Cooling tower
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 air temperature or, in the case of dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature using radiators.
Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations, nuclear power stations 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 draft and induced draft cooling towers.
Cooling towers vary in size from small roof-top units to very large hyperboloid structures that can be up to tall and in diameter, or rectangular structures that can be over tall and long. Hyperboloid cooling towers are often associated with nuclear power plants, 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 to condense and recirculate the water. 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, when in reality the emissions from those towers mostly do not contribute to carbon footprint, consisting solely of water vapor.
History
Cooling towers originated in the 19th century through the development of condensers for use with the steam engine. 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. However, the condensers require an ample supply of cooling water, without which they are impractical. While water usage is not an issue with marine engines, it forms a significant limitation for many land-based systems.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. In areas with available land, the systems took the form of cooling ponds; in areas with limited land, such as in cities, they took the form of cooling towers.
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. 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 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.”
A hyperboloid cooling tower was patented by the Dutch engineers Frederik van Iterson and Gerard Kuypers in the Netherlands on August 16, 1916. The first hyperboloid reinforced concrete cooling tower was built by the Dutch State Mine Emma in 1917 in Heerlen. The first ones in the United Kingdom were built in 1924 at Lister Drive power station in Liverpool, England. On both locations they were built to cool water used at a coal-fired electrical power station.
According to a , the indirect–dew-point evaporative-cooling Maisotsenko 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 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.
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.
Classification by use
Heating, ventilation and air conditioning (HVAC)
An HVAC cooling tower is used to dispose of 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 temperatures. Air-cooled chillers must reject heat at the higher dry-bulb temperature, and thus have a lower average reverse–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 ton of air-conditioning is defined as the removal of. The equivalent ton on the cooling tower side actually rejects about 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 or of water by, which amounts to, assuming a chiller coefficient of performance of 4.0. This COP is equivalent to an energy efficiency ratio 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 heat is supplied by other means, usually from separate boilers.
Industrial cooling towers
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, 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. The circulation rate of cooling water in a typical 700 MWth coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour and the circulating water requires a supply water make-up rate of perhaps 5 percent.If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour A large cooling water intake typically kills millions of fish and larvae annually, as the organisms are impinged on the intake screens. 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, or can also cause an increase in undesirable organisms such as invasive species of zebra mussels 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, whereas surface-only cooling water can be re-used.
Some coal-fired and nuclear power plants 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 circulates about 80,000 cubic metres of water per hour through its cooling tower system.
The world's tallest cooling tower is the tall cooling tower of the Pingshan II Power Station in Huaibei, Anhui Province, China.
Classification by build
Package type
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. There are six types of package cooling towers: dry, closed wet, open wet, and three hybrid systems.Due to their frequent use in or near residential areas, sound level control is a relatively more important issue for package type cooling towers.