In a once-through system, no evaporation takes place. Since there is no increase in concentration of dissolved solids but only an increase in the water temperature, required water treatment is usually significantly reduced. The water is often obtained from a river, lake, or bayou, and aside from rough screening to protect circulating water pumps, removal of suspended solids is rarely practiced. Water treatment is sometimes employed to prevent or minimize scaling and corrosion, but more frequently, its purpose is to mitigate fouling from suspended solids and marine organisms. Discharge water temperature must be compatible with state and federal regulations. Permits for once-through systems, except for coastal (seawater) applications, can be difficult to obtain.
Various antifouling materials including polyacrylates, natural organic materials, and other organic polymers are used to control deposition of solids. The Langelier Saturation Index will provide some indication as to whether the water will tend to be scale- orming, corrosive, or neither. Calcium carbonate, the most common scale in once-through systems, is usually inhibited by applying one or a combination of several depositcontrol materials such as polyphosphates, phosphonates, and polyacrylates. Shock treatment with chlorine may also be used to control biological fouling. As with recirculating water systems, chlorine residuals in once-through cooling water system discharges must comply with environmental regulations.
Corrosion of piping and exchangers in a once-through cooling water system can be a serious problem. Because it is usually impractical to use corrosion inhibitors, the problem is usually handled by using fiberglass reinforced plastic, cementlined, plastic- lined, or other corrosion-resistant pipe and corrosion-resistant condenser and cooler tubing materials, such as 90/10 Cupro-Nickel and titanium.
Corrosion of piping and exchangers in a once-through cooling water system can be a serious problem. Because it is usually impractical to use corrosion inhibitors, the problem is usually handled by using fiberglass reinforced plastic, cementlined, plastic- lined, or other corrosion-resistant pipe and corrosion-resistant condenser and cooler tubing materials, such as 90/10 Cupro-Nickel and titanium.