Practical considerations for saving water in cooling towers
With a nominal efficiency of 0.2 kw per ton, evaporative cooling towers are among the most energy-efficient and cost-effective technologies for rejecting waste heat from air conditioning and other heat-exchange processes to the atmosphere. In the process, however, they use significant volumes of water. This article will discuss practical water-conservation opportunities for cooling towers.
Background
Evaporation (E). To maximize efficiency, one must understand how water is used inside of a cooling tower. For simplicity, consider a direct-evaporative-cooled counterflow tower. Heat is transferred to water, which is sprayed into the tower from above, evaporating into a moving air stream. Fill within the tower slows downward droplet migration and greatly expands the air-interface area, enhancing evaporation. For every 10°F of temperature drop, there is an evaporative loss of approximately 1 percent, equating, on average, to 2.3 to 3.0 (rarely more than 4.0) gpm per 100 tons of capacity, depending on environmental factors. The great majority of water lost consumptively (i.e., not ultimately returned to a sewer outlet) is evaporated, amounting to about 1 percent of tower flow rate. Maintenance of this high evaporative loss is integral to a cooling tower’s operation.
Drift (D). While vital to effective operation, airflow causes a small amount of water to be removed not as vapor, but as droplets or mist. This loss is called drift. Like evaporation, drift is considered consumptive use, albeit use through which heat energy is removed least efficiently. Drift losses depend on the quality of drift reducers and eliminators, tower configuration, and environmental factors and generally range from 0.001 to 0.3 percent of tower flow rate, although losses can be higher if reducers are badly damaged or worn. Despite the temptation to view drift as negligible in cooling-tower calculations--indeed, much of the savings potential in tower water conservation lies elsewhere--with large towers, there are practical conservation opportunities associated with drift reduction.
Blowdown/bleedoff (B) and makeup (M). Water lost through vaporization leaves behind dissolved and suspended substances. If left unchecked, this circuit will lead to basin water with increasing concentrations of total dissolved solids (TDS), any number of other analytes, and, especially, scale-forming compounds. Also, conditions conducive to biofouling and corrosion will arise. In maintaining water quality and controlling scaling and biofouling, a fraction of water is dumped to a sewer (or perhaps to secondary uses), and the same volume is reintroduced to the tower. The water removed from the tower is called blowdown, or bleedoff, while the water that replaces it is called makeup.
In addition to water replacement, standard cooling-tower treatments include pH-lowering acids, scale inhibitors (phosphonates, orthophosphates, and polyposphates), corrosion inhibitors (including sodium silicates, aromatic azoles, and molybdates), and biological-growth inhibitors (oxidizers such as chlorine and bromine and non-oxidizing biocides such as isothiazolin). Inhibitors are best used with coupons to measure corrosion rates.
Most cooling-tower conservation efforts focus on the blowdown-makeup dynamic, as improvement in this area can lead to savings of large volumes of water.
Calculations
Concentration ratio (CR) when a tower is metered and meter reads are recorded. In situations in which makeup and blowdown are metered, consumptive losses can be calculated as follows:
E + D = M − B
This equation can be rearranged a number of ways to solve for unknown variables (e.g., if only blowdown is metered, makeup can be estimated reasonably).
Cycles of concentration often are expressed as "concentration ratio" (equal to makeup divided by blowdown). This is representative of the number of times water in a tower can be cycled before being discharged as blowdown. If only blowdown is metered, concentration ratio can be estimated reasonably as follows:
CR = (B + E + D) ÷ B
Appropriate metering and associated meter reading of at least the inlet or outlet enables the best estimates of concentration ratio. Some municipal water agencies provide "deduct" credit (and even deduct meters) for the consumptive fraction of use, as this water does not end up in the sewer system, burdening the community with the cost of effluent treatment.
Concentration ratio when a tower is unmetered or meter readings are lacking. Cooling towers commonly lack metering or robust monitoring that otherwise would permit direct calculation of concentration ratio. Fortunately, the degradation in water quality endemic to cooling towers provides a fine proxy for calculating concentration ratio, or the ratio of analytes in blowdown water (CB) to analytes in makeup water (CM), or:
CR = CB ÷ CM
TDS and, by proxy, conductivity are common parameters for CB and CM.
Concentration ratio, water efficiency, and improvements. Concentration ratio is used to describe the relative (nonconsumptive) water efficiency of cooling towers. Cooling towers with high concentration ratios are more water-efficient because the dumping of water as blowdown is minimized, and, thus, less makeup is required. On a per-100-ton basis, water use is much higher at low concentration ratios than it is at high ones (Figure 1).
FIGURE 1. Water use vs. concentration ratio (100 tons of cooling).
Water savings associated with elevating concentration ratio can be calculated. Where CR1 is pre-improvement concentration ratio, and CR2 is post-improvement concentration ratio, percent savings can be calculated as follows:
100{(CR2 − CR1) ÷ (CR1[CR2 − 1])}
Water savings associated with a variety of improvements in concentration ratio are presented in Table 1. Clearly, towers with low concentration ratios are the most economical candidates for improvements.
TABLE 1. Makeup savings for select improvements in concentration ratio.
Drift-reduction-improvement calculations. Whereas drift is reported as a percentage of tower flow rate, drift-reduction-improvement savings are calculated as follows:
STFR = TFR(D2 – D1)
where:
STFR = water savings
TFR = tower flow rate
D1 = initial drift-loss percentage
D2 = post-improvement percentage
With the direct measurement of drift rates rare, nominal rates from manufacturers must be relied on for post-improvement percentages. For initial drift-loss percentages, the conservative approach is to use original specifications. Modifications of tower flow rate must be considered separately, as follows:
TFR2D2 – TFR1D1
Final scaling of calculations. Whether you are calculating savings related to concentration-ratio improvements or drift reduction, use correct tonnages, and have a reasonably good estimate of duty factor (DF) or average plant load over the timeframe of interest. Final savings can be estimated as follows:
S = DF(STFR)
where: S = final savings estimate
In the case of multiple towers, one must appropriately weigh the operating time of each for an accurate total estimate of water savings.
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.