Operational and Environmental Consequences of Large Industrial Cooling Water Systems

Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows perpendicular to the air through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum volume. Lastly, a fan forces the air out into the atmosphere.

A distribution or hot water basin consisting of a deep pan with holes or nozzles in its bottom is located near the top of a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material. In a counterflow design, the air flow is directly opposite to the water flow see diagram at left. Air flow first enters an open area beneath the fill media, and is then drawn up vertically.

The water is sprayed through pressurized nozzles near the top of the tower, and then flows downward through the fill, opposite to the air flow. Both crossflow and counterflow designs can be used in natural draft and in mechanical draft cooling towers. Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of make-up volumetric flow rate , evaporation and windage losses, draw-off rate, and the concentration cycles. In the adjacent diagram, water pumped from the tower basin is the cooling water routed through the process coolers and condensers in an industrial facility.

The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water C. The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate.

The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. Fresh water make-up M is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.

A water balance around the entire system is then: Since the evaporated water E has no salts, a chloride balance around the system is: Windage or drift losses W is the amount of total tower water flow that is entrained in the flow of air to the atmosphere. From large-scale industrial cooling towers, in the absence of manufacturer's data, it may be assumed to be:. Cycle of concentration represents the accumulation of dissolved minerals in the recirculating cooling water.

Discharge of draw-off or blowdown is used principally to control the buildup of these minerals. The chemistry of the make-up water, including the amount of dissolved minerals, can vary widely. Make-up waters low in dissolved minerals such as those from surface water supplies lakes, rivers etc. Make-up waters from ground water supplies such as wells are usually higher in minerals, and tend to be scaling deposit minerals. Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping; however, excessive levels of minerals can cause scaling problems.

As the cycles of concentration increase, the water may not be able to hold the minerals in solution. When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water.

Often a professional water treatment consultant will evaluate the make-up water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as water softening , pH adjustment, and other techniques can affect the acceptable range of cycles of concentration.

Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States, many water supplies use well water which has significant levels of dissolved solids. On the other hand, one of the largest water supplies, for New York City , has a surface rainwater source quite low in minerals; thus cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.

Since higher cycles of concentration represent less make-up water, water conservation efforts may focus on increasing cycles of concentration. Disinfectant and other chemical levels in cooling towers and hot tubs should be continuously maintained and regularly monitored.

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Regular checks of water quality specifically the aerobic bacteria levels using dipslides should be taken as the presence of other organisms can support legionella by producing the organic nutrients that it needs to thrive. Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling , the water should be filtered to remove particulates, and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water.

Biofilm can be reduced or prevented by using chlorine or other chemicals. A normal industrial practice is to use two biocides, such as oxidizing and non-oxidizing types to complement each other's strengths and weaknesses, and to ensure a broader spectrum of attack. In most cases, a continual low level oxidizing biocide is used, then alternating to a periodic shock dose of non-oxidizing biocides. Another very important reason for using biocides in cooling towers is to prevent the growth of Legionella , including species that cause legionellosis or Legionnaires' disease, most notably L.

Common sources of Legionella include cooling towers used in open recirculating evaporative cooling water systems, domestic hot water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources include freshwater ponds and creeks. French researchers found that Legionella bacteria travelled up to 6 kilometres 3. That outbreak killed 21 of the 86 people who had a laboratory-confirmed infection.

Drift or windage is the term for water droplets of the process flow allowed to escape in the cooling tower discharge. Drift eliminators are used in order to hold drift rates typically to 0. A typical drift eliminator provides multiple directional changes of airflow to prevent the escape of water droplets. A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for Legionella or water treatment chemical exposure. The CDC does not recommend that health-care facilities regularly test for the Legionella pneumophila bacteria.

Scheduled microbiologic monitoring for Legionella remains controversial because its presence is not necessarily evidence of a potential for causing disease. The CDC recommends aggressive disinfection measures for cleaning and maintaining devices known to transmit Legionella , but does not recommend regularly-scheduled microbiologic assays for the bacteria. However, scheduled monitoring of potable water within a hospital might be considered in certain settings where persons are highly susceptible to illness and mortality from Legionella infection e.

Also, after an outbreak of legionellosis, health officials agree that monitoring is necessary to identify the source and to evaluate the efficacy of biocides or other prevention measures. Under certain ambient conditions, plumes of water vapor fog can be seen rising out of the discharge from a cooling tower, and can be mistaken as smoke from a fire. If the outdoor air is at or near saturation, and the tower adds more water to the air, saturated air with liquid water droplets can be discharged, which is seen as fog. This phenomenon typically occurs on cool, humid days, but is rare in many climates.

Fog and clouds associated with cooling towers can be described as homogenitus , as with other clouds of man-made origin, such as contrails and ship tracks.

This phenomenon can be prevented by decreasing the relative humidity of the saturated discharge air. For that purpose, in hybrid towers, saturated discharge air is mixed with heated low relative humidity air. Some air enters the tower above drift eliminator level, passing through heat exchangers. The relative humidity of the dry air is even more decreased instantly as being heated while entering the tower. The discharged mixture has a relatively lower relative humidity and the fog is invisible. The salt deposition problem from such cooling towers aggravates where national pollution control standards are not imposed or not implemented to minimize the drift emissions from wet cooling towers using seawater make-up.

Similarly, particles smaller than 2. Though the total particulate emissions from wet cooling towers with fresh water make-up is much less, they contain more PM 10 and PM 2. At plants without flue gas purification, problems with corrosion may occur, due to reactions of raw flue gas with water to form acids.

Sometimes, natural draft cooling towers are constructed with structural steel in place of concrete RCC when the construction time of natural draft cooling tower is exceeding the construction time of the rest of the plant or the local soil is of poor strength to bear the heavy weight of RCC cooling towers or cement prices are higher at a site to opt for cheaper natural draft cooling towers made of structural steel.

Some cooling towers such as smaller building air conditioning systems are shut down seasonally, drained, and winterized to prevent freeze damage. Basin heaters, tower draindown, and other freeze protection methods are often employed in cold climates. Operational cooling towers with malfunctions can freeze during very cold weather. Typically, freezing starts at the corners of a cooling tower with a reduced or absent heat load.

Severe freezing conditions can create growing volumes of ice, resulting in increased structural loads which can cause structural damage or collapse. Cooling towers constructed in whole or in part of combustible materials can support internal fire propagation. Such fires can become very intense, due to the high surface-volume ratio of the towers, and fires can be further intensified by natural convection or fan-assisted draft. The resulting damage can be sufficiently severe to require the replacement of the entire cell or tower structure.

For this reason, some codes and standards [38] recommend that combustible cooling towers be provided with an automatic fire sprinkler system. Fires can propagate internally within the tower structure when the cell is not in operation such as for maintenance or construction , and even while the tower is in operation, especially those of the induced-draft type, because of the existence of relatively dry areas within the towers. Being very large structures, cooling towers are susceptible to wind damage, and several spectacular failures have occurred in the past.

Three out of the original eight cooling towers were destroyed, and the remaining five were severely damaged. The towers were later rebuilt and all eight cooling towers were strengthened to tolerate adverse weather conditions. Building codes were changed to include improved structural support, and wind tunnel tests were introduced to check tower structures and configuration.

From Wikipedia, the free encyclopedia. Fan-induced draft, counter-flow cooling tower.

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