Note: Descriptions are shown in the official language in which they were submitted.
130~'3605
HEAT EXCHANGER TUBE HAVING
EMBOS8ED RING BELL AND ~RAZING ~ING
Background of the Invention
This invention relates to l~provement~ in heat e~changer
fabrication, especially for plate-fin heat exchangers which
employ copper tubing. The inventlon i9 more particularly
directed to improvements in the technique of belling the end of a
tube which facilitates the process of expansion, under tension,
of the tubing outward against the heat exchanger fin and thereby
achieve good thermal contact.
The invention is also directed to an improvement in the
assembly of plate-fin type heat exchangers which facilitates the
tension expanding of hairpin tubes into the fin pack, and also
facilitates the connection of return bends to the open end of the
hairpin tubes. In connection with this, the invention is further
directed to improvements of the brazing of heat exchanger tubing
to mating tubing members, to wit, for the mating of hairpin tubes
of a condenser or evaporator coil to associated return bends.
A~ discussed in U.S. Pats. Nos. 4,195,5iO; 4,285,256 and
4,286,486, techniques have been developed for fabrication of
plate-fin heat exchangers for refrigeration systems or automotive
cooling systems. The plate-like fins are stamped from good
thermal conducting materials (such as aluminum). The fins are
stacked with the fin plates aligned one over the other in a
column 80 that their through-apertures are in registry. The
~ assembled fin pack units are then fitted with end baffles or tube
- 25 sheets, and are laced with hairpin tubes. The term "hairpin" or
"hairpin tube" refers to a U-shaped tubular member having two
' '
6~
equally elongated legs that are in generally parallel alignment.
The legs of the hairpin tubes form flow channels that are
perpendicular to the fln plates. The legs of ad~acent hairpin~
are later interconnected by means of short U-~haped tubing bends
referred to as return bends. These members complete the flow of
the circuit through the fin pack unit. To receive the return
bends, the open ends of the hairpin are generally belled, or
expanded to a larger diameter. The return bends are inserted
into the belled openings and are brazed in place to provide a
fluid-tight joint. Also, in order to ensure a good thermal
contact between the tubing and the fins, the legs of the hairpin
tubes are expanded into locking contact against the fin plates by
driving an expanding rod through the tube leg~.
The expansion process is described generally in U.S. Pat.
No. 4,228,573 and also in U.S. Pat. No. 4,584,765. The tube
expansion process is generally carried out by passing an
expansion "bullet", or rounded member of larger diameter than the
tube inside diameter, through the tube from the open ends of the
hairpin tubes. The "bullets" are affixed atop elongated rods
which are driven mechanically or hydraulically into the hairpin
legs. In so-called "compression" expansion, a backing plate is
placed at the tube bends during an expansion to prevent the tubes
from being driven out of the unit as the expansion bullets are
driven through it. As a consequence of this, the tubes are
compressed rearwardly as they are being expanded outwardly by the
expansion tools. This results in tube shrinkage, on the order of
about three percent, 80 that the axial length of each tube can
1;~.36~S
vary dramatically in regard to its neighbor in final assembly.
Because of differences in tube lengths, belling of the tubes is
dificult and generally results ln uneven or mlsaligned bells
being formed in the tube ends. The return bend therefore cannot
always be properly seated within the bells leading to formation
of relatively weak or incomplete brazing in this critical region.
Another problem with compression expansion is that the
coupled force~ of the expansion device and backing plate on the
hairpins can cause the coil unit to bow or to bend. An
elaborate, rigid "iron maiden" coil clamp is required to hold the
coil assembly to prevent this from happening. Even so, a
significant amount of scrap does occur as a result of compression
expansion.
In order to avoid the problems of "compression"
expansion, various proposals have been advanced for "tension"
expansion, that is, supporting the hairpin tubes by the open or
bell end when the expansion tools are driven through. Here, it
has been the practice to bring the open ends of the hairpins a
considerable distance out from the adjacent tube sheet. Then, a
clamping fixture can hold the belled ends of the tube against
axial displacement and expansion tools can be arranged to move
through the open end~ of the tubes to expand the tube walls into
contact against the fin collars and tube sheets of the unit,
while the bells are clamped in place. It has been found that
expansion under tension conditions results in a lesser shrinXage
in tube length, on the order of about one percent.
In addition, tension expansion results in greater
- 3 -
130:3605
evenness in bell height or stand off from the bell-end tube
sheet. This permits the return bends to be properly seated
within all of the bell~ of a partlcular coil unlt. Moreover the
tension ~xpansion operation has much less tendency to bend or bow
out the unit, as the tubes are always under a tension force, and
there is never a force component tending to bow or warp the tube~
Unfortunately, the belled end of a copper tube typically
has only a tiny increase in diameter over the unexpanded tube.
Consequently it is difficult for a gripping device to achieve a
secure grip without damaging the belled end. Further, while it
may be advantageous to support the belled ends of the hair pins
directly from the upper tube plate, the lack of a substantial
shoulder on the bell end makes this proposal difficult.
Another problem of the current techniques for assembling
these fin pack units is that the brazing or soldering tends to
place the metal primarily at the outer end of the bell, and not
at the inner end, where weakness of tube integrity is most
iikely. In hydrostatic tests of these coil units, the weakest
point~ tend to be at the tip of the male end of the return bends,
that is, at the inner end of the bell of the hairpins. This
occurs because if the space between the male end of the return
bend and the bell i8 incompletely filled with braze alloy, or the
braze alloy is incompletely bonded to the joint members, the
hydraulic fluid under pressure tends to peel the inner layer of
the outer member i.e., the bell, away from the outer layer of the
return bend tip. Ideally, therefore, the brazing should occur at
the tip of the return ben-d, i.e., at the base of the bell, but
_ 4 _
, ~
' `' , , ' ~ ' .
1:~0~'36~5
this cannot be achieved using current techniques.
It is currently the practlce to as~emble the return bend~
into the hairpin bells by first placlng a brazing ring over each
leg of the return bend, and then inserting the return bends into
the hairpin bells. After this, the tubing i9 heated sufficiently
to melt the brazing ring, and the brazing material is then drawn
between the return bend and the bell to form a seal. It is clear
that with this technique the brazing operation can be carried OUt
only in a "vertical" or upright mode, that i8, with the fin pack
coils arranged with the bells upwards, 80 that the braze is drawn
by gravity downward into the gap between the hairpin bell and the
return bend. However, the vertical mode i8 not practical for
very large coils or for coil~ in ~hich there may be obstructing
flanges on the tube sheet. In such cases, it may be better to
dispose the entire fin pack coil horizontally or with the return
bends oriented vertically downwards. However, there is no prior
method which will ensure that the brazing material would flow
into the gap between the return bend and the hairpin bells if a
non-upright mode of brazing is used.
Objects and Summary of the Invention
Accordingly, it is an object of this invention to improve
fin pack heat exchanger coils and to improve the methods of
manufacturing them.
It is another object of this invention to eliminate the
problems associated with the gripping of heat exchanger tubes
during the process of expanding them.
¦ rt 8 a rther obj-ct of thi~ tnvontion to reduce the
~ ~ ,.................... I
'~
`~
,
130;~61~5
scrap rate in the manufacture of heat exchanger colls.
It is still a further object of this invention to
facilitate the brazing of the tubular return bend onto the bell3
of their associated hairpin tubes ln a heat exchanger coil.
It is yet another object of this invention to make it
possible to braze the tubular return bends onto the hairpin bells
in orientations other than the vertical or upright orientation,
and to strengthen the resulting brazed joint.
According to one of many possible aspects of this
invention, a heat transfer tube of the type that comprises a
hairpin tube or the like, has an elongated main tube portion and
an end portion which is belled out to a suitable inside diameter
to receive a male tube member, such as a return bend, which has
substantially the same outside diameter as the main portion of
the hairpin tube. The belled end portion of the heat exchanger
tube has a ring or torus formed in it at the junction of the mai~
tube portion with the belled-out end portion. The ring or torus
has a greater diameter than the outer diameter of the end
portion. This forms a shoulder which can be supported by
gripping means, or even by the tube sheet itself, which can
sustain the forces that are encountered during a tension
expansion operation. A ring of brazing material can be situated
within the embossed ring or torus. When the return bend is
inserted into the hairpin bells, the tip of the return bend will
lodge against the ring of brazing material. With this
configuration, when the braze melts, it will flow into the gap
between the bell and the return bend tip at the lower end of the
- 6 - ~
13036r~s
bell. This yields a substantially stronger braze joint than that
resulting from the technique of the prior art descrlbed above.
Moreover, a brazing operation can be carried out automattcally in
any arbitrary orientation of the tubing.
The ends of the hairpin tube can be belled or expanded by
means of a collet device which has a number of radially movable
fingers, and in which an embossing device i8 carried at the ends
of these finger~. The collet i9 expanded outward radially to
enlarge the tube end to form the bell, and, at the same time, to
form the embos~ed ring or torus at the base of the bell. The
embossing device can comprise a wire ring di~posed over the
collet fingers, and preferably can be a ring of brazing wire,
which simply remains in the embossed ring or torus after the bell
expansion operation.
The above and many other objects, features, and
advantages of this invention will be more fully understood from
the ensuing description of a preferred embodiment, which should
be read in connection with the accompanying drawing.
Briof De~cription of the Drawing
Fig. 1 is a partial perspective elevation of a heat
exchanger tube bell, shown with a portion of the associated tube
sheet .
Fig. 2 is an elevational cross-section of the heat
exchanger tube bell of Fig. 1.
Fig. 3 is another elevational cross-section of the heat
exchanger twbe bell of Fig. 1, shown with a brazing ring and an
associated male tubular member.
7- ~
1~036~)5
Figs. 4 and 5 are cross-sectional elevatione of the tube
and a device for forming the bell end in the heat exchanger
tubing.
Fig. 6 i9 a sectional elevation for illustrating and
explaining the tension tube expansion operation according to the
prior art.
Figs 7 and 8 are sectional elevations for explaining
tension tube expan~ion operations according to this invention.
Fig. 9 is a cross-sectional view of another embodiment of
this invention.
Detailed De-cription of th- Preferred Eibodiment
With reference initially to Figs. 1 and 2, a heat
exchanger tube 10, which can here represent the upper end of one
leg of a hairpin tube, has a bell 12 or an expanded end. The
bell 12 i~ generally cylindrical and is usually enlarged just
enough to accommodate the male end of another tube with which it
i8 to be connected. This means that its inside diameter is just
slightly larger than the original outside diameter. At the
junction of the bell 12 with the remainder of the tube 10, there
is a torus or ring 14 embossed into the tube, which provides a
definite shoulder which can seat against a tubing collar of a
tube sheet 16.
As shown in Fig. 3, a ring 18 of brazing wire, for
instance, an alloy of copper and phosphorus, is lodged within the
embossed ring 14. A male end 20 of an as~ociated connecting tube
member is inserted within the bell 16. The male end 20 here has
about the same outside diameter as the unenlarged outside
- 8 -
:::
.
' ` ,~ .' ' ' ' ' ' . ' " '
l~O~G~5
diameter of the tube 10. There i~ a very small clearance between
the inner wall of the bell 12 and the outer wall of the male end
20, 80 that when the bell 12 and ring 14 are heated aufflciently
to melt the brazing wire 18, the brazing materlal i9 drawn up
into thi~ clearance by capillary action. The resulting braze
joint i9 then strongest at the tip~of the male end 20, which
formerly had been the weakest point in other brazing operations.
A~ shown in Fig. 4, the unexpanded heat exchanger tube 10
has its bell 12 formed by means of a suitable tube enlarging
tool. In this example, this tool includes a collet 22 formed of
a number of finger~ 24 which radially surround an axially tapered
expansion pin 26. When the collet 22 i8 inserted into the open
end of the tube 10, the pin 26 is driven axially, thus wedging
the fingers 24 outward and expanding the tube material to form
the bell 12, as shown in Fig. 5. This technique, at least to the
extent described thus far, is generally described in U.S. Pat.
No. 4,584,765. According to this invention, a boss-forming ring
28 is disposed at the distal end of the collet fingers 24. This
ring 28 can either be comprised of rounded members formed on the
end of the fingers 24, or can be a ring of steel wire or a ring
of brazing metal carried on the fingers 24. When the fingers 24
are expanded, as shown in Fig. 5, the boss-forming ring 28 urges
the tube metal outward to emboss the torus or ring 14.
After this, the pin 26 is withdrawn and the collet 22
contracts, as in Fig. 4. The collet is withdrawn to leave the
bell generally as shown in Fig. 1.
There are, of course, other means available for belling
., .... ., ; .. , . ~..... .. . .
~ 605
tubing ends which could provide equivalent result~.
As aforementioned, with the bell-end ring or torus
structure of this invent~on, the process of expanding heat
exchanger tubes under ten~ion 18 described inltially with
reference to Fig. 6, which illustrate~ an existlng technique. In
this technique a heat exchanger tube 10' is provided with a bell
12', which is simply a gradually expanded end portion of the tube
10'. The tube 10' passes through a tube sheet 16, and through a
plurality of fin plates 30. A claw device 32 grips the bell 12'
of the tube. However, because of the notable lack of any
shoulder on the bell 12', it is difficult for the device 32 to
obtain a good purchase on the outside of the bell 12'.
Accordingly, extreme force must sometimes be used, which can
damage the tube 10'. The expansion proces~ is carried out by
lS ramming a "bullet" type expansion device 34 through the
unexpanded tube 10' 80 that its outer surface is expanded into
good thermal contact with the openings in the associated fin
plate~ 30. The bullet device 34 is driven or rammed through the
tube 10' by means of an expander rod 36. While the rod 36 is
driven through the tube 10', the claw 32 maintains tension on the
tube 10'. However, if the claw 32 cannot maintain its grip on
the tube bell 12', the bell can be pushed through the tube sheet
16, necessitating scrapping of the entire coil unit.
As illustrated in Fig. 7, in a tube expansion process
according to the present invention, the claw 32 can grab the tube
behind the embossment or torus 14. This provides a sure grip on
the tube 10' without the need for a great deal of force, and
. :
1~0~ 5
without requiring substantial standoff between the bell 12 and
the tube sheet 16.
As shown in Fiq. 8, lt is posslble ~or the tube expanslon
process to be carried out wlthout a claw device 32, that is, wlth
the tube sheet 16 being directly supported or held, and
transmitting tension directly to, the distal surface of the
embo6sment or torus 14. That is, in the process as illustrated
in Fig. 8, the shoulder of the ring 14 rests directly on the
collar or opening in the tube sheet 16. This mechanical support
of the embossment 14 prevents the tube 10 from being pulled
through the tube sheet opening when the expansion device 34 and
the rod 36 are being driven through the tube 10. With this
configuration, the tube 10 can be provided with a zero standoff
between the tube sheet 10 and the embossment 14. Thi~ permits
lS the use of shorter hairpin tubes, and achieves a greater burst
strength, and a more compact structure.
As shown in Fig. 9, the principles of this invention can
be achieved by an assembly 110 comprising a tube 111 of copper or
other suitable metal in combination with a union 112. The latter
is a cylindrical sleeve with an inner cylindrical surface to
receive tubes of a given outside diameter. A second tube 113 has
an end also received in the union 112. Here there is an annular
embossment 114 midway along the union to define an annular void
serving as means to hold a ring 118 of a wire of fusible metal,
such a~ a braze or a solder. The ring 118 also serves as means
to seat the ends of the two tubes 111 and 113 within the union
`: - 11 -
,
i30~ i05
In all of the above example~, the ring 14 can be of a
braze, a solder, or other fusible material as would be suitable
for the intended end use. Also instead of the embo~ed annulus
14 or 114, the copper tube could be staked or otherwise worXed to
provide means for mechanical retention of the braze or ~older
ring 18.
Thase joints which employ the principles of this
invention are not limited to heat exchanger tubes or joiQts
formed in heat exchanger tubes or joints formed in heat exchanger
fabrication. The invention can be applied to advantage in
joining other metal tubing where increased joint reliability and
ease of brazing or soldering would be desirable. This for
example could be in joining the compressor to the evaporator coil
or the condenser coil. The invention could al80 find
lS advantageous application outside of mechanical refrigeration or
air conditioning apparatus.
While the invention has been described with reference to
selected preferred embodiments, it should be understood that the
invention is not limited to the preferred embodiments, but that
many modifications and variations woul~ become apparent to those
of skill in the art without departing from the scope and spirit
of this invention, as defined in the appended claims.
