GB2070228A - Burner control system - Google Patents

Abstract

This invention is a control system for a burner, for example for a steam boiler, or for providing a supply of hot gas. The fuel and air supply to the burner (15) are controlled by valves (18) and (21) from an actuator (19) in response to a control signal (145) dependent upon burner performance. An additional control is responsive to a signal at (146) responsive to the amount of oxygen in the products of combustion from the burner, and that is used to control the length of a link (125) (Fig. 1) in the linkage between the actuator (19) and the valves (18 and 21) or to control a reference pressure in a bellows controlling a linkage for controlling the air valve (21). <IMAGE>

Description

SPECIFICATION Burner control system This invention relates to a control system for a burner, for example a burner for firing a steam boiler.

In British Patent Specification No.

22156/77 and 9414/78, a burner control system is described which has means for controlling the fuel and/or the air supply automatically in response to demand, and including also means responsive to the amount of oxygen in the products of combustion from the burner for providing an additional control. An object of the present invention is to provide certain improvements to the method of providing such additional control described in that specification.

According to one aspect of the present invention, a burner control system has a linkage for controlling the fuel and/or the air supply automatically in response to burner performance, and includes also means responsive to the amount of oxygen in the products of combustion from the burner arranged automatically to adjust a component in the linkage.

The linkage may include a rod whose length is arranged to be controlled automatically in response to the amount of oxygen in the products of combustion, or the linkage may include a lever pivotally supported between two bellows, one of which is at a pressure dependent on burner performance, while the other is at a reference pressure which is arranged to be automatically adjusted in dependence upon the amount of oxygen.

A signal representing the amount of oxygen in the products of combustion can be arranged to be automatically adjusted in magnitude in dependence upon the boiler demand.

The automatic adjustment might be by means of a potentiometer with a tapping adjusted in dependence on the demand, but preferably is by means of a series of resistors, different ones of which are connected in circuit by the operation of switches controlled by adjustment of the fuel setting.

The linkage for controlling the fuel and/or the air supply will often include a non-linear cam or the equivalent because the desired adjustment of fuel and air valves together, does not foliow a linear relationship. Even with a complicated linkage of that kind, it is a very convenient way of providing the additional control responsive to the amount of oxygen in the products of combustion to adjust automatically a component in the linkage as discussed above.

In British Patent Specification Nos.

22156/77, and 9414/78, the oxygen signal was used to control the setting of a trim damper mounted at the main combustion air inlet, because that is the most convenient position to put a trim damper in an existing system. It suffers from the disadvantage however, that if perhaps due to a fault, the trim damper was completely closed, the burner would be starved of oxygen, and according to a second aspect of the present invention, if a trim damper is to be used rather than a variable component in the linkage for providing the control dependent upon the amount of oxygen, the trim damper is positioned in a part of the combustion air supply in parallel with the part containing a main air flow damper.

The invention may be carried into practice in various ways, and certain embodiments will now be described by way of example with reference to the accompanying drawings; in which Figure 1 is a diagram of one form of air damper and fuel valve control linkage; Figure 2 is a diagram of a bellows and lever system for adjusting combustion air supply; and Figure 3 is a copy of Fig. 1 in the above British Pstent Specification including certain modifications.

In British Patent Specification No.

22156/77 and 29414/78, a boiler control system is described in which the rate of supply of fuel through a metering valve 1 8 is controlled in response to steam demand at 145 by an actuator 19. An input signal to the actuator 1 9 is related to the steam pressure in the outlet from the boiler. A change in the rate of fuel supply requires a change, but not a linearly related change, in the rate of air supply, and a damper 21 is adjusted at the same time by the actuator 1 9 through a linkage 22 with a characteristic which is probably designed by trial and error.

The mechanism is only shown diagrammatically in that British Patent Specification, but in accordance with the present invention, as shown in Fig. 1, the actuator 1 9 can be arranged to turn a jackshaft 1 21 through an operating arm 22 coupled to a crank 1 23 on the shaft 121. A crank 124 on the shaft 121 is coupled to an operating rod 1 25 for driving the damper connected at 21, and the fuel valve control at 1 8 is operated by rotation of the shaft 121 through a cam 126 which has a characteristic arranged to maintain approximately the correct adjustment of the air supply as the fuel supply is adjusted.

In the above British Patent Specification an additional control was provided in response to the oxygen level in the exhaust gases in the stack 24 as analysed by a probe 26, and that additional control operated an actuator 32 for a trim damper 33, if the measured level was outside limits set in a unit 29.

According to the present invention, instead of having the trim damper responding to the oxygen level signal, the operating rod 1 25 is constituted by a variable length actuator, the length of which is set in dependence on the oxygen level signal from the probe 26.

That is a particularly simple way of provid ing the additional control without providing an additional damper, and the control can be applied during only a part of each of a num ber of repeated cycles as described with refer ence Fig. 3 of the above patent specification.

Thus, the actuator 1 25 can receive signals through the contacts T1/1 for a part of a 10 second cycle, but not for the remainder of the cycle. The signal responsive to steam demand for driving the actuator 1 9 could also be applied only during a part of each cycle, although it could also be applied continuously.

In an alternative to the embodiment of Fig.

1, the signal responsive to delivered steam pressure as indicated generally at 145 in Fig.

2, is used to control the fuel valve 18 and is also supplied as an input pressure to one bellows 1 32 of a pair of bellows 1 32 and 133, each having a linkage arm pivotally connected at 1 34 or 1 35 to spaced points along an operating lever 1 36. A link 1 37 is pivotally connected at an intermediate point 1 38 along the lever 1 36 and is used to drive the combustion air flow damper 21.

The other bellows 1 33 receives as its input an air pressure signal constituting a reference pressure.

In such an arrangement the oxygen level signal signal from the probe 1 26 can be arranged to vary the reference pressure in the bellows 1 33 to add in the additional control.

As indicated in Fig. 2, a reference pressure of perhaps 1 5 p.s.i. can be supplied through pressure valve 1 39 to the bellows 1 33 and the constant pressure valve 1 39 can be reset as indicated generally at 141 in response to the oxygen level signal. Once again the oxygen level signal can be applied only during a part of each of successive control cycles.

The systems described with reference to Figs. 1 and 2 produce only two actions, namely a resetting of the fuel valve 18, and a resetting of the combustion air flow damper 21, and in that way they are variants of the system in Fig. 1 of the above British Patent Specification, in which the additional control in response to oxygen level in the stack gases was used to operate a trim damper 33. That arrangement is convenient because the trim damper 33 and its actuator 32 can be added at the air inlet to an existing system without having two have the complications of fitting a damper inside the existing system.

However it has the disadvantage that complete closure of the trim damper 33, for example due to a fault, could starve the boiler of combustion air and if it is not too difficult to fit the additional damper, it would be convenient if, instead of being at the air inlet at 33, it were connected in a by-pass in parallel with the duct 1 6 where the main damper 21 is fitted, as indicated generally at 143 in Fig. 3. It would still be operated by the actuator 32. The operation would be the same providing on overriding control of the combustion air supply, but closure of the trim damper 143 would not in itself starve the burner of combustion air.

With reference to Fig. 3, (which is a modification of Fig. 1 in the above British Patent Specification), and Fig. 1 in the present specification, it will be remembered that an input signal to the actuator 1 9 is derived from the delivered steam pressure at 145. Now the desired level of excess oxygen in the stack exhaust gases tends to be lower at high steam load, than at low steam load, and accordingly the limits set at 29 beyond the probe signal 146 operates the actuator 32 can be arranged to be adjusted automatically in dependence on the steam load. For example, as the shaft 121 in Fig. 1 is rotated by the actuator 19, a potentiometer wiper arm can be reset in a potentiometer in the device 29, so that the value of the excess oxygen signal is adjusted automatically with changing fuel setting.For example, the permitted percentage of oxygen present in the exhaust gases may be 3% at full load, increasing to 5% at 40% load.

In a preferred scheme the shaft 121 has a crank which is arranged to operated a microswitch or a number of micro-switches in turn, as it reaches different angular positions to connect resistors of different values successively in the device 29 to achieve the same effect in step.

In one arrangement the permitted level is 3% from 40% load to full load, and is 5% below 40% load.

The demand signal at 145 may be dependent on steam pressure if the burner is firing a steam boiler; then a potentiometer can be driven by a pressure gauge to give an electrical signal which can be balanced in a bridge circuit by a signal representing a desired pressure.

If the burner was for supplying hot gas for an industrial process for example, a thermocouple could give a demand signal dependent on the gas temperature of the products of combustion, or of secondary air. The oxygen level signal would probably be taken downstream of the burner, but upstream of an entry for secondary air.

Claims (11)

1. A burner control system which has a linkage for controlling the fuel and/or the air supply automatically in response to burner performance, and includes also means responsive to the amount of oxygen in the products of combustion from the burner arranged automatically to adjust a component in the linkage.

2. A control system as claimed in Claim 1 in which the linkage includes a rod whose length is arranged to be controlled automatically in response to the amount of oxygen in the products of combustion.

3. A control system as claimed in Claim 2 in which an actuator is operated in response to burner performance to move a shaft, and there is a link from the shaft to a fuel or an air control including the variable length rod, and a link to an air or a fuel control including a non-linear cam.

4. A control system as claimed in Claim 1 in which the linkage includes a lever pivotally supported between two bellows, one of which is at a pressure dependent on burner performance, while the other is at a reference pressure which is arranged to be automatically adjusted in dependence upon the amount of oxygen.

5. A control system as claimed in Claim 4 in which an actuator responsive to burner performance is arranged to operate a fuel or an air control, and is also arranged to provide in the one bellows a pressure dependent on burner performance.

6. A control system as claimed in any of the preceding claims in which the effect of a signal representing the amount of oxygen in the products of combustion is arranged to be automatically adjusted in magnitude in dependence upon the burner performance.

7. A control system as claimed in any of the preceding claims for a burner for a steam boiler in which a signal responsive to steam pressure is used as a measure of the burner performance.

8. A control system as claimed in any of Claims 1-6 in which a signal dependent upon the temperature of the products of combustion immediately downstream of the burner is used as means responsive to burner performance.

9. A control system as claimed in any of the preceding claims arranged to operate in repeated cycles in each of which either a signal responsive to burner performance, or a signal responsive to the amount of oxygen in the products of combustion, or both, is effective for a part only of each cycle.

10. A control system as claimed in any of the preceding claims including a zirconia cell for providing a signal dependent upon the amount of oxygen in the products of combustion.

11. A burner control system constructed and arranged substantially as herein specifically described with reference to any Figure of the accompanying drawings.