Natural and Mechanical Drafts Cooling Towers

The difference between natural and mechanical cooling towers is the flow of air which passes through the tower. The natural draft tower is that the packing area is design to locate inside the base of large chimney as shown in figure 1. When the packing is in contact with warm cooling water, it makes the density of the air above the packing to be 5% or less than that of the atmosphere. The driving force to overcome the pressure losses that resist the flow of air through the tower is due to the difference between the air in the chimney and the air outside.

Figure 1 Natural draft cooling tower

In the case of mechanical draft towers mainly use fans which are driven by electric motors to produce the flow of air as represented by figure 2, 3 and 4. The location of the fan determines the type of draft cooling tower. The types of mechanical draft cooling tower include; Force draft; that is when the fan is located in the air passage at the base of the tower. Induced draft; that is when is located in the air passage at the top of tower. This type of draft tower uses both axial and centrifugal flow fans. However, the axial fans are normally with induced draft (Singham, 1990).

Figure 2 Forced draft

Figure 3  Induced draft,  mixed flow

Figure 4 Induced draft, cross flow

Figure : Mechanical draft cooling tower layouts