US3267927A

US3267927A - Nozzle mixing burner assembly

Info

Publication number: US3267927A
Application number: US390886A
Authority: US
Inventors: Erwin E Hirschberg
Original assignee: Eclipse Fuel Engineering Co
Current assignee: Eclipse Fuel Engineering Co
Priority date: 1964-08-20
Filing date: 1964-08-20
Publication date: 1966-08-23
Anticipated expiration: 1983-08-23

Description

Aug. 23, 1966 E. E. HIRSCHBERG NOZZLE MIXING BURNER ASSEMBLY 5 Sheets-Sheet 1 Filed Aug. 20, 1964 r m M @M m 5 m m m T 5 r W 4 z 1 l t i 5. QM 1 TN Q 93 v9 1 w mw Q\ m m E l :1 i ax 1l| i w|w lwl (1H .il l l w QQ N9 M9 ww. vm

1966 E. E. HIRSCHBERG 3,267,927

NOZZLE MIXING BURNER ASSEMBLY Filed Aug. 20, 1964 3 Sheets-Sheet 2 INVENTOR. ERWI V E. H/RSCHBERG ?WMW% Aug. 23, 1966 E. HIRSCHBERG NOZZLE MIXING BURNER ASSEMBLY R w m 3 Sheets-Sheet 3 ER W/N E. H/RSCHBERG ATTORNEY I Filed Aug. 20, 1964 United States 3,267,927 NOZZLE MHXENG BURNER ASSEMBLY Erwin E. Hirschherg, Rockford, Ill., assiguor to Eclipse Fuel Engineering Co., Rockford, Ill., a corporation of Illinois Fiied Aug. 2%, 1964, Ser. No. 396,886 12 Claims. (Cl. 126-91) atent cipal among these is, the inability of such a burner to transfer the heat that is generated thereby uniformly to the wall of the tube undergoing heating in proportion to the Btu. input of the burner. This limitation is attributed to excessive turbulence axially along the tube undergoing heating, such turbulence resulting in localized overheating of the tube and consequent tube buckling or other distortion, as well as creating an objectionable noise level.

The present invention is designed to overcome the above-noted limitations that are attendant upon the construction and use of a conventional present-day radiant tube heating burner and, accordingly, the invention contemplates the provision of a relatively simple nozzlemixing burner which will establish a moderate but uniformly distributed degree of turbulence within the associated tube both in the vicinity of the burner and at points remote therefrom.

In carrying out this object, the invention contemplates the provision of a burner assembly including a novel burner casing which establishes a seat for one open end of the particular tube to be fired and through which seat the interior of the casing communicates. The air that is admitted to the casing is caused to follow a centripetal path before it is released forwardly to the adjacent end of the associated tube so that such air enters the tube with a swirling motion and continues to travel forwardly along the tube in a spiral or helical path. Disposed entirely within the tube and at a region spaced an appreciable distance forwardly from the casing is a cylindrical jet-producing gas nozzle which is supplied with raw fuel gas through an elongated gas nipple which is coaxial with the tube and passes centrally through the tube seat, as well as through the air chamber which is established within the casing. The rear end of the gas nipple terminates within the body portion of the casing and cooperates with an adjusting screw by means of which the quantity of the fuel gas that is admitted to the as nozzle 3 through the nipple may be regulated. Means are provided for supporting and accurately centering the gas nozzle within the tube undergoing firing so that a narrow annulus is provided between the nozzle and the adjacent portion of the tube wall. The gas jets which are produced by the gas nozzle are directed radially outwardly into such annulus so that they are picked up by the swirling air passing forwardly through the annulus and combustion is initiated immediately forwardly of the annulus and continues forwardly along the length of the tube undergoing firing.

The release of a swirling body of air into a submerged radiant tube at one end thereof for travel interiorly along the tube in a spiral or helical path, together with the centering of a cylindrical gas nozzle within the tube at a region that is spaced forwardly from such end, the nozzle ice being so designed as to direct a series of gas jets radially into the annulus existing between the nozzle and the adjacent wall of the tube so that the gas is entrained in the forwardly moving swirling stream of air passing through the annulus, constitutes the principal feature of the present invention. By reason of it, a wiping action takes place interiorly of the tube along the entire tube length and short circuiting of the hot gases of combustion centrally along the tube is effectively and efficiently prevented.

Other features of the present invention are based upon structural considerations relating to the design of the gas nozzle and the means whereby the nozzle is fixedly supported and centered within the radiant submerged tube undergoing firing, and also the design of the burner casing so that an effective swirling motion is imparted to the air within the casing before and at the time of release of such air to the radiant tube.

Another feature of the present invention involves the provision of a burner casing block which embodies within it the gas regulating means whereby the quantity of fuel gas that is supplied to the gas nozzle may be effectively controlled, as well as the provision of an effective seat for the adjacent end of the radiant tube to be fired.

The provision of a burner assembly of the character and possessing the features of novelty and advantages briefly outlined above being the principal object of the present invention, numerous other objects and advantages of the invention will readily suggest themselves as the nature of the invention is better understood.

In the accompanying three sheets of drawings forming a part of this specification, one illustrative embodiment of the invention is illustrated.

In these drawings:

FIG. 1 is a side elevational view of a burner assembly constructed according to the present invention, such view showing the assembly operatively applied to a horizontally disposed submerged radiant tube for internal firing thereof, a portion of the tube Wall being broken away in order more clearly to reveal the nature of the invention;

FIG. 2 is a sectional view taken on the line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken substantially on the line 33 of FIG. 2;

FIG. 4 is a sectional view taken on the line 44 of FIG. 3 is the direction indicated by the arrows;

FIG. 5 is a sectional view taken on the line 55 of FIG. 3 in the direction indicated by the arrows; and

FIG. 6 is a fragmentary sectional view, somewhat schematic in its representation, taken vertically and longitudinally through a submerged radiant tube heating installation embodying a plurality of burner assemblies embodying the present invention.

Referring now to the drawings in detail and in particular to FIGS. 1 to 3, inclusive, the burner that is illustrated therein comprises two principal parts or assemblies, namely, a combined burner body and casing 10 (herein after referred to as the casing) and a nozzle assembly 12 (hereinafter referred to as the nozzle). An elongated tubular gas nipple 14 extends between the nozzle and the casing, and when the burner assembly is in its free detached state, serves to support the nozzle from the casing and position the former an appreciable distance forwardly of the latter. The burner is designed for use primarily in connection with the internal firing of a radiant tube such as the tube 16. Accordingly, means are provided whereby one end of the tube may be supported from the casing 10 with the gas nipple 14 projecting into the adjacent end of the tube and serving to maintain the gas nozzle an appreciable distance forwardly of the casing and within the tube 16, all in a manner and for a purpose that will be made clear presently.

As best shown in FIGS. 3, 4 and 5, the casing of the burner assembly is in the form of a casting of generally tear-drop configuration in transverse cross section and includes spaced apart front and rear walls 18 and 2t) and a continuous interconnecting peripheral side wall 22 of relatively short axial extent. One end portion of the front wall 18 is provided with a thickened, forwardly extending, integral block portion 24, and the other end portion of said front wall is provided with a forwardly extending annular attachment flange 26 which threadedly receives the discharge end of an air supply conduit 28 and communicates with the interior of the casing by way of an air inlet opening 3%) in the front wall 18. The rear wall is provided with an air discharge opening 32 which is laterally offset with respect to the air inlet opening and is surrounded by an integral rearwardly extending, annular flange 34. The latter is exteriorly threaded and threadedly receives thereover an adapter ring 36 which is in the form of a seating cup and is shaped to receive the adjacent end of the tube 16. The internal diameter of the annular attachment flange Z6 and the diameter of the air discharge opening 32 are approximately equal.

The block portion 24 of the burner casing is formed with an axially extending bore 38 therethrough and in this bore there is disposed in press-fitted relationship a tubular gas adjusting insert 40. The front end of the insert has a medial bore 42 formed therein. A needle valve 44 in the form of an adjusting screw is threadedly received within a screw-threaded bushing in the front end of the bore 42 and is sealed to the rear end of the wall of the bore by a resilient O-ring 46. The latter is mounted in an annular groove in the rear central portion of the needle valve 44. The rear end region of the insert is provided with an enlarged screw-threaded counterbore 48 which threadedly receives the front end of the tubular gas nipple 14. A diametric passage 50 in the central portion of the insert 46 intersects and communicates with the rear end of the bore 42 and communicates through a centrally-disposed, axially arranged gas port 52 with the counterbore 48 which, in turn, communicates with the front end of the gas nipple 14. The conical rear end 54 of the needle valve 44 cooperates with the gas port 52 in seating relationship in the usual manner of needle valve operation. The front end of the needle valve 44 is slotted as at 56 for cooperation with a suitable turning tool, such as a screw driver or the like. The diametric passage 50 communicates with a transverse passage 58 in the block portion 24 of the casing 10 and the passage 58, in turn, communicates with a threaded gas inlet 60 which threadedly receives the discharge end of a fuel gas supply pipe 62.

The air supply conduit 28 is operatively connected to a motor-driven blower or suitable source (not shown) of air under pressure through a conventional control valve 64 (see FIG. 6), while the fuel .gas supply pipe 62 is connected to a source (not shown) of gas under pressure through a conventional gas cock 66.

Referring now to FIGS. 4 and 5, the peripheral side wall 22 of the burner casing 10 is provided with a substantially semi-circular end wall section 79 of relatively large diameter, and is also provided with a substantially semi-circular end wall section '72 of relatively small diameter. The peripheral side wall of the casing 10 is further provided with two generally tangential interconnecting

side wall sections

74 and 76. The side wall section 74 is substantially flat and merges gradually with the end wall sections and 72. The side wall section 75 also merges gradually with the

end wall sections

70 and 72, but the medial region thereof is formed with an inwardly bowed portion 78 on which there is formed an inwardly extending vane 80. This vane constitutes an extension or continuation of the curvature of the semi-circular end wall section 70 and divides the interior of the casing into a relatively large swirl chamber 82 and a relatively small air inlet chamber 84, the two chambers being in communication through a reduced throat 36. The latter exists by reason of the spacing of the distal or free end 38 of the curved vane 89 from the side wall section 74. It is essential that the cross-sectional area of the throat 86 be as great as the cross-sectional area of the air inlet opening 39, it being somewhat larger in the illustrated form of the casing 14 The curved vane is so designed that a tangent plane at the distal end of the vane extends at an angle of approximately 30 to the plane of the side wall section 74 so that air passing through the throat will be gradually restricted and its velocity increased.

The common axis of the tubular gas adjusting insert 49 and the air discharge opening 32 is offset from the longitudinal axis of the generally cylindrical swirl chamber 82, the direction of offset being such that the vane 80 extends between the air inlet opening 320 and the air discharge opening 32. The axis of the air inlet opening 30, however, is centered within the air inlet chamber 84. Thus, when the burner is in operation, the air which enters the burner casing 14 through the air inlet opening 30 passes through the throat S6 and is first forced against the end wall section 70 and then constrained to follow the curvature of this end wall section due to the involved centrifugal force. Since the axis of the flange 34 that surrounds the air discharge opening 32 is offset from the center of curvature of the end wall section 70, the involved centrifugal forces as the air swirls inwardly toward the air discharge opening in involute fashion becomes increasingly greater so that the air is discharged through the flange in a stable or even manner with an extremely high rotative velocity but with very little forward velocity other than that imparted to it by reason of the pressure of back-up air emerging from the swirl chamber 82. This escaping air is thus caused to hug the inner wall surface of the radiant tube 16 as it approaches the gas nozzle 12 with little forward velocity but with a large component of rotative or circumferential velocity.

The details of the gas nozzle 12 are best illustrated in FIG. 3, the nozzle being comprised of two principal parts, namely, a generally cup-shaped nozzle body 99 and a nozzle cap 91. The nozzle body has a circular end wall 2 which is provided therein with a threaded central opening 93. The latter receives the forward threaded end of the gas nipple 14. The nozzle body 90' also has a cylindrical side wall 94, the open rim region of which threadedly receives thereover the nozzle cap 91. The cap 91 likewise is of cup-shape design and includes an imperforate circular rear end wall 96 and a forwardly extending cylindrical side wall 97. The latter is provided with an outwardly extending annular rim flange 98. The rear region of the cylindrical side Wall 97 of the nozzle cap 91 is formed with a circular row of relatively small gas ports 99 therethrough. Three radial centering screws 100 are threadedly received through the cylindrical side wall 97 and are designed for engagement with the inside face of the radiant tube 16. Lock nuts 102 serve to secure the centering screws in their respective adjusted positions. The three centering screws are spaced equidistantly apart and are arranged in an annular series. The gas nozzle 12 defines an internal gas chamber 103 from which the ports 99 extend radially. The gas that is supplied to the chamber 193 flows radially outwards into the space between the nozzle and the adjacent portion of the tube 16.

The previously mentioned adapter ring 36 embodies a fiat annular seat 104 which is adapted to receive the extreme front end or open rim of the radiant tube 16. Said adapter ring also embodies a short cylindrical side wall 106 which forms a pilot ring for centering the tube against the annular seat 104. Three locking screws 108 extend through screw-threaded holes in the side wall and secure the tube 16 in position within the adapter ring. The three screws are spaced equidistantly apart and are arranged in an annular series.

As previously stated, in the operation of the present burner, the air leaving the swirl chamber 82 through the air discharge opening 32 enters the tube 16 and passes forwardly toward the gas nozzle 12 with a swirling motion which causes it to hug the inner surface of the tube. No combustion takes place in the front region of the tube between the adapter ring 36 and the gas nozzle 12. As the air passes forwardly through the annular space between the gas nozzle 12 and the adjacent portion of the wall of the radiant tube 16, gas is picked up from the jets issuing from the series of gas ports 99 and is constrained to follow the swirling motion of the air. At this initial region of gas entrainment, i.e., within the aforementioned annular space and a very slight distance rearwardly thereof, the mixture is incomplete and little or no combustion takes place. However, as the two gaseous constituents of combustion move forwardly and are thrown radially outwardly into contact with the inner surface of the tube 16, the mixture improves rapidly and at a region a slight distance forwards of the gas nozzle 12, substantially stoichiometric conditions obtain. Thereafter, as the mixture of gases moves rearwardly along the tube 16, there is a tendency for the same closely to hug the cylindrical wall of the tube and exert a wiping action thereon while continuing its swirling path of movement, thus yielding heat evenly to the tube wall for substantially the entire length of the tube.

The burner assembly of the present invention is adapted to be fired either horizontally or vertically since radiant tube installations having either horizontally disposed or vertically disposed tubes are common. However, a majority of installations employ vertical tubes in order to take advantage of the stack effect offered thereby. Such an installation has been fragmentarily and somewhat schematically illustrated in FIG; 6. In this view, only the top and bottom furnace walls 110 and 112 and one furnace side wall 114- have been shown. Likewise, only two adjacent radiant tubes 116 of a multiple tube installation have been illustrated. Preferably, the upper end regions of the tubes 116 are secured as by welding to the top wall 119, while the lower end regions of the tubes are retained by seals 118 in openings 120 in the furnace bottom wall 112 to allow for thermal elongation of the tubes. The tubes terminate a short distance above the top wall 110 and have associated therewith individual constant burning pilot tubes (not shown). The lower end regions of the tubes 116 project a slight distance below the furnace bottom wall so that the entire burner casings are disposed exteriorly of the furnace casing. Experience has shown that the gas nozzles 12 should be positioned a slight distance below the level of the furnace hearth on the bottom wall 112, and thus, in most instances, they will lie within the confines of the associated tube openings 120 so that combustion will take place immediately above the level of the bottom wall 112. The various gas cocks communicate through branch pipes 126 with a common gas manifold 128. Similarly, the various control valves 64 communicate through branch conduits 130 with a common air manifold 132.

Installation of the present burner assembly may be accomplished with comparative ease. The centering screws 160 are not intended to be tightened against the inner surface of the wall of the tube 16 since, in order to protect the burner mounting, it is necessary that there be freedom of relative axial movement between the gas nozzle 12 and the adjacent portion of the tube. For any given installation, the centering screws are adjusted so that the gas nozzle 12 may he slipped into the adjacent end of the tube 16 with a sliding fit yet without undue freedom of lateral shifting movement. The lock nuts 102 are then tightened and the gas nozzle and its associated gas nipple 14 are then introduced endwise into the tube until proper seating of the tube within the adapter ring 36 is attained.

The invention is not to be limited to the exact arrangemeat of parts shown in the accompanying drawings or described in this specification as various changes in the details of construction may be resorted to without departing from the spirit or scope of the invention. Therefore, only insofar as the invention has particularly been pointed out in the accompanying claims is the same to be limited.

Having thus described the invention what I claim as new and desire to secure by Letters Patent is:

1. A gas burner assembly for the interior firing of an open-ended radiant tube, said assembly comprising, in combination, a burner casing having a rear wall provided with an air discharge opening therein designed for register with one open end of the radiant tube to be fired, said casing having another wall provided with an air inlet opening therein, means constraining air introduced into the casing to flow in an involute path and become discharged from said air discharge opening in a rearward direction with a swirling motion and at a low velocity, an elongated tubular gas nipple supported at its front end by said casing and projecting coaxially through said air discharge opening, a generally cylindrical gas nozzle mounted on the rear end of said gas nipple in coaxial relationship with the nipple and providing an internal gas chamber in communication with said gas nipple, said nozzle and nipple being adapted for projection into said tube through said one rear open end thereof, means providing a series of circumferentially spaced radial ports in communication with said internal gas chamber, means for regulably supplying gas to said gas nipple, and means for supplying air to said casing through said air inlet opening.

2. The combination with a cylindrical open-ended radiant tube, of a gas burner assembly for firing said tube internally, said assembly comprising a burner casing having a rear wall provided with an air discharge opening therein in effective register with one open end of said tube, a front wall spaced from said rear wall, and a continuous side wall, said walls defining an internal air chamber, one of said walls being provided with an air inlet opening therein, means within the air chamber constraining air introduced thereinto through said air inlet opening to flow through the chamber in an involute path and become discharged from said air discharge opening in a rearward direction with a swirling motion and at a low velocity, an elongated tubular gas nipple having its front end supported in said front wall and projecting across said air chamber and centrally through said air discharge opening and into the tube in coaxial relation ship with respect thereto, a gas nozzle mounted on the rear end of said nipple, said gas nozzle being generally of cylindrical configuration and including a rear wall, a front wall and an interconnecting generally cylindrical side wall, said walls defining an internal gas chamber in communication with said gas nipple, said gas nozzle being centered within said tube in coaxial relationship, there being a series of circumferentially spaced radial jetproducing gas ports in said cylindrical side wall in communication with the annular space existing between the gas nozzle and the adjacent portion of the wall of the tube, means for regulably supplying gas to said gas nipple, and means for supplying air to said casing through said air inlet opening.

3. The combination set forth in claim 2 and including, additionally, a series of circumferentially spaced centering screws threadedly received in said side wall of the gas nozzle and engaging the wall of said tube for maintaining the gas nozzle centered within said tube.

4. The combination with a cylindrical open-ended radiant tube, of a gas burner assembly for firing said tube internally, said assembly comprising a burner casing having a rear wall, a front wall spaced from said rear wall, and a continuous side wall extending between said rear and front walls, said walls defining an internal aircirculating chamber within the casing, said rear wall being provided with an air discharge opening therethrough, a seating cup mounted on said rear wall exteriorly of the casing and surrounding said air discharge opening, one end of said tube projecting into said seating cup whereby the tube is in open communication with said air discharge opening, said front wall being formed with a forwardly extending thickened block portion, an elongated tubular gas nipple having its front end threadedly received in said thickened block portion, projecting across said air-circulating chamber and centrally through said air discharge opening and into said tube in coaxial relationship therewith, said gas nipple terminating within said tube an appreciable distance rearwardly of said one end of the tube, a generally cylindrical gas nozzle coaxial with said tube and carried at the rear end of said gas nipple, said gas nozzle defining an internal gas chamber in communication with said gas nipple, said gas nozzle being provided with a series of radial ports establishing communication between said internal gas chamber and the annular space existing between the gas nozzle and the wall of the tube, means for supplying air under pres- 7 sure to the interior of said air chamber, there being a gas passage through said thickened block portion in communication with said gas nipple, and means for supplying gas under pressure to said gas passage.

5. The combination set forth in claim 4 and including, additionally, a control valve interposed in said gas passage for regulably controlling the flow of gas therethrough.

6. In a heating installation of the character described, in combination, an open-ended radiant tube, means for directing a stream of air into one open end of said tube and for causing said stream to pass forwardly through the tube at a low velocity and with a swirling motion, an elongated tubular gas nipple projecting centrally and axially into said tube through said one open end and terminating within the tube an appreciable distance forwardly of said one open end, a hollow, generally cylindrical, gas nozzle mounted on the rear end of the gas nipple within the tube and coaxial with the latter, said gas nozzle, in combination with the surrounding wall of the tube, establishing a narrow annular space through which the swirling stream of air issuing from said one open end of the tube is adapted to pass forwardly, the interior of said gas nozzle being in open communication with the rear end of said gas nipple, said gas nozzle being provided with a series of radially disposed jetproducing ports establishing communication between the interior of the nozzle and said annular space, and means for supplying gas to said gas nozzle.

7. In a heating installation, the combination set forth in claim 6 and including, additionally, adjustable centering means for maintaining said gas nozzle centered within said tube.

8. In a heating installation, the combination set forth in claim 6 and including, additionally, a series of no less than three centering screws extending radially between said gas nozzle and the inner surface of the adjacent portion of the tube for maintaining the nozzle centered within said tube, and locking nuts for securing said screws in their adjusted positions.

9. The combination with a cylindrical open-ended radiant tube, of a gas burner assembly for firing said tube internally, said burner assembly comprising a casing having a rear wall, a front wall and a continuous side Wall, said walls defining an internal air-circulating chamber, said side wall presenting two opposed curved end wall sections which merge gradually and tangentially with interconnecting opposed side wall sections, a curved vane projecting inwardly from one side wall section and having its distal end terminating short of the other side wall section, and in combination therewith, establishing a throat, one side of said vane constituting an extension of one curved end wall section and, in combination therewith, establishing a swirl chamber, the other side of said vane, in combination with the other curved end wall section, establishing an air inlet chamber, said swirl chamber and air inlet chamber being in communication with each other through the throat, one of the burner walls being provided with an air inlet opening in communication with the air inlet chamber, the burner rear wall being provided with an air discharge opening in communication with the swirl chamber, the burner front wall being provided with a fuel inlet opening in axial alignment with the air discharge opening, means securing said tube to the casing rear wall with one open end thereof in coaxial register with said air discharge opening, an elongated tubular gas nipple having its front end sealingly connected to the fuel inlet opening, projecting across the swirl chamber, through the air discharge opening centrally thereof, and terminating within the tube an appreciable distance rearwardly of the air discharge opening, and a generally cylindrical gas nozzle on the rear end of said gas nipple, said gas nozzle being provided with a series of circumferentially spaced radially extending ports for directing jets of gas radially outwardly of the nozzle and into the annular space existing between the gas nozzle and the surrounding wall of said tube.

10. The combination set forth in claim 9 and wherein the air inlet opening is in the casing front wall and said air inlet opening is substantially coaxial with the axis of curvature of the curved end wall section which, in part, establishes the air inlet chamber.

11. The combination set forth in claim 9 and including, additionally, a series of no less than three centering screws extending radially between said gas nozzle and the adjacent portion of the surrounding wall of the radiant tube for maintaining the nozzle centered within the tube.

12. The combination set forth in claim 9 and in which a plane of tangency at the distal end of the curved vane extends at a small angle to the other side wall section of the side wall of the burner casing.

References Cited by the Examiner UNITED STATES PATENTS 5/1960 Ferguson 239425 X 6/1964 Keough 12691 X

Claims

1. A GAS BURNER ASSEMBLY FOR THE INTERIOR FIRING OF AN OPEN-ENDED RADIANT TUBE, SAID ASSEMBLY COMPRISING, IN COMBINATION, A BURNER CASING HAVING A REAR WALL PROVIDED WITH AN AIR DISCHARGE OPENING THEREIN DESIGNED FOR REGISTER WITH ONE OPEN END OF THE RADIANT TUBE TO BE FIRED, SAID CASING HAVING ANOTHER WALL PROVIDED WITH AN AIR INLET OPENING THEREIN, MEANS CONSTRAINING AIR INTRODUCED INTO THE CASING THE FLOW IN AN INVOLUTE PATH AND BECOME DISCHARGED FROM SAID AIR DISCHARGE OPENING IN A REARWARD DIRECTION WITH A SWIRLING MOTION AND AT A LOWER VELOCITY, AN ELONGATED TUBULAR GAS NIPPLE SUPPORTED AT ITS FRONT END BY SAID CASING AND PROJECTING COAXIALLY THROUGH SAID AIR DISCHARGE OPENING, A GENERALLY CLYINDRICAL GAS NOZZLE MOUNTED ON THE REAR END OF SAID GAS NIPPLE IN COAXIAL RELATIONSHIP WITH THE NIPPLE AND PROVIDING AN INTERNAL GAS CHAMBER IN COMMUNICATION WITH SAID GAS NIPPLE, SAID NOZZLE AND NIPPLE BEING ADAPTED FOR PROJECTION INTO SAID TUBE THROUGH SAID ONE REAR OPEN END THEREOF, MEANS PROVIDING A SERIES OF CIRCUMFERENTIALLY SPACED RADIAL PORTS IN COMMUNICATION WITH SAID INTERNAL GAS CHAMBER, MEANS FOR REGULABLY SUPPLYING GAS TO SAID GAS NIPPLE, AND MEANS FOR SUPPLYING AIR TO SAID CASING THROUGH SAID AIR INLET OPENING.