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The ubiquitous hyberbolic cooling tower is one of the most evocative sights when the theme of ‘industry’ is discussed. Its plume of steam – assumed to be a more toxic effluent – is a symbol to many of the environmental impact feared from industrial activity. The plume is, of course, a sign of heat going to waste, so it should be a slowly diminishing sight.

Cooling systems for large plants (factories, power stations, etc.) are of two types: once-through and recirculating. The once-through systems are typified by the large power stations situated by the sea or on large rivers, which take in their cooling
water, treat it as necessary, use it and discharge it back whence it came. The circulating systems have a stock of water which is used for cooling, then is itself cooled to dispose of the remaining heat (in, for example, the above-mentioned cooling
towers), and is then returned to stock.

As far as the filtration business is concerned, the cooling water must pass through heat exchangers of some kind, whose basic performance is dictated by clean heat transfer coefficients, but whose actual performance is reduced as deposits of one
kind or another form on the heat exchange surfaces (some of which are in the form of narrow channels, or are otherwise inaccessible). It is the job of the filter system, which must be installed ahead of the heat exchangers, to remove the suspended
material – animal, vegetable or mineral – from the cool water.

Clarification of cooling water is necessary whichever of the two main types of system is in use. Once-through cooling has to deal with whatever quality its source has at the time of abstraction, so has a rather more expansive treatment process,
certainly as far as the intake end is concerned. Recirculating systems on the other hand should have only a small proportion of make-up water to deal with (to make up for the water lost by evaporation), but also have to remove suspended solids
picked up from the cooling flows themselves.

The degree of filtration required will depend upon the quantity of water to be treated, and the quality of treated water necessary to maintain the heat transfer surfaces as clean as possible. A strainer of some kind will be needed as the intake
filter, ahead of a finer filter such as a deep bed system or a multi-bag filter. It may even be necessary to include ultrafiltration as the final step if there is much organic or colloidal matter in the intake water.

The warm water in the cooling tower pond and the accumulated organic material provide perfect growing conditions for bacteria. Outbreaks of Legionnaire’s disease associated with cooling water systems are a cause for serious concern. The conditions under which the bacillus Legionella pneumophila can develop in water systems and be transmitted into the environment are varied and complex. A cooling tower of average size can collect between 2 and 3kg of solid matter every day, including dust, engine exhaust, pollen and insects. Together, these create a biofilm within the system, particularly in the low flow zones of the cooling tower pond, which can act as a food source for bacteria. It is estimated that the cost to UK industry and commerce of this water fouling is in the region of US$1.5 billions per year.
Much work is being undertaken in evaluating the alternatives to the commonly used biocides for the control of biofouling in cooling systems. Alternatives such as bromination, ozonation, ultraviolet treatment and pasteurization are all being
tested. Biocidal efficiency depends, of course, on the quality of the circulating water. Build-up of inorganic and organic debris within the circulating water interferes with biocidal activity and filtration has long been recognized as a way of maintaining biocidal efficiency. Modern filter media and membrane systems permit the removal of bacteria, and so these are becoming an important component of the purification process.

Not all cooling systems are huge, and some are well catered for by use of selfcleaning units such as that shown in Figure 4.8. This is a typical example of an effective automatic filter, which shows three filter elements (here called pods) in use

in parallel. Under normal filtering operation, all filter pods are in use, and the control system monitors the pressure drop between the upper and lower headers. When the pressure drop reaches a preset level, the collector bar is rotated by the positioning motor until a shoe is aligned over the first pod. While the remaining pods continue the filtering operation, the backwash valves are opened to allow a portion of the filtered water to backflush the first pod. In this way, the filter elements are opened and the contaminants are flushed away to waste, usually a backwash collection tank. Each pod is cleaned in this way in turn, and finally the backwash valves are closed and the filter reverts to full filtering mode.

The elements using this type of filtration system are formed from a continuous high grade stainless steel wedge-wire spiral coil. Raised ridges on the upper surface of the coil ensure a precise filtration gap to the required separation rating. Standard ratings are usually 12, 25 and 120m. During self-cleaning, liquid flow is reversed and the compression of the spring is relaxed a trifle. The gap between the turns of the coil thus increases, allowing the contaminants that become compacted into the wedgewire structure to be removed during backwash. An automatic bypass valve is normally installed around the filter. This valve is linked to the control system and opens automatically should the filter become blocked by objects that cannot be backwashed out.

Based on the same wedge-wire screen element, the in-line filter shown in Figure 4.9 is self-cleaning and suitable for filtering industrial water as well as small cooling water

installations. Separation to the required level occurs as the water passes through to the interior of a stainless steel wedge-wire screen. As a build-up of contaminant occurs on the screen a pressure difference is registered and increases to a preset limit, and then initiates the backwashing of one screen, whilst the remainder of the elements continue to clean online. The screens are cleaned individually and in sequence.

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