Crossflow or Counterflow? Choosing a Cooling Tower for ...

By Eric Rasmussen, Senior Product Manager, SPX Cooling Technologies, Inc.

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With efficient heat exchange an important requirement in the design of an HVAC system, the type of cooling tower you specify to support your project&#;s unique cooling goals requires careful consideration. After determining the process parameters required for your application &#; tonnage, range, and approach &#; cooling tower capabilities can be analyzed.

Choosing between crossflow (left) and counterflow (right) cooling towers for your application depends on the factors most important to your project specifications.

Because induced draft crossflow and counterflow cooling towers both have distinct advantages, the design requirements and conditions specific to your application determine the appropriate cooling tower for your project. The fundamental difference between crossflow and counterflow cooling towers is how the air moving through the tower interacts with the process water being cooled. In a crossflow tower, air travels horizontally across the direction of the falling water. In a counterflow tower, air travels vertically upwards in the opposite direction (counter) to the direction of the falling water.

While structural and mechanical components of crossflow and counterflow cooling towers are similar, application-specific design requirements should determine the tower type.

The fundamental difference between crossflow and counterflow cooling towers is how the air moving through the cooling tower interacts with the process water being cooled. Air travels horizontally across the direction of the falling water in a crossflow tower. In a counterflow tower air travels vertically upwards in the opposite direction (counter) to the falling water.

Physical Size - Footprint

Every cooling tower requires a certain volume of air to effectively exchange the heat in the process water. Thus, a cooling tower&#;s plan area and height must be considered with your specific application in mind.

At cooling capacities up to about 750 tons (kW), a counterflow cooling tower with its vertically-stacked components may require less plan area than a crossflow cooling tower. Beyond the 750 ton mark, because crossflow tower modules are stacked vertically at higher tonnages, a counterflow tower offers little to no advantage in footprint versus a crossflow tower and can sometimes take up more plan area.

Depending on the application, a crossflow cooling tower may require less total area than a counterflow tower even at heat loads less than 750 tons because of the location and number of air inlets &#; a crossflow tower has two air inlets compared to four air inlets on a counterflow tower.

Depending on the application, a crossflow cooling tower may require less total area than a counterflow tower because of the location and number of air inlets; a crossflow tower has two air inlets compared to a counterflow tower&#;s four air inlets.

Maintenance

Routine maintenance is essential to extend the life of a cooling tower so maintenance accessibility is a consideration. The method by which air interacts with the process water in each tower type creates two different styles of plenum areas. This space has a direct effect on maintenance accessibility.

In crossflow cooling towers, the air flow is turned from the horizontal air inlet direction to the vertical discharge direction behind the fill media. This creates a tall, easily accessible plenum inside the tower for inspection and servicing of the cold water basin, drift eliminators, motor, drive system, and fan at the top of the cooling tower.

Counterflow cooling towers turn the air from horizontal to vertical flow beneath the fill media. While this gives good access to the cold water basin, the rest of the tower is more compact with lower overall height. This creates limited access to the spray system, eliminators, motor, drive system, and fan.

A crossflow cooling tower&#;s large plenum area allows easy maintenance.

A counterflow cooling tower&#;s stacked components are less accessible for service.

Operating Weight

The overall shipping and operating weights of a crossflow cooling tower may be heavier than a counterflow tower due to the crossflow tower&#;s larger footprint, additional structural supports and steel casing for ease of maintenance access and additional piping for water distribution. However, lighter capacity cranes are typically required to hoist individual modules, which are stacked vertically at higher tonnages. Potential crane and logistical savings must be weighed against the need for additional picks per cell.

Gravity-Fed and Pressurized Water Distribution

A significant design difference between a crossflow and counterflow cooling tower is the method by which water is distributed over the fill media.

In a crossflow cooling tower the process water is pumped to the top of the tower into the hot water distribution basin. The distribution basin is out of the way of the airstream and is gravity fed. The only driving force behind the nozzles is the hydrostatic head of water above the nozzle itself. One advantage of gravity-fed crossflow water distribution is that it can be cleaned while in operation since it is easily accessible from the outside top of the cooling tower.

In a counterflow cooling tower, process water is pumped into a sealed header box. The header box then distributes the water into branch arms and nozzles, creating a pressurized water distribution system. Unlike a gravity-fed system, a counterflow tower&#;s water distribution system requires pumps to be shut off to clean the nozzles and the cold water basin. To inspect and clean nozzles, one must enter a crawl space inside the tower.

Variable Flow and Cold-Weather Operation

There are significant energy savings opportunities if a cooling tower can be operated under variable flow conditions. When the conditions allow (reduced heat load or cool ambient conditions), reducing the flow rate over the cooling tower instead of the process keeps the process operating in its most efficient manner. Variable flow, or &#;turndown,&#; is a way to maximize the effectiveness of the installed cooling tower capacity for any process flow.

Crossflow cooling towers with outboard water inlets and integral inlet louvers handle very high turndown rates (up to 70% or more). Counterflow cooling tower distribution systems are not as easily modified; up to 50% turndown may be achieved but additional pump head may be required.

Cold-weather operation is of paramount importance when choosing a cooling tower to operate in sub-freezing conditions. Ice formation is an ever present danger and can damage tower components including the high efficiency heat transfer fill media. The effects of ice damage can result in higher condenser water return temperatures and increased chiller energy consumption during peak cooling season.

A crossflow cooling tower performs especially well in cold weather. With its gravity-fed water distribution system &#; even with turndown as low as 30% of design flow &#; water can still be evenly distributed across the fill. Even distribution prevents water channeling, ice development, unpredictable performance, scale buildup, and potential damage to the tower. During cold-weather operation, the use of devices such as cups or dams in the hot water basin can keep the heat load toward the weather exposed face of the fill, alleviating ice buildup.

At low-flow operation, a counterflow cooling tower has less head pressure and fewer nozzles to distribute water across the entire cross-section of the fill allowing for uneven distribution. Uneven distribution leads to water channeling, ice development, unpredictable performance, scale buildup, and potential damage to the tower.

Minimum flow rates are both tower type and model specific. Be sure the cooling tower manufacturer understands the minimum anticipated flow rate and confirm the tower can handle the required hydraulic range.

Louvers are designed to keep water within a cooling tower. They prevent splash out which can turn to ice in sub-freezing ambient conditions. Integral louvers incorporated into the fill of some crossflow towers help keep water contained in the fill. This provides no external surface for ice development to occur. Counterflow tower louvers are separate from the fill near the cold water basin. The turbulent water splashing in the cold-water basin can lead to ice accumulation on the louver faces during freezing weather.

Variable Flow Cups - when operating a crossflow cooling tower in cold weather, devices such as cups or dams in the hot water basin keep the heat load toward the side of the fill exposed to the elements.

Heat Transfer Fill

Both counterflow and crossflow fills can vary in shape and size. The appropriate fill for your cooling tower should be based primarily on water chemistry. Suspended solids, biological growth potential, and information about constituents in the process water that can lead to scaling must be determined early in the design process.

Balancing the performance required by a specific fill material and the water chemistry of the process water are the significant factors in choosing the right fill and type of cooling tower for your project. The best fill type for your application, either film fill or splash fill, depends on biological growth potential and the level of suspended solids in your source water. Cooling tower manufacturers publish guidelines that can be used to help determine the quality of your process water source.

High-efficiency PVC film fill is typically used in cooling towers with clean water. This fill is manufactured in cross-corrugated sheets that stretch the falling water into a thin film on the surface of the PVC sheet. The water then interacts with the airflow through the tower to facilitate the heat transfer. Because more surface area for air-to-water contact is available, film fill types are more efficient than splash fills.

Film fill is not appropriate for all applications due to its higher propensity for clogging and fouling. Splash fill is more tolerant of dirty water sources but has lower thermal efficiency that requires a larger structure. This often makes it more costly than film fill type towers for a given load.

Clog-resistant film fill provides a happy medium between the efficacy of high-efficiency film fill and splash fill in both thermal performance and clog resistance.

Summary

Choosing between a crossflow and counterflow cooling tower for your application depends on the factors most important to your project specifications. Both types are effective means to support chillers and achieve efficient evaporative cooling with a few distinct design differences.

Crossflow and Counterflow Cooling Tower Distinctions

• Crossflow Advantages
• Capable of up to 70% turndown
• Performs well in cold-weather applications
• Water distribution system can be cleaned while tower is in operation
• More access for routine maintenance
• Uses high efficiency heat transfer fill

Counterflow Advantages

• Smaller footprint up to ~750 tonnage (with film fill)
• Potentially lower operating weight
• May be easier to install
• Accommodates wide range of fill types to address source water quality

About the Author

Eric Rasmussen is a senior product manager and licensed engineer at SPX Cooling Technologies, Inc., Overland Park, KS.

About SPX Cooling Technologies, Inc.

Contact us to discuss your requirements of cooling tower parts and functions. Our experienced sales team can help you identify the options that best suit your needs.

SPX Cooling Technologies, Inc. is a leading global manufacturer of cooling towers, evaporative fluid coolers, evaporative condensers, industrial evaporators and air-cooled heat exchangers providing cooling solutions, components and technical support for heating, ventilation and air conditioning (HVAC), refrigeration, and industrial process cooling applications for nearly a century. SPX Cooling Technologies and its product brands are part of SPX Corporation. For more information visit www.spxcooling.com.

To read similar Cooling Tower Technology articles visit https://coolingbestpractices.com/index.php/technology/cooling-towers.

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

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.

For more information, please visit air cooling system advantages and disadvantages.