Note: Descriptions are shown in the official language in which they were submitted.
BAKCGROUND OF THE INVENTION
This invention relates to heat transfer tubes and particularly
to tubes which have increased internal surface area as compared to plain
tube. U.S. Patents Z,960,114 and 3,696,863 each disclose a composite
assembly comprising a tube and an externally ribbed or finned insert corP
and discuss the desirability of providing such a core for enhancing the
internal heat transfer coefficient of a tube in certain refrigeration
applications. The earller of the aforesaid patents teaches the locklng
of the inside tube wall and the fins on the core in tight mechanlcal
relationship by means of longitudinal flutes formed in the smooth external
pipe surface in the region between adjacent pairs of internal fins. The
later patent teaches that a rlbbed or finned insert core can be placed in-
side an outer tube which has been previously externally finned. The coreis then mechanically bonded to the previously flnned outer tube by grooving
the annular, transversely arranged external fins in a general longitudinal
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direction at a plurality of locations around the circumference
of the tube. The grooving serves to divide each annular fin into
a plurality of separate fins in a common plane. `
SUMMARY
It is a~ong the objects of the present invention to
provide an improved, externally finned heat transfer tube and
a method of making same wherein a tube having an outer smooth
surface is finned and mechanically locked to a coaxially positioned
- finned insert in a single operation using conventional finning
equipment.
In the preferred method of the present invention, a
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finned insert, usually formed of aluminum, is loosely placed inside
a length of tubing, usually copper. Thé tubing is then finned in
a known manner by fin forming tools, such as the disc type tools
disclosed in U.S. Patent 3,383,893. Since the ribs ~r fins of
the insert support the inner tube wall at a plurality of locations,
the mandrel pin which is normally used during tube finning to prevent
the tube wall from collapsîng can be omitted. The radial inward
pressures applied to the tube by the finning discs serve to reduce
the internal diameter of the tube and force its inner wall into
contact with the outer tips of the ribs or fins on the insert.
Preferably, the finning pressures are sufficient to cause the tips
of the insert ribs to become partially embedded in the inner wall
of the tu~e. The tube is also lengthened during the finning process.
The present învention is defined as a
method of making a heat transfer tube having inner and outer fins
comprising the steps of inserting a core member having a plurality
~- of radially extending longitudinal fins inside a smooth surface
` cylindrical tube and subjecting at least a portion of the outside
surface of the composite assembly to a plurality of finning tools
so as to produce external fins on the tube and reduce its internal
diameter to a dimension less than the external diameter of the finned
core member.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a fragmentary perspective view of a tube
in acco~dance with the present invention;
Fig. 2 is an end cross sectional view taken on line 2-2 of
Fig. l;
Fig.3 is an end view of an insert memberi
Fig. 4 is an end view of a tube member;
Fig. 5 is a partially broken away perspective view showing the
insert of Fig. 3 and the tube of Fig. 4 after they have been telescopically
lo assembled to each other;
Fig. 6 is a partially broken away perspective view of a finning
machine (with one arbor removed for clarity) which can be used to externally
fin the assembly of Fig. 5; and
Fig. 7 is a side view showing the insert and tube assembly of
Fig. 5 following its partial passage to the right through the finning
- apparatus of Fig. 6.
DETAILED DESCRIPTION
- Fig. 1 shows an improved heat transfer tube, in particular a
chiller tube, made in accordance with the present invention. ~he improved
tube indicated generally at 10 comprises an insert portion indicated gen-
erally at 12 (Fig. 3) which is preferably formed of an aluminum alloy such
as type 6063 and extruded into a shape comprising a core portion 14 having
a plurality of ribs or fins 16 extending radially outwardly therefrom. Sur-
rounding the insert member 12 is a tubular member indicated generally at
18 (Fig. 4) which initially has a smooth outer surface 20 and a smooth
inner surface 22. To produce the finished finned tube 10, the insert mem-
ber 12 is placed inside the tube 18 so as to be spaced from the ends of the
tube 18 as shown in Fig. 5. The assembly of Fig. 5 is then placed in a
finning apparatus such as the three arbor structure disclosed in the
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aforementioned Patent No. 3,383,893 or in the four arbor apparatus 33 (one
arbor has been omitted for clarity) shown in Fig. 6.
The finning apparatus 33 includes two sets of variable diameter
finning discs. The final sets of discs 36 are positioned outboard of the
skewed axis arbors 38, 40 and 42 while the initial sets of discs 46 are
positioned inboard of the final discs 36 on the same arbors. The arbors are
positioned on a plurality of cam arms 48 for movement toward and away from
the axis of the tube 18. The assembly shown in Fig. S is transformed into
the shape shown in Fig. 1 by being fed from left to right through the appa-
ratus 33. In order to provide unfinned plane end portions 18', 18" on
the tube, as shown in Fig. 7, the finning discs 36, 46 are not moved into
the tube 18 until a suitable length of plane end tube 18' is moved past
them. At this point, the final discs 36 are brought slowly into contact
with the work until they are at their final depth of penetration. From
this point onward the initial finning discs 46 will proceed to engage the
smooth tube portions proceeding from the left. When fins have been formed
to the desired axial length the cam arms 48 are rotated in order to cause
the finning discs 36, 46 to withdraw from the work piece and leave the
; trailing end of the tubes 18" with a smooth unfinned surface.
Although finning ls usually done with a mandrel bar inslde the
tube to resist the tremendous forces applied by the finning discs we have
found that it is possible to utilize the insert member 12 as a mandrel.
Since the insert 12 can only resist forces applied radially inwardly along
its ribs 16, the inner tube wall 22' tends to assume a straight profile
between adjacent fins 16. Naturally, the outside cross section of the
heat exchange member 10 also tends to assume a similar shape so that it has
a somewhat polygonal configuration. Preferably, the fins 30 are formed to
a sufficient depth to cause enough pressure to be applied to the tube 18
to cause its internal diameter to be reduced to a dimension smaller than
the outer diameter of the fin tips 16' on the insert member 12. This
diameter reduction causes the tube wall to move radially inwardly of the
fin tips sufficiently to cause the fin tips 16' to be embedded in the inner
tube wall 22'. This embedment provides a very firm locking of the insert to
the tube and enhances the transfer of heat between the insert and the tube.
The forces produced by the finning operation normally induces a
twist in the tube being finned. Naturally, the twisting of the tube will
cause a generally identical twisting of the insert member 12. Depending
upon the amount of twist or lack thereof desired in the insert portion of
the final product, it is possible to pretwist the insert in one direction
or another or leave it straight before it is assembled inside the tube.
From the preceding description one can appreciate that the method
of our invention enables a finned insert tube to be made in a single finning
operation rather than require the use of two operations as in the afore-
mentioned prior art patent wherein the tube was finned in a first operation
and then additional grooving operations were necessary to provide a locking
of the insert to the tube. Furthermore, our one-step operation is less
costly and can be done on conventional finning equipment rather than require
additional equipment. Finally, the continuous pressure applied during fin-
ning causes a much firmer engagement between the tube and insert fin than
is possible by longitudinally grooving an already finned tube. This tighter
bonding enhances heat transfer from the ribs of the insert to the metal
body of the tube and thus increases the overall efficiency of the tube.
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