US2195705A - Electric heater - Google Patents

Description

Ap 2, 1940. J. D. MORGAN ELsc'mIc HEATER Filed Nov. 25, 1956 INVENTORV JOHN D. MORGAN ATTORNEY Patented Apr. 2, 1940 UNITED STATES 2,195,705 nmc'rmo naa'raa John D. Morgan, South Orange, N. 1., assignor to Power Patents Company, Hillside, N. 1., a corporation of Maine Application November 25, 1936, Serial No. 112,648

2 Claims.

This invention relates to electric resistance heaters, and'more particularly to metallic rod type electric heaters of several hundred watts capacity adapted for use in heating to high temperatures above 2100 F.

The primary object of the present invention is to provide a metallic rod type electric heater which shall be efficient and possess a long operating life even when used for heating to high temperatures above 2100 F.

The electric heaters which are now generally used for heating to high temperatures employ non-metallic resistors constructed of electrically conductant refractory materials such as carbon and silicon carbide. Such' heaters are complicated in design through having to employ watercooled metal terminals and voltage regulator attachments for varying the current supplied to the heater to hold its temperature constant and.

go to compensate for changes which take place in the conductivity of the resistor throughout its life period. Another object of the present invention is to provide a metallic rod electric heater adapted for high temperature heating which is simplified in design and has substantially uniform electrical conductivity and heating capacity throughout its entire life,

An important feature of the present invention is based on the discovery that metal resistors 80 which have a long life and possess high efliciency when operated at temperatures above 2100 F. should be constructed of a plurality of twisted fine strands of high temperature alloy wire, rather than of a single strand of wire of approximately the same cross-sectional dimensions.

Thus tests have shown that high temperature 8.1- loy wire (chromium-iron-aluminum-alloy) of No. 16 gauge (American wire gauge tables) becomes electrically non-conductive after being heated once or twice to temperatures in the neighborhood of 2200 F. and then cooled, whereas a wire of the same cross-section made up of a plurality of twisted strands of No. 22 gauge 45 chrome-iron-aluminum-wire can be heated repeatedly to temperatures as high as 2500 F. without any noticeable change in its electrical :onductivity.

With the foregoing and other objects and features in view, the invention resides in the 1m:- proved metallic rod type electric heater and process of making same, which is hereinafter described and more particularly defined by the accompanying claims:

In the following description of the invention,

reference will be made to the attached drawing, in which:

Fig. -1 is a view in elevation of a preferred form of low thermal conductivity refractory core for the heater which forms the subject of the 5 present invention;

Fig. 2 is a view in longitudinal section, with parts in elevation, showing a completely assembled heater embodying the core illustrated in Fig. 1;

Fig. 3 illustrates in longitudinal section (with a part in elevation) a modified design of heater with a core having both longitudinal and circumferential grooves.

Fig. 4 is a transverse section taken on the line Q 44 of Fig. 3; and

Fig. 5 illustrates a heater such as shown in Fig. 2, having adielectric glaze to prevent possible grounding under high voltages.

As shown in the drawing, the supporting ele- 2o ment of the electric heater which forms the subject of the present invention comprises a low thermal conductivity ceramic rod it having ends I2 and H of crossesection substantially larger than its mid portion i6, so that the mid portion 25 forms the equivalent of a circumferential annular groove 11 in the design illustrated in Fig. l. The mid-portion ii of the refractory supporting member constitutes the core for an annular heating unit l8 (see Fig. 2) which is built up on so the core in the groove l1 between the inner shoulders of the ends l2 and I 4, by embedding a high temperature alloy metal conductor 20 in a refractory shell 22 of strong highly heat conductant ceramic material. 35

As shown in Fig. 1, the enlarged ends l2 and H are each apertured at 24 and 26 respectively to accommodate weld metal terminals 28 and lead wires 30 for conducting electrical current to and awa;, from the metal resistor 20. The enlarged ends l2 and H of the low thermal conductivity supporting member Ill effectively shield the metal terminals 28 from the heater it to a degree which insures sufliciently cool terminals to withstand the temperatures developed in the zone of the heater, thereby eliminating the necessity of employing water cooling or making any other provision for terminal cooling facilities.

In the modification oi. the heater' illustrated in Figs. 3 and 4, the single circumferential groove H which is illustrated in Fig. 1 is replaced by a plurality of smaller circumferential and longitudinal grooves 32 and 34 respectively. encircling and paralleling the longitudinal axis of the member III. The heaters illustrated in Figs. i

and 4 possess the common feature of a low heat conductant core Ill having enlarged cool ends l2 and I4 for terminals, and having one or more grooves in the periphery intermediate the ends in which are mounted high temperature metal alloy resistor coils 20 embedded in and protected by ceramic shells 22 of high thermal conductivity.

The preferred method of assembling the heater illustrated will now be described.

The low thermal conductivity refractory supporting member is preferably constructed of a zircon aggregate consisting of approximately equal parts of natural grain zircon particles (through 80 mesh) and milled grain zircon particles (through 300 mesh). To this mixture of natural and milled grain zircon there is added about 1% by weight of alumina, 6% by weight of molasses,.and 6% by weight of the finished body of P205, after which the mixture is moistened to a creamy consistency, molded to the shape of the core I0, and slowly baked and finally fired to a temperature in the neighborhood of 2000 F. It will be understood that the principal function of the molasses is to give the article the right degree of adherence during the molding period, and that the constitutents of the molasses are burned out during the firing operation. Such a baked phosphoric acid bonded zircon core is relatively porous and light colored-almost white and possesses a very low thermal conductivity and great strength, as well as the proper color for radiating heat transmitted thereto from the superimposed resistor heater l8.

The high temperature alloy resistor 20 is preferably constructed of not less than four twisted strands of not to exceed No. 22 gauge chromiumiron-aluminum-alloy wire containing 30%-40% by weight of chomium and 5%-8% by weight of aluminum. It will be understood, however, that other high temperature alloy wires such as those of nickel and chromium may be used in making up the metal resistor and the lead wires 30 and terminals 28.

The high thermal conductivity ceramic shell 22 in which the metal resistor 20 is embedded, and which binds the resistor 20 and the supporting core l into a unitary structure, preferably comprises a phosphoric acid bonded zircon-ferrosilicon refractory of the type described in the copending application of John D. Morgan and Russell E. Lowe, Serial No. 87,662, filed June 27, 1936. This preferred zircon-ferrosilicon refractory is dark in color and possesses a thermal conductivity greater than is possessed by commercial silicon carbide. The preferred method of forming the shell consists briefly in mixing 40-50 parts by weight of milled grain zircon (through 300 mesh) with 50-60 parts by weight of 80 mesh ferrosilicon (75%-85% silicon), about 1% of powdered aluminum hydrate, and about 6% by weight of the mixture of 85% ortho-phosphoric acid, dampening the mixture to approximately the consistency of thick cream, slowly digesting the mixture after thorough agitation for a period of several hours to drive of! excess water, crushing the digested product to approximately 80 mesh size, moistening with water to a creamy consistency, and pouring into a mold around the mid portion of the core II to form the embedded metal wire heaters of the type illustrated in Figs. 2 and 4.

The purpose of the glaze which is illustrated in the heater of Fig. is to protect the heater against grounding or short circuitlng when it is supplied with high voltage current. The pre ferred type of glaze is an electrically non-conductant zircon (through 300 mesh) refractory coating bonded with 6%-10% phosphoric acid, then baked and fired to a temperature above 2000 F. This electrically non-conductant thin coating 36 (less than inch thickness) is applied to the outer circumference of the heater 22 as shown in Fig. 5, and the heater is preferably finished by the application of a further thin coating of black zircon-ferrosilicon-refractory 38 for the purpose of providing a complete unit which operates with good radiation.

A metal rod type heater of the class described having a 1500 watt capacity may for example embody a light colored zircon rod l0, ten inches long having enlargedends l2 and ll of one inchdiameter, and a heating zone or core l6 which is /2- /4 inch in diameter and eight inches in length. When the coil 20 is made up of ten twisted strands of No. 26 chrome-iron-aluminum wire embedded inch below the outer surface oi a highly heat conductant black zircon-ferrosilicon shell 22 (see Figs. 2 and 4) the resulting heater develops very rapid surface temperatures due to its refractory core, and can be operated for hundreds of hours through hundreds of repeated beatings and coolings in which temperatures are developed during heating periods in excess of 2400 F. at the outside surface. During such heating operations the metal terminals 28 are protected against overheating by the low heat conductant ends l2 and H, which shield the terminals from the heat developed by heater 22.

The invention having been thus described, what is claimed as new is:

1. An electric heater, comprising a refractory core of low heat conductivity, a metallic electric resistance heating element mounted on said refractory core, a heat conductant dielectric refractory shell embedding the element, and an outer glaze of black zircon-ferrosilicon refractory built up on the said shell.

2. An electric heater as described in claim 1 in which the metallic heating element comprises not less than four twisted strands of chromium-ironaluminum alloy wire, said wire being composed of from 30% to 40% chromium, 5% to 8% aluminum and iron.

1 JOHN D. MORGAN.

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