US2898892A - Heater - Google Patents

Description

Aug. 11, 1959 O. CAMPBELL ET AL HEATER 4 Sheets-Sheet 1 Filed July 22, 1957 INVENTORS Oliver Campbell a Nqrman E. Pennels ATTORNEYS o. CAMPBELL ETL 2,898,892

Aug. 11, 1959 HEATER 4 Sheets-Sheet 2 Filed July 22, 1957 INVENTORS Oliver Campbell BY Normon E. Pennels ATTORNEYS Augo 1959 o. CAMPBELL ETAL 2,898,892

HEATER 4 Sheets-Sheet 3 Filed July -22 1957 IN VENTORS O r m mm %WQ Ma %QA 1959 o. CAMPBELL ETAL 2,898,892

HEATER 4 Sheets-Sheet 4 Filed July 22, 1957 INVENTORS Oliver Campbell BY Norman E. Pennels l 2,898,892 HEATER Oliver Campbell, Whiting, Ind., and Norman E. Pennels, Lansing, Ill., assignors to Sinclair Refining Company, New York, N.Y., a corporation of Maine Application July 22, 1957, Serial No. 673,391 '5 Clains. (Cl. 122-356) Our invention relates to heating fluid materials and in particular provides a heater for a variety of materials commonly encountered in oil refinery service, such as fixed petroleum gases, hydrogen, gas oil and residual ols.

It is a principal object of our invention to provide an oil refinery heater of the tubular type having a direct fired, radiant heating section in which the heat flux distribution about any given heating tube, particularly at the high temperature end of each group of serially interconnected heating tubes is substantially uniform.

It `is a further obejct of our invention to provide an oil refinery heater providing maximum utilization of the space within the furnace shell for the location of double fired heating tubes.

It is still another important object of our invention to provide a 'heating tube positioning arrangement in a refinery heater permitting direct interconnecton of a group of freely supported, serially interconnected radiantly heated tubes with a group of freely supported, serially interconnected convection heated tubes by which differences in expansion of the hotter, radiantly heated tubes and the cooler, convection heated tubes are accommodated.

These and other objects of our invention are basically obtained utilizing a vertical, cylindrical heater construction having an annular, upshot radiant heating furnace section with a central flue extending downwardly through such heating section and in which there are a plurality of upshot bumers spaced about the floor of the radiant heating section. In our heater we employ serally interconnected, vertically positioned heating tubes which extend in single rows in the annular furnace section radially from the flue between adjacent pairs of burners as spokes extend from the hub of a wheel thus providing double ring of each of the heating tubes and assurng minimum ratios of heat release to heating capacity. Each radial row, i.e., bank, of heating tubes can at its inner end include a bank of one or more single fired vertical heating tubes located adjacent to the Wall of the flue and is connected at its inner, inlet end in the upper part of the furnace above the flue -to the outlet end of a bank of one or more vertical heating tubes which extend into the flue where it or they are heated by convection of the flue gases. The heating tube or tubes which are located in the flue are thus utilized for preheating to reduce the number of tubes in any single pass through the heater which must be located in the radiantly heated furnace sections thereby requiring only a single radial bank of double fired tubes in each pass and avoiding the necessity of interconnecting several of the radial banks in a single pass.

In a more specific aspect we contemplate supporting all heating tubes, both those in the convection section and those in the radiant section, only at the bottom of the tubes thus permittng expanson of the tubes as they are heated to take place in an upward direction. By making the outlet tubes in the convection section longer than their associated tubes in the radiant furnace section, that is, by supporting the outlet connection tubes at a level below the floor of the annular radiantly heated furnace section and by connecting the inlet tube of each radiantly heated bank at its upper end to the upper end of the associated outlet convection tube the fact that the convection tube will be heated to a lower temperature and hence will expand proportionately less than their associated tates Patent O tubes in the furnace sections is partly obviated by the greater length of the outlet convection heated tube which is connected to the direct fired bank and partly obviated by the fact that the tubes are free at their upper ends and hence can move laterally as well as vertically to accommodate any actual difference in their total expansions.

For a more complete understanding of the practcal application of the principles of our invention reference is made to the appended drawings in which: I

Figure l is a vertical section of an oil refinery heater constructed in accordance with our invention taken at line 1-1 in Figure 2;

Figure 2 is a cross-section of the heater shown in Figure 1 taken at line 2-2 of Figure l;

Figure 3 is a partially sectioned, plan View of the heater shown in Figure l with the sectioned portion taken along line 3 3 in Figure 1;

Figure 4 is an enlarged, fragmentary sometric view of a portion of the heater shown in Figure 1;

Figure S is an enlarged fragmentary oblque View of another portion of the heater shown in Figure 1;

Figure 6 is a cross-section similar to Figure 2 illustrating a modification of the heater shown in Figure 1; and

Figure 7 is a fragmentary enlarged isometric view of 'another modification in Construction of a heater shown in Figure 1.

Referring to the drawings the reference numeral 10 generally desgnates a cylindrical, upshot oil refinery heater which includes a cylindrical, vertical steel shell 11 completely lined with a layer 12 of refractory brick. Shell 11 is positioned inside of four supporting steel columns 13 attached to shell 11 at intervals and mounted on concrete footings 14. Positioned coaxially within shell 11 is a cylindrical upper flue section 15 constructed of refractory material which terminates at its open upper end 16 in the upper portion of shell 11 substantially below the upper end of shell 11. Flue section 15 is supported on four steel columns 16 mounted on concrete footings with its lower end at approxmately the same level as the lower end of shell 11. Each adjacent pair of columns 16 and 13 have'afl'lxed between them horizontal girders 17 which support a refractory floor 18 of annular shape and which extends across the lower end of shell 11 closing the lower end of shell 11 upon the lower end of 'upper flue section 15.

Upper flue section 15 is attached at its lower end to the upper end of a lower flue section 19, also of generally cylindrical shape and constructed of refractory material. Lower flue section 19 extends downwardly between columns 16 between which it is supported by crossed horizontal girders 20 affixed between diagonally opposite columns 16 at their lower ends. The lower end of lower flue section 19 is enlarged to one `side to a sli ghtly larger diameter just above the bottom of section 19 as indicated by the reference numeral 21 (see particularly Figures 1 and 3). On its side opposte enlarged portion 21, lower ue section 19 opens into a horizontal duct 22 of generally rectangular cross-section which leads laterallyinto an elbow box 23, thence rearwardly to a waste heat boiler 24 of conventional Construction and to exhaust blower 25 driven by a motor 26 which vents into a stack 27.

It Will be noted that the enlargement 21 of the lower portion of lower flue section 19 and the horizontal position of duct 22 beneath heater 10 necessitates passing certain of girders 13 and 16. Since the temperature of the exhaust gases carried through duct 22 is still sufliciently high to recover useful heat, by waste heat boiler 24 for example, it is necessary to protect such girders 13 and 16 from the heat where they are required to extend through lower flue section 20 and duet 22. We have found particularly where I- or H-columns are employed, that such heat protection is best atforded by building a 3 chimney 28 of refractory brick enclosing the column 16 or 13 through the height of duct 22 leaving open the space between the flanges of the column such that air freely can convect'upwardly in such spaces assisting'in cooling the column. i The general' structure of heater -104is completed by a refractory, roof 29 closing the upper end of shell 11 which suitably is supported-by and beneath steel girders 30 extendng between columns 13 and other horizontal cross members indicatedjschematically in the drawing. Centrally roof 29 is provided with a vertical, cylindrical steel port 31 supported' asa hub at the inner ends of horizontal girders 30 and which'is provided with a refractory lining 32. A manhole cover 33; rests over port 31 and is similarly provided with an inner refractory lining 34.

- v Referring more' particularly to Figures 1 and 2, it will be observed that eight burners 35 are located in floor 18, generally equally disposed 'between upper flue section 15 andlining 12 of shell 11 and spaced at equal arcuate intervals (45 degrees) apart concentrically with annular floor 18. Each-burner 35 includes an air register 36 on the under side of floor 18 which can have any conventio'nal form and which can be connected to air preheating systems and the like, if desired. Above each register 36 a conical ho'le 37 is' cut in refractory floor 18 thus defining a nozzle for burner 35. A fuel pipe 38 connected to a suitable fuel system and valves of conventional Construction is brought through air register 38 upwardly to the center of conical opening 37 where pipe 38 terminates. Thus gas or other fuels delivered through pipe 38 when ignited are fired upwardly into the annular furnace chamber 40 formed in heater 10 above floor 18 between lining 12 of shell 11 and refractory upper flue section 15. Additional burners 41 can be mounted in shell 11 above each of burners 35 to provide auxiliary heat release for the upper furnace section 42 which is defined within lining 12 of shell 11 between roof 29 and the upper end 16 of upper flue section 15.

Heating tubes are connected in eight separate passes .through heater 10 leading from a common header 43 to a common outlet pipe 44.

A Header 43 desirably is located adjacent to the enlarged side 21 at the base of lower flue section 19 and is con- .nected to the eight passes through heater 10 by separate valves 45 which lead to inlet pipes 46 which extend through enla'rged portion 21. A number of heating tubes 47 which stand vertically are packed into the flue gas well 48 defined in flue sections' 15 and 19 and are supported at their lower ends in refractory saddles 49 which -rest on a platform 50 supported horizontally in the base of lower flue section 19 upon legs '1 which rest at their lower ends on girders 20. Platform 50 thus holds the lower ends of tubes 47 at about the middle level of duct 22. Because lower flue section 19 is enlarged at its base -21 opposite duct22 and because platform 50 is elevated, 'flue gas traveling downwardly through flue gas well 48 is prevented from channeling at the lower ends of tubes 47 toward duct 22. Thus the flue gases are distributed equally at the base of tubes 47 passing in part directly to duct 22 and in part through enlarged base portion 21 and thereafter under platform 50 into duct 22.

Heating tubes 47 are serially nterconnected in eight `separate banks A, each including an inlet tube 47' and an outlet tube 47". Each of inlet pipes 46, as will' be seen more clearly in Figure 3, extends beneath platform 50 .and is turned upwardly to connect with an inlet heating tube 47'. Each of the heating tubes 47 in each serially nterconnected bank A extends entirely in flue gas we'll 48,

.except that in each serially nterconnected bank A the out- -let -heatng tube 47" extends upwardly 4 heating tube 47 which is to be linked to an adjacent tube similarly provided with a U -bolt 53. Each pair of adjacent U-bolts 53 carrying -between them a linking ring 54 which passes through U-bolts 53 with substantial clearance. The stack of tubes 47 are thus held in their positional relationship to each other *but are free to. move laterally within limits defined by the' clearances of rings 54 in their associated U-bolts 53. V

An additional number of serially nterconnected vertical heating tubes 55 are positioned in annular furnace section 40- resting .at the joined lower ends 55a of adjacent pars in saddles- '56 formed in refractory flooring 18. Tubes 55 also eXtend at their upper ends into upper furnace section 42and are arranged in eight serially interconnected groups, each group being disposed in a radial bank B consisting of a single''ow of tubes 55 extending from upper flue'section 16 outwardly toward shell 11 and ashort arcuate bank C of asingle row of tubes 55 disposed adjacent to the outer wall of upper flue section 15.

The inlet tube 55' of each serially nterconnected group of tubes 55 including a bank B and a bank C is located at the end of bank C remote from its associated bank B and extends upwardly to approximately the same level as the outlet pipe 47" of a bank A of convection section heating tubes 47 towhich tube 55' is connected at its upper end'. Thus the outlet tube 55" of each serially nterconnected group off-heating tubes 55 is the outermost heating tube in the bank B of that group. The number of heating tubes 55 is chosen as an odd number and consequently the outer tube 55" terminates near floor 18 of annular furnace section 40 through which it is connected by an outlet pipe 57 which is in turn connected to common outlet pipe 44 which is Secured to columns 13 and encircles heater 10.

Desirably, adjacent pairs of heating tubes 55 which are connected at their upper ends are linked together by ,links 58 having similar-Construction to links 52. Additionally, it is desirable to link every other pair of heating tubes 55 which are connected at their upper ends by is afixed in roof 29. Thus swinging movement as well as vertical expanson of the -upper ends of heating tubes 55 is permitted within limited amounts.

Each bank B of heating tubes 55 is further arranged such that the vertical plane passing through the axis of the heater 10 nwhich the bank B approximately lies passes medially between a pair of adjacent burners 35 and therefore also medially between a pair of adjacent burners 41. Accordingly, it will be evdent that when burners 35 and 41 are Operating to fire annular furnace section 40, each of the heating tubes 55 in banks B are fired equally from both sides and receive substantially uniform heating. Additionally the tubes 55 in banks C are also radiantly heated by direct firing but on one side only and accordingly receive as in the conventional Construction of furnaces of this type an uneven heat flux pattern. Since the fluid being heated in each pass through heater 10' first. includesconvection heated tubes 47, then single fired radiantheated tubes 55 in banks C and finally double fired radiant heated tubes 55 in banks B, the fact that bank C-heating tubes 55 receive an uneven heat flux pattern is of not a serious consequence as the material being heated at this point does not reach temperatures at which uneven heating could cause such problems as coking and 'the like. It`wll he further observed that at the temperature endof each pass through the furnace substantially uniform heat flux patterns are provided, since the high temperature end of each pass includes the radial banks B which are double fired, and hence coking and other problems attributed -to neven heat flux are minimized. Banks C can be omitted, but are desirable since they are set at angles to their respective tube banks B and provide a more stable tube structure.

Furnace is generally operated in a conventional manner. Thus suitable fuel is supplied to burners 35, and burners 41 if additional heat in the upper section is desired, causing the burners 35 to fire upwardly into annular section 40 and burners 41, if these are also employed, to fire across annular section 40 up into upper section 42. The material to be heated, which can be gas oil, residual oils, hydrogen gas, or almost any other fluid material, is introduced through header 43 and as indicated above flows serially through the eight passes in parallel, first passing through the convection heating tubes 47 in flue gas well 48, then through the single fired radiant heated tubes 55 in the banks C, and lastly through the double fired radiant heated tubes in the banks B, thence into the common outlet pipe 44 from which they are passed to a subsequent refinery operation. Flue gases of course pass upwardly through annular furnace section 40 inwardly toward the center of upper furnace section 42 and then downwardly through flue gas well 48 from which they are withdrawn to stack 27 by blower 25 :through duet 22, box 23 and waste heat boiler 24.

As suggested above tubes 55, because of their location in radiantly heated furnace sections 40 and 42 are heated during operation of heater 10 to substatially higher temperatures than tubes 47 located in convection heated flue gas well 48. Tubes 55, consequently, expand upwardly as they are heated to Operating conditions proportonately greater than tubes 47. The ratio of lengths of inlet tubes 5'5' and of outlet tubes 47" is, accordingly, chosen to approximate the ratio of the temperature differences to which the respective tubes are subjected in warm up to typical Operating condition, such that their respective total expansions will be approximately equal. Since actual service conditions can only be estimated in advance and since a given heater 10 may desirably be employed in different service from time to time some difference in total expansion of tubes 55' and 47" is to be anticipated. This difierence is tolerated in our heater construction by supporting all heating tubes in fixed position at their lower ends only and by securing the tubes at their upper ends in a manner permitting all tubes to move laterally at their upper ends within limited distances. Thus warping of adjacent tubes, even of tubes 47" and 55', can be tolerated, tube fractures reduced, and tube life prolonged.

Referring more particularly to Figure 6 we have illus` trated a modification of a heater designed particularly to accommodate vaporization or partial vaporization of a liquid material in the double fired heating tubes. In Figure 6 substantally the same constructcn is employed and for the sake of Simplicity where the reference numerals correspond to those used with reference to the preceding figures the identical Components are contemplated. Heater 100 shown in Figure 6 diifers from heater 10 of the preceding figures only in the omission of side 'wall burners 41 and in the construction and arrangement of heating tubes. In heater 100 flue gas well 48 is provided with a cylindrical battle 101 positioned vertically and coaxially within flue gas well 48 extending entirely through the length of well 48 up into upper furnace section 42. The purpose of bafiie 101 is to increase the speed of flue gases over convection heated tubes 102 which are located in flue gas well 48 otherwise essentially in the same manner as tubes 47. Thus an inlet heater tube 102,' is connected to header 43 as described above and extends vertically ypward in flue gas well 48 and is connected at its upper end to the next vertical tube 102 through a sequence in turn of thirteen tubes 102 in each convection bank of serially interconnected tubes 102 with the outlet convection heated tube 102" extended upwardly into upper furnace section 42. Each tube 102" is connected at its upper end with the inlet tube 103' of a radial bank D of a single row of serially interconnected vertical heating tubes including an inner group of five tubes 103 having a diameter essentially the same of that convection heated tubes 102 and an outer group of six tubes 104 of substantially larger diameter. The outlet tube 104" of each bank D is connected at its lower end to an outlet pipe 105 of even larger diameter which, like the outlet pipes 57 in the preceding figures, is connected to common outlet pipe 106.

Thus the material to be heated passes through heater 100 including an initial series of small diameter pipes 102 which are located in a convection heated section in well 48, a second group of double fired small diameter tubes 103 in the radiant heated section of heater 100 and finally a third group of double fired large diameter tubes 104 in the radiant heated section. Suitably heater 100 can be employed as a tube still for preheating the feed to a vacuum flashing unit in which partial vaporization of the residual feed stock prior to flashing is accommodated by the larger diameter heating tubes 104. Since a typical vacuum flash unit desirably also includes steam stripping we provide in the modification of Figure 6 a single pass of six larger diameter tubes 107 which are positioned vertically and serially interconnected in flue gas well 48 adjacent to convection heated tubes 102. and adjacent to baie 101. Tubes 107 suitably are employed to superheat stripping steam for the vacuurn flashing unit.

` In the preceding discussion of the drawings little has been said of the phyical Construction of the heater tubes since these are fabricated in the conventional manner and of conventional high temperature alloys, and are similar to those ordinarily use in refinery heater service. Thus the convection heated tubes desirably are provided with longitudinally extended fins of number and size such-that each of the convection heated tubes lies substantially tangent to the next of its serial interconnected band and tagent to those of adjacent banks. One feature, however, of the radiantly heated tubes should be ponted out. Where design considerations indicate a larger number of tubes is a radiantly heated bank B than can be ordinarily accommodated using standard size return bends the arangement of Figure 7 is recommended which permits' adjacent double fired tubes in a given radial bank to be located as close to each other as is required without staggering which introduces uneven heat flux distribution.

Thus in Figure 7 the upper end 55b of a pair of adjacent heater tubes 55 located in a bank B is replaced by the Construction 110 shown in Figure 7 in which a pair of adjacent tubes 55 are connected at their upper ends by construction 110. Construction 110 essentially con sists of a semi-circular standard return bend 111 which is disposed in a vertical plane set at an angle to the vertical radial plane of tubes 55. Return bend 111 is connected at its ends by angularly disposed tube sections 112 to the upper ends of the adjacent tubes 55. Connecting sections 112 have lengths determined by the relative difference in diameter of bend 110 and spacing of pipes 55, since sections 112 suitably turn into return bend 111 and into heater tubes 55 with turns of the same radius as standard return bend 111.

It will be evident from the preceding description that the oil refinery heater of our invention is adapted for a variety of refinery heating problems because of its accommodation to a wide range of outlet temperatures of the fluid being heated. It will also be evident that our heater provides substantial economies in saving of eX pensive, high temperature alloy heating tubes partly because of reduced tube stress due to expansion, partly because double firing at the high temperature end decregses 1 tube temperature for a given outlet temperature of the material being heated, and partly because double firing reduces thetotal tube area required to reach a given outlettemperature.

We claim:-

1. In an oil refinery heater including a vertical, cylindrical furnace shell, a roof positioned across and closing the upper end of said' shell, a vertical, cylindrcal flue positioned centrally within said furnace shell, said flue terminating at the upper end thereof in said shell below said roof and terminating at the lower end thereof below the lower end of said furnace shell, an annular floor positioned across the lower end ofsaid shell closing the lower end of said shell to said flue, whereby said heater includes an annular furnace space defined above said floor between said flue and said, shell, an upper furnace space defined within said shell between said roof and the open upper end of said flue, and a flue gas well' defined within said flue and having a lower portion beneath the level of said floor, a plurality of burners mounted in said annular floor spaced apart at arcuate intervals thereabout between said furnace shell and said flue for directing fuel and air upwardly into said annular furnace space and means for withdrawing flue gas from the lower portion of said flue gas well; the improvement which includes a bank of a single row of serially interconnected heating tubes, each said tube in said bank being disposed vertically extending through said annular furnace space into said upper furnace space, said bank of heating tubes being disposed approxinately in a radial plane within said. cylindrical shell passing medially between an adjacent pair of said burners, a heating tube disposed vertically within said flue gas well interconnected at the upper end thereof in said upper furnace space to the upper end of the. innermost tube of said radial bank, said heating tube within said flue gas well having a greater length than said innermost tube of said radial bank, and means for passing a fluid to be heated serially first through said heating tube disposed within said flue gas well and then outwardly through said radial bank of heating tubes.

2. In an oil refinery heater including a vertical, cylindrical furnace shell, a roof positioned across and closing the upper end of said shell, a vertical, cylindrical flue positioned centrally within said furnace shell, said flue terminating at the upper end thereof in said shell below said roof and terminating at the lower end thereof below the lower end of said furnace shell, an annular floor positioned across the lower end of said shell closing, the lower end of said shell to said flue, whereby said heater includes an annular furnace space defined above said floor between said flue and said shell, an upper furnace space defined: within said shell between said roof and the open upper end of said flue, and a flue gas well defined within said flue and having a lower portion beneath the level of said floor, a plurality of burners mounted in said annular floor spaced apart at arcuate intervals thereabout between said furnace shell and said flue for directing fuel and: air upwardly into said annular furnace space, and means for withdrawing flue, gas from the, lower portion of said flue gas well; the improvement which includes a plurality of banks of, single rows of serially interconnected heating tubes, each said tube in said banks being disposed vertically extending through said annular furnace, space into said upper furnace space, each said bank of heating tubes being disposed approxmately in a different radial plane in said cylndrical, shell, each said plane passing medially between different adjacent pair of said burners, a plurality of heating tubes disposedvertically within said, flue gas; Well, each said tube in said well being nterconnected. at the upper end thereof in said upper furnace space to the upper end of the innermost, tube of a different one of said radial banks, said heating tubes within' said flue gas well having a greater length than saidinnermost tubes of said radial banks to 8 whichthey arerespectively connected, and means f! passing a fluid to be heated serially first in Parallel throughsaid heating tubes disposed within said flue gas well and then in parallel outwardly through said radial banks of heating tubes( 3. The improvement according to claim 2 which further includes a plurality of banks of serially interconnected vertical heating tubes disposed in said furnace spaces adjacent to the outer wall of said flue, each said last named bank being positioned inside of a different one oflsaid plurality of burners between an adjacent pair of said radial banks of heating tubes and being interconnected between the innermost tube in one of said adjacent radial banks and the tube associated therewith in' said flue gas well.

4. The improvement according to claim 2 which further includes a plurality of burners mounted in said furnace shell, each positioned to direct fuel and air into the space between different adjacent pair of said radial banks of said heating tubes above one of said burners insaid floor.

5. In an oil refinery heater including a vertical, cylindrical furnace shell, a roof positioned across and closing the upper end of said shell, a vertical, cylindrical flue positioned centrally within said furnace shell, said flue terminating at the upper end thereof in said shell below said roof and terminating at the lower end thereof below the lower end of said furnace shell, an annular floor positioned across the lower end of said shell closing the lower end of said shell to said flue, whereby said heater includes an annular furnace space defined above said floor between said flue and said shell, an upper furnace space defined within said shell between said roof and the open upper end of said flue, and a flue gas well defined within said flue and having a lower portion beneath the level of said floor, a plurality of burners mounted in said annular floor and spaced apart at arcuate intervals thereabout between said furnace shell and said flue for directing fuel and air upwardly into said annular' furnace space, and means for withdrawing flue gas from the lower portion of said flue gas well; the improvement which .includes a plurality of radial banks of single rows of serially interconnected heating tubes, each said tube in said banks being disposed vertically and extending through said annular furnace space into said upper furnace spaces, each said bank of heating tubes being disposed approxirnately in a difierent radial plane in said cylindrical shell, each said plane passing medially between different adjacent pair of said burners, a plurality of heating tubes disposed vertically within said flue gas Well, each said tube in said well being intercon.- nected' at the upper end thereof in said furnace space to the, upper end of the innermost tube of a different one of said radial banks, said heating tubes within said flue gas well having a greater length than said innermost tubes of said radial banks to which they are respectively connected, said tubes in said radial banks being firmly supported in said floor at the lower ends thereof and spaced beneath said roof at the upper ends of said tubes to permit limited upward expansion thereof, said heating tubes in said flue gas well being supported at lower ends thereof in said lower portion of said well and spaced at upper ends of said tubes beneath said roof to pernit limited expansion thereof, and means for passing a fluid to be heated serially first in paraJlel through said heating tubes disposed within said flue gas well and then in parallel outwardly through said radial banks of heating tubes;

References Citerl in the file of this patent UNITED STATES PATENTS 2,211,903 McCarthy Aug. 20, 1940 2,745,388 Becker May' 15, 1956 2,751,893 Permann June 26, 1956

Images