Clean Cooling Water with Sidestream Sand Filters

In any recirculated system, clean cooling water is of paramount importance and, in power stations with large cooling towers and flow rates, it's essential.

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By Tony Stocker

The purpose of sidestream filtration of a recirculated cooling system is to:

• Reduce the amount of fouling in heat exchangers and other plant and, therefore, reduce running costs.

• Decrease downtime of the units.

• Reduce chemical treatment cost. Increase cycles of concentration in the circulated water system.

• Increase cycles of concentration in the circulated water system.

And, in power stations with large cooling towers and flow rates, clean water is essential.

Filtration accomplishes this task by removing organic matter, airborne dust, sand, silt, scale deposits and turbidity from the circulating water. The sidestream being filtered is only a small portion (usually about 3-5%) of the circulated water from a cooling tower. An important point is that the goal of sidestream filtration is to improve the circulated water so that satisfactory heat transfer may be achieved over longer run lengths on the process units. No attempt is made to make drinking water or achieve 100% removal of

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A single bed filter.

foreign matter from the water. The best proof offered for improvement of heat transfer in circulated water systems is the increase in the amount of time between turnarounds on process units. Without a sidestream filter a unit is sometimes down every 90 days or so in order to clean heat exchangers. With a sidestream filter, however, the average is better than three years between turnarounds.

The AGF Autonomous Valveless Gravity Sand Filter from Superior Filtration Ltd. treats the entire cooling water system and can remove up to 98% of suspended solids ingested by the cooling tower, and thus solving the major problem encountered in open circuit cooling systems. This fine material accumulates in the system and can only be partially removed by blowdown. Efficient sidestream filtration is the only way to keep the water clean. The filtration rate is set to ensure that solids removal equals solids ingress with resultant water quality normally at around 5 mg/Lsuspended solids.

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Typical installation of AGF sand filter on sidestream filtration of power station cooling water circuit.

Being one of the simplest sand filters on the market, the AGF Sand Filter is ideal for cooling water sidestream filtration and operation in zoned hazardous areas. Once installed, the system has virtually no running costs. It's completely automatic and totally self-contained and operates with no instrumentation or backwash pumps. The backwash function of the sand filter is self-initiated and only backwashes itself when necessary, using a pre-stored volume of already filtered water, on the loss of head principle and, therefore, the backwash occurs precisely when it's needed at any time. These filters are renowned for working for long periods without needing any operation or maintenance whatsoever. Filtration is also excellent since conservative ratings are used together with a uniform backwash rate and volume ensuring that the sand bed being backwashed is always maintained in optimum condition.

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A double bed filter configuration

Cooling systems serviced by these filters normally have suspended solids concentrations in the order of 5 mg/L. The water is clear and clean. Even systems where an occasional ingress of hydrocarbons has occurred continue to work efficiently due to a unique construction layout. Worldwide, many installations already exist of the AGF Sand Filter successfully used on applications for sidestream filtration of cooling water systems.

Author's Note: Tony Stocker is with Superior Filtration Ltd., based in London. It also offers the Kleerflo automatic screen filter for once through cooling systems, both fresh and seawater. Contact: +44-20-8989-1171, or

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