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OWOE - Electrical Power Generation - How does a cooling tower work?
  Figure 1 - Cooling Tower Heat Flow Schematic
Figure 1 - Cooling Tower Heat Flow Schematic
Figure 2- Evaporative Cooling Towers
How does a cooling tower work?

All power plants create heat as a byproduct of generating electricity. This is typically in the form of hot water that remains after the steam energy has been used to rotate the steam turbines and then condensed back to liquid state. Some plants simply return this water back to the ocean, a nearby lake, or a dedicated cooling pond. As a result, the waste heat is transferred from the power plant to the external body of water. Plant operating permits typically limit the increase in water temperature in order to protect plants and animals that live in or around the body of water.

Power plants that don't have a large nearby water resource, plus most modern power plants which are prohibited from the practice of "once-through cooling", must utilize a closed water system. In such a system, large cooling towers are used to transfer heat from the water to the air such that the water cools enough to be piped back to the plant and reused. (See Figure 1) The type of heat transfer in a cooling tower is termed "evaporative" in that it allows a portion of the water being cooled to evaporate into a moving air stream to provide significant cooling to the rest of that water stream. To do this the hot water is converted into a spray at the top of the tower, and air is blown upward from the bottom of the tower. The heat from the water stream transferred to the air stream raises the air's temperature and its relative humidity to 100%, and this air is discharged to the atmosphere. The mist that can usually be seen above cooling towers is the moisture that is captured by the air. This results in some water loss that must be made up by a new water source, but it is a relatively small amount compared to the total amount of water circulating through the system. Figure 2 shows the mist coming off a set of evaporative cooling towers.

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