System and method of cooling

Abstract

A cooling system and method for use in industrial plants are disclosed. A hot fluid stream is first cooled in an air-cooled stage to an intermediate temperature, and then to a final temperature in a water-cooled stage. The two-stage cooling method, which has advantages over either air-cooling or water-cooling alone, is well-suited for applications or locations where water use is restricted due to economic or environmental concerns.

Description

BACKGROUND OF THE INVENTION

[0002] Conventional systems for cooling hot fluids in industrial plants or services typically use either air coolers, which do not require the availability of a water resource, or water coolers, which do require a water resource. As an example, a hot fluid may be air-cooled by passing the fluid through an equipment or device, e.g., a heat exchanger, that exchanges heat directly with the surrounding atmosphere. An alternative air cooling configuration may entail passing the hot fluid through an equipment or device that exchanges heat with a colder fluid or a liquid heat exchange medium, e.g., a water/glycol mixture, which in turn passes through another device that exchanges heat with the surrounding atmosphere. In the latter air cooling configuration, the colder fluid is provided as a circulating stream in a closed loop circuit with minimal evaporative loss of the fluid.

[0003] There are two principal types of configurations of water based cooling systems, both of which use a circulating flow of cooling water to cool the hot fluid. In one configuration, the cooling water undergoes heat exchange with the hot fluid, and is then cooled in an evaporative cooling tower before re-circulating back to the process or industrial service. This configuration requires additional water to make up for the evaporative loss to the atmosphere. Water make up is also required to compensate for losses resulting from the need (with this type of system) to purge dissolved mineral matter from the circulating cooling water by means of a blowdown stream, which must be disposed of appropriately. In another configuration, heat is rejected from the cooling water directly to a body of water, which is usually a natural water source such as a river, pond or sea. Both types of water based cooling systems impact the water resource—the first type by virtue of the water consumption and blowdown disposal, and the second type by virtue of the rise in temperature of the body of water to which the heat is rejected.

[0004] Since an air-cooled system is not as efficient as a water-cooled system in terms of the amount of heat transfer per unit surface area, a conventional system using air cooling alone generally requires a much larger surface area for heat exchange than a water-cooled system of comparable cooling capacity. Thus, air-cooled systems as described above are often more expensive than water-cooled systems, particularly if they are used for applications that require lower temperature fluids for more efficient operations. Examples of these applications include, but are not limited to, gas compression, processes that utilize refrigerated or cryogenic equipment, and certain absorption and adsorption processes. However, there may be circumstances that require the use of air-cooled systems for these applications, e.g., in situations where the cost of water is high, or where water use is restricted due to limited availability, environmental concerns or other reasons.

[0005] Therefore, there is an ongoing need to provide alternative cooling systems or methods for use with industrial plants or processes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] While the specification concludes with claims distinctly pointing out the subject matter that the applicants regard as their invention, it is believed the invention would be better understood when taken in connection with the accompanying drawings in which:

[0007] FIG. 1 is a schematic illustration of one embodiment of the present invention having an air-cooled device and a water-cooled device coupled to an evaporative tower;

[0008] FIG. 2 is a schematic illustration of another embodiment of the present invention having an air-cooled device incorporated within a closed loop cooling circuit and a water-cooled device coupled to an evaporative tower;

[0009] FIG. 3 is a schematic illustration of another embodiment of a circulating cooling system having an air-cooled device in series with an evaporative cooling tower;

[0010] FIG. 4 is a schematic illustration of another embodiment of the present invention, with water cooling provided in a “once-through” configuration; and

[0011] FIG. 5 is a schematic illustration of cooling compressed air streams using cooling configurations according to the present invention.

SUMMARY OF THE INVENTION

[0012] The present invention relates generally to a system and method of cooling. One aspect of the present invention provides for a cooling system for use in an air separation plant. The cooling system comprises an air-cooled stage downstream from an equipment in the air separation plant for cooling a hot fluid stream associated with the equipment to a first temperature; and a water-cooled stage downstream from the air-cooled stage for cooling the hot fluid stream further to a second temperature.

[0013] Another aspect of the present invention provides for a method of cooling for use in an air separation plant. The method comprises cooling a hot fluid stream associated with an equipment in the air separation plant to a first temperature using an air-cooled stage; and cooling the hot fluid stream further to a second temperature in a water-cooled stage downstream from the air-cooled stage.

[0014] Yet another aspect of the invention provides for a method of cooling for use in an industrial plant. The method comprises cooling a hot fluid stream originating from an equipment in the industrial plant to a first temperature using an air-cooled stage, the equipment being selected from the group consisting of a compressor and a compressor/expander system; and cooling the hot fluid stream from the first temperature to a second temperature using a water-cooled stage.

DETAILED DESCRIPTION

[0015] The present invention provides generally a system and a method of cooling that can be used in a variety of applications. In particular, the present invention relates to a hybrid cooling system that combines at least one air-cooled system operating at a higher temperature range with at least one water-cooled system operating at a lower temperature range. Depending on the specific applications, such a hybrid system may allow cooling to be achieved at a lower cost than would be achievable by either air-cooled or water-cooled system alone. In general, the air-cooled system may include any suitable air coolers of different designs, while the water-cooled system may include any suitable conventional evaporative systems such as cooling towers, or systems that reject heat to a water source or body of water such as a river, lake or sea. Although embodiments of the invention are discussed with respect to examples associated with equipment in air separation plants, they are more generally applicable to cooling duties associated with any major machinery or equipment, such as those used in a variety of industrial processes or plants.

[0016] As used herein, an “air cooler” or “air-cooled device” refers generally to any heat exchanger whose primary mode of heat rejection to the atmosphere or surroundings is achieved by non-evaporative heat transfer to the atmosphere. It is preferable that there is no evaporative heat transfer in the first cooling stage, i.e., air-cooled stage, although a limited amount of evaporation can be tolerated. Such an air cooler may encompass many different designs, including, for example, some designs in which a limited amount of water may be used to enhance the performance of the air cooler. Thus, a cooling stage incorporating an air cooler that rejects heat to the surrounding atmosphere primarily through a non-evaporative heat transfer process will be referred to as an air-cooled stage. Accordingly, an air-cooled stage may encompass a configuration that includes a closed loop liquid coolant system for transferring heat from the hot process fluid to the air cooler, which then rejects heat to the surrounding atmosphere. Heat exchangers that are part of a system that rejects heat by use of a water resource, either by evaporative cooling, or to a body of water such as a river, lake or sea, will be referred to as “water coolers” or “water-cooled”.

[0017] The hybrid cooling system and method of the present invention are particularly well-suited for applications or locations where water supply is expensive, where water use is restricted by virtue of limited resource or availability, or where water discharge from cooling towers, or heat release into a body of water, may present environmental issues.

[0018] According to embodiments of the present invention, the hybrid cooling system provides cooling in two stages. In the first stage, a hot fluid stream is cooled using an air-cooled system or device. At this stage, the temperature of the hot fluid is relatively high, and the air-cooled system or device can be operated at a correspondingly higher approach temperature (i.e., temperature difference between the outgoing cooled fluid and the incoming ambient air coolant). This allows the bulk of the heat to be removed without the use of a larger air-cooled system, which would otherwise be required for operations at smaller approach temperatures. The hybrid cooling system is generally applicable to cooling any hot fluid stream associated with an equipment in an industrial plant. Thus, the hot fluid stream may be a process fluid originating from a heat source such as certain services or equipment in an industrial plant, e.g., a compressed air stream from a compressor of an air separation plant. In another embodiment, the hot fluid stream may be a suitable heat transfer medium used for heat exchange with the process fluid (with no direct contact between the two fluids), or for cooling the equipment in the industrial plant.

[0019] The second stage of cooling is provided by water cooling, which allows the hot fluid stream to be cooled to an even lower temperature. Since the heat transfer coefficient for water cooling is larger than for air cooling, smaller and less costly water-cooled devices can be used to achieve smaller approach temperatures. Thus, water cooling is generally performed at a smaller approach temperature compared to that of the air cooler. The two cooling stages in the present invention are accomplished in separate cooling systems or devices, as opposed to being integrated within a single cooling system such as a hybrid cooling tower disclosed in U.S. Pat. No. 6,233,941, or a combined wet-dry heat transfer system disclosed in U.S. Pat. No. 4,003,970. The two serially connected cooling stages are used in the present invention for cooling a hot fluid stream via heat transfer with air (directly or indirectly) and then with water in order to achieve a lower final temperature.

[0020] FIG. 1 is a schematic illustration of a cooling system 100 according to one embodiment of the present invention. The cooling system 100 comprises an air-cooled device 110 in series with a water-cooled device 120. As shown in FIG. 1, a hot fluid stream 104 from a heat source 102 is first subjected to cooling in the air-cooled device 110 to an approach temperature given by the difference between Ti (temperature of the hot fluid stream leaving the air cooler) and Tain (incoming air temperature). The hot fluid is then further cooled by the water-cooled device 120 to another temperature Tf.

[0021] As shown in FIG. 1, a circulation pump 124 provides water circulation between the water-cooled device 120 and an evaporative tower circuit 122. The pump 124, water-cooled device 120 and the tower circuit 122 may be considered as forming a water cooling circuit or system 160. The evaporative tower circuit 122 is a conventional circuit that comprises generally various components (not shown) that may include an evaporative tower, a fan to facilitate evaporative cooling, as well as associated components, such as pump, filter and water treatment equipment, intake and discharge structures, among others. An optional chiller 126 may be used to further cool the water being circulated to the water-cooled device 120.

[0022] In this embodiment, one or more heat sources 102 are provided with their own air-cooled devices 110 to satisfy the respective cooling needs, while individual components within the water cooling system 160 such as pump 124 or chiller 126 may be shared with one or more services or equipment requiring cooling. Thus, portions of the cooling water stream may be distributed to equipment 140, 142 requiring cooling, and hot water from such equipment or services will subsequently enter the evaporative tower circuit 122. In general, water supplied to these other services or equipment 140 and 142 may be further cooled by one or more chiller systems, as needed, and may also be cooled in one or more air coolers (not shown) prior to being returned to the water cooling circuit 160.

[0023] As an example, the heat source 102 may be one stage of a compressor in an air separation plant. A hot gas stream 104, after compression by the compressor stage, may have an initial temperature (Th) of about 80° C. The ambient air temperature (Tain) might be about 30° C. The air-cooled device 110 may be a fin fan cooler, which is used to cool the hot gas to an intermediate temperature (Ti) of, for example, about 50° C. The hot gas is subsequently cooled by the water-cooled device 120 to a temperature (Tf) of about 35° C., before passing to another compressor stage or to other equipment in the air separation plant. The water streams entering and exiting the tower circuit 122 typically has temperatures of about 40° C. and about 30° C., respectively.

[0024] It is understood that the operating temperatures cited in this example are meant to be illustrative, and other compressor applications may involve different operating temperature ranges. In general, the hybrid cooling system is applicable to many other processes or wide ranging operating temperature conditions.

[0025] One parameter for characterizing the performance of an air-cooled device is the logarithmic mean temperature difference (LMTD), which is defined as (ΔT1−ΔT2)/ln (ΔT1/ΔT2), where ΔT1 is given by: the temperature difference between the incoming hot fluid and the outgoing air stream (after heat exchange), and ΔT2 is the temperature difference between the outgoing hot fluid (after being air-cooled) and the incoming air stream (before heat exchange). In general, the smaller the LMTD value, the larger is the size requirement of the air-cooled equipment. To achieve a given desired final temperature (Tf) of the hot fluid, the hybrid cooling system allows the air-cooled device to be operated at a higher LMTD than a cooling system using air cooling alone, thus allowing the use of a smaller air cooler than otherwise possible.

[0026] The hybrid cooling approach offers a further advantage, especially in areas with a hot and dry climate. Due to the relatively large evaporative cooling potential (resulting from a large difference between the wet bulb and dry bulb temperatures), the combined air cooling and water cooling approach allows the hot fluid to be cooled to a lower final temperature than is possible if air cooling is used alone.

[0027] FIG. 2 is a schematic illustration of an alternative cooling system 200 according to another embodiment of the present invention. A hot fluid stream 204 generated from a heat source 202 is first subjected to cooling in a first heat exchange section 215. Cooling in this first stage is achieved by a closed loop cooling circuit 210 that incorporates an air cooler 212 for heat rejection. In the closed loop cooling circuit 210, a coolant stream 250 containing a suitable heat transfer fluid is circulated by a circulating pump 214. In general, different heat transfer fluids may be used, e.g., water, or optionally, a water and glycol mixture, among others. The hot fluid stream 204 is cooled from a temperature Th to Tl by heat exchange with the coolant stream 250 in the heat exchange section 215. The warm coolant stream 250 is then cooled by the air cooler 212, which may be a fin fan cooler or other suitable designs. The cooled coolant stream 250 exiting the air cooler 212 is circulated back to the heat exchange section 215. The use of the closed loop cooling circuit 210 in this embodiment is meant to be illustrative. In general, the first cooling stage in this embodiment may be accomplished by other configurations in which a coolant stream is provided to transfer heat from the hot fluid stream 204 to the air cooler 212, which then rejects heat to the surrounding atmosphere.

[0028] After the hot fluid stream 204 from heat source 202 is cooled from its initial high temperature Th to an intermediate temperature Ti, it is subjected to further cooling to a final temperature Tf in a second heat exchange section 225, which is a water-cooled device. The water-cooled device 225 is coupled to a cooling tower circuit 222 similar to that shown in FIG. 1. A circulation pump 224 circulates water between the tower circuit 222 and the water-cooled device 225, thus forming a water cooling circuit or system 260. An optional chiller 226 may also be provided for further cooling of the water in the water cooling system 260.

[0029] In this embodiment, air coolers used for removing heat from different heat sources may be centralized at one or more locations (as opposed to being individually located proximate to each heat source). In addition, aside from serving the cooling requirement for heat source 202, the closed loop circulating circuit 210 with the air cooler 212 may provide cooling to one or more heat sources such as 230. Thus, a coolant stream 232 associated with heat source 230 may be cooled by the air cooler 212, and part of the coolant stream downstream of the pump 214 may be circulated back to this service or equipment 230.

[0030] Furthermore, components in the water cooling circuit 260, e.g., tower circuit 222, pump 224 or chiller 226, may be shared with other services or equipment 240 and 242 requiring cooling. Thus, part of the cooled water stream exiting the tower circuit 222 may be circulated to equipment 240 and 242, and hot water streams 244 and 246 from equipment 240 and 242 may return to the tower circuit 222 for cooling.

[0031] As an example in the context of an air separation plant, the heat source 202 may be a compressor producing a hot compressed gas stream 204 having a temperature Th of about 80° C., and the first and second heat exchange sections 215 and 225 may correspond to two different sections within a shell and tube heat exchanger for cooling the compressed air stream. Alternatively, the first and second heat exchange sections 215 and 225 may also be separate heat exchangers. The ambient air temperature, e.g., around the air cooler 212, may be about 30° C. The coolant stream in the closed loop cooling circuit 210 may be supplied to the heat exchange section 215 at a temperature of about 45° C., and has a temperature between about 55-60° C. after heat exchange with the compressed gas stream 204, which may be cooled to a temperature Ti of about 50° C. In the second cooling stage, temperature of the cooling water entering the water-cooled device 225 is typically about 30° C. and may increase to about 40° C. after heat exchange with the compressed gas stream 204, which is cooled to a final temperature Tf of about 35° C. After cooling, the compressed gas stream 204 is directed to other service or equipment of the air separation plant.

[0032] FIG. 3 is a schematic illustration of another alternative hybrid cooling system 300 according to the present invention. A hot fluid stream 304 from a heat source 302 is cooled from an initial temperature Th to a temperature Tf by heat exchange with a cooling water stream 350 in a heat exchange section 325. The hybrid cooling system 300 comprises an air-cooled device 310 and a water-cooled device 320 connected in series, along with a circulating pump 324 to circulate water between the two cooling devices. In this example, the water-cooled device 320 is an evaporative water tower. Unlike certain prior art teaching, e.g., hybrid cooling tower of U.S. Pat. No. 6,233,941, the air-cooled device 310 and the water tower 320 used in the present invention are distinct cooling devices, as opposed to being incorporated within a single structure. Thus, these devices may be located in disparate locations of a facility which has advantages in overall plant layout.

[0033] In this embodiment, a single circulating water system is used. The water stream 350 containing the amount of heat absorbed from the hot fluid stream 304 is first cooled by the air-cooled device 310 from a temperature Th′ to a temperature Ti′. The air-cooled device 310 may, for example, be a fan-assisted fin cooler, or cooler of other suitable designs. The water stream is then subjected to a second cooling stage provided by the evaporative cooling tower 320. The cooled water stream having a temperature of Tf′ is circulated back to the heat exchange section 325 by the pump 324. An optional chiller 326 may also be used for further cooling of the circulating water.

[0034] As an illustrative example, using the hybrid cooling system 300 at an ambient temperature of 30° C., the hot fluid stream 304 having an initial temperature Th of about 80° C. can readily be cooled to a temperature Tf of about 35° C. In this embodiment, the air-cooled device 310 provides the first stage of cooling for the cooling water stream 350, resulting in a reduction of temperature from about 60° C. to about 45° C. for the cooling water stream 350. The cooling tower 320 removes additional heat from the water stream 350, resulting in a coolant stream temperature Tf′ of about 30° C. This embodiment accommodates a much higher water temperature rise more economically than an individual air-cooled or water-cooled system because the hybrid equipment size requires less surface for a given approach to ambient temperature, thus resulting in lower capital. This embodiment also accommodates a much higher water temperature rise more economically than an individual water-cooled system because the hybrid system requires less makeup water and blowdown which would lead to savings in operating cost.

[0035] Similar to embodiments illustrated in FIG. 1 and FIG. 2, this hybrid cooling system 300 may also be coupled to other services or equipment 336, 338 and 340 requiring cooling. For example, cooled water streams may be distributed to equipment 336 or 338 as appropriate. Optionally, a circulating pump 328 may be used, as required, to provide a cooling water stream to another service or equipment 340 that may be able to utilize a warmer coolant stream at temperature Ti′. Hot water streams from equipment 336, 338 or 340 may be returned to the cooling system 300 at different connection points upstream or downstream the air cooler 310, e.g., at connection points 330 or 334. Some of these optional return paths are shown as dotted lines in FIG. 3. In general, these other services or equipment may utilize cooling provided by one or more of the following: individual air coolers, coolant from the evaporative tower circuit with or without supplemental chilling, or any combinations thereof. Again, it is understood that temperature ranges used in the above examples are meant for illustrative purposes only, and the hybrid cooling system is applicable to high temperature sources as well.

[0036] FIG. 4 illustrates another embodiment of a cooling system 400 comprising an air-cooled system 410 and a water-cooled system 460. The air-cooled system 410 may correspond to an air-cooled device similar to that described in FIG. 1. Alternatively, the air-cooled system 410 may be similar to the closed loop cooling circuit 210 illustrated in FIG. 2, where cooling of a hot fluid stream 404 from a heat source 402 is achieved by heat exchange with a circulating liquid coolant, which in turn is cooled by an air-cooled device. After being cooled from an initial temperature Th to a temperature Tl, the hot fluid stream 404 is further cooled to a final temperature Tf by the water-cooled system 460.

[0037] In this embodiment, the water-cooled system 460 comprises a water-cooled device 420 coupled to a water source 422, which is a body of water such as a river, lake or sea. Water from this water source 422 is provided to the water-cooled device 420 by a water pump 424. After heat exchange with the hot fluid 404 in the water-cooled device 420, the cooling water is discharged into the water source 422. This configuration is sometimes referred to as a “once-through” cooling system. Optionally, a chiller 426 may be used to provide additional chilling of the cooling water prior to entering the water-cooled device 420. Again, cooling water from the water source 422 may be used to provide cooling to other services or equipment 440, 442, as desired. It is understood that this once-through water cooling system may be used as part of the hybrid cooling systems previously illustrated in FIGS. 1-3. That is, the evaporative tower circuits previously shown in FIGS. 1-3 may well be replaced by or incorporate a water source that is open to the atmosphere, such as a river, lake, sea and so on.

[0038] As shown in the above examples, the water-cooled stage comprises an evaporative water tower or an open body of water for heat rejection. Depending on the applications, however, a closed-loop coolant circuit may also be provided in the water-cooled stage. For example, the hot fluid stream may undergo heat exchange with a coolant in the closed-loop circuit, which then exchanges heat with an evaporative water tower or an open body of water for final heat rejection.

[0039] It is also possible, in other embodiments, to arrange the two cooling stages such that both evaporative and non-evaporative heat transfer processes are used within each cooling stage. In these cases, the proportion of evaporative to non-evaporative cooling used in the second cooling stage should be greater than that used in the first stage.

[0040] Embodiments of the present invention are generally applicable to provide cooling for major equipment or machinery in an industrial plant. A hybrid cooling system can be used, for example, as a compressor intercooler between different compressor stages in an industrial plant, or for cooling compressor/expander systems. Under certain circumstances, the hybrid cooling system can also be applied to refrigeration unit condensers. FIG. 5 illustrates schematically several compressor stages 510, 512 and 514 arranged in series to provide sequential compression of a gas stream 520 to increasingly higher pressures. Cooling of the compressed gas streams exiting compressor stages 510 and 512 are achieved by respective intercoolers 530 and 532 having the hybrid cooling designs of the present invention. By cooling the compressed gas streams after each stage of compression, efficient gas compression can be achieved at reduced cost. If desired, the hybrid cooling system may also be used as an aftercooler. Although there will not be a power saving for the compressor itself, capital costs of downstream equipment, e.g., sieve costs for pre-purification units, can be reduced.

[0041] As an example, the air stream 520 may enter compressor stage 510 at a first pressure P1 of about 1 atma and a temperature T1 of about 21° C. After compression, the gas stream may have a pressure P2 of about 2 atma and a temperature T2 of about 100° C. The intercooler 530 having one of the hybrid cooling designs of the present invention is used to cool the compressed gas stream to a lower temperature, e.g., about 26° C. The cooled gas stream is then subjected to further compression in compressor stage 512 to about 4 atma and a temperature of about 105° C. This further compressed air stream can be cooled back to about 26° C. by another intercooler 532 prior to entering the next compressor stage 514, after which, an aftercooler 534 may be used to cool the compressed gas stream to a desired temperature.

[0042] It is understood that the pressure and temperature conditions cited in the above examples are meant for illustrative purpose only. Depending on the compressor designs or specific applications, other pressure and temperature ranges may be obtained for the various gas streams. Furthermore, the hybrid cooling system is generally applicable to a wide range of operating pressure or temperature conditions, and are well-suited for cooling needs associated with machinery or equipment in industrial plants or for cooling process fluids from an air separation plant.

[0043] The hybrid cooling systems provide several advantages over conventional cooling systems. For example, by operating an air-cooled device at a large approach temperature in the first cooling stage, the surface area and the fan power requirements for the air-cooled device may be reduced. This results in a lower cost per unit heat rejection compared to an air-cooled system that is designed for operation at lower approach temperatures. With the water-cooled part of the hybrid system, operation at tight approaches is achievable, which in turn, allows compressor operation to be performed at higher efficiency. Furthermore, lower final temperatures can be achieved (compared to air cooling alone) as a result of evaporative cooling effect associated with the water cooling stage. Systems with air coolers alone normally run with higher approaches, resulting in higher compression costs.

[0044] For cooling systems having evaporative cooling towers, the use of hybrid cooling as described herein allows a considerable reduction in the water make up and blowdown requirements compared to conventional systems using evaporative cooling alone, since a portion of the heat is removed by air cooling. Alternatively, for systems using the once-through water cooling configuration, the hybrid cooling approach allows a reduction in the amount of heat disposed to the body of water. The overall effect is that the hybrid cooling system will have a lower capital and operating cost than air-cooled systems designed in accordance with normal guidelines, but would achieve performance levels close to conventional evaporative systems at reduced makeup requirements and/or environmental impact on the water resource.

[0045] While the present invention has been described with reference to several embodiments, as will occur to those skilled in the art, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention.

Claims

1. A cooling system for use in an air separation plant, comprising: an air-cooled stage downstream from an equipment in said air separation plant for cooling a hot fluid stream associated with said equipment to a first temperature; and a water-cooled stage downstream from said air-cooled stage for cooling said hot fluid stream further to a second temperature.

2. The cooling system of claim 1, wherein said water-cooled stage comprises a water-cooled device, a water source, and a pump for delivering a water coolant stream from said water source to said water-cooled device.

3. The cooling system of claim 2, wherein said water source is at least one of an evaporative water tower or an open body of water.

4. The cooling system of claim 2, further comprising a chiller downstream from said water source but upstream of said water-cooled device.

5. The cooling system of claim 1, wherein said air-cooled stage comprises a non-evaporative air cooler.

6. The cooling system of claim 1, wherein said air-cooled stage comprises a heat exchange section, an air cooler, and a pump for circulating a coolant stream between said heat exchange section and said air cooler; said heat exchange section being configured for heat exchange between said hot fluid stream and said coolant stream.

7. The cooling system of claim 6, wherein said water-cooled stage comprises a water-cooled device, a water source, and a pump for delivering a water coolant stream from said water source to said water-cooled device.

8. The cooling system of claim 7, wherein said water source is at least one of an evaporative water tower or an open body of water.

9. The cooling system of claim 7, further comprising a chiller downstream from said water source but upstream of said water-cooled device.

10. The cooling system of claim 1, wherein said air-cooled stage and said water-cooled stage are coupled in series to a heat exchange section and a circulating pump, said air-cooled stage being downstream from said heat exchange section to provide air cooling of said hot fluid stream to said first temperature.

11. The cooling system of claim 10, wherein said air-cooled stage is a non-evaporative air cooler and said water-cooled stage is an evaporative water tower.

12. The cooling system of claim 10, further comprising a chiller downstream of said water-cooled stage but upstream of said heat exchange section.

13. The cooling system of claim 1, wherein said equipment is selected from the group consisting of a compressor and a compressor/expander system.

14. A method of cooling for use in an air separation plant, comprising: (a) cooling a hot fluid stream associated with an equipment in said air separation plant to a first temperature using an air-cooled stage; and (b) cooling said hot fluid stream further to a second temperature in a water-cooled stage downstream from said air-cooled stage.

15. The method of claim 14, wherein said water-cooled stage comprises a water-cooled device, a water source, and a pump for delivering a water coolant stream from said water source to said water-cooled device for cooling said hot fluid stream to said second temperature.

16. The method of claim 15, wherein said water source is at least one of an evaporative water tower or an open body of water.

17. The method of claim 14, wherein said air-cooled stage is selected from the group consisting of a fin fan air cooler and a liquid circulating circuit; said liquid circulating circuit comprising a heat exchange section, an air-cooled device and a pump for circulating a liquid coolant between said heat exchange section and said air-cooled device.

18. The method of claim 14, wherein said air-cooled stage is operated at a higher approach temperature higher than said water-cooled stage.

19. The method of claim 14, wherein said equipment is selected from a compressor, compressor/expander system, and a refrigeration unit condenser.

20. The method of claim 14, wherein said hot fluid stream is a process stream exiting from said equipment or a coolant stream heated by heat exchange with said process stream.

21. A method of cooling for use in an industrial plant comprising: (a) cooling a hot fluid stream originating from an equipment in said industrial plant to a first temperature using an air-cooled stage, said equipment being selected from the group consisting of a compressor and a compressor/expander system; and (b) cooling said hot fluid stream from said first temperature to a second temperature using a water-cooled stage.

22. The method of claim 21, wherein said hot fluid stream is a compressed air stream from a first compressor, and said method further comprises introducing said compressed air stream at said second temperature into a second compressor.

23. The method of claim 21, wherein said industrial plant is an air separation plant, and said hot fluid stream is a compressed air stream from a compressor.