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Air Cooling for Data Center: Utilization of Ambient Air as Primary Cooling Resource

1. Available Air-Cooling Resources and Applicable Operating Conditions for Data Center

In air-cooled data center designs, the suitability of ambient air for cooling is primarily influenced by outdoor dry-bulb and wet-bulb temperatures and their distribution throughout the year. Regions characterized by high latitude, high elevation, or temperate to cold climates benefit from extended periods of low ambient temperatures, making them particularly well suited for natural cooling solutions. From an engineering perspective, the utilization of air as a cooling resource can be categorized into several operating regimes:

• Low-temperature regime: When outdoor air temperatures are sufficiently lower than the required cooling water supply temperature, the cooling demand can be fully satisfied through air-side heat rejection, eliminating the need for mechanical refrigeration.
• Intermediate regime: As ambient air temperatures approach the upper limit of acceptable cooling water supply temperatures, system performance can be improved by increasing heat exchanger surface area, enhancing airflow rates, or incorporating evaporative assistance. These measures help extend the duration of compressor-free operation.

• High-temperature regime: Under extreme ambient conditions, such as peak summer temperatures or heatwave events, air-side cooling capacity becomes inadequate, and mechanical cooling systems are generally required to maintain continuous and reliable operation.

In air-cooled data center designs, the requirement for mechanical cooling systems is not sized based on average climate conditions, but rather on the most extreme hot-weather conditions experienced over the year. As a result, engineering system design and capacity validation focus on these worst-case conditions, while natural cooling strategies are employed for most operating hours to improve and maximize overall energy efficiency.

2. Typical Air-Cooling Approaches and System Configurations

When ambient air is used as the primary heat rejection medium on the source side, a range of air-based cooling approaches are commonly employed in data center systems. Although these approaches differ in system configuration, they all fundamentally rely on air–fluid or air–air heat exchange mechanisms to dissipate IT heat loads to the surrounding environment.

2.1 Dry Cooler–Based Systems

Dry coolers transfer heat from a closed-loop working fluid (water or glycol solution) to ambient air via finned heat exchangers and forced airflow. This configuration represents one of the most widely adopted air-based cooling solutions due to its simplicity and robustness.
In such systems, the facility water system and the technology cooling system are typically hydraulically separated by plate heat exchangers, enabling effective risk isolation and operational flexibility. Dry cooler–based solutions are well suited for cold or temperate climates and for applications with high reliability and low water dependency requirements.

2.2 Evaporative or Adiabatic Assisted Air Cooling

Evaporative assistance enhances air-side cooling performance by reducing the inlet air temperature through adiabatic humidification, such as spray systems or wetted media installed upstream of air-cooled heat exchangers.
This approach significantly improves cooling effectiveness during higher ambient temperature periods and extends the operating window of natural cooling. However, it also introduces additional considerations related to water consumption, scaling, biological fouling, and freeze protection, and is therefore typically applied as a performance enhancement rather than a standalone solution.

2.3 Direct Air Cooling

In suitable environments, outdoor air may be introduced directly into the data center to remove heat from IT equipment, eliminating intermediate heat transfer stages.
While this approach offers the shortest thermal path and the highest potential energy efficiency, its applicability is constrained by stringent requirements for air quality, humidity control, and environmental stability, limiting its deployment to select locations and operating scenarios.

2.4 Hybrid Air Cooling with Mechanical Refrigeration

To ensure year-round operability and compliance with availability requirements, air-based cooling systems are often integrated with mechanical refrigeration systems.
In this hybrid configuration, air-side cooling is prioritized during favorable ambient conditions, while mechanical cooling systems provide capacity assurance during extreme or unfavorable weather conditions, achieving a balance between energy efficiency and operational reliability.

3. Characteristics and Engineering Considerations of Heat Exchangers in Air-Cooled Systems

Despite differences in system architecture, heat exchangers represent the core components enabling the effective engineering utilization of ambient air as a cooling resource in all air-based data center cooling solutions.

3.1 Low air-side convective heat transfer

The inherently low convective heat transfer coefficient on the air side necessitates large heat transfer surface areas and relatively small approach temperature differences. This, in turn, directly impacts equipment footprint, fan power consumption, and the overall layout of data center cooling systems.

3.2 Functional decoupling via heat exchangers

In air-cooled data center systems, heat exchangers often perform a critical functional decoupling role, separating facility-side and IT-side cooling loops. Plate heat exchangers, in particular, are widely employed to provide hydraulic isolation, mitigate water quality risks, and enable independent optimization of operating temperatures and pressures on each side of the system.

3.3 Design for wide operating envelopes

Air-cooled heat exchangers must be engineered to handle wide operating conditions, including seasonal temperature fluctuations, frost and freeze risks, variable airflow, and partial-load operation. These requirements place stringent demands on thermal design margins, material selection, and control strategies, making heat exchanger performance a key determinant of the overall efficiency and reliability of data center cooling systems.