on the one hand, said envelope (1 ) is made of heat-resistant plastic and, on the other hand, it includes mechanical restraint means (5; 3-30) of said bundle in its axial direction, capable of absorbing the thrust forces resulting from the internal pressure of the fluid which circulates therein and which tends to deform the walls thereof, preventing these forces from being transmitted to the envelope ( 1).

2. Exchanger according to claim 1, characterized in that it comprises a temperature probe (9; 9 ') carried by said casing (1), capable of controlling the shutdown of the burner when the temperature prevailing inside of the envelope, in the vicinity of this probe, exceeds a predetermined threshold.

3. Exchanger according to claim 1 or 2, characterized in that said means of restraint comprises a set of tie rods (5) which extend outside the bundle (2), parallel to the axis (X-X ') of the propeller, and the ends of which are integral with support elements (3, 30) applying against the two opposite faces of the bundle.

4. Exchanger according to claim 3, characterized in that the support element (3, 30) located at one end of the set of tie rods is a thin plate, for example disc-shaped, which is perforated partly central, therefore annular in shape.

5. Exchanger according to claim 4, characterized in that said plate (3) acts as a facade, which partially seals an open face of the envelope, and is fixed to the latter at its periphery, for example by crimping.

6. Exchanger according to claim 5, characterized in that the end portions (50) of the tie rods pass through said facade (3) so as to project slightly outwards and that these end portions (50) are threaded so that they allow a removable mounting of a door (4) against the facade by means of nuts (400).

7. Exchanger according to claim 6, characterized in that said door (4) is integral with the burner (6).

8. Exchanger according to one of claims 4 to 7, characterized in that said tie rods (5) are four in number, substantially arranged in a square, and that the support elements located on the side opposite to said facade consist in a pair of arched or bent flanges (30a, 30b), shaped to closely match the outline of the beam (2) by applying against two diametrically opposite zones thereof, each flange (30a ₅ 30b) being fixed to a pair of neighboring tie rods (5).

9. Exchanger according to one of claims 1 to 8, characterized in that the plastic material constituting the envelope (1) is a composite material based on resin loaded with fibers or glass flakes.

10. Exchanger according to claim 9, characterized in that said resin is a compound of polyphenilene oxide, polystyrene and polypropylene.

11. Exchanger according to one of claims 1 to 10, characterized in that it comprises two bundles of coaxial tubes (2a, 2b) located end to end, and connected to each other, one of which acts as a primary exchanger and the other as a secondary exchanger, a deflector member (7) being interposed between these two bundles, and thus arranged, that the hot gases generated by the burner first pass through the primary exchanger (2a ), crossing the interstices separating its turns from the inside to the outside, then the secondary exchanger (2b), crossing the interstices separating its turns from the outside to the inside, after which they are evacuated via said cuff (12) ..

12. Exchanger according to claim 11, characterized in that said deflector (7) is integral with said bundles of tubes (2a, 2b).

13. Exchanger according to claim 11, characterized in that, the burner (6) being mounted inside said bundle acting as exchanger primary (2a), said deflector (7) has a discoid shape and is integral with the end of this burner, this deflector being provided at its periphery with a thermally insulating seal which is applied against the interior of the bundle.

14. Exchanger according to one of claims 1 to 13, characterized in that said envelope (1) consists of two molded half shells joined and secured to one another, for example by welding.

15. Exchanger according to one of claims 1 to 14, characterized in that it comprises a ferrule (100) disposed outside of said bundle (2) and inside of said envelope (1) made of plastic material , this ferrule (100) providing a heat shield function capable of isolating the latter from the heat emitted by the burnt gases.

16. Exchanger according to claim 15, characterized in that said ferrule (100) is made of a thin stainless steel sheet.

17. Exchanger according to one of claims 15 or 16, characterized in that said ferrule (100) is pressed against the internal surface of said plastic casing (1), but is kept at a certain distance from the latter, for example by means of a series of bosses (101) stamped in the wall of the ferrule (100).

18. Exchanger according to one of claims 15 to 17, characterized in that said ferrule (100) consists of two complementary curved parts (100a, 100b) joined against each other so as to form an annular envelope fitting against the internal surface of said plastic casing (1).

19. Exchanger according to claim 18, characterized in that the facing edges of said curved parts (100a, 100b) have a row of notches (102), approximately semi-circular, or semi-oval, capable of enclosing the portions rectilinear end of the tube, or tubes, constituting) of the winding, when these curved parts (100a, 100b) are joined against each other.