Note: Descriptions are shown in the official language in which they were submitted.
CA 02761537 2011-12-13
1 "A FIRETUBE HAVING THERMAL CONDUCTING PASSAGEWAYS"
2
3 FIELD OF THE INVENTION
4 Embodiments of the invention relate to indirect-fired and direct-fired
heat exchangers and more particularly to firetubes installed in process
vessels, the
6 firetubes having enhanced surface area for heating process fluids.
7
8 BACKGROUND OF THE INVENTION
9 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
11 heat exchanger is fit within a vessel for heating fluids, such as those
commonly
12 handled in oilfield handling and refining operations.
13 In the oilfield, U-shaped "firetubes", referred to as U-tube firetubes or
14 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
16 and tanks. Traditionally the U-tube firetube is made of round steel pipe. A
burner
17 supplies a flame and hot exhaust gases for circulation through the firetube
from an
18 inlet to an outlet. Heat is conducted from the pipe walls to the fluid
contained in the
19 vessel.
In a direct-fired vessel, heat is transferred through the firetube wall
21 immersed directly in a process fluid to be heated, the process fluid being
contained
22 in the vessel and in direct contact with the outside of the firetube. In an
indirect-
23 fired vessel, heat is transferred from the firetube to an intermediate heat
exchange
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1 fluid. A fluid-to-fluid heat exchanger contains the process fluid, the
exchanger being
2 immersed in the heat exchange fluid.
3 Conventional U-tube firetubes have the burner mounted at the gas
4 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
6 U-tube exchanger is generally installed inside the vessel through an oval or
obround
7 shaped manway.
8 The ultimate objective in any fired heating system is to create the
9 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
11 wall and the fluid to be heated on the other side. Use of round pipe to
create the U-
12 tube firetube results in a very inefficient heat exchanger as the surface
area
13 presented to the intended fluid is limited. As a result, a significant
amount of the
14 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
16 operate, waste energy used to generate the heat, typically do not optimally
utilize
17 the heat generated, and release large amounts of waste gas to the
environment.
18 Further, in instances where the process heating requirements change
19 and more process fluids enter the operation than design load, the only
alternative
has been to replace the equipment with larger units.
21 Clearly there is a need for improved heat exchangers which are
22 capable of efficiently and cost effectively transferring thermal input to
fluids to be
23 heated.
2
CA 02761537 2011-12-13
1 SUMMARY OF THE INVENTION
2 Generally, embodiments of firetubes, disclosed herein, have an
3 increased surface area without resulting in an overall increase in the size
of the
4 firetube due to a plurality of thermally conducting passageways which extend
through the firetube and direct fluids to be heated therethrough. Each of the
6 passageways has a wall for heat transfer which adds to the external surface
area of
7 the firetube resulting in the increased surface area. In an embodiment,
fluids are
8 caused to rise through the passageways as a result of a temperature
differential in
9 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
11 and less dense. Embodiments of the firetube are suitable for use in direct
and
12 indirect-fired vessels.
13 Advantageously, where process fluids comprise emulsions of water
14 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
16 relatively small diameter passageways aids in coalescence of like
molecules, which
17 assists in separation of the different constituents a vessel.
18 In a broad aspect, a firetube is adapted to extend horizontally into a
19 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
21 inlet to the gas outlet. The firetube comprises a plurality of passageways,
spaced
22 along the flowpath for passing fluid upwardly therethrough. Each passageway
23 extends generally upwardly from a fluid inlet at a lower portion to a fluid
outlet at an
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1 upper portion and has a thermally conductive wall extending through the
flowpath
2 for conducting heat from hot gases to the fluid passing therethrough.
3 Further, a heat exchanger for a vessel comprises the firetube
4 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
6 firetube being immersed in the process fluid. The firetube is also suitable
for use in
7 an indirect-fired vessel where the fluid is a heat transfer fluid to be
heated by the
8 firetube. In this case, the heat exchanger further comprises a fluid-to-
fluid heat
9 exchanger for flowing the process fluid therethrough, the fluid-to-fluid
heat
exchanger being immersed in the heat transfer fluid. In embodiments the heat
11 transfer fluid is glycol.
12 Embodiments of the firetube are suitable for installing in new vessels
13 or can be used to retrofit existing vessels. As the size of the expanded
surface area
14 firetube is substantially the same as the existing prior art firetube, it
can be simply
installed through the existing manway for flanged connection thereto.
16
17
4
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1 BRIEF DESCRIPTION OF THE DRAWINGS
2 Figures 1A - 1 C illustrate a prior art U-tube firetube, more particularly,
3 Fig. 1A is a plan view of the U-tube shown installed in an
4 manway in a front wall of a vessel, a major portion of the vessel having
been
removed for clarity;
6 Fig. 1 B is a plan view of a front wall of the vessel according to
7 Fig. 1A, illustrating an inlet and an outlet of the U-tube installed in a
front wall
8 of the manway; and
9 Fig. 1 C is an elevation view of the front wall of the oval manway
of Fig. 1 B illustrating the inlet and the outlet;
11 Figure 2A is a plan view of a cross-section of one embodiment of a U-
12 tube firetube installed in a vessel, a major portion of the vessel having
been
13 removed for clarity, the firetube having a dividing wall extending
partially along the
14 firetube and being fit with a plurality of thermally conductive
passageways;
Figure 2B is a side cross-sectional view of one thermally conductive
16 passageway fit to portion of a firetube according to Fig. 2A;
17 Figure 2C is a cross-sectional view of a firetube in a direct-fired vessel
18 incorporating an embodiment of the thermally conductive passageways;
19 Figure 2D is a cross-sectional view of an firetube in an indirect-fired
vessel incorporating an embodiment of the thermally conductive passageways;
21 Figure 3A is a plan view of a cross-section of the U-tube firetube of
22 Fig. 2, wherein the dividing wall is formed by a plurality of plates
between a plurality
23 of the thermally conductive passageways;
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1 Figure 3B is an end cross-sectional view through the firetube of Fig.
2 3A, along section lines A-A;
3 Figure 4 is a plan view of a cross-section of the U-tube firetube of Fig.
4 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;
6 Figure 5 is a perspective view of the firetube according to Fig. 4, the
7 body being rendered as transparent for greater clarity;
8 Figure 6 is a plan view of a cross-section of a U-tube firetube
9 according to another embodiment, the thermally conductive passageways
forming a
tortuous flowpath in the body;
11 Figure 7 is a plan view of a cross-section of a conduit firetube
12 according to another embodiment, suitable for retrofit of a vessel having a
prior art
13 firetube according to Fig. 1A;
14 Figure 8 is a side, cross-sectional view of an embodiment of the
firetube illustrating a variety of possible profiles for the thermally
conductive
16 passageways;
17 Figure 9 is a side, cross-sectional view of the firetube and tubular
18 passageways according to Fig. 3A;
19 Figure 10 is a fanciful illustration of fluid flow through tubular
thermally
conductive passageways, from fluid inlets below the firetube to fluid outlets
above
21 the firetube; and
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1 Figure 11 is a fanciful illustration of the fluid flow through the thermally
2 conductive passageways according to Fig.10 and enhanced by the action of
vortex
3 generators mounted adjacent the passageway inlets.
4
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
6 As shown in Figs. 1A-1C, prior art firetubes 10 are generally U-shaped
7 tubes, having a side-by-side gas inlet 12 and gas outlet 14 at a flanged
connection
8 16 at a front wall 24 of vessel 26. The firetube 10 is generally
manufactured from
9 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
11 obround in shape to accommodate the side-by-side inlet 12 and outlet 14.
The gas
12 inlet 12 connects therethrough to a burner (not shown) for receiving flame
and hot
13 exhaust gases therefrom. The outlet 14 connects to an exhaust stack (not
shown)
14 for exhausting waste gases therefrom. Flanged connections are typically
used
throughout.
16 Firetubes, according to embodiments disclosed herein, can be
17 incorporated in new heat exchange vessels or can be used to retrofit
existing
18 vessels to upgrade and enhance the efficiency of heat transfer therein.
Heat
19 transfer surface area is increased over conventional firetubes by providing
a
plurality of thermally conductive passageways which extend through the
firetube.
21 Fluid in the vessel is heated, not only from the periphery of the firetube
but also
22 through fluid conducted through the passageways.
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1 In more detail, and having reference to Fig. 2A-3B, one embodiment
2 of a firetube 30 comprises a hollow shell or body 32 having body walls 38
for
3 containing and directing hot gases G therethrough. In use, the firetube 30
is fit into
4 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
6 G circulate through a flowpath 42, from a gas inlet 44 at a first end 46 to
a gas outlet
7 48 at a second end 50.
8 As shown in Fig. 2A, when the firetube's gas inlet 44 and gas outlet 48
9 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
11 a burner (not shown) and the outlet 48 is adapted for connection to an
exhaust
12 stack (not shown). The gas inlet 44 and gas outlet 48 are fit to a front or
common
13 header wall 34 secured, such as by flanged connection, to the vessel V. The
14 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
16 firetube 30 has a tube end 36 at a farthest extent from the front wall 34.
17 Hot gases G circulate through the flowpath 42, from the gas inlet 44 to
18 the gas outlet 48, heating the body walls 38 and transferring the heat to
fluid F.
19 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
21 gases. Various internal gas-directing structure are illustrated in Figs.
2A, 3A and 4.
22 The gas-directing structure avoids short-circuiting of the flowpath 42 and
maximizes
23 gas contact with the body walls 38. With reference to Fig. 2A, the gas-
directing
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1 structure can be a first dividing wall 40 extending partially along the
hollow body 32,
2 from a proximal end at the common header wall 34, from a location between
the
3 gas inlet 44 and outlet 48, to a distal end located short of the tube end 36
for
4 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
6 area can be enhanced by further providing a plurality of thermally
conductive
7 passageways 52.
8 Best seen in Fig. 2B, the passageways are spaced apart along the
9 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
11 fluid, to be heated, flows upwardly into the fluid inlet 62 from below the
firetube 30 to
12 exit each passageway 52 at the fluid exit 64 above the firetube 30. The
13 passageways 52 have a thermally conductive, tubular wall 54, typically
formed of
14 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
16 contacting the fluid F. The walls 54 of the plurality of passageways 52
provide
17 additional heat transfer surface over that conventionally provided by the
prior art U-
18 tube firetube.
19 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
21 the cycle. Where no mechanical impetus is provided, the fluid F movement is
like a
22 thermosiphon circulation.
9
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1 Noteably, such an increase in the heat-transferring surface area is
2 accomplished without an increase in the overall size of the firetube 30.
Thus, in an
3 embodiment, the firetube 30 can be installed, as a retrofit, through the
obround
4 manway 22 of an existing vessel V, increasing the vessel's heating
capability over
its original design rating.
6 In an embodiment, as shown in Figs 3A and 3B, a plurality of the
7 thermally conductive passageways 52 can be aligned be integrated with the
dividing
8 wall 40. As shown, the dividing wall 40 is a first wall centrally located
between the
9 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
11 passageways 52 for directing gases G along the passageways 52 to the tube
end
12 36. The plates 60 urge gases G from the gas inlet 44 to the dividing wall's
distal
13 end and back to the gas outlet 48. The plates 60 can be welded between
14 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
16 passing therethrough..
17 The number of passageways 52 fit to the flowpath 42 is a function of
18 the desired or design surface area of the tubular walls 54 while not overly
restricting
19 the flow of gases G therealong.
In another embodiment, shown in Figs. 4 and 5, a second dividing wall
21 40B and third dividing wall 40C are provided, forming two, side-by-side U-
shaped
22 flowpaths 42,42. As shown in Fig. 6, a firetube 30 may or may not have
23 passageways 52 aligned along the first central dividing wall 40. The second
and
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1 third dividing walls 40B, 40C can be connected at distal ends to more
particularly
2 direct the flowpaths. When connected, the second and third dividing walls
40B, 40C
3 form a U-shaped dividing wall 40U wherein a first flowpath 42 is formed from
the
4 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
6 is formed from the gas inlet 44, between the first divider wall 40 and the
body walls
7 38, and to the gas outlet 48 between the first wall 40 and the body walls
38.
8 Returning to Fig. 6, in an embodiment having a centralized dividing
9 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
11 the flow of gas G and increase heat transfer recovered therefrom.
12 Having reference to Fig. 7, alternatively, a plurality of thermally
13 conductive passageways 52 can be retrofitted to the otherwise conventional
prior
14 art U-shaped firetube 10 of Fig. 1 A.
While the plurality of thermally conductive passageways 52 are used
16 to increase the effective surface area of the heat exchanger 30, one of
skill in the art
17 would appreciate that too many or too large a diameter of thermally
conductive
18 passageways 52 may restrict or interfere with the circulation of the hot
exhaust
19 gases G within the heat exchanger 30. Alternatively, too few thermally
conductive
passageways 52 may not increase the surface area sufficiently to increase heat
21 transfer efficiency. Further, if the internal diameter of each thermally
conductive
22 passageways 52 is too small for the fluid F, the flow rate through the
passageways
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1 52 can be ineffective or the passageways could become clogged or plugged by
the
2 fluid F or contaminants therein.
3 In the case of direct-fired systems, shown in Fig. 2C, where the
4 firetube 30 is immersed in a process fluid FP, the passageways 52 could be
prone to
plugging by contaminants entrained within the process fluids Fp passing
6 therethrough. For conventional oilfield operations, Applicant believes that
each of
7 the passageways 52 could have a diameter in the range of from about 15% to
about
8 18% of the diameter of the gas inlet 44 for achieving effective heat
transfer.
9 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-
11 fluid heat exchanger 70 is provided for flowing the process fluid Fp
therethrough, the
12 fluid-to-fluid heat exchanger 70 being immersed in the heat transfer fluid
F. Heat
13 transferred from the gas G to the heat transfer fluid F is transferred the
process fluid
14 F. 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
16 the direct-fired system. Further, in the indirect-fired systems, additional
17 passageways 52 may be added to further increase the surface area and thus,
18 increase the heat transfer efficiency.
19 As shown in Fig. 8, in embodiments, the thermally conductive
passageways 52 can be upright or substantially vertical pipes passing through
the
21 body 32. Having reference to Fig. 9, the thermally conductive passageways
52 can
22 have a variety of shapes or profiles when viewed in cross-section, for
example
23 those profiles including those shown viewed from left to right, having a
narrow fluid
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1 inlet 62 with a wide fluid outlet 64, a wide fluid inlet 62 with a narrow
fluid outlet 64,
2 a narrow fluid inlet 62 and exit 64 with an enlarged intermediate portion,
and one
3 having a uniform profile from inlet 62 to outlet 64.
4 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
6 fluid F in the vessel V below the body 32, and the fluid outlet 64, fluidly
7 communicating with the fluid F above the body 32. The arrangement of the
fluid
8 inlet 62 and outlet 64 permits the fluid F to rise through each passageway
52 and be
9 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
11 temperature differential which exists between the cooler fluid F at the
inlet 62 and
12 the warmed fluid at the outlet 64. The temperature difference would be
sufficient to
13 cause a natural convection current or a thermosiphon effect for urging the
fluid F to
14 circulate through the plurality of passageways 52 and cause circulation of
the fluid F
throughout the vessel V.
16 As the fluid F heats, the fluid F becomes less dense and rises within
17 within each of the passageways 52, passing therethrough, receiving heat
from the
18 tubular wall 54 and rising within the vessel V. The heated fluid F exits
the outlet 64
19 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
21 vessel V establishing a convective circulation.
22 In an embodiment, as seen in Fig. 11, heat transfer can be enhanced
23 from the gas G to the fluid F in the passages 52 by the addition of vortex
generators
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1 80 adjacent one or more of the passageway fluid inlets 62. The vortex
generators
2 80 impart a swirl of the fluid rising within the passageway 52. The swirling
action
3 acts to increase the retention time of the fluid F within the thermally
conductive
4 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
6 fluids, flowing through the passageways 52, to move from the center of the
flow to
7 the outside, effectively creating a laminar flow adjacent the internal
surface 58
8 which aids the heat transfer.
9 Further, as the heated fluid F becomes hotter, a natural separation of
constituents occurs between the dense fluid and less dense fluid. This
phenomenon
11 is particularly advantageous when the fluid F is an unstable emulsion.
12 Applicant believes, this is a useful phenomenon in the case of vessels
13 such as heater-treaters and free water knock-out vessels, where separation
of
14 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
16 the passageways 52, aids in the separation of the hydrocarbons from water.
The
17 constituents of the process fluid FP, particularly the hydrocarbons and the
water,
18 have different viscosities and heat coefficients causing them to expand and
contract
19 at different rates. The expansion and contractions aids in coalescence of
like
molecules which assists in separation of the different constituents.
21
14
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1 In an example, employing embodiments discussed herein, for a
2 process vessel having 2 million British Thermal Unit (BTU) heat exchanger
capacity,
3 the surface area may be increased as much as 50% compared to a conventional
U-
4 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
6 in the vessel V.
7
