EP2359062A1

EP2359062A1 - Closed hearth and regulation method

Info

Publication number: EP2359062A1
Application number: EP09793485A
Authority: EP
Inventors: Anthony Massin
Original assignee: Bodart Et Gonay
Current assignee: BG FIRES
Priority date: 2008-11-18
Filing date: 2009-11-18
Publication date: 2011-08-24
Also published as: WO2010057929A1; BE1018339A3

Abstract

The invention relates to a closed hearth comprising a substantially closed combustion chamber and a system for supplying air and discharging burnt gases that is connected to said combustion chamber and includes an air supply circuit which has a forced air circulation device (18, 30), and a burnt gas discharge circuit. The supply and discharge circuits are in contact over one or more common segments (6) downstream of the forced air circulation device (18, 30) in order to exchange heat between the air and the burnt gases. The closed hearth is characterized in that said system for supplying air and discharging burnt gases also includes a control unit (24) for the forced air circulation device (18, 30), said control unit (24) being connected to a flow rate sensor (25) and being designed to control an air flow rate in such a way that the minimum combustion factor ? in the combustion chamber amounts to 1.5.

Description

"Closed hearth and method of regulation"

The present invention relates to a closed hearth with a system for supplying air and exhausting burnt gases. In the combustion systems of the state of the art, regenerative devices are known for recovering heat from combustion gases in order to preheat the combustion air and thus to improve the efficiency of the combustion system. There are known, for example so-called suction tubes, comprising a central channel for the evacuation of combustion gases and a peripheral channel for the supply of combustion air. Such suction tubes may optionally be provided with an insulation, as illustrated for example in the document EP 1369642, in order to limit heat losses.

Such supply and extraction ducts can operate by simple convection. However, in this case, their effectiveness decreases with the length of the ducts, especially if they include bends. Thus, from a certain length, forced circulation means become necessary to ensure a satisfactory supply and evacuation.

Document DE 3539311 discloses a heating device associated with a suction tube. Hot combustion gases are discharged via the central channel, while combustion air passes through the peripheral channel, in which this air is preheated before entering the combustion chamber. To activate the suction of the fumes and thus the admission of fresh air into the combustion chamber, the free end of the central channel is provided with an exhaust fan.

CA 2260524 discloses a device for introducing a stream of fresh air from the bottom of a flue or chimney. Alternatively, the chimney comprises a jacketed pipe so as to define a central channel for discharging fumes and a peripheral channel for supplying preheated fresh air to the combustion chamber, as well as in the chimney. Fresh air is propelled into the chimney via a fan. This fan is a fan sucking the air to blow it into the chimney.

Document EP 0 013 700 discloses a device for evacuating burnt gases and fresh air intake for gas-tight heaters comprising two concentric ducts, the inner duct being intended for evacuation of flue gases. , while the outer peripheral duct conducts fresh air to the heater. The inner duct is associated with a profiled exhaust hood and an exhaust fan.

The admission of air into the combustion chamber is therefore always obtained by the suction of the combustion gases. This does not allow a precise adjustment of the volume of combustion air admitted, a setting which is advantageous for controlling the temperature and aspect of the combustion flame, as well as the amount of fuel burned.

The document US Pat. No. 4,262,608 describes a system for supplying air and for evacuating burnt gases, comprising an air supply circuit comprising a forced circulation device, and a flue gas evacuation circuit, said circuits for supply and discharge being in contact on one or more common sections to exchange heat between the air and the flue gases. In particular, this system of the state of the art comprises a fan downstream of a peripheral channel of a suction pipe for pushing the combustion air supplied by said peripheral channel to a combustion chamber via a simple pipe, and an extractor sucking fumes upstream of the central channel of the suction pipe to blow them in said central channel. The fan thus acts on a preheated combustion air, while the extractor acts on hot combustion gases.

This system of the state of the art is however intended to be installed on a boiler. The flue gases therefore arrive at the extractor after passing through a heating block, that is to say a gas-liquid heat exchanger in the boiler. Neither their temperature nor, consequently, that of the preheated air, will be more very high.

Closed hearth, however, means hearths, stoves, inserts or cassettes, solid fuel or gas, having a combustion chamber substantially closed but not comprising a gas-liquid heat exchanger. In such closed hearths heat of combustion is therefore mainly released by radiation and / or convection of air, and not by heat exchange with a liquid circuit. In such a closed hearth, the flue gases are normally discharged at a temperature substantially higher than in a boiler. The device for supplying air and exhaust gas burned in US 4,262,608 thus has several disadvantages for use in a closed fireplace. Firstly, the high temperatures of the flue gas and the combustion air preheated by these flue gases in the suction tube could damage the fan, the extractor, and their actuating motor. In addition, while boilers are normally installed in low-traffic areas, closed fireplaces have decorative purposes and are normally installed in well-frequented rooms. The noise generated by the fan, the extractor, and their actuating motor placed near the fireplace will be much more inconvenient.

Thus, all these devices of the state of the art do not provide optimal heat exchange between the combustion air and the flue gases of a closed fireplace.

German patent application DE 30 20 182 A1 discloses a system for supplying air and exhausting burnt gases, comprising an air supply circuit comprising a forced circulation device and a gas evacuation circuit. burned with said supply and discharge circuits in contact with one another common sections for exchanging heat between the air and the flue gases, and a forced circulation device of the air supply circuit upstream of the common heat exchanger section. However, this document does not disclose how the forced air circulation device is regulated. Now, the flow of air supplied by the air supply circuit will determine, with the fuel, the character of the combustion in the closed hearth. Too little airflow can cause less conspicuous flames, and at worst, a generation of toxic carbon monoxide that can even lead to the death of the occupants of the home in which the fireplace is installed in the event of a leak in the home. closed hearth. In boilers, it is known to those skilled in the art to control the flow of fresh air as a function of the concentrations of the combustion products captured by a stoichiometric probe or "lambda probe" in the exhaust gas evacuation circuit. However, such a lambda probe is very expensive, and although in a boiler its use can be economically justified by energy savings, this is more problematic in a closed hearth essentially decorative. In addition, in boilers, in order to obtain a high energy efficiency, the air flow rate is normally adjusted so as to maintain a substantially stoichiometric air / fuel ratio (combustion factor λ = 1). However, with such a combustion factor, the flames are not very visible, whereas in a closed fireplace we mainly look for the decorative effect of the flames. An object of the present invention is therefore to provide bright flames and safe combustion in a closed fireplace, even with air supply and exhaust gas discharge circuits of a certain length.

In at least one embodiment of the invention, a closed hearth comprises: a substantially closed combustion chamber, and a system for supplying air and exhausting burnt gases connected to this combustion chamber and comprising an air supply circuit comprising a forced circulation device, and a burnt gas evacuation circuit, said supply and evacuation circuits being in contact on one or more common sections downstream of the combustion device. forced circulation for exchanging heat between the air and the flue gases and said system for supplying air and exhausting flue gas also comprises a control unit of the forced circulation device connected to a flow sensor and configured to control an airflow such that the combustion factor λ in the combustion chamber is not less than 1.5.

"Combustion factor λ" is the ratio of the mass flow rate of air to the mass flow rate of fuel, divided between the stoichiometric air / fuel ratio. By ensuring an excess of air such that the combustion factor λ in the combustion chamber is not less than 1.5, the control unit of the forced circulation device makes it possible to obtain clearly visible flames and safe combustion. It also allows a good heat exchange or air heating. Normally, flow is defined as mass flow. However, the volume flow could be used alternately.

In addition, by placing the forced circulation device upstream of said heat exchange sections, it is obtained that the forced circulation device acts on the combustion air at a temperature closer to ambient temperature. In this way it is possible to ensure good reliability of the device without having to take expensive and expensive thermal protection or cooling measures. In addition, because of the higher density of this cooler air, the efficiency of the forced circulation device is higher. The regulation of the air flow is also facilitated. Finally, since the forced circulation device is upstream of at least a portion of the common heat exchanger section on the air supply circuit, it may be located at a greater distance from the insert insert type hearth. The apparent noise in the vicinity of the closed hearth can thus be reduced without recourse to the same complex and expensive soundproofing means.

Other sensors are possible, for example to check the content of CO, CO2, etc. combustion gases. The control device can for example in case of bad combustion (too much CO) control the forced circulation device to bring more air into the closed firebox.

Preferably, said at least a portion of a common heat exchanger section located downstream of the forced circulation device may have a length of at least two meters, preferably at least three meters. In this way it is ensured that the preheating of the combustion air is carried out mainly downstream of the forced circulation device.

Preferably, said common heat exchanger section may be a suction tube comprising a central channel for exhausting burnt gases and a peripheral channel for supplying combustion air. In this way, the efficient heat exchange between the flue gas and the combustion air is easily and efficiently provided. In addition, such a suction tube can easily be adapted to a closed hearth, and can also easily give him a path with one or more elbows. However, said common heat exchanger section may alternatively have a different arrangement of the supply and discharge channels, for example a side-by-side arrangement.

More preferably, said forced circulation device may comprise at least one nozzle opening into said peripheral channel. Even more advantageously, said nozzle may be oblique with respect to said peripheral channel. Thus, it also imparts a helical movement to the combustion air, so as to increase the heat exchange between the flue gases in the central channel and combustion air in the peripheral channel.

In a particular embodiment, the forced circulation device may be at least partially integrated in said peripheral channel, so as to reduce the size of the system.

Advantageously, the forced circulation device may comprise at least one fan, which allows a control of the flow rate by controlling the operating speed of the fan. For example, this fan could be a centrifugal fan.

Advantageously, the forced circulation device may comprise at least one ejector. This or these ejectors eject a small amount of air at high speed to blow or propel a larger amount of air. The efficiency of the forced circulation device can thus be improved. Advantageously, the air supply circuit and / or the exhaust gas evacuation circuit may comprise one or more ducts of circular section, but they may also be of different section, such as a square section, a rectangular section. , a hexagonal section, etc. according to the necessities.

In a particular embodiment, the closed hearth is a gas closed hearth, in which the combustion chamber comprises at least one gas burner connected to a fuel gas supply circuit with a flow regulator. Advantageously, in this embodiment, the control unit of the forced circulation device can be configured to control a substantially constant air flow, and the flow regulator of the fuel gas supply circuit have a maximum flow rate such that the combustion factor λ in the combustion chamber is not less than 1.5. The flow regulator of the fuel gas supply circuit could be connected to a user-actuable control to vary the flow of fuel gas supplied to the combustion chamber, of course on a continuous scale, although in a staggered manner, but always with a maximum not exceeding said maximum flow. In this way, it is possible to ensure that the combustion factor is not less than 1.5 in a simple manner and without using a lambda probe, and while regulating the flame. Preferably, during combustion, the flow regulator of the fuel gas supply circuit also has a minimum flow rate with which the combustion factor with said constant air flow rate does not exceed 6. With a large excess of air , the flame can indeed become unstable or even be blown.

In an alternative embodiment, the closed hearth is a solid fuel fireplace, with a combustion chamber configured to receive a solid fuel charge (eg, coal, coke, wood or pellets) and the control unit of the device forced circulation is configured to control a variable air flow as a function of the combustion time. When a solid fuel is burned, the actual fuel input to the reaction of combustion varies during combustion, following pyrolytic generation and evaporation of volatile substances in the solid fuel. It is possible, by analyzing the products of combustion in previous experiments, to establish a combustion profile with the time-dependent development of the airflow required to maintain a combustion factor of greater than 1.5 with predetermined solid fuel charge, and even for this combustion factor to follow a predetermined curve.

Advantageously, the closed hearth further comprises a combustion gas temperature sensor connected to the control unit of the forced circulation device, said control unit of the forced circulation device being configured to control a variable air flow rate according to also the temperature sensed by said temperature sensor. When a solid fuel closed hearth is loaded, it can receive varying amounts of solid fuel. The temperature sensor allows the control unit of the forced circulation device to take into account the amount of fuel received by the closed hearth with said solid fuel charge. The present invention also relates to a method of controlling a closed hearth, with a substantially closed combustion chamber, wherein the flow of air supplied to said combustion chamber by a forced circulation device is controlled so that that the combustion factor λ in the combustion chamber is not less than 1.5. The features and details of the invention will become apparent from the following detailed description in which reference is made to the accompanying drawings.

In these drawings, FIG. 1 is a schematic view of a closed hearth according to an embodiment of the invention, partially in section,

FIG. 2 is a sectional view along the line II-II of FIG. 1; FIG. 3 is a perspective view of an oblique deflector in the form of a fin;

FIG. 4 is a schematic view of a closed hearth according to another embodiment of the invention; FIG. 5 is a diagrammatic view of a fan used in the installation of FIG. 5;

FIG. 6 is a schematic view of a detail of a system for supplying air and exhausting burnt gases according to another embodiment of the invention,

FIG. 7 is a diagrammatic view of a detail of an air supply and exhaust gas evacuation system according to yet another embodiment of the invention,

FIG. 8 is a schematic view of a closed hearth according to yet another embodiment of the invention, partially in section, FIG. 9 shows several curves corresponding to examples of combustion profiles in the closed hearth of FIG. 8 with different fuel loads. Figure 1 shows a closed hearth type "cassette" comprising:

- A combustion chamber 1 substantially closed, said chamber 1 being in this embodiment surrounded by a casing 2 adapted to allow the passage of air 3 in the casing along one or outer walls of the combustion chamber 1 or in contact with the combustion chamber 1, and also comprising a gas burner 4A supplied by a fuel gas supply circuit 4 also comprising a flow regulator 4B connected to a user-actuatable control 50 for selecting, in the lit hearth, between three levels of fuel gas flow Q _min , Q _m oy, and Q _m ax, said envelope 2 also having one or more openings 5 for the passage of air to the combustion chamber 1, a conduit 6 of the concentric suction tube type comprising two concentric ducts 6A, 6B, namely a central duct 6A connected to the combustion chamber 1 so as to serve to evacuate the smoke or combustion gas, and a duct outside 6B defining a peripheral channel 7 connected to the casing 2 for supplying combustion air to the combustion chamber 1, the top of the duct 6 being advantageously provided with a perforated cap 8 with inclined flaps to avoid the rain inlet in the channel 7, and a distributor 9 to control the evacuation of fumes out of the central channel 6A, two flat 10,11 shaped rings extending in the peripheral channel 7 between the wall outside the central duct 6A and the inner wall of the peripheral duct 7B, so as to divide the peripheral channel 7 defined between the ducts 6A and 6B in three portions, namely an upper portion 7A extended by the suction cap 8, a portion lower 7B connected to the casing 2, and an intermediate portion 7C which is sealed with respect to the portions 7A, 7B, the upper portion having near the level of the dish 10 an exit window 12 (just above the dish 10), while the lower portion 78 has an inlet window 13 near the level of the flat 11 (just below the flat 11),

a casing 14 removably mounted on the duct 6, this casing having a channel 15 extending between an inlet opening 16 and an outlet nozzle 17, the inlet opening being intended to be opposite the window 12, while the outlet nozzle 17 is intended to face the window 13, said housing 14 thus serving to place the upper portion 7A in communication with the lower portion 7B of the peripheral channel 7,

- A fan 18 located in a portion of the channel 15 of the housing 14, said fan being rotated by a motor 19, said fan 18 being adapted to draw air from the upper portion 7A to blow in the lower portion 7B through the nozzle 17,

a first series of oblique fin-shaped deflectors situated at a distance of approximately 10 to 30 cm downstream from the nozzle 17, said fins being arranged to induce a movement helical to the air circulating in the lower portion 7B or in a part thereof,

a second series of fins 21 distant from the first series of fins 20 to induce again a helical movement to the air circulating in a portion of the lower portion 7B, the intermediate portion 7C of the channel 7 has an opening 21 located facing a door 22 closing an access window inside the inner duct 6, a plug 23 closing the opening 21, a control unit 24 adapted to control the fuel gas flow regulator 4B, as well as the rotation of the motor 19 and its rotational speed, said control unit 24 being connected by wired and / or wireless connection, for receiving signals from one or more flow sensors 25. Sensors other than the sensor of flow rate 25, such as temperature sensors, pressure sensors, speed sensors, and other actuators, such as valve actuators placed in the air supply, exhaust gas, or gas supply circuits. co may also be connected to the control unit 24. The control unit 24 may advantageously be integrated in the housing 14.

The portions 7A, 7B of the peripheral channel 7 and the channel 15 thus form a combustion air supply circuit to the combustion chamber 1, while the channel or central duct 6A forms a combustion gas evacuation circuit. of the combustion chamber 1. In a possible embodiment the plates 10, 11 would only partially close the passage section of the peripheral channel so as to form an air passage between the portions 7A and 7B which is parallel to the channel 15 of the housing 14.

Figure 2 is a top view of the series of fins 20. These fins extend between the inner conduit 6A and the outer conduit 6B. These fins are advantageously welded to said ducts and are advantageously made of a heat conductive material.

The fins 20 (see FIG. 3) are profiled to generate a substantially helical air movement in the peripheral channel. However, in an alternative embodiment, the peripheral channel 7 may also be devoid of fins, so that the air stream is not helical, but substantially parallel to the axis of the duct 6. The duct 6B may be provided with thermal insulation.

FIG. 4 is a schematic view of an installation according to an embodiment of the invention which is similar to that shown in FIG. 1, except that the housing with fan has been replaced by a series of four small ones. fans 30, integrated in the peripheral channel

7B, and whose axis of rotation is inclined with respect to the axial axis of the duct 6. One of these fans 30 is illustrated in FIG.

Instead of using inclined fans, it is also possible to use turbines placed parallel to the axis of the duct 6, but whose output is associated with an inclined nozzle or a deflector.

FIG. 6 is a schematic view of a part of an installation according to another embodiment of the invention which is also similar to that shown in FIG. 1, except that the fan 18 is located at the top duct 6B, the combustion air entering the fan 18 via a single duct 31.

FIG. 7 is a schematic view of part of an installation according to another embodiment of the invention which is also similar to the previous ones, except that the ducts 6A, 6B are not concentric, but placed side by side, the duct 6A thus serving to supply air, and the duct 6B to the evacuation of combustion gas.

In operation, the combustion chamber 1 of the closed hearth of FIG. 1 is supplied with combustion air by the combustion air supply circuit formed by the portions 7A, 7B of the peripheral channel 7, the channel 15, and the fan 18 of the channel 15, and combustion air by the fuel gas supply circuit 4 via the flow regulator 4B and the burner 4A. An air / fuel mixture is thus formed in the combustion chamber 1 to be burned. The flue gases are then discharged through line 6A.

The control 50 governs the flow regulator 4B of the fuel gas supply circuit

4. A user can thus adjust the intensity of the flame through the control 50 by increasing and decreasing the gas flow through this control 50 and the flow controller 4B. The control unit 24 governs the rotational speed of the motor 19 of the fan 18 according to signals from the flow sensor 25, so as to maintain a substantially constant flow of air, and this independently of the resistance to the flow of air. air of the combustion air supply circuit. The control unit 24 is configured to control an air flow such that, even with the maximum gas flow rate Q _{max that} can be selected by the user with the control 50, the combustion factor λ remains at least equal to at 1.5. For example, Q _max could correspond to a combustion factor λ between 1.8 and 2.4, for example λ = 2, Q _moy at λ = 3.5, and Q _min at λ = 5.

In an alternative embodiment illustrated in Figure 8, the closed hearth is a solid fuel hearth (coal, coke or wood). The combustion chamber 1 comprises a fuel tank 51 for receiving said solid fuel. Apart from the same elements as the closed hearth of Figure 1 (except the fuel gas supply system 4 with the flow regulator 4B and the control 50), this solid fuel fireplace also has a temperature sensor 52 connected to the control unit 24, and, for the air inlet into the combustion chamber 1, lateral openings 5a in addition to the lower openings 5. The supply of air through the lateral openings 5a is controlled by valves 53 connected to the control unit 24 and to close them for example at the beginning of the combustion, when the flames are not yet sufficiently developed so that the air brought by the lateral openings participates in the combustion.

In a solid fuel fireplace, fuel input to the combustion reaction is not as easily adjustable as in a gas fireplace. During the combustion of a solid fuel, some of the solid fuel burns directly on the surface, but at the same time volatile compounds, mainly pyrolysis products, are released by heat and also participate in the combustion.

However, it is possible, by experimentally measuring the evolution of combustion gases as well as the concentrations of the products of combustion (in particular carbon dioxide, carbon monoxide and nitrogen oxides) in this combustion gas throughout the combustion, to deduce both the fuel input and the combustion factor at each moment of combustion. It is thus possible to establish a combustion profile with a curve indicating the mass flow rate of air to be supplied by the fan 18 as a function of the combustion time for a predetermined fuel load in order to maintain a combustion factor λ≥l. 5. Figure 9 illustrates three curves 101, 102, and 103 each indicating the mass flow rate of air as a function of time for a different load. Curve 101 corresponds to a minimum load, curve 102 to average load, and curve 103 to maximum load. In operation, the temperature sensor 52 indicates to the control unit 24 the heat released, thus allowing the unit to control 24 to extrapolate the fuel charge to apply the appropriate combustion profile.

Although the present invention has been described with reference to a specific exemplary embodiment, it is obvious that various modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims.

Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

1. Closed hearth comprising:

a combustion chamber substantially closed, and

an air supply and exhaust gas evacuation system connected to this combustion chamber and comprising: an air supply circuit comprising a forced circulation device (18, 30), and a circuit for supplying combustion gases; discharge of burnt gases, said supply and discharge circuits being in contact on one or more common sections (6) downstream of the forced circulation device (18, 30) for exchanging heat between the air and the gases burned, and the closed hearth being characterized in that said system for supplying air and for exhausting burnt gases also comprises a control unit (24) of the forced circulation device (18, 30) connected to a sensor flow rate (25) and configured to control an air flow rate such that the combustion factor λ in the combustion chamber is not less than 1.5.

2. Closed hearth according to claim 1, wherein the combustion chamber (1) comprises at least one gas burner (4A) connected to a fuel gas supply circuit (4) with a flow regulator (4B).

The closed hearth of claim 2, wherein the control unit (24) of the forced circulation device (18,30) is configured to control a substantially constant air flow rate, and the flow regulator (4b) of the supply circuit in fuel gas (4) has a maximum flow such that the combustion factor λ in the combustion chamber is not less than 1.5.

The closed hearth of claim 1, wherein the combustion chamber (1) is configured to receive a solid fuel charge and the control unit (24) of the forced circulation device (18,30) is configured to control a variable air flow as a function of the combustion time.

The closed hearth of claim 4, further comprising a temperature sensor (26) of the combustion gases connected to the control unit (24) of the forced circulation device (18,30), said control unit (24). ) of the forced circulation device (18,30) being configured to control a variable air flow also according to the temperature sensed by said temperature sensor (26).

6. closed hearth according to any one of the preceding claims, wherein said at least a portion of a common heat exchanger section (6) downstream of the forced circulation device has a length of at least two meters, preferably d at least three meters.

7. Closed hearth according to any one of the preceding claims, wherein said at least one common heat exchanger section (6) is a suction tube having a central channel (6A) for exhausting burnt gas and a peripheral channel (7) d supply of combustion air.

8. closed hearth according to claim 8, wherein said forced circulation device (18) comprises at least one nozzle (17) opening into said peripheral channel (7).

9. closed hearth according to claim 9, wherein said nozzle (17) is oblique with respect to said peripheral channel (7).

10. Closed hearth according to any one of claims 4 to 6, wherein the forced circulation device (30) is at least partially integrated in said peripheral channel (7).

11. A closed combustion air supply and exhaust gas exhaust system according to any one of the preceding claims, wherein the forced circulation device comprises at least one ejector.

12. A method of controlling a closed hearth with a substantially closed combustion chamber, wherein the flow of air supplied to said combustion chamber by a forced circulation device is controlled so that the combustion factor λ in the combustion chamber is not less than 1.5.

13. A control method according to claim 13, wherein said air flow rate is substantially constant and a maximum flow rate of fuel gas supplied to said combustion chamber is such that the combustion factor λ in the combustion chamber is not less than at 1.5.

The control method according to claim 13, wherein the closed hearth is a solid fuel fireplace, and the airflow is controlled according to at least one combustion time.

15. Control method according to claim 15, wherein the air flow rate is controlled also according to the temperature of the combustion gases.