US3425864A

US3425864A - Method for making electric resistance heaters

Info

Publication number: US3425864A
Application number: US473689A
Authority: US
Inventors: Glen H Morey
Original assignee: Templeton Coal Co
Current assignee: Templeton Coal Co
Priority date: 1965-07-21
Filing date: 1965-07-21
Publication date: 1969-02-04
Anticipated expiration: 1986-02-04

Description

METHOD FOR mum ELECTRIC RESISTANCE HEATERS Filed July 21, 1965 G. H. MOREY Sheet INVENTOR. GLEN H. MOREY FV///////////////A I METHOD FOR MAKING ELECTRIC RESISTANCE HEATERS Filed July 21, 1965 G. H. MOREY Feb. 4, 1969 Sheet INVENTOR.

GLEN MOREY United States Patent 3,425,864 METHOD FOR MAKING ELECTRIC RESISTANCE HEATERS Glen H. Morey, Terre Haute, Ind., assignor to Templeton Coal Company, Terre Haute, Ind., a corporation of Indiana Filed July 21, 1965, Ser. No. 473,689 US. Cl. 117215 Int. Cl. H01b 3/12 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method for making electric resistance heaters and is particularly concerned with such a method and apparatus including the plasma spraying of the resistance heating element on a supporting surface, and also including, when desired, the spraying by plasma spraying of electrical insulating material employed in connection with the electric resistance heater.

The spraying of metals, ceramics, and the like by flame spraying devices and the spraying of ceramics by plasma spraying is known but so far as I have been able to determine, the plasma spraying of resistance heating elements has never heretofore been practiced by specific method claimed.

Plasma spraying, in brief, consists of creating a jet of plasma which is directed toward a supporting surface while introducing into the jet the material to be deposited on the surface in finely granulated form. The plasma is created by passing gas through a passage and within which passage the gas is ionized and raised to an elevated temperature by passing an electric current therethrough. Such gases are quite hot, on the order of up to 50,000" F. and issue from the passage at a temperature within the aforementioned range and at relatively high velocity. =When particulate material is introduced into such a jet, the material will be softened by the heat of the jet and will be conveyed by the jet to the surface on which the jet impinges and be deposited on the surface in the form of a layer, the thickness of which can readily be controlled.

With the foregoing in mind, a primary object of the present invention is the provision of a method for employing plasma spraying to effect deposition of electric resistance heating elements on a substrate which may take substantially any form, including metal and electrical insulating materials.

Another object of this invention is the provision of a method for making electric resistance heating elements while simultaneously disposing the electric resistance heating elements in the region where they are to be employed.

Still another object of the present invention is the provision of a method for forming electric resistance heating elements which will be more resistant to deterioration than resistance heating elements made according to the prior art methods.

Still another object of the present invention is the provision of a method for making an electric resistance heat- 3,425,864 Patented Feb. 4, 1969 ing element while placing the electric resistance heating element in intimate thermal contact with the member it is to heat.

The foregoing objects as well as still other objects and advantages of the present invention will become more ap parent upon reference to the following specification taken in connection with the accompanying drawings, in which;

FIGURE 1 is a somewhat diagrammatic perspective view showing an electric resistance heating element being made according to the present invention;

FIGURE 2 is a somewhat schematic longitudinal section through a plasma gun and is [generally indicated by line II-II on FIGURE 1;

FIGURE 3 is a transverse cross-section indicated by line III-III on FIGURE 2;

FIGURE 4 is a sectional view through a support member showing resistance heating element formed thereon according to the present invention;

FIGURE 5 is a perspective view showing a tubular electric heating element constructed according to the present invention;

FIGURE 6 is a fragmentary view showing the appearance of an electric resistance heating element constructed according to the present invention directly on the wall of a structure which is to be heated by the heating element;

FIGURE 7 is a sectional view indicated by line VII- VII on FIGURE 6;

FIGURE 8 is a fragmentary view, partly in section, showing another type of tubular heating element constructed according to the present invention;

FIGURE 9 is a sectional view indicated by line IX]X on FIGURE 8; and

FIGURE 10 is a fragmentary sectional view showing a resistance heating element consisting of a plurality of layers.

Referring to the drawings somewhat more in detail, in FIG. 1, 10 indicates a plasma gun. Connected to the gun is a conduit 12 for the supply of gas thereto, the said supply coming from a pressurized source 14 and being under the control of a valve 16. The gas may comprise substantially any gas, but is preferably in the form of a substantially chemically inert gas so that when the plasma jet issues from the gun, the plasma, in addition to the heating and conveying effect thereof, will also serve as a shielding medium for material therein.

Within gun 10 the gas is converted to plasma b electrical energy supplied to the gun via electric coupling 18.

Associated with the gun are the sources 20 and 22 of finely granulated or particulate material which are connected to the gun near the discharge end thereof by

conduits

24 and 26 in which are disposed flow control elements such as the

valves

28 and 30.

The plasma jet is schematically indicated at 32 and it comprises a relatively high velocity, high temperature jet of plasma in which the particulate material is entrained. This jet is directed toward and impinges upon a support member 34. The support member 34 maybe metal or electrical insulating material or any other sort of receiver which will withstand the temperature of the plasma jet. The entire arrangement of the support member and at least the discharge end of the plasma gun may be contained within an enclosure 36 which may have the atmosphere thereof controlled as by the

conduits

38 and 40 connected thereto. Generally, the enclosure is evacuated and back filled to a pressure of about 1 atmosphere with an inert gas.

Referring to FIGURE 2, it will be seen that the gun 10 comprises a body 42 and extending through the body is a passageway 44 for gas.

Electrodes

46 and 48 are provided which can be energized with electrical energy to develop a spark therebetween to initiate ionization of the gas flowing through the gun.

Following initial ionization, electrons accelerated from the back electrode (cathode) to the front electrode (anode) as a result of the current flowing therebetween cause further ionization of the gas. The continued maintenance of electric current insures that plasma and the intense heat resulting therefrom will issue from the discharge end of the gun defined by the front electrode.

The electric current will maintain the gas ionized and at high temperature, so that the gas discharged at the discharge end 50 of the gun is in the form of intensely hot plasma. Cooling water for the electrodes is conveyed through the gun and conduit means 47 and passage means The temperature of the plasma is sufiicient to soften even ceramic materials and will furthermore soften and sometimes even substantially liquefy metallic materials introduced by way of

conduits

24 and 26 into the gas stream. These materials, when introduced into the gas stream are conveyed thereby to the support member and will be deposited thereon. If the gun is moved, the materials will appear as a layer on the support member of a thickness depending upon the speed of movement of the gun or upon the number of times the gun is caused to traverse a given area of the support member.

The materials may be supplied simultaneously from both of sources 20 and 22 or may be supplied individually from the respective sources, or in succession. For example, one of the sources could comprise insulating material such as ceramic and this material could be deposited on a support member first, to form an insulating base to receive the conductive resistance material. This procedure would be followed where the support member was itself conductive as, for example, where the support member was a metal. On other occasions, the conductive electric resistance heating element could be supplied first, as where the support member was in the form of electrical insulating material, alumina, for example. In this case, the insulating material from the other source might thereafter be supplied to the gun to form an insulating sealing coating over the layer of electric resistance heating material.

Still further, the application could be, first, a layer of insulating material, then a layer of resistance heating material and, finally another layer of insulating material. It is understood that two or more constituents could be supplied to form a mixed layer.

Terminals could be provided on the support member which the electrical insulating material contacts when applied to the support member, or regions of the resistance heating material could be left exposed and electrical connection made thereto after the heater was manufactured.

In FIGURE 4 there is shown a support member 52 having terminals 54 therein and electric resistance heating material 56 applied to the support member by plasma spraying and so applied as to make electrical connection with the terminals 54.

FIGURE 5 shows how a tubular resistance heating element could be fabricated according to the present invention. In FIGURE 5, 60 is a tubular member of electrical insulating material, alumina, for example, which is also resistant to heat. Sprayed on member 60 is a layer 62 of electric resistance heating material. At spaced points along material 62, preferably near the ends thereof, straps 64 are employed to which electric current supply leads 66 are connected so that an electric current can be passed through th resistance heating material 62.

The arrangement of FIGURE 5 has proved to be a particularly advantageous manner of constructing electric resistance heating elements from materials such as molybdenum disilicide. This material has always been diflicult to fabricate into a resistance heating element. It is a conductive ceramic material and has heretofore been made into resistance heating elements by a sintering process. This process forms resistance heating elements which perform satisfactorily up to a certain limiting temperature, but beyond this latter temperature the sintered re- 7 sistance heating elements of the said material become weak and will commence to deform, while, furthermore, some of the bonding agent employed during the sintering process will boil off and the heating elements will commence to become porous and lose strength and effectiveness for this reason. Heating elements of this material constructed as shown in FIG. 5 have the merit that they are supported by the substrate on which they are built up and can thus be operated at higher temperatures and for longer periods of time than the sintered elements according to the prior art, as referred to above.

FIG. 6 shows the wall 70 of a furnace or other enclosure which is to be heated electrically, and directly on this wall is formed an electric resistance heating element 72 by the practice of the present invention. This resistance heating element may be applied by causing the plasma spray gun to traverse the pattern illustrated, but, alternatively, the wall 70 can be masked off and the resistance heating material will then be applied to the exposed portion only of wall 70. Connection to spaced points of the heating element illustrated can be effected as by bolts 74 to which electric leads 76 are connected. A detail of such a connection is illustrated in FIG. 7.

FIGS. 8 and 9 show how a metal pipe 80 could be provided with an electric resistance heating element constructed directly thereon. In these figures, the metal pipe 80 has applied thereto a first layer of electrical insulating material at 82. This layer which is applied by plasma spraying, may comprise, for example, a ceramic.

A second layer 84 is applied by plasma spraying on top of layer 82 and this last-mentioned layer is electrically conductive material. The layer 84 is preferably stepped inwardly from the end of layer '82.

A third layer 86 of electrical insulating material may then be plasma sprayed on top of conductive layer 84 and is also preferably stepped inwardly from the end of layer 84. The exposed end regions of layer 84 receive straps 88 by means of which electrical connection can be made to the conductive layer. Alternatively, layer 86 can be extended to beyond the end of layer 84 and can also be applied over strap 88 except for the terminal portions thereof. Dotted line 90 shows layer 86 applied in this last-mentioned manner.

In FIG. 10 a metal pipe 90 is illustrated and directly on the surface of metal pipe 90 is a layer 92 of ceramic. Directly applied over layer 92 is a layer 94 which is a mixture of ceramic and metal, known in the trade as Cermet and which is a material consisting of ceramic and metal so admixed that the metal is in continuous phase in the ceramic and thereby can con-duct electricity and thereby form an electric resistance heating element.

The final layer illustrated in FIG. 10 at 96 is again ceramic similar to layer 92, and, therefore, non-conductive. The arrangement shown in FIG. 10 could be arrived at by spraying, by the plasma jet, the ceramic continuously on pipe 90 and during a portion of the time that the ceramic is being sprayed on the pipe, metal particles are introduced into the plasma jet in sufiicient quantity to form the conductive layer 94. This can readily be accomplished because the plasma jet can be continued while the supply of particulate material thereto is regulated, including initiating d interrupting the supply of one or more materials to t e plasma jet, or while modifying the proportions of the materials applied relative to each other.

The present invention provides, as will be seen from the foregoing, a relatively simple method and apparatus for making electric resistance heaters characterized in the advantages that electric heaters of improved quality can be made, and that heaters can be formed in situ where they are to be used, and resistance heaters that at the present time cannot be fabricated, or can only be fabricated with great difficulty on account of the temperatures required, can easily be made by the method of the present lnvention.

Other advantages of the present invention are:

(1) It makes possible the use of many materials which are electrically suitable for resistance heating elements which otherwise could not be used because of the mechanical difiiculty of fabricating the material into rods and other shapes.

(2) The heating element applied according to the present invention is spread over the entire surface so that heat emission is uniform.

(3). Because the heating element is spread over a large surface it can operate at a much lower temperature than if it were in the form of rods while at the same time maintaining an adequate temperature of the open space in a furnace or oven, or in a body being heated.

(4) In conventional electrical furnaces the heat must usually be transferred from the heating element through a refractory wall. With the new method the heating element can face into the area to :be heated so that the heat does not have to pass through a refractory wall, thus resulting in a lower temperature for the heating element, longer life thereof, and more effective operation, as well as greater reserve capacity.

It will be understood that this invention is susceptible to modification in order to adapt it to different usages and conditions; and accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

What is claimed is:

1. A method of forming electric resistance coatings on a support having an alumna surface by introducing a chemically inert gas into a closed end of a tubular member of ceramic electric insulation material, conducting a current of electrons into the gas in the tubular member by means of a cathode while conducting electrons away from said gas by means of an anode at a point downstream from the cathode and at a rate sufficient to ionize the gas and convert it into a hot plasma, introducing finely granulated molybdenum disilicide into the plasma at a point downstream of the anode, and directing the current of hot plasma and molybdenum disilicide upon a surface of alumina in an atmosphere of inert gas.

2. The method of claim 1, in which the alumina surface is on the outside of a metal tubular member.

3. The method of claim 1, in which electric terminals are mounted upon the alumina surface before the molybdenum disilicide is applied.

4. The method of claim 3, in which a final coating of alumina is applied upon the molybdenum disilicide coating by introducing powdered alumina into the plasma immediately after the molybdenum disilicide has been applied.

References Cited UNITED STATES PATENTS 3,109,228 11/1963 Dyke et al 117212 XR 3,183,337 5/1965 Winzeler 117-93.1 3,197,335 7/1965 Leszynski 117212 3,269,856 8/1966 Jones 11793.l XR 3,208,835 9/1965 Duncan et a1. 117-217 XR 3,247,579 4/1966 Cattermole et a1. 11793.1 3,279,939 10/1966 Yenni l1793.1 3,347,698 10/1967 Ingham 11793.1

WILLIAM L. JARVIS, Primary Examiner.

US. Cl. X.R.