US2478092A - Metallurgical heating furnace - Google Patents

Description

ZAWMQZ 29 W49, D. EDGE METALLURGICAL HEATING FURNACE 5 Sheets-Sheet l Filed Sept.

W n r 4 JWM/Mm ffm? E565,

Patented Aug. 2, 1949 METALLURGICAL HEATING FURNACE Dexter Edge, Gary, Ind., assignor to Carnegie- IllinoisSteel Corporation, a corporation of New Jersey Application September 27, 1945, Serial No. .618,904

This invention relates to a metallurgical heating furnace and more particularly to such a furnace for annealing coils of strip. Originally, sheets were stacked upon a suitable base and covered with a box to protect them from the action of the air and furnace gases during the annealing cycle. Since development of continuous strip mills, the manufacturing trend is toward anneallng large coils of strip and the ultimate size of such coils is limited only by the capacity of the various furnaces, mills, etc., utilized in processing them up tothe annealing operation. After annealing the coils are subjected to temper rolling to impart stiffness to the product and small differences of temperature existing in any portion of the coils during annealing causes varying degrees of hardness in localized areas and prevents efficient temper rolling. One of the most dim-A cult problems to solve is the inability to obtain a uniform heat throughout the annealed' charge. This has led to the practice of soaking or holding the charge at a predetermined temperature for periods up to 25 hours duration with the hope that normal temperatures within the furnace will raise below normal temperatures of the charge t a degree where the two will become uniformly balanced. rlfhis practice is expensive as it limits output of the furnace and increases annealing costs. The present annealing of coils of strip is almost universally carried out in'radiant tube type furnaces in which coils are stacked upon individual bases with each stack being covered by separate inner covers made of light weight metal. A large furnace cover having a metal framework lined with insulating brick is placed over the rows of inner covers and heated with gas admitted to the radiant tubes which extend along the inside surfaces of the cover. In some instances the furnace cover is also equipped with additional radiant tubes extending between the rows of inner covers. Combustion takes place entirely within the radiant tubes which radiate heat to the covered stacks of coils mounted on the furnace base. In this type of furnace the products of combustion are discharged at temperatures of 1600 to l800 F. resulting in a high loss of fuel. .at the lower of these figures the loss of fuel is 45%., ln addition to thehigh fuel loss, transfer of heat between the products of combustion and the coils is Very poor. First there is a gap between the products of combustion Agenerated by the burners and the inner walls of the radiant tubes. lin some cases, special shaped refractory elements or core busters are inserted in the tubes to dehect the ame toward the portion of the tubes 4 Claims. (Cl. 263-42) facing the inner cover, and these elements ab' sorb acertain .amount of heat. There is also z gap between the radiant tubes and the inner covers through which the heat must pass before reaching the covers. The most commonly adopted practice utilized to improve heat transfer across this gap is to position the heat radiating tubes as closely as possible to theinner covers. This causes the covers to warp-severely and deteriorate to a degree where they are incapable of being used. Between the inner cover and the charge through which the heat is penetratedis a third gap. To improve heat transfer across this gap elevated bases with circulating fans have been resorted to. This arrangement. while improving the conditions of heat transfer to a certain extent, also adds to the maintenance problem of the equipment.

The three gaps described are interposed in relationship directly in the path of heat transfer from the radiant tube flames to the charge thus preventing efcient performance. Each must be separately raised to the convection temperature and as a result, the combination described has been found in practice to prolong the heating cycle two to three times. Inradiant tube and other conventional furnaces, no means are provided to maintain a full atmosphere within the convection zones to assist in efficient transfer of heat so as to obtain ultimate temperatures in the charge with uniformity. No apparent effort has been directed to incorporating the beneficial features inherent to eicent heat transfer into the design and operation of the furnace.

It is an object of this invention to provide a furnace which will eliminate the objectionable heat retarding or heat absorbing parts usually found in such furnaces.

Another object is to provide a method and apparatus for. reducing to a minimum the waste space within the furnace.

Still another object is to provide means for obtaining uniform temperature throughout the charge being annealed.

These and other objects will be more apparent after referring to the following specification and attached drawings, in which:

Figure 1 is a plan view of an annealing furnace;

Figure 2 is a sectional view taken on the line II-II of Figure l.;

Figure 3 is an enlarged sectional view of the coil supporting base; f

Figure 4 is a sectional view on the line IV-IV of Figure 1;

Figure 5 is a bottom View of the seal plate;

vsupporting plate 66.

Figure 6 is a top plan view of the spider base; and

Figure 7 is a graph showing the comparison between heating cycles.

Referring more particularly to the drawings, the reference numeral 2 indicates an annealing furnace having a base 4 and an outer cover 6 which 'is made of the conventional steel framework with a refractory lining 8. Projecting into the furnace cover are six burners I0, three at each end of the furnace. The burners are of the type generally disclosed in my copending application Serial No. 462,183, led October 15, 1942, now Patent No. 2,391,447, of which the present application is a continuation-impart. The burner includes a pipe I2 welded to the steel framework of the furnace cover and surrounding a pipe I4 having fins I6 which bear against the interior wall of pipe I 2 and hold pipe I4 centered therein. One end of pipe I4 extends through pipe I2 toward the furnace while the other end passes through the end I8 of the pipe I2. A second pipe 26 is arranged within pipe I4 and extends through the rear end of the pipe I4. The front end of pipe is supported by a ring 22 having a series of orifices 24 therein. The pipe I4 is connected to an air line 26 by means of an upright pipe 28. The rear end of pipe 20 is connected to a suitable source of fuel gas. Fastened to the rear end of pipe I2 is an upwardly extending pipe 30 which acts as an exhaust for the burnt gases. The combustible mixture from burners I0 is directed into the sealed furnace interior through a refractory tile 32 which has an opening 34 therethrough.

A passage 36 opens into the furnace and is joined laterally to the openingBB.

The furnace base 4 is provided with eight coil supporting bases 38. Each of the bases 38 includes a bearing plate 40 supported on the top of the refractory base 4. Upwardly extending lugs 42 on the plate 40 are spaced around the periphery of the plate and serve to center a support 44. The upstanding leg 46 of the support 44 is provided with spaced apart openings 48 around its periphery. Resting on top of leg 46 is a seal plate 58 having a pluralityof downwardly extending conical ns 52. The seal plate 50 has spaced apart circular anges 54 and 56 extending upwardly'therefrom to provide a sand seal 58. A spider base rests on the top of the plate 50 and is provided with radially extending vertical lugs 62 on its upper surface. The lugs 62 are provided with openings 64 and support the coil Extending through openings in the plate 50 and supported therein are pipes 68 and 'I0 which extend through the base 4. The plates 40 and 44 are provided with circular openings 'I4 and 12, respectively, which permit passage of the pipes therethrough. The spider base 60 and supporting plate 66 are provided with circular openings I6 and 18, respectively, opening 18 being of a diameter equivalent to the internal diametervfof coils of strip 80 which are supported thereon. 'I'he inner cover 82 covers the pile of coils tily and is received in the seal` the cover 6. In lighting the burners the velocity of the air through pipe I4 is reduced to a point where it just barely circulates through the furnace. A long flame igniting tube is inserted through the horizontal portion of the stack 30 into the combustion chamber and the gas is turned on in pipe 20 and ignited. During the warming up period, the quantity of gas and air is regulated until full volume is reached, after which the burner will function in the following manner. Gas is preferably fed through pipe 20 at a velocity of about 230 feet per minute while the pressure of the air and design of the orifices 24 is such that the tip velocity of the air will be approximately 16,000 feet per minute. With these velocities, the flammable mixture formed by the gas and air within the combustion chamber will assume a velocity of approximately 6000 feet per minute. The air entering the furnace is heated by the exhaust gases to approximately 1500" F. so that it expands to approximately three times its normal volume. This ls one of the contributory factors in reducing the velocity in the combustion chamber. By disposing the orifices 24 in a circular path, a vacuum is created within the central portion of the high velocity air stream discharged from the orifices and since the slower moving gas from pipe 20 is discharged into the central portion of the air stream, the gas will be attracted to and become a part of the air stream due to the pick-up action of the vacuum. As a result, a flammable mixture forms which contains a preponderance of initially preheated 1500 air and a relatively small proportional ratio of gas. At this point, the molecules of the mixture become separated but are almost instantaneously reunited by the pushing action imparted by the high velocity of the air which forces the products 0f combustion forward around the furnace as shown by the arrows in Figure l. The products of combustion in passageway 36 are attracted by the pulling eiect of the high velocity air resulting from the multiple of orifices 24 and are caused to enter the high velocity stream of incoming gas and air. The products of combustion not so attracted will be discharged through stack 36, during which discharge the residual heat of the gases will preheat the entering air and gas.

Since air is ordinarily the largest component part of the flammable mixture, the air instead of the fuel is used at a high velocity in order to obtain the maximum push-pull effect within the combustion chamber. of the air current is proportional to the circum- 1 ference thereof, air discharged from six orices as shown, is capable of exerting 2.4 times the moving or displacing effect of a single jet of air equal in area to the six.

Stack 30 performs the additional function of relieving pressure generated within the furnace by the combined forces incidental to expansion of the combustible fuel mixture during burning and inherent'pressure of the fuel stream. vIn this mannerthe hot burning atmospherevwithin the sealed interior of the furnace cover. is caused to circulate at velocities of a magnitude hitherto impossible of accomplishment in prior art furnaces.. y

Prior to applicants invention itwasthought to be impossible to go beyond the critical speed of flame propagation which, with ordinary fuels, is a maximum of approximately feet per minute. Applicant has found that if effective recirculation of combustive gas is provided. it is possible to go beyond the critical speed of name Since the pick-up effect v propagation and apply high velocity air to create thenecessary increase in recirculating rate.' Instead of a visible ame there is only a small pilot orignition flame at the end of the burner tip and the major portion of the gas fuel moving at a slow velocity is picked up with the hot recirculating atmosphere and carried at a high velocity around and around in the combustion chamber. The-surplus of the combusted gas is discharged through the stack. In other Words, the combustion gases are moving so fast and are so thoroughly mixed that the whole atmosphere within the heating chamber does not act in the same manner as the ordinary burning flame, but creates a reddish glow of burning gas particles *a and adjacent air particles which are finely di vided. The recirculating products of combustion l .are sumciently high in temperature to serve as which the system is designed. The gas and air are normally preheated and the discharge of burnt gases is normally on the outside of the gas, but these features are not absolutely necessary to the carrying out of my invention. While a tip velocity of 15,000 feet per minute is ordinarily used, the burner would function if the tip velocity were as little as 5000 feet per minute. It is essential that there be a difference in velocity between the gas and the air since the air must first act to suck in the gas and then subsequently v push it. The discharge of the gas from the furnace is shown to be at the point where the gas and air enter the furnace. l This is important from the standpoint` of obtaining maximum emciency, but the discharge could be at other points as temperatures required may be in a range of from 800 to 2000 F.

Part of the hot furnace gases circulates through the openings 48 in the support 44, thus efliciently transferring heat to the bottom of the stacks of coils. Fins 52 increase the emciency of this heat transfer. Since each charge is` separately surrounded by moving currents of heated gases, it will be quickly raised to annealing temperature which -will be uniform throughout the coils. De oxidizing gas is furnished to the coils through the Y pipe 68 and exhausts through pipe l0. The circulong as the products of combustion return to the point of origin for recirculation.

In other words, the burner l0 operates in a manner comparable to that in my above identined copending application with the exception -that the endless tube loop of the latter is replaced by th'e unoccupied space sealed within the cover 6. Both of these spaces serve as combustion chambers for the fuel and air. The space within the cover is reduced so that there is 15 cubic feet of space for each ton of material be*- ing annealed, this comparing with prior ratios of i9 to 25 cubic feet for each ton of material. This arrangement provides for completely filling all of the heating space with hot atmosphere which is moving rapidly so that heat is very eiciently transmitted to the charge by convection.

In prior art furnaces, attempts have been made to create an active atmosphere under the inner cover by using circulating fans, but these have only been partially successful. By the use of this invention, the entire innery covers heat up almost immediately after the furnace has been ignited, whereas in radiant tube equipped furnaces the tubes themselves must rst be heated to radiance before any heat will be available for work. In this invention a temperature ranging from 1300 to 1500 F. is adequate to heat the coils, whereas in prior art furnaces the radiant tubes must be 4heated to a temperature of from 1600" to 1800 F. The invention could also be used in connection with various heat treatments required for high carbon and alloy steel products where the lation of this gas is shown by the arrows.

Figure 7 compares performance of standard radiant tube equipped annealing furnaces with the furnace of my invention. Curve il represents the maximum and minimum temperatures of the charge registered by top and bottom thermocouples A and B and curve H03 represents the temperature of the inner cover. Corresponding curves 02 and |04 represent performance of standard radiant tube annealing furnaces. In curve H03 the 1400 temperature of the inner cover is reached in 12 hours and remains constant throughout the annealing cycle. Corresponding curve |04 is erratic and it is necessary to maintain the temperature at approximately l500 instead of 1400. In curve IUI, starting .with the eighth hour, temperature B of the bottom of the charge increases at a faster rate than temperature A at the top of the charge and the two temperatures coincide at approximately 1300Q after 30 hours. In curve M12 the temperature B at the bottom of the charge lags behind temperature A at the top during the entire annealing cycle and at'no time during the process will the temperature difference between A and B become less than 90 F.

While one embodiment of my invention has vbeen shown and described, it will be apparent that other adaptations and modications may be made without departing from the scope of the following claims; i

I claim:

l. Metallurgical heating apparatus comprising a furnace for containing a burning mixture of gas and air, said furnace having a refractory wall, a passage through said wall into said furnace, a second passage in said Wall having one end opening into said furnace and its other end joining laterally to the rst passage, superimposed radially spaced gas and air pipes inserted in said first passage with their outlets substantially flush relative one another and adjacent the junction of said passa-ges, and an annular series of velocity increasing orices for the outlet of said air pipe, said gas pipe being inside said air pipe and the latter being radially' spaced inside said first passage.

2. Metallurgical heating apparatus comprising a furnace for containing a burning mixture of gas and air, said furnace having a refractory wall, a passage through said Wall into said furnace, a second passage in said Wall having one end opening into said furnace and its other end joining laterally to the first passage, superimposed radially spaced gas and air pipes inserted in said first passage with their outlets substantially flush relative one another and adjacent the juncsage, and an exhaust stack for the space be- -feetrfor each ton of strip, means for introducing air into said furnace at a velocity of at least 5000 feet per minute, means adjacent said rst means for introducing a fluid fuel into said furnace at `a lower velocity than the air, and means surrounding the second named means for exhausting products of combustionfrom said combustion furnace.

4. A furnace according to claim 3 in which the means for supporting the coils includes a seal plate, said plate being provided with a sand seal for said inner cover, a support, and upstanding legs on said support for supporting said seal plate, said legs having openings therein for permitting circulation of the furnace atmosphere therethrough.

DEXTER EDGE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,017,621 Clarke Feb. 13, 1912 1,306,234 Schutz June 10, 1919 1,390,783 Hering Sept. 13, 1921 1,500,103 Burden et al. July 8, 1924 1,501,532 Egler July 15, 1924 1,617,609 Smith Feb. 15, 1927 2,073,724 Baker Mar. 16, 1937` 2,084,241 Capper June 15.11937 2,163,762 Noack et al June 27, 1939 2,215,081 Hess Sept. 17, 1940 2,250,868 v Huil July 29, 1941 2,391,447 Edge Dec. 25, 1945 FOREIGN PATENTS Number Country Date '490,958 Great Britain 1934 OTHER REFERENCES Industrial Furnaces, vol. II, Trink, 2nd ed., New York, 1942, pages 47 through 55;

Images