EP0499760B1 - Boiler and installation for central heating and/or production of hot water - Google Patents

Description

The invention relates to a boiler and to a central heating and / or domestic hot water production installation. It will find its application in particular in the manufacturers of domestic or industrial heating appliances.

Although the invention has been specially developed in the context of gas boilers, it can also be applied to boilers using other types of fuel and which have the role of ensuring the heating of a heat transfer fluid.

A traditional boiler mainly comprises three parts which can be summarized in a hearth at the base, an intermediate heating body and a flue gas exhaust at the top. The hearth itself consists in particular of gas burners, and a combustion chamber.

The fumes from combustion generally reach a temperature between 1,000 and 1,200 ° C from which a maximum of calories must be removed. It is the role of the heating body which serves as a heat exchanger between the combustion fumes and a heat transfer fluid which is very often water. The heating body may include one or more heat transfer fluid circuits, in domestic applications very often, two circuits are encountered, a heating circuit and a hot water circuit.

At the outlet of the heating body, the temperature of the fumes must have decreased appreciably and an attempt is made to reach approximately 200 ° C. They are evacuated by the flue evacuation pipe.

In the case of boilers with an independent double heating circuit, the heating body consists of an outer casing terminated, at each upper and lower end, by an upper and lower flange through which open tubes capable of being traversed by the combustion fumes. The envelope is waterproof and contains the heat transfer fluid which is used for heating. Between the smoke tubes, if necessary, there is a coil which is used for heating hot water sanitary.

The number and section of the smoke tubes must be determined by the manufacturer to obtain maximum recovery of the calories present in the smoke gases. To increase exchanges, the smoke tubes generally contain turbulators whose role is to slow the speed of passage of gases within the smoke tubes.

The efficiency of a boiler depends on the ability of the heating body to recover the maximum number of calories present in the flue gases. At this level, compromises must be made because, for example, an exchanger, which recovers calories, for a small part by radiation, and for the rest by convection, should have a path for very long smoke with a reduced speed of passage, to recover the maximum of calories. In return, if the smoke is evacuated in poor draft conditions, then there is a poor combustion efficiency with a large percentage of unburnt fuels.

Document FR-A-2,233,572 discloses a gas boiler comprising a combustion chamber, a heating body, as well as a smoke evacuation zone, capable of being connected to a smoke evacuation duct. Said heating body consists of a sealed outer envelope through which are arranged tubes, traversed by the combustion fumes, which have a decreasing section in the direction of the heat flow.

However, the aim pursued by this document is different from that of the present invention since it is sought to create a boiler having balanced heat transfers on the flue gas side to reduce the increase in heat and the thermal stress of the boiler pipes.

For this, this boiler has a configuration particularly so that the burner flames can enter further into the tubes so that most of the combustion takes place in the combustion chamber.

In addition, pollution control legislation requires maximum emissions of carbon monoxide Co under certain conditions of use. As for the French standard, a boiler that uses methane fuel must, with a standardized overload, not emit more than 0.25% of carbon monoxide. In addition, the ideal combustion regime for the fuel corresponds to the so-called stochiometric reaction, also called neutral combustion. This ideal combustion supposes a perfect balance between oxidant and fuel. Consequently, with this heating regime, the parasitic fumes from unburnt products are reduced. Unfortunately, this combustion regime is also unstable at the slightest overload and therefore any adjustment, such as the overload imposed by the standard, causes a significant increase in the emission of carbon monoxide exceeding the standards. Here again a compromise must be found.

A second point which must also be monitored in terms of pollution, is the emanation of nitrogen oxides NOX. Studies have been carried out to determine which factors have a direct influence on nitrogen oxide fumes. One of the determining points is the temperature of the flame which itself depends on the intake of secondary air. In this regard, primary combustion air is defined as the combustion air which is introduced into the burner itself to be mixed with the fuel whereas the secondary air is the air present in the combustion chamber surrounding the flames.

To control nitrogen oxide emissions, it is advantageous to lower the flame temperature and reduce the residence time of the gases at high temperature.

We therefore see divergent requirements imposed on the one hand by a good performance of the operation of the heating body and on the other hand to obtain good combustion and in particular a reduction in pollutants such as carbon monoxide and nitrogen oxide .

In the first case, it is desirable to obtain a low evacuation speed of the burnt gases to allow a prolonged exchange time between heat transfer fluid and combustion fumes while in the second case, on the contrary, a draft must be obtained. high to lower the flame temperature and shorten the residence time of high temperature gases.

The main object of the present invention is to present a boiler, the efficiency of which in terms of heat exchange is preserved compared to existing devices, capable of withstanding high overload conditions specific to the different standards and moreover capable of improving combustion hygienes with reduction of carbon monoxide and nitrogen oxide emissions.

It should be noted that these improvements are made without appreciable changes in the volume of the boiler, the size of which remains substantially similar to the existing installations or even less. In furthermore, from an economic point of view, the manufacturing costs for the heating body of the present invention remain substantially similar with respect to the existing parts.

The heating body of the present invention makes it possible to reconcile totally divergent requirements in terms of heat exchange efficiency and speed of evacuation of combustion fumes.

Other objects and advantages of the present invention will appear during the description which follows which is however given only for information and which is not intended to limit it.

According to the invention, the boiler, comprising a combustion chamber, a heating body, as well as a smoke activity zone, capable of being connected to a smoke evacuation duct, said heating body being constituted of a sealed outer envelope through which are arranged tubes traversed by the combustion fumes, which have a decreasing section in the direction of the flow of combustion fumes, is characterized in that the said tubes are equipped in their lower part with 'tubular appendices on the one hand opening into the combustion chamber and, on the other hand, connected to the smoke tubes.

In one embodiment, the boiler of the present invention uses gas as fuel. Furthermore, the boiler of the present invention is particularly intended for a central heating installation and / or for producing domestic hot water.

• la figure 1 illustre schématiquement en vue de coupe les différentes parties d'une chaudière à gaz,
• la figure 2 représente un délai de réalisation des tubes de fumées.

The invention will be better understood on reading the following description accompanied by attached drawings, among which:

• FIG. 1 schematically illustrates in section view the different parts of a gas boiler,
• Figure 2 shows a delay in producing the smoke tubes.

The present invention relates to a boiler as well as a central heating installation and / or domestic hot water production. She will find in particular its application in the manufacturers of domestic or industrial heating appliances.

The invention has been more specifically developed in the context of gas boilers, however it may very simply extend to boilers using other types of fuel.

The role of a heating body is to recover the calories present in the combustion gases in order to transfer them to one or more circuits of heat transfer fluid. The efficiency of a boiler depends partly on the good conditions under which combustion can take place and on the other hand on the ability of the heating body to take up the calories present in the combustion gases.

With regard to the actual combustion, it is necessary to approach the ideal combustion, that is to say the stochiometric combustion corresponding strictly to the balanced chemical equation with exclusive production of carbon dioxide without emanation of carbon monoxide. For this, it is necessary to have an adequate draft to eliminate the burnt gases as much as possible and introduce sufficient oxygen. It should also be emphasized that these imperatives also correspond to the criteria observed to reduce the emissions of nitrogen oxide NOX.

For the heater, the technical requirements to be met in order to achieve good efficiency are contrary. It is necessary to reduce as much as possible the evacuation speed of the fumes to increase the heat exchange time. In general, the temperature of the flue gases at the inlet is 1,000 to 1,200 ° C and it is 200 ° C at the outlet of the heating body. Most of the heat is exchanged by convection, and the flue tubes are generally fitted with turbulators to slow down the evacuation speed of the fumes and promote the transfer of calories.

Any boiler manufacturer must therefore adopt a compromise between good combustion and adequate draft and a significant transfer of calories with braking in the evacuation of combustion fumes.

Figure 1 shows schematically in section the main components of a boiler, for example gas. At the base are burners 1, placed in a combustion chamber 2 which emits the fumes at a temperature of around 1,000 to 1,200 ° C. It is a traditional design of the boiler known to those skilled in the art.

At the top, the boiler has a combustion gas evacuation zone. For example, an anti-backflow draft cup 3 is placed above the central heating body 4, the said cup is used to recover the smoke emissions leaving the heating body 4 and to direct them towards a discharge duct for the smoke not shown. This is again a traditional organ, the production of which is known to those skilled in the art.

The heating body 4, which is the subject of the present invention, is presented externally in a similar manner to a traditional heating body. In the example chosen, this heating body 4 contains two independent circuits with heat transfer fluid, such as in particular water, namely: it is a heating circuit 5 and a domestic hot water circuit 6.

The heating body 4 is composed of an outer casing 7 terminated at each upper and lower end respectively by an upper flange 8 and a lower flange 9 through which open vertical tubes 10, capable of being traversed by the combustion gases.

The envelope 7 is sealed and contains the heat transfer fluid for the heating circuit 5. With regard to domestic hot water with the circuit 6, the latter is arranged in the heating body in the form of a coil 11 wound around the smoke tubes 10. Turbulators shown diagrammatically in FIG. 1 in dotted lines 12 are placed in the smoke tubes 10 to brake the gases and increase the heat exchange between the heat transfer fluids and the combustion fumes.

According to the main characteristic of the present invention, the boiler has means for increasing the circulation of smoke in the lower part of the heating body 4. This has a direct influence on the combustion operating conditions.

Indeed, so far it is the lower part of the heating body which limits the draft of the boiler. This is where the gases are the hottest, where they occupy the largest volume and therefore where they are most difficult to evacuate.

If a high circulation speed is adopted for the evacuation of the fumes, then the efficiency of the heat exchanger becomes very poor in the upper part of the heating body since the temperature difference between the fumes and the heat transfer fluid is reduced in this region and if the exchange time is very short, there is practically no transfer.

On the contrary, by adopting slow smoke evacuation speeds, there is a good heat exchange, in the upper part of the heating body, where the temperature difference is smaller between the heat transfer fluid and the fumes of combustion, but the burner works in very poor conditions since there is not sufficient draft and there is an increase in the rate of fuel to be burned with emission of carbon monoxide.

In the heating body 4 of the present invention, the speed of passage of the combustion gases is lowered in the lower region of the heating body, that is to say where the transfer of calories takes place under good conditions due to the high temperature difference. Then, the speed of circulation of the fumes is reflected in the upper part of the heating body, where, to obtain a good heat exchange, it is precisely necessary to slow down the speed to increase the time of exchange.

Consequently, the means for increasing the circulation of the fumes in the lower part of the heating body play a favorable role both in terms of the operation of the heat exchange and to promote good combustion at the level of the burners 1.

The means are, for example, in the form of an increase in the cross section of the smoke in the lower part of the heating body 4.

To do this, smoke tubes 10 having a convergent sectional profile can be used. This simple solution is not, however, preferential since it does not allow an improvement in the temperature distribution along the smoke tube.

This is why it is preferred to use smoke tubes 10 which are doubled in their lower part by appendages 13. These appendages 13 on the one hand communicate to the lower part 9 of the heating body 4 and open into the combustion chamber 2 where they make it possible to draw off the combustion fumes at high temperature and on the other hand the appendages 13 are connected to the smoke tubes 10 at their lower part to reinject the fumes into the latter. FIG. 2 diagrams precisely the connection of an appendage 13 to a smoke tube 10.

Preferably, the appendages 13 do not include interior turbulators. This makes it possible to inject, at a certain height of the smoke tubes 10, hot combustion gases taken directly from the combustion chamber. This has two advantages: on the one hand, it facilitates the evacuation of the combustion gases, hence a better yield and less pollution, on the other hand, the temperature distribution in the smoke tube is improved.

Traditionally, the temperature has been reduced in a non-proportional manner in the smoke tube, and with appendix 13 of the present invention. The efficiency of the heat exchange is thereby increased since the area with a large temperature difference is increased.

Of course, the connection height of the appendages 13 on the smoke tubes 10 must be determined by the manufacturer as a function of the geometry of the heating body.

In the example chosen, for a heater body height of 522 mm and for a diameter of 275 mm. the connection height is approximately 100 mm, one fifth of the height of the heating body. Increasing this value, up to 200 mm, the characteristics remain unchanged. Beyond this, there would be a decrease in the efficiency of the heating body because part of the combustion gases would pass too quickly.

On the other hand, a lower connection height, for example of 50 mm, reduces the efficiency of the draft.

In traditional boilers, the heating body acts as a bottleneck for the draft and the secondary air intake cannot be controlled precisely. With the heating body of the present invention, the evacuation of the fumes takes place under good conditions through the heating body and it is possible to regulate the admission of secondary air precisely, for example by rolling. .

This regulation allows a lowering of the flame temperatures and is also accompanied by a rapid evacuation of the flue gases resulting in more limited nitrogen oxide emissions. The coil 11 for domestic hot water is disposed above the appendages 13 and therefore there is no mutual discomfort.

Likewise, it could perfectly be envisaged to provide appendages on some or all of the smoke tubes and / or to connect the appendages 13 at different heights on the main smoke tubes 10.

Claims (7)

1. Boiler comprising a combustion chamber (2), a heating body (4) and a fume activities area (3), suitable for connection to a fume discharge conduit, the said heating body (4) being constituted by a tight outer jacket (7) through which are disposed tubes (10), through which pass the combustion fumes, which have a cross-section that decreases in the direction of flow of the combustion fumes, characterized by the fact that the said tubes (10) are equipped, in their lower portions, with tubular appendages (13) emerging, on one hand, in the combustion chamber (2) and connected, on the other hand, to the fume tubes (10).
2. Boiler according to claim 1, characterized by the fact that the fume tubes (10) are epuipped with turbulence generators (12), and that the appendages (13) do not comprise turbulence generators.
3. Boiler according to claim 1, characterized by the fact that the appendages (13) are connected to the main fume tubes (10) at approximately 1/5 of their height.
4. Boiler according to claim 1, characterized by the fact that the appendages (13) are connected at different heights to the main fume tubes (10).
5. Boiler according to any one of the preceding claims using gas as a fuel.
6. Central heating installation equipped with a boiler according to one of claims 1 to 5.
7. Central heating and domestic hot water production installation equipped with a boiler according to one of claims 1 to 5.

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