US3179392A

US3179392A - Heat and gas barrier for muffle furnaces

Info

Publication number: US3179392A
Application number: US327677A
Authority: US
Inventors: Beck Jacob Howard
Original assignee: BTU Engineering Corp
Current assignee: BTU Engineering Corp
Priority date: 1963-12-03
Filing date: 1963-12-03
Publication date: 1965-04-20
Anticipated expiration: 1982-04-20
Also published as: DE1433717A1; DE1433717B2; GB1084445A

Description

April 20, 1965 J. H. BECK HEAT AND GAS BARRIER FOR MUFFLE FURNACES Filed Dec. 3, 1963 INVENTOR. J. HOWARD BECK BY 4/ wa w- ATTORNEYS United States Patent 3,179,392 HEATAND GAS BARRIER FOR MUFFLE FURNACES Jacob Howard Beck, Waban, Mass., assignorto BTU Engineering Corporation, Waltham, Mass. Filed Dec. 3, 1963, Ser. No. 327,677 Claims. Cl. 263-37) The present invention relates to mufiles for high temperature furnaces and more particularly to a transition mufiie section interposed between two successive gasbearing mullie sections and adapted to function as a gas and heat barrier with respect to said gas-bearing sections.

In many heat treating installations, it is essential to have a furnace with a plurality of stages each designed to perform a particular heat treating operation. Thus, it is quite common to have a furnace with a motile having a first hydrogen-filled section maintained at a first temperature and a second oxygen-filled section maintained at a second temperature. In such a situation it is necessary to provide means for preventing an explosive mixture of the two gases from accumulating in the mufile and it also is usually desirable to maintain a sharp temperature gradient between the two mufiie sections in order to achieve proper process control. The usual approach to the problem of preventing accumulation of explosive mixtures is to provide. an intermediate relatively long section in which is established a controlled flow of an inert gas. The latter dilutes the combustible gases and flushes them out of the system, usually in separate streams, and the hydrogen is burned outside of the furnace by means of a conventional burner. In some cases the inert gas is supplied in the form of a gaseous curtain at the ends of the intermediate section, these curtains preventing the reactive gases from coming together in an explosive mixture. While this approach is satisfactory from the standpoint of combustion control, it usually is inmpable of providing and maintaining a sharp temperature gradient between two mufile sections.

Accordingly, the primary object of the present invention is to provide a transition section adapted to be in corporated between two successive mufile sections each containing a different reactive gas at a different temperature, the transition section effectively preventing migration of gas from one to the other of said successive sections while at the same time helping to preserve a sharp temperature gradient therebetween.

A more specific object of the present invention is to provide a muffle comprising a transition section connecting two heat treating muffle sections, the transition section including (a) means for preserving a sharp temperature gradient between the two heat treating sections and (b) means for withdrawing gas from each of the two heattreating sections and mixing the two withdrawn gases at a point radially removed from the center of the muffle in a chamber where the two gases may be safely combusted. Also provided are means for controlling the temperature in the combustion chamber and means for removing the products of combustion from the system.

Other objects and many of the attendant advantages of the present invention will become readily apparent when reference is had to the following detailed specification which is to be considered together with the accompanying drawings wherein:

FIG. 1 is a fragmentary perspective view of a mufiie embodying the present invention;

FIG. 2 is a vertical sectional view through the center of the apparatus of FIG. 1; and

FIG. 3 is a fragmentary cross-sectional view taken along line 3-3 of FIG. 3.

The device of the present invention is designed to be embodied in a heat treatment furnace of the kind embodying an elongated heat resistant mufile which is surrounded by s'uitable insulation and has an endless conveyor travelling through it on which articles are carried for treatment in successive sections of the mufile. The muffie is of a type having a series of sections each with a particular atmosphere and/ or operating temperature. While the present invention is useable in a variety of heat treating furnaces, it is particularly adapted to furnaces having a mufile with successive sections containing diiferent gases which produce an exothermic reaction when mixed together. A typical case is where the mufile has a first section in which hydrogen is continually circulated over and around the conveyor and the articles thereon at a temperature and a rate suitable for effecting a chemical reduction of the articles, plus a second section on the downstream side of the hydrogen section in which oxygen is circulated at a temperature and at a rate suitable to achieve controlled oxidation of the articles previously subjected to reduction. In such a situation it is essential that the hydrogen and oxygen atmospheres be kept isolated from each other in the mufiie in order to prevent a combustible mixture which can produce a damaging explosion. Additionally, it is usually necessary that the two sections be maintained at different temperatures and that each section have a flat temperature profile along its length so as to attain precise reaction control. The absence of a sharp temperature gradient between the oxygen and hydrogen sections prevents attainment of flat temperature profiles so that accurate reaction control is impossible.

Turning now to FIGS. 1 to 3 there is shown apparatus particularly suitable for furnaces designed to carry out successive reduction and oxidation heat treating operations. FIGS. 1 and 2 show portions of two successive heat treating mufile sections that are separated by a transition section identified generally at 6 which is constructed according to the present invention. The mufiie sections 2 and 4 are generally cylindrical in shape but may have some other convenient cross-sectional configuration. Although not shown it is to be understood that these muifie sections are provided with separate heating means such as surrounding electrical heating coils, whereby they may be heated to different elevated temperatures. These sections are provided with a perforated plate 8 supported by suitable brackets (not shown) which are attached to the interior surfaces of muffle sections 2 and 4. Plate 8 extends through the transition section 6 as seen clearly in FIG. 2. The plate 8 is perforated so as to permitgas introduced below the plate to move up through the plate into the section above it and thereby fill theentire volume of the individual mufiie sections 2 and 4. The plate 8 also supports a perforated endless conveyor 10, only aportion of which is shown. It is to be understood that this conveyor continues along the full length of the rnutlle and is driven by suitable drive means (not shown); The muflle section 2 is adapted to be filled with hydrogen gas while the mufile section 4 is adapted to be filled with oxygen gas. The gases are introduced to the two muffle sections at points remote from the transition section 6 by means of suitable inlet pipes. If desired, each of thesemuffle sections 2 and 4 may also have an outlet pipe located proximate to the transition section 6 so as to minimize the amount of gas which can migrate into the transition section from the adjacent mutlle section. However, such outlet pipes are not essential to the present invention and, therefore, are omitted for the sake of convenience.

It is to be observed that the transition section 6 is substantially larger in diameter than the two muffle sections 2 and 4, thereby providing an increased volume for radial expansion of gases introduced to it from each of the adjacent mufile sections. The transition section 6 basically comprises a pair of opposed

parallel plates

16 and 18 which have center openings and are welded at their center openings to the mufiie sections 2 and 4, respectively. It is to be noted that the plates are curved in cross-section at their center openings so asto provide curving slightly flared connecting portions as shown M20 and 22 between the mufile sections 2 and 4 and the

plates

16 and 18 respectively. These plates are generally circular in shape and are connected at their periphery to a cylindrical plate 24 whichis provided with a plurality of rows of small apertures 26. Surrounding the cylindrical plate 24 is a water jacket comprising inner and

outer wall members

28 and 30 of circular cross section. These wall members are integrally joined to each other and also to the plates 16 and I8at 32 and '34, respectively so as to form an integral

structure comprising plates

16 and 18, cylindrical plate 24 and

wall members

28 and 30. The

wall members

28 and 30 of the water jacket define an annular chamber 38 through which water (or some other suitable non-combustible coolant) 'is circulated via an inlet 40 and a tapered outlet 42. The outlet 42 is located at the bottom of the transition section in approximately 6 oclock position while the inlet 40 is located at approximately the 12 oclock position. While'these positions are not critical. they are preferred. The inner wall member 28 and the cylindrical plate 24 together define an annular combustion chamber 44 which communicates with the interior of the transition'section via'holes 26. Combustion chamber 44 has an outlet 48 which is disposed within outlet 42 of the water jacket. 'By concentrically locating

outlets

42 and 48, there is attained "an aspirator like action whereby coolant emerging from outlet 42 will suck gases (and fluids if there'b'ean'y) out of'the combustion chamber 44 via its outlet 48.

Also forming part of the transition section are several :baflie plates. In the illustrated embodiment there are five bafiie plates 50, 52, 54, 58, and'60 in the order named. All of these plates are attached at their peripheries to the cylindrical plate 24. In this connection, it is to be observed that'the plates are attached to plate 24 at points between's'uccessive rows of openings 26, so that each successivev 'pair'of plates forms a separate annular gasguiding channel which communicates with the combustion chamber via a selected'row o'f'holes 26.

'Ea ch'of the plates 50, 52, 54, 58 and 60 is provided with a relatively large center opening in line with the mufiie sections 2 and '4 and the openings in the

end plates

16 and 18. The openings in plates 50'and 60 are identical in size, as are the openings in plates 52 and 58. However the openings in plates 52 and 58 are slightly smaller than the openings in plates 50 and 60. The effective size of the center opening in plate 54 is still smaller. Plates 50, 52, 58and 60 also are curved in cross-'section'in the region surrounding their center openings as'shown at '62, so as to form a slight dished eifect. Because of the progressive decrease in size of the center openings of. plates 50, 52, and 54 in the order named, and also. because of the dished effect in the region of their center openings, gas migrating into the transition chamber from the'neighbori'ng mufiie section 2 will tend to stratify into dilferent streams which will be channeled radially outward toward the combustion chamber by the baffle plates 50, 52 and 54. Thisaction results from the fact that gases tend to follow the smallest radius. Thus, most of the-gas in a'stream'flowing out of mufile section 2 will tend'to followthepath having the smallest radius, which isclose against the wall of the muflle section and along the inner surface of end plate 16. Anotherportion of the gas will follow a slightly larger radius and'will flow betweengplates 50 and 52. The remaining portion of the gas will tend to follow a still larger radius andwill flow between plates 52 and 54. The stream produced by this process will be richer in the direction of the smaller radius, i.e. the streampassing outwardly between plates 16 and 50 will have a greater gas content than the stream passing outwardly between plates 50 and 52. The same type of gas Stratification occurs with respect to the oxygen gas flowing into the transition section from muffle section 4. This Stratification is helped by the fact that the gases are being continually sucked into thecombustion chamber by the aspirator effect created by

outlets

42 and 48.

This stratifying of gases is also h'elpedalong by additional means which also act to substantially-prevent.

commingling of the hydrogen and oxygen gases in the transition section. This additional'means comprises a hollow toroidal member 66 which is secured to the center baflle plate 54 at the edge defining its center opening and thereby acts to further limit the eifective size of that opening. The inner diameter of toroidal'mernber 66 is large enough to accommodate the endless belt conveyor 10. The toroidal member 66'isprovided with a series of apertures on its inner side and at approximately the 6 oclock position it is connected to a tube 68 which is'dis'posed in a radially-extending slot in plate 54 and which connects to a second tube 70 that projects outside of the transition section. Tube 68 is welded to plate 54 and also to tube 70 which extends through and is welded to plates 16, 50 and 52.

Although not shown, it is to be understood that the tube '70 is connected to a supply of a suitable inert gas under pressure. Preferably the inert gas is nitrogen; however, gases such as argon also are suitable. The nitrogen gas is supplied under a positive pressure relative to the pressure in the transition section 6. The nitrogen fills the toroidal member 66 and exits through the ports on its inner side. The pattern of distribution of the nitrogen gas exiting the toroidal member is shown by the plurality of small arrows in FIGS. 2 and 3. As is'evident from FIGS. 2 and 3, the ports in the toroidal member 66 produce a gaseous curtain of nitrogen which extends across the entire space encompassed by the toroidal member and which also is directed outwardly toward the mufile sections 2 and 4. The presence of this gaseous curtain not only prevents movement of hydrogen into the oxygen muflle section 4 and movement of the oxygen into the hydrogen'muffie section 2, but also minimizes and substantially prevents commingling of hydrogen and oxygen in the transition section by urging them into the channels defined by the various plates 50-60.

The small size of the ports 26 produces controlled flow of oxygen and hydrogen into the combustion chamber 44. Because the combustion chamber'is cooled by the water jacket surrounding it, it is diificult for the resulting mixture in the combustion chamber to react spontaneously.

Accordingly, to initiate combustion, there is provided a suitable igniter, preferably in the form of a spark plug 74, which projects into the combustion chamber near the outlet 48. Coolant is circulated continuously through the water jacket. The cooling elfect of the coolant helps control the combustion reaction so that there is a continuous low-level combustion without risk of violent explosion- The heat generated by the reaction is dissipated 'by the coolant and the combustion products are continuously sucked from the combustion chamber by the coolant flowing out of the outlet 42.

It is to be observed. that the cylindrical plate 24 also operates as an anti-backflash screen. In other words, it prevents the combustion reaction occurring in the chamber 44 from spreading into the channels defined by the plates 16, 50-60 and 18. Confinement of the combustion reaction in the combustion chamber is also helped by the gas stratification mentioned above whereby the greatest concentration of oxygen gas will occur between the plates 60 and 18 and the greatest concentration of hydrogen will occur between the plates 16 and 50, with progressively lower concentrations of oxygen and hydrogen occurring in the succeeding channels defined by the baflie plates and thelowest concentration of each gas occurring in the vicinity of the nitrogen gas curtain provided by the toroidal member 66. As a result there is no accumulation of a combustive mixture in the region of the toroidal member, such a mixture occurring only after the individual gases have entered the combustion chamber 44. The rate at which these gases admix is determined in part by the size of the ports 26 which are made relatively small.

Certain other advantages of the illustrated apparatus should be mentioned. For one thing, the hydrogen is cooled by the cooling effect of the water jacket on the plate 16 so that it enters the combustion chamber 44 below its flash point. A second advantage is that the illustrated transition section makes it possible to maintain a sharp temperature gradient between the two sections 2 and 4. This is achieved by virtue of the fact'that the bafiie plates Sit-6i together with the

end plates

16 and 18 provide conductive paths for rapidly transferring heat to the cooling jacket where the heat is dissipated by continuously circulating the coolant. Additionally, the battles 562-64) function as radiation shields and thereby prevent heat from migrating from the hotter muffle section to the cooler mutlie section. Additional dissipation of heat is achieved by the removal of the gases which have a temperature the same as the muflies from which they are extracted. While the amountof heat contained in the gases is small compared with the heat imparted to the transition section by the mufile walls 2 and 4, it is never theless extracted from the systems so as to facilitate the maintenance of a sharp temperature gradient between the two sections 2 and 4.

The outer diameter of the transition section 6 may vary without affecting the invention, except that it should be larger than the corresponding dimension of the muflie sections 2 tnd 4 which it connects in order to achieve an expansion of the gases. This expansion is helpful in that it produces a pressure reduction which is helpful in allowing the gaseous curtain to assist the baffle members in channeling the hot gases radially outward toward the combustion chamber 54. It is to be understood also that in practice the unit shown in FIG. 1 is mounted within suitable insulation which preferably is encased in an outer metal housing. The type of insulation is not critical and preferably it is in block form. These blocks can be laid up against the outer walls of the transition section as wellas up against the two mufiie sections 2 and 4. It is contemplated also that the mufiie sections 2 and 4 may be relatively long compared with the axial dimension of the transition section 6. In fact, one of the advantages of the present invention is that the transition section 6 functions as a highly efiicient heat barrier even though the spacing between the

plates

16 and 18 is quite small. In ractice, the spacing between the

plates

16 and 18 will be in the order of -12", in comparison with the mufiie sections 2 and 4 which may have a length in the order of several feet.

A further advantage of the present invention is that the trzmsition section shown in the drawing is entirely compatible with existing mufiie structures and is relatively easy to incorporate therein by welding. Construction of the transition section itself also is simple and relatively inexpensive.

Of course, the configuration of the transition section may vary. Thus, it is not necessary for the transition section to be circular in configuration. It may also be square or rectangular. Where a sharp temperature gradient is not too critical, it is possible to use a transition section having an even larger number of bafile plates. This may be desired where the concentration of gases in each of the muflle sections 2 and 4 is relatively high. It is also understood that the gaseous curtain may be produced by means other than a toroidal member. Thus, for example, it is possible to replace the toroidal member with a square or a rectangular hollow ring member having exit holes arranged to provide a gaseous curtain across the aperture defined by its inner surface. However, whatever the configuration of the member producing the gaseous curtain,

it must define an opening sufiiciently large so as not to interfere with movement of the conveyor 10 and also to accommodate the plate 8.

Obviously, many other modifications and variations of the present invention are possible in the light of the foregoing teachings. It is to be understood, therefore, that the invention is not limited in its application to the details of construction and arrangement of parts specifically described or illustrated, and that within the scope of the ap pended claims, it may be practiced otherwise than as spe cifically described or illustrated.

What is claimed is:

1. A transition section for a mufiie having first and second successive sections with a first gas circulating through the first section and a second gas combustible in the presence of said first gas circulating through said second section, said transition section adapted to be interposed between and joined to said first and second sections so as to provide a unitary muffie structure, said transition section comprising a pair of parallel plates having center openings whereby articles to be heat treated in the mufile may be transported from said first section to said second section through said transition section, means dispo-sedbetween said two plates for providing a gaseous curtain effectively isolating the area on one side of said means from the area on the other side of said means, and means connecting said two plates at their peripheries providing (a) separate passageways for withdrawal of said first gas from the region between one of said plates and said gaseous curtain and for withdrawal of said second gas from the region between the other of said plates and said gaseous curtain, (b) a chamber for mixing of said withdrawn gases under controlled temperature conditions, (c) means for combusting said gases in said chamber and (d) means for withdrawing the combustion product of said gases from said chamber.

2. A transition section as defined by claim 1 further including at least one additional plate disposed between each of said two plates and said means for providing a gaseous curtain, said each additional plate having a configuration adapted to promote withdrawal of gases in a direction radial to said transition section.

3. A transition section as defined by claim 2 further wherein said at least one additional plate is curved at its center away from said gaseous curtain providing means.

4. A transition section as defined by claim 2 wherein said at least one additional plate is attached to means forming a wall of said combustion chamber.

5. A transition section as defined by claim 4 wherein said wall has a'plurality of openings communicating with said regions between said pair of parallel plates.

6. A transition section as defined by claim 1 further including a cooling jacket surroundingsaid combustion chamber with a port connecting said combustion chamber and the interior of said cooling jacket, and further wherein said means for withdrawing the combustion product of said gases from said chamber comp-rises an inlet and an outlet means for said jacket and means for circulating a coolant through said jacket via said inlet and outlet means, whereby to create a suction effect at said port which causes said combustion product to be sucked out of said transition section by said coolant.

7. A transition section as defined by claim 2 wherein said means for combusting said gases is an electrically energized igniter at least part of which is disposed within said combustion chamber.

8. A transition section as defined by claim 1 wherein said means for providing a gaseous curtain is a hollow toroidal member, and further including means for supplying an inert gas to said toroidal member, said toroidal member having a plurality of apertures for emitting said inert gas in a pattern effectively forming said gaseous curtain.

9. A transition section for a muffle comprising first and second parallel plates having aligned center openings,

a member disposed between saidtwo plates for producing a gaseous curtain in the space-between said center openings, means securing saidjtwo :plates together and forming a combustion chamber adjacent to and substantia'lly surrounding the peripheries of said plates, means for channeling gases on either side of said gaseous curtainradia lly outward into said combustion chamber,

means for igniting withdrawn gases in said combustion chamben andm'eans for Withdrawing the combustion "product'of 'said'gases from said combustion chamber.

10. A transi's tion section as defined by claim 9 further including means for cooling said cornbustion'chamber.

11. A transition section as defined 'by claim ,10 where said cooling means comprises a'cooling jacket surrounding 'saidcombnstion'chamber.

12. A transition section as defined by claim 9 wherein said means securing said two plates together includes a partition member forming a wall of said combustion chamber, said partition having a series of apertures for causing the gases withdrawn via said channelling means meanscoiinec'tlrig 's'aidtwofplates together attheir periphery, saidm'eansincluding a cylindrical partition efiectively subdividing the space between said two plates into 'a combustionchamber having a combustion initiating elernent therein andan'additionalchamber disposed radially inward of said combustion chamber, means subdividing saidadditional'chamber whereby to form first and-second smaller chambers, means for channeling'gases entering 'said first smaller chamber into said combustion chamber,

means for channeling gases entering said second smaller chamber into said combustion chamber, means 'for preventing the gases from one'of said smaller chambers from entering into the other of said smaller chambers, and means for removing the combustion product from'said combustion chamber;

14. A'transition section'as defined by-claim 13 further including means for cooling said'combustion chamber.

15. A transition section as'defined by claim 13 wherein said means for preventing gases in one of said smaller chambers from entering the other of said smaller chambers comprises means 'for creating a gaseous curtain in a plane parallel to said 'parallel plates.

References Cited bythe Examiner UNITED STATES PATENTS 2,872,175 2/59 Guenzi 2638 3,125,327 3/64 Williams 26315 3,138,372 6/64 Beck 26337 CHARLES SUKALO, Primary Examiner.

JOHN J. CAMBY, Examiner.

Claims

9. A TRANSITION SECTION FOR A MUFFLE COMPRISING FIRST AND SECOND PARALLEL PLATES HAVING ALIGNED CENTER OPENINGS, A MEMBER DISPOSED BETWEEN SAID TWO PLATES FOR PRODUCING A GASEOUS CURTAIN IN THE SPACE BETWEEN SAID CENTER OPENINGS, MEANS SECURING SAID TWO PLATES TOGETHER AND FORMING A COMBUSTION CHAMBER ADJACENT TO AND SUBSTANTIALLY SURROUNDING THE PERIPHERIES OF SAID PLATES, MEANS FOR CHANNELING GASES ON EITHER SIDE OF SAID GASEOUS CURTAIN RADIALY OUTWARD INTO SAID COMBUSTION CHAMBER, MEANS FOR IGNITING WITHDRAWN GASES IN SAID COMBUSTION CHAMBER, AND MEANS FOR WITHDRAWING THE COMBUSTION PRODUCT OF SAID GASES FROM SAID COMBUSTION CHAMBER.