US2231723A - Electric furnace - Google Patents

Description

Feb. 1 1, 1941. H,` JUNG TAL v 2,231,723

ELECTRIC FURNACE Filed Jan. 25, 1939 2 Sheets-Sheet l WITNESSES: INVENTOR Feb. 11, 1941.

H. JUNG ETAL ELECTRIC FURNACE Filed Jan. 25'. 1939 2 Sheets-Sheet. 2

@50M/719g; Scharon/sky.

ATTORNEY Petesied Feb. 11, 1941 ELECTRIC FUENACE 'nm .im ma Gnther schaam, nerim, allignorl to Westinghouse Electric e e Germany.

Manufacturing Company a corporation of Penney hat Pittsbnr h, Pa., ivania t xppiieeuea :man z5. 193s. seem No. mais In Germany January z5, 1938 14 Claims. (Cl. 13--20i This invention pertains to an electric furnace adapted to be electrically heated through the medium of heating elements or conductors ot the rare-metal oxide type such as, for example,

the oxides of the zirconium earths, ytterbium earths, and similar substances capable of re sisting high temperatures. Heating elements of the type referred to are usually non-conducting at ordinary temperatures, or even somewhat above, but have negative temperature coeii'icients so that at the higher temperatures they are satisfactory conductors, especially for furnace-heating elements, and because of their heat resisting characteristics are particularly adaptable for l5 high temperature furnaces.

Heretofore, in the use of such elements in furnaces.- it has been the practice to manually insert auxiliary heaters in the furnace for the purpose of raising the temperatures of the main heating elements of the rare-earth oxidetype until the last becomes sufliciently conducting to maintain by the heat generated therein their low conductivity, after which the auxiliaryy heaters could be removed and the heat for the furnace obtained from the main heating nements.

It is an important object of our invention to provide a construction for an electric furnace heated by rare-earth oxide heating elements which avoldsthe necessity of insertion and removal of the auxiliary heaters of the prior practice.

In accordance with our invention, we provide separate secondary chambers contiguous to the main heating chamber of the furnace but which are insulated to some degree therefrom. The

terminal ends of the main heating elements are disposed in the secondary chambers `together with auxiliary heaters which may be permanently 'fixed inA these chambers. In starting the fur- 40 nace, the auxiliary heaters heat the secondary chambers, thereby raising the temperature of the terminal ends of the main heating elements to render them sufficiently conducting tostart their heating function, whereupon the auxiliary heatt ers can be cut off, that is, the energy to the auxiliary heaters is removed at or about this point,

and we prefer to do this by automatic temperature responsive means. l

. 'I'he use of the main heating elements of the type to which this invention is directed requires l that suitable provisions be made to convey electi'ical4 energy thereta'i.A In accordance with our invention., we provide crucibles adapted to contain a metal which will'melt at temperatures ape proximating those at which the main heating elements become suiliciently conducting to pass heating current. The terminal ends of the main heating elements are turned downward into these crucibles and power leads also extend into the crucibles so that the metal in the crucibles acts 5 as conductors connecting the power leads to the main heating elements. As a precaution against damage to the terminal ends when the metal in the crucibles solidifles as, for example, when the furnace is shut down, we preferto provide a 10 raising and lowering means for the crucibles so that they can be lowered out of contact with the terminal ends when it is expected that the furnace will be shut down, and in starting the furnace they can be raised after the metal in them has becomel molten.

g This can be accomplished manually or automatically, but we prefer to use an automatic temperature responsive means.

It is a further object of our invention to pro- 20 vide protective means for limiting the current to the main heating elements so that they will notedraw excessive current by virtue of their rise in temperature and consequent lowering of resistance. This protective means takes the form 25 of a resistance with a` high positive temperature coefficient, as, for example, an iron wire, which is connected in series with the main heating element, this iron wire being shunted at the lower temperatures at which the main heating element 30 is non-conducting or only slightly conducting.

If desired, the protective resistances may be disposed in a secondary chamber whereby the heat generated in them is utilized.

As before said, the terminal ends of the main 35 heating elements are preferably in the secondary chamber, as are thecrucibles andthe auxiliary heating elements, this construction tending to provide the necessary preheating for, and conduction of current to, the main heating elements. 40 To facilitate the transfer of heat along the main heating elements from its terminal ends, we prefer to make these elements tubular and hollow so that the hollow therein'can act as a path for convection gas-currents whereby the lengths of 45 the elements in the main heating chamber are somewhat indirectly heated. A

Other novel features. objects and elements of our invention will be apparent from the following description thereof taken in connection with 50 the drawings. in which like reference numerals apply to like parts. and in which:

l'igurelisacmsssectionalviewofafurnace embodying our invention, shown diagrammatieeny, 'mi electrical controls therefor, 6l

Fig. 2 is a fragmentary sectional view, mainly of a secondary chamber of the furnace and showing a modification of the control, v Fig. 3 is a fragmentary sectional view of th 5 bottom of the furnace showing still another modification with automatic means for raising and lowering the crucibles of the secondary chambers.

Referring more particularly to Fig. 1, a furnace is indicated in its entirety by the reference numeral 2, and includes a top wall 4, a bottom wall 8, side walls 8 and I0 and the usual end walls, one or both of which may have openings for the insertion and removal of the charges to be heated in the furnace. As shown in the drawings, the side walls 8 and i0 are irregular and include outwardly projecting sections I2 and I4, respectively, and downwardly projecting portions |6 and I8, respectively, and which form, in conjunction with the bottom wall 6, a secondary chamber 20 at the left and a second secondary chamber 22 at the right of the furnace. The walls and portions thereof are constructed in any usual manner, as, for example, of non-metallic 25 refractory material defining a main heating chamber 24 and an outer layer of insulating fire brick or the like.

In order to separate the secondary chambers 20 and 22,from the main heating chamber par- .30 titions'of semi-insulating material, as, :for example, refractory-Vtile walls of relatively small thickness, are built viush with the inner faces of the side walls 8 and i0. Thus the refractory wall 26 completes the enclosure of the secondary chamber 20, and the refractory wall 28 completes the enclosure of the secondary chamber 22, these walls 26 and 28 forming part of the enclosing walls defining the heating chamber 24, as more particularly .shown in Fig. 1.

The main heating elements, indicated in their entirety by the reference numeral 36, are tubular and each comprises a horizontal` portion' 38, extending throughv the heating chamber, and has downwardly-turned, terminal ends 40 in the 46 secondary chamber 20, and 42 in the secondary chamber 22. 'I'hese terminal ends are somewhat enlarged with respect to the portion 38, that is, the cross-sectional area of the terminals is greater than that of the horizontal portions so 50 that the resistance of the terminals is always less than that of the main heating portion 88 whereby the heating will be concentrated in the main heating chamber. In order to facilitate the removal and insertion of the heating elements, we

prefer to form the walls 26 and 28 with remov- A able tiles that are directly above the heating element which can be removed, if necessary, so that the heating elements can be taken out of the furnace through an additional removable section 44 in the wall portion I6. The manner in which such removable sections are formed and supported is not an important feature of our invention, since such expedients are common in the art. Manifestly, the walls l26 and 28 are oi.'v

The bases of the secondary chambers are formed with cavities in which are disposed crucibles 54 and 56 adapted to contain a metal 58 and 60, respectively, and into which the terminal ends of the main heating element 36 dip. The crucibles are sufficiently large so that currentconveying leads 62 for the crucibles 54, and 64 for the crucible 56 can also dip into the metal; these leads extending out of the furnace so that the electric power supply can be connected to 10 them.

In the operation of the structure thus far described, it is manifest that at the start of a heat run, current can be conveyed to the auxiliary heating units 50 and 52 to heat up the secondary 1; chambers 20 and 22 to, say, a temperature of about 1000 C. which is above the melting point of silver which may be contained in the crucibles 54 and 56. When the silver melts, exceptionally satisfactory contact is established between each 20 of the current-conveying leads and their assoelated terminal ends of the heating element.

The heat in the secondary chambers due to the auxiliary elements, accordingly, raises the A temperature of the terminalA endl. to a point 25 whereby the element 86 becomes somewhat conducting, this decrease in resistance being faci1itated by convection gas-currents passing within the hollow of the heating element 86. Once the furnace is started, the semi-insulating, and there- 30 fore somewhat heat conducting, walls 26 and 28 pass sufficient heat from the main heating chamber to maintain the secondary chambers 20 andv 22 at their proper temperature. In one specific embodiment of our invention, the secondary 35 chambers as aforesaid may be kept at a temperature of approximately 1000 C. while the main heating chamber may be at a temperature of 1800 C., or 2000* C., or even higher. l

Each heating unit element 86 `may be formed 40 of zirconium 'oxide tubes which have been impregnated with an aqueous solution of ytterbium nitrate and then fired, during which process the nitrate is converted to ytterbium oxide. This impregnation and firing is repeated until the tube 45 attains the desired amount of both oxides. We can employ a tube which is circular in cross section having an internal diameter of 6 millimeters and an external diameter of 12 millimeters, approximately, and a. length between wall sec- 50 tions 26 and 28 from about 100 to 150 millimeters, or even more and obviously considerably less.

, The manner in which the tubes are disposed in the heating chamber is primarily one of design, and for practical application the tubes may be 5 spaced between intervening piers 68 upon which a charge plate or a charge may be supported.

In Fig. 3, we show wherein the base of the secondary chamber is formed with an extended cavity 1n whose depth is about twice the height of G0' the crucible 12. The crucible has secured thereto a downwardly projecting rod 14 which extends through the'bottom wall 6 and out of the furnace through a packing gland 16, and terminates in a cere 1s.v within the crucible is a metal an, 65 preferably silver, into which a terminal end 82 Vand a current lead 84 are adapted to dip.

In the operation of the modification shown in Fig. 3, the crucible 12 can be assumed in its lowermost position with the silver 88 below the 70 .bottom ends of the terminal end 82 of the main heating element and the downwardly-turned end of thelead 84. When the temperature of the secondary chamber is raised suiiciently by the auxiliary heating means, such as a heating eleassigns ment 86, the metal in the crucible melts. The crucible can now be raised to complete the circuit between the lead 34 and terminal 62. When the furnace is expected to\be shut down, the crucible can be lowered vwhile the silver is still in molten condition so that when the latter hardens or solidies, it cannot damage either the terminal end 02 or the lead 64. n

In accordance with our invention, we prefer to control the furnace automatically, and to this end, with reference more particularly to Fig. 1,

we provide temperature responsive means in the form of a thermocouple 63 or the like extending into the heating chamber 24 and controlling a contactor 90 provided with two cooperating contacts 92 and 94. The contact 62 isof an extended length so that a circuit will be completed between it and the contactor 90 at, say, a tempera- 1 ture of the main heating chamber which assures that the temperature of the secondary chamber is about 1000 C. for this particular embodiment, the circuit being maintained thereafter because of the extended length of the contact 32. At a higher temperature of the main chamber 24, the contactor 90 engages the contact 04 to complete a second control circuit.

rlll'he heat responsive means just described comprises the controlling means for the energizing circuits of the main heating elements 36 and the auxiliary heating units 2l and 22. 'I'he main heating elements may be energized from a power source 96, one conductor 98 of which passes through a pole of the switch |02, through a normally closed contactor |04 offga relay |06 having an operating coil |06, through a conductor H0, and through a normally closed contactor 2 of a relay ||4 having an operating coil H6, from whence the current passes through a conductor ||6 to the supply lead 64. The current then passes through the molten metal 60, to the terminal end 42 of the heating element, and then through the heating element to its other terminal 40, from whence it continues through the molten metal 66 to the other supply lead 62. The current continues by way of the conductor |20, through an overload protective device |22, to a normally closed contactor |24, of the relay |06, through the other pole |26 of the switch |02, and back to the other side of the power source 96.

The auxiliary heating units may be connected, as shown, in series and derive their power from a source of electrical energy |23, the current passing through a. conductor |30, through a pole |32 of a double pole switch |34, through a normally closed contactor |36v of the relay II4, through the heating elements 50 and 52 in series, and back to the source of power |23 through normally closed contacts |36 of the relay ||4, and the other pole |40 of the switch |34.

The relays ||4 and |06 are controlledby the temperature responsve means, one side of eac'h being connected to one side of a power source |42 and the other side of each being connected to the contacts 92 and 94, respectively; the other end of the power source |42 obviously being connected to the contactor 30. The control and energizing circuits are completed by a positive temperature coeillcient resistor |44 connected across the conductors m and ns wnerebyit is snunted in a normally closed position of the contactor ||2, and is inserted in series with the energizing circuit for the main heating element whenever the contactor ||2 is raised, or operated, to circuit-opening position.

In starting the furnace, the switches |02 and |34 are closedcompleting their respective circuits, but inasmuch as the resistance of the heating elements 36 is exceptionally high practically no current will pass through the energizing circuit which may be indicated in its entirety by the reference numeral |46. However, the auxiliary heating elements 50 and 62 are preferably of the metallic Vresistor type such as, for example, an alloy containing nickel and chromium, so that upon closing of the switch |34, these heating units will be energized and thereby raise the temperature of the secondary chambers 20 and 22. As these secondary chambers become heated, the main heating elements 36 become more and more conducting until nally they become sufiiciently conducting to raise the temperature of the heating chamber 24. When the heating chamber-24 reaches the proper temperature, the contact 90 engages the contactor 92 to energize the relay ||4 whereupon the circuit to the auxiliary heating units 50 and 52 is opened at the contactors |36 and |38 and the protective resistance |44 is inserted in series in the energizing circuit |46 by virtue of the raising of the contactor ||2. This does not, however, interrupt the flow of heating current to the elements 36 which continue to convert the electrical energy into heat until the temperature of the heating chamber 24 has risen to a value sufiicient' to cause the contactor 90 to engage the contact 94, thereby completing the circuit to the relay. |06 which operates its contactors |04 and |24 to circuit-opening position. l

The operation of the contactors |04 and |24 obviously interrupts the circuit to the main heating elements, and this indicates that the heating chamber 24 has reached the desired temperature at which the heat-treating process in the furnace is to be carried out, it being understood that the contacts 92 and v94 are suitably adjusted to be engaged by the contactor 90 upon the proper values of temperatures.

Manifestly, the furnace will be maintained at `its operating temperature by the temperature responsive means 88 since when the temperature lowers to the point at which the contactor 90 and contact 94 disengage, the relay |06 will bev In the shut-down of the furnace, the reverse process of control obviously takes place. The control switches |02 .and |34 are opened, thereby cutting off al1 energy both to the main heating elements and the auxiliary heating units, and as the temperature drops in the heating chamber 24, the re1ayr|06 will first be 'deenergized, .and`

then the relay I4, whereupon starting conditions 'are reestablished awaiting only closing of the control switches |02 and |34.

In the modication of iFig. l, the energizing4 circuits for the heating elements and units are all vunder the control of the temperature within the main heating chamber 24 but it is obvious that separate temperature responsive devices may be employed to control the heating units and 02, and in Fig. 2 we show the modification wherein a temperature responsive device |48 is employed responsive to the temperature in a. secondary chamber |58. In this case. the contact 82, of Fig. l, is omitted and instead the temperature responsive means |48 controls the equivalent of the relay ||4. The relay H4 of Fig. 1 has its counterpart in Fig. 2 in the form of a relay ||4' connected to a contact |52 adapted to be engaged by a contactor |54 ofthe heat responsive means 48. 'Ihe relay Ill has an operating coil H6' and normally closed ccntactors |36', |38' and ||2 equivalent, respectively, to the contactors |36, |38 and ||2 of Fig. 1 and in the same circuits.

It is manifest then that the control circuit of Fig. 2 differs from that of Fig. 1, only in that the circuit for the auxiliary heating units in the secondary chambers is controlled by the temperature of the secondary chambers rather than by the temperature in the main heating chamber 24. The temperature responsive device |48 is necessarily operable to energize the relay H4' at temperatures approximately about 1000 C. for the specific embodiment described. It is not believed necessary to go into further detail in the operation of the modication of Fig. 2, since the operation of the control circuits and its correlation to the system of Fig. l is obvious to any one skilled in the art.

The lowering or raising of the crucibles shown in Fig. 3 can obviously ybe manually executed. However, we can also accomplish the same automatically by providing the relay III or H4' of the systems of Fig. 1 and Fig. 2, respectively, with additional normally-open contactors |56 and |58 which are adapted to be closed at the time the operating coil of the relay is energized to open the energizing circuit for the auxiliary heating units and to insert the protective resistance III. In this case, the contactors |56 and |58 `control energizing circuits for two electromagnetic coils |60 and |62 which upon energization attract the cores I8 to raise the crucibles so that they encompass the terminal and heater element ends 82 and 84. Inasmuch as the relays ||4 or H4' in eitherv modification of Figs. 1 .and 2 are energized when the temperature of the secondary chamber is -somewhat above the melting point of the liquid in the crucibles, it is manifest that crucible 'l2 will be maintained in lowermost position during the time the metal therein is being heated to a molten state and is only raised after that state has been reached. Fig. 3 also shows series-connected protective resistances |44 disposed in the secondary chambers. It is also manifest that the embodiment of Fig. 3 can be applied to the furnace of Fig. 1 with the controls of either Fig. 1 or Fig. 2 and is not deemed necessary to burden the description with these obvious modifications.

While we have described our invention in certain preferred forms and controls thereof, it is obvious that many changes may be made within v. the scope and spirit thereof, as, for example, the

chamber `and a. secondary chamber, electrical heating means for saidr furnace comprising an element which is substantially non-conductingat ordinary temperatures but has a negative resistance-temperature coeflicient rendering it conducting at higher temperatures, said element being disposed primarily in said heating chamber but havinga terminal end in said secondary chamber, heat-insulating means between said chambers, and auxiliary heating means in said secondary chamber.

2. Thestructure of claim l in which said element comprises a hollow tube.

3. The structure of claim 1 including means responsive to the temperature in one of said chambers for controlling the heat supplied by said auxiliary heating means.

4. An electric furnace having aV main heating chamber and a secondary chamber, electrical heating means for saidfurnace comprising an element having a negative resistance temperature coeilicient and which is substantially nonconducting at ordinary temperatures, said element being disposed primarily in. said heating chamber but with a terminal end in said secondary chamber, said secondary chamber being adapted to receive an auxiliary heating means for heating said end, means so insulating said secondary chamber from the heat of said main chamber that in operation of the furnace, the temperature of said secondary chamber is below that of the said main chamber but high enough to make said terminal end a conductor.

5. An electric furnace having a hea-ting chamber and two secondary chambers, each of the last being contiguous to said heating chamber, electrical heating means for said furnace coinprising an element disposed primarily in said heating chamber but having downwardly extending terminal ends in each of said secondary chambers, movable crucibles containing a meltable substance, one for each of said terminal ends, means for raising and lowering each of said crucibles, and means for conducting electrical power to said crucibles.

6. An electric furnace having a heating chamber and two secondary chambers, each of the'last being contiguous to said heating chamber, electrical heating means for said furnace comprising an element disposedv primarily in said heating chamber but having downwardly extending 'terminal ends in each of said secondary chambers, movable crucibles containing metal, one for each of said terminal ends, means for raising each of said crucibles to encompass its said associated terminal end when said metal is molten and lowering said Crucible out of contact with the last said terminal end before said molten metal solidies, and means for conducting electrical power to said crucibles.

7. A n electric furnace having a heating chamber and two secondary chambers,- each of the last being contiguous to saidl heating chamber, electrical heating means for said furnace comprising an element disposed partly in said heating chamber, but having terminal ends in said secondary chambers, said element being tubular, and

substantially vnon-conducting at ordinary tem- 9. The structure of claim 5 wherein said ele-` ment is tubular and hollow.

10. The structure of claim 6 wherein said element is tubular and hollow.

11. 'An electric furnace having a heating chamber and two secondary chambers, each of the last being contiguous to said heating chamber, electrical heating means for said furnace comprising an element disposed primarily in said heating chamber but having downwardly extending terminal ends in each of said secondary chambers, movable crucibles, one for each o! said terminal ends and containing metal, means for raising each of said crucibles to encompass its said associated terminal end when said metal is molten and lowering said Crucible out of contact with the last said terminal end before said molten metal solidies, electric power leads extending into said secondary chambers and having downwardly projecting portions in proximity to said terminal ends whereby said crucibles in raised position encompass the said projecting portions also.

12. An electric furnace having a heating chamber and two secondary chambers, each of the last being contiguous to said heating chamber, electrical heating means for said furnace comprising an element disposed partly in said heating chamber, but having terminal ends in said secondary chambers, said element being substantially non-conducting at ordinary temperature but having a negative resistance-temperature coefficient rendering it conducting at higher temperatures, means for supplying electric power to said terminals, said last means including a protective resistance in series with said element, and disposed in said vsecondary chambers, and temperature-responsive means for shunting said resistance at relatively lower temperatures and removing saidshunt at relatively higher temperatures.

i3. An electric furnace having a heating chamber and two secondary chambers, each of the last being contiguous to said heating chamber, electrical heating means for said furnace comprising a hollow, tubular element disposed partly in said heating chamber, but having terminal ends in said secondary chambers, said elementbeing substantially non-conducting at ordinary temperature but having a negative resistance-temperature coeiiicient rendering it conducting at higher temperatures, auxiliary heating means for said secondary chambers, means for supplying electrical power to said terminals, said last means including a protective resistance in series with said element, and means responsive to the temperature in one of said secondary chambers for shunting said resistance at relatively lower temperatures and removing said shunt at relatively higher temperatures, said last means also controlling the heat supplied by said auxiliary heating means.

14. An electric furnace having a heating chamber and a secondary chamber, contiguous to said heating chamber, electrical heating means for said furnace comprising an element having a portion disposed primarily in said heating chamber but having an enlarged downwardly extending terminal end in said secondary chamber, said element being of the refractory metallicoxide type which is a relatively poor conductor of electricity at ordinary temperatures with a negative resistance-temperature coefficient rendering it conducting at higher temperatures, a movable Crucible in said secondary chamber, said crucible containing a metal, such as silver, meltable at an intermediate temperature, means for raising into co-operative relation to said terminal end, and lowering said crucible out of such relation, and means for conducting electrical power to said metal in its molten state.

Hanns JUNG. GNTHER sCHARoWsKY.

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