09-02-2013, 03:39 PM
Water Treatment For Cooling Towers
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INTRODUCTION
Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plants, power stations and building cooling. 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. They are often associated with nuclear power plants in popular culture, although cooling towers are constructed on many types of buildings.
WATER AS TRANSFER FLUID
Water as heat transfer fluid
Swimmers and other water-sport enthusiasts are familiar with the role of water acting as a cooling agent as water evaporates from swimsuits or clothes. To cool a building, the same principle is used. Water is exposed to moving air so that a portion of the water evaporates, which results in cooling the remaining water. The open recirculating loop of the cooling system is comprised of the cooling tower and piping to and from the chiller or heat exchanger. An example of a 1000 gal open recirculating cooling loop where water collects heat from the chiller leaving the chiller at 95 F. Warm water pumped to the cooling tower falls about 10 ft to the basin of the cooling tower where the temperature is 85 F. The 10 F temperature change is the result of evaporation of about 3 gpm from the recirculating water. Recirculation rate (RR) is about 300gpm; the entire volume of the 1000 gal system flows through the cooling tower every 3min.
PROVIDES COMFORT COOLING TO OFFICE BUILDING
Since water is continuously being evaporated and lost as mist, makeup water must be added to the cooling tower--usually to the basin. The water level in the basin is controlled by a float valve so that entering makeup water is provided as needed. Because evaporation removes only water and not dissolved or particulate materials, evaporation cannot be done indefinitely without concomitant "bleed" or "blowdown." Bleed or blowdown is a controlled water loss from the system so that the dissolved materials do not accumulate in the cooling system
WATER AS A SOLVENT
A solute is a material that has been dissolved in a solvent. Solutes may be solids, liquids, or gases. Solvents frequently are liquids. When coffee is made, organic compounds extracted from the coffee grounds are the solutes, and water is the solvent. If sugar is added to coffee, the sugar is an additional solute. The coffee will contain many solutes--ions originally dissolved in the tap water used to make the coffee; numerous different organic compounds from the coffee beans; and the dissolved sugar, a well studied organic compound. Concentrations of solutes in natural waters depend on the geology of the area in which the water is found.
Some regions of the earth, such as the northeastern U.S., have igneous rock as the primary geological component from which soil was made over centuries of weathering. Other regions of the country have more sedimentary rock--limestone being the predominant example.
COOLING TOWER TREATMENT
Cooling towers seem pretty simple to understand. Their name describes what they do – cool water. They do this by evaporation – also an easy concept to grasp. But there's a lot more to think about than simple cooling and evaporating when it comes to cooling towers, especially when considering the way they're made always seems to be evolving, as does the range of uses, from HVAC to industrial.
MONITERING AND DOCUMENTATION
Quantitative information needed to decide whether the system is meeting heat transfer design specifications is difficult to generate because design conditions may not occur. Occasions frequently occur when the system is operating at partial load. Monitoring methods can range from visual observations and periodic
testing by the maintenance staff to on-line continuous monitoring of multiple parameters by various analytical instruments controlled by a computer.