Note: Descriptions are shown in the official language in which they were submitted.
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HEAT EXCHANGER HAVING A COMPACT DESIGN
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to and claims the benefit of U.S. Provisional
Application No. 61/756,784 entitiled "HEAT EXCHANGER HAVING A COMPACT DESIGN"
filed on January 25, 2013, the contents of which are incorporated herein by
reference.
FIELD OF THE INVENTION
The present inventive subject matter generally relates to heat exchanging
devices including heat exchanging tubes.
BACKGROUND OF THE INVENTION
Heat exchangers, are devices for transferring heat from one medium to another,
typically from one fluid to another or to a surrounding environment, without
allowing
the fluids to mix. Some examples are: automobile radiators; air conditioners,
and
steam and hot water radiators, which are used to produce or remove heat. In
order to
prevent mixing of the fluids, or liquids, a barrier is provided between the
two liquids or
media. Many different heat exchanger barrier designs are used. In a "plate and
frame"
design, which is very compact, two liquid streams pass on opposing sides of
one or
more plates. The total heat transfer surface may be increased by increasing
the area of
plates and the number of plates. In a "tube and shell" design, one stream of
liquid flow
passes through tube(s) and the other through the remaining space inside a
shell that
surrounds the tubes.
Though improvements to such heat exchangers have been made over the years,
there remains a need for further improvements that increase efficiency,
improve
performance, reduce cost, and/or reduce the size of heat exchangers.
SUMMARY
A first embodiment of the present inventive subject matter is a heat exchanger
having an economizer configured as a ring of tubes in a periphery of the heat
exchanger. This embodiment includes a cylindrical flue collector and a
manifold at
either end of the cylindrical flue collector. In most embodiments, the
outermost ring of
tubes is an economizer; however, it is conceivable that the innermost ring of
tubes
would be an economizer. The manifold has a plurality of chambers. The manifold
can
be made of steel or plastic and governs fluid flow rate and direction within a
ring of
tubes. At least two rings of heat exchanging tubes, an outer ring and an inner
ring, are
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within the cylindrical flue collector. The rings of tubes are concentric with
each other,
although this is not required.
Fluid flows within the tubes from the outer ring of tubes, which is an
upstream
ring, to the inner ring of tubes, which is the downstream ring. The outer ring
of tubes
preheats fluid within the tubes to a predetermined temperature range and the
inner
further heats the fluid to a temperature higher than the predetermined
temperature
range.
Additional features of this embodiment include multi-dimensional heat exchange
tubes. The heat exchanger tubes have a main section that has a circular cross-
section
and have end sections that have a flattened cross-section. A tube sheet
secures each
of the tubes within the heat exchanger. The tube sheet has holes that engage
respective tubes. The tube sheet can act as a barrier between the manifold and
a tube
chamber within the cylindrical flue collector. The tubes can be welded
directly into the
manifold. The tubes can be finned or finless. If the tubes, are finless, the
ring of tubes
should be equipped with baffles to help enhance the surface area for heat
transfer. In
this embodiment, the baffles would be positioned in gaps between the tubes.
Material
used for the fins is preferably 439 stainless steel; however, the material can
also be a
hybrid of stainless steel and titanium.
A second embodiment of the inventive heat exchanger includes a plurality of
heat exchange tubes. Each heat exchange tube has a main section with a
circular
cross-section, and an end section with a non-circular cross-section. The end
sections of
the heat exchange tubes are positioned proximal to one another in an arc. A
dimension
of the end sections extending along the arc (i.e., tangential to the arc) is
smaller than a
dimension of the end sections extending transverse to the arc, thereby
providing a first
gap between the end sections of the heat exchange tubes that is larger than a
second
gap between the main sections of the heat exchange tubes. The flattened cross-
section
of the tube can be oval, rectangular, or any other shape having a dimension
tangential
to the arc that is shorter than the dimension that is perpendicular to the
arc. A
transition zone exists between the main section and the end section. The tubes
are
welded to a tube sheet at the transition zone.
A third embodiment of the heat exchanger includes a plurality of heat exchange
tubes. Each heat exchange tube has a main section with a circular cross-
section and an
end section with a non-circular cross-section. The end sections of the heat
exchange
tubes are positioned proximal to one another in an arc. And a distance of a
center of
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each tube from an adjoining tube center is approximately one and a half times
or less
than the tube diameter. In this or in any other embodiment, the tubes have
microfins
for enhancing heat transfer between flue gas and water contained within each
tube.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a perspective view of a heat exchanger;
Fig. 2 shows the heat exchanger of Fig. 1 with the outer shell removed;
Fig. 3 shows a cut-away view of a manifold of the heat exchanger of Fig. 1;
Fig. 4 shows a view of a plurality of heat exchange tubes coupled to a tube
sheet;
Fig. 5 shows a perspective view of the tube sheet of Fig. 4;
Fig. 6 shows a perspective view of a tube of the present heat exchanger;
Fig. 7 shows a perspective view of an end of the tube of Fig. 6; and
Fig. 8 shows a perspective view of a header and heat exchange tubes having
attached
baffles.
DETAILED DESCRIPTION
Although the invention is illustrated and described herein with reference to
specific embodiments, the invention is not intended to be limited to the
details shown.
Rather, various modifications may be made in the details within the scope and
range of
equivalents of the claims and without departing from the invention.
One common component of heat exchangers is an economizer, which preheats
fluid that is intended to be heated. An economizer lowers the difference
between the
temperature of the flue gases and the temperature of the fluid exiting the
economizer
to reduce the work necessary to heat water to a target temperature.
Tubular type heat exchangers such as used in economizers or superheaters in
heat recovery steam generators usually utilize pairs of upper and lower
headers which
are connected together by multiple vertically-oriented tubes, so that hot
gases such as
derived from a gas turbine exha st can flow across the tubes to heat fluid
flowing
vertically in the tubes. Such heat exchangers having various tube
configurations are
known; however, such heat exchanger designs utilizing pairs of upper and lower
headers have typically been undesirably expensive, so that improved
configurations
and designs for such heat exchangers have been sought.
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The use of an economizer in a heat exchanger naturally causes the heat
exchanger to require additional material and, therefore, additional space. The
overall
design of a heat exchanger has to account for the additional equipment
required for the
heat exchanger. This problem is exacerbated by the necessity to have access
space to
weld heat exchanger tubes in place. Therefore, it would be extremely
beneficial to
have either a compact economizer design or a compact heat exchanger tube
arrangement or both to limit the amount of space and material consumed by the
entire
heat exchanger.
Fig. 1 is an embodiment of the inventive heat exchanger 2. The heat exchanger
2 includes a shell 4, a water inlet 6 and a water outlet 8. An upper header
10a is
located on an upper end of the heat exchanger 2 and a lower header 10b is
located on
a lower end of the heat exchanger 2. The upper and lower headers 10a and 10b
are for
directing the fluid flow of the heat exchangers throughout
The shell 4 acts as a flue collector and provides an alternate means for
exhaust
of flue gas. Condensate and flue gas is expelled via a port 12 at the bottom
of the shell
4. The shell 4 is preferably made from stainless steel; however, it can be
made from
any other material capable of withstanding plastic deformation from varying
thermodynamic stresses and resistance to acidic condensate. The shell 4 shown
in Fig.
1 is round. However, the shell of the inventive heat exchanger is not required
to be
round, it can be any shape capable of housing a plurality of heat exchanger
tubes
consistent with the tube descriptions provided below.
Inside the shell 4 is a plurality rings of vertical tubes. As shown in Fig. 2.
Two
rings 14 and 16 of vertical tubes are placed within the shell 4. The rings of
tubes are
essentially concentric with the shell 4; however, as mentioned above, it is
not
necessary that these rings be concentric with the shell or even with each
other.
The outer ring of tubes 14 is an economizer ring tubes. The economizer ring
takes advantage of waste heat coming from flue exhaust to preheat water
flowing
through the tubes to a temperature slightly below a target temperature.
Therefore, the
outer ring of tubes 14 is the upstream ring. Fluid flows into the outer ring
of tubes 14
from the inlet 6 to be preheated. It circulates through tube groups until it
is finally
discharged into the inner ring of tubes to be heated to a target temperature.
It is
conceivable that the innermost ring of tubes would be an economizer. For
example, if
flue exhaust is configured to travel along the interior of the shell 4, the
economizer
should be configured as the inner ring to improve heat exchanger efficiency.
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The inner ring of tubes 16 is configured in closer contact with a heat source
to
finish heating the water up to the target temperature. As a result of the
preheating
from flowing through the tube groups of the outer ring of tubes 14, the final
heating is
not as laborious and the heat exchanger is thus more efficient. Like the outer
ring of
5 tubes 14, the inner ring of tubes 16 is divided into tube groups. Thus,
heat makes
multiple passes along the tube groups before it finally passes through the
heat
exchanger outlet 8 and made available for use.
The upper header 10a (also herein referred to as a "manifold") is shown in
Fig.
3. The header is what ultimate determines flow direction 11a and llb of each
tube
group. The header is typically made of steel; however, it is possible to use
plastic or
any other material cap able of withstanding temperature variations resulting
from the
heat exchanger's operation. The header governs fluid flow rate and direction
within a
ring of tubes. The header is compartmentalized with a plurality of waterways
18a and
with a transition waterway 18b. Water flows into the chamber through the inlet
6 and
is directed by waterways 18a of the water into the outer ring of tubes. As
shown in Fig.
3, one of the waterways 18a connect to a tube group having six tubes. However,
it is
not required that the tube groups be a minimum or maximum of six tubes.
Rather, any
number of tubes is possible as long as the most efficient heat transfer is
achieved.
Fluid is directed back and forth along the outer ring of tubes 14 until it
reaches
transition waterway 18b. At that point, fluid is transitioned to the inner
ring of tubes
16. As shown in Fig. 3, the inner ring of tubes is intimately positioned next
to flue 20
for greater heat transfer.
With reference to Fig. 3 and Fig. 4, a tube sheet 22 is provided at an upper
end
and a lower end of the rings of tubes 14 and 16. The ends (or at most an end
portion)
of the tubes within the rings of tubes 14 and 16 engage with through holes 20
within
the tube sheet 18. The tube sheet acts as a barrier between the headers 10a
and 10b
and a tube chamber within the shell 4. Thus, a heat exchange tube chamber is
formed
by the bounds of opposing tube sheets (a second tube sheet 26 is shown in Fig.
2) and
the shell 4. The holes 24 shown in the tube sheet 22 of Fig. 4 are oval or
oblong;
however, this shape is not a requirement. The holes 22 can be round or any
other
shape capable of receiving the tubes of the rings 14 and 16 and a
corresponding weld.
The interaction of the rings of tubes 14 and 16 with the tube sheet 22 is
shown
in Fig. 5. Each ring of tubes 14 and 16 is made of a plurality of individual
tubes 26 that
have a variable cross section. Each heat exchange tube 28 has a main section
with a
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circular cross-section, and an end section with a non-circular cross-section.
The end
sections of the heat exchange tubes are positioned proximal to one another in
an arc
(which ultimately forms a circle). It is preferable that a dimension of the
end sections
extending along the arc (i.e., tangential to the arc) is smaller than a
dimension of the
end sections extending transverse to the arc, thereby providing a first gap
between the
end sections of the heat exchange tubes that is larger than a second gap
between the
main sections of the heat exchange tubes. That way space is provide between
adjacent
tubes 28 to access the tube sheet 22 to provide a weld.
The tube 28 is shown generally in Fig. 6 and includes a main body 30 and end
sections 32 having a flattened cross-section. The flattened end 32 is shown is
being
relatively oval. However, the flattened end 32 can have a rectangular or a
circular
cross section having a size that is smaller than that of the main body cross
section. In
is notable that, with end sections 32 having cross sections smaller than that
of main
body 30, heat transfer can be enhanced as the flowrate through the tube exit
is choked
or at least hindered, which causes the fluid within the tube to be in contact
with the
heated surfaces of the main body 30 of the tube for a greater amount of time
than if
the tube outlet were the same size of the main body 30. The effect should be
that the
shape of the tube end 32 has a dimension tangential to the arc that is shorter
than the
dimension that is perpendicular to the arc.
As shown in Fig. 7, the tube end 32 includes the flattened section 34 and a
transition zone 36. The transition zone 36 is that part of the tube that has a
variable
cross-section between the main body 30 and the tube end 32. Each of the tubes
is
welded to the tube sheet 22 at a point just above the transition zone 36 and
below the
flattened section 34. The flattened tubes allow for welding access so that the
tubes can
be welded to the tube sheet 22. The result is that the distance between tubes
on the
side of the tube sheet having the flattened ends 34 is no less than it would
have been if
the tubes were not flattened. On the other side of the tube sheet 22, however,
the
tubes are allowed to be much closer together thereby redirecting the overall
size of the
heat exchanger or providing the addition of more tubes. Generally, a distance
of a
center of each tube from an adjoining tube center is approximately one and a
half times
or less than the tube diameter. One advantage to bringing the tubes closer
together is
that flue gas velocity over the fin tips is increased, which increases heat
transfer on the
inner row of tubes.
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The tubes are preferably 439 Stainless Steel, which helps to avoid fouling.
Alternatively, the tubes can be a hybrid of 439 stainless steel and titanium
or any other
material that helps to avoid fouling (with or without 439 stainless steel).
The tubes of the heat exchanger can be supplied with fins (not shown) to
increase heat transfer. Due to the close proximity of one tube relative to an
adjacent
tube, the tube should be microfins A higher fin (non-microfin) is possible.
However,
the microfin allows the tubes to be placed closer together, which thereby
increases the
velocity of the gas flowing around the pipes. Since the microfin is a much
smaller size
that a higher fin, its use results in a substantial material cost savings. As
an
alternative to microfins, baffles can be used to direct the flow of hot gas
over the tubes.
As shown in Fig. 8, baffles are positioned in gaps between the tubes. For
example, the
baffles 38 are added around the tubes of the economizer ring 16. Further a
baffle strap
40 can be added to the space between the tubes to help aid the fluid flow
along the
pipe material used for the fins is preferably 439 stainless steel; however,
the material
can also be a hybrid of stainless steel and titanium.
The benefits of the economizer of the present heat exchanger are many. The
configuration of the economizer in combination with the inner ring of tubes
provides the
ability to meet these benefits. The circumferential spacing between the
centerlines of
the inner row of tubes is designed to provide sufficient flue gas velocity
across the fin
tips to provide adequate heat transfer. This is controlled in part by the oval
or elliptical
tube end. The residual heat not absorbed by the inner tube row is absorbed by
the
outer tube row. The tubes of the outer row also have oval tube ends to control
the tube
centerline spacing. The absorption of heat in the outer row of tubes causes
flue gas
condensation. The result is a boiler with a thermal efficiency greater than
90%.
Because the tube ends are welded into the tube sheet, a minimum of one and a
half times the tube diameter spacing is between the tube centerlines in order
to allow
sufficient room to deposit the welding filler metal. When using a micro fin
tube, a
spacing of one and a half times the tube diameter may be too large of a gap
between
tubes. In order to bring the tubes closer while maintaining the one and a half
times the
diameter spacing, the tube end is formed into an oval. The minor radius (rM)
of the
oval is about one fourth the size of the tube diameter. The tube is oriented
so that the
centerline of the tube oval is in line with any line starting from the center
of the round
tube sheet to any tangent point on the tube sheet's outer diameter. The tubes
are
spaced at 1.5 x (2 x rM) and adequate space is provided to deposit weld filler
material.
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The heat exchanger utilizes baffles on the outside of the outer row of tubes
to
increase the velocity of the flue products, resulting in increased heat
transfer into the
fins, through the tube and into the water. The clearance between the fin tips
and baffle
is 0". Further, pliable insulation is added to the hot face of the baffles to
assure that
the baffle contour closely follows the fin profile, minimizing any gaps caused
by
inconsistent fin height or non-straight tubes. The baffle is also insulated on
the cold
side to prevent excessive heat loss through the baffle face directly into the
exhaust
products of combustion. The baffles are held in in place by circumferential
bands
around the outside of the tubes in multiple locations.
It will be understood that many additional changes in the details, materials,
steps and arrangement of parts, which have been herein described and
illustrated to
explain the nature of the invention, may be made by those skilled in the art
within the
principal and scope of the invention as expressed in the appended claims.
While preferred embodiments of the invention have been shown and described
herein, it will be understood that such embodiments are provided by way of
example
only. Numerous variations, changes and substitutions will occur to those
skilled in the
art without departing from the spirit of the invention. Accordingly, it is
intended that
the appended claims cover all such variations as fall within the spirit and
scope of the
invention.
