Successes and Lessons

Located in the Mojave Desert, the Las Vegas Valley has significant indoor cooling requirements. (The July cooling-degree-days average is 796, although it can exceed 900.) From an energy-exchange-efficiency perspective, the area is ideal for evaporative cooling, with a dry-bulb/wet-bulb average Fahrenheit split of 90.3/63.8. (The 99-percent maximum condition is approximately 106/66.) Although the relative lack of water resources negates the potential for its widespread use in the single-family sector, evaporative cooling is the standard for medium and large facilities, including the area’s renowned resorts. Most municipal water is from the Colorado River, a source of quite "hard" water. Southern Nevada Water Authority (SNWA) measurements of water quality at makeup inlets reveal an average calcium-carbonate level over 336 mg per liter and an average TDS level over 648. (Conductivity is 1,042 µS per centimeter.) The average baseline concentration ratio is 2.22. These conditions, combined with supportive, proactive resort-facility managers, have made for excellent opportunities to study and facilitate improvements intended to boost concentration ratio in cooling towers.

The SNWA facilitates cooling-tower improvements through its Water Efficient Technologies (W.E.T.) program, which incentivizes owners to find their own water-conservation solutions, rather than be subjected to "government"-specified mandates. Over the years, the SNWA has worked with many properties (mostly hotels) through the W.E.T. program.

Following are treatments observed to work successfully in field applications in the Las Vegas Valley:

• Highly developed acid injection. The traditional concept behind highly developed acid-injection systems is that by maintaining low pH, scale formation is controlled. The innovation here is that a sophisticated microcomputer system manages all aspects of tower injection so that there is almost no variance in pH. Such a system can require significant room for on-site chemical facilities, and there are associated safety considerations.

Avg. CR1 = 2.33

Avg. CR2 = 3.91

Improvement = 1.58

Experience shows that with very precise, near-continuous acid dosing, much more significant improvement can be realized.

• Ozonation. Ozone is a powerful oxidizing biocide able to rupture bacteria-cell walls thousands of times faster than chlorine. As such, its primary use in cleaning cooling towers appears to be the reduction of the biofilm to which scale sticks. With a native half-life of only about 10 min, ozone usually is produced on site by breaking and reforming oxygen molecules via electrical discharge. Mixed with water, ozone is diffused through a grate inside of a tower basin. The amount of ozone is computer-controlled, with at least one oxidation-reduction-potential (ORP) sensor used to determine when more ozone is required.

Avg. CR1 = 2.15

Avg. CR2 = 3.17

Improvement = 1.02

Ozonation has proven to be a viable technology, provided cooling towers and systems (particularly ORP sensors) are monitored and maintained properly. While ozonation has been employed successfully within the confines of the W.E.T. program, others in Southern Nevada reportedly have experienced nearly catastrophic failures, with too much ozone introduced to cooling towers, damaging valve seals and other components.

Important caveats regarding ozone to consider include:

1) It is not for extremely hot (greater than 104°F) water because of difficulty maintaining dissolved oxygen in solution. Similarly, it is not for high-chemical-oxygen-demand applications, such as petroleum-processing facilities.

2) On-site ozone generators may drive up energy use.

3) Coupons must be used to check corrosion rates.

4) In areas with hard water, ozone probably cannot replace scale inhibitors, although it may be able to reduce their use.

5) Because biofilm removal with ozone is fast (0.1 mg per liter will remove 70 to 80 percent of the film in a tower in three hours), blowdown ports in retrofit installations can "crust up" initially as scale comes out.

• Advanced control technologies. Beyond standard conductivity-based control, a diverse array of elaborate controllers is available. Most measure additional water-quality parameters and may tie into building control systems. All automatically control blowdown and makeup valves.

Avg. CR1 = 1.85

Avg. CR2 = 3.25 (4.50 for "tagged" dispersal-polymer-based control)

Improvement = 1.40

Retrofitting standard cooling-tower control systems with more-sophisticated controllers appears to result in greater cycles of concentration. At this time, however, there is no one definition of what constitutes a "smart" cooling-tower controller.

In Southern Nevada, controllers employing a fluorescent (i.e., "tagged") high-temperature dispersal polymer and an inert tracer material have worked quite well. In comparing the consumption of a polymer with that of an inert material, conditions conducive to the occurrence of scale can be predicted. In response, these controllers adjust basin water quality. According to the sole known manufacturer of a product using this technology, this can save water (as well as reduce heat-exchanger deposits) by permitting a tower to operate at the "edge" of the conceivable performance envelope. The water-savings claim thus far appears supported by SNWA monitoring.

• Drift reduction. Drift reduction usually is associated with repack projects.

Avg. D1 = 0.05 to 0.2 percent

Avg. D2 = 0.001 to 0.005 percent

Improvement: for every 100 tons of capacity, average 70,960 gal. saved annually

In Southern Nevada, consumptive-use reduction is of great interest because water truly is "lost" from the regional water-resource pool (i.e., there is no opportunity for secondary reuse or credit). This dynamic has helped drive innovation in drift reduction in the area.

Conclusion

The W.E.T. program has been quite successful, considering the hardness of Southern Nevada’s water. From a starting average baseline concentration ratio of 2.22, the average participant has moved to a concentration ratio of 3.45 (i.e., on average, blowdown has been reduced by 45 percent). Further, for every 100 tons of capacity, the average participant saves approximately 675,000 gal. a year. The average annual facility savings is 17.7 million gal. Locally, the W.E.T. program has saved more than 1 billion gal., with much of that coming from cooling-tower improvements.

A cooling-tower-retrofit initiative should not end with consideration of the cooling tower. A facility manager should consider anything that reduces peak load and subsequent consumptive losses. Whether it be through physical improvements or better management, the only practical way to reduce evaporative losses is to reduce building heat loads. Just because the field of water conservation inevitably focuses on cooling towers does not mean a facility manager should fail to recognize the critical link between water conservation and ensuring his or her evaporatively cooled facility is as energy-efficient as possible. The most water-efficient cooling tower is the one with minimal need for use.

Kent Sovocool is senior conservation-research analyst for the Conservation Division of the Southern Nevada Water Authority. Previously, he worked as an analytical chemist. He holds a master’s degree in biological sciences and geoscience from the University of Nevada, Las Vegas.

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