The makeup water to the cooling system may appear clean, but even filtered water contains invisible dissolved minerals and some insoluble matter that poses a serious threat to cooling efficiency. These substances include dirt or silt, minerals, gases, and microbiological organisms that, if left untreated, can build up and cause significant reductions in heat transfer efficiency, increased maintenance or even total system shutdown.
Open recirculating systems are prime candidates for contamination problems. Contaminants tend to build up or concentrate in open cooling towers. As the water evaporates, it leaves behind all the contaminants it originally carried into the system. The extra solids gather in the tower basin. As fresh water is added to compensate for the evaporation losses, new contaminants enter the system and add to the total solids. More contaminants are scrubbed from the air by the water as it falls through the tower. These impurities, if left untreated, can lead to a number of serious problems, including:
While open recirculating systems are particularly vulnerable to these problems, once through and closed systems are also subject to these same conditions.
Corrosion is a reaction between a metal and its environment. Heat exchange equipment in cooling systems is made from various metals such as steel, copper, galvanized steel, and stainless steel. If not properly protected, these metals will corrode when exposed to air and water. This destructive process can cause cracks, leaks, and premature failure of system components. Oxygen is the main ingredient in the corrosion process. Since the water in an open recirculating cooling system is saturated with oxygen, an ongoing corrosion control program is required to maintain peak operating efficiency and prolong the useful life of plant equipment.
The minerals in the cooling water, such as calcium carbonate, magnesium silicate, and iron oxide are normally soluble under typical operating conditions, but in high concentrations, they will come out of solution to form hard, dense crystals commonly known as scale. If left untreated, scale deposits form dense, insulating layers on heat transfer equipment. These deposits reduce cooling efficiency and promote corrosion under the scale deposits.
Thousands of tiny organisms can infest a cooling water system through airborne debris, insects, bird droppings, and other sources of bacteria. Bacteria, algae and fungi are particularly troublesome because they can grow into large colonies or populations that plug system components and restrict water flow. Some organisms produce acidic waste products that promote pitting on metal surfaces.
Bacteria produce slimy masses that can grow into large surface deposits. Algae require sunlight for growth and generally inhabit the open cooling tower deck areas. Fungi eat wood fiber, and hence are a threat to wooden components in the system.
Oil, silt, clay and other suspended solids inevitably find their way into cooling water systems. Dirt and debris scrubbed from the air and particulate matter entering through the makeup water are the prime source of foulants. Internally, the rusty by-products of corrosion contribute to fouling deposits. As these impurities accumulate they tend to form large deposits that foul pumps, screens, heat exchangers and other system components.