Finned tube with indented groove base and method of forming same

A tube for heat transfer systems or the like is provided which includes helically or circumferentially disposed, radially extending ribs or fins. The base of the grooves defined between adjacent fins has a plurality of discrete impressions defined therein. The provision of discrete impressions in the groove base improves the heat transfer characteristics of the tube by greatly increasing the outer surface area of the tube relative to the inner surface area of the tube.

BACKGROUND OF THE INVENTION 
The present invention relates to a tube for heat transfer systems or the 
like and, more particularly, to a tube having helically or 
circumferentially disposed, radially extending ribs or fins, wherein the 
base of the grooves between the fins has a plurality of discrete 
impressions defined therein. 
Finned tubes for heat transfer systems are known, generally, as for example 
as shown in U.S. Pat. Nos. 3,791,003 and 3,893,322 and European Patent 
Application No. 0,102,407. In the latter publication, the internal face of 
the tube has an interrupted waviness corresponding to the grooves disposed 
between the fins. Further, individually separated projections of displaced 
tube material are provided on the interrupted waves. The internal surface 
of the tube so provided results in favorable heat transfer properties on 
the tube internal wall. The separate projections correspond to separate 
depressions formed in the groove of the tube outer wall which run in the 
direction of the groove. Though these depressions in the area of the 
groove base increase the surface area of the tube external surface, as 
compared with a non-formed tube, they exert only a limited influence on 
the heat transfer to the tube external face itself. 
Accordingly, it would be desirable to provide a finned tube which 
advantageously influences the heat transfer to the tube external surface. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the heat transfer characteristics 
of a finned tube are improved by providing impressions in the form of fine 
indentations in the tube walls in the area of the groove base. The fine 
indentations are defined so as to distort only the outer surface of the 
tube wall. 
In a preferred embodiment, the fine indentations are defined so as to have 
an axial length which is greater than or equal to the circumferential 
width thereof and, most preferably, the axial length of each indentation 
is less than the fin pitch (fin pitch t.sub.R =the spacing from fin center 
to fin center). 
Furthermore, the center lines of the indentations are disposed as an angle 
.alpha. with the line of its associated grove whereby 
0.degree.&lt;.alpha.&lt;180.degree. and, more preferably, 
10.degree.&lt;.alpha.&lt;170.degree.. In the most preferred embodiment, the 
center lines of the indentations run some what vertical to the line of the 
grooves, i.e. .alpha.=90.degree.. 
Also in accordance with the present invention, the indentations are 
preferably regularly spaced along the line of the groove and approximately 
0.5 to 20 indentations are provided per centimeter of groove length. 
Where the tube wall thickness is relatively small, only a very slight 
indentation is recommended so that distortion of the inner wall surface is 
avoided. Preferably the indentation depth is from about 0.01 to about 1.0 
millimeters and more particularly in the range of 0.05 to 0.5 
millimeters. The indentations may have a cross sectional shape of a V, 
trapezoid, semicircle, or similar cross section. It has been found to be 
most advantageous to combine different cross sectional variants with each 
other along the length of the groove. 
The heat transfer characteristics of the tube external face are further 
improved if the fins of the tube are formed in a substantially T-shape. 
Further, the tube internal face may be substantially smooth. However, the 
heat transfer characteristics are improved by the formation of an internal 
waviness by the distortion of the inner tube surface during the external 
grooving process. 
In accordance with the method of present invention, the fine indentations 
are formed in the base of the grooves of the tube by means of a toothed 
wheel after the fins and grooves have been formed in the tube wall and 
before any distortion, such as the forming of a T-shape, is effected at 
the radially outward most end of the fins. 
Other objects, features, and characteristics of the present invention, as 
well as the methods of operation and functions of the related elements of 
the structure, and the combination of parts and economies of manufacture, 
will become more apparent upon consideration of the following description 
and the appended claims with reference to the accompanying drawings, all 
of which form a part of this specification, wherein like reference 
numerals designate corresponding parts in the various figures.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS 
FIGS. 1 and 2 show a finned tube 1 in accordance with one embodiment of the 
present invention partially broken away and in cross section, 
respectively. T-shaped fins 2 in a helical line are provided on the tube 
external face forming one turn bordering a groove 3 which also runs 
helically. The fin 2 protrudes radially from the tube wall 5, the fin 
peaks 6 being distorted to form a T so that narrowed gaps 7 are formed 
(see the upper gap width S in FIG. 1). 
The spacing between adjacent fins 2 changes continuously such that the 
grooves 3 are basically shaped as rounded off cavities. The fin pitch from 
fin center to fin center is designated with t.sub.R. 
The tube wall 5 has fine indentations 8 in the area of the groove base 3', 
which predominantly run in the tube 1 axial direction and which are 
regular spaced in the circumferential direction of the tube. The 
indentation depth 8 is designated with T (see, in particular, FIG. 2). 
To better illustrate the indentations 8 in the groove base 3', the tube is 
shown in FIG. 3 with the shaped fins 2 partially broken away. 
FIG. 4 shows the indentations 8 in the groove base 3' in an enlarged scale 
with V, trapezoidal and semi-circular cross sections. The deepest points 
of the channel shaped indentations 8 are in each case indicated by center 
lines 8'. 
FIG. 5 clearly shows an angle of the center lines 8' relative to the groove 
3 direction. In the most preferred embodiment, .alpha.= 90.degree.. The 
length of the indentations 8 measured in the direction of the center line 
8' is designated with L, the width with the letter B. L and B in the 
illustrated embodiment are less than t.sub.R. Further, in the preferred 
embodiment, B is less than or equal to L. 
FIG. 6 schematically illustrates on an enlarged scale how each indentation 
8 joins the bottom 4 of the neighbouring fins 2 so that clearly 
discernible corners 4' form in the fin flanks. In addition, the core wall 
thickness W and the depth T of the indentations 8 are shown. The core wall 
thickness reduced by T is designatd with W.sub.R (=residual wall 
thickness), the depth of the indented fin flanks with T.sub.f. Where there 
is a low residual wall thickness W.sub.R, it is preferred that only the 
bottom 4 of neighbouring fins are indented, as seen in FIG. 7 where T=O, 
so that the distortions which define the indentations are limited to the 
tube outer wall. 
The device for the production of a T-shaped finned tube 1 is illustrated in 
FIG. 8. As is apparent, the device can be used with a fixed roller head 
(with the tube turning) or with a rotating roller head (the tube being fed 
axially only). The method of forming a T-shaped finned tube of the present 
invention with a rotating tube will now be explained with reference to 
FIG. 8. 
The device shown in FIG. 8 includes a roller tool 9, a toothed wheel 10, a 
spacer collar 11, a cylindrical smoothing roller 12, a slotter roller 13 
for the fins and a cylindrical upset roller 14 on a tool holder indicated 
with the number 15. Two further tool holders (not shown for clarity) are 
provided, without a toothed wheel 10, each of which is arranged offset 
through 120.degree. in relation to each other about the circumference of 
the tube 1. As is apparent, however, it is possible to use, for example, 
four or more tool holders 15. Each of the tool holders so provided, are 
radially adjustable to accommodate tubes of various diameters. further, 
each is mounted on a locally fixed roller head (not shown). 
The smooth wall tube 1' running-in in the direction of the arrow is set 
into rotary motion by the driven roller tools mounted about the tube 
circumference, the axis of which runs parallel to the tube axis. These 
rolling tools 9 consist of the commonly known arrangement of roller discs 
16 arranged next to each other, whose diameter increases in the direction 
of the arrow so as to form the fins 2' in the tube wall 5, while the tube 
is supported by a roller mandrel 17. More particularly, a diameter 
reduction initially takes place in the front section (pulling-in area). In 
the middle section (finished rolled area) the rolling out of the helically 
formed fins 2' occurs. A toothed wheel 10 is mounted on the tool holder 15 
behind the roller tool 9, the external diameter D of which is larger than 
the external diameter of the last roller wheel 16'. The toothed wheel 10 
has teeth 18 formed parallel to or at an angle relative to the axis 
thereof so that fine indentations 8 can be produced in the area of the 
groove base 3' of the tube wall 5. FIG. 9 shows a cross section through 
the toothed wheel 10 with teeth 18 in greater detail. The external 
diameter is designated with D, the height of the teeth 18 with h.sub.Z. 
In the preferred embodiment, the toothed wheel 10 has approximately 0.5 to 
20 axially parallel or angled teeth 18 per cm of circumference. The teeth 
18 are triangular, trapezoidal and/or semi-circular shaped so that the 
indentations can be formed with various cross-sectional shapes, as was 
discussed above. Further, the teeth have a height h.sub.Z of approximately 
0.01 to 10.0 mm. 
T-shaped fins 2 are formed in a known manner as follows: A spacer collar 11 
is provided adjacent the toothed wheel 10. A smoothing of the fin ends 2' 
is achieved with a smoothing roller 12, so that the fin ends 2' lie on an 
imagined cylinder surface coaxial with the tube center axis 19. A slotter 
roller 13 then slots the fins 2" in the helical direction and 
simultaneously bends them laterally open so that Y-shaped fins 2'" result. 
The Y-shaped fins 2'" are then formed in the radial direction by an upset 
roller 14 into T shaped fins 2. The thickness of the smoothing roller 12, 
slotter roller 13 and upset roller 14 each approximate to the fin pitch 
t.sub.R (between smoothing roller 12 and slotter roller 13 a further 
correction disc 20 is indicated). 
In order to manufacture a finned tube 1 with internal protrusions 21 in 
accordance with a second embodiment of the present invention, a device of 
the type illustrated in FIG. 10 is used in which the rolling mandrel 17 
ends with the last roller disc 16'. In this case a pressure roller 22 
follows the rolling tool 9 in the tool holder 15, whose outside diameter 
is greater than the outside diameter of the final roller disc 16'. 
The groove 3 between the fins 2 is deepened by the pressure roller 22 so 
that protrusions 21 are formed on the internal tube face (internal 
waviness H) due to displaced tube wall material. The indentation of the 
groove base 3' then takes place. The pressure roller 22 and the toothed 
wheel 10 have a smaller thickness than the last roller wheel 16' and the 
toothed wheel 10 has a diameter such that fine indentations may be made 
without further distortion of the internal tube face. 
The advantages and features of the present invention may be more clearly 
recognized by reference to the following example: 
EXAMPLE 
Starting with a smooth tube 1' manufactured from oxygen free Cu (SF-Cu) 
with 18.9 mm external diameter and 1.35 mm wall thickness a finned tube 1 
was produced in accordance with the dimensions shown in the following 
table using a device in accordance with FIG. 10 (for the individual tube 
sizes see in particular FIGS. 1, 5 and 10). 
TABLE 
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Fin pitch t.sub.R 1.35 mm 
Fin diameter d.sub.R 
18.60 mm 
Core diameter d.sub.K 
16.60 mm 
Internal diameter d.sub.I 
14.94 mm 
Fin height h.sub.R 1.00 mm 
Length L of the indentations 8 
0.60 mm 
Width B of the indentations 8 
0.15 mm 
Depth T of the indentations 8 
0.10 mm 
Internal waviness H 0.13 mm 
Core wall thickness W 
0.83 mm 
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The diameter of the roller disc 16' was 36.5 mm. The pressure roller 22 had 
a diameter of 37.0 mm and the toothed wheel 10, with 50 teeth 18 and 
h.sub.Z =5.6 mm height, had an overall diameter D of 37.2 mm. The 50 teeth 
18 on this wheel correspond to about 4 indentations per centimeter of 
groove length. The smoothing roller 12 had a diameter of 34.3 mm, the 
slotter roller 13 a diameter of 35.10 mm and the upset roller 14 a 
diameter of 35.10 mm. 
To allow a comparison with a finned tube with T-shaped fins but without 
indentations 8 on the groove base 3', a device in accordance with FIG. 10 
was used without toothed wheel 10. 
Both tubes were measured in a flooded 20 evaporator operation as single 
tubes (i.e water in the tube, R22 refrigerant external). The finned tube 1 
with indented groove base 3' had a considerably higher performance than 
the comparative tube with a smooth groove base 3'. FIG. 11 illustrates the 
capacity relation Q.sub.T indented/Q.sub.T as a function of the water 
throughput V.sub.W (1h) or the water speed W.sub.W (m/s). As indicated in 
the graph, a capacity increase of approximately up to 20% was achieved. 
For comparative testing, additional finned tubes 1 have been produced with 
a device according to FIG. 10 using toothed wheels 10 having a different 
number of teeth 18 (the other parameters remaining unchanged). The finned 
tubes 1 so produced and having different numbers of indentations 8 per cm 
of groove length, have been tested under the same conditions as in the 
Example, i.e., flooded boiling (water inside tube, refrigerant R22 
outside). 
The results obtained were particularly favorable with tubes having 4 to 13 
indentations per cm of groove length, even more so with 7 to 11 
indentations per cm of groove length, as illustrated by FIG. 12 where the 
performance ratio Q.sub.T indented/Q.sub.T is plotted against the number 
of indentations per cm of groove length, the water flow rate v.sub.W being 
constant at 900 1/h). 
For an indented finned tube 1 as per the Example (4 indentations per cm of 
groove length) and a flow rate v.sub.w of 900 1/h, the performance 
increase of the indented tube over the unindented tube was already 20%. 
This point has been marked P in both FIG. 11 and FIG. 12 for better 
understanding. According to FIG. 12, the performance increase achieved 
with 4 to 13 indentations per cm of groove length (X) is at least 20%, 
with 7 to 11 indentations per cm of groove length (Y) it is more than 30%.