Water cooling

Water cooling is a method of heat removal from components and industrial equipment. Evaporative cooling using water is often more efficient than air cooling. Water is inexpensive and non-toxic; however, it can contain impurities and cause corrosion.

Water cooling is commonly used for cooling automobile internal combustion engines and power stations. Water coolers utilising convective heat transfer are used inside some high-end personal computers to further lower the temperature of CPUs and other components compared to air cooling.

Other uses include the cooling of lubricant oil in pumps; for cooling purposes in heat exchangers; for cooling buildings in HVAC and in chillers.

Water is inexpensive, non-toxic, and available over most of the earth's surface. Liquid cooling offers higher thermal conductivity than air cooling. Water has unusually high specific heat capacity among commonly available liquids at room temperature and atmospheric pressure allowing efficient heat transfer over distance with low rates of mass transfer. Cooling water may be recycled through a recirculating system or used in a single-pass once-through cooling (OTC) system. Water's high enthalpy of vaporization allows the option of efficient evaporative cooling to remove waste heat in cooling towers or cooling ponds. Recirculating systems are open if they rely upon evaporative cooling or closed if heat removal is accomplished in heat exchangers, thus with negligible evaporative loss. A heat exchanger or condenser may separate non-contact cooling water from a fluid being cooled, or contact cooling water may directly impinge on items like saw blades where phase difference allows easy separation. Environmental regulations emphasize the reduced concentrations of waste products in non-contact cooling water.

Water accelerates the corrosion of metal parts and is a favorable medium for biological growth. Dissolved minerals in natural water supplies are concentrated by evaporation to leave deposits called scale. Cooling water often requires the addition of chemicals to minimize corrosion and insulating deposits of scale and biofouling.

Water contains varying amounts of impurities from contact with the atmosphere, soil, and containers. Being both an electrical conductor and a solvent for metal ions and oxygen, water can accelerate corrosion of machinery being cooled. Corrosion reactions proceed more rapidly as temperature increases. Preservation of machinery in the presence of hot water has been improved by addition of corrosion inhibitors including zinc, chromates and phosphates. The first two have toxicity concerns; and the last has been associated with eutrophication. Residual concentrations of biocides and corrosion inhibitors are of potential concern for OTC and blowdown from open recirculating cooling water systems. With the exception of machines with short design life, closed recirculating systems require periodic cooling-water treatment or replacement raising similar concern about ultimate disposal of cooling water containing chemicals used with environmental safety assumptions of a closed system.

Biofouling occurs because water is a favorable environment for many life forms. Flow characteristics of recirculating cooling water systems encourage colonization by sessile organisms using the circulating supply of food, oxygen and nutrients. Temperatures may become high enough to support thermophilic populations of organisms such as types of fungi. Biofouling of heat exchange surfaces can reduce heat transfer rates of the cooling system, and biofouling of cooling towers can alter flow distribution to reduce evaporative cooling rates. Biofouling may also create differential oxygen concentrations increasing corrosion rates. OTC and open recirculating systems are more susceptible to biofouling. Biofouling may be inhibited by temporary habitat modifications. Temperature differences may discourage the establishment of thermophilic populations in intermittently operated facilities, and intentional short-term temperature spikes may periodically kill less tolerant populations. Biocides have been commonly used to control biofouling where sustained facility operation is required.

Chlorine may be added in the form of hypochlorite to decrease biofouling in cooling water systems, but is later reduced to chloride to minimize the toxicity of blowdown or OTC water returned to natural aquatic environments. Hypochlorite is increasingly destructive to wooden cooling towers as pH increases. Chlorinated phenols have been used as biocides or leached from preserved wood in cooling towers. Both hypochlorite and pentachlorophenol have reduced effectiveness at pH values greater than 8. Non-oxidizing biocides may be more difficult to detoxify prior to release of blowdown or OTC water to natural aquatic environments.