WO1998038461A1

WO1998038461A1 - Ignitor for gaseous fuel

Info

Publication number: WO1998038461A1
Application number: PCT/IB1998/000430
Authority: WO
Inventors: Li Ceji
Original assignee: Ranco Of Delaware, Inc.
Priority date: 1997-02-28
Filing date: 1998-02-17
Publication date: 1998-09-03
Also published as: TW389819B; AU6306998A

Abstract

An electrical resistance ignitor formed of nickel alloy wire wound continuously into two open coils nested one within the other. A tubular shield is disposed over the outer of the two coils in closely spaced arrangement. An insulating disk closes one end of the shield and supports the coils and end leads for external connection. Axially extending spacer strips maintain the coils concentrically radially spaced. The ignitor is particularly suitable for series connection with a heat motor in a gas line valve to the burner.

Description

IGNITOR FOR GASEOUS FUEL

BACKGROUND OF THE INVENTION

The present invention relates to ignition of gaseous fuel and particularly to ignition of fuel burners employed in cooking ovens for household use. Currently ignition of gas oven burners is accomplished by positioning an electrical resistance heater adjacent the flame generating ports of the oven burner and opening the gas valve to the burner when the ignitor has reached a temperature sufficient to ignite the gaseous fuel and air mixture flowing from the burner ports. In such an arrangement, the ignitor remains in the flame and is heated by the flame during burner usage. Thus, the ignitor must be able to withstand prolonged exposure to flame temperature for extended service of the oven.

Known resistance ignitors for oven burners are typically fabricated of a filament configured either sinusoidally in a flat array or helically and formed of refractory material such as silicon carbide. Silicon carbide has the known property of functioning as a negative thermistor when electric current is flowed through the filament, i.e., the ignitor resistance decreases with increasing temperature. This negative thermistor or negative slope resistance property of silicon carbide has enabled the ignitor to be connected electrically in series with a heat motor contained within an electric gasline valve. Upon the user closing a switch, for example a switch associated with an oven control thermostat, the ignitor begins heating; and, due to the negative thermistor characteristics of the silicon carbide, the ignitor controls the current through the system as the ignitor temperature changes. Therefore the ignitor reaches ignition temperature before sufficient current can flow in the valve heat motor to cause the valve to open. This arrangement ensures that the valve cannot open before the ignitor reaches ignition temperature.

However, although in widespread usage, this known system has the disadvantage or difficulty in that the resistance value tolerances on the silicon carbide filament and the tolerances on the resistance of the valve heat motor must be tightly controlled to insure that the valve does not open before the ignitor reaches ignition temperature when the power line voltage varies.

In addition, the prolonged exposure of the silicon carbide to flame temperature upon ignition has resulted in the negative slope resistance properties of the silicon carbide ignitor changing or "drifting" over extended service. Changes in the resistance properties or drift of the ignitor over time can create a situation where variations in power line voltage can cause insufficient current flow and failure of the ignitor to reach ignition temperature and thus the system is rendered inoperative.

Furthermore, during the manufacture of silicon carbide ignitor filaments reliance is made upon critical doping of the ceramic mix of the silicon carbide with trace elements prior to firing of silicon carbide refractory material in order to control the negative slope resistance characteristics of the ignitor within desired limits. The combination of these circumstances has resulted in a relatively low yield in the manufacture of silicon carbide resistance ignitors for series use in oven ignition systems and has resulted in a relatively high cost for such ignitors.

Therefore, it has long been desired to find a way to provide an electrical resistance ignitor which can be series connected with the heat motor in the gas valve and which is easy to control in manufacturing and is reliable in prolonged service.

It is known to utilize a coil of electrical resistance wire to ignite a gaseous fuel burner; however, because of the high current density required to achieve gas ignition temperature the metal resistance wire has not proved robust over the extended service life used for ignition of the burner. Thus, it has long been desired to find a way to provide a simple and easy to manufacture electrical resistance ignitor for series connection with a gas valve heat motor which is low in cost and easy to manufacture and yield an extended service life.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electrical resistance ignitor for use in igniting a gaseous fuel burner which utilizes readily available materials and which is reliable and stable in extended service .

It is a further object of the present invention to provide an electrical resistance ignitor which is formed of electrical resistance wire which is commercially available and which is efficiently manufacturable and relatively low in cost.

It is a further object of the present invention to provide an electric resistance ignitor which may be series connected with a heat motor in a gas valve has a relatively short time-to-temperature and is suitable for extended service life as an oven ignitor.

It is a further object of the present invention to provide an electrical resistance ignitor formed of coiled resistance wire which is commercially available and which is sufficiently robust.

The present invention utilizes commercially available electrical resistance wire formed into an outer coil of open or spaced turns with an inner coil of open or spaced turns disposed within the outer coil and having the outer coil surrounded by a shield. In the preferred practice the inner and outer coils are wound of a single continuous length of wire; and, the outer shield comprises a tubular member closely spaced over the outer coil. In the presently- preferred practice the wire is-.formed of nickel alloy, nickel -chromium alloy, iron-chromium alloy or iron-chromium-aluminum alloy; and, the tubular shield is formed of refractory material. The tubular shield protects the ignitor from the flame after ignition; and, the coil-within-a-coil arrangement achieves ignition temperature with sufficiently low current density to provide the desired robustness for extended series life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the ignitor assembly of the present invention;

FIG. 2 is a section view taken along section indicating line 2-2 of FIG. 1 ;

FIG. 3 is an isometric view of the ignitor of FIG. 1 with a portion of the shield broken away;

FIG. 4 is a section view taken along section indicating lines 4-4 of FIG. 1;

FIG. 5 is a view similar to FIG. 2 of an alternate embodiment of the invention; and,

FIG. 6 is a pictorial diagram of the ignitor of the present invention installed in an oven burner ignition system.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 through 3, the ignitor of the present invention is indicated generally at 10 and has a hollow generally cylindrical or tubular shield 12 formed of insulating, and preferably refractory, material with an insert in the form of a disc provided in one end thereof as denoted by reference numeral 13 and which may be formed in place with refractory cement. The heating element indicated generally at 15 of the embodiment 10 comprises a first or outer coil 14 formed of electrical resistance wire and wound with open or space.d coils such that the outer diameter of the coil 14 is spaced closely adjacent the inner surface of the tubular shield 12. In the present practice of the invention the shield is formed of refractory material, but other suitable materials such as metallic material may be employed. The heating element 15 includes a second or inner coil 16 which is also formed of electrical resistance wire and wound with spaced or open coils. In the presently preferred practice, the coils 14, 16 are wound from a single continuous wire with the ends of the wire extending outwardly through apertures 18, 20 formed in the end disk 13, with the wire end leads denoted by reference numerals 22, 24.

Referring to FIG. 4, the coils are preferably maintained in their relative positions with respect to each other by spacer means which in the embodiment 10 comprise a plurality of longitudinally extending spacer strips disposed about the annular space between the coil in circumferentially equally spaced arrangement as denoted by reference numerals 26, 28, 30.

If desired, mounting straps 32, 34 may be provided about the ends of the shield 12 to facilitate mounting the ignitor to a suitable support structure (not shown) .

In the present practice of the invention, it has been found satisfactory to form the coils 14, 16 out of wire selected from the group consisting of nickel alloy, nickel-chromium alloy, iron-chromium alloy, and iron- chromium-aluminum alloy. A commercially available wire of iron, chromium and aluminum carrying the designation Hoskins alloy 875 has been found satisfactory. Wire having a diameter of 0.63 mm has been found particularly suitable. In one embodiment of the invention, the outer coil 14 has a diameter of about 12.5 mm and the inner coil 16 has a diameter of about 4.75 mm; and, the coils 14, 16 have a length of at least 60 mm and at least ten turns or windings. It will be understood however that other sizes, configurations and materials may be employed and the particular sizes and materials are merely illustrative of present practice. The relatively heavy wire of the coils 14, 16 of the present invention results in a relatively low current density; and, this provides the requisite long life in service.

It has been found that the advantage of the present invention is that the outer coil 14 provides a thermal barrier against heat loss from the inner coil 16 as the latter is heating thereby resulting in more rapid heating of the coil 16 for a given current density. This results in the fuel air mixture circulating through the tubular shield 12 and around the coils being ignited more readily and rapidly than if only a single coil is utilized.

Referring to FIG. 5, an alternate embodiment of the invention is indicated generally at 50 and utilizes a tubular refractory shield 52 having a first or outer open or spaced coil 54 received therein with a second or inner coil 56 disposed within the coil 54 and spaced therebetween by longitudinally extending strips such as strips 58, 60 circumferentially spaced about the coils.

The embodiment 50 differs from the embodiment 10 in that the embodiment 50 has the coils 54, 56 formed separately and with adjacent ends thereof attached as indicated generally at 62 by any suitable fastening expedient such as a metal crimp band or a form of weldment. The remaining leads of the two coils 54, 56 extend outwardly through the end of the shield 52 as denoted by reference numerals 64, 66. The embodiment 50 is otherwise identical to the embodiment 10 of FIGS. 1 through 3.

Referring to FIG. 6, the ignitor assembly 10, 50 is shown disposed adjacent an oven burner tube 68 which has a plurality of flame generating ports 70 formed therein and is supplied air through an inlet aspirator 72 and fuel by a gas line 74.

The gas line 74 is connected -to an outlet fitting 76 of a-.thermally operated gas line valve, indicated generally at 78, which has the inlet 80 thereof supplied from a source (not shown) of gaseous fuel as indicated by the black arrow in FIG. 6. The outlet fitting 76 has formed thereon a valve seat 82 against which is closed a resilient elastomeric poppet 84 which is attached to one end of a bi-metal actuator arm 86 which has its opposite end anchored on an insulating block 88 secured to the valve body 90.

A heater coil of resistance wire 92 is coiled about the bi-metal arm 86 with the ends 94, 96 of the coil wire connected through terminal connectors 98, 100 provided on the valve body 90. One terminal 98 is connected to ignitor lead 24; and, the other terminal 100 is connected to a user actuated switch 102 which is connected to one side of a 115 volt AC power line terminal LI. The opposite lead 22 of the ignitor 10, 50 is connected to the other side of the power line at L2. This arrangement connects the ignitor in series with the heating coil 92 for the bi-metal actuator in the valve and is known in the art and forms no part of the invention but is illustrated in order to shown the installation of the invention ignitor. In operation, user closure of switch 102 causes current flow through heater coil 92 and the ignitor coils 14, 16 or 54, 56 until the ignitor has reached ignition temperature. When the ignitor reaches ignition temperature heater 92 then heats arm 86 enough to cause bi-metal arm 86 to warp sufficiently to raise poppet 84 from valve seat 82 and permit gas flow to the burner and effect ignition of the gas and air mixture emanating from burner ports 70.

The present invention thus provides a unique and novel resistance ignitor for igniting flame at a fuel gas burner and which is easily manufactured from commercially available resistance wire. The coil- within-a-coil arrangement of the present invention achieves rapid ignition with a relatively low current density- and is robust and capable of withstanding extended service life with prolonged exposure to flame from the burner by virtue of the coils being shielded from the flame by the tubular shield.

Although the invention has hereinabove been described with respect to the illustrated embodiments, it will be understood that the invention is capable of modification and variation and is limited only by the following claims.

Claims

An ignitor for gaseous fuel comprising: (a a first coil of electrical resistance wire formed in open or spaced windings;

(b) a second coil of lesser diameter in open or spaced winding formed of electrical resistance wire and disposed within said first coil, wherein said first and second coils are electrically series connected; and,

(c) a tubular shield disposed over and relatively closely spaced to said first coil wherein said second coil has a length not greater than said first coil.

The ignitor defined in claim 1, wherein said first and second coils are formed of a common material .

The ignitor defined in claim 1, wherein at least said first coil is formed of nickel alloy material

The ignitor defined in claim 1, wherein at least said first coil is formed of nickel -chromium alloy material .

The ignitor defined in claim 1, wherein one of said first and second coils is formed of material selected from the group consisting essentially of (a) Nickel -Chromium Alloy; (b) Iron-Chromium Alloy; and (c) Iron-Chromium-Aluminum Alloy.

6. The ignitor defined in claim 1, wherein said tubular shield is formed of metallic material.

The ignitor defined in claim 1, wherein said first and second coils are wound of a single continuous wire .

8. The ignitor defined in claim 1, wherein said first and second coils are formed of 0.63 mm diameter wire .

9. The ignitor defined in claim 1, wherein said first coil has a diameter of about 12.5 mm and said second coil has a diameter of about 4.75 mm.

10. The ignitor defined in claim 1, wherein said second coil is formed of at least ten (10) turns.

11. The ignitor defined in claim 1, wherein said tubular shield is formed of refractory material.

12. The ignitor defined in claim 1, wherein said first and second coils have a length of at least 60 mm and at least ten (10) turns . -

13. A method of making a hot surface resistance ignitor for gaseous fuel comprising:

(a) winding a first coil of resistance wire in open or spaced turns ;

(b) winding a second coil of resistance wire in open or spaced turns of lesser diameter than said first coil turns and disposing said second coil within said first coil;

(c) connecting one end of said first coil to one end of said second coil;

(d) disposing said coils within a tubular shield.