US2744152A - Electric furnace with carbonaceous atmosphere - Google Patents

Description

May 1, 1956 s. R. WHITE, .JR

ELECTRIC FURNAcE WITH cARBoNAcEoUs ATMOSPHERE Filed OCC. 28. 1954 Invenfor: Samuel R.Whi{e Jr. lay/.WMA Mn-@ His AHorney United States Patent 2,744,152 ELECTRIC FURNACE lll/HH CARBNACEUS ATMQSPHERE Samuel R. White, Jr., Baliston Spa, N. Y., assigner to General Electric (lomp-any, a corporation of New Yorlr Application October 2S, l'f'd, Serial No. fe'flfsf 6 Claims. (Cl. 13-2tl) My invention relates to electrical furnaces and in particular to furnaces of the controlled atmosphere type in Which the various heating and heat treatment processes are carried out with the charge or charges therein enveloped in an atmosphere of carbonaceous gases.

As is well lqnown in the art, carbonaceous atmospheres of compositions most frequently employed in applications of the aforementioned type generally contain a low percentage of carbon dioxide and a relatively high percentage of carbon monoxide. lt has been observed that in any carbonaceous atmosphere containing any appreciable amounts of carbon monoxide without oxygen being present, free carbon tends to drop out and build up on the heated metallic furnace parts within the furnace enclosure. This free carbon drop out reaction tends to be quite severe over the temperature range of operation of about 800 F. to l300 F., the most critical range being in the vicinity of 900 F. to 1100 F., with the carbon deposits being particularly objectionable on the current carrying elements and electrical terminal members within the furnace walls Where, in a very short time, these deposits become heavy enough to form short circuits between adjacent terminal members or closely positioned conductor members and between such members and the furnace casing.

Under normal operating conditions in temperature ranges normally employed in such furnaces, the electrical conductors and terminals extending through the furnace walls, pass through a temperature zone where the free carbon formation reaction is very severe, and it is therefore with respect to elements in this category in particular that the problem of free carbon deposits has been most pressing.

Such deposits generally do not tend to form on the furnace heating units and the highly heated parts withi the high tern erature zones of the furnace, since these elements are normally out of the temperature range where such deposits form at an objectionable rate. However, in applications where it may be desired to operate the heating elements in a furnace with a carbonaceous atmosphere within the temperature range of about 800 l?. to approximately l300 F., carbon deposits would build up on these elements at an obiectionably rapid rate thereby forming insulating layers on the heaters which adversely affect the heat transfer characteristics thereof.

The above reaction may be represented in chemical symbols as follows:

The problems created by carbon drop out in furnaces having carbonaccous atmospheres have existed fo` some time and various attempts have been made to alleviate the effects of such deposits, such as for instance by enclosing the electrical terminals, conductor elements, and heaters of such furnaces in protective housings, which housings were insulated from the conducting elements so that the carbon deposits could form on the housings without danger of short circuits. With such arrangements, however, the carbo-rx drop out was still not prevented and consequently provision had to be made for allowing removal of such deposits from the `protective housings at regular intervals in order to prevent excessive buildups from interfering with the heat transfer characteristics of the furnace and from mechanically hampering and imped- Mice ing operation thereof. In addition, the provision of such protective housings becomes rather diicult in certain configurations and adds to the manufacturing cost of the furnace.

ln view of the foregoing, it is, accordingly one object of my invention to provide means for greatly reducing the rate of carbon drop out on furnace parts in furnaces having carbonaceous atmospheres.

lt is another object of my invention to provide an improved electrical conductor for use in furnaces having carbonaceous atmospheres, which improved conductor is not subject to the aforementioned disadvantage of excessive carbon formation on the outer surfaces thereof.

l provide, in accordance with one aspect of my invention, means for greatly reducing carbon deposits on metallic surfaces within furnaces having carbonaceous atmospheres therein. l have discovered that certain metals, in particular those commonly employed and accepted as being the most suitable for use as electrical terminals and conductor elements from the standpoint of electrical resistance characteristics and cost and also those commonly employed in outer sheaths of sheathed heaters, tend to act as catalysts to the above reaction and that certain materials such as chromium, are non-catalytic to the carbon drop out reaction. In other words, materials most commonly employed and most suitable for use in heaters and electrical terminals and conductors tend to greatly accelerate the reaction causing carbon drop out whereas certain other metals, such as chromium do not have this effect so that carbon deposits on such metals form at such a low rate as to eliminate the problem.

lily invention therefore contemplates, in accordance with one aspect thereof, the utilization of an electrical conductor or terminal member having an outer surface which is non-catalytic to the above-mentioned carbon drop out reaction. embodiment of my invention illustrated herein, I prefer to employ, from the standpoint of providing the optimum in electrical characteristics as Well as satisfying cost considerations, a composite conductor element having a central conducting core formed of a material chosen from the standpoint of its electrical characteristics, with the outer non-catalytic surface being formed by a coating or plating of a non-catalytic material, such as chromium. It will be seen that my invention is particularly applicable to the current carrying conductors, terminals and heaters passing through the furnace walls and also to those elements within the furnace enclosure which operate Within the above mentioned temperature ranges, since the formation of carbon deposits on Xtures of this type has long been a serious and particularly pressing problem.

@ther objects and advantages of my invention will be apparent from the following detailed description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring to the drawing, Fig. l is a front cross sectional vicw of a furnace utilizing a carbonaceous atmosphere and illustrating one embodiment of my invention; Fig. 2 is a side cross sectional view of the furnace of Fig. l; while Figs. 3 and 4 are side and respectively end enlarged views of one of the terminals of the furnace of Figs. l and 2 illustrating the non-catalytic plating thereon.

Referring to Figs. l and 2, there is shown a furnace of the type commonly referred to as a box furnace which comprises an outer housing 1 and an access door 2 as shown in Fig. 2. The furnace housing 1 and door 2 are provided with a iirst lining of heat insulating material 3 and a second inner lining of a more refractory heat insulating material 4.

The charge is supported in the furnace on a hearth plate 5 and is heated by means of alloy ribbon resistors 6 runln accordance with the particular" assai es ning along opposite sides of the inner furnace walls. Provision is made for the introduction of a carbonaceous atmosphere into the furnace through an inlet pipe 7 which extends through the top of the furnace as shown in Fig. 1. The pressure of the furnace atmosphere is preferably maintained slightly above the atmospheric pressure outside the furnace so that any leakage will take place out of the furnace rather than into the furnace, which latter occurrence would result in contamination of the furnace atmosphere. A leakage vent 8 may be provided to allow initial purging of the furnace interior and to permit a slow leakage during operation thereby providing for a gradual and continuing replacement of spent gases. A thermocouple 9 may also be employed to sense the temperature of the furnace atmosphere and to actuate a suitable control arrangement for controlling the heating current to the heaters o.

VThe heaters 6 are connected to an electrical power source by means of suitable leads 10 connected to terminal members l1 which extend through stufling boxes f2 and into the interior of the furnace as shown. rThe terminal members il are electrically connected to opposite ends of the ribbon resistors 6 as shown in Fig. 2.

Furnaces of the general type thus far described are well known in the art and are employed to perform a wide variety of heating and heat treatment operations. As has been pointed out, in applications where carbonaceous atmospheres are employed, heavy carbon deposits tend to form on the metallic conductors and terminals within the furnace walls and also on those elements and fixtures operating in temperature zones within the range of approximately 800 F. to 1300 F.

1n carrying my invention into elfect in the particular embodiment set forth herein, I provide, as shown in Figs, 3 and 4, a non-catalytic outer surface formed of a plating 13 of chromium on those portions of the terminal members 11 which extend through the furnace walls into the interior of the furnace enclosure. As stated above, I have found that chromium is non-catalytic to the carbon drop out reaction and that by providing a chromium plating on the metallic lixtures in the furnace, such as the terminals lll, the problem of formation of free carbon deposits on these fixtures is, for all practical purposes, eliminated.

I have found that any thickness of plating may be employed with good results but I prefer to utilize a plating thickness in excess of .001 since coatings of lesser thicknesses are subject to damage in normal handling. l have obtained very satisfactory results with a chromium coating of about .005" applied in any well known manner on a standard high temperature conductor or terminal material such as cold rolled steel, in which case the rate at which the carbon drop out reaction occurred was reduced to a negligible value.

My invention may also be employed in connection with eliminating the problem of carbon deposits on heating elements, either of the exposed type or of the sheathed type, and in connection with various other fixtures forming a part of the permanent structural framework of the furnace interior.

It will be observed that one of the advantages of the particular embodiment of my invention illustrated and described herein resides in the fact that with such an arrangement, the conducting core material may be independently selected from the standpoint of providing the most desirable electrical characteristics taking into account the applicable cost considerations, without encountering the heretofore existing problem of excessive carbon deposits.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrically heated furnace comprising a heating enclosure for receiving articles to be treated, means for introducing into said enclosure a carbonaceous atmosphere containing carbon monoxide, heating means corn- .i prising electrical heating elements mounted within said enclosure, and conductor elements extending into said enclosure and connected to said heating elements, said conductor elements having an exterior surface exposed to said atmosphere formed of chromium whereby carbon drop out on said conductor elements from said carbonaceous atmosphere is substantially eliminated.

2. ln combination with an electric furnace having means 'introducing therein a carbonaceous atmosphere conig carbon monoxide, an electrical conductor ele* ment outer surface exposed to said atmosphere in said furnace, at least a portion of said outer surface being formed of chromium whereby carbon drop out on said conductor element from said carbonaceous atmosphere is substantially eliminated.

3. An electrically heated furnace comprising a heating enclosure for receiving articles to be treated, means for introducing into said enclosure a carbonaceous atmosphere containing carbon monoxide, heating means comprising electrical heating elements mounted within said enclosure, terminal means exterior of said furnace connectible to a power source for energizing said heating elements, and conductor means connected to said terminal means and extending into said enclosure from the exterior of said furnace and through a zone wherein the temperature of operations is to be Within a range wherein carbon deposits are likely to form on said conductor means at an objectionable rate, said conductor means having an outer surface formed of chromium at least within said zone whereby forming of said carbon deposits on said conductor means is substantially eliminated.

4. ln combination with an electric furnace having means for introducing therein a carbonaceous atmosphere containing carbon monoxide, a composite electrical conductor element comprising an inner core of electrical conducting material selected from the standpoint of providing the proper electrical characteristics, and an outer plating on said core of chromium for reducing the rate of carbon formation on said conductor element.

5. An electrically heated furnace comprising a heating enclosure for receiving articles to be treated, means for introducing into said enclosure a carbonaceous atmosphere containing carbon monoxide, heating means comprising electrical heating elements mounted within said enclosure, conductor elements extending into said enclosure and connected to said heating elements, and a coating of chromium over at least a portion of said conductor elements for reducing the rate of carbon deposit thereon.

6. An electrically heated furnace comprising a heating enclosure for receiving articles to be treated, means for introducing into said enclosure a carbonaceous atmosphere containing carbon monoxide, heating means cornprising electrical heating elements mounted within said enclosure, electrical terminal means exterior of said furnace connectible to a power source for energizing said heating elements, conductor means connected to -said terminal means and extending into said enclosure from the exterior of said furnace and through a zone within said furnace wherein the temperature of operation is to be Within a range wherein carbon deposits are likely to form on said conductor means at an objectionable rate, and an outer coating of chromium on said conductor means within said zone for reducing the rate of carbon formation on said conductor means.

References (Jited in the le of this patent UNITED STATES PATENTS 1,360,267 Chubb Nov. 30, 1920 1,538,972 Conradty May 26, 1925 1,624,345 Keene Apr. 12, 1927 1,764,045 Kelleher June 17, 1930 2,009,732 Harper et al. July 30, 1935 2,232,327 Hubald Feb. 18, 1941 2,359,157 Roth Sept. 26, 1944

Claims (1)

1. 5. AN ELECTRICALLY HEATED FURNACE COMPRISING A HEATING ENCLOSURE FOR RECEIVING ARTICLES TO BE TREATED, MEANS FOR INTRODUCING INTO SAID ENCLOSURE A CARBONACEOUS ATMOSPHERE CONTAINING CARBON MONOXIDE, HEATING MEANS COMPRISING ELECTRICAL HEATING ELEMENTS MOUNTED WITHIN SAID ENCLOSURE, CONDUCTOR ELEMENTS EXTENDING INTO SAID ENCLOSURE AND CONNECTED TO SAID HEATING ELEMENTS, AND A COATING OF CHROMIUM OVER AT LEAST A PORTION OF SAID CONDUCTOR ELEMENTS FOR REDUCING THE RATE OF CARBON DEPOSIT THEREON.

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