Induced Draft Towers

Induced draft towers pull air into the tower with a fan located at its top. This design is further subdivided into two categories: counterflow and crossflow.

The counterflow design utilizes air inlet louvers at the tower's base to pull the air in and up through the tower where it comes in contact with the falling water at a 180° angle. 

In the crossflow design, louvers are placed along the towers sides, spanning its entire height. In this way, air is introduced perpendicularly to the falling water. 

Note, that in the counterflow tower, the drift eliminators are located at the top; in crossflow towers the eliminators are placed in the tower's middle.

The eliminators are placed just ahead of the fans, in both instances, to prevent windage losses.

The design of the water distribution system provides the most significant difference between the counterflow and crossflow towers. The crossflow design uses risers to convey water to the top of the tower where it is discharged to an open gravity distribution deck.

The water then falls through the orifices and is distributed across the tower. In the counterflow design, water is pumped to a distribution system, consisting of header and lateral piping with spray nozzles, which is located below the top of the tower; the water is then sprayed across the tower.

In terms of cooling efficiency, the counterflow design is more efficient than the crossflow, because the counterflow permits the coldest water to contact the driest air and the warmest water contacts the most humid air. However, the restricted area at the base of the counterflow tends to choke off the flow of high velocity inlet air. The path that air must take, travelling upward against falling water, generates higher static pressure loss. 

Both factors tend to increase fan horsepower compared with a crossflow tower handling equivalent air water flow. But this can be more than offset by use of efficient film type packing that actually lets the counterflow design handle the same cooling duty as a crossflow design with less air volume. And, because of the counterflow's design, it is often the tower of choice when significant cooling water temperature drops are necessary.

In comparison to natural draft towers, which generally have windage losses of between 0.3% and 1.0% of the recirculation rate, mechanical draft towers are designed to minimize windage losses to between 0.005% and 0.3% of the recirculation rate. Typical industry designs have cooling capacities of 10^CF-40°F; this is called "range" and is defined as the difference in temperature between the hot inlet water and the cold basin water. Evaporation is estimated to be responsible for 80%-90% of this cooling, the rest being attributed to convective heat transfer to the surrounding air. The amount of water lost by evaporation can be estimated by applying a factor of 1% of the recirculation rate for each 10°F temperature drop through the tower. We will discuss this estimate and other calculations in more detail in a later section.

A final point we should address on mechanical draft towers is the number of sides that are open for air flow. One, two or four sides can be open, but the counterflow tower is usually the only one found with total (four sided) entry. Towers are usually designed with single entry in order to minimize contamination from a source located on another side, to provide protection in areas where icing is possible and positioning away from the coldest side is desired or in locations in which the construction site is crowded.