Patent Publication Number: US-10309734-B2

Title: Air-to-air heat exchanger bypass for wet cooling tower apparatus and method

Description:
CROSS-REFERENCE 
     This application is related to U.S. Provisional Patent Application Ser. No. 61/877,005, titled “AIR-TO-AIR HEAT EXCHANGER BYPASS FOR WET COOLING TOWER APPARATUS AND METHOD,” filed Sep. 12, 2013, the disclosures of each which are hereby incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates generally to plume abatement for cooling towers or other heat rejection devices and the like. More particularly, the present invention relates to method and apparatus for a cost effective and efficient plume abatement in cooling towers. 
     BACKGROUND OF THE INVENTION 
     In electricity generation using steam driven turbines, water is heated by a burner to create steam which drives a turbine to creates electricity. In order to minimize the amount of clean water necessary for this process, the steam must be converted back into water, by removing heat, so that the water can be reused in the process. In air conditioning systems for large buildings, air inside the building is forced passed coils containing a cooled refrigerant gas thereby transferring heat from inside the building into the refrigerant gas. The warmed refrigerant is then piped outside the building where the excess heat must be removed from the refrigerant so that the refrigerant gas can be re-cooled and the cooling process continued. 
     In both of the foregoing processes, and numerous other processes that require the step of dissipating excess heat, cooling towers have been employed. In wet type cooling towers, water is pumped passed a condenser coil containing the heated steam, refrigerant, or other heated liquid or gas, thereby transferring heat into the water. The water is then pumped to the heat exchange section of the cooling tower and sprayed over a cooling tower media comprised of thin sheets of material or splash bars. As the water flows down the cooling tower media, ambient air is forced passed the heated water and heat is transmitted from the water to the air by both sensible and evaporative heat transfer. The air is then forced out of the cooling tower and dissipated into the surrounding air. 
     Cooling towers are highly efficient and cost effective means of dissipating this excess heat and thus are widely used for this purpose. A recognized drawback to cooling towers, however, is that under certain atmospheric conditions a plume can be created by moisture from the heated water source evaporating into the air stream being carried out of the top of the cooling tower. Where the cooling tower is very large, as in the case of power plants, the plume can cause low lying fog in the vicinity of the cooling tower. The plume can also cause icing on roads in the vicinity of the cooling tower where colder temperatures cause the moisture in the plume to freeze. 
     Efforts have therefore been made to limit or eliminate the plume caused by cooling towers. Such efforts include, for example, a plume abated cooling tower in which ambient air, in addition to being brought in at the bottom of the tower and forced upwards through a fill pack as hot water is sprayed down on the fill pack, is brought into the cooling tower through isolated heat conductive passageways below the hot water spray heads. These passageways which are made from a heat conductive material such as aluminum, copper, etc., allow the ambient air to absorb some of the heat without moisture being evaporated into the air. At the top of the tower the wet laden heated air and the dry heated air are mixed thereby reducing the plume. 
     Another example is a plume prevention system in which the hot water is partially cooled before being provided into the cooling tower. The partial cooling of the hot water is performed using a separate heat exchanger operating with a separate cooling medium such as air or water. The separate heat exchanger reduces the efficiency of the cooling tower and thus should only be employed when atmospheric conditions exist in which a plume would be created by the cooling tower. 
     Another example of a system designed to reduce plume in a wet type cooling tower can be found in the “Technical Paper Number TP93-01”of the Cooling Tower Institute 1993 Annual Meeting, “Plume Abatement and Water Conservation with the Wet/Dry Cooling Tower,” Paul A. Lindahl, Jr., et al. In the system described in this paper, hot water is first pumped through a dry air cooling section where air is forced across heat exchange fins connected to the flow. The water, which has been partially cooled, is then sprayed over a fill pack positioned below the dry air cooling section and air is forced through the fill pack to further cool the water. The wet air is then forced upwards within the tower and mixed with the heated dry air from the dry cooling process and forced out the top of the tower. 
     While the foregoing systems provide useful solutions to the wet cooling tower plume problem, they all require the construction of a complex and costly wet and dry air heat transfer mechanism. Moreover, when such towers operate in “non-plume” abatement mode, more fan energy is expended pull the air through the heat exchange packs, causing the operational costs to the tower to significantly increase. Accordingly, an inexpensive plume abatement method and apparatus is needed wherein the tower may be operated in an “non-abatement” mode without significant cost increase. 
     Another recognized problem with cooling towers is that the water used for cooling can become concentrated with contaminates. As water evaporates out of the cooling tower, additional water is added but it should be readily recognized that contaminants in the water will become more concentrated because they are not removed with the evaporate. If chemicals are added to the cooling water to treat the water these chemicals can become highly concentrated which may be undesirable if released into the environment. If seawater or waste water is used to replace the evaporated water, a common practice where fresh water is not available or costly, salts and solids in the water can also build up in the cooling water circuit As these contaminants become more concentrated they can become caked in between the thin evaporating sheets diminishing the towers cooling efficiency. 
     To prevent the foregoing problem it is a regular practice to “blowdown” a portion of the water with the concentrated contaminants and replace it with fresh water from the source. While this prevents the contaminants in the cooling tower water from becoming too concentrated, there may be environmental consequences to discharging water during the blowdown process. Efforts have therefore been made to reduce the water consumption in cooling towers. 
     U.S. Pat. No. 4,076,771 to Houx, et al. describes the current state-of-the-art in reducing the water consumption in a cooling tower. In the system described in this patent both cooling tower evaporative heat transfer media and a coil section that transfers heat sensibly are provided in the same system. The sensible heat transfer of the coils provides cooling of the process water but does not consume any water. 
     While the foregoing patent represents a significant advancement over prior art cooling towers, it would be desirable if a mechanism were developed for recapturing water from the plume for replacement back into the cooling tower water reservoir which did not require a coil section for sensible heat transfer. 
     A separate problem that has been noted is the desalination of sea water, and purification of other water supplies, to create potable drinking water. Numerous approaches have been developed to remove purified water from a moist air stream. The major commercial processes include Multi-Stage Flash Distillation, Multiple Effect Distillation, Vapor Compression Distillation, and Reverse Osmosis. See “The Desalting ABC&#39;s”, prepared by O. K. Buros for the International Desalination Association, modified and reproduced by Research Department Saline Water Conversion Corporation, 1990. Examples of systems that use low temperature water for desalination or waste heat include the following: 
     “Zero Discharge Desalination”, Lu, et al., Proceedings from the ADA North American Biennial Conference and Exposition, August 2000. This paper provides information on a device that produces fresh water from a cold air stream and a warm moist air stream from a low grade waste heat source. The fresh water is condensed along the walls separating the two air streams. Also, a cold water is sprayed over the warm moist air to enhance condensation. 
     “Open Multiple Effect Desalination with Low Temperature Process Heat”, Baumgartner, et al., International Symposium on Desalination and Water Re-Use, Vol. 4, 1991. This paper provides information on a plastic tube heat exchanger used for desalination that uses cold running water on the inside of the plastic tubes and warm moist air flowing over the exterior of the tubes. The condensate forms on the outside of the cold tubes. 
     Other cooling towers presently in use are specifically designed for water conservation exclusively. For water conservation, such cooling towers wherein dry air is always flowed through the dry path of the cooling tower condensers to condense vapor from the effluent air. While these towers conserve water, thermal performance of the cooling tower typically is affected as the cooling can become inefficient with respect to heat exchange. 
     The typical remedies for increased thermal performance are to increase fan power which increases operating costs, to increase the plan area of the tower which increases capital costs, or both. A design that limits increased fan power or plan area to a modest cost increase is very desirable. The foregoing shows that there is a need for cooling towers or the like that can operate in both plume abatement and non-abatement modes effectively and efficiently providing desired heat exchange in all weather conditions without significantly increasing operational costs. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a cooling tower having a vertical axis is provided, comprising: an evaporative media located at a first position along the vertical axis; a liquid distribution system that distributes hot liquid over said evaporative media; a first heat exchanger module having a first set of passageways in fluid communication with a first flow duct and a second set of passageways in fluid communication with a second flow duct; a second heat exchanger module having a third set of passageways in fluid communication with a third flow duct and a fourth set of passageways in fluid communication with a fourth flow duct; a first bypass flow path that extends between said first heat exchanger module and said second heat exchanger module; and an air current generator that directs air through said first, second, third, fourth sets of passageways and bypass flow path through the cooling tower. 
     In another aspect of the invention, a method for reducing the heat content of an air stream in a cooling tower is provided, comprising the steps of: directing a first air stream through a first set of passageways of a first heat exchanger module; directing a second air stream through a separate, second set of passageways of the first heat exchanger module; directing a third air stream through a third set of passageways of a second heat exchanger module and through a first bypass path; directing a fourth air stream through a separate, fourth set of passageways of the second heat exchanger module and through a second bypass path; and transferring heat from said first air stream into said second air stream. 
     In another aspect of the invention a cooling tower is provide, comprising: means for directing a first air stream through a first set of passageways of a first heat exchanger module; means for directing a second air stream through a separate, second set of passageways of the first heat exchanger module; means for directing a third air stream through a third set of passageways of a second heat exchanger module; means for directing a fourth air stream through a separate, fourth set of passageways of the second heat exchanger module and through a bypass path; and means for transferring heat from said first air stream into said second air stream. 
     In still another embodiment of the present invention, a cooling tower having a vertical axis is provided, comprising: an evaporative media located at a first position along the vertical axis; a liquid distribution system that distributes hot liquid over said evaporative media; a first heat exchanger module having a first set of passageways in fluid communication with a first flow duct and a second set of passageways in fluid communication with said first flow duct; a second heat exchanger module having a third set of passageways in fluid communication with a second flow duct and a fourth set of passageways in fluid communication with said second flow duct; a lifting device that translates said second heat exchange module to a first position and a second position; and an air current generator that directs air through said first, second, third, fourth passageways and bypass flow path through the cooling tower. 
     In yet another embodiment of the present invention, a cooling tower having a vertical axis is provided, comprising: an evaporative media located at a first position along the vertical axis; a liquid distribution system that distributes hot liquid over said evaporative media; a first heat exchanger module having a first set of passageways in fluid communication with a first flow duct and a second set of passageways in fluid communication with a second flow duct; a second heat exchanger module having a third set of passageways in fluid communication with a third flow duct and a fourth set of passageways in fluid communication with a fourth flow duct; a wet path damper disposed in said first flow duct that regulates flow through said first duct; and an air current generator that directs air through said flow ducts and said passageways. 
     There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a cooling tower in a maximum thermal performance mode in accordance with an embodiment of the invention. 
         FIG. 2  is a schematic view of the cooling tower depicted in  FIG. 1  in a plume abatement mode in accordance with an embodiment of the invention. 
         FIG. 3  is a schematic view of the cooling tower depicted in  FIG. 1  and in a partial plume abatement mode in accordance with an embodiment of the invention. 
         FIG. 4  is a schematic view of a cooling tower in accordance with an alternative embodiment of the invention. 
         FIG. 5  is a schematic view of a cooling tower wherein the heat exchange modules are mechanically raised in accordance with an alternative embodiment of the present invention. 
         FIG. 6  is a schematic view of the cooling tower depicted in  FIG. 5  wherein the heat exchange modules are mechanically lowered in accordance with an alternative embodiment of the present invention. 
         FIG. 7  is a schematic view of a cooling tower wherein the heat exchange modules are mechanically rotated in accordance with an alternative embodiment of the present invention. 
         FIG. 8  is a schematic view of a cooling tower depicted in  FIG. 7 , wherein the heat exchange modules are mechanically rotated in accordance with an alternative embodiment of the present invention. 
         FIG. 9  is a schematic view of a cooling tower in accordance with an alternative embodiment of the present invention having wet duct dampers. 
         FIG. 10  is a schematic view of a cooling tower in accordance with another alternative embodiment of the present invention having wet duct dampers positioned at the heat exchange modules. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring now to the figures wherein like reference numerals indicate like elements,  FIGS. 1-3  depict a cooling tower apparatus, generally designated  10  having a support frame assembly  12  and a shroud  14  within which an air current generator (axial fan)  16  operates. The cooling tower  10  is generally comprises a wet, direct cooling section  11  and a dry, indirect cooling section  13  that are separated by an eliminators  15 . The cooling tower  10  includes a plurality of heat exchanges modules  18  positioned and oriented in a series, each in fluid communication with dry air ducts  20  and wet air ducts  22 . The individual exchanger modules  18  preferably have a generally diamond shape or diamond configuration however may alternatively be any functional geometry. As previously discussed, cooling tower  10  also includes a series of ambient or dry air ducts  20  each having air inlets or dampers  38 , along with a series of warm air or wet air ducts  22  through which warm, moist air, or effluent, travels. The ambient air ducts  20  connect to the individual exchanger pack modules  18  as illustrated, so that the air ducts  20  are in communication with the path  17  through of the exchanger module, as indicated by the arrows. The warm air ducts  22  are also connected to the individual exchanger modules as illustrated, however the warm air ducts  22  are in communication with the separate paths  19 . The paths of the passageways  17 ,  19  may flow wet air of effluent through the individual heat exchange modules. Both the ambient air ducts  20  and warm air ducts  22  are positioned below the heat exchanger modules  18 . For convenience a duct is defined as supplying one air path to one module, e.g., the cooling towers bays may be walled off or partitioned. Two adjacent modules may be supplied by one common duct. However, the modules may also be arranged to alternate wet and dry air paths every half bay. For example in  FIG. 1  the ducts are labeled from left to right  22 ,  20 ,  20 ,  22 ,  22 ,  20 ,  20 ,  22 ,  20 ,  20 , and  22 . Alternatively they could be partitioned such that the labels would read from left to right  22 ,  20 ,  22 ,  20 ,  22 ,  20 ,  22 ,  20 ,  22 ,  20 ,  22 , and  20 . 
     Referring now specifically to the to heat exchange modules  18 , in one embodiment of the present invention, each module is a vapor condensing heat exchanger pack. Each heat exchanger module  18  is constructed of thin sheets that are bonded together to form a pack that has a first path  17  and a second path  19  for two different air streams. In a preferred embodiment, the two air streams enter each heat exchanger module  18  at right angles to each other and are kept separate by the thin sheets. 
     The thin sheets are a relatively thin synthetic resin material that are shaped to assist in condensing vapor from a heated water laden air stream passing through passageways  19  and transferring heat to a cool air stream passing through passageways  17 . In a preferred embodiment, the material is 0.005 to 0.040 inches in thickness but is preferably 0.015 to 0.020 inches in thickness. The surface may be textured to provide extended surface area presented to each of the air streams with a minimal amount of resistance to the air stream flow. Other texture patterns may include but not be limited to textures such as dimples similar to golf ball texture and girded texture similar to a screen pattern embossed in the plastic sheet. This increased surface area enhances the heat transfer capabilities of the thin sheet and increases the velocity fluctuations near the sheet surface, which improves the local mixing of the individual air stream. The increased fluctuations and resulting local mixing of the air stream also improves the heat transfer capabilities of the sheet. 
     As depicted, each of the heat exchange modules  18  are offset from one another whereby adjacent modules  18  vary in elevation such that adjacent points of said modules are substantially separated. As illustrated in  FIGS. 1-3 , the cooling tower  10  also includes a series of air bypass doors  24  positioned between adjacent heat exchange modules. While five air bypass doors are depicted, more or less bypass doors  24  may be employed depending upon the size of the tower  10 . Also illustrated in  FIG. 1 , a series of series of dry duct vent doors, generally designated  26  are located at the bottom of each respective dry duct  20 , which operate to control the flow of warm effluent into said ducts  20 . 
     As illustrated in  FIGS. 1-3 , and previously mentioned, the indirect heat exchange modules  18  are located in the indirect cooling section  13  above the direct cooling section  11  which comprises the evaporative media  30  such as fill sheets or any heat exchange media or the like in a counterflow arrangement as illustrated. The direct evaporative section  11  further includes a hot water distribution system  32  that includes a series of conduits and nozzles  33  through which hot water flows. During operation, cool dry air, as represented by the cooling air vectors  34  enters the cooling tower below the evaporative media  30 . 
     Turning now specifically to  FIG. 1 , the cooling tower  10  is illustrated in the maximum thermal performance operation mode or position. By maximal thermal performance, it is understood that the heat exchange modules  18  are offset from one another whereby adjacent modules vary in elevation such that adjacent points of said modules are substantially separated as previously described. In said maximum thermal performance position, each of the bypass doors  24  is open, and similarly each of the vent doors  26  is open while the dry dampers  38  are closed. 
     Accordingly, during operation in this maximum thermal performance mode, hot water from the heat source is pumped through a conduit and to the spray nozzles  33  and sprayed over the evaporative media  30 . Meanwhile the axial fan (or fans)  16  draw airflow of cool ambient air as indicated by the vectors and arrows  34  through the evaporative media  30 . In the evaporative media  30 , the air is heated and moisture is evaporated into the air stream. The heated water laden air is then directed through the dry and wet air flow ducts  20 ,  22  as indicated by the arrows  36 . In this mode ambient air is restricted from entering tower  10  via the dry ducts  20  as the dry dampers  38  are closed. 
     As illustrated by the arrows  36 , the heated water laden air enters and flows through both the dry air ducts  20  and the wet air ducts  22 . Open bypass doors  24  permit a portion of heat water laden air  36  from ducts  20  and  22  to avoid traversing through heat exchange modules  18 . More specifically, the air or effluent bypassing the heat exchange modules  18  reduces the amount of air that must pass through the heat exchanger modules  18  and therefore the air velocity through each module  18  is less and the resulting pressure drop is less. Furthermore, since the air passing through the bypass doors  24  and the heat exchange modules  18  enter into a common plenum  40 , the velocities through the doors  24  and through the modules  18  will adjust to provide a common pressure drop. 
     Turning now specifically to  FIG. 2 , the cooling tower  10  is in plume abatement mode, or partial to the maximum performance mode depicted in  FIG. 1 . By plume abatement mode, it is understood that the air bypass doors  24  are closed along with the dry duct vent doors  26  while the dry dampers  38  are partially or fully opened. During operation, the direct heat exchange section  11  operates similarly as discussed in connection with the maximum thermal mode illustrated in  FIG. 1 . As the heated water laden air or effluent passes through the eliminators  15  and enters the indirect heat exchange section  13 , the vent doors are closed forcing the effluent airflow through the wet ducts  22  and into the heat exchange modules  18 . As previously mentioned, the dampers are partially or fully opened and as the effluent then proceeds through the wet ducts  22  and enters one of the previous described air flow passages  19  of the heat exchange modules  18 . Meanwhile, as previously described, ambient, dry air enters the dry air ducts  20  via the dampers  38  to generate the second air stream. The ambient, dry air is then directed through separate air flow passages  17  of the heat exchanger modules  18 , preferably perpendicular to the flow of the effluent. The ambient, dry air functions to generate a cool surface on the heat exchanger modules  18 , allowing heat to transfer from the first air stream to the second air stream. The ambient, dry air also provides a cool surface on the heat exchanger modules  18  for water vapor from the effluent or first air stream to condense on. The condensate from the effluent may then fall from the exchanger modules  18  of the heat exchange cooling section of the cooling tower. As the two air streams exit the exchange modules  18 , they are combined in the plenum  40  and exit via the shroud  14 . 
     In this mode, the dry dampers  38  on the entrance to the dry air ducts  20  can be fully opened to maximum plume abatement or may be throttled to reduce dry air intake and increase wet section  11  airflow. However, as the ambient temperature rises, the cooling may not be sufficient, but some plume abatement may still be desirable. 
     Turning now to  FIG. 3 , the cooling tower  10  is illustrated in the a partial plume abatement position wherein in the position provides more wet section  11  performance as compared to the position illustrated in  FIG. 2 , while still providing plume abatement. Whereas both the air bypass doors  24  and the dry duct vent doors  26  are closed in the orientation illustrated in  FIG. 2 , the air bypass doors  24  in the wet ducts are open while the dry duct doors  26  and the air bypass doors  24  in the dry ducts are closed in the partial abatement mode illustrated in  FIG. 3 . The dry dampers  38  on the entrance to the dry air ducts can be fully opened to maximum plume abatement or may be throttled to reduce dry air intake and increase wet section airflow. However, as the ambient temperature rises, the cooling may not be sufficient, but some plume abatement may still be desirable. Specifically, in this mode, the dry ambient air flows through the dry ducts  20  and through passages  17  of the heat exchange modules  18  and a portion of wet effluent bypasses the heat exchange modules  18  due to the bypass doors  24  being opened while the rest of the wet effluent flows through passages  19 . Again, the bypassing effluent permits the overall pressure loss through the different air paths to be reduced. Airflow through the wet section is increased which enhances thermal performance. 
     Referring now to  FIG. 4 , an alternative embodiment of the present invention is illustrated. As depicted, the cooling tower  10  is very similar in its operation and function to those embodiments disclosed and described in connection with  FIGS. 1-3 , however in this alternative embodiment the modules  18  are positioned at the same elevation. As illustrated in  FIG. 4 , rather than raising the elevation of alternating heat exchanger modules  18  to create space there between adjacent points of said modules  18 , the modules  18  are alternatively rotated to separate adjacent modules  18  creating bypass pathways  70  which are controlled by the bypass doors  24 . Alternatively, the bay spacing could be increased to create a space between the diamonds without requiring the diamonds to be rotated. The space between diamonds could then be fitted with bypass doors. 
     Turning now to  FIGS. 5 and 6 , another alternative embodiment of the present invention is depicted wherein the heat exchange modules  18  are mechanically raised and lowered via a mechanical linear lifting device  50 . The lifting device  50  may be a driven rod, screw jack, block and tackle, hydraulic cylinders or any other apparatus that allows for the elevation of the modules  18  to be modified.  FIG. 5  depicts the cooling tower  10  in the maximum thermal mode wherein alternating modules  18  are raised such that the modules  18  are positioned at differing elevations creating the bypass paths  52 . In this mode, the dry air dampers  38  are closed while the dry duct vent doors  26  are open allowing the effluent to bypass the modules  18  and accordingly providing maximum thermal performance. 
     Alternatively,  FIG. 6  illustrates the cooling tower in plume abatement mode wherein the modules  18  are positioned at the same elevation, blocking the bypass paths. In this mode, the dry duct vent doors  26  are closed while the ambient air dampers  38  are open allowing the flow of cool air through the dry ducts  20  and into passages  17  of the modules while the wet effluent flows though the wet duct only  22  through the other of the passages  19 . 
     Turning now to  FIGS. 7 and 8 , yet another alternative embodiment of the present invention is depicted. This embodiment is similar to that which is illustrated in  FIGS. 5 and 6 , however instead of moving or translating the heat exchanger modules  18  to different elevations, the modules  18  are rotated to create a bypass route  54 . As illustrated in  FIGS. 7 and 8 , the cooling tower  10  has a lifting mechanical system  56  such as a cable, sheave and linear lifting device. As illustrated in  FIG. 7 , the cooling tower  10  is in the maximum thermal performance mode whereby the heat exchanger modules  18  are rotated upward to provide the bypass path  54 . 
       FIG. 8  alternatively illustrates the cooling tower in the plume abatement mode whereby the modules  18  have been rotated downwardly such that the adjacent points are in close proximity, closing the bypass and forcing the wet effluent to flow through the respective modules while the open dampers  38  allow for the flow of dry ambient air through the dry air ducts  20  and into the modules. 
     Some applications for cooling towers may have a diminished heat load during the cold or winter months of the year. For example, air conditioning systems of buildings may have significantly lower heat load in the winter months as compared to the summer months. Moreover, many processes have minimum cold water temperature limits often called set points. Accordingly, the cold water temperature must be kept at or above the aforementioned set point. 
     In the example of an air conditioning system, the chillers often have set points at 50° F. In freezing climates, an implicit minimum cold water temperature is somewhat above freezing, e.g., 40° F., regardless of the process to avoid ice formation in the cooling system. On very cold days, cooling must be restricted to maintain the minimum set point. This may be accomplished by employing a mechanism, such as the wet dampers of the present invention, that diminish wet section air flow while maintaining or ideally increasing dry air flow. 
     Turning now to  FIGS. 9 and 10 , alternative embodiments of the present invention are depicted wherein cooling towers are illustrated having wet section damper doors. It is noted that while dampers will be discussed in detail in connection with the embodiments illustrated in  FIGS. 9 and 10 , the wet dampers may be employed in each of the embodiments depicted in  FIGS. 1-8 , as needed or as application warrants. As illustrated in  FIGS. 1-8 , damper doors  202 , which restrict wet section air flow, are employed. 
     Turning now to  FIG. 9 , a cooling tower, generally designated  200 , is illustrated having wet duct damper doors  202  that control the air flow traversing through the wet ducts  22 . The tower  200  depicted in  FIG. 9  is similar to that illustrated in  FIG. 2  except that the wet duct damper doors  202  have been partially closed. 
     During normal operation, as illustrated in  FIG. 2  for example, the wet duct damper doors  202  are operated in the open position so as not to significantly restrict air flow traversing through the wet ducts  22 . In the event that the cold water temperature is in danger of falling below the above described set point, the wet duct damper doors  202  may be partially closed, adding resistance to the wet air path, thus diminishing the wet air flow. As the ambient web bulb temperature gets colder, the wet duct damper doors  202  may be incrementally adjusted toward the closed position, thus further restricting wet air flow and maintaining the cold water set point temperature. 
     As can be seen in  FIG. 9 , the wet duct damper doors  202  are shown at the same elevation as the dry duct vent doors. This is a preferred arrangement in particular if a service walkway along the side of the tower is provided. However, alternative arrangements and orientations may be employed wherein the wet duct damper doors  202  may be positioned or located anywhere within the duct or even above to the wet path discharge of the air-to air heat exchangers. Likewise the wet duct damper doors  202  could be located below the wet ducts  22 . 
     Now turning to  FIG. 10 , where damper doors  310  are located on top of the air-to-air heat exchangers, they may serve as flow directional baffles to help with mixing wet and dry air streams. Similar to the embodiment depicted in  FIG. 9 , where the wet duct dampers have been partially closed, this concept also applies to the configurations shown in  FIGS. 6 and 8 . By partially closing the wet dampers to impede flow in wet ducts  22 , wet cooling is reduced and the likelihood of the temperature falling below the minimum cold water temperature (set point) is minimized. 
     In  FIG. 10 , dampers  320  similar to the dry air duct dampers in the figures which are so called louver or blade type dampers may be used. Another alternate embodiment is hinged doors on the wet air path discharge side of the air-to-air heat exchanger. 
     The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirits and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.