Cooling towers Classification by build

Package types

These types of cooling towers are factory pre-assembled and can be easily transported by truck since they are compact machines. Modular towers have limited capacity, so they are usually the first choice for facilities with low heat removal requirements, such as food processing plants, textile mills, some chemical processing plants, or buildings such as hospitals, hotels, shopping malls, automobile factories, etc.Sound level control is a relatively more important issue for encapsulated cooling towers since they are often used in or near residential areas.

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Facilities such as power plants, steel processing plants, oil refineries or petrochemical plants often have field-mounted cooling towers because of their greater heat removal capacity.Field installed cooling towers are usually much larger in size than packaged cooling towers.A typical field-installed cooling tower has a pultruded fiber reinforced plastic (FRP) structure, FRP cladding, ventilation mechanism, and drift eliminators.

Heat transfer methods

• As far as the heat transfer mechanism is concerned, there are mainly:

• Wet or evaporative cooling towers operate on the principle of evaporative cooling.The working coolant (usually water) is a fluid that evaporates and is exposed to the elements.

• Closed circuit cooling towers (also known as fluid coolers) pass the working coolant through a large heat exchanger, usually a radiator, where clean water is sprayed and a fan is applied to induce air.The resulting heat transfer performance approaches that of a wet cooling tower, with the advantage of protecting the working fluid from environmental exposure and      contamination.

• Adiabatic cooling towers spray water onto incoming air or a cardboard mat to cool the air before it passes through an air-cooled heat exchanger.Adiabatic cooling towers use less water than other cooling towers, but do not cool the fluid to near wet-bulb temperatures.Most adiabatic cooling towers are also hybrid cooling towers.

• Dry cooling towers (or dry coolers) are closed circuit cooling towers that separate the operating coolant from ambient air by heat transfer through a heat exchanger,such as in a radiator,using convective heat transfer.They do not use evaporation.

• Hybrid cooling towers are closed circuit cooling towers that can switch between wet or adiabatic and dry operation.This helps balance water and energy savings in all weather conditions.Some hybrid cooling towers can switch between dry, wet and adiabatic modes.

In a wet cooling tower (or open cooling tower), warm water can be cooled to a temperature below the dry bulb temperature of the ambient air if the air is relatively dry (see dew point and humidity).As ambient air is drawn through the water stream, a small portion of the water evaporates, and the energy required to evaporate this portion of the water is taken from the remaining volume of water, thereby reducing the temperature of the water.The evaporated water absorbs approximately 420 kilojoules per kilogram (970 BTU/lb) of thermal energy.Evaporation results in saturated air conditions, reducing the temperature of the water treated by the tower to a value close to the wet bulb temperature, which is lower than the ambient dry bulb temperature, with the difference being determined by the initial humidity of the ambient air.For better performance (more cooling), a medium called padding is used to increase the surface area and contact time between the air and water flow.Splash fill consists of materials placed to interrupt the flow of water that causes splashes.Membrane fill consists of a thin sheet of material (usually PVC) through which water flows. Both methods increase the surface area and contact time between the fluid (water) and gas (air) to improve heat transfer.