Cooling Towers

What is a Cooling Tower?

Cooling Towers are heat rejection devices used for cooling water - or other working medium - to near the ambient wet-bulb air temperature. Cooling towers use evaporation of water to reject heat from processes such as cooling the circulating water used in oil refineries, chemical plants, power plants, steel mills and food processing plants.

An industrial water cooling tower extracts waste heat to the atmosphere though the cooling of a water stream to a lower temperature. Towers that use this process are called evaporative cooling towers.

The process is termed "evaporative” in that it allows a small portion of the water being cooled to evaporate into a moving air stream to provide significant cooling to the rest of that water stream. 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.

Evaporative heat rejection devices such as industrial cooling systems are commonly used to provide significantly lower water temperatures than achievable with "air-cooled" or "dry" heat rejection devices, like the radiator in a car, thereby achieving more cost-effective and energy efficient operation of systems in need of cooling.

Think of the times you've seen something hot be rapidly cooled by putting water on it, which evaporates, cooling rapidly, such as an overheated car radiator. The cooling potential of a wet surface is much better than a dry one.

The industrial water cooling towers vary in size from small roof-top units to very large hyperboloid (hyperbolic) structures that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures that can be over 15 meters tall and 40 meters long. Smaller towers (package or modular) are normally factory-built, while larger ones are typically constructed on site in various materials.

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Types of Cooling Towers

Natural draft, utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. This moist air buoyancy produces a current of air through the tower.

Hyperboloid (aka hyperbolic) cooling towers have become the design standard for all natural draft cooling towers because of their structural strength and minimum usage of material. The hyperbolic form is popularly associated with nuclear power plants, however, this association is misleading, as hyperbolic natural draft cooling towers are often used at large coal-fired power plants as well.

Mechanical draft, which uses power driven fan motors to force or draw air through the tower.

Induced draft Cooling Tower Design: A mechanical draft cooling tower with a fan at the discharge which pulls air through tower. The fan induces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake.

Forced draft Cooling Tower: A mechanical draft cooling tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor power than the equivalent induced draft design. The forced draft benefit is its ability to work with high static pressure.

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