Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a heat exchanger, and particu-
larly to a heat exchanger having replaceable extended heat
e~change surfaces. The invention pertains to a heat exchanger of
the type having a plurality of tubes through which a heated
fluid, such as a liquid or vapor, flows and transfers heat to
extended heat exchange surfaces, such as a plurality of fins, in
thermal contact with the tubes. A cool fluid, such as airr flows
past the fins to remove heat from the extended heat exchange
surfaces.
In known heat exchangers of this type, the fins or other extended
heat exchange surfaces tend to deteriorate over time due to cor-
rosion or mechanical damage. Presently, it is a costly and time
consu~ing task to replace w~rn out heat exchange surfaces. One
aspect of the invention is to provide a replaceable heat exchange
structure for the heat exchanger, whieh may be easily and quickly
removed when it is worn out or when the thermal design of the de-
vice is revised; and replaced by a new structure. The structure
may comprise a surface matrix having a plurality of outwardly
extending fins which is held in thermal contact with the tubes of
the heat exchanger by adhesive or mechanical means or both.
Other disad~antages assoeiated with presently known heat
exchangers include the inefficient flow of the cooling fluid
along the extended heat exchange surfaces resulting in poor heat
transfer, especially when the eooling fluid is a gas; and
potentially damaging vibration eaused by the low velocity wake
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region formed at the downstream sides of the heat exchanger
tubes. Providing a more streamlined cross-sectional
configuration for the tubes of the heat exchanger to which the
heat exchange structure is conformed, results in a more efficient
transfer of heat to the cooling fluid and a reduction in
vibration.
Accordingly, the present invention provides a heat exchanger
comprising a plurality of tubes formed into a bundle, wherein two
or more tubes are arranged linearly side by ,side. A header
structure is positioned at one end of the tube bundle to direct a
flow of heated fluid into at least one tube of the bundle and to
receive a flow of cooled fluid exiting from at least one other
tube of the bundle. Means are located at the other end of the
tube bundle ~or directing the flow of fluid ~rom said one tube to
said other tube of the bundle. One or more heat exchange
structure is positionable about substantially the entire tube
bundle in thermal contact therewith.
The heat exchange structure has a surface matrix which is
provided with a plurality of outwardly extending fins or
receiving heat conducted from the tube bundle to the surface
matrix. The heat exchange structure is positioned about the tube
bundle to receive a flow of cooling fluid about the fins so that
heat ~ay be transferred from the fins to the cooling fluid.
Brief Description of the Drawings
Figure 1 is a perspective view partially broken away of a heat
exchanger in accordance with the invention;
Figures 2-4 are sectional views taken through line 2-2 in Figure
1 showing several preferred configurations of tube bundles in
accordance with the inven~ion;
Figure 5 is a perspective view of a heat exchanger structure
which is formed for attachment about a tube bundle; and
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Figure 6 is a cross-sectional view of a staggered tube bundle
arrangement.
As shown in Fig~ 1, a heat exchanger lO in accordance with the
invention comprises a plurality of tubes 11 arranged in a bundle
12 linearly side by side. The heat exchanger 10 may have several
such tube bundles 12 positioned adjacent one another so that the
tubes ll of each bundle 12 are parallel one another (Fig. 1) or
arranged in a staggered conEiguration.
The tube bundles 12 shown in the drawings each comprise ~our
tubes 11, but the number of tubes 11 forming each bundle 12 is a
matter of design choice. The heat exchanger lO is provided with
a header 15 at one end of the tube bundles 12. The header 15 has
an inlet l6 and an outlet 17. An internal partition 18 may be
provided within the header 15 to direct the flow of fluid from
lS the inlet 16 into a first set of tubes 21 of each bundle 12, and
from a second set of tubes 22 to the outlet 17. At the other end
of the tube bundles 12 means are provided for directing the flow
of fluid from the first set of tubes 21 to the second set of
tubes 22. Such means may comprise a return header ~5 as shown in
Fig. 1, or more direct means such as tubing formed into a U-bend
connecting a tube 11 from the first set 21 to a tube 11 of the
second set 22. The number of tube-pass for the flow of fluid
through the exchanger 10 is a matter of design choice.
Each tube bundle 12 is provided with a heat exchange structure
31. The heat exchange structure 31 preferably comprises a
surface matrix 32 which is provided with a plurality of outwardly
extending fins 33 (see Fig. 5). The heat exchange structure 31
is made of a heat conducting material such as metal so that heat
can be conducted from the tube bundle 12 to the fins 33 where
the heat may be transferred to a passing stream of a cooling
fluid such as air (arrow 39).
The basic heat exchange structure 31 is preferably formed as
shown in Fig. 5 to conform to the exterior surface contour of the
tube bundle 12. The formed heat exchange structure 31 may
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comprise a U-shaped sleeve 34 which may be slid over the tube
bundle 12 and attached thereto by means of a commercially
available heat conductive adhesive or cement 35 and mechanical
means such as a clip 36 at the open end 37 of the sleeve 34
(Fig. S). The heat exchange sleeve 34 is provided abou~ the tube
bundle t2 SQ that the maximum fin surface area is available to
the cooling fluid stream passing through the heat exchanger 10.
A principal advantage of providing each tube bundle 12 with a
heat exchange sleeve 34 as described, is tha~ if necessary, the
sleeve 34 can be removed and replaced with a new sleeve 34 on
site and with minimal shutdown time for the heat exchanger 10.
Thus, a sleeve 34 having fins 33 which have been damaged through
corrosion or mechanical action may be removed by releasing the
mechanical fasteners 36 and dissolving the conductive adhesive or
cement 35. A new sleeve 34 can be attached to the tube bundle 12
and the heat exchanger 10 can quickly be back in operation.
In another aspect of the invention, it has been found that the
efficiency of heat transfer from the fins 33 to the passing
cooling fluid stream may be enhanced by providing a tube cross
section of suitable fluid dynamic shape and by staggering the
alignment of adjacent tube bundles 12. Since the cooling fluid
is commonly air, and the convective heat transfer coefficient for
gases is generally one or two orders of magnitude lower than that
for liquids or vapors, this preferred feature of the invention is
particularly advantageous for heat exchangers in which heat is
transferred to a gas from a liquid or condensing vapor flowing
through the tubes 11 of the device. The selection of an optimum
profile for the assembled tube bundle 12 and heat exchange sleeve
34 can minimize the low velocity wake zone downstream of each
tube 11 and therefore, yield a higher heat transfer coefficient.
For example, the flow geometry of a fluid stream passing around a
tube 11, having a circular cross section is such that in the
~ownstream low velocity wake region, the heat transfer coef-
ficient is only about 10% of the overall value. In addition,
with a tube 11 having a circular cross section, this form drag
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accounts for 40 to 60~ of the downstream pressure drop.
The present invention provides an optimization of the flow
geometry of the cooling fluid passing along the finned surface of
the heat exchanger lO by wrapping a heat exchange sleeve 34 about
a tube b~ndle 12 so that the finned heat exchange structure 31
conforms to the curvature of the exterior surface of the tube
bundle 12. Thus, the heat exchange sleeve 34 is deformed about
the tube bundle 12 to provide a corrugated profile for the fins
33 (see Figs. 2-4). Referring to Fig. 3, the inefficient heat
transfer provided by a tube 11 having a circular cross section
may be significantly improved by utilizing the finned heat
exchange sleeve 34 to streamline the profile of the tubes 11 in
the bundle 12. The sleeve 34 is formed about the tubes 11 of the
bundle 12 with the aid of an interconnecting thermal conductivity
cement 35 so that the circular cross section of the tubes 11
shown in Fig. 3 are altered to an oval or tear drop shape having
more favorable drag characteristics.
Likewise in Fig. 2, the tubes 11 are shaped with circularly
oblong cross sections, and the tubes 11 shown in Fig. 4 are oval
in cross section. Contouring the heat exchange sleeves 34 about
these more favourable aerodynamic shapes results in higher heat
exchange eficiencies between the finned surfaces 33 and the
passing stream of cooling gas or other fluid. For example, the
form drag of an oval tube 11 as shown in Fig. 4 is only about 10
that of a circular tube~ Therefore, use of these and other
streamlined shapes for the tubes 11 of each bundle 12 will result
in an effective increase of the heat transfer surface of 10 to
20% or more and decrease of the downstream pressure drop.
Since most heat exchangers in accordance with the invention will
comprise a plurality of adjacent tube bundles 12, the flow
characteristics of the passing stream of cooling fluid may be
further influenced by arranging the adjacent tube bundles 12 in a
staggered as opposed to a parallel aligned configuration. Thus,
adjacent bundles 12 are offset from one another by about a half
tube pitch dimension while alternating tube bundles 12 are
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arranged in parallel (Fiq. 6). This staggered tube bundle
configuration is felt to be advantageous to give a more even
distribution of cooling fluid over the heat exchange finned
surfaces 33 of the entire heat exchanger 10 than is the case for
bundles 12 arranged in a parallel manner a5 ln Fig. 1.
A further disadvantage of the formation of a low pressure wake
region in the cooling gas downstream of a tube 11 is that the
drag forces cause the tube 11 to vibrate. In prior devices, this
vibration reduces the life of the tube 11, finned tubes and
ultimately the entire heat exchanger 10. The invention provides
for a great reduction in the amount of vibration caused by ~he
passing stream of cooling fluid by securing the individual tubes
11 into bundles 12 which are wrapped by the heat exchange sleeve
34. This composite structure, especially when streamlined as
described above, has a much better resistance to vibration than
is the case for conventional structures.
By way of example only, the heat exchange structures 31 shown in
Fig. 5 may be produced by hot or cold extrusion of metal sheets
or by metal casting techniques. Also, fins 33 may be attached to
a surface matrix 32 by welding, soldering, brazing, embedding or
gluing thereto. The fin densities are preferred to be in the
range of 100 to 800 fins/m with a fin thickness of 0~1 to 1 mm.
The fin height depends on the si2e of the tube 11 to which it is
attached, but typically fin heights range from S to 300 mm in
association with tubes of 5 to 300 mm diameter.
Clearly, the heat exchange structures 31 may be attached to the
tube bundles 12 in any of a number of ways while still achieving
the object of the invention of easy replacement of damaged fins
33. In addition to forming the structures 31 into sleeves 34 as
described above, a heat exchange structure 31 may be formed to
and affixed at each side of a tube bundle 12.
Other variations of the invention will be apparent to the person
skilled in this art. The scope o~ the invention including such
additional embodiments is defined in the following claimsO