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Water cooling

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Cooling tower and water discharge of a nuclear power plant

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

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Advantages

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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.[1] 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,[2] 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.[3]

Disadvantages

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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.[4]

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.[4] Preservation of machinery in the presence of hot water has been improved by addition of corrosion inhibitors including zinc, chromates and phosphates.[5][6] The first two have toxicity concerns;[7] and the last has been associated with eutrophication.[8] Residual concentrations of biocides and corrosion inhibitors are of potential concern for OTC and blowdown from open recirculating cooling water systems.[9] 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.[10]

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.[11] Temperatures may become high enough to support thermophilic populations of organisms such as types of fungi.[12] 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.[13]

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.[14] Non-oxidizing biocides may be more difficult to detoxify prior to release of blowdown or OTC water to natural aquatic environments.[15]

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.[16] 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.[7] Blowdown may also contain chromium leached from cooling towers constructed of wood preserved with chromated copper arsenate.[17]

Total dissolved solids or TDS (sometimes called filterable residue) is reported as the mass of residue remaining when a measured volume of filtered water is evaporated.[18] Salinity indicates water density or conductivity changes caused by dissolved materials.[19] 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.[4]

Some groundwater contains very little oxygen when pumped from wells, but most natural water supplies include dissolved oxygen. Increasing oxygen concentrations accelerate corrosion.[4] Dissolved oxygen approaches saturation levels in cooling towers. It is beneficial in blowdown or OTC water being returned to natural aquatic environments.[20]

Water ionizes into hydronium (H3O+) cations and hydroxide (OH−) anions. The concentration of ionized hydrogen (as protonated water) in a cooling water system is reported as the pH level.[21] 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.[22]

Steam power stations

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The Indian Point Energy Center. Over a billion fish eggs and larvae are killed in its cooling system each year.[23]
Cooling water intake of a nuclear power plant

Few other cooling applications approach the large volumes of water required to condense low-pressure steam at power stations.[24] Many facilities, particularly electric power plants, use millions of gallons of water per day for cooling.[25] 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.[11]

Once-through cooling (OTC) 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.[26] 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.[27]: Ch. A2  More than 1,200 power plants and manufacturing facilities in the U.S. use OTC systems;[28]: 4–4  the intake structures kill billions of fish and other organisms each year.[29] 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 (EPA) to issue regulations on industrial cooling water intake structures.[30] EPA issued final regulations for new facilities in 2001 (amended 2003),[26][31] and for existing facilities in 2014.[32]

Cooling towers

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A Marley mechanical induced draft cooling tower

As an alternative to OTC, industrial cooling towers may use recirculated river water, coastal water (seawater), or well water. Large mechanical induced-draft or forced-draft cooling towers in industrial plants continuously circulate cooling water through heat exchangers and other equipment where the water absorbs heat. That heat is then rejected to the atmosphere by the evaporation of some of the water in cooling towers where upflowing air contacts the downflowing water. The loss of evaporated water into the air exhausted to the atmosphere is replaced by "make-up" fresh river water or fresh cooling water, but the amount of water lost during evaporative cooling may affect the natural habitat for aquatic organisms. Because the evaporated pure water is replaced by make-up water containing carbonates and other dissolved salts, a portion of the circulating water is continuously discarded as "blowdown" water to minimize the excessive build-up of salts in the circulating water; these blowdown wastes may change the receiving water quality.[33]

Internal combustion engines

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The heated coolant mixture can be used to warm the air inside the car by means of the heater core. Also, the water jacket around an engine is very effective at deadening mechanical noises, making the engine quieter.

Open method

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An antique gasoline engine with an evaporative cooler and mesh screen to improve evaporation. Water is pumped up to the top and flows down the screen to the tank.

An open water cooling system makes use of evaporative cooling, lowering the temperature of the remaining (unevaporated) water. This method was common in early internal combustion engines until scale buildup was observed from dissolved salts and minerals in the water. Modern open cooling systems continuously waste a fraction of recirculating water as blowdown to remove dissolved solids at concentrations low enough to prevent scale formation. Some open systems use inexpensive tap water, but this requires higher blowdown rates than deionized or distilled water. Purified water systems still require blowdown to remove the accumulation of byproducts of chemical treatment to prevent corrosion and biofouling.[34]

Pressurization

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Water for cooling has a boiling point temperature of around 100 degrees C at atmospheric pressure. Engines operating at higher temperatures may require a pressurized recycle loop to prevent overheating.[35] Modern automotive cooling systems often operate at 15 psi (103 kPa) to raise the boiling-point of the recycling water coolant and reduce evaporative losses.[36]

Antifreeze

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The use of water cooling carries the risk of damage from freezing. Automotive and many other engine cooling applications require the use of a water and antifreeze mixture to lower the freezing point to a temperature unlikely to be experienced. Antifreeze also inhibits corrosion from dissimilar metals and can increase the boiling point, allowing a wider range of water cooling temperatures.[36] Its distinctive odor also alerts operators to cooling system leaks and problems that would go unnoticed in a water-only cooling system.

Other additives

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Other less common chemical additives are products to reduce surface tension. These additives are meant to increase the efficiency of automotive cooling systems. Such products are used to enhance the cooling of underperforming or undersized cooling systems or in racing where the weight of a larger cooling system could be a disadvantage.[citation needed]

Power electronics and transmitters

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Since approximately 1930 it is common to use water cooling for tubes of powerful transmitters. As these devices use high operation voltages (around 10 kV), the use of deionized water is required and it has to be carefully controlled. Modern solid-state transmitters can be built so that even high-power transmitters do not require water cooling. Water cooling is however also sometimes used for thyristors of HVDC valves, for which the use of deionized water is required.[citation needed]

Liquid cooling maintenance

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CoolIT Rack DCLC AHx liquid cooling solution

Liquid cooling is increasingly used in data centres and other high-density computing systems as increasing heat densities place greater demands on conventional air cooling. Liquid-cooling systems may remove heat at the rack level, within individual computing equipment, or directly from electronic components.[37]

In many data-centre installations, a cooling distribution unit (CDU) transfers heat between the facility cooling system and a secondary technology cooling system. The secondary loop may include supply and return manifolds, hoses, valves, sensors, controllers and quick-disconnect couplings that distribute coolant to racks or individual servers.[37]

Quick-disconnect couplings are used to make liquid-cooled equipment serviceable without draining an entire cooling loop. ASHRAE notes that quick disconnects can allow an individual server or rack to be disconnected and reconnected without coolant leakage while other equipment remains in operation.[37]

The Open Compute Project has developed specifications for universal quick-disconnect connectors used in liquid-cooled electronics. Its Universal Quick Disconnect specification defines a hand-mated connector intended to be drip-free, hot-pluggable and leak-tight under pressure when connected and on both sides when disconnected.[38]

Liquid-cooling systems may use either hand-mated or blind-mated connectors. Blind-mate designs allow connections to be made as equipment is inserted into a rack and require allowance for mechanical tolerances and misalignment. Connector selection also takes into account serviceability, connection force, available space, flow requirements and pressure drop.[39]

Coolant temperature is also controlled to reduce the risk of condensation. ASHRAE describes liquid-cooling systems in which the circulating coolant is maintained above the local dew point so that moisture does not condense on or around electronic equipment.[37]

References

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  1. ↑ Kemmer (1979), pp. 1–1, 1–2.
  2. ↑ Kemmer (1979), pp. 38–1, 38–4, 38-7 & 38-8.
  3. ↑ King (1995), pp. 143, 439.
  4. 1 2 3 4 Betz, pp. 183–184.
  5. ↑ Hemmasian-Ettefagh, Ali (2010). "Corrosion Inhibition of Carbon Steel in Cooling Water". Materials Performance. 49 (3): 60–65. doi:10.5006/MP2010_49_3-60.
  6. ↑ Mahgoub, F.M.; Abdel-Nabey, B.A.; El-Samadisy, Y.A. (March 2010). "Adopting a multipurpose inhibitor to control corrosion of ferrous alloys in cooling water systems". Materials Chemistry and Physics. 120 (1): 104–108. doi:10.1016/j.matchemphys.2009.10.028. ISSN 0254-0584.
  7. 1 2 Kemmer (1979), pp. 38–20, 38–21.
  8. ↑ Goldman & Horne (1983), pp. 153, 160.
  9. ↑ Betz, p. 215.
  10. ↑ Krosofsky, Andrew (18 January 2021). "How to Properly and Safely Dispose of Antifreeze". Green Matters. Retrieved 23 June 2021.
  11. 1 2 Reid (1961), pp. 267–268.
  12. ↑ Jones, E.B.G.; Eaton, R.A. (1969). "Savoryella lignicola gen. et sp. nov. from water cooling towers". Transactions of the British Mycological Society. 52: 161–174. doi:10.1016/S0007-1536(69)80169-5.
  13. ↑ Betz, p. 202.
  14. ↑ Betz, pp. 203–209.
  15. ↑ Veil, John A.; Rice, James K.; Raivel, Mary E.S. "Biocide Usage in Cooling Towers in the Electric Power and Petroleum Refining Industries" (PDF). United States Department of Energy. Retrieved 23 June 2021.
  16. ↑ Betz, pp. 198–199.
  17. ↑ "Leaching of CCA From Treated Wood". National Pesticide Information Center. Retrieved 23 June 2021.
  18. ↑ Franson (1975), pp. 89–98.
  19. ↑ Franson (1975), pp. 99–100.
  20. ↑ "Dissolved Oxygen and water quality". State of Kentucky. Retrieved 23 June 2021.
  21. ↑ Franson (1975), pp. 406–407.
  22. ↑ Betz, pp. 191–194.
  23. ↑ McGeehan, Patrick (12 May 2015). "Fire Prompts Renewed Calls to Close the Indian Point Nuclear Plant". New York Times.
  24. ↑ U.S. Environmental Protection Agency (EPA). (1997). Profile of the Fossil Fuel Electric Power Generation Industry (Report). Washington, D.C. Archived from the original on 5 August 2002. Document No. EPA/310-R-97-007. p. 79.
  25. ↑ EPA (2010). "Partial List of Facilities Subject to Clean Water Act 316(b)."
  26. 1 2 EPA (2014). "Cooling Water Intakes."
  27. ↑ Economic and Benefits Analysis for the Final Section 316(b) Phase II Existing Facilities Rule (PDF) (Report). EPA. 2004. EPA 821-R-04-005.
  28. ↑ Technical Development Document for the Final Section 316(b) Existing Facilities Rule (PDF) (Report). EPA. May 2014. EPA 821-R-14-002.
  29. ↑ Final Regulations to Establish Requirements for Cooling Water Intake Structures at Existing Facilities; Fact sheet (PDF) (Report). EPA. May 2014. EPA 821-F-14-001. Archived (PDF) from the original on 19 June 2020. Retrieved 23 November 2015.
  30. ↑ United States. Clean Water Act, Section 316(b), 33 U.S.C. § 1316.
  31. ↑ EPA. Cooling Water Intake Structures. Final rule: 2001-12-18, 66 FR 65255. Amended: 2003-06-19, 68 FR 36749.
  32. ↑ EPA. "National Pollutant Discharge Elimination System—Final Regulations To Establish Requirements for Cooling Water Intake Structures at Existing Facilities and Amend Requirements at Phase I Facilities" Final rule. Federal Register, 79 FR 48300. 2014-08-15.
  33. ↑ Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley and Sons. LCCN 67019834. (See Chapter 2 for material balance relationships in a cooling tower)
  34. ↑ Betz, p. 192.
  35. ↑ Sturgess, Steve (August 2009). "Column: Keep Your Cool". Heavy Duty Trucking. Retrieved 2 April 2018.
  36. 1 2 Nice, Karim (22 November 2000). "How Car Cooling Systems Work". HowStuffWorks. HowStuffWorks, Inc. Retrieved 20 August 2012.
  37. 1 2 3 4 "Chapter 20: Data Centers and Telecommunication Facilities". ASHRAE Handbook. ASHRAE. Retrieved 29 August 2026.
  38. ↑ "Universal Quick Disconnect (UQD) Specification". Open Compute Project. Open Compute Project Foundation. Retrieved 29 August 2026.
  39. ↑ "Cold Plate Cooling Loop Requirements Rev 2.0". Open Compute Project. Open Compute Project Foundation. Retrieved 29 August 2026.

Bibliography

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  • Handbook of Industrial Water Conditioning (7th ed.). Betz Laboratories. 1976.
  • Franson, Mary Ann (1975). Standard Methods for the Examination of Water and Wastewater (14th ed.). APHA, AWWA & WPCF. ISBN 0-87553-078-8.
  • Goldman, Charles R.; Horne, Alexander J. (1983). Limnology. McGraw-Hill. ISBN 0-07-023651-8.
  • Kemmer, Frank N. (1979). The NALCO Water Handbook. McGraw-Hill.
  • King, James J. (1995). The Environmental Dictionary (3rd ed.). John Wiley & Sons. ISBN 0-471-11995-4.
  • Reid, George K. (1961). Ecology in Inland Waters and Estuaries. Van Nostrand Reinhold.
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