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Cooling Towers

Introduction

Wet cooling towers are heat removal devices used to transfer waste heat from industrial and other processes to the atmosphere. They are used primarily to provide circulatingTVA CoolingTwr.jpg cooling water in large industrial facilities. The circulating cooling water absorbs heat by cooling and/or condensing hot industrial process streams or by cooling hot rotating machinery and other hot equipment within industrial facilities. The cooling towers then transfer that absorbed heat to the atmosphere by evaporating a small part of the circulating water.

Common industrial applications include cooling the circulating water used to remove waste heat from petroleum refineries, natural gas treating plants, petrochemical and other chemical plants, thermal power plants, and large air conditioning systems. Small air conditioning units often use cool air (rather than water) to remove absorbed heat and such systems are referred to as dry cooling.

 

Cooling towers vary in size from small roof-top air-conditioning units to large rectangular structures (as in Figs. 1 and 2) that can be over 20 metres tall and 180 metres long or even larger hyperboloid structures (as in Fig. 3) that can be up to 200 metres tall and 150 metres in diameter.                                             DOE CoolingTwr.jpg


The hyperboloid cooling towers are often associated with nuclear power plants. However, they are also used to some extent in some large industrial facilities. Although the large hyperboloid cooling towers are very prominent, the vast majority of wet cooling towers are quite a bit smaller.

 

How a cooling tower functions


Basically, a cooling tower intimately contacts a flow of warm water with a flow of ambient air which is not saturated with water vapor (i.e., air containing less water vapor than it is capable of containing). That causes a small part of HyperboloidCoolingTwr.jpg the warm water to evaporate and the air absorbs that evaporated water. The heat required to evaporate part of the water is derived from the water itself and thereby causes the water to cool. This process is known as evaporative cooling. The net result is that the air leaving the tower is saturated with water vapor and the unevaporated water leaving the cooling tower has been cooled.[1] [2]

An evaporative cooling tower is referred to as a wet cooling tower or, more often, simply as a cooling tower. Such towers can cool water to a temperature that approaches the wet-bulb temperature of the ambient air. The average ambient air wet-bulb temperature chosen as the design basis essentially determines the size of the cooling tower, and the size of a cooling tower is inversely proportional to the design wet-bulb temperature.

To achieve better performance (i.e., more cooling), a media called fill is used to increase the contact surface area between the air and water flows.[3] Most fill in modern cooling towers is plastic material.

Applications


As discussed earlier above, the primary use of large, industrial cooling towers is to reject the heat absorbed in the circulating cooling water systems used in industrial facilities such as petroleum refineries, natural gas treating plants, petrochemical and other chemical plants, and electric power plants (both thermal and nuclear).

The circulation rate of cooling water in a typical 700 MW conventional coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons per minute)[4] and the system requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour). The largest users of cooling water in an electric power plant are the surface condensers that condense the exhaust steam from the large steam turbines that drive the electricity generators.

If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour[5] 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. Discharging such large amounts of warm water may raise the temperature of the receiving body of water to an unacceptable level for the local ecosystem. A cooling tower serves to dissipate the heat into the atmosphere where wind and air diffusion spreads the heat over a much larger area than warm 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 petroleum crude oil per day (300,000 U.S. barrels per day) circulates about 80,000 cubic metres (352,000 U.S. gallons per minute) of water per hour through its cooling tower system.

This article is devoted to the large-scale cooling towers used in industrial facilities. However, much smaller cooling towers of various types are used for the air-conditioning of office buildings, hotels, sports arenas, food storage facilities and many other commercial establishments.

Cooling tower operational variables


CoolingTwrSchematic.pngQuantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of make-up flow rate, evaporation and drift losses, blowdown rate, and the concentration cycles:[6]

Referring to the schematic diagram in Fig. 4, water pumped from the basin at the bottom of the cooling tower is the cooling water routed through the process stream cooling and condensing heat exchangers 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 the fan-induced upward flow of ambient air. That contact causes a portion of the water (E) to evaporate into water vapor that exits the tower as part of the water saturated air. A small amount of the water also exits with the air as entrained droplets of liquid water called drift losses (D). 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 circulating water from becoming too high, a portion of the water, referred to as blowdown (B, is drawn off for disposal. Fresh water make-up (M) is supplied to the tower basin to compensate for the loss of evaporated water, the drift loss water and the blowdown water.

Defining the various terms:
  • M = Make-up water in m3/hr
  • C = Circulating water in m3/hr
  • B = Blow-down water in m3/hr (also called draw-off)
  • E = Evaporated water in m3/hr
  • D = Drift loss of water in m3/hr (also called windage)
  • 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

A water balance around the entire system is:
  • M = E + D + B

Since the evaporated water (E) has no salts, a chloride balance around the system is:
  • M (XM) = D (XC) + B (XC) = XC (D + B)

and, therefore:[7]
  • XC ÷ XM = Cycles of concentration = M ÷ (D + B) = M ÷ (M – E) = 1 + [E ÷ (D + B)]

From a simplified heat balance around the cooling tower:[8]
  • E = C · ΔT · cp ÷ HV
where:
  • HV = latent heat of vaporization of water, 2,260 kJ / kg
  • ΔT = water temperature difference from tower top to tower bottom, in °C
  • cp = specific heat of water, 4.184 kJ / (kg °C)

Modern cooling towers have demisters known as drift eliminators to reduce the amount of drift losses (D) from large-scale industrial cooling towers. However, some older cooling towers have no drift eliminators. In the absence of manufacturer's data, drift losses may be assumed to be:
  • D = 0.3 to 1.0 percent of C for a natural draft cooling tower without drift eliminators
  • D = 0.1 to 0.3 percent of C for an induced draft cooling tower without drift eliminators
  • D = about 0.005 percent of C (or less) if the cooling tower has drift eliminators

Cycles of concentration


The cycles of concentration represent the accumulation of dissolved minerals in the recirculating cooling water. Blowdown of a portion of the circulating water (from the tower basin) is the principal means of controlling 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).

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 heat exchangers and/or in the cooling tower itself. . 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 in the United States (located in the city of New York) has water that is 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 algacides to prevent growths that could interfere with the continuous flow of the water.[9] Corrosion inhibitors may also be used, but caution should be taken to meet local environmental regulations as some inhibitors use chromates which are toxic.

Air flow generation methods


Wet cooling towers may be categorized in terms of how they generate air flow. These are the methods commonly in use:
  • Natural draft, which uses the so-called stack effect in a tall enclosed structure where the warm air inside the structure naturally rises due to the density differential between the inside warm air and the cooler outside air. Thus, the resulting buoyancy of the inside air relative to the outside air induces an upward flow of air through the cooling tower.[10] Hyperboloid cooling towers (as shown in Fig.3 above) have become the design standard for large natural-draft cooling towers because of their structural strength and minimum usage of material. These designs have become popularly associated with nuclear power plants. However, this association is misleading, as the same kind of cooling towers are often used at large coal-fired power plants as well.
  • Mechanical draft, which uses motor-driven fans to either induce or force air through the tower.
    • Induced draft, which uses a fan at the air exit from the cooling tower to pull or draw air through the tower.
    • Forced draft, which uses a fan at the air intake to the cooling tower to push or force air through the tower.
  • Fan assisted natural draft , which is a hybrid type that appears like a natural draft tower, though airflow is assisted by a fan.


Air-to-water flow arrangements

CounterflowCoolingTwr.png
Wet cooling towers may also be categorized in terms of how they arrange the air-to-water flow.

Counter-flow

In the counter-flow design for a wet cooling tower, the flow of the air is directly opposite to the flow of the water as depicted in Fig. 5 which is a schematic diagram of a counter-flow wet cooling tower.

The air flow first enters an open area beneath the fill material, and then flows up vertically either assisted by a fan or by natural draft. The water is sprayed through pressurized nozzles near the top of the tower, and then flows downward through the fill, opposite to that of the air flow.
  • Advantages of the counter-flow design
    • Using spray nozzles for water distribution makes the cooling tower more freeze resistant.
    • Breakup of the water into sprayed droplets makes the heat transfer more efficient.
  • Disadvantages of the counter-flow design
    • Typically higher initial and long-term cost, primarily due to the water pump requirements.
    • Difficult to use variable water flow, as the spray pattern characteristics may be negatively affected.

Cross-flow

CrossflowCoolingTwr.pngIn the cross-flow design for a wet cooling tower, the flow of the water is perpendicular to the flow of the air as depicted in Fig. 6 which is a schematic diagram of a cross-flow wet cooling tower.

The air flow first enters one or more of the vertical faces of the cooling tower and flows horizontally through the fill material. Water flows vertically downward (perpendicular to the air) through the fill material by gravity. The air continues through the fill material (thus past the water flow) into an open plenum . Lastly, a fan sends the air out into the atmosphere. Warm water distributor basins consisting of a deep pans with holes or nozzles in their bottoms are located near the top of the cross-flow wet cooling tower. Gravity uniformly distributes the water through the holes or nozzles across the fill material.
  • Advantages of the cross-flow design
    • Gravity water distribution allows less costly pumps and maintenance while in use.
    • Typically lower initial and long-term cost, mostly due to lesser pump requirements.
  • Disadvantages of the cross-flow design
    • More prone to freezing.

Terminology

  • Drift or windage: 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.
  • Blowdown or drawoff: The portion of the circulating water flow that is removed so as to maintain the amount of Total Dissolved Solids (TDS) and other impurities at an acceptable low level. Low levels of TDS reduce the risk of scale, biological growth and corrosion. Increasing the amount of blowdown and subsequently increasing the amount of clean make-up water is the most practical method of lowering the TDS level in the circulating water.
  • Plume: The stream of water saturated exhaust air leaving the cooling tower. The plume is visible when the 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.
  • Make-up: The water that is added to the circulating water system to compensate for water losses by evaporation, drift, blow-out, and blowdown.
  • Approach: The approach is the difference between the temperature of the cooled water exiting the tower and the wet bulb temperature of the air entering the tower.
  • Range: The range is the temperature difference between the entering warm water and the exiting cooled water.
  • Fill: The material installed inside the cooling tower to increase the contact surface and contact time between the air and the water, so as to provide more efficient heat transfer. There are two types of fill material: the film type which causes the water to spread into a thin film and the splash type which breaks up the falling water and slows down its vertical flow..
  • Cycles of concentration: The ratio of the soluble pollutants in the circulating water to the soluble pollutants in the make-up water. Each specific tower will have a designated maximum cycle of concentration depending upon the specific make-up water analysis and other local specific parameters.

Legionnaires disease

Legionellosis (referred to Legionnaires' disease) is a dangerous infectious disease caused by bacteria belonging to the genus Legionella. In many outbreaks of that disease, air-conditioning cooling towers have been found to be the source of the disease-causing bacteria. 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.[11] [12] [13] [14]

References

  1. ^ Larry Drbal, Kayla Westra and Pat Boston (1996), Power Plant Engineering, 1st Edition, Springer Publishing, ISBN 0-412-06401-4
  2. ^ Editors: Robert H. Perry (deceased), Don W. Green and James O. Maloney (1986), Perry's Chemical Engineers' Handbook, 6th Edition, McGraw-Hill Publishing, ISBN 0-07-049479-7
  3. ^ Cooling tower fill
  4. ^ Cooling System Retrofit Costs, EPA Workshop on Cooling Water Intake Technologies, John Maulbetsch and Kent Zammit, May 2003
  5. ^ United States Department of Energy, Office of Fossil Energy’s Power Plant Water Management R&D Program, Thomas J. Feeley, Lindsay Green, James T. Murphy, Jeffrey Hoffmann and Barbara A. Carney, July 2005
  6. ^ Milton R.Beychok (1967), Aqueous Wastes from Petroleum and Petrochemical Plants, 1st Edition, John Wiley and Sons, Library of Congress Control Number 67019834 (available in many university libraries)
  7. ^ Same as reference 6
  8. ^ Same as reference 6
  9. ^ Same as reference 6
  10. ^ Editors: I. Mungan and U. Wittek (2004), Natural Draught Cooling Towers, Taylor & Francis Group, ISBN 90-5809-642-4
  11. ^ SPX (Marley) Cooling Technologies, ASHRAE Guideline 12-2000, Minimizing the Risk of Legionellosis
  12. ^ Technical Guidelines for Control and Prevention of Travel Associated Legionnaires' Disease, January 2005, European Working Group for Legionella Infections (EWGLI)
  13. ^ Guidelines for Environmental Infection Control in Health-Care Facilities, Procedure for Cleaning Cooling Towers and Related Equipment (pp. 225-226), U.S. Centers for Disease Control and Prevention (CDC)
  14. ^ Best Practices for Control of Legionella, Cooling Technology Institute

Contributor

  • Milton Beychok