Water cooling

Water cooling is a method of heat removal from components and industrial equipment. Water may be a more efficient heat transfer fluid where air cooling is ineffective. In most occupied climates water offers the thermal conductivity advantages of a liquid with unusually high specific heat capacity and the option of evaporative cooling. Low cost often allows rejection as waste after a single use, but recycling coolant loops may be pressurized to eliminate evaporative loss and offer greater portability and improved cleanliness. Unpressurized recycling coolant loops using evaporative cooling require a blowdown waste stream to remove impurities concentrated by evaporation. Disadvantages of water cooling systems include accelerated corrosion and maintenance requirements to prevent heat transfer reductions from biofouling or scale formation. Chemical additives to reduce these disadvantages may introduce toxicity to wastewater. Water cooling is commonly used for cooling automobile internal combustion engines and large industrial facilities such as nuclear and steam electric power plants, hydroelectric generators, petroleum refineries and chemical plants. Other uses include cooling the barrels of machine guns, cooling of lubricant oil in pumps; for cooling purposes in heat exchangers; cooling products from tanks or columns, and recently, cooling of various major components inside high-end personal computers such as CPUs, GPUs, and motherboards. The main mechanism for water cooling is convective heat transfer. Water cooling is a method of heat removal from components and industrial equipment. Water may be a more efficient heat transfer fluid where air cooling is ineffective. In most occupied climates water offers the thermal conductivity advantages of a liquid with unusually high specific heat capacity and the option of evaporative cooling. Low cost often allows rejection as waste after a single use, but recycling coolant loops may be pressurized to eliminate evaporative loss and offer greater portability and improved cleanliness. Unpressurized recycling coolant loops using evaporative cooling require a blowdown waste stream to remove impurities concentrated by evaporation. Disadvantages of water cooling systems include accelerated corrosion and maintenance requirements to prevent heat transfer reductions from biofouling or scale formation. Chemical additives to reduce these disadvantages may introduce toxicity to wastewater. Water cooling is commonly used for cooling automobile internal combustion engines and large industrial facilities such as nuclear and steam electric power plants, hydroelectric generators, petroleum refineries and chemical plants. Other uses include cooling the barrels of machine guns, cooling of lubricant oil in pumps; for cooling purposes in heat exchangers; cooling products from tanks or columns, and recently, cooling of various major components inside high-end personal computers such as CPUs, GPUs, and motherboards. The main mechanism for water cooling is convective heat transfer. 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 may be open if they rely upon evaporative cooling or closed if heat removal is accomplished in heat exchangers 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 is an ideal cooling medium for vessels as they are constantly surrounded by water that generally remains at a low temperature throughout the year. Systems operating with sea water need to be manufactured from cupronickel, bronze, titanium or similarly corrosion-resistant materials. Water containing sediment may require velocity restrictions through piping to avoid erosion at high velocity or blockage by settling at low velocity. Water accelerates 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 addition of chemicals to minimize corrosion and insulating deposits of scale and biofouling. Water is a favorable environment for many life forms. Flow characteristics of recirculating cooling water systems encourage colonization by sessile organisms to use the circulating supply of food, oxygen and nutrients. Temperatures may become high enough to support thermophilic populations. 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 most susceptible to biofouling. Biofouling may be inhibited by temporary habitat modifications. Temperature differences may discourage 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. Water contains varying amounts of impurities from contact with the atmosphere, soil, and containers. Manufactured metals tend to revert to ores via electrochemical reactions of corrosion. Water can accelerate corrosion as both an electrical conductor and solvent for metal ions and oxygen. 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 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. Total dissolved solids or TDS (sometimes called filtrable residue) is measured as the mass of residue remaining when a measured volume of filtered water is evaporated. Salinity measures 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 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. Water ionizes into hydronium (H3O+) cations and hydroxide (OH−) anions. The concentration of ionized hydrogen (as protonated water) is expressed as pH. Low pH values increase rate of corrosion while 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 may be added to cooling water systems to prevent scale formation if the pH decrease will offset increased salinity and dissolved solids. Concentrations of polyphosphates or phosphonates with zinc and chromates or similar compounds have been maintained in cooling systems to keep heat exchange surfaces clean so a film of gamma iron oxide and zinc phosphate may 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, but residual toxicity in blowdown or OTC water has encouraged reduced chromate concentrations and use of less flexible corrosion inhibitors. Blowdown may also contain chromium leached from cooling towers constructed of wood preserved with chromated copper arsenate.