US2799493A - Furnace construction and its method of operation - Google Patents

Description

July 16, 1957 1.. E. BROWNELL ET L FURNACE CONSTRUCTION AND ITS METHOD OF OPERATION Filed July 14, 1954 INVENTORS 8190mm,

United States Patent M FURNACE CONSTRUCTION AND ITS METHOD OF OPERATION Lloyd E. Brownell, Richard A. Flinn, and Edwin H. Young, Ann Arbor, Mich.

Application July 14, 1954, Serial No. 443,262

7 Claims. (Cl. 266-32) This invention relates to improvements in the construction and operation of a furnace such as a cupola or blast furnace or the like.

An important problem in the operation of such furnaces is to maintain a critical high interior temperature approximating 3000 F. without overheating and disintegrating the furnace sidewalls. The control of carbon, silicon, and other critical elements in the melt depends upon both this temperature level and its stability. In addition the satisfactory production of castings requires the attainment of a proper minimum temperature on tapping, which temperature in turn depends upon the melting zone temperature.

A temperature as high as 3000 F. is quite harmful to the refractory lining used in such furnaces. Therefore it is common practice to cool the hottest sections of the refractory lining in blast furnaces. To a lesser extent this practice is followed in cupola operation. If the refractory lining is not cooled, as for example in the operation of a cupola, the lining is gradually lost by fusion and combination with the slag. This results in a daily shutdown and the costly replacement of the lining. Furthermore when the furnace lining combines with the slag, unpredictable and undesirable slag compositions result which prevent the proper removal of sulfur or phosphorus by the slag.

It is also usually desirable in efiicient cupola and blast furnace operation to preheat the incoming air to a predetermined critical temperature ranging between approximately 600 F. to 1000 F. depending upon such operating conditions as the grade of the iron being melted or the ore being reduced. Preheating of the air has been accomplished heretofore by various types of preheaters, utilizing for example the combustion of exhaust gases from the furnace. cumbersome expensive equipment.

It is important to note in regard to the above that the heat to be removed from the furnace walls is essentially equal to the'heat required to preheat the air for combustion, so that a demand has long existed for suitable and economical means for transferring heat from' the walls to the incoming air, whereby the two operations of cooling and preheating might be performed simultaneously and with considerable economy.

As a means of cooling the furnace walls, it has been common heretofore to circulate water or air through ducts located within or adjacent to the furnace walls. to cool the latter. In such cooling the heat removed from the furnace walls is usually wasted. The heat exchange area afforded by the furnace walls is inadequate to 'heat the circulating air to above 600 F. and at the same time adequately cool the furnace walls. Air is such a poor heat transfer medium that in the case of air cooling excessive quantities of air are required to cool the furnace walls,

resulting in a relatively small increase in the air tempera ture. Thus only a fraction of the cooling air is required to support combustion in the furnace and that fractionv Such preheating is costly and involves 2,799,499 Patented July 16; 1957 must be heated additionally by a suitable preheater in order to obtain the required 600 F. to 1000 F. temperature. The excess cooling air must be exhausted to the atmosphere and wasted. Where water is circulated as the coolant, the water temperature cannot exceed approximately 300 F. without excessive pressure, so that the water coolant cannot be employed to preheat the incoming air to 600 F. In addition apparatus must be employed to cool the water before it can be recirculated as a coolant.

Not only does the circulation of air or water coolants in the furnace sidewalls fail to lend itself to efficient preheating of the incoming air to the desired optimum temperature, but the low temperature at which such coolants must be employed frequently causes localized overcooling of the furnace sidewalls, resulting in the bridging of frozen slag from the sidewalls into the melting zone. The effect of bridging is to retard the flow of air and gases of combustion through the coke and slag, thereby to reduce the temperature in both the combustion zone and in the underlying well for the melt and seriously impair both the refining action and tapping or casting operation. The reduced temperature results in additional bridging, so that the effect is self generating. The obstruction to air flow caused by bridging results in localized air streams of excessive velocity which in turn cause localized oxidation of carbon, silicon, manganese, sulfur and other important elements.

For the foregoing reasons among others, the use of air and water coolants, particularly in cupola operation, has been far from satisfactory. Nevertheless the requirements of maintaining proper refining conditions, including the aforesaid temperature and combustion control and slag composition, while avoiding excessive overheating and destruction of the refractory lining, has long confronted the art with serious problems involving operating economy and efiiciency.

An important object of the present invention is to provide an improved method and apparatus for overcoming the foregoing objections to conventional furnace operation and meeting the problems of temperature and combustion control, whereby the furnace combustion zone is maintained at the desired high operating temperature of approximately 3000 F. and the temperature of the interior surfaces of the furnace walls adjacent the combustion zone is maintained above a comparatively high yet safe and efiicient intermediate operating temperature of approximately 1500 F. Bridging effects are thus either minimized or eliminated entirely. Likewise destruction of the refractory lining is greatly. reduced, assuring a predetermined slag composition. Heat from the furnace walls is conveyed at approximately the aforesaid intermediate temperature to a heat exchanger wherein incoming air to be fed to the furnace is heated from room temperature to the desired inlet air temperature of between 600" F. to 1000" F. The heated air is then fed through tuyeres into the furnace to return the heat thereto that was extracted from the sidewalls. In consequence the furnace is operated with optimum effectiveness and with minimum fuel consumption.

Another object is to provide simple, and improved coolant means in combination with a furnace of the foregoing character for conveying heat from the furnace sidewalls at an intermediate temperature effective to achieve an efiicient heat transfer from the furnace walls to the coolant means and from the latter to the incoming air, thereby to heat the latter efficiently to the optimum temperature for feeding ino the furnace and to facilitate the controls.

Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

In accordance with the method of the present invention, excess heat is conveyed at a desired intermediate temperature from the furnace sidewalls to an air preheater around the sidewalls, the intermediate temperature being selected for a given application between the high interior temperature of the furnace and the temperature of the atmosphere from which the furnace air is supplied. The excess heat at said intermediate temperature within said air preheater is employed to heat air from atmospheric temperature to a desired air feed temperature between said intermediate and atmospheric temperatures. The heated air is then fed as aforesaid into the-furnace to support combustion therein and to maintain the desired controlled furnace operating temperature.

In cupola and blast furnace operation the interior temperature is usually maintained at 2800 F. to 3300" F, the optimum temperature in each case being determined by the furnace operating conditions in accordance with conventional practices and being usually critical within an approximate 2% tolerance. In such operations the aforesaid intermediate and air feed temperatures are usually maintained within the approximate ranges of 1000 F.l500 F. and 600 F.lOO F. respectively. The optimum air feed temperature is determined in each case by the furnace operating conditions in accordance with conventional practice.

Any suitable heat exchanger means known to the art is employed for conveying heat from the furnace sidewalls to the air preheater at the above defined intermediate temperature. The important factor to be noted is that by operating at the temperatures specified, effective differentials exist between the high interior furnace temperature and the temperature range at which the heat exchanger is operated, and between the latter temperature range and the initial atmospheric temperature of the air Supply. Accordingly efficient heat transfer from the furnace walls to the heat exchanger and from the latter to the atmospheric air is readily obtained. In addition the heat conveyed from the furnace walls in cooling the latter is effectively employed to heat the incoming air to the desired air feed temperature, achieving optimum fuel economy and better control of furnace melting conditions.

In a preferred application of the present invention a suitable fluid coolant, such as an eutectic salt mixture or a lead-tin, bismuth, or sodium-potassium alloy by way of example having a boiling point substantially above the intermediate temperature, is circulated within the furnace sidewalls and thence to the air preheater. The sodiumpotassium alloy coolant referred to as NaK having a low melting point of approximately 65 F., and a comparatively high boiling point in excess of 1500 F. is particularly effective because of its exceptional high coeflicient of thermal conductivity, whereby heat is rapidly conveyed from the furnace walls to the air preheater with a minimum of coolannt circulation. In fact where desired, as in cupola furnace operation by way of example, adequate NaK heat transfer area is feasibly employed to cool the furnace sidewalls solely by conduction of heat therefrom to the preheater, so that pumping means for circulating the NaK is not always necessary.

Also by virtue of the thermal characteristics of NaK and its employment as a cool-ant at the above defined comparatively high intermediate temperature, rather than at temperatures conventionally employed for cooling cupola and blast furnace walls, the NaK coolant is feasibly contained in coolant ducts having heat absorbing or wall cooling portions extending along the furnace walls substantially flush with the inner surface of the refractory lining. Thus the coolant is employed as closely as possible to the furnace regions to be cooled, achieving optimum cooling efiiciency and responsiveness to temperature controls necessary for eflicient furnace operation, the temperature of the furnace walls being closely controlled with minimum time lag by regulating the rate of flow of the coolant and the air in the preheater by any suitable pumping means. Accordingly both melting of the refractory lining and bridging are avoided or minimized, thereby to permit increased control over the composition of the melt by increasing the control over the slag composition and the melting zone temperature, and also to permit increased control over the melt temperature to facilitate tapping or casting.

A preferred apparatus adapted for carrying out the foregoing process is illustrated in the drawings wherein:

Fig. l is a fragmentary vertical sectional view taken in the direction of the arrows substantially along the line 11 of Fig. 2, portions of the outer wall of the air prehcater being shownn in elevation.

Fig. 2 is a reduced horizontal section taken in the direction of the arrows substantially along the line 22 of Fig. 1.

Fig. 3 is a fragmentary sectional view taken in the direction of the arrows substantially along the line 33 of Fig. 1.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

An application of the present invention is illustrated by way of example with a cupola-type furnace having an upright cylindrical wall comprising an outer steel shell 10 and a suitable refractory lining 11. The lower portions of the shell 10 comprise an annular steel flange 12 underlying and supporting the lining 11 and supported in turn upon legs or supports 13. A bottom clean out door 14 is hinged at one edge to said flange 12 and suitably secured in the closed position shown by a fastener or clasp 16. A flooring or hearth 17 of sand is provided above the door 14 to support molten grey iron resulting from the furnace operation, the melt being discharged as desired through the opening of spout 18 regulated by refractory plug 19.

Spaced somewhat above the hearth 17 are a number of tuyeres, each comprising a double-walled tubular conduit of annular section. Each tuyere opens radially through the furnace wall and extends inwardly of the lining 11 so as to discharge incoming air inwardly thereof. The opposite annular ends of each tuyere are closed to provide an annular chamber around the central bore or tuyere opening. The annular chamber of each tuyere is filled with a fluid coolant 20a of the type described above and is in communication with opposite ends of a coolant duct 21 also filled with the coolant 20a and located within a lower annular air preheating chamber 22. Each duct 21 comprises an upper radially extending portion and an underlying convoluted portion connected respectively with the upper and lower outer ends of one of each of the annular coolant chambers. Thus each conduit 21 and associated coolant chamber comprises a closed circuit in which the coolant circulates by gravity flow during operation of the furnace. As stated above, the coolant preferably comprises a NaK alloy having an exceptionally high coefficient of thermal conductivity, a melting point of approximately 65 F., and a boiling point in excess of 1500 F. The tuyeres 2t) and ducts 21 preferably comprise a stainless steel alloy adapted to withstand the heat and corrosive action of the NaK and furnace combustion products. By virtue of the high thermal conductivity of the NaK, the inner ends of the tuyeres are protected from the radiant heat of the furnace interior by conduction of the heat into the chamber 22, where the heat is dissipated in preheating the air supply to the furnace, as described below. I

i The lower preheating chamber 22 is closed below and above the conduits 21 by annular steel plates 23 and 24 respectively extending around the furnace wall and secured thereto. The outer wall of the chamber 22 comprises a cylindrical steel shell 25 coaxial with the shell 10 and secured thereto and to the outer edges of the plates 23 and 24. The latter plate carries a number ofsupporting brackets 26 for the conduits 21.

Above the chamber 22 is an annular upper preheating chamber 27 around the furnace and having the plates 24 and 25 for its lower andouter walls respectively. The upper wall of the chamber 27 comprises an annular steel plate 28 connected at its inner and outer edges with the plates 10 and 25 respectively.

Spaced circumferentially around the furnace wall and extending radially therethroughv are a plurality of vertical slots 29 within which are inserted a corresponding plurality of stainless steel coolant ducts 30. Each'ofthe latter comprises an outer convoluted heat dissipating portion within the chamber 27 and a vertical heat absorbing portion 39a substantially flush with the inner surface of the refractory wall 11. As illustrated, each conduit 30 in the present instance comprises an independent closedcircuit and extends through an electromagnetic pump 31 operative to cause circulation of fluid NaK within the associated conduit if desired for control purposes. A number of supporting brackets or hangers 32 depend from the upper wall 28 to support the coolant ducts 30. Enclosing the preheating chambers 22 and 27 is a thermal insulating layer 33.

In the construction shown, each coolant duct 30 comprises a closed circuit independent of the others and is removably inserted through its corresponding sidewall slot 29 to locate its inner heat absorbing portion 30a flush with the interior surface of the lining 11. Accordingly each duct 30 as well as each duct 21, can be removed independently of the others for repairs. To this end access into the chambers 22 and 27 is provided for, as for example by removably securing the outer plate 25 in position.

Before firing the furnace, a refractory paste 34 is preferably packed into the slot 29 behind the conduit portion 30a to prevent escape of furnace gases, and the inner surfaces of the portions 30a are coated with a suitable paste, such as a graphite paste 35. Thereafter when the furnace is fired, molten slag resulting from the furnace operation will freeze around the coolant duct portions 30a to protect the latter from direct radiation from the furnace interior. The thickness of the frozen slag layer will automatically adjust itself to the operating conditions of the furnace. If the frozen slag layer should tend to become too thick, its inner portions will be melted off by the heat of the furnace. If it should tend to become too thin, the resultant increased heat loss to the conduit portions 3011 will cause solidification of more slag.

In order to obtain optimum operating conditions, the cupola furnace temperature as aforesaid is maintained closely at the desired temperature ranging ordinarily from 2800 F. to 3300 F. and the NaK coolant within the tubes 30 is maintained at the aforesaid intermediate temperature. The desired temperature control is achieved by forcing air at ordinary atmospheric temperature into a lower portion of the upper air preheater 27 via inlet 36 by means of a suitable pump or fan which maintains the customary operating pressure within the chamber 27. As the cold incoming air entering through inlet 36 flows past the convoluted conduits 30, it is heated and caused to rise, whereupon the heated air is discharged via outlet 37 to a down-pipe 38 exteriorly of the chamber 27. The pipe 38 opens at a lower end 39 into the lower preheating chamber 22, whereby the heated air is conducted throughthe tuyeres 20 into the furnace.

The number of coolant ducts 30 and the ratio of the lengths .of the heat absorbing portions 30a to the convoluted heat dissipating portions within he chamber 27 are determined to permit adequate cooling of the interior furnace wall while at the same time maintaining the temperature of the coolant at the desired intermediate temperature. A temperature gradient will of course exist along the conduits 21 and 30, but such a gradient will be minimized by the exceptionally high rate of heat conduction through the NaK coolant. Accurate control over the cooling is accomplished by regulating the rate of flow of the coolant, by means of the pumps 31, and also by regulating the amount of cold air supplied through inlet 36.

In order to control the air temperature and to assure that the air is fed through the tuyeres 20 at the desired critical temperature which will range from 600 F. to 1000 F. in accordance with the particular conditions of the furnace operation, a hot air exhaust pipe 40 regulated by damper 41 extends upwardly from the downpipe 38 to permit the exhausting of excess hot air therefrom should the occasion arise. In addition a supplementary cold air inlet duct 42 regulated by damper 43 opens into the lower end of duct 38 adjacent the latters outlet 39-, whereby additional cold air is forced into the lower chamber 22 when desired, either by venturi action or by a suitable fan or pump. Accordingly the air, feed temperature is precisely controlled and the heat withdrawn from the furnace sidewalls in cooling the latter is employed to preheat the furnace air 'supply to the desired optimum air feed temperature, whereby the heat is returned to the furnace with minimum loss. The temperature of the air within the lower chamber is determined so that as it circulates past the ducts 21 and through the tuyeres 20, it will be heated thereby to the desired final temperature and will prevent overheating of the tuyeres 20.

We claim:

1. In combination with a furnace having an annular upright wall portion and a plurality of upright slots extending radially through said wall portion at circumferentially spaced locations, means for conveying heat from said wall portion comprising a plurality of independent coolant ducts, each duct comprising a closed tubular circuit having an upright heat absorbing portion within one of each of said slots adjacent the inner periphery of said wall portion and dimensioned to be removably inserted radially intothe slot from the exterior of said wall portion, each duct extending radially outwardly from the upper and lower ends of the upright portion and having a convoluted heat dissipating portion exteriorly of said wall portion, and a fluid coolant within each duct comprising a NaK alloy having a melting point approximating room temperature.

2. In combination with a furnace having upright wall portions, means for cooling said walls and for preheating air supplied to said furnace comprising an air preheating chamber around said walls, a bathe partitioning said chamber into upper and lower portions, tuyeres opening through said walls into said furnace from the lower portion of said chamber, coolant duct means having heat absorbing portions adjacent the interior of said walls and also having heat dissipating portions within the upper portion of said chamber and connected with said heat absorbing portions, a cold air inlet opening into the upper portion of said chamber, and conduit means for heated air connecting said upper and lower chambers.

3. The combination as set forth in claim 2 wherein each tuyere comprises spaced coextensive inner and outer tubes providing a coolant space circumscribing the inner tube, said tubes having end portions closing the ends of the coolant space therebetween, and comprising in addition coolant duct means having heat dissipating portions within the lower portion of said chamber and connected with said tuyeres in communication with the cool- 7 ant spaces thereof, and a NaK alloy coolant within said duct means and coolant spaces.

4. In combination with a furnace having upright annular wall portions around a heating zone, means for cooling said Walls and for preheating air supplied to said furnace comprising an annular air preheating chamber around said wall portions, the latter comprising the inner wall of said annular chamber and having a plurality of upright slots extending radially therethrough at locations spaced circumferentially around said zone, coolant duct means comprising a plurality of independent conduits, each having an upright heat absorbing portion within one of each of said slots adjacent the inner periphery of said wall portions and dimensioned to be removably inserted radially into its slot from the exterior of said wall portions, each conduitalso having a heat dissipating portion located within said chamber radially outwardly of said upright wall portions, means for forcing air to be heated into said chamber, and air conduit means connected with said chamber to receive heated air therefrom and discharging into said furnace.

5. In the method of operating a cupola furnace having a refractory lining and cooling ducts extending inwardly of said refractory lining from a heat exchanger exteriorly of said furnace, the steps of coating said ducts with a graphite paste, thereafter operating said furnace to melt slag therein and simultaneously cooling the walls thereof to freeze portions ,of the slag on said ducts to protect the latter from the direct radiant heat within said furnace.

6. In the method of operating a cupola furnace, the steps of controlling the composition of the slag and melting zone temperature by circulating a fluid NaK coolant in heat exchange relationship with the interior surface of the refractory lining of said furnace at the region of the melting zone and from thence to the exterior of said furnace, maintaining the temperature of said interior surface below the melting point of said lining and above 1500" F. and also preheating air to an inlet temperature in excess of 600 F. by circulating said air in heat exchange relat'ionship with said NaK exteriorly of said furnace, and thereafter introducing the preheated air at said inlet temperature into said furnace at a region below the said melting zone.

7. In combination with a furnace, means for preheatin'g air supplied to said furnace comprising an air preheating chamber extending around said furnace, tuyeres opening through the walls of said furnace from the lower portion of said chamber, each tuyere comprising spaced coextensive inner and outer tubes providing a coolant space cir'cumscribing the inner tube, said tubes having end portions closing the ends of the coolant space therebetween, and comprising in addition coolant duct means having heat dissipating portions within the lower portion of said chamber and connected with said tuyeres in communication with the coolant spaces thereof, a coolant within said duct means and coolant spaces, and cold air inlet means opening into said chamber.

References Cited in the file of this patent UNITED STATES PATENTS 1,663,977 Foresman Mar. 27, 1928 1,826,293 Weigel Oct. 6, 1931 1,833,130 Roe Nov. 24, 1931 1,922,509 Thurm Aug. 15, 1933 2,078,747 Vial Apr. 27, 1937 2,104,393 Barr et al. Jan. 4, 1938 2,333,654 Lellep Nov. 9, 1943 2,417,345 Beebe Mar. 11, 1947 OTHER REFERENCES Foundry Trade Journal, October 13, 1949, pages 449- 456.

Claims (1)

1. 6.IN THE METHOD OF OPERATING CUPOLA FURNACE, THE STEPS OF CONTROLLING THE COMPOSITION OF THE SLAG AND MELT-ING ZONE TEMPERATURE BY CIRCULATING A FLUID NAK COOLANT IN-HEAT EXCHANGE RELATIONSHIP WITH THE INTERIOR SURFACE OF THE REFRACTORY LINING OF SAID FURNACE AT THE REGION OF THE MELTING ZONE AND FROM THENCE TO THE EXTERIOR OF SAID FURNACE, MAINTAINING THE TEMPERATURE OF SAID INTERIOR SURFACE BELOW THE MELTING POINT OF SAID LINING AND ABOVE 1500* F. AND ALSO PREHEATING AIR TO AN INLET TEMPERATURE IN EXCESS OF 600* F. BY CIRCULATING SAID AIR IN HEAT EXCHANGE RELATIONSHIP WITH SAID NAK EXTERIORLY OF SAID FUR-NACE, AND THEREAFTER INTRODUCING THE PREHEATED AIR AT SAID INLET TEMPERATURE INTO SAID FURNACE AT A REGION BELOW THE SAID MELTING ZONE.

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