The atmospheric type of tower does not use a mechalnical device, such as a fan, to create air flow through the tower. There are two main types of atmospheric towers—large and small. The large hyperbolic towers are equipped with “fill” since their primary applications are with electric power plants. The steam driven alternator has very high temperature steam to reduce to water or liquid state.
Atmospheric towers are relatively inexpensive. They are usually applied in very small sizes. They tend to be energy intensive because of the high spray pressures required. The atmospheric towers are far more affected by adverse wind conditions than are other types. Their use on systems requiring accurate, dependable cold water temperatures is not reocmmended (see Fig. 8-20).
Mechanical-draft towers, such as in Fig. 8-21, are categorized as either forced-draft towers or induced draft. In the forced-draft type the fan is located in the ambient air stream entering the tower. The air is also brought through or induced to enter the tower by a fan as shown in Fig. 8-22. In the induced-draft draws air through the tower by an induced draft.
Forced-draft towers have high air-entrance velocities and low edit velocities. They are extremely susceptible to recirculation and are therefore considered to have less performance stability than induced-draft towers. There is concern in northern climates as the forced-draft fans located in the cold entering ambient air stream can become subject to severe icing. The resultant imbalance comes when the moving air, laden with either natural or recirculated moisture, becomes ice.
Usually forced draft towers are equipped with centrifugal blower-type fans. These fans require approximately twice the operating horsepower of propeller-type fans. They have the advantage of being able to operate against the high static pressures generated with ductwork. So equipped, they can be installed either indoors or within a specifically designed enclosure that provides sufficient separation between the air intake and discharge locations to minimize recirculation (see Fig. 8-23).