US3833208A

US3833208A - Muffle furnace

Info

Publication number: US3833208A
Application number: US00357420A
Authority: US
Inventors: P Andersen
Original assignee: Allegheny Ludlum Industries Inc
Current assignee: Allegheny Ludlum Corp; Pittsburgh National Bank
Priority date: 1973-05-04
Filing date: 1973-05-04
Publication date: 1974-09-03
Anticipated expiration: 1991-09-03

Abstract

A muffle furnace is disclosed which includes an elongated furnace having an elongated chamber or muffle contained therein with the muffle constructed of upper and lower segments connected at their sides to an elongated support which in turn is supported within the furnace by vertical supports spaced along its length. The upper and lower segments of the muffle are catenary-shape in cross section.

Description

United States Patent 191 Andersen [111 3,833,208 1 Sept. 3, 1974 1 MUFFLE FURNACE [75] Inventor: Poul Andersen, Walllngford, Conn.

[73] Assignee: Allegheny Ludlum Industries, Inc.,

Pittsburgh, Pa.

22 Filed: May4, 1973 21 Appl. No.: 357,420

[52] US. Cl. 266/3 R, 432/210 [51] Int. Cl C2ld 9/56 [58] Field of Search 266/3 R; 432/210, 120,

[56] References Cited UNITED STATES PATENTS 2,240,099 4/1941 Schincke 266/3 R X 2,602,653 7/1952 Cope 266/3 R Primary Examiner-Roy Lake Assistant ExaminerDeWalden W. Jones Attorney, Agent, or FirmVincent G. Gioia; Robert F.

Dropkin [5 7 ABSTRACT 6 Claims, 4 Drawing Figures PAIENTEU 31974 SHEET 2 0F 3 PAIENTEUSEP 31w 3,833,208

SHEEI 30$ 3 MUFFLE FURNACE BACKGROUND OF THE INVENTION Muffle furnaces are furnaces constructed so that the materials being heated within them are shielded from flames and insome cases from oxidizing gases. Most muffle furnaces are constructed with compartments or baffles within a fired furnace so that the flue gases may heat the interior portion of the muffle without actually entering it.

Muffle furnaces for continuous processes or for processes wherein the heated piece is very long present special problems. Typical of these processes is bright annealing of metal strip. To bright anneal a metal strip, a shielded chamber must be provided that is long enough for an annealing process to occur while the strip is in transit through the muffle, or if it is not a continuous strip, the muffle must be long enough to con-- tain the strip. Bright annealing of ferrous metals is generally effected at temperatures in the neighborhood of 2,000F.

Presently muffle furnaces for strips are constructed of long tubular muffles supported within elongated furnaces that are suitably fired and suitably insulated. It is essential that the muffles be fabricated of metals having adequate tensile strength at the annealing temperatures, such as the alloy known as INCONEL 600. However, even metals with adequate tensile strength have dimished creep resistance at the high temperature. Creep resistance is theslow distortion of metal when subjected to stresses for a long period of time. Muffles for continuous processing generally must be made of 7 metal to obtain good heat transfer and gas sealing characteristics to provide a reasonably inexpensive structure.

Muffles made of long tubes of INCONEL with circular cross section are supported by saddle-like supports spaced appropriately to maintain the muffles with minimum sagging. The saddles in turn are supported from the floor of the furnace by support legs. The muffles are usually circumferentially corregated for rigidity and provided with interior devices for conveying a strip through them. It is important to have a muffle spaced from the furnace in order to obtain circulation of hot gas around it so that adequate and even heat transfer through the wall may be maintained.

Two serious problems have been encountered in the structures of muffle furnaces for bright annealing. Both of these problems have to do with the diminished creep resistance of the muffle atannealing temperatures. One problem is the tendency of the muffle shell to distort around the saddles in the form of indentations into the muffle shell, caused by localized high stresses on the muffle shell where the saddles support it. The other problem is the tendency of the upper part of the muffle to flatten from a circular to an eliptical shape caused by bending forces introduced into the circular muffle wall by its own weight. Operation of muffle furnaces that are so constructed usually requires rotation of the muffle about its longitudinal axis periodically to permit creep in opposite directions to effect some straightening and thereby to keep the muffle operative.

THE INVENTION This invention is a muffle furnace that is constructed to avoid or greatly mitigate the above noted problems. The muffle furnace of this invention, as in the prior art,

includes an elongated furnace, usually of a refractory brick material having heat generating means such as one'or more burners maintained within it. Also within the furnace is an elongated metal muffle constructed of a lower segment of catenary cross section, an upper segment of catenary cross section and two rigid side support elements, all of which are connected together to form an enclosed chamber. In addition, this invention includes vertical supports within the furnace for holding the muffle above the furnace floor by engaging and supporting the rigid side support elements. 7

The term catenary is used in this specification and the appended claims in its usual sense. A catenary is a curve that would be made by a free hanging chain suspended at both ends. It is of course obvious that reasonable departures from perfection are intended to be included within the scope of the claims herein and that the term catenary is intended to have some latitude. The upper segment of catenary cross section and the lower segment of catenary cross section may also be corregated with circumferential corregations to provide a stiffer wall, although such corregations are less needed than in prior art muffles.

The longitudinal side support elements may be any rigid member that runs substantially the length of the muffle, one on each side, preferably diametrically opposed. The simplest side support member is a tubular member which may form part of the muffle by being welded separately to the top catenary segment and the bottom catenary segment. The supports may also be plate-like elements, I-beams, angles or of any other shape. The criterion for the size and shape of the side support elements is that they must have sufficient rigidity to support the muffle when the side support elements themselves are supported by a series of spaced vertical supports within the furnace.

The vertical supports within the furnace generally are discontinuous in order to promote good circulation of hot gas around the muffle. In simplest form the vertical supports will: be short metal pieces of appropriate square, round or I-beam cross section; however, brick columns or shelves built out from the furnace wall or built onto the furnace floor may also be employed. The vertical supports may be hangers suspended from the top of the furnace. The vertical supports must engage the side supports in a manner that allows for thermal expansion of the muffle.

The structure described above is resistant to creep distortion at high temperatures because its shape conforms to the direction in which the metal is stressed: in this case pure tension in the lower segment and pure compression in the upper segment, with no bending moments introduced at any points of the catenary segment. Supporting the bottom catenary section by its edges instead of by a saddle beneath it provides that all strain on the bottom segment that would cause distortion will resolve itself into a component of force substantially tangential to the cross section at any point. Thus, with the bottom segment made in the curve in which it would hang if it were absolutely flexible, adding increments of flexibility by such means as diminished wall stiffness will not cause its shape to change. Also, removing saddles as supports avoids local stresses that are different thanthose created by the weight of the bottom section of the muffle shell and avoids distortion of the muffle at its point of support.

Similarly, it has been found that the stresses on the upper segment due to its own weight are such that resolution of these forces results in a component that approximately follows the tangent to the catenary-shaped cross section at all points. Thus, the furnace of this invention may be constructed of material with adequate tensile strength to resist the strains imposed on it, and after long periods of use there will be very little or no distortion of the muffle due to diminished creep resistance at the high temperature of use.

DETAILED DESCRIPTION OF THE INVENTION The invention may be better described with reference to the accompanying drawings illustrating several useful embodiments of it.

FIG. 1 is an elevation end view of a furnace embodying this invention in cross section.

FIG. 2 is an elevation end view of another furnace embodying this invention in cross section.

FIG. 3 is an end elevationview of another furnace embodying this invention.

FIG. 4 is a partial elevation side view of the furnace illustrated in FIG. 1 taken along the line 44.

Referring first to FIGS. 1 and 4, there is illustrated a furnace generally designated and constructed of conventional brickwork. The furnace isillustrated in simplified form and is not intended to illustrate wall thicknesses, linings or any other of the conventional construction features of a furnace. The furnace 10 includes a wall 11, a roof 12 and a floor 13 to form a complete enclosure. The furnace is provided with burners 15' positioned in ports 16 so that air-fuel mixtures may be burned within the interior of the furnace 10.

Within furnace 10 a muffle generally designated as 17 is supported. The muffle comprises a lower segment 18 and an upper segment 20, both of which are catenary-shape in cross section. A side support element 21 in the form of a thick-walled tube is connected to the lower segment 18 by weld 22 and to the upper segment by weld 23. The upper and lower segments of the muffle and the portion of the side support 21 between them form a closed chamber that shields material within it from the flames or the hot oxidizing gases within the confines of the furnace l0.

The muffle 17 is supported within the furnace 10 so that there is open space above it, beneath it and around both sides of it. Supporting the muffle 17 is accomplished by vertical supports which engage side support elements 21 between holding elements 19 and maintain them above the floor 13 of the furnace. A

suitable plate 26 is illustrated to hold the vertical support 25 and plate 26 may be adapted to be fixed to the furnace floor 13 and is welded to the base of vertical support 25. Side elements 24 are employed to hold vertical supports 25 upright when the muffle 20 elongates due to thermal expansion and slides along the top of vertical supports 25.

From FIGS. 1 and 4 it is evident that lower segment 18 is a free hanging segment of the muffle l7 suspended between opposing side supports 21, and since its cross section is in the shape of a catenary, it is already in the shape toward which it would move if under sufficient stress to creep, and-as such the use of muffle 17 at high temperature should not affect the shape of lower segment 18.Alth0ugh it is not as evident from inspection, the catenary-shaped cross section of upper segment 20 is also a shape which normal stresses would cause it to assume by creep.

It is also evident from FIGS. 1 and 4 that the manner of supporting muffle 17 on vertical supports 25 does not involve a fixed connection between the two. Since thermal expansion of the muffle may exceed one foot, the arrangement shown where the muffle may slide on the vertical supports will accommodate to such expansron.

FIGS. 2 and 3 illustrate other embodiments of the invention which may be employed. In FIG. 2 lower segment 18 and upper segment 20 of the muffle 17 are connected to a plate-like side support element 21A by

welds

22 and 23. As with the embodiment of FIG. 1, welds 22 and 23 participate in sealing the interior of muffle 17 from the gases within furnace 10. Support element 21A is made rigid with respect to horizontal forces by being connected to muffle 17 and its vertical dimension is sufficient to support the muffle 17. FIG. 2 also illustrates that vertical support 25A which supports the muffle 17 may be constructed of brick. It is contemplated that supports 25A will be small, spaced columns protruding from side wall 11 of the furnace 10 rather than a continuous shelf so that free circulation of hot gases around the muffle 17 is insured. A skid plate 29 is also employed on each column 25A to provide a suitable surface on which side supports 21A can slide when the muffle expands.

FIG. 3 illustrates another embodiment of this invention. In FIG. 3 a lightweight furnace is constructed of a metal shell 30 that is lined with lightweight insulation 31 such as mineral wool. A muffle constructed like the muffle shown in FIG. 2 is maintained, but it is supported from hangers 32. Hangers 32 are illustrated with hook-like bottom elements 33 so that nodirect connection between the side supports 21A and the hangers 32 is required. Thus, muffle 17 may expand and slide with respect to hangers 32. I

Exterior supports 35 may be employed to support the furnace 10 and the muffle 17. The exterior support includes vertical columns 36, a beam 37 and brackets 38. As illustrated, flanges 40 extend from the furnace and rest on brackets 38 whereby the furnace 10 is supported above grade. The muffle 17 is supported independently of the furnace 10 by hanging from beam 37. The height of muffle 17 with respect to furnace 10 may be regulated by nuts 40 operating through springs 41 which operate to absorb shocks in the vertical loading of muffle 17.

An advantage of the embodiment of FIG. 3 is that substantially all of the supporting structure is cool, and accordingly it need not be made of special material or of large cross section. Columns 36, brackets 38 and beam 37 are all much cooler than furnace temperature. Metal shell 30 and flanges 40 are likewise cool. Even hangers 32 are cool except for the very short length within furnace 10.

A particular advantage to the embodiments shown herein is that the muffles are symmetrical about a horizontal plane through the intersection lines of the upper and lower catenary segments. This symmetry permits the muffles to be turned upside down occasionally which substantially increases their effective lives. Even though the catenary shape of lower segment 18 will be maintained throughout its life, there is a tendency for the lower segment to become longer. It is evident that the useful life of muffle 17 could be about doubled if 2. The muffle furnace of claim 1 wherein each of the side support elements is a tube.

3. The muffle furnace of claim 1 wherein continuous welds connect the upper segment and the lower segment to the support elements.

4. The muffle furnace of claim 1 wherein each of the side support elements is a plate.

5. The muffle furnace of claim 1 wherein said vertical supports are hangers.

6. The muffle furnace of claim 1 wherein said muffle is symmetrical about a horizontal plane through the intersection lines of the upper and lower segments.

Claims

1. A muffle furnace comprising: A. an elongated furnace having heat generating means therein, B. an elongated metal muffle constructed of a lower segment of catenary cross section, an upper segment of catenary cross section and opposed rigid side support elements with said upper segment, lower segment and support elements connected to form an enclosed chamber within said furnace, and C. vertical supports within said furnace adapted for supporting the rigid side support elements.

2. The muffle furnace of claim 1 wherein each of the side support elements is a tube.

5. The muffle furnace of claim 1 wherein said vertical supports are hangers.