US1955589A - Method of open hearth furnace operation - Google Patents

Description

Aprf El?? @341 F. E. LEM-w METHOD OF OPEN HEARTH FURNACE OPERATION Filed Sept. 16, 1929 2 Sheefcs-Sheet l JKM WITNESS pl' il?, 3934-. F, E LEAHY 3,955,58@

METHOD 0F OPEN HEARTH FURNACE OPERATION lFiled Sept. 16, 1929 2 Sheets-Sheet 2 WITNESS Patented Apr. 17, 1934 UNITED STATES PATENT OFFICE METHOD OF OPEN HEARTH FURNACE OPERATION Application September 16, 1929, Serial No. 392,769

12 Claims.

The present invention is designed to enhance rapidity of production of open hearth steel without unduly decreasing the normal life of the furnace.

Among the principal objects of the invention, therefore, is the provision of a method of furnace operation which, without increasing the normal rate of deterioration of the furnace, is effective in reducing the time required for the heat treatment of the metal, with the result that by the use of the said method of materially greater amount of steel can be produced during the operative life of the furnace than has heretofore been possible because of the lessened time required for each heat while, on the other hand, the furnace may be maintained in operative condition for a period at least as long, and frequently longer, than substantially similar furnaces when operated in accordance with the best practice heretofore employed.

My invention is therefore advantageous in that, among other things, it reduces the ultimate cost of the steel by enhancing the amount thereof which can be produced during the .operative life of the furnace whereby the proportion of the cost of furnace replacement and operating costs which must be allocated against each ton of the finished product is correspondingly reduced.

My invention is further advantageous in that its employment tends to reduce or substantially eliminate the foaming of the charge during the rening period and which, when present, is inimical to the satisfactory and economical production of open hearth steel.

Other objects, advantages and novel steps and features comprehended by my invention are hereinafter more specifically mentioned or will be apparent to those skilled in the art from the following description of one manner of performing it in connection with which reference may be had to the accompanying drawings.

Heretofore in modern open hearth steel practice it has been customary to utilize throughout the heat treatment of the charge in the furnace a non-luminous or blow torch type of flame derived by burning gas and air mixed prior to combustion for the reason, among others, that a flame of this character has the abilityto adequately penetrate that portion of the charge which consists of metal scrap and which usually is charged last into the furnace and rests in loosely piled condition on the sub-jacent layers of ore, pig iron and limestone which are initially disposed at the bottom of the charge. For the production of this type of flame, the air necessary for combustion is usually separately preheated in whole or in part and then mixed with the gas which is either separately preheated or not, depending upon the kind of gas being used; thus, producer gas is customarily preheated before its mixture with the air while coke oven gas is not. The ensuing combustion of the hot mixture of air and gas results in the production of the said non-luminous flame which is of high temperature but of such quality or character that the percentage of heat transference which it is capable of effecting by radiation is at a minimum although the flame is capable of imparting a high degree of heat transference by convection.

The operation of an open hearthfur-nace is divided into two distinct periods or stages, namely, the melt-down period during which the charge is being reduced to substantial fluidity, and the subsequent refining period.' during which the melted charge is subjected to the further heat treatment necessary or desirable before it can be tapped. During the melt-down period the charge is highly heat absorptive with the result that the greater portion of the heat of the nonluminous flame which impinges more or less directly upon the heaped-up scrap and other portions of the charge is taken up by the latter while the balance of the heat of the flame other than that which eventually leaves the furnace and passes to the stack is absorbed by the furnace walls and roof. Therefore, during the meld-down of the charge which ordinarily requires about one-half of the total operating time of each heat, a non-luminous, blow torch type of flame can be effectively employed at a temperature sufficient to bring about a very rapid melting of the charge without excessive or undue burning of the refractory walls and roof of the furnace as the walls and roof, during this period, absorb but a relatively small proportion of the total heat of the flame.

At the conclusion of the melt-down period, however, the metal of the charge has been reduced to fluid condition and rests in the hearth of the furnace overlaid by a thick layer of slag which is of a highly refractory nature, derived as it is in large part, from the limestone in the charge. The flame passing through the furnace is thus in effect interposed between two refractory juxtaposed surfaces, namely, the slag covering on the top of the charge and the furnace roof, both of which are approximately similarly heat-absorptive, and it is well known that when this condition is reached it becomes necessary to materially reduce the temperature of the flame so as to prevent excessively rapid burning out and deterioration of the furnace. But such reduction immediately and necessarily results in a lessened heat transference to the charge with con- 5 sequent prolongation of the refining stage beyond that which would be required if the temperature of the flame could be maintained at the same point during the refining period as during the meltdown. The furnace operator has therefore hithlC erto been confronted with the problem of keeping the fiame temperature during the refining period at a maximum commensurate with an economical furnace life and the rate of production has thus been necessarily limited to that point at which the furnace deterioration becomes so excessive as to increase the cost of the product against which it must be charged by an amount which renders that product uncommercial.

I have discovered that the length of the refining period may be materially shortened without a disproportionate increase in the wear and tear on the furnace or, in fact, without any material shortening of its normal life by employing different types of flame for the melt-down and for the refining stages respectively, with the result that I am able to effect the melting down of the charge as rapidly and with as little furnace deterioration as in the best commercial practice heretofore employed and to thereafter refine the charge in a much shorter time than and with at least as little furnace deterioration as has heretofore been possible when using a non-luminous or blow torch type of flame for the purpose as heretofore described.

More specifically, and in accordance with my invention, I employ for the melt-down of the charge a high temperature non-luminous fiame which, as above described, has the quality of readily penetrating the charge and of reducing it to fluid condition, without appreciable localized heating and in a minimum of time, and for the subsequent refining of the charge a luminous radiant flame desirably derived from the combustion of a liquid fuel in association with a suitable quantity of air. Among the fuels which may be employed for this purpose are tar, fuel oil, or coke plant residue, for as these fuels are high in hydrocarbons they are among the most desirable for obtaining the luminous name, a quality or characteristic of which is its ability to effect a transference of a large proportion of its heat by radiation as distinguished from convection. By the use of a flame of this type during the refining period, I am enabled to bring about substantially as great heat transference to the 4-fbath while maintaining the flame at a temperature insufficient to unduly deteriorate the furnace as would be possible when using a nonluminous flame of the same temperature as that customarily used during the melt-down period but which, as above pointed out, is so destructive to the furnace when at such temperature as to render impracticable its employment for refining. The radiant, luminous, relatively low temperature flame, however, is capable of satisfactorily heating the charge during its passage through the furnace in a much shorter period of time than a non-luminous ame of like temperature and thus of correspondingly shortening the refining period, with the result that although the luminous fiame be maintained at a temperature not unduly inimical to the refractories of the furnace, the'refining of the charge is completed in a much shorter time than has hitherto been possible when a non-luminous flame of like temperature is employed for the purpose, with corresponding increase in the quantity of steel which can be produced during the normal life of the furnace.

The practice of my invention is in no way confined to any particular form or type of open hearth furnace nor to the use of any particular or specific fuels other than those adapted to produce a non-luminous flame during the meltdown period and a luminous flame during the subsequent refining of the charge. However, to facilitate comprehension of the invention by those skilled in the art, I have illustrated diagrammatically in the accompanying drawings one form of furnace well adapted for the performance of the invention and to which I shall now briefly refer, as well as to the manner of practising the invention in connection with such a furnace arranged by way of example for the employment of coke oven gas for the melting of the charge and a liquid fuel, such as tar or coke plant residue, for the refining thereof.

Referring now more particularly to the drawings, Fig. 1 is a diagrammatic plan view, partially in elevation and partially in horizontal section,

vof the furnace and its associated regenerator or checker chambers and other parts, and Fig. 2 is a longitudinal vertical section substantially on the line 2-2 in Fig. l, like symbols of reference being used to designate the same parts in both figures.

'I'he furnace as shown is of substantially standard construction, comprising the usual hearth 1, roof 2, end walls 3 3 and side walls 4 4, the construction of both ends of the furnace and of the parts associated therewith being similar in accordance with the usual practice. Thus, disposed laterally of each end of the furnace are a pair of checker chambers 5, 6 connected by the usual necks 7, 8 with a common slag pocket 9 disposed beneath the adjacent end of the furnace; from this slag pocket fiues or uptakes l0, 10 extend vertically on either side of the center line of the furnace in front of the adjacent end wall 3 to terminate on opposite sides of a central longitudinally extending wall l2 colloquially known as the dog house. The uptakes 10, l0 are thus effective to direct into the furnace at that end thereof from which it is being fired, the major portion of the air required for combustion and which has been previously preheated in the adjacent checker chambers.

For supplying the fuel to the furnace a burner generally designated as B is disposed in a port or tunnel 13 extending longitudinally through the wall l2 and adjacent end wall. 'Ihis burner is desirably of the aspirating type and comprises a tubular body 14 preferably surrounded near its inner end with a water jacket 15 through which water may be circulated by valve controlled pipes 16, and is so arranged that either gaseous or liquid fuel may be injected into the furnace.

through the burner at will. Thus, as diagrammatically shown, a valve controlled gas pipe 17 is arranged to direct gaseous fuel such as coke oven gas into the funnel-shaped outer end of the burner, while by means of a pipe 18 extending substantially through the burner and terminating adjacent its inner end, liquid fuel such as tarA supplied from a valve controlled pipe 19 commu- L -p nicating with the outer end of the pipe 18 mayA be directed into the furnace. It will of course be understood that the particular form and construction of the burner is a matter of choice so long as, it is capable of fulfilling the requisite i function of selectively directing the desired fuel into the furnace, preferably with an admixture of air aspirated through the burner body from outside the furnace.

At its end remote from the furnace each of the checker chambers 5 and 6 is preferably connected with a common manifold 22 as shown in Fig. l, and this manifold is in turn connected with a stack 23, a stack damper 24 being desirably arranged to control the draft through the stack and, in accordance with the usual practice, a valve 25 of suitable type is interposed between each of the checker chambers 5 and the manifold and a similar valve 26 between it and each of the chambers 6 to facilitate proper distribution of the air and spent gases between the several chambers.

A pair of reversing valves 28, 28 are disposed on opposite sides of the stack connection and are operable to cut oi the ilow of gases to the stack from one side of the manifold or the other and thus enable the furnace to be reversed, while for supplying air under a suitable pressure to the manifold and from thence to the checker chambers and the furnace, a fan or blower F of suitable type and capacity is connected to a Y- shaped conduit 29 whose branches respectively communicate with the manifold on opposite sides of the reversing valves 28, 28', said communication being controlled by valves 30, 30 of any suitable construction.

The use of a fan, blower or other suitable means Y for supplying air under pressure to the manifold,

CII

checker chambers and furnace I regard as desirable in the practice of the present invention for not only is the volume of air thereby increased over that which would' be drawn through the chambers bythe natural stack draft, but the resistance to the entrance of the air to the hearth is also overcome so that the air passes thereto with greater velocity than it would in the absence of the fan. However, under certain conditions it may be found possible to dispense with the use of the fan and to rely solely upon the natural draft created by 'the stack.

In Fig. 1 the furnace is shown with the several valves in the positions which they respectively normally occupy when the furnace is being fired from the left-hand end when viewed as in the drawings, and it will thus be noted that the air from the blower F can pass into the manifold through the valve 30 which is open and from thence into the left- hand checker chambers 5 and 6 through the valves 25, 26 which are also open, the closure of the valve 28 of course preventing any of the air from passing to the stack. From the checker chambers the heated air passes into the common slag pocket 9 through the separate necks 7 and 8, upwardly through the uptakes l0, 10 and thence forwardly and downwardly toward the hearth so as to meet the incoming fuel from the burner somewhat in advance of the inner end thereof and at such point initiates combustion. At the opposite or out-end of the furnace, the spent gases pass downwardly through the uptakes 10, l0, thence through the adjacent checker chambers 5 and 6 and into the manifold, from which they pass to the stack through the open valve 28', the valve 30' which is now closed preventing the spent gases from passing into the conduit 29 as well as the air from the blower from passing into the manifold from the conduit. While the valves 25, 26 are shown open adjacent both ends of the furnace, it will be understood that individual adjustment of these valves may at any time be effected in accordance with the common practice so as to more effectively distribute the air and spent gases in the checker chambers and insure the former entering the furnace at the highest possible temperature.

When the furnace is reversed after a suitable period of operation, the positions of the reversing valves 28, 28 are shifted in the usual way as well as the positions of the valves 30, 30 so as to direct the air int'o the right-hand end, which is now the in-end, of the furnace and permit the escape of the spent gases from the opposite left-hand end, now the out-end thereof.

In accordance with my invention as will now be readily understood, the fuel employed for the operation of the furnace during the melting down of the charge consists of coke oven, natural or other suitable gas introduced to the aspirating burner through the nozzle or pipe 14 and in turn to the furnace by the burner together with a small quantity of coldA aspirated air ,which is drawn through the latter, the bulk of the air requisite for combustion, however, being preheated in the checker chambers and meeting the incoming gaseous mixture in front of the inner end of the burner adjacent the end of the hearth, the ames thus generated being of non-luminous character and of a temperature sufficient to melt the charge in a minimum of time. After the charge has been fully or substantially melted, the supplyof gas is turned off' and a suitable liquid fuel introduced through the burner into the furnace together, preferably, with a small amount of aspirated air, the liquid fuel so injected meeting the incoming and preheated air from the uptakes 10, 10 in front of the inner end of the burner and burning from that point forward with a radiant luminous ame, the temperature of which as above explained is maintained at a point low enough to prevent excessive damage to the refractories of the furnace, until the charge is fully rened and in condition for tapping.

In open hearth furnace operation, the time usually required to melt and refine the charge in. say, a 100-ton furnace, that is, the time required from the beginning of the heat until the metal is in condition to tap, varies from approximately 6 to 1l hours. Under such conditions and With a charge of average composition, the non-luminous flame initially used in melting the charge in accordance with the present invention is ordinarily employed for approximately 3 to 5 hours and the luminous flame for the remainder of the heat, the particular time at which the change in the character of the flame is made, either gradually or more or less abruptly, being largely determined by the temperature of the furnace refractorics and other factors which, in the usual open hearth practice, dictate the time at which the temperature of the flame used for melting should be reduced to prevent excessive deterioration of the furnace and which are fully understood and recognized by operators of open hearth furnaces.

As heretofore pointed out, the interval from the commencement of the heating of a given charge in an open hearth furnace until the charge is in condition for tapping is divided into two distinct periods or stages, namely, that in which the charge is melted, that is, reduced to substantial uidity, herein termed the melt-down period, and that in which the melted charge is subjected to further heat treatment, herein termed the refined period, to remove impurities and properly condition the bath for tapping.

Toward the end of this period certain metallurgical treatments are often resorted to for the purpose of producing certain kinds or grades of steel, but these treatments are distinct from the refinement of the metal itself eifected during the refining period. It has heretofore been the usual practice to reduce the temperature of the melting flame substantially contemporaneously with the completion of the melt-down and thus at about the middle of the total heating time, but in accordance with the particular conditions present, including the character of the charge, condition of the furnace and the like, as well understood in the art, the high temperature melting flame is sometimes employed for a proportionately longer or shorter period. Consequently, in accordance with the present invention, the change-over from the non-luminous melting flame to the luminous refining flame is usually made substantially contemporaneously with the completion of the melt-down when it would ordinarily vbe customary to reduce the temperature, but not to change the character, of the flame, so that the non-luminous and the luminous flames are generally respectively used for about flfty percent. of the total heating period under usual operating conditions, though within the scope of the invention the change-over may be made for any particular heat either earlier or later as dictated by local conditions so long as the non-luminous flame is utilized for the major portion the melt-down period and the luminous flame for the major portion of the subsequent refining period.

It will of course be understood that the reversals of the furnace are effected in the usual way without regard to the particular character of flame being employed at the moment when each reversal takes place, while, moreover, when burning tar or other viscous fuel, it is usually customary to preheat the same outside of the furnace by any suitable means to reduce it to the desired fluidity and to inject a certain amount of steam, supplied through a valve controlled pipe 19' connected with the pipe 18, along with the tar as it passes into the burner, as will be readily understood by those familiar with the art.

While I have referred herein more particularly to the use of my method in connection with a furnace arranged for selectively burning coke oven gas and tar or other liquid fuel when and as required, under certain circumstances it may be desired to utilize producer gas or the like instead of coke oven gas during the melting of the charge, in which case it is only necessary to provide any suitable means for directing this gas into the furnace through the tunnel or port 13 at the temperature at which it is drawn from `the producers, although if desired arrangements may be made for preheating the gas before its introduction into the furnace.

My invention is advantageous not only in the direct reduction of the time required for reiining the charge which it is capable of effecting, but also in the elimination or substantial elimination of foaming of the charge during the refining period and which is a source of material difficulty in the operation of open hearth furnaces in which coke oven gas is employed as a heating medium during that period. This foaming causes a delay in the heat and is also destructive of the furnace itself and particularly of its roof; thus, its elimination or substantial elimination is distinctly desirable, tending as it does to reduction of production costs through speeding up the heat, lessening the wear and tear on the furnace, and facilitating its management and operation.

Moreover, the use of my invention does not effect or require any change in the customary metallurgical operations such, for example, as the addition of ferro-manganese or the like, necessary for producing a desired grade or kind of steel.

While it has heretofore been customary to utilize a non-luminous flame throughout the heat treatment of the charge in an open hearth furnace, and while attempts have been made to similarly employ a luminous flame but without much success because of the tendency of that type of flame to cause localized heating of the cold charge and its inability to satisfactorily penetrate the loosely piled up scrap, as far as I am aware I am the first to use a non-luminous and a luminous flame at different periods in the heat treatment of the charge, and in view of the marked advantages arising therefrom as herein disclosed I consider my invention constitutes a material contribution to the art of open hearth steelmaking. I therefore claim my invention broadly without reference to the particular apparatus or type of furnace which may be utilized in the practice thereof and without confining or limiting the practice of the invention to any particular kind or class of fuels as, Within the scope of the appended claims, any suitable fuels may be employed in its performance.

Having thus described my invention, I claim and desire to protect by Letters Patent of' the United States:

1. In an open hearth steel process, the step of melting down a charge by using a flame of low luminosity, and refining with a flame of relatively high luminosity.

2. In an open hearth steel process, the steps of melting down with a flame of relatively low luminosity and refining with a flame of relatively high luminosity but with a thermal value substantially less than that used during the melting down.

3. In the art of open hearth steel manufacture, that method of furnace operation which comprises the use of a substantially non-luminous flame derived from combustion of gaseous fuel for melting the charge and of a relatively luminous flame derived from combustion of liquid fuel for refining it 4. In the art of open hearth steel manufacture, that method of furnace operation which comprises the steps of melting the charge by means of a substantially non-luminous flame derived from combustion of gaseous fuel, and thereafter refining the charge by means of a radiant relatively luminousflame derived from combustion of liquid fuel.

5. In the art of open hearth steel manufacture, that method of furnace operation which comprises the steps of melting the charge by means of a substantially non-luminous flame derived from combustion of gaseous fuel and thereafter refining the charge by means of a radiant flame of less temperature derived from combustion of liquid fuel.

6. In the art of open hearth steel manufacture, that method of furnace operation which comprises the steps of melting the charge of the furnace through the medium of a substantially non-luminous high temperature flame derived from combustion of gaseous fuel, and then refining the charge through the medium of a luminous radiant ame of relatively low temperature derived from combustion of liquid fuel.

7. A method of operating an open hearth furnace which consists of transmitting heat to the charge during the melting down period of operation primarily by the method of heat transmission by convection and supplying heat to the charge during the remainder of the heat period primarily by the transmission of heat by radiation.

8. In the art of open hearth steel manufacture,

that method of furnace operation which cornprises utilizing a non-luminous flame initially to eiect a substantial melting of the charge and then changing the flame to one of luminous character and continuing the heating of the charge for the remainder of the total heating period by means of such flame until it is in condition to tap.

9. In the art of open hearth steel manufacture, that method of furnace operation which comprises the steps of initially melting the charge by means of a non-luminous flame until the charge is reduced to asubstantially constant level in the bath, and thereafter refining the charge by means of a luminous flame for a period approximating that required to melt it to thereby bring it to condition for tapping.

10. L,In the art of open hearth steel manufacture, that method of furnace operation which comprises the steps of initially subjecting the charge to the action of a non-luminous name until the charge has been reduced to substantially fluid condition and the refractories of the furnace approach a destructive temperature, and thereupon changing the flame to one of luminous character and lower temperature and continuing the heating of the charge therewith until the latter is in condition for tapping.

l1. In the art of open hearth steel manufacture,l that method of furnace operation which comprises the steps of initially utilizing a nonluminous flame for heating the charge for a period approximating one-half of the entire heat, then changing the flame to one of luminous character and utilizing such flame for heating the charge during the entire remainder of the heat until it is in condition for tapping.

12. In the art of open hearth steel manufacture, that method of furnace operation which comprises the steps of utilizing a non-luminous flame for melting the charge until it is reduced to substantially iiuidity and the refractories of the furnace approach a temperature suilicient to cause relatively rapid disintegration, then lowering the ame temperature to a point insuicient to effect such disintegration and changing the ame to one of more luminous character, and continuing the heating of the charge with such ame for the remainder of the heating period and until it is in condition to tap.

FRANK E. LEAHY.

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