Method to reduce air center middle margin turnaround for...

Details Method to reduce air center middle margin turnaround for... Method to reduce air center middle margin turnaround for...

2002-04-23

2003-12-16

Larson, Lowell A. (Department: 3725)

Metal deforming

By use of 'flying tool' engaging moving work

With cutting

C029S890047

Reexamination Certificate

active

06662615

ABSTRACT:

TECHNICAL FIELD
The present invention relates to heat exchangers and more particularly to the formation of the folded fins in a heat exchanger.
BACKGROUND OF THE INVENTION
Heat exchangers for a wide variety of applications, such as radiators, heater cores, condensers, and evaporators are well known in the art and are generally of a similar basic construction. The basic construction of such a heat exchanger typically includes two headers for the input and output of a heat exchanging liquid. A plurality of regularly spaced tubes extend between the two headers to permit the heat exchanging liquid to flow therebetween. A plurality of heat conductive fins, also known as an air center, occupy spaces between each pair of adjacent tubes and are oriented to permit a flow of air therethrough. Most typically, the fins are constructed as a convoluted folded fin, which when viewed from one edge are folded in an accordion-like pattern. The folded fins are bonded, most typically metallurgically bonded, to the sides of the tubes to enhance the heat transfer from the liquid flowing in the tubes to the folded fins so that the excess heat can be convectively transferred to an air stream flowing through the fins. Features of a typical prior art heat exchanger core
10
are illustrated in
FIGS. 1 and 2
.
Prior art heat exchanger core
10
typically comprises a plurality of tubes
12
having an elliptical shape wherein the width of tube ends
16
are generally smaller than the width of a central section
14
of tube
12
such that central section
14
forms a compressible crown. A plurality of air centers
18
formed as accordion-like folded fins are placed between adjacent ones of tubes
12
. Each air center
18
is generally comprised of a plurality of adjacent convolution legs
24
, wherein each convolution leg
24
is connected to a previous leg
24
by top tip radius
20
and to a subsequent convolution leg
24
by bottom tip radius
22
. Each convolution leg
24
can further have a plurality of louvers
26
formed therein for improved heat transfer properties. Prior art core
10
is assembled by abutting a plurality of tubes
12
and air centers
18
in an alternating fashion such that the tip radii
20
,
22
of an air center
18
are abutted to facing sides of adjacent tubes
12
. Since tubes
12
have center sections
14
that are generally wider than tube ends
16
, the arranged tubes
12
and air centers
18
can then be compressed to form core
10
to a desired dimension. By compressing the stacked tubes
12
and air centers
18
, tip radii
20
and
22
can be caused to substantially contact the facing sides of tubes
12
. The compressed core
10
is then processed to bond tip radii
20
and
22
to tubes
12
, most typically by a metallurgical bonding process.
Each air center
18
in prior art core
10
is generally formed by passing a strip of heat conductive metal through a pair of intermeshing star-shaped form rolls similar to the rolls illustrated in FIG.
6
. The intermeshing of the star-shaped form rolls form the generally flat metallic strip into an accordion-like folded fin. A partial cross-section of a prior art form roll
30
is illustrated in
FIG. 3
wherein
FIG. 3
corresponds to the cross-section along the lines
7
—
7
of FIG.
6
. Form roll
30
is generally comprised of a plurality of discs
32
-
36
wherein each disc forms a specific portion of convolution legs
24
and tip radii
20
and
22
. Star-shaped discs
32
-
36
have a plurality of teeth about the circumference wherein each tooth has a top edge
38
and a valley
39
is defined at the bottom convergence of adjacent teeth. The teeth of outer discs
32
and
36
have a plain face
40
to form ends
25
of leg
24
. Discs
33
and
35
are positioned interiorly of discs
32
and
36
, and their teeth have faces
42
for forming louvers
26
in leg
24
. Stripper disc
34
is positioned between discs
33
and
35
. Stripper disc
34
has no teeth and has a diameter substantially less than the diameter circumscribed by valleys
39
. Stripper disc
34
in combination with adjacent discs
33
and
35
define a gap
44
between discs
33
and
35
to permit a stripper finger
46
to be at least partially contained therein. Stripper finger
46
facilitates the removal of formed air center
18
from form roll
30
, and thus remains below valleys
39
to permit the proper intermeshing of teeth from the discs of the two intermeshing form rolls.
The intermeshing form rolls produce three different specific characteristics of the air center
18
; the angle of louvers
26
, the height of air center
18
, and the size of tip radii
20
and
22
. The form rolls operate under minimal clearance to produce the desired effect onto the blank heat conductive strip. The placement of stripper disc
34
corresponds to middle turnaround
28
of air center
18
. Stripper disc
34
does not come in contact with air center
18
but allows for the clearance of stripper finger
46
to enter form roll
30
without creating an interference with the heat conductive strip being formed thereon. The heat conductive strip is trapped by the mating top and bottom form rolls
30
and is in turn drawn over the corresponding edges
38
of the discs to form the top and bottom tip radii
20
and
22
. As the clearance between form rolls is reduced, the bend radii
20
and
22
of the heat conductive strip are also reduced, thereby resulting in sharper tip radii
20
and
22
. As tip radii
20
and
22
become sharper, the height of air center
18
correspondingly becomes higher. The gap
44
created by stripper disc
34
allows that portion of the heat conductive strip in the region between discs
33
and
35
to be pushed toward gap
44
rather than forming a clean bend at the radius. This interaction results in middle turnaround
28
to be at a higher height than the rest of each individual top and bottom tip radii
20
and
22
. Thus, as the mating top and bottom form rolls are setting the corresponding tip radii between them, the material at gap
44
is formed at a sharper radius, resulting in a higher center height of middle margin turnaround
28
of air center
18
.
During assembly, core
10
is compressed to meet a predetermined core package dimension prior to placing a header on the ends of tubes
12
. The height of air centers
18
should be substantially constant from convolution to convolution since a center of excessive height will cause air center
18
to collapse. Similarly, a center height that is too low will cause the air centers
18
to drop out of the core block
10
prior to bonding air centers
18
to tubes
12
. The higher middle margin turnaround
28
could thus interfere with the proper assembly of core
10
. However, as previously discussed, tubes
12
have a compressible crown
14
to permit some compression of tube
12
during assembly. This compressibility allows the increased height of air center middle margin turnaround
28
to be absorbed by the compression of tubes
12
.
In the past, tubes
12
have been fabricated of either welded or extruded construction. However, a folded tube
50
as shown in
FIG. 4
has now been introduced into the construction of heat exchanger cores. Folded tube
50
is designed and formed to have legs
52
and
54
in middle section
56
of tube
50
. Legs
52
and
54
are non-compressible thereby removing the flexibility exhibited by tube
12
having a compressible crown center section
14
. The non-compressibility of center section
56
results from legs
52
and
54
bottoming out on an opposite side of the folded tube
50
. The tube sections between middle portion
56
and ends
58
do however, retain a degree of compressibility. Since legs
52
and
54
align directly with the high middle margin turnaround
28
of air center
18
, the excess height of middle margin turnaround
28
cannot be compensated for since center portion
56
is no longer compressible. Therefore, assembling a heat exchanger core from tubes
50
in combination with air centers
18
having an increased height middle

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