Though an abundant provide of freshwater has been taken for granted in many elements of the world, its availability is turning into less sure, even in North America. Water is a worthwhile resource and commodity that must be efficiently managed to attenuate waste, cut back power consumption, and management price, especially for energy era. The trade should respond by in search of out extra efficient ways to use water, reminiscent of by implementing water recycling and reuse methods, especially for important tools like cooling towers.
The purpose of a cooling tower is to conserve water. It fulfills its purpose by rejecting heat to the environment by convective and evaporative heat switch. As water cascades by means of the cooling tower, it comes into contact with air that is pushed or pulled via the fill by mechanical draft followers. Among the waste heat is transferred from the hotter water to the cooler air by convection. The remainder of the heat is eliminated by evaporation of a small proportion of the recirculated water. The evaporation charge is set by the following equation:
Evaporation (E) = (0.0085) * (Recirculation price, 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 which 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 happen with out restriction, nonetheless, the solubility limit of the dissolved minerals would quickly be reached. When the solubility restrict is reached, dissolved minerals (mostly calcium and magnesium salts) precipitate as an insoluble scale or sludge. This is the off-white, mineral scale that is frequently present in heat exchangers, within the tower fill, or deposited in the sump.
To forestall the tower from overconcentrating minerals, a percentage of the cooling water is discharged to drain. The bleed or blowdown rate is adjusted to manage the focus of dissolved minerals to simply under their solubility restrict. This limit is commonly set and controlled by specific conductance (micromhos/cm) or complete dissolved solids (mg/l) measurements.
The water that is misplaced by evaporation and bleed have to be changed by contemporary make-up to take care of a constant system quantity. Makeup is typically obtained from potable water sources, however it can also come from handled wastewater or recycled water supplies (Figure 7):
Makeup (MU) = Evaporation (E) + Bleed (B) + Uncontrolled losses
7. Water-balancing act. A typical energy plant evaporative cooling system must add makeup water to balance out evaporation and cooling tower blowdown. Supply: Harfst and Associates Inc.
One indicator of cooling tower efficiency is cycles of concentration, or concentration ratio. That is the ratio of the makeup price to the bleed price, MU/B, assuming the uncontrolled losses are negligible. Conversely, growing 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 make-up demand. General, greater cycles of focus translate into better efficiency as measured by a lower in freshwater consumption and wastewater discharge (Figure eight).
Eight. Cooling tower basics. Rising the cycles of concentraion or cooling tower water dissolved mineral content material will decrease the cooling tower blowdown and thereby decrease make-up water requirements. Nevertheless, increased minerals in the water can degrade tower efficiency over time. Supply: Harfst and Associates Inc.
The diminishing returns curve (Figure eight) indicates that major positive factors in water conservation could be achieved by rising the cycles from two to three. As we approach greater cycles, nevertheless, the incremental gains decrease. From a sensible view, windage, leaks, and different uncontrolled losses limit the cycles to a maximum of about 10. That is an affordable goal for most cooling towers and would additional suggest that cooling towers operating under 10 cycles of concentration are lower than a hundred% efficient as measured by make-up consumption and wastewater era (see desk).
Cooling tower efficiency is decided by cycles of concentration. The table data assume that 10 cycles of concentration symbolize 100% cooling tower efficiency for comparability purposes. Source: Harfst and Associates Inc.
These figures counsel that cooling towers that operate at fewer than five cycles of focus (less than 90% efficient) are not attaining their full potential and would benefit from retrofits that would scale back freshwater consumption and lower waste. Towers operating at six to eight cycles are acceptable for many applications. Towers in the 9- to 10-cycles range have reached their peak. Achieving more than 10 cycles could be tough while deriving a reasonable return on funding, unless zero discharge is the last word purpose.
Strategies for Enhancing Performance
Cooling tower cycles might be maximized in a selection of ways. These embody pH adjustment, chemical scale inhibitors, and pretreatment of the tower make-up.
PH Adjustment. Historically, cooling towers operating on excessive-hardness, excessive-alkalinity makeup water utilized pH adjustment with sulfuric acid to maximise cycles of focus. One part of 66° Baume acid is required to neutralize one a part of alkalinity. Sufficient acid is injected into the makeup to keep up the total alkalinity of the cooling water in the range of 50 to one hundred ppm or at a stage that may maintain the pH within the vary of 6.8 to 7.5. The Langelier, Ryznar, or Sensible scaling index is used as an extra control measure to correlate the calcium hardness, whole alkalinity, pH, complete dissolved solids, and temperature to take care of water chemistry on the neutral level of the index (neither scaling nor corrosive).
The problem with using acid to increase cycles is considered one of management. Unintended overfeed circumstances (low pH) make the cooling water very corrosive to system metals. And reducing the M alkalinity removes the natural passivating effect that carbonate and bicarbonate alkalinity have on steel. Operating the cooling tower at pH ranges above eight.5 creates an atmosphere that passivates steel and minimizes corrosion of galvanized steel and copper.
Not like scale deposition, which might be eliminated by chemical or mechanical cleaning, harm caused by acid corrosion can’t be reversed and is very costly to repair. As well as, the handling, transporting, and feeding of concentrated sulfuric acid creates additional environmental, health, and security points.
Chemical Scale Inhibitors. Varied chemical additives and formulations are marketed that improve the solubility of calcium and magnesium salts while at the same time controlling corrosion to within acceptable rates. These chemicals are generally phosphonates (organically certain phosphate compounds), polymers (mono-, co-, and ter-), and organic corrosion inhibitors. These merchandise are used alone or in combination with supplemental acid feed to maximize tower cycles.
Proven efficient in lab assessments and in the sphere, cooling water additives are often limited to retaining calcium and magnesium salts soluble up to a Langelier Index value of about +2.5. Other chemical packages push via the calcium solubility limit by claiming to maintain clear heat transfer surfaces at even larger cycles, regardless of the precipitation of hardness salts, which are chemically conditioned into a fluid, nonadherent sludge that is removed by routine bleed.
Notwithstanding the benefits of a sound chemical remedy program, if the cooling tower cycles are restricted to fewer than 5, significant water savings might be realized by bettering the standard of the tower makeup.
Pretreatment of Cooling Tower Makeup. The primary limiting factor for cycles of focus is calcium hardness. As a normal rule of thumb, the calcium hardness in the cooling tower must be maintained within the range of 350 to 400 ppm on a non-acid remedy program. If the make-up water comprises, say, a hundred ppm calcium hardness, the cycles of concentration are restricted to three.5 to 4.0. That is equivalent to seventy five% to 85% water effectivity. Lowering the calcium hardness to 50 ppm allows the tower to run at seven to eight cycles, which is equal to over 96% water efficiency.
Hardness discount or removing may be completed by lime softening, sodium ion exchange (water softener), or reverse osmosis. Low-hardness makeup is usually available from recycled and reused plant wastewater reminiscent of spent rinse water and steam condensate. Water of any desired hardness might be obtained by the controlled blending of softened water with untreated raw or recycled water.
Benefits of accelerating Focus Cycles
Maximizing cooling tower cycles provides many benefits in that it reduces water consumption, minimizes waste era, decreases chemical therapy necessities, and lowers total operating prices.
As a easy example, a cooling tower dealing with a 1,000-ton load working at 3.5 cycles of focus with a 12F temperature drop across the tower has a make-up demand of sixty one,775 gallons per day (gpd). Growing the cycles to eight has the impact of lowering the makeup demand to 50,four hundred gpd. This reduces the make-up requirement by 18.Four%. The wastewater produced by the cooling tower decreases from 17,640 gpd at three.5 cycles to six,336 gpd at eight cycles, which is equal to a sixty four% lower. And by utilizing much less water, chemical remedy consumption and disposal requirements are proportionately decreased.
Potential value savings fluctuate from plant to plant, relying on the price for uncooked water, waste disposal prices, chemical remedy dosages, and vitality. However, along with the environmental, health, and safety enhancements, the return on investment for enhancing cooling tower efficiency is typically less than one year.