US2543388A - Method of furnace operation - Google Patents

Description

F@H Z"227 1951 N. J. URQUHART HETHOD OFT'URNACE OPERATION 3 Sheets-Sheet 1 Filad Das. 20, 1948 Feb 27 1951 N. J. URQUHART uxamon OF mamcxa OPERATION 3 Sheets-Sheet 2 Filed Das. 20, 1946 Feb, 27, 3951 N. J. URQUHART 2543,388

METHOD OF F'URNACE OPERATION Filed Des. 20, 1946 3 Sheets-Shee't 3 INVENTO 1 Patente Feh 27, 1951 UNITED STATES PATENT OFFICE METHOD OF FURNACE OPERATION Norman J. Urquhart, Scenery Bill, Pa., assignor to Steel Processing Company, Pittsburgh, Pa., a corporation o1 Pennsylvania Application December 20, 1946, Serial N0. 717,509

4 Claims. (GI. 263-52) 'I'his invention relates to a method of furnace operation and r elates particularly to a method of operating a metallurgical furnace for hea'bing solid bodies of metal such as steel ingots, iron and steel castings, blocks and other bodies of iron and steel in a heat-treatment or in preparation for mechanical working.

T corsider initially some pwblems involved in the heating of solid metal bodies, it should be considered that the fundamental heat-absorb- Ing and temperature-attaining properties of a metal body depend upon several factors. One such factor is the met-al cf which the body is composed including the relative purity of such metal 01 the alloy er composition of which it is composed, and in such connection the carbon content of steel may be considewd 120 be of particular importance. Each metal and metal alloy o1 composition has its own particular properties with respect to heat absorption and conductivity and conversely its resistance to heat absorpt-ion and conduction. The mass and shape of the body is a factor in determining the rate cf heat absorption and heat conduction within the body required to bring it uniformly to a desired temperature. There are serious problems involved in supplying an overall heat-inputto a metal body in a manner 170 obtain satisfactory equalized heating to the desired temperature. Because of the variety cf factors involved and for other reasons it has been impractical to plot and accurately to follow a heatin curve for each particular furnace charge.

It is of primary importance to bring the interior structure 01 a charge body to the desired temperature without raising its temperature regionally, as at any portion o1 its surface, to a point at which the metal becomes fluid. The greatest difiiculty in thig connection is encountered in heating packs of sheets or p1ates in which heat must be conducted from the surface of the pack to its interior regions. Great difiiculty has been encountered in raising such composite bodies to an equalized high temperature without producing welding between the Sh88ts or plates in some region of the pack. In an annealing operation performed on material of that sort the greatest; care previously exercised frequently is inadequate a1: all stages of the operation to limit the input of heat to the furnace charge below that at which component elements of the pack become regionally heated to a welding temperature.

In metallurgical heating operations as previously performed it has been impractical to obtain indication of the internal temperature ot an ingot, block, sheet pack 0r other body of the metal ab various stages of the heating operation in such manner as to guide controlled furnace operation. It also has been diflicult and frequently has been found impossible so 130 control the heat-input to a furnace chamber during all stages of a heatin operation that the bodies of metal comprised in the furnace Charge attain each its desired equaliZed high temperature without regional overheating.

It is a primary object of my invention to pro viele a method of furna :e operation which gives indication as to the internal temperature of the metal bodies being heated during the progress of the operation and which provides combustion cf such sort and under such control that heat absorption by the bodies of the furnace charge is facilitated and the heatin is efiected in a manner equalize the temperature throughout the structure of the bodies, without localized overheating of the bodies at any stage cf the furnace operation.

Other objects of my invention are to provide a method of furnace operation cf such sort that the furnace chamber and iurnace charge may be raised to a desired temperature while avoiding substantial combustion against the Walls of the furnace chamber and the furnace charge; to provide a method which gives in the furnace chamber a. furnace atmosphere of high heating Vallle; and to efiect the purpose of the heating operation with economy in fuel as well as in advantageous manner with respect to the effect of such operation on the furnace charge.

Another object of my invention is to obtain the above advantages while utilizing a furnace having such simplicity in structure that instalvlation costs are minimized and overall production is promoted by econom in the floor space occupied by the furnace installation.

Whereas the utility of my method of furnace operation which is emphasized herein relates to the heating of mctal bodies in metallurgical practice, the invention as will be explained has various other important applications. Briefiy and generally stated with respect to the operation of a charged metallurgical furnace, my method consists in sealing the furnace chamber in a manner to provide an inlet for burner products and an outlet orifice for products of combustion. The burner products are supplied d!- rectly to a furnace chamber under positive pressure created by the operation of the burner, and desirably are introduced by way of a combustion tunnel. Such burner products are in approximately stable condition and form an approximately stable mixtme when they enter the Iurnace chamber. Products o! combustion from the furnace chamber are discharged against atmospheric pressure, without the creation of negative pressure or pul1 outwardly of said orifice. The area of the outlet orifice from the iurnace being adjusted to the particular operation which 1s contemplated, normally remains constant during at least the major part of that operation. Combustion is so largely performed before the burner pwducts enter the furnace chamber that the furnace atmosphere a well as the furnace pressure is directly under burner control and the firing is both direct and enonomical.

The furnace chamber being maintained throughout the furnace operation under a positive pressure created by burner operation, such positive pressure is directly under burner control and normally varies in value directly with the volume of burner products introduced 120 the furnace chamber. The furnace atmosphere in the metallurgical heating desirably comprises luminous carbon particl.s which intensify heat transfer within the furnace chamber. The furnace atmosphere remains Stahls throughout the entire range of burner input, so that burner coutrol can be used to vary at will the volume of burner products with control of quantitative heat-input or the temperature of burner products without in either instance departing from the desired atmospheric conditions in the furnace chamber.

T give a theoretical background, serving to clarify essential features of my method, it should b understood that with combustion cf the sort which has been described a selectivity or relativity in the heating and in the temperature conditions of the furnace chamber and the work is obtainable. Let it be assumsd that the furnace chamber is empty, and that by burner Operation the atmosphere in the furnace chamber is brought to a desired temperature. When that temperature has been obtained, it is maintainable merely by a heat-input sufficient to compensate for the temperature; that is, to compensate for heat loss to the outer atmosphere. If then we assume that a 001d charge is introduced, the rate of heat-input being unaltered, the furnace temperature of course drops. Given a long period 01 furnace Operation, however, both the furnace chamber and the charge will again be brought uniformly to the initially selected temperature.

This apparently contradictory circumstance stems from the fact that the charge receives heat directly from the combustion atmosphere which is created by the burner and not by radiation from the interior of the furnace walls. Thus the bodies of the charge are preferentially heated with respect to heat loss through the physical structure of the furnace, so that the charge as weil as the furnace atmosphere must be brought to the initial temperature before the "shed" arrives at its initial value. At this point a recording thermometer will indicate continuing uniformity in the furnace temperature. These results rising from preferential heating of the charge in a suitable combustion atmosphere lead not only t0 fuel economy, but also as a matter of far greater importance form the basis for observing the internal temperature conditions of the charge body and for efiecting accurate coushed of the furnace at that' trol of conditions in the Iurnace chamber and in the charge.

It should be understood, however, that it would be uneconomical in furnace time couduct an ent1re heating operation at such 10W- fire, that is under a heat-input based on the "shed of the furnace at the temperature to which the charge is to be brought. In practice I therefore control the burner input between such 1ow input and a, higher heat-additive burner input, to accelerate the rate of heat supply to the furnace charge. Under optimum conditions, the heat-additive input is effected at high-fire" or ab extra-high-fire. The terms high-fire" and extra-high-flre may be defined respectively as: (a) the rate of heat-input which will most economlcally supply heat to relatively bot work at its maximum rate of heat absorption in that temperature range, and (b) the rate of beatinput, which will most economically supply heat to relatively cold work at its maximum rate of heat absorption in that temperature range.

It should be understood that the terms highfire and extra-high-flre express optimum heat-adclitive burner operation and that d:-.parture from such conditions is possible, provided heat-input is not so excessive that any region of a charge body reaches a melting or welding temperature. Similarly, the optimum heat-input ab low-fire not only is based on the "shed of the furnace at the dasired final temperature of the charge but involves a control very close1y to neutralize that heat loss. Some variation from exactitude in this respect is, bewever, permissible.

With the understanding that the method of my invention may be practiced in furnaces of yvidely varying types, the accompanying drawmgs are illustrative of one apparatus, or installation, embodiment suitable fr the performance 0f my method. In the accompanying drawings:

Fig. I is a vertical sectional view through a furnace suitable for the performance of my method, which is organized as a soaking pit.

Fig. II is a schematic plan view of a complete installation including the soaking pit and various control elements suitably used with it in the performance 015 the method.

Fig. 111 is a schematic diagram illustrating electrical connections for automatically operating a. iurnace in accordance with the method of my lnvention.

Fig. IV is a graphic illustratlon of the control of the valve-operating motors under the influence of two sequentially effective thermocouples.

Referring initially to Fig. I of the drawings, reference numeral I designates generally a furnace having therein a. furnace chamber 2 closed at its upper and by a cover 3, which desirably has an air tight setting on the walls 4 of the furnace provided in suitable manner as by a sand seal 5. As shown, the furnace charge consists of a plurallty of ingots A which stand upon the usual loose heat-insulating bottom 6 provided at the base of the furnace. The outlet orifice from the Iurnace is providecl by a discharge port 1 of fixed dimensions which is shown as located at 9. low level in one wa1l of the furnace and a refractory duct 8 which 1eads to a short Stack 9. In refractory duct 8 there is an adjustable gate In which is used to regulate the area of the outlet oriflce and consequently to regulate pressure with1n the furnace chamber in accordance with the conditions 015 each particular furnace operation. As above indicated. this adjustment is performed prior to the furnace operatlon und normally i.s not changed during the operation, being Set to give positive pressure in the turnace chamber at the lowest burn r 1nput employed during the operation.

In Fig. I an inlet port II is shown a.s' 'jaosltloned in the upper region o1 the same wall through which outlet port 1 1eads. This inlet port II is as shown so dimensioned that it forms all or part o1 a. combustion tuxmel which is eflectively a part of a burner I2. Burner I2 with its combustion tunnel is a burner of such sort that it is capable of providing and introduciug into the furnace chamber under the pressure of the burner a mixture of burner products which because of conditions in the burner and the stage to which combustion proceeds in the tunnel is approximately stable and uniform; und which mixture cf burner products on entry into the furnace chamber gives therein a luminous unstratifled furnace atmosphere. Such burner is typified, as shown, by a. burner in accordance with the disclosure of my application Serial N0. 563,684, which has matured into Patent N0. 2458541 granted January 11, 1949. As shown furnace I is equipp d with two thermo-couples I3 anti Ilmounted in opposite walls l o1 the furnace. 015 these two thermo-couples, thermocouple I3 is placed at a high 1eve1 of the furnace and thermo-couple I4 at the low 1evel thereof.

In the furnace operation, a stable mixture of burner products is introduced into the upper region of chamber 2 under positive burner pressure and at a, temperature of those products somewhat above the temperature to which the charge ultimately is to be raised. The pressure in furnace chamber 2 thus builds up to a degree determined by the setting of damper III, and after circulating through the furnace chamber products of combustion are discharged under the positive pressure which has been established in the furnace chamber. It is to be understood that the relatively low stack 9 does not create any substantial draft, to p1ace the outlet orifice and the furnace chamber under the pu1l o! a.

negative pressure. Because the area of the outlet orifice from the furnace chamber remains constant as pressure builds up under the burner input, the positive pressure in the furnace chember increases in value with increased input volume of the burner products, thus insuring combustion of the desired kind and good heat distribution in the furnace chamber. At low burner input any relief of pressure by increasing the area of the outlet orifice would destroy the calculations based on the furnace shed and thus would destroy the basis for controlling the furnace operation. It is only a1: high pressure build-up under high burner input that any substantial pressure relief may be made during the operation of the furnace without impairing seriougly the effectiveness 015 the operation.

Assuming that the furnace is to be charged with metal bodies such as the steel ingots shown in Fig. I, damper gate 9 is adjusted to maintain positive pressure in the fumace chamber during the heating operation and the empty chamber is brought to the temperature to which the charge uniformly is to be raised. On introduction of the furnace charge the temperature drops anal the burner is controlled to give a definitely heatadditive input. Desirably this heat-input is the extra-high-fire noted above, which gives the maximum heat supply to the furnace charge without raising a.ny region of the charge to a. de-

structive temperature mit! without producing excessive heat loss through the physical structure 01 the Iurnace. Such optimum temperature for supplylng heat to a cold charge is roughly determinable in accordance with known calculations in accordance with the total mass o1. the charge and the mass und composition of the bddy or bodies of which the charge is composed, but is subject to control in accordance with direct observation by way 015 thermo-couple I3 which is to be considered as the primary couple. When the thermometer connected with primary"couple I3 tends t0 show increase in temperature indicating that the rate of heat-input exceeds the absorption rate by the charge, the burner input is cut down to low-fixe which is based on the hea.t shed of the furnace at the desired ultimate temperature o1 the work. Under this heat-input there is heat fiow within the bodies 01' the ingots tending to bring the less directly heated regions of those bodies closer to the temperature cf their more directly heated regions. As this action proceeds und the structure of the ingots proceeds toward temperature equalization there is a, drop in furnace temperature, and when this drop becomes substantial the higher burner input is again restored.

- The cycle is repeated, first dropping heat-input to low-fire and then restoring it to extra.- high-fire until the drop in temperature on the control to low-fire begins definitely to lag, showing that the Centers and lower regions cf the ingots have reached such temperature that the rate of heat transfer within and to the charge is greatly reduced. Because burner products are shown as introduced at the upper region of the furnace chamber, heat is received more directly by the upper regions of the ingots than by their lower regions. When the heating operation ha.s reached the above indicated stage the lower thermo-couple HI which is the secondary couple will cause its thermometer to indicate a selected temperature which is below but approaching the highest desired temperature shown by the thermometer of the primary couple. Burner operation then is controlled between the heat-input cf low-fire which latter is according to the definition which has been given a safe and economical heat-input for a charge which is already at relatively high temperature. The cycle is continued between "low-fire and high-fire until the thermolneter associated with the secondary thermo-coupie records a line closely approaching that scribed by the thermometer of the primary thermo-couple. The heating operation is then to be considered as completed, with the charge fully heat-soaked to the desired temperature equalized throughout each body of the charge.

Positive information that the temperature of a metal body comprised in the furnace charge is of equalized temperature throughout its structure is an important advantage. Particularly is this the case when the work or furnace charge consists of light sheets made into a. pack, or such other laminated structure that the danger of welding between the elements of the pack is a. matter of primary importance. It is a simple matter to determine in such instance at what temperature welding will occur. It is a. difiicult matter, however, to efiect heating with temperature equalization throughout the entire structure of the pack without raising any region of it to that welding temperature.

As above noted, the indication o1 internal and the heat-input of highfire temperature is obtained by basing the low burner input on that rate cf heat-input which will just compensate for normal furnace 1osses at a given temperature. Under such conditions the on1y factor to affect the temperature of the furnace chamber is the charge itself; and if a temperature gradient in any direction exists in the work. the resulting heat-flow to and from the iurnace atmosphere will be indicated as either a rise in temperature 01' fall in temperature, on the recording thermometer. If on the contrary there is no temperature change registered by the recording thermometer. it foilows that there is no temperature gradient within the structure cf the charge bodies, and that the center cf such bodies is at the same temperature as their outer structure. The result is that the control of heat-input and termination cf the heating operation is definitely in accordance with indications given by the furnace charge itself and does not depend upon arbitrary precalculation. With reference particularly to the selective use cf "high-fire and extra-high-fire" it Will be understood that such distinction is based on the fact that metal can' absorb heat much faster when it is cold than when it is hot and it is therefore both safe and economical to employ a higher heat-additive burner input in beginning the heating operation than when that operation is nearing its conclus1on.

As above described, the method cf my invention places the control of the heating operation under the direction of the furna:e charge. Furnace conditions are as has been explained direct1y under the contro1 cf the burner 01 burners of the furnace installation. By the use of automatic equipment the heating operation may be placed directly under contro1 of the furnace charge, by utilizing instruments infiuenced by the condition of the work a s above explained to control the burner or burners cf the furnace.

A complete furnace installation arranged for automatic regulation is shown schematically in Fig. II of the drawings. In Fig. II the furnace and such cf its associated elements as are shown in Fig. I are designated by the Same rcference numerals. Additionally, however, Fig. II shows a fue1 1ine I leading from a sou1ce of fuel supp1y to burner I2 by way of a ratio control I5 and a low velocity air line I'I, which receives air under pressure from a constant pressure blower I8 and which also leads to burner I2 by way cf ratio contro] I6. Ratio contro1 I6 desirably has associated with it a reguiating re1ief attachment such as that disclosed in my Patent N0. 2408114 to give uitimate adjustment between f ue1 and air in accordance With the atmosphere in furnace I. A high pressure air 1ine I9 also leads to burner I2, to provide a fuel-carrying and fuelfilming jet therein.

Both recording therrnometers connected respectively with primary thermo-couple I3 and secondary thermo-coupie I4 are incorporated in a controlling potentiometer comprising relays which will act at set temperatures to control the action of a motor 2I which operates to open and close fuel and air valves cf the ratio control I6. As shown circuits 22 and 23 1ead from the two thermo-couples to the recording and controlling potentiometer 20.

In automatic control the primary thermocouple I3 acts on otentiometer re1ays, the cantacts cf which may be set to open andclose at determined temperatures. Following the cycle of the operation a.s described above with reference to the automatic equipment it will be assumed that as described the Initial action is under control by the high or primary thermocouple I3. 'Ihe potentiometer controller comtacts are set to control burner input to a temperature cf the furnace chamber at "extra-highfire and at low-fire." When the temperature in the furnace chamber tends to exceed the temperature cf extra-high-fire, the potentiometer controller automatically acts through electrical connections 24 to cause motor 2I so to act 011 the valves cf ratio control I6 that the burner input is reduced to a low-fire value. When heat transfer within the bodies of the charge has taken p1ace and the furnace temperature drops below the temperature point at which the contacts cf its relays are set the potentiometer cantroller acts through motor 2I on the valves of the ratio control box to restore the burner input to "extra-high-fire; and as above described this cycle is repeated automatically with the burner ccntrolling combustion conditions in the furnace and the condition of the charge as refiected by furnace conditions controlling the operation cf the burner.

The above described cye1e continues until the temperature recorded by the thermometer associzited with the secondary thermo-couple I4 reaches a predetcrmined point which is be1ow the highest point at which the contacts associated with the prirnary thermo-couple are set. When the thermometer of the secondary couple reaches that point, the potentiometcr controller acts to intrcduce a limit switch into the circuit of motor '2I. That limit switch acta so to 1imit the action cf the motor that the motor stops short cf producing complete opening cf the valves cf the ratio control, the limiting point being such as to give a burner input responding to the input at high-fire which has been described above. The burner is then controlled alternately between "high-fire and low-fire during the remainder of the heating operation, the burner input at the higher heat-aclditive point being such as to insure against injury to the furnace charge because cf excessive heat-input at the temperature to which the charge has been brought. The cycle is continued under such conditions until a condition of the furnace charge is reached at which the 1ine recorded by the secondary thermocouple II closely approaches the line scribed by the primary thermo-couple I3. At that temperature the bodies of which the charge is composed are assumed to have been equalized throughout their structure at the ultimate desired temperature and the heating operation is assumed to have been completed.

Fig. III cf the drawings shows an electrical circuit for efiecting automatic energization of the va1ve motor to produce the above described sequence cf steps. The circuit is shown by means of what might be called a schematic diagram which omits the symbols for relays. motors and the iike.

R1 is a manually-operable general control switch. Hi, H2, L1, Lrz and I-Iz are snap contacts which operate under the influenceof temperature. The interrupters INT1 and INT2 are motor driven and introduce a time element. Contacts L1, L2 and L3 like contacts H are snap contacts. All the L ccntacts give high fue1 supply under low temperature conditions; all the H contacts give low fuel supply under high temperature couditicns. Intermpter INI'; acts to close in balance 9 o1 the couple I3; interrupter INI: closes in balance cf the couple II.

At the beginning of the operation all the H contacts are open and all the L contacts are ciosed. L1 opens up causing the valve mechanism to admit a low flow of fuel. Then H1 closes to give an alternating provislon 015 -tuel flow for extra high fire and tue! flow for low flre, by the making and breaking 01 contacts L1 und H; under the timed interruptions of interrupters INT1. This action continues until contact H; makes its contact at the control point 01 thermocoup1e I4. Relay R is thus energized opening contact R2 to bring SL limit switch into the circuit. H2 contacts then make, causing the valve mechanism to shift to low fuel input. On actuation 01 the valve caused by drop in temperature the valve mechanism opens into a position determined by the setting of limit switch SL to give a fuel input for high flre". The action then is between low flre and high fire for the duration of the heatin g, by the making anti breaking of contacts L2 and Hz under the timed interruptions 01 interrupters INT2. When the interruptions by interrupters INT: fail to produce a substantial temperature drop during the timed period of the interruption, contacts Hz remain c1osed and Iuel flow remains at low fire.

T give a safety factor 11 there should be a. temporary failure of the furna.ce burner er if for any other reason the temperature should drop to a determined limit below the control point o! thermo-couple 4l, contacts L0 make to bring extra high fire again into'existence, switch SL being by-passed.

'I'his described circuit as herein shown is a modification of a standard Leeds & Northrup circuit.

It may be explained in connection with the above, that the designation of the higher thermocouple as the primary thermo-couple and the designation of the lower thermo-couple as the secondary thermo-couple is based upon the organization of the particular furnace structure which is illustratively used. The designation rises from the fact tha.t burner products are introduced into that furnace in an upper region of the furnace, so that the portion of the furnace charge which extends into that region receives the most direct heating effect. The secondary thermo-coupla is placed in a region of the furnace at which the heating eifect is less direct, so that the portion of each body lying in that region tends to remain at a lower temperature than the portion of the body which extends into the upper region 015 the furnace. The distinction is therefore between the relative tendencies toward the aquisition cf high temperature regionally cf the charge bodies, and there must be heat flow within the substance of those bodies from the most directly heated regions thereof to their lass direcly heated regions if they are to attain equalized. temperature. If the furnace were to be so arranged that burner products are introduced in a 1ower region of the furnace, the position of the primary and secondary thermo-couples would be reversed. Also it should be explained that although the furnace illustrating the performance of my method is shown as having a single burner-equipped inlet and a single outlet, a plurality of either or both may in some instances desirably be incorporated in the fumace structure.

It should be explained tha.t whereas I believe the disclosed arrangement 01 automatic electrlc equipment to control burner input to a. turnace to be novel, the automatic electric elements, the presence and function 01 which have been described are in themselves of well knoWn sort commercially available as independent units 01 automatic electrlc equipment.

It has been indicated above that whereas my method has been described with particular reference to a heating operation conducted in a. metallurgical furnace, the method usefully may be performed in utilizing a heating furnace on work o1 variant sort. Thus, for example, the Iurnace charge may be ceramic material. a. bath of glass, preformed glass bodies subjected to an annealing operation er a steam-generating Iurnace. In such latter use the charge is represented by the structure which contains the water from which steam is to be generated and that water itsel1'. In all such operations the controlled input 01 burner products and the obst'arvation and control cf conditions in the furnace and its charge give definite advantage in obtaining an equalized heating of the furnace charge to a desired temperature, without localized or general overheating and with economy in fuel consumption.

Further, it should be borne in mind that in -any specific use of my method control 01 heat und temperature and temperature equalization is obtained by direct as oppbsed to indirect flring of the furnace. Also it should be borne in mind that with the disclosed type of combustion and control of furnace atmosphere conditions, checkwork and recuperators are eliminated; thus decreasing construction costs anti lessening the floor space required for the furnace installation.

I claim as my invention:

1. The method of operating a charged metallurgical furnace to raise the temperature o! a metal body therein uniformly to a desired degree, by introducing directly into the furnace chamber an approximately stab1e mixture of burner products moving ab low velocity and under positive burner pressure, discharging products of coxnbustion from the furnace chamber by way cf an outlet orifice of constant area under a positive pressure in said furnace chamber, and automatically controlling the heating of the furnace charge to a determined equalived temperature in accordance with the temperature in a high temperature zone of the furnace under alternating conditions of low input based on furnace losses at the determined equalized temperature for the charge and a higher heat-additive input volume, with comsequent heat fiow within the body of the charge at said low input until the determined equalized temperature of the charge is maintained under the conditions of low burner input.

2. The method of operating a charged metallurgical furnace to raise the temperatum of an initially reiatively co1d body thcirein uniformly to a desired degree by directly introducing into the furnace chamber an approximately stable mixture of burner products moving at low velocity and under positive burner pressure, discharging products of combustion from the furnace chamber by way of an outlet orifice of constant area under a positive pressure in the said furnace chamber, initially controliing the heating of the furnace charge in accordance with the temperature in a high temperature zone cf the furnace with alternation between a higher heat supply the furnace charge which is the approximate maximum possible without raising any region of the relatively cold charge to a destructive temperature and a1ow heat supply based on furnace losses at a determined equalized temperature for the charge with consequent heat flow within the body o1 the charge until the drop in temperature upon shifi; to low heat supply begins to lag, then shifting to control 01 heating in accordance with the temperature in a low temperature zone 01 the furnace with alternation of heat supply 120 the furnace charge between a heat supply which is the approximate maximum without raising any region of the bot charge to a destructive temperature and the said above defined low heat suppiy until no substantial temperature drop occurs upon shifting from higher heat supply i;o the said above defined low heat suppiy.

3. The method of operating a charged metallurgical furnace to raise the temperature 01 an initially relatively 001d body therein uniformly to a desired degree by directly introducing into the furnace chamber an approximately stab1e mixture of burner products moving at. low veiocity and under positive burner pressure, discharging products of combustion from the furnace chamber by way of an outlet orifice of constant area under a positive pressure in the same furnace chamber, initially controlling the heat 013 the furnace charge automatically in accordance with the temperature in a high temperature zone of the furnace with alternation between a higher heaf; supp1y '00 the furnace charge which is the approximate maximum possible without raising any region of the relatively 001d charge to a destructive temperature and a low heat supply based on furnace losses at a determined equalized temperature for the charge with consequent; heat fiow within the body of the charge until the drop in temperature upon shift to low heat supply begins to lag, then automatically shifting to coutro1 of heating in accordance with the temperature in a low temperature zone 01 the furnace with automatic alternation of heat supply to the fumace Charge between a heat suply which is the approximate maximum without raising any region of the hot Charge to a. destructive temperature and the said above defined low heat supply until 110 substantial temperature drop occurs upon shifting from higher heat supply to the said above deflned low heai: supply.

4. The method 01 operating a charged metallurgical furnace to rais'e the temperature 01 a metal body therein unitormly to a desired degree, by directly introducing into the Iurnace chamber an approximately stable mixture of burner products moving at low velocity and under positive burner pressure, discharging products o! combustion from the fumace chamber by way 01 an outlet orifice of constant area under a positive pressure in said furnace chamber, automatically controlling the heating of the Iurnace charge a determined equalized temperature in accordance with the temperature in a high temperature zone of the furnace under alternating conditions of low input based 011 furnace losses at the determined equalized temperature Ior the charge and a higher heat-additive input volume, with cousequeni; heat flow within the body of the charge at said low input until the determined equalized temperature of the Charge is maintained under the conditions of low burner input, and holding in the furnace chamber a positive pressure 01' a value varying throughout at least the major portion of the furnace operation direccly with the volume of burner input.

NORMAN J. URQUHART.

REFERENCES CITED The following references a.re of record in the file o1. this patent:

UNITED STA'I'ES PATENTS Number Name Date 1,391996 Collins Sept. 27, 1921 1512008 Otis Oct. 14, 1924 1.578,280 Gibson Mar. 30, 1926 1842967 Hegel Jan. 26, 1932 1893,635 Poole Jan. 10, 1933 1906244 Benjamin May 2, 1933 2046,718 Bletz Ju1y 7, 1936 2293550 Kells Aug. 18, 1942 OTHER REFERENCES Pages 194 and 195 of Trinks Industrial Furnaces, vo1 II, Second Edition 1942, published by John Wiley and Sons, New York. New York.

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