U.S. patent number 3,636,607 [Application Number 04/889,123] was granted by the patent office on 1972-01-25 for method of making a heat exchange tube.
This patent grant is currently assigned to United Aircraft Products, Inc.. Invention is credited to Samuel J. DeMarco.
United States Patent |
3,636,607 |
DeMarco |
January 25, 1972 |
METHOD OF MAKING A HEAT EXCHANGE TUBE
Abstract
A method of placing a heat exchange tube into compressive
contact with a concentric fin annulus involving a displacement of
metal by electromagnetic or equivalent forces to achieve a
conforming relation of the tube to the contact tube surface in a
manner providing minimal contact resistance to heat flow.
Inventors: |
DeMarco; Samuel J. (Findlay,
OH) |
Assignee: |
United Aircraft Products, Inc.
(Dayton, OH)
|
Family
ID: |
25394542 |
Appl.
No.: |
04/889,123 |
Filed: |
December 30, 1969 |
Current U.S.
Class: |
29/890.036;
29/455.1; 29/523; 29/419.2; 29/516 |
Current CPC
Class: |
F28F
1/105 (20130101); B21K 25/00 (20130101); Y10T
29/49803 (20150115); Y10T 29/49879 (20150115); Y10T
29/49927 (20150115); Y10T 29/49361 (20150115); Y10T
29/4994 (20150115) |
Current International
Class: |
F28F
1/10 (20060101); B21K 25/00 (20060101); B21d
053/02 (); B23p 015/26 () |
Field of
Search: |
;29/157.3A,421M,455,516,157.3D,157.3R ;113/118A,523 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Campbell; John F.
Assistant Examiner: Reiley, III; Donald C.
Claims
What is claimed is:
1. A method of achieving minimal contact resistance to heat
transfer between a tube wall and a tube contained fin annulus,
including the steps of inserting a compressible fin annulus in a
tube, said annulus comprising corrugated thin metal deformable
material, providing a central support for said annulus, said
support rigidly backing said fin annulus, and applying a radial
compressive force to said fin annulus so that all parts thereof are
in closely contacting compressive relation to the interior wall of
said tube irrespective of irregularities in fin height, the
application of compressive forces being accomplished by displacing
the tube wall radially inwardly so that all parts thereof are in
closely contacting compressive relation to the fin annulus
irrespective of irregularities in fin height, the fin annulus
reacting upon said central support and the tube deforming to
conform to fin irregularities.
2. A method according to claim 1, wherein the displacement of the
tube material is accomplished by electromagnetic forming, a tube
and inserted fin annulus and central support being mounted within
an electromagnetic coil disposing in a fully surrounding relation
to the tube.
3. A method according to claim 1, wherein the fin annulus is
comprised of thin deformable sheet metal corrugated to produce
alternate peaks and valleys, the sheet being formed to a circular
configuration for insertion in the tube to present an outer surface
for contact with the tube wall and an inner surface for contact
with the said central support, inward displacement of tube material
being resisted by the fin material with the tube wall flowing to
conform to and uniformly compressed undulations in the outer fin
surface.
4. A method according to claim 3, wherein the tube wall and the
central support present spaced parallel surfaces and said fin
material having undulating outer and inner surfaces for respective
contact therewith, the displacement of tube material flattening the
inner fin surface upon said central support and deforming the tube
wall into conforming relation to the outer fin surface.
5. A method according to claim 4, wherein the tube displacement is
accomplished electromagnetically simultaneously over its full
length, each part of the tube yielding to applied forces
independently of all other parts.
6. A method according to claim 1, wherein displacement of tube
material is accomplished in an electromagnetic coil, a tube
assembly comprising a tube an inserted fin annulus and a central
support being fully contained within the coil with coil elements in
continuously surrounding relation to the tube, the tube being
substantially without rigid backup except as provided by said
central support through the intermediately disposed deformable fin
material.
7. A method of achieving minimal contact resistance to heat
transfer between a tube wall and a fin annulus contained in an
outer tube, wherein an inner tube is disposed centrally of the fin
annulus in support thereof, characterized by the step of deforming
one of said tubes to reduce the annular space occupied by the fin
annulus and apply a compressive force to said annulus, the
deforming step being an electromagnetic forming process carried out
simultaneously over the length of the fin annulus and accomplishing
a glove fit of the deformed tube wall to the contacting surface of
the fin annulus in which the tube conforms to and adopts a
configuration matching irregularities in the fin surface while
uniformly compressing said surface.
8. A method according to claim 7, wherein the fin annulus is
comprised of corrugated sheet metal material providing adjacent
longitudinal channels defined by alternating peaks and valleys and
connecting beam walls, said walls occupying substantially radial
positions between said tubes, said peaks and valleys being
compressed and said walls being substantially incompressible by the
deforming tube to require the tube substantially to conform to the
encountered fin surface while deforming peaks and valleys insure
close fitting intimate contact between the tube wall and the fin
annulus for minimal contact resistance.
9. A method according to claim 8, wherein the deformed tube wall is
the outer tube, said fin annulus disposing as a compressible means
between said inner and outer tubes, the inner tube providing
backing support for the fin assembly under the inwardly directed
compressive force applied by said outer tube.
Description
BACKGROUND OF THE INVENTION
This invention relates to the art of heat transfer and more
particularly to a method of fabrication of a tube assembly
comprising a tube and an interior fin.
Tube assemblies of the kind to which the invention pertains
comprise a tube usually arranged in a bundle with other like tubes
for a flow of a first fluid over and around the tube exteriors. A
second fluid, different in temperature from that of the first and
separated therefrom, is passed through the tubes. An exchange of
heat occurs through the tube walls. A strip of corrugated metal,
termed a fin, is contoured to an annular shape and inserted in the
tube where it is in common contact with the tube wall and the
internally flowing second fluid. The fin is an extended surface
member, supplementing the tube wall in achieving a conduct of heat.
It has an efficiency determined in large part by the excellence and
continuity of its contact with the tube wall.
In the prior art it has been known to slip fit a fin within a tube.
Also, it has been known metallurgically to bond a fin within a
tube, as by soldering or brazing. In another known method, an inner
tube is placed within the fin annulus and a mandrel drawn through
the inner tube to expand it and thereby seat the fin to the
surrounding wall of the outer tube. All of the known methods suffer
from defects which are reflected in less than optimal heat transfer
and a need for an excess amount of heat transfer surface to perform
to given specifications. Most importantly they are to various
degrees defective in not achieving uniformly minimal contact
resistance to heat transfer between the fin and the tube wall. To
whatever extent the fin lightly engages the tube wall or does not
engage it at all there is a buildup of contact resistance, reducing
the facility with which heat may flow from the fin to the tube wall
or vice versa. Conversely and in accordance with the known
principles of contact resistance, a firm pressural contact of the
fin with the tube wall reduces the resistance to heat flow. In a
slip fit construction, there is no pressured engagement of the fin
with the tube wall. Metallurgical bonding gives good thermal
conductivity but may be difficult to achieve in a confined fin
tube. Gaps in the created joints, as may result from irregularities
in fin height, produce nonuniform heat transfer effects. In a
method expanding an inner tube upon an intervening fin, the applied
compressive forces have inconsistent results because of the
practicable impossibility of forming fin strip material to
precisely uniform heights. Also, some types of fins advantageously
used in heat exchange tubes have built in wave form in their tube
contacting surfaces producing an inherently interrupted contact
with the tube wall. A mandrel drawn through an inner tube may have
a capability of compressing some of the fin surface upon the outer
tube wall but cannot uniformly compress the fin thereon
irrespective of variations in fin height or of fin configuration. A
tube assembly fabricated by the expanded inner tube method will
have numerous areas of individually greater or lesser extent in
which the fin contacts the tube wall lightly or not all all. Heat
transfer in these areas is at a low value, reducing overall heat
transfer efficiency in the tube. Moreover, such areas are
vulnerable to deformation by high-pressure fluid in a manner to
progressively broaden noncontacting areas.
Expanding of the inner tube is, moreover, an indirect means of
compressing the fin material to the outer tube wall. Since contact
resistance at the outer tube wall is of chief concern in a tube
assembly of the class described, expansion of the inner tube is a
less effective means of reaching the desired end than would be
contraction of the outer tube. This, however, is a problem of
less-obvious solution and the use of a die or the like is in any
event subject to the same disadvantages as the forcing of a mandrel
through the inner tube. The die would of necessity be fixed in
diameter and incapable of compelling the tube to deform in
accordance with deformities of the fin surface.
SUMMARY OF THE INVENTION
The present invention has in view a method of fixing a fin annulus
within a tube involving metal displacement, but which is not
limited by the geometry of a particular die or mandrel. A process
of unison deforming is proposed in which an outer or an inner tube
is displaced radially against the resistance of the fin annulus.
The tube wall is allowed to flow freely into a conforming relation
with the fin surface with the result that the entire fin strip is
placed uniformly under compression irrespective of undulations and
irregularities in fin height. Contact resistance is reduced to
minimal levels and is uniformly low over the length of the tube.
According to a feature of the invention, tube wall displacement is
a function of applied electromagnetic forces and may be reflected
in either an expansion of an inner tube or contraction of an outer
tube. According to a further feature, shrinking of the outer tube
is contemplated in order that direct action may be exerted upon
confronting surfaces of the outer tube and the fin material, the
electromagnetic forming process lending itself readily to an
arrangement in which an assembled tube is bodily received within a
forming coil.
Objects of the invention are to provide an improved method
substantially of the class described resulting in improved
utilization of the principle of contact resistance, specifically
advancing the pertaining segment of the heat transfer art.
In the drawings:
FIG. 1 is a detail view in perspective of an outer tube comprised
in a tube assembly produced in accordance with the method of the
invention;
FIG. 2 is a view in perspective of an inner tube useful
alternatively as a backup means and as a means to apply a
compressive force;
FIG. 3 is a view in perspective of a length of fin material as
interposed between the inner and outer tubes;
FIG. 4 is a view in longitudinal section, partly diagrammatic,
showing a tube assembly within an electromagnetic forming coil and
indicating the direction of applied force;
FIG. 5 is a detail fragmentary view like FIG. 4, showing the parts
after completion of the forming operation;
FIG. 6 is a fragmentary view of a tube as shown in FIG. 5, enlarged
with respect thereto; and
FIG. 7 is a view in cross section taken through an assembled
tube.
Referring to the drawings:
A tube assembly as achieved by the method of the invention includes
an outer tube 10 made of a metal of good thermal and electrical
conductivity. The tube is of a uniform diameter, is relatively thin
walled and is open at both ends. Also comprised in the tube
assembly is an inner tube 11 which may be constructed like the tube
10 but which in any event provides a relatively unyielding exterior
for the seating thereon of fin material as will hereinafter more
clearly appear. In the illustrated instance the inner tube is
designed solely for use as a backup means for the fin material. At
least one end thereof is closed, as by crushing or pinching an end
12.
Completing the tube assembly is a strip 13 of fin material. The
strip 13 is comprised of a thin gauge, ductile metal of good
conductivity. Originally in sheet form, it is gathered and crimped
to a corrugated formation to define a series of parallel fins 14 of
longitudinal extent. Each fin comprises a peak portion 15 connected
by inclining beam portions and vertically spaced valley portions 17
to adjacent fins. According to a feature of the invention in its
illustrative form, the fins 14 are formed with a ruffled
configuration, that is, with continuous undulations from side to
side along the length thereof. A fluid flowing through the fins
accordingly is subjected to repeated changes of direction with
turbulent effect yielding increased heat transfer efficiency. As a
product of the laterally formed undulations in the fins, upper and
lower surfaces of the fin strip are simultaneously given a wave
form in which high points 18 and low points 19 succeed one another
in alternating relation from end to end of the strip. In its
production, efforts are made to maintain the fins 14 of a uniform
height over the full area of the strip. Manufacturing limitations
preclude completely consistent results in this connection, however,
so that in addition to the alternating high and low points 18 and
19 fin material may have other areas which are greater or lesser in
height than adjoining areas.
The method of the invention involves a preassembly of the parts in
which fin strip 13 is rolled to an annular configuration about
inner tube 11 with these parts then being inserted as a subassembly
into outer tube 10. With the parts so positioned, the strip 13 has
an outer surface presented for contact with the inner wall of tube
10 and an inner surface presented for contact with the exterior of
inner tube 11. The parts will have a relatively close fit in which
they are frictionally held in an assembled relation. High points on
the fin strip will at least lightly engage confronting tube
surfaces, but the relationship of the fin surfaces to the wall
surfaces is irregular and numerous locations exist of high-contact
resistance, as indicated in FIG. 4.
Referring to FIG. 4, a preassembled tube is inserted in an
electromagnetic forming coil 21. This is a device storing and
releasing electrical energy which assumes the form of magnetic
pressure with respect to a workpiece. In this instance, the forming
coil has a cylindrical shape and is suitably connected to a power
source to draw energy for a period of seconds, store it and then
release the energy in a fraction of a second to do work at a
high-energy rate. The coil is constructed to have a length
exceeding that of the tube so that a fully inserted tube is
completely contained within the coil which overlaps the ends
thereof. Approximately centered within the coil, the tube is
completely and uniformly subject to the magnetic field exerted by
the discharging coil 21. An adapter 22, made of nonconductive
material, is disposed in the coil 21 to receive and position a tube
within the coil. In the process, electrically conductive outer tube
10 becomes the work piece. It is subjected, in response to release
of the stored electrical energy, to a force proportional to the
intensity of the magnetic field and current. The generated force
results in a movement of the conductor, in this instance the tube
10. The coil, since it completely surrounds the tube 10, applies a
force directed radially inwardly so that the material of the tube
is displaced in this direction resulting in a reduction in tube
diameter. However, since the tube is completely surrounded by the
windings of coil 21 all parts of the tube are independently and
equally responsive to the inwardly directed pressure. However,
while the tube is uniformly affected by the electrical discharge it
is free to conform to the underlying surface against which it is
pressed. In accordance with the present inventive concept the tube
is not compressed upon a fixed die but rather upon a relatively
compressible member as defined by the fin strip 13. The contracting
tube applies a compressive pressure to the fin material, squeezing
it between the inner wall of the outer tube 10 and the outer
surface of inner tube 11. Peaks 15 and valleys 17 are to a limited
extent distorted by such compressive forces but the beamlike walls
16 provide effective resistance inhibiting a uniform crushing of
the fin. Fluid passageways as defined by the fins 14 accordingly
remain open. Also, the tube 10 is constrained thereby the flow into
a conforming relation to the high and low points 18 and 19 of the
fin surface as well as to low areas existing in consequence of
manufacturing tolerance. The tube effects a glovelike fit with the
fin strip 13 and may assume externally an irregular or undulating
configuration matching that of the fin strip.
The parts are shown in FIGS. 5, 6 and 7 in the position they assume
following application of the electromagnetic forces. As indicated
in FIG. 5, the tube 10 has decreased in diameter, shrinking
slightly away from adapter 22 and assuming an irregular
configuration, shown in greater detail in the expanded segmental
view of FIG. 6. For purposed of illustration, the undulating
configuration of the tube has been somewhat exaggerated. However,
the conforming configuration assumed by the outer tube does in fact
exist and is effective to apply a substantially uniform compression
to the entire fin strip. The strip is accordingly subjected over
its entire area to a firm, pressural contact with the outer tube
wall assuring a low, uniform level of contact resistance.
The details of means for energizing coil 21 are not shown as being
unessential to an understanding of the method of the invention.
They may include a suitable charging circuit, switching and
capacitor means. Arrows 23 in FIG. 4 illustrate how current
discharging from the coil 21 is directed as an inwardly forming
pulse upon the tube 10.
A formed tube assembly is withdrawn from the forming coil and is
ready for use. Installations thereof may include one in which a
plurality of formed tubes are disposed in a bundle for flow of a
first fluid over and around the tube exteriors. A second fluid, in
separated relation to the first, is controlled and directed to pass
longitudinally through the tubes. The flow is in a annular form,
inner tube 11 being closed. The flowing fluid is in contact with
the inner wall of tube 10 and is also in contact with the material
of fin strip 13. Assuming the fluids to be of different
temperature, a transfer of heat takes place through the tube wall.
The tube wall serves as primary heat transfer surface with respect
to the second fluid in contact therewith. The material of fin strip
13 acts as a secondary surface, conducting heat to or from the tube
wall. The uniform tight fitting engagement of the tube wall with
the fin surface ensures low-contact resistance for uniformly
effective heat transfer.
The invention obviates the need for a forming die. Fin strip 13
assumes the characteristics of a die, and, since it is
compressible, yields for the application of uniform compressive
forces. If found necessary or desirable, dielike inserts may be
placed in the tube ends to define a fixed diameter at these
locations to simplify mounting of the tubes in a header plate or
the like. The fin strip 13 has been disclosed as having a ruffled
configuration and the invention has special merit applied to such a
fin since it provides a method heretofore unknown of achieving
excellent contact resistance over the rippling fin surface as
defined by high and low points 18 and 19. The problem particularly
present in the ruffled fin is to a degree present in all fin
constructions, however, due to a practicable inability to maintain
precise height dimensions over the full area of a completed fin.
The method of the invention accordingly is used with advantage with
fins of other configuration, including straight fins.
The wave form of the fin strip 13 exists, of course, with respect
to both upper and lower surfaces thereof, or with respect to outer
and inner surfaces as the strip is bent to an annular
configuration. Thus in the partly assembled position of the parts,
as shown in FIG. 4, locations of high-contact resistance are
present along the exterior of inner tube 11 as well as along the
interior of outer tube 10. The electromagnetic forming operation
seats the fin strip to the inner tube but with an irregular effect
which it has not been attempted to illustrate in FIGS. 5 and 6. In
the present instance the concern is with the interface between the
fin material and the outer tube 10. Should the reverse be true,
that is should the concern be with the interface between the fin
material and inner tube 11, then an electroforming coil of
expansion effect would be inserted in tube 11. Outer tube 11 would
then serve as a back up while tube 11 deforms to a glove fit with
the inner fin surface.
* * * * *