Cooling tower

Cooling towers are evaporative coolers used for cooling water or other working medium to near-ambient temperature. Cooling towers use evaporation of water to reject heat from the system. They vary in size from small roof-top units to very large hyperbolic structures (as in Image 1) that can be over 120 meters tall and 100 meters in length or rectangular structures (as in Image 2) that can be over 40 meters tall and 80 meters long.

Classification by use

Cooling towers can generally be classifed by use into either industrial or HVAC (air-conditioning) duty.

Industrial cooling towers can by used to reject heat from various sources such as machinery or heated process material.

An HVAC cooling tower is a subcategory rejecting heat generated by a chiller. As heat loads increase, water-cooled chillers are more energy efficient than air-cooled chillers. Large office buildings, hospitals, schools typically use a cooling tower as part of their air conditioning systems.

Generally, industrial cooling towers are much larger than HVAC towers and are entirely erected on site. HVAC cooling towers can be compact enough to factory assemble and ship nearly complete.

Industrial Cooling Towers

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) Cooling System Retrofit Costs EPA Workshop on Cooling Water Intake Technologies, May 2003 and the circulating water requires a supply water make-up 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 Department of Energy Office of Fossil Energy's Power Plant Water Management R&D Program and that 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. 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.

HVAC Cooling Towers

HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller. A ton of air-conditioning is the rejection of 12,000 Btu/hour (12,661 kJ/hour). The equivalent ton on the cooling tower side actually rejects 15,000 Btu/hour (15,826 kJ/hour). This equivalent ton is defined as the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10 °F, which amounts to 15,000 Btu/hour.

Heat transfer methods

With respect to the heat transfer mechanism employed, the main types are:

wet cooling towers or simply cooling towers operate on the principle of evaporation. (see Swamp cooler)
dry cooling towers operate by heat transmission through a surface that divides the working fluid from ambient air.
hybrids are also available.

In order to achieve better performance (more cooling), a media called fill is used to increase the surface area 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 upon which the water flows. Both methods create increased surface area.

Air flow generation methods

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

Natural draft, which utilizes a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. This produces a current of air through the tower. (see Image 1)
Mechanical draft, which uses power driven fan motors to force or draw air through the tower.
- Induced draft: A mechanical draft tower with a fan at the discharge which pulls air through 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. (see Image 2, 3)
- Forced draft: 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 forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and even in some indoor situations. (see Image 4)
Fan assisted natural draft. A hybrid type that appears like a natural draft though airflow is assisted by a fan.

Under certain ambient conditions, plumes of water vapor (fog) can be seen rising out of the discharge from a cooling tower (see Image 1). Ambient conditions dictate the efficiency of any given tower due to the amount of water vapor the air is able to absorb and hold, as can be determined on a psychrometric chart.

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. Increased freezing conditions can create growing volumes of ice, resulting in increased structural loads. During the winter, some sites continously operate cooling towers with 40 °F water leaving the tower.

Categorization by air-to-water flow

Crossflow

Crossflow is a design in which the air flow is directed perpendicular to the water flow (see diagram below). Air flow 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 area. A distribution or hot water basin consisting of a deep pan with holes or nozzles in the bottom is utilized in a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material.

Counterflow

In a counterflow design the air flow is directly opposite of the water flow (see diagram below). Air flow first enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow.

Common to both designs:

The interaction of the air and water flow allow a partial equalization and evaporation of water.
The air, now saturated with water vapor, is discharged from the cooling tower.
A collection or cold water basin is used to contain the water after its interaction with the air flow.

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

Cooling tower as a flue gas stack (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 flue gas stack (chimney). At plants without flue gas purification, this causes problems with corrosion.

Wet cooling tower material balance

Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of makeup flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles: (available in many university libraries)

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)
X_M	= Concentration of chlorides in make-up water (M), in ppmw
X_C	= Concentration of chlorides in circulating water (C), in ppmw
Cycles	= Cycles of concentration = X_C / X_M (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 (X_M) = D (X_C) + W (X_C) = X_C (D + W)

and, therefore:

X_C / X_M = Cycles of concentration = M ÷ (D + W) = M ÷ (M – E) = 1 + ÷ (D + W)

From a simplified heat balance around the cooling tower:

E = C · ΔT · c_p ÷ H_V

where:
H_V	= latent heat of vaporization of water = ca. 2260 kJ / kg
ΔT	= water temperature difference from tower top to tower bottom, in °C
c_p	= specific heat of water = ca. 4.184 kJ / (kg $\cdot$ °C)

Windage (or drift) losses (W) from large-scale industrial cooling towers, 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 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

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.

Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling, the water should also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water.

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

Cooling towers and Legionnaires' disease

Another very important reason for using biocides in cooling towers is to prevent the growth of Legionella which is a Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophilia. The various Legionella species are the cause of Legionnaires' disease in humans and transmission is via exposure to aerosols—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.

French researchers found that Legionella spread through the air up to 6 kilometers from a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais, France. That outbreak killed 21 of the 86 people that had a laboratory-confirmed infection.Airborne Legionella May Travel Several Kilometers (access requires free registration)

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 hold drift rates typically to 0.001%-0.005% of the circulating flow rate. A typical drift eliminator provides multiple directional changes of airflow while preventing the escape of water droplets. A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for Legionella or other chemical exposure.

Many governmental agencies, cooling tower manufacturers and industrial trade organizations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers. Below is a list of sources for such guidelines:

Centers for Disease Control and Prevention - Procedure for Cleaning Cooling Towers and Related Equipment (pages 239 and 240 of 249)
Cooling Technology Institute - Best Practices for Control of Legionella
Association of Water Technologies - Legionella 2003
California Energy Commission - Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants
Marley Cooling Technologies - Cooling Towers Maintenance Procedures
Marley Cooling Technologies - ASHRAE Guideline 12-2000 - Minimizing the Risk of Legionellosis
Marley Cooling Technologies - Cooling Tower Inspection Tips {especially page 3 of 7}
Tower Tech Modular Cooling Towers - Legionella Control
GE Infrastructure Water & Process Technologies Betz Dearborn - Chemical Water Treatment Recommendations For Reduction of Risks Associated with Legionella in Open Recirculating Cooling Water Systems

Cooling Tower Operation In Freezing Weather

Do not operate the tower unattended.J. L. Hermon & Associates, Inc.: Cooling Tower Operation In Freezing Weather

Do not operate the tower without a heat load.

Maintain design water flow rate over the fill.

Manipulate airflow to maintain water temperature above freezing point.Marley: Cooling Tower Operation In Freezing Weather

Cooling towers with malfunctions can freeze. Failures that let smaller amounts of water go the top of a cooling tower can cause a tower to freeze (especially if the fans are running at high speeds). If a roof-mounted cooling tower is allowed to freeze and build up ice, the ice can grow to massive sizes and can result in the tower falling through the roof.

External links

References

Cooling technology | Electric power | Power plants | Oil refineries | Chemical engineering

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