US3812324A

US3812324A - Glow coil ignitor

Info

Publication number: US3812324A
Application number: US00373090A
Authority: US
Inventors: J Faffaelli; R Palmer
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1971-07-08
Filing date: 1973-06-25
Publication date: 1974-05-21
Anticipated expiration: 1991-05-21

Abstract

A glow coil ignitor including a coil of molybdenum disilicide wire having straight end portions of approximately equal lengths, said coil being in the shape of a helix having an approximately constant inside diameter and pitch throughout its entire length. The wire having an approximately constant diameter throughout its entire length, the helix having an odd number of half turns and at least 1 1/2 turns, the wire end portions being connected to and being integral with the ends of said helix, and the axes of said wire end portions being positioned substantially tangent to the axis of said wire at the connections of said wire end portions with the respective ends of said helix.

Description

United States Patent Faffaelli et a1.

[ May 21, 1974 1 1 GLOW COIL IGNITOR [75] Inventors: Joseph Gino Faffaelli, Granada Hills; Reed Albert Palmer, Los Alamitos, both of Calif.

[73] Assignee: International Telephone and Telegraph Corporation, New York, N.Y.

[22] Filed: June 25, 1973 [21] Appl. No.: 373,090

Related U.S. Application Data [62] Division of Ser. No. 160,827, July 8, 1971, Pat. No.

[52] U.S. Cl 219/553, 219/270, 317/98, 338/296 [51] Int. Cl H05b 3/10, F23q 7/10 [58] Field of Search 317/80, 98; 219/260, 268, 219/270, 552, 553; 431/66, 259, 262; 338/296 3,551,083 12/1970 Michaels 431/66 3,017,540 1/1962 Lawser 317/98 3,562,590 2/1971 Mitts et a1 317/98 3,569,787 3/1971 Palmer 317/98 2,149,868 3/1939 Rabezzana 317/98 X 2,487,754 11/1949 Cohn 317/98 X 2,487,753 11/1949 Cohn 317/98 X 2,487,752 11/1949 Cohn 317/98 3,139,558 6/1964 Lindberg 317/98 3,419,704 12/1968 Hunt 219/270 X Primary ExaminerVo1odymyr Y. Mayewsky Attorney, Agent, or Firm-A. Donald Stolzy [5 7] ABSTRACT A glow coil ignitor including a coil of molybdenum disilicide wire having straight end portions of approximately equal lengths, said coil being in the shape of a helix having an approximately constant inside diameter and pitch throughout its entire length. The wire having an approximately constant diameter throughout its entire length, the helix having an odd number of half turns and at least l truns, the wire end portions being connected to and being integral with the ends of said helix, and the axes of said wire end portions being positioned substantially tangent to the axis of said wire at the connections of said wire end portions with the respective ends of said helix,

1 Claim, 15 Drawing Figures GLOW COIL IGNITOR This is a division of copending application Ser. No. 160,827, filed July 8, 1971, now US. Pat. No. 3,774,077. The benefit of the filing date of said copending application is, therefore, hereby claimed for this application.

BACKGROUND OF THE INVENTION This invention relates to the art of heating a resistive wire to the ignition temperature of a gas fuel, and more particularly, to a resistance wire ignitor having an exceedingly long life, mounting or shielding means therefor, and a method of making the same.

In the past, it has been the practice to ignite the surface burners and oven pilot burners of a gas range by placing a coil of platinum wire adjacent a burner, and passing electric current through the wire of a magnitude sufficient to heat the gas to its ignition temperature.

Unfortunately, all suitable materials including platinum wire oxidize so badly in air that they have a useful life while being continuously heated of only about 3 days or less.

A wire made of molybdenum disilicide, MoSi can be heated to the gas ignition temperature by passing current therethrough. When so heated continuously in air, the MoSi wire has a useful life of in excess of 1% years. However, MoSi wire is very brittle and often breaks due to differential thermal expansion with its mounts and with shock, pressure and vibration.

Until the present invention, it was thought that MoSi wire was so brittle that it could not be made or used in any form other than that of a straight wire. This, in itself, made the use of MoSi wire disadvantageous. Unfortunately, such a wire, when employed as a resistance wire ignitor, must be heated to a maximum temperature, usually midway along its length, which is very high in order to provide reliable ignition of natural gas or the like. The very fact that a high wire temperature was required when MoSi wire was used in a resistance wire ignitor even limited the useful life of that ignitor.

Due to differential thermal expansion between a MoSi resistance wire ignitor and its mountings, expensive mountings were required. See US. Pat. No. 3,569,787.

In addition to the foregoing, all the resistance wire ignitors of the prior art required a large number of complicated and expensive component parts.

SUMMARY OF THE INVENTION In accordance with the present invention, the abovedescribed and other disadvantages of the prior art are overcome by providing a coiled wire made of MoSi Although it is unexpected, a way has been devised in which the brittle MoSi wire can be wound into a helix of 2% turns, the helix pitch and diameter being constant throughout its length. The wire has a circular cross section uniform throughout its length. The wire also extends beyond both helix ends the same distance, the extensions both being straight, the axis of each extension being tangent to the helix axis at its opposite ends.

Employing the helix with the extensions, as just described, and utilizing certain relative and/or absolute dimensions extraordinary advantages are derived.

In the first place, it was not expected that protection against the MoSi wire breakage would result in providing a MoSi wire wound in the shape of a helix. Although this was unexpected, it has, in fact, been found that the MoSi wire, when wound in the shape of a helix, actually is more self-protective from breakage due to shock and vibration because the helix, even though the wire itself is still brittle, acts as a coiled spring. Thus, the brittle character of the wire which made it so difficult to handle actually becomes an advantage when the wire is wound into a helix.

Another outstanding advantage of the present invention is that the wire, when wound in the shape of a helix, acts as a spring which can give with differential thermal expansion and, therefore, eliminate breakage thereupon due to the different thermal expansion coefficients of the helix and its mountings.

Still another outstanding advantage of the MoSi helix is that it need not be heated to a temperature as high as that to which a straight MoSi wire must be heated for reliable gas ignition. That is, the maximum temperature required at the point thereof along the helix for reliable gas ignition is substantially below that required of a single straight wire. For this reason, oxidation is reduced and the MoSi helix of the present invention has a substantially longer life than any other resistance wire ignitor including, but not limited to, a straight MoSiwire.

Still another outstanding advantage of the present invention resides in the elimination of voltage regulation equipment. In the past, when straight MoSi resistance wires were used for gas ignition, it was necessary to operate the wire at such an extremely high temperature that voltage regulation was required to prevent the MoSi straight wire from melting down or falling toignite the gas. Regulation thus prevented the input voltage to the straight wire from falling too far due to an increase in current through the ignitor wire caused by a decline in the wire temperature. Wire temperature could thus be reduced by ventilation and gas ignition thereby prevented.

Reliable gas ignition by the MoSi helix maintained at a lower temperature than the straight wire has been proven, wire temperatures in each case being measured by the use of the same optical pyrometer.

The advantage regarding voltage regulation is outstanding because voltage regulation may be completely eliminated by employing the MoSi helix. The cost of special equipment required for voltage regulation in the prior art has, therefore, been eliminated in accordance with the invention.

It will be noted that all of the advantages described hereinbefore and some of the advantages described hereinafter were simultaneously derived from the manufacture of a coiled MoSi wire helix. That is, not just one advantage was derived from this construction, but a great many outstanding advantages were derived simultaneously.

In accordance with another feature of the invention, the helix extensions extend generally in one direction and are located in electrical connector cups integral with prongs for a male electrical plug.

Another feature of the invention resides in spot welding or otherwise bonding a brazing compound inside each cup before the prongs and all of the metal structure connected therewith are fonned. Thus, the prongs are mounted vertical in an oven filled with an inert or hydrogen gas or vacuum. A great many of the ignitors may thereby be batch brazed. The said helix extensions are mounted approximately vertically in the cups and flux applied to the helix extension and cup preparatory to mounting the substantially complete assembly in the oven. By this method, secure inexpensive mountings are provided and bonded to the helix extensions. By using the batch braze, the process is inexpensive.

Another outstanding advantage of the present invention is that the metal of portions integral with the prongs and the prongs themselves are made of 310 stainless steel which has an annealing temperature about the same or somewhat below the brazing temperature of the brazing compound.

In accordance with the foregoing, the brazing step likewise produces an anneal which makes the prongs and their connections soft and pliable to further protect the brittle MoSi helix from breakage due to shock and vibration, for example.

In accordance with the foregoing, the combined brazing and annealing step is economical.

Still a further feature of the invention resides in the use of a necked down portion in the prongs to permit the annealed, dead soft prongs to bend and thereby protect the brittle MoSi helix in the event of a severe shock.

A further feature of the invention resides in the use of a snap-on wind shield for the ignitor. This shield is made from a single metal stamping. It is thus very economical to construct and to assemble. It requires no additional parts whatsoever for assembly. That is, for example, it requires no other fastening elements such as eyelets, screws, bolts, etc.

In accordance with the foregoing, the device of the present invention has a very small number of uncomplicated and inexpensive component parts.

The above-described and other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which are to be regarded as merely illustrative:

FIG. 1 is a front elevational view of a completely as sembled electrical resistance wire ignitor constructed in accordance with the present invention;

FIG. 2 is a side elevational view of the ignitor shown in FIG. 1;

FIG. 3 is a side elevational view identical to that shown in FIG. 1 with only a portion of the ignitor shield shown therewith in dotted lines;

FIG. 4 is a side elevational view of the assembly of FIG. 3 without any shield being shown therewith;

FIG. 5 is an enlarged perspective view of half of an insulator block shown in FIGS. 1,2, 3 and 4;

FIG. 6is a greatly enlarged perspective view of an electrical connector employed with the invention;

FIG. 7 is an enlarged elevational view of an assembly of the half of the insulator block shown in FIG. 5 with two electrical connectors identical to that shown in FIG. 6;

FIG. 8 is a greatly enlarged end elevational view of the resistance wire of the present invention through which current is passed to heat it up for gas ignition;

FIG. 9 is a side elevational view of the wire shown in FIG. 8;

FIG. 10 is a greatly enlarged perspective view of a partial assembly of the present invention, the enlargement being even greater than that shown in FIG. 6;

FIG. 1 I is an elevational view looking at the assembly of FIG. 10 from the front as viewed therein;

FIG. 12 is a vertical sectional view through the assembly taken on the line 1212 shown in FIG. 11;

FIG. 13 is a perspective view of the shield disassembled from the remaining portions of the ignitor of the present invention;

FIG. 14 is a top plan view of a metal stamping from which the shield of FIG. 13 is made; and

FIG. 15 is an assembly view of a plurality of ignitors without shields mounted on a refractory board preparatory to inserting the same into a hydrogen oven for performing the said batch braze, the resistance wires in FIG. 15 not yet being brazed to their corresponding electrical ignitors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I, an electrical ignitor is indicated at 20 having an insulator block 21 which may be used, more or less, as a base. Block 21 is split into two

halves

22 and 23, shown in FIG. 2, along a line 24. Line 24 appears to be off center, and is, but blocks 22 and 23 are actually identical. Their mating surfaces, however, are staggered. The mating surfaces are indicated at 25, 26, 27 and 28 in FIG. 5, only one of the halves of block 21 being shown in FIG. 5.

As shown in FIG. 1, ignitor 20

electrical connectors

29 and 30 which are made of 310 stainless steel.

Connectors

29 and 30 extend completely through block 21, and are fixed in position relative thereto by means to be described.

Connectors

29 and 30 have

lower portions

31 and 32, respectively, which may be plugged into a conventional electrical receptacle to supply electrical current to a resistance wire ignitor 33, shown in FIG. 1. Wire 33 is made of MoSi It is shaped in the form of a helix with a constant pitch and a constant diameter throughout its entire length. Wire 33 is circular in cross section throughout its complete length, and has a diameter constant throughout its complete length.

As shown in FIG. 8, the helix portion of wire 33 begins and ends in a plane perpendicular to the paper of the drawing through a line 34. To each end of the helix.

extensions

35 and 36 are connected. That is,

extensions

35 and 36 are merely portions of wire 33 and thus are integral with the portion thereof that is shaped in the form of a helix.

Portions

35 and 36 have lengths A and B, respectively, where A is equal to B. Note will be taken that the axis of extension 35 is tangent to that portion of the axis of wire 33 which is shaped in the form of a helix. The point of tangency is thus midway between the inner and outer diameters of the helix on each side of the helix axis 37 and at different points in the said plane through line 34. v

In FIG. 8, note will be taken that

extensions

35 and 36, as viewed in FIG. 8, are absolutely vertical. However, this is not true in FIG. 9. Note that there is an.

angle C with the vertical in FIG. 9-that extension 35 makes due to the fact that the helix has a finite pitch and due to the fact that there is true tangency. The angle C is of substantially more than just passing interest, and its importance will be described hereinafter.

The ignitor 20, shown in FIG. 1, also has a shield 38 which is hollow, substantially square in horizontal cross section, except for two

projections

39 and 40, integral therewith and shown in FIG. 13.

Shield 38 may be made from a single metal stamping of flat sheet material of a uniform thickness. Such a stamping is shown at 41 in FIG. 14. A panel 42 provides the top of the shield 38, shown in FIG. 13. Also, the sides of shield 38 are identical. One side is provided by panel 43, the other being provided by the panel 44. Two

panels

45 and 46 enclose another side and

panels

47 and 48 enclose still another side.

Cutouts

49 and 50 provide a substantially circular opening in

panels

45 and 46, respectively, when the shield 38 is assembled. Similarly,

cutouts

51 and 52 provide a circular opening in

panels

47 and 48, respectively, when the shield 38 is assembled.

Panel 43 has circular and

rectangular openings

53 and 54, respectively, therein, circular opening 53 being of a diameter equal to that of a larger circular hole 55 in block 21, shown in FIGS. 3, 5 and 7. Panel 44 has circular and

rectangular openings

56 and 57, respectively, identical to

openings

53 and 54, respectively.

Shield 38 may be snap fit on block 21. Block 21 has bosses 58 and 59, shown in FIG. 3. The surfaces of bosses 58 and 59, shown in FIG. 3, are flat and in the same plane, such plane being parallel to a flat surface 60. Boss 58 has

straight portions

61 and 62 which are inclined with the same taper as the side edges of

shield projections

39 and 40, shown in FIG. 13. The taper is more evident between

lines

63 and 64, in FIG. 1. Similarly, boss 59 has

flat surfaces

65 and 66. The surfaces of

portions

61 and 62 lie in a single plane which is disposed at an angle relative to the single plane in which the surfaces of

portions

65 and 66 lie.

Block 21 has a boss 67 which is substantially square, and which fits inside shield 38 shown in phantom at 38' in FIG. 3.

As shown in FIG. 3, block 21 has

holes

68 and 71 on each side thereof. See also FIG. 5. Hole 68 in FIG. 5 extends completely through the half of the block shown in FIG. 5. However, when two halves are assembled, as shown in FIGS. 1, 2, 3 and 4, holes 68 and 71 do not become aligned. They do not lie in even partial registration. The hole 68 of the block half 22, not shown in FIG. 7, extends around a connector projection 69 whereas insulator block half 23, shown in FIG, 7, has a hole 71 corresponding to hole 68 which has an axis that is horizontally displaced from the axis of the hole 68, shown in FIG. 7. The hole 68 thus has an axis about midway between the vertical surfaces of projection 69 which lie in planes perpendicular to the paper, as viewed in FIG. 7. The axis of hole 71 is thus approximately in the center thereof, spaced substantially more than a hole diameter from that of hole 68 from projection 69.

In FIG. 5, note will be taken that surfaces 25 and 26 lie in the same plane. Similarly, surfaces 27 and 28 lie in the same plane. However, the plane of

surfaces

25 and 26 is parallel to and below that of

surfaces

27 and 28.

Connectors 29. and are identical; however, they are oriented oppositely in FIG. 7. Further, only one connector will be described because they are identical. One such connector is shown in FIG. 6 having an end portion 72 formed in the shape of a compound wedge,

a block portion 73 having a conventional hole 74 therethrough, a first intermediate portion 75, a second intermediate portion 76, a third intermediate portion 77, a fourth intermediate portion 78, a fifth intermediate portion 79 and an assembly 80. The connector shown in FIG. 6 is made from flat sheet stock. That is, both connectors are made of 310 stainless steel of a uniform thickness throughout its area. However, in accordance with one feature of the invention, a strip of brazing compound, shown at 81 in FIGS. 10 and 12, is spot welded or otherwise conventionally bonded to one side of the sheet material from which the connector is made.

Each connector is provided with a projection 82', projection 69 being identical thereto.

Another feature of the invention resides in the use of necked down portion 78 which, as will be described, when employed in a 310 stainless steel connector which has been annealed dead soft, provides shock proofing for the MoSi wire 33.

Connectors

29 and 30 both have assemblies identical to assembly and include, for example, as shown in FIG. 7, upper outside loops 82 and 83, lower outside loops 84 and 85 and

middle loops

86 and 87. Loops 82, 84 and 86 are formed integrally with connector 29.

Loops

83, 85 and 87 are formed integrally with connector 30.

Assembly 80, shown in FIG. 6, shows upper and lower

outer loops

88 and 89, respectively. However, a small portion of a middle loop may also be seen in FIG. 6.

In order to better understand the views of FIGS. 10, 11 and 12, inspection of the views of FIGS. 3 and 4 should be made noting especially the relationship between the wire 33 and the cup ends of

connectors

29 and 30 where loops 82-87 are located. Note that

loops

88, 89 and 90 generally form a cup having a flat bottom surface 90 where loop 89 has been'broken out of the said sheet stock and where the lower end'of wire 33 rests, as shown in FIGS. 10, 11 and 12.

A view of the left end of wire 33, as shown in FIG. 9, is shown in FIG. 8. A view of the right end of wire 33, shown in FIG. 9, would be identical to the view of FIG. 8. The offset of the halves of the block 21 produces an offset in the locations of the upper ends of the connectors. Some further offset is provided because necked down portion 78 is not in the center of portion 77, shown in FIG. 6. The horizontal spacing evident in FIG. 7 is simply produced because block 21 holds

connectors

29 and 30 in that position, the corresponding holding slots for the

connectors

29 and 30 being spaced apart.

For clarity, the reference numerals used in FIG. 6 are also employed in FIGS. 10, 11 and 12. Note in FIGS. 10, 11 and 12 that

loops

88, 89 and 90 generally form a cup to receive an end of wire 33. Comparing FIGS. 3 and 9, note will be taken that, in the view of FIG. 3,

extensions

35 and 36 of wire 33 are disposed at the same angle C relative to a plane transverse to the helix axis. This angle C is also shown in FIG. 12. Note that in FIG. 3, the upper end of connector 29 is actually closer to the viewer than the upper end of connector 30. This offset is provided, as aforesaid, by the offset in the halves of block 21 and in the offset of necked down portion 78 from portion 77 of the connector shown in FIG. 6. The offset is desirable to be able to insert the lower ends of wire 33 into the connector cups without stressing the wire 33. This not only prevents damage or breakage of wire 33 during assembly and during further processing, but also provides a shock proof mounting for wire 33.

The reason for one offset is that wire extension 35 is inclined downwardly, as viewed in FIG. 3, at the rear of the helix, and extension 36 is inclined downwardly in front. Thus, the reason that

extensions

35 and 36 are offset in approximately perpendicular directions is that

extensions

35 and 36 are tangent to the helix portion of wire 33 at the ends thereof on opposite sides thereof.

Notwithstanding the foregoing, it is an advantage that, as shown in FIGS. 4 and 8,

extensions

35 and 36 lie generally in vertical parallel planes tangent to an external cylindrical surface that outlines the helix. Thus, even though the end of wire 33 comes into the cup at angle C, as shown in FIG. 12, looking at it the other way as in FIG. 11, it goes in straight and can thereby make good contact with brazing compound 81, as shown in FIG. 12.

FIGS. l0, l1 and 12 actually illustrate an assembly step in the method of the invention of making the ignitor 20. In the manufacture of the invention,

connectors

29 and 30 are assembled in between block halves 22 and '23, as shown in FIGS. 2 and 7. If desired, sauerelsen or any other conventional refractory or insulator cement may be placed upon the surface facing the viewer in FIG. 7.

With the block halves 22 and 23 and connectors 29 and 30assembled and

connectors

29 and 30 located in the block grooves and in the positions shown in FIG. 7, cement may be inserted into

holes

68 and 71, as shown in FIGS. 3 and 7, respectively. This step may be used with or without the previously described step of applying cement. The same is true of the said previously described step.

After the cement has dried, plugs 91, 92, 93 and 94 FIG. 15.

Portions

31 and 32 and portions corresponding thereto, as shown in FIG. 1, thus extend down into one of eight grooves 96 in board 95. The portion of board 95 between the closest immediate adjacent two of the grooves 96 may be slightly wider than the space between

portions

31 and 32 so that there is a tightor snug fit of the connectors on each side of the two immediate adjacent grooves 96. Alternatively, each groove may be slightly thinner than the thickness of each of the

portions

31 and 32 so as to allow for a snug or tight fit of the

portions

31 and 32 in respective corresponding slots 96.

After

plugs

91, 92, 93 and 94 have been plugged into board 95, wires identical to wire 33 are then assembled therewith as indicated, for example, at 97. Each end of the resistance wire of the MoSi wire ignitor is placed in a cup of a corresponding connector in the manner illustrated in FIGS. 10, 11 and 12. Now, brazing compound 81 is located contiguous to wire 33, as shown in FIG. 12, This is true of the cup of each connector. High temperature brazing flux is applied to each cup area. Board 95 is then placed in a hydrogen or vacuum oven.

The composition of the brazing compound 81 is 18 percent nickel and 82 percent gold. A good bond could be made by raising the temperature of the oven to about l,800 F. However, there is another advantage with the useof this temperature. The useof this temperature causes a dead soft anneal of the 310 stainless are plugged into a refractory board 95, as shown in steel connectors. Thus, the oven temperature is raised to I,800 F. for a period of about 5 minutes. The completed assemblies may be allowed to cool in air and are ready for use immediately thereafter. However, a wind shield is helpful to keep the temperature of wire 33 constant in use. Thus, after shield 38 is fabricated, as shown in FIG. 13, it can be simply snapped over insulator block 21 in the position shown in FIG. 2 with

portions

39 and 40 snuggly fitted in the channels defined between the surfaces of

straight portions

61, 62, and 66 on both halves of the insulator block 21.

Wire 33 may have any diameter, but preferably should be as small as possible for a high resistance, efficient heating and low operating temperature. On the other hand, if wire 33 is too small in diameter, it will be even more likely to break due to any small shock or vibration. A wire diameter of 0.016 inch has been found satisfactory and is preferable in the circumstances.

The inside diameter of the helix, as shown in FIG. 8, preferably is between about 0.12 inch and 0.l3 inch. The pitch of the helix, as indicated at P in FIG. 9, preferably is between'about 0.036 inch and 0.046 inch.

The helix preferably has at least 1% turns and has an odd number of half turns. However, at least 1 /2 turns have been found more satisfactory.

In accordance with the foregoing, it will be appreciated that the helix inside diameter is preferably between about 7 to 9 times as large as the diameter of the wire. Further, the helix pitch must be larger than the wire diameter, and preferably is less than 3 times as large as the wire diameter. More efficient heating can be expected is these limits are retained. The same is true of lower wire temperature.

Preferably, a voltage is applied between

connectors

29 and 30 which is adequate to heat one point on the wire 33 to a predetermined temperature larger than the temperature at any other point thereon, wherein said predetermined temperature is between about l,900 F. and 2,400 F. Reliable ignition may then be achieved with a maximum wire life.

Although a hydrogen oven has been described, it is to be understood that any oven may be employed. However, preferably, the oven is filled with an inert gas including, but not limited to, nitrogen. A vacuum may also be used.

The phrase inert gas is hereby defined herein as including a gas which does not react with the materials placed in the oven in a deleterious way.-

Reduction may be anticipated if hydrogen is used. This is an advantage of hydrogen over some other gases. However, nitrogen may be used, if desired. Inert gas is employed to reduce or eliminate oxidation. Oxidation can occur on the

connectors

29 and 30, the brazing compound 81 and the wire 33.

Due to the fact that the batch braze is perfonned at an oven temperature of l,800 F., a dead soft anneal of the

connectors

29 and 30 is effected because the dead soft annealing temperature of 310 stainless steel is about l,775 F.

The phrase dead soft is hereby defined herein as describing a material or metal which has been annealed to an extent such that it cannot be made any softer to any substantial extent by any kind of further annealing.

Although board 95, shown in FIG. 15, may be made of any desirable refractory material, insulating material or a material of a low specific heat, transite may be preferable because of its low specific heat. Further, board 95 need not be made of a refractory material if it has a low specific heat and will stand a temperature of about l,800 F. for about minutes.

Although outstanding features of the invention are achieved by the use of wire 33 when it is formed at a helix, many features of the invention are not limited to this particular geometric configuration.

Wire ignitor 33 may be formed by heating it to a temperature between about 2,900 F. and 3,000 F. and winding the same on a cylindrical carbon mandrel. Preferably, both the wire and the mandrel are so heated. Heating may be done by the use of three gas torches located in positions equally spaced around the mandrel, the torches being located substantially in a single plane and directed radially inwardly toward the mandrel axis.

Another feature of the invention resides in the use of

holes

53 and 56, shown in FIG. 14, in alignment with hole 55. A bolt may be inserted therein for mounting purposes, if desired.

The phrase dead soft annealing temperature is hereby defined as that temperature above which a metal can be raised, but by the annealing thereof, it cannot be made any softer.

The phrase brazing temperature is hereby defined herein as that temperature to which brazing compound 81 or an equivalent thereof must be raised in order to accomplish a suitable braze.

ln the brazing step, the use of flux may be helpful in attaining suitable wetting of the materials and in preventing oxidation, if desired.

Note will be taken that it is an outstanding advantage of the invention that plugs 91, 92, 93 and 94 may be oriented, as shown in FIG. 15, with all of the connectors extending in an upward vertical direction to easily hold the lower ends of the MoSi resistance wire ignitors for the braze, yet holding the wire ignitors without stressing the same.

Wire 33, when it is first made straight, may be made by any conventional means or method and will be found to have generally a resistivity of 100 ohm circular mils per foot. It may be identical to Kanthal Super ST made by the Kanthal Corporation, if desired. The voltage applied between

connectors

29 and 30 will generally be about 2.6 volts. With this voltage, when the ambient temperature is approximately 70 F., the wire will carry a current of approximately 3.6 amperes.

As stated previously, it is one outstanding advantage of the present invention that for reliable gas ignition, the wire ignitor 33 of the present invention can operate at a lower temperature than a straight wire can. This is unexpected, but, after considerable study, can perhaps now be explained in that reliable gas ignition can be obtained with different peak wire temperatures. Peak temperature is thus hereby defined as the temperature along the length of a wire which is the highest. Note will be taken that the temperature along the wire varies with the length of the wire from connector 29 to connector 30. The peak temperature is likely to be at, or close to, the midpoint to the ends of the wire. In a straight wire, due to a cooling effect by the heat conduction of the

connectors

29 and 30, the total resistance of the wire increases nonlinearly with temperature to such an extreme extent that a voltage regulator must be provided for a straight MoSi wire ignitor. On

the other hand, when the wire is formed into the helix, the adjacent turns of the helix apparently heat each other and form a box inside of which air or gas is heated efficiently. Moreover, the wire of the helix has a more uniform temperature along its length than does a straight wire ignitor. That is, the temperature is not precisely uniform, but it is more uniform because of the mutual heating of adjacent turns. For this reason, the resistance between the connectors through the wire of a MoSi helix varies much more linearly with temperature. Self-regulation is thus produced which does not require additional or more expensive apparatus for accomplishing voltage regulation. In other words, the helix wire ignitor of the present invention with a conventional low cost transformer may be substituted for a MoSi straight resistance wire ignitor and a transformer incorporating means for regulating the wire ignitor voltage.

In accordance with the foregoing, it will be appreci ated that although coiled resistance wire ignitors are old in the art, a great many outstanding advantages are achieved by practicing the present invention and the use of a MoSi helical resistance wire ignitor is not obvious because no one suspected that it could ever be wound into a helix, MoSi being a very brittle material. It is so brittle, it may be broken with approximately the same force as used in breaking pencil lead for automatic pencils.

Note will be taken that the brittle character of the MoSi is, in fact, used because its brittle character makes it a better spring and therefore, in the shape of a helix, more shock and vibration resistant.

It is an outstanding advantage of the present invention that for reliable gas ignition, the peak temperature of the helical MoSi wire of the present invention is substantially lower than that of a straight wire. This means that the wire ignitor of the present invention has a life longer than even a straight MoSi wire.

It is an advantage of the present invention that the portions of

connectors

29 and 30 above the portions thereof identical to necked down portion 78 shown in FIG. 6 and connected to wire

extensions

35 and 36 have a mass greater than that of wire 33. Thus, if

connectors

29 and 30 are dead soft, a shock will cause

connectors

29 and 30 to bend at the said necked down portions thereof and protect wire 33 from damage or breakage.

What is claimed is:

l. A resistance heating element comprising: a coil of molybdenum disilicide wire having straight end portions of approximately equal lengths, said coil being in the shape of a helix having an approximately constant inside diameter and pitch throughout its entire length, said wire having an approximately constant diameter throughout its entire length, said helix having an odd number of half turns and at least 1% turns, said wire end portions being connected to and being integral with the ends of said helix, the axes of said wire end portions being positioned substantially tangent to the axis of said wire at the connections of said wire end portions with the respective ends of said helix, said wire having a diameter of about 0.016 inch, said helix inside diameter being between about 0.12 and about 0.l3 inch, said helix pitch being between about 0.036 inch and about 0.046 inch, said helix having 2% turns.

UNITED STATES PATENT OFFICE CERTIFICATE ()F CORRECTEQN Patent No. 3,812,324 Dated May 21, 1974 xnvenitoyfls) ]'oseph Gino Raffaelli and Reed Albert Palmer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[- On the abstract page, show the name of the first inventor to be Ioseph Gino Raffaelli.

Column 2, line 35, delete "falling" and insert -failing.

Column 4, line 32 after "20" insert -includes Column 8, line 24, delete "1 1/2" and insert ---2 l/2-.

Column 8, line 32, delete "is" and insert -if-.

Signed and sealed this 26th day of November 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents new UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3,812,324 Dated May 21, 1974 hven'tofls) J'oseph Gino Raffaelli and Reed Albert Palmer It is certified that: arms appears in the above-identified patent: and that said Letters Patent are hereby eorrected as shown below:

T. On the abstract page, show the name of the first inventor to be Joseph Gino Raffaelli.

1 Column 2, line 35, delete "falling" and insert -,-fai1ing.

Column 4, line 32, after "20" insert includesn a Column 8, line 24, delete "1 1/2" and insert --2 1/2--.

Column 8, line 32} delete "is" and insert if--.

Signed and sealed this 26th day of November 1974.

(SEAL) Attest: v

MCCOY M. GIBSON JR.- c. MARSHALL DANN Attesting Officer Commi s;sioner of Patents

Claims

1. A resistance heating element comprising: a coil of molybdenum disilicide wire having straight end portions of approximately equal lengths, said coil being in the shape of a helix having an approximately constant inside diameter and pitch throughout its entire length, said wire having an approximately constant diameter throughout its entire length, said helix having an odd number of half turns and at least 1 1/2 turns, said wire end portions being connected to and being integral with the ends of said helix, the axes of said wire end portions being positioned substantially tangent to the axis of said wire at the connections of said wire end portions with the respective ends of said helix, said wire having a diameter of about 0.016 inch, said helix inside diameter being betWeen about 0.12 and about 0.13 inch, said helix pitch being between about 0.036 inch and about 0.046 inch, said helix having 2 1/2 turns.