US2759473A

US2759473A - Radiant tube gas burner

Info

Publication number: US2759473A
Application number: US354132A
Authority: US
Inventors: Herbert M Short
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1953-05-11
Filing date: 1953-05-11
Publication date: 1956-08-21
Anticipated expiration: 1973-08-21

Description

Aug. 21, 1956 H. M. SHORT 2,759,473

RADIANT TUBE GAS BURNER Filed May 11, 1953 INVENTOR ATTORNEY United States Patent RADIANT TUBE GAS BURNER Application May 11, 1953, Serial No. 354,132

6 Claims. (Cl. 126-91) This invention relates to gas burners and more particularly to flame stabilizing means for burners of the diffusion or delayed mixing type wherein separate, contiguous streams of fuel gas and combustion supporting air are delivered in relatively large quantities or volumes to a combustion chamber for producing a relatively long, slow-burning flame.

Such burners are of special utility in the operation of gas-fired heaters of the radiant tube type and the like by reason of the characteristics of the flame. However, it is necessary to incorporate in the burner some means to provide a pilot flame to stabilize the burner flame and keep it ignited. Heretofore, the burner has been complicated by the provision of a pilot burner separately supplied with gas and air premixed in combustible proportions, while attempts to utilize small amounts of the separate air and gas streams directly for piloting the burner flame have proved to be inadequate and ineffectual under varying operating conditions encountered in service, the pilot flame itself usually being unstable and often disappearing entirely when needed most, as under high fuel demand conditions.

It is, therefore, an object of the present invention to provide a burner of the class indicated embodying simple and practical pilot burner means in association with the fuel tube for providing a pilot flame for the burner flame and which is so constructed and arranged as to develop automatically from the separate air and gas streams an ideal gas-air mixture which will serve as the core of the pilot flame, so that such flame itself will be quite stable under varying conditions of mixture, operating rate and types of fuel.

A further object of the invention is to provide a selfpiloting, slow-combustion burner for radiant tube heaters which will positively retain its flame when operating under high fuel consumption rates.

The invention in its present preferred construction is illustrated in the accompanying drawing, wherein:

Fig. 1 is a longitudinal central section through a burner and firing end of a radiant tube heater and embodying the novel features of the invention;

Fig. 2 is a side elevational view, partly in section, of the fuel tube and associated pilot nozzles, removed from the burner,

Fig. 3 is an end view of the structure of Fig. 2, and

Fig. 4 is a sectional view, to a smaller scale, of a portion of the structure of Fig. 1, showing a modification.

Referring to Fig. 1, a portion of a wall of a furnace chamber which is adapted to be heated by means of a gasfired radiant tube heater is indicated at 11. The heater tube 10 may be of the hairpin type, or of any other suitable well known form. The intake or firing end of the tube 14) extends outwardly through the furnace wall and connects with and is enclosed by a gas burner 12, the exhaust end of the tube (not shown) passing through the furnace wall and communicating with an exhaust stack (not shown) in any suitable well known manner.

The burner 12 preferably comprises a two-part casing tion chamber inside the 2,759,473 Patented Aug. 21, 1956 consisting of gland member 13 and cup-shaped body 14, each provided with a circular flange 15 through which bolts 16 extend to draw them up into axial assembly. The gland member 13 encircles the intake end of radiant tube 10 and is removably secured, as by bolts 17, to a burner mounting plate 18 which, in turn, is suitably secured to the usual furnace jacket or shell. Suitable gaskets, as shown, are inserted between the mounting plate and the furnace jacket and the burner casing, respectively.

Gland member 13 is provided with an internal, annular rib 19, one side of which forms a radial seat or land for a packing ring 20 of suitable heat resistant, compressible material. The packing ring 20 is adapted to be engaged by an annular or collar extension 21 on the body member 14 of substantially the same radial dimension as the rib 19. By drawing the body 14 toward the gland member 13 with the bolts 16, the packing ring is compressed and forced into sealing contact with the outer side of the heater tube 10, thereby effectively sealing the joint be tween the burner casing and the heater tube and preventing undesired flow or leakage of gasses into or out of the furnace chamber through the radial clearance space along the outer side of the heater tube.

The end of the gland member which clamps against the mounting plate 18 is provided with a shallow recess or counterbore 32, the bottom of which is defined by the other side of rib 19. This recess accommodates loosely, or with clearance all around, the outer portions of split ring segments 33, the inner portions of which seat in an annular groove 34 formed in the thickened mouth end of the heater tube 10. The ring segments are adapted to be retained on the tube 10, as by tack-welding them together at their contiguous ends. In lieu of the ring segments, a plurality of screw members may be threaded into the heater tube wall in position to cooperate with

members

18 and 19 to restrain tube 10 from creeping into or out of the furnace.

It is to be noted that, by this construction, tube is free to move slightly in radial and axial directions relative to the furnace wall and the fixed burner, thus preventing binding and stressing of the various parts due to differential thermal expansion. Also, when the heater tube is positioned vertically, it will be supported either on the mounting plate 18 or on the rib 19 of the burner casing, depending on whether it extends downwardly or upwardly into the furnace chamber.

Inside the burner, there is provided a central fuel tube which is flanged at one end and adapted to be secured gas tight to the end wall of the burner body 14 by means of bolts 23 and an interposed gasket, as shown. The fuel tube extends axially into the heater or radiator tube 10 a distance beyond the mouth 24 of the latter, so that the starting point of the combustion zone within the heater tube 10 will be located near the inner side of the furnace wall. Also, the fuel tube is of less diameter than the inside diameter of the heater tube, to provide an annular passageway between the two. The burner body has a lateral air inlet and an axial fuel inlet into which are threaded an air supply pipe 25 and a gas supply pipe 26, respectively. Through these supply pipes, fuel gas and combustion supporting air are delivered into the burner, each under suitable pressure and preferably each under suitable volumetric control, as by usual valve means. Within the burner, the air and gas are constrained to flow in parallel passages separated from each other by the wall of the fuel tube throughout the length thereof and form distinct air and fuel gas streams which initially come into contact with each other at the outlet or downstream end of the fuel tube. Consequently, the central fuel gas stream and the annular air stream flow into the combusthe heater heater tube for diffusion or intermixing with little turbulence, resulting in a long, slowburning flame being produced.

Various kinds of fuel gas may be supplied to the burner. However, some gases, principally natural gas due to its high methane content, do not lend themselves readily to complete combustion with diffusion or delayed mixing burners of the class herein disclosed unless the raw gas is initially mixed with a percentage of air, known as primary air. Depending upon various factors, such as gas composition, tube proportions, number of pipe bends or elbows and pipe lengths, etc., the amount of primary air required to prevent soot or lampblack formation in the combustion chamber, irrespective of whether or not adequate total air for complete combustion is supplied, will range from about to 35%. The intermixing of raw gas and primary air, as required, may be effected either externally or internally of the burner. The mixture is still non-inflammable unless additional or secondary air be supplied for combustion. With the exemplary burner illustrated in Fig. 1, raw natural gas and primary air will be mixed externally by well known means and delivered into fuel tube 22 through the supply pipe 26. In case, however, it is desired to effect such mixing internally of the burner, the well known jet inspiration principle may be employed.

In Fig. 4 illustrating a contemplated arrangement utilizing the principle, wherein the parts corresponding to those appearing in Fig. 1 have the same numerals primed applied thereto, the wall of fuel tube 22 is provided with one or a plurality of air ports 36 near the flanged end thereof, the area of which is adapted to be controlled by a sleeve 37 which is adapted to be adjustably moved along the fuel tube by manipulation of a screw or push-pull rod 38 extending out through the end wall of the burner body 14. A spud or jet orifice 39 is provided at the end of the supply pipe 26. Instead of using the air ports, it is contemplated to mount the tube 22 with its flanged end spaced from the end wall of the body 14', so as to permit air to flow around the end of the fuel tube. Of course,

the total air requirements will be supplied to the burner through supply pipe 25. Air from within the burner body 14' will be inspirated into the fuel tube through the air ports (or around the flanged end thereof) by the kinetic energy of the raw gas issuing from the orifice 39. The non-inflammable mixture becomes intermixed by the time it reaches the delivery end of the fuel tube.

Reverting again to Fig. 1, pilot flame producing devices 27 of tubular form are mounted on the delivery end of the fuel tube 22, in order to provide continuous ignition of the constituents of the burner flame and to stabilize the same. Because of the relatively large diameter of the fuel tube, it is desirable to provide a plurality of ignition points around its outlet, two of these pilot tubes 27 being here shown, at opposite sides of the fuel tube. Their axes are disposed parallel to the axis of the fuel tube and preferably have a tapered end portion which projects into the combustion space. In the form illustrated, each pilot tube is easily and cheaply made from a pipe nipple or other suitable tubular member of a length substantially equal to the diameter of the fuel tube and having an inside diameter equal to about one-third of its length. The nipple is squeezed or flattened down for a portion of its length to form a narrow, elongated outlet or slot 28 at one end, the other end providing an unrestricted inlet 29. The tapered end of the nipple is then saw cut or slotted at right angles to outlet 28 to provide lateral discharge slots of narrow, elongated form in the tapered sides of the nipple, these being located to one side of the axis of the nipple. The fuel tube 22 is end slotted to receive the upstream circular end portion of the pilot nipple or tube therein with its inlet 29 facing upstream and abutting the bottom of the fuel tube slot. The pilot tube is then suitably secured in place, as by welding. Before welding, each pilot tube is turned on its axis to bring the long side of its outlet 28 into generally perpendicular or radial relation to the wall of the fuel tube and with the short end of the outlet disposed at the gas side of the wall, as shown in Fig, 3. In addition, each pilot tube is set radially into the tube wall sufiiciently to dispose the discharge slots 30 thereof generally tangential to or in the plane of the tube wall, in which position, the inlet 29 is spanned and divided by the tube wall into gas and air inlets of unequal areas.

In operation, gas and air are delivered to the burner through the

supply pipes

26 and 25, under desired pressure, and they flow along the separate passages in the burner and are discharged into the combustion chamber at the delivery end of the fuel tube 22 where they initially come into contact for combustion at the boundary layer between themselves. The pilot tubes, since they project partly inside and partly outside the fuel tube, operate to scoop gas and air from the respective streams, mix the two and permit the mixture to escape at reduced velocity through their

crossed outlet slots

28 and 30. Once ignited, the low velocity mixtures burn with a stable flame and serve as a sustained ignition source for the gas and air constituents of the burner flame. Ignition of the pilots may be effected in any suitable, well known way, as by means of a spark electrode, for example. In each pilot tube, selected proportions of gas and air are permitted to enter for mixing, the proportioning being controlled by the area ratio of the upstream inlet projected into and outside the fuel tube 22.

The purpose of the outlet slots 30 is to increase the discharge or outlet area of the pilot tube so as to reduce the velocity at which the mixture will leave the tube, thereby stimulating pilot flame stability throughout the operating range of the burner. Disposing slots 30 generally tangential to the wall of the fuel tube 22, as described, results in the presence of flared out or spread pilot flame adjacent the boundary layer between the air and gas constituents of the burner flame. The radial disposition of the outlet 28 assures that even under conditions where the total mixture in the pilot tube may not be the most suitable for flame retention, at some location in the radial passage an ideal gas-air mixture will exist because of the gradual transition from too rich to too lean at opposite ends of the outlet and this ideal mixture somewhere between the ends of the outlet will serve as the core of the pilot flame. It is to be noted that the size of the pilot flames changes automatically when the burner is operated at different levels of fuel consumption, since the amount of gas and air scooped into the pilot tubes to feed the pilot flames varies with the velocity of the main gas and air streams flowing through the burner. This assures adequate and effective piloting of the burner flame when operating the burner under high fuel consumption rates.

The described orientation of the pilot tubes with respect to the fuel tube is the one which is preferred. However, when the burner is used on radient tubes having small diameter throats, there may be too close clearance or even interference between the outer points of the pilot tips and the inside surface of the radiant tube wall. In such case, the pilot tubes may be turned degrees on their longitudinal axes, so that the outlets 28 thereof will be parallel to tangents of tube 22 and the outlet slots 30 radial with respect to the tube 22. With this alternate orientation, the slots 30 may be centered on the axes of the pilot tubes, instead of being offset, if desired. As thus arranged on the fuel tube, the operation of the pilot tubes has proven to be adequately satisfactory.

While in the foregoing, a preferred embodiment has been disclosed and described in detail, it is to be understood that the invention is not limited thereto, but may be otherwise variously embodied within the scope of the following claims.

What is claimed is:

1. A gas burner having an air passage and a gas passage separated from each other by a common wall for delivering separate streams of fuel gas and combustion supporting air to a combustion space at the downstream end of said wall, and means for producing a pilot flame in said combustion space, said means comprising a pilot tube carried by said wall parallel to the stream flow along the wall and terminating in said combustion space, said tube having an open upstream end and a restricted downstream end, the upstream open end of said tube being spanned by said wall with its center disposed on the airstream side of said wall to provide inlet openings of unequal areas at opposite sides of said wall for admittance of proportional amounts of air and gas from the respective streams into said tube for mixing therein, and the downstream end of said tube having an elongated outlet for said mixture to sustain a pilot flame in said combustion zone.

2. A gas burner having an air passage and a gas passage separated from each other by a common wall for delivering separate streams of fuel gas and combustion supporting air to a combustion space at the downstream end of said wall, said wall having an open slot in its downstream end, a pilot tube positioned in said slot and secured to said wall with its axis parallel to the stream flow along the wall and disposed partly to one side and partly to the other side of said wall and terminating in said combustion space, the upstream open end of said tube being spanned by said wall and providing inlet openings at opposite sides of said wall for admittance of air and gas from the respective streams into said tube for mixing therein, and the downstream end of said tube being provided with crossed outlet slots of generally elongated and narrow form for discharge of said mixture to sustain a pilot flame in said combustion zone.

3. A gas burner having an air passage and a gas passage separated from each other by a common wall for delivering separate streams of fuel gas and combustion supporting air to a combustion space at the downstream end of said wall, said wall having an open slot in its said downstream end, a pilot tube having a portion of its length mounted in said slot and secured to said wall and extending in a downstream direction into said combustion space, said tube being disposed partly to one side and partly to the other side of said wall, for admittance of air and gas from the respective streams through the upstream end of the tube and the discharge thereof through the downstream end of the tube to sustain a pilot flame adjacent the boundary layer between the air and gas streams in said combustion space, said downstream end of said tube having an elongated, narrow outlet for the pilot gas and air, the long side of which is generally perpendicular to said wall.

4. A gas burner having an air passage and a gas passage separated from each other by a common Wall for delivering separate streams of fuel gas and combustion supporting air to a combustion space at the downstream end of said wall, a pilot tube having a portion of its length mounted in said end of said wall and extending in a downstream direction into said combustion space, said tube being disposed partly to one side and partly to the other side of said wall, for admittance of air and gas from the respective streams through the upstream end of the tube and the discharge thereof through the downstream end of the tube to sustain a pilot flame adjacent the boundary layer between the air and gas streams in said combustion space, said downstream end of said tube having an elongated, narrow outlet for the pilot gas and air, the long side of which is generally perpendicular to said wall and lateral discharge openings in opposite sides of said pilot tube disposed intermediate the ends of said elongated outlet and substantially in line with the plane of said wall, to increase the discharge area for the gas-air mixture.

5. A self-piloting gas burner for an internally-fired heater tube comprising, in combination with said heater tube, a casing enclosing the intake end of said tube, a fuel tube carried by said casing and extending axially into said heater tube in spaced relation thereto, to form an annular air passage about said fuel tube, means for admitting air into said casing and fuel gas into said fuel tube for flow in separate streams into said heater tube for combustion therein, a pilot tube parallel to said fuel tube and projecting partly inside and partly outside the same to scoop gas and air from the respective streams for mixing in said pilot tube, said pilot tube extending beyond the end of said fuel tube into the combustion zone Within said heater tube and having a narrow, elongated outlet disposed in radial relation with respect to said fuel tube, through which outlet the airgas mixture discharges to sustain a pilot flame, and said pilot tube having lateral discharge openings cooperating with said outlet to increase the discharge area for the 'air gas mixture.

6. A self-piloting gas burner for an internally-fired heater tube comprising, in combination with said heater tube, a casing enclosing the intake end of said tube, a fuel tube carried by said casing and extending axially into the intake end of the heater tube in space relation to the Wall thereof, to form an annular air passage about the fuel tube, means for admitting air into said casing and gas into said fuel tube both under pressure, for flow in separate streams into said heater tube for combustion therein, a plurality of pilot tubes parallel to said fuel tube and disposed about the discharge end thereof and projecting partly inside and partly outside the same to scoop gas and air from the respective streams for mixing in the pilot tubes, each pilot tube having a tapered end portion extended beyond the end of said fuel tube and terminating in a narrow, elongated outlet disposed radially with respect to said fuel tube, through which outlet the air-gas mixture discharges to sustain a pilot flame, and said tapered end portion being provided with elongated lateral discharge slots, located generally tangential to said fuel tube and intersecting with said outlet, to reduce the velocity of the airgas pilot mixture and effect spreading of the pilot flame.

References Cited in the file of this patent UNITED STATES PATENTS 1,760,050 Eickworth May 27, 1930 2,269,699 Stoecker et al Jan. 13, 1942 2,398,611 Beggs Apr. 16, 1946