US3221796A

US3221796A - Self-stabilizing combustion apparatus

Info

Publication number: US3221796A
Application number: US326895A
Authority: US
Inventors: John D Nesbitt
Original assignee: Midland Ross Corp
Current assignee: Midland Ross Corp
Priority date: 1963-11-29
Filing date: 1963-11-29
Publication date: 1965-12-07
Anticipated expiration: 1982-12-07

Description

Filed Nov. 29, 1963 All? INVENTOR. JDHN l7. NESBIT T.

United States Patent SELF-STABILIZING (IQMEUSTIQN APPARATUS John D. Nesbitt, Toledo, Ohio, assignor to Midland-Ross Corporation, Toledo, Ohio, a corporation of Uhio Filed Nov. 29, 1963, Ser. No. 326,895 9 Claims. (Cl. 1581.5)

This invention relates to an improvement in self-stabilizing combustion apparatus of the type described in Canadian Patent 651,907.

In the aforesaid Canadian patent there is described combustion apparatus which is self-stabilizing, i.e., apparatus which may be utilized in low temperature service without the need for a separate pilot burner. According to the teachings of this patent a portion of the stream of fuel, which is introduced axially into the combustion apparatus, is drawn backwards toward the inlet end of the apparatus by virtue of the negative pressure resulting from the tangential admission of a portion of the combustion air requirements to the apparatus at a locus adjacent the inlet end thereof. There is then formed a combustible air-fuel mixture at the interface of the spinning air stream and the portion of the fuel that is drawn backwards toward the inlet end of the apparatus. This airfuel mixture is readily ignitable and, when once ignited, the combustion thereof proceeds with remarkable stability thereby serving to pilot the combustion of the remaining portion of the fuel stream. In a popular embodiment of the invention of the aforesaid Canadian patent a high degree of the combustion of the remainder of the fuel stream is made to occur within the combustion apparatus itself by virtue of the generally radial admission of additional combustion air by means of a plurality of ports in the wall means which serves to define the combustion chamber of the apparatus. Thus, such combustion devices are usually referred to as combustors.

In the operation of combustors of the aforesaid type there have been noted two problems during periods of sustained operation at relatively low inputs, solutions to which it is the object of this present invention to provide. First, at relatively low fuel inputs the forward velocity of the fuel introduced through the fuel inlet means hecomes quite small with the result that an increased quantity of fuel is drawn backwards toward the inlet end of the combustor. When this happens the air-fuel mixture which forms between the spinning air stream and the backwardly flowing fuel stream becomes excessively rich which leads to incomplete combustion of the backwardly flowing fuel stream with the consequent formation of undesirable deposits of carbon on portions of the combustor.

Second, in many instances it is desired to provide any of several common types of flame supervising means to take corrective action in the event the flame of the combustor becomes extinguished. When a combustor of the aforesaid type, equipped with flame supervising means, is operated at relatively low fuel inputs, the rate of heat release is frequently so low that the flame supervising means cannot detect the presence of flame. This is troublesome in that it may lead to unnecessary and undesirable shutdowns of the oven or other device with which the combustor is associated.

For a further understanding of the invention, attention is directed to the following portion of the specification, the drawing and the appended claims.

In the drawing:

FIG. 1 is an elevational view, in section, of a combustor embodying the present invention; and

PEG 2 is a sectional view taken on line 22 of FIG. 1.

A combustor in accordance with the present invention is shown generally at 11 and comprises wall means 12 forming a combustion chamber 13 which is substantially 3,221,795 Patented Dec. 7, 1965 circular in transverse section and which has an inlet end and an outlet end shown, respectively, at 14 and 15. The inlet end 14 of combustion chamber 13 is substantially closed by backplate 16 which is disposed transversely of and preferably, as shown, perpendicular to the axis of combustion chamber 13. An annular air plenum 17 is formed surrounding combustion chamber 13 by means of cylindrical wall 18 in combination with the peripheral portion of backplate 16, an annular frontplate 19', and wall means 12. Air for combustion is delivered to plenum 17 by means comprising a flanged inlet connection 21. Inlet connection 21 is shown as comprising a transversely disposed multiported orifice plate 22 to impart a controlled pressure drop to the supply of combustion air to aid in accurately metering and controlling its rate of delivery. Fuel for combustion is introduced into combustion chamber 13 proximate to inlet end 14 by fuel inlet means comprising a pipe 23 which terminates a short distance downstream of backplate and which is disposed substantially coaxial to chamber 13.

A stream of air, equal to about 410% of the amount required for stoichiometric combustion of the maximum rate of fuel delivery through fuel pipe 23, is introduced tangentially to combustion chamber 13 at a locus closely adjacent inlet end 14 thereof by air inlet means comprising one or more pipes 24 whose axis is directed parallel to a plane disposed generally transversely t0 and preferably substantially perpendicular to the axis of combustion chamber 13 (e.g., the plane of backplate 16). In this manner the air which is introduced through pipe 24 will have not more than a very small component of motion directed toward the outlet end 15 of combustion chamber 13. The tight spinning motion of the air introduced through pipe 24 will create a region of reduced pressure surrounding fuel pipe which will draw a portion of the fuel from pipe 23 backwardly toward backplate 1.6. A readily ignitable fuel-air mixture will form adjacent backplate 16 between the portion of the fuel from pipe 23 that is drawn backwardly toward backplate 16 at the interface between it and the spinning stream of air from air pipe. When once ignited, either by hand or by means such as a spark plug 25, this fuel-air mixture which is being continually generated will burn with great stability to pilot or stabilize the combustion of the remainder of the fuel stream introduced into combustion chamber 13 by means of fuel pipe 23.

In the illustrated embodiment a large degree of the combustion of the remainder of the fuel stream from fuel pipe 23 is made to occur within combustion chamber 13 itself by providing wall means 12 with a plurality of ports 26 to admit air from plenum 17 radially into combustion chamber 13. Ports 26 are preferably arranged in a series of axial stages and are so sized and spaced as to provide combustion air in amounts, and at locations, which will result in sustaining the combustion of the remaining portion of the fuel stream at the maximum rate which will not lead to flame quench mg.

In the operation of combustors of this description for prolonged periods at rates of fuel introduction substantially below the maximum design capacity of the combustor it has been noted that carbon deposits are formed on backplate 16. Carbon deposits are generally undesirable in that they may lead to difiiculty in re-igniting the combustor and in that they may interfere with means provided to monitor the presence of combustor flame, e.g., means comprising a flame rod R in the illustrated embodiment. Additionally, the formation of carbon deposits is particularly undesirable in combustors for certain applications such as combustors for supplying heated air for paint drying and food drying or roasting. In such applications there is the danger that deposited carbon may flake off and pass into contact with the product and thereby detract from its quality.

The reason for the formation of carbon deposits on the backplate of combustors of the foregoing type at low delivery rates is believed to be that the rate of fuel drawn toward the backplate increases as the total rate of fuel delivery decreases due to the fact that the fuel stream has less forward momentum by virtue of its reduced velocity. In line with this reasoning, then, it is believed that fuel-air mixture which forms at the backplate becomes excessively rich at low fuel delivery rateswhich leads to the formation of free carbon in that portion of the reaction zone in contact with backplate 16 with resultant accumulation of carbon particles on backplate 16 (assuming, of course, that the combustor is fired with a carbonaceous fuel, e.g., natural gas).

That the foregoing analysis of the problem is correct is believed to have been established by a solution which has been demonstrated and whose efficacy is analytically consistent with the analysis of the problem. This solution resides in the tangential introduction of a stream of air into fuel pipe 23 at an upstream locus. This is accomplished by providing fuel inlet pipe 23 with a tangential air inlet pipe 27 whose axis is directed transversely of a plane through the axis of fuel inlet pipe 23 and which is so oriented as to cause the air delivered therethrough to rotate in the same direction as the air delivered through pipe(s) 24. The delivery of air to pipe 27 may be accomplished by an air line 28 which bleeds air from flanged air inlet connection 21 of combustor 11 through a bleed connection 29. The rate of air delivery through tangential pipe 27 is preferably small. As a specific example, without being intended as a limitation on the invention, it has been found that the delivery of air at a rate of 220 c.f.h. (cubic feet per hour) has been of considerable utility in combating carbon formation at reduced fuel delivery rates in a combustor sized for a maximum rate of fuel delivery of 2000 c.f.h.

It is believed that the tangential admission of air to fuel inlet pipe 23 is effective in combating carbon deposition on backplate 16 at low delivery rates because the spinning air is preferentially drawn toward the backplate due to its centrifugal momentum thereby reducing the tendency to draw fuel toward the backplate and, additionally, by providing a layer of air adjacent the backplate intermediate the layer of fuel thereby to insure that the mixture which forms adjacent the backplate will not be so rich as to lead to the formation of carbon deposits.

Because of the fact that the tangential delivery of air to fuel inlet pipe 23 through tangential air inlet pipe 27 is of no particular value at relatively high fuel delivery rates there may be provided means, such as a solenoid valve 31., to terminate the delivery of air through line 28 when the delivery of fuel through line 32, as determined by flow meter 33, exceeds a predetermined value.

Another problem connected with the operation of combustors according to the foregoing description at fuel delivery rates substantially below maximum is that of maintaining a locus of flame adjacent the inlet end of the combustor that is sufficiently intense to be detected by flame monitoring means such as means comprising flame rod R. Where the flame is of only marginal intensity the flame monitoring means may, from time to time, not be able to detect the presence of flame and may, unnecessarily, cause the oven, air heater, or the like with which the combustor is associated to be shut off. It has been found that tangential admission of air to fuel nozzle 23 through tangential inlet pipe 27 is of no particular benefit in eliminating such nuisance shutdowns.

It has been found, however, that the stabilizing flame at the inlet end 14 of combustion chamber 13 can be substantially intensified, thereby to avoid nuisance shutdowns of flame-monitored combustors, by introducing an annular, spinning, partially aerated stream of fuel into combustion chamber 13 at a locus very closely adjacent backplate 16. This is accomplished in the illustrated embodiment by providing means comprising a pipe 34 surrounding a portion of fuel inlet pipe 23 and forming, therewith, an annular chamber 35 which opens into combustion chamber 13. Pipe 34 is provided with a tangentially disposed inlet 36 whose axis is disposed in a plane substantially perpendicular to the axis of combustion chamber 13 and which is so oriented as to cause the partially aerated fuel which is delivered therethrough to rotate in the same direction as the air delivered through pipe(s) 24. A partially aerated stream of fuel is delivered to inlet 36 by means of a line 37 from mixing means such as a T connection 38 which receives fuel from a line 39 and air from a branch line 41 from line 23.

In the operation of a combustor comprising such a feature it is desirable that the mass rate of flow of both fuel and air to chamber 35 not be greater than necessary to adequately intensify the stabilizing flame. By way of example only, it has been found, in connection with a combustor fired with natural gas, that the delivery of fuel through line 39 to mixing T 38 at a rate of the order of 2% of the maximum rate of fuel delivery through fuel pipe 23, when coupled with the delivery of air through line 41 to mixing T 38 at a volumetric rate equal from 2 to 3 times the rate of fuel flow to mixing T 38, will result in substantially intensified stabilizing flame conditions as noted by the substantially reduced number of nuisance shutdowns of flame-monitored combustors.

The best mode known to me to carry out this invention has been described above in terms sufiiciently full, clear, concise, and exact as to enable any person skilled in the art to make and use the same. It is to be understood, however, that it is contemplated that other modes of practicing the invention can be made by a skilled artisan without departing from the scope of the invention which is defined only by the appended claims.

I claim:

1. In combustion apparatus comprising, in combination: wall means forming a combustion chamber, said wall means comprising a generally cylindrical wall having a corresponding inside and outside surface and the ends of which cylindrical wall define an inlet and exit end of the combustion chamber, and a backplate aifixed to the cylindrical wall at the inlet end thereof; a fuel delivery pipe having an inlet and exit end and a corresponding inside and outside surface and extending through the backplate into said combustion chamber substantially along the axis of the cylindrical wall and terminating at a point intermediate said inlet and exit ends of said combustion chamber for introducing a stream of fuel sub stantially axial to said combustion chamber; and primary air inlet means connected to the outside surface of said cylindrical wall adjacent said backplate, substantially tangential to the inside surface of said cylindrical Wall, and in a plane disposed substantially perpendicular to the axis of the chamber for introducing a first spinning stream of air comprising a portion of the combustion air requirements of the fuel stream; the improvement comprising secondary air inlet means connected to the outside surface of said fuel delivery pipe, intermediate its inlet and exit ends, substantially tangential to the inside surface of the fuel delivery pipe in a direction for introducing to said fuel delivery pipe and hence to said combustion chamber a second stream of air circumposing the fuel stream and spinning in the same direction as the first stream of air.

2. Apparatus according to claim 1 wherein the generally cylindrical wall of said combustion chamber diverges outwardly from a point intermediate its inlet and exit ends to substantially the exit end thereof.

3. Apparatus according to claim 2 wherein said cylindrical wall means has a plurality of ports at least in the diverging portion thereof and further comprising means for delivering a plurality of radial streams of air to the combustion chamber through the ports to sustain combustion of the fuel stream.

4. In combustion apparatus comprising, in combination: wall means forming a combustion chamber, said wall means comprising a generally cylindrical wall having a corresponding inside and outside surface and the ends of which cylindrical wall define an inlet and exit end of the combustion chamber, and a backplate aflixed to the cylindrical wall at the inlet end thereof; a fuel delivery pipe having an inlet and exit end extending through the backplate into said combustion chamber substantially along the axis of the cylindrical wall and terminating at a point intermediate said inlet and exit ends of said combustion chamber for introducing a stream of fuel substantially axial to said combustion chamber; and primary air inlet means connected to the outside surface of said cylindrical Wall adjacent said backplate, substantially tangential to the inside surface of said cylindrical Wall, and in a plane disposed substantially perpendicular to the axis of the chamber for introducing a spinning stream of primary air comprising a portion of the combustion air requirements of the fuel stream; the improvement comprising means for introducing an annular stream of fuel mixed with a portion of its combustion air through said back plate and into said. chamber adjacent the inlet end thereof, said stream of fuel and air circumposing said fuel stream and spinning in the same direction as said primary air.

5. Apparatus according to claim 4 wherein the generally cylindrical wall of said combustion chamber diverges outwardly from a point intermediate its inlet and exit ends to substantially the exit end thereof.

6. In combustion apparatus comprising, in combination: first wall means forming a combustion chamber, said wall means comprising a generally cylindrical wall having a corresponding inside and outside surface and the ends of which cylindrical wall define an inlet and exit end of the combustion chamber, and a backplate affixed to the cylindrical wall at the inlet end thereof; a fuel delivery pipe having an inlet and exit end extending through the backplate into said combustion chamber substantially along the axis of the cylindrical wall and terminating at a point intermediate said inlet and exit ends of said combustion chamber for introducing a stream of primary fuel substantially axial to said combustion chamber; and primary air inlet means connected to the outside surface of said cylindrical wall adjacent said backplate, substantially tangential to the inside surface of said cylindrical Wall, and in a plane disposed substantially perpendicular to the axis of the chamber for introducing a spinning stream of primary air comprising a portion of the combustion air requirements of the fuel stream; the improvement comprising: second Wall means circumposing and forming with said fuel delivery pipe an annular chamber, closed at its upstream end and in open communication with said combustion chamber through said backplate; and means for introducing secondary fuel and introducing secondary air, sufficient to partially aerate said secondary fuel, to said annular chamber in a manner to deliver a stream of mixed secondary fuel and secondary air into said chamber ad jacent the inlet end thereof and said stream of secondary air and fuel circumposing said primary fuel stream and spinning in the same direction as said primary air.

7. Apparatus according to claim 6 wherein said means for introducing secondary fuel and secondary air to said annular chamber comprises a mixture pipe having an inlet and exit end wherein a mixture of secondary fuel and secondary air is admitted to inlet end of said mixture pipe, and the exit end of said mixture pipe is in communication with said annular chamber and arranged to introduce said mixture tangentially into the annular chamber in a plane perpendicular to the axis of said annular chamber.

8. Apparatus according to claim 7 wherein said second wall means comprises conduit means coaxial with and circumposing said fuel delivery pipe in a manner to form said annular chamber therebetween, said conduit means extending from a point upstream of said backplate and terminating in communication with the inlet of said com bustion chamber, and wherein the exit end of mixture pipe is aflixed to said conduit means in a plane perpendicular to the axis of said annular chamber tangent to said conduit means for introducing said mixture tangentially into said annular chamber.

9. Apparatus according to claim 6 wherein the generally cylindrical wall of said combustion chamber diverges outwardly from a point intermediate its inlet and exit ends to substantially the exit end thereof; and wherein said cylindrical wall means has a plurality of ports at least in the diverging portion thereof, and further comprising means for delivering a plurality of radial streams of air to the combustion chamber through the ports to sustain combustion of the fuel stream.

References Cited by the Examiner UNITED STATES PATENTS 1,073,463 9/1913 Banes -22 X 1,763,047 6/1930 Hepburn 158-106 X 1,870,026 8/ 1932 Rosencrants 110-22 2,051,099 8/1936 Munford 126-91 2,952,307 9/1960 Schramm et al 158-7 3,002,819 10/1961 Brace et a1. 158-76 X FOREIGN PATENTS 1,079,506 5/1954 France.

FREDERICK L. MATTESON, JR., Primary Examiner.

MEYER PERLIN, JAMES W. WESTHAVER,

Examiners.

E. G. FAVORS, Assistant Examiner.

Claims

1. IN COMBUSTION APPARATUS COMPRISING, IN COMBINATION: WALL MEANS FORMING A COMBUSTION CHAMBER, SAID WALL MEANS COMPRISING A GENERALLY CYLINDRICAL WALL HAVING A CORRESPONDING INSIDE AND OUTSIDE SURFACE AND THE ENDS OF WHICH CYLINDRICAL WALL DEFINE AN INLET AND EXIT END OF THE COMBUSTION CHAMBER, AND A BACKPLATE AFFIXED TO THE CYLINDRICAL WALL AT THE INLET END THEREOF; A FUEL DELIVERY PIPE HAVING AN INLET AND EXIT END AND A CORRESPONDING INSIDE AND OUTSIDE SURFACE AND EXTENDING THROUGH THE BACKPLATE INTO SAID COMBUSTION CHAMBER SUBSTANTIALLY ALONG THE AXIS OF THE CYLINDRICAL WALL AND TERMINATING AT A POINT INTERMEDIATE SAID INLET AND EXIT ENDS OF SAID COMBUSTION CHAMBER FOR INTRODUCING A STREAM OF FUEL SUBSTANTIALLY AXIAL TO SAID COMBUSTION CHAMBER; AND PRIMARY AIR INLET MEANS CONNECTED TO THE OUTSIDE SURFACE OF SAID CYLINDRICAL WALL ADJACENT SAID BACKPLATE, SUBSTANTIALLY ANGENTIAL TO THE INSIDE SURFACE OF SAID CYLINDRICAL WALL, AND IN A PLANE DISPOSED SUBSTANTIALLY PERPENDICULAR TO THE AXIS OF THE CHAMBER FOR INTRODUCING A FIRST SPINNING STREAM OF AIR COMPRISING A PORTION OF THE COMBUSTION AIR REQUIREMENTS OF THE FUEL STREAM; THE IMPROVEMENT COMPRISING SECONDARY AIR INLET MEANS CONNECTED TO THE OUTSIDE SURFACE OF SAID FUEL DELIVERY PIPE, INTERMEDIATE ITS INLET AND EXIT ENDS, SUBSTANTIALLY TANGENTIAL TO THE INSIDE SURFACE OF THE FUEL DELIVERY PIPE IN A DIRECTION FOR INTRODUCING TO SAID FUEL DELIVERY PIPE AND HENCE TO SAID COMBUSTION CHAMBER AND A SECOND STREAM OF AIR CIRCUMPOSING THE FUEL STREAM AND SPINNING IN THE SAME DIRECTION AS THE FIRST STREAM OF AIR.