U.S. patent number 3,696,863 [Application Number 05/000,287] was granted by the patent office on 1972-10-10 for inner-outer finned heat transfer tubes.
This patent grant is currently assigned to International Telephone & Telegraph Corporation. Invention is credited to Sung Chul Kim.
United States Patent |
3,696,863 |
Kim |
October 10, 1972 |
INNER-OUTER FINNED HEAT TRANSFER TUBES
Abstract
Inner-outer finned heat transfer tubes are provided for use in
heat exchangers. The heat transfer tubes include a central core
body surrounded by a tubular member. The central core has a
plurality of integral elongated fins extending along the length of
the core and projecting outwardly therefrom. The tubular member has
a plurality of externally protruding fins on the outer surface
thereof.
Inventors: |
Kim; Sung Chul (Des Plaines,
IL) |
Assignee: |
International Telephone &
Telegraph Corporation (New York, NY)
|
Family
ID: |
21690830 |
Appl.
No.: |
05/000,287 |
Filed: |
January 2, 1970 |
Current U.S.
Class: |
156/179; 165/133;
165/181 |
Current CPC
Class: |
F28F
1/26 (20130101); B21C 37/20 (20130101); F28F
1/42 (20130101); F28F 9/22 (20130101); F28F
1/422 (20130101); F28F 1/36 (20130101) |
Current International
Class: |
F28F
1/36 (20060101); F28F 9/22 (20060101); F28F
1/24 (20060101); F28F 1/12 (20060101); B21C
37/15 (20060101); B21C 37/20 (20060101); F28F
1/26 (20060101); F28f 001/42 () |
Field of
Search: |
;165/179,183,184,181,182 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sukalo; Charles
Claims
I claim:
1. A bimetal inner-outer finned heat transfer tube comprising an
elongated core locked within a surrounding elongated tube, said
core having a central body with a plurality of elongated fins
integral with and extending along the length of said core, said
elongated fins projecting outwardly from and perpendicular to the
surface of said core, said elongated fins contacting the inner
surface of said tube, said tube having a plurality of externally
protruding, inwardly arcuate fins which are disposed in planes
extending transversely of said tube and at generally right angles
to the longitudinal axis of said tube, the externally protruding
fins in any of said transverse planes being aligned longitudinally
of said tube in spaced, generally parallel relationship to the
externally protruding fins in any other of said transverse planes,
said externally protruding fins being spaced about the outer
circumference of said tube with intervening grooves, said grooves
extending along the length of said tube and following a slow
helical pattern around said tube.
2. An inner-outer finned heat transfer tube comprising an elongated
tube having incorporated therein an elongated core, said core
having a central body with a plurality of integral elongated fins
projecting outwardly therefrom, and said tube having a plurality of
externally protruding fins on the outer surface thereof which are
disposed in planes extending transversely of said tube and at
generally right angles to the longitudinal axis of said tube, the
externally protruding fins in any of said transverse planes being
aligned longitudinally of said tube in spaced, generally parallel
relationship to the externally protruding fins in any other of said
transverse planes.
3. The inner-outer finned heat transfer tube of claim 2 wherein
said externally protruding fins are spaced about the circumference
of said tube with intervening grooves, said fins being inwardly
arcuate in shape.
4. The inner-outer finned heat transfer tube of claim 2 wherein
said externally protruding fins are spaced about the circumference
of said tube with intervening grooves, said grooves being aligned
in a slow helical configuration around said tube and extending
along the length of said tube.
5. The inner-outer finned heat transfer tube of claim 2 wherein
said elongated core is formed having a spiral twist thereto whereby
spiral flow of fluid through said tube is provided.
6. A heat exchanger having incorporated therein a plurality of
bimetallic inner-outer finned heat transfer tubes, each of said
inner-outer finned heat transfer tubes comprising an elongated core
locked within a surrounding elongated tube, said core having a
central body with a plurality of elongated fins integral with and
extending along the length of said central body, said elongated
fins projecting outwardly from and perpendicular to the surface of
said central body, said elongated fins contacting the inner surface
of said tube, said tube having a plurality of externally
protruding, inwardly arcuate fins which are disposed in planes
extending transversely of said tube and at generally right angles
to the longitudinal axis of said tube, the externally protruding
fins in any of said transverse planes being aligned longitudinally
of said tube in spaced, generally parallel relationship to the
externally protruding fins in any other of said transverse planes,
said externally protruding fins being spaced about the outer
circumference of said tube with intervening grooves, said grooves
extending along the length of said tube and following a slow
helical pattern around said tube.
7. The heat exchanger of claim 6 wherein said grooves of said heat
transfer tubes follow a slow helical pattern around said tube.
8. A heat exchanger having incorporated therein a plurality of
inner-outer finned heat transfer tubes, each of said inner-outer
finned heat transfer tubes comprising an elongated tube having
incorporated therein an elongated core, said core having a central
body with a plurality of integral elongated fins projecting
outwardly therefrom, said elongated fins contacting the inner
surface of said tube, and said tube having a plurality of
externally protruding fins on the outer surface thereof which are
disposed in planes extending transversely of said tube and at
generally right angles to the longitudinal axis of said tube, the
externally protruding fins in any of said transverse planes being
aligned longitudinally of said tube in spaced, generally parallel
relationship to the externally protruding fins in any other of said
transverse planes.
Description
This invention relates to heat exchangers and to the finned heat
transfer tubes employed therein. More particularly, it relates to
inner-outer finned heat transfer tubes for use in heat exchangers
and to methods for producing these tubes.
Heat exchangers are devices for transferring heat from one fluid to
another without allowing the fluids to mix. It is to be recognized
that the term "fluid," as employed herein, is meant to include
liquids, gases, vapors and mixtures thereof.
In tube type heat exchangers, one fluid flows internally to the
tubes and the other fluid flows externally thereto within an outer
shell. Generally, heat transfer between the fluids is accomplished
by means of convection from the hot fluid, whether internal or
external to the tubes, to the solid surface of the tubes, then by
conduction through this solid material and convection from the
other surface of the solid pipe to the cold fluid. The rate of heat
transfer in such systems, known as the overall coefficient of heat
transfer, is the reciprocal of the sum of the individual thermal
resistances encountered in series along the path of heat flow.
The rate at which heat may be transferred from the tube surface to
the main body of fluid is known as the film coefficient. When the
fluid is circulated artifically over the heat-transfer surface, the
value of the film coefficient is governed by the velocity,
temperature difference and physical properties of the fluid, and by
the size, shape, arrangement and nature of the surface.
It has been a continuing problem to maximize the heat transfer
efficiency of tube type heat exchangers. To accomplish this, a
combination of factors must be taken into consideration. For
example, in attaining maximum heat transfer efficiency, the overall
heat transfer coefficients of the unit must be maximized for a
given surface area and fluid temperature difference. This can be
accomplished by improving film coefficients on both sides of the
tubing.
As a practical matter in designing tube type heat exchangers, it is
desirable to promote turbulence in fluid flow within the outer
shell external to the tubing since this factor will tend to
increase the heat-transfer coefficients. Also, the outer and inner
surface area of the heat transfer tubes should be increased to
promote an increased rate of heat transfer. Further, in many
instances, it is very desirable for the heat transfer tubes to have
a low outside to inside surface area ratio, optimally approaching
unity, whereby heat transfer is markedly improved. This low ratio
is particularly desirable in applications where the internal and
external fluids normally produce poor film coefficients, as in the
case of gas-to-gas applications.
From a commercial standpoint, it would be very desirable to provide
tube type heat exchangers which achieve the herein described
objects and features and, in addition, are less costly to produce
and more economical in operation than heretofore proposed heat
exchangers. Additionally, it would be of great value if the heat
exchangers could be produced in a more compact form with
substantially reduced size.
If heat transfer tubes are produced having a unitary metallic body,
considerable precision tooling is required. Consequently, such
tubes have tended to cost more than is desirable considering the
number of such tubes that are required for use in heat exchangers.
Further, the performance of such tubes as regards heat transfer
efficiency has not been sufficient to justify the increased
cost.
Additionally, problems are encountered in utilizing heat transfer
tubes having internal fins within a plain external tube,
particularly for operations wherein the fluids used have poor film
coefficients such as gas-to-gas operations. Such tubes do not
provide adequate film coefficients on both sides of the tubing to
be effective for such application.
Furthermore, heat transfer tubes generally have not possessed
sufficient total surface area per lineal foot; nor have the outside
to inside surface area ratios been low enough to provide
sufficiently improved heat transfer. This is particularly important
for applications wherein both fluids normally produce poor film
coefficients, as is the case with air-to-air applications.
For example, known heat transfer tubes do not provide adequate
performance in gas-to-gas heat exchangers utilizing a refrigeration
cycle to cool compressed air for moisture control Such systems are
commercially important since they are employed for conditioning
compressed air in manufacturing plants and the like, and for use
with pneumatic tools and controls. Consequently, it would be very
advantageous to provide new and improved heat transfer tubes which
can be used effectively in such heat exchange systems having
air-to-air, air-to-refrigerant, or gas-to-gas applications, because
of increased surface area, improved film coefficients on both sides
of the tubing and lower outside to inside surface area ratio.
A further disadvantage of the previous finned heat transfer tubes
is that they have not provided sufficiently increased heat transfer
coefficients. Therefore, it has not been practical to reduce the
number of tubes employed in heat exchangers and correspondingly to
reduce the size of the heat exchangers in which they have been
employed. Accordingly, it would be highly desirable and
economically beneficial to provide new and improved heat transfer
tubes which, because of their improved heat transfer efficiencies,
enable the production of more compact, less costly heat exchange
units.
Accordingly, an object of the present invention is to provide new
and improved heat exchangers of the tube type.
Another object is to provide new and improved shell and tube type
heat exchangers employing unique inner-outer finned heat transfer
tubes. In this connection, these heat exchangers are constructed in
a more economical manner, are more compact, and have improved heat
transfer coefficients.
Another object is to provide new and improved bimetal inner-outer
finned heat transfer tubes for use in heat exchangers which are
more efficient in operation and can be produced more efficiently
and more economically.
Another object is to provide unique inner-outer finned heat
transfer tubes which have increased surface area and improved film
coefficients on both sides of the tubing.
A further object is to provide heat transfer tubes which possess
increased heat transfer efficiencies when the fluids employed have
poor film coefficients. In this regard, an object is to provide
heat transfer tubes having a low outside to inside surface area
ratio whereby heat transfer is improved markedly in applications
wherein the fluids employed normally produce poor film
coefficients.
A still further object is to provide new and improved methods for
producing heat transfer tubes. In this connection, an object is to
provide efficient and economical methods for producing inner-outer
finned heat transfer tubes which are more efficient in
operation.
Yet another object is to provide methods for producing unique
inner-outer finned heat transfer tubes employing low-cost general
purpose tools, rather than high cost precision extrusion tooling
used hitherto in producing finned heat transfer tubes.
In keeping with an aspect of the invention, these and other objects
are accomplished by providing an inner-outer finned heat transfer
tube constructed of an aluminum extruded inner core or spline
having integral elongated fins extending along the length of the
core and projecting outwardly from and substantially perpendicular
to the surface of the core and a copper externally finned tube. The
inner aluminum core is crimp connected to the externally finned
tube to produce a rugged, unitary heat transfer tube structure
having increased heat transfer efficiencies.
Crimping of the aluminum core with the inner fins into the
externally finned tube serves a dual purpose. First, the crimping
operation locks the inner core within the externally finned tube.
Second, the crimping action on the previously externally finned
outer surface causes distortion and bending of the external fins.
These distorted and twisted fins increase the turbulence and fluid
flow between the fins, thus improving the heat transfer.
Preferably, more crimping locations are used than there are fins on
the inner spline. This reduces the degree of distortion of the
tubing and external fins to the most desired level, resulting in
more effective use of the fins.
The unique inner-outer finned heat transfer tubes of the present
invention provide substantially increased heat transfer
coefficients and greatly improved film coefficients. Accordingly,
these inventive heat transfer tubes enable the production of heat
exchangers having greatly reduced size and cost for any given heat
transfer situation.
The abovementioned and other features and objects of this invention
and the manner of obtaining them will become more apparent, and the
invention will be best understood by reference to the following
description of an embodiment of the invention taken in conjunction
with the accompanying drawings in which:
FIG. 1 is a cross-section view which shows the end of a solid
metallic core rod as it appears at the start of a manufacturing
process;
FIG. 2 shows a cross-section of the end of the same core rod after
it has been formed, as by being extruded or drawn through a
die;
FIG. 3 is a cross-section end view which shows a metal tube or
sheath as it appears at the start of a manufacturing process;
FIG. 4 shows a perspective view of the tube of FIG. 3 after an
initial external fin forming step;
FIG. 5 shows the finished heat transfer tube including the crimped
combination of the tube or sheath of FIG. 4 and the finned central
core of FIG. 2;
FIG. 6 is a fragmentary expanded view of the encircled area 6 of
FIG. 5;
FIG. 7 is a cross-section view taken along line 7--7 of FIG. 5
which shows the crimped bimetal inner-outer finned heat transfer
tube; and
FIG. 8 is a perspective view which shows a heat exchanger employing
the inner-outer finned heat transfer tubing of FIG. 6.
To produce the inventive inner-outer finned heat transfer tube, a
bar of metal suitable for forming an internal core member is
extruded or drawn through a die to provide a generally circular
cross-section 10 with a specific diameter. For example, as
received, the metallic bar stock might be nominally circular in
cross-section and approximately three-quarters of an inch in
diameter, but its circumference may be irregular with any diameter
varying by, perhaps, a sixteenth of an inch as compared with any
other diameter. After the extrusion, the diameter could be
five-eights of an inch, and the cross-section is perfectly
circular--within acceptable tolerances.
Suitable metals for use in producing the core member are aluminum,
zinc, magnesium and the like; although, other metals could be used
depending on the specific proposed use.
The circular cross-section, extruded rod 10, FIG. 1, is then formed
into an elongated core member 12 by economical processes such as
the process of extrusion. The core 12 is formed having a solid
central body or rod 14 with a plurality of elongated fins 16
integral with and extending along the length of the central body
14. The fins 16 project outwardly from and substantially
perpendicular to the surface of the central body 14 along a radial
longitudinal plane lying in the principal longitudinal axis of the
central body 14. In the illustrated embodiment of FIG. 2, the core
12 has seven equally spaced elongated fins 16 and, correspondingly,
between these fins 16 a series of seven open spaces 18 are formed.
It will be understood that the number of fins on the core 12 may be
varied as desired to afford the most advantageous ratio of inside
to outside heat transfer areas.
As shown in FIG. 3, an elongated tubular member 20 is provided
having a plain, cylindrical surface. The tubular member 20 is
initially subjected to processing to produce a plurality of
integral upstanding fins 21 which project outwardly about the outer
circumference of the tubular member 20 and lie in a substantially
radial circumferential plane perpendicular to the principal axis of
the tube 20.
As shown in FIG. 4, the upstanding fins 21 are annular in shape and
extend perpendicularly outward from the outer surface of the tube
20 in a thread-like arrangement around the circumference of the
tube 20. However, it will be understood that the upstanding fins 21
may be produced in a variety of shapes and configurations such as,
for example, disconnected disc-like or circular shaped fins.
The manner of forming the upstanding fins 21 is not crucial. For
example, they could be formed on a screw-forming machine somewhat
similar to the manner of threading a bolt. However, I prefer to
roll the tube 20 over a flat die surface in a manner such that the
fins 21 are formed having annular shapes with a slight helical
twist circumferentially disposed along the longitudinal axis of the
tube 20.
The overall inside dimensions of the tubular member 20 should be
slightly larger than the outside dimensions of the radial fins 16
of the core member 12. Thus, the core 12 slips easily inside the
tube 20. However, as illustrated in FIGS. 5 and 7, the core 12
should fit snugly within the tube 20 when it is inserted
therein.
As a practical matter, there is inevitably a somewhat irregular
contact between the inner surfaces of the tube 20 and the elongated
inner fins 16. However, if effective and efficient heat transfer or
flow is to be attained between the fins 16 and the tube 20, the
contact between the fins 16 and the tube 20 must be a nearly
perfect metal to metal contact. This contact must be so perfect and
firm that there is no substantial space between the two surfaces
where there might be a layer of heat insulating gas or air. In
effect, a virtually gas tight seal must be formed between the edges
of the fins 16 and the inside surface of the tube 20.
In the present invention the metal to metal contact of the tube 20
and the fins 16 is attained in such a way that the contact will be
maintained even though the tube 20 and the core 12 may be subjected
to unequal amounts of radial expansion. The way in which this
contact is attained also adapts the tube for bending without
destroying the metal to metal contact of these surfaces. To attain
such contact the parts are so formed and assembled that there is a
constant resilient force acting to maintain such contact.
As illustrated in FIGS. 5 and 7, the inner core or spline 12 is
assembled and locked within the externally finned tube 20 by
inserting the core 12 within the tube 20 and subjecting the
assembly to inward crimping forces at a plurality of positions
along the circumference of the outer surface of the tube 20. This
crimp locking operation is performed so that a nearly perfect mated
core-to-tube combination is provided.
More specifically, FIG. 6 shows an enlarged fragment of the
finished tube 26 taken from the encircled area 6 of FIG. 5.
Initially, as illustrated in FIG. 4, the tube 20 has a plurality of
upstanding annular fins 21 which project outwardly about the outer
circumference of the tube 20 and extend outwardly along a radial
plane thereof. After crimping, as for example, with a hydraulic
tool having somewhat dome shaped fingers, the annular fins 21 are
reshaped to form a plurality of circumferentially disposed
depressed regions 24 with somewhat shelf-like surfaces 22 extending
away from the radial plane. The depressed regions 24, which are
generally semi-circular in cross-section, lie in planes which are
approximately parallel to the outer surface of the tube 20. The
depressed regions 24 on each fin are generally aligned with
corresponding depressed regions on the other fins, as best
illustrated in FIG. 5, to form grooves or flutes 25 which extend
along the length of the tube 20. Preferably, these grooves 25
follow a slow helical or spiral pattern around the tube 20 which
provides improved heat transfer characteristics to the tube 20.
For convenience of expression, this configuration of FIG. 6 is
hereinafter termed the externally protruding, inwardly arcuate fins
21 (or, more simply, the external fins 21) having intervening
grooves 25.
It is to be noted that more crimping locations are employed than
the number of elongated inner fins 16 on the core 12. This promotes
reduced distortion of the external fins 21 resulting in more
effective flow area between adjacent fins 22. However, a degree of
bending and twisting of the fins 21 is highly desirable since the
bent or twisted external fins 21 will promote increased turbulence
in fluid flow between adjacent fins. When the tubes 26 are employed
in a heat exchanger, this increased turbulence causes greatly
improved heat transfer efficiency. Accordingly, it should be
further noted that the crimping force or pressure exerted on the
externally finned tube surface to form the externally protruding
inwardly arcuate fins 21 and to lock the core 12 within the tube
20, also causes controlled bending and twisting of the fins 21.
The tube 20, including the external fins 21, is normally
constructed from copper. However, other materials may be employed
to meet specific conditions that are to be encountered in use.
The materials to be used for the core 12 and the tube 20 may be
rather freely selected from materials having the best and most
desirable characteristics insofar as corrosion resistance, heat
transfer characteristics and cost may be concerned.
In many instances, it is desirable to employ a spiral path for the
fluid that is to pass through the heat exchange tube 26.
Accordingly, in a further embodiment of the present invention, the
core 12 is formed having a spiral twist whereby spiral flow of
fluid through the tube is attained.
In production, the twisted core 12 is formed so that the radial
fins 16 have a spiral form of desired lead. This may be
accomplished readily by application of twisting forces to the core
12. The twisted core 12 is then inserted endwise into an externally
finned tube 20 such as that shown in FIG. 4. Then, crimping forces
are applied to the tube 20 in the manner described hereinbefore to
produce a unique inner-outer finned heat transfer tube having a
spiral or twisted inner core.
In the use of the inner-outer finned heat transfer tube with a
spiral or twisted inner core, the fluid flows within the tube 26
along the several spiral paths 28 defined between the twisted fins
16. When the fluid is a mixture of liquid and gaseous refrigerant
and the rate of refrigerant flow through the tube 26 is relatively
high, the liquid refrigerant is separated by centrifugal force so
as to flow along the inner surfaces of the tube 26, while the
gaseous component of the refrigerant mixture flows along the inner
portions of the space.
From the foregoing, the operation of the inventive heat transfer
tubes now should be apparent. In greater detail, a heat exchanger,
shown generally as 30 in FIG. 8, is provided having an outer shell
32. A plurality of heat transfer tubes, corresponding to tube 26,
are positioned within shell 32. Four ports 34, 36, 38 and 40 are
provided about the surface of shell 32. Ports 34 and 36 are,
respectively, the inlet and outlet ports for the fluid flowing
through the heat transfer tubes 26. Ports 38 and 40 are,
respectively, the inlet and outlet ports for the fluid flowing
within shell 32, external to the tubes 26 through a baffled flow
path, indicated generally by arrow 42.
In operation, fluids such as air or refrigerant are individually
introduced into the exchanger 30 through inlet ports 34 and 38 by
suitable means such as compressors, pumps, blowers, and the like,
not shown. The fluid entering port 34 then circulates through the
tube clearance holes 28 of tubes 26 and is evacuated therefrom
through outlet port 36. The fluid entering port 38 flows through
baffle path 42, thus flowing at about right angles to the tubes 26.
This fluid comes into contact with the externally finned surface of
tube 26 and the fins 21, creating turbulence in the fluid flow
which serves to more completely bring fluid into contact with the
tube surface and thus to improve the heat transfer efficiency of
the unit. The fluid continues to flow through path 42 being exposed
to heat transfer effects throughout the flow path. Eventually, the
conditioned fluid reaches outlet port 40 where it is forced out of
the unit.
In the case of a gas-to-gas operation, assuming that the
temperature of the gas flowing through the tubes 26 is lower than
the the temperature of the gas flowing through path 42, a pressure
drop is created about the outer surface of the tubes 26. This
pressure drop will cause the gas in area 42 to permeate down into
the external fins 21 of tubes 26. Thus, more turbulent flow of gas
through the finned outer surface of tubes 26 is achieved resulting
in improved heat exchange. The cooled gas flowing through path 42
is then forced out of the exchanger 30, for example, as conditioned
compressed air.
The heat transfer tubes of the present invention have been tested
to demonstrate their superior heat transfer capabilities. The
results of these tests showed a 100 percent increase in overall
heat transfer coefficients of the inventive tubes as compared with
plain unfinned heat transfer tubes (i.e., no internal or external
fins). Further comparative tests have demonstrated that the
inner-outer finned tubes of the present invention have overall heat
transfer coefficiencies 67 percent greater than plain external heat
transfer tubes having inner fins.
While the principles of the invention have been described above in
connection with specific apparatus and applications, it is to be
understood that this description is made only by way of example and
not as a limitation on the scope of the invention.
* * * * *