U.S. patent application number 11/660918 was filed with the patent office on 2008-04-17 for multi -channeled flat tube and heat exchanger.
Invention is credited to Yasuhiro Kawatsu, Takahide Maezawa.
Application Number | 20080087408 11/660918 |
Document ID | / |
Family ID | 36000114 |
Filed Date | 2008-04-17 |
United States Patent
Application |
20080087408 |
Kind Code |
A1 |
Maezawa; Takahide ; et
al. |
April 17, 2008 |
Multi -Channeled Flat Tube And Heat Exchanger
Abstract
There is provided a multi-channeled flat tube that includes a
flat outer tube and a plurality of partitions that divide the
inside of the outer tube into a plurality of channels. Each
partition in the multi-channeled flat tube has a mountain-shaped
cross-sectional form composed of two sides, each of the two sides
has a thickness ti and a length a, the partitions are disposed so
that a face-to-face distance between adjacent partitions along
inner surfaces of the outer tube is Li, and a thickness to of the
outer tube satisfies a condition below. By selecting an appropriate
pressure when expanding the multi-channeled flat tube, it is
possible to prevent deformation of the tube outer surface into a
wavy or undulating pattern Li ti a 2 - ti 2 3 .times. a .ltoreq. to
. ##EQU1##
Inventors: |
Maezawa; Takahide; (Nagano,
JP) ; Kawatsu; Yasuhiro; (Nagano, JP) |
Correspondence
Address: |
BLANK ROME LLP
600 NEW HAMPSHIRE AVENUE, N.W.
WASHINGTON
DC
20037
US
|
Family ID: |
36000114 |
Appl. No.: |
11/660918 |
Filed: |
August 31, 2005 |
PCT Filed: |
August 31, 2005 |
PCT NO: |
PCT/JP05/15940 |
371 Date: |
February 23, 2007 |
Current U.S.
Class: |
165/151 ;
138/38 |
Current CPC
Class: |
F28D 1/05366 20130101;
F28F 1/022 20130101; F28F 1/32 20130101 |
Class at
Publication: |
165/151 ;
138/038 |
International
Class: |
F28D 1/04 20060101
F28D001/04; F28F 13/00 20060101 F28F013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2004 |
JP |
2004-252671 |
Claims
1. A multi-channeled flat tube comprising: a flat outer tube; and a
plurality of partitions that divide an inside of the outer tube
into a plurality of channels, wherein each of the plurality of
partitions has a mountain-shaped cross-sectional form composed of
two sides, each of the two sides has a thickness ti and a length a,
the plurality of partitions are disposed so that a face-to-face
distance between adjacent partitions of the plurality of partitions
along inner surfaces of the outer tube is Li, and a thickness to of
the outer tube satisfies a condition below Li ti a 2 - ti 2 3
.times. .times. a .ltoreq. to . Equation .times. .times. 1
##EQU13##
2. The multi-channeled flat tube according to claim 1, wherein the
thickness to of the outer tube also satisfies a condition below 2
.times. Li ti a 2 - ti 2 3 .times. .times. a .gtoreq. to . Equation
.times. .times. 2 ##EQU14##
3. The multi-channeled flat tube according to claim 1, wherein the
thickness to of the outer tube also satisfies a condition below Li
ti 2 .gtoreq. to . Equation .times. .times. 3 ##EQU15##
4. A heat exchanger comprising: a plurality of multi-channeled flat
tubes according to claim 1; and a plurality of fins that are
attached to the plurality of multi-channeled flat tubes and the
plurality of multi-channeled flat tubes passing through the
plurality of fins.
Description
TECHNICAL FIELD
[0001] The present invention relates to the construction of a
multi-channeled flat tube used in a heat exchanger.
BACKGROUND ART
[0002] A plate-fin heat exchanger equipped with a plurality of
plate fins disposed in parallel at regular intervals and a
plurality of tubes disposed so as to pass through such fins is
known as one example of a heat exchanger used in refrigeration
apparatuses, radiators, and the like. One method of manufacturing a
plate-fin heat exchanger includes passing the tubes through the
fins to assemble, and expanding or enlarging the tubes to join the
tubes and fins together. In the expanding the tubes, a rigid rod or
a tube expander is inserted into the tubes to press out and widen
the tubes from the inside. By expanding the tubes, the tubes and
fins are brought into contact.
[0003] The use of multi-channeled flat tubes (multi-channel flat
tubes) in heat exchangers is also known. A multi-channeled flat
tube includes a plurality of partitions provided inside of the tube
to divide the inside of the tube into a plurality of parallel
channels.
[0004] Japanese Laid-Open Patent Publication No. S62-19691
discloses a tube with an oval or rectangular cross-section. By
expanding the surface of the tube bridging between pairs of joints,
the tube surface is pressed onto cooling fins. When the tube (wall
of tube) expands outward, the joints deform from their original
"mountain" shapes (chevron shapes) to flat shapes, thereby
preventing the wall of the tube from contracting or shrinking to
their original positions.
DISCLOSURE OF THE INVENTION
[0005] One aspect of the present invention is a multi-channeled
flat tube including: a flat outer tube; and a plurality of
partitions that divide an inside of the outer tube into a plurality
of channels. Each of the plurality of partitions has a chevron
(mountain-shaped) cross-sectional form composed of two sides. Each
of the two sides has a thickness ti and a length a. The plurality
of partitions are disposed so that a face-to-face distance between
adjacent partitions of the plurality of partitions along inner
surfaces of the outer tube is Li. In addition, a thickness to of
the outer tube satisfies Condition (1) below Equation .times.
.times. 1 Li ti a 2 - ti 2 3 .times. a .ltoreq. to . ( 1 )
##EQU2##
[0006] One method of expanding tube is inserting tube expanders
into channels of a multi-channeled flat tube. Aside from this
method, research has been conducted into introducing a fluid into a
multi-channeled flat tube to raise the internal pressure and
thereby extend the partitions. Regardless of whether a method that
raises the internal pressure of the tube (hereinafter referred to
as "pressurized tube expansion") or expansion using tube expanders
is carried out, it is not preferable for the outer tubes located
between adjacent partitions (hereinafter referred to as "outer
walls") to expand outward so that the tube outer surface deforms
into a wavy or undulating form. This would reduce the contact
surface area between the tube outer surface and the fins and reduce
the heat transfer performance. However, if tube expansion is
stopped before the partitions have extended to reach the desired
size, the desired performance cannot be obtained.
[0007] When examining how partitions that are bent into "mountain"
shapes (chevron shapes, defined here as the shape of the Japanese
hiragana character "Ku" (equivalent to a V shape)) become extended,
the tensile stress that acts from both ends of each partition first
causes the partition to deform so that the angle between both sides
that construct the mountain shape (the V shape) opens up (i.e., the
angle increases). After this, once the angle between the two sides
has reached a certain magnitude, deformation of the partition that
further increases the angle (i.e., deformation that changes the
inclination of the partition, "angle deformation") largely ceases
and the inclination of the partition stops changing (i.e., the
partition becomes straightened out). Hereafter, the tensile stress
that acts from both ends of the partition causes deformation
(hereinafter, "stretch deformation") that reduces the thickness of
the partition. The amount of stress that changes the inclination of
a partition differs to the amount of stress that causes stretch
deformation of the partition and makes the partition thinner, with
less stress being required to change the inclination of the
partition.
[0008] With a multi-channeled flat tube with an outer tube whose
thickness satisfies Condition (1) given above, the outer walls at
parts of the outer tube between partitions will not deform in a
range of pressure that is high enough to change the inclination of
the partitions. Accordingly, a multi-channeled flat tube that
satisfies Condition (1) described above can prevent deformation in
the outer walls until the partitions have become straightened out.
This means that by using an internal pressure in a range where
there is deformation in the inclination of the partitions, the
multi-channeled flat tube can be expanded in a state where
deformation in the tube outer surface into a wavy or undulating
state is prevented.
[0009] The amount of deformation in the inclination of the
partitions varies due to the influence of tolerances for the
multi-channeled flat tube and fluctuations in the applied pressure.
This means that the tube should preferably be expanded with a
maximum pressure (internal pressure) in the range where deformation
occurs in the inclination of the partitions, or a higher pressure.
By expanding the tube with such pressure, it is possible to expand
a tube until a state where the inclined partitions become
substantially straightened out. Accordingly, during expansion, it
is possible to expand the multi-channeled flat tube to a desired
state without having to carry out control of the pressure varying
step-by-step. If the thickness to of the outer tube is small so
that the thickness to of the outer tube is outside the range of
Condition (1), there is the possibility of deformation occurring in
the outer tube before the partitions have become straightened out.
Accordingly, pressure control during tube expansion becomes
difficult.
[0010] Another aspect of this invention is a heat exchanger that
includes: a plurality of multi-channeled flat tubes that satisfy
Condition (1); and a plurality of fins that are attached to the
multi-channeled flat tubes. It is preferable that the plurality of
multi-channeled flat tubes pass through the plurality of fins. As
described earlier, with a multi-channeled flat tube that satisfies
Condition (1), regardless of whether pressurized tube expansion is
carried out using a fluid or whether the tube is expanded using
tube expanders, it will be possible to extend the partitions while
suppressing deformation in the outer walls. This means that it is
possible to raise the contact efficiency after tube expansion
between the plurality of multi-channeled flat tubes and the plate
fins through which the tubes pass and to which the tubes are
attached. Accordingly, it is possible to raise the heat transfer
efficiency of the heat exchanger. In particular, when using
pressurized tube expansion using a fluid, it is not necessary to
insert a tube expander, which makes this method suited to expanding
all of the narrow channels of the multi-channeled flat tubes
substantially uniformly.
[0011] One of other aspects of the present invention is a
multi-channeled flat tube where in addition to Condition (1) above,
the thickness to of the outer tube also satisfies Condition (2)
below Equation .times. .times. 2 2 .times. Li ti a 2 - ti 2 3
.times. a .gtoreq. to . ( 2 ) ##EQU3##
[0012] Here, in terms of making products smaller and lighter, it
would not be economic to increase the thickness of the outer tube
of a multi-channeled flat tube that includes a plurality of
partitions to the thickness of an outer tube required for a flat
tube not equipped with partitions. One merit of using
multi-channeled flat tubes is that since the flat tubes can be made
sufficiently strong by providing partitions inside the tube, it is
possible to reduce the thickness of the outer walls or outer
tube.
[0013] Further one of other aspects of the present invention is a
multi-channeled flat tube where in addition to Condition (1) above,
the thickness to of the outer tube also satisfies Condition (3)
below Equation .times. .times. 3 Li ti 2 .gtoreq. to . ( 3 )
##EQU4##
[0014] A pressure that is equal to or greater than the pressure
used during tube expansion is not a normal operating pressure of a
multi-channeled flat tube. If such pressure were applied during
normal operation, there would be the possibility of further
deformation in the multi-channeled flat tube since this is not how
multi-channeled flat tubes are designed. Accordingly, the pressure
used during tube expansion is an upper limit of the withstand
pressure conditions for normal operation or even higher. Also, the
pressure used during tube expansion is set up to at a pressure
whereby the partitions become straightened out and is not set at a
pressure whereby stretch deformation of partitions, which makes the
partitions thinner, commences. This means it is economical to set
the thickness of the outer tubes so that the outer walls deform at
a pressure where stretch deformation, which makes the partitions
thinner, commences. The heat transfer efficiency also increasing
when thin outer tubes are used.
[0015] In addition, with multi-channeled flat tubes that satisfy
Condition (3), if excessive internal pressure that would cause
stretch deformation of the partitions is applied during pressurized
tube expansion, there is the possibility of the outer walls also
expanding, resulting in the tube outer surfaces deforming.
Accordingly, the state of the tube outer surfaces can also serve as
one factor used when checking the pressure during tube
expansion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 schematically shows a heat exchanger.
[0017] FIG. 2 shows a state where multi-channeled flat tubes and
fins of the heat exchanger shown in FIG. 1 have been joined
together.
[0018] FIG. 3(a) shows a cross section in a length direction of the
multi-channeled flat tubes before tube expansion and FIG. 3(b)
shows a cross section of one of the multi-channeled flat tubes in a
minor axis direction.
[0019] FIG. 4(a) shows a cross section in the length direction of
the multi-channeled flat tubes after tube expansion, FIG. 4(b)
shows a cross section in the minor axis direction of an end part of
one of the multi-channeled flat tubes, and FIG. 4(c) shows a cross
section in the minor axis direction of another part of such
multi-channeled flat tube.
[0020] FIG. 5(a) shows "angle deformation" of partitions and FIG.
5(b) shows "stretch deformation" of the partitions.
[0021] FIG. 6 shows an enlargement of a cross section of a
multi-channeled flat tube.
[0022] FIG. 7 shows changes in the inner diameter of a
multi-channeled flat tube when the internal pressure has been
raised.
[0023] FIG. 8 shows how fluctuations occur in the amount by which
the multi-channeled flat tubes expand due to tolerances and the
like.
[0024] FIG. 9 shows examples of an upper limit and a lower limit
for the thickness of the outer tube of a multi-channeled flat
tube.
BEST MODE FOR CARRYING OUT THE INVENTION
[0025] FIG. 1 schematically shows a heat exchanger that uses
multi-channeled flat tubes. FIG. 2 is a perspective view showing an
enlargement of a state where the multi-channeled flat tubes have
been expanded. The heat exchanger 1 is a plate fin-type heat
exchanger. The heat exchanger 1 has a plurality of plate-like fins
2 disposed in parallel at regular intervals and a plurality of
multi-channeled flat tubes 3 that are disposed in parallel and are
joined to the fins 2 in a state where the multi-channeled flat
tubes 3 pass through the fins 2. Each multi-channeled flat tube
(flat multi-channeled tube, multi-channel flat tube) 3 is
constructed so that the inside of a flat outer tube 21 is divided
into a plurality of parallel channels 14 by a plurality of
partitions 15. End parts 4 at both ends of the multi-channeled flat
tubes 3 are connected to joining holes 19 formed in side walls 9 of
headers 6 and 7 positioned on the left and right sides of the heat
exchanger 1. A heat transfer medium (internal fluid) introduced
from a supply outlet 11 of the header 6 passes through the channels
14 of the multi-channeled flat tubes 3 and is guided or led to an
output outlet 12 of the header 7. When an external fluid B such as
air passes over the heat exchanger 1, the external fluid B contacts
the multi-channeled flat tubes 3 and the fins 2, heat exchange
occurs between the heat transfer medium and the external fluid so
that the heat transfer medium and/or the external fluid is cooled
or heated.
[0026] FIG. 3(a) shows the multi-channeled flat tubes 3 before the
tubes have been expanded. When manufacturing the heat exchanger 1,
the multi-channeled flat tubes 3 are inserted into burring holes 18
provided in advance in the fins 2 to provisionally assemble the
fins 2 and the multi-channeled flat tubes 3 together.
[0027] FIG. 3(b) shows an enlargement of the cross-section of a
multi-channeled flat tube 3. The outer tube 21 of the
multi-channeled flat tube 3 includes outer walls 21w that oppose
each other at the top and bottom of the multi-channeled flat tube
3. The multi-channeled flat tube 3 is a flat tube molded so that
the outer wall 21w that forms the upper wall or top wall of the
outer tube 21 and the outer wall 21w that forms the lower wall or
bottom wall are substantially parallel. A plurality of partitions
15 that connect the upper and lower outer walls 21w and
respectively have cross-section bent like "mountain"-shaped
(V-shaped) to the length direction X of the cross-section of the
multi-channeled flat tube 3 are provided inside each
multi-channeled flat tube 3. These partitions 15 divide the inside
of the flat tube 3 to form the plurality of parallel channels
14.
[0028] The end parts 4 of the multi-channeled flat tubes 3 that
have been assembled so as to pass through the fins 2 are inserted
into the joining holes 19 provided in the headers 6 and 7. These
end parts 4 are joined to the headers 6 and 7 by brazing or another
suitable method. By doing so, the parallel flow channels 14 of the
individual flat tubes 3 are connected via the headers 6 and 7 and
form internal paths through which the heat transfer medium
flows.
[0029] FIGS. 4(a), 4(b), and 4(c) are cross-sectional views showing
the state of the multi-channeled flat tubes 3 after pressurized
tube expansion. FIGS. 5(a) and 5(b) show how the partitions 15 of
the multi-channeled flat tubes 3 are straightened. A compressed
fluid can be supplied via the headers 6 and 7 to the
multi-channeled flat tubes 3. The compressed fluid raises the
internal pressure of the parallel channels 14 and can expand
multi-channeled flat tubes 3 (pressurized tube expansion). By
expanding the tubes 3, as shown in FIG. 4(a), the multi-channeled
flat tubes 3 and the respective fins 2 are tightly joined. With the
heat exchanger 1, the headers 6 and 7 are joined to the end parts 4
of the multi-channeled flat tubes 3 in advance. This means that as
shown in FIG. 4(b), the bent partitions 15 are not straightened at
the end parts 4 of the tubes 3. At other parts, including the parts
where the multi-channeled flat tubes 3 pass through the fins 2, the
bent partitions 15 are straightened as shown in FIG. 4(c).
[0030] Each of the partitions 15 of the multi-channeled flat tubes
3 that is used in the heat exchanger 1 has a thickness ti and each
side out of the two sides forming the mountain shape has a length
a. The gap (face-to-face distance (distance between surfaces))
between the partitions (adjacent partitions) 15 along the inner
surfaces of the outer walls 21w of the flat outer tube 21 is the
distance Li. The thickness to of the outer tube 21 satisfies
Equation (1) below. Equation .times. .times. 1 Li ti a 2 - ti 2 3
.times. a .ltoreq. to . ( 1 ) ##EQU5##
[0031] The expansion of the multi-channeled flat tube 3 with
partitions 15 which in cross-section are bent into mountain shapes
(V-shapes) with two sides 27 of the length a and meeting with an
angle 2.theta. was investigated. As shown in FIG. 5(a), when the
internal pressure is raised, the force that acts on the walls 21w
of the outer tube 21 pulls the partitions 15 that are bent in
mountain shapes outward, thereby causing the partitions 15 to
deform so that the inclination angle (half angle) .theta. of each
side 27 approaches 90.degree.. This deformation changes the
inclination of the partitions and is referred to as "angle
deformation". The angle deformation is assumed to be deformation
that ends or is ceased where a partition 15 becomes straightened
state. The straighten or straitened state of this specification is
the state that a corner (base) 26 where the partition 15 is
connected to the wall 21w of the outer tube 21 and the peak point
25 where the two sides 27 that form the mountain shape meet become
offset substantially by only the thickness ti of the partition 15.
This state is shown in FIG. 6.
[0032] As shown in FIG. 5(b), deformation that occurs thereafter is
assumed to be deformation (referred to as "stretch deformation")
caused by stretching that reduces the thickness of the partitions
15. Accordingly, depending on the applied internal pressure, the
mechanism by which the partitions 15 deform changes. This means
that by applying pressure in a range from a pressure where angle
deformation ends to a pressure where stretch deformation commences,
it is possible to stably cause the partitions 15 to deform until
the partitions 15 become substantially straight.
[0033] FIG. 7 shows the result of measuring the relationship
between the inner height of the tube (the inner diameter or inner
dimension in the minor axis Y) Hi and the internal pressure (the
applied pressure). The solid line A1 shown in FIG. 7 shows measured
values for the case where the plate thickness ti of the partitions
15 is set at 0.19 mm and the dot-dash line A2 shows values produced
by calculating the tangent (tan) from the measured values of the
angle .theta. by which the partitions 15 are bent. As can be
understood from the solid line A1, when the thickness ti of the
partitions 15 is 0.19 mm, the tube height Hi suddenly increases
from a point where the internal pressure exceeds 2 MPa or
thereabouts, showing that angle deformation is occurring for the
partitions 15. When the internal pressure is around 7.2 MPa, the
increasing of amount of deformation in the partitions 15 starts to
fall and when the internal pressure is around 7.5 MPa, the
partitions 15 substantially stop deforming. By applying a pressure
of over 7.5 MPa or so, the partitions 15 become the straightened
state, and even if the pressure is increased further, deformation
of the partitions 15 does not proceed. It is therefore supposed
that angle deformation of the partitions 15 has ended.
[0034] By comparing the calculated values shown by the dot-dash
line A2 in FIG. 7 and the measured values shown by the solid line
A1, it can be seen that angle deformation of the tubes has
substantially ended at the point "tan .theta.=Hi/(2 ti)". As
described earlier, when the peak points 25 of the partitions 15 are
positioned so as to be displaced from the base parts 26 by only the
thickness ti of the partitions 15, it is assumed that the
partitions 15 have been the straightened state and angle
deformation has ended. When judging from the measured values A1,
angle deformation ends at an internal pressure of around 7 to 8
MPa. Accordingly, if the internal pressure used to expand the tubes
is set at around 7 to 8 MPa or higher, it will be possible the
partitions 15 become the straightened state.
[0035] As shown in FIG. 8, there are tolerances for dimensions such
as the thickness ti of the partitions 15. Due to such tolerances,
the relationship between the amount of deformation (i.e., the
height of the tube) Hi and the internal pressure P changes. If the
target value of the height Hi during tube expansion is set at a
position like H2 in FIG. 8 where angle deformation has not ended,
it will be necessary to control the internal pressure P in the
individual tubes so that the height H1 becomes the target value H2.
This kind of tube expansion operation is not economical to carry
out and since there is a fall in the accuracy of the dimensions
after expansion, the manufacturing yield of a heat exchanger that
uses multi-channeled flat tubes also falls. On the other hand, if
the target value of the height Hi during tube expansion is set at a
position like H3 in FIG. 8 where angle deformation has ended, it
will be possible to set the internal pressure during tube expansion
at a value such as P3 in FIG. 8, which can be the pressure at which
angle deformation ends or an even higher pressure. This means that
regardless of any differences between individual tubes, it will be
possible to expand the tubes to the same height Hi by expanding the
tubes with the same pressure. Since the accuracy of the dimensions
of the tubes 3 after expansion is stabilized, the yield and quality
of the heat exchanger 1 that uses the multi-channeled flat tubes 3
are improved.
[0036] In this way, by expanding the multi-channeled flat tubes 3
until the partitions 15 are the straightened state, that is, until
the target value H3 shown in FIG. 8 is reached, it is possible to
expand the multi-channeled flat tubes 3 stably. The pressure
required to expand the tubes in this way is the pressure P3 shown
in FIG. 8, and the thickness to of the outer walls 21w of the outer
tubes 21 is determined so that hardly any deformation is observed
in the tube outer surface at such pressure P3.
[0037] The force (internal pressure) required for angle deformation
of the partitions 15 can be calculated from the force applied when
the partitions 15 become the straightened state as shown in FIG. 6.
Having no intentional deformation of at least the outer walls 21w
of the outer tubes 21 when such internal pressure is applied is one
condition for the present invention. In FIG. 6, when the length of
one side of the partitions 15 is expressed as length a, the
inclination (bent angle) as angle .theta., the face-to-face
distance between adjacent partitions 15 along the inner surfaces of
the outer walls 21w of the flat outer tubes 21 (i.e., the distance
between surfaces of facing partitions 15) as Li, and the height of
the multi-channeled flat tubes 3 (i.e., the inner diameter or inner
dimension in the minor axis direction of the flat outer tubes 21)
as Hi, the stress co produced in the outer walls 21w when the
internal pressure P acts on the outer walls 21w is as follows.
First, since the outer walls 21w can be regarded as fixed beams
(beams whose both ends are fixed respectively) subjected to a
uniformly distributed load of the pressure P along the
inter-partition distance Li, the maximum bending moment Mmax and
the section modulus Z are as shown by Equations (4) and (5).
Accordingly, the maximum stress co applied to the outer walls 21w
is shown by Equation (6). Equation .times. .times. 4 M .times.
.times. max = P Li 2 12 ( 4 ) Equation .times. .times. 5 Z = to 2 6
( 5 ) Equation .times. .times. 6 .sigma. .times. .times. o = M
.times. .times. max Z = P Li 2 12 .times. 6 to 2 = P Li 2 2 .times.
to 2 ( 6 ) ##EQU6##
[0038] A partition 15 is pulled upward and downward by the forces
that act at the bases 26. The bases 26 are subjected to a force
(load) W (where W=applied pressure P.times.pressure-receiving
surface length Li) due to the internal pressure P. The partition 15
is thought to deform due to the bending moment acting on the bases
26 and the peak point 25 where the center of the partition and the
two sides 27 are joined. The partition 15 can be modeled as a fixed
beam (beam whose both ends are fixed) of an effective length 2a and
a thickness ti that receives a concentrated load W' at the center
25 thereof. If the force W is constant, the concentrated load W'
that acts on the partition 15 will be minimized when tan .theta. is
maximized, and therefore calculations can proceed for the
conditions where angle deformation occurs for a minimum
concentrated load W'. The condition that the tan .theta. is
maximized indicates that angle deformation has ended for the
partition 15 and the partition 15 has been the straightened state,
which means that the inclination .theta. can be expressed by
Equation (7). Accordingly, the concentrated load W' can be found as
shown in Equation (8). Equation .times. .times. 7 tan .times.
.times. .theta. = Hi / 2 ti = Hi 2 .times. ti ( 7 ) Equation
.times. .times. 8 W ' = W tan .times. .times. .theta. = P Li Hi 2
.times. ti = 2 .times. ti P Li Hi ( 8 ) ##EQU7##
[0039] The maximum bending moment Mmax during angle deformation of
the partitions 15 is as shown in Equation (9). By using the section
modulus Z in Equation (10), the maximum stress .sigma.i occurring
in the partitions 15 is as shown in Equation (11). Note that since
modeling is carried out based on the cross-sectional form of a
multi-channeled flat tube, stress is calculated in one dimension.
Equation .times. .times. 9 M .times. .times. max = W ' 2 .times. a
8 = 2 .times. ti P Li 2 .times. ( Hi / 2 ) 2 + ti 2 8 .times. Hi =
ti P Li .times. ( Hi / 2 ) 2 + ti 2 2 .times. Hi ( 9 ) Equation
.times. .times. 10 Z = ti 2 6 ( 10 ) Equation .times. .times. 11
.sigma. .times. .times. i = M .times. .times. max Z = ti P Li
.times. ( Hi / 2 ) 2 + ti 2 2 .times. Hi .times. 6 ti 2 = 3 .times.
P Li .times. ( Hi / 2 ) 2 + ti 2 Hi ti ( 11 ) ##EQU8##
[0040] To expand the tubes according to the conditions described
above, the outer walls 21w should not deform in a range of pressure
where angle deformation occurs for the partitions 15. Accordingly,
when the minimum pressure Pmin for expanding the tubes is applied
to the multi-channeled flat tubes 3, the threshold stress
.sigma.lim of the material that constructs the outer walls 21w and
the partitions 15, for example, a metal material including aluminum
and copper, the maximum stress .sigma.i when angle deformation
occurs for the partitions 15, and the maximum stress .sigma.o
applied to the outer walls 21w should satisfy Equation (12)
below.
Equation 12 .sigma.o.ltoreq..sigma.lim.ltoreq..sigma.i (12)
[0041] From Equation (12), it is possible to reach Condition (1)
above. Note that the length a of one side 27 of a partition 15 is
expressed as shown in Equation (13) below.
Equation 13 a= {square root over ((Hi/2).sup.2+ti.sup.2)} (13)
[0042] One merit of multi-channeled flat tubes is that the strength
of the flat tubes is increased by the partitions disposed inside
the tubes, and therefore the outer walls 21w, or in other words,
the outer tubes 21 can be made thinner. Here, suppose the case of a
double-channeled flat tube with two channels 14 that is the minimum
configuration of a multi-channeled flat tube. When the internal
pressure P is applied, the maximum stress co produced in the outer
walls 21w is as shown by Equation (6) given earlier. For a flat
tube with the same internal cross-sectional area as such
double-channeled flat tube, that is, a single flat tube with the
distance 2Li between walls and a tube inner dimension (i.e.,
height) of Hi, to achieve the same strength as the double-channeled
flat tube, it would be necessary to double the thickness to of the
outer walls 21w. This means that if the thickness of the outer
walls 21w is more than double the minimum value calculated using
Equation (1), one of the merits of multi-channeled flat tubes will
be lost. Accordingly, the thickness to of the outer walls 21w
should preferably satisfy Condition (2). Equation .times. .times. 2
2 .times. Li ti a 2 - ti 2 3 .times. a .gtoreq. to ( 2 )
##EQU9##
[0043] In FIG. 8, if the pressure P is increased further, stretch
deformation occurs for the partitions 15, so that the partitions 15
continue to extend while becoming thinner. In this case, it is
difficult to know when the partitions 15 will rupture or break. For
this reason, it is not preferable to expand the multi-channeled
flat tubes 3 using a pressure P that causes stretch deformation to
commence. Also, regarding the pressure used during expanding, it is
not necessary for the outer walls 21w of the outer tubes 21 to
withstand a pressure P thereby the stretch deformation commences
without deforming, since such pressure is at or above the withstand
pressure of the multi-channeled flat tubes for service. In
addition, in view of cost and heat exchanging efficiency, the
thickness to of the flat outer tubes 21 should preferably be thin.
Accordingly, the thickness to of the outer tubes 21 can be such
that the flat outer tubes 21 deform at a pressure that causes
stretch deformation of the partitions 15. In addition, if the outer
walls 21w deform when a pressure P that causes stretch deformation
of the partitions 15 is applied, it will be clear that excessive
pressure is being applied from the external appearance of the
multi-channeled flat tubes 3, and therefore such appearance can be
used as one judgment made to confirm the quality of the
multi-channeled flat tubes 3 and of the heat exchanger 1 that uses
such multi-channeled flat tubes 3.
[0044] When stretch deformation occurs for the partitions 15, the
tensile stress as that acts on the partitions 15 is as shown in
Equation (14). Equation .times. .times. 14 .sigma. .times. .times.
s = P Li ti ( 14 ) ##EQU10##
[0045] It is thought that the outer walls 21w should deform before
stretch deformation occurs for the partitions 15. Accordingly, when
the maximum pressure Pmax for expanding the tubes is applied to the
multi-channeled flat tubes 3, the threshold stress .sigma.lim of
the material of the multi-channeled flat tubes 3, the stress as
when stretch deformation occurs for the partitions 15, and the
maximum stress so applied to the outer walls 21w should satisfy
Equation (15) below.
Equation 15 .sigma.s.ltoreq..sigma.lim.ltoreq..sigma.o (15)
[0046] From Equation (15), it is possible to reach Condition (3),
and the thickness to of the outer walls 21w should more preferably
satisfy this condition. Li ti 2 .gtoreq. to Equation .times.
.times. 3 ##EQU11##
[0047] FIG. 9 shows the thickness to of the outer walls 21w of the
multi-channeled flat tubes 3 relative to the inter-partition
distance Li and the partition wall thickness ti when the target
value of the inner dimension Hi during tube expansion is 1.5 mm.
The plane Cu shown in FIG. 9 shows the upper limit of the thickness
to according to Condition (3), and the plane Cl shows the lower
limit of the thickness to according to Condition (1). If the
multi-channeled flat tubes 3 have outer tubes 21 with a thickness
to in this range, it is possible to set a suitable pressure P3, as
shown in FIG. 8, for expanding the tubes and therefore the tubes
can be expanded with a favorable yield (lower defect rate). In
addition, it is possible to prevent deformation of the outer walls
21w during expansion of the tubes, and therefore it is possible to
achieve a high yield when manufacturing the heat exchanger 1 where
the multi-channeled flat tubes 3 have a stabilized outer form and
achieve a sufficient contact surface area with the fins 2,
resulting in high heat exchanging efficiency. A specific example of
the pressure P3 which is suited to expanding the tubes until the
inner dimension Hi reaches the desired target value within a range
where angle deformation occurs for the partitions 15 without
stretch deformation occurring for the partitions 15 can be set
based on the threshold stress .sigma.lim of the material that
constructs the multi-channeled flat tubes 3. By referring to
Equations (12) and (15), it can be understood that the pressure P3
for expanding the tubes should preferably be set in the range given
below. 2 a 2 - ti 2 3 .times. .times. Li a .times. .sigma. .times.
.times. lim .ltoreq. P .times. .times. 3 .ltoreq. ti Li .times.
.sigma. .times. .times. lim Equation .times. .times. 16
##EQU12##
[0048] Note that as described above, although a procedure for
manufacturing the heat exchanger 1 that has plate-like fins 2 has
been described, the form of the fins is not limited to a plate-like
form and wave-like corrugated fins may be used. In a heat exchanger
that uses corrugated fins, only the parts of the multi-channeled
flat tubes that are attached to the headers need to be expanded and
it is fundamentally unnecessary to expand the parts of the
multi-channeled flat tubes that are joined to the fins. However, it
is possible to expand the multi-channeled flat tubes after the fins
have been joined to increase the contact surface area. When the
insides of the multi-channeled flat tubes are partitioned into
narrow channels by a plurality of partitions, since the individual
channels have small cross-sectional areas, a method that extends
the partitions by increasing the internal pressure by introducing a
fluid into the multi-channeled flat tubes is more suitable than a
method that expands the tubes by inserting tube expanders. However,
by using multi-channeled flat tubes that satisfy Condition (1)
given earlier, regardless of whether a method that raises the
internal pressure (sometimes referred to as "pressurized tube
expansion") or an expansion method that uses tube expanders is
used, it will be possible to prevent the outer walls of the outer
tubes, from deforming due to the internal pressure caused by the
fluid or tube expanders, which would result in the outer surfaces
of the outer tubes becoming wavy or undulating, before the
partitions have extended to reach a desired size. That is, by using
pressurized tube expansion or the like, when the multi-channeled
flat tubes are expanded by internal pressure, it is possible to
extend the partitions while suppressing deformation of the tube
outer walls, and therefore the outer form of the tubes can be
controlled until the multi-channeled flat tubes reach the desired
size. By preventing the tube outer walls from deforming into an
unintended shape, it is possible to achieve a sufficient contact
surface area between the tube outer surfaces and the fins, thereby
improving the heat transfer performance. Accordingly, it is
possible to provide a heat exchanger that has high heat exchanging
efficiency and high reliability.
* * * * *