Method of making a heating body

[57]

ABSTRACT

Electrical resistance heating conductor, surrounded by an electric insulating cylindric layer of borium nitride or beryllium oxide, coated off by outward heat dissipating liner.

Between conductor, intermediate layer and outward liner are at all times shrinking fits sustained. Conductor and liner consist of austanitic stainless steel, molybdenum, nickel, chromium, vanadium, rhenium, tungsten or its alloys.

I Claim, 3 Drawing Figures

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PAIENTEDMAY30 1972

3,665,598

SHEET 1 Or 2

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FIG. 1

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FIG. 2

INVENTOR

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PATENTEDMAY 30 197?

3,665,598

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INVENTOR

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BY

ATTORNEYS

3,665,598

METHOD OF MAKING A HEATING BODY

The invention relates to a heating body, consisting of an electrical resistance element containing a conductor resistance surrounded by an electrical insulating layer, which insulating layer has a metal outer layer placed around it for delivery of the heat produced, in which below working temperature there is a contraction joint between the metal outer jacket and the insulating layer, while in addition, at room temperature, there is a contraction joint between the conductor resistance and the insulating layer.

Heating bodies of this kind, however, still show in present practice the drawback that for numerous applications the heat-loading capacity as expressed in watts/cm^s is not sufficiently high. According to the invention it is possible to attain a high heat-load, viz. a heat-load of the order of about 500 watts/cm^!, at a wall temperature of about 600°-960° C., in that the heating body is executed in such a form of construction that its insulating layer is made of boron nitride, in which construction both the conductor resistance and the metal outer layer are made of austenitic stainless steel, molybdenum, nickel, chromium, vanadium, rhenium, tungsten or their alloys. Beryllium oxide may also be used to advantage as electrical insulating material instead of boron nitride.

It is highly advantageous for this purpose that even at working temperature there is still a contraction joint and hence a surface pressure between the outer jacket and the insulating layer. This surface pressure is very important during the working temperature, as it is only by such pressure that delivery of the heat produced is ensured. This pressure, in conjunction with the close fit of the contraction joint, furthermore ensures that the output geometry of the heating body is maintained.

This is important, because in this way local cavities, often formed as a result of deficient contacting, or a non-symmetrical geometry created by the swaging process to which the element was subjected, can be prevented. Such non-symmetries and local cavities may in practice give rise to hot spots which adversely affect the loading level.

As there is also a contraction joint between the conductor resistance and the insulating layer placed around it, viz. a contraction joint which already exists at room temperature, the loading level may be increased.

This is, however, subject to the condition that even when cold there must be close contact between the conductor resistance and the insulating layer. It is therefore expedient to bring about a light contraction joint between the conductor resistance and the insulating material.

The contraction joint mentioned can most effectively be obtained by making a heating body according to the specified embodiments in such a way as to start with a tube of austenitic stainless steel, molybdenum, nickel, chromium, vanadium, rhenium, tungsten or alloys of these materials, after which this tube is rigidly shrunk around a cylinder of the electrical insulating material used, consisting of boron nitride or beryllium oxide. As this cylinder is a solid one, a contraction joint of this kind with a rigid contraction fit at about 250° C. can be executed without the risk that the boron nitride or beryllium oxide might show dislocations.

After subsequently cooling of the assembly described, the central part of the cylinder of the electrical insulating material used is removed by drilling, so that afterwards a cylindrical jacket of the said electrical insulating material is obtained which, by means of a contraction joint, is surrounded by a cylindrical jacket of austenitic stainless steel, molybdenum, nickel, chromium, vanadium, rhenium, tungsten or their alloys.

Next, after the inner side of the cylindrical jacket of the electrical insulating material used has been finished in a precise manner, this assembly of coaxial cylindrical jackets is shrunk at a temperature of 100° C. with a light contraction fit around a resistance conductor likewise composed of austenitic stainless steel, molybdenum, nickel, chromium, vanadium, rhenium, tungsten or an alloy thereof.

In this connection it should be noted that the high-loaded electrical heating elements known so far have at times failed in practice. This was generally caused by the fact that either local cavities were formed as a result of differences in thermal expansion or by chemical reactions, or because the thickness of the insulating layer was generally not uniform, or because the insulating material was not homogeneous in its characteristics. By applying the above-described method in the production of a heating body according to the invention, the drawbacks attaching to the heating elements known so far are surmounted, since production starts with a cylindrical tube, made of full material, of the electrical insulating material used. In addition there is the advantage arising from the possibility of obtaining the desired necessary surface pressures by a correct choice of the contraction dimension, to be effected with high precision.

The heating bodymade in this way proves capable of delivering a very high heat-flux, resulting in a heat-load up to about 500 watts/cm¹ at a wall temperature roundabout 600°C. -960° C.

By means of the heating body, liquid metals such as sodium, potassium, lithium or their alloys can be heated in a very suitable manner.

In particular, this heating body lends itself for heat-transmission experiment, with cooling by liquid sodium, to be carried out in a nuclear reactor or in an installation provided outside a nuclear reactor for simulation of the latter. This is due to the fact that the metal outer jacket of austenitic stainless steel, nickel, chromium, molybdenum, vanadium, rhenium, tungsten or their alloys is not in any way adversely attacked by the liquid metals.

In many cases the heating bodies according to the invention will have an external diameter of the order of about 6 mm or somewhat more. With this diameter it is possible to obtain a length of such a heating body of about 500 mm.

In the accompanying drawings,

FIGS. 1 and 2 are transverse sectional views of two different heating bodies constructed according to the invention;

FIG. 3 is a transverse sectional view of a third heating body which include a thermo-electric element.

In the accompanying figures, sections have been made over such a heating body.

Item 1 in FIG. 1 denotes the central resistance conductor, 2 is the insulating layer surrounding it and 3 the outer jacket, made of austenitic stainless steel, nickel, chromium, molybdenum, vanadium, rhenium, tungsten or an alloy of these materials.

The embodiment of FIG. 2 differs from the embodiment shown in FIG. 1 only in so far as the central resistance conductor 1 likewise possesses the form of a cylindrical jacket inside which a cylinder 4 is placed, in which other components (not illustrated) may possibly be included. There may, for instance, be channels in it for electrical conductors or for a fluid. Cylinder 4 may be made of a material having qualities suitable for this purpose.

FIG. 3 gives a construction which enters into account for the insertion of thermo-electric elements in the wall of the type of heating element described.

Thermo-electric elements of this particular kind may perform an important function, for instance, in heat-transmission experiments and in fissile lattice testing of nuclear reactors operating with liquid metals, when it is necessary to possess data regarding the wall temperature.

The range of temperatures in which this occurs is usually of the order of 600°C to 900° C. According to a method used up to the present the thermo-electric element 5 was fixed in a groove 8 in the wall of the jacket 3, after which the remaining space was filled with soldering material. A suitable soldering material is found on the market under the name of Coast Metal 52.” This soldering material contains 4.5 percent of silicon, which has a fusion-point lowering effect. The soldering temperature is 1,025° C.

The covering of such thermo-electric elements usually consists of a stainless steel or inconel jacket with a wall thickness of the order of approximately 0.03 mm. Now it has been found

3,665,598 that during the soldering process the jacket of the thermoelectric element has a tendency to pass into solution because of the silicon that is present. According to a further embodiment of the invention an additional tube 6 surrounding the thermo-electric element 5 is provided for the protection of 5 this thin-walled jacket. In this way the risk of breakage of the thermo-electric element is reduced to a minimum.

The materials which enter into account for making the tube 6 are: stainless steel, nickel, chromium and tantalum or an alloy of these materials. 10