Ionisation sensor electrode

Abstract

 The invention relates to an ionisation sensor electrode (9) arranged for detecting the presence of a flame (7) in a combustion space (2) of a combustion apparatus (1). The sensor electrode (9), in operation of said combustion apparatus, is arranged within said combustion space away from said flame. The invention also relates to a combustion apparatus comprising such an ionisation detection sensor. This arrangement reduces the oxidizing rate of the sensor electrode and improves the service life of the sensor electrode and reduces maintenance. Also, less advanced and thus cheaper materials can be applied for the sensor electrode.

Description

FIELD OF THE INVENTION

[0001] The invention relates to an ionisation sensor electrode for detecting the presence of a flame. The invention also relates to a combustion apparatus comprising such an ionisation sensor electrode.

BACKGROUND OF THE INVENTION

[0002] When gases are combusted in combustion devices used for heating purposes, such as central heating installations, water heaters, geysers and furnaces, carbon dioxide and water are formed when air is supplied, e.g. in accordance with the reaction

         CH₄ + 2O₂ → CO₂ + 2H₂O

During such a reaction, free ions and electrons are released as a result of thermal and chemical ionisation processes.

[0003] In the prior art, this reaction is monitored by a flame rod extending into the flame and acting as an ionisation sensor electrode that extends within a combustion chamber and is electrically isolated from the chamber by an insulator. An example is shown in DE 195 02 900. The ionisation sensor utilises said free ions and electrons for detecting the presence of a flame. If an AC voltage is supplied to the ionisation sensor, the free ions and electrons cause a rectifying effect on the applied signal resulting in a DC current and accordingly indicate the presence of a flame.

[0004] The flame rod is exposed to a relatively high temperature of approximately 2000K. Consequently, in an environment of aggressive radicals, the metallic flame rod will oxidize and an oxide layer will be formed on the rod. This layer will eventually prevent detection of the rectification effect and accordingly of the flame presence.

[0005] A careful selection of the materials and alloys thereof for the flame rod makes that present flame rods function well for a reasonable period of time. Maintenance of the electrode may however still be necessary. Further, material ageing caused by oxidation of the metal still presents a problem and may lead to failure of the flame monitoring system.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to provide an ionisation sensor electrode that less easily oxidizes and is capable of detecting the presence of a flame by using the rectification effect.

[0007] To this end, an ionisation sensor electrode arranged for detecting the presence of a flame in a combustion space of a combustion apparatus is provided, wherein said sensor electrode, in operation of said combustion apparatus, is arranged within said combustion space away from said flame.

[0008] The invention also relates to a combustion apparatus comprising a burner bed with one or more flame ports for providing flames in a combustion space, wherein said apparatus further has an ionisation sensor electrode for detecting the presence of a flame, which ionisation sensor electrode, in operation of said combustion apparatus, is arranged in said combustion space away from said flame.

[0009] As the sensor electrode is provided away from the flame, in particular when the sensor electrode is situated underneath or near a lower portion of the flame, it is no longer exposed to the highest temperatures within the combustion space while, surprisingly, it has been found that an appropriate ionisation signal can still be obtained. Further, the sensor electrode is no longer exposed to the aggressive radicals within the flame. Consequently, the oxidizing rate of the sensor electrode is significantly reduced and the service life of the sensor electrode is improved and maintenance is reduced. Also, less advanced and thus cheaper materials can be applied for the sensor electrode.

[0010] In an embodiment of the invention, the sensor electrode is arranged substantially underneath said flame.

[0011] In an advantageous embodiment of the invention, the burner bed is a ceramic burner bed and the ionisation sensor electrode is arranged, and preferably submerged, in said ceramic burner bed. As a ceramic burner bed is a substantially electrically insulating substrate for the sensor electrode, there is no need for further insulating means within the combustion space to ensure proper operation of the sensor electrode.

[0012] In an advantageous embodiment of the invention, a plurality of ionisation sensor electrodes are arranged at various positions in said burner bed underneath a plurality of said flames. This embodiment allows monitoring of the presence of flames over the entire burner bed.

[0013] The invention will be further illustrated with reference to the attached drawings, which schematically shows a preferred embodiment according to the invention. It will be understood that the invention is not in any way restricted to this specific and preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] 

Fig. 1 schematically shows a combustion apparatus comprising a single ionisation sensor electrode provided in a burner bed according to an embodiment of the invention;

Fig. 2 schematically shows a combustion apparatus with a burner bed positioned under an angle.

Fig. 3 schematically shows a combustion apparatus comprising a plurality of ionisation sensor electrodes provided in a burner bed according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0015] Fig. 1 schematically shows a combustion apparatus 1 comprising a combustion chamber 2 determined by a wall 3. A mixture of gas and air is introduced into the combustion chamber 2 via an inlet 4 through a ceramic burner bed 5 with flame ports 6. The combustion reaction is represented by a plurality of flames 7. A heat exchanger device H is positioned in the combustion chamber 2 above the flames 7 and exhaust gasses are output via an outlet 8.

[0016] The combustion reaction takes place at the surface of the flames 7 and free ions and electrons, indicated respectively by the "+" and "-" signs in Fig. 1, are produced. Electrons diffuse away from the flames 7 faster than the ions as their mobility is higher than the mobility of the ions. An electric field builds up until an equilibrium state is present.

[0017] According to an embodiment of the invention, an ionisation sensor electrode 9 extends into the combustion chamber 2 from below and without being directly exposed to the flames 7. By situating the sensor electrode 9 directly below the flames 7, as shown in Fig. 1, the electrode 9 has a lower temperature as compared to a prior art flame rod that directly extends into a flame 7. Further, it is not exposed to aggressive radicals that are only present within the flame 7. Consequently, oxidation of the sensor electrode 9 will occur at a considerably slower rate as compared to a flame rod extending within a flame 7. Surprisingly, experiments have shown that still an ionisation current is detected of a magnitude of approximately 70% of the current measured with a flame rod directly extending into a flame 7. This observation is the more remarkable if one realises that the exposed area of the sensor electrode 9 is considerably less than the area of the conventional rod extending into a flame 7. It was observed that the head of a simple M2 bolt was able to function as a sensor electrode.

[0018] In Fig. 1, the sensor electrode 9 is provided through an opening near a flame port 6 in the ceramic burner bed 5. The sensor electrode 9 is submerged into the burner bed 5 in order to maintain the lowest possible temperature. It should be appreciated that alternatively a distal end D of the sensor electrode 9 may protrude from the burner bed 5 in the direction of the flames 7. Preferably, the distal end D of the sensor electrode 9 terminates before it reaches a flame 7 to avoid high temperatures. It further should be appreciated that other alternatives wherein the sensor electrode 9 is not positioned directly into a flame 7 fall under the scope of the present invention. It should also be appreciated that Fig. 1 is a schematic illustration of a combustion apparatus 1 in cross-section. In practice, the density of flames 7 from the burner bed is considerably higher than shown in Fig. 1 and the sensor electrode 9 will closer approximate a position underneath a flame 7.

[0019] It is noted that, in contrast with the representation in Fig. 1, present day combustion apparatus are often installed such that the flames do not extend in a direction vertically upward, e.g. to avoid dirt entering the combustion chamber 2 via the outlet 8. Often the burner bed 5 is positioned such that the flames 7 extend downwardly under an angle with a horizontal plane. In this position, the sensor electrode 9 according to an embodiment of the invention is still positioned near the origin of the flames 7 underneath these flames, i.e. where the gas flow enters the combustion chamber 3 through the flame ports 6. This is shown schematically in Fig. 2.

[0020] If a voltage is applied from a voltage source 10 to the ionisation sensor electrode 9, the equilibrium state of ions and electrons is disturbed and a current will flow between the ionisation sensor electrode 9 and the wall 3 of the combustion chamber 2, indicating the presence of a flame. If no flame is present, no electrons and ions will be present and a current will not flow on application of a voltage.

[0021] In Fig. 3, a combustion device 1 is shown similar to that of Fig. 1, but with a plurality of sensor electrodes 9 submerged in the ceramic burner bed 5. Consequently, the presence of a flame 7 over the entire burner bed 5 may be monitored.

[0022] The invention is not restricted to the above described embodiment which can be varied in a number of ways within the scope of the claims.

Claims

1. An ionisation sensor electrode (9) arranged for detecting the presence of a flame (7) in a combustion space (2) of a combustion apparatus (1), wherein said sensor electrode, in operation of said combustion apparatus, is arranged within said combustion space away from said flame.

2. The ionisation sensor electrode (9) according to claim 1, wherein said sensor electrode is arranged substantially underneath said flame (7).

3. The ionisation sensor electrode (9) according to claim 2, wherein a distal end (D) of said sensor electrode terminates underneath said flame (7).

4. The ionisation sensor electrode (9) according to one or more of the claims 1-3, wherein said sensor electrode is arranged in a ceramic burner bed (5) with a flame port (6) for said flame (7).

5. The ionisation sensor electrode (9) according to claim 4, wherein said sensor electrode is submerged in said ceramic burner bed (5).

6. A combustion apparatus (1) comprising a burner bed (5) with one or more flame ports (6) for providing flames (7) in a combustion space (2), wherein said apparatus further has an ionisation sensor electrode (9) for detecting the presence of a flame, which ionisation sensor electrode, in operation of said combustion apparatus, is arranged in said combustion space away from said flame.

7. The combustion apparatus (1) according to claim 6, wherein said sensor electrode (9) is arranged substantially underneath said flame.

8. The combustion apparatus (1) according to claim 7, wherein a distal end (D) of said sensor electrode terminates underneath said flame (7).

9. The combustion apparatus (1) according to one or more of the claims 6-8, wherein said burner bed (5) is a ceramic burner bed and said sensor electrode (9) is arranged, and preferably submerged in, said ceramic burner bed.

10. The combustion apparatus (1) according to one or more of the claims 6-9, wherein a plurality of ionisation sensor electrodes (9) are arranged at various positions in said burner bed (5) substantially underneath a plurality of said flames (7).

Drawing