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107 Cards in this Set
- Front
- Back
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What is a Cooling Tower? |
Equipment that uses heat transfer to lower the temperature of the cooling water. |
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How is heat transfer accomplished from the cooling water? |
The actual transfer of heat is accomplished by a sensible heat transfer (heating the air passing through the tower), and by the removal of latent heat as a result of the evaporation of about 5% of the water. As this small portion of water evaporates, the heat required is drawn from the remaining water, thus cooling it. |
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How is water distributed in cooling towers? |
The water to be cooled is distributed in the tower by spray nozzles and splash bars to expose a very large water surface area to atmospheric air thus aiding evaporation. |
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How is circulation of atmospheric air accomplished? |
Circulation of atmospheric air is accomplished by one of the following methods: fans, convection currents, natural wind currents, or induction effects from the water sprays. |
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What are the alternatives to recycling cooling water through a cooling tower? |
1. Use a once-through system with water from a stream or lake 2. Use a cooling pond or natural water body to cool the heated water |
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When is the use of cooling towers common? |
Cooling towers are unquestionably the most common type of heat exchanger used when cooling water is scarce, expensive, and a temperature within 4-6°C (7-10°F) of the ambient wet bulb temperature is required. |
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What is the basic mechanism of cooling towers? |
The water to be cooled is delivered to the top of the tower by a pump and is distributed in such a way that it falls through the tower in a finely divided spray. Air flows through the tower causing part of the water to evaporate. The latent heat of evaporation, carried away by the water vapour leaving the tower, causes a reduction in sensible heat content in the remaining water, resulting in a temperature drop. Some sensible heat is also absorbed by the air flowing through the water spray. After dropping to the bottom of the tower, the cool water collects in a basin and is pumped back to the system again. |
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How is the amount of make up water determined? |
The amount of water lost as water vapour leaving the tower and the blowdown rate determines the amount of "make up" water needed to maintain the tower basin level. |
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What does the rate of heat transfer in cooling towers depend on? |
• Relative velocity of both air and water during contact. • Area of water surface in contact with the air. • Length of contact time between the air and water. • Difference between the inlet water temperature and the inlet air wet bulb temperature (relative humidity of the air). |
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What are the basic components of a cooling tower? |
• Inlet water distributing box or sprays • Packing or fill • Air moving equipment • Inlet air louvres • Drift eliminators • Cooled water basin |
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What are the two classes of Cooling Towers according to the method of air circulation? |
• Natural draft • Mechanical draft |
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What are the two types of Natural Draft Cooling Towers? |
• Atmospheric towers • Chimney towers (which are used mainly in large generating stations) |
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What are Atmospheric Towers? |
Atmospheric towers are those in which the air movement through the tower is dependent on atmospheric conditions. The sides are louvred to direct air flow and reduce water loss by mist.They operate effectively only in locations where there are relatively constant winds and large open spaces. |
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Describe the physical make-up of Chimney Towers. |
Chimney towers are used mainly in large generating stations. This type, also known as a "Hyperbolic Tower", is built using reinforced concrete in sizes up to 25 000 m3/h water circulation rate, with base diameters up to 60 m and heights reaching 90 m. The air inlet, water distribution and fill are similar to a mechanical draft tower, but the great majority of the tower height is purely chimney. |
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How do Chimney Towers work? |
Water falls from a perforated overhead distribution flume or from distribution spray nozzles, down through the packing or decks. It strikes the faces of the tower decking and breaks into fine particles. Air in contact with the hot water is warmed and rises up the chimney; in turn, cooler ambient air is drawn through the louvres and into the base of the tower. During this process, the fine particles of water come in close contact with the air currents, enhancing the process of evaporation and cooling. |
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What should a regular inspection and maintenance schedule of cooling towers consist of? |
• Cleaning the louvres, piping, and nozzles to ensure that they are free of scale, algae, and dirt. • Cleaning the water basin and checking for leaks. • Cleaning the suction screen • Checking that the level control valve is operating properly |
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What are probable causes of High Discharge Water Temperature from the tower? |
Probable causes of this problem include high ambient air temperature, high concentration of solids in the water, restriction of air flow through the tower, and poor water breakup due to worn or dirty nozzles. |
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What are probable causes of a Reduction in Water Flow? |
This may be due to restrictions created by algae, scale, or dirt. |
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What are probable causes of the tower basin or sump overflowing? |
This may be caused by a clogged sump screen, a restriction in the water outlet piping, or an improper level control valve operation. |
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What are probable causes of excessive water drift? |
Excessive wind velocity, broken or missing louvres, or excessive water pressure in the nozzles. |
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What are Mechanical Draft Towers? |
Mechanical draft towers use one or more fans to move large quantities of air through the tower. |
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What are the two subclasses of Mechanical Draft Towers? |
• Forced draft cooling towers • Induced draft cooling towers |
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What are the two types of airflows in Mechanical Draft Towers? |
The air flow in either subclass may be crossflow or counterflow with respect to the falling water. |
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What is Crossflow? |
Crossflow indicates that the airflow is horizontal in the filled portion of the tower. |
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What is Counterflow? |
Counterflow means the air flow is in the opposite direction of the falling water. |
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What is the advantage of Counterflow? |
The counterflow tower occupies less floor space than a crossflow tower but is taller for a given capacity. |
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What is the advantage of Crossflow? |
The principle advantages of the crossflow tower are the low pressure drop (due to less height required) in relation to its capacity and lower fan power requirement leading to lower energy costs. |
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What are the requirements for Mechanical Draft Cooling Towers as it pertains to location? |
All mechanical towers must be located so that the discharge air diffuses freely without recirculating through the tower, and so that air intakes are not restricted. Cooling towers should be located as near as possible to the refrigeration systems they serve, but should never be located below them so as to allow the condenser water to drain out of the system through the tower basin when the system is shut down. |
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Describe the operation of Forced Draft Cooling Towers. |
During operation, the fan forces air at a low velocity horizontally through the packing and then vertically against the downward flow of the water that occurs on either side of the fan. Water entrained in the air is removed by the drift eliminators located at the top of the tower. Vibration and noise are minimal since the rotating equipment is built on a solid foundation. The fans handle mostly dry air, greatly reducing erosion and water condensation problems. |
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How do Induced Draft Cooling Towers work? |
The induced draft tower has one or more fans, located at the top of the tower, that draw air upwards against the downward flow of water passing around the wooden decking or packing. Since the airflow is counter to the water flow, the coolest water at the bottom is in contact with the driest air while the warmest water at the top is in contact with the moist air, resulting in increased heat transfer efficiency. |
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How are Induced Draft Cooling Towers physically oriented? |
The fans at the top discharge the hot, moisture laden air upward and away from the air entering at the bottom of the tower, thus preventing any recirculation of warm air. Warm water from the building enters the distribution system located just under the drift eliminators. The fans and their drive are mounted on the top deck. |
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Describe Crossflow Induced Draft Cooling Towers. |
Crossflow towers provide horizontal air flow as the water falls through the packing. Single and double air flow designs are constructed to suit the job location and operating conditions.
The fans, located at the top, draw air through cells or packing that are connected to a suction chamber partitioned midway beneath each fan. The water falls from the distribution system in a cascade of small drops over the packing and across the horizontal flow of air. The total travel path of the air is longer and there is less resistance to air flow than in the counterflow design. |
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Describe Venturi Induced Draft Cooling Tower. |
The tower basically consists of a venturi-shaped chamber, a spray manifold, and a sump. Neither fill nor fan are required in this tower.
The water to be cooled is injected at the narrow end of the venturi by spray nozzles, inducing a large airflow into the tower which mixes intimately with the fine water spray. Heat transfer by evaporation of a small part of the water takes place while the remaining water drops in temperature. The cooled water falls into the sump and from there flows to the suction of the cooling water circulating pump. The air containing the water vapour leaves the tower via the eliminators and is discharged upward through a cowl. |
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What are the advantages of Venturi Induced Draft Cooling Tower? |
The advantages of this tower are its quietness of operation due to the absence of any moving parts and their associated noise and vibration problems, the elimination of the need for electrical connections, starters, etc., the elimination of fill, and the reduced maintenance requirements. |
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When are Dry Cooling Towers used? |
When cooling water supply is very restricted. |
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How do Dry Cooling Towers work? |
The cooling water passes through finned tubes placed in the tower in banks and cooled by air currents produced by mechanical or natural means. |
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What are the advantages of Dry Cooling Towers? |
Using the closed-circuit method eliminates contact between the water to be cooled and the coolant air. Water loss by evaporation and drift is eliminated and there is no makeup water required. A small example is the automobile radiator. |
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What are the components of the Mechanical Draft Cooling Tower Maintenance? |
1. Lubricate the fan motor every three months, or as specified by the manufacturer, using the recommended lubricant. 2. On V-belt driven fans, lubricate the fan shaft monthly and check the tension on the belt. 3. All tower bolts should be checked monthly. 4. Check the float valve monthly. 5. Clean and repaint corroding exterior metal surfaces annually. The interior of the tower should also be inspected at this time. 6. Fan blades should be cleaned annually and painted if necessary. 7. Fans should be cleaned and balanced when high vibration levels are detected. 8. A water treatment specialist should be consulted if scaling or algae formation is evident. An evaporative type cooling tower must be regularly cleaned to inhibit the growth of bacteria including Legionella Pneumophila (which causes Legionnaires Disease), and thus avoid the risk of sickness or death. Cleaning should be done utilizing proper PPE. |
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Is freezing within cooling towers a major concern? |
Cooling towers do not generally need protection from freezing while operating. An acceptable thin layer of ice may form on the louvres or air intake structure of the tower. |
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When does freezing in cooling towers become a concern? |
If heavy ice forms on the fill jeopardizing the existence and operation of the heat transfer surface, or if heavy ice forms in a support region threatening the tower structure, then action must be taken. |
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What are the two factors that can be controlled to manage freezing? |
Control of both the water flow and the air flow can be useful. |
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How can water flow be managed to protect against freezing? |
A bypass line is used to direct water flow to the water basin beneath the tower instead of directing it over the fill. Total bypass is used during startup in cold weather and a partial bypass may continue during normal operations in especially cold weather. Only when the water in the water basin reaches 27°C (80°F) should the bypass be closed to cause total flow over the fill. |
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How can air flow be managed to protect against freezing? |
Air flow control through the use of two-speed or variable speed fan motors can reduce the amount of cold air passing through the tower and hence reduce the heat loss from the cooling water. Severe ice formation on louvres may require that the fans be reversed for a period of time. This changes the pattern of falling water, bringing a deluge of warm water in contact with ice formations for rapid melting. |
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During shutdown, how can cooling water be protected from freezing? |
When the tower is shut down, the water basin must not freeze. If possible, some heat load and air circulation of the cooling water should be maintained. When these objectives are not possible, other methods need to be employed. A remote sump pump for circulation is an alternative to low load operation. Thermostatically controlled electric heaters or steam coils are sometimes used to prevent the basin water from freezing. The indoor tank or dry basin method allows water to drain continuously from the tower basin to an indoor storage tank from where it is pumped to the refrigeration condensers. A bypass line from the tower supply line is used to drain the supply line and to regulate the cooling water temperature during cold weather and low loads. During a shutdown situation, the bypass line and the main basin drain allow all water to flow into the indoor tank, thus removing all water from the freezing environment above the roof level. |
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What is the purpose of the bleed line in cooling towers? |
A controlled amount of water is bled off to control the build up of scale-forming salts in the water left behind by the evaporating water. Without the bleed line, the chemical concentration would eventually get high enough to cause scale formation and reduce the efficiency of operation. |
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What determines the amount of bleed-off in cooling towers? |
The amount of bleed-off will depend on the cooling temperature range, quantity of water circulated, and the condition of the water supply in the locality. |
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What problems requiring water treatment can occur in cooling towers? |
· Deposits of airborne solid particles |
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Should a regular Power Engineer take charge of water treatment of make-up water in cooling towers? |
A reputable water treatment specialist should be retained to establish and monitor an effective water treatment program. |
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How do airborne particles enter water in cooling towers? |
Since the tower is open to the air, airborne dust and debris can enter the water and eventually plug up the system. |
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How can airborne particles be removed from water in cooling towers? |
Strainers or filters will remove most of this material. They must be cleaned and maintained regularly. |
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Is scaling in cooling towers as big a problem as it is in boilers? |
Scaling in cooling towers is not as severe as in boilers since the operating temperatures are much lower. If the bleed-off mentioned earlier is insufficient to prevent objectionable scale buildup, then a water treatment program is required. |
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In cooling towers what is the main scaling problem? |
In cooling towers, calcium carbonate scaling is the main scaling problem. It normally results from the breakdown under heat of dissolved calcium bicarbonate, a naturally occurring soluble salt.
The reaction is:
Ca(HCO3)2 + heat ---> CaCO3 + H2O + CO2 |
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What does the degree of scaling depend on? |
The degree of scaling depends primarily on the concentration of calcium and bicarbonate ions in solution and the pH of the cooling water. The rate of calcium bicarbonate breakdown and calcium carbonate scaling increases with a rise in the pH and temperature of the cooling tower water. |
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What is the traditional method of treating cooling water to prevent scaling? |
The traditional method of treating cooling tower water to prevent scaling is to add sulphuric acid, causing the following reaction:
Ca(HCO3)2 + H2SO4 ---> CaSO4 + 2CO2 + 2H2O |
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How does calcium sulphate compare to calcium carbonate? |
Calcium sulphate is more soluble than calcium carbonate. Although calcium sulphate scale is harder to remove than calcium carbonate scale, the presence of dissolved calcium sulphate in the cooling water can be tolerated so long as it is kept in solution through proper pH control and bleed-off. Calcium sulphate scaling increases with decreasing pH and increasing temperature. |
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At what pH is cooling tower water kept at under a sulphuric acid treatment program? |
Under a sulphuric acid treatment program, cooling tower water is usually maintained at a pH of between 6 and 7.5 which necessitates additional treatment to prevent corrosion of steel components in the cooling system. |
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What are the 3 alternative methods of preventing formation of calcium scale in cooling water? |
· Calcium hardness is removed from the water through a softening process prior to use. |
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How is softening done in cooling towers? |
Softening is accomplished by using an ion exchange softener or by cold-lime softening which involves the addition of lime (calcium hydroxide-Ca(OH)). Coldlime softening causes the calcium carbonate to precipitate out in a settling tank before the water enters the cooling system. This method is used mostly in industrial situations. |
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How are scale-forming salts kept in solution? |
Specialty chemicals are added that are able to keep scale-forming salts in solution at high concentration levels. The most common chemicals used are polymeric organics (polyaculates) and organic phosphorus compounds. However, some of these compounds can accelerate corrosion of steel and copper. Some chemicals |
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How is impurity precipitated as a removable sludge rather than as a hard scale? |
Soft sludge formation is promoted by crystal modification chemicals, which attach themselves to precipitating solids and prevent the crystals from joining together in a hard scale. These are low molecular weight polymers such as polymaleic acids and sulphonated polystyrenes. The sludge is removed by blowdown from the tower basin or by side-stream filtration. When crystal modifiers are used, the cooling water should appear turbid indicating that they are taking effect. |
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Which chemicals control scale by both dispersion and crystal modification? |
- Lignin/Tannin - Starch/Alginates |
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Which chemicals control scale by solubility? |
- Acids - Polyphosphates - Phosphonates - Phosphate Esters |
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Which chemical controls scale by solubility, dispersion, and crystal modification? |
Low Molecular Weight polymers |
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What is corrosion? |
Corrosion is the loss of metal resulting from direct chemical reaction or by electrochemical action. |
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What is the principal cause of steel corrosion in cooling towers? |
The principle cause of steel corrosion in cooling towers is dissolved oxygen in the cooling water. Under suitable conditions, the oxygen combines with the steel to form rust (iron oxide). |
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What does the rate of corrosion in cooling towers depend on? |
1) The rate of corrosion increases with an increased concentration of dissolved oxygen in the water. Since cooling tower systems continually aerate the water, the amount of dissolved oxygen is high. 2) The rate of corrosion increases as pH decreases. Acidity or even low alkalinity promotes corrosion by increasing both the dissolution rate of the base metal and the oxide formation on the metal surfaces. Surprisingly, even a neutral pH of 7.0 or a slightly alkaline pH of 7.5 can be corrosive. 3) High temperatures will cause corrosion rates to double with every 15-30 degree Celsius rise in temperature up to 70°C. Above 70°C, further temperature increases have relatively little effect on corrosion rates in cooling water systems. |
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What are the basic methods of corrosion control in cooling water systems, used singly or in combination? |
· Use corrosion resistant metals and materials. |
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What are the traditional film-forming chemicals used in cooling towers? |
The traditional film-forming chemicals used are polyphosphates, chromates, and zinc, often used in combination with each other to counterbalance severe sideeffects (such as the tendency of polyphosphate to attack copper and aluminum while it is protecting steel). |
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How do film-forming chemicals prevent corrosion? |
These chemicals form a thin film on the metal preventing dissolved oxygen and carbon dioxide in the water from coming in direct contact with the metal. |
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What is the downside of using chromate based film-forming chemicals in cooling towers? |
While effective at corrosion control, these chemicals cause unacceptable environmental damage when they are released through bleedoff or drainage of the system; therefore combinations that include chromates are now generally discontinued in any systems from which water can escape into the environment. |
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What are nontoxic alternatives for film-forming chemicals in cooling towers? |
Nontoxic alternatives involve zinc, polysilicates, and molybdates. However, they can be more expensive than the traditional treatments. |
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At what speed do various inhibitors form their protective film? |
The speed with which inhibitors form their protective film varies considerably. Chromate is the most rapid, requiring only a few days. Polyphosphate and zinc take about 5 days, while polysilicates and molybdates can take 2 weeks. |
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What is the importance of speed when it comes to inhibitors forming protective films? |
The inhibitor must form a film everywhere on the metal surfaces. If the initial concentration of the inhibitor is too weak, there is a good possibility of severe corrosion occurring at unprotected sites since the entire corrosion potential will be concentrated there. Rapid and severe pitting can occur. |
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What causes biological fouling in recirculating cooling towers? |
Biological fouling of recirculating cooling towers is caused by algae, fungi, and bacteria growth. The threat is uncontrolled growth of these organisms, which is enhanced by the warm water, abundant sunlight, and the oxygen-rich environment found in cooling towers. |
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What are the negative effects of biological fouling in cooling towers? |
These organic growths, called slime, can reduce flow rates in tubes and channels, can release organic acids and waste products that can corrode metals, and can attack and destroy the wooden portions of a tower. |
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What are the two methods of protection against biological fouling? |
· Mechanical methods such as trash bars and strainers. |
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How do chemicals (biocides) kill cells causing biological fouling? |
Chemical methods operate in several ways. Heavy metals penetrate the cell wall and destroy protein groups essential to life support. Oxidizing chemicals such as chlorine irreversibly oxidize protein groups, resulting in a loss of enzyme activity in the cell and hence death of the cell. Nonoxidizing chemicals such as chlorinated phenols pass into the cells and cause precipitation of proteins out of solution inside the cell, causing death. |
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How are biocides added to cooling water systems? |
Biocides are slug-fed at toxic concentrations to the cooling system usually twice a week. This method is called shock feeding and it kills microbiological organisms most effectively by creating a temporary high concentration of a biocide. |
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How do operators protect against microorganisms acquiring immunity to the biocide used? |
Shock feeding one biocide only will result in organisms becoming immune to that particular biocide. To prevent this, once a month an alternate biocide is slug-fed for one week so as to kill off any organisms that are becoming immune to the first biocide. |
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How do operators prevent dead microorganisms from fouling cooling systems? |
To prevent dead microbiological growth from fouling cooling systems, dispersants are added during biocide slug feedings so dead growth will be held in suspension until it is removed through bleed-off. |
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What personal precautions should operators take when handling biocides? |
Operators should be aware that biocides are poisons and extreme caution should be employed when working with them. Precautions include rubber gloves, protective clothing, and eye protection. |
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How is wood deterioration a concern for cooling towers? |
Many cooling towers may contain a lot of wood which is subject to biological and chemical attack, resulting in severe wood deterioration. |
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What is the composition of wood? |
Wood is composed of cellulose, lignin, and natural extractives. Cellulose exists as long fibres giving wood its strength. Lignin cements the cellulose fibres together. The extractives contain the natural compounds that enable wood to resist decay. Unfortunately, the extractives are water soluble and leach away, leaving wood in cooling towers vulnerable to decay. |
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How can chemical attacks effect wood deterioration in cooling towers? |
Chemical attack occurs mainly as delignification. Oxidizing agents such as chlorine and alkaline solutions are the main agents of delignification. |
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How can delignification be controlled in cooling towers? |
Controlling delignification is accomplished through use of nonoxidizing biocides or by keeping the chemical concentration of oxidizing agents sufficiently low. |
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How can biological attacks effect wood deterioration in cooling towers? |
Biological attack of cooling tower wood occurs as a deterioration of the cellulose which is a source of carbon for the growth and development of microorganisms. |
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How can biological attacks on wood in cooling towers be controlled? |
· Choosing woods such as redwood that have a natural resistance to biological attack. |
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What are probable causes of excessive water drift? |
1. Faulty drift elimination 2. Overpumping |
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What is the remedy for faulty drift elimination? |
1. Check to be sure all louvres, eliminator sections, or slats and splash retainers are in place. |
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What is the remedy for overpumping? |
Reduce water flow to tower to design conditions or use larger metering orifices. |
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What are the probable causes of the motor not starting? |
1. Power not available at motor terminals 2. Wrong connections 3. Low voltage 4. Open circuit in motor winding 5. Motor or fan drive stuck 6. Rotor defective 7. Vibration switch tripped or defective |
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What is the remedy for power not being available at motor terminals? |
1. Check power at starter. Correct any bad connections between the control apparatus and the motor. |
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What is the remedy for wrong connections? |
Check motor and control connections against wiring diagrams. |
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What is the remedy for low voltage? |
Check nameplate voltage against power supply. Check voltage at motor terminals. |
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What is the remedy for an open circuit in motor winding? |
Check stator windings for open circuits. |
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What is the remedy for the motor or fan drive being stuck? |
Disconnect motor from load and check motor and gear reducer for cause of problem. |
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What is the remedy for a defective rotor? |
Look for broken bars and rings. |
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What is the remedy for the vibration switch being tripped or defective? |
Reset switch and check for cause of vibration. |
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What are the probable causes of unusual motor noise? |
1. Motor running single-phase |
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What is the remedy for the motor running single-phase? |
Stop motor and attempt to start it. Motor will not start if single-phased. |
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What is the remedy for the motor leads being connected incorrectly? |
Check motor connections against wiring diagram on motor. |
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What is the remedy for the ball bearings making noise? |
Check lubrication. Replace bad bearings. |
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What is the remedy for electric unbalance? |
Check voltage and currents of all three lines. Correct if required. |
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What is the remedy for the air gap not being uniform? |
Check and correct bracket fits |
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What is the remedy for the rotor being unbalanced? |
Rebalance |
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What is the remedy for the cooling fan hitting the guard? |
Reinstall and replace fan. |
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Refer to Table 6 for Cooling Tower Troubleshooting Guide |
It's too long to copy. |
