By DR. DF Duvenhage

1. What are cooling towers, and where are they used?

Evaporative cooling is a well-established and effective method for removing heat from various systems and processes and has been used for over half a century. Its popularity stems from its numerous advantages, including compactness, quiet operation, low energy consumption, and potential for water reuse and savings. Moreover, evaporative cooling systems are simple to operate and maintain, ensuring system efficiency and safety.

Cooling towers find application in a wide range of industries, including:

  • Power generation: Cooling towers are essential in power plants to condense steam and ensure efficient operation.
  • Chemical processing: Chemical reactions often generate significant heat, which needs to be removed by cooling towers.
  • Petrochemical processing: Refining processes in the petrochemical industry require precise temperature control, which is achieved with the help of cooling towers.
  • HVAC systems: Cooling towers are used in large-scale HVAC systems to cool buildings and provide air conditioning.
  • Industrial manufacturing: Many industrial processes generate heat, such as large compressors, mills, lubrication systems, and variable-speed control systems. Cooling towers help maintain optimal operating temperatures for equipment.
  • Data centres: Cooling towers are vital for dissipating heat generated by the high-powered servers in data centres.

Cooling towers effectively reject large amounts of heat to the atmosphere, ensuring optimal operation of various processes and equipment. By removing heat, cooling towers help lower energy consumption in multiple applications. Precise temperature control is crucial for many processes, and cooling towers play a vital role in achieving this. Closed-circuit cooling towers can significantly reduce water consumption and minimise environmental impact.

The specific type of cooling tower best suited for an application depends on various factors.

  • Process requirements: The type of process, the required cooling capacity, and the desired temperature control all impact the selection of the particular cooling tower type or model.
  • Environmental regulations: Water availability, air quality restrictions, and noise limitations must be considered before a cooling tower is selected for an application.
  • Cost considerations: Investment costs, operating costs, and maintenance requirements all contribute to the final selection of a cooling tower type and model..

Cooling towers are versatile and efficient systems for removing heat from various processes and equipment. Their diverse applications and numerous benefits make them vital components in many industrial and commercial sectors. By carefully considering the application’s specific needs and choosing the appropriate type of cooling tower, companies can achieve optimal performance and maintain sustainable operations.

2. Evaporative Cooling Tower Types: More than one way to cool the load

Cooling towers can be generally categorised by the following characteristics:

  1. The type of heat transfer medium;
  2. The type and location of the draft;
  3. The type of contact with water.

Since the heat transfer medium we focus on in this article is water (point 1), we will only focus on different wet, or evaporative, cooling tower configurations due to its superior heat capacity through evaporation.

The type of heat transfer medium

Air

Cooling towers which use only air to cooling a process fluid are typocially referred to as Dry Cooling Tower, or Air-cooled Condensers. Dry cooling towers offer a unique and environmentally friendly solution for industrial applications. Unlike traditional wet cooling towers, they require no water, making them ideal for water-scarce, arid regions and ecologically sensitive areas.

Dry cooling towers rely on air-cooled heat exchangers to dissipate heat from the process fluid. This eliminates the need for water evaporation, resulting in minimal water consumption and plume formation. The process fluid never directly contacts the cooling air, ensuring its purity and preventing contamination. Dry cooling towers come in various configurations, including natural draft, mechanical draft, and hybrid models that combine dry and wet cooling technologies.

Air-cooled cooling towers significantly reduce water usage compared to traditional wet cooling towers. This makes them a sustainable option for water-scarce regions and can offer significant cost savings on water bills. By eliminating water evaporation and plume formation, dry cooling towers have a considerably lower environmental footprint than traditional towers. The closed-loop system eliminates the risk of contact between the process fluid and the cooling air, preventing corrosion and extending the lifespan of the equipment.Therefore, they require less maintenance than wet cooling towers, as they have fewer moving parts and are less susceptible to scaling and water-related issues.

There are, however, certain disadvantages, such as their higher initial cost. This can be a significant barrier for some projects. Dry cooling towers are generally less efficient than wet ones, especially in hot and humid climates. This means they may require larger footprints to achieve the same cooling capacity. Mechanical draft dry cooling towers can generate significant fan noise, which may require additional noise mitigation strategies to comply with noise regulations. In extremely hot and humid conditions, the cooling capacity of dry cooling towers may be insufficient for some applications.

Water

Wet cooling towers are the most widely used type of cooling tower, offering a reliable and cost-effective solution for industrial and commercial applications. They operate by evaporating water to dissipate heat from various processes and equipment.

In the case of open-loop design, the process fluid comes into direct contact with air, allowing efficient heat transfer through evaporation. Generally, water is sprayed over the fill media, creating a large surface area for evaporation. As the water evaporates, the air absorbs heat, cooling the fluid flowing through the tower. The evaporation process produces a visible plume of water vapor, which can be a concern in areas with strict air quality regulations.

Cooling tower plumes https://s33600.pcdn.co/wp-content/uploads/2021/05/cooling-tower-power-plant.jpg

Wet cooling towers are highly efficient at removing heat, making them ideal for applications requiring significant cooling capacity. Compared to dry cooling towers, wet cooling towers are typically cheaper to purchase and install. Wet cooling towers are generally more compact than dry cooling towers, making them suitable for space-constrained installations. Wet cooling towers are relatively easy to operate and require minimal maintenance, yet if this maintenance is addressed, it will be beneficial to the operational efficacy and longevity of the cooling tower.

The major disadvantage with wet cooling towers is that they use a significant amount of water, which can be a concern in water-scarce regions and may incur higher water bills. The evaporation process generates a visible plume, which can contribute to air pollution and increased humidity in the surrounding area. The cooling water can contain minerals that can lead to scaling and corrosion of the tower’s internal components, increasing maintenance requirements. In cold climates, the plume can pose safety concerns by forming ice on nearby structures and equipment. Furthermore, freeze protection might have to be included in areas prone to sub-zero temperatures.

The type and location of the draft

Induced Draft

In this configuration, the fan (1) is located at the top of the cooling tower, “sucking” air in through the inlet louvres (2) around the periphery of the bottom section of the cooling tower. This air then flows upwards, rejecting the higher-temperature air and evaporating moisture into the atmosphere.

Forced Draft

Here, the fan (1) is located on the side of the cooling tower, “pushing” air up through the cooling tower, resulting in the same heat rejection principles as for an Induced Draft configuration.

Comparison of Induced and Forced Draft Cooling Towers

Neither configuration has a significant impact on the sizing of the cooling tower. Instead, the choice between them is based on practical reasons such as noise and space constraints, ease of access to the fan and fan motor and equipment longevity.

The type of contact with water

Open cooling towers

These cooling towers expose process cooling water to the atmosphere. These open towers use an efficient, simple, and economical design. All components in an open system must be compatible with the oxygen introduced via the cooling tower.

Open cooling towers reject heat from water-cooled systems directly to the atmosphere. Hot water from the process enters the cooling tower and is distributed over the cooling tower fill. Air is pulled (induced draft) or pushed (forced draft) through this fill, causing a small portion of the water to evaporate. Evaporation removes heat from the remaining water, which is collected in the cold water basin and returned to the system to absorb more heat.

Their design and operation prioritize efficiency and simplicity, making them a popular choice for many applications. The fill media used in open cooling towers is mainly to maximize the contact surface area between water and air, enhancing the cooling process.

Open-circuit towers achieve high cooling efficiency due to the direct contact between water and air. This makes them ideal for processes requiring significant heat removal. They are also, therefore, best-suited to dry climates, where the relative humidity is lower, thereby decreasing the wet-bulb temperature relative to the dry-bulb temperature. Their relatively simple design translates to lower initial investment costs and easier maintenance compared to other cooling tower technologies. Due to the simple design and higher heat-rejection efficiencies that can be achieved, they require less space than other cooling tower designs, making them suitable for installations with limited space. As a result of the above characteristics, open-circuit towers are readily available from various manufacturers, offering flexibility and cost competitiveness.

The major draw-back from the direct contact of the cooling water with air leads to significant water evaporation, requiring a continuous supply of makeup water. This can be a concern in water-scarce regions and contribute to higher operating costs. The evaporation process generates a visible plume, which can impact air quality and raise humidity levels in the surrounding area. This may require additional mitigation measures to comply with environmental regulations, particularly in urban areas. Minerals in the makeup water can accumulate on the tower’s internal components, as well as process heat exchange equipment where heat is absorbed, leading to scaling and corrosion. This can reduce efficiency and require regular maintenance to address.

Open-circuit cooling towers offer a simple, efficient, and cost-effective solution for various heat rejection needs. However, their high water consumption, environmental impact, and potential for scaling and drift require careful consideration. Choosing open-circuit towers requires balancing their advantages with potential drawbacks and ensuring compliance with environmental regulations and water management practices.

Closed circuit cooling towers

Conversely, these cooling towers completely isolate process cooling fluid from the atmosphere. This is accomplished by combining heat rejection equipment with a heat exchanger equipment, typically tube bundles, in a closed circuit tower. A closed loop system protects the quality of the process fluid, reduces system maintenance, and provides a degree of operational flexibility at a slightly higher initial cost.

Closed circuit cooling towers operate in a manner similar to open cooling towers, except that the heat load to be rejected is transferred from the process fluid (the fluid being cooled) to the ambient air through a heat exchanger. The heat exchanger typically takes the form of a coil of tubes, and serves to isolate the process fluid from the outside air, keeping it clean and contamination free in a closed loop. This creates two separate fluid circuits: (1) an external circuit, in which spray water circulates over the exterior of the coil and mixes with the outside air, and (2) an internal circuit, in which the process fluid circulates inside the coil to be cooled and is returned to the process at a lower temperature. During operation, heat is transferred from the internal circuit, through the coil to the spray water, and then to the atmosphere as a portion of the water evaporates.

By eliminating direct contact, closed-circuit towers slightly reduce water evaporation and water use compared to open-circuit systems. The reduced evaporation results in a significantly smaller or even non-existent plume, minimizing the environmental impact and visual pollution. The closed loop prevents contamination of the process fluid, ensuring its purity and facilitating precise temperature control.

However, compared to open-circuit towers, closed-circuit systems are generally less efficient in heat transfer due to the additional heat exchange step. The more complex design and components of closed-circuit towers result in higher initial investment costs compared to open-circuit systems. The additional components and closed loop require more frequent and specialized maintenance to ensure optimal performance.  Mechanical draft closed-circuit towers can generate significant fan noise, requiring additional sound attenuation measures to comply with noise regulations.

Comparison of Open and Closed Circuit Cooling Towers

3. The anatomy of a cooling tower

While seemingly simple from the outside, a cooling tower is in fact an intricate system comprised of various essential components working to effectively remove heat. In it’s most basic form, evaporative coolers are even used in-doors in hot and dry climates to cool rooms. They work in the same general way and on the same fundamental principle of evaporation as the primary form of cooling.To achieve this, the following basic components are required in a cooling tower, irrespective of its configuration, as discussed above.

  1. Basin: This forms the base of the tower, collecting the cooled water and providing a reservoir for the recirculating water system. Its volume is carefully calculated to maintain adequate water inventory, ensuring consistent performance and preventing pump cavitation. Thermal stratification can occur within the basin, with warmer water settling near the bottom and cooler water rising to the surface.

This necessitates proper mixing techniques to achieve uniform water temperature throughout the basin and optimize cooling efficiency.If the cooling capacity needs to be increased, multiple cooling tower cells (fans and fill sections) can be combined in a common-basin configuration. This provides an advantage in the form of centralised treatment for a large system, instead of separate treatment systems required for separate basins.

To compensate for water losses due to evaporation and drift, makeup water is introduced into the system. The basin serves as a mixing point for the makeup water and the recirculating water, ensuring even distribution and temperature equilibrium before the water re-enters the cooling cycle.

A concrete basin of a field-errected cooling tower.

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Basins of varying sizes of packaged cooling towers.

  1. Fill Media: This is the heart of the tower, consisting of specialized packing materials that provide a large surface area for the hot water to spread and facilitate heat transfer to the air. The primary function of fill media is to maximise the contact surface area between water and air. By employing intricate designs and varied geometries, fill media offer a vast canvas for heat dissipation. Beyond mere surface area, fill media actively facilitates efficient water distribution and interaction with the airflow. Certain designs fragment water into fine droplets, maximising air exposure. Others encourage thin water films to cascade down, mimicking a refreshing shower for the air.

An example of fill media

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  1. Water Distribution System: This network of pipes and nozzles evenly distributes the hot water over the fill media, ensuring optimal contact with the air for efficient cooling. The primary objective of the water distribution system is to deliver a uniform film of water across the entire surface area of the fill media. This necessitates careful hydraulic calculations to overcome pressure drops, ensure consistent flow rates across different sections, and minimize dead zones where water stagnates and contributes to inefficient heat transfer.

At the heart of the system lie the nozzles, meticulously engineered to atomize the water into fine droplets or sheets. Different nozzle types, such as pressure-jet, fan-type, and full-cone, offer varying spray patterns and flow rates, catering to specific fill media designs and cooling requirements. Proper nozzle selection and orientation are crucial for achieving optimal droplet size, ensuring maximum surface area contact with the air, and minimizing drift loss.

Water distribution headers and spray-patterns from the nozzles.

  1. Air Inlet Louvers: These louvers regulate the airflow into the tower, allowing for proper ventilation and preventing water droplets from escaping. The primary function of air inlet louvers is to channel and direct the airflow into the tower. By employing angled blades or vanes, louvers create a controlled airstream that efficiently sweeps across the fill media, maximizing air-water contact and heat exchange. Proper louver design minimizes dead zones within the tower, ensuring all fill media areas experience optimal airflow and contribute to the cooling process.

In certain applications, controlling sunlight exposure within the cooling tower is crucial. For example, excessive sunlight can promote algae growth in the water, leading to increased maintenance and potential biofouling issues. Air inlet louvers can be designed with light-blocking features, such as angled panels or opaque materials, to minimize sunlight penetration and maintain optimal water quality.

  1. Fan System: This system generates airflow through the tower, drawing in fresh air and expelling the heated air. Fans can be axial or centrifugal, each with its own advantages. Axial fans, resembling airplane propellers, generate high airflow volumes at lower pressures, ideal for open-circuit towers. Centrifugal fans, reminiscent of car engine fans, create high pressures with focused airflow, making them better suited for closed-circuit systems.

Modern fan systems often incorporate variable speed drives (VSDs). These allow for adjusting the fan speed based on real-time cooling demands, optimizing energy consumption and reducing noise pollution during periods of lower load.

Since fans are considered roating machinery, they are subject to the typical challenges faced by this class of equipment. Fan blades are subject to immense forces, leading to vibration and potential imbalance. This can cause excessive wear and tear on bearings, shafts, and surrounding components, impacting performance and noise levels. Regular balancing checks and adjustments are crucial. Furthermore, exposure to extreme temperatures, humidity, and corrosive elements can accelerate wear and tear on fan components. Choosing corrosion-resistant materials and implementing preventive maintenance measures are essential.

Centrifugal fans (left) compared to axial fans (right).

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  1. Drift Eliminators: These are mesh-like structures that capture small water droplets entrained in the air, minimizing water loss and drift. Drift eliminators achieve their objective through various mechanisms. Common designs employ mesh screens, baffles, or inclined surfaces to intercept water droplets carried by the airflow. Droplets collide with the eliminator elements, lose momentum, and fall back into the water basin for recirculation.

Effective drift elimination requires a balance between capturing droplets and maintaining desired airflow for heat transfer. Eliminator designs minimize airflow impedance while maximizing droplet capture efficiency. Advanced designs employ computational fluid dynamics simulations to optimize the geometry and placement of eliminator elements.

A top-view of drift eliminators installed in a cooling tower.

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  1. Casing: This encloses the entire structure, protecting the internal components from the environment and ensuring structural integrity. The casing provides the primary structural support for the entire cooling tower, housing the heavy internal components like fill media, basins, and water distribution systems. It must withstand significant loads from water weight, wind forces, and seismic activity.The casing provides the primary structural support for the entire cooling tower, housing the heavy internal components like fill media, basins, and water distribution systems. It must withstand significant loads from water weight, wind forces, and seismic activity. The casing contributes to reducing water loss through drift. Its tight construction and strategic openings help capture entrained water droplets and direct them back into the basin for recirculation.

    The casing can be constructed from a variety of materials, with some of the more popular options being:

    • Concrete: Durable and affordable, concrete casings offer high structural integrity and fire resistance. However, they are heavier and require longer construction times.
    • Steel: Lighter than concrete, steel casings offer faster construction and adaptability to various configurations. However, they require corrosion protection and may be more susceptible to noise emission.
    • Fiberglass Reinforced Plastic (FRP): Lightweight and corrosion-resistant, FRP casings provide good thermal insulation and noise reduction. However, they may have lower structural strength and higher initial cost compared to concrete and steel.
  2. Makeup Water System: This system replenishes the water lost through evaporation, purge (blowdown) and drift, maintaining the water level and ensuring efficient operation. Makeup water can be sourced from various options, including municipal supply, wells, surface water sources like rivers or lakes, or treated wastewater. The choice depends on availability, water quality, and cost considerations.
  3. Control System: This system regulates the operation of various components, including the water distribution system, fan speed, and makeup water supply, optimizing cooling performance and energy efficiency. Typically, a cooling tower is run as a on/off system, directly controlled by the primary process’ SCADA according to predetermined setpoints required to be maintained for the particular process.
  4. Access Doors and Platforms: These provide safe access through the cooling tower casing into the internal areas to maintain and inspect the tower’s enclosed components.

4. Cooling towers: simple in their complexity

By understanding the basic anatomy of a cooling tower and the role of each component, we gain a deeper appreciation for the engineering and technology behind this essential piece of equipment used in diverse applications across industries. Evaporative cooling towers operate on a simple principle but require the harmonious functioning of the various sub-components that are correctly sized, made from the correct materials to handle the conditions, and arranged in the most appropriate configuration to meet the specific application’s needs. This makes the system comprising a cooling tower appear complex. However, each of these sub-components are in their own right, relatively simple in purpose and integration.

This article highlighted an evaporative cooling tower’s primary functioning principle and purpose, considered the different types and configurations, and concluded with an in-depth evaluation of its major sub-components. Future articles will consider the appropriate approach to sizing a cooling tower, factors that need to be considered for maintaining its effective operation and longevity, and an overview of the different economic sectors in Southern Africa that make use of them the most.

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