A variety of such materials as suspended mud, sand, clay, biological matter or even oil may enter a cooling water system through its makeup supply. They usually accumulate in low flow areas, or in locations at which an abrupt change in flow velocity occurs. Therefore, the most sedimentation is found in such places as cooling tower basins and heat exchangers, both in the waterbox area and on the shell sides when product enters on the tube side. The particle size, density and certain water characteristics will affect the probability that a suspended solid will settle and foul a system. The larger and more dense the suspended material, the greater the chance of settling.

The extent of fouling is largely a result of the time afforded for the particle to settle. Because cooling tower basins, waterboxes and shell-side flow areas involve appreciable holding times, the settling of sediment can be extensive. Once these particles have settled, the nature of the resultant deposition depends on the strength of the attractive forces between the particles and the surfaces they contact. If the attractive forces between the particles are strong, the deposit will be dense and well structured; if the forces are weak, the deposit will be soft, or even fluid in nature. Similarly, if the forces between particles and the surfaces they contact are high, the deposit is adhesive and difficult to remove.

Certain foulants, such as river water silt, enter the system in colloidal form in particle sizes ranging from 1.0 to 100 millimicrons. These particles, too small to be seen by the naked eye, often carry similar electrostatic charges which causes a tendency towards continued dispersion rather than precipitation. Introducing ions of opposite charge can cause these colloidal particles to agglomerate and settle in the cooling system. Therefore, nonionic compounds such as surfactants are often used to control the deposition of suspended solids in cooling water systems. These compounds, as well as other frequently used deposit control agents, will be discussed in more detail later

Microbiological growth may be a particularly troublesome foulant in a cooling water system. The microbiological population in a cooling tower often approaches, or exceeds, the control limit for proper tower operation. This condition seriously strains both microbiological and deposit control programs. Slime in a cooling water system is also a deposit. It entraps materials which normally would not have deposited, and leads to greater problems that would otherwise have not occurred. For specific information on microbiological fouling and microbiocides, consult the microbiocide training manual.

Oil often adheres to metal surfaces and acts as a deposit binder. 011 films serve as insulators and can seriously retard heat transfer. In addition, oil acts as a nutrient for microbes, therefore, increasing microbiological activity, fouling and slime binding. Finally, oil films prevent corrosion inhibitors from reaching and passivating metal surfaces

Control of suspended matter by clarification of makeup water is a good example of a method that has successfully minimized fouling by these materials. There may be small but continuous carryover of aluminum and iron floes from the clarifier or post-precipitation of aluminum and iron hydroxides which can foul heat exchange surfaces. This floe carryover may cause a greater deposit problem than the original turbidity. Mechanical methods of deposit control are discussed in more detail later in this section.