WO2008151377A1

WO2008151377A1 - System and method of wetting adiabatic material

Info

Publication number: WO2008151377A1
Application number: PCT/AU2008/000849
Authority: WO
Inventors: Grant David Hall; Robert De Jong
Original assignee: Muller Industries Australia Pty Ltd
Priority date: 2007-06-14
Filing date: 2008-06-13
Publication date: 2008-12-18
Also published as: AU2008261617A1; EP2171362A1; AU2008261617B2; US20100162737A1

Abstract

A moisture recirculation system for evaporatively cooling air, the system including a moisture distribution arrangement (148, 150) which, in use, distributes moisture to an upper portion of moisture absorbent material (135, 140); a trough (155, 160) disposed below the lower most portion of the moisture absorbent material (135, 140) for initially collecting moisture run-off; a sump (165) in fluid communication with the trough (155, 160) for collecting and storing said run-off; and a pump (145) in fluid communication with the sump (165) which, in use, transfers moisture from the sump (165) to the moisture absorbent material (135, 140).

Description

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SYSTEM AND METHOD OF WETTING ADIABATIC MATERIAL

FIELD

The invention relates generally to a system and method of wetting adiabatic material for applications involving the evaporative cooling of air.

BACKGROUND OF THE INVENTION

Generally, in any cooling arrangement that employs adiabatic material for effecting the evaporative cooling of air, the adiabatic material is arranged substantially vertically. Moisture is deposited on an upper portion of the adiabatic material and by the force of gravity, the moisture gradually descends through the adiabatic material and is absorbed into the material.

A sufficient amount of moisture is required to saturate the adiabatic material in order to achieve the maximum evaporative cooling effect possible that results from forcing air through the saturated adiabatic material. As air is forced through the adiabatic material, it is cooled by the action of evaporation. As this occurs, the amount of moisture absorbed into the adiabatic material reduces as the moisture in liquid form is vapourised (i.e. converted to gaseous form) hence cooling the air passing through the adiabatic material by extracting thermal energy from same in order to vapourise the water.

As it is difficult to determine the precise amount of water that will be evaporated as the air is cooled, it is generally accepted that the adiabatic material should remain saturated with moisture and be constantly supplied with additional moisture during the evaporative cooling process. As a result, an over supply of moisture is usually deposited on an upper portion of the adiabatic material and the excess moisture (generally referred to as run-off) is collected in a trough arrangement disposed beneath the lower portion of the adiabatic material. Further, as the trough collects run-off moisture, a pump is employed to extract the run-off moisture from the trough and deposit this runoff moisture onto an upper portion of the adiabatic material, generally through a water distribution arrangement.

During operation, as moisture is evaporated from the adiabatic material, additional moisture is added to the trough (generally referred to as make-up water). The requirement for make-up water is usually monitored by a float valve that, when opened, allows fresh external moisture into the trough and as the level of moisture in the trough rises, the float attached to the valve eventually rises to a level that closes the valve hence terminating the ingress of fresh external moisture into the trough.

The entire arrangement is usually referred to as a water recirculation system and has been used for many years to maintain adiabatic material in a saturated condition to ensure a maximum evaporative cooling effect is achieved.

Whilst the moisture recirculation system described above has been used for many decades, the arrangement does exhibit some disadvantages. For example, the trough disposed below the adiabatic material generally runs the entire length of the adiabatic materia}. In arrangements where there are a relatively large number of adiabatic pads placed side by side, the length of a trough can become relatively long and in order to maintain a positive head of pressure to the pump intake, it becomes necessary to fill the entire length of the trough with a sufficient depth of water to maintain that water pressure. Accordingly, the float valve arrangement is usually set to ensure that the minimum amount of water required to maintain a positive head of water pressure always resides in the trough.

During operation, there is no particular disadvantage associated with this arrangement. However, the water recirculation system looses water through vapourisation as it cools the air forced through the adiabatic material. During the process of vapourisation, any sediment or soluble impurities in the water are not vapourised and hence, over time, the concentration of these impurities gradually increases. The impurities in the water may also include microbial impurities that can collect in the trough and promote the growth of bacteria in the trough.

In order to reduce the fouling of the trough water, it is common practice to "dump" the contents of the trough on a regular basis. Whilst the regular period may differ depending upon the climatic conditions in which the evaporative cooling arrangement resides, it is generally considered an industry practice to dump the trough contents at least once every twenty four (24) hour period. Of course, in the circumstances where a trough is relatively long, and hence contains a relatively large amount of water, then a relatively large amount of water is wasted during the dumping process. For industrial cooling arrangements it is not unusual for a trough to contain 250 to 300 litres of water in order to maintain a positive head of pressure at the pump intake. Accordingly, once every twenty four (24) hour period it is necessary to dump 250 to 300 litres of water.

A further disadvantage of the existing, and well accepted, water recirculation arrangement is the relatively long start-up period that is required to fully saturate the adiabatic material from a dry condition. For example, many industrial cooling arrangements do not activate the water recirculation system until such time as the ambient air is sufficiently warm and thus requires cooling. In these circumstances, upon detecting a sufficiently warm ambient air temperature, the water recirculation system is activated in order to saturate the adiabatic material and commence the evaporative cooling process. In arrangements where the trough has relatively large dimensions, it can take a relatively long period of time to fill the trough with water to a sufficient level to provide the necessary head of pressure at the pump intake. Once achieved, the pump is then activated to extract water from the trough and deposit same on an upper portion of the adiabatic material. In the initial phase, the adiabatic material will absorb most of the water deposited thereon and as water is extracted from the trough via the pump, the float valve will open and allow the ingress of additional external water into the trough thus maintaining the positive head of pressure at the pump intake.

Again, where the trough dimensions are relatively large, it may take some time for the trough to fill to a sufficient depth to safely activate the pump to commence the water recirculation process. Clearly, the delay in filling the trough is affected by both the dimensions of the trough and the available water pressure to the float valve arrangement. In the event that the available water pressure at the float valve is quite low, then it will take a longer period of time to fill the trough to a sufficient level as compared with a higher available water pressure.

In circumstances where a cooling apparatus is considered is to be critical, then it is generally considered necessary to predict the requirement to transition from "dry" mode to "wet" mode cooling. For example, in circumstances where the cooling arrangement is used to act as a heat exchanger for a refrigeration installation that maintains food in a frozen condition, then it becomes necessary for the system to ensure that the requisite' heat exchange capacity is not affected sufficiently to the detriment of maintaining food in the frozen condition. In installations of this type, it is not unusual for the system to predict a future requirement to run a heat exchanger in "wet" mode and in view of the delay associated with transitioning a heat exchanger from dry to wet mode, the water recirculation process may be activated well in advance of the requirement to transition a heat exchanger from dry to wet mode.

Of course, in those circumstances where the prediction of an increased heat exchange requirement is incorrect, the water recirculation process may be falsely commenced which saturates the adiabatic material and fills troughs with water for no reason. This may occur for example where there is a relatively sudden change in the ambient air temperature indicating a significant increase and suggesting the requirement to transition from dry to wet mode cooling. However, such an increase in ambient air temperature may be followed by a substantial and potentially unexpected decrease in ambient air temperature thus not actually requiring a transition of the cooling arrangement from dry to wet mode. At that, time, the water recirculation system would be deactivated and the moisture absorbed into the adiabatic material would be evaporated and not replaced. Unfortunately, having filled the trough unnecessarily, once the regular time period expires, the trough contents would be dumped in accordance with the trough water replacement schedule. In these circumstances, the trough water is unnecessarily wasted. Accordingly, there is a requirement for an improved system and method of wetting adiabatic material that at least ameliorates one or more of the above described disadvantages.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form or suggestion that the prior art forms part of the common general knowledge.

SUMMARY OF THE INVENTION ^'

In one aspect, the present invention provides a moisture recirculation system for evaporatively cooling air, the system including: a moisture distribution arrangement which, in use, distributes moisture to an upper portion of the moisture absorbent material; a trough disposed below the lower most portion of the moisture absorbent material for initially collecting moisture run-off; a sump in fluid communication with the trough for collecting and storing said run-off; and a pump in fluid communication with the sump which, in use, transfers moisture from the sump to the moisture absorbent material.

Preferably, the moisture absorbent material is an adiabatic material which is maintained moist with water. Although water is inexpensive and generally in plentiful supply, in recent times the need to conserve water as much as possible has become well known particularly in view of water restrictions that have been imposed in many parts of the world that are experiencing extended drought conditions. Of course, the water may include additives such as anti- microbial agents and/or any other additives to improve the operation of the water recirculation system.

Preferably, the trough disposed below the lower most portion of the moisture absorbent material is dimensioned such that per unit length the trough will collect and/or hold substantially less run-off as compared with existing trough arrangements that act as the run-off collection and storage means. The trough may act as a temporary and intermediate storage location for run-off water until such time that the water collected in the trough can be transferred to the sump. As the sump is not required to extend the full length of the adiabatic material, it may be substantially smaller and hold substantially less water as compared with existing trough arrangements whilst still maintaining a positive head of pressure at the pump intake.

In an embodiment of the invention, an external source of make-up water is in fluid communication with the moisture recirculation system. In this embodiment, the supply of make-up water is controlled by a valve which is activated and deactivated in accordance with a control system that determines the requirement for make-up water. The make-up water may be supplied to the sump. Alternatively, and preferably, the make-up water is supplied directly to the moisture absorbent material and as run-off water is collected and passed to the sump, the level of stored water in the sump will increase.

Preferably, the pump transfers moisture from the sump to the moisture absorbent material by pumping sump moisture to the moisture distribution arrangement. In this latter embodiment, a standard float λ'alve arrangement is used to monitor the water level in the sump thus ensuring that a positive head of pressure is maintained at the pump intake. The external water is not supplied to the sump but rather is applied directly to the moisture absorbent material and may be transferred through the water distribution arrangement that distributes water to the upper portions of the moisture absorbent/adiabatic material. As a result of by-passing the sump, the make-up- water is directly deposited where it is needed without the normal delay associated with filling the sump and then transferring the make-up water from the sump, through . the pump and subsequently to the water distribution arrangement. Of course, the time required to increase the water level in a relatively small sump is substantially less as compared with existing trough arrangements. However, by applying external make-up water directly to the moisture absorbent material reduces the time required to saturate the moisture absorbent material even further.

Of course, the opening of the float valve which allows the ingress of external water into the water recirculation system ultimately increases the sump contents which in turn will eventually act to close the float operated valve.

Of course, this supply of external water by the Valve to the adiabatic material may occur through a separate water distribution system as compared with the water distribution system that is in fluid communication with the trough and pump arrangement. However, there is no reason why the same water distribution arrangement could not be used for both recirculated water and external make-up water and in an embodiment of the invention, the makeup water is introduced into the water conduit that extends from the pump outlet to the water distribution, system disposed above the adiabatic material.

Embodiments of the invention that incorporate intermediate and temporary run-off water collection arrangements with any collected run-off water being directed to a sump for subsequent transfer by pump to a water distribution arrangement disposed above the adiabatic material may substantially reduce the amount of wasted water particularly where the dimensions of the sump and trough are substantially reduced as compared with, existing trough arrangements. In particular, by using a trough as an intermediate run-off water collection arrangement below the adiabatic material and subsequently transferring run-off water to a sump, the sump may be dimensioned significantly smaller than standard trough arrangements whilst still maintaining the necessary head of pressure at the pump intake. In a prototype arrangement, it is expected that the sump size can be reduced from the usual 250 to 300 litres of water collection and storage that occurs in existing trough arrangements to as little as 40 litres.

Of course, where it is possible to operate a water recirculation system with a smaller sized sump, then the amount of wasted water is commensurately reduced as the regular dumping cycle will only result in the dumping of a smaller quantity of water.

In an embodiment of the invention that directs make-up water to a point in the water recirculation system subsequent to the outlet of the pump and prior to the inlet of the water distribution arrangement, the amount of time required to saturate the moisture absorbent material from a dry condition is reduced as compared with an arrangement where the make-up water is directed into a sump or trough in the first instance and subsequently pumped to the water distribution arrangement, Where make-up water is directed to a point between the outlet of the pump and the inlet of the water distribution arrangement, the time required to transition a cooling apparatus from dry mode to wet mode is substantially reduced as compared with existing systems. Where this delay is sufficiently reduced, the requirement to pre-emptively commence operation of the water recirculation system in anticipation of worsening climatic conditions is obviated. A more responsive water recirculation system that can transfer from dry to wet mode as quickly as possible has the associated benefit of avoiding more instances of false priming of the air cooling system and hence avoids the wastage of water in those instances where a false priming would otherwise occur.

In another aspect, the present invention provides a method of recirculating moisture for evaporatively cooling air including the steps of: applying moisture to an upper portion of moisture absorbent material; initially collecting moisture run-off in a trough disposed below the moisture absorbent material; transferring run-off moisture from the trough to a sump for storage; and transferring moisture from me sump to the moisture absorbent material.

In an embodiment, the moisture is water and make-up water is supplied to the recirculating water system (as required) by supplying water from an external source to the sump. In another embodiment, external make-up water is supplied directly to the moisture absorbent material thereby providing additional run-off water that is eventually collected and stored in the sump. The supply of external make-up water may be controlled by a valve that is activated and/or deactivated according to a control signal that measures the water level in the sump.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying figures in which: Figure 1 is a diagrammatic representation of a cooling system incorporating an existing moisture recirculation system;

Figure 2 is a diagrammatic representation of cooling system including a water recirculation arrangement according to an embodiment of the invention; and Figure 3 is a diagrammatic representation of the cooling system of Figure

2 providing a perspective view of some of the components detailed in Figure 2. DESCRIPT1ON OF ONE OR MORE EMBODIMENTS OF THE

INVENTION

With reference to Figure 1, a diagrammatic representation of an existing cooling system arrangement is provided wherein cooling fluid is passed through cooling coils (25, 30) through an inlet conduit (35) and subsequent to passing through the cooling coils (25, 30) is emitted through an outlet conduit (20). The cooling fluid may be water or a refrigerant fluid that is used to transfer thermal energy such as Freon. Further, where the cooling fluid is water, additives such as Glycol may be added to improve the thermal characteristics of the resulting cooling of fluid. The cooling fluid is supplied to the cooling coils (25, 30) through the inlet conduit (15) for the purpose of cooling the fluid and during the passage through the cooling coils (25, 30) thermal energy is extracted from the cooling fluid such that the fluid emitted through the outlet conduit (20) has a substantially lower temperature and hence may be returned to the cooling apparatus that uses the fluid for the purpose of absorbing and transferring thermal energy.

During periods where the ambient air temperature is sufficiently low, air is drawn through the cooling coils (25, 30) without the operation of the water recirculation system. In this instance, the cooling system (10) is described as running in the "dry" mode and thermal energy is extracted from the cooling fluid solely by the action of passing air through the cooling coils (25, 30) as the cooling fluid (water/refrigerant) passes through the cooling coils (25, 30).

s However, during periods where the ambient air temperature is not sufficiently low, or an increased heat exchange capacity is required that may not be effected by operating a heat exchanger in the "dry" mode, moisture absorbent material in the form of evaporative cooling pads (35, 40) are moistened (preferably with water) in order to effect evaporative cooling of the air prior to the passage of same through the cooling coils (25, 30). - l i ¬

ft- the event that the evaporative air pre-cooler is completely dry and has no water in the troughs (55, 60) then the water make-up Solenoid valve (70) is opened in order to introduce external make-up water into the troughs (55, 60) through conduits (67, 65). The external make-up water is provided to the water make-up Solenoid valve (70) through an inlet conduit (72). A back pressure flow prevention device (73) may be included depending upon local installation regulations.

The troughs (55_; 60) include a water level monitoring device generally in the form of a flotation device that monitors the water level in the troughs (55,

60). Once there is a sufficient water level in the troughs to maintain a positive head of pressure to the inlet of the pump (45) then the pump may be operated to pump water through a conduit (46) and provide same to water distribution arrangements (47, 50) for distribution of the water to the upper portions of the evaporative cooling pads (35, 40).

Of course, as water trickles down through the evaporative cooling pads (35, 40) under the action of gravity the moisture absorbent material (preferably adiabatic material) in the evaporative cooling pad absorbs the water and once saturated any additional water provided to the evaporative cooling pads (35, 40) will run-off the adiabatic material. Ultimately, any run-off water will be collected in the troughs (55, 60). The troughs (55, 60) have an overflow mechanism (80, 85) in the event that there is a continuing supply of run-off water entering the troughs (55, 60) despite the float monitoring device detecting a sufficient water level in the trough and deactivating the water make-up Solenoid valve (70). Over time, as the evaporative cooling system operates, water is evaporated as it cools the ambient air passing through the evaporative cooling pads (35, 40) and any water lost through vaporization is replaced by operation of the water make-up Solenoid valve (70) in conjunction with the float monitoring device in the troughs (55, 60). The moisture recirculation system continues to operate as long as the heat exchanger is required to operate in "wet" mode. A water dump valve (75) is also connected to the troughs (55, 60) by a conduit (65). The water dump valve is operated on a regular basis for the purpose of dumping the contents of the troughs (55, 60) to reduce the potential for the generation and growth of bacteria that may result from an increase in concentration of sediment and/or impurities in the troughs (55, 60).

The particular arrangement detailed in Figure 1 is very common and has been used successfully for many decades. However, as previously described, this standard arrangement embodies various disadvantages including a relatively large trough capacity. In this respect, Figure 1 is an end perspective and the troughs (55, 60) extend the entire length of the evaporative cooling pads (35, 40). In the event that the heat exchanger is relatively long, the sump capacity is commensurably large and in order to maintain a positive head of water pressure at the inlet side of the pump (45) it is necessary to maintain a minimum depth of water in the troughs (55, 60). For a relatively long trough, maintaining the minimum depth may represent a substantial amount of water. Further, a separate disadvantage of existing arrangements is the relatively long period of time that is required to transition the heat exchanger from "dry" to "wet" mode as a result of the supply of external make-up water to the troughs (55, 60).

An embodiment of the invention is detailed in Figure 2 which provides a diagrammatic representation from a similar end perspective as that of Figure 1.

With reference to Figure 2, a cooling fluid that requires cooling is provided to cooling coils (125, 130) through a conduit (115). As the fluid passes through the cooling coils (125, 130) thermal energy is extracted therefrom and cooled fluid is emitted from the bottom of the cooling coils (125, 130). Cooled fluid is returned through a conduit (120). Just as for the arrangement detailed in Figure 1, the cooling system (100) extracts thermal energy from the cooling fluid by passing same through the cooling coils (125, ] 30) whilst passing ambient air through the coils. In the event that the ambient air temperature is not sufficiently low, or an increased heat exchange capacity is required;, the device detailed in Figure 2 is transitioned from "dry" mode to "wet" mode by the application of moisture (preferably water) to the evaporative cooling pads (135, 140) such that the evaporative cooling pads evaporatively cool the air passing through the cooling coils (125, 130).

In the arrangement detailed in Figure 2, when seeking to transition the arrangement to "wet" mode, the water make-up Solenoid valve (170) is activated to allow external water that is supplied through conduit (172) to pass through conduits (146 and 149) until the external make-up water reaches and passes through the water distribution arrangements (148, 150). The external make-up water then trickles down through the adiabatic material of the evaporative cooling pads (135, 140) and is absorbed by same. As ambient air passes through the evaporative cooling pads (135, 140) the air is cooled by the action of evaporation as the water that was initially absorbed by the adiabatic material is then vapour ised and converted from liquid to gaseous form.

In order to ensure that the evaporative cooling pads (135, 140) are completely saturated, a sufficient amount of water is provided to the water distribution arrangements (148, 150) such that water trickles down through the evaporative cooling pads (135, 140) and runs-off the cooling pads into the respective collecting troughs (155, 160). The collecting troughs (155, 160) act as a temporary and intermediate collection of run-off water which is then provided through conduits to the sump (165). The sump does not need to extend the full length of the evaporative cooling pads (135, 340) and may be dimensioned to have a capacity that is substantially smaller as compared with the standard trough capacity (as detailed in Figure 1). The sump (165) collects the run-off water from the. collecting troughs (155, 160) and upon collecting enough run-off water to provide a sufficient head of pressure to the intake of the pump (145) then the pump may be activated to pump run-off water up through conduits (146, 149) and re-distribute the water collected in the sump (165) to the water distribution arrangements disposed above the evaporative cooling pads (M8, 150). A back pressure flow prevention device (147) may be included.

The water make-up Solenoid valve (170) may be activated as a result of a water level monitoring device in the form of a flotation device in the sump (165). A back pressure flow prevention device (171) may be included. In any event, as water is depleted from the evaporative air cooling system, the water level in the sump (165) decreases and when sufficiently low (such that a positive head of pressure will not be maintained at the pump intake) the make- up solenoid valve (170) is activated to introduce replacement make-up water into the system. In the embodiment of Figure 2, the make-up water is deposited directly onto the top of the evaporative cooling pads where it is most directly needed. As run-off is collected in the collecting troughs (155, 160) and passed to the sump (165), the water level in the sump increases.

Again, as for the apparatus detailed in Figure 1, upon expiry of a period of time the water dump valve (175) is activated to release the entire contents of the sump (165) to reduce the likelihood of the generation and growth of bacteria and slime in the sump (165). However, as the sump (165) is dimensioned to have a substantially lower capacity as compared with the sump of a standard arrangement, the amount of water that is wasted as a result of a dumping operation is commensurately substantially less.

In embodiments where the make-up water is provided directly to the water distribution arrangements (148, 150) thus effectively by-passing the sump (165), the arrangement provides even less delay in achieving fully saturated evaporative cooling pads (135, 140) as compared with the existing arrangements.

With reference to Figure 3, a perspective view of the cooling system of

Figure 2 is provided. The same parts in Figures 2 and 3 are identified by use of the same identification number. Figure 3 details various parts of the cooling system in perspective and of particular importance is the extension of the collecting troughs (155. 160) extending the entire length of the evaporative cooling pads (135, 140). Further, the water collected by the collecting troughs (155, 160) is subsequently passed to the sump (165) for collection and storage. As will be noted in Figure 3. the dimensions of the sump (165) are substantially smaller as compared with the dimensions of the collecting troughs (155, 160) and therefore, the sump (165) has a significantly reduced volumetric capacity as compared with the collecting troughs (155, 160). Accordingly, if the troughs (155, 160) were used to collect and store run-off, it would require substantially more water (as compared with the sump (165)) to maintain a minimum head of pressure at the intake of a pump.

In industrial and commercial applications, the evaporative cooling pads (135;, 140) can be relatively large. In these applications, it is not unusual for the evaporative cooling pads (135, 140) to comprise a number of smaller cooling pads that are placed in abutment with one another thus forming a wall that extends for a sufficient length and height to substantially conform with the dimensions of the cooling coils (125, 130). Accordingly, the collecting troughs (155, 160) must extend along the full length of the evaporative cooling pads

(135, 140) in order to collect any water run-off from the evaporative cooling pads (135, 140).

However, in the embodiment of Figures 2 and 3, the collecting troughs (155, 160) may act as a temporary collection and storage means for water runoff and may pass run-off water to the sump (165) for collection and storage. As a result, the volumetric water holding capacity of the collecting troughs (155, 160) can be substantially reduced as compared with existing collection and storage troughs that must both collect and store run-off water and maintain a sufficient head of pressure at a pump intake. Having passed run-off water to the sump (165) the water is pumped (145) up through backflow pressure prevention device (147) and through the conduits to the water distribution arrangements (148, 150) whereby water is distributed to the upper portion of the evaporative cooling pads (135, 140).

The moisture recirculation system and method of the present invention is particularly well suited for retrofitting to existing cooling systems and heat exchangers.

It will also be appreciated by those skilled in the relevant field of technology that the present invention is not limited in scope to any exemplary embodiment but rather, the scope of the present invention is broader so as to encompass other forms of the apparatus.

Claims

1. A moisture recirculation system for evaporativety cooling air, the system including: a moisture distribution arrangement which, in use, distributes moisture to an upper portion of moisture absorbent material; a trough disposed below the lower most portion of the moisture absorbent material for initially collecting moisture run-off; a sump in fluid communication with the trough for collecting and storing said run-off; and a pump in fluid communication with the sump which, in use, transfers moisture from the sump to the moisture absorbent material.

2. A moisture recirculation system according to claim 1 wherein the moisture transferred from the sump to the moisture absorbent material is effected by transferring moisture to the moisture distribution arrangement.

3. A moisture recirculation system according to claim 1 or claim 2 wherein the volumetric moisture storage capacity of the sump is substantially less than the volumetric capacity of the trough.

4. A moisture recirculation system according to any one of the preceding claims wherein an external source of make-up water is in fluid communication with the moisture recirculation system.

5. A moisture recirculation system according to claim 4 wherein the supply of make-up water is controlled by a valve which is activated and deactivated in accordance with a control signal that is generated according to the moisture level in the sump.

6. A water recirculation system according to either claim 4 or claim 5 wherein the make-up water is supplied directly to the moisture absorbent material.

7. A moisture recirculation system according to claim 6 wherein the makeup water is supplied to the moisture distribution arrangement.

8. A moisture recirculation system according to claim 5 wherein the control signal is generated by a float valve arrangement that monitors the moisture level in the sump to ensure that a positive head of moisture pressure is maintained at the pump intake.

9. A moisture recirculation system according to any one of the preceding claims wherein the sump includes a dump valve that, when activated, allows the egress of the contents of the sump for the purpose of emptying the sump of its moisture.

10. A moisture recirculation system according to any one of the preceding claims wherein the moisture is water.

1 1. A moisture recirculation system according to any one of the preceding claims wherein additives are added to the moisture in order to improve the operation of the water recirculation system.

12. A moisture recirculation system according to claim 11 wherein the additive includes an anti-mi crobial agent.

13. A method of recirculating moisture for evaporatively cooling air including the steps of: applying moisture to an upper portion of a moisture absorbent material; initially collecting moisture run-off in a trough disposed below the moisture absorbent material; transferring run-off moisture from the trough to a sump for storage; and transferring moisture from the sump to the moisture absorbent material.

1.4. A method according to claim 13 including the step of supplying make-up moisture to the recirculating moisture by supplying moisture from an external source to the sump.

15. A method according to claim 14 including the step of providing make-up moisture to the moisture absorbent material.

16. A method according to any one of claims 13, 14 or 15 including the step of activating a valve to supply make-up moisture to the recirculating moisture upon receipt of a control signal indicating a requirement for make-up moisture.

17. A method according to claim 16 wherein the control signal is generated according to the moisture level in the sump which is generated to ensure that a minimum head of pressure is maintained at the intake of a pump that transfers moisture from the sump to the moisture absorbent material.

18. A method according to any one of claims 13 to 17 wherein the moisture includes water.

19. A method according to any one of claims 13 to 18 wherein the moisture includes additives to improve the operation of the water recirculation system.

20. A method according to claim 19 wherein an anti-microbial agent is added to the moisture.