How Cooling Towers Work

Cooling towers are objects which remove heat by transferring process waste heat to the atmosphere. This can be done using the evaporation of water to remove process heat and cool the working fluid or by only using air to cool the working fluid. This cooling process is used in the circulating water used in oil refineries, power stations, chemical plants and building cooling. There is a large variation in size: from small roof-top units to very large structures that can be up to 100 meters wide and twice that in length. Smaller towers are usually built in factories, while larger ones are constructed on site.

Heat transfer methods

There are three types of cooling towers. These are:

* Wet cooling towers, which use principles of evaporation. The working fluid and the evaporated fluid, which is usually water, are the same.

* Dry coolers, which transfer heat through a surface that separates the working fluid from surrounding air, such as in a heat exchanger, using the principles of convection. They do not use evaporation.

* Fluid coolers, which are hybrids that pass the working fluid through tubes, upon which clean water is sprayed and a fan-made draft applied. The result is a process which is more closely related to that of a wet cooling tower. The advantage is that it protects the working fluid from environmental exposure.

In wet cooling towers, it is feasible to cool the warm water to a temperature lower than that of the surrounding air, if the air is comparably dry. As air is drawn over water flow, evaporation occurs, which takes heat away from the working fluid.

To achieve better results, i.e. more cooling, a medium called fill is used to increase the surface area between the air and water flows. There are two types of fill, splash and film fill. Splash fill is made of a material which is placed to disrupt the water flow and cause splashing, while in film fill water flows over thin sheets of material. Both increase surface area.

Air flow generation methods

In fluid and dry coolers, air is drawn through the tower. This can be done in three ways:

* Natural draft- Warm, moist air rises due to it having a lower density than the dry, cooler outside air. This movement of warm air causes a current of air through the tower.

* Mechanical draft- This utilizes fans to force or draw air through the tower.

* Fan assisted natural draft, which is a hybrid that looks like a natural draft but with airflow assisted by a fan.

The water that requires cooling has a temperature between 40 and 60 oC, and is cooled by about 10 to 20 oC.It is pumped to the top and flows down many wooden or plastic partitions which turn the water stream into droplets. This increases the surface area