Flame Ionization Control of a Partially Premixed Gas Burner With Regulated Secondary Air

For many years the control of fuel/air ratio of fuel burners for various furnaces or heating appliances has been desired. One particular method is to automatically search for the peak value (maximum or minimum) of a property of the flame or combustion products which is indicative of the fuel/air or oxidant ratio of the fuel being burned in the burner and by various means adjusting the fuel/oxidant ratio in the combustion chamber for complete combustion.

Several years ago Honeywell Inc. developed an FSP1400 Fuel-Air Ratio Sensor described in a Honeywell publication 95-6957-1 of October 1970 which made use of a flame rod for sensing the ionization current in a small flame having the same premixed fuel/air ratio as the main burner. By means of a control apparatus the fuel/oxidant ratio of the burner was adjusted to provide maximum ionization current. The maximum current always occurred at a premixed fuel/oxidant ratio 15% greater than the stoichiometric ratio. Reducing the fuel/oxidant ratio until the current was 80% of maximum gave stoichiometric combustion.

The present invention is concerned with a gas burning furnace in which the control system searches for and maintains the ionization current at a peak value by controlling the fuel and primary air supply to the burner. This results in an excessive amount of fuel. The secondary air supply to the combustion chamber is controlled proportionally to the primary air supply in such a manner that the fuel/oxidant ratio in the combustion chamber is adequate for complete combustion. Secondary air has little or no effect on ionization current. Other properties of flames or combustion products which have peak values at or near the stoichiometric ratio could also be used to monitor fuel/oxidant ratio. These include flame temperature, flame radiation, H.sub.2 O and/or CO.sub.2 levels in the burned gases, etc. Properties of flames or combustion products which have minimum values at or near the stoichiometric ratio could also be used.

FIG. 1 is a schematic showing of a conventional furnace or combustion appliance having a burner in the combustion chamber to which gas or fuel and primary air is supplied. The combustion chamber is then supplied with secondary air for maximum combustion efficiency, and

FIG. 2 is a graphical representation showing the flame rod electrode current for various levels of premixed fuel/oxidant ratio (fuel number or excess air percentage).

Referring to FIG. 1, a furnace or fuel burning heating appliance 10 is shown to have a combustion chamber 11 which is connected to an exhaust flue or stack 12 through which the products of combustion pass to the outside. A burner 13 mounted in the combustion chamber is supplied with fuel or gas through pipe 14 having a burner orifice 15. Primary air to burner 13 is supplied through primary air orifice 20 by a forced draft or a combustion air blower 21. While the combustion air is supplied under pressure by blower 21, with the advent of induced draft furnaces, the combustion air through primary orifice 20 might be induced by a blower in exhaust flue 12 as disclosed in the Lorne W. Nelson, et al, U.S. Pat. No. 4,340,355, issued July 20, 1982. A flame rod 22 is mounted in burner flame 23 and is connected to a conventional fuel/air controller or control system 24 for controlling the output of a gas control or valve 25 and the output of the blower or primary air supply to the burner to maintain a peak flame rod current. Fuel/air controller 24 uses the principle developed by Honeywell some years back as the FSP1400 Fuel-Air Ratio Sensor. The maximum flame ionization current always occurs at a fixed premixed fuel/air ratio, i.e., 15% excess fuel. Fuel/air ratio can then be controlled by maximizing the electrical current of the flame rod 22. A conventional space thermostat 30 is connected to the controller 24 for bringing about operation of the furnace when there is a need for heat in the space to which heat is supplied by furnace 10.

Referring to FIG. 2, when the premixed fuel/oxidant ratio produces a maximum current as shown at 32, the fuel number is in excess of 1.0 and there is an excess of fuel. Such is maintained at the burner by the control of the gas control 25 and the primary air through orifice 20. This maintains undesired combustion performance because the combustible gases of the fuel are burned with insufficient air and incomplete combustion takes place. As the primary air through orifice 20 and the secondary air through orifice 31 are proportionally controlled regardless of the speed of blower 21, by maximizing the ionization current of the flame rod by controller 24, complete combustion in the combustion chamber takes place for maximum efficiency of the furnace. Other characteristic values of properties of the flame or combustion products might be used by the controller 24 such as the characteristic slope of a property shown in FIG. 2.