Closing the loop: Understanding the benefits of fluid coolers

by arslan_ahmed | July 20, 2023 8:00 pm

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By Mihir Kalyani

The cooling tower is perhaps the most broadly and globally recognized single component of a building cooling system. This has led to the incorrect use of the term “cooling tower” to refer to pieces of equipment that provide both latent and sensible heat transfer in buildings.

When comparing open loop cooling towers to closed-circuit coolers, the heat transfer occurs at different locations. Cooling towers generally have polyvinyl chloride (PVC) fill media where the recirculating water and air come in direct contact. The cooled water then collects in a basin and is pumped directly to a chiller, heat exchanger, etc.

Closed-circuit coolers, also known as fluid coolers, have a heat transfer coil bundle(s) containing the process fluid. Water, and sometimes air, comes in direct contact with the coil, but not the process fluid inside the coil. Therefore, the process fluid remains free of atmospheric contaminants such as dirt and debris.

Cooling towers and closed-circuit coolers are very different pieces of equipment, and while they typically look similar and accomplish the same result, they have distinctive advantages and drawbacks.

A closed-circuit cooler has a larger footprint, heavier operating weight, and is more expensive than the well-known open loop cooling tower (if only the base units are compared). However, fluid coolers have numerous benefits for the end user that are often overlooked.

Proponents of fluid coolers often make the point that, while the units are more expensive than open loop cooling towers, a holistic cost comparison generally shows a similar initial investment.  Most cooling equipment manufacturers can provide comparisons between fluid coolers and tower/heat exchanger systems. Traditionally, these comparisons costs fall under the three categories when evaluating the capital investment of a cooling system:

• Equipment costs, such as fluid cooler, cooling tower heat exchangers, pumps, filtration, control panels, etc.
• Rigging, installation, and piping costs such as valves, piping, heat tracing, sensors, and even labor.
• Operation and maintenance costs such as energy consumption, equipment cleaning and maintenance, water treatment, etc.

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The downside of these cost comparisons is they can be arbitrary. Costs for equipment and services fluctuate around the country and around the world. Nonetheless, building owners or mechanical contractors will find a marginal difference between the total cost of purchasing and installing a system with a fluid cooler, versus a cooling tower with a heat exchanger.

The future cost savings of a system using a closed-circuit cooler begin once the system is placed into service. Arguably, the single most significant benefit of a fluid cooler, or at least the advantage most discussed, is its ability to “close the loop.”

What does “closing the loop” mean? Since a cooling tower’s recirculating water, or condenser water, is open to the atmosphere, anything entering the tower from the surrounding area (e.g. leaves, pollen, dust, etc.) has the potential to enter the condenser portion of the chiller—for example, leaving the equipment vulnerable to deposition and fouling. In this example, the chiller’s condenser tubes and the fill media inside the tower are two heat transfer surfaces in an open loop tower system that does not include a heat exchanger.

Conversely, in the case of a closed-circuit fluid cooler, the condenser water loop is sealed to the atmosphere, eliminating the concern and maintenance associated with open loop systems.

Open loop tower and chiller

Figure 1 depicts a system used for a hospital cooling application. In this example, the “open loop” cooling tower is located on the roof with the condenser water piped directly to the chiller.

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The equipment’s service life expectancy and heat transfer efficiency depend largely on the cleanliness of these surfaces. This requires deliberate routine maintenance, water treatment, and filtration for all operational practices to be working in perfect harmony.

Open loop tower and heat exchanger

One strategy used by design engineers to protect the chiller or water source heat pumps (WSHPs) from contaminants within the tower’s recirculating water is to install a plate-and-frame heat exchanger between the cooling tower and chiller or WSHPs, as depicted in Figure 2.

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This example shows a cooling tower system installed at high-rise condominiums using WSHPs.  While installing a heat exchanger achieves the intended purpose of isolating the chiller or heat pumps from the cooling tower, its addition comes with some deficiencies, such as:

• Tight spaces between the plates increases the risk
  of fouling.
• High pressure drops due to the restricted flow path between the plates.
• Heat exchanger approach lowers the system’s heat transfer efficiency.

In this type of system, the water sent to the WSHP 31 C (88 F) from the heat exchanger will be hotter than the tower supply water 29 C (85 F) due to the approach across the heat exchanger.  Consequently, the cooling tower must be oversized to maintain WSHP efficiency levels achieved in a system that lacks a heat exchanger.

Closed circuit fluid cooler

Figure 3 illustrates the use of a closed-circuit cooler paired with WSHPs, though this design would work just as well in a chiller application.

The fluid cooler contains a heat transfer coil bundle(s) which shelters the process fluid inside from environmental impurities. In effect, the heat exchanger has been moved inside the evaporative unit—this provides the advantages of the heat exchanger without its inherent disadvantages.

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The building water loop in Figure 3 is now supplying 31 C (88 F) water directly from the cooler to the WSHP. One will notice the supply water temperature coming from the cooler is -16 C (3 F) higher. This is acceptable because there is no efficiency loss across a plate-and-frame heat exchanger.

One should not simply use the same design conditions to compare a tower/heat exchanger to a cooler. Looking at the impact on the size and cost of the cooler to see if correct temperatures are excluded from the analysis, Table 1 and 2 demonstrate the operating parameters the engineer will need to make a standard closed-circuit cooler selection.

Notice the differences between fluid cooler one versus fluid cooler two:

• 36 percent bigger
• 31 percent heavier
• 25 percent higher in total connected heat pumps

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Real-world examples

Following the key differences, downfalls, and advantages of cooling towers and fluid coolers, and delving into several examples of how fluid coolers have been applied to real-world projects across the country, and the specific design considerations that led engineers to specify fluid coolers over cooling towers.

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Hybrid fluid coolers lower data center operating costs

Hybrid fluid coolers are closed-circuit coolers that can operate with the spray pump off and with only the fan running. While these units look similar to traditional fluid coolers, they have the enormous advantage of being able to operate without the use of water. The coil sits in the air stream so dry capacity can be provided simply from air passing over the coil. They are most often specified when water savings and efficiency are of utmost importance, or when cooling capacity is still needed within a facility when outdoor temperatures are low enough to present the risk of freezing.

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Case study: Data Center

When TiePoint-bkm Engineering Inc. was awarded the project to design a data center in Centennial, Colorado, the senior mechanical engineer, Brian Deleon, and his team members began conducting cooling system analyses and comparing different implementation strategies. Ultimately, the driving force for their design was the city’s strict restrictions on water use—tap fees varied by size. A smaller tap would cost several hundred thousand dollars, while larger sizes would cost more than $1 million.

The following key factors were taken into consideration during the design phase:

• Colorado’s climate—mild summers and cool/
  dry winters.
• Water efficiency—heat rejection equipment capable of running dry.
• Plant payout—freeing up more space in the mechanical room.

The team was able to show the owner the value of hybrid closed circuit coolers. One key factor that could not be ignored was the payback of not using water by running the coolers in dry mode.

Whether it was at night or during the colder seasons, having the flexibility to run the hybrid cooler dry provided substantial initial cost savings
by reducing the tap size for incoming city water.

In addition to increased water efficiency, the data center’s mechanical room and piping also benefited from its innovative design. The engineering firm was able to reduce the footprint of the mechanical room due to the smaller pumps and piping required by the hybrid coolers. Further, heat transfer efficiency is better protected as the entire piping system is not exposed to the atmosphere by the recirculating water.

Even in the event of a water shortage or power hit, the responsiveness of the hybrid coolers and their ability to provide cooling while running completely dry gives this critical facility more resilience in case of emergency.

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Case study: New School

When DLR Group, an integrated design and engineering firm with offices across the U.S., was hired to provide HVAC system design for a new school in Nebraska, its mechanical engineering department looked to a hybrid fluid cooler for heat rejection.

One of the main reasons the use of a fluid cooler was suggested over a traditional cooling tower is because eastern Nebraska has true shoulder seasons. In a climate like this, a heat sink that can operate wet or dry has a real advantage during part-load situations, or when the wet bulb temperature is lower. A heat sink dramatically conserves water and reduces sewer expenses.

A two-cell series fluid cooler was installed for the 14,400 m² (155,000 sf) building. It is set up to provide 281 tonnes (310 tons) of cooling capacity in wet mode, but is capable of 317 tonnes (350 tons), allowing for some future capacity.

The fluid cooler at the new school maintains operation in dry cooling mode for as long as possible, providing sensible cooling to the process fluid within the coil. When the temperature set point can no longer be met during dry operation, the fluid cooler recirculating spray pump is energized, and evaporative cooling is activated. The fluid cooler has the capability of running completely dry while providing 100 percent capacity of the design load with an outdoor dry bulb air temperature of 7.05 C (44.7 F).

By eliminating the need for an open loop cooling tower and its associated components, it offers a substantial maintenance advantage as there is only one piece of equipment to maintain, instead of several.

In this case, first cost was not overlooked either. The total cost for the installed system for cooling towers and fluid coolers of similar capacity are comparable. The systems at the school have performed admirably, occupant comfort is high,
and the school sees desirable operating efficiencies.

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Operation schedule and state regulations

Adiabatic coolers function similarly to dry cooling systems, but with the addition of pre-cooling pads. Water runs over porous media while air is drawn through the pads, lowering the dry bulb temperature of the incoming air; this provides greater heat rejection. Adiabatic units also deliver the required cooling capacity in a smaller footprint and/or lower fan motor horsepower than a completely dry cooler.

If cooling capacity is needed during the winter, closed-circuit coolers are used. If there is no need for cooling capacity in the winter, open cooling towers and a heat exchanger are used.

This simple criterion is the leading consideration in many engineering firm’s decision-making process when deciding between a fluid cooler or cooling tower. Running a hybrid fluid cooler in dry mode (no water flowing) offers the ability to reject heat in the winter, while minimizing or eliminating the risk of ice formation in and around the unit.

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Aside from selecting the appropriate cooler style, there is a crucial design parameter called “the dry bulb switchover temperature” which must be specified to ensure the cooler can run 100 percent dry during colder months. The cooler will run in dry mode when the ambient temperature outside drops below the dry bulb switchover point for that specific cooler.

Two things impacting the dry bulb switchover temperature are surface area of the heat transfer coil and airflow. This results in the end user operating a fluid cooler that does not consume water in the winter, and therefore eliminates the risk of icing.

Dry and adiabatic fluid coolers are becoming more attractive to end users as they can save time and money by not having to register the unit, or test the recirculating water for Legionella, as they would for an evaporative unit such as an open cooling tower or fluid cooler.

Case study: Utah State University

As the name implies, dry coolers avoid the use of water altogether. In a dry cooler, heat from the process loop fluid dissipates through the coil tube surface and comes out to the fins—not through evaporation. Ambient air is drawn across the coil surface by a fan located at the top of the unit. Heat from the process fluid transfers to the air via sensible cooling and discharges to the atmosphere. Dry coolers are best specified where water conservation, reduced maintenance, and eliminating the risk of freezing are the key considerations.

Utah State University’s (USU’s) main campus is perched 1,371 m (4,500 ft) above sea level and is subject to very harsh winter conditions.

The campus district cooling system is served by an open-loop cooling tower capable of 5,443 tonnes (6,000 tons). During winter, when ambient temperatures can fall as low as -34 C (-30 F), the campus cooling load drops more than 90 percent. For roughly half the year, only server rooms and a constant temperature room in the library call for cooling capacity.

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During this time, the cooling towers at USU would freeze solid, building up so much weight that the unit sustained damage. After a decade of this, several of the cells required complete fill media replacement at a tremendous expense, despite efforts to combat the issue.

Ultimately, the decision was made to select a configuration dry cooler. The unit’s competitive price, the availability of stainless-steel construction, correct physical dimensions, and the provision of maximum cooling capacity made it the winning combination.

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Today, the new dry cooler is utilized for “free cooling.” When the ambient air temperature is low enough, the chillers can be turned off and the dry cooler can satisfy the reduced cooling requirement.

The fluid cooler can be used as the university’s sole cooling source from October through April, further eliminating the need to run the cooling tower unnecessarily for half the calendar year.

Between October and April, the open-loop cooling tower system consumed 871 megawatts of electric energy. During this past season, which was a milder winter than usual, the fluid cooler and associated components used 691 megawatts, resulting in a full 21 percent energy reduction.

Moreover, maintenance expenses decreased as well. In addition to reducing damage to the cooling tower and the wear and tear on chillers, using the dry fluid cooler reduced labour hours needed to monitor and tend to the cooling tower system. It also reduced the amount of water and water treatment chemicals needed for the open-loop tower.

Conclusion

The advancement of coil technology and adiabatic pad efficiency, combined with water saving initiatives and Legionella regulations, have catapulted the popularity of fluid coolers.

Closed-circuit coolers lend themselves to higher efficiency and water conservation. Reducing or eliminating chemical use is an environmental and safety consideration, and as new data centers are installed in cooler climates, new cooling solutions are needed. Engineers and facilities management professionals should consider the use of fluid coolers as part of economical, lower-maintenance cooling system designs, whether that be hybrid fluid coolers, adiabatic coolers, or dry coolers.

Author

Mihir Kalyani is global product manager for dry and adiabatic coolers at EVAPCO. His role involves driving the growth and development of EVAPCO’s dry and adiabatic product line, in addition to educating consulting engineers, end users, and sales representatives on the applications and benefits of the products he manages. Kalyani holds a mechanical engineering degree from the University of Maryland, College Park.

Source URL: https://www.constructionspecifier.com/understanding-the-benefits-of-fluid-coolers/

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