Cooling towers operate by evaporative cooling, lowering the temperature of water by forced evaporation. When a portion of water evaporates, it must absorb enough heat from the body of water to vaporize. This is how heat is rejected to the atmosphere, as water vapor from a central cooling tower.
The water that evaporates is pure, because any solids it may have carried in the condenser loop do not evaporate with it. These solids remain in solution and cause the solids concentration to increase. The system must have the capacity to hold these residual solids in solution, or they will precipitate and cause scale buildup inside pipes and pumps. The threat of solids is so severe that it’s necessary to bleed off a volume of water to keep the concentration low. This can be expensive since the building operator must not only purchase the water but pay for the sewage disposal of the wastewater, plus the chemical treatment cost of the replacement water.
Chemical treatment is not completely effective since every time water is extracted from the system expensive chemicals are lost. There are osmotic filters that can rid the water of solids instead of dumping to lower the concentration. This reduces chemical treatment costs.
Smaller systems can benefit from a new class of chemicals that enables the water to hold a greater quantity of chemicals in solution. Whereas these systems can double the solids concentration before scaling starts, special additives can increase this to 20 or more cycles of concentration (e.g., the multiple of original solids content). This means the system needs to be bled ten times less frequently, which can translate into savings on water and sewer bills, as well as chemicals treatment costs.
Of course, there’s always some natural bleed-off, which is water loss from the system by a means other than evaporation. Air forced through the cooling tower to amplify evaporation carries atomized water droplets away. Leaking pumps and valves also lose some water from the loop. These processes continue to occur with the zero bleed-off system and have to be replaced with makeup water. Still, the volume is much less than the one-third to one-half of total volume required to be dumped from conventional systems, every two to six cycles of concentration.
The chemistry of the zero bleed-off treatment implies further savings are possible by also having a solids filtration system, which can further increase the cycles of concentration. The additive chemical bonds with thousands of solid ions before dropping out of solution. This results in larger particles that can be extracted by inexpensive high-efficiency filtration systems, rather than the exotic osmotic technology that is designed to filter microscopic particles.
This hybrid system continuously monitors the chemistry of the circulating fluid, adding chemicals as the larger particles are extracted by the filter, along with the ionized dissolved solids. Thus, there are always free ions in solution to take the harmful solids out of free circulation. The system theoretically never needs to be dumped. Natural bleed-off is sufficient to maintain the system balance.
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Excerpt from Energy Conservation Projects.