US1118387A - Electrical heating device. - Google Patents

Description

E. WEINTRAUB.

ELECTRICAL HEATING DEVICE. APPLICATION FILED OGT.1G. 1913.

1,1 18,387. Patented Nov. 24, 1914.

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Witnesses: Inventor:

His fitter-neg.

UNITED STATES PATENT curios.

EZECHIEL WEINTRAUB, OF LYNN, MASSACHUSETTS, ASSIGNOR T GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

ELECTRICAL HEATING DEVICE.

Specification of Letters Patent.

Patented Nov. 24.. 1914.;

Application filed October 16, 1913. Serial No. 795,559.

furnish a res stor of relatively high electrical resistance, which is cheap, rugged and not easily oxidized at a high temperature. 16 There are at present available for industrial uses, electrical resistance furnaces cap-= able of continuous operation in air at temperatures up to 900 C. For higher temperatures it is necessary to either resort to 20 the very expensive platinum for the resistor,

or to surround the heater by a protective atmosphere. For many purposes, particularly in foundries, a simple rugged furnace 0 erable in air at temperatures up to 1300 b. is desired, for example, for melting copper,

brass and other metals.

In accordance with my present invention, I have provided an electric furnace having a resistor operable continuously in a furnace in air at temperatures up to 1300 .C. and even higher without appreciable oxidation.

My novel resistance unit consists of a refractory core having a rising volt ampere characteristic, and a surrounding shell of silicon or silicon alloy, the latter as a rule having a negative temperature resistance co eficient and the two elements preferably being proportioned to give the resistor unit as a whole an increasing voltage drop with increasing current.

In the accompanying drawings, Figure 1 1 shows in perspective a resistance unit with terminals attached; Figs. 2 and 3 are Seetional views showing the relative size of the core and the outer shell of silicon, also the structure of the terminal and Fig. 4 illustrates in perspective a furnace containing the novel resistor.

Silicon has a high breakdown characteristic and a high negative temperature-coefficient of resistance. Therefore, a voltage high enough to break down the initially high resistance of the silicon is too high for heating a silicon resistor at incandescence. The 5 5 negative temperature coeflioient will cause the conductivity to progressively increase causlng so greata currentto flow as to destroythe resistor. This characteristic of silicon would necessitate either operating the silicon reslstor in series with'large external resistance, co thereby causing waste of energy, or .o perat mg the resistor from a transformer having two windings and shifting connections from; a h1gh voltage Winding to a low voltage Wllldll'lg' after the furnace has been heated up.

In my new form of resistor this high breakdown resistance is eliminated and a resister having preferably a rising volt-am pere characteristic is provided through the nstrumentality of the core which, being a good conductor, initially acts as a heater for the silicon and also prevents the undue rise of current with a rise of temperature. It is 'not absolutely necessary that'the composite 7E conductor have as a whole an increase in voltage drop with an increase in current over the whole range of operating tempera ture for even though it should have at a high temperature a slightly drooping char- 86 acteristic it will require only a very small series resistance inorder to prevent'an abnormal increase of current and dvenheating of the resistor.

In the preferred form of my invention as shownin Figs. 1 and 2, I cast silicon about a core of tungsten although graphite may be used as a core material with almost equal success. The relative size of the core and shell is governed not only by the resistance characteristic of the core but also by other factors such as the heat insulation afforded by the furnace structure. Ordinarily a com- .posite conductor consisting of a tungsten core 1 and a silicon shell 2 which has an 'out- @115 Side diameter of about will require a core about 1 g to in diameter. The silicon protects the tungsten graphite or the refractory conductor from oxidation as silicon at a high temperature becomes coated me with a. thin film of protective oxid.

The terminals constitute an important feature in a silicon resistor as the thin film of silica forming on its surface, particularly when cold, has a very high resistance. Mechanically fitting the terminals to the silicon, no matter how well done, will not prevent the formation of this film. I have found that it can be prevented by casting into intimate contact with the silicon a metal 18 vnonconducting material.

such as copper, aluminum, cupreous alloys or the like.

Qlaims on a terminal for a silicon resistor v Into this cup-shaped shell is cast the metal 1 l which'fiows into intimate contact with the silicon as well-as with the core. The shell 3 may either be left on the finished resistor or removed as desired.

lhe resistance rods thus produced may be inserted in slots or grooves upon the inner surface of a fire brick cylinder 5, the terminals projecting beyond the ends of the cylinder. The rods should be flush with the inner surface of the cylinder 5. Inside of the cylinder is fitted a lining 6'which may consist also of fire-brick or other refractory The'rods are all connected in series, for example, by copper straps or flexible strands 7 as indicated, current being supplied by the terminal conductors 8 and 9. What I claim as new and desire to secure by Letters Patent of the United States, is

1. An electric furnace resistor, comprising-a core and shell electrically connected in parallel in intimate thermal relation with one another, one of said elements consisting of silicon and the other of a refractory material having arising volt-ampere characteristic, said elements being proportioned to give the resistor anincreasing voltage drop with increasing current.

- 2, An electric furnace resistor comprising a core of tungsten and a shell of silicon in close contact therewith.

3. Anelectric furnace resistor comprising a core of tungsten and a shell of silicon in contact therewith, proportioned as to largely parallel therewith, said core being proportioned to substantially compensate the negative temperature coeihcient of the silicon.

" 5. An electric heater comprising a core of refract ry material having a rising voltampere characteristic and a layer of refractory material operable at a higher temperature in the air having a drooping voltampere characteristic surrounding said core and electrically in parallel therewith, said resistance elements 'being proportioned to have as a whole an increasing voltage drop with increasing current.

In witness whereof, I have hereunto set my hand this 9th day of October 1913.

EZECHIEL WEINTRAUB;

Witnesses:

JOHN A. MoMANUs, Jr., Rosana 8mm

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