Cooling towers dissipate heat from recirculating water used to cool chillers, air conditioners, or other process equipment to the ambient air. Heat is rejected to the environment from cooling towers through the process of evaporation. Therefore, by design, cooling towers use significant amounts of water.
Cooling Tower Water Management Overview
The thermal efficiency and longevity of the cooling tower and equipment depend on the proper management of recirculated water.
Water leaves a cooling tower system in one of four ways:
- Evaporation: The primary function of the tower and the method that transfers heat from the cooling tower system to the environment.
- Drift: A small quantity of water may be carried from the tower as mist or small droplets. Drift loss is small compared to evaporation and blowdown and is controlled with baffles and drift eliminators.
- Blowdown: When water evaporates from the tower, dissolved solids (such as calcium, magnesium, chloride, and silica) remain in the recirculating water. As more water evaporates, the concentration of dissolved solids increases. If the concentration gets too high, the solids can cause the scale to form within the system. The dissolved solids can also lead to corrosion problems. The concentration of dissolved solids is controlled by removing a portion of the highly concentrated water and replacing it with fresh make-up water. Carefully monitoring and controlling the quantity of blowdown provides the most significant opportunity to conserve water in cooling tower operations.
- Basin leaks or overflows: Properly operated towers should not have leaks or overflows. Check float control equipment to ensure the basin level is being maintained properly, and check system valves to make sure there are no unaccounted for losses.
How does a cooling tower water treatment system work?
Specific treatment processes vary depending on the requirements of the cooling tower and the quality/chemistry of the feed and circulation water, but a typical cooling tower water treatment system will usually include the following steps:
Cooling tower makeup water intake
Makeup water, or the water replacing bleed and evaporated and leaked water from the cooling tower, is first drawn from its source, which could be raw water, city water, city-treated effluent, in-plant wastewater recycle, well water, or any other surface water source.
Depending on the quality of this water, you may or may not need treatment here. If a water treatment system is needed at this part of the cooling tower water process, it is usually technology that removes hardness and silica or stabilizes and adjusts the pH.
At this point of the process, the proper treatment optimizes the tower evaporation cycles and minimizes the water bleed rate to drain beyond what might be done with chemicals alone.
Filtration and ultrafiltration
The next step is generally running the cooling tower water through some type of filtration to remove any suspended particles such as sediment, turbidity, and certain types of organic matter. It is often useful to do this early on in the process, as the removal of suspended solids upstream can help protect membranes and ion exchange resins from fouling later on in the pretreatment process. Depending on the type of filtration used, suspended particles can be removed down to under one micron.
Ion exchange/water softening
If there’s high hardness in your source/makeup water, there may be a treatment for the removal of the hardness. Instead of lime, a softening resin can be used; a strong acid cation exchange process, whereby resin is charged with a sodium ion, and as the hardness comes through, it has a higher affinity for calcium, magnesium, and iron so it will grab that molecule and release the sodium molecule into the water. These contaminants, if present, will otherwise cause scale deposits and rust.
At this point in the process, there is typically the use of chemicals, such as: corrosion inhibitors (e.g., bicarbonates) to neutralize acidity and protect metal components, algaecides and biocide (e.g., bromine) to reduce the growth of microbes and biofilms
scale inhibitors (e.g., phosphoric acid) to prevent contaminants from forming scale deposits
Thorough treatment prior to this stage can help reduce the number of chemicals needed to treat water at this point in the process, which is ideal considering many chemical treatments can be expensive.
If the cooling tower water is going to be recirculated throughout the system, a side-stream filtration unit will be helpful in removing any problematic contaminants that have entered through drift contamination, leak, etc. A good rule of thumb is that, if you cooling tower water treatment system requires side-stream filtration, about 10% of the circulating water will filter through. It typically consists of a good quality multimedia filtration unit.
The last part of treatment required for cooling tower water is the blowdown or bleed from the tower.
Depending on how much water the cooling plant needs to circulate for proper cooling capacity, plants will choose to recycle and recover the water through some type of post-treatment in the form of reverse osmosis or ion exchange, especially in places where water might be scarce. This allows liquid and solid waste to be concentrated and removed while treated water can be returned to the tower and reused.
If the water from your blowdown needs to be discharged, any discharge your system creates will need to meet all regulatory requirements. In certain areas where water is scarce, there could be large sewer connection fees, and demineralization systems can be a cost-effective solution here, as they can help minimize the cost to connect to water and sewer lines. Also, the discharge of your cooling tower bleed must meet local municipal discharge regulations if your effluent is being returned to the environment or a publicly owned treatment works.
If you have any questions, be sure to visit our website for more information about cooling tower water treatment here. We also have an article you might be interested in about how much a cooling tower water treatment system might cost you and who we recommend as qualified cooling tower water treatment companies for you to consider as you search for all the options available for your plant.