FIRETUBE HAVING THERMAL CONDUCTING PASSAGEWAYS

Abstract

A firetube is immersed in a fluid to be heated and transfers heat from hot gases flowing through the firetube to the fluid surrounding the firetube. The firetube has a plurality of thermally conductive passageways which extend through the firetube for increasing the surface area available for heat transfer. Fluid is conducted through the passageways by a thermosiphon effect resulting from a temperature differential in the vessel, the fluid below the firetube being cooler and denser than fluid above the heat exchanger.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application 61/422,810, filed Dec. 14, 2010, and U.S. provisional application 61/434,258, filed Jan. 19, 2011, the entirety of each of which is incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of the invention relate to indirect-fired and direct-fired heat exchangers and more particularly to firetubes installed in process vessels, the firetubes having enhanced surface area for heating process fluids.

BACKGROUND OF THE INVENTION

It is known to heat process fluids in a variety of vessels, such as ASME code process vessels, atmospheric bath heaters and tanks. Generally, a heat exchanger is fit within a vessel for heating fluids, such as those commonly handled in oilfield handling and refining operations.

In the oilfield, U-shaped “firetubes”, referred to as U-tube firetubes or U-tubes, are common heat exchangers for use in vessels containing fluids to be heated, such as heater-treaters, free water knock-out vessels, and in-line heaters and tanks. Traditionally the U-tube firetube is made of round steel pipe. A burner supplies a flame and hot exhaust gases for circulation through the firetube from an inlet to an outlet. Heat is conducted from the pipe walls to the fluid contained in the vessel.

In a direct-fired vessel, heat is transferred through the firetube wall immersed directly in a process fluid to be heated, the process fluid being contained in the vessel and in direct contact with the outside of the firetube. In an indirect-fired vessel, heat is transferred from the firetube to an intermediate heat exchange fluid. A fluid-to-fluid heat exchanger contains the process fluid, the exchanger being immersed in the heat exchange fluid.

Conventional U-tube firetubes have the burner mounted at the gas inlet end of the firetube. A vent or exhaust stack is connected to the gas outlet. Both the gas inlet and gas outlet are mounted in a common wall of the vessel. The U-tube exchanger is generally installed inside the vessel through an oval or obround shaped manway.

The ultimate objective in any fired heating system is to create the highest thermal input possible for a given space. The thermal input is related in part to the surface area exposed to the hot exhaust gases on one side of the firetube wall and the fluid to be heated on the other side. Use of round pipe to create the U-tube firetube results in a very inefficient heat exchanger as the surface area presented to the intended fluid is limited. As a result, a significant amount of the available heat, imparted by the flame, is lost as hot exhaust gases flow through the firetube and up the stack. Thus, conventional U-tube firetubes are expensive to operate, waste energy used to generate the heat, typically do not optimally utilize the heat generated, and release large amounts of waste gas to the environment.

Further, in instances where the process heating requirements change and more process fluids enter the operation than design load, the only alternative has been to replace the equipment with larger units.

Clearly there is a need for improved heat exchangers which are capable of efficiently and cost effectively transferring thermal input to fluids to be heated.

SUMMARY OF THE INVENTION

Generally, embodiments of firetubes, disclosed herein, have an increased surface area without resulting in an overall increase in the size of the firetube due to a plurality of thermally conducting passageways which extend through the firetube and direct fluids to be heated therethrough. Each of the passageways has a wall for heat transfer which adds to the external surface area of the firetube resulting in the increased surface area. In an embodiment, fluids are caused to rise through the passageways as a result of a temperature differential in the vessel creating a natural convective circulation or thermosiphon effect, the fluids below the firetube being cooler and more dense and the fluids above being warmer and less dense. Embodiments of the firetube are suitable for use in direct and indirect-fired vessels.

Advantageously, where process fluids comprise emulsions of water and hydrocarbons having different coefficients causing them to expand and contract at different rates, the expansion and contraction as the fluid enters and leaves the relatively small diameter passageways aids in coalescence of like molecules, which assists in separation of the different constituents a vessel.

In a broad aspect, a firetube is adapted to extend horizontally into a vessel for heating fluid therein. The firetube has a gas inlet, a gas outlet and at least one flowpath therebetween and conducts hot gases along the flowpath from the gas inlet to the gas outlet. The firetube comprises a plurality of passageways, spaced along the flowpath for passing fluid upwardly therethrough. Each passageway extends generally upwardly from a fluid inlet at a lower portion to a fluid outlet at an upper portion and has a thermally conductive wall extending through the flowpath for conducting heat from hot gases to the fluid passing therethrough.

Further, a heat exchanger for a vessel comprises the firetube according to embodiments of the invention. The firetube is suitable for use in a direct-fired vessel where the fluid is a process fluid to be heated by the firetube, the firetube being immersed in the process fluid. The firetube is also suitable for use in an indirect-fired vessel where the fluid is a heat transfer fluid to be heated by the firetube. In this case, the heat exchanger further comprises a fluid-to-fluid heat exchanger for flowing the process fluid therethrough, the fluid-to-fluid heat exchanger being immersed in the heat transfer fluid. In embodiments the heat transfer fluid is glycol.

Embodiments of the firetube are suitable for installing in new vessels or can be used to retrofit existing vessels. As the size of the expanded surface area firetube is substantially the same as the existing prior art firetube, it can be simply installed through the existing manway for flanged connection thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate a prior art U-tube firetube, more particularly,

FIG. 1A is a plan view of the U-tube shown installed in an manway in a front wall of a vessel, a major portion of the vessel having been removed for clarity;

FIG. 1B is a plan view of a front wall of the vessel according to FIG. 1A, illustrating an inlet and an outlet of the U-tube installed in a front wall of the manway; and

FIG. 1C is an elevation view of the front wall of the oval manway of FIG. 1B illustrating the inlet and the outlet;

FIG. 2A is a plan view of a cross-section of one embodiment of a U-tube firetube installed in a vessel, a major portion of the vessel having been removed for clarity, the firetube having a dividing wall extending partially along the firetube and being fit with a plurality of thermally conductive passageways;

FIG. 2B is a side cross-sectional view of one thermally conductive passageway fit to portion of a firetube according to FIG. 2A;

FIG. 2C is a cross-sectional view of a firetube in a direct-fired vessel incorporating an embodiment of the thermally conductive passageways;

FIG. 2D is a cross-sectional view of an firetube in an indirect-fired vessel incorporating an embodiment of the thermally conductive passageways;

FIG. 3A is a plan view of a cross-section of the U-tube firetube of FIG. 2, wherein the dividing wall is formed by a plurality of plates between a plurality of the thermally conductive passageways;

FIG. 3B is an end cross-sectional view through the firetube of FIG. 3A, along section lines A-A;

FIG. 4 is a plan view of a cross-section of the U-tube firetube of FIG. 3A having a first central divider and additional of the passageways with second and third dividers for forming two generally U-shaped flowpaths in the body;

FIG. 5 is a perspective view of the firetube according to FIG. 4, the body being rendered as transparent for greater clarity;

FIG. 6 is a plan view of a cross-section of a U-tube firetube according to another embodiment, the thermally conductive passageways forming a tortuous flowpath in the body;

FIG. 7 is a plan view of a cross-section of a conduit firetube according to another embodiment, suitable for retrofit of a vessel having a prior art firetube according to FIG. 1A;

FIG. 8 is a side, cross-sectional view of an embodiment of the firetube illustrating a variety of possible profiles for the thermally conductive passageways;

FIG. 9 is a side, cross-sectional view of the firetube and tubular passageways according to FIG. 3A;

FIG. 10 is a fanciful illustration of fluid flow through tubular thermally conductive passageways, from fluid inlets below the firetube to fluid outlets above the firetube; and

FIG. 11 is a fanciful illustration of the fluid flow through the thermally conductive passageways according to FIG. 10 and enhanced by the action of vortex generators mounted adjacent the passageway inlets.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

As shown in FIGS. 1A-1C, prior art firetubes 10 are generally U-shaped tubes, having a side-by-side gas inlet 12 and gas outlet 14 at a flanged connection 16 at a front wall 24 of vessel 26. The firetube 10 is generally manufactured from round, steel pipe which is welded together, using welded mitres 18 for forming the “U” at an end 20. The prior art firetube 10 is connected to a manway 22, typically obround in shape to accommodate the side-by-side inlet 12 and outlet 14. The gas inlet 12 connects therethrough to a burner (not shown) for receiving flame and hot exhaust gases therefrom. The outlet 14 connects to an exhaust stack (not shown) for exhausting waste gases therefrom. Flanged connections are typically used throughout.

Firetubes, according to embodiments disclosed herein, can be incorporated in new heat exchange vessels or can be used to retrofit existing vessels to upgrade and enhance the efficiency of heat transfer therein. Heat transfer surface area is increased over conventional firetubes by providing a plurality of thermally conductive passageways which extend through the firetube. Fluid in the vessel is heated, not only from the periphery of the firetube but also through fluid conducted through the passageways.

In more detail, and having reference to FIG. 2A-3B, one embodiment of a firetube 30 comprises a hollow shell or body 32 having body walls 38 for containing and directing hot gases G therethrough. In use, the firetube 30 is fit into vessel V and immersed in a fluid F contained therein. The body walls 38 form a portion of the surface area for heat transfer from the gas G to the fluid F. Hot gases G circulate through a flowpath 42, from a gas inlet 44 at a first end 46 to a gas outlet 48 at a second end 50.

As shown in FIG. 2A, when the firetube's gas inlet 44 and gas outlet 48 are located side-by-side, a U-shaped flowpath 42 is formed. As is also the case in the prior art, the inlet 44 is adapted for connection to a source of hot gases such as a burner (not shown) and the outlet 48 is adapted for connection to an exhaust stack (not shown). The gas inlet 44 and gas outlet 48 are fit to a front or common header wall 34 secured, such as by flanged connection, to the vessel V. The firetube 30 can be fit through an obround manway (see FIG. 3B) and is cantilevered or otherwise supported to extend generally horizontally from the front wall 36. The firetube 30 has a tube end 36 at a farthest extent from the front wall 34.

Hot gases G circulate through the flowpath 42, from the gas inlet 44 to the gas outlet 48, heating the body walls 38 and transferring the heat to fluid F.

The firetube body 32 can be a U-shaped conduit (See FIG. 7) or a generally open body, the interior of which is then fit with structure for directing the gases. Various internal gas-directing structure are illustrated in FIGS. 2A, 3A and 4. The gas-directing structure avoids short-circuiting of the flowpath 42 and maximizes gas contact with the body walls 38. With reference to FIG. 2A, the gas-directing structure can be a first dividing wall 40 extending partially along the hollow body 32, from a proximal end at the common header wall 34, from a location between the gas inlet 44 and outlet 48, to a distal end located short of the tube end 36 for forming the generally U-shaped flowpath 42 within the body 32. Whether the body 32 is a U-tube conduit (FIG. 7) or fit with one or more dividing walls 40, the surface area can be enhanced by further providing a plurality of thermally conductive passageways 52.

Best seen in FIG. 2B, the passageways are spaced apart along the flowpath 42 and extend through the body 32 from a fluid inlet 62 at lower portion L of the body wall 38 to fluid exit 64 at an upper portion U of the body wall 38. Cooler fluid, to be heated, flows upwardly into the fluid inlet 62 from below the firetube 30 to exit each passageway 52 at the fluid exit 64 above the firetube 30. The passageways 52 have a thermally conductive, tubular wall 54, typically formed of the same material as the body walls 38, forming an external surface 56 in contact with hot gases G flowing through the flowpath 42 and an internal surface 58 for contacting the fluid F. The walls 54 of the plurality of passageways 52 provide additional heat transfer surface over that conventionally provided by the prior art U-tube firetube.

As shown in FIG. 2C, fluid F circulates from the lower portion L to the upper portion U of the body wall 38 and then back down within the vessel to repeat the cycle. Where no mechanical impetus is provided, the fluid F movement is like a thermosiphon circulation.

Noteably, such an increase in the heat-transferring surface area is accomplished without an increase in the overall size of the firetube 30. Thus, in an embodiment, the firetube 30 can be installed, as a retrofit, through the obround manway 22 of an existing vessel V, increasing the vessel's heating capability over its original design rating.

In an embodiment, as shown in FIGS. 3A and 3B, a plurality of the thermally conductive passageways 52 can be aligned be integrated with the dividing wall 40. As shown, the dividing wall 40 is a first wall centrally located between the inlet 44 and outlet 48. Accordingly, the dividing wall 40 can be formed of a plurality of plates 60,60,60 . . . , each plate 60 being connected between adjacent passageways 52 for directing gases G along the passageways 52 to the tube end 36. The plates 60 urge gases G from the gas inlet 44 to the dividing wall's distal end and back to the gas outlet 48. The plates 60 can be welded between passageways 52. In addition, a plurality of the passageways 52 are fit to the firetube 30 along the flowpath 42 for conducting heat from hot gases to the fluid passing therethrough.

The number of passageways 52 fit to the flowpath 42 is a function of the desired or design surface area of the tubular walls 54 while not overly restricting the flow of gases G therealong.

In another embodiment, shown in FIGS. 4 and 5, a second dividing wall 40B and third dividing wall 40C are provided, forming two, side-by-side U-shaped flowpaths 42,42. As shown in FIG. 6, a firetube 30 may or may not have passageways 52 aligned along the first central dividing wall 40. The second and third dividing walls 40B, 40C can be connected at distal ends to more particularly direct the flowpaths. When connected, the second and third dividing walls 40B, 40C form a U-shaped dividing wall 40U wherein a first flowpath 42 is formed from the gas inlet 44, between the first and second divider walls 40,40B, and to the gas outlet 48 between the first and third divider walls 40,40C, and a second flowpath 42 is formed from the gas inlet 44, between the first divider wall 40 and the body walls 38, and to the gas outlet 48 between the first wall 40 and the body walls 38.

Returning to FIG. 6, in an embodiment having a centralized dividing wall 40, without passageways 52 integrated therein, a plurality of non-aligned passageways 52 are distributed laterally across the flowpath 42 to access more of the flow of gas G and increase heat transfer recovered therefrom.

Having reference to FIG. 7, alternatively, a plurality of thermally conductive passageways 52 can be retrofitted to the otherwise conventional prior art U-shaped firetube 10 of FIG. 1A.

While the plurality of thermally conductive passageways 52 are used to increase the effective surface area of the heat exchanger 30, one of skill in the art would appreciate that too many or too large a diameter of thermally conductive passageways 52 may restrict or interfere with the circulation of the hot exhaust gases G within the heat exchanger 30. Alternatively, too few thermally conductive passageways 52 may not increase the surface area sufficiently to increase heat transfer efficiency. Further, if the internal diameter of each thermally conductive passageways 52 is too small for the fluid F, the flow rate through the passageways 52 can be ineffective or the passageways could become clogged or plugged by the fluid F or contaminants therein.

In the case of direct-fired systems, shown in FIG. 2C, where the firetube 30 is immersed in a process fluid F_P, the passageways 52 could be prone to plugging by contaminants entrained within the process fluids F_P passing therethrough. For conventional oilfield operations, Applicant believes that each of the passageways 52 could have a diameter in the range of from about 15% to about 18% of the diameter of the gas inlet 44 for achieving effective heat transfer.

In the case of indirect-fired systems, shown in FIG. 2D, the firetube 30 is immersed in a substantially clean, heat transfer fluid such as glycol. A fluid-to-fluid heat exchanger 70 is provided for flowing the process fluid F_P therethrough, the fluid-to-fluid heat exchanger 70 being immersed in the heat transfer fluid F. Heat transferred from the gas G to the heat transfer fluid F is transferred the process fluid F_P. Having minimized risk of clogging of the passageways 52, as clean fluid F flows therethrough, the passageways 52 could be made with a smaller diameter than in the direct-fired system. Further, in the indirect-fired systems, additional passageways 52 may be added to further increase the surface area and thus, increase the heat transfer efficiency.

As shown in FIG. 8, in embodiments, the thermally conductive passageways 52 can be upright or substantially vertical pipes passing through the body 32. Having reference to FIG. 9, the thermally conductive passageways 52 can have a variety of shapes or profiles when viewed in cross-section, for example those profiles including those shown viewed from left to right, having a narrow fluid inlet 62 with a wide fluid outlet 64, a wide fluid inlet 62 with a narrow fluid outlet 64, a narrow fluid inlet 62 and exit 64 with an enlarged intermediate portion, and one having a uniform profile from inlet 62 to outlet 64.

With reference to FIGS. 10 and 11, each of the plurality of thermally conductive passageways 52 has the fluid inlet 62, fluidly communicating with the fluid F in the vessel V below the body 32, and the fluid outlet 64, fluidly communicating with the fluid F above the body 32. The arrangement of the fluid inlet 62 and outlet 64 permits the fluid F to rise through each passageway 52 and be heated during its passage therethrough. Applicant believes that the fluid to be heated F is circulated through the firetube 30 and vessel V as a result of a temperature differential which exists between the cooler fluid F at the inlet 62 and the warmed fluid at the outlet 64. The temperature difference would be sufficient to cause a natural convection current or a thermosiphon effect for urging the fluid F to circulate through the plurality of passageways 52 and cause circulation of the fluid F throughout the vessel V.

As the fluid F heats, the fluid F becomes less dense and rises within within each of the passageways 52, passing therethrough, receiving heat from the tubular wall 54 and rising within the vessel V. The heated fluid F exits the outlet 64 at a temperature greater than that of the nominal vessel temperatures and releases heat thereto. As heated fluid F transfers its heat, the fluid F begins to sink within the vessel V establishing a convective circulation.

In an embodiment, as seen in FIG. 11, heat transfer can be enhanced from the gas G to the fluid F in the passages 52 by the addition of vortex generators 80 adjacent one or more of the passageway fluid inlets 62. The vortex generators 80 impart a swirl of the fluid rising within the passageway 52. The swirling action acts to increase the retention time of the fluid F within the thermally conductive passageways 52, permitting more efficient transfer of heat to the fluid F therein. Further, it is believed that the vortex generators 80 cause more cooler or dense fluids, flowing through the passageways 52, to move from the center of the flow to the outside, effectively creating a laminar flow adjacent the internal surface 58 which aids the heat transfer.

Further, as the heated fluid F becomes hotter, a natural separation of constituents occurs between the dense fluid and less dense fluid. This phenomenon is particularly advantageous when the fluid F is an unstable emulsion.

Applicant believes, this is a useful phenomenon in the case of vessels such as heater-treaters and free water knock-out vessels, where separation of hydrocarbons and water can also occur. Applicant believes that the effect of the fluid F entering the passageways 52, followed by an expansion of the fluid F leaving the passageways 52, aids in the separation of the hydrocarbons from water. The constituents of the process fluid F_P, particularly the hydrocarbons and the water, have different viscosities and heat coefficients causing them to expand and contract at different rates. The expansion and contractions aids in coalescence of like molecules which assists in separation of the different constituents.

In an example, employing embodiments discussed herein, for a process vessel having 2 million British Thermal Unit (BTU) heat exchanger capacity, the surface area may be increased as much as 50% compared to a conventional U-tube which is sized to be installed in the same size manway. The increased surface area is directly reflected in the increased heat which can be transferred to the fluid F in the vessel V.

Claims

1. A firetube adapted to extend horizontally into a vessel for heating fluid therein, the firetube having a gas inlet, a gas outlet and at least one flowpath therebetween, the firetube conducting hot gases along the flowpath from the gas inlet to the gas outlet, the firetube comprising:

a plurality of passageways spaced along the flowpath for passing fluid upwardly therethrough, each passageway extending generally upwardly from a fluid inlet at a lower portion of the firetube to a fluid outlet at an upper portion of the firetube and having a thermally conductive wall extending through the flowpath for conducting heat from the hot gases to the fluid passing therethrough.

2. The firetube of claim 1 wherein the flowpath is generally U-shaped from the gas inlet to the gas outlet.

3. The firetube of claim 2 wherein the generally U-shaped flowpath comprises

a U-tube conduit having the gas inlet adjacent the gas outlet at a common header wall, the plurality of passageways being spaced along the U-tube conduit.

4. The firetube of claim 2 wherein the firetube is a hollow body having body walls, the gas inlet being adjacent the gas outlet at a common header wall, the generally U-shaped flowpath comprising:

at least one dividing wall extending from between the gas inlet and the gas outlet, partially along the hollow body from the common header wall and toward a tube end, wherein

the gases are directed to flow along the U-shaped flowpath from the gas inlet, about a distal end of the at least one dividing wall, and to the gas outlet.

5. The firetube of claim 2 wherein at least some of the plurality passageways are distributed laterally across the flowpath.

6. The firetube of claim 4 wherein

at least some of the plurality of passageways are integral with the at least one dividing wall.

7. The firetube of claim 6 wherein passageways integral with the at least one dividing wall are substantially aligned and connected therebetween by plates to urge gas along the U-shaped flowpath.

8. The firetube of claim 1 wherein the flowpath comprises two side-by-side flowpaths.

9. The firetube of claim 8 wherein the firetube is a hollow body having enclosing body walls, the gas inlet being adjacent the gas outlet at a common header wall, the two, side-by-side flowpaths comprise:

a first dividing wall, intermediate the gas inlet and the gas outlet, and extending from the common header wall toward a tube end,;

a second dividing wall extending from the common header wall intermediate the gas inlet; and

a third dividing wall extending from the common header wall intermediate the gas outlet,

wherein gases flow along the two, side-by-side flowpaths from the gas inlet, about distal ends of the first, second and third dividing walls and to the gas outlet,

10. The firetube of claim 9 wherein:

the distal end of the second dividing wall and the distal end of the third dividing wall art are connected; and wherein

a first flowpath of the side-by-side flowpaths is formed from the gas inlet, between the first and second divider walls, and to the gas outlet between the first and third divider walls, and

a second flowpath of the side-by-side flowpaths is formed from the gas inlet, between the first divider wall and the body walls, and to the gas outlet between the first divider wall and the body walls.

11. The firetube of claim 1 wherein the thermally conductive passageways are pipes extending substantially vertically through the firetube.

12. The firetube of claim 1 further comprising a vortex generator at one or more of the passageway fluid inlets.

13. A heat exchanger for a vessel comprising the firetube of claim 1, wherein

the vessel is a direct-fired vessel and the fluid is a process fluid to be heated by the firetube, the firetube being immersed in the process fluid.

14. The heat exchanger of claim 13 wherein each of the plurality of passageways has a diameter from about 15% to about 18% of a diameter of the inlet.

15. A heat exchanger for a vessel comprising the firetube of claim 1, wherein

the vessel is an indirect-fired vessel and the fluid is a heat transfer fluid to be heated by the firetube, the heat exchanger further comprising a fluid-to-fluid heat exchanger for flowing a process fluid therethrough, the fluid-to-fluid heat exchanger being immersed in the heat transfer fluid.

16. The heat exchanger of claim 15 wherein the heat transfer fluid is glycol.

17. The heat exchanger of claim 13 wherein

the firetube is obround in cross-section and is installed through an obround manway formed in the vessel.