Incorporating an efficient filtration system into a cooling tower can extend the life of the tower and lower maintenance costs, thereby providing a significant return on resources (ROR). Additional benefits can include:
- Improved cooling system efficiencies. A
buildup of solid contaminants in cooling towers can increase the “fouling
factor” and significantly increase energy consumption. Solid contaminants also
can clog small spray nozzle orifices, causing poor distribution throughout the
distribution system. Filtration minimizes deposits on heat transfer surfaces
and prevents nozzle clogging.
- Increased life of the cold water basin, which might, in
turn, increase the life of the entire cooling tower. A thick layer of
solid contaminants in the cold water basin reduces the ability of treatment
chemicals such as corrosion inhibitors or oxidizers to reach the basin.
Filtered cooling tower water prevents solid contaminants from building up in
- Reduced treatment chemical costs. A layer of
solid contaminants in the cold water basin provides an excellent environment
for bacteria and algae growth. Also, suspended solid contaminants can buffer
the beneficial effects of treatment chemicals and can make additional chemical
treatments necessary. Filtering the cooling tower water prevents solid
contaminant contribution to bacteria and algae growth and minimizes the need
for additional chemicals.
- Reduced maintenance. A layer of solid
contaminants will eventually need to be removed from the cold water basin.
Filtering the cooling tower water will minimize the need for manual cleanings.
- Reduced downtime for cleaning. In most cases, an
efficient water filtration system can reduce downtime by more than 80 percent.
Contamination SourcesThree primary sources are responsible for most common cooling tower contaminants:
- Ambient air dirt load, including windblown sand,
soot and all types of organic debris. Cooling towers make excellent
air scrubbers. Relatively high volumes of air pass through cooling towers, and
most of the contaminants in the air end up in the cooling tower basin.
- Circulation water buildup. Calcium carbonate
scale that forms in the tower can flake off. Treatment chemical residue and
algae also can build up in the circulation water.
- Makeup water. Depending on the source and water quality, the makeup water also can contribute to contaminant buildup.
In closed-loop water circulation systems such as those used in cooling towers, filtration systems can be sized to filter around 10 percent of the full stream, which is known as side-stream filtration (figure 1). With this amount of the full stream filtered, the water is cleaned and its particle load condition stabilizes in a short amount of time (typically two to three days). In areas where debris is more prevalent, the side-stream amount can be increased to handle the higher particulate load. In severe cases, a full-stream system also can be implemented.
Filtration TechnologiesSeveral different technologies are used to filter cooling tower water. These include:
- Centrifugal separators.
- Sand media.
- Automatic self-cleaning screens.
- Cartridges or bags.
Centrifugal Separators. Centrifugal separators can remove up to 90 percent of the particles that have a specific gravity greater than water and are larger than 70 micron.1 This means that any particles that are organic or lighter than water, or any particles smaller than 70 micron, most likely will not be removed in a single pass. Depending on the system, smaller particles and higher percentages of larger particles might be removed with multiple passes through the separator. However, those particles must remain in suspension for longer periods of time, which requires higher particle loads to be maintained in the cooling tower water.
Centrifugal separators are cleaned either by purging the particle-collection chamber or by allowing a continuous small flow from the collection chamber. They are often an integral part of a filtration system that uses other technologies for the more reliable removal of smaller particles. In those cases, the separator removes the larger and heavier particles, while the other technologies remove the remaining finer and lighter particles that pass through the separator.Sand Media Filters. Sand media filters require a relatively large footprint, and their flush waste is high compared to other filters. For instance, a typical sand media system might require 1,200 gal of waste to back-flush, while the equivalent size of some self-cleaning screen filter technologies would use only 30 gal. Back-flushing sand media takes at least three minutes, and the tank being flushed must be completely offline during the back-flush process. For this reason, a sand media system requires at least two tanks to accommodate the flushing process.
Sand media filtration can remove particles that are 10 micron or smaller and is excellent for removing organic particles. However, it can be difficult to back-flush heavy particles such as dirt or sand that are collected from the tower without losing some of the media. Occasional replacement of the media is one of the maintenance requirements of sand media filtration systems.
Microfiltration Systems. Where microfiltration is required for specialized cooling systems, cartridge, bag or membrane systems are available that can remove both organic and inorganic particles down to 0.5 micron without requiring back-flushing. However, this technology can be expensive because the cartridges, bags or membranes might need to be changed frequently. In addition, these types of filters often require a prefilter in front of the cartridge housing to remove the larger particulate and reduce the total suspended solids (TSS), thereby maximizing the life of the cartridge or bag.
Self-Cleaning Screen Filters. Automatic self-cleaning screen filters can remove organic and inorganic particulate down to 10 micron, making them an increasingly common choice for cooling tower filtration applications. These systems provide the least flushing discharge or wastewater during the cleaning process. Some automatic self-cleaning screen filter technologies use only a few gallons of water in each flushing cycle. Unlike media filtration, some automatic self-cleaning screen technologies provide continuous screen filtration during the screen-cleaning process, thereby requiring only a single filter for continuous filtration. Automatic self-cleaning screen filters also are used as a prefilter in front of microfilters such as cartridges, bags or membranes.
System DesignA filtration system can be designed to meet any level of filtration quality desired. The most important prerequisite in specifying a filtration system is to define the requirements of the cooling tower in which the system will be used, as well as the quality of the water to be filtered. Other design features to look for include:
- The ability to remove both organic and inorganic suspended
- Uninterrupted filtration during the flush or screen cleaning
- Flush flow rates in the range of 5 to 10 percent of the filter’s
total flow rate. Lower flush flow rates compared to the total flow will
minimize wasted treatment chemicals as well as makeup water requirements.
- Short cleaning cycles. Cleaning cycles that last longer than 15 sec
can waste water and decrease profits.
- Simplicity. Fewer moving parts and simpler controls will require
less maintenance and training. Look for the simplest drive mechanisms --
hydraulic if possible, because adding additional electrical costs doesn’t help
- Efficient filtration. A 25 micron filtration degree is the most common for cooling tower water; however, the required filtration might vary depending on the tower location, local conditions and filtration objectives. Reduce the TSS only to the desired cost effective level -- don’t over-filter.
Most importantly, purchase the level of filtration that will achieve the highest ROR -- and therefore will give you the greatest return on your investment.
*In 2016, VAF Valve and Vilter, Arvada, Colo., was acquired by Evoqua. Learn more about VAF at https://www.evoqua.com/en/brands/VAF-Filtration/.
- Ambient air dirt load, including windblown sand, soot and all types of organic debris. Cooling towers make excellent air scrubbers. Relatively high volumes of air pass through cooling towers, and most of the contaminants in the air end up in the cooling tower basin.