Air vs. Water Cooled Chillers

Which chiller type is best for data centers: air cooled, or water cooled? There’s no single answer. HVAC system designers need to ask a lot of questions before advocating one type of chiller over another. The obvious equipment choice is sometimes over-ruled by unique project factors including the available square footage, backup power strategies, local regulations limiting power and water use, or the owner’s sustainability priorities.

Ideally, data centers will consult with the HVAC company on the front end of the design process to make sure the solution meets short- and long-term objectives. If you’re the one making the decision on behalf of a data center, it helps to understand the pros and cons that are inherent to each type of chiller.

Capacity: Packaged air-cooled chillers are typically available in sizes ranging from 7.5 to 500 tons [25 to 1,580 kW]. Packaged water-cooled chillers are typically available from 10 to 4,000 tons [35 to 14,000 kW]. In other words, water-cooled chillers can deliver higher cooling capacity with fewer units and a smaller footprint.

Maintenance: Air-cooled chillers eliminate the need for cooling towers. Water-cooled chillers require cooling towers, which have critical maintenance demands: water treatment, chiller condenser-tube cleaning, tower mechanical maintenance, and freeze protection. Systems that use open cooling towers must have a water treatment program to prevent contaminants such as bacteria and algae. Cooling towers also evaporate water so you will need access to a large supply of make-up water to replace the evaporated volume.

Location: The pursuit of cheap power and lower taxes is changing the face of the data center industry, placing huge facilities in remote locations far removed from the users. If placed in location where temperature drop below freezing, air-cooled chillers can be easier to operate , because they eliminate the problems associated with operating a cooling tower in severe winter conditions. Cooling towers may require special control sequences, basin heaters, or even an indoor sump for safe operation in freezing weather.

Energy efficiency: Water-cooled chillers are typically more energy efficient than air-cooled chillers. The refrigerant condensing temperature in an air-cooled chiller is dependent on the ambient dry-bulb temperature. The condensing temperature in a water-cooled chiller is dependent on the condenser-water temperature, which is dependent on the ambient wet-bulb temperature. Since the wet-bulb temperature is often significantly lower than the dry-bulb temperature, the refrigerant condensing temperature (and pressure) in a water-cooled chiller can be lower than in an air-cooled chiller. The lower condensing temperature, and therefore lower condensing pressure, means that the compressor needs to do less work and, subsequently, consumes less energy. This efficiency advantage may lessen during nighttime operation because the dry-bulb temperature tends to drop faster than the wet-bulb temperature when the sun goes down.

Delivery & Installation: Most air-cooled chillers are “packed systems.” The system, including the condenser, compressor, and evaporator, is designed and configured at the factory for optimal performance and reliability, which reduces design and delivery time and simplifies installation. Water-cooled chillers have the added complexities of condenser-water piping, pumps, cooling towers, and controls.

Longevity: With the continued technology advances and growth in more data processing, the data center infrastructure needs to be able to support these advancements while delivering consistent performance. In general, air-cooled chillers last 15 to 20 years while water-cooled chillers last 20 to 30 years.

Partially, it’s because water-cooled chillers are typically installed indoors and operate at lower condenser fluid pressure, while air-cooled chillers operate outdoors, at higher condenser pressure.

Water conservation: Water availability, cost, treatment requirements as well as potential additional construction complexity all play a role in system selection. Since air-cooled chiller do not require water, they are often a preferred choice especially in locations where there is a water shortage, or the water is very expensive.

Water is one of two major resources data centers consume [1] . Combined, US data centers were responsible for consumption of 626 billion liters of water in 2014, which includes both water consumed directly at data center sites and water used to generate the electricity that powered them that year. However, as the report points out, far more water is used to generate electricity that powers data centers than to cool them so chiller efficiency again comes back into consideration.

In summary, air-cooled chiller advantages include lower maintenance costs, a prepackaged system for easier design and installation, and better performance in freezing temperature. Water-cooled chiller advantages include greater energy efficiency, larger capacities, and longer equipment life. Yet, the best choice for the project may also depend on an entirely different set of decisions criterial. It’s important for data centers and system designers to take all factors into consideration to make sure the chiller that ultimately gets specified balances all the objectives over the long term.

[1] Sverdlik, Yevgeniy, DataCenter Knowledge, July 2016, “Here’s How Much Water All US Data Centers Consume.”  https://www.datacenterknowledge.com/archives/2016/07/12/heres-how-much-water-all-us-data-centers-consume