US2006685A - Electric furnace - Google Patents

Description

July 2, 1935. w E. MOORE 2,006,685

' ELECTRIC FURNACE Filed Oct. 24, 1931 5 Sheets-Sheet 1 lull/222m, M0021 Juiy 2, 1935.

W. E. MOORE ELECTRIC FURNACE Filed Oct. 24, 1951 3 Sheets-Sheet 2 101mb, if 700 Juiy Z, 1935. W. E. MOORE 2,006,685

ELECTRIC FURNACE Filed Oct. 24, 1951 5 Sheets-Sheet 3 JKKMQWWZ William EJ1002 6 Patented July 2, 1935 g UNITED STATES PATENT. OFFICE ELECTRIC FURNACE William E. Moore, Pittsburgh, Pa., assignor to Pittsburgh Research Corporation, Pittsburgh, Pa.

Application October 24, 1931, Serial No. 570,957

13 Claims. (Cl. 1320) My invention relates to improvements in elec- Figure 16 is a vertical section through the retric furnaces and more particularly to furnaces sistor support, adapted for annealing or similar heat treating Figure 17 is a similar view of a slightly modified operations. form of resistor support, An important object of my invention is the pro- Figure 18 is a detail of the resistor supporting 5 vision of an annealing or similar iurnace which spool, showing two forms thereof, and, will withstand the stress and strain of use, which Figure 19 is a detail of an alternative form of is portable and which may be easily taken apart. tie strip.

A further object of my invention is to provide In the drawings, wherein for the purpose of a. furnace of this character which may be readily illustration is shown a preferred embodiment of 10 and effectively sealed. I my invention the numeral 20 designates the fur- Another object of my invention is the provision nace base which is preferably portable and has a of a furnace which will accommodate expansion brick hearth 2| adapted to p oj p d y into and contraction without being subjected to undue the cover 22. The cover 22 is removable and has 5 strain. a removable roof 23 secured thereto by means of 5 Still another object of my invention is to pro- U-bolt fasteners 24 which engage fla 0n the vide means for assisting and guiding the furnace roof and on the cover body as Seen in Figures 3 cover into position on its base. and 4.

A further object of my invention is to provide The base 20 is of structural steel welded con- 20 means for determining the temperature of the struction, provided with insulating brick layers, articles being treated. and the hearth 2| is of fire brick. In some treat- Astill further object of my invention is the proments it is desirable to have a hearth plate vision of a novel resistor support for use with my p d of heat resistant arranged on D furnace. of the brick hearth.

25 Yet another object of myinvention is the novel The base is provided with an inner channel 25 distribution of the resistor elements about the 5 fi d other 5 1 and W h an Oute walls of the furnace. channel 26 in which a suitable material such as Other objects and advantages of my invention a Oil, P h other material y be sed will be apparent during the course of the followas a 30 ing description. The heating cover is of welded steel construc- 30 In the accompanying drawings, which form 9, tion, and is built in two sections, the upper secpart of this specification, and wherein like charon or ro ein r ily rem v l to ive a actors of reference denote like or co din cess to the interior of the furnace. The joint parts throughout the same, between the roof 23 and cover 22 is made gas as Figure 1 is a longitudinal sectional view of my i h y m ns of a s i e ton u d ro v improved furnace showing the roof removed, construction seen in Figure 4, and by a suitable Figure 2 is a transverse sectional view thereheat resistant gasket thmugh, Thecover 22 and its roof 23 are lined with a Figure 3 is a fragmentary elevation showing Suitable refractory brick and heat insulating one end portion of the furnace, material. This refractory material is subjected 40 Figure 4 is a. fragmentary transverse section to severe mechanical stresses due to the raising therethrough, 'and lowering and transporting of the cover by Figure 5' is a diagrammatic view showing the the usual shop crane. There are also severe location of the guide posts and expansion joint, thermal stresses due to the fact that the interior Figures 6, 7 and 8 are detail views of the exf the furnace is maintained at annealin term 45 pension joint, perature while the steel case of the shell is but Figure 9 is a plan view partly in section, showlittle above room temperature. The sudden ing the arrangement of the sealing coil, chilling of certain parts of the furnace due to Figures 10, 11 and 12 are detail views showing transferring of the heated cover from one base the location, arrangement and construction of to another results in thermal stresses being set 50 the tie strips, up in the cover. These stresses have heretofore Figures 13 and 14 are diagrammatic views showresulted in failure of the wall so that large poring the distribution of the resistor elements, tions of the brick work became loose and fell out,

. Figure 15 is a perspective view showing the denecessitating a shut down of the furnace to retails of the resistor and resistor support, build the side walls. I have found, however, 55

side of the steel case of the furnace-.cover and extending into the brick work between the mortar joints, that the walls are substantially an.- chored while at the same time sufficient freedom of motion is allowed to permit of the normal expansion and contraction of the brick work in the regular heating and cooling cycle of the furnace operation. I v

The tie strips are shown in position in Figures 1 and 2 and in detail in Figures 10, 11 and 12. The strips are formed of metal so thin that it cannot be successfully welded to the furnace shell. Their ends are wrapped about a metal pin 28 of substantial cross section, the ends of which project beyond the edges of the tie strip 29. The pin 28 has suflicient metal weight so that it may be welded or otherwise successfully attached to the furnace shell and its rectangular cross section prevents unrolling of the tie strip 29. Teeth 30 are provided by forming diagonal cuts in the strip and bending the teeth in opposite directions. These teeth insure a firm anchoring of the strips in the furnace walls.

In place of the teeth 30, any other suitable means for anchoring the strips to the furnace walls may be employed such as serrations or the like. In Figure 19 I have shown an alternative form of strip 29' which is corrugated transversely at 30' to produce an anchorage of the strip to the furnace walls. Screws 28 are utilized for securing the strips to the furnace shell, although other securing means may obviously be employed. The end of the strip 29' is folded upon itself and bent at an angle to present a reinforced tab for the screws or other fastening 28'. Preferably the tie strips 29 and 29 are formed from a heat resistant metal having a low heat conductivity so as to minimize the heat flow from the hot metal of the furnace to the shell thereof.

Heat insulating and refractory bricks are used in the inner walls of the furnace. The bricks at present available for this purpose with a sufliciently high heat insulating value are quite low in shear strength. To remedy this deficiency I have provided at intervals throughout the walls, bricks 3i of a higher shear strength such as standard fire bricks set on end as a header. This arrangement of header bricks materially improves the mechanical strength of the wall by tying in the brick layers to one another without too adversely affecting the heat insulating value of the wall as a whole.

The walls may be two or more bricks thick. The use of the header bricks extending across the two layers of wall, tie these layers securely together, but if the usual heat-insulating brick were used for this purpose it would break or shear due to differential expansion between the two brick walls. strength bricks has been found to securely anchor the walls in place.

The inner portions of the bottom steel casing designated at 32 are subjected to higher temperatures during the heating of the furnace than are the outer surfaces of the casing, so that the inner edge expands and tends to-break the welded construction at diagonally opposite corners of the steel structure of the bottom of the casing. Strengthening of these corners by a heavier metal construction at this point merely causes the cracks to occur at some weaker point. This cracking is avoided by the provision of a ing material below its melting point.

The use of higher shear bottom 32 and to the lower portion of the end plate 32. The slot 33 is arranged centrally of the plates 34 and 35 and suitable packing, such as asbestos millboard, is arranged between the bottom 32, end plate 32' and the plates 34 and 35, as seen at 36 in Figure 8 to render the joint gas tight. The slip joints are arranged adjacent diagonally opposite corners of the bottom plate 32 as indicated at 31 in Figure 5.

In the use of this furnace, one or more of the bases 20 are provided. the sheets or other material to be treated are stacked upon the hearth as at 38, and the cover 22 lowered upon the base. It is important in handling this type of furnace that only one man will be needed to handle the shop crane and only one man will be needed to guide the cover into position when lowering it over the charge on one of the bases. To accomplish this I provide two guide posts 39 extending above the base at any suitable points, such as diagonally opposite corners, as seen in Figure 5. Suitable vertically arranged sheaths 40 on the furnace cover slidably receive the guide posts 39. The posts 39 preferably extend above the charge so that the cover may be centered before being lowered over the charge, to

prevent contact of the resistance elements with the charge. One of the posts 39 extends a substantial distance above the other post so that the operator may guide the taller post into its sheath, and then go to the other side of the furnace and guide the sheath onto the other post. The same effect can be produced by having the posts the same size, but having one of the sheaths extended lower than the other.

An ordinary sand seal may be used in the channel 25 but the channel 26 is provided with a low melting point alloy or a high melting point compound sealing material. Circulation pipes 4| are provided in the channel 26 for the circulation of steam or other heating medium for melting the sealing material, and for the circulation of a cooling medium to maintain the seal- It will be seen that this seal renders the furnace gas tight.

In annealing operations it is desirable to know the temperature of the material being treated, and for this purpose I provide my furnace with means for arranging one or more thermocouples within the furnace with wires therefrom extended outside of the furnace. To accomplish this it is necessary that one or more thermocouples be extended through the furnace hearth 2| in such a way that removal of the heating ing, the lower part of the tube is filled with some An angle platehigh flash point oil such as transformer oil, palm oil, or other suitable material.

Any number of these thermocouple openings may be built into each base so that the thermocouples can be inserted in the charge at the top and bottom and center or edge of the stack 28, as desired. Where an inner cover is used over the charge, the thermocouple openings are arranged to emerge from the hearth inside of such cover.

In the heat treating of material piled vertically on the hearth of this type of furnace, if the heater elements are uniformly distributed over the wall surfaces the upper parts of the charge will attain a higher temperature than the lower parts of the charge due to the eifect of convection and re-radiation from the arched roof and other factors. Heretofore it has been the practice to avoid this overheating of the top of the furnace by the use of two or more zones of heater elements each separately controlled so that the ribbons in the higher zones are carried at a lower temperature than ribbons in the lower zone to compensate for this tendency to overheating of the top of the furnace. Such an arrangement is disadvantageous, however, and I propose to obtain uniform heating of the charge by a novel distribution of a single zone of heating elements.

An ordinary and advantageous use of my invention is for heat treating sheets of metal stacked horizontally on base 2! with the edges to the side-walls. The heat conductivity of such charge is very much greater horizontally than vertically, and for that reason I preferably place my heating elements principally on 1 16 side walls of the furnace with a small portion on the end walls to compensate for equal heating there. I also preferably locate the major portion of the resistors of the heating elements below the horizontal center of the charge so that the radiation and convection had of the charge is approximately the same in the bottom portions of the charge and the top portions of the charge. I also prefer to so distribute the heat on the sidewalls to avoid over-heating the corners. Irregular heating tends to buckle the sheets or over-oxidize the edges of the corners which by my invention I have avoided.

In Figure 13 I have shown one method of accomplishing this result. The heating element in the form of a resistor ribbon 44 is supported by suitable insulating supports 45 arranged in upper, lower and middle rows. In the arrangement shown in Figure 13, for every resistor loop that extends from the lower to the upper row of supports, two loops extend only to the middle row of supports. As a consequence, over two thirds of the heating element surface is arranged below the horizontal center line of the furnace. It is obvious that other arrangements of the resistor loops may be used to obtain the desired distribution.

The usual practice is to use the same proportion of heating elements on the end walls as on the side walls but this has been found to be impracticable, due to overheating of the ends of the charge in this type of furnace. If any appreciable amount of heat is supplied by the end walls, the end portions of the charge are supplied with heat from three sides, whereas the central portions of the charge are supplied with heat from only two sides, resulting in very unequal heating of the charge. To remedy this defect I propose to heat the charge from two sides only and produce at the end walls only enough heat to compensate for the heat losses of the end walls. This is accomplished by providing only a few resistor loops on the end walls, as seen in Figure 14; where the loops are arranged in the center and at the bottom of the end walls where most of the heat losses occur. Any desired distribution could be used, however.

In Figures 15, it, 17 and l8 1 have shown a resistor support which is well adapted for this type of furnace. Insulating and. refractory blocks 46 are set into the walls of the furnace cover and carry heat resistant metal pins N which project beyond the inner surface of the furnace cover and slidably receive electrical insulating spools M which are held in place by suitable fasteners such as iii-shaped clips ts fitting in suitable notches in the ends of the pins ll. In the form of the invention shown in Figures 15 and 16, the block 46 is split horizontally and provided with a bore to receive the pin and communicating with a vertical bore 50. The pin it is provided with a bent end which extends into the bore to prevent removal of the pin when the block sections are assembled. Additional bores 5| may be provided for securing the block sections together or to receive the pin if a different position of the pin is desired.

The spool may be formed in one piece as shown in Figure 17 and at the right of Figure 18, or it may be split at its center to facilitate assembling, as seen in Figure 16 and at the left of Figure 18. The center portion of the spool is polygonal in cross section to present knife edge contacts to the resistor ribbon, and thereby reduce conduction losses to the support and permit a less hampered radiation from the resistor.

It will .be seen that the spools 48 may be readily removed or replaced, by removing the fastener 49 and sliding the spool from the pin, without removing the block 46 from the wall.

In Figure 17 the insulating block 46' is formed in one piece and a lag screw 41, is used in place of the pin 41. The screw 41' may be removed and replaced without removing the block 46' from the wall. The head of the screw 41' retains the spool in osition.

The removable furnace roof is an important feature of my furnace construction because it contributes to ease in building the furnace and in repairing the furnace walls and resistor elements. When it is necessary to repair the furnace or replace resistor elements, cooling of the furnace is accomplished very rapidly when the roof is removed, and a great deal of time is saved in this operation.

While I have shown and described the pre-- ferred embodiment of my invention it is to be understood that various changes in the size, shape and arrangement of parts may be resorted to without departing from the spirit of my invention or the scope of the subjoined claims.

Having thus described my invention what I claim and desire to protect by Letters Patent is:

1. In an electric heat treating furnace, a base, a removable cover therefor, said cover comprising a metallic shell lined with a plurality of layers of refractory bricks, and tie strips of metal having substantially less thickness than the metal of the shell, said strips having a piece of.metal of substantially greater thickness than the tie strip secured at one end thereof and attached to the shell, the strips having portions of their edges struck. up to form anchoring members.

2. A tie strip for electric furnace linings comprising an anchoring cross piece of polygonal cross section and of heavy metal, and a thin metallic strip having its end bent about the anchoring piece, said strip having portions struck up to form anchoring members, and said strip being substantially narrower than the cross piece, wherebythe cross piece 'may be attached to a furnace shell.

3. In an electric heat treating furnace, a base,-

a removable cover therefor, said cover comprising a metallic shell lined with a plurality of layers of refractory bricks, header bricks arranged at intervals in said lining, and metallic tie strips attached to said shell and extending into-said lining.

4. In an electric heat treating furnace, a base, a removable heating cover therefor, said cover comprising a metallic shell lined with a plurality of layers of refractory bricks, header bricks arranged at intervals in said lining, metallic tie strips welded to said shell and extending into said lining, and a removable roof for said heating cover.

5. In an electric heat treating furnace, a base, a removable heatingcover therefor comprising a metallic shell lined with refractory material, the bottom of said shell being provided with an expansion joint comprising a slit in the shell and a metal-plate bolted to the shell in a position to cover the slit, there being a layer of fibrous material arranged between the shell and the metal plate.

6. In an electric heat treating furnace, a base, a heating cover adapted to be placed on said base, guide posts extending vertically above said base on opposite sides thereof, one of ,said posts extending a substantial distance above the other,

and means on said cover to engage and be guided by said posts.

7. In an electric heat treating furnace, a base, a heating cover adapted to be placed on said base, guide posts extending vertically above said base on opposite sides thereof, guide sheaths. on said cover adapted to engage the posts when the cover-is lowered onto the base, and means whereby one of the posts engages its sheath before the other post engages its sheath, when the cover is lowered into position.

8. In an electric heat treating furnace, a base, a heating cover adapted to removably rest on said base, a removable roof for said cover, a flange on said cover adjacent the top thereof, a flange on the roofat a point which is spaced from the cover flange in the assembled position of the roof and cover, a lifting member, and means'connecting the lifting member with the flanges on the cover and roofto permit their removal from the base as a unit.

9. In an electric heat treating furnace, a base, a heating cover adapted to removably rest on said base, a removable roof for said cover, a

flange on said cover adjacent the top thereof, a flange on the roof at a point which is spaced from the cover flange in the assembled position of the roof and cover, a lifting member, and means connecting the lifting member with the flanges on the cover and roof to permit their removal from the base as a unit, said connecting means engaging the lower surfaces of said flanges, whereby lifting strains are transmitted to both the cover and the roof;

10. In an electric heat treating furnace, a base, a heating cover adapted to removably rest on said base, a removable roof for said cover, a flange on said cover adjacent the top thereof, a flange on the roof at a point which is spaced from the cover flangein the assembled position of the roof and cover, a lifting-member, means connecting the lifting member with the flanges on the cover and roof to permit their removal from the base as a unit, and sealing means arranged between the base and cover and between the roof and cover.

11. In an electric furnace, a base adapted to support the furnace charge, a removable heating cover therefor, and electric heating elements supported along the sides of the heating cover, said heating elements extending upward from points adjacent the bottom of the cover, and

the greater portion of said heating elements being disposed below the horizontal center of the on said base, aremovable roof on said cover, said roof and cover each comprising a metallic shell and refractory material arranged therein, a se curing member connecting the metallic shells of said cover and roof together and permitting their removal from the base as a single unit, said securing member being provided with means to be engaged by a lifting device and having abutments engaging the shells of the cover and roof to transmit the lifting strains directly to the metallic shells of the cover and roof.

13. In an electric furnace having a shell and a refractory lining therefor composed of bricks, a tie strip comprising a wide, thin, flexible metal strip of low heat conductivity, said strip being attached to the furnace shell and extending into said lining between the bricks thereof, said strip having a large surface contact with the brick lining whereby heat will not be conducted by the strip to the furnace shell, the strip being deformed whereby it will securely anchor the brick lining to the shell while yielding to diiferential expansion of the furnace shell and refractory lining.

. WILLIAM E, MOORE.

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