US20120318009A1

US20120318009A1 - Cooling tower

Info

Publication number: US20120318009A1
Application number: US13/423,941
Authority: US
Inventors: Bernard F. Duesel, Jr.; Michael J. Rutsch
Original assignee: Heartland Technology Partners LLC
Current assignee: ELECTRUM PARTNERS LLC; Heartland Technology Partners LLC
Priority date: 2007-01-19
Filing date: 2012-03-19
Publication date: 2012-12-20

Abstract

A cooling tower includes an evaporator vessel, a gas delivery tube partially disposed within the evaporator vessel and a liquid inlet that provides liquid to the evaporator vessel. A weir is disposed within the evaporator vessel adjacent the gas delivery tube to form a first liquid circulation path between a first weir end and a wall of the evaporator vessel and a second fluid circulation path between a second weir end and another wall of the evaporator vessel. During operation, gas introduced through the tube mixes with the liquid and the combined gas and liquid flow at a high rate with a high degree of turbulence along the first and second circulation paths defined around the weir, thereby promoting vigorous mixing and intimate contact between the gas and the liquid. This turbulent flow leads to a more efficient and complete heat and mass transfer between the gas and the liquid.

Description

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 11/625,022, filed on Jan. 19, 2007, the entire specification of U.S. patent application Ser. No. 11/625,022 is hereby incorporated by reference herein.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to cooling towers, and more specifically, to evaporative or wet cooling towers.

BACKGROUND

Cooling towers of the evaporative type are used to cool water to levels that approach ambient air temperatures. Because the water discharged from cooling towers may be recycled back to the cooling tower from processes that apply the water for cooling purpose, cooling towers are most generally used for the purpose of conserving water resources. Common applications for the cooling water produced in cooling towers include removing heat from critical components or controlling the operating temperature of engines, electrical generators, refrigeration compressors (e.g., air conditioning) and a broad range of industrial processes across a broad spectrum of industries. Natural draft and mechanical draft cooling towers are the two types of cooling towers that are most commonly used in commercial service.
As evaporative coolers, natural draft and mechanical draft cooling towers rely on the transfer of both sensible at latent heat between water and air to cool water by rejecting heat to air. Sensible heat is transferred primarily by conduction as heat flows from the warmer water to the cooler air, and latent heat is transferred through evaporation of a portion of the water into the air. The evaporative effect provides the preponderance of heat transfer in evaporative cooling processes, typically about 80%. Theoretically, heat exchange between the water and air within the cooling tower could continue until the air becomes saturated with moisture (i.e., reaches 100% relative humidity) at the adiabatic saturation temperature of the cooling system. Thus, the lowest theoretical temperature that the water can be cooled to is limited by the inlet temperatures of the air and water and the moisture content (humidity) of the inlet air. In practice, the outlet water temperature from a cooling tower may generally be brought to within approximately 10 degrees Fahrenheit of the wet bulb temperature of the incoming air stream.
Because cooling towers rely on heat and mass transfer between water and air and both conductive heat transfer and evaporation proceed fastest when the two phases are in direct contact, the efficiency of a cooling tower is critically dependent on bringing the air and water into intimate contact over a large area of interfacial surface between the two phases. Likewise, given that intimate contact over a finite interfacial surface area exists, increasing the degree of mixing or turbulence within the water and air phases increases both the rate of conductive heat transfer and evaporation (mass transfer). However, the particular method used to create interfacial surface area and the means by which the air is forced to flow over the surface area affect the size of the cooling tower and compact designs are generally more desirable considering: the value of space within industrial facilities; the fact that many cooling towers are installed on rooftops; and the negative aesthetic impact of larger units.
Natural draft cooling towers are used almost exclusively to cool large power generating plants due in part to the tremendous size of such towers and the high circulating rates of the cooling water required to justify the costs of building the towers, usually in range of 100,000 to 200,000 gal/min. Natural draft cooling towers use very large concrete chimneys to introduce air into the system and may be 400 feet or more in height for large applications such as providing cooling water for nuclear power plants. Packing or fill located within the chimney of a natural draft cooling tower is typically used to create a large amount of extended surface area for contacting the water and air phases within the chimney. A water distribution system, usually spray nozzles or a weir system, is normally used to uniformly distribute the incoming warm water over the top of the packing or fill. Thus distributed, the water flows downward over the extended surface area within the chimney of the natural draft cooling tower as a thin film under the force of gravity. Air within the chimney that is contact with the surface of the warm water picks up both heat and moisture from the water causing the density (i.e., weight per unit volume) of the air to decrease. This decrease in density creates a buoyancy effect that causes the air to rise upward through and out of the top of the chimney while higher density ambient air is drawn into openings located at the bottom of the chimney. This flow (natural draft) through the chimney of the natural draft cooling tower continues for as long at the driving forces of differential temperature between warm water and cooler air and the ability of the air to absorb moisture persist within the chimney.
Mechanical draft cooling towers, on the other hand, generally use fans to force air over circulated water as the water flows downward under the force of gravity over the same types of packing or fill as used in natural draft cooling towers. Mechanical draft cooling towers are scalable and generally range in height from approximately 10 feet to more than 40 feet, and from a few gallons of cooling fluid per minute to hundreds or thousands of gallons of cooling fluid per minute depending on the particular cooling requirements. Because mechanical draft cooling towers rely on fans to force air to flow over the film of water distributed over the extended surface area of the packing or fill, the rates of heat and mass transfer per unit of surface area can be increased based on the ability of the fans to directly induce greater turbulence to the air stream and thereby indirectly induce greater turbulence to the water through disturbance of the surface of the water film by the air flowing over it. Thus, at the cost of using higher pressure or higher volume fans, mechanical draft cooling towers can achieve higher efficiency per unit of interfacial contact area than that of natural draft cooling towers. Due to improved efficiency, mechanical draft cooling towers are generally more compact than natural draft cooling towers per unit of cooling, but more expensive to purchase, operate and maintain per unit of cooling.
Of course, typical cooling towers have inefficiencies. Some entrained droplets of water escape with the air as it rises through the cooling tower. This phenomenon is known as drift or windage. To mitigate this problem, known systems employ drift eliminators, typically in the form of a series of baffle-like devices to collect the entrained droplets and return them to the water phase. Additionally, the evaporation of water causes dissolved solids within the cooling fluid to become more concentrated. Eventually, without corrective action, the solids will begin to precipitate out of the cooling fluid and join with additional particles of solids that are brought into the cooling tower with the air causing fouling of the system.
Each of these problems requires mitigation in the form of periodic cleaning and replacement of the concentrated water with fresh water in order to maintain dissolved solids at acceptable levels. Normally, In order to maintain the level of concentration within the cooling tower at acceptable levels, a portion of the cooling water is continuously withdrawn as waste (blow down) and an equal portion of fresh water is continuously added to the cooling tower.
Submerged gas evaporators, also known as submerged gas reactors and/or combination submerged gas evaporator/reactor systems in which gas is dispersed within the liquid phase are used to concentrate wastewater streams by evaporation prior to disposing of any unwanted compounds within the wastewater. Such submerged gas evaporators may be useful as cooling towers as well. U.S. Pat. No. 5,342,482, which is hereby incorporated by reference, discloses a common type of submerged combustion gas evaporator, in which gas is delivered though an inlet pipe to a dispersal unit submerged within the liquid to be evaporated. The dispersal unit includes a number of spaced-apart gas delivery pipes extending radially outward from the inlet pipe, each of the gas delivery pipes having small holes spaced apart at various locations on the surface of the gas delivery pipe to disperse the combustion gas as small bubbles as uniformly as practical across the cross-sectional area of the liquid held within the processing vessel. According to current understanding within the prior art, this design provides desirable intimate contact between the liquid and the combustion gas over a large interfacial surface area while also promoting thorough agitation of the liquid within the evaporation vessel.
Because submerged gas evaporators disperse gas into a continuous liquid phase, for a given ratio of gas to liquid at a particular pressure the volume of the required space within the equipment that is used to bring the two phases into contact is the minimum possible and generally a much smaller volume than that required in gas-liquid contacting devices used in conventional cooling towers where the gas is the continuous phase and the liquid is dispersed into the gas stream either as droplets or thin moving films flowing over the extended surfaces of packing or fill.
However, during the evaporation process dissolved solids within the liquid phase become more concentrated often leading to the formation of precipitates that are difficult to handle. In certain cases precipitation of solids can lead to the formation of large crystals or agglomerates that can block passages within processing equipment, such as the gas exit holes in the system described in U.S. Pat. No. 5,342,482. Generally speaking, liquid streams that cause deposits to form on surfaces and create blockages within process equipment are called fouling fluids
Unlike conventional cooling towers where heat and mass are transferred from the liquid phase as it flows over the extended surface of fill or packing, heat and mass transfer within submerged gas evaporators takes place at the dynamic renewable interfacial surface area of a discontinuous gas phase dispersed within a continuous liquid phase and there are no solid surfaces upon which deposits can accumulate.
Submerged gas evaporators also tend to mitigate the formation of large crystals because dispersing the gas beneath the liquid surface promotes vigorous agitation within the evaporation zone, which is a less desirable environment for crystal growth than a more quiescent zone. Further, active mixing within the evaporation vessel tends to maintain precipitated solids in suspension and thereby mitigates the formation of potential blockages that are related to settling and/or agglomeration of suspended solids.
However, mitigation of crystal growth and settlement is dependent on the degree of mixing achieved within a particular submerged gas evaporator, and not all submerged gas evaporator designs provide adequate mixing to prevent large crystal growth and related blockages. Therefore, while the dynamic renewable heat transfer surface area feature of submerged gas evaporators eliminates the potential for fouling fluids and/or precipitates to coat extended surfaces, conventional submerged gas evaporators are still subject to potential blockages and carryover of entrained liquid within the exhaust gas flowing away from the evaporation zone.
Regardless of the type of submerged gas evaporator, in order for the process to continuously perform effectively, reliably and efficiently, the design of the submerged gas evaporator must include provisions for efficient heat and mass transfer between gas and liquid phases, control of entrained liquid droplets within the exhaust gas, mitigating the formation of large crystals or agglomerates of particles and maintaining the mixture of solids and liquids within the submerged gas evaporation vessel in a homogeneous state to prevent settling and agglomeration of suspended particles.

SUMMARY OF THE DISCLOSURE

A cooling tower includes an evaporator vessel and one or more tubes partially disposed within the evaporator vessel, which are adapted to transport a gas (e.g., air) into the interior of the evaporator vessel. The evaporator vessel has an inlet that transports warm liquid to be cooled into the vessel at a rate that maintains the liquid level inside the evaporator vessel at a predetermined level. The evaporator vessel may include an open top or an exhaust stack to allow gas to flow away from the vessel. In addition, the cooling tower includes one or more weirs that at least partially surround the tube or tubes and are submerged in the liquid to create a liquid circulation path formed by the space between each weir or each weir and the interior wall surface of the evaporation vessel and each tube. In one embodiment, each weir is open at both ends and forms a lower circulation gap between a first one of the weir ends and a bottom wall of the evaporator vessel and an upper circulation gap between a second one of the weir ends and the normal operating level of the surface of the liquid within the evaporation vessel
During operation, gas introduced through each tube mixes with the liquid in a first confined volume formed by a weir, or a weir and the interior wall surface of the evaporation vessel and the tube, and the mixture of gas and liquid flows at high volume with a high degree of turbulence along the circulation path defined around the weir, thereby imposing a high degree of mixing action between the gas and the liquid and any suspended particles within the liquid. Shear forces within this two-phase or three-phase turbulent flow region that result from the high density liquid phase overrunning the low density gas phase create extensive interfacial surface area between the gas and the liquid that favors minimum residence time for mass and heat transfer between the liquid and gas phases to come to equilibrium compared to conventional gas dispersion techniques. Still further, vigorous mixing created by the turbulent flow hinders the formation of large crystals of precipitates within the liquid and, because the system does not use small openings to introduce the gas into the liquid, clogging and fouling are significantly reduced or entirely eliminated. Still further, the predominantly horizontal flow direction of the liquid and gas mixture over the top of the weir and along the surface of the liquid within the evaporator vessel enables the gas phase to disengage from the liquid with minimal entrainment of liquid droplets due to the significantly greater momentum of the much higher density liquid that is directed primarily horizontally compared to the low density gas with a relatively weak but constant vertical momentum component due to buoyancy. The cooled liquid may be pumped through an outlet port to any number of heat exchange systems for the purpose of cooling equipment, the heated liquid may then be returned to the cooling tower for additional cooling.
In addition, a method of cooling a liquid using a submerged gas evaporator includes providing the warm liquid to be cooled to a vessel at a rate sufficient to maintain the surface of the liquid at a predetermined level within the vessel, supplying a gas to the vessel, and mixing the gas and cooling fluid within the vessel by causing the gas and liquid to flow around a weir or multiple weirs, within the cooling tower to thereby transfer heat and mass between the gas and liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a cooling tower constructed in accordance with the teachings of the disclosure.

FIG. 2 is a cross-sectional view of a second cooling tower including a baffle.

FIG. 3 is a cross-sectional view of a third cooling tower having a tubular shaped weir.

FIG. 4 is a top plan view of the cooling tower of FIG. 3.

FIG. 5 is a cross-sectional view of a fifth cooling tower having multiple gas tubes and weirs.

FIG. 6 is a cross sectional view of a sixth cooling tower having multiple cooling chambers within a single vessel.

FIG. 7 is a cross-sectional view of a plurality of cooling towers connected with one another in series to form a multi-stage cooling system.

DETAILED DESCRIPTION

Advantages of cooling towers according to the disclosure may be realized by substituting such cooling towers for conventional cooling towers in most cooling applications. Wherever cooling towers according to the disclosure are employed, conventional means may be employed to control the flow of the gas and the flow of cooling fluid through the cooling tower and, if required, to post-treat the cooling liquid and/or gas. Likewise, most other conventional means of controlling cooling tower systems to meet the requirements of a particular application may be employed. Also, multiple cooling towers according to the disclosure may be connected in series or parallel configurations to meet the cooling demand of a particular application.
Referring to FIG. 1, a cooling tower 10, in the form of a submerged gas evaporator, includes a blower 20 that provides gas under positive pressure to an evaporator vessel 30 through a gas supply tube or gas inlet tube 22 having sparge or air exit ports 24 at or near an end 26 thereof. The evaporator vessel 30 has a bottom wall 31 and a liquid outlet port 32. a liquid inlet port 34 is disposed in one side of the evaporator vessel 30 and enables a mixture of the liquid 35, makeup liquid (not shown) and any chemical additives (not shown), which may be used to control corrosion and/or biological growth, to be provided into the interior of the evaporator vessel 30. Additionally, a weir 40, which is illustrated in FIG. 1 as a flat or solid plate member having a first or lower end 41 and a second or upper end 42, is disposed within the evaporator vessel 30 adjacent the gas inlet tube 22. Although the evaporator vessel 30 and gas supply tube 22 are generally shown herein as cylindrical in shape, one skilled in the art will realize that many other shapes may be employed for these elements. The weir 40 defines and separates two volumes 70 and 71 within the evaporator vessel 30. As illustrated in FIG. 1, a gas exit port 60, disposed in the top of the evaporator vessel 30, enables gas to exit from the interior of the evaporator vessel 30. Disposed at or near a junction of the gas exit port 60 and the evaporator vessel 30 is a demister 61. The demister 61 removes droplets of liquid that are entrained in the gas phase as the gas disengages from the liquid phase at the surface 80 of the liquid 35. The demister 61 may be a vane-type demister, a mesh pad-type demister, or any combination of commercially available demisters elements. Further, a vane-type demister may be provided having a coalescing filter to improve demisting performance. The demister 61 may be mounted in any orientation and adapted to a particular evaporator vessel 30 including, but not limited to, horizontal and vertical orientations.
In the cooling tower of FIG. 1, the blower 20 supplies gas through a line 51 to the evaporator vessel 30. Moreover, the liquid is supplied through the liquid inlet 34 by a pump (not shown in FIG. 1) at a rate sufficient to maintain the surface 80 of the liquid 35 within the evaporator vessel 30 at a predetermined level, which may be set by a user. A level sensor and control 82 may be used to determine and control the rate that the liquid 35 is supplied through the inlet port 34.
As illustrated in FIG. 1, the weir 40 is mounted within the evaporator vessel 30 to form a lower circulation gap 36 between the first end 41 of the weir 40 and the bottom wall 31 of the evaporator vessel 30 and to form an upper circulation gap 37 between the second end 42 of the weir 40 and the surface 80 of the liquid 35 (or the top wall of the evaporator vessel 30). As will be understood, the upper end 42 of the weir 40 is preferably set to be at or below the surface level 80 of the liquid 35 when the liquid 35 is at rest (i.e., when no gas is being introduced into the vessel via the gas inlet tube 22). In some situations, it may be possible to set the upper end 42 of the weir 40 above the at rest level of the liquid 35, as long as introduction of the gas via the gas inlet tube 22 still causes liquid to flow over the upper end 42 of the weir 40. In any event, as illustrated in FIG. 1, the weir 40 also defines and separates the confined volume or space 70 in which the sparge ports 24 are located from the volume or space 71. If desired, the weir 40 may be mounted to the evaporator vessel 30 via welding, bolts or other fasteners attached to internal side walls of the evaporator vessel 30.
During operation, a pressurized mixture of gas from the line 51 is forced to flow under pressure into and through the gas inlet tube 22 before reaching the sparge or exit ports 24. The gas exits the gas inlet tube 22 through the sparge ports 24 into the confined volume 70 enclosed by the weir 40, the gas inlet tube 22 and the side wall of the evaporation vessel 30, causing the gas to be dispersed into the liquid 35 within the evaporator vessel 30. Generally speaking, gas exiting from the sparge ports 24 mixes with the liquid 35 within the confined volume 70 and causes a high volume flow pattern to develop around the weir 40. The velocity of the flow pattern and hence the turbulence associated with the flow pattern is highest within the confined volume 70 and at the locations where the liquid flows through the upper gap 37 and the lower gap 36 of the weir 40. The turbulence within the confined volumes 70 and 71 significantly enhances the dispersion of the gas into the liquid 35 which, in turn, provides for efficient heat and mass transfer between the gas and the liquid 35. In particular, after exiting the sparge ports 24, the gas is dispersed as a discontinuous phase into a continuous liquid phase of the liquid 35 forming a gas/liquid mixture within the confined volume 70. The mass per unit volume of the gas/liquid mixture at any point within the confined volume 70 is significantly less than the average mass per unit volume of the mixture of gas and the liquid 35 in the volume 71 due to the large difference in mass per unit volume of the liquid compared to the gas, typically on the order of approximately 1000 to 1. This difference in mass per unit volume creates a difference in static hydraulic pressure between the gas/liquid mixture in the confined volume 70 and the liquid phase in the volume 71 at all elevations within the vertical extent of the weir. This imbalance in static hydraulic pressure forces the liquid 35 to flow from the higher pressure region, i.e., the volume 71, to the lower pressure region, i.e., the confined volume 70, at a rate that overcomes the impressed static hydraulic pressure imbalance and creates flow upward through the confined volume 70.
Put another way, the dispersion of gas into the liquid 35 within the confined volume 70 at the sparge ports 24 develops a continuous flow pattern that draws liquid 35 under the bottom edge 41 of the weir 40 through the lower circulation gap 36, and causes the mixture of gas and liquid 35 to move through the confined volume 70 and toward the surface 80 of the liquid 35. Near the surface 80, the gas/liquid mixture reaches a point at which the imbalance of static hydraulic pressure is eliminated. Generally speaking, this point is at or near the upper circulation gap 37 formed between the second end 42 of the weir 40 and the surface 80. At the balance point, the force of gravity, which becomes the primary outside force acting on the gas/fluid mixture, gradually reduces the vertical momentum of the gas/liquid mixture to near zero. This reduced vertical momentum, in turn, causes the gas/liquid mixture to flow in a predominantly horizontal direction over the second end 42 of the weir 40 (through the circulation gap 37 defined at or near the surface 80 of the liquid 35) and into the liquid 35 within the volume 71.
This flow pattern around and over the weir 40 affects the dispersion of the gas into the liquid 35 and, in particular, thoroughly agitates the continuous liquid 35 within the evaporator vessel 30 while creating a substantially horizontal flow pattern of the gas/liquid mixture at and near the surface 80 of the continuous liquid phase of the liquid 35. This horizontal flow pattern significantly mitigates the potential for entrained liquid droplets to be carried vertically upward along with the dispersed gas phase as the dispersed gas phase rises through the liquid phase due to buoyancy and finally disengages from the continuous liquid phase of the liquid at the surface 80 of the liquid 35.
Also, the mixing action created by the induced flow of liquid and liquid/gas mixtures within both the confined volume 70 and the volume 71 hinders the formation of large crystals of precipitates, which generally requires a quiescent environment. By selectively favoring the production of small incipient particles of precipitates, the mixing action within evaporator vessel 30 helps to ensure that suspended particles formed in the evaporation process may be maintained in suspension within the liquid phase circulating around the weir 40, which effectively mitigates the formation of blockages and fouling within the cooling tower. Likewise, because relatively small particles that are readily maintained in suspension are formed through precipitation, the efficiency of the evaporator is improved over conventional evaporation systems in terms of freedom from clogging and fouling and the degree to which the cooling liquid may be concentrated.
In addition, as the circulating liquid 35 within volume 71 approaches the bottom wall 31 of the evaporator vessel 30, the liquid 35 is forced to flow in a predominantly horizontal direction and through the lower gap 36 into the confined volume 70. This predominantly horizontal flow pattern near the bottom wall 31 of the evaporator vessel 30 creates a scouring action at and above the interior surface of the bottom wall 31 which maintains particles of solids including precipitates in suspension within the circulating liquid while the cooling tower is operating. The scouring action at and near the bottom wall 31 of the evaporator vessel 30 also provides means to re-suspend settled particles of solids whenever the cooling tower is re-started after having been shutdown for a period of time sufficient to allow suspended particles to settle on or near the bottom wall 31.
Cooling towers generally employ a process that affects evaporation by contacting a gas with a liquid or liquid mixture, which may be a compound, a solution or slurry. Within an evaporation process that uses a submerged gas evaporator, heat and mass transfer operations occur simultaneously at the interface formed by the dynamic boundaries of the discontinuous gas and continuous liquid phases. Thus, all submerged gas evaporators include some method to disperse gas within a continuous liquid phase. The system shown in FIG. 1 however integrates the functions of dispersing the gas into the liquid phase, providing thorough agitation of the liquid phase, and mitigating entrainment of liquid droplets with the gas phase as the gas disengages from the liquid. Additionally, the turbulence and mixing that occurs within the evaporator vessel 30 due to the flow pattern created by dispersion of gas into liquid within the confined volume 70 reduces the formation of large crystals of precipitates and/or large agglomerates of smaller particles within the evaporator vessel 30.
FIG. 2 illustrates a second embodiment of a cooling tower 110, which is very similar to the cooling tower 10 of FIG. 1 and in which elements shown in FIG. 2 are assigned reference numbers being exactly 100 greater than the corresponding elements of FIG. 1. Unlike the device of FIG. 1, the cooling tower 110 includes a baffle or a shield 138 disposed within the evaporator vessel 130 at a location slightly above or slightly below the liquid surface 180 and above the second end 142 of the weir 140. The baffle or shield 138 may be a flat plate shaped and sized to conform generally to the horizontal cross-sectional area of the confined volume 170. Additionally, if desired, the baffle 138 may be mounted to any of the gas inlet tube 122, the evaporator vessel 130 or the weir 140. The baffle 138 augments the force of gravity near the balance point by presenting a physical barrier that abruptly and positively eliminates the vertical components of velocity and hence, momentum, of the gas/liquid mixture, thereby assisting the mixture to flow horizontally outward and over the weir 140 at the upper circulation gap 137. The baffle enhances the mitigation of entrained liquid droplets with the gas phase as the gas disengages from the liquid. Furthermore, the blower 120 is disposed on the gas exit port 60 in this embodiment, thus providing gas to the evaporator vessel 130 under negative pressure i.e., via suction.
As will be understood, the weirs 40 and 140 of FIGS. 1 and 2 may be generally flat plates or may be curved plates that surround the gas tubes 22 and 122 or extend across the interior of the evaporator vessel 30 between different, such as opposite, sides of the evaporator vessel 30. Basically, the weirs 40 and 140 create a wall within the evaporator vessels 30 and 130 defining and separating the volumes 70 and 71 (and 170 and 171). While the weirs 40 and 140 are preferably solid in nature they may, in some cases, be perforated, for instance, with slots or holes to modify the flow pattern within the evaporator vessel 30 or 130, or to attain a particular desired mixing result within the volume 71 or 171 while still providing a substantial barrier between the volumes 70 and 71 or 170 and 171. Additionally, while the weirs 40 and 140 may extend across the evaporator vessels 30, 130 between opposite walls of the evaporator vessels 30, 130, they may be formed into any desired shape so long as a substantial vertical barrier is formed to isolate one volume 70 (or 170) closest to a gas inlet tube 22 from the volume 71 (or 171) on the opposite side of the weir 40, 140.
FIG. 3 illustrates a cross-sectional view of a further cooling tower 210 having a weir 240 that extends around a gas inlet tube 222. A blower delivers approximately 3,500 standard cubic feet per minute (scfm) of gas to the gas inlet tube 222. While the dimensions of the cooling tower 210 are exemplary only, the ratios between these dimensions may serve as a guide for those skilled in the art to achieve a desirable balance between three desirable process results including: 1) preventing the formation of large crystals of precipitates and/or agglomerates of solid particles while maintaining solid particles as a homogeneous suspension within the liquid by controlling the degree of overall mixing within evaporator vessel 230; 2) enhancing the rates of heat and mass transfer by controlling the turbulence and hence interfacial surface area created between the gas and liquid phases within confined volume 270; and 3) mitigating the potential of entraining liquid droplets in the gas as the gas stream disengages from the liquid phase at the liquid surface 280 by maintaining a desirable and predominately horizontal velocity component for the gas/liquid mixture flowing outward over the second end 242 of the weir 240 and along the surface of the liquid 280 within the evaporator vessel 230.
As illustrated in FIG. 3, the cooling tower 210 includes an evaporator vessel 230 with a dished bottom having an interior volume and a vertical gas inlet tube 222 at least partially disposed within the interior volume of the evaporator vessel 230. In this case, the gas inlet tube 222 has a diameter of approximately 20 inches and the overall diameter of the evaporator vessel 230 is approximately 120 inches, but these diameters may be more or less based on the design capacity and desired result as relates to both gas and liquid flow rates and the type of blower device (not shown in FIG. 3) supplying gas to the cooling tower. In this example the weir 240 has a diameter of approximately 40 inches with vertical walls approximately 26 inches in length. Thus, the weir 240 forms an annular confined volume 270 within the evaporator vessel 230 between the inner wall of the weir 240 and the outer wall of the gas inlet tube 222 with cross sectional area of approximately 6.54 square feet and volume of approximately 14.18 cubic feet. In the embodiment of FIG. 3, twelve sparge ports 224 are disposed near the bottom of the gas inlet tube 222. The sparge ports 224 are substantially rectangular in shape and are, in this example, each approximately 3 inches wide by 7¼ inches high or approximately 0.151 ft²in area for a combined total area of approximately 1.81 ft²for all twelve sparge ports 224.
As will be understood, the gas exits the gas inlet tube 222 through the sparge ports 224 into a confined volume 270 formed between the gas inlet tube 222 and a tubular shaped weir 240. In this case, the weir 240 has a circular cross-sectional shape and encircles the lower end of the gas inlet tube 222. Additionally, the weir 240 is located at an elevation which creates a lower circulation gap 236 of approximately 4 inches between a first end 241 of the weir 240 and a bottom dished surface 231 of the evaporator vessel 230. The second end 242 of the weir 240 is located at an elevation below a normal or at rest operating level of the liquid within the evaporator vessel 230. Further, a baffle or shield 238 is disposed within the vessel 230 approximately 8 inches above the second end 242 of the weir 240. The baffle 238 is circular in shape and extends radially outwardly from the gas inlet tube 222. Additionally, the baffle 238 is illustrated as having an outer diameter somewhat greater than the outer diameter of the weir 240 which, in this case, is approximately 46 inches. However, the baffle 238 may have the same, a greater or smaller diameter than the diameter of the weir 240 if desired. Several support brackets 233 are mounted to the bottom surface 231 of the evaporator vessel 230 and are attached to the weir 240 near the first end 241 of the weir 240. Additionally, a gas inlet tube stabilizer ring 235 is attached to the support brackets 233 and substantially surrounds the bottom end 226 of the gas inlet tube 222 to stabilize the gas inlet tube 222 during operation.
During operation of the cooling tower 210, gas is ejected through the sparge ports 224 into the confined volume 270 between the outer wall of the gas inlet tube 222 and the inside wall of the weir 242 creating a mixture of gas and liquid within the confined volume 270 that is significantly reduced in bulk density compared to the average bulk density of the fluid located in the volume 271 outside of the wall of the weir 240. This reduction in bulk density of the gas/liquid mixture within confined volume 270 creates an imbalance in head pressure at all elevations between the surface 280 within evaporator vessel 230 and the first end 241 of the weir 240 when comparing the head pressure within the confined volume 270 and head pressure within the volume 271 outside of the wall of the weir 240 at equal elevations. The reduced head pressure within the confined volume 270 induces a flow pattern of liquid from the higher head pressure regions of volume 271 through the circulation gap 236 and into the confined volume 270. Once established, this induced flow pattern provides vigorous mixing action both within the confined volume 270 and throughout the volume 271 as liquid from the surface 280 and all locations within the volume 271 is drawn through the circulation gap 236 and upward through the confined volume 270 where the gas/liquid mixture flows outward over the second end 242 of the weir 240 and over the surface 280 confined within the evaporator vessel 230.
Within confined volume 270, the induced flow pattern and resultant vigorous mixing action creates significant shearing forces that are primarily based on the gross difference in specific gravity and hence momentum vectors between the liquid and gas phases at all points on the interfacial surface area of the liquid and gas phases. The shearing forces driven by the significant difference in specific gravity between the liquid and gas phases, which is, generally speaking, of a magnitude of 1000:1 liquid to gas, cause the interfacial surface area between the gas and liquid phases to increase significantly as the average volume of each discrete gas region within the mixture becomes smaller and smaller due to the shearing force of the flowing liquid phase. Thus, as a result of the induced flow pattern and the associated vigorous mixing within the confined area 270, the total interfacial surface area increases as the gas/liquid mixture flows upward within confined volume 270. This increase in interfacial surface area or total contact area between the gas and liquid phases favors increased rates of heat and mass transfer between constituents of the gas and liquid phases as the gas/liquid mixture flows upward within confined volume 270 and outward over the second end 242 of the weir 240.
At the point where gas/liquid mixture flowing upward within confined volume 270 reaches the elevation of the surface 280 and having passed beyond the second edge 242 of the weir 240, the difference in head pressure between the gas/liquid mixture within the confined volume 270 and the liquid within volume 271 fluid is eliminated. Absent the driving force of differential head pressure and having the confining effect of the wall of the weir 240, gravity and the resultant buoyancy of the gas phase within the liquid phase become the primary forces affecting the continuing flow patterns of the gas/liquid mixture exiting the confined space 270. The combination of the force of gravity and the impenetrable barrier created by the baffle 238 in the vertical direction eliminates the vertical velocity and momentum components of the flowing gas/liquid mixture at or below the elevation of the bottom of the baffle 238 and causes the velocity and momentum vectors of the flowing gas/liquid mixture to be directed outward through the gap 239 created by the second end 242 of the weir 240 and the bottom surface of the baffle 238 and downwards near the surface 280 within the evaporator vessel 230 causing the continuing flow pattern of the gas/liquid mixture to assume a predominantly horizontal direction. As the gas/liquid mixture flows outwards in a predominantly horizontal direction, the horizontal velocity component continually decreases causing a continual reduction in momentum and a reduction of the resultant shearing forces acting at the interfacial area within the gas/liquid mixture. The reduction in momentum and resultant shearing forces allows the force of buoyancy to become the primary driving force directing the movement of the discontinuous gas regions within the gas/liquid mixture, which causes discrete and discontinuous regions of gas to coalesce and ascend vertically within the continuous liquid phase. As the ascending gas regions within the gas/liquid mixture reach the surface 280 within the evaporator vessel 230, buoyancy causes the discontinuous gas phase to break through the surface 280 and to coalesce into a continuous gas phase that is directed upward within the confines of the evaporator vessel 230 and into the vapor exhaust duct 260 under the influence of the differential pressure created by the blower or blowers supplying gas to the cooling tower 210.
FIG. 4 is a top plan view of the cooling tower 210 of FIG. 3 illustrating the tubular nature of the weir 240. Specifically, the generally circular gas inlet tube 222 is centrally located and is surrounded by the stabilizer ring 235. In this embodiment, the stabilizer ring 235 surrounds the gas inlet tube 222 and essentially restricts any significant lateral movement of the gas inlet tube 222 due to surging or vibration such as might occur upon startup of the system. While the stabilizer ring 235 of FIG. 4 is attached to the support brackets 233 at two locations, more or fewer support brackets 233 may be employed without affecting the function of the cooling tower 210. The weir 240, which surrounds the gas inlet tube 222 and the stabilizer ring 235, and is disposed co-axially to the gas inlet tube 222 and the stabilizer ring 235, is also attached to, and is supported by the support brackets 233. In this embodiment, the confined volume 270 is formed between the weir 240 and the gas inlet tube 222 while the second volume 271 is formed between the weir 240 and the cylindrical side wall of the evaporator vessel 230. As will be understood, in this embodiment, the introduction of the gas from the exit ports 224 of the gas inlet tube 220 causes liquid to flow in an essentially toroidal pattern around the weir 240.
Some design factors relating to the design of the cooling tower 210 illustrated in FIGS. 3 and 4 are summarized below and may be useful in designing larger or smaller cooling towers. The shape of the cross sectional area and length of the gas inlet tube is generally set by: the allowable pressure drop, the configuration of the evaporator vessel, the costs of forming suitable material to match the desired cross sectional area and the characteristics of the blower that is coupled to the cooling tower. However, it is desirable that the outer wall of the gas inlet tube 222 provides adequate surface area for openings of the desired shape and size of the sparge ports which in turn admit the gas to the confined volume 271. For a typical cooling tower the vertical distance between the top edge 242 of the weir 240 and the top edge of the sparge ports should be not less than about 6 inches and preferably is at least about 17 inches. Selecting the shape and, more particularly, the size of the sparge port 224 openings is a balance between allowable pressure drop and the initial amount of interfacial area created at the point where the gas is dispersed into the flowing liquid phase within confined volume 271. The open area of the sparge ports 224 is generally more important than the shape, which can be most any configuration including, but not limited to, rectangular, trapezoidal, triangular, round, oval. In general, the open area of the sparge ports 224 should be such that the ratio of gas flow to total combined open area of all sparge ports should at least be in the range of 1,000 to 18,000 acfm per ft², preferably in the range of 2,000 to 10,000 acfm/ft²and more preferably in the range of 2,000 to 8,000 acfm/ft², where acfm is referenced to the operating temperature within the gas inlet tube. Likewise, the ratio of the gas flow to the cross sectional area of the confined volume 270 (CSA_{confined volume}) should be at least in the range of 400 to 10,000 scfm/ft², preferably in the range of 500 to 4,000 scfm/ft²and more preferably in the range of 500 to 2,000 scfm/ft². Additionally, the ratio of the cross sectional area of the evaporator vessel 230 (CSA_vessel) to the cross sectional area of the confined volume 270 is preferably in the range from three to one (3.0:1) to two-hundred to one (200:1), is more preferably in the range from eight to one (8.0:1) to one-hundred to one (100:1) and is highly preferably in the range of about ten to one (10:1) to fourteen to one (14:1). These ratios are summarized in the table below. Of course, in some circumstances, other ratios for these design criteria could be used as well or instead of those particularly described herein.

TABLE 1

	Preferred	Acceptable	Preferred
Ratios	Embodiment	Range	Range

Acfm:Total CSA_{sparge ports}	2,000-8,000	1,000-18,000	2,000-10,000
	acfm/ft²	acfm/ft²	acfm/ft²
Scfm:CSA_{confined volume}	500-2,000	400-10,000	500-4,000
	scfm/ft²	scfm/ft²	scfm/ft²
CSA_{confined volume}:CSA_vessel	10:1-14:1	3.0:1-200:1	8.0:1-100:1

The embodiment of a cooling tower shown in FIG. 5 includes multiple gas tubes 522 and multiple weirs 540. The evaporator vessel 530 may include more than one gas tube 522 and/or more than one weir 540 to increase cooling capability without a significant increase in the size of the cooling tower 510.
The embodiment of a cooling tower shown in FIG. 6 includes multiple gas tubes 822 and multiple weirs 840 within a single vessel 830, like the embodiment shown in FIG. 5. However, the embodiment shown in FIG. 6 is arranged to provide sequential (multi-stage) cooling by having a series of baffles 892 a, 892 b that divide the vessel into three cooling chambers 894 a, 894 b, 894 c. Of course, more or less than three cooling chambers may be used depending on the amount of space available and the required level of cooling. Generally, the more cooling chambers 894 a, 894 b, 894 c, the greater the cooling of the liquid 890. The liquid enters the vessel 830 into the first cooling chamber 894 a. The liquid 890 is sequentially cooled in each subsequent cooling chamber 894 b, 894 c further reducing temperature in each cooling chamber 894 a, 894 b, 894 c. The liquid 890 is removed from the last cooling chamber 894 c at its lowest level of temperature. The additional cooling accomplished in each chamber is due in part to the introduction of fresh gas into each cooling chamber 894 a, 894 b, 894 c.
Turning now to FIG. 7, a multi-stage cooling system is illustrated that comprises first and second cooling towers 910, 910′ that are connected in series. In other embodiments, more than two cooling towers may be connected in series. In yet other embodiments, a plurality of cooling towers may be connected in parallel. The gas inlet tube 922′ of the second cooling tower 910′ is fluidly connected to the gas exit port 960 of the first cooling tower 910. Thus, gas exiting the first cooling tower is used as an input gas for the second cooling tower. By fluidly connecting a plurality of cooling towers 910, 910′ in series, gas exiting the system (i.e., gas exiting the last cooling tower in the series) may be brought very close to its adiabatic saturation temperature, which increases overall efficiency of the cooling system.
In the example of FIG. 7, gas exiting the first cooling tower 910 may be within about 10° F. of its adiabatic saturation temperature while gas exiting the second cooling tower 910′ may be within about 5° F. of its adiabatic saturation temperature due to the additional evaporation that occurs in the second cooling tower 910′. Each successive cooling tower would bring the exiting gas closer to its adiabatic saturation temperature. For example, by adding a third cooling tower, the exiting gas may be within about 2° if its adiabatic saturation temperature.
Generally speaking a series of smaller cooling towers (such as the cooling towers illustrated in FIG. 7) may be used to replace a single larger cooling tower. A series of smaller cooling towers may have a smaller overall footprint, or the series of smaller cooling towers may be stacked to conserve space and thus, the series of smaller cooling towers be placed in confined spaces that cannot accommodate a single larger cooling tower. A series of smaller cooling towers connected in series may be used in other space-limited locations, such as on rooftops of buildings to cool liquids in HVAC systems, such as water. Additionally, because head pressures of the fluids within the evaporator vessels 910, 910′ are not additive in nature, a smaller blower or induction fan may be used for a series of smaller cooling towers than a blower that would be required for a single larger cooling tower having a similar cooling capacity. For example, if each of the respective evaporator vessels 910, 910′ had a 17 inch difference between the bottom of the gas inlet tube 922, 922′ and the top of liquid in the vessel 910, 910′ (which would result in a 17 inch head pressure), the blower would only need to be sized to overcome a single 17 inch head pressure because the head pressure of the second evaporator vessel 910′ does not add to the head pressure of the first evaporator vessel 910. Thus, a series of smaller cooling towers may use a smaller blower (and thus less energy) than a single larger cooling tower having a similar cooling capacity. Similarly, a series of smaller cooling towers may use a blower that is approximately the same size as a blower that would be required for a single cooling tower in the series. The blower for the series of smaller cooling towers need only be marginally larger in that the blower for the series of smaller cooling towers would need to overcome only the additional fluid friction generated by the additional connection pipes and gas inlet tubes.
The cooling system of FIG. 7 illustrates embodiments of individual cooling towers similar to those illustrated in FIGS. 3 and 4. However, any of the disclosed cooling tower embodiments may be connected in series or parallel to produce a cooling system. Moreover, various disclosed embodiments of cooling towers may be mixed and connected in series produce a cooling system.
Generally speaking, in any of the disclosed cooling towers, the liquid has a temperature that is greater than a temperature of the gas flowing through the gas inlet tube. The gas inlet tube may be connected to a source of cool gas and the fluid inlet may be connected to a source of warm liquid. The temperature of the liquid at the fluid inlet may be greater than the temperature of the gas entering the gas inlet tube. In some embodiments the temperature of the liquid may be between about 1° F. and about 60° F. greater than the temperature of the gas. In other embodiments, the temperature difference may be between about 5° F. and about 40° F., and more specifically between about 10° F. and about 30° F.
It will be understood that, because the weir and gas dispersion configurations within the cooling towers illustrated in the embodiments of FIGS. 1-7 provide for a high degree of mixing, induced turbulent flow and the resultant intimate contact between liquid and gas within the confined volumes 70, 170, 270, etc., the cooling towers of FIGS. 1-7 create a large interfacial surface area for the interaction of the liquid and the gas provided via the gas inlet tube, leading to very efficient heat and mass transfer between gas and liquid phases. Furthermore, the use of the weir and, if desired, the baffle, to cause a predominantly horizontal flow pattern of the gas/liquid mixture at the surface of the fluid mixture mitigates or eliminates the entrainment of droplets of cooling liquid within the exhaust gas. Still further, the high degree of turbulent flow within the evaporator vessel mitigates or reduces the formation of large crystals or agglomerates and maintains the mixture of solids and liquids within the evaporator vessel in a homogeneous state to prevent or reduce settling of precipitated solids. This factor, in turn, reduces or eliminates the need to frequently clean the evaporator vessel and allows the evaporation process to proceed to a very high state of concentration by maintaining precipitates in suspension. In the event that such solids do form, however, they may be removed via the outlet port (e.g., the outlet port 32 of FIG. 1) using a conventional valve arrangement. As known to those skilled in the art of operating cooling towers, removal (blowdown) of liquid and replacement with fresh (makeup) liquid at a controlled rate may be employed to maintain the level of solids within the liquid at a desirable equilibrium concentration of total suspended and/or dissolved solids.
While several different types cooling towers having different weir configurations are illustrated herein, it will be understood that the shapes and configurations of the components, including the weirs, baffles and gas entry ports, used in these devices could be varied or altered as desired. Thus, for example, while the gas inlet tubes are illustrated as being circular in cross section, these tubes could be of any desired cross sectional shape including, for example, square, rectangular, oval, etc. Additionally, while the weirs illustrated herein have been shown as flat plates or as tubular members having a circular cross-sectional shape, weirs of other shapes or configurations could be used as well, including weirs having a square, rectangular, oval, or other cross sectional shape disposed around a gas inlet tube, weirs being curved, arcuate, or multi-faceted in shape or having one or more walls disposed partially around a gas inlet tube, etc. Also, the gas entry ports shown as rectangular may assume most any shape including trapezoidal, triangular, circular, oval, or triangular. Furthermore, the weirs need not be solid surfaces and may be perforated or latticed if desired. Further, virtually any liquid may be used in the cooling towers disclosed herein (e.g., anti-freeze solutions).
Additionally, various elements illustrated in the individual embodiments illustrated in FIGS. 1-7 are compatible with other embodiments. For example, the process fluid level sensor and control 82 of the embodiment of FIG. 1 may be incorporated into any of the other embodiments to maintain a desired level of liquid within the evaporator vessel. Similarly, the blowers of the embodiments illustrated in FIGS. 1, 3, 5, and 6 may be located on the gas exit port, like the blower 120 of the embodiment illustrated in FIG. 2, to act as an induction fan, or vice versa. One advantage of using an induction fan over a positive pressure blower is that an induction fan lowers the static pressures within various components of the cooling towers, which would cause the liquids to evaporate at lower temperatures, thus enhancing cooling due to evaporation.
While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the methods and apparatus disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.

Claims

1. A cooling tower comprising:

an evaporator vessel having an interior for holding a liquid;

a gas inlet tube disposed within the evaporator vessel for transporting a gas into the interior of the evaporator vessel;

a weir disposed within the evaporator vessel adjacent to the gas inlet tube;

a gas exit port for transporting gas from the interior of the evaporator vessel; and

a liquid inlet for supplying the liquid to the interior of the evaporator vessel;

wherein the weir includes a first weir end and a second weir end and is disposed within the evaporator vessel to define a first circulation gap between the first weir end and a first wall of the evaporator vessel and to define a second circulation gap between the second weir end and a second wall of the evaporator vessel, liquid within the evaporator vessel being caused to circulate through the first and second circulation gaps when gas is introduced into the evaporator vessel from the gas tube, and

wherein a temperature of gas that is transported into the interior of the evaporator vessel through the gas inlet tube is less than a temperature of liquid supplied to the interior of the evaporator vessel through the liquid inlet.

2. The cooling tower of claim 1 further including a baffle disposed proximate the second circulation gap and generally perpendicular to the weir.

3. The cooling tower of claim 1, wherein the gas tube includes a gas exit disposed below a surface of the cooling liquid when the liquid is disposed within the evaporator vessel.

4. The cooling tower of claim 3, further including a plurality of gas exits disposed in the gas tube, wherein each gas exit is substantially rectangular in shape.

5. The cooling tower of claim 1, wherein the weir comprises a tubular member disposed around the gas tube.

6. The cooling tower of claim 5, wherein the tubular member is disposed co-axial to the gas tube.

7. The cooling tower of claim 1, further including a blower attached to the gas tube.

8. The cooling tower of claim 7 wherein the blower supplies gas to the evaporation vessel under positive pressure.

9. The cooling tower of claim 1 wherein the blower supplies gas to the evaporation vessel under negative pressure.

10. The cooling tower of claim 1 further comprising a plurality of gas tubes.

11. The cooling tower of claim 1 further comprising a plurality of weirs.

12. The cooling tower of claim 1 further comprising a plurality of blowers.

13. A method of cooling a liquid in a cooling tower having a weir disposed within an evaporator vessel, the weir defining first and second volumes within the evaporator vessel and a gas inlet tube extending into the evaporator vessel into the first volume, the method comprising:

supplying a liquid at a first temperature to the evaporator vessel at a rate sufficient to maintain a liquid surface level in the evaporator vessel at or above a first end of the weir when the cooling tower is operating;

providing gas at a second temperature through the gas inlet tube to force the gas through an exit in the gas inlet tube to cause mixing of the gas and the liquid within the first volume by creating a circular flow of liquid from the first volume around the first end of the weir into the second volume and from the second volume around a second end of the weir and into the first volume; and

removing gas through a gas exit port in the evaporator vessel,

wherein the first temperature is greater than the second temperature.

14. The method of claim 13, further including removing liquid with suspended solid particulate from the evaporator vessel.

15. The method of claim 13, wherein mixing of the gas and the liquid causes the liquid to at least partially evaporate, thereby lowering a temperature of the liquid within the evaporator vessel.

16. A cooling system for cooling a liquid, the cooling system including a plurality of cooling towers fluidly connected to one another,

at least one cooling tower comprising:

an evaporator vessel having an interior for holding a liquid;

a weir disposed within the evaporator vessel adjacent to the gas inlet tube;

wherein a temperature of gas transported into the interior of the evaporator vessel through the gas inlet tube has a temperature that is less than a temperature of liquid supplied to the interior of the evaporator vessel through the liquid inlet.

17. The cooling system of claim 16, wherein at least two cooling towers in the plurality of cooling towers are fluidly connected in series.

18. The cooling system of claim 17, wherein a gas exit port of a first cooling tower in the plurality of cooling towers is fluidly connected to a gas inlet tube of a second cooling tower in the plurality of cooling towers.

19. The cooling system of claim 16, wherein gas flows through the plurality of cooling towers under negative pressure.

20. The cooling system of claim 16, wherein gas exiting the cooling system is within about 5° F. of its adiabatic saturation temperature.