Data centers rely heavily on the capability and optimal performance of heat rejection equipment being utilized to function efficiently. When conserving water is a primary goal, careful attention must be given to the heat rejection equipment being specified.

Data center cooling applications can be served by either an open-loop cooling tower or a closed-circuit cooler. The requirements of the application, location of the facility, local utility costs, water availability, and environmental considerations are the primary factors when choosing between the two.

When comparing open-loop (evaporative) cooling towers to closed-circuit coolers, the primary difference is where heat transfer occurs. Cooling towers generally contain PVC fill media where the recirculating water and air come in direct contact. The cooled water then collects in a basin and is pumped to a heat exchanger, chiller, or other equipment being cooled. When the system is in operation, some amount of makeup water is constantly required to replenish the water lost due to evaporation.

With closed-circuit coolers, also known as fluid coolers, the process fluid is never exposed to the atmosphere because it’s contained within the coil bundle. There is a secondary, local, recirculating water loop, which uses a pump located in the unit's basin to circulate water over the coil bundle. The water from the recirculating loop evaporates on the surface of the coil, removing heat from the process fluid. Because these two fluid loops are isolated by the coil bundle, the process fluid remains free of atmospheric contaminants, dirt, and debris.

Despite their similar exterior appearance, cooling towers and closed-circuit coolers are very different pieces of equipment with distinct advantages and limitations. This is especially true when determining which option is best suited for a specific cooling application.

Types of fluid coolers

It’s important to make a distinction among the types of closed-circuit coolers: fully evaporative, hybrid, adiabatic, and dry. Despite their operational differences, each offer the same primary advantages inherent to closed-circuit coolers — cleaner recirculating fluid, fewer components within the cooling system, and great potential to reduce water consumption.

Evaporative coolersare closest in appearance to open-loop cooling towers due to their size potential and their availability in induced- and forced-draft configurations. The process fluid is pumped through the coil bundles by the pumping system on-site, and there is a smaller recirculating pump mounted on the cold-water basin in the unit. This smaller pump recirculates water over the coil bundles as the fans pull ambient air across the bundles, resulting in evaporative heat rejection on the surface of the coils. A make-up valve in the unit’s basin replenishes the water lost by evaporation. Evaporative coolers can operate dry when the ambient dry bulb temperatures are low enough.

Hybrid coolers are evaporative coolers with finned coils to maximize heat transfer surface area and significantly increase the number of hours during the year the unit can achieve the site’s heat rejection requirements while operating completely dry. This means that the spray pump mounted on the unit can remain off for a greater portion of the year, while the fans draw ambient air across the finned coil bundles and cool the process fluid completely sensibly (dry). Once cooling demand exceeds the dry cooling capacity of the system, the spray pump is energized to wet the coil bundles. This increases the heat rejection capacity of the system by providing latent (evaporative) heat transfer. Some hybrid coolers feature an additional finned dry coil, which is piped in series with the finned evaporative coil.

This additional dry coil serves a couple of purposes.

1. The hottest water enters the dry coil prior to flowing into the evaporative coil, which means that a portion of the total heat load is rejected completely dry, even during the peak summer months. This reduces the total evaporation during wet operation and contributes to lowering the total annual water consumption greatly.
2. The dry coil adds sensible heat to the air that has picked up humidity from the latent cooling process occurring on the evaporative coil, abating plume in the process.

Dry coolers are precisely what the name implies. These systems contain coil bundles that provide only sensible cooling. There is no wet cooling element. Dry coolers are best suited for cooling applications where conserving water is the No. 1 priority. Dry coolers require significantly more connected fan power and heat transfer surface area relative to their evaporative and hybrid counterparts.

Adiabatic coolers are dry coolers with entering air pre-cooling pads. Once the cooling demand exceeds the dry cooling capacity of the system, a valve opens to dampen the adiabatic element. As incoming air passes through the damp pad, the dry bulb temperature of the air is depressed to approach the ambient wet bulb temperature before coming in contact with the unit’s coil.

This increases what’s called the “approach,” or the difference between entering air dry bulb temperature and leaving fluid temperature, increasing the cooling capacity of the unit significantly, or allowing the unit to provide leaving fluid temperatures lower than the ambient dry bulb. All this occurs with very minimal water consumption. Adiabatic units also deliver the required cooling capacity in a smaller footprint and/or lower fan motor horsepower than a dry cooler.

Specific benefits

If only the base units are compared, closed-circuit coolers have a larger footprint, heavier operating weight, and carry a larger initial cost than open-loop cooling towers of the same capacity. However, fluid coolers have numerous benefits for end users that are often overlooked. Over time, these benefits eclipse the difference in upfront cost. The priority of those benefits, some of which are listed below, is determined by site-specific needs and conditions.

Water conservation — An increasing consideration for many facilities today, closed-circuit coolers use less water than open-loop cooling towers. In some cases, they consume a great deal less water or no water at all. Depending on location, this can result in reduced or eliminated utility tap fees. Consider that, for example, the cost savings for data centers, which, on average, use roughly 3 to 5 million gallons of water per day.

Reduced water treatment — When water use is reduced or eliminated, so is the need for water treatment. Treating recirculating water for either a closed- or open-loop cooling system carries several expenses: the initial investment of the treatment system, the treatment chemical cost (if chemical in nature), the power to operate the treatment system, and the labor to service and maintain it. By eliminating or curtailing water use, the associated treatment costs fall accordingly.

Less maintenance, more uptime — Industrial processes benefit from having very limited downtime. Cooling systems that employ closed-circuit coolers typically do not require a heat exchanger between the heat rejection unit and the process loop because the fluid is isolated from the atmosphere. When uptime is critical, one less component that can fail — or one less component that requires service — becomes an advantage.

Round-the-clock, 24/7 operation — Data center cooling systems are unique from HVAC cooling applications in that the cooling demand fluctuates little throughout a 24-hour period. A closed-circuit cooler’s ability to operate dry, unlike a cooling tower, allows a hybrid or adiabatic system to capitalize on lower ambient temperatures overnight. Here too, water is only used once the system’s sensible cooling capacity has been exceeded. Depending on the specified capacity of the cooling system, nighttime ambient temperatures are often low enough to keep the system running in dry mode.

Cold climate performance — In a space cooling application, the heat rejection load falls dramatically over the winter or is eliminated altogether. This is not the case for data center cooling loads. Open-loop cooling towers are extremely difficult to maintain during winter conditions and often sustain damage as a result of freezing. Because closed-loop systems can operate dry, low ambient temperatures are an advantage, rather than a disadvantage. Also, if extreme cold is expected, propylene or ethylene glycol can be added to provide freeze protection.

Material selection — Several different materials can be used to manufacture the coil bundles within many closed-circuit coolers. This allows the coolers to be used in a wide variety of applications, including those in caustic environments. Hot-dipped galvanized steel is the most common coil material, but Type 304 and 316 stainless steel are also options. Stainless steel has much greater corrosion resistance. Facilities in coastal regions, for example, benefit from the use of stainless steel.

Reduced need for low water temperatures — Many cooling systems that serve data center applications are designed for high outlet water temperatures — 100° to 110°F instead of the 85° to 95° required by most air conditioning applications where evaporative cooling systems are typically installed. The higher the outlet water temperature that can be tolerated, the more opportunity there is to conserve water with a closed-circuit cooler.

Open loop versus closed circuit

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. There are a diverse number of cooler versus tower/heat exchanger cost comparison analyses available. These comparisons traditionally list costs under the following three categories when evaluating the capital investment of a cooling system.

The downside of these cost comparisons is that they can be arbitrary. Costs for equipment and services fluctuate around the country and around the world. Nonetheless, facility owners or specifying engineers 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 utilizing a closed-circuit cooler begins once the system is placed into service. Arguably the single most significant benefit of a fluid cooler, or at least the advantage that’s most frequently discussed, is its ability to “close the loop.” What does that mean?

Because a cooling tower’s recirculating water, or condenser water, is open to the atmosphere, anything that enters the tower from the surrounding area (leaves, pollen, dust, etc.) has the potential to enter the condenser portion of the cooling loop. This can leave the equipment vulnerable to deposition and fouling.

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.

Sealed systems

A fluid cooler contains heat transfer coil bundles that shelter the process fluid inside from environmental impurities. In effect, the heat exchanger has been moved inside the cooler. This provides the advantages of the heat exchanger without its inherent disadvantages.

Specifiers should avoid the temptation to use the same design conditions to compare a tower/heat exchanger to a cooler. Let’s look at the impact on the size and cost of the cooler if correct temperatures are excluded from the analysis. Figures 1 and 2 show the operating parameters an engineer or equipment sales representative will need to make a standard closed-circuit cooler selection.

Now that we’ve discussed the key differences between cooling towers and fluid coolers, let’s look at several examples of how fluid coolers have been applied to real-world projects across the country, and the specific design considerations that ledengineers to specify fluid coolers over cooling towers.

Case in point: A Colorado data center

TiePoint-bkm Engineering Inc. was awarded a data center design project in Centennial, Colorado. Senior mechanical engineer, Brian Deleon,and his team members began conducting cooling system analyses while comparing different implementation strategies. Ultimately, the driving force for their design was the city’s strict limitation of water use. Tap fees varied by size. A smaller tap would cost hundreds of thousands of dollars, while larger sizes would exceed $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 layout — a need to minimize mechanical space.

Deleon and his team were able to show the owner the value of hybrid closed-circuit coolers.

“The payback of not using water by running the coolers in dry mode was too significant to be ignored,” said Deleon.

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.

Each Evapco eco-ATWB-H at the data center contains a dry coil (Arid Fin-Pak) in the upper casing section that pre-cools the process fluid prior to being introduced into the evaporative coil below. The dry coil, on average, covers 20% of the heat load. This directly relates to water savings, as there is now less heat that needs to be rejected from the evaporative coil when the unit runs with the spray pump on.

In addition to increased water efficiency, the data center’s mechanical room and piping also benefited from TiePoint’s innovative design.

“We were able to reduce the footprint of the mechanical room due to the smaller pumps and piping required by the hybrid coolers,” Deleon said. “Also, 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.

Case in point: An Illinois injection molding process

Roger Sauter from Bullock Logan & Associates worked with Evapco’s design team and professionals at Ram Mechanical Services Inc. to design a heat rejection system serving a large, 24-hour injection molding process in northern Illinois.

“Their manufacturing facilities are loaded with open-loop molding machines that require year-round heat rejection,” said Sauter. “Any time year-round heat rejection is required in this area, there are huge concerns about freeze-ups causing downtime to production that must run three shifts without fail. The customer had many painful stories about their previous systems and failures, and they didn't want to go down that road again. Also, the owner needs to be environmentally responsible, and wanted to use as little water as possible for heat rejection.”

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

• Northern Illinois climate — freezing was a serious concern.
• Water efficiency — owners insisted on using as little water as possible for heat rejection.
• Zero downtime — the cooling system must operate continuously around the clock, with zero downtime.

Although there are heat rejection alternatives with a lower upfront cost, none of them could present a solution to the freeze risk or the need for reduced water use as well as an adiabatic system could.

“We chose to close the outside loop using a glycol solution,” said Sauter. “Using a heat exchanger, the outside loop is isolated from the open loop inside, which uses straight water. If the indoor loop had been closed, a heat exchanger would not have been necessary.”

To meet the cooling load at the manufacturing campus, both single- and double-stack cooler configurations were used. In addition to dramatically reducing water consumption, adiabatic systems do not require a treatment system for the water used by the adiabatic element.

“Of all our ideas, this one carried the largest initial cost, but the customer saw the benefits and wanted it,” said Sauter. “The outdoor heat rejection equipment only uses water for a small number of hours in the summer while producing outlet temperatures low enough for the process.”

Case in point: Utah State University

Utah State University’s main campus in Logan, Utah, is perched 4,500 feet 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 6,000 tons. During winter, when ambient temperatures can fall as low as minus 30°F, the campus cooling load drops by more than 90%. For roughly half the year, only server rooms and a constant temperature room in the library call for cooling capacity. 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.

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

• Reduced Cooling load — The cold climate, paired with the much lower cooling load during the winter months, meant that very little cooling capacity was needed for half the year.
• Northern Utah’s cold climate — The new cooling system was needed for cooling during the winter months, meaning that it could not incorporate evaporative or adiabatic elements.
• Energy and water savings — The primary goal of the cooling system expansion was to remove the cooling tower from service during the winter, but water and energy savings were also of interest.

“I reached out to rep firm Midgley-Huber Inc.,” said Reid Olsen, USU central energy plant manager. “They visited the site and collected load information before suggesting the installation of a dry cooler to handle the small winter cooling load.”

Ultimately, the decision was made to select an Evapco eco-Air series V-configuration dry cooler. 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.

“We stopped using the open-loop tower unless absolutely necessary,” continued Olsen. “We’re able to utilize the Evapco fluid cooler as our sole cooling source from October 1 through April 1. We’d been running the cooling tower unnecessarily for half the calendar year.”

Between October and April, the open-loop cooling tower system consumed 871 MW of electric energy. During this past season, which was a milder winter than usual, the fluid cooler and associated components used 691 MW; a full 21% energy reduction. Additionally, the new dry cooler provides untold maintenance and repair costs on the larger cooling tower, along with great reduction in water and water treatment expense.

Operation schedule and state regulations

“If cooling capacity is needed during the winter, we use closed-circuit coolers,” said Craig Huston, principal of Huston Engineering. “If there’s no need for cooling capacity in the winter, we use open cooling towers and a heat exchanger.”

That sounds simple, but it’s the leading consideration in Huston’s decision-making process when deciding between a fluid cooler or cooling tower.

“Running a hybrid fluid cooler in dry mode (no water flowing) allows us to reject heat in the winter while minimizing or eliminating the risk of ice formation in and around the unit,” he said.

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

Two things impact the DB switchover temperature:

1. Surface area of the heat transfer coil.
2. Airflow.

Huston and his team of engineers calculate, then specify a DB switchover temperature. This results in the end user operating a fluid cooler that does not consume water in the winter, therefore eliminating the risk of icing.

“Dry and adiabatic fluid coolers are becoming more attractive to end users in our area,” Huston said. “Our clients 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.”