US2253920A

US2253920A - Radiant tube heater

Info

Publication number: US2253920A
Application number: US249605A
Authority: US
Inventors: Arthur H Vaughan
Original assignee: Electric Furnace Co
Current assignee: Electric Furnace Co
Priority date: 1939-01-06
Filing date: 1939-01-06
Publication date: 1941-08-26
Anticipated expiration: 1958-08-26

Description

Allg- 26, '1941- A. H. vAuGl-IAN 2,253,920

lRADIANT TUBE HEATER med Jan. e, 1959 2 sheets-sheet 1 m N m mum \,\\\\\\\\\\\\\\\\\\\\.\\\\\\\\\\\\\\\\\\\\\\\\w\\ Patented Aug. 26, 1941 RADIANT TUBE HEATER Arthur Vaughan, Salem, Ohio, assignor to Thel Electric Furnace Company, Salem, Ohio, a corporation of Ohio Application January 6. 1939,'Serial No. 249,605

21 Claims.. (Cl. 126-91) ation of heat from the radiant tube is substantially uniform throughout the effective length of the tube. These attempts have not proved altogether satisfactory. I have found that such uniformity of temperature distribution may be obtained by employing a secondary air pipe within the combustion tube and providing the secondary air pipe with air holes spaced along the air pipe in a manner which is more particularly pointed out hereinafter.v 'Ihe distribution of the air holes is n'ot uniform along the air pipe but is in accordance with a graph or curve which` I have found to produce substantially uniform heating throughout 'the effective length of the combustion tube. This will be referred to in detail later on. 'I'he non-uniform distribution of the air holes prevents the occurence of local hot spots, and the invention also provides bailles adjacent the air holes for preventing direct impingement of the air onl the combustion tube and creates turbulence of the fuel and air so as to aid in uniform and complete combustion.

In the accompanying drawings, which illustrate a preferred embodiment of my invention,

Fig. 1 is a fragmentary transverse` vertical I section of a furnace' embodying my radiant tube heater;

Fig. 2 is a fragmentary longitudinal sectional view of a heater, the view being on a larger scale than Fig, 1;

Fig. 3 is a transverse vertical section through the heater taken on the line III-III of Fig. 2;

and

Fig. 4 is a diagram showing correct and incorrect hole distribution in the secondary air pipe, together with the resulting temperature distribution obtained with each type of air hole distribution.

Referring now more particularly to the accompanying drawings, the radiant tube heater comprises a combustion tube 2 which, as shown, is

'4 in the side -

walls

5 and 5a of the furnace;

greater part of its length. The ends of the combustion tube extend through openings 3 and The left hand end of thecombustion tube, as viewed in Fig. 1, has a tapered portion 6 and a portion 1 of smaller diameter than the main portion of the tube, the portion 1 extending outwardly beyond the f rnace wall. A sealing means 8 is provided for :making a tight seal between the furnace wall and the extension 1 while allowing contraction a d expansion of the combustion tube. A .burn r 9 extends into the left hand end of the co bustion tube, the space between the burner an extension l being sealed by a The b rner is. connected to a supply (not shown). trol apparatus is preferably of the type referred to in my above mentioned application which maintains subst ntially constant the relative amounts of fuel, rimary air and secondary air. The non-uniform spacing of the vair holes in the air pipe as co templated in the present invention is, howev independent of any particular type of fuel control, it being adapted for use with various t es of controls. y

The right hand e d of the combustion tube extends through the\ opening I in the furnace wall and has a flan II welded to it as indicated by the referenc numeral I2. This flange may be bolted or oth rwise secured to a flange I3 secured to the fu ace wall so as to clamp sealing material I4 between the two flanges. The end of the combustion tube is provided with a. chimney I5 for delivering the products of combustion. z

A secondary air pipe"A I6 is located within the combustion tube. Itvextends through the right hand end ofthe combustion tube, that is the end of the combustion tube opposite to that at which the burner 9 is located. The secondary air pipe extends substantially throughout the effective length of the combustion tube and is provided at intervals along its length with series of air holesv I1, these series of air holes being distributed non-uniformly, as will be described in detail later. adjacent each of the series of air holes for preventing direct impingement of the secondary air on the combustion tube. These .baiilesre shown more in detail in Figs. 2 and 3. Each of the bailies consists of a hub I9 which is welded as indicated by the reference numeral 20 to the outside of the secondary air pipe I6, and Wing portions 2|. Some or all of the bailles may be straight and of uniform diameter throughout the provided with centering lugs 22 which fit loose- The fuel con- A baille I8 is located ly in the combustion tube 2 and locate the air pipe approximately centrally therein.

The baffles are so located along the air pipe that the air stream issuing from each hole i1 strikes the inner surface of the wing portion 2 l. In this manner a blow pipe action upon the wall of the combustion tube 2 is prevented while the shielding effect of the radial portion 23 of the wing is suillcient to avoid destructive acton upon the wing itself. The wings cause turbulent flow of the moving gases in the space between the air pipe and combustion tube which improves the mixing action and the transfer of heat tothe combustion tube wall.

Any convenient number of wings may be formed upon a baille but for radiant tubes of ordinary size, say up to v5" outside diameter, I prefer to use four. Where onlv two holes l1 are used at any point along the length of the secondary air tube, they are located diametrically opposite one another, the baille wings with which no holes are associated functioning only to produce turbulence. In case there are two adjacent baflies each with only two air holes associated with them, I stagger the holes, so that the center line of the pair .corresponding to one baille is at right angles to adjacent pairs. Groups of four or more holes may likewise be staggered, though I have not found this necessary. Other forms of baiiies may be used in place of the particular type described but they should be such as to protect the combustion tube from direct air jet impingement and to produce a,l desirable amount of turbulence.

The secondary airbefore issuing through the openings I1 is preheated by the waste products of combustion flowing out of the right hand end of the combustion tube. In Fig. ,1, the secondary air is preheated in a preheater or recuperator 25 located intermediate an air supply pipe 26 and the secondary air pipe I6. The recuperator isof larger diameter than the secondary air pipe so as to cause the products of combustion to ow into intimate contact with its outer wall. Inside of the recuperator is a cylindrcally shaped baille 21 which causes the incoming secondary air to flowinto intimate contact with the wall of the recuperator. A considerable degree of preheat is thus obtained, with appreciable reduction in the fuel requirements and the further advantage of greater ease in disposing of the cooler waste gases. I have found it entirely practicable to reduce the waste gas temperature to 1200L7 F. or lower when operating at a tube surface temperature of 1700o F. 'Ihe heat corresponding to this reduction is obviously returned as useful heat to the interior of the tube.

In the preferred .manner of operating the heater, the burner 9 delivers a fuel-rich mixture of gas and air to the interior of the combustion tube 2. This mixture generally contains from 50 to 65% of the air required for complete combustion. The additional air necessary for complete combustion is supplied through the secondary air pipe I6, the secondary air issuing from the openings il and mixing with the gases flowing through inner face of the' wall 5a is reached. The resultI is the liberation of heat throughout the entire active length of theA heater with far better uniformity of tube temperature than would result if a complete mixture of fuel and air were admitted through the burner.

I have found that the desired general heat distribution from end to end of the heater depends upon the admission of secondary al1l at the proper rate as the gases pass from inlet to outlet end of the heater and that this is by no means a uniform rate per unit of length in the usual forms where a uniform rate of heat output is desired. I have found also that if unshielded holes are employed in the' secondary air pipe, the combustion tube is subjected to local overheating opposite these holes due to the blow pipe action of the issuing jets. Furthermore, the mixing of combustible gases and secondary air tends to be incomplete.

I shall now refer more particularly to the arrangement of air openings in the secondary air pipe required to produce uniform heating throughout the effective length of the combustion tube. These openings may be of various sizes and of other than round form but drilled holes of uniform size are usually preferable from both designing and manufacturing standpoints. Such holes will, therefore, be assumed for the purposes of this description. One would expect that a substantially uniform spacing of holes through the greater partroi' the length of the pipe IIB would produce a. desirable uniformity of useful heat liberation. I have found', however, that such is the space between the combustion tube and the Y not the case. The heat distribution is affected by a number of variables, including the following:

Varying temperature fof secondary air discharged by orices at different points inthe air pipe length. 1

Amount of preheat obtained in the recuperator. Increasing velocity of flow of gases as the outlet end of the heater is approached. Equilibria of partial combustion reactions as affected by operating temperature and other factors.

ticable any theoretical calculations and I have found it necessary to determine the air hole spacing by empirical methods. Suchmethods are not burdensome since a generally similar spacing applies in most-"cases and li' a, heater is o be built of a new size or for new operating onditions not differing widelyfrom previous experience, a spacing can be assumed which will require no more than minor changes after experimental operation. f

Referring now more particularly to Fig. 4, at the top of the figure is-shown a diagram of a radiant heater tube assembly, drawn to scale as regards dimensions in the y direction of tube length. At the bottom (in solid lines) is shown diagrammatically the air hole distribution which is actually used with satisfactory results in one furnace built under my direction. Adjacent the solid-line diagram is plotted (in dot-'and-dash lines) a uniform air hole distribution along the secondaryair pipe which I have found to give unsatisfactory results. In the central part of the figure are shown tube temperature Acurves illustrating the type of temperature distribution (though not necessarily the exact values) resulting from both kinds of air hole distribution.

In the actual heater represented, a primary mixture of about 61/2 parts air and one part of gas is introduced through the burner 9. The

These variables are such as to render imprac-A composition of the gas is approximately as follows:

Methane, CH'. 1 --per cents.. 84.6 Ethane, 0114's do -13.4 Nitrogen, Na'. do 2.0

Air/gas ratio for 4perfect combustion, ap-

' to line a. whose crests lie on or near the arbitrarily assumed curve. I usually place two orv -more holes at a. given location, since a single hole is not likely to aect the full periphery of the combustion tube vv vith satisfactory uni- -frmlty.

proximately 10.3

Approximately 4.8 parts of secondary air are introduced through the secondary air pipe I6. The total represents excess air over that required for perfect combustion. The initial stage of `lcombustion occurs in the space', designated A, between thev burner nozzle 9. and the first point of admission of secondary air, which is at the second baille I8, and designated lila. There are no air holes. associated with. the first baffle, which serves merely to center the pipe and will be readily understoodthat .tooA high a temperature at a givenpoint indicates that'too much to assist thel combustion reaction by' .producing turbulence. At each of the second,'third, fourth,

fifth and sixth baiiles two holes are located, and;

at each of the remaining onesthere are fourlholes. All holes aregf" diameter. lProgressive Ifurther combustion takes place following each stage of secondary air admissionuntil the vcombustible .content of the fuel has been exhausted. In the lower part of Fig. 4, the horizontal axis or abscissa representsdistances inA inches measured fromk the inner face of the furnace wall 5 adjacent the fuel inlet. The vertical axis or ordinate represents the numb'er of air holes (and also obviously the total air ho1`e,area) between the inner face of the furnace wall B and any given point on the air pipe. The solid zig zag line a shows the distribution of air holes actually used and corresponds to the air hole distribution shown in the upper part of Fig. 4.1

This line rises vertically at each hole location;

a distance representing the number of holesat that location. The crests of the-zig zag line have been connected by a series of straightlines b approximating a smooth curve which is upwardlyconcave. Under these conditions, it' will be obvious that the rate of secondary air admission to the'combustion tube, per unit of length, increases Aas the waste gas outlet is approached.

In a similar manner, the dot-and-dash linesv d and e represent a uniform hole .distribution along the secondary air pipe. Thesolid line c .f lin the. middle portion of Fig. 4 shows tube temperatures such as observed under operatingconditions when non-uniform -airhole-distribution as shown by curve b is employed. This represents highly satisfactory heater performance. The

secondaryair has been-admitted ahead'of that point, `and too low -atemperature indicates too little-air. f

. In the `:preferred embodimentof .the invention as described, a mixture of gas and air is introduced into one end of the combustion tube and they required additional amount of secondary air for lcompleting combustion is introduced through the secondaryfair pipe.l However, my` invention may' also be practiced whenfuel only, rather than 'a man mixture of fuel andain'is supplied at the i et end of the combustion tube. With fuels 4oi.' relativelyhigh' hydrocarbon content, including natural gas, commercial propane and butane, retort coal gas, and the like, such a method of firinglleads to ."cracklng ofthe hydrocarbons in the initial combustion zone, with deposit of solid carbon. 'When using'suchfuels. serious diillculty from accumulation 4koi carbon can be avoidedvonly by the use-of .primary\ air, usually 50% or more of the-required total. How,- ever, certain producergases whose combustibl.

cori'tentis largely CO and .Hz, are substantially through the increased amount of waste heat.re.

covery. The present invention isequally advantageous in this, case, because the greaterv quanvtity of air introduced through the air pipe would otherwise actually increase the variation in air temperature between different locations and the blowpipe effect of the air jets.

vIt will be obvious that the same empirical procedure may be used for determining: air hole disdot-and-dash line .f is a tube temperature curve such as obtained by the uniform air hole distribution represented by curve e. It will be noted that. curve f represents a very undesirable temperature distribution, since the temperature. ad-

jacent the mid-portion of the combustion tube v is much higher than adjacent the fuel inlet' end. Furthermore, the temperature at the end where the waste gases exit is much4 lower than thev `temperatureat the fuel entrance end.; As 'com-v paredswith this, the curve cshows substantially uniform temperaturesthroughout,the-length of the combustion tube.

In designing a new base, pon an arbitrary ,curve similar to curvej b and .having such upward concavity as expe? jrience. indicates to' be approximately correct. Air ,holes are tien located. along the secondary air'pipefas det rmined by a/zig/,zag line similar f' s, l, I f' f size of radiant tube 'heaterf I firs establisha tentative air hole distribution tribution' as when employing primary air.

It will be seen from the construction which has been described'that theair hole distribution along thelength of' the secondary air pipe is not uniform but is graduated so that the secondary 'air is admitted `to' the combustion zone at a greater rate per` unit of combustion zone length near /the' outlet than near the inlet end ofthe 1ciomloustion zone. The secondary air outlet orices th differences in temperature of the secondary air discharged at various points along the secon aryairr'zipel.l I6: Thus, as" shown in Fig. l,`the ain jets'fiss'uingfrox'n the openingsk I'l'located adjacent, the right hand end of the air pipe are not y heated to as high/a temperature asthat issuing from the openings adjacent the left hand end of the/air ipe. There are only two air openingsinl ea'ch series at,y the leftl hand en whereas ther 'aregfour openings ineachserie adjaeent the rlghtf'hand end. The increased areaor the secondary air openings fa'dja'cent the right hand' are so proportioned as Ito compensate for' y end compensates for the lower temperature of the air issuing from these openings. It will be noted further that the total area of the air holes lying to the left of the mid-portion of the secondary air pipe is less than the total area of the air hole openings lying to the right of the midportion.

As previously pointed out, the curve B shown at the bottom of Fig. 4 is upwardly concave. The curve is of generally constantly increasing slope from the beginning toward the end of the curve. This type of curve has been found in actual practice to produce substantially uniform temperature distribution throughout the effective length of the combustion tube under the conditions which have been referred to. In determining the area and distribution of air holes to be used in an air pipe for a radiant heater of a dierent size from that already constructed, the location and number or area oi' secondary air orifices at each location should be such as to produce a Vcurve similar to curve b. This is done by employing a graph on which the ordinate represents the total air hole area between the face of the furnace wall adjacent the fuel inlet and the point of the air tube in question. The abscissa represents the distance between the furnace wall face just referred to and the point on the secondary air pipe where the hole or holes are to be located. The place of location and the total air hole area are then selected so that when the points are plotted on the graph they form a curve which is upwardly concave.

In the illustrated embodiment of the invention, I use a perforated air pipe. Instead of a single perforated air pipe, I may use several smaller pipes terminating in open ends at different points in the combustion zone and when an air pipe is referred to in the claims it is intended to include such construction.

Although I have illustrated and described a preferred embodiment of my invention, it is to be understood that the invention is not limited thereto but may be otherwise embodied or practised Within the scope of the following claims.

I claim:

1. In a heater for furnaces and the likeja'n imperforate elongated outer tube having a fuel inlet end portion, an outlet end portion, and a portion enclosing a combustion zone intermediate said end portions, means for supplying fuel to the combustion zone at the inlet end thereof, and a conduit traversing said outer tube through the outlet end thereof and through themajor part of the length of the combustion zone, said conduit having air discharge orifices at points within the combustion zone so disposed as to discharge air at a progressively greater rate per unit of combustion zone length near the outlet than near the inlet end of the combustion zone.

.2. In a heater for furnaces and the like, an imperforate elongated outer tube having a fuel inlet end portion, an outlet end portion, and a portion enclosing a combustion zone intermediate said end portions, means for supplying a predetermined mixture of fuel and air insufficient for complete combustion to the combustion zone at the inlet end thereof, and a conduit traversing said outer tube through the outlet end thereof and through the major part of the length of the combustion zone, said conduit having air discharge orifices at points Within the combustion zone so disposed as to discharge air at a progressively greater rate per unit of combustion zone length near the outlet than near the inlet end ofthe combustion zone.

3. In a heater for furnaces and the like, an imperforate elongated outer tube having a fuel inlet end portion, an outlet end portion, and a portion enclosing a combustion zone intermediate` said end portions, means for supplying fuel to the combustion zone at the inlet end thereof, and means for supplying air to the combustion zone for progressive combustion of the fuel, said means including a conduit entering through an end portion of said outer tube and traversing at least a substantial part of the length of the combustion zone, said conduit having air outlets so disposed as to discharge progressively greater amounts of air in relation to the combustion zone length near the point of air entry than remote therefrom.

4. A radiant tube heater for furnaces and the like, comprising an imperforate combustion tube, means for supplying combustible material to one end of said tube, an air pipe within said tube and provided with air holes arranged at intervals therealong for supplying air flowing in a direction countercurrent to the flow of products of combustion, the sum of the areas of the holes from the mid-portion to the air inlet end of the air pipe exceeding the sum ofthe areas of the holes from the mid-portion to the end of the air pipe remote from the inlet end.

5. A radiant tube heater for furnaces and the like, comprising an imperforate combustion tube, means for supplying combustible material to one end of said tube| an air pipe within said tube and provided with air holes spaced therealong, the spacing of the holes lengthwise of the pipe and the air hole area at any point along the air tube being so related that when distances measured from the inner face of the furnace wall adjacent the fuel inlet to the air hole points on the air pipe are plotted as abscissa and the total air hole area between said face and the point of the air tube in question are plotted as ordinates, the curve of the plotted points is upwardly concave.

6. A radiant tube for furnaces and the like. comprising an imperforate combustion tube, means for supplying combustible material to one end of said tube, an air pipe within said tube and provided with air holes arranged at intervals therealong, the total area of such air holes from the end of the 'air pipe remote from the air inlet end of the pipe to a mid-zone therein being less than the total area of air holes from the mid- Zone to the inlet end of the pipe, the distribution and the area of the air holes being such that a graph plotting the sum of the areas of the air holes on its ordinate against the distance between a given point on the air pipe and the inner face of the furnace wall adjacent the fuel inlet on its abscissa is a curve of generally constantly increasing slope from the beginning toward the end of the curve.

7. A radiant tube heater for furnaces and the like, comprising a combustion tube, means for supplying combustible material to one end of said tube, an air pipe within said tube and provided with air holes spaced therealong, and baiiles located adjacent the air holes for preventing divrect irnpingement of the air on the combustion tube and for creating turbulence of the combustible material `and air.

il. An air tube for an internally fired heating tube and adapted to extend through the outlet and the major portion of the combustion zone of said heating tube, said air tube having a plurality of spaced discharge orifices within the combustion zone and means associated with said orices 'for deflecting and distributing the air issuing 10. In an internally fired heating tube, means for obtaining a desired temperature distribution in theheat-radiating portion of the tube, said means including an internally disposed air conduit having orifices for discharging air for combustion into the combustion space and baffles associated with said orice for deflecting and distributing said air after discharge.

11. In an internally fired heating tube, means for obtaining a desired temperature distribution in the heat-radiating portion of the tube, said means including an internally disposed air conduit having orifices for discharging air for combustion into the combustion space and baffles associated with said orifices for deecting and distributing said air after discharge, said orifices being so located in respect to the length of said heat-radiating portion as to compensate for the temperature of the air discharged therefrom.

12. A radiant tube heater for furnaces and the like, comprising an imperforate combustion tube,

means for supplying combustible material to one end of said tube, an air pipe within said tube and provided with air holes spaced therealong, the spacing of the holes lengthwise of the' pipe and the air hole area at any point along the air tube being so related that when distances measured from the inner face of the furnace wall adjacent the fuel inlet to the air hole points on the air pipe are plotted as abscissa and the total air hole area between said face and the point of the air tube in question are plotted as ordinates, the curve of the plotted points is upwardly concave, and bailies located adjacent the air holes for preventing direct impingement of the air on the the inner face of the furnace wall adjacent the fuel inlet to the air hole points on the air pipe are plotted as abscissa and the total air hole area between said face and the point of the air tube in question are plotted as ordinates, the curve of the plotted points is upwardly concave.

14. A radiant type heater for furnaces and the like, comprising an imperforate combustion tube, means for supplying combustible material to one end of said tube and discharging products of combustion from the opposite end of said tube, an air pipe within said tube and provided with air holes arranged at intervals therealong throughvso related that when distances measured from out the maior portion of the combustion tube, the air hole area increasing progressively per unit of air tube length inthe direction of flow of products of combustion and throughout the major portion of the combustion tube.

15. A radiant type heater for heat treatin furnaces containinga controlled atmosphere for the work being treated,'said heater comprising an imperforate combustion tube, having a length at least several times its diameter, means for supplying combustible material to one end of said combustion tube and discharging products of combustion from the opposite end of said tube, an air pipe within said combustion tube for supplying air iiowing in a direction countercurrent to the flow of products of combustion, said air pipe provided with air holes arranged at intervals therealong throughout the major portion of the air pipe length, the air hole area increasing progressively per unit of air pipe length in the direction of flow of products of combustion.

16. A radiant type heater for heat treating furnaces containing a controlled atmosphere for the Work being treated, said heater comprising an imperforate combustion tube having a length at least several times its diameter, means for supplying combustible material to one end of said combustion tube and burning said material in contact with said tube and discharging products of combustion from the opposite end of said tube, an air pipe within said combustion tube for supplying air flowing in a direction countercurrent to the flow of products of combustion, said air pipe provided with air holes arranged at intervals therealong throughout the major portion of the air pipe length, the air hole area increasing progressively per unit of air pipe length in the direction of flow of products of combustion.

17. A radiant type heater for heat treating furnaces containing a controlled atmosphere for the work being treated, said' heater comprising an imperforate combustion tube having a length at least several times its diameter, said combustion tube having its external heat radiating surface freely exposed to the furnace atmosphere, means for supplying combustible material to one end of said combustion tube and discharging products of combustion from the opposite end of said tube, an air pipe within said combustion tube for supplying air flowing in a direction countercurrent to the flow of products of combustion, said air pipe provided with air holes arranged at intervals therealong throughout the major portion of the air pipe length, the air hole area increasing progressively per unit of air pipe length in the direction of flowof products of combustion.

18. A radiant type heater for heat treating furnaces containing a controlled atmosphere for the work being treated, said heater comprising an imperforate combustion tube having a length at least several times its diameter, said combustion tube' having its external heat radiating jsurface located so as to freely supply heat to the furnace, means for supplying combustible material to one end of said combustion tube and discharging products of combustion from the opposite end of said tube, an air pipe Within said combustion tube for supplying air owing in a direction countercurrent to the flow of products of combustion, said air pipe provided with air holes arranged at intervals therealong throughout a substantial portion of the combustion tube, the air hole area increasing progressively per unit of air pipe length in the direction of iiow of products of combustion and throughout the perforated portion of said air pipe.

19. Heating means for furnaces comprising a combustion tube having inlet and outlet end portions and an imperforate heat-radiating portion enclosing a combustion zone intermediate said end portions, said heat-radiating portion having a length at least several times its diameter, means for supplying combustible material to the inlet end of said combustion zone and for discharging products of combustion through said outlet end portion, Iand a secondary air pipe extending through said outlet end portion into and through a substantial portion of said combustion zone, said secondary air pipe having air holes spaced therealong throughout substantially its entire length within the combustion zone, the area ot said air holes per unit length of said pipe increasing constantly throughout said length in the direction of flow of products of combustion.

20. A radiant tube heater for furnaces and the like, comprising a combustion tube, means for supplying combustible material to one end of said tube,v an air pipe within said tube and provided with air holes spaced therealong, baiiies located adjacent the air holes for preventing direct impingement ot the air on the combustion tube and for creating turbulence of the combustible material and'air, and means tor spacing said bailles fromvthe combustion tube.

21. A radiant tube heater for furnaces and the like, comprising a combustion tube, means for supplying combustible material to one end of said tube, an air pipe within said tube and provided with air holes spaced therealong, bafiles located adjacent the air holes for preventing direct impingement ofthe air on the'combustion tube and for creating` turbulence of the combustible ma- 'terial and air, and centering lugs associated with at least one of said baiilesY for spacing the bailies 20 from the combustion tube.

ARTHUR H. VAUGHAN.