US2910285A - Heat treating furnace - Google Patents

Description

Oct. 27, 1959 F. E. HARRIS 2,910,285

HEAT TREATING FURNACE Original Filed Aug. 29, 1952 I 2 Sheets-Sheet 1 EA;- m

INVENTOR. flag/Z. Haw/Z3 Oct. 27, 1959 F. E. HARRIS 2,910,285

HEAT TREATING FURNACE Original Filed Aug. 29, 1952 I 2 Sheets-Sheet 2 United States HEAT TREATiN G FURNACE 'ZFloydJHarr-is, Flint, Mich., .ass'ignor to The Dow FurnaceCompanvDetroit, Mich., a corporation of Michi- Continuation of application Serial No. 589,325, June 4, 1956, which isn continuation of application seriai No. 307,035, August 29, 1952. This application September 30, 1958, Serial No. 765,699

18 Qlaims. (Cl. 263-15) This invention in general rel-ates to heat treating furnaces and has particular reference to a novel and highly efficient and improved radiant heating tube therefor.

I This application is a continuation of my copending application Serial No. 589,325, filed June 4, 1956, which inturn-is a continuation of application Serial No. 307,- 035., filed August 29, 1952, for Heat Treating Furnace.

Radiant heating tubes of the type used in controlled atmosphere heat treating furnaces are subjected to severe temperature conditions in use, and while the'art has long recognized the desirability of firing such tubes .in such a manner as to avoid hot spots which cause the tubes to burn out, this objective has not been satisfactorily achieved, particularly in tubes which must be operated athigh temperatures of the order of 1800 to 1900 F.

ant heating atube structure which overcomes the foregoing objections.

According to my invention it is possibleto fire the heating tube so as to avoid hot spots and so as uniformly and efficiently to utilize the radiantheat transfer surface exposed within the furnace chamber.

A principal object of the invention, therefore, is to provide va new and improved radiant heating tube for heat treating furnaces.

Qther and further objects of the invention will be apparent from the following description and claims and may be understood by reference to the accompanying drawings, of which there are two sheets, which by way of illustration show preferred embodiments of the invention .and what I now consider to be the best mode .in which I have contemplated applying the principles of my invention. Other embodiments of the invention may be used without departing from the scope of the present invention as set forth in the appended claims.

.In the drawings:

Fig. 1 is a vertical .cross sectional view of a heat treatingfurnace embodying my invention;

Fig. 2 is a vertical sectional View of the furnace taken at right angles to the view shown in Fig. 1;

Fig. 3 is an enlarged vertical sectional view, partly broken away, of one form of radiant heating tube embodying my invention;

Fig. 4 is a cross sectional view taken along the line 4-4-of Fig. 3;

iFig. 5 is an enlarged vertical sectional view showing 'azmodified form of heating tube embodying my invention;

1 .Fig. 6 is 'a cross sectional view taken along "the line r Ice 2,910,285

Patented Oct. 27., 195.9

vFig. .7 is an elevational view. of part of the burner assembly.-

As shownin Figs. 1 and 2, a heat treating furnacesembodying my invention comprises heat insulating walls .10

defining a furnace chamber 12 provided with an access opening .14. A roller track indicated generally at :16

forms a support .for a work box 18, the box having a perforate bottom and being open at the top and adapted forholding a batchof pieces of work to be heat treated.

Depending upon the size of the furnace chamber 12 and the heating temperature desired therein, there is provided one or more radiant heater tubes '20, there being between the two pairs of radiant heater tubes 20.

An electric motor driven centrifugal type offan, indicated generally at 22, depends from the roof of the furnace chamber 12 and is arranged to draw atmosphere in the furnace chamber upwardly through the batch of work contained within the work box 18 and to discharge the same radially outwardly where the furnace wallsand cooperating baffles cause such atmosphere to flow downwardly over the heating tubes 20 so as to be heated thereby, and thence upwardly through the track 16 and .throughand'o-ver the work contained in the box 18. The atmosphere is recirculated within the furnace chamber -for asufiicient length of time, depending upon the -speci fications of the treatment desired, so as to bringthe work contained within the box 18 to control temperature and then to effectthe'desired surface treatment thereof.

Beforethe work is inserted into the chamber 12 the furnace is brought to the desired temperature and provided .with the desired controlled gaseous atmosphere which is maintained during the heat treating operation.

Thework box 18 may be inserted in and removed from the chamber 12. through the access opening 14 which is closed by a door 2 A vestibule 26 encloses the door and the outer end-of the access opening 14 and may be provided with a pair of vertically movable roller tracks 28 and 30, commonly referred to as an elevator. The tracks 28 and Stl aresupported upon chains 32 which are adapted to he wound upon or unwound from a motor driven dnum 34 so'that either of the tracks 28 or 30 may .be arranged in line with the track 16in the furnace chamher so that work may be pushed into or removed'from the furnace chamber.

Work in the box 18 on the track 28 may be lowered into a quench tank 36 for quenching the work before removal from the vestibule through the door controlled opening 38. On the other hand, work in the box 18 on the track 30 may be elevated in the vestibule so as to slow cool thesame, the upper part of the vestibule forming a slow cool chamber indicated generally at 40. The bottom end of the vestibule is sealed by the liquid in the quench tank, and the vestibule may be provided with the same atmosphere as in the furnace, or with any other desired atmosphere.

The furnace door 24 is mounted on chains 42 which maybe wound upon or unwound from a motor driven drum 44 for closing or exposing the access opening 1-4 The vestibule door 46 may be moved between its open and closed positions by an air operated cylinder 48. The liquid in the quench tank may be circulated by a motor drivenpropeller 50, and the temperature of the liquid in the quench tank may be controlled so as to obtain any desired temperature thereof.

The furnace structure may be of the type disclosed and claimed in the application of John A. Dow, Serial No. 746,277, filed May 6, 1947, for Heat Treating Furnace and Method. 'I'he heater tube construction the furnace chamber.

' As shown in Figs. 3 and 4, the radiant heater tube construction comprises a U-shaped metal alloy tube mounted on a plug 54 which is removably secured within an opening in a wall (the roof) of the furnace chamber,

heat insulating refractory material therebetween. One leg 56 of the tube comprises a firing leg, while the other leg 58 comprises an exhaust leg, and the legs are connected by a return bend 60.

As shown, a pipe or tube 62 projects through the firing leg 56 and is provided with an opening 64 communicating with the furnace chamber 12. The tube 62 may comprise a supply passage through which the controlled gas atmosphere is supplied to the furnace chamber 12, or it may form the retort of a catalytic gas generator, in which event the tube 62 will be filled with a mass of a catalyst 65 such as nickel coated pieces of refractory or nickel shot.

The tube 62 provides a retort in which the gas supplied thereto in the presence of the catalyst 65 and under the influence of the heat generated in the firing leg 56 produces a gas of proper analysis for the process being carried on in the furnace chamber 12, and such gas is discharged into the furnace chamber through the outlet gas opening 64.

For carbo-nitriding or cyaniding, the controlled gas atmosphere in the furnace chamber may have approximately the following composition: 40% hydrogen; 40% nitrogen; 19 /t% carbon monoxide; and a trace of carbon dioxide and ammonia.

The generator tube 62 may be arranged to maintain the atmosphere within the furnace chamber 12 of the desired composition, although other components of such atmosphere might be introduced into the furnace other than through the tube or generator 62. Preferably, each of the radiant heating tubes 20 has a generator or gas supply tube 62 associated with the firing leg thereof, although if desired the generator or supply tube 62 may be disposed in the exhaust leg 58 or both legs may be provided with such a tube. For some operations it may not be necessary that all of the heating tubes be provided with a generator or gas supply tube 62.

The tube 62 at its lower end is welded into a neck in the return bend 60 in such manner that the products of combustion within the tube 20 will not escape into The outer end 66 of the generator or supply tube 62 is adapted to be connected to a suitable gaseous supply, and if the tube 62 functions merely as a supply tube, such gaseous supply will be an independent atmosphere generator. The space within the firing leg 56 and the return bend 60 and outside of the tube 62 comprises an annular combustion chamber, although according to the invention combustion will be substan tially completed in firing leg 56. If desired, the tube 62 need not be used as a generator or gas supply pipe and in such event the tube 62 need not project through the return bend as shown in Fig. 3, but should extend downwardly in the firing leg 56 below the lower end 80 of the tube 68. For example, in an assembly where approximately four feet of the U-tube is exposed inside the furnace chamber, I would extend tube 62 downwardly below the lower end 80 of tube 68 at least one foot and such shortened tube 62 should be closed at either end thereof, or it could be a solid bar.

It is desirable that combustion of the fuel supplied to the tube 26 occur entirely and uniformly within the firing leg 56 and the return bend 60, in order to avoid hot spots in the wall of the tube and to efiiciently utilize the heat radiating surface of the tube 20 exposed within the chamber 12. It is also desirable that combustion begin in the firing leg 56 at a point in the tube the plug comprising inner and outer metal plates with just below that part surrounded by the plug 54. In order to achieve this, I employ a tube 68 which projects into the firing leg 56 to a point closely adjacent to the inner face of the heat insulating wall through which the tube 20 extends. This tube 68 in conjunction with the tube 62 and the firing leg 56 defines an annular secondary air supply passage 70 surrounded by an annular fuel supply or premix passage 72. The tube 68 outside of the furnace is provided with a fitting 74 which fits around the tube 62 and is secured to the end of the firing leg 56 and forms a means by which secondary air at the desired pressure is supplied under suitable controls to the secondary air passage 70 through the inlet port 76 and through which fluid fuel, for example, natural gas, is supplied under suitable controls and at the desired pressure to the fuel passage 72 through the inlet port 78. Y

The gas supplied through the passageway 72, frequently referred to as premix, does not have sufficient air to support combustion even at low velocity. However, such premix together with the secondary air supplied through the passage 70 begins mixing at the junction of the passages 70 and 72 to form a combustible mixture immediately below such junction, the junction being at the lower end 80 of the tube 68. The passage 72 is provided with means comprising a plurality of elongated spiral rods 82 which separate the fuel supply passage 72 into a plurality of spiral paths and which causes the premix after it emerges from the passage 72 to flow in a spiral path around the annular stream of secondary air emerging from the air supply passage 70 for mixture therewith. The spiral rods 82 not only afford a directional effect to the premix, but also promote uniform distribution of the premix around the passageway 72 and uniform distribution of the combustible mixture around and throughout the length of the firing leg 56. This arrangement slows up the velocity of the combustible mixture through the firing leg and the whirling action produced thereby increases the velocity of propagation of the flame. Such formation and uniform distribution of a combustible mixture as just described at the junction of the passages 70 and 72 and throughout the firing leg 56, insures that combustion will occur at such junction and uniformly throughout the firing leg, and be substantially completed before the gases pass into the exhaust leg.

The rods 82 extend for a suificient length around the tube 68 so that the gaseous fuel flowing through the spiral paths is substantially free of turbulence as it emerges from the lower end of the passage 72, and the rods 82 are preferably constructed and arranged so that they will create as little turbulence and eddy currents in the gas flow as possible. The rods 82, together with the outer surface of the tube 68 and the inner surface of the firing leg 56, are constructed and arranged so as to deliver the gaseous fuel into the combustion chamber with uniform distribution of the fuel around the annular secondary air stream and so that gradual mixing of the fuel with the secondary air will begin just below the tube end 80 and will progressively continue throughout the firing leg 56 so that the combustible mixture will begin burning just below the plug 54 and will burn uniformly throughout the firing leg 56 so that the tips of the flame will just curl around the return bend 60. In other words, the whirling action imparted to the premix before it emerges from the passageway 72 promotes the distribution of the mixture in the firing leg and makes it possible to produce combustion at a uniform rate throughout substantially the entire extent of the firing leg below the tube end 80. This uniformity of firing obtained reduces the possibility of hot spots and makes for uniformity of temperature throughout the full extent of the firing leg, and this in turn makes it possible to fully utilize the heat radiating surface of the firing leg for heating the furnace chamber and the work contained therein. The arrangemeat preferably is such that the velocity of the mixture at the junction of the air and fuel supply passages is greater than the rate of flame propagation in a still mixmay be provided with a plurality of spiral rods, such as the rods 82, forsepa'rating the air stream into a plurality 6f spiral paths and which will cause the secondary air after it emerges from the passage 72 to flow in a spiral path in the same direction of flow as the 'spiral'path of new of the premix, and this preferably is employed when the shortened tube 62 is used.

In the modification illustrated in Figs. :5 and 6, in stead of employing a 'U-shaped tube, I employ a straight tube 90 which is closed at its lower end by plate 95. The tube 90 may be provided with a generator or supply tube 62, as shown in Fig. 3, provided with an outlet opening into the furnace chamber through which the atmosphere generated or passing through the tube 62 passes into the furnace chamber, or it may be provided with a shortened tube '62 as previously described.

In the 'case of Fig. 5, a tube 92 nested within the tube 90 and arranged in spaced relation with such tube and the tube '652, defines a leg 94 on the outside of the tube 92 and an exhaust leg 96 on the inside of the tube 92. The tube 90 is also fprovided with a tube 98 corresponding' with the tube 68 which cooperates withthe tube 90 and the tube 92 to form an annular secondary air supply passage 100 surrounded by an annular fuel supply passage l d 2 In this case,eachof the passages 100 and '102 is provided with spiral means in the form of three rods 104 or 106 which separate the passages 100 and 102 into a plurality of spiral paths, the rods 1% functioning to impart a spiraling action to the fuel so as to cause it to spiral after it emerges from the passage 102, and the rods 1'04 functioning to impart a spiraling action to the secondary air 'so as to cause it to Spiral after it emerges from the air supply passa'ge 1 00, both 'the fuel and air spiraling in the same direction and gradually intermixing so as to provide a combustible mixture throughout the length of firing leg 94 and which will burn at a substantially uniform rate throughout the extent of the firing leg 94.

The combustible mixture formed at the junction of the passages 100 and 102 and throughout the firing leg '94 "will begin burning at such junction and will burn at a substantially uniform rate through the length of the firing deg 94. The tips of the flame, however, should just curl around the lower end of tube 92 and the fuel supply just before or in the return bend and in the exhaust leg should be controlled so that the flame will not impinge on the plate 95. Firing of "such mixture is substantially complete before the gases pass into the exhaust leg 96. 'A fitting 108 surrounding "the tube 92 cooperates therewith and with the end of the tube '90 to supply secondary "air at the desired pressure to the passageway 100 through "the Lpbrt 110 and premix at the desired pressure to the passage 102 through the port 1 12.

The mixing of the premix and secondary air and the combustion thereof in the radiant heater tube 90 Will be substantially the sameas that which obtains in the tube 20. It should be noted, however, that in the modification of Fig. 5, the exhaust gases will preheat the secondary air. While -I prefer to introduce the premix outside of and around the secondary air and believe that the heat generated by the combustion thereof is morequickly dissipated through the tube walls, the secondary air might be introduced o'utside or and around the premix by intel- 7 a 6 changing the fuel and air supply connections to the tube fittings 74 and 108.

' Without the means for imparting a spiraling action to the fuel stream and/ or the air stream, at high settings of the fuel and air controls, combustion will occur beginning and even above the discharge end of the exhaust leg, and such combustion is not uniform and even, with the result that premature burn-outs of the. radiant tubes are frequently experienced. In addition, the heat radiating surface of the firing tube was not fully utilized, with the result that the desired heat output could not be obtained from such heating tubes, except by overfiring the same which accelerates the premature destruction thereof. I

While I have illustrated and described preferred embodiments of my invention, it is understood that these are capable of modification, and Itherefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes and alterations as fall within the purview of the following claims.

I claim:

1. In a furnace, a heat insulating Wall defining a't least part of a furnace chamber, tubular means projecting through said Wall and into said chamber, said tubular means providing an elongated annular combustion chamber with an inlet end and an outlet, means including a tube associated with the inlet end of said combustion chamber and tubular means and forming an annular secondary air supply passage to the combustion chamber surrounded by an annular fuel supply passage to said combustion chamber, means for supplying an incombustible fuel mixture under pressure to one end of said fuel supply passage for discharge from the other end thereof into said combustion chamber, means for supplying .secondary air under pressure to one end of said air supply passage for discharge from the other end thereof into said combustion chamber, said tube terminating at a point closely adjacent the inner face of said heat insulating Wall, and spiral means in said fuel supply passage separating the same into a plurality of spiral paths and which function to cause the fuel mixture as it emerges from the 'fuel passage into said combustion chamber to how in a spiral path around the stream of secondary air to form'a combustible mixture at the interface and throughout the length of the combustion chamber whereby gradual mixing to the fuel mixture and air will be initiated at the junction of the fuel and air passages and continued throughout the length of said combustion chamber, said spiraling action functioning to increase substanbustible mixture, said tubular casing of said generator having a gas outlet into said furnace chamber through which gas generated .in said generator is supplied thereto.

4. Apparatus according to claim 1 whereintubingencloses the space surrounded by said annular combustion chamber and communicates with said furnace chamber and forms apassage through which a gaseous atmosphere is supplied to said. furnace chamber.

5. Ina-furnace, a radiant heater having an elongated tubular combustion chamber provided with an inlet end and an exhaust, means including a tube associated with the inlet end of said combustion chamber and forming a secondary air supply passage to said combustion chamber, and a fiuid fuel supply passage to said combustion tially the velocity of propagation of the flame of said chamber, one around the other, means for supplying an incombustible fuel mixture under pressure to one end of said fuel supply passage for discharge from the other end thereof into said combustion chamber, means for supplying secondary air under pressure to one end of said air supply passage for discharge from the other end thereof into said combustion chamber, and spiral means in one of said passages separating one of the same into a plurality of spiral paths and functioning to cause the gas as it emerges from said one of said passages to flow in a spiral path adjacent the stream of gas supplied by the other of said passages to form a combustible mixture at the interface and throughout the length of the combustion chamber whereby gradual mixing of the fuel mixture and air will be initiated at the junction of the fuel and air passages and continued throughout the length of said combustion chamber, said spiraling action functioning to increase substantially the velocity of propagation of the flame of said combustible mixture during combustion while decreasing the velocity of the mixture through the combustion chamber.

6. Apparatus according to claim wherein said fuel passage surrounds said air passage. I

7. Apparatus according to claim 5 wherein said combustion chamber is formed in part by a U-shaped tube into which said tube projects.

8. Apparatus according to claim 5 wherein said combustion chamber is formed by and between two nested straight tubes, the outer tube being closed at one end, one end of the inner tube being spaced from the closed end of the outer tube and forming a passage through which the products of combustion are exhausted from said combustion chamber, said last-mentioned passage being in heat exchange relation with one of said supply passages.

9. Apparatus according to claim 5 wherein said combustion chamber is formed by two nested straight tubes, the outer tube being closed at one end, one end of the inner tube being spaced from the closed end of the outer tube and forming a passage through which the products of combustion are exhausted from said combustion chamber, said last-mentioned passage being in heat exchange relation with one of said supply passages, and a tubular gas generator casing disposed within said inner tube and heated by the products of combustion therein.

10. A radiant heater for a furnace chamber comprising a tubular combustion chamber provided with an inlet end, means including two nested spaced tubes associated with said inlet end of said combustion chamber to form a secondary air supply passage to said combustion chamber and a fluid fuel supply passage to said combustion chamber, one around the other, means for supplying an incombustible fuel mixture under pressure to one end of said fuel supply passage for discharge from the other end thereof into said combustion chamber, means for supplying secondary air under pressure to one end of said air supply passage for discharge from the other end thereof into said combustion chamber, and spiral means in one of said supply passages separating the same into a plurality of spiral paths and functioning to cause the gas as it emerges from said one of said supply passages to flow in a spiral path adjacent the stream of gas supplied to the combustion chamber through the other of said supply passages to form a combustible mixture at the interface and throughout the length of the combustion chamber whereby gradual mixing of the fuel mixture and air will be initiated at the junction of the fuel and air passages and continued throughout the length of the combustion chamber, said spiraling action functioning to increase substantially the velocity of propagation of the flame of said combustible mixture during combustion.

11. Apparatus according to claim 10 wherein said tubular combustion chamber comprises a U-shaped tube.

12. Apparatus according to claim 10 wherein said heater comprises a straight tube and wherein the inner phere generator casing arranged within said inner tube so as to be heated by the products of combustion generated in said combustion chamber.

14. A radiant heater for a furnace chamber comprising a tube forming a tubular combustion chamber provided with an inlet end, means including a tubular member associated with the inlet end of said combustion chamber and tube to form a secondary air supply passage to said combustion chamber and a fluid fuel supply passage to said combustion chamber, one around the other, means for supplying a stream of an incombustible fuel mixture under pressure to one end of said fuel supply passage for discharge from the other end thereof into said combustion chamber, means for supplying a stream of secondary air under pressure to one end of said air supply passage for discharge from the other end thereof into said combustion chamber, flow directing means other than the confining walls of the combustion chamber and passages acting on such streams to so direct the flow thereof throughout the combustion chamber as to form a combustible mixture at the interface between such streams and throughout a substantial part of the length the combustible mixture during combustion, and a gas atmosphere generator having a casing arranged within said tubular member so as to be heated by the products of combustion generated in said combustion chamber.

15. A radiant heater for a furnace chamber according to claim 14 wherein a tube arranged between said tubular member and said generator casing forms at least part of an exhaust passage from said combustion chamber, said exhaust passage being in heat exchange relation with one of said streams. I

16. A radiant heater for a furnace chamber comprising a straight tube forming a tubular combustion chamber provided with an inlet end, means including two nested spaced straight tubular members associated with the inlet end of said combustion chamber and tube to form a secondary air supply passage to said combustion chamber and a fluid fuel supply passage to said combustion chamber, one around the other, means for supplying an in combustible fuel mixture under pressure to one end of said fuel supply passage for discharge from the other end thereof into said combustion chamber, means for supply ing secondary air under pressure to one end of said air supply passage for discharge from the other end thereof into said combustion chamber, a gas atmosphere generator having a casing arranged Within the inner one of said tubular members and so as to be heated by the products of combustion generated in said combustion chamber, the space between the inner one of said tubular members and said gas generator casing forming at leastpart of an exhaust passage from said combustion chamber in heat exchange relation with one of said streams and said generator.

17. A radiant heater for a furnace chamber according to claim 14 wherein a tube arranged between said tubular member and said generator casing forms at least part of an exhaust passage from said combustion chamber, said exhaust passage being in heat exchange relation with one of said streams, and wherein said tube and tubular member are straight throughout their length.

18. A radiant heater for a furnace chamber comprising a straight tube forming a tubular combustion chamber provided with an inlet end, means including two nested spaced straight tubular members associated with the inlet end of said combustion chamber and tube to form a secondary air supply passage to said combustion chamber and a fluid fuel supply passage to said combustion chamber, one around the other, means for supplying an incombustible fuel mixture under pressure to one end of said fuel supply passage for discharge from the other end thereof into said combustion chamber, means for supplying secondary air under pressure to one end of said air supply passage for discharge from the other end thereof into said combustion chamber, a tubular means aranged within the inner one of said tubular members and so as to be heated by the products of combustion genstreams, said tube, tubular members, and tubular means being straight throughout their length.

References Cited in the file of this patent UNITED STATES PATENTS Lacy et a1. Jan. 1, 1935 Dow May 19, 1953

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