Note: Descriptions are shown in the official language in which they were submitted.
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hAn~N~r
Docket No. 655.00882 (Index 895)
METHOD OF MAKING A TUBE
FOR A HEAT EXCHANGER
FIIJ,L.D OF 'fHE INVENTION
This invention relates to heat exchangers, alld more particularly, to
so-called oval or flattened dimpled tubes employed in such heat exchangers and
methods of making the same.
BA('KGfOUND OI' 'fI-Il: INVENTION
So-called flattened or oval cross-section, dimpled tubes have long
been employed in heat exchangers of various types. In such tubes, dimples are
lonncc( in the side walls of the tubes to extend partially or wholly across
the interior
of the tube in a direction across the tube minor dimension. In many cases, the
dimples arc employed for the purpose of inducing turhulcncc in the heat
exchange
Iluid (lowing within the tube to thereby increase the rU~ ul~ heat transfer.
Ira other
cases, opposed dimples are bonded to one another or dimples are bonded to the
opposing side wall of the tube for the purpose of providing pressure
resistance to the
tuhcs.
More specifically, with weight being a concern in many applications,
particularly in vehicular applications, and to reduce material costs, tubes of
this sort
are made with extremely thin walls and consequently may be incapable of
supporting
the tube against distortion caused by pressure of the fluid within the tube.
This is of
particular concern when the tubes are used in higher pressure applications
such as
in condensers for refrigeration systems, etc. In particular, because the tubes
are
flattened, when subjected to pressure, they tend to "go round" unless they are
supported against the forces causing such distortion. In some cases, plate
fins are
employed on the exterior of the tubes to girdle the tubes and prevent such
distortion
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while in others, inserts are located within the tubes and bonded to opposed
side walls
for the same purpose. In still others, internally directed dimples are brazed
to one
another or to the opposite side wall of the tube to provide the required
strength.
Because of its relatively low density and relative high thermal
conductivity, aluminum is increasingly becoming the material of choice in
forming
such tubes. Aluminum tubes are easily brazed to other heat exchanger
components
such as headers and fins. 'thus, commonly a dimpled, aluminum tube is formed
by
placing dimples in a strip used to form the tube while the strip is generally
t7at or
planar. Through forming techniques of a conventional nature, the strip is
formed
into a generally round shape and the sides of the strip brought together. High
frequency induction welding is employed along the edges that are in abutment
to
weld the Drip into a closed tube. lvrther sizing of the tube from a basically
round
tube to the conventional flattened or oval cross-section shaped tube then
takes place.
1)IlllpleS formed in the strip while (latare brought into substantial abutment
with one
another.
'the strip itself is formed of aluminum base metal and clad with an
aluminum based braze alloy on both sides thereof'. l3ra~ing flux is introduced
into
the interior of the tube and when the tube is assembled into a heat exchanger
and
held in place by a fixture, and subject to a brazing operation, braze alloy
material on
the interior of the tube flows at the dimples and joins opposed dimples
together.
One problem with forming dimpled tubes in this fashion is the fact
that the strip of which the tube is formed must be braze clad on both sides
thereof.
Braze cladding on one side is required to assure the availability of braze
alloy during
the brazing process where the dimples are in contact with one another so as to
bond
the two together. It is also required on the opposite side of the strip which
will be
the exterior of the tube, to allow the strip to be brazed to headers, and even
more
particularly, to f ns. Strip stock that is provided with braze cladding on
both sides
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is more expensive than strip stock that is clad with braze alloy on only one
side.
However, according to present techniques, the former is required to achieve
brazing
of the internally directed dimples to one another.
Another difficulty is the fact that brazing flux must be present on the
interior of the tube in order to assure that the dimples will braze to one
another.
Brazing tluxes typically employed in an aluminum brazing operation, such as
the
NocolokC~ process leave residues after the brazing process is completed. These
flux
residues make it difficult to employ the process on tubes having passages of
relatively small hydraulic diameter since the pflSSageS Illay beC()nle
partially or
wholly plugged by the flux residue. Moreover, introducing the flux residue
into the
interior of the tube can be challenging and the tube interiors may require a
subsequent cleaning operation to minimise the accumulation of braiing flux
therein.
The present invention is directed to overcoming one or more of the
abcwe problems.
SUMMARY OF THE INVENTION
It is the principal object of the invention to provide a new and
improved method of forming a dimpled tube. (t is also an object of the
invention
to provide a tube for a heat exchanger made according to the inventive method.
The highly preferred method of making a heat exchanger tube
?0 according to the invention includes the steps of:
a) providing an elongated strip of aluminum base material and
having opposed edges and a braze alloy on only one side thereof;
b) forming dimples in the strip such that the dimples project to
the side of the strip opposite the strip one side and the dimples on one side
of a
predetermined fold line located between the opposed edges will align with the
dimples on the other side of the fold line;
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Docket No. 655.00882 (Index 895)
c) placing slits extending through the strip at the apexes of the
dimples on at least one side of the fold line;
d) folding the strip along the fold line and bringing the edges into
contact with one another while causing the apexes of aligned ones of the
dimples to
abut one another;
e) welding the edges to each other to form a tube; and
f) causing the braze alloy to flow from the strip one side through
the slits to the interior of the tube and braze the apexes of the aligned
dimples to one
another.
Even more preferably, step f) is proceeded by the step of applying a
flux to the strip one side at at least the vicinity of the dimples having the
slits.
Preferably, the dimples arc nominally frusta-conical.
In some cases, perforations other than slits may be used while in
crihcrs, dimples may be located on only one side of the fold line and brought
into
contact with the opposite internal side wall of the tube.
Also contemplated is a new and improved dimpled tube made by the
method of the invention.
Other objects and advantages will become apparent from the
following specification taken in connection with the accompanying drawings.
DI:SCRIP'rION OF TI-IE DRAWINGS
Fig. 1 is a side elevation of a typical heat exchanger employing
dimpled tubes made according to the invention;
Fig. 2 is a perspective view of a section of aluminum sheet stock to
be ultimately folded upon itself to form the tube;
Fig. 3 is a view similar to Fig. 2 but showing the section of sheet
stock folded upon itself to form a completed tube section; and
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Docket No. 655.00882 (Index 895)
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Fig. 4 is an enlarged, sectional view of a dimpled tube made
according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The method ofthe present invention is intended to produce aluminum
based tubes of so-called flattened or oval cross-section which include
internally
directed dimples. Tubes of this sort are ideally suited For employment in heat
exchangers of various types and particularly those where internally directed
dimples
are employed for inducing turbulence within the interior of the tube and
wherein the
tube is required to have some degree of internal resistance to pressure of a
heat
exchange fluid passing through the tube. Typically, the tubes will be useful
in the
tahrication of vehicular radiators, charge air coolers, heat exchangers used
in air
conditioning or refrigeration systems, etc. However, it is to be particularly
noted that
no limitation to use of any of the foregoing listed heat exchangers is
intended. It
should be sufficient to note that the tubes are ideally suited for use in any
heat
exchanger application wherein dimples are employed for inducing turbulence
and/or
for providing pressure resistance.
With reference to Uig. l, a typical heat exchanger made according to
the invention includes opposed, spaced, parallel headers which may be in any
known
form. I~or example, the headers 10, 12 may be tubular, formed of one or more
pieces. Alternatively, the headers I 0 and 12 may be formed of header plates
capped
with tanks in a conventional fashion.
As shown in Fig. l, an inlet is schematically indicated at 14 and
extends to the header 10. An outlet, schematically illustrated at 16 extends
from the
header 12. A plurality of tubes 18, only some of which are shown, extend
between
the headers 10, 12 and are in fluid communication with the interior thereof.
The
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tubes 18 will be of so-called or flattened or oval cross-section and extend
through
slots (not shown) in facing sides of the headers l0, 12 and brazed in place.
Serpentine fins 20 are located between adjacent ones of the tubes 18
and brazed thereto in a conventional fashion. However, it is to be
particularly noted
that plate fins could be used if desired.
All of the components are formed of an aluminum based material so
that they may be brazed together in a conventional fashion as, for example, by
the
so-called Nocolok~ process which employs potassium fluoro aluminate compounds
as a brazing flux.
It should also be noted that while inlet 14 is indicated as extending
to one of the headers 10 and the outlet 16 extends from the header 12,
describing a
so-called single pass heat exchanger, baffles may he placed in one or the
other or
both of the headers 10, 12 to provide as many different passes as may be
desired.
Thus, the inlet 14 and the outlet I 6 may be located in a single one of the
headers 10,
12 rather than in separate headers, depending upon the number of passes
provided.
Turning to Fig. 2, tubes 18 are formed of an elongated strip 22 of
aluminum based sheet stock. The strip 22 has opposed edges 24, 26 which may be
curled upwardly in the same direction through about 90 ° at each of the
edges 24, 26
as illustrated.
Generally centrally of the strip is a pre-determined fold line 28. In
actuality, the fold line 28 is imaginary and represents an area on which the
strip 22
will be bent along its length to bring the edges 24 and 26 into contact with
one
another. Of course, if desired, the fold line 28 could actually be located on
the strip
22 by any suitable marking technique from scoring to the application of ink or
the
like.
Between the fold line 28 and the edge 24, a series of dimples 30 are
formed in at least one row that extends longitudinally of the strip 22. The
dimples
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Docket No. 655.00882 (Index 895)
30 are spaced from one another, nominally fcusto-conical in shape, and
according to
a preferred embodiment, imperforate. They may be formed by any suitable
stamping
or drawing technique as the strip 22 is passed through forming equipment of a
conventional nature employed to form the strip 22 into a completed tube.
A second row of spaced dimples 32 is formed between the fold line
28 and the edge 26. The dimples 32 are located so as to align with the dimples
30
when the strip is folded approximately 180° on the fold line 28. The
dimples 32 are
also generally frusto-conical in shape and their apexes are provided with
openings
34.
'The strip 22 is, as mentioned previously, aluminum based material.
On its upper surface 36, bare aluminum is present. That is to say, the upper
surface
36 is free of brace alloy, although the SaillC Illay bC OXICII%,ed somewhat as
is well
known. The opposite side of the strip 38 is clad in braze alloy which will
typically
be an aluminum based compound containing some silicon or other materials which
lowers its melting point slightly below the mc1t111g pollll ol' the base
aluminum ol~
which the strip 22 is formed. This allows the braze alloy to become a liquid
and
flow before the aluminum of the base strip 22 softens and flows. The
metallurgy of
such a system is well known and will not be described further herein.
n section of completed tube is illustrated in Fig. 3. Here, the edges
''0 24 and 26 have been brought into abutment with one another and bonded to
each
other as, for example, by high frequency induction welding. In addition, the
apexes
of the dimples 30 and 34 have been brought into contact with one another and,
by
means to be described in greater detail, bonded to one another. Of course, the
tubes
18 will typically be substantially longer than that shown in Fig. 3 which
represents
only a short section of a completed tube 18.
Turning now to Fig. 4, the strip 22 is illustrated with its edges 24, 26
abutted against one another and with a weld 40 formed at the point of abutment
to
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complete a tube. Also seen is the layer 42 of braze alloy on the side 38 of
the strip
22. It will be noted that the side 36 is free of braze clad alloy.
The apex 44 of one of the dimples 32 has been brought into abutment
with the apex ,46 of one of the dimples 30.
S It will be seen that the openings 34 in the apexes 44 thus align with
the apexes 46. Fillets 48 of braze material bond the apexes 44, 46 together as
well
as seal the openings 34. The fillets 48 are formed by braze material from the
layer
42 on the side 38 of the strip entering the interior of the tube through the
openings
34 to wet the apex 46 of the dimples 30. Upon solidification, the fillets 48
are
formed.
In the usual case, a strip 42 will be unwound from a coil or the like
and run through a stamping or drawing machine to form the dimples 30, 32 as
well
as the curl at the edges 24 and 26. Through roll forming techniques or the
like, the
strip 22 is bent generally about the fold lint and elsewhere to cause the
edges 24, 26
to be brought into abutment with one another where they are subject to welding
to
form the weld 40. The resulting tube will be generally round in shape and may
be
flattened about the weld 40 and the fold line 46 to assume the configuration
generally illustrated in I~ig. 3, thereby bringing the dimples 30, 32 into
contact with
one another. In some cases, the imperforate dimple 30 may be omitted and the
~0 perforated dimples 32 brought into direct contact with the facing interior
wall of the
tube 18. The welding operation will generally occur before the dimples 30, 32
are
abutted but can take place at the same time or afterwards as desired.
At this or any subsequent time, flux may be applied to the exterior of
the tube at least in the vicinity of the dimples 30, 32 and the tube subject
to a brazing
operation. The braze clad material 42 on the surface 38 of the tube will
liquify and,
along with the flux, flow through the openings 34 to the interior of the tube
and wet
the apex 46 of the dimple 30 and ultimately form the fillets 48. Preferably,
however,
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prior to that occurring, the tube is assembled in a fixture along with the
headers 10,
12, subject to fluxing and then to brazing temperature to achieve the linished
heat
exchanger with the dimples 30, 32 bonded to one another.
While the openings 34 may take on a variety of shapes from circular
S to oval to other configurations, it is preferred to form them as slits such
as the slits
shown at 34 in the drawings. In the course of forming the strip 22 as
mentioned
previously, it has been determined that slits are substantially easier to form
than
circular openings such that the forming process need not be slowed down by any
need to actually remove material to form the openings 34. That is to say, a
slit
opening is basically one that penetrates fully through the strip 22 at the
apex 44 of
the dimples 32 without the removal of any material. I Iowevcr, where some
decrease
in assembly line speed can be tolerated, the openings 34 may be lormed by
other
than slitting with material actually removed at the apexes 44.
The openings 34 thus provide access to the interior of the tube in the
vicinity of the apexes 46 on the dimples 30 to allow both the braze alloy from
the
layer 42 and flux to enter at the precise point where the bonding between
dimples 30,
32 is to occur. As a consequence of the foregoing, the strips 22 need be braze
clad
only on one side rather than on both sides. Consequently, material costs for
the tubes
is reduced.
Furthermore, fluxing of the interior of the tube is avoided entirely
since the flux flows into the joint from the exterior of the tube. Thus,
problems
associated with flux in the passages and the need to form a flux removing
cleaning
operation are avoided as well.
The resulting tube therefore provides the advantages of known
dimpled, flattened tubes while being produceable at a lower cost and avoiding
any
problems associated with the flux residue. The dimples in the tube provide the
desired turbulence and/or pressure resistance for many applications.
