Cooling tower

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Cooling towers are structures for cooling water or other working medium to near-ambient temperature. 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 and other industrial facilities. The circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling tower amounts to about 71,600 cubic meters an hour (315,000 U.S. gallons per minute) [1] and the circulating water requires a supply water makeup rate of perhaps 5 percent (i.e., 3,600 cubic meters an hour).

If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic meters an hour [2] and that amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuosly 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. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water.

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 also have very large cooling tower systems. A typical large refinery processing 40,000 metric tonnes of crude oil per day (300,000 barrels per day) circulates about 80,000 cubic meters of water per hour through its cooling tower system.

Image:Didcot power station cooling tower zootalures.jpg With respect to the heat transfer mechanism employed, the main types are:

In a wet cooling tower the warm water can be cooled to a temperature lower than ambient, if the ambient air is relatively dry. (see: Dew point).

With respect to drawing air through the tower there are three types of cooling towers:

  • Natural draft, which utilizes a tall chimney
  • Fan assisted natural draft
  • Mechanical draft (or forced draft) which uses power driven fan motors to force or draw air through the tower

Under certain ambient conditions, plumes of water vapor (fog) can be seen rising out of a wet cooling tower (see image).

Contents

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Cooling tower as a chimney

At some modern power stations, equipped with flue gas purification like the Power Station Staudinger Grosskrotzenburg and the Power Station Rostock the cooling tower is also used as a chimney. At plants without flue gas purification this causes problems with corrosion.

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Wet cooling tower material balance

Main article: Cooling tower system

Quantitatively, the material balance around a wet cooling tower system is governed by the operational variables of makeup flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles:

Image:CoolingTower.gif

M = Make-up water in m³/hr
C = Circulating water in m³/hr
D = Draw-off water in m³/hr
E = Evaporated water in m³/hr
W = Windage loss of water in m³/hr
X = Concentration in ppmw (of any completely soluble salts … usually chlorides)
XM = Concentration of chlorides in make-up water (M), in ppmw
XC = Concentration of chlorides in circulating water (C), in ppmw
Cycles = Cycles of concentration = XC / XM (dimensionless)
ppmw = parts per million by weight

In the above sketch, water pumped from the tower basin is the cooling water routed through the process coolers and 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 (W) and some of the water (E) to 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 salts 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 (D) for disposal. Fresh water makeup (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.

A water balance around the entire system is:

M = E + D + W

Since the evaporated water (E) has no salts, a chloride balance around the system is:

M (XM) = D (XC) + W (XC) = XC (D + W)

and, therefore:

XC / XM = Cycles of concentration = M ÷ (D + W) = M ÷ (M – E) = 1 + [E ÷ (D + W)]

From a simplified heat balance around the cooling tower:

E = C · ΔT · cp ÷ HV
where:  
HV = latent heat of vaporization of water = ca. 2260 kJ / kg
ΔT = water temperature difference from tower top to tower bottom, in °C
cp = specific heat of water = ca. 4.184 kJ / kg / °C

Windage losses (W), in the absence of manufacturer's data, may be assumed to be:

W = 0.3 to 1.0 percent of C for a natural draft cooling tower
W = 0.1 to 0.3 percent of C for an induced draft cooling tower
W = about 0.01 percent of C if the cooling tower has windage drift eliminators

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

The chemistry of the makeup water including the amount of dissolved minerals can vary widely. Makeup waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Makeup waters from ground water supplies (wells) are usually higher in minerals and tend to be 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.

As the cycles of concentration increase the water may not be able to hold the minerals in 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 water treatment consultant will evaluate the makeup 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 the majority of water supplies are well waters and have significant levels of dissolved solids. On the other hand one of the largest water supplies, New York City, has a surface supply quite low in minerals and cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.

(Note: Draw-off and blowdown are synonymous. Windage and drift are also synonymous.)

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Frequency of Cleaning

Cooling towers should be cleaned and disinfected at least twice a year. Normally this maintenance will be performed before initial start-up at the beginning of the cooling season and after shut-down in the fall. Systems with heavy bio-fouling or high levels of Legionella may require additional cleaning.

Legionella is a Gram negative bacterium, including species that cause Legionellosis or Legionnaires' disease

Any system that has been out of service for an extended period should be cleaned and disinfected. New systems require cleaning and disinfecting because construction material residue can contribute to Legionella growth.

After a cooling tower has been drained and cleaned, some experts recommend an 'on line' disinfection using a minimum of 5 ppm free halogen residual while running the pumps for at least 6 hours before operating the fans.

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Design - Best Practices Guide for Cooling Towers

High-efficiency drift eliminators are essential for all cooling towers. Cross-flow cooling tower drift eliminators rated at 0.005% drift rates are available for retro-fits of existing towers. Counter-flow cooling tower drift eliminators rated at 0.001% drift rates are available for retro-fits of existing towers. Older systems can usually be retrofitted with high-efficiency models. A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for exposure. Other important design features include easy access or easily disassembled components to allow cleaning of internal components including the packing (fill). Enclosure of the system will prevent unnecessary drift of water vapor, and other design features to minimize the spray generated by these systems are also desirable.

The fiberglass and stainless steel cooling towers are rated for higher levels of chlorine than the galvanized towers. Some manufacturers recommend that their galvanized cooling towers not be exposed to chlorine levels above 1 ppm. There was a report of a trend in Europe to install more heat exhangers and fewer cooling towers because of the risks of Legionella.

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Legionellae and temperature

  • Below 68 °F: Legionellae can survive but are dormant
  • 68 to 122 °F (20 to 50°C): Legionellae growth range
  • 95 to 115 °F (35 to 46°C): Ideal growth range
  • Above 122 °F (50 °C): They can survive but do not multiply
  • At 131 °F (55 °C): Legionellae die within 5 to 6 hours
  • At 140 °F (60 °C): Legionellae die within 32 minutes
  • At 151 °F (66 °C): Legionellae die within 2 minutes
  • 158 to 176 °F (70 to 80 °C): Disinfection range
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External links

es:Torre de refrigeración fr:Tour de réfrigération nl:Koeltoren ja:冷却塔 ru:Градирня fi:Jäähdytystorni

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