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{{short description|Device which rejects waste heat to the atmosphere through the cooling of a water stream}}
{{for|the historical Iranian architectural element|windcatcher}}
{{Use dmy dates|date=June 2017}}
[[File:Evaporative Cooling Tower.jpg|thumb|right|A typical evaporative, forced draft open-loop cooling tower rejecting heat from the condenser water loop of an industrial chiller unit]]
[[File:Didcot power station cooling tower zootalures.jpg|thumb|Natural draft wet cooling [[Hyperboloid structure|hyperboloid towers]] at [[Didcot Power Station]] (UK)]]
[[File:Power station Westfalen. Cooling towers.jpg|thumb|Forced draft wet cooling towers (height: 34 meters) and natural draft wet cooling tower (height: 122 meters) in [[Westphalia]], Germany]]
[[File:Cooling tower power station Dresden.jpg|thumb|Natural draft wet cooling tower in [[Dresden]] (Germany)]]

A '''cooling tower''' is a device that rejects [[waste heat]] to the [[atmosphere]] through the cooling of a [[coolant]] stream, usually a water stream, to a lower temperature. Cooling towers may either use the [[evaporation]] of water to remove heat and cool the working fluid to near the [[Wet-bulb temperature|wet-bulb air temperature]] or, in the case of ''dry cooling towers'', rely solely on air to cool the working fluid to near the [[Dry-bulb temperature|dry-bulb air temperature]] using [[Radiator|radiators]]. 

Common applications include cooling the circulating water used in [[oil refineries]], [[petrochemical]] and other [[chemical plant]]s, [[thermal power station]]s, [[nuclear power station]]s and [[HVAC]] systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are [[Natural convection|natural draft]] and [[Forced convection|induced draft]] cooling towers.

Cooling towers vary in size from small roof-top units to very large [[hyperboloid structure]]s that can be up to {{convert|200|m|ft}} tall and {{convert|100|m|ft}} in diameter, or rectangular structures that can be over {{convert|40|m|ft}} tall and {{convert|80|m|ft}} long. Hyperboloid cooling towers are often associated with [[nuclear power plant]]s,<ref>{{cite web|website=CleanEnergy Footprints (cleanenergy.org)|url=http://blog.cleanenergy.org/2012/12/31/identifying-nuclear-reactors-in-google-earth/|title=Identifying Nuclear Reactors in Google Earth|date=31 December 2012|access-date=19 May 2014|archive-date=23 October 2014|archive-url=https://web.archive.org/web/20141023052158/http://blog.cleanenergy.org/2012/12/31/identifying-nuclear-reactors-in-google-earth/|url-status=dead}}</ref> although they are also used in many coal-fired plants and to some extent in some large chemical and other industrial plants. The [[steam turbine]] is what necessitates the cooling tower. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from [[air conditioning]]. Cooling towers are also often thought to emit [[smoke]] or harmful fumes by the general public and environmental activists, when in reality the emissions from those towers mostly do not contribute to [[carbon footprint]], consisting solely of [[water vapor]].<ref>{{Cite web |date=2007-02-15 |title=Myth of cooling towers is symptomatic of global warming information shortage |url=https://www.rsc.org/news-events/articles/2007/02-february/cooling-towers/ |access-date=2022-03-02 |website=[[Royal Society of Chemistry]] |language=en-GB}}</ref><ref>{{Cite web |last= |first= |date=2017-07-24 |title=What you need to know about nuclear cooling towers |url=https://nuclear.duke-energy.com/2017/07/24/blog_post-20170724 |access-date=2022-03-02 |website=[[Duke Energy]] {{!}} Nuclear Information Center |language=en}}</ref>

==History==
[[File:Barnard's fanless self-cooling tower.jpg|thumb|upright|A 1902 engraving of "Barnard's fanless self-cooling tower", an early large evaporative cooling tower that relied on [[Natural Draft|natural draft]] and open sides rather than a fan; water to be cooled was sprayed from the top onto the radial pattern of vertical wire-mesh mats.]]

Cooling towers originated in the 19th century through the development of [[Condenser (heat transfer)|condenser]]s for use with the [[steam engine]].<ref name=ICS1902a>{{cite book |author=International Correspondence Schools |date=1902 |title=A Textbook on Steam Engineering |publisher=International Textbook Co. |location=Scranton, Pa. |at=33–34 of Section 29:"Condensers" |url=https://archive.org/details/textbookonsteame04inteiala}}</ref> Condensers use relatively cool water, via various means, to condense the steam coming out of the cylinders or turbines. This reduces the [[back pressure]], which in turn reduces the steam consumption, and thus the fuel consumption, while at the same time increasing power and recycling boiler water.<ref name="Croft 1922">{{cite book |editor-last=Croft |editor-first=Terrell |date=1922 |title=Steam-Engine Principles and Practice |publisher=McGraw-Hill |location=New York |pages=283–286 |url=https://archive.org/details/steamengineprinc00crofrich}}</ref> However, the condensers require an ample supply of cooling water, without which they are impractical.<ref name="Heck 1911" /><ref name="The Engineer 1906">{{cite journal |last=Watson |first=Egbert P. |date=1906 |title=Power plant and allied industries<!--Title on cover image Google books--> |journal=The Engineer (With Which is Incorporated Steam Engineering) |volume=43 |issue=1 |pages=69–72 |publisher=Taylor Publishing Co. |location=Chicago |url=https://books.google.com/books?id=cKUiAQAAMAAJ}}</ref> While water usage is not an issue with [[Marine steam engine|marine engines]], it forms a significant limitation for many land-based systems.{{Cn|date=May 2021}}

By the turn of the 20th century, several evaporative methods of recycling cooling water were in use in areas lacking an established water supply, as well as in urban locations where municipal water mains may not be of sufficient supply, reliable in times of high demand, or otherwise adequate to meet cooling needs.<ref name=ICS1902a /><ref name="The Engineer 1906" /> In areas with available land, the systems took the form of [[cooling pond]]s; in areas with limited land, such as in cities, they took the form of cooling towers.<ref name="Heck 1911">{{cite book |last=Heck |first=Robert Culbertson Hays |date=1911 |title=The Steam Engine and Turbine: A Text-Book for Engineering Colleges |publisher=D. Van Nostrand |location=New York |pages=569–570 |url=https://archive.org/details/steamengineturbi00heck}}</ref><ref name="Snow 1908" />

These early towers were positioned either on the rooftops of buildings or as free-standing structures, supplied with air by fans or relying on natural airflow.<ref name="Heck 1911" /><ref name="Snow 1908">{{cite book |last=Snow |first=Walter B. |date=1908 |title=The Steam Engine: A Practical Guide to the Construction, Operation, and care of Steam Engines, Steam Turbines, and Their Accessories |publisher=American School of Correspondence |location=Chicago |pages=43–46 |url=https://archive.org/details/steamenginepract00amerrich}}</ref> An American engineering textbook from 1911 described one design as "a circular or rectangular shell of light plate—in effect, a chimney stack much shortened vertically (20 to 40 ft. high) and very much enlarged laterally. At the top is a set of distributing troughs, to which the water from the condenser must be pumped; from these it trickles down over "mats" made of wooden slats or woven wire screens, which fill the space within the tower".<ref name="Snow 1908" />

[[File:Staatsmijn Emma Koeltoren III - Brunssum - 20260911 - RCE.jpg|thumb|left|200px|[[Frederik van Iterson|Van Iterson]] cooling tower, 1918]]
A [[hyperboloid structure|hyperboloid]] cooling tower was patented by the Dutch engineers [[Frederik van Iterson]] and [[Gerard Kuypers]] in the Netherlands on August 16, 1916.<ref>NL/GB Patent No. 108,863: {{cite web |url=http://v3.espacenet.com/publicationDetails/biblio?KC=A&date=19180411&NR=108863A&DB=EPODOC&locale=en_V3&CC=GB&FT=D |title=GB108863A Improved Construction of Cooling Towers of Reinforced Concrete. |website=Espacenet, Patent search |access-date=2023-12-03}}</ref> The first hyperboloid reinforced concrete cooling towers were built by the [[Staatsmijn Emma|Dutch State Mine (DSM) Emma]] in 1918 in [[Heerlen]].<ref>{{cite web |url=https://www.gluckauf.nl/emma/koeltorens-van-de-stm-emma |title=Koeltorens van de Staatsmijn Emma |website=Glück Auf |language=Dutch |access-date=2023-12-03 }}</ref> The first ones in the United Kingdom were built in 1924 at [[Lister Drive power station]] in [[Liverpool]], England.<ref name="PopMech1930">{{cite magazine|title=Power Plant Cooling Tower Like Big Milk Bottle|url=https://books.google.com/books?id=p-IDAAAAMBAJ&pg=PA201|magazine=Popular Mechanics|date=February 1930|publisher=Hearst Magazines|page=201|issn=0032-4558}}</ref> On both locations they were built to cool water used at a coal-fired electrical power station.

According to a [https://scholar.google.com.tr/scholar?q=INTEGRATED+INDUSTRIAL+WASTEWATER+REUSE+VIA+THE+MAISOTSENKO+CYCLE+HEAT+RECOVERY+PROCESS Gas Technology Institute (GTI) report], the indirect–dew-point evaporative-cooling Maisotsenko Cycle (M-Cycle) is a theoretically sound method of reducing a working fluid to the ambient fluid’s dew point, which is lower than the ambient fluid’s wet-bulb temperature. The M-cycle utilizes the psychrometric energy (or the potential energy) available from the latent heat of water evaporating into the air. While its current manifestation is as the M-Cycle HMX for air conditioning, through engineering design this cycle could be applied as a heat- and moisture-recovery device for combustion devices, cooling towers, condensers, and other processes involving humid gas streams.

The consumption of cooling water by inland processing and power plants is estimated to reduce power availability for the majority of thermal power plants by 2040–2069.<ref name="van VlietWiberg2016">{{cite journal|last1=van Vliet|first1=Michelle T. H.|last2=Wiberg|first2=David|last3=Leduc|first3=Sylvain|last4=Riahi|first4=Keywan|title=Power-generation system vulnerability and adaptation to changes in climate and water resources|journal=Nature Climate Change|volume=6|issue=4|year=2016|pages=375–380|issn=1758-678X|doi=10.1038/nclimate2903|bibcode=2016NatCC...6..375V}}</ref>

In 2021, researchers presented a method for steam recapture. The steam is charged using an ion beam, and then captured in a wire mesh of opposite charge. The water's purity exceeded [[EPA]] potability standards.<ref>{{Cite web|last=Irving|first=Michael|date=2021-08-04|title=MIT steam collector captures pure water for reuse in power plants|url=https://newatlas.com/environment/steam-collector-water-reuse-power-plants/|url-status=live|access-date=2021-08-09|website=New Atlas|language=en-US|archive-url=https://web.archive.org/web/20210804032517/https://newatlas.com/environment/steam-collector-water-reuse-power-plants/ |archive-date=4 August 2021 }}</ref>

==Classification by use==
{{More citations needed section|date=May 2021}}
===Heating, ventilation and air conditioning (HVAC)===
[[File:Loop Shopping Centre Exterior I.jpg|thumb|Two HVAC cooling towers on the rooftop of a shopping center (Darmstadt, Hessen, Germany)]]
{{main|HVAC}}

[[File:Induced Draft Cooling Tower for HVAC.jpg|thumb|FRP cooling towers installed on roof top]]

[[File:Cooling Tower Fill Material.jpg|thumb|right|Cell of a cross-flow type cooling tower with fill material and circulating water visible]]

An [[HVAC]] (heating, ventilating, and air conditioning) cooling tower is used to dispose of ("reject") unwanted heat from a [[chiller]]. Liquid-cooled chillers are normally more energy efficient than air-cooled chillers due to [[heat rejection]] to tower water at or near [[wet-bulb temperature]]s.  Air-cooled chillers must reject heat at the higher [[dry-bulb temperature]], and thus have a lower average reverse–[[Carnot cycle|Carnot-cycle]] effectiveness. In hot climates, large office buildings, hospitals, and schools typically use cooling towers in their air conditioning systems.  Generally, industrial cooling towers are much larger than HVAC towers.
HVAC use of a cooling tower pairs the cooling tower with a liquid-cooled chiller or liquid-cooled condenser. A [[Refrigeration#Capacity ratings|''ton'' of air-conditioning]] is defined as the removal of {{convert|12,000|BTU/h|kW}}. The ''equivalent ton'' on the cooling tower side actually rejects about {{convert|15,000|BTU/h|kW}} due to the additional waste-heat–equivalent of the energy needed to drive the chiller's compressor. This ''equivalent ton'' is defined as the heat rejection in cooling {{convert|3|usgal/minute|L/minute|abbr=off}} or {{convert|1,500|lb/h|kg/h}} of water by {{convert|10|F-change|C-change}}, which amounts to {{convert|15,000|BTU/h|kW}}, assuming a chiller [[coefficient of performance]] (COP) of 4.0.<ref>{{cite book|last1=Cheremisinoff|first1=Nicholas|title=Handbook of Chemical Processing Equipment|date=2000|publisher=Butterworth-Heinemann|isbn=9780080523828|page=69}}</ref> This COP is equivalent to an energy efficiency ratio (EER) of 14.

Cooling towers are also used in HVAC systems that have multiple water source [[heat pumps]] that share a common piping ''water loop''. In this type of system, the water circulating inside the water loop removes heat from the condenser of the heat pumps whenever the heat pumps are working in the cooling mode, then the externally mounted cooling tower is used to remove heat from the water loop and reject it to the [[atmosphere]].  By contrast, when the heat pumps are working in heating mode, the condensers draw heat out of the loop water and reject it into the space to be heated.  When the water loop is being used primarily to supply heat to the building, the cooling tower is normally shut down (and may be drained or winterized to prevent freeze damage), and heat is supplied by other means, usually from separate [[boiler]]s.

===Industrial cooling towers===
[[File:Open loop Cooling Tower for DG Set.jpg|thumb|]]
[[File:Cenk Endustri Field Erected Industrial Cooling Tower.JPG|thumb|Industrial cooling towers for a power plant]]
[[File:Industrial Counterflow Cooling Towers.jpg|thumb|Industrial cooling towers for fruit processing]]
Industrial cooling towers can be used to remove heat from various sources such as machinery or heated process material.  The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating [[cooling water]] systems used in [[power plants]], [[oil refinery|petroleum refineries]], [[petrochemical]] plants, [[natural gas]] processing plants, food processing plants, semi-conductor plants, and for other industrial facilities such as in condensers of distillation columns, for cooling liquid in crystallization, etc.<ref>
{{Cite report |title=Profile of the Fossil Fuel Electric Power Generation Industry |url=http://www.epa.gov/compliance/resources/publications/assistance/sectors/notebooks/fossil.html |author=U.S. Environmental Protection Agency (EPA) |year= 1997 |publisher= United States }} Document No. EPA/310-R-97-007. p. 79.</ref> The circulation rate of cooling water in a typical 700 MW<sub>th</sub> [[coal-fired power plant]] with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 US gallons per minute)<ref>[http://www.epa.gov/waterscience/presentations/maulbetsch.pdf Cooling System Retrofit Costs] EPA Workshop on Cooling Water Intake Technologies, John Maulbetsch, Maulbetsch Consulting, May 2003</ref> and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour, equivalent to one cubic metre every second).

If that same plant had no cooling tower and used '''once-through cooling'''<!--bad name; evaporative is also once-through--> water, it would require about 100,000 cubic metres an hour<ref>Thomas J. Feeley, III, Lindsay Green, James T. Murphy, Jeffrey Hoffmann, and Barbara A. Carney (2005). [http://204.154.137.14/technologies/coalpower/ewr/pubs/IEP_Power_Plant_Water_R&D_Final_1.pdf "Department of Energy/Office of Fossil Energy's Power Plant Water Management R&D Program."] {{webarchive|url=https://web.archive.org/web/20070927104349/http://204.154.137.14/technologies/coalpower/ewr/pubs/IEP_Power_Plant_Water_R%26D_Final_1.pdf |date=27 September 2007 }} U.S. Department of Energy, July 2005.</ref> A large cooling water intake typically kills millions of [[fish]] and [[larvae]] annually, as the organisms are impinged on the intake [[fish screen|screens]].<ref>The [[Indian Point Energy Center]] cooling system kills over a billion fish eggs and larvae annually. {{cite news |last=McGeehan |first=Patrick |date=2015-05-12 |title=Fire Prompts Renewed Calls to Close the Indian Point Nuclear Plant |url=https://www.nytimes.com/2015/05/13/nyregion/fire-prompts-renewed-calls-to-close-the-indian-point-nuclear-plant.html |newspaper=New York Times}}</ref> A large amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem. Elevated water temperatures can kill fish and other aquatic organisms (see ''[[thermal pollution]]''), or can also cause an increase in undesirable organisms such as invasive species of [[zebra mussel]]s or [[algae]]. 

A cooling tower serves to dissipate the heat into the atmosphere instead, so that wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water. Evaporative cooling water cannot be used for subsequent purposes (other than rain somewhere), whereas surface-only cooling water can be re-used.
Some coal-fired and [[nuclear power plant]]s located in [[coastal]] areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems.

Petroleum refineries may also have very large cooling tower systems. A typical large refinery processing 40,000 metric tonnes of crude oil per day ({{convert|300000|oilbbl}} per day) circulates about 80,000 cubic metres of water per hour through its cooling tower system.

The world's tallest cooling tower is the {{convert|210|m|ft}} tall cooling tower of the [[Pingshan Power Station|Pingshan II Power Station]] in [[Huaibei]], Anhui Province, China.<ref>{{cite web|url=https://www.hamon.com/cases-studies/largest-cooling-tower-in-the-world/#:~:text=Pingshan%20II%20is%20the%20largest,coal%20fired%20unit%20ever%20built |title=Pingshan II: the largest Cooling Tower in the world designed by Hamon |publisher=Hamon.com |date= |accessdate=2023-01-14}}</ref>
[[File:Field erected cooling tower.JPG|thumb|Field-erected cooling tower]]

==Classification by build==
{{Unreferenced section|date=April 2021}}
=== Package type ===
[[File:Field_Errected_Cooling_Towers.jpg|thumb|Field-erected cooling towers]]
[[File:Brotep_Eco_coolin_tower.jpg|thumb|Brotep-Eco cooling tower]]
[[File:Package_type_cooling_tower.JPG|upright|thumb|Package cooling tower]]
These types of cooling towers are factory preassembled, and can be simply transported on trucks, as they are compact machines. The capacity of package type towers is limited and, for that reason, they are usually preferred by facilities with low heat rejection requirements such as food processing plants, textile plants, some chemical processing plants, or buildings like hospitals, hotels, malls, automotive factories, etc.

Due to their frequent use in or near residential areas, sound level control is a relatively more important issue for package type cooling towers.

=== Field-erected type ===
Facilities such as power plants, steel processing plants, petroleum refineries, or petrochemical plants usually install field-erected type cooling towers due to their greater capacity for heat rejection. Field-erected towers are usually much larger in size compared to the package type cooling towers.

A typical field-erected cooling tower has a [[Pultrusion|pultruded]] [[fiber-reinforced plastic]] (FRP) structure, FRP [[Cladding (construction)|cladding]], a mechanical unit for [[air draft]], and a drift eliminator.

==Heat transfer methods==
{{Unreferenced section|date=April 2021}}
With respect to the [[heat transfer]] mechanism employed, the main types are:
* '''Wet cooling towers''' or '''open-circuit Cooling Tower''' or '''evaporative cooling towers''' operate on the principle of [[evaporative cooling]].  The working [[coolant]] (usually water) is the evaporated fluid, and is exposed to the elements.
* '''Closed circuit cooling towers''' (also called '''fluid coolers''') pass the working coolant through a large [[heat exchanger]], usually a [[radiator]], upon which clean water is sprayed and a fan-induced draft applied.  The resulting heat transfer performance is close to that of a wet cooling tower, with the advantage of protecting the working fluid from environmental exposure and contamination. 
* '''[[Adiabatic]] cooling towers''' spray water into the incoming air or onto a cardboard pad to cool the air before it passes over an air-cooled heat exchanger. Adiabatic cooling towers use less water than other cooling towers but do not cool the fluid as close to the wet bulb temperature. Most adiabatic cooling towers are also hybrid cooling towers.
* '''Dry cooling towers''' (or '''dry coolers''') are closed circuit cooling towers which operate by [[heat transfer]] through a heat exchanger that separates the working coolant from ambient air, such as in a radiator, utilizing convective heat transfer. They do not use evaporation and are air-cooled heat exchangers.
*'''Hybrid cooling towers''' or '''wet-dry cooling towers''' are closed circuit cooling towers that can switch between wet or adiabatic and dry operation. This helps balance water and energy savings across a variety of weather conditions. Some hybrid cooling towers can switch between dry, wet, and adiabatic modes. Thermal efficiencies up to 92% have been observed in hybrid cooling towers.<ref>{{Cite journal|last=Gul|first=S.|date=2015-06-18|title=Optimizing the performance of Hybrid: Induced-Forced Draft Cooling Tower|url=http://www.piche.org.pk/journal/index.php?journal=jpiche&page=article&op=view&path%5B%5D=192|journal=Journal of the Pakistan Institute of Chemical Engineers|language=en|volume=43|issue=2|issn=1813-4092}}</ref>

In a wet cooling tower (or open circuit cooling tower), the warm water can be cooled to a temperature ''lower'' than the ambient air dry-bulb temperature, if the air is relatively dry (see [[dew point]] and [[psychrometrics]]). As ambient air is drawn past a flow of water, a small portion of the water evaporates, and the energy required to evaporate that portion of the water is taken from the remaining mass of water, thus reducing its temperature. Approximately {{convert|970|BTU/lb|kJ/kg|order=flip}} of heat energy is absorbed for the evaporated water. Evaporation results in saturated air conditions, lowering the temperature of the water processed by the tower to a value close to [[wet-bulb temperature]], which is lower than the ambient [[dry-bulb temperature]], the difference determined by the initial humidity of the ambient air.

To achieve better performance (more cooling), a medium called ''fill'' is used to increase the surface area and the time of contact between the air and water flows. ''Splash fill'' consists of material placed to interrupt the water flow causing splashing. ''Film fill'' is composed of thin sheets of material (usually [[polyvinyl chloride|PVC]]) upon which the water flows.  Both methods create increased surface area and time of contact between the fluid (water) and the gas (air), to improve heat transfer.

==Air flow generation methods==
{{More citations needed section|date=May 2021}}
[[File:Stairway to nowhere^ - geograph.org.uk - 455003.jpg|thumb|Access stairs at the base of a massive hyperboloid cooling tower give a sense of its scale (UK).]]
With respect to drawing air through the tower, there are three types of cooling towers:
* '''[[Natural draught|Natural draft]]'''{{dash}}Utilizes buoyancy via a tall chimney.  Warm, moist air ''naturally'' rises due to the density differential compared to the dry, cooler outside air. Warm [[Lighter than air#Water vapour|moist air]] is less dense than drier air at the same pressure.  This moist air buoyancy produces an upwards current of air through the tower.{{Multiple image
| direction         = vertical
| total_width       = 220
| image1            = Inside of a Natural Draft Cooling Tower.jpg
| image2            = Inside of a Natural Draft Cooling Tower 3.jpg
| caption2          = Inside views from a natural draft cooling tower
}}
* '''Mechanical draft'''{{dash}}Uses power-driven fan motors to force or draw air through the tower.
** '''Induced draft'''{{dash}}A mechanical draft tower with a fan at the discharge (at the top) which pulls air up through the tower. The fan ''induces'' hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of ''recirculation'' in which discharged air flows back into the air intake.  This fan/fin arrangement is also known as ''draw-through''. 
** '''Forced draft'''{{dash}}A mechanical draft tower with a blower type fan at the intake. The fan ''forces'' air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The benefit of the forced draft design is its ability to work with high [[static pressure]]. Such setups can be installed in more-confined spaces and even in some indoor situations. This fan/fin geometry is also known as ''blow-through''.
* '''Fan assisted natural draft'''{{dash}}A hybrid type that appears like a natural draft setup, though airflow is assisted by a fan.

===Hyperboloid cooling tower===
On 16 August 1916,<ref name="FinancialTimesCollins">{{cite news |last1=Collins |first1=Michael |title=Falling giants: Britain's vanishing cooling towers |url=https://www.ft.com/content/1a1d5f45-c699-4876-af21-fe02c9cb998a |access-date=11 February 2022 |work=Financial Times |date=31 July 2020}}</ref> [[Frederik van Iterson]] took out the UK patent (108,863) for ''Improved Construction of Cooling Towers of [[Reinforced concrete|Reinforced Concrete]]''.<ref>{{cite patent |country=GB |number= 108863|status=Expired |title=Improved Construction of Cooling Towers of Reinforced Concrete |pubdate=11 April 1918|gdate= |fdate=9 August 1917 |pridate= |inventor= |invent1= van Iterson, F. K. T. |invent2= Kuypers, G|assign1= |assign2= |class= |url=}}</ref> The patent was filed on 9 August 1917, and published on 11 April 1918. In 1918, [[DSM (company)|DSM]] built the first hyperboloid natural-draft cooling tower at the [[Staatsmijn Emma]], to his design.

[[hyperboloid structure|Hyperboloid]] (sometimes incorrectly known as [[hyperbolic paraboloid|hyperbolic]]) cooling towers have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material.<ref name="krex-Chen">{{cite journal |last1=Chen |first1=HB |title=analysis and design of a hyperbolic (sic) cooling tower |journal=[[K-State Research Exchange]] |date=1976 |url=https://krex.k-state.edu/dspace/bitstream/handle/2097/8903/LD2668R41976C49.pdf |access-date=10 February 2022 |publisher=[[Kansas State University]] |quote=Master's Degree}}</ref><ref>{{cite web |last1=Anderson |first1=Kevin |title=Nuclear Cooling Towers |url=https://large.stanford.edu/courses/2015/ph241/anderson-k1/ |website=PH241: Introduction to Nuclear Energy |publisher=[[Stanford University]] |access-date=10 February 2022 |archive-url=https://web.archive.org/web/20170503182850/https://large.stanford.edu/courses/2015/ph241/anderson-k1/ |archive-date=3 May 2017 |date=February 26, 2015 |quote=Submitted as coursework for PH241, Stanford University, Winter 2015}}</ref><ref name="sciencing-4899957">{{cite web |last1=Lee |first1=Kevin |title=How Does a Cooling Tower Work? |url=https://sciencing.com/cooling-tower-work-4899957.html |website=Sciencing |access-date=10 February 2022 |language=en |date=April 24, 2017}}</ref><ref name="drax.com">{{cite web |title=The inside of a cooling tower looks like no place on earth |url=https://www.drax.com/power-generation/inside-cooling-tower-looks-like-no-place-earth/ |website=[[Drax Power Station]] |access-date=10 February 2022 |date=29 November 2018}}</ref> The hyperboloid shape also aids in accelerating the upward [[convection|convective]] air flow, improving cooling efficiency.<ref>{{cite web |title=Cooling Towers Explained: How does a cooling tower work? |url=https://www.engineeringclicks.com/cooling-tower/ |website=EngineeringClicks |access-date=10 February 2022 |date=13 September 2018}}</ref><ref>{{cite web |title=Cooling tower design and construction |url=https://www.designingbuildings.co.uk/wiki/Cooling_tower_design_and_construction |website=designingbuildings.co.uk |access-date=10 February 2022 |language=en-gb}}</ref> These designs are popularly associated with [[nuclear power plant]]s.  However, this association is misleading, as the same kind of cooling towers are often used at large coal-fired power plants and some geothermal plants as well. The [[steam turbine]] is what necessitates the cooling tower. Conversely, not all nuclear power plants have cooling towers, and some instead cool their working fluid with lake, river or ocean water.

==Categorization by air-to-water flow==
{{Unreferenced section|date=April 2021}}
===Crossflow===
[[File:Crossflow diagram.svg|thumb|left]]
[[File:factory assembled crossflow.jpg|thumb|Mechanical draft crossflow cooling tower used in an HVAC application]]
[[File:Crossflow cooling tower.jpg|thumb|Package crossflow cooling tower]]Typically lower initial and long-term cost, mostly due to pump requirements.
Crossflow is a design in which the airflow is directed perpendicular to the water flow (see diagram at left). Airflow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum volume. Lastly, a fan forces the air out into the atmosphere.

A ''distribution'' or ''hot water basin'' consisting of a deep pan with holes or ''nozzles'' in its bottom is located near the top of a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material. [http://www.deltacoolingtowers.in/counter-flow-vs-cross-flow.html Cross Flow V/s Counter Flow]

Advantages of the crossflow design:
* Gravity water distribution allows smaller pumps and maintenance while in use.
* Non-pressurized spray simplifies variable flow.

Disadvantages of the crossflow design:
* More prone to freezing than counterflow designs.
* Variable flow is useless in some conditions.
* More prone to dirt buildup in the fill than counterflow designs, especially in dusty or sandy areas.

===Counterflow===
[[File:Cooling tower - showers.jpg|thumb|Showers inside cooling tower]]
[[File:Counterflow diagram.svg|thumb|left]]
[[File:Crossflow Cooling tower.jpg|thumb|Forced-draft counter-flow package-type cooling tower]]
In a counterflow design, the air flow is directly opposite to the water flow (see diagram at left). Air flow first enters an open area beneath the fill media, and is then drawn up vertically. The water is sprayed through pressurized nozzles near the top of the tower, and then flows downward through the fill, opposite to the air flow.

<br />
Advantages of the counterflow design:
* Spray water distribution makes the tower more freeze-resistant.
* Breakup of water in spray makes heat transfer more efficient.
<br />
Disadvantages of the counterflow design:
* Typically higher initial and long-term cost, primarily due to pump requirements.
* Difficult to use variable water flow, as spray characteristics may be negatively affected.
* Typically noisier, due to the greater water fall height from the bottom of the fill into the cold water basin

===Common aspects===
Common aspects of both designs:
*The interactions of the air and water flow allow a partial equalization of temperature, and evaporation of water.
*The air, now saturated with water vapor, is discharged from the top of the cooling tower.
*A "collection basin" or "cold water basin" is used to collect and contain the cooled water after its interaction with the air flow.

Both crossflow and counterflow designs can be used in natural draft and in mechanical draft cooling towers.

==Wet cooling tower material balance==
{{More citations needed section|date=May 2021}}
Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of make-up [[volumetric flow rate]], [[evaporation]] and windage losses, draw-off rate, and the concentration cycles.<ref name="Beychok">{{cite book | author=Beychok, Milton R. | title=Aqueous Wastes from Petroleum and Petrochemical Plants |edition=1st | publisher=John Wiley and Sons | year=1967 | lccn=67019834 | title-link=Aqueous Wastes from Petroleum and Petrochemical Plants }}</ref><ref name="Beychok2">{{cite journal| author=Milton R. Beychok|date=October 1952|title=How To Calculate Cooling Tower Control Variables|journal= Petroleum Processing|pages=1452–1456}}</ref>

In the adjacent diagram, water pumped from the tower basin is the cooling water routed through the process coolers and [[condenser (steam turbine)|condensers]] in an industrial facility.  The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower.  As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. That contact causes a small amount of the water to be lost as windage or drift (W) and some of the water (E) to [[evaporation|evaporate]]. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved [[salt (chemistry)|salt]]s behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off or blown down (D) for disposal. Fresh water make-up (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.

[[File:CoolingTower.png|frame|right|Fan-induced draft, counter-flow cooling tower]]

Using these flow rates and concentration dimensional units:
{| border="0" cellpadding="2"
|-
|align=right| '''M'''
|align=left|= Make-up water in m<sup>3</sup>/h
|-
|align=right| '''C'''
|align=left|= Circulating water in m<sup>3</sup>/h
|-
|align=right| '''D'''
|align=left|= Draw-off water in m<sup>3</sup>/h
|-
|align=right| '''E'''
|align=left|= Evaporated water in m<sup>3</sup>/h
|-
|align=right| '''W'''
|align=left|= Windage loss of water in m<sup>3</sup>/h
|-
|align=right| '''X'''
|align=left|= Concentration in [[parts per notation|ppmw]] (of any completely soluble salts ... usually chlorides)
|-
|align=right| '''{{math|X<sub>M</sub>}}'''
|align=left|= Concentration of [[chloride]]s in make-up water (M), in ppmw
|-
|align=right| '''X<sub>C</sub>'''
|align=left|= Concentration of chlorides in circulating water (C), in ppmw
|-
|align=right| '''Cycles'''
|align=left|= Cycles of concentration = X<sub>C</sub> / X<sub>M</sub> (dimensionless)
|-
|align=right| '''ppmw'''
|align=left|= parts per million by weight
|}

A water balance around the entire system is then:<ref name=Beychok2/>

:{{math|1={{var|M}} = {{var|E}} + {{var|D}} + {{var|W}}}}

Since the evaporated water (E) has no salts, a chloride balance around the system is:<ref name=Beychok2/>

:{{math|1= {{var|MX{{sub|M}}}} = {{var|DX{{sub|C}}}} + {{var|WX{{sub|C}}}} = {{var|X{{sub|C}}}}({{var|D}} + {{var|W}})}}
:<math>M X_M = D X_C + W X_C = X_C (D + W)</math>

and, therefore:<ref name=Beychok2/>

:<math> {X_C \over X_M} = \text{Cycles of concentration} ={ M \over (D + W)} = {M \over (M - E)} = 1 + {E \over (D + W)}</math>

From a simplified heat balance around the cooling tower:

:<math> E = {C \Delta T c_p \over H_V} </math>

{| border="0" cellpadding="2"
|-
|align=right|where:
|&nbsp;
|-
|align=right| '''H<sub>V</sub>'''
|align=left|= latent heat of vaporization of water = 2260 kJ / kg
|-
|align=right| '''{{math|ΔT}}'''
|align=left|= water temperature difference from tower top to tower bottom, in °C
|-
|align=right| '''c<sub>p</sub>'''
|align=left|= specific heat of water =  4.184 kJ / (kg<math>\cdot</math>°C)
|}

Windage (or drift) losses (W) is the amount of total tower water flow that is entrained in the flow of air to the atmosphere. From large-scale industrial cooling towers, in the absence of manufacturer's data, it may be assumed to be:

:'''W''' = 0.3 to 1.0 percent of C for a natural draft cooling tower without windage drift eliminators
:'''W''' = 0.1 to 0.3 percent of C for an induced draft cooling tower without windage drift eliminators
:'''W''' = about 0.005 percent of C (or less) if the cooling tower has windage drift eliminators
:'''W''' = about 0.0005 percent of C (or less) if the cooling tower has windage drift eliminators and uses sea water as make-up water.

===Cycles of concentration===
Cycle of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Discharge of draw-off (or blowdown) is used principally to control the buildup of these minerals.

The chemistry of the make-up water, including the amount of dissolved minerals, can vary widely. Make-up waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Make-up waters from [[ground water]] supplies (such as [[water well|wells]]) are usually higher in minerals, and tend to be [[Limescale|scaling]] (deposit minerals). Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping; however, excessive levels of minerals can cause scaling problems.

[[File:Cycles-Of-Concentration.png|thumb|Relationship between cycles of concentration and flow rates in a cooling tower]] As the cycles of concentration increase, the water may not be able to hold the minerals in [[Solution (chemistry)|solution]]. When the [[solubility]] of these minerals have been exceeded they can [[precipitate]] out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the [[heat exchangers]]. The temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water.  Often a professional [[Industrial water treatment|water treatment]] consultant will evaluate the make-up water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as [[water softening]], [[pH]] adjustment, and other techniques can affect the acceptable range of cycles of concentration.

Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States, many water supplies use well water which has significant levels of dissolved solids. On the other hand, one of the largest water supplies, for [[New York City water supply system|New York City]], has a surface rainwater source quite low in minerals; thus cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.

Since higher cycles of concentration represent less make-up water, [[water conservation]] efforts may focus on increasing cycles of concentration.<ref>{{cite web |url = http://energy.gov/eere/femp/best-management-practice-cooling-tower-management |title = Best Management Practice Cooling Tower Management |publisher = Department of Energy |date = 30 April 2005 |website = Energy.gov |access-date= 16 June 2014 }}</ref> Highly treated recycled water may be an effective means of reducing cooling tower consumption of potable water, in regions where potable water is scarce.<ref name="Diego">{{cite journal | last = San Diego County Water Authority | date = July 2009 | title = Technical Information for Cooling Towers Using Recycled Water | url = http://www.sdcwa.org/sites/default/files/files/water-management/recycled/techinfo-cooling-towers.pdf | website = www.sdcwa.org | publisher    = San Diego County Water Authority | access-date  = 18 June 2014 }}</ref>

==Maintenance==
Clean visible dirt & debris from the cold water basin and surfaces with any visible biofilm (i.e., slime).{{citation needed|date=December 2019}}

Disinfectant and other chemical levels in cooling towers and hot tubs should be continuously maintained and regularly monitored.<ref>{{cite web|url =https://www.cdc.gov/legionella/downloads/toolkit.pdf |title=Developing a Water Management Program to Reduce Legionella Growth & Spread in Buildings: A Practical Guide to Implementing Industry Standards |date = 5 June 2017|page= 13 {17 of 32}| publisher =CDC }}</ref>

Regular checks of water quality (specifically the aerobic bacteria levels) using [[Dip slide|dipslides]] should be taken as the presence of other organisms can support legionella by producing the organic nutrients that it needs to thrive.{{citation needed|date=December 2019}}

===Water treatment===
{{see also|Industrial water treatment}}
Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and [[fouling]], the water should be [[Filter (water)|filtered]] to remove particulates, and also be dosed with [[biocide]]s and [[algaecide]]s to prevent growths that could interfere with the continuous flow of the water.<ref name=Beychok/> Under certain conditions, a [[biofilm]] of micro-organisms such as bacteria, fungi and algae can grow very rapidly in the cooling water, and can reduce the heat transfer efficiency of the cooling tower. Biofilm can be reduced or prevented by using [[sodium chlorite]] or other [[chlorine]] based chemicals. A normal industrial practice is to use two biocides, such as oxidizing and non-oxidizing types to complement each other's strengths and weaknesses, and to ensure a broader spectrum of attack. In most cases, a continual low level oxidizing biocide is used, then alternating to a periodic shock dose of non-oxidizing biocides.{{citation needed|date=December 2019}}

==== Algaecides and biocides ====
Algaecides, as their name might suggest, is intended to kill algae and other related plant-like microbes in the water. Biocides can reduce other living matter that remains, improving the system and keeping clean and efficient water usage in a cooling tower. One of the most common options when it comes to biocides for your water is bromine.<ref name="auto">{{Cite web|title=Cooling Tower Chemicals – Robinson India|date=23 July 2021 |url=https://robinsonindia.com/water-treatment-chemicals/cooling-tower-chemical/|access-date=2021-07-23|language=en-US}}</ref>

==== Scale inhibitors ====
Among the issues that cause the most damage and strain to a water tower's systems is scaling. When an unwanted material or contaminant in the water builds up in a certain area, it can create deposits that grow over time. This can cause issues ranging from the narrowing of pipes to total blockages and equipment failures.<ref name="auto"/>

The water consumption of the cooling tower comes from Drift, Bleed-off, Evaporation loss, The water that is immediately replenished into the cooling tower due to loss is called Make-up Water. The function of make-up water is to make machinery and equipment run safely and stably.{{Cn|date=May 2021}}

====Legionnaires' disease====
[[File:Legionella pneumophila (SEM) 2.jpg|thumb|''Legionella pneumophila'' (5000 × magnification)]]
{{further|Legionellosis|Legionella}}
[[File:Cooling Tower Algal Mat.jpg|thumb|right|A multitude of microscopic organisms such as bacterial colonies, fungi, and algae can easily thrive within the moderately high temperatures present inside a cooling tower.]]
Another very important reason for using biocides in cooling towers is to prevent the growth of ''[[Legionella]]'', including species that cause [[legionellosis]] or Legionnaires' disease, most notably ''L. pneumophila'',<ref>{{cite book|editor1-last=Ryan|editor1-first=K.J.|editor2-last=Ray|editor2-first=C.G.|title=Sherris Medical Microbiology|edition=4th|publisher=McGraw Hill|year=2004|isbn=978-0-8385-8529-0}}</ref> or ''[[Mycobacterium avium]]''.<ref>[http://wwwnc.cdc.gov/eid/content/4/3/pdfs/v4-n3.pdf Centers for Disease Control and Prevention – Emerging Infectious Diseases] (page 495)</ref> The various ''Legionella'' species are the cause of Legionnaires' disease in humans and transmission is via exposure to [[particulate|aerosol]]s—the inhalation of mist droplets containing the bacteria. Common sources of ''Legionella'' include cooling towers used in open recirculating evaporative cooling water systems, domestic hot water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources include freshwater ponds and creeks.<ref name=Cun2016>{{cite journal|last1=Cunha|first1=BA|last2=Burillo|first2=A|last3=Bouza|first3=E|title=Legionnaires' disease.|journal=Lancet|date=23 January 2016|volume=387|issue=10016|pages=376–85|pmid=26231463|doi=10.1016/s0140-6736(15)60078-2|s2cid=28047369}}</ref><ref name=CDC2016Start>{{cite web|title=Legionella (Legionnaires' Disease and Pontiac Fever) About the Disease|url=https://www.cdc.gov/legionella/about/index.html|website=CDC|access-date=17 June 2017 |date=26 January 2016}}</ref>

French researchers found that ''Legionella'' bacteria travelled up to {{convert|6|km|mi}} through the air from a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais, France. That outbreak killed 21 of the 86 people who had a laboratory-confirmed infection.<ref>[https://web.archive.org/web/20080922134426/http://www.medscape.com/viewarticle/521680 Airborne Legionella May Travel Several Kilometres] (access requires free registration)</ref>

Drift (or windage) is the term for water droplets of the process flow allowed to escape in the cooling tower discharge. Drift eliminators are used in order to hold drift rates typically to 0.001–0.005% of the circulating flow rate. A typical drift eliminator provides multiple directional changes of airflow to prevent the escape of water droplets. A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for ''Legionella'' or water treatment chemical exposure. Also, about every six months, inspect the conditions of the drift eliminators making sure there are no gaps to allow the free flow of dirt.<ref>{{Cite web|url=https://deltacooling.com/resources/faqs/preventing-legionnaires-disease-in-cooling-towers|title=How do you Prevent Legionnaires' Disease in Cooling Towers? {{pipe}} Delta Cooling Towers, Inc.|date=4 June 2017|website=Manufacturers of Cooling Towers & Systems by Delta Cooling Towers, Inc.}}</ref>

The [[US Centers for Disease Control and Prevention]] (CDC) does not recommend that health-care facilities regularly test for the ''Legionella pneumophila'' bacteria. Scheduled microbiologic monitoring for ''Legionella'' remains controversial because its presence is not necessarily evidence of a potential for causing disease. The CDC recommends aggressive disinfection measures for cleaning and maintaining devices known to transmit ''Legionella'', but does not recommend regularly-scheduled microbiologic assays for the bacteria. However, scheduled monitoring of potable water within a hospital might be considered in certain settings where persons are highly susceptible to illness and mortality from ''Legionella'' infection (e.g. [[hematopoietic stem cell]] transplantation units, or solid organ transplant units).  Also, after an outbreak of legionellosis, health officials agree that monitoring is necessary to identify the source and to evaluate the efficacy of biocides or other prevention measures.<ref>{{Cite web|url=https://www.cdc.gov/infectioncontrol/guidelines/index.html|title=Guidelines Library {{pipe}} Infection Control {{pipe}} CDC|date=9 September 2020|website=www.cdc.gov}}</ref>{{Failed verification|date=January 2021}}

Studies have found ''Legionella'' in 40% to 60% of cooling towers.<ref>{{Cite web|url=http://www.cti.org/downloads/WTP-148.pdf|title=Cooling Tower Institute, July 2008. Page 5 of 12, column 1, paragraph 3. Most professional and government agencies do not recommend testing for ''Legionella'' bacteria on a routine basis.|access-date=14 July 2008|archive-date=13 May 2021|archive-url=https://web.archive.org/web/20210513193135/http://www.cti.org/downloads/WTP-148.pdf|url-status=dead}}</ref>

==Terminology==
{{More citations needed section|date=May 2021}}
[[File:Floor of cooling tower, Iru Power Plant, 2.jpg|thumb|Fill plates at the bottom of the [[Iru Power Plant]] cooling tower (Estonia).  Tower is shut down, revealing numerous water spray heads.]]

{{anchor |Windage|Drift}}
*'''Windage''' or '''Drift'''{{dash}}Water droplets that are carried out of the cooling tower with the exhaust air. Drift droplets have the same concentration of impurities as the water entering the tower. The drift rate is typically reduced by employing baffle-like devices, called drift eliminators, through which the air must travel after leaving the fill and spray zones of the tower.  Drift can also be reduced by using warmer entering cooling tower temperatures.
{{anchor |Blow-out}}
*'''Blow-out'''{{dash}}Water droplets blown out of the cooling tower by wind, generally at the air inlet openings. Water may also be lost, in the absence of wind, through splashing or misting. Devices such as wind screens, louvers, splash deflectors and water diverters are used to limit these losses.
{{anchor |Plume}}
*'''Plume'''{{dash}}The stream of saturated exhaust air leaving the cooling tower. The plume is visible when water vapor it contains condenses in contact with cooler ambient air, like the saturated air in one's breath fogs on a cold day. Under certain conditions, a cooling tower plume may present fogging or icing hazards to its surroundings. Note that the water evaporated in the cooling process is "pure" water, in contrast to the very small percentage of drift droplets or water blown out of the air inlets.
{{anchor |Draw-off|Blow-down}}
*'''Draw-off''' or '''blow-down'''{{dash}}The portion of the circulating water flow that is removed (usually discharged to a drain) in order to maintain the amount of [[total dissolved solids]] (TDS) and other impurities at an acceptably low level.  Higher TDS concentration in solution may result from greater cooling tower efficiency. However the higher the TDS concentration, the greater the risk of scale, biological growth, and corrosion. The amount of blow-down is primarily regulated by measuring by the [[electrical conductivity]] of the circulating water. Biological growth, scaling, and corrosion can be prevented by chemicals (respectively, biocide, sulfuric acid, corrosion inhibitor). On the other hand, the only practical way to decrease the electrical conductivity is by increasing the amount of blow-down discharge and subsequently increasing the amount of clean make-up water.
{{anchor |Zero bleed for cooling towers|Zero}}
* '''Zero bleed for cooling towers''', also called '''zero blow-down for cooling towers''', is a process for significantly reducing the need for bleeding [[water]] with residual [[solid]]s from the system by enabling the water to hold more solids in solution.<ref name=WilliamHClark1997 >{{citation | author=William H Clark | year=1997 | title=Retrofitting for energy conservation | publisher=McGraw-Hill Professional | isbn=978-0-07-011920-8  | page=66 }}</ref><ref name=InstituteofIndustrialEngineers1981-1982 >{{citation | author=Institute of Industrial Engineers 1981–  | year=1982 | title=Proceedings, Volume 1982 | publisher=Institute of Industrial Engineers/American Institute of Industrial Engineers  | page=101 }}</ref><ref name=Mathie1998 >{{citation | last=Mathie | first=Alton J. | year=1998 | title=Chemical treatment for cooling water | publisher=Fairmont Press | isbn=978-0-88173-253-5  | page=86 }}</ref>
{{anchor |Make-up}}
*'''Make-up'''{{dash}}The water that must be added to the circulating water system in order to compensate for water losses such as evaporation, drift loss, blow-out, blow-down, etc.
{{anchor |Noise}}
*'''Noise'''{{dash}}Sound energy emitted by a cooling tower and heard (recorded) at a given distance and direction. The sound is generated by the impact of falling water, by the movement of air by fans, the fan blades moving in the structure, vibration of the structure, and the motors, gearboxes or drive belts.
{{anchor |Approach}}
*'''Approach'''{{dash}}The approach is the difference in temperature between the cooled-water temperature and the entering-air [[Wet-bulb temperature|wet bulb temperature]] (twb). Since the cooling towers are based on the principles of evaporative cooling, the maximum cooling tower efficiency depends on the wet bulb temperature of the air.  The wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor
{{anchor |Range}}
*'''Range'''{{dash}}The range is the temperature difference between the warm water inlet and cooled water exit.
{{anchor |Fill}}
*'''Fill'''{{dash}}Inside the tower, fills are added to increase contact surface as well as contact time between air and water, to provide better heat transfer. The efficiency of the tower depends on the selection and amount of fill. There are two types of fills that may be used:
**'''Film type fill''' (causes water to spread into a thin film)
**'''Splash type fill''' (breaks up falling stream of water and interrupts its vertical progress)
{{anchor |Full-Flow Filtration|Full-Flow}}
*'''Full-flow filtration'''{{dash}}Full-flow filtration continuously strains particulates out of the entire system flow. For example, in a 100-ton system, the flow rate would be roughly 300 gal/min. A filter would be selected to accommodate the entire 300 gal/min flow rate. In this case, the filter typically is installed after the cooling tower on the discharge side of the pump. While this is the ideal method of filtration, for higher flow systems it may be cost-prohibitive.
{{anchor |Side-Stream Filtration|Side-Stream}}
*'''Side-stream filtration'''{{dash}}Side-stream filtration, although popular and effective, does not provide complete protection. With side-stream filtration, a portion of the water is filtered continuously. This method works on the principle that continuous particle removal will keep the system clean. Manufacturers typically package side-stream filters on a skid, complete with a pump and controls. For high flow systems, this method is cost-effective. Properly sizing a side-stream filtration system is critical to obtain satisfactory filter performance, but there is some debate over how to properly size the side-stream system. Many engineers size the system to continuously filter the cooling tower basin water at a rate equivalent to 10% of the total circulation flow rate. For example, if the total flow of a system is 1,200 gal/min (a 400-ton system), a 120 gal/min side-stream system is specified.
{{anchor |Cycle of concentration|Cycle}}
*'''Cycle of concentration'''{{dash}}Maximum allowed multiplier for the amount of miscellaneous substances in circulating water compared to the amount of those substances in make-up water.
{{anchor |Treated timber|Treated}}
*'''Treated timber'''{{dash}}A structural material for cooling towers which was largely abandoned in the early 2000s. It is still used occasionally due to its low initial costs, in spite of its short life expectancy. The life of treated timber varies a lot, depending on the operating conditions of the tower, such as frequency of shutdowns, treatment of the circulating water, etc. Under proper working conditions, the estimated life of treated timber structural members is about 10 years.
{{anchor |Leaching}}
*'''Leaching'''{{dash}}The loss of wood preservative chemicals by the washing action of the water flowing through a wood structure cooling tower.
{{anchor |Pultruded FRP|Pultruded}}
*'''Pultruded FRP'''{{dash}}A common structural material for smaller cooling towers, [[fibre-reinforced plastic]] (FRP) is known for its high corrosion-resistance capabilities. Pultruded FRP is produced using [[pultrusion]] technology, and has become the most common structural material for small cooling towers. It offers lower costs and requires less maintenance compared to reinforced concrete, which is still in use for large structures.

==Fog production==
[[File:Clouds, natural and otherwise - geograph.org.uk - 778331.jpg|thumb|Fog produced by [[Eggborough power station]]]]
Under certain ambient conditions, plumes of water vapor can be seen rising out of the discharge from a cooling tower, and can be mistaken as smoke from a fire. If the outdoor air is at or near saturation, and the tower adds more water to the air, saturated air with liquid water droplets can be discharged, which is seen as fog. This phenomenon typically occurs on cool, humid days, but is rare in many climates. Fog and clouds associated with cooling towers can be described as homogenitus, as with other clouds of man-made origin, such as [[contrail]]s and [[ship track]]s.<ref>{{cite news|last1=Sutherland|first1=Scott|title=Cloud Atlas leaps into 21st century with 12 new cloud types|url=https://www.theweathernetwork.com/news/articles/cloud-atlas-leaps-into-21st-century-with-12-new-cloud-types/80685/|access-date=24 March 2017|work=The Weather Network|agency=Pelmorex Media|date=23 March 2017}}</ref>

This phenomenon can be prevented by decreasing the relative humidity of the saturated discharge air. For that purpose, in hybrid towers, saturated discharge air is mixed with heated low relative humidity air. Some air enters the tower above drift eliminator level, passing through heat exchangers. The relative humidity of the dry air is even more decreased instantly as being heated while entering the tower. The discharged mixture has a relatively lower relative humidity and the fog is invisible.{{Cn|date=May 2021}}

== Cloud formation  ==
Issues related to applied meteorology of cooling towers, including the assessment of the impact of cooling towers on cloud enhancement were considered in a series of models and experiments. One of the results by [[Krzysztof Edward Haman|Haman's group]] indicated significant dynamic influences of the condensation trails on the surrounding atmosphere, manifested in temperature and humidity disturbances. The mechanism of these influences seemed to be associated either with the airflow over the trail as an obstacle or with vertical waves generated by the trail, often at a considerable altitude above it.<ref name="Haman1989">{{Cite journal
 | last1       = Haman
 | first1      = Krzysztof E.
 | last2       = Malinowski
 | first2      = Szymon P.
 | title       = Observations of cooling tower and stack plumes and their comparison with plume model "ALINA"
 | journal     = Atmospheric Environment
 | volume      = 23
 | issue       = 6
 | pages       = 1223–1234
 | year        = 1989
 | publisher   = Elsevier
| doi = 10.1016/0004-6981(89)90149-2
 | bibcode = 1989AtmEn..23.1223H
 }}
</ref>

==Salt emission pollution==
When wet cooling towers with seawater make-up are installed in various industries located in or near coastal areas, the drift of fine droplets emitted from the cooling towers contain nearly 6% sodium chloride which deposits on the nearby land areas. This deposition of sodium salts on the nearby agriculture and vegetative lands can convert them into [[Soil salinity control|sodic saline]] or [[Alkali soils|sodic alkaline soils]] depending on the nature of the soil and enhance the [[Sodium adsorption ratio|sodicity]] of ground and surface water. The salt deposition problem from such cooling towers aggravates where pollution control standards are not imposed or not implemented to minimize the drift emissions from wet cooling towers using seawater make-up.<ref name="wet cooling">[http://www.ec.gc.ca/inrp-npri/default.asp?lang=En&n=2ED8CFA7-1 Wet Cooling Tower Guidance For Particulate Matter, Environment Canada] {{Webarchive|url=https://web.archive.org/web/20150403110105/http://www.ec.gc.ca/inrp-npri/default.asp?lang=En&n=2ED8CFA7-1 |date=3 April 2015 }}, Retrieved on 2013-01-29</ref>

[[Respirable suspended particle|Respirable suspended particulate matter]], of less than 10 [[micrometre|micrometers]] (μm) in size, can be present in the drift from cooling towers. Larger particles above 10&nbsp;μm in size are generally filtered out in the nose and throat via cilia and mucus but particulate matter smaller than 10&nbsp;μm, referred to as PM<sub>10</sub>, can settle in the bronchi and lungs and cause health problems. Similarly, particles smaller than 2.5&nbsp;μm, (PM<sub>2.5</sub>), tend to penetrate into the gas exchange regions of the lung, and very small particles (less than 100 nanometers) may pass through the lungs to affect other organs. Though the total particulate emissions from wet cooling towers with fresh water make-up is much less, they contain more PM<sub>10</sub> and PM<sub>2.5</sub> than the total emissions from wet cooling towers with sea water make-up. This is due to lesser salt content in fresh water drift (below 2,000&nbsp;ppm) compared to the salt content of sea water drift (60,000&nbsp;ppm).<ref name="wet cooling" />

==Use as a flue-gas stack==
[[File:Chimney included in cooling tower.jpg|upright|thumb|left|Flue-gas stack inside a natural draft wet cooling tower]]
[[File:Power station Duisburg Walsum.JPG|thumb|Flue gas stack connection into a natural draft wet cooling tower]]
At some modern power stations equipped with [[flue-gas desulfurization|flue gas purification]], such as the [[Großkrotzenburg Power Station]] and the [[Rostock Power Station]], the cooling tower is also used as a [[flue-gas stack]] (industrial chimney), thus saving the cost of a separate chimney structure. At plants without flue gas purification, problems with corrosion may occur, due to reactions of raw flue gas with water to form [[acid]]s.{{Cn|date=May 2021}}

Sometimes, natural draft cooling towers are constructed with structural steel in place of concrete (RCC) when the construction time of natural draft cooling tower is exceeding the construction time of the rest of the plant or the local soil is of poor strength to bear the heavy weight of RCC cooling towers or cement prices are higher at a site to opt for cheaper natural draft cooling towers made of structural steel.{{Cn|date=May 2021}}

==Operation in freezing weather==
{{Unreferenced section|date=April 2021}}
[[File:Песочин ТЭЦ5 Градирни VizuIMG 2181.JPG|thumb|Large [[hyperboloid]] cooling towers made of structural steel for a power plant in Kharkiv (Ukraine)]]
Some cooling towers (such as smaller building air conditioning systems) are shut down seasonally, drained, and winterized to prevent freeze damage.

During the winter, other sites continuously operate cooling towers with {{convert|4|°C|°F|abbr=on}} water leaving the tower.  Basin heaters, tower draindown, and other freeze protection methods are often employed in cold climates. Operational cooling towers with malfunctions can freeze during very cold weather. Typically, freezing starts at the corners of a cooling tower with a reduced or absent heat load. Severe freezing conditions can create growing volumes of ice, resulting in increased structural loads which can cause structural damage or collapse.

To prevent freezing, the following procedures are used:
* The use of water modulating by-pass systems is not recommended during freezing weather. In such situations, the control flexibility of variable speed motors, two-speed motors, and/or two-speed motors multi-cell towers should be considered a requirement.
* Do not operate the tower unattended.  Remote sensors and alarms may be installed to monitor tower conditions.
* Do not operate the tower without a heat load.  Basin heaters may be used to keep the water in the tower pan at an above-freezing temperature. Heat trace ("heating tape") is a resistive heating element that is installed along water pipes to prevent freezing in cold climates.
* Maintain design water flow rate over the tower fill.
* Manipulate or reduce airflow to maintain water temperature above freezing point.

==Fire hazard==
Cooling towers constructed in whole or in part of combustible materials can support internal fire propagation. Such fires can become very intense, due to the high [[surface-volume ratio]] of the towers, and fires can be further intensified by natural convection or fan-assisted draft.  The resulting damage can be sufficiently severe to require the replacement of the entire cell or tower structure. For this reason, some [[model building code|code]]s and standards<ref>[[National Fire Protection Association]] (NFPA). NFPA 214, [http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=214 ''Standard on Water-Cooling Towers''] {{Webarchive|url=https://web.archive.org/web/20100617095816/http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=214 |date=17 June 2010 }}.</ref> recommend that combustible cooling towers be provided with an automatic [[fire sprinkler system]]. Fires can propagate internally within the tower structure when the cell is not in operation (such as for maintenance or construction), and even while the tower is in operation, especially those of the induced-draft type, because of the existence of relatively dry areas within the towers.<ref>NFPA 214, [http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=214 ''Standard on Water-Cooling Towers.''] {{Webarchive|url=https://web.archive.org/web/20100617095816/http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=214 |date=17 June 2010 }} Section A1.1</ref>

==Structural stability==
Being very large structures, cooling towers are susceptible to wind damage, and several spectacular failures have occurred in the past. At [[Ferrybridge power station]] on 1 November 1965, the station was the site of a major [[structural failure]], when three of the cooling towers collapsed owing to vibrations in {{convert|85|mi/h|km/h|abbr=on}} winds.<ref>{{Cite news|url=https://www.bbc.com/news/uk-england-leeds-35927009|title=Ferrybridge C Power Station officially closes after 50 years|work=BBC News |date=31 March 2016}}</ref> Although the structures had been built to withstand higher wind speeds, the shape of the cooling towers caused westerly winds to be funneled into the towers themselves, creating a [[vortex]]. Three out of the original eight cooling towers were destroyed, and the remaining five were severely damaged. The towers were later rebuilt and all eight cooling towers were strengthened to tolerate adverse weather conditions. Building codes were changed to include improved structural support, and [[wind tunnel]] tests were introduced to check tower structures and configuration.{{Citation needed|date=June 2017}}

==See also==
{{div col|colwidth=30em}}
* [[List of tallest cooling towers]]
* [[Alkali soils]]
* [[Architectural engineering]]
* [[Deep lake water cooling]]
* [[Evaporative cooler]]
* [[Evaporative cooling]]
* [[Fossil fuel power plant]]
* [[HVAC|Heating, ventilating and air conditioning]]
* [[Hyperboloid structure]]
* [[Mechanical engineering]]
* [[Nuclear power plant]]
* [[Power station]]
* [[Spray pond]]
* [[Water cooling]]
* [[Willow Island disaster]]
{{div col end}}

==References==
{{Reflist|30em}}

==External links==
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{{Commons}}
*[http://www.cti.org/whatis/coolingtowerdetail.shtml What is a cooling tower?] {{Webarchive|url=https://web.archive.org/web/20100507124703/http://www.cti.org/whatis/coolingtowerdetail.shtml |date=7 May 2010 }} – Cooling Technology Institute
*[http://www.nucleartourist.com/systems/ct.htm "Cooling Towers" – includes diagrams] – Virtual Nuclear Tourist
*[https://web.archive.org/web/20150403110105/http://www.ec.gc.ca/inrp-npri/default.asp?lang=En&n=2ED8CFA7-1 Wet cooling tower guidance for particulate matter, Environment Canada.]
*[https://www.lonelyplanet.com/news/2017/02/15/photographer-europes-abandoned-cooling-towers/ Striking pictures of Europe's abandoned cooling towers] by Reginald Van de Velde, Lonely Planet, 15 February 2017 (see also excerpt from [https://www.bbc.co.uk/news/av/world-europe-38029636/see-inside-giant-cooling-towers radio interview], ''[[World Update]]'', BBC, 21 November 2016)

{{Electricity generation}}
{{HVAC}}
{{Wastewater}}
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{{DEFAULTSORT:Cooling Tower}}
[[Category:Cooling towers| ]]
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[[Category:Heating, ventilation, and air conditioning]]
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