GB2237665A - Boiler control - Google Patents

Abstract

A gas-fired heating boiler comprises a gas burner 1, a heat exchanger 6 healed by the gas burner and through which is passed a fluid to be heated, a fan 4 for providing combustion air to the burner 1, and a control circuit for controlling the speed of the fan 4 and gas flow to the burner 1 in dependence upon the temperature of the heated fluid, the flow of air from the fan 4 and the pressure of gas supplied to said burner. The actual fluid temperature measured at 10 is compared at 11 with a reference, any difference being input to a comparator 12 for controlling the speed of fan 4. An airflow sensor 5 provides feedback to the comparator 12 and also provides an input to a comparator 13 for comparison with a signal from a gas pressure sensor 14, any difference controlling a valve 3. A shutdown signal I may be generated if the difference between the signals from sensors 5, 14 exceeds a threshold. <IMAGE>

Description

Heating Boilers This invention relates to heating boilers and more specifically to gas-fired heating boilers. The invention is especially applicable to central heating boilers which may be of conventional form or may take the form of a combination central heating boiler.

There is a demand nowadays for gas-fired boilers to operate as efficiently as possible over a wide range of operating conditions, which may include poor installation or siting of the boiler, possibly resulting in insufficient water flow or inadequate flueing.

The present invention relates to a gas-fired heating boiler which includes an improved form of control system which can take account of different operating conditions.

According to the present invention there is provided a gas-fired heating boiler comprising a gas burner, a heat exchanger heated by said burner and through which is passed a fluid to be heated, a fan for providing combustion air to said burner, and a control circuit for controlling the speed of said fan and the gas flow to said burner in dependence upon the temperature of said heated fluid, the flow of air from said fan and the pressure of the gas supplied to said burner.

In a preferred arrangement according to the present invention there will be provided a temperature sensor for sensing the temperature of said heated fluid, a first comparator for comparing the output of the temperature sensor with a reference value, an air flow sensor for sensing the flow of combustion air to said burner, and a second comparator for comparing the output of the air flow sensor with the output of the first comparator and for controlling the speed of said fan, and may also comprise a gas flow control valve for controlling the gas flow to said burner, a gas pressure sensor for sensing the gas burner pressure, and a third comparator for comparing the output of the gas pressure sensor and the output of the air flow sensor and for controlling the setting of said gas flow control valve.

Advantageously, a fourth comparator may be provided for comparing the output of the air flow sensor with the output of the gas pressure sensor and for shutting down the boiler if the difference between said outputs exceeds a predetermined amount.

In addition means may be provided, operable on the air flow sensor output applied to said third comparator, for correcting for system non-linearity.

In carrying out the invention it may be arranged that said comparators take the form of differential amplifiers; said temperature sensor takes the form of a negative temperature coefficient thermistor; said air flow sensor senses the velocity of the combustion air; in which case the velocity of the combustion air may be sensed using a pressure difference arrangement; said gas flow control valve takes the form of a modulating gas valve; said gas pressure sensor takes the form of a diaphragm device incorporating a capacitance detector arrangement or takes the form of a solid state sensor; and/or that said solid state sensor comprises a piezoresistive strain gauge.

An exemplary embodiment of the invention will now be described reference being made to the accompanying single figure drawing which is a diagrammatic/schematic representation of a gas-fired heating boiler in accordance with the present invention.

The gas-fired heating boiler shown in the drawing comprises a gas burner 1 which is supplied with gas from a gas input 2 via a modulating gas valve 3.

Combustion air for the gas burner 1 is derived from a variable speed fan 4 the air flow from which is measured by an air flow sensor 5. The air flow sensor 5 may take any convenient form and may be arranged to sense the velocity of the air directly or it may deduce the air velocity from the pressure difference between two points of measurement.

The gas burner 1 is arranged to heat a heat exchanger 6, through which a suitable fluid, e.g. water is caused to flow via inlet pipe 7 and outlet pipe 8.

Products of combustion from the gas burner 1 are fed to a flue outlet 9.

In order to ensure that the heating boiler thus far described is operated efficiently over a wide range of operating conditions, a control circuit is provided for controlling the gas and air flow to the gas burner 1 in order to maintain the temperature of the fluid in the outlet pipe 8 at a required level.

The control system comprises a fluid temperature sensor 10, typically a negative temperature coefficient thermistor, which is mounted in close proximity to the outlet pipe 8, the output from which is applied to a comparator 11, typically a differential amplifier, in which it is compared with a reference signal R which is set at a level dependent upon the required fluid temperature level. Typically the reference level R may be derived from a potentiometer which constitutes a temperature setting control of the boiler.

The output from the comparator 11, which is indicative of whether the actual fluid temperature as measured by the fluid temperature sensor 10 is higher or lower than that corresponding to the reference signal R, is applied to a second comparator 12, typically a differential amplifier, in which it is compared with the output from the air flow sensor 5.

The output from the comparator 12 is used to control the speed of the variable speed fan 4.

The output from the air flow sensor 5 is also applied to a third comparator 13, typically a differential amplifier, in which it is compared with the output of a gas pressure sensor 14 which is connected to measure the gas pressure applied to the gas burner 1. The output from the comparator 13 is applied as a control signal to the modulating gas valve 3 which controls the gas flow to the gas burner 1. The gas pressure sensor 14 may take any convenient form and may take the form of a diaphragm device incorporating a capacitance detection system, or a solid state sensor in which, for example, a piezoelectric strain gauge registers the pressure imposed thereon.

A typical operating sequence of the heating boiler thus far described will now be given.

Assume that the heating boiler is set for operation with fan speed low and the gas burner 1 lit but at a low setting so that the outgoing fluid temperature is low (i.e. at a temperature below that corresponding to the reference R). At this time the output from comparator 11 will indicate a "heat" requirement and will afford an output to comparator 12.

The output from comparator 11 is compared with the output from the air flow sensor 5 in comparator 12 which affords an output to the fan 4 causing it to increase the flow of combustion air.

The output from the air flow sensor 5 is applied to the comparator 13 along with the output from the gas pressure sensor 14, the output from which controls the modulating gas valve 3 to increase the gas flow to the gas burner 1 in accordance with air flow set by the fan 4. By this means, the flow of gas to the burner 1 is at all times related to the flow of air from the fan 4.

The increased gas flow to the burner 1 increases the heat output from it, thereby causing the heat exchanger 6 to be heated so that the fluid flowing therethrough to the outlet pipe 8 is heated.

As the temperature of the fluid in the outlet pipe 8 reaches the level demanded by the temperature reference R, the output from comparator 11 changes to indicate to comparator 12 the reduced air requirement.

The control loop around the fan 4 and air flow sensor 5 thus reduces the fan speed accordingly until the actual air flow equals the required air flow. The reduced air flow signal from the air flow sensor 5 causes the gas control loop around comparator 13 to reduce the gas flow and therefore the heat input to the heat exchanger 6. As the control system forms a closed control loop, the temperature in the outlet pipe 8 is stabilised at a level corresponding to the reference R.

It has been found in the heating boiler which has been described that in order to maintain the required air/gas ratio over a wide operating range, it may be necessary to provide some correction to the system response as the output levels vary. This can be achieved by inserting a non-linearity correction device shown in dotted outline 15 between the air flow sensor 5 and the input to the comparator 13. The device 15 may be based on analogue circuitry where the characteristic curve required is made up of discreet sections with different slopes, or could be based on a software "look-up" table where the required gas rate at any air flow is stored in memory.

It will be appreciated that the major functional components i.e. the gas valve 3 and fan 4 are contained within their own closed control loop, and the system is inherently immune to moderate variations in the performance of these components, as long as the sensors 5, 10 and 14 produce reliable outputs.

The advantages of the system thus far described can be appreciated by considering what happens when the flue resistance is progressively increased, thereby restricting the overall air flow through the device.

This can occur in practice if the flue was made very long or followed a very tortuous route to the outside, or if the flue becomes restricted. As the flue resistance is increased, the air flow sensor/fan control loop will react to increase the speed of the fan 4 to maintain the actual air flow at the rate demanded by comparator 11. When sufficient resistance is included to cause the fan 4 to reach maximum speed the air flow will naturally fall. This fall will be registered on the signal from the air flow sensor 5 to the gas control loop and so cause the gas pressure applied to the burner 1 to correspondingly drop. Thus the heat output of the boiler is automatically limited if the flue system is unable to supply sufficient air for full rate combustion.

The control system of the heating boiler which has been described is dependent upon the outputs from sensors 5, 10 and 14 for its correct operation. If a sensor produces an incorrect signal, the operation of the boiler will be impaired and may prove hazardous.

If this occurs it is advantageous to provide means for shutting down the boiler. This can be effected in the heating boiler shown in the drawing by providing a fourth comparator 16, typically a differential amplifier, which compares the outputs from the air flow sensor 5 and gas pressure sensor 14 and affords a shutdown signal I if the difference between them exceeds a predetermined amount. The shut down signal I may be used in any convenient way to ensure that the heating boiler is shut down.

It is envisaged that the heating boiler control system which has been described may operate alone as described or may be incorporated as part of a full sequence control circuit which provides "start-up" from a completely shut-down condition. It is particularly advantageous in that it includes an apparent "intelligence" which continually monitors the changing operating conditions of the boiler and makes consequent adjustments, and this makes for ease of installation of the boiler and obviates, for example, the need to determine the flue resistance which exists in any particular installation.

Claims (14)

1. A gas-fired heating boiler comprising a gas burner, a heat exchanger heated by said burner and through which is passed a fluid to be heated, a fan for providing combustion air to said burner, and a control circuit for controlling the speed of said fan and the gas flow to said burner in dependence upon the temperature of said heated fluid, the flow of air from said fan and the pressure of the gas supplied to said burner.

2. A boiler as claimed in claim 1, comprising a temperature sensor for sensing the temperature of said heated fluid, a first comparator for comparing the output of the temperature sensor with a reference value, an air flow sensor for sensing the flow of combustion air to said burner, and a second comparator for comparing the output of the air flow sensor with the output of the first comparator and for controlling the speed of said fan.

3. A boiler as claimed in claim 2, comprising a gas flow control valve for controlling the gas flow to said burner, a gas pressure sensor for sensing the gas burner pressure, and a third comparator for comparing the output of the gas pressure sensor and the output of the air flow sensor and for controlling the setting of said gas flow control valve.

4. A boiler as claimed in claim 3, comprising a fourth comparator for comparing the output of the air flow sensor with the output of the gas pressure sensor and for shutting down the boiler if the difference between said outputs exceeds a predetermined amount.

5. A boiler as claimed in claim 3 or claim 4, comprising means operable on the air flow sensor output applied to said third comparator for correcting for system non-linearity.

6. A boiler as claimed in any of claims 2 to 5, in which said comparators take the form of differential amplifiers.

7. A boiler as claimed in any of claims 2 to 6, in which said temperature sensor takes the form of a negative temperature coefficient thermistor.

8. A boiler as claimed in any of claims 2 to 7, in which said air flow sensor senses the velocity of the combustion air.

9. A boiler as claimed in claim 7, in which the velocity of the combustion air is sensed using a pressure difference arrangement.

10. A boiler as claimed in any of claims 3 to 9, in which said gas flow control valve takes the form of a modulating gas valve.

11. A boiler as claimed in any of claims 3 to 10, in which said gas pressure sensor takes the form of a diaphragm device incorporating a capacitance detection arrangement.

12. A boiler as claimed in any of claims 3 to 10, in which said gas pressure sensor takes the form of a solid state sensor.

13. A boiler as claimed in claim 12, in which said solid state sensor comprises a piezoresistive strain gauge.

14. A gas-fired boiler substantially as hereinbefore described with reference to the accompanying drawing.