Cooling water can be provided either by a once-through or a closed-loop system. Once iihrough systems normally take fresh and cool water from a river, lake or seawater and pump it directly to heat exchangers. This stream is just filtered on trash racks and screens, before circulating water pumps suction side, to remove suspended bulky materials and regularly, no additional treatment for suspended solids is required. Eventually, if the presence of organic matter and microorganisms is significant, that can be mitigated by shock chlorination. This kind of cooling utility system is subject to intense fouling, erosion and corrosion, because it is not treated, and special materials, like copper alloys, ferritic stainless steels and titanium, can be used for heat exchangers. Environmental risks and regulation are making this practice almost outdated and forbidden in most parts of the world.
Closediioop water cooling requires a much more complex treatment. Water evaporation is demanded to recover low temperature, so hot cooling water has to be recirculated across towers, producing high concentrations of dissolved solids. When these materials reach solubility limits and concentration levels are elevated, scaling, fouling and corrosion processes, over cooling circuit components, increase substantially.
A specific internal water treatment is required. Chemical products must be injected to control higher solids concentrations effects and biological growth. Blowdown is necessary to control the concentration of dissolved solids within prescribed permitted limits, to control suspended solids in circulating water and minimize sediments accumulation in the cooling tower basin.
Treatment characteristics can be classified as cleaning or scaling. In cleaning treatments, general parameters like pH are maintained at a level in which heat exchanging surfaces are kept clean, in a kind of constant mild pickling. On the other hand, scaling treatment is less aggressive to metal surfaces, allowing some rusting and fouling to form. Control limits depend on make -up water quality, which is basically service or industrial water, and internal treatment option.
Concerning energy efficiency, the choice of treatment can bring about different outcomes. In the first option, better heat transfer coefficients are achieved, due to surface smoothness and reduced fouling, but it may imply faster equipment degradation and eventual sudden stoppages occasioned by leaks. Selection of thicker piping for this condition may reduce overall heat transfer coefficients and operational energy costs, while increasing initial capital investment. The scaling option can proportionate longer life to pieces of equipment, but reduce heat transfer. Fouling allowance can determine the need of additional spare heating transfer area in exchangers, eventual maintenance cleaning might be needed. Balancing these options depends on the financial availability for investments versus operational and ongoing maintenance costs.
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