Manufacturers need a steady supply of clean, cool water to keep their plants operating. Unfortunately, waterborne particles often get in the way, plugging heat exchangers, clogging nozzles, reducing pipe capacity and shutting down production lines.
“Cooling towers make good air washers,” says Lee Munday, senior mechanical engineer at one of Milliken & Co.’s chemical plants in Blacksburg, S.C. “They’ll pull all sorts of stuff out of the air.”
Particles can enter the cooling equipment at any point in an open- or closed-loop system from a number of different sources, including the surrounding air, the fresh water source, pipe corrosion or the process materials themselves. These particles cause two types of problems. One is the erosion and plugging caused by the particles. But even more damaging can be the combined effect of those particles with the dissolved minerals in the cooling water. For example, calcium and magnesium dissolved in the water act as chemical agents to bond particles to the metal surfaces in the cooling system. These deposits are particularly a problem on hot surfaces, such as molds and heat exchangers, but they also affect pipes, tubing, sensors and other components. Chemical analyses have shown that the calcium and magnesium make up less than 2 percent of deposits; the rest is composed of airborne particles, rust, sand, biological organisms and other contaminants.
These deposits build up quickly, reducing the effective pipe size (figure 1). As the scale continues to grow, it restricts the pipe size even further, and higher pressures are required to maintain the water flow. The deposits also effectively increase the pipe wall thickness, lowering the rate of heat transfer in heat exchangers and molds.
Removing the dissolved minerals will eliminate the fouling problems and will keep the particles from becoming cemented in place; however, pure water is highly corrosive and will leach minerals out of the pipes. A more cost-effective option is to filter out the particles that comprise 98 percent of the pipe deposits.
Companies increasingly are turning to self-cleaning filters to improve the quality of their cooling water. Such filters incorporate a self-cleaning mechanism that allows an uninterrupted flow of filtered water even while the cleaning takes place. The procedure uses a fraction of the water that normal backflushing requires.
In one self-cleaning filter design, the dirty water flows in around the outside of a coarse filter, which removes the larger particles (figure 2). This prefiltered water then flows to the other end of the filter housing, where it passes from the inside to the outside of the final filter and then through the outlet. Over time, as the filter removes particles from the water, those particles clog the screen, reducing water flow and water pressure. At the filter outlet is an adjustable pressure differential switch, typically set at 5 to 7 lb. When that threshold is crossed, the switch opens the small flush valve that initiates the cleaning procedure.
Rather than taking the full water flow to initiate a complete backwash, self-cleaning filters contain an array of small cleaning nozzles positioned around a central shaft. Opening the flush valve lowers the water pressure within the cleaning unit, and the nozzles vacuum the dirt from the inside surface of the filter screen. A hydraulic motor and piston cause the nozzles to rotate and move axially to cover the entire screen surface in 5 to 10 sec. At that point the flush valve closes, and the cleaning mechanism returns to its starting position. Other than a small control voltage for the differential pressure sensor and to actuate the flush valve, all motions involved in cleaning the filter are performed using the existing water pressure.
End-to-End FiltrationSelf-cleaning filters can be used alone or in conjunction with other types of filters to eliminate particulates at different points in the cooling system.
For example, Jones Dairy Farm, a family-owned meat products company in Fort Atkinson, Wis., uses self-cleaning filters on its inlet water. The company has been selling sausage, ham and bacon since the late 1800s. It uses 200,000 gal of well water daily from its municipal utility.
“The city water contains variable amounts of silt, sand and dirt,” says David Mikolainis of Fluid Handling Inc., the Milwaukee firm that specified the filters. “Being a food processing facility, they wanted to keep that out of the plant.”
The plant was already using cartridge filters, but these required frequent changes. The original plan was to supplement these filters with a centrifugal separator, but this solution wouldn’t have met Jones Dairy’s stringent requirements.
“They wanted it to be an automatic operation and they wanted to have a 5-micron retention,” Mikolainis explains. “They did not want to have to change bags or cartridges, and other screen and disc technologies were not capable of filtering down to 5 micron.”
Mikolainis specified four Tekleen ABW4-LP automatic self-cleaning filters with 5-micron screens to run in parallel and provide an 800 gal/min flow. These were installed as sediment prefilters to the existing cartridge filters in order to cut down on maintenance costs.
“Previously we had to replace the cartridge filters quite frequently,” says plant engineer David Mroch. “But with the Tekleens on the incoming water line, we don’t have to do this as often.”
The Plastek Group - a privately held plastic injection molding firm with three plants in Erie, Pa., and others in Sao Paolo, Brazil; Manchester, England; and Caracas, Venezuela - had enough clean water from the local utility. The company’s problem was debris getting into the water at the cooling tower. The May-to-June cottonwood season was a particular problem for the Erie plants, with shredded fibers from the seedlings making it downstream. But there was also the usual array of organic and inorganic material year round.
“If a bird got in our cooling tower, there was a chance that some of that bird would end up going through our pumps and possibly wind up clogging equipment on our shop floor,” says Ray O’Donnell, Plastek’s corporate facilities manager.
The company had tried sand and gravel filters, but since these were sidestream rather than full flow, organic material was bypassing the filters and making its way into the presses. To get complete water filtration, the company replaced the sand filters with three Tekleen ABW 10-P filters (1,750 gal/min) on 10" lines and an ABW 12-LP filter on a 12" 3500 gal/min line coming from the cooling towers. All the filters have 200-micron screens. O’Donnell says that these filters passed the cottonwood test. The plant wraps a prefilter media belt around the cooling towers to catch most of the seedlings, but the self-cleaning filters catch the rest.
“We have always taken pride in having good water quality,” says O’Donnell. “This was the next generation of filtration, and it has resulted in decreasing maintenance costs and fewer preventive maintenance issues regarding water filtration.”