Note: Descriptions are shown in the official language in which they were submitted.
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COPPER ALLOY FOR HEAT EXCHANGER TUBE
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. provisional patent application
no.
61/224,671, filed on July 10, 2009, the disclosure of which is incorporated
herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention pertains generally to copper alloys and use of
the copper
alloys in tubes for heat exchangers. Specifically, the invention pertains to a
high strength
copper alloy tube that has desirable pressure fracture strength and
processability properties.
The alloy is suitable to reduce thickness, and therefore, conserves material,
for existing air
conditioning and refrigeration (ACR) heat exchangers, and is suitable for use
in a heat
exchanger using a cooling medium such as C02-
BACKGROUND OF THE INVENTION
[0003] Heat exchangers for air conditioners may be constructed of a U-shaped
copper
tube bent like a hairpin and fins made from aluminum or aluminum alloy plate.
[0004] Accordingly, a copper tube used for the above type heat exchanger
requires
suitable conductivity, formability, and brazing properties.
[0005] HCFC (hydro-chlorofluorocarbon)-based fluorocarbons have been widely
used for cooling media used for heat exchangers such as air conditioners.
However, HCFC
has a large ozone depleting potential such that other cooling media have been
selected for
environmental reasons. "Green refrigerants", for example, CO2, which is a
natural cooling
medium, have been used for heat exchangers.
[0006] The condensing pressure during operation needs to be increased to use
CO2 as
a cooling media to maintain the same heat transfer performance as HCFC-based
fluorocarbons. Usually in a heat exchanger, pressures at which these cooling
media are used
(pressure of a fluid that flows in the heat exchanger tube) become maximized
in a condenser
(gas cooler in C02). In this condenser or gas cooler, for example, R22 (a HCFC-
based
fluorocarbon) has a condensing pressure of about 1.8 MPa. On the other hand,
the CO2
cooling medium needs to have a condensing pressure of about 7 to 10 MPa
(supercritical
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state). Therefore, the operating pressures of the new cooling media are
increased as compared
with the operating pressure of the conventional cooling medium R22.
[0007] Due to the increased pressure and to some loss of strength due to
brazing in
some tube forming processes, conventional copper materials have to be made
thicker thereby
increasing the weight of the tube and therefore the material costs associated
with the tube.
[0008] What is needed is a heat exchanger tube that has high tensile strength,
excellent processability and good thermal conductivity that is suitable for
reducing the wall
thickness, and therefore, the material costs, for ACR heat exchangers and that
is suitable for
withstanding high pressure applications with new "green" cooling media such as
CO2.
BRIEF SUMMARY OF THE INVENTION
[0009] The present invention provides a copper alloy, for use in heat
exchanger tubes,
having, for example, high tensile strength, excellent processability and good
thermal
conductivity.
[0010] In one aspect the present invention is a copper alloy composition,
which
includes the following where the percentages are by weight. The composition
comprises
copper (Cu), nickel (Ni) and tin (Sri). In one embodiment, the alloy has a
composition of 99%
copper by weight, 0.5% nickel by weight and 0.5% tin by weight, represented as
CuNi(0.5)Sn(0.5). In another embodiment, nickel is present in the range of
0.2% to 1.0%, tin
in the range of 0.2% to 1.0%, and the remainder includes Cu and impurities.
The composition
optionally comprises phosphorus in the range of 0.01% to 0.07%.
[0011] In another aspect, the present invention provides tubes for ACR
applications
comprising the copper alloy composition. In yet another aspect of the present
invention, the
alloy composition is formed into tubes for ACR applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Figure 1. Graphical representation of relative metal value per feet vs.
copper
price for a presently used alloy, C122, at standard wall thickness compared
with an alloy of
the present invention CuNi(0.5)Sn(0.5) at reduced wall thickness.
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[0013] Figure 2. Graphical representation of tensile strength and conductivity
for
tested alloys as a function of Ni and Sn contents. Sn has a greater influence
on both strength
and conductivity.
[0014] Figures 3(a) - (c). Graphical representation of various views of a tube
according to an embodiment of the present invention. Figure (a) is a
perspective view; Figure
(b) is a cross-section of the tube of (a) as viewed along a longitudinal axis;
and Figure (c) is a
cross-section of the tube of (a) and (b) as viewed along an axis normal to the
longitudinal
axis.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The present invention provides a high strength alloy which can, for
example,
reduce the wall thickness and therefore reduce the cost associated with
existing ACR tubing
and/or provide ACR tubing capable of withstanding the increased pressures
associated with
cooling media such as CO2. By, high strength it is meant that the alloy and/or
tube made from
the alloy has at least the levels of tensile strength and/or burst pressure
and/or cycle fatigue
failure set out herein. The copper alloy can provide savings in material,
costs, environmental
impact and energy consumption.
[0016] In order to provide a copper alloy for a heat exchanger tube, which
can, for
example, be used with cooling media such as CO2, the selected alloy should
have appropriate
material properties and perform well with regard to processability. Important
material
properties include properties such as, for example, burst pressure/strength,
ductility,
conductivity, and cycle fatigue. The characteristics of the alloy and/or tube
described herein
are desirable so they can withstand ACR operating environments.
[0017] High tensile strength and high burst pressure are desirable tube
properties
because they define what operating pressure a tube can withstand before
failing. For example,
the higher the burst pressure, the more robust the tube design or for a given
burst pressure
minimum the present alloy allows for a thinner wall tube. A correlation exists
between tensile
strength and burst pressure. The alloy and/or tube comprising the alloy has,
for example, a
material tensile strength of a minimum of 38 ksi (kilo-pound per square inch).
The material
tensile strength can be measured by methods known in the art such as, for
example, the
ASTM E-8 testing protocol. In various embodiments, the alloy and/or tube
comprising the
alloy has a material tensile strength of 39, 40, 41 or 42 ksi.
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[0018] Ductility of the alloy and/or a tube made from the alloy is a desirable
property
because, in one embodiment, tubes need to be bent 180 degrees into hairpins
without
fracturing or wrinkling for use in the coil. Elongation is an indicator of
material ductility. The
alloy and/or tube comprising the alloy has, for example, an elongation of a
minimum of
40 %. The elongation can be measured by methods known in the art such as, for
example, the
ASTM E-8 testing protocol. In various embodiments, the alloy and/or tube
comprising the
alloy has a minimum elongation of 41, 42, 43, 44, 45, 46, 47, 48, 49 or 50%.
[0019] Conductivity is a desirable property because it relates to heat
transfer
capability and therefore, it is a component of the efficiency of an ACR coil.
Also,
conductivity can be important for tube formation. The alloy and/or tube
comprising the alloy
has, for example, a conductivity of a minimum of 35% IACS. The conductivity
can be
measured by methods known in the art such as, for example, the ASTM E-1004
testing
protocol. In various embodiments, the alloy and/or tube comprising the alloy
has a minimum
conductivity of 36, 37, 38, 39, 40, 45, 50, 55, 60 or 65% (IACS).
[0020] The alloy and/or tube has, for example, at least equal resistance to
cycle
fatigue failure as the current alloy in use, e.g., C122 as shown in Table 2.
Further, it is
desirable that the alloy and/or tube has, for example, at least equivalent
resistance against one
or more types of corrosion (e.g., galvanic corrosion and formicary corrosion)
as the current
alloy in use, e.g., C122.
[0021] In one embodiment, a tube comprising an alloy of the present invention
has
improved softening resistance (which can be important for brazing) and/or
increased fatigue
strength relative to a standard copper tube, e.g., a tube made from C 122.
[0022] In one embodiment, a tube depicted in Figures 3(a) - (c) with reduced
wall
thickness t (relative to a tube comprising a conventional alloy, e.g., C 122)
comprising the
present alloy has equal or improved burst pressure and/or cycle fatigue
relative to tube
comprising a conventional alloy, e.g., C122. For example, the tube wall
thickness of a tube of
the present invention is minimized relative to a standard tube, e.g. a C122
tube, which
reduces total material cost, and both tubes exhibit the same burst pressure.
In various
embodiments, the tube wall thickness is at least 10, 15 or 20% less than a
C122 tube, where
both tubes have the same burst pressure. The burst pressure can be measured by
methods
known in the art such as, for example, CSA-C22.2 No. 140.3 Clause 6.1 Strength
Test - UL
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207 Clause 13. The cycle fatigue can be measured by methods known in the art
such as, for
example, CSA-C22.2 No. 140.3 Clause 6.4 Fatigue Test - UL 207 Clause 14.
[0023] The alloy of the present invention can be fabricated according to
methods
known in the art. During the alloy fabrication process and/or tube formation
process, it can be
important to control the temperature. Control of temperature can be important
in keeping the
elements in solution (preventing precipitation) and controlling grain size.
For example,
conductivity can increase and formability can suffer if processed incorrectly.
[0024] For example, to maintain both the desired grain size and prevent
precipitate
formation in the alloy fabrication and/or tube formation processes, heat
treatment in the
production process will occur over a short time such that the temperature of
the alloy and/or
tube will be between 400-600 C with a rapid (e.g., 10 to 500 C/second)
upward and
downward ramping of the temperature.
[0025] It is desirable that alloy and/or tube made from the alloy have a
desired grain
size. In one embodiment, the grain size is from 1 micron to 50 microns,
including all integers
between 1 micron and 50 microns. In another embodiment, the grain size is from
10 microns
to 25 microns. In yet another embodiment, the grain size is from 10 microns to
15 microns.
The grain size can be measured by methods known in the art such as, for
example, the ASTM
E-1 12 testing protocol.
[0026] The alloy compositions of the present invention include the following
where
relative amounts of the components in the alloy are given as percentages by
weight. The
ranges of percentage by weight include all fractions of a percent (including,
but not limited
to, tenths and hundredths of a percent) within the stated ranges.
[0027] In one embodiment, the composition comprises copper, nickel, tin, and,
optionally, phosphorus. The nickel is present in the range of 0.2% to 1.0%,
and more
specifically in the range of 0.3% to 0.7%; tin in the range of 0.2% to 1.0%,
and more
specifically in the range of 0.3% to 0.7%; and its remainder includes copper
and impurities.
In one embodiment, the composition of the alloy is CuNi(0.5)Sn(0.5). In
another
embodiment, the composition of the alloy is CuNi(0.5)Sn(0.5)P(0.020).
[0028] The impurities can be, for example, naturally-occurring or occur as a
result of
processing. Examples of impurities include, for example, zinc, iron and lead.
In one
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embodiment, the impurities can be a maximum of 0.6 %. In various other
embodiments, the
impurities can be a maximum of 0.5, 0.45, 0.3, 0.2 or 0.1%.
[0029] Phosphorus is present, optionally, in the range of 0.01% to 0.07%, and
more
specifically in the range of 0.015% to 0.030%, or at 0.02%. Without intending
to be bound by
any particular theory, it is considered that inclusion of an appropriate
amount of phosphorus
in the alloy increases the weldability of the alloy by effecting the flow
characteristics and
oxygen content of the metal, while addition of too much phosphorus leads to
poor grain
structure and unwanted precipitates.
[0030] In one embodiment the composition consists essentially of Cu, Ni and Sn
in
the aforementioned ranges. In another embodiment the composition consists
essentially of
Cu, Ni, Sri and P in the aforementioned ranges. In various embodiments,
addition of
components other than copper, nickel, tin (and phosphorus in the case of the
second
embodiment) does not result in an adverse change of greater than 5, 4, 3, 2 or
I% in
properties of the alloys of the present invention such as, for example, burst
pressure/strength,
ductility, conductivity, and cycle fatigue.
[0031] In one embodiment, the composition of the alloy consists of Cu, Ni, Sn
and P
in the aforementioned ranges. In another embodiment, the composition of the
alloy consists
of Cu, Ni, Sn and P in the aforementioned ranges.
[0032] The alloy of the present invention may be produced for use by various
processes such as cast and roll, extrusion or roll and weld. The processing
requirement
includes, for example, brazeability. Brazing occurs when the tubes are
connected as described
below.
[0033] Generally, in the roll and weld process the alloy is cast into bars,
roll reduced
to thin gauge, heat treated, slit to size, embossed, formed into tube, welded,
annealed, and
packaged. Generally, in the cast and roll process the alloy is cast into
"mother" tube, drawn to
size, annealed, machined to produce inner grooves, sized, annealed, and
packaged. Generally,
in the extrusion process, the alloy is cast into a solid billet, reheated,
extrusion pressed, drawn
and grooved to final dimensions, annealed and packaged.
[0034] In one aspect the present invention provides tubes comprising a copper-
nickel-
tin alloy (described herein). In one embodiment, the tubes are from 0.100 inch
to 1 inch in
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outer diameter, including all fractions of an inch between 0.100 inch and 1
inch, and have a
wall thickness of from 0.004 inch to 0.040 inch, including all fractions of an
inch between
0.004 and 0.040 inch. One advantage of the present invention is that thinner
walled tubes can
be used in ACR applications. This leads to reduced materials costs (see Figure
1).
[0035] In one embodiment, the tubes comprising the copper-nickel-tin alloy
(described herein) are used in ACR applications. It is desirable that the
tubes have sufficient
conductivity (e.g., so that the tubes can be joined by welding) and
formability (e.g., ability to
be shaped, e.g., bent, after formation of the tube). Also, it is desirable
that the tubes have
properties such that the tube can have internal groove enhancement.
[0036] One example of a process suited for the alloy of the present invention
is a heat
exchanger coil having tubes formed with a roll and weld process. In an initial
step, the copper
alloy of the present invention is cast into slabs followed by hot and cold
rolling into flat
strips. The cold rolled strips are soft annealed. The soft annealed copper
alloy strips are then
formed into heat exchanger tubes by means of a continuous roll forming and
weld process.
Before the roll forming and welding process the tubes may be provided with
internal
enhancements such as grooves or ribs on the inside wall of the tube as will be
evident to those
of ordinary skill in the art. The tubes are formed in a continuous roll and
weld process and the
output may be wound into a large coil. The large coil may then be moved to
another area
where the coil is cut into smaller sections and formed into the U or hairpin
shape.
[0037] In order to construct a heat exchanger, the hairpin is threaded into
through-
holes of aluminum fins and a jig is inserted into the U-shaped copper tube to
expand the tube,
thereby closely attaching the copper tube and the aluminum fin to each other.
Then the open
end of the U-shaped copper tube is expanded and a shorter hairpin similarly
bent into a U-
shape is inserted into the expanded end. The bent copper tube is brazed to the
expanded open
end using a brazing alloy thereby being connected to an adjacent hairpin to
make a heat
exchanger.
[0038] The following Example is presented to further describe the present
invention
and is not intended to be in any way limiting.
EXAMPLE
[0039] Copper alloys with different Ni and Sri contents were produced in pilot
scale
and mechanical and physical properties tested, see Table 1.
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[0040] The results was plotted versus the amount of Ni or Sri, see Figure 2.
All tested
alloys meet a desired minimum conductivity of 35 % IACS. The mechanical
properties of a
minimum tensile strength of 38 ksi is achieved for all tested alloys. In order
to meet desired
strength and conductivity the composition should be from 0.2% to 1.0 % by
weight for both
Ni and Sri.
[0041] Material of a composition of 0.5 % Ni and 0.5 % Sn (CuNi(0.5)Sn(0.5)
was
produced in full production scale and formed to tubes using the roll and weld
method. The
tubes were produced both in standard wall thickness (e.g., 0.0118 inches) and
with 13 %
lower wall thickness. Mechanical properties of the tubes were tested using
ASTM and UL
(e.g., UL testing protocols and compared with tubes made of "present use"
copper alloy
C 12200 with standard wall thickness. The results are shown in Table 2. The
alloy of the
invention CuNi(0.5)Sn(0.5) has higher strength and higher burst pressure in
standard wall
thickness. For tubes produced with reduced wall thickness the burst pressure
for an alloy of
the present invention (CuNi(0.5)Sn(0.5)) is still higher compared with C122 at
standard wall
thickness.
Table 1. Mechanical properties and conductivity for tested alloys at different
Ni and Sri
contents.
Alloy Ni Sn P TS E TS E Electrical
no Parallel Parallel Transverse Transverse Conductivity
(%) (%) (%) (ksi) (%) (ksi) (%) (% IACS)
A 0.2 0.2 0.026 39.4 48.0 38.0 47.9 65
B 0.5 0.5 0.020 41.1 44.2 40.9 46.7 51
C 1.0 0.5 0.028 42.2 44.1 42.2 46.3 42
D 0.8 1.2 0.025 46.6 46.3 46.0 47.9 35
E 1.7 1.2 0.020 50.0 43.0 - - 35
Table 2. Mechanical properties of tubes made of an alloy of the invention
(CuNi0.5Sn0.5)
compared with current standard alloy C 12200 (Cu-DHP).
Alloy Wall Grain Tensile Elongation Burst Conductivity Cycle
thickness of size strength pressure Fatigue
tube (mm) (ksi) (%) (psi) (% IACS)
CuNiO.5SnO.5 Standard 0.015 39.7 46 2450 52 Pass
CuNiO.5SnO.5 87 % of 0.015 39.7 50 1980 52 Pass
standard
C12200 Standard 0.020 34.7 47 1950 83 Pass
[0042] While the invention has been particularly shown and described with
reference
to specific embodiments, it should be understood by those having skill in the
art that various
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changes in form and detail may be made therein without departing from the
spirit and scope
of the present invention as disclosed herein.
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