Cooling water fouling deposits

To increase equipment reliability and plant efficiency, corrosion inhibitors are used in boiler and cooling water programs to control fouling and deposition on critical heat-transfer surfaces. In cooling systems, corrosion inhibition is commonly achieved through the use of passivators, which encourage the formation of a protective metal oxide film on the metal surface ( 1). ... 

The cooling tower, which is an efficient air scrubber can easily become a catchall for contaminants resulting from the location of the tower or from the industrial process. In arid areas, ingress of sand contributes to fouling, which reduces efficiency and contributes to biofilm and under-deposit corrosion. In coastal areas, sand laden with chlorides can cause corrosion of stainless steel components and impair chemical corrosion inhibitor performance. Heavy industries, such as steel or aluminum manufacture, produce severely contaminated cooling water resulting from direct contact with metal slags and lubricants. 

The practical heat-transfer coefficient is the sum of all the factors that contribute to reduce heat transfer, such as flow rate, cocurrent or countercurrent, type of metal, stagnant fluid film, and any fouling from scale, biofilm, or other deposits. The practical heat-transfer coefficient ((/practical) is, in reality, the thermal conductance of the heat exchanger. The higher the value, the more easily heat is transferred from the process fluid to the cooling water. Thermal conductance is the reciprocal of resistance (/ ), to heat flow ... 

Similarly, any fouling will reduce efficiency, and 5 mil (5 thousandths of an inch) of a scale or deposit may well reduce efficiency by 20 to 30%. A small amount of scale can greatly increase refrigeration compressor horsepower or cooling water pumping requirements. 

NOTE In this chapter there is discussion concerning some individual aspects of corrosion, fouling, and deposition, but every attempt is made to link them together into a coherent whole. It is also incumbent upon the reader and water treatment practitioner to consider each specific problem or aspect of water management in the light of the complete, dynamic, cooling water system. 

Corrosion, fouling, and deposition processes of some degree most probably will have been steadily taking place for some time prior to the evaluation being made. If an index or model is to be used, the makeup water source should always be the starting point and cycled-up cooling water composition calculated from this point (despite the problems of accurately assessing the cycled-up water pH). 

There has clearly been a gradual decline in the effectiveness of the cooling water treatment program and the cooling system is badly fouled, both from algal/bacterial slimes and from mineral deposits. The process contamination and variability of makeup water quality has exacerbated the situation. 

All kinds of interactions and combinations are possible. In cooling water, for instance, the deposits are likely to include micro-organisms, particles, scale, and corrosion products. In flue gas systems, the fouling may be because of particle deposition, chemical reaction, and corrosion. In crude oil processing, the deposits may be products of reaction, combined with particle deposition and corrosion. 

Maximum outlet temperatures for cooling water usually are dictated by the chemistry of the cooling water. Most cooling water contains chlorides and carbonates consequently temperatures at the heat transfer surfaces must not exceed certain values in order to minimize formation of deposits or scale, which reduces heat transfer and leads to excessive corrosion. In addition, velocity restrictions must be imposed and observed to prevent corrosion and fouling as a result of sedimentation and poor venting. Stagnant conditions on the water side must always be avoided. In some plants, water consumption is dictated by thermal pollution restrictions. 

In general the discussion will concern aqueous systems, largely because it is within these systems that much work has been carried out to develop suitable additives, principally for cooling water application. Because aqueous systems are common to many industries, and the potential need for suitable fouling mitigation is widespread, the financial incentive has been present to develop suitable and effective additives. The principles may be applied to other systems but it is unlikely that if the process stream is unique, a suitable additive will have been developed and tested. The development of a chemical that will restrict or prevent, a particular fouling problem may be expensive and the development cost unjustified. Under these circumstances either the deposition problem has to be tolerated or a different technique applied (see Chapters 15 and 16). 

Veale, M.A., 1984, Control of deposits in cooling water systems, in Fouling and Heat Exchanger Efficiency. Continuing Education Course. Instn. Chem. Engrs., University of Leeds. 

There are a number of features in the design of cooling water plant that need attention in order to reduce or even eliminate the incidence of fouling. The extent of the individual mechanisms of particulate deposition, corrosion, scale formation and biofouling, will depend on the quality of the cooling water, the temperature and the material of construction. 

The choice of material of construction will influence the extent of possible corrosion and may affect the retention of other deposits, for instance corrosion resistant titanium tubes have been known to foul more rapidly than some stainless steel alloys under similar conditions. It is unlikely in large cooling water systems, e.g. power station condensers, that treatment of the heat exchange surfaces will be considered as a feature for the reduction of fouling, due to the relatively high costs involved including initial cost and maintenance. 

The rate of fouling deposits inside the cooling water tubes can increase exponentially as water velocity decreases. Water velocity should be at least... 

The pH of cooling water naturally increases due to evaporation. Sulfuric acid is added as required to control the pH at 6.5-7.0. An excursion of high pH (9-10) will cause a rapid increase in inorganic fouling deposits in the exchanger tubes. 

What is happening in cooling water In correlation with cycles of concentration, dissolved solids concentrate, which lead to corrosion and deposition problems. Deposition is based on two mechanisms - scaling and fouling. 

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