US2125588A

US2125588A - Electric furnace

Info

Publication number: US2125588A
Application number: US25244A
Authority: US
Inventors: Raymond R Ridgway
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1935-06-06
Filing date: 1935-06-06
Publication date: 1938-08-02
Anticipated expiration: 1955-08-02

Description

Aug. 2, i938. R R. RHDGWAY ELECTRIC FURNACE Filed June 6, 1935 4 Sheets-Sheet l s E m if;

vw MN Aug. 2, w3- RR. RmGWAY 2,112.5;588

ELECTRIC FURNACE y Filed June 6, 1935 4 Sheets-Sheet 2 Au@ 29 w38o H. R. RHDGWAY ELECTRIC FURNACE Filed June 6, 1935 4 Sheets-Shes?I 4 w n, n M W Patented Aug. 2, 1938 UNITED STATES PATENT oFFlcE ELECTRIC FURNACE Application June 6, 1935, Serial No. 25,244

8 Claims.

'I'he invention relates to electric furnaces, particularly of the resistance type and, with regard to its more specific features, to an electric furnace having pressure molding apparatus.

One object of the invention is to provide a furnace which will operate continuously at temperatures up to 2500 deg. C. Another object of the invention is to provide a low voltage carbon resistor furnace which Will operate at a high power factor so that an electrical control of temperature can be maintained where the wattage absorption of the furnace is proportionate to the change of voltage and where the wattless component of the current is kept at a minimum. Another object of the invention is to provide a carbcn resistor tube furnace adapted for large power inputs manufactured of metallic parts yet having no appreciable power losses from magnetic hysteresis and eddy currents. Another object of the invention is to provide a furnace construction to receive an easily replaceable tube resistor. Another object i the invention is to provide a furnace construction adapted to insure long life to a tube resistor. Another object of the invention is to provide a gas-tight container for an loxidizable tube arranged so as to prevent destruction of the tube by reaction with air and at the same time providing for expansion and contraction of the tube which occurs during heat- 30mg and cooling of the furnace. Another object of the invention is to provide for the maintenance of continuous electrical contact and constant contact resistance with the resistor and at the same time permitting expansion and contraction of the tube freely in the container shell. Another object of the invention is to provide a high temperature furnace adapted for the maintenance of a long uniform temperature zone which is at the same time adapted to the application of mechanical pressure on the furnace contents and the simultaneous exact measurement of the temperature. Another object of the invention is to provide a furnace which will operate at a high degree of thermal efficiency while conforming to the requirements of the other objects listed above. Another object of the invention is to provide an improved furnace for carrying out the process disclosed in the copending application of Ridgway and Bailey, Serial No. 694,502 filed October 20, 1933. Another object of the invention is to provide combined pressure apparatus and furnace apparatus which may be easily and quickly manipulated for loading and unloading. Another object of the invention is to provide an electric '5J furnace having a sufficient seal. Another object of the invention is to provide a resistance type furnace in which adequate provision is made for cooling the members that support the resistance element. Another object of the invention is to provide a furnace construction of extremely low inductance as well as of low ohmic resistance. Another` object of the invention is to distribute the current evenly around the electrodes in order to attain uniform temperatures. Other objects will be in part obvious or in part pointed out hereinafter.

The invention accordingly consists in the features of construction, combinations of elements and arrangements of parts, as will be exemplified in the structure to be hereinafter described and the scope of the application of which will be indicated in the following claims.

1n the accompanying drawings, in which is shown a preferred embodiment of my invention,

Figure l is a view partly in axial section and partly in elevation of the furnace and pressure apparatus;

Figure 2 is a view partly in end elevation and partly in cross-section along the lines 2-2 of Figure 3;

Figure 3 is an axial' sectional view on an enlarged scale of one end of the furnace;

Figure iis a cross-sectional view taken on the line @-4 of Figure 3;

.Figure 5 is a cross-sectional view taken on the line 5--5 of Figure 3;

Figure 6 is a fragmentary enlarged sectional view on an axial plane of a gland ring and associated parts;

Figure 7 is a plan view of a pressure plunger showing its articulation to a screw shaft;

Figure 8 is a cross-sectional View taken on the line 0--8 of Figure l;

Figure 9 is'a detailed sectional view of the bus bars and their connection to the furnace;

Figure i0 is an enlarged front elevation of the central' portion of the furnace showing the sighting tube for an optical pyrometer and associated structure;

Figure l1 is a fragmentary axial-sectional view illustrating a manner of using the furnace apart from the pressure apparatus;

Figure 12 is an isometric view of the entire apparatus.

Similar reference characters refer to similar parts throughout the several views of the drawings.

Referring rst to Figures l and l2, I provide a pair of

standards

20, 20, as shown in Figure 1, which may be similar but oppositely oriented and Intersecting the

standards

24, 23 at the plane marked by the

lines

22, 22 on Figure 12 are generally U-shaped supports 23 comprising plane sheet metal ends 24. outside sheet metal plates 2l. and inside sheet metal plates' 23, the latter being bent into circular cylindrical segments, and either the plates 2l or 23 being bent around the tops oi' the U, and the entire structure constituting a support for a cylindrical furnace from which it cannot accidentally roll on'. This structure described is strong and rigid and of very low specific heat. The inside pair of sheet metal ends 24 may be integral with the corresponding sides of the pyramidal standards 23 if desired, and the various parts may be welded together, the entire frame structure being more clearly understandable from inspection of Figure 12 than is possible by way of verbal description.

Extending through the upper ends of the U-shaped supports 23 and also through the lower portion of the supports are three supporting tubes 21. 'Ihese tubes 21 lend further strength and rigidity to the supporting structure; they also constitute a mounting for thrust members for the pressure apparatus to be described. These thrust members preferably take the form of three tubes 28 which are slidable in the supporting tubes 21.

yReferring now to Figure 8, resting on the inside cylindrical plates 23 are chairs 33. 33 :in the form of inverted Us. There are a pair of these chairs 30 in each support 23, thus making four in all, and upon the inclined tops thereof I alx insulating and heat-resistant pads 3l.

Resting on top of the pads 3| are a pair of

aluminum cylinders

32 and 33 connected together by ilanges 34 and 3l and bolts 3l. An insulatlng ring 31 is interposed between the

flanges

34 and 35, and as better shown in Figure 9 insulating sleeves 38 are provided surrounding the bolts 36. As shown in Figures 1 and 12 a rectangular portion 39 is formed in the otherwise cylindrical wall of each

cylinder

32 and 33, and through the openings formed thereby access may be had to the interior of the cylinders as for the introduction of heat insulating material the nature of which will be presently described. Covers 40 normally cover these openings.

Referring now particularly to Figure 3, closing the otherwise open end of each cylinder are an nular end plates 4i. 'I'hese annular plates 4| may be made of aluminum or other suitable may terial and may be welded to the

cylinders

32 and 33 respectively. By the provision of aluminum cylinders partly closed by aluminum end plates, I have provided a non-magnetic material surrounding the central heating chamber of my furnace, and at the same time the material is electrically conductive and I use the

cylinders

32 and 33 for passing a heavy heating current into the furnace by a cylindrical path, the advantages of. which will presently be pointed out.

Referring now also to Figure 8, the annular plates 4I have integral cylindrical portions 43 geometrically projected from the inside bounding circles of the plates. The portions 43 conmamas stitute supports for the heating resistance element and the electrodes. Slidably mounted in the cylindrical extensions 43 are cylindrical rings 4l having radial inward extensions 43. One of the features of the invention is the provision of means permitting the expansion and contraction of the heating tube hereinafter described without fracture thereof. Another feature of the invention is the accomplishment of the foregoing with a gas-tight seal. I provide gland rings 41 which support the tube hereinafter referred to and also constitute the seal, expansion and contraction being taken care of by sliding of the rings 43 in the extensions 43. I have found, however, that .in order to prevent seizing of the rings 43 in the cylindrical extensions 43 it is desirable to water-cool the gland rings 41. Accordingly these rings are water-cooled in a manner which will be more fully pointed out hereinafter.

Referring now particularly to Figure 6, the construction of a gland ring 41 and its connection to a cylindrical ring 4l is therein disclosed. I provide a pair of insulating rings 43 which, because of the high temperatures encountered due to direct radiation from the heating tube, I prefer to make of mica, Transite or the like. The rings 43 are located on opposite sides of the radial inward extension 43 and bolts 43 extend through both rings 43, through the extension 44, and into the gland ring 41 securing these parts together, but the gland ring 41 being insulated from its support the cylindrical ring 43. I may also provide insulating tubes 49a of mica or the like surrounding the bolts 49. Thus each gland ring 41 is insulated from the

aluminum shells

32 and 33 respectively. Excepting as hereinafter noted the furnace is symmetrical and every part on the left is duplicated by a part on the right and this holds true up to the bus bars and to the thrust ing elements of the pressure apparatus. so I V shall now continue to describe the left-hand end of the furnace using the singular excepting where there are a number of identical parts at each end. Referring now to Figures 1 and 3, the furnace is a resistance type of furnace and the principal heating element thereof is a graphite tube 3l (there is only one such tube) which extends the length of the combined

cylinders

32 and 33 and projects slightly therebeyond at each end thereof. The graphite tube 50 is supported by the gland ring 41 which has radial inward extensions Bi defining an annular space in which is located an annular ridge 52 integral with the graphite tube 50, the lt being a loose one and this construction constituting a labyrinth seal,.which is filled with the material which illls the

cylinders

32 and 33. Thus this gland prevents entry of air into the furnace and at the same time supports the tube 50 by loose connection in order to avoid breakage due to the high heat employed and consequent expansion and warping of some of the parts.

The entire assembly constituting the tube l, the water-cooled gland ring 41, and the cylindrical member 45 with its extension 46 is free to move inside of the cylinder 43 as the graphite tube 50 expands under the influence of the heat generated therein, until the end of the ring 4l contacts with limiting stops 53 bolted to the annular plate 4i, as shown in Figure 3; the gap 54 is large enough to take care of all the expansion of the one-haii'. of the tube 50 that takes place in practice, while progressive creeping of the tube 33

electrodes

50, 50, 50 to each end of the graphite tube 50. The electrodes 50 are segmental and of identical shape. Each one subtends an arc of approximately 120 deg., and as better shown in Figure 4, each is provided at its two ends withabutting flanges 6i, there being six such flanges for the three segments collectively, and each set of three segments being flexibly connected together by bolts 62 passing through the flanges 5i and nuts 63 which engage springs 6I; thus the segmental electrodes 60 are maintained in firm contact with the tube 58 but when the diameter of the tube 58 increases on account of thermal expension, the electrodes expand with it despite the fact that they are water-cooled. The electrodes 80 are likewise formed with radial flanges 85,.each` one connecting a pair of anges 6I and displacing nearly one hundred and twenty degrees, and

the flanges 65 receive electric current from the4

cylinders

32 and 33, as will now be described.

Referring now to Figure 3, circumferentially around and upon the outside of each of the

cylinders

32 and 33, I solder copper plates 1l), and to these plates 1li I bolt cable terminals 1I. preferably of copper, by means of bolts 1.2. The terminals 1I are mounted on the ends of cables 13, the other ends of which extend into clamps 14 bolted to the flanges 65. The cabl-es 13 are preferably of copper stranded cable, which provides a path of extremely low resistance for the current, also being mechanically flexible, and the arrangement of these cables can be understood from reference to Figure 2 which shows twelve of them, four extending to each electrode 50.

Referring now to Figures 1, 8 and 9, I provide upwardly extending wings 15, integral with the

flanges

34 and 35 respectively and the insulating ring 31 has a similar upward extension 15, and to these wings I fasten bus bars 11 and 18. The bus bars 11 and 18 are each of them branching bus bars, and of slightly dierent shape and interlaced construction, as shown in Figure 1, and the form adopted insures that they and connecting leads or bars shall have low inductive reactance. As illustrated in Figure 9 in detail, copper plates 19 are soldered to the aluminum wings 15 and the bus bars 11 and 18 are held to these copper plates 19 by means of four bolts 8| surrounded by insulating tubes 82 so that the bus bars 11 and 18 are insulated from each other. Each bolt 8l is further provided with insulating washers 83 and a nut 84 to complete the insulation and to hold the parts together, this entire construction being clearly shown in Figures 8 and 9. The bus bars 11 and 18 preferably are made of copper, and I note that by providing copper plates soldered to the

aluminum cylinders

32 and 33 and extensions thereof, a very exc-client conducting path for the electric current is provided, much superior to a direct contact between aluminum and copper without the soldering, as aluminum quickly oxidizes and the lm of aluminum oxide found on the surfaces of exposed aluminum parts is not an extremely good conductor of electricity.

For molding boron carbide, in order to make articles thereof, I found that a temperature of around 2200 deg. C. had to be attained and therefore a reasonable requirement of the furnace was that it should be able to operate continuously at temperatures up to 2500 deg. C. So far as I am at present aware, there are few materials which will withstand such high temperatures and the outstanding class of substances is carbonaceous substances. I have found that electric furnace graphite of the brand known as Acheson's graphite .is the material which is most reliable and constant in its properties within the temperature range desired for the resistance tube. Graphite, however, is not strong, relatively speaking, and therefore I make the tube of substantial cross-section, particularly as it is subject to mechanical strains in the furnace. Graphite, however, has low electrical resistance when compacted into a tube or the like and therefore a mechanically strong resistor tube will have lowl electrical resistance if the furnace is of reasonable size. Therefore, in the case of a furnace large enough to mold fair sized articles, insomuch as there is a limit to the length of the furnace for mechanical reasons, the heating tube is of low resistance, and `in order to attain high temperatures and reach them in a practical length of time, a high power (energy input rate) on the resistor is required. Because of the high power and the low resistance of a practicable, resistor tube, high currents at low voltages are encountered. Since the R is so low the nductance L must be kept at a minimum value if high power factors are to result in the furnace.

This and allied objects ,of the invention are attained in the construction already described because the current paths in the

aluminum shells

32 and 33 are not localized and the current path may be thought of as a series of elements along the cylindrical shell, the current direction being instantaneously always opposite to that in the carbon resistor tube. It will be observed that this feature would be attained even though the bus bars were not located at a mid point of the furnace as disclosed. Insomuch as the heating tube 50 is as close to the aluminum shell as it may be consistent with heatl insulation requirements, the magnetic flux induced by the current in one of these parts does not give reactance to the flow of the current in the other part. Furthermore it will be seen that no continuous metallic current paths link the magnetic flux induced by the flow of the heavy currents.

Because of the radial distribution of the cables `13 around the circumference of the furnace, only a fractional part of the current links the flux induced in the space inside the cylindrical shells. tance is attained which is theoretically possible. Although the cables 13 form a slight loop in extending to the water-cooled electrodes 60, vthey are quite short and I have found by experimentation -that the slight increased voltage required for end-connections such as described is less than the losses incurred in having an actual sliding electrical contact in the position of the gland ring 41.

Although I prefer to make the

cylinders

32 and 33 of aluminum because it provides good electrical conductivity and mechanical strength with low weight and low specific heat and absence of magnetic effects, nevertheless it should be noted that because the geometry of the furnace is practically noninductive these cylinders might be made even of steel or iron without large losses. However, as stated, aluminum shells are preferred.

Aand thus the furnace utilizes a very high per- Thus substantially the minimum induc-` centage of the kva. input in heating the resistance element 50. So far as operation of the furnace is concerned, the current might be direct current, but in commercial practice alternating current is the one chiefly met with, and for high power installations at the lowvoltages it is the only one commercially practical, and therefore the elimination of inductive effects is of great importance from a commercial standpoint as even in the case of frequencies as low as 25 cycles per second, with the amount of current which I found it desirable to use, inductive reactance is a factor of prime importance and the avoidance thereof measureably increases the efficiency of the furnace, and the accuracy of temperature control.

Considering now the cooling of the electrodes and the gland rings, and referring first to Figure 1, I provide a water connection 9i) in the form of a union having a valve 9| which is connected by piping 92 to a T-union 93 where the Water branches, by way of feed pipes 94 extending to each end of the furnace. At the exhaust end, I provide a union 96 having a valve 91 which receives water from a pipe 98 extending from a T-union 99 that receives water from pipes |00 'extending to each end of the furnace. By means of the valves 9| and 91, the flow of Water may be controlled, and by restricting the flow by the valve 91, the cooling chambers may be maintained full o'f water.' Distribution and return of cooling Water is the same at each end of the furnace.

Referring now to Figure 3, it will be seen that the pipe `94 extends upwardly beyond the axis of the tube 50 and there has a U-bend. Referring now to Figure 2, the delivery end of the pipe 94 is therein shown and it is connected to a T- union |02 with branching extensions |03 and |04, the former leading up and the latter leading down. The piping is distributed around a circle which includes the electrodes described, and still referring to Figure 2, at the top of the circle the extension |03 extends away from the plane of this view along a line parallel to the axis of the furnace and branches into feed pipes |05 and |06.

'This connection is fragmentally shown in Figure 3, and as therein shown, the feed pipe |05 extends downwardly and into one of the three electrodes 60, being the left-hand upper electrode as they are viewedin Figure 2. Referring now to Figure 2, the feed pipe |06 extends downwardly into the right-hand upper electrode 60. Water passages are formed in all three electrodes, a passage |01a in the left-hand upper electrode extending downwardly through the radial fiange 65 and into the base of the electrode G0, then along its entire length and outwardly. This passage |01a may be formed by milling radial slots in the electrode at opposite ends of its deg. contour, then milling an arcuate slot connecting these radial slots near the inner periphery of the segmental electrode, then closing the slots at the outside with welded segmental rings and bars.

The downwardly extending feed pipe |04 extends backwardly and then inwardly to deliver water to a passage |01c the delivery end of which is connected to a return pipe |2, thus cooling the lower electrode 60. The pipe |09 extends radially. then axially, then radially again, and finally into a segment coaxial with the furnace, and pipes ||0 and |2 are Joined by a union to this segment |09, and thus water flows both ways in the segment |09 to a T-union III which connects to the exhaust pipe |00.

At the T-union |02 is a third pipe Il 5 extending in the third dimension from pipes 04, |03 and |04, and this leads water to the gland ring 41 in a manner that can be better appreciated by reference to Figures 3, 5 and 6. Figure 5 shows the pipe ||5, and Figure 6 illustrates one end of it which ext'ends through to the annular space Ill in the ring 41. An exhaust pipe ||1, as shown in Figure 4, and also in Figure 3, leads water to the union H3, and thus to the exhaust pipe |00. This gland ring 1 is preferably a brass casting, and the passage l I5 may be made with a core.

Referring now to Figure 1, I provide at one end of the furnace movable pressure apparatus and at 'the other end thereof adjustable apparatus to take the thrust. Referring to the lefthand side of Figure l, the tubes 28 support a spider |20 having parallel bores through three bosses |2| on the ends of the three arms thereof, and having a central hub. |22 in which is a nut |23 secured thereto. Extending through the nut |23 is a screw shaft |24 on the left-hand end o! which is fastened a. hand wheel |25.

Referring now to Figure 3, the right-hand end of the screw shaft |24 is turned down, and fastened thereon by means of a pin I 26 is a collar 21 having a boss |28 as better shown in Figure '7,

Upon the threaded support |33 I mount a graphite pressure plunger |38, which has an internally threaded bore |31 for this purpose. The parts shown in Figure 7, with the exception of the screw shaft 24, are duplicated at the righthand end of the furnace, there being a second graphite pressure plunger |36 at the right-hand end of the furnace connected, however, to a piston rod |40. By reason of the articulation of the graphite plungers |36 as described, they may be swung through approximately deg. when the plungers are withdrawn from the furnace, and in order to facilitate this, desirably I provide a handle |4| fastened to the pin |32 at each end, each pin |32 being secured to the ears |3I as by means of cross-pins |42. To take the thrust on the screwshaft |24 I provide transverse pins |43 extending through the bosses |2| and the shafts 28. It will be seen that when the parts are in the position shown in Figure 7, the thrust upon the plungers |36 is taken by the plate |30 and transmitted to the collar 21 and thence to the screw shaft |24 at one end of the furnace, or to the piston rod |40 at the other end of the furnace. For loading the furnace, the plunger |30 may be rapidly withdrawn by means of the hand wheel |25 at the left-hand end of the machine, or by the pneumatically actuated apparatus at the right-hand end of the machine, and by the provision for swinging the plungers |36 through 180 deg. I am' enabled to make the entire apparatus more compact yet allowing ready access to the inside of the resistance tube 50.

Considering now the pneumatic pressure apparatus by which high pressures may be exerted upon a substance to be molded under heat and pressure in the resistance tube 50, this apparatus may be supported upon the tubular shafts 23 at the right-hand side of the furnace as shown in Figure i.

As in the copending application previously referred to, an important use for the furnace is to mold powdered boron carbide (B4G) under heat and pressure thereby forming a boron carbide article of great density, strength, and uniformity of crystalline structure. In the table below I give a typical example of instantaneous values of electrical quantities for a furnace constructed in accordance with the invention.

In order to conserve the heat generated in the furnace and in order to prevent the apparatus from becoming overheated, I ll the cylinders' 32 and 33 with an .inert material, and inl order to avoid oxidation or other chemical aotivity this inert material should be, as nearly as possible, of the same substance as the resistance element 50. Graphite being the substance which I prefer to use for the resistance element, I have found that powdered carbon, such as lamp black or the like is the substance which I prefer to use to confine the heat in the resistance element 50. Therefore I -ll the

cylinders

32 and 33 with powdered carbon, and this seeps into the space between the resistance element 50 and the ridges 52 thereof and tl gland ring 41, and whatever oxidation takes plates, takes place only at the opening of this gland. Any slight amount of air in the powdered carbon is converted into carbon monoxide or carbon dioxide and the reaction goes no further. I may charge the cylinders with carbon through the covers 40, and in order to facilitate removal of all carbon therefrom I have provided screw threaded plugs |15 in the bottom of the cylin-

V ders

32 and 33.

It is one of the features of this invention that carbon black may be used without electrical leakage at the low voltage practically attained through the specic non-inductive features, as the sole insulating material loosely packed inside of the cylinders. This carbon black provides heat insulation of great efficiency which is non-conducting at the voltages used. It is well known that at temperatures as high as the operating temperature of this furnace, heat losses by radiation are enormous. Carbon black acts as a radiation screen and at the same time prevents conduction and convection. Any gas leaks in the joints of the furnace permitting oxygen in the air to enter the compartment will be reacted upon by the finely divided carbonaceous filling material to absorb the oxygen by combining with it. Furthermore, in case carbon monoxide is formed and explodes, the covers 40 provide what is in the nature of a safety valve as they simply lift with the explosion which thus does no damage to the-apparatus.

In the operation of this furnace it is desirable that a check be made upon the heat developed Instantaneous values of electrical quantities 25 cycles 00 cycles vPower Am- Miinput res Volts cmhms l k'wpe z x er. z X RF.

startofrun 61.2 5,880 10.6 1.71 1.8 .332 .98a 1.94 .79s .912 At molding temperature 8.28 1,600 6.2 3.235 3.25 .332 .995 3.33 .798 .971

In the above table Z and X are given in miin the graphite tube 50. To this end, and recrohms while P. F. represents power factor. ferring now to Figures 8, 10 and 12, I provide The remarkable thing about the electrical data is the high power factor achieved in this furnace using such large currents at very low voltage. This is achieved by the features hereinbefore described.

an orifice |60 in the tube 50 which is in line with the tapered' bore |8| in a graphite tube |82 "Whose axis is perpendicular tothat of the tube 50 and which is supported, as better shown in Figure 8, at the inside end by a countersunk portion in the side of the tube l0 and at the outside end by means of a graphite plug |84 which is also countersunk to receive the end of the tube |82 as shown in Figure 8. The plug |84 has a bore |85 and is in turn supported by a countersunk portion in a cylindrical block having a bore |88 which is in alignment with the bores |0| and |86. The block |58 may be made of suitable electrical and heat insulating material. In Figure 8 I show the Athree bores |8|, |85 and |88 blocked by a plug which is inserted into a countersunk portion of the outside of the cylindrical block |88. |8| may be made of the same substance as the block |88 and may be readily removed. When it is removed an optical pyrometer may be used to determine the heat of the mold |10.

Referring now to Figures 10 and 12, it will be observed that the

flanges

34 and 35 merge into semi-cylindrical portions and |95 which support the block |88 as shown. The semi-cylindrical portions |95 and |85 do not contact each other but are separated by the insulating ring l1.

Referring now to Figure l1, I show a modification of my furnace in which any article may be fused or otherwise heat treated but not under pressure. In this embodiment of the invention the furnace is of the same construction as that already described, and the piston and cylinder unit as well as the screw shaft |24 and operating mechanism may be used for the purpose of readily opening and closing the furnace, or lsimplified mechanism may be substituted therefor. In place of the graphite plungers |36 I provide graphite plugs 200 which are hollow the greater part of their length as shown in Figure 11, and have the inner ends of their bores blocked bf; plugs 20|. The remainder of the interior of the plugs 200 may be filled with lamp black 202 or other form of carbon or the like. By this con'- struction I provide effective heat insulation for the furnace at low expense. In the center of this furnace I have shown a crucible 205 representative of a container for an article or substance being fused under heat but not under pressure.

It is a feature of the invention that the total water-cooled area in contact with the graphite tube is maintained at alminimum. 'I'his is attained by the combination of the narrow sliding gland 41 and the narrow electrodes 50. If the electrical contacts were allowed to slide to take care of expansion and contraction, much larger areas would have to be provided and greater heat losses incurred.

There are definite practical limitations in the manufacure of graphite resistors of great length in proportion to their cross-sectional area. Since it is desirable in a furnace of the type described to have the temperature gradient fall continuously from the point of highest temperature, which is in the center of the furnace, to the ends wherethe water-cooled electrodes are attached, a long zone of constant temperature in the center of the furnace requires a tube which is relatively llong in proportion to its cross-sectional area.

'I'he amount of the tube wasted on the ends for the electrodes and for the glands should therefore be kept to the minimum, and by the provision of narrow glands and electrodes a substantial saving and improvement have been effected.

Considering now the operation of my furnace, having selected a suitable graphite mold |10, and plugged up one end thereof with a graphite plunger |12, I fill the mold from the other end This plug then insert the other plunger |12. At this time the plunger |35 is withdrawn from the furnace and is away from the opening of the graphite resistance tube 50. being swung back on its fulcrum |32. The piston |45 is to the right so that the other plunger |35 is out of the tube 50 and it may also be swung out of the way. It may be desirable to cold-pressthe plungers |12 against the boron carbide in the mold |10 with a slight pressure to facilitate handling. Whether this is done or not, the entire unit may be readily lntroduced into the graphite tube 50 and placed approximately in the middle thereof, as by means of any long rod. Graphite blocks |13 are then inserted in the furnace and moved up against the plungers |12. The plunger |38 is now lowered and by spinning the hand wheel |25 it is caused to enter the furnace.

By careful control of the handle |61, after lowering the other plunger |35 to horizontal position, this plunger may be now introduced into the furnace. At this time the parts may be carefully watched so that the plunger |35 is moved just far enough to eliminate open spaces between the various elements in the tube 50 without undue shock to the moving plunger. At this time it should be ascertained that the position of the two plungers |38 is approximately the same at both ends of the furnace, or if the positions of these plungers respectively are different, adjustment may be made by turning the hand wheel .|25. The valves 9| and 91 should be now opened to insure the ow'of cooling water, and after the valve 9| is opened wide the valve 81 should be closed slightly to squeeze out air blocks.

The current may be now applied and the air should be now turned on by means of the valves |6| and |53 until the reading on the gauge |58 is such as to give the desired pressure per square inch against the boron carbide |1| being molded. As heretofore stated, all the factors of heat, pressure, time, and the like may be varied, and there will be a corresponding variation in the final article. However, results can be duplicated and the furnace is quite universal for the production of many different types of articles. I contemplate that in many cases it will be desirable to attach a pointer to the piston rod |40 which may extend to a scale marked on one of the rods 2B, for example. Any suitable type of pointer or indicator may be used and it may be removable or adjustable or both.

An optical pyrometer may be used through the medium of the pyrometer tube described, in order that the operator may control the temperature as desired. In order to hasten the` cooling, a stream of water may be turned upon the outside of the

cylinders

32 and 33. The pressure may be 'reduced at an instants notice bymanipulation of the valve |6I.

The furnace described may be easily operated and controlled on account of the means described for moving the plungers |36 and swinging them out of the way. Furthermore, a very high heat may be generated as already indicated, and despite the high heat and pressure the graphite tube 50 is not destroyed for itsl expansion is taken care of at the gaps 54 and the pressure is confined to a single axis. In fact, it will be noted that no couple whatsoever is generated by the pressure means employed, there being no component of force in any direction other than a horizontal direction and the pressure as well as the thrust being along a single axis. Furthermore, by reason of the distribution of the rods 28 at equal distances from the axis of the tube 5D, all strains and forces are balanced and there is no pressure against the

cylinders

32 or 33 or any of the parts connected to the electrodes. Radial expansion of the graphite tube can take place Without fracturing any part due to the expansibility of the electrode structure. Furthermore, the flow oi current is cylindrical and radial thus substantially avoiding inductance effects. The insides of the

cylinders

32 and 33 may be readily reached at any time and it will be noted that the resistance tube, the heat insulating structure and the electric current carrying structure, as well as the water coolant connections, are freely suspended so as to be virtually independent, in a structural sense, from the pressure apparatus. This further reduces undesired strains and stresses and also'facilitates the dismantling of the furnace or the replacement of parts whenever desired.

It will thus be seen that there has been pro- ,vided by this invention an apparatus in which the various objects hereinbeforel mentioned are successfully achieved. As various possible ern bodiments may be made of the above invention and as many changes may be made in the ein bodiment above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

1. In an electric furnace, a casing, a tubular resistance element, a support for the resistance element permitting slight axial movement thereof relative to the casing. a multi-segment electrode, radial iianges projecting from the opposite ends of each of the segments, and spring means holding said flanges together whereby to hold the segments rmly against the tubular resistance element yet allowing the resistance element to expand when heated. 1

2. In apparatus as claimed in claim l, the combination with parts therein specified of pipes to convey cooling medium to each of said segments.

3. In an electric furnace, a graphite tube, a pair of cylinders surrounding said graphite tube, an insulating annulus separating said cylinders, conductors to convey current separately to said cylinders, annular discs closing the ends of said cylinders, supporting sealing glands around said graphite tube in the form of hollow annuli supported bysaid annular discs, separate annular electrodes clamped to said tube, and connections to lead current to the electrodes from said cylinders.

4. In an electric furnace, a graphite tube, a pair of cylinders surrounding said graphite tube, an insulating annulus separating said cylinders, conductors to convey current separately to said cylinders, annular discs closing the ends of said cylinders, supporting sealing glands around said graphite tube in the form of hollow annuli supported by said annular discs, hollow annular electrodes, means to convey cooling medium to each of the four hollow annuli, and means to "convey current from the cylinders 'to the electrodes.

5. In an electric furnace, a furnace casing, a support for said casing, pressure apparatus comprising a piston and cylinder unit and a thrust taking unit, and connecting means between said piston and cylinder unit and said thrust taking unit supported by said support but free to move horizontally independently thereof, whereby the casing is free from any of the pres-Il sure forces.

6. In an electric furnace, a symmetrical, approximately cylindrical metallic casing divided into two parts along a median line, a pair of interlaced parallel bus bars, one connected to each of said parts, annular insulation separating said parts, a central axial tube of resistance material in said casing, radial electric connecting means conveying current from the ends of the -respective parts of said casing, an electrode structure rigidly clamped to said tube and connected to said radial electric connecting means, and separate supporting means for said tube allowing sliding motion for expansion and contraction oi' said tube, the construction constituting a substantially non-inductive furnace having a high power factor by reason of non-inductiveness and rigidly clamped electrodes avoiding fracture of the tube by reason of the sliding action. y

7. In an electric furnace, a tube of fragile resistance material, a. furnace casing surrounding the tube, supporting means for the tube connected to the casing and supporting the tube rigidly in a radial direction but permitting movement in an axial direction, a sealing gland around said tube, a multi-section electrode clamped to said tube including resilient means permitting the expansion thereof, and conductors connected to the electrode to convey current thereto, the resiliently pressed multi-section electrode constitutlng a good electrical contact without danger of fracturing the tube and a sliding support permitting expansion and contraction without fracturing the tube yet sealing the inside of th furnace to prevent the escape of heat.

8. In apparatus of the class described, a cylinder and piston unit, a plurality of parallel rods. a spider connecting the cylinder to the plurality of parallel rods, a thrust taking member including a nut and a screw, a second spider connecting the nut to the plurality of parallel rods, a furnace casing located between the rods and a. support for the casing supporting also the parallel rods, the rods being free to move in the direction of their axes independently of the casing and the support, whereby the rods take the pressure o! the piston and the strain is not exerted on the casing.

RAYMOND vR. RIDGWAY.