GB2230332A

GB2230332A - A burner start-up method

Info

Publication number: GB2230332A
Application number: GB9008449A
Authority: GB
Inventors: Joachim Berg; Thomas Kupka; Bernd Winkelmann
Original assignee: Joh Vaillant GmbH and Co
Current assignee: Vaillant GmbH
Priority date: 1989-04-13
Filing date: 1990-04-12
Publication date: 1990-10-17
Anticipated expiration: 2010-04-12
Also published as: CH683636A5; NL9000824A; GB9300749D0; BE1004109A6; GB2261063B; GB2230332B; GB9008449D0; DE4011690A1; GB2261063A

Description

A method of burning a -fuel-air mixture and a burner arrangement for

working such a method.

The present invention relates to a method of 5 burning a fuel-air mixture in a combustion chamber supplied with a stream of combustion air.

Experience shows that when an aforementioned mixture burns after a cold start, the proportion of air is initially too high, and sensitive fan burners react unfavourably thereto. More particularly the mixture becomes less combustible during an aforementioned cold start.

An aim of the present invention therefore is to provide a particularly combustible mixture during the starting phase of the combustion process, such as to ensure a strong ionization stream with a low flame, with consequent high reliability when starting the burner, even when the burner is under partial load and the proportion of fuel is throttled accordingly, and/or poorly- ignitable fuel is used.

Accordingly, a first aspect of the present invention is directed to a method as set out in the opening paragraph of the present specification, in which, during a starting phase of combustion, the combustion chamber is supplied with a proportion of combustion air which is less than a proportion of air provided for subsequent combustion.

1 Thus the proportion of air can be set at the optimum value for the burner when hot.

This deliberate reduction in the air coefficient during the starting phase of combustion gives a desired effect, that is supplying the burner with an easily ignitable mixture, even when the burner power is set at a low value.

A second aspect of the present invention is directed to a burner arrangement for working the method according to the present invention, comprising a fan which serves to supply the combustion chamber with combustion air, the fan being provided with a motor drive, in which a switching circuit is provided to control the speed of the fan motor, so as to reduce it during the starting-up of the burner.

Such a burner arrangement may be provided with various different means for reducing the air coefficient during the starting phase by using different constructions for the switching circuit.

For example, in a very simple construction, the switching circuit of the fan motor can comprise a series resistor which is switched on only during the starting phase to operate the fan motor at a low speed, and which can be bypassed by a bypass line.

In order to limit the reduction in the air coefficient to- the period of the starting phase, a switching contact can be provided which connects the bypass line to the fan motor at the end of the starting phase and interrupts that circuit in the starting phase only.

A switching circuit of this construction can be controlled by a time function element or by a temperaturesensitive switching element, for example a thermo-element. In both cases the duration of the starting phase can be limited by the element concerned.

Alternatively a temperature-sensitive series resistor, for example a "high-temperature conductor", may be included in the switching circuit of the fan motor. Such a resistor decreases in value as the temperature increases and thus smoothly adapts the power output of the fan motor to the increasing air requirement of the burner.

is Such a series resistor can be disposed at a place where it is gradually heated when the burner starts, either directly or indirectly by being disposed on a heated component, for example on a waste-gas Collecting hood of a combustion chamber of the burner so as to be indirectly heated by the component.

In order to supply a particularly combustible air/fuel mixture at the starting phase of the combustion process, so as to ensure a strong ionization current at a low flame in the region of a flame monitoring device and consequent high reliability on starting the burner, even when the burner is under partial load and the proportion of fuel is correspondingly throttled and/or poorly- ignitable fuel is used, a relatively low level proportion of combustion air is provided in a pre-scavenging phase and continues into an ignition phase to the end of a preset safety period, after which the supply of combustion 5 air is increased.

The result is a reduction in the air coefficient, so that during the starting phase the burner is supplied with a particularly easily-ignited mixture before and during the ignition phase. This ensures reliable ignition even under very difficult conditions, for example by virture of the use of a poorly-combustible fuel.

Another problem may occur with a burner in which the power at which it operates has to be throttled considerably during operation.

Therefore, the proportion of combustion air may be reduced even when the burner is in operation, in dependence upon a signal derived from the condition of the flame in the combustion chamber, preferably an ionization signal, the reduction occurring when the value of the ionization or other variable falls below a predetermined threshold.

In this connection, in the case of a burner supplied with a fuel-air mixture via a gas valve 'and a fan and having at least one ionization monitoring electrode disposed in its neighbourhood and adapted to have a high voltage applied to it, the gas valve and the f an may be connected to a control circuit which has an input connected to the monitoring electrode and which reduces the speed of the fan motor when the ionization in the stream of burning mixture detected by the monitoring 5 electrode falls below a given value.

Such features may simplify the construction of a burner arrangement or the control thereof.

Examples of methods, in accordance with the present invention, of burning a fuel-air mixture and examples of burner arrangements for performing such methods are shown in the accompanying drawings, in which; Figures 1 and 3 show respective circuit diagrams of two different switching circuits of such burners; Figures 2 and 4 show the circuit diagrams of Figures 1 and 3 respectively, with modifications and associated burners shown diagrammatically; Figure 5 shows diagrammatically a cross-section of a modified burner arrangement; and Figures 6 and 7 are graphs showing the voltage applied to a fan motor of the arrangement shown in Figure 5, and also an ionization signal for cold-start operation and burn operation, plotted against time.

With references to Figures 1 to 5, circuits are shown each with a fan or blower motor 1 having a circuit 2 to which is applied a voltage U and which is closed by a switch contact 3 when required, for example to supply the burner with combustion air. Figures 2 and 4 in addition show a burner 4 which is mounted inside a combustion chamber 6 enclosed by a wall 5 and also containing a heat exchanger 7, for example a water heater. The combustion chamber 6 merges via a waste-gas collecting hood 8 into a waste-gas outlet 9 which opens into a chimney or directly to the atmosphere.

In the Figure 1 embodiment, the switching circuit 2 contains a series resistor 11 which can be switched in only in the starting phase, can be bypassed by a bypass line 10, and, when switched in, reduces the power supply to the fan motor 1. The circuit 2 further contains a switching contact 12 which connects the bypass line 10 to the fan motor 1 at the end of the starting phase, which breaks the circuit during the starting phase only, and which is controlled by a time function element 13 via a control lead 14 as shown in Figure 1.

During the starting phase, which is controlled as a function of time and is limited in time in the manner described, the switching contact 12 closes the switching circuit 2 via the series resistor 11, which ensures that the power supply to the fan motor 1 is of a relatively low value so that only a comparatively small proportion of combustion air is supplied to the combustion chamber 6 of the burner 4.

By contrast, when the switching circuit 12 is in the position shown in Figures 1 and 2, corresponding to normal operation of the burner when hot, the full voltage 1 is i U is applied to the fan motor 1, to supply the latter with the full current via the bypass line 10 so as to operate the fan 1 at full power, and so as to supply the combustion chamber 6 with a predetermined quantity of air.

Figure 2 shows a modification of the switching circuit 2 shown in Figure 1 where the switching circuit 12 is controlled by a temperature-sensitive switching element, preferably a thermo-element 15, which is disposed in the waste-gas collecting hood 8 of the combustion chamber 6 of the burner 4. That element 15 is therefore indirectly heated by waste gases from the burner 4, and is connected by a control line 16 to a coil 17 of an electromagnetic switching contact of the switch 12.

After the burner 4 starts up a thermo-element 15 s continuously heated and, after a certain time when a predeterm ined temperature of the element 15 is reached, the switching contact 12 closes the bypass line 10 so 1Chat the fan motor 1 provides the burner 4 with a maximum supply of combustion air, being the optimum proportion.

In Figures 3 and 4 the switching circuit 2 contains a temperaturesensitive series resistor 18 which may be constituted by a hightemperature conductor and which has an electrical resistance which constantly reduces in value as the temperature of the resistor increases, and thus first throttles the power of the fan motor 1, when cold, but later, upon heating increases the speed of the motor 1 until full power is reached.

This temperature-sensitive series resistor 18 is positioned where it is constantly heated, either directly or indirectly, when the burner 4 is operating.

In Figure 4 the temperature-sensitive series resistor 18 is mounted on the waste-gas collecting hood 8 of the combustion chamber 6 of a burner 4 having the construction shown in Figure 2 and is thus, when the burner 4 starts, indirectly heated through this component, which may be part of a water-heater.

In Figure 5, a burner 4 comprising a porous ceramic burner plate is mounted in a combustion chamber 6 which also contains a heat exchanger 7. The burner 4 is supplied with fuel gas via gas valve 22 by means of which the gas supply can be adjusted. The burner 4 is further supplied with air in a controllable manner by means of a fan 1.

Ignition electrodes 20 and an ionization monitoring electrode 21 are also disposed near the burner 4 and to both of which can be applied a high electrical voltage.

Electrodes 20 and 21 are connected to a control circuit 23 which controls the gas valve 22 and the fan 1.

During a cold start. as shown in Figure 6, the control circuit 23 operates as follows: During a pre- scavenging phase, in which the combustion chamber 6 is freed from ignitable mixture, the fan motor voltage and consequently the speed of the fan 1 is reduced. As the gas valve 22 is opened immediately prior to combustion, the speed of the fan 1 is held relatively low for a predetermined safety period during which the burner 4 should ignite, and for an additional time if required, after which the speed of the fan 1 is increased continuously or stepwise.

Advantageously the speed of the fan 1 is increased from the time at which the ionization signal from the electrode 21 exceeds a preset threshold value Is when the temperature rises in the neighbourhood of the monitoring electrode 21, as shown by a comparison between the two graphs in Figure 6.

As can be seen from Figure 7, as soon as the ionization signal falls below a threshold value Is after the burner power is reduced, the fan motor voltage and consequently the speed of the fan 1 is reduced, whereas the fan motor voltage is increased as soon as the ionization signal again exceeds the threshold value Is. The increase may be a step function of time, so that the fan motor voltage is abruptly increased, or it may be a more gradual increase as shown by the broken lines.

The number of times such a decrease and increase of the f an motor voltage U nay occur in this way may be limited to, for example five times per hour.

1

Claims

1. A method of burning a fuel-air mixture in a combustion chamber supplied with a stream of combustion air, in which, during a starting phase of combustion, the combustion chamber is supplied with a proportion of combustion air which is less than a proportion of air provided for subsequent combustion.

2. A method according to claim 1, in which the relatively low supply of combustion air is provided in a pre-scavenging phase and continues during an ignition phase to the end of a predetermined safety period, after which the supply of combustion air is increased.

3. A method according to claim 1, in which the 15 relatively low supply of combustion air is provided during operation of the burner in dependence upon a signal derived from the flame in the combustion chamber, preferably an ionization signal, the reduction occurring when the measured value falls below a preset threshold.

4, A method according to any preceding claim in which the increase in the proportion of combustion air is made in stages at the end of a reduction phase.

5. A method according to claim 3, in which within a burner cycle, the number of occurrences of a period in which the proportion of combustion air is at a relatively low level, is limited.

6. A burner arrangement for working the method 2k according to claim 1, comprising a fan which serves to supply the combustion chamber with combustion air, the fan being provided with a motor drive, in which a switching circuit is provided to control the speed of the fan motor, 5 so as to reduce it during the starting-up of the burner.

7. A burner arrangement according to claim 6, in which the switching circuit of the fan motor contains a series resistor which can be switched in during the starting phase so that the fan is operated at a relatively low speed, and which can be bypassed by a bypass line.

8. A burner arrangement according to claim 7, in which a switching contact is provided to connect the bypass line to the fan motor after the starting phase, and to interrupt the circuit during the starting phase, and in which the switching contact is controlled by a time function element.

9. A burner arrangement according to claim 7, in which a switching contact is provided to connect the bypass line to the fan motor after the starting phase, and to interrupt the circuit during the starting phase, and in which the switching contact is controlled by a temperature-sensitive switching element, for example a thermo-element.

10. A burner arrangement according to claim 6, in 25 which the switching circuit of the fan motor contains a temperature-sensitive series resistor, for example a hightemperature conductor, the value of the resistance of 4 which is reduced as its temperature increases.

11. A burner arrangement according to claim 10, in which the series resistor is disposed at a place where it is heated when the burner is started.

12. A burner arrangement according to claim 11, in which the series resistor is disposed on a component which is heated when the burner is started, f or example on a waste-gas collecting hood of a combustion chamber of the burner.

13. A burner arrangement for working the method according to claim 2, adapted to be supplied with a fuel air mixture via a gas valve and a fan, and having an ionization monitoring electrode disposed in its neighbourhood and adapted to have a high electrical voltage applied to it, in which the gas valve and the fan are connected to a control circuit which has an input connected to a monitoring electrode and which reduces the speed of the fan motor when the ionization detected by the monitoring electrode falls below a given value.

14. A burner arrangement substantially as described herein with reference to any one of Figures 1 to 4, or with reference to Figures 5 to 7, of the accompanying drawings.

15. A method of burning a fuel-air mixture in a combustion chamber, substantially as described herein with reference to any one of Figures 1 to 4, or with reference to Figures 5 to 7, of the accompanying drawings.

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