US2851018A - Steam generating unit with corner fired furnace and gas recirculation - Google Patents

Description

Sept. 9, 1958 L. w. HELLER 2,851,013

. STEAM GENERATING UNIT WITH CORNER FIRED FURNACE AND GAS RECIRCULATION Filed April 30, 1953. 2 Sheets-Sheet l ATTORNEY Sept. 9, 1958 w. HELLER 2,851,013

STEAM GENERATING UNIT WITH CORNER FIRED FURNACE AND GAS REGIRCULATION Filed April 30. 1953 2 Sheets-Sheet 2 INVENTOR Zia 4s fi/(f/EALEK BY/Pw-W ATTORNEY United States Patent 'STEAM GENERATING UNIT wrrn CORNER p rgnn FURNACE AND GAS RECIRCULA- Application April 30, 1953, Serial No. 352,227

6 Claims. (Cl. 122-479) This invention relates to the construction and operation of vapor generators and more particularly, of fuel burning steam generating and superheating units operating at high steam pressures and temperatures.

The invention is particularly directed to apparatus for and a method of varying the radiant transmission of heat to vapor generating tubes positioned in the furnace walls of a unit of the character described, by the recirculation to the furnace of relatively cool gaseous products of combustion withdrawn from a position beyond convection heat absorbing surface. The invention is more particularly directed to a special arrangement for introducing the recirculated flue gases for the described purpose to a furnace in which the fuel is supplied by so-called corner tangential firing burners arranged at one or more levels in the furnace.

While the invention may be advantageously utilized for affecting the rate of heat transfer to furnace wall cooling tubes for superheat temperature control, it may be separately or concurrently used when burning a slagforrning fuel to modify the temperature of the gases flowing from the furnace outlet to convection heat absorbing surfaces for the purpose of avoiding or mitigating slag accumulation on such surfaces.

It is well known that furnace gas temperatures and the rate of radiant transfer of heat from furnace gases can be regulated by the introduction of recirculated partially cooled gaseous products of combustion into the furnace. It is also known that to introduce such gases into the furnace admixed with the air supplied to burn the fuel, so dilutes the oxygen content with inert gases that a retardation of combustion is to be expected. This is particularly true when the fuel burners used, particularly for burning a fuel like pulverized coal, are of a long flame type as compared with short flame type burners providing an intense turbulence between the combustion air and the air-borne fuel by the particular construction of the burners. If the quantity of recirculated gas admixed with the secondary combustion air and introduced through the burner ports is a high percentage of the air, the dilution may be such that it may be difficult, if not impossible, to maintain ignition of the pulverized fuel-primary air streams.

In accordance with the invention the recirculated gases are introduced into the furnace in proximity to the points of introduction of the fluid fuel and combustion air through burner assemblies in the corners of a tangentially fired polygonal furnace in such a manner that the radiant heat transmission is reduced without hazarding ignition or unduly delaying combustion. The consequent reduction of heat absorption by the vapor generating furnace wall tubes of a steam generating and superheating unit permits a greater amount of heat to be carried out of the furnace to the convection heat absorbing surfaces, such as the steam superheater, and can be utilized in effecting a regulation of the convection superheating.

On the other hand, when recirculated gas is introduced 2,851,018 Patented Sept. 9, 1958 to the furnace and the furnace is operated at high rates of heat liberation, a reduction of the furnace outlet temperature of the combined newly developed products of combustion and the recirculated gases is to be expected. Such a reduction in gas temperature under such heat release rates is advantageous where it is desired to limit the furnace exit gas temperature in order to reduce slagging difiiculties with slag-forming fuels, such as pulverized coal.

The Kreisinger et al. Patent 2,363,875, Nov. 28, 1944, Kruger Patent 2,243,909, June 3, 1941, and an article by Elno M. Powell entitled Tilting Burners Provide Flexible Furnace Performance in themagazine Combustion of June 1945, pp. 36-39, disclosesymmetrical arrangements of variable direction burners positioned at the corners of a polygonal section furnace for-firing a slag-forming fuel, such as pulverized coal, in directions substantially tangential to an imaginary circle in the furnace. The invention is applicable to fuel fired furnaces of the character described having fixed corner burners, as well as to such furnaces having horizontally movable burners, such as illustrated in the Kruger patent, or vertically tiltable burners, such as illustrated in the Kreisinger patent and Powell article.

The various features of novelty which characterize the invention are pointed out with particularity in'theclaims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described preferred embodiments of the invention.

Of the drawings:

Fig. l is a sectional elevation of a steam generating and superheating unit embodying the invention;

Fig. 1 is a fragmentary sectional elevation of a similar unit with a modified form of furnace bottom;

Fig. 2 is a partly diagrammatic horizontal view taken on the line 22 of Fig. 1;

Fig. 3 is a partly diagrammatic horizontal view taken on the line 33 of Fig. 1;

Fig. 4 is an enlarged fragmentary horizontal schematic view taken through one of the gas ports; and

Fig. 5 is a vertical section taken on the line 5-5 of Fig.4.

In the construction illustrated in Figs. 1 and 2 to 5, a vertically elongated furnace chamber 10 of square crosssection is defined by a vertical front Wall 12, opposite side walls 14 and 16, and a rear wall 18. The furnace chamber is designed for dry ash operation, having a hopper bottom 20. A nose baffle 22 extends inwardly from the upper part of the rear Wall 18 and unites with the furnace side Walls and roof 24 to define a lateral outlet gas pass 26 which opens into a downfiow gas pass 28. The boiler illustrated includes an upper steam and Water drum 30 to which the upper ends of closely spaced bare steam generating tubes 32 and 34, lining the front and rear walls 12. and 18 of the furnace '10 respectively, and tubes 36 along the side Walls are connected. The lower ends of the above mentioned tubes are connected to headers 38, 40, and 42 respectively and these headers are connected to the water space of the steam and water drum by suitable downcomer pipes (not shown) to supply water for upward flow through the furnace Wall tubes. The tubes lining the front and rear walls have their lower portions converging to define the hopper bottom 20, from which a central transverse ash discharge opening 44 opens to a subjacent ash collecting pit (not shown). The furnace lateral gas outlet is screened by extensions of the rear wall tubes 34. The downfiow gas pass 28 is connected to one side of a rotary regenerative type air 3 heater 46, the opposite side of which is connected to a gas outlet duct 48.

Saturated steam from the steam Space of the drum 30 is conducted by a transverse row of tubes 50, which extend along the rear wall of the 'downflow gas pass to a superheater inlet header 52. A counterfiow primary superheater section formed by vertically spaced banks 54 of laterally spaced multiple-loop tubes, is positioned in the upper portion of the gas pass 28 and discharges to a transverse steam outlet header 56. The outlet header 56 is connected to a conduit 58, which includes a spray attemperator 60, with an inlet header 62 of a pendant type parallel flow secondary superheater section 64 positioned in the lateral gas pass 26 adjacent the furnace outlet. The secondary superheater includes laterally spaced blanks of serially connected multiple-loop tubes, the inlet ends of which are connected to the header 62 and the outlet ends to an outlet header 68 from which the highly superheated steam is conducted to a point of use. Vertically spaced serially connected banks 66 of economizer tubes are arranged across the downflow pass downstream of the primary superheater section 54.

As indicated in Fig. l, pulverized fuel is supplied to the burners by four pulverizers 70, each pulverizer having two valve controlled discharge conduit connections 72, each supplying a stream of pulverized coal and primary air to the nozzle of a pulverized coal burner 74, as hereinafter described. The furnace of Fig. 1 is provided with two vertically spaced sets of pulverized fuel burners 74 74 each set comprising four burners positioned at the corners of the furnace as shown in Fig. 2. The two pulverized coal and primary air conduits 72 from each pulverizer 70 are arranged to supply fuel to corresponding diagonally opposite burners 74 or 74' at the same level. The valves 76 in these conduits provide a selective control of the flow through the individual discharge conduits. Secondary air for the combustion of the airborne pulverized fuel from the pulverizers is supplied under pressure by a forced draft fan 78 which discharges into the air heater 46 as indicated. Preheated secondary air from the air heater is conducted under pressure by a duct 80 and branch ducts 81 and '83 controlled by dampers 82 to the burnerports 84.

With the operation of the pulverizers receiving raw coal and carrier air and directing streams of primary air and entrained pulverized coal through the conduits 72 connected to the burner nozzles of one or both sets of burners, and with the flow ofapreheated secondary air through the secondary air duct '80 andthrou'ghth'e burner ports 84 surrounding the burner fuel nozzles 74 and 74*, as shown by Fig. 2, mixing of the fuel and carrier air stream with the preheated secondary air is attained to such a degree that ignition of the fuel is readily obtained and the fuel burned as it is carried along the flow path towards the furnace chamber gas outlet.

No attempt has been made to illustrate the details of construction and mounting of the fuel burner nozzles 74 and 74 insofar as the introduction of fuel and combustion air to the furnace is concerned, as the prior art mentioned adequately discloses the manner in which such burner nozzles would be constructed and arranged 'to satisfactorily introduce fuel and air into a furnace chamber from corner points in directions substantially tangential to an imaginary circle in the furnace chamber for creating a clockwise rotating burning mass of combustible elements within the furnace chamber. Either or both sets of buel burners are constructed in accordance with the disclosures of the mentioned prior art, so that the direction of the fuel and air streams can be adjusted or altered with respect to the horizontal or in relation to the vertical furnace Walls to vary the position of the burning fuel mass in the furnace chamber.

In accordance with the invention and as shown in Figs. 1, 2 and 3, relatively cool heating gases are withdrawn from a positionin the heating gas flow path downstream of the economizer tube banks 66. A recirculating gas fan 86 having a damper controlled inlet duct 88 connected to the downflow gas pass 28 at a position downstream of the economizer and ahead of the air heater 46 is provided. As indicated diagrammatically in Figs. 1 and 3, the fan discharges recirculated gas under pressure through dampercontrolled ducts 90 and branch ducts 92 having individual dampers 96 to a corresponding recirculated gas discharge port 94 at one corner of the furnace. The four gas ports are arranged at a level intermediate the elevations of the two sets of burners 74 and 74 Openings for the recirculated gas ports 94, as well as for the burner ports 84 at the respective furnace corners, are provided by bending intermediate tube lengths, such as sections 32 and 36 of tubes 32 and 36 respectively out of the corresponding wall plane. A frame 95 is mounted-0n pivots 97 at the upper and lower ends of the frame whereby the frame may be rotated about a vertical axis by manipulation of a lever 98, which is attached to an extension of the pivot 97 extending through the upper wall of the gas port. Additionally a plurality of vertically spaced vanes 99 are positioned transversely of the frame, each being carried by one of the pivots 93 in the side walls of the frame 95. The series of vanes 99 are ganged together by a rod 100, so that tilting of an intermediate vane 99 by a flexible adjustment member 102, which while flexible enough to permit lateral movement at its lower end with pivoted movement of the frame, is stiff enough to provide for tilting of the connected vanes, by adjustment of the vertical position of the member 102 in a sleeve 106 extending through the top of the chamber.

By the positioning of lever 98, the direction of the recirculated gases discharged through the port and the frame 95 can be adjusted, as shown, to enter at an angle of 45 degrees to the adjacent walls or the direction may be at an acute angle to either wall. Further, with the pivotable adjustment of the vanes 99, the gases may be directed in a horizontal direction inwardly through the port 94 or the vanes may be adjusted to angles inclined upwardly or downwardly, so that the recirculated gases flowing through the ports 94 may be directed either upward toward't'he upper portion of the furnace or downward toward the hopper bottom of the furnace.

It will be appreciated that with the sets of burners at spaced elevations above and below the elevation of the set of recirculated gas ports 94 and fuel burners of one or 'both sets constructed and operable in accordance with the proposals of the previously mentioned art, the direction of fuel and air introduction can be varied independently of the direction of recirculated gas introduction. However, through the independent adjustment of the direction of recirculated gas introduction, particularly in relation to the fresh products of combustion or the adjacent water cooled furnace walls, as may be desired, the maximum or'minimum effect of the introduction of recirculat'ed gas can be obtained, either as a diluent to temper the gases leaving the furnace, or in a manner to effect a regulation of the radiant heat transferred to the furnace walls whereby the heat content of the gases leaving the furnace maybe controlled. The above described combination of adjustable direction burners at two levels and adjustable direction means intermediate those levels to introduce recirculated gas into the furnace, provide a fiexible arrangement whereby modification of furnace gas exit temperatures or heat transfer to surrounding furnace wall tubes, maybe advantageously accomplished with a corner firing furnace installation.

For example, when the unit of Figs. 1 and 2 to 5 is operated at high load, both sets of fuel burners will be tilted downwardly to cause the lower portion of the furna'ce to be highly effective in absorbing radiantly transmitted heat, to the endthat thetgases leaving the furnace chamber will have been cooled to an optimum tempera- 3 ture. Should the heat transfer to the furnace wall tubes be inadequate or should it be desirable to reduce the height of the furnace in order to reduce costs, an effective way of tempering the gas to a lower optimum furnace exit temperature would be by introducing recirculated gases in regulable amounts with the vanes 99 in the gas ports 94 tilted upwardly and horizontally turned so that the recirculated gases will be directed upwardly closely adjacent the wall. This provides a minimum mixing with the fresh products of combustion originating from the fuel and airintroduced downwardly by the upper set of burners 74 The maximum tempering effect on the gases leaving the furnace chamber is attained by this setting.

On the other hand, when the unit is to be operated at an extremely low load, it is expected that the upper set of fuel burners 74 will be fired and tilted to direct the flame upward as taught by the prior art. While the lower portion of the furnace below the active combustion zone with this burner arrangement will not absorb heat at a high rate, absorption of the water cooled walls about the lower portion of the furnace will be further reduced by tilting the vanes 99 and adjusting the gas port frame 95 so that the cooled recirculated gases will be directed downwardly in the lower portion of the furnace to maintain the gases in the lower furnace portion at a minimum heat radiating temperature.

Fig. 1 illustrates the adjustable recirculating gas directing port means as adapted to a slag-forming fuel fired furnace for collecting and withdrawing the fuel ash in a molten condition. In such installations it is customary to design and proportion the furnace so that the temperature in the lowermost portion will be above the ash fusion temperature, so that fluidity of the slag will be maintained even though the load and rate of heat liberation may be reduced to relatively low values. The modified furnace shown by Fig. 1, has a horizontally extending floor 110 spaced closely adjacent the elevation of the lower set of burners. The floor and the adjacent side walls of the furnace are suitably constructed to collect and retain slag in a molten condition for periodic discharge to slag disposal apparatus, all of which is well known in the art.

The furnace bottom of Fig. 1, when used with the fuel, air and recirculated gas provisions of Fig. 1, is so positioned subjacent the lowermost set of fuel burners, that the heat from the fuel introduced through these burners will be released in such proximity that the fluidity of the slag accumulated on the floor will be maintained at the desired fractional load heat liberation rate. In all other respects the two sets of burners are arranged as shown in Fig. l, the lower set of burners 74 being selectively supplied with fuel from two of the pulverizers while the upper set 74 is supplied with fuel from the remaining two pulverizers.

In an installation like Fig. 1*- the primary advantage of recirculated gas introduction is in regulably tempering the gases before leaving the furnace in order to control slagging of the convection heat absorbing surface. With such an installation comprising a furnace as shown by Fig. 1, fuel burners, recirculated gas equipment, and a superheater boiler as per Fig. l, the adjustable gas port frame and the vanes will be adjusted for maximum load condition, when both sets of burners are used, so as to obtain the minimum furnace gas exit temperature, there 6 maximum amount of heat may pass from the furnace to the convection superheater. Under such circumstances the lowermost set of burners will direct the fuel and combustion air streams horizontally or at a small downwardly inclined angle, while recirculated gas from the set of gas ports at the higher elevation will have their damper frames and vanes adjusted to direct the recirculated gas in an upwardly inclined direction along the furnace walls, whereby the radiant heat absorption by the walls in the upper portion of the furnace will be reduced, leaving a greater quantity of heat to be carried by the combined recirculated and fresh combustion gases flowing from the furnace to the convection superheater.

While in accordance with the provisions of the statutes 1 have illustrated and described herein the best forms of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

l. A vapor generating and superheating unit comprising walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a convection heating gas pass opening to the upper end theneof, vapor generating tubes lining the walls of said furnace chamber substantially throughout their height, a convection heated vapor superheater in said gas pass, means for firing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners in the lower part of said furnace chamber walls, the burners of each group being independently operable and symmetrically arranged in the walls of said furnace chamber to discharge in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace chamber for creating a turbulent rotating body of burning fuel in the lower part of the furnace chamber in position to radiate heat to said vapor generating tubes, means for independently varying the angle to the horizontal of the direction of discharge of the fuel burners in each of said groups so as to raise or lower the location of the rotating being no reason to attempt to reduce heat absorption by the furnace walls.

When such a unit is operated at the lower end of the load range, with a minimum of heat input, preferably with only the lowermost set of burners delivering fuel and air, it is important that the temperature in the lowermost furnace zone be maintained at a high value in order to insurefiuidity of slag therebelow, while it is-also important to reduce the radiant heat transfer to the wall tubes in the upper portion of the furnace, so that the body of burning fuel, and means for withdrawing heating gases from said gas pass at a point downstream of said vapor superheater and introducing the withdrawn gas in separate streams into said furnace chamber comprising recirculated gas inlet ports in the corners of said furnace chamber at a level intermediate said upper and lower fuel burner groups, and gas directing means in each of said gas inlet ports constructed and independently operable to vary the angle to the horizontal of the angle of discharge of the recirculated gas streams.

2, A vapor generating and superheating unit comprising walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a convection heating gas pass opening to the upper end thereof, vapor generating tubes lining the walls of said furnace chamber substantially throughout their height, a convection heated vapor superheater in said gas pass, means for firing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners in the lower part of said furnace chamber walls, the burners of each group being independently operable and symmetrically arranged in the walls of said furnace chamber to discharge in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace chamber for creating a turbulent rotating body of burning fuel in the lower part of the furnace chamber in position to radiate heat to said vaporgenerating tubes, means for independently varying the angle to the horizontal of the direction of discharge of the fuel burners in each of said groups so as to raise or lower the location of the rotating body of burning fuel, and means for withdrawing heating gases from said gas pass at a point downstream of said vapor superheater and introducing the withdrawn gas in separate streams into said furnace chamber comprising recirculated gas inlet ports in the corners of said furnace chamber at a level intermediate said upper and lower fuel burner groups, and gas directing means in each of said gas inlet ports constructed and independently operable to vary the angle to the horizontal of the angle of discharge of the recirculated gas streams and to vary the axis of discharge of each of said recirculated gas streams relative to the vertical axis of said imaginary cylinder.

3. A vapor generating and superheating unit comprising walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a convection heating gas pass opening to the upper end thereof, vapor generating tubes lining the walls of said furnace chamber substantially throughout their height, a convection heated vapor superheater in said gas pass, means for firing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners in the lower part of said furnace chamber walls, the burners of each group being independently operable and symmetrically arranged in the walls of said furnace chamber to discharge in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace chamber for creating a turbulent rotating body of burning fuel in the lower part of the furnace chamber in position to radiate heat to said vapor generating tubes, means for independently varying the angle to the horizontal of the direction of discharge of the fuel burners in each of said groups so as to raise or lower the location of the rotating body of burning fuel, and means for withdrawing heating gases from said gas pass at a point downstream of said vapor superheater and introducing the withdrawn gas in separate streams into said furnace chamber comprising recirculated gas inlet ports in the corners of said furnace chamber at a level intermediate said upper and lower fuel burner groups, and gas directing means in each of said gas inlet ports having a series of vertically spaced pivoted vanes operable to vary the angle to the horizontal of the angle of discharge of the recirculated gas streams and pivotally mounted to vary the axis of discharge of each of said recirculated gas streams relative to the vertical axis of said imaginary cylinder.

4. In a vapor generating and superheating unit comprising walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a convection heating gas pass opening to the upper end thereof, vapor generating tubes lining the walls of said furnace chamber substantially throughout their height, a convection heated vapor superheater in said gas pass, means for firing said furnace chamber comprising upper and lower vertically spaced groups of fuel burners in the lower part of said furnace chamber walls, the burners of each group being independently operable and symmetrically arranged in the walls of said furnace chamber to discharge in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace chamber for creating a turbulent rotating body of burning fuel in the lower part of the furnace chamber in position to radiate heat to said vapor generating tubes, and means for withdrawing heating gases from said gas pass at a point downstream of said vapor superheater and introducing the withdrawn gas in separate streams into said furnace chamber comprising recirculated gas inlet ports in the corners of said furnace chamber at a level intermediate the upper and lower fuel burner groups, the method of operating said unit at high loads which comprises introducing the fuel to be burned through both groups of fuel burners, tilting said burners downwardly to lower the level of the rotating body of burning fuel in the furnace chamber, and directing the streams of recirculated gas upwardly in said furnace chamber at an angle to the walls thereof to maintain a body of recirculated gas in the furnace chamber between the rotating body of burning fuel and the furnace walls.

5. in a vapor generating and superheating unit comprising walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a convection heating gas pass opening to the upper end thereof, vapor generating tubes lining the walls of said furnace chamber substantially throughout their height, a convection heated vapor superheater in said gas pass, means for firing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners in the lower part of said furnace chamber walls, the burners of each group being independently operable and symmetrically arranged in the walls of said furnace chamber to discharge in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace chamber for creating a turbulent rotating body of burning fuel in the lower part of the furnace chamber in position to radiate heat to said vapor generating tubes, and means for withdrawing heating gases from said gas pass at a point downstream of said vapor superheater and introducing the withdrawn gas in separate streams into said furnace chamber comprising recirculated gas inlet ports in the corners of said furnace chamber at a level intermediate the upper and lower fuel burner groups, the method of operating said unit at fractional loads to reduce the radiant heat absorption of the vapor generating tubes and increase the heat content of the heating gas contacting with the vapor superheater which comprises introducing substantially all of the fuel to be burned through the upper group of fuel burners, tilting said burners upwardly to raise the level of the rotating body of burning fuel in the furnace chamber, and directing the streams of recirculated gas downw'ardly in said furnace chamber at an angle to the walls thereof to maintain a body of recirculated gas in the lower part of the furnace chamber below the rotating body of burning fuel.

6. In a vapor generating and superheating unit comprising walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a convection heating gas pass opening to the upper end thereof, vapor generating tubes lining the walls of said furnace chamber substantially throughout their height, a convection heated vapor superheater in said gas pass, means for firing said furnace chamber comprising upper and lower vertically spaced groups of slag-forming fuel burners in the lower part of said furnace chamber walls, the burners of each group being independently operable and symmetrically arranged in the walls of said furnace chamber to discharge in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace chamber for creating a turbulent rotating body of burning fuel in the lower part of the furnace chamber in position to radiate heat to said vapor generating tubes, and means for withdrawing heating gases from said gas pass at a point downstream of said vapor superheater and introducing the withdrawn gas in separate streams into said furnace chamber comprising recirculated gas inlet ports in the corners of said furnace chamber at a level intermediate the upper and lower fuel burner groups, the method of operating said unit at fractional loads to maintain slag collecting in the bottom of the furnace chamber in a molten condition which comprises introducing substantially all of the fuel to be burned through the lower group of fuel burners, tiiting said burners downwardly to lower the level of the rotating body of burning fuel in the furnace chamber, and directing the streams of recirculated gas upwardly in said furnace chamber at an angle to the walls thereof in mixing relation with the 9 -10 gaseous products of combustion above the body of burn- 2,013,565 Lundgren Sept. 3, 1935 ing fuel. 2,229,643 De Baufre Jan. 28, 1941 2,363,875 Kreisinger et a1. Nov. 28, 1944 References Cited in the file of this patent 2,781,746 Armacost et a1. Feb. 19, 1957 UNITED STATES PATENTS v 5 FOREIGN PATENTS 1,739,594 Jackson Dec. 17, 1929 827,384 Germany Jan. 10, 1952 1,860,366 Lucke May 31, 1932 523,870 Great Britain July 24, 1940

Images