Cooling Towers: Choosing Between Counterflow and Crossflow

Choosing which cooling tower is best for your project can be intimidating. There are so many aspects to consider, such as size, material, and configuration. Of those three, the first two are pretty self-explanatory. When talking about configurations, however, understanding the differences is the first step in choosing the tower that best works for your HVAC system. The main difference between the two configurations is how air flows throughout and ultimately cools the water during its journey.

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Crossflow Cooling Towers

As hot water enters a crossflow cooling tower, it flows downwards solely due to gravity. Spray nozzles in the tower rely on this gravity rather than external pressurization to distribute the water throughout the fill. As water flows downward, the air is blown horizontally across the water, thus cooling it as it falls. This “cross” between water and air is what gives this configuration its name.

Counterflow Cooling Towers

Rather than relying on gravity to distribute the water throughout the cooling tower – in a counterflow design, that water is distributed downwards throughout the fill via low-pressure nozzles. Airflow in this configuration travels vertically, directly against the flow of water.  There is generally a fan above the nozzles which pulls the air upwards.  Once the water has been cooled, it then drains down to a cold water basin location at the bottom of the tower.

Which Configuration is Best?

So, why would you choose one configuration over the other? It’s going to depend on a variety of factors – the biggest being available space. Counterflow towers tend to require less area on the ground. For buildings with limited floor space (or rooftop space), this option may work best. These towers tend to be taller, so keeping that in mind when thinking about any height restrictions of the space is advised. Crossflow towers are also lighter in weight compared to crossflow towers. So, for areas that have operating weight restrictions, this may be the best option.

Other than the size variable, efficiency and serviceability also play key roles when deciding on a cooling tower configuration. Crossflow towers generally offer more efficiency and are easily maintained over time, thus giving them extra appeal for projects where the space issue isn’t a concern. What makes them more easily serviced is that crossflow towers often have a built-in (or option to do so) service platform. From that platform, workers can access the hot water basin to perform preventive maintenance measures or assess any issues. In fact, even when in operation, workers can inspect certain components – including drift eliminators. In counterflow towers, access to the nozzles is more difficult (impossible while operating). Thus, any maintenance that needs to be done relies on shutting down the tower for some time. This can halt operations or productivity in certain situations.

Also, because crossflow towers don’t utilize pressurization for water distribution, operating costs tend to be less. Energy is saved by not relying on pressurized nozzles to distribute the water.

Environmental considerations should be considered as well when choosing a configuration. For those in areas where noise is a concern, crossflow towers are likely the better choice. The distance between the bottom of the fill and the cold water basin in these towers is pretty small. So, not a lot of noise is generated by falling water droplets. In crossflow towers, on the other hand, water droplets fall into the basin from a much higher distance. This generates more noise – especially in larger towers. There are methods you can take to reduce this noise, such as utilizing sound attenuators.

Climate plays a role as well. One thing you want to avoid is water freezing inside the fill of a cooling tower. In areas that see a lot of cold weather, this is more easily done with a crossflow tower. The reasoning behind this is that water distribution in crossflow towers leads to much less channeling of water inside the fill that could turn to ice when temperatures fall below freezing.

In hot climates, counterflow towers may be a better option to avoid algae growth inside the tower. Crossflow towers leave a part of the plenum directly exposed to sunlight. The heat this generates stimulates algae growth, which must be carefully monitored and regularly treated.

Cooling Tower Experts – You Don’t Need to Choose Alone

Fully understanding enough about each type of cooling tower in order to make an informed decision on the best option is difficult. Luckily, the folks at Cooling Tower Experts know these decisions can be daunting. Our goal is to help make the process easier. Our experts bring years of industry knowledge and field experience, working with cooling towers and HVAC systems across multiple industries. We have truly become experts and want to share that knowledge with our customers. Our services range from project consultation to training to service and maintenance. Depending on how much or little help you need, we’ve got an option for you. The biggest things you need to worry about when replacing an existing tower or installing a new one are choosing the best option and then having a service/maintenance plan in place to keep that tower running efficiently for years. We’d be happy to get you on the right track – contact us to start your project today!

Table of Contents

Cooling towers are indispensable for cooling process water and keeping equipment from overheating. They are useful in industrial facilities such as oil refineries, chemical plants and thermal power stations, and they are common in manufacturing facilities and many buildings with HVAC systems.

Cooling towers come in various types, and it's important to choose the correct one for your plant's needs. In the guide below, we'll explain how these apparatuses work and discuss some of the different types of cooling towers.

Cooling Towers and How They Work

A cooling tower is a heat rejection device. It works by bringing air and water into contact to cool the water and release unwanted heat into the atmosphere. Cooling towers are useful in industrial processes because industrial equipment tends to generate tremendous amounts of heat. Facilities need reliable ways to dissipate that heat to keep their working environments cool and reduce the risk of breakdowns and fire.

Cooling towers come in a variety of sizes, with some as small as a few square feet and some several hundred feet large. The different sizes enable different load configurations. Cooling towers also come in different shapes and interior designs, which we'll discuss in more detail below.

All cooling towers perform the same primary function — to increase the surface area over which air and water interact. A larger surface area means more efficient evaporation, and more efficient evaporation means faster cooling.

How does that process work? Typically, hot industrial process water flows toward the cooling tower and enters at the top. The water then flows down through the cooling tower. As it does, equipment within the tower spreads the water out over a large surface area, often by converting the water into small droplets or thin films that have a larger surface area than deep water in a tank. The increased water-to-air contact boosts heat transfer through evaporation.

The water flows through the cooling tower, losing heat along the way, until it reaches the sump at the bottom. The sump sends most of the cold water back to cool the hot machinery. When heat transfer from the equipment heats the water again, the water flows back to the cooling tower, and the process repeats.

Essential Components of Cooling Towers

Below are a few essential components of many cooling towers:

• Fans: A cooling tower may contain large fans that circulate significant volumes of air. Though not all cooling towers require fans, many models use them to create and direct airflow through the tower. They may push or pull air through the tower, and they may be axial or centrifugal depending on the specific needs of the application. Axial fans are more efficient, whereas centrifugal fans are quieter and can deal with higher levels of static pressure.
• Fill: Fill, also called wet deck or surface, typically consists of textured polyvinyl chloride (PVC) that is integral to the cooling tower function. It usually features ridges with open spaces for the air and water to travel through. Its purpose is to allow water to collect on it, thereby maximizing the surface area of the water and facilitating heat transfer between the water and the air. Fill can come in a couple of different types. Film-type fill increases the water's surface area by spreading it into a thin film. Splash-type fill increases the water's surface area by breaking a falling stream of water into smaller droplets.
• Spray nozzles: Spray nozzles in the cooling tower can also be useful in increasing the surface area of the water. In some types of cooling towers like counterflow towers, spray nozzles disperse small droplets of water into the air. The spray helps ensure a uniform distribution of the water over the fill, and the droplets provide a large surface area for air contact.
• Distribution basin: The distribution basin, or hot water basin, is often used in crossflow cooling towers. A distribution basin takes the place of the spray nozzles by distributing the hot water evenly throughout the tower. It sits atop the tower and typically consists of a pan with holes or nozzles along its base. Hot water flows in through the top of the tower, and the holes or nozzles release it evenly over the fill material below.
• Collection basin: The collection basin, or cold water basin, sits at the bottom of the tower to collect the water after it has cooled. In field-built models, these basins are often built of concrete to support the tremendous weight of the water coming down the tower.
• Inlets and outlets: Inlets and outlets in the cooling tower take in cool air from the environment and release the warm air after it has absorbed the water's heat.
• Drift eliminators: Drift, or water loss, in a cooling tower is undesirable, but it sometimes occurs when droplets of water escape into the outlet and flow out with the exiting air. Drift eliminators help keep the water secure in the tower. They point the airflow in multiple directions to prevent it from whisking water away.

Types of Cooling Towers

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Cooling towers come in a few unique designs that use different technologies to cool process water. Below, we'll discuss some of the different cooling tower types.

The cooling industry typically categorizes cooling towers in multiple ways, including:

• Whether their air flows horizontally or vertically
• Whether they use mechanical fans or natural convection
• Where and how their fans are positioned

A single cooling tower may fall into more than one of the categories listed below — like a counterflow induced draft cooling tower or a crossflow forced draft cooling tower.

If you're looking for a reliable cooling tower, you'll have several reputable brands to choose from. EVAPCO, ENEXIO, Baltimore Aircoil Company (BAC), Cooling Tower Systems, American Cooling Tower and many others manufacture trustworthy, quality products.

1. Crossflow

Crossflow cooling towers get their name because the air they use cuts perpendicularly across the flow of water. Crossflow towers use splash fills that allow incoming air to flow horizontally through the cooling tower. At the same time, gravity sends hot water flowing down from distribution basins at the top of the tower.

Crossflow cooling towers offer the advantage of great height, and they are some of the simplest models to maintain. Because they use gravity to aid air-to-water contact, they can use smaller pumps, so they are cost-effective and easy to maintain — even while in use. And because their spray is non-pressurized, they allow for more variable water flow.

They are more prone to freezing than counterflow towers, though, and they can be more inefficient. Their design also makes their fill more likely to become clogged with dirt or debris, especially in windy, sandy and dusty regions.

EVAPCO's AXS cooling towers are good examples of induced draft crossflow towers.

2. Counterflow

Counterflow cooling towers get their name because the air and water enter from opposite ends of the tower. In a counterflow cooling tower, as in a crossflow tower, water flows down from the top of the tower. In this case, though, the air also moves vertically across the splash fill, from the bottom of the tower to the top. Because the airflow is upward, counterflow towers cannot use gravity-flow basins, so these towers use pressurized spray nozzles to distribute the water over the splash fill.

Counterflow towers are more modest in size than crossflow towers, which means they can sometimes provide greater efficiency. And because of their spray distribution, they offer more resistance to freezing than crossflow towers. The extensive surface area of the large volume of spray they produce also makes heat transfer more efficient.

However, the greater energy expenditures and larger pumps required to push air against the flow of water can also lead to operational inefficiencies and increased utility bills. Counterflow towers also sometimes struggle with variable water flow because it can impede the tower's spray characteristics. And they can often be noisier than their crossflow counterparts because the water has farther to fall from the bottom of the fill into the collection basin.

BAC's Series V cooling towers are good examples of counterflow towers.

3. Natural Draft

Unlike mechanical cooling towers such as induced and forced draft models, natural draft or passive draft cooling towers use natural convection. Air flows naturally through the tower, and differences in air density create specific patterns of movement.

The cold, dry air flowing into the tower is less dense than the warm, moist air flowing out after contact with the hot water, so the warm air naturally rises while the cold air falls. These movements create a stable, constant pattern of air circulation that helps cool incoming water and release heat. Natural draft towers often feature steep chimney architecture to enhance the natural vertical flow of air.

One particularly effective type of natural draft cooling tower is the hyperbolic cooling tower. These cooling towers use a chimney-stacking design to let the dry, cool outside air push the warm, moist air. The bottom of the tower contains splash fill, and the cool air moving upward cools the water spraying over it.

Hyperbolic towers offer numerous benefits. Their hyperbola shape helps direct the flow of air upward, enhancing their efficiency. They also typically provide impressive structural integrity and strength while requiring only modest amounts of materials in their construction. They are common structures at industrial facilities like coal-fired power plants.

ENEXIO manufactures good examples of natural draft cooling towers.

4. Induced Draft

Induced draft cooling towers use mechanical means — such as fan systems — to move air through the tower. An induced draft tower typically has fans located at the top of the air outlet. These fans pull cool air through the tower. They get their name from the induction of warm, moist air out of the discharge outlet.

One of the benefits of an induced draft cooling tower is that the force of the induction means the air is moving at a high velocity when it exits the tower. That high velocity sends the air far enough away to prevent unwanted recirculation.

EVAPCO's AT cooling towers and SUN cooling towers are good examples of induced draft counterflow towers, as are BAC's PT2 cooling towers.

5. Forced Draft

A forced draft tower is similar to an induced draft tower, but the placement of its fans is different. A forced draft tower typically has fans located in the air intake rather than the air outlet. These fans, located on the sides or at the base of the tower, push air directly into the tower instead of pulling it.

Forced draft cooling towers take air in at a high velocity, but they tend to discharge it as a lower velocity, since friction slows the air as it passes through the tower. This lower velocity means forced draft towers are more susceptible to undesirable air recirculation. Their design also makes them costlier and more inefficient to run because they require more power. And like crossflow towers, forced draft towers are more susceptible to freezing than other types of towers.

However, forced draft cooling towers are particularly useful in indoor facilities because they handle high pressure exceptionally well. This ability makes them well suited to smaller indoor spaces.

EVAPCO's LSTE cooling towers and LPT cooling towers are good examples of forced draft counterflow towers.

Methods of Cooling Tower Assembly

Cooling towers have two main assembly methods. Factory employees may assemble the towers directly in the production factory and then ship them to their sites, or workers may assemble the towers at the sites.

Factory-Assembled

In factory-assembled cooling towers, employees at the production factory put the tower together. Once it is complete, they transport it intact to the location that will use it. This preassembly process generally works best for smaller towers, since larger towers can become unwieldy to ship or sustain damage in transit. Factory-assembled towers are useful in modestly demanding applications such as food-processing plants, automotive facilities and cooling systems across a range of industries.

EVAPCO, for instance, makes several different models of factory-assembled cooling towers.

Field-Erected

If a cooling tower would be too large, fragile or difficult to ship, the receiving location may opt to erect it on site. In that case, the manufacturer or supplier generally provides the labor, and the workers usually build the tower close to the building where industrial processes take place.

Field-erected towers can make use of either crossflow or counterflow designs. Because they have few limits on their size and can be much larger, they are often useful in industrial applications that draw substantial amounts of power.

ENEXIO manufactures many field-erected cooling towers.

Click here for part 2, which focuses on the efficiency of each type of cooling tower.

Whether you have a crossflow, counterflow, natural draft, induced draft or forced draft cooling tower, it will likely require cleaning at some point. Keeping your cooling tower clean and well maintained is essential for several reasons. It helps prevent scale and corrosion, and it helps prevent the accumulation of microorganisms that could spread illnesses like Legionella. Effective cleaning and maintenance help increase your facility's efficiency, reduce repair frequency, save money and prevent catastrophic breakdowns and disease outbreaks.

To see the benefits of chemical water treatment for your cooling tower, make Chardon Labs your trusted provider. Our extensive years of experience and industry expertise mean we can select the chemicals necessary to get your cooling tower operating at peak performance. We'll provide a complimentary assessment of your current system, deliver and add the chemicals, dispose of the containers safely and provide ongoing maintenance at a fixed yearly price that eliminates surprise bills.

Contact us today to learn more.

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