Electric resistance heating device

Abstract

A heating device includes electric resistance wire and a glass plate cover for the wire, the wire being made of molybdenum disilicide, having a positive temperature coefficient of resistance, and comprising at least two series-interconnected sections of differing cross-sectional areas. The wire is supported on the flat surface of a refractory body, and is adjacent to but spaced from the glass plate cover, the body enclosing both sections with the side of the glass plate cover which faces the wire. The two sections of differing cross-sectional areas may be proportioned relative to each other and to the electric power used, so that upon initial energization, the section of smaller cross-sectional area receives maximum power and almost immediately flashes to incandescence while converting the space enclosed by the body and the glass plate cover into a heating chamber heating the section of larger cross-sectional area which, with increasing temperature, increases in resistance to lower the current through the section of lower cross-sectional area, the power consumption of the device then stabilizing at a lower value. Incorporation of the device into the horizontal flat top of a cooking stove, for heating a cooking vessel on top of the glass plate mainly by radiation, provides for a substantially instantaneous high cooking heat followed by stabilization of the temperature.

Description

BACKGROUND OF THE INVENTION

Electric resistance heated cooking stoves using hot plates comprising a coil of refractory insulated, metal alloy electric resistance wire enclosed by a metal sheath, have been commercially successful but have suffered from the objection of slow heating upon initial energization. This has given gas stoves with their substantially instantaneous heating a competitive advantage.

To overcome such slow heating, the prior art has proposed the use of an electric hot plate comprising a transparent glass plate cover with open or unsheathed metal alloy electric resistance wire beneath the cover with the wire diameter and length proportioned relative to the domestic power supply voltage, so that with initial energization the wire substantially immediately becomes incandescent, cooking utensils on the cover being heated mainly by direct radiation. Insofar as is known, this has been unsuccessful because the excessive wire temperatures following initial energization, result in an impractical short service life of any cooking stove incorporating such a hot plate construction. Hermetic enclosure of the wire to permit its operation in a vacuum in the manner of an electric lamp, has been considered to be impractical.

Electric resistance materials of non-metallic nature, capable of operating at substantially higher temperatures than metal alloys, have been available. Silicon carbide is one example, but this has the disadvantage of being slow-acting, its electrical resistivity being highest when cold. That is to say, silicon carbide has a negative temperature coefficient of resistance.

An apparently more suitable material is one of the refractory metal silicides, particularly molybdenum disilicide. As commercially available, this material is in the form of approximately 90% by weight of molybdenum disilicide with a balance of ceramics. It has a positive temperature coefficient of resistance, and if made in the form of wire with a cross-sectional area, or diameter, to length ratio suitable for use with the domestic power supply, nominally in the area of 120 volts in the U.S., it should substantially immediately flash to incandescence when initially energized.

However, if such wire is used in the form of a layer of coils, loops, etc., beneath a glass plate, with the intention of providing a transparent glass plate type of hot plate for a domestic cooking stove, there is the problem of controlling the power so that upon reaching incandescence with the consequent increase in electrical resistivity, overheating is avoided. Such overheating could result in destructive heating of the glass plate cover, and if continued, destruction of the wire.

All electric heating devices are sold in a highly competitive market, and this is particularly true in the case of domestic cooking stoves. This prohibits the use of industrial control systems such as are used to control molybdenum disilicide rod-type industrial furnace heating applications.

One object of the present invention is to use molybdenum disilicide wire, or wire made of other refractory metal silicides should they become commercially available, in applications where the wire operates behind a cover through which the heat is passed, such as a glass plate cover, with such a wire heating element capable of inherently flashing to incandescence when initially energized, but which will thereafter stabilize and operate at a predetermined desired power consumption without using control equipment of the industrial type.

SUMMARY OF THE INVENTION

According to the invention, the above object is attained by making the refractory metal silicide wire, such as molybdenum disilicide, in the form of at least two series-interconnected sections of differing cross-sectional areas and substantially enclosing both of these sections together with the side of the cover which faces the wire.

The cover particularly contemplated is the previously referred to transparent glass plate and the means used to enclose the two wire sections and the adjacent side of the glass plate, is a refractory body forming a flat surface on which the wire is formed as a single layer comprising loops or coils of the wire. More than two sections of the differing cross-sectional areas may be used; the differing sections are preferably butt-welded directly end-to-end. Preferably the just-mentioned body is made from a compacted fibrous refractory material, the wire, necessarily being of small cross-sectional area as to all of the sections, being hooked or stapled to this flat surface by molybdenum disilicide fastening elements. If wire made of other refractory metal silicides is used, the fastening elements should be made of corresponding material.

The various sections of differing cross-sectional areas have these areas and their lengths proportioned relative to the voltage of the available power source so that upon initial energization the sections of smaller cross-sectional size receive maximum power and become incandescent substantially instantaneously. The space in which the wire is enclosed by the cover or glass plate on one side and the refractory body on the other side, is proportioned in size so that this space, ordinarily containing air, becomes rapidly heated, this, together with the electric resistance heating, heating the section or sections of larger cross-sectional area and thus increasing the resistance in the circuit of the various series-interconnected sections to an extent resulting in operation of all of the sections, and the power consumption, becoming quickly stabilized at any desired value. In this way the device is made inherently self-stabilizing, although providing substantially instantaneous high temperature production, and all without using external control equipment of any kind.

The described use of hooks or staples is primarily because of shipping problems. Once the device is operating horizontally, as in a domestic cooking stove application, there is no need for fastening of the wire, but this need does exist during handling involved by shipment, and possibly by other conditions.

To eliminate the fastening elements, and using the body with its flat surface made of fibrous refractory material, the flat surface on which the wire is placed, may be roughened or up inherently made fuzzy; that is to say, a surface having upstanding refractory fibers can be provided. BBeing thin and upstanding, when the wire is placed on these fibers, energization of the wire at the plant where the device is manufactured, such as would ordinarily be done for testing, results in temperatures fusing the fibers so that with cooling of the wiire the wire adheres to the fibers and is anchored in position without the need for the elements described. The type of refractory referred to is commercially available in a form which in its compacted condition satisfactorily resists the molybdenum disilicide wire operating temperature but which when in the form of upstanding fibers, fuses under such temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS up or Being wire

Specific examples of the invention are illustrated by the accompanying drawings in which:

FIG. 1 is a top view showing the glass plate cover broken away to more clearly reveal the construction;

FIG. 2 is a cross section taken on the line 2--2 in FIG. 1;

FIG. 3 in cross section shows a fragment of the FIG. 2 construction and illustrates the attachment of the wire to the upstanding fibers as previously described; and

FIG. 4 is a top view of another example of the invention, this showing this construction prior to the application of the cover such as the glass plate .

DETAILED DESCRIPTION OF THE INVENTION

Having reference first to FIGS. 1 and 2, the molybdenum disilicide resistance wire is shown as comprising three sections 1, 2 and 3, the section 1 being the one of larger diameter or cross sectional area and butt-welded end-to-end to the sections 2 and 3 which are of smaller diameter or cross sectional area relative to that of the section 1. The points where the sections are interwelded can be located by the connections of the wires 4 which are provided with a switch 5 for the purpose of shunting out the section 1 made of the wire of larger size, should this be desired.

The cover is shown as a flat glass plate 6 having any of the compositions suitable for elevated temperature service, and means are shown for substantially enclosing the sections 1 through 3 together with the side of the glass plate 6 facing the wire, this means comprising the body 7 and an annulus 7a which spaces the flat surface 7b of this body from the inside of the glass plate 6. In this way the previously described space 8 is formed. The mutually opposing faces of the inside of the glass plate 6 and the flat surface 7b are spaced rather closely together, but the wire should not contact the glass cover plate. As shown, the wire is formed as a single layer without overlapping wire portions.

The sections 1 through 3 are, of course, in series with each other, terminals 9 being provided for connection with the electric power supply.

The supporting or enclosing body 7 is made of vacuumformed ceramic fibers made by Johns-Manville under the tradename "FIBERCHROME" and consists basically by weight of 55% SiO.sub.2, 40.5% Al.sub.2 O.sub.3 and 4% of Cr.sub.2 O.sub.3, the fibers being bonded together by means of an inorganic binder forming the balance. This material is rated for maximum operating temperatures of 2700.degree.F. The body 7 may be made of other corresponding commercially available materials and the spacer 7a shown by FIG. 2 could be integral with the portion providing the flat surface 7b.

By abrasion or roughening of the surface 7b its fibers may be made to upstand as shown at 7c in FIG. 3. When the device is fabricated in the horizontal position with the wire on the surface 7a roughened as described, the wire may be energized to flash its temperature above the melting temperature of the fibers 7c, the heating then being promptly terminated. This results in fusion, or at least incipient fusion, of the fibers on which the wire rested, cooling bonding the wire to the fibers so that the wire cannot shift out of position during shipment of the device, or other handling. To effect such flash heating, the switch 5 shown by FIG. 1 may be closed so that the stabilizing wire section 1 is out of the circuit. This fastening of the wire may be done either before or after application of the glass cover plate 6.

An experimental device according to this invention has been reduced to practice substantially as disclosed by FIGS. 1 and 2. The part 7 was made of a 1 3/8 inch thick pad of the specific ceramic fiber material previously described, and the part 7a was formed by a 1/2 inch thick flat pad of the same material, placed between the flat pad 7 and the glass plate 6 and having a suitable cut-out hole. Using molybdenum disilicide wire, the wire forming the section 1 had a diameter of 0.031 inch and the sections 2 and 3, made of the same material, had a diameter of 0.024 inch. The total length of the wire, including all of the sections, was designed for use with the usual U.S. domestic power voltage or around 120 volts.

The design of the above was such that when the section 1 was cut out of circuit, as by closing the switch 5 in the illustrated instance, the unit consumed 1140 watts at a voltage of 120 volts. With the section 1 in the circuit with the sections 2 and 3, the sections 2 and 3 almost immediately reached incandescence and as the temperature built up inside of the space 8 the power stabilized at approximately 880 watts, representing continuous operation. Under such stabilized conditions, the wire was incandescent substantially throughout all of the sections.

FIG. 4 is provided to show an instance when the wire is in the form of a continuous spiral coil, the sections of smaller cross sectional area being shown at 10 and those of larger cross-sectional area being shown at 11. In this instance there is a large multiplicity of the differing sections, all interconnected in series by butt-welding of the sections end-to-end.

As required for cooking purposes, the device of the present invention may be provided with a suitable voltage controller such as represented by currently available, inexpensive solid state controllers, such as incorporate silicon controlled rectifiers. Also, sections may be cut in and out of circuit as indicated.

The flat surface of the body 7 should be a fibrous material surface to reduce heat transfer from the wire to the body as much as possible. Intimate contact of the wire with a flat, solid ceramic surface, for example, results in so much loss of heat by conduction into what is then a heat-sink, as to prevent the described operation. If it was possible, it would be best to support the wire without it losing any heat to the supporting medium.

Mention has been made of 120 volt operation. The device can be designed for other line voltages, such as 240 volts for example.

The terminals 9 are shown as being larger than the wire sections 2 and 3. This is because at the present time "cold" terminals are required for connection with the disilicide wire. For the same reason the wires 4 are shown connected via similar terminals 4a.

Claims

What is claimed is:

1. An electric cooking stove assembly comprising a flat glass plate forming a cooking surface area, a convoluted molybdenum disilicide wire forming a layer positioned substantially parallel with and below said plate and covering an area substantially co-extensive with said cooking surface area, said wire having opposite ends for connection with electric power and having at least one portion between said ends, which portion has a larger cross-sectional size than the balance of the wire, said wire forming a continuous circuit from one of its ends to its other end with said portion in series with the balance of the wire, a refractory body forming a flat surface extending substantially co-extensively with said layer of wire and having upstanding refractory fibers on which said layer of wire is supported and with the fibers fused to the wire, and a refractory annulus peripherally surrounding said layer of wire and interconnecting said glass plate and refractory surface so as to substantially enclose a space containing said layer of wire.

2. The assembly of claim 1 in which said molybdenum disilicide wire is convoluted in zigzag form.

3. The assembly of claim 1 in which said molybdenum disilicide wire is convoluted in spiral coil form.