US2600094A - Apparatus for annealing annular coils of sheet metal - Google Patents

Description

c. CONE 2,600,094

APPARATUS FOR ANNEALING ANNULAR cons OF SHEET METAL June 10, 1952 Filed April 15, 1948 IN VEN TOR Car/"0H Cone Patented June 10, 1952 I APPARATUS FOR ANNEALING ANNULAR COILS OF SHEET METAL Carroll Cone, near Toledo, Ohio, assignor to 'Sur- I face Combustion Corporation, Toledo, Ohio, a

corporation of Ohio Application April 13, 1948, Serial No. 20,785

1 The present invention relates to a bell type furnace and a method of operation for annealing annular coils of sheet metal in a protective atmosphere maintained about the charge or stack of coils by a bell-type metal cover disposed thereover. In a furnace of this type the heat for the annealing operation is applied to the external side of the cover. Hence the heat is applied to the coils solely by radiation from the cover except for such heat as may be transferred from the cover to other parts of the coils by circulation of the gaseous medium- (the protective atmosphere) within the cover. Conversely, cooling of the coils is effected by dissipation of heat through the walls of said cover. The object of the present invention is to provide for the circulation of said gaseous medium in such a way to provide for the heating and cooling of saidgaseous medium in such a way that the heat transferred by' convection should represent a substantial portion of the total heat required to be applied to coils during the annealing operation.

For a consideration of what I consider to. be novel and my invention, attention is. directed to the following specification and the claims appended thereto.

In the accompanying drawing forming part of this specification:

Fig. 1 is a vertical section of an annealing furnace embodying the present, invention.

Fig. 2 is an enlarged fragmentary horizontal section on line 22 of Fig. 1.

Fig. 3 is a plan view with a portion broken away of a channeled spacer element adapted to be positioned between the annular coils to provide gas passages therebetween.

Fig. 4 is a fragmentary end view of Fig. 3.

The furnace comprises a stand I on which a plurality of annular coils ll of sheet metal may be supported one above the other with their cores in vertical alinement whereby the charge is in the form of a stack having a central gas passage. During the heating and cooling operations the coils are protected against oxidation by a protective atmosphere in 'a-bell-type metal cover l2 adapted to be disposed over the charge to form with the furnace base 13 on which the coil stand is supported a substantially gas-tight heat-treating chamber. The seating portion of the inner cover l2 preferably comprises areinforcing band [9 of angle cross-section, having a horizontal portion welded gas tight to the side-wall and a vertical portion depending therefrom about which loose sand is disposed to reduce gas leakage from said chamber to a minimum.

3 Claims. (Cl. 263-40) The coil stand l0 comprisesa top plate [4 having a central aperture, and a lower plate [5 separated from the top plate by a plurality of radially extending spacer ribs It to" provide a plurality of radially extending gas passages between said plates. The lower plate rests on a refractory pedestal which constitutes part of the furnace base. A radial flow fan I! is mounted between the plates I l and J5 coaxial with the aperture in the upper plate; hence the direction of flow ofprotective atmosphere is downwardly towards the fan and thence radially therefrom. The 'fan is driven by an electric motor [8 appropriately secured to the underside of the furnace base.

Heat for the annealing operation is provided by a portable heating hood 20 disposed over the metal cover 12. On completion 'of the heating operation, the heating hood is removed but the protective cover 12 is left in place; hence cooling of the coils is by dissipation of heat through the walls of said cover.- The heat producing means in the hood 20 is of secondary importance but will ordinarily consist of heat radiating elements 21 such as internally fired tubes.

The furnace thus far described involves nothing new and is typical of a conventional bell-type annealing furnace. I

In the present invention, the adjacent ends of the coils are separated from each other by an annular metal separator 22; hence any heating or cooling of the separator will result in similar heating or cooling of the ends of the coils in contact therewith. Each separator comprises a plurality of open-ended gas passages 23 between the inner and outer rims thereof so that gaseous medium is free to flow through said passages. In the present invention the internal heat transfer area of the coil separators is from 1.5 to 2.0 times the net area of the adjacent coil ends, this increased area being provided by the combined area of the side walls 24 of said passages, it being understood that the height and number of said walls are .the determining factors.

The fan I! is sufficiently powerful to produce sufficient partial vacuum in the core of the stack of coils to cause the gaseous medium about the stack. to'flow throughseparator passages. 23 with avelocity between 2000 and 3000 feet per minute. A closure plate 25 on top of the stack restricts the flow of gaseous medium through and around the stack to the desired minimum.

As best shown in Fig. 2, the sidewall of the protective cover 12 is corrugated to provide a contact area for the gaseous medium inside of the cover that is between 1.75 and 2.25 times the projected area of said wall thereby permitting a correspondingly greater amount of heat to be exchanged between said wall and said atmosphere. The furrows and ridges of said corrugations are vertical. The use of a reinforcing band IQ. of angle cross-section makes the vertically corrugated cover substantially rigid by preventing the corrugations from spread-- ing accordion-fashion, and also provides a depending blade-type seal.

From the foregoing explanation it will beappreciated that there are two distinctheat trans-.. fer processes involved in heating vor cooling the coil charge. Radiation heat transfer, on the one hand, occurs primarily between the inner surface of the protective cover and the outer wraps of the coils, in quantity which varies directly with the projected inner surface of the cover, and at a rate which increases rapidly with temperature. The rate of heating byradiation heat transfer is severely. limited by the rated heat transfer from. the. outer wraps of the coils to the center thereof; Convection heat transfer, on theother hand, delivers heat energy primarily to or from the coil .ends in contact with thework separators used .inthe preferred arrangement. The quantity ofheat transferred by convection varies withthe developed-rather than the projected area of the inside. coversurface, the effective surface or heat transfer area of .the coil separators, and the-power andefficiency of the circulating fan.

Since convection heat transfer depends upon wiping heat offof the innercover, and delivering it .tothe coilsas by a separator, a proper design of r the inner cover together. with a correspondingly; careful choice of a coil separator anda circulating fan will be required to. obtain maximumbenefits from each.

Because of the basic differences in thetwo heating effects, the results: secured in temperature uniformity, heating-and cooling-time requirements and :theshape f the time-temper atureqcurvesecured duringheating and cooling may be drastically modified by changingtherelative amounts of heat;transferred-.by -radiation and convection. In g eneral,;thechange; to more heating; by convection increasesoveralL heat transfer; effects since radiationheating; isnot necessarily. reduced: by the. expedients used to promote. convection heat transfer;- However,- such: expedients represent additional items 1 of investment and operating; cost, to the extent that.;.the determination ofithe. mostieconomical distribution of the heat :transfer burden between these. two processesbecomes a..very important step-.in the design of coil heating equipment; hence as already stated, Ihave found it desirable to; design the equipmentto satisfy the following. empirical relations: a

1; The. internal heat transfer. area of coil sepa: rators. should total between .1.5.-. and.2.01; times the net area of adjacent coilzends.

2.; The velocity of gas circulation througlnthe open passages. through. such' coil. separators shouldcbebetween 2000 and 3000feetper minute.

3. Theidevelopedinsidesurface area .of. the protective cover. shouldbe. between. 1.7 5: and 2.25 times the projected area-of thatsurfaca;

I;believe that with the-combination of heatin efiects:as;described,-, and-used-in accordancewith the, preferredratios defined above, Lobtairr. the mosteconomical operation with the bcstmetallurgioa-l results What I claim is:

1. Apparatus for annealing a stack of coils of sheet metal in a protective atmosphere comprising, in combination, a base for supporting said stack. of coils, separators for spacing the coils in saidstack, said separators being. provided with passages for flow of said protective atmosphere between. the interior and the exterior of said stack, a gas-tight cover for said stack having asingle vertically corrugated cylindrical side Wall closely surrounding said stack, means comprising a power-driven fan for circulating said protective atmosphere at high velocity along the channels formed' by said corrugations and throughthe interior of said stack, said atmosphere also flowing through the passages in said separators, and means for applying heat directly to the exterior of the corrugations forming the channels. along which said atmosphereis. circulated; said corrugated cylindrical side wall providing-transfer of heat-tosaid coils by conduction; ata; substantially: greater rate than a; smooth cylindrical side wall, and providingtransfer of ;heat to .said-coils'by radiation at-substantially, as great. a ,rate as a smooth cylindrical sidewall.

2. Apparatusxaccording to. claim 1, comprising a. Supporting. ring; having. a horizontal flange secured to :the foot 101? the corrugated cylindrical side wall ofthe-cover, and'havinga depending: cylindrical; portion servingas a; bladefor engagement with'thebase. to confine the protective atmosphere within 1 the cover.

3. Inaprotectivecatmosphere annealing: furnace comprising. heating 1means,:.a' base support-v ing a=stack of coils .of sheet metal; separators between said .coils that providepassages between the interior and the exteriorof .said stack, and a high-capacity: power-drivenf an for; circulating the. protective atmosphereithrough'. the interior of said stack and through the passages in. said separators; the combination therewith of .a gastight cover, interposed between said heating. meanssandrsaid stack of coils,. havinga single vertically corrugated cylindrical side wall closely REEERENCESZ CITED;

The following referencesare'of record in the-

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