EP4075061B1 - Arrangement for reducing the consequences of a flashback in a premix burner of a heater - Google Patents

Description

The invention relates to an arrangement for reducing the consequences of a flashback in a burner that is operated with a mixture of a fuel gas and air (premix burner), in particular for a heater.

Hydrogen as a fuel gas or as an admixture to fuel gases is becoming increasingly important, and great efforts are being made to equip new or existing heating devices to operate with it. This is not just about large systems, but also about wall-mounted devices for heating water and generally about heating devices for heating buildings and/or providing warm water. Burners for such purposes often have a burner body made of perforated sheet metal and can be designed as so-called flat or cylinder burners. Round holes are usually used, but elongated slots and other shapes of holes are also possible. The dimensions of the holes as well as their number/area unit and arrangement influence the flame stability during combustion in a combustion chamber as well as combustion hygiene (completeness of combustion and production of pollutants). Depending on the properties of the fuel gas used, it can happen with a given burner that the flame does not burn on or just above a burner surface, but instead migrates into the perforated plate and the fuel gas-air mixture (hereinafter also referred to simply as a mixture) is already unintentionally on the Inside of the perforated plate ignited. This can, especially if it occurs frequently, lead to thermal overloading of components in the burner and/or in one lead upstream mixture path. The risk of such back-ignition, known as flashback, is particularly great if the flame speed of the respective fuel gas-air mixture is higher than the outflow speed at the burner holes/slots. This is especially true when using pure hydrogen as fuel gas or fuel gas with a hydrogen content of e.g. B. 10% or more is the case.

DE 10 2019 101 329 A1 concerns the regulation of the mixing ratio of combustion air and fuel gas in a hot water device or a heater. Devices and a method are specified which enable precise control of the mixing ratio of combustion air to fuel gas (lambda value) by means of non-invasive measurement, even for different compositions of the fuel gas, in particular when hydrogen is added. This is made possible with an optical filter and optical sensor present on or in the housing.

DE 1 238 412 B discloses various configurations of a safety device for preventing the formation of gas mixtures and for stopping explosions in fuel gas lines, consisting of the safety elements flame arrester, gas backflow valve and pressure relief valve. The gas backflow valve is designed in such a way that it can shut off the gas flow in any direction of flow and the pressure relief valve is held in its open position after it opens suddenly. This ensures that if the gas-carrying hoses are destroyed, further gas supply is immediately interrupted.

The basic use of a safety valve is also suggested in the context of combustion and the risk of flashbacks with other fuel gases, such as for oxyfuel combustion, see e.g. b. DITARANTO M ET AL: "Combustion instabilities in sudden expansion oxy-fuel flames", COMBUSTION AND FLAME, ELSEVIER SCIENCE PUBLISHING CO., INC., NEWYORK, NY.; US, AMSTERDAM, NL, Vol. 146, No. 3, August 1, 2006 (2006-08-01), pages 493-512, XPO27878623 ISSN: 0010-280 , on which the two-part form of claim 1 is based.

It therefore makes sense, on the one hand, to minimize the risk of a flashback through suitable operating parameters and yet to quickly detect any flashbacks that occur and to stop the supply of fuel gas, for which various sensors and the evaluation of their signals can be used. On the other hand, studies have shown that in the event of a flashback, the volume of mixture in the burner body ignites completely very quickly and then first triggers a pressure wave in the mixture path in front of the burner, followed by a flame front, so that the mixture in the mixture path also ignites and the greater the hydrogen content in the fuel gas, the more violently it burns and develops heat. This can lead to damage to the fan, particularly in premix burners where the fuel gas is mixed in before a fan, especially if it contains parts (e.g. a fan wheel) made of plastic, which is generally the case.

To date, the mixture path in heaters has mainly been optimized with regard to the supply of mixture to the burner in order to keep the energy consumption for the supply (and thus the performance of the fan) low. There were no special design measures to control flashback or to reduce its consequences.

The object of the present invention is to at least partially solve the problems mentioned with reference to the prior art. In particular, constructive ones should be Measures must be taken on and/or in the mixture path to reduce the consequences of a flashback and thus help to avoid damage to components of the mixture path and/or blower.

An arrangement according to the independent claim is used to solve this problem. Advantageous refinements and further developments of the invention are specified in the dependent claims. The description, especially in connection with the drawing, illustrates the invention and gives further exemplary embodiments.

An arrangement contributes to this by reducing the consequences of a flashback of a flame into a mixture path of a premix burner in a heater, which can be operated with a mixture of fuel gas and air, with a pressure relief valve being arranged in the mixture path between a burner body and a fan, which is so (dimensioned and) set so that it is (always) closed during normal operation of the premix burner.

The heater, in particular a condensing boiler, can be operated with hydrogen or a hydrogen-containing fuel gas.

Furthermore, the mixture path has a branch at which mixture from the fan can flow in laterally from a fan branch, but a pressure wave running in the opposite direction essentially or predominantly (only) reaches the pressure relief valve. Such a branching, which in terms of flow technology produces only small losses in normal operation, has completely different properties for a returning pressure wave than for the normal flow of the mixture. In this way, the effect of the pressure wave on the pressure relief valve can be increased and its response can be accelerated. Accordingly, a subsequent flame front can also be guided past the fan branch.

The pressure relief valve has little influence on normal operation, but opens in the event of overpressure due to a pressure wave, such as can be triggered by a flashback.

The pressure relief valve is preferably arranged and/or designed in such a way that it can be directly hit by a pressure wave triggered by a flashback and (thereby automatically) opened. Direct means here in particular that there are no deflections or installations between the point of origin of a pressure wave (burner body) and the pressure relief valve.

In particular, the pressure relief valve is (directly) opposite an inlet of the mixture path into the burner body, whereby it is hit frontally, for example, by a pressure wave emanating from the burner body and can respond very quickly. A subsequent flame front can then run through the open pressure relief valve.

Typically, the pressure relief valve can open to an outlet area behind it, which is either fireproof and pressure-resistant or pressure-equalizing. It is possible for the outlet area to be at least partially fireproof, pressure-resistant and pressure-equalizing. For safety reasons, the run-out area will generally not be the open environment, but rather a closed and/or protected area in which there are no flammable materials or materials that can be changed by expected high temperatures. The larger the volume of the compensation area, the faster the pressure is reduced. The pressure reduction can be supported by suitable installations, in particular metallic sieves, perforated sheets, offset walls and the like. These also lead to a rapid reduction in the gas temperature.

In particular, the branch itself and/or the blower branch are shaped in such a way that a pressure wave from the burner body can essentially or predominantly run past the blower branch to the pressure relief valve. A confluence angle of the fan branch that is suitable for the respective space conditions can contribute to this. A narrowing of the fan branch near the branch also increases the speed of the mixture there during normal operation, which reduces the likelihood of the mixture in the fan branch igniting when a flame front passes by.

Opening the pressure relief valve can therefore practically always end combustion in the mixture path. Since temporarily, even when the pressure relief valve closes again, no combustible mixture (but only combustion gases) flows to the burner, the flame in the combustion chamber usually goes out, which can be detected by a flame monitor and leads to the fuel supply being switched off if the flame has not already flashed back was detected in another way.

A schematic embodiment of the invention, to which it is not limited, and the operation of the method will now be explained in more detail with reference to the drawing. It shows:
Fig. 1 : Schematic of part of a heater with a pressure relief valve in the mixture path.

Fig. 1 shows schematically parts of a heater 1, in particular a condensing boiler, which can be operated with hydrogen or a hydrogen-containing fuel gas. The heater 1 has a combustion chamber 2 with a housing 10 in which a flame 13 burns during operation, the heat of which is dissipated by a heat exchanger 12. The flame 13 is generated by a premix burner 16, which is arranged in the combustion chamber 2 and has a burner body 3 with holes 4 (or differently shaped openings, e.g. slots). A mixture of air and fuel gas is supplied to the burner body 3 through an inlet 11. The inlet 11 is often designed as a kind of door in the housing 10, in which the burner body 3 is attached. The mixture is guided by a fan 7 via a mixture path 5 to the burner body 3. The blower is usually attached to a blower flange 9 with a blower outlet 8. During trouble-free operation, there is a mixture flow 14 (indicated by light arrows) through the mixture path 5 from the fan 7 to the burner body 3. In the event of a flashback, the entire mixture contained in the burner body 3 ignites practically explosively and initially generates a pressure wave opposite the mixture flow 14 (their Direction 15 is indicated with dark arrows) followed by a flame front which, without countermeasures, would run through the entire mixture path 5 and then also reach the blower 7 and could possibly damage it due to an increase in pressure and/or temperature. To avoid this, the mixture path 5 is specially designed with a branch 18, at which a blower branch 19 opens into a substantially straight connection between the inlet 11 and a pressure relief valve 6. The pressure relief valve 6 can be opened by a pressure wave to a fireproof and pressure-absorbing or balancing outlet area 17. The opening takes place so quickly and for such a long time that the subsequent flame front runs out and the flame goes out (and thus also the flame 13 in the combustion chamber 2 ). Through a special design of the branch 18 it can be achieved that the pressure wave practically does not get into the blower branch 19 and damages existing components or the blower 7. Even a flame front following the pressure wave does not ignite the mixture in the blower branch 19 (at least the heat generated in the burner body 3 and the part of the mixture path 5 that has been passed through so far does not reach the blower 7), especially if the blower branch 19 is suitably designed. This can be achieved by a narrowing near the branch 18 and/or by a suitable confluence angle. The exact embodiment of the outlet area 17 depends on various boundary conditions. In any case, the run-out area 17 must be located in such a way that there is no danger to property or people in the area. It must be fireproof and can ensure rapid pressure reduction due to the size of its volume (with a closed outlet area 17) and/or through suitable pressure-reducing means (perforated sheets, sieves, labyrinth structures, etc.). A large surface area and thermal conductivity (with a high heat capacity at the same time) in the outlet area 17 also contribute to this, which can quickly compensate for high temperatures of the inflowing gas and thus reduce the volume of the gas. Even with a closed outlet area 17, pressure equalization to the environment can be provided. The pressure relief valve 6 thus primarily ensures that a pressure wave triggered by a flashback cannot cause damage to the blower 7. However, it generally also means that flames in the mixture path 5 go out and combustion cannot continue there until the fuel gas supply is switched off. Since the flame front runs past the blower branch 19 into the outlet space 17 and there is initially no combustible mixture available behind the flame front and all components in the blower branch 19 are cold, the arrangement described leads to the flames going out even before the fuel gas supply is switched off (which happens very quickly after a flame flashback, but not as quickly as the flame front runs to the fan 7).

The present invention makes it possible to significantly reduce the consequences of a flame flashback using simple and robust means and, in particular, to protect a fan 7 from it and to ensure that the flames 13 are quickly extinguished.

Reference symbol list

1

heater

2

combustion chamber

3

burner body

4

Holes/slits

5

mixture path

6

Pressure relief valve

7

fan

8th

Fan outlet

9

blower flange

10

Housing

11

Inlet into the burner body

12

Heat exchanger

13

flame

14

Mixture flow

15

Direction of a pressure wave/flame front

16

Premix burner

17

Run-out area

18

branch

19

Fan branch

Claims (9)

1. Arrangement for reducing the consequences of a flashback of a flame (13) into a mixture path (5) of a premix burner (16) in a heater (1) which is operable with a mixture of hydrogen or a hydrogen-containing fuel gas as fuel gas and air, the arrangement comprising:

   - a premix burner (16),

   - a mixture path (5) of the premix burner (16),

   - a burner body (3) of the premix burner (16),

   - a blower (7),

   - a pressure relief valve (6),

   wherein the pressure relief valve (6) is arranged in the mixture path (5) between the burner body (3) and the blower (7), and is set in such a way that it is closed during normal operation of the premix burner (16),

   characterised in that

   the mixture path (5) has an intersection (18) at which mixture from the blower (7) can flow in laterally from a blower branch (19), but a pressure wave running in the opposite direction enters the pressure relief valve (6).
2. Arrangement according to claim 1, wherein the pressure relief valve (6) is arranged and designed to be directly struck and opened by a pressure wave triggered by a flashback.
3. Arrangement according to claim 2, wherein the pressure relief valve (6) is situated opposite an inlet (11) of the mixture path (5) into the burner body (3).
4. Arrangement according to claim 2 or 3, wherein there are no baffles or built-in components between the point of origin of the pressure wave, i.e. the premix burner (16), and the pressure relief valve (6).
5. Arrangement according to any one of the preceding claims, wherein the pressure relief valve (6) can open to an outlet region (17) therebehind which is either fireproof and pressure-resistant or pressure-compensating.
6. Arrangement according to the preceding claim 5, wherein the outlet region (17) is pressure-compensating and is a closed and/or protected region.
7. Arrangement according to the preceding claim 6, wherein said outlet region (17) has built-in components suitable for pressure reduction, in particular metallic screens, perforated plates, staggered walls, labyrinth structures.
8. Arrangement according to any one of the preceding claims, wherein at least the intersection (18) or the blower branch (19) is shaped such that a pressure wave from the burner body (3) can substantially travel past the blower branch (19) to the pressure relief valve (6).
9. Heater (1) having an arrangement according to any one of the preceding claims, wherein the heater (1) is a condensing heater configured for operation with hydrogen or a hydrogen-containing fuel gas.

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