Hyperbolic Towers

Hyperbolic towers are natural draft units built of concrete which can be several hundred feet in height and of equal width at the basin; a shape which allows for optimum air flow and structural stability.

Hyperbolic towers may be a counterflow or crossflow design. 

In the crossflow design, water is distributed outside the tower with the initial contact between air and water taking place prior to entry into the tower itself. In counterflow hyperbolic towers, water distribution occurs inside the structure.

Air flow through hyperbolic towers is produced by differences in air density; dry bulb temperatures and relative humidity also play an important role. In operation, heavier outside air is drawn in from around the base of the tower. It displaces lighter saturated air in the tower, forcing it up and out the top. In fact, it works much like a conventional chimney, except that water saturation rather than heat causes the changes in air density that are responsible for air movement.

Unlike the mechanical draft tower whose fan moves fixed volume of air regardless of its density, a hyperbolic unit's air flow varies with changing atmospheric conditions.

Because of the height and diameter necessary to gain the natural draft required for a given capacity and because they utilize such expensive materials as prestressed concrete, the construction costs of hyperbolic towers are high, as compared to mechanical draft towers. The absence of fans, however, results in lower operating costs than conventional towers. 

They are economical when a plant can amortize the construction costs over an extended period of time. Utilities have found the use of hyperbolic towers advantageous in terrain, where the height of the towers raises the plume to eliminate recirculation and contact with the surrounding load areas. Industrial plants, which have shorter process payout times and much smaller heat loads, and thus smaller plumes, usually construct mechanical draft units.