US1749762A - Tunnel kiln and method of operating the same - Google Patents

Description

Maurik'4 11,/- 1930. F. A. J, Frrz GERALD En" m, 1,749,762

TUNNEL KILN AND METHOD OF OPEATING THE SAME- 5 sheets-sheer 2 Filed June l() 0 1925 March 1l, 1930.

F. A. J. FWZ GERAL ET AL I TUNNEL KILN AND METHOD OF OPERATING THE -SAME Filed June lfh7 1925 5 Sheets-Sheet 3 March 1l, 1930.

F'. A. J. FWZ GEHEEL@ ET AL,

TUNNEL MLN AND METHOD DE OPERATING TEE SAME Eile@ Jun@ im, w25 Smeets-sheet 4 March 1l, 1930,

' F A. J. FETE GERALD E? AL ,749,762

TUNNEL KILN AND METHOD OF OPERATING THE SAME Filed Jun@ 1G 'i925 5 sheets-sheet 5 @gi 5y/4.,

l im

Patented Mar. 11, 1930 Legea UNITED srirrss PATENr OFFICE rRANcIs A. J. FITz GERALD, OE NIAGARA rA'Lris, NEW YORK, ANn JAMES KELLEHER, OF CHIPPAWA, ONTARIO, CANADA, AssIeNORs To HARPER ELECTRIC FURNACE CORPORATION, AA CORPORATION or NEW YORK' TUNNEL KIIIN IlEtlHClID F PEBATlNG THE SAME Appncatinnpied mi@ iai-leas. serial No. sense.

1o where the highest Atemperatures are required it is necessary to use a carbonaceous material such, for example, as carbon or graphite. Except in unusual cases, it is necessary to enclose the'carbonaceous resistor in a chamber or compartment, in order to protect it from oxidation. In a tunnel kiln, where the tunnel can be kept completely filled with a reducing or absolutely inert atmosphere, the carbonaceous resistor may be exposed,but inthe usual cases the atmosphere of the tunnel will be oxidizing and if. the resistor is exposed to such atmosphere it will rapidly deteriorate. A When the earbonaceous resistor is enclosed. in any manner, Whether by means of a'protective coating or by enclosure' in a compartment keptiilled with an inert or reducing gas, or maintained in a gas-tight condition toI prevent ingress of air, the thermo-efficiency of the furnace is correspondingly diminished 3o since heat radiated bythe resistor must be forced throughwalls of the enclosurebefore it .reaches the material passing through the tunnel for heat-treatment. The construction and maintenance of such a heating means involves 3 5 expense that is saved when the resistor is of such a nature that it may be allowed to radiate directly'to the material under treatment.

Let lit be assumed by way of illustration that an electric tunnel is used' for heat-treat- 40 ment of material which must be raised to a maximum temperature of 15,00o C. 'lo reach this temperature 1t 1s necessary to use a carbonaceous resistor. In accordance with. our

inventiomithe carhonaceous resistor is only {15 used at temperatures where its use is imperative. At the entrance end of the tunnel and. for a certain distance a metallic resistor is used. We find that the metallic resistor may be used to raise the material to a temperature of 900o to 1000o C. Beyond the metallic resister we employ a carbonaceous resistor which extends to that part ofthe kiln where tnetemperatureof the material reaches the desired point of 15000 C. We however do not conine ourselves to the combination of metallic and carbonaceous resistors, for we may use instead of the metallic resistor a resistor having a silicon carbide base, or the like, in which case the transition from' such resistor to the carbonaceous resistor may be at a point where the material under treatment reaches a temperature of 1200 or 1300o C.,.or We may use a combination consisting of a metallic resistor.

to a material temperature of 900 to 1000o C., a'silicon carbide resistor or the like to a material temperature or i200 tol300 C., and a carbonaceous resistor for higher temperatures.

@ther features of our invention comprise improvements in the resistor terminals and in the construction of thechamber enclosing the carbonaceous resistor, and an iinproved means for preventing .or minimizing the deterioration of the carhonaceous resistor when subjected to continneduse.

Numerous other objects and advantages ot our invention will appear from the following description,- taken in connection with the accompanying drawing, wherein Fig. l is a diagrammatic view of an electric -furnace, partly in section on the line l of Fig. 2, partly in elevation and partly broken away, showing one embodiment of our invention.

llig. 2 is a section taken on the line 2- of Fig. l.

.- lFig. 3 is a transverse section taken on the line 3-3 of Fig. l.

Fig. '4L is a detail elevation, partly in section, showing an electric terminal passing through the wall of the furnace and adapted to supply current to the heating resistor.

lllig. 5 is a detail elevation of a portion of the structure shown in Fig. 4 showing a slight modification.

llig. 6 is a sectional view taken on the line e-e er rig. s.

llig. Z is a view similar to Fig. 5 but showing a still. :further modiication.

. f Fig. 13 is a detail transverse section through the furnace showing an improved construction of the septum or partition which divides the chamber containing the carbonaceous resistor from the chamber which contains Vthe material to be heated.

Fig. 14 is a section ltaken on the line 14-14 of Fig. 13.

Figs. 15 and 1.6 are views similar to Fig. 14 but showing separate modifications of the septum construction.

Fig. 17 is a diagrammatic view showing an improved means for electrically connecting the heating resistor with the walls of its enclosing chamber so as'to prevent deterioration of the resistor when subject to continued use in the chamber.

Fig. 18 is a detail section taken on the line 18-18 of Fig. 17.

Fig. 19 is a View similar to Fig. 17 but showing a modification.

Fig. 20 is a view taken on the line .20-20 of Fi 19.

Re erring to the drawing, the walls 10 of the furnace form an interior space which is divided into a chamber 11 adapted to contain the material being treated, a chamber 12 containing the carbonaceous resistor 13, which chamber is closed from the chamber 11 by a septum 14, and a chamber 16 containing a metallic' resistor or the like 15, .which is adapted to radiate heat directly on the material being treated through an opening 20 forming a connection between the chambers 11 and 16.

The heat-producing element 13 is preferably a so-called graphitized carbon zigzag resistor formed by cutting a series of blindstaggered slits 3() transversely ofv a graphitized carbon slab. The resistor 13 is preferably supported in carbonaceous terminals 32, 33 projecting through the side wall of the furnace. The construction of these terminals will. be more particularly described here,- after. In the construction shown, the resistor 13 is supported between its terminal connections. A source of current 35 may be emplo ed to supply current to the terminals 32 an 33 for the purpose of heating the resistor 13. Y

The resistor chamber 12 is bounded by relai tively massive roof slabs or tiles 36, side slabs 37and floor plates forming the septum 14,

all supported by a series of silicon carbide uprights 40 and foot-blocks 41, leaving an interposed gap 43 in the chamber 11 connect? `ing with an enlarged runway 44 to receive a car or series of cars 45 mounted on wheels 4 6 and whose upper plates or platforms 47 are formed of silicon carbide. These cars are adapted to be moved through the chamber 11 in the direction of the arrow A, at the proper rate to permit the material or articles carried by the cars to receive the proper heat treatment. The silicon carbide septum 14 forms a heat-transferring medium between the chambers 12 and 11. Paralleling the outer surfaces of the chamber walls are wall gaps lled with sand 49 of silicon carbide. The chamber 16 may be lined similarly to the chamber 12, except as previously stated such chamber 16 opens directly into chamber 11. The outer roof of the furnace may be suitably formed to receive pulverized nonheat-conductive refractory material, and the entire furnace may then be further enveloped with material most effective to resist the outward iiow of low temperature heat.

The metallic or similar resistor 15 may be of any desired form and may be supported by terminals 50 and 51 extending through the wall of the furnace, which terminals are suitabl connected with a source of current 54 for heating the resistor 15.

In forming the silicon carbide lining for the chambers 11 and 12, as well as the septum 14, we preferably employ blocks of a silicon carbide composition which have been prepared from recrystallized silicon carbide. These blocks are then placed in por sition to form the linings and septum of thc furnace, and they are then electrically fired as a result of which they coalesce at their junctures. We may provide means such as a tube for supplying to the chamber 12, containing the carbonaceous resistor 13, an inert gas.

In Figs. 13 to 16 inclusive, we have shown several forms of septum construction designed to prevent leakage of air into the resistor chamber`or leakage of gas outwardly from the chamber. The septum shown in Figs. 13 and 14 consists of alower series of plates 62 and an upper series of plates -63 ranged that the 'unctures between successive plates of the lower series are in a staggered relationship with the junctures of the upper series. It will be apparent that if a joint between two of the plates 62-is not perfect, a plate 63 of the upper series which covers this joint will effectively compensate for this defect, and similarly the plates of the lower series will compensate for a defect ying upon the lower series and so arin a joint of the upper series and form an :amat/ea l 3 cooling they will contract. is the'plates rest on the Walls of the furnace, they are liable to stickpto the Walls with the result that openings in the joints will be formed', but since each joint is covered by a plate this opening, which is relatively small, will be effectively closed.

Fig. 15 represents a joint between two thick plates Where the saine principle is embodied. Plates 'and '66 are so made that when theyjare "butted together they have on their upper lsurface a recess slightly larger than the refactory insert G8. rlhis insert, Which is thers'ame length as the plates, is rubbed into place in the recess and a tight joint made with packing materal 69, Which is similar in composition to the plates 65, G6 and the insert 68.

When the distance to be spanned by the insert is great, a construction shovvn in Fig. 16 may be employed, Wherein the bottoms of the recesses formed by plates 65 and 66 are inclined downwardly toward the center and the insert plates 68 are formed Wit-lidovvn- Wardly tapered lower faces adapted to fit the bottoms of the recesses. rl`his construc-A tion is inefect a combination of the structures shown in Figs. 11i and 15. Each of the structures disclosed may be used in any one furnace, depending upon the mechanical necessities of the furnace, and combinations of the above may be used Without departingA from they spirit of the invention.

ln Figs. l to `12 inclusive, Wehave shown certain improved forms of electric terminals Which are adapted to support the heating resistor and to supply current to such resistor through the furnace Walls. rlhe constructions disclosed in these figures comprise the subject matter of a divisional application which has eventuated in U. S. Patent No.f1,657,671, granted January 31, 1928.

1n electric furnacesof the resistance type itis often desirable to use a terminal havingdifferent cross section and shape throughout its length. formed of Acheson graphite or some other forml of carbon. When the portion of the' terminal inside the furnace has to be larger, due to mechanical necessity, than actually 1 required for carrying current, the cost of making these terminals out of'large blocks ofgraphite becomes great. The heat loss is also greater through a large heavy terminal. 1n the forms of terminals shown in Figs. to 8, We have provided an improved construc- `tion whereby the terminals may be made of two or more differently shaped pieces, such pieces giving the desired mechanical design, good electrical properties, and permit-- .ting easy' Water-cooling facilities, which are generally necessary to prevent oxidation of the terminal. l `Referring to Fig. 4, the resistor 13 is adapted to be supported on aresistor-carrv- These terminals are oftenL ing block 75 of squared cross section, Which in turn is connected, both mechanically and electrically, to the cylindrical portion 76 of the terminal by means of a threaded tapered extension 77 formed on the cylindrical portion 76 and which iits into a tlireaded'recess in the resistor-carrying block 75. This joint, although .mechanically andelectrically good, is still a poor conductor of heat and serves to prevent considerable heat loss from the resistor 13. This joint may, if desired, be formed as shown in Figs. 5 and 6, wherein the cylindrical member 76 carries a tapered lug 8O adapted to be seatediua recess lin the resistor-carrying block, and the two portions held together with carbonaceous half-pins 81. ln case it is desired to form the terminal portion Whichpasses'through the furnace Wall of rectangular section, it may be formed as shown at in Figs. 7 and 8, and secured in the angular recess S3 by the half-pins 81.

1n order to prevent the outer portion of the terminal from burning, the coil of pipe 85 is Wound around the portion 7 6 as shown in 1, and lead or other easily cast metal is poured around it so as to make good heating contact between portion 76 and the coil 85. lllater enters this coil at 88 and leaves it at 89. Electrical connection'witli the portion 7 6 may be made at 90 by soine suitable means.

rlfhe lead surrounding the'coil 85 may have its outer surface cast to any shape suitable to be buil-t into the refractory briclrworlr'or the furnace. Ft will be seen how readily this form of construction maybe suited to lit different furnaces Vand the shapes employed may be altered rlvithout departing from the general scope of the invention.

lffthe cross section of the copper pipe 85 is made sutliciently large, vthe current required by the resistor 13 may be conducted through pipes 85 to the terminal portion 76. This permits -an even shorter terminal and also allovvs the Whole to be bedded solidly in the briclrivork and eliminates the necessity of the electrical connection 90.

in Figs. 9 to 12, We have shown another form of terminal which is provided with an improved means for securingthe resistor thereto and which is further provided vvith cooling means and means whereby an inert gas may be supplied to the resistor chamber through the body of the terminal. This terminal consists of' Acheson graphite or some other carbonaceous material and is provided near its inner 'end with an inner pocket or space 100 into which the end of the resistor 13 fr coil of pipe 102 designed to carry of the terminal Which-is subjected to the oxidizing eeets of gases or material below a vcut or moulded into the terminal and has an outlet 106 which, when the terminal is built Ainto the kiln, is inside the chamber in which the resistor is located and an inlet 107 opening outside of the kiln. This passage is employed to supply an inert gas or vapor to the resistor chamber. The passage 105 may, if

desired, be lined with some hard inert substance such as porcelain, glass or other material to prevnt erosion of the terminal by 'the rapidly moving gases.

As stated, the resistor 13 is seated in the recess or pocket 100 of the terminal, and in order to permit the placingand removing of this resistor in an easy manner we provide a removable plate 110 of the same material as the terminal, which is adapted to be seated in bevelled or dove tailed grooves formed in the side walls of the recess 100. As shown in Fig. 12, a wall 111 is left at one side of the recessl100.

When it is desired to place a new resistor in the terminal, plate 110 is removed, the resistor dropped into the recess 100, and the space between the end ofthe resistor and the terminal packed with some such material as graphite, so 'as to make good electrical contact. In placing this packing, the web or wall 111 is of great help and the web also helps toreenforce the terminal at that part of the recess. When the sides are packed, the plate 110 is slipped into position and the space 112, Fig. 12, 1s packed in a similar manner with graphite.

While we havedescribed the terminals disclosed in Figs. 4 to 12 as adapted for use in the furnace of the type'shown in Fig. 1, it will be apparent that s uch terminals are of general application and may be employed for a variety of purposes as Will suggest themselves to those skilled in the art.

In Figs. 17 to 20, we have shown an improved means whereby the life of the caronaceous resistor is greatly prolonged. It has been found that when a carbonaceous resistor is operated for a considerable length of time, deterioration occurs even though the resistor is fully protected from oxidation by being immersed in an inert atmosphere. The material of which the resistor is composed undergoes a change in` character after a certain amount of use and when such use is continued for-a sufficient length of timethe resistor fails by breaking, although no appreciable oxidation has taken place.' We have discovered that this deterioration of the resistor can be prevented by providing means for electrically connecting the radiating surface of the resistor with its surroun i ngs.

insulated from such walls, if one terminal of the resistor is electrically connected to the walls this deterioration is less marked, particularly toward the end of the resistor at which this electrical connection is made. If

the center of the resistor is electrically connected with the walls of the chamber, as by means of a carbon block in contact with both resistor and walls, the deterioration is less marked, particularly in those parts near the center of the resistor, and if all parts of the resistor throughout its length are electrically connected with the walls of the chamber the deterioration is effectively prevented.

Referring to Figs. 17 and 18, the resistor 13 is connected with the walls of the chamber 12 by a row of carbon rods 120 resting on supports 121 mounted on the bottom of the chamber 12. This construction, because of the high contact resistance between the rods themselves, between the rods and the resistor, and between the rods and the supports 121reduces to aminimum the leakage of heating current from the resistor to the Walls of the chamber.

In the form shown in Figs. 19 and 20, we employ two layers of rods 120 beneath the resistor 13.

While not shown in the drawings, it is preferable to place one or more layers of rods on the upper side of the resistor and completely cover it, the lower layer being sufficiently long to come in contact with the side walls of the resistor chamber.

While we do not desire to limit ourselves to any particular theory as to how the constructions shown in Figs. 17 to 20 prevent the deterioration of the resistor, it appears that such deterioration is due to an electric discharge from the resistor to the surrounding uwalls of the resistor chamber, and the structures disclosed. operate to eliminate or prevent this discharge, to some extent at least, by equalizing the potentials of the resistor and the walls of the chamber. It will be evident to those familiar with the constructionof electric furnaces that the method of protection described above is by no means the only one that can-be used, for many other arrangements can be devised without departing from the general principles of the inven tion. v

We claim:

1. In an electric kiln furnace, a chamber through whichmaterial to be heated to a. high temperature' is moved, a resistor exposed to the atmosphere of said chamber and adapted to heat such material to a temperature less than said high temperature, and an enclosed resistor adapted to heat said mate amarte,

rial from said second-named temperature to said high temperature.

2. 1n an electric kiln furnace, a chamber throughwhich material `to beheated to a high temperature is moved,a resistor exposedA to the'atmosphere of said chamber and adapted to heat such material to a vtemperature less than said high temperature', a resistor adapted to heat said material from said second-I named temperature to said'high temperature,

resistor adapted to. heat said material from said second-named temperature 'to said high temperature, a silicon carbide enclosure for said ysecond-named resistor, and means to forman inert atmosphere within said enclosure.

Il. In an electric kiln furnace, a chamber through which material to be heated to a high temperature is moved, a metallic resistor exposed to the'atmosphere of said chamber and adapted to heat such material to a temperature less than said high temperature, and an enclosed carbonaceous resistor adapted yto heat said material from said second-named temperature to said high temperature..

5. In an electric-kiln furnace, a chamber, means forl moving material to be heated to a high temperature through said chamber, a metallic resistor located above the path 'of said material and adapted to radiate heat -directly'to said material for the purpose of heating said material to a temperature below `said high temperature, a carbonaceous resistor located above the path of said material as it moves from under the influence of said first-named resistor, said carbonaceous resistor being adapted to raise the temperature l of said material to said high temperature, a

gas-tight chamber enclosing said carbonaceousresistor and formed with walls spaced fromsaid resistor, said walls being formed of dense, self-sustaining refractory material of high heat-conductivity.l

6. The method of heating material to aA high temperature which consists in heating such material to a lower temperature by direct radiation from an exposed metallic resistor and subsequently raising the temperature of said material to said high temperature by means of a carbonaceous resistor by radiation-and conduction through the walls of a chamber containing said resistor while maintaining said resistor in an inert atmosphere. K

47. An electric furnace having a chamber,

,fa carbonaceous resistor self-supported in said 'chamber and a connecting means between said resistor and the walls of said chamber of high electrical resistance and of low heat resistance. I l

8. in electric furnace having a chamber, a resistor' supported in said chamber spaced from the walls thereof, and a separately supported means within said chamber to equalize the potentials of said resistor and the walls of said chamber throughout substantially the length of said resistor without obstructing the iiow of heat from said resistor to the walls of said chamber.

9. .ein electric furnace having a chamber, a carbonaceous resistor supported in said chamber spaced from the walls thereof, and carbonaceous material separately supported within said chamber from said resistor forming a high resistance electrical connection betweensaid resistor and the walls' of said chamber without obstructing the How of heat from the resistor to the walls ofv said chamber. I

l0., An electric lfurnace having a chamber containing an' inert atmosphere, a carbonaceous resistor supported in said chamber spaced from the walls thereof, and carbonaceous material within said chamber separately supported from said resistor forming a high resistance electrical connection loetween said resistor and the walls of said chamber without obstructing the 'iiow of heat from the resistor to the walls of said chamber. i,

ll. An electric furnace having a chamber, a graphite resistor in said chamber, and a series of carbonaceous rods resting against said resistor and forming a high-resistance current path between said resistor' and the walls of said chamber.

l2. Ain electric furnace having a chamber, a carbonaceous resistor in said chamber, and means forming a high resistance electrical connection with a plurality of points along the length of said resistor and with the walls of said chamber without obstructing the flow of heat from the resistor to the walls of said chamber.

i3. An electric furnace having a chamber, a carbonaceous resistor in said chamber spaced from the walls thereof, and a plurality of carbonaceous rods resting in contact with substantially all portions of the eX- ternal surface ofsaid resistor and forming an electrical connection between said portions and the walls of said chamber.

le. A n electric resistor furnace having a chamber, a resistor formed of a carbonaceous block in said chamber, a terminal for said resistor extending through the wall of said chamber, said terminal being formed with a passage therethrough for supplying fluid to said chamber.

l5. An electric resistor furnace having a chamber, a resistor in said chamber, a terminal for said resistor formed of a carbonaceous block extending through the wall of said chamlr, szld blolkfbeing folrrned a ere roug or supp yin u1 topszlacamber, said assage having aglining 5 of hard, inert materia 16. A carbonaoeous resistor and a carbonaceous terminal therefor formed with a recein which one end of said resistor is seated, and an electrically contacting pulverized car- 10 bonaceous material between the end of said resistor an the walls of said recess.

FRANCIS A. J FITZ GERALD. JAMES KELLEHER. v

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