This is the most comprehensive guide to cooling towers on the internet.
Here‘s what you‘ll learn:
Cooling towers are specialized heat exchangers that efficiently remove excess heat from water primarily via latent heat dissipation through evaporation as it interacts with an airstream. Besides the cooling effect from evaporation, water is also cooled by sensible heat transfer due to the temperature gradient between the air and water. Unlike conventional heat exchangers that utilize conduction and convection, such as shell and tube or plate heat exchangers, cooling towers achieve cooling by enabling direct interaction between water and air.
To fully grasp the upcoming topics, it is beneficial to become acquainted with some fundamental terminologies associated with cooling towers.
This term describes the temperature difference in water between the inlet and outlet points of the cooling tower.
This is the pace at which heat is removed from the water.
The volume of fresh water needed to replenish the water that has evaporated from the system.
The very small droplets of water that are carried away by the airstream during contact with the water.
The exiting stream of warm air and water vapor from the cooling tower, recognized as the effluent mix.
The variance between the temperature of the water exiting the tower and the wet bulb temperature of the introduced air.
Referring to the ambient air temperature at complete saturation, or 100% relative humidity. It is measured using a psychrometer, which involves covering a thermometer bulb with a thin film of moisture and exposing it to airflow. This reading is typically lower than a standard thermometer reading, contingent on the relative humidity.
The deliberate removal of water from the system to eliminate accumulated solids from evaporation, as well as sludge from impurities and bacterial growth.
Occurs when the ejected air mixes back into the incoming air stream, lowering the cooling tower's operational efficacy.
Cooling tower parts may be further broken down into structural, mechanical, and electrical. Structural components are static equipment such as the basin, tower framework, fan deck, casing, and louvers. Mechanical parts, on the other hand, are the rotating equipment: fans, driveshafts, and speed reducers. Electrical parts consist of motors and their controls.
Sometimes called the wet deck or surface, the fill increases the surface area within the cooling tower to maximize the contact and interaction time between the air and the water, while minimizing airflow resistance.
The distribution system varies based on the cooling tower design concerning air-to-water flow. In cross-flow cooling towers, a gravity-flow distribution system is used, where water is distributed from above the fill. Counter-flow cooling towers, on the other hand, use pressurized water spray systems.
Drift eliminators prevent water droplets from escaping by creating abrupt changes in the air stream’s path. As the air stream moves through the eliminators, large water droplets collide with the walls, causing them to fall back into the cooling tower.
Air intake louvers help prevent water splash-out, reduce noise, and block debris. Found commonly in cross-flow cooling towers, they are located above the cold water basin, at the bottom of the panel where the fill is packed, and encircle the tower.
The casing provides structural housing for the cooling tower and transmits loads to the tower frame, while also containing the water within the tower.
The fans force air either in or out of the cooling tower, depending on the type of draft required for the application. The main driver is the motor. Torque from the motor is transferred by the driveshaft to the fan or to a gearbox. Large cooling towers with big and heavy fans require gearboxes to increase the torque, which eliminates the need for heavy motors, and in turn eliminates the need for a stronger structure. In some cases, belt and pulley assemblies are used instead of gear boxes.
The fan stack, also known as the fan cylinder or fan cover, improves fan efficiency by providing a well-designed air inlet that ensures smooth airflow into the fan. It also aids in discharging air at higher elevations, which reduces recirculation and interference.
This component supports the fan cylinders and acts as a structural element to distribute loads throughout the tower frame.
Valves are essential for regulating the water level inside the cooling tower. The types of valves used include isolation valves, flow-control valves, and make-up regulator valves.
The collection basin, situated at the bottom of the cooling tower, is designed to gather water that has fallen but not evaporated or lost through drift. It also typically serves as the cooling tower’s foundation and is where chemical treatments for the circulating water are added.
This is a deep pan or small basin with holes or nozzles positioned above the fill. It is a component of the distribution system in a cross-flow cooling tower, where gravity helps evenly distribute hot water across the fill material.
The tower frame supports the whole cooling tower and transmits all loads to the foundation. Common materials used for the frame are concrete and wood. Fiberglass and stainless steel are often used.
Cooling towers are typically classified as follows:
Air Flow Generation: Cooling towers differ in how air is introduced into the system, which can be through natural, mechanical, or hybrid draft methods. Mechanical draft cooling towers are further categorized into forced draft and induced draft types.
Natural Draft: Natural draft cooling towers utilize no mechanical drivers or fans to create air flow through the cooling tower. This cooling tower takes advantage of the difference in ambient air densities below and above the tower. Air flow is created as the denser air at the bottom of the tower travels to a lower pressure area above the tower. These towers are inexpensive but can only be installed outdoors. Also, these towers have lower reliability as they are more affected by ambient wind and temperature changes.
Forced Draft: As the name suggests, this type of cooling tower uses fans or blowers to force air into the cooling tower. Air flow has high entrance velocity as it is being forced by the blower. As it passes through the tower, air flow slows down. Thus, performance is less stable compared to induced draft towers due to recirculation. Forced draft cooling towers are used in indoor applications where high static pressure is a concern.
Induced Draft: These have their fans located at the top that draws (or induces) air from the air intake louvers at the bottom or sides of the tower. Contrary to forced draft cooling towers, this arrangement has low entrance and high exit velocity, which results in reduced recirculation. These types of cooling towers are widely used in industrial plants requiring stable performance.
Hybrid Draft: Its operation is the same as natural draft towers, but equipped with fans to augment air flow. Hence, they are also referred to as fan-assisted natural draft cooling towers. The fans in this setup have lower horsepower compared to forced and induced draft fans. Because of the additional draft, there is no need to construct a tall tower which may be economically impractical for a given application.
Air-to-Water Flow: This classification distinguishes cooling towers as cross-flow or counter-flow types, based on the method by which water interacts with the air stream.
Cross-Flow: In cross-flow configuration, air flows horizontally through the fills across the downward fall of water. A distribution basin distributes the hot water to evenly fall into the fill by gravity through nozzles or orifices. The action of gravity eliminates the need for a pressurized-spray system. Maintenance is easier for cross-flow cooling towers since the distribution system can be sectionalized and serviced separately, eliminating downtime.
Counter-flow: In this type, air flows parallel but opposite to the fall of water. Counter-flow cooling towers have a pressurized-spray distribution system. The need for this pressurized system puts counter-flow cooling towers at a disadvantage. Nevertheless, counter-flow types take less space than cross-flow for the same cooling load.
The table below outlines the characteristics of each type.
| Areas of Consideration | Cross-flow | Counter-flow |
| 1. Size | • Takes more space but can be constructed lower than counter-flow | • Takes a smaller area than cross flow |
| 2.Maintenance and Operation |
• Access to nozzles is available anytime • For towers using induced draft fans, access to the tower fills and drift eliminator is possible anytime |
• Inspection on fills and drift eliminator can only be done during shut down. • No access to distribution system while in operation |
| 3. Water Flow Rate | • Flow can be varied by replacing the orifices installed in the nozzles while in operation | • Flow cannot be adjusted since there is no access to the nozzles |
| 4. Pumping Energy | • Pressurized water system is not required, reducing electricity cost | • Water pressure is required to ensure proper atomization of water droplets |
| 5. Water Distribution |
• Potential orifice clogging • Distribution basin is prone to biological fouling |
• Spray distribution improves water droplet size, which increases heat transfer |
Heat Transfer: As mentioned in Chapter 1, heat removal from water occurs through two primary methods: latent heat loss via evaporative cooling and sensible heat transfer. Sensible heat transfer in a cooling tower can occur through conduction, convection, or a combination of both. Classification by heat transfer methods distinguishes cooling towers based on how these principles are utilized.
Fluid or Closed Circuit Cooling Towers: In this system, the returning hot water from consumers passes through tubes or coils where sensible heat transfer happens. Outside of these tubes, water is sprayed, similar to wet cooling towers. Both latent and sensible heat is removed to the sprayed water by coming into contact with the air stream. The main advantage of this system is that water used by consumers is free from contamination.
Build or Construction: This classification categorizes cooling towers based on their construction methods or how the structure is assembled or manufactured.
Designing a cooling tower involves considering numerous factors, including psychrometry, heat, and mass balance. Beyond analyzing the properties of the incoming and outgoing air and water, one must also evaluate the tower's physical characteristics, such as its ability to create a natural draft, its structural integrity, and the susceptibility of its components to fouling and corrosion. This chapter addresses the key parameters that influence the performance of cooling towers.
Cooling tower efficiency = (CW return temperature - CW supply temperature) / (CW return temperature - Air wet bulb temperature) × 100%
Consequently,
Cooling tower efficiency = Range / (Range + Approach) × 100%
From these, it can be seen that a cooling tower with a smaller approach is more efficient. Cooling towers usually have a 5 to 10⁰F approach. While a small approach is desired, investment cost may be impractical since the size of the cooling tower increases exponentially as the approach is being lowered.
Usually, the range and cooling water flow rate are the parameters being balanced. This is because the heat load is already given from consumer demand, and ambient air wet-bulb temperature may not be manipulated. Increasing the range will make the cooling tower efficient. This can be done by increasing the cooling water return temperature, or by lowering the cooling water supply temperature. In either of the cases, usually, one temperature is constant due to the requirement of end users. Of these two options, increasing the cooling water return temperature is more practical since the temperature difference between air and water in contact will be much larger. The larger the temperature difference, the more heat can be dissipated.
If the only option is to lower the cooling water supply temperature, the result will also lower the approach. In turn, the design will require a much larger tower.
Wet-bulb Temperature: This is a significant parameter for cooling towers relying on evaporative cooling. Design wet-bulb temperatures depend on existing site conditions. Thus, careful site surveys must be conducted, especially during summer months when the ambient temperature and relative humidity are high. A designer must consider publications from engineering and scientific organizations such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the National Oceanic and Atmospheric Administration for the unique, worst-case design conditions for a given location.
From the previous point, it is seen that a high ambient wet-bulb temperature will decrease the approach. Thus, at locations where there are high wet-bulb temperature conditions present, larger cooling towers are required for a given cooling load.
Consumer Heat Load: The size and cost of a cooling tower is proportional to the heat load. Cooling towers are usually designed using the maximum consumer heat load or cooling demand. They will then be the rated capacity of the cooling tower. However, there are times when there is low demand for cooling. In these cases, the tower will operate at a lower efficiency.
In order to save energy, one method is to use fan speed control. Heat transfer rate is increased by higher air velocity. If not much heat transfer or evaporative cooling is needed, fan speeds can be reduced. This can be done by using variable speed drive motors, two-three speed fan motors, and adjustable pitch fan blades. Another option is to design a multi-cell cooling tower. In this case, one cell may be on standby during off-hours or at times of low demand.
Cooling towers, like any other equipment, require regular inspection and maintenance to ensure they deliver the necessary cooling efficiently and to help extend their service life. The following are some recommended maintenance and inspection practices for cooling towers.
During the inspection, check for the following:
Any cracks or damage on the basin, casing, fan deck, and tower frame. Address all inspection findings accordingly.
The CTI Standard 201 "establishes a program whereby the Cooling Technology Institute certifies that all models within a line of Evaporative Heat Rejection Equipment from a specific manufacturer will perform thermally according to the manufacturer’s published ratings" (CTI.org, 2018). A CTI certification ensures that a cooling tower has been inspected by a CTI-licensed testing agent and meets both CTI standards and the manufacturer's specifications.
As water evaporates in the cooling tower, impurities become more concentrated. When make-up water is added, it also evaporates, leaving behind additional impurities. These dissolved minerals eventually accumulate as scale on the components of the cooling tower that are in contact with the water. This scaling affects not only the cooling tower but also associated equipment, such as heat exchangers and condensers.
In addition to scaling, biological fouling can occur on surfaces. Evaporative cooling towers are especially susceptible to biological fouling because the water scrubs microbes from the airstream. The concentrated minerals resulting from evaporation create an ideal environment for microbial growth.
Water treatment methods vary depending on the application and water quality parameters. There are various proprietary water treatment chemicals and filtration systems available that address a range of issues. The following are some common methods for treating cooling water.
In any industrial plant, heat is generated by equipment used in various processes. Removing this undesirable heat is a common requirement in industrial manufacturing. The same applies to commercial and residential buildings, where cooling for comfort, refrigerated storage, and equipment preservation are necessary. Without effectively removing or rejecting this excess heat, machinery, equipment, and air conditioning systems will not function properly.
Cooling towers are a popular choice for heat rejection due to their high efficiency. They come in various sizes depending on the application, ranging from small chiller units for residential use to 200-meter tall structures for power generation plants. Below are some common applications of cooling towers.
HVAC is used for comfort cooling of residential and commercial areas. Heat generated from people, equipment (computers, servers, etc.), lighting, solar radiation, and outdoor ambient air is absorbed by the cooling system and rejected to the cooling tower.
This application is used for cold storage in industries such as food and beverage, pharmaceuticals, and air and gas generation. It functions similarly to an HVAC system, where a refrigeration unit absorbs heat from a closed space and transfers that heat to the cooling tower for rejection.
Power generation plants utilize steam as the working fluid. To generate power, water is heated into steam using coal, natural gas, or nuclear radiation, and this heat is converted into mechanical energy. However, not all of this heat can be converted into energy and must be removed to complete the steam cycle. Cooling towers play a crucial role in this process by removing the excess heat.
This is similar to a power plant. Condensers, heat exchangers, and cooling jackets all absorb heat from processes. This heat is then carried by water to be rejected through the cooling tower.
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