U.S. patent application number 16/057821 was filed with the patent office on 2019-03-07 for heat exchanger.
This patent application is currently assigned to KEIHIN THERMAL TECHNOLOGY CORPORATION. The applicant listed for this patent is KEIHIN THERMAL TECHNOLOGY CORPORATION. Invention is credited to Mana KOBAYASHI, Hiroshi OTSUKI, Kazuyuki TAKAHASHI.
Application Number | 20190072344 16/057821 |
Document ID | / |
Family ID | 65364248 |
Filed Date | 2019-03-07 |
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United States Patent
Application |
20190072344 |
Kind Code |
A1 |
TAKAHASHI; Kazuyuki ; et
al. |
March 7, 2019 |
HEAT EXCHANGER
Abstract
A heat exchanger includes exchange tubes formed from an aluminum
extrudate and fins made of an aluminum bare material. The wall of
each heat exchange tube is composed of a main body portion made of
an Al alloy forming the aluminum extrudate, and a covering layer
made of an Al--Si--Zn alloy and covering the main body portion. A
diffusion layer containing Zn and Si diffused from the Al--Si--Zn
alloy is formed in an outer surface layer portion of the main body
portion. A low potential portion whose spontaneous potential is the
lowest, and a high potential portion whose spontaneous potential is
60 mV or more higher than that of the low potential portion, are
present within a range between an outermost surface of the wall and
a deepest portion of the diffusion layer such that the low
potential portion is located toward the outermost surface of the
wall.
Inventors: |
TAKAHASHI; Kazuyuki;
(Oyama-shi, JP) ; OTSUKI; Hiroshi; (Oyama-shi,
JP) ; KOBAYASHI; Mana; (Oyama-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KEIHIN THERMAL TECHNOLOGY CORPORATION |
Oyama-shi |
|
JP |
|
|
Assignee: |
KEIHIN THERMAL TECHNOLOGY
CORPORATION
Oyama-shi
JP
|
Family ID: |
65364248 |
Appl. No.: |
16/057821 |
Filed: |
August 8, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28D 2021/0084 20130101;
F28F 2255/16 20130101; F28F 9/0251 20130101; F28F 21/084 20130101;
F28D 1/05375 20130101; F28F 19/06 20130101 |
International
Class: |
F28F 19/06 20060101
F28F019/06 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2017 |
JP |
2017-169933 |
Claims
1. A heat exchanger comprising: a plurality of heat exchange tubes
formed from an aluminum extrudate; and fins made of an aluminum
bare material, each disposed between adjacent heat exchange tubes,
and joined to the corresponding heat exchange tubes by a brazing
material, wherein each heat exchange tube has a wall composed of a
main body portion made of an Al alloy forming the aluminum
extrudate, and a covering layer made of an Al--Si--Zn alloy and
covering an outer surface of the main body portion; a diffusion
layer in which Zn and Si contained in the Al--Si--Zn alloy forming
the covering layer are diffused is formed in an outer surface layer
portion of the main body portion of the wall of each heat exchange
tube; and a low potential portion whose spontaneous potential is
the lowest, and a high potential portion whose spontaneous
potential is 60 mV or more higher than that of the low potential
portion, are present within a range between an outermost surface of
the wall of each heat exchange tube and a deepest portion of the
diffusion layer such that the low potential portion is located
toward the outermost surface of the wall.
2. The heat exchanger according to claim 1, wherein within the
range between the outermost surface of the wall of each heat
exchange tube and the deepest portion of the diffusion layer, the
spontaneous potential of the wall lowers from the outermost surface
of the wall toward the main body portion up to the low potential
portion, and the spontaneous potential of the wall increases from
the low potential portion toward the main body portion up to the
high potential portion.
3. The heat exchanger according to claim 1, wherein the brazing
material for joining the heat exchange tubes and the corresponding
fins is composed of Al contained in the Al alloy forming the
aluminum extrudate, and Si of Si powder caused, before joining, to
adhere to surfaces of the heat exchange tubes.
4. The heat exchanger according to claim 1, wherein the Al--Si--Zn
alloy serving as the covering layer of each heat exchange tube is
composed of Al contained in the Al alloy forming the aluminum
extrudate, Si of Si powder caused, before joining, to adhere to the
surface of the heat exchange tube, and Zn of Zn powder caused,
before joining, to adhere to the surface of the heat exchange tube.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a heat exchanger. More
particularly, the present invention relates to a heat exchanger
which is used as a condenser for a car air conditioner mounted on a
vehicle such as an automobile.
[0002] In this specification and claims, the term "aluminum"
encompasses aluminum alloys in addition to pure aluminum. Also,
materials represented by chemical symbols represent pure materials,
and the term "Al alloy" means an aluminum alloy.
[0003] In this specification, the term "spontaneous potential" of a
material refers to the electrode potential of the material within
an acidic (pH: 3) aqueous solution of 5% NaCl with respect to a
saturated calomel electrode (S.C.E.), which serves as a reference
electrode.
[0004] A heat exchanger having the following structure has been
widely known and used as a condenser for a car air conditioner. The
heat exchanger has a plurality of flat heat exchange tubes formed
from an aluminum extrudate, header tanks, corrugated aluminum fins,
and aluminum side plates. The flat heat exchange tubes are disposed
at predetermined intervals in their thickness direction such that
they have the same longitudinal direction and their width direction
coincides with an air-flow direction. The header tanks are disposed
at opposite longitudinal ends of the heat exchange tubes such that
their longitudinal directions coincide with the direction in which
the heat exchange tubes are juxtaposed. Opposite ends of the heat
exchange tubes are connected to the corresponding header tanks.
Each of the fins is disposed between adjacent heat exchange tubes
or on the outer side of the heat exchange tube at each of opposite
ends, and is brazed to the corresponding heat exchange tube(s). The
side plates are disposed outward of the fins at opposite ends and
are brazed to the corresponding fins. Each of the header tanks is
composed of a tubular tank body formed of aluminum and closing
members formed of aluminum. The tank body is formed by bending,
into a tubular shape, an aluminum brazing sheet having a brazing
material layer on each of opposite sides thereof and brazing
opposite side edges of the sheet which are butted against each
other. The tank body has openings at opposite ends thereof. The
closing members are brazed to the opposite ends of the tank body so
as to close the openings at the opposite ends. The tank body has a
plurality of tube insertion holes elongated in the air-flow
direction and spaced from one another in the longitudinal direction
of the tank body. An end portion of each heat exchange tube is
inserted into the corresponding tube insertion hole and is brazed
to the tank body.
[0005] The present applicant has proposed a method of manufacturing
the above-described heat exchanger (see Japanese Patent Application
Laid-Open (kokai) No. 2014-238209). The proposed method includes
steps of: preparing heat exchange tubes and fins; adhering Zn
powder and flux powder to outer surfaces of the heat exchange
tubes; and brazing the heat exchange tubes and the corresponding
fins and forming a Zn diffused layer in an outer surface layer
portion of each of the heat exchange tubes. Each of the heat
exchange tubes has a wall thickness of 200 .mu.m or less and is
formed from an aluminum extrudate made of an alloy containing Mn in
an amount of 0.2 to 0.3 massa, Cu in an amount of 0.05 mass % or
less, and Fe in an amount of 0.2 mass % or less, the balance being
Al and unavoidable impurities. Each of the fins is formed from a
brazing sheet composed of an aluminum core material, and a coating
material formed of an aluminum brazing material and covering the
opposite sides of the core material. In the step of adhering Zn
powder and flux powder to the outer surfaces of the heat exchange
tubes, a dispersing liquid is prepared by mixedly dispersing flux
powder, and Zn powder having an average particle size of 3 to 5
.mu.m and a largest particle size of less than 10 .mu.m in a
binder. The dispersing liquid is applied to the outer surface of
each of the heat exchange tubes, and the liquid component of the
dispersing liquid is vaporized so as to adhere the Zn powder and
the flux powder to the outer surface of each heat exchange tube
such that the Zn powder adhesion amount becomes 1 to 3 g/m.sup.2,
the flux powder adhesion amount becomes 15 g/m.sup.2 or less, and
the ratio of the flux powder adhesion amount to the Zn powder
adhesion amount (the flux powder adhesion amount/the Zn powder
adhesion amount) becomes 1 or higher. In the step of brazing the
heat exchange tubes and the corresponding fins, the heat exchange
tubes and the fins in an assembled condition are heated so as to
braze the heat exchange tubes and the corresponding fins through
utilization of the flux powder adhering to the outer surfaces of
the heat exchange tubes and the coating material of the fins and to
melt the Zn powder adhering to the outer surfaces of the heat
exchange tubes for diffusing Zn in outer surface layer portions of
the heat exchange tubes so as to form Zn diffused layers in the
respective outer surface layer portions of the heat exchange
tubes.
[0006] In manufacture of the heat exchanger by the method described
in the publication, the heat exchange tubes and the corresponding
fins are joined by a brazing material melted out from the coating
material of the brazing sheet for forming the fins.
[0007] A conceivable method for further enhancing the corrosion
resistance of the fins in the heat exchanger manufactured by the
method described in the publication is to use fins made of an
aluminum bare material in place of the fins formed from an aluminum
brazing sheet. In this case, the described method may be modified
such that, in addition to the Zn powder, Si powder is caused to
adhere to the outer surfaces of the heat exchange tubes, and the
heat exchange tubes and the corresponding fins are joined by a
brazing material composed of Al contained in the Al alloy forming
the aluminum extrudate from which the heat exchange tubes are
formed, and Si of the Si powder caused, before joining, to adhere
to the surfaces of the heat exchange tubes.
[0008] However, in the heat exchanger manufactured by such a
method, the corrosion resistance of the heat exchange tubes may
become insufficient.
SUMMARY OF THE INVENTION
[0009] An object of the present invention is to solve the
above-described problem and to provide a heat exchanger in which
heat exchange tubes exhibit excellent corrosion resistance.
[0010] A heat exchanger according to the present invention
comprises a plurality of heat exchange tubes formed from an
aluminum extrudate and fins made of an aluminum bare material, each
disposed between adjacent heat exchange tubes, and joined to the
corresponding heat exchange tubes by a brazing material. Each heat
exchange tube has a wall composed of a main body portion made of an
Al alloy forming the aluminum extrudate, and a covering layer made
of an Al--Si--Zn alloy and covering an outer surface of the main
body portion. A diffusion layer in which Zn and Si contained in the
Al--Si--Zn alloy forming the covering layer are diffused is formed
in an outer surface layer portion of the main body portion of the
wall of each heat exchange tube. A low potential portion whose
spontaneous potential is the lowest, and a high potential portion
whose spontaneous potential is 60 mV or more higher than that of
the low potential portion, are present within a range between an
outermost surface of the wall of each heat exchange tube and a
deepest portion of the diffusion layer such that the low potential
portion is located toward the outermost surface of the wall.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a perspective view showing the overall structure
of a condenser for a car air conditioner to which a heat exchanger
according to the present invention is applied;
[0012] FIG. 2 is an enlarged sectional view partially showing the
wall of a heat exchange tube of the condenser of FIG. 1; and
[0013] FIG. 3 is a graph showing spontaneous potentials at
different depths from the wall outermost surface of a single heat
exchange tube to which a fin is brazed in an experimental
example.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0014] An embodiment of the present invention will next be
described with reference to the drawings. In the embodiment, a heat
exchanger according to the present invention is applied to a
condenser for a car air conditioner.
[0015] FIG. 1 shows the overall structure of a condenser for a car
air conditioner to which a heat exchanger according to the present
invention is applied, and FIG. 2 shows the structure of a main
portion of the condenser.
[0016] Notably, in the following description, the upper, lower,
left-hand, and right-hand sides of FIG. 1 will be referred to as
"upper," "lower," "left," and "right," respectively.
[0017] As shown in FIG. 1, a condenser 1 for a car air conditioner
includes a plurality of flat heat exchange tubes 2 formed from an
aluminum extrudate, corrugated fins 3 each formed of an aluminum
bare material, a pair of header tanks 4 and 5 formed of aluminum,
and side plates 6 formed from an aluminum brazing sheet. The heat
exchange tubes 2 are disposed at predetermined intervals in the
vertical direction (the thickness direction of the heat exchange
tubes 2) in such a manner that their longitudinal direction
coincides with the left-right direction and their width direction
coincides with an air-passing direction. The corrugated fins 3 are
disposed between adjacent heat exchange tubes 2 and on the outer
sides of the uppermost and lowermost heat exchange tubes 2, and are
brazed to the corresponding heat exchange tubes 2. The header tanks
4 and 5 are disposed at a predetermined interval in the left-right
direction in such a manner that their longitudinal direction
coincides with the vertical direction (the direction in which the
heat exchange tubes 2 are juxtaposed). Left and right end portions
of the heat exchange tubes 2 are connected to the header tanks 4
and 5. The side plates 6 are disposed on the outer sides of the
uppermost and lowermost corrugated fins 3, and are brazed to the
corresponding corrugated fins 3. Air flows in a direction indicated
by an arrow W in FIG. 1.
[0018] The left header tank 4 is divided by a partition plate 7
into upper and lower header sections 4a and 4b, at a position
higher than the center of the left header tank 4 in the height
direction. The right header tank 5 is divided by another partition
plate 7 into upper and lower header sections 5a and 5b, at a
position lower than the center of the right header tank 5 in the
height direction. A refrigerant inlet (not shown) is formed at the
upper header section 4a of the left header tank 4, and an aluminum
inlet member 8 having an inflow passage 8a communicating with the
refrigerant inlet is brazed to the upper header section 4a. A
refrigerant outlet (not shown) is formed at the lower header
section 5b of the right header tank 5, and an aluminum outlet
member 9 having an outflow passage 9a communicating with the
refrigerant outlet is brazed to the lower header section 5b.
Refrigerant having flowed into the upper header section 4a of the
left header tank 4 through the inflow passage 8a of the inlet
member 8 flows rightward within the heat exchange tubes 2 located
above the partition plate 7 of the left header tank 4, and flows
into an upper portion of the upper header section 5a of the right
header tank 5. The refrigerant then flows downward within the upper
header section 5a, flows leftward within the heat exchange tubes 2
whose vertical positions are located between the partition plate 7
of the left header tank 4 and the partition plate 7 of the right
header tank 5, and flows into an upper portion of the lower header
section 4b of the left header tank 4. The refrigerant then flows
downward within the lower header section 4b, flows rightward within
the heat exchange tubes 2 located below the partition plate 7 of
the right header tank 5, and flows into the lower header section 5b
of the right header tank 5. The refrigerant then flows to the
outside of the condenser 1 through the outflow passage 9a of the
outlet member 9.
[0019] Each of the left and right header tanks 4 and 5 is formed
from an aluminum pipe having a brazing material layer on at least
an outer surface thereof; for example, a tubular member formed by
bending an aluminum brazing sheet having a brazing material layer
on each of opposite sides thereof into a tubular shape and brazing
side edges thereof which overlap each other. Each of the left and
right header tanks 4 and 5 is composed of a tank body 11 having a
plurality of tube insertion holes elongated in the air-flow
direction, and aluminum closing members 12 brazed to the opposite
ends of the tank body 11 so as to close the openings at the
opposite ends. A detailed illustration of the tank body 11 is
omitted. Also, the tank body 11 may be formed from a tubular
aluminum extrudate having a brazing material thermally sprayed to
an outer circumferential surface thereof.
[0020] Preferably, each heat exchange tube 2 is formed from an
extrudate formed of, for example, an Al alloy containing Cu in an
amount of 0.4 to 0.5 mass % and Mn in an amount of 0.1 to 0.3 mass
%, the balance being Al and unavoidable impurities. The Al alloy is
usually used for forming a heat exchange tube formed from an
extrudate.
[0021] As shown in FIG. 2, each heat exchange tube 2 has a wall 30
composed of a main body portion 31 and a covering layer 32. The
main body portion 31 is made of an Al alloy forming the aluminum
extrudate. The covering layer 32 is made of an Al--Si--Zn alloy and
covers the outer surface of the main body portion 31. A diffusion
layer 33 is formed in an outer surface layer portion of the main
body portion 31 of the wall 30 as a result of diffusion of Zn and
Si contained in the Al--Si--Zn alloy forming the covering layer
32.
[0022] Preferably, the wall 30 of each heat exchange tube 2 has a
thickness of 200 .mu.m or less. The thickness of the wall 30 of the
heat exchange tube 2 may not be uniform, but may differ locally.
The expression "the wall 30 has a thickness of 200 .mu.m or less"
means that a thickest portion of the wall 30 has a thickness of 200
.mu.m or less.
[0023] A low potential portion whose spontaneous potential is the
lowest, and a high potential portion whose spontaneous potential is
60 mV or more higher than that of the low potential portion, are
present within a range between an outermost surface 34 of the wall
30 of each heat exchange tube 2 and a deepest portion 35 of the
diffusion layer 33 such that the low potential portion is located
toward the outermost surface 34 of the wall 30. For example, within
the range between the outermost surface 34 of the wall 30 and the
deepest portion 35 of the diffusion layer 33, the spontaneous
potential of the wall 30 lowers gradually from the outermost
surface 34 of the wall 30 toward the main body portion 31 up to the
low potential portion, and the spontaneous potential of the wall 30
increases from the low potential portion toward the main body
portion 31 up to the high potential portion.
[0024] Cu contained in the alloy forming the aluminum
extrudate-made heat exchange tubes 2 has the effect of improving
the corrosion resistance of the main body portion 31 of each heat
exchange tube 2. However, at a Cu content of less than 0.4 massa,
the effect is not yielded. At a Cu content in excess of 0.5 massa,
the sacrificial corrosion effect of the diffusion layer 33 for the
main body portion 31 deteriorates. That is, the diffusion layer 33
in which Zn is diffused has the effect of lowering the spontaneous
potential of the diffusion layer 33 and is formed so as to serve as
a sacrificial corrosion layer for the main body portion 31.
However, at a Cu content in excess of 0.5 mass %, the effect of Zn
becomes insufficient, resulting in a failure to sufficiently lower
the spontaneous potential of the diffusion layer 33. Therefore,
preferably, the Cu content is 0.4 to 0.5 mass %. Also, Mn contained
in the alloy forming the aluminum extrudate-made heat exchange
tubes 2 has the effect of improving the strength of the heat
exchange tubes 2. However, at an Mn content of less than 0.1 mass
%, the effect is not yielded. At an Mn content in excess of 0.3
mass %, extrusion workability deteriorates. Therefore, preferably,
the Mn content is 0.1 to 0.3 mass %.
[0025] In some cases, the alloy forming the aluminum extrudate-made
heat exchange tubes 2 contains, as unavoidable impurities, Si in an
amount of 0.2 mass % or less, Fe in an amount of 0.2 mass % or
less, Mg in an amount of 0.05 mass % or less, Cr in an amount of
0.05 mass % or less, Zn in an amount of 0.05 mass % or less, and Ti
in an amount of 0.05 mass % or less. In some cases, the content of
these unavoidable impurities is zero. At excessively high Si and Fe
contents, the corrosion resistance of the heat exchange tube 2
deteriorates. At an excessively high Zn content, the spontaneous
potential of the heat exchange tube 2 lowers, resulting in a change
in potential balance in relation to peripheral components. At an
excessively high Ti content, the cost increases. Further, in some
cases, unavoidable impurities other than Si, Fe, Mg, Cr, Zn, and Ti
are contained such that individual contents are 0.05 mass % or less
(including zero mass %) and such that the total content is 0.15
mass % or less.
[0026] Preferably, each of the corrugated fins 3 is formed of, for
example, an Al alloy containing Mn in an amount of 1.0 to 1.5 mass
% and Zn in an amount of 1.2 to 1.8 mass %, the balance being Al
and unavoidable impurities. The Al alloy forming the corrugated
fins 3 is an ordinary alloy used as a bare material for forming
fins.
[0027] Mn contained in the alloy forming the corrugated fins 3 has
the effect of improving the strength of the corrugated fins 3.
However, at an Mn content of less than 1.0 mass %, the effect is
not yielded. At an Mn content in excess of 1.5 mass %, workability
deteriorates. Therefore, the Mn content is set to 1.0 to 1.5 mass
%.
[0028] Zn contained in the alloy forming the corrugated fins 3 has
the effect of appropriately maintaining potential balance with the
heat exchange tubes 2. However, at a Zn content of less than 1.2
mass %, the effect is not yielded. At a Zn content in excess of 1.8
massa, corrosion of the corrugated fins 3 becomes intensive.
Therefore, the Zn content is set to 1.2 to 1.8 mass %.
[0029] In some cases, the Al alloy forming the corrugated fins 3
contains, as unavoidable impurities, Si in an amount of 0.6 mass %
or less, Fe in an amount of 0.5 mass % or less, Cu in an amount of
0.05 mass % or less, and Cr in an amount of 0.12 mass % or less. In
some cases, the content of these unavoidable impurities is zero. At
excessively high Si, Fe, and Cu contents, the corrosion rate of the
corrugated fins 3 increases. Further, in some cases, unavoidable
impurities other than Si, Fe, Cu, and Cr are contained such that
individual contents are 0.05 mass % or less (including zero mass %)
and such that the total content is 0.15 mass % or less.
[0030] The condenser 1 is manufactured by a method described
below.
[0031] First, there are prepared the heat exchange tubes 2 formed
from an extrudate made of the Al alloy described above, the
corrugated fins 3 made of the Al alloy described above, the side
plates 6, the partition plates 7, a pair of tubular aluminum header
tank body intermediates having a brazing material layer on at least
the outer surfaces thereof, the closing members 12, the inlet
member 8, and the outlet member 9. The header tank body
intermediates have a plurality of tube insertion holes formed
therein.
[0032] A dispersing liquid is prepared by mixedly dispersing flux
powder, Zn powder, and Si powder in a binder. The Zn powder has an
average particle size of 3 to 5 .mu.m and a maximum particle size
of less than 10 .mu.m. The Si powder has an average particle size
of 2 to 6 .mu.m and a maximum particle size of less than 10 .mu.m.
The flux powder is of, for example, fluoride-based noncorrosive
flux containing a mixture of KAlF.sub.4 and KAlF.sub.5 as a main
component. The binder is, for example, a solution prepared by
dissolving acrylic resin in 3-methoxy-3-methyl-1-butanol. Notably,
in order to adjust the viscosity of the binder, a diluent of, for
example, 3-methoxy-3-methyl-1-butanol is added to the dispersing
liquid.
[0033] Next, the dispersing liquid is applied to the outer surface
of each heat exchange tube 2, and the liquid component of the
dispersing liquid is vaporized so as to cause the Zn powder, the Si
powder, and the flux powder to adhere to the outer surface of each
heat exchange tube such that the Zn powder adhesion amount becomes
4 to 6 g/m.sup.2, the Si powder adhesion amount becomes 3 to 6
g/m.sup.2, and the flux powder adhesion amount becomes 6 to 24
g/m.sup.2. A method of causing the Zn powder, the Si powder, and
the flux powder to adhere to the outer surface of each heat
exchange tube 2 is as follows: the dispersing liquid is applied to
the outer surface of each heat exchange tube 2 by a spraying
process, and subsequently, each heat exchange tube 2 is dried
through application of heat for vaporizing the liquid component of
the dispersing liquid, thereby causing the Zn powder, the Si
powder, and the flux powder to adhere to the outer surface of each
heat exchange tube 2; alternatively, the dispersing liquid is
applied to the preheated outer surface of each heat exchange tube 2
by a roll coating process, and subsequently, each heat exchange
tube 2 is dried through application of heat for vaporizing the
liquid component of the dispersing liquid, thereby causing the Zn
powder, the Si powder, and the flux powder to adhere to the outer
surface of each heat exchange tube 2.
[0034] As a result of adhesion of the Zn powder, the Si powder, and
the flux powder to the outer surface of each heat exchange tube 2,
a flux powder layer containing the Zn powder and the Si powder is
formed on the outer surface of the heat exchange tube 2. In the
flux powder layer, the Zn powder and the Si powder are uniformly
dispersed.
[0035] Next, the paired header tank body intermediates having the
tube insertion holes formed therein are disposed at a predetermined
interval; the closing members 12 are disposed at the opposite ends
of both header tank body intermediates; and the partition plates 7
are disposed in the respective header tank body intermediates. The
heat exchange tubes 2 and the fins 3 are alternately disposed, and
opposite end portions of the heat exchange tubes 2 are inserted
into the corresponding tube insertion holes of the header tank body
intermediates. The side plates 6 are disposed outward of the fins 3
at opposite ends, and the inlet member 8 and the outlet member 9
are disposed in place.
[0036] Next, header tank intermediates composed of the header tank
body intermediates, the closing members 12, and the partition
plates 7, the heat exchange tubes 2, the fins 3, the side plates 6,
the inlet member 8, and the outlet member 9 are temporarily fixed
together, thereby yielding a provisional assembly.
[0037] Next, the provisional assembly is placed in a brazing
furnace and is heated to a predetermined temperature within the
brazing furnace. Notably, flux is applied beforehand to components
other than the heat exchange tubes 2 as needed by a publicly known
method such as brushing.
[0038] In the course of increasing the temperature of the
provisional assembly, first, the flux powder of the flux powder
layer melts, thereby breaking oxide films on the outer surfaces of
the heat exchange tubes 2, oxide films on the outer surfaces of the
corrugated fins 3, oxide films on particle surfaces of the Si
powder, and oxide films on particle surfaces of the Zn powder.
Next, Si and Zn diffuse in the outer surface layer portions of the
heat exchange tubes 2 to thereby form a brazing material of
Al--Si--Zn alloy having a low melting point in the outer surface
layer portions of the heat exchange tubes 2. The brazing material
brazes the heat exchange tubes 2 and the corrugated fins 3. The
remainder of the brazing material not used for brazing becomes the
covering layers 32, and Zn and Si contained in the Al--Si--Zn alloy
of the covering layers 32 diffuse to thereby form the diffusion
layers 33. At the same time, molten flux on the outer surfaces of
the heat exchange tubes 2 flows and spreads. Also, molten Zn flows
and spreads; as a result, Zn diffuses in the outer surface layer
portions of the heat exchange tubes 2, thereby forming Zn diffused
layers. By this procedure, the condenser 1 is manufactured.
[0039] In each heat exchange tube 2 of the thus-manufactured
condenser 1, as described above, the wall 30 includes the main body
portion 31, the covering layer 32, and the diffusion layer 33
formed in the outer surface layer portion of the main body portion
31. A low potential portion whose spontaneous potential is the
lowest, and a high potential portion whose spontaneous potential is
60 mV or more higher than that of the low potential portion, are
present within the range between the outermost surface 34 of the
wall 30 and the deepest portion 35 of the diffusion layer 33 such
that the low potential portion is located toward the outermost
surface 34 of the wall 30.
[0040] On the basis of the results of a test which will be
described next, the following limitation is imposed on each heat
exchange tube 2: a low potential portion whose spontaneous
potential is the lowest, and a high potential portion whose
spontaneous potential is 60 mV or more higher than that of the low
potential portion, are present within the range between the
outermost surface 34 of the wall 30 and the deepest portion 35 of
the diffusion layer 33 such that the low potential portion is
located toward the outermost surface 34 of the wall 30.
[0041] There were prepared heat exchange tubes formed from an
extrudate made of an Al alloy containing Cu in an amount of 0.42
massa, Mn in an amount of 0.16 mass %, Si in an amount of 0.12 mass
%, Fe in an amount of 0.11 mass %, and Ti in an amount of 0.01 mass
%, the balance being Al and unavoidable impurities, and corrugated
fins formed from a bare material of an Al alloy containing Si in an
amount of 0.77 mass %, Fe in an amount of 0.24 mass %, Mn in an
amount of 1.68 mass %, Zn in an amount of 1.60 mass %, and Zr in an
amount of 0.11 mass %, the balance being Al and unavoidable
impurities. The Al alloy forming the heat exchange tubes contains
unavoidable impurities other than Si, Fe, and Ti such that
individual contents are 0.05 mass % or less and such that the total
content is 0.15 mass % or less. Each heat exchange tube has a wall
thickness of 180 .mu.m, and each corrugated fin has a thickness of
70 .mu.m.
[0042] Further, there were prepared fluoride-based noncorrosive
flux powder containing a mixture of KAlF.sub.4 and KAlF.sub.5
(KAlF.sub.5 content in the mixture: 10 to 40 mass %) in an amount
of 90 mass % or more, Zn powder (zinc oxide accounts for 5 mass %
of the total weight of the Zn powder) having an average particle
size of 3 to 5 .mu.m and a maximum particle size of less than 10
.mu.m, Si powder having an average particle size of 2 to 6 .mu.m
and a maximum particle size of less than 10 .mu.m, a binder in the
form of a solution prepared by dissolving acrylic resin in
3-methoxy-3-methyl-1-butanol, and a diluent of
3-methoxy-3-methyl-1-butanol. The Zn powder, the Si powder, and the
noncorrosive flux powder were mixedly dispersed in the binder and
the diluent, thereby yielding a dispersing liquid. The weight
ratios of all the components of the dispersing liquid are as
follows: Zn powder: 14.1 parts by weight; Si powder: 10.6 parts by
weight; noncorrosive flux powder: 21.1 parts by weight; binder: 9.2
parts by weight; and diluent: 45.0 parts by weight.
[0043] Next, after the dispersing liquid was applied by spraying to
the outer surfaces of the heat exchange tubes, the heat exchange
tubes were dried within a drying machine for vaporizing the liquid
component of the dispersing liquid so as to cause the Zn powder,
the Si powder, and the flux powder to adhere to the outer surfaces
of the heat exchange tubes such that the Zn powder adhesion amount
becomes 4 to 6 g/m.sup.2, the Si powder adhesion amount becomes 3
to 6 g/m.sup.2, and the flux powder adhesion amount becomes 24
g/m.sup.2 or less.
[0044] Subsequently, the heat exchange tubes and the corrugated
fins were alternately stacked and were heated in a nitrogen gas
atmosphere within a furnace for brazing the heat exchange tubes and
the corrugated fins. The heat exchange tubes and the corrugated
fins were heated for 6.3 minutes such that the heat exchange tubes
had a substantial temperature of 580.degree. C. or higher and a
maximum temperature of 600.7.degree. C.
[0045] One heat exchange tube was cut off from a brazed assembly of
the heat exchange tubes and the fins, and the spontaneous potential
was measured at different depths from the outermost surface 34 of
the wall 30. FIG. 3 shows the results of the measurement. The
thickness of the wall 30 was 180 .mu.m. As shown in FIG. 3, a low
potential portion whose spontaneous potential was the lowest within
the range between the outermost surface 34 of the wall 30 and the
deepest portion 35 of the diffusion layer 33 was located at the
position indicated by straight line A; i.e., at a depth of 7 .mu.m
from the outermost surface 34. The deepest portion 35 of the
diffusion layer 33 was located at a depth of 100 .mu.m from the
outermost surface 34 of the wall 30. It is found from the results
shown in FIG. 3 that a low potential portion whose spontaneous
potential is the lowest, and a high potential portion whose
spontaneous potential is 60 mV or more higher than that of the low
potential portion, are present within the range between the
outermost surface 34 of the wall 30 and the deepest portion 35 of
the diffusion layer 33 such that the low potential portion is
located toward the outermost surface 34 of the wall 30.
[0046] Further, after the CCT test was carried out on the brazed
assembly of the heat exchange tubes and the fins for 240 days, one
heat exchange tube was cut off, and the depth of corrosion of the
wall 30 of the heat exchange tube from the outermost surface 34 was
measured. The measured maximum corrosion depth was 53.0 .mu.m,
indicating that corrosion stopped at the high potential portion
present in the diffusion layer 33. Also, a remaining portion of the
wall 30 of the heat exchange tube after the CCT test has a
thickness of 100 .mu.m or more, indicating that the heat exchange
tube has sufficient corrosion resistance.
[0047] On the basis of the above-mentioned test results, the
following limitation was imposed on each heat exchange tube 2: a
low potential portion whose spontaneous potential is the lowest,
and a high potential portion whose spontaneous potential is 60 mV
or more higher than that of the low potential portion, are present
within the range between the outermost surface 34 of the wall 30
and the deepest portion 35 of the diffusion layer 33 such that the
low potential portion is located toward the outermost surface 34 of
the wall 30.
[0048] The present invention comprises the following modes.
[0049] 1) A heat exchanger comprising a plurality of heat exchange
tubes formed from an aluminum extrudate; and fins made of an
aluminum bare material, each disposed between adjacent heat
exchange tubes, and joined to the corresponding heat exchange tubes
by a brazing material, wherein each heat exchange tube has a wall
composed of a main body portion made of an Al alloy forming the
aluminum extrudate, and a covering layer made of an Al--Si--Zn
alloy and covering an outer surface of the main body portion; a
diffusion layer in which Zn and Si contained in the Al--Si--Zn
alloy forming the covering layer are diffused is formed in an outer
surface layer portion of the main body portion of the wall of each
heat exchange tube; and a low potential portion whose spontaneous
potential is the lowest, and a high potential portion whose
spontaneous potential is 60 mV or more higher than that of the low
potential portion, are present within a range between an outermost
surface of the wall of each heat exchange tube and a deepest
portion of the diffusion layer such that the low potential portion
is located toward the outermost surface of the wall.
[0050] 2) The heat exchanger described in par. 1), wherein within
the range between the outermost surface of the wall of each heat
exchange tube and the deepest portion of the diffusion layer, the
spontaneous potential of the wall lowers from the outermost surface
of the wall toward the main body portion up to the low potential
portion, and the spontaneous potential of the wall increases from
the low potential portion toward the main body portion up to the
high potential portion.
[0051] 3) The heat exchanger described in par. 1) or 2), wherein
the brazing material for joining the heat exchange tubes and the
corresponding fins is composed of Al contained in the Al alloy
forming the aluminum extrudate, and Si of Si powder caused, before
joining, to adhere to surfaces of the heat exchange tubes.
[0052] 4) The heat exchanger described in any of pars. 1) to 3),
wherein the Al--Si--Zn alloy serving as the covering layer of each
heat exchange tube is composed of Al contained in the Al alloy
forming the aluminum extrudate, Si of Si powder caused, before
joining, to adhere to the surface of the heat exchange tube, and Zn
of Zn powder caused, before joining, to adhere to the surface of
the heat exchange tube.
[0053] According to the heat exchangers of pars. 1) to 4), the fins
are formed of an aluminum bare material and thus exhibit an
improved corrosion resistance as compared with the case of a heat
exchanger having fins formed of an aluminum brazing sheet.
[0054] Also, each heat exchange tube has a wall composed of a main
body portion made of an Al alloy forming the aluminum extrudate,
and a covering layer made of an Al--Si--Zn alloy and covering an
outer surface of the main body portion; a diffusion layer in which
Zn and Si contained in the Al--Si--Zn alloy forming the covering
layer are diffused is formed in an outer surface layer portion of
the main body portion of each heat exchange tube; and a low
potential portion whose spontaneous potential is the lowest, and a
high potential portion whose spontaneous potential is 60 mV or more
higher than that of the low potential portion, are present within
the range between an outermost surface of the wall of each heat
exchange tube and a deepest portion of the diffusion layer such
that the low potential portion is located toward the outermost
surface of the wall. Therefore, corrosion from the outer surface of
the wall of each heat exchange tube stops at the high potential
portion. Accordingly, the corrosion depth can be made shallow,
whereby the corrosion resistance of the heat exchange tubes is
improved. As a result, the thickness of the wall of each heat
exchange tube can be decreased, whereby the weight of the heat
exchange tubes can be decreased, and thus, the weight of the heat
exchanger using the heat exchange tubes can be decreased.
* * * * *