US3502419A - Flame-proving ignition system for gas burners - Google Patents

Description

March 24, 1970 R. L. PERL 3,502,419

FLAME-PROVING IGNITION SYSTEM FOR GAS BURNERS Filed Nov. 5, 1967 IIO V INVENTOR RICHARD L. PERL ATTORNEYS United States Patent Ofiice 3,502,419 Patented Mar. 24, 1970 3,502,419 FLAME-PROVING IGNITION SYSTEM FOR GAS BURNERS Richard L. Perl, Mansfield, Ohio, assignor to The Tappan Company, Mansfield, Ohio, a corporation of Ohio Filed Nov. 3, 1967, Ser. No. 680,458 Int. Cl. F23n 5/00 US. Cl. 431-66 7 Claims ABSTRACT OF THE DISCLOSURE A gas burner ignition system using a hot wire ignitor in association with a control thermistor, the igmtor being initially heated electrically to the ignition temperature for the burner fuel and heating the thermistor to lower the resistance of the same. When the ignitor is at operating temperature, the decrease in the thermistor resistance is used to open the fuel supply valve to the burnerpafter a short interval following the fuel supply, the electric heating of the ignitor is interrupted so that the thermistor will be held at its elevated control temperature only if the burner has ignited and generates sufiicient heat.

This invention relates to an improved electric ignition system for a gas oven burner which also controls the flow of the gaseous fuel to the burner and is flame-proving in the sense that such flow is interrupted if the burner fails to ignite or if after ignition the flame is extingu shed.

It is a primary object of the invention tOJJl'OVlde a relatvely simple and economical system on this order m which a heated coil or element is used to effect the lgllltion of the burner and the supply of fuel to the latter is dependent upon the element being heated to and held at or above the ignition temperature.

Another object is to provide such a system wherein the initial heating of the ignition coil is accomplished by electric circuit means and the first permitted opening of the fuel supply to the burner is delayed for an interval after such heating of the coil.

Another object of the invention is to provide temperature responsive interruption of the initial electrical C11- cuit heating of the ignition coil to test for burner flame and close the fuel supply line to the burner if the flame is not detected.

An additional object is to provide a system as last noted above employing a control resistance for the temperature responsive interruption in circuit relation with valve means regulating the fuel delivery to the burner, with the valve means closed in the event the heating of the element is not maintained by burner flame after the initial electric heating of the same.

Other objects and advantages of the present invention will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principle of the invention may be employed.

In said annexed drawing:

FIG. 1 is a first schematic diagram showing an ignition system in accordance with the present improvement applied to the control of an oven gas burner; and

FIG. 2 is a second schematic diagram showing a modified form of the system and controlling selectively both upper and lower burners within an oven.

Referring now to the drawing in detail, the gas burner 10 in the first diagram is supplied with the usual gaseous fuel mixture through a line 11 in which there is a thermal relay valve designated generally by reference numeral 12.

Such valve comprises a seat 13 and a cooperable poppet 14 mounted on the end of a bimetallic leaf or arm 15. The arm 15 normally is in the condition shown in full line and, when heated substantially above room temperature to a predetermined valve-opening temperature, warps to the position approximately indicated by the dashed lines and withdraws the poppet from the seat to permit the through flow of the fuel to the burner.

The heating of the bimetallic arm 15 to thus actuate the valve is accomplished by an electric resistance element 16 wrapped about a section of the arm as shown, or in other proximate disposition relative to the same, and this element is connected in a circuit which is energized at a potential of about 24 volts through a first secondary winding 17 of a supply transformer designated generally by reference numeral 18. The transformer at the prima y side is connected through an adjustable thermostatic controller T to the usual household electric energy supply, and the bimetal heater coil 16 is in electrical series with a thermistor 19 shown as enclosed within the metal-sheathed probe 20 adjacent a ported section of the burner 10 and in the raw gas stream issuing from the same when fuel is delivered to the burner. The thermistor 19 is in electrically insulated relation within the probe sheath, and the bimetal heating circuit thus comprises only the resistance 16, of suitable heater wire and having the normal positive temperature coeflicient of resistance, and the thermistor 19 in series, the latter having a negative temperature coefficient of resistance.

An ignitor coil 21, made, for example, of platinum, platinum alloy or molybdenum disilicide wire, is wrapped about the probe sheath and has an electrical connection to the same at one end 22 and through the conductive sheath to a terminal 23 at the distal end of the probe and thus relatively away from the burner flame. Ignitor coil 21 is energized at about 5 volts by a second secondary winding 24 of the transformer 18 through normally closed switch contacts 25, 26. Contact 25, to which probe terminal 23 is connected, includes an arm 27 in close parallel relation to the bimetallic leaf 15, while the latter carries the other contact 26 for normal engagement with contact 25 as shown in the non-actuated condition of the leaf. Flexure of the leaf 15 when heated will first withdraw poppet 14 from the valve seat 13 to permit fuel to flow to the burner and, with continued heating, subsequently withdraw contact 26 from contact 25 to interrupt the energization of the ignitor coil 21. The arm 27 is preferably also bimetallic as shown for fast response and to compensate for ambient temperature influences on the leaf 15.

Operation of the burner in this first system is initiated by manual closure of the thermostatic controller T, which can of course be of any suitable type to respond to the temperature within the oven in which the burner is located. Both the bimetal heater-thermistor circuit and the ignitor coil circuit are energized at the voltage levels indicated, with the current in the first restricted by the normally high resistance of the thermistor at room temperature. The thermistor circuit response can be made adjustable as, for example, by the inclusion of a variable resistor R. The ignitor coil 21 heats very rapidly and raises the thermistor temperature to diminish its effective resistance and increase the heat generated in the bimetal heater 16. The parameters are so related that the ignitor coil will be heated to a temperature safely above that at which the gaseous fuel mixture employed in the burner will ignite when the bimetallic leaf has been sufiiciently heated to move the poppet 14 away from the valve seat 13 and permit the fuel to be delivered to the burner.

With proper operation of all circuits and components, the raw gas issuing from the burner section adjacent the ignitor assembly flows over and is ignited by the hot coil, and the resulting flame of course also heats the probe and hence the thermistor. At the point of ignition, the contacts and 26 remain closed and maintain energization of the ignitor coil 21, but upon predetermined further heating of the bitmetal leaf 15, these contacts are opened to deenergize coil 21. Accordingly, the heating of the thermistor 19 at this stage is solely dependent upon the burner flame and, if the latter is not present or too weak to be elfective, the thermistor cools rapidly and reduces the current flow through the bimetal heater 16 to a normal level insuflicient to hold the bimetal flexed. The poppet moves against the seat and cuts off the fuel supply to the burner, but since the thermostatic controller setting still calls for the burner to operate, the system will again attempt ignition upon the closure of the contacts 25 and 26 as the bimetal leaf relaxes and ignition or re-ignition can occur if some temporary fault clears, for example, in the supply of the fuel.

The same type of ignition and flame-proving are utilized in the second diagrammed system for controlling the operation of a single cavity cooking oven having an upper or broil burner 30 and a lower or bake burner 31. As will be more fully described below, the combustible mixture can selectively be delivered from a common supply 32 through a first solenoid valve 33 and line 34 to the upper burner 30 or through a second solenoid valve 35 and line 36 to the lower burner 31. Again the basic adjustable control is provided by a conventional temperature controller TC. The upper burner in this case is served by a top ignitor coil 39 With which a sheathed thermistor 40 is associated, while a bottom ignitor coil 41 and thermistor 42 are associated with the lower burner. The internal thermistor elements as well as the coils are connected to the metal sheaths, these devices being commercially available, and, as is evident from the wiring, the two thermistors are in parallel between circuit points 43 and 44. Point 43 is connected to one side of a secondary transformer winding 45 having an output of about 24 volts and the further circuit point 44 is connected to the other side of such winding through a variable resistance 46 and the coil of a relay 47 in series.

Also connected across the secondary winding 45 is a resistance heating element 48 in a circuit including normally closed movable contact 49 and fixed contact 50 of the relay 47, the former being moved in response to energization of the relay coil. Another contact 51 of the relay which is engaged by the movable contact 49 when the relay is energized at operating level is connected to the movable contact 52 of a selector switch designated generally by reference numeral 53. This selector switch can be manually adjusted to move the contact 52 into engagement with one fixed contact 54 connected to one side of the operating coil of solenoid valve 33 or another fixed contact 55 which is connected to one side of the coil of the second solenoid valve 34, with both of these coils at their other sides commonly connected to the transformer secondary winding 45 at the end thereof opposite the end which is connected to the relay movable contact 49.

The resistance heater 48 is an actuator for a switch comprising bimetal members 56, 57 which are normally disengaged, connected in a circuit from a second supply conductor L2 of the energy source to the primary 58 of an ignition transformer 58 and through the controller TC to the supply conductor L1. The ignition transformer has a secondary winding 59 connected to energize the top and bottom ignitor coils 39 and 41 in parallel at approximately 3 volts as indicated. The primary winding 60 of the 24 volt transformer is also connected across the conductors L1, L2 through the controller TC.

In the operation of this second diagrammed system, the temperature controller TC will be set to the desired temperature to be maintained in the oven and closed and selector switch 53 adjusted suitably to select between the upper and lower burners. It will first be assumed that the user desires to conduct a broiling operation in the oven and hence to employ the upper burner 30, with controller TC closed to a broil position, and switch contact 52 moved into engagement with the cooperable contact 54 in the circuit of the solenoid valve 33 in the line to the upper burner.

The closure of the controller energizes the transformer windings 60, 45 and hence both the secondary circuit including the bimetal heater 48 and closed relay contacts 49, 50 and the further secondary circuit which includes the top and bottom thermistors, adjustable resistance 46 and relay 47. Since the thermistors are initially at room temperature, their resistance values are high and the current level in this circuit is insufiicient to actuate the relay. However, the heater 48 does actuate the bimetallic switch contacts 56, 57 into engagement and a circuit is thereby completed through the ignition transformer 58, 59 to energize the two ignition coils 39, 40 in parallel. These coils become almost immediately heated and the two thermistors 40, 42 heated thereby are brought up to operating or control temperature with attendant decrease in resistance. When the resistance of the circuit of the thermistors reaches the control level, the increasing current flow through the coil of relay 47 actuates the movable contact 49 from engagement with the fixed contact 50 in the circuit of the bimetal heater 48 into engagement with the other fixed contact 51 to complete the energization circuit for the coil of the solenoid valve 33 through the selector switch 53.

With termination of the heating of th bimetallic contactors 56, 57 by such relay actuation, these contacts relax and open to interrupt the energization of the ignition transformers 58, 59. At the time this occurs, the top burner should have ignited, in which case the flame maintains the heating of the top thermistor 40 and hence actuation energization of the relay 47. However, if the burner for some reason did not ignite or if, once ignited, is extinguished or becomes inetfectively weak although the oven is still set for operation, the thermistor circuit will be unable to hold the relay actuated and the latter will return to the full line condition illustrated and terminate the opening energization of the solenoid valve 33 to cut off the fuel supply to the burner 30. It will be appreciated in this instance as well that the system will attempt re-ignition as long as the.oven controls are set for operation, but the gas supply will be blocked unless and until the burner functions properly.

The lower or bake burner 31 of the oven is controlled in the same manner, with the operation in this case established by again setting and closing the temperature controller TC and closing the contacts 52, 53, the latter in the circuit of the coil of the second solenoid valve 35 through which the fuel is delivered to the bake burner. The heater 48 is again energized to close the bimetal contacts 56, 57 and complete the energization circuit for the ignition transformer. The cycle continues as previously set forth with, in effect, the bottom burner associated ignitor, thermistor and solenoid valve substituted for the corresponding top components in the previous description of the operation of the latter. It will also be apparent that adjustment of the resistance 46 provides variation of the potential applied commonly to the thermistors.

The above-described use of the coils and thermistors in electrical parallel provides a reduction of the initial or cold elfective resistance and hence a relatively higher potential available at the relay for the closing action. Less potential is required to hold the relay closed and this levelcan be realized by the response of one of the thermistors to proper burner flame,

This arrangement of course requires only a single ignition transformer as compared to two and added selector controls in a system with independent ignition circuits for the burners.

Both diagrammed ignition systems accordingly utilize basically a hot-wire ignitor element in association with a control resistance responsive to heat initially developed by electric circuit means, with this heating required to reach a level indicative of igniting condition of the wire element. The ignitor heating to. this condition results in the opening of the fuel supply to the burner and, after a time interval, the electric heating is interrupted, with continuation of the fuel supply dependent upon the existence of burner flame adequate to maintain the control resistance at its effective operating temperature.

It will be appreciated that a comparable circuit could be designed with employment of suitable positive temperature coeflicient of resistance elements in lieu of the described thermistors, and also that the initial electric heating of such control element or elements might be differently provided. For example, the ignition coils have been disclosed illustratively as being made of platinum or platinum alloy because it has been determined that such material effects catalytic ignition, i.e. causing the gas stream to ignite at less than incandescing temperatures, and such a coil may be indirectly heated rather than by a circuit for direct resistance heating as illustrated. One such arrangement could use a separate small heater of standard wire effective to heat the thermistor probe and an unconnected platinum coil wound about the latter. It is also possible that the thermistor or control resistance itself can in a proper circuit generate suflicient heat to bring such an unconnected platinum coil to the ignition temperature.

A positive time delay in opening the ignitor circuit relative to first supply of the fuel, in the action of the bimetallic leaf in the first diagram and in the bimetallic contactor in the second diagram, is preferred although an ignitor coil might be employed which will remain hot enough long enough for comparable testing of the flame by the noted interruption of the initial electric heating of the element, either directly or indirectly, coincident with the opening of the fuel line to the burner. The assured delay is of particular significance in burner systems which operate on gaseous fuels, such as methane, which do not as readily combine with air as other fuels, for example, butane.

It will be further understood that the disclosed relay functions can be served also by solid state controls having equivalent operating modes.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed' I, therefore, particularly point out and distinctly claim as my invention:

1. In combination with first and second gas burners, and a gaseous fuel supply therefore including valve means operable selectively to supply th fuel to the first or the second burner; a flame-proving electric ignition system for the burners, comprising an ignitor element positioned in the discharge of the fuel mixture from each burner and adapted to be heated to a temperature which Will cause it to ignite the mixture, a control resistor in heat transfer relation to each ignitor element and associated burner, the electrical resistance of the control resistors being variable with changes in temperature, and circuit means for heating the ignitor elements and controlling operation of the fuel supply valve means in accordance with changes in the resistances of the control resistors, said circuit means initially heating the ignitor element of the selected burner by opening the valve means for fuel supply thereto upon predetermined change in the resistante of the associated resistor, interrupting such initial heating to test for burner flame heating of said resistor, closing the valve means in response to a resistance change due to cooling of said resistor, the two ignitor elements and the two resistors being connected in electrical parallel relation.

2. In combination with a gas burner, electric hot wire ignition means for the burner including a component the resistance of which varies appreciably with change in its temperature, relay means in circuit with said ignition means so that current flow through the former is influenced by variation in the resistance of the latter, valve means controlled by the relay means for supplying gaseous fuel to the burner, means for electrically energizing the ignition means to heat the same to the ignition temperature of the fuel and to produce by such heating a predetermined change in the resistance of said component, the relay means being operative in response to said predetermined change of resistance to cause the valve means to open and thereby supply the fuel to the burner, and means for interrupting effective energization of the ignition means after a delay interval following the opening of the valve means to test for burner flame heating of said component sufficient to maintain the relay-operating resistance value of the same.

3. The combination set forth in claim 1, wherein the burners are disposed within a cooking oven respectively for baking and broiling therein, the two ignitor elements are both energized in all'burner operations, and the circuit means responds to the effective resistance of the parallelconnected control resistors.

4. The combination set forth in claim 2, including means for delaying opening of the valve means for an interval following initial energization of the ignition means.

5. The combination set forth in claim 2, wherein the ignition means comprises an ignitor element, which is heated electrically to the fuel ignition temperature, and 'a control resistor as the variable resistance component.

6. The combination set forth in claim 5, wherein the control resistor is in heat transfer relation to the ignitor element for heating of the former by the latter to produce the pre-determined change in the resistance thereof.

7. The combination set forth in claim 6, including means for delaying opening of the valve means for an interval following initial energization of the ignition means.

References Cited UNITED STATES PATENTS 2,549,633 4/1951 Ottmar 43166 X 3,151,661 10/1964 Matthews 43166 X 3,393,038 7/1968 Burkhalter et al 431-66 EDWARD G. FAVORS, Primary Examiner

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