Cooling tower system
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The primary use of large, industrial cooling tower systems is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants and other industrial facilities. The absorbed heat is rejected to the atmosphere by the evaporation of some of the cooling water in mechanical forced-draft or induced draft towers (as seen in the image to the right) or in natural draft hyperbolic shaped cooling towers as seen at most nuclear power plants.
Overall material balance relationships
Here are the governing relationships for the makeup flow rate, the evaporation and windage losses, the draw-off rate, and the concentration cycles in a wet cooling tower system: (available in many university libraries)
| M | = Make-up water in m³/hr |
| C | = Circulating water in m³/hr |
| D | = Draw-off water in m³/hr |
| E | = Evaporated water in m³/hr |
| W | = Windage loss of water in m³/hr |
| X | = Concentration in ppmw (of any completely soluble salts … usually chlorides) |
| XM | = Concentration of chlorides in make-up water (M), in ppmw |
| XC | = Concentration of chlorides in circulating water (C), in ppmw |
| Cycles | = Cycles of concentration = XC / XM (dimensionless) |
| ppmw | = parts per million by weight |
In the above sketch, 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. That contact causes a small amount of the water to be lost as windage (W) and some of the water (E) to evaporate. 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. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off (D) for disposal. Fresh water makeup (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:
- M = E + D + W
- M (XM) = D (XC) + W (XC) = XC (D + W)
- XC / XM = Cycles of concentration = M ÷ (D + W) = M ÷ (M – E) = 1 + [E ÷ (D + W)]
- E = C · ΔT · cp ÷ HV
| where: | |
| HV | = latent heat of vaporization of water = ca. 2260 kJ / kg |
| ΔT | = water temperature difference from tower top to tower bottom, in °C |
| cp | = specific heat of water = ca. 4.184 kJ / (kg · °C) |
Windage losses (W), in the absence of manufacturer's data, may be assumed to be:
- W = 0.3 to 1.0 percent of C for a natural draft cooling tower without windage drift eliminators
- W = 0.1 to 0.3 percent of C for an induced draft cooling tower without windage drift eliminators
- W = about 0.01 percent of C (or less) if the cooling tower has windage drift eliminators
Cycles of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Draw-off (or blowdown) is used principally to control the buildup of these minerals.The chemistry of the makeup water including the amount of dissolved minerals can vary widely. Makeup waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Makeup waters from ground water supplies (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 makeup 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 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. Only the waters with the lowest levels of dissolved minerals can be allowed to concentrate above 7 cycles without depositing minerals. In the United States the majority of water supplies are well waters and have significant levels of dissolved solids. On the other hand, one of the largest water supplies, New York City, has a surface supply quite low in minerals and cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.
Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling, the water should also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water.
(Note: Draw-off and blowdown are synonymous. Windage and drift are also synonymous.)
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