US3488133A - Protected hot wire ignition system - Google Patents

Description

Jan. 6, 1970 R. 1.. PERL.

PROTECTED HOT WIRE IGNITION SYSTEM Original Filed Oct. 18, 1967 ooeouanaooooouoo INVENTOR RICHARD L. PERL ATTORNEYS United States Patent 3,488,133 PROTECTED HOT WIRE IGNITION SYSTEM Richard L. Perl, Mansfield, Ohio, assignor to The Tappan Company, Mansfield, Ohio, a corporation of Ohio Continuation of application Ser. No. 676,149, Oct. 18, 1967. This application Jan. 9, 1969, Ser. No. 791,872

Int. Cl. F23n /00 US. Cl. 43166 9 Claims ABSTRACT OF THE DISCLOSURE An ignition system for a gas burner using a high temperature wire ignitor, preferably of molybdenum disillcide, having two coils of different diameter and resistance value, the two in electrical series. The smaller diameter coil is the ignition element and the other a ballast .coil arranged next to the burner and between the same and the ignition coil. When the burner is turned on, current flows through the coils and a current relay in series which opens a solenoid valve in the gas line to the burner; the ignition current fiow heats the ignition coil to ignite the raw gas from the burner while the ballast is relatively cool. The ballast coil closer to the flame after ignition is heated by the same to increase its resistance and cut down the current flow in the ignition circuit to prolong the coil life.

This application is a continuation of application Ser. No. 676,149, filed Oct. 18, 1967, now abandoned.

DISCLOSURE This invention relates to an electric ignition system for gas burners, such as employed in domestic ovens and cook tops, in which the igniting element is a wire in a continuous direct resistance heating circuit.

This type of ignition has generally been avoided in such appliances because of fairly rapid deterioration and failure of the elements or coils which have been tested, primarily due to overheating when placed at good ignition locations, and the difficulty has been greatly emphasized in the current development of the so-called selfcleaning gas ovens in which a temperature of approximately 1000 F. may be reached in a cleaning cycle. Resort has thus been had to spark ignitor systems for such ovens, which are relatively more complicated, expensive and subject to malfunction and,v according at least to some designs, are continuously throughout the whole period of the burner operation.

There has been some improvement in hot-wire ignitors in the sense of new materials with superior operating characteristics, with the molybdenum disilicide wire trademarked Kanthal notable in this respect since it has an operating temperature of about 3000 F. and good life. However, this wire is very brittle which makes it difficult to work in the forming of an ignitor from the same, and the problem of overheating would still exist in the normal relation to the burner within the oven.

It is a primary object of the present invention, accordingly, to provide such a hot-wire ignition system for a gas oven burner in which the ignition element is protected from the usual overheating due to the burner operation and the life of the element thereby prolonged.

Another object is to provide such a system in which the flow of the gaseous mixture to the burner is dependent upon integrity of the ignition element and, in the continuing sense, upon combustion of the mixture being initiated and maintained.

It is also an object of the invention to provide an improved configuration or mechanical design for such a hot-wire ignition element.

3,488,133 Patented Jan. 6, 1970 ice Other objects and advantages of the present invention will become apparent as the following description pro ceeds.

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 schematic diagram of a gas oven ignition system in accordance with the present invention;

FIG. 2 is an elevation of a hot-wire ignitor used in this system, on an enlarged scale;

FIG. 3 is a side elevation of the ignitor;

FIG. 4 is a simplified and fragmented plan view of a fixture employed in forming the hot-wire element; and

FIG. 5 is a cross-sectional view of the FIG. 4 assembly as shown therein by the line 5-5.

Referring now to the drawing in detail, the full system shown schematically in FIG. 1 is applied to th control of a cooking oven comprising an upper burner 10 and a lower burner 11, with the specific forms of such burners being without particular significance in the present improvements. The oven structure itself can likewise be of any suitable form and accordingly is not illustrated.

The operation of the two burners and hence the representative oven is basically programmed by a temperature controller T which comprises in its simplest form a thermostatically actuated switch contactor 12 and first and second adjustable switch contacts 13 and 14, respectively, the adjustment of which is effected by manual setting of the controller. The contactor cooperates with fixed contacts 15 in a line from the energy source terminal 16 to the oven control circuit to be more fully described for control of the energization of the latter in accordance with the oven temperature. The contactor is shown as operated by a thermostat the sensor 17 of which is suitably disposed within the oven; other specific temperature responsive devices could obviously be utilized and will in all cases provide for manual setting or adjustment of the desired temperature in the usual manner.

The control circuit extends from the contacts 15 to a normally open movable contact 18 of a current relay 19 the fixed contact 20 of which is connected by a wire 21 to the first movable switch contact 13 of the temperature controller. This contact 18 can selectively be moved to engage either a contact 22 from which a wire 23 extends to one side of a first solenoid valve V1 or another stationary contact 24 connected by wire 25 to one side of a second solenoid valve V2. The other sides of the two solenoid valves are shown as having a common ground connection 26.

The gaseous fuel mixture to the burners is delivered from a suitable supply line 27 through a manual valve V to the fuel lines 28 and 29 which extend respectively to the upper and lower burners. The solenoid valves V1 and V2 are located in the branch lines 28 and 29 and, accordingly, the adjustment of the switch contact 13 in the setting of the temperature controller T conditions one or the other of the burners for operation, for example, with the upper burner to be used for broiling and the lower burner selected for baking as is conventional.

The relay 19, which obviously must be energized to operate the selected solenoid valve V1 or V2, is in a separate circuit at a lower voltage, for example 6 volts, derived from a stepdown transformer 30. The secondary of this transformer is connected to one end of the relay coil and to upper and lower wire ignition devices desig nated generally by reference numerals 31 and 32. The

latter are respectively connected to stationary contacts 33 and 34 associated with the second controller adjustable contact 14 which is, in turn, connected to the other end of the relay coil. The relay 19 is therefore in series with one or the other of the ignitors depending upon the adjustment of the controller contact 14 in the programming of the oven operation.

The ignitors 31 and 32 are respectively disposed at ignition locations for the upper and lower burners, with each positioned in the path of raw gas issuing from a section of the associated burner when the mixture is supplied to the same and effective to ignite the mixture at such place by being heated to a sufiiciently high temperature as a result of the flow of electric current through the same. The details of the preferred form for the ignitor will be further described below, but it will be appreciated from the schematic of the system that it is impossible to operate either burner unless the associated ignitor is energized. For example, in the circuit condition shown in full lines in FIG. 1, the controller T has been set for operation of the lower or bake burnerll and, the setting also closing the thermostatic contactor 12, a bottom ignition circuit is completed by closure of the contacts 14 and 34 through relay coil 19 and ignitor 32 in series. If the latter is intact, the current flow through this circuit effects the desired heating of the ignitor to the temperature for ignition and the relay contacts 18 and 20 are closed to complete the circuit through the contacts 13 and 24 for energization of the solenoid valve V2. Similarly, adjustment of the controller T to close contacts 13 and 22 readies the solenoid valve V1, the upper ignitor circuit is energized through closure of the contacts 14, 33 and the relay 19 is again closed for opening actuation of the valve V1. In either case, a break in the ignitor element would preclude operation of the relay 19 and thus protect against any flow of the fuel mixture to the selected burner.

Each ignitor device 31 and 32 comprises two serially connected coils, and these are preferably formed of the previously noted molybdenum disilicide or equivalent wire. As shown more in detail in FIGS. 2 and 3, which can be taken to illustrate the upper ignition device 31 upon the understanding that the lower one is identical, the wire is formed into a first coil 35 with a given crosssectional area or wire diameter and a second coil 36 of significantly larger wire diameter. The first coil is the actual ignition element, while the second is a ballast coil, and their respective resistance values are such that the current in the ignition circuit causes the ignition coil 35 rapidly to heat to incandescence without comparable heat development in the series ballast coil 36. For example, the ignition coil can be brought by such direct resistance heating quickly to an igniting temperature in excess of 2000 F., while the ballast coil temperature does not exceed half of this value and can be as low as 500 F. or less. It is also significant that the ignitor is arranged relative to the burner so that the ballast coil 36 is closest to the same and hence the flame, as indicated at F in FIG. 3, while the burner is operating.

With this arrangement, when the raw gas first issues from the port adjacent the ignitor, it flows over the ballast coil 36 and initially has a cooling effect on the same, which further serves to hold this coil relatively cool until the burner ignites. The wire has of course a positive temperature coeflicient of resistance and, after the burner ignites, the ballast coil will be significantly heated by the flame with the result that the current in the circuit is decreased. The ignition coil temperature drops by reason of the currentdrop and the useful life of the same is thereby extended, while higher and more positive ignition temperatures, for example on the order of 2400 F., can initially be reached.

It is preferred that the two coils 35 and 36 be formed from a single length of the wire, such as the noted molybdenum disilicide, and the de ired variation in d ameter can be accomplished in a number of ways. Such variation can, for example, be realized by extruding the wire, pressing, rolling, or chemically etching at a variable rate. It is particularly difficult to work this type of wire, especially to provide the coil form which is most eflicient for the intended application, but coiling can be accomplished by closely controlled high temperature heating in the manner indicated in FIGS. 4 and 5. In these figures, a length of the wire W is shown as gripped in a holder 37 at one end and resting on the top of a ceramic body 38 in which there is a gaseous fuel chamber 39 with a top opening. A ceramic post 40 extends vertically through the chamber and projects above the top surface of the body to define a relatively narrow annular port for the chamber at which the mixture is burned to provide a sleeve of flame 41 closely about the projecting part of the post. The wire W is brought against the post 40 and heated over a small section by the flame to a temperature of about 2800 F., which will permit bending without fracture at the very localized heated area. The wire is then manipulated to be progressively warped around the post with progressive heating of the same in small increments and the coil or coils thus formed.

The ignitor device 31 comprises an inner insulative body 42 through which first and second terminal wires 43, 44 extend from upper looped ends to lower ends in sockets 45 and 46 where they are connected to insulated supply wires 47, 48. The ends of the ignition- ballast coils 35, 36 are attached to the looped terminal ends by high-temperature solder or welding, while a third wire loop 49 is provided irfthe insulation body 42 to support the mid-point of the coils as shown, without electrical connection.

, The insulation body 42 is received within an inner metal housing 50 which includes an upper cap 51 serving as a shield for the coils. This cap has open ends with the edges at the top and sides at each end cut and bent inwardly to form tabs 52 to partially obscure and thus protect the coils. The inner housing fits within an outer housing 53 and the latter is shown as slotted at its sides for projection of spring arms 54 provided in the inner housing 50. These outwardly biased arms are employed for gripping of the device within a separate mounting, not shown, to hold the same at the selected ignition location adjacent the burner.

It will therefore be seen that high-temperature metals and alloys, which are inherently brittle, difiicult to work, and; still subject to too rapid deterioration in excessive heat exposure can be applied to gas burner hot wire ignition by the disclosed improvements with the needed reliability and the desired economy particularly as compared to available spark ignition systems.

In the preferred form of the ignitor, the above coiling operation would of course be preceded by the working described as providing the two sections of varying diameter in a single length of the wire, but two separate lengths of such different diameter could if desired be employed respectively for the ballast and ignition sections with the sange operative effect and control function. Where more than one ignitor is used as in the described oven system, they may be connected in electrical series and both therefore operated if any burner is to be ignited, with this modification of the circuit eliminating the low voltage switching in the previously disclosed parallel arrangement of such devices. It will also be appreciated that in this case as well the desired protection against operation due to a break in the ignitor or ignitors is assured by the series relation to the relay controlling the solenoid gas valves, with hazard as a result of shorting relatively inconsequential in such low voltage circuitry.

I claim:

1. In combination with a burner to which a combustible gaseous mixture is supplied and issues therefrom at a combustion outlet, an electric ignition system therefor, comprising a high resistance ignitor wire section, a ballast wire ction of lower resistance and having a posi ive t pe a ture coefficient of resistance connected in electrical series circuit relation to the ignitor section, means for mounting the ignitor wire section at an ignition location relative to the stream of the raw mixture issuing from the outlet when supplied to the burner and the ballast section in exposure to the resulting burner flame for heating thereby, and means for flowing electric current through the wire sections at a value so related to the diifering resistances thereof that the ignitor section heats rapidly by direct resistance heating to the temperature at which the same ignites the raw mixture which the ballast section is heated to an appreciably lesser degree, the exposure of the ballast wire to heating by the burner flame after ignition substantially raising the temperature and resistance thereof to reduce the current flow through the sections while the burner is operating.

2. The combination set forth in claim 1, wherein the ballast section is located in the path of the raw mixture issuing from the burner outlet between the latter and the ignitor wire section.

3. The combination set forth in claim 1, including control means for preventing the supply of the mixture to the burner in the event of a break in the circuit of the wire sections.

4. The combination set forth in claim 3, wherein the control means includes valve means in a mixture supply line to the burner and means responsive to current flow in the wire section circuit for opening said valve means.

5. The combination set forth in claim 1, wherein the wire sections are made of a material having the characteristics of molybdenum disilicide.

6. An electric ignitor for a gas burner, comprising wire having discrete sections of appreciably different electrical resistance values and a positive temperature coeflicient of resistance, such that current flow therethrough produces relatively low direct heating of the section with the lesser resistance in the interval required to heat the other section directly to a high temperature sufiicient to ignite the combustible gas mixture with which the burner is operated, and means for mounting the wire With said sections thereof proximate to each other for exposure of both to the gas mixture issuing from the burner and the flame resulting from ignition of the same.

7. An ignitor as set forth in claim 6, wherein the wire is made of a material having the characteristics of molybdenum disilicide.

8. An ignitor as set forth in claim 6, wherein the sections are successive coils of the wire of different wire diameter.

9. An ignitor as set forth in claim 8, wherein the wire is made of a material having the characteristics of molybdenum disilicide.

References Cited UNITED STATES PATENTS 10/1944 Weber 43166 7/1965 Miller et al 43166 US. Cl. X.R.

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