U.S. patent application number 16/478079 was filed with the patent office on 2020-10-08 for heat exchangers having brazed tube-to-fin joints and methods of producing the same.
The applicant listed for this patent is HEAT TRANSFER TECHNOLOGIES. Invention is credited to Daniel BACELLAR, John R.H. BLACK, Cara S. MARTIN, Dennis M. NASUTA, Reinhard RADERMACHER, Yoram L. SHABTAY.
Application Number | 20200318911 16/478079 |
Document ID | / |
Family ID | 1000004925741 |
Filed Date | 2020-10-08 |
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United States Patent
Application |
20200318911 |
Kind Code |
A1 |
SHABTAY; Yoram L. ; et
al. |
October 8, 2020 |
HEAT EXCHANGERS HAVING BRAZED TUBE-TO-FIN JOINTS AND METHODS OF
PRODUCING THE SAME
Abstract
Heat exchangers and methods of producing thereof having fins
with slots formed therethrough, and a continuous tube having
parallel tube runs connected by reverse bends to define a
serpentine coil that traverses back and forth through the slots
formed in the fins. Each fin has surface enhancements and is
metallurgically joined to corresponding portions of the tube at the
slots with brazed joints therebetween.
Inventors: |
SHABTAY; Yoram L.; (Prospect
Heights, IL) ; BACELLAR; Daniel; (Silver Spring,
MD) ; MARTIN; Cara S.; (Columbus, MD) ;
NASUTA; Dennis M.; (Washington, DC) ; RADERMACHER;
Reinhard; (Silver Spring, MD) ; BLACK; John R.H.;
(Barrington, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HEAT TRANSFER TECHNOLOGIES |
Prospect Heights |
IL |
US |
|
|
Family ID: |
1000004925741 |
Appl. No.: |
16/478079 |
Filed: |
December 14, 2018 |
PCT Filed: |
December 14, 2018 |
PCT NO: |
PCT/US2018/065743 |
371 Date: |
July 15, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62599047 |
Dec 15, 2017 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28D 1/0477 20130101;
F28F 1/325 20130101 |
International
Class: |
F28D 1/047 20060101
F28D001/047; F28F 1/32 20060101 F28F001/32 |
Claims
1. A heat exchanger comprising: fins having dogbone-shaped slots
formed therethrough, each of the slots defining one or more
circular portions interconnected with and intersected by a
rectilinear portion that has a width that is narrower than a
diameter of the circular portions, each of the circular portions
having an incomplete circular perimeter and a collar bordering the
incomplete circular perimeter; at least one tube having reverse
bends forming at least two parallel tube runs to define a
serpentine coil that traverses back and forth through the slots
formed in the fins; and surface enhancements located between the
slots thereof and located along the rectilinear portions thereof;
wherein the collars of the slots are metallurgically joined to
corresponding portions of the tube with brazed joints.
2. The heat exchanger of claim 1, wherein each of the slots defines
an entire dogbone shape comprising a pair of the circular portions
interconnected with and intersected by the rectilinear portion
therebetween.
3. The heat exchanger of claim 1, wherein each of the collars has a
reflare portion that defines a surface that bends away from a
central axis of the collar and the circular portion bordered
thereby.
4. The heat exchanger of claim 1, wherein each of the collars
defines a gap between the incomplete circular perimeter thereof and
the corresponding portion of the tube and the gap is filled by one
of the brazed joints.
5. The heat exchanger of claim 1, wherein each of the slots has
only one of the circular portions and the rectilinear portion
defines an opening at an edge of the fin.
6. The heat exchanger of claim 5, wherein each of the fins
comprises a second slot having a single circular portion and a
single rectilinear portion that defines an opening at the edge of
the fin.
7. The heat exchanger of claim 5, wherein each of the collars has a
reflare portion that defines a surface that bends away from a
central axis of the collar and the circular portion bordered
thereby.
8. The heat exchanger of claim 5, wherein each of the collars
defines a gap between the incomplete circular perimeter thereof and
the corresponding portion of the tube and the gap is filled by one
of the brazed joints.
9. The heat exchanger of claim 1, wherein the tube includes at
least two adjacent circuits each having a first connection
configured to feed a fluid into the circuit and a second connection
configured to discharge the fluid from the circuit, wherein at
least one of the first and second connections is shared by the at
least two adjacent circuits to define a split connection.
10. The heat exchanger of claim 9, wherein the tube is a continuous
tube that defines the serpentine coil in its entirety, and the
split connection is located at a corresponding one of the reverse
bends in the tube.
11. The heat exchanger of claim 9, wherein the tube comprises a
plurality of continuous tubes that define the serpentine coil.
12. A method of producing a heat exchanger, the method comprising:
providing fins having dogbone-shaped slots formed therethrough,
each of the slots defining one or more circular portions
interconnected with and intersected by a rectilinear portion that
has a width that is narrower than a diameter of the circular
portions, each of the circular portions having an incomplete
circular perimeter and a collar bordering the incomplete circular
perimeter, the fins having surface enhancements located between the
slots thereof and located along the rectilinear portions thereof;
forming at least one tube having reverse bends and at least two
parallel tube runs to define a serpentine coil; assembling the fins
and the tube such that the tube traverses back and forth through
the slots formed in the fins, the assembling of the fins and the
tube including inserting each of the reverse bends through a
corresponding one of the slots; and performing a brazing operation
on the fins and the tube such that a braze material clad on at
least one of the tube and fins melts and the collars of the slots
are metallurgically joined to corresponding portions of the tube
with brazed joints formed by the braze material.
13. The method of claim 12, wherein each of the slots defines an
entire dogbone shape comprising a pair of the circular portions
interconnected with and intersected by the rectilinear portion
therebetween.
14. The method of claim 12, wherein each of the collars defines a
gap between the incomplete circular perimeter thereof and the
corresponding portion of the tube, and the brazing operation causes
the brazed material to fill the gap.
15. The method of claim 12, wherein each of the collars has a
reflare portion that defines a surface that bends away from a
central axis of the collar and the circular portion bordered
thereby.
16. The method of claim 12, the method further comprising stacking
the fins prior to the assembling of the fins and the tube such that
at least some of the fins contact corresponding collars of the fins
adjacent thereto.
17. The method of claim 12, further comprising providing a hole in
at least a first of the reverse bends and providing a connection
fluidically coupled to the hole configured to either feed a fluid
into or discharge the fluid from a pair of circuits of the tube
coupled to opposite ends of the first reverse bend.
18. The method of claim 17, wherein the hole is formed in the first
reverse bend after the first reverse bend is formed.
19. The method of claim 17, wherein the hole is formed in the first
reverse bend before the first reverse bend is formed.
20. The method of claim 17, wherein the hole has an elliptical
shape before the first reverse bend is formed and a circular shape
after the first reverse bend is formed.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention generally relates to brazing and heat
exchanger technologies. The invention encompasses methods of
producing heat exchangers by brazing serpentine round tubes to a
plurality of fins.
[0002] Conventional round tube and plate fin heat exchangers of the
type commonly found in air conditioning, and conventional
serpentine round tube and plate fin heat exchangers of the type
commonly used in household refrigerator evaporators, generally
include tubes and fins joined only by mechanical interference fit
tube-to-fin joint ("mechanical joints"). This often results in
relatively weak tube-to-fin joints which lead to common
shortcomings including an overall weak structure, limits to fin
density, limits on enhancements to fins, limits on heat exchanger
size, and poor surface contact resulting in high thermal
resistance. In addition, conventional air conditioning type
tube-fin heat exchangers using hairpin-shaped tubes ("hairpins")
and U-shaped return bends ("U" bends) have a relatively large
number of tube brazing joints between tube sections resulting in a
high potential for refrigerant leakage. Direct refrigerant leakage
from HVAC&R systems constitutes a significant quantity of
greenhouse gas emissions. Serpentine round tube heat exchangers
using "dogbone" type plate fins (fins with elongated
"dogbone"-shaped slots) have a relatively small number of tube
brazed joints but lack fin enhancements and require mechanical
joints, and cannot be used in air conditioning and larger
refrigeration equipment, for example, due to weak structure and
poor performance.
[0003] In view of the above, it can be appreciated that there are
certain problems, shortcomings or disadvantages associated with the
prior art, and that it would be desirable if an improved heat
exchanger were available that was capable of at least partly
overcoming or avoiding these problems, shortcomings or
disadvantages.
BRIEF DESCRIPTION OF THE INVENTION
[0004] The present invention provides heat exchangers and method of
producing the same that include brazed tube-to-fin joints with a
significantly reduced number of tube brazed joints, increased
strength, and increased fin contact area relative to conventional
heat exchangers with mechanical interference-fit tube-to-fin
joints.
[0005] According to one aspect of the invention, a heat exchanger
is provided that includes fins having "dogbone"-shaped slots formed
therethrough that each defines one or more circular portions
interconnected with and intersected by a rectilinear portion that
has a width that is narrower than a diameter of the circular
portions. Each circular portion has an incomplete circular
perimeter and a collar bordering the incomplete circular perimeter.
At least one tube is provided having reverse bends forming at least
two parallel tube runs to define a serpentine coil. The tube
traverses back and forth through the slots formed in the fins. The
perimeters of the collars are metallurgically joined to
corresponding portions of the tube with brazed joints, and the fins
each comprise surface enhancements located between the slots
thereof and located along the rectilinear portions thereof.
[0006] According to another aspect of the invention, a method of
producing a heat exchanger includes the steps of providing fins
having "dogbone"-shaped slots formed therethrough such that each
slot defines a one or more circular portions interconnected with
and intersected by a rectilinear portion therebetween that has a
width that is narrower than a diameter of the circular portions.
Each circular portion has an incomplete circular perimeter and a
collar bordering the incomplete circular perimeter. The fins each
comprise surface enhancements located between the slots thereof and
located along the rectilinear portions thereof. The fins are
assembled with at least one tube having reverse bends forming at
least two parallel tube runs to define a serpentine coil such that
the tube traverses back and forth through the slots formed in the
fins, wherein either the fins or the tube is formed of a material
clad with a braze material, and then performing a brazing operation
on the fins and the tube such that the collars are metallurgically
joined to corresponding portions of the tube with brazed joints
formed by the braze material.
[0007] Another aspect of the invention is where a heat exchanger as
described above comprises a hole in at least a first of the reverse
bends and a connection fluidically coupled to the hole is
configured to either feed a fluid into or discharge the fluid from
a pair of circuits of the tube coupled to opposite ends of the
first reverse bend.
[0008] Technical effects of heat exchangers and methods of
producing the same as described above preferably include the
ability to provide heat exchangers with improved tube-to-fin joint
strength by utilizing a continuous serpentine tube-type heat
exchanger with fins having surface enhancements that can perform as
well as or better than current state-of-the-art HVAC&R,
hairpin-type tube or serpentine type tube heat exchangers. Such
high-performance serpentine tube-type heat exchangers have brazed
tube to fin joints and a significantly reduced number of tube
brazed joints, resulting in increased heat exchanger strength and
reduced risk of refrigerant leakage. Such a design has the
potential to reduce the number of brazed joints by more than 70%
while maintaining thermal performance comparable to conventional
heat exchangers with mechanical joints.
[0009] Other aspects and advantages of this invention will be
appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For convenience, consistent reference numbers are used
throughout the drawings to identify the same or functionally
equivalent elements, but with an alphabetical suffix (A, B, or C,
etc.) added to distinguish the particular embodiment from other
embodiments of the invention.
[0011] FIGS. 1 and 2 schematically represent nonlimiting first and
second embodiments of heat exchangers comprising one or more
serpentine tubes in accordance with certain aspects of the
invention.
[0012] FIG. 3A schematically represents fluid flow in a prior art
heat exchanger, FIG. 3B schematically represents fluid flow in a
heat exchanger that utilizes a serpentine tube in accordance with a
nonlimiting aspect of the invention, and FIG. 3C represents a
detailed view of an optional split adapter used in the construction
of the heat exchanger configuration of FIG. 3B.
[0013] FIGS. 4A through 4D represent methods of producing holes in
round bends of a serpentine tube in accordance with certain aspects
of the invention.
[0014] FIG. 5A represents locations of multiple brazed joints in a
prior art non-continuous "hairpin" tube type heat exchanger. Each
hairpin has a mating return "U" bend resulting in two brazed
connections for each hairpin. FIG. 5B represents a heat exchanger
that utilizes a continuous serpentine tube having brazed joints at
only the inlet and outlet thereof in accordance with a nonlimiting
aspect of the invention.
[0015] FIGS. 6A and 6B schematically represent serpentine tubes
having different degrees of orientation in accordance with
nonlimiting aspects of the invention.
[0016] FIG. 7 represents tube flow configurations for heat
exchangers equipped with serpentine tubes in accordance with
nonlimiting aspects of the invention.
[0017] FIG. 8 represents partial views of two fins having surface
enhancements and different types of dogbone-type slots formed
therein for receiving a serpentine tube in accordance with
nonlimiting aspects of the invention.
[0018] FIG. 9 schematically represents plan and cross-sectional
views of a portion of either fin of FIG. 8 and a collar formed on
the fin to surround a circular portion of the slot in the fin.
[0019] FIGS. 10A and 10B represent methods of assembling serpentine
tubes with fins having the types of slots represented in FIG.
8.
[0020] FIG. 11 shows images of brazed tube-to-fin joints of a
serpentine heat exchanger produced using a brazing method in
accordance with certain aspects of the invention.
[0021] FIG. 12 shows images of mechanical interference-fit
tube-to-fin joints of a prior art non-brazed serpentine heat
exchanger.
[0022] FIG. 13A represents partial plan and end views of a prior
art fin configured for receiving an individual round tube of a
prior art heat exchanger, and FIGS. 13B and 13C represent partial
plan and end views of two nonlimiting enhanced fins for receiving a
portion of a serpentine tube in accordance with nonlimiting aspects
of the invention.
[0023] FIG. 14 represents plan and end views of a fin with surface
enhancements having a non-limiting circular portion with a
100-degree perimeter in accordance with another nonlimiting aspects
of the invention.
[0024] FIG. 15 is a graph plotting simulation results of a
continuous serpentine tube-type heat exchanger of the type
represented in FIGS. 1 and 2, compared to a baseline air
conditioning heat exchanger conventionally formed with
hairpin-style tubes and fins with surface enhancements.
DETAILED DESCRIPTION OF THE INVENTION
[0025] FIG. 5A schematically represents typical locations of
multiple brazed joints in a noncontinuous serpentine tube 14A of a
prior art non-continuous "hairpin" round tube-type heat exchanger.
The serpentine tube 14A comprises hairpins 46 and U-shaped tube
sections (U-bends) 48. Each hairpin tube 46 is inserted into a
stack of fins (not shown) with round holes and collars. The tubes
46 are then mechanically expanded to interface and contact the
collars. To complete the liquid circuitry, the U-bends 48 are
brazed to ends of each hairpin tube 46 to form multiple brazed
joints 47 as shown. Each return U-bend 48 results in two brazed
joints 47. FIG. 5B represents a single continuous serpentine tube
14B capable of use in heat exchangers in accordance with
nonlimiting aspects of this invention. In contrast to the
non-continuous serpentine tube 14A produced from the hairpin tubes
46 of FIG. 5A, the continuous serpentine tube 14B of FIG. 5B only
requires brazed joints 17 at its inlet and outlet. Various
embodiments of the invention will be described below as
incorporating a continuous serpentine tube of the type represented
in FIG. 5B, in part because the ability to reduce the number of
brazed joints 17 reduces the likelihood of refrigerant leaks.
[0026] FIGS. 1 and 2 schematically represent nonlimiting examples
of heat exchangers 10A and 10B that utilize aspects of the
continuous serpentine tube 14B of FIG. 5B. Each heat exchanger 10A
and 10B comprises one or more round continuous serpentine tubes 14,
each having multiple reverse (180 degree or U-shaped) bends
(elbows) 16 and parallel straight tube runs 18. In the nonlimiting
embodiment of FIG. 1, multiple continuous serpentine tubes 14 are
used to define multiple discrete fluid circuits of the heat
exchanger 10A, and the tubes 14 are connected to define what may be
referred to as a serpentine coil. In the nonlimiting embodiment
shown in FIG. 2, a reduced number of brazed joints is achieved with
a single continuous serpentine tube 14 that defines a serpentine
coil and multiple discrete fluid circuits of the heat exchanger
10B. In large heat exchangers such as found in large AC or
refrigeration units, for example, the multiple discrete fluid
circuits of the heat exchangers 10A and 10B offer the advantages of
additional circuitry connections and reduced pressure drop as
compared to a long continuous serpentine tube of the type
represented in FIG. 5B. In both embodiments, the tubes 14 traverse
back and forth through series of slots (not shown) formed in a
plurality of parallel fins 12, and each fin 12 is metallurgically
joined with a brazing material to corresponding tube runs 18 of the
tubes 14 to define brazed joints at the slots of the fin 12. The
heat exchangers 10A and 10B are configured to transfer heat between
a fluid (hereinafter referred to as a liquid as a matter of
convenience) flowing through their respective coils with another
fluid (hereinafter referred to as a gas as a matter of convenience)
flowing through the stack of fins 12 (for example, in a direction
perpendicular to the plane of the images of FIGS. 1 and 2) for the
purpose of heating or cooling the liquid and/or gas. The flow
directions of a liquid through the tubes 14 are represented with
arrows in FIGS. 1 and 2.
[0027] The continuous serpentine tube-type heat exchangers 10A and
10B of FIGS. 1 and 2 may be assembled by inserting a complete
serpentine tube 14 into a stack of fins 12 each having elongated
"dogbone" shaped slots (i.e., slots having a pair of generally
circular portions and a rectilinear portion therebetween that is
narrower than the circular portions). In this case, the leading
edge of the serpentine tube 14 is narrowed (e.g., flattened
slightly) to allow the serpentine tube 14 to be inserted through
the slots in the fins 12. As discussed in greater detail below, the
fins 12 of the continuous serpentine tube-type heat exchangers 10A
and 10B are also formed to have surface enhancements at locations
along and between their dogbone slots to promote the heat transfer
efficiency of the heat exchangers 10A and 10B.
[0028] As described in more detail below, the heat exchangers 10A
and 10B comprise certain features and aspects that are believed to
provide comparable or improved performance relative to conventional
heat exchangers that have serpentine round tubes and non-enhanced
fins, or have hairpin-tubes with enhanced fins that are joined
mechanically to yield what are referred to herein as mechanical
interference-fit tube-to-fin joints, or more simply mechanical
joints. Exemplary improvements are represented in FIG. 15, which
shows simulation results of a continuous serpentine tube-type heat
exchanger of the type described above in reference to FIGS. 1 and
2, compared to a baseline air conditioning heat exchanger
conventionally formed with hairpin-style tubes and fins with
surface enhancements. The heat exchangers 10A and 10B may provide
improvements for a wide variety of applications including air
conditioning, refrigeration, heat pumps, and other devices that use
a heat exchanger to transfer heat between a gas and a liquid.
[0029] FIG. 1 represents an example of a joining configuration for
serpentine tube heat exchangers where parallel circuitry is desired
in order to reduce liquid pressure drop, wherein each tube 14 of
the serpentine coil individually has a connection for receiving the
liquid and a connection for discharging the liquid from the
discrete fluid circuit defined by the tube 14. The heat exchanger
of FIG. 2 represents an example of advantageously reducing the
number of brazed joints as compared to the heat exchanger 10A of
FIG. 1 by using a split joining configuration wherein multiple
circuits (in FIG. 2, pairs of adjacent circuits) receive the liquid
through a shared split connection 19A and multiple circuits (in
FIG. 2, one pair of adjacent circuits) discharge the liquid through
a shared split connection 19B, thus resulting in fewer overall
feeding/discharging connections required. The split joining
configuration of FIG. 2 provides for a significant reduction in
joints per circuit and a potential reduction in refrigerant
pressure drop and further reduced risk of refrigerant leakage.
[0030] FIG. 3A schematically represents a row of prior art hairpins
46 prior to their assembly and brazing to return "U-bends" to yield
a discontinuous serpentine tube with multiple braze joints. In
contrast, FIGS. 3B and 3C schematically represent a continuous
serpentine tube 40 with adjacent bends, one of which is equipped
with a straight tube section 42 forming a split joining
configuration that can function as an inlet or outlet (indicated as
an outlet in FIG. 3B). FIG. 3C schematically represents an isolated
view of a bend of the serpentine tube 40 that was produced to
include the straight tube section 42 during investigations leading
to the present invention. To accept the tube section 42, a hole was
formed in the bend via electrical discharge machining (EDM) and
then expanded to form a collar (not shown) to accept the tube
section 42. The tube section 42 was then partially inserted into
the hole so that the collar firmly held the tube section 42 in
place prior to brazing.
[0031] As the method of FIGS. 3B and 3C effectively forms a split
connection, the technique can also be used to form the split
connections of a continuous serpentine tube similar to that of FIG.
2. That is, EDM may be used to provide a hole directly in one or
more of the bends 16 of the continuous serpentine coil, and the
holes may undergo additional forming to produce collars into which
straight tube sections 42 may be inserted. The straight tube
sections 42 and bends 16 are then metallurgically joined with
brazed joints formed during a subsequent brazing process.
[0032] Manufacturing an integral serpentine tube 14 for the split
joining configuration of FIG. 2 may be a challenge with respect to
tools, manufacturing order, shape, and sizes. Conventional drilling
is preferably avoided due to the risk of depositing metal dust and
debris inside the tube 14. Therefore, techniques such as
puncturing, piercing, and EDM cutting are preferably used. FIGS. 4A
through 4D represent four methods of providing holes in a bend of a
serpentine tube (such as the U-shaped bends 16 of the tube 14 in
FIG. 2). FIG. 4A depicts a first method in which partial holes are
cut in edges of a sheet before rolling the sheet into a tube, which
can then undergo bending to form a U-shaped bend. FIG. 4B depicts a
second method in which a hole is punched in a tube after the tube
has been bent to form a U-shaped bend. Preferably, the bend is
punctured to form the hole after the tube has been brazed to fins
to avoid hole tapping with any cladding material on surfaces of the
tube 14. A particular challenge for this method is having a tool
that fits within the confined space between the inside of the bend
and the fins. FIG. 4C depicts a third method in which a round hole
is punched in a straight round tube before the tube is bent to form
a U-shaped bend. Puncturing the tube is relatively easy, but the
round hole becomes elliptical once the tube is bent to form the
bend. Additionally, hole locations need to be precisely calculated
to match the coil design. FIG. 4D depicts a fourth method in which
an elliptical hole is punched in a straight round tube before the
tube is bent to form a U-shaped bend, and the hole acquires a
rounder shape when the tube is bent to form the bend.
[0033] The previously described serpentine tubes 14 (FIGS. 1 and
2), 40 (FIGS. 3B and 3C), and 14B (FIG. 5B) define serpentine coils
that essentially entirely lie in a single plane, such that adjacent
bends 16 of the coil are oriented at an angle .theta. of 0 degrees,
as represented by the tube 14 in FIG. 6B. Alternatively, adjacent
bends 16 of the serpentine coil may be oriented at a corrugation
angle .theta. other than 0 degrees (slanted) such that the coil
"zigzags" back and forth between what can be described as separate
banks of tubes that lie in separate but parallel planes, as
represented by the tube 14 in FIG. 6A. In the example of FIG. 6B,
dogbone slots formed in fins to be assembled with the tube 14 are
parallel to the longitudinal axis of each fin. In the example of
FIG. 6A, dogbone slots formed in fins to be assembled with the tube
14 are inclined at the corrugation angle .theta. to the
longitudinal axis of each fin. The heat exchangers 10A and 10B may
include one or more banks of tubes. For a heat exchange comprising
multiple banks of tubes, either or both coil configurations of
FIGS. 6A and 6B may be used. As represented in FIG. 7, the coil
configuration of FIG. 6B can be utilized in full-cross counter flow
heat exchangers, while the coil configuration of FIG. 6A can be
utilized in semi-cross parallel or counter flow heat
exchangers.
[0034] As noted above, slots are formed in the fins 12 to receive
the serpentine tubes 14 or tube runs 18 of the heat exchangers 10A
and 10B. The method of assembly will be affected by both the type
of serpentine coil used (e.g., FIG. 6A or 6B) and the type of slots
provided in the fins 12. According to a preferred aspect of the
invention, FIG. 8 represents two nonlimiting types of slots (Type 1
and Type 2) that may be formed in the fins 12 of the heat
exchangers 10A and 10B to receive their serpentine tubes 14. The
fin 12 identified as Type 1 has a "dogbone" slot 22 aligned
parallel to the longitudinal length of the fin 12 and defined by a
pair of generally circular portions 24 intersected and
interconnected by an intermediate rectilinear portion 26
therebetween that is narrower than the circular portions 24. The
fin 12 identified as Type 2 has partial "dogbone" slots 23 each
aligned perpendicular to the longitudinal length of the fin 12 and
defined by a single circular portion 24 intersected by a
rectilinear portion 27 that is narrower than the circular portion
24 and defines an opening at a downstream edge of the fin 12. As a
result of intersecting the rectilinear portion 26 or 27, the
circular portions 24 of the slots 22 and 23 do not have complete
circular perimeters, but instead each circular portion 24 has an
incomplete circular perimeter that spans an arc of greater than 180
degrees. According to a preferred aspect of the invention, the
circular sections 24 are not required to form mechanical
interference-fit joints with the tubes 14 with which they are
assembled. Instead, and as schematically represented in FIG. 9, the
circular portions 24 of the slots 22 and 23 can be sized to define
a radial gap 33 between the tube 14 and fin 12 of greater than 0 mm
up to about 0.1 mm, for example, from about 0.05 to 0.1 mm, which
must be bridged by a brazed joint. In another embodiment, a Type 2
fin slot 23 may be complete and similar to a Type 1 slot 22. In
such embodiment, the Type 2 fin is similar to Type 1 except that
the slot orientation is perpendicular to the Type 1 slot.
[0035] As also represented in FIG. 9, a collar 32 borders at least
a portion of, and preferably the entire, incomplete circular
perimeter of each circular portion 24 of the slots 22 and 23, so
that an inner surface of the collar 32, and not just the inner edge
of the circular portion 24, faces the outer circumference of the
tube 14 and is available as a heat transfer path between the tube
14 and fin 12. The collars 32 can be formed by material of the fin
12 that has been bent away from adjoining surfaces of the fin 12.
As represented in FIG. 9, the collars 32 may be preferably formed
to have a reflare section 34, defined herein as a surface that
bends away from the central axis of the collar 32 and the circular
portion 24 it borders. The reflare sections 34 of the collars 32
allow the fins 12 to be stacked one against the other without the
fins 12 telescoping into each other, which allows for the assembly
of heat exchangers 10A and 10B with increased fin densities
relative to, for example, conventional refrigerator low-fin density
evaporators using serpentine tube designs.
[0036] FIG. 10A schematically represents the manner in which a
serpentine tube 14 can be inserted into a stack of the Type 1 fins
12 of FIG. 8 in the transverse (z-axis) direction of the fins 12.
This insertion method is essentially the same as would be employed
to assemble a serpentine tube with a stack of conventional fins.
However, the interference fit required to create the mechanical
joints between a tube and conventional fins requires a relatively
high force to insert the tube through a stack of fins. This
necessitates the use of fins that are stiff, which discourages the
presence of surface enhancements on the fins. In contrast, the gap
33 between the fins 12 and tube 14 of this invention reduce the
force required to insert the tube 14 into a stack of the Type 1
fins 12 of FIG. 8. As a result, the fins 12 can be equipped with
surface enhancements 36 capable of significantly increasing the
thermal efficiency of the fins 12. Investigations leading up to the
present invention showed that gap 33 between the fins 12 and a tube
14 of up to about 0.1 mm can be easily bridged by a braze
material.
[0037] For the Type 2 fin 12 of FIG. 8, the same assembly technique
cannot be performed without modifications to the serpentine tube
14. FIG. 10B represents two methods of inserting serpentine tubes
14 into a Type 2 fin 12 of FIG. 8. One method involves lateral
insertion of the tube 14 in the x-axis direction of each fin 12,
and the other method involves bending the series of the bends 12 on
one end of the tube 14 so that it can move through the slots 23 in
the transverse (z-axis) direction of each fin 12. In these figures,
solid white arrows indicate the directions of tube insertion. The
serpentine tube lead area is slightly flattened to allow the tube
14 to penetrate the "dogbone" shape slot 23. For example, the tube
leading bends in FIG. 10A are slightly flattened to allow
penetration of the serpentine tube 14 through the rectilinear
portion 27 of the slot 23.
[0038] Once the tubes 14 and fins 12 have been assembled and prior
to brazing, the entire heat exchanger assembly may be de-greased to
remove process oils if needed. Then the entire heat exchanger is
placed in a furnace and heated at appropriate temperatures and
durations to form complete brazed joints between the tubes 14 and
fins 12. The components of the heat exchangers 10A and 10B may be
formed of various materials suitable for brazing and producing
brazed joints between the tubes 14 and fins 12. For example, either
the tubes 14 or fins 12 may be formed of a material having a
cladding material thereon that contains a source of the braze
material. As a nonlimiting example, the tubes 14 may be formed of
an aluminum alloy having a clad layer formed of a 4000-series
aluminum-silicon alloy as the braze material. Such alloys typically
contain about 10 to 12% silicon and the clad layer thickness
accounts for about 10% of the wall thickness of the tube 14.
[0039] Fluxing of the heat exchanger assembly prior to brazing can
be performed using various methods capable of depositing a small
controlled amount of flux on the components of the assembly. For
example, one such method involves mixing a flux powder with
isopropyl alcohol and spraying the resulting flux mixture on the
assembly while the flux powder is kept in suspension. Another such
method involves pouring a mixture of a flux powder and water on the
assembly. After the flux mixtures are applied, the heat exchanger
assembly is preferably dried, for example, using forced hot air or
in a drying furnace.
[0040] Preferably, the brazing operation is performed in a furnace
that contains an inert atmosphere, for example, nitrogen gas, to
reduce the likelihood of oxidation of the alloy from which the
tubes 14 are made. A controlled atmosphere brazing furnace (CAB) is
particularly well suited for this purpose, though other aluminum
brazing equipment and methods may be used. The temperature profile
of the brazing operation may include a rapid temperature increase
up to the point where the flux melts. The flux removes any oxide
layer from the surfaces of the heat exchanger components and allows
the cladding material, as the source of the braze material, to
properly flow and wet the surfaces to be joined by the braze
material. The temperature of the assembly preferably continues to
increase to the liquidus temperature of the braze material.
Tube-to-fin joints are created as the braze material flows to
create the required braze joint fillets. The creation of the
fillets is also aided by capillary action of the gap 33 between the
tube 14 and fins 12, which is preferably not greater than about 0.1
mm to reduce the risk of forming incomplete fillets if there is
insufficient braze material.
[0041] The assembly is held at the liquidus temperature of the
braze material for a duration dependent on a few factors. For
example, the size of the heat exchanger assembly and how
homogeneous the temperature distribution is within the furnace.
Afterwards, the resulting brazed heat exchanger is moved from the
brazing chamber to a cooling zone of the furnace where the brazed
heat exchanger is cooled before exiting the furnace. If removal of
the dried flux residue is desired, the heat exchanger can be
washed.
[0042] In an investigation leading to the present invention, sample
heat exchanger assemblies were produced using aluminum alloy (Alloy
3003) tubes having 7.2 or 8 mm outer diameters and 0.6 mm wall
thicknesses, and an outer clad layer containing an aluminum-silicon
eutectic brazing alloy (Alloy 4045) and 1% zinc. The clad layer
constituted about 10% of the wall thickness. The tubes were
assembled with fins resembling the Type 1 fin represented in FIG. 8
as having a "dogbone" slot and circular portions bordered by
collars with a reflare section, as described above in reference to
FIG. 9. The reflare sections of the collars were intentionally
formed to promote stacking of the fins without the fins telescoping
into each other and to allow for the building of heat exchangers
10A and 10B with increased fin densities and homogeneous fin
spacing relative to, for example, conventional low-density, no
enhancement fin refrigerator evaporators using serpentine tube
designs. As also described in reference to FIG. 9, the sizes of the
slots and collars of the fins were such that radial gaps of up to
0.1 mm were present between the fins and the tubes assembled
therewith.
[0043] The samples were coated with a standard aluminum brazing
flux. It was observed that even when the fins were stacked closely
such that their collars contacted adjacent fins, the flux was able
to penetrate and coat surfaces of the tubes and fins within the
areas of contact between the tubes and the fin collars. In
particular, the flux appeared to penetrate and coat the required
area of the collar and tube through the remaining open area within
each slot. During the brazing process, the samples were brazed in a
nitrogen atmosphere furnace following a temperature profile of a
temperature rise up to 605.degree. C., a soak time of about one
minute, and cooling to about 150.degree. C., after which the
samples were removed from the furnace and air cooled to room
temperature. Examination of the brazed samples showed successful
brazed joints were achieved with complete fillets inside and around
each fin collar, filling the gaps between the collars and tubes.
Therefore, it was concluded that the reflare sections of the
collars were beneficial to stacking the fins to increase fin
densities without the fins telescoping into each other and that
flux could penetrate to the required areas to provide a good braze
joint.
[0044] FIG. 11 shows images of brazed joints of a serpentine heat
exchanger produced during the investigation. For contrast, FIG. 12
shows images of mechanical joints of a conventional non-brazed
serpentine heat exchanger. Without brazed joints, there are gaps
between the tube and fins in FIG. 12, and the gaps can be larger
than the fin thickness. In contrast, the brazed joints completely
fill the gaps between the tube and fins in FIG. 11. Even where the
gap between a fin and the tube was large, the braze material was
able to completely bridge the gap. The brazed joint established a
much wider contact area that extended from beyond the tip of the
"pseudo-collar" to the tip of the braze material between the tube
and the "elbow" of the "pseudo-collar." This wider contact area
between the fins and tubes improves the heat transfer and therefore
increases the performance of the heat exchanger. This increased
contact surface area compensates for the lack of fin surface area
due to the large dogbone slot 22, compared with a conventional fin
having a circular hole (FIG. 13A).
[0045] Torsion tests conducted on brazed and mechanical joint
samples such as shown in FIGS. 11 and 12, respectively, indicated
that the brazed sample was rigid due to its brazed joints, whereas
the mechanical joint sample allowed the tube run to twist within
its fins even under a very small torsional load, too small to
measure with available equipment.
[0046] As noted above, conventional heat exchangers having tubes
and fins joined mechanically (mechanical joints) require fins that
are relatively stiff in order to prevent the fins from collapsing
when a linear tube run or serpentine tube is inserted through the
holes in the fins during assembly. In contrast, the fins 12
described above in reference to FIGS. 8 and 9 do not require an
interference fit during assembly with a tube. Instead, the gap 33
between the fins 12 and tube 14 assembled therewith provide a
sufficient radial clearance of about 0.05 to 0.1 mm, such that the
force required to introduce the tube 14 into the fin 12 during
assembly is lower than with heat exchangers requiring an
interference fit. Therefore, while the brazed heat exchangers 10A
and 10B may include smooth planar fins 12 of the types used on
conventional heat exchangers, they may also or alternatively
include fins 12 having surface enhancements that significantly
increase fin efficiency. Such enhancements have been determined to
allow fins 12 having dogbone-shaped slots 22 or 23 to perform
competitively with conventional fins having circular
interference-fit holes despite a reduced surface area due to the
presence of the rectilinear portion 26 or 27 of the slot 22 or
23.
[0047] FIG. 13A represents a fragment of a fin 12A of a type used
in conventional mechanically-expanded tube heat exchangers used in
air-conditioners, having multiple hair-pin tubes, multiple brazed
CU'' bends, and fully circular (360 degree) holes 20 with louvers
28 between each hole 20 that serve as surface enhancements. FIGS.
13B and 13C represent two nonlimiting embodiments by which surface
enhancements can be incorporated into brazed serpentine tube heat
exchangers of the type disclosed above (e.g., FIGS. 1 and 2) to
improve the performance of the heat exchangers such that they
perform equal to or better than conventional air-conditioner heat
exchangers. Such enhancements include but are not limited to slits
(raised lances), louvers 28 (corrugated lances) and winglets 30
(turbulence generators). FIGS. 13B and 13C represent fragments of
fins 12B and 12C having "dogbone" shaped slots 22 and rectilinear
portions 26 that subtend an angle of about 120 degrees of their
respective circular portions 24. FIG. 13B represents the fin 12B as
having eight louvers 28 between the slots 22 and twelve winglets 30
adjacent to the slot 22. FIG. 13C represents the fin 12C as having
twelve upper louvers 28A between the slots 22 and eight lower
louvers 28B adjacent to the slots 22. The upper and lower louvers
28A and 28B do not necessarily have the same dimensions and
characteristics. The end views of each fin 12A, 12B, and 12C show
relative angles of the louvers 28. Fins 12 having other enhancement
configurations are foreseeable and within the scope of the
invention.
[0048] FIG. 14 represents a modified version of the fin 12C of FIG.
13C, labeled in FIG. 14 as 12D, wherein the rectilinear portions 26
are narrowed such that they subtend an angle of about 100 degrees
of their respective circular portions 24. This modification
increases the tube-to-fin contact area at the collar (not shown).
The lower louvers 28B also include larger dimensions since there is
additional space available. Notably, narrowing the rectilinear
portion 26 requires the tube 14 to be further flattened in order to
be inserted into the slot 22. As the tube 14 flattens, its
cross-sectional area reduces, thus increasing a local flow
resistance and resulting in a pressure drop penalty.
[0049] Brazing serpentine tube-to-fin joints as described herein
provides significantly increased joint strength compared to
mechanical joints. This improved strength allows for the production
of larger heat exchangers, particularly for air-conditioning and
large refrigeration systems which were previously limited in size
due to mechanical joints in the prior art heat exchanger. The
brazed joints also provide increased surface contact area and
therefore increased thermal conductivity between tube and fin thus
improving heat exchanger performance. Brazed joints also allow for
the introduction of surface enhancements like louvers on the fin
surface since little pressure is exerted on the fins during
insertion into the serpentine tube.
[0050] While the invention has been described in terms of a
specific or particular embodiment, it should be apparent that
alternatives could be adopted by one skilled in the art. For
example, the heat exchanger 10 and its components could differ in
appearance and construction from the embodiment described herein
and shown in the drawing, functions of certain components of the
heat exchanger 10 could be performed by components of different
construction but capable of a similar (though not necessarily
equivalent) function, various materials could be used in the
fabrication of the heat exchanger 10 and/or its components, and the
heat exchanger 10 could be installed in various types of heating,
cooling, or electrical systems. In addition, the invention
encompasses additional or alternative embodiments in which one or
more features or aspects of a particular embodiment could be
eliminated. Accordingly, it should be understood that the invention
is not necessarily limited to any embodiment described herein or
illustrated in the drawing. It should also be understood that the
phraseology and terminology employed above are for the purpose of
describing the disclosed embodiment, and do not necessarily serve
as limitations to the scope of the invention. Therefore, the scope
of the invention is to be limited only by the following claims.
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