EP3957910A1 - Method and assembly for the pneumatic mixture formation in a premix burner - Google Patents

Abstract

The invention relates to a method and an arrangement for the pneumatic formation of a mixture for a burner (3) with a fan (2) for generating an air flow which is fed to the burner (3) via a throttle point (6), with upstream or in the Throttle point (6) fuel gas can be added via a fuel gas control valve (6), with a reference pressure measuring point (13) between fan (2) and throttle point (6) in an inlet stage (8) influencing the air flow upstream of the throttle point (6), which has a Control line (14) is connected to the fuel gas control valve (9). In particular, the preliminary inlet stage (8) is designed in such a way that a reduced reference pressure (Pref) can be taken at the reference pressure measuring point (13), the preliminary inlet stage (8) having a narrowed area (16) in which at least one pressure extraction opening (12) is present is, which is connected to the reference pressure measuring point (13). Each pressure tapping opening (12) can have a chamfer (15) at least on a downstream edge (17). The present invention allows a reduced reference pressure (Pref) to be taken from a heater (1) for regulating the supply of fuel gas, as a result of which pressure losses in the exhaust gas duct (10) of the heater (1) no longer have a negative effect and reliable activation of a fuel gas control valve ( 9) is possible even with changing pressure losses.

Description

The invention relates to a method and an arrangement for the pneumatic formation of a stable mixture in a premix burner for the combustion of fuel gas with ambient air. Known burners, including those in so-called gas condensing boilers in particular, are supplied with ambient air by a blower, and fuel gas is added in a predeterminable amount via a fuel gas control valve. The invention relates to arrangements in which fuel is added on the pressure side of the fan. The mixture is then ignited in a combustion chamber and then used in a stable, controlled combustion process to generate heat, usually to provide warm service water and/or heat for a heating system. A mixture of air and combustible gas only burns safely and in an environmentally friendly manner if the mixing ratio of air to combustible gas (the lambda air ratio) is in an ignitable and environmentally compatible range. If this is not the case, (partially) unburned combustible gas may escape into the environment, possibly including carbon monoxide and/or an ignitable mixture may even accumulate at an undesirable location, which may pose a risk of explosion. For this reason, special attention is paid to the formation of the mixture of ambient air and fuel gas.

According to the prior art, at the beginning of a combustion process, a blower is first switched on, which directs a predeterminable air flow via a throttle point, in particular a Venturi nozzle, into a burner. Combustion gas is added to this air flow at a combustion gas inlet, usually in front of or in the Venturi nozzle, which is done by opening a combustion gas shut-off valve and via a combustion gas control valve. Around to ensure that fuel gas is not added too early when the blower is started up or in the wrong amount during operation, there is a control line that forwards a reference pressure Pref, which is measured between the blower and the fuel gas inlet, to the fuel gas control valve so that it only opens when sufficient air is conveyed by the blower and allows a quantity of combustion gas proportional to the reference pressure to pass within a certain control range or a pressure Pout equal to the reference pressure Pref is set at the outlet of the combustion gas control valve. So here Pout = Pref. Conversely, the fuel gas control valve closes if for some reason not enough air is conveyed because the reference pressure is then too low. However, the control range is also left when the difference between the reference pressure and a supply pressure Pin of the fuel gas, the z. B. can be about 15 mbar [millibar], is too small, so the reference pressure is too large in relation.

This described pneumatic regulation thus depends, among other things, on the quality or type of measurement of the reference pressure. In the case of very high pressure losses, which depend on the respective burner output, e.g. B. can arise in the exhaust system, the reference pressure formed in this way can become so high that it almost reaches the inlet pressure Pin of the fuel gas available at the fuel gas control valve (e.g. 15 mbar) and thus leaves the control range. The consequence of this is that the flow of combustible gas mixed with the flow of air tends to zero and accordingly the burner cannot be supplied with enough combustible gas for combustion, which can lead to undesired consequences up to and including shutting down the system.

For such a pneumatic control according to the prior art, it is the object of the present invention to change the reference pressure decrease in such a way that influences, in particular due to pressure losses downstream of the throttle point (Venturi nozzle), do not have too great an influence on the reference pressure and at least one for the control required pressure difference between reference pressure and supply pressure of the fuel gas during operation is available as reliably as possible.

A method and an arrangement as well as an inlet preliminary stage according to the independent claims are used to solve this problem. Advantageous refinements and developments of the invention are specified in the respective dependent claims. The description, in particular in connection with the figures, illustrates the invention and specifies further exemplary embodiments.

In the method according to the invention for forming a mixture of air and fuel gas for a premix burner, a fan generating an air flow which is fed to the burner via a throttle point, in particular a venturi nozzle, and fuel gas is added to the air flow before or in the throttle point via a fuel gas control valve a reference pressure measuring point for taking a reference pressure between the blower and the throttle point is connected by means of a control line to the fuel gas control valve for forwarding the reference pressure, the reference pressure is taken in a pressure-reducing manner in an inlet preliminary stage. The reference pressure is still proportional to the static pressure caused by the blower in front of the throttling point, but by using an inlet preliminary stage, the pressure is reduced somewhat and the disturbing influence of pressure losses in the downstream system is relatively reduced. In the simplest case, such an influencing (reduction) of the reference pressure can be achieved by measuring this pressure at a point where the air flow is faster (and therefore the pressure is lower) than at a conventional reference pressure measuring point. Typical nozzle shapes result in such areas of faster flow. In this case, the reference pressure Pref taken in the forward stage is made up according to the formula Pref=Pstat+Pdyn, where Pstat is the static pressure in front of the throttle point, which represents the amount of air conveyed by the blower and Pdyn is a flow rate dependent dynamic (negative) pressure.

In a preferred embodiment of the invention, the preliminary inlet stage is directly upstream of the throttling point, of course in a streamlined manner, so that there are no unnecessary pressure losses at the connection point. The preliminary inlet stage has a narrowed area in which the reference pressure is extracted by means of at least one pressure extraction opening. Since a flow becomes faster and the pressure lower in a narrowed area through which flow occurs, the desired reduction in the reference pressure can be achieved , since the reduced reference pressure is almost exclusively dependent on the static part of the air flow generated by the blower.

Two or more pressure tapping openings are particularly advantageously arranged in the narrowed area and connected to one another to form a reference pressure measuring point. This can be achieved, for example, in that all pressure tapping openings open into an annular space around the narrowed area, from which the reference pressure is then also tapped.

The invention also provides an arrangement for the pneumatic formation of a mixture for a burner for the combustion of fuel gas with ambient air, with a fan being present for generating an air flow which is fed to the burner via a throttle point, in particular a Venturi nozzle, with upstream or in the Throttle point can be mixed with a fuel gas control valve fuel gas, further a reference pressure measuring point between fan and throttle point is present, which is about a control line is connected to the fuel gas control valve and the reference pressure measuring point is arranged in an inlet preliminary stage influencing the air flow upstream of the throttle point. According to the invention, the reference pressure measuring point is not arranged at a point with a flow that is as uniform as possible, as was customary up to now, but in an inlet preliminary stage that changes the flow. It is important that the presence of the preliminary inlet stage allows the reference pressure measuring point to be located in an area in which the pressure is lower than that of the undisturbed flow. There are many designs that locally reduce the pressure, e.g. B. throttle points, nozzles, venturi nozzles etc.

The inlet preliminary stage is therefore preferably designed in such a way that a reduced reference pressure can be taken at the reference pressure measuring point, which is then fed to the fuel control valve. This is calibrated accordingly for the changed pressure range of the reference pressure, which, however, still has the desired proportionality to the air flow, so that the desired amount of fuel continues to be mixed with the air flow. The condition Pout = Pref is thereby modified so that in general Pout > Pref applies, since Pout should remain the same but Pref decreases.

In a preferred embodiment of the invention, the preliminary inlet stage has a narrowed area in which there is at least one pressure tapping opening which is connected to the reference pressure measuring point. Up to now, such pressure tapping openings have mostly been bores, but other shapes can also be produced with modern production techniques. It is only important that the pressure of the flow can be reliably taken at the opening.

In a special embodiment, each pressure tap opening has a chamfer at least on a downstream edge. This is particularly beneficial when no laminar flow develops in the constricted area, which is to be expected. A chamfer on each inflow edge can then help to avoid additional turbulence and pressure fluctuations. Since pressure tapping openings can also be located in regions running obliquely to the direction of flow (that is to say tapering conically), it is particularly advantageous to bevel at least the inner edges of the pressure tapping openings located downstream. However, the entire edges can also be chamfered.

The preliminary inlet stage is preferably arranged directly upstream of the throttling point and forms a nozzle or venturi nozzle through the narrowed area, to which the throttling point is connected in a flow-optimal manner. In this way, the combination of preliminary inlet stage and throttling point can be built in a compact manner and with little disruption to the flow. It is also possible to integrate both into one component.

The invention also relates to a preliminary inlet stage for a burner, the preliminary inlet stage having a narrowed area from which at least one pressure tapping opening branches off and is connected to a reference pressure measuring point. For example, the constricted area can exhibit a reduction in the cross-sectional area available for air flow by 5 to 40%, preferably 10 to 20%. Such an inlet preliminary stage can be easily retrofitted or integrated into new systems and allows a reduced reference pressure, which is not disturbed by subsequent pressure losses in a system, to be taken, with which a fuel gas control valve can be reliably controlled.

Fig. 1:

Eine erfindungsgemäße Vorrichtung zur pneumatischen Gemischbildung und

Fig. 2:

ein Detail aus Fig. 1, nämlich die Form einer Druckentnahmeöffnung.

A schematic embodiment of the invention, to which it is not limited, and the functioning of the method according to the invention will now be explained in more detail with reference to the drawing. They represent:

Figure 1:

A device according to the invention for pneumatic mixture formation and

Figure 2:

a detail out 1 , namely the shape of a pressure release port.

figure 1 shows schematically an arrangement according to the invention, namely a heating device 1 with its details that are essential for understanding the invention. A blower 2 sucks in ambient air via an air intake pipe 4 and guides it to a burner 3. Behind the blower 2 and in front of the burner 3, the air flows through a throttle point 6, in particular in the form of a Venturi nozzle, in which combustible gas is mixed with the air flow. This passes via a fuel gas shut-off valve 5 from a supply line with the supply pressure Pin to a fuel gas control valve 9 and from there with an outlet pressure Pout to the throttle point 6. The fuel gas control valve 9 is connected to a reference pressure measuring point 13 via a control line 14. The reference pressure Pref taken there determines whether and to what extent the fuel gas control valve 9 opens in order to admix an amount of fuel gas corresponding to the air flow. A control unit 7 controls and regulates the processes taking place in the heater via signal lines (not shown). Flames emerging from the burner 3 heat a heat exchanger 11 via a heat exchanger 11, a heating circuit and/or service water circuit not shown here. Any exhaust gases are discharged into the environment via an exhaust duct 10 .

While according to the prior art the reference pressure Pref is simply taken between the blower 2 and the throttle point 6 , according to the invention this is done in a preliminary inlet stage 8 which has a narrowed area 16 . The reference pressure Pref is removed there via at least one pressure removal opening 12 . This is lower because of the faster flow in the narrowed area 16 . As stated above, different conditions in the burner 3 and/or the exhaust gas duct 10 (or different ambient conditions or different outputs of the burner) lead to pressure losses, which, however, can no longer lead to unfavorable consequences due to the effect of the narrowed area 16 and the resulting lower reference pressure Pref. The lower reference pressure Pref can be compensated for by calibrating the fuel gas control valve 9 accordingly. However, there is no longer a risk that the reference pressure Pref will come close to the supply pressure Pin of the fuel gas, which is why an insufficient supply of fuel gas is no longer to be feared even with relatively high pressure losses in the gas routing system 10 .

The design of the preliminary inlet stage 8 with one or more pressure tapping openings 12 leaves a great deal of design freedom. In the present schematic exemplary embodiment, two pressure tapping openings 12 are shown as an example, which open into an annular space and are connected to the reference pressure measuring point 13 via this. The pressure tapping openings 12 are arranged in a narrowed area 16 in which the air stream flows faster. The pressure tapping openings 12 can already be in a conical area that is still narrowing or in a cylindrical area that is already narrowed. In general, but especially in the first case, it is advantageous to provide the inner edges of the pressure tapping orifices 12 with a chamfer 15, at least in a downstream part.

This configuration with chamfer 15 is enlarged again in 2 shown. There, the pressure tapping opening 12 shown is in a conical part of the narrowed area 16.

The present invention makes it possible to obtain a reduced reference pressure for controlling the fuel gas supply in a heater, as a result of which pressure losses in the exhaust gas duct of the heater no longer have a negative effect and a reliable Control of a fuel gas control valve is possible even with changing pressure losses.

reference list

1

heater

2

fan

3

burner

4

air intake pipe

5

fuel gas shut-off valve

6

Throttle point (Venturi nozzle)

7

control unit

8th

pre-enema stage

9

fuel gas control valve

10

exhaust duct

11

heat exchanger

12

pressure vent

13

reference pressure measuring point

14

control line

15

chamfering

16

constricted area

17

Downstream edge of the pressure tap

Pref

reference pressure

Pin code

Fuel gas supply pressure

pout

Output pressure of the fuel gas downstream of the fuel gas control valve

Claims (9)

A method for forming a mixture of air and fuel gas for a burner (3), with a fan (2) generating an air flow which is fed to the burner (3) via a throttle point (6), and the air flow upstream of or in the throttle point ( 6) via a fuel gas control valve (9) fuel gas is added, with a reference pressure measuring point (13) for taking a reference pressure (Pref) between the blower (2) and the throttle point (6) by means of a control line (14) with the fuel gas control valve (9) for forwarding the Reference pressure (Pref) is connected and wherein the removal of the reference pressure (Pref) takes place in a pressure-reducing manner in an inlet preliminary stage (8). Method according to Claim 1, in which the preliminary inlet stage (8) is connected directly upstream of the throttle point (6) and has a narrowed area (16) in which the reference pressure (Pref) is tapped by means of at least one pressure tapping opening (12). Method according to Claim 2, in which two or more pressure tapping openings (12) are arranged in the narrowed region (16) and connected to one another to form a reference pressure measuring point (13). Arrangement for the pneumatic formation of a mixture for a burner (3) for the combustion of fuel gas with ambient air, a blower (2) being present for generating an air flow which is fed to the burner (3) via a throttle point (6), with before or in the throttle point (6) via a fuel gas control valve (6) Combustion gas can be added, with a reference pressure measuring point (13) between the blower (2) and the throttle point (6), which is connected to the combustion gas control valve (9) via a control line (14) and the reference pressure measuring point (13) in a the inlet preliminary stage (8) influencing the air flow is arranged upstream of the throttle point (6). Arrangement according to Claim 4, in which the preliminary inlet stage (8) is designed in such a way that a reduced reference pressure (Pref) can be taken at the reference pressure measuring point (13). Arrangement according to Claim 4 or 5, in which the preliminary inlet stage (8) has a narrowed region (16) in which there is at least one pressure tap opening (12) which is connected to the reference pressure measuring point (13). Arrangement according to Claim 6, in which each pressure tap opening (12) has a chamfer (15) at least on a downstream edge (17). Arrangement according to one of claims 6 to 7, wherein the preliminary inlet stage (8) is arranged directly upstream of the throttle point (6) and forms a nozzle or Venturi nozzle through the narrowed area (16), to which the throttle point (6) adjoins in a flow-optimal manner. Inlet preliminary stage (8) for a burner (3), the inlet preliminary stage (8) having a narrowed area (16) from which at least one pressure tap opening (12) branches off and is connected to a reference pressure measuring point (13).

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