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Antifouling strategies and corrosion control in cooling circuits
The settlement of macrofouling in cooling canals and the formation of biofilm on surfaces of heat exchangers are the two main causes that dramatically reduce the functionality of the cooling circuits, especially in the marine environment. A power plant, for instance, can loose more than 5% of its efficiency due to biological films and, at worst, the power units must be shut down if macrofouling plugs the tubes of condensers.The efficiency and availability of a power plant depend to a great extent on the integrity and cleanness of the condenser. These are reasons why specific solutions have been adopted, as continuous mechanical cleaning by foam balls, corrosion-resistant alloys, such as titanium and stainless steel, etc. The mechanical cleaning of the systems and a water filtration (by rotative filters and mussle filters) are the most commonly used processes. They includes the continuous cleaning of the pipes of condensers by foam balls or brushes, manual cleaning, use of trash rakes, filters with meshes of different widths. Physical methods can be regularly used for the anti-fouling treatment of industrial systems, requires an appropriate design of the cooling systems, consisting in:– maintaining of velocities high enough to avoid the fixation of organic organisms (v >2 m/s);– temperature increase raising the temperature of the cooling water (40 C° for few hours). Non-toxic coatings and paints, which reduce the fixation of the organisms, reinforce the velocity effect and facilitate cleaning can be used, but they are not generally applicable under water and must be renewed every 4 to 5 years. The main biocide used for industrial cooling circuits is still chlorine. However the needs of such products, a growing attention is paid concerning public health and on possible environmental damages deriving from the use of biocides treatments and by-products. It is generally produced on site by sea water electrolyses. This process avoids the risk involving the transport of NaOCl by truck. The chlorination can be made in low doses, on continuous or discontinuous depending on many factors such as meteclimatic characteristics of the site, cooling circuit design and biofouling typology. The concentration in free chlorine in the condenser reaches normally 0.2- 1 mg/L, but in the discharge it is generally lower than 0.1 mg/L, being 0.2 mg/L the regular limit set by local regulations. In some areas, the use of chlorine was banded (in Italy and in The Netherland, for instance). Alternative biocide product can be also applied as, ClO2 and ozone. In this context, innovative treatments and monitoring tools can in several ways be a valid help to plant operators. A review of new solutions and some specific experiences related to their industrial application are presented.
31 Dicembre 2012
Risparmio di energia nei settori: civile, industria e servizi (EFFICIENZA 2012)