Natural draft cooling towers are a highly efficient and widely used method for cooling water in various industrial processes, such as thermal power plants. The evaporation process within these towers is a complex and intricate phenomenon that involves the interplay of several key factors. In this comprehensive guide, we will delve into the technical details of how water is evaporated in natural draft cooling towers, providing you with a deep understanding of this process.
Water Distribution in Natural Draft Cooling Towers
The water cooling process in a natural draft cooling tower begins with the distribution of the warm water from the industrial process. The warm water is pumped from the power plant or industrial facility to the top of the cooling tower, where it is evenly dispersed over the fill area through a network of spray nozzles. The fill area is designed to maximize the surface area of the water, facilitating efficient heat transfer and evaporation.
The spray nozzles are strategically positioned to ensure an even distribution of the warm water across the entire crosssection of the tower. This uniform distribution is crucial for achieving optimal cooling performance, as it prevents the formation of hot spots or uneven air flow patterns within the tower.
Evaporative Cooling Mechanism
As the warm water cascades down through the fill, a portion of it undergoes evaporation due to the heat transfer from the water to the surrounding air. This evaporative cooling process is the primary mechanism by which natural draft cooling towers reduce the temperature of the water.
The rate of evaporation is influenced by several factors, including the temperature and humidity of the air, the surface area of the water, and the air velocity within the tower. The latent heat of vaporization, which is the energy required to convert liquid water into water vapor, plays a crucial role in this process.
The evaporation of water from the surface of the fill causes the remaining water to cool down, as the heat required for the phase change is extracted from the water itself. This cooling effect is further enhanced by the continuous air flow through the tower, which carries away the water vapor and replenishes the air with drier, cooler air.
Air Flow and Stack Effect
The driving force behind the air circulation in a natural draft cooling tower is the stack effect, which is a result of the difference in air density between the hot, moist air at the top of the tower and the cooler, drier air at the base.
As the air inside the tower is heated by the warm water, it becomes less dense and rises, creating a natural draft that pulls in cooler air from the base of the tower. This continuous air flow ensures a steady supply of fresh air for the evaporative cooling process, while also carrying away the water vapor.
The height of the cooling tower is a critical factor in the effectiveness of the stack effect. Taller towers, typically ranging from 100 to 200 meters, generate a stronger stack effect, leading to higher air velocities and more efficient cooling.
Heat Transfer and Fill Design
The fill design in a natural draft cooling tower is optimized to maximize the contact surface area between the water and the air, thereby enhancing the heat transfer efficiency. Two common fill designs are:

Film Fill: This design uses a series of thin, corrugated sheets that create a large surface area for the water to flow over, promoting efficient heat and mass transfer.

Splash Fill: This design uses a series of splash bars or grids that break up the water flow, creating a large surface area for airwater interaction and evaporation.
The fill material is typically made of durable, corrosionresistant materials, such as PVC or ceramic, to withstand the harsh operating conditions within the cooling tower.
In addition to the fill design, the cooling tower may also incorporate drift eliminators, which are devices that reduce the amount of water droplets carried out of the tower by the exhaust air, minimizing water losses and operational costs.
Calculating Water Evaporation Rate
The rate of water evaporation in a natural draft cooling tower can be calculated using the following formula:
m_water_evaporated = m_water_circulated x (h_fg / h_in)
Where:
– m_water_evaporated is the mass of water evaporated (kg/s)
– m_water_circulated is the mass flow rate of water circulated in the cooling tower (kg/s)
– h_fg is the latent heat of vaporization of water at the wet bulb temperature (kJ/kg)
– h_in is the enthalpy of water at the inlet temperature (kJ/kg)
To use this formula, you will need to know the following parameters:
– Mass flow rate of water circulated in the cooling tower
– Inlet water temperature
– Wet bulb temperature of the air
By plugging in these values, you can calculate the mass of water that is evaporated in the cooling tower, which is a crucial metric for understanding the tower’s cooling capacity and efficiency.
Technical Specifications of Natural Draft Cooling Towers
Natural draft cooling towers typically have the following technical specifications:
Specification  Range 

Height  100 to 200 meters 
Base Diameter  Up to 100 meters 
Materials  Steel and reinforced concrete 
Fill Design  Film fill or splash fill 
Drift Eliminators  Reduce water losses and operational costs 
Sump  Depressed portion of the collecting basin for drawing cold water 
Distribution System  Ensures even water distribution to spray nozzles 
These specifications highlight the large scale and complex engineering involved in the design and construction of natural draft cooling towers, which are essential for their efficient and reliable operation.
Conclusion
Natural draft cooling towers are a highly effective and energyefficient solution for cooling water in various industrial processes. The evaporation process within these towers is a complex interplay of water distribution, evaporative cooling, air flow, and heat transfer, all of which are carefully engineered to maximize the cooling capacity and minimize operational costs.
By understanding the technical details and principles behind the water evaporation in natural draft cooling towers, you can gain a deeper appreciation for the engineering feats that make these structures so crucial in modern industrial operations.
References
 An Introduction to Water Cooling Towers – CED Engineering
 Evaporative cooling of water in a natural draft cooling tower
 Natural Draft Cooling Towers – savree.com
 How Natural Draft Cooling Towers Work (Stack Effect) – YouTube
 Cooling Towers – Student – Cheresources.com Community