EP0037333B1 - Gas heater with closed combustion circuit - Google Patents

Description

The present invention relates to gas boilers with a sealed combustion circuit called "with vented

There are on the market such boilers, which are generally placed against a wall and in elevation. Their power does not exceed, in practice, 70 KW because, above this value, there is no combination of boiler / burner to satisfy the problem of operation in "suction cup".

The main objective of the present invention is to create a boiler with a small footprint and low cost price, which can be operated in a sealed combustion circuit with powers much higher than those of known vacuum boilers.

To this end, the subject of the present invention is a gas boiler comprising an exchanger constituted by tubes with tabs connected at their ends to water inlet and outlet manifolds, at least one burner having a tubular body, one of which part pierced with multiple holes extends in the space defined by the finned tubes, a gas inlet, a combustion air inlet, and a flue gas exhaust pipe, characterized in that it comprises a inner box with removable cover, the exchanger being housed in said box with its finned tubes arranged vertically, said box also housing said burner which is arranged vertically and enters the box through the cover thereof, the burner having a portion retained by a sealing ring above the trunk lid and being open at its top and closed at its base below the multiple holes; an outer casing arranged vertically, surrounding the inner trunk and the upper part of the burner, this outer casing being provided at its upper part with a removable cover, the outer casing forming a sealed pre-hearth comprising a space formed between the inner trunk and the inner face of the outer casing, the gas inlet passing through the upper part of the outer casing and being connected to the upper part of the burner, a safety and control device being mounted on the gas inlet, the combustion air inlet being connected to the upper part of the outer casing, and the flue gas discharge pipe being connected to the inner trunk and passing through the outer casing; and means on the combustion air inlet for injecting pressurized air into said space, from which pressurized air enters the burner through the open top thereof, said burner comprising in its part upper a gas and air mixer entering the burner, this mixer comprising an annular jacket surrounding the tubular body of the burner and to which said gas inlet is connected, a ring of gas injection holes provided in the body tubular burner in front of said annular jacket and a core for adjusting the air inlet section in the burner, slidably mounted for this purpose at the top of the burner and which is removable, to allow easy cleaning in situ of inside the burner.

When the combustion air is taken outside, the boiler operates in a sealed combustion circuit, the air suction and burnt gas outlet pipes being able to be located close enough to each other so that the possible wind has no influence on the combustion air flow.

Fresh air can also be drawn into the boiler room, the combustion product evacuation pipe then being connected to a chimney.

The "pressurized" pre-hearth that surrounds the boiler on all sides prevents any leakage of combustion products from spreading in the boiler room. In addition, between the envelope and the trunk, the injected air circulates by thermosyphon effect, from top to bottom along the envelope and from bottom to top along the trunk with self-regulation of its circulation speed, and it therefore also acts as a very effective heat-insulator making it possible to obtain a very low temperature of the outer walls of the envelope and it removes the heat from the security and control devices that can be housed there.

Gas boilers comprising a burner with a perforated tubular body extending between the finned tubes of an exchanger are known from DE-A-2 100 344, which moreover relates to a water heater rather than a boiler, and by FR-A-2 314 448. These boilers are not housed in a sealed envelope completely surrounding them, forming around them a pre-hearth, into which the combustion air is injected under pressure with the abovementioned advantages . According to said FR-A-2 314 448, the burner and the exchanger are moreover arranged horizontally and the boiler operates in vacuum, the fan provided at one end of the boiler sucking the combustion gases to discharge them into the chimney. DE-A-2 162 139 relates to a boiler, not of gas but of liquid fuel, comprising a combustion chamber provided with a fuel injection nozzle and into which the combustion air is drawn; the pre-hearth located between the outer shell of the boiler and the shell of the exchanger surrounding the combustion chamber is therefore under vacuum.

As for FR-A-2 411 365, it relates to a gas water heater, the burner of which has a ramp and not a perforated tubular body and the fin exchanger of which is arranged horizontally at the top of a chamber heating above the gas rail; in this embodiment, there is also no waterproof outer envelope surrounding the habil lage in which are housed the burner, the heating chamber and the exchanger and which would form around this covering a pre-hearth in which the combustion air would be sent under pressure.

In addition, according to the present invention, the air-gas mixture of the burner or burners moves up and down, the fresh air and the gas being injected at the top. Therefore, not only the maintenance of the burner is easy, but also there occurs as mentioned above, for the bypass air between the casing and the trunk, a natural circulation by thermosiphon with self-regulation, which ensures both the proper cooling of the outer casing without overloading the fan power and pre-heating the air supplied to the burner, thus recovering calories increasing the overall efficiency of the boiler. Such a top-down movement of air and gas was not provided for in the previous boilers described in the aforementioned documents.

The exchanger is preferably made up of tubes with vertical fins arranged around or on either side of the burners, these tubes being connected at their ends to water inlet and outlet manifolds. High power is thus obtained, with a small footprint. GB-A-1 324 636 describes a boiler in which burners are arranged in line, the finned tubes of the exchanger being also arranged in line on either side of the burners. However, this boiler, which is of a conventional type, does not include an interior trunk and an outer casing forming around the trunk a sealed pre-hearth, into which the combustion air is sent under pressure, and the fins of the tubes. are too far apart to properly cool the flue gases; it follows that the efficiency of this boiler is insufficient.

The exchanger of the present boiler can be combined with a tube used for the supply of hot water for sanitary or industrial purposes for example.

The burner (s) are supplied with air and gas in substantially stoichiometric proportions. They advantageously comprise on their perforated tubular body sealing rings, the distribution of the thermal flow being ensured by partial and suitable sealing of the holes.

The boiler is particularly suitable for supplying heating installations whether or not combined with a hot water supply service.

According to a particular embodiment of the boiler according to the invention, its exchanger is divided into two parts in the vertical direction by a refractory floor, which allows it to play, in the part which is located above this floor and which contains the burner (s), its classic role of exchanger, and in the part which is located below the floor, and where it receives cold water, both a role of exchanger and a role of combustion product condenser.

This configuration of the exchanger further improves the efficiency of the boiler according to the invention.

Everyone is aware that the good performance of boilers is a determining factor in the field of energy savings.

214 kcal étant la chaleur exothermique.The boilers currently built have their output pushed to their almost extreme limit. The only reason that prevents us from achieving a truly maximum efficiency is that the products of combustion take away calories because of their temperature. These combustion products are nitrogen, C0 ₂ and especially water vapor, the weight of which is relatively considerable; 1.611 kg per m ³ of natural gas burned according to the reaction scheme below:

214 kcal being exothermic heat.

It is therefore important to be able to recover as much as possible of the calories carried away by the combustion gases and most of the water vapor whose condensation causes 516 cal / kg (2 159.710 J) to be recovered - latent heat of vaporization .

To achieve this result, it suffices to pass the burnt gases mixed with the water vapor through an exchanger placed on the outlet of the boiler.

This can be made up of smooth or finned tubes in which the return water from the radiators circulates. The condensation phenomenon begins as soon as the temperature of this water drops below 59 ° (dew point).

The recovery of the heat contained in the combustion gases begins as soon as the temperature of the return water is lower than that of the burnt gases.

The price of this exchanger is relatively high, which limits its use.

This drawback is eliminated with this new configuration of the exchanger which makes it possible to provide the boiler according to the invention with an exchanger-condenser, and this, without much impact on the cost price of the boiler.

The aforementioned DE-A-2 100 344 relates to a gas water heater, the exchanger, which includes finned tubes arranged vertically, is divided into two parts in the vertical direction by a deflector placed above the burner, itself mounted at the base of the water heater; the latter therefore operates in the opposite direction to the present boiler, with a cold water inlet at its top and a hot water outlet at its base. The upper part of the exchanger, located above the deflector, acts as an exchanger and condenser for the water vapor contained in the com bustion, but the condensed water is evaporated in the low and hot region of the exchanger where it falls, which cancels the recovery of the thermal energy of the condensation water, which had taken place in the high region of the exchanger. This water heater with a structure reversed with respect to that of the present boiler does not therefore offer the particular advantage of the latter, when it is provided for condensation.

In accordance with other particular embodiments of the present invention, advantageous structural modifications may be made to the burner (s) and to the fins of the exchanger.

The first modification to the burner consists in providing additional air inlet orifices in the region of the burner body which follows the zone of the mixer.

The advantage of this improvement lies in the fact that a fairly large part of the combustion air which enters these orifices - the diameter of which will be judiciously calculated - is deducted from that which passes through the mixer. However, the main pressure drop in the combustion air circuit is precisely in the mixer area. Thus, without changing the total amount of air which is introduced into the burner, and by passing less air through the mixer, the pressure drop in air circulation is reduced, resulting in lower air pressure in the front foyer. It is therefore possible to use a less powerful fan, which saves electrical energy and reduces the construction price. Furthermore, the air introduced through said orifices creates turbulence favorable to the air-gas mixture.

The second modification consists in making the manifold independent of the burner body, which allows the latter to be easily assembled and reassembled without dismantling the mixer, which is integral with the arrival of the gas.

A double wall mixer will be provided for this purpose, the internal wall of which forms a cylinder which is coaxial with the body of the burner.

The burner body is capable of sliding with gentle friction inside the aforementioned cylinder.

The gas leaves the mixer through orifices arranged in a ring and formed in this internal wall. It will preferably be arranged for these orifices to open above the burner body so that the gas penetrates more freely into it, otherwise it would be necessary to also provide perforations in the body itself.

Advantageously, provision will be made for additional air inlet orifices, which is the subject of the previous modification. In this case, the extended internal wall of the mixer and the burner body will have facing orifices for the introduction of this additional air.

The modification announced above which may be made to the exchanger consists in modifying the arrangement of the fins of the tubes of this exchanger so that the fins of a tube are offset in height relative to those of the neighboring tube, which allows to bring together the different tubes constituting the exchanger.

In exchangers where the water tubes are arranged either in a ring or in lines, the fins of a tube are all located at the same level as those of the neighboring tubes and the fins of the exchanger which are in the same plane are almost ready to touch. V-baffles should be placed on the outside of the tubes to ensure that the combustion flames reach the maximum surface area of the fins.

The aforementioned modification of the arrangement of the fins forces the flames and hot gases to directly lick a large part of the section of the fins, without the use of baffles.

Furthermore, this arrangement makes it possible, on the one hand, for the same number of tubes, to reduce the volume of the exchanger, thereby resulting in a reduction in the dimensions of the boiler and thereby a reduction in its cost price, and on the other hand, for the same size (same diameter of an exchanger with crown tubes), to accommodate a greater number of tubes (as a rule 25% more), which contributes to improving the efficiency of the boiler .

It goes without saying that if a condensing zone of the water vapor resulting from combustion is provided, as explained above, the fins of the section of the exchanger-condenser can be arranged in the advantageous manner which comes to be defined.

• La figure 1 est une vue en plan avec coupe horizontale partielle d'une chaudière conforme à l'invention.
• La figure 2 est une coupe par II-II de la figure 1.
• La figure 3 est une vue analogue à la figure 1, mais représentant une variante.
• La figure 4 est une coupe par IV-IV de la figure 3.
• La figure 5 est une vue schématique développée d'un échangeur agencé en vue de la fourniture d'eau chaude à usage domestique.
• La figure 6 est une vue en coupe verticale d'une chaudière équipée d'un échangeur-condenseur, conformément à un mode de réalisation de l'invention particulièrement intéressant.
• Les figures 7 et 8 représentent chacune une vue en coupe verticale d'une variante de réalisation d'un brûleur équipant la chaudière selon l'invention.
• La figure 9 est une vue en coupe horizontale partielle de l'échangeur de la chaudière conforme à la figure 1 et la figure 10 est une vue analogue à la précédente, montrant une variante intéressante de la disposition relative des ailettes.

Several embodiments of the boiler according to the present invention will be described in more detail below by way of non-limiting indication, with reference to the attached drawing in which:

• Figure 1 is a plan view with partial horizontal section of a boiler according to the invention.
• Figure 2 is a section through II-II of Figure 1.
• Figure 3 is a view similar to Figure 1, but showing a variant.
• Figure 4 is a section through IV-IV of Figure 3.
• Figure 5 is a schematic developed view of an exchanger arranged for the supply of hot water for domestic use.
• Figure 6 is a vertical sectional view of a boiler equipped with a heat exchanger-condenser, according to a particularly interesting embodiment of the invention.
• Figures 7 and 8 each show a vertical sectional view of an alternative embodiment of a burner fitted to the boiler according to the invention.
• Figure 9 is a partial horizontal sectional view of the heat exchanger of the boiler con shape in Figure 1 and Figure 10 is a view similar to the previous one, showing an interesting variant of the relative arrangement of the fins.

In the embodiment shown in Figures 1 and 2, the boiler has a sealed vertical envelope 1, which can be placed on the ground by means of a base 1a and is closed at its upper part by a removable cover 1 b. The cross section of the envelope 1 can be arbitrary, square for example.

The envelope 1 contains a trunk 2, of smaller cross section and of lower height, arranged so that a free space is provided on all of its faces. The trunk 2 is provided with a removable cover 2a.

It contains an exchanger formed by tubes 3, arranged vertically along the generatrices of a cylinder, as shown in Figure 1, between two annular collectors 4. The water to be heated enters the lower collector by a tube 5 and leaves the collector upper by a tube 6. As can be seen in FIG. 2, the tubes 5 and 6 pass through, with sealing, the walls of the trunk 2 and of the envelope 1.

The bottom of the trunk 2 and the cover 2a are pressed against the exchangers by means of annular bosses 2b with which they are provided (FIG. 2). The tubes 3 are lined, over their entire length, with fins 3a intended to increase the heat exchange surface. In addition, vertical V baffles 3b are arranged outside the tubes 3, as shown in Figure 1 in order to cause the gases to lick said tubes.

The lid 2a of the trunk 2 has an axial circular orifice 2c through which is inserted, in the axis of the exchanger, a burner 7 which has, in its upper part, a sealing ring 7a which comes to rest on this lid 2a (figure 2).

At its upper part, outside the trunk 2, the burner comprises a mixer 8, composed of an annular jacket which surrounds the tubular body of the burner.

The gas arrives via a lateral tube 9, which crosses with tightness the wall of the envelope 1 and on which are mounted, inside the envelope 1, the devices 9a for regulation and control.

The gas passes into the burner body through a ring of injection holes 10 located at the top of the mixer.

The air inlet section of the latter is adjusted by means of a cylindrical-conical core 11, provided with an upper flange 11 a and which can be made to penetrate more or less into the body of the burner 7.

This body extends in the trunk 2 to the bottom of the lower manifold. It is closed at its base and pierced over its entire portion, opposite the tubes 3, multiple rings of small holes 12 through which the mixture of air and gas exits. This exit through multiple small holes prevents flashback.

In order to ensure that the fins 3a of the exchanger receive the same amount of heat over the entire height of the tubes 3 despite the convection movements of the burnt gases in the vertical direction, the perforated portion of the burner is provided with sealing rings 13 that we bring more or less close to each other, in order to identify the number of holes it takes. Only two rings 13 have been shown in Figure 2 so as not to complicate the drawing.

The upper part of the casing 1 is connected to a fan 14 which pressurizes the fore-hearth 15 which constitutes said part, as well as the annular space 16 which surrounds the trunk 2 and the lower part 17, located under this chest.

The burnt gases are collected in the space 18 where they arrive after having passed between the tubes 3, 3a and they leave the trunk 2 by a lateral pipe 19 which crosses with tightness the wall of the envelope 1 then the wall M.

Fresh air is supplied to the fan 14 by a pipe 20 which also passes through the wall M.

The boiler that has just been described operates as follows: the fan 14 sucks in the fresh air through the pipe 20 and pressurizes the spaces 15, 16 and 17 of the envelope 1 which forms before the hearth. This air is discharged into the mixer 8 where it mixes with the gas leaving the injection holes 10.

After ignition, the mixture burns around the burner 7, passes between the tubes 3, 3a, bypassing them, thanks to the baffles 3b, reaches the space 18 and exits through the pipe 19.

The pipes 19 and 20 opening out substantially in the same vertical plane and at a short distance from each other, the wind which is possibly exerted on their orifices makes the differential pressures of inlet and outlet of the air constant. . In this case, there is an overpressure in the fore-hearth, which has no appreciable effect on the pressure differences and, therefore, on the combustion air flow.

In the variant of FIGS. 3 and 4, the finned tubes 3, 3a of the exchanger are arranged on two parallel lines and vertical screens 3c are provided at the ends of these lines, between them so as to force the gases to pass between the tubes.

The upper collector has two compartments 4a and 4b, which communicate respectively with one and the other of the lines of tubes, the water entering taking place at 5 in compartment 4a and the exit at 6 from compartment 4b . The water therefore circulates from top to bottom in the right tubes and from bottom to top in the left tubes, as indicated by arrows in FIG. 4. Three burners 7 are arranged vertically and in line between the two lines of tubes 3. They are supplied by the tubing 9 by means of a manifold 9b.

The operation is the same as that of the embodiment described above.

If one wishes to produce hot water, for example for domestic, sanitary or industrial purposes, without being obliged to go through an external exchanger, it is sufficient, whatever the variant adopted for the boiler, to pass a tube 21 in the tubes 3 and the collectors 4. The inlet of the water to be distributed takes place at 22 and the outlet at 23, FIG. 5.

The tube 21 is preferably made of copper or stainless steel. The heat exchange is very active due to the large contact surface and the high water speeds on both sides. The volume of the boiler remains the same.

The advantages of the present invention are manifold.

The overpressure that constantly prevails in the envelope 1 around the trunk 2 prevents any leakage of burnt gas from spreading in the boiler room.

The presence of air in the spaces 15, 16 and 17 dispenses with the need for heat-insulating products on the walls of the envelope 1. In fact, the heated air in the lower spaces 16 and 17 rises in the envelope where it mixes in space 15 with the fresh air blown by the fan. This results in a circulation by thermosiphon which on the one hand prevents excessive heating of the air and on the other hand, ensures a heating of the air which enters the mixer 8. The calories thus recovered participate in a better overall yield of the boiler. The energy produced by the fan is also saved. The control and regulation devices 9a work well because they are cooled by the arrival of fresh air in the upper space 15 where they are placed.

The mixture of air and gas can be metered stoichiometrically in the mixer (s) 8, which makes it possible to have a very short flame, hence an extremely reduced focus capacity. The central part of the mixer (s), constituted by the cylindrical-conical core 11 is easily removable and allows easy access to the body of the burner. However, it is inside this body and on the small holes 12 that dust can accumulate. After removing the cover 1b and the core 11, a simple brushing precipitates the dust at the bottom of the burner which has been extended for this purpose downwards under the perforated portion. We can thus dispense with a filter on the fresh air intake, a filter that would be more difficult to clean than the burner. In addition, an abnormal fouling of the interior of the burner is signaled by the air flow controller which automatically switches off the boiler. The disassembly of the burner presents no difficulty once the cover 1b of the envelope has been removed.

The exchanger, placed vertically, offers advantages: in the embodiment of Figures 1 and 2, the entry of water from the bottom and its outlet from the top allow a complete air purge. In addition, all the tubes being traversed by water at the same temperature, no tension problem due to differences in expansion can arise.

Whatever the embodiment adopted, the installation is very simple since it suffices to pass the pipes 19 and 20 outside, their outside orifice preferably being protected by a grid.

If one does not want or cannot benefit from the sealed circuit, it suffices to connect the pipe 19 to a chimney, the fan then drawing air into the boiler room.

The fan can be calculated so that a significant residual pressure is ensured at the outlet of the combustion products. It is thus possible to considerably reduce the section of the chimney or the section of the pipes 19 and 20 which connect the boiler to the outside when the sealed circuit is used in its entirety.

It was possible to build, according to the invention, a boiler of 200 KW which measured on the ground 0.50 × 0.45 m and had a height of 1.05 m. This volume is approximately one-seventh that of a conventional gas boiler. The weight is correspondingly reduced, the boiler can be transported in the rear trunk of a light passenger car.

The boiler shown in Figure 6 is in accordance with a particular embodiment of the invention. Like the boiler shown in FIG. 1, it comprises a sealed vertical envelope 1 which can be placed on the ground by means of a base 1 a, and which contains a trunk 2 whose cover 2a has an axial circular orifice 2c by which is introduced a burner 7 which has, in its upper part a sealing collar 7a which comes to rest on this cover 2a. The burner 7 comprises an air-gas mixer 8, located outside the trunk 2 in which the gas arrives through a lateral tube 9 on the path of which the regulating, control and safety devices 9a are arranged. The air is supplied by a fan 14, which pressurizes the fore-hearth 15, the annular space 16 surrounding the trunk 2 and the lower part 17 located under this trunk. The burner 7 extends inside the trunk 2, substantially over half of its height or more, by a cylindrical ramp pierced with multiple rings of small holes 12 (approximately 8/10 mm in diameter) through which the air mixture exits - ignited gas, sealing strips 13 also being provided. The trunk 2 contains an exchanger formed by tubes 3 with fins 3a, arranged vertically between two annular collectors 4, and in a ring around the burner ramp 7.

According to this particular embodiment of the invention, these tubes 3 extend beyond this ramp.

The water to be heated enters the lower collector by a tube 5 and leaves the upper collector by a tube 6.

A refractory floor 24 located below from the bottom of the burner 7 in the space limited by the tubes 3 to which it is fixed by any appropriate means, separates the interior of the exchanger 3 into two parts, the upper part 24a constituting the exchanger itself and the lower part 24b receiving in 5 the return water (cold water) and functioning as an exchanger-condenser.

For this purpose, the combustion gases (including water vapor) leaving part 24a are reintroduced laterally in part 24b, the water to be heated engaging in part 24a after having recovered the heat of condensation in the part 24b, thereby improving the efficiency of the boiler.

The lower manifold 4 is located at a certain distance from the bottom of the boot 2. It rests on a plate 25 having a central opening 26 and a lateral opening 27 leading to a vertical pipe 28 conveying the burnt gases to the outside and ending at this effect by an outlet bend 29 approximately halfway up the trunk 2.

The plate 25 and the bottom of the trunk 2 delimit a sealed container 30 having a lateral tube 31 for the evacuation of the condensation water.

The path of the burnt gases, including the water vapor, therefore conforms to the arrowed path 32. The condensation water is collected at 31 and can be recovered as distilled water.

FIG. 7 shows a burner 7 with the annular jacket of the mixer 8 and the cylindrical-conical core 11 making it possible to adjust the air inlet section in the mixer, this burner 7 having, according to this variant embodiment, the characteristic of present a series of air intake holes 33 (for example a ring of holes) located between the flange 7a and the mixing zone 8. A better efficiency of the burner has been observed for the reasons which have been explained above in the preamble.

FIG. 8 illustrates another variant of construction of the burner according to which the annular jacket 8 which constitutes the air-gas mixer is double-walled, the external wall 8a not having undergone any modification and the internal wall 8b forming a cylinder which is coaxial with the body of the burner and which also extends to the trunk 2 of the boiler where it carries a flange 8c which comes to rest on the lid 2a of the trunk 2.

The internal wall 8b has a ring of gas injection holes 10, which open a little above the top of the burner body.

The latter slides with gentle friction in the tube 8b; it carries at its upper part a tab 36 which can be formed by an extension of its wall and which is perforated to allow the passage of a positioning pin 37, which also passes through the wall 8b of the mixer.

During a maintenance visit, the operator removes the pin 37 and extracts the burner body to check and clean it. Its replacement is very easy since it suffices to do the reverse operation.

The gas injection holes 10 can also be checked without having to disconnect the gas supply there too.

It will also be noted that additional air inlet orifices 33 could be envisaged as in the variant according to FIG. 7, orifices 33a situated opposite the orifices 33 having to be made in the wall 8b.

Furthermore, with regard to the exchanger of the boiler according to the invention, the tubes 3 of which are arranged either in a ring around a single burner (FIG. 1 or in lines (FIG. 3), its fins 3a will be located in the same horizontal plane being almost touching, as can be seen in detail in Figure 9. In this case, so that the combustion flames interest the maximum surface area of the fins 3a, baffles 3b must be placed, which force these very hot flames or gases to bypass the tubes and their fins 3a before exiting through the slots 34. To avoid this drawback, the tubes 3 can be arranged as in FIG. 10, the fins 3a of a tube being offset in height with respect to the fins 3a of the adjacent tubes 3, and the outer edge of each fin 3a almost touching the neighboring water tubes 3. This arrangement forces the flames and hot gases to lick a large part of the section of the fins, which allows to delete without disadvantage baffles 3b.

Claims (16)

1. Gas boiler capable of functioning in a closed combustion circuit, comprising an exchanger constituted by finned tubes (3) connected at their ends to collectors (4) for entry and exit of water, at least one burner (7) comprising a tubular body, of which a part pierced by a plurality of holes (12) extends into the space defined by the finned tubes, a gas inlet, a combustion air inlet and an outlet pipe for burnt gas, characterised by comprising an interior casing (2) with a removable cover (2a), the exchanger (3, 4) being housed in the said casing with its finned tubes (3) disposed vertically, the said casing also housing the said burner (7), which is disposed vertically and extends into the casing through the cover thereof, the burner having a portion retained by a sealing ring (7a) above the cover (2a) of the casing and being open at its top and closed at its base below the plurality of holes; an exterior housing (1) arranged vertically around the interior casing and the upper part of the burner, this exterior housing being provided at its upper part with a removable cover (1b), the exterior housing forming a sealed prechamber comprising a space (15, 16, 17) located between the interior casing and the internal surface of the exterior housing, the gas inlet (9) extending through the upper part of the exterior housing and being connected to the upper part of the burner, a safety and control device (9a) mounted in the gas inlet, the combustion air inlet (20) being connected to the upper part of the exterior housing, and the outlet pipe (19) for burnt gas being connected to the interior casing and extending through the exterior housing; and means (14) in the combustion air inlet for injecting air under pressure into the said space, wherein the air under pressure enters the burner through the open top thereof and circulates between the housing and the casing thermosyphonically and in a self-regulating manner, the said burner comprising at its upper part a mixer (8) for gas and air entering the burner, this mixer comprising an annular casing surrounding the tubular body of the burner and to which the said gas inlet is connected, a ring of holes for injection of gas being provided in the tubular body of the burner opposite the said annular casing and a core (11), for regulating the air-inlet cross-section into the burner, being slidably mounted for this purpose on the top of the burner and being removable to permit easy cleaning of the interior of the burner.

2. Boiler according to claim 1, characterised in that the finned tubes (3) are arranged in parallel lines, in a manner known per se, several burners (7) being arranged in a line between the lines of tubes (3).

3. Boiler according to claim 1 or 2, characterised in that protective rings (13) are provided on the perforated part of the tubular body of the burner, such that the finned tubes (3) receive the same quantity of heat over their entire height despite the convection movements on the burnt gases in the vertical direction.

4. Boiler according to any one of claims 1 to 3, characterised in that the safety and control device (9a) mounted in the gas inlet is placed in the upper portion of the pre-chamber, where it regulates the arrival of fresh air.

5. Boiler according to any of claims 1 to 4, characterised in that the exchanger (3, 4) comprises a circuit (21-23) allowing the provision of hot water for sanitary or industrial purposes.

6. Boiler according to any of claims 1 to 5 having a closed combustion circuit, characterised in that the means for injecting combustion air under pressure are a fan (14), the inlet tube (20) of which extends from the exterior and adjacent the region where the burnt gas outlet tube (19) discharges outside the exterior housing (1), such fan discharging directly into the said space located between the exterior housing and the interior casing, at a point located above the interior casing and serving to pressurise the said sealed pre- chamber.

7. Boiler according to any of claims 1 to 6, characterised in that the fresh air is redirected by a fan (14) arranged to ensure a residual pressure in the burnt gas outlet which allows a reduction in the cross-section of the tubes (19, 20) or of a chimney, if any.

8. Gas boiler according to any of claims 1 to 7, the exchanger (3, 4) of which is divided into two parts in the vertical direction by a refractory floor (24), one of these parts containing the burner(s) (7), characterised in that the part (24a) of the exchanger located above the said floor contains the burner(s) (7) and a has a standard purpose for an exchanger, in that the part (24b) of the exchanger located beneath the said floor both receives cold water and also acts as an exchanger and condenser for the products of combustion which are formed in the vicinity of the upper part (24a) of the exchanger-condenser and are directed towards the lower part (24b), in that the refractory floor (24) is located under the bottom of the burner(s) (7) in the space defined by the tubes (3) of the exchanger, to which it is fixed by any appropriate means, the products of combustion leaving the standard upper part (24a) being reintroduced laterally into the lower part (24b) and the water to be heated being contained in the upper part (24a) after recovery, in the lower part (24b), of the heat of condensation of the water vapour contained in the products of combustion, and in that, in the lower part of the heater, a sealed tank (30) is formed, which receives the products of combustion, burnt gas then being conveyed to the exterior by an outlet pipe, whilst the collected water of condensation is removed by a tube (31).

9. Gas boiler according to claim 8, characterised in that the tank (30) is defined at its upper part by a plate (25) on which the lower collector (4) of the exchanger-condenser rests and which has a central opening (26) for the inlet of combustion products derived from the zone (24b) and a lateral opening (27) leading into a vertical pipe (28) which directs the burnt gas towards the exterior and terminates for this purpose in an outlet elbow (29) adjacent the mid-height of the casing (2).

10. Gas boiler according to any of claims 1 to 9, characterised in that the tubular body of the burner(s) (7) has a series of supplementary orifices (33) for the entry of air into the region downstream of the air-gas mixing zone (8).

11. Gas boiler according to any of claims 1 to 10, characterised in that, for each burner, the air-gas mixer (8) is formed independently of the body of the burner.

12. Gas boiler according to claim 11, characterised in that the mixer (8) has double walls, the internal wall (8b) forming a cylinder which is coaxial with the body of the burner and which extends as far as the casing (2) of the burner where it carries a collar (8c) resting on the cover (2a) of the casing (2), the said body being slidable with low friction on the interior of the cylinder (8b).

13. Gas boiler according to claim 12, characterised in that the gas inlet orifices (10) formed in the wall (8b) discharge above the body of the burner in such a manner that the gas enters freely into the air-gas mixing zone.

14. Gas boiler according to claim 11 or 12 or 13 and according to claim 10, characterised in that the wall (8b) has perforations (33a) located opposite the perforations (33) of the burner (7), for the inlet of additional air.

15. Gas boiler according to any of claims 12 to 14, characterised in that the body of the burner carries at its upper part a leg (36) which can be formed by a prolongation of its wall and which is perforated to receive a locating pin (37) which also extends through the wall (8b) of the mixer.

16. Gas boiler according to any of claims 1 to 15, characterised in that the fins (3a) of a tube (3) of the exchanger (3, 4) are offset in height with respect to the fins (3a) of the adjacent tubes, the exterior edge of each fin (3a) nearly touching the neighbouring water tubes (3).

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