GB2291700A

GB2291700A - Heating appliance with catalyst-coated burner tubes

Info

Publication number: GB2291700A
Application number: GB9514860A
Authority: GB
Inventors: Elfriede Herzog; Konrad Weber
Original assignee: Joh Vaillant GmbH and Co
Current assignee: Vaillant GmbH
Priority date: 1994-07-25
Filing date: 1995-07-20
Publication date: 1996-01-31
Anticipated expiration: 2015-07-20
Also published as: DE19527767A1; CH690282A5; DE19527767B4; GB2291700B; NL1000853C1; GB9514860D0

Description

A heating appliance The present appliance comprising a exchanger through

which 2291700 invention relates to a heating catalyst-coated burner and a heat a heat exchange fluid flows.

In previously proposed such heating appliances the problem encountered was designing the burner such that an optimum transfer of heat from the burner to the heat exchanger fluid is achieved.

It is an aim of the present invention to provide a heating appliance which is of the construction set out in the opening paragraph of the present specification and in which the heat exchange fluid is optimally heated.

Accordingly, in a heating appliance according to the present invention, the burner comprises catalyst-coated burner tubes, which communicate at one end with a mixture supply tube, through which a mixture of fuel gas and air flows, and at the other end with an exhaust gas space, and means are provided for cooling the burner tubes.

The proposed measures.have the result that the cooling tubes are exposed to the radiant heat from the burner tubes on all sides so that the heat exchange fluid can be heated at a correspondingly high rate.

The features of claim 2 result in a very simple dissipation of heat from the burner tubes and, as a result, in a low emission of pollutants by the burner.

The features of claim 3 afford the advantage that the heating appliance has a simple design.

By the features of claim 4 a radiation of heat into the environment is substantially obviated.

The features of claim 5 permit an ignition of mixture of the burner tubes from the exhaust gas duct so that back-flowing flames may rise in the burner tubes to heat the latter. The ignition is effected in the exhaust gas space at a location at which the mixture first leaves the reaction tubes and which is sufficiently spaced from the exhaust gas exit. In that case it will be sufficient to ensure that the velocity of flow of the mixture during the igniting phase is lower than the velocity of the flame. Because there is no catalytic coating in that one-third portion of the burner tubes which is nearer to the exhaust gas space, the flames developed during the ignition are unlikely to damage the coating. However, the burner can be operated like a normal burner to produce flames when the catalytic coating has possibly been damaged.

The flame and the gas are conducted in mutually opposite directions. It is thereby ensured that there will be no excessive combustion temperatures which might damage the material because a mixture of fuel gas and exhaust gas must pass through the flame front. This involves a dilution of the mixture so that there is an equilibrium between the exhaust gas and the mixture or fuel gas. This will stabilize the flame which is developed during the ignition process. The burner can be ignited in the exhaust gas space.

By heating the burner from a separate ignition 3 will be achieved is space, the features of claim 6 will afford the advantage that the velocity of flow of the mixture can be reduced by a throttling of the by- pass during the ignition.

By the features of claim 7 the mixture and the heat exchange fluid are conducted in opposite directions so that a particularly effective heat transfer Advantageously, the fins permit a dissipation of heat from the exhaust gases at a high rate. In that case the heating appliance may be operated as a condensing appliance in which there is a condensation from the exhaust gases.

By the features of claim 8 the heating appliance can be modulated in operation and in dependence on the instantaneous heat demand more or fewer burner tubes may be supplied with the mixture. The control is simply effected by a corresponding increase or decrease of the pressure of the mixture in the mixture supply tube and, if necessary, also in the return flow line for the heat exchange fluid, which may comprise water or, for example a heat transfer oil, which is conducted in a closed circuit and, for instance, transfers heat through a further heat exchanger to a different fluid.

By the features of claim 9 a correspondingly high reliability in operation is ensured and the flashbackpreventing nets will also act to homogenize the mixture that is composed of a fuel gas and air, thus ensuring that the mixture will uniformly flow in contact with the surface of the catalyst.

is The features of claim 10 result in a sufficiently high pressure potential being available for the pre-mixing and for overcoming the total flow resistance of the burner.

By the features of claim 11 the heat exchange fluid surrounding the chambers which surround the burner tubes will be heated quickly through the heated chambers. The walls of the chambers are made of a material having a high thermal conductivity.

The features of claim 12 afford the advantage that a high proportion of the energy which has been released will be converted to radiant energy so that there will be a very favourable transfer of heat to the heat exchange fluid through the walls of the chamber, the said walls being heated by the radiant heat.

The features of claim 13 result in a very high utilisation of the heat exhaust gases and allow the heating appliance to be operated also as a condensing appliance so that an additional convective heat exchanger will no longer be required.

In that case the exhaust gas space will desirably be provided with a drain for condensate.

The features of claim 14 result in a very uniform supply to the several burner tubes.

The features of claim 15 allow an ignition of the burner and of the mixture in a separate ignition space during the ignition phase. In that case the velocity of flow of the mixture may be decreased during the ignition phase.

This can be effected simply in that the valves in the by-pass are closed. Alternatively, mixture of the burner tubes may be ignited from the exhaust gas space so that the counterflowing flames can rise in the burner tubes to heat the latter and thus to initiate the catalytic reaction of the mixture. However, in that case it will be essential that the velocity of flow of the mixture is lower than the velocity of the flames during the ignition phase. The burner tubes will also be heated by heat conduction from the exhaust gas side.

The features of claim 17 result in a highly compact design of the heating appliance. The proposed arrangement of the exhaust gas duct will permit a utilisation of the heat of the exhaust gases.

Examples of a heating appliance embodying the present invention will now be described with reference to the accompanying drawings, in which:

Figures 1 and 2 are longitudinal sectional views illustrating respective such examples; Figure 3 is a cross-sectional view of the appliance taken on line III-III in Figure 1; Figure 4 is a schematic side view of a separate ignition space; and to 7 illustrate further such examples. Figures, like reference characters Figures 5 In all designate like details.

The heating appliance in accordance with the is present invention comprises a return f low line 1 and a flow line 2, which serve to conduct a heat exchange fluid, such as water, and are interconnected by a register of cooling tubes 3, which are made of a material which has a high thermal conductivity and heat capacity, such as copper tubes having a dull black coating, or similar materials having a high surface emissivity.

The cooling tubes 3 extend through burner tubes 4, which at an end 18, which is nearer to the flow line 2, communicate with a mixture supply tube 5, through which flows a mixture of fuel gas and air.

The gas is mixed with the air, which is drawn in by a fan 25, in a mixture supply tube 5.

The burner tubes 4 are made of metal or ceramic and on their inside surface 20 are provided with a catalytic coating 6, which extends from an end 18 of the burner tubes 4 that is adjacent to the mixture supply tube, along about 213 of the length of the burner tubes 4. The burner tubes 4 and the entire burner are provided on the outside with an insulation 7 made of heat-insulating material, in order to minimise the radiation losses.

The burner tubes 4 open into an exhaust gas space 10, which is provided downstream thereof with an exhaust gas duct 11. The return flow line 1 protrudes into the exhaust gas space 10 and is provided with fins 12, which also protrude into the exhaust gas space 10. The exhaust gas space 10 is provided, at the bottom, with a condensate drain 13. The return flow line 1 extends in the exhaust 7 gas space 10, as a result of which the unburnt mixture and the exhaust gas, on the one hand, and the heat exchange fluid flowing through the cooling tubes 3, on the other hand, are conducted in mutually opposite directions.

A modulation of the operation of a catalytic burner would involve a high expenditure. As a result, burner tubes may separately be put into operation in dependence on the heat demand if a valve 8 controlled by a pressure -responsive diaphragm is provided in the mixture supply line between the several burner tubes. In case of a high heat demand, the gas valve 22 will allow gas and air to flow at a high rate into the available burner tubes. In response to a rise of the pressure above a critical value, the diaphragm-controlled valve 8 will open so that an additional burner tube is then used for a catalytic combustion of the gas.

Valves 8 controlled by pressure-responsive diaphragms are installed in the mixture supply tube 5 upstream of the second and following burner tubes 4 with respect to the direction of flow of the mixture.

In that case the pressure in the mixture supply tube 5 may be correspondingly varied to supply the mixture to more or fewer burner tubes 4. The valves open in response to progressively increasing pressures towards the closed end of the mixture-conducting tube 5.

Additional diaphragm valves 9 controlled by pressure responsive diaphragms may be provided in the return flow line 1 upstream of the second and further - 8 subsequent cooling tubes 3 with respect to the direction of flow. The said diaphragm valves 9 are put into operation and opened at the same time as the diaphragm- controlled valves in the mixture supply space. The diaphragmcontrolled valves open in response to a pressure in excess of the predetermined pressure and the pressure which causes each diaphragmcontrolled valve to open increases towards the closed end of the return flow line 1.

The operation of the heating appliance can continuously be modulated by means of the diaphragmcontrolled valves 8 in the mixture supply tube 5.

In such an operation the reaction tubes which are open at the time are ignited in the conventional manner from the exhaust gas-collecting space as described hereinbefore.

If the appliance is not ignited from the exhaust gas space 10 be means of required ignition electrodes 21, an ignition space 14 will be provided in the mixture supply tube 5 and will precede the f irst burner tube 4 with respect to the direction of f low of the mixture and will contain baffles 15 for deflecting the flowing mixture.

A temperature-controlled valve 17 is provided in the by-pass 16 of the mixture supply tube 5. That by-pass 16 is parallel to the ignition space 14.

During the ignition phase the valve 17 opens only for the supply of gas to the ignition space. The mixture flows into the burner 24 and is ignited by the ignition electrodes 21.

The exhaust gas is deflected by the baffles 15 and flows through the supply line 16 down-stream of the valve 17 and through the catalytic burner. When the burner, provided with operating temperature, valve, is actuated so the catalytic coating, reaches the the valve 17, which is a change-over that the gas-air mixture will then directly flow into the catalytic burner through the supply line 16 without having been combusted in a separate ignition space.

The means for supplying gas to the burner tubes 4 are preceded by flashback-preventing nets 19, which have a mesh size that is smaller than the flame-arresting distance. Said nets serve also to homogenize the mixture and the velocity of flow of the mixture throughout the crosssection of the burner tube.

In a preferred embodiment shown in Figure 5 the heating appliance comprises a boiler 31, which is elongate in a top plan view and in its lower portion contains an exhaust gas space 32. Chambers 33 protrude upwardly from the exhaust gas space 32 and are closed at their top ends 54.

A return line 34 for conducting a fluid to be heated protrudes into the exhaust gas space 12 and is provided with fins 35. The return flow line 34 communicates with spaces 36, the said spaces 36 being provided between the chambers 33 and the walls of the boiler 31 and are filled with the fluid.

The burner tubes 38 are connected to a mixture is supply tube 37, which extends through the top ends 54 of the chambers 33. The burner tubes 38 are centrally disposed in the chambers 33 and surrounded by heat exchanger surfaces. The burner tubes 38 are provided with a catalytic coating 39, which extends from the port 55 that is connected to the mixture supply tube 37, in which a mixture of fuel gas and air is conducted. The mixture is ignited on the outside of the catalyst-coated cylinder.

The spaces 36 filled with the heat exchange fluid also communicate with a flow line 40.

The exhaust gas space 32 communicates with an exhaust gas duct 41 and is provided with a condensate drain 42.

The mixture is catalytically reacted on the burner tubes 38 to generate heat, which effects a radiant heating of the walls 56 of chambers 33. Said walls 56 are made of a material having a high thermal conductivity, such as copper. the walls 56 of the chambers are surrounded by a heat exchange fluid, such as water, and dissipate the heat to the heat exchange fluid. As a result, the mixture on the one hand and the heat exchange fluid, which flows from the return line 34 to the flow line 40 under the action of a pump, on the other hand, f low in mutually opposite directions.

i.n the embodiment shown in Figure 6 the chambers 33 are arranged along a circular line and the exhaust gas pipe 41 provided with fins 43 is disposed at the centre of that circle.

1 This will afford the advantage that an additional heat exchanger for utilising the residual heat of the exhaust gas is no longer required.

The burner tubes 38 comprise a wire net, expanded is metal, or a porous ceramic body having a regular or irregular structure, and are also provided with a catalytic coating 39 and each is surrounded by a heat exchanger tube. The burner tube 38 contains a gas distributor, which comprises a wire net, expanded metal, or a porous ceramic body having a regular or irregular structure. The heat which is generated by the catalytic reaction is transferred mainly by radiation to the surrounding heat exchangers.

The residual heat of the exhaust gas is dissipated, on the one hand, by convection over the flow paths leading to the exhaust gas-collecting space and, on the other hand, by the fins provided on the exhaust gas tube.

In the embodiment shown in Figure 6 the boiler 31 is provided with a cover 45, through which the mixture supply tubes 37 and the exhaust gas tube 41 extend in sealing contact.

In the embodiment shown in Figure 7 the burner tubes are supplied with the mixture through a manifold 46, to which the burner tubes 38 are connected at their ends.

The burner tubes 38 comprise a ceramic or metallic body, which on its inside surface is coated with a catalyst. The outside surface of the ceramic or metallic body serves to radiate the heat which has been generated by - 12 the catalytic reaction.

Each burner tube 38 is surrounded by a heat exchanger tube. Adjacent to the exhaust gas duct the heat exchanger tubes may be provided with fins for a utilisation of the residual heat of the exhaust gas. The heat exchanger provided with the integrated catalytically catalyst-coated burner tubes 38 is gastightly closed above the gas duct.

The gas-air mixture flows through the manifold 46 and the burner tubes 38 into the chamber 33.

The said manifold 46 communicates via an ignition space 47 with a mixer 48. The ignition space 47 is provided with baffles 49, by which the flowing mixture is deflected. Air is blown by a fan 50 into the mixer 48, which is provided downstream thereof with a gas line 51 and communicates with the manifold 46 by a by-pass 52, which contains a temperaturecontrolled valve 53.

The burner may alteratively be ignited by an ignition effected in the exhaust gas duct 32.

In that case the velocity of flow of the gas must be lower than the velocity of the flames so that the exhaust gas can rise into the burner tubes 38.

In that case flow passes through the ceramic burner tube inwardly from the outside. As soon as the catalyst has reached the light-off temperature, the catalytic reaction inside the burner tube 38 will be initiated and the flame in the gas space 32, 33 will automatically be extinguished. The gas-air mixture must i 13 then flow through the flow passage inside the burner tube 38.

During the ignition phase of the mixture a bypass 23 is closed until the burner tubes 38 have been heated to the operating temperature and the reaction of the mixture may then catalytically be continued. As a result, the burner tubes are correspondingly heated so that the mixture can react catalytically.

14

Claims

Claims is 1. A heating appliance comprising a catalyst-coated burner and a

heat exchanger through which a heat exchange fluid flows, in which the burner comprises catalyst-coated burner tubes, which communicate at one end with a mixture supply tube, through which a mixture of fuel gas and air flows, and at the other end with an exhaust gas space, and means are provided for cooling the burner tubes.
2. A heating appliance according to claim 1, in which cooling tubes through which a cooling fluid flows, extend through the burner tubes.
3. A heating appliance according to claim 1 or claim 2, in which the cooling tubes constitute a register, which communicates with a flow line and a return flow line.
4. A heating appliance according to any preceding claim, in which the burner tubes are provided, on their outside, with heat-insulating material.
5. A heating appliance according to any preceding claim, in which the burner tubes are free of a catalytic coating in the onethird of their length which is nearer to the exhaust gas space.
6. A heating appliance according to any preceding claim, in which the mixture supply tube contains a temperature-controlled valve, which controls a by-pass leading to an ignition space which is provided with baffles.
7.

4 A heating appliance according to any preceding - is
8 claim, in which the return flow line extends in the exhaust gas space and is provided with fins.

A heating appliance according to any preceding claim, in which pressurecontrolled valves, which preferably comprise diaphragms, are provided adjacent to the return flow line and the mixture supply line.
9. A heating appliance according to any preceding claim, in which check valves are provided upstream of the burner tubes with respect to the direction of flow of the mixture.
10. A heating appliance according to any preceding claim, in which a fan is provided in an air supply tube.
11. A heating appliance according to claim 1, in which the burner tubes are provided in chambers, which protrude from and open into the exhaust gas space and define a space which is filled with the heat exchange fluid and communicates with a return flow line and a flow line.
12. A heating appliance according to claim 11, in which the burner tubes optionally comprise porous bodies, such as wire nets, expanded metal, ceramic bodies having a regular and/or irregular pore structure, or silicone carbide, and the burner tubes are surrounded by radiant tubes, which are preferably made of special steel, silicon carbide or a different material having a high thermal conductivity and corrosion resistance.
13. A heating appliance according to claim 11 or claim 12, in which the return flow line protrudes into the exhaust gas space and is preferably provided with fins.
14. A heating appliance according to any one of claims 11 to 13, in which the burner tubes are adapted to be supplied at one end with the mixture through a manifold, which is preceded by a mixer provided with gas and air is inlets.
is. A heating appliance according to claim 14, in which an ignition space, which is provided with baffles for enforcing a deflection of the flowing mixture, is provided between the mixer and the manifold and is adapted to be bridged by a by-pass which is provided with a temperaturecontrolled valve.
16. A heating appliance according to any one of claims 11 to 15, in which the exhaust gas space is provided with a condensate drain.
17. A heating appliance according to any one of claims 11 to 16, in which the chambers provided with the burner tubes are arranged along a circular line in a boiler and the exhaust gas space is preferably provided with a central exhaust gas duct which is provided with fins.
18. A heating appliance, substantially as described herein with reference to and as shown in Figures 1, 3 and 4, or Figures 2 and 3, or Figures 5, 6 or 7 of the accompanying drawings.