Cooling Tower


 
Specific treatment processes vary depending on the requirements of the cooling tower and 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 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.
 
Chemical addition
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 amount of chemicals needed to treat water at this point in the process, which is ideal considering many chemical treatments can be expensive.
 
Side-stream filtration
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.
 
Blowdown treatment
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 discharge, 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.
 
Choosing the Right Cooling Tower Filtration System
Industrial manufacturing facilities typically require cooling equipment to cool down chillers, heat exchangers, products and other devices, and this equipment needs to be protected from contaminants found in the cooling water.  In addition, environmental regulations for protecting water sources require that manufacturers return the utilized cooling water to its source in a non-pollutant state. Cooling tower filtration systems are essential for safeguarding equipment and to abide by these regulations.
Here are some other valuable benefits of cooling tower filtration:
  • Improved heat transfer rates in water cooling system for longer period
  • Reduced water consumption
  • Improved chemical effectiveness
  • Reduced chemical consumption
  • Reduced maintenance and energy costs
  • Increased equipment lifetime

What is Water Filtration for Cooling Towers?

The above-mentioned regulations require that cooling systems be chemically treated and monitored on a regular basis, specifically for corrosion, scaling, suspended solids and biological fouling.

Corrosion, scaling and biological fouling can be controlled by routine water treatment, but the suspended solids can be more challenging as they can reach the system in various ways, such as, production contaminants, pipe corrosion and variations in the raw water composition. Airborne particles are also an issue and, in some cases, can lead to high dirt loads. If left untreated, suspended solids can cause poor energy transfer and obstructions and have an adverse effect on the efficiency of the chemical treatment. In severe cases, suspended solids may lead to expensive replacements, and production efficiency and productivity would be impacted if the system were to be shut down for repairs.

Filtration plays an important role in protecting cooling towers and is divided into 3 categories: side stream water filtration, full stream water filtration and make-up water filtration.

Side Stream Water Filtration:

A percentage (5-15%) of the recirculating cooling water volume flows through a water loop, reducing the overall load of solids. This loop then passes through a filtration system.

Full Stream Water Filtration:

A filtration unit is installed on the discharge side of the pump, after the cooling tower, and continuously filters all of the recirculating water in the system.

Make-up Water Filtration:

 

Water is lost during the cooling process and make-up filtration process is used to replenish the water reservoir used for cooling. There are four reasons for this water loss:

  • Naturally occurring evaporation
  • Blowdown/Bleeding: to reduce salinity and particle concentration
  • Drift: Mist and/or tiny water drops in small quantities may be carried from the cooling tower, but water loss is less significant than in blowdown and evaporation.
  • Overflows or basin leaks: Cooling towers should be examined and properly maintained to avoid overflows and leaks.

Cooling Water Filtration Technologies

Making an informed decision about which filtration technology is the right one for your system is essential. It is important to understand the general application requirements, including filtration degrees, as well as reviewing the advantages / disadvantages and costs of each technology.

Centrifugal Separators

Centrifugal separators, also known as hydrocyclones, use centrifugal forces to separate heavy solids larger than 70 micron from water. They are frequently used together with a filtration system that can remove finer particles, but centrifugal separators do not stop organic particles.

Traditional Sand Filters

Sand/media filters route the cooling water to the media bed. There are various types of filter beds, such as, sand, crushed glass, basalt and more. As the water passes through the bed, the particles get trapped and the filtered water then goes to the bottom of the tank to be discharged through the outlet.

Automatic Screen Filters

Automatic screen filters use multiple screens which trap the suspended solids in the cooling water as it passes through them. Filtered water then flows through the filter outlet. The gradual dirt buildup (cake) on the inner screen’s surface begins the cleaning process.

Polymeric Disc Filters

Disc filters are made up of series of flat grooved discs that are stacked together under pressure. As the unfiltered water flows through the discs, trapping the particles. As more particles are trapped, the filter’s pressure differential increases, beginning the cleaning process.

Cartridges & Bag Filters

Cartridge and bag filters are flexible solutions for side-stream filtration. However they are consumable and require manual replacement. This should be taken into consideration as the disposable nature of these filters will increases expenses over time.

High Efficiency Media Filters

Like the sand filters mentioned above, high efficiency media filtration utilizes sand and crushed glass as the media, but the sand characteristics differ here. Another difference is the cross-flow patterns produced by the water injectors, sweeping over the media bed surface. This causes some of the water to flow in parallel to the top layer for submicron filtration and prevents media fouling and channeling through the media. Contaminants within and on the media are removed by an automatic backwash cycle.

Review of Side Stream Filtration System Characteristics

Why Cooling Water is So Complex

Cooling water poses four significant challenges and if not controlled, over time, all can reduce performance and efficient use energy.

  1. Biological contamination: By design, the cooling tower environment is high in temperature and humidity, making it the perfect breeding ground for bacteria. Even the slight accumulation of biofilm inside components can dramatically reduce efficiency.
  2. Corrosion: When the air, sodium and additional chemicals present in water are not monitored, they can erode metal and result in leaks in equipment throughout the process and in cooling towers.
  3. Fouling: When dirt, dust and/or debris clog the cooling surfaces or pipes and the water cannot flow as needed, the air is not cooled as required. This leads to wasted energy, as the system works harder to attain the same outcome.
  4. Scaling: During the heat exchange process, various minerals, such as calcium, may accumulate on the heat exchange surfaces, creating layers of scaling. If not removed, scaling will increase operating costs.

 

​Selecting the Right Cooling Tower Filtration System


Here are some factors to take into account when contemplating which type of cooling tower water filtration would best meet the requirements and provide the best filtration performance:

  • Location: Where in the cooling process will the filtration system be installed?
  • Footprint: Are there any limitations on installation space?
  • Flow rate: What is the system’s flow rate? Is the preferred pressure variable or steady?
  • Filtration degree: What is the micron size of the particles that need to be filtered?
  • Particle characteristics: Sand, algae, etc.
  • Costs (purchase, maintenance, operations, backwash, energy): Knowing this will assist in the decision.

 

 
 
 

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