US2036788A - Electric heating unit - Google Patents

Description

p 1935- c. c. ABBOTT 2,036,788

ELECTRIC HEATING UNIT Filed March 6, 1934 Tnvent'ov: Chcwies C. Abbott,

l His AttOTTNQg.

Patented Apr. 7, 1936 UNITED STATES PATENT OFFICE ELECTRIQ HEATING UNIT New York Application March 6, 1934, Serial No. 314,286

3 Claims.

This invention relates to electric heating units, more particularly to electric heating units of the sheathed type, and it has for its object the provision of an improved heating unit of this 5 character.

This invention has particular application to sheathed heating units of the type described and claimed in my United States Patent No. 1,367,341 dated February 1, 1921. As there described, this type of heating unit comprises a resistance conductor encased by a metallic sheath and embedded in and supported in spaced relation with respect to the walls 01 the sheath by a powdered heat refractory electrically insulating material,

5 such as magnesium oxide, compacted to a hard dense mass by reducing and elongating the sheath, as by rolling or swaging.

The metallic sheath of this heater is formed of a metal which is capable of being materially as reduced and elongated, such as steel, in order that the insulating material can be compacted into a mass sufliciently dense to readily conduct heat from the resistance conductor to the sheath.

Q5 In certain applications, it has been found that sheaths formed of steel and other like commercially practical metals capable of being Worked sufiiciently to compact the insulating material will corrode or deteriorate and after a time fail so completely. For example, it has been found that heaters having steel sheaths corrode and ultimately fail when used to heat water in certain applications, such as in the cooling systems of mercury arc rectifiers.

35 In this application, it has been discovered that pure nickel will not corrode or deteriorate asdoes the steel sheath, but it has been very difficult and practically impossible in certain cases to make an electric heating unit with a 40 pure nickel sheath because nickel cannot be worked mechanically so as to compact the insulating material sufficient-ly. When subjected to the necessary reducing operations, the nickel splits up and cracks open, thereby ruining the 45 heater.

In accordance with this invention, the heating unit is made with a sheath formed of steel, or some other mechanically strong metal capable of being worked sufiiciently to compact the in- 50 sulating material. After the sheath has been assembled with the resistance conductor and its insulating material and the sheath has been reduced and elongated in order to compact the insulating material, the sheath is slipped into 55 or encased by a second sheath formed of nickel.

(Cl. Mil-6'3) This second sheath is then reduced and elongated just enough to cause it to grip tightly the inner sheath so as to effect a good thermal and mechanical connection between the two sheaths.

For a more complete understanding of this invention, reference should be had to the accompanying drawing in which Fig. l is an elevation partly in section of an electric heating unit embodying this invention; and Fig. 2 is a secit tional view taken through the line 2-2 of Fig. l and looking in the direction of the arrows.

Referring to the drawing, this invention has been shown as applied to a sheathed heating unit of the type described and claimed in my ill above ment'oned Patent No. 1,367,341 intended to be used to heat water in the cooling system of a mercury arc rectifier. Briefly, this heater comprises a helical resistance conductor l6 encased by a metallic sheat it formed of any suitable material, such as steel. The helical resistance element ill is embedded in and supported in spaced relation with respect to the sheath ll by means of suitable heat refractory electrically insulating material l2, such as powdered magnesium oxide. The powdered magnesium oxide i2 is compacted to a hard dense mass so as to readily conduct heat from the resistance element ill to the sheath M by reducing and elongating the metallic sheath ll. This may be accomplished by swaging or rolling. The ends of the sinuous resistance element iii are electrically and mechanically connected to suitable terminal members l3, the left-hand terminal only being shown.

As pointed out previously, the steel metallic sheath M will deteriorate and corrode in certain water heating applications, such as in cooling systems for mercury arc rectifiers and the like. The heater has a comparatively short life when used in such applications.

In accordance with this invention, the sheath ii is covered by a second sheath H3, which is formed of any suitable metal which will not disintegrate or corrode in the presence of moisture and water, such as substantially pure nickel. Preferably, and as shown in Fig. 1, the sheath l4 will have substantially the same length as has the sheath ll.

The steel sheath ll is assembled with the nickel sheath i4 after the sheath H has been assembled with its heating unit and the magnesium oxide l2, and the latter has been compacted. Thus, in making the heater thus far described, the resistance conductor 10 is me- 5 chanically and electrically connected with its terminals II. This may be done conveniently in the manner described and claimed in my Patent No. 1,494,938, dated May 20, 1924. As there described, each terminal I3 is provided with a threaded portion l5 upon which the associated end turns of the helical resistance conductor are turned or screwed. The diameter of the helix is somewhat less than the diameter of the terminal at the roots of the thread so that the helix is extended somewhat when turned on. When the conductor subsequently contracts by reason 01' its resiliency on the threaded portion of the terminal, it will make a very good electrical and mechanical connection with the terminal.

Alter the conductor has been assembled with its terminals, it is threaded into the sheath II and is supported in a central position within the sheath by its terminals which are supported in any suitable manner, not illustrated. After this, the sheath II is loaded with the magnesium oxide l2 and then is reduced and elongated so as to firmly compact the oxide within the sheath, as described in my above-mentioned Patent No. 1,367,341. As pointed out previously, the sheath ll may be reduced and elongated in order to compact the oxide I! by rolling or swaging.

After this operation, the sheath II is fitted into or encased by the sheatlr I4. Preferably, the sheath l4 will have an internal diameter which will just permit the sheath II to he slid easily into it after the sheath II has been reduced and elongated to compact the oxide. Then the nickel sheath I4 is reduced and elongated just enough to cause it to tightly grip the inner sheath II. In a specific example,- the sheath H has an original internal diameter Just sufficient to permit the sheath II to be slid into it, and then its diameter is reduced by approximately .010 or .015 inch. This reduction will be sufiicient to cause the outer sheath H to grip the inner sheath ll sufiiciently tightly to effect a good thermal and mechanical connection between these members.

Arranged within the end portions of the inner sheath II are masses of electrically insulating sealing material l6, arranged to seal the heater against the pusage of fluids, such as moisture and gases, through the ends of the heater. While any suitable sealing material l6 may be used, it is preferable to use glass having a high electrical resistance at the operating temperature of the heater. In making the seals, glass tubes having substantially the diameter of inner sheath ll, after it has been reduced, will be threaded upon the terminals l3, as shown in Fig. 1, then will be heated substantially to the plastic state, and then while pressure is applied allowed to cool. The glass l6 thus treated will be caused to adhere to the terminals I3 and to the surrounding inner walls of the sheath H with fluid impervious joints.

After the seals have thus been formedat the ends of the heater, suitable electrically insulating washers I! are threaded on the terminals so as to cover the .sealing material It and the end walls of the sheaths H and I4. These washbut I prefer to anchor them to the terminals by providing nicks in the opposite sides of the terminals into which the metal of the connectors is forced or compressed. It will be understood that the members I8 will be formed of a suitable relatively soft material, such as brass, so that it will lend itself to the flattening or compressing operation. This method of securing the connectors to the terminals is described and claimed in my above-mentioned Patent No. 1,494,938.

Each lug or connector 18 is provided with a threaded portion "a upon'which are threaded several nuts 2|. These nuts serve to secure suitable leads 22 to the connectors. Preferably, cupshaped washers 23 will be interposed between the inner nuts 2| and the leads 22, as clearly shown in Fig. 1.

While I have shown a particular embodiment of my invention, it will be understood, of course. that I do not wish to be limited thereto since many modifications may be made, and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An electric heater comprising a metallic sheath capable of being materially reduced and elongated, a resistance element within said sheath, heat refractory electrically insulating material within said sheath embedding said resistance element and compacted to a hard dense mass by elongating and reducing said sheath, a second metallic sheath formed mainly of nickel covering said first named sheath and fitted tightly thereto in good thermal relation with it s by reducing and elongating said second sheath on said first sheath.

2. An electric heating unit comprising a steel sheath, powdered heat refractory electrically insulating material within said sheath compacted to a hard dense mass by reducing and elongating the sheath, a resistance conductor within said sheath embedded in said mass out of contact with said sheath, terminals connected to said resistance conductor projecting from said steel sheath, a second sheath formed substantially of pure nickel encasing said steel sheath and elongated and reduced in diameter sufilciently to give it a good mechanical and thermal relation with said steel sheath, and masses of electrically insulating sealing material within the end portions of said steel sheath surrounding said terminals and having fluid impervious joints with said terminals and the surrounding walls of said sheath.

3. An electric heater comprising a metallic sheath capable of being materially reduced and elongated, a resistance element within said sheath, heat refractory electrically insulating materialwithin said sheath embedding said resistance element and compacted to a hard dense mass by reducing and elongating said sheath, and a second sheath formed of a selected material, which is incapable of being worked mechanically to reduce and elongate it sufficiently to compact said insulating material, covering said first sheath and reduced and elongated sufficiently to cause it to firmly grip said first sheath, whereby a good mechanical and thermal relation is established between said sheaths.

CHARLES C. ABBO'I'I.

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