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The term "cooling tower" is used to describe both direct (open circuit) and indirect (closed circuit) heat rejection equipment. Cooling towers are heat-transfer units, used to remove heat from any water-cooled system. The cooled water is then re-circulated (and thus, recycled) back into the system. Since the process water is re-circulated, the mineral concentration increases as a result of the evaporation. When the water contains two times the original mineral content, it is said to contain two cycles of concentration. When it contains three times the original mineral content, it has three cycles of concentration and so on. The other half of the cooling loop takes heat from the process through a heat exchanger, condenser or water jacket.
Performance of this unit is critical for efficient and economical operation. The heat transfer surfaces must be kept as clean as possible to assure maximum heat transfer. As the mineral content in the cooling water increases, the potential for scaling and corrosion increases, threatening efficient operation of the heat exchanger.
The heat transfer surface is the hottest area in contact with the cooling water. The solubility of calcium carbonate (CaCO3 found in cooling water) is an inverse function, with respect to temperature, causing calcium carbonate scaling to occur on the heat transfer surfaces. The accumulation of a CaCO3 scale layer reduces the heat transfer capability, causing corrosion and creating hot spots, resulting in thermal stress. All of which affect the efficiency and life span of the heat exchanger. One basic way of preventing scaling, is to bleed off or blowdown a fraction of recycled water and replace it with fresh water (make-up water) of a lower mineral concentration. A complete analysis of the make-up water is required to determine the maximum concentration of minerals that can be tolerated without causing deposits to form.
A direct, or open circuit cooling tower is an enclosed structure with internal means to distribute the warm water fed to it over a labyrinth-like packing or "fill." The fill provides a vastly expanded air-water interface for heating of the air and evaporation to take place. An indirect, or closed circuit cooling tower involves no direct contact of the air and the fluid, usually water or a glycol mixture, being cooled. Unlike the open cooling tower, the indirect cooling tower has two separate fluid circuits. One is an external circuit in which water is re-circulated on the outside of the second circuit, which is tube bundles (closed coils) which are connected to the process for the hot fluid being cooled and returned in a closed circuit. Air is drawn through the re-circulating water cascading over the outside of the hot tubes, providing evaporative cooling similar to an open cooling tower.
The purpose of a water treatment program is to maximize the cycles of concentration while minimizing scaling, corrosion and microbiological growth. The most important factor is scaling, which is typically caused by the over saturation of calcium compounds in the cooling water.
While the most common usage of cooling towers has traditionally been in the air conditioning industry, many other industries can benefit from cooling tower technology such as plastics, dry cleaning, manufacturing, petroleum refining, electrical generation.
In the past, cooling was accomplished by using available water from nearby lakes, rivers or municipal systems on a once through basis. Problems that were encountered included plugging of the heat exchanger by suspended solids (slit or mud) and biological growth within the equipment. The expense of the equipment involved and the increasing restrictions by the EPA, have placed a much greater emphasis on the treatment and re-use of water by means of re-circulating cooling towers. This has significantly reduced industry's demand for fresh water and the quantity of effluent produced.
Cooling towers are heat-transfer units, used to remove heat from any water-cooled system. The type of heat removal in a cooling tower is termed "evaporative" in that it allows a small portion of the water being cooled to evaporate into a moving site stream to provide significant cooling to the rest of the water stream. The cooled water is then re-circulated (and thus, recycled) back into the system. Cooling towers may either use the evaporation of water to reject process heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to near the dry-bulb air temperature. The wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Where as the dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. In construction, it is an important consideration when designing a building for a certain climate.
A re-circulating cooling system re-uses the same water by passing it through heat exchangers, cooling canals or cooling towers to remove the heat that has been transferred into it from equipment or industrial processes. Re-circulating cooling towers affect cooling by evaporation of water, and also by direct heat exchange with air passing through the tower. The basic operating principle is relatively straightforward, but the associated heat transfer equipment varies widely in both cost and complexity.
Problems encountered in cooling systems are not usually with the equipment, but with the water that is used, there are three problems which are common to all cooling systems: they are corrosion, scale and biological growth. All water contains some level of impurities which cause scale and corrosion in the heat exchanger equipment. In the tower itself, the combination of air and warm water provide an ideal environment for biological growth. Dust and other particles (dissolved solids) can also be introduced into the cooling tower depending on prevailing environmental conditions, adding to maintenance and treatment requirements.
Corrosion is caused by impurities in the cooling water reacting with the metal in the system component. The result is a loss of metal which weakens piping and equipment until leaks or ruptures occur.
These impurities can be categorized into two classes:
Biocides are added on a time basis, such as once per day, twice per week or some other interval. A common practice is to alternate feeding two different types of biocide, not allowing the biological growth to become resistant to one or the other. This "shock" treatment is very successful in controlling undesirable biological growth. An alternate method is to use chlorine as the biocide, monitoring its active level using an ORP analyzer.
Dispersants are added to prevent coagulation/flocculation of suspended solids. They are fed in an alternating fashion with inhibitors, one after the other until the demand is satisfied.
Most large air conditioning systems, and many industrial processes, require the use of water to cool some other fluid in a heat exchanger. The heat in the water is dissipated by cascading it down the inside of a cooling tower and blowing air through it. A good deal of the water will thereby be lost due to evaporation, but almost all of the dissolved solids will be left behind.
Over a period of time the level of dissolved solids will steadily increase until a number of undesirable things begin to happen, including corrosion and scaling. To maintain the efficiency of the heat exchanger and to protect the expensive equipment, it is necessary to "dump" some of the contaminated water and replace it with fresh. Conductivity is the measurement of choice for this blowdown application, while a pH measurement is used to measure and control the alkalinity level of the water in order to prevent corrosion.
Most towers can be adequately controlled using just conductivity and pH instrumentation. The "logic" circuit shown in the figure disables the chemical feeds during blowdown of the tower. This saves costly chemicals by not allowing them to be fed down the drain with the solids being removed during blowdown. More complex cooling tower systems include the addition of biocides, inhibitors and dispersants to further protect the equipment.
Transmitter: 4-wire conductivity measurement system SC450G 2-wire conductivity measurement system FLXA21
Option 1: SC42 Conductivity Sensor (fittings available for Flow-Thru, Insertion, or Immersion installations)
Option 2: SC4A Conductivity Sensor (fittings available for Insertion, Sanitary, or Retractable installations)
Transmitter: 4-wire pH/ORP measurement system PH450G 2-wire conductivity measurement system FLXA21
Option 1: FU20 pH/ORP Combination Electrode (fittings available for Flow-Thru, Insertion, or Immersion installations
Option 2: F*20 Insertion or Flow-thru assembly series with individual measure, reference and temperature electrodes for pH/ORP (i.e. SM21-AG4, SR20-AP24 and SM60-T1; SC21C- AGC55 and SM60-T1)
*Note: For additional information on this application contact the Yokogawa Analytical Product Marketing Department
The FLEXA™ series analyzers are used for continuous on-line measurements in industrial installations. With an option for single or dual sensor measurement, they are the most flexible two-wire analyzer available.
The PH450 pH and ORP analyzer is a multivariable analyzer that combines pH with temperature and ORP (Redox) measurement, which can be utilized through different output functions: two mA current outputs, four independent SPDT contact outputs and HART.
All-In-One pH and ORP digital SMART sensor that keeps the motto "Simple is best" while combing the sensor with built-in intelligence and direct digital communication.
The FU20 and FU24, all-in-one pH and ORP, sensors show how Yokogawa applies the motto "Simple is best" to sensor technology.
The measurement of specific conductivity in aqueous solutions is becoming increasingly important for the determination of impurities in water. Yokogawa has designed a full range of precision sensors and instruments to cope with these measurements, even under extreme conditions.
Yokogawa has invested considerable design and development time in producing a full range of fittings with particular emphasis on designs that reduce installation and maintenance time and consequently save operation costs.
The model ISC40 sensors are designed for use with the EXA ISC analyzers. This combination exceeds all expectations for conductivity measurement in terms of reliability, accuracy, rangeability and price performance.
The heart of a pH measuring loop is the electrode system. Yokogawa has designed a wide range of electrodes to ensure this heart keeps beating under the most severe conditions.
Conductivity sensors and electrodes are used to measure process conductivity, resistivity, WIFI, demineralizer water, RO water, percent concentration, boiler blowdown and TDS. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper electrode/sensor selection is critical for optimal measurement results.
pH and ORP meters, analyzers and transmitters are used for continuous process monitoring of pH and ORP to ensure water/product quality, monitor effluent discharge, batch neutralization, pulp stock, scrubbers, cooling towers, chemical, water/wastewater treatment and many other applications.
pH electrodes and sensors are the sensing portions of a pH measurement. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper pH electrode/sensor selection is critical for optimal measurement results.
Contact a Yokogawa Expert to learn how we can help you solve your challenges.