Water heating device and method for measuring a flame current in a flame in a water heating device

Abstract

The invention relates to a water heating device, comprising a burner (20) and a flame current measuring device (100) for measuring a flame current, which measuring device comprises two electrodes and a voltage source (14), wherein each of the poles (18, 19) of the voltage source is connected to one of the electrodes. The water heating device further comprises a heat exchanger (40) which is electrically insulated relative to the burner. The burner and the heat exchanger here form the electrodes of the flame current measuring device. The heat exchanger functioning as electrode can be earthed (41). The measured flame current can be used to determine the excess air factor of the combustion. The water heating device can further comprise an air/fuel controller for controlling the air/fuel ratio, wherein the air/fuel controller uses the determined excess air factor to control the air/fuel ratio. The invention also relates to a method for measuring a flame current in a flame.

Claims

1. Water heating device, comprising: a burner, a flame current measuring device for measuring flame current to determine the excess air factor of the combustion, which measuring device comprises two electrodes and a voltage source, wherein each of the poles of the voltage source is connected to one of the electrodes, a heat exchanger which is electrically insulated relative to the burner, wherein the burner and the heat exchanger form the electrodes of the flame current measuring device, and characterized by an air/fuel controller for controlling the air/fuel ratio, wherein the air/fuel controller uses the determined excess air factor to control the air/fuel ratio.

2. Water heating device as claimed in claim 1, characterized by an ionization-based safety for closing the fuel supply to the burner when no flame is present between the burner and heat exchanger, wherein the ionization-based safety comprises the flame current measuring device and determines on the basis of the measured flame current whether a flame is present.

3. Water heating device as claimed in claim 1, characterized in that the voltage source applies an alternating potential difference to the two electrodes and measures the flame current in both directions.

4. Method for measuring a flame current in a flame in a water heating device comprising a burner and a heat exchanger electrically insulated therefrom, the method comprising of: applying a potential difference between the burner and the heat exchanger, and measuring a current which begins to flow as a result of the applied potential difference, characterized in that the heat exchanger is connected to the earth potential.

5. Method as claimed in claim 4, characterized by the step of determining an excess air factor on the basis of the measured flame current.

6. Method as claimed in claim 5, characterized in that the burner is provided with a mixture of air and fuel in an air/fuel ratio, and the method further comprises the step of controlling the air/fuel ratio on the basis of the determined excess air factor.

7. Method as claimed in claim 4, characterized in that the applied potential difference is an alternating potential difference, and the method further comprise the steps of: measuring the flame current in both directions; determining whether there is a flame present between the burner and the heat exchanger by establishing that the flame currents measured in both directions are not substantially the same; and closing the fuel supply to the burner if there is no flame present between the burner and heat exchanger.

Description

(1) Further embodiments and advantages are described with reference to the figures, in which

(2) FIG. 1 shows schematically a prior art flame current measuring device;

(3) FIG. 2 shows an electrical equivalent-circuit diagram of the flame in the flame current measuring device of FIG. 1;

(4) FIG. 3 shows schematically a flame current measuring device according to the present invention; and

(5) FIG. 4 shows a perspective view with exploded parts of a water heating device with a flame current measuring device according to the invention.

(6) A preferred embodiment of the invention comprises a burner 20 and a heat exchanger 40. When an air/gas mixture flows out of the burner and the mixture is ignited, a flame 30 then burns. Owing to the combustion hot gases flow along heat exchanger 40 and relinquish their heat thereto. Heat exchanger 40 comprises a guide, for instance in the form of a tube 44, through which water flows. Cold water is supplied through a feed 42. Heat exchanger 40 relinquishes heat to the water in tube 44, whereby the water is heated. Hot water leaves heat exchanger 40 via discharge 46.

(7) Burner 20 and heat exchanger 40, which are electrically insulated relative to each other, form the electrodes of a flame current measuring device 100. In the shown example heat exchanger 40—just as other non-current-carrying metal components of the water heating device—is connected to the earth potential via a line 41. Burner 20 on the other hand is electrically insulated from the surrounding construction, and particularly from heat exchanger 40. Both burner 20 and heat exchanger 40 comprise an electrically conductive material, for instance aluminium, copper or steel. Heat exchanger 40 comprises a material which is thermally conductive, for instance aluminium, copper or steel. The burner and the heat exchanger are each connected to a pole of a series connection of an alternating voltage source 14 and a capacitor 16. The alternating voltage source 14 ensures that an alternating electric field is created between burner 20 and heat exchanger 40. Capacitor 16 separates the alternating voltage component from the direct voltage component caused by flame 30.

(8) Due to the heat of the combustion in the flame 30 a part of the gases in and around flame 30 ionizes. Under the influence of the electric field between burner 20 and heat exchanger 40 the charged particles will be displaced and a small leakage current will flow between the two electrodes, burner 20 and heat exchanger 40. The extent of this leakage current is determined by, among other factors, the completeness of the combustion, and thereby by the excess air factor λ. The excess air factor λ is determined on the basis of the measured flame current.

(9) Because the alternating voltage source 14 generates an alternating voltage, the electric field is alternating and the leakage current is likewise alternating. The leakage currents are not the same in both directions. The consequence is that over the series connection of the alternating voltage source 14 and capacitor 16 there is an alternating voltage on clamps 18 and 19 which has a direct current offset. (The flame itself additionally also functions to some extent as a weak voltage source.) This direct current component can be measured over capacitor 16. As soon as a direct current component is detected over these clamps, this means that a flame is burning between burner 20 and heat exchanger 40. The signal at clamps 18 and 19 is transmitted to a conventional circuit (not shown here) for ionization-based safety, wherein a comparator looks at whether the direct current component rises above a threshold voltage. If this is the case, then flame 30 is still burning and the valve in the gas feed may remain open. As soon as the comparator determines that the direct current component falls below the threshold value, the valve is no longer actuated, closes and the gas feed is shut off.

(10) In addition, the signal at clamps 18, 19 is used to control the gas/air ratio of burner 20. As stated, the flame current represents an indication of the completeness of the combustion, and thereby of the excess air factor λ. The excess air factor λ can thus be determined on the basis of the signal detected at clamps 18, 19, after which an air/fuel controller (not shown here) connected to clamps 18, 19 compares the thus determined factor λ to a desired value of the excess air factor. On the basis of this comparison the fuel supply and/or the air supply is then controlled so as to set a desired air/fuel ratio. In practice the air/fuel controller intervenes in the fuel supply by operating the gas block.

(11) FIG. 4 shows a practical embodiment of a water heating device according to the invention. The distance between burner 20 and heat exchanger 40 is highly exaggerated here; in reality burner 20 is located close to the heat exchanger in a recessed space 43 formed by having the fins 45 of heat exchanger 40 protrude relatively less far outward. Shown clearly in the figure is that burner 20 has a relatively large surface area and extends over substantially the whole width of heat exchanger 40. A large flame current is hereby generated, so that a strong signal will thus be present at clamps 18, 19. This provides for a reliable flame detection and stable control of the gas/air ratio. The detection is in this way also less sensitive to an exact correct placing of the “electrodes” than in the case of a measuring pin. In addition, the sensitivity to ambient influences, for instance soot deposition, is greatly decreased due to the large surface area of the burner 20 functioning as electrode.

(12) The embodiments described above and shown in the drawings are only exemplary embodiments by way of illustration of the present invention. Many modifications to and combinations of the shown and described exemplary embodiments are possible within the invention. The exemplary embodiments must not therefore be interpreted as being limitative. The protection sought is defined solely by the following claims.