TECHNICAL FIELD
[0001] This invention relates to heat exchangers in general, and specifically to a novel
construction for a fabricated heat exchanger tube.
BACKGROUND OF THE INVENTION
[0002] Cross flow automotive heat exchangers, such as radiators, condensers, and heater
cores have, for decades, followed the same general design of a basic core bordered
by two side tanks or header tanks. The basic core consists of a plurality of parallel
flow tubes, stacked with brazed corrugated air fins between, the ends of which tubes
are brazed leak tight into regularly spaced slots in the header tanks. The header
tanks feed a flow medium into and out of the tubes, while air is blown across the
tubes and air fins in a perpendicular or "cross" flow direction. Basic flow and heat
transfer formula, also well known for decades, determine the optimum size of the flow
tubes, as well, so that the biggest choice that a designer has to make is simply the
best and most economical method of manufacturing the tubes. That choice, in turn,
is partially driven by the method of assembling and manufacturing the core.
[0003] One of the two standard manufacturing methods for the tubes are the one piece extruded
tube, in which a billet of hot metal, generally aluminum, is forced through a die
that gives a constant cross section to the tube all along its length. Any part, produced
in one, integral piece is generally thought to be more economical than a multi part
piece, but, as noted, other considerations may apply. Extruded tubes have proven difficult
to surface coat with braze material. Consequently, the surface coating of braze material
necessary to braze all parts of the core together must generally be applied to the
corrugated fin material that contacts the outside of the extruded, one piece tube.
Such braze material is abrasive and deleterious to the fin forming machinery, and
the fin material must be made thicker and heavier than otherwise needed in order to
allow successful braze material coating. Fabricated, multi piece tubes, on the other
hand, are formed from flat stock that can easily be coated with braze material first,
obviating the need to coat the fins.
[0004] In the case of tubes that are subjected to a fairly high internal pressure, it has
been the practice to incorporate an internal strengthening member inside the tube,
to act in tension to hold the walls together. Such members also divide the tube interior
into multiple, smaller passages, with the obvious improvement in heat transfer that
results therefrom. Extruded tubes use simple, integral dividing and strengthening
ribs, which cannot practically be made as anything but straight, uninterrupted walls.
Fabricated high pressure tubes have far more potential design variations, since the
internal member can be, but need not absolutely be, made as a separate piece. In addition,
the outer shell or walls of the tube can and has been made in a variety of ways, which
are outlined below.
[0005] The simplest design for a fabricated tube that does not need internal reinforcement
for internal pressure resistance is simply a folded shell, with a live hinge on one
side and a seam on the other. An example may be seen in USPN 4,470,452. Adding a corrugated
web on the inside can easily be done before the tube is folded, as seen in Japanese
patent 57-66389, a patent which also illustrates the equivalence between the extruded
and fabricated tube design. A design that attempts to combine the advantages of fabricated
and extruded designs uses a single piece of metal stock folded in a general Z shape,
with the center of the Z being corrugated to provide the inner web, and the top and
bottom of the Z folded down over the center corrugation from opposite directions to
form integral outer walls of the tube. An example may be seen in USPN 2,757,628. While
one piece, such a design is limited insofar as both the central corrugation and top
and bottom walls must have the same material and thickness, meaning that the integral
internal web may well be thicker and heavier than it would otherwise have to be. In
addition, the internal web will inevitably be coated with the same braze material
as the outer integral walls. A variation of the Z design bends the two outer walls
only half way down over the width of the tube, into abutment with a single central
wall, giving only two, rather than several, divided compartments in the tube. An example
is shown in USPN 4,633,056, which also shows that the edges of the outer walls, where
they are brazed to the single central wall, may either be sharp, or bent over in a
curved foot, the latter obviously giving more surface to surface braze contact, although
requiring an extra bending step in processing.
[0006] Very early on, it was recognized that a simple, strengthened version of the hollow
fabricated tube, divided into n distinct chambers, could be made with (n-1) separate
pieces of metal stock by bending the separate pieces over with short 90 degree inner
walls welded to one another along the surface, with edges that abut the inner surface
of the tube. As such, the welded inner walls act as spacers and strengtheners for
the tube as a whole. An example may be seen in UK patent 1,149,923. The simplest version
of this basic design is simply two (n) chambers, with only one (n-1) piece of metal
stock, bent back on itself to the center in a general B shape, with only two adjacent
90 degree inner walls, centrally located. While this simple design does not provide
particularly compact flow paths, it is stronger than a single chamber, hollow tube.
With the later common use of braze material coated metal stock, it was possible to
eliminate the separate welding step for the abutted 90 degree edges, which would naturally
braze to one another as the surface material melted, as shown in published Japanese
Patent Application 63242432A. While the design shown there uses sharp edged inner
walls, it is also known to provide inwardly bent feet to the otherwise sharp edges.
These may be either curved feet, as taught by USPN 4,633,056 noted above, or perpendicular
and flattened feet, as shown in USPN 6,004,461. Compared to sharp edges, the integral,
outwardly bent feet provide more surface in mutual contact between the central stiffening
walls and the opposed inner surface of the tube.
[0007] It is also known to incorporate an integral corrugation within the basic B tube design,
analogous to the integral corrugation in the simple folded tube of USPN 2,757,628.
An example may be seen in USPN 5,441,106, in which the inwardly bent curved feet described
above are, in effect, extended on out to create two halves of an internal web. Such
a design has the same basic drawbacks as the integral corrugation design shown in
USPN 2,757,628, in that the inner web and outer walls must be of the same thickness
and material, and will be inevitably coated with braze material, as well. A common
feature of internal corrugated webs in all fabricated tubes, so far as is known, is
that the corrugations are regular or symmetric. This gives both a uniform size for
all of the flow paths (but for the outboard pair, which are often inevitably smaller
in cross section), and gives a uniform internal pressure resistance to the tube all
across its width.
SUMMARY OF THE INVENTION
[0008] The invention is a novel tube construction that has the central strengthening feature
of the B tube design described above, but with divided flow paths provided by a specially
designed, separate inner corrugated web.
[0009] In the preferred embodiment disclosed, the outer shell of the tube is formed in a
general "B" shape, with two 90 degree walls that abut at the center. Preferably, the
edges of the abutting 90 degree walls are curved upwardly, rather than being sharp.
Unlike other fabricated tubes, however, the edges of the 90 degree walls do not directly
contact the inner surface of the tube. Instead, a corrugated inner web is placed inside
the tube as it is folded down, and is captured between the under surface of the 90
degree wall edges and the opposed inner surface of the tube. The corrugated web, rather
than being regular and symmetric, has a widened and flattened central channel that
allows it to be captured without deforming the corrugations to either side. Preferably,
both the inner and outer surfaces of the outer tube are braze coated, so that the
inner web need not be.
[0010] In the braze operation, one side of the web channel brazes to the undersurface of
the 90 degree wall edges, and the other side of the web channel brazes to the opposed
inner surface of the tube, solidly anchoring and locating the web within the tube.
The net effect is that the abutted 90 degree walls strengthen the tube, even without
direct contact across both sides of the tube. The web can be formed with any desired
thickness, independent of the outer tube wall thickness and, as noted, need not be
braze coated, though it can be. Small, divided flow paths inside the tube are created
both by the regular corrugations located to either side of the central web channel,
and by the location of the abutted 90 degree walls within the central web channel.
The decoupling of the web and tube material allows the optimal material to be independently
used for both, but the end result is similar to a one piece extruded tube in terms
of strength and function.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] These and other features of the invention will appear from the following written
description, and from the drawings, in which:
Figure 1 is a perspective view of the end of a preferred embodiment of a tube made
according to the invention;
Figure 2 is an end view of a piece of tube stock prior to the manufacturing operation;
Figure 3 is an end view of the tube stock after a first bending operation;
Figure 4 is an end view of the tube stock after a second bending operation;
Figure 5 is an end view of the tube stock after a third bending operation;
Figure 6 is an end view of the tube stock after a fourth bending operation, and showing
the web;
Figure 7 is an end view of the tube stock after a fifth bending operation, and showing
the web in place;
Figure 8 is an end view of the tube stock after a sixth bending operation, and showing
the web in place and anchored down.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0012] Referring first to Figure1, an end view of a preferred embodiment of a tube according
to the invention, indicated generally at 10. Tube 10 is a brazed, fabricated tube,
having only two basic components, one of which is an outer shell formed with two inner
chambers, like the so called "B tube" configuration described in UK patent 1,149,923
noted above. As such, n=2 (the number of inner chambers), and only n-1, or one, piece
of tube stock is needed to form the outer shell, in a manner described in more detail
below. Specifically, the outer shell, though unitary, can be conceptualized as a single,
full width lower wall 12 spaced from a pair of upper walls 14 which preferably, but
not necessarily, are equal in width. Upper walls 14 are integral to a pair of equal
height, abutted 90 degree walls 16, each of which terminates in a curved, out turned
foot 18. The abutted 90 degree walls 16 form a central seam running the entire length
of the outer shell of tube 10, and form a central strengthening member therefor. While
the coincidental provision of two divided chambers within tube 10 would provide some
heat transfer advantage, by the obvious expedient of providing a greater ratio of
conductive perimeter surface per enclosed volume, that effect is minimal, for such
a minimal subdivision of the inner volume. The primary advantage of the abutted 90
degree walls 16, as in the UK patent noted above, is simply the additional stiffening
and strengthening and outer shell, and the location of the inevitable at least one
seam down the central upper surface of the tube 10, rather than down the side edge.
The end of tube 10 is ultimately brazed into a header slot, as with any headered cross
flow heat exchanger, and it is easier to control the geometry of the slot-tube end
braze interface along the width of the slot, rather than at the edge of the slot.
[0013] Referring next to Figures 1 and 5, the other basic component of tube 10 is a corrugated
inner web, a preferred embodiment of which is indicated generally at 20. Web 20 would
likely be same basic material as the outer shell of tube 10, or at least similar enough
to prevent a significant galvanic differential. However, web 20 need not be identical
to the outer shell of tube 10, since it is not integral therewith. Therefore, it can
be, preferably, thinner, as shown, and need not be coated with braze material on its
outer surface (though it can be, as described in more detail below). Specifically,
web 20 has a width W1 and is formed with a series of corrugations 22 which may be,
but need not be, generally sinusoidal and regular in shape, with rounded crests and
sloped sides. They could also be more pointed at the crests, or completely squared
off with vertical sides, if desired. Most significantly, the entire width of web 20
is not comprised of regular, symmetrical corrugations, as is conventional. Instead,
a widened, intermediate channel 24 of width C, is formed, which is flattened at the
bottom, and open at the top, for a purpose described below. Preferably, the channel
24 is also central to the web 20, with an equal number of regular corrugations 22
located to either side, though, again, it need not absolutely be centrally located.
[0014] Referring next to Figures 2 through 5, the initial steps in the manufacture of the
outer shell of tube 10 are illustrated. A single, flat piece of flat metal stock S
is braze coated on at least one surface, that which will ultimately comprise the outer
surface of tube 10 and, preferably, on the other surface, as well, though not necessarily
on more than the outer surface. Most likely, stock S would be pulled from a continuous
coil of stock, and run through a progressive series of rollers, that would continually
and gradually form it into the subsequent shapes illustrated, rather than being bent
incrementally in individual dies. The first step in the gradual formation of the final
shell shape is the bending of the curved feet 18, each of which has a total width
F, shown in Figure 3. Next, as shown in Figure 4, the 90 degree walls 16 are bent
to shape, each with a total height H, which will ultimately determine the inner height
of final tube 10. The two upper walls 14 are partially bent up, leaving the lower
wall 12 in the center. As disclosed, each upper wall 14 is preferably one half the
total outside width W2 of lower wall 12, "upper" and "lower" being terms of convenience,
of course. Before the upper walls 14 are bent too close together to prevent it, web
20 would be fed in between them by a suitable apparatus.
[0015] Referring next to Figures 6 through 8, after the web 20 is fed in, the upper walls
14 are bent progressively farther over and, eventually, the web 20 settles onto the
inner surface of lower wall 12. The web width W1 is comparable to the width W2 of
lower wall 12, less by approximately twice the wall thickness of stock S, so as to
facilitate the location of web 20 inside of tube 10. Eventually, the upper walls 14
are bent over far enough to abut, and the under surfaces of the feet 18 pass by the
crests of the two inner most corrugations 22 and down to engage the upper surface
of web channel 24, anchoring its lower surface to the inner surface of lower wall
12. The height H of the 90 degree walls 16, added to the thickness of the material
of web 20, is set so as to assure that the tops and bottoms of the web corrugations
22 make close contact, without crushing, with the inner surfaces of both the upper
walls 14 and the lower wall 12. The total width of the out turned feet 18 is just
slightly less than the width C of web channel 24, so as to assure a close fit into
the channel 24 without binding, but still serving to help positively locate the web
20 accurately within the interior of tube 10, with a limited side to side play.
[0016] Referring finally to Figures 1 and 8, the fully nested and abutted composite of the
bent metal stock S and separate, anchored inner web 20 are brazed together in a conventional
braze oven to complete tube 10. This is best done as part of an entire core with tubes
10. As is typical, braze material melted from and near the interfaces of the abutted
component surfaces is drawn by capillary action into those closely abutted interfaces,
later hardening to create strong bonds. There are several possible combinations for
the braze coating of these contacting surfaces. Both surfaces of the tube stock S
could be coated, and web 20 not at all. Or, only the outer surface of the tube stock
S could be coated, and both sides of the web 20 coated. When a total core is brazed,
clad material on the air centers could provide what was needed for a bolted walls
16, so bare tube stock S could be used. Or, all surfaces could be coated, on both.
Any such combination would provide a supply of melted braze material to the various
interfaces. For example, the under surfaces of the feet 18 would braze to the upper
surface of the flattened channel 24, and the under surface of channel 24 would braze
to the inner surface of tube lower wall 12, ultimately securing the upper walls 14
to the lower wall 12. The abutted 90 degree walls alone 16 add a degree of strengthening
to the tube 10, and the presence of the intermediate bonded corrugations add to that
strengthening, depending on the thickness of the material of web 20. That thickness
cannot be varied in the design shown in USPN 5,441,106, where the thickness of outer
shell and inner web are inevitably the same. One of the advantages of decoupling the
outer shell from the inner web is that, for example, a very thin web 20 could be used
in a low pressure tube, where additional strength was not needed. Another advantage
is the ability to not coat the web 20 with the rather abrasive braze material that
tends to wear on the machines that typically produce corrugations like 22. Regardless
of the thickness and consequent inner pressure resistance potential of web 20, it
serves to subdivide the interior of tube 10 into multiple flow paths, with the attendant
increase in the ratio of conductive surface area (wetted perimeter) to flow area that
improves efficiency. The location of the abutted 90 degree walls, within the web channel
24, serves to produce two subdivided flow channels, in and of itself.
[0017] Variations of the preferred embodiment disclosed could be made. The width F of the
curved feet 18 could be varied, in absolute terms, but making the width of the feet
18 together approximately equal to the width of two corrugations 22 serves to subdivide
the channel 24 into two flow paths approximately equal to the size of the flow paths
created by each of the corrugations 22, and so yields a measure of structural symmetry
across the entire width of tube 10. The degree of curvature of the feet 18 could be
made more or less, but flattened edges, or sharp edges, instead of a curvature would
not be preferred. Such edges would not braze as well to the upper surface of the channel
24, and would not be as likely to fold past the adjacent web corrugations 22 without
binding as the upper walls 14 were folded down. As disclosed, the abutted 90 degree
walls are central, and the upper walls 12 consequently of equal width, but they could
be shifted to one side or the other, if desired, especially if the relative width
of the feet 18 and the corrugations 22 noted above were maintained, since the effect
on the inner structural symmetry of the tube 10 would not be severe. Therefore, it
will be understood that it is not intended to limit the invention to just the embodiment
disclosed.