Patent Description:
Air-cooled heat exchangers such as fluid coolers and condensers reject heat to the atmosphere. These devices reject heat by sensible heating of the ambient air; therefore the lowest temperature they can achieve is some temperature above the ambient dry bulb temperature. By use of adiabatic cooling, the ambient air can be cooled to a temperature approaching the wet bulb temperature. This pre-cooled air is then used to reject heat. By use of adiabatic cooling, a dry-cooling heat exchanger can be made smaller (less expensive) or can cool to a lower temperature (more energy efficient) or some combination of the two.

There are two typical ways that adiabatic cooling is performed. One way is to cool the air with saturated pads. Thick pads are placed at the inlet to the air-cooled heat exchanger. These pads are saturated with water. When incoming air is drawn across these pads, some of the water is evaporated and the air is cooled. Although these pads are in widespread use, they have several drawbacks. To full saturate the pads, a heavy stream of water needs to be run over the pads. Most of this water is not evaporated and is either sent to drain or recirculated. Sending this water to drain is very inefficient, while recirculation requires another system to treat and periodically drain the water. Additionally, these pads are made of a material that absorbs water and they have a life expectancy of only a few years before needing to be replaced. Furthermore, the pads are left in place year round, even when adiabatic cooling is not used. The pads cause a resistance to air flow and require higher fan horsepower all year round.

The second typical way to generate adiabatic cooling is by the use of misting nozzles. Misting nozzles generate small droplets of water that quickly evaporate thus cooling the air. Misting nozzles spray water at a lower rate than water is streamed over the saturated pads, thus there is no need for a recirculation system and less water is used. The nozzles do not cause any resistance to air flow, so fan horsepower is kept at a minimum. One issue with misting nozzles is that the minerals that are contained in the spray must pass through the coils and these minerals can cause issues. In a pad system these minerals stay with the excess water that is sent over the pads or is trapped on the pads themselves.

To prevent scaling, particularly of calcium carbonate, soft water or softened water must be used with misting nozzles. If hard water is sprayed, scale can form at the nozzles and on the coils. To minimize this problem many manufacturers severely limit the number of hours that the adiabatic sprays can be run each year. Scaling can be avoided by using softened water. Softening replaces the +<NUM> valence cations in the water with sodium. Sodium salts are highly soluble and thus will not form a scale. The concern with softened water is that all of the anions that were present in the hard water are still present in the softened waters. These anions, particularly chloride, sulfate, and hydroxide, can be very corrosive to the coils and fins. This is particularly true if the salts are allowed to stay on the coils for extended period of time. To minimize these corrosion effects many manufacturers limit the number of hours that the adiabatic sprays can be run each year with softened water.

The solution for running extended hours with an adiabatic spray system is to use very low mineral water. Typically reverse osmosis ("RO") water is used for these extended-hour systems. Low-cost RO systems are available that can provide sufficient RO water to operate a cell at a reasonable cost. These low-cost units operate off of domestic water pressure without the need of a separate high-pressure pump. These RO devices should be fed softened water for best membrane life. The RO will remove most of the sodium ions as well as most of the corrosive anions. The resulting water is often less corrosive than rainwater to the materials of construction of the heat exchanger.

There are issues with using these low-cost RO systems for adiabatic cooling. One is that these systems are inefficient on water use. The table below illustrates the output of a low-cost, high-volume RO. Fully <NUM>% of the raw softened water is discarded in order to generate <NUM>% clean water.

Another issue is that even though a single unit is not too expensive, a single unit can provide sufficient misting for only about a single cell; most units will have <NUM> or more cells thus requiring multiple RO units. A known adiabatic pre-cooling system for a heat exchange unit is disclosed in <CIT>. Another example of a device for cooling air with softened water is disclosed in the document <CIT>.

An embodiment of the invention provides a method to use softened water for adiabatic cooling without severely limiting the hours of operation each year. According to the invention, softened water may be used to provide adiabatic cooling over extended hours, with a periodic reverse osmosis "RO" flush of the coils. In another embodiment of the invention, the RO-reject stream from generating the pure water for the RO flush may be combined with softened water and used for adiabatic cooling thus using the RO-reject water for cooling instead of discarding. In another embodiment, particularly for small units, no softened water is used directly. According to this embodiment, the cooling system operates with RO-reject water for the spray, while storing the RO-purified water ("RO-permeate"). The system then switches to RO-pure with additional flow added to flush the coil while RO-reject is stored. In both of these embodiments no RO-reject is discarded. A first aspect of the invention is defined in claim <NUM>, and a second aspect of the invention is defined in claim <NUM>.

<FIG> illustrates one embodiment of the invention. In this embodiment tap water or different source water is sent to a softener <NUM>. The softener is only necessary if the source water is moderately hard or harder. The softener operates by ion exchange to replace calcium and magnesium ions in the source water with sodium ions. The softened water <NUM> is then fed to a reverse osmosis device <NUM> ("RO"). The RO <NUM> shown in <FIG> is a standard commercially available device that operates on source-water pressure. A more complex RO system with a high pressure pump may be used, but this type of RO system is usually too expensive for an adiabatic system.

The RO reject water <NUM> with concentrated minerals is directed to the RO-Reject storage tank <NUM>; the RO permeate <NUM> is directed to the RO-Permeate storage tank <NUM>. A spray pump <NUM> is connected to receive water from either the RO-Reject storage tank <NUM> or the RO-Permeate storage tank <NUM> depending on the position of valves <NUM> and <NUM>. The spray pump <NUM> provides flow to the misting nozzles <NUM> for cooling. When operating from the RO-Reject tank <NUM>, the nozzles <NUM> will mist high mineral containing water but not scale-forming water since the scale forming minerals have been removed by softening. Some of the minerals may deposit on the coil and fins and if left could result in corrosion. To prevent this corrosion, pure mineral free water (RO-Permeate) <NUM> is periodically used to flush the coil via flush pump <NUM> removing any minerals that may have deposited on the fins and coils. Optionally some of this RO permeate water <NUM> could be sent to the nozzles for additional cooling by opening valve <NUM> and closing valve <NUM>. Both the spray nozzle line <NUM> and the coil flush line <NUM> could be configured with a UV system <NUM> to minimize the potential for the growth of pathogenic bacteria such as Legionellae. The system also is configured to allow complete drainage when not in use to eliminate the risk of biological growth in stagnant water or freezing. In this design <NUM>% of the water sent to the RO <NUM> is utilized either for cooling or flushing the coil.

The system also is configured to allow complete drainage via valves <NUM>, <NUM>, and <NUM> and drain <NUM> when not in use to eliminate the risk of biological growth in stagnant water or freezing.

<FIG> illustrates another embodiment of the invention. In this embodiment tap water or different source water is sent to a softener <NUM>. The softener <NUM> is only necessary if the source water is moderately hard or harder. The softener <NUM> operates by ion exchange to replace calcium and magnesium ions in the source water with sodium ions. The softened water <NUM> is then fed to a reverse osmosis device <NUM> ("RO"). The RO <NUM> shown in <FIG> is a standard commercially available device that operates on source-water pressure. A more complex RO system with a high pressure pump may be used, but this type of RO system is usually too expensive for an adiabatic system.

The RO-Reject water <NUM> is sent to a storage tank <NUM> where it combines with additional softened water <NUM>. This combined softened/RO reject water <NUM> is used for cooling by sending to the spray pump <NUM>. Since all of the water has been softened, this water will not result in scaling on the fins. When operating from the RO-Reject/softened-water tank <NUM>, the nozzles <NUM> will mist high mineral containing water but not scale-forming water since the scale forming minerals have been removed by softening. Some of the minerals may deposit on the coil and fins and if left could result in corrosion. To prevent this corrosion, pure mineral free water (RO-Permeate) <NUM> is periodically used to flush the coil.

The RO-Permeate water <NUM> is sent to a pressurized storage tank <NUM> via low pressure pump <NUM>. The pressure in the storage tank <NUM> may be maintained and/or adjusted via bladder <NUM>, pressure switch <NUM> and low pressure pump <NUM>. Because storage tank <NUM> is pressurized, a smaller RO unit can be used and run at night or other times that adiabatic cooling is unnecessary. Periodically this RO-permeate water <NUM> is used to flush the coils removing any minerals that may have deposited on the fins and coils.

Both the spray nozzle line <NUM> and the coil flush line <NUM> may be configured with a UV system <NUM> to minimize the potential for the growth of pathogenic bacteria such as Legionella. The system also is configured to allow complete drainage via valves <NUM> and <NUM> and drains <NUM> when not in use to eliminate the risk of biological growth in stagnant water or freezing. In this design not only is <NUM>% of the water sent to the RO used either cooling or flushing, but fewer systems or smaller RO units are needed as the RO-permeate water <NUM> is used only to flush the coils.

<FIG> illustrates another embodiment of the invention. This embodiment is similar to the one in <FIG> except that the RO-rej ect water <NUM> is sent to drain <NUM>. By sending the RO-reject water to drain <NUM>, the system can be greatly simplified as the RO-reject/softened-water storage tank <NUM> and float control valve <NUM> (<FIG>) can be eliminated. The disadvantage is that the RO-reject water is discarded. Some of the reject water can be recovered if the RO is operated when the spray pump <NUM> is energized. By use of an auxiliary pump <NUM> or aspiration and additional drain valve <NUM>, the RO-reject water <NUM> could be combined with the softened water <NUM> and used for cooling.

The fundamental problem that is corrected by this invention is the corrosion of fins and coils caused by extensive use of softened water. For cost and heat-transfer abilities aluminum and aluminum alloys are extensively used in air-cooled heat exchangers. Aluminum is very sensitive to pH both high and low (amphoteric). For corrosion protection, often the aluminum is coated which adds cost, reduces heat transfer, and is still subject to corrosion at the inevitable holidays in the coating. Aluminum is very resilient to aqueous corrosion at near neutral pH. If the water leaving the softener is not near neutral (<NUM> to <NUM>) then that water must be pH adjusted before use. Fortunately most water used for adiabatic cooling will fall within this pH guideline.

Aluminum is also subject to corrosion by salts that have dried on the surface. Most of these salts are hygroscopic and will absorb sufficient moisture from the atmosphere when the relative humidity is greater than <NUM>%. Thus corrosion can occur even in seemingly dry conditions.

Claim 1:
A water supply system for delivering water used for adiabatically cooling air in a heat exchanger, comprising:
a water softener (<NUM>) configured to receive water from a water source;
a reverse osmosis device (<NUM>) configured to receive softened water (<NUM>) from said water softener;
an RO-permeate storage tank (<NUM>) configured to receive RO-permeate water (<NUM>) from said reverse osmosis device;
a flush pump (<NUM>) configured to deliver RO-permeate water (<NUM>) from said RO-permeate storage tank (<NUM>) to surfaces of a coil of said heat exchanger via a coil flush line (<NUM>);
an RO-reject storage tank (<NUM>) configured to receive RO-reject water (<NUM>) from said reverse osmosis device (<NUM>);
a RO-permeate water line (<NUM>) comprising a valve (<NUM>) configured to deliver RO-permeate water from the RO-permeate storage tank (<NUM>) to a spray pump (<NUM>);
a RO-reject water line (<NUM>) comprising a valve (<NUM>) configured to deliver RO-reject water from the RO-reject storage tank (<NUM>) to the spray pump (<NUM>);
spray nozzles (<NUM>) receiving water from the spray pump (<NUM>) via a spray nozzle line (<NUM>).