Method for controlling a heating unit as well as a heating unit and a computer program product for carrying out the control method

Abstract

The present invention relates to a method for controlling a heating unit comprising a burner (1) with a burner housing (2), an ionization electrode (7) associated with the burner (1), and a voltage supply (8) for applying an alternating voltage between the ionization electrode (7) and the burner housing (2), said method comprising the method steps: applying an alternating voltage between the ionization electrode (7) and the burner housing (2) by means of the voltage supply (8) and correcting the output of the voltage supply (8) in the event of parasitic leakage flows. The object of the present invention is in particular to improve the reliability when ascertaining the air-fuel ratio via the ionization current.

Claims

1. A method for controlling a heating unit comprising a burner (1) with a burner housing (2), an ionization electrode (7) associated with the burner (1), and a voltage supply (8) for applying an alternating voltage between the ionization electrode (7) and the burner housing (2), comprising the steps of: applying an alternating voltage between the ionization electrode (7) and the burner housing (2) by means of the voltage supply (8), and correcting the output of the voltage supply (8) in the event of parasitic leakage flows, wherein a voltage which is actually applied to the ionization electrode (7) is measured, compared to a set point and, if necessary, is adjusted to the set point.

2. The method according to claim 1, wherein the output of the voltage supply (8) is increased with rising load points of the gas heating unit.

3. The method according to claim 1, wherein the correcting of the output of the voltage supply (8) is carried out in such a way that the detected ionization current for each load point can clearly be assigned to an air ratio at which the burner (1) is operated.

4. The method according to claim 1, wherein the alternating voltage which is actually applied to the ionization electrode (7) is kept substantially constant throughout the load range.

5. The method according to claim 1, wherein the output of the voltage supply (8) is lowered with rising load point.

6. The method according to claim 1, wherein an ionization current set point curve is known for each applied alternating voltage and the air ratio is determined on the basis of the known ionization current set point curve and the applied alternating voltage.

7. A heating unit, comprising: a burner (1) with a burner housing (2), an ionization electrode (7) associated with the burner (1), a voltage supply (8) for applying an alternating voltage between the ionization electrode (7) and the burner housing (2), and a control unit which corrects a voltage supply (8) in the event of parasitic leakages, the control unit being designed in such a way that it has a measuring unit by means of which the voltage actually applied to the ionization electrode (7) is measured, and the control unit compares the voltage actually applied to the ionization electrode (7) with a set point and, if necessary, adjusts it to the set point.

8. The gas heating unit according to claim 7, wherein the control unit is designed in such a way that the output of the voltage supply (8) is raised or lowered with rising load points of the gas heating unit.

9. The gas heating unit according to claim 7, wherein the burner has a cylindrical surface which is provided with a perforation structure.

10. A computer program product with computer-executable instructions for executing the method according to claim 1.

Description

(1) Advantageous developments of the invention are explained in more detail by means of a below explained embodiment in conjunction with the drawing, wherein:

(2) FIG. 1a shows a schematic view of a gas burner, wherein the gas burner housing is switched to positive potential and an ionization electrode is switched to negative potential,

(3) FIG. 1b shows a schematic view of the same burner with reversed polarity,

(4) FIG. 1c shows the voltage curve over time and the idealized ionization current between burner and ionization electrode in the flame,

(5) FIG. 2 shows an equivalent circuit diagram of a burner of a heating device with an alternating voltage supply,

(6) FIG. 3a shows an ionization current dependency on the load point of the prior art heating device as well as

(7) FIG. 3b shows an ionization current dependency on the heat load point with a feedback control according to the invention,

(8) FIG. 4 shows a voltage characteristic curve without the feedback control according to the invention as well as a voltage characteristic curve in the feedback control according to the invention.

(9) FIG. 1a shows, by way of diagram, a burner 1, which is part of a heating unit (not shown).

(10) The burner 1 has a cylindrical burner housing 2 having a front-side opening 3. A gas nozzle 4 is arranged inside the burner housing 2 and concentrically thereto and slightly set back in relation to the front-side opening 3. Air flows into the burner housing 2 and gas flows into the gas nozzle 4 from a rear side of the burner housing 2. The gas from the nozzle 4 is mixed with the air in a mixing zone 5, arranged in front of the nozzle and inside the burner housing.

(11) The gas-air mixture is ignited by means of an igniter (not shown), and a flame 6 is produced, which extends from the housing through the front-side opening 3. An ionization electrode 7 arranged on the front side in front of the opening 3 is provided within the flame.

(12) An alternating voltage is applied between the ionization electrode 7 and the burner housing 2 (cf. FIG. 1c). The applied alternating voltage is between 20 and 75 volt; further preferred values are between 20 and 150 V, in particular between 30 and 100 V, most preferably 130 V.

(13) In a variant which is not shown in FIG. 1, the burner 4 has a cylindrical surface which is provided with a perforation structure. Therefore, the gas-air mixture flows over the cylindrical surface and through the perforation structure.

(14) A flame area is thus formed on the surface and is stabilized in particular by the perforation structure. A more constant profile of the ionization current set points is achieved for a constant air ratio by an appropriate selection of the perforation structure. This is advantageous for the feedback control process and also aspects, such as air ratio constancy, in the case of modulation.

(15) A frequency is preferably 50 Hz, further preferred regions are between 30 and 150 Hz, in particular between 40 Hz and 100 Hz, most preferably 50 Hz+/10 Hz.

(16) The alternating voltage is generated by a voltage supply 8 and is appropriately applied between the ionization electrode 7 and the burner housing 2. The applied alternating voltage is preferably between 20 and 200 V, in particular between 90 and 150 V, most preferably 130 V+/10 V. The output of the voltage supply can be regulated.

(17) The voltage supply 8 is preferably accommodated in a control unit of the heating unit, which is not shown. This control unit can contain a control unit by means of which the method according to the invention is carried out.

(18) As shown in succession in FIGS. 1a and 1b, a current flows when the plus pole of the voltage supply 8 is coupled to the burner housing 2 and the minus pole of the voltage supply 8 is coupled to the ionization electrode 7, and in the reverse case, as shown in FIG. 1b, no current flows when the burner housing 2 is switched to a negative potential and the ionization electrode is switched to a positive potential since the electrodes e.sup. in the flame flow with the ions l.sup.+ to the ionization electrode 7 where the ions l.sup.+ are discharged, i.e. neutralized.

(19) This schematic diagram shows the idealized behavior of the rectification.

(20) The ionization electrode 7 and the burner 2 can have any geometry but these two devices have to be arranged in relation to one another in such a way that an ionization current is produced between the ionization electrode 7 and the burner as a result of the rectifying effect of the flame 6.

(21) Alternatively to the gas burner, it is e.g. also possible to use an oil burner or a burner for another fuel.

(22) FIG. 1c correspondingly shows the idealized current flow as compared to the applied voltage over time. As is clear from this figure, the flame 6 has a rectifying effect.

(23) It has surprisingly been shown that in real heating units the resistance in the heating unit, in particular between the ionization electrode and the burner housing, is complex and not only of ohmic nature. This leads to parasitic resistances which in addition to the ionization current through the burner flame are responsible for another parasitic current flow.

(24) A corresponding equivalent circuit diagram of a real burner 1 is shown e.g. in FIG. 2, this burner also having a measuring circuit 9, by means of which, as described below, the voltage actual applied between the ionization electrode 7 and the burner housing 2 is measured and the voltage supply 8 is correspondingly readjusted on this basis.

(25) The voltage supply 8 is shown by way of diagram on the left-hand side of FIG. 2 and has a resistance R.sub.innen.

(26) An equivalent circuit diagram of the burner 6 is shown on the right-hand side of FIG. 2. The idealized flame 6 itself, which includes the rectifying effect, is formed by the diode D and by the flame resistance R.sub.Flamme. Said figure shows a parasitic resistance Z.sub.Flamme, which is connected in parallel thereto and is responsible for a parasitic current flow on the basis of the operating parameters, such as load, lambda value and type of gas.

(27) The parasitic resistance Z.sub.Flamme is complex and therefore, as a sort of impedance, it is also labeled with the common reference sign Z as used in connection with coils. The resistance has an ohmic portion and also a capacitive portion. It was found that the burner flame has the ohmic portion and also a capacitor effect.

(28) An oscillating circuit between the ohmic portion and the capacitive portion is formed in the burner flame, in particular within high load ranges, which reduces the ionization voltage compared to the idealized image or causes the ionization voltage to collapse.

(29) The arrow in FIG. 2 labeled with the reference sign 10 shows by way of diagram that the voltage supply 8 in the method according to the invention is controlled by means of the actually measured voltage of the ionization electrode 7.

(30) FIG. 3a shows an ionization current dependency on the load point for different lambda values without the control according to the invention, i.e. without the output stabilization, and FIG. 3b shows an ionization current dependency on the load point for different lambda values with the control according to the invention, i.e. with the output stabilization.

(31) Starting at the top, the lines in FIGS. 3a and 3b correspond to the lambda values of 1.04, 1.14, 1.24, 1.34, 1.54, which are shown on the right-hand sides in the corresponding figures, i.e. the air excess increases in the graphs from top to bottom.

(32) As can be seen e.g. in FIG. 3a at a low load point of 10%, the measured ionization current is increased with increasing lambda substantially inversely thereto (vertical section at 10% load point). The change in the ionization current is inversely proportional to the change in the air ratio.

(33) The values plotted on the Y-axis are current values (amperage in A). The lower the corresponding lambda value, the higher the respectively measured ionization current.

(34) The measured ionization current shall be described below with a predetermined preadjusted voltage at the voltage supply 8 for the lambda value of 1.34 (4.sup.th line from the top in FIG. 3a).

(35) When the load point is increased from about 10% to about 40%, the measured ionization current will increase.

(36) In a further increase in the load point, however, the ionization current first plunges between about 50% and about 75%. This drop of the measured ionization current between ionization electrode 7 and burner housing 2 is due to the fact that a parasitic current flow occurs. As a result, the voltage actually applied between the ionization electrode 7 and burner 1 drops and the ionization current in the flame is lowered correspondingly.

(37) As shown in FIG. 3a, the two curves for the lambda value of 1.14 and 1.04 intersect at the 75% load point (cf. the upper two lines in FIG. 3a; 2.sup.nd point from the right on the respective graphs in FIG. 3): Although the lambda values are different, the same ionization current is measured.

(38) Therefore, it is no longer possible to draw conclusions from the ionization current about the corresponding air ratio and/or the lambda value.

(39) The hatched area (region without sensitivity) of 50% to 100% and between the lines for an air ratio of 1.04 and 1.14, which is shown in FIG. 3a, therefore does not show any air ratio sensitivity.

(40) This means that the ionization current cannot be used in this load range for determining the air ratio. Such load ranges can be as follows: above 30%, preferably above 50%, in particular above 70% but below 100%. The described values can each be an upper limit and lower limit.

(41) FIG. 3a shows three different ranges. Up to a load point of 10%, the current surges (at least for lambda values of 1.34 and more). This range is referred to as a range of unfavorable sensitivity because a measurement within this range can contain considerable defects. In addition to this range and the above described range without sensitivity, in particular the characteristic line for lambda 1.34 has an unfavorable characteristic curve within the range of the peak.

(42) However, FIG. 3b shows the same dependency for the corresponding seven lambda values with the control according to the invention. In so far as the actual voltage measured on the ionization electrode 7 is measured and this voltage is e.g. kept constant in accordance with the load point, the lines of the ionization current dependency on the load point no longer intersect for the corresponding lambda values.

(43) For example, as soon as there is a parasitic resistance or leakage current, the output of the voltage supply 8 is upregulated.

(44) In this way, it is also possible to clearly determine the air ratio for low lambda values of below 1.14 since the corresponding lines in FIG. 3b do not intersect. The corresponding graphs for the individual lambda values in FIG. 3b all slightly increase. Only the graph for the lambda value of 1.3 slightly drops between about 50% and 70% of the load point. Nevertheless, there is no intersection or contact between the individual graphs.

(45) In particular, this is due to the fact that the corresponding voltage value actually applied to the ionization electrode 7 is adjusted.

(46) FIG. 4 shows a comparison of a dependency of the applied voltage (voltage adjusted at the voltage supply) on the ionization current. At the line referred to as a, the applied voltage is always constant even if the ionization current is lowered due to the leakage currents with equal load point. In the method according to the invention (cf. line b in FIG. 4), the voltage supplied by the voltage source is increased in the case of an ionization current lowered on account of occurring leakage currents. As a result, a constant actual voltage is applied between the ionization electrode 7 and the burner.

LIST OF REFERENCE SIGNS

(47) 1 burner 2 burner housing 3 opening 4 gas nozzle 5 mixing zone 6 flame 7 ionization electrode 8 voltage supply 9 measuring circuit 10 control D diode R.sub.Flamme resistance Z.sub.Flamme leakage resistance