GB2261063A

GB2261063A - A burner arrangement.

Info

Publication number: GB2261063A
Application number: GB9300749A
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: 1993-01-14
Publication date: 1993-05-05
Anticipated expiration: 2013-01-14
Also published as: CH683636A5; GB2230332A; NL9000824A; GB9300749D0; BE1004109A6; GB2261063B; GB2230332B; GB9008449D0; DE4011690A1

Description

-121 A 3137 - 1 A burner arrancement The present invention relates to a

burner arrangement for 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 poorlyignitable fuel is used.

In our co-pending application No. 9008449.2 (Publication No. 2,230,332A), there is described and claimed 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, and 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.

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.

The present invention seeks to provide a burner for carrying out such a method effectively.

Accordingly the present invention is directed to a burner arrangement 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, in which the switching circuit of the fan motor contains a temperature- sens it ive series resistor, for example a high-temperature conductor, the value of the resistance of which is reduced as its temperature increases, and in which the series resistor is disposed on a component which is heated when the burner is started, for example on a waste-gas collecting hood of a combustion chamber of the burner.

The temperature-sensitive series resistor, for example a "hightemperature conductor", included in the switching circuit of the fan motor decreases in value as the temperature increases and thus smoothly adapts the power output of the fan motor to the increasing air requirenent of the burner. The series resistor is 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 wastegas collecting hood of a combustion chamber of the burner so as to be indirectly heated by the component.

In the present invention the switching circuit of the fan notor comprises a series resistor which is switched on only during the starting phase to operate the fan motor at a low speed, and which preferably 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 temperature-sensitive switching element, for example a thermo-elenent. In both cases the duration of the 4 - starting phase can be limited by the element concerned.

By virtue of a relatively low level proportion of combustion air being provided in a pre-scavenging phase and continuing into an ignition phase to the end of a preset safety period, after which the supply of combustion air is increased, a particularly combustible air/fuel mixture at the starting phase of the combustion process, is supplied. This ensures 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.

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 fan 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 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 corribustion 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 vaste-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 notor 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 tine and is linited 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 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 is continuously heated and, after a certain time when a predetermined temperature of the element 15 is reached, - 8 the switching contact 12 closes the bypass line 10 so that 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- s ens it ive 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 f an 1.

is 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 prescavenging 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 f rom Figure 7, as soon as the ionization signal falls below a threshold value Is after the burner power is reduced, the fan notor voltage and consequently the speed of the fan 1 is reduced, whereas - 10 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 fan motor voltage U nay occur in this way may be limited to, for example five times per hour.

- 11

Claims

1. A burner arrangement comprising a fanwhich 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, in which the switching circuit of the fan motor contains a temperature-sensitive series resistor, for example a high-temperature conductor, the value of the resistance of which is reduced as its temperature increases, and in which the series resistor is disposed on a component which is heated when the burner is started, for example on a waste-gas collecting hood of a is combustion chamber of the burner.

2. A burner arrangement substantially as described herein with reference to any one of the Figures of the accompanying drawings.