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.
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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.
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.
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.
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.
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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.
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.
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
Counterflow Advantages
About the Author
Eric Rasmussen is the Director of Product Quality and licensed engineer at SPX Cooling Technologies, Inc., Overland Park, KS.
About SPX Cooling Technologies, Inc.
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.
It’s important to regularly touch base on the basics of cooling towers to provide anyone new to the industry with vital information, but it’s also a good reminder for those who have been in the industry for many years. There are three basic flute geometry designs for modular film fills: cross flutes (CF), offset-vertical flutes (OF), and vertical flutes (VF). Each geometry has its own set of advantages and disadvantages in relation to fouling resistance and thermal performance.
Cross Flutes
Cross-fluted designs have been the industry standard for over 30 years. The nominal 30° from vertical flute orientation – 60° angle included between flutes on adjacent sheets – maximizes turbulence and air-water mixing, and high rates of heat transfer are created in relatively shallow fill sections (6’ and less).
Another important feature is this geometry’s ability to redistribute the falling water laterally so that a multi-layer fill section made up of alternating horizontal rows can fully “wet-out” the entire packed volume. This makes the cross-fluted geometry very thermally efficient but not very resistant to fouling. Because of the angled flutes, water film velocity is slowed and deposition of solids can readily occur. It is for this reason that Brentwood discourages CF fills in water that has a high degree of fouling potential.
Offset-Vertical Flutes
Like CF fills, the offset-vertical flute geometry allows for a high degree of air-water turbulence, and therefore, high heat transfer rates. A differentiating factor between the two fill types is that offset-fluted fills offer lower airside airflow resistance (pressure drop) than CF fills. The vertically oriented flutes allow for high water film velocity, thus allowing for a higher degree of fouling resistance than CF fills. Since the flutes are offset, the water can migrate laterally, as it does in CF fills, but to a lesser degree. For highly fouling water, however, OF designs should be considered only if achieving maximum thermal performance is required.
Vertical Flutes
Vertically oriented flute geometry allows the highest water film velocity and the highest degree of fouling resistance. New VF designs allow for high heat transfer rates but not as high as those of CF and OF designs. The high velocity water film significantly reduces the attachment of slime-forming bacteria, and therefore, the accumulation of silt. This type of media is recommended by Brentwood when the source water has strong fouling potential and thermal performance can be compromised.
The above references are for counterflow fill designs and their macro-structure. The same basics can be applied to film fills as well as for newer modular splash packs, or trickle packs. There has been a recent uptick in the requests for information regarding the fouling potential between a cross-fluted designed trickle pack and an offset vertical designed splash pack, like the HTP25. Even though these products may look very anti-fouling, the underlying principles of why packs foul is still at play. Cross-fluted designs still increase thermal performance at the expense of water velocity through the packs as compared to a vertically offset design due to the reduced water velocity and corresponding shear stresses.
Future blogs will dive deeper into fouling and we will be sharing some preliminary results from the Brentwood R&D fouling chamber, so keep an eye out!