During process cooling, the cooling water absorbs large volumes of airborne particulate while the cooling tower acts as an air scrubber, filtering the surrounding air. With time, these fine particles can build up and settle within the system, adversely affecting cooling performance while lowering the life of wetted subcomponents. Typically, 85 percent of the suspended solids found in cooling water and hot water loops are smaller than 5 microns in size.[1] Scientific studies have shown that these fine particles tend to be the adherent contaminants that cause fouling of the cooling tower water, chilled water and heat exchangers, thereby reducing cooling system efficiency.[2] High volumes of fine suspended particles will not always be eliminated solely by using a chemical treatment program. Water filtration, therefore, is becoming a vital addition.

Among water filtration methods, side-stream filtration is used to filter a portion of the system’s circulating water. Side-stream filtration can remove debris and particles adversely affecting the cooling loop and its components, including the heat exchangers, chillers and tower. Over time, filtering 1 to 10 percent of the cooling loop flow results in the filtration of the total volume of water every 12 to 72 hours. The actual filtration rate, of course, depends on the application and filtration technology used.

Side-stream filtration technology generally falls into general categories: screen filters, centrifugal filters, bag and cartridge filters, sand/media filters and high efficiency media filters. Filter technology is evaluated based on its performance in filtering fine particulate (ranging from 1 to 50 microns). Choosing the appropriate side-stream filtration system can yield significant savings for processors.

Side-stream filtration technology includes screen filters, centrifugal filters, bag and cartridge filters, sand/media filters and high efficiency media filters. Such technology is evaluated based on its performance when filtering fine particulate.

6 Signs Your Cooling Tower Would Benefit from Filtration

Observing the following conditions in your cooling water system are signs that your facility would benefit from using a side-stream filtration system:

• The makeup water source is high in suspended solids, iron, biological and organic activity.
• There is a concern for biological growth even though a strong biocide treatment is in effect.
• Frequent cleaning of the heat exchangers and chillers is necessary. (Regular cleaning should occur approximately every 12 to 16 months.)
• There are signs of excessive rates of corrosion within the water loop and its components.
• There is a loss of heat transfer and efficiency in the overall cooling system.
• A high buildup of solids occurs within the sump or fill of the cooling tower.

Once the need for filtration is identified, the next step is to choose the filtration technology.

One filtration system has a high capacity media filter that combines crossflow dynamics with microsand media to achieve submicron filtration. This technology offers higher filtration rates than traditional media filters while capturing finer particulate.

Choosing Side-Stream Filtration for Cooling Towers

Separators, traditional media filters, automatic screen filters, cartridge and bag microfilters and high efficiency media filtration systems all offer effective ways of removing particles from the circulating water. Each of these filtration technologies offers differing benefits and associated costs. Understanding your application and the available technologies is necessary in choosing an appropriate side-stream filter for your cooling tower.

Factors that affect side-stream filtration selection include:

• The quality of the makeup water added to the system.
• The type of contaminants already in or being generated within the system.
• The operational duty or objective of the system.
• The current chemical treatment program.
• The cycles of concentration at which the system is operating.
• The maintenance and operating requirements.
• Budget constraints.

A closer look at each technology will highlight the benefits and disadvantages of each. Working with a filtration supplier can help narrow the choices as well.

Centrifugal Separators. Also called hydrocyclones, centrifugal separators generally are the least expensive in terms of equipment costs. The separator utilizes centrifugal forces, spinning the water to separate the heavier particulate from the water. They often are selected in applications with budgets constraints, but their ability to filter fine particles is limited.

Centrifugal separators can remove up to 90 percent of the particles that have a specific gravity equal to sand and are larger than 70 micron. Unfortunately, particles that are organic or lighter than water, or particles smaller than 70 microns, will not be removed. Centrifugal separators are cleaned by purging the particle-collection chamber or allowing a continuous flow of water from the collection chamber to a nearby drain.

Centrifugal separators often are used in conjunction with another filtration system; typically, one that is capable of removing smaller and lighter particles. The technology’s footprint and compact design makes it appealing for retrofit projects. The simplicity of the technology — not trapping particles that clog or damage its components — means it has few maintenance requirements.

Sand Filters. Traditional sand or media filters direct cooling water onto the surface of the media bed (one or multiple layers of media). As the cooling water passes through the media, particles are captured within the layers of the media bed. The water then flows from the media by gravity, passing into a drain at the bottom of the filter tank before discharging through an outlet pipe.

Such filters have the capacity to remove organic particulate greater than 10 microns. The system’s performance in removing fine particles highly depends on the media used within the filter bed. The media is cleaned by a backwashing cycle. It is important to note that backwashing heavy particles such as dirt, sand or calcium carbonate collected from the tower can lead to the loss of some media. Periodic replacement of the media is one of the maintenance requirements of sand media filtration systems.

These units require a relatively large footprint, which can present a design challenge in retrofit projects with limited space. The size of sand filters also can yield a second concern: water usage. If the side-stream filtration effectively removes particulate and the cooling water remains clean, with time, the backwash cycles will be reduced and water usage will be decreased. In systems where particulate and debris are continually introduced, these systems use more water when compared to the other side-stream filtration technologies.

The nature of microsand and crossflow technology has aided in the creation of a system that occupies a smaller footprint and weighs less than traditional sand media filters.

Automatic Screen Filters. In automatic screen filters, water passes through a weave-wire screen-filtering element. Solid particles are captured by the screen. The screen-filtering element acts as a barrier, removing organic and inorganic particulate down to 10 microns.

Automatic self-cleaning screen filters are seeing wider use in industrial applications. It often is the filter technology chosen for cooling tower filtration applications requiring removal of particles 10 microns and larger. This is due, in part, to the fact that during the self-cleaning process, automatic screen filters yield the least flushing discharge. Screen filters also provide uninterrupted filtration during the cleaning process, requiring only a single filter for continuous filtration.

Cartridges and Bag Filters. Where demanding or specialized fine filtration is required, microfiltration systems are available. These systems can achieve removal of both organic and inorganic particles down to 0.1 microns without requiring a backwash cycle. They also are one of the more flexible side-stream solutions, adjusting the level of micron filtration as needed.

While effective, the cartridges and bags require monitoring and manual replacement. These consumables can become costly with time. When frequent replacement is necessary, such systems require a prefilter inline before the bag or cartridge. Doing so will aid in removing the larger particulate, reducing the total suspended solids (TSS), and maximize the life of the microfiltration system.

High Efficient Media Filters. High efficiency media filters are similar to traditional sand filters in that sand is used as the filtration media. The size, shape and weight of the sand (microsand) used in high efficiency systems, however, allow for improved filtration efficiencies. The injectors also differ: They generate cross-flow patterns that sweep the media surface. This sweeping motion causes a portion of the water to flow parallel to the top layer of the media. This allows for high efficiency submicron filtration performance while preventing media fouling at the surface and water channeling through the media.

Contaminants trapped upon and within the microsand media (depth filtration) are removed using an automatic backwash cycle. The system’s backwash cycle requires less flow and a shorter duration than traditional sand or multimedia filters. The result is a technology that removes particles down to submicron levels at higher flow rates than traditional media filters. High efficiency media filters also require less water for backwash, and the filtration system occupies a smaller footprint. It can be used for horizontally stacked vessels, doubling filtration capacity within the same footprint.

Filtration Benefits

In many process cooling applications, it is important to understand that your water and the suspended solids it contains are adversely influencing the overall system. For such applications, side-stream filtration can yield a high return on investment for facility and maintenance managers.

While the benefits listed below may not be applicable for all applications and environments, these are some of the most common.

When fine particles are filtered through the system, the chemical treatment program will yield improved results.

A cleaner system requires fewer resources to maintain its integrity, keeping the overall system performing as intended. Potentially, this offers decreased downtime and increased uptime.

The removal of abrasive suspended solids aids in reducing the potential for erosion and corrosion.

Over time, users of side-stream filtration systems may experience reduced maintenance costs and chemical consumption. PC

Sources

1. Results based on multiple laser particle distribution tests completed in-house.

2. U.S. Department of Energy (2011). Cooling Towers: Understanding Key Components of Cooling Towers and How to Improve Water Efficiency. DOES/PNNL-SA75820.