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.
Mechanism
Advantages
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 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.Disadvantages
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.
Concentrations of polyphosphates or phosphonates with zinc and chromates or similar compounds have been maintained in cooling systems to keep heat exchange surfaces clean enough that a film of gamma iron oxide and zinc phosphate can inhibit corrosion by passivating anodic and cathodic reaction points. These increase salinity and total dissolved solids, and phosphorus compounds may provide the limiting essential nutrient for algal growth contributing to biofouling of the cooling system or to eutrophication of natural aquatic environments receiving blowdown or OTC water. Chromates reduce biofouling in addition to effective corrosion inhibition in the cooling water system, but residual toxicity in blowdown or OTC water has encouraged lower chromate concentrations and the use of less-flexible corrosion inhibitors. Blowdown may also contain chromium leached from cooling towers constructed of wood preserved with chromated copper arsenate.
Total dissolved solids or TDS is reported as the mass of residue remaining when a measured volume of filtered water is evaporated. Salinity indicates water density or conductivity changes caused by dissolved materials. Probability of scale formation increases with increasing total dissolved solids. Solids commonly associated with scale formation are calcium and magnesium both as carbonate and sulfate. Corrosion rates initially increase with salinity in response to increasing electrical conductivity, but then decrease after reaching a peak as higher levels of salinity decrease dissolved oxygen levels.
Some groundwater contains very little oxygen when pumped from wells, but most natural water supplies include dissolved oxygen. Increasing oxygen concentrations accelerate corrosion. Dissolved oxygen approaches saturation levels in cooling towers. It is beneficial in blowdown or OTC water being returned to natural aquatic environments.
Water ionizes into hydronium cations and hydroxide anions. The concentration of ionized hydrogen in a cooling water system is reported as the pH level. Low pH values increase the rate of corrosion; high pH values encourage scale formation. Amphoterism is uncommon among metals used in water cooling systems, but aluminum corrosion rates increase with pH values above 9. Galvanic corrosion may be severe in water systems with copper and aluminum components. Acid can be added to cooling water systems to prevent scale formation if the pH decrease will offset increased salinity and dissolved solids.
Steam power stations
Few other cooling applications approach the large volumes of water required to condense low-pressure steam at power stations. Many facilities, particularly electric power plants, use millions of gallons of water per day for cooling. Water cooling on this scale may alter natural water environments and create new environments. Thermal pollution of rivers, estuaries and coastal waters is a consideration when siting such plants. Water returned to aquatic environments at temperatures higher than the ambient receiving water modifies aquatic habitat by increasing biochemical reaction rates and decreasing the oxygen saturation capacity of the habitat. Temperature increases initially favor a population shift from species requiring the high-oxygen concentration of cold water to those enjoying the advantages of increased metabolic rates in warm water.Once-through cooling systems may be used on very large rivers or at coastal and estuarine sites. These power stations put the waste heat into the river or coastal water. These OTC systems thus rely upon an ample supply of river water or seawater for their cooling needs. Such facilities are built with intake structures designed for bringing in large volumes of water at a high rate of flow. These structures tend to also pull in large numbers of fish and other aquatic organisms, which are killed or injured on the intake screens. Large flow rates may trap slow-swimming organisms including fish and shrimp on screens protecting the small bore tubes of the heat exchangers from blockage. High temperatures or pump turbulence and shear may kill or disable smaller organisms that pass through the screens entrained with the cooling water. More than 1,200 power plants and manufacturing facilities in the U.S. use OTC systems; the intake structures kill billions of fish and other organisms each year. More-agile aquatic predators consume organisms impinged on the screens; and warm water predators and scavengers colonize the cooling water discharge to feed on entrained organisms.
The U.S. Clean Water Act required the Environmental Protection Agency to issue regulations on industrial cooling water intake structures. EPA issued final regulations for new facilities in 2001, and for existing facilities in 2014.