U.S. patent application number 15/601295 was filed with the patent office on 2017-11-30 for furnace coil modified fins.
This patent application is currently assigned to NOVA Chemicals (International) S.A.. The applicant listed for this patent is NOVA Chemicals (International) S.A.. Invention is credited to Leslie Wilfred Benum, Jeffrey Stephen Crowe, Jeffrey Thomas Kluthe, Evan Geevouy Mah, Grazyna Petela.
Application Number | 20170343301 15/601295 |
Document ID | / |
Family ID | 58800866 |
Filed Date | 2017-11-30 |
United States Patent
Application |
20170343301 |
Kind Code |
A1 |
Petela; Grazyna ; et
al. |
November 30, 2017 |
FURNACE COIL MODIFIED FINS
Abstract
The present disclosure provides for thick fins on the surface of
coils or tubes in a steam cracking furnace. The fins have a
thickness at their base from 1/4 to 3/4 of the radius of the
furnace tube. The fins have grooves or protuberances on not less
than about 10% of a major surface. The fins help increase the
radiant heat taken up by the tube from the walls and combustion
gases in the furnace.
Inventors: |
Petela; Grazyna; (Calgary,
CA) ; Benum; Leslie Wilfred; (Red Deer, CA) ;
Mah; Evan Geevouy; (Calgary, CA) ; Kluthe; Jeffrey
Thomas; (Lacombe, CA) ; Crowe; Jeffrey Stephen;
(Calgary, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NOVA Chemicals (International) S.A. |
Fribourg |
|
CH |
|
|
Assignee: |
NOVA Chemicals (International)
S.A.
Fribourg
CH
|
Family ID: |
58800866 |
Appl. No.: |
15/601295 |
Filed: |
May 22, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 30/00 20130101;
C22C 19/058 20130101; F28D 2021/0075 20130101; C22C 19/07 20130101;
F28F 1/38 20130101; F28F 1/12 20130101; F28F 1/025 20130101; C10G
9/20 20130101; C22C 19/056 20130101; F28F 1/14 20130101; F28F
2215/10 20130101; F28D 2021/0056 20130101 |
International
Class: |
F28F 1/12 20060101
F28F001/12; F28F 1/02 20060101 F28F001/02 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2016 |
CA |
2930827 |
Claims
1. 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.degree. relative to the
major axis of said fin, said fin having on at least one major
surface an array selected from: outwardly open grooves in a regular
or semi-regular pattern covering at least 10% of the surface area,
said grooves having a depth of less than a quarter of the maximum
thickness of the fin; protuberances having a base 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-regular pattern covering at least 10% of the
surface area of at least one major surface of said fin.
2. The furnace tube according to claim 1, wherein the array covers
not less than one quarter of at least one major surface of the
fin.
3. The furnace tube according to claim 2, wherein the fin has a
thickness at it base from 1/3 to the radius of the furnace
tube.
4. The furnace tube according to claim 3, wherein the fin has a
cross section in the form of an outwardly extending parabola,
parallelogram, an "E" shape, or a blunted "V".
5. The furnace tube according to claim 4, wherein the array
comprises grooves having a depth from a eighth to a tenth of the
maximum thickness of the fin.
6. The furnace tube according to claim 5, wherein the grooves are
in a form selected from an outwardly open V, a truncated outwardly
open V, an outwardly open U, and an outwardly open parallel sided
channel.
7. The furnace according to claim 3, 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.
8. The furnace tube according to claim 7, wherein the protuberance
has a shape selected from: a tetrahedron; 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.
9. The furnace tube according to claim 5, wherein the fin forms a
transverse plate in the form of a circle, ellipse, or an N sided
polygon.
10. The furnace tube according to claim 7, wherein the fin forms a
transverse plate in the form of a circle, ellipse, or an N sided
polygon.
11. The furnace tube according to claim 5, wherein the fin is a
longitudinal fin having a cross section in the form of an outwardly
extending parabola, parallelogram, or an "E" shape.
12. The furnace tube according to claim 7, wherein the fin is a
longitudinal fin having a cross section in the form of an outwardly
extending parabola, parallelogram, or an "E" shape.
13. The furnace tube according to claim 1, wherein the furnace tube
and the fin comprise the same metal composition.
14. The furnace tube according to claim 13, comprising 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.
15. The furnace tube according to claim 14, 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 %.
16. The furnace tube according to claim 13, 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 % 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 %.
17. The furnace tube according to claim 16, 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 %.
18. The furnace tube according to claim 13, comprising from 20 to
38 weight % of chromium from 25 to 48, weight % of Ni.
19. The furnace tube according to claim 18, 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 % and the balance
substantially iron.
20. A cracking furnace comprising a radiant section having furnace
tubes according to claim 1.
21. A method of cracking a paraffin comprising passing the paraffin
in a gaseous state through the radiant section of a cracking
furnace according to claim 20, at a temperature from 600.degree. C.
to 1000.degree. C. for a time from 0.001 to 0.01 seconds.
Description
[0001] The present disclosure 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 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.
[0002] 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.
[0003] In the cracking of a feed comprising paraffins, typically
C.sub.2-4 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 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.degree.
C.-950.degree. 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.degree. C. to 950.degree. C., in some cases from 800.degree. C.
to 900.degree. C. at a pressure from 200 to 500 kPa in some cases
from 250 kPa to 550 kPa.
[0004] The interior of the radiant section of the furnace is lined
with heat absorbing/radiating refractory, and is heated typically
by gas fired burners.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] U.S. Pat. No. 5,950,718 issued Sep. 14, 199 to Sugitani et
al. assigned to Kubota Corporation provides one example of this
type of technology.
[0009] 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.
[0010] U.S. Patent Application Publication No. 20030015316
published Jan. 23, 2003 in the name of Burkay teaches a heat
exchanger tube having internal fins and external fins. 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.
[0011] U.S. Pat. No. 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.
[0012] U.S. Pat. No. 7,096,931 issued Aug. 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.
[0013] U.S. Patent Application Publication No. 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.
[0014] U.S. Pat. No. 8,790,602 issued Jul. 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.
[0015] U.S. Pat. No. 7,743,821 issued Jun. 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 disclosed herein.
[0016] U.S. Pat. No. 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.
[0017] The present disclosure seeks to provide thick or substantial
fins for furnace tubes having on at least one major surface an
array selected from: 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-regular pattern covering at least 10% of the surface area of
at least one major surface of said fin.
[0018] 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.degree. relative to the major axis of said fin,
said fin having on at least one major surface an array selected
from 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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".
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] In a further embodiment, there is provided furnace a tube
wherein the array comprises protuberances having:
[0031] i) a maximum height from 3 to 15% of the base of the
fin;
[0032] ii) a contact surface with a fin, or a base, which main
dimension is 0.1%-10% of the fin thickness;
[0033] iii) a geometrical shape which has a relatively large
external surface containing a relatively small volume.
[0034] In a further embodiment, there is provided a furnace tube
wherein the protuberance has a shape selected from:
[0035] a tetrahedron;
[0036] a Johnson square pyramid;
[0037] a pyramid with 4 isosceles triangle sides;
[0038] a pyramid with isosceles triangle sides;
[0039] a section of a sphere;
[0040] a section of an ellipsoid; and.
[0041] a section of a tear drop;
[0042] a section of a parabola
[0043] In a further embodiment, there is provided a furnace tube
wherein the furnace tube and the fin comprise the same metal
composition.
[0044] In a further embodiment, there is provided a furnace tube,
and fin(s) comprising 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.
[0045] 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
%.
[0046] 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 %
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
%.
[0047] 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 %.
[0048] 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.
[0049] 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 % and the balance substantially iron.
[0050] In a further embodiment, there is provided a cracking
furnace comprising a radiant section having furnace tubes as
above.
[0051] 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.degree. C. to 950 .degree. C. for a time
from 0.001 to 0.01 seconds, and separating the resulting olefins
from the feed and co-products
[0052] The present disclosure also provides any and all
combinations of the foregoing embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] FIG. 1 shows a furnace tube with longitudinal fins of the
present disclosure modified with grooves on the surface.
[0054] FIG. 2 shows a fin of the present disclosure modified with
protuberances of the present disclosure
[0055] FIG. 3 is a graph showing the per cent increase in the
surface area of the fin modified with different protuberances of
the present disclosure.
NUMBERS RANGES
[0056] Other than in the operating examples or where otherwise
indicated, all numbers or expressions referring to quantities of
ingredients, reaction conditions, etc. used in 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 desired. 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.
[0057] Notwithstanding that the numerical ranges and parameters
setting forth the broad scope of the disclosure 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.
[0058] 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.
[0059] 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
skilled in the art readily understand, the amounts of the
components actually used will conform to the maximum of 100
percent.
[0060] 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.
[0061] As used in this specification "fin height" refers to the
distance the fin extends away from the external surface of the
furnace tube.
[0062] In some embodiments, the furnace tubes have fins which have
high integrity, good stress resistance and are quite thick. In some
embodiments, the fins will have a thickness at their base of not
less than about 33% of the radius of the furnace tube, for example,
about 40%, or for example 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 about 5, or from about 1 to about 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 .degree., or about 10.degree. or less inward relative to
the center line of the fin. The edge of the fin may be flat,
pointed (at a 30.degree. to 45.degree. 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, for longitudinal fins, the fin
cross section may be "E" shaped (monolith with parallel
longitudinal extensions (having parallel grooves).
[0063] 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%, for example greater than 75%,
or for example 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.
[0064] 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.degree. to 25.degree. 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.
[0065] 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.
[0066] 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 3/4, or from about 1/3 to 3/4 or in some instances 1/3 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 about 5, or from about 1 to
about 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.degree., or about 10.degree. or less
inward relative to the center line of the fin. The tip or leading
edge of the fin may be flat, tapered (at a 30.degree. to 45.degree.
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.degree. and
30.degree. , or for example 45.degree. . In some case the fin may
end in a flat surface perpendicular to the surface of the tube.
[0067] A furnace tube or pass having grooved fins will be described
in accordance with FIG. 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.
[0068] In a further embodiment, the fins may comprise an array of
protuberances.
[0069] FIG. 2 shows a fin 20 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.
[0070] 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, the protuberances may cover from 40 to 100%, or from
50% to 100%, or 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.
[0071] 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. In some embodiments,
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:
[0072] a tetrahedron (pyramid with a triangular base and 3 faces
that are equilateral triangles);
[0073] a Johnson square pyramid (pyramid with a square base and
sides which are equilateral triangles);
[0074] a pyramid with 4 isosceles triangle sides;
[0075] 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);
[0076] a section of a sphere (e.g. a hemi sphere or less);
[0077] 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.
[0078] 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);
[0079] 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.
[0080] 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
machining the external surface of a cast fin such as by the use of
knurling device for example a knurl roll.
[0081] The above protuberances are closed solids.
[0082] A protuberance may have a height (L.sub.z) above the surface
of the fin from 3% to 15% of the maximum thickness of the fin, and
all the ranges in between, for example from 3% to 10% of the
maximum thickness of the fin.
[0083] 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).
[0084] 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 is undesirable. 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 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.
[0085] FIG. 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.
[0086] The size of the protuberance must be carefully selected. In
some embodiments 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, in some
embodiments, the "tail" may point down, in line with the overall
direction of flue gas flow, when the coil is positioned in the
furnace.
[0087] 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.
[0088] 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.
[0089] 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 % 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 %.
[0090] 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 %.
[0091] 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 (for example at least 85%, or in other
embodiments at least 95% iron), the sum of the components adding up
to 100 weight %.
[0092] 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.
[0093] 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.
* * * * *