Atmospheric cooling tower

There are two recognized codes for thermal testing of cooling towers Atmospheric Water-... 

Cooling-Tower Plumes. An important consideration in the acceptabiHty of either a mechanical-draft or a natural-draft tower cooling system is the effect on the environment. The plume emitted by a cooling tower is seen by the surrounding community and can lead to trouble if it is a source of severe ground fog under some atmospheric conditions. The natural-draft tower is much less likely to produce fogging than is the mechanical-draft tower. Nonetheless, it is desirable to devise techniques for predicting plume trajectory and attenuation. 

Coil Shed Towers - These are composed of a combination structure of a cooling tower installed on top of a substmcture that contains atmospheric section coils (refer to Figure 7). 

A leak in another heat exchanger allowed flammable gas to enter a cooling-water return line. The gas was ignited by welding, which was being carried out on the cooling tower. The atmosphere had been tested before work started, five hours earlier (see Section 1.3.2). 

Most cooling towers are built of redwood or cypress. However, special conditions and atmospheres dictate other types of construction. 

Figure 9-99. Atmospheric cooling tower. Used by permission of The Pritchard Corp. (now, Black and Veatch Pritchard).

Figure 9-100. Atmospheric spray tower, air flow aspirated by pressure-spray water distribution system. Usually applied in small sizes. Used by permission of Hensley, John C. (ed), Cooling Tower Funds.-menteUs, 2nd Ed. (1985), The Mariey Cooling Tower Co., a United Dominion Company.

The economics of forced and induced draft cooling tower operation require a study of fan and water pump horsepower and usually dictate a fan static pressure requirement not to exceed 0.75-1.0 in. of water. For atmospheric and natural draft towers the economics of pumping water are still very important. This means that the ground area must be so selected as to keep the height dovm while not dropping the unit rates so low that performance becomes poor. This then, is a balance of ground area versus total deck height. Pritchard [16] presents an... 

Figure 9-130. Atmospheric cooling tower water loss for various wind velocities. Used by permission of Plastics Technical Service, The Dow Chemical Co., Midland Mich, with data added from Fuller, A. L, et al. Chemical Engineering Progress, V. 53, No. 10 (1957) p. 501 all rights reserved.

Cooling towers and evaporative condensers release into the atmosphere fine droplets of water, which may carry sources of contamination such as algae and bacteria. Many of these thrive at the temperatures to be expected in water cooling systems and one of them, Legionella pneumophila, has been identified as a particular hazard to health. Cooling apparatus should be cleaned and disinfected frequently to reduce these risks of contamination and should not be located where water droplets can be drawn into ventilation air intakes. 

Electrolytic zinc smelters contain up to several hundred cells. A portion of the electrical energy is converted into heat, which increases the temperature of the electrolyte. Electrolytic cells operate at temperature ranges from 30 to 35°C (86 to 95°F) at atmospheric pressure. During electrowinning a portion of the electrolyte passes through cooling towers to decrease its temperature and to evaporate the water it collects during the process. 

Another complication that often arises is loss of water from the system. This could be, for example, the loss of water to effluent from a hosing operation or the evaporative loss to atmosphere from a cooling tower, neither of which becomes available for reuse. To illustrate how water losses can be accounted for, suppose that an operation is added to those in Table 26.5 with a maximum inlet concentration of 80 ppm and a flowrate of 10 t-h-1, all of which is lost. 

In the United States, about 80% of the 23 million kg of technical PCP produced annually — or about 46% of worldwide production — is used mainly for wood preservation, especially utility poles (Pignatello etal. 1983 Kinzell etal. 1985 Zischke etal. 1985 Choudhury etal. 1986 Mikesell and Boyd 1986 USPHS 1994). It is the third most heavily used pesticide, preceded only by the herbicides atrazine and alachlor (Kinzell et al. 1981). Pentachlorophenol is a restricted-use pesticide and is no longer available for home use (USPHS 1994). Before it became a restricted-use pesticide, annual environmental releases of PCP from production and use were 0.6 million kg to the atmosphere from wood preservation plants and cooling towers, 0.9 million kg to land from wood preservation use, and 17,000 kg to aquatic ecosystems in runoff waters of wood treatment plants (USPHS 1994). There are about 470 wood preservative facilities in the United States, scattered among 45 states. They are concentrated in the South, Southeast, and Northwest — presumably due to the availability of preferred timber species in those regions (Cirelli 1978). Livestock facilities are often constructed of wood treated with technical PCP about 50% of all dairy farms in Michigan used PCP-treated wood in the construction of various components of livestock facilities (Kinzell et al. 1985). The chemical is usually applied to wood products after dilution to 5% with solvents such as mineral spirits, No. 2 fuel oil, or kerosene. More than 98% of all wood processed is treated with preservative under pressure about 0.23 kg of PCP is needed to preserve 1 cubic foot of wood (Cirelli 1978). Lumber treated with PCP retains its natural appearance, has little or no odor, and can be painted as readily as natural wood (Wood et al. 1983). 

The order of process items in the layout spacing recommendations (Table 19) is quite similar. The flares are on the top of the list. The next unsafe are cooling towers, boilers and compressors. Storage tanks under pressure appear next. After that come low pressure and atmospheric storage tanks and pumps of flammable liquids. The last and safest group is formed of equipment handling nonflammable and nontoxic materials. 

Contamination of cooling tower water with flammable or combustible process leaks potentially resulting in a flammable atmosphere in and around the cooling tower... 

Further, steam and gas may escape into the atmosphere from wells undergoing tests, and from blow-out drillholes. Some gas may also escape in solution in the surplus condensate flowing from the cooling towers. 

Various poisonous chemical components in geothermal steam escape into the atmosphere from electric power plants via ejector exhausts, cooling towers, silencers, and drains and traps. These compounds include H2S, B, Hg, As, and Rn. Other compounds of environmental concern in geothermal steam, although not poisonous, include C02 and CH4. Apparently, not much attention has been paid to airborne poisons in geothermal steam other than H2S. This noxious gas has an unpleasant smell when present in low and harmless concentrations. When more strongly concentrated, H2S can paralyse the olfactory nerves and thus becomes odourless, and eventually lethal. 

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