FR2846075A1 - Condensation heat exchanger for domestic application, has bundles of tubes mounted inside casing made of heat-resistant plastic material, containment mechanism to ensure mechanical containment of bundle for absorbing thrust loads - Google Patents

Abstract

The heat exchanger has a helically bound bundle (2) of tubes fixedly mounted inside a casing fitted with a pipe for evacuating gases generated by a burner (6). The casing (1) is made of a heat-resistant plastic material. A containment mechanism comprising a tie rod (5) and an annular plate (30) ensures mechanical containment of the bundle in an axial direction to absorb thrust loads resulting from fluid internal pressure.

Description

The present invention relates to a condensing heat exchanger,

  associated - directly or indirectly - with a burner, particularly

gas or fuel.

  This exchanger is intended in particular to equip a gas boiler 5 for domestic applications, for supplying a central heating circuit and / or 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 tube bundle (s) of flattened section, the The type described in EP-B-0 678 186, to which reference may be made if necessary.

  EP-B-0 678 186 discloses a heat exchanger element which consists of a tube of thermally good conducting material, in which a coolant, 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 plane faces which are spaced from the faces of the adjacent turn of the tube. a gap of constant width, this width being substantially smaller than the thickness of said cross section, the spacing between two adjacent turns being further calibrated by means of spacers, which consist of

  by bosses formed in the wall of the tube.

  This document also discloses heat exchangers having a plurality of elements as described above, which are arranged in different ways in the various disclosed embodiments.

  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, secondly, the fluid to be heated, such as water, which circulates inside thereof.

  Indeed, during its passage through the gap between the turns, in an approximately radial direction, the flow of hot gas comes into contact with a relatively large area of the wall of the exchanger element.

  The present invention more particularly aims to provide a condensing heat exchanger, of the general type described above, whose heat exchange elements are bundles of flat tubes such as those

  known from the aforementioned EP-B-0 678 186.

  The envelope constituting the known condensing apparatus of the kind described above, like the tube (s), is made of metal, generally made of stainless steel. The use of metal, and in particular of stainless steel, is indeed suitable for resisting both mechanically the stresses due to the expansions occurring within the winding of tube (s) and chemically to corrosion.

  emanating from fumes (gas burned) and condensates.

  In this respect, it should be pointed out, as an 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, in the order of 2.5 to 3.5 bar, ie 2.5 × 10 5 to 3.5 × 10 5 Pa.

  For safety reasons, the tubular bundle is advantageously designed to withstand a pressure of 4.5. 105 Pa.

  The initially planar side walls of the tubes tend to bulge, the magnitude 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, through the bosses forming spacers that 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, as a result of 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 bar.

  The total elongation is proportional to the number of windings mounted end to end which constitute the beam of the exchanger.

  Of course, by increasing the wall thickness of the tubes it is possible to reduce the amplitude of the deformation. Unfortunately, oversizing the thickness significantly increases the weight of the apparatus. It also poses manufacturing problems of the tubular elements, by hydroforming,

  process that requires extremely high working pressures.

  To oppose the elongation and withstand the axial thrust resulting from the internal pressure of the fluid flowing in the beam, the solution so far used is to adopt a metal casing (serving as support for the two ends of the beam), whose thickness and mechanical strength are chosen such that they prevent the axial expansion of said bundle under the effect of the internal pressure, or at least limit it to an acceptable amplitude, compatible with the limit of elastic deformation of the 'envelope.

  This type of exchanger gives satisfaction on the technical level, especially in terms of performance.

  However, it is relatively heavy, which can pose difficulties for the operator during its transport and handling during its installation, 10 and its cost price is relatively high, because it is necessary to resort (in order to withstand the mechanical stresses and chemical attacks of fumes and condensates) to a high-quality metallic material shell, such as

stainless steel.

  The objective underlying the present invention is to reduce substantially both the weight and the cost price of the apparatus, proposing to provide it with an envelope which, although in a substantially less noble material and less expensive, in this case the plastic material, does not pose a problem of resistance of a chemical or mechanical order, in consideration of the dilation problem

axial recalled above.

  The condensing heat exchanger which is the subject of the invention

  is intended to be associated with a gas or fuel 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 material. good conductor and has a flattened and oval cross section, whose major axis is perpendicular, or approximately perpendicular to that of the helix, while the width of the gap separating two adjacent turns is constant and significantly lower than the thickness of said cross section, this beam being fixedly mounted inside a gas-impermeable envelope, means 30 being provided for circulating a fluid to be heated, in particular cold water, inside ( s) tube (s) constituting (s) said bundle, this envelope having a sheath for evacuation of the gas burnt, this exchanger being so arranged that the hot gases generated by the burner cross radially, or appr 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 material and:

  on the other hand, the exchanger comprises means for mechanically retaining said beam in its axial direction, capable of absorbing the thrust forces resulting from the internal pressure of the fluid circulating therein and which tends to deform the walls thereof, avoiding that these efforts are transmitted to the envelope.

  Thus, the two roles hitherto devolved to the envelope are dissociated, that is to say serve as an enclosure for the circulation and the evacuation of hot gases, as well as for the collection and evacuation of condensates, and, on the other hand ensure the mechanical holding of the tube bundle.

  Furthermore, according to a certain number of advantageous, but not limiting, features of the invention: The exchanger comprises a temperature probe carried by said envelope, able to control the shutdown of the burner when the temperature prevailing within the envelope, in the vicinity of this probe, exceeds a predetermined threshold.

  Said restraining means comprise a set of tie rods which extend outside the beam, parallel to the axis of the helix, and whose ends are integral with support elements applying against the two faces 20 opposite of the beam.

  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, of

annular form therefore.

  Said plate serves as a facade, which partially closes an open face of the envelope, and is fixed thereto at its periphery, for example by crimping.

  The end portions of the tie rods pass through said facade so as to protrude slightly outwardly and these end portions are threaded such that they permit removable mounting of a door against the facade by means of nuts.

  Said door is integral with the burner. The said tie rods are four in number, substantially arranged in a square, and the support elements situated on the opposite side to the said facade consist of a pair of arcuate or bent flanges, shaped to conform as closely as possible to the contour of the beam. applying against two diametrically opposite zones of

  this one, each flange being attached to a pair of adjacent tie rods.

  The constituent plastic material of the envelope is a composite material based on resin loaded with fibers or glass flakes.

  The resin is a compound of polyphenylene 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 secondary exchanger, a deflector member being interposed between these two beams, and thus arranged, that the hot gases generated by the burner first pass through the primary exchanger, through the interstices separating its turns from the inside to the outside, and then the secondary exchanger, crossing the interstices

  separating his turns from outside to inside.

  The baffle is integral with said bundles of tubes. Since the burner is mounted inside the beam which acts as a primary heat exchanger, said baffle has a discoid shape and is integral with the end of the burner, this baffle being lined at its periphery with a gasket.

  thermally insulating which is applied against the inside of the beam.

  Said envelope consists of two molded half-shells contiguous and secured to one another, for example by welding.

  Other features and advantages of the invention will become apparent from the description and the accompanying drawings which represent, by way of examples only.

  non-limiting of the 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 tube bundle 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 lower; FIG. 6 is a side view of the exchanger of FIG. 5, illustrating the mode of contention of the beam which is implemented therein; - Figure 7 shows these contention means seen from the front, and schematically; FIG. 8 is a detailed view showing a possible variant of the temperature detector that can be implemented, replacing that shown in FIG. 5; FIG. 9 illustrates the operation of the apparatus of FIG. 5; FIGS. 10, 11 and 12 are views similar to those of FIGS. 1, 2 and 3, respectively, showing a third embodiment of a

  exchanger according to the invention, without a burner.

  The exchanger shown in FIGS. 1 and 2 comprises a shell, or envelope, 1 which delimits an enclosure inside which is fixedly mounted a tubular bundle 2, which consists of a helicoidal winding, of axis X-X '

  a group of tubes arranged end to end and connected in series.

  They are straight flattened tubes whose long sides are perpendicular to the X-X 'axis.

  Bosses 200 provided on the large faces of the tubes act as spacers, delimiting between each turn a valuable gap.

calibrated, 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 helicoidal tubular elements connected in series are provided in which the fluid to be heated

  flows from left to right.

  Collectors 15, 16, which are fixed to the casing 1 allow the connection of the apparatus, in a conventional manner, to a conduit for supplying the fluid

  cold, which must be warmed up, and discharge of the hot fluid.

  These collectors also ensure the transfer of fluid into

  circulation, from a tubular element to the neighboring winding.

  Each tubular element has straight end portions, that is to say of rectilinear axis, and of progressively variable section, the part of which

  end-point is circular.

  In the example illustrated in FIG. 2, the two end portions

  are arranged parallel and located on the same side of the winding.

  It may 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 end portions of a tubular winding extend in the same plane, their mouths being directed in the opposite direction. one of the other, according to a layout conforming to that illustrated in the figure

  24 of 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 5 provided in the casing 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 roto-molding or injection molding.

  The envelope is made of two half-shells which are heat-sealed to each other after the tubular bundle has been installed inside one of them.

  The envelope 1 is open on one of its sides, in this case on the side to the left, if we consider FIG.

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

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

  The rear wall of the casing has the reference 11; it has a recess 110 which, as will be seen later, forms a channel allowing the passage of the burnt gases and fumes and channeling them to a cuff

evacuation 12.

  Of course, the orifice 13 is connected to a condensate discharge pipe, while the sleeve 12 is connected to a flue gas discharge duct, for example a chimney flue. These conduits are not represented

in the figures.

  The open side of the envelope is closed by a front element 3. The latter is fixed on its entire periphery by a flange 30 which is crimped

  hermetic gas to a peripheral bead 14 bordering the entrance of the envelope.

  A seal, for example 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 heat-resistant plastic material, and an inner plate 41 of insulating material, for example to

ceramic base.

  These two plates are traversed in central part by an opening which is traversed by a burner 6, for example a gas burner, which is secured

  with the door 4 by means not shown.

  Suitable means connected to the burner 6 make it possible to supply the apparatus with a mixture of gas fuel and air, such as propane + air. These means may consist in particular of a fan attached to the door, suitable for blowing the gas mixture into the burner, or a duct

flexible plugged into the door.

  The burner 6 is a cylindrical tube with a closed end, whose wall is pierced with a multitude of small holes which allow the fuel mixture to pass radially out of the tube.

  The outer surface of this wall constitutes the combustion surface. An ignition system of known type, not shown, for example comprising a spark generator electrode, 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 tubular bundle 2 is subdivided into two parts, one 2a located to the left of a deflector 7, and the other 2b to the right thereof.

  The deflector 7 is a disc made of thermally insulating material, for example based on ceramic; it is carried by a plate-shaped frame 70, made of stainless steel, whose peripheral edge is inserted between two turns

adjacent to the beam.

  We are dealing here with a double heat exchanger, as shown in the figure

  8 of the aforementioned European patent, which provides an excellent performance.

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

proper heating.

  According to an essential feature of the invention, the turns of the tubular bundle 2 are firmly held against each other by means of a mechanical restraint system.

  This is, in this case, a set of four tie rods 5, constituted by cylindrical rods of stainless steel, and which are associated with elements

  support for each of the two opposite ends of the beam.

  As seen in Figure 2, the tie rods 5 are arranged at the four corners 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 an annular disc plate 30, 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 10 tie rods 5 are fixed to the front 3 of which has been mentioned above.

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

  Nuts 500 screwed on these threaded portions 50 ensure the tensioning of the tie rods, so as to forcefully apply (from the right to the left) the plate 30 against the last turn of the beam 2 and, correspondingly, (in

  opposite direction) the facade 3 against the first turn of this beam.

  The beam 2 is thus axially compressed with force between the bearing elements 3 and 30.

  It will be noted that the end portions 50 are relatively long; 20 they exceed beyond the nuts 500 on a non-negligible length, as we

can see it in figure 3.

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

  For this purpose, the plate 40 constituting the door, whose diameter is greater than that of the insulating portion 41, is traversed by four holes allowing

portion commitment 50.

  Fixing is provided by nuts 400, which are advantageously self-locking nuts, to reduce the risk of inadvertent loosening, in particular.

  particularly under the effect of vibrations ..

  An annular lip seal 42 housed in a suitable groove formed in the plate 40 makes it possible to apply the latter in a sealed manner

  fumes against the external face of the facade 3.

  As seen in Figure 2, the tie rods 5 are arranged outside the beam 2.

  With the observation of Figure 3, it is clear that the assembly consisting of the front 3, the tie rods 5 and the end support elements 3, 30,

constitutes an autonomous whole.

  The expansions that tend to occur under the effect of the internal pressure prevailing in the tube of the winding 2 are counteracted by the tie rods and the bearing elements which absorb the forces of the axial thrust integrally.

  There is no impact of this thrust against the wall of the envelope containing this set.

  The tubular bundle can be held in place within the casing simply by interlocking the end portions of the tubes 20, 21 in the housings provided in the casing to receive them.

  Note also that is provided above the rear area of the winding 2 a deflecting partition 8, which partially overlaps the rear annular plate 30, to its central opening 300.

  This partition advantageously participates in the good maintenance of the beam inside the envelope.

  It is attached to the inner wall of the envelope and extends obliquely under the sleeve 12. It preferably has an arcuate shape, of circular arcuate contour, surrounding the upper zone of the bundle.

  The hot gases generated by the burner 6 firstly pass through the first portion 2a of the beam 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 can not escape immediately by the sleeve 12.

  They must cross the rear part 2b of the exchanger (located on the right of the deflector plate 7), this time from the outside towards the inside, realizing a

  preheating the water flowing through the tube bundle.

  Finally, the cooled gases escape via the rear channel delimited by the wall 110, to join the exhaust duct 12.

  The constituent plastic material of the envelope is chosen to withstand continuously temperatures of the order of 150 to 160 ° C.

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

  As a particularly suitable type of resin include a compound of polyphenylene oxide, polystyrene and polypropylene, such a material being suitable to withstand the chemical attack of hot fumes

and condensates.

  The wall of the casing 1 may be relatively thin, for example of thickness between 4 and 8 mm, because it is not exposed to significant mechanical stresses. For maintenance, it is easy to access the inside of the front part of the exchanger, which is the only one actually exposed to fouling of fumes; it suffices to unscrew the nuts 400 and remove axially

  the assembly constituted by the door 4 and the burner 6 which is integral therewith.

  After cleaning, setting up this set is just as easy. These disassembly and reassembly operations have no effect on the restraint operated by the tie rods 5, which remain active despite the removal.

momentary of the door.

  In an alternative embodiment, of this device, it would be possible to

  fix the disc deflector 7 at the end of the burner 6.

  In this case, the door 4, the burner 6 and the baffle 7 would form a removable assembly in block, which would allow access for cleaning to the entire interior space of the winding, including in the portion back which preheats.

  Of course, in this case, it would be necessary to provide all around the deflector disk 7 a ring seal, highly heat resistant, coming to bear against the inner surface of the beam for

  avoid the direct passage of gases at this level, to part 2b.

  In the second embodiment of the invention, which is illustrated in FIGS. 5 to 7, there is a configuration similar to that just described, the apparatus being however turned 180 degrees (frontage on the right of

Figure 5).

  Elements identical or similar to those of the first embodiment have been assigned the same reference numbers, and will not be given to

  again an explanation as to their nature and function.

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

  As already stated, the rectilinear end portions of the tubes 35 extend tangentially to the winding, their axes being contained in the same

  longitudinal plane, arranged laterally (see Figure 6).

  Furthermore, on the opposite side to the front 3, the tie rods 5 are fixed not to an annular plate 30, but a pair of flat bent rods 30a, 30b, whose central zones bear against an angular sector, surface

  relatively 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, in

  marrying closely two diametrically opposite areas of the winding.

  Note (see Figure 5) that the partition 8 has a recess 80 located above the tubular winding, close to the tubes at the outlet 10 of the portion 2a constituting the main heat exchanger.

  In this recess is mounted a temperature sensor 9. It is a thermal cut-out, which is mounted sealingly with respect to the casing. For this purpose, the probe 9 is advantageously held in place by means of a circlip in a stainless steel bowl fitted in the recess 80, which is open downwards, a suitable seal ensuring

  the seal between the bowl and the wall of the recess 80.

  This probe is connected to the control of the burner, and is adapted to cause the burner to stop when the detected temperature exceeds a predetermined threshold, for example of 1600 C. Abnormal superheating may occur accidentally, for example in case of absence of water in the tubes or in case of poor circulation of

  water in the tubes, for example because of a clogging of one of them.

  In the absence of any safety, there could be a very significant rise in the temperature of the flue gases exiting the tubes placed around the burner, and which come into contact with the inside of the plastic casing. In fact, the fumes would not transmit enough of their

heat to the tubes.

  It could arise then a problem of mechanical resistance of the plastic material and a serious deterioration, see an inflammation of the envelope. In the variant illustrated in FIG. 8, the probe, referenced 9 ',

  has a fuse element 92 ', sensitive to heat.

  The electrical supply circuit of the boiler is connected to two lugs 90 'and 91' which are connected via this thermo fuse 92 '.

  In the event of abnormal rise in temperature, for example above 160.degree. C., the melting of this element 92 'breaks the electrical circuit between the two.

  lugs 91 ', 90', causing the shutdown of the burner control.

  FIG. 9 illustrates the circulation of the hot gases generated by the burner 6, which is fed with a fuel mixture G + A. After ignition of the burner, the latter generates scorching gases, for example at a temperature of 1000 ° C., which propagates radially outward as symbolized by the arrows Fi.

  These flaming gases pass through the interstices of the first part of the exchanger 2a, radially, from the inside to the outside (arrows F2).

  During this passage, a large part of the heat of the flaming gases is transmitted via the wall of the tubes to the water circulating therein, so that the temperature of the hot gases at the outlet of the beam portion 2b is - as

  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 brightening gases Fl.

  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 F3.

  An additional part of the heat is thus transmitted to the water circulating in the tubes. The temperature of the gases which escape from the apparatus (arrows F4 and F5), as an indication is of the order of 65 to 70 C. As for the water, it is warmed generally from the ambient temperature to a temperature of the order of 80 C. Of course, the flow of water is against the flow of smoke, the preheating occurring in the zone 2b of the heat exchanger and the heating

proper in zone 2a.

  In the embodiment shown in FIGS. 10 to 12,

  the exchanger has no burner.

  The casing comprises an inlet sleeve E for hot gases, which originate from an external source.

  This sleeve opens into the winding of tubes 2. This is a provision similar to that which is the subject of the

  Figure 19 of the aforementioned European patent.

  The same reference numerals have been used to designate elements identical to those of the first embodiment, where appropriate assigned to

  the "premium" index when the elements are similar but not identical. This is a simple exchanger (without preheating).

  The hot gases which penetrate into the inner enclosure of the envelope, via the sleeve E, escape radially from the inside to the outside of the tube bundle 2, heating the fluid flowing therethrough; gas

  cooled escape through the sleeve 12.

  The tubular elements constituting the winding can be arranged in parallel, the inlet and outlet manifolds 15 'respectively 16'

  ensuring their collection and distribution either at the inlet or at the outlet of the tubes.

  The envelope 1 'is made of plastic. The means for mechanical restraint of the beam are similar to those of the first embodiment.

  They comprise 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 sleeve E is a disk whose center has an opening 300 corresponding to the gas inlet passage delimited by the sleeve E.

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

  To prevent the bottom wall of the casing facing the plate 3 ', which is exposed to the hot gases, a clearance j is provided between these two elements. Of course, this apparatus 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; it could have a flat or hemispherical shape while remaining attached to the door. The weight gain obtained by the use of a plastic envelope is of the order of 20% compared with a similar device, having the same

  performance, but whose envelope is metallic.

Claims (14)

  1. Condensation 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 conductive material and has a flattened and oval straight section, the major axis of which is perpendicular, or approximately perpendicular, to that (X- X ') of the helix, while the width of the gap separating two adjacent turns is constant and considerably smaller than the thickness of said straight section, this beam being fixedly mounted inside an envelope (1 ) impervious to gases, means 10 being provided for circulating a fluid to be heated, in particular cold water, within (s) tube (s) constituent (s) of said beam (2), this envelope (1) having a cuff (12) for evacuation of the gas burned, this exchanger being thus arranged that the hot gases generated by the burner (6) pass radially, or approximately radially, through the interstices 15 between its turns, characterized in that, on the one hand, said envelope ( 1) is   made of heat-resistant plastic material and that, on the other hand, it comprises means (5; 3-30) for mechanically retaining said bundle along its axial direction, capable of absorbing the thrust forces resulting from the internal pressure of the fluid that circulates and tends to deform the walls, avoiding that these efforts are transmitted to the envelope (1).   2. Heat exchanger according to claim 1, characterized in that it comprises a temperature sensor (9; 9 ') carried by said casing (1), adapted to control 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 restraining means comprises a set of tie rods (5) which extend outside the beam (2), parallel to the axis (X-X ') of the propeller, and whose ends are integral with support elements (3, 30) applying against both opposite sides of the beam.   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, of form annular therefore.   5. Exchanger according to claim 4, characterized in that said plate (3) serves as a facade, which partially closes an open face of   the envelope, and is attached to the latter at its periphery, for example by crimping.   6. Exchanger according to claim 5, characterized in that the 5 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 such 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 the support elements located on the opposite side to said facade consist in a pair of arcuate or bent flanges (30a, 30b), shaped to fit at most the contour of the beam (2) by applying against two diametrically opposed regions thereof, each flange (30a, 30b) being attached to a pair of adjacent rods (5).   9. Exchanger according to one of claims 1 to 8, characterized in that the plastic material constituting the casing (1) is a composite material based on resin filled with fibers or glass flakes.   10. Exchanger according to claim 9, characterized in that said resin is a compound of polyphenylene 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 of secondary exchanger, a deflector member (7) being interposed between these two beams, and thus arranged, that the hot gases generated by the burner first pass through the primary exchanger (2a), passing through the interstices separating its turns from the inside to the outside, then the secondary heat exchanger (2b), passing through the interstices separating its turns from outside inwards, after which they are   evacuated via said sleeve (12).   12. Exchanger according to claim 11, characterized in that said baffle (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 beam acting as primary exchanger (2a), said baffle (7) has a discoid shape and is integral with the end of this burner, this deflector being trimmed at its periphery with a heat seal   insulation that is applied against the inside of the beam.   14. Exchanger according to one of claims 1 to 13, characterized in that said casing (1) consists of two molded half-shells   contiguous and secured to each other, for example by welding.