US2744945A - Graphite furnace electrode for a stabilized arc - Google Patents

Description

y 1956 H. v. JOHNSON GRAPHITE FURNACE ELECTRODE FOR A STABILIZED ARC Filed March 11, 1955 INVENTOR HARRY V. JOHNSON ATT RNEY United States Patent" GRAPHITE FURNACE ELECTRODE FOR A STABILIZED ARC Harry V. Johnson, Niagara Falls, N. Y., assignor to Union Carbide and Carbon Corporation, a corporation of New York Application March 11, 1955, Serial No. 493,637

4 Claims. (Cl. 13--18) This invention relates to a graphite electrode for an arc furnace and has for an object to provide more .eflicient utilization of energy by an electrode that makes possible a greater average power input, is adapted to have a more steady arc, and one which will prolong the life of furnace refractories.

One conspicuous difference between the arc in a furnace having an electrode of amorphous carbon known as a carbon electrode and that of crystalline carbon known as a graphite electrode has been arc stability. The furnace electrode for a carbon arc becomes pointed and the arc is quite steady at the electrode tip. With graphite electrodes, which have largely and gradually superseded the carbon electrodes for arc furnaces during the last several decades, the arc moves around over a more blunt end of an electrode flashing from its lower side walls as well as and perhaps more often than from the bottom. With the usual alternating current are from a graphite electrode this condition of instability has been found to be accompanied by the are frequently not being struck during melt down of scrap in a steel furnace for several consecutive half cycles with the result the power input to the furnace is not as high as is desired. The radiant heat from an are having a lateral component of its length outside the side Wall surfaces of the electrode has a deleterious effect upon the furnace side walls and especially shortening the life of the furnace roof. Such roof is usually possessed of a shorter life expectancy than are the side walls.

One reason for the stability of the carbon arc is believed to be the low thermal conductivity and negative coefficient of electrical resistance in contrast with graphite which has a higher thermal conductivity and a positive coefiicient of electrical resistance above about 400 C. Thus when an arc is struck with a carbon electrode, the spot of arc origination becomes hotter and therefore more conducive to perpetuation of the discharge from the same spot. It is known that are discharge is more easily accomplished from a hot spot than from a cold one. The combination of the lowered path of electrical resistance, with low thermal conduction of heat from the place of origin of the are from a carbon electrode has assisted in the persistence of the hot spot, resulting in the are being struck in succeeding half cycles from the same spot. With a conventional graphite electrode the combination of higher thermal conductivity with a positive coefficient of electrical resistance results in rapid cooling of the spot from which an arc is struck and the arc of the next half cycle or more being struck from some other spot. Motion picture studies have shown that much of the time the graphite arc extends somewhat laterally from a lower side wall or rounded corner surface of the electrode. Arcs of this latter type are what are believed to affect the roof adversely by radiant heat. 1

According to this invention a steadier graphite arc has been produced. One factor contributing to this result is the use of a hollow graphite electrode. Since a large hole is expensive in elimination of conductive graphite 2,744,945 Patented May 8, 1956 during extrusion of the electrode, at first the problem was thought to involve an investigation of what mini mum size hole was adequate to provide a stable are. Research with a graphite electrode only 2 inches in diameter revealed that a longitudinal hole only A of an inch in diameter was capable of producing a stable arc but that a longitudinal hole only A: of an inch in diameter was incapable of holding the arc steady. The exact reasons for the difference are not fully understood. It is entirely possible that a quarter inch hole in any size of graphite electrode should be capable of stabilizing an arc. However, when it is recalled that a common 20 inch graphite electrode is 72 inches long and one 24 inches in diameter has a usual length of 96 inches, the difiiculty of drilling a small hole of that length may be appreciated when it is said a rigid small drill 6 to 8 feet and longer does not exist. The problem in one view becomes one in finding an appropriate size of hole which is large enough to stabilize the arc and be capable of being drilled commercially in a straight enough direction for the hole in one electrode, section to be reasonably aligned with a hole in a connected section.

By reason of arc stability the arc efiiciency in heating and power input is raised. When the discovery of arc stability was first made its commercial importance was not appreciated because the consumption of graphite was observed to be larger than was the case with a solid electrode of the same size.

Recently a further advantage has been discovered to reside in arc stability from a graphite electrode having at least its lower end provided with a longitudinal hole, and such is believed to reside in the fact that although the consumption of graphite is more rapid than is the case with the solid electrode, the heat and power input are enough larger than the increased consumption of graphite to make the stable arc graphite-electrode commercially worthwhile. Two causes are believed to contribute to such greater heating. One is that the walls at the lower end of a longitudinal hole cannot radiate heat away rapidly without the result they become the hottest part of the electrode, support the arc and become more conductive to an electron discharge or are formation. Such is believed to cause an arc to form earlier and last longer in each half cycle than is true of the solid electrode. This means more energy input. A second cause was found in a steel furnace where the are from a solid electrode was interrupted more frequently and for more successive half cycles with a solid than with a hollow electrode, as oscillograph studies showed and this was especially true during an early part of the melt down of scrap.

The usual connecting nipple presents further dilficulty because it has been regarded as the structurally weaker element in the modern furnace electrode. This nipple extends for approximately 10% to 15% of the total electrode length so that if an arc is to be stabilizedfor the entire electrode length the nipple must also be capa- One section- 10 of a graphite electrode for an arc furnace is provided with a longitudinal hole 11. Adjacent the lower end of the usual socket for a connecting nipple is an imperforate integral diaphragm 12 formed by taking care not to drill all the way through the entire electrode length. This diaphragm needs to be only thin enough to prevent convection currents of air from rising through the electrode and out its upper end causing too rapid an oxidation of the graphite. Its minimum thickness may be as small as one eighth or a quarter of an inch because when such diaphragm approaches the lower end of the electrode it must be capable of being eroded through before the arc reaches the nipple in order that the arc may be continually stabilized. The hole 11 gradually becomes larger as the are supporting end is approached. While the cross sectional shape of the lower end of the electrode is not always the same, that illustrated is perhaps typical. Adjacent the arc, investigation has shown the hole 13 to be enlarged due possibly to oxidation and other causes under the intense heat. Motion picture studies have shown the are not to wander as it did before over the lower outer side walls of the electrode and elsewhere. 1

As a result of this invention the precise reasons for greater are stability have been determined. The lower inner surface 14 near the bottom of the electrode is where the arc strikes and moves around these inner wall surfaces. Arc stability is due to inability of inner wall surfaces to radiate heat away as rapidly as do the outer wall surfaces with the result that these inner surfaces 14 become hotter and therefore better adapted for electron emission than any other electrode surface in spite of the superior thermal conductivity and positive coefiicient of electrical resistance of graphite as compared with amorphous carbon.

The size of the hole 11 when drilled depends upon the electrode length in practice. For an electrode 96 inches long a drill no smaller than A to 1 inch in diameter will be found desirable on account of inevitable fiexure in a smaller size drill functioning as a cantilever beam. The upper limit on drill size is largely economic and due to the high cost of a quantity of graphite being removed as well as the increase in electrical resistance of the electrode having a large hole. Common sizes of graphite electrodes cost several hundred dollars to produce. A factor affecting price is not only quantity of graphite but general bulk due to the long covered heating and cooling periods that such electrodes have to have in their manufacture. For such reason no consumer wants to pay for uneconomic bulk nor for graphite that cannot be consurned and utilized. Possibly a hole 3 inches in diameter may be an upper economic limit for the larger sizes of electrodes, as large as 40 inches in diameter and about 110 inches long.

The nipple 15 is of the usual graphite type and has a longitudinal hole 16 drilled nearly but not quite all the way through the entire length. A diaphragm 1'7 is located preferably at the upper end of the nipple. With the diaphragm visible the usual workman will arrange the nipple to have the diaphragm 17 at the upper end. However no serious inconvenience will be encountered if the diaphragms 12 and 17 are placed next to each other.

When there is a diaphragm 12 at an upper end of each electrode section, it is not necessary for the nipple to have a diaphragm 17. Usually an electrode in operation will include two sections connected by a nipple and in such event the controlling consideration is that somewhere there should always be a diaphragm to preclude any updraft and rapid oxidation of the bore. The upper electrode section 18 will be of the same construction as the lower one. Radial reservoirs 19 for pitch are preferably provided as described in the copendiug application of Johnson et al. Serial No. 461,714, filed October 12, 1954, for Pitch Cartridge for Electrode Joint, now Patent No. 2,735,705. A half inch hole 16 has been found satisfactory for many nipples.

Conservatively and roughly it may be said an arc stabilizing hole in an electrode has resulted in about 10% more graphite being consumed in a given time during operation of a steel furnace having a 2 to 4 inch hole in a 14 inch diameter electrode than did a solid graphite electrode of the same size, but effected about a 15% more heat and electrical energy input. This same hollow electrode required approximately minutes to melt down a charge of scrap as compared with minutes for a solid graphite electrode to melt down the same amount of scrap in the same furnace. An oscillograph showed approximately .93 as the ratio of actual to theoretical power input for this hollow electrode as compared with about .84 as the same ratio for the same mentioned solid electrode. Although this shortening of time may seem small, nevertheless when looked at through the eyes of those steel manufacturers running their are furnaces almost continually, a saving of 10 minutes per charge will soon amount to large and substantial savings when all costs of operation including wages and interest on the investment are added up and balanced against a small increase in cost of electrodes. A steel plant has reported a noticeable reduction in injury to the roof, which is believed to be due "olely to the are being stable and extending nearly sti at down as opposed to the unstable, wandering arc which radiated more energy upward causing the life of refractories in the roof of the furnace to be shortened.

The difference in power input between holes of 2 inches, 3 inches, and 4 inches in a graphite electrode was found to be comparatively small. Where a solid 14 inch graphite electrode delivered 4360 kilowatt hours per hour with a given electrical connection, the electrode with a hole 2 inches in diameter delivered 4790 kilowatt hours per hour for the same furnace, and the same size electrodes with 3 and 4 inch holes delivered 4830 and 4860 kilowatt hours per hour respectively in the same furnace for melting scrap.

Perhaps as great differences as were found were discovered to exist in open circuit cycles shown in an oscillograph record made during an early part of meltdown using a 2 inch hole in a 14 inch graphite electrode and a solid graphite electrode of the same size in the same furnace. During the first five minutes of meltdown 41 occasions were shown to exist when the circuit was open and only 16 such open circuit occasions occurred when the electrode had a 2 inch hole under substantially the same conditions in the same steel furnace. During a period from 5 until 30 minutes after starting meltdown, the same solid electrode was shown to have 33 noticeable periods of current interruption while the same hollow electrode had none. The oscillograph showed 20 such periods of interruption in cur rent during a period from 10 until 15 minutes after starting with a solid electrode and no such interruptions with the electrode with the 2 inch hole. During these entire 15 minute periods the electrode having the 2 inch hole showed 37% more power input than could be supplied through the solid electrode due to those periods of interruption.

During the entire period of melting down a charge of. steel scrap the solid 14 inch graphite electrode consumed 7 inches of its length, the electrode with the 2 inch hole consumed about 7.5 inches of its length in the shorter time required for a charge of the same size in the same furnace, the electrode having the 3 inch hole consumed about 8 inches of its length per charge and the electrode with the 4 inch hole about 8.5 inches per charge.

While the present electrode may be designated as hollow, it will be understood that it is only the lower portion of an electrode section or nipple that needs to be con tinually and such hollow portion is believed to be effective only when the diameter of'the hole has been enlarged at about the portion 13 and below and the temperature of the walls of the hole raised to a heat that will be more conducive to an electron discharge than will the outer walls of the electrode.

The present invention should not be confused with the disclosure of Speiden 1,007,151, dated October 31, 1911,

who proceeded on the erroneous assumption that carbon and graphite electrodes were equivalents. They may have been so in 1910 when that patent was filed and few if any graphite electrodes were used in arc furnaces in this country. Today the art eloquently testifies to the non-equivalence of carbon and graphite when a large preponderance of arc furnaces in steel mills use graphite electrodes because of their superior thermal and electrical characteristics. The graphite electrode is more conductive electrically and all portions are more thermally conductive than carbon. The hollow electrodes of Speiden in all but his Fig. 7 were unnecessarily ineflicient by reason of the non-uniform oxidation and erosion that will take place in his non-circular holes. He lacked any appreciation of the need for a hole of small size because that illustrated has a diameter of more than 50% of the electrode diameter. No claim is made herein to any hole broadly, nor to a large hole in an electrode of any size or shape. The impractical nature of Speidens suggestions are evidenced by the built-up nature of his graphite electrode. No such built-up graphite electrodes are known today. Speidens purpose set forth in lines 16 to 19 of page 1, namely to aflord a wider distribution of current to the bath fllan could be had from the same mass disposed in the usual round or rectangular shape, was founded upon error so far as graphite electrodes are concerned but upon truth as to carbon electrodes. The true state of affairs is that the arc moves around the electrode and furnace too much with a graphite electrode but is so steady with. a carbon electrode that it may need to be moved around the perimeter of a carbon electrode having a large opening of any shape of cross section.

Neither should this invention be confused with the disclosure of Chappell 2,603,669, July 15, 1952, for Large Electrode with Thermal Stress Relief. Outside slots 11 and 11a are believed to cause inefliciency in graphite consumption and do not prevent instability of the arc while the inside slots 12 and 12a are inefiicient causing oxidation and erosion for a considerable length of the electrode resulting in the hole 13a of Fig. 3 being brought to a shape near the arc in which a number of cusps exist in a cross section taken transversely as well as cusps being formed near the arc in a longitudinal cross section. Stated in another way a non-circular shape to a hole causes an inefiicient use of graphite because of erosion and oxidation in the lower part of the electrode. Only a hole which is initially substantially circular can efficiently stabilize an are from a graphite electrode. The slots 12a in Fig. 3 of Chappell prevent the hole 13a from being ever circular functionally. This Chappell patent 6 has the slots 12 and 12a 2% to 4% of the cross sectional area of that electrode and the hole 13 of a diameter which is 10% to of the electrode diameter. The invention claimed herein distinguishes both functionally as well as numerically and structurally from this non-equivalent Chappell suggestion.

Each of the terms oxidation and erosion is used herein to designate an eating away of the graphite from any cause.

I claim:

1. The combination with a pair of graphite arc furnace electrode sections and a threaded nipple connecting said sections, each of the sections and nipple being provided with a longitudinal hole of a size which is at least large enough to stabilize an arc and reduce the likelihood of its being formed on an outer lower side surface of an electrode section, and a graphite diaphragm in at least one of a nipple and electrode section, said diaphragm being of a thickness such that erosion will penetrate the diaphragm by at least the time it reaches the lower end of the electrode from which an arc is formed.

2. A combination according to claim 1 in which the longitudinal hole in the nipple is smaller than that in the electrode section connected by said nipple.

3. A combination according to claim 1 in which said diaphragm is in each electrode section and also in said nipple.

4. The combination of a pair of graphite arc furnace electrode sections and a threaded graphite nipple joining such sections, each of the electrode sections having therein a longitudinal hole large enough to stabilize an are being formed between said electrode and material in the furnace, and substantially reduce the presence of an arc on a lower outer side surface, said hole having a diameter of but a minor portion of the electrode diameter, substantially circular in cross section and contiguous to the electrode longitudinal axis and said nipple providing a diaphragm to preclude any updraft through said hole.

References Cited in the file of this patent UNITED STATES PATENTS 1,007,151 Speiden Oct. 31, 1911 1,091,559 Brown Mar. 31, 1914 1,115,027 Seabury Oct. 27, 1914 1,640,735 Soderberg Aug. 30, 1927 1,912,560 Wiles June 9, 1933 2,603,669 Chappell July 15, 1952

Claims (1)

1. 4. THE COMBINATION OF A PAIR OF GRAPHITE ARC FURNACE ELECTRODE SECTIONS AND A THREADED GRAPHITE NIPPLE JOINING SUCH SECTIONS, EACH OF THE ELECTRODE SECTION HAVING THEREIN A LONGITUDINAL HOLE LARGE ENOUGH TO STABILIZE AN ARC BEING FORMED BETWEEN SAID ELECTRODE AND MATERIAL IN THE FURNACE, AND SUBSTANTIALLY REDUCE THE PRESENCE OF AN ARC ON A LOWER OUTER SIDE SURFACE, SAID HOLE HAVING A DIAMETER OF BUT A MINOR PORTION OF THE ELECTRODE DIAMETER, SUBSTANTIALLY CIRCULAR IN CROSS SECTION AND CONTIGUOUS TO THE ELECTRODE LONGITUDINAL AXIS AND SAID NIPPLE PROVIDING A DIAPHRAGM TO PRECLUDE ANY UPDRAFT THROUGH SAID HOLE.

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