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.

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