US2721735A - Tubular heater with partial flue gas recirculation and heating method - Google Patents

Description

Oct. 25, 1955 K. PERMANN 2,721,735

TUBULAR HEATER WITH PARTIAL FLUE GAS RECIRCULATION AND HEATING METHOD Filed OOC. 23, 1951 2 Sheets-Sheet l His ATfornzg Oct. 25, 1955 K. PERMANN 2,721,735

TUBULAR HEATER WITH PARTIAL F LUE GAS RECIRCULATION AND HEATING METHOD Filed Oct. 23, 1951 2 Sheets-Sheet 2 Fig. 2.

\veni'or= Karl Permann United States Patent TUBULAR HEATER WITH PARTIAL FLUE GAS RECIRCULATION AND HEATING METHOD Karl Permann, Oakland, Calif., assignor to Shell Development Company, Emeryville, Califi, a corporation of Delaware Application October 23, 1951, Serial No. 252,746

12 Claims. (Cl. 263-41) This invention relates to improvements in combustion furnaces for heating fluids flowed through heating tubes and to an improved method of heating such fluids, wherein the heating tubes are arranged within a heating zone or chamber so as to cause combustion gases to flow substantially longitudinally with respect to the heating tubes, and wherein a part of the combustion gases are recirculated to the burners. The heater and method according to the invention are, for example, particularly useful in the carrying out of endothermic chemical reactions, such as the dehydrogenation of alcohols or other gaseous hydrocarbons which takes place at about 1000 F., in the conversion of ethylene dichloride into vinyl chloride and hydrochloric acid at about 960 F., the cracking of hydrocarbon oil, and catalytic dehydrogenation processes.

It is an object of the invention to provide an improved method and heater of the character described that are capable of effecting uniform and controlled heating of fluids within the tubes, wherein different sections of the tubes are heated by separate currents of combustion gases flowing in opposite directions. In such a heater and method one section of the tubes may be used to bring the fluids up to the desired reaction temperature while the other section may be used to supply heat to the reaction stream while it is undergoing an endothermic reaction.

It is a further object to provide a method and heater of the character described wherein uniform and controlled heating of heat sensitive fluids is effected and the tubes containing the fluids are effectively shielded from flame impingements and excessive wall temperatures, while providing adequate heat absorption rates.

Further objects are to improve the heat distribution to the tubes by symmetry in design and forced high rate of gas movement; to provide for flexibility in operation by using divided gas flow and partial recirculation of combustion gas; and to effect high thermal efficiency by providing two heating zones or chambers and discharging to the stack the flue gas from only one of these chambers after adequate utilization of the heat capacity of such gas.

Further objects will become apparent from the following description.

In summary, according to this invention, elongated heating tubes are mounted longitudinally within two elongated terminally adjacent heating zones enclosed by walls and the fluid to be heated is passed through the tubes from their inlet ends to the discharge ends to traverse the heating zones in succession. Hot combustion gas is generated by one or more burners in a separate combustion chamber, which preferably surrounds the heating zones at the juxtaposed ends thereof and comunicates therewith by passages permitting influx of combustion gas with a circumferential velocity component. Combustion gas that has already passed through one of the heating zones is returned to the combustion chamber, preferably tangentially so as to set up a rotating annulus, to dilute and cool the hot combustion gas. The diluted combustion gas enters the heating zones at the juxtaposed ends thereof and passes as two currents in opposite directions in contact with the heating tubes towards the remote ends where the gas is withdrawn. The part of the divided gas stream which is withdrawn at one end is returned to the combustion zone to dilute the hot combustion gases as described above, while the other part of the gas stream is discharged as flue gas.

The method and apparatus will be described in greater detail with reference to the accompanying drawing showing one illustrative embodiment thereof, wherein:

Figure 1 is a plan view, partly in section, of a heater according to the invention suitable for practicing the method; and

Figures 2 and 3 are transverse sections taken on correspondingly numbered lines on Figure 1.

Referring to the drawings in detail, 10 represents a wall structure in the form of a cylindrical shell having a horizontal axis and defining within itself two elongated, terminally juxtaposed heating zones or chambers A and B that are fully open to each other. The shell may be lined with insulating and refractory material, as shown. The ends of the shell are fitted to end ring sections 11 and 12 which form end closures for the heating chambers at the remote ends thereof. The ring 11 has a tangential outlet opening 13 communicating with a stack 14 for the dis charge of flue gas from the chamber A. A damper 15 in the stack breaching regulates the discharge of flue gas. The ring 12 has a tangential outlet opening 16 for the discharge of flue gas from the chamber B communicating with a fan 17 for recirculating the gas, the fan being driven by an electric motor 18. The heating tubes 19, having fins 19a on the parts thereof situated in the chamber A, extend longitudinally through the heating chambers and axially through the end walls of the ring sections 11 and 12; they are connected at their inlet and outlet ends to headers 20 and 21, respectively. The heating tubes are preferably arranged in a circle concentric with the axis of the shell and in close proximity from the inner surface thereof but spaced therefrom to provide room for header fittings and to permit combustion gases to circulate on all sides thereof. The heating chamber A at which the fluid is introduced into the tubes may be called the inlet zone and the other chamber B the discharge zone. A gas disperser 22 or 23 is mounted coaxially within each chamber and inside of the circle of heating tubes, supported from the shell at intervals by supports 24. The dispersers are circular in cross section and increase progressively in cross-sectional area toward the remote ends of the heating chambers, as shown, being spaced apart axially in the vicinity of the combustion chamber, which is described below. It is advantageous to provide a helical vane 25 on the disperser 23 in order to secure a higher heat transfer effect. These dispersers may, if desired, be protected by heat resistant material, such as castable high temperature ceramic, not shown.

The shell has a peripheral opening 26 located between the juxtaposed ends of the heating chambers. A pair of annular end walls 27, 28 and a peripheral wall 29 define an annular combustion chamber that is in communication with the heating chambers through the opening 26. The wall 29 has a tangential inlet 30 and a plurality of circumferentially spaced burners 31 that are preferably inclined with respect to the radii to the burners to emit hot combustion gases with a circumferential velocity component in the same circumferential direction as that of the gas admitted through the inlet 30. The helical vane 25 is also formed to permit gas rotating about the axis of the heating chamber to continue rotation in flowing toward the discharge end. The burners may be oil or gas burners, as desired; in the illustrated embodiment they are gas burners, provided with auxiliary equipment for mixing gas and air, from supply lines 32 and 33, respectively. Air under suitable pressure is supplied from a fan 34 driven by an electric motor 35 and fuel gas under pressure is supplied from a source, not shown, through a pipe 36 and a ratio valve 37 that is connected also to the blower output to regulate the rate of gas flow in accordance with the rate of air flow. The burners are preferably of the premixing type emitting short flames and may have cylindrical muffles 33. A gas return duct 39 connects the discharge side of the fan 17 to the tangential inlet 36. Header boxes it and 1 enclose the headers 20 and 21 to retain heat.

In operation, the fluid to heated, for example, re actants capable of undergoing an endothermic reaction, is supplied to the header 29, passed through the tubes 19, and discharged through the header 21. T he burners 31 being in operation, the hot combustion gas generated thereby is promptly diluted with colder t bustion gas admitted through the inlet 39 to form a rotating annulus of diluted combustion gas which enters the heating zones with a circumferential velocity component through the opening 26. No flame enters the heating zones due to the ring shape of the combustion chamber, wherein complete mixing of hot combustion gases and diluent gas takes place, and the tubes 19 are heated solely by convection. One portion of the diluted combustion gas flows from the place of introduction through the inlet zone A to the remote end thereof and is discharged through the stack 14; this portion is equal in amount to the quantity of hot combustion gas generated by the burners but consists, of course, in part of recycled flue gas. The remaining portion of the introduced rotating diluted combustion gas flows through the discharge zone B to the remote end thereof, whereat it is withdrawn through the outlet 16 and is returned to the combustion chamber by means of the fan 17 and return duct 35; having been cooled by contact with the heating tubes l9 this recycled combustion gas is capable of reducing materially the temperature of the hot combustion gas upon being mixed therewith in the combustion chamber. In flowing through the heating chambers the rotating gases follow a generally helical path about the gas dispersers, thereby insuring rapid flow of the gases relatively to the heating tubes, insuring higher rate of heat transfer and uniform heating of the several tubes. This helical movement is insured by the vanes which are of particular value on the disperser 23 for two reasons: Since the fan 37 induces positive circulation of the gas traversing the zone B, the vanes promote rotation of the gas about the disperser and the pressure head of the fan is sufficient to overcome drag due to this movement-a condition not encountered in the zone A where the disperser 22 is located, unless an induced draft fan is provided in the stack, the provision of such a fan being, of course, not excluded from the scope of the invention. The second reason is that, as shown in the drawing, the zone B is usually made longer than the zone A although this relation of lengths, too, is subject to variations in design as will be explained below; hence, there is usually a greater tendency for the gases in the zone B to lose their rotational velocity due to drag against the heating tubes, and this tendency is overcome by providing the vanes.

Each of the two portions of the introduced diluted combustion gas undergoes a reduction in temperature in approaching the remote end of its respective heating zone; however, the gradients of these two currents are not necessarily the same and they can be controlled for any given rate of flow of fluid through the heating tubes at a given inlet temperature by changing the rate of heat input to the burners and the rate of recirculation through the fan 17. By operating the fan 17 at a higher rate the total quantity of gases flowing through the zone B may be increased, resulting in a smaller temperature gradient therein; operating the burners at a hi her rate similarly reduces the gradient in the zone A. in general, it is preferred to operate the zone A with a temperature gradient that is considQably greater than the gradient in the other zone,

whereby the exhaust gases vented to the stack 14 are considerably cooler than the recycled gases; this brings about improved emciency without requiring that gases in contact with the heating tubes 19 in the zone B be at a reduced temperature.

When the heater is used for carrying out chemical reactions, the heating tubes 19 may be regarded as comprising two sections, a preheating section near the inlet header 2% and a reaction section near the discharge heador 21. The boundary between these sections is not sharply defined but is usually in the vicinity of the combustion chamber. Hence, the relative lengths of the heating zones are selected having regard to heat transfer rates and the reaction time required.

It follows from the foregoing that control of the gas movement as to quantity and temperature is easily effected during operation by regulating the amount of heat release from the burners, the speed of the recycle fan 17 and the setting of the damper 15 in the stack-breaching. The heating gas inside the heater is held at or slightly above atmospheric pressure, e. g., at a gauge pressure of onehalf inch of water, and is advantageously neutral in composition, i. e., there is complete combustion of the fuel. The operation of the heater is, therefore, highly flexible.

While a specific, preferred embodiment has been shown, it is to be understood that changes may be made without departing from the inventive concept; thus, it is not in every case necessary to arrange the heating tubes in parallel for single pass, and other known flow arrangements may be used, and temperature control instruments can be applied to make the controls fully automatic.

1 claim as my invention:

1. A process for heating a fluid stream while controlling the intensity of heating independently at successive stages of the heating comprising the steps of flowing said stream through elongated tubes having different sections thereof contained in two separate, confined, elongated, terminally juxtaposed convection heating zones; burning fuel in a combustion zone communicating with both said heating zones at the juxtaposed ends thereof; diluting the resulting hot combustion gas with colder combustion gas; introducing different portions of the resulting diluted gas into the heating zones at the juxtaposed ends thereof; flowing said portions of the diluted gas through the heating zones in directions away from said juxtaposed ends toward the remote ends thereof in contact with the sections of said elongated tubes therein to impart heat thereto; withdrawing the gas from the heating zones at said remote ends thereof; and recycling gas withdrawn at the remote end of one of said convection heating zones to the combustion zone, as the said colder combustion gas in regulated amount independently of gas withdrawn from the other convection heating Zone, thereby to control the intensity of heating in the said one convection heating Zone independently of the intensity of heating in the other convection heating zone.

2. A process for heating a fluid stream while controlling the intensity of heating independently at successive stages of the heating comprising the steps of flowing said stream through elongated tubes having different sections thereof contained in two separate, confined, elongated, terminally juxtaposed convection heating zones; burning fuel substantially completely in an annular combustion zone communicating with both said heating zones at the juxtaposed ends thereof and exterior thereto and thereby generating hot combustion gas; introducing colder combustion gas tangentially into said annular combustion zone and thereby diluting the hot combustion gas and forming a rotating annulus of diluted combustion gas; introducing different portions of said rotating annulus of diluted gas into said heating zones at the juxtaposed ends thereof without impingement of flame on the said tubes; flowing said portions of the introduced gas through the heating zone in directions away from said juxtaposed ends toward the remote ends thereof in generally helical paths and in contact with the sections of said elongated tubes therein to impart heat thereto; withdrawing the gases from the heating zones at said remote ends thereof; and recycling gas withdrawn at the remote end of one of said convection heating zones as the said colder combustion gas to the annular combustion zone in regulated amount independently of gas withdrawn from the other convection heating zone, thereby to control the intensity of heating in the said one convection zone independently of the intensity of heating in the other convection heating zone.

3. A process for heating a fluid stream of reactants to reaction temperature and thereafter supplying endothermic reaction heat thereto at a substantially uniform temperature'at an independently controlled heating intensity comprising the steps of flowing said stream of reactants through elongated tubes having inlet and discharge sections thereof contained in two separate, confined, elongated, terminally juxtaposed convection heating zones, said tubes having inlet ends and discharge ends at the ends of the heating zones that are remote from the juxtaposed ends, said fluid stream being introduced into the inlet ends of the tubes; burning fuel substantially completely in a combustion chamber that is external to and communicates with said heating zones at the juxtaposed ends thereof to generate hot combustion gas; diluting said hot combustion gas with colder combustion gas; introducing a portion of the resulting diluted gas in amount approximately equal to the amount of said hot combustion gas generated into the heating zone that contains the inlet sections of the tubes at the juxtaposed ends thereof; flowing said portion through said heating zone in contact with the inlet sections of the elongated tubes and thereby heating said reactants to reaction temperature; withdrawing said portion of gas from the said heating zone at the remote end thereof; introducing the remaining portion of the diluted gas into the other heating zone at the juxtaposed end thereof; flowing said remaining portion of the gas through the latter heating zone in contact with the discharge sections of the elongated tubes and thereby supplying endothermic reaction heat to said reactants; withdrawing the said remaining portion of the gas from the latter heating zone at the remote end thereof; returning the latter withdrawn portion of gas to the combustion zone as the said colder combustion gas; and regulating the amount of said remaining portion of the diluted gas to control the intensity of heating of the said other convection heating zone independently of the intensity of heating of the inlet ends of the tubes.

4. The process according to claim 3 wherein the said colder'combustion gas is introduced into the combustion gas in a tangential direction to set up a rotating annulus of diluted combustion gas; both of said portions of diluted gas are introduced into the respective heating zones with a rotary motion directly from the inside of said rotating annulus; and both of said portions of diluted gas are flowed through the respective heating zones with substantially helical motions.

5. A heater for fluid material comprising an enclosing wall defining a pair of terminally juxtaposed, elongated convection heating chambers; a plurality of elongated heating tubes situated within and extending longitudinally through both said chambers for the passage of said fluid material; a combustion chamber situated at the terminally juxtaposed ends of said chambers and communicating with both said chambers at said ends thereof; one or more burners in said combustion chamber for generating hot combustion gas; outlets situated at the ends of the heating chambers that are remote from said juxtaposed ends for withdrawing combustion gases at said remote ends; a gas return duct interconnecting the outlet of one of said heating chambers to the combustion chamber for returning gas withdrawn at the remote end of said heating chamber to dilute and cool the hot combustion gases; and means for regulating the flow of said withdrawn gas through the gas return duct independently of gas withdrawn from the other of said heating chamber, for controlling the intensity of heating in said one heating chamber independently of the intensity of heating in the said other heating chamber.

6. A heater according to claim 5 wherein the heating chambers are of unequal lengths, the heating chamber having the outlet thereof connected to said return duct being longer than the other heating chamber.

7. A heater for fluid material comprising a generally cylindrical enclosing wall defining a pair of terminally juxtaposed, elongated heating chambers; a plurality of elongated heating tubes situated within and extending longitudinally through both said chambers for the passage of said fluid material; an annular combustion chamber situated peripherally about said wall at the juxtaposed ends of the heating chambers and communicating with both heating chambers at said ends thereof through one or more passageways permitting the entry of gas into the heating chambers with a circumferential velocity component; one or more burners in said combustion chamber for generating hot combustion gas; a tangential inlet for colder combustion gas for said combustion chamber whereby colder combustion gas introduced therethrough will promptly dilute and cool said hot combustion gas and form a rotating annulus of diluted combustion gas in the combustion chamber; outlets situated at the ends of the heating chambers that are remote from said juxtaposed ends for withdrawing combustion gases at said remote ends; and means for returning gas withdrawn at the remote end of one of said heating chambers to said tangential inlet.

8. A heater according to claim 7 wherein said combustion chamber has a plurality of burners, each burner being inclined with respect to the radius to the respective burner so as to discharge hot combustion gas with a circumferential velocity component in the circumferential direction in which said colder combustion gas is admitted into the combustion chamber through the tangential inlet.

9. A heater according to claim 7 wherein the combustion chamber has a peripheral wall and two annular ends walls, each end wall extending radially outwardly from one juxtaposed end of a heating chamber, said heating chambers being fully open to each other at said ends thereof, the combustion chamber open to the heating zones between the inner portions of said annular end walls about the entire periphery and said burners are of the type producing short flames, whereby the diluted gas admitted into the heating zones is substantially free from flame.

10. A heater for fluid material comprising a generally cylindrical enclosing wall defining a pair of terminally juxtaposed elongated convection heating chambers that are fully open to each other at the juxtaposed ends thereof; a plurality of elongated heating tubes situated adjacent said wall Within and extending longitudinally through both said chambers for the passage of said fluid material; a peripheral opening in said enclosing wall between the juxtaposed ends of the heating chambers; a pair of annular walls extending outwardly from said opening and connected by a peripheral wall spaced outwardly from the enclosing wall and defining an annular combustion chamber; a tangential inlet for colder combustion gas in said peripheral wall; a plurality of circumferentially spaced burners in said combustion chamber for generating hot combustion gas; outlets situated at the ends of the heating chambers that are remote from said juxtaposed ends for withdrawing combustion gases at said remote ends; means including a return duct and a blower for returning gas withdrawn at the remote end of one of said heating chambers in regulated amount independently of gas withdrawn from the other chamber to said tangential inlet for diluting and cooling the hot combustion gases and forming a rotating annulus of diluted combustion gases within the combustion chamber, whereby the intensity of heating in said one heating chamber can be controlled independently of the intensity of heating in the said other heating chamber; and a gas disperser within at least one heating chamber situated substantially at the axis thereof radially inwardly from the heating tubes.

11. A heater according to claim 10 wherein the gas disperser is circular in cross section and is situated in the heating chamber, the remote end of which is connected to said return duct, said disperser having a helical vane at the periphery thereof.

12. A heater according to claim 10 wherein the crosssectional area of the gas disperser increases progressively from the juxtaposed end of the heating chamber to the 15 remote end thereof.

References Cited in the file of this patent UNITED STATES PATENTS Choinski Sept, 13, 1927 Newhouse Mar. 19, 1929 McCann June 11, 1929 Besta July 2, 1929 Leamon Oct. 31, 1933 Ramseyer May 18, 1948 Huber Apr. 17, 1951 Lorenzo June 5, 1951 Lacerenza Mar. 25, 1952 Carson July 20, 1954 FOREIGN PATENTS Great Britain 1904

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