High-efficiency centrifugal sand filters can provide cleaner cooling water, maximizing quality while minimizing process interruptions and maintenance costs.
Process cooling operations often demand an uninterrupted stream of filtered water. If the water supply is unreliable, contaminated or maintenance-intensive, processes suffer. Polluting particles can contribute to equipment corrosion, spur the growth of harmful bacteria, and cause system fouling and a reduction in system efficiency. These problems invariably translate into diminished product quality, process interruptions and exorbitant maintenance costs.
“Since no two water sources are the same, and all processes differ to varying degrees, those tasked with designing or modifying water-treatment systems can only be successful if they balance the requirements of the system that will use the water against the physical and chemical analysis of the water to be treated,” advises Phil D’Angelo, vice president of technical development for JoDan Technologies Ltd., a consulting firm based in Glen Mills, Pa., that works with industrial and utility clients to resolve water-related problems. “There are a lot of conditions and requirements to consider: the turbidity of the water, hardness, system pressures, process purity requirements, volume demands, filter backwashes and system maintenance. Certainly, in a process-filtration system, minimizing downtime for maintenance is a major consideration.”
Many process water systems require pretreatment of the water taken from surface or ground water sources. Post-filtration process water often is used in combination with other water-treatment technologies such as softeners, demineralizers or membranes in a continuous flow mode, feeding process equipment. For many of these applications, JoDan Technologies considers centrifugal sand filtration to be a valuable technology. A centrifugal filtration system uses a combination of in-situ fine-sand centrifugal separation combined with downflow sand filtration, which ensures greater filtration efficiency than traditional downflow sand filters.
Sand-Based Filtration Solutions
Sand filters often are deployed in cooling towers and other process cooling applications, mainly because they tend to be more economical than other filtering systems such as membranous solutions or systems using reverse osmosis. However, conventional sand filters have drawbacks. As static systems, they absorb solids and gradually become impregnated by them, requiring a sand filter media change on a roughly biannual basis. The filtering medium typically is a fairly dense grain to ensure that the sand will not clog the system. The dense-grain sand filters are sometimes less effective at trapping extremely small-size particles.
Centrifugal sand filters force suspended solids to accumulate near the inside walls of the tank and remove them with an automatic backwash cycle (figure 1). This design makes it possible to use a much finer grain of sand to remove small particles (down to 0.45 µm) with filtration up to 20 gal/min/ft2 without the risk of clogging the system. Figure 2 compares the operation of a conventional sand filter with a centrifugal system.
“With the centrifugal technology’s turbulence strategy, the work of chemical dispersing agents is enhanced, inhibiting the elements that contribute to corrosion and bacteria growth,” says Martine Mènard, marketing manager for Sonitec Inc., Holyoke, Mass. Because the system effectively traps particles, the amount of chemistry required (especially bacteria-killing biocides) can be reduced by as much as 50 percent.
Energy efficiency improves along with production quality. For example, temperature is a critical factor in robotic auto-welding. Cooling water loops are used to provide cooling to the welding tips. A variation of just a few degrees, often caused by fouling, can be enough to increase the frequency of welding tip replacement and cause possible downtime. A sand filter utilizing centrifugal separation and automatic backwash can provide a consistent reduction in total suspended solids (TSS), thereby reducing fouling due to the suspended iron typically found in welder water systems.
Case In Point
One example of the benefits of centrifugal water filtration for process applications can be found in a recent JoDan project at an electric power generation facility located in the northeastern United States. A relatively small plant that generates 100 MWe from landfill (methane) gas, the plant provides approximately $100,000 worth of electricity per hour during summer peak periods and therefore can afford little downtime.
The power plant uses two 800 psi boilers that run on softened water from large-pack-bed softeners, which require clean influent water. Previously, the plant had used a large clarifier to which ferric chloride, sodium hypochlorite and caustic were added to flocculate the suspended materials, control biological contaminants, and add some alkalinity for boiler water chemistry control. The clarified water then passed through existing pack-bed softeners at a rate of approximately 250 gal/min.
“The system was not only supplying water for producing electricity, it was also supplying process steam to a local steel mill,” D’Angelo explains. “The steel mill dropped its requirement for steam, so the flow rates dropped from 250 gal/min to approximately 50 gal/min total. Consequently, the system they had in place was hydraulically too large for the flow requirements.”
JoDan designed a filtration and softener package that would use the utility’s existing pack-bed softeners in an intermittent process mode. However, instead of using the clarifiers in front of the softeners, a centrifugal sand filter supplied by Sonitec Inc. was suggested.
“We proposed a 150 gal/min centrifugal sand filter composed of three vessels, used in processing mode,” says D’Angelo. “Although the vessels only needed to be 30" in diameter, because of our experience with water-treatment equipment, we decided to be on the conservative side and made them 36" in diameter. This approach increased the available filtration surface area, which optimized the linear velocity [in gal/min/ft2] and maximized particle loading to a differential pressure end point, which is clearly the correct approach for any filter design.”
Following installation of the new water-treatment and filtration system, river water now comes into the plant based on the soft-water storage tank level. The river water is pretreated with chemicals before going into the centrifugal filtration system. The filtered water then flows into a 72,000-gal filtered water storage reservoir. Before entering the boiler, the water passes through a 1 µm absolute pleated filter as well as redox media from KDF Fluid Treatment Inc., Three Waters, Mich., to remove chlorine. From there, it flows into the packed-bed water softeners and, finally, into the soft-water storage tank at 150 gal/min. When the soft-water storage tank is full, the system shuts down until the water level is reduced to two-thirds, and the process starts over again.
“When the filtration system reaches about a 9 to 10-psi differential, each of the three filtration vessels automatically backwashes in sequence,” says D’Angelo. “One of the benefits is that even while a given filter vessel is backwashing, two other filter vessels are on-line. Because they are sized at 36" diameter, they can easily handle the 150 gal/min demand. As a result, the power plant has no downtime, which is of vital importance in any process application.”
D’Angelo adds that the filter vessels are backwashed using chlorinated soft water, which controls biologicals. “When you backwash a sand filter, you backwash from the bottom up,” he says “It is advantageous to add chlorine or bromine upflow through the filter sand bed in order to avoid any buildup of biological materials. By doing it this way, we’re keeping that sand clean all of the time and reducing maintenance requirements and costs.”
The new utility plant water treatment system has been running consistently since May 2006. “It has performed extremely well since startup,” D’Angelo says. The process and the results would be similar when pulling cooling tower makeup water directly from a river, De’Angelo says.
Other plants also are reaping the benefits of centrifugal sand filtration. For example, a Kansas City soft-drink bottler uses a centrifugal filtration system because it is much more compact than the multimedia filtration system that was used previously. “Because the centrifugal technology operates much more efficiently, including a much higher velocity, it takes up perhaps 70 percent less space,” says a company spokesperson. “While our previous filter operated at 4 to 5 gal/min/ft2 of surface area, the centrifugal sand filter operates at 15 to 17 gal/min/ft2. Plus, it uses 70 percent less water for backwashing than our previous multimedia filter system.”
The BOC Group Inc., an industrial gas manufacturer in Cayce, S.C., uses a centrifugal sand filter as part of its equipment cooling apparatus. “The system is closed-loop. It sends water to the cooling compressors we use in the plant via very small tubes. This is where the system earns its stripes. It removes small particles from the system that might otherwise clog the pipes and make heat transfer more difficult,” says Patrick Browder, an on-site technician. Before installing the centrifugal sand filter, BOC had to remove the coolers and send them off for cleaning and reinstallation. According to Browder, it took 10 days on average to get them back - an inconvenience exacting a hefty penalty in lost money and downtime.
For many process cooling applications, clean water is crucial. Centrifugal sand filters can achieve fine filtration at a high rate, using less water for backwashing compared to other sand filter designs. They also allow for a reduction in the amount of water-treatment chemicals used. With centrifugal sand filtration technology, companies can ensure clean, clear cooling water, along with an efficient, high-quality, low-maintenance process.