US2799491A

US2799491A - Furnace for production of controlled furnace atmosphere with recuperative preheating

Info

Publication number: US2799491A
Application number: US475953A
Authority: US
Inventors: Frank A Rusciano
Original assignee: Metallurgical Processes Co
Current assignee: Metallurgical Processes Co
Priority date: 1954-12-17
Filing date: 1954-12-17
Publication date: 1957-07-16
Anticipated expiration: 1974-07-16

Description

July 16, 1957 F. A. RUSCIANO 2,799,491

FURNACE FOR PRODUCTION OF CONTROLLED FURNACE ATMOSPHERE WITH RECUPERATIVE PREHEATING Filed Dec. 17, 1954 5 Sheets-Sheet l INVENTOR. F A RUSCIANO /1/l www AT TORNEY 2,799,491 CE ATMOSPHERE 5 summer-smeet s www@ AT ORNEY F. A. RUSCIANO July 16, 1957 FURNACE FOR PRODUCTION 0F CONTROLLED FURNA WITH'RECUPERATIVE PREHEATING Filed Dec. 17, 1954 F. A. RusclANo 2,799,491

WITH RECUPERATIVE: PREREATTNG 5 Sheets-Sheet 4 July 16, 1957 FURNACE FOR PRODUCTION 0F CONTROLLED FURNACE ATMOSPHERE Filed Deo. 17, 1954 W4 ,M4/,e4

AT ORNEY July 16, 1957 F. A. RusclANo 2,799,491

, FURNACE FOR PRODUCTION 0F CONTROLLED FURNACE ATMOSPHERE WITH RECUPERATIVE PREHEATING Filed Dec. 17, 1954 5 Sheets-Sheet 5 //j/A/H i INVENTOR. F.A.RU$CIANO ATTORNEY United States Patent O FURNACE FOR PRODUCTION F CONTROLLEH) FURNACE ATMOSPHERE WITH RECUPERA- TIVE PREHEATENG Frank A. Rusciano, New York, N. Y., assigner to Metailurgical Processes Co., Newark, N. J., a corporation of New Jersey Application December 17, 1954, Serial No. 475,953

11 Claims. (Cl. 266-5) This invention relates to a method for producing rapid and efficient heating of metal to high temperatures under non-scaling conditions and to a furnace structure by which such method may be practiced.

In an application of F. A. Rusciano et al., Serial No. 347,716, led April 9, 1953 and entitled Method and Apparatus for Producing Controlled Furnace Atmospheres a method is disclosed for producing a non-scaling atmosphere to which the work is subjected during heating and which is composed of the direct products of combustion of the furnace burners. Briefly, this method comprises adjusting the air-fuel ratio of the burners so that the reaction products, when the reactions are carried to completion, will have a CO2/CO and H2O/H2 ratio of such value that the sum thereof is equal to unity. The reactions resulting from such a mixture, while producing an atmosphere which is completely non-oxidizing to steel, release a relatively small percentage of the available B. t. u. of the fuel, of the order of 20%V to 25%, and therefore, produce a low heating rate and a restricted upper temperature limit. In the process of the aforesaid application, these limitations are overcome by the subsequent combustion of this protective atmosphere gas externally of the work chamber and out of contact with the work but in heat transfer relation thereto.

The present invention utilizes the above mentioned feature of the foregoing application, one of its object being to increase the operating efciency of the process.

A further object is to provide a novel furnace structure for effecting the secondary combustion of the protective atmosphere gases and for making available to the work a greater proportion of the heat generated by said secondary combustion.

Other objects and advantages will hereinafter appear.

In accordance with the present invention the heating of the metal is conducted in two stages, namely a preheating stage and a final heating stage. The preheat stage may increase the metal temperature up to from 1000 F. to l500 F. whereas in the final heating the metal may attain a temperature as high as 2500 F. In the preferred form of the invention both the preheating and the nal heating are carried out in the rich combustible protective atmosphere although, as Will be described, the initial heating, up to temperatures of the order of 1200 F., may be effected in a normal combustion atmosphere since no appreciable scaling occurs up to such temperatures, particularly when employing a rapid heating rate. Moreover any slight oxidation which may occur in the preheating step may be reduced in the subsequent high temperature heating in the non-oxidizing atmosphere. In carrying on the present process both the primary and secondary combustion are completed in heat transfer relation to the metal in its final heating stage. Thereafter, the products of the secondary combustion, which are at a temperature substantially equal to or higher than the final temperature of the 2,799,491 Patented July 16, 1957 ICC metal being heated, are utilized to heat the work in the preheat stage. Since the temperature of the metal being heated in this stage may range from room temperature up to 1200 F. or possibly 1500 F., a large proportion of the residual heat of these combustion products will be transferred to the work. The process is applicable either to continuous or batch type furnaces.

In one form of the furnace illustrated, it comprises a linear or straight through heating chamber in which the work passes from one end to the other. This chamber is divided into two heating zones, the first extending from the charging opening to a point at which the work will have attained in the normal travel period, the desired preheat temperature. While 1200 F. to 1500 F. has been mentioned, it is to be understood that these temperatures are only by way of example, the selected preheating temperature normally being dependent upon the most economical length of the preheat zone. The final heating zone extends from the end of the preheating zone to the discharge end of the furnace. This second zone is provided with a hot atmosphere produced by the reaction of combustion of a fuel and air mixture having a suiiicient deficiency of air required for complete combustion, of the order of 50%, so as to render the atmosphere non-scaling to the work at the elevated temperature attained in the zone. This non-scaling atmosphere is generated by burners directly associated with this zone. The preheat zone is in open communication with the iinal heat zone and will receive some of the atmosphere generated therein. However, the venting arrangement of the furnace is designed to supply the major portion of this atmosphere directly to a second combustion chamber disposed directly over the final heating zone and separated therefrom by a relatively thin partition or arch of good heat conducting material. This atmosphere is highly combustible, still containing about to 80% of the B. t. u. content of the original fuel. This potential heat is converted into sensible heat by completing the combustion thereof with additional air in the second combustion chamber whereby the additional heat thus generated may be utilized to increase the temperature and heating rate of the iinal heating zone. The products of this secondary combustion are thereafter passed in heat transfer relation to the work in the preheating zone.

The invention will be more fully understood by reference to the laccompanying drawings in which:

'Fig 1 is a central vertical longitudinal section of a continuous furnace embodying the features of the invention;

Fig. 2 is a vertical transverse sectional view of the furnace, taken on the line 2--2 of Fig. 1;

'Fig 3 is a vertical transverse sectional View, taken on the line 3-3 of Fig. l;

Fig. 4 is a central vertical longitudinal sectional view of a modified form of furnace;

Fig. 5 is a vertical transverse sectional View, taken on the line 5 5 of Fig. 4;

Fig. 6 is a central vertical longitudinal sectional view of a batch type furnace embodying the present invention; and

v Fig. 7 is a transverse sectional view taken on the line 7--7 of Fig. 6.

Reference will first be made to Figs. l, 2 and 3 in which a continuous furnace of the gravity or rolldown conveyor type is shown. This furnace is composed of refractory lbrickworlr and includes an inclined floor 10,

opposite side walls

11 and 12, an arched roof 13 and

opposite end walls

14 and 15. The end wall 15 is provided with a charging slot 16 and end wall 14 has a similar discharge slot 17, the work W, shown as round bar stock, being conveyed from the charging slot to the discharge slot by gravity, rolling upon spaced rails 18. In the ease of flat stock, suitable pusher mechanism may be employed to force the work along the rails. Other forms of conveyance for the work may be provided, such as a belt or chain conveyor.

The charging and discharging slots 16 and 17 are provided with doors 20 and 21, respectively, adapted to be opened and closed by conventional mechanism, not shown.

An arched partition 22 extends transversely across the furnace, the lower end thereof being spaced above the floor a sufficient distance to permit passage of the work W therebeneath. The partition 22 serves to partially separate the furnace into two sections and aids in the support of a pair of thin ceramic partitions 23 and 24, arched transversely across the furnace intermediate the floor 10 and roof 13, and thus with the partition 22 divides the interior of the furnace into four

inter-connected chambers

25, 26, 27 and 28. Chamber 25 forms the low or preheating zone of the furnace, communicating with chamber 26 which serves as the final or high heat zone. Vents 29 extend from adjacent the floor of the high heat zone 26, upwardly through the opposite side Walls 11 and 12 (Fig. 3) and discharge into the upper chamber 27 through ports 35. A group of vents 3l (Figs. l and 3) extend upwardly through the end wall 14 above the discharge slot 17 and also discharge into chamber 27 through ports 32. A third group of vents 33 extend upwardly through the end wall above the charging slot 16, these vents communicating with a horizontal passageway 34 which continues around in the

side walls

11 and 12 and discharges into chamber 27 through ports 35 (Fig. 2).

Air jet nozzles

36, 37 and 38 are provided adjacent to the ports 32 and 35, respectively, in such manner as to induce a flow of gas from chamber 26 through the

vents

29 and 31 and from chamber 25 through the vent 33.

Passageways 40 extend through the upper portion of the arch 22, permitting relatively unrestricted passage of gases from chamber 27 to chamber 28, and chamber 28 is provided with a transverse vent slot 4l at the forward end thereof adjacent to the end wall 15.

A series of burners 42 are disposed in the

side Walls

11 and 12 beneath the arched partition 24 and above the level of the work W. These burners are directed substantially tangentially to the partition 24 so that the burner products will have scrubbing engagement with the lower face of this partition.

Burners 42 provide the entire fuel supply for creating the protective atmosphere in the

heating chambers

25 and 26 and for the heating requirements of the furnace. The air-fuel mixture supplied to the burners 42, is one which on complete reaction will produce an atmosphere completely non-scaling to steel. phere, as stated, will be one in which the sum of the CO2/CO and H2O/H2 ratios is approximately one. The actual CO2/CO and H2O/H2 ratios will vary with temperature each being about 0.5 at l500 F. At 2100 F. the CO2/CO ratio will be about 0.3 and the H2O/H2 ratio about 0.7. The proper air fuel mixture for obtaining this ratio is largely independent of the operating temperature of the furnace, being controlled primarily by the ratio of molecular carbon to hydrogen in the fuel. Gaseous fuels having a C/H2 ratio below 1.0 will be restricted to from 50% to 54% of the air required for complete combustion in order to achieve the desired nonscaling condition, whereas the light oils, which have a C/Hz ratio of between 1.0 and 2.0 will produce the desired C02/CO and H2O/H2 ratio summation with an air deficiency of from 40% to 50%. These rich mixtures release as useful heat only about to 25% of the available B. t. u. content of the fuel and develop temperatures only of the general order of 2100 F. Under normal furnace combustion condition they are This form of atmosincapable of reacting to completion without some thermal assistance and hence incapable of producing the desired non-scaling ratios. It is for this reason that the burners 42 are directed upwardly against the partition 24, which, as will appear, is maintained at a temperature several hundred degrees above the unassisted reaction temperature of the required air-fuel mixture. A portion of the reaction products created in the nal heat zone 26 will be drawn into the zone by the vents 33 and thus maintain the protective atmosphere therein and under normal conditions of operation the amount of air supplied to the induction nozzles 38 associated with vents 33 will be only suflicient to maintain the chamber 25 full against air infiltration about the door 20. The great majority of this rich atmosphere, which still contains from 75% to 80% of the available B. t. u. content of the fuel, is vented directly into chamber 27 through the

vents

29 and 31 under the suctional effect of the air nozzles 36 and 37, respectively. These nozzles are so spaced from their respective ports that the suctional effect produced by the air required to complete the combustion of the atmosphere gases will maintain the

chambers

25 and 26 under a slight positive pressure. Chamber 27 thus forms second combustion chamber to which an air-fuel mixture is supplied by the ports 32 and 35. Temperatures of the order of 2600 F. to 2800" F. are thus attainable in chamber 27. In order that a substantial portion of this heat will be transferred to the lower work heating chamber 26, the arch partition 24 is preferably composed of thin interlocking refractory tiles composed of a material of good heat conductivity, such as silicon carbide. This hot arch thus serves to heat the work in the chamber 26 both by radiation and by transfer of heat to the incoming burner products as they sweep over the hot surface thereof. Work temperatures as high as 2500 F. are thus obtainable in chamber 26.

Following this secondary combustion in chamber 27 the products thereof are drawn into chamber 28 by the vent 41 and thus pass over the second arch partition 23 so as to impart a further portion of their heat to the incoming work in the preheat zone. Since the work in this zone is relatively cool there will be a high heat differential between

chambers

28 and 25 and hence a rapid heat transfer to the latter. Thus the gases vented from slot 4 1 will be at a considerably reduced temperature, depending upon the desired length of the preheat zone. This decrease in temperature of the secondary combustion gases between ports and vent 41 represents the gain in operating eicieney due to the provision of the preheat zone. In furnace having a Work temperature of the order of 2500 F. such a decrease in the exhaust gas temperature of 500 F. represents an increase in operating heating eticiency of about 100%.

In order to further utilize the heat of the exhaust gases from vent 41 an air supply conduit 42, for providing pressurized air to the

nozzles

36, 37 and 38, is positioned directly above the slot 41 transversely of the furnace, whereby preheating of the air will occur. Air is supplied centrally to this conduit through supply pipe 43 and is conducted to

nozzles

36, 37 and 38 by

manifolds

44 and 45 extending along each side of the furnace.

Individual tubes

46 and 47 supply the nozzles 36, at opposite sides of the furnace, from

manifolds

44 and 45, respeetively. Air for nozzles 37 are supplied by tube 48 u from a secondary manifold 49 which is connected at its opposite ends to manifolds 44 and 45, through normally closed

electric valves

51 and 52, respectively. By-

passes

53 and 54 extending around

valves

51 and 52 are provided with

manual valves

55 and 56, respectively, whereby a predetermined low air supply is fed to the nozzles 37 at all times. Nozzle 38 at one side of the furnace is connected to the manifold 44 through a normally closed electric valve 57 and tubing 58, with a normally open by-pass 59 disposed around the valve 57. Nozzle 38 at the opposite side of the furnace is similarly connected to manifold 4S through an electric valve 61 and by-pass 62, by tubing 63. By virtue of this arrangement a minimuni supply of air is provided to nozzles 37, 38 and 38' which maintain a suction in the door slot vents 31 and 33, respectively, sufficient to draw into these vents any oxidizing products of combustion which may be produced in the door slots by seepage of air about the doors 21 and 20. Vents 33, as previously stated, also serve to maintain a low circulation of the non-scaling atmosphere from chamber 26 into chamber 25.

Electric valves

57 and 61 are in circuit with a door operated switch 64 adapted to be closed, when the charging door is opened, to energize these valves to open position, thereby to augment the normal air supply to

nozzles

38 and 38 and thus increase the suction in vents 33 to remove a larger volume of combustion products produced in the door slot 16 in the open condition of door 20. This increased suction also prevents flare out of the combustible atmosphere gases and burning at the door opening. .Electric valves 51 and S2, associated withI nozzles 37 are energized in a like manner by a switch 65 upon opening of the discharge door 21 whereby to similarly increase the suctional effect in the vents 31 during the open period of this door.

In Figs. 4 and 5 I have shown a modified furnace in which the heat radiating partition 23 of Fig. l is omitted. In this embodiment the completely combusted gases pass directly from the secondary combustion chamber 27 into the preheat chamber 2S. The arched roof 13 is corbeled down to adjacent the upper level of the work to bring the vented gases down over .the incoming cold work and thus lincrease the rate of heat transfer thereto. These gases are vented near the charging door 20 by an elongated vent slot 41.

The exhaust gases entering chamber 25 through the openings in the arch partition 22 are, of course, scaling to steel at elevated temperature. However, no appreciable oxidation occurs up to about 1200c F., particularly when the heating period is short. The length of the chamber 25', therefore, should not be greater than that required to bring the work up to a temperature of this order in the time of travel therethrough. However, temperatures somewhat above this non-scaling range may be tolerated as the work approaches the bulkhead 22 since there will be a small ow of non-scaling gas from chamber 26' to chamber 2S beneath this partition which serves to blanket the work in chamber 21 adjacent thereto. It is desired to reduce this flow to a minimum, however, and for this purpose the bulkhead 22 is fabricated with a flat lower face 70 closely spaced above the work so as to restrict the free space beneath the bulkhead to a small proportion of the effective area of the vent ports 29. The effective area of these ports is, of course, the actual area times a factor dependent upon the suctional effect of the air jet nozzles 36 and 37. With proper design it is possible to obtain factors up to l0. It is preferred to design the vents 29 to obtain approximately 90% venting therethrough and 10% or less beneath the wall 22.

The combustible gas entering chamber 25 directly from chamber 26 contains a large percentage of carbonmonoxide and hydrogen which will burn on contact with the outerv air at the slot 41. To avoid this external burning and to utilize the available heat therein, I provide air jets 71 tiring into the chamber 25' from both ,sides thereof.

A supplemental vent 72 has been provided in the roof 'of the furnace adjacent to the secondary combustion chamber outlet 40'. This vent is normally covered by an adjustable refractory 73 so that the vent may be parytially opened to by-pass a portion of the combustion products should there be a tendency to heat the work in chamber 25 above the safe non-scaling temperature. A similar adjustable refractory 74 is provided for the 6 vent 41' to assist in the control of the relative flow through vents 41 andV 72.

It is obvious that if it should be necessary to stop the movement of the work through zone 25 for any appreciable period with a portion of the load still in this zone, such work might become undesirably heated to above the safe non-scaling temperature. To avoid scaling under such conditions vent 41' may be completely closed by the refractory 74 and vent 72 opened fully. Since the oxidizing combustion gases entering chamber 25 through opening 40 will be considerably higher in temperature than the non-scaling gases entering beneath the wall 22', the non-scaling gases will tend to remain beneath the oxidizing combustion gases and form a protective atrnosphere about the work. It will, of course, be understood that during such idling periods the volume of fuel and air supplied to the burners 42 will also be reduced in accordance with normal practice.

In Figs. 6 and 7 a batch type furnace of the duplex type is shown. This furnace comprises two work heating chambers 75 and 76, separated by a partition 77; two secondary combustion chambers 78 and 79 which open freely into each other through an opening 80 in the partition wall 77 and are separated from the work heating chambers by thin arched partitions 81 and 82 composed of a good heat conducting refractory; and a series of reaction tubes 83 to 88 which extend across the furnace within the secondary combustion chambers between the

side walls

89 and 90. Each of the reaction tubes abuts against a burner block 91 at one end thereof and communicates at the other end with a passageway 92 which opens into the work heating chambers 75 and 76 through ports 93. The burner blocks 91 are provided with suitable burners 94, alternate tubes being fired from opposite ends. Thus,

tubes

84, 86 and 88 have their burners and burner blocks arranged in side wall as shown in Fig. 7 whereas tubes 83, 85 and 87 have their burners and burner blocks disposed in side wall 92.

The air-fuel mixture supplied to burners 94 is one which on complete reaction will produce a non-scaling atmosphere or if desired, a carburizing atmosphere, that is a mixture having about 50% or less aeration. The supplemental heat required for completing the reactions is obtained by secondary combustion of the products of these reactions with additional air in the chambers 78 and 79. The gaseous products formed in tubes 83 to 88 are passed into the chambers 75 and 76 to provide the protective or work treating atmosphere. Ports 95 adjacent to the hearth or floor 96 of the furnace open into vents 97 which communicate with chambers 78 and 79 through burner tunnels 98, air jet nozzles 99 being positioned to direct a flow of air into the tunnels 98 in such manner as to produce a suctional effect in the vents 97 soV as to pull the rich combustible atmosphere from the Work chambers and induce it, with supplemental air of combustion, into the secondary combustion chambers, Vents 101 and 102 are provided in the roof 103 for chambers 78 and 79, respectively, and vents 104 and 105 for the Work chambers 75 and 76 are provided in the end walls 106 land 107, respectively. Vents 101 and 104 may be closed individually or together by a sliding refractory 108 and vents 102 and 105 are similarly closed by a refractory 109.

Access to the chambers 75 and 76 for insertion and removal of work is obtained through door openings 110 and 111, respectively, normally closed by

doors

112 and 113, respectively.

In the normal operation of the furnace the chambers 75 and 76 will operate alternately as preheat and iinal heat chambers. Thus, if it be assumed that at a particular time chamber 76 is operating as a preheat chamber and chamber 75 as a final heating chamber, the valve block 108 will be positioned as shown to close both vents 101 and 104. At such time the burners associated with tubes 83 to 85 will be operating to supply the desired air-fuel mixture to these tubes Whereas the burners associated with tubes 85 to 88 will be turned od. The reaction products from tubes 83 to 85 will pass into chamber 75 to protect the work therein and impart heat thereto. Since the normal reaction temperature of the mixture employed in the tubes S3 to 85 is only of the order of 2100 F. whereas the combustion temperature of the secondary combustion produced outside of the tubes in chamber 78 is of the order of 2600" F. to 2800 F. the reaction products within the tubes will absorb a large portion of this external heat, thus insuring the completion of the reactions within the tubes and effectively transmitting the transferred heat into the work heating chamber. The use of the high temperature reaction tubes, as compared with burners firing directly into the work chamber, as shown in Figs. l to 5 is particularly desirable when employing oil fuel in order to completely combine the carbon content ot the fuel with the meager air supply employed, and thus prevent soot formation. The hot products in passage to the vent ports 95 pass over the work in chamber 75 giving up a portion of its heat thereto. It is then mixed with additional air by the nozzles 99 associated with the vent ports 97 of chamber 75 and is burned in chamber 78, thus supplying the high heat head referred to for heating the tubes 83 to 85. The arch partition also is highly heated and thus serves as a radiant heat source for the work in chamber 75.

The combustion products in chamber 78, which ordinarily would be vented to the outer atmosphere, are now used to prelieat the work in chamber 76, by passing the same into chamber 79, and over the arch S2 from which the work in chamber 76 will be heated by radiation. When it is desired to keep the work being preheated from contact with these combustion gases, they may be directly vented from the furnace by opening the vent 102. However, up to temperatures of about 1200 F. the oxidizing effect of these gases is .negligible to the work and during, at least this portion of the preheat I prefer to close vent 102 and thus to force the combustion products into chamber 76 through the fiues 97 associated with this chamber whereby they will pass directly in contact with the work for greater heating efficiency. They are then vented from chamber 76 by the flue 105. After the work has attained a temperature at which oxidation would occur in this atmosphere fiue 105 may be closed and vent 102 opened to continue its heating from the radiant arch only until the work in chamber 75 has reached its final temperature. At this time the burners associated with tubes 83 to 85 will be shut off, refractory 108 adjusted to open vent 101, refractory 109 moved to close vents 102 and 105, the burners 99 associated with tubes 85 to S8 turned on and the air supply to nozzles 99 associated with fiues 97 of chamber 76 turned on. Chamber 76 then serves a final heat chamber to bring the work therein up to its final temperature. The hot work may now be removed from chamber 75 and a cold load inserted. The refractory 108 is then adjusted to close vent 101 and open vent 106 whereby the secondary combustion products will be passed through chamber 75 to aid in the preheating thereof.

When high production is desired it will be understood that both work chambers may be operated at high temperature, that is with burners 94 of both sets of tubes in full operation and air supplied to the nozzles 99 of the fines 97 of both chambers. Under this condition of operation fines 104 and 105 will both be closed and vents 101 and 102 will both be open so that the secondary combustion products will vent directly from chambers 78 and 79 without passing through chambers 75 or 76.

Obviously, numerous other modifications of the lapparatus in which the process herein described may be carried out will occur to those skilled in the art without departing from the essential features of the invention. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope 8 of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

What is claimed is:

l. A furnace for the heating of metal comprising a work heating chamber having -a preheating section and a final heating section, means for supplying to said final heating section the hot primary products of combustion of a mixture of fuel and air having a large deciency of air for complete combustion, -a combustion chamber disposed in direct heat transfer relation to said final heating section, means for venting at least a portion of said primary combustion products from said final heating section into said combustion chamber, means for adding additional air to said vented products whereby to produce secondary combustion thereof in said combustion chamber and means for passing the products of said secondary combustion in heat transfer relation to said preheating section.

2. A furnace constructed in accordance with claim l in which said combustion chamber is separated from said final heating section by a partition composed of -a refractory having good heat conducting characteristics.

3. A furnace constructed in accordance with claim 1 having means for venting a portion of the primary combustion products through said preheating section.

4. A furnace constructed in accordance with claim 1 having venting means for said preheating section whereby a portion of the primary reaction products are drawn through said preheating section.

5. A furnace constructed in accordance with claim 1 in which said preheating section is provided with a partition composed of a material having good heat conducting characteristics and means for passing said secondary combustion products in Contact with one side of said partition externally of said preheating section.

6. A furnace constructed in accordance with claim 3 having means for introducing air into said preheating section for secondary combustion of the primary combustion products vented therethrough.

7. A furnace constructed in accordance with claim 1 in which said secondary combustion products are passed through said preheating section.

8. A furnace constructed in accordance with claim 1 in which said means for supplying primary reaction products to said final heating section comprises burners directed into said final heating section.

9. A furnace for the heating of metal having a first work heating chamber -and a second work heating chamber, a partition at least partially separating said first and second work heating chambers, a third chamber separated from said first work heating chamber by a partition composed of good heat conducting material, a fourth chamber separated from said second work heating chamber by a partition composed of good heat conducting material, means for supplying primary combustion products to said first work heating chamber having a large deficiency of air for complete combustion, venting means extending from said first work heating chamber to said third chamber for passage of said primary products to said third chamber, means for supplying air to said vented products for secondary combustion thereof in said third chamber, means for passing said secondary products of combustion from said third to said fourth chamber and means for venting said secondary products from said fourth chamber.

l0. A furnace constructed in accordance with claim 9 in which said means for venting the fourth chamber includes said second work heating chamber.

ll. A furnace constructed in accordance with claim 9 having means for supplying primary combustion products to said second work heating chamber having a large deciency of air for complete combustion, means for venting said primary products to said fourth chamber, means for adding supplemental air to said vented products for secondary combustion in said fourth chamber and means for venting said third chamber to the external atmosphere, whereby said rst and second work heating chamber may be operated alternately as preheating and nal heating chambers.

References Cited in the le of this patent UNITED STATES PATENTS 975,077 Rockwell Nov. 8, 1910 10 Dreein Mar. 4, 1941 Holcraft Mar. 18, 1952 Ness Apr. 12, 1955 Rusciano Sept, 18, 1956 FOREIGN PATENTS Great Britain Oct. 22, 1925

Claims

1. A FURNACE FOR THE HEATING OF METAL COMPRISING A WORK HEATING CHAMBER HAVING A PREHEATING SECTION AND A FINAL HEATING SECTION, MEANS FOR SUPPLYING TO SAID FINAL HEATING SECTION THE HOT PRIMARY PRODUCTS OF COMBUSTION OF A MIXTURE OF FUEL AND AIR HAVING A LARGE DEFICIENCY OF AIR FOR COMPLETE COMBUSTION, A COMBUTION CHAMBER DISPOSED IN DIRECT HEAT TRANSFER RELATION TO SAID FINAL HEATING SECTION, MEANS FOR VENTING AT LEAST A PORTION OF SAID PRIMARY COMBUSTION PRODUCTS FROM SAID FINAL HEATING SECTION INTO SAID COMBUSTION CHAMBER, MEANS FOR ADDING ADDI TIONAL AIR TO SAID VENTED PRODUCTS WHEREBY TO PRODUCE SECONDARY COMBUSTION THEREOF IN SAID COMBUSTION CHAM-BER AND MEANS FOR PASSING THE PRODUCTS OF SAID SECONDARY COMBUSTION IN HEAT TRANSFER RELATION TO SAID PREHEATING SECTION.