中央空调冷却塔英文文献翻译 第5页
fogging, or plume, and carryover. Fogging occurs during cooler weather when moist warm air ejected from the tower comes into contact with the cold ambient air, condenses, and forms fog. Fog from cooling towers can limit visibility and can be an architectural nuisance. Carryover is when small droplets of entrained water in the air stream are not caught by the drift eliminators and are ejected in the exhaust air stream. These droplets then precipitate out from the exhaust air and fall to the ground like a light mist or rain (in extreme cases). Carryover or drift contains minerals and chemicals from the water treatment in the tower and can cause staining or discoloration of the surfaces it settles upon. To mitigate problems with fog or carryover, as with recirculation, the designer should consider nearby traffic patterns, parking areas, prevailing wind direction, large glass areas, or other architectural considerations.
Operation and Maintenance
Winter Operation
If chillers or refrigeration equipment are being used in cold weather, freeze protection should be considered to avoid formation of ice on or in the cooling tower. Capacity control is one method that can be used to control water temperature in the tower and its components. Electric immersion heaters are usually installed in the tower sump to provide additional freeze protection. Since icing of the air intakes can be especially detrimental to tower performance, the fans can be reversed to de-ice these areas. If the fans are operating in extremely cold weather, ice can accumulate on the leading edges of the fan blades, which can cause serious imbalance in the fan system. Instrumentation to detect out-of-limits vibration or eccentricity in rotational loads should be installed. As with any operational equipment, frequent visual inspections during extreme weather are recommended.
Water Treatment
The water circulating in a cooling tower must be at an adequate quality level to help maintain tower effectiveness and prevent maintenance problems from occurring. Impurities and dissolved solids are concentrated in tower water because of the continuous evaporation process as the water is circulated through the tower. Dirt, dust, and gases can also find their way into the tower water and either become entrained in the circulating water or settle into the tower sump. To reduce the concentration of these contaminants, a percentage of the circulating water is drained or blown-down. In smaller evaporatively cooled systems, this process is called a bleed-off and is continuous. Blow-down is usually 0.8 to 1.2% of the total water circulation rate and helps to maintain reduced impurity concentrations and to control scale formation. If the tower is served with very poor water quality, additional chemical treatments might be needed to inhibit corrosion, control biological growth, and limit the collection of silt. If the tower installation presents continuing water quality problems, a water treatment specialist should be consulted.
Legionellosis
Legionellosis has been connected with evaporative condensers, cooling towers, and other building hydronic components. Researchers have found that well-maintained towers with good water quality control were not usually associated with contamination by Legionella pneumophila bacteria. In a position paper concerning Legionellosis, the Cooling Tower Institute (CTI, 1996) stated that cooling towers are prone to colonization by Legionella and have the potential to create and distribute aerosol droplets. Optimum growth of the bacteria was found to be at about 37°C (99°F) which is an easily attained temperature in a cooling tower.
The CTI proposed recommendations regarding cooling tower design and operation to minimize the presence of Legionella. They do not recommend frequent or routine testing for Legionella pneumophila bacteria because there is difficulty interpreting test results. A clean tower can quickly be reinfected, and a contaminated tower does not mean an outbreak of the disease will occur.
Maintenance
The cooling tower manufacturer usually provides operating and maintenance (O&M) manuals with a new tower installation. These manuals should include a complete list of all parts used and replaceable in the tower and also details on the routine maintenance required for the cooling tower. At a minimum, the following should also be included as part of the maintenance program for a cooling tower installation.
• Periodic inspection of the entire unit to ensure it is in good repair.
• Complete periodic draining and cleaning of all wetted surfaces in the tower. This gives the opportunity to remove accumulations of dirt, slime, scale, and areas where algae or bacteria might develop.
• Periodic water treatment for biological and corrosion control.
• Continuous documentation on operation and maintenance of the tower. This develops the
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