Feedback control for modulating gas burner

• US 20060105279A1
• Filed: 11/18/2004
• Published: 05/18/2006
• Est. Priority Date: 11/18/2004
• Status: Active Grant

• Current Assignee: Ademco Inc. (Resideo Technologies, Inc.)
• Original Assignee: Honeywell International Inc.
• Inventors: Strand, Rolf L., Munsterhuis, Sybrandus
• Granted Patent: US 7,241,135 B2
• Time in Patent Office: Days
• US Class Current: 431/18
• CPC Class Codes: F23N 1/022 using electronic means
• CPC Class Codes: F23N 2223/10 Correlation
• CPC Class Codes: F23N 2227/20 Calibrating devices
• CPC Class Codes: F23N 2233/08 with variable speed
• CPC Class Codes: F23N 2235/16 variable flow or proportional valves
• CPC Class Codes: F23N 5/123 using electronic means

Abstract

A modulating gas burner control system using closed loop feedback from a flame rod which provides a signal that varies with gas pressure and which provides combustion air fan control to accurately control the heat from the system without use of expensive control valves.

Claims

1. In a burner control system having a gas supply, a valve receiving gas from the gas supply and controlling gas flow, an air source to be mixed with the gas, a burner for producing a flame from the gas/air mixture, a flame rod for detecting the presence of flame, a heat exchanger for receiving the burning gas/air mixture, a combustion fan for driving the gas/air mixture through the heat exchanger and a pressure switch for determining the gas/air pressure in the combustion fan, the improvement comprising:

a controller for receiving a signal from the flame rod and producing a control signal to the valve in accordance with a predetermined relationship between the flame rod signal and the gas pressure so as to position the valve in accordance with a desired gas pressure and wherein the controller provides for calibration of the flame rod from time to time by determining the gas pressure at a maximum opening of the valve and determining the gas pressure at smaller openings therefrom.

2. Apparatus according to claim 1 further including a connection from the pressure switch to the controller to provide the controller with a signal indicative of the flow through the combustion air fan so that the controller produces a control signal to the combustion air fan in accordance with a desired fan speed.

3. Apparatus according to claim 1 wherein the predetermined relationship is a linear equation of the form:

Desired Flame Current=K*Firing Rate+Offset

4. Apparatus according to claim 1 further including means modulating the airflow to provide a desired gas/air ratio for the gas/air mixture.

5. Apparatus according to claim 4 wherein the airflow is modulated by modulating the fan speed of the combustion fan.

6. Apparatus according to claim 5 wherein the RPM of the combustion fan is supervised with an RPM sensor and a signal indicative thereof is presented to the controller and the controller produces an output to the combustion fan to control the speed thereof.

7. Apparatus according to claim 6 wherein the maximum setting for the airflow is calibrated by increasing the RPM of the combustion fan until a predetermined set point for the pressure switch is reached which corresponds to a maximum load.

8. Apparatus according to claim 7 wherein the airflow from the predetermined set point is proportional to the RPM of the fan at a predetermined temperature.

9. A burner control system comprising:

a valve to produce a controlled gas flow, and having a gas output pressure from the valve varying with valve opening;

a burner receiving a gas/air mixture from the gas flow and a source of air to provide a gas/air mixture which is to be ignited to produce a flame;

a flame rod for detecting the presence of flame to produce a signal which also varies with the gas output pressure from the valve;

a heat exchanger receiving the flame and directing the burning gas/air mixture downstream;

differential pressure responsive means producing a signal indicative of a downstream pressure of the burning gas/air mixture; and

a controller for receiving the signal from the flame rod and the signal from the differential pressure responsive means to produce a control signal to the valve in accordance with a predetermined relationship between the flame rod signal and the signal from the differential pressure responsive means so as to position the valve in accordance with a desired gas pressure and wherein the controller provides for calibration of the flame rod from time to time by determining the gas pressure at a maximum opening of the valve and determining the gas pressure at smaller openings there from.

10. The burner control system of claim 9 further including a combustion fan for driving the gas/air mixture through the heat exchanger.

11. The burner control system of claim 10 wherein the differential pressure responsive means includes pick up points located proximate the combustion fan.

12. The burner control system of claim 11 wherein the controller produces a control signal to the combustion air fan in accordance with a desired fan speed.

13. Apparatus according to claim 12 wherein the airflow is modulated by modulating the fan speed of the combustion fan.

14. Apparatus according to claim 13 wherein the RPM of the combustion fan is supervised with an RPM sensor and a signal indicative thereof is presented to the controller and the controller produces an output to the combustion fan to control the speed thereof.

15. Apparatus according to claim 14 wherein the maximum setting for the airflow is calibrated by increasing the RPM of the combustion fan until a predetermined set point for the pressure switch is reached which corresponds to a maximum load.

16. Apparatus according to claim 15 wherein the airflow from the predetermined set point is proportional to the RPM of the fan at a predetermined temperature.

17. A burner control system comprising:

a gas supply;

a valve receiving gas from the gas supply and controlling gas flow;

an air source to be mixed with the gas to produce a gas/air mixture;

a burner for producing a flame to produce a burning gas/air mixture;

a flame rod for detecting the presence of flame;

a heat exchanger for receiving the burning gas/air mixture

a combustion fan for driving the gas/air mixture through the heat exchanger and directing heat to a desired space;

a pressure switch for determining the gas/air pressure in the combustion fan; and

a controller for receiving a signal from the flame rod and producing a control signal to the valve in accordance with a predetermined relationship between the flame rod signal and the gas pressure so as to position the valve in accordance with a desired gas pressure and wherein the controller provides for calibration of the flame rod from time to time by determining the gas pressure at a maximum opening of the valve and determining the gas pressure at smaller openings therefrom.

18. The method of controlling the flow of gas from a valve in a combustion control system including the steps of:

A. sensing the gas pressure from the valve from a signal produced by a flame rod;

B. sensing the differential pressure of the gas air mixture with pressure pick up means located proximate the combustion fan;

C. comparing the gas pressure signal produced by the flame rod with the differential pressure sensed to provide a desire pressure signal,

D. controlling the opening of the valve in accordance with the desired pressure signal, and,

E. calibrating the flame rod from time to time by determining the gas pressure at a maximum opening of the valve and determining the gas pressure at smaller openings there from.

19. The method of claim 18 further including the step of:

A1. Igniting the gas/air mixture and directing the burning gases past the flame rod to a heat exchanger.

20. The method of claim 19 including the step of:

A2. directing the burning gasses downstream with the combustion fan.

21. The method of combustion control comprising the steps of:

A. directing gas from a source to a gas valve to produce a gas output;

B. mixing the gas output with air and directing the mixture of gas and air to a burner for ignition;

C. directing the gas/air mixture to a heat exchanger past a flame rod to produce a flame rod output signal when the gas/air mixture is burning;

D. directing the burning gas/air mixture in the heat exchanger to a combustion air fan for exhaust;

E. sensing the gas/air pressure with differential pressure pick up means at the combustion air fan to produce a sensor signal;

F. comparing the sensor signal with the flame rod output signal to produce a control signal;

G. adjusting the opening of the valve in accordance with the control signal;

H. calibrating the flame rod from time to time by determining the gas pressure at a maximum opening of the valve and determining the gas pressure at smaller openings there from; and

I. modulating the airflow by modulating fan speed of the combustion fan.