May. 06, 2024
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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.
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 (3295kW), 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.
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.
Related links:For more information, please visit cooling tower splash fill.
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 a senior product manager 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.
To read similar Cooling Tower Technology articles visit https://coolingbestpractices.com/index.php/technology/cooling-towers.
Cooling towers are used to remove heat from water used in water-cooled applications. The heat is rejected to the atmospheric air that is passed through the system fill in a crossflow or counterflow method. A small percentage of the hot water from the system evaporates, cooling the remaining water which falls into the cold-water basin which is then pumped from the cold water basin to the system to absorb more heat in a continuous process.
It’s important to consider the efficiency of the cooling towers ability to reject heat for your project requirements. When you have the project requirements in the way of total tons, range and approach, then you can begin to analyze the cooling towers that would be appropriate for your water-cooled application.
Crossflow and Counterflow Cooling TowerInduced draft crossflow and counterflow cooling towers both offer advantages and disadvantages based on specific project requirements and site conditions.
The obvious difference is in the method by which the air moves through the cooling tower in relationship to the cooling water, which is indicated by its description of a being crossflow or counterflow.
With a crossflow tower the air travels horizontally perpendicular) to the water traveling vertically down to the cold basin. With a counterflow cooling tower, air travels in the opposite direction vertically upward against the stream of falling water.
Air is required in sufficient amounts to accomplish the heat transfer requirements at the design conditions and at varying amounts for less than design conditions. Consideration must be made for this volume of air and the space required in the design of the tower to ensure the proper volume of air effectively interacts with the cooling water.
Cooling Tower Footprint – Crossflow vs. CounterflowCooling tower manufactures will have different footprint thresholds where the vertically configured counterflow goes from the smaller footprint to the larger footprint. Because of the nature of the counterflow tower where the air and water interact vertically, this allows for a smaller footprint most of the time in towers around 800 tons or less. As cooling towers get larger than 800 tons, the advantage could switch to counterflow.
Site conditions will determine whether the physical size of the cooling towers footprint is more important, or any height restrictions imposed by site factors.
In order to keep the cooling tower operating for efficiently and for its useful life it’s important to provide periodic maintenance. This will require that you consider tower access as a factor in the design and location of the tower.
Crossflow Cooling Tower Maintenance and AccessAccess to the internal working parts various between the two designs. With the crossflow tower most of the internal parts are easily accessible as the fill material surrounds the exterior portions of the insides, allowing for access to the fan, motor, drift eliminators and cold-water basin. With larger crossflow cooling towers, the manufactures have an option to have ladders and platforms preinstalled for easy access to the motor and drive.
Counterflow Cooling Tower Maintenance and AccessAccess to the components of a counterflow tower are less friendly to the maintenance personnel because the overhead horizontal fill material provides a barrier between various components. The fan, motor, eliminators and spray system have limited access because they are located above the horizontal fill making then inaccessible from the cold-water basin.
Because of the crossflow towers larger footprint, it’s shipping and operating weight will often be greater than that of the counterflow tower. The bigger footprint of the counterflow tower can require additional structural support legs.
With the crossflow tower there is a hot-water basin above the fill that distributes the water that is pumped to the top of the tower. The water in the hot-water basin than makes its way through the distribution nozzles by gravity without the force of the pump. This allow for easier maintenance of the nozzles since the water is gravity fed and out of the air stream.
The design of the counterflow tower and the method by which the water descends in direct opposition to the vertically induced air, necessitates the need to pressurize the water distribution. Since the water is under pressure through the nozzles this requires that you shutoff the pump in order to clean or service the distribution system.
Another design and maintenance consideration are the method by which the water is distributed over the fill material.
With many jurisdiction and states increasing energy efficiency requirements, the use of variable flow increases energy savings. As the tonnage requirement drops it’s wise to have the tower vary is flow in order to save energy.
Capacity can be increased basically three different ways. One way is by increasing the footprint of the tower, the second method would be to increase its height, and the third way would be to increase the HP of the fan motor to get more CFM out of the same tower size before having to go to the next size tower. These are some of the decisions that will need to be made based on project conditions.
Crossflow cooling towers are better at turndown than counterflow because of the inherent features of their water distribution methods.
Cooling towers are rated based on standard conditions of 95ºF (35.0ºC) entering water temperature to an 85ºF (29.4ºC) leaving water temperature at a 78ºF (25.6ºC) entering wet-bulb temperature. This correlates to 3 GPM of water per nominal ton. For non-standard conditions seek the assistance of your local cooling tower sales representative and their access to tower sizing software.
Cooling tower manufactures offer options on the type of fill based on factors such as the condition or chemistry of the water and the potential for biological growth. It’s important to prevent scaling which will reduce the capacity and ability of the tower to efficiently reject heat.
The use of PVC for fill material is common due to its low cost and effectiveness in clean water applications. If the water quality is not satisfactory than the use of splash fill might be required which will also increase the cost due to it be less efficient at heat transfer, which will also increase the size of the tower to meet the same project requirements. The use of splash fill is less prone to clogging due to dirty water as opposed to PVC Film which can clog more easily.
There are anywhere from one or more fans that provide for the air movement through the towers. The most energy efficient being the axial fan which sits at the top of the tower. The use of centrifugal fans are more commonly used in forced draft fluid coolers.
Your project requirements and site conditions will determine the best option of either a crossflow or counterflow cooling tower including any consideration for maintenance.
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