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# Has anyone heard of a cooling tower 'universal rating chart'?

## I am studying for my PE exam and came across a sample problem that refers to using a 'universal rating chart' for a cooling tower operating at 20,000 gpm and 90-80-70 deg. F and changing the temperatures to 125-90-80 deg. F. I have not been able to find something like this and the chart that the cooling tower guru at work found in a vendor catalog isn't quite it. If anyone can direct me to a website or a reference/standard/book, etc. that would have it, I would GREATLY appreciate it! I figure it might be handy to have down the road anyway since I am a mechanical engineer in the power industry. Thanks!

Asked By: engr_tam - 10/12/2007
Best Answer - Chosen by Asker
This article has so info and charts with comparions for wet
and dry bulb formulas. Also Wikipedia, the free encyclopedia has a lot of Formulas under Cooling Towers.
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Cooling Towers: Design and Operation Considerations

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Cooling towers are a very important part of many chemical plants. They represent a relatively inexpensive and dependable means of removing low grade heat from cooling water.

ctowers1.gif (3535 bytes)

Figure 1: Closed Loop Cooling Tower System

The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling.

Types of Cooling Towers
Cooling towers fall into two main sub-divisions: natural draft and mechanical draft. Natural draft designs use very large concrete chimneys to introduce air through the media. Due to the tremendous size of these towers (500
ft high and 400 ft in diameter at the base) they are generally used for water flowrates above 200,000 gal/min. Usually these types of towers are only used by utility power stations in the United States. Mechanical draft cooling towers are much more widely used. These towers utilize large fans to force air through circulated water. The water falls downward over fill surfaces which help increase the contact time between the water and the air. This helps maximize heat transfer between the two.

Types of Mechanical Draft Towers

ctowers2.gif (12304 bytes)

Figure 2: Mechanical Draft Counterflow Tower Figure 3: Mechanical Draft Crossflow Tower

Mechanical draft towers offer control of cooling rates in their fan diameter and speed of operation. These towers often contain several areas (each with their own fan) called cells.

Cooling Tower Theory
Heat is transferred from water drops to the surrounding air by the transfer of sensible and latent heat.

ctowers3.gif (3317 bytes)

Figure 4: Water Drop with Interfacial Film

This movement of heat can be modeled with a relation known as the Merkel Equation:
ctowers4.gif (1257 bytes)
(1)

where:
KaV/L = tower characteristic
K = mass transfer coefficient (lb water/h ft2)
a = contact area/tower volume
V = active cooling volume/plan area
L = water rate (lb/h ft2)
T1 = hot water temperature (0F or 0C)
T2 = cold water temperature (0F or 0C)
T = bulk water temperature (0F or 0C)
hw = enthalpy of air-water vapor mixture at bulk water temperature
(J/kg dry air or Btu/lb dry air)
ha = enthalpy of air-water vapor mixture at wet bulb temperature
(J/kg dry air or Btu/lb dry air)

Thermodynamics also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air:
ctowers5.gif (1432 bytes) (2)

(3)

where:
L/G = liquid to gas mass flow ratio (lb/lb or kg/kg)
T1 = hot water temperature (0F or 0C)
T2 = cold water temperature (0F or 0C)
h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature (same units as above)
h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature (same units as above)

The tower characteristic value can be calculated by solving Equation 1 with the Chebyshev numberical method:
ctowers6.gif (3725 bytes)
(4)

ctowers7.gif (3927 bytes)

Figure 5: Graphical Representation of Tower Characteristic

The following represents a key to Figure 5:
C' = Entering air enthalpy at wet-bulb temperature, Twb
BC = Initial enthalpy driving force
CD = Air operating line with slope L/G
DEF = Projecting the exiting air point onto the water operating line and then onto the
temperature axis shows the outlet air web-bulb temperature

As shown by Equation 1, by finding the area between ABCD in Figure 5, one can find the tower characteristic. An increase in heat load would have the following effects on the diagram in Figure 5:
1. Increase in the length of line CD, and a CD line shift to the right
2. Increases in hot and cold water temperatures
3. Increases in range and approach areas
The increased heat load causes the hot water temperature to increase considerably faster than does the cold water temperature. Although the area ABCD should remain constant, it actually decreases about 2?or every 10 0F increase in hot water temperature above 100 0F. To account for this decrease, an "adjusted hot water temperature" is usd in cooling tower design.

ctowers8.gif (7822 bytes)

Figure 6: Graph of Adjusted Hot Water Temperatures

The area ABCD is expected to change with a change in L/G, this is very key in the design of cooling towers.

Cooling Tower Design
Although KaV/L can be calculated, designers typically use charts found in the Cooling Tower Institute Blue Book to estimate KaV/L for given design conditions. It is important to recall three key points in cooling tower design:
1. A change in wet bulb temperature (due to atmospheric conditions) will not change the tower characteristic (KaV/L)
2. A change in the cooling range will not change KaV/L
3. Only a change in the L/G ratio will change KaV/L

ctowers9.gif (112606 bytes)

Figure 7: A Typical Set of Tower Characteristic Curves

The straight line shown in Figure 7 is a plot of L/G vs KaV/L at a constant airflow. The slope of this line is dependent on the tower packing, but can often be assumed to be -0.60. Figure 7 represents a typical graph supplied by a manufacturer to the purchasing company. From this graph, the plant engineer can see that the proposed tower will be capable of cooling the water to a temperature that is 10 0F above the wet-bulb temperature. This is another key point in cooling tower design.
Cooling towers are designed according to the highest geographic wet bulb temperatures. This temperature will dictate the minimum performance available by the tower. As the wet bulb temperature decreases, so will the available cooling water temperature. For example, in the cooling tower represented by Figure 7, if the wet bulb temperature dropped to 75 0F, the cooling water would still be exiting 10 0F above this temperature (85 0F) due to the tower design.
Below is the summary of steps in the cooling tower design process in industry. More detail on these steps will be given later.
1. Plant engineer defines the cooling water flowrate, and the inlet and outlet water temperatures for the tower.
2. Manufacturer designs the tower to be able to meet this criteria on a "worst case scenario" (ie. during the hottest months). The tower characteristic curves and the estimate is given to the plant engineer.
3. Plant engineer reviews bids and makes a selection

Design Considerations
Once a tower characteristic has been established between the plant engineer and the manufacturer, the manufacturer must design a tower that matches this value. The required tower size will be a function of:
1. Cooling range
2. Approach to wet bulb temperature
3. Mass flowrate of water
4. Web bulb temperature
5. Air velocity through tower or individual tower cell
6. Tower height
In short, nomographs such as the one shown on page 12-15 of Perry's Chemical Engineers' Handbook 6th Ed. utilize the cold water temperature, wet bulb temperature, and hot water temperature to find the water concentration in gal/min ft2. The tower area can then be calculated by dividing the water circulated by the water concentration. General rules are usually used to determine tower height depending on the necessary time of contact:

Approach to Wet Bulb (0F)

Cooling Range (0F)

Tower Height (ft)
15-20 25-35 15-20
10-15 25-35 25-30
5-10 25-35 35-40

Other design characteristics to consider are fan horsepower, pump horsepower, make-up water source, fogging abatement, and drift eliminators.

Operation Considerations

Water Make-up
Water losses include evaporation, drift (water entrained in discharge vapor), and blowdown (water released to discard solids). Drift losses are estimated to be between 0.1 and 0.2?f water supply.

Evaporation Loss = 0.00085 * water flowrate(T1-T2) (5)
Blowdown Loss = Evaporation Loss/(cycles-1) (6)
where cycles is the ratio of solids in the circulating water to the
solids in the make-up water
Total Losses = Drift Losses + Evaporation Losses + Blowdown Losses (7)

Cold Weather Operation
Even during cold weather months, the plant engineer should maintain the design water flowrate and heat load in each cell of the cooling tower. If less water is needed due to temperature changes (ie. the water is colder), one or more cells should be turned off to maintain the design flow in the other cells. The water in the base of the tower should be maintained between 60 and 70 0F by adjusting air volume if necessary. Usual practice is to run the fans at half speed or turn them off during colder months to maintain this temperature range.

You can download a small DOS program that will calculate the tower characteristic or cold water temperature for a given tower based on a few inputs. Download here!

Click Here!

References:

1. The Standard Handbook of Plant Engineering, 2nd Edition, Rosaler, Robert C., McGraw-Hill, New York, 1995

2. Perry's Chemical Engineers' Handbook, 6th Edition, Green, Don W. et al, McGraw-Hill, New York, 1984

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The Chemical Engineers' Resource Page, 1442 Goswick Ridge Road, Midlothian, VA 23114, Email
All material is Copyright © 2004, www.cheresources.com
Cooling towers areevaporative coolers used for cooling water or other working medium to near the ambient wet-bulb air temperature. Cooling towers use evaporation of water to reject heat from processes such as cooling the circulating water used in oil refineries, chemical plants, power plants and building cooling, for example. The towers vary in size from small roof-top units to very large hyperboloid structures (as in Image 1) that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures (as in Image 2) that can be over 40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site.
Contents
[hide]

* 1 Classification by use
o 1.1 HVAC
o 1.2 Industrial
* 2 Heat transfer methods
* 3 Air flow generation methods
* 4 Categorization by air-to-water flow
o 4.1 Crossflow
o 4.2 Counterflow
* 5 Cooling tower as a flue gas stack (Industrial chimney)
* 6 Wet cooling tower material balance
* 7 Cooling towers and Legionnaires' disease
* 8 Cooling tower malfunctions
o 8.1 Cooling Tower Operation In Freezing Weather
* 9 Some commonly used terms in the cooling tower industry
* 10 See also
* 11 External links
* 12 References

 Classification by use

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

 HVAC

An HVAC cooling tower is a subcategory rejecting heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at near wet-bulb temperatures. Air-cooled chillers must reject heat to the dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. Large office buildings, hospitals, schools typically use one or more cooling towers as part of 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 water-cooled chiller or water-cooled condenser. 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 about 15,000 Btu/hour (15,826 kJ/hour) due to the heat-equivalent of the energy needed to drive the chiller's compressor. 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, or a chiller coefficient-of-performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 13.65.

 Industrial

Industrial cooling towers can be used to reject 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 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 metres an hour (315,000 U.S. gallons per minute)[1] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour).

If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres 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 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 metres of water per hour through its cooling tower system.

The world's tallest cooling tower is the 200 metre tall cooling tower of Niederaussem Power Station.

 Heat transfer methods
Image 3: Mechanical draft crossflow cooling tower used in an HVAC application
Image 3: Mechanical draft crossflow cooling tower used in an HVAC application

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.
* Dry coolers operate by heat transmission through a surface that divides the working fluid from ambient air. They thus rely mainly on convection heat transfer to reject heat from the working fluid, rather than evaporation.
* Fluid coolers are hybrids that pass the working fluid through a tube bundle, upon which clean water is sprayed and a fan-induced draft applied. The resulting heat transfer performance is much closer to that of a wet cooling tower, with the advantage provided by a dry cooler of protecting the the working fluid from environmental exposure.

In a wet 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 air is drawn past a flow of water, the two flows attempt to equalize. The air, if not saturated, absorbs additional water vapor, leaving less heat in the remaining water flow.

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 buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Warm moist air is less dense than drier air at the same temperature and pressure. This moist air buoyancy produces a current of air through the tower.
* Mechanical draft, which uses power driven fan motors to force or draw air through the tower.
o 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. This fan/fill arrangement is also known as draw-through. (see Image 2, 3)
o 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. This fan/fill geometry is also known as blow-through. (see Image 4)

Image 4: A forced draft cooling tower
Image 4: A forced draft cooling tower

* Fan assisted natural draft. A hybrid type that appears like a natural draft though airflow is assisted by a fan.

Hyperboloid (aka hyperbolic) cooling towers (Image 1) have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperbolic form is popularly associated with nuclear power plants, due to media coverage at Three Mile Island. However, this association is misleading, as hyperbolic natural-draft cooling towers are often used at large coal-fired power plants as well.

 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.

Image:Crossflow_diagram.PNG

 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.

Image:Counterflow_diagram.PNG

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 (Industrial 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 (industrial chimney). At plants without flue gas purification, this causes problems with corrosion.

 Wet cooling tower material balance

Main article: Cooling tower system

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:[3]

image:CoolingTower.png
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\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 and fouling, the water should be filtered and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water.[3] For closed loop evaporative towers, corrosion inhibitors may be used, but caution should be taken to meet local environmental regulations as some inhibitors use chromates.

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 and Legionnaires' disease

Further information: Legionellosis and Legionella

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. pneumophilia[4]. 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 kilometres 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.[5]

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?.005?f 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 PreventionPDF (1.35 MiB) - Procedure for Cleaning Cooling Towers and Related Equipment (pages 239 and 240 of 249)
* Cooling Technology InstitutePDF (76.2 KiB) - Best Practices for Control of Legionella
* Association of Water TechnologiesPDF (964 KiB) - Legionella 2003
* California Energy CommissionPDF (194 KiB) - Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants
* SPX Cooling TechnologiesPDF (119 KiB) - Cooling Towers Maintenance Procedures
* SPX Cooling TechnologiesPDF (789 KiB) - ASHRAE Guideline 12-2000 - Minimizing the Risk of Legionellosis
* SPX Cooling TechnologiesPDF (80.1 KiB) - Cooling Tower Inspection Tips {especially page 3 of 7}
* Tower Tech Modular Cooling TowersPDF (109 KiB) - Legionella Control
* GE Infrastructure Water & Process Technologies Betz DearbornPDF (195 KiB) - Chemical Water Treatment Recommendations For Reduction of Risks Associated with Legionella in Open Recirculating Cooling Water Systems

 Cooling tower malfunctions

Under certain ambient conditions, plumes of water vapor (fog) can be seen rising out of the discharge from a cooling tower (see Image 1), 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 -- what we see as fog. This phenomenon typically occurs on cool, humid days, but is rare in many climates.

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 (note: this assumes that the ice 'grows' beyond the typical liquid volume).

Typical methods to circumvent freezing are: air flow through the tower is reduced, a basin heater is installed, a heater is installed indoors on the water loop, a drain system or remote basin design is used, and in some cases where evaporative closed loop towers are used the tower spray water is drained completely.

 Cooling Tower Operation In Freezing Weather

Wikipedia, the free encyclopedia.
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 continuously operate cooling towers with 40 °F water leaving the tower. Basin heaters, tower draindown, and other freeze protection methods are often employed in cold climates.

* Do not operate the tower unattended.[6]

* Do not operate the tower without a heat load. This can include basin heaters and heat trace. Basin heaters maintain the temperature of the water in the tower pan at an acceptable level. Heat trace is a resistive element that runs along water pipes located in cold climates to prevent freezing.

* Maintain design water flow rate over the fill.

* Manipulate airflow to maintain water temperature above freezing point.[7]

 Some commonly used terms in the cooling tower industry

* Drift - 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.

* Blow-out - 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.

* Plume - 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.

* Blow-down - The portion of the circulating water flow that is removed in order to maintain the amount of dissolved solids and other impurities at an acceptable level. It may be noted that higher TDS (total dissolved solids) concentration in solution results in greater potential cooling tower efficiency. However the higher the TDS concentration, the greater the risk of scale, biological growth and corrosion.

* Leaching - The loss of wood preservative chemicals by the washing action of the water flowing through a wood structure cooling tower.

* Noise - 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, and the motors, gearboxes or drive belts.

* Approach - The approach is the difference in temperature between the cooled-water temperature and the entering-air 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.

* Range - The range is the temperature difference between the water inlet and water exit.

* Fill - Inside the tower, fills are added to increase contact surface as well as contact time between air and water. Thus they provide better heat transfer. The efficiency of the tower also depends on them. There are two types of fills that may be used:
o Film type fill (causes water to spread into a thin film)
o Splash type fill (breaks up water and interrupts its vertical progress)

 See also

* Cooling tower system
* Cooling water
* Power station
* Fossil fuel power plant
* Deep lake water cooling
* HVAC
* Mechanical engineering
* Architectural engineering

 External links

* J.C. Hensley (Editor) (2006). Cooling Tower Fundamentals, 2nd Ed., SPX Cooling Technologies.
* Cooling Towers: Design and Operation Considerations
* What is a cooling tower?
* nucleartourist.com - Includes diagrams

 References

1. ^ Cooling System Retrofit Costs EPA Workshop on Cooling Water Intake Technologies, John Maulbetsch, Maulbetsch Consulting, May 2003
2. ^ Department of EnergyPDF (297 KiB) Office of Fossil Energy's Power Plant Water Management R&D Program
3. ^ a b Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants, 1st Edition, John Wiley and Sons. LCCN 67019834. (available in many university libraries)
4. ^ Ryan K.J.; Ray C.G. (editors) (2004). Sherris Medical Microbiology, 4th Edition, McGraw Hill. ISBN 0-8385-8529-9.
5. ^ Airborne Legionella May Travel Several Kilometers (access requires free registration)
6. ^ J. L. Hermon & Associates, Inc.: Cooling Tower Operation In Freezing Weather
7. ^ SPX Cooling Technologies: Operating Cooling Towers in Freezing WeatherPDF (1.45 MiB)

Retrieved from ""http://en.wikipedia.org/wiki/Cooling_tower"

Categories: Cooling technology | Power stations | Oil refineries | Chemical engineering | Building engineering | Mechanical engineering | HVAC
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