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.

Description

BACKGROUND

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.