Note: Descriptions are shown in the official language in which they were submitted.
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FURNACE COIL MODIFIED FINS
FIELD OF THE INVENTION
The present invention relates to the field of cracking paraffins to olefins
and
more particularly to substantial fins on the external surface of the process
coil(s) in the
radiant section of a cracking furnace. The fins may be transverse (horizontal)
or
longitudinal. The fins have an array selected from the group consisting of
upwardly or
outwardly open grooves having a depth of less than a quarter of the maximum
thickness of the fin; or protuberances having a base not exceeding 10 % of the
maximum thickness of the fin, and a height not exceeding 15% of the maximum
thickness of the fin or both, in a regular or semi-regular pattern covering at
least 10%
of the surface area of at least one major surface of the fin.
BACKGROUND OF THE INVENTION
The field of heat exchanger designs is replete with applications of fins to
improve the heat transfer. Typically this is heat transfer by forced
convection
mechanism. Heat transfer by forced convection takes place between a solid
surface
and fluid in motion, which may be gas or liquid, and it comprises the combined
effects
of conduction and convection. This type of heat transfer occurs in most of the
conventional heating systems, either hot water or electric, and industrial
heat
exchangers.
In the cracking of a feed comprising paraffins, typically C2-4 paraffins, such
as
ethane, or naphtha, or mixtures thereof, the feed typically together with
diluent steam
is fed into a cracker comprising a series of pipes or tubes passing through
several
sections of a furnace. First the feed passes through the tubes in the
convection
section of the furnace where exhaust gasses flowing from the downstream
radiant
section of the furnace heat the external surfaces of the tubes. There, the
feed is
heated to a temperature at or near the level at which cracking may begin. Then
the
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feed flows to the tubes in the radiant section of the furnace where the tubes
are
primarily heated by radiation from the refractory walls of the furnace and
from
combustion gases generated by burners typically mounted in the floor or walls
of the
radiant section. Some forced convection heating of the tubes is also provided
by the
combustion gases. Feed is heated in the furnace radiant section up to a
temperature
of about 800 C - 950 C. At these temperatures, the feed undergoes a number
of
reactions, including a free radical decomposition (cracking), reformation of a
new
unsaturated product and the coproduction of hydrogen. These reactions occur
over a
very short period of time that corresponds to the feed residence time in a
coil. The
residence time is typically from about 0.01 to about 10 seconds, in some cases
from
0.01 to 2 seconds in some cases from 0.01 to 1 second. The reactants may be
heated to temperatures from 750 C to 950 C, in some cases from 800 C to 900
C at
a pressure from 200 to 500 kPa in some cases from 250 kPa to 550 kPa.
The interior of the radiant section of the furnace is lined with heat
absorbing/radiating refractory, and is heated typically by gas fired burners.
The cracked gas exits the radiant section of a furnace and then passes through
a transfer line exchanger to a quencher to rapidly cool the product stream to
a
temperature at which the reaction stops. The resulting product stream is then
separated into various components such as ethylene, propylene etc.
There is a drive to improve the efficiency of cracking furnaces as this
reduces
process costs and greenhouse gas emissions. There have been two main
approaches to improving efficiency: the first by improving heat transfer to
the furnace
coils, i.e. from flame, combustion gases and refractory walls to the external
surface of
a process coil; and the second by improving heat transfer within the coil,
i.e. from the
coil internal walls into the feed flowing inside the coil.
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One of the methods representing the second approach, is the addition of
internal fins to the inner walls of the furnace coil, to promote the
"swirling" or enhanced
mixing of the feed within the coil. This improves the convective heat transfer
from the
coil walls to the feed as the turbulence of the feed flow is increased and the
heat
transferring surface of the hot inner wall of the coil is increased as well.
United States patent 5,950,718 issued Sept. 14, 199 to Sugitani et al.
assigned
to Kubota Corporation provides one example of this type of technology.
The papers "Three dimensional coupled simulation of furnaces and reactor
tubes for the thermal cracking of hydrocarbons", by T. Detemmerman, G.F.
Froment,(
Universiteit Gent, Krijgslaan 281, b9000 Gent¨ Belgium, mars-avri, 1998); and
"Three dimensional simulation of high internally finned cracking coils for
olefins
production severity", by Jjo de Saegher, T. Detemmerman, G.F. Froment,
(Universiteit
Gent1, Laboratorium voor Petrochernische Techniek, Krijgslaan 281, b-9000
Gent,
Belgium,1998 provide a theoretical simulation of a cracking process in a coil
which is
internally finned with helicoidal and longitudinal fins (or rather ridges or
bumps). The
simulation results are verified by lab scale experiments, where hot air flows
through
such internally finned tubes. The papers conclude that the tube with internal
helicoidal
fins performs better then with internal longitudinal fins and that the results
for "a tube
with internal helicoidal fins are in excellent agreement with industrial
observations".
However, no experimental data are provided to support these conclusions. There
is
also no comparison made to the performance of a bare tube, with no internal
ribs or
fins. The authors agree that one potential disadvantage of such coils with
internal fins
is that carbon deposits may build up on the fins, increasing the pressure drop
through
the tube.
United States patent application 20030015316 published Jan 23, 2003 in the
name of Burkay teaches a heat exchanger tube having internal fins and external
fins.
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There is no teaching or suggestion in Burkay that the external fins should
have
additional grooves on their external surface. The patent application teaches
away
from the subject matter of the present application.
NOVA Chemicals United States patent 7,128,139 issued Oct. 31, 2006 teaches
external annular fins on the cracking furnace coil to increase convection heat
exchange to the coil. The patent fails to teach or suggest the fins have
further
grooves on the major external surface of the fins.
United States patent 7,096,931 issued August 29, 2006 to Chang et al.
assigned to ExxonMobil Research and Engineering Company teaches an externally
finned heat exchanger tube in a slurry reaction (Fischer Tropsch synthesis).
In the
reaction, a slurry of CO and hydrogen in a hydrocarbyl diluent containing
catalyst,
flows over the external surface of heat exchanger tubes containing flowing
cooling
water. The heat exchanger tubes has ribs having an aspect ratio of less than
5.
There is no teaching or suggestion in the patent that the fins have further
grooves on
their major external surface.
United States patent application 2012/0251407 published in the name of Petela
et al., assigned to NOVA Chemicals (International) S.A. teaches longitudinal
fins on
furnace tubes in the radiant section of a cracking furnace. The fins do not
have
grooves on their surface. Paragraph 54 teaches the thickness of the fin at its
base.
Typically the fin has a thickness at its base from 6% to 25% of the diameter
of the
tube, preferably from 7.5% to 15% of the diameter of the furnace tube.
United States patent 8,790,602 issued July 29, 2014 to Petela et al., assigned
to NOVA Chemicals (International) S.A. teaches furnace tubes or coils used in
the
radiant section of a cracking furnace having protuberances on their surface.
The
patent does not teach or suggest fins having protuberances on the surface of
the coils
used in the radiant section of the furnace.
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United States patent 7,743,821 issued June 29, 2010 to Bunker et al., assigned
to General Electric Company teaches a heat exchanger tube having an annular
fin
which is dimpled, mechanically or molded, on at least a portion of its major
surface.
The heat exchanger is used to cool gas or air (i.e. air conditioners). The
heat
exchanger is primarily concerned with convective heat exchange rather than
radiant
heat exchange. The heat exchanger is not comparable to the tubes in a cracking
furnace. There is no written disclosure of the wall thickness of the heat
exchanger
tube, or the thickness of the fin. From the figures the dimples appear to be
about a
half to a third the thickness of the fin which is significantly greater than
the maximum
of one quarter of the thickness of the fin required in the present invention.
United States patent 8,376,033 issued Feb. 19, 2013 to Robidou et al.,
assigned to GEA Batignolles Technologies Thermiques teaches a comparable fin
in a
convection heat exchanger except that the grooves are of diminishing depth
from the
base of the fin to the external edge. The patent teaches that the fin may have
a
thickness at its inner edge (base) from about 0.4 to 1 mm and a thickness at
its outer
edge from 0.15 to 0.4 mm (Col. 5 lines 25-30). The patent also teaches that
the
grooves may have a depth (thickness) between 0.4 and 1.5 mm. The grooves seem
to have a thickness of about half the thickness of the fin. Again these fins
are for
convective heating and not for radiant heating as in a cracking furnace.
The present invention seeks to provide thick or substantial fins for furnace
tubes having on at least one major surface an array selected from the group
consisting of: upwardly or outwardly open grooves having a depth of less than
a
quarter of the thickness of the fin; or protuberances having a base with the
main
dimension not exceeding 10 % of the maximum thickness of the fin, and a height
not
exceeding 15% of the maximum thickness of the fin; or both, in a regular or
semi-
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regular pattern covering at least 10% of the surface area of at least one
major surface
of said fin.
SUMMARY OF THE INVENTION
In one embodiment there is provided a furnace tube having on its external
surface one or more thick fins having a thickness at its base from 1/4 to 3/4
of the of
the radius of said furnace tube and having parallel sides or sides with an
upward
inward taper of less than 15 relative to the major axis of said fin, said fin
having on at
least one major surface an array selected from the group consisting of
outwardly open
grooves in a regular or semi-regular pattern covering at least 10% of the
surface area
of said grooves having a depth of less than a quarter of the maximum thickness
of the
fin; and protuberances having a base not exceeding 10 % of the maximum
thickness
of the fin, and a height not exceeding 15% of the maximum thickness of the
fin; or
both in a regular or semi-regular pattern covering at least 10% of the surface
area of
at least one major surface of said fin.
In a further embodiment there is provided a furnace tube wherein the grooves
have a depth from a eighth to a tenth of the maximum thickness of the fin.
In a further embodiment there is provided a furnace tube wherein the array of
grooves covers not less than one quarter of at least one major surface of the
fin.
In a further embodiment there is provided a furnace tube wherein the grooves
are in the form of an outwardly open V, a truncated outwardly open V, an
outwardly
open U, and an outwardly open parallel sided channel.
In a further embodiment there is provided a furnace tube wherein the fin forms
a transverse plate in the form of a circle, ellipse, or an N-sided polygon.
In a further embodiment the base of the fins has a thickness from a third to
one
half of the radius of the furnace tube.
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In a further embodiment there is provided a furnace tube wherein the fin is a
longitudinal fin having a cross section in the form of an outwardly extending
parabola,
parallelogram, or "E" shape (monolith with parallel longitudinal channels) or
a blunted
"V".
In a further embodiment there is provided a furnace tube wherein the array of
grooves covers not less than one quarter of at least one major surface of the
fin.
In a further embodiment there is provided a furnace tube wherein the grooves
have a depth from a eighth to a tenth of the maximum thickness of the fin.
In a further embodiment there is provided a furnace tube wherein the grooves
are in the form of an outwardly open V, a truncated outwardly open V, an
outwardly
open U, an outwardly open parallel sided channel.
In a further embodiment there is provided a furnace tube having horizontal
fins
being spaced apart at least two times the external diameter of the furnace
tube.
In a further embodiment there is provided a furnace tube having longitudinal
fins the base of said fins covering from one third to a half of the radius of
the furnace
tube.
In a further embodiment there is provided furnace a tube wherein the array
comprises protuberances having:
i) a maximum height from 3 to 15% of the base of the fin;
ii) a contact surface with a fin, or a base, which main dimension is 0.1%-
10% of the fin thickness ;
iii) a geometrical shape which has a relatively large external
surface
containing a relatively small volume.
In a further embodiment there is provided a furnace tube wherein the
protuberance has a shape selected from the group consisting of:
a tetrahedron;
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a Johnson square pyramid;
a pyramid with 4 isosceles triangle sides;
a pyramid with isosceles triangle sides;
a section of a sphere;
a section of an ellipsoid; and.
a section of a tear drop;
a section of a parabola
In a further embodiment there is provided a furnace tube
wherein the furnace tube and the fin comprise the same metal composition.
In a further embodiment there is provided a furnace tube, and fin(s)
comprising
from about 55 to 65 weight A of Ni; from about 20 to 10 weight % of Cr; from
about 20
to 10 weight A of Co; and from about 5 to 9 weight % of Fe and the balance
one or
more of the trace elements.
In a further embodiment there is provided a furnace tube, and fin(s) further
comprising from 0.2 up to 3 weight % of Mn; from 0.3 to 2 weight % of Si; less
than 5
weight % of titanium, niobium and all other trace metals; and carbon in an
amount of
less than 0.75 weight % the sum of the components adding up to 100 weight %.
In a further embodiment there is provided a furnace tube, and fin(s)
comprising
from 40 to 65 weight % of Co; from 15 to 20 weight % of Cr; from 20 to 13
weight % of
Ni; less than 4 weight A of Fe and the balance of one or more trace elements
and up
to 20 weight % of W the sum of the components adding up to 100 weight %.
In a further embodiment there is provided a furnace tube, and fin(s) further
comprising from 0.2 up to 3 weight % of Mn; from 0.3 to 2 weight % of Si; less
than 5
weight A of titanium, niobium and all other trace metals; and carbon in an
amount of
less than 0.75 weight %.
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In a further embodiment there is provided a furnace tube, and fin(s)
comprising
from 20 to 38 weight % of chromium from 25 to 48, weight % of Ni.
In a further embodiment there is provided a furnace tube, and fin(s)
further comprising from 0.2 up to 3 weight % of Mn, from 0.3 to 2 weight % of
Si; less
than 5 weight % of titanium, niobium and all other trace metals; and carbon in
an
amount of less than 0.75 weight A and the balance substantially iron.
In a further embodiment there is provided a cracking furnace comprising a
radiant section having furnace tubes as above.
In a further embodiment there is provided a method of cracking a paraffin
comprising passing the paraffin in a gaseous state through the radiant section
of a
cracking furnace as above at a temperature from 600 C to 950 C for a time
from
0.001 to 0.01 seconds, and separating the resulting olefins from the feed and
co-
products
The present invention also provides any and all combinations of the foregoing
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a furnace tube with longitudinal fins of the present invention
modified with grooves on the surface.
Figure 2 shows a fin of the present invention modified with protuberances of
the
present invention.
Figure 3 is a graph showing the per cent increase in the surface area of the
fin
modified with different protuberances of the present invention.
DETAILED DESCRIPTION
Numbers ranges
[1] Other than in the operating examples or where otherwise indicated,
all numbers
or expressions referring to quantities of ingredients, reaction conditions,
etc. used in
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the specification and claims are to be understood as modified in all instances
by the
term "about." Accordingly, unless indicated to the contrary, the numerical
parameters
set forth in the following specification and attached claims are
approximations that can
vary depending upon the properties that the present invention desires to
obtain. At
the very least, and not as an attempt to limit the application of the doctrine
of
equivalents to the scope of the claims, each numerical parameter should at
least be
construed in light of the number of reported significant digits and by
applying ordinary
rounding techniques.
[2] Notwithstanding that the numerical ranges and parameters setting forth
the
broad scope of the invention are approximations, the numerical values set
forth in the
specific examples are reported as precisely as possible. Any numerical values,
however, inherently contain certain errors necessarily resulting from the
standard
deviation found in their respective testing measurements.
[3] Also, it should be understood that any numerical range recited herein
is
intended to include all sub-ranges subsumed therein. For example, a range of
"1 to
10" is intended to include all sub-ranges between and including the recited
minimum
value of 1 and the recited maximum value of 10; that is, having a minimum
value
equal to or greater than 1 and a maximum value of equal to or less than 10.
Because
the disclosed numerical ranges are continuous, they include every value
between the
minimum and maximum values. Unless expressly indicated otherwise, the various
numerical ranges specified in this application are approximations.
[4] All compositional ranges expressed herein are limited in total to and
do not
exceed 100 percent (volume percent or weight percent) in practice. Where
multiple
components can be present in a composition, the sum of the maximum amounts of
each component can exceed 100 percent, with the understanding that, and as
those
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skilled in the art readily understand, the amounts of the components actually
used will
conform to the maximum of 100 percent.
As used in this specification the term outwardly when referring to the grooves
is
outward relative to the major plane of the fin which they are on.
As used in this specification fin height refers to the distance the fin
extends
away from the external surface of the furnace tube.
In accordance with the present invention the furnace tubes have fins which
have high integrity, good stress resistance and are quite thick. Typically,
the fins will
have a thickness at their base of not less than about 33% of the radius of the
furnace
tube, typically about 40%, desirably not less than about 45%, in some
embodiments
up to 50% of the radius of the tube. The fins are thick or stubby. They have a
height
to maximum width ratio of from about 0.5 to 5, typically 1 to 3. The sides
(edges) of
the fin may be parallel or be lightly tapered inward toward the external edge
of the fin.
The angle of taper should be no more than about 15 , typically about 100 or
less
inward relative to the center line of the fin. The edge of the fin may be
flat, pointed (at
a 30 to 450 angle from each surface), or have a blunt rounded nose. The fins
may
have a cross section shape in the form of an outwardly extending parabola,
parallelogram, of a blunt "V" shape In some cases, preferably for longitudinal
fins, the
fin cross section may be "E" shaped (monolith with parallel longitudinal
extensions
(having parallel grooves).
In one embodiment at least one major surface of the fin has an array of
outwardly open grooves in a regular or semi-regular pattern covering at least
10% of
the surface area of at least one major surface of the fin (e.g. top or bottom
for
horizontal fins or sides for longitudinal fins), said grooves having a depth
of less than a
quarter, in some instances from a eighth to a tenth of the maximum thickness
of the
fin. The array may cover not less than 25%, in some cases not less than 50%,
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preferably greater than 75%, most preferably greater than 85 % up to 100% of
the of
the surface area of one or more the major surfaces of the fin. The array could
be in
the form of parallel lines, straight or wavy, parallel to or at an angle from
the major
axis of the fin, crossed lines, wavy lines, squares, or rectangles. The
grooves may be
in the form of an outwardly open V, a truncated outwardly open V, an outwardly
open
U, and an outwardly open parallel sided channel.
The fins may be transverse or parallel (e.g. longitudinal) to the major axis
of the
furnace tube. The transverse fins could be at an angle from about 0 to 25
off
perpendicular relative to the major axis of the furnace tube. However, it is
more costly
and difficult to make transvers fins at an angle off perpendicular to the
major axis of
the tube. The transverse fins may have a shape selected from a circle, an
ellipse, or
an N sided polygon where N is a whole number greater than or equal to 3. In
some
embodiments N is from 4 to 12. The major surface(s) for the transverse fins
are the
upper and bottom face of the fin. Transverse fins should be spaced apart at
least two
times in some instances from 3 to 5 times, the external diameter of the
furnace tube.
The longitudinal fins may have a shape of a parallelogram, a part of an
ellipse
or circle and a length from about 50% of the length of the furnace tube
(sometimes
referred to pass) in the radiant section up to 100% of the length of the
furnace tube in
the radiant section and all ranges in between.
The base of the longitudinal fin may be not less than one quarter of the
radius
of the furnace tube, in some instances from 1/4 to 1/4 typically from about
1/3 to 3/4 or in
some instances1/3 to 5/8 in other instances from 1/3 to 1/2 of the radius of
the furnace
tube. The fins are thick or stubby. They have a ratio of height to maximum
width of
from about 0.5 to 5, typically 1 to 3. The sides (edges) of the fin may be
parallel or be
lightly tapered inward toward the tip of the fin. The angle of taper should be
no more
than about 15 , typically about 10 or less inward relative to the center
line of the fin.
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The tip or leading edge of the fin may be flat, tapered (at a 300 to 45 angle
from the
top and bottom surfaces of the fin), or have a blunt rounded nose. The leading
edge of
the longitudinal fin will typically be parallel to the central axis of the
furnace tube. In
cases where the fin extends less than 100% of the length of the furnace tube
the
leading edge of the fin will for the most part be parallel to the central axis
of the
furnace tube and then angle in to the furnace tube wall at an angle between
about 60
and 30 typically 45 . In some case the fin may end in a flat surface
perpendicular to
the surface of the tube.
A furnace tube or pass having grooved fins will be described in accordance
with figure 1. The furnace tube 1 comprises a central channel 2 and an annular
wall
3. The fins 4 and 5 in this embodiment are straight sided and do not angle or
taper
inwardly to the tips 6 and 7. The fins bear on their surface a series of
parallel grooves-
channels 10.
In a further embodiment of the invention the fins may comprise an array of
protuberances.
Figure 2 shows a fin 20 of the present invention having its surface 21 covered
with one or more protuberances. The protuberances may be in the shape of a
square
pyramid 23, an equilateral cone 24 or a hemisphere 25. The protuberances may
be
applied by casting or machining the fin, or by using a knurl roll so that the
surface 21
of the fin has a textured surface.
The array of protuberances can cover from 10% to 100% (and all ranges in
between) of the external surface of the fin. In some embodiments of the
invention, the
protuberances may cover from 40 to 100%, typically from 50% to 100%, generally
from 70% to 100% of the external surface of the fin radiant coil. If
protuberances do
not cover the entire surface of the fin, they can be located at the bottom,
middle or top
of the fin.
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A protuberance base is in contact with the external coil surface. A base of a
protuberance has an area not larger than from 0.1%-10% of the maximum
thickness
of the fin. Preferably, the protuberance have geometrical shapes having a
relatively
large external surface that contains a relatively small volume, such as for
example
tetrahedrons, pyramids, cubes, cones, a section through a sphere (e.g.
hemispherical
or less), a section through an ellipsoid, a section through a deformed
ellipsoid (e.g. a
tear drop) etc. Some useful shapes for a protuberance include:
a tetrahedron (pyramid with a triangular base and 3 faces that are equilateral
triangles);
a Johnson square pyramid (pyramid with a square base and sides which are
equilateral triangles);
a pyramid with 4 isosceles triangle sides;
a pyramid with isosceles triangle sides (e.g. if it is a four faced pyramid
the
base may not be a square it could be a rectangle or a parallelogram);
a section of a sphere (e.g. a hemi sphere or less);
a section of an ellipsoid (e.g. a section through the shape or volume formed
when an ellipse is rotated through its major or minor axis); and.
a section of a tear drop (e.g. a section through the shape or volume formed
when a non uniformly deformed ellipsoid is rotated along the axis of
deformation);
a section of a parabola (e.g. section though the shape or volume formed when
a parabola is rotated about its major axis ¨ a deformed hemi- (or less)
sphere), such
as e.g. different types of delta-wings.
The selection of the shape of the protuberance is largely based on the ease of
manufacturing the fin. One method for forming protuberances on the fin surface
is by
casting in a mold having the shape of the protuberance in the mold wall. This
is
effective for relative simple shapes. The protuberances may also be produced
by
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machining the external surface of a cast fin such as by the use of knurling
device for
example a knurl roll.
The above protuberances are closed solids.
A protuberance may have a height (Lz) above the surface of the fin from 3% to
15% of the maximum thickness of the fin, and all the ranges in between,
preferably
from 3% to 10% of the maximum thickness of the fin.
In some embodiments the concentration of the protuberances is uniform and
essentially covers the external surface of the fin. However, the concentration
may
also be selected based on the radiation heat flux at the location of the coil
pass (e.g.
some locations may have a higher heat flux than others ¨corners).
In designing the protuberances care must be taken so that they adsorb more
radiant energy than they may radiate. This may be restated as the transfer of
heat
through the base of the protuberance into the fin surface must exceed that
transferred
to the equivalent surface on a bare smooth fin at the same operational
conditions. If
the concentrations of the protuberances become excessive and if their geometry
is not
selected properly, they may start to reduce heat transfer, due to thermal
effects of
excessive conductive resistance, which defeats the purpose of the invention.
The
properly designed and manufactured protuberances will increase net radiative
and
convective heat transferred to a fin, and subsequently to a coil from
surrounding
flowing combustion gasses, flame and furnace refractory. The positive impact
of
protuberances on radiative heat transfer is not only because more heat can be
absorbed through the increased fin external surface so the contact area
between
combustion gases and fin is increased, but also because the relative heat loss
through the radiating fin surface is reduced, as the fin surface is not smooth
any
more. Accordingly, as a protuberance radiates energy to its surroundings, part
of this
energy is delivered to and captured by other protuberances, thus it is re-
directed back
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to the fin surface. The protuberances will also increase the convective heat
transfer to
a fin, due to increase in fin external surface that is in contact with flowing
combustion
gas, and also by increasing turbulence along the fin surface, thus reducing
the
thickness of a gaseous boundary layer adjacent to the fin surface.
Figure 3 is a plot of the percent increase in the area of the surface 21 of
the fin
20 when the protuberances are an equilateral pyramid 26, a square pyramid 23,
an
equilateral cone 24 and a hemisphere 25, having a main dimension 'a' (side
length of
a pyramid or diameter for a cone or hemisphere) in mm.
The size of the protuberance must be carefully selected. Generally, the
smaller
the size, the higher is the surface to volume ratio of a protuberance, but it
may be
more difficult to cast or machine such a texture. In addition, in the case of
excessively small protuberances, the benefit of their presence may become
gradually reduced with time due to settlement of different impurities on the
fin
surface. However, the protuberances need not be ideally symmetrical. For
example
an elliptical base could be deformed to a tear drop shape, and if so shaped
preferably the "tail" may point down, in line with the overall direction of
flue gas flow,
when the coil is positioned in the furnace.
Another important advantage of the fins with grooves or protuberances is that
although the fin has the increased contact surface, its weight might be
reduced.
The fins and the furnace tube may comprise the same material. In some
embodiments the fins are easiest to cast as part of the furnace tube. In other
embodiments the fins may be cast separately and welded in place.
The tube and the fin(s) may comprise from about 55 to 65 weight % of Ni; from
about 20 to 10 weight % of Cr; from about 20 to 10 weight % of Co; and from
about 5
to 9 weight % of Fe and the balance one or more of the trace elements. The
alloy
from which the tube and fins are made may further comprising from 0.2 up to 3
weight
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CA 02930827 2016-05-25
% of Mn; from 0.3 to 2 weight % of Si; less than 5 weight % of titanium,
niobium and
all other trace metals; and carbon in an amount of less than 0.75 weight % the
sum of
the components adding up to 100 weight %.
The furnace tube and fins may comprise from 40 to 65 weight % of Co; from 15
to 20 weight % of Cr; from 20 to 13 weight % of Ni; less than 4 weight % of Fe
and the
balance of one or more trace elements and up to 20 weight % of W the sum of
the
components adding up to 100 weight %. The alloy from which the furnace tube
and
fins are made may further comprise from 0.2 up to 3 weight % of Mn; from 0.3
to 2
weight % of Si; less than 5 weight % of titanium, niobium and all other trace
metals;
and carbon in an amount of less than 0.75 weight % the sum of the components
adding up to 100 weight %.
The furnace tube and fins may comprise from 20 to 38 weight % of chromium
from 25 to 48, weight % of Ni. The alloy from which the furnace tube and fins
may be
made may further comprise from 0.2 up to 3 weight % of Mn, from 0.3 to 2
weight % of
Si; less than 5 weight % of titanium, niobium and all other trace metals; and
carbon in
an amount of less than 0.75 weight % and the balance substantially iron, the
sum of
the components adding up to 100 weight %.
The grooves or protuberances could be machined on the surface of the cast fin.
In some embodiments it is preferred to cold roll (at a temperature below the
recrystallization temperature of the steel) the fin to produce the grooves
/protuberances without removing any material. This may be particularly useful
where
the fins are substantially flat.
The grooves or protuberances could be in a geometric pattern such as
longitudinal or transverse parallel lines, diagonal lines, a cross hatch
pattern, squares,
rectangles, circles, ellipses, etc. The pattern could be regular or semi
¨regular.
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