When there is a need for larger cooling ranges and closer approaches, the natural-draft or atmospheric towers might be considered. Fill installed in natural-draft cooling towers increases the time of contact between water and air.

Various types of fill and spacing are utilized, and tower heights vary in relation to the extent of cooling to be accomplished.The cooling is dependent on the efficiency of the fill and the air velocity through the cooling tower as the water descends through the fill.

The advantages are (1) no electric power is required except for pumping head and (2) no mechanical equipment is necessary, reducing maintenance requirements.

The disadvantages are (1) atmospheric towers have limited capacities, since they are solely dependent on ambient atmospheric conditions, (2) water loss as a result of high wind velocities can be appreciable, and (3) a rather high pumping head is required to allow for maximum air–water contact time.

The large natural-draft hyperbolic cooling towers are found only in utility power station service in the United States. The economics of plant designs will favor the mechanical-draft cooling towers because of the rather short amortization period.

Natural-draft towers perform better when wet-bulb temperatures are low and relative humidity is high or if demand is higher in winter. A combination of low design wet-bulb temperature and high inlet and exit water temperatures would enhance the operation of a hyperbolic tower.

Because of the size of these units, 500 ft (155 m) high and 400 ft (122 m) in diameter at the base, they are more practical when the circulating cooling water flow rate is about 200,000 gal/min and higher.

Mechanical-draft towers have a positive control of air delivery through the fill by the use of large-diameter fans. Therefore, they can be designed for close control of cold-water temperature. Counterflow and crossflow designs are indicated in Figs. 3.17 and 3.18.

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