According to the U.S. Office of Energy Efficiency & Renewable Energy, installing a sidestream filtration system to cleanse cooling tower water will help maintain water efficiency across facilities. Although this applies to many types of applications and throughout industries, systems that are exposed to a large amount of airborne particulates are at a greater risk for compromised efficiency and system failure.
Cooling towers provide process cooling throughout a broad range of applications in the industrial sector, including manufacturing and power generation. They also are an essential aspect of many refrigeration systems, providing temperature regulation for comfort in places of work, residences and service centers. Regardless of application, cooling towers are located at the point in a cooling system where unwanted heat is released into the atmosphere through evaporation. Keeping the heat transfer surfaces of a cooling tower system clean is widely recognized as the best way to ensure efficient operation.
Because of the operating environment of cooling towers, and because of the nature of their technology, cooling towers are vulnerable to the elements. They are susceptible to particulates that are introduced by the wind. As air quality and wind conditions change, cooling towers undergo wide variations in particulate loading. Operation can be significantly affected by the quality of the water making up the system.
Atmospheric particulate matter can originate from dust storms, living vegetation, fires and industrial processes, which may all, at various times, contribute to patterns of particle loading in cooling towers. The mineral dusts of materials such as airborne soils/sand, ash and cement — comprised of oxides and carbonates — can contribute to higher particle loading in cooling tower water.
Cooling Tower Filtration
It is commonly known that poor water quality, including high particle loading, can lead to common problems — corrosion, scaling, fouling and microbiological activity —within an open-recirculating cooling tower system. These problems are interdependent to the extent that prevention of one may help reduce the magnitude of the others.
|See the related web exclusive, "Water Analysis at Superior Industries," that shows the results of the laboratory tests run on Superior Industries cooling towers using cooling tower water filtration devices from Forsta.|
Cooling tower filtration systems pull water from the sump, filter out particulates and return the cleaner water to the tower. This allows the system to function more efficiently, requiring less additional makeup water and chemicals.
An effective filtration system lowers the particulate levels in the cooling water, which directly reduces fouling. Because microbiological organisms will feed on organic particulates, reduction of particulates also corresponds directly with a reduction in biological growth. It follows that filtration will prevent corrosion that occurs as a result of microbiological growth. It also helps prevent scaling, which occurs as a byproduct of fouling and corrosion. The fact of the matter is simple: filtration helps minimize all the risks associated with cooling tower operation. When designed properly, a filtration system will save on water, energy, time and money.
Superior Industries’ Cooling Tower Problem
Superior Industries International Inc. is an OEM supplier of cast aluminum road wheels for the automotive industry. Superior operates five major manufacturing facilities and employs approximately 4,000 people in the United States and Mexico. The facilities produce aluminum wheels for the vehicle platforms of automobile and light-truck manufacturers.
Superior’s Fayetteville plant in Arkansas had a cooling tower exposed to dusty atmospheric conditions — precisely the type of conditions that make cooling towers susceptible to the introduction of unwanted particulates. The company needed a low maintenance solution to remove and prevent further pipe-scale debris from the open cooling tower. Other goals included reducing machine downtime due to water issues and improving cooling ability.
To address these concerns, a filtration system had to be designed to handle to the highest influx of particulates without disrupting operation. The goal was to eliminate downtime and reduce maintenance while protecting the downstream casting machines. With plans to expand the total number and production capacity of the casting machines, it was more important than ever to ensure system protection and automation.
At Superior’s plant, the casting machines comprise a key component in the direct-chill casting process. With poor water quality leaving the cooling tower, the casting machines were vulnerable to cumulative buildup that steadily compromised their function. The machines had to be shut down for regular maintenance, and, in the worst case, could have potentially failed altogether.
The automated cleaning cycle, coupled with the ability to operate on system pressure alone, prompted Superior to select Forsta self-cleaning filters for its Fayetteville plant.
To help Superior meet its objectives, sizing the filtration system was important. With self-cleaning water filters, care must be taken to understand the levels of particle loading in the water source. The degree of filtration must be selected appropriately so the filter will function successfully in the operating environment. In other words, the screen must be selected in correspondence with the upper value of inlet particle loading. (Total suspended solids, or TSS, typically is measured in parts per million or ppm or milligrams per liter).
Superior’s Kyle Gunn, the capital improvement project manager, worked with Forsta engineers to define Superior’s operating environment and parameters. Together, they settled on a goal to remove pipe scale and debris down to 50 micron. Gunn explained, “Forsta self-cleaning water filters seemed like the ideal choice for cooling applications like ours. With aging pipes and cooling tower, we wanted to stay ahead of any buildup.”
Now at Fayetteville, water flows from the cooling tower, to a pump, through the Forsta self-cleaning filters and then to the casting machines. The plant uses a total of three Model B6-180 filters in parallel, with 50 micron screens, to filter the process and cooling water generated from their 5000-gpm cooling tower. A main header feeds three pipes. Each pipe has its own filter and feeds a different set of casting machines. Each of the plant’s filters flow 350 to 400 gpm of water at 125 to 155 psi at a temperature of 76°F (24°C). The filters perform a backwash sequence once every 12 hours, and they have a daily backwash volume of just 30 to 50 total gal.
Gunn said, “Now, we have less water problems in the casting machines, meaning more production and less downtime.”
A total of 18 casting machines operate at Fayetteville. According to David Hamm, onsite engineer at Superior, “Depending on the cycle at any one point, a different number of filters/amount of water will be flowing to the machines. The filters help keep the casting machines running flawlessly.”
Water Analysis: Before and After Filtration
U.S. Water Services Inc., St. Michael, Minn., provides service to Superior’s Fayetteville plant. U.S. Water’s onsite representative oversees the cooling towers and boiler water management programs, utilizing chemical, equipment, engineering and operational data to find and remove potential problems and improve the overall conditions of these systems.
U.S. Water notes that “…effective filtration helps with overall water quality, decreasing the likelihood of deposits throughout the system.” At the Fayetteville plant, U.S. Water personnel collected and analyzed water samples at both the inlet and outlet of the Forsta filters. The samples were sent to U.S. Water’s in-house analytical laboratory to assess the total suspended solids and particle size distribution. The results demonstrated the particle-removal efficiency of the filters. (For complete graphs and charts from the reports, see the “Water Analysis at Superior Industries” web exclusive on www.process-cooling.com.)
With the 50 micron screens installed at Superior, TSS downstream of the filters was reduced from 28.53 ppm to 7.35 ppm. For particles larger than 50 microns, particle reduction was 100 percent, from 17.66 ppm to 0 ppm after filtration.
Not only were 100 percent of particles larger than 50 microns removed, but the level of TSS in the 5 to 50 micron size range fell as well. In fact, in the 5 to 50 micron size, TSS levels fell by 25 to 50 percent. This is due to the filter-cake effect. The filter-cake effect simply describes the fact that degree of filtration becomes finer as a screen accumulates dirt. Or, put another way, pore size decreases as pressure differential increases. This explains the 25 percent reduction in particles at the 5 to 15 range, for example.
The positive onsite report from engineers at Superior, combined with the data collected by U.S. Water’s analytical department, shows that the Forsta self-cleaning water filters are suited for cooling water filtration applications.
Even a small amount of particles larger than 50 micron in cooling tower water can drastically reduce the efficiency of a cooling system. Superior Industries found automatic filtration to be the most effective and sustainable way of removing the harm caused by this unwanted dirt. If your cooling tower water is loaded with airborne particulates, it may benefit from full-flow or sidestream filtration as well.