Water used for cooling operations can come from a variety of sources including surface water (streams, lakes, or impoundments), groundwater, seawater, estuarine water, or reclaimed water. In selecting a water source, the major requirement is that an adequate supply of water is available to accomplish cooling. The water quality of the source dictates the need for pre-treatment and the feasibility of recycling the water with or without sidestream treatment. Water quality also influences the economic feasibility of using the water source for boiler feedwater or closed recirculating systems (Baron et al., 2000: Wijesingheetal., 1996).
Because of the wide range of water sources that can be employed for cooling operations, water quality guidelines have been developed (Crook et al., 1994). A summary of these guidelines is given in Table 3.2 for cooling and boiler water. Water quality guidelines are based on prevention or minimisation of corrosion, scale or fouling. Boiler water quality requirements are more stringent than those for cooling water due to the relatively higher temperature and pressure conditions needed for steam production. In general, boilers that operate under higher temperature and pressure conditions require higher quality feedwater, particularly if the heat source is in direct contact with the tube, as in watertube designs. Toprevent operational problems it is important to control the salt content of boiler water, as generally indicated by the conductivity, which then generally determines the operational pressure (Fig. 3.3). Under low pressures, boilers can tolerate conductivity levels over 5000 pS cm-l. However, as the operating pressure increases, the salt concentration becomes more critical. Thus, system design is based on the composition of the water available for use as boiler water. Other critical water quality components for boiler water include alkalinity, silica, iron, manganese and copper (Fig. 3.4). Two operational problems are associated with high levels of alkalinity. As the water temperature increases, carbon dioxide is released increasing the potential for corrosion. In addition, carbonates can contribute to foaming that leads to deposits in the superheater, reheater and/or turbines (Puckorius, 199 7; Troscinski and Watson, 19 70; Vanderpool, 2001).