US3291104A

US3291104A - Tubular heater

Info

Publication number: US3291104A
Application number: US491625A
Authority: US
Inventors: Merle A Zimmerman
Original assignee: WASTE HEAT ENGINEERING CORP
Current assignee: WASTE HEAT ENGINEERING CORP
Priority date: 1965-09-30
Filing date: 1965-09-30
Publication date: 1966-12-13
Anticipated expiration: 1983-12-13

Description

Dec. 13, 1966 M. A; ZIMMERMAN TUBULAR HEATER 2 Sheets-Sheet l Filed Sept. 30, 1965 /Wef/e f4. Z Wma/Man INVENTOR.

BY nam/ef OOOOOOOOOOMOOOO QOOO MOOOOOOOOOO l \Wu M Dec. 13, 1966 M. A. ZIMMERMAN TUBULAR HEATER 2 Sheets-Sheet 2 Filed Sept. 50, 1965 Mer/e A'. Z #nwe/mon /l TTORNE YS nited States Patent O 3,291,304 TUBUILAR HEATER Merle A. Zimmerman, Shreveport, iba., assigner to Waste Heat Engineering Corp., a corporation of Texas Filed Sept. 30, 1965', Ser. No. 491,625 7 Claims. (@Cl. 122-240) The present invention relates to a fired heater enclosure and a plurality of fired heater sections and a combination of burners and tube banks therefor.

Various forms of fired heaters have been heretofore proposed, such as, by way of example, that as shown in United States Patent Nos. 2,396,200, 2,527,410, and 3,066,656. The present invention may be considered an improvement in heater construction to those mentioned above, as will be more fully described hereinafter. In the heating of fluids, such as hydrocarbons, hydrocarbon oils, and the like, it is desirable to accomplish the heating of the fluids in a manner to avoid undesirable effects thereof. To accomplish this, it is desirable that the heating of the liu-ids be substantially uniform and not occur in local arcas on the tube banks through which the fluid is conducted, which localized heating causes hot spots which may cause deleterious effects on the fluid being heated. Also, it is desirable to accomplish heating in the fluids while simultaneously avoiding flame impingement upon the tubes through which the fluid is flowing to reduce the possibility of forming hot spots.

To properly accomplish the heating of fluids in an efficient manner, the heated fluids are caused to flow through a series of tubes which are heated -by radiation from the combustion haines without impingement of the flames on the heated tubes and/ or with convection from the flow of the products of combustion against the tube surfaces.

One problem as illustrated in Patent No. 2,396,200 is that the heater does not provide adequate control for the path of flame travel. and is therefore subject to flame impingement on the tube banks.

Another problem in the prior art and as illustrated in Patent No. 3,066,656 is that the heater has two setting walls in planes substantially parallel to each bank of heated tubes. The setting walls of the heater are further very expensive because of the added cost of two walls instead of one wall.

The heater shown in Patent No. 2,527,410 illustrates the problem that as tube banks are heated by luminous radiation from one side only, relatively nonuniform heat transfer rates will be produced and this requires an excessive amount of heated tube .surface which results in an excessive cost for the tubes as well as an excessive cost for the setting walls.

An object of the present invention is to provide a fired heater that is economical with respect to the cost of the radiant tube banks or walls of tubes and whose radiant tube banks have a high average heat transfer rate relap tive to the maximum spot transfer rate.

Another object of the present invention is to provide a plurality of stacked tubes in a red heater enclosure wherein the stacked tubes are heated uniformly on both sides to increase the rate of heat transfer and thereby reduce costs of construction.

Yet another object of the present invention is to provide in one embodiment two parallel, vertical banks of spaced heated tubing with only two exterior heater walls which are parallel to the two tube banks wherein the use of two walls in place of the four heater setting walls ordinarily provided for two tube lbanks effects a substantial reduction in the cost of the heater.

It is another object of the present invention to accomplish heating of the fluids Wh-ile simultaneously avoiding flame impingement on the tubes through which the .ibldbli Patented Dec. i3, i966 ICC fluid is flowing to reduce the possibility of forming hot spots.

Another object of the present invention is to provide a new and improved fired heater construction which has an initial cost much less than other fired heaters having tube banks or walls heated from only one side.

Other objects and advantages of the present invention will become more readily apparent from a consideration of the following description and drawings, wherein:

FIG. l is a top cross-sectional, schematic representation of the first embodiment of the invention;

FIG. 2 is a front cross-sectional, schematic representation also representing the first embodiment; l

FIG. 3 is a second embodiment of the invention similar to FIG. 2 but with banks of tubes placed in a different position; and

FIG. 4 is a third embodiment of the invention and similar to FIG. l illustrating a plurality of fired heater sections -with a common convection section -or unit.

Briefly, the present invention relates to a new and improved fired heater enclosure which greatly reduces cost of construction of fired heater enclosures because the radiant tube banks are heated from both sides which thereby produces a high uniform heat transfer rate and a combination of conventional burners and radiant type burners used in conjunction with a plurality of combustion chambers formed by the tube banks.

Attention is directed to FIG. l of the drawings wherein, as illustrated, the structure referred to as the fired heater enclosure is generally designated by the numeral 2. As illustrated in FIGS. l and 2, the fired heater enclosure comprises a fired heater section 3 having side or

end walls

3a, 3b, 3c, and 3d and bottom and top walls 3e and 3f, respectively. The fired heater section 3 is formed of suitable refractory materials such as fire brick or the like and is not critical to the present invention. Any suitable, well-known, and standard practice of forming the structure may be employed.

The red heater section 3, by reason of its closed side walls, bottom and top form the space referred to generally Iby the numeral 4. It will be noted that spaced rows of tubes, illustrated at 5, are provided in the space 4 to divide the space 4 into separate combustion chambers 6, '7, and 8. It is to be understood that the tubes in each row are joined at their ends and form one or more continuous fluid conducting paths in each row. Also, as illustrated in FIG. 2, chamber 6 adjacent side wall 3b; chamber 8 is adjacent side wall 3a; and chamber '7 is intermediate chambers 6 and 8.

On the side walls 3a and 3b is secured a plurality of radiant type burners 9. The radiant type burners 9 are of the type construction to spread th-e flames therefrom parallel to the Walls to avoid llame impingement on the plurality of stacked tube walls or tube banks 5. The radiant type, short flame burners 9 are suitable for a relatively small heat liberation per burner and preferably may be fueled with .gas or va clean fluid oil to insure rapid combustion. On each of the end or side Walls 3c and 3d, there `are provided a plurality of conventional type burners 10 which, as illustrated in FIGS. 1 and 2, are positioned to be immediately adjacent Ithe intenmediate combustion `chamber 7. The burners `10 are of the conventional type such as, for example, long flame burners for either gas or `fuel oil and should produce a llame length conductive lto uniform heat liberation along both of the tube banks S. The burners 10 do not cause flame impingement on the tube banks S since flue gas from the radiant type burners 9 in the outer combustion chambers 6 and 8 flows between the tube banks after combustion is completed .and into the intermediate combustion charnber 7. The combined fl-ue gases then flow up -into a convection section 1-2 which will be discussed hereafter.

- As mentioned above, the tube banks 5 are formed by tubes 11 which are joined at their ends to form continuous fluid conducting paths and which also form divider Walls between the

chambers

6, 7, and 8. The tubes 11 also serve to conduct fluid therethrough, which fluid is 'heated lby the burners 9 and 10 in the various chambers.

As illustrated in FIG. 2, the first embodiment is also provided with -a convection section or convection unit 12 which 'has been mentioned above and which acts as a common discharge for the gas passed therethrough -to be emitted from the stack 13. Bel-ow the stack 113 and in the space Iprovided for lthe convection -unit or convection section 12 is a plurali-ty of connected tube sections 12a for heating fluid therein by the stack gases. It can be appreciated that the convection tube section 12a as well as the tubes 11 may be connected lin series so that the fluid is first preheated in the convection section 12a and then conducted to tubes 11 rfor final heat. In some instances, convection section 12 may serve las a preheater for a fiuid stream which is conducted to tubes |11 for final heat. A spaced row of tube banks 5 may als-o be connected -to receive and heat separate fluid streams or process streams. For example, tube bank 5a may be connected to receive one fluid stream and tube bank 5b may be connected to receive a second fiuid stream,

The fired heater section 3 includes side and end walls 3a through 3d and bottom and top walls 3e and 3f of any suitable construction well known in the art. The combustion or heating 'gases are generated by the burners 9 and 10 and fiow through the chambers to accomplish their heating function and are then discharged at the top 3f through the convection section or -unit 12 and into the stack 13. As shown, the tube walls or tube banks 5 are formed by vertical tubes which are horizontally spaced or aligned, but they may also extend Ihorizontally and be aligned vertically if desired, as illustrated on FIG. 3.

Therefore, as illustrated in the first embodiment, each tube bank 5 is heated yfrom both sides to produce a uniform heat transfer rate around the periphery of the tubes. 'Fhis result produces a high average heat transfer rate and lallows -great economic savings in the cost of the tube banks. It is, of cofurse, well known that a single row of tubes heated from both sides has an average heat transfer -rate substantially lgreater than a row of tubes adjacent a Wall heated from one side only.

As illustrated in FIG. 3, the second embodiment is provided with a fired heater enclosure 102 which is illustrated as having two fired heater sections 1013 and 3'. Fired heater sections 103 and 103' have common outside, top, and -bottom walls with the side walls 3a and 3b and the bottom 3e illustrated in FIG. 3. The side walls 3a and 3b are provided with a plurality of radiant type burners 109 which operate substantially like lthe radiant type, short flame burners of the first embodiment. The second embodiment is also provided with a plurality of conventional type burners 110 which are illustrated as being secured and positioned in the fioor 3e. The fired heater section 103 is provided with one row of tubes or tube banks 10'5b Iand 105C. The fired heater sections -103 and 103' are separated by a center wall 3g which, when combined with the tube banks 105a, 105b, and 105C divides the fired heater enclosure 102 into four

combustion chambers

106, 107, 108, and 114 which perform the same function as the combustion chambers `6, 7, and 8 of the first embodiment. It should be readily apparent that the fired heater sections 103 and `103' are constructed in substantially the same manner as the heater section 3 of the first embodiment and that the tube bank 105a is runiformly heated from both sides which produces a greater heat transfer rate and that the double row of tube banks 1051) and `105e are provided when it is desired to heat an additional number of uid streams. It should also be noted that the conventional type burners 110 which are positioned in the bottom Wall 3d are randomly placed on eac'h side of the center wall 3g to provide heat for each of the tube banks a, 10511, and 105C.

As illustrated in FIG. 4, there is provided a third embodiment with a fired heater enclosure generally designated at 202, which is provided with fired heater sections 203 and 203'. It should be noted that each of the fired heater sections 203 and 203' is provided with a plurality of

tube banks

205 and 205 which are -formed in substantially the same way as the tube banks 5 of the first embodiment and, as illustrated, each of the fired heater sections is provided with four sets of the tube banks 205 and 205' with the tube bank sets 205e and 205]; lying substantially in a common plane, and the two tube bank sets 205e and 205d also lying substantially in a common plane. Of course, it is to be understood that the four sets of tube banks in fired heater section 203 are constructed in substantially the same manner las the tube sets and fire heater sections 203.

As illustrated in FIG. 4, the fired heater section 203 is provided with side walls 203a `through 203d'; and the fired heater section 203 is provided with side Walls 203a through 203d which are `formed substantially like the side walls of the first embodiment as illustrated in FIG. 1. Of course, it is to be understood that the fired heater sections 203 and 203' are further provided with a bottom wall (not numbered) and a top wall (not shown). Each of the fired heater sections 203 `and 203 are further provided with

chambers

206, 207, 208, 206', 207', and 208', respectively, which are formed by the four sets of tube banks y205 and 205', respectively, in each of the fired heater sections 203 and 203'. Each of the

combustion chambers

206, 207, 208, 206', 207', and 203' perform substantially .the same lfunction as the

combustion ohambers

6, 7, and 8 of the first embodiment.

Of course, each of the fired heater sections 203 and 203' are provided with radiant, short fiame burners 209 and 209', respectively, and are further provided with the conventional type burners 210 and 210', respectively, which perform the sazme function as the radiant, short flame type burners 9 and conventional type burners .10 of the first embodiment and are secured to side walls 203g, zosb, 20311', zosa', 203C, zioad, 203C', and zosd', respectively, in substantially the same manner as the burners of the first embodiment.

It is to be understood that additional fired heater sections may be provided if necessary for the heating requirements.

As illustrated in FIG. 4, the third embodiment is provided with `a common convection section or convection unit 212 which communicates with each of the fired heater sections 203e and 203d' and is immediately adjacent and partially formed by the

side walls

203b and 203a of the fired heater sections. Therefore, the third embodiment is capable of heating many fluid streams or process streams using the same convection unit 212 and is, of course, much cheaper in construction because of the common convection unit.

It should also be noted that the heating sections need not be arranged in opposed pairs but may be staggered if desired.

It will also be apparent that two radiant sections as shown in FIG. 1 may be aligned end to end with space therebetween for accommodation of burners 10 and a common convection section extended over the two radiant sections.

Broadly, the present invention relates to a fired heater enclosure which may consist of a plurality of fired heater sections with spaced apart tube banks therein adapted to be uniformly heated around the periphery thereof.

What I claim is:

1. 'Ihe combination of burners and tube banks in a Ifired heater enclosure having four side walls for controlled heating of process streams comprising:

(a) a pair of spaced horizontal tube banks, each of said banks having an inner and outer side and positioned in the enclosure parallel -to two of the side walls wherein said outer sides yof each of said pair of tube banks face one of each of the parallel side walls;

(b) said tube banks thereby forming three combustion chambers in the enclosure wherein one of said charnbers is adjacent one of the parallel side walls and the second of said chambers is adjacent t-he other of the parallel side walls and the third of said three chambers is intermediate said first and second chambers;

(c) a plurality of short fiame radiant type burners secured to both of the parallel side walls for heating said first and second chambers, respectively, and for heating said outer sides of each of said pair of parallel tube banks facing the parallel side walls, respectively; and

(d) a plurality of long flame burners mounted with the other two of the four side walls of the enclosure at spaced intervals, said long rialme burners being positioned adjacent said intermediate third chamber to enable said burners to thereby uniformly heat said third chamber and said inner side of each of said tube banks which thereby enables substantially uniform heating of said inner and outer sides of each of said tube banks which facilitates substantial unifonmity of heatingy thr-oughout said three chambers of the fired heater enclosure.

2. In a fired heater enclosure having four side walls for controlled heating of process streams comprising:

(a) two sets of a plurality of hollow tubes having their axes lying substantially in a c-ommon plane wherein each of said sets forms two walls of hori- Zontal tubes;

(b) each of :said walls of tubes having an inner and outer side and bein-g positioned in the enclosure parallel to two of the side walls wherein said outer side of each of said walls of tubes faces one of each of the parallel walls;

(c) said walls of tubes forming three combustion` chambers in the enclosure wherein one of said charnbers is adjacent one of the parallel side walls and the second of said chambers is `adjacent the other of the parallel side walls and the third of said three chambers is intermediate said first and second charnbers;

(d) a plurality `of short flame radiant type burners secured to both of the parallel side walls for heating said first and second chambers, respectively, and for uniformly heating said outer sides of each of said walls of tubes facing the parallel side walls of the enclosure, respectively; and

(e) a plurality of long flame burners mounted with the other two of the four side walls of the enclosure, said burners being positioned adjacent said intermediate Ithird chamber to thereby uniformly heat by said third chamber and said inner side of each of said walls of tubes, thereby enabling uniform heating of said inner and outer sides of each of said walls of tubes and said three chambers within the fired heater enclosure.

3. The combination of burners and tube banks in a fired heater enclosure having four side walls and a bottom wall for controlled heating of process streams comprising:

(a) Ithree sets of a plurality of hollow tubes having their axes lying substantially in a common plane, leach of said sets of hollow tubes forming three Ihorizontal tube banks; (b) two of said tube banks being positioned substantially adjacent each other to form a ydouble row of said tube banks;

(c) the third of said tube banks being substantially spaced from said double row of tube banks;

(d) said tube banks positioned in the enclosure parallel to two of the side walls and forming three heat combustion chambers in the enclosure wherein one of said chambers is adjacent one of the parallel side walls and the second of said chambers is adjacent the other parallel side walls and the third of said' three chambers is intermediate said rst and second chambers;

(e) a plurality of short flame -radiant type burners secured to each of the parallel side walls for uniformly heating said double row of tube banks and said third tube bank, respectively, and for heating said first and second chambers, respectively; and

(f) a plurality of long flame burners secured to the bottom wall of the enclosure and positioned to face said intermediate third chamber wherein said third chamber is uniformly heated by said burners and wherein the sides of said double row of tube banks and said third tube bank are uniformly heated which facilitates uniformity of heating throughout said three chambers of the fired lheater enclosure.

4. The combination of burners and tubes in a four sided, bottom walled fired heater enclosure for controlled heating of process streams comprising:

(a) three sets of a plurality of hollow tubes having their axes lying substantially in a common plane, each of said three sets thereby forming three tube banks;

(b) two of said tube banks positioned substantially adjacent each other to form a double row of tube banks;

(c) said third tube bank spaced from said double row of tube banks;

(d) said tube banks positioned in the enclosure parallel to two of the side walls;

(e) an intermediate Wall secured to the bottom of the enclosure and extending upwardly relative to the enclosure and positioned parallel to and between said double row tube bank and said third tube bank wherein four chambers are thereby formed in the enclosure, said first and fourth chambers adjacent one of each of the outer parallel said walls, respectively, and said second and third chambers intermediate therebetween .and further divided by said intermediate wall;

(f) a plurality of short fiame radiant type burners secured to each of the parallel side walls of the enclosure for heating said first and fourth chambers, respectively, and for heating the sides of said double row tube banks and said third tube bank facing said first and fourth chambers, respectively; and

(g) two sets of a plurality of long flame burners, each of said sets secured to the bottom wall and the other two of the four side walls of the enclosure and one of each of said sets positioned on each side of said center wall wherein said second yand third combustion chambers are thereby heated and the sides of said double row tube banks and said third tube bank adjacent said second and third combustion chambers, respectively, are thereby heated which facilitates heating of said first, second, third, and fourth combustion chambers of the fired heater enclosure.

5. The combination of burners and tubes for a plurality of four sided, bottom and top walled red heater sections for controlled heating of a plurality of process streams comprising:

(a) four sets of a plurality of hollow tubes positioned in each of the sections, each of said sets of said tubes having their axes lying substantially in a common plane wherein each of said four sets forms four spaced tube banks;

(b) each of said four tube banks being positioned par- .allel to two of the side walls of each of the sections in which each of said four tube banks are positioned and forming three combustion chambers in each of o and wherein the sides of each of said plurality of tube banks adjacent said intermediate sections are uniformly heated which facilitates uniformity of heating throughout said three chambers of each of the the sections wherein one of the chambers is adjared heater sections; and

cent the other parallel side walls, and the third of (e) a common convection unit secured to the top of said three combustion chambers is intermediate said each of the fired heater sections, said concevtion unit first and second chambers; forming an opening in the top of each of the fired (c) a plurality of short flame radiant type burners heater sections to permit removal of the gas from secured to the two parallel side walls in each of the 10, each of the sections.

sections for heating said first and second chambers, respectively, and for heating the sides of each of said four tube banks facing the adjacent parallel side walls in each of the sections;

(d) a plurality of long ame burners mounted with 7. The combination of burners and tubes for a plurality of four sided, bottom and top walled fired heater sections for controlled heating of a plurality of process streams comprising:

(a) a plurality of sets of a plurality of hollow tubes the other two of the four sided wall of each of the sections and positioned adjacent said intermediate third chamber wherein said third chamber is uniformly heated by said conventional burners, and

positioned in each of the sections, each of said sets of tubes forming spaced horizontal tube banks;

(b) each of said tube banks being positioned substantially parallel to two of the side walls of each of wherein the sides of each of said four sets of tube the plurality of sections in which each of said tube banks adjacent said intermediate section are unibanks is positioned and forming three combustion formly heated which facilitates uniformity of heatchambers in each of the sections wherein one of the ing throughout said three chambers of each of the chambers is adjacent one of the parallel walls, the fired heated sections; and second of said chambers is adjacent the other parallel (e) a common convection unit secured to the top of side walls, and the third of said three combustion each of the tired heater sections, said convection unit chambers is intermediate said first and second chamforming an opening in the top of each of the tired bers; heater sections to permit removal of the gas from (c) a plurality of short flame radiant type burners each of the sections. secured to the two parallel side walls in each of the 6. The combination of burners and tubes for a plurality sections for heating said first and second chambers,

of four sided, bottom and top walled fired heater secrespectively, and for heating the sides of each of tions for controlled heating of a plurality of process said tube banks facing the adjacent parallel side streams comprising: walls in each of the sections;

(a) a plurality of sets of a plurality of hollow tubes (d) a plurality of long ame burners secured to one positioned in each of the sections, each of said sets of the other two walls of each `of the sections and of said tubes having their axes lying substantially positioned adjacent said intermediate third chamin a common plane and each of said plurality of ber wherein said third chamber is uniformly heated sets forms -a pluarlity of spaced horizontal tube banks; by said conventional burners and wherein the sides (b) each of said plurality of tube banks being posi- 0f each of said plurality of tube banks adjacent said tioned parallel to two of the side walls of each of the intermediate section are uniformly heated which sections in which each of said plurality of tube banks facilitates uniformity of heating throughout said is positioned and thereby forming three combustion three Chambers 0f Cach 0f the fed heater Sections; chambers in each of the sections wherein one of the and chambers is adjacent one of the parallel walls, the (e) a common convection unit secured to the top of second of said chambers is adjacent the other pareach of the fired heater sections and positioned adjaallel side walls, and the third of said three combuscent the opposite wall in each of the fired heater section chambers is intermediate said first and second tiOnS from Which the plurality of said conventional chambers; burners are secured, said convection unit forming (c) a plurality of short flame radiant type burners an opening in the top of each of the fired heater secured t0 the two parallel Side walls in each 0f the 50 sections for removing gases from each of the sections.

sections for heating said first and second chambers, respectively, and for heating the sides of each of said plurality of tube banks facing the adjacent parallel side walls in each of the sections;

References Cited by the Examiner UNITED STATES PATENTS (d) a plurality of long flame burners mounted with 3003481 10/1961 Throckmorton et al' 122-356 the other two of the four side walls of each of the 3105467 10/1963 Gnth 122-356 3,182,638 5/1965 Lee et al. 122--240 sections said burners being positioned adjacent said intermediate third chamber wherein said third chamber is uniformly heated by said conventional burners ENNETH W SPRAGUE Prlmmy Examiner'

Claims

1. THE COMBINATION OF BURNERS AND TUBE BANKS IN A FIRED HEATER ENCLOSURE HAVING FOUR SIDE WALLS FOR CONTROLLED HEATING OF PROCESS STREAMS COMPRISING: (A) A PAIR OF SPACED HORIZONTAL TUBE BANKS, EACH OF SAID BANKS HAVING AN INNER AND OUTER SIDE AND POSITIONED IN THE ENCLOSURE PARALLEL TO TWO OF THE SIDE WALLS WHEREIN SAID OUTER SIDES OF EACH OF SAID PAIR OF TUBE BANKS FACE ONE OF EACH OF THE PARALLEL SIDE WALLS: (B) SAID TUBE BANKS THEREBY FORMING THREE COMBUSTION CHAMBERS IN THE ENCLOSURE WHEREIN ONE OF SAID CHAMBERS IS ADJACENT ONE OF THE PARALLEL SIDE WALLS AND THE SECOND OF SAID CHAMBERS IS ADJACENT THE OTHER OF THE PARALLEL SIDE WALLS AND THE THIRD OF SAID THREE CHAMBERS IS INTERMEDIATE SAID FIRST AND SECOND CHAMBERS; (C) A PLURALITY OF SHORT FLAME RADIENT TYPE BURNERS SECURED TO BOTH OF THE PARALLEL SIDE WALLS FOR HEATING SAID FIRST AND SECOND CHAMBERS, RESPECTIVELY, AND FOR HEATING SAID OUTER SIDES OF EACH OF SAID PAIR OF PARALLEL TUBE BANKS FACING THE PARALLEL SIDE WALLS, RESPECTIVELY; AND (D) A PLURALITY OF LONG FLAME BURNEERS MOUNTED WITH THE OTHER TWO OF THE FOUR SIDE WALLS OF THE ENCLOSURE AT SPACED INTERVALS, SAID LONG FLAME BURNERS BEING POSITIONED ADJACENT SAID INTERMEDIATE THIRD CHAMBER TO ENABLE SAID BURNERS TO THEREBY UNIFORMLY HEAT SAID THIRD CHAMBER AND SAID INNER SIDE OF EACH OF SAID TUBE BANKS WHICH THEREBY ENABLES SUBSTANTIALLY UNIFORM HEATING OF SAID INNER AND OUTER SIDES OF EACH OF SAID TUBE BANKS WHICH FACILITATES SUBSTANTIAL UNIFORMITY OF HEATING THROUGHOUT SAID THREE CHAMBERS OF THE FIRED HEATER ENCLOSURE.