U.S. patent application number 12/988520 was filed with the patent office on 2011-07-21 for evaporative cooling tower performance enhancement through cooling recovery.
Invention is credited to Jarrell Wenger.
Application Number | 20110174003 12/988520 |
Document ID | / |
Family ID | 41199486 |
Filed Date | 2011-07-21 |
United States Patent
Application |
20110174003 |
Kind Code |
A1 |
Wenger; Jarrell |
July 21, 2011 |
Evaporative Cooling Tower Performance Enhancement Through Cooling
Recovery
Abstract
A method of enhancing evaporative cooling towers of various
types. Such cooling towers have a flow of water, an air intake
stream of ambient air and an air exhaust such that the flow of
water is cooled by ambient air from the air intake and evaporating
a portion of that water flow into the ambient air, and the air
discharge stream for the ambient air and a portion of evaporated
water from the water flow. The method provides a closed cycle
coolant channel having a heated heat discharge portion and a cooled
heal sink, placing the cooled portion at the air intake, placing
the heated portion in the flow of the air at the air exhaust. The
ambient air flow at the intake is cooled by the closed cycle
coolant channel, reducing its wet bulb temperature and increasing
the capability of the cooling tower to cool the flow of water.
Inventors: |
Wenger; Jarrell; (Golden,
CO) |
Family ID: |
41199486 |
Appl. No.: |
12/988520 |
Filed: |
April 18, 2009 |
PCT Filed: |
April 18, 2009 |
PCT NO: |
PCT/US09/41056 |
371 Date: |
April 8, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61046036 |
Apr 18, 2008 |
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Current U.S.
Class: |
62/121 |
Current CPC
Class: |
F28C 1/06 20130101; F28C
1/14 20130101; F28D 15/0266 20130101 |
Class at
Publication: |
62/121 |
International
Class: |
F28C 1/00 20060101
F28C001/00 |
Claims
1. A method of enhancing an evaporative cooling tower, the cooling
tower having a flow of water, an air intake for receiving an air
intake stream of ambient air, such that the flow of water is cooled
by subjecting the flow of water to the air intake stream of ambient
air and evaporating a portion of that flow of water, and an air
exhaust for an air discharge stream and a portion of evaporated
water from the flow of water carried by the air discharge stream,
the method including a) providing a closed cycle heat transfer
system having a heat discharge portion and a heat sink portion, b)
subjecting the heat discharge portion to the air discharge stream
and the portion of evaporated water, c) subjecting the heat sink
portion to the air intake stream of ambient air, whereby the heat
is transferred to the air discharge stream and the air intake
stream of ambient air is cooled by the closed cycle heat transfer
system, reducing its wet bulb temperature and increasing the
capability of the cooling tower to cool the flow of water.
2. The method of claim 1 wherein the closed circuit heat transfer
system is selected from a group of heat exchange systems including
a thermosiphon, a heat pipe, a pumped fluid loop, a parallel plate
heat exchanger, and a heat wheel.
3. The method of claim 1 wherein the closed circuit heat transfer
system is a heat pipe.
4. The method of claim 1 wherein the water can be cooled to a
temperature which is lower than the wet bulb temperature of the
ambient air at the air intake.
5. The method of claim 1 wherein the water can be cooled to a
temperature which approaches the dew point temperature of the
ambient air at the air intake.
6. The method of claim 1 wherein the air exhaust is above the air
intake.
7. The method of claim 6 wherein the closed circuit heat transfer
system is a heat pipe or a thermosiphon.
8. The method of claim 1 wherein the closed circuit heat transfer
system is a heat wheel.
9. A method of cooling a supply of water for a process with a
predetermined cooling load and temperature comprising, providing a
water evaporation cooling tower having an ambient air intake to
receive an air intake stream from the atmosphere, an air exhaust to
discharge an air discharge stream from the cooling tower, and the
supply of water to be evaporatively cooled by the air stream
passing from the air intake to the air exhaust, transferring heat
from the cooling load to the air discharge stream, providing a
closed circuit heat transfer system having a heat sink portion and
a heat discharge portion, placing the heat sink portion in the
ambient air intake stream and placing the heat discharge portion in
the air exhaust stream, whereby the dry bulb temperature of the air
discharge stream is lower than the dry bulb temperature of the air
intake stream such that heat is transferred from the air intake
stream to the air discharge stream, lowering the dry bulb and wet
bulb temperature of the air intake stream, whereby the air stream
flow is reduced to satisfy the predetermined load and temperature
requirements, whereby the amount of water evaporated from the
supply of water is decreased relative to an identical cooling tower
without the provided closed circuit heat transfer system.
10. The method of cooling a process of a predetermined cooling load
as set forth in claim 9 wherein the cooling load is from a
refrigeration system, a power plant condenser, an industrial
process or a building space.
11. The method of cooling a process of a predetermined cooling load
as set forth in claim 9 wherein the closed circuit heat transfer
system comprises one selected from the group consisting of heat
pipes, thermosiphons, heat wheels, parallel plate heat exchangers
and pumped fluid loops.
Description
BACKGROUND
[0001] This invention deals with methods and systems for enhancing
the performance of, and potentially the range of uses for,
otherwise standard evaporative cooling towers. More specifically,
this invention teaches boosting cooling output by using a heat
transfer system or device, such as an array of thermosiphons or
heat pipes to pre-chill the incoming ambient air ducted through the
evaporation section of the cooling tower by transferring heat from
this incoming air to the flow of cool, humid exhaust air coming out
of the evaporation section. This has the effect of reducing the wet
bulb temperature of the incoming air and ultimately reducing the
temperature of the working fluid, which is typically water either
in a closed loop or in a basin; the closed loop evaporative fluid
cooler can be used to cool any of a number of industrial fluids and
would be described as an evaporative condenser in the case where
the fluid undergoes a phase change from vapor to liquid. This
working fluid temperature reduction depends on the effectiveness of
transferring heat from the air coming into the cooling tower intake
to air at the exhaust of the cooling tower, the ambient air
conditions (dry bulb and wet bulb temperatures) and the heat load
on the working fluid.
SUMMARY
[0002] Accordingly, disclosed is an apparatus and method of
enhancing an evaporative cooling tower 10 having an air intake
stream 22 for ambient air, a flow of water, means for cooling the
flow of water by subjecting the flow of water to ambient air from
the air intake and evaporating a portion of that flow of water into
the ambient air, and an air exhaust for the air and a portion of
evaporated water from the flow of water. The enhancements disclosed
include providing a closed cycle heat transfer system having a
portion to be heated, sometimes called a "heat sink portion", and a
portion to be cooled, sometimes called a "heat discharge portion",
placing the heat discharge portion to be cooled in the air
discharge stream, placing the heat sink portion to be heated in the
air intake stream. In this way, the flow of ambient air is cooled
by the closed cycle heat transfer system, thus reducing its wet
bulb temperature and increasing the capability of the cooling tower
to cool the flow of water through evaporation into the flow of air.
This closed circuit heat transfer system could comprise one or more
thermosiphons, heat pipes, pumped fluid loops, parallel plate heat
exchangers, or heat wheels, also known as rotating recuperators.
Ideally, the water is cooled to a temperature which is lower than
the wet bulb temperature of the ambient air at the air intake, even
to a temperature which approaches the dew point temperature of the
ambient air at the air intake. In the situation where the air
exhaust is above the air intake, the closed circuit heat transfer
system is preferably a passive heat pipe or a thermosiphon. Where
the evaporative cooling tower system is designed from the ground
up, that is specifically to take the best advantage of the closed
cycle cooling system, the closed cycle cooling system would
preferably be of the heat wheel or rotating recuperator type.
BRIEF DESCRIPTION OF THE FIGURES
[0003] FIG. 1 is a schematic view of an open cooling tower 10 with
water distribution 18 system, here shown as a spray device, and
high surface area fill 20 emptying into a basin 24 of water used as
the working fluid, together with the closed cycle cooling system
30.
[0004] FIG. 2 is a schematic view of an alternative system where
water distribution sprays a closed loop containing a working fluid,
together with the closed cycle cooling system.
[0005] FIG. 3 is a psychrometric chart produced using the computer
program "Psychrometric Analysis, Version 6" by the American Society
of Heating, Refrigeration and Air-Conditioning Engineers, Inc.
(ASHRAE). This figure shows the projected performance of a standard
commercially available cooling tower used for a 5.6 degree Celsius
process range and the same cooling tower with its projected
performance according to my invention.
[0006] FIG. 4 is a similar psychrometric chart from the ASHRAE
computer program showing performance of the same standard cooling
tower for a 2.2 degree Celsius process range and that cooling tower
with its projected performance according to my invention.
[0007] FIGS. 5a and 5b show families of sample cooling tower
performance curves at various ambient air conditions and a 75% heat
exchange effectiveness rating with the projected cold water
production performance and relative evaporation according to the
invention for 5.6 degree and 2.2 degree Celsius ranges
respectively. The wet bulb depression parameter is the difference
between the ambient dry bulb and wet bulb temperatures.
[0008] FIGS. 6a and 6b show families of sample cooling tower
operating curves for the same process range temperatures, ambient
air conditions and heat exchange effectiveness rating for the case
where the capacity and cold water temperatures are matched to those
achieved by the standard cooling tower by modulating its air flow.
The curves show the fan speeds that are required relative to that
of the standard cooling tower; the evaporative water consumption
characteristic relative to that the standard cooling tower is
numerically similar to the relative fan speeds.
[0009] Cooling of a working fluid through evaporation of water is a
conventional strategy. This evaporative cooling process works
especially well in relatively dry environments where the air has
significant capacity to absorb moisture as evaporating water
undergoes a phase change from liquid to vapor. In a cooling tower
with no heat load on the cooled water, the temperature of the
cooled water, and thus the temperature of the working fluid in
intimate contact with the cooled water would reach the wet bulb
temperature corresponding to the particular ambient air temperature
and humidity. This theoretical limit is not reached in practice
since a heat load is typically placed on the cooled water. It is
known however that since the purpose of a cooling tower is to
provide cooled water, the lower the wet bulb temperature of the
incoming air, the cooler the water can become and thus the more
useful the cooled water becomes for cooling a building or an
industrial process. Performance of a cooling tower is often
described as "approach to wet bulb"; if the cooling tower produces
cooled water that is 8 degrees above the wet bulb temperature of
the ambient air flowing into the tower, the cooling tower is
described as achieving an 8 degree approach to wet bulb.
[0010] The primary strategies to improve cooling tower capacity
have been to move larger masses of ambient air through the tower,
and to increase the surface area exposed to the air flow of the
water to be evaporated. Reducing the cooling load on a cooling
tower improves (reduces) the approach to wet bulb; for a given
cooling tower with a fixed water flow rate, a reduced load
translates into a reduced operating process range (the difference
between working fluid inlet and outlet temperatures). For a given
load, the same effect can be achieved by utilizing an oversized
cooling tower. However, even an infinitely large cooling tower can
not cool water below the wet bulb temperature of the ambient air
drawn into the cooling tower. Cooling tower enhancements have been
developed to pre-cool entering water through heat exchange with
ambient air when that ambient air is sufficiently cool. This
feature has the effect of reducing the process range (load) that is
imposed on the tower, thereby improving the approach to wet bulb
and also reducing water consumption, but can not be used in hot
ambient air conditions.
[0011] Other cooling tower enhancements have been developed to use
some of the cooled process water to pre-cool ambient air entering
the cooling tower to reduce its wet bulb temperature. From the
standpoint of potential cold water temperature achieved by the
cooling tower, this approach has essentially the same impact of the
current invention. However, unlike the enhancement discussed here,
using the cooled process water for pre-cooling consumes significant
cooling tower capacity, thus substantially reducing the useful
(remaining) cooling tower output. At some high ambient temperature
conditions, the portion of the cooling tower capacity required for
such pre-cooling can equal the overall output of the cooling tower
such that it produces no net useful cooling effect.
[0012] No one has previously used a closed loop heat transfer
system to move heat to the relatively cool (but relatively high
humidity) air exiting the tower to help reduce the temperature (and
wet bulb temperature) of the relatively warm (but low humidity) air
being drawn into the cooling tower, as will be detailed. This
enhancement results in expanding the theoretical performance limit
of the cooling tower from wet bulb temperature to the generally
lower dew point temperature without sacrificing capacity and while
decreasing evaporative water consumption.
[0013] Accordingly, shown in FIG. 1 is a schematic of a cooling
tower 10 with a cooling load L transferring heat to the cooled
process to the working water 12. The air entering the cooling tower
inlet 22 is shown coming in from the lower left. The air being
drawn up through the cooling tower by a conventionally powered fan
14, although this fan could be supplemented or eliminated by using
convective flow, especially in extremely large hyperbolic towers
where the air inlet stream 22 enters the tower through a peripheral
inlet at grade and the air exhaust stream 16 exits the tower
between 300 and 500 feet above grade. A spray 18 of the working
water circulates downwardly in a counter flow manner through the
fill material 20, which operates to increase the surface area of
the water and increase contact with the warm dry air rising up
through the fill. A portion of water evaporates, cooling the
remaining liquid water, which collects in the basin 24 and is used
for cooling the Load L, which could be a building or for other
cooling tasks such as absorbing heat rejected from the load L from
a refrigerating system an industrial process or from a power plant
condenser.
[0014] Also shown schematically is a heat recovery or transfer
system comprising a closed circuit heat transfer system 26 of known
type that runs in series with but separate from this evaporative
water cooling cycle. A heat discharge coil 28 is positioned in the
exit airflow 16 and sheds heat into that outgoing airflow. The heat
is circulated from a heat sink coil 30 in contact with the incoming
airflow entering the air inlet 22, thus cooling that entering
airflow. This closed circuit heat transfer system could be made up
of one or more thermosiphons, heat pipes, pumped fluid loops,
parallel plate heat exchangers, or heat wheels.
[0015] More particularly, this closed circuit heat transfer system
26 could be a pumped liquid loop, parallel plates or a heat wheel,
but preferably at least for retrofit installations, it should be
one or a series of parallel heat pipes or thermosiphons of known
form, taking advantage of the fact that the working fluid in such
heat pipes would normally be drawn downwardly by gravity after it
is condensed by the relatively cool exhaust air stream 16 exiting
the cooling tower, running down to the warm heat recovery coil at
the air inlet. Here the heat pipe working fluid evaporates, or the
thermosiphon fluid temperature increases, absorbing some of the
heat from and thus cooling incoming air and lowering its wet bulb
temperature. Cooling this incoming air before sending it through
the cooling tower and exposing it to the counter-flowing water in
the cooling tower will be of considerable benefit in providing
cooled water to the basin.
[0016] FIG. 2 schematically shows a slightly different standard
type cooling tower 10 having a pipe surface area coil 32 containing
working fluid 12 used for process cooling or refrigeration load L.
This cooling coil is bathed in water spray 18, which also falls in
a counter flow manner through ambient air pulled through cooling
tower by the fan 14 (with or without convective enhancement as
discussed above) and into a basin 24. Here again, the enhancing
system 26 is schematically shown with a heat recovery coil 28 in
the cool exhaust airflow 16 and a heat sink coil 30 conditioning
the warm air before it enters the tower through an air inlet. As in
the system shown in FIG. 1, the closed loop heat transfer system 26
would be a system selected from one or more heat pipes,
thermosiphons, or heat wheels, depending on the orientation and
separation distance of the incoming air stream to be cooled and the
exhaust air stream into which the heat from the selected closed
loop heat transfer system will be shed.
[0017] Turning to FIGS. 3 and 4, the enhanced performance of the
inventive cooling process and apparatus illustrated in FIGS. 1 and
2 will become apparent. FIG. 3 is a normal temperature
psychrometric chart at sea level. The performance curve labeled
"Standard Cooling Tower . . . " shows the typical airside process
of a relatively standard commercially available cooling tower
mechanism of the types shown in FIGS. 1 and 2. For this example,
air entering the cooling tower is presumed to be 37.8 degrees
Celsius at about 22% relative humidity, with a wet bulb temperature
of 21.1 degrees Celsius and a dew point of 12.2 degrees Celsius. As
air passes through the cooling tower, it absorbs moisture and is
typically cooled below the ambient air temperature. With a 5.6
degrees Celsius process range, the sample cooling tower output air
temperature is shown to be 27.1 degrees Celsius, with the water
output temperature stabilizing at about 26.0 degrees Celsius. The
performance curve labeled "Enhanced CT 75% Effectiveness . . . "
shows projected performance of an identical cooling tower with a
heat recovery system as disclosed previously. Here this heat
recovery system is assumed to have a 75% heat exchange
effectiveness rating. That is, it the combination of heat recovery
coils in air inlet and exhaust that is able to move 75% of the heat
from the inlet air to the outlet air. This would cool the inlet air
temperature, once the overall system had stabilized, from 37.8
degrees Celsius to this new temperature of about 28.1 degrees
Celsius. This 75% effectiveness is shown on the chart as 75% of the
difference between the new stabilized outlet temperature of about
27.7 degrees Celsius, and the temperature of the ambient air
entering the heat recovery coil of 37.8 degrees Celsius.
[0018] Thus, the enhanced system would result in reducing the
temperature of the working fluid to about 23.8 degrees Celsius--a
2.2 degrees Celsius cooling benefit. Even allowing for the capital
and maintenance costs of the thermosiphons and/or heat pipe
installation, and the small but measurable restriction on air flow
caused by imposing the heat exchange surfaces of these heat
transfer devices, this 2.2 degrees Celsius drop in water
temperature would be a substantial efficiency enhancement. The
inventive apparatus could permit reducing the scale of a typical
cooling tower and thus the overall energy needed to move the air,
water, etc., or an existing tower system could be operated with
reduced airflow for a reduction in fan power draw, since the
cooling load on the cooling tower working fluid could be more
easily satisfied. A lower cooling water temperature can also
improve the energy performance of a refrigeration process, by
reducing the energy consumed, or of a power production process, by
increasing the energy produced. The cooling tower enhancement also
reduces water evaporation and hence required water makeup relative
to the standard cooling tower as will be discussed in detail with
reference to FIGS. 5 and 6.
[0019] FIG. 4 shows similar curves but here the process range is at
2.2 degrees Celsius. This 2.2 degrees Celsius range is equivalent
to increasing the cooling tower capacity relative to the load, and
reduces the approach to wet bulb of the standard cooling tower.
Here, the analysis is similar and the presumed 75% effective heat
exchange is also applied. The 2.2 degrees Celsius range however
permits the cooling tower outlet temperature to be proportionally
lower and thus when at the equilibrium the enhanced curve shows
that the cold water temperature, is projected to reach 19.5 degrees
Celsius. As in the system modeled in FIG. 3, this compares quite
favorably to the 23.3 degrees Celsius cold water for the identical
cooling tower without the enhancement, resulting in a 3.8 degree
Celsius cooling benefit. The cold water temperature is also 1.5
degrees Celsius below the ambient wet bulb temperature, which is
not possible with the standard cooling tower. The lower cold water
temperature achieved has potential to provide building cooling
using the evaporative process alone, without the need for
refrigeration. Currently used evaporative cooling processes for
buildings typically add moisture to the building air, partially
counteracting the comfort benefit of cooling.
[0020] This performance enhancement is also illustrated in FIGS. 5a
and 5b, showing 2.2 and 5.6 degree Celsius range performance curves
for a standard cooling tower, and 2.2 and 5.6 degree Celsius data
for the enhanced tower. For the 5.6 degree Celsius case with 16.7
degrees Celsius of wet bulb depression and an ambient wet bulb
temperature of 21.1 degrees Celsius, the cold water temperature
achieved is 23.8 degrees Celsius, matching the conditions shown in
FIG. 3, with predicted water evaporation being 84% of that for the
standard cooling tower. For the 2.2 degree Celsius case with 16.7
degrees Celsius of wet bulb depression and an ambient wet bulb
temperature of 21.1 degrees Celsius, the cold water temperature
achieved is 19.5 degrees Celsius, matching the conditions shown in
FIG. 4, with predicted water evaporation being 61% of that for the
standard cooling tower.
[0021] FIGS. 6a and 6b illustrate the performance enhancement from
the alternative operating strategy of matching the capacity and
cold water temperature of the enhanced cooling tower to that of the
standard cooling tower. The figure shows 2.2 and 5.6 degree Celsius
range performance curves for a standard cooling tower, and 2.2 and
5.6 degree Celsius speed curves for the enhanced tower. For the 5.6
degree Celsius case with 16.7 degrees Celsius of wet bulb
depression and an ambient wet bulb temperature of 21.1 degrees
Celsius, the required fan speed is 75% and the predicted water
evaporation 75% relative to the standard cooling tower. For the 2.2
degree Celsius case with 16.7 degrees Celsius of wet bulb
depression and an ambient wet bulb temperature of 21.1 degrees
Celsius, the required fan speed is 52% and the predicted water
evaporation 50% relative to the standard cooling tower.
[0022] Thus explained, the proposed enhanced method and cooling
tower systems can easily be adapted to a wide range of applications
along with direct building cooling, industrial process cooling and
the like.
[0023] The thus enhanced cooling tower would reduce the amount of
water evaporated for and equivalent working load and ambient air
temperature, since the cooling water would be subjected to a flow
of air at less than ambient temperatures when the system is
operating at steady state. While any of the chosen closed cycle
coolant systems will restrict somewhat the flow of air into and out
of the cooling tower, the reduction in fan efficiency should be
slight in comparison to the overall benefit in cooling
effectiveness. This is especially evident in retrofit systems which
have or are provided in the retrofit project with a variable speed
motor. Instead of achieving a decrease in working water temperature
for a given load, the fan speed could be reduced to proportionally
reduce the air flow while maintaining the same capacity and cold
water temperature as the standard cooling tower. Since the power
consumption of the electric motor, assuming negligible losses from
the variable speed controller circuitry, is a function of the
volume of air moved per unit of time to the third power. Although
there would be a slight increase in full speed fan power with the
addition of the closed cycle heat exchange system, if the cooling
requirements of the load could be satisfied by running the fan at
half of the original speed, the electric power needed to run the
fan would drop to one eighth of that required for full speed
operation. FIG. 6 graphically shows this.
[0024] Air conditioning systems in large building often utilize
cooling towers for heat rejection from a water-chilling
refrigeration system. Since the cost to operate such refrigeration
systems is quite high, such cooling systems are often configured to
allow generation of appropriate temperature chilled water directly
from the cooling towers, without operating the chillers, during
mild conditions of ambient air dry bulb and wet bulb temperatures.
This is termed "free cooling" because the relatively energy hungry
refrigeration system is not needed. By providing such a cooling
system with the disclosed enhancements, the number of hours of such
"free cooling", that is cooling that does not require operating the
refrigeration system, could be increased by hundreds of hours per
year, resulting in cost savings and a rapid payback for the
enhancements discussed.
[0025] The increased cooling effectiveness of the subject method
would also aid in retrofitting existing systems with old style
supplemental refrigeration systems such as those discussed above.
Many existing systems use CFCs (chlorinated fluorocarbon)
refrigerants, which are known to harm the ozone layer.
Environmentally acceptable replacement refrigerants can be used in
place of the CFCs, but use of the replacement refrigerants is known
to decrease the efficacy of such refrigeration systems such that
either the cooling towers must be replaced with larger, higher
capacity units, or entire refrigeration system must be replaced. By
retrofitting an existing cooling tower with the disclosed enhanced
system, the existing refrigeration system, even with the
environmentally acceptable refrigerant, could potentially meet the
cooling loads without replacing the existing cooling tower.
* * * * *