An abundant supply of fresh water is taken for granted in many parts of the world. This Utopian view is rapidly changing; however, as greater demands are placed on what is fast becoming a limited resource. As experts publicly debate the impact of global warming on climatic conditions, drought and local water shortages, the general public’s awareness of these problems is increased. And as concerns over water quality and quantity issues grow, government regulatory controls expand in an attempt to resolve conflicts caused by water shortages, restrictions on use and owners’ rights. This is forcing industry to seek out more efficient ways to use water such as implementing water recycling and reuse strategies. What emerges from this is the view that water is, in fact, a valuable commodity that needs to be efficiently managed to minimize waste, reduce energy consumption and control cost.
A cooling tower is a common water conservation device. Cooling towers serve process heat exchangers, mechanical chillers, turbine condensers, and air compressors in utility, industrial, institutional and commercial plants. Unfortunately, a majority of these cooling towers are not operated at peak efficiency. As a result, owners and operators fail to achieve the full economic and environmental benefits afforded by this water conservation tool.
This article discusses some basic methods for improving cooling tower performance as measured by a reduction in water consumption, wastewater discharge, chemical requirements, and energy usage. These improvements in efficiency translate into dollars and cents savings that offer an attractive return on investment.
COOLING TOWER OPERATION
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 intimate 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:
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 bulk of the tower water. If this were to occur without restriction, however, the solubility limit of the dissolved minerals would soon be reached. At this point, 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 over-concentrating, 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 with fresh makeup to maintain a constant system volume. The makeup is generally obtained from potable water sources, but may also come from treated wastewater or recycled water supplies.
Makeup (MU) = Evaporation (E) + Bleed (B) + Uncontrolled losses
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 windage and uncontrolled losses are negligible)
Cycles of concentration (C) = Makeup (MU) / Bleed (B)
Cycles of concentration are also estimated by the ratio between specific conductance of the cooling water and the makeup water.
From these relationships, the amount of bleed required to maintain a specific cycle of concentration is determined by:
Bleed (B) = Evaporation (E) / (Cycles (C) – 1)
If the Evaporation (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 fresh water withdrawal and wastewater production.
As we approach higher cycles, however, the incremental gains are less. 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. This would further suggest that cooling towers that operate at less than 10 cycles of concentration are less than 100% efficient as measured by makeup consumption and wastewater generation.
The following chart illustrates the tower efficiency as determined by cycles of concentration. In this case we take 10 cycles to represent 100% water efficiency.
|
Cycles of Concentration |
Percent water efficiency |
|
10 |
100% |
|
9 |
98.8% |
|
8 |
97.4% |
|
7 |
96.4% |
|
6 |
93.8% |
|
5 |
90.0% |
|
4 |
84.4% |
|
3 |
75.0% |
|
2 |
56.3% |
These figures suggest that cooling towers that operate at less than 5 cycles of concentration (less than 90% efficient) are not achieving their full potential and would benefit from retrofits that would reduce water consumption and decrease waste. Towers operating at 6 to 8 cycles are acceptable for most applications. Towers in the 9 to 10 cycles range have reached their peak. Pushing beyond 10 cycles would be difficult to achieve within a reasonable return on investment expectation unless zero discharge is the ultimate goal.
STRATEGIES FOR INCREASING TOWER PERFORMANCE
Cooling tower cycles can be maximized in a variety of ways. These include pH adjustment, chemical scale inhibitors, and pretreatment of the tower makeup.
pH adjustment: Traditionally, cooling towers operated on high hardness, high alkalinity makeup water utilize pH adjustment with sulfuric acid to maximize cycles of concentration. One part of 66o Baume acid is required to neutralize one part of alkalinity. Sufficient acid is injected into the makeup to maintain the total (or M) alkalinity of the cooling water in the range of 50 to 100 ppm or at a level that will maintain the pH within the range of 6.8 to 7.5. The Langelier, Ryznar or Practical scaling index is used as an additional control measure to correlate the calcium hardness, total alkalinity, pH, TDS, and temperature to maintain the water chemistry at the neutral point of the index (neither scaling nor corrosive).
The problem with using acid to increase cycles is one of control. Accidental overfeed conditions (low pH) make the cooling water very corrosive to system metals. Also, reducing the M alkalinity removes the natural passivating effect that carbonate and bicarbonate alkalinity has on steel. Operating the cooling tower at pH levels above 8.5 creates an environment that passivates steel and minimizes corrosion of galvanized steel and copper.
Unlike scale deposition that can be removed by chemical or mechanical cleaning, damage caused by acid corrosion is very expensive to repair. In addition, the handling, transporting, and feeding of concentrated sulfuric acid creates additional environmental, health and safety issues.
Chemical scale inhibitors: Various chemical additives and formulations are marketed that enhance the solubility of calcium and magnesium salts while at the same time control corrosion to within acceptable rates. These chemicals are generally phosphonates (organically bound phosphate compounds), polymers (mono-, co-, and ter-) and organic corrosion inhibitors. These products can be used alone or in combination with supplemental acid feed to maximize tower cycles.
Proven effective in lab tests and in the field, cooling water additives are generally limited to keeping calcium and magnesium salts soluble up to a Langelier Index value of about +2.5. Other chemical programs push through the calcium solubility limit by claiming to maintain clean heat transfer surfaces at even higher cycles despite the precipitation of hardness salts, which are chemically conditioned into a fluid, non-adherent sludge that is removed by routine bleed.
Notwithstanding the benefits of a sound chemical treatment program, if the cooling tower cycles are limited to less than 5, significant water savings can be realized by improving the quality of the tower makeup.
Pretreatment of cooling tower makeup: The primary limiting factor for cycles of concentration is calcium hardness. As a general rule of thumb, the calcium hardness in the cooling tower should be maintained within the range of 350 to 400 ppm on a non-acid treatment program. If the makeup water contains, say, 100 ppm calcium hardness, the cycles of concentration are restricted to 3.5 to 4.0. This is equivalent to 75 to 85% water efficiency. Reducing the calcium hardness to 50 ppm, allows the tower to run at 7 to 8 cycles, which is equivalent to over 96% water efficiency.
Hardness reduction or removal can be accomplished by lime softening, sodium ion exchange (water softener) or reverse osmosis. Low hardness makeup is often available from recycle and reuse of plant wastewater such as spent rinse water and steam condensate. Water of any desired hardness can be obtained by the controlled blending of softened water with untreated raw or recycled water.
BENEFITS OF HIGHER CYCLES OF CONCENTRATION
Maximizing cooling tower cycles offers many benefits in that it reduces water consumption, minimizes waste generation, decreases chemical treatment requirements, and lowers overall operating costs.
As a simple example, a cooling tower handling a 1000 ton load operating at 3.5 cycles of concentration with a 12 oF temperature drop across the tower has a makeup demand of 61,775 gallons per day (GPD). Increasing the cycles to 8 has the impact of decreasing the makeup demand to 50,400 GPD. This reduces the makeup requirement by 18.4%. The wastewater produced by the cooling tower decreases from 17,640 GPD at 3.5 cycles to 6,336 GPD at 8 cycles, which is equivalent to a 64% decrease. And by using less water, chemical treatment consumption and disposal requirements are proportionately reduced.
Potential cost savings vary from plant to plant depending on the cost for raw water, waste disposal costs, chemical treatment dosages, and energy. Nevertheless, in addition to the environmental, health and safety improvements, the return on investment to improve cooling tower efficiency is generally less than one year.
SUMMARY AND CONCLUSIONS
Increasing cooling tower cycles offers performance, environmental and economic benefits. Reducing fresh water withdrawals and wastewater discharge is good for the environment. And the associated benefit of reducing water, chemical and energy costs helps maintain employment and create profits for the stakeholders.
Overall, using less water to provide the same level of service is the greenest and generally the least expensive supply option for companies. This simple enhancement in cooling tower operation will also yield long-term improvements in plant performance and reliability.
Water Technology Report 