Note: Descriptions are shown in the official language in which they were submitted.
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TABBED TRANSFER FINS AND AIR-COOLED HEAT EXCHANGERS
WITH TABBED FINS
Contractural Origin of the Invention
The United States Government has rights in this invention under Contract No.
DE-AC36-9960-10337 between the United States Department of Energy and the
National Renewable Energy Laboratory, a Division of the Midwest Research
Institute.
Cross Reference to Related Auplications
This application claims the benefit of U.S. Provisional Application No.
60!486,071, filed July 10, 2003, which is incorporated by reference herein in
its entirety.
l0 Background of the Invention
Field of the Invention.
The present invention relates generally to heat exchangers that utilize fins
or
plates on or in contact with tubes, pipes, or plates to transfer heat away
from the working
fluid in the tubes, pipes, or plates, and more particularly, to heat transfer
fins, and heat
exchangers or condensers that include such fins, that include a plurality of
tabs
extending from the fins to provide enhanced heat transfer on the air side of
the heat
exchanger with low and acceptable increases in pressure drop.
Relevant Background_
Heat exchangers are used extensively in industrial and consumer applications,
2 0 and typically employ two moving fluids, one fluid being hotter than the
other, to transfer
heat to the colder fluid. Many heat exchangers currently in use, such as in
air
conditioners, automotive radiators, process industry air-cooled condensers,
and boilers,
transfer heat between a gas and a single or mufti-phase liquid. Typically,
such heat
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exchangers include a number of liquid conduits, e.g., circular, oval, or flat
tubes, pipes,
or plates, that are positioned within a shell or housing that defines a gas
flow passage or
chamber. The heat exchanger uses a fan or blower to force a gas, e.g., air, to
flow within
the gas flow chamber in a perpendicular (i.e., cross-flow) or parallel (i.e.,
counter-flow)
direction relative to the liquid conduits. The resulting heat transfer between
the liquid
and the gas is directly proportional to the heat transfer surface area between
the liquid
and the gas, to the temperature difference between the liquid and the gas, and
to the
overall heat transfer coefficient of the heat exchanger. The overall heat
transfer
coefficient is defined in terms of the total thermal resistance to heat
transfer between the
gas and the liquid, and it is dependent on a number of characteristics of the
heat
exchanger design, such as the thermal conductivity of the material used to
fabricate the
conduit and the local film coefficients along the conduit, i.e., measurements
of how
readily heat can be exchanged between the gas and the exterior surfaces of the
conduit.
Although gas-liquid heat exchangers are widely used, the heat transfer
effectiveness of these heat exchangers is low. The low heat transfer
effectiveness leads
to relatively high operating and capital costs for gas-liquid heat exchangers
because a
greater number of units and/or larger capacity units that require more power
must be
used to obtain a desired heat transfer. For example, air-cooled geothermal
power plants
operate at low temperature differences between the gas and the liquid and, in
these
2 o power plants, more than 25 percent of the cost of producing electricity is
the expense of
purchasing and operating gas-liquid heat exchangers or condensers. As a result
of these
high costs, continuing efforts are being made to improve heat transfer
effectiveness of
gas-liquid heat exchangers while at the same time controlling the
manufacturing and
operating cost to increase the likelihood that new heat exchanger designs will
be adopted
2 5 by industry and consumers.
Geothermal plants provide one example of a situation in which there is often
not
a sufficient supply of water or other cooling liquid for evaporative cooling,
and heat
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must be rejected to atmospheric air. This heat rejection is accomplished
through the use
of large air-cooled condenser units in which air is forced through several
rows of long
individually firmed tubes by large fans, i.e., a gas-liquid heat exchanger or
condenser is
employed. Each of the tubes carrying the hot working fluid has fins on their
outer
surfaces in order to provide a large heat transfer surface axes. Finned-tube
heat
exchangers have been used for many years to improve the gas-side heat transfer
rate by
increasing the heat transfer surface area available for contacting the gas as
it flows
through the heat exchanger. In general, finned-tube heat exchangers are cross-
flow heat
exchangers that include a number of tubes, i.e., conduits, for carrying the
liquid
l0 fabricated from aluminum, copper, steel, or other high thermal conductivity
materials.
The tubes pass through and contact a series of parallel, high thermal
conductivity
material sheets or plates, i.e., fins, which provide an extended heat transfer
area for the
tubes. The overall heat transfer area is based on the number and size of the
included
fins. The fins are separated a fixed distance, i.e., a fin separation
distance, and define
relatively parallel channels that direct the gas flow across and among the
tubes. Heat
transfer occurs as the gas flows through the channel and contacts the surface
of the fins
and as the gas contacts the outer surfaces of the tubes. The highest heat
transfer rate on
a flat surface like a flat fin occurs at the leading edge of the surface and
decreases with
distance from the leading edge as a boundary layer develops and thickens
causing the
2 o local heat transfer coefficient to decrease.
However, although finned-tube heat exchangers are widely used because they
are relatively inexpensive to produce and do not create a large pressure drop,
there are
several operational drawbacks to finned-tube heat exchangers. For example,
finned-tube
heat exchangers have low heat transfer coefficients on large portions of the
fins due to
2 5 the development of thick boundary layers. Additionally, these heat
exchangers have
poor heat transfer in the wake or shadowed regions behind tubes as a majority
of the gas
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flowing over a tube does not contact the back side of the tube or contact the
portion of
the fin surface that is shadowed by the tube.
In an attempt to increase the effectiveness of finned-tube heat exchangers,
efforts
have been made to vary the surface and overall geometry of the parallel fins
to interrupt
gas boundary layers or to make it more difficult for thick boundary layers to
form on the
fins. For example, finned-tube heat exchangers have utilized triangular or s-
shaped
wavy fins to enhance the heat transfer coefficient by disrupting boundary
layer
development and, also, by increasing the available heat transfer area.
Alternatively, the
surface geometry of flat, parallel fins can be enhanced, as is often done in
refrigerant
condensers, by slitting the fin three or four times in the areas of the fin
between the
tubes, thereby interfering with boundary layer development by creating offset
surfaces
on the fin that cause repeated growth and wake destruction of boundary layers.
A
number of heat exchangers have been developed that include structures on the
fin
surfaces that are designed to create turbulence in the channel between the
fins to break
up the boundary layer and increase heat transfer. Generally, these structures
have been
configured with a major portion of their surface area, such as winglets,
vortex
generators, and the like, facing the flowing gas or directed toward or into
the gas flow
path, e.g., to have a large profile relative to the gas flow path within the
fin channel.
However, the larger the profile or "form" placed in the flow path of the gas,
including
2 o the liquid tubes, the larger the pressure drop in the cooling gas as form
drag is increased,
which is generally an undesirable and often unacceptable result.
While some of the above changes in the fin surface and fin shape may provide
somewhat higher heat transfer coefficients in finned-tube heat exchangers, the
design
changes also result in unacceptably large increases in pressure drop on the
gas side of
2 5 the heat exchanger that require increased expenditures on fan power.
Additionally,
many of these design changes have not been adopted due to unacceptably high
manufacturing costs in producing the fins or due to increased maintenance
costs as some
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of the fin surface structures snag or collect debris often found in unfiltered
air often used
in air-cooled heat exchangers.
Hence, there remains a need for a more effective finned-tube, gas-liquid heat
exchanger that provides improved heat transfer capabilities on the gas side of
the
exchanger while creating an acceptable increase in the pressure drop for the
gas passing
through the tubes and fins and while controlling manufacturing and maintenance
costs.
Disclosure of the Invention
The present invention addresses the above problems by providing an improved
design for heat transfer fins that enhances the heat transfer rate on the gas
or air side of
heat exchangers with relatively low increase in pressure drop. Briefly, the
fins include
numerous tabs or secondary fins that are bent upward and downward from the
body of
the fin at a selected bend angle (such as between about 70 and 110 degrees and
more
typically, about 90 degrees). In this manner, the material of the fin body is
retained for
use in heat transfer with the air or gas flowing over the fins. Preferably,
all or a majority
of the tabs are aligned with the flow paths) of the cooling gas to minimize
the creation
of turbulence and pressure drop (i.e., by minimizing creation of flow drag by
only
"showing" the tab's leading edge to the flowing gas).
For example, the tabs may be substantially planar and aligned with their
surfaces
parallel to the main flow path or simple flow path or line (or in some cases,
the local
2 0 flow paths) of the cooling gas relative to the fin. In a first embodiment,
the tabs are
positioned with their planar surfaces perpendicular to a leading edge of the
fin to align
the tabs substantially parallel with the main flow path of gas across the fm.
In a second
embodiment, some or all of the tabs are positioned to be more aligned with
local flow
paths or with streamlines to guide air flowing in the channel between fins to
reduce the
2 5 size of wakes behind tubes and to reduce pressure drop relative to the
first embodiment
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by producing less turbulent flow. In the second embodiment, the tabs may be
positioned
substantially parallel to or angled less than about 5 degrees relative to the
streamlines.
This is achieved by positioning the tabs at various, differing offset angles,
e.g., 0 degrees
(or substantially parallel to the simple flow path), 10 degrees as measured
from either
side of the simple flow path, and the like. The offset angles are typically
less than about
20 degrees and more preferably less than about 10 degrees as measured from
either side
of the simple flow path with the offset angle, at least in some embodiments,
being
selected to be substantially parallel (such as within 5 to 10 degrees or less
to being
parallel) to the local stream line or flow path. In this manner, heat transfer
is
significantly enhanced by reducing the thickness of the thermal boundary layer
on each
tab and by placing heat transfer surface area in contact with cooler portions
of the
flowing gas (for a cooling application), e.g., the surface area of the tabs
extends outward
into cooler portions of the flow channel between adjacent fins.
The tabs of the fin serve four main functions. First, the tabs are preferably
arranged so as to serve as a plurality of sites for starting new boundary
layers. This is
achieved generally by offsetting the tabs (or adjacent rows of the tabs) such
that
downstream tabs are not shadowed by upstream tabs. Second, the tabs are
preferably
positioned relative to the flowing gas to enhance heat transfer. More
particularly, the
tabs typically have a tab height as measured from the surface of the fin body
that allows
2 0 the tab to extend out into the region of high air flow rate and cool air
(in the case of
cooling applications), i.e., forming on both sides of the fin body. In one
embodiment,
the fin height is selected to between about 40 and 50 percent (e.g., about one
half of the
size of the channel between adjacent fins, i.e., a fin separation distance and
tabs are
extended outward from both sides of the fin body. In other embodiments, the
fin height
2 5 is greater than 50 percent with one specific embodiment using a tab height
of about two
thirds or about 67 percent of the fin separation distance. In this manner, the
tabs place
fin material into the coolest portion of the gas flowing on both sides of the
tab. Third,
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the openings in the main fin surface disrupt the boundary layer on that
surface thus
enhancing heat transfer. Fourth, due to their angles, their flow resistance,
and the
channels they create, the tabs direct air flow so that the fin surface is more
uniformly
covered and relatively stagnant wake regions behind tubes are reduced. To
achieve
these functions, the tabs are formed by punching holes in the fin body but
retaining a
connection to the fin body on at least one edge. The material is then bent
upward and/or
downward relative to the fin body to extend at a bend angle from one or both
of the
surfaces of the fin body, i.e., to allow the tabs to extend into the boundary
layers that
form on one or both sides of a fin.
According to one aspect of the invention, a method is provided for fabricating
heat transfer fins for heat exchangers. The method comprises providing a plain
fin, such
as an aluminum fin typically utilized in finned-tube heat exchangers. A tab
pattern is
selected or provided for the particular fin to define the quantity, size, and
location of
heat transfer tabs on the fin. The tab pattern selection may comprise
performing a
variety of flow and heat transfer tests on the fin implementing a number of
potential tab
patterns to obtain a useful pattern to enhance heat transfer while not
unacceptably
increasing pressure drop. With a tab pattern selected, a punch mechanism or
tool can be
fabricated or provided based on the pattern. The punch mechanism can be
adapted for
punching the tabs in one operation with tabs extending from one or both sides
of the fin
2 0 body. The method continues with forming, such as with the punch mechanism,
the heat
transfer tabs defined by the tab pattern by creating openings or holes in the
fin by
removing material from the fin body while retaining a connecting edge between
the fin
body and the removed material or tab body. The forming comprises bending the
removed fin body material along the connector edge to a bend angle, such as 90
degrees,
2 5 relative to one of the two sides of the fin body. The tab pattern is
configured such that
all or a majority of the tab bodies are aligned parallel (or within about 10
to 20 degrees)
to a simple flow path (i.e., a directional line drawn perpendicular to the
leading edge of
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the fin body) or are aligned parallel (or within about 5 to 10 degrees) of
local flow paths.
In one embodiment, the tab pattern is configured such that the surface area of
the
removed material or tab bodies is such that the tabbed fin has porosity of
less than about
50 percent and more typically between about 15 and 30 percent. In some
embodiments,
about half of the tabs extend from one side of the fin body while the
remaining tabs
extend from the second side of the fin body. Preferably, the tabs on each side
of the fin
body are arranged in the tab pattern such that adjacent upstream and
downstream tabs
(or proximal and distal tabs relative to a fin body leading edge) are offset
to avoid
shadowing of downstream tabs. The tabs are also arranged in such a way that
they do
1 o not adversely interfere with the tabs on adjacent fins. Further, their
pattern encourages
uniform flow over the main fin and maximized heat conduction within the fin.
Brief Descriution of the Drawings
Figure 1 is a simplified heat exchanger according to the present invention
illustrating one configuration in which tabbed fins or plates (such as those
shown in later
figures) can be employed to enhance heat transfer on the air or gas side;
Figure 2 illustrates two fins according to the invention, one that is
partially
punched or has less tab density and one that is fully fabricated or has higher
tab density,
and a template that can be used for producing a tool to fabricate the fins
shown;
Figure 3 is a partial cross section of a set of fins (prior to placement on
tubes)
2 0 illustrating a side view of the fins, i.e., a view showing the planar
surface of the tabs
extending outward in this embodiment from both sides of the fins, in
accordance with
the present invention;
Figure 4 is a partial sectional view of a heat exchanger similar to that shown
in
Fig. 3 illustrating more clearly the mating of the fins to a liquid tube and
one exemplary
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configuration for the tabbed fins of the present invention (with fewer fins
shown than
would typically be included for ease of illustration);
Figure 5 is another partial sectional view of a pair of fins according to the
invention illustrating selection of the height of the tabs to reach the region
of high air
flow and coolest air (in a cooling application);
Figure 6 is a top view of a portion of a fin fabricated according to the
present
invention which is useful for illustrating that the tabs in adjacent rows are
offset from
each other to enhance heat transfer and for illustrating one useful pattern of
the tabs on a
fin relative to the fin collars/tube openings or to the tubes in a heat
exchanger assembled
according to the invention;
Figure 7 is a cross sectional view of the fin shown in Fig. 6 taken at line 7-
7
illustrating one arrangement for tabs on a fin (e.g., extending from both
sides or surfaces
of the fin body) and showing the bend angle and height of the tabs, in
accordance with
the present invention;
Figure 8 is a perspective view of another fin fabricated according to the
invention and further illustrating the concept of offset tabs utilized in most
fin
embodiments;
Figure 9 is a flow diagram for air or gas relative to a fin, such as the fin
shown in
Fig. 6 that can be utilized for designing the tab pattern on a fm, such as for
selecting the
2 0 alignment of the tabs relative to the leading edge of the fin body, in
accordance with the
presentinvention;
Figure 10 is a perspective view of a portion of another embodiment of a tabbed
fin according to the invention showing the use of tabs positioned at offset
angles to align
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the tabs substantially parallel with local flow paths or streamlines and to
redirect flow to
reduce the size of wakes and distribute the flow more uniformly over the fin
surface;
Figure 11 is a view of one of the heat transfer surfaces or sides of the
tabbed fin
of Fig. 10 provided to better illustrate the use of a number of offset angles
to align all or
most of the fins with local flow paths and to illustrate the use of
rectangular tabs (e.g.,
shown by the holes created by the removal of material from the fin body to
form the tab
bodies), i11 accordance with the present invention;
Figure 12 is an illustration of flow within a channel adjacent to a fin
without tabs
as modeled with a bubbler device showing typical path lines and showing
typical wakes
formed behind heat transfer tubes, in accordance with the present invention;
Figure 13 is an illustration of the invention similar to Fig. 12 showing
modeled
flow within a channel adjacent a tabbed fin according to the invention having
rectangular tabs aligned parallel to the simple flow path and providing
reduced wakes
behind tubes;
Figure 14 is another illustration of the invention similar to Fig. 12 showing
modeled flow within a channel adjacent another tabbed fin according to the
invention
again having square tabs but with at least a portion of the tabs positioned at
small offset
angles relative to the simple flow path to direct flow and to create reduced
wakes
compared to the fin of Fig. 13;
2 0 Figures 15-17 illustrate graphically the tested performance of one
embodiment
of tabbed fins of the present invention compared with performance of plain
fins both in
an 8 fin per inch (FPS configuration and with performance of the tabbed fin
design as
indicated by a set of ratios commonly used for heat transfer design to compare
a new
design with an older or original design; and
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Figures 18-20 illustrate graphically the tested performance of the tabbed fin
of
Figures 16 and 17 in a 10 FPI configuration again comparing performance of the
tabbed
fin to a plain fin and providing values of heat transfer design ratios
achieved with the
tabbed fin, in accordance with the present invention.
Detailed Description of the Preferred Embodiments
The present invention is directed to heat exchanger fins employing tabs to
increase heat transfer effectiveness, heat exchangers or condensers
incorporating such
tabbed fins or plates such as air-cooled, finned-tube heat exchangers, and
methods of
making tabbed fins. Generally, each fin of the invention includes a
multiplicity of small
tabs or secondary fins that are formed by punching material (i.e., metal) out
of the main
fin body (i.e., creating a hole or opening) and the punched material is bent
outward away
from the main fin body in one or both directions from the surface of the fin.
To
minimize or control the creation of a vortex or increased pressure drop, the
tabs are
generally planar and aligned with the direction of the fluid, e.g., air,
flowing over the
fins in the channel between adjacent fins. In other words, a leading edge of
the tab first
contacts the flowing gas and the substantially planar body of the tab is
aligned
substantially parallel to the gas flow path or direction over the fin.
In some embodiments, it is assumed that there is one flow direction through
the
fins, such as perpendicular to the leading edge of the fins, and all of the
fins are aligned
2 o parallel to this flow direction. These embodiments can be described as
tabbed fins with
tabs positioned at a zero offset angle relative to the simple flow path (i.e.,
a line drawn
perpendicular to the leading edge of the fin body) such that the planar tab
bodies are
substantially parallel to the simple flow path. In other embodiments, two or
more flow
directions within the fin channel are identified, and fin tabs in different
locations of the
2 5 fin are aligned with these different flow paths to better limit creation
of pressure drop.
The different flow paths can be termed "local flow paths" or "local
streamlines" and in
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these embodiments, a portion or all of the tabs are positioned at offset
angles greater
than zero relative to the simple flow path (such as less than 20 degrees and
more
preferably less than about 10 degrees offset). In some cases, a portion or
subset of the
tabs are aligned somewhat, such as less than about 5 percent or about 5
degrees, off of
the local flow direction or local flow path in order to redirect flow into
areas of low flow
such as the area behind or shadowed by a tube, whereby heat transfer is
enhanced
without creating a vortex at the tab.
The tabs can take many shapes, such as square, rectangular, triangular, semi-
circular, and a combination of these shapes, and are generally bent outward at
about a
right angle relative to the body of the fin but a smaller bend angle can be
utilized to
practice the invention. The tabs can also be curved, e.g., into an L-shape or
U-shape, to
place more tab area into the most advantageous flow regions. Tabs in adjacent
rows are
preferably offset from each other so as to avoid shadowing of subsequent tabs
and to
promote the creation of multiple boundary layers within the channels. The tabs
have a
height defined by the amount of material removed from the fin body, and this
tab height
is generally (but not necessarily, such as when a bend angle of less than 90
degrees is
utilized) less than the separation distance between fins, i.e., fin separation
distance, and
in many embodiments, the tab height is selected to be about one half and three
fourths of
the fm separation distance, e.g., two thirds or 67 percent, to place a large
portion of
2 0 surface area of the tab bodies within the coolest air flowing between the
fins, i.e., at
about the top of the boundary layer formed by the fin body. As will become
clear from
the following description, the use of tabbed fms according to the invention
can
significantly enhance the air-side heat transfer coefficient in finned-tube
heat
exchangers, with some tests indicating an increase of up to approximately 100
percent
2 5 with a corresponding increase in pressure drop of approximately 60 percent
relative to
smooth or plain fins. Additional test results are provided and explained with
reference
to Figures 15-20 that, briefly, indicate significant increases in heat
transfer coefficients
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for tabbed fins according to the invention over plain fins with smaller
corresponding
increasesin pressure drop.
In the following description, the use of tabbed fins according to the
invention is
explained most fully with finned-tube arrangements in which the fins or plates
are
arranged in a parallel fashion. However, it should be understood that the
invention
covers the use of tabs on many other arrangements of fins than just the ones
shown. For
example, it is anticipated that tabbed fins would be useful with helically
wound fins.
Additional applications of tabs to fins, whether or not the fins are applied
to tubes, will
be understood by those skilled in the art and are considered within the
breadth of the
1 o invention and following description.
Figure 1 illustrates a simplified heat exchanger 100 that may be configured
with
tabbed fins according to the present invention. The heat exchanger 100 is a
finned-tube
heat exchanger or condenser and is shown in simplified form for ease of
description.
Generally, the heat exchanger 100 would further ,include a housing enclosing
fins and
tubes and, at least in part, defining air flow channels, i.e., causing the air
or other gas to
flow between the fins and to define a gas inlet and a gas outlet. The heat
exchanger 100
further would include one or more fans to draw (or push) air across the tubes
and
between the fins. These components are well known and hence, it is not
believed
necessary to illustrate or describe these components further to allow one
skilled in the
2 0 arts to understand and practice the invention. The heat exchanger 100
transfers heat
energy from one fluid, i.e., the fluid in, FIN, to another fluid or gas, i.e.,
the air in, AIN,
which results in a cooler fluid being output, i.e., the fluid out, Four, and a
hotter fluid or
gas being output, i.e., the air out, AoUT, from the heat exchanger 100. Of
course, the
fluids being cooled may be a gas or liquid or any mixture thereof, and the
invention
2 5 applies to heating as well as cooling.
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Referring again to Figure l, the heat exchanger 100 includes a plurality of
plates
or fins 110 that are arranged in a parallel manner and separated a fin
separation distance.
The fins 110 are tabbed, not shown in Figure 1 for simplicity's sake but
described fully
in the following paragraphs and shown in detail in subsequent figures, to
enhance heat
transfer on the gas or air side of the exchanger 100. The fins 110 are
typically pressed or
fit together and spaced apart a fin separation distance by a set of tube
collars 115
provided on one (or both) sides of each tube opening in the fin 110. The tube
collars
115 are sized to receive the liquid tubes 120, 130, such as copper, steel, or
other metal
tubes or pipes, which may have a circular cross section or another cross
sectional shape
such as oval, flat, rectangular, and the like (with the size, shape, location,
material, and
other properties of the tube not being limited to the invention). The collars
115 provide
the heat transfer surface between the tubes 120, 130 and the fins 110 and are
often press
fit onto the tubes, such as by inserting the tubes 120, 130 and then over
pressurizing the
tubes 120, 130 to cause the tubes 120, 130 to expand and contact the collars
115. Of
course, fins may be attached to tubes in other ways including welding, brazing
and the
like and winding or wrapping (as is the case in foil fins that are helically
wound onto
tubes).
The incoming air, AIN, is passed through the channels between the fins 110 and
strikes the leading edges of the plurality of secondary fins or tabs (which
are shown in a
2 0 rows that are diagonally offset in Figures 2-9 but other patterns can be
used such as
those in Figures 10 and 11), thereby significantly increasing the amount of
heat
transferred from the incoming liquid, FIN, via the tubes 120, 130, to the
outgoing air,
AOUT~ Additionally, tabs on the fins 110 typically are all substantially
perpendicular to
the leading edges 140. The tabs, or at least a portion of the tabs, can also
be arranged or
2 5 aligned at different angles, i.e., offset angles, chosen such that during
operation of the
heat exchanger 100 the tabs direct flow smoothly around the tubes 120, 130
(and collars
115). In this manner, the tabs on the fins 110 can be utilized to shrink the
wakes behind
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the tubes 120, 130. It should be further noted that although tubes 120, 130
are shown in
Figure 1, the tabbed fin concept of the present invention could also be used
in other heat
exchangers, including those that do not use tubes. For example, but not as a
limitation,
tabs similar to the ones described herein may be used on serpentine-like fins
in plate
heat exchangers.
Referring now to Figure 2, portions of fins (or test fin sections)
manufactured
according to the present invention are shown along with a template useful in
creating a
punch tool for forming the holes and tabs in the fin bodies. The fin sections
shown were
formed for 1/Z-inch tubing but, of course, the size of the tube may be varied
to practice
1 o the invention (e.g., it is anticipated that the tabbed fins will be useful
with 1-inch tubing
that is often used in heat exchangers). The fin 210 is formed from a metal
(such as
aluminum, copper, or other metal useful on the air or gas side of a heat
exchanger) sheet
that is fully manufactured or punched with numerous tabs extending outward
from both
sides or surfaces of the fin body, such as with about 50 percent extending
upward and
about 50 percent extending downward. Although it may not be clear from Figure
2, the
tabs remain attached to the body of the fin 210, 220 along a seam or
connecting portion,
i.e., to retain the heat transfer material and surface area of the original
fin body, and axe
bent outward so when the fin 210, 220 is used in a heat exchanger the tabs
extend into
the air stream flowing over the fin 210, 220 (as is explained in detail
below). The tabs
2 0 thus act as secondary fins or secondary fin surfaces.
As shown, a relatively large portion of the fin body surface area has been
removed to form the tabs. For example, the surface area of the fin body
removed or
used to form the tabs may be selected from the range of 0 to 50 percent, and
more
preferably between 10 and 40 percent, and in one preferred embodiment, the
surface
2 5 area removed is about 20 to 25 percent of the fin body surface area, i.e.,
20 to 25 percent
of the original fin body surface area or material is used to form the tabs.
While initially
it may appear that the area used to form the tabs should be maximized, there
are
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limitations to how much material can or should be removed from the fin body.
More
particularly, the removal of fin body material reduces the volume of or mass
of the fin
body that is available to conduct the heat from the tube collar and tube
contacting the
collar to the tabs. Hence, testing may likely be required to identify for a
particular fin
and tube arrangement the amount of fin body surface area that should be used
in forming
the tabs. The amount of material removed defines the surface porosity of the
fins 210,
220, and this porosity may be varied to practice the invention and may vary
with fin
materials and the makeup of the fluids passed through the heat exchanger. In
one
embodiment, the fin porosity is selected to be between 20 and 30 percent with
tested
embodiments utilizing about 25 percent porosity. The number and size of the
tabs may
be varied while maintaining a desired porosity with larger tabs resulting in
fewer tabs
and smaller tabs resulting in fins with more tabs. Also, the amounts of heat
transfer
improvement and pressure drop increase can be controlled by varying the tab
dimensions (height and length) and the number of tabs or porosity.
The tabs shown are generally square (as can be seen clearly from the material
removed to form the tab openings or holes) but numerous other shapes can be
utilized,
such as rectangular (such as shown in Figures 10 and 11), triangular,
trapezoidal (e.g.,
with 0 to 10 degree or more angled leading and trailing edges) and semi-
circulax. The
tabs can also be bent into shapes such as L-bends and U-bends. As will be
explained
2 o below, the size of each tab is typically dictated by its height, i.e.,
what distance the tab is
to extend away from the fin body, and its length. For example, if a tab is
square and the
height is selected to be one half of the distance between adjacent fins, then
the sides of
the square tab would each have a length equal to the tab height or one half
the fin
separation distance. The tabs are shown to be arranged in relatively linear
rows ilz fin
2 5 210 and are shown to be more dense in areas of the fin in which flow is
expected to be
higher or highest, e.g., between tubes, in front of tubes, and away from
housing wall
surfaces. The fin 220 illustrates the use of a much smaller percentage of the
fin body
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surface area to form tabs, and is also useful for showing an embodiment of the
invention
in which the fins are not necessarily arranged in neatly linear rows but,
instead,
"downstream" (e.g., tabs more distal from a leading edge of the fin body) tabs
are offset.
In other words, downstream tabs are generally not positioned immediately
behind an
upstream tab or in the same air flow path to minimize shadowing and to
encourage
development of new boundary layers by each tab. Item 230 is a template useful
for
designing a punch tool for creating the tabs in a plain fin with darkened or
colored holes
indicating tabs to be punched to extend from a first surface of the fin body
and the other
holes indicating tabs punched the other direction. Again, the template 230 is
useful for
showing that downstream tabs are offset from upstream tabs. Arrangement of the
tabs is
also done with due consideration for providing heat conduction paths in the
base fin
from the tube to outer regions of the fin.
Figure 3 illustrates a sectional view of a series of fins formed according to
the
present invention. As shown, air, Ate, enters the channel formed between
adjacent fin
bodies and the heated air, AoUT, exits the other end of the fin channel or air
flow
chamber of the heat exchanger. The view of Figure 3 can be thought of as a
side view
and shows that in this embodiment the fins include a plurality of tabs that
extend
outward from the fin body on both sides. As shown, the tabs are relatively
planar with a
square cross section. The planar portion of the tabs (i.e., the larger surface
area portion
2 0 of the tabs) are bent away from the fin body to be substantially parallel
to the direction
of the air, AIN, through the channel between adjacent fin bodies or
substantially parallel
to the simple flow path in the channel. The fins or fin bodies are separated
by a distance
(typically determined by the height of the tube collars) that is shown as
DFB~r SEPAx~TTON
or DF.s.. In the illustrated embodiment, the tabs have a height (as measured
from the fin
2 5 body surface to the distal edge of the tab) that is less than about half
of the DFB~r
SEPARATION~ ~ this manner, the tabs in one fin do not typically contact tabs
in adjacent
fins (which may cause assembly problems) but can be positioned within the
cooler
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portion of the air, AIN, such as at the top of the boundary layer (distal to
the fin body
surface). In other embodiments (not shown), the tabs extend from only one side
of the
fin body, and in some embodiments (such as those shown in Figures 10 and 11),
the fin
tabs extend outward more than one half of the fin separation, DFIN SEPaRaTION~
to a
distance of about two thirds of the DFB~r SEP.~TTON~ Another embodiment is to
have fins
with tabs punched out of one side of the fin only and with these fins arranged
back-to
back such that the combined fin has everywhere conduction paths for heat
transfer (i.e.,
no openings completely through the fins exist). It is also possible to combine
these tabs
with some tabs that reach all the way across the gap to serve as spacers. In
addition,
some of the tabs can be twisted to introduce vorticity.
Figure 4 illustrates in more detail a sectional view of a heat exchanger of
the
present invention. As shown, a pair of fin bodies 420 are placed in contact
with each
other via tube collars 422 and the collars are press-fit onto a tube 430 that
is used for
carrying a hot fluid, FIN through the heat exchanger. The pair of fins are
separated by a
distance, DF.s., and the tabs 404, 410 shown on the fin bodies 420 extend
outward from
both surfaces 424, 428 of the fin bodies 420 a tab height, HT~. As discussed
previously, the tab height (or distance at which they extend when not
configured with a
bend angle of approximately 90 degrees relative to the body surface 424, 428)
is
typically selected to be less than the distance separating the fins, DF,s,,
and more
2 0 typically, as shown, is selected to be about 40 to 50 percent of this
distance, DF.s.. In
other embodiments, the tab height, HT~, is greater than 50 percent of the
distance, DF,s.a
to place more surface area of the tabs 404, 410 into the cooler portion of the
air flow
(rather than just the tip of the tabs 404, 410), and in one embodiment, the
tab height,
HT,~, is selected to be between about 50 and 75 percent and more preferably
about two
2 5 thirds or about 67 percent of the distance, DF.s..
The tab 404 is shown to include a leading edge 406 and a trailing edge 408.
The
tab 404 is bent or formed in a manner that positions the leading edge 406 to
contact the
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incoming air, AIN, and such that the planar area of the tab 404 is
substantially parallel to
the flow path of the air, AIN. In some embodiments, the tabs 404, 410 are
substantially
parallel to the simple flow path while in other embodiments, some, a majority,
or all of
the tabs 404, 410 are arranged substantially parallel (such as within about 5
to 10
degrees) to the local flow paths or streamlines. Tab 410 is shown to be
generally square
in shape but to include rounded shoulders 414 such that its leading edge is
less likely to
snag or catch debris in the incoming air, AIN, that might clog the air flow
channel
between the fins and reduce heat transfer and/or increase pressure drop. Some
of the
tabs can be more semicircular in shape indicating that the shape of the tabs
404, 410 can
vary on differing fins or within a single fin to practice the invention. The
tabs can also
bend at angles less than 50 degrees. In some situations, it may be
advantageous to have
tabs bend downward in the direction of gravity to facilitate water drainage
from the fin
surface that could result from rain, dew formation (as occurs in the case of
an
evaporator), and spray cooling enhancement.
Figure 5 is an enlarged view of air flow in a channel between a pair of tabbed
fins of the present invention. As shown, the fin bodies 520, 521 have tabs 510
extending at a height, HT~,, from the fin surfaces 528, 529. The fin bodies
520, 521 are
separated by a fin separation distance, DF,s., and the tab height, HT~, is
selected to be
about one half of the fin separation distance, DF,s,. The tab height may be
varied and the
2 0 illustrated embodiment is one useful embodiment. In typical finned tube
heat
exchangers, a boundary layer is formed on each fin surface 528, 529 as the
incoming air,
AIN, flows through the fins 520, 521 from the fin body leading edge 530, 531
toward a
fin body trailing edge 532, 533. As can be seen, the boundary layers extend
outward
from the fin surfaces 528, 529 creating an insulting layer that ends at about
a midpoint
2 5 between the fins 520, 521. Hence, it is desirable to have the tabs 510
extend at a height,
HT~, that allows the fin material, i.e., the material in the tabs 510, to
extend into (and in
some cases, through) the outer portions of the main boundary layers and into
the coolest
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air flowing through the fins 520, 521. Again, the tabs 510 are illustrated as
being
substantially planar with a square shape with rounded or smoothed shoulders
514 to
reduce snagging or filtering of debris in the air, AIN. In one embodiment, the
shoulders
514 have a radius of 0.009 inches but, of course, other larger and smaller
radii may be
implemented.
Figure 6 is a top view (or side view) of a fin or fin portion 600 fabricated
according to the invention. As shown, when the fin 600 is in use, the cool
air, AIN,
flows across a leading edge 611 of the fin 600 then across a first surface 610
of the fin
body. The air then contacts a fin collar (and included tube not shown) 636
causing the
creation of a wake or low pressure area 640 behind the collar 636 (and tube)
and then,
the air continues along the fin surface 610 until it passes over the trailing
edge 612
where it is expelled as hotter air, Ao~, of the heat exchanger in which the
fin 600 is
installed. The fin 600 includes numerous tabs that are formed by punching out
sections
of the fin body (but leaving the material attached on one edge, e.g., a
proximal edge or a
tab seam or connector) and bending or hinging the material upward away from
the
surface 610 or downward away from the opposite surface (not shown) of the fin
600.
For example, a first tab 614 is bent downward relative to the surface 610 and
the
material removed from the body or surface 610 forms a tab opening or hole 616
adjacent
the tab 614. A second tab 620 extends upward relative to the surface 610 at a
2 0 substantially right angle with the removed material (i.e., the material
retained in tab 620)
creating a tab hole or opening 622 adjacent the tab 620. As shown, the tab
holes 616,
622 (and corresponding tabs 614, 620) are substantially square in shape but
other
embodiments of fins of the invention may utilize other shapes.
As shown, the tabs are arranged generally in rows that extend substantially
2 5 parallel to the leading and trailing edges 611, 612 of the fin 600. Note,
that this
particular configuration is not required but is useful for ease of tab pattern
selection
(such as relative to amount of surface area to be utilized), for ease of
manufacturing, and
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for assuring that tabs are positioned to achieve a desired sequentially offset
arrangement.
The offset feature of the invention can be seen by looking at the tabs in Rows
1-4 and
particularly the four tabs of Rows 1-4 shown with the dashed line 630 that can
be said to
be corresponding or adjacent tabs in adjacent ones of the Rows 1-4. Row 1 can
be
thought of as the first row or most upstream row of tabs with Row 2 being the
second
row and immediately downstream row relative to Row 1 (or adjacent to Row 1).
The
tabs shown by line 630 in Rows 1 and 2 can be seen to be offset from each
other.
Likewise, the tab in element 630 in Row 3 is offset from the tab in element
630 in Row
2 (immediately upstream or in the adjacent row), and the tab in element 630 in
Row 4 is
offset from the tab in Row 3.
The amount of offset can vary to practice the invention, with the offset shown
being one useful embodiment, e.g., the opening and tab in the downstream row
is
positioned substantially in the space between adjacent openings/tabs in the
upstream
row. Note, also, that the tabs in the element 630 are offset on a "diagonal"
and this
pattern is continued in several additional rows of tabs. However, other offset
patterns
may be utilized as long as corresponding tabs in adjacent rows are offset from
each
other. Preferably, the offset pattern is selected so as to provide a spacing
between
similarly positioned tabs, such as by skipping a number of rows before placing
a tab in a
similar position within a row (e.g., as shown in Figure 6, a pattern of 8 rows
is used with
2 0 7 rows provided as a "spacer" before repeating a row pattern).
Figure 6 also illustrates that the pattern of tabs may be relatively uniform
or, as
shown, denser in areas of anticipated high flow of the cooling gas or air.
Hence, as
shown, there are fewer tabs placed behind the fin collar 636 (and other
collars) where a
wake is created by the collar 636 and, therefore, later installed tube (not
shown). In this
2 5 manner, a larger percentage of the tabs (and therefore, the area or
material taken of the
fin body 610) are placed in locations in between adjacent fins were heat
transfer is most
likely to occur effectively. Another way of stating this configuration is that
the fm 600
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is configured to have areas of higher porosity in areas upstream of tubes 636
than
immediately downstream (e.g., areas of lower porosity in wake areas). Higher
porosity
or more tabs may also be provided between pairs of tubes to place more heat
transfer
surface in higher flow areas of channels between fin pairs. The spray pattern
is also
selected with due consideration to its impact on heat conduction within the
fin.
In some embodiments, a subset (such as a small percentage such as 10 percent
or
less) of the tabs are purposely not aligned with the main air flow through the
fin (i.e., not
perpendicular to a leading edge of the fin) but are instead skewed or angled
relative to
the main or simple flow path or flow direction of the cooling gas so as to act
as
directional vanes. Typically, these directional vane tabs are positioned near
(e.g., beside
and/or slightly behind) the collars (and inserted tubes) of the fins to direct
the air or gas
flow into areas that otherwise would be starved for flow such as in the wake
region
behind the collar/tube. In other cases, the direction vane tabs may be
fiu~ther upstream to
begin diversion of flow to the wake area prior to the tube collar and tube so
that the flow
redirection can be more gentle, i.e., less dramatic or turbulent. In one
embodiment, at
least some of the tabs near the collar 636 are angled to direct some air flow
from the
main flow path into the wake region 640. Preferably, the angle for the
directional vane
tabs is selected so as to avoid or minimize the creation of vortices behind
these tabs so
as to control increases in pressure drop, e.g., the angle may be less than
about 5 degrees
2 o relative to the local flow paths or streamlines and the like. Unlike a
delta winglet pair,
the tabs in this embodiment gently direct the flow into the wake regions
without causing
turbulence. The reduction in wake size reduces form drag and overall pressure
drop
while at the same time providing better heat transfer coverage in the wake
region behind
the tubes/collar.
2 5 The introduction of the fins, although parallel to the simple flow path or
local
flow paths or streamlines, does alter the flow of the cooling gas relative to
the fin, such
as by increasing friction and by creating multiple thermal boundary layers
within the gas
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flow channel or passage. In some embodiments, patterns of the tabs are
selected with
the express purpose of gently redirecting flow of the cooling gas. For
example, the
pattern of the tabs are selected purposely to fold or direct more of the
cooling gas into
the wake areas and areas of low pressure or flow between two fins. In one
case, the tabs
are offset in diagonal patterns to gently orient (e.g., with minimal
turbulence) toward the
wake regions behind collars and tubes.
In still other embodiments of the invention (not shown), fins are fabricated
according to the invention so as to generate at least some vortices in the
cooling air or
gas. In one vortex generating application, the tabs illustrated and described
in the
invention are utilized in combination with delta winglet pairs, such as near
the tube
collars or other areas of low flow. In this manner, the beneficial effects of
the tabs of the
present invention and of winglet pairs are combined to enhance heat transfer.
The
amount of pressure drop can be controlled by limiting the number of winglet
pairs
utilized, and/or this embodiment may be employed when a higher pressure drop
is
acceptable. In another vortex generating application, tabs that are sharply
angled (such
as over 5 degrees up to 90 degrees) relative to the main or local flow paths
of the cooling
gas are included on a fm. Typically, in these embodiments of the invention,
the majority
of tabs would remain aligned parallel with the main or local flow paths with a
minority
or small number of unaligned tabs being added in strategic locations, such as
locations at
2 0 which winglet pairs are often employed or other locations at which it is
desirable to
create turbulence.
Figure 7 illustrates a section of the fin 600. As shown, the tab 620 is formed
by
bending material upward from the surface 610 to form a hole or opening 622 in
the fin
600. Tab 614 adjacent to tab 620 in the tab row is formed by bending material
2 5 downward from surface 610 to form the hole or opening 616. The tabs 614,
620 are
bent away from the fin (e.g., from surface 610 for fin 620) at a selected
angle, i.e., a
bend or punch angle, ~, that is typically selected to be about 90 degrees for
ease of
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manufacturing but similar or better heat transfer results may be achieved at
smaller bend
angles such as between 30 and 90 degrees. As discussed earlier, tabs bent
downward or
in the direction of gravity can enhance water drainage. The tabs 614, 620 have
a tab
height, HT,~, that is shown to be about one half of the height of tube collar
636, which
typically corresponds to the separation distance between adjacent fins. As
discussed
previously, the tab height, HT~, may be smaller or more preferably, larger
such as 50 to
80 percent and in some tested cases, was about 67 percent to place a larger
portion of the
tab body or tab surface area within the cooler regions of air or gas flow in
the channel
between adjacent fins.
Figure 8 illustrates another embodiment of a fin 800 formed according to the
presenting invention. The fin 800 may be useful in heat exchangers in which
fins are
utilized without tubes. The fin 800 also illustrates a more regular pattern of
tabs and
openings than in Figure 6 with the offset clearly being on a "diagonal," i.e.,
with tabs in
adjacent rows being offset a selected distance from the immediately upstream
row.
Figure 9 is a graph 900 of the exemplary flow paths of a gas at about 1 to 3
meters/second across the surface of a fin such as the fin 600 in Figure 6. As
can be seen,
most of the flow is along a path generally between the tubes or perpendicular
to the
leading edge of the fin. Note, that over much of the fin surface the flow is
predominantly in one direction with the path lines changing only slightly with
distance
2 o from the fin surface. Hence, except very near the tubes or in the wake
regions, aligning
the tabs on a fm along the main flow direction or simple flow path is useful
for
significantly increasing the heat transfer coefficient of a fm while avoiding
or controlling
introduction of drag or pressure drop. Such knowledge of the flow led to the
design of
the tabbed fins shown in Figures 2-8 in which the tabs are generally planar
and aligned
2 5 with the flow, i.e., positioned with their larger planar surfaces parallel
to the main or
simple flow path shown in Figure 9.
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While machining costs and pressure drops may be increased, some embodiments
of the invention can be fabricated with the tabs aligned more particularly
with the flow
path of gas corresponding to the location of the tab, e.g., alignment along
the local flow
path lines. In other words, the tabs may be arranged with many differing
alignments to
suit the flow in that particular region of the fin. It is expected, though,
that the
placement of the tabs in the fin would change the flow relative to the fin and
it may take
numerous iterations to "match" such a tab alignment to the flow. Additionally,
flow
patterns vary with other parameters such as gas or air velocity and the like,
although
laboratory tests have demonstrated that one tab pattern can work well over a
wide range
of air flow rates.
An exemplary tabbed-fin was fabricated according to the invention (similar to
that shown in Figure 6) to allow the effectiveness of the above-described
invention to be
tested. To fabricate small test cores for transient testing, a punching tool
was used in
conjunction with a template from a CAD program that allowed concurrent
punching
(i.e., punching of tabs in both directions away from a fin body). Aluminum fin
material
was used to fabricate fins for testing, as is commonly used for finned-tube
condensers
with %a-in. tubes. An example of the template and fins used in the test are
shown in
Figure 2 as template 230 and fins 210.
Measurements at air flow of 3 m/s in early fin samples showed that the tabbed
2 0 fins provided 68% more air-side heat transfer and had a 33% higher
pressure drop than
similar untabbed fins. For comparison purposes, small cores made up of
advanced fin
materials were also tested, e.g., wavy fins and louvered fins. These cores
were of a
different fin density, but it was believed useful to compare the performance
of the
tabbed fins to the plain fins for these other arrangements designed for
enhanced heat
2 5 transfer. Table 1 shows the test results for these cores and compares the
results to the
results obtained for the tabbed fins of the invention. Note that the wavy fins
and
louvered fins were in a test core containing only one row of tubes. Also, the
tabbed fin
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design had fins spaced at only 5 fins per inch (0.20 inch spacing) because
that was the
stock available. The tab-forming tool used produced tabs that are 0.050 inch
on a side,
so each tab extended only one-quarter way across the gap whereas it is
believed that tabs
that extend halfway or more across the gap perform better.
Percent Change in ~ Percent Change
Enhancement Type Heat in
Transfer Pressure Dro
Wavy fins one tube -5 45
row
Louvered fins 56 136
one tube row
Tabbed fins (3 tube 6~ 33
rows)
Table 1. Percent changes in heat transfer and pressure drop for
enhanced fins on %2-inch tubes compared to a reference of the same
geometry but with plain fins (3 mls approach velocity).
to
As discussed above, some embodiments of the invention comprise tabbed fins in
which some or all of the tabs are arranged to be generally aligned with or to
be
substantially parallel (such as within 5 to 10 degrees or less) to the local
flow path or
local streamlines. Additionally, some embodiments may comprise tabs each with
a
body that is rectangular in shape so as to provide a more desirable aspect
ratio to
enhance heat transfer rates relative to the same overall porosity of a tabbed
fin. With
these design ideas in mind, a tabbed fin (or fin portion) 1000 is shown in
Figures 10 and
11 that utilizes rectangular tabs extending from both sides of a fin body. The
tabs are
arranged at offset angles relative to the simple flow path of the fin 1000,
i.e., a line
2 0 drawn perpendicular to the leading edge 1020 of the fin body 1010, so as
to align with
the normal flow paths and avoid flow disruption and its associated pressure
drop.
Further, the tabs can be oriented at slight angles to the usual local path
lines to help
direct the flow into the stagnant wake regions behind the tubes.
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As shown, the fin 1000 comprises a fin body 1010 with a plurality of tabs 1030
extending at bend angles of about 90 degrees from a first fin surface 1014 and
a plurality
of tabs 1040 extending in an opposite direction at bend angles of about 90
degrees from
a second fin surface 1018. Generally, the tabs 1030, 1040 are alternated in
each tab row,
such as rows 1050, 1054, 1058 so as to create an overall tab pattern in which
corresponding tabs or upstream/downstream tab pairs are offset or purposely
not aligned
along a line drawn perpendicular to the leading edge 1020 of the fin body 1010
so as to
avoid shadowing downstream tabs with upstream tabs. In other words, heat
transfer
achieved in downstream tabs is enhanced by not placing the downstream tabs
directly in
wakes or vortices created by upstream tabs. Wherever possible, consistent with
the
above considerations, the hinged edge of a tab is located closest to the tube
to minimize
the heat conduction path length.
The fin 1000 further comprises tube collars 1026 for mating with heat transfer
or
fluid tubes (not shown). ,The tube collars 1026 define fin separation
distances when the
fin 1000 is mated with another fin in a heat exchanger (such as exchanger 100
in Figure
1). Each tab 1030, 1040 is generally rectangular (with or without a curved
corner having
a small radius to reduce collection of debris at sharp corners) with a tab
body 1060
having a leading edge 1062 that initially contacts flowing gas or air. The tab
body 1060
further has a top edge 1064 distal to the fin body 1010 and a trailing edge
1066.- During
2 0 use, it will be appreciated that higher heat transfer rates are achieved
proximal to the
leading edge 1062 but that the entire surface of the tab body 1060 contributes
to transfer
heat achieved by the tabs 1030, 1040.
Figure 11 illustrates more clearly that the tabs 1030, 1040 are fabricated
with a
bend angle of about 90 degrees measured relative to the fin body surfaces
1014, 1018 so
2 5 as to extend substantially perpendicularly from the fin body 1010. Smaller
or greater
bend angles can be used but one of about 90 degrees is useful for extending
the tabs
1030, 1040 fiarther out into the cooler air flowing across the fin 1000 with
less material
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removal. As shown, the fin body 1010 has a porosity of about 25 percent that
is
achieved with fewer tabs 1030, 1040 than used in the fin arrangements of
Figures 2-9,
which used small, square tabs.
A simple flow line, Ls;mpie r~oW, is illustrated as showing generally air flow
across
the fin 1000 during its use in a heat exchanger. As can be seen, the tabs
1030, 1040 are
positioned at offset angles, B1, 6z, relative to the simple flow path or line,
Lsimple Flow, fat
enables the tabs 1030, 1040 to align with the usual local flow direction and,
further, to
direct the flow more uniformly over the fin surface and into the wake regions.
As
shown, the offset angles, Bl, B2, relative to the simple flow line, Lsimle
r~oW, are about 10
1 o degrees but the offset angles, 91, Ba, may be larger (such as up to about
20 to 30 degrees)
or smaller. The measurement of the offset angles, B1, Ba, is provided as an
absolute
value or variance from the simple flow line, Ls;mpie r~oW" but could be also
provided, as
shown, as 10 degrees, 170 degrees, 190 degrees, and 350 degrees for the
various tabs
1030, 1040 on the fin 1000. A streamline or line representing a local flow
path,
Ls~.eam~ine, is also shown in Figure 11 relative to the tabs 1030, 1040. The
tabs .1030,
1040 are preferably aligned or positioned on the body 1010 such that the tabs
1030,
1040 are substantially parallel to (e.g., within about 5 degrees of) the local
flow path or
streamline, Lstrea~ine. Owing to their flow resistance and pattern and also,
to their local
angles, the tabs 1030, 1040 on the body 1010 act to direct flow, i.e., change
the shape
2 o and location of local flow paths from those found in an untabbed or plain
fin, to reduce
wakes behind tube collars 1026 and tubes (not shown) while reducing drag or
pressure
drop increases associated with adding tabs 1030, 1040. The tab pattern of fin
1000 was
obtained through some trial and error because the introduction of tabs 1030,
1040
altered the shape and location of local flow paths, Ls~.eam~ine~ and several
iterations were
2 5 required to obtain the alignment of the tabs 1030, 1040 with the resulting
streamlines,
Lscreamnne. Those spilled in the art will understand that the resulting
streamlines,
Lscreamnne, may vary with the shape, size, and number of the tabs 1030, 1040,
the
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location, shape, and number of the tube collars 1026, the speed of the air or
gas flow
across the fin 1000, and other parameters. However, it is believed that the
teaching
provided in this description can readily be extended to many other tube and
fin
configurations.
To more fully describe the invention, it may be useful to more fully describe
one
fabricated embodiment of the fin 1000 of Figures 10 and 11 and provide typical
test
results. One particular embodiment or implementation of the fin 1000 was
configured
for a finned tube heat exchanger (such as, but not limited to, the heat
exchanger 100 of
Figure 1) that has 8 fins per inch (FPn. However, the fins 1000 can readily be
used in
different FPI exchangers such as 10 FPI exchangers. In the particular 8 FPI
embodiment
of the fin 1000, the tab length as measured along the top edges 1064 was 0.120
inches.
The fin separation in the heat exchanger (not shown) was about 0.115 inches
and the
tabs 1030, 1040 were designed to have a tab height as measured along the
leading and
trailing edges 1062, 1066 of tab body 1060 of about one half the fin spacing
or about
0.0575 inches. However, as discussed earlier, it is preferable in some cases
to have a tab
height that is greater to place more area of the tab surface in cooler
portions of air flow,
and in these cases, the tab height may be about two thirds or more of the fin
separation
or about 0.0771 inches in the 8 FPI embodiment. The corners are rounded with a
radius
of about 0.009 inches. The tab edges, such as edges 1062, 1066 are not
perpendicular as
2 0 in a typical rectangle but are instead angled inward at about 5 degrees
giving the tab
body 1064 a trapezoidal shape or substantially rectangular shape. The offset
angles, , B1,
B2, were set at about 10 degrees in this particular embodiment of the fin
1000.
To model and understand flow of air through channels defined by two adjacent,
tabbed fins, a bubbler device was utilized by the inventors. In Figure 12, a
drawing
2 5 1200 representing flow visualization photographs is provided for a plain
or untabbed fin
1210. The bubbler device creates numerous bubbles that flow in fluid passing
over the
fin 1210 and the bubbles help to define or allow visualization of local flow
paths or
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streamlines, identified as element 1218. Air coming in, AirIN, in a plain fin
would pass
over leading edge 1212 of the fin and generally flow in streamlines 1218 that
are
generally perpendicular to the front edge 1212 until the streamlines 1218 pass
between
tube collars 1220 and out of the channel as hotter air, Airo~. At this point,
the local
flow paths are directed inward away from and around the tube collars 1220 and
create
wakes 1230, which have are relatively long with a wake length, LW~ 1, of about
1 to
1.5 inches with %2-inch tube collars or tubes at 3 meter/second flow. The
streamlines
1218 are also disrupted by inner tube collars 1230 and another wake 1240 is
created
with stagnant, separated flow and again the wake 1240 has a length, LWpKF, a,
that may
be about 1 to 1.5 inches. The creation of wakes 1230, 1240 is important
because in the
wakes 1230, 1240 heat transfer with the tube/tube collars and/or fin 1210 is
dramatically
reduced when compared with other areas of the fin 1210 or when compared with
the
front of the tube/tube collars because of the stagnant flow in these regions.
An
important design goal associated with implementing tabbed fins, therefore, is
to reduce
the size of the wakes 1230, 1240 or to at least increase heat transfer in
these areas of the
fin 1210.
Figure 13 illustrates with flow visualization 1300 the effect the tabs 1314
have
on air flowing across, AirIN, a tabbed fin 1310 before being discharged as
AirouT. The
tabbed fin 1310 is configured with square tabs 1314, such as 0.050-inch by
0.050-inch
2 0 tabs, that are aligned substantially perpendicular to the front or leading
edge 1312 of the
fin 1310 so as to be generally parallel to the simple flow path of the air
relative to the fin
1310. The pattern used in fin 1410 is similax to that shown in Figures 2 and
6. The
local streamlines 1318 are shown less compressed between the tube
collars/tubes 1320
as the flow channels and pressure drop caused by the tabs 1314 act to
distribute the flow
2 5 more uniformly and redirect flow forcing at least a portion to flow more
directly behind
the tube/tube collars 1320. As a result, the side wakes 1330 have a length,
LW,~ l, that
is shorter, such as about 0.5 to 1 inch rather than 1 to 1.5 inches.
Similarly, the pattern
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of tabs 1314 near the inner tube/tube collar results in a wake 1340 that has a
wake
length, LW~ a, that is significantly smaller.
Figure 14 illustrates with flow visualization 1400 the usefulness of a tabbed
fin
1410 comprising at least some tabs 1414 that are bent or positioned at an
offset angle
relative to the simple flow path. The pattern of the tabs 1414 is the same as
in the fin
1310 of Figure 13 except that a portion of the tabs 1414 have been bent at an
offset
angle between about 10 and about 15 degrees relative to the simple flow path.
In this
manner, the incoming air, AirIN, is allowed to flow in local streamlines 1418
but is also
directed to flow into the wakes 1430 and 1440 so as to create wakes with
lengths, LW,~
1 and LW~ Z, that are significantly shorter than the wakes of a plain fin and
even shorter
than tabbed fins for which the tabs are not positioned at an offset angle (or
the offset
angle is zero). As a result, the air flowing out, Airo~, is able to absorb
more heat from
contact with the tabs 1414 and the tube/tube collars 1420 and fm surfaces. As
will be
understood, numerous patterns of the tabs 1414 can be used to reduce wake size
by
directing the air flow but an important consideration is the trade off between
redirecting
air flow to reduce wake size and the undesirable increase in drag and pressure
drop that
can occur with tabs with too large an offset angle. Generally, tabbed fins
according to
the invention utilize offset angles that are relatively small, such as less
than 15 degrees,
so that the tabs remain substantially parallel with the simple flow path and
present a
2 0 relatively small contact profile with the flowing gas or air.
Testing of a set of fins 1000 shown in Figure 10 and 11 was performed by the
inventors in a 8 FPI configuration and in a 10 FPI configuration with
comparison
provided to plain fins in the same configurations. The results are provided in
the graphs
in Figures 15-20. In the 8 FPI configuration, the fm separation distance or
gap was
2 5 0.115 inches for the plain and tabbed fins and was 0.09 inches in the 10
FPI
configuration for both types of fins. The tabs had a height of 0.0575 inches
which
resulted in the 8 FPI configuration testing tabs extending 50 percent of the
fin separation
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distance while in the 10 FPI the fins extended to approximately two thirds of
the gap or
more particularly about 64 percent of the fin separation distance. This
resulted with fins
in adjacent fins actually overlapping by about 0.025 inches, but the tabs were
arranged
so as not to contact or abut when the heat exchanger or test assembly was
fabricated.
The porosity of the tabbed fins was approximately 25 percent.
Turning to Figure 15, a graph 1500 is provided of the results of testing a
plain,
aluminum fin at varying air flow velocities. Determined heat transfer
coefficients and
isothermal pressure drops are plotted in the graph versus varying air flow
velocities,
which are typical of conventional air cooled condensers. Line 1510 represents
values of
the heat transfer coefficient for the plain fin while line 1520 represents
values of the
pressure drop in the plain fin, 8 FPI configuration. As shown, in the range of
2 to 3 m/s
face velocity the heat transfer coefficient ranges from about 38 to 50 W/maK
while the
pressure drop ranges from about 22 to 43 Pa.
Figure 16 provides a graph 1600 similar to graph 1500 showing values of the
heat transfer coefficient for a tabbed fm according to the invention (see
Figures 10 and
11) with line 1610 and values of pressure drop with line 1620 at varying face
velocities.
As can be seen, there is a significant improvement in the heat transfer
coefficient for the
tabbed fin compared with the plain fin. Specifically, in the same face
velocity range of 2
to 3 m/s, the heat transfer coefficient ranges from about 65 to about 85
W/maK. This
2 0 represents about a 70 percent increase in the heat transfer coefficient
for the tabbed fin
relative to the plain fin of similar size, material, and thickness. The "cost"
of this added
efficiency or effectiveness of the tabbed fin is an increase in pressure drop.
However, as
shown, the pressure drop for the tabbed fin in the 2 to 3 m/s face velocity
range is only
about 31 to 62 Pa which represents a relatively low increase in pressure drop
of about 50
2 5 percent that would likely be an acceptable tradeoff for the significant
increase in the fins
heat transfer rate and in effectiveness of heat exchangers incorporating such
tabbed fins.
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Figure 17 illustrates a graph 1700 of ratios comparing design parameter values
of the tabbed fin with the plain or original fin plotted against the varying
face velocities
used during the testing of the fins. As shown, line 1710 represents values of
a
dimensionless heat transfer coefficient called the Colburn j factor for the
tabbed fin
compared with the plain fin. Line 1720 represents values of the friction
factor (i.e., a
dimensionless pressure drop for internal flow) for the tabbed fin over the
valued for the
plain fin. As can be seen from these two ratios, the heat transfer coefficient
is
consistently greater than for the plain fin but so is the pressure drop. Line
1730
represents values of a ratio of the above two ratios. This ratio is
significant in that the
ratio is greater than one over the various face velocities indicating that the
increase in
heat transfer coefficient values is always greater than the corresponding
increase in
pressure drop. Finally, line 1740 illustrates values for the design equation
of
(J/Jo)/(F/Fo)~(1/3) that is utilized by heat transfer designers in determining
whether a
design change, such as tabbed fins, can provide more heat transfer at the same
fan
power.
Figures 18-20 provide results of testing similar to those provided in Figures
15-
17 but for a fin configuration of 10 FPI. Figure 18 illustrates a graph 1800
of the results
of a plain fin configured at 10 FPI with line 1810 representing heat transfer
coefficient
values determined for the plain fm at various face velocities. When face
velocities vary
2 0 range from 2 to 3 m/s, the heat transfer coefficient ranges from about 40
to 48 W/m2K.
Line 1820 represents pressure drop values for the varying face velocities, and
in the
range of 2 to 3 m/s face velocity ranges from about 28 to 55 Pa.
Figure 19 provides a graph 1900 showing similar test results for a fin tabbed
as
shown in Figures 10 and 11 with a porosity of 25 percent and placed in a 10
FPI
2 5 configuration. As shown with line 1910, the heat transfer coefficient is
greatly
improved ranging from about 80 to about 98 W/m2K when the face velocity ranges
from
2 to 3 m/s. This represents an increase in the heat transfer coefficient for
the tabbed fin
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relative to the plain fin of about 100 percent. Line 1920 represents the
corresponding
pressure drop, which in the 2 to 3 mls face velocity range has a value of
about 45 Pa to
about 85 Pa. The "cost" is then an increase in pressure drop for the tabbed
fin of about
55 to 60 percent, which as with the 8 FPI configuration would be an acceptable
price to
pay for such a large enhancement of heat transfer effectiveness for the air
side fins and
for heat exchangers incorporating such tabbed fins. Figure 20 illustrates a
graph 2000 of
design ratios similar to graph 1700 of Figure 17 with lines 2010, 2020, 2030,
and 2040
representing values of heat transfer design ratios. As with the 8 FPI design,
the tabbed
fin testing in a 10 FPI configuration shows through the values of the design
ratios in
graph 2000 that the tabbed fin concept of the present invention, and
particularly, the
pattern of the fins in Figures 10 and 11, has merit and should be considered
further for
inclusion in heat exchangers, and especially, for inclusion in air cooled heat
exchangers
in which fan power is a limited resource.
As can be seen, the tabs may be selected to have a height as measured from the
fin body that varies significantly to practice the invention, but that will
typically be
selected to be about the fin separation distance or less to avoid problems in
fabricating
the heat exchanger. More typically, the tab height is selected to remove
adequate
material from the fin body to have the tab body extend out into the cooler
flowing air.
Generally, testing has shown that it is desirable for the edge of the tab
distal to the fin
2 o body to extend beyond the top of the boundary layer so as to place a
significant portion
of the tab body surface area in the very coolest air. With this in mind, most
embodiments of the invention utilize a tab height in the range of about 25 to
75 percent
of the gap or fin separation distance. For an 8 FPI exchanger, the gap is
about 0.115
inches and the tab height is selected from the range of about 0.029 inches to
about 0.087
2 5 inches. More typically, the tab height is selected from the range of about
40 to 67
percent of the gap. In these cases, the tab height is in the range of about
0.0460 inches to
about 0.077 inches for the 8 FPI exchanger. In a 10 FPI heat exchanger, the
gap is
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smaller at about 0.09 inches and hence, the tab heights would be selected from
the
overall range of about 0.022 inches to about 0.0675 inches while the narrower
range is
about 0.036 inches to about 0.06 inches.
While generally the tabs or a majority of the tabs are aligned so as to
control
pressure drop, it may be useful in some embodiments of the invention to have a
subset
of the tabs purposely arranged or configured to generate turbulence. These
turbulence
generating tabs may, for example, be arranged with offset angles greater than
10
degrees, e.g., at or near 90 degrees but often at lower offsets, and may be
located on the
fm in select areas to create higher heat transfer in otherwise low heat
transfer areas (such
as behind the collars and/or tubes) or may be interspersed among the other
tabs. The
number of tabs in the subset relative to the other tabs may vary widely to
practice the
invention and will typically be driven by allowable pressure drop for a heat
exchanger
application.
In other embodiments (not shown), a transition to turbulence can be promoted
by
the configuration of the tabs and base fin by manipulating surface roughness
or texture.
Generally, the tabs and base fin have low surface roughness, e.g., are smooth
metal. In
some embodiments, though, surface roughness of the base metal is increased to
a
desired amount to cause adjacent flow to begin to transition to turbulent
flow. The
surface roughness can be thought of as a surface treatment and may include (or
be
2 0 replaced by) dimples or other surface treatments that alter the surface
texture from
smooth to a level of roughness that promotes turbulent flow. In one
embodiment, the
surface treatment is applied only to tabs (or portions of each tab or a subset
of the tabs)
while in others the treatment is applied only to the fin and/or tubes. In
other cases, the
surface treatment may be applied to all of these components or any
combination.
2 5 The above disclosure sets forth a number of embodiments of the present
invention. Other arrangements or embodiments, not precisely set forth, could
be
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practiced under the teachings of the present invention and as set forth in the
following
claims. Particularly, the use of tabs aligned and configured as discussed in
the above
description is readily applicable with fm arrangements other than parallel
plate
arrangements. For example, air-cooled condensers are often configured with
tubes upon
which fin material (such as aluminum foil) is helically wound. Such condensers
may
readily incorporate tabbed fins to enhance heat transfer, such as by punching
the fin
material prior to winding the material onto the tube. The tabs illustrated in
the figures
are generally planar with a rectangular cross-section when viewed from the
leading
edge. Other tab cross sections can readily be envisioned and are considered
within the
l0 breadth of the above disclosure. For example, tabs with an upside down "L"
cross-
section can be substituted for the illustrated tabs and may be useful for
placing greater
tab surface area in the cooler air flow while not unacceptably increasing drag
and/or
manufacturing costs. Other tab cross sections include a stepped cross section,
a wavy or
serpentine cross section, an L-bend, a U-bend, and the like. Fin bodies can be
fabricated
with tabs extending from both sides by forming a composite fin from two fins
with tabs
extending from one side and their planar surfaces abutting.
