271 Introduction

Hyperbolic cooling towers are large, thin shell reinforced concrete structures that contribute to environmental protection and to power generation efficiency and reliability. As shown in Figure 27.1, they may dominate the landscape but they possess a certain esthetic eloquence due to their doubly curved form. The operation of a cooling tower is illustrated in Figure 27.2. In a thermal power station, heated steam drives the turbogenerator that produces electric energy. To create an efficient heat sink at the end of this process, the steam is condensed and recycled into the boiler. This requires a large amount of cooling water, whose temperature is raised and then recooled in the tower.

In a so-called "wet" natural draft cooling tower, the heated water is distributed evenly through channels and pipes above the fill. As the water flows and drops through the fill sheets, it comes into contact with the rising cooler air. Evaporative cooling occurs and the cooled water is then collected in the water basin to be recycled into the condenser. The difference in density of the warm air inside and the colder air outside creates the natural draft in the interior. This upward flow of warm air, which leads to a continuous stream of fresh air through the air inlets into the tower, is protected against atmospheric turbulence by the reinforced concrete shell. The cooling tower shell is supported by a truss or framework of columns bridging the air inlet to the tower foundation.

There are also "dry" cooling towers that operate simply on the basis of convective cooling. In this case, the water distribution, the fill, and the water basin are replaced by a closed piping system around the air inlet, resembling, in fact, a gigantic automobile radiator. While dry cooling towers are doubtless superior

FIGURE 27.1 Computer vision of the lignite power plant at Niederaussem.
FIGURE 27.2 Thermal power plant with cleaned flue gas injection.

from the point of view of environmental protection, their thermal efficiency is only about 30% of comparable wet towers. If the flue gas is cleaned by a washing technology, it is frequently discharged into the atmosphere by the cooling tower upward flow. This saves reheating of the cleaned flue gas and the construction of a smoke stack (see Figure 27.2).

Figure 27.3 summarizes the historical development of natural draft cooling towers. Technical cooling devices first came into use at the end of the 19th century. The well-known hyperbolic shape of cooling towers was introduced by two Dutch engineers, Van Iterson and Kuyper, who in 1914 constructed the first hyperbolic towers, which were 35 m high. Soon, capacities and heights increased until around 1930,

Niederaussem — D 1998;

200.00 m

151.00m

Niederaussem — D 1966;

117.00m

68.00m 37

35.00m

25.00 m

Niederaussem — D 1998;

200.00 m

151.00m

Niederaussem — D 1966;

117.00m

68.00m 37

35.00m

25.00 m

Valenciennes — F 1904 FIGURE 27.3 Historical development of natural draft cooling towers.
FIGURE 27.4 Cooling tower, Gundremmingen, Germany.

when tower heights of 65 m were achieved. The first such structures to reach higher than 100 m were the towers of the High Marnham Power Station in Britain.

In the 1970s, cooling towers for nuclear power plants exceeded elevations of 160 m, having key dimensions as given in Figure 27.4. The increase of efficiency at French nuclear stations led to tower heights close to 190 m, demonstrated by the highest cooling tower for nearly one decade in Civaux, France.

After the German reunification, a generation of new power plants in the lignite belt of the former East Germany replaced the old equipment there, with all having cooling towers with heights close to 180 m. Recently in Niederraussem, near Cologne, Germany, a lignite power station with 965 MW of net capacity and achieving a degree of efficiency of 43% started energy production. The Niederraussem cooling tower with a height of 200 m presently is the world's highest [1]. For all of these new tall towers, the shell is proportionally thinner than an egg.

In the near future, if a renewal program for nuclear power stations in the United States begins, the expected cooling tower heights will easily exceed 200 m, both for better control of thermal pollution and for savings of nuclear fuel.

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