U.S. patent application number 09/792902 was filed with the patent office on 2001-10-11 for internal grooved tube and manufacturing method thereof.
Invention is credited to Hashizume, Toshiaki, Mori, Yasutoshi, Sumitomo, Tetsuya, Yamamoto, Koji.
Application Number | 20010027859 09/792902 |
Document ID | / |
Family ID | 18571923 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010027859 |
Kind Code |
A1 |
Yamamoto, Koji ; et
al. |
October 11, 2001 |
Internal grooved tube and manufacturing method thereof
Abstract
An internal grooved tube according to the present invention
comprises a large number of fine spiral grooves on the inside
surface in parallel arrangement, wherein the grooves are formed to
assure that the ratio of a groove width in the tube axial direction
to a groove height is in the range of 1 to 2. A lead angle .theta.
of the grooves to the tube axis is preferably limited to 26 to 35
degrees. A method of manufacturing an internal grooved tube
according to the present invention comprises the steps of inserting
a grooved plug into a blank tube rotatably, and then pressing the
blank tube against the outside surface of the grooved tube with
several balls revolving both around the circumference of the blank
tube and on its axis in a location of the grooved plug inserted,
while drawing out the blank tube longitudinally in one direction,
wherein the number of balls is limited to 2 to 3.
Inventors: |
Yamamoto, Koji; (Tokyo,
JP) ; Sumitomo, Tetsuya; (Tokyo, JP) ; Mori,
Yasutoshi; (Tokyo, JP) ; Hashizume, Toshiaki;
(Tokyo, JP) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
92660
US
|
Family ID: |
18571923 |
Appl. No.: |
09/792902 |
Filed: |
February 26, 2001 |
Current U.S.
Class: |
165/133 ;
165/181; 165/183 |
Current CPC
Class: |
Y10T 29/49382 20150115;
F28F 1/40 20130101; B21C 37/207 20130101; Y10T 29/49373 20150115;
Y10T 29/49384 20150115 |
Class at
Publication: |
165/133 ;
165/181; 165/183 |
International
Class: |
F28F 013/18; F28F
019/02; F28F 001/20; F28F 001/14; F28F 001/36 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2000 |
JP |
2000-050090 |
Claims
What is claimed is:
1. An internal grooved tube, comprising: a large number of fine
spiral grooves formed on an inside surface in parallel arrangement;
wherein said grooves are formed to assure that the ratio of a
groove width W in the tube axial direction to a groove height H is
in the range of 1 to 2.
2. An internal grooved tube according to claim 1, wherein a lead
angle .theta. of said grooves to the tube axis is limited to 26 to
35 degrees.
3. A method of manufacturing an internal grooved tube comprising
the steps of: inserting a grooved plug having a large number of
fine spiral grooves on the outside surface into a blank tube
rotatably; and pressing the peripheral wall of the blank tube
against the outside surface of the grooved plug with several balls
revolving both around the circumference of the blank tube and on
its axis in a location of the grooved plug inserted, while drawing
out the blank tube longitudinally in one direction; wherein the
number of balls is limited to 2 to 3.
4. A method of manufacturing an internal grooved tube according to
claim 3, wherein a lead angle .theta. of said grooves of the
grooved plug to the axis is limited to 26 to 45 degrees.
5. A method of manufacturing an internal grooved tube according to
claim 3, wherein the direction of revolution of the balls is
allowed to match the direction of rotation of the grooved plug.
6. A method of manufacturing an internal grooved tube according to
claim 4, wherein the direction of revolution of the balls is
allowed to match the direction of rotation of the grooved plug.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an internal grooved tube used as a
heat exchanger tube for a heat exchanger of a refrigerator and an
air conditioner or the like and a method of manufacturing such an
internal grooved tube, and more specifically, to an internal
grooved tube having a large number of fine spiral grooves (or fins)
formed on the inside surface in parallel arrangement at a certain
pitch and a method of manufacturing such an internal grooved
tube.
[0003] 2. Description of the Prior Art
[0004] The promotion of miniaturization, higher performance and
energy conservation has been made as to a heat exchanger. In this
connection, as an internal grooved tube to meet such demands, in
Japanese Patent Laid-open No. 8-21696, for instance, there is
proposed a heat exchanger tube having spiral grooves of a great
height on the inside surface and fins of a sharp vertical
angle.
[0005] As a method of manufacturing an internal grooved tube, in
Japanese Patent Laid-open No. 54-37059, there is disclosed a method
of manufacturing a heat exchanger tube by the steps of inserting a
grooved plug having a large number of fine spiral grooves on the
outside surface into a blank tube rotatably, then pressing the
blank tube against the outside surface of the grooved plug with a
plurality of rolls arranged to revolve both around the
circumference of the blank tube and on its axis in a location of
the grooved plug inserted, while drawing out the blank tube in one
direction, and then using a holder to hold the roll axis for
stabilizing the rotation of the rolls.
[0006] As a method for high-speed machining of an internal grooved
tube, in Japanese Patent Laid-open No. 55-103215, there is
disclosed a method of manufacturing a heat exchanger tube by the
steps of inserting the grooved plug as described the above into a
blank tube rotatably, and then pressing the blank tube against the
outside surface of the grooved plug with balls densely arranged to
revolve both around the circumference of the blank tube and on its
axis in a location of the grooved plug inserted, while drawing out
the blank tube in one direction.
[0007] The internal grooved tube disclosed in Japanese Patent
Laid-open No. 8-21696 meets the requirements of spiral grooves of a
great height and fins of a sharp vertical angle, permitting the
achievement of the intended objects. However, with greater groove
height (fin height), it is necessary to increase a thickness of a
tube in proportion to the groove height, resulting in an increase
in tube weight. Besides, large crushes of fins formed in the tube
occur in tube expansion (by press-fitting a rod provided with a net
ball at the tip for tube expansion to fix the tube to aluminum
fins) for incorporating the tube into the heat exchanger, and as a
result, the grooves formed to be of a great height could not often
take satisfactory effect.
[0008] Among the internal grooved tube manufacturing methods, the
method of permitting the planetary revolution of a plurality of
rolls having axes held by the holder around the circumference of
the blank tube in a location of the grooved plug inserted as
disclosed in Japanese Patent Laid-open No. 54-37059 described the
above requires a lubricating mechanism between the roll and the
roll axis, in addition to the holder, for revolution of the rolls
at high speed to increase a machining speed, resulting in an
increase in roll diameter and also a complication of structure. For
that reasons, an increase in number of revolutions of the rolls
hinders the stability of the revolution of the roll and its
rotation axis, and therefore, it is not possible to hold a stable
orbit of revolution, resulting in a difficulty in increasing a
grooving (rolling) speed.
[0009] In order to solve the above problems, the technique of
arranging the balls densely, instead of the rolls, around the
grooved plug location of the blank tube to be drawn out is
developed, as disclosed in Japanese Patent Laid-open No. 55-103215
described the above. When the balls are in use in this manner, the
balls and the blank tube make point-contact each other, permitting
stable and higher-speed machining. Then, with an increase in number
of balls, the balls might normally revolve around the circumference
of the blank tube in a shorter period in the state of being pressed
against the circumference of the blank tube to form the grooves on
the inside surface of the tube by rolling, permitting more improved
grooving workability, together with higher machining speed.
[0010] However, when the grooves of the grooved plug have a large
lead angle to the axis, breakage (tear-off) of the blank tube
occurs in process of machining to hinder higher-speed machining in
spite of adding more balls. Thus, there has been a limit to
manufacture of a high-performance heat exchanger tube having a
large lead angle to the tube axis.
SUMMARY OF THE INVENTION
[0011] After having made various trials and errors, the present
inventors found out the fact that the heat transfer performance of
an internal grooved tube is at its highest when a width of each
internal groove in the tube axial direction (the longitudinal
direction) in the heat exchanger tube has a fixed relation to a
groove height, resulting in the proposal of the present invention.
It is an object of the present invention to provide an internal
grooved tube, which permits the realization of higher performance,
lightweight and miniaturization, without the need for greater
internal groove height (greater fin height).
[0012] Another object of the present invention is to provide an
internal grooved tube manufacturing method, which makes it possible
to machine a heat exchanger tube satisfying the above object
smoothly at high speed without causing breakage.
[0013] To attain the above objects, according to the present
invention, there is provided an internal grooved tube, which
comprises a large number of fine spiral grooves formed on the
inside surface in parallel arrangement, wherein the ratio of a
groove width W of each groove in the tube axial direction to a
groove height H is in the range of 1 to 2. A lead angle .theta. of
the above grooves to the tube axis is preferably limited to 26 to
35 degrees.
[0014] To attain the above objects, according to the present
invention, there is provided an internal grooved tube manufacturing
method, which comprises the steps of inserting a grooved plug
having a large number of fine spiral grooves on the outside surface
into a blank tube rotatably, and pressing the peripheral wall of
the blank tube against the outside surface of the grooved plug with
several balls revolving both around the circumference of the blank
tube and on its axis in a location of the grooved plug inserted,
while drawing out the blank tube longitudinally in one direction,
wherein the number of balls is limited to 2 to 3.
[0015] A lead angle .theta.' of the grooves of the grooved plug to
the axis is preferably limited to 26 to 45 degrees, and the
direction of revolution of the balls is preferably allowed to match
the direction of rotation of the grooved plug.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing and other objects and features of the
invention will become apparent from the following description of
preferred embodiments of the invention with reference to the
accompanying drawings, in which:
[0017] FIG. 1 is a partially enlarged development showing an
embodiment of an internal grooved tube according to the present
invention;
[0018] FIG. 2 is a schematic sectional view showing an apparatus
for illustrating an embodiment of a method of manufacturing an
internal grooved tube according to the present invention;
[0019] FIG. 3 is a partially sectional view showing the direction
of metal flow in a blank tube when the direction of revolution of
balls and the direction of rotation of a grooved plug are reversed
in the method of manufacturing an internal grooved tube according
to the present invention;
[0020] FIG. 4 is a partially sectional view showing the direction
of metal flow in a blank tube when the direction of revolution of
balls and the direction of rotation of a grooved plug are matched
in the method of manufacturing an internal grooved tube according
to the present invention;
[0021] FIG. 5 is a graph showing the relation of the number of
balls for grooving to a drawing force in a manufacture step of the
internal grooved tube, when the internal grooves have a small lead
angle to the tube axial direction and when those have a large lead
angle;
[0022] FIG. 6 is a graph showing the relation of the number of
balls to a grooving speed (a drawing speed), when heat exchanger
tubes according to an example of the present invention and those
according to a comparative example were manufactured; and
[0023] FIG. 7 is a graph showing the relation among a variation in
lead angle of the grooves of a grooved plug to the axis, the number
of balls and the results of grooving in the manufacture step of the
internal grooved tube.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] Embodiment of Internal Grooved Tube
[0025] A heat exchanger tube 1 made of copper, copper alloy or
other highly heat-conductive metal materials has a large number of
fine spiral grooves 10 on the inside surface in parallel
arrangement.
[0026] Each groove 10 is formed to assure that the ratio of a
groove width W in the tube axial direction L to a groove height H
may be in the range of 1 to 2, and that a lead angle .theta. of the
grooves to the tube axis may be limited to 26 to 35 degrees.
[0027] The heat exchanger tube 1 having an outer diameter of about
7 mm is preferably 0.2 to 0.3 mm in bottom thickness T, 0.2 to 0.3
mm in groove height H, and 10 to 30 degrees in a vertical angle
.alpha. of each fin 11 between the adjacent grooves 10.
[0028] Firstly, since the groove width W of each internal groove 10
in the tube axial direction L is equal to or twice as much as the
groove height H, the internal grooved tube in the embodiment
permits the sufficient growth of swirls occurring as shown by an
arrow a in FIG. 1 when a flow of refrigerant (a flow in the tube
axial direction) collides with the fins 11, resulting in the
improvement of heat transfer performance.
[0029] That is, the optimum condition for the sufficient growth of
swirls occurring in collision between the refrigerant and the fins
11 to fill the grooves with the swirls of refrigerant is that the
groove width W of each internal groove 10 in the tube axial
direction L should be equal to or twice as much as the groove
height H.
[0030] Secondly, since the improvement of heat transfer performance
is attained on the basis of the swirl effects of the refrigerant in
the grooves, there is no need for excessive groove height (fin
height) H, resulting in a reduction in heat exchanger tube weight.
Besides, a tube expansion step required for incorporation of the
tube into a heat exchanger permits less crushes of fins.
[0031] Thirdly, since the lead angle .theta. of the grooves 10 to
the tube axis is limited to 26 to 35 degrees, the heat exchanger
tube in the embodiment permits a relatively large collision between
the refrigerant and the fins 11 without hindering the flow of
refrigerant in the tube axial direction to excess, and the growth
of refrigerant swirls in the grooves may be further hastened,
resulting in the further improvement of heat transfer
performance.
[0032] The most appropriate space (a space close to the apex of
fins) for the growth of refrigerant swirls may be attained when the
vertical angle .alpha. of each fin 11 between the adjacent grooves
10 is limited to 10 to 30 degrees, resulting in the further
improvement of heat transfer performance.
[0033] When the ratio of the groove width W in the tube axial
direction L to the groove height H is less than 1, the groove width
W in the tube axial direction L is considered to be so small that
the refrigerant swirls in the grooves 10 might not be grown enough
to reach the groove bottom, resulting in the degradation of heat
transfer performance.
[0034] On the other hand, when the ratio of the groove width W in
the tube axial direction L to the groove height H exceeds twice,
the groove width W is considered to be much greater than the size
of the refrigerant swirls grown in the grooves 10 to permit
formation of a portion making no contact with the refrigerant in
the grooves 10, resulting in the hindrance of heat transfer
acceleration.
[0035] Embodiment of Manufacturing Method In FIG. 2, reference
numeral 4 denotes a drawing die, and 5 is a floating plug. A
small-diameter grooved plug 2 is connected rotatably to the tip of
the floating plug 5 through a tie rod 50. A large number of fine
grooves 20 having a lead angle .theta.' of 26 to 45 degrees to the
axis are formed on the outside surface of the grooved plug 2 in
parallel arrangement.
[0036] Two or three balls 3 capable of revolution and rotation in
the state of being pressed against the grooved plug 2 are installed
at uniform angular intervals in a location where the grooved plug 2
is installed.
[0037] A finishing die 6 is installed in a location on the further
downstream side of the grooved plug 2.
[0038] After setting the tip part of a blank tube la made of copper
alloy having an outer diameter of 12.5 mm, for instance, in the
drawing die 4, the floating plug 5 is set in the blank tube la as
shown in FIG. 2 to supply lubricating oil of relatively high
viscosity to an upstream portion of the floating plug 5 in the
blank tube 1a. Subsequently, while continuously supplying
lubricating oil of low viscosity to a contact portion between the
blank tube 1a and the balls 3 after drawing out the blank tube la
in the right direction in FIG. 2, each ball 3 is operated to
revolve around the blank tube la at a speed of about 10000/rpm in
the state of being pressed against the outside surface of the blank
tube 1a. The direction of revolution of the balls 3 is allowed to
match the direction of rotation of the grooved plug 2.
[0039] The blank tube 1a is firstly subjected to reduction by
drawing with the drawing die 4 and the floating plug 5, and the
grooves 20 of the grooved plug 2 are transferred to the inside
surface of the blank tube 1a while the blank tube 1a is further
subjected to reduction by rolling with the grooved plug 2 and the
balls 3. Thereafter, the blank tube is finished after being
subjected to further reduction down to about 7 mm in outer diameter
by sinking with the finishing die 6.
[0040] In the method of manufacturing the internal grooved tube
according to the embodiment, when the lead angle .theta. of the
grooves 20 on the outside surface of the grooved plug 2 to the axis
is limited to 45 degrees, the lead angle 0 of the grooves 10 in the
heat exchanger tube 1 to the tube axis comes to about 35 degrees in
the reverse direction of the lead angle .theta.' of the grooves
20.
[0041] In the manufacturing method, there is no need to insert the
finishing die 6 into the tube after forming the grooves 10 in some
cases. In this case, the lead angle .theta. of the grooves 10 in
the tube axial direction and the lead angle .theta.' of the grooves
20 of the grooved plug 2 are of the same value, while being
reversed.
[0042] According to the manufacturing method of the embodiment,
since the number of balls 3 is limited to 2 to 3, it is possible to
manufacture the internal grooved tube having the structure as
described in the above embodiment smoothly at high speed without
causing breakage.
[0043] The internal grooved tube may be manufactured more smoothly
at higher speed by allowing the direction of revolution of the
balls 3 to match the direction of rotation of the grooved plug
2.
[0044] A description will now be given of the reasons. As shown in
FIGS. 3 and 4, for instance, assuming that the blank tube 1a is
drawn out in the direction indicated by an arrow b and the twist
direction of the grooves 20 of the grooved plug 2 is as shown in
the drawings, the grooved plug 2 makes rotation in the direction as
indicated by an arrow c through the movement of the blank tube 1a
in the drawing direction b, together with the operation of the
balls 3. In this place, when the direction of revolution of the
balls 3 as shown by an arrow d in FIG. 3 and the direction c of
rotation of the grooved plug 2 are reversed, the metal flow in the
blank tube 1a occurs as shown by an arrow f through the movement of
the blank tube 1a, together with the operation of the balls 3. In
this case, since the metal flow direction f crosses the grooves 20
of the grooved plug 2 at a large angle, metal hardly flows into the
grooves 20. That is, a high flow resistance to the metal flow is
offered. On the other hand, when the direction of revolution of the
balls 3 as shown by an arrow e in FIG. 4 and the direction c of
rotation of the grooved plug 2 are matched, the metal flow in the
blank tube 1a occurs as shown by an arrow g. In this case, since
the metal flow direction g crosses the grooves 20 of the grooved
plug 2 at a small angle, the metal smoothly flows into the grooves
20. That is, the flow resistance to the metal flow is reduced.
EXAMPLE 1
[0045] As shown in Table 1, with variations in a lead angle .theta.
of the grooves 10 in the tube to the tube axis, heat exchanger
tubes of sample Nos. 1 to 7 as the examples, in which the ratio of
the groove width W in the tube axial direction L to the groove
height H is in the range of 1 to 2, were manufactured, together
with heat exchanger tubes of sample Nos. 8 to 19 as comparative
examples, in which the ratio of the groove width W to the groove
height H is in the range of less than 1 to more than 2. Then, the
condensation performance of the above heat exchanger tubes was
measured.
[0046] Table 1 shows the condensation performance rate when the
condensation performance (reference) of the heat exchanger tube of
sample No. 8 as the comparative example is assumed to be 1. In each
heat exchanger tube other than those of sample Nos. 17 and 18, a
copper tube having an outer diameter of 12 mm was used as a blank
tube, which was then subjected to finishing into a tube having an
outer diameter of 7 mm.
[0047] Rolling required for the above example may not apply to
manufacture of the heat exchanger tubes of sample Nos. 17 and 18 as
the comparative examples, in which the lead angle .theta. of the
internal grooves to the tube axis exceeds 45 degrees. Thus, the
above heat exchanger tubes were manufactured by the steps of
forming the grooves on one surface of a metal strip by rolling with
a grooved roll and a leveling roll, then molding the resultant
metal strip in the shape of a tube using a group of molding rolls
such that the grooved surface faces the inside, and then welding a
butted part of the metal strip for construction of a tube, which
was then subjected to finishing into a tube having an outer
diameter of 7 mm.
[0048] As shown in Table 1, the heat exchanger tube in each example
achieves condensation performance higher by 27% or above than the
heat exchanger tubes of sample Nos. 15, 19 showing the condensation
performance attained to the highest level among the heat exchanger
tubes as the comparative examples. In particular, the heat
exchanger tubes (of sample Nos. 1, 3, 4, 6 and 7), in which the
lead angle .theta. of the internal grooves to the tube axis is more
than 26 degrees, among the heat exchanger tubes as the examples
achieve the higher condensation performance.
1 TABLE 1 Groove width Groove Groove Condensation Sample W in tube
axial height twist performance No. direction H angle .theta. W/H
rate Example of 1 0.26 0.26 35 1.00 2.00 the invention 2 0.37 0.22
23 1.70 1.65 3 0.28 0.20 35 1.40 1.90 4 0.46 0.23 30 2.00 1.95 5
0.36 0.24 23 1.50 1.70 6 0.48 0.25 26 1.90 1.95 7 0.46 0.23 31 2.00
1.80 Comparative 8 1.05 0.21 18 5.00 1.00 example 9 0.88 0.20 15
4.40 1.10 10 0.55 0.24 20 2.30 1.15 11 0.68 0.20 25 3.40 1.10 12
0.67 0.21 30 3.20 0.90 13 0.44 0.20 28 2.20 1.25 14 0.33 0.15 40
2.20 1.10 15 0.56 0.20 28 2.80 1.30 16 1.35 0.27 15 5.00 1.20 17
0.24 0.27 55 0.88 1.00 18 0.17 0.22 61 0.79 0.80 19 0.26 0.30 45
0.85 1.30
EXAMPLE 2
[0049] A blank tube consisting of a copper tube having an outer
diameter of 12 mm was used to manufacture two kinds of heat
exchanger tubes, which are 0.23 mm in groove height H, 0.46 mm in
groove width W in the tube axial direction and respectively 20 and
31 degrees in lead angle .theta. of the grooves to the tube axis,
according to the same conditions except that the number of
machining balls varies from 2 to 6 without the need for a finishing
die. Then, a change of drawing force was measured as to both the
above heat exchanger tubes.
[0050] The results are shown in FIG. 5, in which the horizontal
line is denoted as the number of balls and the vertical line as a
drawing force rate. As shown in FIG. 5, in case of the heat
exchanger tube having a relatively small lead angle .theta. (20
degrees) of the internal grooves, the drawing force increased at a
substantially fixed rate with an increase in number of balls. On
the other hand, in case of the heat exchanger tube having a large
lead angle .theta. (31 degrees) of the grooves, the use of four or
more balls results in an increase in drawing force more rapidly
than that when two or three balls were in use.
EXAMPLE 3
[0051] A blank tube consisting of a copper tube having an outer
diameter of 12 mm was used to manufacture a heat exchanger tube of
sample No. 7 (a lead angle .theta. of the grooves before finish
drawing is 36 degrees, while a lead angle .theta. of the grooves
after finish drawing is 31 degrees) as the example, together with a
heat exchanger tube of sample No. 16 (a lead angle .theta. of the
grooves before finish drawing is 20 degrees, while a lead angle
.theta. of the grooves after finish drawing is 15 degrees) as the
comparative example according to the same conditions except that
the number of machining balls varies from 2 to 6. Then, a critical
(maximum) grooving speed (drawing speed) was measured as to both
the heat exchanger tubes.
[0052] Incidentally, the heat exchanger tube of sample No. 7 as the
example was manufactured on condition that the direction of
revolution of the balls and the direction of rotation of the
grooved plug are matched and also on condition that both the
directions are reversed. On the other hand, the heat exchanger tube
of sample No. 16 as the comparative example was manufactured on
condition that the direction of revolution of the balls and the
direction of rotation of the grooved plug are reversed.
[0053] The results are shown in FIG. 6. In FIG. 6, in manufacture
of the heat exchanger tube of sample No. 16 as the comparative
example having a relatively small lead angle .theta. of the
grooves, a critical grooving speed gradually increased at a
substantially fixed rate with an increase in number of balls. On
the other hand, when the heat exchanger tube of sample No. 7 as the
example was manufactured by the use of four balls at the same
grooving speed as the case of using three balls, the breakage of a
tube occurred in process of machining.
[0054] Further, when the direction of revolution of the balls was
allowed to match the direction of rotation of the grooved plug, the
critical machining speed was improved more than that when both the
directions were reversed.
EXAMPLE 4
[0055] A copper tube having an outer diameter of 12 mm was used to
manufacture a heat exchanger tube, which is 0.23 mm in groove
height H, 0.46 mm in groove width W in the tube axial direction, 10
mm in outer diameter and 3000 m in length, by the use of grooved
plugs having groove lead angles .theta.' varying from 10 to 50
degrees by only rolling without the need for finish sinking on
condition that the number of machining rolls varies from 2 to
6.
[0056] In FIG. 7, the number of balls in the critical machining
speed is represented by .circle-solid., the number of balls in the
machining speed lower than the critical machining speed is by
.smallcircle., and a failure in grooving (a case where the breakage
of the tube occurred in process of machining) is by .times.,
respectively.
[0057] As a result, in case of the grooved plugs having the groove
lead angle .theta.' of 10 to 25 degrees, the machining speed
reached the maximum by the use of four to six balls. On the other
hand, in case of the grooved plugs having the groove lead angle
.theta.' of 26 to 45 degrees, the machining speed reached the
maximum by the use of two to three balls, whereas the breakage of
the tube occurred in process of machining when four or more balls
were in use. Further, in case of the grooved plug having the groove
lead angle .theta.' of more than 45 degrees, the breakage of the
tube occurred in process of machining even by slowing down the
machining speed, resulting in a failure of machining.
[0058] It is found from the results shown in FIGS. 5 to 7 that in
manufacture of the internal grooved tube as described in the above
example by rolling, the use of two to three machining balls less
than those required for the prior art permits a mass production of
internal grooved tubes smoothly with high workability without the
need for an increase in drawing force to excess.
[0059] According to the internal grooved tube according to the
present invention, since the groove width W of each internal groove
10 in the tube axial direction L is equal to or twice as much as
the groove height H, this internal grooved tube permits the
sufficient growth of swirls occurring as shown by the arrow a in
FIG. 1 in collision between the refrigerant flow (the flow in the
tube axial direction) and the fins 11, resulting in the improvement
of heat transfer performance.
[0060] Further, since the improvement of heat transfer performance
is attained on the basis of the swirl effects of the refrigerant in
the grooves, there is no need for excessive groove height (fin
height) H, resulting in a reduction in heat exchanger tube weight.
Besides, tube expansion required for incorporating the heat
exchanger tube into the heat exchanger permits less crushes of
fins.
[0061] Further, when the lead angle .theta. of the grooves 10 in
the tube to the tube axis is limited to 26 to 35 degrees, the heat
exchanger tube of the present invention permits a relatively large
collision between the refrigerant and the fins 11 without hindering
the flow of the refrigerant in the tube axial direction to excess,
and the growth of refrigerant swirls in the grooves may be further
hastened, resulting in the further improvement of heat transfer
performance.
[0062] According to the method of manufacturing the internal
grooved tube according to the present invention, the number of
balls 3 is limited to 2 to 3, resulting in smooth high-speed
manufacture of the internal grooved tube according to the present
invention without causing the breakage.
[0063] When the direction of revolution of the balls 3 is allowed
to match the direction of rotation of the grooved plug 2, it is
possible to manufacture the internal grooved tube according to the
present invention more smoothly at higher speed.
* * * * *