Pumps and Systems, August 2009

Cooling towers, an effective and common means for industrial process heat extraction, are not a closed-loop system. As a result, air and water-borne particulate sucked in the system can eventually foul valuable downstream equipment such as spray nozzles, welding operations, chillers, heat exchangers and small bore piping in cooling circuits.

In fact, many plants must shut down cooling water systems often to clean the cooling tower and downstream equipment, which is time consuming and expensive.

Although some basic solutions to this problem are available, including rudimentary water strainers, side stream filtration (which involves filtering a small percentage of the total water volume) and point-of-use filters for specific critical equipment, using full stream filtration to protect all downstream equipment has emerged as an alternative for cooling tower water systems.

It's a Dirty Job

"A cooling tower will suck in a lot of air from the environment," says John Flaherty, president of Delta Cooling Towers, Inc. (Rockaway, N.J.). "Any foreign material or elements in the air can easily get sucked into the tower, and a portion of that will either get dissolved into the water or remain suspended in the water."

Water flowing through cooling towers can be contaminated from many sources, including ambient air, makeup water sources, corrosion and rusting of metal cooling tower "skin," and residue picked up from processes. Airborne contaminants include leaves, paper, debris and other pollution.

As a result, air and water-borne particulate sucked into the cooling tower system can eventually foul valuable downstream equipment such as spray nozzles, welding operations, chillers, heat exchangers and small bore piping in cooling circuits.

"A cooling tower works through evaporation, so you are continuously making up whatever cooling system water has been evaporated," explains Flaherty. "Many plants use city water, which is comparatively clean. But if you are using lake, river or runoff water, you will have to address problems of suspended and dissolved matter that can damage downstream equipment or make it less efficient."

More efficient technology for cooling tower water filtration dates back to the 1960s when the multi-element, automatic, self-cleaning strainer was introduced. This design provides an alternative to sand filters, centrifugal separators and basket type strainers. Unlike those designs, which have limitations in particle size filtration and can require frequent maintenance, the multi-element, self-cleaning strainer can provide continuous removal of suspended solids down to 25 microns.

While multi-element strainers may be used in a side-stream application, full stream configuration is normally recommended since it provides complete filtration of water. Unlike the side-stream filters sometimes used with cooling towers, the full-stream use of a multi-element strainer enables cooling systems to remain online continuously because it backwashes elements in sequence, one at a time. Therefore, all the remaining elements are available for continuous straining. With such full flow straining capabilities, plant equipment is provided positive protection at all times.

Conversely, due to limitations in straining area, other strainer designs and point of use filters can become clogged quickly. When that occurs, cleaning, media replacement or backwashing is necessary, which adversely affects productivity and maintenance costs. If maintenance is not performed in a timely manner, spray nozzles, heat exchangers and other production equipment can become fouled and less effective or even damaged.

The full-stream use of a multi-element strainer enables cooling systems to remain online continuously because it backwashes elements in sequence, one at a time. Therefore, all the remaining elements are available for continuous straining.

Another significant feature of the multi-element design is in the design of the backwash mechanism. With automatic basket strainers, for example, the backwash mechanism comes in close or direct contact with the straining media. This can be problematic, as large, suspended solids often encountered in raw water can become lodged between the straining media and the backwash arm. The result is damage and/or rupture of the straining media that can compromise downstream equipment.

The multi-element design uses a tube sheet that separates the straining media from the backwash mechanism. This prevents the backwash mechanism from coming into direct contact with the media and eliminates the possibility of damage to the elements resulting in reduced maintenance and downtime.

When considering strainer technology for a cooling tower's water system, continuous, full stream filtration provides protection for the cooling tower as well as all downstream equipment and piping. The automatic, self-cleaning, multi-element strainer is one possible solution for filtration.

Tags: August 2009 Issue , Filters

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