Water Management
Though an abundant supply of freshwater has been taken for granted in many parts of the world, its availability is becoming less certain, even in North America. Water is a valuable resource and commodity that needs to be efficiently managed to minimize waste, reduce energy consumption, and control cost, especially for power generation. The industry must respond by seeking out more efficient ways to use water, such as by implementing water recycling and reuse strategies, especially for critical equipment like cooling towers.
The purpose of a cooling tower is to conserve water. It fulfills its purpose by rejecting heat to the atmosphere by convective and evaporative heat transfer. As water cascades through the cooling tower, it comes into contact with air that is pushed or pulled through the fill by mechanical draft fans. Some of the waste heat is transferred from the warmer water to the cooler air by convection. The remainder of the heat is removed by evaporation of a small percentage of the recirculated water. The evaporation rate is determined by the following equation:
Evaporation (E) = (0.0085) * (Recirculation rate, R) * (Temperature differential across tower, dT)
The water that is evaporated from the tower is pure; that is, it doesn’t contain any of the mineral solids that are dissolved in the cooling water. Evaporation has the effect of concentrating these dissolved minerals in the remainder of the tower water. If this were to occur without restriction, however, the solubility limit of the dissolved minerals would soon be reached. When the solubility limit is reached, dissolved minerals (most commonly calcium and magnesium salts) precipitate as an insoluble scale or sludge. This is the off-white, mineral scale that is frequently found in heat exchangers, in the tower fill, or deposited in the sump.
To prevent the tower from overconcentrating minerals, a percentage of the cooling water is discharged to drain. The bleed or blowdown rate is adjusted to control the concentration of dissolved minerals to just below their solubility limit. This limit is commonly set and controlled by specific conductance (micromhos/cm) or total dissolved solids (mg/l) measurements.
The water that is lost by evaporation and bleed must be replaced by fresh makeup to maintain a constant system volume. Makeup is typically obtained from potable water sources, but it may also come from treated wastewater or recycled water supplies (Figure 7):
Makeup (MU) = Evaporation (E) + Bleed (B) + Uncontrolled losses
7. Water-balancing act. A typical power plant evaporative cooling system must add makeup water to balance out evaporation and cooling tower blowdown. Source: Harfst and Associates Inc.
One indicator of cooling tower efficiency is cycles of concentration, or concentration ratio. This is the ratio of the makeup rate to the bleed rate, MU/B, assuming the uncontrolled losses are negligible.
Cycles of concentration are also estimated by the ratio of the specific conductance of the cooling water and the makeup water:
Cycles of concentration (C) = MU / B
From these relationships, the amount of bleed required to maintain a specific cycle of concentration is determined by:
B = E / (C – 1)
If E is held constant, reducing the bleed causes the cycles to increase. Conversely, increasing the bleed causes the cycles to decrease. Operating the cooling tower at maximum cycles of concentration reduces the amount of water sent to drain and thereby decreases the freshwater makeup demand. Overall, higher cycles of concentration translate into greater efficiency as measured by a decrease in freshwater consumption and wastewater discharge (Figure 8).
8. Cooling tower basics. Increasing the cycles of concentraion or cooling tower water dissolved mineral content will decrease the cooling tower blowdown and thereby decrease makeup water requirements. However, increased minerals in the water can degrade tower performance over time. Source: Harfst and Associates Inc.
The diminishing returns curve (Figure 8) indicates that major gains in water conservation can be achieved by increasing the cycles from two to three. As we approach higher cycles, however, the incremental gains decrease. From a practical view, windage, leaks, and other uncontrolled losses limit the cycles to a maximum of about 10. This is a reasonable goal for most cooling towers and would further suggest that cooling towers operating below 10 cycles of concentration are less than 100% efficient as measured by makeup consumption and wastewater generation (see table).
Cooling tower efficiency is determined by cycles of concentration. The table data assume that 10 cycles of concentration represent 100% cooling tower efficiency for comparison purposes. Source: Harfst and Associates Inc.
These figures suggest that cooling towers that operate at fewer than five cycles of concentration (less than 90% efficient) are not achieving their full potential and would benefit from retrofits that would reduce freshwater consumption and decrease waste. Towers operating at six to eight cycles are acceptable for most applications. Towers in the nine- to 10-cycles range have reached their peak. Achieving more than 10 cycles would be difficult while deriving a reasonable return on investment, unless zero discharge is the ultimate goal.
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