US3162239A

US3162239A - Flame arrestor burner

Info

Publication number: US3162239A
Application number: US105339A
Authority: US
Inventors: Jeffery D Irons
Original assignee: Union Tank Car Co
Current assignee: Union Tank Car Co
Priority date: 1961-04-25
Filing date: 1961-04-25
Publication date: 1964-12-22
Anticipated expiration: 1981-12-22

Description

Dec. 22, 1964 J. D. IRONS 3,162,239

FLAME ARRESTOR BURNER Filed April 25, 1961 3 Sheets-Sheet 1 I08 INVENTOR. 58 22 4 J D. Irons 80 2Q BY KM Dec. 22, 1964 J. D. IRONS 3,162,239

FLAME ARRESTOR BURNER Filed April 25, 1961 3 Sheets-Sheet 2 L8? 0} o o O Q 0 0 o IN VEN TOR.

J D. Irons Dec. 22, 1964 J. D. IRONS FLAME ARRESTOR BURNER 3 Sheets-Sheet 3 Filed April 25, 1961 8 FIG 7 FIG INVENTOR J D Irons United States Patent 3,162,23 Patented Dec. 22, 1964 3,162,239 FLAME ARRESTGR BURNER Jefiery D. Irons, Tulsa, Gkltn, assignor to Union Tank Car Company, Chicago, EL, a corporation of New Jersey Filed Apr. 25. 1951, Ser. No. 105,339 2 Claims. (Cl. 158-112) The present invention relates to flame arrestor burners and, more particularly, to burners that must be used in areas where combustible external atmospheres might exist.

It is an object of the present invention to provide a new and improved flame arrestor burner.

It is another object of the present invention to provide a flame arrestor burner which eliminates the possibility of igniting a combustible external atmosphere because of its operation.

It is a further object of the present invention to provide a flame arrestor burner that does not rely upon natural draft to draw air into it.

It is yet a further object of the present invention to provide a flame arrestor burner which is designed to contain an explosion or the like and which prevents the explosion from reaching an external combustible atmosphere.

It is still another object of the present invention to provide a new and improved flame arrestor burner having separate and sealed off mixing and combustion chambers.

It is an object, in accordance with the previous object, to isolate any possible explosion to the combustion chamber, thereby preventing combustion of fuel in a mixing chamber.

It is still a further object to provide a flame arrestor burner having separate mixing chambers for main and pilot burner assemblies, respectively.

The above and other objects are realized in accordance with the present invention by providing a new and improved flame arrestor burner. The burner is particularly adapted to be used with fired equipment, such as boilers, indirect heaters, tubestill heaters and the like. Since the above fired equipment is used in areas in which combustible atmospheres occur, it is imperative that any explosion, flashback, or the like be contained by and confined within the flame arrestor burner; thus any explosion or the like is not exposed to the combustible atmospheres with its attendant ignition. To this end, the burner is so constructed that its mixing chamber, wherein the fuel and air is mixed for passage into the burner nozzle, and the combustion chamber, wherein the mixture of fuel and air are burned, are separated and isolated from one another. Accordingly, in the event of an explosion, flashback, or the like in the combustion chamber, the combustible activity is restricted to the combustion chamber and is prevented from contacting the mixing chamber and, particularly, reaching the fuel. In another aspect of the invention, separate mixing chambers are provided for the main and pilot burner assemblies.

The invention, both as to its organization and method of operation, taken with further objects and advantages thereof, will best be undestood by reference to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a prespective view of a fired equipment, shown with one embodiment of a burner embodying the features of the present invention;

FIG. 2 is an enlarged end elevational view of the burner of FIG. 1;

FIG. 3 is a sectional view, shown partially in elevation, taken along line 33 of FIG. 2;

FIG. 4 is a fragmentary sectional view of a modified form of the burner of FIG. 1;

FIG. 5 is an end elevational view of another embodiment of the burner of FIG. 1;

FIG. 6 is a sectional view, shown partially in elevation, taken along line 66 of FIG. 5;

FIG. 7 is an end elevational of yet another embodiment of the burner of FIG. 1; and

FIG. 8 is a sectional view, shown partially in elevation, taken along line 88 of FIG. 7.

Referring now to the drawings, and in particular to FIG. 1, there is illustrated a piece of fired equipment, for example, an indirect heater, identified generally by reference numeral 10. The indirect heater 10 is used either in the shop or in the field and performs the function of heating oil well product or the like. In this connection, the indirect heater It) includes a piping arrangement through which the oil well product passes and is heated. The piping arrangement is immersed in water which fills the indirect heater 10 and is heated by a fire tube which, in a preferred embodiment, is of the reversible type. The fire tube is in turn, heated by a flame arrestor burner embodying the features of the present invention and generally identified by reference numeral 12. As shown, the burner is suitably mounted on an end wall 14 of the indirect heater 10 and is oriented in a generally vertical position. A suitable stack 15 is also supported from the end wall 14 of the indirect heater in and serves to transport flue gases between the heater 1t) and the atmosphere.

The burner 12 embodies both a main burner assembly 16 which is periodically operable and a pilot burner assembly 18 which is continuously operable to assure ignition of the main burner assembly when operated. In this connection, the pilot burner assembly is continuously supplied with fuel from a suitable fuel storage tank 20 via a conduit 22. On the other hand, the main burner assembly 16 is supplied with suitable fuel from the storage tank 20 via conduit 24 under the control of a thermostatic system 26. The system 26 includes a suitable heat responsive bimetallic element associated with the heater it) and a control valve located in the conduit 24. Hence, in response to the call for heat by the thermostatic system 26, fuel is supplied to the main burner assembly to and, since the pilot assembly 18 is continuously burning, ignition of the fuel in the main burner assembly 16 is assured. On the other hand, when the desired temperature is achieved, the thermostat system 26 operates to cut off the supply of fuel to the main burner assembly 16, with the result that the flame pro duced by the main burner assembly 16 is extinguished.

Referring now more particularly to the constructional details of the burner 12, one embodiment of the burner 12 is disclosed in FIGS. 2 and 3, a second embodiment of the burner 12 is disclosed in FIGS. 5 and 6, and a third embodiment of the burner 12 is disclosed in FIGS. 7 and 8. Referring first to the FIGS. 2 and 3 embodiment, the burner comprises a casing 30 which is of generally L-shaped configuration. The burner 12 is vertically oriented and is fixedly secured to the end wall 14 of the indirect heater It so that a portion of the casing 30 extends into an opening 33 defined in the wall 14 of the indirect heater 16. Specifically, there is provided a radially extending flange 32 which abuts against the wall and is suitably secured to the end wall 14 by welding or the like. It should be appreciated that the flange 32 comprises the only means for attaching the burner 12 to the indirect heater It).

More specifically, the casing 3d comprises a horizontal tubular section 34 and. a vertical tubular section 36 which are right angularly related to one another. 7

sections

34 and 36 enclose and support the main burner assembly 16 and the pilot burner assembly 1-8 in a manner described hereinbelow. The right end, of the tubular section 34, asshown in FIG. 3, extends. into the fire tube The tubular provided in the indirect heater 1%, as does therespective ,7

burner nozzle of the main burner and

pilotburner asseme blies

16 and 18. On the other hand, the section 36 extends upwardly from the left end of the section 34 and is located adjacent to and parallel with the end wall 14 of the indirect heater 1d. The section 36, similar to the section 34, contains portions of the main burner and

pilot burner assemblies

16 and 18 respectively and, in this connection, includes a partition 33 for defining a main mixing chamber 40 and a pilot mixing chamber 41. The upper end of the section 36, as well as the main mixing chamber41, comprises a main burner primary air inlet 42 within which is located a main burner flame trrestor'cell 43 The function of theflame arrestor cell 43, as is well known, is to prevent an explosion, flash back, flame or combustible material from passing out of the upper end ofthe casing 36, yet at the same time to permit passage of air into the mixing chamber t-ilso that an appropriate amount of air can be mixed with theburner fuel to sup-' port its combustion when burned by the main burner assembly 16. Hence, an air-fuel mixer or injector 44is appropriately supported within the mixing chamber in V such a manner that the passage of fuel through themixer 44 causes air to be drawn into the mixing chamber 4% the section'36 tocoact with the'upper part of the flamearrestor cell 43. The clamping device 45 is readily ad justable to permit removal of the flame'arrestor cell 43 for the purpose of replacement, cleaning, or the like. The flame arrestor cell 43 is protected from inclement weather or the like by a shield 46 which is appropriately supported by the clamping device 45. As shown, the cell 46includes a depending skirt 47 which has a larger diameter made of a suitable air impervious material. By this arrangement, any interaction between the continuously operating pilot burner assembly 18 and the intermittently operating main burner assembly 16 is substantially eliminated. a

As best shown in FIG. 3, the section 34 defines a socalled combustion chamber 54 which is in communication with the fire tube or the like in the indirect heater 1%. As is wellknown, the mixture of air and fuel, leaving the main and

pilot burner assemblies

16 and 18, is more efficiently consumed when secondary air is supplied to the burning mixture. To this end, a secondary air inlet 56 is provided in the left endof the section 34. Similar to theabove described

air inlets

42 and 43, a flame arrestor cell 58 is suitablyrpositioned in the air inlet 56. Specifically, the cell 58 includes a casing 69 which is suitably secured by fasteners 62 or the like .to a radial flange 64 appropriately secured by welding or the liketo the end of the tubular section 34. The flame arrestor cell 58 is internally constructed similar to' flame arrestor cells 43 and and performs the identical functions of preventing In accordance with a feature of the present invention,

themixing chambers

40 and 41 are isolated from the combustion chamber 54 for the purpose of further pre-.

venting any interaction between the burning of the fuel j mixture in the combustion chamber 54 and the mixing processes existing in the mixing chamber 40 and'41. In

this connection, the mixing chamber 41, and hence the mixing chamber 40, is sealed ofl from the combustion chamber 54 by a partition 66. In the embodiment'illustrated, the partition 66 comprises a portion of the wallof the tubular section 34, which partition .66 is appropriately secured by welding or the like to the lower end tube sec- "tion 36. As described in greater detail hereinafter, the

" between the mixing chamber 41 and combustion champartition 66 is suitably apertured to accommodate por tions of'both the main burner assembly 16 of the pilot burner assembly 18. In order to maintaina gas-tight seal ber. 54, the 'portionsof the main burner and pilot burner assemblies are suitably, sealed to the partition 66.

Referring now more specifically to the main burner assembly 16, as shown in FIG. 3, fuel is supplied via conduit 24 toa coupling 68 which is located in an aperture than the section 36 and whichextends below the upper end of the section 36. Hence, primary air entering the mixing chamber 40 passes upwardly between the skirt 47 and the section 36 and then downwardly through the flame arrestor cell 43 into the mixing chamber 40.

In a similar manner, the pilot burner mixing chamber. 41 is provided with a primary air inlet 48 which is defined by a suitable aperture provided in the wall section 36. A collar '49 is suitably secured by welding or the like adjac'ent the primary air inlet 48 and serves to support a flame arrestor cell 50 in registry with the primary air inlet. 48. Similar to the flame arrestor cell 43, a clamping device 51 is supported from the collar 49 to coact with the outer end of flame arrestor cell 50. The .fl'ame arrestor cell 50 functions identically to the flame arrestor cell 43 and, thus, functions to' prevent an explosion, flash back, or the like from passing out of the primary air' 70 defined in a section 36. In order to obtain a gas-tight I and flame-tight sealbetween thecoupling68 and section 36, the coupling 68 is appropriatelysecured. by-welding inlet 48, while at the same time permitting passage of primary air into the mixing chamber 41.

As in the case of the main burner assembly 16, an airfuel mixer or injector 52 0f the pilot burner assen'qblylS I and suitablymixed with burner fuel.

It will beappreciated that the air-fuel. mixers 44 and v 52 function in exactly the same manner but each is provided with primary air from a separate source. Moreover, the mixing

chambers

40 and 41 are separated and sealed off from one another by the partition 38 which is or the like. A conduitf72 is connected'to'the coupling 68 and extends inwardly of the mixing chamber 40. into engagement with the air-fuel mixer 44. The conduit 72 fixedly securesthe upper end of the air-fuel mixer 44 within the section 36 and, hence, within the mixing chamber 40. The air-fuel mixer 44 comprises a conventional venturi tube, which by virtue of the passage of fuel through it produces a suction-which acts to draw air into the mixer 44throu'gh suitable slots 74. Particularly, the

suction causes air. to be drawn through theprimary air inlet 42, into the mixing chamber 40 and into the mixer 44 through the slots 74 to be mixed with the fuel passing through the mixer 44. The resultant mixture passes out of the mixer 44 and into a conduit 76 which is suitably supported from the'section 34 by a bracket 81. The conduit 76 includes an angled conduit 80 which is suitably secured to the partition 66 and, thereby, supports the lower end of the mixer 44 and also includes a straight conduit 77 that is threaded on its right 'end to accommodate a burner nozzle or tip 82; By this arrangement,

the mixture of fuel and air passes out of the burner nozzle 82 and is ignited, in a manner Well known, by the includes an air-fuel mixer 52 which is suitably connected to the supply conduit 22 by a coupling 84 and associated conduit 86. The coupling 84, similar to the coupling 68, is suitably secured to the wall of the section 36 so as to provide both a gas-tight seal and a flame-tight seal. Similar to the mixer 44, the mixer 52 produces a suction for drawing air through the primary air inlet 43, into the mixing chamber 41, and into the mixer 52 through suitable openings 88 for mixing with the burner fuel. The resultant air-fuel mixture leaves the mixer 52 and passes into a conduit 90 which is threadedly connected at its upper end to the mixer 52 and at its lower end to a coupling 92 suitably sealed in the partition 65. By the above arrangement, the mixer 52 is supported in the tubular section 36, and hence the mixing chamber 41, by the conduits 86 and 9t) and their associated

couplings

84 and 92. Additional conduit means comprising conduit 94, coupling 96, and conduit d8 interconnect the coupling 92 with a pilot burner nozzle 10%; specifically, the conduit 5 8 is supported from the section 34 by the same bracket 81 that supports the conduit 76 and is threaded at its right end to accommodate the pilot nozzle 1%. As seen in FIG. 3, the length of the conduit 93 is such that the pilot nozzle 100 is located immediately adjacent to the main nozzle 82, with the result that whenever the air-fuel mixture passes out of the burner nozzle 82 it is readily ignited by the flame of the pilot nozzle 10%.

It will be appreciated that in normal operation of the burner 12 fuel is continuously supplied to the pilot burner assembly 18 whereby the air-fuel mixture is continuously burned to produce a so-called pilot flame at the pilot nozzle 1%. Accordingly, when the main burner assembly 16 is supplied with fuel, in response to a call for heat by the thermostatic system 26, the pilot nozzle Nil positively assumes that the air-fuel mixture flowing out of the main nozzle 82 is ignited.

It, for any reason, an explosion, flash back, or the like occurs in the fire tube of the indirect heater It) or in the combustion chamber 54 of the burner 12, it is entirely contained in the combustion chamber 54. Specifically, the flash back, explosion, or the like is prevented from contacting the atmosphere (which might be combustible) by the flame arrestor cell 53 and, further, is prevented from contacting the fuel passing through either of the

mixers

44 or 52 by the partition 66.

Furthermore, it should be appreciated that improved performance of the burner 12 is achieved by separating and sealing off the main burner mixing chamber 49 and the pilot burner mixing chamber 41. Specifically, the pilot burner assembly 18 does not become starved for air when fuel is not supplied to the main burner as is the case when the pilot burner assembly is located in the combustion chamber. Hence, it will be appreciated that a burner built in accordance with the present invention is extremely safe in operation and is capable of extremely dependable and reliable performance.

A modified form of the first embodiment is illustrated in FIG. 4. The modified embodiment, with one exception, is identical in all respects to the embodiment disclosed in FIGS. 2 and 3 and, in this connection, similar reference numerals are used to identify similar components. The one exception or difference resides in the use of a secondary air shield 102 in the combustion chamber 54. More specifically, the secondary air shield 162 is cylindrical in configuration and extends throughout the entire length of the combustion chamber from a point adjacent to the secondary air inlet 56 to the end of the burner nozzle 82. As shown, the secondary air shield 1tl2 includes a suitable aperture 104 for accommodating the angled conduit 8% and is concentrically related to the conduit 77 by suitable supporting members lll and 1&8. By the use of the shield 102, the secondary air enters the flame arrestor cell 58 and passes directly into the secondary air shield 192. Hence, secondary air is conducted directly to the end of the main nozzle 82, thereby assuring that air is available to support the combustion of the air-fuel mixture leaving the main nozzle 82. In addition, since the end of the shield 102 is disposed between the main nozzle 82 and the pilot burner nozzle 1%, secondary air is conducted to the main nozzle 82 exclusively, i.e. little or no secondary air is conducted to the pilot burner nozzle 1%.

Considering now the embodiment illustrated in FIGS. 5 and 6, it is generally similar in construction and operation to the above described embodiment shown in FIGS. 2 and 3, with the exception that the pilot burner assembly 118 is located entirely within the mixing chamber 54. The FIGS. 5 and 6 embodiment is identical in many respects to the FIGS. 2 and 3 embodiment and, to avoid unnecessary duplication of description, the same reference numerals are used on components corresponding to those embodied in the FIGS. 2 and 3 embodiment. Briefly, the H65. 5 and 6 embodiment includes the casing 30, primary air flame arrestor cell 43, main burner assembly 16, and flame arrester cell 53 associated with the com bustion chamber 54. In contrast to the FIGS. 2 and 3 embodiment, the air inlet 56 provides primary air to the pilot burner assembly 118 and also secondary air to both the main and pilot burner assemblies 116 and 118. In this connection, the pilot burner assembly 118 includes a coupling 129 in communication with the pilot conduit 22 and located in an aperture 122 defined in the wall of the section 34, the coupling 12% being suitably secured by Welding or the like to the section 34 so as to provide a gastight and flame-tight seal. The assembly 113 includes a conduit 124 that interconnects the coupling 12d with an air-fuel mixer 152. The air-fuel mixer 152 is connected to the main pilot burner nozzle 1% via a conduit 126 which is suitably supported from the casing 34 by the support member 81.

The partition 66 can have only a single opening to ac commodate the angled conduit d d of the main burner assembly 16 or, alternatively, the partition 66 can have two openings to accommodate both the main and pilot burner assemblies shown in FIGS. 2 and 3. In the latter case, a suitable plug 12% can be appropriately sealed in the opening provided for the pilot burner assembly 18.

The function and performance of the FIGS. 5 and 6 embodiment is generally similar to that of the FIGS. 2 and 3 embodiment and in the interest of avoiding unnecessary duplication, an extended discussion is omitted.

The P163. 7 and 8 embodiment is generally similar to the H68. 2 and 3 embodiment in the sense that separate mixing chambers are provided for the main and pilot burner assemblies, whereas the FIGS. 7 and 8 embodiment is similar to the FIGS. 5 and 6 embodiment in the sense that onry the mixer portion of the main burner as sembly 16 is disposed within the vertical section 36. In the interest of the description, the same reference numerals are used on components corresponding to those embodied in the FIGS. 2 and 3 embodiment.

In contrast to both the FIGS. 2 and 3 embodiment and the FlGS. 5 and 6 embodiment, there is additionally provided a casing member 21.6) which accommodates the pilot burner assembly 213. The casing member 210 is inclined to the vertical and has its upper end disposed within an opening 214 defined in the lower portion of the section 3 1-, such that the casing member 21% depends downwardly at an angle to the section 34. As shown, the upper end of the casing member 216 is curved and suitably apertured to accommodate, in sealed relation, a burner nozzle 2% of the pilot burner assembly 218, whereby the mixing chamber 216 defined by the casing member 2th is entirely sealed off from the combustion chamber 54'.

At the lower end of the casing member 216 is provided a primary air inlet 212 within which is disposed a flame arrestor cell 220 of the type described above. The supply conduit 22 is attached to a coupling 222 disposed in an opening 224 defined in the wall of the casing member 210. As above, the coupling member 222 is secured by Welding or the like to the casing member 213 so as to provide both a gas-tight and flame-tight seal. The .coupling 222 is connected to a mixer 252 by a coupling 226 1 of the same type as embodied in the FIGS. 5 and 6 embodiment. The mixer 252 is connectedito the burner nozzle 200 by a conduit 228 which is of substantially shorter combustible external environment; which burner comprises a tubular housing that defines a first mixing chamber, a second mixing chamber sealed from said first mixlength than the conduit 126 in the FIGS. 5 and 6 embodi-- 1 By this arrangement, a separate mixing chamber 216 is provided for the pilot burner assembly 213. Hence, primary air is separately supplied to the pilot burner assembly ing chamber anda combustion chamber sealed from said first and second mixing chambers; a venturi-type main burner assembly including an 'air-fuelmixer disposed within said first mixingchamber, a main burner outlet disposed Within said combustion chamber, and mixture con- 218 whereas secondary air is provided to the pilot burner assembly through the flame arrestor cell 58 in the same all such modifications and improvements'as fall within'the' true spirit and scope of the invention. a

' What is desiredto be claimed and secured by Letters".

Patent of theUnited States is;

1. A flame arrestor burner that is safely operable in a combustible external environment; which burner comprises a tubular housing thatdefines a first mixing chamher, a second mixing chamber, sealed, from said first mixing chamber and a combustion chamber sealed from said first and second mixing chambers; a venturi-type main burner assembly including an air-fuel mixer disposed within said first mixing chamber anda main burner outlet disposed within said combustion chamber and conin said combustion chamber adjacent said main burner 3 outlet and connected to said pilot burner mixer so as to receive the output therefrom; flame arresting air inlet means provided in said first mixing chamber for, supplying primary air to said air-fuel mixer of said main burner assembly; 'fiame arresting air inlet means provided in said second mixing chamber for supplying primary air veying means connecting said main burner mixer to said main burner outlet, said mixture conveying means of said main burner assembly extending from said first mixing chamber to said combustion chamber through the chamber defining walls of said housing and in sealed relation thereto; a venturi-type pilot burnerassembly including an air-fuel mixer disposed within said second mixing chamher, a pilot burner outlet disposed within said combustion chamber adjacent said main burner outlet and mixture conveying means-connecting said pilot burner mixer to said pilot burner outlet, said mixture conveying means of said pilot burner assembly extending from said second mixing chamber to said combustion chamber. through the chamber defining wall of said housing and in sealed relation thereto; flame arresting air inlet means provided in said first mixing chamber for supplying primary air to said air-'fuel mixer of' said main burner assembly; fiame arresting air inlet means provided in said second mixing chamber for supplying primary air to said air-fuel mixer of saidpil'ot burner assembly; and flame arresting air inlet means provided in said combustion chamber for supplying secondary air to said -main burner outlet and said pilot burner outlet.

7 References Cited in the file of this patent UNITED STATES PATENTS.

1,295,086 Eddison Feb; 18, 1919 e 1,329,052 Lukasz'ewski Jan. 27, 1920 1,372,713 Maxwell Mar. 29, 1921 1,841,463 Barber et al Jan. 19, 1932 2,241,583 Campbell May 13,1941 2,362,972 Brownback Nov. 21, 1944 2,445,059 Garretson July 13, 1948 2,531,139 Lily et al. Nov. 21,' 1950 2,625,992 Beck Jan. 20, 1953 2,870,829 Williams Jan. 27, 1959 2,941,525 Harshfield June 21, 1960 2,997,040 Bold Aug. 22, 1961 3,079,242 1963 Glasgow Feb. 26,

Claims

1. A FLAME ARRESTOR BURNER THAT IS SAFELY OPERABLE IN A COMBUSTIBLE EXTERNAL ENVIRONMENT; WHICH BURNER COMPRISES A TUBULAR HOUSING THAT DEFINES A FIRST MIXING CHAMBER, A SECOND MIXING CHAMBER SEALED FROM SAID FIRST MIXING CHAMBER AND A COMBUSTION CHAMBER SEALED FROM SAID FIRST AND SECOND MIXING CHAMBERS; A VENTURI-TYPE MAIN BURNER ASSEMBLY INCLUDING AN AIR-FUEL MIXER DISPOSED WITHIN SAID FIRST MIXING CHAMBER AND A MAIN BURNER OUTLET DISPOSED WITHIN SAID COMBUSTION CHAMBER AND CONNECTED TO SAID MAIN BURNER MIXER SO AS TO RECEIVE THE OUTPUT THEREFROM; A VENTURI-TYPE PILOT BURNER ASSEMBLY INCLUDING AN AIR-FUEL MIXER DISPOSED WITHIN SAID SECOND MIXING CHAMBER AND A PILOT BURNER OUTLET DISPOSED WITHIN SAID COMBUSTION CHAMBER ADJACENT SAID MAIN BURNER OUTLET AND CONNECTED TO SAID PILOT BURNER MIXER SO AS TO RECEIVE THE OUTPUT THEREFROM; FLAME ARRESTING AIR INLET MEANS PROVIDED IN SAID FIRST MIXING CHAMBER FOR SUPPLYING PRIMARY AIR TO SAID AIR-FUEL MIXER OF SAID MAIN BURNER ASSEMBLY; FLAME ARRESTING AIR INLET MEANS PROVIDED IN SAID SECOND MIXING CHAMBER FOR SUPPLYING PRIMARY AIR TO SAID AIR-FUEL MIXER OF SAID PILOT BURNER ASSEMBLY; AND FLAME ARRESTING AIR INLET MEANS PROVIDED IN SAID COMBUSTION CHAMBER FOR SUPPLYING SECONDARY AIR TO SAID MAIN BURNER OUTLET AND SAID PILOT BURNER OUTLET.