The need for cooling capacity is increasing globally. An expanding population and an ever-growing dependence on data increases the need for process cooling, centralized space cooling, and data center cooling. Meanwhile, in many places, water scarcity is a massive issue.
In conventional, industrial cooling applications, the use of water for heat rejection is critical. Cooling towers and most evaporative fluid coolers depend heavily on water to reject waste heat to the atmosphere.
Taking blowdown and drift losses into account, evaporative cooling systems use approximately 3 gpm of water per 100 tons of cooling capacity. A 1,000-ton industrial cooling application running around the clock consumes approximately 15.7 million gallons of water annually, assuming a constant year-round load. As ambient air temperatures rise globally and the climate becomes more severe, the demand for water increases.
Oftentimes, when facility managers find out how much water is consumed for an evaporative cooling system, they look to dry cooling systems as the answer. This seems like an idealistic response before becoming aware of the obstacles that dry cooling systems pose for large facilities.
Eliminating the use of water from the cooling process entirely dramatically increases total connected fan horsepower (and energy consumption), initial investment, and/or mechanical footprint — often to the point of being impractical or impossible.
So, is there a happy medium?
Adiabatic coolers are closed-circuit coolers that primarily operate dry. However, these coolers have the unique ability to supplement the cooling capacity of the equipment through the use of pre-cooling pads, which require small quantities of water for a finite period in the year. Typically used in the hottest summer months, these pads lower the dry bulb temperature of the incoming air, providing greater heat rejection from the heat exchanger coils. This allows the unit to provide sensible (dry) heat transfer for as long as possible while capitalizing on the latent (evaporative) heat transfer provided by the adiabatic pads once the unit’s dry cooling capacity has been exceeded.
These adiabatic pads, made of absorbent material, are dampened when the ambient conditions exceed the unit's ability to satisfy the cooling load. Water never comes in contact with the coil itself, greatly reducing unit maintenance and extending its life cycle. As incoming air passes through the damp pad, the dry bulb temperature of the air is depressed before coming in contact with the unit’s coil. This increases what’s called the “approach,” or the difference between entering air temperature and leaving fluid temperature, increasing the cooling capacity of the unit with very minimal water consumption.
There are benefits to dry and adiabatic cooling systems beyond water savings though. Water treatment chemicals are eliminated, maintenance is significantly reduced, and water utility tap fees are often reduced.
It’s also worth noting that there are areas in North America where the use of evaporative cooling systems has come under heavy scrutiny. For example, the Las Vegas Valley Water District Board of Directors recently approved a water conservation measure stating that, after Sept. 1, 2023, no new permits in the water district’s service area will be issued for commercial or industrial buildings that plan to use evaporative cooling. Replacing evaporative cooling systems with alternative technologies is also being incentivized by the Southern Nevada Water Authority.
The downside of installing an adiabatic cooling system in place of a conventional design is that, to meet the same capacity, adiabatic systems use more power and cover a larger footprint in addition to carrying a higher per-ton initial equipment cost.
For these reasons, facility managers, owners, and engineers must carefully examine a number of site-specific considerations during the early design phase of any cooling application — whether new or for replacement.
Facilities that gain the greatest advantage from adiabatic cooling systems are those that wish to remain in dry cooling operation as long as possible while supplementing their cooling capacity with small amounts of water. This may be the result of a maximum utility tap size, local restriction on water consumption, high cost of water, leaving fluid temperature requirements, high ambient conditions, or simply an environmental stance taken by the company.
This approach allows the dry portion of the system to be undersized for the total cooling load. The adiabatic portion of the system is then used to supplement capacity during peak ambient conditions, reducing the physical size and initial cost of the dry cooler(s).
Many cooling systems that serve processing or data center applications are designed for high return water temperatures; 100° to 110°F, instead of the 85° to 95° required by most air conditioning applications. Dry coolers do not transfer heat as effectively as evaporative coolers and can’t cost-effectively provide leaving fluid temperatures as low as evaporative systems. Because of this, dry coolers provide a sound solution for high-temperature cooling applications. Supplementing the dry system with an adiabatic component trims the peaks from the cooling load during the warmest days of the year.
Of course, there needs to be physical space sufficient for dry and adiabatic heat rejection. These units require more space per ton of cooling capacity than evaporative units.
Even that challenge, however, can be reduced to a degree. To mitigate the space challenge that dry and adiabatic systems can pose, some companies have released stackable units — offering more heat rejection capacity while occupying roughly half the space of a conventional model.
In addition to saving mechanical space, the stacked configuration reduces wiring and consumes slightly less water than a single stack unit of the same capacity.
“If cooling capacity is needed during the winter, we typically specify closed-circuit coolers,” said Craig Huston, principal of Huston Engineering, in Troy, New York.“If there’s no need for cooling capacity in the winter, we use open-loop cooling towers and a heat exchanger.”
That sounds simple, but it’s the leading consideration in Huston’s decision-making process when selecting between a fluid cooler or cooling tower.
“Running a hybrid fluid cooler — like an adiabatic system — in dry mode allows us to reject heat during the winter in dry mode, eliminating the risk of ice formation in and around the unit. Then, in the summer, the cooling load can be met with the adiabatic element.”
Aside from selecting the appropriate type of cooler, there’s a crucial design parameter called the “dry bulb switchover temperature” that must be specified to ensure the cooler can operate 100% dry during the cold months. The cooler will run in dry mode when the ambient outdoor temperature drops below the dry bulb switchover point for that specific cooler.
The dry bulb switchover temperature is determined by the surface area of the heat transfer coil and the airflow volume across the coil.
Huston and members of his engineering team calculate, then specify, a dry bulb switchover temperature. This results in the end user operating a fluid cooler that does not consume water in the winter,completely 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 with an evaporative unit. Water treatment and the cost of associated chemicals are also eliminated.”
Some m aintenance
While adiabatic systems require a great deal less maintenance than evaporative cooling units, it is still a consideration. Adiabatic pads are constructed of an impregnated cellulose material that resembles robust cardboard. The media is engineered to withstand wetting and drying cycles.
Adiabatic pad life expectancy is largely determined by the quality of water being used and the frequency with which the adiabatic component is energized. Site maintenance can take steps to extend pad life expectancy, like softening, descaling, washing, etc. But, pads are a degradable component of the system and have an average life cycle of five years. It’s worth noting that adiabatic systems do not require a sanitary drain, and some facilities opt to recirculate the water.
Cooling Technology Institute certification
It’s critical that business owners and mechanical engineers understand the importance of Cooling Technology Institute (CTI) certification, and that not all fluid coolers are CTI certified.
CTI certification provides independent, third-party validation of thermal performance claims made by heat rejection equipment manufacturers. Cooling tower test codes have been in place for decades, but only recently have dry coolers been added to CTI’s certification process. In 2018, CTI published an acceptance test code, ATC 105-DS to test the thermal performance of dry coolers. In September of 2022, CTI included dry coolers in Standard 201, the prevalent thermal performance certification standard that includes open cooling towers and evaporative fluid coolers.
Currently, the base dry performance of dry and adiabatic coolers is being certified by CTI, but the performance of adiabatic pad systems is not currently included in the test code. This is expected to change in 2023, as CTI is currently in the process of developing a test code for the adiabatic pad systems.
Specifying CTI-certified equipment relieves liability from the mechanical engineer and eliminates the need to build margin of error into the specification.
The dry and adiabatic cooling market is exploding globally. This is due to the rise in data center construction, the need for more process cooling, and accelerating demand to conserve water. In the U.S., the government is incentivizing domestic manufacturing, which increases the need for cooling domestically.
Demand for dry and adiabatic coolers certainly has not usurped the market for evaporative coolers because there’s more demand for cooling equipment than manufacturers can produce. That said, the increase in demand for water-sensitive cooling systems is rising at a staggering pace.
Many professionals in the cooling industry expect sweeping water use code changes within the next few decades. If this occurs — and current trends already allude to it — dry and adiabatic heat rejection is potentially future-proof.