Note: Descriptions are shown in the official language in which they were submitted.
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MULTI-CROSS SECTIONAL FLUID PATH CONDENSER
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0001] The present invention relates to refrigeration system air-cooled
condensers.
DESCRIPTION OF THE BACKGROUND
[0002] A typical refrigeration system condenser consists of multiple,
serpentine heat
transfer fluid paths (or circuits) such that the superheated heat transfer
vapor entering each
circuit (path) will be condensed completely prior to leaving the heat exchange
device. Figure
3 illustrates an example of a prior art condenser tube bundle. The condenser
consists of
approximately 50 serpentine tubes, with one inlet header and one outlet
header. Vapor enters
the upper header (inlet) and is dispersed into all 50 tubes, all having the
same diameter. For
the entire fluid flow path, the number of the tubes remains constant, and the
cross-sectional
area of each tube remains constant. At the bottom of the tube bundle, the
condensed
refrigerant is collected at the outlet header.
SUMMARY OF THE INVENTION
[0003] The overall heat transfer coefficient is primarily controlled by the
external
heat transfer coefficient and at other times by the internal film heat
transfer coefficient. At
each circuit entrance (or path), the entire volume exists in a gaseous (or
vapor) state. The
initial vapor velocity at each circuit entrance is significant resulting in a
high internal pressure
drop per incremental fluid circuit length which in turn provides a significant
internal film heat
transfer coefficient. The external heat transfer coefficient governs heat
removal in this portion
of each circuit. As heat transfer continues between the refrigerant and air
along each circuit
length and the heat transfer fluid (still in a vapor state) reaches
saturation, the vapor begins to
condense. As a result, and continuing along each circuit length, the vapor
volume and
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velocity decrease. The vapor exit velocity for each circuit is virtually nil ¨
the heat transfer
fluid in liquid form exits the condenser. The continuous reduction in vapor
velocity along
each fixed cross sectional area circuit length decreases the internal film
heat transfer
coefficient. Moreover, the internal film heat transfer coefficient prior to
approaching the exit
region of each circuit limits the condenser's potential or overall heat
transfer capability.
[0004]
Applicant has observed certain deficiencies in the prior art, including that
while the volume and velocity of vapor is a maximum at the entrance of the
first pass, there is
little or no vapor velocity in the last pass. The significant inlet vapor
volume produces a high
refrigerant pressure drop in the first pass due to the high vapor velocity.
This in turn limits the
refrigerant mass flow rate per tube (or circuit/path). Conversely, the very
low vapor velocity
in the last pass adversely affects the internal film heat transfer coefficient
and thus reduces
the condenser's total heat transfer capability.
[0005] The
present invention ameliorates heat transfer deficiency of the prior art as
well as high initial refrigerant pressure drop in the first pass by providing
multi-cross
sectional fluid paths (circuits) for condensation coupled with segmented
headers in lieu of
return bends. Thus at the entrance of each circuit when the vapor volume is
significant, a
larger cross-sectional area is provided for each circuit. The larger total
initial cross sectional
area reduces the internal pressure drop and the vapor velocity while
maintaining the internal
film heat transfer coefficient above the external heat transfer coefficient.
As the vapor volume
decreases along each circuit length as a result of condensation, the total
cross sectional area is
reduced to maintain a threshold internal film heat transfer coefficient that
is equal to or
greater than the external heat transfer coefficient. This decrease in total
cross sectional area
may be accomplished by incorporating a multiple pass circuit selection coupled
with a greater
total cross sectional area for the initial fluid path in comparison to later
passes. This
arrangement lowers the initial heat transfer fluid pressure drop per
incremental circuit length
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with minimal heat transfer sacrifice in the first pass. Moreover, it
significantly improves the
condenser's heat transfer deficiency by increasing the internal film heat
transfer coefficient in
the later passes in comparison to the prior art single cross-sectional area
circuit devices.
Overall, the multi-cross sectional condenser of the invention provides greater
heat rejection at
a lower heat transfer fluid pressure drop. The multi-cross sectional fluid
path condenser of the
invention can be implemented using larger tubes in the first pass and smaller
tubes in
subsequent passes, or by using more tubes in the first pass and fewer tubes in
subsequent
passes, or by some combination of the two, that is reducing both the number of
tubes and the
cross-sectional area of the tubes in with each subsequent pass.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Figure 1 is a cutaway perspective view of a evaporative refrigerant
condenser.
[0007] Figure 2 shows the principal of operation of an evaporative
refrigerant
condenser.
[0008] Figure 3 shows a prior art evaporative refrigerant condenser tube
bundle.
[0009] Figure 4a is a plan view photograph of a mockup of a multi-cross
sectional
area tube bundle (also referred to herein as "heat exchange bundle") according
to an
embodiment of the invention;
[0010] Figure 4b is a formal drawing corresponding to the photograph of
Figure 4a
[0011] Figure 5a is a perspective view photograph of the mockup shown in
Figure 5a.
[0012] Figure 5b is a formal drawing corresponding to the photograph of
Figure 5a
[0013] Figure 6a is the plan view photograph of Figure 2a, with arrows to
show the
refrigerant flow path.
[0014] Figure 6b is a formal drawing corresponding to the photograph of
Figure 6a.
[0015] Figure 7a is a labeled version of the photograph of Figure 5a.
[0016] Figure 7b is a formal drawing corresponding to the photograph of
Figure 7a.
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[0017] Figure 8
is a sketch of an embodiment of the invention having four condenser
sections.
[0018] Figure 9
is a perspective view of an embodiment of the invention having three
condenser sections arranged so that the inlet header, outlet header and
intermediate headers
are all on the same side of the device.
DETAILED DESCRIPTION
[0019] This
invention relates particularly to condenser coil bundles used in refrigerant
condensers, and particularly (although not exclusively) in evaporative
refrigerant condensers
of the type shown in Figures 1 and 2 configured to indirectly transfer heat
between a
superheated refrigerant and ambient air, operative in a wet mode or a dry mode
as described
below depending on ambient atmospheric conditions, such as temperature,
humidity and
pressure.
[0020] The
apparatus 10 includes a fan 100 for causing air to flow through the
apparatus, and as shown schematically in FIG. 1, sitting atop housing 15. At
normal ambient
atmospheric conditions where freezing of the cooling liquid, typically water,
is not of
concern, air is drawn into the plenum 18 of the apparatus via air passages at
the bottom of the
unit through the open air intake dampers, and enters the evaporative heat
transfer section 12
where heat transfer takes place involving the distribution of water from a
water distribution
assembly 90 driven by a pump 96. When the ambient temperature and the
temperature of the
cooling liquid fall to indicate a concern of freezing the cooling liquid, the
distributor
assembly of cooling liquid is turned off
[0021] Prior
art refrigerant coil assemblies 20 have a generally parallelepiped overall
shape of six sides retained in a frame 21 and has a major/longitudinal axis
23, where each
side is in the form of a rectangle. The coil assembly 20 is made of multiple
horizontal closely
spaced parallel, serpentine tubes connected at their ends to form a number of
circuits through
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which the refrigerant flows. Each individual circuit within the coil assembly
is a single,
continuous length of coil tubing that is subjected to a bending operation
which forms the
tubing into several U-shaped rows that are in a generally vertical and equally-
spaced
relationship from each other, such that each circuit has a resultant
serpentine shape.
[0022] The coil
assembly 20 has an inlet 22 connected to an inlet manifold or header
24, which fluidly connects to inlet ends of the serpentine tubes of the coil
assembly, and an
outlet 26 connected to an outlet manifold or header 28, which fluidly connects
to the outlet
ends of the serpentine tubes of the coil assembly. The assembled coil assembly
20 may be
moved and transported as a unitary structure such that it may be dipped, if
desired, if its
components are made of steel, in a zinc bath to galvanize the entire coil
assembly.
[0023] The
refrigerant gas discharges from the compressor into the inlet connection
of the apparatus. Heat from the refrigerant dissipates through the coil tubes
to the water
cascading downward over the tubes. Simultaneously, air is drawn in through the
air inlet
louvers at the base of the condenser and travels upward over the coil opposite
the water flow.
A small portion of the water evaporates, removing heat from the system. The
warm moist air
is drawn to the top of the evaporative condenser by the fan and discharged to
the atmosphere.
The remaining water falls to the sump at the bottom of the condenser where it
recirculates through the water distribution system and back down over the
coils.
[0024] The
invention constitutes a change and improvement over the prior art
wherein instead of tube bundles comprising a single cross-sectional area
throughout the entire
refrigerant flow path through the coil, the indirect heat exchange section has
multiple
sections, each having different cross-sectional areas, decreasing as the
refrigerant travels
through the heat exchange section.
[0025] Figures
4a, 5a, 6a and 7a are photographs of a mockup of a multi-cross-
sectional area refrigerant condenser according to an embodiment of the
invention. Figures 4b,
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5b, 6b and 7b are formal drawings corresponding to Figures 4a, 5a, 6a and 7a,
respectively.
A first condenser section 103 includes plurality of straight tubes 105 having
a first total cross-
sectional area. While round tubes are shown in the mock-up, tubes of any
shape, size and
feature may be used according to the invention, Indeed, any passage capable of
permitting
refrigerant flow and heat exchange may be adapted for use in connection with
the invention
in the place of the tubes shown in the Figures, including microchannel plates
and other
conduit structures. For the sake of the description of the invention with
reference to the
mock-ups shown in Figures 4a, 5a, 6a and 7a, the term "tube" will be used, but
it should be
understood that the words "passage," or "conduit" may be substituted for the
word "tube" in
the description herein, whatever the construction, provided that it can convey
refrigerant and
permit heat exchange between refrigerant inside and air outside.
[0026] As used
herein, the term "total cross-sectional area" refers to the sum of the
cross-sectional areas of the individual tubes in a condenser section. The term
"total cross-
sectional area" as used herein is not calculated to include the area between
tubes in a
condenser section. The cross-sectional area of each straight tube 105 in first
condenser
section 103 may be the same as or different from one-another, but the sum of
the cross-
sectional areas of all straight tubes 105 in first condenser section 103
equals the first total
cross-sectional area. The tubes in first condenser section 103 are preferably
finned. Each
straight tube 105 in the first condenser section 103 terminates at one end at
inlet header or
manifold 107 and at terminates at a second end at intermediate header or
manifold 109.
[0027] A second
condenser section 111 includes a second plurality of straight tubes
113 having a second total cross-sectional area. The cross-sectional area of
each straight tube
113 in second condenser section 111 may be the same as or different from one-
another, but
the sum of the cross-sectional areas of all straight tubes 113 in second
condenser section 111
equals the second total cross-sectional area. The second total cross-sectional
area is less than
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the first total cross-sectional area. The cross-sectional area of each
straight tube 113 in the
second condenser section may be the same or different from the cross-sectional
area of each
straight tube 105 in the first condenser section, but the cross-sectional area
of each straight
tube 113 in the second condenser section is preferably less than cross-
sectional area of each
straight tube 105 in the first condenser section. The number of tubes in the
second condenser
section may be the same or different from the number of tubes in the first
condenser section,
but is preferably less. The length of the tubes in the second condenser
section may optionally
be shorter than the length of the tubes in the first condenser section (as
shown for example in
Figures 4a and 4b). The tubes in second condenser section 111 are preferably
finned.
[0028] The
second condenser section receives refrigerant from the first condenser
section via intermediate header or manifold 109. As shown, for example in
Figures 4a and
4b, each straight tube 113 in the second condenser section terminates at one
end at
intermediate header or manifold 109 and terminates at a second end at outlet
header or
manifold (not shown).
[0029]
Alternatively, third, fourth and fifth or more condenser sections may be
present. Figure 8 is a representation of an embodiment of the invention having
four
condenser sections. According to these embodiments, a second intermediate
header or
manifold 115 directs refrigerant to a third condenser section 117, and each of
said third 117,
fourth 119, and fifth or more condenser sections are each constructed of a
plurality of straight
tubes, and each of said third, fourth, and fifth or more condenser sections
each have a total
cross-sectional area that is less than a cross-sectional area of an
immediately upstream
condenser section.
[0030] Each of
the straight tubes in said third, fourth, and fifth or more condenser
section is connected at one end to an immediately upstream condenser section
by an
intermediate header or manifold, and at a second end to another intermediate
header or
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manifold 121 (if there is a subsequent condenser section) or to an outlet
header or manifold
123.
[0031] Figure 9
shows an alternate embodiment of the invention in which the inlet
header, outlet header and intermediate headers are all arranged on the same
side of the device,
and each condenser section contains two sets of straight lengths of tubes
connected at an end
opposite the header end by U-bends. Accordingly, inlet header 201 receives
superheated
refrigerant vapor and distributes it to first set of straight tubes 203 in a
first condenser section
205. The first set of straight tubes 203 are connected at an opposite end to a
second set of
straight tubes 207 in said first condenser section by U-bends 209. The first
and second set of
tubes in the first condenser section have the same number of tubes and the
tubes have the
same diameter. U-bends 209 have approximately the same cross-sectional
size/diameter as
the first and second set of tubes in said first condenser section. The side of
the second set of
tubes in the first condenser section are connected at an end opposite the U-
bend end to first
intermediate header 211. First intermediate header then delivers the
refrigerant to the second
condenser section 213 having a second condenser first set of tubes 215 and a
second
condenser second set of tubes 217 connected at an opposite end from said
intermediate
header by another set of U-bends 219. The first and second set of tubes in
said second
condenser section have the same cross-sectional dimensions and are equal in
number. The U-
bends 219 connecting the first and second set of tubes in the second condenser
section
likewise have approximately the same cross-sectional dimensions as the first
and second set
of tubes they connect. The second condenser section second set of tubes 217
terminate at a
second intermediate header 221. The second intermediate header 221 receives
refrigerant
from the second condenser section set of tubes 217 and direct it to the third
condenser section
223. The third condenser section first set of tubes 225 are connected at a
first end to the
second intermediate header and at an opposite end to yet another set of U-
bends (not shown)
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that are in-turn connected to a first end of third condenser section second
set of tubes 227.
The third condenser section second set of tubes 227 are connected at the
header end to outlet
header 229. The tubes of each condenser section are progressively smaller
while (according
to the embodiment shown in Figure 9) the number of tubes in each condenser
section is
equal. However, as with the embodiments described above, the size of the tubes
could be left
the same, and the number of tubes could be reduced, so that the total cross-
sectional area of
each condenser section is smaller than the first section, and is preferably
smaller than each
upstream section.
[0032] By
increasing the number of circuits (tubes) in the first condenser section and
increasing the cross-sectional area of each tube in the first condenser
section the invention
can reduce the inlet vapor velocity more than 50% and thus reduce the
refrigerant pressure
drop to less than 25% of the original value. Moreover, the entrance vapor
velocity, per
circuit, is sufficient to establish an internal film heat transfer coefficient
greater than the
external heat transfer coefficient while limiting the internal pressure drop
for the heat
rejection intended. The subsequent decrease in total cross sectional area will
occur after the
first path or even later in the heat transfer fluid path depending upon
operating conditions.
The number of tubes in the second condenser section may be adjusted to
additionally lower
vapor velocity which in turn reduces refrigerant pressure drop. The second
group also
exhibits a reduced total cross sectional area then the first group in this
illustration and thus
maintains vapor velocity prior to entering the last reduction in cross
sectional area. A third
condenser section may have further reduced cross sectional area to re-
establish the vapor
velocity prior to exiting the condenser. It is most preferred that each
condenser section
incorporate smaller or same as, cross sectional area paths in comparison to
the initial circuits.
In doing so, the fluid (vapor) velocity is re-established such that the
associated internal film
heat transfer coefficient is greater than that leaving the initial total cross
sectional area
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provided coupled with initial circuit quantity. Multi-cross sectional
interfaces are preferably
utilized throughout the condenser as needed via segmented headers (see, e.g.,
Figs. 4a, 4b and
Fig. 9) such that the heat transfer fluid (vapor) velocity can be maintained
(on average)
leading into the final pass. There are many permutations regarding the path
cross sectional
area coupled with number of paths per section that can be used with this
invention to
optimize performance. Iterative calculations can be performed depending upon
the operating
conditions, refrigerant and heat rejection requirements. There are other
advantages with this
invention including lower refrigerant inventory as well as better condenser
efficiency due to
reduced refrigerant pressure drop.
