DECSA Explains What Is A Cooling Tower And How Does It Work

The cooling tower is a heat dissipation system that, based on the principle of evaporative cooling, cools down water (or water and glycol) through its exchange with air.

Evaporative cooling - used for some time now in the industrial and air conditioning (HVAC) market - is a simple principle. Based on the latent heat of water's evaporation, it reduces the temperature of the process water by evaporating a small part of it. For example, one can note that, through the evaporation of one kg of water, an average of 600 kcal/h is dissipated (for more details, see also: What factors influence the cooling tower's performance ?).

Industrial or conditioning processes

The wet bulb temperature is usually lower than the ambient temperature by about 4 to 6°C

The reference parameter for heat exchange is not the ambient temperature but the wet bulb temperature, i.e., the temperature that can be determined with the psychrometric diagram, knowing the ambient temperature and the relative humidity. The wet bulb temperature is usually lower than the ambient temperature by about 4 to 6°C (or even higher): a water cooling tower allows to cool down water to a temperature lower than the ambient temperature.

The cooling towers are therefore essential in those industrial or conditioning processes with thermal cycles such as food applications, hospitals, chemical-pharmaceuticals, and automotive. These processes require water to be cooled and reintegrated into the production process to complete the cycle and allow industrial production to continue.

Recovery of recirculated water

As well explained in a video published by Assoclima, the Association of Air Conditioning Manufacturers of ANIMA Confindustria Meccanica Varia - of which Decsa is also a member - "Cooling towers are the most reliable, well consolidated and efficient heat dissipation system available on the market." Compared to other cooling systems, cooling towers have a consumption of up to four times lower, thus ensuring significant energy savings.

As the amount of water evaporated in an evaporative cooling tower varies between 3% and 4%, these cooling systems ensure recovery of recirculated water of 96-97% and, therefore, low water consumption.

Industrial cooling towers

There are two families of industrial cooling towers:

1. Open circuit cooling towers where the heat exchange takes place with direct contact between water and air. In this way, hot water is directly cooled down by its contact with air.
2. Closed-circuit cooling towers where heat exchange occurs through a pipe heat exchange. There is no direct contact between the hot water and the environment.

How does an open circuit cooling tower work

As mentioned above, open circuit cooling towers are cooling systems that, by bringing water and air into direct contact, exploit the operating principle of evaporative cooling, ensuring low energy consumption. Water cooling towers function as heat exchangers: 

1. The open-circuit cooling tower receives a large mass of process water at high temperatures (e.g., 40°C) from the user that needs to be cooled.
2. The 40°C hot water is sprayed uniformly from above onto the fill pack through spray nozzles. 
3. The fill pack is a section inside the cooling tower where heat exchange occurs with countercurrent air. Its function is to expand the useful surface area of the heat exchange and to allow a small amount of water to evaporate.
4. When the air directly meets warm water, the evaporation of a small amount of water takes place. The remaining cool water cooled down to a lower temperature (e.g., 30°C) can be recirculated in the industrial process.
5. The cooled water is pumped into the water basin of the cooling tower, from where pipes connected to pumps enable the process cycle to restart.

Note that the evaporation - albeit limited - of some water in open circuit cooling towers implies the reintegration of this water into the process.

Research and Development department

Large heat exchange surface provided by the fill packs inside the tower favors the cooling process

As mentioned, the large heat exchange surface provided by the fill packs inside the tower favors the cooling process. Due to the availability of a state-of-the-art Research and Development department, has managed to improve the heat exchange inside its cooling towers using increasingly high-performance fill packs and balancing the L/G ratio in the best possible way. Thus, it is possible to obtain cold water outlet temperatures that are increasingly close to the reference wet bulb temperature values.

Note that the air inside the cooling tower can be sucked in through axial fans or pushed in through centrifugal fans. To satisfy all requests, Decsa offers its customers a wide range of solutions, with axial or centrifugal open circuit cooling towers. For more details, visit cooling tower pages at open circuit SQA, TMA-EU, and TMR.

How does a closed circuit cooling tower work

Closed circuit cooling towers are also systems that use the principle of evaporative cooling to cool down a stream of process water through the air.

Compared to an open-circuit system, closed-circuit cooling towers do not bring hot water into direct contact with the air. Here, the water remains inside the heat exchanger, thus isolated from the external environment. This means that all water remains inside a closed system, cleaned and free from contamination. 

Closed-circuit cooling towers

Closed-circuit cooling towers, therefore, have two separate circuits: 

• A primary circuit, where warm water circulates within the exchange coil.
• A secondary circuit inside the heat exchanger of the cooling tower, where the water meets the outside air and cools down. Then the cycle can begin again.

Due to the isolation of water from the external environment, closed-circuit cooling towers are especially suitable where hot water cannot be polluted by external agents. By avoiding the evaporation of water and the subsequent need to reintegrate it, these towers ensure that the water is never contaminated and that components of the industrial process are not damaged.

That said, the traditional process of a closed-circuit cooling tower includes the following steps:

1. The cooling tower gets a large amount of high-temperature hot water (e.g., 40°C) from the user which needs to be cooled. 
2. Hot water with a temperature of 40°C that needs to be cooled circulates within the coils of a bundle of pipes; at the same time, secondary circulation takes place in the cooling tower.
3. The spray nozzles evenly distribute the water extracted from the basin of the cooling tower onto the heat exchanger above the coils, while the air is blown or sucked in by the fans.
4. The airflow from the fans cools down the water in the secondary circuit and thus - indirectly - the thermal energy of the hot water.

Available in both models

By using highly efficient "full surface" HDGS heat exchanger coils and optimizing the L/G balance, Decsa cooling towers ensure high-performance efficiency and reliability. 

Available in both models with axial fans (REF-A) and models with centrifugal fans (REF-C), these cooling towers are best integrated where quietness and reliability are key points in equipment selection. For more details, visit the cooling tower pages at closed-circuit: REF-A and REF-C.