US2601667A - Tube heater with flue gas recirculation and heating method - Google Patents

Description

June 24, 1952 K. PERMANN 2,601,667

TUBE HEATER WITH FLUE GAS RECIRCULATION AND HEATING METHOD Filed Feb. 12, 1948 5 Sheets-Sheet l INVENTOR KARL PERMANN BY W/v WM HIS ATTORNEY June 1952 K. PERMANN 2,601,667

TUBE HEATER WITH FLUE GAS RECIRCULATION AND HEATING METHOD Filed Feb. 12, 1948 3 Sheets-Sheet 2 INVENTOR: KARL PERMANN HIS ATTORNEY June 24, 1952 K. PERMANN 2,601,667

TUBE HEATER WITH FLUE GAS RECIRCULATION AND HEATING METHOD Filed Feb. 12, 1948 s Sheets-Sheet 5 36 ;E I1 I35 IOZ FIG. 3

IMVENTOR KARL PERMANN HIS ATTOR NEY TUBE HEATER WITH FLUE GAS RECIRCULA- TION AND HEATING METHOD Karl Permann, Oakland, Calif., assignor to Shell Development Company, San Francisco, Calif.,

Patented June 24, 1952 a corporation of Delaware Application February 12, 1948, Serial N 0. 7,987 15 Claims. (01. 122-275) This invention relates to improvements in combustion furnaces for heating fluids flowed through tubes, and to an improved method of heating such fluids, wherein the heating tubes are arranged flue gas is withdrawn from the heater, insuch relation to the Venturi constriction as to facilitate regulation of the recirculated gas.

Still further objects of the invention will be within a heating space so as to cause hot com- 5 apparent from the following description, taken in bustion gases to flow longitudinally with respect connection with the drawings forming a part of to the heating tubes, and wherein the combustion this specification, wherein: ases are recirculated. The heater and method Fig. 1 is a vertical sectional view of a preferred according to this invention are, for example, parembodiment of the invention, parts being shown ticularly useful in the carrying out of endothermic in el vati n; chemical reactions, such as the dehydrogenation Fig. 2 is a sectional view taken on line 2-2 of of alcohols or other gaseous hydrocarbons which Fig. 1; takes place at about 1000 F. Other examples are Fig. 3 is a vertical sectional view showing a the conversion of ethylene dichloride into vinyl modified construction; and chloride and hydrochloric acid at about 960 F" Fig. 4 is a sectional view taken on line 4-4 of the cracking of hydrocarbon oil, and catalytic de- Fig, 3, hydrogenation processes. Briefly, according to the instant invention com- In heaters of this class difficulty is frequently bustion gases are recirculated within the enexperienced because of uneven heating of the a closure of the heater proper by means of the jet tubes, resulting in local overheating. It is aleffect of a gas, such as saturated or superheated ready known to provide a more gentle hea steam, air, or combustion gases which have been by recirculating the flue gases, but this has recompressed. I prefer to inject steam as the jet sulted in operating difficulties. With the heaters gas to avoid formation of an oxidizing atmosheretofore known the use of steam injectors for phere, and because it is readily available in fureffecting circulation was not satisfactory because nace installations. However, the invention is not the primary and recirculated combustion products were not always completely mixed and different tubes were not heated equally.

It is an object of this invention to provide an improved tube heater and an improved method of heating flowing fluids wherein heat of substantially uniform intensity is distributed among the several heating tubes, and the tubes are heated substantially evenly alongtheir lengths.

It is a further object to provide an improved tube heater and an improved method of heating flowing fluids wherein recirculation of combustion products or flue gas is induced by the injection of a gas, such as steam, in a flow arrangement which will insure substantially even distribution of heat to the several heating tubes. Ancillary thereto, it is anobject to provide an improved method of heating and an improved heater wherein the flue gas recirculation rate can be easily controlled.

Still another object is toprovide a heater having a combustion chamber fitted with one or more Venturi constrictions downstream from the burner or other source of combustion gases and having a nozzle for expanding and discharging a gas,

such as steam or air or combustion gas, into the.

Venturi throat in -'a manner to induce circulation of the flue gas through the combustion chamber, in combination with flow passage ways limited to the use of steam and any gas, including air, may be used as the jet gas. The jet gas is ejectedfrom one or more expanding nozzles and directed as one or more jets through one or more Venturi throats, thereby entraining combustion gases. The hot combustion gases, from any source, such as a liquid fuel burner, are commingled with the recirculated flue gas after the latter has passed in contact with the heating tubes and before flowing through the Venturi throat. The stream of hot combustion gases is therefore, successively augmented with recirculated flue gas and with jet gas and, as a result, undergoes a decrease in temperature. The augmerited and partially cooled stream is then passed through one or more elongated convection heating passages containing the heating tubes through which the fluid to be heated is passed, the flow of the augmented gas stream being parallel to the axis of the heating tube and either concurrently or counter-currently to the direction of flow of the fluid in the tubes. The reduced temperature and increased volume of the augmented as stream minimize the temperature gradient along the heating tubes and also equalize the heat transfer rate from the gas to theheating tubes along the length and about the circumferfor flowing theresulting augmented stream of ence of the heating 1illbescombustion gases in heat exchange with the heaty my method f mixing he fresh combusing tubes and to effect the recirculation of flue 131011 gases with the recirculated a s prior to gas through the combustion chamber. Ancillary passage t h e Venturi throat (whereby the thereto, it is an object to arrange the discharge Venturi throat is disposed upstream from the port, through which excess or non-recirculated heating passages) insures a complete mixing of 3 the fresh and the recycled combustion gas and the jet gas; this results in a more uniform heating of the heating tubes.

Excess gases are withdrawn from the circulating gas stream at any desired point in the circuit, and I prefer to withdraw them (approximately at a rate equal to the sum of the 'rate of admission of fresh combustion gases and jet gas) just after the gas has traversed the heating tubes.

The method will be understood more completely from a consideration of the apparatus illustrated in the drawings, which illustrate certain preferred embodiments of the invention, it being understood that the invention is not limited to the specific arrangement of flow passages shown therein.

Referring to Figs. 1 and 2, the heater is supported on a concrete foundation I having a central or inner wall la supporting a horizontal bulkhead l'b lined on its upper face with refractory fire bricks supporting a'plurality, for example, five burners '-2. These burners receive combustion air via opening lc. The wall la is continued upwardly by a refractory brick section 3, a checker brick section 6, and a refractory section 5 having slots 5a near the top. The space above the burners 2 and within these sections is herein referred to as the combustion space. The combustion space is fitted with a transverse partition or Venturi block 6 having a plurality, for example, five'perforations 6a forming Venturi throats. The lower portions of these throats may be lined with metallic sheathing as shown. Expansion nozzles 1 are mounted beneath each of the Venturi openings and disposed to discharge jet gas upwardly substantially tangent to the walls of the Venturi throats.

An outer furnace wall is provided surrounding the sections and '5 and spaced therefrom. This wall may be supported in any conventional man ner. As shown in Figs. 1 and 2 it is suspended near a steel framework comprising H--columns 8 resting on the foundation l and supporting vertical angle sections 9 through gusset plates ill. Thelan'gl'e sections 9 support arcuate angles H from which fire resistant tile blocks 12 are sus pended by well known means, not shown, including suspension lugs. A sheathing of heat insulating material l3 surrounds the blocks 12 and is also suspended from the angles l I Horizontal l b'eam's l4 extend across the top of the furnace, being ecured to the H-columns 8 and forming supports for'the suspended refractory roof I 5' and for'the heating tubes is and f1, suspended by collars Ida and l'la. These tubes extend vertically Within the annular connection heating space between the combustion chamber walls and the outer furnace wall, extending downwardly through openings in a lower, annular refractory wall l8, supported from the steel framework. The heating tubes are sealed to the roof [5- and lower wall 18 by means of refractory bushings l9.

The lower portion of the outer wall has a series of openings 20 communicating with an annular flue box 21 connected at one side to a stack 22. A damper 23 is provided in the connection between the box 2| and stack.

The nozzles I are connected by a manifold 24 to' a steam line 25 which is connected to a source of saturated or superheated steam, not shown. The valve 26 is open and the valve '2'! is closed. For superneating and/ or generating the steam I may provide a steam coil 28 in the stack or in the The tubes are 4 flue box 21; in this case the valve 26 is closed and the valve 21 is open.

The fluid to beheated is/passed from inlet 29 to a manifold 30 connected to the bottoms of the heating tubes 5. In this embodiment the heating tubes are arranged to vaporize liquid and the "tubes Hi function as preheaters and are of rela tively small diameter to effect the turbulence necessary for efficient heat transfer. The vapors pass into the iiia'in heating tubes l'l via U-connector '31. The tubes I! are of relatively large diameter toacco'mmodate vapors and provide the necessary space velocity for the particular use to which thefurnace is put. The fluid is discharged via radial pipes 32a to a circular exit header 32.

'In theop'era-t'ion of the heater, hot combustion gases are 'g'enerated 'by the burners 2 'andpassed upwardly through the combustion chamber,- radially 'outwardly'thi'oug h the slots 5a downwardly through the-annularheating space enclosing the heating tubes,and out through the flue box 21 and stack '22-. A portion of the flue gas is recirculat'ed through the checkerbr-ic-k 4, augmenting the-sfiream-of combustion gases and cooling the latter. circa-neon is induced by the action of the nozzles and the Venturi block- 6, wherein the augmented stream is entrained and still further'auginented by the 'jet gas. The pressure in theup'per portion-ofthe heater-above the Venturi block, may be atmespnene or slightly above atmespheric, while the pressure beneath the Venturi block may be somewhat less than atmospheri'c, although the invention is not limited to operation at-such pressures. V

The recirculation rate is regulated by the rate o-f-flow o-f jet-gas issuing from the nozzles I and by the dam er 23. By recirculation rate I mean the weight ratio or the flue-gas flowing radially'i-n through the checker br'iokto the cornbus-tio'n gases generated by the burners- 2. reeirculatiori rate' ma be varied, depending upon the desired temperature of the mixed gases flowing downwardly through the annular heating space. 12y way of exampn, andw-ithoutlimiting the invention thereto, it'may' be stated that the circulation rate may in typical installations be between about 1:0.5 and 1+3. Thus, when the recirculation rate is I12, the quantity of gas passed through the heating space is between 3 and 4 times as great as the quantity generated (consisting-of one part or fresh combustion gases, two parts of recirculated flue and a small amount, usually less than one part, of jet gas).

Hea'tlosses are minimized in this heater by placing the hottest porn-en inside the inner, cylind'rical sections t 5 and by venting thee'xcess flue gas into the ride box 2-! at its lowest temperature. It is, furthen desirable toaveid- "ex-- cess combustion air in-order to improve the thermal efficiency and to avoid formation of an oxidizi ng gasfmi-xture which rapidly deteriorates the tubes and brickwork.-

The augme ted tream-or combustion gas is usually cooled lh' sowin through the Venturi throats because the jet-"gasis cooler than the gas stream entrained thereby.

Thecentral location or ar combustion-chamher and the recirculation of flue "gas make it possible to use less resistant walls for the outer walls and minimizes heat lost by'r'adiatio'n. Thehating tubes are evenly spaced in the annular heating space whereby the individual tubes at dilfrent'pbints' of the circumference of the heater receive an Lemma-mount of heat. This axial symmetry, while particularly useful with The the heater disclosed, is also useful in cases where the Venturi throat and jet nozzle 1' are omitted and the flow of flue gas is effected by thermosyphon action or by fans or the like, and the invention, in one aspect, resides in this arrangement of flow channels, regardless of whether means for inducing circulation are provided or of the type of such mean employed. I

An important feature of the invention is the arrangement whereby an augmented stream of heating gas is flowed along the heating tubes at a reduced temperature, whereby the temperature gradient lengthwise of the tube surface is reduced and the heat transfer rate in the direction of the length and circumference of the heatin tubes is nearly equalized. Another important feature resides in the location of the Venturi throats immediately downstream from the point of juncture of the fresh, hot combustion gases and the recirculating flue gas. This arrangement insures complete mixing of the fresh and recycled gases and the jet gas and results in the more uniform heating of the tubes.

By suspending the heating tubes from the top by collars resting on the exterior steel framework they are easily removed from the top, it being only necessary to disconnect them from the bottom manifolds. By connecting the preheating tubes I6 to the main heating tubes I1 at the top.

wherein no vertical movement due to thermal expansion takes place, the provision of fluid-tight connections are facilitated.

The heater may be modified in several respects. For example, the discharge of excess flue gases may be effected from the top or from an intermediate level of the heating space, whereby the heating of the tubes may be varied to meet particular conditions. The outer walls may be made entirely of refractory or of insulating material, instead of containing a layer of each. Further, it is not necessary to arrange the combustion chamber vertically or to place the heating tubes entirely around the combustion chamber; however, the arrangement shown is preferred in that it provides a symmetrical arrangement and thereby promotes even distribution of heating gases. Finally, the suspension of the outer furnace wall and the tubes from a steel framework is not a part of the invention, and I may employ any other construc tion such as, for example, that illustrated in Figs. 3 and 4.

Referring to Figs. 3 and 4, the furnace comprises a concrete foundation IOI with a cylindrical portion IOIa lined with refractory fire bricks I03 and mm and insulating brickwork I33, and provided with a plurality of premix burners I02 adapted to generate combustion gases by burning a previously partly or fully mixed stream of fuel and air. Above this brickwork there are checker brickwork I04 and a cylindrical refractory section I05 having slots I05a at the top. The section I05 has a plurality, e. f., five, radial fins I051) extending outwardly and subdividing the annular heating space into a plurality of smaller heating spaces. A single Venturi block I06 having a throat I06a is fitted within the section I05 and provided with a plurality of nozzles I01 for expanding jet gas into the Venturi throat. I v

The outer furnace wall comprises checker brickwork I 34 and a cylindrical wall II2 which may be of suitable heat-resistant concrete or of refractory brick. Thefurnace top H5 rests on the wall H2 and is covered by heat insulating material I35 and a steel plate I36. Heating tubes II! extend vertically within the vertical heating spaces between the walls: I05 and H2 and fins H151) and are suspended from the steel plateIISb by collars Illa. The tubes are connected at the top to a manifold I310 and at the bottom to a circular header I32 via radial connectors I32a.

The flue exit duct is in the form of an annular flue box I2I in communication with the heating space through the checker brickwork I34. It is connected to astack I22 through a passageway fitted with a damper I23. The nozzles I01 are supplied with jet gas by a manifold I24, pipe I25 and valve I26. The heater is operated in the same manner as that according to Figs. 1 and 2, except that no preheating tubes are provided. The fluid to be heated may be fiowed either upwardly or downwardly. through the heating tubes, depending upon the type of heating desired. For example, in endothermic reactions, wherein the conversion temperature is to be maintained as uniformly as possible, it is usually important to avoid too great a difference in temperature between the reacting gas mixture and the heating gas in the part of the heating tube where the reaction takes place, i. e., the outlet part of the tube. At the inlet end greater differences in temperature are permissible Without causing any detrimental effect. In such a case heating in concurrent flow with the heating gas is most suitable and the reaction mixture is flowed downwardly through the tubes Ill. For other applications of the heater countercurrent flow may, of course, be more appropriate, and the mixture may be flowed upwardly through the heating tubes.

I claim as my invention:

1. A tube heater comprising an enclosing wall structure forming a central vertical combustion chamber, a source of hot combustion gases near the bottom of said combustion chamber, an outer wall spaced from and surrounding said wall structure to form a vertically elongated heating space therebetween, vertical convection heating tubes located entirely outside of said enclosing Wall structure and within said heating space, passageways interconnecting the combustion chamber and the heating space both at the top and at the bottom, means located within said combustion chamber for inducing upward movement of combustion gases and flue gas upwardly within said combustion chamber, whereby gases may circulate through said chamber and space, and a flue for discharging flue gas.

2. The tube heater according to claim 1 wherein there are a plurality of interconnecting passageways at each end of the heating space and the heating tubes and the passageways are spaced substantially evenly about the combustion chamber.

3. The tube heater according to claim 1 wherein the heating tubes are arranged in pairs, the

tubes ofeach pair being interconnected at the one vertical end of the heating space.

4. A tube heater comprising an enclosing wall structure forming a central vertical combustion chamber, a source of hot combustion gases near the bottom of said combustion chamber, an outer wall spaced from and surrounding said wall structure to form a vertically elongated heating yspace therebetween, a plurality of radial partitions extending from the enclosing wall structure to said outer wall and subdividing said eponoev heating space into a plurality of circumferentially contiguous vertical smaller heatingspaces, vertical convection heating tubes locatederrtirely outside of said enclosing wall structure and within said smaller heating spaces, passageways interconnecting the combustion chamber and the smaller heating spaces both at the top and at the bottom, means located within saidvcombustion chamber for inducing. upward movement of combustion gases and flue gas upwardly within said combustion chamber, whereby gases may circulate through said chamber and-'smalier heating spaces, and a flue for discharging flue gas.

5. A tube heater comprisingan enclosing. Wall structure forming a combustion "chamber, a source of hot combustion gases near oneend of the chamber, a second wallstructureformi'ng, to?- gether with said first wall-structure, ane'lon'gated convection heating space, convection heating tubes within the heating space and locatedflto be shielded by said enclosing wallstructurefrom direct radiation from the said source of "combustion gases extending longitudinally with respect to the heating space, passageways interconnecting the ends of said heating space with spaced points in said combustion chamber, a Venturi con-'- striction within said combustion chamber inter mediate said spaced points. nozzle means. for discharging a fluid jet into the throat of: said Venturi constriction, and a flue for discharging flue gas.

6. A tube heater comprising a first inner wall of refractory material forming a vertical com-- bustion chamber, a source of hot combustion gases near the bottom of said combustion chamher, a second outer wall of insulating ref-ractory material surrounding said first wall and spaced therefrom to form a vertically elongated heating space therebetween, vertical convection heating tubes located entirely outside of said first inner wall and within said heating space, passageways interconnecting the ends of said heating space with spaced points in said combustion chain ber above said source of combustion gases a Venturi constriction within said combustion :chamber intermediate said spaced pointsnozz-le means for discharging a fluid :jet upwardly into the throat of said Venturi constriction, and. a1 flue for discharging. flue gas.

'7. The tube heater according toclaimfi wherein the flue is connected to the bottom of= said heating space. I

8. The tube heater according to claimfi wherein there are a plurality of passageways atv each end of the heating space, the heating tubes and passageways are spaced substantially evenly about the combustion chamber andthe-flue comprises an annular flue box surrounding the-outer wall and in communication with said heating space through a plurality of passageways. disposed circumferentially with respect-totheheating space.

9. A tube heater comprising. an inner wa-li structure of refractory material form-inga vertical combustion chambensaid wallstructurecomprising an open checkerwork section near the bottom and a solid section above said checkerwork section, a plurality of circumferentially spaced openings at the top of said solid: section, a Venturi block withinsaid solid section. having one or more Venturi. constrictions, a source of hot combustion gases'beneath said.openrcheclrer work section, an outer wauoi insulating one fractory material surrounding said inner wall and spaced therefrom to form a vertically elongated heating space therebetween, said space being in communication with the combustion chamber through said open checkerwork sec tion and "said openings, a plurality of vertical convection heating tubes Within said heating space spaced substantiallyevenly about the combustion chamber, a second, outer wall structure surrounding. said inner wall and forming a flue box, passage-ways interconnecting said heating spa-ceand said flue box at a plurality of points, av flue for discharging flue gas from said flue box, and a damper for controlling the discharge of the flue gas. 1

10. The heater according to claim 9 wherein the flue box is located near the bottom of the heating space.

11. A method of heating flowing media comprising'flowing the medium to be heated through a: tube, flowing an augmented stream of combustion gases, described hereafter, through an elongated convection heating space longitudinally with respect to said tube, thereafter admixing at least a portion of the combustion gases with fresh, hot combustion gases, to form an augmented stream of combustion gases, forcing flow of the augmented stream toward said convection space through the entraining action of a jet, thereby establishing a circulating stream of combustion gases, and removing a portion of said circulating stream. V

12. The method according to claim 11 wherein the portion of the circulating stream is removed from the. augmented stream immediately after passage thereof. through the convection space;

13. The methoda'ccording to claimll wherein the flow oftheaugmented stream is induced by injecting a jet of a gas intoa Venturi constriction.

:14,- A method: of. heating flowing media comprising flowing-the medium to be heated through a plurality of vertical tubes, flowing an augmerited: stream of combustion gases, described hereafter, downwardly throughfa vertically elongated convectionheating space along said heating tubes, thereafter admixing a portion of the combustion gases with fresh, hot combustion gases, to form an augmented stream" :oi combustion gases, inducing upward flow oi the aug merited streain externally of said convection space toward the top of said convection space by means o'fthe entraining action 'of a jet, thereby establishing a circulating stream of combustion gases, andreihoving a portion of said circulat ingstream.

I 15. The-method according'to claim 14 wherein the upward flow'ofthe augmented stream is inducedbyinjectingja jet of steam-into a Venturi constriction. 7

PERM'ANN.

Resonances CITED The fol-lowingi'eference'sare of record: in: the file "of thispatentt' UNITED STATES PATENTS

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