US20070296093A1

US20070296093A1 - Cooling Tower

Info

Publication number: US20070296093A1
Application number: US11/664,202
Authority: US
Inventors: Kevan Russel-Smith
Original assignee: Individual
Current assignee: Halliburton Energy Services Inc
Priority date: 2004-09-29
Filing date: 2005-09-28
Publication date: 2007-12-27
Also published as: WO2006035299A3; WO2006035299A2; AU2005288660A1; AU2005288660B2; WO2006035299B1

Abstract

A method of operating an evaporative cooling tower includes contacting water with air in a cooling zone to cool the water and heat the air and cooling the heated air to condense water therefrom, thereby to reduce water loss from the cooling tower. Typically, the cooling of the heated air is by means of evaporative cooling employing a refrigerant.

Description

THIS INVENTION relates to a cooling tower. In particular, it relates to a method of operating a cooling tower, and to a cooling tower operable in accordance with the method.
In accordance with a first aspect of the invention, there is provided a method of operating an evaporative cooling tower which includes
contacting water with air in a cooling zone to cool the water and heat the air; and
cooling the heated air to condense water therefrom, thereby to reduce water loss from the cooling tower.
The cooling of the heated air may be by means of evaporative cooling employing a refrigerant, such as Freon. The evaporative cooling of the heated air may be provided by a compression refrigeration system. Instead, the evaporative cooling of the heated air may be provided by an absorption refrigeration system.
The evaporative cooling tower may be a natural draught cooling tower with upwardly moving air and the cooling of the heated air is thus being effected at a particular elevation or level in the cooling tower. The method may include, above the level of the cooling of the heated air, heating the upwardly moving air to promote the draught in the cooling tower.
The refrigeration system may include a condenser and an evaporator, arranged in a circuit. The evaporator may be at a low elevation inside the cooling tower to cool the upwardly moving air and remove water therefrom by condensation, and the condenser may be located above the evaporator to heat the upwardly moving air, which is dried air, to promote the draught in the tower. There may be several such circuits, each with its own compressor, condenser, etc.
The cooling tower typically employs filling with drift eliminators, which may be plate-like, above the filling in the tower. The evaporator may comprise one or more coils in or on one or more drift eliminators and the condenser may comprise one or more coils in or on one or more drift eliminators located above the evaporator coil or coils.
In accordance with another aspect of the invention, there is provided, in a natural draught cooling tower of the type used for large scale water cooling as in the cooling of water used by thermal power stations, and which comprises a typically concrete hollow shell which is circular in plan view and has walls which are concave and parabolic in side view outline, the bottom of the tower being raised from ground level on stilts or the like to provide a circumferentially extending air inlet at its lower end and the tower containing filling at its lower end and a water distribution system for distributing water to be cooled onto the filling from which it drains into a reservoir under the tower while air circulates from the inlet up over the filling and out of the top of the tower by natural convection to cool the water, there is provided the method of operation which comprises, above the filling, cooling the upwardly moving air to condense water therefrom, thereby to reduce water loss from the tower at its top.
The method may include, above the level of cooling of the upwardly moving air, heating the upwardly moving air to promote the draught in the cooling tower.
The cooling of the upwardly moving air may be by evaporative cooling provided by a compression refrigeration system or an absorption refrigeration system. The refrigeration system typically includes a condenser and an evaporator arranged in a circuit. The evaporator may be located above the filling in the tower to cool the upwardly moving air and remove water therefrom by condensation, and the condenser may be located above the evaporator to heat the upwardly moving air, which is dried air, to promote the draught in the tower.
The cooling tower typically employs drift eliminators above the filling in the tower. The evaporator of the refrigeration circuit may comprise one or more coils in or on one or more drift eliminators, and the condenser of the refrigeration circuit may comprise one or more coils in or on one or more drift eliminators, located above the evaporator coil or coils.
According to a further aspect of the invention, there is provide an evaporative cooling tower which includes a cooling zone for contacting water with air to cool the water and heat the air, and a refrigeration or cooling system for cooling the heated air from the cooling zone for condensing water therefrom.
In one embodiment of the invention, the refrigeration system is a compression refrigeration system for the evaporative cooling of the air by means of a refrigerant. The compression refrigeration system may be arranged to heat the cooled air at a level above that at which the refrigeration system cools said air.
The compression refrigeration system typically comprises one or more cooling circuits, each comprising a condenser and an evaporator, in association with a refrigerant compressor and an expansion valve.
Each cooling circuit may thus include a compressor which is driven by a turbine and optionally via a gearbox. The gearbox, when present, will be between the turbine and the compressor. The turbine may be driven by an incoming stream of water to be cooled as it enters the cooling tower.
In another embodiment of the invention, the refrigeration system is an absorption refrigeration system for the evaporative cooling of the air by means of a refrigerant. The refrigerant is a gas soluble in a liquid solvent. The absorption refrigeration system may be arranged to heat the cooled air at a level above that at which the absorption refrigeration system cools said air.
The absorption refrigeration system typically comprises one or more cooling circuits, each comprising a generator, a condenser and an evaporator, in association with an expansion valve. Each circuit may have its generator located outside the cooling tower.
The cooling tower typically includes filling and drift eliminators. Each evaporator may comprise coils in or on one or more drift eliminators above the filling in the tower, and each condenser may comprise coils in or on one or more drift eliminators above the filling, the condenser coils being at a higher level than the evaporator coils.
In accordance with yet a further aspect of the invention, there is provided a natural draught cooling tower of the type used for large scale water cooling as in the cooling of water used by thermal power stations, and which comprises a typically concrete hollow shell which is circular in plan view and has walls which are concave and parabolic in side view outline, the bottom of the tower being raised from ground level on stilts or the like to provide a circumferentially extending air inlet at its lower end and the tower containing filling at its lower end and a water distribution system for distributing water to be cooled onto the filling from which it drains into a reservoir under the tower while air circulates from the inlet up over the filling and out of the top of the tower by natural convection to cool the water, the cooling tower further including a refrigeration system for cooling upwardly moving air in the tower above the filling, for condensing water therefrom.
In one embodiment of the invention, the refrigeration system is a compression refrigeration system for the evaporative cooling of the upwardly moving air by means of a refrigerant. The compression refrigeration system may be arranged to heat the cooled upwardly moving air at a level above that at which the compression refrigeration system cools said air.
The compression refrigeration system typically comprises one or more cooling circuits, each comprising a condenser and an evaporator, in association with a refrigerant compressor and an expansion valve. There may be several such cooling circuits.
Each cooling circuit may thus include a compressor which is driven by a turbine and optionally via a gearbox. The gearbox, when present, will be between the turbine and the compressor. The turbine may be driven by an incoming stream of water to be cooled as it enters the cooling tower.
In another embodiment of the invention, the refrigeration system is an absorption refrigeration system for the evaporative cooling of the air by means of a refrigerant, which will be a gas soluble in a liquid solvent. The absorption refrigeration system may also be arranged to heat the cooled upwardly moving air at a level above that at which the absorption refrigeration system cools said air.
The absorption refrigeration system typically comprises one or more cooling circuits, each comprising a generator, a condenser and an evaporator, in association with an expansion valve. There may be several such cooling circuits. Each circuit may have its generator located outside the cooling tower.
Cooling towers of the type in question typically employ plate-like drift eliminators above the filling in the tower. Each evaporator may comprise coils in or on one or more drift eliminators above the filling in the tower, and each condenser may comprise coils in or on one or more drift eliminators above the filling, the condenser coils being at a higher level than the evaporator coils.
The evaporator coils may be separated from the condenser coils by being on separate drift eliminators or by means of an air gap or insulating material if they are on the same drift eliminator, to avoid direct heat transfer therebetween, as the drift eliminators are preferably of heat-conductive material such as copper to promote cooling and heating respectively by the evaporator coils and the condenser coils.
The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which
FIG. 1 shows a part-sectional side elevation of a cooling tower in accordance with the invention;
FIG. 2 shows diagrammatically a compression refrigeration circuit for the cooling tower of FIG. 1;
FIG. 3 shows in side elevation a diagrammatic representation of a drift eliminator for the cooling tower of FIG. 1, and forming part of the circuit of FIG. 2;
FIG. 4 shows a sectional end elevation in the direction of line IV-IV in FIG. 3, of the drift eliminator of FIG. 3;
FIG. 5 shows diagrammatically an absorption refrigeration circuit for another embodiment of the invention;
FIG. 6 shows in side elevation a diagrammatic representation of another embodiment of a drift eliminator; and
FIG. 7 shows a sectional end elevation in the direction of line VII-VII in FIG. 6, of the drift eliminator of FIG. 6.
In FIG. 1 of the drawings, reference numeral 10 generally designates a cooling tower in accordance with the invention. The cooling tower 10 is a natural draught cooling tower of the type used for large scale water cooling as at a thermal power station. The cooling tower 10 comprises a hollow concrete shell 12 which is circular in plan view and in side elevation has walls with a profile which is concave and parabolic. The bottom 14 of the shell 12 is raised from the ground 16 to provide a circumferentially extending air inlet 18. The shell 12 is supported on stilts 20. The lower end of the tower, up to about the height of the inlet 18, is packed with filling 22 made e.g. of wood, asbestos, or the like. The filling is permeable to air flow and water flow and provides a packing material over which water can flow downwardly in use of the cooling tower 10, to provide a large air/water surface area for evaporative cooling. The filling 22 is supported on upright supports 21 which also act to support the water distribution system, drift eliminators, etc., described hereunder.
The cooling tower has a main hot water inlet pipe 23, which extends e.g. from a power station, radially inwardly at ground level into the bottom of the tower 10, to a central position where it has a riser 24. From the top of the riser 24 a water distribution system extends, shown diagrammatically in FIG. 1 by a plurality of radially extending pipes 26 connected to a plurality of sprays 28. The radial pipes 26 have a plurality of branch pipes (not shown), and the radial pipes and branch pipes are provided with the sprays 28, so that, at the level of the top of the filling 22, there is a multiplicity of downwardly directed, evenly distributed sprays 28, for spraying water from the pipe 23 to be cooled, downwardly onto the filling 22. Under the tower 10 is provided a reservoir 30 which empties into a cooled water outlet pipe 32 for returning cooled water to the power station (typically via pumps which are not shown). Above the level of the pipes 26 and the sprays 28 is provided a plurality of drift eliminators 34. These drift eliminators provide a multiplicity of elongated flattened plates, spaced from one another in parallel arrays, the longitudinal direction of each drift eliminator extending substantially horizontally, with the plane of each drift eliminator extending substantially vertically. Thus, if each drift eliminator 34 is regarded as an elongated rectangular flat metal plate, its long edges will be horizontal and its short edges will be vertical.
In FIG. 2, the riser 24 is shown. At the upper end of the riser 24 there is a manifold 36 feeding into the radial pipes 26. A turbine 38 is shown in the riser 24, having an output shaft 40 extending via a gearbox 42 to a compressor 44. The compressor 44 forms part of a compression refrigeration circuit 46 employing Freon as a refrigerant. The circuit 46 includes a condenser 48, an evaporator 50 and an expansion valve 52, the condenser 48 being- connected between the expansion valve 52 and the outlet of the compressor 44, and the evaporator 50 in turn being connected between the expansion valve 52 and the inlet to the compressor 44.
With reference to FIGS. 3 and 4, a drift eliminator is designated 34. The drift eliminator 34 comprises a pair of copper sheets attached together face-to-face, and shaped so that the drift eliminator 34 has two sets of tubes 54 extending along opposite longitudinal edges thereof, the drift eliminator 34 being elongate rectangular in outline. In use, the drift eliminator 34 will be arranged so that its longer edges extend horizontally, as mentioned above, and as shown in FIGS. 3 and 4, so that the upper set of tubes 54 forms part of the evaporator 50 of the circuit 46 shown in FIG. 2. A slot 56 extends centrally along the drift eliminator 34, to resist undesirable direct heat transfer from the tubes 54 of the condenser 48, to the tubes 54 of the evaporator 50.
It is contemplated that, in use, a plurality of compression refrigeration circuits, say four in number, with be provided in the cooling tower 10, of the type shown at 46 in FIG. 2. Each of these circuits 46 will have its compressor 44 driven by the gearbox 42. The compressor 44 of each circuit will have its outlet connected to a multiplicity of the drift eliminators 34, located for example in a 90° sector of the tower when seen in plan view. The upper sets of tubes 54 of the drift eliminators 34 to which said compressor is connected, will thus form the condenser 48 of the circuit 46 in question. These tubes 54 will feed into a common flow line, and will pass through the associated expansion valve 52, whence refrigerant will return via the evaporator 50 to the compressor 44. The evaporator 50, similarly, comprises the various lower sets of tubes 54 in the drift eliminators 34 to which that compressor 44 is connected.
In use, hot water enters the cooling tower 10 via the pipe 23. It is distributed in the cooling tower 10 along the riser 24, the radial pipes 26 and their branch pipes, and issues via the sprays 28 downwardly onto the filling 22. This hot water flows downwardly over the filling, and into the reservoir 30, whence it is removed via the pipe 32 to be returned to its source such as the power station mentioned above.
As the water flows downwardly over the filling 22, it heats the surrounding air, which through natural convection moves upwardly through the shell 12 of the tower 10 to issue at the top of the tower. Convective natural circulation of air is thus set up in the tower 10, the top of the tower acting as the air outlet, and air entering through the circumferential or peripheral inlet 18 at the bottom of the shell 12. Cool air entering the tower from the outside of the tower thus cools the water flowing downwardly over the filling 22, both by removing sensible heat from the water, and also by removing latent heat from the water, by evaporating a proportion of the water.
In accordance with the invention, the water entering the tower via the riser 24 drives the turbine 38, which in turn drives the compressors 44 of the various refrigeration circuits 46. Flow of refrigerant along said circuits 46 results in evaporative cooling in the evaporators 50 of the circuits, and in condensation and rejection of heat in the condensers 48 of the circuits 46.
Air flowing upwardly from the top of the filling 22 and over the drift eliminators 34 (see FIG. 1.) will have been heated by the water falling over the filling 22, and will contain a substantial proportion of water vapour. The cooling effected by the evaporators 50 will cool said rising air, resulting in condensation of a proportion of the water therein, which falls down onto the filling 22. This cooled and dried air, upon rising further, will pass over the condensers 48 of the circuits 46, and will then again be heated.
It will thus be appreciated that use of the refrigeration circuits 46 results in a reduced water loss with air from the top of the tower 10, and that the reheating of the cooled dried air by the condensers 48 will serve to promote convective natural circulation of air through the tower 10, and will at least partially offset any reduction in such convective circulation caused by the cooling of air by the evaporative coolers 50. In this regard it will be appreciated that each drift eliminator 34 forms part of an associated refrigeration circuit 46, its upper tubes 54 forming part of the associated evaporator 50. As evaporation will be taking place at a lower temperature than condensation, there will be a temperature difference between the tubes 54 a the top of the drift eliminator 34 and the tubes 54 at the bottom thereof, and for this reason the slot 56 is provided to reduce direct heat transmission by conduction between said upper and lower tubes 54. Instead, it will be appreciated that some other form of heat insulation may be provided, or, indeed, the lower tubes 54 and the upper tubes 54 may be provided on separate copper sheets, acting as drift eliminators.
Instead of using a compression refrigeration system 36, the cooling tower 10 may employ an absorption refrigeration system. In FIG. 5, an absorption refrigeration circuit 80 for an absorption refrigeration system is shown. The circuit 80 comprises an absorber 82 connected by a flow line 84 having a pump 86 to a heat exchanger 88. Flow from the pump 86 through the heat exchanger 88 continues along flow line 90 to a boiler or generator 92 which is connected to a source of heating water (not shown) by flow lines 94, 96. A solvent return line 98 from the generator 92 passes through the heat exchanger 88 to the absorber 82. A flow line 100 for refrigerant from the generator 92 extends to a condenser 102, an evaporator 104 and an expansion valve 106, the condenser 102 being connected between the expansion valve 106 and the generator 92, and the evaporator 104 in turn being connected between the expansion valve 106 and the absorber 82. The valve 106 is between the condenser 102 and the evaporator 104.
With reference to FIGS. 6 and 7, another embodiment of a drift eliminator for the tower 10 is designated 200. The drift eliminator 200 comprises a pair of copper sheets attached together face-to-face, and shaped so that the drift eliminator 200 has two sets of tubes 202 extending along opposite longitudinal edges thereof, the drift eliminator 200 being elongate rectangular in outline. In use, the drift eliminator 200 will be arranged so that its longer edges extend horizontally, as mentioned above, and as shown in FIGS. 6 and 7, so that the upper set of tubes 202 forms part of the condenser 102 of the absorption refrigeration circuit 80 shown in FIG. 5, and the lower set of tubes 202 forms part of the evaporator 104 of the circuit 80 shown in FIG. 5. The drift eliminator 200 has a central longitudinally extending corrugation 204 so that when a plurality of the drift eliminators are packed side-by-side in series in closely spaced relationships their corrugations can nest in one another. Flow between these side-by-side eliminators in a direction parallel to their faces and parallel to their short edges is thus interrupted by the corrugations 204 which reduce or prevent water droplets entrained in the flow from passing between the eliminators 200, the droplets striking said corrugations 204. One or more longitudinal slots 206 may be provided in each eliminator 200, in or alongside its corrugation 204, to resist undesirable direct heat transfer from the tubes 202 of the condenser 102 to the tubes 202 of the evaporator 104.
It is contemplated that, in use, a plurality of absorption refrigeration circuits, say four in number, will be provided for the cooling tower 10, of the type shown at 80 in FIG. 5. Each of these circuits 80 will have its condenser 102, valve 106 and evaporator 104 located inside the tower 10; and will have its absorber 82, pump 86, heat exchanger 88 and generator 92 located outside the tower 10. The flow line 100 will extend from the inside of the tower to the outside of the tower, to the absorber 82 and generator 92, the flow lines 84, 90 and 98 being located outside the tower 10. The flow lines 94, 96 extend to a heat source outside the tower 10 as described in more detail hereunder. The generator 92 of each circuit will have its refrigerant outlet (flow line 100) connected to a multiplicity of the drift eliminators, in this particular instance to a quarter of the drift eliminators, located for example in a 90° C. sector of the tower 10 when seen in plan view. The upper sets of tubes 202 of the drift eliminators 200 to which said generator 92 is connected, will thus form the condenser 102 of the circuit 80 in question. These tubes 202 will feed into a common flow line 100, and will pass through the associated expansion valve 106, whence refrigerant will return via the evaporator 104 to the absorber 82. The evaporator 104, similarly comprises the various lower sets of tubes 202 in the drift eliminators 200 to which that flow line 100 is connected.
In use, hot water enters the cooling tower 10 to be cooled by air, as hereinbefore described. In accordance with the invention, an external heat source, such as waste heat from a power station boiler, is used to heat water to drive the absorption refrigeration circuits 80. Thus in each circuit 80 heated water under pressure passes along flow line 94, which can be a suitably lagged pipe, to the generator 92. A suitable refrigerant gas, dissolved in a relatively high concentration in a suitable solvent, enters the generator 92 via flow line 90. Heat from water from the flow line 94 boils the refrigerant gas out of the solvent in the generator 92. Water from the flow line 94, after cooling in the generator 92, returns along flow line 96 to the heat source. Refrigerant gas passes along the flow line 100 to the condenser 102 where it is condensed, through the expansion valve 106 and to the evaporator 104 where it is evaporated. Solvent depleted of refrigerant from the generator 92 passes along flow line 98 through the heat exchanger 88 where it is cooled and from which it passes onto the absorber 82. In the absorber 82 it absorbs refrigerant gas from the flow line 100 and evaporator 104. Solvent, containing re-dissolved refrigerant gas passes from the absorber 82 along flow line 84 via pump 86 to heat exchanger 88 where it is heated, and thence via flow line 90 to the generator 92. In this regard it is contemplated that the hot water flowing to the generator 92 along flow line 94 will typically be heated by a heat source comprising hot ash from the power station boiler, passing through suitable heat exchangers there, heating coils or the like (not shown). Flow of refrigerant along said circuits 80 results in evaporative cooling in the evaporators 104 of the circuits, and in condensation and rejection of heat in the condensers 102 of the circuits 80.
Air flowing upwardly from the top of the filling 22 and over the drift eliminators 200 will have been heated by the water falling over the filling 22, and will contain a substantial proportion of water vapour. The cooling effected by the evaporators 104 will cool said rising air, resulting in condensation of a proportion of the water therein, which falls down on to the filling 22. This cooled and dried air, upon rising further, will pass over the condensers 102 of the circuits 80, and will then again be heated.
It will thus be appreciated that use of the refrigeration circuits 80 results in a reduced water loss with air from the top of the tower 10, and that the re-heating of the cooled dried air by the condensers 102 will serve to promote convective natural circulation of air through the tower 10, and will at least partially offset any reduction in such convective circulation caused by the cooling of air by the evaporative coolers 104. In this regard it will be appreciated that each drift eliminator 200 forms part of an associated refrigeration circuit 80, its upper tubes 202 forming part of the associated condenser, and its lower tubes 202 forming part of the associated evaporator. As evaporation will be taking place at a lower temperature than condensation, there will be a temperature difference between the tubes 202 at the top of the drift eliminator 200 and the tubes 202 at the bottom thereof, and for this reason the slots 206 are provided to reduce direct heat transmission by conduction between said upper and lower tubes 202. Instead, it will be appreciated that some other form of heat insulation may be provided, or, indeed, the lower tubes 202 and the upper tubes 202 may be provided on separate copper sheets, acting as drift eliminators.
The invention is intended for use where availability of water can be critical, as in thermal power stations, which have a limited water supply, and which have to close down should this water supply run out. In this regard it will be appreciated that the power to drive the circuits 80 is obtained from incoming hot water in the line 94, and eventually must be obtained from waste heat from the power station. It is however believed that, where water shortage is a critical factor, the present invention can have substantial utility and in this regard it should be noted that the invention requires no additional water consumption, bearing in mind that flow lines 94, 96 form part of a closed water circuit. The power to drive the compressors 44 of the circuits 46 is obtained from the incoming hot water in the risers 24, and eventually must be obtained by the pumps pumping such incoming water. However, if desired, the power for the compressors 44 may be provided in some other fashion, for example by means of a suitable electric motor, conveniently located outside the tower 10, and connected to the gearbox 42 of FIG. 2, or the like.
Although the invention, i.e. the cooling of air above the filling to resist water loss, has been described with reference to natural draught cooling towers, it will be appreciated that this method is in principle equally applicable to forced draught cooling towers and the invention accordingly extends thereto.

Claims

1. A method of operating an evaporative natural draught cooling tower which has upwardly moving air, the method including

contacting water with air in a cooling zone to cool the water and heat the air;

cooling the heated air by means of evaporative cooling provided by a compression refrigeration system or an absorption refrigeration system, with the refrigeration system including a condenser and an evaporator arranged in a circuit, the evaporator being at a low elevation inside the cooling tower to cool the upwardly moving air and remove water therefrom by condensation; and

above the level of cooling of the heated air, heating the upwardly moving dried air with the condenser, to promote the draught in the tower.

2. The method as claimed in claim 1, in which the cooling tower employs filling with drift eliminators above the filling in the tower, the evaporator comprising one or more coils in or on one or more drift eliminators and the condenser comprising one or more coils in or on one or more drift eliminators located above the evaporator coil or coils.

3. In a natural draught cooling tower of the type used for large scale water cooling as in the cooling of water used by thermal power stations, and which comprises a typically concrete hollow shell which is circular in plan view and has walls which are concave and parabolic in side view outline, the bottom of the tower being raised from ground level on stilts or the like to provide a circumferentially extending air inlet at its lower end and the tower containing filling at its lower end and a water distribution system for distributing water to be cooled onto the filling from which it drains into a reservoir under the tower while air circulates from the inlet up over the filling and out of the top of the tower by natural convection to cool the water, there is provided the method of operation which comprises, above the filling, cooling the upwardly moving air to condense water therefrom, thereby to reduce water loss from the tower at its top, and above the level of cooling of the upwardly moving air, heating the upwardly moving air to promote the draught in the cooling tower, the cooling of the upwardly moving air being by evaporative cooling provided by a compression refrigeration system or an absorption refrigeration system, with the refrigeration system including a condenser and an evaporator arranged in a circuit, the evaporator being located above the filling in the tower to cool the upwardly moving air and remove water therefrom by condensation, and the condenser being located above the evaporator to heat the upwardly moving air, which is dried air, to promote the draught in the tower.

4. The method as claimed in claim 3, in which the cooling tower employs drift eliminators above the filling in the tower and the evaporator of the refrigeration circuit comprises one or more coils in or on one or more drift eliminators, and the condenser of the refrigeration circuit comprises one or more coils in or on one or more drift eliminators, located above the evaporator coil or coils.

5. An evaporative cooling tower which includes a cooling zone for contacting water with air to cool the water and heat the air, and a refrigeration or cooling system for cooling the heated air from the cooling zone for condensing water therefrom, the refrigeration system being a compression refrigeration system for the evaporative cooling of the air by means of a refrigerant, the refrigeration system being an arranged to heat the cooled air at a level above that at which the refrigeration system cools said air.

6. The cooling tower as claimed in claim 5, in which the refrigeration system comprises one or more cooling circuits, each comprising a condenser and an evaporator.

7. The cooling tower as claimed in claim 6, which includes filling and drift eliminators, each evaporator comprising coils in or on one or more drift eliminators above the filling in the tower, and each condenser comprising coils in or on one or more drift eliminators above the filling, the condenser coils being at a higher level than the evaporator coils.

8. The cooling tower as claimed in claim 6, in which each cooling circuit includes a compressor which is driven by a turbine and optionally via a gearbox, the gearbox, when present, being between the turbine and the compressor, and the turbine being driven by an incoming stream of water to be cooled as it enters the cooling tower.

9. An evaporative cooling tower which includes a cooling zone for contacting water with air to cool the water and heat the air, and a refrigeration or cooling system for cooling the heated air from the cooling zone for condensing water therefrom, the refrigeration system being an absorption refrigeration system for the evaporative cooling of the air by means of a refrigerant, the refrigeration system being arranged to heat the cooled air at a level above that at which the refrigeration system cools said air.

10. The cooling tower as claimed in claim 9, in which the refrigeration system comprises one or more cooling circuits, each comprising a generator, a condenser and an evaporator, with each circuit having its generator located outside the cooling tower.

11. The cooling tower as claimed in claim 9, which includes filling and drift eliminators, each evaporator comprising coils in or on one or more drift eliminators above the filing in the tower, and each condenser comprising coils in or on one or more drift eliminators above the filling, the condenser coils being at a higher level than the evaporator coils.

12. A natural draught cooling tower of the type used for large scale water cooling as in the cooling of water used by thermal power stations, and which comprises a typically concrete hollow shell which is circular in plan view and has walls which are concave and parabolic in side view outline, the bottom of the tower being raised from ground level on stilts or the like to provide a circumferentially extending air inlet at its lower end and the tower containing filling at its lower end and a water distribution system for distributing water to be cooled onto the filling from which it drains into a reservoir under the tower while air circulates from the inlet up over the filing and out of the top of the tower by natural convection to cool the water, the cooling tower further including a refrigeration system for cooling upwardly moving air in the tower above the filling, for condensing water therefrom,

13. The cooling tower as claimed in claim 12, in which the refrigeration system is a compression refrigeration system for the evaporative cooling of the upwardly moving air by means of a refrigerant, the refrigeration system being arranged to heat the cooled upwardly moving air at a level above that at which the refrigeration system cools said air.

14. The cooling tower as claimed in claim 13, in which the refrigeration system comprises one or more cooling circuits, each comprising a condenser and an evaporator.

15. The cooling tower as claimed in claim 14, in which each evaporator comprises coils in or on one or mote drift eliminators above the filling in the tower, and each condenser comprises coils in or on one or more drift eliminators above the filling, the condenser coils being at a higher level than the evaporator coils.

16. The cooling tower as claimed in claim 15, in which each cooling circuit includes a compressor which is driven by a turbine and optionally via a gearbox, the gearbox, when present, being between the turbine and the compressor, and the turbine being driven by an incoming stream of water to be cooled as it enters the cooling tower.

17. The cooling tower as claimed in claim 12, in which the refrigeration system is an absorption refrigeration system for the evaporative cooling of the air by means of a refrigerant, the refrigeration system being arranged to heat the cooled upwardly moving air at a level above that at which the refrigeration system cools said air.

18. The cooling tower as claimed in claim 17, in which the refrigeration system comprises one or more cooling circuits, each comprising a generator, a condenser and an evaporator, with each circuit having its generator located outside the cooling tower.

19. The cooling tower as claimed in claim 18, in which each evaporator comprises coils in or on one or more drift eliminators above the filling in the tower, and each condenser comprises coils in or on one or more drift eliminators above the filling, the condenser calls being at a higher level than the evaporator coils.